Effect of Ion Exchange and Dehydration Temperature on the

Jul 10, 2004 - 4 Engineering Drive 4, Singapore 117576 ... capacities of the three gases in the Na form of ETS-4 were comparable, and Sr exchange did ...
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Ind. Eng. Chem. Res. 2004, 43, 5281-5290

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Effect of Ion Exchange and Dehydration Temperature on the Adsorption and Diffusion of Gases in ETS-4 R. P. Marathe, K. Mantri, M. P. Srinivasan, and S. Farooq* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576

Samples of small-pore Engelhard titanium silicate ETS-4 and its Sr-exchanged variant, patented by Engelhard Corporation, were successfully synthesized in our laboratory, and the characteristic signatures of their crystal structures were confirmed by X-ray diffraction (XRD). Thermogravimetric analysis (TGA) was carried out to study their thermal behavior and stability, and crystal morphology was examined using a scanning electron microscope (SEM). The synthesized fine crystals were pelletized by pressure binding without the addition of any binder material. Equilibrium and uptake measurements for oxygen, nitrogen, and methane on Na- and Sr-exchanged ETS-4 were then carried out in a constant-volume apparatus. The equilibrium capacities of the three gases in the Na form of ETS-4 were comparable, and Sr exchange did not seem to significantly affect these capacities. However, Sr exchange appreciably increased the uptake rate of nitrogen and reduced that of methane, thereby increasing the kinetic selectivity to a promising level. Capacity and uptake measurements were also conducted on the Srexchanged ETS-4 dehydrated at different temperatures. It was observed that increasing dehydration temperature progressively lowered the capacities and also slowed the kinetics of adsorption. Probable reasons for the observed changes in equilibrium capacity and uptake are discussed in detail. 1. Introduction Microporous and nanoporous materials are extensively used in the chemical process industries. Their major applications are in separations by adsorption and as catalysts, nanostructured hosts, and chemical sensors.1 Adsorption separation processes can be classified into two major groups according to the mechanism of separation, namely, equilibrium-controlled and kinetically controlled. Equilibrium controlled separation processes depend on the equilibrium selectivities of the adsorbents for the components in the feed mixture. Kinetic selectivity stems from difference in uptake rates of the components in the feed. The adsorbate molecules are nearly the size of the adsorbent pores in the case of kinetic separations. A majority of the known commercial adsorption processes are equilibrium-controlled and use zeolites, activated carbon, silica gel, and activated alumina. The kinetically controlled separation processes are more recent and have resulted from the development of the carbon molecular sieve (CMS) adsorbent. Nitrogen production from air by pressure swing adsorption (PSA) on CMS is the most commonly cited industrial adsorption process that exploits kinetic selectivity. CMS and small-pore zeolites are known to offer kinetic selectivity to gas mixtures such as oxygen/argon, methane/nitrogen, and methane/carbon dioxide. Separation by steric exclusion can be viewed as the extreme limit of kinetic selectivity. Steric exclusion requires uniform pore size and, therefore, is only possible in zeolites. One such example is the separation of nparaffins from their branched isomers in 5A zeolite. CMS adsorbents have a pore size distribution in the range of 3-5 Å with a mean of about 4 Å. A modest * To whom correspondence should be addressed. Tel.: (65)6874-6545. Fax: (65)6779-1936. E-mail: [email protected].

increase in kinetic selectivity is possible by manipulating the pore size distribution in the activation step.2 Pore size adjustment in zeolites by ion exchange is a well-established technique.3 Attempts have also been made to tailor zeolite pores by chemical vapor deposition.4 However, the pore size changes obtained by these methods are in the range of 1 Å, which is not sufficient to realize a desirable high kinetic selectivity for separating mixtures of small molecules. The search for new materials for adsorption and catalysis has led to titanium silicate molecular sieves of uniform pore size, obtained by replacing aluminum with titanium in the crystalline aluminosilicate structure of conventional zeolites. Chapman and Roe5 reported one of the earliest studies in titanium silicates. Promising examples in this new family of molecular sieves are ETS-4, ETS-10 and ETS-14, all of which have been developed and patented by Engelhard Corporation. ETS stands for Engelhard titanium silicate (aNa2O/ bTiO2/ySiO2/zH2O). Some potential uses of ETS-10 and ETS-14 are metal uptake from aqueous waste streams,6 VOC removal from humid air streams,7 and milder oxidation that is more substrate selective.8 Small-pore ETS-4 is structurally very interesting and is related to mineral zorite, although not its synthetic counterpart.9 Although the Na form of this material shows a low thermal stability, it has been suggested that ion exchanging Na+ with bivalent ions such as Sr2+, Ba2+, Ca2+, Mg2+, etc., yields improved thermal stability.10 In addition, the structure of these ion-exchanged forms of ETS-4 can be contracted in a controlled manner that results in finer tuning of the pore size than is possible in zeolites. The structural contraction results from progressively increasing the dehydration temperature.11 The net effect of this property is that the pore size of ion-exchanged ETS-4 can be adjusted to within 0.1 Å.

