Study of the CA-treated ZSM-22 zeolite with enhanced catalytic performance in the hydroisomerization of long-chain n-dodecane

In this study, Pt catalysts on a series of ZSM-22 zeolites treated with citric (CA) solution with different molar concentrations (Pt/ZSM-22-CA-x, x = 0,1, 2, and 3) were prepared, and evaluated in the hydroisomerization of n-dodecane. The CA-treated ZSM-22 zeolite had little change in the morphology, regardless of the molar concentration of the CA solution used. The prepared Pt catalysts on CA-treated ZSM-22 zeolites exhibited enhanced catalytic activity in n-dodecane hydroisomerization, compared to the original ZSM-22 zeolite supported Pt catalyst (89.7% vs. 19.6%) under the same reaction conditions. This was mainly due to the increased Bronsted (B) sites on the treated ZSM-22 zeolites, accelerating the skeletal rearrangement rate. Moreover, Pt/ZSM-22-CA-2 and Pt/ZSM-22-CA-3 exhibited the maximal, similar catalytic activity and yield, but different distribution for C12-isomers. This phenomenon can be explained by the different ratios between accessible Pt sites and B acidic sites (CPt/CH+), and a higher CPt/CH+ for Pt/ZSM-22-CA-3 (0.36) than Pt/ZSM-22-CA-2 (0.30) meant that the probability of iso-alkene intermediates for hydrogenation saturation on the Pt sites was increased before diffusing to the next B acid, inhibiting the further isomerization and cracking.