Effect of zeolites on the alkylation of aromatics with alkanes using a Pd nanoparticle/solid acid cooperative catalytic system

Abstract

The direct alkylation of benzene with alkanes is an effective method for alkylbenzene production. Our group previously discovered that a mixture of supported Pd nanoparticles and solid acids effectively promoted the alkylation of benzene with alkanes. Herein, the alkylation of toluene with n-heptane was catalyzed by physical mixture of H-mordenite and Pd nanoparticles supported on hydrotalcite to afford the corresponding C7 alkylation product with 87% selectivity and 14% toluene conversion. The reaction slightly proceeded in the absence of Pd nanoparticles or H-mordenite, indicating cooperative catalysis by the two different solid catalysts. Moreover, the high stability of the Pd nanoparticles on hydrotalcite was confirmed via reuse experiments and transmission electron microscopy (TEM) analysis. The catalyst mixture was reused at least three times without any loss of product yield, and after three reuses, TEM analysis revealed that the size of the Pd nanoparticles following the initial catalytic reaction was similar to that of the catalyst. Scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) analysis of the recovered catalyst mixture revealed the preservation of Pd nanoparticles on the hydrotalcite surface, as well as the close positioning of the two different catalyst particles, thus suggesting interparticle hydrogen transfer. The structure of the solid acid strongly affected the alkylation product selectivity. For example, H-mordenite showed high selectivity for the n-heptane alkylation product with a C7 alkyl chain, whereas the selectivity changed with other zeolites. This cooperative catalytic system can be applied to the alkylation of other substituted benzenes, such as xylenes and phenols, with good selectivity toward the desired alkylation product.