Article pubs.acs.org/JPCA
The Role of Catalysis in Alkanediol Decomposition: Implications for General Detection of Alkanediols and Their Formation in the Atmosphere Manoj Kumar* and Joseph S. Francisco Department of Chemistry, University of NebraskaLincoln, 639 North 12th Street, Lincoln, Nebraska 68588, United States
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S Supporting Information *
ABSTRACT: Quantum chemical calculations have been carried out to investigate the gas-phase structure, stability, and decomposition of the two simplest alkanediols, methanediol and 1,1-ethanediol, in the presence of various catalysts. Three different conformers for monomeric alkanediols namely cis, trans, and trans′ were considered. The calculations reveal that alkanediols may exist not only as monomers but also as dimers that have high binding energies of 7−11 kcal/mol due to hydrogen bonding among the oxygenate functionalities. Some of these dimers have high dipole moments and, thus, may be more easily detected experimentally than the monomers of alkanediols. For the decomposition of alkanediols, the calculations dominantly favor dehydration over dehydrogenation. The relatively low barrier for the decomposition of 1,1-ethanediol suggests that the structure of an alkanediol plays a role in its decomposition. Though the dehydration of alkanediols with or without water catalyst involves large barriers, organic and inorganic acids, the hydroperoxyl radical catalytically influences the reaction to such an extent that the dehydration reaction either involves significantly reduced barriers or essentially becomes barrierless. Considering that alkanediols contain hydroxyl groups and their dimers have high binding energies, the gas-phase dehydration may be self-driven. Because acids are present in significant amounts in the troposphere, results suggest that diol dehydration may be facile under atmospheric conditions.
I. INTRODUCTION
and CH2OH radicals is another synthetic pathway for the alkanediol formation in astrophysical ices.15 The alkanediol chemistry also plays a crucial role in the prebiotic synthesis of amino acids such as glycine in the interstellar medium. The reactions of formaldehyde with amines have been suggested to account for the interstellar formation of hexamethylenetetramine, which is one of the dominant products when ice mixtures are photolyzed and heated.14,16−18 Alkanediols are also important industrial intermediates in the manufacturing of resins, plastics, adhesives, and many other commercial products. In the aqueous phase, alkanediols are spontaneously formed by the hydration of aldehydes at ambient conditions.19 Analytical experiments reveal that in a 5% by weight aqueous solution of formaldehyde, approximately 80% methanediol is present.19,20 However, in hot water above 200 °C, the chemical equilibrium
Alkanediols and their dehydrated analogues are key organic entities impacting various fields such as atmosphere, aqueousphase chemistry, industry, and interstellar medium. In the atmosphere, alkanediols have been implicated as potential seed precursors for aerosol formation.1,2 Formaldehyde, which is produced by the dehydration of methanediol, is the most abundant carbonyl compound,3 and an important intermediate in the photochemical oxidation of hydrocarbons in the atmosphere. 3−9 The C 1 organic chemistry is also of considerable interest to the field of astrophysics. Many oxygenates, including alcohols, aldehydes, and carboxylic acids, have been identified in the interstellar environment.10−13 Methanediol CH2(OH)2, which is the simplest alkanediol, is also believed to form in grain surface reactions triggered by the UV or cosmic ray processing of ice mantles.13,14 The hydration of formaldehyde is the most probable mechanism for the alkanediol formation in these mantles, which is also supported by recent theoretical calculations.1,2 The reaction between OH © XXXX American Chemical Society
Received: August 6, 2015 Revised: September 2, 2015
A
DOI: 10.1021/acs.jpca.5b07642 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.jpca.5b07642
Scheme 1. Two Main Decomposition Pathways for an Alkanediol
is dominantly shifted in favor of formaldehyde.21, The alkanediols are capable of polymerizing into different poly(oxymethylene)glycols HO(CH2O)nH, depending upon the temperature, concentration, and pH of the solution.22 Methanediol has also been explored as the useful model compound in various theoretical investigations for the study of the anomeric effect.23−30 Therefore, a detailed understanding of alkanediol chemistry is of broader significance. The decomposition of alkanediols leads to either aldehydes or carboxylic acids (Scheme 1). The alkanediol decomposition has received significant experimental and theoretical attention over the past few years. The chemistry of the simplest alkanediol, methanediol, has been the focal point of most of these studies. The reaction rate constant for the dehydration of methanediol under ambient as well as industrial conditions has been measured using various chemical scavengers. LeHénaff observed k = 4.5 × 10−3 s−1 at 293 K.31 Bell, Evans, and Evans obtained a similar k = 5.1 × 10−3 s−1 at 298 K.32 The flow measurements of Sutton and Downes predicted k = 4.4 × 10−3 s−1 at 295 K.33 Los et al. found k = 5.7 × 10−3 s−1 at 298 K with pulse polarography.34 Funderburk et al. measured k = 4.2 × 10−3 s−1 at 298 K using carbazides and hydrazine as trapping agents.35 Recently, Winkelman et al. have measured the kinetics of the methanediol dehydration in aqueous sulfite/bisulfite buffer solutions at the temperatures relevant in industrial formaldehyde absorbers (T = 293−333 K).36 The calculated rate constant for the dehydration of methanediol at 293 K, k = 5.7 × 10−3 s−1 agrees with previous literature estimates at 293− 298 K.31−35 The experimental reaction rate constant for the methanediol dehydration at 293−333 K temperatures was described by an Arrhenius-type equation, k = 4.96 × 107e(−6705±300/T) s−1. From the temperature coefficient, the activation energy for reaction is estimated to be 13.3 ± 0.6 kcal/mol. Regression of the rate constants resulted in ΔH‡ = 12.7 ± 0.6 kcal/mol and ΔS‡ = −25.4 ± 2.1 (cal/mol)/K for the dehydration reaction. Theoretical calculations have also played an important role in improving our fundamental understanding of the decomposition mechanism of alkanediols, especially that of methanediol. Quantum mechanical calculations and ab initio molecular dynamics simulations have been used to gain insight into the dehydration mechanism for the uncatalyzed and watercatalyzed reactions. Böhm et al. used semiempirical PM3 as well as ab initio Hartree−Fock and second-order Møller−Plesset calculations to examine the dehydration of methanediol with and without water catalyst.37 The barrier for the uncatalyzed dehydration was estimated to be 45.1 kcal/mol at the MP2/631G** level of theory, which was lowered by 16.5 kcal/mol in the catalytic presence of a single water molecule. The semiempirical PM3 calculations led to an unreasonable description of the water-catalyzed reaction, as the barrier of the reaction was about 2.0 kcal/mol higher in the presence of water. Kent et al. used the fourth-order Møller−Plesset, coupled cluster, and infinite-pressure RRKM rate calculations to investigate the thermodynamic and kinetic stability of
methanediol under the laboratory and interstellar conditions.38 The activation energy for the unimolecular decomposition of methanediol at 300 K was calculated to be 42.8 and 44.6 kcal/ mol at the MP4/cc-pVTZ//MP2/cc-pVDZ and CCSD(T)/ccpVTZ//MP2/cc-pVDZ levels of theory, respectively. The endothermicity of the dehydration reaction at 298.15 K and 1 atm pressure was predicted to be 7.6 and 8.1 kcal/mol at the MP4 and CCSD(T) levels, respectively. Methanediol in the gas phase was found to be thermodynamically stable at typical temperatures for interstellar mantles (
