Thermal Degradation of Aqueous Piperazine for CO2 Capture: 2

Article pubs.acs.org/IECR

Thermal Degradation of Aqueous Piperazine for CO2 Capture: 2. Product Types and Generation Rates Stephanie Anne Freeman† and Gary Thomas Rochelle*,† †

Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: The generation of degradation products at 135 to 175 °C was investigated for concentrated, aqueous piperazine (PZ) loaded with CO2. From 135 to 175 °C, N-formylpiperazine, ammonium, N-(2-aminoethyl)piperazine, and 2-imidazolidone were found to be the most abundant products. These species accounted for 63% of nitrogen and 49% of carbon lost as PZ and CO2 during degradation. Thermal degradation of PZ is believed to be initiated by the nucleophilic attack of PZ at the α-carbon to a protonated amino function on H+PZ to create a ring opened PZ structure. H+PZ was found to be the active and likely initiating species required for the initial reactions of thermal degradation. Further SN2 substitution reactions can produce a variety of products. CO2 is not required for thermal degradation to proceed but is reduced to create formate or formyl amides, reacts with amines to form stable ureas, and dictates the overall product mix. The mechanism for CO2 reduction to formate or formyl amides is not clear but indicates the severity of thermal degradation conditions.

1. INTRODUCTION Amine-based absorption-stripping is the current state-of-the-art technology for postcombustion carbon dioxide (CO2) capture from coal-fired power plants. A crucial step in utilizing this technology, however, is solvent selection. The most desirable solvent for this application should have fast CO2 absorption rates, high CO2 capacity, high heat of absorption for CO2, limited or controllable oxidation and thermal degradation, low amine volatility, and favorable physical properties. The traditional baseline solvent investigated for this application is monoethanolamine (MEA). In comparison with MEA, concentrated, aqueous piperazine (PZ) is an advanced solvent that has double the CO2 absorption rates and CO2 capacity, limited oxidation and thermal degradation rates, and lower amine volatility.1−4 The rate of thermal degradation of a CO2 capture amine is a critical parameter in solvent selection. Solvents can spend over one-third of the residence time of an industrial system at temperatures well above 100 °C. Thermal degradation in the stripping section of a CO2 capture system will reduce CO2 capacity, increase the steam requirement for stripping, and increase the cost of amine replacement, reclaiming, and disposal. Thermal degradation can also lead to environmental issues relating to the unknown health effects and reactivity in the environment of many of the degradation products.3,5−8 Minimizing thermal degradation is a crucial design criterion for an effective amine-based absorption-stripping system. The thermal degradation rate of concentrated PZ in terms of the pertinent process conditions has been presented in our previous study.9 The thermal stability of PZ has been well established with this study and others.1,9−11 Given the higher expected cost of PZ compared to less expensive alternatives (i.e., MEA or N-methyldiethanolamine (MDEA)), minimizing even low rates of thermal degradation may still prove important. Equally important is understanding what products are generated and through which pathways in order to © 2012 American Chemical Society

adequately assess environmental risks, reclaiming options, and associated costs. The generation of formate, formyl amides, and ethylenediamine (EDA) during PZ thermal degradation has been discussed previously for only a limited set of conditions.10 An extensive study of the thermal degradation products of PZ has not been undertaken previously in the literature as has been done for other gas-treating amines such as MEA,12−16 diethanolamine (DEA),17−25 and MDEA.21,26−28 The discussion below presents all of the thermal degradation products that have been identified for concentrated PZ. A discussion of possible pathways to explain the generation of the major products is also included. In a few cases, products that are suspected but not yet fully confirmed are presented along with analytical evidence of their role in degradation.

2. METHODS AND MATERIALS 2.1. Aqueous Solution Preparation and Total Inorganic Carbon (TIC) Assay. Concentrated, aqueous piperazine solutions were prepared as described in detail previously.1,2,10,29 Aqueous PZ solutions were heated to dissolve solids and gravimetrically sparged with CO2 to attain the desired CO2 loading. Anhydrous piperazine (IUPAC: 1,4diazacyclohexane, CAS 110-85-0, purity 99%, Acros Organics N.V., Geel, Belgium) and CO2 (CAS 124-38-9, purity 99.5%, Matheson Tri Gas, Basking Ridge, NJ) were obtained from commercial sources and used with distilled, deionized water for experimental solutions. Acidification of PZ solutions was performed by first melting water and PZ until an aqueous solution was obtained. Sulfuric acid (H2SO4, 36N, Fisher Scientific Worldwide, Hampton, NH) was added dropwise to PZ solution until the desired concentration was achieved. Received: Revised: Accepted: Published: 7726

August 26, 2011 April 27, 2012 April 30, 2012 April 30, 2012 dx.doi.org/10.1021/ie201917c | Ind. Eng. Chem. Res. 2012, 51, 7726−7735

Industrial & Engineering Chemistry Research

Article

confirmed using IC-MS analysis and commercially available standards to verify peak position and molecular weight of each product. In most cases, suspected products were not fully confirmed because standards were not commercially available. No product was identified without a standard for comparison (i.e., MS library matching was not used). 2.5. High-Performance Liquid Chromatography. Highperformance liquid chromatography (HPLC) was used to quantify 2-imidazolidone (2-Imid) using a Dionex UltiMate 3000 LC unit with LPG-3400SD quaternary analytical pump, WPS-3000SL analytical split-loop autosampler, TCC-3000SD thermostatted column compartment, and VWD-2100 single channel variable wavelength detector quantifying at 210 nm (Dionex) as described previously in detail.1 Separation was achieved in an Acclaim Polar Advantage II reversed-phase analytical column with guard column (C18, 5 μm, 120 Å, 4.6 × 250 mm, Dionex) using a variable gradient of acetonitrile (CH3CN) and methanol (MeOH) in analytical grade water. Details of the eluent method are as follows: pure water for 4 min, ramp to to 5% MeOH/95% water at 10 min, ramp to 50% MeOH/50% water at 20 min, and finally ramp to 20% CH3CN/50% MeOH/30% water at 30 min. 2.6. Nuclear Magnetic Resonance (NMR) Spectroscopy. Quantitative 13C and 1H Nuclear Magnetic Resonance (NMR) spectroscopy was performed using 125 MHz Varian Iova NMR for 13C analysis and 500 MHz Varian Inova NMR for 1H NMR (Agilent Technologies, Santa Clara, CA). Samples for NMR analysis were prepared according to previous work.30 Samples composed of 10 wt % D2O (purity 99.9%, Cambridge), 1 wt % dioxane (Fisher), and the balance experimental sample were mixed well and 2 mL of solution was transferred to a 5.0 mm OD × 4.2 mm ID × 7 in NMR tube (Item No. 507PP-7, Wilmad Glass, Vineland, NJ). When working with sensitive samples containing 13C-labeled CO2, the end of the glass tube was sealed with an open flame.

