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Adsorption equilibria of benzoic acid on silica gel from supercritical carbon dioxide
J. of Supercritical Fluids 54 (2010) 237–242
Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Adsorption equilibria of benzoic acid on silica gel from supercritical carbon dioxide Berta Carola Pérez Molina, Monika Johannsen ∗ Technische Universität Hamburg – Harburg, Institut für Thermische Verfahrenstechnik, Eissendorfer Straße 38, 21073 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 19 May 2010 Accepted 19 May 2010 Keywords: Adsorption isotherm Benzoic acid Silica gel Carbon dioxide
a b s t r a c t Supercritical fluids, especially carbon dioxide, are increasingly used as carriers for adsorption–desorption processes, particularly in the pharmaceutical industry. Nevertheless, equilibria data for such processes are rather limited. Therefore, in this work, the adsorption equilibria of benzoic acid onto non-modified silica gel from scCO2 were evaluated by Supercritical Fluid Chromatography applying the Peak Maxima method. Solubility of benzoic acid in scCO2 was enhanced by addition of 2-propanol. The effects of modifier content, temperature and pressure on the solute loading were investigated. Experimental data were best described by the cubic Hill isotherm model, which accounted for the change of curvature of the elution profiles observed as concentrations in the mobile phase increased. For the concentration range reached in this study (up to 6 mg/mL), adsorption of benzoic acid was favoured at low modifier contents, high temperatures and low pressures, conditions at which the solvating power of the modified scCO2 decreased. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Although supercritical fluids were discovered more than 175 years ago, their first commercial applications were only implemented in the late 1970s with the production of decaffeinated coffee and hops aroma. Since then supercritical fluid technology has been extensively applied to many processes (extraction, upgrading and purification, cleaning, particle design, chromatography, dyeing and impregnation, reaction), being CO2 the most commonly used supercritical fluid due to its low critical temperature, low price and abundant availability [1,2]. For those applications involving adsorption–desorption processes, the use of supercritical fluids as solvents is of great interest due to their low viscosity and high diffusivity, which allow a rapid equilibration and micropore penetration of the fluid phase. The design, feasibility and optimisation of such adsorption processes normally requires the knowledge of the thermodynamic equilibria between the mobile phase, the stationary phase and the components of the mixture to be separated [3,4]. Nevertheless few studies have been reported on adsorption isotherms from supercritical fluids [5–16]. The adsorption equilibrium of toluene and ethylene benzene from scCO2 on activated carbon was investigated [5,6]. Breakthrough curves and adsorption equilibria of a mixture of 13
∗ Corresponding author. E-mail address: [email protected] (M. Johannsen). 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.05.015
terpenes from scCO2 on silica gel at different temperatures and constant CO2 density were studied [7]. The single-compound adsorption behaviour was represented by Langmuir isotherms, and competitive adsorption of terpenes and scCO2 was observed. Adsorption of scCO2 on silica gel was determined gravimetrically [8]. The significant amounts of CO2 adsorbed on the solid surface showed that CO2 cannot be regarded as a non-adsorbed substance. Adsorption of Vitamin D3 from CO2 modified with ethanol was measured on a modified silica gel by the Peak Maxima method [9] and fitted by the Hill isotherm model. Adsorption of benzoic acid, salicylic acid and acetylsalicylic acid on alumina, silica gel and amberlite from saturated CO2 fluid phases were studied by a static method and simulated by non-linear partial least squares [10,11]. Single and multicomponent adsorption isotherms of ␣- and ␦-tocopherol from a mixture scCO2 – 2-propanol were analysed on two unmodified silica gels using perturbation chromatographic experiments [12,13]. Experimental data for single component isotherms were fitted by the cubic Hill expression, and showed an anti-Langmuir behaviour with increasing concentration. Competitive mixture adsorption was predicted by means of the Ideal Absorbed Solution Theory (IAST). Adsorption of o-hydroxybenzoic acid from scCO2 on the impregnation of poly(methyl methacrylate) (PMMA) was studied using a stirred batch reactor [14]. Equilibria data were correlated by the Toth equation. Adsorption equilibria of eicosapentaenoic acid ethyl ester and docosahexaenoic acid ethyl ester from scCO2 on silica gel were studied using the elution by characteristic point (ECP) method [15]. Isotherm data were fitted by the Langmuir model. Determination of adsorption isotherms of
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maxima as [4]: Nomenclature Q C tR tM εt qs b1 b2 b3 w/w
solute loading in the stationary phase (mg/mL) solute concentration in the mobile phase (mg/mL) retention time (min) hold-up time (min) total porosity saturation loading in the stationary phase (mg/mL) coefficient of Hill isotherm (mL/mg) coefficient of Hill isotherm (mL2 /mg2 ) coefficient of Hill isotherm (mL3 /mg3 ) weight ratio
Indices i max
component at peak maxima
Abbreviations scCO2 supercritical carbon dioxide UV ultraviolet
tRi (ci,max ) − tM εt dQi = · tM 1 − εt dCi Ci,max
