介孔磷酸铌一锅法高效催化木糖制备糠醛

木糖转化到糠醛一般包括两步: 首先在酶、碱或路易斯(L)酸的催化作用下异构化木糖到木酮糖, 接下来木酮糖在酸的作用下脱水得到糠醛. 针对木糖水相脱水一步制备糠醛, 利用十六烷基三甲基溴化铵(CTAB)为模板剂, 借助软模板合作策略制备了一种抗水的新型固体酸催化剂, 介孔磷酸铌, 并利用X射线衍射(XRD)、N₂吸脱附、透射电镜(TEM)、氨气程序升温脱附(NH₃-TPD)和吡啶吸附傅里叶变换红外(Py-FTIR)光谱对材料的结构和酸性质进行了表征. 研究发现介孔磷酸铌不仅具有很高的比表面积(>200 m²·g^-1), 比较窄的孔径分布(3.5nm), 同时还具有很强的L酸性和布朗斯特(B)酸性. 通过L酸催化的木糖异构化为木酮糖/来苏糖和B酸催化的木酮糖/来苏糖进一步脱水得到糠醛, 实现了一步由木糖到糠醛的高效转化. 为了优化反应条件, 考察了水溶液中反应温度、投料质量比及反应时间对木糖转化率和糠醛收率的影响, 在最佳的反应条件下, 木糖的转化率为96.5%, 糠醛的收率达49.8%. 进一步地, 为了提高收率且易于分离, 利用4-甲基-2-戊酮(MIBK)/NaCl水溶液(体积比为7:3)作为反应混合溶剂, 使糠醛收率提高到68.4%.

One-Pot Catalytic Conversion of Xylose to Furfural on Mesoporous Niobium Phosphate

The conversion of xylose to furfural normally involves two steps: the isomerization of xylose to xylulose catalyzed by an enzyme, a base or a Lewis acid, followed by the acid-catalyzed dehydration of xylulose to furfural. To allow a more efficient single-step conversion, a new water-tolerant solid acid catalyst, mesoporous niobium phosphate was synthesized. This synthesis was performed using a soft template approach, with cetyltriethylammonium bromide (CTAB) as the template. The structure and properties of the catalyst thus synthesized were investigated by X-ray diffraction (XRD), N₂ sorption, transmission electron microscopy (TEM), temperature-programmed desorption of NH₃ (NH₃-TPD), and pyridine sorption FTIR (Py-FTIR). These studies determined that the niobium phosphate not only had a large surface area (>200 m²·g^-1) and narrow pore size distribution (3.5 nm), but also had relatively strong Lewis and Brønsted acidity. This catalyst was found to be capable of producing furfural via a simple one-pot process, including the isomerization of xylose to xylulose and subsequent dehydration. The influence of several variables including temperature, mass ratio of xylose/catalyst, and reaction time on the extent of xylose conversion and furfural yield were studied. Under optimal conditions, the yield of furfural in aqueous solution reached 49.8% with 96.5% xylose conversion. It was further determined that both the yield and the separation of furfural could be improved by employing a methyl isobutyl ketone (MIBK)/water (volume ratio 7:3) biphase containing NaCl in the aqueous phase, resulting in a 68.4% yield.