Supported Absorption of CO2 by Tetrabutylphosphonium

DOI: 10.1002/chem.200501015 Supported Absorption of CO2 by Tetrabutylphosphonium Amino Acid Ionic Liquids Jianmin Zhang,[a, b] Suojiang Zhang,*[a] Kun Dong,[a] Yanqiang Zhang,[a] Youqing Shen,[b] and Xingmei Lv[a] Introduction In view of sustainable development and green chemistry, in- vestigation of efficient methods for capturing CO2 from flue gas, produced by combustion of fossil fuels in which CO2 concentration varies from 3 to 14%, is significant as it con- cerns both the conversion of CO2 as C1 feedstocks into in- dustrially useful compounds and environmental issues, such as the reduction of the greenhouse effect. During the past two decades, the chemical fixation and conversion of CO2 have been attracting increasing interest. One of the most widely utilized processes for CO2 recovery is chemical ab- sorption of CO2 by aqueous amines to form carbamates. [1–6] However, this process has many disadvantages: 1) The uptake of water into the gas stream requires an additional drying process. 2) The loss of volatile amines and the evapo- ration of water make the process energy and cost intensive. 3) The desorption of CO2 by heating causes serious corro- sion and other operational difficulties. Ionic liquids are novel media composed of ions. The negli- gible volatility of these liquids results in a noncontaminated target gas, making them especially attractive for the absorp- tion of gases. For example, Brennecke and co-workers stud- ied CO2 solubility in imidazolium-based ionic liquids [7–9] under high pressures. The high CO2 solubility of ionic liq- uids when compared to methane provides them with the po- tential to be utilized in the separation of CO2 from natural gas. The interaction between CO2 and imidazolium-type ionic liquids, attributed to the activity of H-2 in the imidazo- lium ring, accounts for its higher solubility of CO2. [10–11] Due to the unique “self-designable” character of ionic liquids, al- kaline groups, such as -NH2, can be attached to ionic liquids to give a zwitterion after absorption of CO2. [12] This kind of “task specific ionic liquid” can help to overcome the limita- tions of aqueous amine. As another kind of useful adsorb- ent, the amidine structure was applied in the reversible ab- sorption of CO2. [13] The absorption of CO2 can transform the substances that contain an amine group from molecular compounds to inner zwitterions. Amino acids have been used as both anions and cations in ionic liquids. Shan and co-workers[14] reacted amino acids (AA) with equimolar amounts of H2SO4 and obtained [AAH]ACHTUNGTRENNUNG[HSO4]-type ionic liquids. If the amount of H2SO4 Abstract: A new type of “task specific ionic liquid”, tetrabutylphosphonium amino acid [P(C4)4][AA], was synthe- sized by the reaction of tetrabutylphos- phonium hydroxide [P(C4)4][OH] with amino acids, including glycine, l-ala- nine, l-b-alanine, l-serine, and l-lysine. The liquids produced were character- ized by NMR, IR spectroscopies, and elemental analysis, and their thermal decomposition temperature, glass tran- sition temperature, electrical conduc- tivity, density, and viscosity were re- corded in detail. The [P(C4)4][AA] sup- ported on porous silica gel effected fast and reversible CO2 absorption