An ATR-FTIR Study on the Effect of Molecular Structural Variations on the CO₂ Absorption Characteristics of Heterocyclic Amines, Part II

Abstract

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.

1. Introduction

The dominant sources of anthropogenic CO₂ emission are fossil-fuel combustion and industrial processes.^{[1, 2]} Sequestration of this CO₂ is now a major target for reduction of atmospheric CO₂ levels. While it is clear that any significant long-term reduction in greenhouse gas emissions must involve changing our approach to energy production and consumption, technologies are required to reduce levels in the short term. Coal-fired power stations are the largest point-source emitters of CO₂ in Australia and worldwide.^[3] The prospect of integrating post-combustion CO₂ capture (PCC) technology in both existing and new coal-fired power stations offers the potential to lower CO₂ emissions in the face of existing and predicted growth in the number of coal-fired power stations.^[4]

Currently, aqueous amine-based PCC is viewed as the most promising and near-ready technology for the reduction of CO₂ emissions from coal-fired power stations. PCC involves separating CO₂ from a flue gas stream by chemical absorption and re-releasing CO₂ from the absorbent by heating in a two-step process for subsequent storage or industrial use. PCC is industrially proven with absorbents such as aqueous monoethanolamine (MEA), used for decades for CO₂ removal from gas streams in small-scale commercial processes such as ammonia production and natural-gas processing.^{[5, 6]}

Despite being an established technology, deployment of current industry-standard technology (30 wt % aqueous MEA) on a large scale applies a considerable efficiency penalty to the power generation process. Regeneration of PCC absorbent is energy-intensive^[7] and will result in up to 25 % reduction in the net efficiency of a coal-fired power plant.^{[8, 9]} Clearly, the absorption/regeneration characteristics of the amine-based PCC absorbent will influence the economic feasibility of this technology. One approach to reducing the energy requirements and cost of the PCC process is the development of more cost effective and better performing amines. There is considerable scope to develop absorbents that show higher CO₂ absorption, lower regeneration costs and greater chemical stability, particularly in the face of an increasing move towards demonstration-scale PCC plants.

The CO₂ absorption/desorption by aqueous amine-based absorbents has been, and continues to be, extensively studied in the search for improvements in PCC efficiency. The CO₂ absorption/desorption process is shown schematically in Figure 1. Typically, CO₂ reacts with aqueous amines to generate carbamate (1), bicarbonate (2) and a protonated amine. The amine substitution pattern affects the products produced. Primary amines such as MEA and secondary amines like diethanolamine (DEA) and piperazine (PZ) react with CO₂ under aqueous conditions to form a carbamate derivative R₁R₂NCOO⁻ (Figure 1). The 2:1 reaction stoichiometry restricts CO₂ absorption capacity of primary and secondary amines to a theoretical upper limit of 0.5 mol CO₂/mol amine. However, it has been demonstrated that the carbamate can be hydrolysed at high CO₂ loadings (>0.5 mol CO₂/mol amine) to produce bicarbonate and regenerate a free amine,10 which allows for slightly improved absorption capacities (Figure 1). Given the chemical stability of carbamates formed from 1° and 2° amines, hydrolysis does not occur at an industrially relevant rate,11 with the exception of heterocyclic secondary amines such as piperidine, which have been demonstrated to form a labile carbamate that is readily hydrolysed.^[10]

Figure 1.

Reaction mechanism leading to carbamate (1) formation for the reaction of CO₂ with primary (R₁R₂NH, where R₁ or R₂=H) and secondary (R₁R₂NH) amines (top),^[10] which occurs in the absorption step of the PCC process (bottom).

Tertiary amines (R₁R₂R₃N) cannot react directly with CO₂ to form carbamates.^[12–^14] Tertiary amines are believed to act as catalysts facilitating the hydrolysis reaction between CO₂ and OH⁻ to form bicarbonate.^{[12, 14, 15]} This 3°-amine pathway is kinetically and thermodynamically less favourable than carbamate formation.^[16] Bicarbonate formation is advantageous in consuming only one molecule of amine per molecule of CO₂, allowing for increased CO₂ absorption capacities.

CO₂ absorption by aqueous amines is a reversible process, and the degree of reversibility is amine-dependent. Amines that form stable carbamates exhibit faster reaction rates, but a larger input of energy is required for absorbent regeneration. Conversely, amines that form more bicarbonate than carbamate exhibit slower reaction rates and require less energy for regeneration. Recent technological advances have allowed for the convenient and rapid analysis of these chemical species to be carried out in situ during the PCC absorption/desorption cycle by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. In particular, the ability of ATR-FTIR to distinguish carbamate from bicarbonate formation accelerates the screening of potential PCC amines.^{[10, 17]}

We recently reported the application of ATR-FTIR in a model PCC absorbent system with substituted piperidines.^[10] Herein we report on the in situ CO₂ absorption characteristics of a series of heterocyclic diamines (Figure 2): piperazine (PZ), 2,6-dimethyl- and 2,5-dimethylpiperazine (2,6-DMPZ and 2,5-DMPZ), hexahydropyrimidine (HHPY), 2-methylhexahydropyrimidine (MHHPY), 2,2-dimethylhexahydropyrimidine (DMHHPY) and hexahydropyridazine (HHPZ).

Figure 2.

Chemical structures of the heterocyclic amines examined for CO₂ absorption properties.

2. Results and Discussion

2.1. Infrared Spectral Analysis

The effect of structural variations on the CO₂ absorption characteristics of the heterocyclic diamines shown in Figure 2 were assessed in relation to the IR-identifiable products, that is, carbamate versus bicarbonate absorbance; CO₂ absorption capacity, defined as moles of CO₂ absorbed per mole of amine in solution (mol CO₂/mol amine); and the initial rate of CO₂ absorption (mol CO₂/mol amine min⁻¹). Full details of our experimental approach is given in the Experimental Section and also in our previous work.^[10] Each experiment was conducted until equilibrium was established and a maximum CO₂ loading achieved. This was amine-dependent but typically required 45–90 mins. Calculations were also performed to investigate the electronic/steric effects of the structural variations on the amine-carbamate derivatives.

