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Hydroxide based decomposition pathways of alkyltrimethylammonium cations
Journal of Membrane Science 399–400 (2012) 49–59
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Hydroxide based decomposition pathways of alkyltrimethylammonium cations Joseph B. Edson a , Clay S. Macomber b , Bryan S. Pivovar b , James M. Boncella a,∗ a b
Materials, Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States National Renewable Energy Laboratory, Golden, CO 80401, United States
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 18 January 2012 Accepted 22 January 2012 Available online 1 February 2012 Keywords: Tetraalkylammoniumcations Decomposition Fuel cell membranes Evolved gas analysis
a b s t r a c t A systematic study that altered the number of -hydrogen atoms susceptible to Hofmann elimination and introduced increased steric hindrance of substituted (ethyl, n-propyl, isobutyl, and neopentyl) alkyltrimethylammonium cations was performed. The mechanism of the thermal decomposition of these four ammonium cations in deuteroxide form was studied using evolved gas analysis (EGA) because of their potential importance in alkaline membrane fuel cells or electrolyzers. The products of the decomposition reactions are in many cases the expected Hofmann elimination products (trimethylamine and olefins), however, as the number of -hydrogen atoms decrease or they become more sterically encumbered (from the addition of adjacent methyl groups), nucleophilic attack of hydroxide on the methyl groups increases in relative importance. The use of deuterated water and deuteroxide in our study shows that deprotonation of the tetraalkylammonium ions establishes a rapid equilibrium between the nitrogen ylide species that is formed by methyl group deprotonation and water that scrambles deuterium into the methyl groups of the amine. The results of this work show that at high temperature and low water content tetraalkylammonium hydroxide salts are relatively unstable in membranes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction While an enormous effort has been directed towards the development of proton exchange membrane fuel cells (PEMFCs) in recent years, the development of alkaline fuel cell versions of PEMFCs is relatively much less explored [1]. Alkaline membrane fuel cell (AMFC) systems have traditionally been ignored by the fuel cell community due to the concerns over carbonate formation from the reaction of OH− ions with CO2 contaminants in the oxidant gas stream leading to the formation of carbonate/bicarbonate (CO3 2− /HCO3 − ) [2–4]. However, alkaline membrane fuel cells (AMFCs) can offer some advantages over their PEMFC counterparts. The basic medium allows for the use of non-precious electrode catalysts made from inexpensive metals such as Fe/Co/Ni/Ag versus the use of the precious and expensive Pt used in PEMFCs [5–9]. Another concern with AMFCs is the stability of the membrane at high pH necessary for good hydroxide ion conductivity, especially at elevated temperatures [10,11]. Early studies on membrane development have focused on the use of ammonium cations [NR4 ]+ tethered to a polymer backbone for hydroxide ion transport. In this regard, a number of recent publications have focused on the development and testing of these ammonium cation tethered polymers for membrane use [12–23]. While many of these efforts have been directed towards the
∗ Corresponding author. Tel.: +1 505 665 0795. E-mail address: [email protected] (J.M. Boncella). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.01.025
polymer membrane stability at high pH and temperature, very little has been done to study the effect of these variables on the stability of the ammonium cations themselves. Previous work has shown that the hydroxide ion in the membrane can react with ammonium cations via nucleophilic attack or as a Bronsted base. Furthermore, the formation of nitrogen ylide species through reversible deprotonation/protonation of methyl groups attached to the nitrogen cations was observed offering another reaction pathway for decomposition [24–26]. We have previously reported the detailed mechanism of thermal decomposition of tetramethylammonium hydroxide, [NMe4 ][OH], using thermogravimetric analysis (TGA) as well as evolved gas analysis (EGA) with the identity and quantities of the evolved gases analyzed by Fourier-transform infrared (FTIR) spectroscopy and mass spectrometry (MS) [24]. While [NMe4 ][OH] showed good stability under the conditions studied due to its compactness and the lack of -hydrogen atoms susceptible to Hofmann elimination, it is impractical for fuel cell usage as a free ion not tethered to the polymer membrane. Ammonium cations are typically tethered to a polymer backbone via post polymerization modification of the polymers containing pendant alkyl chloride functional groups through reaction with trimethylamine to give substituted alkyltrimethylammonium functional groups, [RNMe3 ]+ . The cations presented in this study are analogues of tethers that could be used in anion exchange polymers. Most polymer systems studied to date have used a benzyl group as the point of attachment (R = Bn), however there have been recent reports of systems utilizing aliphatic points of cation attachment such as the neopentyl
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Table 1 Alkyltrimethylammonium cations studied. Cation
Name
# of -hydrogens
Ethyltrimethylammonium (ETMA)
3
nPropyltrimethylammonium (PTMA)
2
Isobutyltrimethylammonium (ITMA)
1
Neopentyltrimethylammonium (NTMA)
0
(Np)-like groups used recently by Coates and coworkers [17,20]. Both the benzyl and neopentyl linkages result in no -hydrogen atoms present to undergo Hofmann elimination due to the assumed severe instability of these cations. However, this limits the number of potential polymer backbones. Here, we present a fundamental and systematic study of the stability of [RNMe3 ]+ cations where the number of -hydrogens susceptible to Hofmann elimination and the steric hindrance to this reaction are probed. We have chosen to use the substituted trimethyl version of ammonium cations in this study for various reasons. The first of which is it allows us to focus on a single substitution point of attack for the decomposition reactions (for which the methyl amine groups are not directly associated). Additionally, the synthesis of the singly substituted trimethylammoniums is straightforward and does not have significant steric issues associated with quanternizing the nitrogen. Finally, the use of three methyl groups allows for the more compact and hydrophilic ammonium cations to be studied. In practical materials, increasing the size and hydrophobicity would be expected to play a detrimental role on water uptake and conduction of membrane materials employing these cations. In Table 1 we show the four cations employed in this study with nomenclature used in this paper as well as the number of -hydrogen atoms susceptible to Hofmann elimination. The mechanism of thermal decomposition of ethyltrimethylammonium deuteroxide, [EtNMe3 ][OD]·xD2 O, n-propyltrimethylammonium deuteroxide, [n-PrNMe3 ][OD]·xD2 O, iso-butyltrimethylammonium deuteroxide, [i-BuNMe3 ][OD]·xD2 O, and neopentyltrimethylammonium deuteroxide, [NpNMe3 ][OD]·xD2 O, was studied using TGA and EGA. The deuteroxides were chosen for this study as it has been previously reported that nitrogen ylide formation on the methyl group of the ammonium cations has a low barrier and the use of the deuteroxide counter ions enables the detailed observation of these species via isotopic scrambling in the decomposition products [24]. The formation of ylides can be important in degradation for these or similar cations, particularly when similar cations are placed into polymeric systems where the likelihood of ylide species further reacting with the polymer backbone or other cations/tethers is greatly enhanced. 2. Experimental 2.1. Materials Ethyltrimethylammonium iodide, 1-bromopropane, 1-bromo2-methylpropane, neopentylamine, trimethylamine (45% in H2 O), silver (I) oxide, methyl iodide, potassium carbonate, acetonitrile,