when compared with bubbling CO2 into the bulk of the ionic liquid. No changes in absorption capacity and kinetics were found after four cycles of absorption/ desorption. The CO2 absorption capaci- ty at equilibrium was 50 mol% of the ionic liquids. In the presence of water (1 wt%), the ionic liquids could absorb equimolar amounts of CO2. The CO2 absorption mechanisms of the ionic liq- uids with and without water were dif- ferent. Keywords: absorption · amino acids · carbon dioxide fixation · ionic liquids · SiO2 [a] Dr. J. Zhang, Prof. Dr. S. Zhang, K. Dong, Y. Zhang, Dr. X. Lv Research Laboratory of Green Chemical Engineering and Technology, Institute of Process Engineering Chinese Academy of Sciences, PO Box 353 Beijing 100080 (P. R. China) Fax: (+86)10-8262-7080 E-mail : sjzhang@home.ipe.ac.cn [b] Dr. J. Zhang, Prof. Dr. Y. Shen Chemical and Petroleum Engineering Department College of Engineering, University of Wyoming 1000 E. University Ave. Laramie, WY 82071 (USA) Chem. Eur. J. 2006, 12, 4021 – 4026 K 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 4021 FULL PAPER was decreased by a half, [AAH]2ACHTUNGTRENNUNG[SO4]-type ionic liquids were obtained. This type of ionic liquid can not effectively absorb CO2. Recently, ionic liquids with amino acids as anions were synthesized by neutralization between [emim] [OH] and amino acids.[16] Optimistically, the H+ in the new ionic liquid have been removed, resulting in an active -NH2 group, which would be promising for CO2 absorption. Phosphonium [PR1R2R3R4]+ is another type of cation used for room temperature ionic liquids (RTILs).[17–21] Phos- phonium is more stable in a basic environment at high tem- peratures than nitrogen-containing cations, such as imidazo- lium.[22] In this work, a series of phosphonium ionic liquids with amino acids as anions [P(C4)4][AA] were synthesized. The ionic liquids were further supported on porous silica gel and their CO2 absorption was investigated. The rates of CO2 absorption of the supported ionic liquids were much higher than those of the viscous ionic liquids themselves. Results and Discussion The glass transition temperature and melting point are im- portant physical properties of ionic liquids, and are deter- mined by structural features of the anion and cation, such as hydrogen bonding, dipoles, and the distance between them. From Table 1, it can be seen that there is an order of Tg,[P(C4)4][Ser]>Tg,[P(C4)4][Lys]>Tg,[P(C4)4][Gly]>Tg,[P(C4)4][Ala]> Tg,[P(C4)4][b-Ala] . It is the distance between the anion and cation that plays the most important role in the order of [P(C4)4]- ACHTUNGTRENNUNG[Gly]> [P(C4)4]ACHTUNGTRENNUNG[Ala]> [P(C4)4]ACHTUNGTRENNUNG[b-Ala]. A shorter distance between the anion and cation leads to a higher attracting ability and a higher glass transition temperature. In the structure of [P(C4)4]ACHTUNGTRENNUNG[Ser], there is one additional OH com- pared to [P(C4)4] ACHTUNGTRENNUNG[Ala]; therefore, it can be surmised that the additional hydrogen bond contributed to its higher glass transition temperature. This is in accord with Tg,[emim][Ser]> Tg,[emim][Ala] , reported earlier. [16] As for [P(C4)4] ACHTUNGTRENNUNG[Lys], there are two -NH2 groups in its anion structure. Although [Lys] � is a larger anion compared to other amino