An ATR-FTIR study on the effect of molecular structural variations on the CO₂ absorption characteristics of Heterocyclic Amines: part II

This paper reports on an ATR-FTIR spectroscopic investigation of the CO₂ absorption characteristics of a series of heterocyclic diamines: hexahydropyrimidine (HHPY), 2-methyl and 2,2-dimethylhexahydropyrimidine (MHHPY and DMHHPY), hexahydropyridazine (HHPZ), piperazine (PZ) and 2,5- and 2,6-dimethylpiperazine (2,6-DMPZ and 2,5-DMPZ). By using in situ ATR-FTIR the structure–activity relationship of the reaction between heterocyclic diamines and CO₂ is probed. PZ forms a hydrolysis-resistant carbamate derivative, while HHPY forms a more labile carbamate species with increased susceptibility to hydrolysis, particularly at higher CO₂ loadings (>0.5 mol CO₂/mol amine). HHPY exhibits similar reactivity toward CO₂ to PZ, but with improved aqueous solubility. The α-methyl-substituted MHHPY favours HCO₃− formation, but MHHPY exhibits comparable CO₂ absorption capacity to conventional amines MEA and DEA. MHHPY show improved reactivity compared to the conventional α-methyl-substituted primary amine 2-amino-2-methyl-1-propanol. DMHHPY is representative of blended amine systems, and its reactivity highlights the advantages of such systems. HHPZ is relatively unreactive towards CO₂. The CO₂ absorption capacity C_A (mol CO2/mol amine) and initial rates of absorption R_IA (mol CO₂/mol amine min⁻¹) for each reactive diamine are determined: PZ: C_A =0.92, R_IA=0.045; 2,6-DMPZ: C_A =0.86, R_IA=0.025; 2,5-DMPZ: C_A =0.88, R_IA=0.018; HHPY: C_A =0.85, R_IA=0.032; MHHPY: C_A =0.86, R_IA=0.018; DMHHPY: C_A =1.1, R_IA=0.032; and HHPZ: no reaction. Calculations at the B3LYP/6-31+G** and MP2/6-31+G** calculations show that the substitution patterns of the heterocyclic diamines affect carbamate stability, which influences hydrolysis rates.