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Hydrocarbon sorption properties of pure silica MCM-22 type zeolite
Microporous and Mesoporous Materials 40 (2000) 305±312
www.elsevier.nl/locate/micromeso
Hydrocarbon sorption properties of pure silica MCM-22 type zeolite Hongwei Du a,1, Mohan Kalyanaraman b, Miguel A. Camblor c, David H. Olson a,* a
Department of Chemical Engineering, University of Pennsylvania, 311A Towne Building, 220 S. 33rd Street, Philadelphia, PA 19104, USA b Exxon Mobil Research & Engineering, P.O. Box 480, Paulsboro, NJ 08066, USA c Instituto de Tecnologia Quimica (CSIC-UPV), Universidad Politecnica de Valencia, Avda. Los Naranjos s/n, 46071 Valencia, Spain Received 3 April 2000; accepted 5 July 2000
Abstract The adsorption properties of the MWW compositional end member, ITQ-1, have been explored empirically and via computer simulation using n-hexane, 3-methylpentane, 2,3-dimethylbutane, p-xylene and ethylbenzene as adsorbate probes. n-Hexane and 3-methlypentane diuse rapidly and both have a sorption limit of four molecules per unit cell, three of which are in the large channel cavities. 2,3-dimethylbutane sorbs more slowly through the 10-ring windows; the limiting three molecules per unit cell are all in the large cavities. The two aromatics adsorb in both channel systems, with p-xylene diusing rapidly and ethylbenzene more slowly but much faster than 2,3-dimethylbutane. No clear distinction between intra- and extra-crystalline adsorption was observed. Computer modeling indicated distinctions between the two channel systems. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Sorption properties; ITQ-1; Hydrocarbons; Computer simulation
1. Introduction MCM-22, having MWW topology, is one of the most interesting of the recent new zeolites [1], possessing two independent, 10-ring de®ning pore systems. In addition, its typical thin platelet morphology results in high external surface area. One pore system, referred to as the inter-layer region *
Corresponding author. Fax: +1-215-573-2093. E-mail address: [email protected] (D.H. Olson). 1 Present address: Department of Chemical Engineering, Northwestern University, 2145 Sheridan Road, E136, Evanston, IL 60208, USA.
and at z 0, contains large cylindrical cavities in size. The second pore system, the 7.1 18 A intra-layer system at z 1=2, has sinusoidal channels and lacks any large cavity regions. Both systems are accessed by 10-ring windows having The external surface diameters of 4.0 5:5 A. deep, 12-ring diameter pockets that may has 9 A contain tetra-coordinated aluminum atoms fully connected to the framework. These aluminum atoms would be the source of high acid activity. Indeed, even though the intra-crystalline pore regions are accessed via 10-rings, there have been several reports showing catalytic shape selectivity intermediate between that of 10- and 12-ring
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zeolites, indicating that external surface acid sites are contributing to the catalytic conversion reactions [2±4]. The adsorption properties of MCM-22 are also aected by this complex array of surfaces. Many adsorption studies have been directed at understanding the complex adsorption system of MCM-22. Roth and coworkers have reported on the capacities and relative adsorption rates of several hydrocarbons [5]. They found that 135 mg/ g n-hexane is adsorbed at 25°C and P =P0 0:5 and that 3-methylpentane adsorbs eight times faster than 2,2-dimethylbutane. Using molecular mechanics calculations, Perego et al. found high energy barriers, 377 and 234 kJ/mol, for the diusion of cumene through the 10-rings of the sinusoidal and large cavity system, respectively [6]. From a molecular dynamics study of the diusion of nheptane and 2-methylhexane at 450 K, Corma et al. concluded that the linear n-heptane diuses rapidly in both pore systems but that branched heptane diusion is severely restricted in both pore systems [7]. Extending this study, Sastre et al. [8] reported that 2-methylhexane diusion is severely restricted even at 650 K. Both observe no movement of this molecule through the 10-rings connecting the large cavities. Roque-Malherbe et al. found that benzene, toluene and ethylbenzene have about the same diusion coecients over the 300±450 K range whereas that of o-xylene is about four orders of magnitude lower [9]. Corma et al. ®nd a toluene sorption capacity of 135 mg/g at 315 K [10]. High 1,3,5-trimethylbenzene sorption capacity for an MCM-22 preparation, at P =P0 0:5, is reported by Ravishankar et al. [11]. Eder et al. [12] have reported the heats of adsorption and Henry's constants for several small hydrocarbons on MCM-22. The importance of the external surface and restricted diusion of o-xylene has been demonstrated by Wendelbo et al. [13]. However, empirical adsorption studies on the pure silica analog of MCM-22, ITQ-1, have not been reported yet. The importance for such a study is (1) ITQ-1 [14] is free of framework aluminum and associated cations, allowing structure speci®c properties to dominate. (2) ITQ-1 is hydrophobic imparting to it the potential for the removal of hydrocarbons from water rich media [15]; in this regard, the adsorption selectivities peculiar to
MWW topology [16±18] are of interest. (3) ITQ-1 employed here has a much larger crystal size than the MCM-22 materials studied earlier, minimizing external surface adsorption and in turn, leading to a better understanding of the intra-crystalline adsorption behavior of MCM-22. As part of this study, we have examined the sorption properties of ITQ-1 with several hydrocarbons of practical interest. Adsorption rates, capacities and isotherms of these sorbates on ITQ1 are reported. Computer simulations were employed to indicate hydrocarbon sorption capacities and their distribution in the two channel systems. 2. Experimental The ITQ-1 zeolite used in this study was prepared as described earlier [14]. X-ray diraction, SEM and TEM examination of the product indicated high purity. Crystal size measurements, from a set of TEM observations, indicated 1.5 lm diameter and 0.027 lm thickness. SEM and TEM photographs are shown in Fig. 1(a) and (b). Adsorption measurements were made using a computer controlled thermogravimetric balance consisting of a TA51 thermobalance and associated TA-2000/PC control system. This 1 atm, gas ¯ow through electrobalance system was controlled via Macintosh based LabView control software, Kinetic Systems Interface, mass ¯ow controllers and Eurotherm temperature controller. Isotherms were ®t with the Langmuir equation Q Qs kP = 1 kP