2.1.1. Piperazine (PZ)

Our investigations commenced with parent heterocyclic diamine PZ (Figure 2). As can be seen from the partial (1750–950 cm⁻¹) FTIR spectrum collected during a typical CO₂ absorption experiment with an aqueous PZ solution (1.5 mol L⁻¹), five major FTIR peaks evolve during CO₂ absorption (Figure 3). The carbamate (NCOO⁻) derivatives of heterocyclic monoamines have been identified as giving rise to several strong absorbance bands in the 1600–1260 cm⁻¹ region, including the asymmetric , 1600–1500 cm⁻¹) and symmetric , 1450–1350 cm⁻¹) vibrations of the COO⁻ moiety and the N–COO⁻ stretching vibration , 1300–1260 cm⁻¹) of the NCOO⁻ derivative.^[10] The protonated amine (NH₂⁺) generated on absorption of CO₂ was found to give rise to an absorbance band in the 1479–1474 cm⁻¹ region (NH₂⁺ bending mode). In an amine/CO₂/H₂O system, the bicarbonate (HCO₃⁻) species was identified as giving rise to a broad peak in the 1360–1354 cm⁻¹ region (ν_sC–O).^{[10, 17]} Assignment was based on the spectral data acquired for 1-methylpiperidine (tertiary amine)/CO₂/H₂O and 2-amino-2-methyl-1-propanol (AMP)/CO₂/H₂O systems. It is known that the absorption of CO₂ by aqueous AMP (α-dimethyl-substituted MEA derivative) leads to the formation of mostly HCO₃⁻ with no significant NCOO⁻ formation.^{[14, 18, 19]} Herein these peaks can be related to the vibrational modes of the potential ionic reaction products, including PZ-carbamate (PZ-COO⁻), protonated PZ (PZ-H⁺) and bicarbonate (HCO₃⁻). PZ, being a secondary diamine, should react with CO₂ in solution to form NCOO⁻, predominately in the form of a protonated PZ-COO⁻ derivative (⁺H₂NR₁R₂NCOO⁻).^{[20, 21]} One amine moiety acts as the absorption site for CO₂, and the other as a proton acceptor. PZ has also been reported to form the dicarbamate (⁻OOC-PZ-COO⁻), which was detected by ¹H and ¹³C NMR spectroscopy, at CO₂ loadings of 0.2–0.8 mol CO₂/mol amine.^{[20, 21]}

Figure 3.

Partial IR spectral profile of an aqueous solution of PZ (1.5 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.92 mol CO₂/mol amine.

The FTIR spectra for the PZ/CO₂/H₂O system closely resembles that we previously reported for the piperidine/CO₂/H₂O system, differing only in slight shifts in key IR stretching frequencies.10 At low levels of absorbed CO₂ the PZ/CO₂/H₂O system exhibits the (1524 cm⁻¹), (1432 cm⁻¹) and (1276 cm⁻¹ and 1294 cm⁻¹) of the PZ-COO⁻ derivative and the NH₂⁺ vibration of PZ-H⁺ (1470 cm⁻¹). These peaks shift to 1546, 1425 and 1289 cm⁻¹, respectively, with increasing CO₂ absorption levels (Figure 3). As anticipated, a near-linear relationship between cumulative CO₂ absorption and IR peak intensity is observed for the spectral peaks assigned to , and PZ-H⁺. Increased peak absorbance is concomitant with the rate of NCOO⁻ formation at the reaction onset plateauing as a maximum CO₂ loading of 0.92 mol CO₂/mol amine is approached (Figure 4). This near-linear relationship differs from that observed for the bands at 1276 and 1294 cm⁻¹. From the data presented in Figures 3 and 4 the primary absorbance emerged at 1276 cm⁻¹ and was the dominant peak, but only at CO₂/mol amine loadings of 0.4–0.5 mol CO₂/mol amine. At amine loadings greater than 0.5 CO₂/mol amine the absorbance decreases correspondingly with a sharp increase in intensity of the absorbance band at 1294 cm⁻¹ and a frequency shift to 1289 cm⁻¹. This trend in peak absorbance in the 1294–1276 cm⁻¹ region is attributed to formation of ⁻OOC-PZ-COO⁻. The IR absorbance of PZ/CO₂/H₂O in this region differs from those of all other heterocyclic amine and diamine systems thus far reported, and the remaining subset of secondary heterocyclic diamines analysed in this study (see below) displays only a single absorbance band in the 1283–1272 cm⁻¹ region.

Figure 4.

Relationship between cumulative CO₂ absorption of an aqueous solution of PZ (1.5 mol L⁻¹) and IR absorbance for the bands assigned to the vibrational modes of NCOO⁻ and HCO₃⁻.

As the IR stretching frequencies of PZ-dicarbamate had not been previously reported we turned to computational approaches to facilitate the assignment of key vibrational modes of PZ-COO⁻, in particular . Calculations were performed at the B3LYP/6-31+G** and MP2/6-31+G** levels (gas phase, Spartan ‘08).^[22]

The B3LYP/6-31+G** calculations assigned PZ- to a single band at 1282 cm⁻¹ (no scaling), while MP2/6-31+G** positioned this band at 1284 cm⁻¹ (no scaling), similar in shape, but not as broad, as that which initially emerges at 1276 cm⁻¹ in Figure 3. For the ⁻OOC-PZ-COO⁻ species B3LYP/6-31+G** gave two sharp absorbances at 1297–1266 and 1348–1345 cm⁻¹, which correlated well with MP2/6-31+G** calculated positions of 1302–1274 cm⁻¹ and 1364–1355 cm⁻¹. These values correlate well with the experimentally observed peaks at values of 1266, 1276 and 1294 cm⁻¹, with the latter two shifting to 1289 cm⁻¹ with CO₂ absorption. The B3LYP/6-31+G** and MP2/6-31+G** calculations confirm our peak assignments for the PZ-CO₂ carbamate absorption species above.

The evolution of a weak broad absorbance band in the 1360–1350 cm⁻¹ region of the PZ/CO₂/H₂O IR spectral profile (Figure 3) was assigned to ν_sC–O of HCO₃⁻. This absorbance band was far less prominent than that we observed for the piperidine/CO₂/H₂O system. Additionally, this absorbance in the PZ system does not follow the trend observed with the corresponding piperidine system; that is, the depletion of , and absorbance bands with concomitant increase in the HCO₃⁻ absorbance band for CO₂ loadings greater than 0.5 mol CO₂/mol amine.^[10] The increase in HCO₃⁻ absorbance in the piperidine system is attributable to hydrolysis of the initially formed carbamate, which strongly suggests, consistent with the IR data presented herein, that the PZ system forms a hydrolysis-resistant carbamate.

2.1.2. 2,6- and 2,5- Dimethyl-Substituted Piperazine Derivatives (2,6-DMPZ and 2,5-DMPZ)

The effect of alkyl substituents on the CO₂ absorption characteristics of PZ was examined with 2,6-dimethylpiperazine (2,6-DMPZ) and 2,5-dimethylpiperazine (2,5-DMPZ) (Figure 2). The IR spectral profiles obtained for 2,6-DMPZ/CO₂/H₂O and 2,5-DMPZ/CO₂/H₂O are shown in Figures 5 and 6, respectively.

Figure 5.

a) Partial IR spectral profile collected for an aqueous solution of 2,6-DMPZ (1.5 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.86 mol CO₂/mol amine. b) Relationship between the cumulative CO₂ absorption and IR absorbance for 2,6-DMPZ.

Figure 6.

