Catalytic dehydration of xylose to furfural: vanadyl pyrophosphate as source of active soluble species

The acid-catalysed, aqueous phase dehydration of xylose (a monosaccharide obtainable from hemicelluloses, e.g., xylan) to furfural was investigated using vanadium phosphates (VPO) as catalysts: the precursors, VOPO₄·2H₂O, VOHPO₄·0.5H₂O and VO(H₂PO₄)₂, and the materials prepared by calcination of these precursors, that is, γ-VOPO₄, (VO)₂P₂O₇ and VO(PO₃)₂, respectively. The VPO precursors were completely soluble in the reaction medium. In contrast, the orthorhombic vanadyl pyrophosphate (VO)₂P₂O₇, prepared by calcination of VOHPO₄·0.5H₂O at 550 °C/2 h, could be recycled by simply separating the solid acid from the reaction mixture by centrifugation, and no drop in catalytic activity and furfural yields was observed in consecutive 4 h-batch runs (ca. 53% furfural yield, at 170 °C). However, detailed catalytic/characterisation studies revealed that the vanadyl pyrophosphate acts as a source of active water-soluble species in this reaction. For a concentration of (VO)₂P₂O₇ as low as 5 mM, the catalytic reaction of xylose (ca. 0.67 M xylose in water, and toluene as solvent for the in situ extraction of furfural) gave ca. 56% furfural yield, at 170 °C/6 h reaction.     

Acidic cesium salts of 12-tungstophosphoric acid as catalysts for the dehydration of xylose into furfural

Cesium salts of 12-tungstophosphoric acid, Cs(x)H(3-x)PW(12)O(40) (Cs(x)PW), in the bulk form or supported on medium-pore MCM-41 (3.7 nm) or large-pore (9.6 nm) micelle-templated silicas are active solid acid catalysts for the cyclodehydration of xylose into furfural, in a toluene/water solvent system (T/W) or in dimethyl sulfoxide (DMSO). The catalytic results are comparable to those obtained using sulfuric acid, under similar reaction conditions. The initial activities increase in the order H(3)PW(12)O(40)相似文献   

Optimization of sugarcane bagasse conversion by hydrothermal treatment for the recovery of xylose

This work aims at the valorization of sugarcane bagasse by extracting xylose which is destined to the production of xylitol after purification and hydrogenation. Our approach consists in applying the principle of biorefinery to sugarcane bagasse because of its hemicellulose composition (particularly rich in xylan: (92%)). Optimizing of the thermal treatment was investigated. A treatment at 170 °C for 2 h was found optimal, with higher solubilzation of hemicellulose than that at 150 °C and lower degradation of sugar monomers than 190 °C. Recovery of xylose was high and the purity of xylose solution (78%) allows expecting an easy purification and separation of xylose before hydrogenation. Analysis of thermal hydrolyzates shows the presence of xylan oligomers and polymers with large distribution of DPs. This fraction should be submitted to enzymatic treatment to recover more xylose monomer.     

Theoretical insight into the conversion of xylose to furfural in the gas phase and water

The antioxidant properties of some phenolic Schiff bases in the presence of different reactive particles such as ^•OH, ^•OOH, (CH₂=CH−O−O^•), and ^-•O₂ were investigated. The thermodynamic values, ΔH _BDE, ΔH _IP, and ΔH _PA, were used for this purpose. Three possible mechanisms for transfer of hydrogen atom, concerted proton−electron transfer (CPET), single electron transfer followed by proton transfer (SET-PT), and sequential proton loss electron transfer (SPLET) were considered. These mechanisms were tested in solvents of different polarity. On the basis of the obtained results it was shown that SET-PT antioxidant mechanism can be the dominant mechanism when Schiff bases react with radical cation, while SPLET and CPET are competitive mechanisms for radical scavenging of hydroxy radical in all solvents under investigation. Examined Schiff bases react with the peroxy radicals via SPLET mechanism in polar and nonpolar solvents. The superoxide radical anion reacts with these Schiff bases very slowly.