acid anions, the hydrogen bond plays an important role in determining its melting point. The fact that all the glass transition tempera- tures of the phosphonium amino acids are lower than the [emim][AA]-type maybe due to the larger volume of the phosphonium cations. From Table 1 it can be seen that the density of phosphoni- um amino acids follows a similar order to the glass transi- tion temperature, that is, 1[P(C4)4][Ser]>1[P(C4)4][Lys]>1[P(C4)4] [Gly]>1[P(C4)4][b-Ala]>1[P(C4)4][Ala] , suggesting that the micro in- teraction is determined by the same mechanism. The viscosi- ty of the ionic liquids is strongly affected by the side alkyl chains of the amino acids anions. The anions with more complex structures, such as [Lys]� and [Ser]� , have a greater viscosity. The electric conductivity is determined by viscosi- ty. The larger the viscosity, the smaller the electric conduc- tivity. The absorption of CO2 by [P(C4)4]ACHTUNGTRENNUNG[b-Ala]-SiO2, [P(C4)4]- ACHTUNGTRENNUNG[Gly]-SiO2, and [P(C4)4]ACHTUNGTRENNUNG[Ala]-SiO2 were investigated and the results are presented in Figure 1. This data suggests that the absorption equilibriums can all be reached in less than 100 min. The absorbed CO2 was released in a vacuum at room temperature over several hours. Due to the large sur- face area of silica gel (approximately 500 m2g�1), the ab- sorption rate of CO2 was significantly increased. Four cycles of absorption/desorption were repeated and no changes in the absorption capacity or the rates were observed. Accord- ing to the mechanism proposed by James,[12] one mol of NH2- can absorb half one mol of CO2. In Figure 1, the great- er than one half mol absorption of CO2 may be a result of physical absorption. We also observed that the mass of the sample of [P(C4)4][AA]-SiO2 saturated by CO2 slightly but continuously decreased at room temperature and atmos- pheric pressure. The IR spectra of [P(C4)4]ACHTUNGTRENNUNG[b-Ala], [P(C4)4]ACHTUNGTRENNUNG[Gly], and [P(C4)4]ACHTUNGTRENNUNG[Ala] contain only one peak at approximately n˜= 1600 cm�1, corresponding to the CO2 � group in the AA anion structure (Figure 2). Upon exposure to CO2, a new peak appeared at approximately n˜=1660 cm�1, correspond- ing to the formation of a new CO2H group (Figure 3). The - NH- absorption peak was over- lapped with that of SiO2 at n˜= 3300 cm�1. Thus, pure [P(C4)4]- ACHTUNGTRENNUNG[b-Ala] was characterized by IR before and after absorption of CO2 to increase the clarity of the peak change from NH2- to - NH-. In addition, there was a new peak at n˜=1697 cm�1 Table 1. Properties of the phosphonium amino acid ionic liquids. [P(C4)4] ACHTUNGTRENNUNG[Lys] [P(C4)4] ACHTUNGTRENNUNG[Ser] [P(C4)4] ACHTUNGTRENNUNG[Gly] [P(C4)4] ACHTUNGTRENNUNG[Ala] [P(C4)4] ACHTUNGTRENNUNG[b-Ala] Tg [K] 208.01 211.70 198.33 197.66 196.14 Td-N2 [K] 498 493 473 475 476 1 [g cm�3] 0.9730 0.9910 0.9630 0.9500 0.9590 h [mPas] 744.71 734.20 232.85 226.69 244.71 s [10�4 s cm�1] 1.04 1.68 4.85 4.18 4.04 Figure 1. Cycles of CO2 absorption of [P(C4)4][AA]-SiO2. &= [P(C4)4]- ACHTUNGTRENNUNG[Gly]-SiO2, *= [P(C4)4] ACHTUNGTRENNUNG[Ala]-SiO2, and ~= [P(C4)4] ACHTUNGTRENNUNG[b-Ala]-SiO2. www.chemeurj.org K 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 4021 – 40264022 (Figure 4) and a N�H stretch at n˜=3234 