The nitrogen transformation and controlling mechanism of NH3 and HCN conversion during the catalytic pyrolysis of amino acid

Pyrolysis can convert biomass into sustainable chemicals and fuels, but the nitrogen compounds in its non-lignocellulosic components may produce hazardous NOx and its precursors. In this paper, catalytic pyrolysis of glutamic acid (Glu), used as a representing compound for biomass nitrogen source, was investigated at bench scale to study the transformation of the nitrogen element and the mechanism of Ammonia (NH3) and Hydrogen Cyanide (HCN) formation. The yields of the liquid, gaseous and solid products and the distribution of nitrogen compounds were systematically analysed. The experimental results showed that catalytic pyrolysis with H-ZSM-5 could increase the nitrogen compounds in the gas product from 38.54 % to 53.95 %, while it can reduce the nitrogen in the tar and char by 69.22 % and 58.07 %, respectively. The result of Density Functional Theory (DFT) study showed that the deamination to ternary cyclic ethers was the optimum pathway for NH3 formation. The formation of HCN has been dominated by the removal of side chain groups (reaction path 8). The energy barrier of the reaction path 9 was reduced to a level comparable to the energy barrier of path 8 during the catalytic pyrolysis, which may compete with each other. The 8T cluster model of the H-ZSM-5 catalyst revealed that the hydrogen bond formed between the acid sites and the reactants played an significant role in decreasing the reaction energy barrier. The experimental and computational results demonstrated the controlling mechanism of acid sites on the production of NH3 and HCN compounds and provided fundamental understanding to nitrogen transformation during the biomass pyrolysis.