ARTICLE pubs.acs.org/JPCA
A Revisited Picture of the Mechanism of Glycerol Dehydration Teodoro Laino,*,† Christian Tuma,† Alessandro Curioni,† Evan Jochnowitz,‡ and Steffen Stolz‡,z †
IBM Research Zurich, S€aumerstrasse 4, CH-8803 R€uschlikon, Switzerland Philip Morris International R&D, Quai Jeanrenaud 5, CH-2000 Neuch^atel, Switzerland z University of Twente, Faculty EEMCS, P.O. Box 217, NL-7500 AE Enschede, The Netherlands ‡
bS Supporting Information ABSTRACT: The dehydration mechanism of neutral glycerol in the gas phase was investigated by means of metadynamics simulations. Structures, vibrational frequencies, Gibbs free energy barriers, and rate constants at 800 K were computed for the different steps involved in the pyrolytic process. In this article, we provide a novel mechanism for the dehydration of neutral glycerol, proceeding via formation of glycidol with a barrier of 66.8 kcal/mol. The formation of glycidol is the rate limiting step of the overall decomposition process. Once formed, glycidol converts into 3-hydroxypropanal with a barrier of 49.5 kcal/mol. 3-Hydroxypropanal can decompose further into acrolein or into formaldehyde and vinyl-alcohol with barriers of 53.9 and 35.3 kcal/mol, respectively. These findings offer new insights to available experimental data based on glycerol pyrolysis studies performed with isotopic labeling and on the interpretation of the chemistry of glycerol and sugars in pyrolytic conditions.
’ INTRODUCTION Carbohydrates are the primary constituents of most biomass materials and due to their ubiquitous presence their chemistry is of paramount importance. However, while the reactivity of sugars at ambient conditions was explored in the last century, the understanding of the fundamental underlying reaction mechanisms involved in the pyrolysis of the sugars is still basically undiscovered. The lack of knowledge about chemical processes involved in the pyrolysis of carbohydrates is surprising since thermal degradation processes occur widely wherever biomasses are combusted1 or food is cooked at high temperatures,2 not to mention its relevance in the formation of cigarette smoke.3 As already outlined in several studies,47 the reason for such a lack of understanding can be explained by the inherent difficulties in studying the combustion mechanisms in sugars due to the complexity arising from the large number of adjacent hydroxyl groups. Despite chemical differences, glycerol (propan-1,2,3-triol) has been largely established as the smallest compound possessing much of the functionalities of a carbohydrate but still small enough to make its behavior at high temperature more readily understood. Moreover, since it is a small molecule, it allows the application of high level and time-consuming potential energy exploration tools to characterize thermal decomposition pathways. Last, it is worth noting that the thermal decomposition of glycerol is an important topic itself. In fact, its wide presence in animal fats and vegetable oils exposes glycerol contained in food to possible thermal decomposition processes during heat treatments.8 Glycerol pyrolysis has been the subject of several experimental studies. The formation of acrolein (2-propenal), formaldehyde, r 2011 American Chemical Society
and acetaldehyde depends on the temperature6 and on the presence of water either as steam6,9 or as supercritical liquid.10,11 Recently, Paine et al.7 investigated pyrolysis mechanisms using isotopic labeling and found that the decomposition pathway is entirely unimolecular in nature. They proposed three different, highly competitive mechanisms depending on the presence of catalysts or impurities. Two studies recently published4,5 contributed to the understanding of glycerol pyrolysis using theoretical approaches. Nimlos et al.4 studied the dehydration of neutral and protonated glycerol reporting a high barrier for 1,2-dehydration (∼71 kcal/mol), which was lower for a pericyclic 1,3-dehydration (∼65 kcal/mol) and considerably lower in the case of the protonated glycerol. They concluded their study by stating that the decomposition of neutral glycerol can occur only at a relatively high temperature such as in pyrolysis or combustion. From a mechanicistic point of view, they inspected, based on chemical intuition, different reaction mechanisms: 1,2-dehydration and the pericyclic 1,3-dehydration for neutral glycerol and the formation of oxirane or oxetane intermediates for protonated species. Nimlos et al.4 did not consider the possible formation of oxirane intermediates for the neutral glycerol decomposition. More recently, Sun et al.5 investigated the possibility for neutral glycerol to dehydrate into glycidol (an oxirane intermediate). The results of their calculations suggest a barrier of ∼59 kcal/mol, also requiring high temperatures for glycerol to decompose into glycidol. Received: February 1, 2011 Revised: March 17, 2011 Published: March 31, 2011 3592
