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Effect of the polar modifiers on supercritical extraction efficiency for template removal from hexagonal mesoporous silica materials: Solubility parameter and polarity considerations
Separation and Purification Technology 118 (2013) 120–126
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Effect of the polar modifiers on supercritical extraction efficiency for template removal from hexagonal mesoporous silica materials: Solubility parameter and polarity considerations Zhen Huang a,⇑, Jing-huan Li a, Hui–shan Li b, Hui Miao a, S. Kawi c, A.H. Goh c a b c
Tianjin University of Commerce, Tianjin 300134, PR China Department of Auto Engineering, Military Transportation Academy, Tianjin 300161, PR China Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, 117576 Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 31 January 2013 Received in revised form 25 June 2013 Accepted 29 June 2013 Available online 8 July 2013 Keywords: Supercritical CO2 extraction Hexagonal mesoporous materials Modifier identity Template removal Solubility parameter Polarity
a b s t r a c t The introduction of polar modifiers usually can make supercritical CO2 extraction more attractive in various potential applications. In this work, modified supercritical CO2 extraction of the dodecylamine template has been performed on freshly synthesized mesoporous HMS materials with the use of 10 different polar modifiers. The effects of the polar modifiers on the extraction efficiencies of the amine template and structural properties of resultant HMS materials have been investigated in detail. Among the ten modifiers used, formic and acetic acids, water and methanol give reasonably high extraction efficiencies and the effects of the modifier identity on extraction efficiencies have been discussed in terms of solubility parameter and dipole moment of the modifiers. Methanol seems to be much better than the other modifiers for the SCCO2 extraction of dodecylamine from HMS materials since the resulted materials have well preserved highly ordered mesoporous structure and possessed much large specific surface area and pore volume as reflected by XRD spectra and nitrogen adsorption/desorption results. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Supercritical fluid extraction (SFE) has been exploited for a diverse range of practical applications because supercritical fluids have unique properties intermediate between those of gases and liquids. Because of the gas-like transport properties and liquid-like densities, supercritical fluids are more attractive and thus have extensively been investigated for various porous materials fabrications [1–6] or diverse bioactive species extraction from natural produces [7–10]. In terms of mass transfer, it is known that the diffusivity of the supercritical fluid is about 10–100 times greater than that of liquid and that the viscosity of a supercritical fluid is generally comparable to that of a gas but about 100 times lower than that of a liquid [11]. This means that it is much easier for supercritical fluids to penetrate pores with lesser resistance than for liquid solvents. In terms of the solvent strength, supercritical fluids (0.2–0.9 g/cm3) have not much lower density than liquids (0.6–1.6 g/cm3), thus still possessing appreciated solvating power to the solute [11]. The solvent strength of a supercritical fluid solvent can be adjusted or tuned with temperature or pressure [11] and it may plausibly be quantitatively represented by a commonly applied ⇑ Corresponding author. E-mail address: [email protected] (Z. Huang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.047
concept, i.e., the Hildebrand solubility parameter [12]. The Hildebrand solubility parameter is a measure of the cohesive energy density of a substance and is useful for indicating the solvency behavior of a specific solvent. A role of thumb says that when the solvent–solute solubility parameter difference is within 4 MPa1/2, the solute should be fairly soluble in it. The solubility parameter of a substance, d, at different temperatures and pressures was calculated using the following equation:
d¼
DEv ap 1=2 DHv ap RT 1=2 ¼ V V
ð1Þ
where DEv ap and DHv ap are the molar internal energy and molar enthalpy of vaporization, respectively. V is the molar volume of the substance, R is the universal gas constant, and T is given temperature. Usually, for solid or liquid substances, it could be assumed that the value of d is constant if the temperature range considered is not broad. However, Eq. (1) is not applicable to supercritical fluids since the enthalpy of vaporization is no longer meaningful. Promisingly, Giddings et al. [13] have established an empirical correlation of the solubility parameter for supercritical fluids. The correction can be written in SI units by Marcus [14] as follows.
d ¼ 3:02 Pc1=2 qr
ð2Þ
Z. Huang et al. / Separation and Purification Technology 118 (2013) 120–126
where Pc is in units of MPa, qr is the reduced density (=q/qc) and d has units of MPa1/2. Using Eq. (2) along with equations of state, Marcus [14] estimated the d values at qr = 1 .0 and Tr = 1.2 for more than 30 supercritical fluids. Prof. Singhal and his group have promisingly attempted to relate the solubility parameter of modified SCCO2 to extraction performances [9,10]. Further, Machida et al. [15] have addressed that the controllable solubility parameters of supercritical solvents through varying temperature and/or pressure could give them wide applicability when either used as solvents with/without a modifier or co-solvents. Among a variety of supercritical solvents, supercritical CO2 (SCCO2) is the most widely used because it is environmentally benign, inexpensive, essentially nontoxic, inflammable, and allows supercritical operation at low pressures and near room temperature. However, carbon dioxide is a non-polar molecule so that it is not a particularly good extraction solvent for polar organic compounds, as reflected in our previous studies [16–20]. Therefore, a polar modifier is usually required for SCCO2 to extract polar organic species. The addition of a small amount of a polar modifier could significantly increase the solvating power of supercritical CO2 to targeted organic chemicals. For a mixture consisting of SCCO2 and a modifier, the solubility parameter for the mixed fluid is determined as follows [9,10]:
dmix ¼ uCO2 dCO2 þ um dm
ð3Þ
where u is the volume fraction, and dmix and dm are the solubility parameter of the mixture and that of the modifier, respectively. The u can be estimated from the flow rates of liquefied CO2 and the modifier by assuming the excess volume is equal to zero, which is a good approximation for a small amount of the modifier. Besides enhancing the solvent strength of SCCO2, the polar modifier added may interact with the analyte/matrix complex so as to lower the activation energy of analyte desorption [21], promote the accessibility of remote sites to the extraction fluid and facilitate the transport of the analyte to the bulk fluid [22], and covering the matrix active sites and thereby preventing the re-adsorption of analyte back to those active sites [23]. For these reasons, the introduction of polar modifiers is not only able to dramatically increase the solvent strength of in SCCO2 to polar organic compounds but also can make SCCO2 extraction more attractive transport properties in various potential applications. In this work, we have investigated to the SCCO2 extraction for removing the organic template from freshly synthesized hexagonal mesoporous silica (HMS) with the introduction of 10 different polar modifiers. Adding a polar modifier seems to be a must because in our earlier studies it has been known that pure SCCO2 could not extract any organic templates from freshly prepared siliceous mesoporous materials MCM-41, MCM-48, SBA-1 and SBA-3 powders [16–20], due to the low solubility of polar organic templates in SCCO2. The HMS used was synthesized by using dodecylamine as the templating agent. The effects of these modifiers on dodecylamine removal were analyzed in terms of Hildebrand solubility parameter in detail. The structural properties of processed porous materials were also investigated as well.
2. Experimental 2.1. Materials The sources and purity of all compounds used in the present study are given in Table 1. These materials were used as received without further purification.
