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Ind. Eng. Chem. Res. 1995,34, 4170-4173
Spectroscopic and Simulation Studies of Adsorption and Removal of Hydrocarbons Using Novel Materials Carolyn k Koh,*?+ Robert I. NooneyJ Michael Maddox? and Keith E. Gubbins' Department tif Chemistry, King's College, University of London, Strand, London WC2R 2LS, U.K., and School of Chemical Engineering, Cornell University, Zthaca, New York 14853
We report adsorption and spectroscopic measurements for mixtures involving low molecular weight hydrocarbons (methane, propane, isobutane) and carbon dioxide and also for pure methane; where possible, comparisons with molecular simulation are made. mPo4-5 and VPI-5 were found to selectively adsorb isobutane in isobutandCO2 gas mixtures and propane in propane/ methane gas mixtures, while little or no selectivity was observed for isobutane in isobutane/ propane on APo4-5. In-situ diffise reflectance infrared spectroscopy has been used to study the adsorbate-adsorbent and adsorbate-adsorbate interactions of trace gases on VPI-5. This provided an insight into the extents of adsorption and strengths of the intermolecular adsorbateadsorbent interactions.
Introduction Adsorption onto solid substrates (e.g., activated carbons, zeolites, aluminas, and silicas; No11 (1992)) provides one of the most effective means for removal of trace pollutants (inorganics, volatile organic compounds (VOC), etc.) from gaseous streams (Kumar and Golden (1990, No11 (1992), Ruhl(19931, Stenzel(1993)). With appropriate choice of adsorbent and operating conditions, a high degree of purification can be achieved for many pollutant streams, since the method is very selective. The process is often reversible, and recovery and reuse of the adsorbed material are therefore usually possible, leading to improved cost effectiveness; this is in contrast to thermal or catalytic oxidation, another widely used method for VOC removal (Moretti and Mukhopadhyay (1993)). While present adsorbents and adsorption processes are effective for many purposes, recent model calculations by our group and others have shown that the present technology is far from optimal (Jiang et al. (1994)). In this paper we report the initial stages of a study in which we investigate the selective adsorption behavior of mixtures of hydrocarbon gases in VPI-5 and AlPo4-5. Adsorption characteristics of AlPO4-5 with a ring of 10 tetrahedral atoms and a pore diameter of 9.1 A (Wilson et al. (1982))are compared with VPI-5 which has a ring of 18 tetrahedral atoms and a pore diameter of 12.1 A (Davis et al. (1988, 1989)). Grand canonical Monte Carlo (GCMC) simulation studies of the adsorption and heat of adsorption of simple gases and benzene in aluminophosphate materials were performed using simple Lennard-Jones intermolecular potentials for both the fluid-fluid and fluidsolid interactions (cf. Cracknell et al. (19931, Koh et al. (1994))and compared with gas chromatography-mass spectrometry (GC-MS) experimental data. The nature of the adsorbate-adsorbent interactions can be experimentally examined using mid- and near-infrared diffise reflectance measurements of these inorganic materials (Ferraro and Rein (1985)). The aluminum-oxygen bonds of aluminophosphates can be examined by midIR studies to define pore vibrations, to show which linkages are structure sensitive (i.e., to define the degree
' University of London. e-mail:
[email protected]. Fax: 171-873-2380. + Cornell University.
of crystallinity), and to study species adsorbed in these porous materials. Near-IR diffuse reflectance infrared FT spectroscopy (DRIFTS) measurements indicate the location of water molecules in the lattice and how they are bound.