For the determination of adsorption isotherms, theoretical adsorption models (e.g. Langmuir, Freundlich, and Toth) in their derivative form are evaluated to best fit the experimental data obtained from Eq. (1). Fitting parameters are then applied in the corresponding isotherm equation to calculate the adsorption isotherm in the concentration range studied. Single component adsorption behaviour was well described in this study by the Hill adsorption isotherm. This model was derived by Hill [21] from statistical thermodynamics, and allows describing isotherms of various types, including those with inflexion points. The cubic form of the Hill model and its derivative were here applied (Eqs. (2) and (3)). Q =
qs b1 c + 2b2 c 2 + 3b3 c 3 3 1 + b1 c + b2 c 2 + b3 c 3
2. Fundamentals Thermodynamic equilibrium of adsorption–desorption processes is normally described by adsorption isotherms. Due to the complex interactions between adsorbent and solute, these thermodynamic functions cannot be predicted up to now by mathematical methods and must be determined experimentally. Different techniques have been developed for this purpose including static and dynamic methods [4,17]. The selection of a particular method depends mostly on the specific system (price and availability of pure substances, column efficiency, the possibilities of quantitative detection), on the available equipment and on accuracy and time constraints [4]. In the present work, the Peak Maxima method has been applied for the determination of the adsorption isotherms of benzoic acid from scCO2 . This is a simple and efficient method which provides good accuracy at low and medium concentrations [9,17–20]. Compared to other methods (i.e. gravimetry, frontal analysis, pertubation, elution by characteristic point), the Peak Maxima method also requires smaller amounts of sample, and neither column equilibration nor very high efficiency columns are needed [4]. By the Peak Maxima method, the column is systematically overloaded by injection of samples of increasing concentration. Chromatograms exhibiting both compressive and dispersive part are obtained by elution. Assuming instant equilibrium between the fluid phase and the stationary phase (equilibrium theory of chromatography), the slopes of the isotherm at the peak maxima dQ/dC(Ci,max ) can be calculated from the retention times at the peak
(2)
qs b1 + b1 b2 c 2 + 4b1 b3 c 3 + 4b2 c + b2 b3 c 4 + 9b3 c 2 dQ = 2 3 dC (1 + b c + b c 2 + b c 3 ) 1
artemisin on silica gel from scCO2 by frontal analysis has been reported [16]. Improved competitive adsorption of CO2 on silica gel was observed as pressure increased. Adsorption isotherms were correlated by Freundlich model corrected by means of a Real Adsorption Solution Theory (RAST). In this work, the adsorption equilibria of benzoic acid on silica gel from scCO2 were determined by Supercritical Fluid Chromatography using the Peak Maxima method. The chosen solute was considered as a model substance for the supercritical adsorption of low vapour pressure (i.e. low volatility/high sublimation temperature) and thermally-labile compounds. The effects of temperature, pressure and modifier content governing the adsorption process at supercritical conditions were analysed.
(1)
2
(3)
3
3. Experimental 3.1. Materials Carbon dioxide (CO2 ) with purity higher than 99.95% (grade 3.5) was purchased from Westfalen GmbH (Bochum, Germany). 2-Propanol (analytical grade), which was applied in this work as modifier, was donated by BASF (Ludwigshafen, Germany). Benzene (GC purity >99.5%) for solute dissolution was supplied by Fluka Chemika (Buchs, Switzerland) and n-hexane (purity >95%) was obtained from Lab-Scan Analytical Sciences (Sowinskiego, Poland). Benzoic acid of analytical reagent grade (purity >99.5%) was purchased from Merck (Darmstadt, Germany). A commercial pre-packed analytical column (250 mm × 4.6 mm) kindly donated by Kromasil® was used for the experiments. The column was packed with non-modified silica gel with a particle size of 5 m, a specific surface area of 540 m2 /g and a mean pore size of 80 Å. 3.2. Experimental apparatus The Peak Maxima method was chosen to measure the adsorption isotherms of benzoic acid on silica gel. The experimental apparatus for adsorption analysis is shown in Fig. 1. It mainly consists of a fluid preparation unit; a chromatographic system coupled to a UV detector, and pressure compression and expansion modules. In this experimental setup, gaseous CO2 supplied from a storage tank at approximately 7 MPa is initially condensed and then compressed to 30–35 MPa by a pneumatically driven pump. The pressure is then decreased to nearly the inlet pressure by a pressure reducer valve. The liquefied CO2 is afterwards passed through a 0.5 m sintered metal filter to remove impurities. Both CO2 and modifier are then fed to the system by high pressure syringe pumps which allow a free pulsation supply and constant volumetric flow rates. The fluid streams are mixed in a dynamic mixing chamber and heated up to the desired pressure in an oven where they reach the supercritical state. The column and the switching valves are also placed inside the oven. The pressure is controlled by a back pressure valve located in the pressure expansion module. Samples of benzoic acid of different concentration are injected manually into the system by a six-port position valve and carried by the supercritical fluid through the silica gel column. The elution
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239
Fig. 1. Flow sheet of experimental setup for measuring of adsorption isotherms.
profiles at the outlet of the column are detected by a UV detector equipped with a high pressure flow cell and recorded by chromatographic software for further analysis. The supercritical fluid is depressurised to about 1 MPa by heated backpressure regulators and the precipitated eluted substances and modifier are collected in a container.
