Search for the Microwave Nonthermal Effect in Microwave Chemistry: Synthesis of the Heptyl Butanoate Ester with Microwave Selective Heating of a Sulfonated Activated Carbon Catalyst

The heptyl butanoate ester was synthesized from butanoic acid and heptanol in a heterogeneous medium in the presence of sulfonated activated carbon (AC-SO3H) catalyst particles subjected to microwave irradiation, which led to higher conversion yields (greater product yields) than conventional heating with an oil bath. The advantage of the microwaves appeared only when the moisture content in the butanoic acid batch(es) was high, suggesting that, unlike conventional heating, the reverse reaction caused by the moisture content and/or by the byproduct water was suppressed by the microwaves. This contrasted with the results that were found when carrying out the reaction in a homogeneous medium in the presence of the 2,4,6-trimethylpyridinium-p-toluene sulfonate (TMP-PTS) catalyst, as product yields were not improved by microwave heating relative to conventional heating. The removal of moisture/water content in the reaction solution was more pronounced when the reactor was cooled, as the reaction yields were enhanced via selective heating of the heterogeneous catalyst. A coupled electromagnetic field/heat transfer analysis gave credence to the selective heating of the AC-SO3H catalyst, which was further enhanced by cooling the reactor. It was deduced that unforeseen impurities and local high-temperature fields generated on the surface of small fine catalyst particles may have had an effect on the microwave chemistry such that the associated phenomena could be mistaken as originating from a nonthermal effect of the microwaves. Accordingly, it is highly recommended that impurities and selective heating be taken into consideration when examining and concluding the occurrence of a microwave nonthermal effect.