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2.2. Synthesis of HMS materials The hydrothermal procedure for synthesizing purely siliceous HMS materials has been described in earlier works [19,20]. Typically, 25.1 g of dodecylamine was weighed in a 1000 mL polypropylene bottle and mixed with 265 mL of absolute ethanol and 266 mL deionized water. The resultant mixture was vigorously stirred for about 15 min to completely dissolve dodecylamine. 106.2 g of tetraethylorthosilicate was then added into the solution under continuous stirring. Agitation was stopped after 5 min and the obtained mixture was allowed to statically react under room temperature of around 22 °C for 18 h. The resultant solid was washed with deionized water and recovered by filtration through a Buchner funnel. After repeating washing and filtrating for about four times, the moist solid was transferred to a ceramic dish and dried at 30–40 °C for 2 days. After drying, the final product was sieved into the desired powder with a particle size between mesh 40 and 60. 2.3. SCCO2 extraction of the template The as-synthesized HMS materials were subsequently subjected to extraction using modifier-enhanced SCCO2 to remove the amine template used. The experimental procedure can be refereed elsewhere [16,19,20]. High purity carbon dioxide, after liquefied, was fed via a pump into a premixing coil that is placed in a temperature-controlled oven. The liquid modifier was introduced into the system by a syringe pump. The premixing coil was sufficiently long to ensure that the modifier and CO2 could be thoroughly mixed prior to entering the extraction unit. The resultant CO2/ modifier mixture was then fed into a 5.0 mL extraction unit where approximately 0.5 g of as-synthesized powder was loaded for each run. A 0.5 lm filter was placed at each end of the vessel to eliminate entrainment. The template-loaded CO2/modifier leaving the extraction vessel was depressurized to air, leading to the deposition of the dissolved template. After each extraction run, the whole system was flushed using pure SCCO2 for 0.5 h to remove any modifier retained in the vessel or precipitated template along the tubing in order to avoid blocking subsequent runs. The experiment values were obtained at each condition at least 2 or 3 times to render the experimental errors be less than 3.0%. 2.4. Materials characterization Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were simultaneously carried out on a Shimadzu DTG-60 analyzer. A comparison of the weight losses between as-synthesized and SFE-processed samples was performed to determine the extraction efficiency under various experimental conditions. The extraction efficiency is defined as a ratio of the amount of the organic template removed to the amount of the template originally existing in the powder. For each run, approximately 5–13 mg of the sample was loaded and heated to 550 °C with a heating rate of 10 °C/min and at a flow rate of 35 mL/min air. According to Tanev and Pinnavaia [24], weight losses in these three ranges can be accounted for separately, as given in Table 2. Based on the greatest weight loss occurred in the range 150– 300 °C, the organic amine template included in the HMS sample could be readily estimated. It may be noted that the moisture content varies from sample to sample, thus the sample dry weight, not including the mass of the adsorbed water and organics, should always be used as the basis for calculating the extraction efficiency. The mesostructural properties of the HMS samples were determined by using powder XRD analysis and Nitrogen adsorption/ desorption measurements. XRD analysis was carried out by means of a Shimadzu XRD-6000 Spectrometer, from 1.5° to 20° by using
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Table 1 Source and purity of all compounds used. No.
Compound
Source
Specification
1 2 3 4 5 6 7 8 9 10 11 12 13
Carbon dioxide Methanol Ethanol 2-Propanol Formic acid Acetic acid Acetone Chloroform Dichloromethane Triethylamine Water Tetraethylorthosilicate Dodecylamine
Tianjin Sifang Gas Co. Ltd., China Sigma–Aldrich Chemicals Hayman Specialty Products Sigma–Aldrich Chemicals Fluka Chemicals Sigma–Aldrich Chemicals Tedia Company, Inc. Sigma–Aldrich Chemicals Sigma–Aldrich Chemicals Tedia Company, Inc. Self-made Aldrich Chemicals Fluka Chemicals
99.99% Anhydrous, 99.98% Absolute, 99.95% Anhydrous, 99.5% Puriss. p.a. 98%(T) ACS reagent, P99.7% HPLC/Spectro, 99.97% Anhydrous, P99% ACS reagent, P99.5% 99.5 + %, GC grade Ultra pure deionzed Reagent grade, 98% Puriss., P99.5%(GC)
Table 2 Explanation to TGA weight loss of siliceous HMS materials. Temperature range (°C) 20150 150300 300500
Reasons for weight loss observed Desorption of water and other volatile organics Decomposition and combustion of organic amine template Dehydroxylation of the surface of HMS
Cu Ka radiation (1.5406 Å), 40 kV and 40 mA with a scanning rate of 2°/min. Nitrogen adsorption/desorption isotherms of the samples, after outgassed under vacuum for 5 h at 200 °C, were immediately measured on Quantachrome Autosorb-1 using high purity N2 at 77 K as the adsorbate. The BET specific surface area was computed using the adsorption data in the relative pressure range of 0.05–0.3. The total pore volume was estimated on the basis of the amount adsorbed at a relative pressure close to unity by assuming that all the pores were fully filled with liquid nitrogen. The mesopore size distribution was obtained from the desorption branch of N2 isotherms applying the BJH model and the BJH pore size was taken as the peak value of the BJH pore size distribution. Fourier transform infrared (FTIR) analysis was performed to compare the chemical identity of the template extracted with that of the original powder. The collected template was dried to remove any solvent residual and then mixed with KBr powder and pelletized before placed into a sample holder. A reference sample was also prepared by grinding KBr powder with the original sample. The FTIR analysis spectra were obtained in the frequency range of 400–4000 cm1 using a Bruker Alpha-H infrared spectrophotometer. 3. Results and discussion SFE of the amine template from HMS has been carried out using ten different liquid modifiers in conjunction with SCCO2 at 65 °C, 15.0 MPa or 85 °C and 10.0 MPa. The modifiers involved are methanol, ethanol, 2-propanol, formic acid, acetic acid, acetone, chloroform, dichloromethane, triethylamine and water. The flow rates of liquefied CO2 and liquid modifier investigated were in the span of 1.8 and 0.2 mL/min, respectively. The extraction results obtained in 1.0 h are shown in Table 3. 3.1. Pure SCCO2 extraction Similarly as reported elsewhere [19,20], the zero extraction was observed for the dodecylamine template removal from HMS materials when pure SCCO2 was used. The reason for it could be due to the occurrence of a chemical reaction. As addressed by Francis [25],
Table 3 SCCO2 extraction efficiency obtained in 1 h using different modifiers. Modifier
Methanol Ethanol 2-Propanol Formic acid Acetic acid Acetone Chloroform Dichloromethane Triethylamine Water
Extraction efficiency (%) 65 °C + 15.0 MPa
85 °C + 10.0 MPa
60.4 52.3 43.1 76.9 61.2 38.3 21.6 25.7 18.9 64.3
95.7 [19] 79.5 49.6 98.2 86.8 41.5 26.3 37.4 23.2 96.4
the salt forming reaction may have taken place between the weakly acidic CO2 and the moderately basic aliphatic amine. Ashraf-Khorassani and Taylor [26] have pointed out that only tertiary aliphatic and aromatic amines could be extracted using SCCO2 whereas primary and secondary aliphatic amines could react with CO2 and form insoluble products known as the carbamic acid. Being a primary amine, dodecylamine might possibly react with CO2 and impede its dissolving in SCCO2, and this may account for the complete absence of extraction and thereby justifies the need of a modifier.
3.2. Modifier enhanced SCCO2 extraction Table 3 shows the SCCO2 extraction efficiencies obtained by using 10 different modifiers in 1.0 h under 65 °C and 15.0 MPa. It can be seen that after adding polar modifiers the template dodecylamine could be more readily extracted from the porous matrix, with the extraction efficiency ranging from 18.7% to 76.9% for the different modifiers used. This is understandable since the polarity of SCCO2 can be increased after introduced with a polar modifier and its solvating power to the polar template enhanced, and then more dodecylamine could have been extracted. If based solely upon the extraction efficiency, formic acid seems to be the best modifier while acetic acid, water and methanol are the better ones, these four species are found to result in the extraction efficiency higher than 60%. On the other hand, the others like acetone, chloroform, dichloromethane and triethylamine are much poorer for in removing the template and the removal efficiency achieved is less than 30%. These extraction results might be related to the polarity of the modifiers and Table 4 shows the dipole moment of these modifiers [27]. However, it can be seen from Tables 3 and 4 that the extraction performance of added modifiers is no fully consistent with the
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Z. Huang et al. / Separation and Purification Technology 118 (2013) 120–126 Table 4 Dipole moment and solubility parameter of pure modifiers, CO2 and dodecylamine.a Compounds
dipole moment (Debye)
d (MPa)1/2
Acetone Water Methanol Acetic acid Ethanol Dichloromethane 2-Propanol Formic acid Chloroform Triethylamine CO2 Dodecylamine
2.88 1.85 1.70 1.70 1.69 1.60 1.58 1.42 1.04 0.66 0.00 1.20
20.05 47.86 29.66 25.77 26.39 19.84 24.34 27.61 19.02 14.93 10.45b,9.86c 18.00
Table 5 Solubility parameter values of different modifer/SCCO2 mixtures.a Modifier
Formic acid Water Acetic acid Methanol Ethanol 2-Propanol Acetone Dichloromethane Chloroform Triethylamine a
a b c
Dipole moment at 25 °C from [27]; d from [28]. Obtained at 65 °C + 15.0 MPa using Eq. (2), respectively. Obtained at 85 °C + 10.0 MPa using Eq. (2), respectively.
