J. Phys. Chem. B 2005, 109, 3257-3261
3257
Effects of Site Occupancy, Cation Relocation, and Pore Geometry on Adsorption Kinetics in ETS-4 R. P. Marathe, S. Farooq,* and M. P. Srinivasan Department of Chemical & Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576 ReceiVed: August 27, 2004; In Final Form: December 13, 2004
Strontium-exchanged titanium silicate, Sr-ETS-4, is a new molecular sieve with promises for exciting applications in gas separation. The literature mentions poor thermal stability and low adsorption capacity as the drawbacks of the as-synthesized Na-ETS-4 and therefore the need for cation exchange. Upon ion exchange, Sr-ETS-4 shows appreciable kinetic selectivity between methane and nitrogen, which can be improved further by controlled dehydration. In this study, we trace the changes at the molecular level behind this improvement. By combining distributed information in the literature, it is shown that the governing factors are changes in pore geometry with progressive dehydration, changes in ion occupancy, and relocation of cations due to Sr exchange.
Introduction Small-pore titanium silicates, generally known as ETS-4, constitute a new class of adsorbents that have the potential for many new applications in industrial gas separation. Synthesized in the Na form, the cation-exchanged variants are more suitable for separation applications. Na-ETS-4 has been reported to exhibit low thermal stability and poor adsorption properties. Exchanging the sodium with divalent cations such as strontium or barium yields improved stability and accurate pore size control. Sr-ETS-4 shows a higher kinetic selectivity for methanenitrogen separation than does Na-ETS-4.1 Though size-selective separation of nitrogen from methane has been reported,2 the structural changes in the adsorbent leading to the size-selective separation are yet to be explained in full detail in relation to the observed adsorption kinetics. Three possible effects have been mentioned, namely, dehydration-induced framework contraction, loss of crystallinity, and change in ion occupancy, leading to relocation of Sr2+ cations in the framework.3 However, the reasons for slow uptake of gases in Na-ETS-4 and the factors leading to faster uptake in Sr-exchanged ETS-4 are not immediately clear from the above possible effects. Equilibrium isotherms and the kinetics of oxygen, nitrogen, and methane in Na-ETS-4 and three samples of Sr-ETS-4 dehydrated at various temperatures were reported in a recent paper from this laboratory.1 The effects of cation exchange and higher dehydration temperature on the uptake rates were investigated. In this paper, we assimilate scattered structural information in the literature to elucidate the effects of ion occupancy, cation relocation, and pore size shrinkage with increasing dehydration temperature on the observed adsorption kinetics of gases in Na-ETS-4 and Sr-ETS-4 measured in this laboratory. Synthesis and Characterization Na-ETS-4 was synthesized with an initial gel composition of 4.42Na2O:0.95K2O:TiO2:5.71SiO2:81.88H2O. Na-ETS-4 was * To whom correspondence should be addressed. Phone: 65-6874 6545. Fax: 65-6779 1936. E-mail:
[email protected].
exchanged with strontium and barium, and the samples were subsequently regenerated at increasing temperatures. The experimental details are described elsewhere.1 The products were characterized physically with scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The SEM images and the crystalline phase identification by XRD were in close agreement with the literature reports.4,5 The samples were pelletized by pressure binding without adding any external binder. Equilibrium isotherms and uptakes of oxygen, nitrogen, and methane over a range of experimental temperatures and pressures were measured volumetrically. The Na-ETS-4 sample was regenerated at 175 °C, while the Sr-ETS-4 samples were regenerated at 190, 270, and 310 °C (hereafter named Sr190, Sr270, and Sr310, respectively). Results and Discussion Effect of a Cation on Water Content and Stability. The percent water losses with temperature for Na-ETS-4, Sr-ETS4, and Ba-ETS-4 are shown in Figure 2. As reported earlier,1 the structural shrinkage in ETS-4 is a direct consequence of the water loss. Na-ETS-4 shows a continuous drop in water content with increasing temperatures, and the profile becomes practically flat at about 210 °C. The Sr-ETS-4 and Ba-ETS-4, on the other hand, show a reduced rate of water loss with an increase in temperature, indicating enhanced thermal stability. The crystallinity is also correspondingly higher, which means that higher thermal stability delays structural collapse in the Sr- and Ba-exchanged Na-ETS-4 compared to the Na form. Figure 3 shows the water content (%) in ion-exchanged Phillipsite6 and our ETS-4 samples as a function of the cation radius. As may be seen from the figure, in both cases, upon exchange with a monovalent or a divalent cation, the water content decreases with increasing ionic radius. The water content in ion-exchanged Phillipsite is in the order Li > Na > K > Rb > Cs. A closer observation of the figure shows that, in Phillipsite, for a comparable ionic radius, exchange with a divalent cation results in a structure with higher water content than exchange with a monovalent cation. It has been reported
10.1021/jp0461192 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/29/2005
3258 J. Phys. Chem. B, Vol. 109, No. 8, 2005
Marathe et al.
