Reaction pathway in vapor-phase synthesis of pyrazinyl compounds from glycerol and 1,2-propanediamine over ZnO-based catalysts

ZnO-based catalysts on Al2O3 as support are prepared and modified with different metal elements through impregnation, and first applied in synthesis of pyrazinyl compounds from glycerol and 1,2-propanediamine (PDA) in the present work. The results indicate that the produced pyrazinyl compounds mainly include 6-hydroxymethyl-2-methylpyrazine (6-HMP), 5-hydroxymethyl-2-methylpyrazine (5-HMP), 2,6-dimethylpyrazine (2,6-DMP) and 2,5-dimethylpyrazine (2,5-DMP). Characterization results show that ZnO species in catalysts provide acid–base pairs (Zn2+–O2−), catalysts modified by Ce, Ca, Mn and Mo exhibit spillover phenomenon of hydrogen. According to the effect of Zn amount and modification on the physicochemical and catalytic property of ZnO-based catalysts, the cyclization pathway of glycerol with PDA is proposed. The formation of 2,6- and 2,5-DMP is thought to directly take place over acid sites through glycerol dehydration and further cyclization with the activated PDA, also through hydrodehydration of 6- and 5-HMP, respectively. 6- and 5-HMP are mainly produced by the interaction of activated PDA molecules on acidic sites with dihydroxypropanal or dihydroxyacetone from glycerol dehydrogenation on basic sites.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (203 K)Download as PowerPoint slideHighlights► ZnO-based catalysts provide weak Lewis acid–strong Brønsted base pairs. ► Base sites favors glycerol activation through dehydrogenation. ► Acid sites catalyze glycerol dehydration and activate 1,2-propanediamine (PDA). ► 5- or 6-hydroxymethyl-2-methylpyrazine is from dehydrogenized glycerol and PDA. ► 2,5- or 2,6-dimethylpyrazine can be directly from dehydrated glycerol and PDA.