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Langmuir 1997, 13, 1020-1025
Surface Properties of Silica-Based Adsorbents Measured by Inverse Gas-Solid Chromatography at Finite Concentration† Marek Pyda and Georges Guiochon* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197 Received October 30, 1995. In Final Form: January 29, 1996X The adsorption energy distributions (AEDs) of methanol and dichloromethane on samples of the same silica unmodified and modified with octadecyldimethylsilyl (C18) were estimated from isotherm data obtained by inverse gas chromatography at finite concentration. Wall-coated open tubular columns were used as they provide a high efficiency and a low pressure drop, allowing the acquisition of accurate isotherm data. The adsorption isotherms were obtained from nonlinear elution profiles using the characteristic points method of chromatography. The numerical estimation of AEDs was made by applying the expectationmaximization method to the experimental, unfitted adsorption isotherm data. The adsorption energy distributions obtained exhibit two peaks of adsorption energy. The height of the higher energy peak is reduced after chemical modification of the silica surface.
Introduction The determination of adsorption energy distributions (AEDs) has become a conventional approach for the study of the surface heterogeneity of unmodified and modified silica samples. This determination is conveniently made using inverse gas-solid chromatography at finite concentration to measure the adsorption isotherms, as an intermediate step.1-3 The classical method of deriving isotherm data from the profile of the diffuse boundary of an overloaded elution band in gas-solid chromatography is known as elution by characteristic points (ECP). This technique uses the model of ideal chromatography (which assumes that the column has an infinite efficiency) to account for the profile of bands obtained with actual columns, i.e., with columns having a finite efficiency. Thus, ECP includes an important model error. The practical importance of this error is minimized by using high efficiency columns.3,4 Previous results demonstrate that isotherm data acquired with columns having an efficiency of less than 1000 theoretical plates are worthless and that data acquired with columns having less than 10 000 plates are fraught with too much error at high partial pressures (corresponding to low adsorption energies) to give any meaningful AED.5,6 This raises a major experimental issue. We need an efficient column. However, it is possible to make a packed gas chromatography column with an efficiency of 10 000 plates only with particles having a size of about 100 µm. With coarser particles the length becomes impractical. With finer particles, the pressure drop becomes prohibitive. This sets unacceptable conditions, especially for the study of fine adsorbent particles. The alternative is the †
Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Solids Surface; Academic Press: New York, 1992. (2) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, The Netherlands, 1988. (3) Guiochon, G.; Golshan-Shirazi, S.; Katti, A. M. Fundamentals of Preparative and Nonlinear Chromatography; Academic Press: Boston, MA, 1994. (4) Guan, H.; Stanley, B. J.; Guiochon, G. J. Chromatogr. 1994, 659, 27. (5) Stanley, B.; Guiochon, G. Langmuir 1994, 10, 4278. (6) Stanley, B.; Guiochon, G. Langmuir 1995, 11, 1735.
