Organic template removal from hexagonal mesoporous silica by means of methanol-enhanced CO2 extraction: Effect of temperature, pressure and flow rate

Separation and Purification Technology 77 (2011) 112–119

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Organic template removal from hexagonal mesoporous silica by means of methanol-enhanced CO2 extraction: Effect of temperature, pressure and flow rate Zhen Huang a,∗ , Li Xu a , Jing-Huan Li a , S. Kawi b , A.H. Goh b a b

Tianjin Key Laboratory of Refrigeration Technology, Department of Packaging Engineering, Tianjin University of Commerce, Tianjin 300134, 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 17 June 2010 Received in revised form 11 November 2010 Accepted 27 November 2010 Keywords: Supercritical fluid extraction Mesoporous materials Methanol Amine removal

a b s t r a c t Supercritical fluid extraction was evaluated for removing the amine surfactant from hexagonal mesoporous silica materials. In this study, methanol-enhanced supercritical CO2 extraction was carried out on freshly synthesized materials by means of a dynamic method. Experiments were performed at pressure ranging from 10.0 to 25.0 MPa, temperature from 45 to 105 ◦ C, CO2 flow rate from 0.45 to 4.5 ml/min, and methanol flow rate from 0.05 to 0.5 ml/min. The influence of these parameters on the extraction has been evaluated in terms of the surfactant recovery. The results show that by using this method up to 96% of the total amine surfactant used is extracted out of as-synthesized mesoporous materials within 1 h extraction whereas only 78% of the surfactant can be removed by liquid ethanol extraction. The materials processed by supercritical CO2 extraction are found to possess better structural properties as reflected by the X-ray diffraction analysis and N2 adsorption results, and thus have higher thermal and hydrothermal stability than those obtained by liquid ethanol extraction or high temperature calcination. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Supercritical fluids (SCFs) have several features making them suitable solvents for the extraction of specialty chemicals. Most notably they can solubilize nonvolatile components at nearambient temperatures and can be completely separated from the solute via a pressure reduction. Among various SCFs, CO2 is the most widely used supercritical solvent since it has relatively low critical parameters, and it is non-toxic, non-flammable, non-corrosive, inexpensive and easy to handle. All these advantages have rendered supercritical CO2 extraction a promising and attractive alternative to separate thermally sensitive components from natural products [1–10]. For instance, Chen et al. have examined the feasibility of using supercritical CO2 (SCCO2 ) to extract triglycerides from plant seeds [1,5]. Their studies show that extraction recovery of triglyceride has markedly reached more than 96%. Moreover, Cheah et al. have compared the performance of supercritical fluid extraction (SFE) with that of pressurized liquid extraction on the recoveries of target compounds and activity of Magnolia officinalis bark extracts and found that extracts obtained by SFE demonstrated greater antioxidant bioactivity [3]. Besides selective extraction of bioactive components from natural produces, SFE can also be used to remove certain toxic components from solid matrices. Liang et al. have evaluated the use of SCCO2 extraction to remove major aris-

∗ Corresponding author. Tel.: +86 22 26686251. E-mail address: [email protected] (Z. Huang). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.11.029

tolochic acid components from two Chinese herbs for the purpose of improving the anti-inflammatory performance in disease treatment [11]. Their work has demonstrated that the content of toxic components in herbs has been greatly removed by 60–81% after SFE process. Similarly, Wang and Guan have attempted to remove toxic arsenite and arsenate from parent matrix by SCCO2 extraction coupled with ion-pairing method [12]. More recently, SCCO2 modified by suitable chelating agents has been developed to remove transition metal ions from various solid matrices. The most attractive features of this new technology include high solubilities of chelating agents and resultant metal complexes in SCCO2 [13,14]. Apart from extraction of targeted compound from various natural matrices, supercritical fluid technology has also been received ever-increasing interests in synthesizing new mesoporous materials for specific applications [15–17]. In the present work, we have attempted to SCCO2 extraction to remove the organic template from freshly synthesized hexagonal mesoporous silica (HMS). To date, a few studies have been performed on a variety of inorganic mesoporous materials for removing the organic template by modified SCCO2 extraction [18–23]. It should be noted that for successfully synthesizing these porous inorganic materials, specific organic compounds like quaternary ammonium salts [18–21], or poly(ethylene oxide)-based copolymers [22], primary amines [23], must be used as a templating agent to form well-ordered porous structure. However, these organic species must be removed to yield the void pores after the synthesis for subsequent application investigations. Usually, these organic compounds are directly burned off at high temperature, but using this way will waste large amounts

