The influence of nanopowder metal oxides on the methylating activity of dimethyl carbonate in analytical pyrolysis

Nanopowder aluminum oxide, silicon oxide, titanium silicate and aluminium titanate were tested as active solids for pyroysis methylation with dimethyl carbonate (DMC) in combination with gas chromatography and mass spectrometry (Py–GC–MS). Pyrolysis methylation was applied to triglycerides (soybean oil, 1,3-palmitin-2-stearin), poly(3-hydroxybutyrate) (PHB), poly(ɛ-caprolactone) (PCL), ferulic acid and 5-hydroxymethyl-2-furaldehyde. Analyses were performed by means of a resistively heated filament pyrolyser on the sample dissolved into DMC and adsorbed onto few milligrams of the metal oxide. Among the selected materials, only titanium silicate (TiSiO4) promoted the formation of methyl esters of butenoic acids from PHB and hexenoic acids from PCL. The other metal oxides enabled the pyrolytical transesterification of triglycerides to fatty acid methyl esters (FAMEs), but TiSiO4 provided the highest FAME yields with the best precision, as confirmed by quantitative off-line pyrolysis experiments. According to the proposed mechanism, TiSiO4 assisted the DMC methylation of carboxylic acids produced by thermal degradation, improving their detection by GC analysis of the corresponding methyl esters. However, unsaturated compounds were subjected to double bond isomerisation, while ferulic acid and 5-hydroxymethyl-2-furaldehyde were not methylated at significant levels.