Application of a Pyroprobe-Deuterium NMR System: Deuterium Tracing and Mechanistic Study of Upgrading Process for Lignin Model Compounds

In this study, a pyroprobe-deuterium (H-2) NMR system has been used to identify isotopomer products formed during the deuteration and ring opening of lignin model compounds. Several common model compounds for lignin and its upgraded products, including guaiacol, syringol, toluene, p-xylene, phenol, catechol, cyclohexane, methylcyclohexane, and methylcydopentane, have been examined for selective ring opening. Similar pathways for upgrading of toluene and p-xylene has been found, which will undergo hydrogenation, methyl group elimination, and ring opening process, and benzene, cyclohexane, and methylcyclohexane have been found as major intermediates before ring opening. Very interestingly, the 2H NMR analysis for the deuterium-traced ring opening of catechol on Ir/gamma-Al2O3 is almost identical to the ring opening process for phenol. The ring opening processes for guaiacol and syringol appeared to be very complicated, as expected. Benzene, phenol, toluene, cyclohexane, and methylcyclohexane have been determined to be the major products.