Vu Hue-Tong, Harth Florian M, Wilde Nicole
Institute of Chemical Technology, Universität Leipzig, Leipzig, Germany.
Front Chem. 2018 May 16;6:143. doi: 10.3389/fchem.2018.00143. eCollection 2018.
A systematic silylation approach using mono-, di-, and trichlorosilanes with different alkyl chain lengths was employed to enhance the hydrothermal stability of zeolite Y. DRIFT spectra of the silylated zeolites indicate that the attachment of the silanes takes place at surface silanol groups. Regarding hydrothermal stability under aqueous-phase processing (APP) conditions, i.e., pH ≈ 2, 473 K and autogenous pressure, the selective silylation of the zeolite surface using monochlorosilanes has no considerable influence. By using trichlorosilanes, the hydrothermal stability of zeolite Y can be improved significantly as proven by a stability test in an aqueous solution of 0.2 M levulinic acid (LA) and 0.6 M formic acid (FA) at 473 K. However, the silylation with trichlorosilanes results in a significant loss of total specific pore volume and total specific surface area, e.g., 0.35 cm g and 507 m g for the silylated zeolite Y functionalized with n-octadecyltrichlorosilane compared to 0.51 cm g and 788 m g for the parent zeolite Y. The hydrogenation of LA to γ-valerolactone (GVL) was conducted over 3 wt.-% Pt on zeolite Y (3PtY) silylated with either n-octadecyltrichlorosilane or methyltrichlorosilane using different reducing agents, e.g., FA or H. While in the stability test an enhanced hydrothermal stability was found for zeolite Y silylated with n-octadecyltrichlorosilane, its stability in the hydrogenation of LA was far less pronounced. Only by applying an excess amount of methyltrichlorosilane, i.e., 10 mmol per 1 g of zeolite Y, presumably resulting in a high degree of polymerization among the silanes, a recognizable improvement of the stability of the 3 PtY catalyst could be achieved. Nonetheless, the pore blockage found for zeolite Y silylated with an excess amount of methyltrichlorosilane was reflected in a drastically lower GVL yield at 493 K using FA as reducing agent, i.e., 12 vs. 34% for 3PtY after 24 h.
采用一种系统的硅烷化方法，使用具有不同烷基链长度的一氯硅烷、二氯硅烷和三氯硅烷来提高Y型沸石的水热稳定性。硅烷化沸石的漫反射红外傅里叶变换光谱表明，硅烷的附着发生在表面硅醇基团上。关于水相处理（APP）条件下的水热稳定性，即pH≈2、473K和自生压力，使用一氯硅烷对沸石表面进行选择性硅烷化没有显著影响。通过使用三氯硅烷，Y型沸石的水热稳定性可以得到显著提高，这在473K下于0.2M乙酰丙酸（LA）和0.6M甲酸（FA）的水溶液中进行的稳定性测试中得到了证明。然而，用三氯硅烷进行硅烷化会导致总比孔容和总比表面积显著损失，例如，用正十八烷基三氯硅烷功能化的硅烷化Y型沸石的总比孔容和总比表面积分别为0.35 cm³/g和507 m²/g，而母体Y型沸石分别为0.51 cm³/g和788 m²/g。在使用不同还原剂（如FA或H₂）的情况下，在经正十八烷基三氯硅烷或甲基三氯硅烷硅烷化的Y型沸石（3PtY）上进行LA加氢制γ-戊内酯（GVL）反应。虽然在稳定性测试中发现用正十八烷基三氯硅烷硅烷化的Y型沸石具有增强的水热稳定性，但其在LA加氢反应中的稳定性远不明显。只有通过施加过量的甲基三氯硅烷，即每1g Y型沸石10mmol，这可能导致硅烷之间高度聚合，才能实现3PtY催化剂稳定性的明显改善。尽管如此，在493K下使用FA作为还原剂时，过量甲基三氯硅烷硅烷化的Y型沸石出现的孔堵塞反映在GVL产率大幅降低上，即24小时后3PtY的GVL产率为12%，而未硅烷化的为34%。