Partial IR spectral profile of an aqueous solution of 2,5-DMPZ (1.5 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.88 moles CO₂ per mole of amine.

The IR spectral data collected for the 2,6-DMPZ/CO₂/H₂O system (Figure 5 a and b) are almost identical to those recorded for the equivalent PZ system. While the and bands were less intense and only a single absorbance band evolves in the 1289–1276 cm⁻¹ region, all major absorbances of the 2,6-DMPZ system are within 6 cm⁻¹ of those of the PZ system: at 1526 cm⁻¹; at 1425 cm⁻¹; at 1279 cm⁻¹ for 2,6-DMPZ-COO⁻; NH₂⁺ bending of 2,6-DMPZ-H⁺ at 1464 cm⁻¹; and HCO₃⁻ absorbance at 1354 cm⁻¹. The and absorbances shift to 1550 and 1289 cm⁻¹, respectively, with increasing CO₂ absorption. The lack of a second absorbance is reflected in the relationship between absorbance and cumulative CO₂ absorption (Figure 5).

Due to steric congestion arising from the two α-CH₃ moieties, initial CO₂ absorption most likely occurred at the less hindered and more nucleophilic amine moiety, resulting in NCOO⁻ formation. This reduced nucleophilicity and hence reactivity towards CO₂ hindered dicarbamate formation, correlating with the observation of a single peak in the IR spectrum. The reduced prevalence of dicarbamate formation resulted in increased hydrolysis and HCO₃⁻, as evidenced by rapid growth of the ν_sC–O band at 1354 cm⁻¹ (Figure 5 a, b). The α-dimethylamine moiety acted catalytically, in a manner analogous to that reported for sterically hindered amines, to accelerate formation of HCO₃⁻.^[10]

The subtle structural variations between 2,6-DMPZ and 2,5-DMPZ resulted in a significant change in the IR profile. In the case of the 2,5-DMPZ/CO₂/H₂O system the most dominant feature is HCO₃⁻ absorbance, as evidenced by the intense peak in the 1400–1300 cm⁻¹ region. There is little evidence to support formation of a stable carbamate (Figure 6).^{[10, 17]}

2.1.3. Hexahydropyrimidine (HHPY)

The HHPY/CO₂/H₂O system displayed some similarity with the PZ/CO₂/H₂O system in terms of signal positioning but with evidently weaker signals, due in part to the lower concentration of amine (HHPY) available for this study. Notwithstanding this, Figure 7 shows evolution of at 1570–1520 cm⁻¹, at 1427 cm⁻¹ and at 1293 cm⁻¹ of HHPY-COO⁻; the NH₂⁺ bending mode of HHPY-H⁺ at 1479 cm⁻¹; and HCO₃⁻ absorbance at 1354 cm⁻¹. The HCO₃⁻ absorbance band was more prominent than that observed for the PZ/CO₂/H₂O system, that is, HHPY forms a more labile NCOO⁻ derivative that is more susceptible to hydrolysis.

Figure 7.

Partial IR spectral profile collected for an aqueous solution of HHPY (1.5 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.85 mol CO₂/mol amine.

2.1.4. 2-Methylhexahydropyrimidine (MHHPY)

The IR spectrum of the MHHPY/CO₂/H₂O system is dominated by the broad HCO₃⁻ absorbance band in the 1400–1300 cm⁻¹ region (Figure 8), which is characteristic of α-substituted amines such as AMP.^{[10, 17]} MHHPY is the methyl-substituted analogue of HHPY. Given the intensity of the HCO₃⁻ band in Figure 8, MHHPY was readily hydrolysed under the study conditions, with HCO₃⁻ formation dominating on absorption of CO₂.

Figure 8.

Partial IR spectral profile collected for an aqueous solution of MHHPY (1.5 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.86 mol CO₂/mol amine.

2.1.5. 2,2-Dimethylhexahydropyrimidine (DMHHPY)

Given the structural similarity between DMHHPY and MHHPY, we anticipated predominant HCO₃⁻ formation on CO₂ absorption by DMHHPY. However the IR profile obtained for the DMHHPY/CO₂/H₂O system (Figure 9 a) was significantly different to that obtained with MHHPY (Figure 8) and the HHPY and PZ systems (Figures 7 and 3, respectively). Here DMHHPY is acting more in keeping with a blended-amine PPC absorbent system. Close examination of the in-house synthesized DMHHPY revealed the presence of unconverted 1,3-diaminopropane (DAP) which had been unavoidably carried forward to the final product. Hence, the contamination of DMHHPY with DAP explains the observed blended-system-like IR profile (Figure 9 a).

Figure 9.

Partial IR spectral profile of an aqueous solution of a) synthesised DMHHPY (1.5 mol L⁻¹) and b) DAP (0.6 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 1.10 and 0.95 mol CO₂/mol amine, respectively.

Re-examination of the IR spectrum of the DMHHPY/CO₂/H₂O system identified the NH bending mode of 1,3-diaminopopane at 1602 cm⁻¹. While 1,3-diaminopropane [approximately 37 %, 1.91 g (¹H NMR)] was the minor component within the DMHHPY (63 %, 3.24 g)/CO₂/H₂O system, it formed predominately the corresponding carbamate (DAP) on CO₂ absorption.

To allow potential deconvolution of the DAP and DMHHPY signals in the original DMHHPY/CO₂/H₂O IR profile, data were collected separately for a DAP/CO₂/H₂O system at a DAP concentration of 0.6 mol L⁻¹ (Figure 9 b). It is apparent that the original DMHHPY system is dominated by the reactivity of DAP (cf. Figure 9 a, b). Both systems show evolution of (1565 and 1568 cm⁻¹, respectively), (1440 cm⁻¹) and (1328 and 1330 cm⁻¹, respectively) of the DAP-COO⁻ derivative and the NH₃⁺ bending mode of protonated DAP (1492 cm⁻¹). For the blended DMHHPY/CO₂/H₂O system, weaker absorbance bands were also observed to emerge at 1385 and 1370–1350 cm⁻¹ at CO₂ loadings above 1.0 mol CO₂/mol amine. These new peaks are consistent with NCOO⁻ hydrolysis and HCO₃⁻ formation. Carbamate hydrolysis was not observed for the pure DAP/CO₂/H₂O system.

In the initial DMHHPY/CO₂/H₂O system (Figure 9 a) the DAP-COO⁻ absorbance bands dominate the IR spectrum. However in the DAP/CO₂/H₂O systems the carbamate absorbances are considerably weaker, the initial effect of which was thought to be that the DAP concentration on the DMHHPY system appears to be significantly higher than the 0.6 mol L⁻¹ evident in Figure 9 b. However, a similar difference in intensity between the carbamate absorbance bands of a blended AMP (2.4 mol L⁻¹)/PZ (0.6 mol L⁻¹) system (Figure 10, further described below) versus an unblended PZ (0.6 mol L⁻¹) system was also observed (Figure 11). Carbamate absorbance in the unblended PZ system was found to be considerably weaker than that observed for the AMP/PZ blended system, despite equivalent PZ concentrations (6 mol L⁻¹). Based on the percentage concentrations determined by ¹H NMR spectroscopy the ratio of DMHHPY to DAP in the blended system was 0.95/0.85 mol L⁻¹ (total concentration 1.8 mol L⁻¹).

