Cooperative Activation of Cellulose with Natural Calcium

Abstract

Naturally occurring metals, such as calcium, catalytically activate the intermonomer β-glycosidic bonds in long chains of cellulose, initiating reactions with volatile oxygenates for renewable applications. In this work, the millisecond kinetics of calcium-catalyzed reactions were measured via the method of the pulse-heated analysis of solid and surface reactions (PHASR) at high temperatures (370–430 °C) to reveal accelerated glycosidic ether scission with a second-order rate dependence on the Ca 2+ ions. First-principles density functional theory (DFT) calculations were used to identify stable binding configurations for two Ca 2+ 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 the mechanism with calcium cooperativity to the experimental barrier (48.7 ± 2.8 kcal mol –1 ) suggests that calcium enhances the reactivity through a primary role of stabilizing charged transition states and a secondary role of disrupting native H-bonding.

Activation of Cellulose with Alkaline Earth Metals

“…Reaction Experiments. The α-cyclodextrin films were reacted under controlled conditions (time, temperature) by the experimental method of pulse-heated analysis of solid reactions (PHASR), as previously described [25,27] . Thin film samples (<20 μm characteristic length) were rapidly heated to reaction temperature (370 -430 ⁰C) in 20-30 milliseconds, after which a 2000 Hz PID controller maintained the sample at reaction temperature for 20 milliseconds up to 2.0 seconds.…”

“…Such an explicit model is identical to the model that was used in our previous study that examined the influence of Ca 2+ on cellulose activation kinetics. [25] The generalized gradient approximation of the PBE functional was used in conjunction with D2 corrections to account for dispersive interactions . [33,34,35] The projector augmented wave (PAW) pseudopotential was used to construct plane waves with an energy cut-off of 396 eV.…”

“…In the previous study with calcium salts, it was shown that the activation kinetics are independent of the anionic counterion identity of the salt (e.g., Cl -, NO 3 -). [25] The independence of catalytic kinetics on the anionic identity was attributed to the anions leaving the cellulose melt as volatile acid vapors before pyrolysis reactions begin. As a result of the release of volatile acids, the active catalytic site was proposed to be a Lewis-base/Lewis-acid/Lewisbase bridge of O --Ca +2 -O -.…”

“…These ions coordinate with cellulose in several positions including near the glycosidic bond oxygen atom, where they can catalyze chain scission. Experiments measuring the kinetics of cellulose activation with calcium catalyst doped into the surrogate cyclodextrin determined that calcium ions work in pairs; one ion breaks the hydrogen bonds of the C6 hydroxyl groups of a cellulose monomer, and the second calcium ion acts as a Lewis acid catalyst and stabilizes glycosidic bond cleavage [25] . By this mechanism, calcium lowers the barrier of cellulose activation (E a,Ca ~49 kcal mol -1 ) and rapidly accelerates the rate of cellulose conversion.…”

Porous membranes of quaternized chitosan composited with strontium-based nanobioceramic for periodontal tissue regeneration

“…43 The calcium ions from the apatite lattice may disrupt the native hydrogen-bonding network through the interaction of calcium with the deprotonated hydroxyl groups at C 2 and C 6 positions of adjacent chains, similar to that occur in cellulose molecule in presence of calcium ions. 44 The presence of calcium ion interacting to Micromorphology. The scanning electron microscopy images of the samples (QC-SA-0, QC-SA-1.5, and QC-SA-2.0) at a magnification of 400X are shown in Figure 5.…”