Figure 1. Schematic representation of the ETS-4 structure: (a) view in the a-b plane showing titania chains and the 12-membered ring (12MR), (b) view in the a-c plane showing 8MR along the b direction. Reprinted with permission from ref 12. Copyright 2004 Elsevier.
Figure 4. Linear range fractional uptakes at 283.15 K in Na-ETS-4 (dehydrated at 175 °C) and Sr-ETS-4 (dehydrated at 190, 270, and 310 °C): (a) oxygen, (b) nitrogen, and (c) methane.
Figure 2. Water loss profiles of Na-ETS-4, Sr-ETS-4, and Ba-ETS-4 with heat treatment at high temperature.
Figure 3. Water content in ion-exchanged zeolite and ETS-4. “di” and “mono” denote samples exchanged with divalent and monovalent cation, respectively.
that the divalent cation-exchanged Phillipsite samples exhibit lower thermal stability. However, in the case of ETS-4, the situation is reversed. Sr- and Ba-exchanged ETS-4 have lower water content (14.7% and 12%, respectively) than does Na-ETS-4, which contains 15.3% water. The thermal stability, as shown in Figure 2, is in the order Ba-ETS-4 > Sr-ETS-4 > Na-ETS-4. It may, therefore, be concluded that the energy required to remove water from the zeolite channels is a function of the ionic radius and the ionic valency. Also, in the case of
both Phillipsite and ETS-4, a more hydrated framework exhibits lower thermal stability. Adsorption Kinetics. Parts a-c of Figure 4 show the uptakes of oxygen, nitrogen, and methane, respectively, in Na-ETS-4 and Sr-ETS-4 dehydrated at progressively higher temperatures. The uptakes were measured in the linear range of the isotherm (low adsorbate concentration) at 283.15 K in a constant volumetric setup. A pressure step of about 100 mbar was used to remain within the linear range of the isotherm. From Figure 4, it can be easily seen that the uptakes are progressively slower at higher dehydration temperatures in all four adsorbents. However, the effect is more pronounced for the kinetics of nitrogen than for oxygen and methane in the Sr-ETS-4 samples. Whereas the oxygen and methane uptakes show small drops with increasing dehydration temperature, nitrogen uptake is slowed considerably in Sr270 and Sr310. Structure of Na-ETS-4. There have been attempts to elucidate the structure of ETS-4. Braunbarth7 mentioned that the framework of Na-ETS-4 is made up of SiO4 tetrahedra at the corners and TiO6 octahedra, unlike conventional zeolites where both silicon and aluminum are tetrahedrally coordinated. Also, the molecular sieving and adsorptive properties are enabled and facilitated by a channel system such as that in zeolites. Kuznicki et al.8 and Nair et al.3 have discussed the framework contraction in strontium-exchanged ETS-4. They extracted the unit cell and profile fittings from the diffraction pattern of ETS-4 using the Le Bail extraction algorithm. The peak shape was modeled by a pseudo-Voigt function. The locations of cations and water molecules were ascertained by comparing the Fourier
Adsorption Kinetics in ETS-4
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3259
TABLE 1: Atom/Ion Occupancies in ETS-4 Samplesa atom/molecule
Na-ETS-4 (175 °C)
Na1 Na2 Sr1 Sr2 Sr3 W1 W2 W3 W4 W5
0.50 0.84 0 0 0 0.50 0.49 0 0.35 0.22
a
Sr150
Sr200
Sr250
Sr300