difference in the modifier polarities in terms of dipole moment. Of all these modifiers, acetone has the highest dipole moment value and is expected to given the best performance but the template removed is only about 38.3%. Likewise dichloromethane (1.60 Debye) has a higher polarity than isopropanol (1.58 Debye), but when used as a polar modifier to SCCO2 it has resulted in much lower extraction efficiency of 25.7% than isoporpanol which has led to an extraction efficiency of 43.1%. Interestingly, formic acid seems to be the most efficient modifier and results in the highest extraction efficiency of 76.9% at 65 °C + 15.0 MPa but it does not bear the highest polarity. The reason for it may be interpreted in term of the acid–base reaction occurring between the rather strong formic acid and the basic amine template [25]. It is well known that the effectiveness of any SCCO2 extraction process depends on the ability of the supercritical solvent to compete with the matrix for the analyte [11]. The salting-forming reaction possibly occurred could have elevated to the affinity of the analyte for the supercritical solvent instead of the porous inorganic matrix and then boosted the extraction efficiency. Due to the acid–base reaction, the chemical identity of the template have altered and this might have crippled the hydrogen bond interactions existing initially between the matrix pore wall and the analyte template, thereby facilitating the formic acid-modifier extraction. Rather similarly, acetic acid has also produced very high extraction efficiency and it is higher than that obtained by using ethanol although these two modifiers have almost the same dipole moment. The effectiveness of these modifiers could be correlated with the solubility parameters of the modifiers as well. Table 4 also lists the Hilderbrand solubility parameter values abstracted from the literature [28] for all the chemicals of interest. For the template dodecylamine, the Hilderbrand solubility parameter (ds) value is 18.00 (MPa)1/2 at 25 °C and assumed to change not too much when ⁄temperature varies from 25 to 85 °C. The dmix values of supercritical CO2 modified with the various modifiers at 65 °C, 15.0 MPa or 85 °C + 10.0 MPa are calculated by Eq. (2) and tabulated in Table 5. For simple description, it is better to express the solvent–solute solubility parameter difference in terms of |ds–dmix| and the calculated |ds–dmix| values are also given in Table 4. Since it is believed that the value of |ds–dmix| is the smaller, the solute can be dissolved in the solvent more readily [13]. To examine its validity, Table 5 is re-plotted in Fig. 1 by following the descending order of extraction efficiency and the values of |ds–dmix| are correspondently plotted in the same figure. As shown in Fig. 1, the extraction performances for most modifier-enhanced SCCO2 systems could approximately follow the decreasing sequence of the |ds–dmix| values. That is to say, higher extraction efficiency can be achieved if the |ds–dmix| value is lower. The exception to the above observations includes three modifiers of water, formic acid and acetic acid.
65 °C + 15.0 MPa
85 °C + 10.0 MPa
dmixture
|dS–dmSF|
|dS–dmSF|
10.76 12.79 10.58 10.97 10.64 10.44 10.01 9.99 9.90 9.49
7.24 5.21 7.42 7.03 7.36 7.56 7.99 8.01 8.10 8.51
12.17 14.19 11.98 12.37 12.05 11.84 11.41 11.39 11.31 10.90
5.83 3.81 6.02 5.63 5.95 6.16 6.59 6.61 6.69 7.10
All the values are in unit of (MPa)1/2.
For formic acid and acetic acid, they both give unexpectedly high extraction efficiencies but these results are not agreeing well with their relatively large |ds–dmix| values as clearly shown in Fig. 1. As discussed earlier, the effectiveness of these two modifiers could be explained as the consequence of the acid–base reaction probably occurring between the acid and amine. In the case of watermodified SCCO2, despite its lowest |ds–dmix| value, it could not yield an exceptionally good performance as expected. But it is still the second best modifier among all studied and its unsatisfactory extraction performance may be explained from two aspects. Firstly, the presence of water in SCCO2 may have induced a certain degree of pore collapse in the HMS sample, similar to that report for other mesoporous materials [16,17], thereby obscuring the template and obstructing the extraction process. Water modifier has been reported to boost the SFE of solute from montmorillonite clay owing to its ability to swell the matrix, thus allowing better penetration of the matrix by the supercritical solvent [22]. However, perhaps due to this swelling function of water, the pore collapse has been unfortunately occurred in MCM-41 [17] or MCM-48 [16] when water is used as the modifier for the SCCO2 extraction of the organic template. Secondly, the extraction performance of water modifier may be related to the existence of the two phases in the CO2/water system [29]. Actually, the extraction fluid exists as two phases under current conditions: liquid water phase with dissolved CO2 and SCCO2 phase with little water. Then liquid water might delay the template extraction process if trapped in the collapsed pores even it is possibly a good solvent for the amine analyte. Of the 10 modifiers, the well-performed ones could include formic acid, acetic acid, water, methanol and ethanol as they have resulted in appreciated extraction efficiency of higher than 50%. Thus these five modifiers were reexamined, along with the other modifiers, at extraction temperature of 85 °C and pressure of 10.0 MPa as methanol-modified SCCO2 extraction has been shown to result in the extraction efficiency of over 95% in the proceeding paper [19]. The results thus obtained are also listed in Table 3. As seen from this table, the resultant extraction efficiencies are 98.2%, 86.8%, 79.5% and 96.4% for formic acid, acetic acid, ethanol, and water respectively. The differences in extraction performance for these modifiers are consistent with those discussed above. Again, the formic acid seems to be the most effective modifier for amine removal as reflected by the highest extraction efficiency. Further examination of Table 3 shows that the extraction efficiencies are considerably higher at 85 °C than those obtained at 65 °C for all 10 polar modifiers. These significant increases in the efficiencies are possibly due to the combined effects of temperature and modifier. Increasing the extraction temperature can render the amine template more volatile and provide more energy for overcoming the strongly interaction between the template molecules and sample matrix and desorbing the template, subsequently increasing
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Fig. 1. Dependence of extraction efficiency and |dS–dmSF| on the 10 modifiers used for 1 h SCCO2 extraction at 65 °C and 15.0 MPa.
