Search

We carried out a mechanistic investigation of the dehydration of fructose catalyzed by a series of proton‐exchanged faujasite zeolites. Using a combination of analytical tools (HPLC, GC, NMR), we could solve and quantify most of the variety of products formed, including (i) cyclic intermediates (ii) transient species (monomeric and dimeric) (iii) byproducts (oligomeric species and humins). The results prove that the reaction goes through a cyclic mechanism, and we could propose a kinetic model matching most of our experimental results and describing the interplay between the different categories of products. By comparing the performances of different catalysts with varying properties, we surprisingly find that increasing the concentration of Brønsted acid sites induces lower catalytic activity. Contrary to general expectations, the mesoporous volume seems to be the most determining parameter for the activity of the catalysts over the set of catalysts evaluated here, which may be explained by the importance of allowing the reactant a significant accessibility to the active sites. [ABSTRACT FROM AUTHOR]

The reaction steps involved in the 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid conversion by means of H2O2 were investigated employing a dedicated computational protocol based on density functional theory. The catalytic environment of choice was a molecular model representing a portion of the halloysite nanotube outer surface, functionalized by an organosilane, the 3‐aminopropyltriethoxysilane, whose amino group bonds one gold atom. At this stage of the investigation, the process was fully detailed in terms of the interactions between the reaction intermediates and the catalyst, and the reaction standard free energies. In addition, the energy barriers of the elementary steps involving the hydrogen migration from the adsorbed organic species to the gold atom were analyzed. On the basis of the interaction geometries, a certain distinction among the preferred reaction path can be inferred as a function of the net negative charge characterizing the catalyst outer surface. Since the inner surface of halloysite can represent the acid environment needed to obtain 5‐hydroxymethylfurfural through dehydration of fructose, the present study is framed in a wider research field where the possibility to consider functionalized halloysite as one‐pot reactor for the valorization of biomass is explored. [ABSTRACT FROM AUTHOR]

Glucose to fructose transformation is an essential transformation for the success of biorefinery. Herein, the epitaxial‐grafting of Nb in UIO‐66 significantly improved the fructose yield. Post‐synthesis incorporating Nb in the UIO‐66 phenomenally improved the glucose conversion from 31.1 % to 71.8 % and fructose selectivity from 38.2 % to 71.2 %. The incorporation of Nb was confirmed from the FTIR, HR‐TEM investigation, EDX analysis, and elemental mapping. The XPS analysis distinctly stated the successful incorporation of Nb and generation of Zr−O−Nb oxo clusters in the UIO‐66 framework. The NH3‐TPD provided distinguishable information about the chemical interaction of Nb with the UIO‐66 framework that resulted in the increase of the acid strength after the Nb incorporation in the UIO‐66 catalyst. The content of Nb and reaction parameters, especially temperature and duration, control the glucose to fructose isomerization over glucose to mannose epimerization. The structure‐activity relationship was established, and a reaction mechanism was proposed. The catalyst was recycled five times with only a lower decrease in the fructose yield (47.3 % to 42.2 %) than several reported MOFs catalysts. The activity of the developed catalyst was higher or similar to the reported catalysts. The successful grafting of Nb demonstrates that by swiftly modifying the acidity of the catalyst, a superior activity can be achieved. [ABSTRACT FROM AUTHOR]

The dehydration of fructose to selectively produce 5‐HMF was catalytically developed in a new type of ionic liquids based on triazoles supported on alumina (Al2O3). The synthesis of triazoles by 1,3‐dipolar cycloaddition allows for creating a tailor‐made triazole, that was eventually used to design catalysts based on triazolium‐based ionic liquids. In this work, four catalysts based on triazolium‐ionic liquids supported on Al2O3 were synthesized to produce 5‐HMF from fructose in DMSO as the reaction solvent. According to their molecular structure, the catalysts were tagged as TR1/Al2O3, TR1‐HSO4/Al2O3, TR2/Al2O3, and TR2‐HSO4/Al2O3. [ABSTRACT FROM AUTHOR]