Figure 10.

Partial IR spectral profile of an aqueous solution of an AMP/PZ blend (2.4/0.6 mol L⁻¹, respectively) as CO₂ is absorbed to a maximum loading of 1.00 mol CO₂/mol amine.

Figure 11.

Partial IR spectral profile of an aqueous solution of a) unblended AMP (3 mol L⁻¹) and b) unblended PZ (0.6 mol L⁻¹) as CO₂ is absorbed to a maximum loading of 0.84 and 0.92 mol CO₂/mol amine, respectively.

For comparative purposes an AMP/PZ blended amine system (2.4 mol L⁻¹/0.6 mol L⁻¹, respectively) was also investigated. Similarly to the blended DMHHPY absorbent, the AMP/PZ blend consists of an amine that forms predominately HCO₃⁻ on absorption of CO₂ (AMP, major constituent) and an amine that forms predominately NCOO⁻ (PZ, minor constituent). A similar trend in IR absorbance was observed in the spectral data collected for the AMP/PZ/CO₂/H₂O system (Figure 10) to that described above for the blended DMHHPY/CO₂/H₂O system. Despite PZ being the minor constituent of the amine blend, the PZ-carbamate absorbance bands dominated the IR spectral profile. Figure 10 shows the evolution of ν_ascoo- (1533 cm⁻¹), ν_scoo- (1421 cm⁻¹) and (1276 cm⁻¹ and shifts to 1263 cm⁻¹) of the PZ-COO⁻ derivative and the NH₂⁺ vibration of PZ-H⁺ (1471 cm⁻¹). HCO₃⁻ absorbance was seen to emerge in the 1386–1330 cm⁻¹ region after a CO₂ loading of about 0.5 mol CO₂/mol amine. For comparison Figure 11 a and b present the IR spectral profile for an unblended AMP/CO₂/H₂O system (3 mol L⁻¹) and PZ/CO₂/H₂O system (0.6 mol L⁻¹), respectively. The HCO₃⁻ absorbance band was more prominent in the IR spectra of the AMP/PZ/CO₂/H₂O system compared to that of the DMHHPY/CO₂/H₂O system. This was most likely due to the difference in amine concentrations, with the AMP/PZ blend having a total concentration of 3 mol L⁻¹ and the DMHHPY/1,3-diaminopropane blend a total concentration of 1.5–1.8 mol L⁻¹.

2.1.6. Hexahydropyridazine

HHPZ absorbed no CO₂ during a typical CO₂ absorption/FTIR experiment. HHPZ is a hydrazine derivative that is reported to have a pK_a value of 7.9,^[23] which is much lower than that of PZ (9.73),^[24] HHPY (9.75)^[25] or 2,5-DMPZ (9.66).^[26] The low basicity of HHPZ compared to the other diamines (pK_a>9.5) would significantly reduce the reactivity of the amine towards CO₂.

2.2. Absorption Capacity and Absorption Rate

Having established the ability of our diamines to absorb CO₂, the initial absorption rate R_IA and absorption capacity C_A were determined. The R_IA value was measured by a thermal gravimetric analysis (TGA) method, and C_A was measured simultaneously with the IR spectral data (see Experimental Section). These data are presented in Table 1. For comparison, the reactivity of conventional absorbents MEA, DEA and AMP (α-dimethyl-substituted MEA) are also included.

Table 1.

Measured absorption capacity C_A at 40 °C for an amine concentration of 1.5 mol L⁻¹ and initial absorption rate R_IA at 40 °C and an amine concentration of 1.5 mol L⁻¹ for aqueous solutions of PZ, 2,6-DMPZ, 2,5-DMPZ and synthesised amines HHPY, MHHPY, DMHHY and HHPZ. For comparison, the reactivity of conventional absorbents such as MEA, DEA and AMP (1.5 mol L⁻¹, unless otherwise stated) are included.

Amine	CA[a]	RIA[b]
PZ	0.92	0.045
2,6-DMPZ	0.86[c]	0.025[d]
2,5-DMPZ	0.88	0.018[e]
HHPY	0.85	0.032
MHHPY	0.86	0.018
DMHHPY	1.33	0.032
HHPZ	0	0
MEA	0.56[12], [f]	0.027
DEA	0.60[12], [f]	0.015
AMP	0.84[12], [f]	0.006

^[a]Mol CO₂/mol amine; data used to calculate C_A were measured in the absorption reactor/FTIR system.

^[b]Mol CO₂/mol amine, min⁻¹; data used to calculate R_IA were measured by microscale TGA. Initial absorption rates were calculated by using linear regression to determine the slope of the absorption capacity curve. R²≥0.995.

^[c]A precipitate formed during CO₂ absorption/FTIR.

^[d]A precipitate formed during the CO₂ and N₂ runs of the TGA experiment.

^[e]A precipitate formed during the CO₂ run of the TGA experiment. This could be mainly due to the evaporation of water.

^[f]3 mol L⁻¹ concentration analysed.

The current industry-standard PCC amine MEA returned C_A=0.56 mol CO₂/mol amine and R_IA=0.027 mol CO₂/mol amine min⁻¹. From the data amassed for PZ, 2,6-DMPZ, HHPY and DMHHPY in Table 1, superior C_A and R_IA values were observed for all these diamines relative to MEA. Superior C_A values were also observed for 2,5-DMPZ and MHHPY, but with lower R_IA values. HHPZ did not react with CO₂, and DMHHPY was blended with DAP. Diamine C_A values ranged from 0.85 (HHPY) to 0.92 (PZ) mol CO₂/mol amine and R_IA values from 0.018 (2,5-DMPZ) to 0.045 (PZ) mol CO₂/mol amine min⁻¹. While in absolute terms PZ was the standout pure diamine with the highest C_A (0.92 mol CO₂/mol amine) and R_IA (0.045 mol CO₂/mol amine min⁻¹), C_A and R_IA are not the sole factors to be considered in determining the most efficient PCC diamine absorbent; aqueous solubility and stability of the carbamate also play a role. HPPY displays higher water solubility than PZ, on the basis of observations when preparing 1.5 mol L⁻¹ amine solutions. HHPY was readily soluble at this concentration, as opposed to PZ, which required heating and stirring for dissolution. Additionally, HHPY displays high C_A (0.85 mol CO₂/mol amine) and R_IA (0.032 mol CO₂/mol amine min⁻¹; Table 1) and showed clear evidence of formation of a hydrolysis-susceptible carbamate (see above and Figure 7).

