Transition Metal Catalysts for the Glycerol Reduction:
Recent Advances

Francesca Coccia, Nicola d'Alessandro, Andrea Mascitti, Evelina Colacino,
Lucia Tonucci

► To cite this version:

Francesca Coccia, Nicola d'Alessandro, Andrea Mascitti, Evelina Colacino, Lucia Tonucci. Transition Metal Catalysts for the Glycerol Reduction: Recent Advances. ChemCatChem, 2024, 16 (8), pp.e202301672. 10.1002/cctc.202301672. hal-04990559

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# Transition Metal Catalysts for the Glycerol Reduction: Recent Advances

Francesca Coccia,[a] Nicola d'Alessandro,[b, c] Andrea Mascitti,[b] Evelina Colacino,[d] and Lucia Tonucci*[a]

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Glycerol (GL) represents a widespread agro-industrial waste. Its valorization is pivotal for a sustainable society because GL is a renewable compound deriving from biomass, but it has a high oxygen to carbon ratio, compared with feedstock used in the energy and chemistry sectors. Oxygen-poor derivatives are easily and immediately transferable to the industry, avoiding a deep and pressing modification of the plants. From this perspective, keeping the carbon content but with an oxygen content reduction, we could effectively obtain the enhancement of the recovery and the use of GL converting it

into attractive industrial building-blocks. In this Review, we present and discuss the up-to-date results about the chemical reduction of GL into products with 3 carbon and 0, 1, or 2 oxygen atoms. The focus is on the transition metal (TM) catalysts that have made the hydrogenation reactions of GL possible, partitioning the metals into early and late, based on their position in the periodic table. This discussion will contribute to select and develop new catalysts aimed at the improvement of the yield and of the selectivity in the hydrogenation reactions of GL.

# 1. Introduction

The energy transition represents the passage from a society based on an energy mix centered on fossil fuels to one with low or zero carbon emissions, better if based on renewable resources. In recent years, the topic of decarbonization has been given serious consideration, specifically for the electrification of consumption, which involves replacing electricity produced from fossil fuels with electricity generated from renewable sources. This challenge will be the greatest of the entire century; therefore the total replacement of carbonaceous resources (e. g., with hydrogen) will certainly contribute to the decarbonization of our Planet, but, in a short time, it will also be necessary to associate other strategies to change not the materials for energy use but simply their origin.[1] For example, ethanol can be obtained from ethene by catalyzed hydration reaction or even by sugar fermentation. Its synthetic catalytic production decreased significantly since the 1980s until a value of 7% in 2010[2] due to the increase in the prices of fossil resources and also to the change of approach to environmental sustainability and renewable feedstocks. At the same time, the fermentative pathway from sugars, starchy and lignocellulosic materials, better if available as wastes, noticeably increased. Moreover, it has recently been proposed the reverse synthetic hydration reaction able to furnish polyethylene starting from the catalytic dehydration of bioethanol.[3]

Generically, a large part of industrial carbonious waste is polar compounds with relatively high oxygen to carbon ratio (O/C), at least compared with similar materials used for energy purposes. Green Chemistry Principles, and particularly the number 2 (atom economy), impose to save any atom when passing from reagents to products; however, losing carbon or oxygen atoms is not the same, since oxygen is ubiquitously available. On the other hand, oxygen-poor derivatives will be much more eligible for actual and future industrial applications. Therefore, we can propose an innovative perspective that aims to achieve less polar compounds from any feedstocks keeping selectively the carbon content and reducing the oxygen content (Figure 1).

To reach this goal, several strategies were applied. Thermal processes are widely diffused, often assisted by homogeneous or heterogeneous catalysts.[4] Liquid biomasses like vegetable oils, as such or after a hydrolytic step to free fatty acids, were successfully deoxygenated to form hydrocarbons by using, for example, a γ-alumina supported Ni/Mo catalyst, at 300–400 °C, in H₂ atmosphere.[5,6] More recently even biomass of different nature, like lignocellulosic materials, were subjected to controlled oxygen removal to obtain a series of functional high-value low-oxygenated olefins or hydrocarbons.[7,8] Several contributions on rhenium catalyzed deoxydehydration reaction (DODH, formal removal of a H₂O₂ unit from a diol) appeared recently in literature (Scheme 1).

[a] Dr. F. Coccia, Dr. L. Tonucci
Department of Philosophical, Educational and Economic Sciences
University “G. d’Annunzio” of Chieti-Pescara
Via dei Vestini, 66100 Chieti, Italy
E-mail: lucia.tonucci@unich.it

[b] Prof. N. d’Alessandro, Dr. A. Mascitti
Department of Engineering and Geology
University “G. d’Annunzio” of Chieti-Pescara
Via dei Vestini, 66100 Chieti, Italy

[c] Prof. N. d’Alessandro
Uda-TechLab, Research Center
University “G. d’Annunzio” of Chieti-Pescara
66100 Chieti, Italy

[d] Prof. Dr. E. Colacino
ICGM, Univ Montpellier, CNRS, ENSCM
Montpellier 34296, France

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© 2023 The Authors. ChemCatChem 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.

<figure><img src="image_7.png" /><figcaption>Figure 1. The correlation between the number of O and the number of H atoms, per C atom, in selected organic compounds deriving from biomass.</figcaption></figure>
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<figure><img src="image_9.png" /><figcaption>Scheme 1. Catalyzed DODH reaction of a diol to obtain its olefin derivative.</figcaption></figure>

The value of this reaction is that many natural polyols, often available as waste, can be transformed into low oxygenated derivatives (or even hydrocarbons) useful as monomers in the polymer industry, as fuel or as building-blocks.[9,10]

In this review, we have focused our attention just to the smallest natural polyol, widespread in nature, since it is a primary constituent of lipids. Glycerol (GL) molecule is the second simplest sugar alcohol and the first available in

considerable amounts in nature. However, we are strongly convinced that the remarks of this review can be applied also to other important natural sugar alcohols with a more complex structure, like erythritol, xylitol or sorbitol.

As for ethanol, even for GL, its release on the world market changed during the last fifty years. It is obtained mainly from two routes: i) via petrochemical sources (synthetic GL) or ii) as a by-product of oleochemical production (bioglycerol). Regarding its industrial availability, the transition from the i) to the ii) route occurred about the 2000s when biodiesel started to be introduced massively on the market to replace part of the fossil diesel fuel, so making available a large amount of bioglycerol, equivalent to about 10% w/w of the transesterified lipids. This abundance, other than to contribute suppressing almost totally the synthetic route from propene, prompted the industries to reverse the former Solvay technology, and transform it to a

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Francesca Coccia, M.Sc. in Chemistry and Pharmaceutical Technologies, received her Ph.D. in Chemical Sciences from University “G. d’Annunzio” of Chieti-Pescara (Italy), then she moved to Milan University (Italy) in the Benaglia group, working about the organic catalysts supported on different materials. In 2018 she started a postdoc position in CNR (National Research Council) of Macromolecules at Milan for the functionalization of natural polymers; now she is Assistant Professor of Inorganic Chemistry at the University “G. d’Annunzio” of Chieti-Pescara. Her research focuses on materials functionalization, Green Chemistry and catalysis by metal nanoparticles.

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Nicola d'Alessandro obtained his Ph.D. in Chemistry at the Photochemistry Research Group of University of Pavia (Italy), studying the photochemical reaction between aromatic nitriles and benzylic donors. He then joined the Mariano Research group at the University of Maryland (US) and the von Sonntag Research Group at the Max Planck Institute für Strahlenchemie (Mulheim, Germany). After a brief experience at the University of Torino (Italy), in 1997 he moved to the University “G. d'Annunzio” of Chieti-Pescara (Italy) where, since 2005, he is Chemistry Associate Professor. His main research activity concerns the chemical transformations of widely-available agro-by-products catalyzed by transition metals.

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Andrea Mascitti, M. Sc. in Chemistry and Advanced Chemical Methodologies at University of Camerino (Italy), received his Ph.D. in Biomolecular and Pharmaceutical Sciences from University “G. d’Annunzio” of Chieti-Pescara (Italy) where he is now a Research fellow. His research focuses on Green Chemistry, the valorization of agro-industrial waste and metal catalysis.

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Evelina Colacino received her double Ph.D. (with European Label) in 2002 at the University of Montpellier II (France) and the University of Calabria (Italy). Associate Professor of Organic Chemistry at the University of Montpellier (France), she promotes sustainability in Higher Education by integrating green chemistry in academic curricula, with a special focus on the fundamentals and the practice of mechanochemistry. Her main research activities concern the development of sustainable approaches by mechanochemistry to prepare value-added compounds for the industry, with a special focus on Active Pharmaceutical Ingredients. Green catalytic methods in non-conventional media using enabling technologies are also explored. She chairs of the European Programme COST Action CA18112 MechSustInd (Mechanochemistry for Sustainable Industry) and the Horizon Europe Project IMPACTIVE (Innovative Mechanochemical Processes to synthesize green ACTIVE pharmaceutical ingredients)

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Lucia Tonucci, M.Sc. in Chemistry at University of Bologna (Italy), received her Ph.D. in Chemical Sciences at University “G. d’Annunzio” of Chieti-Pescara (Italy) where she is now Assistant Professor of Inorganic Chemistry. She is the Academic Coordinator for Public Engagement at the University “G. d’Annunzio” and is engaged in the dissemination about Green and Sustainable Chemistry, and Science and Society. Her research focuses on the Green Chemistry and the valorization of agro-industrial waste by metal catalysts and photocatalysts for chemistry and energy sectors.
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method to obtain green epichlorohydrin from natural GL (Epicerol technology; Solvay first and Technip Energies subsequently).[11] In 2018–2022, the annual global production of crude GL was around 6 Mton;[12] the major contribute comes from biodiesel industry (60%), increasing to 80% in EU production with an amount of 1 Mton.[13]

Within this review, deoxygenated GL products, such as a series of C₃ compounds having 0, 1, or 2 oxygen atoms (Figure 2), crucial as green industrial feedstocks, will be considered and the TM-catalyzed technologies able to obtain them from GL, published from 2017 on will be described. We stated that 2-hydroxypropanal, glycidol and propanoic acid were not reported as significative products from GL chemical reductions, at least in this time range. So, we don't cite them in this review.

The presence of a catalyst is mandatory for deoxygenation reactions on polyols. Excluding the biodeoxygenation reactions, it remains to consider the TMs as the unique candidates to perform green and sustainable catalytic procedures. TM examined and named in the paragraph titles of this review are only those reported in scientific literature as effective catalysts for the deoxygenation reactions of GL to C₃ products, at least in the time interval in question. TMs are characterized by partially filled *d* orbital subshells in the free elements and, for most of them, also for their cations. Unlike the main group elements, the TMs also exhibit horizontal chemical similarities. Electronegativity increases from left to right and from top to bottom, making differently reactive the left side (early) and right side (late) elements. The review is organized, comparing these two groupings, to answer a crucial question: "What is the reason for late TMs being used more frequently? Is it because of their peculiar reactivity, mainly due to their electronic properties, or, simply, because some metals, extensively used in the past,

<figure><img src="image_16.png" /><figcaption>**Figure 2.** C₃ compounds from the deoxygenation of GL, considered in this review. The color of the arrows reflects the main reaction pathways to the product: red, hydrogenolysis; green, oxydehydration; pink, deoxydehydration; orange, dehydration; grey, not present in literature in the range time.</figcaption></figure>

continue to be used along well-standardized paths rather than testing new solutions with the risk of having major failures?"

## 2. Early Transition Metals

### 2.1. Titanium

In the large world of GL deoxygenation reactions, titanium (Ti sponge average price: 7.26 $/kg)[14] has been used mostly as active support.[15] The success of titanium oxide is due to its acidity since Lewis Acid sites (LAS) are needed for the C–O cleavages in the dehydration reactions; however, too many of these sites could also promote C–C cleavage through an acidic cracking mechanism.[16] Villa et al. reported the counter proof of this evidence: bimetallic Au-Pt catalysts were tested in the GL hydrogenolysis, using oxide supports with different acid-base properties (such as TiO₂ and MgO).[17] The medium strength of TiO₂ LAS led to the best performance in terms of activity, stability of the catalytic system and selectivity towards propane-1,2-diol (1,2-PD), avoiding the formation of ethanol, methanol or propanols by over-hydrogenolysis of 1,2-PD.[17]

Diols, such as 1,2-PD and propane-1,3-diol (1,3-PD), are useful products, largely employed in polymer industries (e.g., packaging, fiber, filler) that can be obtained by hydrogenolysis of GL.[18,19] The reaction is a variegated process that can be conducted, for example, on a metal-acid bifunctional catalyst, able to promote dehydration, *via* intermediates as hydroxyacetone (Hac) and 3-hydroxypropanal (3-HPA), followed by a hydrogenation reaction occurring on metal sites.[19]

It was observed that increased aqueous GL conversion (from 53% to 74%) and 1,3-PD yield (from 20% to 25%) were obtained if TiO₂ was used as support for Pt/WO₃-ZrO₂ catalyst, carrying out the reaction in a vertical stainless steel fixed-bed reactor at 523 K and 50 bar H₂ pressure.[20] An explanation of this improvement could be the introduction of TiO₂ that increased the Brønsted acidity of the catalyst: WOₓ species were linked, through O bridges, to Zr but also to Ti, with a larger electronegativity than Zr. It was noted a linear correlation between the 1,3-PD yield and the concentration of the Brønsted acid site (BAS) and the 1,2-PD yield and the amount of LAS. In this case, the generation of *in situ*-BAS could be explained with the H₂ dissociation on the Pt surface forming H⁺ atoms and then, during H⁺ spillover, the electron-donation from H⁺ to the LAS of the support produced H⁺.[20] Even other parameters, such as properties of Pt and of the supports, can affect the generation of BAS. Besides these aspects, also the temperature can play an important role, especially in increasing the GL conversion. Moreover, when Pt loading was augmented from 1% to 3%, the GL conversion raised, but the selectivity of the reaction changed, leading to 1-propanol (1-PO) instead, while with a Pt loading of more than 4%, the substrate transformation decreased, due to the aggregation of Pt particles.[20] The morphology of the TiO₂ support affected the conversion and the selectivity: using rutile to obtain a Pt-WOₓ/TiO₂, a high conversion was observed (81%) with an acceptable 1,3-PD selectivity (51%) but, using anatase, only 7% of GL was
<figure><img src="image_17.png" /></figure>

converted mainly to 1-PO (62% selectivity).[21] Using hollow
mesoporous SiO₂-TiO₂ nanosphere catalysts, it was possible to
load and distribute Pt nanoparticles and WOₓ nanoclusters very
well into the solid support structure.[22] This Pt/WOₓ/SiO₂-TiO₂
fully catalyzed the GL conversion (85%), with a high 1,3-PD
selectivity (54%) at 423 K and 40 bar H₂ pressure. During the
recycle tests, the selectivity slightly decreased because of the
over-hydrogenolysis to 1-PO. It is possible also to dope Pt/TiO₂
with Cr, Mo or W oxides to tune the products of hydrogenolysis
at 513 K and 30 bar H₂: in the presence of Cr and Mo, the main
product was 1,2-PD (80% and 60%, selectivity); with W, 1,3-PD
was obtained with 50% selectivity.[23]

TiO₂ was used as a support for the deoxygenation of GL using Cu-based catalysts. The results were compared with those obtained in the presence of more acidic support such as Al₂O₃.[24] The catalytic activity of Cu/TiO₂ and Cu/Al₂O₃ catalysts led to the same conversion; however, the selectivity was different. In particular, Cu/Al₂O₃ catalyst was more selective to 1,2-PD and HAc compared Cu/TiO₂, mainly leading to 1-PO. The addition of NaOH (5 wt%) in the reaction mixture with Cu/TiO₂ increased the selectivity for 1,2-PD to 92%, with 52% of GL conversion (at 473 K and 40 bar H₂), since the presence of hydroxide ion promoted the dehydration of glyceraldehyde, formed by dehydrogenation of GL, giving 2-hydroxyacrolein as intermediate compound (Scheme 2).[24]

Recently, monometallic Cu/γ-Al₂O₃ or Cu/TiO₂ catalysts were reported to be able to significantly increase the reaction selectivity towards the formation of HAc, at 553 K in gas phase and atmospheric pressure under N₂. The results confirmed that this reaction was catalyzed by both the metal and by acid sites of the solid support.[25] In particular, 50% of HAc and 20% of pyruvaldehyde (PAI) selectivities were obtained over Cu/TiO₂ catalyst, at a lower extent compared to Cu/γ-Al₂O₃ catalyst (70% and 40%, respectively). The reason for this outcome relies on the nature of the support: indeed, TiO₂, presents both acidic and basic sites, differently than this home-made alumina, which presents very strong acid sites. However, both catalysts suffered from deactivation, caused by the copper oxidation. The reactivation was feasible by thermal treatment at 723 K, followed by a reduction step with H₂/Ar.[25]