Furan carboxylic acids are useful chemicals in various industries. In this work, biocatalytic production of furan carboxylic acids was reported with high productivities by cofactor‐engineered Escherichia coli cells. NADH oxidase (NOX) was introduced into E. coli harboring aldehyde dehydrogenases (ALDHs) to promote intracellular NAD+ regeneration, thus significantly enhancing ALDH‐catalyzed oxidation. These engineered biocatalysts were capable of efficient aerobic oxidation of a variety of aromatic aldehydes. More importantly, they exhibited high substrate tolerance toward toxic furans. E. coli co‐expressing vanillin dehydrogenase and NOX (E. coli_CtVDH1_NOX) enabled efficient oxidation of 250 mM of 5‐hydroxymethylfurfural (HMF) to 5‐hydroxymethyl‐2‐furancarboxylic acid (HMFCA), providing a productivity of 3.7 g/L h. With E. coli_CtVDH2_NOX as catalyst, up to 240 mM of furfural and 5‐methoxymethylfurfural (MMF) could be smoothly oxidized. 2‐Furoic acid (FCA, 227 mM) and 5‐methoxymethyl‐2‐furancarboxylic acid (MMFCA, 287 mM) were produced in fed‐batch synthesis, providing the productivities of 2.0 and 5.6 g/L h, respectively. [ABSTRACT FROM AUTHOR]

The selective hydroconversion of 5‐hydroxymethylfurfural (HMF) to biofuels is currently highly sought‐for. While the literature has demonstrated that this reaction is possible on promoted Ni catalysts, we show here that a monometallic, non‐promoted Ni/SBA‐15 catalyst, prepared by incipient wetness impregnation, can convert HMF to 2,5‐dimethylfuran (DMF) and to 2,5‐dimethyltetrahydrofuran (DMTHF) at 180 °C, in a consecutive way. Through a control over reaction time, high yields to DMF (71 %, at conversion of 93 %) or DMTHF (97 %, at conversion of 100 %) can be achieved. Kinetic modelling suggests a preferential route to DMF via 5‐methylfurfural (MFFR) as intermediate, though the route via 2,5‐bis(hydroxylmethyl)furan (BHMF) is also present. The favored route in the experimental conditions involves the hydrogenolysis of the hydroxyl group of HMF as first step, followed by the hydrogenation of the aldehyde function, to methylfurfuryl alcohol (MFOL). It is suggested a higher reaction rate of hydrogenation or hydrogenolysis of the side group is linked to the presence of a methyl group in the molecule. No hydrogenation of the furan ring is detected on the intermediates. [ABSTRACT FROM AUTHOR]

5‐Hydroxymethylfurfural (HMF) is an important bio‐based chemical, which has been recognized as a key platform chemical for furan‐based polymers, biomass‐derived fuels, fine chemicals, and pharmaceuticals. Catalytic conversion of fructose into HMF is an important reaction, and the development of an efficient heterogeneous catalysts with high selectivity towards HMF is highly desired. In this study, several porous ionic crystals with a Keggin‐type polyoxometalate [PW12O40]3− and a macrocation [Cr3O(OOCCH2CN)6(H2O)3]+ as building blocks are synthesized, and the structures are solved by single crystal X‐ray diffraction analysis. The catalytic activities are basically dependent on the porosity, and the compound with a mesopore (aperture: 3 nm×2 nm) showed the highest HMF yield (30 % at 353 K in methanol/toluene solvent). [ABSTRACT FROM AUTHOR]