Methods
A. Molecular Simulation. The structure of the pore was taken from DLS (distance least squares) refined atom positions in the literature, calculated from X-ray measurements for the APo4-5 and dehydrated VPI-5 structures (Annen et al. (1991), Bennett et al. (1983)). Grand canonical Monte Carlo (GCMC) simulations were performed using the procedure of bulk fluids (Allen and Tildesley (1987)). The algorithm of the simulation, including types of trials and acceptance probabilities, was similar to that used in our previous work on inert gases adsorbed onto aluminophosphate materials (Cracknell et al. (1993)). The bulk chemical potential p obtained from the grand canonical ensemble was converted into a bulk pressure, P , by the use of the ideal gas law and then converted to reduced pressures, PIP" (Po= vapor pressure). The ensemble average of the number of gas molecules per unit cell was taken from the simulation and plotted against the reduced pressure to give the adsorption isotherm. Periodic boundary conditions were applied a t the ends of the boxes. A total of 1 x lo6 configurations was used to equilibriate each run, and averages were collected over a further 2 x lo6 configurations (cf. Bennett et al. (1983), Cracknell et al. (1993)). The GCMC method solves the problem of uncertainty in the number of particles needed t o be modeled in the system. It also easily satisfies the condition of equilibrium with the bulk vapor using, as input to the simulation, the chemical potential of the bulk vapor, which is calculated separately from a known equation of state for the model fluid. The Monte Carlo method requires only the configurational energy of the system, rather than energy and forces, as would be required by molecular dynamics. B. FTIR Spectroscopy. A variable-temperature and high-pressure cell with a zinc selenide hemispherical window was placed in a diffuse reflectance infrared spectrometer (DRIFTS)system (Spectra-Tech). Infrared spectra were recorded over the range 4000-500 cm-l
0888-5885/95/2634-417Q~09.00/0 0 1995 American Chemical Society
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/ .
0
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'B
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Figure 2. Adsorption of methane in VPI-5 at 77 and 95 K from experiment (dashed line) and GCMC (solid line and points).
3000
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2880
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cni'
Figure 1. Diffuse reflectance IR spectrum of ethane gas in activated VPI-5 at 300 K.
using an Analect (RFX 40)Fourier transform infrared spectrometer. AluminophosphateKBr matrix and neat aluminophosphate powders were analyzed. C. Selective Adsorption Measurements. Selective adsorption isotherm data for gas mixtures were measured at 298 K using a KRATOS MS89O gas chromatograph-mass spectrometer (GC-MS). Each isotherm was measured a t fxed T , P, and bulk gas composition as a function of time. Prior to all adsorption measurements, samples were outgassed a t 623 K for 5 h, with a vacuum of Torr. Adsorption of pure methane gas on VPI-5 was measured at 77 K using a Coulter Adsorptometer instrument. A pressure range of 10-3-10 Torr was covered. Prior t o all adsorption measurements, samples were outgassed at 623 K for 24 h with a vacuum of Torr.
Results and Discussion
FTIR Spectroscopy. The perturbations of methane and ethane molecules by Lewis acid sites of activated aluminophosphate material were identified by diffuse reflectance IR spectroscopy (see Figure 1). In addition t o the physically adsorbed species, the spectra reveal adsorbed methane and ethane species, represented by the broad bands shifted to lower frequencies, i.e., 28602790 cm-l. Adsorption of paraffins has also been observed in aluminas and zeolites (Bennett et al. (1983)). Adsorption Data: GCMC Simulations and Experiment. The adsorption isotherms obtained for methane in VPI-5 and AlPo4-5 (P= 0.64,T* = kT/cR, where E R is the well depth of the L-J fluid-fluid interaction) from GCMC simulations are given in Figure 2.The experimental adsorption data obtained for methane in VPI-5 shows qualitative agreement with the simulation data. Close agreement was obtained at low coverage; however, adsorption at higher coverage was predicted to be considerably greater than experiment. Previous work has indicated pore blocking in the real material t o be a major contribution toward this effect (Cracknell et al. (19931,Cracknell and Gubbins (19921,Koh et al. ( 1994)).
Reaction Tlme/minutes
Figure 3. Selectivity for isobutane and C02 in isobutane/COz mixtures at 298 K isobutaneNP1-5 (0);CO2NpI-5 (0);isobutane/ aP04-5 (0);COdAlPo4-5 (m). Bulk pressure = 5 x lo2 Torr; mole fraction of isobutane = 0.5; mole fraction COz = 0.5.
