Phosphoric acid intercalated Mg–Al hydrotalcite-like compounds for catalytic carboxylation reaction of methanol in a continuous system

• Mg–Al HTs with intercalated phosphate are prepared by co-precipitation.
• These solids catalyze the thermodynamically unfavoured carboxylation reaction.
• They improve the low activity of others reported heterogeneous catalysts.
• Positive effect in activity of high P/Mg molar ratio and ultrasonic-treatment.
• The selectivity depends on the temperature due to the formation of DME.

Mg–Al-hydrotalcite-like catalysts (HTlcs) with different amount of P/Mg molar ratio were prepared via co-precipitation method and calcined at 450 °C. The synthesized catalysts were tested in the direct-carboxylation reaction of methanol in gas phase in a continuous system such as chemical route for CO2 valorization. Activities around 2% with total selectivity towards the dimethyl carbonate (DMC) was obtained at moderate temperatures (150 °C) with both Mg/Al mixed oxides (HTO) and phosphated Mg/Al mixed oxides (HTc-9c). Even though the conversion increased until 16% at higher temperatures (200 °C), the selectivity with both catalysts (HTO and HTc-9c) decreased due to the decomposition of DMC to dimethyl ether (DME). Nevertheless, the catalyst with P showed less DMC decomposition and a higher selectivity towards the desired product. This is explained by the presence of orthophosphate species bonded to the Al3+ metals of the HTs (OP(OAl)3) which give to the catalyst a higher structural stability and specific acid properties.In addition, the solids were characterized in-depth by XRD, ICP, NH3-TPD, FTIR, Raman, 27Al and 31P MAS NMR spectroscopy and SEM.

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