Recently, 5‐hydroxymethyl furfural (5‐HMF) became into the spotlight as one of the most promising biorenewable platforms available from carbohydrates. Among other versatile derivatives of 5‐HMF its dimer 5,5′‐(oxy‐bis (methylene)) bis‐2‐furfural (OBMF) was recognized as a useful building block for the production of polymers and pharmaceuticals. Herein we explored the synthesis of OBMF under solvent free conditions applying vacuum as a green tool for water removal. Several readily available acidic resins, silica gel supported inorganic acid, acidic zeolites and Lewis acids were studied as promoters for this transformation. Under these conditions, SiO2/CF3SO3H exhibited the best performance allowing the synthesis of OBMF in 55 % yield in only 1 min reaction time. Similar results were achieved using Amberlyst 15 and SO2/PPA resulting in 51 % yield in 20 min and 46 % yield in 10 min, respectively. A new strategy based on solvent free self‐etherification of 5‐HMF towards the synthesis of 5,5′‐[oxybis(methylene)] bis‐2‐furfural (OBMF) was studied. This approach allowed up to 55 % yield using readily available acid catalysts and vacuum as a facile environmental benign water removal method. [ABSTRACT FROM AUTHOR]

Within the last decade, interest in using biphasic systems for producing furans from biomass has grown significantly. Biphasic systems continuously extract furans into the organic phase, which prevents degradation reactions and potentially allows for easier separations of the products. Several heterogeneous catalyst types, including zeolites, ion exchange resins, niobium‐based, and others, have been used with various organic solvents to increase furan yields from sugar dehydration reactions. In this minireview, we summarized the use of heterogeneous catalysts in biphasic systems for furfural and 5‐hydroxymethylfurfural production from the past five years, highlighting trends in chemical and physical properties that effect catalytic activity. Additionally, the selection of an organic solvent for a biphasic system is extremely important and we review and discuss properties of the most commonly used organic solvents. Reviewing biomass media: There is a significant amount of research available on the topic of furan production from sugars and biomass in biphasic solvent systems. Herein, we present trends in heterogeneous catalyst properties and solvent selection that emerged from papers published within the last five years. [ABSTRACT FROM AUTHOR]

Abstract: A new mechanism for glucose dehydration to HMF without the intermediate isomerization to fructose is discussed for surface models of anatase TiO2 using periodic density functional theory calculations. Activation of the glucose at glucose's C3‐OH position by titania starts the reaction resulting in adsorbed 3‐deoxyglucosone. The mechanistic study reveals two possible pathways for the acyclic form of glucose. Comparison of different surface models shows that the presence of tetrahedral Ti4+ species on a defective TiO2(101) anatase surface is essential for explaining the activity. Synergy between a strong Lewis acidic Ti site and a vicinal basic oxygen site of a TiOH group is essential to establish the direct conversion of glucose to HMF. IR spectroscopy of adsorbed chloroform in the presence of water confirms the presence of water tolerant Lewis acid sites in close proximity to basic sites. In conventional glucose dehydration, Lewis acid sites and proton donors act as catalytic sites for efficient sequential isomerization‐dehydration to HMF, while direct dehydration to HMF requires the cooperation of Lewis acid‐base pairs. [ABSTRACT FROM AUTHOR]

Abstract: Catalytic transformation of biomass‐derived compounds to different platform chemicals and liquid fuel is a prominent way to reduce the global dependence on fossil resources. In past few decades, biomass‐derived furans such as 2‐furfuraldehyde (furfural) and 5‐hydroxymethyl‐2‐furfural (HMF) have received outstanding attention because of their wide applications in the production of various industrially important value‐added chemicals and fuel components. Various catalytic systems and methodologies have been extensively explored for the transformation of these furans to a wide range of products including open ring diketones, ketoacids, alcohols and long chain alkanes. This Review is aimed to provide an extensive overview of the recent developments of several high‐performing heterogeneous catalysts for the catalytic upgradation of the key biomass‐derived furans (furfural and HMF) to value‐added chemicals. Moreover, the role of these catalysts in the catalytic transformations including hydrogenation, decarbonylation, oxidation, hydrogenolysis and ring opening reactions, and the mechanistic pathways are also highlighted in this Review. [ABSTRACT FROM AUTHOR]

