A Novel Method For The Preparation Of Micro/mesoporous Composite Molecular Sieves

Micro/mesoporous molecular sieves possess two-fold pore structure and combine the advantages of mesoporous molecular sieves (large pore) and microporous molecular sieves (strong acidity and high hydrothermal stability). To adjust the pore diameter and acidity of composite materials, the materials with different pore structure and acidity distribution can be used.Recently, study on the micro/mesoporous composite molecular sieve has focused on the synthesis of mesopores with higher hydrothermal stability and stronger acidity using zeolite seeds with zeolite primary and secondary buildingunits as silica-alumina source, in which the key factor is the preparation of zeolite seeds. However, most zeolites, especially for some important zeolites with shape-selectivity such as MOR, etc., cannot obtain their corresponding liquid aluminosilicate nanoclusters at present time via the "seeds" method. Therefore, the effective assembly of different zeolite seeds with surfactants was not available for such zeolites and the assembly method mentioned above is limited. In order to solve this problem, a general approach, ie. dissolution of zeolite into alkaline solution, for the preparation of silica-alumina source with zeolite primary and secondary building units has been developed in this paper for the first time. Using this method, any zeolite with higher Si/Al ratio that can partly dissolved into alkaline solutions can be used to produce zeolite building units. Two kinds of micro/mesoporous molecular sieves have been obtained by using MOR zeolite as silica-alumina source in this dissertation.The first kind of micro/mesoporous molecular sieves is the composite of two objective materials of microporous MOR zeolite and MCM-41 mesoporous molecular sieves. Zeolite MOR was partly dissolved into alkaline solutions by controlling the pH value and the dissolution time. There were a large number of secondary building units characteristic of MOR besides the small zeolite crystals. With the addition of surfactant CTAB, the gel would condense around the self-assembling aggregate of CTAB and many secondary building units in the initially dissolved aluminosilicate could be introduced into the mesoporous walls of the composite. In addition, the undissolved zeolite MOR crystals served as holders for mesoporous phase. Thus the micro/mesoporous composite molecular sieve was obtained. All the characterization results show that the composite is quite different from the mechanical mixture. The composite material possesses much thicker mesoporous walls (The mesoporous wall thickness for conventional MCM-41 is 1.40run, and the wall thickness for mesopore in the composite is 2.22 nm), which arises from the successful introduction of secondary building units of zeolite MOR into the mesoporous walls of the composite. Therefore, the mesophase in the composite shows much higher hydrothermal stability and stronger acidity in comparison to the MCM-41 in the mechanical mixture, and the composite shows much higher catalytic activity than the mechanical mixture. It can be anticipated that different micro/mesoporous composite molecular sieves, which contains distinctive pore systems and acid sites, can be obtained by using kinds of zeolites. Additionally, the relative amounts of meso- and microstructure in the composite were able to be controlled by varying the Na/Si ratio in the system.The second composite is the objective material MCM-41 with certain properties of the zeolite MOR. Zeolite MOR was dissolved into alkaline solutions with higher concentration and the obtained silica-alumina solution was separated withthe undissolved zeolite crystals. Then the dissolved MOR gel was assembled with surfactant CTAB and many secondary building units in the initially dissolved aluminosilicate could be introduced into the mesopore walls. Compared to the conventional MCM-41, the mesoporous M-MCM-41 from dissolved zeolite MOR shows much thicker mesoporous walls, certain amount of micropores, much higher hydrothermal stability, and lower structural order. At the same time, M-MCM-41 shows stronger acidity than conventional MCM-41 and has good catalytic activity in acid catalytic reaction, especially in reaction with large molecules.