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Activation of Cellulose via Cooperative HydroxylCatalyzed Transglycosylation of Glycosidic Bonds Vineet Maliekkal, Saurabh Maduskar, Derek Saxon, Mohammadreza Nasiri, Theresa M. Reineke, Matthew Neurock, and Paul J. Dauenhauer ACS Catal., Just Accepted Manuscript • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018
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ACS Catalysis
Activation of Cellulose via Cooperative Hydroxyl-Catalyzed Transglycosylation of Glycosidic Bonds Vineet Maliekkal1,†, Saurabh Maduskar1,† , Derek J. Saxon2, Mohammadreza Nasiri2, Theresa M. Reineke1,2, Matthew Neurock1, Paul Dauenhauer1, * 1
University of Minnesota, Department of Chemical Engineering, Amundson Hall, 425 Washington Ave SE, Minneapolis, MN 55455, USA. 2 University of Minnesota, Department of Chemistry, Smith Hall, 207 Pleasant Street SE, Minneapolis, MN 55455, USA. *Corresponding Author:
[email protected] †Authors contributed equally. Abstract. The thermal activation of cellulose by initial glycosidic bond cleavage determines the overall rate of conversion to organic products for energy applications. Here, the kinetics of ether scission by transglycosylation of β-1,4-glycosidic bonds was measured using the ‘pulse-heated analysis of solid reactions’ (PHASR) method from 400-500 °C. Levoglucosan (LGA) formation from cellulose was temporally resolved over the full extent of conversion, which was interpreted via a coupled reactant-product evolution model to determine an apparent barrier of LGA formation of 27.9 kcal mol-1. In parallel, LGA formation from the glucose monomer of cellobiosan was measured at temperatures between 380-430 °C by isotopically labeling the 13C1 carbon; an apparent activation energy of LGA formation was measured as 26.9 ± 1.9 kcal mol-1. The unusually low activation barrier for LGA formation at lower temperature is in agreement with previous PHASR studies for cellulose breakdown and is indicative of catalytic rather than thermal C-O bond activation. A catalytic mechanism was proposed wherein vicinal hydroxyl groups from neighboring cellulose sheets promote transglycosidic C-O bond activation. First principle density functional theory (DFT) calculations showed that these vicinal hydroxyl groups cooperatively act to create an environment that (a) stabilizes charged transition states and (b) aids in proton transfer, thus leading to reduced activation barriers for transglycosylation. Models incorporating intra-sheet H-bonding of cellulose were also used to establish their influence on kinetics. The calculated apparent barrier (29.5 kcal mol-1) agreed well with the experimental apparent activation energy (26.9 ± 1.9 kcal mol-1) and establishes the dominant mode for cellulose activation and subsequent levoglucosan formation at lower temperatures (< 467 °C) as site-specific, vicinal hydroxyl-catalyzed transglycosylation. Keywords: Cellulose, Levoglucosan, Hydroxyl-catalyzed, Transglycosylation, Cellobiose, Cellobiosan
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Introduction. Cellulose from lignocellulosic biomass1–3 is the most abundant polymer in the world providing voluminous renewable carbon-based feedstock.4 At elevated temperatures, cellulose fragments into smaller components of one-to-six carbons which can be catalytically upgraded as a chemical route for chemicals and fuels.5,6 The kinetics of cellulose activation and volatile product formation has historically been described through lumped reaction kinetic models,7,8 which fail to describe the elementary reaction steps associated with initiating cellulose chemistry. Cellulose activation has been proposed to proceed via a fast initiation reaction which produces a short-lived intermediate called “active cellulose”9,10. This is in contrast to earlier models for cellulose fragmentation which described product formation from cellulose in a single reaction.11,12 Evidence for an “active cellulose” kinetic intermediate was recently obtained from diverging rate data of cellulose conversion above 467 °C;13 however, the molecular mechanisms of cellulose chain-breaking chemistry leading to activation remain unknown.14,15 Cellulose is comprised of long linear chains of anhydro-glucopyranose monomers which can be activated via the scission of 1,4-β-glycosidic bonds.15,16
Three general mechanisms of glycolysis,
