Thermal decomposition of model compound of algal biomass

Predicting the Decomposition Mechanism of the Serine α-Amino Acid in the Gas Phase and Condensed Media

Comprehending the nitrogen combustion chemistry during the thermal treatment of biomass demands acquiring a detailed mechanism for reaction pathways that dictate the degradation of amino acids. Serine (Ser) is an important α-amino acid that invariably exists in various categories of biomass, most notably algae. Based on density functional theory (DFT) coupled with kinetic modeling, this study presents a mechanistic overview of reactions that govern the fragmentation of the Ser compound in the gas phase as well as in the crystalline form. Thermokinetic parameters are computed for a large set of reactions and involved species. The initial decomposition of Ser is solely controlled by a dehydration channel that leads to the formation of a 2-aminoacrylic acid molecule. Decarboxylation and deamination routes are likely to be of negligible importance. The falloff window of the dehydration channel extends until the atmospheric pressure. Bimolecular reactions between two Ser compounds simulate the widely discussed cross-linking reactions that prevail in the condensed medium. It is demonstrated that the formation of the key experimentally observed products (NH3, CO2, and CO) may originate from direct bond fissions in the melted phase of Ser prior to evaporation. A constructed kinetic model (with 24 reactions) accounts for the primary steps in the degradation of the Ser molecule in the gas phase. These steps include dehydration, decarboxylation, deamination, and others. The kinetic model presents an onset decomposition temperature of 700 K with the complete conversion attained at ∼1090 K. Likewise, the model portrays the temperature-dependent increasing yields of CO2 and NH3. The results presented in this work offer a detailed analysis of the intricate chemical processes involved in nitrogen transformations, specifically in relation to amino acids. Amino acids play a crucial role as the primary nitrogen carriers in biomass, such as microalgae and protein-rich biomass.

Formation mechanism of NOx precursors during the pyrolysis of 2,5-diketopiperazine based on experimental and theoretical study

Incineration of food waste leads to the release of NO_x pollutants, whereas the formation mechanism of the NO_x precursors (HCN, NH₃, and HNCO) during the initial pyrolysis process is far from well-studied, limiting the source control on NO_x release. In this work, 2,5-diketopiperazine (DKP) was selected as the N-containing model compound to study the formation mechanism of NO_x precursors in food waste pyrolysis, by combining experiments and density functional theory (DFT) calculations. The C1-N2 bond broken via the N2-to-N5 H-transfer possesses the lowest energy barrier, together with the largest reaction rate constants in the range of 400-800 °C. NH₃ can be easily generated with low energy barriers and high rate constants at low temperatures (below 630 °C). Whereas, the rate constants of the pathways for HCN formation will exceed those for NH₃ generation in the range of 630-740 °C. In addition, the DKP pyrolysis can also lead to the formation of HNCO with a very low energy barrier, and it can convert into HCN and NH₃ through further hydrogenation and decomposition. These calculation results are exactly consistent with the experimental results that NH₃ was the main precursor in the range of 400-600 °C, and the yield of HCN exceeded that of NH₃ when the temperature was over 600 °C. Our current work on the formation mechanism of NO_x precursors during the pyrolysis of DKP can provide theoretical guidance for the development of NO_x control technology in the pyrolysis/combustion process of organic waste.

Density functional theory-based investigation of HCN and NH3 formation mechanisms during phenylalanine pyrolysis

As sludge pyrolysis produces large amounts of toxic NH3 and HCN, many works have studied nitrogen transfer during this process, commonly employing amino acids as models of sludge protein. Herein, density functional theory is used to probe the production of HCN and NH3 during the pyrolysis of phenylalanine as a model, revealing the existence of two formation paths for each gas. In the first (lower-energy-barrier) NH3 formation path, the hydrogen bonding-assisted transfer of carboxyl group hydrogen to the amino group is followed by direct NH3 generation via decarboxylation, and the second (higher-energy-barrier) path features decarboxylation followed by the transfer of carboxyl group hydrogen to the adjacent carbon atom to form phenethylamine, the deamination of which affords NH3 and styrene. For HCN, the first (lower-energy-barrier) path features C2-C3 bond cleavage to afford dehydroglycine, which further decomposes to produce HCN, while in the second path, the decomposition of phenylalanine into phenethylamine, CO, and H2O is followed by internal hydrogen transfer in phenethylamine to generate HCN. The overall energy barrier of the two HCN formation paths exceeds that of NH3 formation paths, i.e., phenylalanine is more prone to afford NH3 than HCN upon pyrolysis.

Mechanistic formation of hazardous molecular heterocyclic amines from high temperature pyrolysis of model biomass materials: cellulose and tyrosine

Background

Research inventories on the co-pyrolysis of major biomass components such as cellulose with amino acid materials is scarce in literature despite the fact that such studies are critical in understanding toxic product relations from high temperature cooking, combustion of bio-fuels, cigarette smoking and forest fires. This paper explores, quantitatively, the yields of heterocyclic nitrogenated molecular reaction products of grave mutagenetic concern from the co-pyrolysis of model biomass materials; tyrosine and cellulose. Research has established that heterocyclic amines such as isocyanates are mutagens as well precursors for asthma, and other respiratory disorders.

Methods

An equimassic mixture of tyrosine and cellulose (50 ± 2 mg) by weight were pyrolyzed in a tubular quartz reactor in flowing nitrogen at 1 atm. Besides, varying combinations of tyrosine and cellulose in the ratios 3:1 and 1:3 were also explored for comparison. The reaction time was set at 2 s so as to simulate combustions events in nature. The pyrolysate was collected over 5 mL dichloromethane and characterized using a gas chromatograph coupled to a mass spectrometer detector.

Results

Evidently, it was noted that 1-methylindazole was released in high yields at 300 °C, constituting ~ 300 µg in the entire pyrolysis temperature range (200–700 °C). Nonetheless, isoindazole gave the highest yield ~ 730 µg while 1-naphthyl isocyanate gave a total yield of ~ 336 µg in the same temperature range. Remarkably, the change in char yield between 300 and 450 °C for the pyrolysis of 25% tyrosine in 75% cellulose was found to be ~ 48% whereas the change in char yield for the pyrolysis of 75% tyrosine in 25% cellulose was 49%.

Conclusion

The char and tar yields considered important residues of biomass burning have been reported in this study and found to be consistent with other research output in literature. The striking similarities of % yield of char across all temperatures for various combinations was the most significant observation in this investigation—char yield was independent of the mixing ratio during pyrolysis. From a mechanistic standpoint, it was noted that tyrosine inhibited cellulose based nitrogenated products. Thus N-products dominated the O-products.