10.1021/ie049818+ CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

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As such, ETS-4 has the potential to further improve the kinetic selectivity of separations mentioned earlier, as well as to open opportunities for new and exciting applications. The patent granted to Engelhard Corporation12 is a vital source of information on the method of preparation of this crystalline molecular sieve. In the patent, Kuznicki12 reported the synthesis of ETS-4 with TiCl3 as the titanium source and an alkali metal hydroxide such as sodium hydroxide or sodium silicate solution as the source of sodium oxide, Na2O. Other alkali metal hydroxides such as potassium can also be used. The pore size was reported to be 3-4 Å, with a Si/Ti mole ratio in the product of 0.1-1. The pH of the reaction mixture must be on the alkaline side, as even mildly acidic reaction media failed to yield pure, crystalline titanium silicates. There are other reports on the preparation of ETS-4 discussing the effects of synthesis conditions on the product purity and its physical characterization.13-15 Pavel and co-workers15 reported that the crystallization field of ETS-4 is large and highlighted the importance of strict control over reaction conditions, e.g., the pH and the number of moles of Na2O and TiO2 in the initial gel, to get the desired product. Depending on the Na/Ti ratio in the starting gel, pure ETS-10, pure ETS-4, or a mixture of the two was obtained. The optimum Na/Ti ratio was about 3 for obtaining pure ETS-4, and the optimum pH for the formation of ETS-4 was between 12 and 12.8. If the pH fell below this range, ETS-10 was formed as an impurity. At a low Na2O content, the transition from ETS-10 to ETS-4 was gradual, and there was a region where the two phases coexisted. Higher alkalinity favored the formation of ETS-4. The size of ETS-4 crystals increased monotonically with time, whereas the crystal growth rate went through a maximum. Du et al.16 synthesized the synthetic analogue of zorite and found that, as the Si/Ti ratio was lowered, i.e., as the titanium content was increased, the rates of nucleation and crystal growth increased. At Si/Ti < 2, the crystallinity was low, and impurities were obtained, whereas at Si/Ti > 14.28, ETS-4 was produced with other unidentified phases. As the Si/Ti ratio was increased, the product yield dropped significantly. However, at high values of Si/Ti, large crystallites could be obtained. Kim and co-workers17 studied the effect of temperature on the crystallization of ETS-4 by carrying out the reaction for 24 h at three different temperatures and found that the crystallization curves at the three temperatures exhibited sigmoidal shapes with distinct nucleation and crystal growth stages. Their results also indicated that sodium acted as a structure-directing agent for the formation of ETS-4. In other words, high alkalinity led to a denser phase. Nastro et al.18 synthesized a highly crystalline ETS-4 product using TiCl4 as the titanium source. They mentioned that the addition of KF modified the prepolymerization of the gel and enhanced the solubility of the titanium silicate. The substitution of KF with KCl gave a reduced product yield, whereas replacing NaOH with KOH gave an amorphous product. Therefore, they called ETS-4 sodium molecular sieve because potassium presented a hindrance to its synthesis. Naderi and Anderson19 investigated the thermal stability of ETS-4 and its phase transformation into narsarsukite. They visualized the structure of ETS-4 as sheets containing titania chains separated by TiSi4O11

