An Investigation of Some Sterically Hindered Amines as Potential

Ind. Eng. Chem. Res. 1997, 36, 1779-1790

1779

An Investigation of Some Sterically Hindered Amines as Potential Carbon Dioxide Scrubbing Compounds Robert J. Hook† Department of Defence, Defence Science and Technology Organisation, Aeronautical and Maritime Research Laboratory, P.O. Box 4331, Melbourne 3001, Australia

In order to improve the efficiency of the carbon dioxide cycling process and to reduce amine emissions, a series of nonvolatile amino acid salts with sterically hindered amine groups were investigated to determine their potential as direct replacements for monoethanolamine (MEA) in submarine-based CO2 scrubbers. Absorption from atmospheres containing various levels of CO2 was measured to assess the total capacities and absorption rates of amine solutions. The regeneration rates and extent of CO2 desorption were established by heating these solutions. 13 C NMR spectroscopy was used to establish reaction products and solution compositions after both absorption and desorption. Methyl groups substituted adjacent to the amine were found to increase solution absorption capacities but with an overall reduction in absorption rate. Poor absorption rates at low CO2 levels and precipitation problems would prevent the R-dimethylamines examined from being used in existing submarine scrubbers. These amines, however, show potential as replacements in industrial CO2 scrubbing processes. Introduction

Scheme 1

For many years amines such as monoethanolamine (MEA, 1) have been used in industrial processes to remove carbon dioxide from gas streams (Blauwhoff et al., 1984; Tontiwachwuthikul et al., 1991; Niswander et al., 1993). Similarly, MEA-based CO2 scrubbers have been used to suppress carbon dioxide levels that build up in enclosed atmospheres such as in submerged submarines (Ravner and Blachly, 1962; Gustafson, 1968). As is the case for their industrial counterparts, the carbon dioxide absorption in these submarine atmosphere scrubbers is driven by the low-temperature reaction of CO2 with the amine groups in aqueous MEA solutions. The CO2 is subsequently desorbed by heating and then expelled from the submarine. Carbon dioxide reacts with aqueous solutions of primary or secondary amines, reaching an equilibrium of carbamate, bicarbonate, and carbonate (Scheme 1). There have been conflicting chemical mechanisms proposed to describe the reaction process (Sartori and Savage, 1983; Maddox et al., 1987; Crooks and Donnellan, 1990). However, it is clear from reaction rates that the initial absorption reaction is the formation of the carbamate (or at least the precursor intermediate (Sartori and Savage, 1983)), which can then undergo hydrolysis to produce the bicarbonate and, if conditions such as pH are suitable, the carbonate species. The degree of hydrolysis of carbamate is determined by parameters such as amine concentration, solution pH, and the chemical stability of the carbamate (Caplow, 1968; Ewing et al., 1980; Chakraborty et al., 1988). At low temperatures the equilibria in Scheme 1 will favor the formation of carbamate and bicarbonate, but upon heating this equilibrium favors the liberation of amine and carbon dioxide. It can be noted from Scheme 1 that the maximum absorption of CO2 is achieved when all of the absorbed CO2 exists as bicarbonate. This is due to the requirement of the carbamate and carbonate species for 2 mol of amine/mol of CO2 reacted, while a one-to-one ratio exists for the bicarbonate. It is expected that a solution †

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containing a greater proportion of bicarbonate will also undergo desorption at a greater rate and produce a “leaner” (lower total carbamate/bicarbonate/carbonate concentration) desorbed solution (Sartori and Savage, 1983; Tontiwachwuthikal et al., 1991). It is therefore desirable to achieve maximum hydrolysis of the carbamate in the CO2 absorbed scrubber solutions. The MEA scrubbers are not without their problems and limitations. Although MEA is reasonably nonvolatile, during its handling and the scrubber operation some of this material and its more volatile decomposition products enter the submarine atmosphere. Given the toxicity data available for MEA and other such amines, it would be advantageous if a nonvolatile alternative could be identified as a replacement (Ravner and Blachly, 1962; Gustafson, 1968; Sax, 1986). The instability of MEA under scrubbing conditions has been widely reported, with the decomposition of the amine resulting in reduced scrubbing efficiency, the production of ammonia, increased viscosity, and excessive foaming (Ravner and Blachly, 1962; Gustafson, 1970; Niswander et al., 1993). This oxidative degradation is particularly prevalent in submarine scrubbers where the desorber heating element is in direct contact with the susceptible carboxylated amine. The solution close to the element is significantly hotter than the overall solution temperature. This contrasts with industrial scrubbers where steam-heated boilers are used rather than direct heating. Power is at a premium on nonnuclear submarines. The need for power conservation coupled with the likelihood of future reductions in acceptable submarine atmospheric CO2 levels makes it desirable to be able to improve the efficiency of the scrubbing units. The aim of this work is to identify an amine replacement for

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Published 1997 by the American Chemical Society

