Grafted Poly(1→4-β-glucan) Strands on Silica: A Comparative Study of Surface Reactivity as a Function of Grafting Density

Grafted poly(β-glucan) (β-glu) strands on the surface of silica are synthesized with varying degrees of grafting density, and display an amorphous-like environment via ¹³C CP/MAS NMR spectroscopy. Thermal gravimetric analysis of these materials under oxidative conditions shows increased β-glu thermal stability with higher degrees of grafting density. The range of temperature stability between the most and least hydrogen-bound grafted β-glu strands spans 321 to 260 °C. This range is bound by the combustion temperature previously measured for crystalline and amorphous cellulose, with the former having greater oxidative stability, and is likely controlled by the extent of hydrogen bonding of a grafted β-glu strand with the underlying silica surface. When using these materials as reactants for glycosidic bond hydrolysis, the total number of reducing ends formed during reaction is quantified using a BCA colorimetric assay. Results demonstrate that the material with greatest interaction with silica surface silanols undergoes hydrolysis at an initial rate that is 6-fold higher than the material with the lowest degree of such interaction. The role of the surface as a reactive interface that can endow oxidative stability and promote hydrolysis activity has broad implications for surface-catalyzed processes dealing with biomass-derived polymers

Understanding the Role of Defect Sites in Glucan Hydrolysis on Surfaces

Though unfunctionalized mesoporous carbon consisting of weakly Brønsted acidic OH-defect sites depolymerizes cellulose under mild conditions, the nature of the active site and how this affects hydrolysis kineticsthe rate-limiting step of this processhas remained a puzzle. Here, in this manuscript, we quantify the effect of surface OH-defect site density during hydrolysis catalysis on the rate of reaction. Our comparative approach relies on synthesis and characterization of grafted poly­(1→4-β-glucan) (β-glu) strands on alumina. Grafted β-glu strands on alumina have a 9-fold higher hydrolysis rate per glucan relative to the highest rate measured for β-glu strands on silica. This amounts to a hydrolysis rate per grafted center on alumina that is 2.7-fold more active than on silica. These data are supported by the lower measured activation energy for hydrolysis of grafted β-glu strands on alumina being 70 kJ/mol relative to 87 kJ/mol on silica. The observed linear increase of hydrolysis rate with increasing OH-defect site density during catalysis suggests that the formation of hydrogen bonds between weakly Brønsted acidic OH-defect sites and constrained glycosidic oxygens (i.e., those juxtaposed adjacent to the surface) activates the latter for hydrolysis catalysis. Altogether, these data elucidate crucial structural requirements for glucan hydrolysis on surfaces and, when coupled with our recent demonstration of long-chain glucan binding to mesoporous carbon, present a unified picture, for the first time, of adsorbed glucan hydrolysis on OH-defect site-containing surfaces, such as unfunctionalized mesoporous carbon

Fundamental Insights into the Nucleation and Growth of Mg–Al Layered Double Hydroxides Nanoparticles at Low Temperature

Nucleation and growth of the Mg–Al layered double hydroxide nanoparticles are controlled by varying the nucleation temperature in conjunction with a fast addition of metal precursor solution. Nanoparticles of the size between 20 and 200 nm are obtained, while avoiding the use of organic structure directing agents and forgoing aging process. Nanoparticles self-assembly in solution is discussed and correlated to the agglomeration process

Glucan Adsorption on Mesoporous Carbon Nanoparticles: Effect of Chain Length and Internal Surface

The adsorption of cellulose-derived long-chain (longer than ten glucose repeat units on size) glucans onto carbon-based acid catalysts for hydrolysis has long been hypothesized; however, to date, there is no information on whether such adsorption can occur and how glucan chain length influences adsorption. Herein, in this manuscript, we first describe how glucan chain length influences adsorption energetics, and use this to understand the adsorption of long-chain glucans onto mesoporous carbon nanoparticles (MCN) from a concentrated acid solution, and the effect of mesoporosity on this process. Our results conclusively demonstrate that mesoporous carbon nanoparticle (MCN) materials adsorb long-chain glucans from concentrated acid hydrolyzate in amounts of up to 30% by mass (303 mg/g of MCN), in a manner that causes preferential adsorption of longer-chain glucans of up to 40 glucose repeat units and, quite unexpectedly, fast adsorption equilibration times of less than 4 min. In contrast, graphite-type carbon nanopowders (CNP) that lack internal mesoporosity adsorb glucans in amounts less than 1% by mass (7.7 mg/g of CNP), under similar conditions. This inefficiency of glucan adsorption on CNP might be attributed to the lack of internal mesoporosity, since the CNP actually possesses greater external surface area relative to MCN. A systematic study of adsorption of glucans in the series glucose to cellotetraose on MCN shows a monotonically decreasing free energy of adsorption upon increasing the glucan chain length. The free energy of adsorption decreases by at least 0.4 kcal/mol with each additional glucose unit in this series, and these energetics are consistent with CH−π interactions providing a significant energetic contribution for adsorption, similar to previous observationsin glycoproteins. HPLC of hydrolyzed fragments in solution, ¹³C Bloch decay NMR spectroscopy, and GPC provide material balance closure of adsorbed glucan coverages on MCN materials. The latter and MALDI-TOF-MS provide direct evidence for adsorption of long-chain glucans on the MCN surface, which have a radius of gyration larger than the pore radius of the MCN material

Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn–Na–W/SiO₂ Catalyst

The work here presents a thorough evaluation of the effect of Mn–Na–W/SiO₂ catalyst surface parameters on its performance in the oxidative coupling of methane (OCM). To do so, we used microporous dealuminated β-zeolite (Zeo), or mesoporous SBA-15 (SBA), or macroporous fumed silica (Fum) as precursors for catalyst preparation, together with Mn nitrate, Mn acetate and Na₂WO₄. Characterizing the catalysts by inductively coupled plasma–optical emission spectroscopy, N₂ physisorption, X-ray diffraction, high-resolution scanning electron microscopy–energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and catalytic testing enabled us to identify critical surface parameters that govern the activity and C₂ selectivity of the Mn–Na–W/SiO₂ catalyst. Although the current paradigm views the phase transition of silica to α-cristobalite as the critical step in obtaining dispersed and stable metal sites, we show that the choice of precursors is equally or even more important with respect to tailoring the right surface properties. Specifically, the SBA-based catalyst, characterized by relatively closed surface porosity, demonstrated low activity and low C₂ selectivity. By contrast, for the same composition, the Zeo-based catalyst showed an open surface pore structure, which translated up to fourfold higher activity and enhanced selectivity. By varying the overall composition of the Zeo catalysts, we show that reducing the overall W concentration reduces the size of the Na₂WO₄ species and increases the catalytic activity linearly as much as fivefold higher than the SBA catalyst. This linear dependence correlates well to the number of interfaces between the Na₂WO₄ and Mn₂O₃ species. Our results combined with prior studies lead us to single out the interface between Na₂WO₄ and Mn₂O₃ as the most probable active site for OCM using this catalyst. Synergistic interactions between the various precursors used and the phase transition are discussed in detail, and the conclusions are correlated to surface properties and catalysis