1
or the Langmuir equation plus a Henry law type term to account for adsorption on the external surface Q Qs kES P kP = 1 kP :
2
In both cases, both the ks and the Qs were used as ®tting variables, the latter being the limiting high pressure adsorption for the temperature of the isotherm. Typically, Eq. (1) was used for curve ®tting; Eq. (2) was applied only to the two or three lowest temperature isotherms where external surface adsorption begins to have an observable effect. Tables 1±3 list the resulting curve ®tting
H. Du et al. / Microporous and Mesoporous Materials 40 (2000) 305±312
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Fig. 1. (a) SEM photograph showing platelet morphology, and (b) TEM shows hexagonal crystal with a diameter of 1.5 lm. Table 1 Langmuir's constants and saturation capacities for the adsorption of n-hexane on ITQ-1 Temperature (°C) 30 45 60 75 90 105 120 135 150 165
k (Torrÿ1 ) a
(1.65) (0.81)a 0.378 0.246 0.158 0.0918 0.0554 0.0383 0.0208 0.0150
Qs (mg/g) a
(91.9) (88.7)a 84.8 81.5 79.0 76.0 72.9 68.4 67.7 62.1
Table 2 Langmuir's constants and saturation capacities for the adsorption of 3-methylpentane on ITQ-1 Temperature (°C)
k (Torrÿ1 )
Qs (mg/g)
30 45 60 75 90 105 120 135 150 165
(2.83)a (1.17)a (0.770)a (0.436)a 0.216 0.185 0.115 0.0732 0.0425 0.0311
(90.7)a (86.0)a (81.2)a (76.4)a 70.7 65.3 60.8 56.9 54.6 47.8
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ38:1 kJ/mol.
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ36:4 kJ/mol.
parameters. Isosteric heats of adsorption were computed using the Clausius±Clapeyron equation. The computer simulations were done using the molecular simulation software Cerius2 (version 3.8 from Molecular Simulations Inc., San Diego) using the Sorption [19] module. The zeolite framework is held ®xed, and the sorbate molecules are placed into the three-dimensional framework. While the molecules can be created, translated, rotated or destroyed, the module treats them still as rigid bodies. The Burchart1.01 [20]±Dreiding2.21 [21] combined force ®eld was used for the simulations. All user-de®ned parameters used were set to the default value. The zeolite framework is assumed to
be fully siliceous, and the model did not take account of any coulombic interactions. We used a 2 2 1 supercell for better visualization of the pockets, a temperature of 273 K, and a pressure of 75 Torr for all simulations. Iterations were typically stopped at 10 million or at invariant energy. The adsorption models presented here represent molecular arrangements that are feasible from a steric standpoint, i.e., there are no short atom± atom distances that would produce high repulsive energies. The molecular positions are energetically feasible. However, their positions have not been re®ned to produce the lowest energy arrangement of the molecules in the channels.
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Table 3 Langmuir's constants and saturation capacities for the adsorption of p-xylene on ITQ-1 Temperature (°C)
k (Torrÿ1 )
Qs (mg/g)
30 45 60 75 90 105 120 135 150
(17.7)a (9.9)a 5.86 4.38 2.80 1.85 1.26 0.73 0.52
(86.4)a (80.8)a 73.1 68.3 65.5 59.9 52.6 47.8 40.8
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ29:7 kJ/mol.
3. Results and discussion As indicated above, the unique combination of a dual pore structure and the high external surface area typical of MCM-22 make distinction between intra- and extra-crystalline adsorption very challenging. Adding to this complication is deep pockets on the external the existence of 9 A surface that may have adsorption properties similar to that of the intra-crystalline region. However, the larger crystal size of the pure silica isotype of MWW, ITQ-1, reduces the later adsorption, simplifying this problem. Also, simulated adsorption computations have provided further guidance in making this distinction, primarily by indicating intra-crystalline adsorption capacities. The computations have also indicated the distribution of molecules between the two channel systems. 3.1. Adsorption of C6 parans As shown in Fig. 2, the rate adsorption of light, normal parans, exempli®ed by n-hexane, is fast in MWW type zeolites, reaching equilibrium in 6.7 min (t1=2 20 s1=2 ) at 30°C. This agrees with reported n-hexane adsorption results for MCM-22 [11]. The rate of adsorption of singly branched parans, e.g., 3-methylpentane is also fast, its rate being indistinguishable from that of n-hexane within the response limitations of our measure-
Fig. 2. Rate of adsorption of n-hexane, 3-methylpentane and 2,3-dimethylbutane on ITQ-1 at 30°C and P =P0 0:39: amount adsorbed versus square root of time.
ments. In contrast, the doubly branched C6 paran 2,3-dimethylbutane diuses much more slowly, reaching equilibrium in 325 min (t1=2 140 s1=2 ) at 30°C. Other work shows that gemdimethyl parans, such as 2,2-dimethylbutane diuse even more slowly in ITQ-1 and other MWW type zeolites [5]. Note that the desorption curves for n-hexane and 3-methylpentane show a break at four molecules per unit cell (Fig. 2); the break is ascribed to a change in overall desorption rate at the completion of the relatively rapid external desorption and the continuation of the slower intra-crystalline desorption. This indicates that either four molecules per unit cell is the intracrystalline adsorption limit and higher adsorption levels represents external surface adsorption or that at higher levels of intra-crystalline adsorption, the energetics are less favorable. In either case, the intracrystalline adsorption limit for n-hexane and 3-methylpentane is at least four molecules per unit cell. Modeling calculations indicate that for both molecules, the energetics are favorable for the adsorption of four molecules per unit cell. The modeling also indicates that in each case, there are three molecules in the large cage and one molecule per unit cell in the sinusoidal channel system. The adsorption curve for 2,3-dimethylbutane levels o at three molecules per unit cell. This may be taken as an adsorption limit. Simulated ad-
H. Du et al. / Microporous and Mesoporous Materials 40 (2000) 305±312
Fig. 3. Adsorption isotherms for n-hexane on ITQ-1 over the temperature range 30±165°C.
