Modeling the impact of solid surfaces in thermal degradation processes.

Glycerol (GC, propane-1,2,3-triol), propylene glycol (PG, propane-1,2-diol), and triacetin (TA, propane-1,2,3-triyl triacetate) (Figure 1) are commonly used as solvents, food additives, and humectants in the food and tobacco industries. Their use often involves high-temperature conditions, in which thermal decomposition reactions occur, exposing the human body to potentially harmful substances formed during pyrolytic processes. For this reason, it is of great importance not only to know their pyrolysis but also to study the underlying reaction mechanisms. A detailed picture of the energetics and kinetics of the pyrolytic process is the key to making reliable predictions for the products formed under certain external conditions. Recently we investigated the pyrolysis of GC, PG, and TA in the gas phase. For GC, we provided a novel overall decomposition scheme whose characteristic is the formation of glycidol as the first intermediate representing the overall rate-limiting step. The investigation of PG dehydration led to the discovery of propylene oxide as a similar intermediate during the rate-limiting step. Regarding TA, we presented a scheme describing its complex thermal decomposition pattern for the first time, in which the rate-limiting step is predicted to consist of the initial elimination of acetic acid. For both PG and TA, we characterized major decomposition products and the corresponding energetics, complementing theoretical results with qualitative data from experiments. Unfortunately, for all the aforementioned substances, a comparison with published experimental data, if available, is a complex task. Experimental studies are always performed under specific catalytic conditions that are totally different from a clean pyrolytic gas-phase scenario. Gas-phase simulations of pyrolytic steps offer ideal conditions to study a broad range of possible chemical degradation processes in detail. However, it is also clear that carbohydrate and fat pyrolysis chemistry occurs widely wherever biomass is combusted or food is cooked. These rather complex situations can hardly be simplified by investigating such chemical compounds under pure gas-phase conditions. In fact, the presence of solid particles and the interactions of organic compounds with a variety of solid surfaces may strongly affect the energetics of the degradation processes, leading to pyrolytic patterns that are totally different to what is known for the gas phase. Several experimental studies related to surface catalyzed degradation reactions of GC have been reported, describing optimal conditions to convert GC either into acrolein or acetol. One reason for these extensive investigations is that GC became available widely as a byproduct of the transesterification of fats into biodiesel. Regarding the reactivity of GC in the presence of catalytically active surfaces, only few computational studies exist in the literature: density functional theory simulations on the reactivity of GC over zeolites and transition metals. More recently, Vlachos and coworkers reported a QM/MM investigation on the acid catalyzed dehydration of polyols in liquid water. However, to the best of our knowledge, neither experimental data nor computational results for the pyrolysis of PG and TA over solid surfaces/materials are available in the literature. Nonetheless, the information available from publications in this field is relevant only to conditions extremely different from those found in everyday life. The latter involve the exposure of GC, PG, and TA to carbon and metal-oxide particles, as in cigarettes, or when pots and pans are used in high-temperature food-preparation processes. For this reason, our study focuses on the rate-limiting steps of the gas-phase decomposition of GC, PG, and TA predicted in our recent works and simulates how barrier heights change when these reactions occur at the surfaces of different solids, characterizing the key features that Figure 1. Chemical structures of glycerol (GC), propylene glycol (PG), and triacetin (TA).