1780 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

MEA which exhibits an improved CO2 cycling ability and which also addresses the problems of volatility, toxicity, and degradation. Reducing Carbamate Stability. It has been demonstrated that the introduction of substituents at the carbon adjacent to the amine group creates the carbamate instability required to result in enhanced hydrolysis (Chakraborty et al., 1986). For such compounds the solution level of bicarbonate is increased, allowing greater CO2 loading of the amine solutions. Sartori and Savage (1983) suggested that this instability was due to the steric hindrance created by these R-substituents. Chakraborty et al. (1988) examined the electronic effects of such substituents and have proposed that substitution at the R-carbon atom results in interaction of the ΠMe and ΠMe* methyl group orbitals with the lone pair of the nitrogen. This interaction reduces the charge at the nitrogen, making it a softer base which results in a weakening of the N-H bond. These effects allow a greater level of hydrolysis by the hydroxide (hard base) in solution. The steric hindrance would be expected to slow the rate of the initial reaction with CO2 to some extent, but as 1 mol of amine is released upon hydrolysis of the carbamate, the level of amine available for reaction with CO2 is increased, thus causing a counteracting increase in rate. Sartori and Savage (1983) also state that sterically hindered amines have “unique capacity and rate advantages in CO2 absorption processes”. It has been reported that the sterically hindered amine solutions can be desorbed to a greater extent than their nonsubstituted counterparts, thus producing a leaner solution, which will result in a greater mass transfer upon reabsorption (Sartori and Savage, 1983). 2-Amino-2-methyl-1-propanol (AMP, 2) is the R-dimethylated derivative of MEA. It has been reported that when AMP reacts with CO2, no carbamate is observed in solution, with almost total hydrolysis to bicarbonate (Chakraborty et al., 1986). The CO2 solubilities in AMP solutions are higher than those for MEA solutions at 40 °C but lower at 80 °C (Tontiwachwuthikal et al., 1991). These facts suggest that AMP has the potential to be a superior absorber of CO2 at low temperature and a superior desorber of CO2 at the higher temperatures. It is also claimed that such amines exhibit superior degradation resistance in comparison to those which are unsubstituted (Tontiwachwuthikal et al., 1991). Another amine which has attracted interest is the potassium salt of N-methylalanine (Alkazid M, 3). Alkazid M has been compared to MEA and reported to exhibit superior oxidation stability and, due to its ionic structure, is less volatile (Goan, 1960). It is also suggested that 3 has favorable toxicity characteristics (Goan, 1961). This amine has been assessed in the laboratory, has undergone extensive testing in a fullscale laboratory-based scrubber, and has been tested for its performance in an operational scrubber onboard a submarine in active service (Cassidy, 1995). Overall it has performed only slightly less efficiently than MEA, but its characteristics are encouraging. In an attempt to take advantage of the favorable characteristics of MEA (1), AMP (2), and Alkazid M (3), amines 4-8 (Figure 1) have been investigated to determine their efficacy as CO2 “cycling” (absorbing/ desorbing) compounds. All incorporate the potassium carboxylate function to reduce volatility, with amines 6-8 having two R-substituted methyl groups. In an effort to establish a clear structure-efficacy relation-

Figure 1. Chemical structures of amines 1-8.

ship, 6-8 are compared with similar amines with fewer methyl substituents (4 and 5) as well as amines 1-3. As it is the rates of absorption and desorption that are most critical (rather than merely the total level of absorption at equilibria), the kinetics of these processes were followed at various levels of CO2 concentrations. Nuclear magnetic resonance (NMR) spectroscopy was utilized in order to study the reaction products and establish the levels of all species in solution. Experimental Section Chemistry. All chemicals (except where noted otherwise) were purchased from Aldrich Chemical Co. Monoethanolamine (1) and 2-amino-2-methyl-1-propanol (2) were used for absorption experiments without further purification. N-Substituted aminecarboxylic acid precursors to carboxylate salts 3, 7, and 8 were prepared using a procedure similar to that described by Fu and Birnbaum (1953). For example, (2-bromopropionic acid (50 mL, 0.56 mol) was added dropwise under N2 to a stirred solution of 40% by weight methylamine in water (225 mL, 2.61 mol), while the mixture was kept at Alkazid M (2) > 6 (2), AMP (2) > 7 (3) > 8 (3). Observations of the adverse effect of steric hindrance on amine-CO2 reaction rates have been made previ-

Figure 3. Profiles of the absorption of 100% CO2 by 2.5 M aqueous solutions of amines 1-8. The symbols do not represent data points. The levels at which carbonate precipitation commenced are marked by asterisks.

ously (Sartori and Savage, 1983). Clearly any introduced steric hindrance around the amino group reduces the initial rate of reaction even though in time these hindered amines absorb to a higher level. The position of substitution is important, as is illustrated by the comparison of Alkazid M (3) with AMP (2) and 6. Each of these amines has two sterically hindering groups but the N-methylated R-monosubstituted 3 absorbs at a faster rate than the R-disubstituted species. The slow absorption of the N-substituted, R-dimethylated amines 7 and 8 relative to 6 indicates that the presence of three bulky groups around the reaction site causes too much of a physical barrier, thus greatly impeding the reaction. This steric effect swamps the increase in reaction rate expected due to the secondary amines having a higher pKa than the primary amine. A comparison of the curves for 7 and 8 shows that for R-dimethylated amines the larger N-substituent slows the absorption rate. It is not only the rate of absorption at low loading levels which is important. Although “lean” solutions of MEA absorb CO2 at a greater rate than the other amines, as the CO2 loading increases at levels above 0.3 mol/mol the absorption rate of MEA decreases rapidly (in comparison to the hindered amines) due to its lower saturation level. The implications of these results on

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1783

Figure 4. Profiles of the CO2 absorption from 4.7% CO2 in air for 2.5 M aqueous solutions of amines 1-8. The symbols do not represent data points.

Figure 5. Profiles of the CO2 absorption from 1.1% CO2 in air for 2.5 M aqueous solutions of amines 1-8. The symbols do not represent data points.

the specific submarine scrubber application will be discussed in a later section. Absorption from Low-Level-CO2 Atmospheres. Figures 4 and 5 show the CO2 absorption of 2.5 M amine solutions for atmospheres of 4.7% and 1.1% CO2 in air, respectively. At these lower levels of CO2 the trends in absorption efficacy of amines 1-8 are similar to those observed for the pure CO2 experiments. The absorption curves of solutions of the hindered amines begin to level out at lower CO2 loading values when the atmospheric CO2 level is reduced. Although these amines absorb more CO2 than MEA, they do not approach the theoretical value of 1 mol of absorbed CO2/mol of amine at the lower CO2 concentrations. This is to be expected as the equilibrium solubility depends on the atmospheric CO2 partial pressure. The performance (based on rates of absorption) of some of these amines relative to MEA diminishes with atmospheric CO2 concentration. This can be illustrated by comparing the rate at which CO2 loading levels are reached by the amines. For example, in the time taken for the MEA solution to absorb to a level of 0.5 mol/mol (100% CO2, 2.5 M solutions), amines 2-8 reach levels of 0.43, 0.45, 0.49, 0.52, 0.44, 0.28, and 0.20 mol/mol, respectively. At an atmospheric level of 1.1% CO2 these values (relative to 0.5 for MEA) are 0.25, 0.48, 0.55, 0.50, 0.32, 0.23, and 0.06 mol/mol (see Figures 3 and 5).

