Gas-phase hydrogenation of furfural into value-added chemicals: The critical role of metal-based catalysts

Furfural (FF: aldehyde derivable from lignocellulosic biomass) has been widely recognized as a versatile building block for eco-friendly and sustainable applications to reduce industrial reliance on fossil-fuel carbon sources. Hydrogenation of FF, in particular, is recognized as one of the most effective routes for producing various value-added chemicals (e.g., furfuryl alcohol and 2-methylfuran). The gas-phase FF hydrogenation reaction offers economic and environmental advantages over its liquid-phase counterpart in conversion efficiency, product selectivity, and kinetics. The operation of the former does not require high hydrogen pressures or hazardous solvents while not generating undesirable by-products (due to reduced selectivity toward the ring-opening reaction). In this context, the utility of noble and non-noble metal catalyst systems has been recognized for their potential to induce effective FF hydrogenation in the gas phase. The present review addresses current understandings and recent developments in research on gas-phase FF hydrogenation and the factors governing the performance of metal-based catalysts (e.g., materials and surface chemistry; conversion efficiency; product selectivity; and the mechanisms, pathways, and kinetics of the associated reactions). Current shortcomings and research avenues are also discussed to help establish a roadmap for future development of the gas-phase FF hydrogenation technology and associated disciplines. Overall, the present review is expected to offer much-needed insights into the scalability of metal-based catalytic systems for efficient FF hydrogenation in the gas phase.

Cu/SiO2 synthesized with HKUST-1 as precursor: high ratio of Cu+/(Cu+ + Cu0) and rich oxygen defects for efficient catalytic hydrogenation of furfural to 2-methyl furan

Cu/SiO2 is one of the most promising catalysts for the furfural (FF) hydrogenation reaction but suffers from the difficulty of tailoring the microstructure and surface properties. Herein, we developed a MOF-derived Cu/SiO2 catalyst (Cu/SiO2-MOF) for FF hydrogenation to 2-methyl furan (2-MF). In comparison with Cu/SiO2 catalysts prepared from ammonia evaporation (Cu/SiO2-AE) and traditional impregnation (Cu/SiO2-TI), the copper species in Cu/SiO2-MOF could not only be anchored on the silica surface via forming Cu-O-Si bonds but also exposed many more active sites. In this way, a higher ratio of Cu+/(Cu+ + Cu0) and richer oxygen defects were constructed via strong metal-support interactions, which were responsible for the superior catalytic performance. In addition, it was found that the solvent effect on product distribution played an important role in adjusting the selectivity to 2-MF and cyclopentanone (CPO). The present work not only provides a deep insight into the catalytic mechanism of Cu/SiO2-MOF for the FF hydrogenation reaction but also sheds light on the design and synthesis of highly efficient catalysts for other heterogeneous catalysis fields.

The Hydrogenation of Crotonaldehyde on PdCu Single Atom Alloy Catalysts

Recyclable PdCu single atom alloys supported on Al₂O₃ were applied to the selective hydrogenation of crotonaldehyde to elucidate the minimum number of Pd atoms required to facilitate the sustainable transformation of an α,β-unsaturated carbonyl molecule. It was found that, by diluting the Pd content of the alloy, the reaction activity of Cu nanoparticles can be accelerated, enabling more time for the cascade conversion of butanal to butanol. In addition, a significant increase in the conversion rate was observed, compared to bulk Cu/Al₂O₃ and Pd/Al₂O₃ catalysts when normalising for Cu and Pd content, respectively. The reaction selectivity over the single atom alloy catalysts was found to be primarily controlled by the Cu host surface, mainly leading to the formation of butanal but at a significantly higher rate than the monometallic Cu catalyst. Low quantities of crotyl alcohol were observed over all Cu-based catalysts but not for the Pd monometallic catalyst, suggesting that it may be a transient species converted immediately to butanol and or isomerized to butanal. These results demonstrate that fine-tuning the dilution of PdCu single atom alloy catalysts can leverage the activity and selectivity enhancement, and lead to cost-effective, sustainable, and atom-efficient alternatives to monometallic catalysts.

