Catalytic Performance of HZSM-5–Al2O3 Nanocomposites in Glycerol Conversion

Abstract

The effect of the components’ ratio in HZSM-5–Al2O3 compositions on their acidic and catalytic characteristics during glycerol transformation was investigated. It was found that the introduction of alumina into HZSM-5 zeolite resulted in a reduction of Brønsted acidity and the emergence of new Lewis acid sites at the phase boundary. The main products of glycerol transformation at 300°C–340°C over studied catalysts were acetaldehyde and allyl alcohol, and their selectivity reaches 81% and 84%, respectively. Therewith, the products of direct glycerol dehydration, acrolein and acetol, are formed in small amounts, which probably indicates their rapid conversion into acetaldehyde and allyl alcohol by sequential hydrogen transfer reactions. In the absence of hydrogen in inflow, glycerol or its intermediates can act as hydrogen donors. Increasing the Al2O3 content in the composites led to a decrease in allyl alcohol formation and an increase in acetaldehyde production with minimal formation of direct dehydration products. The relationship between acid properties and product selectivity in glycerol conversion with varying HZSM-5–Al2O3 component ratios is reflected in acetaldehyde selectivity increasing with increasing Brønsted acidity, while allyl alcohol selectivity follows the concentration of Lewis acid sites. The results also indicate that coke deposition during glycerol conversion predominantly occurs on Lewis acid sites, with all catalytic reactions on these sites contributing equally to the coking. Thus, the key role of Lewis acid sites in coke formation during catalysis has been established.