Although the unsubstituted 4 and the R-monomethylated Alkazid M (3) have retained their reactivity relative to MEA, the performance of the R-dimethylated amines is reduced. These results suggest that these sterically hindered amines are better suited for absorbing CO2 from atmospheres with high CO2 partial pressure. Crystallization of Solution Species. A problem with the potassium carboxylate salts 3-8 is that upon reaction with CO2 the doubly-charged protonated amines form salts (carbonate and bicarbonate) with lower solubility than those of MEA and AMP, thus resulting in precipitation. The integration of quantitative (inversegated) 13C NMR experiments performed on these precipitates redissolved in water indicates that there are 2 mol of amine present/mol of the carbon dioxide species. Hence, the precipitates are the carbonate salts [(KO2CR′NH2R+)2CO32-]. Figure 3 shows the levels of carbon dioxide loading at which crystallization occurred for the 2.5 M amine solutions absorbing from a 100% CO2 atmosphere. No precipitation was observed with MEA, AMP, and 4, while the other amines precipitated in the following order: Alkazid M (precipitation at a CO2/amine of 1.0 mol/mol), 5 (0.94), 7 (0.90), 8 (0.50), and 6 (0.32). While at this concentration the presence of the R-methyl or the potassium carboxylate functional groups alone is not


Figure 6. Profiles of reabsorption of 100% CO2 by 2.5 M solutions of amines 1-8 (a and c) and the desorption profiles of these same solutions (b and d). Note that the absorption commences from the equilibrium level reached during desorption. The representative scrubber operating range (0.2-0.45 mol of CO2/mol of amine) is identified by the dotted lines. The symbols do not represent data points.

sufficient to cause precipitation of AMP and 4, all amines with both of these features form precipitates at some CO2 loading level. The introduction of the methyl groups R to the amine reduces the level of carbon dioxide that can be absorbed into solution prior to precipitation. This is illustrated by comparing the series 4 (no R-methyl, no precipitation), 5 (one R-methyl, precipitation at a loading of 0.94 mol/mol), and 6 (two R-methyl groups, 0.32). A substituent on the amine (i.e., secondary amine) reduces the changes of precipitation. This is reflected in the CO2 loading at the precipitation point for the primary amines compared with their N-substituted derivatives (i.e., 6 (0.32) with 7 (0.90) and 5 (0.94) with 3 (1.00)). It is unclear whether the superior solubility characteristics for 7 (N-CH3) in comparison with 8 (N-CH2CH2OH) are due to the size of the N-substituent or the presence of the hydroxyl group in 8. Due to the lower loading levels reached in the 1.1% and 4.7% CO2 experiments, crystallization only occurred for amine 6 in these cases. Carbon Dioxide Desorption and Cycling Ability of Amine Solutions For an amine to perform successfully as a regenerative carbon dioxide cycler the desorption reaction must achieve two aims. The desorption process must take place at a satisfactory rate, and it must produce a

sufficiently lean amine solution to allow a favorable reabsorption rate. Desorption of CO2 Saturated Amine Solutions. The desorption curves obtained by boiling (100 °C) the CO2 saturated solutions of amines 1-8 are presented in Figure 6. These desorption profiles show an initial delay of 1 min, this being the time taken for the solution to reach 60-70 °C and to start the desorption process. The relative order of desorption performance of these 2.5 M solutions of the amines may be obtained in several ways. If only the kinetics of the desorption reaction are to be considered, then calculating the CO2 released during the first 5 min of desorption gives an order of AMP (2, 0.69 mol of CO2 released/mol of amine) > Alkazid M (3, 0.62) > 8 (0.49) > 6 (0.46) > 5 (0.43) > MEA (1, 0.38) > 4 (0.32) = 7 (0.31). With the exception of 7, the trend is that the amines which allow the greatest absorption of carbon dioxide into solution, by generating the most bicarbonate, produce the fastest CO2 stripping upon heating. This is not unexpected given that these solutions contain a greater concentration of carbonated species. Alternatively, if the total desorption values at equilibrium are considered, the order of decreasing CO2 regeneration follows a trend similar to that for the 5 min data; however, the N-methyl-R-dimethylamine (7) shifts to between 8 and 6. With this analysis 7 now fits the pattern expected as it performs in a manner similar to that of the other substituted amines.


The full implications that the desorption and reabsorption results have on the efficiency of the carbon dioxide scrubbing process will be discussed in a later section. At this stage it will suffice to say that desorption in the scrubber does not commence with a CO2saturated amine solution. This means that analysis of the results must take into account the latter stages of the desorption process. If the final level of “CO2 species” remaining in solution (i.e., how lean a solution is produced) is the assessment criterion, then the order of performance in order of increasing residual solution CO2 species is AMP (2, 0.10 mol of carbamate/bicarbonate/carbonate remaining in solution/mol of amine) < 8 (0.15) < MEA (1, 0.18) = 7 (0.20) < Alkazid M (3, 0.25) = 6 (0.27) < 4 (0.36) < 5 (0.41). At first glance there seems to be no clear pattern linking amine structure with the level of CO2 species remaining in the desorbed solutions. However, if the carboxylate salts(3-8) are considered separately from the alcohol amines (1 and 2), then a trend emerges. In general, solutions of those amines with a higher degree of steric hindrance are regenerated to a lower CO2 loading level. For example, 8 reaches a level of 0.15 mol/mol, while 4 and 5 reach only 0.36 and 0.41 mol of CO2/mol of amine, respectively. Similarly, the R-dimethylamine AMP (2) is desorbed to a level of 0.1 mol/mol, while MEA (1) reaches only 0.2 mol/mol. A comparison of the effect of the potassium carboxylate group (CO2-K+) relative to the hydroxymethylene group (HOCH2) may be made by comparing 2 with 6 and 1 with 4. The CO2 loading levels reached after desorption are 0.10 and 0.27 mol/mol for AMP (2) and 6, respectively, and for MEA (1) and 4, they are 0.18 and 0.37 mol/mol, respectively. Clearly, in both the R-dimethylamine and R-unsubstituted amine cases the carboxylates fail to desorb to the levels reached by the alcohols. Similarly, when a comparison is made of the data obtained for the loss of CO2 (decrease in CO2 loading) during the first 5 min of desorption, the results for AMP (2) and 6 are 0.69 and 0.46 mol/mol, and for 1 and 4 they are 0.38 and 0.32 mol/mol. In each case the amines with carboxylate functional groups (6 and 4) desorb at a slower rate. It should be noted that both MEA (1) and 4 commenced desorption with approximately a 76% CO2 loading, which consisted of 31% carbamate and 69% bicarbonate species. Similarly, solutions of 2 and 6 commenced desorption with CO2 levels of 95% and 98%, respectively, with no detectable carbamate in either case. Despite these similarities their desorption characteristics are clearly different. No explanation has been found to account for the slow rate of stripping of CO2 from the solution of amine 7. Reabsorption and Cycling of CO2. In order to simulate the MEA scrubber processes, the amine solutions were subjected to sequential CO2 absorption/ desorption cycling. As each initial absorption (as described in the previous section) commences with pure amine and all subsequent cycles commence from the desorption equilibrium, attention must be focused on these reabsorption curves. All amines (1-8) successfully underwent CO2 cycling as each successive absorption and desorption were, within experimental error, identical. The reabsorption curves obtained experimentally by using the equilibrated desorbed solutions to absorb carbon dioxide from a 100% CO2 atmosphere are shown in Figure 6. The importance of efficient desorption is made clear by these curves. The failure of some amine solutions to produce a sufficiently lean solution