cm�1 after absorp- tion of CO2, but the peak at n˜=3359 cm �1 disappeared, indi- cating that this N�H bond may react with a CO2 molecule. In addition, as the resonance of CO2 � in the amino acid anion of the above three ionic liquids is at approximately d=157 ppm in the 13C NMR spectrum (400 MHz, DMSO), a new resonance for CO2H does not appear. However, the peak area at d=157 ppm increases in size after absorption of CO2. Absorption of CO2 by using these ionic liquids with a small amount of water (1%, mass fraction) was also investi- gated. The viscosity was low and the liquid was stirred with greater ease at the beginning of the absorption, but the vis- cosity gradually increased and finally became very high and the transparent liquid became cloudy. In Figure 5, it can be seen that [P(C4)4]ACHTUNGTRENNUNG[Gly] with water can absorb almost 13 wt% of CO2, which is close to the theoretical absorption capacity (13.52%, based on a 1:1 mol ratio between the ionic liquid and CO2), in 200 mins. If more water is added, the ionic liquid saturated with CO2 separates into a solid phase and a liquid phase. The residue liquid finally ob- tained, after drying in vacuum at 353.15 K for 12 h, is more viscous than pure [P(C4)4]ACHTUNGTRENNUNG[Gly]. There was even trace of a white powder at the bottom of the glass container. As can be seen in Figure 6, the IR spectrum of CO2 saturated 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 1 2 3 Figure 2. IR spectrum of ionic liquids supported on porous SiO2 before absorption of CO2. 1) [P(C4)4] ACHTUNGTRENNUNG[Ala]-SiO2, 2) [P(C4)4] ACHTUNGTRENNUNG[Gly]-SiO2, and 3) [P(C4)4] ACHTUNGTRENNUNG[b-Ala]-SiO2. 1000 1500 2000 2500 3000 3500 4000 3 2 1 1594 1582 1578 1658 1662 1658 Figure 3. IR spectrum of ionic liquids supported on porous SiO2 after ab- sorption of CO2. 1) [P(C4)4] ACHTUNGTRENNUNG[Ala]-SiO2, 2) [P(C4)4] ACHTUNGTRENNUNG[Gly]-SiO2, and 3) [P(C4)4] ACHTUNGTRENNUNG[b-Ala]-SiO2. a b 1593 1697 3234 3275 3359 500 1000 1500 2000 2500 3000 3500 4000 4500 Figure 4. IR spectra of [P(C4)4] ACHTUNGTRENNUNG[b-Ala] a) before and b) after absorption of CO2. Figure 5. CO2 absorption of [P(C4)4] ACHTUNGTRENNUNG[Gly] with water (1% mass fraction). 500 1000 1500 2000 2500 3000 3500 4000 4500 b a Figure 6. IR spectrum of [P(C4)4] ACHTUNGTRENNUNG[Gly] after absorption of CO2 a) with and b) without water. Chem. Eur. J. 2006, 12, 4021 – 4026 K 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemeurj.org 4023 FULL PAPERSupported Absorption of CO2 [P(C4)4]ACHTUNGTRENNUNG[Gly] (with small amount of water) does not contain a peak at approximately 1690 cm�1, suggesting that the ab- sorption mechanism is different. In the 13C NMR (400 MHz, DMSO) spectrum of [P(C4)4]ACHTUNGTRENNUNG[Gly] (with small amount of water) after absorption, there is a new resonance at d= 175.422 (markedly peaked, greater area) and 171.468 ppm (small peak and area), representing [HCO3] � and [CO3] 2�, respectively, in addition to the resonance at d= 157.588 ppm, corresponding to the original CO2 � group in the amino acid anion. As for the micro absorption mechanism, it may be compli- cated with or without water. Possible mechanisms for the absorption processes for which the ionic liquids do not con- tain water are shown in Schemes 1 and 2. In the