The adsorption profiles of COdisobutane, Codpropane, isobutane/methane, isobutane/propane, propane/ methane, and gas mixtures on VPI-5 and AlPo4-5 are shown in Figures 3-7. Both aluminophosphate molecular sieves, VPI-5 and AlPo4-5, displayed a low affinity for carbon dioxide, while a higher affinity was shown for the short-chain hydrocarbons,isobutane and propane (see Figures 3 and 4). The large selectivities observed for isobutane (Figure 5) may be explained by considering the increased number of fluid-wall intermolecular interactions a t the near cylinder pore structure of AlPo4-5 and VPI-5. The experimental competitive adsorption results for COdisobutane, Codpropane, and methane/propane were in qualitative agreement with recent GCMC simulations (Koh et al. (1995)). These results also correlate with previous GCMC calculations of the density profiles of L-J fluids adsorbed in Alp0 pores (Cracknell et al. (1993)).In these calculations a pore was divided into three regions, 1,2,and 3. By the symmetry of the VPI-5 space group (Pa3crn) the space in the rest of the pore belongs to one of the three regions, and statistics are collected over the whole pore. Region 3 corresponds to the corner of the hexagonal pore, and reinforcement of the solid-fluid forces leads to adsorbent molecules preferring to occupy these sites, where
4172 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 0.1-
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o
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Reaction Timelminutes
Figure 4. Selectivity of COz and propane in Codpropane mixtures at 298 K, C O m I - 5 (0);propaneNP1-5 (m);cOdAlP04-5 (0);propandAlPO4-5 (0).Bulk pressure = 5 x lo2 Torr; mole fraction of propane = 0.5; mole fraction C02 = 0.5. 0
Reaction Time / minutes
g
Figure 6. Selectivity for isobutane and propane in isobutanel propane mixtures at 298 K: isobutaneNP1-5 (0);propaneNP1-5 (A);isobutane/AlP04-5 (0);propane/AlP04-5 (0).Bulk pressure = 5 x 102 Torr; mole fraction of isobutane = 0.5; mole fraction propane = 0.5.
0
0
1
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Reaction Time I minutes
Figure 5. Selectivity for isobutane and methane in isobutane/ methane mixtures at 298 K isobutaneNP1-5 (0);methaneNP1-5 ( 0 ) ;isobutane/AlP04-5 (A);methanelAlP04-5 (A). Bulk pressure = 5 x I O 2 Torr; mole fraction of isobutane = 0.5; methane = 0.5.
the potential is strongest. Lower adsorptions were observed in region 2 due to steric hindrance. No selectivity was observed for isobutane from an isobutane/propane gas mixture on W I - 5 (see Figure 6).
Reaction Time I minutes
Figure 7. Selectivity for propane and methane in propanel methane mixtures at 298 K methaneNP1-5 (0);propaneNP1-5 (0);methane/AlPO4-5 (A); propandAlPO4-5 (A).Bulk pressure = 5 x lo2 Torr; mole fraction of propane = 0.5; mole fraction methane = 0.5.
This can be explained by considering the similar branched shapes of isobutane and propane molecules, and the similarities in their uff parameters: udisobu-
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4173 tane) = 5.341 A, udpropane) = 5.061 A; e&(isobutane) = 217 K, edpropane) = 206 K (from Hirschfelder et al. (1964)). In contrast, APo4-5 was found t o selectively adsorb isobutane from a propanehsobutane mixture. The pore sizes of AP04-5 and WI-5 must therefore play a role in the selectivity in this mixture. Nicholson et al. (1995) have recently shown that the competitive adsorption behavior for methaneharbon dioxide mixtures is dependent on the micropore size. Figure 7 shows the strong selectivity for propane from a propane/methane gas mixture on VPI-5 and APo4-5. Extremely large selectivities (in many cases SZ" exceeding lo5; Sf = (XdXl)por&&l)bulk, where xi is the mean mole fraction of component i in the pores and yi is the mole fraction of i in the coexisting bulk phase) were shown from recent density functional theory studies to be possible in monodisperse porous materials, when the optimal conditions of temperature, pressure, and pore size were used (Jiang et al. (1994)). The optimum pore size for methane/propane mixtures is H* = H/Um = 2.37 or H = 9.05 A, where H is the pore width and Om is the diameter of the methane molecule, 3.81 A. This width corresponds t o the value that just accommodates one layer of the trace molecules. The density functional theory calculations showed that a larger pore width also produced a local maximum in the selectivity (H* = 3.40, H = 13 A). The second maximum in SZ"was calculated to be generally smaller than that for the one trace layer. The pore widths for APo4-5 (H = 7.6 A) and VPI-5(H = 12.1 A) would be expected from these calculations to display a SZ" value of -loz (where component 2 is propane and component 1is methane). The experimental selectivity profiles suggest that VPI-5has a slightly lower selectivity for propane in the propanelmethane mixture than APo4-5. This may be explained by considering that the pore diameter of VPI-5 is less than the second local maximum of 13 A predicted by density functional theory (DFT),with an SZ" value slightly lower than the corresponding value for APo4-5.
