Effect of silicon source on synthesis and physicochemical properties of MCM-22 zeolite

The effects of silicon source on the synthesis and physicochemical properties of MCM-22 zeolite were studied by using silica sol, fumed silica, silicic acid and silica gel as synthesis materials under dynamic hydrothermal conditions. MCM-22 zeolite with high crytallinity and purity was synthesized by using silica sol, silica, silicic acid and silica gel as silicon sources. The average sizes of zeolite particle are 190, 220 and 750 nm, respectively. Three silicon sources can affect the aggregate morphology of the zeolites, which are grain dispersion, grain semi dispersion and grain aggregation. The distribution of acid strength of the three groups of samples are almost same, all of them have more medium and strong acidity, and the total acid content of zeolite synthesized by fumed silica is the highest. MCM-22 obtained from silica sol and silicic acid has higher B/L acid ratios in the range of medium strong acid. The results of NMR show that the main aluminum in the samples is skeleton aluminum without obvious non-skeleton aluminum. MCM-22 zeolite can not be obtained when silica gel is used as silicon source because silica gel can adsorb the free water in the preparation system, suppressing the hydrothermal reaction. The reaction conditions should be carefully set up when silica gel is used as silicon source to prepare zeolite.