0.1970
0.1969
0.1970
0.1969
0.2536 0.5600 0.1727
0.2314 0.5121 0.3452
0.2236 0.5167 0.3475
0.2444 0.5125 0.3469
0.3713
0.1928
0.1005
0.055
Data taken from Kuznicki et al.8 and Cruciani et al.9
TABLE 2: Contact Distances (Å) for Water Molecules in the 8MRa W1‚‚‚W2 W2‚‚‚W4 W2‚‚‚W5 a
3.037 1.839 3.008
W4‚‚‚O2 W4‚‚‚W5 W5‚‚‚O1
3.030 1.326 2.680
W5‚‚‚O2
2.594
Data taken from Cruciani et al.9
difference maps of the ion- and water-occupied structure with those of an empty ETS-4 framework model with no cations or water molecules. Rietveld refinement of the framework structure was carried out, and the ion occupancies in the framework and extraframework positions were obtained. The framework of Na-ETS-4, as shown in Figure 1, has been resolved and reported to be composed of titania (O-Ti-O) chains running along the [010] direction (which is the crystallographic b axis). The titanium in these chains is octahedrally coordinated and connected in the [001] direction (i.e., c axis) by silicate tetrahedra. The titania chains are bridged in the [100] direction (i.e., a axis) by titanosilicate bridging units containing a five-coordinated titanium atom that is bonded to the four silica tetrahedra and to an apical oxygen. However, Cruciani et al.9 reported that this titanium in the chains is a six-coordinated atom. The structure is disordered in the sense it is faulted in the [100] and [001] directions, that is, along the a and c axes. It has been described as an intergrowth or superimposition of four hypothetical pure polymorphs differing in the arrangement of the titanosilicate bridging units.3,7,8 The superimposition model, first proposed by Sandomirskii and Belov,10 holds because the faulting probabilities in the a and c directions are close to 50%. Braunbarth et al.7 report that any combination of the polymorphs along the c direction yields structures with blocked 12-ring pores. In other words, the 12-membered ring (12MR) is always blocked and is therefore inaccessible to diffusing molecules. This faulting, however, does not block the 8-ring pores (8MR) in the b direction. Thus, the channels formed by the 8MR along the b direction are the sole channels for transport of diffusing molecules. Transport Paths in Na-ETS-4. The slow kinetics of adsorption in Na-ETS-4 and the dramatic effect upon Sr exchange can be explained by studying the pore structure together with the location and distribution of cations and water molecules in the structure. Nair et al.,11 Cruciani et al.,9 and Braunbarth et al.7 have mentioned two Na+ cation sites in the Na-ETS-4 structure. These sites are named Na1 and Na2. Water molecules occupy four sites (namely, W1, W2, W4, and W5) in Na-ETS-4 and three in Sr-ETS-4 (namely, W2, W4, and W5). Oxygen atoms occupy seven sites, namely, O1, O2, O3, O4, O5, O6, and O7. Locations of Na+ cations and water molecules are shown in Figure 1. The Na1 site is in the titanosilicate double ring D6MR and is coordinated to four framework oxygens (O3 × 4) and two water molecules (W1 × 2). The other site (Na2)
Figure 5. Schematic representation of the eight-membered ring (8MR) in Na-ETS-4 showing framework oxygen atoms and water molecules. Possible hydrogen bonding is indicated with dotted lines.