after desorption means that they do not commence reabsorption at a favorable point in the absorption curve. This is most noticeable for amines 4 and 5 (see Figure 6a). As in the case of the absorption of CO2 into pure amine solutions, the data for the reabsorptions allow the amines to be assessed in a number of ways. For the reabsorption of 100% CO2 the order of decreasing total CO2 absorbed per cycle is AMP (2, 0.85 mol of CO2/ mol of amine) > Alkazid M (3, 0.79) = 8 (0.78) > 7 (0.72) = 6 (0.71) > MEA (1, 0.56) > 5 (0.53) > 4 (0.42). As mentioned previously, the rate of absorption is a vitally important parameter and the assessment of this characteristic results in a somewhat different order of efficacy. These data are particularly relevant as amine solutions in the CO2 scrubbing process do not reach the fully saturated equilibrium. When the absorption capacity in the first 10 min of reabsorption is considered (as may be the case in a CO2 scrubber), the order of reducing amine efficacy becomes MEA (1, 0.29 mol of CO2/mol of amine) > Alkazid M (3, 0.24) > 6 (0.22) = AMP (2, 0.21) > 7 (0.14) = 4 (0.13) = 5 (0.12) > 8 (0.09). It is more difficult to see a relationship between solution efficacy and amine structure for these results. The reason for this is that there is more than one process involved, that is, the reabsorption results depend on both the absorbing ability of the amine solution and the extent to which desorption has taken place. The effects of the groups substituted adjacent to the amine group can be summarized as generally being unfavorable for absorption rate but favorable for increasing total absorption and favorable for reducing residual carbonated species in solution. Hence, as expected, in the case of reabsorption, the “hindered amine” solutions still absorb more CO2 than the unsubstituted amine solutions. However, the opposing effects of the substituents in decreasing absorption rate but increasing total desorption have resulted in the reabsorption rates for the relatively nonhindered 4 and 5 being poor, whereas the absorption rates of unreacted 4 and 5 were high in comparison to solutions of the other amines. MEA (1) remains well placed in the order of reabsorption rate, but this is due to the good desorption level attained for the alcohols in comparison to the carboxylated salts which was noted above. Figure 7 shows the CO2 reabsorption curves for the 2.5 M amine solutions absorbing from the 1.1% CO2 atmosphere. These curves were generated by replotting the “first absorption cycle” data beginning from the minimum levels of CO2 loading reached during desorption. The order of decreasing CO2 absorbed during the first 100 min of reabsorption for these amine solutions is MEA (1, 0.22 mol of CO2/mol of amine) > Alkazid M (3, 0.20) > 4 (0.18) > 5 (0.15) > 6 (0.11) ) AMP (2, 0.11) > 7 (0.07) > 8 (0.02). Alkazid M aside, there is a good correlation between the presence of substituents and a reduction in the reabsorption rate. Alkazid M seems to have a balance of sterically hindering substituents which result in both good absorption and desorption characteristics relative to the other potassium carboxylate salts. Consideration of the CO2 cycling data, as displayed in Figures 6 and 7, reveals that overall the potassium carboxylate sterically hindered amines are likely to be poorer performers than MEA when CO2 scrubber conditions are applied. The absorption rates for 2.5 M solutions of 7 and 8 (at all CO2 levels) and AMP (2) and 6 (at low CO2 levels) are poor in comparison with that


Figure 8. 13C NMR spectra of a 2.5 M aqueous solution of 4 (a) after equilibrated absorption of 100% CO2 and (b) after the desorption equilibrium was reached.

Figure 7. Profiles of the CO2 reabsorption from 1.1% CO2 in air by 2.5 M solutions of amines 1-8. The representative scrubber operating range (0.2-0.45 mol of CO2/mol of amine) is identified. The symbols do not represent data points.

of MEA (1). Poor desorption characteristics are exhibited by the solutions of 4, 5, and 7, and there is only mediocre desorption for 6. Crystallization is a potential problem, particularly for solutions of 6 and 8. NMR Spectroscopic Determination of Solution Species Nuclear magnetic resonance (NMR) spectroscopy allows identification of the individual atoms of all species in a solution, and, under appropriate conditions, it can be used quantitatively as well as qualitatively. 13C NMR spectroscopy was used to study the “CO2-absorbed” and “desorbed” solutions of the amines in order to establish the varying levels of chemical species in solution. In particular, quantifying the levels of carbamate and bicarbonate (see Scheme 1) makes it possible to determine the fate of the absorbed CO2. Amines 2, 3, and 5-8 were selected for their anticipated ability to form “carbonated solutions” containing only the bicarbonate species, and the quantitative NMR experiments enable their efficacy in achieving this to be investigated. CO2 Cycling of Solutions of 4: Quantitating Carbamate and Bicarbonate/Carbonate. Figure 8 presents an example of the 13C NMR spectroscopic

