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

Nb-Based Catalysts for the Valorization of Furfural into Valuable Product through in One-Pot Reaction

Nb-based catalysts supported on porous silica with different textural properties have been synthesized, characterized, and tested in the one-pot reaction of furfural to obtain valuable chemicals. The catalytic results reveal that the presence of fluoride in the synthesis, which limits the growing of the porous silica, limits diffusional problems of the porous silica, obtaining higher conversion values at shorter reaction times. On the other hand, the incorporation of NbOx species in the porous silica provides Lewis acid sites and a small proportion of Brönsted acid sites, in such a way that the main products are alkyl furfuryl ethers, which can be used as fuel additives.

Function of Brønsted and Lewis acid sites in xylose conversion into furfural

In this work, the xylose conversion and the selectivity to furfural were assessed over mesoporous sulfonic silica SBA-15-(X)SO3H catalysts doped with metal ions (X = Al(iii), Ti(iv) or Zr(iv)). The type and amount of acid sites were analyzed by adsorption of pivalonitrile. The SBA-15-(X)SO3H materials show Lewis acid sites (LAS) and two types of Brønsted acid sites (BAS) with different strengths. Type I (BAS I) belongs to terminal silanol groups, type II (BAS II) is ascribed to hydroxyl groups bonded to sulfur or transition metal, and the LAS is related to M-O bonds. Optimal reaction conditions for the most active catalyst (SBA-15-(Zr)SO3H) were 120 minutes of reaction at 160 °C, 20 wt% of catalyst, and 2.5% of xylose/solvent. Additionally, a kinetic study was carried out to calculate the rate constants, the activation energy, and the pre-exponential factor for the xylose dehydration reaction. It was found that the selectivity to furfural in sulfonic silica SBA-15-(X)SO3H catalysts was directly related to the BAS II fraction. While LAS negatively impacts the selectivity to furfural leading to the undesired reaction between furfural and xylose obtaining humins as secondary products.

Uranium-Doped Zinc, Copper, and Nickel Oxides for Enhanced Catalytic Conversion of Furfural to Furfuryl Alcohol: A Relativistic DFT Study

Transition metal oxides (TMOs) and actinide ones (AnOs) have been widely applied in catalytic reactions due to their excellent physicochemical properties. However, the reaction pathway and mechanism, especially involving TM–An heterometallic centers, remain underexplored. In this respect, relativistic density functional theory (DFT) was used to examine uranium-doped zinc, copper, and nickel oxides for their catalytic activity toward the conversion of furfural to furfuryl alcohol. A comparison was made with their undoped TMOs. It was found that the three TMOs were capable of catalyzing the reaction, where the free energies of adsorption, hydrogenation, and desorption fell between −33.93 and 45.00 kJ/mol. The uranium doping extremely strengthened the adsorption of CuO-U and NiO-U toward furfural, making hydrogenation or desorption much harder. Intriguingly, ZnO-U showed the best catalytic performance among all six catalyst candidates, as its three reaction energies were very small (−10.54–8.12 kJ/mol). The reaction process and mechanism were further addressed in terms of the geometrical, bonding, charge, and electronic properties.

Porous SiO2 Nanospheres Modified with ZrO2 and Their Use in One-Pot Catalytic Processes to Obtain Value-Added Chemicals from Furfural

Porous SiO2 nanospheres were modified with different loadings of ZrO2 to obtain catalysts with a Si/Zr molar ratio from 2.5 to 30. These materials were characterized by X-ray diffraction, transmission and scanning electron microscopies, N2 adsorption-desorption at -196 °C, X-ray photoelectron spectroscopy and pyridine and 2-6-dimethylpyridine thermoprogrammed desorption. The characterization of these catalysts has revealed that a high proportion of Zr favors the formation of Lewis acid sites, which are implied in catalytic transfer hydrogenation processes, whereas the low Brönsted acidity promotes a dehydration reaction, being possible to give rise to a large variety of products from furfural through consecutive reactions, such as furfuryl alcohol, i-propyl furfuryl ether, i-propyl levulinate, and γ-valerolactone, in a range of temperature of 110-170 °C and 1-6 h of reaction.

Catalytic Transfer Hydrogenation and Acid Reactions of Furfural and 5-(Hydroxymethyl)furfural over Hf-TUD-1 Type Catalysts

Heterogeneous catalysis, which has served well the petrochemical industry, may valuably contribute towards a bio-based economy by sustainably enabling selective reactions to renewable chemicals. Carbohydrate-containing matter may be obtained from various widespread sources and selectively converted to furanic platform chemicals: furfural (Fur) and 5-(hydroxymethyl)furfural (Hmf). Valuable bioproducts may be obtained from these aldehydes via catalytic transfer hydrogenation (CTH) using alcohols as H-donors under relatively moderate reaction conditions. Hafnium-containing TUD-1 type catalysts were the first of ordered mesoporous silicates explored for the conversion of Fur and Hmf via CTH/alcohol strategies. The materials promoted CTH and acid reactions leading to the furanic ethers. The bioproducts spectrum was broader for the reaction of Fur than of Hmf. A Fur reaction mechanism based on literature data was discussed and supported by kinetic modelling. The influence of the Hf loading and reaction conditions (catalyst load, type of alcohol H-donor, temperature, initial substrate concentration) on the reaction kinetics was studied. The reaction conditions were optimized to maximize the yields of 2-(alkoxymethyl)furan ethers formed from Fur; up to 63% yield was reached at 88% Fur conversion, 4 h/150 °C, using Hf-TUD-1(75), which was a stable catalyst. The Hf-TUD-1(x) catalysts promoted the selective conversion of Hmf to bis(2-alkoxymethyl)furan; e.g., 96% selectivity at 98% Hmf conversion, 3 h/170 °C for Hf-TUD-1(50).

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.

gem-Diol-Type Intermediate in the Activation of a Ketone on Sn-β Zeolite as Studied by Solid-State NMR Spectroscopy

Lewis acid zeolites have found increasing application in the field of biomass conversion, in which the selective transformation of carbonyl-containing molecules is of particular importance due to their relevance in organic synthesis. Mechanistic insight into the activation of carbonyl groups on Lewis acid sites is challenging and critical for the understanding of the catalytic process, which requires the identification of reaction intermediates. Here we report the observation of a stable surface gem-diol-type species in the activation of acetone on Sn-β zeolite. ¹³ C, ¹¹⁹ Sn, and ¹³ C-¹¹⁹ Sn double-resonance NMR spectroscopic studies demonstrate that only the open Sn site ((SiO)₃ Sn-OH) on Sn-β is responsible for the formation of the surface species. ¹³ C MAS NMR experiments together with density functional theory calculations suggest that the gem-diol-type species exhibits high reactivity and can serve as an active intermediate in the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reaction of acetone with cyclohexanol. The gem-diol-type species offers an energy-preferable pathway for the direct carbon-to-carbon hydrogen transfer between ketone and alcohol. The results provide new insights into the transformation of carbonyl-containing molecules catalyzed by Lewis acid zeolites.

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.

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.