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. NH3-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.

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► Tunable SiO2 deposition on medium (1–1.5 μm) and small (200 nm) sized HZSM-5.
► Change in external Brønsted acidity due to SiO2 modification is elaborated.
► Effects of surface SiO2 deposition and P–Ni doping on surface acidity are clarified.
► Pre-existing extra Na+ has impacts on zeolite surface acidity and diffusivity.
► Silylation together with P–Ni doping is highly para-selective for toluene alkylation.