Z. Huang et al. / Separation and Purification Technology 77 (2011) 112–119

2.1. Preparation of hexagonal mesoporous silica Purely siliceous HMS was synthesized according to the method described in the literature [24,25]. Mole ratio composition used to give the final synthesis gel is tetraethylorthosilane:dodecylamine:H2 O:ethanol = 1:0.27:29.6:9.09. Based on the formula given, 25.03 g of dodecylamine (99.5%, Fluka) was weighed in a 1000 ml polypropylene bottle and mixed with 265 ml of ethanol (99.95%, Hayman) and 296 ml deionized water. The resultant mixture was vigorously stirred for about 15 min to completely dissolve dodecylamine. 104.2 g of tetraethylorthosilane (98%, Aldrich) 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 product was washed with deionized water and recovered by filtration through a Buchner funnel. The washing and filtrating process was repeated for about 4 times. The moist solid was then put in an evaporation dish and dried in an oven at 30–40 ◦ C for 2 days. The final product was sieved into a desired particle size between that using mesh no. 40 (0.425 mm) and 60 (0.250 mm).

8

3

9

6

5

1

7 Fig. 1. Schematic of the experimental apparatus used to remove the organic template from molecular sieves via the SCCO2 -modifier extraction: (1) CO2 cylinder; (2) chiller; (3) liquid pump; (4) switching valve; (5) premixing coil; (6) extraction vessel; (7) oven; (8) back pressure regulator; (9) collection tube; (10) modifier reservoir; (11) modifier pump.

where approximately 0.5 g of as-synthesized powder was loaded for each run. A 0.5 ␮m filter was placed at each end of the vessel to eliminate entrainment. The template-loaded CO2 /modifier leaving the extraction vessel was depressurized to atmospheric pressure, resulting in the deposition of the dissolved template. After 1 h extraction, the whole system was flushed for 0.5 h in order to remove any entertainer retained in the vessel or precipitated surfactant along the tubing. After the sample was collected, the same condition was then run again. Apart from SFE, the calcination and liquid solvent extraction were also considered for comparison purpose. In the former case, a small amount of dried powder was calcined at a heating rate of

a

0.35 0.30

2.2. Template extraction

0.25 0.20 0.15 0.10 0.05 0.00 0

0.1

0.2

0.3

0.4

0.5

0.6

Methanol flow rate(ml/min)

b

15

110

90

Pc (MPa)

The as-synthesized HMS materials were subsequently subjected to extraction using methanol-enhanced SCCO2 to remove the organic template used. The experimental procedure can be refereed elsewhere [18,19]. A schematic of the experimental setup is shown in Fig. 1. It has been known that pure SCCO2 cannot extract the template from freshly prepared siliceous MCM-41, MCM-48, SBA-1 and SBA-3 powders [18–21,23]. Thus, adding a polar modifier is a must due to the low solubility of polar organic surfactants in CO2 . For that reason, methanol (99.98%, Tedia) was selected as a polar liquid modifier in this study to enhance the solvating power of CO2 . The liquid methanol was directly introduced into the system by a syringe pump at different flow rates. The critical parameters for CO2 /modifier mixtures can be estimated from the literature [26,27] and given in Fig. 2. High purity carbon dioxide (99.99%, Tianjin Sifang Gas Co. Ltd., China), after liquefied at −5 ◦ C, was pumped with a constant flow rate mode into a premixing coil that is placed in a temperature-controlled oven. The premixing coil was sufficiently long to ensure that the modifier and CO2 could be thoroughly mixed prior to entering the extraction unit. The fluid of modifier and CO2 mixture was then fed into a 5.0 ml extraction unit

4

2

12 70

T c ( oC)

2. Experimental

10 11

Methanol molar fraction

of used templates and meanwhile produce terrible smells and toxic gases. Even worse, some works have evidenced that the calcination method may have resulted in collapsed or distorted pore structures as reflected by the disappearance or depletion of X-ray diffraction (XRD) peaks after calcination [24]. As such, SFE is advantageous in removing the organic template and retaining specific mesoporous structures since it works at mild temperatures. In this paper, the HMS used was synthesized by using dodecylamine as the templating agent. Methanol was utilized as a polar modifier to enhance the solvating power of pressurized CO2 fluid to the organic template. The amine removal from the HMS by means of SCCO2 extraction has been reported previously where, however, only one isotherm and two isobars are given [23]. Here, the extraction was carried out in the temperature range of 45–105 ◦ C and at pressures of 10.0–25.0 MPa. Note that the range of temperature and pressure is spanned from the subcritical to the supercritical region of the solvent mixture, which is less frequently investigated. The flow rates of liquefied CO2 and liquid modifier on the surfactant recovery were also investigated as well.

