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.
Reaction-time resolved IR spectroscopy highlights the role of CO and surface –OCH3 in the MTO conversion catalysed by CoAPO-18 with maximised concentration of acidic sites.
A spectroscopic and computational insight in the defective nature of the acetylene dicarboxylic acid based Ce-MOF, having UiO-66 topology and denoted as Ce-UiO-66-ADC MOF.
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