Experimental solutions made for NMR analysis were made as described above, but with 13CO2 (purity 99%, Cambridge Isotope Laboratories, Andover, MA) rather than natural CO2. The CO2 loading of aqueous solutions was quantified using a total inorganic carbon (TIC) assay that has been described previously.1,2,10,30 Phosphoric acid was used to liberate CO2 from solution, while an infrared detector (Horiba Instruments Inc., Spring, TX) was used to quantify CO2 concentration. A calibration curve generated from an inorganic carbon standard (Ricca Chemical Company, Pequannock, NJ) was used to calculate CO2 concentration, which is reported in units of mol CO2/mol alkalinity (mol CO2/mol equivalent amine). 2.2. Thermal Degradation Cylinders. Thermal degradation was performed in 5 in. long 316SS cylinders with Swagelok end-caps as described previously.1,2,10,12 Multiple cylinders were filled with the amine solution of interest, sealed according to Swagelok specifications, and placed in forced convection ovens for up to 72 weeks. Cylinders were removed from the oven periodically to sample the experiment. The amine solution was removed from sealed cylinders and analyzed for PZ, other amines, if blended, and CO2 concentration. 2.3. Cation and Anion Ion Chromatography and Amide Quantification. Cation IC was used to quantify PZ and other amine concentrations using a Dionex ICS-2100 Integrated Reagent-Free IC system with AS autosampler, 4-mm Cationic Self-Regenerating Suppressor (CSRS), and conductivity detector as described in detail previously (Dionex Corporation, Sunnyvale, CA).1,2,10 Separation was achieved in an IonPac CG17 guard column (4 × 50 mm) and IonPac CS17 analytical column (4 × 250 mm) using a gradient of methanesulfonic acid in analytical grade water. Anion IC was used to quantify carboxylate ion concentrations using a Dionex ICS-3000 modular Dual Reagent-Free IC system with AS autosampler, 4-mm Anionic SelfRegenerating Suppressor (ASRS), Continuously Regenerated Anion Trap Column (CR-ATC), Carbonate Removal Device (CRD), and conductivity detector as described previously.1,2,10 Separation was achieved in an IonPac AG15 guard column (4 × 50 mm) and IonPac AS15 analytical column (4 × 250 mm) using a gradient of potassium hydroxide in analytical grade water. Base hydrolysis (1 g of 5 N NaOH to 1 g of sample) for at least 24 h was used to separate amides into their corresponding carboxylate ion and amine as described previously.1,10 Hydrolyzed samples were analyzed using anion and cation IC to determine the increase over unhydrolyzed samples due to the presence of amides. Anion species are reported as the total concentration resulting from the sum of the carboxylate ion and the carboxylate ion due to the presence of amides quantified in the hydrolyzed sample. 2.4. Coupled Ion Chromatography−Mass Spectrometry (IC-MS). A Dionex ICS-2100 ion chromatography system (Dionex) was coupled with a Thermo Finnegan TSQ Mass Spectrometer with a triple-stage quadrupole and electro-spray ionization (Thermo Fisher Scientific, Waltham, MA). The chromatographic separation proceeded as described above for cation IC using a methanesulfonic acid gradient in analytical grade water. The eluent exiting the conductivity detector of the ICS-2100 was fed directly to the inlet of the ESI system of the MS. Because of this, the CSRS suppressor was not operated in recycle, or autosuppression, mode, but auxiliary water was added to the back of the suppressor membrane to flush waste from the suppressor. All identified cation products were

3. RESULTS AND DISCUSSION 3.1. Thermal Degradation Products. A variety of molecules are generated when concentrated, loaded PZ is exposed to elevated temperature. Experiments were performed up to 175 °C, above expected operating conditions, with the intention of accelerating or maximizing thermal degradation in order to more easily quantify the reactions occurring. The experimental approach was to use high levels of degradation in order to study the products generated and then apply that knowledge to the initial rates and causes of degradation in real systems. Thermal degradation of concentrated PZ generates amines, amides, carboxylate ions, imidazolidones, and ureas as degradation products, while aldehydes, oxazolidones, and polymeric compounds may also be generated, although these have not been positively quantified in degraded PZ. As an example of what would be expected in PZ thermal degradation, the results from the degradation of 8 m (m) PZ (approximately 4.1 mol PZ/kg solution) with 0.3 mol CO2/mol alkalinity at 165 °C for 20 weeks will be presented. This temperature is of specific interest to new PZ projects that call for stripper or twostage flash operation around 150 °C. Any deviations from the products and relative concentrations seen in this experiment will be discussed in section 3.2 as regards the effect of process conditions. The degradation of CO2-loaded 8 m PZ for 20 weeks at 165 °C resulted in a loss of 1730 mmol PZ/kg and 888 mmol CO2/ 7727

dx.doi.org/10.1021/ie201917c | Ind. Eng. Chem. Res. 2012, 51, 7726−7735


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Table 1. Identified and Suspected Thermal Degradation Products of Concentrated PZa

Status1: I − fully identified and quantified, INQ − identified but not quantified, and S − suspected based on IC-MS, but no commercial standard available. a