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In this article, we employ the metadynamics12 technique in order to explore in a much more thorough way the potential energy surface related to the pyrolytic decomposition of glycerol in vacuum. This exploration provides an alternative reaction mechanism for glycerol pyrolysis based on the possibility of glycerol to dehydrate into glycidol. We discuss the thermodynamic and kinetic properties of the different reaction steps with particular attention to the effect of temperature, providing rate constants k(T) obtained from canonical transition-state theory.1317
’ METHODS Metadynamics simulations,12 in contrast to standard quantum chemical approaches where results strongly depend on the chemical intuition employed to build the model, sample in a more unbiased way the potential energy space with respect to a set of internal (collective) variables. Therefore, it can be considered a useful exploratory tool offering the advantage of providing reactive trajectories that follow the minimum energy path projected in the space of collective variables. Free-energy surfaces were sampled according to the following strategy: the different systems have been first thermalized by molecular dynamics in an NVT ensemble at the temperature of 800 K using a density functional theory (DFT, PBE18) Hamiltonian. Upon completion of the thermalization process, determined by the averages and the fluctuations of the potential energy, we have started the metadynamics12 exploration, employing a set of appropriately chosen collective variables to sample the free energy space of glycerol, glycidol, and 3-hydroxypropanal, searching for low-barrier reactive events. Collective variables employed were all related to the coordination of oxygen with hydrogens and carbons, describing a water molecule either connected to the glycerol moiety or a dehydrated glycerol. The functional form employed to describe the coordination variable, as implemented in the CPMD19 code is sðO1 , C1 , C2 , C3 Þ
8 9 > RO1 C 1 6 R O 1 C2 6 R O 1 C3 6 > > > > > 1 1 1 = 1< R0 R0 R0 ¼ 12 þ 12 þ 12 > 2> R O 1 C1 R O 1 C2 R O 1 C3 > > > > 1 1 ; :1 R0 R0 R0
ð1Þ Here ROiCj are the distances between a selected oxygen (i) and the carbon atoms (j) of the glycerol moiety. R0 is the equilibrium distance of an oxygencarbon bond in glycerol, equal to 1.55 Å. Reactive trajectories were optimized using a climbing image nudged elastic band (NEB)20 approach with 20 images sampling the reactive path. These calculations were performed using the CP2K code21 and a semiempirical PM622 Hamiltonian. Starting from selected points of the PM6 optimized reactive paths, stationary points corresponding to reactants, transition structures, and products were refined using density functional theory (DFT) first with the PBE18 and, subsequently, the PBE023 parameter free exchange-correlation functionals. The choice of refining the PBE results with PBE0 is driven by the higher systematic accuracy of hybrid functionals in estimating barrier heights compared to pure gradient corrected functionals.24 In these calculations a plane wave basis set with a 100 Ry kinetic energy cutoff was employed combined with norm conserving atomic pseudopotentials.25 Periodic images of the molecules were decoupled using a Poisson solver26 with cubic computational
boxes of 15 Å edge length. Convergence was achieved in structure relaxations when all Cartesian force components became smaller than 2.0 105 atomic units. PBE0 force constant matrices (Hessians) were computed by numerical differentiation of analytical forces. Translations and rotations were projected from the Hessian. Stationary points were characterized on the basis of corresponding vibrational frequencies. Reactants and products of each reactive step had no imaginary frequencies while transition structures had exactly one imaginary frequency. In addition, transition structures were inspected by intrinsic reaction coordinate (IRC) calculations27 in both directions in order to retrieve either the reactants or the products. These calculations were performed using the CPMD code.19 The evaluation of reaction rate constants k(T) was done within canonical transition-state theory formalism kB T ΔG‡ exp kðTÞ ¼ h RT
! ð2Þ
where kB is the Boltzmann constant, T is the temperature (800 K), h is the Planck constant, ΔG‡ is the activation Gibbs free energy of the reaction, and R is the gas constant. Finite temperature entropy and energy contributions were obtained from standard statistical mechanics approaches.
