Conversion of methyl cinnamic acid into nitrogenous compounds during pyrolysis revealed using 15N isotopic labeling

The co-pyrolysis of the coal-like model compounds cinnamic acid and its derivatives with NH4Cl or 15NH4Cl was investigated at medium temperatures (500 °C, 600 °C, 700 °C). The resulting products were examined by GC-MS to analyze the pyrolysis process, and the validation was assisted by using nuclear magnetic resonance (NMR) detection and computational simulation, and the mechanism of product formation. Relatively high yields of pyrolysis products were observed at 600 °C. The special structure of cinnamic acid, especially the triple conjugated moiety with a benzene ring, double bond and carboxyl group is amenable to electron transfer during its pyrolysis with NH4Cl, which leads to cinnamonitrile or quinoline derivatives. However, the free radical reaction is extremely rapid, and the existing detection methods are limited to the analysis of the 1equilibrium reaction results at a certain time point. By adding 15NH4Cl for isotopic tracing, the 15N-substituted product after isotopic exchange was successfully captured. It was found that when there is no substituent group on the benzene ring, the reaction products were both quinoline and cinnamonitrile, and cinnamonitrile was slightly more than quinoline. For methyl, the pyrolysis of both 2-methylcinnamic acid and 3-methylcinnamic acid resulted in the corresponding methylcinnamonitrile as the main product due to the combined control of the reaction by the electronic and site-resistant effects of methyl, but 8-methylquinoline was also detected among the pyrolysis products of 3-methylcinnamic acid with NH4Cl, whereas the pyrolysis of 4-methylcinnamic acid yielded 7-methylquinoline as the main product. This experiment reveals the evolution of the products of cinnamic acid and its derivatives during pyrolysis with NH4Cl. At the same time, it offers a method for the preparation of substituted cinnamonitriles and quinolines using pyrolysis, while also validating the reaction mechanism using isotopic labeling.