S0743-7463(95)00541-5 CCC: $14.00
use of wall-coated open tubular columns.5-9 The use of these columns for isotherm determinations has some most important advantages over the use of either the classical packed columns or the classical static techniques. First, it is easy to achieve a high chromatographic efficiency, allowing the determination of accurate isotherms from the overloaded elution bands. Second, the low column pressure drop results in a small correction of the retention volumes for gas compressibility. The James and Martin correction factor assumes the column permeability to be constant, an assumption which is highly questionable with long packed columns. Procedures used to pack gas chromatography (GC) columns10 are not effective at providing a high degree of consolidation.11 It is frequent to observe that retention times are significantly different if the direction of the gas stream is reversed. This phenomenon is easily explained by an unsymmetrical distribution of the local column permeability. Other advantages of the use of wall-coated open tubular columns are that small adsorbent particles can be used without causing an excessive pressure drop, that small amounts of adsorbent and adsorbate are needed, and that high adsorption energies can be measured by operating at moderate temperatures. The adsorption isotherms were obtained from nonlinear elution profiles using the ECP method. The global isotherm q(p) was calculated by integration along the diffuse boundary of the corrected retention volume, VR VM, which is derived from the retention time and the flow rate5,12
q(p) )
1 Ms
∫0y
j(VR - VM) dc 1-ψ
(1)
where q is the amount of vapor adsorbed at equilibrium with the partial pressure p, Ms is the mass of adsorbent contained in the wall-coated open tubular column, y is the (7) Roles, J.; Guiochon, G. J. Chromatogr. 1992, 591, 233-243. (8) Roles, J.; Guiochon, G. J. Chromatogr. 1992, 591, 245-265. (9) Pyda, M.; Stanley, B. J.; Xie, M.; Guiochon, G. Langmuir 1994, 10, 1573. (10) Guiochon, G.; Guillemin, C. Quantitative Gas Chromatography; Elsevier: Amsterdam, The Netherlands, 1988. (11) Guiochon, G.; Sarker, M. J. Chromatogr., A 1995, 704, 247. (12) Conder, J. R.; Young, C. L. Physico-Chemical Measurements by Chromatography; Wiley: New York, 1979.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 5, 1997 1021
mole fraction of the solute at the column outlet, VR and VM are the retention volume and the holdup volume of the column, respectively, calculated as
VR - VM ) (tR - tM) F(y)
(2)
The symbol j in eq 1 is the pressure gradient correction factor (or James and Martin correction factor) with correction for nonideal gas behavior12
[
j ) J32 1 +
]
y2p0B22 2 (J3 - 1) RT
(3)
where
m(Pm - 1)
Jmn )
(4)
n(Pn - 1)
where P is the inlet to outlet pressure ratio. ψ in eq 1 is a correction term5,12
ψ ) ajy
(5)
with
a)
[
1 + k(1 - yJ21) 1 + k(1 - yJ32)
1+
]
2yp0B22(1 - yJ21) RT
Experimental Section
(6)
where k is the mass distribution coefficient or retention factor at infinite dilution. In these equations, the use of the coefficients j, k and the second virial coefficient, B22, corrects effectively the adsorption data for the effects of the pressure drop, the real gas behavior, and the sorption effect. P is close to 1 because the pressure drop used in this work is 0.345 bar (5 psi), hence P = 19/14 ) 1.35, leading to a value of J 23 of 0.84. The correction made to the retention volumes is small and cannot be grossly incorrect. This is one of the important advantages of using open tubular rather than packed columns. In principle, a complete adsorption isotherm can be obtained from one single chromatogram. However, several bands should be acquired with different size samples, in order to ascertain that the diffuse boundaries of the band profiles (their rear boundary in the case of a convex upward isotherm3) do overlap. Failure for the profiles to overlap indicates that the experimental setup is incorrect or that the mass transfer kinetics is too slow. Accurate isotherm data cannot be obtained in such a case, and the adsorption energy values derived from an incorrect isotherm are meaningless. The AED, f(E), is calculated from the adsorption isotherm using the fundamental integral equation
q(p) )
∫Θ(E) f(E) dE
Rudzinski and Everett1 and by Jaroniec and Madey.2 Because it has the advantage of giving an unbiased estimate of the AED, the expectation-maximization (EM) method13 was used for the numerical estimation of these functions from the experimental, unfitted adsorption isotherm data. The adsorption properties of probe compounds on silica are related to the existence of at least two kinds of atom groups on the surface, hydrophilic silanols (Si-OH) and hydrophobic siloxanes (Si-O-Si), and to the topology of the surface which is responsible for the accessibility of these groups. They also depend on the size and shape of the adsorbates. It is known from previous studies that the surface of silica changes from hydrophilic to hydrophobic upon modification with alkyl chains.10,14,15 This reaction involves the replacement of silanol groups by Si-O-Si(CH3)2-CnH2n+1 groups. According to Jelinek and Kovats,15 a maximum of half of the silanol groups are replaced. The actual fraction depends on the yield of a difficult reaction. This work reports on a new series of measurements of the adsorption energy distribution of methanol and dichloromethane on a porous silica and on a sample of the same material whose surface was modified by reaction with octadecyldimethylchlorosilane.