5-Hydroxymethylfurfural (HMF) is a pivotal chemical in the utilization of biomass as a chemical feedstock. For this approach the vast majority of methods require access to pure HMF from biomass before its utilization, which incorporates one or more purification steps that obstruct industrial implementation. We describe an operationally simple and versatile method that encompasses both the dehydration of several carbohydrate substrates into HMF and subsequently its direct conversion into various high value-added furan derivatives in a one-pot process. To that end, we selected the aldol condensation of HMF with methyl isobutyl ketone (MIBK) as a model reaction. With the optimized conditions of solvent and base catalyst in hand, the one-pot dehydration of fructose and the condensation sequence with various ketones afforded the products in good overall yields over two steps. The scope of the process was expanded to more complex carbohydrates, such as glucose, lactose, sucrose, cellobiose, and inulin. Moreover, by fine-tuning the reaction conditions, the one-pot methodology in which we used inulin as the substrate was highly efficient if applied to various other important reactions, such as oxidation, reduction, and Cannizzaro and Baylis-Hillman reactions, which rendered the corresponding furan derivatives in high overall yields. [ABSTRACT FROM AUTHOR]

The synthesis of 5,5′-[oxybis(methylene)]bis-2-furfural (OBMF) from 5-hydroxymethylfurfural (5-HMF) was studied using bulk and alumina-supported Preyssler heteropolyacids (HPAs). The formation of OBMF was related to the amount of Brønsted acid sites, and the lowest yield of OBMF was obtained with supported HPAs. However, the Lewis acidity of the HPA supported on Al2O3 favored the formation of 2,5-dimethylfurane. The effects of solvent, catalyst loading, temperature, and reaction time on the selectivity to OBMF from 5-HMF were studied to optimize OBMF production using bulk Preyssler HPAs; a yield of 84 % to OBMF was obtained at 5 h and 343 K. These results demonstrate that bulk Preyssler HPA is a good candidate for OBMF synthesis under mild reaction conditions. [ABSTRACT FROM AUTHOR]

Novel polymeric solid catalysts, Au@(polythiophene-polythiophene oxides) (Au@PTh-PThO x, x=1-2), are prepared by chemical over-oxidative polymerization of thiophene in the presence of Au nanoparticles. These organocatalysts show significantly higher activity than their liquid analogs (i.e., DMSO, a frequently-used solvent and catalyst) for the catalytic dehydration of fructose into 5-hydroxymethylfurfural (HMF) and exhibit a complete inhibiting effect on HMF rehydration to undesired levulinic acid in low-boiling solvents. We demonstrate that the sulfoxide (-SO) group in Au@PTh-PThO x is more active than the sulfone (-SO2) group, although the formation of the former could be maximized by lowering the concentration of surfactant or improving the concentration of oxidant used in catalyst preparation. The highest HMF yields of 72.6 % and 39.7 % are achieved over Au@PTh-PThO catalyst in 1,4-dioxane and water, respectively. This catalyst could be completely recycled for use in water without activity loss. [ABSTRACT FROM AUTHOR]

Furan derivatives, such as 5-hydroxymethylfurfural (HMF) and furfural, obtained from renewable feedstocks are sustainable alternatives to petroleum-based building blocks for the manufacture of chemical products. This study shows for the first time the applicability of the catalytic fast pyrolysis (CFP) of glucose over tailored zeolites for the production of HMF and furfural. In particular, we demonstrate that Lewis-acid-containing faujasite zeolites attained by alkali metal-cation exchange exhibit combined yields to these platform molecules of 17 %. Despite the fact that further research is required on the process assessment at a larger scale and under more realistic conditions, CFP has potential for the production of biomass-derived commodity chemicals. [ABSTRACT FROM AUTHOR]

