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An ATR‐FTIR Study on the Effect of Molecular Structural Variations on the CO 2 Absorption Characteristics of Heterocyclic Amines, Part II
Author(s) -
Robinson Kelly,
McCluskey Adam,
Attalla Moetaz I.
Publication year - 2012
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201200066
Subject(s) - chemistry , amine gas treating , reactivity (psychology) , piperazine , diamine , aqueous solution , absorption (acoustics) , hydrolysis , medicinal chemistry , fourier transform infrared spectroscopy , solubility , organic chemistry , medicine , physics , alternative medicine , pathology , quantum mechanics , acoustics
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.