Mechanistic studies of the role of formaldehyde in the gas-phase methylation of phenol

The gas-phase reaction between phenol and formaldehyde was studied using Brønsted acidic and basic catalysts to gain insight into the components of phenol methylation with methanol. Formaldehyde and methylformate form from methanol during phenol methylation over basic catalysts. In the basic-catalyzed methylation of phenol with formaldehyde, the prevailing phenolic products were o-cresol and 2,6-xylenol; the same products were obtained in the methylation with methanol. When formaldehyde was used with the acidic catalyst, the distribution of products was similar to that obtained over basic catalysts. Comparing these results with those of methylation with methylformate, methyliodide, and methanol shows that with basic catalysts, formaldehyde was the true methylating agent in the reaction between phenol and methanol. This indicates that the transformation of methanol differentiates the reaction pattern in phenol methylation over basic or acidic catalysts. The role of the catalyst is to generate the methylating species; the nature of the latter then determines the type of products obtained in phenol methylation, whereas the catalyst type (either acidic or basic) has little influence on it.