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Levulinic esters, synthesized by the esterification of biomass-derived levulinic acid with various alcohols, is an important chemical that plays an essential role in the fields of biomass fuel additives, organic synthesis, and high value-added products. In the present work, the catalytic esterification of levulinic acid with n-butyl alcohol was selected as a typical model reaction to investigate the catalytic performance of an inexpensive commercial catalyst, titanium oxide nanoparticles. The influences of reaction time, reaction temperature, and catalyst loading on the conversion of levulinic acid to n-butyl levulinate were systematically examined through single-factor experiments. Additionally, the optimization of the reaction conditions was further investigated by a Box–Behnken design in response to the surface methodology. The desired product, n-butyl levulinate, with a good yield (77.6%) was achieved under the optimal conditions (reaction time of 8 h, reaction temperature of 120 °C, and catalyst dosage of 8.6 wt.%) when using titanium oxide nanoparticles as catalysts. Furthermore, it was found that addition of water to the catalytic system facilitated the reaction process, to some extent. This study reveals that the nanosized TiO2 material, as an efficient solid acid catalyst, had good catalytic performance and stability for the esterification of levulinic acid after six consecutive uses. [ABSTRACT FROM AUTHOR]

Hydrogenase is the key point of H 2 -based biotechnology. However, the O 2 -sensitivity largely hinders its applications in biofuel cells and biological H 2 production. Therefore, substantial breakthrough on understanding the molecular basis of O 2 -sensitivity and developing more O 2 -tolerant hydrogenases are urgently required. In this study, we found adding extra cysteines to the vicinity of the proximal Fe–S cluster to the NiFe active centre could largely enhance oxygen tolerance of hydrogen-evolving hydrogenase 3 from Klebsiella oxytoca HP1 ( Ko Hyd3), through homologous sequence comparison and site-directed mutagenesis. Ratio of aerobic hydrogen yield to anaerobic hydrogen yield (RHH) of Gly47Cys (Gly47 was replaced with Cys47), Gly50Cys, Gly113Cys, Gly120Cys and Gly50Cys–Gly120Cys (double exchange) were increased by 46.99%, 42.15%, 59.19%, 44.74% and 78.72%, respectively, comparing with that of wild type. Moreover, TiO 2 - Ko Hyd3 (Gly47Cys, Gly50Cys, Gly113Cys, Gly120Cys and Gly50Cys–Gly120Cys) particles acted well in UV light-driven H 2 production from water. These results revealed that extra cysteines nearby Fe–S clusters had significant effects on oxygen tolerance of Ko Hyd3. It also provided a promising way to produce O 2 -tolerant hydrogenase as biocatalysts in biofuel cells or H 2 production by photolysis of water. [ABSTRACT FROM AUTHOR]

Biofuels production from lignocellulose hydrolysates by microbe fermentation has merited attention because of the mild reaction conditions involved and the clean nature of the process. In this work, xylulokinase (XK) and xylose isomerase (XI) were overexpressed in Klebsiella oxytoca HP1 to enhance hydrogen production by the fermentation of xylose. The recombinant strains exhibited higher enzyme activity of XI or XK compared with the wild strain. Hydrogen production from pure xylose, xylose/glucose mixtures and bamboo stalk hydrolysate was significantly enhanced with the overexpression of XI and XK in K. oxytoca HP1 in terms of total hydrogen yield (THY), hydrogen yield per mole substrate (HYPM) and hydrogen production rate (HPR). The HYPM of K. oxytoca HP1/xylB and K. oxytoca HP1/xylA reached 1.93 ± 0.05 and 2.46 ± 0.05 mol H2/mol xylose, respectively in pure xylose, while the value for the wild strain was 1.68 ± 0.04 mol H2/mol xylose. The xylose consumption rate (XCR) for the recombinant strain was significantly higher than that for the wild strain, particularly in the early stage of fermentation. Relative to the wild type, hydrogen yield (HY) from 1 g of preprocessed bamboo powder of HP1/xylB and HP1/xylA increased by 33.04 and 41.31%, respectively. It was concluded that overexpression of XK or XI was able to promote hydrogen production from xylose and xylose/glucose mixtures by simultaneously increasing the utilization efficiency of xylose and weakening the inhibitory effect of glucose on xylose use. In addition, the results indicated that overexpression technology was an effective way to further increase hydrogen production from lignocellulosic hydrolysates. [ABSTRACT FROM AUTHOR]