A high performance of TiO₂ used as support in GL
dehydration was evidenced even by Ginjupalli *et al.* They
synthesized tungstate metal phosphate (WOₓ/MP; M=Al, Zr, Ti)
solid supported acid catalysts for vapor phase GL dehydration
to acrolein (Acr) and HAc operating at 593 K, under atmospheric
pressure in a vertical fixed-bed quartz reactor. Their outcomes
(100% GL conversion and 80% Acr selectivity) demonstrated
that WOₓ/TiP was the most selective and stable catalyst
(compared to those supported on AlP or ZrP) owing to the

acceptable amount of surface acidity and the substantial
number of pores available on its surface.[26] By DFT (Density
Functional Theory) calculation, it was demonstrated that the
adsorption through the central hydroxyl group of GL on the
TiO₂ anatase surface had the best geometry and energy
(−30.91 kcal/mol) to promote the dehydration reaction leading
to Acr.[27] Dehydration of GL to Acr was also studied over the
widely used Keggin-type heteropolyacids (i. e., H₄SiW₁₂O₄₀xH₂O and H₄PVMo₁₁O₄₀xH₂O) supported on several metal oxides (TiO₂,
Al₂O₃, ZrO₂, AlTiZr oxide).[28] TiO₂ resulted the best support for
the active H₄SiW₁₂O₄₀: Acr was obtained selectively at 498 K by
complete conversion of aqueous GL.[28]

The good dispersion of nano-size metal particles on titania
surface represents an additional advantage to its use, as
demonstrated by investigating GL conversion and selectivity to
propanols, 1-PO + 2-PO (2-propanol), at 493 K and atmospheric
pressure, as a function of metal phosphate supported platinum
catalysts (Pt/AlP, Pt/TiP, Pt/ZrP and Pt/NbP).[29] Among all
catalysts investigated, Pt/TiP showed a significant catalytic
performance in one-step vapor phase hydrogenolysis of GL to
propanols (100% conversion of GL and 97% selectivity for
propanols, 1-PO 87% and 2-PO 10%). The authors reported that
the activity of this catalyst was due to its acidity and to the
good dispersion of platinum particles on the TiO₂ surface. The
catalyst was reusable only one time with a weak decay in its
catalytic activity.[29]

The use of TiO₂ as support fostering the activity of the
catalyst is valuable in the perspective of reducing the amount
of expensive critical raw materials, usually needed for this
transformation. For example, iridium nanoparticles stabilized
over modified ReOₓ on rutile TiO₂ were effective for the GL
hydrogenolysis to 1,3-PD (67% selectivity with 35% conversion).[30] Although the support was not directly involved
in the reaction mechanism, it joined to stabilize and to
uniformly disperse the Ir nanoparticles and to favor the contact
between the Ir species and ReOₓ.[30]

2.2. Vanadium

Vanadium (average market price: 176.16 $/kg)[31] is the 20th most abundant element in the Earth's crust and the 6th one among the TMs.[32] In aqueous solution, the most accessible oxidation states range from +2 (d³) to +5 (d⁰); therefore, V¹l compounds could act as reducing agents, while V⁰ compounds as oxidizing agents.[33] The vanadium affinity for oxygen leads to passivation of the metal surface in air; this characteristic enhances the catalyst activity and selectivity towards the oxidation reaction step.[34] It is noteworthy that several selective oxygen transfer reactions in the presence of vanadium complexes were reported,[35] also with polyoxometalates.[36]

Acrylic acid (AAc) is produced mostly by propene oxidation
in industrial synthesis, and Acr constitutes a transient
compound.[34] From a green perspective, GL oxydehydration
could be a competitive route with that from petrochemicals to
obtain AAc, a key compound for the polymer industry: plastics,
fibers, paints, polishes, coatings, and adhesives.[37] The use of a

<figure><img src="image_18.png" /><figcaption>**Scheme 2.** Mechanism of GL hydrogenolysis and dehydration of glyceraldehyde under basic conditions.</figcaption></figure>
<figure><img src="image_19.png" /></figure>

catalyst with BAS allows to dehydrate GL to 3-HPA and Acr first, subsequently oxidized to AAc in the presence of the metal sites of the catalyst having mild oxidative capability (Scheme 3). Chieregato *et al.* suggested that even the presence of LAS is a very important factor contributing to the oxidation step of Acr, making possible its coordination with the nucleophilic oxygen species (i.e., O<sup>2-</sup>) present on the surface and generated by V ions, likely V<sup>5+</sup>.<sup>[38]</sup>

Vanadium oxide catalysts constitute the right compromise because they are sufficiently acidic to catalyze the dehydration step, but not so acidic to inhibit the desorption of AAc.<sup>[38]</sup> On the contrary, the presence of highly active redox-metals, such as noble metals, could cause direct oxidation of GL and decomposition of Acr to CO<sub>x</sub> rather than the controlled oxidation to AAc (Scheme 4).<sup>[39,40]</sup>

Mixed Mo<sub>x</sub>V<sub>y</sub>O<sub>z</sub> oxides, supported on acidic ZSM-5 zeolite, were able to produce AAc from GL, with low coke deposition and high stability of the catalyst.<sup>[41]</sup>

The dehydration of GL to Acr, in the presence of solid acid catalysts, is generally viable but, in several cases, their deactivation, due to coke deposition, remains a huge barrier. However, the transformation of GL to Acr and AAc was investigated at 573 K, using mixed vanadium and molybdenum oxides supported on alumina.<sup>[42]</sup> Their catalytic activity was recovered due to the vanadium mediated degradation of coke. Recently, a Keggin-type vanadium-substituted phosphomolybdic acid supported on mesoporous MCM-41 was reported for the catalytic dehydration of GL to Acr in a fixed-bed tubular reactor at 498 K and 1 bar of pressure, under N<sub>2</sub> flux. The authors observed that the Acr selectivity depended on the vanadium-substituted phosphomolybdic acid loading. The selectivity raised to 68% increasing the amount of vanadium phosphomolybdic acid to 40%, which also enhances BAS,

suggesting that the selectivity of the reaction to Acr depends on the Brønsted acidity.<sup>[43]</sup>

Mo-V mixed oxides were hydrothermally synthesized in the presence of ionic surfactants (i.e., sodium dodecyl sulfate and cetyltrimethylammonium bromide) which led to the formation of nanorod-type crystals.<sup>[44]</sup> This peculiar morphology favored the redox process bringing better catalyst performance. The catalysts produced with both surfactants presented higher productivity of AAc and good catalytic stability compared to the reference catalyst prepared without surfactants, with no coke formation and a considerable decrease in CO<sub>x</sub> evolution during 6 h of reaction (from 66% to 36%). The catalyst synthesized in the presence of sodium dodecyl sulfate showed the best catalytic results with 100% of conversion and 57% of AAc selectivity at 593 K and atmospheric pressure in a fixed-bed reactor.<sup>[44]</sup> Vieira *et al.* achieved a one-pot oxydehydration of GL (AAc selectivity was about 46%) in the presence of 3D and 2D vanadosilicates, with structures similar to ferrierite and ITQ-6 zeolites, but without framework aluminum.<sup>[45]</sup>

A vanadium catalyst (NH<sub>4</sub>VO<sub>3</sub>) was used to catalyze GL DODH producing 22% allyl alcohol (AA) and small amounts of Acr (Scheme 5) and propanal.<sup>[46]</sup> This catalyst afforded yield and selectivity comparable to the methyltrioxorhenium (MTO) in the same reaction conditions, under 1 H<sub>2</sub> bar pressure at 548 K in neat GL.<sup>[46]</sup>

Silicates modified with vanadium and niobium were also tested at 523 K using H<sub>2</sub>O<sub>2</sub> as an oxidizing agent for the dehydration/oxidation of GL. The catalytic test showed GL conversion up to 95% with good selectivity to AA (if commercial GL is used) or acetone (if waste GL is used).<sup>[47]</sup> The catalysts assumed a needle morphology, with small particle size (two distributions: 55 and 160 nm) raising the amount of exposed niobium.<sup>[47]</sup> Almeida *et al.* supported vanadium oxide on beta zeolite (Si/Al = 25 with 4% V) and used it for the one-pass gas-phase conversion of GL to AA without any external reductant.<sup>[48]</sup> The presence of zeolite allowed the dehydration to Acr thanks to its acid sites. Carrying out the reaction at 593 K under atmospheric pressure, in a fixed-bed flow type quartz reactor, it was possible to reach a 30% of selectivity for AA with a 20% of GL conversion. Modifying the acidity of the vanadium beta-zeolite, it was possible to address the conversion to the AA and AAc which resulted favored by a low acidity density and the presence of redox sites; a high presence of acid sites promoted a dehydration of GL to Acr.<sup>[49,50]</sup>

Another example of flow equipment was used for gaseous GL direct conversion to AA without any sacrificial additive.<sup>[51]</sup> Porous MoFe/x-KIT-6 (x = V, Nb, or Ti) oxide catalysts with high specific surface area were prepared, through *in situ* hydrothermal and wet impregnation methods. At 613 K under atmospheric pressure, all the catalysts showed high activity in

<figure><img src="image_20.png" /><figcaption>Scheme 3. Oxydehydration reaction pathway of GL to AAc.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Scheme 4. The direct oxidative dehydration of GL over WVNbO catalyst to Acr e AAc.</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Scheme 5. The proposed catalyzed mechanism for the catalyzed formation of Acr.</figcaption></figure>
<figure><img src="image_23.png" /></figure>

the gaseous GL catalytic conversion with AA (31% yield), Acr (17%) and acetaldehyde (13%) as the main products.[51]

Among all catalysts, MoFe/V-KIT-6 showed improved catalytic stability and good regeneration performance besides higher GL conversion (92%) and AA selectivity (31%), while the total selectivity of AA, Acr, acetaldehyde and HAc reached about 70%. The reasons for this success could be ascribed to the strong interactions between the V-KIT-6 and MoFe oxides reducing the surface acid density, as well as providing the ordered mesoporous structure.[51]

In Table 1 a summary of the V-based catalysts, reported in vanadium paragraph, and their selectivity in AAc, Acr and AA were reported.

## 2.3. Chromium

Chromium is the 7th most abundant element on Earth (average market price: 9.794 $/kg),[52] most of it resides in the core and mantle.[53] It's a common groundwater and soil contaminant, particularly in industrial areas, and is listed by the US Environmental Protection Agency as one of 129 priority pollutants.[54] There are few research works in the literature of last years about the use of chromium in catalyzed GL conversion to C₃ deoxygenated compounds, probably due to its toxicity. In 2017 the adsorption, configurations and energetics of the reactants,

intermediates and final products of the GL hydrogenolysis on metallic Cu were studied and compared with spinel CuCr₂O₄, by DFT experiments.[55] They observed that the formation of HAc (by cleavage of the terminal hydroxyl group), as intermediate, is thermodynamically and kinetically favored to that of 3-HPA (by cleavage of the central hydroxyl group), for both catalysts, leading to 1,2-PD as main reaction product (Scheme 6). The Cu species are the real active catalyst for the C–O cleavage, whereas the Cr atom gives to GL a stable adsorption site to facilitate C–O cleavage by the copper in the dehydration step, that is the rate-limiting step. The dehydration path from GL to HAc (upper in Scheme 6) resulted thermodynamically and kinetically favorite in these conditions. The hydrogenation of HAc to 1,2-PD on the CuCr₂O₄ surface is thermodynamically and kinetically more favored than that on the Cu surface, thanks to the formation of bonds between Cr and oxygens, and the formation of hydrogen bonds stabilizes the transition states. These data were supported by experimental results of the reaction carried out under 40 bar H₂ and at 493 K for 12 h in water solution. The authors declared 32% of GL conversion and a 1,2-PD yield of 24%.[55]

Recently, chromium silicates with MFI structure were tested as heterogeneous catalysts for gaseous GL conversion into acetaldehyde and Acr as main products. The reaction was performed at 773 K under atmospheric pressure in a continuous-flow glass fixed-bed reactor, leading to high GL conversion (97%) to acetaldehyde and Acr in a 30% and 10% yield, respectively.[56]

<table><caption>**Table 1.** Vanadium catalysts in the hydrogenation of GL.</caption><thead><tr><th>Catalyst</th><th>Main Product</th><th>Selectivity (mol%)</th><th>Ref.</th></tr></thead><tbody><tr><td>Mo-V mixed oxides with ionic surfactants</td><td>AAc</td><td>57</td><td>[44]</td></tr><tr><td>Vanadosilicates</td><td>AAc</td><td>46</td><td>[45]</td></tr><tr><td rowspan="2">Mo<sub>x</sub>V<sub>y</sub>O<sub>z</sub> oxides on acidic ZSM-5</td><td>AAc</td><td>18</td><td rowspan="2">[41]</td></tr><tr><td>Acr</td><td>28</td></tr><tr><td>V<sub>2</sub>MoO<sub>8</sub> on alumina</td><td>Acr</td><td>75</td><td>[42]</td></tr><tr><td>Vanadium-substituted phosphomolyblic acid on mesoporous MCM-41</td><td>Acr</td><td>68</td><td>[43]</td></tr><tr><td>VO<sub>x</sub> on beta zeolite</td><td>AA</td><td>30</td><td>[48]</td></tr><tr><td>VO<sub>x</sub> on beta zeolite</td><td>AAc</td><td>46</td><td>[50]</td></tr><tr><td>Porous MoFe/x-KIT-6 (x = V, Nb, or Ti) oxide</td><td>AA</td><td>31</td><td>[51]</td></tr><tr><td>NH<sub>4</sub>VO<sub>3</sub></td><td>AA</td><td>22</td><td>[46]</td></tr><tr><td>Vanadium-niobium silicates</td><td>AA</td><td>14</td><td>[47]</td></tr></tbody></table>

<figure><img src="image_24.png" /><figcaption>**Scheme 6.** The potential dehydration–hydrogenation mechanisms of GL hydrogenolysis; in the presence of spinel CuCr₂O₄, the upper pathway is thermodynamically and kinetically favored. Adapted with permission from Y. S. Yun, T. Y. Kim, D. Yun, K. R. Lee, J. W. Han, J. Yi, *ChemSusChem* 2017, 10, 442–454. Copyright 2017 John Wiley and Sons.</figcaption></figure>

## 2.4. Zirconium

An approach to make the sustainable reactions more suitable for industrial applications is to carry out them in flow mode, using supported catalysts.[57] This approach allows to reuse the catalyst and avoids metal leaching into the reaction medium. Performing the reaction in a fixed-bed reactor is a guarantee of safety, especially if hazardous reagents are employed, such as H₂, used in many studies of GL deoxygenation. Performing the reaction in very small channels of a reactor or microreactor, the consumption (amount) of these dangerous reagents is very low per minute and so the chemical risk decreased. The possibility to carry out the reaction in continuum, feeding the reactor with a mixture of reagents (without catalyst) through a syringe pump, allows to achieve high percentage of productivity during the time. The advantages of this method comparing to the conventional batch reactions are higher surface-to-volume ratio, easier heat and mass transfer resulting in more efficient and sustainable reaction, and easier control and scale-up.[58] In literature there are many examples of deoxygenation reactions of GL performed in continuous-flow fixed-bed reactors in the presence of zirconium as catalyst (Zr sponge average market price: 26.42 $/kg).[59] It was used as acid-modified for Co-Al catalysts, to increase catalyst acid strength and metal dispersion, leading to a high selectivity for 1,2-PD (85% with 72% of GL conversion) in water and in a vertical fixed-bed reactor at 503 K and 35 bar.[60]
<figure><img src="image_25.png" /></figure>

A sequential double hydrogenation reaction in flow was also presented by Luo *et al.*, using modified amorphous zirconium phosphate supported Pt catalysts.[61] The acid sites of the catalysts promoted the selective dehydration of GL into Acr, whereas the Pt sites facilitated $H_2$ dissociation and sequential hydrogenation of Acr into 1-PO (24% yield). The addition of tungsten to the catalyst improved the results (81% 1-PO yield at 543 K and 20 bar $H_2$) increasing the acidity of the catalyst and gaining a better dispersion of Pt nanoparticles (Scheme 7).[61]

The reaction conditions (e.g., the operating temperature), and not only the type of catalyst, can also affect the selectivity of GL transformations. Srinivasa Rao's group studied the conversion of GL to Acr using modified zirconium phosphate (ZrP), often used as a water-tolerant and high thermal stable solid acid catalyst.[62] The authors linked the reaction temperature and the catalyst acidity by the varying P/Zr ratio in the GL dehydration in flow: both conversion of GL and selectivity for Acr increased until full conversion of GL and 63% Acr and 2% HAc yield, with increasing P/Zr molar ratio from 0.33 to 2.00 at 590–600 K and atmospheric pressure of $N_2$. Instead, at lower temperatures, HAc yield increased (up to 19%), due to variations in the activation energies pathways.[62]

GL hydrogenolysis carried out in a vertical fixed-bed stainless steel reactor was also reported with ZrP as support for metallic Ru and $CoO$.[63] When $Ru^0$ was added to $CoO$ supported on ZrP, 80% of propanal with the quantitative GL conversion was obtained, in a one-step process, with very satisfactory catalyst stability after 50 h at 543 K and 20 bar $H_2$. The main product was propanal, instead of Acr, since $Co-O$ species can absorb $C=O$ bond more strongly than ZrP, leading to the Acr hydrogenation while the presence of Ru nanoparticles, able to dissociate the $H_2$ molecule, promote the catalytic performances of $CoO$ in the GL hydrogenolysis.[63]

The partial substitution in MgAl layered double oxide with Zr enhanced the catalytic activity of this solid compound varying the ratio between acid and basic sites and improving the formation of *in situ* $H_2$.[64] This modified MgAl was tested in the catalytic hydrogenation of GL at 453 K in 4 h resulting in a higher conversion compared to that observed with MgAl without Zr (37% rather than 13%). Unfortunately, the 1,2-POD selectivity was not high (7%) because of the over-hydro-

genation of the diol to ethanol and acetaldehyde, the main final product.[64]

## 2.5. Niobium

Niobium is a TM found in nature not as a pure element but in minerals (Nb average market price: 88.08 $/kg).[65] It shows acidic characteristics so that it is employed as Lewis acid in several different types of reactions (e.g., alkylation, dehydration of polyols, oxidation/dehydrogenation reactions).[66] With five electrons in its outer valence shell, niobium displays rich redox chemistry through oxidation states ranging from -1 to +5.[67] The most common compound of niobium, in nature, is the pentoxide, $Nb_2O_5$, able to catalyze the oxydehydration reaction of GL into Acr or AAc.[66]

Several are the examples of catalysis involving Nb with W. La Salvia *et al.* synthesized Nb-W-O and V-W-O bronze catalysts onto mesoporous KIT-6 as inert support.[68] The catalyst containing niobium promoted the dehydration of GL to Acr having own acid sites, while the catalyst containing vanadium presented also redox sites, necessary to obtain AAc.

