Catalytic effects of hierarchical zeolites for toluene methylation with methanol

Abstract

The work presented in this dissertation addresses the catalytic effect of hierarchical zeolite in toluene methylation with methanol. Benzene, toluene, and xylene are the basic raw materials for various petrochemical products. Industrially, surplus toluene is usually converted into a more valuable xylene via either transalkylation or methylation process. Among toluene-to-xylene processes, toluene methylation using methanol has been extensively studied owing to its high catalytic selectivity towards xylene. Nevertheless, the toluene conversion is ca. 40% over various conventional zeolite catalysts and further improvement is demanded for the prevalent commercialization of this process. The present work was able to determine that the diffusion path-length of a zeolite catalyst could strongly affect the toluene conversion. Thus, the effect of the zeolite catalyst’s crystal thickness on the catalytic performance has been investigated in detail. Several MFI zeolites with different crystal thickness in the range of 2.5–200 nm were hydrothermally synthesized using appropriate structure-directing agents and characterized using various techniques. The zeolites were evaluated as the catalysts for toluene methylation using methanol. The catalytic performances of the MFI zeolite catalysts were observably enhanced progressively as the zeolite’s thickness decreased. A toluene conversion as high as 72% could be obtained over the MFI nanosheet catalyst with a crystal thickness of 2.5 nm, which was the best result reported thus far.

The catalytic performance was greatly dependent on the zeolite crystal thickness according to the catalytic results because the toluene methylation using methanol often competed with the methanol-to-hydrocarbon (MTH) conversion, which was an undesirable side reaction. Thus, the remarkable high catalytic performance of the nanosheet catalyst was attributable to the low selectivity of the side products via the MTH route. Therefore, regulation of the crystal thickness in the zeolite could be considered a viable means of enhancing the catalytic performance for toluene methylation using methanol.