Effects of controlled SiO₂ deposition and phosphorus and nickel doping on surface acidity and diffusivity of medium and small sized HZSM-5 for para-selective alkylation of toluene by methanol

In this work the controlled SiO₂ deposition on the HZSM-5 of medium (M-) and small (S-) sized grains is accomplished through the mild steam-pretreatment of zeolites (to modify the density of surface OH groups) followed by chemical liquid deposition (CLD) process. The amount of deposited SiO₂ is tunable by varying the conditions of steam-pretreatment and/or CLD process, which allows quantitative correlating the changes of sample characteristics with the amount of deposited SiO₂. NH ₃-TPD, pyridine and 2,6-DBT-pyridine adsorption infrared spectroscopic studies revealed the effect of surface SiO₂ deposition on strength (weak and strong), type (Brønsted and Lewis), and location (external) of acid sites of HZSM-5. The external Brønsted acid sites of more than 75% are diminished upon SiO₂ deposition of 13 wt%. Due to pre-existing extra Na⁺ in S-HZSM-5, less amount of SiO₂ deposition or P-doping will cause more obvious enhancement in para-selectivity on S-HZSM-5. The present study revealed that with the integrated modifications of SiO₂ deposition and P-Ni doping on S-HZSM-5, toluene conversion of 31% and para-selectivity of 91% can be achieved. It was also found that doping of 5 wt% Na and 6 wt% P to M-HZSM-5 (free of silylation) can give p-xylene selectivity of ∼80% at toluene conversion of ∼26%. SiO₂ deposition can more obviously reduce the strong acid sites, and P-Ni doping seems also to affect Lewis acidity. Low density of Brønsted (external)/Lewis acid sites and controlled diffusivity of HZSM-5 would be favorable for p-xylene generation in toluene alkylation with methanol.