The PZ analogues 2,6-DMPZ and 2,5-DMPZ displayed lower R_IA values of 0.025 and 0.018 mol CO₂/mol amine min⁻¹, respectively. As these analogues differ only in the number of methyl substituents (PZ has none) and their positioning, these data suggested that introduction of methyl moieties had an adverse effect on the initial rate of CO₂ absorption. 2,6-DMPZ has both methyl groups α to a single NH group, while 2,5-DMPZ has one methyl group α to each NH group. The measured R_IA values indicate that the effect of addition of α-methyl groups is cumulative, with R_IA dropping from 0.032 (PZ) to 0.025 (2,6-DMPZ) to 0.018 mol CO₂/mol amine min⁻¹ (2,5-DMPZ). Concurrent with the reduction in R_IA was an increased prevalence towards HCO₃⁻ formation for 2,5-DMPZ (see above and Figure 6). The reactivity of 2,6-DMPZ towards CO₂ was found to be similar to that of MEA, with the exception of a higher C_A value. The propensity of 2,5-DMPZ for HCO₃⁻ formation was similar to that observed with MHHPY (see above and Figure 8), which was reflected in the almost identical C_A (0.88 and 0.86 mol CO₂/mol amine respectively) and R_IA (0.018 and 0.018 mol CO₂/mol amine min⁻¹, respectively) values obtained for these amines. The reactivity of 2,6-DMPZ towards CO₂ was found to be similar to that of MEA, with the exception of a higher C_A value. 2,6-DMPZ was found to form predominantly carbamate on absorption of CO₂ (Figure 5), similar to HHPY (Figure 7). While the propensity for carbamate hydrolysis and subsequent HCO₃⁻ formation is also similar to that observed with HHPY (cf. Figures 5 and 7), which was reflected in the almost identical C_A (0.88 and 0.85 mol CO₂/mol amine, respectively) values, 2,6-DMPZ returned a lower R_IA value (0.025 and 0.032 mol CO₂/mol amine min⁻¹, respectively). Despite forming predominately HCO₃⁻, the initial absorption rates obtained for both MHHPY and 2,5-DMPZ were much higher than that obtained for AMP and comparable to those of MEA and DEA (Table 1).

Of the diamines examined, DMHHPY exhibited the highest C_A (1.1 mol CO₂/mol amine) value and an R_IA value higher than that of MHHPY and comparable to that of HHPY. This is an artefact of the serendipitous blending with DAP, which contributes significantly to the observed CO₂ absorption capacity. Aqueous DAP has C_A=0.95 mol CO₂/mol amine. The bicarbonate-forming DMHHPY further promotes CO₂ absorption, resulting in C_A>1.0 mol CO₂/mol amine. The 1° amino groups of DAP contribute towards DMHHPY’s increased initial reaction rate compared to MHHPY.

2.3. Effect of Structural Variations on Carbamate Structures

The effect of diamine structural variation on the ability to form stable carbamates was examined at the B3LYP/6-31+G** and MP2/6-31+G** levels of theory. Geometry optimisations were initially performed on PZ, 2,6-DMPZ, 2,5-DMPZ, HHPY, MHHPY, DMHHPY and HHPZ. Table 2 lists selected atomic properties including electrostatic potential (ESP) partial charges on the amino nitrogen atoms and the exposed area on the nitrogen atoms for these diamines. The trends in results obtained at the two levels of theory were found to be in good agreement with one another.

Table 2.

ESP charges and exposed areas on the nitrogen atoms [Ų] for optimised forms of the diamines analysed herein.

Amine		ESP charge on N		Exposed area on N [Å2]
		B3LYP	MP2	B3LYP	MP2
PZ	N1	−0.584	−0.640	4.71	4.80
	N2	−0.584	−0.640	4.71	4.80
2,6-DMPZ	N1	−0.718	−0.752	4.21	4.25
	N2	−0.752	−0.778	4.70	4.82
2,5-DMPZ	N1	−0.746	−0.785	4.45	4.54
	N2	−0.746	−0.785	4.45	4.54
HHPY	N1	−0.874	−0.907	4.91	4.96
	N2	−0.874	−0.907	4.91	4.96
MHHPY	N1	−0.851	−0.881	4.77	4.83
	N2	-0.728	-0.759	4.57	4.65
DMHHPY	N1	−0.959	−0.979	4.50	4.57
	N2	−0.957	−0.982	4.51	4.57
HHPZ	N1	−0.525	−0.551	6.82	6.89
	N2	−0.386	−0.402	5.71	5.77

Diamines can react with CO₂ in aqueous solution to form three possible forms of carbamate species: amine carbamate (HNR₁R₂NCOO⁻), protonated amine carbamate (⁺H-HNR₁R₂NCOO⁻) and dicarbamate. Of these three forms, ⁺HHNR₁R₂NCOO⁻ was expected to be the main reaction product, with one amino group acting as the binding site for CO₂ and the other as a proton acceptor. Geometry optimisations were next performed for the protonated amine-carbamate (⁺H-HNR₁R₂NCOO⁻) and amine-carbamate (HNR₁R₂NCOO⁻) species. For the lowest-energy conformer of each ⁺HHNR₁R₂NCOO⁻ and HNR₁R₂NCOO⁻ derivative, Table 3 provides the calculated N–COO⁻ bond length (r_N–C [Å]), r_C1–O1/r_C1–O2 [Å] (Figure 12) and ESP partial negative charges on both oxygen atoms as a measure of charge delocalisation.

Table 3.

Calculated N–COO⁻ bond lengths r_N–C [Å], r_C1–O1/r_C1–O2 [Å] and ESP partial charge on both oxygen atoms for optimised geometries of the ⁺HHNR₁R₂NCOO⁻ and HNR₁R₂NCOO⁻ derivatives of the subset of diamines analysed.

