Degradation of cellulose by hydrogen chloride gas under elevated pressure in a custom-built reactor

Acid-catalysed hydrolysis of cellulose was investigated in a custom-built reactor where dry HCl gas could be dosed at a pressure range between 0.1 - 1.0 bar. The mechanism of degradation proceeds by HCl adsorption on the fibre surface, followed by dissociation of HCl in a thin water layer covering the fibres, resulting in subsequent hydrolysis. The effect of water content was also investigated by conditioning the fibres in diverse relative humidity conditions prior to the hydrolysis. The degree of polymerisation (DP) dropped quickly to levelling-off degree of polymerisation (LODP) at room temperature, signalling that all disordered segments in cellulose microfibrils can easily be hydrolysed with HCl gas. Further hydrolysis presumably from the crystallite ends was detected at higher pressures at prolonged time periods (several days). Curiously, higher water contents (up to 10%) in the fibres resulted in significant humin formation from the glucose generated in cellulose hydrolysis. Such levels of humin are rarely detected from reaction mixtures with conventional aqueous acids. The results showed that behaviour of cellulosic fibres under pressurized HCl gas is fundamentally different from the well-established conditions in aqueous hydrolytic systems. By proper understanding on what governs the hydrolysis and its side reactions, HCl gas has potential to become a viable reagent in the future biorefinery. Moreover, gas is easier to recycle and the dry reaction product is easier to purify than in the case of aqueous systems.