# Catalytic Approaches for CO 2 Conversion to Value-Added Products: An Overview of Life Cycle Assessment Studies

Anastasia Pappa, Cuong Pham-Huu, Spiros Papaefthimiou, Spyridon Zafeiratos

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Anastasia Pappa, Cuong Pham-Huu, Spiros Papaefthimiou, Spyridon Zafeiratos. Catalytic Approaches for CO 2 Conversion to Value-Added Products: An Overview of Life Cycle Assessment Studies. Advanced Energy and Sustainability Research, 2025, 6 (8), 10.1002/aesr.202400399. hal-05226797

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# Catalytic Approaches for CO₂ Conversion to Value-Added Products: An Overview of Life Cycle Assessment Studies

Anastasia Pappa, Cuong Pham-Huu, Spiros Papaefthimiou, and Spyridon Zafeiratos*

The urgent need to address climate change has driven efforts to develop sus-
tainable strategies for environmental mitigation. Among these, the catalytic and
electrocatalytic conversion of CO₂ into value-added products using renewable
energy holds significant promise. E-fuels, produced through heterogeneous cat-
alytic processes involving CO₂ and renewable hydrogen, exemplify this potential,
offering sustainable alternatives. Life cycle assessment (LCA) is a critical tool to
evaluate the environmental impacts of CO₂ utilization technologies, providing a
comprehensive analysis of a broader sustainability metrics. This review synthesizes
findings from selected LCA studies, focusing on the environmental impacts of CO₂
conversion processes, particularly those utilizing heterogeneous catalysis and
electrochemical reduction. The goal is to provide practical insights and recom-
mendations to help technology developers identify pathways with the lowest
environmental impact and optimize sustainable CO₂ utilization technologies. It is
highlighted that despite the widely recognized advantages of CO₂-based processes,
the environmental benefits cannot be guaranteed, while the carbon intensity of the
electricity source used significantly affects the outcomes. The review identifies
possible improvement strategies associated with electricity sources, CO₂ capture
methods, catalytic processes, and H₂ production pathways, as the electrification of
the chemical sector shows great potential for enormous greenhouse gas (GHG)
emission mitigation despite the emerging challenges.

**1. Introduction**

Global warming and climate change have raised concerns in
recent decades and prompt both industries and the scientific
community investigations to combat environmental degradation.

A. Pappa, C. Pham-Huu, S. Zafeiratos
Institute of Chemistry and Processes for Energy
Environment and Health (ICPEES)
ECPM, UMR 7515 CNRS-University of Strasbourg
25 rue Becquerel, 67087 Strasbourg Cedex 02, France
E-mail: spiros.zefiratos@unistra.fr

S. Papaefthimiou
School of Production Engineering and Management
Technical University of Crete
73100 Chania, Greece

<figure><img src="image_2.png" /></figure>

The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/aesr.202400399.

© 2025 The Author(s). Advanced Energy and Sustainability Research published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aesr.202400399

In this context, CO₂ catalytic or electrocatalytic conversion to value-added products—such as commodity chemicals and fuels—using renewable energy emerges as a highly promising strategy. Utilizing CO₂ as a feedstock instead of treating it as waste not only reduces greenhouse gas (GHG) emissions but also paves the way for a new era of a low-carbon economy.[1] When paired with CO₂ derived from direct air capture (DAC), the environmental benefits are even more significant, potentially leading to carbon-neutral processes.[2,3]

E-fuels, or electrofuels, are a special
category of synthetic fuels produced using
electricity generated from renewable
resources. Typically consists of hydrocar-
bons in either liquid or gaseous form,
created through heterogenous catalytic pro-
cesses involving CO₂ and H₂ derived from
water electrolysis. Examples of these fuels
are e-methane, e-methanol, e-ethanol,
e-gasoline, and e-diesel. By integrating
electricity generated from renewable
resources such as wind, solar, hydropower,
or even nuclear energy, e-fuels appear as a
promising solution toward sustainability

in the energy sector, especially in regions with abundant access
to them. The concept of a circular economy highlights the impor-
tance of using the resources that we already possess, turning
even the most damaging ones, like CO₂, to valuable products
maximizing their possible environmental benefits. However,
the feasibility and the actual environmental benefits of such prac-
tices are still in question, as the need for new infrastructure and
the limitations associated with this type of energy pose certain
challenges.[4,5]

In this framework, catalytic processes are explored even fur-
ther, as they give the opportunity to chemically transform CO₂
into various products that can later be used as fuels, chemicals
or other materials. In this way, it is possible to minimize our
dependency on fossil fuels and to mitigate our GHG emissions
potentially achieving even negative ones. This would mean new
prospects both for environmental and economic growth as new
greener markets will develop to produce but also to promote
these products.[1]

Life cycle assessment (LCA) becomes an indispensable
method to measure the environmental performance of these pro-
duction processes and to evaluate their actual feasibility. By mea-
suring the emitted CO₂ and quantifying the results in various
impact categories, we can have an accurate representation of
how each product influences the environment, offering us a more holistic view. With LCA, it is thus possible to comprehend each stage of the life cycle of a product, from raw material acquisition to production, use, and end-of-life. The step-by-step framework of LCA allows for the assessment of all phases of a product separately, indicating specific parts of the process that need to be modified to achieve optimized environmental benefits.[6,7] Early LCAs published in 2013 highlighted the potential environmental benefits of using captured CO₂ as a feedstock for chemical synthesis compared to traditional fossil-fuel-based processes.[8,9] Since then, significant progress has been made across a broad spectrum of emerging technologies related to CO₂ utilization. These advancements span various domains, including CO₂ sourcing, H₂ production efficiency, electricity generation, and processes for converting CO₂ into value-added products. The vast number of possible combinations creates a highly complex landscape, making it challenging to identify the most beneficial configurations and conditions. These combinations must address not only CO₂ emission reductions and climate change mitigation but also other less widely considered impact categories related to human health and environmental sustainability.

The goal of this article is to provide insights and recommendations derived from selected LCA studies, helping to identify the pathways with the lowest environmental impact in terms of GHG emissions and other LCA categories. This review does more than summarize the latest studies in the field; it takes a distinctive approach by carefully selecting works that include detailed descriptions of the electricity sources, chemical process, and catalytic systems involved in the LCA analyses. Particular emphasis is placed on heterogeneous catalytic conversion processes, specifying details such as catalyst types whenever possible, information vital for technology developers and process engineers. Such critical details are often omitted in similar reports,[10–13] which focus on the environmental impact of e-fuels without thoroughly analyzing the chemical aspects of the processes. Conversely, some studies primarily examine production technologies while overlooking the life cycle of the chemicals produced.[14] Additionally, some reviews prioritize the economic evaluation of the technologies[11,15] treating environmental consequences as a secondary concern.[16]

The organization of this review also differs from prior works, as it categorizes findings based on the desired CO₂ conversion products and provides an overview of the associated chemical and catalytic processes. Whenever possible, conventional production methods are compared with the emerging technologies, to assess whether the environmental performance aligns with expectations. Additionally, methodological choices in selected examples are clarified to help readers visualize the key factors influencing reported GHG emissions estimates.

Overall, this article primarily targets technology developers working on catalysts and catalytic processes, who may not be experts in LCA methodologies. It seeks to discuss the environmental impacts associated with each CO₂ conversion process individually aiming to bridge the gap between LCA findings and practical applications, offering valuable guidance for advancing sustainable technologies and further process optimization.

## 2. Key Details in the Approach used for this Review

For the bibliographic research and data collection, the article utilizes three platforms: “Web of Science”, “Google Scholar”, and “Scopus”. Specific keywords were used to narrow down all the necessary and suitable studies for this review article: (“LIFE CYCLE ASSESSMENT” OR “LCA”) AND (“CO₂ CATALYTIC CONVERSION”), (“LIFE CYCLE ASSESSMENT” OR “LCA”) AND (“E-FUELS” OR “ELECTRO-FUELS”), (“LIFE CYCLE ASSESSMENT” OR “LCA”) AND (“P2G” OR “P2L” OR “P2X” OR “PtG” OR “PtL” OR “PtX”). We initially identified more than 100 results, which we refined to 49 based on criteria aligned with our review objectives, specifically requiring that the articles provide details on the catalytic process considered in the LCA. Since the primary goal was to focus specifically on LCAs of chemicals and fuels derived exclusively from catalytic CO₂ conversion—excluding all non-catalytic methods—we ultimately selected a total of 47 LCAs for consideration.

The sustainability of the CO₂ conversion process heavily relies on the electricity grid, making the electricity mix a crucial factor in LCA calculations. The term electricity mix (or power generation mix) refers specifically to the various sources used to produce electricity. It should not be confused with the energy mix, which describes all energy sources used to meet a region's total energy needs, including transportation, industry, and housing. Evidently in countries like Sweden and France, where more than 90% of the electricity mix comes from low-carbon sources[17] (mainly nuclear and renewable sources), the CO₂ conversion using electricity is more sustainable. Figure 1a illustrates that the majority of LCA studies used in this article focus on the European electricity mix, which is also the geographical location with the highest share of electricity mix from low-carbon sources.[17]

Figure 1b presents the distribution of various chemicals synthesized from CO₂ using heterogeneous catalytic processes. Methane-based products, synthetic fuels derived from carbon monoxide and H₂ mixtures via the Fischer–Tropsch synthesis (FTS) and primary alcohols, are central to LCA studies. This reflects both the practical production importance and alignment with current research interests in CO₂ conversion technologies, a trend further corroborated by the volume of publications and research patterns highlighted in a recent review on CO₂ conversion.[18]

To better understand the relationships among the various LCAs included in this review, a bibliometric analysis was conducted using VOSviewer software.[19] Figure 2a presents a network visualization of keyword co-occurrences within the selected papers. The network consists of 24 keywords, chosen from the most relevant terms cited in the articles and meeting a minimum threshold of three occurrences. Each keyword is represented as a node, with node size proportional to its frequency. Links between nodes illustrate keyword co-occurrence within the cited literature, with link thickness indicating the degree of association between connected keywords. Related nodes are classified into 4 clusters with distinct colors. The keywords found in the same cluster are more closely related and frequently co-occur within the analyzed articles. The terms “life cycle assessment”
<figure><img src="image_3.png" /><figcaption>**Figure 1.** a) Geographical distribution map indicating the continents of origin for the electricity mix used in LCA studies on catalytic CO₂ conversion to chemicals. b) Percentage breakdown of various chemicals and fuels produced via catalytic CO₂ conversion analyzed in the 49 LCA studies.</figcaption></figure>

and “CO₂ capture and utilization” are central across all articles, as might be expected, followed by “hydrogen”, “power-to-gas”, “energy”, “fuels”, and “methanol”. This suggests that most LCA studies on CO₂ valorization focus primarily on fuel production, with significantly fewer studies addressing applications where CO₂ is used as feedstock in processes aimed at producing commodity chemicals.

Figure 2b illustrates the network of links between countries, providing insight into bibliographic coupling among them. The map reveals a strong scientific presence from Italy, Germany, Canada, England, China, and the USA, each contributing numerous studies. Line thickness indicates the frequency of connections, with the USA, Italy, and England showing particularly strong bibliographic coupling. However, it appears that not all leading countries in the field are interacting in the same manner. Thus, although Germany and Canada show similar levels of research activity, they interact less with each other, appearing in separate clusters, whereas the USA and China exhibit a stronger interconnection. The map also highlights that the USA, Italy, Germany, South Korea, and Switzerland are highly interconnected, suggesting potential collaborations or shared research interests.

**Table 1** provides an overview of the works included in this review, summarizing critical information about the CO₂ conversion processes and the LCA methodologies employed in each study. Key details about the products, renewable energy sources, reference processes or energy inputs, catalysts used, regional

context, functional units (FU), databases, system boundaries, software tools, and calculation methodologies are systematically presented to facilitate comparison and analysis.