is in the seven-membered ring (7MR) provided by the titania chains and the bridging titanosilicate units.9,11 It is coordinated to three oxygens of the titania chain, two oxygens of the titanosilicate unit, and two water molecules (W1, W2). Cruciani et al.9 and Nair et al.11 report about 84% and 75% Na2 site occupancy, respectively, suggesting that this site can be occupied even when the chain bridging titanium (and therefore the 7MR) is absent. The occupancy would be only 50% if it depended on the presence of titanium. There is no sodium cation in the 8MR. According to Nair at al.,11 W1 coordinates with Na1 or is hydrogen bonded to the framework oxygen O3 (O3‚‚‚HW1OW1-HW1‚‚‚O3). Nair et al.11 and Braunbarth et al.7 have reported accurate Na‚‚‚W1 contact distances based on ETS-4 single-crystal analysis. W2 also coordinates to the Na1 cation and by hydrogen bonding to the apical oxygen atom O7. The W4 and W5 water molecules are not coordinated to Na1 or Na2 in Na-ETS-4 or to any cation in Sr-ETS-4. Table 1 shows the occupancies of the sodium cations and water molecules in Na-ETS-4, as reported by Cruciani et al.9 In addition, it shows the occupancies of the sodium cations, strontium cations, and water molecules in Sr-ETS-4 dehydrated at various temperatures.8,9 It can be seen that, in Na-ETS-4, the occupancies of water molecules W1 and W2 are high (about 50%) while W4 and W5 have lower occupancies in the 8MR. Site W3 is unoccupied. However, Nair et al.11 found from single-crystal data analysis that the occupancies of W4 and W5 are lower than those reported by Cruciani et al.9 Contact distances between the water molecules in the 8MR, calculated by Cruciani et al.,9 are given in Table 2. The contact distances of W4 and W5 molecules from the oxygen atom at O2 and their contact angles suggest the possibility of hydrogen bonding9,11 such as O2‚‚‚HW4-OW4-HW4‚‚‚O2, as shown in Figure 5. The figure was drawn on the basis of the contact distances and hydrogenbonding scheme proposed by Cruciani et al.9 and Nair et al.11 Also, it was discussed earlier in relation to Figure 2 that the Na-ETS-4 structure is more hydrated than its ion-exchanged counterparts and the high propensity for hydrogen bonding between the extraframework water molecules and framework oxygen atoms makes the structure thermally less stable. From Figure 5, which illustrates the water sites in the 8 MR of NaETS-4, and from Table 2, it should be clear that some or all the four water sites (W1, W2, W4, W5) in the 8MR will be occupied at any given time. It can be inferred further from the occupancies shown in Table 1 that 8MRs containing no water molecules will be extremely scarce. The 8MR is, therefore, either fully blocked or partially blocked, and the diffusing molecules are either entirely excluded or greatly hindered. Hence, the diffusing molecules have to traverse a more tortuous
3260 J. Phys. Chem. B, Vol. 109, No. 8, 2005 path of least resistance, which in this case are the pores with less degree of blockage. Ion Occupancy and Cation Relocation in Sr-ETS-4. The framework of Sr-ETS-4 differs from that of Na-ETS-4 primarily in the distribution of cations and water molecules.11 The coordination of Na1 in Sr-ETS-4 is similar to that in Na-ETS4, though its occupancy is lower since most of the Na1 cations are exchanged with strontium to balance the charge. In general, the cation sites of Na-ETS-4 and Sr-ETS-4 are similar to one another. However, there is a difference in that Sr-ETS-4 exhibits an ordering of cations.7 There is one site (Sr1) for the Sr2+ ion in Sr-exchanged ETS-4. The other two sites (Sr2 and Sr3) are created by relocation of the Sr2+ ions after heat treatment, as discussed later in the section. The Sr2+ cations preferentially occupy the cation sites coordinated to both the chain and the chain-bridging titanium polyhedra (previously designated as Na2). In verifying the site-ordering hypothesis, Braunbarth et al.7 redistributed the cations between both sites in their structure refinement and always obtained a higher residual. Nair et al.11 have shown that the sodium cations at Na2 sites are replaced by strontium (denoted as Sr1), resulting in shorter bond distances with oxygen and water molecules owing to the larger size of the Sr2+ ions (1.32 Å) compared with the Na+ ions (0.95 Å). It has also been shown that the Sr1 site has lower