results obtained for CO2 cycling amines. Figure 8a shows the spectrum obtained for an equilibrated CO2saturated solution of 2.5 M potassium glycinate (4). Figure 8b shows the same solution after undergoing desorption. The peaks have been identified, and peak areas enable the species to be quantitated. The CO2-absorbed spectrum shows the presence of carbamate (signal at 164.7 ppm) and exchanging bicarbonate and carbonate (160.8 ppm). The chemical shifts of aqueous solutions of sodium bicarbonate and sodium carbonate are 158.1 and 165.7 ppm, respectively. The position of the peak at 160.8 ppm suggests a predominance of bicarbonate in this mixture. Data from a pH against carbonate/bicarbonate concentration diagram (Morel, 1983) also suggest that at this pH (approximately 8 for the fully absorbed solution) bicarbonate would be the major species. Calculations using the volume of CO2 absorbed and NMR peak intensities show that the equilibrium mixture is composed only of carbamate, bicarbonate, and the protonated amine and that no carbonate or unreacted amine are present. The absence of unreacted or unprotonated amine is reflected by the chemical shift values of peaks f and g (173.2 and 42.0 ppm, respectively) which are similar to those observed for glycine in aqueous solution (173.5 and 42.5 ppm for -O2CCH2NH3+; see Breitmaier and Voelter, 1978). The “desorbed solution” spectrum (Figure 8b) shows an increased level of carbamate (164.7 ppm) and a small peak for the bicarbonate/carbonate species (165.8 ppm). The chemical shift 165.8 ppm suggests that this signal is due almost solely to carbonate. This shift is due to the increase in solution pH as desorption takes place (Morel, 1983). Analysis shows that only 26% of the potassium glycinate (4) species is regenerated, with 69% of 4 existing as the carbamate and the protonated counterion and a further 5% being protonated counterion for the residual carbonate. These data explain the poor reabsorption capacity of the 2.5 M solution of 4 noted above. Effect of Amine Structure on “Absorbed Solution” Species Distribution. Each of the CO2-absorbed solutions (for absorption from 1.1%, 4.7%, and 100% CO2) was examined by 13C NMR spectroscopy, and the levels of solution species were calculated (Table 1). The amines are listed in order of increasing number of groups substituted onto or adjacent to the reaction site. Examination of the data for absorption from pure CO2 shows that, for the unsubstituted amines (MEA and 4), 31% of the absorbed CO2 exists in the form of carbamate, with 69% being bicarbonate. The introduction of

100% (164.9 ppm) 100% (163.7 ppm) 0% 0%

a 2.5 M solutions. b All data taken after 20 h of absorption. c CO loadings (mol of CO absorbed/mol of amine) obtained from the NMR data and from the absorption values (in parentheses). 2 2 The fraction of CO2 absorbed by the amine solution which is subsequently desorbed upon heating. d

77% 87% (166.5 ppm) 13% (164.0 ppm)

78% 87%

72% 54% 100% (164.9 ppm) 0% 93% (164.7 ppm) 7% (165.8 ppm)

22% (18%) 35% (37%) (41%) 23% (25%) (10%) (27%) 20% (20%) 11% (15%) 69% (160.9 ppm) 69% (160.8 ppm) 90% (160.5 ppm) 100% (160.4 ppm) 100% (160.6 ppm) 100% (160.4 ppm) 100% (158.4 ppm) 100% (158.5 ppm) 24% (161.4 ppm) 78% (76%) 31% (164.9 ppm) 30% (160.8 ppm) 76% (78%) 31% (164.7 ppm) 54% (161.1 ppm) 100% (94%) 10% (163.7 ppm) 92% (161.4 ppm) 100% (100%) 0% 100% (162.2 ppm) (95%) 0% 100% (163.8 ppm) 94% (98%) 0% 100% (162.4 ppm) 93% (95%) 0% 100% (165.2 ppm) 87% (92%) 0% 76% (164.6 ppm) 70% (164.3 ppm) 46% (163.7 ppm) 8% (163.9 ppm) 0% 0% 0% 0% 62% (64%) 64% (74%) 84% (85%) 84% (87%) 80% (84%) 83% (91%) 72% (88%) 44% (52%) 16% (162.2 ppm) 17% (161.5 ppm) 37% (162.0 ppm) 81% (162.7 ppm) 100% (163.8 ppm) 100% (162.7 ppm) 100% (164.6 ppm) 100% (164.8 ppm) 84% (164.6 ppm) 83% (164.3 ppm) 63% (163.7 ppm) 19% (163.9 ppm) 0% 0% 0% 0% 56% (60%) 52% (65%) 70% (71%) 67% (77%) 55% (63%) 70% (81%) 51% (59%) 20% (30%) ...CH2NH2 ...CH2NH2 ...CH(R1)NH2 ...CH(R1)NHR2 ...C(R1)(R2)NH2 ...C(R1)(R2)NH2 ...C(R1)(R2)NHR3 ...C(R1)(R2)NHR3 1 (MEA) 4 5 3 (Alkazid M) 2 (AMP) 6 7 8

0 0 1 2 2 2 3 3

HCO3-/CO32carbamate HCO3-/CO32carbamate

reactive site aminea

4.7% CO2 experimentsb 1.1% CO2 experimentsb

no. of form of CO2 in solution and δ values form of CO2 in solution and δ values bulky total CO2 total CO2 carbamate HCO3-/CO32loadingc carbamate HCO3-/CO32groups loadingc

total CO2 loadingc

form of CO2 in solution and δ values

100% CO2 experimentsb

total CO2 loadingc

desorbed solutions

Table 1.