     

Thermodynamics of the conversion of aqueous xylose to xylulose

The thermodynamics of the conversion of aqueous xylose to xylulose has been investigated using high-pressure liquid chromatography (HPLC) and microcalorimetry. The reaction was carried out in aqueous phosphate buffer over the pH range 6.8-7.4 using solubilized glucose isomerase with MgSO(4) as a cofactor. The temperature range over which this reaction was investigated was 298.15-342.15 K. A combined analysis of both the HPLC and microcalorimetric data leads to the following results at 298.15 K for the conversion process: DeltaG degrees = 4389 +/- 31 J mol(-1), DeltaH degrees = 16090 +/- 670 J mol(-1), and DeltaC(p) degrees = 40 +/- 23 J mol(-1) K(-1). The temperature dependence of the equilibrium constant for the reaction is expressed as R ln K = -4389/298.15 +16090[(1/298.15)-(1/T)]+40[(298.15/T)-1 + ln(T/298.15)]. Comparisons are made with literature data.     

Expression of Azotobacter vinelandii soluble transhydrogenase perturbs xylose reductase-mediated conversion of xylose to xylitol by recombinant Saccharomyces cerevisiae

Pyridine nucleotide transhydrogenase is a metabolic enzyme transferring the reducing equivalent between two nucleotide acceptors such as NAD⁺ and NADP⁺ for balancing the intracellular redox potential. Soluble transhydrogenase (STH) of Azotobacter vinelandii was expressed in a recombinant Saccharomyces cerevisiae strain harboring the Pichia stipitis xylose reductase (XR) gene to study effects of redox potential change on cell growth and sugar metabolism including xylitol and ethanol formation. Remarkable changes were not observed by expression of the STH gene in batch cultures. However, expression of STH accelerated the formation of ethanol in glucose-limited fed-batch cultures, but reduced xylitol productivity to 71% compared with its counterpart strain expressing xylose reductase gene alone. The experimental results suggested that A. vinelandii STH directed the reaction toward the formation of NADH and NADP⁺ from NAD⁺ and NADPH, which concomitantly reduced the availability of NADPH for xylose conversion to xylitol catalyzed by NADPH-preferable xylose reductase in the recombinant S. cerevisiae.     

Molecular-level insights into furfural hydrogenation intermediates over single-atomic Cu catalysts on magnesia and silica nanoclusters

This paper addresses the geometrical, charge, topological, and thermochemical data for the adsorption of the relevant furan species during hydrogenation of furfural to furfuryl alcohol over Cu/SiO₂ and Cu/MgO catalysts. The calculations indicated that the binding of Cu to magnesia was stronger than that on silica. The results also indicated that the formation of an alkoxide intermediate is more favoured than that of a hydroxyalkyl species. The binding energetic data were in general agreement with the geometrical parameters, electron densities, and molecular orbital energy levels. The QTAIM data revealed the closed-shell interactions between Cu and O atoms on both of the catalysts. Finally, the analysis of the partial charges on the atoms revealed that the Cu atom acquires a positive charge upon interaction with the carbonyl group owing to a π-back-donation from Cu to the C?=?O bond.     

Biotransformation of furfural and 5-hydroxymethyl furfural by enteric bacteria

Summary A survey was conducted with seventeen enteric bacterial strains (including the generaKlebsiella, Enterobacter, Escherichia, Citrobacter, Edwardsiella andProteus) to examine their ability to transform furfural and 5-hydroxymethyl furfural (5-MHF). The enteric bacteria were able to convert furfural to furfuryl alcohol under both aerobic and anaerobic conditions in a relatively short incubation time of 8 h. 5-HMF was transformed by all the enteric bacteria studied to an unidentified compound postulated to be 5-hydroxymethyl furfuryl alcohol, which had an absorbance maximum of 222 nm. These bacteria did not transform furfuryl alcohol or 2-furoic acid. The enteric bacteria did not use furfural, 5-HMF, furfuryl alcohol or 2-furoic acid as sole source of carbon and energy. Biotransformation of furfural and 5-HMF was accomplished by co-metabolism in the presence of glucose and peptone as main substrates. The rate of transformation was similar under both aerobic and anaerobic conditions. These transformations are likely to be of value in the detoxification of furfurals, and in their ultimate conversion to methane and CO₂ by anaerobic digestion.     

Dynamic consolidated bioprocessing for direct production of xylonate and shikimate from xylan by Escherichia coli

Numerous value-added chemicals can be produced using xylan as a feedstock. However, the product yields are limited by low xylan utilization efficiency, as well as by carbon flux competition between biomass production and biosynthesis. Herein, a dynamic consolidated bioprocessing strategy was developed, which coupled xylan utilization and yield optimization modules. Specifically, we achieved the efficient conversion of xylan to valuable chemicals in a fully consolidated manner by optimizing the expression level of xylanases and xylose transporter in the xylan utilization module. Moreover, a cell density-dependent, and Cre-triggered dynamic system that enabled the dynamic decoupling of biosynthesis and biomass production was constructed in the yield optimization module. The final shake flask-scale titers of xylonate, produced through an exogenous pathway, and shikimate, produced through an endogenous pathway, reached 16.85 and 3.2 g L⁻¹, respectively. This study not only provides an efficient microbial platform for the utilization of xylan, but also opens up the possibility for the large-scale production of high value-added chemicals from renewable feedstocks.     