diethyl ether, dimethylsulfoxide-d6 (DMSO), methanol-d4 (MeOD) and deuterium oxide (D2 O, 99.95% atom D) were purchased from commercial sources and used as received. All alkyltrimethylammonium deuteroxides were stored under an argon atmosphere in tightly sealed vessels at −30 ◦ C prior to experiments. Ethyltrimethylammonium deuteroxide (1) – To a 100 mL round bottom flask under an active argon flow was added ethyltrimethylammonium iodide (1.06 g, 4.92 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (2.24 g, 9.65 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization to yield 0.54 g of the white solid [EtNMe3 ][OH]·xH2 O. Preparation of [EtNMe3 ][OD]·xD2 O was achieved by dissolution of [EtNMe3 ][OH]·xH2 O in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.50 g of [EtNMe3 ][OD]·xD2 O as a white solid. 1 H NMR (D2 O, 300 MHz): 3.28 (q, 2H, J = 7.2 Hz, CH2 ), 2.97 (s, 9H, N(CH3 )3 ), 1.24 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (DMSO, 75 MHz): 63.6 (CH2 ), 53.7 (N(CH3 )3 ), 9.4 (CH3 ). n-Propyltrimethylammonium bromide (2) – A 100 mL round bottom flask was charged with 1-bromopropane (5.8 mL, 63.8 mmol), trimethylamine (45% in H2 O, 20 mL, 126 mmol), and 20 mL acetonitrile. The reaction was heated at 40 ◦ C for 5 h and then cooled to room temperature. The solvent was removed via rotary evaporation to leave an oily residue. Diethyl ether was added to precipitate the product, which was filtered and washed with additional ether. The product was dried in vacuo overnight to yield 10.1 g (87%) of a white solid. 1 H NMR (MeOD, 300 MHz): 3.61 (m, 2H, CH2 N(CH3 )3 ), 3.42 (s, 9H, N(CH3 )3 ), 2.03 (m, 2H, H3 CCH2 ), 1.22 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 69.1 (CH 2 N(CH3 )3 ), 53.7 (N(CH3 )3 ), 17.5 (H3 CCH2 ), 10.9 (CH3 ). n-Propyltrimethylammonium deuteroxide (3) – To a 100 mL round bottom flask under an active argon flow was added npropyltrimethylammonium bromide (1.88 g, 10.3 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (4.75 g, 20.5 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization and the product, [n-PrNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.87 g of [n-PrNMe3 ][OD]·xD2 O as a colorless oily solid. 1 H NMR (D2 O, 300 MHz): 3.16 (m, 2H, CH2 N(CH3 )3 ), 2.98 (s, 9H, N(CH3 )3 ), 1.70 (tq, 2H, J = 7.8, 4.2 Hz, H3 CCH2 ), 0.87 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (DMSO, 75 MHz): 66.2 (CH2 N(CH3 )3 ), 51.7 (N(CH3 )3 ), 15.7 (H3 CCH2 ), 10.5 (CH3 ). Isobutyltrimethylammonium bromide (4) – A 100 mL round bottom flask was charged with 1-bromo-2-methylpropane (6.0 mL, 55.2 mmol), trimethylamine (45% in H2 O, 18 mL, 114 mmol), and 20 mL acetonitrile. The reaction was heated at 40 ◦ C for 5 h and then cooled to room temperature. The solvent was removed via rotary evaporation to leave an oily residue. Diethyl ether was added to precipitate the product, which was filtered and washed with additional ether. The product was dried in vacuo overnight to yield 8.76 g (81%) of a white solid. 1 H NMR (MeOD, 300 MHz): 3.33 (d, 2H, J = 5.1 Hz, CH2 N(CH3 )3 ), 3.21 (s, 9H, N(CH3 )3 ), 2.20 (dt, 1H, J1 = 6.9 Hz, J2 = 5.1 Hz, HCCH2 ), 1.14 (d, 3H, J = 6.9 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 75.1 (CH2 - N(CH3 )3 ), 54.2 (N(CH3 )3 ), 25.0 (H3 CCH), 23.2 (CH3 ). Isobutyltrimethylammonium deuteroxide (5) – To a 100 mL round bottom flask under an active argon flow was added 4 (1.29 g, 6.58 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (3.05 g, 13.2 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug
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of celite. The water was then removed via lyophilization and the product, [i BuNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.61 g of [i BuNMe3 ][OD]·xD2 O as a colorless oily solid. 1 H NMR (MeOD, 300 MHz): 3.29 (d, 2H, CH2 N(CH3 )3 ), 3.20 (s, 9H (showing D incorporation), N(CH3 )3 ), 2.28 (m, 1H, (H3 C)2 CH), 1.17 (d, 6H, J = 6.6 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 75.1 (CH2 N(CH3 )3 ), 53.9 (N(CH3 )3 ) (appears as a multiplet from deuterium scrambling), 25.1 ((H3 C)2 CH), 23.4 (CH3 ). Neopentyltrimethylammonium iodide (6) – To a 100 mL round bottom flask was added neopentylamine (3 mL, 25.6 mmol), potassium carbonate (4.70 g, 34 mmol), and iodomethane (5.1 mL, 82.0 mmol) in 40 mL of ethanol. The reaction was stirred overnight and the excess potassium carbonate was removed by filtration. The solvent was removed in vacuo to give a yellow solid. Recrystallization from isopropanol gave 4.24 g (64%) of a white crystalline solid. 1 H NMR (DMSO, 300 MHz): 3.37 (s, 2H, CH2 N(CH3 )3 ), 3.19 (s, 9H, N(CH3 )3 ), 1.11 (s, 9H, (H3 C)3 C CH2 ). 13 C NMR (MeOD, 75 MHz): 77.8 (CH2 N(CH3 )3 ), 56.3 (N(CH3 )3 ), 34.4 ((H3 C)3 C CH2 ), 30.2 ((H3 C)3 C CH2 ). Neopentyltrimethylammonium deuteroxide (7) – To a 100 mL round bottom flask under an active argon flow was added 6 (2.13 g, 8.30 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (3.84 g, 16.6 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization and the product, [NpNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 1.36 g of [NpNMe3 ][OD]·xD2 O as a white crystalline solid. 1 H NMR (MeOD, 300 MHz): 3.29 (s, 2H, CH2 N(CH3 )3 ), 3.20 (s, 9H (showing D incorporation), N(CH3 )3 ), 1.20 (s, 9H, C(CH3 )3 ). 13 C NMR (MeOD, 75 MHz): 78.0 (CH2 N(CH3 )3 ), 55.9 (N(CH3 )3 ) (appears as a multiplet from deuterium scrambling), 34.3 ((H3 C)3 C), 29.97 ((H3 C)3 C).
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Fig. 1. Schematic diagram of EGA experiment.
Experimental parameters for the headspace gas chromatography–mass spectroscopy [GC–MS] decomposition experimentation consist of the implementation of a Thermo Scientific Trace GC Ultra in conjunction with an ISQ single quadrupole MS calibrated using a perfluorotributylamine standard. Sample preparation involved loading microliters/micrograms of sample into inert gas purged 10 mL headspace vials, sealed with high performance Teflon septa. The vials were subsequently heated in an incubator/agitator oven at 120 ◦ C for pre-determined amounts of time. Gas sampling was achieved using a Triplus HS autosampler outfitted with a heated gas-tight syringe set at 150 ◦ C. 2 mL of headspace gas was collected from the vial while in the incubator and injected into a split/splitless [S/SL] injector set to splitless mode. The species in the gas were separated on an Agilent CP-Volamine column during a GC temperature profile with an isothermal hold at 40 ◦ C for 8 min, followed by a ramp of 20 ◦ C min−1 to 250 ◦ C, and an isothermal hold at 250 ◦ C for 3.5 min. Mass spectra were collected on the ISQ operating in electron ionization [EI] mode, electron energy of 70 eV, and scanning from 1 to 500 amu. After background subtraction a NIST search was performed to identify species and appropriate standards were run for confirmation of spectra and retention times.
2.2. Instrumentation 3. Results and discussion Evolved gas analysis (EGA) was performed on the decomposition of the materials to identify products in an effort to determine reaction pathways. Mass loss, gas speciation, quantification and isotopic measurements were carried out using a collection of instrumentation coupled together with heat traced stainless steel transfer lines. Decomposition reactions were performed in a horizontal large furnace Mettler-Toledo 851 TGA/SDTA thermogravimetric analyzer (TG). The reaction zone was constantly purged with nitrogen at 40 sccm as measured by an Agilent ADM1000 flowmeter. Evolved gases from the decompositions were carried by the purge gas from the outlet of the TG and flowed through a Thermo-Electron 380 Fourier transform infrared spectrometer (TG–FTIR) equipped with a Nicolet 2 m heated gas cell (KBr windows). At the inlet to the gas cell a Pfeiffer Thermo-Star mass spectrometer (TG–MS) sampled the gas via a stainless steel capillary at a rate of 1 sccm in addition to a Varian 4-channel CP-4900 gas chromatograph (GC) via a stainless steel capillary (Fig. 1). FTIR difference spectra were taken at a resolution of 0.5 cm−1 after subtraction of the KBr and UHP N2 background blank. The solid starting materials were contained in 900 L alumina crucibles. In non-isothermal experiments crucibles were filled with ∼25 mg of sample material, forming a thin layer to minimize mass transfer related problems. Non-isothermal temperature ramp rates consisted of 0.15 and 0.25 ◦ C min−1 starting from 30 and an endpoint of 150 ◦ C. Isothermal experiments initiated at an equilibrium temperature of 30 ◦ C for 10 min followed by a rapid ramp from 30 to 120 ◦ C and were followed by an isothermal segment at 120 ◦ C for 30 min.
3.1. Decomposition studies of [EtNMe3 ][OD]·xD2 O (1) The presence of -hydrogen atoms in 1 would suggest that decomposition through Hofmann elimination to form ethylene and trimethylamine would be the primary pathway for thermal decomposition. Scheme 1 illustrates a number of viable decomposition pathways for the thermal decomposition of the ETMA deuteroxide. Specifically one can envision decomposition occurring through 3 potential pathways. Previous studies on the thermal decomposition of tetramethylammonium deuteroxide, [Me4 N][OD]·5D2 O,demonstrated that formation of a nitrogen ylide species through reversible deprotonation/protonation of [Me4 N]+ occurred prior to any further decomposition [24]. Thus, one cannot rule out the formation of similar ylide species in the decomposition of 1. Furthermore, direct nucleophilic attack of the deuteroxide ion on the cation is certainly possible. In order to understand the thermal decomposition process, the identity of the gases evolved during the decomposition of 1 were analyzed by a combination of FTIR spectroscopy, mass spectrometry, and gas chromatography. A thermal curve (TGA) of the decomposition of 1 when heated at 0.15 ◦ C min−1 starting at 30 ◦ C is shown in Fig. 2. In stark contrast to prior decomposition studies of [Me4 N][OD]·5D2 O where the onset of decomposition is not observed until some water of hydration is lost (2 molecules) and the temperature is above 110 ◦ C [24], the decomposition of 1 begins immediately upon heating from
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Scheme 1. Thermal decomposition of ETMA deuteroxide.
