Dissolution Mechanism of Crystalline Cellulose in H₃PO₄As Assessed by High-Field NMR Spectroscopy and Fast Field Cycling NMR Relaxometry

Abstract

Many processes have been proposed to produce glucose as a substrate for bacterial fermentation to obtain bioethanol. Among others, cellulose degradation appears as the most convenient way to achieve reliable amounts of glucose units. In fact, cellulose is the most widespread biopolymer, and it is considered also as a renewable resource. Due to extended intra- and interchain hydrogen bonds that provide a very efficient packing structure, however, cellulose is also a very stable polymer, the degradation of which is not easily achievable. In the past decade, researchers enhanced cellulose reactivity by increasing its solubility in many solvents, among which concentrated phosphoric acid (H(3)PO(4)) played the major role because of its low volatility and nontoxicity. In the present study, the solubilization mechanism of crystalline cellulose in H(3)PO(4) has been elucidated by using high- and low-field NMR spectroscopy. In particular, high-field NMR spectra showed formation of direct bonding between phosphoric acid and dissolved cellulose. On the other hand, molecular dynamics studies by low-field NMR with a fast field cycling (FFC) setup revealed two different H(3)PO(4) relaxing components. The first component, described by the fastest longitudinal relaxation rate (R(1)), was assigned to the H(3)PO(4) molecules bound to the biopolymer. Conversely, the second component, characterized by the slowest R(1), was attributed to the bulk solvent. The understanding of cellulose dissolution in H(3)PO(4) represents a very important issue because comprehension of chemical mechanisms is fundamental for process ameliorations to produce bioenergy from biomasses.

Thermal transformation of micro-crystalline cellulose in phosphoric acid

“…Namely, disruption of intra-molecular hydrogen-bonds is crucial to increase the rate of cellulose degradation and enhance yield of glucose achievement (Xiang et al 2003). In previous papers, it has been reported that phosphoric acid is able to withdraw intra-molecular H-bonds in cellulose through formation of phosphor-ester linkages (Zhang et al 2006;Conte et al 2009). Due to this, cellulose dissolves in H 3 PO 4 , thereby consenting achievement of a homogeneous liquid solution (Zhang et al 2006;Conte et al 2009).…”

“…)-glucose (CAS N. 492-62-6), and acetonitrile HPLC grade were purchased from Sigma-Aldrich Ò (Milan, Italy) and used without further purification. The characterization of the cellulose is reported elsewhere (Conte et al 2009). …”

“…Many efforts have been made to identify solvents for cellulose in order to develop methods for the achievement of fermentable glucose (Jayme and Lang 1963;Wei et al 1996;Isogai and Atalla 1998;Pagliaro 1999;Jin et al 2007;Rinaldi et al 2008;Conte et al 2009;Zakrzewska et al 2010) and for the exploitation of fast and highly productive chemical hydrolyses which must be also economically suitable (Wei et al 1996;Pagliaro 1999;Aguilar et al 2002;Sun and Cheng 2002;Knill and Kennedy 2003;Zhang et al 2006;Zhao et al 2007;Sun et al 2007Sun et al , 2009Zhang et al 2010;Jiang et al 2011).…”

“…In the last years, dissolution in phosphoric acid (a weak mineral acid, non toxic and safer to use as compared to other inorganic acids) has been increasingly considered as a simple and economic method for cellulose pretreatment before its degradation to glucose (Pagliaro 1999;Zhang et al 2006Zhang et al , 2007Conte et al 2009;Li et al 2009). Cellulose swells in 71-80% phosphoric acid solutions, whereas at higher H 3 PO 4 concentrations dissolution occurs (Wei et al 1996;Pagliaro 1999;Zhang et al 2006;Conte et al 2009).…”

“…Cellulose swells in 71-80% phosphoric acid solutions, whereas at higher H 3 PO 4 concentrations dissolution occurs (Wei et al 1996;Pagliaro 1999;Zhang et al 2006;Conte et al 2009). Up to now, it has not been reported cellulose conversion to glucose in H 3 PO 4 solutions, although acid hydrolyses at temperatures ranging from 80 to 250°C (e.g.…”