Absolute calibration of UV detector was performed by flushing the detector cell with a series of solutions of known concentration. The resulting UV detector response was calibrated by a polynomial function [12,23,24]. The calculation of the coefficients was carried out in MATLAB® [25] by minimisation of the difference between the estimated and the known injected amounts of benzoic acid (Least Squares method).
3.3. Experimental procedure
4. Results and discussion
Adsorption isotherms of benzoic acid were measured at different modifier contents, temperatures and pressures. Modifier contents and temperatures were varied from 2 to 10 wt% and from 318.15 to 328.15 K, respectively. Mean pressure in the column was set to 14, 16 and 18 MPa. Pressure drop through the adsorption column was less than 1.2 MPa for all conditions. Pressure and temperature variations were controlled to be within ±0.2 MPa and ±1 K, respectively. Solubility of benzoic acid in scCO2 was enhanced by the addition of a modifier. In this work, 2-propanol was selected based on the solubility parameter criteria and the fact that smaller molecules, such as methanol, have higher accessibility to active sites. Total flow rates were in the range of 1.5–2.0 mL/min. Densities of CO2 were calculated by the Span and Wagner equation of state using the Dynamic Link Library for CO2 developed by Ruhr Universität Bochum [22]. A 50 L sample loop was applied to overload the column producing non-symmetric, non-Gaussian peaks even at small concentrations. For injection of benzoic acid into the system, different dissolution solvents were tested: n-hexane, cyclohexane, carbon tetrachloride, benzene, 2-propanol and tetrahydrofuran. Samples were prepared in benzene since no distortion and no band splitting of the main peak was observed with increasing injection volumes. Apart from the Peak Maxima experiments, the hold-up volume of the system and porosity of the column were analysed. This was performed by injection of n-hexane which it is considered not to be adsorbed on the silica packing. For the determination of the hold-up volume, the column was replaced by a pipe connector of negligible volume.
4.1. Process parameters The system hold-up volume (without column) was determined from five consecutive chromatograms and averages to Vsystem = 0.600 mL. Porosity was derived from 10 chromatograms for each operating condition. For the Kromasil® silica gel a slight dependence on temperature and pressure was observed as previously reported by Poplewska et al. [26] for octadecyl-bonded silica, and by Lübbert [12] for non-modified silica, respectively. Porosity values ranged from 0.613 to 0.684. For both, system hold-up volume and porosity, relative standard deviation of hold-up times was less than 1%. Column efficiency obtained by injection of a 5 L solution of benzoic acid (1.0 mg/mL) was higher than 4000 theoretical plates. 4.2. Adsorption isotherms Typical elution profiles of benzoic acid are shown in Fig. 2. A change of the curvature from a Langmuir type to an anti-Langmuir type was discernable as the concentration of benzoic acid increased. This behaviour was observed for all operating conditions and it may be related to an energetic heterogeneity (i.e. uneven distribution of surface energy) of the adsorbent [27] or to dimerisation of benzoic acid in the mobile phase [28,29] with solute concentration. According to the work of Sohn and Kim [27] on the adsorption of solutes onto solid surfaces in solution systems, the concentration of solute affects both adsorption and desorption stages. Then, benzoic acid may exhibit a tendency to adsorb onto more active
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Fig. 2. Elution peaks of benzoic acid with increasing concentration obtained at 16 MPa, 328.15 K and X2-propanol = 5% (w/w) (Cinj = 79.80/40.15/20.23/10.06/7.46/5.06/2.52/1.07 mg/mL).
Fig. 4. Isotherms of benzoic acid for different modifier contents at 16 MPa and 328.15 K (including experimental points).
Fig. 5. Isotherms of benzoic acid for different temperatures at 16 MPa and X2-propanol = 5% (w/w). Fig. 3. Isotherm slope fitting of benzoic acid at 16 MPa, 328.15 K and X2-propanol = 5% (w/w).
sites where pre-adsorbed molecules, such as 2-propanol or CO2 , can be easily displaced. Additionally, solute–solute interactions in the mobile phase (dimerisation of benzoic acid) lead to the formation of solvent–cosolvent–solute hydrogen bonded complexes. The concentration of such complexes with solute concentration might also affect how benzoic acid monomeric and dimeric species are adsorbed onto the silica gel inducing a shift of the adsorption shape. Unusual change in the shape of the isotherm with concentration was also reported by Diankov et al. [14] for adsorption of hydrobenzoic acid onto poly(methyl methacrylate) in scCO2 . The effect was mainly attributed to the co-existence of glassy and rubbery parts in the polymers exhibiting different solute sorption properties. The slopes of the different isotherms were calculated from the measured retention times according to Eq. (1). For the studied operating conditions, all resulting experimental data were best fitted by the cubic Hill isotherm model in its derivative form as illustrated in Fig. 3 for the experimental data reported in Fig. 2. Standard devi-
ations t for the fitting of the local total derivatives dQi /dCi were estimated from Eq. (4) and calculated values were less than 1%.