30 ◦ C. Complete decomposition of the material is observed by 68 ◦ C at the given heating rate. This drastic difference can be attributed to a number of factors. Namely, the exact extent of the solvation level of the 1 starting material could not be determined. Whereas [Me4 N][OD]·5D2 O is a commercially available, stable, crystalline solid, 1 had to be prepared via synthetic methods and all attempts to crystallize the material were unsuccessful. Despite this, an analysis of the gases produced by FTIR spectroscopy and GC showed the formation of only ethylene and trimethylamine (TMA) as reaction products (Fig. 2) in equal concentrations. This is consistent with Hofmann elimination as the only decomposition pathway. The initial TMA decomposition product observed in the mass spectrum at 30 ◦ C is free of deuterium scrambling and is representative of undeuterated TMA. Above 40 ◦ C, isotopic scrambling becomes evident and when the isotopic composition of the TMA produced was analyzed as a function of increasing temperature the isotopic distribution of TMA reached an equilibrium distribution within 2–3 ◦ C. Fig. 3 shows the mass spectrum of TMA that is
Fig. 2. Evolved gas analysis (EGA) of [EtNMe3 ][OD]·xD2 O thermal decomposition.
produced when 1 decomposes above 40 ◦ C. The deuterium scrambling above 40 ◦ C is also consistent with the appearance of C–D stretches in the FTIR spectrum of the evolved gases. A time evolved 3D FTIR plot is shown in Fig. 4 that also shows the appearance of C–D stretches beginning at a temperature of 40 ◦ C (bands at approximately 2190 and 2070 cm−1 ) [27]. The molar mass of undeuterated TMA is 59 amu, with 58 amu being the major signal in the spectrum [28]. It is apparent from the mass spectral data in Fig. 3 that heavier mass isotopomers make up the majority of the observed species. These heavier mass species arise from the exchange of deuterium from D2 O in the sample with protons on the methyl groups of the ETMA ions as the material is decomposing. This occurs via formation of nitrogen ylide species, which have also been observed during
Fig. 3. Mass spectrum of the trimethylamine produced in the ramped decomposition of [EtNMe3 ][OD]·xD2 O.
J.B. Edson et al. / Journal of Membrane Science 399–400 (2012) 49–59
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Fig. 4. Evolution of the IR spectra of evolved gases from 1 as a function of time.
the decomposition of tetramethylammonium deuteroxide [24]. At least two factors, time and temperature (water content being an additional potential variable) play a role in the observed isotopic scrambling of TMA. This data alone is insufficient to talk about specific rates or energy barriers of the two processes; however, it does confirm that ylide scrambling occurs to a substantial extent. An additional difficulty in interpreting similar data is that H/D isotopic exchange will reach equilibrium and after this point, while the phenomenon continues, no further change can be witnessed in the observed mass spectrum. The combined results from the thermal decomposition of 1 described above give a clear view of the mechanism for decomposition. Referring back to Scheme 1 it was proposed that 3 pathways were viable for [EtNMe3 ][OD]·xD2 O decomposition. The only observed decomposition products are TMA and ethylene, which come from Hofmann elimination. However, the observation of deuterium scrambling into the TMA above 40 ◦ C suggests a rather low barrier for reversible ylide formation. The absence of the formation of methanol, ethyldimethyl amine, or dimethyl ether (DME), as indicated by the lack of peaks in the GC of the product gases, suggest that the pathway for SN 2 attack has too high of barrier to be accessed under the experimental conditions. These results are consistent with computational results that show the Hofmann elimination reaction to be of much lower activation barrier than the nucleophilic attack of the deuteroxide ion on the cation [26]. However, while an ylide species could not be identified by computational methods, the experimental results demonstrate that formation of this species has a relatively low activation barrier.
Fig. 5. Thermal curve of the decomposition of [n-PrNMe3 ][OD]·xD2 O.
very first samples of TMA collected. In fact, analysis of 3 by 1 H NMR spectroscopy reveals that deuterium scrambling into the methyl groups on the nitrogen occurs during its synthesis indicating a low barrier for scrambling. Fig. 6 shows the mass spectrum of the TMA that is produced when 3 decomposes. The deuterium scrambling can also be observed through the appearance of C–D stretches in the FTIR spectrum (Fig. 6). The results obtained are nearly identical to the observations for the decomposition of 1. The only observed decomposition products are TMA and propylene, which come from Hofmann elimination. In an attempt to try to overcome the SN 2 activation barrier, 3 was isothermally decomposed at 120 ◦ C and the resultant evolved gases were analyzed as before. The material is rather unstable at this temperature and was completely decomposed within 5 min. As previously observed in the controlled heating runs, propylene and trimethylamine were the only observable products. These results are consistent with results that show the Hofmann elimination reaction to be of much lower activation barrier than the nucleophilic attack of the deuteroxide ion on the cation. However, reversible ylide species formation has a relatively low activation barrier as evidenced from the deuterium scrambling observed immediately in the FTIR and MS spectra.
3.2. Decomposition studies of [n-PrNMe3 ][OD]·xD2 O (3)
3.3. Decomposition studies of [i BuNMe3 ][OD]·xD2 O (5)
We performed similar thermal degradation studies with 3, in which one -hydrogen atom on 1 is replaced with a methyl group. Scheme 2 illustrates the viable decomposition pathways for the thermal decomposition of the PTMA deuteroxide. A TGA curve of the decomposition of 3 when heated at 0.15 ◦ C min−1 starting at 30 ◦ C is shown in Fig. 5. Similar to the decomposition of 1, [n-PrNMe3 ][OD]·xD2 O begins to decompose immediately at 30 ◦ C. Complete decomposition of the material is observed by approximately 90 ◦ C at the given heating rate. As expected, the material is more stable than 1 presumably due to the presence of the -methyl group, which may hinder Hofmann elimination or the fact that only two rather than three -hydrogen sites exist for Hofmann elimination. An analysis of the decomposition products by GC and FTIR spectroscopy revealed the formation of propylene and trimethylamine as the only reaction products, which again is consistent with Hofmann elimination as the only decomposition pathway. Analysis of the trimethylamine given off by mass spectrometry revealed that deuterium scrambling is observed immediately in the
[i BuNMe3 ][OD]·xD2 O represented the next step in replacing a -hydrogen atom with a methyl group. Scheme 3 illustrates the viable decomposition pathways for the thermal decomposition of the ITMA deuteroxide. The thermal decomposition of 5 was carried out at a heating rate of 0.25 ◦ C min−1 and the TGA curve is shown in Fig. 7. This material also begins to decompose immediately starting at 30 ◦ C. However, in this case at 95 ◦ C the TGA curve levels off until approximately 110 ◦ C at which point the material rapidly decomposes. Analysis of the reaction products below 95 ◦ C shows only the formation of Hofmann elimination products (isobutylene and trimethylamine). Deuterium scrambling is observed immediately at 30 ◦ C with the mass spectrum of the trimethylamine showing a similar isotopic scrambling pattern as seen before. However, above 100 ◦ C the appearance of isobutyldimethylamine is observed in the mass spectrum, which can only be produced via nucleophilic attack of the deuteroxide ion on the methyl group of the ammonium cation, or by further reaction of the nitrogen ylide species with D2 O, which are indistinguishable from each other. In both the TMA and
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Scheme 2. Thermal decomposition of PTMA deuteroxide.
isobutyldimethylamine evolved gases, the majority species consist of heavier mass isotopomers from deuterium scrambling (Fig. 8). When the decomposition was performed at such a heating rate (0.15 ◦ C min−1 ) that the material completely decomposed before reaching 100 ◦ C the only reaction products were isobutylene and trimethylamine. However, if the reaction was performed isothermally by ramping the TGA from 30 to 120 ◦ C immediately and holding at 120 ◦ C the material decomposes in 10 min and the formation of isobutyldimethylamine is again observed in the mass spectrum (along with trimethylamine and isobutylene). The TGA curves for these runs are shown in Fig. 9. To gain further insight, 5 was decomposed isothermally at 120 ◦ C in a vial and the headspace was analyzed by GC-MS. The reaction products were separated by GC and MS confirmed the identity of each. The gas chromatogram is shown in Fig. 10. As expected, the only observed products were those from Hofmann elimination and SN 2 attack/ylide decomposition (MeOD, dimethyl ether (DME), TMA, isobutylene, and isobutyldimethylamine).
Because of the low barrier to nitrogen ylide formation, deuterium scrambling was observed in each of the individual decomposition products. Closer examination of the isobutyldimethylamine mass spectrum reveals a characteristic distribution of heavier isomers (Fig. 8). Furthermore, this deuterium isotopic scrambling pattern is observed in each of the daughter fragmentation species arising from methyl group loss. Fragmentation of isobutyldimethylamine will also produce an iminium ion [((CH3 )2 N CH2 )]+ , which has a molecular ion peak at 58 amu [29]. One would expect this species to have a distribution of deuterium ions incorporated and thus, it is impossible to distinguish this species from TMA produced through Hofmann elimination using mass spectrometry without the aid of gas chromatography to separate these products. The combined results from the thermal decomposition of 5 described above give a clear view of the mechanism for decomposition. Referring to Scheme 3 it was proposed that 3 pathways were viable for the decomposition of 5. It is clear that the only observed decomposition products are TMA and isobutylene when
Fig. 6. (a) Mass spectrum of the trimethylamine produced in the ramped decomposition of [n-PrNMe3 ][OD]·xD2 O. (b) Evolution of the IR spectra of evolved gases from 3 as a function of time.
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Fig. 7. Thermal curve of the decomposition of [i BuNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.25 ◦ C min−1 .
the decomposition is carried out such that the temperature does not reach 90–95 ◦ C before 5 is completely decomposed. Above this temperature, the formation of isobutyldimethylamine, dimethylether and methanol indicate that the SN 2 nucleophilic attack of the deuteroxide ion on the ammonium cation can now compete with Hofmann elimination. These results are not surprising, and what one would expect as the steric hindrance around the -hydrogen is increased effectively increasing the activation barrier to Hofmann elimination. 3.4. Decomposition studies of [NpNMe3 ][OD]·xD2 O (7) Completely removing all -hydrogens and replacing with methyl groups, as in 7, was the final step for this study. Scheme 4 illustrates the expected thermal decomposition pathway for 7,
Fig. 8. Mass spectrum of the TMA (top) and isobutyldimethylamine (bottom) produced in the ramped decomposition of [i BuNMe3 ][OD]·xD2 O.