113

9 50

6

0

0.1

0.2

0.3

0.4

0.5

0.6

30

Methanol flow rate(ml/min) Fig. 2. Critical parameters of CO2 /methanol mixtures involved in this work: (a) CO2 flow rate is 1.8 ml/min and the density of pressurized CO2 at 6.0 MPa and −5 ◦ C obtained is 790 kg/m3 by interpolating the literature data [26]; the density of methanol is 791 kg/m3 ; the mixture composition value under different methanol flow rates is evaluated by using the arithmetic mixing method. (b) The critical parameters are calculated from those published data in the literature [27].

Z. Huang et al. / Separation and Purification Technology 77 (2011) 112–119

120

1200

110

1000

100

800

90 600 80 400

70

200

60 50

DTA(µV)

Weight loss(%)

114

0

100

200

300

400

500

0 600

Temperature (ºC) Fig. 3. TGA and DTA curves of as-synthesized HMS powders.

2 ◦ C/min and maintained at 650 ◦ C for a period of 4 h. In the latter, around 0.5 g of the sample powder was washed in 110 ml of ethanol at 50 ◦ C under agitation for 1 h before recovery by filtration. 2.3. Characterization of as-synthesized and SFE-treated powders Powder XRD analysis was carried out on the as-synthesized and SFE-processed materials by means of a Shimadzu XRD-6000 Spectrometer. The characterization was performed from 1.5 to 8◦ by ˚ 40 kV, and 40 mA with a scanning using Cu K␣ radiation (1.5406 A), rate of 2◦ per min (2). Nitrogen adsorption/desorption isotherms were measured on Quantachrome Autosorb-1 using high purity N2 at 77 K as the adsorbate. Before the measurements, the samples were outgassed under vacuum for 5 h at 200 ◦ C. 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. Based on the characteristic XRD peak position and BJH pore size, the pore wall thickness could be estimated. The sample usually exhibits a peak at around 2 = 2.2◦ , which corresponds to the 100 plane of the hexagonal unit cell. Using Bragg’s law and assuming first order diffraction, the structural parameter dh k l can be readily obtained from equation  = 2dh k l sin . For Cu K␣ radiation, the ˚ Thus the d1 0 0 value can be resulted wavelength  is 1.5406 A. from the diffraction angle. Next, the interpore distance (a0 , the distance between the center of two pores) √ may be calculated assuming hexagonal unit cell size a0 = 2d1 0 0 / 3. Finally, the pore wall thickness for each sample is simply obtained by subtracting the BJH pore size from the a0 value. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed, using a simultaneous Shimadzu DTG-60 analyzer (±10 ␮g) on both the as-synthesized and the SFEprocessed samples of about 5–10 mg, to determine the extraction recovery under various operating conditions by comparing their weight losses. The extraction recovery is defined as a ratio of the amount of the organic surfactant removed to the amount of the surfactant originally existing in the powder. The samples were heated to 550 ◦ C with a heating rate of 10 ◦ C/min and at a flow rate of 35 ml/min air. Fig. 3 shows the TGA and DTA results of the as-synthesized Si-HMS. As can be seen, the weight loss curve could be divided into three temperature ranges [24,25]. The first-stage before 150 ◦ C can be assigned to the desorption of water or other volatile organics adsorbed in molecular sieve apertures, and the weight loss is calculated to be 9.2% of the