2 1 (dq/dc)theor − (dq/dc)exp k k t (%) = × 100 exp N (dq/dc)
(4)
k
Since the maximum concentration reached in this study for benzoic acid in the fluid phase (6 mg/mL) is far away from saturation conditions, isotherms are only valid in the concentration range they were fitted to. Solubility data reported in the literature for benzoic acid, at 328.15 K, in scCO2 with ethanol as cosolvent (2 mol%) vary from ∼12.8 mg/mL to ∼32.1 mg/mL, at pressures ranging from 13 to 20 MPa [30]. Parameters of the cubic Hill isotherm model are reported in Table 1. Adsorption isotherms of benzoic acid from scCO2 on silica gel are shown in Figs. 4–6. The solid line indicates the region where fitted isotherms are based on measured data. Although the Hill model described better the experimental data compared to the
Table 1 Hill isotherm parameters and goodness of fit. Pressure (MPa)
Temperature (K)
16 16 16 16 14 18
328.15 328.15 328.15 318.15 328.15 328.15
X2-propanol (%, w/w) 2 5 10 5 5 5
b1 (mL/mg)
b2 (mL2 /mg2 )
b3 (mL3 /mg3 )
qs (mg/mL)
r2
t (%)
2.87E−01 1.16E−01 8.37E−02 1.85E−01 8.66E−02 1.71E−01
2.29E−02 4.00E−03 2.69E−03 7.69E−03 3.36E−03 7.02E−03
5.66E−03 4.91E−04 1.82E−04 1.97E−03 4.24E−05 1.41E−03
94.37 81.67 60.87 48.72 116.80 56.13
0.9846 0.9754 0.9682 0.9721 0.9897 0.9451
0.38 0.35 0.45 0.63 0.68 0.67
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5. Conclusions Using the Peak Maxima method, adsorption isotherms of benzoic acid on a non-modified silica gel from scCO2 modified with 2-propanol were measured. Elution profiles diverted from Langmuir to anti-Langmuir fronts as concentration increased, probably due to an energetic heterogeneity of sites and dimerisation of benzoic acid in the fluid phase. Isotherms were determined at different modifier content, temperature and pressure, showing a linear adsorption trend in the concentration range reached in this study (up to 6 mg/mL). In general, adsorption of benzoic acid was greatly affected by the solvating power of the modified scCO2 , being favoured at low modifier contents, high temperatures and low pressures. Acquired data can be used for design and simulation of adsorptive separation processes of carboxylic acids. Fig. 6. Isotherms of benzoic acid for different pressures at 328.15 K and X2-propanol = 5% (w/w).
Acknowledgements The authors gratefully acknowledge the partial financial support of this research by the Evonik Stiftung and the assistance of Dr. M.H. Chuang during the initial operation of the equipment.
linear model, as seen in Fig. 3, equilibrium loadings of the low volatile solute show an almost linear dependence on the concentration of benzoic acid in the modified scCO2 . Same dependence was reported by Diankov et al. [14] for the adsorption of salicylic acid on poly(methyl methacrylate) from scCO2 at low-to-medium concentrations. As shown in Fig. 4, at definite pressure and temperature, the adsorption amount of benzoic acid decreased as the concentration of modifier added to CO2 increased. This decrease of the isotherm slopes observed in Fig. 4 can be partially attributed to the following effects: first, an increase of the solvating power of CO2 caused by the modifier [30], which increases the bonding force between the solute and the fluid phase. Second, an increased adsorption of the modifier on the silica gel surface (competitive solute-modifier adsorption). And third, an increased dimerisation of benzoic acid in the fluid phase as the concentration of the modifier increases [29]. Adsorption isotherms of benzoic acid on silica gel at different temperatures (318.15 and 328.15 K) when pressure and modifier content were kept constant at 16 MPa and 5% (w/w), respectively, are shown in Fig. 5. It is noticeably seen that adsorption of benzoic acid was favoured at higher temperatures as the adsorbed amount of solute increased as temperature rised. This effect can be related to the decreased density of the fluid phase with increasing temperatures [31]. Like so, solute solubility is reduced and adsorption of benzoic acid onto the solid surface is enhanced. Moreover, by increasing temperature, not only the diffusion resistance decreases due to a reduced viscosity of the fluid phase, but also dimerisation of benzoic acid diminishes [32]. Adsorption capacity of silica gel for benzoic acid was also studied at different pressures (14, 16 and 18 MPa) at a temperature of 323 K and a modifier content of 5% (w/w). As seen from the corresponding isotherms shown in Fig. 6, adsorption amount of benzoic acid on silica gel decreased as the pressure increased. The difference in surface loading between 16 and 18 MPa is better distinguished from the chromatograms (Cinj = 7.5 mg/mL) enclosed in Fig. 6. The decreasing adsorption trend with pressure can be partially explained by an increased density of the fluid phase [29]. This leads to a higher solvating power of the modified scCO2 [30] and reduces therefore adsorption. Also competitive adsorption of scCO2 on silica gel is enhanced with the increase of pressure [14,16]. Compared to adsorption of other compounds on silica gel from supercritical solutions, such as ␣- and ␦-tocopherol [12,13], the adsorbed amount of benzoic acid on silica gel from modified scCO2 was significantly lower by a factor of 5–10.
