Improved Transalkylation of C₁₀ Aromatics with 2-Methylnaphthalene for 2,6-Dimethylnaphthalene Synthesis over Pd-Modified Zeolites with Different Framework Structures

Abstract

Pd-HZSM-5 zeolites were prepared and, respectively, tested as catalysts in the transalkylation of C 10 aromatics (C 10 A) with 2methylnaphthalene (2-MN) for synthesizing 2,6-dimethylnaphthalene (2,6-DMN). Each one of these Pd-zeolites exhibited a significantly improved transalkylation reactivity (compared with its parent precursor) in H 2 reaction atmosphere, likely attributed to a hydrogen spillover from metal Pd to zeolite and the subsequent generation of surface-active hydrogen species, which greatly enhanced the proton-transfer efficiency in the surface transalkylation catalyzed by strong protonic acid sites. Meanwhile, it is speculated that the surface-active hydrogen species also accelerated the secondary DMNs' isomerization and simultaneously caused a coincident increase in the selectivities (in DMNs) of 2,6-DMN and β,β-DMNs with a higher diffusion rate in pores or pockets. The 0.1 wt % Pd-HBeta with 12-ring large pores, more strong Bronsted acid sites, a high Lewis acid density, and a higher palladium dispersion showed a higher 2-MN conversion, DMN selectivity, and 2,6-DMN yield than those of the other three 0.1 wt % Pd-modified zeolites. Because of the strong protonic acid density, the Pd dispersion of 0.1 wt % Pd-HMCM-22 or 0.1 wt % Pd-HMOR was lower, and 0.1 wt % Pd-HZSM-5 had a lower palladium dispersion and a repulsion of its smaller pores to C 10 A reactants. Moreover, the reasonable distributions of metal-Pd sites and acidity on the 0.1 wt % Pd-HBeta catalyst not only prevented the severe naphthalene ring loss resulting from the side reactions of hydrogenation and subsequent ring opening and cracking, but also endowed it with a remarkably improved catalytic stability due to the fact that a reasonable catalytic hydrogenation effectively suppressed the formation of large coke molecules and restrained a fast coke deposition. Finally, the 2-MN conversion of >81% and the 2,6-DMN yield of >9.8% were obtained over 0.1 wt % Pd-HBeta during a 25 h onstream reaction, depending on both an efficient internal diffusion of reactants/products in its three-dimensional 12-ring pores, where enough active sites were accessible, and a high-efficiency surface-concerted catalysis of metal-Pd sites and acid sites.