actual wet mass of the sample. The second range (150–300 ◦ C), i.e., the greatest weight loss of 30.5%, is due to the decomposition and combustion of the organic amine template associated with Si–O in the sieve apertures. The third range (300–500 ◦ C) is a gradual drop of 6.3% in weight, which is mostly attributed to a small amount of dehydroxylation of the surface of HMS mesoporous materials. Hence, only about 54.0% of the actual mass can be truly attributed to that of the molecular sieve. Interestingly, the endothermic DTA signal of the last temperature range is much more significant than that of the second range even though the mass loss in the last range is only one-fifth of that in the second range. The possible explanation is that the combustion of the hydroxy groups on the pore surface evolves much more heat than that of the amine surfactant. Neglecting the mass loss arising from the adsorbed water and organics, the template mass can be calculated as about 33.6% of the dry mass of the tested sample. Following this way, the weight percentage of the retained template is obtained for Si-HMS samples having undergone SFE. By sheer comparison of the two percentages, the extraction recovery under various operating conditions such as temperatures and pressures could be determined. It should be noted that the dry weight, instead of the actual weight of the wet sample, should always be used as the basis for comparison because the moisture content varies from sample to sample. Each sample was measured twice by DTG-60 to give a recovery value with a 0.5% deviation or smaller. Since there are two samples for each SFE extraction condition, thus the amine recovery was obtained by averaging the measurements for the two samples. The maximum deviation observed for all the recovery data is less than 3.0%, giving a good indication of the reliability of the method used and the expected accuracy of experimental results obtained. 3. Results and discussion The removal process of amine surfactant, from freshly synthesized HMS powder matrices in packed extraction vessels by mans of methanol-enhanced SCCO2 extraction, is similar to those adopted for natural produce extraction [28]. For investigating the influence of the operating parameters, such as pressure, temperature, flow rate, and modifier content, on the surfactant recovery, it is better to fully understand the whole extraction process. In this work, the extraction may comprises diffusion of CO2 and methanol solvent molecules from the bulk fluid into the mesopores, adsorption of solvent molecules on the pore wall surface, formation of solute–solvent associates, desorption of solute from the pore wall, dissolution of the solute to fluid phase, formation of the solute clustering in the pores, convective transport of the dissolved solute, diffusion through the pores, and finally transport to the bulk fluid. Therefore, the process of supercritical extraction may be controlled either thermodynamically or dynamically, or both, in terms of solubility control, bulk fluid mass transfer control or interpore mass transfer limitation. 3.1. Effect of temperature Table 1 presents the experimental results obtained in 1 h under different pressures and temperatures. Fig. 4a shows the recovery vs. temperature isobars for 1 h extraction of the amine surfactant using CO2 modified by methanol. The extraction flow rate was 1.8 ml/min for CO2 and 0.2 ml/min for methanol, respectively, and this corresponds to a molar flow ratio of about 85:15. For such a mixture, the values of critical parameters are found to be about 50 ◦ C and 9.5 MPa, as given in Fig. 2. As shown in Fig. 4a, all the isobaric curves possess two maximum points, one at 50 ◦ C and the other at about 85 ◦ C. In the temperature range of 45–50 ◦ C, the extraction fluid passes through the subcritical region to critical point region. In the sub-

Z. Huang et al. / Separation and Purification Technology 77 (2011) 112–119 Table 1 Extraction recovery from as-synthesized HMS powders under different temperatures and pressures.a T (◦ C)

P(MPa)

45 50 55 60 65 70 75 80 85 90 95 100 105

10.0

12.5

15.0

17.5

20.0

22.5

25.0

75.2 91.9 65.1 53.2 76.0 86.8 90.3 94.9 95.7 93.5 89.2 85.0 80.8

65.8 77.0 60.1 48.2 59.0 61.3 65.7 67.4 72.2 69.1 65.1 61.8 57.8

59.0 69.5 55.5 41.0 50.0 53.7 55.6 58.9 61.8 56.7 54.5 47.6 44.3

75.9 93.1 59.8 60.1 64.3 68.1 70.5 76.5 80.3 84.3 82.8 79.1 73.5

80.2 91.7 64.0 60.2 61.4 65.1 73.9 77.5 82.3 76.4 73.7 70.8 68.9

70.5 90.2 62.0 58.2 62.9 63.2 66.9 70.0 76.9 73.8 71.1 69.1 67.6

78.6 93.9 66.2 61.1 68.5 73.9 79.3 83.9 91.9 92.0 90.0 87.9 86.2

a Conditions: liquefied CO2 with flow rate = 1.8 ml/min, methanol modifier with flow rate = 0.2 ml/min, time = 1 h.

critical domain, the fluid entering the extraction vessel consists of a liquid methanol and a methanol-entrained SCCO2 phase. In terms of mass transfer, the viscosity of a SCF solvent is generally comparable to that of a gas but about 100 times lower than that of a liquid whereas its diffusivity lies in the middle of those of a gas and a liquid [28]. This means that it is much easier for mass transfer to take place in SCFs than in liquids, and that the presence of the liquid methanol phase could greatly increase the level of mass transfer resistance present in the system. However, the modifier methanol has a positive effect on the surfactant solubility. It is well known that the solvating power of a fluid is an increasing function

Amine recovery (%)

a

100

80

10.0 MPa 12.5 MPa

60

20.0 MPa 25.0 MPa

40 40

60

80

100

120

140

Temperature (ºC)

Amine recovery (%)

b

100

80

50 o C 60 o C 75 oC o 85 C o 105 C

60

40

10

15

20

25

30

Pressure (MPa) Fig. 4. Temperature and pressure dependence of amine recovery from assynthesized HMS powders: liquefied CO2 with flow rate = 1.8 ml/min, methanol modifier with flow rate = 0.2 ml/min, time = 1 h.