Enhancing the catalytic performance of Co-N-C derived from ZIF-67 by mesoporous silica encapsulation for chemoselective hydrogenation of furfural

Developing Cr-free and non-noble metal catalysts with high activity, selectivity and durability for chemoselective hydrogenation of furfural to furfuryl alcohol is highly desirable yet challenging. In this study, we design a hollow mesoporous Co-N-C@mSiO₂ nanostructure derived from ZIF-67 via the encapsulation-pyrolysis strategy. The Co-N-C@mSiO₂ catalyst exhibits excellent catalytic performance in the furfural hydrogenation towards furfuryl alcohol with good stability, and is much better than the Co-N-C catalyst originating from plain ZIF-67 and other reported transition metal catalysts. Characterization methods and control experiments show that Co-N_x species rather than Co metal should be catalytically active sites for the above reaction. The enhanced performance is associated with abundant Co-N_x active sites, good mass transport, and the SiO₂ shell protection. This work provides a novel and facile strategy for preparing highly efficient non-precious metal catalysts to replace Cr-based and noble metal catalysts for furfural hydrogenation.

Catalysts Based on Ni(Mg)Al-Layered Hydroxides Prepared by Mechanical Activation for Furfural Hydrogenation

Ni(Mg)Al-layered hydroxides with molar ratios of (Ni + Mg)/Al = 2, 3, 4 and Ni/(Ni + Mg) = 0.1, 0.3, 0.5, 0.7 were synthesized by mechanochemical activation. It has been proven that the phase composition of the samples was presented by a single hydrotalcite phase up to Ni/(Ni + Mg) = 0.5. For the first time, catalysts based on Ni(Mg)Al-layered hydroxides prepared by a mechanochemical route have been studied in the reaction of furfural hydrogenation. The correlation between furfural conversion, the selectivity of the products, and the composition of the catalysts was established. The effect of phase composition, surface morphology, and microstructure on the activity of the catalysts was shown by XRD, SEM, and TEM. It was found that catalysts with Ni/(Ni + Mg) = 0.5 have the highest furfural conversion. Herewith, the product selectivity can be regulated by the (Ni + Mg)/Al ratio.

The Role of Copper in the Hydrogenation of Furfural and Levulinic Acid

Currently, there is a great interest in the development of sustainable and green technologies for production of biofuels and chemicals. In this sense, much attention is being paid to lignocellulosic biomass as feedstock, as alternative to fossil-based resources, inasmuch as its fractions can be transformed into value-added chemicals. Two important platform molecules derived from lignocellulosic sugars are furfural and levulinic acid, which can be transformed into a large spectrum of chemicals, by hydrogenation, oxidation, or condensation, with applications as solvents, agrochemicals, fragrances, pharmaceuticals, among others. However, in many cases, noble metal-based catalysts, scarce and expensive, are used. Therefore, an important effort is performed to search the most abundant, readily available, and cheap transition-metal-based catalysts. Among these, copper-based catalysts have been proposed, and the present review deals with the hydrogenation of furfural and levulinic acid, with Cu-based catalysts, into several relevant chemicals: furfuryl alcohol, 2-methylfuran, and cyclopentanone from FUR, and γ-valerolactone and 2-methyltetrahydrofuran from LA. Special emphasis has been placed on catalytic processes used (gas- and liquid-phase, catalytic transfer hydrogenation), under heterogeneous catalysis. Moreover, the effect of addition of other metal to Cu-based catalysts has been considered, as well as the issue related to catalyst stability in reusing studies.

Preparation of Highly Active Cu/SiO2 Catalysts for Furfural to 2-Methylfuran by Ammonia Evaporation Method

Biomass plays an important role in the green manufacture of high value-added chemicals. Among them, the conversion of furfural (FFA) into 2-methylfuran (2-MF), catalyzed by a copper-chromium catalyst, is important in its industrial application. However, the use of chromium is limited due to its toxicity and pollution of the environment. In this paper, a Cu/SiO2 catalyst, prepared by the ammonia evaporation method, shows a better catalytic performance compared with that prepared by the co-precipitation method for the vapor-phase hydrodeoxygenation of FFA. The selectivity of 2-MF is higher than 80% with almost a complete conversion of FFA. Combined with the characterizations, the superiority of the ammonia evaporation method is attributed to the reduction of highly dispersed copper species and the increased Cu+/(Cu+ + Cu0) ratio due to the formation of a large content of copper phyllosilicate during the preparation. Moreover, Cu+ sites can act as a weak acid site, which improve the surface acidity of the catalyst and facilitate the formation of 2-MF. This new catalytic system provides a feasible and promising strategy for the industrial preparation of 2-MF from FFA, and effectively utilizes biomass resources to promote the development of biomass industry.