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Contents lists available at ScienceDirect
The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu
Adsorption equilibria of benzoic acid on silica gel from supercritical carbon dioxide Berta Carola Pérez Molina, Monika Johannsen ∗ Technische Universität Hamburg – Harburg, Institut für Thermische Verfahrenstechnik, Eissendorfer Straße 38, 21073 Hamburg, Germany
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 19 May 2010 Accepted 19 May 2010 Keywords: Adsorption isotherm Benzoic acid Silica gel Carbon dioxide
a b s t r a c t Supercritical fluids, especially carbon dioxide, are increasingly used as carriers for adsorption–desorption processes, particularly in the pharmaceutical industry. Nevertheless, equilibria data for such processes are rather limited. Therefore, in this work, the adsorption equilibria of benzoic acid onto non-modified silica gel from scCO2 were evaluated by Supercritical Fluid Chromatography applying the Peak Maxima method. Solubility of benzoic acid in scCO2 was enhanced by addition of 2-propanol. The effects of modifier content, temperature and pressure on the solute loading were investigated. Experimental data were best described by the cubic Hill isotherm model, which accounted for the change of curvature of the elution profiles observed as concentrations in the mobile phase increased. For the concentration range reached in this study (up to 6 mg/mL), adsorption of benzoic acid was favoured at low modifier contents, high temperatures and low pressures, conditions at which the solvating power of the modified scCO2 decreased. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Although supercritical fluids were discovered more than 175 years ago, their first commercial applications were only implemented in the late 1970s with the production of decaffeinated coffee and hops aroma. Since then supercritical fluid technology has been extensively applied to many processes (extraction, upgrading and purification, cleaning, particle design, chromatography, dyeing and impregnation, reaction), being CO2 the most commonly used supercritical fluid due to its low critical temperature, low price and abundant availability [1,2]. For those applications involving adsorption–desorption processes, the use of supercritical fluids as solvents is of great interest due to their low viscosity and high diffusivity, which allow a rapid equilibration and micropore penetration of the fluid phase. The design, feasibility and optimisation of such adsorption processes normally requires the knowledge of the thermodynamic equilibria between the mobile phase, the stationary phase and the components of the mixture to be separated [3,4]. Nevertheless few studies have been reported on adsorption isotherms from supercritical fluids [5–16]. The adsorption equilibrium of toluene and ethylene benzene from scCO2 on activated carbon was investigated [5,6]. Breakthrough curves and adsorption equilibria of a mixture of 13
∗ Corresponding author. E-mail address: [email protected] (M. Johannsen). 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.05.015
terpenes from scCO2 on silica gel at different temperatures and constant CO2 density were studied [7]. The single-compound adsorption behaviour was represented by Langmuir isotherms, and competitive adsorption of terpenes and scCO2 was observed. Adsorption of scCO2 on silica gel was determined gravimetrically [8]. The significant amounts of CO2 adsorbed on the solid surface showed that CO2 cannot be regarded as a non-adsorbed substance. Adsorption of Vitamin D3 from CO2 modified with ethanol was measured on a modified silica gel by the Peak Maxima method [9] and fitted by the Hill isotherm model. Adsorption of benzoic acid, salicylic acid and acetylsalicylic acid on alumina, silica gel and amberlite from saturated CO2 fluid phases were studied by a static method and simulated by non-linear partial least squares [10,11]. Single and multicomponent adsorption isotherms of ␣- and ␦-tocopherol from a mixture scCO2 – 2-propanol were analysed on two unmodified silica gels using perturbation chromatographic experiments [12,13]. Experimental data for single component isotherms were fitted by the cubic Hill expression, and showed an anti-Langmuir behaviour with increasing concentration. Competitive mixture adsorption was predicted by means of the Ideal Absorbed Solution Theory (IAST). Adsorption of o-hydroxybenzoic acid from scCO2 on the impregnation of poly(methyl methacrylate) (PMMA) was studied using a stirred batch reactor [14]. Equilibria data were correlated by the Toth equation. Adsorption equilibria of eicosapentaenoic acid ethyl ester and docosahexaenoic acid ethyl ester from scCO2 on silica gel were studied using the elution by characteristic point (ECP) method [15]. Isotherm data were fitted by the Langmuir model. Determination of adsorption isotherms of
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maxima as [4]: Nomenclature Q C tR tM εt qs b1 b2 b3 w/w
solute loading in the stationary phase (mg/mL) solute concentration in the mobile phase (mg/mL) retention time (min) hold-up time (min) total porosity saturation loading in the stationary phase (mg/mL) coefficient of Hill isotherm (mL/mg) coefficient of Hill isotherm (mL2 /mg2 ) coefficient of Hill isotherm (mL3 /mg3 ) weight ratio
Indices i max
component at peak maxima
Abbreviations scCO2 supercritical carbon dioxide UV ultraviolet
tRi (ci,max ) − tM εt dQi = · tM 1 − εt dCi Ci,max
