Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.
ChemSusChem. 2017 May 22;10(10):2226-2234. doi: 10.1002/cssc.201700297. Epub 2017 Apr 28.
Reductive catalytic fractionation of biomass has recently emerged as a powerful lignin extraction and depolymerization method to produce monomeric aromatic oxygenates in high yields. Here, bifunctional molybdenum-based polyoxometalates supported on titania (POM/TiO ) are shown to promote tandem hydrodeoxygenation (HDO) and alkylation reactions, converting lignin-derived oxygenated aromatics into alkylated benzenes and alkylated phenols in high yields. In particular, anisole and 4-propylguaiacol were used as model compounds for this gas-phase study using a packed-bed flow reactor. For anisole, 30 % selectivity for alkylated aromatic compounds (54 % C-alkylation of the methoxy groups by methyl balance) with an overall 72 % selectivity for HDO at 82 % anisole conversion was observed over H PMo O /TiO at 7 h on stream. Under similar conditions, 4-propylguaiacol was mainly converted into 4-propylphenol and alkylated 4-propylphenols with a selectivity to alkylated 4-propylphenols of 42 % (77 % C-alkylation) with a total HDO selectivity to 4-propylbenzene and alkylated 4-propylbenzenes of 4 % at 92 % conversion (7 h on stream). Higher catalyst loadings pushed the 4-propylguaiacol conversion to 100 % and resulted in a higher selectivity to propylbenzene of 41 %, alkylated aromatics of 21 % and alkylated phenols of 17 % (51 % C-alkylation). The reactivity studies coupled with catalyst characterization revealed that Lewis acid sites act synergistically with neighboring Brønsted acid sites to simultaneously promote alkylation and hydrodeoxygenation activity. A reaction mechanism is proposed involving activation of the ether bond on a Lewis acid site, followed by methyl transfer and C-alkylation. Mo-based POMs represent a versatile catalytic platform to simultaneously upgrade lignin-derived oxygenated aromatics into alkylated arenes.
生物质的还原催化分级最近已经成为一种强大的木质素提取和解聚方法，可以高产率地生产单体芳香族含氧化合物。在这里，负载在二氧化钛上的双功能钼基多金属氧酸盐（POM/TiO ）被证明可以促进串联加氢脱氧（HDO）和烷基化反应，将木质素衍生的含氧芳烃转化为高产率的烷基苯和烷基苯酚。特别是，苯甲醚和 4-丙基愈创木酚被用作该气相研究的模型化合物，使用填充床流动反应器。对于苯甲醚，在 7 小时的运行时间内，H PMo O /TiO 上观察到 30%的烷基化芳香族化合物（通过甲基平衡的甲氧基 C-烷基化 54%）的选择性和 72%的 HDO 总选择性，转化率为 82%。在类似条件下，4-丙基愈创木酚主要转化为 4-丙基苯酚和烷基化 4-丙基苯酚，烷基化 4-丙基苯酚的选择性为 42%（77%的 C-烷基化），4-丙基苯和烷基化 4-丙基苯的总 HDO 选择性为 4%，转化率为 92%（7 小时的运行时间）。更高的催化剂负载量将 4-丙基愈创木酚的转化率推至 100%，并导致 41%的丙基苯、21%的烷基化芳烃和 17%的烷基化苯酚（51%的 C-烷基化）的更高选择性。结合催化剂表征的反应性研究表明，路易斯酸位与相邻的布朗斯台德酸位协同作用，同时促进烷基化和加氢脱氧活性。提出了一种反应机制，涉及路易斯酸位上醚键的活化，然后是甲基转移和 C-烷基化。基于钼的 POM 代表了一种多功能的催化平台，可以将木质素衍生的含氧芳烃同时升级为烷基芳烃。
