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An investigation of MoP catalysts for alcohol synthesis Zaman, Sharif Fakhruz


Molecular simulation and experimental methods have been used to assess the catalytic behavior of MoP in the conversion of synthesis gas (CO, CO₂, H₂) to oxygenated hydrocarbons. The potential energy surface of synthesis gas conversion to methane and methanol was investigated on a Mo₆P₃ cluster model of the MoP catalyst. The potential energy surface (PES) for CH₄ formation was determined to be: COad → CHOad → CH₂Oad → CH₂OHad → CH₂.ad+H₂Oad → CH₃.ad+H₂Oad → CH₄+H₂O and for CH₃OH : COad → CHOad → CH₂Oad → CH₂OHad → CH₃OHad. The hydroxymethyl (CH₂OH) species was a common reaction intermediate for both CH₄ and CH₃OH formation and the simulation predicted selective formation of CH₄ rather than CH₃OH from syngas over MoP. The cluster model was modified to investigate the effect of a SiO₂ support and a K promoter. Both SiO₂ and K decreased the activation energy for methanol formation. However, the activation energy for methanol formation remained higher than the activation energy for C-O bond cleavage. The high adsorption energy of methanol and the formation of geminal dicarbonyl species on the K-Mo₆P₃-Si₃O₉ cluster suggested the possibility of the formation of higher oxygenates. The conversion of syngas to alcohols was also investigated on 5, 10, and 15 wt% MoP supported on silica, with 0, 1, and 5 wt% K added as a promoter The major products were acetaldehyde, acetone and ethanol. Low selectivities to methanol (<5 C atom %) and methane (< 10 C atom%) were observed on the 5%K-10%MoP-SiO₂ catalyst. The product distribution obtained over the K-MoP/SiO₂ catalyst was distinct from that reported in the literature over other Mo-based catalysts. Addition of Rh to the K-MoP/SiO₂ catalyst improved the stability of the catalyst. However, the Rh increased the selectivity to hydrocarbon products, especially CH4, while only increasing the selectivity to ethanol marginally. The reaction kinetics of the major products were described by a simple empirical power law. The activation energies of ethanol and acetaldehyde were very similar, suggesting that both originated from the same surface intermediate.

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