Table 2. Nitrogen and Carbon Mass Balance Closure for Thermally Degraded PZ at 165 °C for 20 Weeks nitrogen balance degradation products

PZ lost in 20 weeks CO2 lost in 20 weeks TOTAL mass lost N-formyl PZ NH4+ AEP 2-Imid HEP EPZ EDA MPZ formate total acetate TOTAL

product (mmol/kg)

N (mmol/kg)

1729 888

3458 3458 1115 496 355 218 122 109 106 41 0 0 2561

558 496 118 109 61 54 53 21 108 36

carbon balance

lost N (%) 32.2 14.3 10.3 6.3 3.5 3.1 3.1 1.2 74.1

C (mmol/kg) 6916 888 7804 2787 0 710 327 367 327 106 103 108 71 4904

lost C (%) 35.7 9.1 4.2 4.7 4.2 1.4 1.3 1.4 0.9 62.8

and carbon mass balance achieved after 20 weeks at 165 °C is presented in Table 2 to demonstrate the concentration of products achieved and the relative concentration or importance of each product. The mass balances are presented as the percent that each product represents in terms of the loss nitrogen or carbon from PZ and CO2 disappearance. A balance

kg or 40.8% and 32.3% of the initial concentrations, respectively. In this experiment, 10 significant, fully identified degradation products were quantified. A summary of the degradation products for the thermal degradation of PZ is presented in Table 1. Minor products and those suspected but not fully confirmed are also included in Table 1. The nitrogen 7728



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Anion IC and NMR is shown in Table 3. The NMR quantification was based on the assumption that the formate

of 100% would indicate that all of the lost PZ and CO2 were recovered as quantified products. Over 74% of nitrogen and 62% of carbon lost in PZ and CO2 were recovered in the quantified degradation products. The most abundant products are N-formylpiperazine (FPZ), ammonium (NH4+), N-(2-aminoethyl)piperazine (AEP), and 2-imidazolidone (2-Imid). These four products account for 63% of nitrogen and 49% of carbon, a majority of mass lost during degradation. Minor products positively identified include N-(2-hydroxyethyl)piperazine (HEP), N-ethylpiperazine (EPZ), ethylenediamine (EDA), N-methylpiperazine (MPZ), formate, acetate, and acetyl amides. Another product, N-(2-aminoethyl)-2-imidazolidone (AEI), was positively identified through IC-MS and a commercial standard but was not quantified. Because they are significantly more volatile than PZ, ammonia, EPZ, and MPZ are degradation products that could have significant environmental impact if they escape the water wash system treating flue gas exiting the absorber. 3.2. Effect of CO2 on Degradation Product Generation. The presence of CO2 in loaded amine solutions has been shown to have a significant influence on the overall rate of thermal degradation.6,12,14,24,31 This has been established for concentrated PZ solutions in previous work by the authors.9 The effect of CO2 on degradation products, however, has not yet been explored for concentrated PZ. The following sections detail the influence of CO2 on the generation of degradation products and the overall product mix through experiments targeted at investigating specific species and degradation processes. 3.2.1. Generation of Formate in Oxygen-Poor Thermal Degradation. The generation of formate during oxygen-poor thermal degradation has not been fully explored. Although formate production during thermal degradation has been reported previously,10 formate and other carboxylate ions, commonly referred to as heat stable salts, are generally seen as oxidation products.32−34 In fact, formate has been seen in the oxygen-poor thermal degradation of a variety of CO2 capture amines: cyclic diamines (PZ, MPZ, 2-methylpiperazine), cyclic amines (piperidine, morpholine, pyrrolidine, hexamethyleneimine), straight chain amines (EDA, hexamethylenediamine, 3(methylamino)propylamine), and alkanolamines (MEA, MDEA) (some unpublished work).11,31,35 In all cases, the formate production is too great to be explained by the presence of headspace oxygen. The only significant source of oxygen in all of these solutions is the CO2 molecule itself. Therefore, it is hypothesized that formate is generated from the reduction of a CO2-containing molecule during high temperature degradation. To determine the source of formate observed after degradation, 8 m PZ with 0.3 mol 13CO2/mol alkalinity was degraded at 175 °C for 6 weeks. IC analysis of the 13CO2 and natural CO2 control experiments showed similar behavior in PZ, formate, total formate, and other products with 42% and 37% loss in PZ, respectively, observed. Samples taken at 2 and 6 weeks were analyzed using quantitative 13C NMR. NMR analysis of neat PZ spiked with 30 to 300 mmol formate/kg confirmed identification of a peak between 170.19 and 170.30 ppm belonging to the carbonyl in formate that matched the expected chemical shift of sodium formate in D2O.36 The formate carbonyl was shifted further downfield and separated from carbonyls representing amides and carbamates. Quantification of the formate peak in degraded samples was performed based on the peak representing dioxane. A comparison of the formate concentration observed with

Table 3. Comparison of Formate Quantified Using Anion IC and NMR in Degraded PZ with 13CO2 and Natural CO2 formate concentration expt 13

CO2

natural CO2

deg. time weeks

anion IC (mmol/kg)

NMR − 13C (mmol/kg)

NMR − natural C (mmol/kg)