### 3. Purpose and Methodology of Life Cycle Assessment Studies

LCA evaluates the environmental aspects and potential impacts of a product, such as resource consumption and the environmental effects of pollutant emissions, throughout its life cycle.[20] A comprehensive LCA considers all stages, including raw material extraction and preprocessing (cradle), transportation of materials to the manufacturing site, component production, system assembly, transportation to market (gate), the use phase, and end-of-life processes. End-of-life may involve transporting used equipment to a waste treatment facility, such as a landfill (grave) or recycling/material recovery (back to cradle). LCA studies can vary in scope, and thus, they can be 1) partial, focusing on specific lifecycle phases like cradle-to-gate; 2) semicomplete, including landfilling or partial recycling (i.e., cradle-to-grave); or 3) complete, covering all phases and incorporating material upcycling based on circular economy (CE) principles, i.e., cradle-to-cradle.[21] LCA has been extensively applied to evaluate the environmental impacts of energy production from both renewable and nonrenewable sources.[22-24] These studies are typically conducted following the guidelines established by the
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) VOSviewer co-occurrence map combined with unit analysis identifying keywords in the articles cited in this review. b) VOSviewer bibliographic coupling with unit analysis identifying countries.</figcaption></figure>
<table><caption>Table 1. Overview of key information on CO₂ conversion processes and the corresponding LCA methodologies utilized in the studies reviewed in this article.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO₂ source</th><th>Main CO₂ conversion process</th><th>Reference process/ energy</th><th>H₂ source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>Syngas</td><td>German mix 2050,</td><td>DAC</td><td>SOEC</td><td>Small-scale SMR</td><td>-SOEC</td><td>Ni/YSZ</td><td>Germany</td><td>1 kg of syngas produced</td><td>Ecoinvent 3.5 and GaBi Professional 9.2 database</td><td>Cradle-to-gate</td><td>GaBi 9.2.0.58</td><td>Recipe version 2016 v1.1 Midpoint (Hierarchist-)</td><td>[34]</td></tr><tr><td>Syngas</td><td>US mix, Wind</td><td>-</td><td>SMR and rWGS</td><td>-</td><td>SRM</td><td>-</td><td>USA</td><td>1GJ</td><td>Ecoinvent 3.6 database</td><td>Cradle-to-gate</td><td>SimaPro 9.0</td><td>-</td><td>[36]</td></tr><tr><td>e-MeOH</td><td>Hydro, solar, wind</td><td>Flue gases</td><td>rWGS, CO MeOH synth. CO₂ MeOH synth. vs EC-MeOH</td><td>Industrial</td><td>AEL</td><td>Cu-Zn-Al-Zr and CuGa₂</td><td>Global</td><td>1 kg MeOH</td><td>Ecoinvent database</td><td>Cradle-to-gate</td><td>SimaPro 9.0</td><td>ReCiPe 2016 (H)lv 1.03</td><td>[47]</td></tr><tr><td>e-MeOH</td><td>Eu-Mix (30% renewable), PV</td><td>Flue gases</td><td>rWGS, CO MeOH synth. CO₂ MeOH synth. vs EC-MeOH</td><td>Industrial</td><td>AEL</td><td>CuO/ZnO/Al₂O₃</td><td>EU</td><td>1 g MeOH</td><td>Ecoinvent 3. Agrifootprint</td><td>Cradle-to-gate</td><td>Simapro 8</td><td>-</td><td>[52]</td></tr><tr><td>e-MeOH</td><td>Canada local and total mix</td><td>Flue gases</td><td>CO₂ MeOH synth., DRM, TRM</td><td>Industrial MeOH synth. from syngas</td><td>PEM</td><td>Cu/ZnO/Al₂O₃</td><td>Canada</td><td>-</td><td>-</td><td>Cradle-to-gate</td><td>GREET v1.30 (2019.net)</td><td>-</td><td>[54]</td></tr><tr><td>e-MeOH</td><td>Canada local and total mix</td><td>-</td><td>E-CRM, TRM, CO₂ MeOH synth.</td><td>Industrial MeOH synth. from syngas</td><td>-</td><td>Fe-based</td><td>Canada</td><td>-</td><td>Ecoinvent</td><td>-</td><td>OpenLCA version 1.11.0</td><td>Tool for Reducing and Assessing Chemical and Other Environmental Impacts (TRACI) and ReCiPe</td><td>[55]</td></tr><tr><td>e-MeOH</td><td>Solar heat</td><td>-</td><td>Solar thermo-chemical</td><td>Industrial MeOH synth. from syngas</td><td>-</td><td>Cu/ZnO/Al₂O₃</td><td>-</td><td>-</td><td>-</td><td>Cradle-to-gate</td><td>-</td><td>-</td><td>[56]</td></tr><tr><td>SAF, e-diesel, e-MeOH, gasoline from MeOH, DME</td><td>Solar heat</td><td>DAC</td><td>Solar thermo-chemical</td><td>Biofuels, e-fuels</td><td>-</td><td>CuO/ZnO/Al₂O₃</td><td>Spain, Chile Marocco</td><td>1 GJ of fuel</td><td>Ecoinvent 3.8 database</td><td>Well-to-wheel</td><td>-</td><td>-</td><td>[57]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/ energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>e-MeOH to formalin, DME, MTBE, biodiesel</td><td>EU+grid-mix, wind, hydropower</td><td>Flue gases</td><td>CO<sub>2</sub> MeOH synth.</td><td>—</td><td>—</td><td>—</td><td>EU</td><td>1 ton of MeOH derived product</td><td>GaBi database</td><td>Cradle-to-gate</td><td>GaBi, version 8</td><td>ReCIPe</td><td>[58]</td></tr><tr><td>e-MeOH, e-CH<sub>4</sub>, gasoline, diesel</td><td>—</td><td>Flue gases, DAC</td><td>CO<sub>2</sub> MeOH synth., Sabatier, FTS, SOEC co-electrolysis</td><td>—</td><td>AEL</td><td>Ni catalyst</td><td>USA, Canada, Germany, Australia, and Japan</td><td>—</td><td>—</td><td>Cradle-to-gate</td><td>CREET</td><td>—</td><td>[59]</td></tr><tr><td>e-MeOH, DME</td><td>PV</td><td>Not considered in the LCA</td><td>CO<sub>2</sub> MeOH synth., CO<sub>2</sub> DME synth.,</td><td>SMR-H2, gasoline and diesel produced without renewable energy</td><td>Not specified</td><td>Ni/Rh/Al<sub>2</sub>O<sub>3</sub> and Pd/ZnO/Al<sub>2</sub>O<sub>3</sub> for DME and Co/ZnO/Al<sub>2</sub>O<sub>3</sub> for MeOH</td><td>Netherlands</td><td>Energy content of each fuel, i.e., 1 GJ of fuel produced</td><td>Ecoinvent database (v.3.3, 2016)</td><td>Cradle to grave, from cradle-to-factory gate plus end-of-life</td><td>Gambi 5</td><td>TRACI, CML, EDIP, Ecoindicator</td><td>[60]</td></tr><tr><td>e-CH<sub>4</sub></td><td>Wind, PV EU mix</td><td>—</td><td>Sabatier</td><td>SRM, oil refining</td><td>Ni-based</td><td>Ni-based</td><td>EU-27 countries</td><td>1 MJ of H<sub>2</sub> or CH<sub>4</sub></td><td>GaBi Professional database</td><td>Cradle-to-gate</td><td>GaBi 5 (PE International, GaBi Version 5, 2013)</td><td>TRACI, CML, Ecoindicator, EDIP</td><td>[61]</td></tr><tr><td>e-CH<sub>4</sub></td><td>PV, wind, Swiss mix, EU mix</td><td>Flue gases, DAC, biogas</td><td>Sabatier</td><td>wholesale electricity market</td><td>PEM, AEL</td><td>Not specified</td><td>Switzerland</td><td>1 MW ht of combustion heat</td><td>Ecoinvent database version 3.1</td><td>—</td><td>Simapro</td><td>ILCD 2011 Midpoint</td><td>[67]</td></tr><tr><td>e-CH<sub>4</sub></td><td>PV, wind, Swiss mix, EU mix</td><td>Flue gases, DAC, biogas</td><td>Sabatier</td><td>—</td><td>PEM, AEL</td><td>Ni-based</td><td>Switzerland</td><td>1 MJ of H<sub>2</sub>, 1 km distance traveled by compressed NG, 1 kWh of electricity</td><td>Ecoinvent</td><td>—</td><td>SimaPro 8.0.4.30</td><td>ILCD 2011 MidPoint</td><td>[68]</td></tr><tr><td>e-CH<sub>4</sub></td><td>PV</td><td>—</td><td>Sabatier</td><td>Portuguese electricity mix</td><td>—</td><td>Ni-based catalyst</td><td>Europe</td><td>1 mole of methane produced per hour</td><td>Not specified</td><td>Cradle-to-gate</td><td>Simapro V8.5.2 PhD version</td><td>ILCD 2011</td><td>[69]</td></tr><tr><td>e-CH<sub>4</sub></td><td>Wind</td><td>Flue gas</td><td>Sabatier</td><td>traditional NG production</td><td>PEM</td><td>Ni/MgAl<sub>2</sub>O<sub>4</sub></td><td>Germany</td><td>Production of 1 GJ fossil-based natural gas</td><td>Ecoinvent v3</td><td>Cradle-to-gate</td><td>SimaPro v9</td><td>Recipe Midpoint</td><td>[70]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>e-CH<sub>4</sub></td><td>PV, wind, South Africa mix</td><td>flue gases, DAC</td><td>Sabatier</td><td>Present and future electricity mixes of South Africa</td><td>–</td><td>AEL, PEM, SOEC</td><td>Ni-based catalyst</td><td>Europe and Africa</td><td>1 MJ</td><td>USDA database</td><td>Cradle-to-gate</td><td>OpenLCA</td><td>TRACI, CMI, EDIP, Ecoindicator</td><td>[72]</td></tr><tr><td>e-CH<sub>4</sub></td><td>PV, on- and off-shore wind</td><td>DAC</td><td>SOEC co-electrolysis and syngas methanation</td><td>–</td><td>SOEC</td><td>Ni/MgAl<sub>2</sub>O<sub>4</sub></td><td>Ni-based catalyst</td><td>North America and Asia Pacific region</td><td>1 MJ of renewable methane</td><td>Ecoinvent version 3.8</td><td>Cradle-to-gate</td><td>SimaPro version 9.4</td><td>ReCiPe 2016</td><td>[73]</td></tr><tr><td>e-CH<sub>4</sub></td><td>Onshore wind farms, Italian electricity mix</td><td>flue gas</td><td>Sabatier</td><td>Fossil and bio CH<sub>4</sub> production</td><td>PEM</td><td>Ni-based catalyst</td><td>Italy</td><td>1 kg of methane</td><td>Ecoinvent 3.5 cut-off mode</td><td>Cradle-to-gate</td><td>–</td><td>The Environmental Footprint (EF) 2.0</td><td>[74]</td></tr><tr><td>e-CH<sub>4</sub></td><td>Onshore wind farms</td><td>CLC from coal</td><td>Sabatier</td><td>Italian electricity mix</td><td>–</td><td>Ni/Al<sub>2</sub>O<sub>3</sub></td><td>Italy</td><td>o 1 kWh of electricity generated and 0.49 kg of methane product</td><td>Ecoinvent database, version 3.5 cut-off model</td><td>–</td><td>Not specified</td><td>Environmental Footprint (EF) 2.0</td><td>[78]</td></tr><tr><td>e-CH<sub>4</sub></td><td>Spanish renewable mix 2030</td><td>CLC of biomass</td><td>Sabatier</td><td>Electricity generated from fossil fuels</td><td>PEM</td><td>Not specified</td><td>Spain, Germany and EU</td><td>An electrolyzer powered by surplus energy from 500 MWe of renewable sources for a year.</td><td>GaBi database</td><td>Cradle-to-grave</td><td>GaBi 9.5 Pro</td><td>GaBi database</td><td>[77]</td></tr><tr><td>e-CH<sub>4</sub>, e-NH<sub>3</sub></td><td>PV, Italian mix</td><td>CO<sub>2</sub> and N<sub>2</sub> from flue gas</td><td>Sabatier and Haber-Bosch</td><td>Conventional electricity grid usage</td><td>PEM</td><td>Fe-based (Haber-Bosch) and Ni/Zr/Al<sub>2</sub>O<sub>3</sub> (Sabatier)</td><td>Italy</td><td>1 kg of treated flue gas</td><td>Ecoinvent</td><td>Cradle-to-gate</td><td>SimaPro v8.4.0.0</td><td>Cumulative Energy Demand (CED) assessment method.</td><td>[79]</td></tr><tr><td>e-CH<sub>4</sub>, e-NH<sub>3</sub></td><td>Wind, German mix 2020</td><td>CO<sub>2</sub> and N<sub>2</sub> from flue gas</td><td>Sabatier and Haber-Bosch</td><td>German 2020 electricity mix</td><td>PEM, AEC, SOEC</td><td>Fe-based (Haber-Bosch) and Ni-based (Sabatier)</td><td>Germany</td><td>Total amount of valorizable flue gas produced from NG powerplants in Germany in 2019</td><td>Ecoinvent v3.5</td><td>Cradle-to-gate</td><td>SimaPro v9.2.</td><td>–</td><td>[80]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>SAF, e-naphta, kerosine, gasoil, baseoil</td><td>PV</td><td>DAC</td><td>FTS</td><td>GTL and BTL as reference technologies</td><td>CuO/ZnO/Al<sub>2</sub>O<sub>3</sub> for RWGS for FTS</td><td></td><td>Spain</td><td>1 MJ of liquid fuel</td><td>Ecoinvent 2.2</td><td>Gate to end use</td><td>CMLCA version 5.2 software</td><td>CML2001 impact methodology</td><td>[90]</td></tr><tr><td>SAF, naphtha, diesel, kerosene</td><td>German mix, Saudi Arabia mix, Wind, PV</td><td>–</td><td>FTS</td><td>–</td><td>SOEC</td><td>Not specified</td><td>Germany</td><td>Gram CO<sub>2</sub> equivalent per MJ of Diesel</td><td>version 3.1 of the ecoinvent database</td><td>–</td><td>OpenLCA version 1.6.3</td><td>CML 2001, CED</td><td>[91]</td></tr><tr><td>SAF, e-diesel, gasoline, kerosene, light hydrocarbons, and waxes</td><td>Wind, PV, nuclear, grid mix</td><td>DAC, flue gas</td><td>FTS</td><td>Fossil diesel production</td><td>PEM</td><td>Co/Al<sub>2</sub>O<sub>3</sub></td><td>–</td><td>1 kg of FT e-diesel.</td><td>Ecoinvent v.3.5.</td><td>Cradle-to-gate</td><td>SimaPro v9.2</td><td>ReCiPE 2016 v1.03</td><td>[92]</td></tr><tr><td>SAF</td><td>Wind, nuclear, Norwegian import electricity</td><td>DAC</td><td>FTS</td><td>UK grid</td><td>AEL</td><td>Co-Pt/γ-Al<sub>2</sub>O<sub>3</sub></td><td>UK</td><td>1 MJ of SAF</td><td>Ecoinvent 3.6</td><td>Well-to-wake</td><td>SimaPro V.9.3.0.3</td><td>–</td><td>[93]</td></tr><tr><td>SAF</td><td>German mix 2015, wind, PV</td><td>DAC</td><td>FTS</td><td>Fossil-based Jet A-1 fuels</td><td>SOEC, PEM</td><td>–</td><td>Germany</td><td>1 MJ</td><td>Sphera's LCI database, GaBi database, 2019 version, 2020 GaBi database through correspondence with Sphera</td><td>Well-to-wake</td><td>GaBi version 9</td><td>–</td><td>[94]</td></tr><tr><td>SAF, kerosene, diesel, and naphtha</td><td>Nuclear</td><td>CO<sub>2</sub> from bio-EtOH plants</td><td>FTS</td><td>Fossil-based low-sulfur diesel and jet fuel</td><td>SOEC</td><td>BaCeO<sub>2</sub>ZrO<sub>2</sub>Y<sub>0.16</sub>ZnO<sub>0.04</sub>O<sub>3</sub> for RWGS and a Co-based catalyst for FTS, Pr/SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> for hydrocracking</td><td>Braidwood and Prairie Island</td><td>1 MJ of FT-fuel energy (LHV)</td><td>–</td><td>Well-to-wheel</td><td>GREET 2022</td><td>–</td><td>[95]</td></tr><tr><td>e-CH<sub>4</sub>, syncrude</td><td>Wind, thermal power plant</td><td>flue gas</td><td>Combined P2L and P2G</td><td>–</td><td>AEL, PEM, SOEC, AEM</td><td>Fe-based catalyst</td><td>–</td><td>10 000 m<sup>3</sup> of flue gas input the CO<sub>2</sub> capture unit</td><td>–</td><td>Cradle-to-gate</td><td>–</td><td>–</td><td>[97]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/ energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>Shipping fuels (e-CH<sub>4</sub>, e-NH<sub>3</sub>, e-MeOH, biofuels)</td><td>PV</td><td>DAC</td><td>Sabatire FTS, Haber-Bosch</td><td>—</td><td>PEM</td><td>BaCe<sub>0.2</sub>Zr<sub>0.8</sub>Y<sub>0.16</sub>Zn<sub>0.04</sub>O</td><td>Global</td><td>GHG emissions, including CH<sub>4</sub>, N<sub>2</sub>O and CO<sub>2</sub> Per kWh</td><td>—</td><td>Well-to-tank, and tank-to-wake,</td><td>GREET</td><td>—</td><td>[101]</td></tr><tr><td>Shipping fuels (e-MeOH, e-NH<sub>3</sub>, and waste-derived biofuels)</td><td>Wind</td><td>—</td><td>Sabatire FTS, Haber-Bosch</td><td>Fossil fuel oil</td><td>—</td><td>Ni-based catalyst</td><td>—</td><td>The propulsion energy requirement of the global fleet over 1 year, amounting to 5.3 EJ year<sup>-1</sup></td><td>Ecoinvent 3.7.1</td><td>—</td><td>Sphere LCA for Experts software</td><td>ReCiPe 2016 Hierarchical methodology</td><td>[102]</td></tr><tr><td>Shipping fuels (e-CH<sub>4</sub>, e-MeOH, and e-diesel and advanced biofuels)</td><td>renewable energy sources, EU mix 2050</td><td>—</td><td>Sabatier, FTS</td><td>Conventional fossil fuels</td><td>PEM, AEL</td><td>—</td><td>Germany, Europe, USA, Switzerland</td><td>1 MJ</td><td>Ecoinvent 3.8</td><td>Well-to-wake</td><td>—</td><td>—</td><td>[103]</td></tr><tr><td>Shipping fuels (e-MeOH, e-NH<sub>3</sub>)</td><td>Onshore wind, EU mix, Swedish and, German mixes</td><td>—</td><td>—</td><td>Fossil-based combustion engines</td><td>—</td><td>TiO<sub>2</sub></td><td>EU</td><td>One round trip of a RoPax vessel from Gothenburg to Kiel</td><td>Ecoinvent 3.7.1</td><td>Cradle to grave</td><td>—</td><td>Environmental Footprint 3.0 method</td><td>[104]</td></tr><tr><td>e-CH<sub>4</sub>, e-MeOH to polymers</td><td>Wind, German electricity mix</td><td>DAC, Flue gas</td><td>P2G and P2L followed by polymer synthesis</td><td>Fossil-fuel based processes</td><td>Not specified</td><td>Ni-Fe catalysts</td><td>Germany</td><td>1 kg CH<sub>4</sub>, syngas, or MeOH, 1 kg of POM, PE, or PP for polymers</td><td>Ecoinvent 3.1</td><td>Cradle-to-gate</td><td>OpenLCA 1.4.1</td><td>—</td><td>[105]</td></tr><tr><td>e-CH<sub>4</sub>, e-MeOH, e-diesel to polymers</td><td>Hydro, PV, nuclear, geothermal (for heat), wind, PV, Swiss, and EU mix (for electricity).</td><td>DAC</td><td>P2G and P2L and SOEC coelectrolysis, followed by polymer synthesis</td><td>Electricity originating from a coal plant</td><td>PEM, SOEC</td><td>Ni-based catalyst from 19% nickel and 81% aluminum alloy</td><td>Europe</td><td>For polymers: kg of polymer produced and used. For transportation: passenger kilometer (or pkm). For</td><td>Ecoinvent v3.8</td><td>Cradle-to-grave</td><td>GREET model</td><td>—</td><td>[106]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/ energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>e-diesel</td><td>Hydro-dominated grids, Nuclear, Wind, PV</td><td>DAC</td><td>FTS</td><td>—</td><td>PEM</td><td>Cobalt-based catalyst</td><td>Canada, France, USA, Argentina</td><td>power generation: 1 kW h of electricity<br>Per ton (t) of CO<sub>2</sub> 162 captured from the atmosphere and per megajoule (MJ) of synthetic fuel combusted.</td><td>Ecoinvent</td><td>Cradle-to-grave</td><td>SimaPro 8.0.2</td><td>—</td><td>[112]</td></tr><tr><td>e-diesel</td><td>PV</td><td>anaerobic digestion</td><td>FTS</td><td>—</td><td>—</td><td>Not specified</td><td>UK</td><td>1 L of liquid fuels</td><td>Ecoinvent 2.2</td><td>Cradle-to-grave</td><td>GaBi V6.4</td><td>CML 2001 method</td><td>[113]</td></tr><tr><td>alkane-alcohol blends (HyFIT)</td><td>Biomass/ electricity from today's EU mix, wind power, global mix 2030</td><td>DAC and biomass</td><td>hydroformylation and rWGS, DRM, and Sabatier</td><td>Fossil diesel and BEVs</td><td>PEM</td><td>Iron-based catalyst/iron-based</td><td>Europe</td><td>The provision of 1 km of transportation in a heavy-duty van</td><td>ecoinvent</td><td>Well-to-wheel</td><td>—</td><td>Environmental Foot-print 3.0 method</td><td>[114]</td></tr><tr><td>e-CH<sub>4</sub>, e-MeOH, e-DME/OME,</td><td>Wind and solar PV electricity</td><td>—</td><td>—</td><td>Conventional Method</td><td>—</td><td>Cu/Zn/Al-based catalyst/zelite/ iron or cobalt-based catalyst</td><td>Europe</td><td>vehicle kilometers (vkm), heavy trucks (middle, ton-km (lkm)), and aircraft (right, passenger-km (pkm))</td><td>Ecoinvent. Life Cycle Inventory Database, v3.7</td><td>Cradle-to-grave</td><td>GREET</td><td>—</td><td>[111]</td></tr><tr><td>Formic Acid</td><td>PV, wind farms</td><td>—</td><td>—</td><td>—</td><td>—</td><td>Ru and phosphine-based catalysts</td><td>Europe (Spain?)</td><td>1 kg of FA</td><td>Ecoinvent 3.3 database</td><td>Cradle-to-grave</td><td>GaBi Professional</td><td>—</td><td>[132]</td></tr><tr><td>Formic Acid</td><td>PV</td><td>—</td><td>—</td><td>Fossil-based synthesis</td><td>—</td><td>Not specified</td><td>—</td><td>1 kg of HCOOH (85% wt purity).</td><td>Ecoinvent 3.9 database</td><td>Cradle-to-grave</td><td>OpenLCA 1.11.0</td><td>ReCiPe 2016 v1.1 Midpoint (H)</td><td>[118]</td></tr></tbody></table>
<table><caption>Table 1. Continued.</caption><thead><tr><th>Products</th><th>Energy source</th><th>CO<sub>2</sub> source</th><th>Main CO<sub>2</sub> conversion process</th><th>Reference process/ energy</th><th>H<sub>2</sub> source</th><th>Catalyst or electrocatalyst</th><th>Region</th><th>FU</th><th>Database</th><th>Boundaries</th><th>Software</th><th>Calculation Methodology</th><th>Reference</th></tr></thead><tbody><tr><td>e-EOH</td><td>Hydro, solar, wind</td><td>DAC</td><td>EC-EOH</td><td>Grain fermentation</td><td>–</td><td>Carbon-based</td><td>Queensland, Australia</td><td>1 kg of ethanol 95% m/m</td><td>Australian Life Cycle Initiative (AusCl) VI 35.</td><td>Cradle-to-gate</td><td>SimaPro version 9.0.0.48</td><td>CML-IA 2016, version 4.7, ReCiPe Midpoint hierarchist 2016 version 1.03</td><td>[121]</td></tr><tr><td>e-EOH</td><td>Hydro, PV, wind, EU mix</td><td>–</td><td>EC-EOH</td><td>Thermochemical and the biochemical processes</td><td>–</td><td>Cu catalyst</td><td>Denmark</td><td>1 kg of ethanol with 99.5 wt% purity</td><td>Ecoinvent v3.8 database</td><td>Cradle-to-gate</td><td>SimaPro version 9.3.0.2</td><td>ReCiPe 2016 Midpoint methodology</td><td>[122]</td></tr><tr><td>Oxymethylene ethers (OMEx)</td><td>Hydro/solar</td><td>DAC, biomass</td><td>Multistep catalytic processes</td><td>Condensation with formaldehyde, fossil diesel</td><td>PEM</td><td>Copper/zinc oxide-based catalysts</td><td>China/EU</td><td>1 kg or 1 L of produced OMEx fuel</td><td>ecoinvent</td><td>Cradle-to-gate</td><td>–</td><td>ReCiPe 2016</td><td>[124]</td></tr><tr><td>dimethyl ether (DME), OMEs and MeOH</td><td>German mixes in 2021 and 2030, Wind</td><td>Flue gas</td><td>Multistep catalytic processes</td><td>Fossil diesel</td><td>AEL</td><td>CuO/ZnO/Al<sub>2</sub>O<sub>3</sub></td><td>Germany</td><td>1 L diesel-equivalent</td><td>GaBi professional database/ ecoinvent 3.9.1 version</td><td>Well-to-tank</td><td>GaBi</td><td>EF3.1 methodology</td><td>[125]</td></tr><tr><td>Ethylene</td><td>Singapore mix, wind</td><td>Flue gas</td><td>EC-C<sub>2</sub>H<sub>2</sub></td><td>–</td><td>–</td><td>n.a.</td><td>CuO-derived Cu catalyst</td><td>Singapore</td><td>–</td><td>Cradle-to-gate</td><td>–</td><td>–</td><td>[128]</td></tr><tr><td>Ethylene, Formic Acid,</td><td>Wind, PV, hydro, biomass</td><td>–</td><td>EC-C<sub>2</sub>H<sub>2</sub></td><td>Conventional Processes</td><td>n.a.</td><td>Cu-based electrocatalyst</td><td>n.a.</td><td>Global</td><td>1 kg FA and 1 kg ethylene</td><td>Ecoinvent 3</td><td>SimaPro 9</td><td>ReCiPe 2016</td><td>[129]</td></tr><tr><td>Ethylene</td><td>PV and wind</td><td>DAC</td><td>EC-C<sub>2</sub>H<sub>2</sub></td><td>Conventional method</td><td>n.a.</td><td>Cu-based electrocatalyst</td><td>n.a.</td><td>–</td><td>Provision of 10 000 tons of C<sub>2</sub>H<sub>4</sub> year<sup>-1</sup></td><td>IDEA Ver. 3.2.0</td><td>IDEA</td><td>–</td><td>[130]</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Typical methodological stages of a Life Cycle Assessment. Reproduced with permission.[20] Copyright 2022, Elsevier.</figcaption></figure>