13C

NMR Generated Data for the Carbon Dioxide Absorbed (1.1%, 4.7%, and 100% CO2 Experiments) and Desorbed Amine Solutions

form of CO2 in solution and δ values

% soln CO2 desorbedd


one R-methyl group in amine 5 resulted in the carbamate being reduced to a level of 10% with a corresponding increase in CO2 loading from 0.76 to 0.94 mol of CO2/ mol of amine. The disubstituted Alkazid M (3) and each of the R-dimethylamines (2, 6-8) showed no detectable levels of carbamate and, as expected, had the highest CO2 loading values. Examination of the data for absorption from the lower level CO2 atmospheres reveals the same trend, with only the R-dimethylamines showing no trace of their respective carbamate species. These results suggest that the two methyl groups R to the amine reduce the stability of the carbamate as required but that the effect of one R-methyl group is insufficient for full conversion to bicarbonate. This effect is particularly apparent in the lower CO2 level data where carbamate is reduced from 84% (1) and 83% (4) to 0% (2 and 6) by the addition of two R-methyl groups. Effect of Atmospheric CO2 Concentration on Absorbed Solution Species Distribution. A comparison of the performance of each amine in absorbing CO2 from different atmospheric concentrations shows that as the percentage of CO2 decreases there is a significant increase in the percentage of carbamate in solution and a corresponding decrease in CO2 loading. This is probably due to the increase in absorption rate at higher CO2 levels which increases the rate of carbamate formation and hence favors the hydrolysis reaction forming bicarbonate. It should be noted that, although all of the 100% CO2 absorption curves had reached equilibrium when the spectra were produced (after 20 h of reaction), for the lower levels of CO2 (1.1% and 4.7%) most amine solutions had not stopped absorption. This was most noticeable with the slower absorbing amines (7 and 8) which had not reached equilibrium. As the CO2 scrubbing process does not allow solutions to absorb to equilibrium, this failure to equilibrate is not of concern and does not prevent a meaningful performance assessment. The bicarbonate/carbonate chemical shift values in Table 1 decrease as the level of atmospheric CO2 is decreased. This trend shows that the bicarbonate/ carbonate balance is swung more toward carbonate at the lower CO2 levels. The lower levels of CO2 in solution in these cases results in a higher solution pH, which would be expected to shift the equilibrium more toward carbonate. Precipitation of Carbonates. The NMR data clarify the cause of the carbonate precipitation. The results show that, as expected, the R-methylamines cause a faster conversion of carbamate to bicarbonate. Consequently, more bicarbonate is formed before the solution pH has been reduced, and therefore at these higher pHs more bicarbonate is converted to the carbonate. This results in a higher level of carbonate in solution at lower CO2 loadings, and the critical precipitation level is reached earlier in the absorption process. However, there are other factors contributing to the precipitation. It is apparent that the protonated counterions formed by the alcohols MEA and AMP produce more soluble carbonate salts than those formed with the potassium carboxylates 3-8. There are also structural determinants which alter the solubility of the carbonate formed with the protonated amine as the counterion. Such amine structural influences may determine the differing solubilities of the carbonates formed with the R-dimethylamines 6-8. Overall, it is difficult to predict

1788 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 2. Levels of Carbamate, Bicarbonate, and Carbonate in CO2 Cycled Solutions CO2 saturated solutions amine solutiona 1 (MEA) 4 3 (Alkazid M) 7 8

desorbed solutions

carbamate HCO3-/CO32- carbamate HCO3-/CO32(mmol) (mmol) (mmol)b (mmol)c 6.0 5.9 0 0 0

13.4 13.0 24.9 23.1 21.6

5.5 8.1 0.7 0 0

0 0.6 5.0 5.0 2.7

a Total level of amine in solution is 24.9 mmol. b Solution pH and NMR δ values suggest that HCO3- is the major species in this equilibrium. c Solution pH and NMR δ values suggest that CO32- is the major species in this equilibrium.

which of these effects will dominate and determine the loading level at which precipitation occurs. NMR Spectra of Desorbed Solutions: Effect of Amine Structure on Desorption. 13C NMR spectra were generated for the equilibrated desorbed solutions of amines 1, 3, 4, 7, and 8. Table 1 lists the solution compositions both before and after desorption. If the carboxylate salts are considered separately from MEA (1), a relationship between structure and desorption efficacy is apparent. As the degree of substituted groups is increased, the proportion of carbamate (compared to HCO3-/CO32-) remaining in the equilibrated desorbed solution is decreased, and consequently a lower level of “CO2” is retained in solution. This is reflected by an increase in the proportion of “CO2-absorbed solution species” which are desorbed and also as a decrease in the final CO2 loading (Table 1). It is interesting that MEA defies this trend as it has relatively good desorption characteristics despite 100% of the CO2 remaining after desorption being in the form of the carbamate. This reinforces the observation made in the previous section that for amines with similar degrees of substitution the alcohols (1 and 2) exhibit superior desorption characteristics relative to the carboxylate salts. It is apparent that a greater degree of stability is imparted onto the carbamate by the carboxylate group relative to the hydroxymethyl group of 1 and 2. This is further supported by the data in Table 2 which lists the levels of carbamate and bicarbonate/carbonate in molar terms rather than as percentages. Although CO2 absorbed solutions of MEA (1) and potassium glycinate (4) have similar levels of all species, desorption slightly reduces the level of carbamate for MEA but increases the carbamate level for 4. Upon desorption of the saturated Alkazid M (3) solution, 0.7 mmol of carbamate is present when none was detected in the fully absorbed solution (Table 2). No carbamate was detected in the desorbed solutions of the R-methylamines 7 and 8 where the two substituent groups appear to reduce the carbamate stability. Detailed mechanistic information describing the processes that take place during the desorption of CO2 from these solutions is unknown. Bicarbonate is relatively easily decomposed by heating and would be expected to decompose at a significant rate at temperatures above 60 °C. Upon decomposition and subsequent loss of CO2, solution pH would rise and residual bicarbonate would be converted to the more stable carbonate species (as reflected by the chemical shift values in Table 1). The carbamates can undergo acid-catalyzed cleavage to reform amine and carbon dioxide (Ewing et al., 1980) or can be hydrolyzed to bicarbonate. Due to solution pH the former process is unlikely to occur for any solutions other than those that are highly saturated.

