Effect and Mechanism of Aluminum(III) for Guaiacol-Glyoxylic Acid Condensation Reaction in Vanillin Production.

3-methoxy-4-hydroxymandelic acid (VMA) was the critical intermediate for the synthesis of vanillin by the glyoxylic acid method. Meanwhile, a valuable byproduct (2-hydroxy-3-methoxy-mandelic acid, -VMA) was obtained during the reaction. Al was found to be a helpful catalyst in increasing the selectivity for VMA and -VMA. In the presence of Al, the selectivity for VMA and -VMA increased from 83 to 88% and from 3 to 8%, respectively, while that of the helpless byproduct 2-hydroxy-3-methoxy-1,5-mandelic acid (di-VMA) decreased from 14% to less than 4%. The kinetics based on the kinetic equation of the condensation reaction was studied by the initial concentration method. The results indicated that the involvement of Al could reduce the activation energy of the reaction on the basis of the Arrhenius equation. Combined with thermogravimetric analysis, Fourier transform-infrared spectroscopy, and H NMR research, Al was found to interact with guaiacol through Al-O and Al···H, which further improved the selectivity of the VMA and -VMA and reduced the selectivity of di-VMA by adding the electronegativity of the ortho- and para-positions of hydroxyl groups of guaiacol.