hydrolysis, and transglycosylation have been proposed to describe glycosidic bond cleavage.17 Glycolysis of the ether linkage cleaves the polymer and produces intermediates including unsaturated pyrans and furans.18 Hydrolysis cleaves the ether linkage to produce two hydroxyl groups and a glucose chain end.19 Transglycosylation involves the breaking of the 1,4-β-glycosidic bond and formation of a new bridging bond between C1 and the C6 hydroxyl group, yielding a chain end with levoglucosan (LGA).15,20,21 LGA (1,6-anhydro-β-D-glucopyranose), a six-carbon anhydrosugar, is the most abundant product of cellulose pyrolysis with yields as high as 70%,22. Studies also report that captured intermediate of cellulose decomposition contains short-chain (two-to-eight monomer) anhydro-oligomers with LGA end groups.23,24 This indicates that transglycosylation is a probable reaction mechanism in cellulose activation. Mechanisms by which LGA forms from cellulose reactive intermediates are depicted in Scheme 1 including radical (Scheme 1A), ionic (1B), and concerted (1D) mechanisms. A mechanism of homolytic glycosidic cleavage was proposed based on increased favorability of radical formation reactions at higher temperatures.25 Ponder et al.26 and Lowary et al.27 also proposed a heterolytic mechanism that proceeded in two steps and involved the formation of an ionic intermediate. The concerted one-step transglycosylation mechanism includes the simultaneous formation of the C6-O-C1 ether bridge and breaking of the glycosidic bond.15,20,21 This concerted mechanism has been calculated to have an activation barrier in the range of 4653 kcal mol-1 ,15,20,21 which is lower than barriers for a homolytic/heterolytic cleavage.
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ACS Catalysis
Scheme 1. Mechanisms of transglycosylation to produce levoglucosan from cellobiosan, the simplest anhydro-sugar. A. Radical mechanism25. B. Ionic mechanism26,27. C. 2 step mechanism28. D. Concerted transglycosylation mechanism15,20,21. E. Atom identification scheme used consistently throughout the study.
Insights into the cellulose activation mechanism can be established by comparing ab initio calculated activation energies with experimentally measured reaction barriers. Global cellulose conversion data obtained by thermogravimetric analysis (TGA) has been previously used to extract experimentallyfitted kinetic parameters for cellulose breakdown. Activation energies obtained through such techniques however are inconsistent and vary over a wide range (10-68 kcal mol-1)7,25. One major reason for this inconsistency is that such techniques only measure the change in weight of the reacting material with time, which obscures the details about individual molecular pathways and reaction events such as cellulose activation. More recently, microreactor experimental methods have provided molecular kinetics of cellulose conversion.29,30 Thin-film31 and pulsed-film13,32 reaction techniques, including the “pulse-heated analysis of solid reactions” (PHASR) method, permit time-resolved quantification of the conversion of cellulose and the evolution of individual oxygenated products. In recent studies, Zhu et al.17 used the PHASR method to obtain pulsed-film thermal decomposition kinetics of cellulose and cyclodextrin, a cellulose surrogate. These studies revealed high activation energy
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(Ea 53.7+1.1 kcal mol-1), high pre-factor breakdown kinetics at higher temperatures (above 467 °C) which appear to agree with the concerted transglycosylation mechanism. At low temperatures, the reaction proceeds via a new low-temperature mechanism, whereby the rates are substantially higher than those extrapolated from the rates depicted in Figure 1A which proceed with an apparent barrier of 53.7+1.1 kcal mol-1. This low temperature regime is characterized by an unusually low barrier of Ea 23.2+1.9 kcal mol-1 and a pre-factor that is nearly nine orders of magnitude lower than those calculated in the higher temperature regime (Figure 1A). The low activation barrier (~23 kcal mol-1) found at low temperature is inconsistent with purely thermal C-O bond activation kinetics and appears more consistent with heterogeneous catalytic C-O bond activation.33,34 Hydrolysis has been proposed as a potential initiation mechanism for this regime. The proclivity of water to evaporate immediately at these conditions, however, makes this pathway improbable. Westmoreland and Seshadri extended these ideas suggesting that water as well as other small alcoholic products could promote the formation of LGA from glucose.35 The calculated barriers reported in these studies, however, are significantly higher than the experimental values and provide no explanation for the low pre-exponential factor. As such, there remains a need for detailed understanding of the mechanism responsible for the activation of cellulose in the low temperature kinetic regime (