Scheme 3. Thermal decomposition of ITMA deuteroxide.
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Fig. 9. (a) Thermal curve of the decomposition of [i BuNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.15 ◦ C min−1 . (b) isothermal decomposition at 120 ◦ C.
which one would expect only SN 2 attack is the viable decomposition pathway due to the absence of -hydrogen atoms. The thermal decomposition of 7 was carried out at a heating rate of 0.15 ◦ C min−1 and the TGA curve is shown in Fig. 11. This material shows a gradual decomposition beginning at 30 ◦ C with a dramatic mass loss observed above ∼65 ◦ C resulting in complete decomposition by 77 ◦ C. Deuterium scrambling is observed immediately at 30 ◦ C with the FTIR spectrum of the evolved gases showing C–D stretches similar to those previously observed. The reaction products observed were methanol, dimethyl ether, and neopentyldimethylamine, which can only be produced via nucleophilic attack of the deuteroxide ion on the methyl group of the ammonium cation, or by direct further decomposition from the nitrogen ylide species, which are indistinguishable from each other.
To gain further insight 7 was decomposed isothermally at 120 ◦ C in a vial and the headspace was analyzed by GC–MS. The reaction products were separated by GC and MS confirmed the identity of each. The gas chromatogram is shown in Fig. 12. In addition to the expected products (MeOD, DME, neopentyldimethylamine), the appearance of three minor products was also observed (labeled 1, 2, and 3 (Fig. 12)). Analysis of the mass spectrum of each of these individual components showed molecular ions consistent with products having molecular formulas of C5 H10 O (peak 1) and C6 H12 N (peaks 2 and 3). The mass spectrum of peak 1 is identical to that of neopentanal. Despite analyzing a number of amine standards, the identity of the minor species of peaks 2 and 3 could not be determined. We are currently investigating the identity and source of these very minor degradation products to determine
Scheme 4. Thermal decomposition of NTMA deuteroxide.
J.B. Edson et al. / Journal of Membrane Science 399–400 (2012) 49–59
57
Fig. 10. GC trace for the isothermal decomposition of [i BuNMe3 ][OD]·xD2 O at 120 ◦ C.
what implications they may have on neopentyltrialkylammonium cation stability and results will be reported in due course. Because of the low barrier to nitrogen ylide formation, deuterium scrambling was observed in each of the individual decomposition products. A close look at the neopentyldimethylamine mass spectrum reveals a characteristic distribution of heavier isomers with this isotopic scrambling pattern observed in each of the daughter fragments, including the iminium ion [((CH3 )2 N CH2 )]+ , which is also observed in the decomposition of 5. These combined results for the thermal decomposition of [NpNMe3 ][OD]·xD2 O show that the major decomposition productions arise from SN 2 attack/ylide reaction. Quite unexpected was the formation of the other three minor reaction products of which one was identified as neopentanal. The identity of the other 2 minor products could not be determined. None of these products can be explained by any of the decomposition pathways highlighted
Fig. 11. Thermal curve of the decomposition of [NpNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.15 ◦ C min−1 .
in Scheme 4, which illustrates that further experimentation must be conducted on the stability of tetraalkylammonium and other cations used for alkaline fuel cell membranes if all decomposition pathways are to be understood. 4. Summary With these results in hand, a comparison of decomposition mechanisms between the different alkyltrimethylammonium cations can be made. Looking at the change in mass with changing temperature as a function of temperature for each of the cations, the decompositions of each alkyltrimethylammonium can be directly compared because they were decomposed at a standard heating rate (0.15 ◦ C min−1 ). This plot is shown in Fig. 13. Of note, the ETMA deuteroxide displays peak decomposition at approximately
Fig. 12. GC trace for the isothermal decomposition of [NpNMe3 ][OD]·xD2 O at 120 ◦ C.
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Table 2 Summary of the degradation products for the alkyltrimethylammonium cations studied. Cation
Ylide scrambling
Hofmann elimination
Ylide/SN 2 decomposition
Peak degradation temperature (◦ C)
G‡ Hofmann barrier (kcal/mol)
G‡ SN 2 barrier (kcal/mol)
Yes
Yes
No
60
17.5
24.7
Yes
Yes
No
79
22.9
25.9
Yes
Yes
Yes
74
21.7
25.7
Yes
NA
Yes
74
NA
25.0
5. Conclusions We have extended our previous work in which we examined the decomposition of tetramethylammonium ion to the degradation of tetralkylammonium cations that may be more relevant to the functional groups used on polymer membranes for alkaline fuel cells. The overall picture that emerges from these studies is that Hofmann elimination is the preferred decomposition pathway for ammonium cations bearing -hydrogens, and ylide formation scrambles protons from water (or D2 O) into the trimethylamine that is formed with a low activation barrier. Blocking the -hydrogen positions by successively adding methyl groups increases the stability of the cation, but in the case of [i-BuNMe3 ][OD]·xD2 O, a second decomposition pathway is observed (nucleophilic attack or ylide decomposition). Completely blocking the -hydrogen position, as in the case of [NpNMe3 ][OD]·xD2 O, results in decomposition occurring almost exclusively through SN 2 attack/ylide reaction, however, further reaction products were observed that have not yet been identified. These results represent the extreme conditions of potential fuel cell operation (effectively zero percent relative humidity). Further studies will focus on decomposition of these materials under controlled humidity conditions. Fig. 13. Change in mass over the change in temperature as a function of temperature for ETMA, PTMA, ITMA, and NTMA deuteroxide decompositions at a heating rate of 0.15 ◦ C.
60 ◦ C while the PTMA, ITMA, and NTMA deuteroxides show peak decomposition temperatures at 79, 74, and 74 ◦ C, respectively. The presence of more -hydrogens in the ETMA cation in addition to the decreased steric bulk for Hofmann elimination can explain its relatively low peak decomposition temperature. Increasing steric bulk by adding adjacent methyl groups (and also decreasing the number of -hydrogens) results in increased cation stability as highlighted by the increase in peak decomposition temperature. Interestingly, NTMA (bearing no -hydrogens) shows a peak decomposition temperature similar to the PTMA and ITMA cations indicating that SN 2 attack of hydroxide on the methyl groups (or ylide decomposition) for NTMA is of similar activation energy to Hofmann elimination for PTMA and ITMA. These results are consistent with the trends observed in density functional theory (DFT) calculations investigating the degradation pathways of these cations under fully hydrated conditions [30]. Table 2 summarizes the data from this report and contains preliminary DFT calculated G‡ values for Hofmann elimination and SN 2 attack pathways of the cations studied in which a detailed full report will be reported in due course [30].
Acknowledgements This project was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering and contract # DE-AC36-08GO28308 to NREL. References [1] J.R. Varcoe, R.C.T. Slade, Prospects for alkaline anion-exchange membranes in low temperature fuel cells, Fuel Cells 5 (2005) 187–200. [2] G.F. McLean, T. Niet, S. Prince-Richard, N. Djilali, An assessment of alkaline fuel cell technology, Int. J. Hydrogen Energy 27 (2002) 507–526. [3] M. Cifrain, K.V. Kordesch, Advances, aging mechanism and lifetime in AFCs with circulating electrolytes, J. Power Sources 127 (2004) 234–242. [4] E. Gulzow, M. Schulze, Long-term operation of AFC electrodes with CO2 containing gases, J. Power Sources 127 (2004) 243–251. [5] S. Gamburzev, K. Petrov, A.J. Appleby, Silver–carbon electrocatalyst for air cathodes in alkaline fuel cells, J. Appl. Electrochem. 32 (2002) 805–809. [6] E. Gulzow, M. Schulze, G. Steinhilber, Investigation of the degradation of different nickel anode types for alkaline fuel cells (AFCs), J. Power Sources 106 (2002) 126–135. [7] N. Wagner, M. Schulze, E. Gulzow, Long term investigations of silver cathodes for alkaline fuel cells, J. Power Sources 127 (2004) 264–272. [8] X.G. Li, G. Liu, B.N. Popov, Activity and stability of non-precious metal catalysts for oxygen reduction in acid and alkaline electrolytes, J. Power Sources 195 (2010) 6373–6378. [9] X.G. Li, B.N. Popov, T. Kawahara, H. Yanagi, Non-precious metal catalysts synthesized from precursors of carbon, nitrogen, and transition metal for oxygen reduction in alkaline fuel cells, J. Power Sources 196 (2011) 1717–1722.