the extraction efficiency at high temperatures. In the meantime, the polar modifier at high temperature may strongly interact with the sample matrix and then largely lower the template desorption energy, yielding readily desorption of the template and partition of more templates to the extraction fluid [21–23]. Similar results have been reported for extraction using SCCO2 with modifiers at high temperature could be the most efficient SFE method for extracting polycyclic aromatic hydrocarbons from environmental samples [30]. 3.3. Properties of processed HMS samples Besides TGA analysis, the samples treated by various SCCO2modifier extraction systems were also examined by using the XRD and N2 adsorption/desorption characterizations. Table 6 gives structural properties of HMS samples treated at 65 °C and 15.0 MPa by using different SCCO2-modifer extraction systems. It is known that the template if not extracted or removed from the pores of the HMS samples has exerted a considerable amount of impact on the material structural characteristics such as pore diameter, pore volume and specific surface area. In view of that, the samples treated with different SCCO2-modifer extraction were directly subjected to nitrogen adsorption test. From Table 6, it can be seen that various structural parameters are heavily dependent on the degree of extraction. Among the modifiers used, formic acid has led to the highest extraction efficiency of 76.9% and consequently yielded the highest parameter values. On the contrary, triethylamine has resulted in the lowest efficiency of 18.9% and subsequently produced the lowest vales of the structural parameters in terms of pore diameter, pore volume and specific surface area. These results imply that the template molecules near the pore
Table 6 Structural properties of the HMS samples treated in 1 h with ten modifer/SCCO2 mixtures under 65 °C and 15.0 MPa. Modifier identity
BJH pore size (Å)
BET surface area (m2/g)
Total pore volume (cm3/g)
Formic acid Water Acetic acid Methanol Ethanol 2-Propanol Acetone Dichloromethane Chloroform Triethylamine
27.8 27.4 27.5 27.1 26.3 25.6 24.7 25.3 24.1 24.7
931 825 782 749 658 477 375 203 146 124
0.812 0.739 0.686 0.643 0.601 0.432 0.354 0.182 0.135 0.117
mouth are, most probably, removed prior to the inner ones. Otherwise, the diffusion and subsequent adsorption of nitrogen onto the pore surface would have been hindered, causing measured pore diameter, pore volume and specific surface area to fall down. Thus it is a need to remove all the template before the mesoporosity of the materials can be revealed. The effect of remaining amine on the structural stability of the HMS sample has also been investigated by considering the sample that has undergone methanol-SFE and 95.7% of the template has been removed. For comparison, the same SFE-treated sample was treated by further calcination to ensure the complete removal of the template. These two samples were then immersed in hot water at 96 °C for 3 days, followed by nitrogen adsorption testing. The test results show that the sample without calcination experiences a very obvious collapse of mesoporosity but not for the sample with calcination. This is further proved by the fact that the specific surface area of the former is only 467 m2/g but for the latter it is 1210 m2/g. These observations indicate that the structural stability of the HMS, like its mesoporosity, may only be full materialized after the complete removal of the template, and that the mesoporous structure may be probably weaker if still supported by the amine template and therefore more prone to collapse when exposed to moisture. Table 7 shows N2 adsorption/desorption results for the HMS samples obtained at 85 °C and 10.0 MPa with formic acid, water, acetic acid, methanol and ethanol, respectively. It should be noted that for these SFE samples, the remaining template was all burned away at temperature of 650 °C for about 2 h before subjecting the samples to nitrogen adsorption and XRD. The XRD spectra results, nitrogen adsorption/desorption isotherms and resultant pore size distributions for these samples are shown in Figs. 2–4, respectively. In previous work [19], the HMS has been reported to possess a BET surface area and pore volume of 1072 m2/g and 1.04 cm3/g for the directly calcined sample, and 871 m2/g and 0.87 cm3/g for the liquid extraction sample, respectively. Obviously, using a modifier
Table 7 Structural properties of the HMS samples treated in 1 h with five modifer/SCCO2 mixtures under 85 °C and 10.0 MPa. Modifier identity
BJH pore size (Å)
BET surface area (m2/g)
Total pore volume (cm3/g)
Formic acid Acetic acid Methanol Ethanol Water
29.5 27.5 27.8 26.5 28.4
1172 1084 1210 987 886
1.34 1.14 1.46 1.04 0.86
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Fig. 2. XRD spectra of the HMS powders treated by 1 h different modifier/SCCO2 extractions at 85 °C and 10.0 MPa.
Fig. 4. Pore size distributions of the HMS samples treated by 1 h different modifier/ SCCO2 extractions at 85 °C and 10.0 MPa.
Fig. 3. Nitrogen adsorption/desorption isotherms of the HMS samples treated by 1 h different modifier/SCCO2 extractions at 85 °C and 10.0 MPa.
of formic acid, acetic acid or methanol has rendered much larger surface area and pore volume as seen from Table 7. As can be graphically seen in Fig. 2, XRD results also prove that for the three modifiers, highly ordered mesoporous structure could still be well preserved for the HMS materials after mild modified SCCO2 extraction. From the standpoint of extraction efficiency and product structural properties, formic acid seems to be the best modifier. However, the use of formic acid could possess three main disadvantages. Firstly, it could react and then modify the chemical identity of the organic template, subsequently rendering the recycling of the template complicated. Secondly, the use of acid under high pressure may be highly corrosive and therefore detrimental to the proper operation of both the pumps and the extraction vessel. Thirdly, unlike methanol and water, the use of formic acid as a modifier has been observed to cause the HMS particles to lump together and form sticky gel. As a result, the amount of the HMS product collected from the extraction vessel is lowered since some HMS has been stuck to the inner wall of the vessel and therefore lost. On the other hand, the use of water as a modifier may be plausibly attractive since the extracted template could be readily
obtained as an aqueous solution and possibly reused directly for hydrothermal synthesis purpose. However, the structure fragility of HMS materials has been observed after subjecting to watermodified SCCO2 extraction. As shown in Table 7 and Figs. 2–4, water modifier seems to have caused a certain degree of pore collapse in the HMS sample. This can be evidenced by (1) a drop in the specific surface area of the water-modified HMS to below 900 m2/g, (2) the insignificant peak observed for the XRD pattern as well as (3) two peaks exhibited in the pore size distribution curve where the first peak probably denotes the characteristic pore size of the sample while the second rather mild one is mainly arising from water-induced pore contraction. These results all exclude the possibility of using water as a properly-selected modifier for amine removal from HMS materials with SCCO2 extraction despite its low cost and high availability. In view of XRD and N2 adsorption/desorption results given in Table 7 and Fig. 2, methanol could be selected as a modifier since much ordered mesoporous structure of HMS could be perfectly maintained. This conclusion can also be deduced from our previous works [19,20]. For the purpose of recycling the amine template, the chemical identity for the amine template obtained from HMS methanol-modified SCCO2 extraction was examined. Fig. 5 shows
Fig. 5. FTIR spectra of original dodecylamine and the sample obtained by methanol/ SCCO2 extraction.
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FTIR spectra characterizing the original dodecylamine and the one obtained from HMS by using methanol-modified SCCO2 extraction. The excellent agreement between these two spectra in 400– 4000 cm1 confirms their structural consistency, indicating that dodecylamine extracted by methanol-modified SCCO2 has experienced no change in its chemical structure and might be reused for future synthesis of HMS materials, hence, minimizing its production cost. For these reasons, it may be deduced that methanol is the most suitable modifier because it could preserve the chemical identity of the extracted template and the well-preserved mesoporosity of HMS materials.