(7)
where Θ(E,p) is the local adsorption isotherm. In the present work, we have chosen to use the Langmuir equation. In this work, one single assumption was made to derive the AEDs, the use of the Langmuir model to account for the local adsorption isotherm. The experimental data regarding the global adsorption isotherm, q(p), have been used without any fitting to any analytical form for the function f(E). The general mathematical problem of solving eq 2 is difficult. Many investigations using different approaches have been so far unsuccessful in providing either an analytical or a satisfactory numerical solution. These studies have been reviewed in detail by
I. Materials. 1. Adsorbents. Two different adsorbents were used. An unmodified silica (S), Impaq RG1010Si (BTR Separations, Wilmington, DE), was used having a specific surface area of 366 m2/g, determined by BET, a mean pore diameter of 101 Å, and an average particle diameter of 8.8 µm. The specific surface area was calculated assuming the conventional value of the crosssection area of the nitrogen molecule, 16.2 Å2, although there a value of 13.5 Å2 might have been more correct.15 A modified silica (SC18), Impaq RG1010Si (BTR Separations, Wilmington, DE), was used made of the same silica as S which has been bonded with (C18) octadecyldimethylsilyl groups. After the manufacturer, the proportion of total bonded organic carbon determined by elemental analysis is 22.0%. Assuming that the bonded groups are Si(CH3)2C18H37, this translates into a bonding density of 2.7 µmol m-2. 2. Adsorbates. Methanol and dichloromethane (all HPLC grade, obtained from Baxter) were used as probes to study the silica surfaces. II. Equipment. The experiments were performed with a Perkin-Elmer Model 8500 gas chromatograph (Norwalk, CT), equipped with a flame ionization detector (FID). The chromatograph was interfaced to the computer via on A/D board for digital acquisition of the analog signal of the detector. The chromatograph has been previously modified7,8 and was used as described in these previous papers. The most important of these changes was the use of a split injection of the adsorbate. An aliquot goes to the column under study, another to a calibration open tubular gas-liquid column, and the rest goes to a vent, to avoid the injection of too large a sample on either columns.5,7,8 The split ratio between the two open tubular columns is constant and carefully measured. The calibration column (DB-5, 0.32 mm i.d., 30 m long, from J&W Scientific, Folsan, CA) allows an accurate quantitation of the amount injected. It gives a Gaussian peak profile, recorded after elution of the adsorbate sample on the column studied. The open tubular columns studied in this work were 15-16 m long, 0.53 mm i.d., made from quartz tubes from Polymicro Technologies (Phoenix, AZ). The silica samples were introduced in empty capillary columns according to the coating procedure described in earlier publications.7,16 Hamilton syringes (0-5 µL and 5-20 µL, Reno, NV) were used for manual injection of the (13) Stanley, B. J.; Guiochon, G. J. Phys. Chem. 1993, 97, 8098. (14) Unger, K. K. Porous Silica, its Properties and Uses as a Support in Liquid Chromatography; Elsevier: Amsterdam, The Netherlands, 1979. (15) Jelinek, L; sz. Kovats, E. Langmuir 1994, 10, 4225. (16) Halasz, I.; Horvath, Cs. Nature 1963, 197, 72.