International Standardization Organization (ISO) and more specifically in ISO 14040 and ISO 14044.[20] The LCA process is performed iteratively through four distinct phases, as outlined in **Figure 3**.

The first phase involves establishing the goal and scope of the study, as well as selecting the functional unit (FU) and defining the system boundaries. The FU serves as the basis for comparing similar products by focusing on the overall product function rather than a specific physical property. Typically, the FU is time bounded and accounts for the expected duration of use and desired quality under specific conditions. The clear definition of system boundaries is critical, as it determines the type of LCA being conducted, whether it follows a cradle-to-gate, cradle-to-grave, or cradle-to-cradle approach.[25]

The second phase involves creating an inventory of all energy, materials, and environmental inputs and outputs throughout the product's life cycle analysis (Life Cycle Inventory, LCI). Data collection includes identifying and quantifying resource use, emissions, and production processes. This data is then scaled to the predetermined functional unit for the system under study. However, challenges such as limited data availability and quality are common, particularly in studies involving nonstandardized procedures. Other obstacles include geographic variations in raw material quality, energy sources, production methods, and associated environmental impacts.[26]

The third phase of an LCA focuses on assessing the potential environmental impacts of the inputs and outputs identified during the inventory analysis (Life Cycle Impact Assessment, LCIA). Impacts are grouped into categories and converted into environmental impact scores using characterization factors, which can be derived at either the midpoint or endpoint level.

Midpoint indicators address specific environmental issues, such as climate change or acidification, while endpoint indicators provide aggregated impacts across three broad categories: 1) human health, 2) biodiversity, and 3) resource scarcity. This phase plays a key role in assessing sustainability within an LCA study.[27]

The last step involves interpreting and synthesizing the results from the inventory analysis and impact assessment to inform decision-making. This step includes comparing findings with previous studies to ensure alignment with the literature. Sensitivity analysis may also be conducted to verify the consistency and robustness of the results. The ISO framework emphasizes the iterative nature of this process. If the findings are insufficient for drawing conclusions, earlier LCA steps may need to be revisited and refined until the study's goals are fully met.[28]

Depending on the scope of the LCA study, the life cycle stages of energy production systems may include some or all of the following: 1) fuel consumption (including the nonconsumable portion of the produced fuel) and its transportation to the plant, 2) facility construction, 3) facility operation and maintenance, and 4) dismantling. LCA is the preferred methodology for comparing the environmental performance—both strengths and weaknesses—of various energy technologies, including renewable energy systems. The core concept of adopting a life cycle perspective in power generation is to recognize that environmental impacts are not solely due to the power production process but also stem from the production chains of components, materials used, energy carriers, and associated services. By employing LCA, a product is analyzed throughout its entire life cycle, that is, from cradle-to-grave.
Conducting an LCA for renewable energy systems, for example, requires careful planning in areas such as goal and scope definition, selection of the functional unit reference system, defining system boundaries, and accurately inventorying and allocating emissions for products and by-products. The primary aim of LCA is to support and educate stakeholders involved in the development of a product, enabling them to make better-informed decisions. LCA is commonly used to identify hot spots in a process, compare improvement scenarios for an existing product, design new products, select the best option from multiple alternatives, and more.[21]

## 4. General Concept of CO₂ Catalytic Conversion to Value-Added Chemical and Fuels

Current CO₂ conversion technologies typically involve three main stages: CO₂ capture, H₂ generation, and CO₂ catalytic or EC conversion into value-added products. In the CO₂ capture unit, CO₂ is captured either directly from the air (DAC) or from sources like biogas and industrial flue gases using various capture technologies. In the H₂ generation unit, water (H₂O) is split into H₂ and O₂ using renewable electricity through four primary electrolysis methods: alkaline electrolysis (AEL), anion exchange membrane (AEM) electrolysis, polymer exchange membrane (PEM) electrolysis, and solid oxide electrolysis cell (SOEC). AEL, AEM, and PEM cells operate at low temperatures (below 90 °C) and are thus referred to as low-temperature electrolysis, while SOEC operates at around 800 °C, making it a high-temperature electrolysis technology. Figure 4a–d provides a schematic representation of the main components and basic operation principles of each cell, while Figure 4e highlights the main technical characteristics of each technology.

In the majority of LCAs reviewed in this article, the CO₂ utilized in the processes is captured either from ambient air or from flue gases generated by chemical or energy industry

plants. Figure 5 summarizes and categorizes the various CO₂ capture technologies, highlighting the primary materials and processes employed in each. A detailed overview of current CO₂ capture technologies can be found in two recent comprehensive review papers;[29,30] therefore, no additional technical details will be provided here.

In the CO₂ transformation unit, two heterogeneous catalytic processes are used in most cases. Power-to-gas (P2G) and power-to-liquid (P2L) pathways are based on the so-called Sabatier (or methanation) and FTS heterogeneous catalytic reactions, respectively (for more details, please refer to the relevant section of this article). Typically, these reactions are preceded by another heterogeneous catalytic reaction, the reverse water-gas shift (rWGS) reaction, which converts CO₂ to CO, as CO is significantly more reactive in subsequent synthesis processes. Alternatively, CO₂ can be converted to CO through EC methods: CO₂ electroreduction, which produces pure CO, and the coelectrolysis of CO₂ and H₂O, yielding a mixture of CO and H₂ known as synthesis gas or syngas. This syngas serves as a valuable feedstock for fuel synthesis processes. By carefully selecting the reaction conditions (pressure, temperature, etc.) and primarily the type of catalyst, other chemicals like methanol can be produced from the same feedstocks. It is worth noting that petrochemical refineries produce a relatively small number of chemical building blocks, primarily short-chain olefins and aromatics. These compounds, along with synthesis gas, serve as the foundation for the carbon family, which comprises more than 100 000 chemical substances found in final products.[31]

The review is organized into seven sections distinguished according to the CO₂ conversion product. This division starts with the synthesis gas production and covers e-MeOH, e-CH₄, sustainable aviation fuels (SAF), marine fuels, polymers, and a last section including other, less commonly studied chemicals. Since the majority of studies involved several reaction products, we will discuss the primary product in detail in its corresponding section and simply reference them in the other sections.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Schematic representation of the main components and the basic operation principles of a) alkaline cells (AEL), b) AEM cells, c) PEM cells, d) SOEC, and e) the main technical characteristics of each technology. Adapted with permission.[98] Copyright 2022, Elsevier.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5. The various CO₂ capture technologies, along with the associated materials and processes involved. Reproduced with permission.[29] Copyright 2024, MDPI.</figcaption></figure>

# 5. LCA Studies Focus on CO₂ Conversion to Value-Added Products

## 5.1. Production of Synthesis Gas

### 5.1.1. Introduction

Due to the high thermodynamic stability of CO₂, it is often advantageous to first convert it into CO, which is significantly more reactive for downstream applications. The resulting CO can then be combined with H₂ to produce syngas, a versatile intermediate that can be utilized in various applications. Syngas can be converted into fuels, such as synthetic diesel, used in the production of chemicals like methanol, or further processed to extract pure H₂. Several conventional processes are used to produce syngas in the industry, with the most widely utilized method today being steam methane reforming (SMR, R. 1),[32,33] largely due to the abundant availability of natural gas.

$$CH_4 + H_2O \rightarrow CO + 3H_2 \quad (R. 1, SMR) \quad (1)$$

Due to growing environmental concerns, methods relying on fossil resources are gradually being phased out in favor of more sustainable and environmentally friendly approaches. LCA studies play a crucial role in identifying environmental hotspots, enabling the development of targeted strategies to minimize or eliminate these impacts.

### 5.1.2. LCA of Syngas Production via Thermocatalytic and Electrocatalytic Pathways

LCA was used to analyze syngas production by coelectrolysis of CO₂ and H₂O in a high-temperature SOEC (see Figure 4d).[34] The SOEC stack consisted of electrode-supported cells featuring a lanthanum strontium cobalt ferrite anode, an 8 mol% YSZ electrolyte, and a nickel cermet (NiO/8YSZ) cathode. CO₂ obtained via DAC and H₂O in the form of steam were cofed to the cathode side of the cell, where the electrolytic production of H₂ and CO occurred simultaneously (R. 2). Operation at high-temperature

offers lower electrical energy consumption and increases efficiencies compared to low-temperature electrolysis.[35]

$$CO_2 + H_2O + 4e^- \rightarrow CO + H_2 + 2O^{2-} \quad (2) \\ (R. 2, coelectrolysis on oxygen ion conducting electrolyte)$$

The assumed electricity mixes were projected based on the German electricity grid mix for 2050, considering two different GHG emission scenarios and two variations in hourly availability. For comparison, a small-scale fossil-fuel-based steam methane (CH₄) reforming process (SMR, R. 1) was used as a benchmark.

The “cradle-to-gate” assessment (i.e., excluding the use of syngas) revealed that high-temperature coelectrolysis has a lower global warming potential (GWP) and reduced fossil fuel depletion impact compared to the reference process. Furthermore, it can achieve a negative net GWP when powered entirely by renewable energy. Although wind and photovoltaic (PV) systems do not emit CO₂ during electricity generation, their construction phase has a significant environmental footprint. This results in impacts such as human toxicity, acidification, particulate matter formation, and metal depletion being higher compared to those of the SMR process.

A recent study[36] investigated syngas production using a combined SMR (R. 2) and reverse water-gas shift (rWGS) (R. 3) thermocatalytic (TC) process. A special focus was placed on how shifting from traditional natural gas (NG) combustion heating of the catalytic reactor to resistive electric heating (or joule heating)—aligned with the ongoing trend of industrial electrification[37]—affects the sustainability of the SMR-rWGS process.

$$CO_2 + H_2 \rightarrow CO + H_2O \quad (R. 3, reverse water-gas shift) \quad (3)$$

In their scenario, NG from the US market and steam (H₂O) react in the SMR reactor to produce an H₂ and CO mixture with a H₂:CO ratio of 5:1. This mixture is then enriched with an external CO₂ supply (makeup CO₂) and used as feedstock in the rWGS reactor. The rWGS reactor produces a H₂:CO mixture
with a significantly higher CO content, making it more suitable for further use in chemical synthesis processes, such as methanol and diesel production. Unfortunately, the source of CO₂ for the rWGS reactor is not specified in the study. Two electricity sources are considered for electrifying the reactor heaters: the current US electricity mix, which is produced with about 25% from low-carbon sources, and onshore wind-powered electricity. The results showed that CO₂ emissions decrease by ≈60% when both SMR and rWGS reactors are heated using wind-generated electricity. Furthermore, the electrification of SMR has a much higher impact on overall GHG emission reductions than the rWGS process.

In contrast, using the conventional electricity mix for furnace electrification resulted in the highest environmental impacts, significantly greater than those associated with using NG as fuel. In such indirect heating system, combustion must generate temperature far above the reaction temperature to allow the necessary inward heat flux to the catalyst bed for operating the reaction.[38,39] The heat transfer must also overcome the thermal resistance of the reactor wall and the catalyst bed, generally constituted by insulator carriers, which contributes to another heat loss factor. No significant improvement was observed in the environmental impact categories related to human health and ecotoxicity when comparing conventional and green electricity. This suggests that transitioning to green electricity does not necessarily result in substantial benefits in these specific areas. Moreover, it was found that a higher H₂ percentage in the produced syngas leads to greater environmental impacts.

It should be noted that in addition to the TC and EC approaches, syngas can also be produced using other methods, such as the thermochemical pathway discussed later.[40] Syngas production is a central process in the conversion of CO₂ and plays a key role in the environmental impact. However, in the LCA analyses reviewed below, syngas production is integrated into a broader framework where it serves as an intermediate for synthesizing fuels and chemicals. Consequently, its production is assessed within the context of the overall process, whether it's a cradle-to-gate (from raw material extraction to final disposal or recycling) or cradle-to-grave (from raw material extraction to the point of use) analysis.

### 5.1.3. General Conclusions of LCA Studies on Syngas Production

The LCA analysis of syngas production through TC and EC pathways reveals significant potential for environmental benefits, especially when powered by renewable energy sources. High-temperature coelectrolysis with SOECs and the electrification of SMR-rWGS reactor heating systems can significantly reduce GHG emissions. However, these benefits are realized only when the processes are powered by clean energy sources, as the use of conventional electricity can offset these advantages and potentially increase the overall environmental impact. Challenges such as environmental impacts from renewable infrastructure and inefficiencies in heat transfer systems persist. Syngas production, integral to CO₂ conversion processes, should be optimized within broader lifecycle systems to maximize sustainability.

## 5.2. Production of e-Methanol

### 5.2.1. Introduction

Methanol (MeOH) is a multifunctional chemical with diverse applications. It can be directly utilized as a fuel, solvent, or feedstock for the production of a wide range of chemicals, such as formaldehyde and olefins. MeOH's high-octane number and clean-burning properties make it a promising alternative fuel, particularly for high-compression engines.[41] Replacing gasoline or diesel with MeOH yields substantial environmental benefits, primarily through the significant reduction of nitrogen oxides (NOx), sulfur oxides (SOx), and nanoparticle emissions. MeOH is also increasingly favored in the marine sector, where its lower emission profile makes it a cleaner alternative to diesel.[42] The MeOH market demand is influenced by several factors, including fuel prices and environmental policies, but it is generally expected to rise with a compound annual growth rate of 2.6%.[43]

The current industrial MeOH production route starts with the SMR reaction (R. 1) in which, NG, primarily CH₄, reacts with steam to produce H₂ and CO, forming syngas (i.e., CO/H₂). The syngas is catalytically converted to MeOH over Cu/ZnO/Al₂O₃ catalysts[44] following the reaction

$$CO + 2H_2 \rightarrow CH_3OH \quad (R. 4, methanol synthesis from CO) \quad (4)$$

The products are then sent through a purification system consisting of a separator, flash drum, and distillation column to remove unreacted gases and water.[45]

Alternatively, to reduce reliance on fossil fuels, MeOH can also be produced from CO₂ feedstock through various methods, including heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis, and electrocatalysis.[46] Although none of these methods have yet been applied at large industrial scale, research into heterogeneous catalytic hydrogenation and the EC reduction of CO₂ has reached higher Technology Readiness Levels (TRL), supporting LCA studies of these processes.[46,47] Since electricity is required for H₂ production in both catalytic hydrogenation (using a CO₂/H₂ mixture) and CO₂ electroreduction, MeOH produced by these methods is commonly referred to as electromethanol or e-MeOH. When renewable energy sources and sustainable feedstocks are utilized in production, it may also be termed renewable MeOH.

