촉매를 이용한 이산화탄소 고정화 및 고리형 카보네이트 합성반응에 대한 계산화학적 해석

Abstract

In this study, a computational chemistry methodology called as molecular modeling has been applied to explain several experiment results mechanistically. More specifically, probable reaction pathways have been extensively investigated based on energetics and the most probable reaction pathways have been proposed. The reaction chosen for this study is to remove carbon dioxide, known as a primary green house gas, by an epoxide via the carbon dioxide fixation to produce carbonates. This reaction inherently needs the use of catalysts because it has a significantly high activation barrier (55~59 kcal/mol). Among various types of catalysts, we have studied in zeolitic imidazolate framework (ZIF)-90, polystyrene-supported quaternized ammonium salt, KI/KI-glycine, dimethylethanolamine (DMEA). This study have predicted the reaction mechanisms with considered catalysts and provided theoretical bases for experimental results. 1) ZIF-90 ZIF is a class of metalorganic framework (MOF) to resemble the phases of zeolites. Unlike MOFs, ZIFs have excellent catalytic activity without co-catalyst. Although we used the smallest repeated section of ZIF-90 to save the calculation time (so-called, cluster approximation), a very possible reaction pathway for carbon dioxide fixation was identified where Zn and the aldehyde functional ligand in ZIF-90 play a major role. 2) Polystyrene quaternized ammonium salt Similar to the ZIF-90 case, only a styrene monomer was considered for calcualtion because polystyrene is an infinite chain polymer. The proposed reaction pathway presented a close interaction between amine (–NH) ligand in ammonium salt and oxygen in epoxide. 3) KI/KI-glycine KI is a type of metal halide salts. The catalyst efficiency of metal halides is highly dependent upon the presence of co-catalysts. An interesting experimental findings was that amino acid alone and metal halide alone give 12% and 20% conversion at the maximum but a binary mixture of amino acid and metal halide show a significantly improved catalytic performance. Although the calculated activation barriers for KI (metal halide only) and KI-glycine (binary mixture of metal halide and amino acid) were similar, it was found that the intermediate for the KI-glycine case is located very stable before carbon dioxide insertion. In other words, the whole reaction proceeds more forwardly for the KI-glycine case, explaining a high yield in experiments 4) Dimethylethanolamine (DMEA) DMEA is a kind of alkanolamine that does not contain metal or halide salts. The calculation results showed that the activation barrier is considerably lowered (~42 kcal/mol) only by a reaction between amino and hydroxyl groups. The introduction of computational chemistry has a greater value in combining experiment and theory. One of its advantages is to reduce trial-and-errors in experiments because we possibly predict future experimental results. Another advantage is to identify exact reaction mechanisms by performing calculations for every step of the whole reaction and comparing with real experimental data. It thus presents a theoretical interpretation for experimental results. The computational chemistry approach used in this study is very general and so expected to be used in other various chemical processes.