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Quaternaries as Intermediates in the Thermal and Oxidative Degradation of Alkanolamines Stephen A. Bedell,* Clare M. Worley, Rida Sadek Al-Horr, and David A. McCrery The Dow Chemical Company, 2301 North Brazosport BouleVard, Freeport, Texas 77566
Mass spectral evidence is presented for the formation of quaternaries in the high temperature (140 °C) autoxidation of MDEA. Additionally it is demonstrated that the quaternary reacts with neutral amines in a manner consistent with an SN2 mechanism. Rate studies are presented which could be extended to include a systematic variation of nucleophiles, quaternaries, temperature, and solvent composition and allow precise modeling of thermal degradation. Introduction Amine degradation is important to understand for the development of flue gas CO2 capture processes. It is probable that amine degradation will not be completely eliminated and therefore must be managed. A recent paper1 discusses the implications of degradation products on cyclic capacity, kinetics, corrosion, energy, and other properties necessary to understand for the proper operation of the CO2 capture plant. Three modes of tertiary amine degradation occurring in flue gas CO2 capture have been previously identified:2 direct oxidation by metal ions, thermal disproportionation, and autoxidation. A fourth route, involving irreversible CO2 reactions,3,4 can occur when primary or secondary amines are used or when such amines are formed by the degradation of tertiary amines. This group previously reported5 on batch reactions in which N-methyldiethanolamine (MDEA) was thermally degraded in stainless steel bombs at 182 °C. Triethanolamine (TEA) and N,N-dimethylethanolamine (DMEA) were the major products noted and during the early course of the reaction, molar quantities of those products were close to equal to the amount of MDEA degraded. A mechanism for this disproportionation (involving exchange of methyl and hydroxyethyl groups) was suggested which involves the formation of a quaternary intermediate. It was further demonstrated that addition of a N-tetramethyl quaternary salt results in an increase in the rate of the MDEA disproportionation reaction. Continued reaction showed that the reaction is substoichiometric, and probably catalytic in quaternary salt. In the utilization of an N-tetraethyl quaternary salt, the formation of mixed ethyl, methyl, and hydroxyethyl amines demonstrated the participation of the quaternary in these exchange reactions. Mechanistically, the group exchange probably occurs through an SN2 mechanism as suggested by Snyder6 for alkyl benzyl amines. The dealkylation of quaternary salts by MEA was reported by Hu¨nig and Baron7 who studied the effect of different quaternary alkyl groups. Their work showed the slowest rates for more sterically hindered alkyl groups and the fastest rates for beta olefins. This order of reactivity is consistent with an SN2 transition state which involves sp2 hybridization of the alkyl group.8 Such a mechanism explains the higher thermal stability of N-ethylethanolamines relative to N-methylethanolamines reported by Lichtfers.9 Hu¨nig and Baron7 showed a rate constant for deethylation that was 50% of that for demethylation. Though formation of a quaternary via elimination of ethylene oxide has * To whom correspondence should be addressed. Phone: 979-2382011.
been suggested, it is more likely that a protonated amine plays a role similar to the quaternary. It has been reported10 that thermal degradation of MDEA occurs faster when partially loaded with the acid gases H2S or CO2. Very small amounts of protonated amine are always present, even in unloaded solution. If the protonated amine can convert to a quaternary via a pathway like that in Figure 1, larger amounts of quaternary could form in solution. In aerobic studies and in actual flue gas plants, these thermal degradation reactions will be coupled with oxidative pathways. The degree of thermal degradation will depend on the temperatures encountered in the process. This report presents evidence for the production of quaternaries in an oxidative degradation of MDEA at a temperature representative of the high range for
Figure 1. Proposed mechanism for thermal disproportionation of protonated and quaternary salts.
