木质素在不同Ru/Nb基催化剂上解聚和脱氧加氢制芳烃化合物（英文）
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摘要
木质素高效转化为芳烃是木质素利用的一个非常重要的过程,一般通过解聚和脱氧加氢反应来实现.我们曾发现NbOx物种在木质素及其模型化合物的C–O键活化和断裂的过程中发挥了至关重要的作用.本文分别选择两种商业的铌基材料(HY-340和NbPO-CBMM)和实验室自制的具有层状结构的氧化铌材料(Nb_2O_5-Layer)为载体,制备了负载型Ru催化剂,将其用于木质素及其模型化合物的催化转化.同时,为了尽量避免Ru与铌基载体相互作用的影响,制备了较为均匀的Ru纳米胶粒并吸附于铌基载体上,得到Ru@铌基催化剂,并用于木质素模型化合物—对甲酚的催化转化反应中.研究表明,木质素在所有的Ru/铌基催化剂上都可以得到比较高的C7–C9碳氢化合物的收率.其中,在Ru/Nb_2O_5-Layer催化剂上C_7–C_9碳氢化合物的摩尔收率为99.1%,选择性为88.0%.采用X射线衍射、N_2吸附-脱附、热重分析、吡啶吸附红外光谱(Py-FTIR)、X射线光电子能谱(XPS)以及CO化学吸附等技术表征了Ru/铌基催化剂的性能与铌基材料的性质、金属Ru颗粒大小及其表面电子状态之间的关系.Py-FTIR结果表明, Nb_2O_5-Layer材料上几乎没有Br?nsted酸,但含有最多的Lewis酸,而NbPO-CBMM上的Lewis酸量最低.结合催化性能数据发现,单体产物收率与铌基载体的Lewis量成正相关关系,其中以Ru/Nb_2O_5-Layer催化剂上最高.CO化学吸附和XPS结果表明,不同的铌基载体负载的金属Ru的分散度和电子状态都有差异.在Ru/Nb_2O_5-Layer上Ru的分散度最好,颗粒尺寸最小,木质素转化得到的芳烃选择性最好;在Ru/HY-340和Ru/NbPO-CBMM上,虽然Ru的分散度相近,但其表面电子状态不同, Ru/HY-340上的金属态Ru带有更多的正电荷δ+,其得到芳烃的选择性高于Ru/Nb PO-CBMM.由此可见, Ru的颗粒尺寸和表面电子状态会影响芳烃选择性.对甲酚催化转化反应结果表明, Ru颗粒大小相同时,铌基载体性质会影响对甲酚转化率;而同一铌基载体上, Ru颗粒大小则影响芳烃选择性,较小的Ru颗粒有利于芳烃的生成.
Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbOx species played a key role in the activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial niobic acid(HY-340), niobium phosphate(Nb PO-CBMM) and lab-made layered niobium oxide(Nb_2O_5-Layer) were chosen as supports to study the effect of Br?nsted and Lewis acids on the activation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C_7–C_9 hydrocarbons(82.3%–99.1%). What's more, Ru/Nb_2O_5-Layer affords the best mole yield of C_7–C_9 hydrocarbons and selectivity for C_7–C_9 aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.
Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbOx species played a key role in the activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial niobic acid(HY-340), niobium phosphate(Nb PO-CBMM) and lab-made layered niobium oxide(Nb_2O_5-Layer) were chosen as supports to study the effect of Br?nsted and Lewis acids on the activation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C_7–C_9 hydrocarbons(82.3%–99.1%). What's more, Ru/Nb_2O_5-Layer affords the best mole yield of C_7–C_9 hydrocarbons and selectivity for C_7–C_9 aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.
引文
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