Commercial Cu2 Cr2 O5 Decorated with Iron Carbide Nanoparticles as a Multifunctional Catalyst for Magnetically Induced Continuous-Flow Hydrogenation of Aromatic Ketones

Copper chromite is decorated with iron carbide nanoparticles, producing a magnetically activatable multifunctional catalytic system. This system (ICNPs@Cu₂Cr₂O₅) can reduce aromatic ketones to aromatic alcohols when exposed to magnetic induction. Under magnetic excitation, the ICNPs generate locally confined hot spots, selectively activating the Cu₂Cr₂O₅ surface while the global temperature remains low (≈80 °C). The catalyst selectively hydrogenates a scope of benzylic and non‐benzylic ketones under mild conditions (3 bar H₂, heptane), while ICNPs@Cu₂Cr₂O₅ or Cu₂Cr₂O₅ are inactive when the same global temperature is adjusted by conventional heating. A flow reactor is presented that allows the use of magnetic induction for continuous‐flow hydrogenation at elevated pressure. The excellent catalytic properties of ICNPs@Cu₂Cr₂O₅ for the hydrogenation of biomass‐derived furfuralacetone are conserved for at least 17 h on stream, demonstrating for the first time the application of a magnetically heated catalyst to a continuously operated hydrogenation reaction in the liquid phase.

Efficient and chemoselective hydrogenation of aldehydes catalyzed by well-defined PN3-pincer manganese(II) catalyst precursors: an application in furfural conversion

Well-defined and air-stable PN³-pincer manganese(II) complexes were synthesized and used for the hydrogenation of aldehydes into alcohols under mild conditions using MeOH as a solvent. This protocol is applicable for a wide range of aldehydes containing various functional groups. Importantly, α,β-unsaturated aldehydes, including ynals, are hydrogenated with the CC double bond/CC triple bond intact. Our methodology was demonstrated for the conversion of biomass derived feedstocks such as furfural and 5-formylfurfural to furfuryl alcohol and 5-(hydroxymethyl)furfuryl alcohol respectively.

Recent advances in the conversion of furfural into bio-chemicals through chemo- and bio-catalysis

Furfural is a promising renewable platform molecule derived from hemi-cellulose, which can be further converted to fossil fuel alternatives and valuable chemicals due to its highly functionalized molecular structure. This mini-review summarizes the recent progress in the chemo-catalytic and/or bio-catalytic conversion of furfural into high-value-added chemicals, including furfurylamine, C₆ carboxylic acid, i.e., furandicarboxylic acid, furfural alcohol, aromatics, levulinic acid, maleic acid, succinic acid, furoic acid, and cyclopentanone, particularly the advances in the catalytic valorization of furfural into useful chemicals in the last few years. The possible reaction mechanisms for the conversion of furfural into bio-chemicals are summarized and discussed. The future prospective and challenges in the utilization of furfural through chemo- and bio-catalysis are also put forward for the further design and optimization of catalytic processes for the conversion of furfural.

Density Functional Investigation of the Conversion of Furfural to Furfuryl Alcohol by Reaction with i-Propanol over UiO-66 Metal-Organic Framework

Carbonyl C═O bond reduction via catalytic transfer hydrogenation (CTH) is one of the essential processes for biomass conversion to valuable chemicals and fuels. Here, we investigate the CTH of furfural to furfuryl alcohol with i-propanol on UiO-66 metal-organic frameworks using density functional theory calculations and linear scaling relations. Initially, the reaction over two defect sites presented on Zr-UiO-66, namely, dehydrated and hydrated sites, have been compared. The hydrated active site is favored over that on the dehydrated active site since the activation free energy of the rate-determining reaction step occurring on the hydrated active site is lower than that occurring on the dehydrated active site (14.9 vs 17.9 kcal/mol). The catalytic effect of exchanged tetravalent metals (Hf and Ti) on Zr-UiO-66 is also considered. We found that Hf-UiO-66 (13.5 kcal/mol) provides a lower activation energy than Zr-UiO-66 (14.9 kcal/mol) and Ti-UiO-66 (19.4 kcal/mol), which corresponds to it having a higher Lewis acidity. The organic linkers of UiO-66 MOFs play a role in stabilizing all of the species on potential energy surfaces. The linear scaling relationship also reveals the significant role of the UiO-66 active site in activating the carbonyl C═O of furfural, and strong relationships are observed between the activation free energy, the charge of the metal at the MOF active sites, and the complexation energies in reaction coordinates.