Carbamate derivative	rN−C [Å]		rC–O1/rC–O2 [Å]		ESP charge on O1/O2
	B3LYP	MP2	B3LYP	MP2	B3LYP	MP2
H+-PZ-carbamate	1.513	1.507	1.247/1.247	1.256/1.256	−0.690/−0.690	−0.681/−0.681
PZ-carbamate	1.471	1.477	1.257/1.257	1.264/1.264	−0.743/−0.743	−0.739/−0.739
H+-2,6-DMPZ-carbamate[a]	1.508	1.505	1.248/1.249	1.256/1.256	−0.688/−0.689	−0.680/−0.680
2,6-DMPZ-carbamate[a]	1.469	1.477	1.258/1.258	1.264/1.264	−0.772/−0.772	−0.767/−0.767
H+-2,6-DMPZ-carbamate[b]	1.523	1.527	1.247/1.247	1.255/1.255	−0.691/−0.692	−0.699/−0.698
2,6-DMPZ-carbamate[b]	1.461	1.463	1.260/1.260	1.266/1.266	−0.753/−0.753	−0.749/−0.749
H+-2,5-DMPZ-carbamate[c]	1.505	1.503	1.249/1.247	1.259/1.253	−0.699/−0.665	−0.702/−0.656
2,5-DMPZ-carbamate[c]	1.465	1.470	1.259/1.258	1.266/1.264	−0.750/−7.35	−0.746/−0.731
H+-HHPY-carbamate	1.507	1.503	1.227/1.277[d]	1.235/1.284[d]	−0.641/−0.735[d]	−0.625/−0.736[d]
HHPY-carbamate	1.475	1.478	1.257/1.259	1.263/1.267	−0.762/−0.767	−0.744/−0.751
H+- MHHPY-carbamate	1.417	1.417	1.211/1.354[d]	1.211/1.354[d]	−0.650/−0.772[d]	−0.640/−0.782[d]
MHHPY-carbamate	1.463	1.469	1.258/1.260	1.264/1.266	−0.753/−0.770	−0.746/−0.759
H+-DMHHPY-carbamate	1.555	1.550	1.235/1.244	1.243/1.253	−0.653/−0.699	−0.648/−0.702
DMHHPY-carbamate	1.501	1.500	1.252/1.261	1.258/1.268	−0.758/−0.819	−0.743/−0.816
H+-HHPZ-carbamate	1.390	1.396	1.215/1.350[d]	1.221/1.353[d]	−0.575/−0.660[d]	−0.566/−0.670[d]
HHPZ-carbamate	1.460	1.470	1.254/1.263	1.260/1.267	−0.741/−0.759	−0.736/−0.758

^[a]Isomer 1: R₂=R₃=CH₃ and R₁=R₄=H (Figure 12).

^[b]Isomer 2: R₁=R₄=CH₃ and R₂=R₃=H (Figure 12).

^[c]R₂=R₄=CH₃ and R₁=R₃=H (Figure 12).

^[d]Hydrogen bonding between O2 of the carbamate moiety and a proton of the NH₂⁺ group (Figure 12).

Figure 12.

Chemical structural associated with Table 3 with regards to r_N–C, r_C1–O1/r_C1–O2. a) 2,6-DMPZ (1. R₂=R₃=CH₃, R₁=R₄=H and 2. R₁=R₄=CH₃, R₂=R₃=H) and 2,5-DMPZ (R₂=R₄=CH₃, R₁=R₃=H). b) HHPY (R₁=R₂=H), MHHPY (R₁=CH₃, R₂=H) and DMHHPY (R₁=R₂=CH₃). c) HHPZ.

As anticipated, methyl substitution significantly increased the partial negative charges on the amino groups of 2,6-DMPZ (N1 −0.718; N2 −0.752), 2,5-DMPZ (N1, N2 −0.746), MHHPY (N1 −0.851; N2 −0.728) and DMHHPY (N1 −0.959; N2 −0.957) relative to PZ (N1, N2 −0.584) and reduced the exposed area on the amino nitrogen atom of 2,6-DMPZ (N1 4.21 Ų; N2 4.70 Ų), 2,5-DMPZ (N1, N2 4.45 Ų), MHHPY (N1 4.77 Ų; N2 4.47 Ų) and DMHHPY (N1 4.50 Ų; N2 4.51 Ų) relative to PZ (N1, N2 4.71 Ų) at the B3LYP level. These changes in electronic effects both have impact on the resonance structure of the carboxylate moiety and hence overall stability of the carbamate derivative (see below).

Studies herein (Figure 3 and 4) demonstrated that PZ forms a hydrolysis-resistant NCOO⁻ derivative, and thus the optimised geometries of H⁺-PZ-COO⁻ and PZ-COO⁻ were used as the baseline against which the remaining amine NCOO⁻ derivatives were compared. For both H⁺-PZ-COO⁻ and PZ-COO⁻ resonance stabilisation of the carboxylate moiety is evident, with calculations showing identical charges on O1 and O2 as well as displaying identical r_C–O values. The r_C–O values of 1.247 and 1.257 Å reveal partial double-bond character for H⁺-PZ-COO⁻ and PZ-COO⁻, respectively (Table 3). The N–COO⁻ bond length of both PZ-COO⁻ species was of single-bond character [standard single-bond r_N–C in PZ is 1.466 Å (B3LYP) and 1.465 Å (MP2)]. H⁺-2,6-DMPZ-COO⁻ and 2,6-DMPZ-COO⁻ also exhibit this stable resonance structure, allbeit with slightly shorter N–COO⁻ bond lengths. These findings are in keeping with our IR studies on 2,6-DMPZ and PZ (Figure 5), which gave very similar outcomes, with the exception of the emergence of a small HCO₃⁻ absorbance band. The protonated and unprotonated 2,6-DMPZ-COO⁻ forms of isomer 2 (Figure 12), the minor reaction component, contributed to HCO₃⁻ formation.

H⁺-HHPY-COO⁻ displays lower levels of charge delocalisation across the two oxygen atoms. Changes in charge distribution and bond length were noted with r_C–O1=1.227 Å and r_C–O2=1.277 Å, which were mirrored in the change in electron density at O1 (−0.641) and O2 (−0.735) (B3LYP). The shift in electron distribution was much less pronounced in the HHPY-COO⁻ species with an r_C–O1 of 1.257 Å and a r_C–O2 of 1.259 Å which was mirrored in the change in electron density at O1 (−0.762) and O2 (−0.767) (B3LYP). The N–COO⁻ bond lengths of H⁺-HHPY-COO⁻ (1.507 Å) and HHPY-COO⁻ (1.475 Å) are similar to that of the PZ-COO⁻ species (1.471 Å). These findings are in keeping with our IR studies (Figure 7), in which HHPY was identified as forming a more labile NCOO⁻ derivative that is more susceptible to hydrolysis than that of PZ. The lowest energy conformer obtained for H⁺-HHPY-COO⁻, as opposed to HHPY-COO⁻ species, exhibited intramolecular hydrogen bonding between the COO⁻ group and the NH₂⁺ moiety. The low-energy conformers of H⁺-MHHPY-COO⁻ and H⁺-HHPZ-COO⁻ were also found to exhibit the same intramolecular hydrogen bonding. The resonance structure of the H⁺-MHHPY-COO⁻ was less delocalised with r_C–O1=1.211 Å and r_C−O2=1.354 Å, which was mirrored in the changes in electron density at O1 (−0.650) and O2 (−0.772) (B3LYP). The N–COO⁻ bond length of H⁺-MHHPY-COO⁻ was found to be much shorter (1.417 Å). The shift in electron distribution of the carboxylate resonance structure was again much less pronounced in the MHHPY-COO⁻ species compared to the H⁺-MHHPY-COO⁻ species. MHHPY-COO⁻ has a shorter N–COO⁻ bond length (1.463 Å) than PZ-COO⁻ (1.471 Å) and HHPY-COO⁻ (1.475 Å). MHHPY forms predominantly HCO₃⁻ on absorption of CO₂, as does 2,5-DMPZ. Both species display shorter N–COO⁻ bonds and lower levels of resonance stabilisation.