115

of its density and the density of a liquid (0.6–1.6 g/cm3 ) is always higher than that of a SCF solvent (0.2–0.9 g/cm3 ) [28]. Hence the presence of the liquid phase may enhance the solvating power of the fluid as a whole. Furthermore, it may be noted that the solubility of a polar solute in a SCCO2 solvent can be greatly enhanced by the addition of a polar modifier [28]. As reported elsewhere [29], the dipole moment values are around 0.0, 1.70 and 0.90 Debye for carbon dioxide, methanol and the surfactant, respectively. Therefore, methanol seems to promote the dissolution of the amine analyte in SCCO2 , leading to high extraction recovery. As temperature increases the mass transfer resistance will be considerably reduced since (1) high temperature can promote the rate of solvent transfer into the matrix pores and the subsequent migration of the analyte to the bulk fluid [30]. (2) High temperature could also aid in overcoming the activation energy of desorption, thereby boosting the extraction recovery [28,31]. (3) The content of the liquid phase drops until it becomes totally eliminated at the mixture critical temperature, thus leading to appreciated decrease in mass transfer resistance. Therefore, the reduction in mass transfer resistance, together with the increase in the solute solubility, leads to the obvious increase in amine recovery as temperature increases to the mixture critical temperature of 50 ◦ C. High extraction recovery of about 90% has been achieved at 50 ◦ C over the pressure range investigated as shown in Fig. 4a, which is similar to that reported earlier [23]. This is attributed to, besides the considerable mass transfer obtained, the relatively appreciated solubility arising from high fluid density achieved around the critical point [28,32,33]. With further increase in temperature from 50 ◦ C, the surfactant recovery is observed to decrease drastically to the minimum point of 60 ◦ C. The recovery at this temperature varies in the range of 50–70%, dependent on the pressure used. This may be explained as follows. As pointed by Eckert and Knutson [34], intensive solvent molecules are clustering or agglomerating around the solute molecule in the supercritical solution, leading to the presence of large amounts of solvent/solute clusters. Subsequently, these clusters lead to a significant drop in the diffusion coefficient because of the difficulty associated with moving the entire clusters [35,36]. On the other hand, the temperature effect on the solute solubility should also be taken into account. It is known that the extraction fluid is highly compressible in the vicinity of the critical point, thus a slight increase in the temperature will decrease its density significantly and then its solvating power. However, the solubility drop from the density decrease could be cancelled, at least partially, by enhanced vaporization of the solute as it is increasing as temperature increases. Once the former is dominant over the solute vaporization, it gives rise to the diminution in the solute solubility, i.e., around the critical point the solute solubility may decrease drastically with increasing temperature at constant pressures [35]. Therefore, the decreased diffusivity and solubility diminution may account for the immense fall in the extraction recovery obtained. As seen from Fig. 4a, the decrease in recovery lasts for only about 10 ◦ C, possibly due to the significant reduction in both hindered diffusion and solubility diminution. This is because that (1) the solute-centered clustering effect may be diminished progressively by the increase in temperature whereas the transport of solutecentered clusters can be promoted simultaneously; (2) the solute vaporization is considerably enhanced and turns dominant over the density decrease effect as temperature increases further, leading to rise in solubility. As a combined effect of the above two aspects, a surge in the surfactant recovery takes place with increasing temperature. But temperature is over about 80–85 ◦ C, the recovery is found to drop down. The reason for it may possibly be due that the mass transfer limitation in the mesopores turns to be worsen with increasing temperature since (1) it is known that the porous HMS material is featured by its 2-dimensional tubular structure and the

Z. Huang et al. / Separation and Purification Technology 77 (2011) 112–119

forward flow of the surfactant-loaded fluid in the mesopores may be constrained; (2) the as-synthesized HMS powders was randomly stacked together to form the extraction bed, leading to the disorderedly distribution of the 2-D parent matrices and subsequent increase in the mass transfer resistance. Besides, the formation of channeling and solvent bypass and insufficient fluid–solute contact may be unfavorable to the analyte desorption from the pore wall and the subsequent transport of the dissolved analyte from the mesopores to the bulk fluid as temperature increases further. Therefore the analyte mass transfer is possibly constrained by the nanosized porous spaces and get worse at higher temperature over 85 ◦ C.