4. Conclusion In this work, for supercritical CO2 removing the templating agent dodecylamine from newly synthesized hexagonal mesoporous silica (HMS) materials, 10 polar modifiers have been examined at 65 °C, 15.0 MPa or 85 °C and 10.0 MPa with the flow rates of 1.8 and 0.2 mL/min for CO2 and modifier, respectively. The modifier effects on extraction efficiency and structural properties of resultant materials are then discussed in detail. The results show that the amine extraction efficiency is strongly dependent on the identity of the modifier used for enhancing supercritical CO2 extraction. The differences in extraction performance for 10 modifiers cannot be interpreted in terms of the dipole moment of the modifier but the solvent–solute solubility parameter difference rule. The exception to the rule includes three modifiers of water, formic acid and acetic acid, and these three modifiers along with methanol have led to reasonably high extraction efficiencies of more than 85%. The high performance observed for two acids may be attributed to the possible amide formation reaction between amine and modifier. For water modifier, the extraction performance may be influenced by the induced pore collapse and the presence of some liquid water trapped in the collapsed pores. Nitrogen adsorption/desorption and XRD spectra results show that the modifiers have strong effects on the physical properties of resultant HMS materials. Among these polar modifiers used, methanol apparently seems to be the most suitable modifier for the SCCO2 extraction of dodecylamine from HMS materials because aqueous modifier could destroy the material mesoporous structure as reflected by XRD spectra while acid modifiers could be corrosive to the extraction vessel and cause lumping of HMS. Using methanol as the modifier, the extracted template retains unmodified as identified with FTIR and may be possible for future reuse. The resultant HMS materials could have well preserved highly ordered mesoporous structure and thus possess much larger specific surface area of 1210 m2/g and higher pore volume of 1.46 cm3/g than those obtained for the other modifiers. Acknowledgements The authors would like to thank the financial support of National Natural Science Foundation of China (NNSFC-20676107) and of State Education Ministry of China (SRF Project for ROCS, SEM). References [1] R.M. Couto, T. Carvalho, L.A. Neves, R.M. Ruivo, P. Vidinha, A. Paiva, I.M. Coelhoso, S. Barreiros, P.C. Simões, Development of Ion-JellyÒ Membranes, Sep. Purif. Technol. 106 (2013) 22–31.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Effect of the polar modifiers on supercritical extraction efficiency for template removal from hexagonal mesoporous silica materials: Solubility parameter and polarity considerations Zhen Huang a,⇑, Jing-huan Li a, Hui–shan Li b, Hui Miao a, S. Kawi c, A.H. Goh c a b c
Tianjin University of Commerce, Tianjin 300134, PR China Department of Auto Engineering, Military Transportation Academy, Tianjin 300161, PR China Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, 117576 Singapore, Singapore
a r t i c l e
i n f o
Article history: Received 31 January 2013 Received in revised form 25 June 2013 Accepted 29 June 2013 Available online 8 July 2013 Keywords: Supercritical CO2 extraction Hexagonal mesoporous materials Modifier identity Template removal Solubility parameter Polarity
a b s t r a c t The introduction of polar modifiers usually can make supercritical CO2 extraction more attractive in various potential applications. In this work, modified supercritical CO2 extraction of the dodecylamine template has been performed on freshly synthesized mesoporous HMS materials with the use of 10 different polar modifiers. The effects of the polar modifiers on the extraction efficiencies of the amine template and structural properties of resultant HMS materials have been investigated in detail. Among the ten modifiers used, formic and acetic acids, water and methanol give reasonably high extraction efficiencies and the effects of the modifier identity on extraction efficiencies have been discussed in terms of solubility parameter and dipole moment of the modifiers. Methanol seems to be much better than the other modifiers for the SCCO2 extraction of dodecylamine from HMS materials since the resulted materials have well preserved highly ordered mesoporous structure and possessed much large specific surface area and pore volume as reflected by XRD spectra and nitrogen adsorption/desorption results. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Supercritical fluid extraction (SFE) has been exploited for a diverse range of practical applications because supercritical fluids have unique properties intermediate between those of gases and liquids. Because of the gas-like transport properties and liquid-like densities, supercritical fluids are more attractive and thus have extensively been investigated for various porous materials fabrications [1–6] or diverse bioactive species extraction from natural produces [7–10]. In terms of mass transfer, it is known that the diffusivity of the supercritical fluid is about 10–100 times greater than that of liquid and that the viscosity of a supercritical fluid is generally comparable to that of a gas but about 100 times lower than that of a liquid [11]. This means that it is much easier for supercritical fluids to penetrate pores with lesser resistance than for liquid solvents. In terms of the solvent strength, supercritical fluids (0.2–0.9 g/cm3) have not much lower density than liquids (0.6–1.6 g/cm3), thus still possessing appreciated solvating power to the solute [11]. The solvent strength of a supercritical fluid solvent can be adjusted or tuned with temperature or pressure [11] and it may plausibly be quantitatively represented by a commonly applied ⇑ Corresponding author. E-mail address: [email protected] (Z. Huang). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.047
concept, i.e., the Hildebrand solubility parameter [12]. The Hildebrand solubility parameter is a measure of the cohesive energy density of a substance and is useful for indicating the solvency behavior of a specific solvent. A role of thumb says that when the solvent–solute solubility parameter difference is within 4 MPa1/2, the solute should be fairly soluble in it. The solubility parameter of a substance, d, at different temperatures and pressures was calculated using the following equation:
d¼
DEv ap 1=2 DHv ap RT 1=2 ¼ V V
ð1Þ
where DEv ap and DHv ap are the molar internal energy and molar enthalpy of vaporization, respectively. V is the molar volume of the substance, R is the universal gas constant, and T is given temperature. Usually, for solid or liquid substances, it could be assumed that the value of d is constant if the temperature range considered is not broad. However, Eq. (1) is not applicable to supercritical fluids since the enthalpy of vaporization is no longer meaningful. Promisingly, Giddings et al. [13] have established an empirical correlation of the solubility parameter for supercritical fluids. The correction can be written in SI units by Marcus [14] as follows.
d ¼ 3:02 Pc1=2 qr
ð2Þ
Z. Huang et al. / Separation and Purification Technology 118 (2013) 120–126
where Pc is in units of MPa, qr is the reduced density (=q/qc) and d has units of MPa1/2. Using Eq. (2) along with equations of state, Marcus [14] estimated the d values at qr = 1 .0 and Tr = 1.2 for more than 30 supercritical fluids. Prof. Singhal and his group have promisingly attempted to relate the solubility parameter of modified SCCO2 to extraction performances [9,10]. Further, Machida et al. [15] have addressed that the controllable solubility parameters of supercritical solvents through varying temperature and/or pressure could give them wide applicability when either used as solvents with/without a modifier or co-solvents. Among a variety of supercritical solvents, supercritical CO2 (SCCO2) is the most widely used because it is environmentally benign, inexpensive, essentially nontoxic, inflammable, and allows supercritical operation at low pressures and near room temperature. However, carbon dioxide is a non-polar molecule so that it is not a particularly good extraction solvent for polar organic compounds, as reflected in our previous studies [16–20]. Therefore, a polar modifier is usually required for SCCO2 to extract polar organic species. The addition of a small amount of a polar modifier could significantly increase the solvating power of supercritical CO2 to targeted organic chemicals. For a mixture consisting of SCCO2 and a modifier, the solubility parameter for the mixed fluid is determined as follows [9,10]:
dmix ¼ uCO2 dCO2 þ um dm
ð3Þ
where u is the volume fraction, and dmix and dm are the solubility parameter of the mixture and that of the modifier, respectively. The u can be estimated from the flow rates of liquefied CO2 and the modifier by assuming the excess volume is equal to zero, which is a good approximation for a small amount of the modifier. Besides enhancing the solvent strength of SCCO2, the polar modifier added may interact with the analyte/matrix complex so as to lower the activation energy of analyte desorption [21], promote the accessibility of remote sites to the extraction fluid and facilitate the transport of the analyte to the bulk fluid [22], and covering the matrix active sites and thereby preventing the re-adsorption of analyte back to those active sites [23]. For these reasons, the introduction of polar modifiers is not only able to dramatically increase the solvent strength of in SCCO2 to polar organic compounds but also can make SCCO2 extraction more attractive transport properties in various potential applications. In this work, we have investigated to the SCCO2 extraction for removing the organic template from freshly synthesized hexagonal mesoporous silica (HMS) with the introduction of 10 different polar modifiers. Adding a polar modifier seems to be a must because in our earlier studies it has been known that pure SCCO2 could not extract any organic templates from freshly prepared siliceous mesoporous materials MCM-41, MCM-48, SBA-1 and SBA-3 powders [16–20], due to the low solubility of polar organic templates in SCCO2. The HMS used was synthesized by using dodecylamine as the templating agent. The effects of these modifiers on dodecylamine removal were analyzed in terms of Hildebrand solubility parameter in detail. The structural properties of processed porous materials were also investigated as well.
2. Experimental 2.1. Materials The sources and purity of all compounds used in the present study are given in Table 1. These materials were used as received without further purification.