’ RESULTS AND DISCUSSION In addition to the three decomposition pathways proposed by Nimlos et al.,4 we complete the picture with an energetic competitive scheme, leading either to acrolein or formaldehyde/ acetaldehyde. It is worth noting that in our simulations we sampled the free energy landscapes according a set of collective variables describing the elimination of water only; for this reason, although similar in transition state energies, we were sampling regions of the phase space different from the ones inspected by Nimlos et al.4 A similar reactivity4 can be achieved by enforcing a free energy sampling involving both water elimination and the corresponding carboncarbon bond breaking. The new decomposition mechanism of glycerol can be sketched into four different reaction steps: formation of glycidol via 1,2-water elimination (i); conversion of glycidol into 3-hydroxypropanal (ii); decomposition of 3-hydroxypropanal into vinyl-alcohol and formaldehyde (iii) or into acrolein and water (iv). In the following paragraphs we will review these steps. Glycerol can form glycidol via 1,2-elimination of water (Scheme 1, reaction 1). This reaction path has been recently investigated by Sun et al.5 who computed kinetics and thermodynamics for several structures with different relative configurations of the atoms involved. Refining the reactive trajectory using DFT, we computed the thermodynamic and kinetic quantities reported in Table 1 in good agreement with previously published results.5 With a Gibbs free energy barrier of 66.8 kcal/mol (T = 800 K), this reaction step is to be considered the rate limiting step. Upon opening of the epoxy ring, glycidol converts into 3-hydroxypropanal via the transition structure depicted in Scheme 1, reaction 2. This reaction step is extremely fast; it proceeds via formation of a transient carbocationic species (secondary carbocation). Hydrogen present on atom C1 of glycidol is transferred to the secondary carbon atom concurrently to the formation of the carbonyl group. Metadynamics runs provided no evidence of the possibility to form a keto group via formation of a primary 3593
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Scheme 1. Reaction Profile for the Pyrolysis of Glycerol
Table 1. DFT (PBE0) Total Energy, ΔE‡, Zero-Point Vibrational Energy Corrected, ΔE‡0, Enthalpy, ΔH‡(T), and Gibbs Free Energy, ΔG‡(T), Barrier Heights (in kcal/mol) As Well As Rate Constants k(T) (in 1/s) Computed for the Different Reaction Steps of Glycerol Pyrolysis Shown in Scheme 1 (T = 800 K) step
ΔE‡
ΔE‡0
ΔH‡(T)
ΔG‡(T)
k(T)
1
70.9
67.1
66.9
66.8
9 106
2
53.2
48.4
47.7
49.5
5 101
3
36.7
33.4
32.7
35.3
4 10þ3
4
58.3
53.8
54.0
53.9
3 102
carbocation which is well-known to be less stable than a secondary carbocation. 3-Hydroxypropanal may decompose into two different paths. The most favorable in terms of barrier height is the conversion into formaldehyde and the enolic form of acetaldehyde (Scheme 1, reaction 3). This reaction represents the fastest process in the decomposition of glycerol and is driven by the acidity of the hydroxyl group. Although energetically less favorable, 3-hydroxypropanal can follow a different decomposition pathway. It can decompose into water and acrolein (Scheme 1, reaction 4). Because of the larger barrier height, the formation of acrolein should be observed only at high temperatures where this process competes with the decomposition into formaldehyde and vinyl-alcohol. The formation of acrolein is driven by the acidity of the hydrogen atom at the R-carbon atom, which is known to be less acidic than a hydroxyl group. The pyrolytic studies of glycerol in steam6,9 and supercritical water10,11 cannot be directly compared to the present computational results. In fact, either supercritical water or steam at high temperature may affect the decomposition pathways, because water itself is more acidic at high temperature. Only recently, Paine et al.7 studied the decomposition of pure glycerol in the gas phase. Unfortunately, while investigating extensively the mechanisms they did not focus on any measurement of kinetic data. Interestingly enough, from a quantitative point of view, our findings complete the energetics for
mechanism B proposed by Paine7 in which the center carbon in glycerol ultimately ends up as the methyl group in acetaldehyde, thus providing strong support to their outlines.
’ CONCLUSIONS We have proposed a completely new decomposition pathway for glycerol via formation of glycidol. The present computational study shows that the mechanism is isoenergetic and therefore highly competitive with the ones already published in literature and offers a complete picture of its pyrolytic chemistry without assuming any enol-type intermediate. Since no experimental studies have ever hypothesized the pyrolytic decomposition of glycerol via glycidol formation, this work provides a substantial improvement to the interpretation of the chemistry of glycerol and sugars in pyrolytic conditions. ’ ASSOCIATED CONTENT
bS
Supporting Information. A comparison between PBE/ PBE0 barrier heights, Cartesian coordinates, and vibrational frequencies obtained for the transition structures and intermediates are available. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: þ41 (0) 44 724 8933. Fax: þ41 (0) 44 724 8958.
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