Structural studies of bacterial cellulose through the solid-phase nitration and acetylation by CP/MAS 13C NMR spectroscopy

Abstract

The solid-phase nitration and acetylation processes of bacterial cellulose have been investigated mainly by CP/MAS 13 C NMR spectroscopy to clarify the features of these reactions in relation to the characterization of the disordered component included in the microfibrils. CP/MAS 13 C NMR spectra of bacterial and Valonia cellulose samples are markedly changed as the nitration progresses, in a similar way to the case of cotton linters previously reported; and the relative reactivity of the OH groups in the glucose residues is found to decrease in the order of O(6)H>O(2)H>O(3)H. Moreover, the nitration rate and mode greatly depend on the concentration of nitric acid in the reaction media. At dilute and medium concentrations, the O(6)H groups in the crystalline and disordered components are subjected to nitration at nearly the same rate, indicating that these two components are distributed almost at random in the entire region of each microfibril. The preferential penetration of nitric acid into each microfibril also occurs prior to nitration at the medium concentration, resulting in an increase in the mole fraction of the disordered component. In contrast, all OH groups undergo nitration very rapidly at the higher concentration, although nitration levels off to a certain extent for O(3)H groups. In solid-phase acetylation, no regio-selective reactivity is observed among the three kinds of OH groups, which may be due to the characteristic reaction that proceeds in a very thin layer between the acetylated and nonacetylated regions in each microfibril. The almost random distribution of the disordered component in the entire region of the microfibrils is also confirmed in this solid-phase acetylation. On the basis of these results, the mechanism of the solid-phase reactions and the microfibril structure are discussed.

Conversion of cellulose Iα to Iβ via a high temperature intermediate (I-HT) and other cellulose phase transformations

“…Entropic effects may play a role in the relative stability of polymorphs at a given temperature. Hydrogen bond and primary alcohol conformational disorder exist in cellulose Ia and Ib, and may affect the relative stability of these polymorphs (Langan et al 2005;Yamamoto et al 2006). With molecular simulation, achieving adequate sampling of conformational space while calculating accurate energies for each conformation can be conflicting goals.…”

Nitrocellulose Synthesis from Miscanthus Cellulose

“…The nitrogen content was quantified by the ferrous sulfate method [14], which relies on saponifying NC with concentrated sulfuric acid and on reducing the formed nitric acid with iron (II) sulfate to nitrogen oxide that generates, in excess of iron (II) sulfate, a [Fe(NO)]SO 4 complex compound that turns the solution yellow-pink. The degree of substitution (DS) was calculated by the following equation [15]: DS = (1.62 3 N)/(14 À0.45 3 N). The NC viscosity was determined by measuring the flow time of the 2 % NC solution in acetone out of a VPZh-1 capillary column.…”

Statistical approach for solid-state NMR spectra of cellulose derived from a series of variable parameters

“…This diversity is the main reason for the difficulty of the NMR spectral analysis of cellulose. In previous reports, spectral analysis has been performed by preparation with reagents 6 and by changing the measurement temperature. 7 PCA has also been used in the spectral analysis of cellulose to investigate the effects of acid hydrolysis and solvent exchange on the spectral features of different cellulose substrates, 8 but to our knowledge there is no previous example of the application of PCA to NMR spectral data collected during experiments in which only one parameter was changed in a pulse sequence, such as a relaxation time measurement sequence.…”