Synthesis And Calculation Of New Molecular Sieves Prepared Using Hydroxide Hexane Dual Ammonium As Structrue Directing Agent

Double quaternary ammonium salt or double quaternary ammonium hydroxide is a kind of surface active agent that has rich framework structure and high charge density. In recent years, because of its good biodegradability, low toxicity, and good oriented advantages in the synthesis of multidimensional new zeolites, it has become a research focus. ITQ-13 molecular sieve is a microporous material with a three-dimensional pore structure in which it is composed by 9-ring and ten-ring structural systems. Its special pore structure with a shape selective effect is unique in catalytic cracking reaction. It shows extensive application prospect in the selectivity of propylene, the conversion of aromatics, dewaxing of lube oil and so on. However, its difficult synthesis and high cost are become the most important issues today. SAPO-53 is a two-dimensional microporous zeolite formed intersectionally by 8-ring channel. It is a cage structure with a size of 0.31×0.43 nm, and its channel size is similar to that of SAPO-34 but with more rich pores. Thus, SAPO-53 molecular sieve is expected to have good selectivity in the reactions from methanol to olefins (MTO) and from methanol to propylene (MTP). But the disadvantages of using methylamine as a structure-directing agent to synthesize SAPO-53 are a long synthetic time, large dosage, poor purity, and environmental pollution, so these have severely restricted its further development at the moment.In this study, SAPO-53 and ITQ-13 molecular sieve were hydrothermally synthesized with hydroxide hexane dual ammonium (R(OH)2) as template. Ion exchange method was used to change the hexamethonium bromide (HMBr2) and to obtain hydroxide hexane dual ammonium, meanwhile, to explore the best raw materials recipe and optimal technological conditions of its exchange. Using R(OH)2 as the structure directing agent in the silicon germanium system and HMBr2 as template in the alkaline fluoride ion system plus with seed synthesized ITQ-13 and its heteroatom molecular sieve. ITQ-13 molecular sieve synthesized by R(OH)2 and HMBr2 were then characterized and compared systematically. The result shows that AMTQ-13 is more difficult to synthesize comparing to ITQ-13 and B-ITQ-13 molecular sieve. While using HMBr2 as the template, the synthesis of ITQ-13 does not require a basification of quaternary ammonium salt, this can further reduce the synthetic steps. Furthermore, the density functional theory (DFT) in the quantum mechanics calculation was applied to study the distribution of 9 different T positions of Si substituted isomorphously by Ge, B, Al in ITQ-13 zeolite. By comparing the substitution energies of Ge, B, Al and the proton affinities of B, Al after substitution, the location and difficult degree for heteroatom substitution were also explored from the microscopic to understand its acidic strength and sites. When Ge, B, Al substituting Si into the framework of ITQ-13 molecular sieve, it has found that the most likely locations are T7 and T6 positioned at the orifice intersection as well as T5 of double four membered ring. Proton affinity analysis shows that the PA value of Al is smaller to that of B, indicating that after Al substitution proton acid is stronger than that of B.SAPO-53 molecular sieve has been synthesized by adding Si source under the system of aluminum phosphate, and its property is compared with those prepared by using methylamine (CH3NH2) as a structural guide agent.The properties of synthesized zeolites have been characterized by using a series of analytic technologies such as:XRD, SEM-EDX, FT-IR, MASNMR, electrophoresis, N2 adsorption desorption, TG-DTG, UV-Vis DRS and so on. Also we have examined the ratios of Si/Al and P/Al, dosage of template agent, crystallization time and temperature, as well as the effects of quaternary ammonium salt (HMBr2) and quaternary ammonium alkali (R(OH)2) used as template agents, and other influence conditions on the synthesis of SAPO-53 molecular sieve. Through studying the reaction kinetics and the charge variation of gel surface in the synthesis process, it is more easily to achieve and optimize the synthesis conditions. In order to improve the acidity and catalytic performance, Co-SAPO-53 molecular sieve has been synthesized by incorporating heteroatom.Through a series of characterization, also comparing to SAPO-53 molecular sieve prepared with CH3NH2, it can be seen that using R(OH)2 to synthesize SAPO-53 molecular sieve has some good advantages, for example:high crystallinity, short crystallization time, high purity, low synthetic difficulty, good repeatability, and pollution-free to the environment. Moreover, the apparent growth rate of SAPO-53 is greater than its nucleation rate, thus, the nucleation process is the rate controlling step.In a word, this paper shows that ITQ-13 has been successfully synthesized by a simplified experimental step using HMBr2 as a structure directing agent, the isomorphous replacement and acid of ITQ-13 have been also microscopically studied. Meanwhile, a new type of silicoaluminophosphate SAPO-53 molecular sieve has been also synthesized in this study, which provides a new idea for the development of molecular sieve synthesis process.