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2.2. Synthesis of HMS materials The hydrothermal procedure for synthesizing purely siliceous HMS materials has been described in earlier works [19,20]. Typically, 25.1 g of dodecylamine was weighed in a 1000 mL polypropylene bottle and mixed with 265 mL of absolute ethanol and 266 mL deionized water. The resultant mixture was vigorously stirred for about 15 min to completely dissolve dodecylamine. 106.2 g of tetraethylorthosilicate was then added into the solution under continuous stirring. Agitation was stopped after 5 min and the obtained mixture was allowed to statically react under room temperature of around 22 °C for 18 h. The resultant solid was washed with deionized water and recovered by filtration through a Buchner funnel. After repeating washing and filtrating for about four times, the moist solid was transferred to a ceramic dish and dried at 30–40 °C for 2 days. After drying, the final product was sieved into the desired powder with a particle size between mesh 40 and 60. 2.3. SCCO2 extraction of the template The as-synthesized HMS materials were subsequently subjected to extraction using modifier-enhanced SCCO2 to remove the amine template used. The experimental procedure can be refereed elsewhere [16,19,20]. High purity carbon dioxide, after liquefied, was fed via a pump into a premixing coil that is placed in a temperature-controlled oven. The liquid modifier was introduced into the system by a syringe pump. The premixing coil was sufficiently long to ensure that the modifier and CO2 could be thoroughly mixed prior to entering the extraction unit. The resultant CO2/ modifier mixture was then fed into a 5.0 mL extraction unit where approximately 0.5 g of as-synthesized powder was loaded for each run. A 0.5 lm filter was placed at each end of the vessel to eliminate entrainment. The template-loaded CO2/modifier leaving the extraction vessel was depressurized to air, leading to the deposition of the dissolved template. After each extraction run, the whole system was flushed using pure SCCO2 for 0.5 h to remove any modifier retained in the vessel or precipitated template along the tubing in order to avoid blocking subsequent runs. The experiment values were obtained at each condition at least 2 or 3 times to render the experimental errors be less than 3.0%. 2.4. Materials characterization Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were simultaneously carried out on a Shimadzu DTG-60 analyzer. A comparison of the weight losses between as-synthesized and SFE-processed samples was performed to determine the extraction efficiency under various experimental conditions. The extraction efficiency is defined as a ratio of the amount of the organic template removed to the amount of the template originally existing in the powder. For each run, approximately 5–13 mg of the sample was loaded and heated to 550 °C with a heating rate of 10 °C/min and at a flow rate of 35 mL/min air. According to Tanev and Pinnavaia [24], weight losses in these three ranges can be accounted for separately, as given in Table 2. Based on the greatest weight loss occurred in the range 150– 300 °C, the organic amine template included in the HMS sample could be readily estimated. It may be noted that the moisture content varies from sample to sample, thus the sample dry weight, not including the mass of the adsorbed water and organics, should always be used as the basis for calculating the extraction efficiency. The mesostructural properties of the HMS samples were determined by using powder XRD analysis and Nitrogen adsorption/ desorption measurements. XRD analysis was carried out by means of a Shimadzu XRD-6000 Spectrometer, from 1.5° to 20° by using
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Table 1 Source and purity of all compounds used. No.
Compound
Source
Specification
1 2 3 4 5 6 7 8 9 10 11 12 13
Carbon dioxide Methanol Ethanol 2-Propanol Formic acid Acetic acid Acetone Chloroform Dichloromethane Triethylamine Water Tetraethylorthosilicate Dodecylamine
Tianjin Sifang Gas Co. Ltd., China Sigma–Aldrich Chemicals Hayman Specialty Products Sigma–Aldrich Chemicals Fluka Chemicals Sigma–Aldrich Chemicals Tedia Company, Inc. Sigma–Aldrich Chemicals Sigma–Aldrich Chemicals Tedia Company, Inc. Self-made Aldrich Chemicals Fluka Chemicals
99.99% Anhydrous, 99.98% Absolute, 99.95% Anhydrous, 99.5% Puriss. p.a. 98%(T) ACS reagent, P99.7% HPLC/Spectro, 99.97% Anhydrous, P99% ACS reagent, P99.5% 99.5 + %, GC grade Ultra pure deionzed Reagent grade, 98% Puriss., P99.5%(GC)
Table 2 Explanation to TGA weight loss of siliceous HMS materials. Temperature range (°C) 20150 150300 300500
Reasons for weight loss observed Desorption of water and other volatile organics Decomposition and combustion of organic amine template Dehydroxylation of the surface of HMS
Cu Ka radiation (1.5406 Å), 40 kV and 40 mA with a scanning rate of 2°/min. Nitrogen adsorption/desorption isotherms of the samples, after outgassed under vacuum for 5 h at 200 °C, were immediately measured on Quantachrome Autosorb-1 using high purity N2 at 77 K as the adsorbate. The BET specific surface area was computed using the adsorption data in the relative pressure range of 0.05–0.3. The total pore volume was estimated on the basis of the amount adsorbed at a relative pressure close to unity by assuming that all the pores were fully filled with liquid nitrogen. The mesopore size distribution was obtained from the desorption branch of N2 isotherms applying the BJH model and the BJH pore size was taken as the peak value of the BJH pore size distribution. Fourier transform infrared (FTIR) analysis was performed to compare the chemical identity of the template extracted with that of the original powder. The collected template was dried to remove any solvent residual and then mixed with KBr powder and pelletized before placed into a sample holder. A reference sample was also prepared by grinding KBr powder with the original sample. The FTIR analysis spectra were obtained in the frequency range of 400–4000 cm1 using a Bruker Alpha-H infrared spectrophotometer. 3. Results and discussion SFE of the amine template from HMS has been carried out using ten different liquid modifiers in conjunction with SCCO2 at 65 °C, 15.0 MPa or 85 °C and 10.0 MPa. The modifiers involved are methanol, ethanol, 2-propanol, formic acid, acetic acid, acetone, chloroform, dichloromethane, triethylamine and water. The flow rates of liquefied CO2 and liquid modifier investigated were in the span of 1.8 and 0.2 mL/min, respectively. The extraction results obtained in 1.0 h are shown in Table 3. 3.1. Pure SCCO2 extraction Similarly as reported elsewhere [19,20], the zero extraction was observed for the dodecylamine template removal from HMS materials when pure SCCO2 was used. The reason for it could be due to the occurrence of a chemical reaction. As addressed by Francis [25],
Table 3 SCCO2 extraction efficiency obtained in 1 h using different modifiers. Modifier
Methanol Ethanol 2-Propanol Formic acid Acetic acid Acetone Chloroform Dichloromethane Triethylamine Water
Extraction efficiency (%) 65 °C + 15.0 MPa
85 °C + 10.0 MPa
60.4 52.3 43.1 76.9 61.2 38.3 21.6 25.7 18.9 64.3
95.7 [19] 79.5 49.6 98.2 86.8 41.5 26.3 37.4 23.2 96.4
the salt forming reaction may have taken place between the weakly acidic CO2 and the moderately basic aliphatic amine. Ashraf-Khorassani and Taylor [26] have pointed out that only tertiary aliphatic and aromatic amines could be extracted using SCCO2 whereas primary and secondary aliphatic amines could react with CO2 and form insoluble products known as the carbamic acid. Being a primary amine, dodecylamine might possibly react with CO2 and impede its dissolving in SCCO2, and this may account for the complete absence of extraction and thereby justifies the need of a modifier.
3.2. Modifier enhanced SCCO2 extraction Table 3 shows the SCCO2 extraction efficiencies obtained by using 10 different modifiers in 1.0 h under 65 °C and 15.0 MPa. It can be seen that after adding polar modifiers the template dodecylamine could be more readily extracted from the porous matrix, with the extraction efficiency ranging from 18.7% to 76.9% for the different modifiers used. This is understandable since the polarity of SCCO2 can be increased after introduced with a polar modifier and its solvating power to the polar template enhanced, and then more dodecylamine could have been extracted. If based solely upon the extraction efficiency, formic acid seems to be the best modifier while acetic acid, water and methanol are the better ones, these four species are found to result in the extraction efficiency higher than 60%. On the other hand, the others like acetone, chloroform, dichloromethane and triethylamine are much poorer for in removing the template and the removal efficiency achieved is less than 30%. These extraction results might be related to the polarity of the modifiers and Table 4 shows the dipole moment of these modifiers [27]. However, it can be seen from Tables 3 and 4 that the extraction performance of added modifiers is no fully consistent with the
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Z. Huang et al. / Separation and Purification Technology 118 (2013) 120–126 Table 4 Dipole moment and solubility parameter of pure modifiers, CO2 and dodecylamine.a Compounds
dipole moment (Debye)
d (MPa)1/2
Acetone Water Methanol Acetic acid Ethanol Dichloromethane 2-Propanol Formic acid Chloroform Triethylamine CO2 Dodecylamine
2.88 1.85 1.70 1.70 1.69 1.60 1.58 1.42 1.04 0.66 0.00 1.20
20.05 47.86 29.66 25.77 26.39 19.84 24.34 27.61 19.02 14.93 10.45b,9.86c 18.00
Table 5 Solubility parameter values of different modifer/SCCO2 mixtures.a Modifier
Formic acid Water Acetic acid Methanol Ethanol 2-Propanol Acetone Dichloromethane Chloroform Triethylamine a
a b c
Dipole moment at 25 °C from [27]; d from [28]. Obtained at 65 °C + 15.0 MPa using Eq. (2), respectively. Obtained at 85 °C + 10.0 MPa using Eq. (2), respectively.