1022 Langmuir, Vol. 13, No. 5, 1997
Pyda and Guiochon
Table 1. Adsorption Capacity and Adsorption Energy Distributions of the Methanol and Dichloromethane on Unmodified Silica (S) and Modified by C18 (SC18) at Different Temperatures adsorbent methanol S SC18 dichloromethane S SC18
T (°C)
qm × 10-5 (mol/g) (0.5
E1 (kJ/mol) (0.2
q1 × 10-5 (mol/g)
E2 (kJ/mol) (0.1
q2 × 10-5 (mol/g)
50 50 70
24.2 22.0 5.6
53.5 52.7 53.6
1.1 0.97 0.26
64.0 58.6 61.1
0.11 0.042 0.052
47.3 51.9
0.16 0.036
56.5 61.1
0.016 0.023
40 40
8.20 3.15
probes into the chromatograph. Measurements of the weight of silica contained in the columns were performed using an analytical semi-micro balance (Sartorius Corp., Bohemia, NY) with (0.01 mg accuracy. More information about the instrumentation has been provided in previous publications.4-9 III. Experimental Procedures. The two columns prepared were first preconditioned in the oven of the chromatograph at 320 °C for 2-3 days under a constant helium stream with an inlet pressure of 0.069 bar (1.0 psi). Then they were weighed and found to contain 9.7 mg of S and 6.6 mg of SC18, respectively. The experimental conditions for the acquisition of the overloaded elution bands required for the determination of the isotherms were the following. The oven temperature was constant, set between 40 and 70 °C. Helium was used as the carrier gas, with an inlet pressure Pi ) 0.345 bar (5 psi). The temperature of the injection port was 150 °C, and that of the FID was 350 °C. The data were acquired on two channels, at the maximum detector sensitivity and at a low sensitivity in all experiments. The split ratio between the calibration and tested columns was around 1/10 but measured accurately each day. The flow rate through the studied column was between 4 and 8 mL/min and the column efficiency was always well in excess of 20 000 plates. The volumes of probes injected were between 2 and 15 µL (vapor). Series of experiments were conducted under the same conditions to record the chromatographic band profiles for different probe amounts. In all cases, the rear diffuse boundaries of the band profiles overlapped nicely (see Figures 1 and 2). The FID signal, R(V) in volts, at low and high detector sensitivity was converted into partial pressure of the adsorbate according to the expression
p ) S2 R(v)
(8)
where S2 is the detector response factor in atm/V. The corrected retention times (tR - tM) of each partial pressure are calculated and transformed into retention volumes, knowing the flow rate. The global adsorption isotherm q(p) was calculated using eqs 1 to 6 and the adsorption energy distributions is derived from eq 7, using the expectation-maximization method as previously described.13 In contrast with most previous approaches17,18 to AED calculations, no assumptions were made regarding the functional nature of f(E) and no fitting of the experimental adsorption data to any isothermal model were made. The single assumption made regards the local adsorption isotherm, Θ(E,p), taken as the Langmuir model:
Θ(E,p) )
p p + K exp(-E/RT)
(9)
In this equation, K ) Pv exp(Λ/RT) is the Henry constant of adsorption, in the form suggested by Jaroniec,2 in which Pv is the saturation vapor pressure of the probe at the experimental temperature T, Λ its molar heat of vaporization, and R the ideal gas constant.
Results and Discussion I. Adsorption Isotherms. The experimental data were acquired under the experimental conditions listed in Table 1 where T is the temperature of experiment, qm (17) Jagiello, J.; Ligner, G.; Papirer, E. J. Colloid Interface Sci. 1990, 131, 128. (18) Roles, J.; Guiochon, G. J. Phys. Chem. 1991, 95, 4098.
Figure 1. Experimental band profiles of methanol on unmodified silica (S) at 50 °C, at low detector sensitivity. Overlays of the rear diffuse boundaries of the profiles were obtained by injection of volumes of 3 and 4 µL. Experimental conditions were as follows: column length, 1597.6 cm; mass of silica, 9.7 mg; inlet pressure, 1.317 atm; gas hold-up time, 0.520 min; column efficiency around 20 000 plates.
is the total monolayer adsorption capacity calculated from the AEDs, and q1 and q2 are the adsorption capacities corresponding to the first and second peaks of the AED, with adsorption energies E1 and E2, respectively. (Note that qm . q1 + q2, the difference being the adsorption capacity of the sites corresponding to the low adsorption energy band of the AED.) Typical chromatograms illustrating the excellent overlap of the rear diffuse boundaries of the band profiles are shown for methanol in Figures 1 (S) and 2 (SC18). The same quality of results were obtained with the other probes (not shown). It was the same as demonstrated in previous publications.5-9 The adsorption isotherms data used in the AED determination were derived from the profile of the largest peak, for self-consistency. Some of the isotherms obtained are shown in Figures 3 and 4. Figure 3 compares the isotherms of methanol on SC18 at 50 and 70 °C. Figure 4 compares the adsorption isotherms of methanol at 50 °C on S and SC18. All these adsorption data are highly reproducible and quite accurate (Figures 1 and 2). The parameters characterizing the adsorption of the three probes studied are summarized in Table 1.
