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Effect of a Vacuum on the Fast Pyrolysis of Cellulose: Nature of Secondary Reactions in a Liquid Intermediate

M. Brennan Pecha, Jorge Ivan Montoya, Farid Chejne, Manuel Garcia-Perez
Industrial & engineering chemistry process design and development 2017 v.56 no.15 pp. 4288-4301
cellulose, crosslinking, decarboxylation, evaporation, furfural, gas chromatography, gases, high performance liquid chromatography, hydrogen, ionization, liquids, mass spectrometry, methanol, oils, oligosaccharides, process design, pyrolysis
Pyrolysis of thin films of cellulose was performed at five pressures from 4 mbar (vacuum) to 1 bar at 500 °C in a modified pyroprobe captive sample reactor designed to minimize gas-phase secondary reactions. Known molecule products were identified and quantified by gas chromatography (GC), GC/mass spectrometry (MS), and high-performance liquid chromatography, and yields were calculated. Vacuum allowed the evaporation of cellobiosan and promoted the release of heavy oligosaccharides by thermal ejection (cellotriosan, etc.), reducing in this way the residence time of the oligomeric products in the liquid intermediate and, consequently, their secondary reactions. The char yield increased from 3.6 to 8.3% in the range of pressures studied. The unknown heavy fractions in the oil were studied in more detail by high-resolution mass spectrometry using positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results illustrated a dominant dehydration reaction pathway in the liquid intermediate to form distinct parent molecules from which fragmentation reactions occur. Notable reactions were decarboxylation, loss of glycoaldehyde, 5-(hydroxymethyl)furfural, and 1,4:3,6-dianhydro-α-d-glucopyranose, and gain of hydrogen, methanol, and water. Furthermore, as the pressure increased, cross-linking and polyaromatic ring formation in the liquid intermediate were enhanced. A new cellulose pyrolysis reaction scheme stressing the secondary reactions in the liquid intermediate was proposed to reflect the results of this study.