difference in the modifier polarities in terms of dipole moment. Of all these modifiers, acetone has the highest dipole moment value and is expected to given the best performance but the template removed is only about 38.3%. Likewise dichloromethane (1.60 Debye) has a higher polarity than isopropanol (1.58 Debye), but when used as a polar modifier to SCCO2 it has resulted in much lower extraction efficiency of 25.7% than isoporpanol which has led to an extraction efficiency of 43.1%. Interestingly, formic acid seems to be the most efficient modifier and results in the highest extraction efficiency of 76.9% at 65 °C + 15.0 MPa but it does not bear the highest polarity. The reason for it may be interpreted in term of the acid–base reaction occurring between the rather strong formic acid and the basic amine template [25]. It is well known that the effectiveness of any SCCO2 extraction process depends on the ability of the supercritical solvent to compete with the matrix for the analyte [11]. The salting-forming reaction possibly occurred could have elevated to the affinity of the analyte for the supercritical solvent instead of the porous inorganic matrix and then boosted the extraction efficiency. Due to the acid–base reaction, the chemical identity of the template have altered and this might have crippled the hydrogen bond interactions existing initially between the matrix pore wall and the analyte template, thereby facilitating the formic acid-modifier extraction. Rather similarly, acetic acid has also produced very high extraction efficiency and it is higher than that obtained by using ethanol although these two modifiers have almost the same dipole moment. The effectiveness of these modifiers could be correlated with the solubility parameters of the modifiers as well. Table 4 also lists the Hilderbrand solubility parameter values abstracted from the literature [28] for all the chemicals of interest. For the template dodecylamine, the Hilderbrand solubility parameter (ds) value is 18.00 (MPa)1/2 at 25 °C and assumed to change not too much when ⁄temperature varies from 25 to 85 °C. The dmix values of supercritical CO2 modified with the various modifiers at 65 °C, 15.0 MPa or 85 °C + 10.0 MPa are calculated by Eq. (2) and tabulated in Table 5. For simple description, it is better to express the solvent–solute solubility parameter difference in terms of |ds–dmix| and the calculated |ds–dmix| values are also given in Table 4. Since it is believed that the value of |ds–dmix| is the smaller, the solute can be dissolved in the solvent more readily [13]. To examine its validity, Table 5 is re-plotted in Fig. 1 by following the descending order of extraction efficiency and the values of |ds–dmix| are correspondently plotted in the same figure. As shown in Fig. 1, the extraction performances for most modifier-enhanced SCCO2 systems could approximately follow the decreasing sequence of the |ds–dmix| values. That is to say, higher extraction efficiency can be achieved if the |ds–dmix| value is lower. The exception to the above observations includes three modifiers of water, formic acid and acetic acid.
65 °C + 15.0 MPa
85 °C + 10.0 MPa
dmixture
|dS–dmSF|
|dS–dmSF|
10.76 12.79 10.58 10.97 10.64 10.44 10.01 9.99 9.90 9.49
7.24 5.21 7.42 7.03 7.36 7.56 7.99 8.01 8.10 8.51
12.17 14.19 11.98 12.37 12.05 11.84 11.41 11.39 11.31 10.90
5.83 3.81 6.02 5.63 5.95 6.16 6.59 6.61 6.69 7.10
All the values are in unit of (MPa)1/2.
For formic acid and acetic acid, they both give unexpectedly high extraction efficiencies but these results are not agreeing well with their relatively large |ds–dmix| values as clearly shown in Fig. 1. As discussed earlier, the effectiveness of these two modifiers could be explained as the consequence of the acid–base reaction probably occurring between the acid and amine. In the case of watermodified SCCO2, despite its lowest |ds–dmix| value, it could not yield an exceptionally good performance as expected. But it is still the second best modifier among all studied and its unsatisfactory extraction performance may be explained from two aspects. Firstly, the presence of water in SCCO2 may have induced a certain degree of pore collapse in the HMS sample, similar to that report for other mesoporous materials [16,17], thereby obscuring the template and obstructing the extraction process. Water modifier has been reported to boost the SFE of solute from montmorillonite clay owing to its ability to swell the matrix, thus allowing better penetration of the matrix by the supercritical solvent [22]. However, perhaps due to this swelling function of water, the pore collapse has been unfortunately occurred in MCM-41 [17] or MCM-48 [16] when water is used as the modifier for the SCCO2 extraction of the organic template. Secondly, the extraction performance of water modifier may be related to the existence of the two phases in the CO2/water system [29]. Actually, the extraction fluid exists as two phases under current conditions: liquid water phase with dissolved CO2 and SCCO2 phase with little water. Then liquid water might delay the template extraction process if trapped in the collapsed pores even it is possibly a good solvent for the amine analyte. Of the 10 modifiers, the well-performed ones could include formic acid, acetic acid, water, methanol and ethanol as they have resulted in appreciated extraction efficiency of higher than 50%. Thus these five modifiers were reexamined, along with the other modifiers, at extraction temperature of 85 °C and pressure of 10.0 MPa as methanol-modified SCCO2 extraction has been shown to result in the extraction efficiency of over 95% in the proceeding paper [19]. The results thus obtained are also listed in Table 3. As seen from this table, the resultant extraction efficiencies are 98.2%, 86.8%, 79.5% and 96.4% for formic acid, acetic acid, ethanol, and water respectively. The differences in extraction performance for these modifiers are consistent with those discussed above. Again, the formic acid seems to be the most effective modifier for amine removal as reflected by the highest extraction efficiency. Further examination of Table 3 shows that the extraction efficiencies are considerably higher at 85 °C than those obtained at 65 °C for all 10 polar modifiers. These significant increases in the efficiencies are possibly due to the combined effects of temperature and modifier. Increasing the extraction temperature can render the amine template more volatile and provide more energy for overcoming the strongly interaction between the template molecules and sample matrix and desorbing the template, subsequently increasing
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Fig. 1. Dependence of extraction efficiency and |dS–dmSF| on the 10 modifiers used for 1 h SCCO2 extraction at 65 °C and 15.0 MPa.
