4 years ago

Enhanced Methanol to Olefin Catalysis by Physical Mixtures of SAPO-34 Molecular Sieve and MgO

Enhanced Methanol to Olefin Catalysis by Physical Mixtures of SAPO-34 Molecular Sieve and MgO
Wei-Ke Chang, Ying-Qian Cao, Ya Wang, Jay B. Benziger, Sheng-Li Chen, Yu-Li Gao, Qi Zhang
Reaction rates, product distributions, and catalyst deactivation are reported for SAPO-34, MgO, MoO3, and physical blends of these materials in fixed-bed reactors. Methanol is converted to small olefins (C2 and C3 primarily) over SAPO-34. A small fraction of the methanol oligomerizes further, forming coke that deactivates the catalyst. Methanol is dehydrogenated to CO and H2 over MgO catalysts. Methanol is oxidized to CO2 over MoO3 catalysts. Physically blending SAPO-34 with MgO or MoO3 reduces the reaction selectivity for the methanol to olefins (MTO) reaction. Physical mixing of SAPO-34 and MgO favorably extends the catalyst lifetimes by reducing the rate of coke formation on SAPO-34. This is explained by opening up parallel reaction pathways involving oxygenated reaction intermediates. In contrast, SAPO-34 deactivation is more rapid when SAPO-34 is physically blended with MoO3. MgO by itself shows almost no catalytic activity for methanol. However, when MgO is sandwiched between layers of SAPO-34, the catalytic activity and production distribution from methanol is significantly altered to favor carbon oxides in comparison to SAPO-34 by itself. MgO is only catalytically active when it is sandwiched between layers of SAPO-34, demonstrating that reaction intermediates are transported from SAPO-34 to MgO and back again. Blends of MoO3 and SAPO-34 showed that catalyst deactivation was more rapid in comparison to SAPO-34 alone; methanol was oxidized to formaldehyde over MoO3, which was transported to SAPO-34, where the acidic SAPO-34 catalyst is assumed to polymerize the formaldehyde. The data indicate that acid-catalyzed reactions critical to MTO, oligomerization, and scission reactions occur on SAPO-34. MgO-catalyzed methanol dehydrogenation and MoO3-catalyzed methanol oxidation. Physical blends of catalysts can open up new reaction pathways through coupling of different catalyst functionalities that may provide a simple and convenient method for tuning catalytic performance.

Publisher URL: http://dx.doi.org/10.1021/acscatal.7b01285

DOI: 10.1021/acscatal.7b01285

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