Working in flow at atmospheric pressure, full GL conversion was achieved leading to Acr in 70% yield using Nb-W catalyst at 573 K. On the contrary, V-W-based catalyst promoted the one-pot oxydehydration of GL, catalyzing two different steps: i) the GL dehydration into Acr onto acid sites and ii) the oxidation of Acr into AAc *via* redox sites ($V^{4+}/V^{5+}$). In this case, the temperature required to speed up the reaction was higher (600 K), also producing an increased amount of carbon oxides and heavy compounds (e.g., GL oligomers and ketals) with about 30% selectivity for AAc. Thanks to the interactions between metal and support, that resulted in a decrease in the number of strong acid sites, the supported Nb-W and V-W catalysts onto KIT-6 showed better Acr and AAc selectivities, respectively. These supported metal oxides minimized the degradation to AAc (with Nb-W) and to other compounds (with V-W) and the selectivity rose from 70% to 78% for Acr and from 19 to 31% for AAc.[68]

In the presence of a series of hydrothermal prepared bronze W-Nb-O catalysts, the selectivity to Acr decreased, favoring the formation of by-products, when the Nb-content in the catalysts increased.[69] A strong presence of Nb promoted a structural change of the oxides (pseudocrystalline phases) increasing the proportion of LAS on the surface, thus promoting the degradation of Acr to heavy compounds.[70]

Niobium was also used to extend the tungstate zirconia catalysts life-stability.[71] The optimal catalytic performance was obtained in presence of $NbWO_x$/ZrO$_2$ (3% $Nb_2O_5$) with 81% GL conversion and 72% Acr selectivity, after 24 h reaction at 563 K and atmospheric pressure, in a vertical fixed-bed tubular quartz reactor. The W-Zr catalyst contained both LAS and basic sites. Doping the catalyst with Nb led to an improvement in catalytic activity, probably due to the neutralization of some basic sites on the $ZrO_2$ support by niobium addition.[71]

Fine tuning the ratio of acid/base sites is fundamental to achieve a good compromise between catalytic behavior and

<figure><img src="image_26.png" /><figcaption>**Scheme 7.** Reaction mechanism for GL hydrogenolysis to 1-PO with $Pt/WO_x$-ZrP catalyst. Adapted with permission from R. Luo, X. Zhao, H. Gong, W. Qian, D. Li, M. Chen, K. Cui, J. Wang, Z. Hou, *Energy & Fuels* 2020, 34, 8707–8717. Copyright 2020 American Chemical Society.</figcaption></figure>
<figure><img src="image_27.png" /></figure>

catalyst's long life. Among the physical-chemical superficial characteristics of a catalyst (many of which could be altered during the calcination step), the presence of acid or basic sites is a property that can be modulated, as seen above. The 8 wt% Nb₂O₅ supported on zirconium doped mesoporous silica (Si/Zr = 5 molar ratio) underwent phosphoric acid treatments varying the Nb/P molar ratio between 0.1 and 1, and modifying acid properties of the catalyst surface.[72] The best results in terms of both selectivity to Acr and catalyst stability were seen when the acid sites had low and moderate strength (Nb/P = 0.2). This synthesized catalyst allowed for achieving a quantitative GL conversion and an Acr yield of 74% after 2 h of time-on-stream, at 623 K, under atmospheric pressure, in a fixed-bed continuous-flow stainless steel reactor. Moreover, the spent catalyst was reactivated by thermal treatment at 823 K without loss of activity.[72]

The impregnation of Nb₂O₅ by Mo/Co or Co/V imperceptibly enhanced the selectivity to Acr (from 74% to 75% and 73%, respectively) but changed the distribution of by-products favoring the formation of the industrially important AAc (10 and 3%) rather than C₂ and C₁ aldehydes, working at 573 K.[73]

The strength and balance between the BAS and LAS are also affected by catalyst preparation method. Stawicka *et al.* synthesized niobium containing SBA-15 by two different methods: one starting from ammonium niobate(V) oxalate and SBA-15 by impregnation method of 15 or 25 wt% of Nb, the other mixing Nb₂O₅ with silica and then calcinating it at 773 K.[74] They found only weak LAS and BAS in the mixing prepared catalyst, while the catalyst obtained by impregnation showed strong BAS, especially for 25% Nb one. Since it is known that the strength of BAS, not its high concentration, promotes the dehydration reaction, to reach a better selectivity of Acr (61 instead 43%), the authors reduced the number of acid sites by silylation, with a considerable decrease in coke formation (from 47% to 27%) carrying out the reaction at 623 K in continuum mode. The reason for this high yield and selectivity of Acr could be ascribed not only to high strength and low concentration of BAS, but also to the proximity of BAS and LAS on the catalyst surface.[74]

Other C₃ products were obtained in the presence of Nb-based catalysts using GL as starting material. Products of reduction can be achieved after Nb doping of other metals so that Nb₂O₅ promotes GL adsorption on catalyst surface and then the interaction of the organic with the metal. 1,2-PD was obtained using Ru/Al₂O₃ (boehmite phase) doped with Nb₂O₅ which played a stabilizing role for Ru nanoparticles avoiding their aggregation.[75] When Nb/Ru molar ratio is about 1 (with Ru 3 wt%), even the smaller Ru nanoparticles (10 nm) were very well dispersed on the Nb-incorporated alumina matrices, allowing the achievement of good results (GL conversion 29% and 1,2-PD selectivity 26%) working in flow continuum mode in H₂/N₂ atmosphere at 531 K and 45 bar.[75]

The Nb addition to a Pd-Zr-Al catalyst favors the reduction of the PdO species and its dispersion on the catalyst surface; at the same time, Nb can enhance the acidity of the catalyst and consequently contribute to increasing the GL conversion in aqueous solution from 37 to 69% and selectivity for 1,2-PD

from 78 to 85%, at 473 K and 35 bar H₂; thanks to its high stability, good reusability was observed.[76]

When the doping of 2% Nb was done into a WO structure supporting Pt catalyst, 1,3-PD was the main hydrogenolysis product (30% selectivity with 40% aqueous GL conversion at 433 K, 10 H₂ bar for 12 h of reaction).[77] Employing a simple Pt/WO catalyst, the 1,3-PD yield decreased (from 14 to 8%) increasing H₂ pressure (from 10 to 50 bar) since an over-reduction occurred. However, the addition of Nb to the solid catalyst inhibited the overreduction of W species; in the presence of this Nb-added catalyst, carrying out the reaction at the same temperature and reaction time and increasing H₂ pressure to 50 bar, the 1,3-PD yield was not affected.[77]

Oliveira's research group prepared and tested different catalysts to transform GL to AA. They synthesized a niobium-vanadium-modified silicate and a niobium oxyhydroxide for the simultaneous dehydration/oxidation of GL with hydrogen peroxide at 523 K resulting in a quantitative conversion of the substrate and 14% and 46% AA selectivity, respectively.[47,78]

## 2.6. Molybdenum

Molybdenum (commonly in the form of sulfides and oxides) is used as a heterogeneous catalyst since it usually can control the rate and selectivity of a wide range of chemical reactions, such as hydrotreating and oxidation. Its average market price is 62.01 $/kg.[79] Molybdenum borides, carbides, nitrides and phosphides are identified as effective catalysts for a range of reactions, i.e. hydrodeoxygenation (HDO), hydro-denitrogenation, hydrodesulfurization and syngas conversion.[80]

Recently, the conversion of gaseous GL into AA over Fe-Mo and Cs-Fe-Mo/HZSM-5 (with SiO₂/Al₂O₃ = 30 and 1500) catalysts was tested in a packed-bed continuous-flow reactor at 623 K and atmospheric pressure.[81] The reaction showed 52% of AA selectivity at 94% GL conversion with Cs-Fe-Mo/HZSM-5 catalyst. The success of this catalyst was attributed to the presence of basic sites, balanced with acidic sites, to the high mesopore volume (0.072 cm³/g, only 0.031 cm³/g in the case of the HZSM-5) and mesopore surface area (103 m²/g, only 40 m²/g in the HZSM-5) and to the synergetic interaction between metal species and ZSM-5 zeolite support (in the upper reaction of Scheme 8, the formation of AA was reported; in the other sites all the by-products were displayed).[81]

CeO₂ was also used as support of the bimetal oxides MoFe: the catalyst showed good stability throughout the gaseous conversion of GL to AA, probably due to the small nanoparticle sizes (7–17 nm) and low degree of CeO₂ crystallinity.[82]

HDO reaction can be directed toward different products modifying the surface of molybdenum carbide catalyst to reduce the oxophilicity of Mo by Cu monolayers.[83] Mo₂C was very active in breaking C–O bonds to form propene, but the authors managed to change products distribution depending on the amount of Cu added. Therefore, in the presence of Cu, it was possible to form AA and propanal and, increasing Cu amount, the Cu/Mo₂C surface cleaved only one C–O bond to obtain mainly HAc.[83] Different products were produced by gas
<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /><figcaption>**Scheme 8.** Full reaction pathway of GL to AA in the presence of the Cs-Fe-Mo/HZSM-5 catalyst by dehydration and transfer hydrogenation. Adapted with permission from A. Kostyniuk, D. Bajec, P. Djinović, B. Likozar, *Chem. Eng. J.* 2020, 397, 125430. Copyright 2020 Elsevier.</figcaption></figure>

phase HDO over a NiMoSₓ/Al₂O₃ catalyst, depending on reaction time (range between 3 and 360 s) at 673 K and 18 bar H₂ in a packed-bed flow reactor. At short contact times, the main products were unsaturated oxygenates including AA (41% yield), Acr (9%), HAc (21%), and acetaldehyde (4%) while the products detected at longer contact times were alkanes/alkenes (80% yield), COₓ (12%), and saturated alcohols (4%).[84] The authors tried to find a solution to catalyst deactivation by coke, flowing H₂S. With a 2100 ppm H₂S co-feed, the NiMoSₓ catalyst showed a 12-time decrease in the deactivation rate for deoxygenation and a 6-time decrease in the deactivation rate for hydrogenation.[84] The production of AA was correlated with the redox sites activity and the surface moderate acid density of catalysts.[85] The catalyst MoFe-0.3/KIT-6, having a good balance between moderate acidity (0.478 µmol m⁻²) and weak reducibility of catalysts, exhibited a remarkable yield of 27% of AA at 94% conversion of GL without external hydrogen donors.[85]

Mo-based catalysts supported on black or activated carbon catalyzed propene formation *via* one-step GL HDO reaction. Here the use of H₂ was found essential for selective propene production because it converted the intermediate 2-propenol into propene impeding the 2-propenol isomerization to propanal. Moreover, a pre-treatment of the Fe-Mo catalyst at 770 K with H₂ was crucial because it produced the Mo⁴⁺ and Mo⁵⁺ active and selective species.[86]

MoO₃ was used as an acidic modifier in a Ni based catalyst (Ni₂P/Al₂O₃), proposing a one-pot two-step synthesis of propene from pure GL in a fixed-bed reactor by hydrogenolysis of GL to propanol and catalytic cracking of propanol to propene with

ZSM-5 catalyst, achieving 88% of the desired product at 523 K without H₂.[87] Molybdenum species were used again as modifier: the metal was added on a zirconia-based Pt catalyst to promote liquid-phase GL hydrogenolysis.[88] The different acidity of several modifiers (e.g., W, B, and Mo) were compared, highlighting the correlation between GL conversion and catalyst Lewis acidity at initial 6 bar H₂ and 433 K: Pt//W/ZrO₂ was the most active catalyst leading to 42% 1,3-PD with a not so high conversion (around 20%), followed by Pt//Mo/ZrO₂ that produced 49% 1,2-PD but with a conversion of only 7%, while Pt//B/ZrO₂ was nearly inactive.[88] DFT was used to study the role of α-MoO₃ in the GL conversion to propanal, then its hydrogenation to 2-PO and 1-PO and, finally, dehydration to propene.[89] The authors underlined that this HDO took place on surface vacancy defects of the catalyst by an interaction between Mo species and oxygen of GL: the three-step reaction (dehydration, keto–enol equilibrium and hydrogenation) was found selective to the C–O cleavage instead of C–C one. Exploiting this positive interaction between Mo and GL, the use of 8.7 wt% Mo on black carbon was reported to produce 71% of propene over full GL conversion in vapor phase under continuous-flow conditions at 553 K and 30 H₂ bar; other identified products were the reaction intermediates as propanal, 1-PO and 2-PO, as previously proposed by the DFT.[90]

Bezerra *et al.* promoting the gas phase oxidative dehydro-
tion of GL to Acr (at atmospheric pressure and 573 K, in a
vertical packed-bed borosilicate reactor) found that its good
selectivity was dependent also on the acidity of vanadium and
molybdenum oxides supported on Al₂O₃.[42] Unfortunately, the
Mo catalyst suffered of higher deactivation by coke
deposition.[42]

A study reported several TM hydrogen phosphates sup-
ported on meso-HZSM-5 as a hybrid catalyst for the Acr
formation *via* GL dehydration. The catalytic activity and
selectivity were correlated to the physical (pore characteristics
of support) and chemical (moderate strength and appropriate
amount of BAS) properties of catalysts.[91] The pore volume of
meso-HZSM-5 were 0.28 cm³/g, larger than that of commercial
HZSM-5 (0.15 cm³/g); in the synthesized meso-HZSM-5, the
micropores (<1 nm) appeared quantitatively diminished and
the mesopores (2–4 nm) increased. More strong acid sites are
presented in the phosphates supported on meso-HZSM-5; by
temperature programmed desorption of ammonia, the strong
acid percentage reached 80% in the Mo₁/₃HPO₄/meso-HZSM-5
from 41% of the commercial HZSM-5. The yields of the GL
dehydration were found dependent on the employed TM
hydrogen phosphates (CuHPO₄, Mo₁/₃HPO₄, ZnHPO₄, NiHPO₄,
MnHPO₄). The protons provided by hydrogen phosphate acted
as a source of strong BAS mediated by the support, which
greatly enhanced the yield of Acr. Indeed, the acidic modified
TM-HPO₄/meso-HZSM-5 showed high Acr yields (≥ 70%), with
Mo₁/₃HPO₄/meso-HZSM-5 produced 72% of Acr.[91]

Wu *et al.* reviewed in depth many studies about the
formation of AAc from GL.[92] Among binary metal oxides (Mo-V-
O, W-V-O, Mo-V-W-O, W-V-Nb-O), Mo-V-O catalysts appeared
very active to promote one-pot oxydehydration of the GL to
AAc in the continuous gas-phase (Scheme 9).[92]
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /><figcaption>**Scheme 9.** Oxydehydration of GL to AAc, catalyzed by Mo-V-O catalyst. Adapted with permission from S. T. Wu, Q. M. She, R. Tesser, M. Di Serio, C. H. Zhou, *Catal. Rev. - Sci. Eng.* 2020, 62, 481–523. Copyright 2020 Taylor and Francis Group.</figcaption></figure>

The gaseous atmosphere played an important role in the AAc formation because a raising selectivity and catalyst stability were observed with 100% O₂, due to the equilibrium between V⁴⁺ and V⁵⁺ species.[⁹³] The efficiency and the stability of MoVW oxides can also be raised by adding surfactants in the hydrothermal synthesis of the catalysts.[⁴⁴] MoV₂O₈, among the molybdenum vanadium mixed catalyst, was presented as the most active species for the AAc synthesis at 593 K and atmospheric pressure, since the cycle of reduction and oxidation of the vanadium during the reaction caused pronounced dynamic formation of oxygen vacancies, resulting in 97% conversion of GL and 32% selectivity.[⁹⁴]

In the oxydehydration of GL, also bifunctional catalyst formed from Keggin-type molybdovanadophosphoric acid, H₃⁺·xPV₂Mo₁₂ₓO₄₀ (x=0–3), and ZSM-5 zeolite was proposed to obtain at 623 K high yields of AAc (35%) and Acr (60%).[⁹⁵]

## 2.7. Tantalum

Tantalum is a refractory metal (Ta powder average market price: 324.14 $/kg)[⁹⁶] used for different purposes in alloys with many other metals.[⁹⁷] It is oxophilic and frequently present as Ta(V) in the coordination compounds. Due to the oxide film on its surface, tantalum is known to be very corrosion resistant, but it is attacked by HF.[⁹⁸]

To the best of our knowledge, only in recent works, the use of tantalum in deoxygenated reactions of GL was reported. Pt supported on WOₓ-modified tantalum oxide was synthesized as a catalyst for hydrogenolysis of GL to produce 1,3-PD.[⁹⁹] The crystalline T-phase Ta₂O₅ was chosen to act as support for Pt owing to thermal and hydrothermal stability and for its high similarity with W (in electronegativity and ionic radius); in that way, the WOₓ dispersion on the support surface can be enhanced.[⁹⁹]

The WOₓ species acted as anchoring sites for Pt without formation of Pt nanoparticles, so an atomically Pt dispersion could be formed on the surface. This peculiarity increased the H₂ absorption capability. The positively charged Pt species in Pt⁶⁺/WOₓ/T-Ta₂O₅ enabled H₂ heterolytic dissociation to H⁶⁺ and H⁵⁻ species, and H⁵⁺ species spilled over the surface of support *via* Pt-O-W interfacial, producing BAS.[⁹⁹,¹⁰⁰] Operating in water solution at 433 K and 50 bar H₂ pressure, a 90% conversion of GL was reached and a selectivity for 1,3-PD of 45%. This catalyst was reused for 4 successive runs, evidencing only a small decrease in GL conversion from 90% to 79%, without changes in the selectivity toward 1,3-PD.[⁹⁹]

Recently, a mesoporous tantalum phosphate was used in the dehydration of gaseous GL to obtain Acr in a down-flow fixed-bed reactor at room pressure and 573 K under N₂.[¹⁰¹] The catalyst, which exhibited mainly Ta⁵⁺ species, showed BAS and LAS that favored the formation of the product with >70% selectivity (at 85% conversion).

