Influence of impregnated catalyst on the phenols production from pyrolysis of hardwood, softwood, and herbaceous lignins

The production of phenols through catalytic pyrolysis of different types of lignin was investigated. Samples from eucalyptus (hardwood), pine (softwood), and sugarcane bagasse (herbaceous) were impregnated with the catalysts, Al2O3, ZnO, NaOH and KOH, in quantities equivalent to 1% of the lignin mass. Pyrolysis was conducted using thermogravimetric analyser (TGA), and the released phenols were captured in thermal desorption (TD) tubes for quantification by gas chromatography/mass spectrometry (GC/MS). From the TGA profiles, some modifications in the primary mechanisms were observed, especially with the hydroxide catalysts. In comparison to non-catalysed lignin pyrolysis, KOH produced the most statistically significant effect on the total phenols yields from sugarcane bagasse (S-S) lignin, with increases up to +26%, while NaOH increased the yield from Eucalyptus (E-K) lignin by + 40%. Syringol was the main S-type phenol product, with the highest yield increase by NaOH from E-K lignin (+ 90%) and a final yield of 1.86 wt.%. NaOH also produced the highest yield increases of G-type phenols from E-K lignin, especially 4-vinylguaiacol (+ 39% and yield of 2.80 wt.%). The increased production of primary compounds (S-type and G-type) by these hydroxides was probably correlated with the enhanced depolymerisation in the first decomposition stages, as observed by TGA. Oxide catalysts had limited effects on the production of S-type and G-type compounds but were more selective towards the production of P type phenols, especially ZnO with S-S lignin (+ 36%) due to the enhancement of demethoxylation reaction.