The synthesis process involving heterogeneous catalytic hydrogenation is commonly referred to as P2L (power-to-liquid). The P2L process involves two main steps: first, green H₂ production via water electrolysis powered by renewable electricity (such as solar or wind); second, combining H₂ with CO₂ over a Cu or Pd-based catalysts to produce e-MeOH through the catalytic reaction

$$CO_2 + 3H_2 \rightarrow CH_3OH + H_2O \quad (R. 5, MeOH synthesis from CO₂ or CO₂ \qquad (5) \\ \text{hydrogenation to MeOH})$$

The P2L method, in principle, has a lower carbon footprint (CF) compared to traditional MeOH production based on fossil
fuels. This is primarily because it utilizes renewable energy sources and captures CO₂ from the atmosphere or industrial processes, effectively recycling carbon emissions and reducing overall GHG emissions associated with fuel production.[48]

MeOH can also be synthesized via the electroreduction of CO₂ (CO₂RR, R. 6) typically conducted at ambient temperature and pressure without the need for H₂ feed.[46,49–51] The CO₂RR is usually performed in proton-conducting electrolytic cells that consist of two compartments, allowing for a variety of possible products depending on the catalyst used and the applied voltage. The reaction for the EC reduction of CO₂ to MeOH is as follows

$$CO_2 + 6H^+ + 6e^- \rightarrow CH_3OH + H_2O \quad (6) \\ (R. 6, \text{electroreduction of } CO_2 \text{ to MeOH})$$

### 5.2.2. LCA of e-MeOH Synthesis from Thermo- and Electro catalytic Pathways

The environmental impact of MeOH synthesis from CO₂ via TC and EC processes, activated by temperature or electrons respectively, has been assessed in two recent studies.[47,52] Both studies involved capturing CO₂ from flue gases, while for the TC process, H₂ was produced via AEL. The TC conversion process was modeled based on the reactions R. 3–5. For the EC pathway, MeOH synthesis was directly carried out from CO₂ (without requiring H₂) in a flow electrochemical cell similar to that shown in Figure 6a. Copper-based mixed oxide catalysts and electrocatalysts were considered in both studies, although the specific compositions were different. For all MeOH synthesis methods, the LCA boundaries included product separation and purification stages following a cradle-to-gate approach.

In the study by Guzman et al.[52] the comparison of TC and EC pathways was based on experimental data collected at the laboratory scale, using the same Cu/Zn/Al-based catalyst for both processes. Although Cu/Zn/Al catalysts are traditionally used in industrial heterogeneous catalysis for MeOH synthesis,[44] the authors showed that depositing this catalyst on carbon support can enhance its electrical conductivity, enabling its use in EC MeOH production.[52] The European energy mix, consisting of ≈30% renewable energy, was evaluated alongside scenarios using fully renewable electricity. The LCA results reveal that in lab-scale experiments the environmental impact of the EC process was lower than that of the catalytic. However, this trend changes when scaling up, as energy-intensive separation and purification processes must be considered. Consequently, at a MeOH production rate of ≈3 kg h⁻¹, the environmental impact of EC and TC processes becomes comparable. In the best scenario, both processes achieve a CF of about 8.1 kg CO₂eq kg⁻¹ MeOH. It should be noted that the dominant impact in the scaled-up processes is electricity usage. Therefore, the more renewable the electricity from the country's electricity mix, the lower the environmental impact these technologies could achieve. In a scenario with 100% renewable energy usage, it is possible to achieve CF of up to 62% lower.

In a more recent LCA study, the environmental impact of TC and EC processes was benchmarked against the conventional industrial MeOH synthesis, which converts NG and steam into

CO₂ and H₂ through multiple catalytic reforming steps, including R. 5.[47] Recently developed Cu-Zn-Al-Zr mixed oxide and CuGa₂ intermetallic catalysts[53] were considered for TC and EC processes, respectively. A specific geographical location was not considered; instead, global regional data were assumed, for example, for grid electricity. Figure 6b depicts the CF for MeOH production as a function of the electricity-related carbon emissions, for the three MeOH production methods considered. The graph clearly shows that the electricity source plays a dominant role in determining the carbon emissions of emerging processes, whereas the conventional pathway remains virtually unaffected. Notably, the EC method, under its current performance, cannot compete with the conventional MeOH production process in terms of carbon emissions due to its high electricity consumption. This trend is consistent across all evaluated environmental impact categories, including global warming (GW), terrestrial ecotoxicity (TE), human noncarcinogenic toxicity (HNC), and fossil resource scarcity (FS). This suggests that the transition to greener technology is questionable as we cannot observe direct environmental benefits. The high energy demand should be reduced to achieve better environmental performance across all impact categories. This contradicts the conclusion of reference,[52] which suggested the potential sustainability of the EC process.

The TC method becomes advantageous when the carbon intensity of the electricity mix decreases to 0.102 kg CO₂eq kWh⁻¹ or lower. Under scenarios employing 100% renewable energy—particularly hydropower, which provides high efficiency and reliability—the TC pathway demonstrates significantly better sustainability compared to the conventional industrial method. Sensitivity analysis reveals that adopting a renewable electricity mix can reduce environmental impacts by 41% for TC pathway and 88% for EC pathway.[47] These findings underscore the critical importance of low-carbon electricity in enhancing the sustainability of emerging MeOH production technologies, highlighting the potential of renewable energy to transform their environmental performance.

The authors of[54] analyzed three emerging syngas production technologies starting from CO₂ in order to identify the most sustainable among them for MeOH synthesis. These technologies include direct CO₂ hydrogenation to MeOH based on R. 5 (CCU), dry reforming of methane (DRM, R. 7), and tri reforming of CH₄ (TRM), in which CO₂ is introduced to the feed together with steam, oxygen, and methane, combining three different reforming reactions. The impact of CO₂-based technologies was benchmarked against the conventional industrial MeOH synthesis process, which uses NG as a feedstock.

$$CH_4 + CO_2 \rightarrow 2CO + 2H_2 \quad (R. 7, \text{dry reforming of methane}) \quad (7)$$

CO₂ was captured from the emissions of the cermet industry, while H₂ for the hydrogenation reactions was produced using PEM cells.

The LCA indicated that the DRM is not a viable option for MeOH production due to lower efficiency and higher emissions compared to conventional processes. In addition, it was demonstrated that CCU and TRM processes become advantageous only when electricity GHG emissions fall below 0.17 kg
<figure><img src="image_8.png" /><figcaption>**Figure 6.** a) A schematic representation of the flow electrochemical cell used for the electrochemical MeOH production from CO₂. Methanol, along with other reaction products, is collected at the "Liquid Product Out" outlet of the cell. Adapted with permission.[53] Copyright 2022, Wiley-VCH GmbH. b) Comparison of CFs for MeOH production using conventional, thermochemical, and electrochemical processes. The x-axis represents the electricity emissions intensity, while the inset illustrates the carbon intensity of electricity generated from solar, wind, and hydropower sources. Adapted with permission.[47] Copyright 2023, Elsevier. c) The impact of electricity generation GHG on the GHG emissions of MeOH production for conventional, direct CO₂ hydrogenation to MeOH (CCU) and tri-reforming of CH₄ (TRM) pathways, Adapted with permission.[47] Copyright 2021, Elsevier. d) GHG emissions of the three MeOH production pathways in different countries, reproduced with permission.[47] Copyright 2023, Elsevier.</figcaption></figure>

CO₂eq kWh⁻¹. Beyond this threshold, the conventional process, with its lower electricity dependency, becomes the minimum GHG emissions pathway. When powered by renewable energy sources, CCU emerges as the preferred option over TRM, enabling negative GHG emissions. However, as electricity carbon intensity rises, TRM becomes more favorable. Notably, in countries like Brazil and Norway, where renewable or low-carbon electricity is abundant, CCU presents the most environmentally friendly choice, as depicted in Figure 6d.

The same research group performed an LCA to evaluate the environmental impact of a novel electrified combined reformer (E-CRM) method for MeOH production in comparison to the TRM process using syngas and a Cu/ZnO/Al₂O₃ catalyst, CO₂ hydrogenation to MeOH (R. 5), and conventional NG-based process.[55] A key difference between the E-CRM and TRM processes is the method of reactor heating. In the TRM process, heat is generated by burning unreacted gases with added oxygen, while the E-CRM method uses an electric heater, similar to the considerations in ref. [36]. This approach eliminates the need for oxygen supply and allows for the recycling of most unreacted gases back into the reformer as feedstock, thereby reducing costs. Given that the electricity mix is critical in such LCA studies, the authors evaluated three regions in Canada, each with a unique electricity mix profile: Quebec, where electricity is primarily derived from renewable sources like hydropower and

wind; Ontario, which utilizes a mix of nuclear, hydropower, NG, and wind; and Alberta, which relies heavily on fossil fuels. The LCA results reveal that in regions with access to low-carbon or renewable electricity, CO₂ hydrogenation demonstrates the lowest environmental impact. Conversely, the conventional NG-based method performs better in regions where fossil fuels dominate the electricity mix. However, when considering Canada's national average electricity mix, the E-CRM process stands out as the option with the lowest environmental impact among the processes evaluated. TRM systematically underperformed regardless of the electricity mix, due to its high dependence on electricity for electrolytic hydrogen production and its lower efficiency compared to conventional methods.

### 5.2.3. LCA of *e*-MeOH Synthesis via Solar Thermochemical Syngas Production from CO₂ and H₂O

Solar thermochemical syngas production is a noncatalytic process that uses concentrated solar energy to split H₂O and CO₂ into syngas. In the solar thermochemical cycle, a solar concentrating system focuses sunlight onto an array of solar receivers at the top of a central tower.[40] The concentrated solar radiation generates high temperatures, typically exceeding 1500 °C, which are used to heat a redox reactor. In this reactor, a reducible oxide,
such as cerium oxide (CeO₂), is used to split H₂O and CO₂ into H₂ and CO. In particular, CeO₂ is initially thermally reduced to CeO₂-x, and in a subsequent step, it is reoxidized by CO₂ and/or H₂O to produce CO and/or H₂, according to the reactions

$$CeO_{2-x} + xCO_2 \rightarrow CeO_2 + xCO \quad (R. 8) \quad (8)$$

$$CeO_{2-x} + xH_2O \rightarrow CeO_2 + xH_2 \quad (R. 9) \quad (9)$$

An early LCA study examined conventional MeOH production from NG utilizing solar energy in a thermochemical system.[56] The LCA shows that the solar-driven process offers significant advantages in terms of GWP, acidification potential (AP), and nonrenewable primary energy consumption (PCI). This improvement appears as a very promising solution to significantly reduce the environmental footprint of the processes. Specifically, the solar-driven process can reduce the impact in the GWP category by up to 2.5 times and decrease the impact in the PCI category by ≈50 times compared to the conventional synthesis method. It should be noted that in the process proposed in reference,[56] the electricity is used to operate gas compressors during the amine-based CO₂ capture, and not to produce H₂ by electrolysis, as is the case in most later studies. This distinction underscores differences in energy use strategies, which can significantly influence the overall environmental performance of the process.

In a more recent study,[57] the solar thermochemical production of numerous e-fuels from syngas formed from air-captured H₂O and CO₂ was analyzed. The research explores three geographical locations, Tabernas (Spain), Sierra Gorda (Chile), and Ouarzazate (Marocco), with high direct normal irradiation, employing a baseline heliostat field reflective area of 1 km² to produce: jet fuel and diesel through FTS, MeOH from syngas, gasoline from MeOH, dimethyl ether (DME), and H₂. Also, two different scenarios are examined: a near-term future (until 2030) and a long-term one (after 2030). The results suggest that when fuels are produced using solar energy through a thermochemical route, there is a reduction in environmental impacts. The GHG emissions are comparable to those of advanced biofuels derived from wheat straw and forestry residues, as well as future e-fuels synthesized from CO₂ and H₂ using wind-powered or PV electricity.

### 5.2.4. LCA of MeOH as a Feedstock for Synthesis of Fuels and Chemicals

Although MeOH can be used directly in many applications, it is also a feedstock for producing long-chain hydrocarbons such as diesel. In this context, Galusnyak et al.[58] assessed the impact of MeOH production and its subsequent conversion into formaldehyde (formalin), DME, methyl tert-butyl ether (MTBE), and bio-diesel. The process involves MeOH synthesis through direct CO₂ hydrogenation (R. 5), but the type of catalyst used is not specified in the study. CO₂ is captured from the cement industry using methyl-di-ethanol-amine (MDEA), and H₂ is produced by water electrolysis using three different sources of electricity: the EU grid mix, wind power, and hydropower. According to the LCA, regardless of the electricity source, conversion of MeOH to bio-diesel has the lowest environmental impact among the products

studied. This is due to the reduced amount of MeOH required, the high purity of the product, and the higher cost of by-products compared to other hydrocarbons. The greatest impact on the GWP of the process is from the electricity used for H₂ production. Therefore, it is not surprising that using hydropower for both H₂ generation and MeOH production results in the best environmental performance across all investigated cases.

Morimoto et al.[59] examined the net CO₂ emissions associated with the synthesis of MeOH, CH₄, gasoline, and diesel using captured CO₂ and H₂ produced via AEL powered by renewable electricity. The study considered two CO₂ capture technologies: 1) conventional flue gas treatment with amines, currently used in industry, and 2) DAC, representing a future scenario. In both scenarios, MeOH is synthesized directly from CO₂ and H₂ feedstocks through the P2L method. For CH₄ synthesis, the CO₂ methanation or Sabatier reaction is used

$$CO_2 + 4H_2 \rightarrow CH_4 + 2H_2O \quad (10) \\ (R. 10, CO_2 methanation or Sabatier reaction)$$

Gasoline is produced from MeOH using methanol-to-gasoline (MTG) technology, a multistep catalytic process that converts MeOH into gasoline-range hydrocarbons. Diesel, on the other hand, is synthesized from syngas via FTS according to the reaction

$$(2n + 1)H_2 + nCO \rightarrow CnH_{2n+2} + nH_2O \quad (R. 11, FTS) \quad (11)$$

The syngas production methods differ between the conventional and future scenarios. In the conventional scenario, CO₂ is first converted to CO by reacting with H₂ through the catalytic rWGS reaction (R. 3). The resulting CO is then mixed with additional H₂ to reach the desired syngas composition for downstream synthesis. In the future scenario, syngas is produced electrocatalytically via the coelectrolysis of CO₂ and H₂O in a SOEC, which simultaneously converts CO₂ and H₂O into CO and H₂ according to R. 2, similar to ref. [34]. The study also analyzed the impact of regional characteristics on carbon capture and utilization (CCU) by conducting calculations for five countries: the United States, Canada, Germany, Australia, and Japan. The LCA found that the highest reduction in CO₂ emissions was achieved for diesel production, with an average of 6.33 kg-CO₂ kg⁻¹ of fuel, while MeOH showed the lowest reduction, averaging 3.43 kg-CO₂ kg⁻¹ of fuel. The transition to DAC and renewable energy significantly improved CO₂ emission reductions, with average reductions of 47% for MeOH, 99% for CH₄, 545% for gasoline, and 621% for diesel. In the future scenario, Germany is projected to have the lowest CO₂ emission reduction costs for MeOH and diesel, Australia for CH₄, and Canada for gasoline. The study concludes that H₂ production has the greatest influence on environmental impact, even when renewable energy is used.

An LCA[60] was conducted to evaluate the environmental performance of CO₂-based fuels (DME and MeOH) in comparison to H₂ produced either via conventional SMR (R. 1) or through water electrolysis powered by renewable energy. The study considered the full carbon cycle, including emissions at end-of-life when the fuel is combusted for transport. Results indicated that H₂ produced via renewable energy-based electrolysis is the most
efficient option both in terms of carbon and energy use. It is note-
worthy that MeOH requires only 25% of the energy compared to
DME. However, both of them produce as much CO₂ as they
remove. Regarding DME, its GWP does not exhibit much differ-
ence compared to that of the SMR-H₂, gasoline, and diesel pro-
duced without renewable energy. Consequently, DME does not
offer any environmental benefits, while MeOH reduces the GWP
by up to 10% compared to SMR-H₂, gasoline, and diesel. The
authors concluded that when CO₂ is sourced from fossil-based
resources, such as NG, the synthesis of CO₂-based fuels offers
limited practical benefits due to an unfavorable balance between
cost and environmental impact. To improve the CF of CO₂-based
fuels, it is recommended to use CO₂ from nonfossil sources,
such as biomass combustion or DAC. This shift could signifi-
cantly enhance the sustainability of CO₂-based fuel production
and its relevance as a low-carbon alternative.

5.2.5. *General Conclusions of LCA Studies on e-MeOH Production*

It is concluded that producing e-MeOH by integrating renewable
energies and sustainable technologies can significantly alter the
CF of the processes in a positive way. In all the LCAs that were
analyzed, it is proved that there is a great reduction in GHG
emissions, while some configurations managed to reach even
an 88% reduction in GHG emissions compared to the conven-
tional methods relying on fossil fuels. There was shown to be a
high dependency between the source of the electricity and the
environmental performance of the e-MeOH production. When
wind or solar power is employed to generate electricity, the over-
all emissions appear reduced. In the meantime, using hydro-
power to cover the energy demands of processes such as H₂
production yields the most outstanding results and directly
affects the GHG emissions rate during the e-MeOH production
process.

