Low Temperature Microwave Driven C1 Reactions

The selective heating observed by Spencer et al. in 1946 by microwave irradiation is the same observation that is showing up in recent articles published on microwave driven reactions. The unique way in which microwaves heat some materials selectively (reagents and catalysts) depending on their physical properties facilitates lower observed reaction temperatures, lower energy consumption, and higher reaction rates when compared to traditional thermal heating. Because of these advantages of microwave heating there has been an increase in publications on the topic over the last 25 years, from 3 publications in 1986 to 4000 in 2011. In early literature the observed microwave efficiencies were explained by the reality that microwaves instantaneously heat systems whereas convective heating requires time to reach the desired temperature. The literature debate now hinges around if there is a "special microwave" (non thermal) effect. In the studies presented here, two microwave driven chemical reaction systems are investigated. A heterogeneously catalyzed one in which methanol is oxidized over microwave absorbing solid catalyst. In the second, the direct absoption of microwaves by carbon is used to drive the highly energy intensive carbon-steam reaction for the production of synthesis gas. Our initial catalytic studies focused on oxidation reactions that fit two criteria. One is that they are of interest as pathways to industrially important commodity or fine chemicals. The second is that there is some literature precedent that suggests that oxides such as spinels will catalyze these transformations. In the former system we have developed microwave specific catalysts materials based on magnetic spinel oxides to accomplish difficult oxidation processes. For the particular system of chromite spinels of the general form MCr2O4 (M=Fe2+, Co2+, Cu2+) we were able to oxidize methanol to formaldehyde in aqueous solutions under very mild conditions. In our studies of the carbon-steam reaction, which is the primary reaction in coal gasification, we identified a reaction that is driven thermally at high temperatures whose reactants are strongly microwave absorbing. In this study our objective was to heat the reagents using microwave irradiation in order to drive the reaction at a lower temperature than traditional heating methods. We are able to directly heat the carbon to temperatures at which it reacts with steam (also generated using the microwaves) to produce synthesis gas (CO + H2). In our work we observe that microwave heating contributes to increased reaction rates, which in one case is attributed to efficient heating and in the other case a "microwave effect". Microwave heating is underutilized by the scientific community. With the possibility of realizing a microwave effect and the assurance of highly efficient heating, researchers should consider using microwave heating as a possible substitute to the traditional convective heating that is used in many labs.