秸秆预处理对土壤微生物量及呼吸活性的影响

冬小麦秸秆经8．0g·L^-1H2O2(pH11．0)溶液、12．5g·L^-1 NaOH溶液或H2SO4溶液浸泡8h并80℃烘干后,与无机N一起加入土壤,进行室内25℃恒温培养试验,在不同时间测定土壤微生物量C、N和CO2释放速率。结果表明,培养前期,秸秆预处理使土壤微生物量C数量增加了1．0～1．4倍,但降低了土壤微生物的呼吸活性;培养后期,NaOH和H2SO4处理使土壤微生物量C分别下降了28％和42％,但增加了土壤微生物的呼吸活性;H2O2处理则使土壤微生物量N增加90％;土壤微生物区系中的真菌比例在不同时刻有所增加,表明将秸秆预处理后施入土壤,将对土壤中微生物数量和呼吸活性产生一定影响。     

Engineering of a matched pair of xylose reductase and xylitol dehydrogenase for xylose fermentation by Saccharomyces cerevisiae

Metabolic engineering of Saccharomyces cerevisiae for xylose fermentation has often relied on insertion of a heterologous pathway consisting of nicotinamide adenine dinucleotide (phosphate) NAD(P)H-dependent xylose reductase (XR) and NAD⁺-dependent xylitol dehydrogenase (XDH). Low ethanol yield, formation of xylitol and other fermentation by-products are seen for many of the S. cerevisiae strains constructed in this way. This has been ascribed to incomplete coenzyme recycling in the steps catalyzed by XR and XDH. Despite various protein-engineering efforts to alter the coenzyme specificity of XR and XDH individually, a pair of enzymes displaying matched utilization of NAD(H) and NADP(H) was not previously reported. We have introduced multiple site-directed mutations in the coenzyme-binding pocket of Galactocandida mastotermitis XDH to enable activity with NADP⁺, which is lacking in the wild-type enzyme. We describe four enzyme variants showing activity for xylitol oxidation by NADP⁺ and NAD⁺. One of the XDH variants utilized NADP⁺ about 4 times more efficiently than NAD⁺. This is close to the preference for NADPH compared with NADH in mutants of Candida tenuis XR. Compared to an S. cerevisiae-reference strain expressing the genes for the wild-type enzymes, the strains comprising the gene encoding the mutated XDH in combination a matched XR mutant gene showed up to 50% decreased glycerol yield without increase in ethanol during xylose fermentation.     

Acid and enzymic hydrolysis of chaotropically pretreated millet stalk,acha and rice straws and conversion of the products to ethanol

Successful attempts have been made using acid and enzyme hydrolyses and chaotropic ions to solubilize some common but valuable sources of cellulose which are somewhat inaccessible to useful conventional degradative reagents/systems. Millet stalk, acha and rice straw (15% w/v) were incubated at 50°C for 1 h in 4 m hydrochloric acid saturated in lithium chloride, then hydrolysed for 45–60 s at 100°C, to yield maximally 79.2, 84.0 and 74.7% hydrolysis products for the three straws, respectively. Samples incubated in 1 m hydrochloric acid, containing saturated lithium chloride for 24 h at 27°C, washed thoroughly, freeze dried, then hydrolysed (0.5% w/v) with 0.4% w/v Trichoderma viride cellulase MVA 1284 [see 1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4] gave 16.4, 34.4 and 52.8% hydrolysis products for millet stalk, acha and rice straw, respectively. In general conversion of hydrolysed products direct to ethanol may be difficult as yeasts cannot grow in high salt concentrations resulting from acid hydrolyses. Growth response curves for two strains of Saccharomyces cerevisiae show the results of prior removal of chaotropic ions for successful growth of the organisms on the hydrolysates.     

