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

4 years ago

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