ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2014, том 48, № 6, с. 639-644
УДК 66.081
PHOSPHONIUM-BASED IONIC LIQUIDS GRAFTED ONTO SILICA
FOR CO2 SORPTION
© 2014 J. M. Zhu",b, F. Xinb, Y. C. Sunb, X. C. Dongb
aThe Key Laboratory of Coal-based CO2 Capture and Geological Storage, Jiangsu Province(CUMT),
Xuzhou, Jiangsu 221008, P.R. China bSchool of Chemical Engineering and Technology, China University of Mining & Technology,
Xuzhou, Jiangsu 221116, P.R. China zhujiamei@hotmail.com Received 19.02.2013
Phosphonium-based ionic liquids with varied alkyl chain lengths bearing different anions (Cl , BF4 and
PFg-) were prepared and grafted on the commercial silica surface. The modified particles were characterized via FTIR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and CO2 sorption/desorption. The effects of the type and activation temperature of silica support, the mole fraction and structure of the grafted ionic liquids on CO2 sorption were discussed in detail. The results indicated that CO2 sorption capacity of the sorbents had little impact on the chain length of the cation, while mainly depended on the anion types. The relationship between the initial mole fraction of ionic liquids and CO2 sorption properties had a non-linear character. Phosponium-cation hexafluorophosphate grafted on type A SiO2 activated under 426 K at ionic liquids/SiO2 feed ratio of 1/1 showed the highest CO2 sorption capacity (6.35 wt %) at 273 K and 100 kPa.
Keywords: ionic liquids, silica, CO2 sorption, CO2 capture.
DOI: 10.7868/S0040357114060153
INTRODUCTION
There is a high demand on the reduction of greenhouse gases across the world. At present, post combustion CO2 capture using aqueous amine-based systems has been considered as the most practical candidates due to the high reactivity and low absorbent cost [1]. However, such amine-based systems have significant drawbacks including insufficient carbon dioxide capture capacity, equipment corrosion and high energy consumption during regeneration [2]. Nowadays, ionic liquids (ILs) are regarded as alternative or next-generation CO2-selective separation media. Nitrogen-based cation, in particular N-methylimidazolium, has been the subject of many of the publications in the field [3, 4]. Phosphonium cation-based ILs are also available and more attractive for higher thermal and chemical stability and they are usually much cheaper than imidazolium-based ionic liquids. Recently, researchers have developed a class of stable phosphonium ILs which possess high CO2 absorption capacity [5—7].
An important drawback for much discussed in the case of common ILs is their quite high viscosity resulting in mass transfer limitations. Moreover, this is unattractive from an economical point of view due to the high cost of most ILs. To overcome these problems,
the immobilized ILs on supports would be effective because of the highly developed porous structure of supports [8, 9]. The combination of the advantage of ILs with those of supports, usually silica or other inorganic and organic polymers is of particular interest. Silica is the most common substance on earth and shows a large specific surface area, so ILs supported on silica surface have lately attracted much attention in CO2 capture. Zhang [10] reported that a series of ami-no-acids based and dual amino-functionalized phos-phonium ionic liquids supported on porous silica gel presented very optimistic results in higher CO2 sorption efficiency. Therefore, immobilizing ILs on porous support is believed to be a promising strategy for CO2 capture.
The sorbents were usually prepared by deposition of ILs onto the silica, in which no covalent bonds are formed and there is a problem of losing ionic liquids in process. Another problem is a certain amount of the pores may be blocked by ILs resulting in decreasing specific surface area. Ren [11] synthesized several N-(3-aminopropyl)aminoethyl tributylphosphoni-um amino acid salts supported on silica and found that under the desorption condition the weight loss of the ILs increased linearly with time and reached 15 and
EtO4
/Si' EtO I OEt
silica
Cl +
R. .R d , EtO\
p^ dry toluene V*"
I 12o°C EtO^ I
R OEt
"O\ ,
/Si
-O^ I
OEt
R = C4H9 or C8H17 X = BF4 or PF6
^(RhX"^
silica
4P+(R)3Cl- + SiO2
■O\ ,
/Si
-O I
OEt
~P+(R)3Cl-
Fig. 1. Scheme for the synthesis procedure of immobilized phosphonium-based ionic liquids via grafting method on silica gel.
7.4% in 96 h for Sorb-Lys and Sorb-Gly, respectively. The results also showed that with increasing the ILs/silica ratio the nanoscale pores of immobilization silica diminished gradually, which leading to decrease the sorption capacity. To improve the material recycling, covalent bonding is formed between modified reagent and silanol groups on support surfaces via grafting methods [12—14]. This chemical force (anchored mediate) is obviously stronger than the physical adsorption. Moreover, the ILs mainly spread on the surface of the microporous particles and the porous structure of support can be maintained. The ILs grafted supports have been proved to create active and stable catalysts [15—17], but these materials, especially solid phosphonium-based sorbent, have been tested for CO2 capture to a less extent.