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[20] H.A. Kostalik, T.J. Clark, N.J. Robertson, P.F. Mutolo, J.M. Longo, H.D. Abruna, G.W. Coates, Solvent processable tetraalkylammonium-functionalized polyethylene for use as an alkaline anion exchange membrane, Macromolecules 43 (2010) 7147–7150. [21] E.E. Switzer, T.S. Olson, A.K. Datye, P. Atanassov, M.R. Hibbs, C. Fujimoto, C.J. Cornelius, Novel KOH-free anion-exchange membrane fuel cell: performance comparison of alternative anion-exchange ionomers in catalyst ink, Electrochim. Acta 55 (2010) 3404–3408. [22] D. Valade, F. Boschet, B. Ameduri, Grafting polymerization of styrene onto alternating terpolymers based on chlorotrifluoroethylene, hexafluoropropylene, and vinyl ethers, and their modification into ionomers bearing ammonium side-groups, J. Polym. Sci., Part A: Polym. Chem. 48 (2010) 5801–5811. [23] J.H. Wang, J. Wang, S.H. Li, S.B. Zhang, Poly(arylene ether sulfone)s ionomers with pendant quaternary ammonium groups for alkaline anion exchange membranes: preparation and stability issues, J. Membr. Sci. 368 (2011) 246–253. [24] C.S. Macomber, J.M. Boncella, B.S. Pivovar, J.A. Rau, Decomposition pathways of an alkaline fuel cell membrane material component via evolved gas analysis, J. Therm. Anal. Calorim. 93 (2008) 225–229. [25] S. Chempath, B.R. Einsla, L.R. Pratt, C.S. Macomber, J.M. Boncella, J.A. Rau, B.S. Pivovar, Mechanism of tetraalkylammonium headgroup degradation in alkaline fuel cell membranes, J. Phys. Chem. C 112 (2008) 3179–3182. [26] S. Chempath, J.M. Boncella, L.R. Pratt, N. Henson, B.S. Pivovar, Density functional theory study of degradation of tetraalkylammonium hydroxides, J. Phys. Chem. C 114 (2010) 11977–11983. [27] see K.L. Feilberg, D.W.T. Griffith, M.S. Johnson, C.J. Nielsen, The 13 C and D kinetic isotope effects in the reaction of CH4 with Cl, Int. J. Chem. Kinet. 37 (2005) 110–118, and references 5–9 therein. [28] G. Hvistend, K. Undheim, High resolution mass spectrometry of trimethylamine, Org. Mass Spectrom. 3 (1970) 821–824. [29] R.D. Bowes, A. Maccoll, Low-energy, low-temperature mass spectra 4-saturated alkylamines, Org. Mass Spectrom. 20 (1985) 331–335. [30] H. Long, K. Kim, B.S. Pivovar, Degradation pathways for tetraalkylammonium cations in alkaline membrane fuel cells: a DFT study, in preparation.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Hydroxide based decomposition pathways of alkyltrimethylammonium cations Joseph B. Edson a , Clay S. Macomber b , Bryan S. Pivovar b , James M. Boncella a,∗ a b
Materials, Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States National Renewable Energy Laboratory, Golden, CO 80401, United States
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 18 January 2012 Accepted 22 January 2012 Available online 1 February 2012 Keywords: Tetraalkylammoniumcations Decomposition Fuel cell membranes Evolved gas analysis
a b s t r a c t A systematic study that altered the number of -hydrogen atoms susceptible to Hofmann elimination and introduced increased steric hindrance of substituted (ethyl, n-propyl, isobutyl, and neopentyl) alkyltrimethylammonium cations was performed. The mechanism of the thermal decomposition of these four ammonium cations in deuteroxide form was studied using evolved gas analysis (EGA) because of their potential importance in alkaline membrane fuel cells or electrolyzers. The products of the decomposition reactions are in many cases the expected Hofmann elimination products (trimethylamine and olefins), however, as the number of -hydrogen atoms decrease or they become more sterically encumbered (from the addition of adjacent methyl groups), nucleophilic attack of hydroxide on the methyl groups increases in relative importance. The use of deuterated water and deuteroxide in our study shows that deprotonation of the tetraalkylammonium ions establishes a rapid equilibrium between the nitrogen ylide species that is formed by methyl group deprotonation and water that scrambles deuterium into the methyl groups of the amine. The results of this work show that at high temperature and low water content tetraalkylammonium hydroxide salts are relatively unstable in membranes. © 2012 Elsevier B.V. All rights reserved.
1. Introduction While an enormous effort has been directed towards the development of proton exchange membrane fuel cells (PEMFCs) in recent years, the development of alkaline fuel cell versions of PEMFCs is relatively much less explored [1]. Alkaline membrane fuel cell (AMFC) systems have traditionally been ignored by the fuel cell community due to the concerns over carbonate formation from the reaction of OH− ions with CO2 contaminants in the oxidant gas stream leading to the formation of carbonate/bicarbonate (CO3 2− /HCO3 − ) [2–4]. However, alkaline membrane fuel cells (AMFCs) can offer some advantages over their PEMFC counterparts. The basic medium allows for the use of non-precious electrode catalysts made from inexpensive metals such as Fe/Co/Ni/Ag versus the use of the precious and expensive Pt used in PEMFCs [5–9]. Another concern with AMFCs is the stability of the membrane at high pH necessary for good hydroxide ion conductivity, especially at elevated temperatures [10,11]. Early studies on membrane development have focused on the use of ammonium cations [NR4 ]+ tethered to a polymer backbone for hydroxide ion transport. In this regard, a number of recent publications have focused on the development and testing of these ammonium cation tethered polymers for membrane use [12–23]. While many of these efforts have been directed towards the
∗ Corresponding author. Tel.: +1 505 665 0795. E-mail address: [email protected] (J.M. Boncella). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.01.025
polymer membrane stability at high pH and temperature, very little has been done to study the effect of these variables on the stability of the ammonium cations themselves. Previous work has shown that the hydroxide ion in the membrane can react with ammonium cations via nucleophilic attack or as a Bronsted base. Furthermore, the formation of nitrogen ylide species through reversible deprotonation/protonation of methyl groups attached to the nitrogen cations was observed offering another reaction pathway for decomposition [24–26]. We have previously reported the detailed mechanism of thermal decomposition of tetramethylammonium hydroxide, [NMe4 ][OH], using thermogravimetric analysis (TGA) as well as evolved gas analysis (EGA) with the identity and quantities of the evolved gases analyzed by Fourier-transform infrared (FTIR) spectroscopy and mass spectrometry (MS) [24]. While [NMe4 ][OH] showed good stability under the conditions studied due to its compactness and the lack of -hydrogen atoms susceptible to Hofmann elimination, it is impractical for fuel cell usage as a free ion not tethered to the polymer membrane. Ammonium cations are typically tethered to a polymer backbone via post polymerization modification of the polymers containing pendant alkyl chloride functional groups through reaction with trimethylamine to give substituted alkyltrimethylammonium functional groups, [RNMe3 ]+ . The cations presented in this study are analogues of tethers that could be used in anion exchange polymers. Most polymer systems studied to date have used a benzyl group as the point of attachment (R = Bn), however there have been recent reports of systems utilizing aliphatic points of cation attachment such as the neopentyl
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Table 1 Alkyltrimethylammonium cations studied. Cation
Name
# of -hydrogens
Ethyltrimethylammonium (ETMA)
3
nPropyltrimethylammonium (PTMA)
2
Isobutyltrimethylammonium (ITMA)
1
Neopentyltrimethylammonium (NTMA)
0
(Np)-like groups used recently by Coates and coworkers [17,20]. Both the benzyl and neopentyl linkages result in no -hydrogen atoms present to undergo Hofmann elimination due to the assumed severe instability of these cations. However, this limits the number of potential polymer backbones. Here, we present a fundamental and systematic study of the stability of [RNMe3 ]+ cations where the number of -hydrogens susceptible to Hofmann elimination and the steric hindrance to this reaction are probed. We have chosen to use the substituted trimethyl version of ammonium cations in this study for various reasons. The first of which is it allows us to focus on a single substitution point of attack for the decomposition reactions (for which the methyl amine groups are not directly associated). Additionally, the synthesis of the singly substituted trimethylammoniums is straightforward and does not have significant steric issues associated with quanternizing the nitrogen. Finally, the use of three methyl groups allows for the more compact and hydrophilic ammonium cations to be studied. In practical materials, increasing the size and hydrophobicity would be expected to play a detrimental role on water uptake and conduction of membrane materials employing these cations. In Table 1 we show the four cations employed in this study with nomenclature used in this paper as well as the number of -hydrogen atoms susceptible to Hofmann elimination. The mechanism of thermal decomposition of ethyltrimethylammonium deuteroxide, [EtNMe3 ][OD]·xD2 O, n-propyltrimethylammonium deuteroxide, [n-PrNMe3 ][OD]·xD2 O, iso-butyltrimethylammonium deuteroxide, [i-BuNMe3 ][OD]·xD2 O, and neopentyltrimethylammonium deuteroxide, [NpNMe3 ][OD]·xD2 O, was studied using TGA and EGA. The deuteroxides were chosen for this study as it has been previously reported that nitrogen ylide formation on the methyl group of the ammonium cations has a low barrier and the use of the deuteroxide counter ions enables the detailed observation of these species via isotopic scrambling in the decomposition products [24]. The formation of ylides can be important in degradation for these or similar cations, particularly when similar cations are placed into polymeric systems where the likelihood of ylide species further reacting with the polymer backbone or other cations/tethers is greatly enhanced. 2. Experimental 2.1. Materials Ethyltrimethylammonium iodide, 1-bromopropane, 1-bromo2-methylpropane, neopentylamine, trimethylamine (45% in H2 O), silver (I) oxide, methyl iodide, potassium carbonate, acetonitrile,
