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
Authors: Ping Lu, Zhaoyang Fei, Lei Li, Xinzheng Feng, Weijie Ji, Weiping Ding, Yi Chen, Weiming Yang, Zaiku Xie
Abstract:
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 paraselectivity 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.
Keywords:
SiO2 deposition
Phosphorus
Nickel
Doping
Alkylation
Toluene
Methanol para-Xylene
Published in: Applied catalysis A: General (Volume 453, Pages 1-402, 26 February 2013)
Publisher: Elsevier
ISSN Information: 0926-860X
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
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