## 2.8. Tungsten

Tungsten species (W average market price: 38.19 $/kg)[¹⁰²] are often employed as co-catalysts in the valorization processes of GL: they can not only enhance the acidity of the catalyst or increase the number of BAS but also improve the dispersion of other active metals. The presence of BAS can promote the hydrogenolysis of GL to 1,3-PD, by absorption of one -OH moiety and subsequent dehydration and hydrogenation steps or by a dehydration to AA, which is easily converted to 3-HPA and hydrogenated.[¹⁰³]

Garcia-Fernandez et al. observed that polytungstates (WOₓ) have the appropriate moderate BAS to transform GL to 1,3-PD selectively, establishing structure-activity relationship of these supported bimetallic catalysts (Pt/WOₓ on aluminum oxide materials and on a HZSM-5 zeolite), where the proximity of Pt promoted the hydrogenation of the intermediate carbocation into 1,3-PD (56% selectivity with catalyst on alumina).[¹⁰⁴] Raising the strength of BAS (HZSM-5 as support), 1,3-PD (21% selectivity) was transformed to 1-PO (35% selectivity) and propane (33% selectivity), while the Pt dispersion made the catalyst more active in terms of reagent conversion (from 8% to 16%) but less selective since even 1,2-PD was produced (1,3-PD/1,2-PD ratio from 3.1 to 2.6).[¹⁰⁴] *In situ* growth of hydrate WO₃, on the surface of γ-Al₂O₃, encouraged a regular dispersion of Pt species producing their strong interactions with WOₓ, and converted 77% of GL to 1,3-PD at 453 K; when this catalyst was synthesized by traditional wet impregnation/calcination, only 39% was transformed.[¹⁰⁵] The stabilization of metal single atoms on tungsten oxide and their catalytic properties in H₂ activation were discussed. Pt species on WOₓ increased catalyst stability and facilitated the dissociation of H₂ producing BAS (H⁶⁺) under H₂ atmosphere after spillover of hydrogen to WOₓ.[¹⁰⁶] Shi et al. observed the influence of increased acidity and metal dispersity, for the Pt/TiO₂ catalyst when doped with WOₓ, on the reagent conversion at the expense of the selectivity: at high conversion, 1-PO was formed with 33%
<figure><img src="image_32.png" /></figure>

selectivity, detrimental to the formation of 1,3-PD, obtained with 22% selectivity.[107] It was also pointed out that the polymerization degree of the WOₓ domains affected the 1,3-PD yield.[108] WOₓ enhanced significantly 1,3-PD formation rate at 453 K and 80 bar, especially after Mn doping of Pt-WOₓ/ZrO₂ catalyst, since Mn was able to generate a large amount of medium polymerized WOₓ. The authors described Pt-(WOₓ)ₙ-H as the catalytically active structure.[108] The high activity of the oligomeric WOₓ species was outlined also using a Pt/W/SBA-15.[109]

Wang *et al.* observed that Mg doping on the Pt/WOₓ-ZrO₂ catalyst surface inhibited high polymerization degrees of WOₓ and, furthermore, the leaching of tungsten, greatly improving catalyst stability.[110] Pt/0.50Mg/WOₓ-ZrO₂ catalyst exhibited superior stability with 53% conversion in 25 h time-on-stream at 423 K and 40 bar.[110] Pt/H-WO₃ (with high oxygen vacancies) exhibited higher GL conversion (64%) and 1,3-PD selectivity (43%) than that (38% conversion, 33% selectivity) with Pt/L-WO₃ (low oxygen vacancies), carrying out the reaction at 433 K and 50 bar.[111] This was explained considering that Pt/H-WO₃ can more easily be coordinated by two terminal hydroxyl groups of a GL molecule, while the secondary OH group can be protonated by the H⁺ generated by H₂ on Pt.[111] Liu *et al.* determined that the interface of Pt-WOₓ is regarded as the active site for the selective dissociation of C–O bond and, for this reason, the authors’ issue was to increase the affinity of Pt and W.[112] It is possible to change the catalyst structure of Pt-WOₓ/SiO₂ by modulating the tungsten amount: only the catalyst modified by appropriate small amount of W (W/Pt = 0.25) can work with success at 80 bar of H₂ and 413 K, reaching 83% of GL conversion and 57% of 1,3-PD selectivity.[112] At 413 K and 10 bar H₂, 81% GL conversion and 51% 1,3-PD selectivity were obtained employing Au-promoted Pt/WOₓ catalysts, characterized by a high metal dispersion on WOₓ.[113]

The use of a bifunctional catalyst (metal/BAS surface) can provide a double effect driving on dehydration and hydrogenation steps together: Ni/WO₃–TiO₂ catalyst afforded 1-PO and 2-PO in total yield of 94% at 513–543 K and H₂ pressure of 20–36 bar.[114] The strong acidity of the catalyst promoted the dehydration while Ni nanoparticles allowed the hydrogenation reaction. Even the H₂ pressure influenced the yield of 1-PO: using 10Ni-30HSiW/α-Al₂O₃ at 40 bar, 85% selectivity was calculated.[115] The H₂ pressure preserved the zero-state of nickel, necessary for the selectivity in the hydrogenation step, while reduced the presence of W⁶⁺ and, consequently, the acidity of the catalyst and the conversion of GL (55% from 65% at 10 bar).[115]

Trying to shift the dehydration reaction product to Acr using a WO₃/ZrO₂ catalyst in flow mode at 553 K and 5 bar, with H₂ as the gas carrier, Hulteberg *et al.* focused their research on changing the catalyst pore structure, removing pores with diameter < 45 Å by thermal sintering at temperatures above 1073 K.[116] In this way, it was possible to obtain a longer life catalyst with a lower specific surface area (11% decreased), more open pore structure (pores larger than 45 Å), a lower absolute acidity but higher specific acid density (from 1.85·10⁻⁶ to 2.05·10⁻⁶ mmol/m²).[116] In a different study, the results

obtained when using WO₃-SiO₂ and WO₃-ZrO₂-SiO₂ catalysts were compared for the gas phase GL dehydration to Acr.[117] The reaction was carried out in a fixed-bed reactor, under aerobic atmospheric pressure and in a temperature range of 553–653 K. The yields of Acr were between 40 and 90%, strongly depending on the catalyst composition (W and/or Zr amount). Usually, the yield decreased when Zr amount increased, maybe due to the formation of other oxidative by-products, e.g. COₓ. The authors observed also a parallelism between the selectivity to Acr and the proportion of BAS in catalysts.[117] Dehydration of GL to Acr was achieved using a WO₃/ZrO₂@SiC catalyst, under microwave irradiation, with better results compared to conventional electric heating: at 523 K the Acr selectivity reached over 70% with complete GL conversion under microwave, against 54% by conventional heating.[118] The microwave was used again to prepare, by sol-gel method, a 25%WO₃-SiO₂ that catalyzed the Acr formation (54% selectivity at full conversion) in a fixed-bed reactor at 573 K.[119] The microwave raised the concentration of medium-strength BAS and led to a high amount of cubic WO₃ crystals which increased the catalyst stability.

WO₃/Al₂O₃ catalyst, with 20 wt% WO₃ loading, showed, in the dehydration of GL, higher activity and durability (assessed up to 5 h) than other similar supported WO₃ catalysts and zeolites.[120] By carrying out the reaction at 588 K in a fixed-bed reactor, the number of BAS and mesopores in the catalyst were the same before and after the reaction, extending the catalyst life also under O₂ flow and raising to 90% Acr yield.[120] Recently, tungsten-based heteropolyacids were supported on non-ordered mesoporous silica to catalyze the dehydration to Acr.[121] The Acr formation rate showed a good linear relationship with respect to the BAS concentration and the total acid concentration (Brønsted and Lewis) until H₃PW₁₂O₄₀ loading 30 wt%. This relationship changed for higher percentage loading, maybe for a worst dispersion of heteropolyacids that did not allow the linear increase of acid sites number. The authors reported an Acr formation rate in the range of 3.0–5.0 mmol h⁻¹ g⁻¹ working in a vertical fixed-bed reactor at atmospheric pressure and 573 K.[121]

A hydrothermally synthesis of two different W-V-O catalysts was reported in literature: the hexagonal tungsten bronze (h-WO₃) phase and monoclinic phase (m-WO₃).[122] The h-WO₃ showed a significantly higher concentration of acid sites than m-WO₃, making h-WO₃ active for AAc production (yields of Acr and AAc were 24% and 32%, respectively), while m-WO₃ led to the formation of COₓ. This gas-phase oxydehydration of GL was conducted in a fixed-bed reactor at atmospheric pressure and 593 K.[122]

## 2.9. Rhenium

Rhenium (Re average market price: 2,395.82 $/kg)[123] is a metal with very specific catalytic performances showing several oxidation states, frequently in very stable forms.[124] For other metals, indeed, it is not common to have stable high oxidation state derivatives at room temperature (e.g., Mn) but Re₂O₇ is
<figure><img src="image_33.png" /></figure>

commonly used, in water or oxygenated organic solvents (where it is very soluble), to prepare numerous supported catalysts at 298 K. By taking a brief look at the rhenium chemistry which occurs in any catalytic applications, it is interesting to evidence its peculiarity to activate the C–O bond, then facilitating the bond-cleavage. In this regard, it was demonstrated that the ReO₂⁻ species, when in its gaseous form, can extract an oxygen atom from CO₂ producing CO and ReO₃⁻.[¹²⁵] As we are considering only reduction reactions, hydrogenolysis and DODH are certainly the two widely diffused reactions where this metal is employed as a catalyst. Hydrogenolysis of GL leads to 1,2-PD or 1,3-PD and Re is often used in association with other TMs. A bimetallic supported Re-Cu/ZnO system was prepared via wet impregnation: the presence of Re improved the distribution of metal oxide on the support while the presence of Cu decreased the Brønsted acidity of the catalyst.[¹²⁶] The supported catalytic system was active for hydrogenolysis of GL in a continuous-flow fixed-bed reactor in a temperature range of 423–523 K and a H₂ pressure of 60 bar. At 423 K, the selectivity to 1,2-PD was excellent (about 100%) but, increasing temperature or H₂ pressure, the formation of minor alcohols, 1-PO (8% selectivity at 523 K) and ethylene glycol (45%) occurred.[¹²⁶] It is worthy of mention also the work by Varghese *et al.*, which investigated the formation of 1,3-PD when the supported ReOₓ-Ir/SiO₂ catalyst was used.[¹²⁷] Despite we cited the same research in the iridium section, here we highlight the role of Re in the improvement of the 1,3-PD yield (with 1:1 = Re:Ir) to the detriment of 1,2-PD (with Ir only). The reduced Brønsted acidic ReOₓ clusters, dispersed on the Ir nanoparticle surface, facilitated dissociative attachment of GL addressing the reaction to a preferential formation of the primary propoxide, with a consequent formation of 1,3-diol. In the presence of a H-ZSM-5-supported similar catalyst, at mild temperatures (393–453 K), the over-hydrogenolysis of diols was inhibited; however, raising pressure from 20 to 80 bar, the conversion and 1,3-PD selectivity were higher (from 8 to 10% and from 14 to 35%, respectively).[¹²⁸]

As potential DODH products, we can mention AA that can be obtained in good yield employing MTO, Re₂O₇, ReO₃, or other rhenium oxides.[¹²⁹] A remarkable AA yield (91%) was obtained at 443 K using 10 wt% ReOₓ/Al₂O₃, with 2-hexanol as both reductive and solvent agent.[¹³⁰] The authors confirmed the heterogeneity of the involved catalysis mechanism since the reaction stopped upon removal of the ReOₓ/Al₂O₃ by hot filtration and they also evidenced the absence of the metal leaching. The catalyst reuse tests demonstrated its activity for 3 reaction cycles. Mechanistically, the authors proposed a catalysis involving the redox-couple ReV/ReVI. Mesoporous ceria supports, prepared by a nanocasting process with silica as hard template, were impregnated with 10 wt% ReOₓ and used in DODH reaction at 448 K using 2-hexanol or 4-methyl-2-pentanol as solvent and hydrogen donor.[¹³¹] Quantitative conversion of GL was observed with 88% yield in AA; also this catalyst was recycled 3 times without loss of activity.