For the determination of adsorption isotherms, theoretical adsorption models (e.g. Langmuir, Freundlich, and Toth) in their derivative form are evaluated to best fit the experimental data obtained from Eq. (1). Fitting parameters are then applied in the corresponding isotherm equation to calculate the adsorption isotherm in the concentration range studied. Single component adsorption behaviour was well described in this study by the Hill adsorption isotherm. This model was derived by Hill [21] from statistical thermodynamics, and allows describing isotherms of various types, including those with inflexion points. The cubic form of the Hill model and its derivative were here applied (Eqs. (2) and (3)). Q =
qs b1 c + 2b2 c 2 + 3b3 c 3 3 1 + b1 c + b2 c 2 + b3 c 3
2. Fundamentals Thermodynamic equilibrium of adsorption–desorption processes is normally described by adsorption isotherms. Due to the complex interactions between adsorbent and solute, these thermodynamic functions cannot be predicted up to now by mathematical methods and must be determined experimentally. Different techniques have been developed for this purpose including static and dynamic methods [4,17]. The selection of a particular method depends mostly on the specific system (price and availability of pure substances, column efficiency, the possibilities of quantitative detection), on the available equipment and on accuracy and time constraints [4]. In the present work, the Peak Maxima method has been applied for the determination of the adsorption isotherms of benzoic acid from scCO2 . This is a simple and efficient method which provides good accuracy at low and medium concentrations [9,17–20]. Compared to other methods (i.e. gravimetry, frontal analysis, pertubation, elution by characteristic point), the Peak Maxima method also requires smaller amounts of sample, and neither column equilibration nor very high efficiency columns are needed [4]. By the Peak Maxima method, the column is systematically overloaded by injection of samples of increasing concentration. Chromatograms exhibiting both compressive and dispersive part are obtained by elution. Assuming instant equilibrium between the fluid phase and the stationary phase (equilibrium theory of chromatography), the slopes of the isotherm at the peak maxima dQ/dC(Ci,max ) can be calculated from the retention times at the peak
(2)
qs b1 + b1 b2 c 2 + 4b1 b3 c 3 + 4b2 c + b2 b3 c 4 + 9b3 c 2 dQ = 2 3 dC (1 + b c + b c 2 + b c 3 ) 1
artemisin on silica gel from scCO2 by frontal analysis has been reported [16]. Improved competitive adsorption of CO2 on silica gel was observed as pressure increased. Adsorption isotherms were correlated by Freundlich model corrected by means of a Real Adsorption Solution Theory (RAST). In this work, the adsorption equilibria of benzoic acid on silica gel from scCO2 were determined by Supercritical Fluid Chromatography using the Peak Maxima method. The chosen solute was considered as a model substance for the supercritical adsorption of low vapour pressure (i.e. low volatility/high sublimation temperature) and thermally-labile compounds. The effects of temperature, pressure and modifier content governing the adsorption process at supercritical conditions were analysed.
(1)
2
(3)
3
3. Experimental 3.1. Materials Carbon dioxide (CO2 ) with purity higher than 99.95% (grade 3.5) was purchased from Westfalen GmbH (Bochum, Germany). 2-Propanol (analytical grade), which was applied in this work as modifier, was donated by BASF (Ludwigshafen, Germany). Benzene (GC purity >99.5%) for solute dissolution was supplied by Fluka Chemika (Buchs, Switzerland) and n-hexane (purity >95%) was obtained from Lab-Scan Analytical Sciences (Sowinskiego, Poland). Benzoic acid of analytical reagent grade (purity >99.5%) was purchased from Merck (Darmstadt, Germany). A commercial pre-packed analytical column (250 mm × 4.6 mm) kindly donated by Kromasil® was used for the experiments. The column was packed with non-modified silica gel with a particle size of 5 m, a specific surface area of 540 m2 /g and a mean pore size of 80 Å. 3.2. Experimental apparatus The Peak Maxima method was chosen to measure the adsorption isotherms of benzoic acid on silica gel. The experimental apparatus for adsorption analysis is shown in Fig. 1. It mainly consists of a fluid preparation unit; a chromatographic system coupled to a UV detector, and pressure compression and expansion modules. In this experimental setup, gaseous CO2 supplied from a storage tank at approximately 7 MPa is initially condensed and then compressed to 30–35 MPa by a pneumatically driven pump. The pressure is then decreased to nearly the inlet pressure by a pressure reducer valve. The liquefied CO2 is afterwards passed through a 0.5 m sintered metal filter to remove impurities. Both CO2 and modifier are then fed to the system by high pressure syringe pumps which allow a free pulsation supply and constant volumetric flow rates. The fluid streams are mixed in a dynamic mixing chamber and heated up to the desired pressure in an oven where they reach the supercritical state. The column and the switching valves are also placed inside the oven. The pressure is controlled by a back pressure valve located in the pressure expansion module. Samples of benzoic acid of different concentration are injected manually into the system by a six-port position valve and carried by the supercritical fluid through the silica gel column. The elution
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Fig. 1. Flow sheet of experimental setup for measuring of adsorption isotherms.
profiles at the outlet of the column are detected by a UV detector equipped with a high pressure flow cell and recorded by chromatographic software for further analysis. The supercritical fluid is depressurised to about 1 MPa by heated backpressure regulators and the precipitated eluted substances and modifier are collected in a container.
