Tuneable Mesoporous Materials from α‐D‐Polysaccharides

Abstract

The abundancy, low cost and unique chemistry of polysaccharides makes them excellent candidates from which to develop novel mesoporous materials. Utilisation of the natural chirality inherent in all polysaccharides potentially allows the preparation of a unique class of materials, which may ultimately find use in high-value applications such as asymmetric synthesis or enantiomeric separation. The direct preparation of mesoporous a-d-polysaccharides is a new area and still represents a significant challenge with regard to controlling and manipulating properties of the material including pore volume, particle morphology and crystallinity. Conversely, inorganic mesoporous materials are well described and their use in a wide range of applications has received extensive attention since the first MCM-41 material was described. Morphological manipulation, control of textural property and functional group moderation have opened up whole new fields of research in catalysis and adsorbents. Therefore, to optimise the preparation of novel polysaccharide materials for specific applications, an understanding of the formation of the nanoporous network is still required. a-d-Polysaccharides are typically derived from a variety of different starch botanical sources (i.e. potato, corn (maize) or rice), each reflecting a specific polysaccharide composite, molecular-weight distribution and thermochemical character. The complex structure and heterogeneity of the polymer molecular weight in the naturally occurring composites makes the preparation of tuneable mesoporous materials a significant challenge. Therefore, to enable controlled and predictable preparation, an understanding of the influence of the individual components, amylose and amylopectin, on material properties must be understood. From our initial experiments, the critical step in material preparation is a thermally assisted hydration/plasticisation of the hydrogen-bond network of the polysaccharide, resulting in gelatinisation of the starch granule and the formation of an aquagel phase. There has been much discussion in the literature with regard to network formation in starch gels. However, little discussion has thus far been made on the influence of gel type or structure on the resulting properties of porous materials. Amylose and amylopectin are contrasting polymers. Amylose is an amorphous linear a ACHTUNGTRENNUNG(1–4)-linked polyglucopyranose, whilst the larger amylopectin is composed of aACHTUNGTRENNUNG(1–4) and about 5% of aACHTUNGTRENNUNG(1–6) linkages, which generate a branched and crystalline polysaccharide in its naturally occurring form. In nature, starch polysaccharides form compact supramolecular granule structures composed of alternating crystalline and amorphous zones. It might be expected with regard to reproducibility and tuneability that the gelatinisation step will be diffusion-dependent and potentially generate metastable material states. One approach to understand this kinetic challenge is to use the gelatinisation temperature as a parameter to manipulate polysaccharide response, water diffusion and polymer recrystallisation. Gels prepared at different temperatures should therefore exhibit different network structures of varying metastable and stable states that impart differing textural properties in the resulting solid. Our initial results indicate that the most promising method to regulate gelatinisation temperature is to employ microwave-assisted heating. Microwaves are known to couple efficiently with polar solvents with high dielectric constants such as water to afford rapid penetrative heating, which in turn reduces gelatinisation time, the propensity for glycosidic bond cleavage and polysaccharide depolymerisation. Furthermore, recent results suggest that microwaves may induce different starch granule responses as compared to conventional heating methods. Herein, we report on mesoporous materials from amylose, amylopectin and mixtures thereof (natural and synthetic). The properties of the material are dependent on the preparation temperature, thus allowing regulation of the textural properties, particle morphology and polysaccharide ordering of this exciting class of material. Nitrogen porosimetry was used to monitor the observed transformation from native polysaccharide to porous product. To establish the validity of this technique and to demonstrate that the key step in the preparation of the material was gelatinisation, methylene blue was used as a probe of the polysaccharide aquagel network. Calculation of the surface area by the Langmuir isotherm gave a value of 180 mg , in good agreement with BET values from N2 adsorption data, with surface areas approaching 200 mg 1 for all samples (Table 1). 9] Application of the BJH model demonstrated the lack of a defined porous structure accessible to nitrogen within the native starch granule (Figure 1A). Nitrogen porosimetry analysis shows the mesoporous nature of these polysaccharide-derived materials, which exhibit type IV H3 isotherms (Figure 1B). By this simple preparation strategy, a material with a bimodal pore size distribution was produced, with the majority of porosity lying in the mesopore region (2–50 nm region). The surface area shows no correlation with the preparation temperature, whilst the remaining textural parameters are more variable with distinct trends observed for mesopore and micropore volume (Table 1 and Figure 2A). [a] R. J. White, Dr. V. L. Budarin, Prof. J. H. Clark Green Chemistry Centre of Excellence Department of Chemistry, University of York York, YO10 5DD (UK) Fax: (+44)1904-432705 E-mail : jhc1@york.ac.uk Supporting information for this article is available on the WWW under http://www.chemsuschem.org or from the author.