Authors: Romy Ettlinger | Quim Peña | Stefan Wuttke
Source: Advanced Functional Materials (article, 2024-03-11)
Field: Chemistry — Inorganic Chemistry

Nano‐to‐Macroscale Reticular Materials to Address Societal Challenges Reticular materials are among the most recent synthetic functional porous materials.[1, 2] Over the last years, reticular materials, such as metal–organic frameworks (MOFs), zeolite imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs), have increasingly attracted high academic (research) and industry attention. Reticular chemistry provides high material design versatility due to the possibility of self-assembling a broad range of diverse molecular (inorganic and/or organic) building block units into porous frameworks – with the intrinsic potential to achieve molecular-level precision across different length scales (nano-, meso-, and macro-scale).[1, 3] As a result, reticular materials have been often claimed to “be limited only by one's imagination” and the adjectives “highly diverse and ever-growing” are widely used these days. Yet, about three decades after the first reports on reticular materials,[1, 4-6] it is time to look at their evolution and the academic and industrial impact, and to critically ask ourselves: “What are the intrinsically unique features of reticular materials and how can we promote the transfer of scientific knowledge into industrial, technological and societal impact?” To understand where the field stands today, we assessed the generated academic and industrial (market) impact over the years 1984–2023. By first looking at the academic output of reticular materials using publication-based statistics (Figure 1a–d), the year-by-year analysis of the number of publications underscores the exponentially increasing research interest in these materials, with a total of almost 48 000 publications since the 1990s, and about 20% of those reported in 2023 alone (Figure 1a). Concretely, MOFs are among the most explored sub-class of reticular materials and account for 77% of all these publications. Breaking the 48 000 publications down by the authors´ country affiliation shows the international repercussion of reticular materials, with more than 70% of the world's countries represented (NB: in 2023, reports originate de facto from 140 different countries of all the official 195 countries) (Figure 1b,c). In this context, the People's Republic of China, the United States of America, and the Republic of India are in the top three of the world's countries based on the absolute number of published documents on reticular materials (Figure 1b), together accounting for more than 75% of the publications in the field by 2023. Within European countries, the United Kingdom has published the most, followed by Germany, Spain, and France. Interestingly, the ranking of the 14 top-publishing countries (top 10% of countries with most publications on reticular materials) changes if considering the number of publications per number of residents, now with Singapore – quite outstandingly – being the country with the highest number of published documents per million residents (Figure 1c).[7] Regarding the different subject areas in which research on reticular materials has been reported, chemistry (28.3%), materials science (20.5%), and chemical engineering (15.2%) together account for more than 60% of the publications on the topic (Figure 1d), while the rest are distributed in multiple different areas, including engineering (9%), physics and astronomy (7.9%), environmental science (5.5%), energy (5.4%), biochemistry, genetics and molecular biology (4.6%), medicine (0.8%) and pharmacology, toxicology and pharmaceutics (0.7%). The diversity in the different topic categories evidences the high interdisciplinary research interest and application potential of reticular chemistry and the corresponding nano-to-macroscale materials. Beyond the academic (research) outcomes in the form of scientific publications, reticular materials have already started to gain more and more industrial relevance and are being increasingly explored toward real-world applications (Figure 1e).[8] This can be illustrated by the raising trend in the last years in the global market size of one of the most prevalent sub-classes of reticular materials, MOFs (Figure 1e). After reaching US$ 124 million in 2020,[9] US$ 271.1 million in 2021,[10] US$ 308.26 million in 2022,[11] and US$ 420.19 million in 2023,[11] the market size of MOFs is expected to grow to US$ 1396.2 million by 2029,[12] which corresponds to a compound annual growth rate of ≈25% since 2020. Similarly, more than 40 companies and spin-offs have been founded during the last 5–10 years aiming to explore industrial feasibility of MOFs (and other reticular materials) for a range of diverse applications, such as sensing, water harvesting, gas storage, and separation.