diethyl ether, dimethylsulfoxide-d6 (DMSO), methanol-d4 (MeOD) and deuterium oxide (D2 O, 99.95% atom D) were purchased from commercial sources and used as received. All alkyltrimethylammonium deuteroxides were stored under an argon atmosphere in tightly sealed vessels at −30 ◦ C prior to experiments. Ethyltrimethylammonium deuteroxide (1) – To a 100 mL round bottom flask under an active argon flow was added ethyltrimethylammonium iodide (1.06 g, 4.92 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (2.24 g, 9.65 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization to yield 0.54 g of the white solid [EtNMe3 ][OH]·xH2 O. Preparation of [EtNMe3 ][OD]·xD2 O was achieved by dissolution of [EtNMe3 ][OH]·xH2 O in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.50 g of [EtNMe3 ][OD]·xD2 O as a white solid. 1 H NMR (D2 O, 300 MHz): 3.28 (q, 2H, J = 7.2 Hz, CH2 ), 2.97 (s, 9H, N(CH3 )3 ), 1.24 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (DMSO, 75 MHz): 63.6 (CH2 ), 53.7 (N(CH3 )3 ), 9.4 (CH3 ). n-Propyltrimethylammonium bromide (2) – A 100 mL round bottom flask was charged with 1-bromopropane (5.8 mL, 63.8 mmol), trimethylamine (45% in H2 O, 20 mL, 126 mmol), and 20 mL acetonitrile. The reaction was heated at 40 ◦ C for 5 h and then cooled to room temperature. The solvent was removed via rotary evaporation to leave an oily residue. Diethyl ether was added to precipitate the product, which was filtered and washed with additional ether. The product was dried in vacuo overnight to yield 10.1 g (87%) of a white solid. 1 H NMR (MeOD, 300 MHz): 3.61 (m, 2H, CH2 N(CH3 )3 ), 3.42 (s, 9H, N(CH3 )3 ), 2.03 (m, 2H, H3 CCH2 ), 1.22 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 69.1 (CH 2 N(CH3 )3 ), 53.7 (N(CH3 )3 ), 17.5 (H3 CCH2 ), 10.9 (CH3 ). n-Propyltrimethylammonium deuteroxide (3) – To a 100 mL round bottom flask under an active argon flow was added npropyltrimethylammonium bromide (1.88 g, 10.3 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (4.75 g, 20.5 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization and the product, [n-PrNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.87 g of [n-PrNMe3 ][OD]·xD2 O as a colorless oily solid. 1 H NMR (D2 O, 300 MHz): 3.16 (m, 2H, CH2 N(CH3 )3 ), 2.98 (s, 9H, N(CH3 )3 ), 1.70 (tq, 2H, J = 7.8, 4.2 Hz, H3 CCH2 ), 0.87 (t, 3H, J = 7.2 Hz, CH3 ). 13 C NMR (DMSO, 75 MHz): 66.2 (CH2 N(CH3 )3 ), 51.7 (N(CH3 )3 ), 15.7 (H3 CCH2 ), 10.5 (CH3 ). Isobutyltrimethylammonium bromide (4) – A 100 mL round bottom flask was charged with 1-bromo-2-methylpropane (6.0 mL, 55.2 mmol), trimethylamine (45% in H2 O, 18 mL, 114 mmol), and 20 mL acetonitrile. The reaction was heated at 40 ◦ C for 5 h and then cooled to room temperature. The solvent was removed via rotary evaporation to leave an oily residue. Diethyl ether was added to precipitate the product, which was filtered and washed with additional ether. The product was dried in vacuo overnight to yield 8.76 g (81%) of a white solid. 1 H NMR (MeOD, 300 MHz): 3.33 (d, 2H, J = 5.1 Hz, CH2 N(CH3 )3 ), 3.21 (s, 9H, N(CH3 )3 ), 2.20 (dt, 1H, J1 = 6.9 Hz, J2 = 5.1 Hz, HCCH2 ), 1.14 (d, 3H, J = 6.9 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 75.1 (CH2 - N(CH3 )3 ), 54.2 (N(CH3 )3 ), 25.0 (H3 CCH), 23.2 (CH3 ). Isobutyltrimethylammonium deuteroxide (5) – To a 100 mL round bottom flask under an active argon flow was added 4 (1.29 g, 6.58 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (3.05 g, 13.2 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug
J.B. Edson et al. / Journal of Membrane Science 399–400 (2012) 49–59
of celite. The water was then removed via lyophilization and the product, [i BuNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 0.61 g of [i BuNMe3 ][OD]·xD2 O as a colorless oily solid. 1 H NMR (MeOD, 300 MHz): 3.29 (d, 2H, CH2 N(CH3 )3 ), 3.20 (s, 9H (showing D incorporation), N(CH3 )3 ), 2.28 (m, 1H, (H3 C)2 CH), 1.17 (d, 6H, J = 6.6 Hz, CH3 ). 13 C NMR (MeOD, 75 MHz): 75.1 (CH2 N(CH3 )3 ), 53.9 (N(CH3 )3 ) (appears as a multiplet from deuterium scrambling), 25.1 ((H3 C)2 CH), 23.4 (CH3 ). Neopentyltrimethylammonium iodide (6) – To a 100 mL round bottom flask was added neopentylamine (3 mL, 25.6 mmol), potassium carbonate (4.70 g, 34 mmol), and iodomethane (5.1 mL, 82.0 mmol) in 40 mL of ethanol. The reaction was stirred overnight and the excess potassium carbonate was removed by filtration. The solvent was removed in vacuo to give a yellow solid. Recrystallization from isopropanol gave 4.24 g (64%) of a white crystalline solid. 1 H NMR (DMSO, 300 MHz): 3.37 (s, 2H, CH2 N(CH3 )3 ), 3.19 (s, 9H, N(CH3 )3 ), 1.11 (s, 9H, (H3 C)3 C CH2 ). 13 C NMR (MeOD, 75 MHz): 77.8 (CH2 N(CH3 )3 ), 56.3 (N(CH3 )3 ), 34.4 ((H3 C)3 C CH2 ), 30.2 ((H3 C)3 C CH2 ). Neopentyltrimethylammonium deuteroxide (7) – To a 100 mL round bottom flask under an active argon flow was added 6 (2.13 g, 8.30 mmol) and 15 mL H2 O with a Teflon coated magnetic stir bar. The solution was cooled to 0 ◦ C and Ag2 O (3.84 g, 16.6 mmol) was added. The reaction was allowed to come to room temperature over a period of 2 h and then was filtered through a plug of celite. The water was then removed via lyophilization and the product, [NpNMe3 ][OH]·xH2 O, was dissolved in deuterium oxide followed by solvent evaporation via dynamic vacuum at room temperature. This process was repeated three times to afford 1.36 g of [NpNMe3 ][OD]·xD2 O as a white crystalline solid. 1 H NMR (MeOD, 300 MHz): 3.29 (s, 2H, CH2 N(CH3 )3 ), 3.20 (s, 9H (showing D incorporation), N(CH3 )3 ), 1.20 (s, 9H, C(CH3 )3 ). 13 C NMR (MeOD, 75 MHz): 78.0 (CH2 N(CH3 )3 ), 55.9 (N(CH3 )3 ) (appears as a multiplet from deuterium scrambling), 34.3 ((H3 C)3 C), 29.97 ((H3 C)3 C).
51
Fig. 1. Schematic diagram of EGA experiment.
Experimental parameters for the headspace gas chromatography–mass spectroscopy [GC–MS] decomposition experimentation consist of the implementation of a Thermo Scientific Trace GC Ultra in conjunction with an ISQ single quadrupole MS calibrated using a perfluorotributylamine standard. Sample preparation involved loading microliters/micrograms of sample into inert gas purged 10 mL headspace vials, sealed with high performance Teflon septa. The vials were subsequently heated in an incubator/agitator oven at 120 ◦ C for pre-determined amounts of time. Gas sampling was achieved using a Triplus HS autosampler outfitted with a heated gas-tight syringe set at 150 ◦ C. 2 mL of headspace gas was collected from the vial while in the incubator and injected into a split/splitless [S/SL] injector set to splitless mode. The species in the gas were separated on an Agilent CP-Volamine column during a GC temperature profile with an isothermal hold at 40 ◦ C for 8 min, followed by a ramp of 20 ◦ C min−1 to 250 ◦ C, and an isothermal hold at 250 ◦ C for 3.5 min. Mass spectra were collected on the ISQ operating in electron ionization [EI] mode, electron energy of 70 eV, and scanning from 1 to 500 amu. After background subtraction a NIST search was performed to identify species and appropriate standards were run for confirmation of spectra and retention times.
2.2. Instrumentation 3. Results and discussion Evolved gas analysis (EGA) was performed on the decomposition of the materials to identify products in an effort to determine reaction pathways. Mass loss, gas speciation, quantification and isotopic measurements were carried out using a collection of instrumentation coupled together with heat traced stainless steel transfer lines. Decomposition reactions were performed in a horizontal large furnace Mettler-Toledo 851 TGA/SDTA thermogravimetric analyzer (TG). The reaction zone was constantly purged with nitrogen at 40 sccm as measured by an Agilent ADM1000 flowmeter. Evolved gases from the decompositions were carried by the purge gas from the outlet of the TG and flowed through a Thermo-Electron 380 Fourier transform infrared spectrometer (TG–FTIR) equipped with a Nicolet 2 m heated gas cell (KBr windows). At the inlet to the gas cell a Pfeiffer Thermo-Star mass spectrometer (TG–MS) sampled the gas via a stainless steel capillary at a rate of 1 sccm in addition to a Varian 4-channel CP-4900 gas chromatograph (GC) via a stainless steel capillary (Fig. 1). FTIR difference spectra were taken at a resolution of 0.5 cm−1 after subtraction of the KBr and UHP N2 background blank. The solid starting materials were contained in 900 L alumina crucibles. In non-isothermal experiments crucibles were filled with ∼25 mg of sample material, forming a thin layer to minimize mass transfer related problems. Non-isothermal temperature ramp rates consisted of 0.15 and 0.25 ◦ C min−1 starting from 30 and an endpoint of 150 ◦ C. Isothermal experiments initiated at an equilibrium temperature of 30 ◦ C for 10 min followed by a rapid ramp from 30 to 120 ◦ C and were followed by an isothermal segment at 120 ◦ C for 30 min.