In our IR studies DMHHPY was found to be representative of a blended amine system. Nonetheless, optimised geometries of the H⁺-DMHHPY-COO⁻ and DMHHPY-COO⁻ were still analysed, and both exhibited reduced resonance in the carboxylate moiety with r_C–O1=1.235 Å and r_C–O2=1.244 Å, which was mirrored in the changes in electron density at O1 (−0.653) and O2 (−0.699), and with r_C–O1=1.252 Å and r_C–O2=1.261 Å, which was mirrored in the changes in electron density at O1 (−0.758) and O2 (−0.819) (B3LYP), respectively. Given the structural similarity of DMHHPY and MHHPY, DMHHPY was expected to form predominantly HCO₃⁻ on absorption of CO₂.

Experimentally HHPZ was unreactive towards CO₂. Calculations revealed H⁺-HHPZ-COO⁻ to have a significantly shorter N–COO⁻ bond length of 1.390 Å (significant double-bond character), as well as the largest displacement in electron distribution of the carboxylate resonance structure with r_C−O1=1.215 Å and r_C–O2=1.350 Å, which was mirrored in the changes in electron density at O1 (−0.575) and O2 (−0.660) (B3LYP). This was much less pronounced in the HHPZ-COO⁻ species; nonetheless, in a diamine system it is typical for one amino group to act as binding site for CO₂ while the other is protonated. These data support our experimental observations.

3. Conclusions

A series of heterocyclic diamines (PZ, 2,6-DMPZ, 2,5-DMPZ, HHPY, MHHPY, DMHHPY and HHPZ) have been evaluated as potential PCC absorbents by in situ ATR-FTIR spectroscopy. Of these diamines, PZ displayed both the highest CO₂ absorption capacity (C_A=0.92 mol CO₂/mol amine) and highest initial absorption rate (R_IA=0.045 mol CO₂/mol amine min⁻¹). These values represent a significant enhancement over currently used amines such as MEA. PZ forms a hydrolysis-resistant carbamate, as well as a dicarbamate. This behaviour is unique to PZ. Hydrolysis of the carbamate derivative of HHPY was observable in the IR spectra collected during CO₂ absorption. HHPY displayed similar CO₂ absorption characteristics to PZ, but with a higher propensity for HCO₃⁻ formation. The introduction of α-methyl substituents increased the propensity towards carbamate hydrolysis and HCO₃⁻ formation. Additionally α-methyl substitution decreased R_IA, with PZ analogues 2,6-DMPZ and 2,5-DMPZ displaying lower R_IA values of 0.025 and 0.018 mol CO₂/mol amine min⁻¹, respectively. Increasing the number of methyl groups α to the NH group also increases the rate of HCO₃⁻ formation. Despite forming predominately HCO₃⁻, the R_IA of MHHPY (0.018 mol CO₂/mol amine min⁻¹) was much higher than that of the corresponding α-dimethyl substituted 1° amine AMP (R_IA=0.006 mol CO₂/mol amine min⁻¹) and comparable with that of the industrially relevant MEA and DEA. The serendipitously blended DAP/DMHHPY exhibited the highest C_A (1.1 mol CO₂/mol amine) and excellent R_IA (0.032 mol CO₂/mol amine min⁻¹). HHPZ was found to be relatively unreactive towards CO₂. In all instances our calculations at the B3LYP/6-31+G** and MP2/6-31+G** levels of theory supported our experimental observation. Finally, we propose that HHPY offers the best compromise between high CO₂ absorption capacity, carbamate formation, hydrolysis to HCO₃⁻ and water solubility for future use in model, and potentially pilot-scale, PCC systems.

Experimental Section

General: All starting materials were purchased from Sigma Aldrich and used without further purification. Solvents were bulk and distilled prior to use. ¹H and ¹³C NMR spectra were recorded on a Bruker Avance AMX 300 MHz spectrometer at 300.1315 and 75.4762 MHz, respectively. Chemical shifts δ are reported relative to internal standards. Mass spectra were recorded on a Shimadzu LCMS-2010 EV spectrometer and obtained by the ESI method. IR spectra were recorded by ATR-FTIR on a Mettler-Toledo ic10 FTIR: spectrometer.

Preparation of Hexahydropyrimidine (HHPY): Formaldehyde (37 wt % solution, 1.5 mol, 126 g) was added dropwise to an ice-cooled, stirred aliquot of anhydrous 1,3-diaminopropane (1 mol, 74.1 g) over 30 mins. The reaction mixture was then stirred for 24 h at room temperature and cooled in an ice bath, and NaOH was added (1 mol, 40 g). The organic layer was transferred to a Dean–Stark apparatus and azeotropically distilled with cyclohexane (100 mL). The cyclohexane was removed in vacuo and the residue fractionally distilled to afford anhydrous hexahydropyrimidine (17 g, 20 %) as a clear oil (b.p. 57–60 °C/27 mbar, lit. ^{[25, 27]}: b.p. 58–60 °C/20 mm Hg). ¹H NMR (300 MHz, CDCl₃): δ=1.36 (2 H, m), 1.96 (2 H, br s), 2.82 (4 H, t, J=5.5 Hz), 3.63 ppm (2 H, s); ¹³C NMR (75 MHz, CDCl₃): δ=28.61, 45.65, 62.79 ppm; MS (ESI+): m/z 87 (M+1); FTIR: ν_NH 3267 cm⁻¹.

Preparation of 2-Methylhexahydropyrimidine (MHHPY): Acetaldehyde (0.215 mol, 9.5 g) in diethyl ether (100 mL) was added dropwise to ice-cooled 1, 3-diaminopropane (0.2 mol, 14.8 g). The reaction mixture was then stirred over K₂CO₃ (0.4 mol, 55.4 g) for 24 h at room temperature. The solvent was removed in vacuo and the residue fractionally distilled to afford 2-methylhexahydropyrimidine (17.5 g, 87 %) as a clear oil (b.p. 56–59 °C/30 mbar, lit. ^[28] b.p. 60 °C/30 mm Hg). ¹H NMR (300 MHz, CDCl₃) δ=0.80 (3 H, d, J=6.0 Hz), 1.09 (2 H, m), 2.48 (2 H, m), 2.76 (4 H, m), 3.25 ppm (1 H, q, J=6.0 Hz); ¹³C NMR (75 MHz, CDCl₃) δ=22.75, 26.80, 45.41, 67.09 ppm; MS (ESI+: m/z 101 [M+1]; FTIR: ν_NH 3266 cm⁻¹.

