MAPO-18 Catalysts for the Methanol to Olefins Process: Influence of Catalyst Acidity in a High-Pressure Syngas (CO and H₂) Environment

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 CO 2 , CO, and H 2 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 CH 3 OH/CO/CO 2 /H 2 . Co-feeding high pressure H 2 with methanol improved the catalyst activity over time, but simultaneously led to the hydrogenation of olefins (olefin/paraffin ratio < 0.5). Co-feeding H 2 /CO/CO 2 /N 2 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/g cat , 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.

Catalyst sites and active species in the early stages of MTO conversion over cobalt AlPO-18 followed by IR spectroscopy

“…The sample under investigation is a cobalt containing AlPO-18 (CoAPO-18), synthesized at the Oslo University and preliminary characterized as reported in ref. 29. Two SAPO-18 catalysts with different silicon contents were selected for comparison (synthesis and characterization of SAPO-18 are also reported in ref.…”

“…Two SAPO-18 catalysts with different silicon contents were selected for comparison (synthesis and characterization of SAPO-18 are also reported in ref. 29). The elemental analysis performed by ICP-OES revealed for CoAPO-18 a Co/(Al + P) ratio equal to 0.65.…”

“…† The spectra have in common the presence of an intense signal due to ν(OH), corresponding to the BAS at 3620 cm −1 and 3580 cm −1 for SAPO-18 and CoAPO-18, respectively. 29 The very first interaction with methanol vapours (orange spectrum in Fig. 6 and purple in Fig.…”

“…The probe molecule interaction experiments were carried out following procedures reported in the literature. 29,31 Experiments of FT-IR spectroscopy "in continuo" (i.e. repeated, not delayed in situ measurements) have been performed using a Bruker Invenio R spectrophotometer with an MCT cryodetector working at 40 kHz, equipped with an AABSPEC 2000 multi-mode FT-IR cell operating in transmission mode, allowing high temperature and high pressure in situ measurements.…”

“…Catalytic data obtained when running the conversion at 350 °C can be found in ref. 29, for the sake of comparison. Herein, the description is focused on the transformation of active species before and during the mechanism of HCP, starting from the earliest interactions of methanol with active sites.…”