0 2 6 0 2 6

0.27 127.2 153.9 0.21 112.4 135.6

0 66 132 0 -

0 6120 12,160 0 101

peak (identified at 170.23 ppm) represented a formate population that was entirely 13C, rather than the usual assumption that peak areas represent the naturally occurring 13 C in a molecule. The calculation performed assuming the formate population is from naturally occurring carbon is also shown in the final column of Table 3. Although the NMR quantification demonstrates error, as expected when quantifying peaks vastly different in size on a single spectrum, it is clear that the formate carbon is 13C and is derived from CO2 in solution during thermal degradation. Direct reduction of CO2 is not known to proceed in this manner under normal conditions. The mechanism for the production of formate is not known at this time but likely proceeds through the oxidation of another molecule, likely PZ, to a product that has yet to be identified and fits within the missing mass in the mass balances. This finding implies that formate made from thermal degradation processes does not result directly from the fragmentation of the PZ molecule and does not have the same impact on CO2 capacity and overall performance as formate derived from oxidation mechanisms. 3.2.2. Equilibrium of Formate and Formyl Amides. A corollary to the previous section is the interaction of formate and formyl amides in solution. Both formate and formyl amides were found in every thermally degraded PZ solution in this study. In the experiment described in section 3.1, FPZ was the only formyl amide detected. This was so except for a few cases with severe levels of PZ loss where appreciable concentrations of other formyl amides were found. Both formate and FPZ were found in the initial stages of thermal degradation without a lag in either species in all cases. It is not known which species is formed first, but most likely one is generated and reacts quickly with water or PZ to produce the complementary species. Equilibrium, as shown in eq 1, is occurring between formate and FPZ where both are final products and the concentrations depends on degradation conditions. It has been shown previously that amides can be converted to their respective carboxylate ion and amine molecules through a hydrolysis, which is the basis of the alkaline hydrolysis for quantifying amides.37 The hydrolysis can be catalyzed by either acid or base and could occur readily in either direction at high temperature.38 It is well established that FPZ can be produced through the reaction of PZ with active reactants in high temperature environments. For example, Horrom and colleagues produced FPZ by combining methyl formate and PZ, while Duranleau reacted CO with catalyst at high temperature to produce FPZ and N,N′-diformylpiperazine.39,40 7729



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CO2-containing species such as PZ carbamate (PZCOO−) without creating H+PZ. After 10 weeks at 175 °C, only 2.5% of the initial PZ was lost in the HCO3− experiment compared to 29% loss of PZ in 8 m PZ with 0.1 mol CO2/mol alkalinity degraded in the same manner. The absence of H+PZ was found to severely limit thermal degradation indicating this is a crucial, initiating species in the degradation pathways for PZ. The k1 values are compared in Table 4 to demonstrate the effect of HCO3− or absence of H+PZ. 3.2.4. Influence of CO2 on Product Mix through Urea Generation. The presence of CO2 was also found to influence which products were generated. A comparison of the degradation products generated with only acidification to those generated with typical CO2 loading illustrates the influence of CO2. First, only negligible concentrations of formate were produced in the presence of H+PZ (absence of CO2). After 15 weeks at 175 °C, only 7.7 mmol formate/kg was generated compared to over 700 mmol formate/kg generated in a comparable CO2-loaded experiment. This finding agrees with the NMR results presented in section 3.2.1 and further solidifies the finding that formate is derived directly from CO2. The accumulation of EDA as a thermal degradation product is also influenced by the presence of CO2, as shown in Figure 2.

The interconversion of formate and FPZ was tested by spiking 8 m PZ with 0.3 mol CO2/mol alkalinity and 200 mM of formate or FPZ. The ratio of these species was monitored over 5 days at 175 °C, as shown in Figure 1. In both solutions,

Figure 1. Equilibrium of formyl amides and formate at 175 °C in 8 m PZ with 0.3 mol CO2/mol alkalinity spiked with 200 mM formate (○) or FPZ (●); line at a ratio of 2.5 drawn for reference.

the spiked component reacted with either PZ or water to produce the other, indicating that neither is an intermediate for the other, but both are final products that can interconvert. In both cases, the solution had reached equilibrium ratio near 2.5 within 1 day at 175 °C, and neither direction of the equilibrium appeared to be faster than the other. 3.2.3. Acidification and H+PZ as a Reactive Species. PZ acidified with sulfuric acid was degraded in the absence of CO2 to determine the effect of CO2 on product generation. The addition of sulfuric acid created reactive protonated PZ species such as H+PZ and, to a lesser extent, H+PZH+, in the absence of CO2. PZ does not significantly degrade in the absence of CO2, but acidified PZ thermally degraded in the absence of CO2 due to the presence of protonated molecules.9 After 15 weeks at 175 °C, 8 m PZ with either 0.3 mol H+/mol alkalinity or 0.3 mol CO2/mol alkalinity lost 34% or 70% of the initial PZ, respectively. Degradation occurred without CO2 in the presence of H+PZ but at a rate roughly half as fast as loaded degradation. The k1 values for these experimental conditions are compared in Table 4 to demonstrate the effect of acidification. As a corollary, 8 m PZ with 0.1 mol KHCO3/mol alkalinity was degraded to determine if degradation would occur in the absence of H+PZ. HCO3− was added instead of CO2 to create

Figure 2. Generation of EDA in 8 m PZ at 175 °C with 0.3 mol H+ (●) or CO2 (○) per mole alkalinity.

In this figure, four repeated experiments with CO2 (open points) are shown as a single data set to give an idea of repeatability and the overall trend. In the presence of H+PZ, EDA is generated as a degradation product and accumulates in solution. In the presence of CO2, EDA is generated quickly, to match the faster overall degradation rate, but then plateaus at 40 to 60 mmol EDA/kg. The explanation for the limited EDA accumulation in CO2loaded degradation is the generation of a urea. In the presence of CO2, EDA has been found to react quickly to 2-Imid, its internal, cyclic urea with a high yield without catalyst from 150 to 180 °C.41,42 Wu and colleagues suggested two dehydration mechanisms in which the EDA reacts with CO2 to form an EDA carbamate (EDACOO−) and is then followed by a dehydration step through either a dialcohol or isocynate intermediate.42 Either scheme is possible for the primary aliphatic structure of EDA, and the overall reaction is an internal cyclization as shown in eq 2. Once EDACOO− is formed, the proximity of the secondary amino function of EDA makes 2-Imid the most likely product. No evidence of cross-

Table 4. Thermal Degradation Rate (k1) for 8 m PZ with Varying Additives at 175 °C comparison +

effect of H

effect of HCO3−

additive

additive conc (mol/mol alk)

k1 × 109 (s−1)

