Abstract
The transition from integrated petrochemical complexes toward decentralized chemical plants utilizing distributed feedstocks calls for simpler downstream unit operations. Less separation steps are attractive for future scenarios and provide an opportunity to design the next-generation catalysts, which function efficiently with effluent reactant mixtures. The methanol to olefins (MTO) reaction constitutes the second step in the conversion of CO2, CO, and H2 to light olefins. We present a series of isomorphically substituted zeotype catalysts with the AEI topology (MAPO-18s, M = Si, Mg, Co, or Zn) and demonstrate the superior performance of the M(II)-substituted MAPO-18s in the conversion of MTO when tested at 350 °C and 20 bar with reactive feed mixtures consisting of CH3OH/CO/CO2/H2. Co-feeding high pressure H2 with methanol improved the catalyst activity over time, but simultaneously led to the hydrogenation of olefins (olefin/paraffin ratio < 0.5). Co-feeding H2/CO/CO2/N2 mixtures with methanol revealed an important, hitherto undisclosed effect of CO in hindering the hydrogenation of olefins over the Brønsted acid sites (BAS). This effect was confirmed by dedicated ethene hydrogenation studies in the absence and presence of CO co-feed. Assisted by spectroscopic investigations, we ascribe the favorable performance of M(II)APO-18 under co-feed conditions to the importance of the M(II) heteroatom in altering the polarity of the M–O bond, leading to stronger BAS. Comparing SAPO-18 and MgAPO-18 with BAS concentrations ranging between 0.2 and 0.4 mmol/gcat, the strength of the acidic site and not the density was found to be the main activity descriptor. MgAPO-18 yielded the highest activity and stability upon syngas co-feeding with methanol, demonstrating its potential to be a next-generation MTO catalyst.
Highlights
The industrial process typically operates at 350−500 °C and 1 bar, using SAPO-34 (12-/8-ring CHA topology) and ZSM-5 (10-ring MFI topology) catalysts.[3−5] High selectivity toward C2−C4 olefins achieved with SAPO-34 is due to its topology which limits product effusion to molecules smaller than 3.8 Å.6
The reaction path is dominated by the dual-cycle mechanism (Figure S1), in which alkenes and arenes [the hydrocarbon (HC) pool species] are methylated and subsequently cracked or dealkylated to form light olefins.[3,8−10] The cycles are connected through hydrogen transfer reactions, that are core to HC pool initiation by first C−C bond formation, as well as deactivation by coke formation.[1,11−13] In addition to the zeolite/zeotype topology,[14−17] the number/strength/distribution of acidic sites,[18−21] lattice defects, and crystal size/ morphology influence methanol to olefins (MTO) catalyst performance.[22−24] Reaction conditions such as temperature, methanol partial pressure, and contact time[25,26] are paramount to optimal catalyst performance
The MAPO-18s were prepared via hydrothermal synthesis using DIPEA as the structure-directing agent.[45]
Summary
The conversion of methanol to olefins (MTO) using zeolite/ zeotype catalysts provides a viable key step to the production of chemicals from alternative carbon raw materials, including natural gas, CO2, biomass, and municipal waste.[1,2] The industrial process typically operates at 350−500 °C and 1 bar, using SAPO-34 (12-/8-ring CHA topology) and ZSM-5 (10-ring MFI topology) catalysts.[3−5] High selectivity toward C2−C4 olefins achieved with SAPO-34 is due to its topology which limits product effusion to molecules smaller than 3.8 Å.6 On the other hand, high selectivity toward propene is attained with ZSM-5 by the recycling of products and operation at higher temperatures (>500 °C) to facilitate the cracking of the hydrocarbon products.[3]. The reaction path is dominated by the dual-cycle mechanism (Figure S1), in which alkenes and arenes [the hydrocarbon (HC) pool species] are methylated and subsequently cracked or dealkylated to form light olefins.[3,8−10] The cycles are connected through hydrogen transfer reactions, that are core to HC pool initiation by first C−C bond formation, as well as deactivation by coke formation.[1,11−13] In addition to the zeolite/zeotype topology,[14−17] the number/strength/distribution of acidic sites,[18−21] lattice defects, and crystal size/ morphology influence MTO catalyst performance.[22−24] Reaction conditions such as temperature, methanol partial pressure, and contact time[25,26] are paramount to optimal catalyst performance.
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