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 CO(2) 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 CO(2) 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 CO(2) loadings (>0.5 mol CO(2)/mol amine). HHPY exhibits similar reactivity toward CO(2) to PZ, but with improved aqueous solubility. The α-methyl-substituted MHHPY favours HCO(3)(-) formation, but MHHPY exhibits comparable CO(2) 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 CO(2). The CO(2) absorption capacity C(A) (mol CO(2)/mol amine) and initial rates of absorption R(IA) (mol CO(2)/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.