Exploring high-performance base-metal approaches for the sustainable production of chemicals from biomass is presently attracting immense interest and is truly important to promote their industrialized application. Herein, CoO x-N/C and α-MnO2 were combined as a base-metal catalyst that can achieve high yields of furan-2,5-dimethylcarboxylate (FDMC, 95.6 %) for the catalytic oxidative esterification of 5-hydroxymethylfurfural (HMF) without basic additive. The reaction proceeds through fast conversion of HMF to diformylfuran (DFF) with α-MnO2 and subsequent transformation of DFF to FDMC by CoO x-N/C. Quantitative X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations indicated that the pyridinic-N present in doped carbon could behave as a Lewis base to promote the abstraction of hydrogen for the oxidative esterification reaction. Consequently, CoO x-N/C is a high performance catalyst for the synthesis of FDMC from DFF in a neutral medium. [ABSTRACT FROM AUTHOR]

Deep eutectic solvents (DES) have emerged as promising solutions in many areas of chemistry. DES have many of the advantages of ionic liquids (e.g., low vapor pressure, nonflammability, tunability, etc.) but have lower costs, straightforward syntheses, and are often biodegradable. In this minireview, we discuss briefly the applications of DES in areas such as biotransformations, organocatalysis, and combinations of these that lead to multistep processes. DES can be used as solvents, cosolvents, performance additives for bio- and organocatalysis, tools to anchor organocatalysts selectively, or for extraction/separation. Several DES have been used both as solvents and organocatalysts at the same time. Given their promising features, it may be expected that many applications in these areas will appear in the coming years. [ABSTRACT FROM AUTHOR]

Levulinic acid (LA) was used as a catalyst for the first time to produce 5-hydroxymethylfurfural (5-HMF) and furfural (FAL) from pinewood and eucalyptus sawdust in a mono- or biphasic solvent system. 2-Methyltetrahydrofuran was used as a co-solvent with water in different ratios and temperatures (140-200 °C). Highest yields of 5-HMF and FAL were obtained at 180 °C and 2 h reaction time; however, at 160 °C, high yields of C6 and C5 sugars were obtained. Both hydrolysis and dehydration steps were accelerated in the MTHF/water biphasic system compared to pure aqueous phase. In particular, 1:2 w/w ratio of MTHF/water resulted in the highest yield of 5-HMF and FAL, whereas 2:1 w/w ratio showed highest yield of C6 and C5 sugars. Increasing the ratio of MTHF/water resulted in a higher fraction of dehydrated products extracted into the organic phase. LA as a catalyst is beneficial because it is miscible in both the phases and the presence of LA favours the equilibrium towards 5-HMF production. [ABSTRACT FROM AUTHOR]

Organic structure-directing agent (OSDA)-free zeolite Beta with high Al content exhibit remarkably high catalytic performance in the conversion of glucose to 5-hydroxymethylfurfural (HMF) by virtue of their appropriate acid properties; specifically, a sufficient number of Lewis acid sites were generated by calcination of the NH4-form zeolite, while the original Brønsted acid sites were substantially maintained, with both acid sites being in close proximity. The OSDA-free Beta catalyst, having a large number of Brønsted acid sites and sufficient Lewis acid sites, showed superior catalytic performance to a physical mixture of each type of acid catalyst with a similar number of each acid site. This behavior could be ascribed to the high reactivity and slow intrazeolitic diffusion of fructose. The structure of the Al species active in the isomerization of glucose to fructose is discussed based on relationships between catalytic activities and changes in Al species by the calcination, changes that were observed by 27Al magic-angle spinning (MAS) NMR and IR spectroscopies. [ABSTRACT FROM AUTHOR]

Sustainable supply of energy is one of the biggest challenges today. The conversion of energy from any abundant and renewable resources would be an ideal solution for this ever increasing demand of sustainable energy. Biomass provides a potential energy alternative through the platform chemical 5-hydroxymethylfurfural (HMF), which is considered as a sustainable source for liquid fuels and commodity chemicals. Herein, we report the synthesis of a nanoporous polytriphenylamine (PPTPA-1) having high surface area (1437 m2 g−1) by a simple one-step oxidative polymerization pathway. Upon sulfonation of PPTPA-1, the sulfonated polymer SPPTPA-1 showed very high surface acidity and it has been successfully employed as a solid acid catalyst for direct conversion of sugar to HMF. Both PPTPA-1 and SPPTPA-1 materials have distinct nanofiber morphologies and they are characterized thoroughly by using powder XRD, FTIR spectroscopy, 13C solid state magic-angle-spinning NMR spectrometry, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and N2 sorption techniques. We have optimized the HMF yields by using different carbohydrate sources and estimated the recycling efficiency of the catalyst. [ABSTRACT FROM AUTHOR]

