Synthesis, Post-treatment Of MCM-22 Family Zeolites And Their Catalytic Performance Of Them In Methane And Propane Non-oxidative Aromatization

MCM-n microporous molecular sieves is a family with MWW topology stucture layer zeolites, including MCM-22, MCM-49,MCM-56,MCM-36,ITQ-1 and ITQ-2. Due to their special pore network, MCM-n zeolites have showed unusual catalytic performance in various chemical proesses. Currently, it is great of interest to synthesize various MCM-n zeolties with high crystalline degree under amplificatory conditions. In previous work, we obtained MCM-49,MCM-22,MCM-56 material with very high quality under the optimal conditions, and successfully synthesized MCM-49 zeolite by using 10 L high-pressure kettle. However, further work is still required to acquire the good quality and high stability of MCM-36 and ITQ-2 zeolites in less time by the modified method, and to synthesize MCM-49, MCM-56 and MCM-22 zeolties under more amplificatory conditions.The synthesis of zeolites usually involves the presence of structure-directing agents (e.g. quaternary ammonium salts), to form the crystalline Si–Al–M oxides. There exist validated protocols based on seeds, from which the crystals grow under hydrothermal conditions. In this way the use of the organic template is minimized. However, detemplation is still required. After hydrothermal synthesis the template has to be removed by calcination at high temperature. The oxidation generates a lot of heat (thermal shock), which produces structural inhomogeneities such as inclusions, voids and cracks. New approach for detemplation was requested.Benzene and other aromatics products are important intermediate in the petrochemical industry. Considering the prospect, scientific research and utilization of resource, aromatization of natural gas (mostly contenting methane) and LPG (including propane and butane) with low cost is one of the main routes to provide aromatics products. Many researches have focused on the study of the reactions, and found that Mo or Zn-supported MCM-22 and MCM-49 zeolites shows excellent catalytic performance in non-oxidative aromatization of methane and propane and that some valuable viewpoints were obtained. However, more researches are required to clarify the relationship between Mo species, the structure, acid-base properties and their catalytic performance for understanding the reaction mechanism. Particularly, a systemic study on the role of MCM-22's external surface acid sites in non-oxidative aromatization can be of great benefit to obtain new materials that surpass even MCM-22's superior ability, leading to important advance in aromatization synthesis.The first purpose of this thesis is studying the synthesis methods of MCM-36 and ITQ-2 zeolites, then optimizing the conditions of crystallization process. We also attempt to synthesize MCM-49, MCM-56 and MCM-22 zeolties by using 30 L high-pressure kettle. Moreover, a new method of detemplation—post-treatment with H₂O₂ were found, Through this method, template of MCM-22 precursor, MCM-56 and MCM-49 were removed. In addition, we verified and deeply investigated the connection of the acidic properties of zeolites, the dispersion of Mo species, the pore systems and catalytic performances on methane or propane aromatization.The main results obtained are as follows:1. Synthesis and characterization of molecular sieves in the presence of template HMIWe repeated the method of the synthesis of MCM-36, and systematically investigated the influences to area surface and porous distribution by the altering swelling agents, the weight compose and hydrolyzation. The result showed that hydrolyzation promoted the formation of mesopore and decreased micropore area surface. In addition, we attempted to synthesize MCM-36 zeolite by swelling MCM-49 precursor, and found that after hydrolyzing pillared material, MCM-36 zeolite with mesopore were successfully synthesized.On the basis of the synthesis of MCM-49 zeolites with reaction equipment of 10L cubage, we have enlarged the scale of sythesis conditions to produce more and better zeolite crystal. The result showed that the good quality nano MCM-22, MCM-49 and MCM-56 zeolites can be obtained with reaction equipment of 30L cubage. The zeolites synthesized with reaction equipment of 30L cubage satisfied the needs of large applications.We attempted to synthesize MCM-22 zeolite without Al species by MCM-22 and MCM-49 zeolites as crystal seeds. But it is unsuccessful and the finall zeolite is nonasil(HMI) porosil. Various synthesis parameters, including the type of crystal seeds, silica source, reaction temperature and the concentration of Na+ ion, are further investigated in HMI-SiO2-H2O system. It was found that the addition of crystal seeds could considerably shorten the time of crystallization in the presence of appropriate amount of Na+ ions and that different kind of crystal seeds could accelerate the crystal growth with