Using soluble catalytic systems, the DODH to obtain AA was reported in the presence of several rhenium compounds, among them oxides, phosphine and halide derivatives, in

alcoholic solvent or neat conditions.[¹²⁹] Lupacchini *et al.* found that, independently of the starting Re species (e.g., oxides, iodides, phosphine), based on IR and XPS measurements, the formation of a Re-alkoxide precipitate was observed and was proposed to be the active catalytic species for all the used Re-catalysts, in alcoholic solvents or in neat conditions; in these last tests GL acted, simultaneously, as reactant, solvent and reducing agent.[¹²⁹] The reactions in solvent were conducted in 2,4-dimethyl-3-pentanol at 413 K, under air or H₂ flux. The best yields of AA were observed in solution with ReO₃ (91%) and MTO (86%) and under hydrogen gas flux. Cp²¹ReO₃ (Cp²¹= 1,3-di-tert-butylcyclopentadienyl) was reported as the catalyst in 3-octanol under N₂ atmosphere at 408 K able to convert quantitatively and selectively the GL to AA.[¹³²] In this work, the real catalyst species was not identified but the authors observed, during the reaction, the dissociation of the Cp ligands. An improvement in DODH was obtained using ionic liquids as reaction media.[¹³³] When benzyl or imidazolyl derivative was used, good yields of alkene derivatives were observed, and it was possible to reuse the rhenium catalyst, seeing only a slight decrease in product yield. The preserved activity of the rhenium catalyst is probably due to the involvement of the ionic liquid with the resulting suppression of the polymerization of by-products (e.g., HAc, lactic acid, and glyceraldehyde, recovered for the first time at the end of DODH reaction).[¹³³]

# 3. Late Transition Metals

## 3.1. Iron

Iron has been considered an attractive metal, mainly due to the redox cycle Fe³⁺/Fe²⁺, but also because it is an element easily available, at low cost (pig iron average market price: 0.471 $/kg)[¹³⁴] and less toxic than vanadium, tungsten or molybdenum.[¹³⁵] Many articles in literature reported the use of phosphates as catalyst for the hydrogenation of GL since they showed better catalytic behavior than other supports such as zeolites, and they suffer less deactivation. This was confirmed during the gas phase GL dehydration on mesoporous AlPO₄ and modified AlPO₄ with a small amount (1 wt%) of a TM (Co, V, Fe), calcined at 623 K and 723 K.[¹³⁶] The best GL conversion (around 90%) was obtained for the Fe catalysts, followed by those with Co (75%) and V (50%), performing the reaction in a continuous-flow fixed-bed microreactor at 553 K under atmospheric pressure. The main product obtained with Fe catalysts was Acr (44% selectivity with FePO₄).[¹³⁶] The crystallinity and the acidity of the iron phosphates were crucial: a quantitative conversion of GL with 82% Acr selectivity was observed only in the presence of the amorphous Fe catalyst.[¹³⁷]

Despite their easy deactivation, several examples of zeolites, used as support for iron, were reported. Diallo *et al.* prepared catalysts for the GL transformation to Acr with different iron contents and different Si/Al ratio.[¹³⁸] The role of Fe was just to reduce the catalyst deactivation by altering the amount and the nature of the coke components. In this way, and with high Si/Al
<figure><img src="image_34.png" /></figure>

ratio, the Acr selectivity was about 80% performing the reaction at 593 K under atmospheric pressure.[138] H,Fe-MCM-22 was studied as catalyst for gas-phase oxydehydration of GL to AAc and Fe³⁺ oligomers and iron oxide clusters were identified as the main iron species contained in the catalyst structure.[135] The authors observed an inverse relationship between the content of iron with the acid sites density and the textural properties: the catalysts containing the lowest iron amount (1.2%) had shown better yields to AAc (25% instead of 13% with Fe 4.5 wt%). To obtain a higher GL conversion, a balance between acid and textural properties was tuned: 93% GL conversion with 57% AAc selectivity was obtained using 1.2 wt% of Fe in the catalyst and working in a fixed-bed vertical reactor under oxygen feed, using both to warrant the regeneration of Fe³⁺ and to avoid the formation of carbonaceous deposits on the catalytic surface. The evidence that Fe³⁺ tetrahedral species, contained in catalytic framework, were more selective to AAc, while Fe²⁺Oₓ clusters in extra-framework position increased Acr selectivity was indicated.[139] The authors synthesized Fe-containing zeolites through two different processes, based on hydrothermal synthesis or post-synthesis preparation. The catalyst obtained by post-synthesis method, with Si/Al molar ratio = 45, showed the reduction of crystallite size and the enhancement of the external surface, leading to 99% GL conversion and 96% selectivity to Acr (no AAc amount), after 30 h on-stream at 593 K and atmospheric pressure. The catalyst obtained by hydrothermal process showed almost the same activity but different selectivity (Acr 71%, AAc 24%) thanks to the Fe³⁺ tetrahedral species in the framework catalyst.[139]

Martín *et al.* presented Fe, Ni and FeNi oxides supported on commercial mesoporous alumina as catalysts in the dehydration-dehydrogenation reaction of gas phase GL at 573 K and ambient pressure.[140] The authors observed that, in these bimetallic catalysts, the presence of nickel reduced the iron oxidation state, enhancing the redox properties of the solid. Moreover, the combination of the two metals changed the selectivity towards the AA (90% GL conversion and 90% AA selectivity) whereas with the monometallic oxides, 88% HAc selectivity was reported.[140] The addition of potassium species to α-Fe₂O₃ yielded a loss of LAS and, in the presence of formic acid, morphology changes of solid were observed with the formation of BAS.[141] Using these catalysts in the dehydration of GL at 623 K, AA was obtained as the main product (40% yield, Acr 10%, others 25%).

The use of iron in combination with other TMs was also described: the electron interaction between Pt and Fe was the cause of enhancement of 1,2-PD yield from GL hydrogenolysis.[142] Pt/Al₂O₃ and Pt/Fe₂O₃-Al₂O₃ catalysts having different Fe/Pt ratios were compared: thus, by increasing Fe loading, it was possible to go from 16% GL conversion and 14% 1,2-PD selectivity (without iron) to 94% GL conversion and 51% 1,2-PD selectivity (Al/Fe = 1/1 atomic ratio), under N₂ pressure at 523 K.[142] The same trend was underlined by Zhang *et al.*: the Fe(OH)O species in Pt-Fe³⁺ enhanced BAS and the faded Pt-H binding strength, thus resulting in 3-fold enhanced selectivity toward ethylene glycol and 1,2-PD (combined selectivity of around 70% when the conversion of GL up to

60%), compared to monometallic Pt supported on zeolite catalysts.[143] A synergistic effect between Fe and Co was also found: Co was able to improve the activation of hydrogen, and Fe improved the acidity of Co-Fe alloy resulting in the formation of AA, from dehydration - hydrogenation, and 1-PO, via consecutive hydrogenation of AA, in case of higher reaction temperature and hydrogen pressure.[144]

## 3.2. Cobalt

Despite its sensitivity to the temperature (Co powder average market price: 29.595 $/kg),[145] cobalt was also used for deoxygenation of GL, being a very active catalyst hydrogenation.

Cobalt oxide (10 and 20 wt% Co) supported on synthetic mesoporous aluminum phosphate and Al-Co phosphates (10 wt%–37 wt% Co) were prepared and used in oxydehydration of gaseous GL to AAc, via Acr, at 553 K in a continuous-flow fixed-bed reactor in presence of O₂.[146] Comparing the Co catalysts with the corresponding Fe and V ones, the Co-based were the best catalysts, able to produce AAc thanks to the balance between redox and acid properties.[146] Mesoporous AlPO₄ was modified with a lower percentage of Co (1 wt%), and compared it to that with other TMs (Cu, Cr, Fe): Co catalyst showed the maximum yield to Acr (54% with almost quantitative conversion of GL) at 553 K, using a continuous-flow fixed-bed microreactor.[147] The performance was better for Co on AlPO₄ comparing to Co supported on some commercial zeolites, like H-ZSM-5 or H-Y.[147]

Two examples reported the use of Cu-Co catalysts supported on alumina for the continuous hydrogenolysis of GL to 1,2-PD.[148,149] Raju *et al.* tested the reaction at atmospheric pressure by moving the temperature in the range of 453–513 K.[148] Co doping induced a fast reduction of CuO species, and the active interactions Cu-Co led to high Cu dispersion (from 35% without Co to 50% in 10%Cu-7%Co) and metal surface area (from 234 m²g⁻¹ without Co to 338 m²g⁻¹ in 10% Cu-7%Co). The catalyst with 10%Cu-7%Co on Al₂O₃ converted completely GL with 77% selectivity to 1,2-PD and it turned out to be very stable.[148] Different Co-Cu/Al₂O₃ with a fixed Cu loading of 7 wt% but a variable Co loading from 0 to 1.5 wt% were prepared, to evaluate the effect of Co.[149] The presence of Co increased the hydrogenation rate of GL up to the plateau due to the raising number of acid sites and to the geometry of the metal species but, above 1.3 wt% of Co, a decreasing of catalytic activity was observed, owing to the formation of aluminates and less acidic sites. However, at 493 K and 5 bar of H₂, the main product was always 1,2-PD.[149] Also in the absence of Cu, a Co/Al₂O₃ was employed in water solution but the conversion and 1,2-PD selectivity were 54% and 76% carrying out the reaction in a fixed-bed flow reactor at 503 K and 35 bar.[150]

The presence of Co⁰ nanoparticles is very important: a hydrotalcite-derived Co₂-Ca₄-Al₃ catalyzed several steps for the formation of 1,2-PD.[151] XRD characterization revealed the formation of a CaCO₃ layer on the surface of the catalyst that reduced the number of strong basic sites, enhanced the
<figure><img src="image_35.png" /></figure>

number of moderate basic sites, and helped to avoid the metals leaching during the reaction, carried out at 483 K and 40 bar H₂ in a continuous-flow fixed-bed reactor. So, the product was obtained with excellent selectivity (91%) at complete GL conversion.[151]

Cobalt aluminate nanoparticles were also used as catalyst in the gas phase GL transformation. They were prepared by flame spray pyrolysis and employed for the catalytic reaction at 573 K and 1 bar H₂.[152] After 3 h reaction, 35% of GL conversion to a mixture of HAc (19%), PAI (23%), lactic acid (8%) and lactide (27%) as main products, was observed.[152]

### 3.3. Nickel

Although nickel is not a noble metal (Ni briquettes average market price: 18.268 $/kg),[153] nevertheless it can catalyze the hydrogenolysis of GL in a good way. Recently, a monometallic Ni catalyst, supported on CeO₂, was used at 20 bar and 503 K to obtain 1,3-PD.[154] The same catalyst was prepared by two methods: wet impregnation and by ultrasound, the last one being the most successful because of its homogenous distribution, smaller crystallite size, bigger surface area and low reduction temperature. In the presence of the sonochemically synthesized catalyst, 54% GL conversion, 1,3-PD (53% selectivity), 1,2-PD (15%), 1-PO (19%) were obtained.[154]

A series of bimetallic oxides (Ni, Cu or Zn with Zr) was tested supporting them on H-beta zeolite with loading of 5-20 wt% via wet impregnation.[155] The most active catalyst in the GL hydrogenolysis was Ni-Zr that gave 1,3-PD selectivity of 14% over 73% GL conversion at 473 K and 41 H₂ bar.[155] In order to modify the acidic characteristic of a catalyst employed in the production of glycols, ion exchange was a viable pathway. Decreasing acidity of the bifunctional catalyst 10Ni-30H₄SiW/Al₂O₃ was reported by the exchange of H⁺ with Cs⁺, without altering the Keggin structure.[156] The catalysts with Cs ions almost did not influence the GL conversion but affected the products distribution. The authors, working at 513 K and 60 bar H₂, observed that with 10Ni-30Cs1H₃SiW₁₂O₄₀/Al₂O₃ it was possible to reach the best selectivity for 1,3-PD and 1-PO and that, in the absence of Ni, the catalyst activity was poor. However, 10Ni-30Cs₄SiW₁₂O₄₀, with all the protons substituted by Cs⁺, had low selectivity for 1-PO and high selectivity for 1,2-PD and ethylene glycol.[156]

Although even the Cu/Al₂O₃ was an effective catalyst in the hydrogenolysis of GL to 1,2-PD at 523 K and 40 bar H₂ (full conversion and 70% yield), it is interesting to spare the hydrogen gas, obtaining the same very good results.[157] Freitas et al. obtained 25% yield of 1,2-PD, at the same conditions, but under an inert atmosphere, using CuNi/Al₂O₃ or CuNi/ZSM-5, prepared by wet impregnation and containing 20 wt% of CuO and NiO.[157] The formation of a physical alloy of the two metals was detected by XRD: Ni had the role of promoting the formation of H₂ *in situ*, consumed to hydrogenate HAc, the intermediate obtained during the first-step of dehydrogenation, leading to the desired 1,2-PD in the second one-pot step (about 25% of yield with both supports).[157] Yield and conversion were

improved when a copious amount of NaOH (NaOH/GL = 0.5 molar ratio) was added to the reaction mixture containing CuNi/Y as catalyst: the GL conversion raised from 17 (0 mol NaOH) to 97% and the 1,2-PD yield from 3 to 32% in a fixed-bed at 533 K in absence of H₂.[158] The preparation of 1,2-PD was also investigated using 20 wt% Cu-Ni (1:1)/γ-Al₂O₃ bimetallic catalyst, charged in a packed-bed down-flow tubular reactor.[159] Full GL conversion and excellent 1,2-PD selectivity (90%) at 493 K and 7.5 bar were observed when the catalyst was previously calcined at 673 K. Calcination temperature was the key factor to the formation of bimetallic Cu-Ni and metal-support phases leading to high metal dispersion (about 20%) and surface area (30 m²g⁻¹), and small average crystallite size (≤ 20 nm). The authors concluded that, at 673 K, total acidic strength was maximum, diminishing with higher temperature because of metal particles sintering.[159] The dispersion of metal particles onto the solid support and the formation of the suitable acid sites can be gained also with the addition of Ag, like in Ni-Ag/γ-Al₂O₃, active and selective towards 1,2-PD formation.[160]

A comparison between alumina and silica, used as support for Re-promoted nickel catalysts, synthesized by wet impregnation and used in the transformation of GL to 1,2-PD, was published.[161] The reactions were performed in a continuous-flow fixed-bed reactor in a temperature range of 523–598 K and at H₂ pressure of 60 bar. The alumina catalyst was found to be more active than the silica at any temperature in the range but with the formation of mono-alcohols at higher temperature or pressure. The results of this Re-promoted nickel catalyst on the alumina were ascribed to higher metallic surface areas and BAS number.[161] Moreover, the addition of Pt to Ni catalysts seems to be helpful: a Pt-Ni/γ-Al₂O₃ catalyzed the hydrogenolysis to 1,2-PD (52% selectivity and 71% conversion) at 513 K under 10 bar of inert gas.[162]

A Ni-WO₃, supported on modified-sepiolite, was successful in the hydrogenolysis of GL (88% conversion) at 453 K and 20 bar H₂ in the presence of NaOH, producing 1,2-PD with 97% selectivity.[163]

To improve the activity of Ni catalysts supported on a silica-carbon composite in the formation of 1,2-PD, Gatti *et al.* used two alternative strategies.[164] The first strategy consisted in a modification of the catalyst acidity functionalizing the carbon support by oxidation at 353 K with HNO₃ to increase the number of LAS. The second strategy relies on a modification of the metallic phase, producing a NiZn alloy by the addition of small quantities of Zn (1–1.8%). In this last case, 1,2-PD was obtained with 87% of selectivity and 59% GL conversion at 533 K and 20 bar H₂ pressure (85% selectivity and 52% conversion with modified support catalyst).[164] A MgO/Ni₃C catalyst, also in absence of H₂ and of solvent, was usefully employed to transform 74% GL to 1,2-PD (67% selectivity) and HAc (13%) at 513 K.[165] The catalyst was recovered, washed and calcined to reuse it with sufficient activity for 6 cycles diminishing the conversion of 20%.[165]

Ni-based catalyst supported on TiO₂ was also investigated: Ni/Cu/TiO₂ catalyst efficiently catalyzed the hydrogenolysis of GL to 1,2-PD, under N₂ atmosphere (35 bar) and using 2-PO as
<figure><img src="image_36.png" /></figure>

the hydrogen source.[166] Compared to the same catalyst supported over alumina (Ni/Cu/Al₂O₃), the use of titania as support gave a better selectivity to 1,2-PD (74% vs 66% without H₂, 87% vs 83% with 35 H₂ bar) and conversion (85% vs 78% without H₂, both 100% with 35 H₂ bar) at 500 K, due to the high Cu dispersion (10.4% vs. 5.1%) and the right Ni/Cu atomic ratio (1.30 vs. 1.13) on catalytic surface. Unfortunately, metal particles sintering and absorption of organic species on the surface led to a rapid deactivation of the catalyst.[166] Mixed oxide of Cu-Cr doped with NiO on γ-Al₂O₃ was tested for gas phase GL hydrogenolysis at 473 K and 3–5 bar in a flow system reactor to produce 1,2-PD and HAc, as by-product.[167] In this case, the glycol selectivity was directly related to temperature and pressure increasing up to 95% with 85% GL conversion (at 5 bar and 493 K).[167]

Nanometric Ni phosphides, in presence of citric acid, demonstrated to act as a chelating agent, proved to be a good catalyst to produce 1,2-PD under N₂.[168] After the calcination step, the residual amounts of citric acid assisted the dispersion of metal particles avoiding the sintering, and supported the initial reduction of the Ni species.

1-PO was obtained in a two-step reaction, using two Ni-based catalysts: first, Ni/γ-Al₂O₃, and then, nickel on carbon-based support modified with phosphorus.[169] It was noted that the excellent selectivity (71% at 533 K and 65 bar) was due to the surface acidity of the second catalyst, correlated with the amount of phosphate species content.[169] The same research group prepared also Ni phosphide on mesoporous carbon to catalyze the transformation of aqueous GL, allowing 74% of 1-PO selectivity with full conversion at 533 K and 20 bar H₂.[170] They noted a different activity for several contents of the phosphate precursor, Al(H₂PO₄)₃: when it was between 4 and 10 wt%, the catalyst was more acidic and it was turned out more active, while with a higher content, the catalyst activity increased but the selectivity decreased, due to the strong contribution of dehydration reactions on the support acid sites.[170]

A bimetallic catalytic system, such as Ni/WO₃ supported on TiO₂ or ZrO₂, was prepared by co-precipitation of WO₃-ZrO₂ or WO₃-TiO₂, and catalyzed the GL conversion to 1-PO and 2-PO in a flow system.[171] Working at 523 K and hydrogen pressure of 30 bar, the total yield of propanols was 94% over full GL conversion, with strong prevalence of 1-PO (80% with catalyst supported on zirconia and 88% with catalyst supported on titania).[171] The high dehydrating activity needed to transform GL to carbonylic compounds in the first steps of the reaction was secured by the presence of BAS in the catalyst structure, while the presence of Ni nanoparticles enhanced the extent of hydrogenation to propanols (Scheme 10).[171]

Nickel-substituted tungstophosphoric acid on Y-ASA (porous amorphous silica-alumina) was used as catalyst in the GL dehydration.[172] The presence of nickel enhanced the BAS and LAS of the catalyst and, consequently, its selectivity for the Acr production gaining 74% at 593 K and atmospheric pressure in a fixed-bed stainless steel reactor.[172]

<figure><img src="image_37.png" /><figcaption>**Scheme 10.** Mechanism of GL hydrogenolysis to propanols on a bifunctional catalyst (Ni/WO₃ supported on TiO₂ or ZrO₂). Adapted from A. A. Greish, E. D. Finashina, O. P. Tkachenko, L. M. Kustov, *Molecules* 2021, 26, 1–16.</figcaption></figure>

### 3.4. Copper

Copper is one of the most utilized TM catalysts in synthetic organic chemistry, thanks to its economic (and environmental) advantages arising from its high Earth abundance, compared to precious TM catalysts (Cu average market price: 8.357 $/kg).[173] Also in this review, copper is cited in a lot of papers since it has been employed alone or, often, in combination with other metals, reaching excellent results.