On the Role of Protonic Acid Sites in Cu Loaded FAU31 Zeolite as a Catalyst for the Catalytic Transformation of Furfural to Furan

The aim of the present paper is to study the speciation and the role of different active site types (copper species and Brønsted acid sites) in the direct synthesis of furan from furfural catalyzed by copper-exchanged FAU31 zeolite. Four series of samples were prepared by using different conditions of post-synthesis treatment, which exhibit none, one or two types of active sites. The catalysts were characterized by XRD, low-temperature sorption of nitrogen, SEM, H₂-TPR, NMR and by means of IR spectroscopy with ammonia and CO sorption as probe molecules to assess the types of active sites. All catalyst underwent catalytic tests. The performed experiments allowed to propose the relation between the kind of active centers (Cu or Brønsted acid sites) and the type of detected products (2-metylfuran and furan) obtained in the studied reaction. It was found that the production of 2-methylfuran (in trace amounts) is determined by the presence of the redox-type centers, while the protonic acid sites are mainly responsible for the furan production and catalytic activity in the whole temperature range. All studied catalysts revealed very high susceptibility to coking due to polymerization of furfural.

Earth-abundant 3d-transition-metal catalysts for lignocellulosic biomass conversion

Transformation of biomass to chemicals and fuels is a long-term goal in both science and industry. However, high cost is one of the major obstacles to the industrialization of this sustainable technology. Thus, developing catalysts with high activity and low-cost is of great importance for biomass conversion. The last two decades have witnessed the increasing achievement of the use of earth-abundant 3d-transition-metals in catalysis due to their low-cost, high efficiency and excellent stability. Here, we aim to review the fast development and recent advances of 3d-metal-based catalysts including Cu, Fe, Co, Ni and Mn in lignocellulosic biomass conversion. Moreover, present research trends and invigorating perspectives on future development are given.

Elucidation of copper environment in a Cu-Cr-Fe oxide catalyst through in situ high-resolution XANES investigation

Copper containing materials are widely used in a range of catalytic applications. Here, we report the use of Cu K-edge high resolution XANES to determine the local site symmetry of copper ions during the thermal treatment of a Cu-Cr-Fe oxide catalyst. We exploited the Cu K-edge XANES spectral features, in particular the correlation between area under the pre-edge peak and its position to determine the local environment of Cu2+ ions. The information gained from this investigation rules out the presence of Cu2+ ions in a tetrahedral or square planar geometry, a mixture of these sites, or in a reduced oxidation state. Evidence is presented that the Cu2+ ions in the Cu-Cr-Fe oxide system are present in a distorted octahedral environment.

Highly Efficient Ir-CoOx Hybrid Nanostructures for the Selective Hydrogenation of Furfural to Furfuryl Alcohol

Decoration of noble metals with transition-metal oxides has been intensively studied for heterogeneous catalysis. However, controllable syntheses of metal-metal oxide heterostructures are difficult, and elucidation of such interfaces is still challenging. In this work, supported IrCo alloy nanoparticles are transformed into supported Ir-CoO_x close-contact nanostructures by in situ calcination and following selective reduction. Relative to Ir/Al₂O₃, Ir-CoO_x/Al₂O₃ shows greatly enhanced activities for the hydrogenation of furfural derivatives to the corresponding furfuryl alcohol derivatives with more than 99% selectivity and demonstrates significantly improved activities and selectivity for hydrogenations of α,β-unsaturated aldehydes to α,β-unsaturated alcohols. The modification of Ir surfaces with CoO_x prevents Ir nanoparticles from growing, achieving high thermal and catalytic stabilities. Theoretic calculation suggests that the better catalytic performance of Ir-CoO_x/Al₂O₃ is ascribed to the Ir-CoO_x interaction, which promotes the absorption of furfural as well as desorption of furfuryl alcohol, resulting in enhanced catalytic activities.