CO2 CO2 H+ CO2 KHCO3

0 0.3 0.3 0.1 0.1

7.0 132 47 66 4.5 7730



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ureas. The generation of minor products such as HEP (peak 3), MPZ (peak 4), and an unknown (peak 9) is accelerated in the presence of CO2 due to the higher level of overall degradation. 3.3. Effect of Temperature and PZ Concentration on Product Generation. The rate of thermal degradation is enhanced with an Arrhenius dependence on temperature, as shown previously.1,10 Increased temperature increases both the rate of degradation and the rate of product generation. The second order rate constant, k2, for PZ loss is compared to the initial, linear rate of total formate generation in Figure 4 for 8 m

product ureas, such as a urea of PZ and EDA, was found. It is likely that further generation of ureas is occurring. For example, AEI, the internal, cyclic urea of diethylenetriamine (DETA), was identified as a product of PZ thermal degradation. DETA itself was not quantified, but the rapid rate of thermal degradation of DETA at low temperatures has been well documented, and it would not be expected to accumulate at 175 °C.6,12

Another urea may be generated by the reaction of CO2 with 1-[2-[(2-aminoethyl)amino]ethyl]piperazine (AEAEPZ) to produce the internal urea of AEAEPZ (1-[2-(piperazinyl)ethyl]-2-imidazolidinone). AEAEPZ and its internal urea are not fully confirmed products due to the lack of a commercially available standard. Their presence, however, is suspected based on IC-MS results and the behavior of the acidified PZ degradation experiments. The influence of CO2 is evident in the behavior of AEAEPZ and its urea. A portion of the cation IC chromatogram for 8 m PZ with 0.3 mol CO2 or H+/mol alkalinity degraded at 175 °C for 15 weeks is shown in Figure 3. This section of the chromatogram Figure 4. Comparison of the second order rate constant, k2, for PZ loss (●) and initial, linear rate of total formate production (○) for 8 m PZ with 0.3 mol CO2/mol alkalinity.

PZ with 0.3 mol CO2/mol alkalinity. The detailed regression to obtain k2 for each experimental temperature can be found in the accompanying manuscript.9 Although previously analyzed using a first order rate constant, additional data for PZ thermal degradation demonstrated a better fit to second order behavior, as discussed in Section 3.5 of this paper.1,9,10 The rate of total formate generation has the same temperature behavior as the loss of PZ with similar activation energies, EA, between 204 and 213 kJ/mol. Temperature only serves to accelerate the degradation mechanisms for PZ and does not significantly affect which products are generated. PZ concentration, on the other hand, does affect which degradation products are made due to the increased concentration of free PZ and PZ species as the PZ concentration increases. The ratio of formyl amides to formate generated is compared for 4 to 20 m PZ with 0.3 mol CO2/mol alkalinity at 175 °C in Figure 5. The ratio increases with increased PZ concentration, which is expected with higher concentrations of free PZ according to Le Chatelier’s principle. With higher free PZ concentrations, the equilibrium presented in eq 1 is driven toward the production of FPZ. The concentration of EDA that accumulates in each case is increased from 36 mmol/kg in degraded 4 m PZ to over 100 mmol/kg in degraded 20 m PZ. For all PZ concentrations, EDA is a clear intermediate, but the reaction with CO2 to form 2-Imid is slowed in higher concentration solutions. 3.4. Pathways To Explain Products. An overall set of pathways has been developed that can explain the major products generated during thermal degradation. The purpose is to suggest the types of reactions believed to be occurring and offer examples of how particular degradation products may be formed.

Figure 3. Comparison of amines detected with cation IC for 8 m PZ degraded with 0.3 mol CO2 (bottom, thin line) or H+/per alkalinity (top, thick line) at 175 °C. Peak identities: 1 − EDA, 2 − PZ, 3 − HEP, 4 − MPZ, 5 − EPZ, 6 − AEP, 7 − AEAEPZ Urea, 8 − AEAEPZ, 9 − not determined, 10 − PEP.

is where diamine, triamines, and higher order amines elute. The acidified experiment experienced only half as much thermal degradation as the loaded experiment, but the differences due to the presence of CO2 are clear. In the absence of CO2, EDA (peak 1 in Figure 3), AEP (peak 6), and two larger polyamines (peaks 8 and 10) accumulate. In the presence of CO2, peak 7 accumulates, while peak 8 is not present. Based on IC-MS analysis and peak position on the cation IC chromatogram, peaks 7, 8, and 10 were tentatively identified as the urea of AEAEPZ, AEAEPZ, and 1,1′-(1,2-ethanediyl)bis-piperazine (PEP), respectively. As with AEAEPZ and its urea, PEP is not fully confirmed due to the lack of a commercial standard. Based on the acidification experiment, EDA, EPZ (peak 5), AEP, AEAEPZ, and PEP are produced due to the presence of H+PZ and do not require CO2 for their generation. When CO2 is added, the AEAEPZ and EDA react to form their internal 7731



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CO2 will also populate the solution with the carbamates formed by reversible reaction with primary and secondary amine functions. The carbamates rapidly equilibrate so that the respective free amine is still available for SN2 reactions and free CO2 is available for urea formation. Once AEAEPZ and its urea are formed, a variety of other SN2 type reactions can occur to generate degradation products. The reaction mixture is quickly complicated when all of the potential nucleophiles and attack sites are considered. Only PZ will be considered as the attacking nucleophile, but analogous reactions could potentially be occurring by other nucleophiles in solution such as amine degradation products (e.g., EDA or AEP). Until a significant amount of PZ has been lost to degradation, it is the most likely nucleophile due to its abundance in solution. For example, the SN2 reaction between PZ and protonated AEAEPZ (or AEAEPZ urea) could generate EDA and PEP or two AEP molecules (eq 5). Nucleophilic attack at the α-carbon to the terminal, protonated amino function will produce ammonium NH4+ and a quinternary amino molecule.

Figure 5. Ratio of formyl amides to formate produced during degradation of 4 (●), 8 (○), 12 (■), and 20 m PZ (□) with 0.3 mol CO2 per mole alkalinity at 175 °C.