the extraction efficiency at high temperatures. In the meantime, the polar modifier at high temperature may strongly interact with the sample matrix and then largely lower the template desorption energy, yielding readily desorption of the template and partition of more templates to the extraction fluid [21–23]. Similar results have been reported for extraction using SCCO2 with modifiers at high temperature could be the most efficient SFE method for extracting polycyclic aromatic hydrocarbons from environmental samples [30]. 3.3. Properties of processed HMS samples Besides TGA analysis, the samples treated by various SCCO2modifier extraction systems were also examined by using the XRD and N2 adsorption/desorption characterizations. Table 6 gives structural properties of HMS samples treated at 65 °C and 15.0 MPa by using different SCCO2-modifer extraction systems. It is known that the template if not extracted or removed from the pores of the HMS samples has exerted a considerable amount of impact on the material structural characteristics such as pore diameter, pore volume and specific surface area. In view of that, the samples treated with different SCCO2-modifer extraction were directly subjected to nitrogen adsorption test. From Table 6, it can be seen that various structural parameters are heavily dependent on the degree of extraction. Among the modifiers used, formic acid has led to the highest extraction efficiency of 76.9% and consequently yielded the highest parameter values. On the contrary, triethylamine has resulted in the lowest efficiency of 18.9% and subsequently produced the lowest vales of the structural parameters in terms of pore diameter, pore volume and specific surface area. These results imply that the template molecules near the pore
Table 6 Structural properties of the HMS samples treated in 1 h with ten modifer/SCCO2 mixtures under 65 °C and 15.0 MPa. Modifier identity
BJH pore size (Å)
BET surface area (m2/g)
Total pore volume (cm3/g)
Formic acid Water Acetic acid Methanol Ethanol 2-Propanol Acetone Dichloromethane Chloroform Triethylamine
27.8 27.4 27.5 27.1 26.3 25.6 24.7 25.3 24.1 24.7
931 825 782 749 658 477 375 203 146 124
0.812 0.739 0.686 0.643 0.601 0.432 0.354 0.182 0.135 0.117
mouth are, most probably, removed prior to the inner ones. Otherwise, the diffusion and subsequent adsorption of nitrogen onto the pore surface would have been hindered, causing measured pore diameter, pore volume and specific surface area to fall down. Thus it is a need to remove all the template before the mesoporosity of the materials can be revealed. The effect of remaining amine on the structural stability of the HMS sample has also been investigated by considering the sample that has undergone methanol-SFE and 95.7% of the template has been removed. For comparison, the same SFE-treated sample was treated by further calcination to ensure the complete removal of the template. These two samples were then immersed in hot water at 96 °C for 3 days, followed by nitrogen adsorption testing. The test results show that the sample without calcination experiences a very obvious collapse of mesoporosity but not for the sample with calcination. This is further proved by the fact that the specific surface area of the former is only 467 m2/g but for the latter it is 1210 m2/g. These observations indicate that the structural stability of the HMS, like its mesoporosity, may only be full materialized after the complete removal of the template, and that the mesoporous structure may be probably weaker if still supported by the amine template and therefore more prone to collapse when exposed to moisture. Table 7 shows N2 adsorption/desorption results for the HMS samples obtained at 85 °C and 10.0 MPa with formic acid, water, acetic acid, methanol and ethanol, respectively. It should be noted that for these SFE samples, the remaining template was all burned away at temperature of 650 °C for about 2 h before subjecting the samples to nitrogen adsorption and XRD. The XRD spectra results, nitrogen adsorption/desorption isotherms and resultant pore size distributions for these samples are shown in Figs. 2–4, respectively. In previous work [19], the HMS has been reported to possess a BET surface area and pore volume of 1072 m2/g and 1.04 cm3/g for the directly calcined sample, and 871 m2/g and 0.87 cm3/g for the liquid extraction sample, respectively. Obviously, using a modifier
Table 7 Structural properties of the HMS samples treated in 1 h with five modifer/SCCO2 mixtures under 85 °C and 10.0 MPa. Modifier identity
BJH pore size (Å)
BET surface area (m2/g)
Total pore volume (cm3/g)
Formic acid Acetic acid Methanol Ethanol Water
29.5 27.5 27.8 26.5 28.4
1172 1084 1210 987 886
1.34 1.14 1.46 1.04 0.86
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Fig. 2. XRD spectra of the HMS powders treated by 1 h different modifier/SCCO2 extractions at 85 °C and 10.0 MPa.
Fig. 4. Pore size distributions of the HMS samples treated by 1 h different modifier/ SCCO2 extractions at 85 °C and 10.0 MPa.
Fig. 3. Nitrogen adsorption/desorption isotherms of the HMS samples treated by 1 h different modifier/SCCO2 extractions at 85 °C and 10.0 MPa.
of formic acid, acetic acid or methanol has rendered much larger surface area and pore volume as seen from Table 7. As can be graphically seen in Fig. 2, XRD results also prove that for the three modifiers, highly ordered mesoporous structure could still be well preserved for the HMS materials after mild modified SCCO2 extraction. From the standpoint of extraction efficiency and product structural properties, formic acid seems to be the best modifier. However, the use of formic acid could possess three main disadvantages. Firstly, it could react and then modify the chemical identity of the organic template, subsequently rendering the recycling of the template complicated. Secondly, the use of acid under high pressure may be highly corrosive and therefore detrimental to the proper operation of both the pumps and the extraction vessel. Thirdly, unlike methanol and water, the use of formic acid as a modifier has been observed to cause the HMS particles to lump together and form sticky gel. As a result, the amount of the HMS product collected from the extraction vessel is lowered since some HMS has been stuck to the inner wall of the vessel and therefore lost. On the other hand, the use of water as a modifier may be plausibly attractive since the extracted template could be readily
obtained as an aqueous solution and possibly reused directly for hydrothermal synthesis purpose. However, the structure fragility of HMS materials has been observed after subjecting to watermodified SCCO2 extraction. As shown in Table 7 and Figs. 2–4, water modifier seems to have caused a certain degree of pore collapse in the HMS sample. This can be evidenced by (1) a drop in the specific surface area of the water-modified HMS to below 900 m2/g, (2) the insignificant peak observed for the XRD pattern as well as (3) two peaks exhibited in the pore size distribution curve where the first peak probably denotes the characteristic pore size of the sample while the second rather mild one is mainly arising from water-induced pore contraction. These results all exclude the possibility of using water as a properly-selected modifier for amine removal from HMS materials with SCCO2 extraction despite its low cost and high availability. In view of XRD and N2 adsorption/desorption results given in Table 7 and Fig. 2, methanol could be selected as a modifier since much ordered mesoporous structure of HMS could be perfectly maintained. This conclusion can also be deduced from our previous works [19,20]. For the purpose of recycling the amine template, the chemical identity for the amine template obtained from HMS methanol-modified SCCO2 extraction was examined. Fig. 5 shows
Fig. 5. FTIR spectra of original dodecylamine and the sample obtained by methanol/ SCCO2 extraction.
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FTIR spectra characterizing the original dodecylamine and the one obtained from HMS by using methanol-modified SCCO2 extraction. The excellent agreement between these two spectra in 400– 4000 cm1 confirms their structural consistency, indicating that dodecylamine extracted by methanol-modified SCCO2 has experienced no change in its chemical structure and might be reused for future synthesis of HMS materials, hence, minimizing its production cost. For these reasons, it may be deduced that methanol is the most suitable modifier because it could preserve the chemical identity of the extracted template and the well-preserved mesoporosity of HMS materials.
4. Conclusion In this work, for supercritical CO2 removing the templating agent dodecylamine from newly synthesized hexagonal mesoporous silica (HMS) materials, 10 polar modifiers have been examined at 65 °C, 15.0 MPa or 85 °C and 10.0 MPa with the flow rates of 1.8 and 0.2 mL/min for CO2 and modifier, respectively. The modifier effects on extraction efficiency and structural properties of resultant materials are then discussed in detail. The results show that the amine extraction efficiency is strongly dependent on the identity of the modifier used for enhancing supercritical CO2 extraction. The differences in extraction performance for 10 modifiers cannot be interpreted in terms of the dipole moment of the modifier but the solvent–solute solubility parameter difference rule. The exception to the rule includes three modifiers of water, formic acid and acetic acid, and these three modifiers along with methanol have led to reasonably high extraction efficiencies of more than 85%. The high performance observed for two acids may be attributed to the possible amide formation reaction between amine and modifier. For water modifier, the extraction performance may be influenced by the induced pore collapse and the presence of some liquid water trapped in the collapsed pores. Nitrogen adsorption/desorption and XRD spectra results show that the modifiers have strong effects on the physical properties of resultant HMS materials. Among these polar modifiers used, methanol apparently seems to be the most suitable modifier for the SCCO2 extraction of dodecylamine from HMS materials because aqueous modifier could destroy the material mesoporous structure as reflected by XRD spectra while acid modifiers could be corrosive to the extraction vessel and cause lumping of HMS. Using methanol as the modifier, the extracted template retains unmodified as identified with FTIR and may be possible for future reuse. The resultant HMS materials could have well preserved highly ordered mesoporous structure and thus possess much larger specific surface area of 1210 m2/g and higher pore volume of 1.46 cm3/g than those obtained for the other modifiers. Acknowledgements The authors would like to thank the financial support of National Natural Science Foundation of China (NNSFC-20676107) and of State Education Ministry of China (SRF Project for ROCS, SEM). References [1] R.M. Couto, T. Carvalho, L.A. Neves, R.M. Ruivo, P. Vidinha, A. Paiva, I.M. Coelhoso, S. Barreiros, P.C. Simões, Development of Ion-JellyÒ Membranes, Sep. Purif. Technol. 106 (2013) 22–31.
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