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) is one of the most attractive reactions to establish biomass-based sustainable chemical processes. Supported Au and Pt catalysts have mainly been reported for this reaction, but excess amounts of base additives are generally required, which makes the process less green. Here, we demonstrate that Pt nanoparticles loaded on functionalized carbon nanotubes (CNTs) can catalyze the aerobic oxidation of HMF to FDCA in water without any base additives. Kinetic studies suggest a tandem reaction mechanism via 2,5-diformylfuran and 5-formylfurancarboxylic acid intermediates. It has been clarified that the oxygen-containing functional groups, in particular carbonyl/quinone and/or phenol groups, on CNT surfaces play crucial roles in FDCA formation. These functional groups could enhance the adsorption of HMF as well as the reaction intermediates from water and might facilitate hydrogen transfer. [ABSTRACT FROM AUTHOR]

Selective hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural were performed with carbon nanotube-supported bimetallic NiFe (NiFe/CNT) catalysts. The combination of Ni and Fe in an appropriate atomic ratio of Ni/Fe (2.0) significantly increased the selectivity to 2,5-furandimethanol or 2,5-dimethylfuran depending on the reaction temperature. The selectivities to 2,5-furandimethanol and 2,5-dimethylfuran were as high as 96.1 % at 383 K and 91.3 % at 473 K, respectively. The characterization results confirmed that bimetallic particles with sizes less than 7 nm were formed on the catalyst. Several key molecules related to 5-hydroxymethylfurfural transformation were used to investigate the product distribution and reaction pathway. The results indicated that the formation of NiFe alloy species is beneficial to the selective cleavage of the CO bond. Recycling experiments showed that the catalyst can be easily separated with a magnet and reused several times without significant loss of activity. [ABSTRACT FROM AUTHOR]

Herein, polyaniline-grafted vanadyl acetylacetonate [VO(acac)2] was prepared and used as an effective catalyst for the oxidation of 5-hydroxymethylfurfural (HMF) under atmospheric oxygen pressure. A high HMF conversion of 99.2% was obtained after 12 h at 110 ℃, and 2,5-diformylfuran (DFF) was obtained in 86.2% yield. More importantly, the one-pot conversion of fructose into DFF was also realized with two binary catalysts: Amberlyst-15 and polyaniline-VO(acac)2. Amberlyst-15 was used for the acid-catalyzed dehydration of fructose into HMF, followed by the in situ oxidation of HMF catalyzed by polyaniline-VO(acac)2. DFF could be obtained in a yield of 42.1% from fructose by using the one-pot reaction, whereas two consecutive steps gave a higher DFF yield of 71.1%. Owing to the facile catalyst preparation and low cost, this method showed a promising potential for the synthesis of DFF from abundant carbohydrates. [ABSTRACT FROM AUTHOR]

Nanosized phosphated TiO2 catalysts with different phosphate contents were synthesized and tested for the conversion of glucose to 5-hydroxymethylfurfural. The resulting materials were characterized by using N2-adsorption, XRD, inductively coupled plasma atomic emission spectroscopy, X-ray spectroscopy, TEM, temperature-programmed desorption of ammonia, and FTIR spectroscopy of pyridine adsorption techniques to determine their structural, bulk, surface, and acid properties. We found that TiO2 nanoparticles catalyzed this reaction under mild conditions in a water-butanol biphasic system. The incorporation of phosphorus into the TiO2 framework remarkably enhances the target product selectivity, which is ascribed to increased surface area, enhanced acidity, as well as thermal stability resulting from the TiOP bond formation. Under optimal reaction conditions, phosphated TiO2 was found to exhibit excellent catalytic performance, which resulted in 97 % glucose conversion and 81 % HMF yield after 3 h of reaction at 175 °C. More importantly, the catalyst showed good stability and could be reused for several reaction cycles. [ABSTRACT FROM AUTHOR]