sorption indicates that three molecules per unit cell is feasible with all adsorption taking place in the large cavities. Adsorption isotherms for n-hexane in the temperature range 30±165°C are shown in Fig. 3. The 30°C and 45°C isotherms were ®tted using a combined Langmuir±Henry law Eq. (2) whereas all higher temperature curves were ®tted using the Langmuir relationship (1). The former is an attempt to take account of external surface adsorption. In these ®ttings, both the ks and the limiting adsorption capacity, Qs , were re®ned as the latter were not measured directly in our experiments. Plots of these variables as a function of 1=T (K) and T (°C), respectively, show linear relationships (Figs. 4 and 5). The slope from the former plot leads to DH ÿ38:0 kJ/mol. Isosteric heats of adsorption, qst , obtained from the isotherms are essentially constant with loading having an average value of ÿ46.9 kJ/mol. This value for Al-free, large cavity ITQ-1 zeolite agrees well with the value, ÿ53 kJ/mol, reported by Eder et al. [22] for the large pore zeolite HY (3.0 meq H(Al)/g). After adjusting for the additional 7 kJ/mol that has been attributed to adsorption on the Bronsted acid sites, the values are essentially identical [23]. However, these two values are substantially lower than the value of ÿ69 kJ/mol reported for the aluminum containing HMCM-22 [12]. As expected, all are lower than the ÿ82 kJ/mol reported for the smaller
309
Fig. 4. Plot of log Langmuir constant k versus 1=T (K) for nhexane on ITQ-1 over the temperature range 30±165°C.
Fig. 5. Plot of saturation capacity, Qs , for n-hexane on ITQ-1 versus T (°C) over the temperature range 30±165°C.
pore HZSM-5 [22] where there is greater molecule framework interaction. A comparison of the n-hexane adsorption isobars for ITQ-1 and ZSM-5 are informative. While both are 10-ring zeolites, they have distinctly different channels systems. Also, typically ZSM-5 materials are free of signi®cant contributions of the external surface to the adsorption properties. For ITQ-1, such eects are likely to contribute. This latter dierence is re¯ected in their isobars shown in Fig. 6. The isobar for ZSM-5 ¯attens out at its pore ®lling level as the temperature drops below 100°C (P =P0 0:04) whereas the curve for ITQ-1 continues to increase in this region due to contributions from the external surface.
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Fig. 6. n-Hexane adsorption isobars, at 72 Torr n-hexane, for ITQ-1 and ZSM-5.
Fig. 8. 3-methylpentane adsorption isobars, at 72 Torr n-hexane, for ITQ-1 and ZSM-5.
A family of 3-methylpentane adsorption isotherms is shown in Fig. 7. Again qst , calculated from the isotherms, is nearly constant with loading having an average value of 49.4 kJ/mol, very close to the value found for n-hexane. A plot of Qs , the high pressure adsorption limit versus T (°C) reveals a 0°C intercept essentially the same as the value for n-hexane, i.e., 100 and 98 mg/g, respectively. The slope for 3-methylpentane is sharper, however, with Qs dropping o more rapidly with increasing temperature. Thus, Qs at 150°C is 55 and 68 mg/g for 3-methylpentane and n-hexane,
respectively. This indicates a more eective adsorbate±zeolite framework interaction for the linear than for the branched paran. A comparison of the 3-methylpentane isobars for ITQ-1 and ZSM-5 reveals a linear increase in Q with temperature for the former and a leveling out at four molecules per unit cell for ZSM-5 before moving higher at temperatures below 100°C (Fig. 8). The latter corresponds to ®lling each of the four channel intersections. Again, the high external surface area of the former and weaker interactions at loadings above four per unit cell for the latter obscures any complete pore ®lling phenomena. 3.2. Adsorption of C8 aromatics
Fig. 7. Adsorption isotherms for 3-methylpentane on ITQ-1 over the temperature range 30±165°C.
The adsorption behavior of p-xylene is of interest because of its petrochemical use and because ZSM-5, also having a channel structure de®ned by 10-ring openings, shows high para selectivity in reactions such as toluene disproportionation. This selectivity is due in large part to the large dierence in diusivity of p-xylene compared with the m- and o-xylene isomers. Evidence for modest para selectivity has been reported for MCM-22 [2,4,16]. The adsorption of p-xylene in ITQ-1 is fast, reaching equilibrium in 15 min (t1=2 30 s1=2 ) at 50°C. For MCM-22, other workers have reported fast diusion of p-xylene relative to m-xylene and/or
H. Du et al. / Microporous and Mesoporous Materials 40 (2000) 305±312
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Fig. 9. Adsorption isotherms for p-xylene on ITQ-1 over the temperature range 30±150°C.
Fig. 10. Rate of adsorption of ethylbenzene on ITQ-1 at 50°C and P 4:1 Torr: amount adsorbed versus square root of time.