Surface Properties of Silica-Based Adsorbents
Langmuir, Vol. 13, No. 5, 1997 1023
Figure 4. Comparison of the adsorption isotherms of methanol on the initial (S) (solid line) and the modified SC18 silica (dotted line) at 50 °C.
Figure 2. Experimental band profiles of methanol on octadecylmodified silica (SC18) at 60 °C, at low detector sensitivity. Overlays of the rear diffuse boundaries of the profiles were obtained by injection of volumes of 8, 5, 4, and 2 µL. Experimental conditions were as follows: column length, 1654.8 cm; mass of silica, 6.59 mg; inlet pressure, 1.313 atm; gas holdup time, 0.861 min; column efficiency around 20 000 plates.
Figure 3. Effect of temperature on the adsorption isotherms of methanol on modified silica (SC18) at 50 °C (solid line) and 70 °C (dotted line).
1. Effect of the Temperature on the Saturation Capacity. Figure 3 compares the adsorption isotherms of methanol on the modified silica surface (SC18) at two different temperatures, 50 and 70 °C. From the values of the parameters derived from these isotherms, it appears that the saturation capacity of this adsorbent decreases substantially with increasing temperature. The monolayer capacity at 50 °C was found to be qm ) 22.0 × 10-5
mol/g, a value which became only qm ) 5.6 × 10-5 mol/g at 70 °C. A similar drop of the adsorption capacity was observed for dichloromethane (Table 1). 2. Effect of a Chemical Modification of the Surface of Silica on Adsorption Isotherms. It is generally recognized that the silanol groups on the surface of silica are responsible for the adsorption of polar molecules with high-energy adsorption centers. In order to deactivate the corresponding adsorption sites, the surface of silica can be modified chemically, by grafting atom groups generating only dispersive intermolecular forces. In our case, octadecyl groups were bonded to the silica surface to change its adsorption properties. The adsorption capacity of methanol and dichloromethane on the surface was decreased after the chemical modification (see Table 1). Figure 4 compares the adsorption isotherms for methanol on the silica surface before and after modification. As seen in Table 1, the saturation capacity is reduced after bonding to the alkylsilane. This decrease in the saturation capacity is only about 10% for methanol but is as high as a factor 2.5 for dichloromethane. A similar result was reported previously for diethyl ether.6,9 We note also a marked change in the shape of the adsorption isotherms. In the case of methanol, there is an inflection point for the isotherm on silica. This feature disappears after silanization. The monolayer adsorption capacity (qm) for methanol is much higher than for dichloromethane (Table 1). II. The Adsorption Energy Distributions. The adsorption energy distributions were derived from the unfitted experimental adsorption isotherms acquired and just described. The influences of the temperature and the chemical modification of the surface on the behavior of the AEDs are illustrated by the data presented in Figures 5-7 and in Table 1. Two high-energy peaks are observed in each of the distributions calculated. They are detached in front of a low-energy band which is incomplete. It was not possible to obtain the complete profile of this broad low-energy band of the AEDs. This would have required the measurement of the isotherm adsorption data up to values of the relative partial pressures which are higher than those which can be reached with the ECP method. Unfortunately, measurements done by frontal analysis by characteristic points (FACP) did not give results consistent with those of ECP
1024 Langmuir, Vol. 13, No. 5, 1997
Figure 5. Effect of temperature on the AED of methanol on modified silica (SC18) at 50 °C (solid line) and 70 °C (dotted line).
