Yao Fu

Hydrolysis of cellulose to glucose by solid acid catalysts

As the main component of lignocelluloses, cellulose is a biopolymer consisting of many glucose units connected through β-1,4-glycosidic bonds. Breakage of the β-1,4-glycosidic bonds by acids leads to the hydrolysis of cellulose polymers, resulting in the sugar molecule glucose or oligosaccharides. Mineral acids, such as HCl and H2SO4, have been used in the hydrolysis of cellulose. However, they suffer from problems of product separation, reactor corrosion, poor catalyst recyclability and the need for treatment of waste effluent. The use of heterogeneous solid acids can solve some of these problems through the ease of product separation and good catalyst recyclability. This review summarizes recent advances in the hydrolysis of cellulose by different types of solid acids, such as sulfonated carbonaceous based acids, polymer based acids and magnetic solid acids. The acid strength, acid site density, adsorption of the substance and micropores of the solid material are all key factors for effective hydrolysis processes. Methods used to promote reaction efficiency such as the pretreatment of cellulose to reduce its crystallinity and the use of ionic liquids or microwave irradiation to improve the reaction rate are also discussed.

A one-pot approach for conversion of fructose to 2,5-diformylfuran by combination of Fe3O4-SBA-SO3H and K-OMS-2

We have demonstrated an efficient, selective and environmentally benign heterogeneous catalyst (K-OMS-2) for aerobic oxidation of 5-HMF to 2,5-DFF. In addition, a combination of Fe3O4-SBA-SO3H and K-OMS-2 successfully catalyzed direct synthesis of 2,5-DFF from fructose via acid-catalyzed dehydration and successive aerobic oxidation in one-pot reaction.

Hammett equation and generalized Pauling's electronegativity equation

Substituent interaction energy (SIE) was defined as the energy change of the isodesmic reaction X-spacer-Y + H-spacer-H --> X-spacer-H + H-spacer-Y. It was found that this SIE followed a simple equation, SIE(X,Y) = -ksigma(X)sigma(Y), where k was a constant dependent on the system and sigma was a certain scale of electronic substituent constant. It was demonstrated that the equation was applicable to disubstituted bicyclo[2.2.2]octanes, benzenes, ethylenes, butadienes, and hexatrienes. It was also demonstrated that Hammetts equation was a derivative form of the above equation. Furthermore, it was found that when spacer = nil the above equation was mathematically the same as Paulings electronegativity equation. Thus it was shown that Hammetts equation was a derivative form of the generalized Paulings electronegativity equation and that a generalized Paulings electronegativity equation could be utilized for diverse X-spacer-Y systems. In addition, the total electronic substituent effects were successfully separated into field/inductive and resonance effects in the equation SIE(X,Y) = -k(1)F(X)F(Y) - k(2)R(X)R(Y) - k(3)(F(X)R(Y) + R(X)F(Y)). The existence of the cross term (i.e., F(X)R(Y) and R(X)F(Y)) suggested that the field/inductive effect was not orthogonal to the resonance effect because the field/inductive effect from one substituent interacted with the resonance effect from the other. Further studies on multi-substituted systems suggested that the electronic substituent effects should be pairwise and additive. Hence, the SIE in a multi-substituted system could be described using the equation SIE(X1, X2, ..., Xn) = Sigma(n-1)(i=1)Sigma(n)(j=i+1)k(ij)sigma(X)isigma(X)j.

Catalytic conversion of biomass-derived levulinic acid to valerate esters as oxygenated fuels using supported ruthenium catalysts

The development of the catalytic conversion of biomass-based platform molecules into oxygenated fuel molecules is of great significance in order to reduce the dependence on fossil resources and to solve environmental problems. Alkyl valerate esters were proven to have the potential to be renewable additives of gasoline and diesel. In this work, we studied the hydrogenation of levulinic acid (LA) to valerate esters over supported Ru catalysts, and found that the acidity was an important factor for the catalyst performance. A bifunctional catalyst Ru/SBA-SO3H was developed as an active catalyst, and a highest yield of 94% to ethyl valerate (EV) was achieved. The catalyst was characterized by nitrogen adsorption/desorption methods, X-ray power diffraction (XRD), transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The effects of reaction conditions were comprehensively investigated and probable reaction pathways were proposed and verified. The conversion of LA to various alkyl valerate esters can also be catalyzed by the bifunctional catalyst. In addition, supported Cu and Ni catalysts were also screened under similar reaction conditions as Ru-based catalysts, and the combination of Ni/SBA-15 and SBA-SO3H exhibited activity for the conversion of LA to EV.

Catalytic Conversion of Cellulose into Levulinic Acid by a Sulfonated Chloromethyl Polystyrene Solid Acid Catalyst

A novel solid acid catalyst, sulfonated chloromethyl polystyrene (CP) resin (CP‐SO3H‐1.69), was synthesized by partially substituting chlorine groups (Cl) of CP resin with sulfonic group(SO3H). This new type solid acid contains not only acid sites, but also cellulose‐binding sites (Cl). A high yield of levulinic acid up to 65.5 % was obtained by converting microcrystalline cellulose over CP‐SO3H‐1.69. The high catalytic activity of CP‐SO3H‐1.69 was attributed to high amount of sulfonic group and chlorine on the catalyst, which is essential to keep the catalyst with great affinity to substrate.