occupancy compared to the other two sites. The Sr2 site is a result of the movement of the Sr2+ cations to the 6MR along the a-c plane. The third site (Sr3) is located in the 8MR on the a-c plane (Figures 1b and 6). Since the 8MR is the only channel for diffusing molecules in ETS-4, the availability of these channels plays a very important role in understanding the transport behavior in Sr-ETS-4. Upon Sr exchange and subsequent heat treatment, W1, W4, and W5 sites occupying water molecules are freed up and water occupancy in the W2 site is reduced. While water loss opens up some previously blocked 8MR channels, occupancy of some Sr3 sites blocks some other 8MR channels. It is quite obvious from Table 1 that reduced water content opens up more 8MRs in Sr-ETS-4 than new blockage due to cation relocation. The net effect is more open 8MR channels, resulting in faster diffusion of oxygen and nitrogen upon exchange with Sr. However, it is not completely clear why methane uptake is not enhanced. Pore Contraction. Figure 7 shows a schematic representation of the 8MRs in which the Sr3 site and water sites are unoccupied. These open 8MRs form the principal channels for diffusion in Sr-ETS-4. The schematic representation was constructed using the oxygen-oxygen contact distances given by Nair et al.3 and Kuznicki et al.8 As shown, the 8MR appears as a more “regular” octagon in Sr190 and the pore changes geometry owing to nonuniform contraction at higher dehydration temperatures. The O5-O5 distance (D1), O1-O1 distance (D2), and O2-O2 distance (D3) decrease nonuniformly with increasing temperature. D1 and D3 decrease nearly monotonically with temperature from 190 to 310 °C, while D2 actually increases with temperature (from 4.09 Å at 190 °C to 4.57 Å at 270 and 310 °C). Consequently, the pore contracts in two directions and elongates along the third direction. The 8MR distorts from a regular octagonal shape to a near “quadrilateral”. The dimension D3 being the smallest dictates the access or hindrance to diffusing molecules in the 8MR. With increasing contraction, the access through the pore is more and more inhibited. This results in reduced uptake rates of diffusing gases reported in Figure 4. With increasing dehydration temperature, there is some relocation of Sr2+ ions. Structural simulation results of Nair et
Marathe et al.
Figure 6. Schematic representation of the 8MR in Sr-ETS-4 blocked by a Sr2+ ion at the center in (a) Sr190, (b) Sr270, and (c) Sr310. The dimensions (Å) are taken from Nair et al.3
al.3 show that a high occupancy of the Sr1 site is not favorable in the heat-treated Sr-ETS-4, resulting in migration of Sr2+ cations from the Sr1 and Sr2 sites to the Sr3 site. The relocation of the cations changes the occupancies of the three sites as shown in Table 1. However, it is clear that, after the initial relocations between 150 and 200 °C, there is only a small change in the position of the cations and the site occupancies. In Sr190, Sr270, and Sr310, the Sr1 site has occupancy of about 23% while that of Sr2 and Sr3 is higher, and this promotes higher framework stability.9 Interpolation of the data in Table 1 reveals that the Sr3 site, located in the 8MR, has a fractional occupancy of about 0.3447 in Sr190, 0.3472 in Sr270, and 0.3467 in Sr310. Thus, about 34% of the 8MR channels have Sr2+ cations occupying the Sr3 site. Figure 6 shows the 8MR with the Sr3 site occupied by the Sr2+ cation. The contact distances between the Sr3 cation and the oxygen atoms are also shown in the figure for Sr190, Sr270, and Sr310, which suggests that the cation in the 8MR blocks access to all three diffusing molecules under consideration, namely, oxygen, nitrogen, and methane. In other words, oxygen, nitrogen, and methane molecules with kinetic diameters of 3.40, 3.68, and 3.82 Å, respectively, are sterically excluded from the Sr3-occupied 8MRs. The open 8MR channels are the only accessible conduits for diffusing molecules. Since the Sr3 site occupancy becomes practically constant after 200 °C, there is no new blockage of the 8MR channel as the dehydration is continued. On the other hand, the water content of Sr-ETS-4 decreases with increasing dehydration temperature,
Adsorption Kinetics in ETS-4