o-xylene [9]. The qst value computed from isotherm data (Fig. 9) is 53.6 kcal/mol, somewhat higher than that found above for the hexanes. This relatively low value is taken as a consequence of the large diameter of the large cavity compared with the diameter of the p-xylene molecule. By inspection, the adsorption isotherms suggest a three molecule per unit cell limit (73.5 mg/g). Simulated adsorption indicates the adsorption of at least three molecules of p-xylene per unit cell, one in the sinusoidal channel system and two in each of the large cavities of the inter-layer system. The adsorption properties of ethylbenzene are of special interest because the aluminum containing counterpart of ITQ-1, MCM-22, is used commercially for the synthesis of ethylbenzene via the alkylation of benzene with ethylene. Both benzene and ethylene are projected to diuse rapidly in materials having the MWW topology. Ethylbenzene adsorbs at a rate intermediate between that of p-xylene and 2,3-dimethylbutane, reaching equilibrium in 135 min (t1=2 90 s1=2 ) at 50°C (Fig. 10). This time is 13 min at 120°C. In view of this, it is of interest that a recent paper provides data suggesting that in this process, a majority of the catalysis is on the external surface of the zeolite crystals [17]. This raises the question of why, if the products and reactants diuse reasonably fast, does the external surface sites play a dominant role (Recall that the crystal size of MCM-22 is signi®-
cantly smaller than that of ITQ-1). The recent work by Lercher et al. suggests a probable cause [18]. In an in situ infrared study of the alkylation of toluene with methanol over HZSM-5, they found that the rapidly diusing molecules escaped the crystal channels quickly, whereas slowly diffusing molecules, e.g., m-xylene, o-xylene and trimethylbenzenes, build up to a relatively high steady state value. In the EB synthesis process, any m- and o-diethylbenzene would be trapped, possibly building up to reaction quenching levels, leaving only the external surfaces as the active catalytic surface.
4. Conclusions Adsorption of linear and mono-branched C6 parans is fast in ITQ-1 and shows a limit of four molecules per unit cell. Computer simulations indicate an upper limit of three molecules in each large cage plus another one in the sinusoidal channels. By contrast, the adsorption of di-branched C6 parans is more hindered with a much lower rate of adsorption and a lower adsorption capacity of three molecules per unit cell (all in the large cage). Computer simulations suggest dibranched C6 parans reside in only one of the pore systems in MWW type zeolites, a conclusion
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to be taken into account in studies of catalytic reactions involving these species as reactants or products. The adsorption of p-xylene is fast and shows an upper limit of three molecules per unit cell (two in each large cage and one in the sinusoidal channel). This result contrasts with the restricted diusion of o-xylene [13] and may thus explain the moderate para selectivity found with MCM-22 in the disproportionation of toluene [3]. Finally, the adsorption of ethylbenzene is moderately fast, and hence, the proposed high or sole contribution of catalytic sites on the external surface for the synthesis of ethylbenzene [17] is tentatively attributed to diusion restriction imposed by diethylbenzene products blocking the pores.
Acknowledgements We wish to acknowledge and thank the Mobil Technology Company (MTC) for their support of this project. We also acknowledge and thank W. Borghard, J. Santiesteban, and Steve Brown, all of the MTC, for many helpful discussions. References [1] M.E. Leonowicz, J.A. Lawton, S.L. Lawton, M.K. Rubin, Science 264 (1994) 1910. [2] A. Corma, C. Corell, J. Perez-Pariente, J.M. Guil, R. GuilL opez, S. Nicolopoulos, J.G. Calbet, M. Vallet-Regi, Zeolites 16 (1996) 7. [3] P. Wu, T. Komatsu, T. Yashima, Micropor. Mesopor. Mater. 22 (1998) 343. [4] J. Weitkamp, U. Weiss, S. Ernst, in: H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (Eds.), Catalysis by Microporous Materials, Studies of Surface Science and Catalysis, vol. 94, Elsevier, Amsterdam, 1995, p. 363.
[5] W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, S.B. Mcullen, in: H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (Eds.), Catalysis by Microporous Materials, Studies of Surface Science and Catalysis, vol. 94, Elsevier, Amsterdam, 1995, p. 301. [6] C. Perego, S. Amarilli, R. Millini, G. Bellussi, G. Girotti, G. Terzoni, Micropor. Mater. 6 (1996) 395. [7] A. Corma, C.R. Catlow, G. Sastre, J. Phys. Chem. B 102 (1998) 7085. [8] G. Sastre, C.R.A. Catlow, A. Chica, A. Corma, J. Phys. Chem. B 104 (2000) 416. [9] R. Roque-Malherbe, R. Wendelbo, A. Mifsud, A. Corma, J. Phys. Chem. B 99 (1995) 14064. [10] A. Corma, C. Corell, J. Perez-Pariente, J.M. Guil, R. GuilL opez, S. Nicolopoulos, J.G. Calbet, M. Vallet-Regi, Zeolites 16 (1996) 7. [11] R. Ravishankar, D. Bhattacharya, N.E. Jacob, S. Sivasanker, Micropor. Mater. 4 (1995) 83. [12] F. Eder, Y. He, G. Nivarthy, J.A. Lercher, Recl. Trav. Chim. Pays-Bas 115 (1996) 531. [13] R. Wendelbo, R. Roque-Halherbe, Micropor. Mater. 10 (1997) 231. [14] M.A. Camblor, C. Corell, A. Corma, M.-J. Dõaz-Caba~ nas, S. Nicolopoulos, J.M. Gonz alez-Calbet, M. Vallet-Regi, Chem. Mater. 8 (1996) 2415. [15] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512. [16] A. Corma, C. Corell, F. Llopis, A. Martõnez, J. PerezPariente, Appl. Catal. A 115 (1994) 121. [17] J.C. Cheng, T.F. Degnan, J.S. Beck, Y.Y. Huang, M. Kalyanaraman, J.A. Kowalski, C.A. Loeher, D.N. Mazzone, Science and Technology in Catalysis, Kodansha, Tokyo, 1999, p. 53. [18] G. Mirth, J.A. Lercher, J. Catal. 132 (1991) 244. [19] Cerius2 software, Molecular Simulations Inc., San Diego, CA, www.msi.com. [20] E.V. Burchart, Ph.D. Thesis, Studies on Zeolites: Molecular Mechanics, Framework Stability and Crystal Growth, 1992 (Table I, Chapter XII). [21] S.L. Mayo, B.D. Olafson, W.A. Goddard III, J. Phys. Chem. 94 (1990) 8897. [22] F. Eder, M. Stockenhuber, J.A. Lercher, J. Phys. Chem. B 101 (1997) 5414. [23] F. Eder, J.A. Lercher, J. Phys. Chem. B 101 (1997) 1273.
