The Synthesis, Characterizations And Properties Of Novel Micro/mesoporous And Bi-micro/mesoporous Composite Molecular Sieves

Micro/mesoporous composite molecular sieves with bimodal pore structure has been a research hotspot in the material science field, since it can combine both advantages of strong acidity and high hydrothermal stability of microporous molecular sieve and tunable aperture of mesoporous molecular sieve, and show potential application prospect in petroleum cracking area. To deeper understanding of composite mechanism between the molecular sieves of micropore and mesopore, the micro/mesoporous binary composite molecular sieves of Beta/MCM-41 was synthesized in simple system, and their growth and evolvement process in different synthesis phases, as well as their composite mechanism based on thermodynamics and interaction forces were studied. In addition, in order to obtain various pore structures and more reasonable acid distribution, a new kind of ternary composite molecular sieves of bi-micro/mesopore (Y-Beta/MCM-41) was developed further. Its synthesis mechanism and regularity in acidic or basic systems were studied respectively. This is a challenging work because the synthesis system is much complexity, and the results have potential benefit to development of oil domain .The research work in this thesis includes the three sections :In Section 1:The composite molecular sieves of Beta/MCM-41 with different silica alumina ratios and bimodal pore structure were synthesized by two-step crystallization with tetraethylammonium hydroxide and cetyltrimethylammoniumbromide as templates. The structure of products was characterized by XRD, N₂ adsorption/desorption. The acid content and catalytic performance of Beta/MCM-41 with different silica alumina ratios were investigated by Py-IR and cracking reaction of n-heptane, respectively. It was found that the order of mesoporous phase in composite molecular sieves was improved with the increasing of silica alumina ratio. However the crystallinity of microporous phase, acid content and the percent conversion of n-heptane were decreased. It indicates that the performance of composite molecular sieves can be modified by changing the silica alumina ratio in synthesis process. The growth and evolvement process of Beta and MCM-41 in Beta/MCM-41 during crystallization was studied by SEM and TEM. It was found the phase composition and morphology of Beta/MCM-41 could experience: tabular crystal (Beta attached on MCM-41), sandwich (Beta wrapped in MCM-41), sandwich (MCM-41 debris wrapped in Beta), and circinal Beta particle (transformation from MCM-41 to Beta). The composite and transformation mechanism of Beta/MCM-41 was discussed based on the results.In Section 2: The bi-micro/mesoporous ternary composite molecular sieves of Y-Beta/MCM-41 with hierachical pore structure were first synthesized by overgrowth in basic system, in virtue of mixed cationic-nonionic surfactants (CTAB/OP-10) as template, zeolites Y and Beta as precursor, S⁺_S0I^-as assembly route. Various factors effected on the synthesis of Y-Beta/MCM-41 were studied systematically and the optimal synthesis condition was obtained (CTAB/SiO₂= 0.15, CTAB/OP-10 = 6:1,pH = 11.1,Si/A1= 25,W_Y+Beta/W_SiO2(g/g) = 0.534, W_Y/W_Beta(g/g) =1:1,T = 100℃,t = 2 d). The results show that the activity ofα-methylnaphthalene cracking on the composite molecular sieves (Y-Beta/MCM-41) was two times of that on the mechanical mixture. When the composite molecular sieves were synthesized in basic system, the proportion of the two microporous phases of Y and Beta could be adjusted at any ratio, but the proportion of microporous phase and mesoporous phase, and silica alumina ratio of microporous phase could only be in a certain range. The ternary composite molecular sieves with low silica alumina ratio could be obtained in basic system by desilication.In Section 3:In order to obtain ternary composite molecular sieves with wide silica alumina ratio of microporous phases or high silica alumina ratio, the composite molecular sieves of Y-Beta/MCM-41 were first synthesized by overgrowth and dealuminization in acidic system, in virtue of mixed cationic-nonionic surfactants (CTAB/OP-10) as template, zeolites Y and Beta as precursor, S⁺_S0X^-I⁺,as assembly route respectively. Various factors effected on the synthesis of Y-Beta/MCM-41 in acidic system were studied systematically and the optimal synthesis condition were obtained (CTAB/SiO₂= 0.28,CTAB/OP-10 = 7:1, pH =1.8,W_Y+Beta/W_SiO2(g/g) = 0.794,W_Y/W_Beta(g/g) = 2:1,T= 100℃,t = 2 d). The proportion between microporous phase and mesoporous phase, the proportion between two microporous phases and silica alumina ratio of microporous phase in composite molecular sieves could be adjusted by controlling synthesis condition. The activity of a-methylnaphthalene cracking on Y-Beta/MCM-41 synthesized in acidic system was 1.55 times of that on the mechanical mixture. The composite molecular sieves of Y-Beta/MCM-41 synthesized in acidic system had high hydrothermal stability (600℃,100 % steam,6 h).Compared composite molecular sieves synthesized in basic and acidic systems, it was found that they could make up insufficiencies each other in many aspects, which could provid many kinds of optional catalytic materials for practical application.