Magnetic Resonance Studies of Hierarchically Ordered Replicas of Wood Cellular Structures Prepared by Surfactant-Mediated Mineralization

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, nonuniform porosity with irregular pore structures is inferred from similar data for 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.

Hierarchically porous materials: synthesis strategies and structure design

“…A rich variety of natural materials with hierarchically porous architectures are utilized as direct biotemplates owing to their low cost and environmentally-friendly properties. Of these materials we cite bacterial threads, 414 echinoid skeletal plates, 415 eggshell membranes, 416 insect wings, 417 pollen grains, 418 plant leaves, 419 wood celluloses, 420,421 protein aggregates, 422 spider silk, 423 diatom, 87,424 and other bio-inspired systems. 146,425 Some studies have used native or chemically treated plant components as templates to fabricate many hierarchically porous materials.…”

“…Infiltration of nanoparticles or solids is the key step in the synthesis process for fully replication of all hierarchical levels of biological materials. 421,429 Coating inorganics to the surface of biotemplates followed by calcination at high temperature to remove the template is also a versatile strategy to obtain biomorphic hierarchical porous materials. [179][180][181] Hierarchical meso-/macroporous TiO 2 /SiO 2 composite materials fabricated by using LBL coating of TiO 2 nanoparticles on the surface of diatomite using phytic acid as molecular binder has been demonstrated.…”

129Xe NMR on Porous Materials: Basic Principles and Recent Applications

“…[ 46,73 ] Further applications for the investigation of porous anode materials for Li ion batteries [ 74,75 ] and silica replicas of wood structures can be found. [ 76 ] Recently, it has been shown how the combination of the results of 129 Xe EXSY and PFG NMR diffusometry can yield information about porosity of materials over several length scales. [ 77 ]…”

Xenon NMR as a Probe for Microporous and Mesoporous Solids, Polymers, Liquid Crystals, Solutions, Flames, Proteins, Imaging

“…Xe NMR data confirm a highly ordered and uniform structure with interconnected porosity in the positive silica wood replicas prepared under acidic conditions. In contrast, non-uniform porosity with irregular pore structures is inferred from similar data for negative silica wood replicas prepared under basic conditions [39].…”

“…Based on the analysis of the spectra, a model for the possible mutual location of organic matter and iron compounds in natural soils was suggested. Another study used hyperpolarized Xe to characterize the hierarchically ordered positive and negative replicas of wood cellular structures prepared using surfactant templating methods [39]. Xe NMR data confirm a highly ordered and uniform structure with interconnected porosity in the positive silica wood replicas prepared under acidic conditions.…”