Controllable synthesis of nano-ZSM-5 catalysts with large amount and high strength of acid sites for conversion of methanol to hydrocarbons

Abstract

Abstract Nano-ZSM-5 with large amount and high strength of acid sites were successfully synthesized by hydrothermal crystallization of pure-silica nucleus in NaAlO2 alkali solution. The concentration of NaAlO2 alkali solution was decreased by increasing the H2O/Al ratio from 200 to 2600, which strongly determined the acidic properties and crystal size of the obtained nano-ZSM-5. Low concentration of NaAlO2 alkali solution was found unfavorable for the incorporation of Al into the bulk of ZSM-5 crystal in crystallization process. Hence, more Al distributed at the exterior of crystals, which enhanced the accessibility of the acid sites during reaction. Large amount and high strength of acid sites were produced for nano-ZSM-5 by increasing H2O/Al ratio. When the H2O/Al ratio increased from 200 to 2600, the total amount of acid sites increased from 0.59 to 0.81 mmol g−1, much higher than conventional value of 0.50 mmol g−1. In addition, the acid strength increased and the ratio of Bronsted/Lewis acid sites were sharply increased from 2.3 to 4.1 when the H2O/Al ratio increased. Moreover, the size of nano-crystal was also decreased by above decreasing the concentration of the NaAlO2 alkali solution. When the H2O/Al ratio was 2600, the obtained sample possessed total acid amount of 0.81 mmol g−1 and crystal size of 60 nm, respectively. Methanol to hydrocarbon (MTH) reaction results showed that the increased acid sites of the catalyst decreased its catalytic lifetime to some extent, but the aromatics selectivity was tunable obviously. As for the catalyst prepared with H2O/Al ratio of 2000, the aromatics selectivity of the obtained catalyst reached to 66.0%. This paper provided a method for the fabrication of small-size nano-ZSM-5 with large amount and high strength of acid sites and regulated Al distribution for MTH reaction.