In the desorption experiment the desorbed CO2 is collected and held above the desorbing solution. This allows reabsorption from this CO2 atmosphere to take place until an equilibrium is reached with the heated solution. The levels of carbamate formation reached under these circumstances would be less than those reached at lower temperatures. However, for nonhindered amines there is little conversion to bicarbonate of these relatively stable (in comparison to the hindered amines) carbamates until levels approach a loading of 0.5 mol of carbamate/mol of amine (Sartori and Savage, 1983), and an equilibrium with a significant quantity of residual carbamate is produced. As noted above the carbamate produced by amine 4 appears to be more stable than that produced by MEA, and as a consequence it equilibrates to a higher level at the desorption temperature (100 °C). Due to the instability of the carbamates formed by the R-dimethylamines, there is no trace of carbamate in the desorbed solutions of 7 and 8. The residual level of carbonate in these solutions shows the equilibrium reached by the high-temperature absorption and desorption processes. The small quantity of carbamate present in the desorbed solution of Alkazid M implies that the carbamate is sufficiently stable at the higher pH of this solution (relative to the CO2-absorbed solution) to remain in the equilibrium. The lack of any carbamate in the fully absorbed solution highlights this change in carbamate stability with solution conditions. Implications for Amine-Based Carbon Dioxide Scrubbers The monoethanolamine-based scrubber operates with a process of continuous carbon dioxide absorption and desorption of the MEA solution. This process means that the solution absorbing the CO2 is not fully regenerated nor is the solution sent to the desorber saturated with CO2. The MEA scrubber operation has been optimized, and consequently the MEA solutions cycle, on average, over a narrow CO2 loading range. In order to achieve a reasonable reaction rate, the absorption must take place in the steeper range of the MEA absorption curves. To allow a more realistic interpretation of the hindered amine results, they will therefore be compared with MEA over a representative range of 0.2-0.45 mol of CO2/mol of amine for both absorption and desorption (see Figures 6 and 7). This range has been chosen to include the current submarine-based MEA scrubber operating range, but this does not mean that other ranges should not be considered. The optimum operating range for any particular solution can only be determined in a full-scale scrubbing unit under final operating conditions. Submarine-based scrubbers are required to absorb carbon dioxide at far lower partial pressures than those found in most industrial applications. Typically, the input atmosphere would have 0.5-1.5% CO2. For the submarine application the 1.1% CO2 results are clearly the most relevant, but the higher CO2 level absorption data are of value for land-based operations. Although the experiments reported here were conducted at a slightly lower absorber temperature and lower amine concentration than those used in the scrubbers, these results enable assessment of the relative performance of the amines. Consideration of both the absorption and desorption processes is essential. When the level of CO2 in the atmosphere undergoing scrubbing is high, the MEA


absorption and desorption processes occur at similar rates in the scrubbing range being considered (Figure 6). Under these circumstances desorption is the critical process, as the majority of power consumed by these scrubbers is used in heating the solution. However, for low CO2 level atmospheres the MEA absorption rate decreases by an order of magnitude and improvements in absorption efficacy become more relevant. Reabsorption results for amines 2-8 are generally poor when compared to MEA (1). Trends are the same at both high and low levels of carbon dioxide, with all amines failing to equal MEA in absorption rate in the representative range examined. When absorption from a pure CO2 atmosphere is considered (Figure 6), it can be seen that Alkazid M (3) and the R-dimethylamines AMP (2) and 6 absorb at a comparable rate to MEA. However, at low CO2 levels only Alkazid M approaches the absorption rate of MEA. The results are more positive for desorption where AMP (2) shows the highest rate and most complete desorption. Desorption characteristics for solutions of 8 and Alkazid M (3) are also at least as good as MEA (Figure 6). The poor desorption of solutions of 4 and 5 subsequently causes poor reabsorption due to the inability of these solutions to reach a sufficiently lean desorbed state. Consideration must be given to the possibility of adjusting scrubber operating parameters to allow the absorption/desorption processes to take place over either a higher or lower CO2 loading range. When operating in different ranges, amines such as 4-6 (higher ranges) and AMP (2) (lower range) have the potential to exhibit improved cycling ability. Increasing desorber temperature may produce a leaner desorbed solution for 4 and 5, thus allowing better reabsorption. A reduced CO2 loading range would be desirable as it would lead to lower solution viscosity and a reduced likelihood of crystallization. However, as the regenerator consumes most of the power used by the scrubber and as reduction in power consumption is vitally important in submarines, it would be advantageous to avoid a CO2 loading range that included the bottom of the desorption curves, where the desorption rate is slowest. The propensity for carbonate salts to crystallize from solution produces a potentially serious engineering problem. It would be difficult to pump suspensions around the scrubber and to avoid solid buildups and the resultant blockages (for example, on filters). This could possibly be overcome by taking measures such as increasing absorption temperature or by “shifting” the operating CO2 loading range. However, it would always be difficult to avoid crystallization at some points in the scrubber system. Such problems render R-dimethylamines 6 and 8 unusable. Under the experimental conditions (2.5 M amine, 22 °C) solutions of Alkazid M (3), 5, and 7 do not form precipitates until they reach a CO2 loading approaching saturation and no precipitates are formed for MEA, AMP, and 4. Thus, although 3, 5, and 7 could be used to extract CO2, careful monitoring to assess potential crystallization points would be required. It is concluded that, of the amines examined, only Alkazid M (3) is likely to fulfill the submarine-based scrubber requirements. Under the experimental conditions this amine performs only marginally less efficiently than MEA, with its low volatility and prospective greater stability in comparison with MEA offering advantages. Although AMP (2) exhibits encouraging

