Development of highly active and selective catalyst for side-chain alkylation of toluene with methanol to styrene

Abstract

The side-chain alkylation of toluene with methanol has been widely investigated as an alternative pathway for styrene production because of the potential benefits such as large availability and low cost of reactant feedstocks. In this study, we comprehensively investigated the effects of acid-base properties and microporous structures of zeolite catalysts. We synthesized X zeolite catalysts (FAU structure) with and without the controlled addition of free Cs oxide ($Cs_2O$) and boron promoter ($B_2O_3$). A zeolite catalysts (LTA structure) containing secondary mesoporosity were also prepared. A zeolite has too narrow micropore apertures (<0.40 nm) and toluene (kinetic diameter: 0.59 nm) cannot access the micropores. This means that the reaction can occur only at the mesopore surface of the catalysts. Therefore, these materials with similar chemical compositions can be used as ideal model catalytic systems to clearly understand the role of micropores. Our results revealed a significant dilemma in the design of catalyst for this reaction. The main active sites required for side-chain alkylation of toluene are base sites. However, the catalysts having sufficient basicity have limited Lewis acidity, which is ineffective for stabilizing toluene under elevated temperatures. Fortunately, the microporous structure of zeolites provides additional secondary interactions (e.g., van der Waals interaction) to toluene, which remarkably increases the catalytic activities of basic zeolites. In contrast, the model catalysts synthesized using mesoporous A zeolite showed no toluene adsorption at all due to the absence of any accessible microporosity. Consequently, the A zeolite catalysts which have similar acid-base properties to those of the X zeolite catalysts showed no detectable toluene alkylation activity. The results clearly showed the importance of secondary interactions (or solvation effects) in zeolite catalysis for toluene side-chain alkylation. Based on the present results, we believe that developing an effective strategy for increasing the affinity between toluene and the basic catalyst surface would be a key step in designing more advanced catalysts for the side-chain alkylation of toluene.