in the high concentration range, suggesting uncorrected for problems arising in vapor-rich gas streams.15 In Table 1 are included the monolayer adsorption capacity calculated from the AEDs, qm, the mean adsorption energy of the two peaks observed, E1 and E2, respectively, and the amounts of probe adsorbed corresponding to the areas of the first and second peaks, q1 and q2, respectively. Generally qm * q1 + q2 and the difference between these two quantities is the contribution of the low-energy band to the monolayer capacity. This contribution, qm, is usually much more important than q1 + q2. The conditions required for the correct calculation of the AEDs using the EM method were discussed in a previous paper.8 Briefly, there are two conditions. First, the adsorption energy must be at least 10 kJ/mol greater than the heat of vaporization of the probe. Second, lateral interactions between molecules in the adsorbed layer must be negligible compared to the heat of adsorption. 1. Effect of Temperature on the AEDs. Figure 5 compares the AEDs of methanol on the modified silica (SC18) at 50 and 70 °C. As expected and as found in a previous study of different brands of porous silica for chromatography,9 the shape and position of the AED peaks remain nearly the same when the temperature is increased while the contributions of the high-energy peaks to the monolayer adsorption capacity decrease most rapidly. 2. Effect of a Chemical Modification of the Surface on AEDs. The influence of the chemical modification of the surface on the AED is illustrated in Figures 6 and 7 and in Table 1. Figure 6 shows that, after silanization, the two high-energy peaks of the AED of methanol are shifted to lower values on the energy scale, from 53.5 and 64 kJ/mol for S to 52.7 and 58.6 kJ/mol for SC18. Upon silanization, the highest peak nearly disappears, its area decreasing 20 times. In the same time, the adsorption capacity of the second high-energy peak is reduced by hardly 10%. Figure 7 compares the AEDs of dichloromethane for the modified and the unmodified silicas. By contrast with the result obtained for methanol, there is a significant shift of both high-energy peaks toward higher adsorption energies, by approximately 7%. The adsorption capacities corresponding to both peaks are changed. Due to the chemical modification of the surface, the capacity of the highest energy peak is increased by 50% while that of the
Pyda and Guiochon
Figure 6. Comparison of the AED of methanol on initial (S) (dashed line) and modified SC18 silica (dotted line) at 50 °C.
Figure 7. Same as Figure 6 for dichloromethane at 40 °C .
second energy peak is decreased by a factor 4.5. A similar behavior was observed for the AEDs of dichloromethane on different silica at previous results.9 For diethyl ether, the results are slightly different.5,9 The distribution is shifted by approximately 8.4 kJ/mol toward lower energies, in agreement with the vaporization energy of diethyl ether being 8.4 kJ/mol lower than that of methanol. The two high-energy peaks have a higher capacity than for methanol. This capacity decreases upon derivatization but their energy increases by a few kJ/mol. Conclusions The results of this work can be summarized as follows: The use of wall-coated open tubular columns rather than conventional packed columns in IGC at finite concentration allows a considerable improvement in the precision and accuracy of the adsorption data obtained to characterize the properties of the heterogeneous surface of adsorbents. This is demonstrated by the excellent overlap of the experimental chromatograms obtained at different sample amounts injected. The expectation-maximization method allows convenient numerical estimation of the parameters of the
Surface Properties of Silica-Based Adsorbents
adsorption energy distributions, without introducing any spurious information. This method works best with the unfitted experimental adsorption isotherms data. All experimental results exhibit two high-energy peaks in front of an incomplete low-energy band. In most cases, it is not possible to acquire adsorption data in the range of partial pressures corresponding to the low-energy region of the AED. This shape of the AEDs suggests that there should be at least two and possibly three types of energy sites on the surface of the studied silica.
Langmuir, Vol. 13, No. 5, 1997 1025
The experimental results show also that modification of the silica surface by bonding with alkyl chain C18 causes a marked reduction of the surface activity of the adsorbent, a marked reduction of its adsorption capacity, and a shifting of the high-adsorption-energy bands of methanol toward lower energies. Increasing the temperature reduces the adsorption capacity for both the initial and the modified silica surface. LA950541F
