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.
Highlights
Guaiacol (2-methoxyphenol) is a significant intermediate in fine chemical production, especially for the synthesis of vanillin (4-hydroxy-3-methoxybenzaldehyde) [1,2]
We found that the initial catalytic activities of metal phosphates follow the sequence of cerous phosphate (CP) > Lanthanum phosphate (LP) > magnesium phosphate (MP) > aluminium phosphate (ALP) > Zinc phosphate (ZP)
Metal phosphates M-P-O (M = La, Ce, Mg, Al, Zn) catalysts were prepared by precipitation method
Summary
Guaiacol (2-methoxyphenol) is a significant intermediate in fine chemical production, especially for the synthesis of vanillin (4-hydroxy-3-methoxybenzaldehyde) [1,2]. Guaiacol is prepared in liquid-phase by using dimethyl sulphate or alkyl halides as methylating agents and sodium hydroxide as homogeneous catalyst, the methylating agents are expensive, corrosive, and toxic [3]. Vapour-phase methylation of catechol (1, 2-dihydroxybenzene) with non-toxic methanol or dimethyl carbonate (DMC). Various heterogeneous catalysts have been investigated in vapour-phase methylation of catechol with methanol, including metal oxides [6,7], sulphates [2], metal phosphates [5,8,9], and supported heteropoly acids [10]. The catalytic performance of these catalysts is greatly affected by the operation conditions, catalyst structures, acid and/or base properties. A common problem is that most of the investigated catalysts exhibited unsatisfactory catalytic performance
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