Absolute calibration of UV detector was performed by flushing the detector cell with a series of solutions of known concentration. The resulting UV detector response was calibrated by a polynomial function [12,23,24]. The calculation of the coefficients was carried out in MATLAB® [25] by minimisation of the difference between the estimated and the known injected amounts of benzoic acid (Least Squares method).
3.3. Experimental procedure
4. Results and discussion
Adsorption isotherms of benzoic acid were measured at different modifier contents, temperatures and pressures. Modifier contents and temperatures were varied from 2 to 10 wt% and from 318.15 to 328.15 K, respectively. Mean pressure in the column was set to 14, 16 and 18 MPa. Pressure drop through the adsorption column was less than 1.2 MPa for all conditions. Pressure and temperature variations were controlled to be within ±0.2 MPa and ±1 K, respectively. Solubility of benzoic acid in scCO2 was enhanced by the addition of a modifier. In this work, 2-propanol was selected based on the solubility parameter criteria and the fact that smaller molecules, such as methanol, have higher accessibility to active sites. Total flow rates were in the range of 1.5–2.0 mL/min. Densities of CO2 were calculated by the Span and Wagner equation of state using the Dynamic Link Library for CO2 developed by Ruhr Universität Bochum [22]. A 50 L sample loop was applied to overload the column producing non-symmetric, non-Gaussian peaks even at small concentrations. For injection of benzoic acid into the system, different dissolution solvents were tested: n-hexane, cyclohexane, carbon tetrachloride, benzene, 2-propanol and tetrahydrofuran. Samples were prepared in benzene since no distortion and no band splitting of the main peak was observed with increasing injection volumes. Apart from the Peak Maxima experiments, the hold-up volume of the system and porosity of the column were analysed. This was performed by injection of n-hexane which it is considered not to be adsorbed on the silica packing. For the determination of the hold-up volume, the column was replaced by a pipe connector of negligible volume.
4.1. Process parameters The system hold-up volume (without column) was determined from five consecutive chromatograms and averages to Vsystem = 0.600 mL. Porosity was derived from 10 chromatograms for each operating condition. For the Kromasil® silica gel a slight dependence on temperature and pressure was observed as previously reported by Poplewska et al. [26] for octadecyl-bonded silica, and by Lübbert [12] for non-modified silica, respectively. Porosity values ranged from 0.613 to 0.684. For both, system hold-up volume and porosity, relative standard deviation of hold-up times was less than 1%. Column efficiency obtained by injection of a 5 L solution of benzoic acid (1.0 mg/mL) was higher than 4000 theoretical plates. 4.2. Adsorption isotherms Typical elution profiles of benzoic acid are shown in Fig. 2. A change of the curvature from a Langmuir type to an anti-Langmuir type was discernable as the concentration of benzoic acid increased. This behaviour was observed for all operating conditions and it may be related to an energetic heterogeneity (i.e. uneven distribution of surface energy) of the adsorbent [27] or to dimerisation of benzoic acid in the mobile phase [28,29] with solute concentration. According to the work of Sohn and Kim [27] on the adsorption of solutes onto solid surfaces in solution systems, the concentration of solute affects both adsorption and desorption stages. Then, benzoic acid may exhibit a tendency to adsorb onto more active
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Fig. 2. Elution peaks of benzoic acid with increasing concentration obtained at 16 MPa, 328.15 K and X2-propanol = 5% (w/w) (Cinj = 79.80/40.15/20.23/10.06/7.46/5.06/2.52/1.07 mg/mL).
Fig. 4. Isotherms of benzoic acid for different modifier contents at 16 MPa and 328.15 K (including experimental points).
Fig. 5. Isotherms of benzoic acid for different temperatures at 16 MPa and X2-propanol = 5% (w/w). Fig. 3. Isotherm slope fitting of benzoic acid at 16 MPa, 328.15 K and X2-propanol = 5% (w/w).
sites where pre-adsorbed molecules, such as 2-propanol or CO2 , can be easily displaced. Additionally, solute–solute interactions in the mobile phase (dimerisation of benzoic acid) lead to the formation of solvent–cosolvent–solute hydrogen bonded complexes. The concentration of such complexes with solute concentration might also affect how benzoic acid monomeric and dimeric species are adsorbed onto the silica gel inducing a shift of the adsorption shape. Unusual change in the shape of the isotherm with concentration was also reported by Diankov et al. [14] for adsorption of hydrobenzoic acid onto poly(methyl methacrylate) in scCO2 . The effect was mainly attributed to the co-existence of glassy and rubbery parts in the polymers exhibiting different solute sorption properties. The slopes of the different isotherms were calculated from the measured retention times according to Eq. (1). For the studied operating conditions, all resulting experimental data were best fitted by the cubic Hill isotherm model in its derivative form as illustrated in Fig. 3 for the experimental data reported in Fig. 2. Standard devi-
ations t for the fitting of the local total derivatives dQi /dCi were estimated from Eq. (4) and calculated values were less than 1%.