Linked strategy for the production of fuels via formose reaction

Formose reaction converts formaldehyde to carbohydrates. We found that formose reaction can be used linking the biomass gasification with the aqueous-phase processing (APP) to produce liquid transportation fuel in three steps. First, formaldehyde from syn-gas was converted to triose. This was followed by aldol condensation and dehydration to 4-hydroxymethylfurfural (4-HMF). Finally, 4-HMF was hydrogenated to produce 2,4-dimethylfuran (2,4-DMF) or C9-C15 branched-chain alkanes as liquid transportation fuels. In the linked strategy, high energy-consuming pretreatment as well as expensive and polluting hydrolysis of biomass were omitted, but the high energy recovery of APP was inherited. In addition, the hexoketoses via formose reaction could be converted to HMFs directly without isomerization. A potential platform molecule 4-HMF was formed simultaneously in APP.

Selective hydrodeoxygenation of lignin-derived phenols to alkyl cyclohexanols over a Ru-solid base bifunctional catalyst

Cyclohexanol and alkyl cyclohexanol are important chemical intermediates. It is meaningful to prepare cyclohexanols from non-fossil-based biomass. Here we report Ru/ZrO2–La(OH)3, a metal-solid base bifunctional catalyst, to show its excellent performance on the partial hydrodeoxygenation of lignin-derived phenols. Guaiacol could be converted to cyclohexanol with a 91.6% yield in water. Alkyl phenols with one or two methoxy groups were converted into alkyl cyclohexanols with yields over 86.9%. The catalyst had good activity of removing a methoxy group and retaining a hydroxyl group. In this catalyst, Zr and La interacted with each other to form a mixed (hydr)oxide, thus making ZrO2–La(OH)3 a stable support. Ru was highly dispersed on the ZrLa support. The pathway from guaiacol to cyclohexanol was investigated and proposed as two parallel ways, demethoxylation followed by hydrogenation (I), the saturation of the aromatic ring through hydrogenation and then demethoxylation through direct hydrogenolysis (II).

Towards the sustainable production of pyridines via thermo-catalytic conversion of glycerol with ammonia over zeolite catalysts

In this study, renewable pyridines could be directly produced from glycerol and ammonia via a thermo-catalytic conversion process with zeolites. The major factors, including catalyst, temperature, weight hourly space velocity (WHSV) of glycerol to catalyst, and the molar ratio of ammonia to glycerol, which may affect the pyridine production, were investigated systematically. The optimal conditions for producing pyridines from glycerol were achieved with HZSM-5 (Si/Al = 25) at 550 °C with a WHSV of glycerol to catalyst of 1 h−1 and an ammonia to glycerol molar ratio of 12 : 1. The carbon yield of pyridines was up to 35.6%. The addition of water to the feed decreased the pyridine yield, because water competed with glycerol on the acid sites of the catalyst and therefore impacted the acidity of the catalyst. After five reaction/regeneration cycles, a slight deactivation of the catalyst was observed. The catalysts were investigated by N2 adsorption/desorption, XRD, XRF and NH3-TPD and the results indicated that the deactivation could be due to the structure changes and the acid site loss of the catalyst. The reaction pathway from glycerol to pyridines was studied and the main pathway should be that glycerol was initially dehydrated to form acrolein and some by-products such as acetaldehyde, acetol, acetone, etc., and then acrolein, a mixture of acrolein and acetaldehyde, or other by-products reacted with ammonia to form imines and finally pyridines.

Direct production of indoles via thermo-catalytic conversion of bio-derived furans with ammonia over zeolites

In this study we demonstrate that indoles can be directly produced by thermo-catalytic conversion of bio-derived furans with ammonia over zeolite catalysts. MCM-41, β-zeolite, ZSM-5 (Si/Al = 50) and HZSM-5 catalysts with different Si/Al ratios (Si/Al = 25, 50, 63, 80) were screened and HZSM-5 with an Si/Al ratio of 25 showed the best reactivity for indole production due to the desired pore structure and acidity. Temperature displayed a significant effect on the product distribution. The maximum yield of indoles was obtained at moderate temperatures around 500 °C. The weight hourly space velocity (WHSV) of furan to catalyst investigation indicated that a lower WHSV could cause the overreaction of furan over the catalyst to produce more aniline and pyridines, while a higher WHSV would cause the incomplete reaction of furan. Because ammonia served as both a reactant and a carrier gas, to supply sufficient reactants and keep the desired reaction time, an appropriate ammonia to furan molar ratio was important for furan conversion to indoles. Under optimized conditions, the highest total carbon yield of indoles and their selectivity in the N-containing chemicals were 32% and 75%, respectively. 2-Methylfuran and the mixture of furan and 2-methylfuran were also studied, which demonstrated that more alkyl indoles could be selectively obtained via the coupling reaction of different bio-derived furans. Ring opening of the furan is a more favorable mechanism compared to the Diels–Alder mechanism, and the pyrrole reacting with furan is the more favorable pathway compared to pyrrole reacting with pyrrole based on our experimental and theoretical calculations.

Selective Hydrogenation of Phenol to Cyclohexanone over Pd–HAP Catalyst in Aqueous Media

The production of pure cyclohexanone under mild conditions over catalysts with high reactivity, selectivity, compatibility, stability, and low cost is still a great challenge. Here we report a hydroxyapatite‐bound palladium catalyst (Pd–HAP) to demonstrate its excellent performance on phenol hydrogenation to cyclohexanone. Based on catalyst characterization, the Pd nanoclusters (≈0.9 nm) are highly dispersed and bound to phosphate in HAP. Only basic active sites on HAP surface are detected. At 25 °C and ambient H2 pressure in water, phenol can be 100 % converted into cyclohexanone with 100 % selectivity. This system shows a universal applicability to temperature, pH, solvent, low H2 purity, and pressure. The catalyst reveals high stability to be recycled without deactivation or morphology change; and Pd nano‐clusters barely aggregate even at 400 °C. During the reaction, HAP adsorbs phenol, and Pd nanoclusters activate and spillover H2. The mechanism is also investigated, proposed, and verified.