Thermocatalytic degradation of lignin monomer coniferyl aldehyde by aluminum–boron oxide catalysts

Abstract

Two aluminum–boron oxide catalysts were produced via a sol–gel method at pH 3 and 4 during the solution mixing step of the synthesis, these materials were employed in thermocatalytic degradation of coniferyl aldehyde (CA), which was used as a probe molecule of the lignin polymeric molecule and is comprised of the repetitive monomers coniferyl, sinapyl, and paracoumaryl. The two synthesized catalysts were mostly amorphous and mesoporous, aiding in permeability and percolation of CA. A commercial catalyst was compared (Pt/alumina at 1 wt%) with both catalysts synthesized in this work by kinetic tests by varying the CA concentration and inlet temperature. Under the same reaction conditions, the commercial catalyst showed higher activity than the aluminum–boron oxide catalysts, but the synthetic catalysts presented a wider variety of organic products than the commercial catalyst. In particular, two high-value products, isomers of eugenol and isoeugenol, were yielded in higher percentages. The experimental reaction rate data was fit to a Langmuir–Hinshelwood model, and kinetic parameters were analyzed, revealing how the adsorbed CA molecules on the catalytic surface had higher mobility with the synthesized catalyst compared with the commercial catalyst, the value of $\Delta S_{\mathrm{ads}}^{0}$ for the synthetic catalysts were $-$5.48 and $-$4.31 J/mol-K and for the commercial catalyst $-$37.17 J/mol-K.

Keywords

• Coniferyl aldehyde
• Aluminum–boron oxide
• Thermocatalytic
• Lignin
• Biomass
• Eugenol