Vapour-Phase Selective O-Methylation of Catechol with Methanol over Metal Phosphate Catalysts

Abstract

Guaiacol (2-methoxyphenol), an important fine chemical intermediate, is conventionally synthesized by liquid-phase processes with expensive, corrosive and toxic methylating agents such as dimethyl sulphate and dimethyl iodide. Recently, vapour-phase alkylation of catechol (1, 2-dihydroxybenzene) with methanol for the synthesis of guaiacol in the presence of heterogeneous catalysts has received more attention, as the route is economical and environmentally friendly. However, most of the investigated catalysts exhibited unsatisfactory catalytic performance for industrial applications. In this study, five metal phosphates M-P-O (M = La, Ce, Mg, Al, Zn) catalysts were synthesized and tested in the selective O-methylation of catechol with methanol. Among these catalysts, cerium phosphate (CP) showed the highest catalytic activity and guaiacol yield. Lanthanum phosphate (LP) was the second most active, which was still obviously better than magnesium phosphate (MP) and aluminium phosphate (ALP). Zinc phosphate (ZP) was not active in the reaction. Relevant samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), N2 adsorption-desorption, temperature programmed desorption of NH3 or CO2 (NH3-TPD, CO2-TPD). The suitable acid-base properties contribute to the superior catalytic performance of CP. Long-term stability and regeneration tests were also studied.