It should be noted that the influence of speciation on all of the reactions discussed is currently unknown. The reactions described below that include attacking reactive sites or functional groups being attacked should be considered as just the active site discussed, while the rest of the molecule may be in a different form (i.e., a free amino function, protonated amino function, or carbamate). Reactions likely occur with any combination of the molecules present, with the rate of reaction being dependent on the individual reacting species. Overall, a particular pair may dominate, such as PZ and H+PZ, but all of the species are likely involved in quick reaction and equilibrium steps. 3.4.1. Secondary SN2 Substitution Reactions. Nucleophilic substitutions, as described by SN2 reactions, are believed to be the most prominent type of degradation reaction due to the strong nucleophilic nature of PZ and its substituted products. In the SN2 reactions of PZ, the α-carbon to an amino function or protonated amino function is subject to attack that can occur at either secondary or tertiary amines. SN2-type substitution reactions have been observed in thermally degraded alkanolamines such as MDEA where protonation of amines or quaternaries are present or through arm-switching type reactions in 7 m MDEA/2 m PZ.43,44 The first step in high temperature degradation of PZ is most likely the attack of H+PZ at the α-carbon by another PZ molecule that proceeds through an intermediate structure to produce AEAEPZ, a ring-opening product of 2 PZ molecules, as shown in eq 3.

3.4.2. Elimination Reactions. Another type of reaction believed to be occurring in thermally degraded PZ is elimination at a tertiary amine. In elimination reactions, two substituents are removed from a molecule, usually to produce an alkene, and rely on the presence of a good leaving group.45 SN2 reactions are usually favored over second order E2 eliminations except when basicity increases, at higher temperatures or when the nucleophile is poor, among other circumstances.45 In the case of PZ thermal degradation, it is possible that both SN2 and E2 eliminations are occurring simultaneously because of the high temperature and mix of nucleophiles and leaving groups. Hofmann elimination at a quaternary amine function to form an alkyl and a tertiary amine is possible. Hoffman elimination of protonated PEP would produce a PZ and a 1-ethenylpiperazine molecule (eq 6). Anti-Markovnikov hydration of 1-ethenylpiperazine would produce HEP (eq 6). The generation of HEP or other hydroxyethyl molecules in other systems studied usually occurred through SN2 substitution of molecules originally containing hydroxyethyl arms, unlike PZ.12,14,43,44

In the presence of CO2, AEAEPZ will react with CO2 in solution to produce its internal urea as mentioned previously. It is believed that both of these molecules are present in solution and exist in equilibrium based on the behavior of EDA and 2Imid. This equilibrium is shown in eq 4.

3.4.3. Urea Generation. Urea generation is likely an important reaction type in the thermal degradation of PZ. 2Imid and AEI have been confirmed as products with the urea of 7732



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Table 6. Comparison of k2 for a Range of CO2 and PZ Concentration for Thermal Degradation at 175 °C

AEAEPZ suspected to accumulate in CO2 -loaded PZ degradation. Any ethylenediamine-type structure, with two amino functions separated by an ethylene, should create its internal urea when exposed to CO2. This is analogous to oxazolidone formation in the presence of ethanolamine structures and CO2 such as has been described in detail for MEA, DEA, and MDEA thermal degradation.12,14,44 3.5. Overall Reaction Kinetics. Previous work on the subject of PZ thermal degradation has utilized a first order rate law and rate constant, k1, to describe the loss of PZ. However, the proposed SN2 mechanisms for the initial steps of thermal degradation suggest a second order rate law and rate constant, k2, to describe PZ loss (eq 3). A second order rate law relating the concentration of PZ, CPZ to the k2 is shown in eq 7, with the solved form in terms of CPZ, k2, and the initial concentration of PZ, CPZ,0, shown in eq 8 dCPZ = k 2C2PZ dt

(7)

1 1 = + k 2t CPZ CPZ,0

(8)

−

PZ conc (m)

PZ conc (mol/kg)

CO2 loading (mol/mol alk)

4 8 8 8 8 8 8 12 20

2.1 4.8 4.5 4.5 4.2 4.0 4.1 5.2 5.6

0.3 0 0.1 0.2 0.3 0.4 0.47 0.3 0.3

k2 (mmol/kg-s) 1.12 1.48 1.95 2.77 5.65 6.17 1.34 6.17 8.21

× × × × × × × × ×

10−10 10−12 10−11 10−11 10−11 10−11 10−11 10−11 10−11

In order to verify the SN2 mechanism for PZ thermal degradation, the second order rate law was fit to experimental data for 8 m PZ with 0.3 mol CO2/mol alkalinity from 135 to 175 °C. Values for k2 were regressed from experimental data and are compared to k1 values previously derived assuming first order rate behavior of PZ. Values for k1 and k2 are shown in Table 5 along with the coefficient of determination (r2) for Figure 6. Effect of CO2 loading on k2 for 8 m PZ at 175 °C with a linear trendline drawn through 0 to 0.4 mol CO2/mol alkalinity data.

Table 5. Comparison of Rate Constants and Coefficient of Determination (r2) for First and Second Order Kinetics for the Thermal Degradation of 8 m PZ with 0.3 mol CO2/mol Alkalinity T (°C) 135 150 165 175

k1 (1/s) 9.69 6.12 3.14 1.32

× × × ×

10−10 10−9 10−8 10−7

2

r of k1 0.574 0.960 0.956 0.984

k2 (kg/mmol-s) 2.39 1.49 1.28 4.94

× × × ×

10−13 10−12 10−11 10−11

CO2/mol alkalinity in a near linear manner but then drops off between 0.4 and 0.47 mol CO2/mol alkalinity. The speciation is complex between 0.3 and 0.5 mol CO2/mol alkalinity and is impacting the rate of thermal degradation. Predicted speciation of 8 m PZ at 175 °C has been reported previously and shows an increase in the concentrations of H+PZCOO− and HCO3− with a decrease in the concentration of H+PZ above a loading of 0.3 mol CO2/mol alkalinity.1 The SN2 substitution reactions discussed in Section 3.4.1 are believed to be a primary driver of thermal degradation and are initiated with substitution at a protonated amino function. Speciation is predicted based on an undegraded, neat PZ solution, but the results could be extended to imply that the overall concentration of protonated PZ functions, as represented by H+PZ in the simulation, are reduced at very high CO loadings. Therefore, at high CO2 loadings, there are fewer protonated PZ functions available for SN2 substitution, so the overall thermal degradation rate decreases, as shown in Figure 6.