J. Phys. Chem. B, Vol. 109, No. 8, 2005 3261 continues to slow. Therefore, it may be reasonable to argue that the reduction in the uptake of oxygen, nitrogen, and methane with increasing dehydration temperature primarily results from 8MR contraction (pore size reduction). Conclusion This study makes use of available information on cation relocation and pore contraction to explain the observed difference in gas diffusion in Na-ETS-4 and its strontium ionexchanged variant Sr-ETS-4. The existence of four sites for water molecules and their relatively higher occupancy levels, and the location of the Na+ ions, render low thermal stability and unattractive gas diffusion properties to Na-ETS-4. Exchanging Na+ with Sr2+ results in a net increase in open 8MR channels, the only transport path facilitating the transport of oxygen and nitrogen. Although dehydration of Sr-ETS-4 at higher temperatures causes relocation of the Sr2+ cations in the structure and a change in occupancies of the Sr and water sites, pore contraction appears to be the dominant effect leading to reduction in uptake rates as dehydration temperature is progressively increased. References and Notes
Figure 7. Dimensions (Å) of open (accessible) 8MRs in Sr-ETS-4: (a) Sr190, (b) Sr270, and (c) Sr310. Note the distorted contraction with increasing dehydration temperature. The dimensions are taken from Nair et al.3
which is reflected in the reduced occupancies of the water sites (see Figure 2 and Table 1). The occupancy of water sites (W2) in Sr-ETS-4 is much lower than in Na-ETS-4, as may be read from Table 1. Though accurate occupancy values of all three water sites are not reported in the literature, it may definitely be concluded from the available information compiled in the table that they are considerably lower at higher dehydration temperatures. The water content and the occupancy of water sites in the 8MR decrease with increasing dehydration temperature. Consequently, the partial or total blockage of 8MR due to water molecules is less in increasingly dehydrated Sr-ETS4. Therefore, the diffusional hindrance due to water molecules is in the order Sr190 > Sr270 > Sr310. Though the steric hindrance to diffusion offered by the water molecules decreases with increasing dehydration temperature, the gas diffusion rate
(1) Marathe, R. P., Mantri, K., Srinivasan, M. P., S. Farooq. Effect of ion exchange and dehydration temperature on the adsorption and diffusion of gases in ETS-4. Ind. Eng. Chem. Res. 2004, 43 (017), 5281-5290. (2) Kuznicki, S. M.; Petrovich, I. Desai, B. T. Small-pored crystalline titanium molecular sieve zeolites and their use in gas separation processes. U.S. Patent 6,068,682, 1990. (3) Nair, S., Tsapatsis, M., Toby, B. H., Kuznicki, S. M. A study of heat-treatment induced framework contraction in strontium-ETS-4 by powder diffraction and vibrational spectroscopy. J. Am. Chem. Soc. 2001, 123, 12781-12790. (4) Chapman D. M.; Roe A. L. Synthesis, characterization and crystal chemistry of microporous titanium-silicate materials. Zeolites 1990, 10, 730-737. (5) Philippou, A.; Anderson, M. W. Structural investigation of ETS4. Zeolites 1996, 16 (2/3), 98-107. (6) Ribeiro, R. R., Rodrigues, A. E., Rollmann, L. D., Naccache, C. Zeolites: Science and Technology; Martinus Nijhoff Publishers: The Hague, The Netherlands, 1984; p 48. (7) Braunbarth, C.; Hillhouse, H. W.; Nair, S.; Tsapatsis, M.; Burton, A.; Lobo, R. F.; Jacubinas, R. M.; Kuznicki, S. M. Structure of strontium ion-exchanged ETS-4 microporous molecular sieves. Chem. Mater. 2000, 12 (7), 1857-1865. (8) Kuznicki, S. M.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R. M.; Braunbarth, C. M.; Toby, B. H.; Tsapatsis, M. A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules. Nature 2001, 412 (6848), 720-724. (9) Cruciani, G., De Luca, P., Nastro, A., Pattison, P. Rietveld refinement of the zorite Structure of ETS-4 molecular sieves. Microporous Mesoporous Mater. 1998, 21, 143-153. (10) Sandomirskii, P. A.; Belov, N. V. SoV. Phys. Crystallogr. 1979, 24, 686-693. (11) Nair, S.; Jeong, Hae-Kwon; Chandrasekaran, A.; Braunbarth, C. M.; Tsapatsis, M.; Kuznicki, S. M. Synthesis and Structure Determination of ETS-4 Single Crystals. Chem. Mater. 2001, 13 (11), 4247-4254. (12) Miraglia, P. Q., Yilmaz, B., Warzywoda, J., Bazzana S., Sacco, A., Jr. Morphological and surface analysis of titanosilicate ETS-4 synthesized hydrothermally with organic precursors. Microporous Mesoporous Mater. 2004, 69, 71-76.