3.1. Decomposition studies of [EtNMe3 ][OD]·xD2 O (1) The presence of -hydrogen atoms in 1 would suggest that decomposition through Hofmann elimination to form ethylene and trimethylamine would be the primary pathway for thermal decomposition. Scheme 1 illustrates a number of viable decomposition pathways for the thermal decomposition of the ETMA deuteroxide. Specifically one can envision decomposition occurring through 3 potential pathways. Previous studies on the thermal decomposition of tetramethylammonium deuteroxide, [Me4 N][OD]·5D2 O,demonstrated that formation of a nitrogen ylide species through reversible deprotonation/protonation of [Me4 N]+ occurred prior to any further decomposition [24]. Thus, one cannot rule out the formation of similar ylide species in the decomposition of 1. Furthermore, direct nucleophilic attack of the deuteroxide ion on the cation is certainly possible. In order to understand the thermal decomposition process, the identity of the gases evolved during the decomposition of 1 were analyzed by a combination of FTIR spectroscopy, mass spectrometry, and gas chromatography. A thermal curve (TGA) of the decomposition of 1 when heated at 0.15 ◦ C min−1 starting at 30 ◦ C is shown in Fig. 2. In stark contrast to prior decomposition studies of [Me4 N][OD]·5D2 O where the onset of decomposition is not observed until some water of hydration is lost (2 molecules) and the temperature is above 110 ◦ C [24], the decomposition of 1 begins immediately upon heating from
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Scheme 1. Thermal decomposition of ETMA deuteroxide.
30 ◦ C. Complete decomposition of the material is observed by 68 ◦ C at the given heating rate. This drastic difference can be attributed to a number of factors. Namely, the exact extent of the solvation level of the 1 starting material could not be determined. Whereas [Me4 N][OD]·5D2 O is a commercially available, stable, crystalline solid, 1 had to be prepared via synthetic methods and all attempts to crystallize the material were unsuccessful. Despite this, an analysis of the gases produced by FTIR spectroscopy and GC showed the formation of only ethylene and trimethylamine (TMA) as reaction products (Fig. 2) in equal concentrations. This is consistent with Hofmann elimination as the only decomposition pathway. The initial TMA decomposition product observed in the mass spectrum at 30 ◦ C is free of deuterium scrambling and is representative of undeuterated TMA. Above 40 ◦ C, isotopic scrambling becomes evident and when the isotopic composition of the TMA produced was analyzed as a function of increasing temperature the isotopic distribution of TMA reached an equilibrium distribution within 2–3 ◦ C. Fig. 3 shows the mass spectrum of TMA that is
Fig. 2. Evolved gas analysis (EGA) of [EtNMe3 ][OD]·xD2 O thermal decomposition.
produced when 1 decomposes above 40 ◦ C. The deuterium scrambling above 40 ◦ C is also consistent with the appearance of C–D stretches in the FTIR spectrum of the evolved gases. A time evolved 3D FTIR plot is shown in Fig. 4 that also shows the appearance of C–D stretches beginning at a temperature of 40 ◦ C (bands at approximately 2190 and 2070 cm−1 ) [27]. The molar mass of undeuterated TMA is 59 amu, with 58 amu being the major signal in the spectrum [28]. It is apparent from the mass spectral data in Fig. 3 that heavier mass isotopomers make up the majority of the observed species. These heavier mass species arise from the exchange of deuterium from D2 O in the sample with protons on the methyl groups of the ETMA ions as the material is decomposing. This occurs via formation of nitrogen ylide species, which have also been observed during
Fig. 3. Mass spectrum of the trimethylamine produced in the ramped decomposition of [EtNMe3 ][OD]·xD2 O.
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Fig. 4. Evolution of the IR spectra of evolved gases from 1 as a function of time.
the decomposition of tetramethylammonium deuteroxide [24]. At least two factors, time and temperature (water content being an additional potential variable) play a role in the observed isotopic scrambling of TMA. This data alone is insufficient to talk about specific rates or energy barriers of the two processes; however, it does confirm that ylide scrambling occurs to a substantial extent. An additional difficulty in interpreting similar data is that H/D isotopic exchange will reach equilibrium and after this point, while the phenomenon continues, no further change can be witnessed in the observed mass spectrum. The combined results from the thermal decomposition of 1 described above give a clear view of the mechanism for decomposition. Referring back to Scheme 1 it was proposed that 3 pathways were viable for [EtNMe3 ][OD]·xD2 O decomposition. The only observed decomposition products are TMA and ethylene, which come from Hofmann elimination. However, the observation of deuterium scrambling into the TMA above 40 ◦ C suggests a rather low barrier for reversible ylide formation. The absence of the formation of methanol, ethyldimethyl amine, or dimethyl ether (DME), as indicated by the lack of peaks in the GC of the product gases, suggest that the pathway for SN 2 attack has too high of barrier to be accessed under the experimental conditions. These results are consistent with computational results that show the Hofmann elimination reaction to be of much lower activation barrier than the nucleophilic attack of the deuteroxide ion on the cation [26]. However, while an ylide species could not be identified by computational methods, the experimental results demonstrate that formation of this species has a relatively low activation barrier.
Fig. 5. Thermal curve of the decomposition of [n-PrNMe3 ][OD]·xD2 O.
very first samples of TMA collected. In fact, analysis of 3 by 1 H NMR spectroscopy reveals that deuterium scrambling into the methyl groups on the nitrogen occurs during its synthesis indicating a low barrier for scrambling. Fig. 6 shows the mass spectrum of the TMA that is produced when 3 decomposes. The deuterium scrambling can also be observed through the appearance of C–D stretches in the FTIR spectrum (Fig. 6). The results obtained are nearly identical to the observations for the decomposition of 1. The only observed decomposition products are TMA and propylene, which come from Hofmann elimination. In an attempt to try to overcome the SN 2 activation barrier, 3 was isothermally decomposed at 120 ◦ C and the resultant evolved gases were analyzed as before. The material is rather unstable at this temperature and was completely decomposed within 5 min. As previously observed in the controlled heating runs, propylene and trimethylamine were the only observable products. These results are consistent with results that show the Hofmann elimination reaction to be of much lower activation barrier than the nucleophilic attack of the deuteroxide ion on the cation. However, reversible ylide species formation has a relatively low activation barrier as evidenced from the deuterium scrambling observed immediately in the FTIR and MS spectra.
3.2. Decomposition studies of [n-PrNMe3 ][OD]·xD2 O (3)
3.3. Decomposition studies of [i BuNMe3 ][OD]·xD2 O (5)
We performed similar thermal degradation studies with 3, in which one -hydrogen atom on 1 is replaced with a methyl group. Scheme 2 illustrates the viable decomposition pathways for the thermal decomposition of the PTMA deuteroxide. A TGA curve of the decomposition of 3 when heated at 0.15 ◦ C min−1 starting at 30 ◦ C is shown in Fig. 5. Similar to the decomposition of 1, [n-PrNMe3 ][OD]·xD2 O begins to decompose immediately at 30 ◦ C. Complete decomposition of the material is observed by approximately 90 ◦ C at the given heating rate. As expected, the material is more stable than 1 presumably due to the presence of the -methyl group, which may hinder Hofmann elimination or the fact that only two rather than three -hydrogen sites exist for Hofmann elimination. An analysis of the decomposition products by GC and FTIR spectroscopy revealed the formation of propylene and trimethylamine as the only reaction products, which again is consistent with Hofmann elimination as the only decomposition pathway. Analysis of the trimethylamine given off by mass spectrometry revealed that deuterium scrambling is observed immediately in the
[i BuNMe3 ][OD]·xD2 O represented the next step in replacing a -hydrogen atom with a methyl group. Scheme 3 illustrates the viable decomposition pathways for the thermal decomposition of the ITMA deuteroxide. The thermal decomposition of 5 was carried out at a heating rate of 0.25 ◦ C min−1 and the TGA curve is shown in Fig. 7. This material also begins to decompose immediately starting at 30 ◦ C. However, in this case at 95 ◦ C the TGA curve levels off until approximately 110 ◦ C at which point the material rapidly decomposes. Analysis of the reaction products below 95 ◦ C shows only the formation of Hofmann elimination products (isobutylene and trimethylamine). Deuterium scrambling is observed immediately at 30 ◦ C with the mass spectrum of the trimethylamine showing a similar isotopic scrambling pattern as seen before. However, above 100 ◦ C the appearance of isobutyldimethylamine is observed in the mass spectrum, which can only be produced via nucleophilic attack of the deuteroxide ion on the methyl group of the ammonium cation, or by further reaction of the nitrogen ylide species with D2 O, which are indistinguishable from each other. In both the TMA and
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Scheme 2. Thermal decomposition of PTMA deuteroxide.
isobutyldimethylamine evolved gases, the majority species consist of heavier mass isotopomers from deuterium scrambling (Fig. 8). When the decomposition was performed at such a heating rate (0.15 ◦ C min−1 ) that the material completely decomposed before reaching 100 ◦ C the only reaction products were isobutylene and trimethylamine. However, if the reaction was performed isothermally by ramping the TGA from 30 to 120 ◦ C immediately and holding at 120 ◦ C the material decomposes in 10 min and the formation of isobutyldimethylamine is again observed in the mass spectrum (along with trimethylamine and isobutylene). The TGA curves for these runs are shown in Fig. 9. To gain further insight, 5 was decomposed isothermally at 120 ◦ C in a vial and the headspace was analyzed by GC-MS. The reaction products were separated by GC and MS confirmed the identity of each. The gas chromatogram is shown in Fig. 10. As expected, the only observed products were those from Hofmann elimination and SN 2 attack/ylide decomposition (MeOD, dimethyl ether (DME), TMA, isobutylene, and isobutyldimethylamine).
