Reaction Pathways for the Pyrolysis of Glycerol, Propylene Glycol and Triacetin in the Gas Phase and at Solid Surfaces


Authored by  C Tuma*, T Laino*, A Curioni*, E Jochnowitz, S Stolz

Presented at CPMD 2011     
* This author is not affiliated with PMI.

Abstract

Glycerol, propylene glycol, and triacetin are commonly used as solvents, food additives, and humectants in the food and tobacco industries. Their use often involves potential exposure to high temperatures. Using metadynamics simulations we sampled the free energy spaces of these substances for gas phase low-barrier reactive events. We refined the decomposition pathways found by locating reactants, transition structures, and products on the PBE and pbe0 energy surfaces. We also performed frequency calculations to characterize the stationary points found and to derive Gibbs free energy barrier heights and rate constants for a temperature of 800 k. For the pyrolysis of glycerol we have predicted the formation of an epoxide intermediate (glycidol) as the overall rate limiting step connected to a barrier of 66.8 kcal/mol. While formaldehyde and vinyl alcohol are found as the main decomposition products, we find the formation of acrolein to be competitive at very high temperatures only [1]. For propylene glycol we propose a novel dehydration scheme leading to the formation of propanal and only minor amounts of acetone. Similar to the case of glycerol we have predicted the formation of an epoxide intermediate (propylene oxide) as the overall rate limiting step with a barrier of 66.5 kcal/mol. For triacetin we present the first computational study related to its complex decomposition pattern. It is characterized by stepwise eliminations of acetic acid or acetic anhydride and many different intermediates. The overall rate limiting step corresponds to an initial elimination of acetic acid with a barrier of 62 kcal/mol rendering triacetin thermically slightly less stable than glycerol and propylene glycol. To study the impact of the presence of solids we have simulated the decomposition reactions at surfaces of amorphous carbon, amorphous silica and crystalline zirconia and compare to the situation in the gas phase. While silica and zirconia hardly influence the barrier height for the rate limiting steps, amorphous carbon shows a strong tendency for chemisorption reactions which results in decomposition pathways not predicted for the gas phase. 

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