Chloromethane Transformation To Light Olefins Over Molecular Sieve Catalyst And Reaction Mechanism

Light olefins production from nature gas is an attractive subject considering the shortage of petroleum in the world. Among the processes for natural gas utilization, methane transformation to light olefins via chloromethane as intermediate is a potential and an alternative route, which could be realized under mild condition and in less steps. In the present study, we try to develop catalyst for chloromethane transformation to light olefins and investigate the possible mechanism of this reaction.Various zeolites and SAPO catalysts with difference channel structure and acidity have been employed in the chloromethane transformation. The acidity and channel structure of the catalysts affect the activity and selectivity. SAPO-34 molecular sieve presents high selectivity for light olefins production and good conversion, proving its application potential in chloromethane transformation. Silicon incorporation is the key for acid site generation of SAPO molecular sieves. The acidity and reaction behavior could be modified by varying the Si content of SAPO-34. When using SAPO-34 with different Si incorporation in chloromethane transformation, we find the conversion and product distribution varied with Si content. Isomorphous substituted MeAPSO-34 was synthesized by hydrothermal method. The incorporation of Metal into SAPO-34 can vary the selectivity to light olefins. For better catalytic performance, two methods of metalincorporated, isomorphous substitution and impregnation, are farther investigated in details. With hydrothermal method, a series of MgAPSO-34 are synthesized. Isomorphous Mg substitution of Al is proved. Chloromethane conversion over MgAPSO-34 showed improved C₂⁼-C₃⁼0 selectivity. Mg/SAPO-34, prepared with Mg impregnation, present higher initial selectivity to light olefins than SAPO-34.Differential pulse reaction equipment is used to investigate the reaction mechanism of chloromethane conversion. For the first time, the induction period of chloromethane conversion is proved. Cyclo-organic surface species, forming in induction period and behaving as reaction center for light olefins assembly, are determined by mass spectra and FT-IR. Those observation allow us to propose a realistic reaction mechanism—"Hydrocarbon pool"mechanism.