different degree. Particularlly, nonasil(HMI) with high crystallinity level could be obtained using heterogeneous crystal seeds (i.e. MCM-22, MCM-49).2. Post-treatment of MCM-n microporous molecular sieves for detemplation.Detemplation of zeolite was often carried out by conventional thermal calcination. In the process it could always generate a lot of heat (thermal shock), which produced structural inhomogeneites such as inclusions, voids and cracks. And it was especially serious to zeolite, such as MCM-56, having bad thermal stability. In this work, we made an attempt to remove the template of MCM-49,MCM-56,MCM-22 and ITQ-2 zeolites by direct treatment by using H₂O₂ at relatively low temperature. With this approach, unlike calcination, the pristine structure of the material was well preserved and no extra-framework aluminum was formed. After the post-treatment by H₂O₂, MCM-49 and MCM-56 zeolites presented more micropous area surface and more acid centers than those by calcination. By treating MCM-22 precursor using H₂O₂, the pristine MWW structure and morphology of MCM-22(P) was well preserved and a new layered material (ITQ-2-like) bearing MWW structure could also be obtained. And through oxidation of H₂O₂, ultrasonic oxidation of H₂O₂ and ultrasonic calcination to swell MCM-22 precursor, respectively, we found ITQ-2 obtained by the method of ultrasonic oxidation of H₂O₂ had the largest micropore and mesopore area surface.3. Relationship between Mo species, zeolite structure, acid-base properties and catalytic performance of Mo/MCM-n microporous molecular sievesHMCM-49 zeolites were prepared with acid properties being modified by selectively NH4+ ion-exchange, and the catalytic performance of 2-20%Mo/HMCM-49 were investigated for the purpose to clarify the relationship between Mo species, zeolite structure, acid-base properties and catalytic performance. It was found out that with increasing Mo content, conversion of methane was increased. And when the structure was unchanged, increasing the acid sites of HMCM-49 promoted the selectivity of benzene (without amounting coke), while the content of Mo species did not influence the selectivity of benzene.Compared with Mo/H-MCM-56(C), the sample of Mo/H-MCM-56(H₂O₂) showed higher conversion of methane and yield of benzene, with lower selectivity to benzene and higher selectivity to coke. Concerning the fact that H-MCM-56(H₂O₂) has more 10-ring sinusoidal channels, 12-ring pockets and acidity, it could be concluded that the 10-ring sinusoidal channels in MWW structure zeolites had shape selectivity to the major product benzene and that the 12-ring pockets located on the external and the acid sites promoted the formation of coke. In comparison with the catalytic results of Mo/H-MCM-49 or Mo/H-MCM-22, the catalytic activity and benzene selectivity of Mo/H-MCM-56 was lower, but the formation rate of coke is higher. Considering the fact that MCM-56 possesses majority of 12 MR cups on the crystal exterior and the proportion of its surpercage was lower than those in MCM-22 or MCM-49, we could further confirm that the supercages in family of MWW-type zeolites have a positive role to produce benzene, and the large external surface was benefit to the increase of coke.The role of molybdenum species, acidity and different pore sites of zeolite supports were more deeply investigated by methane aromatization over Mo/HMCM-22, Mo/HITQ-2-like and Mo/HITQ-2 which were obtained by various detemplation methods and had different acid capacity. It was found that the order of catalytic performance was the following: Mo/HMCM-22 > Mo/HITQ-2-like >Mo/HITQ-2. Concerning the analysis of N2 adsorption-desorption and IR spectra of d3-acetonitrile adsorption, it could be concluded that supercages not only had positive role to the conversion of methane and the production of benzene but also keep the activity and the lives of the reaction; and the ordered layers could decrease the formation of coke and promote catalytic activities.4. Study of the catalytic performance of propane aromatization over Zn/MCM-n microporous molecular sieves.Catalytic dehydroaromatization of propane over MCM-56 with different Mo contents were investigated under various reaction conditions in this paper. The optimal propane conversion could reach 40% with BTX selectivity of 50% and aromatics selectivity of 65% under following typical conditions: wt. 3⁸% Zn content catalyst, the reaction temperature between 550oC and 600oC and feed space velocity between 1400¹600ml/g*h. And then we studied catalytic properties of Zn/HMCM-49, Zn/HMCM-22 and Zn/HITQ-2-like obtained by various methods. It was found that the order of catalytic performance is following: 6%Zn/H MCM-49(H₂O₂)>6%Zn/H MCM-49(C)>6%Zn/H ITQ-2-like>6%Zn/H MCM-22. Concerning the analysis of N2 adsorption-desorption and IR spectra of d3-acetonitrile adsorption, it could be concluded that 12-ring cup and acidity promoted the conversion of propane.