a Amine recovery (%)

116

100 80 60 o

85 C 20.0 MPa

40 20 0

0

1

2

3

4

5

3.2. Effect of pressure Fig. 4b illustrates the recovery isotherms obtained in 1 h at different pressures by means of methanol modified CO2 . As can be seen, these isotherms exhibit a drastic drop in the recovery as pressure increases from 10.0 MPa and produce a minimum value around 15.0 MPa. It is known from Fig. 2 that the CO2 -methanol mixture used has a critical pressure of 9.5 MPa, thus the sharp drop in recovery from 10.0 MPa may be understandable in terms of the strong clustering effect and the high fluid density taking place near the critical pressure. As discussed earlier, in the vicinity of the critical pressure, the strong solute–solvent associating interactions as well as the relatively large solvating power of the fluid is dominant and may have induced a favorable partition of the analyte form the matrix into the supercritical solvent. This effect, in turns, may have effectively prevented the subsequent re-adsorption of the extracted analyte back to the matrix, resulting in very high surfactant recovery at 10.0 MPa. However, it may be noted that the efficient mass transfer may be constrained as the condition is close to the critical region due to the slow diffusion of solvent-solute clusters. As pressure increases from near the critical pressure, the clustering induced mass transfer becomes less constrained whereas the solute dissolution could get favorable because of increased solvating power associated with elevated pressure. The extraction recovery thus obtained is observed to decrease till about 15.0 MPa and then increase thereafter. The recovery is expected to increase continuously as a result of enhanced solvent density arising from increased pressure. However, it actually turns to drop from 18.0 MPa to a minimum at around 22.5 MPa. This is perhaps a balanced effect of both the solvating power increase and the mass transfer constraint. As the pressure continues to increase, the fluid becomes more viscous and its diffusivity drops very much, thus the net solute transport has subsequently slowed down. Compared to the effect of increased solubility, the effect of the mass transfer resistance might be much more pronounced, leading to a decrease in recovery. Beyond 22.5 MPa, the mass transfer limitation may be overcome by the significant increase in solubility, causing the increase in overall extraction recovery again. 3.3. Effect of CO2 flow rate Fig. 5a shows the dependence of 1 h extraction recovery on the CO2 flow rate at 85 ◦ C, 20.0 MPa and a 9:1 volumetric flow ratio of liquefied CO2 and liquid methanol. The CO2 flow rate considered ranges up to 4.5 ml/min, and the methanol flow rate employed varies from 0.05 to 0.5 ml/min correspondently. Usually, it is reasonable that increasing the solvent flow rate could result in an increase in extraction recovery since the amount of solvent used is large and thus more analyte can be extracted. However, it is not totally correct over the whole CO2 fluid rate range for such a dynamic system, possibly due to the mass transfer limitation.

b

100

Amine recovery (%)

CO 2 flow rate (ml/min)

80 60 o

80 C 20.0 MPa

40

o

60 C 15.0 MPa 20 0

0

0.1

0.2

0.3

0.4

0.5

0.6

Methanol flow rate (ml/min) Fig. 5. Amine recovery obtained in 1 h under various fluid flow rates: (a) at a 9:1 volumetric flow ratio of liquefied CO2 and liquid methanol; (b) liquefied CO2 at a constant flow rate of 1.8 ml/min.

It can be seen from Fig. 5a, that at low flow rates (<1.8 ml/min), the amine removal is enhanced as the CO2 flow rate increases. This is understandable since there could be adequate contact time between the fluid and the matrix at low flow rates. As such, the analyte desorption from the matrix and the analyte transfer from the pores into the bulk fluid could be readily realized without any pronounced constraint [30,31]. Instead, the amine recovery is mainly governed by the solubility of the analyte in the SCF. At low flow rate, the fluid is fully saturated with the analyte before leave the extraction vessel and using low flow rate is extensively accepted for dynamically measuring the equilibrium solubility in SCFs. Hence, increase in the CO2 flow rate leads to more solvent being available for dissolving the analyte and subsequently, a corresponding elevation in the surfactant recovery. As the CO2 flow rate increases further beyond 1.8 ml/min, the amine recovery begins to level off. At high flow rates, the fluid–solute contact may be insufficient, leading to the unfavorable desorption of the analyte form the pore wall. Besides, the channeling and solvent bypass might readily be formed in the extraction bed and subsequently render the analyte to dissolve into the bulk solvent become insufficient. Thus, the modified CO2 fluid is not fully saturated with the analyte before leaving the extraction vessel. Under such circumstance, continuous increase in the fluid rate has resulted in no further increase in the analyte extraction. Therefore, there is a need to find the optimum CO2 flow rate and that is 1.8 ml/ml for present work. Above this value, the supercritical solvent becomes unsaturated and does not carry the maximum amount of the analyte that it can dissolve. 3.4. Effect of modifier flow rate Fig. 5b shows how the percentage removal of the template varies with different flow rates of the methanol modifier at the conditions