Recent catalytic routes for the preparation and the upgrading of biomass derived furfural and 5-hydroxymethylfurfural

Furans represent one of the most important classes of intermediates in the conversion of non-edible lignocellulosic biomass into bio-based chemicals and fuels. At present, bio-furan derivatives are generally obtained from cellulose and hemicellulose fractions of biomass via the acid-catalyzed dehydration of their relative C6-C5 sugars and then converted into a wide range of products. Furfural (FUR) and 5-hydroxymethylfurfural (HMF) are surely the most used furan-based feedstocks since their chemical structure allows the preparation of various high-value-added chemicals. Among several well-established catalytic approaches, hydrogenation and oxygenation processes have been efficiently adopted for upgrading furans; however, harsh reaction conditions are generally required. In this review, we aim to discuss the conversion of biomass derived FUR and HMF through unconventional (transfer hydrogenation, photocatalytic and electrocatalytic) catalytic processes promoted by heterogeneous catalytic systems. The reaction conditions adopted, the chemical nature and the physico-chemical properties of the most employed heterogeneous systems in enhancing the catalytic activity and in driving the selectivity to desired products are presented and compared. At the same time, the latest results in the production of FUR and HMF through novel environmental friendly processes starting from lignocellulose as well as from wastes and by-products obtained in the processing of biomass are also overviewed.

Ruthenium Nanoparticles Intercalated in Montmorillonite (nano-Ru@MMT) Is Highly Efficient Catalyst for the Selective Hydrogenation of 2-Furaldehyde in Benign Aqueous Medium

Chemoselective hydrogenation of 2-furaldehyde to furfuryl alcohol using green solvents is an important research area to get eco-friendly fuels and fine chemicals. Herein, we report ruthenium nanoparticles (~1.8 nm) intercalated in montmorillonite as an efficient catalytic system, which can selectively hydrogenate 2-furaldehyde in a benign aqueous medium. The complete conversion was observed at 40 °C with 1 MPa H2, the selectivity of furfuryl alcohol being >99%, and turnover number 1165. After a catalytic run, the montmorillonite-supported ruthenium nanoparticles can be recycled and reused without losing their activity and selectivity.

Towards the Green Synthesis of Furfuryl Alcohol in A One-Pot System from Xylose: A Review

In the pursuit of establishing a sustainable biobased economy, valorization of lignocellulosic biomass is increasing its value as a feedstock. Nevertheless, to achieve the integrated biorefinery paradigm, the selective fractionation of its complex matrix to its single constituents must be complete. This review presents and examines the novel catalytic pathways to form furfuryl alcohol (FuOH) from xylose in a one-pot system. This production concept takes on chemical, thermochemical and biochemical transformations or a combination of them. Still, the bulk of the research is targeted to develop heterogeneous catalytic systems to synthesize FuOH from furfural and xylose. The present review includes an overview of the economic aspects to produce this platform chemical in an industrial manner. In the last section of this review, an outlook and summary of catalytic processes to produce FuOH are highlighted.

Tuning the synthesis of polymetallic-doped ZIF derived materials for efficient hydrogenation of furfural to furfuryl alcohol

Cu, Co and Zn modified N-doped porous carbons (CuCo/Zn@NPC) are prepared using a polymetallic homogeneous doping and self-templating method as high performance non-noble metal catalysts for the hydrogenation of furfural (FF) to furfuryl alcohol (FAL). The CuCo/Zn@NPC-600 catalyst after treatment at 600 °C shows a superior catalytic activity with nearly 100% conversion of FF and an almost 100% selectivity of FAL using H2 at 140 °C. Meanwhile in the catalytic transfer hydrogenation (CTH) using 2-propanol as a H-donor, the conversion of FF reaches 95.8% and the selectivity of FAL is 99.1%. The results show that the Zn dopant leads to 37.3 times higher yield on the CuCo/Zn@NPC-600 catalyst than that on CuCo@NPC-600, and 2.3 times higher than that on Co/Zn@NPC-600 with Cu dopants. The efficient activity of the CuCo/Zn@NPC-600 catalyst is mainly due to the highly dispersed metal nanoparticles, the advanced porous structure resulting from Zn escape from the precursor template, and the synergistic effect between Cu and Co. Furthermore, the CuCo/Zn@NPC-600 catalyst exhibits good recyclability in FF hydrogenation in four cycle tests. The advanced synthesis method using a homogeneous doping and self-templating strategy sheds light on preparing effective catalysts for hydrogenation of biomass-based compounds.