The effect of salts and Brønsted acids on the Lewis acid (CrCl3 ⋅6 H2O)-catalyzed glucose dehydration to 5-hydroxymethylfurfural (HMF) in aqueous media are described. We show that the reaction with bromide salts in place of chlorides leads to higher HMF yields. The influence of salts can be attributed to the anions in solution, specifically to the bromide anions enhancing the fructose dehydration step. Additionally, we demonstrate that the reaction kinetics are governed strongly by acidity. Although the fructose dehydration step is accelerated by the addition of Brønsted acids, even on a catalytic scale, a significant retardation of the glucose conversion rate results in a substantial drop in HMF yields. The suppression in glucose-to-fructose isomerization rate with increasing acidity is ascribed to the restrained formation of the chromium-glucose chelate complex during the reaction. [ABSTRACT FROM AUTHOR]

Herein, we report a new reaction pathway to produce hydroxymethylfurfural (HMF) from cellulose under mild reaction conditions (140-190 °C; 5 m M H2SO4) in polar aprotic solvents (i.e. THF) without the presence of water. In this system, levoglucosan is the major decomposition product of cellulose, followed by dehydration to produce HMF. Glucose, levulinic acid, and formic acid are also produced as a result of side reactions with water, which is a by-product of dehydration. The turnover frequency for cellulose conversion increases as the water content in the solvent decreases, with conversion rates in THF being more than twenty times higher than those in water. The highest HMF yield from cellulose was 44 % and the highest combined yield of HMF and levulinic from cellulose was 53 %, which are nearly comparable to yields obtained in ionic liquids or biphasic systems. Moreover, the use of a low boiling point solvent, such as THF, facilitates recovery of HMF in downstream processes. [ABSTRACT FROM AUTHOR]

We have previously shown that 2,5-dimethylfuran (DMF) can be produced selectively from 5-hydroxymethylfurfural in up to 80 % yield via catalytic transfer hydrogenation with 2-propanol as a hydrogen donor and Ru/C as a catalyst. Herein, we investigate the active phase of the Ru/C catalyst by using extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and high-resolution TEM analyses. The results reveal that RuO2 is the dominant phase in the fresh (active) catalyst and is reduced to metallic Ru during the reaction with the hydrogen produced in situ from 2-propanol. The deactivation of the catalyst is correlated with the reduction of the surface of RuO2. Reactivity studies of individual phases (bulk RuO2 and reduced Ru/C catalysts) indicate that RuO2 mainly catalyzes the Meerwein-Ponndorf-Verley reaction of 5-hydroxymethylfurfural that produces 2,5-bis(hydroxymethyl)furan and the etherification of 2,5-bis(hydroxymethyl)furan or other intermediates with 2-propanol and that the reduced Ru/C catalyst has moderate hydrogenolysis activity for the production of DMF (30 % selectivity) and other intermediates (20 %). In contrast, a physical mixture of the two phases increases the DMF selectivity up to 70 %, which suggests that both metallic Ru and RuO2 are active phases for the selective production of DMF. The oxidation of the reduced Ru/C catalyst at different temperatures and the in situ hydrogen titration of the oxidized Ru/C catalysts were performed to quantify the bifunctional role of Ru and RuO2 phases. The mild oxidation treatment of the Ru/C catalyst at 403 K could activate the catalyst for the selective production of DMF in up to 72 % yield by generating a partially oxidized Ru catalyst. [ABSTRACT FROM AUTHOR]