pillars. Calcination above 200 °C resulted in a partial loss of structure followed by a complete loss of crystallinity above 500 °C, when the material became completely amorphous according to the XRD results. Mintova et al.20 studied the effect of lithium, calcium, and potassium instead of sodium in the initial gel and concluded that all of these cations have a strong influence on the reaction kinetics and also on the crystal morphology. Mihailova et al.21 investigated the vibrational spectra of ETS-4 and ETS-10. They used Raman and infrared absorption spectra and X-ray diffraction data to specify the structural geometry of ETS-4. They described the structure of ETS-4 as comprising two types of atoms: silicon-oxygen, whose atoms are arranged in strips of eight-membered rings of tetrahedra, and titanium-oxygen, whose atoms are arranged as pyramids of chains of octahedra. Most of the aforementioned studies focused on the synthesis of ETS-4 and the influence of starting ingredients and synthesis conditions on the kinetics of the crystallization process. There are several other reports where the X-ray diffraction patterns of the synthesized materials were studied.17,22 The patents mentioned in the preceding paragraphs, a recent review article by Tsapatsis,1 and an article by Mitariten23 highlight the potential of ETS-4 and its ion-exchanged variants as adsorbents for many separation applications. Mitariten23 has also reported the successful operation of a PSA process for upgrading natural gas by separating nitrogen from methane on a pore-size-adjusted titanium silicate molecular sieve. However, available information on the equilibrium and kinetics of gas adsorption in this new material is surprisingly limited. Only a few isotherms of oxygen, nitrogen, methane, and ethane have been reported by Kuznicki et al.11 and Tsapatsis.1 It is therefore clear that very little is known in the open literature about the equilibrium and kinetics of gas adsorption in either the sodium form of ETS-4 (Na-ETS4) or any of its ion-exchanged variants. These fundamental investigations are central to the understanding of the mechanism behind each separation process. It should also be mentioned that ETS adsorbents are neither sold commercially nor are they available for academic exploration. A study has been undertaken in this laboratory first to synthesize the sodium form of ETS-4 (Na-ETS-4) and its ion-exchanged variants and then to investigate systematically the unary and binary adsorption and diffusion of gases in the synthesized adsorbents. In this article, we report the synthesis of Na-ETS-4 and strontium-exchanged ETS-4 (Sr-ETS-4), as well as the pure-component equilibrium and kinetics of oxygen, nitrogen, and methane adsorption in the synthesized samples. The impact of the dehydration temperature on the equilibrium and uptakes of these gases in Sr-ETS-4 is explored. These results provide insight into the mechanism of methane/nitrogen separation in Sr-ETS-4 and also provide a database for developing suitable equilibrium and kinetic models for process calculations. 2. Experimental Section A. Synthesis of ETS-4. In the present study, ETS-4 was synthesized using TiCl3 (30% solution in HCl, Acros Organics, Morris Plains, NJ) as the titanium source and sodium silicate solution (28.6% SiO2, 8.82% Na2O, 62.58% H2O, Merck) as the silicon source. NaOH pellets (Merck) were added to a solution of sodium silicate in