www.elsevier.nl/locate/micromeso
Hydrocarbon sorption properties of pure silica MCM-22 type zeolite Hongwei Du a,1, Mohan Kalyanaraman b, Miguel A. Camblor c, David H. Olson a,* a
Department of Chemical Engineering, University of Pennsylvania, 311A Towne Building, 220 S. 33rd Street, Philadelphia, PA 19104, USA b Exxon Mobil Research & Engineering, P.O. Box 480, Paulsboro, NJ 08066, USA c Instituto de Tecnologia Quimica (CSIC-UPV), Universidad Politecnica de Valencia, Avda. Los Naranjos s/n, 46071 Valencia, Spain Received 3 April 2000; accepted 5 July 2000
Abstract The adsorption properties of the MWW compositional end member, ITQ-1, have been explored empirically and via computer simulation using n-hexane, 3-methylpentane, 2,3-dimethylbutane, p-xylene and ethylbenzene as adsorbate probes. n-Hexane and 3-methlypentane diuse rapidly and both have a sorption limit of four molecules per unit cell, three of which are in the large channel cavities. 2,3-dimethylbutane sorbs more slowly through the 10-ring windows; the limiting three molecules per unit cell are all in the large cavities. The two aromatics adsorb in both channel systems, with p-xylene diusing rapidly and ethylbenzene more slowly but much faster than 2,3-dimethylbutane. No clear distinction between intra- and extra-crystalline adsorption was observed. Computer modeling indicated distinctions between the two channel systems. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Sorption properties; ITQ-1; Hydrocarbons; Computer simulation
1. Introduction MCM-22, having MWW topology, is one of the most interesting of the recent new zeolites [1], possessing two independent, 10-ring de®ning pore systems. In addition, its typical thin platelet morphology results in high external surface area. One pore system, referred to as the inter-layer region *
Corresponding author. Fax: +1-215-573-2093. E-mail address: [email protected] (D.H. Olson). 1 Present address: Department of Chemical Engineering, Northwestern University, 2145 Sheridan Road, E136, Evanston, IL 60208, USA.
and at z 0, contains large cylindrical cavities in size. The second pore system, the 7.1 18 A intra-layer system at z 1=2, has sinusoidal channels and lacks any large cavity regions. Both systems are accessed by 10-ring windows having The external surface diameters of 4.0 5:5 A. deep, 12-ring diameter pockets that may has 9 A contain tetra-coordinated aluminum atoms fully connected to the framework. These aluminum atoms would be the source of high acid activity. Indeed, even though the intra-crystalline pore regions are accessed via 10-rings, there have been several reports showing catalytic shape selectivity intermediate between that of 10- and 12-ring
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zeolites, indicating that external surface acid sites are contributing to the catalytic conversion reactions [2±4]. The adsorption properties of MCM-22 are also aected by this complex array of surfaces. Many adsorption studies have been directed at understanding the complex adsorption system of MCM-22. Roth and coworkers have reported on the capacities and relative adsorption rates of several hydrocarbons [5]. They found that 135 mg/ g n-hexane is adsorbed at 25°C and P =P0 0:5 and that 3-methylpentane adsorbs eight times faster than 2,2-dimethylbutane. Using molecular mechanics calculations, Perego et al. found high energy barriers, 377 and 234 kJ/mol, for the diusion of cumene through the 10-rings of the sinusoidal and large cavity system, respectively [6]. From a molecular dynamics study of the diusion of nheptane and 2-methylhexane at 450 K, Corma et al. concluded that the linear n-heptane diuses rapidly in both pore systems but that branched heptane diusion is severely restricted in both pore systems [7]. Extending this study, Sastre et al. [8] reported that 2-methylhexane diusion is severely restricted even at 650 K. Both observe no movement of this molecule through the 10-rings connecting the large cavities. Roque-Malherbe et al. found that benzene, toluene and ethylbenzene have about the same diusion coecients over the 300±450 K range whereas that of o-xylene is about four orders of magnitude lower [9]. Corma et al. ®nd a toluene sorption capacity of 135 mg/g at 315 K [10]. High 1,3,5-trimethylbenzene sorption capacity for an MCM-22 preparation, at P =P0 0:5, is reported by Ravishankar et al. [11]. Eder et al. [12] have reported the heats of adsorption and Henry's constants for several small hydrocarbons on MCM-22. The importance of the external surface and restricted diusion of o-xylene has been demonstrated by Wendelbo et al. [13]. However, empirical adsorption studies on the pure silica analog of MCM-22, ITQ-1, have not been reported yet. The importance for such a study is (1) ITQ-1 [14] is free of framework aluminum and associated cations, allowing structure speci®c properties to dominate. (2) ITQ-1 is hydrophobic imparting to it the potential for the removal of hydrocarbons from water rich media [15]; in this regard, the adsorption selectivities peculiar to
MWW topology [16±18] are of interest. (3) ITQ-1 employed here has a much larger crystal size than the MCM-22 materials studied earlier, minimizing external surface adsorption and in turn, leading to a better understanding of the intra-crystalline adsorption behavior of MCM-22. As part of this study, we have examined the sorption properties of ITQ-1 with several hydrocarbons of practical interest. Adsorption rates, capacities and isotherms of these sorbates on ITQ1 are reported. Computer simulations were employed to indicate hydrocarbon sorption capacities and their distribution in the two channel systems. 2. Experimental The ITQ-1 zeolite used in this study was prepared as described earlier [14]. X-ray diraction, SEM and TEM examination of the product indicated high purity. Crystal size measurements, from a set of TEM observations, indicated 1.5 lm diameter and 0.027 lm thickness. SEM and TEM photographs are shown in Fig. 1(a) and (b). Adsorption measurements were made using a computer controlled thermogravimetric balance consisting of a TA51 thermobalance and associated TA-2000/PC control system. This 1 atm, gas ¯ow through electrobalance system was controlled via Macintosh based LabView control software, Kinetic Systems Interface, mass ¯ow controllers and Eurotherm temperature controller. Isotherms were ®t with the Langmuir equation Q Qs kP = 1 kP
1
or the Langmuir equation plus a Henry law type term to account for adsorption on the external surface Q Qs kES P kP = 1 kP :
2
In both cases, both the ks and the Qs were used as ®tting variables, the latter being the limiting high pressure adsorption for the temperature of the isotherm. Typically, Eq. (1) was used for curve ®tting; Eq. (2) was applied only to the two or three lowest temperature isotherms where external surface adsorption begins to have an observable effect. Tables 1±3 list the resulting curve ®tting
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Fig. 1. (a) SEM photograph showing platelet morphology, and (b) TEM shows hexagonal crystal with a diameter of 1.5 lm. Table 1 Langmuir's constants and saturation capacities for the adsorption of n-hexane on ITQ-1 Temperature (°C) 30 45 60 75 90 105 120 135 150 165
k (Torrÿ1 ) a
(1.65) (0.81)a 0.378 0.246 0.158 0.0918 0.0554 0.0383 0.0208 0.0150
Qs (mg/g) a
(91.9) (88.7)a 84.8 81.5 79.0 76.0 72.9 68.4 67.7 62.1
Table 2 Langmuir's constants and saturation capacities for the adsorption of 3-methylpentane on ITQ-1 Temperature (°C)
k (Torrÿ1 )
Qs (mg/g)
30 45 60 75 90 105 120 135 150 165
(2.83)a (1.17)a (0.770)a (0.436)a 0.216 0.185 0.115 0.0732 0.0425 0.0311
(90.7)a (86.0)a (81.2)a (76.4)a 70.7 65.3 60.8 56.9 54.6 47.8
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ38:1 kJ/mol.