first mecha- nism (Scheme 1), CO2 attacks the free electron pair of the N atom on the -NH2 group and forms a hydrogen bond O� H···N with the NH2 group of another AA � . A second possi- bility is that one of the H atoms in the NH2 group is taken as a proton by the original CO2 � to form a new CO2H group (Scheme 2). A different mechanism is proposed for absorption proc- esses that used ionic liquids containing a small amount of water (Scheme 3, this mechanism is the same as that for the absorption of CO2 by aqueous amines). When this mixture, saturated with CO2, is heated in vacuum, another reaction may occur (Scheme 4) in which 2 ACHTUNGTRENNUNG[HCO3] � decomposes into 1H2O, 1CO2, and [CO3] 2�, possibly forming [P(C4)4]2ACHTUNGTRENNUNG[CO3]. Conclusion [P(C4)4]ACHTUNGTRENNUNG[Ser], [P(C4)4]ACHTUNGTRENNUNG[Gly], [P(C4)4]ACHTUNGTRENNUNG[Ala], [P(C4)4] ACHTUNGTRENNUNG[b-Ala], and [P(C4)4]ACHTUNGTRENNUNG[Lys] were synthesized from [P(C4)4][Br] and the corresponding amino acids by the use of anion exchange resin and neutralization. The CO2 absorption of the ionic liquids supported on porous SiO2 is fast and reversible with a capacity of 1CO2/2 [P(C4)4][AA]. The proposed mecha- nism suggests that a CO2 molecule attacks the N atom of the NH2 group and results in -NHCO2 � formation, during which one H atom leaves and forms a new CO2H with the NHCO2 � or the original CO2 � in the amino acid anion. The CO2H group formed a hydrogen bond with the electron pair of NH2 in another amino acid anion for which the N atom was inert to reaction with CO2. In the presence of water (1%, mass), the [P(C4)4][AA] can absorb an equimolar amount of CO2 by a different mechanism. Experimental Section Glycine, l-serine, l-alanine, l-b-alanine, l-lysine, ethanol, and anion ex- change resin �711(Cl) were of analytical grade and were produced by the Beijing Chemical Reagent Plant. Tetrabutylphosphonium bromide solution was provided by CYTEC CANADA INC. All aqueous solutions were prepared with deionized water. Anion exchange resin-711(Cl) was pretreated by hydrochloric acid (2m) before use. Then, this resin was transformed from Cl-type into OH-type by passing NaOH solution (5m, 10 mLmin�1) through the resin column (l=100 cm, r=3 cm) until Cl� could not be detected with AgNO3/HNO3 solution. As the anion ex- change resin (OH) is not stable at temperatures higher than 313.15 K, NaOH solution must be used after it is cooled. Excessive NaOH solution was washed by deionic water. Tetrabutylphosphoium bromide solution was diluted (2m) and then transformed into tetrabutylphosphoium hy- droxy solution, which was concentrated through rotation evaporation at 323.15 K (pure tetrabutylphosphoium hydroxy is not stable and its color may change from colorless to black after several days). The concentra- tion of OH� was determined by titration with HCl solution. Then, the tetrabutylphosphoium hydroxy solution reacted with a slight excess of amino acid (without further purification) solution through neutralization at room temperature for 24 h. After being dried at 323.15 K under vacuum, ethanol was added to the residue and the solution was agitated completely so that the excess amino acids were deposited. After filtra- tion, the ethanol was removed by evaporation. The liquid product ob- tained