The effects of growth modifiers on the catalytic performance of hierarchical Pd/ZSM-23 bifunctional catalysts for n-hexadecane hydroisomerization

Hydroisomerization of n-alkanes catalyzed by metal-zeolite bifunctional catalysts is an important process to produce clean biofuels. The employment of appropriate growth modifiers during the synthesis of hierarchical zeolites to increase the iso-alkane yield has captured much attention. In this work, cetyltrimethylammonium bromide (CTAB) and polydiallyl dimethyl ammonium chloride (PDDA), representatives of cationic surfactants and cationic polyelectrolytes, are added to the initial gel to one-pot synthesize hierarchical ZSM-23 zeolites (Z23-xPC and Z23-yPP) via the bottom-up strategy. The corresponding Pd/ZSM-23 catalysts are prepared by loading 0.5 wt% Pd on microporous and hierarchical ZSM-23 and tested for the hydroisomerization of n-hexadecane. Due to the cationic adsorption of CTAB or steric hindrance effect of PDDA, Z23-xPC and Z23-yPP samples all demonstrate enhanced mesoporosity compared with the conventional microporous Z23-P sample. The diffusivity experiments indicates that the D/L2 values decrease in the order of Pd/Z23-5PP > Pd/Z23-3PC > Pd/Z23-P, suggesting that the hydrocarbons diffusion in the channel of hierarchical ZSM-23 samples is evidently improved. In addition, the Brønsted acid densities of the Z23-xPC and Z23-yPP samples are also evidently reduced compared to that of the microporous Z23-P sample because of the proportion decreases of framework Al atoms at T2-T5 sites. Compared with the microporous Pd/Z23-P catalyst, the appropriate addition amount of growth modifiers can evidently improve the yield of iso-hexadecane products and limit the cracking process. Especially for the Pd/Z23-5PP catalyst, the cracking process is effectively limited and the maximum iso-hexadecane yield of 75.4% is obtained at an n-hexadecane conversion of 88.8% at 290 °C, which is the highest among all catalysts. Therefore, this work has provided an effective method for the simple preparation of bifunctional catalysts with improved diffusion performance and more favorable metal–acid balance.