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deionized water and stirred. Then, TiCl3 solution was added dropwise, and the blackish gel was stirred vigorously for 30 min. Following this, KF‚2H2O (Nacalai Tesque Inc., Kyoto, Japan) was added to the black gel, and the mixture was stirred for 1 h until a homogeneous gel was obtained. The Si/Ti ratio was 5.7:1. The reaction was carried out in a Teflon-lined stainless steel autoclave in a Carbolite tube furnace (CTF 12/100/900). The crystallization was done at 150 °C for 7, 14, and 21 days. The product was a white paste with a small amount of supernatant clear liquid of pH 11.4, as measured with a pH meter (Mettler Toledo 320). The paste was washed several times with deionized water and dried in a vacuum oven at 100 °C for 24 h. Exchange with Strontium. To validate and investigate the higher thermal stability and other reported merits of the ion-exchanged variants of ETS-4, the NaETS-4 was exchanged with Sr in the following manner. Ten grams of the Na-ETS-4 was mixed with 300 mL of a 0.5 M solution of SrCl2‚6H2O and stirred for 1 h at 80 °C. The solution was then allowed to settle, and this clear liquid was decanted from the top. These steps were repeated 5 times to ensure a high degree of exchange. Energy dispersive X-ray (EDX) analysis confirmed that >97% exchange of Na+ with Sr2+ was achieved. The ionexchanged product was dried overnight at 100 °C and then calcined at three different temperatures of 190, 270, and 310 °C. B. Characterization. The particle morphologies of the Na-ETS-4 and Sr-ETS-4 samples were studied with a SEM (JEOL scanning electron microscope, model 5600 LV). The crystalline phase identification of the products was done by X-ray diffraction (XRD) (Shimadzu X-ray diffractometer, model Lab-X 6000) using Cu KR radiation in the 5-50° 2θ range. Thermogravimetric analysis (TGA) (TA Instruments, TGA 2050 thermogravimetric analyzer) was done from room temperature to 600 °C. Fourier transform infrared (FTIR) (Bio-Rad FTS 3500 ARX FTIR mainframe) analysis was carried out on the outgoing TGA stream. C. Adsorption Studies. Adsorption equilibrium and uptake measurements were carried out for oxygen, nitrogen, and methane over Na-ETS-4 regenerated at 175 °C and Sr-ETS-4 regenerated at 190, 270, and 310 °C. The Sr-ETS-4 samples are hereafter denoted Sr190, Sr270, and Sr310, respectively. The measurements were made at 263.15, 273.15, 283.15, and 303.15 K. Sample Preparation. The dried ETS-4 samples were pelletized in a pellet die set that was 13 mm in diameter (Specac, model 3000) using a hydraulic press (Carver, model 3912) under a load of 6 t for 10 min. SrETS-4 was also pelletized by the same procedure. The pellets were cut into smaller pieces with mean dimensions (H × W × D) of 3.55 mm × 3.05 mm × 2.87 mm for adsorption and diffusion studies. For more effective regeneration of the samples, the following procedure was employed. The dried adsorbent samples were heated in situ to the desired preset temperature (175 °C for Na-ETS-4 and 190, 270, and 310 °C for Sr-ETS4) using a portable furnace equipped with an accurate temperature controller. During heating to the desired temperature, vacuum was pulled for 2 h using a portable vacuum pump (Vacuubrand, diaphragm vacuum pump, model MD 4C). Particular care was taken to flush the system with helium for 10 min to increase the effectiveness of regeneration. This was followed by pulling vacuum again for another 2 h. This sequence of pulling

vacuum (for 2 h) followed by flushing with helium (for 10 min) was repeated 6 times to ensure complete regeneration of the adsorbent. Constant-Volume Method. The isotherm and uptake measurements were done by the constant-volume method. The constant-volume apparatus is shown in Figure 1. The apparatus basically consisted of two cylindrical chambers, called test and dose chambers, connected by an on/off solenoid valve (Asco/Joucomatic, model 71235S) controlled with a dc power supply (Topward Electric Instruments, model TPS-4000). The use of a solenoid valve helped minimize the dynamics associated with manual opening, which could affect the early part of the uptake curve. The early part of the uptake data is very crucial for distinguishing between possible transport mechanisms in the micropores. The adsorbent was placed in the test chamber. A pressure transducer (Endress+Hauser, model PMP131-A2201R4S, range of 0-300 psig) was connected to the dose side to measure the initial and final system pressures, which were needed to carry out the mass balance and calculate the equilibrium adsorbed amount. In addition to this absolute pressure transducer, two differential pressure transmitters (Validyne, model P55D-1-N-2-38-S-4-A, range of 0-8 psig), DPT1 and DPT2, were installed on the dose and the test sides and their reference ports were connected to a common cylinder. The differential pressure transmitters were necessary to track the change in pressure on the two sides of the solenoid valve until the system reached new equilibrium following the introduction of a known pressure step. The equilibrium and uptake measurements were carried out in the same experimental run with the difference being the step size. Samples of about 10-12 g of Na-ETS-4 and Sr-ETS-4 were used in the adsorption experiments. The weights were taken after regeneration. The densities of the samples based on external volume (i.e., not including the helium pore volume) were 1.32 g/cm3 for Na-ETS-4 and 1.53 g/cm3 for Sr190, Sr270, and Sr310. D. Experimental Procedure. The Na-ETS-4 adsorbent was regenerated at 175 °C and the three Sr-ETS-4 samples at 190, 270, and 310 °C under vacuum. After regeneration of a particular sample on the test side, the valve connecting the cylinders to the vacuum pump was closed, and the system was brought to the desired experimental temperature using the constant-temperature bath. The system pressure and temperature were noted, and the solenoid valve was closed to separate the test side from the dose side. A known amount of adsorbate gas was then added to the dose side through the connecting valve, and some time was allowed for the gas to attain the system temperature. Let this new pressure of the dose side be denoted by P0+ d (j) when the temperature stabilized to Ts. The solenoid valve was turned on and left in that position to allow the pressures on the dose and test sides to be equalized and the system to reach new equilibrium. For equilibrium measurements, the step size was limited by the desired frequency of data points in the experimental pressure range of the isotherm. Hence, the step size was limited to ∼0.5 bar when the target was only to obtain an equilibrium data point. As previously mentioned, a single experimental run gave isotherm as well as kinetic data. The uptakes reported in this paper are differential uptakes of the adsorbate gases in the

