Value-additive utilization of agro-biomass: preparation of cellulose triacetate directly from rice straw as well as other cellulosic materials†
Non-forage crop residues like rice straw are among the most important and pertinent renewable biomass available for utilization for producing biofuels, specialty chemicals, aromatics etc. Recovery of cellulose from them and its processing for products are two key steps of value addition to these materials that are available in abundance. Of these, rice straw is perhaps the most recalcitrant of all. In this study, production of cellulose triacetate (CTA) from cellulose by three different (alternate) practical methods are reported. It has been demonstrated that cellulose in the form of microcrystalline cellulose (MCC), other celluloses commercially available from chemical suppliers, cotton linters, and rice straw as such (directly after powdering) are suitable as substrates for the reaction. The methods make use of either acetic acid, acetyl chloride or isopropenyl acetate (IPA) as the acetyl donor in the presence of methane sulfonic acid (a cheap and green reagent) as the acid catalyst. The conventional acetyl donor agent, acetic anhydride is also a suitable alternative reagent for the purpose. The post-reaction work-up procedure involves neutralization and filtration as the only two simple successive steps. The processes reported thus eliminate the need for using restricted/toxic chemicals (acetic anhydride and pyridine, respectively) and provide cellulose tri-O-acetate of high purity in excellent yields. Preparation of CTA directly from rice straw has been achieved in >70% yield, representing acetylation of the straw cellulose into its triacetate to an extent of 90%. With IPA as the acyl donor, acetone was a valuable and recoverable co-product. Although not demonstrated in this study, the residue from the acetylation reaction can be utilized for the recovery of value-added materials such as fine silica.