desorption characteristics, its rate of CO2 absorption at low partial pressures is likely to prohibit its use. However, at higher levels of CO2, as may be found in industrial processes, AMP is potentially superior to MEA (Figure 6). The results show that the combination of the potassium carboxylate group and the R-dimethylamine moiety, as in 2 and 6-8, fails to produce the desired increase in CO2 extracting efficacy. Conclusions Under the experimental conditions solutions of the sterically hindered R-dimethylamines show enhanced CO2 absorption capacities in comparison to MEA; however, the increased substitution around the amine decreases absorption rates. Two methyl groups substituted R to the amine reduce the stability of the subsequent carbamate, fully shifting the CO2 absorption equilibrium to bicarbonate. One substituted R-methyl is insufficient to produce total carbamate hydrolysis. When the atmospheric CO2 level is decreased, the CO2 absorption efficiency of the R-dimethylamines is reduced relative to MEA, reducing their potential efficacy in a submarine-based scrubber. The problem of carbonate salt crystallization would be likely to prohibit the use of many of these amines as a direct replacement for MEA. Due to the rapid formation of bicarbonate early in the absorption process, those amines with the greatest CR-substitution generally exhibit the worst precipitation problems. However, N-substituted amines produce reaction products with greater solubility. Due to their high CO2 loading capacity, the CRsubstituted amines exhibit high initial rates of desorption, and within the series of carboxylate salts (3-8), leaner desorbed solutions are produced for those amines with greater steric hindrance. It is apparent that the carbamates produced by the carboxylate salt amines are more stable than those produced by the alcohol amines. CO2-saturated solutions of the alcohols, MEA and AMP, desorb at a superior rate and produce leaner solutions than the corresponding carboxylate salts. Although all of the amines investigated were found to successfully cycle CO2, with the exception of MEA (1) and Alkazid M (3), each suffered problems with either absorption, desorption, or precipitation of salts. The R-dimethylamine function is successful in driving the absorption equilibrium to bicarbonate, but an alternative to the potassium carboxylate group is required to achieve reduced volatility and solubilization of all reaction species. Only Alkazid M (3) is likely to fulfill submarine-based scrubber requirements. R-Dimethylamines may be more suited to use in industrial processes where CO2 partial pressures are higher. Literature Cited Blauwhoff, P. M. M.; Versteeg, G. F.; Van Swaaij, W. P. M. A Study on the Reactions Between CO2 and Alkanolamines in Aqueous Solutions. Chem. Eng. Sci. 1984, 39 (2), 207. Breitmaier, E.; Voelter, W. 13C NMR Spectroscopy, Methods and Applications in Organic Chemistry, 2nd ed.; Verlag Chemie: New York, 1978; p 276. Caplow, M. Kinetics of Carbamate Formation and Breakdown. J. Am. Chem. Soc. 1968, 90 (24), 6795. Cassidy, S. Personal communication, Wellman Process Engineering, U.K., 1995. Chakraborty, A. K.; Astarita, G.; Bischoff, K. B. CO2 Absorption in Aqueous Solutions of Hindered Amines. Chem. Eng. Sci. 1986, 41 (4), 997.

1790 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Chakraborty, A. K.; Bischoff, K. B.; Astarita, G.; Damewood, J. R., Jr. Molecular Orbital Approach to Substituent Effects in Amine-CO2 Interactions. J. Am. Chem. Soc. 1988, 110, 6947. Cocker, W. Preparation of the Simpler R-Alkylamino-acids. Part 1. J. Chem. Soc. 1937, 1693. Crooks, J. E.; Donnellan, J. P. Kinetics of the Reaction between Carbon Dioxide and Tertiary Amines. J. Org. Chem. 1990, 55, 1372. Ewing, S. P.; Lockshon, D.; Jencks, W. P. Mechanism of Cleavage of Carbamate Anions. J. Am. Chem. Soc. 1980, 102, 3072. Fu, S.-C. J.; Birnbaum, S. M. The Hydrolytic Action of Acylase I on N-Acylamino Acids. J. Am. Chem. Soc. 1953, 75, 918. Goan, J. C. Alkazid M as a Regenerative Carbon Dioxide Absorbent. In The Present Status of Chemical Research in Atmosphere Purification and Control on Nuclear-Powered Submarines; Miller, R. R., Piatt, V. R., Eds.; NRL Report 5465; Naval Research Laboratory: Washington, DC, 1960; Chapter 12. Goan, J. C. Alkazid M. In The Present Status of Chemical Research in Atmosphere Purification and Control on Nuclear-Powered Submarines; Piatt, V. R., Ramskill, E. A., Eds.; NRL Report 5630; Naval Research Laboratory: Washington, DC, 1961; Chapter 7. Gustafson, P. R. CO2 Absorption Properties of Some New Amines. In The Present Status of Chemical Research in Atmosphere Purification and Control on Nuclear-Powered Submarines; Miller, R. R., Piatt, V. R., Eds.; NRL Report 6722; Naval Research Laboratory: Washington, DC, 1968; Chapter 5. Gustafson, P. R. CO2 Absorption Properties of Some New Amines. In Chemical Research in Nuclear Submarine Atmosphere Purification; Piatt, V. R., Ramskill, E. A., Eds.; NRL Report 7037; Naval Research Laboratory: Washington, DC, 1970; Chapter 5.

Maddox, R. N.; Mains, G. J.; Rahman, M. A. Reactions of Carbon Dioxide and Hydrogen Sulfide with Some Alkanolamines. Ind. Eng. Chem. Res. 1987, 26 (1), 27. Morel, F. M. M. Principles of Aquatic Chemistry; John Wiley & Sons: New York, 1983; p 132. Niswander, R. H.; Edwards, D. J.; DuPart, M. S.; Tse, J. P. A More Energy Efficient Product for Carbon Dioxide Separation. Sep. Sci. Technol. 1993, 28 (1-3), 565. Ravner, H.; Blachly, C. H. Studies on Monoethanolamine (MEA). In The Present Status of Chemical Research in Atmosphere Purification and Control on Nuclear-Powered Submarines; Piatt, V. R., White, J. C., Eds.; NRL Report 5814; Naval Research Laboratory: Washington, DC, 1962; Chapter 5. Sartori, G.; Savage, D. W. Sterically Hindered Amines for CO2 Removal from Gases. Ind. Eng. Chem. Fundam. 1983, 22, 239. Sax, N. I., Ed. Hazardous Chemicals Information Annual No. 1; Van Nostrand Reinhold Information Services: New York, 1986; p 30. Tontiwachwuthikul, P.; Meisen, A.; Lim, C. J. Solubility of CO2 in 2-Amino-2-methyl-1-propanol Solutions. J. Chem. Eng. Data 1991, 36, 130.

Received for review September 9, 1996 Revised manuscript received January 6, 1997 Accepted January 23, 1997X IE9605589

X Abstract published in Advance ACS Abstracts, March 1, 1997.

An Investigation of Some Sterically Hindered Amines as Potential

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