2 1 (dq/dc)theor − (dq/dc)exp k k t (%) = × 100 exp N (dq/dc)
(4)
k
Since the maximum concentration reached in this study for benzoic acid in the fluid phase (6 mg/mL) is far away from saturation conditions, isotherms are only valid in the concentration range they were fitted to. Solubility data reported in the literature for benzoic acid, at 328.15 K, in scCO2 with ethanol as cosolvent (2 mol%) vary from ∼12.8 mg/mL to ∼32.1 mg/mL, at pressures ranging from 13 to 20 MPa [30]. Parameters of the cubic Hill isotherm model are reported in Table 1. Adsorption isotherms of benzoic acid from scCO2 on silica gel are shown in Figs. 4–6. The solid line indicates the region where fitted isotherms are based on measured data. Although the Hill model described better the experimental data compared to the
Table 1 Hill isotherm parameters and goodness of fit. Pressure (MPa)
Temperature (K)
16 16 16 16 14 18
328.15 328.15 328.15 318.15 328.15 328.15
X2-propanol (%, w/w) 2 5 10 5 5 5
b1 (mL/mg)
b2 (mL2 /mg2 )
b3 (mL3 /mg3 )
qs (mg/mL)
r2
t (%)
2.87E−01 1.16E−01 8.37E−02 1.85E−01 8.66E−02 1.71E−01
2.29E−02 4.00E−03 2.69E−03 7.69E−03 3.36E−03 7.02E−03
5.66E−03 4.91E−04 1.82E−04 1.97E−03 4.24E−05 1.41E−03
94.37 81.67 60.87 48.72 116.80 56.13
0.9846 0.9754 0.9682 0.9721 0.9897 0.9451
0.38 0.35 0.45 0.63 0.68 0.67
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5. Conclusions Using the Peak Maxima method, adsorption isotherms of benzoic acid on a non-modified silica gel from scCO2 modified with 2-propanol were measured. Elution profiles diverted from Langmuir to anti-Langmuir fronts as concentration increased, probably due to an energetic heterogeneity of sites and dimerisation of benzoic acid in the fluid phase. Isotherms were determined at different modifier content, temperature and pressure, showing a linear adsorption trend in the concentration range reached in this study (up to 6 mg/mL). In general, adsorption of benzoic acid was greatly affected by the solvating power of the modified scCO2 , being favoured at low modifier contents, high temperatures and low pressures. Acquired data can be used for design and simulation of adsorptive separation processes of carboxylic acids. Fig. 6. Isotherms of benzoic acid for different pressures at 328.15 K and X2-propanol = 5% (w/w).
Acknowledgements The authors gratefully acknowledge the partial financial support of this research by the Evonik Stiftung and the assistance of Dr. M.H. Chuang during the initial operation of the equipment.
linear model, as seen in Fig. 3, equilibrium loadings of the low volatile solute show an almost linear dependence on the concentration of benzoic acid in the modified scCO2 . Same dependence was reported by Diankov et al. [14] for the adsorption of salicylic acid on poly(methyl methacrylate) from scCO2 at low-to-medium concentrations. As shown in Fig. 4, at definite pressure and temperature, the adsorption amount of benzoic acid decreased as the concentration of modifier added to CO2 increased. This decrease of the isotherm slopes observed in Fig. 4 can be partially attributed to the following effects: first, an increase of the solvating power of CO2 caused by the modifier [30], which increases the bonding force between the solute and the fluid phase. Second, an increased adsorption of the modifier on the silica gel surface (competitive solute-modifier adsorption). And third, an increased dimerisation of benzoic acid in the fluid phase as the concentration of the modifier increases [29]. Adsorption isotherms of benzoic acid on silica gel at different temperatures (318.15 and 328.15 K) when pressure and modifier content were kept constant at 16 MPa and 5% (w/w), respectively, are shown in Fig. 5. It is noticeably seen that adsorption of benzoic acid was favoured at higher temperatures as the adsorbed amount of solute increased as temperature rised. This effect can be related to the decreased density of the fluid phase with increasing temperatures [31]. Like so, solute solubility is reduced and adsorption of benzoic acid onto the solid surface is enhanced. Moreover, by increasing temperature, not only the diffusion resistance decreases due to a reduced viscosity of the fluid phase, but also dimerisation of benzoic acid diminishes [32]. Adsorption capacity of silica gel for benzoic acid was also studied at different pressures (14, 16 and 18 MPa) at a temperature of 323 K and a modifier content of 5% (w/w). As seen from the corresponding isotherms shown in Fig. 6, adsorption amount of benzoic acid on silica gel decreased as the pressure increased. The difference in surface loading between 16 and 18 MPa is better distinguished from the chromatograms (Cinj = 7.5 mg/mL) enclosed in Fig. 6. The decreasing adsorption trend with pressure can be partially explained by an increased density of the fluid phase [29]. This leads to a higher solvating power of the modified scCO2 [30] and reduces therefore adsorption. Also competitive adsorption of scCO2 on silica gel is enhanced with the increase of pressure [14,16]. Compared to adsorption of other compounds on silica gel from supercritical solutions, such as ␣- and ␦-tocopherol [12,13], the adsorbed amount of benzoic acid on silica gel from modified scCO2 was significantly lower by a factor of 5–10.
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