Abstract
In this study, we highlighted how the catalytic effect of alkali metals on xylan pyrolysis is strongly affected by the adopted doping approach. Thermogravimetric and pyrolysis tests, up to 973 K and at a heating rate of 7 K/min, were conducted on a set of potassium- or sodium-doped xylan samples containing controlled amounts of KCl or NaCl introduced, starting from a demineralized xylan sample, through a conventional wet impregnation approach. Pyrolysis product yields from xylan-doped samples were compared with those related to the demineralized xylan sample. The performances of the doping procedure were assessed through a comparison with the data collected on raw xylan and a xylan sample doped with potassium ions by a cationic exchange approach. The results showed that the introduction of potassium ions by wet impregnation using a chloride salt negligibly affected the pyrolytic behaviour of the demineralized sample and indicated that the doping approach based on wet impregnation using chloride salts is not appropriate for the study of the effect of alkali metals on the pyrolysis of polysaccharides bearing acidic functional groups as xylan.
Highlights
In pyrolytic processes, the biomass is heated to moderate temperatures, 675–875 K, in the absence or with a very low amount of oxygen to produce solid, liquid, and gaseous products
Pyrolysis product yields from xylan-doped samples were compared with those related to the demineralized xylan sample. e performances of the doping procedure were assessed through a comparison with the data collected on raw xylan and a xylan sample doped with potassium ions by a cationic exchange approach. e results showed that the introduction of potassium ions by wet impregnation using a chloride salt negligibly affected the pyrolytic behaviour of the demineralized sample and indicated that the doping approach based on wet impregnation using chloride salts is not appropriate for the study of the effect of alkali metals on the pyrolysis of polysaccharides bearing acidic functional groups as xylan
At first, the results of TG and slow pyrolysis tests on samples containing growing KCl amounts prepared by wet impregnation were compared in order to investigate the role of the metal ion load. is approach follows the one previously used by some of the authors to investigate the effect of potassium doping by wet impregnation on cellulose pyrolysis [17]. e results disclosed in the first part of this section have been strengthened taking into account the effect of the cation type
Summary
The biomass is heated to moderate temperatures, 675–875 K, in the absence or with a very low amount of oxygen to produce solid, liquid, and gaseous products. Slow pyrolysis is a process in which the heating rate is kept slow (approximately 0.1–10 K/s) leading to higher char yields at the expense of liquid and gaseous products. Fast pyrolysis is characterized by faster heating rates (about 10–200 K/s) and, among the different pyrolysis processes, it is considered the best one for producing liquids or gases. Inherent alkali and alkaline earth metallic (AAEM) species are known to affect both the temperature and the mechanism of biomass thermal decomposition [23,24,25]. Patwardhan et al [12, 18] found that earth alkali metals were more effective in enhancing the formation of 2-furaldehyde and char at the expense of the other dehydration reaction products during the pyrolysis of both cellulose and hemicellulose. The effect of counterion was assessed showing a clear effect of Cl− in decreasing the levoglucosan yield favouring the production of furanic derivatives [12]
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