Nickel-Catalysed Vapour-Phase Hydrogenation of Furfural, Insights into Reactivity and Deactivation

Furfural is a key bioderived platform molecule, and its hydrogenation affords access to a number of important chemical intermediates that can act as “drop-in” replacements to those derived from crude oil or novel alternatives with desirable properties. Here, the vapour phase hydrogenation of furfural to furfuryl alcohol at 180 °C over standard impregnated nickel catalysts is reported and contrasted with the same reaction over copper chromite. Whilst the selectivity to furfuryl alcohol of the unmodified nickel catalysts is much lower than for copper chromite as expected, the activity of the nickel catalysts in the vapour phase is significantly higher, and the deactivation profile remarkably similar. In the case of the supported nickel catalysts, possible contribution to the deactivation by acidic sites on the catalyst support is discounted based on the similarity of deactivation kinetics on Ni/SiO2 with those seen for less acidic Ni/TiO2 and Ni/CeO2. Powder X-ray diffraction is used to exclude sintering as a primary deactivation pathway. Significant coking of the catalyst (~ 30 wt% over 16 h) is observed using temperature programmed oxidation. This, in combination with the solvent extraction analysis and infrared spectroscopy of the coked catalysts points to deactivation by polymeric condensation products of (reactant or) products and hydrocarbon like coke. These findings pave the way for targeted modification of nickel catalysts to use for this important biofeedstock-to-chemicals transformation.

Gas-Phase Hydrogenation of Furfural to Furfuryl Alcohol over Cu-ZnO-Al2O3 Catalysts Prepared from Layered Double Hydroxides

Several layered double hydroxides (LDHs) with general chemical composition (Cu,Zn)1−xAlx(OH)2(CO3)x/2·mH2O have been synthesized by the co-precipitation method, maintaining a (M2+/M3+) molar ratio of 3, and varying the Cu2+/Zn2+ molar ratio between 0.2 and 6.0. After calcination and reduction steps, Cu/ZnO/Al2O3 catalysts were synthesized. These catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 thermoprogrammed reduction (H2-TPR), N2 adsorption-desorption at −196 °C, N2O titration, X-ray photoelectron miscroscopy (XPS), NH3-thermoprogramed desorption (NH3-TPD) and CO2- thermoprogrammed desorption (CO2-TPD). The characterization data revealed that these catalysts are mainly meso-and macroporous, where Cu, ZnO and Al2O3 are well dispersed. The catalytic results show that these catalysts are active in the gas-phase hydrogenation of furfural, being highly selective to furfuryl alcohol (FOL) and reaching the highest FOL yield for the catalyst with a Cu2+/Zn2+ molar ratio of 1. In an additional study, the influence of the aging time on the synthesis of the LDHs was also evaluated. The catalytic data revealed that the use of shorter aging time in the formation of the LDH has a beneficial effect on the catalytic behavior, since more disordered structures with a higher amount of available Cu sites is obtained, leading to a higher yield towards FOL (71% after 5 h of time-on-stream at 210 °C).

Technology Trends of Catalysts in Hydrogenation Reactions: A Patent Landscape Analysis

The purpose of this review is to present an overview of the patent landscape for catalysts used in hydrogenation reactions. Based on patent data extracted from PatBase®, we use predefined patent classifications as well as a keyword-based search for our analyses. The results indicate that the number of patent families that protect heterogeneous catalysts grows twice as fast as that for their homogeneous counterparts. Furthermore, the data show a shift towards abundant and non-toxic elements in heterogeneous catalysis, while the noble metals continue to dominate the patent landscape of homogeneous catalysis. A subsequent geographical analysis reveals that the high growth rates in heterogeneous catalysis, especially for nickel and iron, are driven by China. Conversely, patenting activities with regard to homogeneous catalysts mainly take place in the USA, the EU, and Japan. The subsequent keyword-based search illustrates the continuous industrial relevance of enantioselective hydrogenation and transfer hydrogenation, as well as the rapidly increasing body of patents in hydrodeoxygenation. Setting these finding into context, we present and apply two concepts that are commonly used in patent analyses, namely the technology life cycle and the S-curve. We conclude that hydrogenation catalysis has not reached its peak economic relevance yet and will continue to spark valuable patents and innovations in the future.

