A series of propylsulfonic (MCM-SOH) and octyl co-functionalised propylsulfonic (MCM-Oc-SOH) catalysts have been prepared by post modification of MCM-41 with mercaptopropyltrimethoxysilane (MPTS) to achieve SOH surface coverages spanning the range 0.12-1 monolayer. Within the MCM-Oc-SOH series, samples with submonolayer MPTS coverages were further grafted with octyltrimethoxysilane to cap bare hydroxyl sites and tune the hydrophobicity of the support. For the MCM-SO H series NH calorimetry revealed acid strength increases as a function of sulfonic acid loading, with -ΔH(NH ) increasing from 87 to 118 kJ mol. In contrast, MCM-Oc-SOH exhibits a dramatic enhancement of acid strength for submonolayer SOH coverages, with -ΔH(NH ) found to increase to 103 kJ mol. In line with these acid strength measurements the per-site activity of the MCM-SOH series in the esterification of butanol with acetic acid was found to increase with SOH content. Incorporation of octyl groups further promotes esterification activity of all the samples within the MCM-Oc-SOH series, such that the turn over frequency of the sample with the lowest loading of SOH more than doubles. Molecular dynamic simulations indicate that the interaction of isolated sulfonic acid groups with the pore walls is the primary cause of the decrease in acid strength and activity of submonolayer samples within the MCM-SOH series. Incorporation of octyl groups results in a combination of increased hydrophobicity and lateral interactions between adjacent sulfonic acid head groups, resulting in a striking enhancement of acid strength and esterification activity.
Bibliographical noteWe thank the Engineering and Physical Sciences Research Council for financial support (EP/E013090/1; EP/F063423/1; EP/G007594/1) and the award of a Leadership Fellowship (AFL).
Dacquin, J-P., Cross, H. E., Brown, D. R., Düren, T., Williams, J. J., Lee, A. F., & Wilson, K. (2010). Interdependent lateral interactions, hydrophobicity and acid strength and their influence on the catalytic activity of nanoporous sulfonic acid silicas. Green Chemistry, 12(8), 1383-1391. https://doi.org/10.1039/c0gc00045k