was dried at 373.15 K under vacuum for 2 d. 1H NMR, 13C, 31P, and IR spectroscopies were performed to determine the structure of the phosphonium amino acids. Due to the activity of the hydrogen atoms in - NH2, its peak could not be easily observed by 1H NMR spectroscopy; however, IR spectroscopic data indicated the existence of -NH2. Further elemental analysis of C, H, N, and P (Elementar Vario EL, Germany) in- dicated that the elemental ratio of phosphonium amino acids agrees well with their predicted structure. Differential Scanning Calorimetry (DSC 2010, Thermal Analysis, USA) and TG-DTA (Netzsch STA 449, Germa- ny) were performed to determine the melting points (glass transition temperature) and the decomposition temperature. [P(C4)4] ACHTUNGTRENNUNG[Gly]: 1H NMR (400 MHz, DMSO, 25 8C, TMS): d=0.92 (t, J= 8.0 Hz, 12H; CH3), 1.34–1.52 (m, 16H; (CH2)2), 2.13–2.23 (m, 8H; P- CH2), 2.64 ppm (s, 2H; N-CH2-CO2); 13C NMR (400 MHz, DMSO, Scheme 1. First proposed absorption mechanism between [P(C4)4] ACHTUNGTRENNUNG[b-Ala] and CO2. Scheme 2. Second proposed absorption mechanism between [P(C4)4]- [b-Ala] and CO2. Scheme 3. Proposed absorption mechanism between [P(C4)4] ACHTUNGTRENNUNG[Gly]-H2O and CO2. Scheme 4. Proposed CO2-desorption mechanism of [HCO3][P(C4)4]. www.chemeurj.org K 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 4021 – 40264024 S. Zhang et al. 25 8C): d=175.7, 46.9, 23.9, 23.2, 18.0, 17.5, 13.8 ppm; 31P NMR (400 MHz, DMSO, 25 8C): d=33.88 ppm; IR (KBr): n˜=3354, 3276, 3182, 2958, 2931, 2873, 1577, 1465, 1409, 1379, 1341, 1312, 1290, 1237, 1099, 1051, 1037, 1006, 969, 918, 907, 871, 825, 750, 723, 650, 592 cm�1; elemen- tal analysis (%) calcd for: C 64.77, H 11.99, N 4.2, P 9.29; found: C 63.92, H 11.85, N 4.1, P 9.18. [P(C4)4] ACHTUNGTRENNUNG[Ala]: 1H NMR (400 MHz, DMSO, 25 8C, TMS): d=0.92 (t, J= 8.0 Hz, 12H; CH3-C3-P), 0.99 (d, J=4.0 Hz, 3H; CH3-C-N), 1.32–1.52 (m, 16H; (CH2)2), 2.12–2.27 (m, 8H; P-CH2), 2.78 ppm (q, J=10 Hz, 1H; CH); 13C NMR (400 MHz, DMSO, 25 8C): d=177.3, 52.3, 23.8, 23.2, 18.0, 17.5, 13.9 ppm; 31P NMR (400 MHz, DMSO, 25 8C): d=33.92 ppm; IR (KBr): n˜=3353, 3278, 2958, 2932, 2873, 1591, 1465, 1382, 1346, 1315, 1234, 1099, 1072, 1006, 969, 921, 908, 828, 772, 723, 646, 528 cm�1; ele- mental analysis (%) calcd for: C 65.61, H 12.09, N 4.03, P 8.91; found (%): C 65.45, H 12.11, N 4.02, P 8.89. [P(C4)4] ACHTUNGTRENNUNG[Ser]: 1H NMR (400 MHz, DMSO, 25 8C, TMS): d=0.92 (t, J= 8.0 Hz, 12H; CH3), 1.30–1.60 (m, 16H; (CH2)2), 2.12–2.26 (m, 8H; CH2- P), 2.78 (q, J=8.0 Hz, H; CH-O), 3.14 (t, J=12 Hz, H; CH-N), 3.26 ppm (q, J=4.0 Hz, H; CH-O); 13C NMR (400 MHz, DMSO, 25 8C): d=177.1, 66.4, 55.5, 23.9, 17.7, 13.9 ppm; 31P NMR (400 MHz, DMSO, 25 8C): d= 33.85 ppm; IR (KBr): n˜=3354, 3187, 2959, 2932, 2873, 1600, 1465, 1384, 1318, 1233, 1202, 1099, 1046, 1006, 969, 920, 909, 841, 766, 723, 559 cm�1; elemental analysis (%) calcd for: C 62.72, H 11.55, N 3.85, P 8.52; found: C 62.60, H 11.58, N 3.84, P 8.51. [P(C4)4] ACHTUNGTRENNUNG[b-Ala]: 1H NMR (400 MHz, DMSO, 25 8C, TMS): d=0.91 (t, J=8.0 Hz, 12H; CH3), 1.35–1.60 (m, 16H; (CH2)2), 1.86 (t, J=8.0 Hz, 2H; CH2-CO2), 2.15–2.30 (m, 8H; CH2-P), 2.52 ppm (t, J=8.0 Hz, 2H; CH2-N); 13C NMR (400 MHz, DMSO, 25 8C): d=175.4, 42.5, 23.7, 17.6, 13.6 ppm; 31P NMR (400 M