It is also interesting that even though in lab scale it was noted
that electrochemical methods were more beneficial from an envi-
ronmental aspect, when scaling up to higher mass production,
the differences in the impacts are normalized due to separation
and purification methods. In larger applications, the energy
demands required for the processes can surpass the optimized
results that occur in controlled lab environment. The geographi-
cal area of the e-MeOH production also significantly affects
the environmental performance of the fuels. When the produc-
tion process takes place in regions that have access to plenty
renewable energy resources and can use them to produce high
amounts of energy, the environmental impacts are mitigated
compared to regions relying on fossil fuels. Consequently, local
energy mixes should be reconsidered, particularly for processes
such as CO₂ hydrogenation.

**5.3. e-Methane (Synthetic Natural Gas)**

**5.3.1. Introduction**

Methane (CH₄), the primary component of NG, is widely used
across various sectors due to its high energy content and rela-
tively clean combustion. In the transportation sector, compressed
NG (CNG) and Liquefied NG (LNG) are utilized as vehicle

fuels.[61,62] CH₄ also serves as a fuel for combustion heating in
NG furnaces, boilers, and water heaters, as well as for electricity
generation in power plants. Additionally, it is frequently used as
a feedstock for the production of fuels such as H₂, MeOH,
and NH₃.

Currently, most CH₄ is extracted from NG fields through drilling and hydraulic fracturing. As we transition to a low-carbon economy and seek independence from fossil fuels, synthetic methane (e-CH₄) emerges as an attractive alternative. e-CH₄ is produced using renewable electricity in a catalytic process known as methanation or Power-to-Gas (P2G). This process combines green H₂ (generated through water electrolysis powered by renewable electricity) with captured CO₂ via the Sabatier reaction (R. 10). The resulting CH₄ is chemically identical to NG but does not rely on fossil fuel sources. Therefore, CH₄ production from CO₂ might be also considered as an alternative strategy for CO₂ storage and sequestration. As NG, e-CH₄ can be employed as a fuel for heating, power generation, and as a chemical feedstock, while it also serves as a fuel for vehicles.[63] With the global methane market projected to grow at an annual rate of nearly 10%,[64] several LCA studies have been conducted to assess the environmental impact of e-CH₄.

e-CH₄ is often confused with synthetic natural gas (SNG) in LCA literature. Both terms refer to methane-rich mixtures (>90% CH₄) that can serve as alternatives to NG. Historically, however, SNG specifically described CH₄ derived from carbon-based sources like coal or biomass through gasification, typically without using green H₂.[65] In contrast, as mentioned above e-CH₄ is produced through a low-carbon process that combines CO₂ with green hydrogen, resulting in a significantly lower CF. In this article, SNG is produced using a low-carbon process and thus is effectively identical to e-CH₄.

5.3.2. LCAs of Catalytic CO₂ Conversion to e-CH₄

Processes Utilizing the Sabatier Reaction: An early LCA study examined the production of H₂ and CH₄ via the P2G process, powered by wind and PV electricity, with a specific focus on the GWP indicator.[61] This study compared the environmental impact of P2G with conventional methods, such as NG steam reforming, crude oil processing, and NG extraction. The findings revealed that the CF of the electricity source is a crucial determinant of the P2G process's environmental impact. Specifically, a GWP threshold was identified for the electricity used in order for P2G to offer environmental benefits over conventional technologies. For H₂ production, the electricity's GWP should not exceed 190 g CO₂ kWh⁻¹, while for CH₄ production, it should not exceed 113 g CO₂ kWh⁻¹. If these limits are exceeded, the GWP impact of P2G-produced H₂ and CH₄ does not improve upon that of conventional production methods. At the time of assessment (2015), the EU electricity mix exceeded this threshold, resulting in GWP scores 147% higher than those from conventional H₂ production by SMR. However, with a significant increase in renewable energy in the EU—≈30% rise since 2015 (from around 18 to 24% of the total energy mix[66])—the balance has shifted toward a reduced environmental impact for the P2G process. This suggests that the viability of P2G improves as the share of renewables continues to grow.
In two studies published concurrently in 2017, researchers from the Paul Scherrer Institute in Switzerland investigated the environmental impact of the P2G process for producing H₂ and CH₄ in comparison with conventional production technologies.[67,68] The LCA analysis considered multiple stages of the P2G process: electricity supply, H₂ production through electrolysis (using both PEM and AEL), CO₂ sourcing from industrial flue gasses, DAC and biogas, methane synthesis through the Sabatier reaction (R. 10), gas processing, and the final combustion of the gases for heat production. A key finding was that the electricity supply for electrolysis contributed over 90% of the environmental impact. Comparing the GHG emissions of H₂ production via electrolysis powered by wind versus PV-generated electricity, electrolysis driven by wind-electricity exhibits significantly lower GHG emissions.[68]

Among the various electricity sources examined, Swiss wind power emerged as the most environmentally favorable option. In contrast, electricity from the Swiss grid had a significant impact on ionizing radiation-related human health effects, largely due to the contribution of nuclear power in the energy mix. Moreover, electrolysis powered by the EU grid electricity demonstrated the highest overall environmental impact, underscoring the importance of low-carbon electricity sources in minimizing the footprint of P2G systems.[67] This finding aligns with a more recent LCA study of P2G systems, which indicated that the electricity source has a greater impact than the biomass source in most impact categories.[69] Notably, when electricity mix is predominantly fossil fuel-based, P2G technology results in higher GHG emissions compared to conventional technologies.[68]

Although P2G systems have the potential to reduce GHG emissions compared to conventional fossil fuel-based technologies, they may perform less favorably in other environmental impact categories.[68] For H₂ production, the high demand for renewable electricity results in increased impacts in the categories of human toxicity, cancer, and freshwater ecotoxicity. This is largely due to the substantial quantities of steel and aluminum required for constructing wind and PV plants, as well as the copper and fossil fuels used in the Swiss electricity grid. As with CH₄ production, the P2G technology performs worse than conventional technologies in four impact categories: human toxicity cancer and human toxicity noncancer, freshwater ecotoxicity, and resource depletion (mineral, fossil, and renewable).[68] The poorer performance of CH₄ production compared to H₂ production is due to the lower efficiency of the methanation process, as well as the additional energy and materials required for CO₂ capture. The authors highlighted that while scaling up P2G systems could enhance economic feasibility, it would have minimal effect on environmental impacts, which are predominantly driven by electricity consumption, largely unaffected by system scale.[67]

De Weireld and coworkers investigated the production of e-CH₄, referred to as SNG in their work, using CO₂ captured from cement plant flue gas following the P2G process.[70,71] They considered a PEM electrolyzer with an efficiency range of 65–85%, powered by wind-generated electricity, for H₂ production. The Sabatier reaction for CH₄ production took place over a 15% Ni/MgAl₂O₃ catalyst at temperatures between 300 and 400 °C and pressures ranging from 3 to 10 bar. The LCA results indicated that CH₄ production via P2G reduces overall GHG emissions by 76% and significantly mitigates

fossil resource scarcity by over 80%. However, environmental impacts of the P2G process were found to be higher than those of conventional NG production from fossil fuels in certain categories. Notably, freshwater eutrophication, terrestrial acidification, and mineral resource scarcity seemed to have worse environmental performance under this pathway. These findings were subject to an uncertainty range of 26–47% due to variable input parameters. Given the amount of uncertainty associated with this approach, further examination of the processes is required through collection of detailed data and scenario analysis. The study concluded that while the P2G process does not qualify as a fully negative-emission technology, its environmental impacts are substantially lower than those of conventional methods. This creates many opportunities and leaves space for further improvement.

A recent LCA analyzed the environmental performance P2G systems used to produce transport fuels through water electrolysis, methanation, and CO₂ separation, excluding transportation and storage.[72] The study evaluated three electrolyzer types—AEL, PEM, and SOEC—and examined electricity sources including solar PV, wind, and current (2020) and projected South African grid mixes. Two CO₂ sources were considered: capture from a coal-fired power plant using amine scrubbing, and DAC technology. Methane synthesis was performed by combining H₂ and CO₂ over a nickel-based catalyst. Key findings include the following: 1) Electricity source impact: Using renewable energy sources like solar and wind significantly reduces the environmental impact of P2G systems compared to traditional fossil fuel-based methods. Conversely, using South Africa's current and projected electricity mix results in higher environmental impacts than the conventional method. 2) Comparison of H₂ and CH₄ production: Producing H₂ has a ≈20% lower GWP than CH₄, due to the additional energy and CO₂ capture requirements for CH₄. Wind-powered electrolysis results in significantly lower GWPs compared to solar PV. 3) Air quality benefits: Using wind or solar for CH₄ synthesis reduces photochemical smog by 77 and 50%, respectively. 4) Fuel cell vehicles: H₂-powered fuel cell vehicles have 98% lower CF than internal combustion diesel-powered vehicles. CH₄-powered vehicles offer a moderate reduction of 29% compared to gasoline-powered vehicles. 5) Electrolyzer impact: The SOEC electrolysis has a higher environmental impact compared to AELand PEM electrolyzers, due to the high electricity consumption and the use of nickel in the cathode electrode. 6) CO₂ source and other impacts: The source of CO₂ (flue gas or DAC) has a limited impact on the overall environmental performance when using nonrenewable electricity. However, P2G systems can lead to increased sulfur oxide emissions and abiotic depletion.

The study highlighted the critical role of renewable electricity in minimizing environmental impacts for P2G processes and suggests that alternative electrolyzer materials and designs could further improve sustainability.

**Process Utilizing Syngas Methanation:** In an alternative approach of e-CH₄ production, Choe et al.[73] conducted an LCA study on e-CH₄ production via syngas methanation (R. 12), which is practically the reverse reaction of SMR

$$CO + 3H_2 \rightarrow CH_4 + H_2O \quad (R. 12, \text{syngas methanation}) \quad (12)$$
In their scenario, CO₂ is provided by DAC and syngas is pro-
duced through CO₂ and H₂O coelectrolysis (R. 2) using SOEC
powered by renewable-based electricity (see **Figure 7a**). The elec-
tricity is sourced from three sources: PV, onshore wind, and off-
shore wind energy. The study evaluated the climate change
impact in terms of CO₂ emissions within a cradle-to-gate system
boundary. The authors found that electricity generated from
onshore and offshore wind resulted in a net CO₂ emission reduc-
tion of ≈112 g CO₂-eq kWh⁻¹. Based on their findings, they con-
cluded that while these technologies are not yet mature, e-CH₄
production combined with DAC has the potential to be a signifi-
cant contributor to climate change mitigation. In particular, by

replacing fossil-fuel-based NG with e-CH4, global CO2 emissions could be reduced by as much as 11.36 Gt. As illustrated in Figure 7b, the impact of this transition would be particularly significant in regions that are major consumers of NG, such as North America, where the contribution to CO2 reduction would be substantial. However, in regions like Africa, where NG consumption is lower, the impact would be less pronounced.

Processes Integrating Carbon Capture Technologies in e-CH4
Production: An LCA of a novel reactor configuration for catalytic methanation was reported in ref. [74]. In this process, CO2 is captured from flue gas by reacting it with calcium oxide (CaO) to form calcium carbonate (CaCO3). The CaCO3 is then calcined

<figure><img src="image_9.png" /><figcaption>**Figure 7.** a) Schematic representation of renewable methane production with renewable syngas produced by solid oxide electrolysis, b) Comparison of CO₂ emissions during conventional NG and renewable methane consumptions. Adapted with permission.[73] Copyright 2022, Elsevier.</figcaption></figure>
at high temperatures to release CO₂, which is subsequently fed, along with H₂ produced from PEM electrolysis, into a catalytic reactor. The H₂ is generated using electricity sourced from the Italian electricity mix. A commercial Ni-based catalyst is used in the methanation reactor to produce CH₄ via the Sabatier reaction (R. 10). The energy required for heating and purifying the gas streams is also considered in the LCA. For comparison, fossil CH₄ production and bio-CH₄ production from maize silage were used as reference processes. In these cases, H₂ is produced through water electrolysis, utilizing excess electricity from onshore wind farms.

Compared to the conventional methods, the novel process resulted in CH₄ production with a significantly lower environmental impact across the majority of impact categories including terrestrial acidification, photochemical ozone formation, marine and terrestrial eutrophication, resource use-energy carriers, and respiratory inorganic impacts categories.

The PEM unit, consuming a substantial amount of wind-generated electricity, emerged as the most significant factor influencing environmental consequences, ranging from 35% in ionizing radiation to 100% in water scarcity, resource use-mineral and mental, land use, and cancer human health. Overall, the proposed layout had improved environmental performance in most categories, with exceptions noted in marine and terrestrial eutrophication, ozone depletion, and water scarcity.

Chemical looping combustion (CLC) is an advanced combustion technology designed for CO₂ capture. It operates through a two-step process as schematically shown in the block diagram in **Figure 8**. In the first reactor, the fuel is combusted with a metal oxide (such as iron oxide or copper oxide), known as an oxygen carrier. The oxygen carrier provides the necessary oxygen for fuel combustion, producing CO₂ and water vapor. A key advantage of CLC is that the CO₂ is easily separated from the combustion gases, as it does not mix with nitrogen from the air, as occurs in conventional combustion processes. The reduced metal oxide is then transferred to a second reactor, where it is reoxidized by

air, regenerating the oxygen carrier. The process enables combustion without direct contact between the fuel and atmospheric oxygen, facilitating the capture of CO₂ in a concentrated form for easier storage or utilization.[75,76] P2G process for CH₄ production, utilizing CO₂ supplied by the CLC, was explored in two LCA studies.[77,78]

A 2020 study was the first to assess the environmental impact of a P2G process using CO₂ captured from a CLC system.[78] In this specific two-stage reactor setup, coal was combusted in a CLC reactor with a CuO/ZrO₂ oxygen carrier to produce a CO/CO₂ gas stream. This gas stream was then converted into CH₄ via the Sabatier reaction over a Ni/Al₂O₃ catalyst, using H₂ generated from water electrolysis powered by onshore wind energy. When compared to a conventional system (electricity from the Italian grid and CH₄ from NG), the proposed P2G-CLC system demonstrated significant improvements in the climate change mitigation impact category with 25% fewer GHG emissions. Also reduced exposure to ionizing radiation with up to 40% fewer radionuclides was noticed as it was reflected through the ionizing radiation impact category. However, it performed less favorably in other impact categories, with impacts up to 28 times higher for the noncancer human health effects and 22 times higher for the mineral and metal depletion. The study identified the production and disposal of the CuO/ZrO₂ oxygen carrier as the primary source of these negative impacts, contributing up to 100% to the overall environmental footprint. To mitigate these issues, the researchers proposed two strategies: 1) reusing oxygen generated through electrolysis to reoxidize the oxygen carrier in the CLC reactor and 2) minimizing the usage of the oxygen carrier or developing more sustainable alternatives to CuO/ZrO₂.

Navajas et al. presented a more optimistic outlook on the potential of CLC systems integration to P2G.[77] They compared the environmental impacts of the PtG systems utilizing CO₂ obtained from chemical CLC of biomass. The oxygen carrier in the CLC was either synthetic CuO-based materials or

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Schematic block diagram of the P2M system with integrated CLC unit as the CO₂ source. Main components of the process: 1) H₂ production unit, 2) CLC unit (CO₂ generation), and 3) methanation unit (catalytic reactor for CH₄ production). Reproduced with permission.[77] Copyright 2022, Elsevier.</figcaption></figure>
FeTiO₃ minerals. Methane is produced via the Sabatier reaction, while the system employed a PEM electrolysis unit consuming renewable energy following the Spanish renewable mix until 2030, consisting of 30% wind energy, 47% solar, and 23% hydro-power. LCA suggested that integration of CLC technology has the potential to achieve negative GWP values, meaning that this process would facilitate the removal of previously emitted CO₂.

The integration of CLC in P2G not only reduces the GWP but also significantly enhances various LCA impact categories, like resource depletion, ozone depletion, human health, acidification, and eutrophication. Two primary factors drive this improvement: 1) capturing CO₂ eliminates the need for NG extraction, thereby conserving valuable resources and reducing associated environmental impacts, and 2) a distinctive feature of CLC is its ability to release nitrogen as N₂, preventing the formation of harmful NOₓ emissions commonly linked to biomass combustion. This reduction in air pollutants significantly benefits human health and the environment.

The choice of electricity source for H₂ production played a crucial role in the final GWP, with electricity generated from fossil fuels having the most significant impact. Compared to the case of CO₂ collected from power plant emissions, this process was found to less severe effect on the GWP.