Conclusions Adsorbate-adsorbent interactions which occur during the adsorption of lower order hydrocarbons in aluminophosphates have been investigated using diffuse reflectance IR spectroscopy. Selective adsorption measurements have shown that significant selectivities can be obtained for a hydrocarbon molecule in a twocomponent mixture on VPI-5 and APo4-5. GCMC molecular simulations for adsorption of methane at 77 K have been found to be in qualitative agreement with experimental adsorption measurements.
Acknowledgment This work was supported by a NATO collaborative grant and a grant from the U.S.Department of Energy (NO.DE-FG02-88ER13974). Nomenclature Alp0 = aluminophosphate VOC = volatile organic compounds GCMC = grand canonical Monte Carlo simulation H* = reduced pore width !P = reduced temperature
Literature Cited Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford University Press: Oxford, 1987; pp 126-131. Annen, M. J.; Young, - D.; Davis, M. E. J . Phys. Chem. 1991,95, 1380-1383. Bennett, J. M.; Cohen, J. P.; Flanigen, E. M.; Pluth, J . J.; Smith, J. V. ACS SymD. Ser. 1983.218.109-118. Choudhary, V."R., Akolekar, D. B. J. Catal. 1987,103,115-125. Cracknell, R. F.; Gubbins, K. E. J . Mol. Liq. 1992,54, 261-271. Cracknell, R.; Koh, C. A.; Thompson, S. M.; Gubbins, K. E. Mater. Res. SOC.Symp. Proc. 1993,290,135-146. Davis, M. E.; Saldarriaea, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988,331,698-699. Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J . M. J. Am. Chem. SOC.1989,111,3919-3924. Ferraro, J. R.; Rein, A. J. In Fourier Transform Infrared Spectroscopy, Applications to Chemical Systems; Ferraro, J . R.; Basile, L. J., Eds.; Academic Press: New York, 1985. Curtiss, C. F.; Bird, R. B. MoZecular Theory of Hirschfelder, J. 0.; Gases and Liquids; Wiley: New York, 1964. Jiang, S.; Balbuena, P. B.; Gubbins, K. E. J. Phys. Chem. 1994, 98,2403-2411. Koh, C. A.; Zollweg, J. A.; Gubbins, K. E. Studies in Surface Science, Characterization of Porous Solids III; Rouquerol, J., Rodriguez-Reinoso, F., Sing, K. S. W., Unger, K. K. Eds.; Elsevier: Marseille, France, 1994; Vol. 87, pp 61-70. Koh, C. A.; Nooney, R. I.; Tahir, S. Proc. Am. Chem. Eng. Mtg. 1995, submitted. Kumar, R.; Golden, T. C. Gas Sep. Purif 1991,51,21-26. Moretti, E. C.; Mukhopadhyay, N. Chem. Eng. Prog. 1993,89(No. 7), pp 20-26. Nicholson, D.; Gubbins, K. E., to be published, 1995. Noll, K. E.; Gounaris, V.; Hou, W. S. Adsorption Technology for Air and Water Pollution Control;Lewis Pub.: Chelsea, MI, 1992. Ruhl, M. J. Chem. Eng. Prog. 1993,89 (No. 7), pp 37-41. Stenzel, M. H. Chem. Eng. Prog. 1993,89(No. 41, pp 36-43. Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flanigen, E. M. J . Am. Chem. SOC.1982,104,1146-1147.
Received for review April 3, 1995 Revised manuscript received September 5, 1995 Accepted September 19, 1995* IE950231S
* Abstract published in Advance A C S Abstracts, November 15, 1995.