Hence, the modification of silica grafted with phosphonium-based ILs was synthesized and the CO2 sorption capacity was investigated. The objective of this work seeks to develop a comprehensive understanding of the factors that affect the CO2 separation.
EXPERIMENTAL
Chemicals. Tri-n-octylphosphine (97%) and tri-n-butylphosphine (99%) were purchased from Stream Chemicals. 3-Chloropropyltriethoxysilane (97%) was provided by TCI Shanghai. Sodium tetrafluoroborate, potassium hexafluorophosphate, toluene (dried through Na), acetone and ethyl ether were of analytical grade and were produced from Chemical Reagent Co. of Shanghai, China. These chemicals were used as received. Two kinds of porous SiO2 (0.5—1.5 mm in particle size), type A with an average pore size 2.0—3.0 nm and type B with 4.5—7.0 nm, were purchased from Qingdao Silica Gel Factory, and were pretreated at 423 or 773 K for 3.5 h.
Measurements. The IR spectra were obtained by using a Thermo Scientific Nicolet 380 infrared spectrometer. The morphologies were examined on S-30000N scanning electron microscope. Thermogravimetric analy-
sis (TGA) was performed on a NETZSCH-STA409C under N2 atmosphere with a heating rate of 10° C/min.
The adsorption isotherm of CO2 was measured by Quantachrome Autosorb-1-MP, from which CO2 adsorption capacity was obtained. The porous particles were dried and degassed at 373 K under vacuum for 3 h to remove moisture or other volatile contaminants. CO2 was introduced into the chamber as adsorbate under the pressure of1.33 |Pa to 100 kPa at 273 K. DFT analysis was used to calculate the pore size distribution (PSD) and the pore structure parameter.
Synthesis Methods. The preparation of the phospho-nium ILs grafted on silica gel was carried out according to the literature [18] in a three-step reaction, as illustrated in Fig. 1. Firstly, anhydrous toluene (80 mL) and 3-chloropropyltriethoxysilane (12 g, 0.05 mol) were added to a round bottom Schlenk flask flushed with nitrogen. Tri-n-octylphosphine or tri-n-bu-tylphosphine was then added via syringe. The reaction mixture was stirred at 383 K for 40 h. After that solvent was evaporated to form the phosphonium ionic liquid (chloride form) and product was dried at 353 K under vacuum for 5 h. Then the activated silica gel (1 g) was added to anhydrous toluene (50 mL) solution containing prepared grafting agent under a nitrogen atmosphere. The reaction mixture was kept at 363 K for 24 h under stirring. The solid was collected by filter and extracted by Soxhlet extraction using acetone for 24 h. The product was further dried at 353 K under vacuum for 5 h. The weight ratio of immobilized phosphonium ILs to silica support (ILs/SiO2) was controlled in the range from 0.5/1 to 1/1 by adjusting the feed ratio. Finally, grafted silica (1 g) and sodium tetrafluoroborate (2.35 g, 0.022 mol) or potassium hexafluorophosphate (4.05 g, 0.022 mol) were dispersed in acetonitrile and stirred at room temperature for 35 h. The solid was washed with acetonitrile and distilled water. The final product was dried at 353 K for 5 h under vacuum conditions.
IR (vmax, cm-1) of sample PH-9 was as follows: 2955, 2920, 2850 cm-1 (C-H, aliphatic); 1377 cm-1 (P-C); 1120, 1070 cm-1 (Si-O-Si); 1084 cm-1 (B-F).
I I I I I I I I I I I I I I I I I I I I I I
(c) x900 50 цш (d) x900 50 цш
Fig. 2. Scanning electron micrographs of silica-immobilized phosphonium ionic liquids: (a) sample PH-6; (b) sample PH-8; (c) sample PH-9; (d) sample PH-10.
IR (vmax, cm 1) of sample PH-10 was as follows: 2918, 2853 cm-1 (C-H, aliphatic); 1377 cm-1 (P-C); 1130, 1082 cm-1 (Si-O-Si); 851 cm-1 (P-F).
RESULTS AND DISCUSSION
SEM Analysis. The scanning electron micrographs (SEMs) of the internal morphology after grafting are shown in Fig. 2. It can be clearly seen that the porous morphology of ILs-SiO2 was basically maintained after the phosphonium-based ILs immobilization. However, the sample PH-8 prepared with type B SiO2 had less developed porous morphology, while other samples prepared with type A SiO2 had well developed pore structure.
Thermogravimetric Analysis. The weight-loss curves of blank silica and modified sample PH-9 are shown in Fig. 3. For the unmodified silica, mass loss is attributed exclusively to the loss of adsorbed water molecules as well as the condensation of silanol groups [19]. Two main steps were also observed on the TG curves of the modified materials. The first step takes place in the temperature range 298-473 K and the second one in the temperature range 473-773 K. The mass loss up to 473 K corresponds to liberation of pre-adsorbed water and gas. The second step of thermal decomposition in the te
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