Copper chromite and Cu/Al₂O₃ catalysts had very different physical and chemical properties; however, in the hydrogenolysis of GL, carried out in a trickle-bed reactor, 97% selectivity of 1,2-PD and complete conversion of GL were obtained for both catalysts at 503 K and 14 bar H₂.[174] This comparable activity would be related to the presence of CuO species in both catalysts, as seen by XPS. The authors observed also that higher temperatures produced a decrease in the selectivity to 1,2-PD due to the formation of different by-products: minor alcohols in presence of Cu/Al₂O₃, and HAc and 3-HPA in the case of copper chromite.[174] In the range 473–493 K, a catalyst consisting of Cu-Cr oxides with NiO supported on γ-Al₂O₃ led to an increased selectivity for 1,2-PD, in spite of HAc.[167] The reactivity of copper chromite catalyst in the hydrogenolysis of GL was also studied by DFT showing that the interactions with the substrate were due to both Cr-O and hydrogen bonds.[55] In particular, after the adsorption of the substrate on Cr, the C–O cleavage was favored and the Cu atom promoted the dehydration step to HAc, being the rate-determining step of GL hydrogenolysis.[55]

Lower conversion (72%) and 1,2-PD selectivity (86%) were achieved with the catalyst Cu-Ni/Al₂O₃ at 483 K and 45 H₂ bar with a pseudo-first-order kinetics with respect to GL concentration.[175] Moreover, the kinetic study showed that dehydration of GL to HAc took place on the acidic centers of the catalyst whereas the hydrogenation of HAc to 1,2-PD took place on the metallic sides.[175]

The performance of Cu/Al₂O₃, compared with individual copper particles, was studied in liquid-phase hydrogenolysis, obtaining a linear relation of the reaction rate with specific surface area of copper.[176] Both catalysts, at 473 K and under pressure of 20 bar, gave 98% 1,2-PD selectivity, but the presence of support was able to prevent intensive sintering of
<figure><img src="image_38.png" /></figure>

the active phase of the catalysts.[176] The addition of a co-metal such as Ba, Ce or La on γ-Al₂O₃ was a very efficient method to avoid any sintering event, stabilizing the Cu particle size during reaction, consequently increasing the lifetime of the catalyst to 70 h, while the bare Cu catalyst exhibited marked deactivation after only 50–60 h reaction.[177] Furthermore, carrying out the reaction at 473 K and 75 bar in a fixed-bed reactor, the selectivity to 1,2-PD grew up to 96% if the co-metal was Ba, even if the conversion was modest (18%).[177] The geometry of active phase in Cu/Al₂O₃ was indicated as not crucial as its chemical composition.[178] This statement was deduced from the observed higher activity of the Cu₂O phase in comparison with the CuO phase of the catalyst and from the almost same results for catalysts characterized by different particle sizes (20–140 nm) but close chemical composition, with a 98% selectivity for 1,2-PD, at 473 K.[178]

Vila *et al.* performed DRIFT and ¹H NMR spectra revealing that the enediol species (1,2- and 2,3-enediol), formed in the hydrogenation reaction of GL, were mainly stabilized at the surface of the reduced Cu/γ-Al₂O₃ catalyst (Scheme 11).[179] Therefore, an increase of surface metallic copper in the catalyst should be a focus to improve the 1,2-PD selectivity by the stabilization of 2,3-enediol. Cu-based oxide, derived from hydro-talcite-like structured catalyst, that exhibited a metallic surface area of 21 m²/g, catalyzed the formation of 1,2-PD (84% selectivity and conversion) at 40 H₂ bar and 483 K; the same catalyst, but with a metallic surface area of 12 m²/g, transformed only 57% of aqueous GL (1,2-PD selectivity, 69%).[180]

Starting from Cu and Zn hydroxides as precursors, a copper carbide (CuₓC) composite, with Cu/Zn = 8, was obtained by thermal treatment/hydrogen reduction.[181] In flow, GL was quantitatively converted and 92% 1,2-PD selectivity at 513 K and 20 H₂ bar was observed. This activity, 10 times over than that of metal Cu in the same conditions, was due to its dispersion in the presence of ZnO and to the acidity provided from Zn species.[181] The fine dispersion of copper oxide particles

onto Mg₉Al₂.₇-Ga₂.₃O₂ helped the catalytic activity giving a full conversion of gaseous GL to 1,2-PD at 493 K and 5 bar.[182]

The monometallic Cu catalyst supported on γ-Al₂O₃ or on TiO₂ was tested at 553 K under N₂ atmosphere for the production of HAc, resulting more active than the bimetallic system (i.e., Cu-Ni) supported on the same solids.[25] The best results between copper (5 wt%) on γ-Al₂O₃, ZrO₂, or SiO₂ were obtained when ZrO₂ was the support, due to the highest quantity of acidic sites, with metallic copper more homogeneously distributed and dispersed at the catalyst surface.[183] The use of SiO₂ in these cases was never advantageous due to its insufficient acidity. However, Mazarío *et al.* modified the acidity changing the size of CuO nanoparticles: so, CuO appeared more dispersed on SiO₂ increasing their activity and their acid sites density.[184] This catalyst fully converted GL to HAc (selectivity > 80%) using CuO of 2.9 nm on commercial fumed silica in a tubular fixed-bed reactor at 533 K.[184] Cu(10 wt%)/SBA-15 catalyst was prepared by simple grinding and used for liquid-phase GL hydrogenolysis at 503 K and 40 H₂ bar affording 97% selectivity of 1,2-PD (on 90% of conversion) thanks to Cu-O-Si-O interface structures (not highly present on the catalyst prepared by impregnation).[185] These interface structures were the origin of the LAS, useful to dehydrate GL to HAc, the potential intermediate in hydrogenolysis, that was hydrogenated onto the close Cu⁰ sites.[185] The search for greening these hydrogenolysis reactions led to use ethanol as an *in situ* hydrogen donor; in the presence of a Cu-Al-Zn catalyst, synthesized by co-precipitation, 1,2-PD was produced at 473 K and 30 N₂ bar with 69% selectivity.[186]

MgO was also used as support for copper by preparing Cu and Cu-Ru catalyst on MgO with different amounts of Cu and Ru but with a total metal loading of 10 wt%.[187] The best results (98% conversion, 97% 1,2-PD selectivity) were obtained with 6Cu-4Ru/MgO catalyst in a flow reactor at 493 K and 8 bar, while at atmospheric pressure, the main product was HAc (80% yield; 1,2-PD 15% yield). The catalytic activity was attributed to a good synergy between the copper and ruthenium that was a reservoir of hydrogen to spillover it to the Cu.[187] Not-hazardous Cu-based mixed oxides derived from hydrotalcites were also used as catalysts in a continuous-flow fixed-bed reactor.[188] Cu-Mg-AlOₓ led to 60% HAc selectivity with almost full GL conversion at 513 K and under atmospheric pressure. FT-IR and XPS measurements evidenced that the presence of Cu, especially the most active Cu⁺ species, was essential to carry out the dehydration with high reaction rates.[188] A series of copper-based catalysts supported over magnesium oxide or hydroxide fluoride were tested in gas phase at 573 K and atmospheric pressure.[189] The best catalyst (Cu-MgF₂) led to 82% conversion and HAc in 45% yield, due to an adequate number of LAS and the ability to stabilize the Cu⁺ species, while the catalytic activity was very low for Cu-MgF(OH) and Cu-MgO.[189]

Montmorillonite is an active support that shows BAS and LAS and it can support several metallic species; in particular, Cu₂O/montmorillonite, prepared by hydrothermal method, catalyzed the oxydehydration and hydrogenolysis of GL, using H₂O₂ and dimethylformamide as solvent, at 353 K and room

<figure><img src="image_39.png" /><figcaption>Scheme 11. Mechanism of GL hydrogenolysis to 1,2-PD over Cu/γ-Al₂O₃.</figcaption></figure>
<figure><img src="image_40.png" /></figure>

pressure.[190] The authors obtained 72% selectivity to AAc and 10% 1,2-PD with full GL conversion. They proposed a mechanism involving dehydration of GL to HAc on the acid sites of montmorillonite forming 1,2-PD, which was converted to AAc by dehydration on acid sites and oxidation steps by Cu₂O.[190]

The search for high specific surface area supports with particular morphologies is a trend study to understand and improve catalytic performances. Spherical hybrid ZnO/PAAH mesospheres (PAAH = polyacrylic acid) were developed from the 3D self-assembly of ZnO nanoparticles.[191] In order to keep a high specific surface area, the synthesis of Cu supported on ZnO/PAAH was carried out.[191] The ethanol washing and calcination steps contributed to an increase in the specific surface area by decreasing Van der Waals interactions between particles. This characteristic was the reason for the no-sintering copper process, during the reaction, even when a high Cu loading (35%wt) was used. The catalyst showed 70% conversion and 90% selectivity for 1,2-PD performing GL transformation at 473 K and 30 H₂ bar.[191] The authors prepared also Cu/ZnO/ZnAl₂O₄/PAAH by sol-gel method where Cu nanoparticles had 4–13 nm size, obtaining full conversion with selectivity 80–90%.[192]

An interfacial bimetallic (Pt and Cu) synergetic effect at the atomic scale, boosted by the SAA (single atom alloy) approach, was reported.[193] PtCu-SAA catalyst was prepared from a hydro-talcite precursor leading to quantitative GL conversion and full selectivity for 1,2-PD at 473 K and 20 H₂ bar. The TOF value was 8–120-fold larger than that of metal catalysts ever reported in the same reaction conditions. Theoretical calculations identified the active sites being at the interface of PtCu-SAA, where a single Pt atom enabled the cleavage of C–H bond on the central carbon and a close Cu atom was responsible for terminal C–O bond breaking.[193]

Ardila et al. reported the preparation and the use of bimetallic Cu-Pd/TiO₂-Na catalyst in the HDO reaction.[194] They obtained 93% 1,2-PD selectivity at 60% of substrate conversion, working in aqueous phase at 493 K and 7 bar H₂. The excellent selectivity was imputed to the formation of CuPd alloy nanoparticles in the catalyst. The presence of the alloy avoided the leaching of Cu particles stabilizing the solid; furthermore, Pd species promoted the generation and the activation in situ of hydrogen from GL reforming.[195]

Lately, copper Keggin-like catalysts were used to produce propanols: Cu with tungstosilicic acid, supported on γ-Al₂O₃, in continuous-flow fixed-bed reactor at 613 K and 50 H₂ bar, converted 98% GL with 78% total selectivity (71% 1-PO, 7% 2-PO).[196] However, the formation and deposition of condensation products, mostly polyaromatic, deactivated this catalyst that could be protected varying its synthesis procedure with a major amount of tungstosilicic acid.[196]

### 3.5. Zinc

Zinc is not a very expensive metal: its average market price is 3.261 $/kg.[197] Many examples of copper-zinc catalysts in hydrogenolysis reaction of GL were reported.[198] Pandey et al.

modulated the acidity and basicity of the catalyst, inserting (basic or acid) doping metal oxides in the catalytic composition, to disclose their correlation with the successful of catalytic performances.[199] Among all studied catalysts, Cu-Zn/MgO was the most selective for vapor-phase hydrogenolysis to 1,2-PD: it provided 98% conversion of GL with 89% selectivity at 493 K and 7 bar H₂ thanks to its basicity, its active metal surface and its high metal dispersion.[199] Biswas's research group identified, other than in the appropriate basic character of Cu-Zn/MgO, also in the high amount of hydrogen spillover on the surface and in the easy reduction of CuO in the presence of Zn species, the excellent activity of these catalysts.[200,201]

A fixed-bed reactor with a ceramic membrane (Figure 3) led to the formation of microbubbles-in-liquid over the solid Cu-ZnO at 473 K, affecting the mixture system and the good selectivity for the 1,2-PD (> 92%).[202] The classic Cu/ZnO was prepared from Cu₃(benzene-1,3,5-tricarboxylic acid)₂·9.4(H₂O), through calcination and reduction steps, and it was constituted by ZnO nanoparticles dotted on Cu hybrid (Cu₁,₁/ZnO).[203] This catalyst was tested at 523 K in a fixed-bed reactor: the GL was fully converted to 1,2-PD with selectivity over 80%.

Cu/ZnO based catalysts are promising catalysts, but they suffer of deactivation during hydrogenation. The reason seemed to be the zinc species leaching by acid attack, followed by the copper sintering.[204] Moreover, the presence and the amount of initial water in the reaction mixture influenced the final yield and the lifetime of the catalyst.[205]

In some cases, Zn support was doped with Al, to increase its acidity and adsorption capacity for metals. Chimentão et al. compared Cu and Ni supported on ZnO with or without Al.[206] The addition of Al promoted the GL conversion: after 4 h of reaction, a conversion of 100% was observed with Ni/ZnAl (with Ni/ZnO, only 40% conversion). On the other hand, at the same reaction time, a conversion of 50% was observed for Cu/ZnAl (with Cu/ZnO, 30%). The authors described the presence

<figure><img src="image_41.png" /><figcaption>**Figure 3.** The fixed-bed reactor with the membrane dispersion for the continuous heterogeneous catalytic hydrogenolysis of GL. Adapted with permission from *Ind. Eng. Chem. Res.* 2018, 57, 1, 158–168. Copyright 2018 American Chemical Society.</figcaption></figure>
<figure><img src="image_42.png" /></figure>

of a NiZn alloy as the suppressor of the small molecules
formation (i.e., methane) and the production of lactide as the
main product (22% yield at 60% conversion) while, in the case
of Cu supported on ZnAl and ZnO, HAc formation (29% and
26% respectively) was observed.[206]

Yfanti *et al.* carried out two reactions in parallel: the liquid phase GL HDO and the methanol reforming over a Cu:Zn:Al catalyst at 523 K.[207] As a result, under poor hydrogen conditions (methanol/GL low ratio), GL dehydrogenated following the glyceraldehyde route to 1,2-PD, which further slightly hydrodeoxygenated to 1-PO. The last one dehydrogenated back to HAc with high H₂ concentrations (methanol/GL high ratio).[208] In a Cu-Al-Zn system, consisting mainly of hydrotalcite, the presence of basic sites was found to be mandatory for the GL hydrogenation, and the copper species, in the reaction carried out without hydrogen, proved to be essential for increasing the 1,2-PD selectivity.[186]

A Cu-Zn-Mg-Al-O catalyst was also employed in the preparation of 1,2-PD, resulting active and selective when in presence of NaOH; unfortunately, this catalyst was not reusable because of the sintering of ZnO and Cu that deactivated it.[209] Several ZnPd nanoparticles supported on the Al-doped ZnO were also prepared by modifying the calcination temperature of the supports: increasing the temperature from 623 K to 1023 K, a clear reduction of the activity (GL conversion from 70 to 25% and 1,2-PD selectivity from 93 to 79%) was observed.[210]

PdZn alloy was supported on monoclinic ZrO₂ to catalyze GL hydrogenolysis in water solution obtaining 1,2-PD at 493 K and 60 bar H₂.[211] The kinetically and selectively relevant step on metal surfaces was the α-C–H cleavage in 2,3-dihydroxypropanoxide to glyceraldehyde (step 3, Scheme 12), the intermediate compound to form the 1,2-PD (Scheme 13). Zn showed higher oxophilicity than Pd promoting the abstraction of α-H atom synergistically by adjacent Pd site.[211]

The authors calculated ΔG of the C–H cleavage: in presence of the PdZn alloys, it was lower than the value for Pd alone by 17 kJmol⁻¹.