The effect of xylose reductase genes on xylitol production by industrial Saccharomyces cerevisiae in fermentation of glucose and xylose

The co-production of xylitol and ethanol from agricultural straw has more economic advantages than the production of ethanol only. Saccharomyces cerevisiae, the most widely used ethanol-producing yeast, can be genetically engineered to ferment xylose to xylitol. In the present study, the effects of xylose-specificity, cofactor preference, and the gene copy number of xylose reductase (XR; encoding by XYL1 gene) on xylitol production of S. cerevisiae were investigated. The results showed that overexpression of XYL1 gene with a lower xylose-specificity and a higher NADPH preference favored the xylitol production. The copy number of XYL1 had a positive correlation with the XR activity but did not show a good correlation with the xylitol productivity. The overexpression of XYL1 from Candida tropicalis (CtXYL1) achieved a xylitol productivity of 0.83 g/L/h and a yield of 0.99 g/g-consumed xylose during batch fermentation with 43.5 g/L xylose and 17.0 g/L glucose. During simultaneous saccharification and fermentation (SSF) of pretreated corn stover, the strain overexpressing CtXYL1 produced 45.41 g/L xylitol and 50.19 g/L ethanol, suggesting its application potential for xylitol and ethanol co-production from straw feedstocks.     

Production of furans from rice straw by single-phase and biphasic systems

5-Hydroxymethylfurfural (HMF) and furfural, both of which can be derived from renewable sources, are key components for the production of different chemicals and fuels. In this study, rice straw, a cheap, abundant, and mainly unused agricultural waste, is converted to furans by a dilute acid hydrolysis process. The highest yield of HMF in a single-phase hydrolysis was 15.3 g/kg straw, attained at 180 °C during 3 h with 0.5% sulfuric acid, while the maximum yield of furfural, 59 g/kg straw, was obtained at 150 °C during 5 h. Different extracting solvents, including 2-PrOH, 1-BuOH, methyl isobutyl ketone (MIBK), and acetone at 180 °C for 3 h as well as tetrahydrofuran (THF) at 150 °C for 5 h were examined in biphasic systems. Use of the solvents generally improved the production of HMF compared to the single aqueous phase process. The best results of HMF production, more than 59 g/kg straw, were obtained in the systems containing either 2-PrOH or 1-BuOH. Using THF as an extracting solvent, a relatively high furfural yield, 118.2 g/kg straw, was obtained, and 96% of furfural produced in this system was extracted into THF during the process.     

Characterization of the degree of polymerization of xylooligomers produced by flowthrough hydrolysis of pure xylan and corn stover with water

Mechanisms that control xylan removal during pretreatment of lignocellulosic biomass are not well understood. For example, although hemicellulose hydrolysis is virtually always assumed to follow first-order homogeneous kinetics, the increase in xylan removal with flow rate for flowthrough pretreatment systems is inconsistent with the predictions for such models, and better information is needed to understand the causes of such discrepancies. Thus, new methods were developed to follow the fate of xylooligomers with degrees of polymerization of up to 30, a range not possible before, for water-only flowthrough pretreatment of oat spelt xylan and corn stover for temperatures of 200–240 °C. Material balances based on the oligomer release profiles produced by batch and flowthrough operations could be closed, suggesting the methods were quite accurate. However, the results also showed that increasing the flow rate from 0 to 2 and then 25 mL/min affected the size distribution of the xylan oligomers (DP < 30) released from corn stover but not from oat spelt xylan and also increased overall hemicellulose sugar solubilization. One explanation for these difference is that lignin and lignin–xylan compounds in particular play an important role in the hydrolysis of lignocellulosic biomass.     

Production of xylitol by recombinant Saccharomyces cerevisiae containing xylose reductase gene in repeated fed-batch and cell-recycle fermentations

Xylitol is a well-known sugar substitute with low-calorie and anti-cariogenic characteristics. An effort of biological production of xylitol from xylose was made in repeated fed-batch and cell-recycle fermentations of recombinant Saccharomyces cerevisiae BJ3505/δXR harboring the xylose reductase gene from Pichia stipitis. Batch fermentation with 20 g/l xylose and 18 g/l glucose resulted in 9.52 g/l dry cell mass, 20.1 g/l xylitol concentration and approximately 100% conversion yield. Repeated fed-batch operation to remove 10% of culture broth and to supplement an equal volume of 200 g/l xylose was designed to improve xylitol production. In spite of a sudden drop of cell concentration, an increase in dry cell mass led to high accumulation of xylitol at 48.7 g/l. To overcome loss of xylitol-producing biocatalysts in repeated fed-batch fermentation, cell-recycle equipment of hollow fiber membrane was implemented into a xylitol production system. Cell-recycle operation maintained concentration of the recombinant cells high inside a bioreactor. Final dry cell mass of 22.0 g/l, 116 g/l xylitol concentration, 2.34 g/l h overall xylitol productivity were obtained in cell-recycle fermentation supplemented with xylose and yeast extract solution, which were equivalent to 2.3-, 5.8- and 3.8-fold increases compared with the corresponding values of batch-type xylitol production parameters.     