Processes Integrating Catalytic CO₂ and N₂ Conversion to e-CH₄ and e-NH₃: Recent studies have assessed the sustainability of processes that involve capturing CO₂ and N₂ from flue gas, combining them with electrolytically produced H₂, and subsequently converting them into CH₄ and NH₃ via the Sabatier and Haber-Bosch reactions, respectively.[79,80] The integrated process involves four main stages, schematically presented in Figure 9: 1) capture and separation of CO₂ and N₂ from industrial flue gases, 2) H₂ production via water electrolysis, and two heterogeneous catalytic processes: 3) the Sabatier reaction to convert CO₂ and H₂ mixtures into e-CH₄, and 4) the Haber-Bosch process to synthesize e-NH₃ from N₂ and H₂ mixtures. Together, these steps enable the efficient conversion of industrial emissions into valuable fuels and chemicals. The same catalytic processes were considered in the two LCA studies.[79,80]

In the Sabatier reaction (R. 10), CO₂ reacts with H₂ over a Ni-based catalyst to produce CH₄, while in the Haber-Bosch reaction (R. 13), N₂ reacts with H₂ over a Fe-based catalyst to yield NH₃

$$N_2 + 3H_2 \rightarrow 2NH_3 \quad (\text{R. 13, Haber-Bosch reaction}) \quad (13)$$

In the earlier study,[79] the authors considered idealized flue gas streams containing only CO₂ and N₂, excluding other common components such as O₂, H₂O, SO₂, and NOₓ. The study considered two electricity sources: a PV system and the Italian electricity grid mix, with a PEM-based electrolyzer used for H₂ production. The LCA results revealed that using PV electricity reduces total CO₂ emissions to half of those associated with grid electricity. Additionally, in the case of PV electricity, NH₃ production contributes ≈70% of the total CF of the flue gas treatment process, while CH₄ production accounts only for 30%. This analysis underscores the environmental advantage of renewable electricity sources, especially PV, in reducing CO₂ emissions and highlights that NH₃ production is the dominant contributor to the CF of this process.

In a more recent study,[80] the flue gas was assumed to originate from a combined-cycle NG power plant in Germany. CO₂ and N₂ in the flue gas stream were separated using a membrane separator, with the authors noting that the Sabatier process is tolerant to N₂ impurities, as nitrogen behaves as an inert gas, but CO₂ impurities should not be fed into the Haber-Bosch process. Hydrogen was produced using three water electrolysis technologies—PEM, AEC, and SOEC—along with two electricity sources: the German 2020 electricity mix and wind-powered electricity (see Figure 9). This resulted in six different scenarios: PEMEC-Grid, PEMEC-Wind, AEC-Grid, AEC-Wind, SOEC-Grid, and SOEC-Wind. In their LCA approach, the authors employed the Planetary Boundaries framework (PB-LCA), which assesses the absolute environmental impact of the technology on Earth's natural limits and human health. Wind power scenarios significantly outperformed the business as usual (BAU) baseline, where current practices, technologies, and policies continue unchanged, demonstrating lower environmental impacts. Among the electrolyzer types considered for H₂ production, SOEC-Wind had the least environmental footprint. Compared to BAU, wind-powered scenarios nearly halved the overall environmental impact. While impacts in the Human Health category increased by around 85%, they remained lower than those from the German grid-based scenario. This German electricity mix contributed over 90% of the total environmental impacts, while in the wind-powered scenario, this percentage dropped to 50%.

### 5.3.3. General Conclusions for e-CH₄

The production of e-CH₄ using renewable resources offers significant environmental advantages over conventional fossil fuel-based methods. By powering the -CH₄ production process with renewable energy sources, it is possible to substantially reduce GHG emissions. In particular, electricity generated from wind power seems to have the most beneficial environmental impacts in GWP reduction. Innovative technologies like P2G systems, integrated CO₂ and N₂ conversion, and calcium looping units for CO₂ capture can further enhance the environmental profile of e-CH₄ production. Some studies even suggest that transitioning to renewable resources could lead to negative CO₂ emissions, effectively removing carbon dioxide from the atmosphere. Additionally, the choice of electrolysis technology for H₂ production significantly influences environmental impacts. However, there is no consensus in the literature on which of the main technologies—AEL, PEM, or SOEC—is preferable. Another critical factor is the source of CO₂, which greatly affects environmental performance. When e-CH₄ is produced using a combination of DAC and renewable energy sources, the environmental benefits are maximized. However, it's important to consider potential tradeoffs.

In some impact categories, such as human health and resource scarcity, producing e-CH₄, even with renewable electricity, may lead to higher impacts. Therefore, a careful assessment and prioritization of different impact categories is essential. Moreover, considering the substantial disparity in the scale of NG pipelines (spanning several thousand kilometers) compared to hydrogen pipelines (limited to only a few thousand kilometers), converting hydrogen into e-CH₄ emerges as an efficient strategy. This approach not only enables the storage of hydrogen
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Schematic representation of the integrated production of CH₄ and NH₃ from CO₂ and N₂ sourced from flue gases, using H₂ produced via three different water electrolysis technologies. CH₄ synthesis follows the Sabatier reaction, while NH₃ synthesis is based on the Haber–Bosch process. Adapted with permission.[80] Copyright 2022, American Chemical Society.</figcaption></figure>

in chemical form but also simplifies its transportation over long distances, leveraging existing NG infrastructure.

**5.4. Sustainable Aviation Fuels (SAF)**

**5.4.1. Introduction**

The electrification of the transportation sector is the key to reducing CO₂ emissions; however, due to logistical and safety issues, this approach is difficult to implement in the aviation industry, necessitating alternative decarbonization strategies. In particular, a new category of products has been studied as an alternative to fossil fuels for aviation, called sustainable aviation fuels (SAF). The term SAF refers to kerosene-range fuels (C₈–C₁₆) which achieve at least a 50% reduction in life cycle GHG emissions compared to conventional jet fuel (e.g., Jet A-1).[81] According to a feasibility assessment study for the potential development of SAF, their production in Europe can reach

5–10% of total European jet fuel consumption by 2030 and up to 85% by 2050.[82,83] Although several green strategies are currently exploited for SAF production, such as biochemical,[84–86] the most technologically advanced approach is the P2L process for conversion of biosourced syngas produced by the gasification of organic feedstocks, to kerosene via FTS.[87] In the P2L process, captured CO₂ is combined with H₂ produced via water electrolysis and converted into liquid hydrocarbons through FTS, as shown in **Figure 10**. The main functional units in FTS are as follows: 1) the rWGS reactor which enables the catalytic conversion of the CO₂/H₂ mixture into syngas (CO and H₂), which serves as the feedstock for FTS; 2) the FTS reactor where syngas is transformed into liquid hydrocarbons, such as naphtha, kerosene, and diesel, with compositions similar to those derived from crude oil; and 3) upgrading and separation units used to refine and isolate the target liquid hydrocarbons from the mixture of FTS products, ensuring the final product meets the required fuel specifications.
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Schematic overview of the power-to-liquid (P2L) process for converting CO₂ into liquid hydrocarbons via FTS. CO₂, captured from various sources, is combined with H₂ produced by electrolysis using diverse electricity sources. This gas mixture undergoes transformation in the FTS unit, comprising an rWGS reactor for syngas production and an FTS reactor for synthesizing liquid hydrocarbons. The final stage includes upgrading and refining units to isolate and purify the products to meet the required standards. Adapted with permission.[92] Copyright 2022, CS Sustainable Chem. Eng.</figcaption></figure>

FTS is a well-established catalytic process used for transforming CO and H₂ gases into a range of hydrocarbons. The chain length and composition of the hydrocarbon depend on the operating conditions, such as temperature, pressure, type of catalyst, and reactor design, and can be manipulated for an optimal subsequent refining stage of the products. Low operation temperatures (<250 °C) produce higher molecular weight products and therefore are better adapted for SAF synthesis. Cobalt-based catalysts are considered the most efficient catalysts for low-temperature FTS.[88,89]

### 5.4.2. LCAs of Catalytic CO₂ Conversion to SAF

An early LCA study, conducted in 2014, examined the production of liquid hydrocarbon fuels from CO₂ and solar energy using FTS.[90] The authors did not specifically mention SAF in their analysis, as the term was not yet widely used. Instead, they referred to these fuels as “solar fuels”, describing the production of synthetic fuels from CO₂ using only solar energy. The study’s findings were not very encouraging for SAF application. The authors concluded that, in most scenarios analyzed, producing liquid hydrocarbon fuels from CO₂ with existing technologies required significantly more energy than conventional hydrocarbon fuels. Improvements in life cycle CO₂ emissions were observed only when solar energy was generated via PVs and atmospheric CO₂ was captured through DAC. However, the

authors acknowledged that future technological advancements could potentially reverse this outcome.

The predominant importance of the electricity source was also highlighted in a later study that examined how different energy sources affect the environmental impact of the PtL process.[91] Four electricity sources were considered: the German grid, the Saudi Arabia grid, and electricity from wind and PV sources installed in Germany. According to the findings, environmental advantages are achieved only through the use of wind or PV-derived electricity. The wind turbine had the lowest values for climate chance potential, with the PV, German grid, and Saudi Arabia Grid ranking next in order. Regarding acidification, the wind energy yielded the best results, while the Saudi Arabia grid performed the worst.

Reflecting the growing interest in the field, four LCA studies analyzing the environmental impact of SAF production via the P2L pathway were published between 2023 and 2024.[92–95] Despite using the same P2L production pathway, these studies varied in key assumptions regarding electricity sources, electrolysis technologies, carbon capture methods, and the specific FTS products considered. They also addressed different LCA impact categories, highlighting how differing methodological choices can influence the overall environmental assessment of SAF.

Guillén-Gosálbez and coworkers analyzed the production of diesel, gasoline, kerosene, light hydrocarbons, and waxes,[92] generated by P2L process. The study examined electricity generated from various sources, including wind, solar, nuclear, and
conventional electricity mix, using a PEM electrolyzer, alongside
two ways of CO₂ capture: DAC and coal power plant CO₂ capture
(COAL). Their combination resulted in eight different scenarios
that were compared with the fossil diesel production as bench-
mark. The results underscored that the electricity source used for
hydrogen production is a determinant of the process's environ-
mental impact. The authors ranked the environmental impact of
various electricity sources for fuel production as follows: conven-
tional electricity > solar energy > wind > nuclear energy. The
utilization of CO₂ from DAC is essential for balancing the CF
of the entire process. On a cradle-to-gate basis, the LCA analysis
showed that only scenarios combining wind or nuclear power
with CO₂ from DAC resulted in a lower environmental impact
than fossil diesel production, achieving net-negative carbon
emissions (−1.94 kg CO₂-eq kg⁻¹ diesel for wind and
−3.87 kg CO₂-eq kg⁻¹ for nuclear). Among these, e-diesel pro-
duced via nuclear-DAC offered the most favorable environmental
outcome. However, on a cradle-to-grave basis—considering fuel
combustion—the overall lifecycle impact remains carbon-
positive. The authors concluded that nuclear-DAC is the only sce-
nario with no adverse impacts on human health categories, while
all scenarios improve resource conservation by reducing fossil
resource dependence.

Rojas-Michaga et al.[93] examined a similar scenario for SAF production, involving a DAC unit, H₂ from water electrolysis, and a FTS plant. The selected DAC technology was the so-called low-temperature DAC, while H₂ is produced by an AEL powered by offshore wind electricity. The FTS reaction was catalyzed by Co-Pt/γ-Al₂O₃, known for its selectivity toward kerosene production.[89] The P2L process was optimized by reusing heat and water resources among the various processes. The LCA analysis revealed that the GWP of the SAF produced through this pathway is lower than fossil jet fuel and meets the UK aviation mandate for a 50% emissions reduction compared to fossil fuels. Among all the processes involved in SAF synthesis, electricity has the most significant impact. For example, a scenario using the current UK grid electricity mix resulted in a 1000% increase in GWP compared to offshore wind-generated electricity. In terms of water footprint, SAF exhibits a higher one than conventional fossil-based jet fuels. This difference in water consumption is primarily due to the refinery's cooling water requirements and the electricity's water footprint.

Another LCA study of SAF production assessed three electric-
ity sources, two H₂ production methods, and two DAC technol-
ogies to evaluate the environmental impact of different SAF
pathways relative to conventional jet fuel (Jet A-1).[94] The elec-
tricity sources included the German grid mix, onshore wind
farms, and PV arrays, all situated in Germany using data from
2015. H₂ production from water was evaluated using both
high-temperature SOEC and low-temperature PEM electrolysis,
while CO₂ capture employed both low-temperature and high-
temperature DAC methods. The study analyzed four SAF pro-
duction pathways, each using a unique combination of CO₂ cap-
ture and H₂ production technologies. Among these, the pathway
utilizing SOEC-derived H₂ powered by wind energy and CO₂
from low-temperature DAC showed the lowest environmental
impact across all LCA categories, reducing GHG emissions by up
to 89% compared to conventional jet fuels. PV electricity reduced

GWP relative to Jet A-1, but the German grid mix increased
impacts across all categories compared to conventional fuels.

P2L fuels showed additional environmental advantages by avoiding SO₂ and H₂SO₄ emissions and reducing those of NOₓ, which led to lower acidification and eutrophication impacts. However, PV-based electricity resulted in higher impacts in several impact categories such as acidification, eutrophication, photochemical ozone creation potential, and land transformation compared to Jet A-1 fuels. In terms of freshwater consumption, the combination of SOEC and low-temperature DAC demonstrated significantly lower impacts meaning it requires less water throughout its production process compared to the alternative technologies. Similar variations in impact levels occurred with wind power use, with differences depending on the specific P2L-kerosene pathway selected. These findings underline the importance of carefully evaluating energy sources and production methods in order to minimize environmental impacts during the transition to more sustainable aviation fuels.

Delgado et al.[95] analyzed the P2G process for producing kero-
sene, diesel, and naphtha using CO₂ captured from bioethanol
plants and transported via pipeline to an FTS production facility,
similar to that of Figure 10. Fossil-based low-sulfur diesel and jet
fuel are used as benchmark fuels. The required H₂ is produced
through high-temperature SOEC powered by a nearby nuclear
power plant. The SOEC's high electrical efficiency of ≈80%,
combined with nuclear energy's provision of low-cost, high-tem-
perature steam, makes this setup particularly advantageous for
H₂ production by significantly boosting overall efficiency. In
LCA was assumed close proximity between the CO₂ capture,
H₂ production, and FT synthesis facilities, reducing the environ-
mental impact associated with transport. Nuclear plants with
electricity generation capacities of 100, 400, and 1000 MW were
analyzed. For the rWGS and FT reactions, BaCe<sub>0.2</sub>Zr<sub>0.6</sub>
Y<sub>0.16</sub>Zn<sub>0.04</sub>O<sub>3</sub> and cobalt-based catalysts were used, respectively.
The results demonstrate a significant reduction in “Well-to-
Wheel” GHG emissions, reaching up to −25 g CO₂eq MJ⁻¹
when steam is directly generated for nuclear plant cooling.
This represents a nearly 130% decrease compared to the
85 g CO₂eq MJ⁻¹ required for conventional jet fuel production.
Notably, the scale of the nuclear plant has minimal impact on
these emission reductions. Nuclear energy’s inherent advantages
for electricity generation, including near-zero CO₂ emissions and
a minimal fuel requirement, contribute to the improved GHG
emissions. Moreover, its independence from external factors
ensures a reliable and continuous power supply, making it a
highly promising technology for mitigating environmental
impact.[96]

5.4.3. *Integrated P2L and P2G Process*

Gao et al.[97] conducted an exergy-based LCA on an integrated P2L and P2G system designed to simultaneously produce synthetic crude oil and e-CH₄. This integrated process schematically shown in **Figure 11** involves the FTS reaction (P2L), with a portion of the unreacted gas stream diverted to a second reactor for the Sabatier reaction (P2G). The analysis assumed a common CO₂ capture method—chemical absorption from cement plant flue gas. The impact of four different water electrolysis
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 11.** a) Process of CO₂ capture unit using monoethanolamine-solvent chemical absorption. b) Process flow diagram of integrated P2L and P2G units. c) Processes of different water electrolysis technologies. Adapted with permission.[97] Copyright 2023, Elsevier.</figcaption></figure>
technologies (AEL, PEM, SOEC, and AEM) on the P2L/P2G system was examined. A thermal power plant supplied the electricity for heating and pressurizing, while wind power provided the electricity for electrolysis. For SOEC, NG was used to provide the necessary heat for water conversion to steam. While the study mentioned the use of a Fe catalyst in the FTS, it did not specify the catalyst used for the Sabatier reaction. Additionally, it remains unclear whether the rWGS reaction, essential for converting CO₂ to CO, was considered in the analysis.

The results indicated that the integrated P2L/P2G system using AEL electrolysis achieved the lowest GWP and performed best in impact categories such as acidification potential (AP), which measures the contribution of emissions like sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) to acid rain and eutrophication potential (EP), which assesses the excessive input of nutrients into water bodies, leading to harmful ecological effect. However, SOEC technology excelled in ozone depletion potential (ODP) and human toxicity potential (HTP) due to its reduced renewable electricity consumption which minimizes the environmental impacts associated with power generation.

The superior performance of AEM electrolysis in AP and EP was attributed to its higher operational pressure and stack efficiency, leading to lower compressor and electrolyzer electricity demand. The electrolyzer emerged as the primary contributor to environmental impacts, highlighting its significant role in exergy loss. Interestingly, the assumed AEL and SOEC efficiencies were 97.3 and 63.9%, respectively, which appears to contradict typical values in the literature, where SOEC typically exhibits higher efficiencies than AEL.[98] This discrepancy in assumed efficiencies could affect the interpretation of results and suggests a potential area for further analysis.

### 5.4.4. General Conclusions on SAF

Summarizing the different studies regarding LCA of SAF, it becomes clear that the type of electricity impacts the GWP in a significant way. Off-shore wind, nuclear, and solar power result in significantly lower GWP compared to conventional electricity sources relying on fossil fuels. Integrating renewable resources helps improve almost all impact categories, with off-shore wind turbines and nuclear power emerging as the most promising options. It is important to note that the manufacturing and end-of-life stages of the equipment required for the DAC process and the cooling network contribute significantly to the environmental impacts, and they need to be taken into consideration in order to reduce these impacts.

Conversely, the water footprint of SAF appears higher than that of conventional jet fuels, primarily due to the additional water required in various stages of SAF production, and no significant improvement is observed in terms of risks to human health as the production and combustion of SAF still involve certain chemicals and pollutants that can pose similar health risks as conventional fuels. Apart from the fact that renewable resources present a much lower environmental impact, they also have a smaller energy demand. The different technologies used for H₂ production and CO₂ capture can notably affect the sustainability of SAF production.

