Activation of Cellulose with Alkaline Earth Metals

Alkaline earth metal ions accelerate the breaking of cellulose bonds and control the distribution of products in the pyrolysis of lignocellulose to biofuels and chemicals. Here, the activation of cellulose via magnesium ions was measured over a range of temperatures from 370 to 430 ⁰C for 20 to 2000 milliseconds and compared with activation of cellulose via calcium, another naturally-occurring alkaline earth metal in lignocellulose materials. The experimental approach of pulse heated analysis of solid/surface reactions (PHASR) showed that magnesium significantly catalyzes cellulose activation with a second order rate dependence on the catalyst concentration. An experimental barrier of 45.6 ± 2.1 kcal mol-1 and a pre-factor of 1.18 x 1016 (mmol Mg2+ / g CD)-2 * s-1 was obtained for the activation of α-cyclodextrin (CD), a cellulose surrogate, for catalyst concentrations of 0.1 to 0.5 mmol Mg+2 per gram of CD. First principles density functional theory calculations showed that magnesium ions play a dual role in catalyzing the reaction by breaking the hydrogen bonds with hydroxymethyl groups and destabilizing the reacting cellulose chain, thus making it more active. The calculated barrier of 47 kcal mol-1 is in agreement with the experimentally measured barriers and similar to that for calcium ion catalysts (~50 kcal mol-1)

Cooperative Activation of Cellulose with Natural Calcium

Naturally occurring metals such as calcium catalytically activate the inter-monomer β-glycosidic bonds in long chains of cellulose initiating reactions to volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium catalyzed reactions were measured via the method of pulse-heated analysis of solid/surface reactions (PHASR) at high temperature (370-430 °C) to reveal accelerated glycosidic ether scission with a second order rate dependence on Ca2+ ions. First principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca2+ ions that demonstrated accelerated transglycosylation kinetics with an apparent activation barrier of 50 kcal mol-1 for a cooperative calcium catalyzed cycle. The agreement of mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol-1) suggests that calcium enhances reactivity through a dual role of disrupting native H-bonding and stabilizing charged transition states

Five Rules for Measuring Biomass Pyrolysis Rates: Pulse-Heated Analysis of Solid Reaction Kinetics of Lignocellulosic Biomass

Pyrolytic conversion of lignocellulosic biomass utilizes high temperatures to thermally fragment biopolymers to volatile organic compounds. The complexity of the degradation process includes thousands of reactions through multiple phases occurring in less than a second. In this work, the requirements are established for measuring the reaction kinetics of high temperature (>400 °C) biomass pyrolysis in the absence of heat and mass transfer limitations. Additionally, experimental techniques must heat and cool biomass samples sufficiently fast to elucidate the evolution of reaction products with time while also eliminating a substantial reaction during the heating and cooling phases, preferably by measuring the temperature of the reacting biomass sample directly. These requirements are described with the PHASR (pulse-heated analysis of solid reactions) technique and demonstrated by measuring the time-resolved evolution of six major chemical products from loblolly pine pyrolysis over a temperature range of 400 to 500 °C. Differential kinetics of loblolly pine pyrolysis are measured to determine the apparent activation energy for the formation of six major product compounds including levoglucosan, furfural, and 2-methoxyphenol

Ring Activation of Furanic Compounds on Ruthenium-Based Catalysts

We employed a combination of isotopic labeling experiments, density functional theory calculations, and first-principles microkinetic modeling to investigate the mechanism of H/D exchange of furanic platform molecules. Alkylated furans (e.g., 2-methylfuran (2-MF)) exhibit appreciable H/D exchange, but furan and oxygenated furanics (e.g., furfuryl alcohol) do not. Detailed mass fragmentation pattern analysis indicates H/D exchange only occurs at unprotected α-carbons. Simulations show that, in the presence of coadsorbed toluene (solvent), the most likely pathway involves Ru surface mediated scission of the C–O bond in the furan ring at the unsubstituted carbon atom, followed by dehydrogenation, deuteration, and ring-closure steps. The degree of H/D exchange reaction depends mainly on the adsorption strength of exchange intermediates: strongly bound compounds, e.g., furan and furfuryl alcohol, inhibit H/D exchange via site blocking and slow desorption, whereas alkylated furans are sterically repelled by the solvent freeing up catalyst sites for exchange at the unsubstituted α-carbon of the furan ring. The binding strength of exchange intermediates is governed by interaction of the substituent group both with the surface and with the coadsorbed solvent molecules. The proposed H/D exchange mechanism on metal catalysts, which involves the opening of furan ring, is in stark contrast to the Brønsted catalyzed ring activation and suggests a possible pathway for the formation of ring-opening products and for rational selection of solvents

Ring activation of furanic compounds on ruthenium-based catalysts

Περίληψη: We employed a combination of isotopic labeling experiments, density functional theory calculations, and first-principles microkinetic modeling to investigate the mechanism of H/D exchange of furanic platform molecules. Alkylated furans (e.g., 2-methylfuran (2-MF)) exhibit appreciable H/D exchange, but furan and oxygenated furanics (e.g., furfuryl alcohol) do not. Detailed mass fragmentation pattern analysis indicates H/D exchange only occurs at unprotected α-carbons. Simulations show that, in the presence of coadsorbed toluene (solvent), the most likely pathway involves Ru surface mediated scission of the C–O bond in the furan ring at the unsubstituted carbon atom, followed by dehydrogenation, deuteration, and ring-closure steps. The degree of H/D exchange reaction depends mainly on the adsorption strength of exchange intermediates: strongly bound compounds, e.g., furan and furfuryl alcohol, inhibit H/D exchange via site blocking and slow desorption, whereas alkylated furans are sterically repelled by the solvent freeing up catalyst sites for exchange at the unsubstituted α-carbon of the furan ring. The binding strength of exchange intermediates is governed by interaction of the substituent group both with the surface and with the coadsorbed solvent molecules. The proposed H/D exchange mechanism on metal catalysts, which involves the opening of furan ring, is in stark contrast to the Brønsted catalyzed ring activation and suggests a possible pathway for the formation of ring-opening products and for rational selection of solvents.Παρουσιάστηκε στο: The Journal of Physical Chemistry