Because of the low barrier to nitrogen ylide formation, deuterium scrambling was observed in each of the individual decomposition products. Closer examination of the isobutyldimethylamine mass spectrum reveals a characteristic distribution of heavier isomers (Fig. 8). Furthermore, this deuterium isotopic scrambling pattern is observed in each of the daughter fragmentation species arising from methyl group loss. Fragmentation of isobutyldimethylamine will also produce an iminium ion [((CH3 )2 N CH2 )]+ , which has a molecular ion peak at 58 amu [29]. One would expect this species to have a distribution of deuterium ions incorporated and thus, it is impossible to distinguish this species from TMA produced through Hofmann elimination using mass spectrometry without the aid of gas chromatography to separate these products. The combined results from the thermal decomposition of 5 described above give a clear view of the mechanism for decomposition. Referring to Scheme 3 it was proposed that 3 pathways were viable for the decomposition of 5. It is clear that the only observed decomposition products are TMA and isobutylene when
Fig. 6. (a) Mass spectrum of the trimethylamine produced in the ramped decomposition of [n-PrNMe3 ][OD]·xD2 O. (b) Evolution of the IR spectra of evolved gases from 3 as a function of time.
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Fig. 7. Thermal curve of the decomposition of [i BuNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.25 ◦ C min−1 .
the decomposition is carried out such that the temperature does not reach 90–95 ◦ C before 5 is completely decomposed. Above this temperature, the formation of isobutyldimethylamine, dimethylether and methanol indicate that the SN 2 nucleophilic attack of the deuteroxide ion on the ammonium cation can now compete with Hofmann elimination. These results are not surprising, and what one would expect as the steric hindrance around the -hydrogen is increased effectively increasing the activation barrier to Hofmann elimination. 3.4. Decomposition studies of [NpNMe3 ][OD]·xD2 O (7) Completely removing all -hydrogens and replacing with methyl groups, as in 7, was the final step for this study. Scheme 4 illustrates the expected thermal decomposition pathway for 7,
Fig. 8. Mass spectrum of the TMA (top) and isobutyldimethylamine (bottom) produced in the ramped decomposition of [i BuNMe3 ][OD]·xD2 O.
Scheme 3. Thermal decomposition of ITMA deuteroxide.
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Fig. 9. (a) Thermal curve of the decomposition of [i BuNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.15 ◦ C min−1 . (b) isothermal decomposition at 120 ◦ C.
which one would expect only SN 2 attack is the viable decomposition pathway due to the absence of -hydrogen atoms. The thermal decomposition of 7 was carried out at a heating rate of 0.15 ◦ C min−1 and the TGA curve is shown in Fig. 11. This material shows a gradual decomposition beginning at 30 ◦ C with a dramatic mass loss observed above ∼65 ◦ C resulting in complete decomposition by 77 ◦ C. Deuterium scrambling is observed immediately at 30 ◦ C with the FTIR spectrum of the evolved gases showing C–D stretches similar to those previously observed. The reaction products observed were methanol, dimethyl ether, and neopentyldimethylamine, which can only be produced via nucleophilic attack of the deuteroxide ion on the methyl group of the ammonium cation, or by direct further decomposition from the nitrogen ylide species, which are indistinguishable from each other.
To gain further insight 7 was decomposed isothermally at 120 ◦ C in a vial and the headspace was analyzed by GC–MS. The reaction products were separated by GC and MS confirmed the identity of each. The gas chromatogram is shown in Fig. 12. In addition to the expected products (MeOD, DME, neopentyldimethylamine), the appearance of three minor products was also observed (labeled 1, 2, and 3 (Fig. 12)). Analysis of the mass spectrum of each of these individual components showed molecular ions consistent with products having molecular formulas of C5 H10 O (peak 1) and C6 H12 N (peaks 2 and 3). The mass spectrum of peak 1 is identical to that of neopentanal. Despite analyzing a number of amine standards, the identity of the minor species of peaks 2 and 3 could not be determined. We are currently investigating the identity and source of these very minor degradation products to determine
Scheme 4. Thermal decomposition of NTMA deuteroxide.
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Fig. 10. GC trace for the isothermal decomposition of [i BuNMe3 ][OD]·xD2 O at 120 ◦ C.
what implications they may have on neopentyltrialkylammonium cation stability and results will be reported in due course. Because of the low barrier to nitrogen ylide formation, deuterium scrambling was observed in each of the individual decomposition products. A close look at the neopentyldimethylamine mass spectrum reveals a characteristic distribution of heavier isomers with this isotopic scrambling pattern observed in each of the daughter fragments, including the iminium ion [((CH3 )2 N CH2 )]+ , which is also observed in the decomposition of 5. These combined results for the thermal decomposition of [NpNMe3 ][OD]·xD2 O show that the major decomposition productions arise from SN 2 attack/ylide reaction. Quite unexpected was the formation of the other three minor reaction products of which one was identified as neopentanal. The identity of the other 2 minor products could not be determined. None of these products can be explained by any of the decomposition pathways highlighted
Fig. 11. Thermal curve of the decomposition of [NpNMe3 ][OD]·xD2 O starting at 30 ◦ C with a heating rate of 0.15 ◦ C min−1 .
in Scheme 4, which illustrates that further experimentation must be conducted on the stability of tetraalkylammonium and other cations used for alkaline fuel cell membranes if all decomposition pathways are to be understood. 4. Summary With these results in hand, a comparison of decomposition mechanisms between the different alkyltrimethylammonium cations can be made. Looking at the change in mass with changing temperature as a function of temperature for each of the cations, the decompositions of each alkyltrimethylammonium can be directly compared because they were decomposed at a standard heating rate (0.15 ◦ C min−1 ). This plot is shown in Fig. 13. Of note, the ETMA deuteroxide displays peak decomposition at approximately
Fig. 12. GC trace for the isothermal decomposition of [NpNMe3 ][OD]·xD2 O at 120 ◦ C.
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Table 2 Summary of the degradation products for the alkyltrimethylammonium cations studied. Cation
Ylide scrambling
Hofmann elimination
Ylide/SN 2 decomposition
Peak degradation temperature (◦ C)
G‡ Hofmann barrier (kcal/mol)
G‡ SN 2 barrier (kcal/mol)
Yes
Yes
No
60
17.5
24.7
Yes
Yes
No
79
22.9
25.9
Yes
Yes
Yes
74
21.7
25.7
Yes
NA
Yes
74
NA
25.0
5. Conclusions We have extended our previous work in which we examined the decomposition of tetramethylammonium ion to the degradation of tetralkylammonium cations that may be more relevant to the functional groups used on polymer membranes for alkaline fuel cells. The overall picture that emerges from these studies is that Hofmann elimination is the preferred decomposition pathway for ammonium cations bearing -hydrogens, and ylide formation scrambles protons from water (or D2 O) into the trimethylamine that is formed with a low activation barrier. Blocking the -hydrogen positions by successively adding methyl groups increases the stability of the cation, but in the case of [i-BuNMe3 ][OD]·xD2 O, a second decomposition pathway is observed (nucleophilic attack or ylide decomposition). Completely blocking the -hydrogen position, as in the case of [NpNMe3 ][OD]·xD2 O, results in decomposition occurring almost exclusively through SN 2 attack/ylide reaction, however, further reaction products were observed that have not yet been identified. These results represent the extreme conditions of potential fuel cell operation (effectively zero percent relative humidity). Further studies will focus on decomposition of these materials under controlled humidity conditions. Fig. 13. Change in mass over the change in temperature as a function of temperature for ETMA, PTMA, ITMA, and NTMA deuteroxide decompositions at a heating rate of 0.15 ◦ C.
60 ◦ C while the PTMA, ITMA, and NTMA deuteroxides show peak decomposition temperatures at 79, 74, and 74 ◦ C, respectively. The presence of more -hydrogens in the ETMA cation in addition to the decreased steric bulk for Hofmann elimination can explain its relatively low peak decomposition temperature. Increasing steric bulk by adding adjacent methyl groups (and also decreasing the number of -hydrogens) results in increased cation stability as highlighted by the increase in peak decomposition temperature. Interestingly, NTMA (bearing no -hydrogens) shows a peak decomposition temperature similar to the PTMA and ITMA cations indicating that SN 2 attack of hydroxide on the methyl groups (or ylide decomposition) for NTMA is of similar activation energy to Hofmann elimination for PTMA and ITMA. These results are consistent with the trends observed in density functional theory (DFT) calculations investigating the degradation pathways of these cations under fully hydrated conditions [30]. Table 2 summarizes the data from this report and contains preliminary DFT calculated G‡ values for Hofmann elimination and SN 2 attack pathways of the cations studied in which a detailed full report will be reported in due course [30].
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