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Figure 1. Schematic diagram of constant-volume setup.

linear range of the isotherm (low loading). The uptakes were measured with a very small step size (0.05-0.1 bar) and the pressure transients on the two sides were recorded through a data acquisition system. The temperature rise observed on the test side for differential uptake measurements was always within 1 °C, and its impact on the kinetics was insignificant. E. Processing of Equilibrium Data. Assuming that the ideal gas law was valid in the pressure range for the gases in question, the following mass balance is applicable for the jth equilibrium step

Vd ∞ {P0+ ) d (j) - P (j)} RgTs

Vu - Va + ∆n(j) (1) RgTs

{P∞(j) - P∞(j - 1)}

for j ) 1, 2, 3, .... In eqs 1 and 2, Vd, Vu, and Va are the

∆n(j) ) n(j) - n(j - 1)

(2)

volumes of the dose side, test side, and adsorbent

particles, respectively. ∆n(j) is the number of moles adsorbed by the adsorbent particles as a result of pressure perturbation in step j. n(j) is the total number of moles adsorbed up to the jth step and is in equilibrium with the adsorbate at pressure P∞d (j) [) P∞u (j)]. The ETS-4 adsorbent particles used in this study were prepared by pressure binding of the synthesized crystals. Hence, the adsorbent samples had a bidispersed pore structure: intercrystalline macropores and crystal micropores on the order of a few angstroms. At equilibrium, the adsorbate gas in the macropores is the same as that in the bulk phase. The equilibrium adsorbed concentration per unit microparticle volume, qc, is related to the adsorbed concentration per unit macroparticle volume, qp, by the following equation

qp ) pc + (1 - p)qc

(3)

In the above equation, p is the macropore voidage, and c is the equilibrium gas-phase concentration. In this study, a macropore voidage of 0.4 was assumed for all the adsorbent samples. In terms of the variables used

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Figure 3. Yield and crystallinity of Na-ETS-4.

Figure 2. (a) SEM image of Na-ETS-4. (b) XRD pattern of NaETS-4.

to describe equilibrium data processing, qp ) n(j)/Va and c ) P∞(j)/RgTs. All isotherms measured in this study are presented as qc vs c plots. F. Processing of Kinetic Data. The segment of the isotherm covered in a differential step change is taken to be linear, and the kinetic parameter is assumed to remain constant over the small segment. The fractional uptake plots were obtained by combining the pressure responses from the two sides according to the following equation

mt ) m∞

(Vu - Va) Vd

∞ [P0+ d (j) - Pdt(j)] - [Put(j) - P (j - 1)]

(Vu - Va) Vd (4)

∞ ∞ ∞ [P0+ d (j) - P (j)] - [P (j) - P (j - 1)]

3. Results and Discussion A. Product Characterization. Scanning Electron Microscopy (SEM). The SEM image presented in Figure 2a shows the crystals of Na-ETS-4 as clusters of plates/needles. This is in good agreement with the structure reported in the literature.24,5,17 Valtchev and co-workers24 described the morphology as sheaf-like spheroidal aggregates. Chapman and Roe5 reported their product as highly intergrown and intertwined aggregates. That ETS-4 has a dense and globular structure was also reported by Kim et al.17 In the present study, the agglomerates grew, with time, to about 10-15 µm, and the individual platelike crystals had approximate dimensions of 5 µm × 5 µm × 1 µm after 14 days of crystallization. It was difficult to obtain the exact size of individual crystal in the aggregate, but it is clear that they were elongated and layered in structure. X-ray Diffraction (XRD). The XRD pattern of the product ETS-4 is shown in Figure 2b. It matches well with published reports13,9,22 and shows distinct peaks at 2θ ) 7.5°, 12.7°, and 30°, which constitute the characteristic signatures of ETS-4 framework. Figure 3 shows the yield and crystallinity of ETS-4 as functions of the crystallization time. As can be seen in this figure, the yield of ETS-4 was about 14-15% at the end of 14 days of reaction. Multiple synthesis runs were carried out to check and confirm reproducibility. The product crystallinity was evaluated as the ratio of the sum of the areas of the strongest two XRD peaks of the sample