2

r of k2 0.578 0.954 0.938 0.991

each regression used to determine each rate constant. First and second order kinetics appear to fit the data well with the r2 values being similar for the same experiment. As shown in eq 3, the initial step of the thermal degradation mechanism is believed to be the generation of AEAEPZ through a bimolecular reaction of two PZ molecules. The second order rate law fits the experimental data as well and corroborates the proposed pathway. 3.5.1. Effect of CO2 and PZ Concentration on k2. Since the second order rate law agrees with the mechanism suggested above, the effect of PZ and CO2 concentration on k2 was evaluated. The k2 for experiments ranging from 4 to 20 m PZ and 0 to 0.47 mol CO2/mol alkalinity are summarized in Table 6. Both CO2 and PZ concentration affect the k2 value to a lesser extent than temperature, as seen previously with a k1 analysis.1,9 The k2 value ranges from 5.7 × 10−11 to 1.1 × 10−10 mmol/kg-s for 4 to 20 m PZ degraded at 175 °C with 0.3 mol CO2/mol alkalinity. The concentration of PZ produced a minimum in k2 near 8 m PZ, while k1 determined previously showed an increase with increasing PZ concentration. The effect of CO2 concentration, or CO2 loading, on k2 is seen in Figure 6. The rate constant increases from 0 to 0.4 mol

4. CONCLUSIONS From 135 to 175 °C, FPZ, NH4+, AEP, and 2-Imid were found to be the most abundant thermal degradation products of concentrated PZ. They accounted for 63% of nitrogen and 49% of carbon lost as PZ and CO2 during degradation. Overall, 74% of nitrogen and 63% of carbon loss during degradation was recovered in quantified degradation products. Thermal degradation of PZ is believed to be initiated by the nucleophilic attack of PZ at the α-carbon to a protonated amino function on H+PZ to create AEAEPZ. H+PZ was found to be the active and likely initiating species required for the 7733



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(15) Polderman, L. D.; Dillon, C. P.; Steele, A. B. Why M.E.A. Solution Breaks Down in Gas-Treating Service. Oil Gas J. 1955, 54, 180−183. (16) Yazvikova, N. V.; Zelenskaya, L. G.; Balyasnikova, L. V. Mechanism of side reactions during removal of carbon dioxide from gasses by treatment with monoethanolamin. Zh. Prikl. Khim. 1975, 48 (3), 674−676. (17) Chakma, A.; Meisen, A. Corrosivity of Diethanolamine Solutions and Their Degradation Products. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25 (4), 627−630. (18) Chakma, A.; Meisen, A. Degradation of Aqueous DEA Solutions in a Heat-Transfer Tube. Can. J. Chem. Eng. 1987, 65 (2), 264−273. (19) Hsu, C. S.; Kim, C. J. Diethanolamine (DEA) Degradation under Gas Treating Conditions. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24 (4), 630−635. (20) Meisen, A.; Kennard, M. L. DEA Degradation Mechanism. Hydrocarbon Process. 1982, 61 (10), 105−108. (21) Blanc, C.; Grall, M.; Demarais, G. The Part Played by Degradation Products in the Corrosion of Gas Sweetening Plants Using DEA and MDEA. Presented at The Gas Conditioning Conference, Oklahoma City, OK, 1982. (22) Kennard, M. L.; Meisen, A. Control DEA Degradation. Hydrocarbon Process. 1980, 60 (4), 103−106. (23) Kennard, M. L.; Meisen, A. Gas-Chromatographic Technique for Analyzing Partially Degraded Diethanolamine Solutions. J. Chromatogr. 1983, 267 (2), 373−380. (24) Kennard, M. L.; Melsen, A. Mechanisms and Kinetics of Diethanolamine Degradation. Ind. Eng. Chem. Fundam. 1985, 24 (2), 129−140. (25) Polderman, L. D.; Steele, A. B. Why Diethanolamine Breaks Down In Gas-Treating Service. L. Reid Gas 1956, 49−56. (26) Dawodu, O. F.; Meisen, A. Degradation of alkanolamine blends by carbon dioxide. Can. J. Chem. Eng. 1996, 74 (6), 960−966. (27) Chakma, A.; Meisen, A. Methyl-diethanolamine degradation Mechanism and kinetics. Can. J. Chem. Eng. 1997, 75 (5), 861−871. (28) Closmann, F.; Rochelle, G. T. Degradation of aqueous methyldiethanolamine by temperature and oxygen cycling. Energy Procedia 2011, 4, 23−28. (29) Freeman, S. A.; Rochelle, G. T. Density and Viscosity of Aqueous (Piperazine + Carbon Dioxide) Solutions. J. Chem. Eng. Data 2011, 56 (3), 574−581. (30) Hilliard, M. D. A Predictive Thermodynamic Model for an Aqueous Blend of Potassium Carbonate, Piperazine, and Monoethanolamine for Carbon Dioxide Capture from Flue Gas. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2008. (31) Zhou, S.; Chen, X.; Nguyen, T.; Voice, Alexander, K.; Rochelle, G. T. Aqueous Ethylenediamine for CO2 Capture. ChemSusChem 2010, 3 (8), 913−918. (32) Rooney, P. C.; DuPart, M. S.; Bacon, T. R. Oxygen’s role in alkanolamine degradation. Hydrocarbon Process. 1998, 77 (7), 109− 113. (33) Sexton, A. J. Amine Oxidation in CO2 Capture Processes. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2008. (34) Sexton, A. J.; Rochelle, G. T. Reaction Products from the Oxidative Degradation of Monoethanolamine. Ind. Eng. Chem. Res. 2011, 50 (2), 667−673. (35) Voice, A. K.; Vevelstad, S. J.; Chen, X.; Nguyen, T.; Rochelle, G. T. Aqueous 3-methylamino-propylamine for CO2 Capture. Submitted for publication, 2012. (36) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62, 7512−7515. (37) Koike, L.; Barone, J. S.; Godinho, O. E. S.; Aleixo, L. M.; Reis, F. D. M.; Fernandes, E. C.; Barbosa, J. B. N-FormyldiethanolamineA New Artifact in Diethanolamine Solutions. Chem. Ind. (London, U.K.) 1987, 17, 626−627. (38) Brown, B. R. The Organic Chemistry of Aliphatic Nitrogen Compounds; Clarendon Press: Oxford, 1994; Vol. 28.