[13] The exponential growth in research publications, together with the increasing economical investment and expected market size growth rate, illustrate the high interest in this type of materials, both at the level of fundamental research and (industrial) applicability. However, despite the application potential of reticular chemistry, the number of nano-, micro-, and macro-scale reticular structures crossing the research-to-innovation technology gap remains relatively low. To unlock the technological potential of reticular materials to address societal challenges, it is time to reflect on their developmental and industrial feasibility, commercial value, and their intrinsic ability to address unmet societal needs in order to progress beyond the state-of-the-art. Synthetic Procedures. The first step toward improving industrial production of reticular materials relies on scalable, cost-effective and, ideally, also environmentally friendly production protocols. To develop more environmentally sustainable synthetic protocols, R. Ettlinger et al. assess how to adopt the 12 fundamental principles of Green Chemistry for the synthesis of reticular materials and report on how to maximize the value of the Earth's available resources by incorporating strategies such as re-use, re-generate, or re-cycle and by performing life cycle assessments (article number 2304660). In addition to this, new synthetic routes are also constantly being developed to produce reticular materials with potentially new properties and performances. While reticular chemistry is typically characterized by its ability to generate (porous) structures with high crystallinity, S. Horike et al. examine the less common but equally interesting amorphous glassy states of network-forming coordination polymers (CPs) and MOFs; by summarizing potential synthetic strategies, shape adaptability, and synergistic effects of multi-scale hierarchical CP/MOF glass materials, and by evaluating their properties for different applications, such as gas permeability, ionic and electronic conductivity, optics, catalysis, and battery technology (article number 2307226). Characterization and Modeling Methods. Optimal (control over) material performance strongly depends on its structural and physicochemical properties. Increasing the level of accuracy in material characterization is crucial not only to further understand and predict structure–property–performance relationships, but also to consequently enhance control over material production and performance, overall maximizing its applicability. In this regard, E. Ploetz et al. review the advantages and limitations of non-destructive vibrational techniques, such as Raman spectroscopy, and develop a tutorial guide to showcase how Raman spectroscopy can be employed to precisely characterize MOF systems and study their properties (article number 2307518). In addition, the use of computational and (molecular) modeling approaches is becoming a more and more valuable tool to, besides minimizing material resources and costs, manage and process huge amounts of generated (research) data, optimize material structure and properties, and, eventually (ideally) predict the performance. R. Q. Snurr et al. describe the role of such atomic- and molecular-level modeling methods in MOF research, particularly on the use of density functional theory (DFT), and Monte Carlo and molecular dynamics simulations (article number 2308130). They also highlight how the design and development of MOF structures can be optimized for production and performance by integrating theoretical-experimental MOF research, and by incorporating machine learning techniques into quantum and classical simulations. Scale Up Production. Despite the extensive amount of research data generated about reticular materials, these have been mostly obtained at (confined) laboratory bench-scale settings. However, to eventually translate MOFs, ZIFs, COFs, and other materials into technological products and real-world applications, it is key to evaluate and control material properties during (industrial) scale-up production. G. Mouchaham, F. Nouar, and C. Serre et al. underscore the importance of implementing robust and reproducible processes for large-scale synthesis and manufacturing of MOFs to ensure cost-effective commercial development – also with high emphasis on the need for techno-economic analyses and life-cycle assessments (article number 2309089). Processing Protocols. Similarly, and in addition to sustainable and industrial-scale synthesis protocols, the resulting materials have to be processed and transformed from powders into macroscopic, usable products. P. Albacete, M. Asgari, and D. Fairen-Jimenez et al. review the state-of-the-art of the processability of reticular materials (particularly MOFs) toward industrial use, highlight the importance of developing robust strategies for material shaping and densification, and offer a rational guide on how to self-shape dense monoliths for reticular materials as a promising strategy toward ensuring control on the performance during scale-up production (article number 2305979). Approaching the material processing from another angle, S. Wuttke and J. M. Chin et al. report on multilength scale hierarchy in MOFs, give recommendations on how to master hierarchical structuring based on the current synthetic toolbox as well as fabrication techniques, highlight where MOF hierarchical materials can be most impactful and critically analyze the arising advantage for different applications, such as catalysis, gas storage, and optical