Besides methanol-enhanced SCCO2 extraction, the template removal from the HMS was also considered by using liquid extraction and calcination. The samples treated by these three methods were examined by using the XRD and N2 adsorption/desorption characterizations. The XRD spectra and nitrogen adsorption/desorption isotherms thus obtained are shown in Figs. 6 and 7, respectively. Note that for the SFE sample, 96% of the surfactant can be removed under the extraction condition of 85 ◦ C and 10.0 MPa whereas the surfactant is totally burned off at high temperature by means of calcination. In the case of liquid extraction to remove the surfactant, 110 ml of ethanol was employed to wash 0.5 g of the as-synthesized HMS sample for 1 h. The volume of ethanol used is roughly equal to that of liquid CO2 plus methanol used in the SFE process. However, only 78% of the amine used can be extracted, mostly probably due to the mass transfer resistance encountered during the transport of the liquid solvent in and out of the pores. From this regards, employing SFE is superior to using the liquid solvent extraction. As can be graphically seen in Fig. 6, three HMS samples exhibit rather similar XRD patterns, in good agreement with those reported in the literature [24,25]. But the characteristic peak is much sharper for the SFE sample, indicating that much ordered mesoporous

Liquid extraction Calcined 1

2

3

4

5

6

7

8

9

2 Theta(degree) Fig. 6. XRD spectra of HMS powders treated by different methods.

structure of HMS could be perfectly maintained after the SFE treatment, resulting in more enhanced mesoporosity than the other two cases. In the meantime, the 2 position for the HMS characteristic peak is much lower for the SFE as well, suggesting a relatively larger pore size for the resultant material after SFE treatment. Fig. 7 shows the nitrogen adsorption/desorption results of the HMS samples. The isotherms exhibit a typical inflexion in the relative pressure (P/P0 ) range of 0.1–0.3 and show a small hysteresis loop between adsorption and desorption branches in the high relative pressure range, similar to that reported for mesoporous materials [24,25]. The SFE sample may possess higher quality in well-framed mesoporous structure than the other two, as reflected by the steeper and taller isotherm in the P/P0 region of 0.1–0.5 [25]. All materials show a well-defined hysteresis loop in the nitrogen adsorption/desorption isotherms, indicating the pres-

a

900

SFE processed Pore volume (cm3/g, STP)

3.5. Properties of HMS materials

SFE processed

600

Liquid extraction Calcined

300

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P 0

b Desorption Dv(d)(a.u.)

of 80 ◦ C, 20.0 MPa and 60 ◦ C 15.0 MPa. The flow rate of liquefied CO2 was maintained at 1.8 ml/min and only the flow rate of methanol was varied. The latter has been increased to as high as 0.5 ml/min, but for practical application of SFE, the use of such high concentration of the modifier may not be wise, due to a significant change in the critical parameters of the extraction solvent and complicating the entire extraction process. The critical parameters for the mixture are graphically presented in Fig. 2. It is seen that as the flow rate of methanol increases, the critical temperature monotonically increases from 31.1 ◦ C of pure CO2 to about 90.9 ◦ C and the critical pressure from 7.38 MPa to 14.42 MPa. The two extraction temperatures used are roughly correspondent to the methanol flow rates of 0.28 and 0.42 ml/min, respectively. Hence, as the methanol flow rate increases the system turns from the supercritical to the critical, and then to the subcritical region. As shown in Fig. 5b, the extraction recovery linearly and rapidly increases with the methanol flow rate, and then gradually slows down to the peak near the critical temperature. The dependence observed is understandable since as the methanol flow rate increases, i.e., more amounts of modifier is added into the system, the solvating power of the solvent mixture to the amine surfactant is greatly improved, rendering the amine readily extracted out of the matrix. The extraction recovery seems to be completely solubility controlled. This may be clearly explained in terms of the polarity of the solvent. Since the dipole moment of the amine analyte is inbetween those of methanol and SCCO2 [29], hence, increasing the methanol content in the extraction fluid could make the molecular interactions among the CO2 /methanol fluid and the amine analyte strong, thereby elevating the solvating power and boosting the amine recovery. With more methanol continuously added, the system approaches the critical region and the cluster-induced mass transfer constraint takes into effect. Thus as a combined effect of both the mass transfer and the solubility limitations, the extraction recovery reaches a peak around the critical temperature. Further increasing the methanol flow rate may lead to a transition of the fluid from supercritical to subcritical region and subsequently the amine recovery turns to drop as due to the presence of liquid methanol phase, which could impose a great deal of mass transfer resistance to the system due to the relatively low diffusivity and high viscosity of liquid [28].