3.6. Ruthenium

Despite its relatively expensive nature (average market price: 15,140 $/kg),[212] ruthenium is widely used in catalysis and for industrial purposes because of its high activity. Freshly prepared Ru/Mg(OH)₂ catalyst showed the best catalytic activity in the GL hydrogenolysis compared to commercial Ru/Mg(OH)₂ or Ru/MgO (synthesized or commercial): the synthesized catalyst containing Mg(OH)₂ displayed a cotton-like morphology; Ru was highly dispersed on the support and strong basic active sites were present, leading 1,2-PD with 65% selectivity and 24% GL conversion at 483 K and 30 bar H₂ pressure.[213] Poor results were observed in the presence of catalyst containing commercial Mg(OH)₂: 6% conversion and 57% selectivity of 1,2-PD.[213] Mane *et al.* underlined the correlation between the selectivity in GL hydrogenolysis and the precursor used to synthesize the catalyst: Ru/C catalyst prepared from RuCl₃·3H₂O

1. Adsorption of glycerol on metal surface to form adsorbed glycerol species

<figure><img src="image_43.png" /></figure>

2. Dissociation of glycerol* to form a bond alkoxide species (2,3-dihydroxypropanoxide) and a bound H atom

<figure><img src="image_44.png" /></figure>

3. a-C-H cleavage of 2,3-dihydroxypropanoxide to form absorbed glyceraldehyde species

<figure><img src="image_45.png" /></figure>

4. Glyceraldehyde desorption

<figure><img src="image_46.png" /><figcaption>**Scheme 12.** Proposed steps for the hydrogenolysis of GL on metal surface to glyceraldehyde. Adapted with permission from Q. Sun, S. Wang, H. Liu, ACS *Catal.* 2017, 7, 4265–4275. Copyright 2017 American Chemical Society.</figcaption></figure>

<figure><img src="image_47.png" /><figcaption>**Scheme 13.** Full metal catalyzed pathway of GL hydrogenolysis to 1,2-PD in aqueous solution. Adapted with permission from Q. Sun, S. Wang, H. Liu, ACS *Catal.* 2017, 7, 4265–4275. Copyright 2017 American Chemical Society.</figcaption></figure>

showed high activity in C–C cleavage (ethylene glycol selectiv-
ity 56% and 1,2-PD 23%) while, in the case of Ru(NO)(NO₃)₃
precursor, low conversion (< 30%) but higher selectivity to 1,2-
PD (44%) and lower to ethylene glycol (42%) were observed at
450–510 K and 52 H₂ bar.[214] By reducing the Ru catalysts in the
presence of H₂ rather than with NaBH₄, the C–C cleavage
products were suppressed and only C₃ compounds were
obtained. In this case, both the reaction conditions and the
reaction method to prepare the catalyst influenced the proper-
ties (texture, active surface area, acidity) of the catalyst itself.[214]
Employing Ru–WOₓ/C, it was possible to use milder temperature
and pressure of the hydrogenolysis (423 K and 5 bar) with 73%
conversion and quantitative 1,2-PD selectivity because the Ru
nanoparticles activity was tuned by the acidity of the WOₓ
species at Ru/W molar ratio = 4.[215]

Ru-Mo catalyst supported on carbon nanotubes was used in the presence of phosphotungstic acid at 473 K and 60 H₂ bar.[216] 1,2-PD was produced with 53% selectivity (51% conversion): the presence of Mo oxides promoted the selective adsorption of the secondary -OH of GL and the phosphotungs- tic acid helped the initial dehydration.[216] The same group
<figure><img src="image_48.png" /></figure>

prepared Ru-Mo supported on active carbon modified by
phosphotungstic acid: at 40 bar and 473 K, the main products
were 1,2-PD (74% selectivity) and 1,3-PD (21 %). [217]

The addition of copper to Ru nanoparticles, dispersed on multiwall carbon nanotubes by wet impregnation, significantly affected the reducibility of the metallic species, promoting the C–O cleavage.[218] The conversion was limited (18%) but a remarkable 1,2-PD selectivity (93%), at 473 K and 50 H₂ bar, was obtained.[218] Recently, Pd-Ru nanoparticles were supported on a series of metal oxides and zeolitic supports with the aim of understanding the correlation between the catalytic activity (conversion and 1,2-PD yield) and the support acidity.[219] It was observed that C₃ products (instead of C₂ and C₁) were favored at 20 bar H₂ and 438 K, using catalysts displaying a moderate density of strong acid sites (130 NH₃ µmol g⁻¹ on 230 NH₃ mmol g⁻¹ of total acidity) as in TiO₂, giving 55% GL conversion and 1,2-PD with 50% selectivity.[219] Significative differences using Ru or Ru-Cu modified-zirconia catalysts were found in the hydrogenolysis at 453 K and 25 bar H₂: the Ru-based converted 30% of the aqueous GL (1,2-PD selectivity 60%) and the Ru-Cu transformed only 14% but with a very high selectivity (89%).[220] The superficial presence of copper (added on surface after ruthenium) addressed the reaction towards the C–O cleavage protecting the C–C bond from the Ru action.[221]

A series of Ru, Os, Pd, Cu catalysts supported on fluorine-
doped tin oxide were tested towards the formation of 1,2-PD:
as a result, Ru-based catalyst displayed the best activity at
423 K, with a 94% selectivity and a full conversion of GL, under
20 bar H₂.[222]

3.7. Palladium

Pd is a TM used more in the GL oxidation than its reduction,
nevertheless, some articles are present in the literature about
this topic (Pd average market price: 41,000 $/kg).[223] Rosas et al.
compared LaY and Pd/LaY catalysts (with HY zeolite) in the
dehydration reaction of GL.[224] The most active catalyst was Pd/
LaY getting 90% GL conversion and leading to Acr and HAc as
main products. The presence of small amounts of Pd on the
surface of the catalyst positively promoted the elimination of
coke precursors as a result of the reduction reactions in the
presence of H₂ at atmospheric pressure. The conversion of GL
increased with the raise in temperature as well as the acid sites
concentration, leading Acr in 95% yield at 573 K.[224] Even Pd
and PdO nanoparticles (3.8 nm) supported on SBA-15 were able
to dehydrate GL to Acr (64% selectivity with 90% conversion)
at 593 K and 2 bar in a continuous fixed-bed reactor.[225] The
activity of the catalyst was maintained up to 20 h and after,
gradually, its catalytic sites were deactivated by metal sintering
and carbon depositions.

Pd nanoparticles were loaded into sulfate-functionalized
metal-organic frameworks for hydrogenolysis purposes, obtain-
ing a moderate conversion of GL (24%) with full selectivity for
1,2-PD.[226] This catalyst contained BAS, on the sulfate species of
metal-organic frameworks, useful for dehydration step, as well
as the Pd for the dehydrogenation step.

Pd species, in a PtPd on activated carbon, were responsible
of the activity of the catalyst in the hydrogenolysis at 473 K and
10 N₂ bar (1,2-PD selectivity: 45%).[227] The results were due to
the strong interactions between metal and support promoting
the formation of Pd carbide that influenced the electronic
arrangement of Pt.[227]

Hydrogenolysis of GL to 1-PO and 2-PO was performed at vapor phase in a fixed-bed reactor by using bifunctional 1–4 wt% Pd/MoO3–Al2O3 catalyst prepared by wetness impregnation method.[228] The synergic interaction between the Pd and MoO3 on Al2O3 support and its weak-moderate acidity were the key factors determining its catalytic activity. At 483 K and 1 bar H2 pressure, 2 wt% Pd/MoO3–Al2O3 catalyst was the most active, with 88% GL conversion and 91% selectivity to propanols.[228]

3.8. Silver

Only few and early works were reported in literature using silver
as catalyst for GL conversion (Ag powder average market price:
887.16 $/kg).[229] Lari et al. deposited Ag nanoparticles onto a
hierarchical ZSM-5 zeolite, to create a bifunctional catalyst to be
employed in the gas phase GL dehydration–hydrogenation to
AA via Acr using a continuous fixed-bed reactor.[230] The role of
silver was to reduce the Acr into AA (selectivity 20%) in the
presence of H₂. Furthermore, the activity of Ag degraded the
coke deposited over the solid prolonging the life of the catalyst.

3.9. Iridium

Iridium, as noble metal (its average market price: 188,360 $/
kg),[231] is able to activate the H₂, keeping its metallic properties;
therefore monometallic and multimetallic metal species, com-
bined with acidic components, were frequently used in the
preparation of metal–acid bifunctional catalysts for the hydro-
genolysis of GL to 1,2- and 1,3-PD (Scheme 14, pathway 3).[232]

1,3-PD was produced when using ReOₓ-Ir/SiO₂ as catalyst,
and then transformed in 1-PO by a successive dehydroxylation

<figure><img src="image_49.png" /><figcaption>Scheme 14. Proposed mechanisms of GL hydrogenolysis to produce pro-
panediols.</figcaption></figure>
<figure><img src="image_50.png" /></figure>

pathway, due to partially reduced ReOₓ clusters, finely dispersed and directly interacting with the metallic Ir nanoparticles. In the highly concentrated GL solution, it was possible to achieve 50% reagent conversion thanks to more favorable substrate-catalyst interactions. However, the same result could be obtained by adding sulfuric acid in the reaction mixture because it increased GL amount within the silica pores, favoring the dissociation of GL to a primary propoxide. Therefore, working at 393 K with an initial hydrogen pressure of 80 bar, the selectivities were: 35% 1,3-PD, 18% 1,2-PD, 32% 1-PO, and 10% 2-PO.[127] Recently, 1,3-PD selectivity was related to Ir amount in bimetallic catalyst Ir/ReOₓ: higher Ir loading amount (20 wt%) was, lower ratio of Re/Ir was, more active the catalyst was, leading to 1,3-PD in 32% yield, with a 47% selectivity and 69% GL conversion, at 80 bar H₂ and 393 K.[233] It was pointed out that the absence of Ir-Re alloy and the presence of ReOₓ clusters at low valence attached to metallic Ir were responsible for this outcome.[234]

HZSM-5 was also reported as support for Ir-based catalysts involved in dehydrogenation of GL. Wan *et al.* prepared it *via* grinding-assisted impregnation method and attempted it as an effective and recyclable catalyst for the aqueous-phase hydrogenolysis of GL to 1,3-PD (75% selectivity at 453 K and 80 H₂ bar) in the absence of acid additives.[235] Ir dispersions, proper Ir⁰/Ir³⁺ ratios and, more importantly, the amounts of overall acid/Brönsted acid sites seemed to influence reactivity and selectivity. To achieve good results, the size of IrOₓ particles should be small (1.7 nm) and the Ir⁰/Ir³⁺ ratio should be 40/60.[235] Moreover, the use of rutile TiO₂ as support for Ir-ReOₓ led to a reasonable yield of 1,3-PD (36%): indeed, a high density of Ir particles over the TiO₂ surface, covered with the ReOₓ cluster, resulted in highly active Ir-ReOₓ interface.[30]

An attractive and safer method to carry out the reaction of GL deoxygenation is to generate H₂ *in situ*. The addition of Pt in the Ir-ReOₓ/SiO₂ catalyst (0.5 wt%) led to the formation of two main active sites for C—O hydrogenolysis: Pt-Ir alloy and Ir-ReOₓ species were able to catalyze aqueous phase reforming at 463 K, providing 81% GL conversion with 56% HAc and 19% 1,2-PD selectivity without external hydrogen sources.[233] The addition of Fe to Ir/TiO₂ (rutile) catalyzed the formation of 1,2-PD (selectivity, 67% at full GL conversion) after 42 h at 453 K and 80 H₂ bar and the overhydrogenolysis to propanols after 144 h (about 30% selectivity both 1-PO and 2-PO).[236] The active catalyst was constituted by the interface between Ir-Fe alloy and FeOₓ species.

### 3.10. Platinum

Pt is a noble metal (its average market price: 32,535,700 $/kg)[237] mainly useful in the hydrogenolysis of GL.[238] The effects of several pore sizes of Pt/WOₓ/Al₂O₃ catalyst was studied in the hydrogenolysis reaction.[239] The littlest pores (6 nm vs. 40 nm) resulted in superior Pt dispersion (32% vs. 29%), smaller metal crystallite size (3.5 nm vs. 3.9 nm) and a great number of active metallic sites (0.47 mmol NH₃ g⁻¹ vs. 0.32 mmol NH₃ g⁻¹); but the diffusional constraint within pore channels inhibited the transport of reactant and product, lowering one third GL

conversion and 1,3-PD yield.[239] Using a catalyst with the same metal loading but with large pores, 1,3-PD was obtained in a 33% yield from aqueous GL (78% conversion) at 493 K, 60 H₂ bar, after 5 h reaction.[239] The catalyst was proposed as bifunctional active site, formed by closely interacting Pt species, that activated the hydrogen, and W⁶⁺-O-Al sites, necessary for the adsorption and activation of GL.[240] Good results in the production of 1,3-PD were obtained even in the presence of Pt-WOₓ/α-Al₂O₃ calcinated at high temperature.[241] Its preparation method allowed to disperse Pt as single atom on solid surface, with more Pt and WOₓ interactions. Therefore, the catalytic efficiency (conversion of 60% and selectivity of 44% for 1,3-PD and 27% for 1-PO), obtained at 453 K, 50 H₂ bar for 24 h, was attributed to the *in situ* generated BAS due to the strong interactions between the isolated WO₄ species and Pt.[241] The deactivation of this catalyst, however, seems to be due to the aggregation of Pt particles during the reactions,[242] that could be prevented by Re addition.[243] The temperatures of preparation are crucial: calcining WOₓ/Al₂O₃ at 1073 K, WOₓ species were connected to form W-O-W clusters and, after the Pt impregnation, a new calcination was conducted at 873 K and, then, the catalyst was reduced at 723 K.[244]

As seen, the combination of Pt and W is successful for the formation of 1,3-PD. García-Fernández *et al.* prepared Pt/WOₓ/γ-Al₂O₃ catalysts to elucidate the role of Al and W species to bind GL terminal OH groups.[245] Employing *in situ* and *ex situ* ATR-IR spectroscopy, it was demonstrated that the bond with W resulted to be the strongest one, leading to a 38% yield of 1,3-PD at 473 K, 45 bar of H₂ and after 4 h reaction.[245] The presence of an oxophilic metal, as W, together with Pt, acted as an anchoring site for the terminal hydroxyl groups of GL and protonating the secondary hydroxyl group, leading to the formation of 1,3-PD as main product.[245] The optimum Pt/W atomic ratio was reported around 1/2-1/4.[246] The hydrogenolysis of aqueous GL was carried out in a fixed-bed down-flow stainless steel reactor at 453 K and 50 bar, obtaining 80% GL conversion and 1,3-PD in 28% yield (35% selectivity) in the presence of 6Pt/12.9 W/Al₂O₃. Hydrogen spillover occurred in the easy to reduce WOₓ domains thanks to the presence of very well dispersed Pt species.[246] Pt single atoms on tungsten oxide were accredited as greatly stabilized when trapped into hollow site near WOₓ(001) region and negatively charged, to favor the H₂ homolytic cleavage to metal-H species, as demonstrated by both DFT studies and experimental results.[106] The creation of Pt-H onto the support caused the formation of BAS, essential to the 1,3-PD selectivity.[106] Even the texture of support (Al₂O₃) resulted to be critical for the performance of the Pt/WOₓ/Al₂O₃ catalysts. Two-dimensional nanosheets with an open pore structure turned out to have the best morphology of alumina for hydrogenolysis to 1,3-PD in comparison with the rod- or the spindle-shaped structures.[247] A bifunctional Pt-WO₃ supported TiP (titanium phosphate) catalyst was used to obtain 1,3-PD (51% selectivity at 85% of conversion) at 483 K and room pressure in a fixed-bed quartz reactor thanks to the strong interactions between Pt and WO₃ species and to the high number of BAS.[248]
<figure><img src="image_51.png" /></figure>

W and Al were incorporated in SBA-15 using it as support for Pt nanoparticles.[249] The catalyst displayed synergistic and equilibrated BAS and LAS, contributing to relevant GL conversion (66%) and 1,3-PD selectivity (50%) at 60 bar H₂ and 433 K after 12 h reaction. The 64% conversion and 57% 1,3-PD selectivity were reported with Pt-WOₓ/SiO₂ at 433 K and under 80 bar of H₂.[250] The high catalytic efficiency originated from the strong Brönsted acidity, due to the appropriate polymerization degree of WOₓ and to its synergism with Pt metal. As DFT studies underlined, medium polymeric WOₓ domains (dimeric HW₂O₇ cluster can delocalize more negative charge to the wider Pt surface, leading to a much stronger BAS), with no reduction of catalyst, showed the strongest Brönsted acidity and the lowest apparent activation energy in the formation of 1,3-PD. The importance of strong BAS evidenced that, in these conditions, the hydrodeoxygenation mechanism was involved in the 1,3-PD formation.[250]

Recently, 1,3-PD 63% was reported as yield at 423 K under 40 bar H₂ pressure, in the presence of a tungsten-doped siliceous mesostructured cellular foams-supported Pt catalyst (Pt/W-1.5MCFs).[251] Pt species gave spillover hydrogen atoms to Wv on the interface between the Pt nanoparticles and the W-MCFs. This peculiarity, combined with the particular 3D catalyst structure, was the explanation for the good results of these W-MCFs.[251]

In order to maximize the Pt-O-W interface with well dispersed Pt, low-polymeric WOₓ on the Ta₂O₅ surface were reported as support for Pt: a thin layer of the WOₓ-Ta₂O₅ solid solution on the Ta₂O₅ surface was obtained by calcinating the catalyst at 1173 K, leading to 48% GL conversion and 1,3-PD 40% selectivity, at 50 H₂ bar and 333 K after 12 h reaction.[252]