## 5.5. Life Cycle Assessments of CO₂ Conversion to Marine Fuels

The shipping industry currently contributes about 2% of global energy-related CO₂ emissions, primarily due to its reliance on carbon-intensive fuels.[99] According to the International Maritime Organization (IMO), adopting alternative low- or zero-carbon marine fuels is essential to meet the target of Net Zero Emissions by 2050.[100]

Lee et al.[101] conducted an LCA of nine different production pathways for four alternative shipping fuels. Their analysis encompassed all stages from feedstock extraction to combustion in the ship's engine, as illustrated schematically in Figure 12. The authors calculated the CO₂-eq emissions of each fuel by multiplying the mass of CO₂, N₂O, and CH₄ by the GWP of each gas. Particularly, e-MeOH was produced from H₂ generated from solar electricity and CO₂ from DAC using a BaCe₀.₂Zr₀.₆Y₀.₁₆ Zn₀.₀₄O catalyst. Their analysis suggested that biomass-based FT-diesel, e-methanol, and e-ammonia are the most environmentally friendly options, with GHG reductions of 92, 88, and 87%, respectively. However, the authors noticed that considering current carbon prices using those fuels is not cost-effective.

A comparative LCA study of the impact of H₂, ammonia, MeOH, and waste-derived biofuels as potential shipping fuels is reported in ref. [102]. The goal was to determine the ability of the above fuels to contribute to the shipping industry's target of 100% GHG emission reduction. Green hydrogen, waste-derived biodiesel, and bio-MeOH were identified as having the most significant decarbonization potential. Compared to heavy fuel oil, these alternatives could reduce emissions by 74–81, 87, and 85–94%, respectively. However, the study highlights that none of these fuels are currently produced at a scale large enough to meet the shipping industry's energy demands. Additionally, while these alternative fuels hold significant potential for decarbonization, relying on them alone will not achieve the goal of a 100% reduction in emissions by 2050. To reach this ambitious target, new vessel designs and complementary decarbonization strategies, such as slow steaming and wind propulsion, will also be essential.

The maritime transport sector was also a key focus of reference,[103] which investigated the potential of bio-fuels produced using grid electricity mix across various European countries to achieve GHG emissions lower than those of conventional fossil fuels. Hydrogen produced by electrolysis, can serve directly as an energy carrier or be combined with carbon sources (mainly CO₂) to produce methane and MeOH. The LCA indicated that transitioning to biofuels could indeed reduce GHG emissions compared to fossil fuel production, but only in countries with low-carbon electricity grids. A threshold of 100 g CO₂eq kWh⁻¹ was identified for this reduction. The paper further explored the expanded use of renewable energy for biofuel production, considering Europe's projected electricity mix in 2050. The authors concluded that to meet the FuelEU Maritime goal of reducing GHG emissions by up to 75%, the production of biofuels must be linked to national electricity mixes with GHG intensities below 17 g CO₂eq kWh⁻¹. According to the authors, this target is achievable due to the growing dominance of renewable electricity in many European national grids.
<figure><img src="image_16.png" /><figcaption>**Figure 12.** Schematic representation of the various stages in the marine fuels life cycle from feedstock extraction, conversion and transportation, storage, and combustion in the ship engine. Reproduced with permission.[101] Copyright 2024, MDPI.</figcaption></figure>
In an effort to decarbonize the shipping sector, Grahn et al.[104] conducted a comparative analysis of the environmental and economic impacts of three emerging ship propulsion technologies: 1) internal combustion engines powered by MeOH, NH₃, and H₂, each produced through electrified processes; 2) fuel cells fuelled by NH₃ and H₂ to generate on-board electricity; and 3) battery systems. These alternative technologies were benchmarked against the currently dominant approach: conventional internal combustion engines powered by fossil-based maritime fuels. The study was set in 2030, assuming a 25 year operational life for the ships. Europe was chosen as the geographical location of the prospective life cycle assessment (pLCA). A total of four processes require electricity: fuel production and liquefaction, component manufacturing, mooring at Gothenburg port, and mooring at Kiel port, each utilizing a different electricity provider. The European electricity mix is employed for component manufacturing, while Swedish and German electricity mixes are used for the mooring from Gothenburg and Kiel, respectively. However, for fuel production, only electricity originated onshore wind is utilized. The results indicate that in all pathways, there is a possible reduction in climate change. In the case of carbon-based energy carriers, that is, MeOH, the most contributing process appears to be the combustion phase, while in the carbon-free ones (i.e., H₂ and NH₃), the most affecting processes are the fuel production phase and the electricity generation. Mitigation in several impact categories such as acidification, ozone depletion, photochemical oxidation, and particulate matter was noticed in all decarbonization methods. The GWP is highly affected in the options with low energy conversion efficiency as greater amounts of electricity are necessary. If the carbon intensity of the electricity mix is less than 150 gCO₂eq kWh⁻¹, in all non-fossil-fuel options, a lower GWP is observed compared to reference case throughout their entire life span.

## 5.6. Life Cycle Assessments of CO₂ Conversion to Polymers

Hoppe et al.[105] explored the utilization of CO₂ and H₂ in the production of CH₄, MeOH, and syngas, with a focus on their subsequent use as precursors for polyethylene (PE), polyoxymethylene (POM), and polypropylene (PP) synthesis. In their LCA, the authors evaluated two processes for synthesizing thermoplastic polymers. The first process utilizes CO₂ and electrolytic H₂, integrating the P2G and P2L technologies to produce CH₄, MeOH, and syngas which serve as feedstock for polymer synthesis (Figure 13a, top). The second process (Figure 13a, bottom) represents the conventional approach, relying on fossil fuels. In the CO₂-based method, CO₂ was captured from various sources, including air, cement plants, and municipal waste incineration facilities. H₂ was produced via electrolysis, although the study does not specify the exact electrolyzer technology considered. Wind power was used for H₂ production, while the German electricity mix was applied for other processes. The catalytic reactions considered in the CO₂-based polymer synthesis are as follows: the Sabatier reaction (R. 10), followed by the SMR (R. 1) and finally the CO₂-methanol synthesis (R. 14).

$$CO_2 + 3H_2 \rightarrow CH_3OH + H_2O \quad (R. 14, syngas methanation) \quad (14)$$

Figure 13b illustrates the gain or loss in global warming impact (GWI) for all products and CO₂ sources analyzed in this study. With the exception of CH₄ and MeOH, the production of all other chemicals using the CO₂-based synthesis method results in a positive GWI, indicating increased CO₂ emissions. However, for most products, the CO₂-based synthesis generally exhibits lower GWIs compared to the fossil fuel-based method. A notable exception is polyolefins (PP and PE) produced using CO₂ from DAC, which shows the opposite trend. The highest GWI gain is observed when CO₂ is captured from cement plants, while the DAC path consistently demonstrates the lowest. This can be attributed to the significant heat and electricity demands associated with the DAC process. Regarding material-oriented indicators, raw material input (RMI) and total material requirement (TMR) are lower for chemicals produced from fossil fuels than those produced from CO₂. Once again, CO₂ capture from cement plants offers the best performance among the various CO₂ sources. The sensitivity analysis underscored the critical role of renewable electricity in mitigating the GWI of the process.

Nishikawa et al.[106] conducted an LCA study exploring the conversion of CO₂ into CH₄, MeOH, and diesel, which are subsequently used for polymer production, transportation, and power generation. A unique aspect of this study is its focus on different transformation pathways for CO₂, including electrochemical processes like the coelectrolysis of CO₂ and H₂O in SOEC (R. 2) and heterogeneous catalytic processes (thermochemical) such as the reverse water-gas shift (rWGS) reaction (R. 3). The required hydrogen for these catalytic processes is generated either by low-temperature PEM or high-temperature SOEC. The authors define two main scenarios. The first is the “purely electrochemical” scenario, where CO₂ and H₂O undergo coelectrolysis to produce syngas, which is then converted to the target products via TC reactions (R. 11, 4, and 10). The second, referred to as the “hybrid” scenario, combines electrochemical processes with thermochemical (heterogeneous catalytic) reactions (R. 10, 14, 3, and 11). Importantly, both scenarios involve TC reactions, as even in the “purely electrochemical” pathway, the syngas produced requires subsequent TC conversion to reach the final product.

The study highlights that the source and consumption of electricity during the conversion process are key factors influencing GHG emissions. Other influential factors include processing unit specifics, such as SOEC degradation rates, energy demand for H₂ production, electrolyzer type, and FTS conversion efficiency. An interesting aspect of this work is the sensitivity analysis focusing on the electricity source parameter. Several scenarios were examined, including a low-carbon scenario powered by renewable energy sources such as hydropower, solar PV, wind, and nuclear energy, with geothermal heat as a supplement. In contrast, the high-carbon scenario relied on electricity from coal plants and heat from combined heat and power (CHP) plants. The results of the sensitivity analysis demonstrated that using renewable electricity sources could reduce GHG emissions by up to 115% compared to the reference scenario, whereas the high-carbon scenario could increase GHG emissions by up to 77%.
<figure><img src="image_17.png" /><figcaption>(b) GWI of CO₂-Based and Conventional Polymer Production Pathways</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 13.** a) Overview of the pathways analyzed in the LCA for CO₂-based (top) and conventional (bottom) synthesis of POM, PE, and PP polymers. b) GWI for production of basic chemicals and polymers from different CO₂ sources compared to the conventional method. MTO: MeOH-to-olefins; PE: polyethylene; POM: polyoxymethylene; PP: polypropylene. Adapted with permission.[105] Copyright 2022, Wiley-VCH GmbH.</figcaption></figure>

**5.7. LCAs of Other Chemical and Fuel Production from CO₂**

**5.7.1. Introduction**

In addition to CO, CH₄, MeOH, SAF, and polymers, the production of several other chemicals and fuels through the catalytic conversion of CO₂ has been evaluated in LCA studies. These include e-diesel produced via the FTS process using iron and cobalt-based catalysts,[107] electrochemically produced e-ethanol using a copper-based catalyst,[108] oxymethylene ethers (OMEs), and formaldehyde.[109,110] It is important to note that while the production of these chemicals was also considered in some of the previously mentioned studies, the LCAs reviewed in

this section specifically focus on the production of these particular compounds.

**5.7.2. e-Diesel and Alkane-Alcohol Blends as Possible Diesel Substitutes**

Diesel is a popular hydrocarbon-based fuel primarily composed of alkanes, cycloalkanes, and aromatics. Traditionally, diesel is produced from crude oil through fractional distillation and refining processes. In the pursuit of more sustainable energy solutions, alternative diesel fuels derived from renewable sources while maintaining a similar chemical composition are being
explored. One promising alternative is synthetic diesel, often referred to as e-diesel, produced through the FTS process using captured CO₂ and green H₂.[¹¹¹] This approach offers the potential to close the carbon cycle and reduce GHG emissions.

Lieu et al.[¹¹²] conducted a cradle-to-grave LCA study to assess the GHG emissions associated with the complete process of e-diesel production, from raw material supply (e.g., extraction of materials for the FTS catalyst) to end use (e.g., e-diesel combustion in vehicles). The LCA incorporated data from a pilot DAC plant in Canada and FTS data obtained from the literature. The study estimated that, under conditions of low emissions from electricity use, the entire process emits only 0.51 gCO₂eq per g of CO₂ captured from the air—representing a highly favorable scenario. The authors further calculated that an electricity emissions factor below 139 gCO₂eq kWh⁻¹ is necessary for this pathway to achieve a climate advantage over conventional diesel fuel. Consequently, e-diesel synthesis facilities should be located in areas with extremely low grid emissions factors or, preferably, colocated with newly built renewable energy sources. Interestingly, consistent with previous studies, the analysis showed that replacing NG with electricity in high-temperature thermal treatments can reduce GHG emissions by up to 60%. Among electricity sources, wind power was identified as having the lowest environmental impact per unit of exergy produced. A distinctive aspect of this study is its detailed consideration of the environmental impact associated with the production of cobalt-based catalysts used in FTS. It was found that catalyst production contributes only 2% to the total emissions, a relatively minor share.

The sustainability of utilizing CO₂ sourced from anaerobic digestion of sewage sludge for production of transportation fuels via the FTS was evaluated in ref. [113]. Although FTS produces a wide range of hydrocarbon products, which require subsequent separation, the study does not precise the composition of the fuels produced via the FTS and they are simply described as “liquid fuels”. For large-scale plants (1670 t day⁻¹), the FTS fuels outperform fossil diesel in all environmental impacts across all the designs, with several impacts being net-negative. The only exceptions are the ozone depletion category, for which fossil diesel is the best option, and GWP, which is lower for fossil diesel for some process designs. Optimizing the systems reduces the GWP of FTS fuels in the best case by 70% below that of fossil diesel.[¹¹³]

Recently, an innovative process has emerged, combining FTS (R. 11) with hydroformylation and hydrogenation reactions to produce alkane-alcohol blends, acronymized as HyFiT, as potential diesel substitutes.[¹¹⁴] A comprehensive well-to-wheel LCA was conducted to compare the environmental impact of HyFiT fuel with both fossil diesel and battery electric vehicles (BEVs). The syngas required for the FTS reaction was sourced from either nonfood competitive biomass (bio-HyFiT) or CO₂ captured through DAC (CO₂-HyFiT). CO₂ was transformed to syngas by three different methods: rWGS (R. 3), DRM (R. 7), and Sabatier reaction (R. 10). Hydrogen was produced via water electrolysis using a PEM cell, powered by the projected global electricity mix in 2030. The CF of bio-HyFiT fuel was found to be lower than that of fossil diesel and even competitive with BEVs across a range of electricity impact scenarios. In contrast, CO₂-HyFiT fuel requires low-carbon electricity (below 65

gCO₂eq kWh⁻¹) to outperform diesel. HyFiT fuel also exhibited improved performance in other environmental categories compared to both diesel and BEVs. Specifically, compared to diesel, HyFiT demonstrated lower consumption of energy carriers, as well as reductions in ozone depletion, ionizing radiation, and CF. When compared to BEVs, it exhibited decreased resource use of minerals and metals, along with lower acidification and freshwater eutrophication impacts. However, in the land use category, bio-based HyFiT performs worse, as expected due to its reliance on biomass.

### 5.7.3. Formic Acid

Formic acid (HCOOH) is the simplest carboxylic acid and a versatile chemical with several applications ranging from food processing to chemical industry. It has also gained attention as a promising liquid organic hydrogen carrier (LOHC) due to its low toxicity and low flammability under ambient conditions.[¹¹⁵] In conventional MeOH carbonylation production processes, formic acid is synthesized through a two-step reaction: first, MeOH reacts with CO in the presence of a catalyst to form methyl formate (HCOOCH₃) followed by hydrolysis to yield HCOOH. Since 2022, two reviews and one perspective paper have been published providing comprehensive summaries of LCA studies on HCOOH synthesis from CO₂.[¹¹⁶⁻¹¹⁸] The authors of reference[¹¹⁶] highlighted that most assessments neglect the impact of CO₂ storage and transportation to the HCOOH synthesis site. Additionally, they identified electricity as a critical factor in developing an energy-efficient pathway for CO₂ utilization and highlighted the promising potential of CO₂-to-HCOOH conversion.

Very recently, Ghazi et al. reviewed the sustainability of the electrochemical conversion of CO₂ to HCOOH.[¹¹⁷] The authors acknowledged that the electrochemical conversion of CO₂ to HCOOH has currently a larger environmental footprint, due to its high energy consumption, compared to conventional production methods. However, they emphasized that this process holds significant potential for future implementation. They suggested that advancements in materials and engineering, including optimization of design and operational parameters, could enhance its sustainability. Crucially, increasing the share of green electricity sources, particularly using solar power, could play a pivotal role in making the process more environmentally friendly and economically viable.

The electrochemical conversion of CO₂ to HCOOH was also a key focus in reference.[¹¹⁸] As in previous studies, electricity was identified as the most critical factor influencing the process. The study highlighted that only by utilizing low-carbon electricity sources can the process achieve a GWP impact comparable to conventional fossil-fuel-based production routes; otherwise, a net increase in GWP is observed.

Additionally, the study demonstrated that GWP is not the only challenge. The fossil-based route outperforms the CO₂-based process in other LCA categories, such as land use and water consumption because of the required infrastructure and the processes associated with extraction and refining. Consequently, the authors emphasized that significant technological advancements are essential to achieve net environmental benefits from
this process particularly in minimizing resource consumption
and improving overall sustainability.

5.7.4. *e*-Ethanol

Ethanol (EtOH) is one of the most extensively utilized chemicals in both domestic and industrial applications and serves as a key component in numerous possesses. Different synthesis routes have been advanced for its production, and a lot of research has focused on its exploitation. Despite being a newly introduced technology, the EC CO₂ reduction reaction for EtOH production (EC-EtOH) has proved its capability as an alternative synthesis route.[119,120] Two recent LCA studies examined the sustainability of EC-EtOH process in comparison to conventional strategies.[121,122]

The earlier of the two studies[121] compared the EC-EtOH
process with EtOH produced via grain fermentation across
nine LCA impact categories, including GWP, eutrophication,
and acidification. The results indicated that the EC-EtOH
process becomes competitive with the fermentation in terms
of GWP when the electricity used has a carbon intensity below
63 g CO₂eq kWh⁻¹. However, with electricity mixes that have a
lower share of renewable energy, the EC-EtOH process is less
favorable.

