Effect of substitution of Fe3+/Cr3+ on the alkylation of phenol with methanol over magnesium-aluminium calcined hydrotalcite

Abstract

Hydrotalcite (HT)-like compounds with the general formula, Mg0.8M(III)0.2(OH)2(CO3)0.1·yH2O with M(III)=Al, Fe and Cr (Mg/Al atomic ratio 4) were synthesized by coprecipitation method using a mixture of NaOH/Na2CO3 as precipitant. The Powder X-ray diffraction (PXRD) of aluminium and iron containing samples (M(III)=Al, Fe) showed single phase corresponding to HT when a mixture of HT and MgCrO4·7H2O phase was detected for chromium containing sample (M(III)=Cr). These samples after calcination at 450°C/8 h/air, were used as catalyst for vapour phase alkylation of phenol with methanol. MgAl 4.0-CHT showed nearly 100% phenol conversion around 400°C and the reaction was found to proceed predominantly through the formation of anisole (O-alkylation). Isomorphous substitution of Al3+ by Fe3+ or Cr3+ in the HT framework resulted in a decrease in phenol conversion in the order MgAl 4.0-CHT MgCr.0-CHT MgFe 4.0-CHT and the alkylation reaction was found to proceed exclusively at C-centres (C-alkylation) to give o-cresol as a major product. The catalytic performance for the methylation of phenol has been correlated with acid-base properties of the catalysts as determined by test reactions such as decomposition of 2-methyl-3-butyne-2-ol (MBOH) and cyclohexanol.