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ36:4 kJ/mol.
parameters. Isosteric heats of adsorption were computed using the Clausius±Clapeyron equation. The computer simulations were done using the molecular simulation software Cerius2 (version 3.8 from Molecular Simulations Inc., San Diego) using the Sorption [19] module. The zeolite framework is held ®xed, and the sorbate molecules are placed into the three-dimensional framework. While the molecules can be created, translated, rotated or destroyed, the module treats them still as rigid bodies. The Burchart1.01 [20]±Dreiding2.21 [21] combined force ®eld was used for the simulations. All user-de®ned parameters used were set to the default value. The zeolite framework is assumed to
be fully siliceous, and the model did not take account of any coulombic interactions. We used a 2 2 1 supercell for better visualization of the pockets, a temperature of 273 K, and a pressure of 75 Torr for all simulations. Iterations were typically stopped at 10 million or at invariant energy. The adsorption models presented here represent molecular arrangements that are feasible from a steric standpoint, i.e., there are no short atom± atom distances that would produce high repulsive energies. The molecular positions are energetically feasible. However, their positions have not been re®ned to produce the lowest energy arrangement of the molecules in the channels.
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Table 3 Langmuir's constants and saturation capacities for the adsorption of p-xylene on ITQ-1 Temperature (°C)
k (Torrÿ1 )
Qs (mg/g)
30 45 60 75 90 105 120 135 150
(17.7)a (9.9)a 5.86 4.38 2.80 1.85 1.26 0.73 0.52
(86.4)a (80.8)a 73.1 68.3 65.5 59.9 52.6 47.8 40.8
a Values extracted from higher temperature parameter relationships. Isotherm curve ®tted using Eq. (2) and Qs , kES , and k as variables. All other curves ®tted using Eq. (1) and Qs , and k as variables. Heat of adsorption from ks ÿ29:7 kJ/mol.
3. Results and discussion As indicated above, the unique combination of a dual pore structure and the high external surface area typical of MCM-22 make distinction between intra- and extra-crystalline adsorption very challenging. Adding to this complication is deep pockets on the external the existence of 9 A surface that may have adsorption properties similar to that of the intra-crystalline region. However, the larger crystal size of the pure silica isotype of MWW, ITQ-1, reduces the later adsorption, simplifying this problem. Also, simulated adsorption computations have provided further guidance in making this distinction, primarily by indicating intra-crystalline adsorption capacities. The computations have also indicated the distribution of molecules between the two channel systems. 3.1. Adsorption of C6 parans As shown in Fig. 2, the rate adsorption of light, normal parans, exempli®ed by n-hexane, is fast in MWW type zeolites, reaching equilibrium in 6.7 min (t1=2 20 s1=2 ) at 30°C. This agrees with reported n-hexane adsorption results for MCM-22 [11]. The rate of adsorption of singly branched parans, e.g., 3-methylpentane is also fast, its rate being indistinguishable from that of n-hexane within the response limitations of our measure-
Fig. 2. Rate of adsorption of n-hexane, 3-methylpentane and 2,3-dimethylbutane on ITQ-1 at 30°C and P =P0 0:39: amount adsorbed versus square root of time.
ments. In contrast, the doubly branched C6 paran 2,3-dimethylbutane diuses much more slowly, reaching equilibrium in 325 min (t1=2 140 s1=2 ) at 30°C. Other work shows that gemdimethyl parans, such as 2,2-dimethylbutane diuse even more slowly in ITQ-1 and other MWW type zeolites [5]. Note that the desorption curves for n-hexane and 3-methylpentane show a break at four molecules per unit cell (Fig. 2); the break is ascribed to a change in overall desorption rate at the completion of the relatively rapid external desorption and the continuation of the slower intra-crystalline desorption. This indicates that either four molecules per unit cell is the intracrystalline adsorption limit and higher adsorption levels represents external surface adsorption or that at higher levels of intra-crystalline adsorption, the energetics are less favorable. In either case, the intracrystalline adsorption limit for n-hexane and 3-methylpentane is at least four molecules per unit cell. Modeling calculations indicate that for both molecules, the energetics are favorable for the adsorption of four molecules per unit cell. The modeling also indicates that in each case, there are three molecules in the large cage and one molecule per unit cell in the sinusoidal channel system. The adsorption curve for 2,3-dimethylbutane levels o at three molecules per unit cell. This may be taken as an adsorption limit. Simulated ad-
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Fig. 3. Adsorption isotherms for n-hexane on ITQ-1 over the temperature range 30±165°C.
