Further progress into the thermal characterization of HCN polymers

摘要

In order to further investigate the thermal characterization of hydrogen cyanide polymers, in this study we focus on the thermogravimetry (TG) of three polymer samples obtained from different synthetic routes. The first one was synthesized from an aqueous solution of equimolar amounts of sodium cyanide (NH_4Cl) and ammonium chloride (NaCN). The second one was produced from the polymerization of HCN gas in water, HCN being obtained from the reaction of NH_3, CH_4 and air over a platinum catalyst at 1100 ℃; and the last sample was synthesized from anhydrous HCN, produced by the action of sulfuric acid on sodium cyanide. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and a coupled TG-mass spectrometer (MS) were used to evaluate the thermal behavior of these black polymers, which are of significant interest in prebiotic chemistry but also in extraterrestrial chemistry. All the samples present a similar thermal behavior. Following a dehydration process, three pyrolysis stages are observed, which occur over the temperature ranges of 150-300 ℃, 300-550 ℃ and above 550 ℃. However, we observe that the char-forming reactions at higher temperatures, in the carbonization stage, is strongly dependent on the HCN polymer sample and the presence or absence of oxygen. During each stage, the same volatilization processes take place regardless of the type of HCN polymer sample. Deamination and dehydration reactions dominate in the low temperature range while thermal depo-lymerization, releasing HCN, and a denitrogenation process occur mainly at elevated temperatures. The release of isocyanic acid (or cyanic acid) and formamide are also identified within a wide range of temperatures. We discuss and interpret the thermal decomposition behavior to be related to the polymerization mechanisms in accordance with recent studies. We outline that the thermal profiles could be a good fingerprint of this complex macromolecular prebiotic material, whose structure still remains uncertain, despite of their intensive studies over the last fifty years.