Highly Selective Reduction of Bio-Based Furfural to Furfuryl Alcohol Catalyzed by Supported KF with Polymethylhydrosiloxane (PMHS)

Hydrogenation of bio-based furfural (FUR) to furfuryl alcohol (FFA) is tremendously expanding the application of biomass in many industries such as resins, biofuels, and pharmaceuticals. However, mass manufacture of FFA from FUR is restrained by strict requirements of reaction conditions and expensive catalysts. In this work, an economical and benign catalytic system, containing an easily prepared and reusable catalyst 5 wt.% KF/ZrO2 and a low-cost hydrogen source polymethylhydrosiloxane (PMHS), was developed to be efficient for the hydrogenation of FUR to high-value FFA under mild conditions. The catalyst reactivity was found to be remarkably influenced by the support acid-base properties and KF loading doge. In the presence of 5 wt.% KF/ZrO2, a high FFA yield of 97% and FUR conversion of 99% could be obtained at 25°C in just 0.5 h, which was superior to those attained with other tested catalysts. The KF/ZrO2 catalyst could be recycled at least five times, with the FFA yield slightly decreasing from 97% to 71%. The spare decrease in FFA yield is possibly attributed to the catalyst pore blocking, as clarified by SEM, BET, XPS, and ICP-MS measurements of the fresh and reused catalysts.

Chemoselective and efficient catalytic hydrogenation of furfural by iridium and ruthenium half-sandwich complexes

A recyclable homogeneous iridium complex for the selective synthesis of furfuryl alcohol from furfural without additional solvent and hydrogen gas.

A theoretical insight into furfural conversion catalyzed on the Ni(111) surface

Biomass-derivatives, e.g., furfural, have been widely reported to become new-generation renewable sources of chemicals and fuels. However, it is too complicated to understand the product selectivity of furfural conversion in diverse reactions. Accordingly, by using density functional theory calculations, both the hydrodeoxygenation and decarboxylation of furfural on the Ni(111) surface to form furan, 2-methylfuran, furfuryl alcohol, tetrahydrofuran, and tetrahydrofurfuryl alcohol have been thoroughly investigated. On the basis of the minimum energy path, furfural decarbonylation leads to the formation of furan via F-CHO + 2H → F + CO + 2H → F + CO + H → F-H, and then tetrahydrofuran forms via sequential hydrogenation on the carbon atoms of the furan ring, while furfuryl alcohol (F-CHO + 2H → F + CHOH + H → F-CH₂OH) can be obtained via furfural hydrogenation. More importantly, 2-methylfuran tends to form through the hydrodeoxygenation reaction, and tetrahydrofurfuryl alcohol is generated via furfural hydrogenation, which is realized with furfuryl alcohol identified as the likely intermediate. Overall, among all these products, furan is a dominant product. More importantly, it has been found that different types of metal doping will also lead to different adsorption configurations of the reactants. These findings should provide guidance in catalyst design for converting furfural to value-added products.

Heterogeneous Catalytic Upgrading of Biofuranic Aldehydes to Alcohols

Heterogeneous catalytic conversion of lignocellulosic components into valuable chemicals and biofuels is one of the promising ways for biomass valorization, which well meets green chemistry metrics, and can alleviate environmental and economic issues caused by the rapid depletion of fossil fuels. Among the identified biomass derivatives, furfural (FF) and 5-hydroxymethylfurfural (HMF) stand out as rich building blocks and can be directly produced from pentose and hexose sugars, respectively. In the past decades, much attention has been attracted to the selective hydrogenation of FF and 5-hydroxymethylfurfural using various heterogeneous catalysts. This review evaluates the recent progress of developing different heterogeneous catalytic materials, such as noble/non-noble metal particles, solid acids/bases, and alkali metal salts, for the efficient reduction of bio-based furanic aldehydes to alcohols. Emphasis is laid on the insights and challenges encountered in those biomass transformation processes, along with the focus on the understanding of reaction mechanisms to clarify the catalytic role of specific active species. Brief outlook is also made for further optimization of the catalytic systems and processes for the upgrading of biofuranic compounds.