applications (article number 2308376). To maximize the benefits of rational design of reticular materials, M. Kamkar et al. evaluate the adoption and shaping of MOFs for the next-generation MOF-based electromagnetic interference shields by systematically analyzing shielding mechanisms, current design strategies and challenges; highlighting the unique molecular-, nano-, micro-, and macro-scale structural features of MOFs that might help achieve a balanced electronic–magnetic performance (article number 2304473). Application Performance. Ultimately, but arguably one of the most important and first aspects to consider toward successful technological development, the design and potential application of (reticular) materials must aim to address current societal challenges and unmet needs. Identifying the societal challenges in which nano-to-macroscale reticular materials can outperform currently existing technologies is crucial to enhance their technological and commercial value, and thereby promote the innovation-to-technology transfer and generate societal impact. Several fine-tuned and shaped reticular products have already found their way into different fields of application – mostly subjected to gas storage, separation, and water adsorption and treatment. In other cases, such as energy production and storage (e.g., supercapacitors) or biomedicine, the translational and commercial value of reticular materials is still hampered by several conceptual (fundamental) and translational barriers that remain to be addressed. Gas storage, separation, and sensing. For their utilization for gaseous fuel storage, carbon capture, and water harvesting, D. Zhao et al. present representative examples of MOFs and COFs with a special focus on the relationship between their structures and properties (article number 2307778). Similarly, Q. Ma and B. Wang et al. report on MOF-based sensors and filters for pollutant detection and air quality remediation by summarizing different fabrication methods, the required qualities, and properties of these multi-functional materials and by discussing the remaining challenges, such as the compatibility of MOF and substrate, their stability, the cost-efficiency, and limited control over synthetic scale (article number 2304773). Regarding gas separation, A. Knebel et al. give insights on how reticular materials, such as MOFs, COFs, porous organic cages, and metal–organic cages and polyhedra, can be utilized as membrane components for gas separation by comparing different material types and by outlining the importance of control over architecture (e.g., pores with narrow openings) (article number 2306202). The authors also highlight the value of such cost-efficient, reticular material-based membranes for efficient gas separation technologies. To directly capture CO2 gas from the atmosphere, O. K. Farha et al. review the potential of MOFs by showing promising research lab data with high CO2 capturing capacities, by discussing issues with synthesis scale-up without affecting the resulting material performance, and by critically assessing the obstacles toward implementation of CO2-capturing MOF technology (article number 2307478). Also focusing on CO2, S. Krause and B. V. Lotsch et al. shed light on the use of COFs as sustainable photocatalysts for photochemical CO2 gas reduction, by analyzing design and synthetic strategies as well as their functionalization/tailoring for optimal catalytic performance, and by highlighting the need of a catalytic system that is log-term stable and capable of both oxidation and reduction half-reactions (article number 2309060). Water Adsorption and Treatment. Another relevant research direction where reticular materials are being increasingly explored is the control over the concentration of water present in the air, quite prominently for water harvesting. In this context, F. Kapteijn et al. evaluate MOFs as water sorbents in heat reallocation, water harvesting and humidity control, discuss their unique step-like water isotherm at MOF-specific relative pressures that enable easy loading and regeneration over a small range of temperatures and pressure conditions, and review the required steps for their practical implementation (article number 2304788). Working in aqueous phases, F. Ahmadijokani and H. Molavi et al. report on different COF-MOF hybrids for the treatment of wastewater by reviewing their synthesis and design possibilities, by underlining how combining both can maximize their performance for contaminant sensing, adsorptive removal or catalytic photodegradation, and by discussing remaining challenges in their (industrial) development (article number 2305527). Beyond water, but still within the liquid phase, another important research direction for reticular materials is the production of (potable) freshwater from seawater. In their manuscript, S. Dutta and S. Wuttke et al. overview the prospects of MOFs in this regard, discuss various fabrication strategies and the performance of MOF-integrated membrane materials in desalination processes such as reverse osmosis and forward osmosis, and highlight the promise of MOFs as the next-generation desalination technology (article number 2304790). Energy Storage (Supercapacitors). Focusing on energetic applications, A. Walsh and A. C. Forse et al. review the use of MOFs as electrode materials with supercapacitor performances and capacitances that exceed those of conventional materials, discuss relevant obstacles, such as limited cycle lifetimes, poor rate performances, and lack of understanding of degradation mechanisms, and highlight the need of innovative design principles to improve supercapacitor energy storage device performances (article number 2308497). Biomedicine. Finally, reticular materials, particularly MOFs, are also being extensively evaluated for medicinal applications. Different potential indications have been explored, mostly related to cancer, but also for antibacterial or regenerative medicine purposes. Yet, the translational success rate of these materials in (bio)medicine has been, to date, limited. In this regard, Q. Peña et al. critically discuss potential barriers and avenues to clinical translation (such as the difficulty in attracting interest from clinicians and pharmaceutical industry, lack of rigorous preclinical evaluation and data analysis, challenging upscaling and manufacturing protocols, and non-standardized characterization and quality control), and highlight potential niche perspectives of MOFs for biomedical applications such as gas delivery, detoxification purposes and as active metallo-(immuno) therapeutic platforms (article number 2308589). Taken together, it is undeniable that reticular materials have already exhibited high structural and physicochemical versatility, as well as increasing potential toward several applications. However, their research has been (and is still) mainly focused on specific lab-confined applications, under (typically) non-standardized, restricted, and hardly controllable experimental conditions. This focus hinders the eventual industrial and technological development of nano-to-macroscale reticular materials to address societal challenges and generate societal benefit. To unlock their full technological potential, more inter-disciplinary and inter-laboratory collaborations, as well as closer cooperations between academia and industry (including, eventually, investors) are essential to foster the transition of reticular materials into manufacturable, beneficial, and profitable products. The international and multi- and inter-disciplinary nature of all the contributions to this special issue (Figure 2c) reflects the fact that the field of reticular materials is pushing the frontiers of knowledge toward many different directions. In this special issue collection, the authors lay the groundwork for novel ideas of not only the design, synthesis, and characterization of the next-generation nano-to-macroscale reticular materials, but also toward promoting their technological, manufacturing, and commercial value. We are sincerely grateful for their contribution into this special issue in Advanced Functional Materials. Finally, we would like to extend our gratitude to the whole editorial board of Advanced Functional Materials, especially Dr. Richard Murray, for their strong support and for making this special issue happen. The authors declare no conflict of interest. Romy Ettlinger obtained her Ph.D. at the University of Augsburg (Germany) in the research group of Dr. Hana Bunzen, establishing metal–organic framework (MOF) nanoparticles and iron oxide MOF core–shell nanoparticles for their application as nanocarriers for highly toxic arsenic-based drugs or as a theranostic agents. Subsequently, she joined the group of Prof. Russell E. Morris as postdoctoral research fellow at the University of St Andrews (United Kingdom), developing functionalized porous hybrid framework materials and composites for medical and environmental applications. Currently, she is a Junior Research Group Leader at the Technical University of Munich (Germany). Her research focuses on resource recovery and treatment of water using porous composite materials. Quim Peña obtained his Ph.D. degree in Chemistry from the Universitat Autònoma de Barcelona (Spain) and Aix-Marseille Université (France) in 2019, focusing on the study of metal-based anticancer compounds. Since 2020, he is a postdoctoral researcher in the Department of Nanomedicine and Theranostics at the Institute for Experimental Molecular Imaging (RWTH Aachen University Hospital, research focuses on the use of chemistry and for of and with the aim of treatment and, promoting clinical is currently of the of a European on metal–organic framework materials and he is in different scientific such as the of the of the European for Molecular and the including its Wuttke the research group for at the Institute of Chemistry at the University of Munich Currently, he is at the for Materials, and focus is the design, synthesis, and of MOFs and their to diverse applications. the he to a understanding of the chemical and processes in the synthesis, and application of these hybrid materials. Reticular connective tissue ; Nanotechnology ; Macro ; Materials science ; Engineering physics ; Computer science ; Engineering Metal-Organic Frameworks: Synthesis and Applications ; Covalent Organic Framework Applications ; Advanced Photocatalysis Techniques