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Intensity

Z. Huang et al. / Separation and Purification Technology 77 (2011) 112–119

SFE processed

Liquid extraction Calcined 10

30

50

70

90

Pore size(Å) Fig. 7. Nitrogen adsorption/desorption isotherms (a) and pore size distribution (b) of HMS samples treated by different methods.

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Table 2 Physical properties of HMS samples obtained by different treatments. Samples a

Calcination Liquid extractionb SFEc a b c

˚ BJH pore size (A)

BET surface area (m2 /g)

˚ Pore wall thickness (A)

Total pore volume (cm3 /g)

24.9 26.2 27.8

1072 871 1210

18.3 18.5 19.4

1.04 0.87 1.46

By calcination. By liquid methanol extraction. By 1 h SCCO2 /methanol extraction at 85 ◦ C and 10.0 MPa.

ence of substantial interparticle mesoporosity composed of the non-crystalline intra-aggregate voids and spaces. On the basis of N2 adsorption data, the structural properties are summarized in Table 2 for HMS upon calcination, liquid extraction and SFE. As ˚ can be seen, directly calcined HMS has a BJH pore size of 24.9 A, and its BET surface area and total pore volume are 1072 m2 /g and 1.04 cm3 /g, respectively. After liquid ethanol extraction, HMS exhibits a surface area of 871 m2 /g, pore volume and 0.87 cm3 /g, ˚ The much lower surface area and pore voland a pore size of 26.2 A. ume thus obtained is probably related to that remarkable amount of the amine analyte has still retained in the HMS material after treated by liquid ethanol extraction. The value of these two structural parameters could become higher if the extraction procedure was repeated at least twice or a subsequent calcination was taken [25]. In the case of using the methanol-SFE process, HMS has a ˚ which is larger than those of calcined HMS mesopore size of 27.8 A, ˚ and ethanol-processed HMS (26.2 A). ˚ The smallest pore (24.9 A) size for the calcined HMS may result from the mesopore contraction caused by high temperature calcination. Apart from the pore size, the specific surface area (1210 m2 /g) and total pore volume (1.46 cm3 /g) are much larger than those of HMS samples treated by high temperature calcination or liquid extraction, possibly related to well-preservation of mesoporous structure after mild methanolmodified SCCO2 extraction. Besides, the HMS has a thicker pore wall for the SFE treatment, as shown in Table 2. Considering the more ordered mesoporous structure and thicker pore wall obtained from the SFE treatment, the resultant HMS may be expected to afford higher thermal and hydrothermal stability for practical applications. These differences in structural properties seem to suggest the methanol-modified SFE is a feasible and efficient technique for surfactant removal from HMS materials. 4. Conclusions The extraction of pressurized CO2 /methanol has been performed on as-synthesized porous siliceous HMS powders over wide ranges of pressure, temperature and flow rate. The following conclusions may be drawn from this work: (1) The surfactant recovery obtained 1 h of extraction varies in the range of 30–96%, strongly dependent on the extraction pressure, temperature and flow rates employed. (2) The isobaric recovery is found to have two maximum values at about 50 and 85 ◦ C, respectively. The former is mainly due to the appreciated solubility obtained around the critical point while the latter is attributed to the pronounced effect of the mass transfer resistance. (3) Each recovery isotherm has two minimum values at about 15.0 and 22.5 MPa, respectively, due to that the decrease in efficient mass transfer rate has cancelled the positive effect of increased solvent density on solubility. At low pressure the slow diffusion of solvent-solute associated clusters takes control while the viscous fluid becomes dominant at high pressure. (4) The surfactant recovery is observed to increase with the CO2 fluid rate but it levels off at high fluid rate because there is no

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