PNNL

Abstract

Hierarchically ordered positive and negative replicas of wood cellular structures prepared using surfactant templating methods under acidic and basic conditions have been studied by means of continuous flow hyperpolarized 129Xe NMR, solid-state MAS 13C, 29Si, and 2D WISE NMR spectroscopic techniques. 129Xe NMR data confirm a highly ordered and uniform structure with interconnected porosity in the positive silica wood replicas (SWR(+)) prepared under acidic conditions. In contrast, non-uniform porosity with irregular pore structures is inferred from similar data for in negative silica wood replicas (SWR(-)) prepared under basic conditions. This contrasts with results from N2 adsorption and TEM measurements that indicate regular nanoporous channels in both cases. From 13C MAS NMR spectra, significant leaching of wood lignin was found to occur under acidic but was not evident for samples subjected to basic treatment. 29Si MAS NMR spectra revealed higher hydroxylation levels for the silica replica prepared under acidic conditions compared to those observed in samples prepared under basic conditions. 2D WISE NMR experiments showed increased mobility of protons associated with the organic functional groups in cellulose after acidic treatment when compared with the dry and base-treated wood. Reported NMR data provide compelling evidence for the proposed mechanism of surfactant directed mineralization of wood cellular structures in acidic and basic solutions.