Figure 4. SEM image of Sr190.

under consideration to that of the most crystalline sample obtained during the study separated from the amorphous phase by sonification.13,15 The crystallinity increased steadily with reaction time and reached a value of 97% after 14 days. From 14 to 21 days of crystallization, the product crystallinity remained fairly constant, but the yield decreased marginally. Therefore, the product obtained after 14 days of crystallization was chosen for equilibrium and kinetic measurements. The Sr-ETS-4 samples, Sr190, Sr270, and Sr310 (numbers indicating dehydration temperatures), were characterized by SEM and XRD. Figure 4 shows the SEM image of Sr190. Upon strontium exchange, there is no discernible change in the external morphology of the crystal agglomerates. The crystals are seen as clusters of plates resembling those of Na-ETS-4 (Figure 2a). The XRD patterns of Sr-ETS-4 are represented in Figure 5. The XRD pattern shown in Figure 5a for Sr190 closely resembles that reported in the literature.25 As can be noted in Figure 5a, Sr-ETS-4 shows clear and distinct peaks at 2θ ) 7.6°, 12.7°, and 30°. These peaks are nearly identical to the corresponding ones in case of Na-ETS-4 shown in Figure 2b. For Sr-ETS-4, the peaks at 2θ ) 24.7°, 28.9°, and 34.6° appear to have intensified, the peak at 16.8° has disappeared, and a new peak was detected at 2θ ) 39.2°. In ETS-4, as in zeolites, the cations such as Na+ or Sr2+ are chargebalancing ions and are not a part of the framework. They are extraframework cations loosely attached at the cation-deficient sites. Therefore, two Na+ ions were replaced by one Sr2+ upon strontium exchange, with no alteration of the framework structure. Consequently, the characteristic signatures at 2θ ) 7.6°, 12.7°, and 30° remain unchanged in position before and after exchange. Strontium, having a single oxidation state of +2, can form only one type of oxide, SrO2, during calcination. Its presence should be reflected as a new peak in the XRD pattern. The new peak at 2θ ) 39.2° might have arisen from strontium oxide, SrO2, formed upon calcination of the exchanged sample. The disappearance of the peak at 2θ ) 16.8° might be attributed to the removal of an oxide of sodium by strontium exchange. For Sr270, all peaks, notably those at 2θ ) 12.86°, 29.22°, and 30.0° showed greatly reduced intensity. Further reduction in peak intensity was seen in

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Figure 5. XRD patterns of Sr-ETS-4: (a) Sr190, (b) Sr-270, (c) Sr-310.

case of Sr310. In Sr310, the peaks with diminished intensity were at 2θ ) 12.82° and 30.0°. Thus, higher regeneration temperatures resulted in a reduction in peak intensities, but the positions of the peaks remained unchanged. B. Heat Treatment and Thermal Stability. Xu et al.22 investigated the energetics of ETS-4, and by comparing the heats of formation from oxides and the heats of formation from elements of the two phases, they studied the thermodynamic stability of ETS-4 at 298 K and 1 atm. They compared the stability of ETS-4 with that of ETS-10 and reported that ETS-4 was thermodynamically more stable than ETS-10 at room temperature. This behavior can largely be attributed to the higher degree of hydration of ETS-4. The hydration of extraframework cations is an exothermic process, and thus, it might serve as the driving force for stabilizing hydrated ETS-4 at low temperatures. Thermogravimetric analysis (TGA) results for NaETS and Sr-ETS-4 are reported as percent water loss versus temperature in Figure 6. It is clear that NaETS-4 is thermally less stable than Sr-ETS-4. It loses its loosely bound moisture at about 100 °C. Beyond 100 °C, the entropy of dehydration becomes strongly positive.22 The higher the water content, the greater the change in the free energy of formation with temperature. Therefore, the more hydrated framework of NaETS-4 exhibits low stability and begins to amorphize at a relatively low temperature of 200 °C. This is mainly because Na-ETS-4 contains mostly strongly bound water, removal of which allows the structure to begin to collapse. The strontium-exchanged form of ETS-4 is thermally more stable. Sr-ETS-4 loses the loosely bound and surface water up to 100 °C, and structural water bound to cations at temperatures above 300 °C. It has been reported10 that, up to 250 °C, the framework contraction is reversible, and the crystals regain the original crystalline structure by readsorbing atmospheric water. The framework is contracted irreversibly