10.1021/ie100938e 2010 American Chemical Society Published on Web 07/12/2010
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regeneration and presents reactivity effects for the nucleophilic attack of these type of quaternaries by various amines. Experimental Section Autoclave Degradation Experiments. Though experiments to study only thermal degradation were always made under anaerobic conditions, the particular sample that was analyzed by electrospray ionization mass spectrometry was subjected to air during the degradation. Twenty mL of 50 wt % MDEA (remainder was water) solution was placed in a 100-mL Hastalloy C autoclave. Temperature was controlled at 140 °C with an initial total absolute air pressure of 2.17 MPa. After correction for solvent vapor pressure this resulted in initial pO2 ) 0.38 MPa. This reaction was run for 4 h with stirring. Ion Chromatography. Separation was done using ion exchange chromatography (IC) with suppressed conductivity detection and mass spectrometry. A Dionex ICS-3000 reagentfree ion chromatography (RFIC) system was used. The ICS3000 system consisted of a DP dual pump module, EG eluent generator module and DC detector/chromatography module with a single temperature zone configuration. The EG was equipped with an EluGen II methanesulfonic acid (MSA) cartridge (P/N 058902) for electrolytic production of MSA eluent. The cartridge was preceded by a continuously-regenerated cation trap column, CR-CTC (P/N 066262) for added purification of the deionized water prior to eluent generation. Chromeleon 6.8 chromatography management software was used for system control. Samples were injected into a 10-µL loop using an AS autosampler. The loop was mounted onto one of the two 6-port injection valves in the ICS 3000 DC module. Chromatographic separation was achieved using a gradient of MSA eluent (0.5 to 11 mM) at 0.25 mL/min on an IonPac CS18 (2 × 250 mm) separator column preceded by an IonPac CG18 (2 × 50 mm)
guard column. The column effluent was passed through a CSRS 300 self-regenerating suppressor used in the external water mode prior to the conductivity detector and the mass spectrometer. Chromatographic conditions were as follows: Columns: Dionex IonPac CG18, 2 × 50 mm (P/N 062880); IonPac CS18, 2 × 250 mm (P/N 062878) Eluent: 0.5 mM MSA at -7.0 min for column equilibration; 0.5 mM MSA at 0.0 min; 1.0 mM MSA at 10.0 min; 4.0 mM MSA at 18.0 min; 11.0 mM MSA from 24.0 to 40 min. Eluent source: EGC II MSA with CR-CTC Flow rate: 0.25 mL/min Temperature: 30 °C (lower and upper compartment) Injection volume: 10 µL Detection: suppressed conductivity with mass spectrometry, CSRS 300 Suppressor: CSRS 300 2-mm, external water 9 mA Electrospray Ionization Mass Spectrometry. Mass spectrometry analyses were performed using a Waters LCT Premier XE LC/MS system. The suppressed eluent from the ion chromatograph was introduced to the mass spectrometer via an electrospray ionization (ESI) probe interface. The sample stream was split such that the flow to the ESI probe was approximately 0.2 mL/min. Conditions for the Waters/Micromass Technologies LCT TOF mass spectrometer were as follows: ESI conditions: Ionization mode: positive ion Source block temp: 110 °C Desolvation temp: 250 °C Capillary voltage: 1500 V Cone voltage: 30 V Desolvation gas (He): 600 L/h MS conditions:
Figure 2. Ion chromatogram with suppressed conductivity detection and positive ion electrospray ionization reconstructed total-ion-current chromatogram (RTICC). Components of interest elute at 29.44 and 30.32 minutes in the RTICC.
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Figure 3. Positive ion ESI mass spectrum obtained for the components eluting at 29.44 and 30.32 min.
Resolution: 10,000 (W mode) Scan: 40-800 Da Rate: 1.5 s/scan MCP: 2400 V Lock mass: leucine-enkephalin, 556.2771 Da Kinetic Measurements. Solutions of amine, solvent, and quaternary salts were prepared and portioned into multiple thickwalled vials fitted with Teflon-lined septa and magnetic stirrers. A temperature-controlled heated block (Pierce Reacti-Therm) with stirring control (stir rate >200 rpm) was kept at 150 ( 2 °C. The solution vials were loaded into the block with a timing that assured the block temperature did not decrease significantly when cool vials were loaded. At periodic time intervals a vial was removed from the block and quenched in cool water. Gas chromatographic measurements were made on the solution in each vial to determine the amount of product, monitoring either concentrations of the tertiary amine formed from the quaternary or the alkylated amine formed from the nucleophile. Most kinetic reactions were run only once. Repetitions of three different reaction combinations were run and showed a reproducibility within 5%. Amine solution samples were analyzed using an Agilent 6890N GC with split-splitless inlet, autosampler, and FID detector. A DB-Wax column was used. Internal standard method was used with a multiple point calibration for each component. Calibration points covered the expected component ranges when samples were diluted by a factor of 20 using methanol as the solvent. The limit of detection is >0.01 wt %. The identity of the measured components was confirmed by GC/MS using the same column type and known standards. The reproducibility on each determination was