Spectroscopy Identification of the Bimetallic Surface of Metal-Organic Framework-Confined Pt-Sn Nanoclusters with Enhanced Chemoselectivity in Furfural Hydrogenation

Research and development in bimetallic nanoparticles have gained great interest over their monometallic counterparts because of their distinct and unique properties in a wide range of applications such as catalysis, energy storage, and bio/plasmonic imaging. Identification and characterization of these bimetallic surfaces for application in heterogeneous catalysis remain a challenge and heavily rely on advanced characterization techniques such as aberration-corrected electron microscopy and synchrotron X-ray absorption studies. In this article, we have reported a strategy to prepare sub-2 nm bimetallic Pt-Sn nanoclusters confined in the pores of a Zr-based metal-organic framework (MOF). The Pt-Sn nanoclusters encapsulated in the Zr-MOF pores show enhanced chemoselectivity from 51 to 93% in an industrially relevant reaction, furfural hydrogenation to furfuryl alcohol. The presence of bimetallic Pt-Sn surfaces was investigated by a surface-sensitive characterization technique utilizing diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO to probe the bimetallic surface of the encapsulated ultrafine Pt-Sn nanocluster. Complementary techniques such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were also used to characterize the Pt-Sn nanoclusters.

Influence of the Incorporation of Basic or Amphoteric Oxides on the Performance of Cu-Based Catalysts Supported on Sepiolite in Furfural Hydrogenation

Cu-based catalysts supported on sepiolite have been tested in vapor-phase hydrogenation of furfural. The incorporation of basic or amphoteric metal oxides (magnesium oxide, zinc oxide, or cerium oxide) improves the catalytic behavior, reaching a maximum furfural conversion above 80% after 5 h of reaction at 210 °C. In all cases, the main product is furfuryl alcohol, obtaining 2-methylfuran in lower proportions. The incorporation of these metal oxide species ameliorates the dispersion of metallic Cu nanoparticles, increasing the number of available Cu0-sites, which enhances the catalytic performance. The presence of acid sites favors the hydrogenolysis of furfuryl alcohol towards 2-methylfuran, although it also causes an increase of carbon species on its surface, which is associated with the catalytic deactivation of the catalyst along the time-on-stream.

Influence of Structure-modifying Agents in the Synthesis of Zr-doped SBA-15 Silica and Their Use as Catalysts in the Furfural Hydrogenation to Obtain High Value-added Products through the Meerwein-Ponndorf-Verley Reduction

Zr-doped mesoporous silicas with different textural parameters have been synthesized in the presence of structure-modifying agents, and then characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption at −196 °C, NH₃ thermoprogrammed desorption (NH₃–TPD), CO₂ thermoprogrammed desorption (CO₂–TPD), and X-ray photoelectron spectroscopy (XPS). These porous materials were evaluated in the furfural hydrogenation through the Meerwein-Ponndorf-Verley (MPV) reaction. The catalytic results indicate that the catalyst synthesized under hydrothermal conditions and adding a pore expander agent is more active and selective to furfuryl alcohol. However, the Zr-doped porous silica catalysts that were synthesized at room temperature, which possess narrow pore sizes, tend to form i-propyl furfuryl and difurfuryl ethers, coming from etherification between furfuryl alcohol (FOL) and isopropanol molecules (used as H-donor) by a S_N2 mechanism.

Exploiting the Synergetic Behavior of PtPd Bimetallic Catalysts in the Selective Hydrogenation of Glucose and Furfural

Mono and bimetallic catalysts based on Pt and Pd were prepared by a co-precipitation method. They were tested in liquid phase hydrogenation reactions of glucose and furfural at low temperature and pressure. The bimetallic PtPd/TiO2 catalyst proved to be an efficient material in selectively hydrogenating glucose to sorbitol. Moreover, high furfural conversion was attained under relatively soft conditions, and the furfuryl alcohol selectivity was strongly affected by the chemical composition of the catalysts. Furfuryl alcohol (FA) was the major product in most cases, along with side products such as methylfuran (MF), furan, and traces of tetrahydrofuran (THF). These results showed that the PtPd bimetallic sample was more active relative to the monometallic counterparts. A correlation between the catalytic results and the physicochemical properties of the supported nanoparticles identified key factors responsible for the synergetic behavior of the PtPd system. The high activity and selectivity were due to the formation of ultra-small particles, alloy formation, and the Pt-rich surface composition of the bimetallic particles supported on the TiO2 nanowires.