A highly selective aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) into 2,5-diformylfuran (DFF) by employing a magnetically separable 2,2,6,6-tetramethylpiperidine- N-oxide (TEMPO) catalyst (Fe3O4@SiO2-TEMPO) under metal- and halogen-free conditions is described. Quantitative yields of DFF with excellent selectivities as high as >99 % were achieved under several versatile reaction conditions in a catalytic system consisting of Fe3O4@SiO2-TEMPO as a catalyst, tert-butyl nitrite as a co-catalyst, acetic acid as an additive, and toluene as a solvent. The results clearly demonstrate that the present catalytic system for selective conversion of 5-HMF into DFF is comparable or even superior to the many systems based on transition-metal catalysts. In addition, the catalyst can be successfully recovered and reused in five consecutive reaction runs by an external magnetic source with keeping its catalytic performance and selectivity. [ABSTRACT FROM AUTHOR]

The catalytic performance of zeolite-supported vanadia catalysts was examined for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in organic solvents such as N, N-dimethylformamide (DMF), methyl isobutyl ketone, toluene, trifluorotoluene and DMSO. Catalysts based on the four different zeolite supports H-beta, H-Y, H-mordenite, and H-ZSM-5 with 1-10 wt % vanadia loading were prepared and characterized by nitrogen physisorption, X-ray powder diffraction, scanning electron microscopy, ammonia temperature-programmed desorption, Raman spectroscopy and UV/Vis spectrophotometry. The H-beta zeolite catalysts were found to contain highly dispersed vanadium oxide species at all loadings, and provided the highest reaction selectivity towards DFF and the lowest metal leaching of the examined systems. In particular, 1 wt % V2O5/H-beta was found to be a stable, recyclable, and non-leaching catalyst for the production of DFF under mild conditions in DMF as solvent, although with low DFF yield. To increase the yield, oxidation of HMF at elevated pressures was also investigated with this catalyst. Under optimized conditions, a reaction selectivity towards DFF of >99 % at 84 % HMF conversion was obtained, albeit with some contribution from lixiviated species to the total catalyst activity. [ABSTRACT FROM AUTHOR]

A complementary computational and experimental study of the reactivity of Lewis acidic CrCl2, CuCl2 and FeCl2 catalysts towards glucose activation in dialkylimidazolium chloride ionic liquids is performed. The selective dehydration of glucose to 5-hydroxymethylfurfural (HMF) proceeds through the intermediate formation of fructose. Although chromium(II) and copper(II) chlorides are able to dehydrate fructose with high HMF selectivity, reasonable HMF yields from glucose are only obtained with CrCl2 as the catalyst. Glucose conversion by CuCl2 is not selective, while FeCl2 catalyst does not activate sugar molecules. These differences in reactivity are rationalized on the basis of in situ X-ray absorption spectroscopy measurements and the results of density functional theory calculations. The reactivity in glucose dehydration and HMF selectivity are determined by the behavior of the ionic liquid-mediated Lewis acid catalysts towards the initial activation of the sugar molecules. The formation of a coordination complex between the Lewis acidic Cr2+ center and glucose directs glucose transformation into fructose. For Cu2+ the direct coordination of sugar to the copper(II) chloride complex is unfavorable. Glucose deprotonation by a mobile Cl− ligand in the CuCl42− complex initiates the nonselective conversion. In the course of the reaction the Cu2+ ions are reduced to Cu+. Both paths are prohibited for the FeCl2 catalyst. [ABSTRACT FROM AUTHOR]

Cover Feature: Structure-Function Relationships in Fructose Dehydration to 5-Hydroxymethylfurfural under Mild Conditions by Porous Ionic Crystals Constructed with Analogous Building Blocks (ChemCatChem 16/2019) B The Cover Feature b shows a mesoporous ionic crystal composed of a Keggin-type polyoxometalate and a macrocation with Cr(III), catalyzing fructose dehydration to 5-hydroxymethylfurfural (HMF), which is an important bio-based chemical, without any apparent by-products at mild conditions. In their Communication, Sayaka Uchida and co-workers synthesize three ionic crystals with analogous building blocks but different pore sizes, and showed that the catalytic activities were basically dependent on the porosity. [Extracted from the article]