Preparation of 2, 2-Dimethylhexahydropyrimidine (DMHHPY): A solution of acetone (0.215 mol, 12.5 g) in diethyl ether (100 mL) was added dropwise to ice-cooled 1,3-diaminopropane (0.2 mol, 14.83 g). The reaction mixture was then stirred over K₂CO₃ (0.4 mol, 55.4 g) for 24 h at room temperature. The solvent was removed in vacuo and the residue fractionally distilled to afford 2, 2-dimethylhexahydropyrimidine (16.17 g, 71 %) as a clear oil (b.p. 50–54 °C/25 mbar, lit. ^[28]: b.p. 65 °C/25 mm Hg). ¹H NMR (300 MHz, CDCl₃): δ=0.90 (6 H, s), 1.05 (2 H, m), 1.45 (2 H, s), 2.60 ppm (4 H, t, J=5.7 Hz); ¹³C NMR (75 MHz, CDCl₃): δ=27.26, 36.87, 39.99, 64.31 ppm; MS (ESI+): m/z 115 [M+1]; FTIR: ν_NH 3271, 1602 cm⁻¹.

Preparation of Hexahydropyridazine (HHPZ): Dibromobutane (0.1 mol, 21.6 g) was added dropwise to a stirred solution of diethylhydrazine dicarboxylate (0.1 mol, 17.6 g), K₂CO₃ (0.2 mol, 27.7 g) and acetonitrile (100 mL) at room temperature. The reaction mixture was then heated to reflux for 24 h, cooled and filtered. The solvent was removed in vacuo and the residue fractionally distilled to afford 1, 2-dicarbethoxyhexahydropyridazine (18.5 g, 80 %) as a clear oil (b.p. 110–114 °C/2–3 mbar, lit. ^[29]: 106–114 °C/3 mm Hg). ¹H NMR (300 MHz, CDCl₃): δ=1.07 (6 H, m), 1.49 (4 H, s), 2.74 (4 H, br s), 3.97 ppm (4 H, m); ¹³C NMR (75 MHz, CDCl₃): δ=14.36, 23.19, 44.76, 61.93, 155.15 ppm; MS (ESI+): m/z 231 [M+1]; FTIR: ν_C=O 1706 cm⁻¹.

1, 2-Dicarbethoxyhexahydropyridazine (0.13 mol, 30.1 g) was added to a solution of NaOH (0.72 mol, 20.8 g), water (20 mL) and methanol (150 mL). The reaction mixture was then heated to reflux for 20 h, cooled and the precipitated inorganic salt removed by filtration. The filtrate was heated to reflux for a further 20 h, cooled, filtered and the solvent removed in vacuo. The residue was extracted with dichloromethane (40 mL×3), concentrated in vacuo and fractionally distilled to afford hexahydropyridazine (6.75 g, 60 %) as a yellow oil (b.p. 38–40 °C/11 mbar, lit. ^[30]: b.p. 38 °C/8 mm Hg). ¹H NMR (300 MHz, MeOD): δ=2.39 (4 H, m), 3.73 (4 H, m), 5.72 ppm (2 H, s); ¹³C NMR (75 MHz, MeOD): δ=23.06, 47.00 ppm; MS (ESI+): m/z 87 [M+1]; FTIR: ν_NH 3269 cm⁻¹.

Microscale Thermogravimetric Analysis (TGA): A Setaram Labsys TG-DTA/DSC thermogravimetric analyser (TGA) was used in isothermal mode at 40 °C to analyse aqueous CO₂/amine reactivity on a microscale (100 μL). 1.5 mol L⁻¹ aqueous amine solutions were exposed to a gas stream of 15 vol % CO₂ (>99.9 % purity, BOC Australia) in N₂ at atmospheric pressure. A gas flow rate of 30 mL min⁻¹ was used for all experiments.

To determine total CO₂ uptake two separate TGA experiments were performed for each amine. The first experimental run determined the mass loss due to evaporation when the test solution was exposed to a 100 % N₂ gas stream. The second experimental run determined the mass increase of the test solution when exposed to CO₂ (15 vol % CO₂ in N₂ gas stream) over the same length of time. Each experiment was performed on a fresh 100 μL aliquot of the test solution in a 100 μL alumina crucible (Setaram). From the data collected an absorption curve was then calculated for each amine by subtracting the mass at time t of the evaporation run from the mass at time t of the absorption run. Initial absorption rates could then be calculated by using linear regression to determine the slope of the absorption curve.

Absorption/FTIR Experiments: The absorption reactor apparatus used to analyse aqueous CO₂/amine reactivity has been described previously.^[10] Briefly, a gas stream of 13 vol % CO₂ (>99.9 % purity, BOC Australia) in N₂ with a flow rate of 1.8 L min⁻¹ was bubbled through a 1.5 mol L⁻¹ aqueous amine solution in a glass reactor vessel, maintained at 40 °C by a temperature-controlled water bath (Techne). The CO₂ content of both the gas inflow and outflow was measured by using a Horiba VA 3000 CO₂ analyser. The difference between the CO₂ concentration of the reactor gas inflow and gas outflow was used determine the amount of CO₂ absorbed by the amine solution (mol CO₂/mol amine). Each experiment was run until the measured CO₂ concentration in the outflow returned to the original percentage value, that is, equilibrium was reached. A typical run lasted between 45 and 90 mins. Solution volumes of 30 mL were used for all experiments.

For the duration of each absorption experiment an ATR diamond-tipped IR probe, coupled via a mirrored K6 conduit to an ic10 FTIR spectrometer (Mettler-Toledo), was immersed in the aqueous amine solution. In situ IR measurements were obtained simultaneously with the CO₂ absorption measurements, with the FTIR spectrometer set to continuously collect spectra for the duration of the absorption experiment over the spectral range of 4000–650 cm⁻¹. Each spectrum was recorded as the average of 256 scans over a sampling interval of fifteen seconds with a resolution of 4 cm⁻¹. The amines investigated were synthesised according to the procedures detailed above, with the exception of commercially available PZ, 2,6-DMPZ and 2,5-DMPZ (Sigma-Aldrich).

Computational Details: The computational software package Spartan ‘08 was used to calculate and compare optimised geometries (gas phase) of the heterocyclic diamines and their carbamate derivatives.^[22] First, molecular mechanics calculations using the MMFF94 force field and Monte Carlo search algorithm were used to obtain a set of low-energy conformers for each amine and carbamate molecule. Each subset of low-energy conformers were then re-submitted as a geometry optimisation at the B3LYP/6-31+G** and MP2/6-31+G** levels to obtain an equilibrium geometry corresponding to an energy minimum (characterized by a gradient <0.001). Vibrational analysis was performed for all optimised geometries to ensure that they correspond to local minima, that is, there are no imaginary frequencies.