initial reactions of thermal degradation. Further SN2 substitution reactions then proceed to produce NH4+, AEP, EDA, and PEP. In the presence of CO2, AEAEPZ and EDA react to form their internal ureas, AEAEPZ urea and 2-Imid, which accumulate as stable products. CO2 is not required for thermal degradation to proceed but is reduced to create formate or formyl amides, reacts with amines to form ureas, and dictates the overall product mix. The mechanism for CO2 reduction to formate or formyl amides is not clear but indicates the severity of thermal degradation conditions and has proved to be an important mechanism in the degradation of a variety of CO2 capture amines.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The Luminant Carbon Management Program provided support for this work. REFERENCES

(1) Freeman, S. A. Thermal Degradation and Oxidation of Aqueous Piperazine for Carbon Dioxide Capture. PhD Dissertation, The University of Texas at Austin, Austin, TX, 2011. (2) Freeman, S. A.; Dugas, R.; Van Wagener, D.; Nguyen, T.; Rochelle, G. T. Carbon dioxide capture with concentrated, aqueous piperazine. Int. J. Greenhouse Gas Control 2010, 4 (2), 119−124. (3) Nguyen, T.; Hilliard, M.; Rochelle, G. T. Amine volatility in CO2 capture. Int. J. Greenhouse Gas Control 2010, 4 (5), 707−715. (4) Rochelle, G. T.; Chen, E.; Freeman, S.; Van Wagener, D.; Xu, Q.; Voice, A. Aqueous piperazine as the new standard for carbon dioxide capture technology. Chem. Eng. J. 2011, 171 (3), 725−733. (5) Eide-Haugmo, I.; Brakstad, O. G.; Hoff, K. A.; Sørheim, K. R.; daSilva, E. F.; Svendsen, H. F. Environmental impact of amines. Energy Procedia 2009, 1, 1297−1304. (6) Eide-Haugmo, I.; Lepaumier, H.; Einbu, A.; Vernstad, K.; daSilva, E. F.; Svendsen, H. F. Chemical stability and biodegradability of new solvents for CO2 capture. Energy Procedia 2011, 4, 1631−1636. (7) Veltman, K.; Singh, B.; Hertwich, E. G. Human and environmental impact assessment of postcombustion CO2 capture focusing on emissions from amine-based scrubbing solvents to air. Environ. Sci. Technol. 2010, 44 (4), 1496−1502. (8) Karl, M.; Wright, R. F.; Berglen, T. F.; Denby, B. Worst case scenario study to assess the environmental impact of amine emissions from a CO2 capture plant. Int. J. Greenhouse Gas Control 2011, 5 (3), 439−447. (9) Freeman, S. A.; Rochelle, G. T. Thermal degradation of aqueous piperazine for CO2 capture: 1. Effect of process conditions and comparison of thermal stability of CO2 capture amines. Ind. Eng. Chem. Res. 2012, in press, DOI: 10.1021/ie201916x. (10) Freeman, S. A.; Davis, J.; Rochelle, G. T. Degradation of aqueous piperazine in carbon dioxide capture. Int. J. Greenhouse Gas Control 2010, 4 (5), 756−761. (11) Freeman, S. A.; Rochelle, G. T. Thermal degradation of piperazine and its structural analogs. Energy Procedia 2011, 4, 43−50. (12) Davis, J. Thermal Degradation of Aqueous Amines Used for Carbon Dioxide Capture. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2009. (13) Davis, J.; Rochelle, G. T. Thermal degradation of monoethanolamine at stripper conditions. Energy Procedia 2009, 1 (1), 327−333. (14) Lepaumier, H.; Picq, D.; Carrette, P.-L. New Amines for CO2 Capture. I. Mechanisms of Amine Degradation in the Presence of CO2. Ind. Eng. Chem. Res. 2009, 48, 9061−9067. 7734



Article

(39) Duranleau, R. G.; Knifton, J. F.; Speranza, G. P. Method for the Preparation of Formamides, U.S. Patent 4,565,866, Jan. 21, 1986. (40) Horrom, B. W.; Freifelder, M.; Stone, G. R. 1-Formylpiperazine and Related Compounds. J. Am. Chem. Soc. 1954, 77, 753−754. (41) Bhanage, B. M.; Fujita, S.-i.; Ikushima, Y.; Arai, M. Synthesis of cyclic ureas and urethanes from alkylene diamine and amino alcohols with pressurized carbon dioxide in the absence of catalysts. Green Chem. 2003, 5, 340−342. (42) Wu, C.; Cheng, H.; Liu, R.; Want, Q.; Hao, Y.; Yu, Y.; Zhao, F. Synthesis of urea derivatives from amines and CO2 in the absence of catalyst and solvent. Green Chem. 2010, 12, 1811−1816. (43) Bedell, S. A.; Worley, C. M.; Al-Horr, R. S.; McCrery, D. A. Quaternaries as Intermediates in the Thermal and Oxidative Degradation of Alkanolamines. Ind. Eng. Chem. Res. 2010, 49 (16), 7147−7151. (44) Closmann, F. B. Oxidation and thermal degradation of methyldiethanolamine/piperazine in CO2 capture. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2011. (45) McMurry, J. Organic Chemistry, 5th ed.; Brooks/Cole: Pacific Grove, CA, 2000.

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