Several studies focused on the properties and the activity of the Pt-WOₓ deposited on Zr solid supports as catalysts for GL hydrogenolysis.[253-255] Pt-WOₓ was deposited on two different crystal phases of ZrO₂: when tetragonal ZrO₂ was used, the Pt particles size was smaller (0.59 nm vs. 0.72 nm) and the number of BAS higher than in the case of monoclinic phase.[254] These aspects drove the hydrogenolysis toward 49% 1,3-PD yield and 80% conversion at 413 K and 80 bar H₂ pressure (the values with monoclinic catalyst were cut in half).[254]

By an easy deposition method, Pt was incorporated onto preformed Au/WO₃ catalyst, to study the effect of gold as doping agent in Pt/WOₓ catalyst.[256] Even in this case, several effects (e.g., Pt well dispersed and in low-valence state, assistance in the redox cycle) for selective hydrogenolysis of GL to 1,3-PD led to 54% selectivity but lower GL conversion (30%) at 50 bar and 453 K.[256]

Wen et al. studied the effect of Ru in the Pt/WOₓ/Al₂O₃ catalyst for GL hydrogenolysis.[257] 2Pt-1Ru/WOₓ/Al₂O₃ catalyst afforded a 90% GL conversion, compared to 2Pt/WOₓ/Al₂O₃ catalyst (70% GL conversion), with 40% selectivity for 1,3-PD, when performing the reaction at 90 bar and 443 K. Ru addition and its interaction with Pt species could restrain the condensation of metals preventing the growth of the nanoparticle sizes. Moreover, Ru was able to promote the redox cycle of tungsten and to increase the adsorption of hydrogen.[257]

Using the coimpregnation-calcination method, the Pt-1Li₂B₄O₇/WOₓ/ZrO₂ was prepared modifying Pt/WOₓ/ZrO₂ with Li₂B₄O₇.[258] This catalyst showed increased activity and stability (200 h) during the hydrogenolysis of GL due to good Pt dispersion and high BAS. Hydrogenolysis of GL was carried out at 423 K and 40 H₂ bar in a fixed-bed stainless steel reactor where the catalyst was put in the center of the reaction tube and fixed with quartz sand at both ends. After 100 h, a high conversion of 91% and a 45% yield of 1,3-PD were obtained.[258]

Even by changing the structural characteristic of support, i.e. using ordered mesoporous alumina instead of the commonly used γ-Al₂O₃ in the synthesis of PtNPs-HSiW/Al₂O₃, it was possible to increase Pt dispersion (from 15 to 35%) and the Brönsted acidity (from 12 to 30 µmol/g).[259]

The structural modifications influenced 1,3-PD production, doubling it. Therefore, working at 473 K and 40 H₂ bar in batch reactor, the GL conversion and the 1,3-PD selectivity were 60% and 33% respectively; in the recycle runs, the catalyst retained its activity despite the leaching.[259]

WO₃-Al₂O₃-SiO₂ was indicated as active support for Pt nanoparticles: it was synthesized by sol-gel method followed by Pt deposition for impregnation.[260] The catalyst showed high selectivity for 1,3-PD, around 56%, on 48% GL conversion in batch reactor, at 60 H₂ bar and 433 K. The authors presented a potential mechanism via acid-catalyzed dehydration and following hydrogenation on metal sites (Scheme 15).[260]

62% of 1,3-PD selectivity and 94% GL conversion were obtained in vapor phase, using sulfuric acid-activated montmorillonite clay as support for 2 wt% Pt nanoparticles, at 473 K, 1 H₂ bar in a fixed-bed reactor.[261] The good selectivity was due to the presence of strong BAS, coming from the acid activation of the support.

Von-Held Soares et al. compared Pt, Pd and Ni supported on Fe₃O₄ in the GL hydrogenolysis to 1,2-PD at 493 K.[262] The activity order for the tested catalysts was Pt > Pd > Ni, directly

<figure><img src="image_52.png" /><figcaption>**Scheme 15.** Proposed mechanism of the 1,3-PD preparation over Pt on WO₃-Al₂O₃-SiO₂. Adapted from S. Feng, B. Zhao, L. Liu, J. Dong, *Ind. Eng. Chem. Res.* 2017, 56, 11065–11074. Copyright 2017 American Chemical Society.</figcaption></figure>
<figure><img src="image_53.png" /></figure>

related to the stability of the reduced nanoparticles. The main
advantage of Fe₃O₄-supported catalysts was their easy removal
from the reaction mixture by using a magnet, that can facilitate
their potential reuse and recycle. In the reactions with Pt
catalyst, the conversion was 81% and the selectivity to 1,2-PD
was 79% (HAc 6% and 1-PO 5%).[262] The interactions between
the metal and the FeOₓ support were studied to find the
appropriate catalyst to transform GL into 1,3-PD (78% selectiv-
ity) in the absence of H₂ at 513 K.[263] It was important that the
catalyst, 2.5%Pt/FeOₓ, was reduced at 473 K to form electron-
rich Pt near the oxygen vacancy and the iron cation (Pt⁶⁻-Ov⁻
Fe²⁺) that were the active sites.[263]

Significant results of GL HDO (83% conversion) and 1,2-PD selectivity (43%) were reported, also without H₂, using trimetallic 2%Ru-2%Re-2% Pt/C catalyst and MgO as solid base promoter, carrying out the reaction at 493 K and under 20 N₂ bar.[264]

In the past, the hydrogenolysis was proposed in presence of
a base to facilitate the dehydrogenation steps to 1,2-PD;
recently, a theoretical and experimental study about the very
positive effect of alkaline earth metal cations with Pt/C in the
GL reaction was published.[265]

Considering that the hydrogenolysis of the GL (to diols)
with metal/Al₂O₃ is essentially driven by the interaction
between the acid sites of alumina and the metal species, Du
et al. introduced a method to increase the metal-acid interface
making the catalyst more active.[266] The authors synthesized
porous Al₂O₃ coating onto a Pt/Al₂O₃ catalyst using atomic layer
deposition. The catalyst promoted 90% GL conversion, leading
to 1,2-PD with 64% selectivity at 493 K and a pressure of H₂ of
80 bar.[266]

The dimension of metal nanoparticles is critical for the
catalysis: the size of Pt nanoparticles influenced the reaction
mechanism and, consequently, the selectivity of the products.
For example, using 2% Pt/Al₂O₃, it was possible to tune the
products varying the nanoparticles size, from 1,2-PD (with 3-
4 nm Pt) to hydrogen gas (with 2 nm Pt).[267] Pt nanoparticles of
2 nm, but supported on a CeO₂ modified with ZrO₂, were used
to catalyze the HDO at 443 K and 10 bar H₂ obtaining 1,2-PD
selectively (74%).[268]

Zhang et al. published GL hydrogenolysis by Pt-In alloy
catalyst, prepared by impregnation of indium and platinum
precursors onto the MgAl support, followed by calcination and
reduction process.[269] At 473 K and 20 H₂ bar, the authors
obtained full GL conversion and a remarkable selectivity for 1,2-
PD (91%). The active site was established as Pt⁶⁻ for adsorbing
GL through the primary C–O and breaking the appropriate
terminal C–H bond, preserving the C–C bond.[269]

Sulfates-doped Pt-WOₓ/TiO₂ catalyst achieved 100% con-
version of GL, with 36% selectivity to 1,3-PD and 57%
selectivity to 1-PO at 393 K and 40 bar.[270] Its activity, higher
than the no-doped catalyst, was ascribed not only to the better
dispersion of Pt and WOₓ species and more BAS, but also to its
stability due to the action of sulfate in the redox cycle, and then
in the prevention of leaching of both Pt and WOₓ species.

Amorphous zirconium phosphate-supported Pt catalyst was
modified with the incorporation of WOₓ, with the purpose of

enhancing the acidity (mainly LAS) and the dispersion of Pt
nanoparticles.[271] In particular, in the presence of Pt/7WOₓ-ZrP
catalyst, 81% yield of 1-PO at full conversion of GL in a
continuous-flow fixed-bed reactor at 543 K and 20 H₂ bar was
obtained. This catalyst showed stability for 70 h of reaction.
Moreover, by calcination at air, it was possible to regenerate it
obtaining an active catalyst for 50 h, with similar conversion
and yield.[271] Similar propanol selectivity, by 34% conversion,
was reported for a Pt onto Zr-Al mixed oxides (Pt/Zr₀.₇Al₀.₃O₇),
with a > 80% selectivity to 1-PO at 503 K and 60 bar H₂.[272]

3.11. Gold

The fascinating catalytic activity of gold (its average market
price: 67,860 $/kg)[273] in coordination and organometallic
chemistry has been attracting researchers for fifty years, as
evidenced even in the reactions of deoxygenation of GL.[274]

A two-step synthesis of AAc from GL was proposed: in the
first step, AA is produced by a formic acid catalyzed DODH
reaction, while in the second step, after switching the pH from
acidic to basic, the oxidation of AA to AAc occurred in the
presence of Au/CeO₂ with 87% yield.[275] This cascade reaction
was realized to overcome the difficulty of separation of AA from
formic acid residue in the first step, due to their similar boiling
points, while it is not difficult to separate AAc from formic acid
by distillation (their boiling points are sufficiently different). The
newly formed AAc can undergo hydration, side reaction that
can be avoided using low reaction temperature (298 K), high
oxygen pressure (10 bar) and a dilute AA concentration (0.1 M).
Between the different shapes of CeO₂ tested (e.g., rods,
octahedra, cubes) as support, octahedra showed the best
results and were very stable during recycling (up to the fifth
run, 80% yield).[275] WOₓ was proposed as support for AuPt-
based catalysts: in the presence of AuPt/WOₓ catalyst, 81% GL
conversion and 52% 1,3-PD selectivity were obtained at 413 K,
10 bar H₂.[113] The presence of Au modified the structure of Pt/
WOₓ and provided more activated W species for the formation
of frustrated Lewis-pair analogs, namely for more catalytic
*in situ* generated BAS.[113]

When using Au-Pt/WOₓ/Al₂O₃, at 453 K and 50 bar, the
conversion of GL was 78% obtaining 1,3-PD with 55%
selectivity.[276] The addition of small amounts of Au, that likely
were present as single atoms and forming Au-WOₓ clusters
interacting with Pt, diminished the strong metal support
interactions between Pt and WOₓ sites. Platinum species were
more exposed and active in the H-spillover and, therefore, in
the formation of strong BAS. The good performance only
slightly decreased, reusing the catalyst (fourth run: 1,3-PD
selectivity 56%).[276] AuPt/TiO₂ proved to be more active than
AuPt/MCM41, AuPt/SiO₂, AuPt/H-mordenite, AuPt/sulfated ZrO₂
in the formation of 1,2-PD (selectivity 90% at 68% conversion of
GL), while AuPt/MgO worked very well (selectivity > 90%), but
it deactivated quickly.[17] The superiority of AuPt/TiO₂ was due
to the acid strength of the support, related in inverse
proportion to catalysts activity; also the small size (4 nm) of Au
nanoparticles was essential.[17]
<figure><img src="image_54.png" /></figure>

## 4. Conclusions and future perspectives

As just seen in the previous sections, several catalytic routes to convert the GL into useful $C_3$ compounds can be proposed to the industrial world, reaching out also the Sustainable Development Goals by United Nations.[277]

To better understand which metals are more suitable for obtaining safer, cheaper, and widely diffused low polar $C_3$ derivatives, we explored the up-to-date published data, simply grouping TMs in: early (from Sc to Mn group) and late (from Fe to Zn group). In both Figures, 4 and 5, we reported the number of articles (art), obtained from a scientific database, using, as research keys, the name of each metal associated to each single $C_3$ derivative and to GL in the time range 2017–2023.

It is not immediate to rationalize the reports since, for example, they involve both the use of metal as real catalyst and as support, or systems with a synergic catalytic effect between them.

Even considering what has just been said, the use of Cu to obtain 1,2-PD represents the most studied process (242 art) and, furthermore, 1,2-PD is the $C_3$ chemical reported in the

largest number of studies (926 art, of which 173 with early and 753 with late metals). An overview on the total number of art highlights how their number, per single metal (average value), is clearly in favor of late TMs (early: 859 art / 9 TMs = 95.44; late: 1864 art / 11 TMs = 169.45). The traditional explanation for this difference is the strong resistance of early TMs to undergo redox cycling because of their weak electronegativity (e.g., 1.4 for Sc vs 1.9 for Cu).[278] Nevertheless, we can affirm that, apart from Fe, which is the most abundant TM found on the Earth's crust (51,000 g/ton),[279] the other most abundant metals are represented on the early region (Ti, 6,300 g/ton; Mn, 930 g/ton; Cr, 200 g/ton; Zr, 190 g/ton),[279] and, furthermore, they are endowed of a relatively low toxicity. These two last considerations suggest that, probably, the catalytic studies involving the less toxic and more abundant early TMs, to obtain low oxygenated derivatives from natural wastes, must be increased and that the actual gap in scientific literature is a chance, and not a limitation, to develop innovative and, above all, sustainable catalytic technologies.

In our research group, for several years, selective deoxygenation studies on natural polyols have been carried out using an early TM (i.e., rhenium),[10,125,129,133,280] which can be, in our opinion, a starting point for moving to analogous (from the catalytic point of view) metals, such as Mo or V.[281–284]

However, efficient deoxygenation performance is not solely determined by the choice of the metal catalyst. As strongly demonstrated by this review, the actual applications mainly employ traditional thermochemical methodologies to transform polyols to low-oxygenated, carbon-equivalent, chemicals.

From the literature, the alternative energy-suppliers, e.g. photochemistry, mechanochemistry or electrochemistry, were scarcely utilized.

Marcì et al. obtained Acr irradiating, with UV light, a water suspension of GL and Keggin heteropolyacids supported on $TiO_2$.[285] This one example demonstrates that photochemistry can be a feasible alternative to conventional productions of fine and commodity chemicals. Several TM catalysts, described in previous sections, like Keggin heteropolyacids, $WO_x$ or $TiO_2$, are photoactive and could be used, in the future, as photocatalysts to deoxygenate the GL.[286]

It is worth noticing that in some cases, composite catalysts could be prepared by solid-state grinding, leading to mesoporous composites where the precursors species were introduced into the channels of the mesoporous material, with an improved interaction between the metal precursors and the silica template (removed after calcination of the catalyst). In most of the cases, catalysts prepared by (manual) grinding showed a high BET surface area compared to the traditional methods, which results, generally speaking, in catalytic system outperforming confronted to those prepared by conventional solvent-based methods. However, to the best of our knowledge, deoxygenation of GL to access $C_3$ compounds activated by grinding or by milling remains unexplored, the only and still very few, examples being limited to the preparation of the catalysts needed for the reaction. The reasons behind this gap might be due to the difficulties in accessing dedicated milling devices to handle gases under pressure, other than the lack of

<figure><img src="image_55.png" /><figcaption>**Figure 4.** Number of papers in literature of early TMs as catalysts for the deoxygenation of GL to $C_3$ compounds.</figcaption></figure>

<figure><img src="image_56.png" /><figcaption>**Figure 5.** Number of papers in literature of late TMs as catalysts for the deoxygenation of GL to $C_3$ compounds.</figcaption></figure>
<figure><img src="image_57.png" /></figure>

cross-fertilization of knowledge. Therefore, it could be expected that, in the future, these limitations will be overcome and new process conditions, based on mechanical activation, will be disclosed for GL deoxygenation to value-add C₃ chemicals.[287] Examples of preparation of C₃ chemicals from GL also include activation by ultrasounds.[288]

Even electrochemical synthetic methods represent a valid alternative to provide energy in the deoxygenations. The delay in the use of such technology may be due to several reasons: i) a suitable electrochemical device is necessary and its cost and maintenance are not particularly cheap; ii) to promote an efficient electron transfer in solution, an electrolyte usually must be introduced together with a solvent that can be not particularly green (toluene, THF, etc.).[289]

The knowledge gained so far about the metal catalysts must prompt us to deeply explore the TMs world, which is not fully understood, to improve, even with the use of alternative energies, the transformations of waste polyols to fine chemicals without carbon loss.

## List of Abbreviations

1-PO Propane-1-ol

2-PO Propane-2-ol

1,2-PD Propane-1,2-diol

1,3-PD Propane-1,3-diol

3-HPA 3-Hydroxypropanal

AA Allyl alcohol

AAc Acrylic acid

Acr Acrolein

art Articles

BAS Brønsted acid site

DODH Deoxydehydration

DFT Density functional theory

GL Glycerol

HAc Hydroxyacetone

HDO Hydrodeoxygenation

LAS Lewis acid site

MTO Methyltrioxorhenium

PAI Pyruvaldehyde

PAAH Polyacrylic acid

SAA Single atom alloy

TM Transition metal

## Acknowledgements

The research project was partially supported by the FSE-REACT-EU, PON Ricerca e Innovazione 2014-2020 DM 1062/2021, Cod: MUR 53-G-14753.

## Conflict of Interests

The authors declare no conflict of interest.

**Keywords:** glycerol · hydrogenation · early transition metal · late transition metal · catalysis · deoxygenation

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Manuscript received: December 18, 2023

Revised manuscript received: December 22, 2023

Accepted manuscript online: December 28, 2023

Version of record online: January 12, 2024