Interdependent lateral interactions, hydrophobicity and acid strength and their influence on the catalytic activity of nanoporous sulfonic acid silicas

A series of propylsulfonic (MCM-SO₃H) and octyl co-functionalised propylsulfonic (MCM-Oc-SO₃H) catalysts have been prepared by post modification of MCM-41 with mercaptopropyltrimethoxysilane (MPTS) to achieve SO₃H surface coverages spanning the range 0.12-1 monolayer. Within the MCM-Oc-SO₃H 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 ₃H series NH₃ calorimetry revealed acid strength increases as a function of sulfonic acid loading, with -ΔH_ads(NH ₃) increasing from 87 to 118 kJ mol^-1. In contrast, MCM-Oc-SO₃H exhibits a dramatic enhancement of acid strength for submonolayer SO₃H coverages, with -ΔH_ads(NH ₃) found to increase to 103 kJ mol^-1. In line with these acid strength measurements the per-site activity of the MCM-SO₃H series in the esterification of butanol with acetic acid was found to increase with SO₃H content. Incorporation of octyl groups further promotes esterification activity of all the samples within the MCM-Oc-SO₃H series, such that the turn over frequency of the sample with the lowest loading of SO₃H 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-SO₃H 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.