sorption indicates that three molecules per unit cell is feasible with all adsorption taking place in the large cavities. Adsorption isotherms for n-hexane in the temperature range 30±165°C are shown in Fig. 3. The 30°C and 45°C isotherms were ®tted using a combined Langmuir±Henry law Eq. (2) whereas all higher temperature curves were ®tted using the Langmuir relationship (1). The former is an attempt to take account of external surface adsorption. In these ®ttings, both the ks and the limiting adsorption capacity, Qs , were re®ned as the latter were not measured directly in our experiments. Plots of these variables as a function of 1=T (K) and T (°C), respectively, show linear relationships (Figs. 4 and 5). The slope from the former plot leads to DH ÿ38:0 kJ/mol. Isosteric heats of adsorption, qst , obtained from the isotherms are essentially constant with loading having an average value of ÿ46.9 kJ/mol. This value for Al-free, large cavity ITQ-1 zeolite agrees well with the value, ÿ53 kJ/mol, reported by Eder et al. [22] for the large pore zeolite HY (3.0 meq H(Al)/g). After adjusting for the additional 7 kJ/mol that has been attributed to adsorption on the Bronsted acid sites, the values are essentially identical [23]. However, these two values are substantially lower than the value of ÿ69 kJ/mol reported for the aluminum containing HMCM-22 [12]. As expected, all are lower than the ÿ82 kJ/mol reported for the smaller
309
Fig. 4. Plot of log Langmuir constant k versus 1=T (K) for nhexane on ITQ-1 over the temperature range 30±165°C.
Fig. 5. Plot of saturation capacity, Qs , for n-hexane on ITQ-1 versus T (°C) over the temperature range 30±165°C.
pore HZSM-5 [22] where there is greater molecule framework interaction. A comparison of the n-hexane adsorption isobars for ITQ-1 and ZSM-5 are informative. While both are 10-ring zeolites, they have distinctly different channels systems. Also, typically ZSM-5 materials are free of signi®cant contributions of the external surface to the adsorption properties. For ITQ-1, such eects are likely to contribute. This latter dierence is re¯ected in their isobars shown in Fig. 6. The isobar for ZSM-5 ¯attens out at its pore ®lling level as the temperature drops below 100°C (P =P0 0:04) whereas the curve for ITQ-1 continues to increase in this region due to contributions from the external surface.
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Fig. 6. n-Hexane adsorption isobars, at 72 Torr n-hexane, for ITQ-1 and ZSM-5.
Fig. 8. 3-methylpentane adsorption isobars, at 72 Torr n-hexane, for ITQ-1 and ZSM-5.
A family of 3-methylpentane adsorption isotherms is shown in Fig. 7. Again qst , calculated from the isotherms, is nearly constant with loading having an average value of 49.4 kJ/mol, very close to the value found for n-hexane. A plot of Qs , the high pressure adsorption limit versus T (°C) reveals a 0°C intercept essentially the same as the value for n-hexane, i.e., 100 and 98 mg/g, respectively. The slope for 3-methylpentane is sharper, however, with Qs dropping o more rapidly with increasing temperature. Thus, Qs at 150°C is 55 and 68 mg/g for 3-methylpentane and n-hexane,
respectively. This indicates a more eective adsorbate±zeolite framework interaction for the linear than for the branched paran. A comparison of the 3-methylpentane isobars for ITQ-1 and ZSM-5 reveals a linear increase in Q with temperature for the former and a leveling out at four molecules per unit cell for ZSM-5 before moving higher at temperatures below 100°C (Fig. 8). The latter corresponds to ®lling each of the four channel intersections. Again, the high external surface area of the former and weaker interactions at loadings above four per unit cell for the latter obscures any complete pore ®lling phenomena. 3.2. Adsorption of C8 aromatics
Fig. 7. Adsorption isotherms for 3-methylpentane on ITQ-1 over the temperature range 30±165°C.
The adsorption behavior of p-xylene is of interest because of its petrochemical use and because ZSM-5, also having a channel structure de®ned by 10-ring openings, shows high para selectivity in reactions such as toluene disproportionation. This selectivity is due in large part to the large dierence in diusivity of p-xylene compared with the m- and o-xylene isomers. Evidence for modest para selectivity has been reported for MCM-22 [2,4,16]. The adsorption of p-xylene in ITQ-1 is fast, reaching equilibrium in 15 min (t1=2 30 s1=2 ) at 50°C. For MCM-22, other workers have reported fast diusion of p-xylene relative to m-xylene and/or
H. Du et al. / Microporous and Mesoporous Materials 40 (2000) 305±312
311
Fig. 9. Adsorption isotherms for p-xylene on ITQ-1 over the temperature range 30±150°C.
Fig. 10. Rate of adsorption of ethylbenzene on ITQ-1 at 50°C and P 4:1 Torr: amount adsorbed versus square root of time.