Mo et al.[122] employed LCA as a method to better understand how the electrochemical EtOH production process affects the environment compared to thermochemical and biochemical processes, and to evaluate its overall impact in a quantitative scale. Special focus has also been given to examining the potential substitution of the EU electricity mix with electricity generated from renewable resources, such as hydropower, solar power, wind power, bioenergy, and geothermal energy and exploring the feasibility of this approach. The LCA was supplemented with a sensitivity analysis to investigate the extent to which replacing the conventional electricity mix with electricity generated from renewable sources would impact the total score of the three processes across the impact categories. Although there are various sources of renewable energy production in Europe, the sensitivity analysis mainly focused on the potential of electricity provided by hydropower, solar, and wind energy. It revealed that the two conventional processes were almost unaffected by the substitution of the electricity grid with renewable electricity. Impacts originating from biomass cultivation, heat utilities, transportation, CO₂ emissions, and discharge of flue gas and wastewater not only remained the same, but also outweighed all the beneficial contributions provided by the substitution. In the case of using wind power, the environmental impact in terrestrial ecotoxicity potential (TETP) increased for all three processes due to the high demand for copper wires necessary to support this energy source.

Contrary to the two conventional processes, the electrochemi-
cal process had negative impact indicators, when supplied by
renewable electricity. There was a significant reduction ranging
from 108 to 750%, with wind power being an exception at 73%,
indicating that the impacts caused by electricity usage are almost
eradicated. Among the renewable sources, hydropower appears
to be the most promising renewable source for mitigating nega-
tive environmental consequences with a remarkable reduction of

4274%, followed by geothermal at 3297% and solar energy at
3144%. The study finally revealed that among all the impact cat-
egories, TETP, human toxicity noncancer potential, ionizing
radiation potential, GWP, and fossil resource scarcity potential
were the most impactful, having the highest scores, particularly
TETP, which had a notably higher value compared to other indi-
cators. Consequently, future studies on ethanol production
should focus on these specific impact categories for effectively
mitigating the environmental impacts associated with the pro-
duction process.

5.7.5. Oxymethylene Ethers (OMEs)

Oxymethylene ethers (OMEs) are a group of organic compounds being investigated as potential synthetic fuels and fuel additives owing to their clean-burning properties.[123] The synthesis of ethers involved complex chemical processes that fall outside the scope of this review. An early LCA on OME-based fuels, focused on an OME₁-diesel-blend[124] (the number in the OME designation refers to the number of oxymethylene groups (—CH₂O—) in the molecule). The study considered two different pathways: the conventional route involving condensation with formaldehyde, and a novel direct route utilizing CO₂ and H₂. The CO₂ used in the process is sourced either from biogas or through DAC technology. The LCA aims to assess the environmental benefits of OME₁-blends over fossil diesel and compare conventional OME₁ production methods relying on MeOH and formaldehyde, to a more sustainable approach using CO₂. In the most promising scenario, the efficacy of the OME₁ almost reaches the ideal blending component and mitigates GWI as well as impacts related to the fossil depletion category. The potential benefits of OME₁-blends over diesel depend on using electricity mix from low-GWI, low-CED (cumulated energy demand) sources for electrolysis. Thus, renewable electricity and grid mixes with a large share of renewable or nuclear electricity appear to be convenient sources of electricity. Since the conventional synthesis route uses less hydrogen, it is less sensitive to the electricity sources used. Surprisingly, NOₓ and soot emissions decreased beyond the proportional reduction in diesel fuel in their mixture, suggesting a synergistic effect between OME₁ and diesel. Overall, when renewable energy is employed in the production of OME, then OME₁-diesel blends can significantly mitigate the negative impacts on both human health and the environment.

A recent well-to-tank LCA study compared the environmental impact of DME, OMEs, and MeOH, all produced from CO₂ and H₂ feedstock via the P2L processes.[125] The CO₂ was captured from a lignite power plant, and H₂ was generated through alkaline water electrolysis (AEL). Three electricity sources were evaluated: the German electricity mix for 2021, the German electricity mix projected for 2030, and wind power. MeOH was synthesized using the CO₂-based methanol synthesis reaction (R. 14) with a CuO/ZnO/Al₂O₃ catalyst. The results were benchmarked against fossil diesel produced through conventional refining pathways.

The study found that using the 2021 German electricity mix resulted in five to eight times higher GHG emissions for all PtL fuels compared to conventional diesel production. However, the
2030 German electricity mix is expected to significantly reduce GHG emissions by a factor of 3. When electricity is sourced entirely from wind power, GHG emissions decrease by up to 15 times compared to the 2021 mix. In total, DME and MeOH exhibited considerably lower environmental impacts than OME3-5 due to the lower energy intensity of their production processes.

In addition to GWP, the study evaluated 12 additional LCA impact categories, including human toxicity, particulate matter, terrestrial acidification, and freshwater ecotoxicity. For most categories, fossil diesel outperformed synthetic fuels, except for MeOH. Notable exceptions were human toxicity (noncancer) and freshwater ecotoxicity categories, where synthetic fuels, particularly those produced using wind power, demonstrated better performance due to their lower emissions and reduced environmental impact during production.

### 5.7.6. Ethylene

Ethene ($C_2H_2$), commonly known as ethylene, is a versatile chemical widely used as a building block in various industrial processes, especially in the production of polymers. Traditionally, ethylene is produced via steam cracking, an energy-intensive process that involves breaking larger hydrocarbons derived from fossil fuels by heating them to high temperatures in the presence of steam.[126] However, the growing emphasis on decarbonizing the chemical industry has stimulated the search for alternative production methods. One promising approach is the electrochemical conversion of $CO_2$ to $C_2H_2$ (EC-C$_2$H$_2$) which utilizes captured $CO_2$ as a feedstock and relies on low-carbon electricity.[127] This method has the potential to reduce greenhouse gas emissions associated with ethylene production, aligning with sustainability goals. It is important to clarify that, unlike technologies relying on syngas, such as FTS, the EC-C$_2$H$_2$ process does not require H$_2$. Instead, the electricity in this process is directly utilized for the electroreduction of $CO_2$ in devices similar to those depicted in **Figure 14a**.

Three LCA studies on ethylene production via the electrochemical route are reported in the last 4 years.[128–130] All studies adopt a cradle-to-gate LCA approach, beginning from $CO_2$ supply up to the final ethylene output. They consider a similar electrochemical cell configuration (see Figure 14a), employing copper-based electrocatalysts for $CO_2$ electroreduction. However, differences exist in the processes analyzed and the LCA methodologies applied. It should be noted that EC-C$_2$H$_2$ technology is still in a low TRL. Therefore, technological advancements, particularly in the efficiency of electrochemical conversion devices, will impact the conclusions of the assessment.[130]

In a 2020 paper, Khoo et al.[128] explored the EC-C$_2$H$_2$ at both laboratory and industrial scales, considering facilities producing 1 g and 1 ton of C$_2$H$_2$, respectively. $CO_2$ was sourced from a power plant, with energy supplied either by the Singapore electricity mix (primarily NG-based) or renewable sources, predominantly wind power. Processes associated with electrocatalyst production were not included in LCA, which, as subsequent studies showed, can have a substantial environmental impact. The study's findings on GHG emissions aligned with patterns observed in most LCA studies: reduction in net $CO_2$ emissions

<figure><img src="image_19.png" /><figcaption>**Figure 14.** a) Schematic representation of the basic operational principles of an electrochemical cell for the electrochemical reduction of $CO_2$ to C$_2$H$_2$ (EC-C$_2$H$_4$ cell). b) Normalized sensitivity of the EC-C$_2$H$_4$ cell performance indicators with respect to net GHG emissions. Green and orange bars indicate the reduction or increase in net emissions when the respective indicators are altered. The sensitivities of cell current density and consumables lifetime were found to be negligible and were therefore excluded from the determination of benchmark values. FE stands for Faradaic efficiency. Adapted with permission.[130] Copyright 2024, Wiley-VCH, GmbH.</figcaption></figure>

was only achieved when renewable energy sources were used, while conventional electricity grids led to a net increase in GHG emissions.

Additionally, industrial-scale EC-C$_2$H$_2$ production showed potential negative effects on other LCA impact categories, such as acidification potential (AP) and photochemical ozone creation potential. These impacts stem from increased emissions of acidifying pollutants, such as sulfur and nitrogen oxides, as well as ozone-forming compounds like volatile organic compounds (VOCs) and nitrogen oxides ($NO_x$). Finally, the authors emphasized that the scale of the process influenced the results in complex ways, with additional variables such as energy demands, material efficiencies, and emissions, making it unsuitable to simply extrapolate small-scale findings linearly to industrial scales.

A 2022 study[129] considered both conventional and sustainable electricity sources—namely hydropower, wind, solar, and biomass—while expanding the LCA model to include the synthesis of catalysts and the preparation of electrodes. The findings revealed that ethylene C$_2$H$_2$ production via EC-C$_2$H$_2$ offers limited environmental benefits in terms of GHG emissions.
However, when renewable energy sources are utilized, signifi-
cant reductions are observed in several impact categories.
Human health damage decreases due to lower emissions of
harmful pollutants. Ecosystem damage is mitigated through
reduced acidification and resource depletion. Additionally, petro-
chemical resource use is significantly lowered, as the process
becomes less reliant on fossil fuel-derived inputs compared to
conventional production methods.

A notable finding was that the preparation of Cu-based electro-
catalysts accounted for the highest environmental impact among
all processes, contributing 55% of the total impact. This under-
scores the critical need for further research into novel electroca-
talysts and more sustainable preparation methods to improve the
overall environmental performance of the EC-C₂H₂ process.

Recent research has highlighted inconsistencies in net GHG emissions across the previous LCA studies on EC-C₂H₂ processes.[130] This discrepancy is attributed to the rapid advancements in electrochemical CO₂ conversion to C₂H₂ technology, which directly impacts calculated GHG emissions. To address this, the authors aimed to establish benchmark performance parameters for EC-C₂H₂ to achieve a target net GHG emission of -0.5 t-CO₂eq/t-C₂H₄. The analyzed EC-C₂H₂ process involves the following steps: CO₂ capture from a DAC unit, CO₂ purification in an electrochemical device (CO₂ enrichment), electrochemical reduction of CO₂ to C₂H₂ in an electrochemical cell, and finally cryogenic purification of the cell's output mixture and combustion of the byproducts. The process is powered by a combination of PV and wind energy.

Through sensitivity analysis, four critical factors (indices) were identified as key determinants of the overall process's GHG emissions and their minimum values required to achieve the target of -0.5 t-CO₂eq/t-C₂H₄ were set (see Figure 14b), in particular, 1) a minimum of 3.5 V in the electrochemical cell voltage (V<sub>ECO2R</sub>); 2) a minimum of 70% in the Faradaic efficiency (FE), meaning that 70% of the total charge passed through the cell is used for the desired reaction rather than side reactions; 3) a minimum conversion rate of 20%, the proportion of CO₂ converted to ethylene relative to total CO₂; and 4) the CO₂ capturing energy related to the enrichment cell voltage should not exceed 2.2 GJ/t-CO₂. While the electrochemical performance metrics reported in the literature are typically within 10% of the benchmarked values, a significant challenge lies in the high degradation rate of the current EC-C₂H₂ systems, which falls short of the 1 year system lifetime benchmark.

**5.8. Conclusions on LCAs on other Fuels**

Producing e-fuels through catalytic conversion is a very promis-
ing approach for achieving sustainability. All of the e-fuels dis-
cussed are produced using specific catalytic methods in order to
reach carbon neutrality and negative GHG emissions. The pro-
duction of e-diesel is optimized in terms of environmental
impacts when DAC is used in combination with renewable ener-
gies. Particularly the use of an electric calciner leads to improved
environmental performance by minimizing dependence on fos-
sil fuels with even better results achieved through the integration
of wind power. In the case of formic acid, whether they are syn-
thesized through homogenous catalysis or with electrochemical

reduction of CO₂, various environmental benefits are observed. It
has been proven that when renewable energies are used, the pro-
duction leads to fewer carbon emissions compared to conven-
tional formic acid. For syngas production, the integration of
renewable energies leads to a significant reduction of the envi-
ronmental impacts compared to conventional electricity based
on fossil fuels. Interestingly, for ethanol production, the transi-
tion from conventional energy sources to renewable ones had
only a minor effect on the environmental outcomes.

The overall environmental performance of the process not only was not improved by a noticeable level but on the contrary when using wind power some impact categories became worse because of the high demand for copper wires. This raises many concerns about balancing the environmental benefits of renewable energies with the drawbacks of changing or using specific equipment.

**6. Future Perspectives and Main Limitations**

Catalytic and EC processes for converting CO₂ into value-added products using renewable energy offer a promising approach to mitigating climate change and transitioning to a low-carbon economy. However, the complexity of these systems, influenced by factors such as CO₂ sourcing, hydrogen production technologies, and electricity inputs, poses challenges in identifying the optimal solutions with the lowest environmental impacts. LCA is a critical tool for evaluating the environmental feasibility of emerging technological processes. This paper reviewed recent LCA studies encompassing the full spectrum of available technologies and configurations for catalytic CO₂ conversion, detailing critical characteristics of each, including catalyst types, electricity sources, and chemical pathways. By categorizing findings based on CO₂ conversion products and comparing emerging technologies to conventional methods, the review enables the identification of critical factors that dominate the environmental impact of each process.

Early LCA studies investigating the environmental impact of using captured CO₂ as a feedstock for chemical synthesis highlighted significant potential benefits compared to traditional processes reliant on fossil resources. However, nearly a decade later, the works reviewed in this paper have made it evident that the environmental advantages of CO₂-based processes are not always guaranteed, as the current performance of most emerging technologies remains insufficient to consistently achieve this target. Beyond the technological bottlenecks associated with these processes, the carbon intensity of the electricity used—whether indirectly for H₂ production or directly in CO₂ electroreduction—emerges as the critical factor determining whether these emerging technologies can truly be environmentally sustainable. However, given that many of the technologies considered in the LCAs reviewed in this article are still in the development stage and have not yet reached the maturity required for large-scale industrial implementation, it is challenging to apply the results of this study in a definitive or conclusive manner. Instead, the paper identifies key characteristics and differences across the various strategies (e.g., electricity sources, CO₂ capture technologies, catalytic processes, and hydrogen production
methods) which can help identify potential areas for improvement and further development.

Nevertheless, the presented results clearly highlight the benefits of electrification in the chemical sector, which is one of the three major contributors to GHG emissions, alongside steel and cement production. While electrification can reduce GHG emissions, its full potential can only be realized by considering all emission sources and energy input availability. Significant GHG reductions are expected with the adoption of new operational processes that imply efficient heat management, since most of the catalytic reactions involved in CO₂ transformation take place at high temperatures. That said, it is important to emphasize that heat management alone cannot drastically alter the overall environmental cost of the process. Additional factors must also be considered, such as the role of external stimuli in the catalytic or EC conversion (e.g., electric or magnetic fields) and innovations like defective catalysts, to fully leverage the potential of electrification in the chemical industry. When the CF of available electricity is high, H₂ production can be achieved through alternative methods like catalytic methane pyrolysis or using nonbiomass waste (plastic), as outlined in the Hydrogen Europe Report 2024.[131] Additionally, using onsite biomethane in methane pyrolysis can significantly reduce the overall environmental impact.

## Acknowledgements

A.P. and S.Z. acknowledge the financial support of the French Agence Nationale de la Recherche (ANR), under the projects DuCaCO₂ (grant no. 236294).

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

**Anastasia Pappa:** data curation (lead); methodology (equal); validation (equal); visualization (supporting); writing—original draft (equal); writing—review and editing (supporting). **Cuong Pham-Huu:** validation (supporting). **Spiros Papaefthimiou:** validation (supporting); writing—original draft (supporting); writing—review and editing (supporting). **Spyridon Zafeiratos:** conceptualization (lead); funding acquisition (lead); methodology (lead); project administration (lead); resources (lead); supervision (lead); validation (lead); visualization (lead); writing—original draft (lead); writing—review and editing (lead).

## Keywords

carbon capture and utilization, CO₂ catalytic conversion, electrolysis, greenhouse gas emissions, life cycle assessment, renewable electricity

Received: November 24, 2024

Revised: February 26, 2025

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Anastasia Pappa is a PhD student in Chemical Sciences at the University of Strasbourg, France, conducting her research at the Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES). She holds a degree in Production Engineering and Management from the Technical University of Crete, Greece. Her work focuses on the development and life cycle assessment of heterogeneous catalysts for CO₂ electrified conversion, with the aim of advancing sustainable technologies for carbon capture and utilization.
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**Cuong Pham-Huu** is Research Director at the Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES, UMR 7515 CNRS, University of Strasbourg). His research interest focuses on the electrification of chemical sector by combining carbon and/or supported metal catalysts and induction heating for operating new catalytic processes with low carbon footprint. Details related to his research can be found at https://scholar.google.fr/citations?user=t-7F8s4AAAAJ&hl=fr

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**Spiros Papaefthimiou**, Professor at the Technical University of Crete, specializes in renewable energy, energy efficiency, and smart materials for energy applications. His research focuses on photovoltaics, solar thermal systems, sustainability-circular economy, energy policies, and market mechanisms. He has authored over 60 peer-reviewed papers, with more than 3500 citations with an h-index of 34. He has been actively involved in more than 50 research projects and several international conferences and academic collaborations. He serves as President of the Hellenic Association for Energy Economics and was ranked among the world's top 2% scientists (2022–2024) in Energy by Stanford University.

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**Spyridon Zafeiratos** is a Research Director at the French National Centre for Scientific Research (CNRS) and head of the Catalysis and Materials Department at ICPEES, a joint institute of CNRS and the University of Strasbourg, France. His research focuses on sustainable energy technologies, including CO2 conversion and hydrogen production via solid oxide electrochemical cells, with an emphasis on designing and characterizing innovative materials. His recent work explores the electrification of catalytic processes and the life cycle assessment of catalytic technologies, aiming to develop efficient, environmentally sustainable solutions for the energy transition.