Direct Synthesis of Acetic Acid from CH4-CO2 over Pd/LDH at Low Temperature
The direct synthesis of acetic acid from CH4 and CO2 is an effective method for converting CH4 and reducing CO2 emissions in a 100％atom economy reaction.Hydrotalcite is widely used in CH4 and CO2 conversion reactions due to their large specific surface area,high thermal stability,good dispersion of active metals,and suitable acid-base properties,while Pd is often considered as the active center for C—C bond coupling.Herein,a series of Pd/LDH catalyst with Pd loading of 5％(mass fraction)was prepared by ion-exchange method using magnesium-aluminum hydrotalcite(MgAl-LDH)with Mg/Al molar ratios of 1.5,3,5,7 and 9 as the carrier,and applied for the direct synthesis of acetic acid from CH4-CO2 by a two step-wise technique.The structure and surface acidity-alkalinity of the catalyst were analyzed by XRD,ICP,N2 adsorption-desorption,XPS,NH3-TPD,CO2-TPD and in situ DRIFTS.The results showed that the yield of acetic acid over MA5 catalyst with Mg/Al molar ratio of 5 was the highest(61.8 μmol·g-1cat·h-1),and acetic acid was the only product in the liquid phase product.Its yield was positively correlated with the amount of medium strong acid and surface Pd0/(Pd2++Pd0)ratios.The rapid inactivation of the catalyst was due to the collapse of the hydrotalcite structure,which resulted in a significant decrease in the amount of medium strong acid.In situ DRIFTS experiments showed that CH4 was firstly dissociated to form CHx*intermediate and H proton on the catalyst surface,then CO2 was directly inserted into CHx* to form CHxCOO*,with further hydrogenated to yield acetic acid,or CO2 was combined with H protons to form COOH* intermediates which were then coupled with CHx* to form CHxCOOH*,and finally hydrogenated to obtain acetic acid.
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