Zeolite MCM-22 was first synthesized by Mobil researchers in 1990. It presents a peculiar structure with two independent pore systems, both accessible through 10-membered rings. In recent years, the synthesis of zeolite MCM-22 has become one of the increasing interests due to its important applications in a variety of catalytic processes. In general, MCM-22 is synthesized in a hydrothermal system containing an alumina source, a silica source and a template. Very often the product obtained is dependent on the silica source selected. Although TEOS was the most preferred organic silica source for the synthesis of zeolite because of its monomeric nature and moderate rate of hydrolysis, no attempt to the synthesis of MCM-22 using TEOS as silica source appears so far. On the other hand, the physicochemical properties of zeolites are strongly influenced by the framework composition. Zeolite MCM-22 can be readily crystallized when the framework SiO2/Al2O3 molar ratio is 20-50. Actually, intensive efforts targeting at the expansion of the SiO2/Al2O3 ratio out of this range have been tried, however, a substantial increase of the SiO2/Al2O3 ratio directly by the conventional synthesis is difficult. Therefore, the direct hydrothermal synthesis of high-silica MCM-22 zeolite is still a rich field of research.

In this work, pure MCM-22 zeolite with high crystallinity was facile synthesized via a novel two-step sol-gel route using TEOS as silica source. This involved the first step of the hydrolysis of TEOS catalyzed in a strong acid media with a low pH value of less than 2, followed by the second step of the hydrothermal reactions of the hydrolyzed precursor with template and alumina source in a base media with a high pH value of 11-12. This synthesis strategy is very different from traditional synthetic methods, where the simultaneous hydrolysis and crystallization reactions occur in a multi-component system under the same basic conditions. Various parameters that significantly influenced the synthesis results during both the hydrolysis reaction of TEOS in acid media and the hydrothermal reaction in base media were investigated in detail. The produced samples were characterization by XRD, ICP, and SEM techniques. It is demonstrated that the this new two-step synthesis route can lead to the well crystallized zeolite MCM-22 in a broad window of reaction conditions. The increase of temperature and pH value for the hydrolysis of TEOS favors the shortening of crystallization time and the decrease of particle size of MCM-22 product. Moreover, by adjusting the hydrothermal reaction parameters, such as increasing the crystallization temperature, the alkalinity of the synthesis gel mixture and decreasing the SiO2/Al2O3 ratio of the synthesis gel mixture, the crystallization is accelerated substantially. Also, rose-like or lamellar-like morphologies of MCM-22 particles with different sizes can be obtained by changing these reaction parameters. The distinguished advantages of this synthesis strategy are the shortness of crystallization time, low cost of template, and the possibility of directly synthesis of very high silica zeolite MCM-22 with a SiO2/Al2O3 ratio up to 100.

Keywords: hydrothermal synthesis, zeolite MCM-22, tetraethyl orthosilicate

Acknowledgements: This work is supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732)