Optimized extraction by cetyl trimethyl ammonium bromide reversed micelles of xylose reductase and xylitol dehydrogenase from Candida guilliermondii homogenate

The intracellular enzymes xylose reductase (XR, EC 1.1.1.21) and xylitol dehydrogenase (XD, EC 1.1.1.9) from Candida guilliermondii, grown in sugar cane bagasse hydrolysate, were separated by reversed micelles of cetyl trimethyl ammonium bromide (CTAB) cationic surfactant. An experimental design was employed to optimize the extraction conditions of both enzymes. Under these conditions (temperature = 5 degree C, hexanol: isooctane proportion = 5% (v/v), 22 %, surfactant concentration = 0.15M, pH = 7.0 and electrical conductivity = 14 mScm(-1)) recovery values of about 100 and 80% were achieved for the enzymes XR and XD, respectively. The purity of XR and XD increased 5.6- and 1.8-fold, respectively. The extraction process caused some structural modifications in the enzymes molecules, as evidenced by the alteration of K(M) values determined before and after extraction, either in regard to the substrate (up 35% for XR and down 48% for XD) or cofactor (down 29% for XR and up 11% for XD). However, the average variation of V(max) values for both enzymes was not higher than 7%, indicating that the modified affinity of enzymes for their respective substrates and cofactors, as consequence of structural modifications suffered by them during the extraction, are compensated in some extension. This study demonstrated that liquid-liquid extraction by CTAB reversed micelles is an efficient process to separate the enzymes XR and XD present in the cell extract, and simultaneously increase the enzymatic activity and the purity of both enzymes produced by C. guilliermondii.     

Fermentation of xylose and hemicellulose hydrolysates by an ethanol-adapted culture ofBacteroides polypragmatus

Bacteroides polypragmatus type strain GP4 was adapted to grow in the presence of 3.5% (w/v) ethanol by successive transfers into 1% (w/v)d-xylose media supplemented with increasing concentrations of ethanol. The maximum specific growth rate of the ethanol-adapted culture (=0.30 h^-1) was not affected by up to 2% (w/v) ethanol but that of the non-adapted strain declined by about 50%. The growth rate of both cultures was limited by nutrient(s) contained in yeast extract. The ethanol yield of the adapted culture (1.01 mol/mol xylose) was higher than that (0.80 mol/mol xylose) of the non-adapted strain. The adapted culture retained the ability to simultaneously ferment pentose and hexose sugars, and moreover it was not inhibited by xylose concentrations of 7–9% (w/v). This culture also readily fermented hemicellulose hydrolysates obtained by mild acid hydrolysis of either hydrogen fluoride treated or steam exploded Aspen wood. The ethanol yield from the fermentation of the hydrolysates was comparable to that obtained from xylose.This paper is issued as NRCC No. 26338     

The enhancement of xylose monomer and xylotriose degradation by inorganic salts in aqueous solutions at 180 degrees C

The inorganic salts KCl, NaCl, CaCl2, MgCl2, and FeCl3, and especially the latter, significantly increased xylose monomer and xylotriose degradation in water heated to 180 degrees C with unaccountable losses of xylose amounting to as high as 65% and 78% for xylose and xylotriose, respectively, after 20 min incubation with 0.8% FeCl3. Furthermore, losses of both xylose and xylotriose were well described by first order homogeneous kinetics, and the rate constants for xylose and xylotriose disappearance increased 6- and 49-fold, respectively, when treated with 0.8% FeCl3 solution compared to treatment with just pressurized hot water at the same temperature. Although the addition of these inorganic salts produced a significant drop in pH, the degradation rates with salts were much faster than could be accounted for by a pH change. For example, the rate constants for the disappearance of xylose and xylotriose with 0.8% FeCl3 were 3-fold and 7-fold greater, respectively, than for treatment with very dilute sulfuric acid at the same pH. In addition, xylose losses were greater than could be accounted for by just furfural production, suggesting that other degradation products were also formed, and xylose losses to unidentified compounds increased significantly with the addition of FeCl3. The unidentified compounds could be formed through aqueous furfural resinification and condensation reactions that are accelerated by FeCl3, but the actual mechanisms are still not clear.