About the Influence of Doping Approach on the Alkali Metal Catalyzed Slow Pyrolysis of Xylan

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.

Inherent Metal Elements in Biomass Pyrolysis: A Review

“…As found for cellulose, also in the case of xylan-doped samples, many authors detected an increase of char and gas yields and a decrease of the condensable fraction yield. ,,,− The presence of metal salts promoted the formation of char, water, furfural, light oxygenated species, and gas at the expense of xylose, anhydro xylose, and dianhydro xylose. ,,,, …”

“…The xylan samples have been doped mainly with AAEMs. Both organic (acetate, formate) and inorganic salts (chlorides, nitrate, carbonates, oxides) have been used, ,,− and the techniques used for doping were the same proposed for the cellulose: wet impregnation, dry mixing, and cation exchange. ,,− However, the water solubility exhibited by xylan hampers a correct execution of the water impregnation procedure. Indeed, as underlined by Collard et al the impregnation solution cannot be removed by filtration but only by evaporation, leaving also precipitated salts in the sample .…”

“…Indeed, as underlined by Collard et al the impregnation solution cannot be removed by filtration but only by evaporation, leaving also precipitated salts in the sample . Differently, the doping approach based on cationic exchange , takes advantage of the xylan water solubility ensuring a tight interaction between the metal ions and the polysaccharide, and it avoids the presence of counterions that can hinder the catalytic effect of the added cations.…”

“…Giudicianni and colleagues demonstrated that beechwood xylan can be completely demineralized, and the glucuronic acid units present in its structure convert in the neutral form by using a cationic Dowex resin in the H-form . The same authors showed that the demineralized beechwood xylan can be further doped with growing amounts of K + or Na + by using the same cationic Dowex resin in a K-form or Na-form, respectively, , and that such a procedure was more effective than wet impregnation for the investigation of the catalytic role of alkali ions in xylan pyrolysis …”

“…It is likely that the cationic exchange approach ensures a more intimate contact and a tighter interaction between the polysaccharidic chain and the metal ions through ionic bonding. Moreover, a mitigation effect of the counterion (Cl – ) cannot be excluded when doping is performed through wet impregnation …”