o-xylene [9]. The qst value computed from isotherm data (Fig. 9) is 53.6 kcal/mol, somewhat higher than that found above for the hexanes. This relatively low value is taken as a consequence of the large diameter of the large cavity compared with the diameter of the p-xylene molecule. By inspection, the adsorption isotherms suggest a three molecule per unit cell limit (73.5 mg/g). Simulated adsorption indicates the adsorption of at least three molecules of p-xylene per unit cell, one in the sinusoidal channel system and two in each of the large cavities of the inter-layer system. The adsorption properties of ethylbenzene are of special interest because the aluminum containing counterpart of ITQ-1, MCM-22, is used commercially for the synthesis of ethylbenzene via the alkylation of benzene with ethylene. Both benzene and ethylene are projected to diuse rapidly in materials having the MWW topology. Ethylbenzene adsorbs at a rate intermediate between that of p-xylene and 2,3-dimethylbutane, reaching equilibrium in 135 min (t1=2 90 s1=2 ) at 50°C (Fig. 10). This time is 13 min at 120°C. In view of this, it is of interest that a recent paper provides data suggesting that in this process, a majority of the catalysis is on the external surface of the zeolite crystals [17]. This raises the question of why, if the products and reactants diuse reasonably fast, does the external surface sites play a dominant role (Recall that the crystal size of MCM-22 is signi®-
cantly smaller than that of ITQ-1). The recent work by Lercher et al. suggests a probable cause [18]. In an in situ infrared study of the alkylation of toluene with methanol over HZSM-5, they found that the rapidly diusing molecules escaped the crystal channels quickly, whereas slowly diffusing molecules, e.g., m-xylene, o-xylene and trimethylbenzenes, build up to a relatively high steady state value. In the EB synthesis process, any m- and o-diethylbenzene would be trapped, possibly building up to reaction quenching levels, leaving only the external surfaces as the active catalytic surface.
4. Conclusions Adsorption of linear and mono-branched C6 parans is fast in ITQ-1 and shows a limit of four molecules per unit cell. Computer simulations indicate an upper limit of three molecules in each large cage plus another one in the sinusoidal channels. By contrast, the adsorption of di-branched C6 parans is more hindered with a much lower rate of adsorption and a lower adsorption capacity of three molecules per unit cell (all in the large cage). Computer simulations suggest dibranched C6 parans reside in only one of the pore systems in MWW type zeolites, a conclusion
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to be taken into account in studies of catalytic reactions involving these species as reactants or products. The adsorption of p-xylene is fast and shows an upper limit of three molecules per unit cell (two in each large cage and one in the sinusoidal channel). This result contrasts with the restricted diusion of o-xylene [13] and may thus explain the moderate para selectivity found with MCM-22 in the disproportionation of toluene [3]. Finally, the adsorption of ethylbenzene is moderately fast, and hence, the proposed high or sole contribution of catalytic sites on the external surface for the synthesis of ethylbenzene [17] is tentatively attributed to diusion restriction imposed by diethylbenzene products blocking the pores.
Acknowledgements We wish to acknowledge and thank the Mobil Technology Company (MTC) for their support of this project. We also acknowledge and thank W. Borghard, J. Santiesteban, and Steve Brown, all of the MTC, for many helpful discussions. References [1] M.E. Leonowicz, J.A. Lawton, S.L. Lawton, M.K. Rubin, Science 264 (1994) 1910. [2] A. Corma, C. Corell, J. Perez-Pariente, J.M. Guil, R. GuilL opez, S. Nicolopoulos, J.G. Calbet, M. Vallet-Regi, Zeolites 16 (1996) 7. [3] P. Wu, T. Komatsu, T. Yashima, Micropor. Mesopor. Mater. 22 (1998) 343. [4] J. Weitkamp, U. Weiss, S. Ernst, in: H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (Eds.), Catalysis by Microporous Materials, Studies of Surface Science and Catalysis, vol. 94, Elsevier, Amsterdam, 1995, p. 363.
[5] W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, S.B. Mcullen, in: H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (Eds.), Catalysis by Microporous Materials, Studies of Surface Science and Catalysis, vol. 94, Elsevier, Amsterdam, 1995, p. 301. [6] C. Perego, S. Amarilli, R. Millini, G. Bellussi, G. Girotti, G. Terzoni, Micropor. Mater. 6 (1996) 395. [7] A. Corma, C.R. Catlow, G. Sastre, J. Phys. Chem. B 102 (1998) 7085. [8] G. Sastre, C.R.A. Catlow, A. Chica, A. Corma, J. Phys. Chem. B 104 (2000) 416. [9] R. Roque-Malherbe, R. Wendelbo, A. Mifsud, A. Corma, J. Phys. Chem. B 99 (1995) 14064. [10] A. Corma, C. Corell, J. Perez-Pariente, J.M. Guil, R. GuilL opez, S. Nicolopoulos, J.G. Calbet, M. Vallet-Regi, Zeolites 16 (1996) 7. [11] R. Ravishankar, D. Bhattacharya, N.E. Jacob, S. Sivasanker, Micropor. Mater. 4 (1995) 83. [12] F. Eder, Y. He, G. Nivarthy, J.A. Lercher, Recl. Trav. Chim. Pays-Bas 115 (1996) 531. [13] R. Wendelbo, R. Roque-Halherbe, Micropor. Mater. 10 (1997) 231. [14] M.A. Camblor, C. Corell, A. Corma, M.-J. Dõaz-Caba~ nas, S. Nicolopoulos, J.M. Gonz alez-Calbet, M. Vallet-Regi, Chem. Mater. 8 (1996) 2415. [15] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, J.V. Smith, Nature 271 (1978) 512. [16] A. Corma, C. Corell, F. Llopis, A. Martõnez, J. PerezPariente, Appl. Catal. A 115 (1994) 121. [17] J.C. Cheng, T.F. Degnan, J.S. Beck, Y.Y. Huang, M. Kalyanaraman, J.A. Kowalski, C.A. Loeher, D.N. Mazzone, Science and Technology in Catalysis, Kodansha, Tokyo, 1999, p. 53. [18] G. Mirth, J.A. Lercher, J. Catal. 132 (1991) 244. [19] Cerius2 software, Molecular Simulations Inc., San Diego, CA, www.msi.com. [20] E.V. Burchart, Ph.D. Thesis, Studies on Zeolites: Molecular Mechanics, Framework Stability and Crystal Growth, 1992 (Table I, Chapter XII). [21] S.L. Mayo, B.D. Olafson, W.A. Goddard III, J. Phys. Chem. 94 (1990) 8897. [22] F. Eder, M. Stockenhuber, J.A. Lercher, J. Phys. Chem. B 101 (1997) 5414. [23] F. Eder, J.A. Lercher, J. Phys. Chem. B 101 (1997) 1273.