Figure 6. (a) Water-loss curves of Na-ETS-4 and Sr-ETS-4. (b) TGA and crystallinity plots of Na-ETS-4. (c) TGA and crystallinity plots of Sr-ETS-4.

upon treatment at higher temperatures of about 330 °C. This is accompanied by a large decrease in crystallinity. The material becomes almost completely amorphous when heated beyond 400 °C. From the XRD plots of Sr190, Sr270, and Sr310 (Figure 5), it is evident that, with higher regeneration temperatures, there is a systematic reduction in the peak intensity. Together with the observation that the position of the peaks is virtually unchanged, this points to the shrinkage of the crystal lattice dimensions. This suggests that shrinkage can be seen as a consequence of the loss of strongly bound moisture from the crystal structure. TGA results expressed as percent water loss versus temperature in Figure 6a show a drop in the moisture content of the samples with heating. The reduced rate of water loss in Sr-ETS-4 in the 200-350 °C range is very clear, which explains its improved thermal stability over NaETS-4. Water loss (%) and the corresponding change in crystallinity with temperature are compared for NaETS-4 and Sr-ETS-4 in parts b and c, respectively, of Figure 6. The profile for Na-ETS-4 is in good agreement with that reported by Pavel et al.15 Their results showed the crystallinity of Na-ETS-4 dropping sharply after about 175 °C, reaching a value of about 17% at 300 °C. In the present study, the shape of the water-loss profile seems to match very well with the change in crystallinity profile in both the cases. The nearly identical trends of the two plots for both samples are a strong indication that the structural shrinkage in ETS-4 is related to water loss. As already mentioned, the sample crystallinity is estimated as the ratio of the sum of the two strongest peaks of the sample to that of the most

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Figure 7. Equilibrium isotherms of oxygen on ETS-4: (a) Na-ETS-4, (b) Sr190, (c) Sr270, (d) Sr310.

Figure 8. Equilibrium isotherms of nitrogen on ETS-4: (a) Na-ETS-4, (b) Sr190, (c) Sr270, (d) Sr310.

crystalline sample in the present study. Thus, a drop in crystallinity provides a quantitative measure of the extent of structural shrinkage. Moreover, TGA-FTIR analysis of the outgoing TGA stream revealed the presence of only the -OH group in the effluent (water loss), further confirming the conclusion that higher regeneration temperature causes only moisture loss from the structure. The fall in crystallinity indicates that it is the strongly bound structural water that is lost, causing the structure to partly collapse. Thus, it is eminently important and logical to investigate the effect of this water loss and shrinkage on the adsorption and transport properties of various gases in ETS-4 samples. Accordingly, equilibrium and uptake measurements of oxygen, nitrogen, and methane on Na-ETS4,

Sr190, Sr270, and Sr310 were obtained at different temperatures in the pressure range of 0-10 bar. C. Equilibrium Measurements. Figures 7-9 show representative adsorption isotherms for oxygen, nitrogen, and methane on Na-ETS-4, Sr190, Sr270, and Sr310, in the temperatures range 263.15-303.15 K. As can be seen in Figures 7a, 8a, and 9a, oxygen and nitrogen have comparable capacities on all of the adsorbents whereas methane has a lower isotherm on Na-ETS-4. No reasonable equilibrium selectivity appears to exist between either oxygen/nitrogen or the much desired nitrogen/methane. The figures also show the effect of strontium exchange on the gas capacities of the adsorbent upon exchanging Na-ETS-4 with strontium. Only a marginal increase in capacity is seen

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Figure 9. Equilibrium isotherms of methane on ETS-4: (a) Na-ETS-4, (b) Sr190, (c) Sr270, (d) Sr310.

Figure 10. Experimental uptake curves of oxygen, nitrogen, and methane on ETS-4 samples at low coverage at 273.15 K: (a) Na-ETS-4, (b) Sr190, (c) Sr270, (d) Sr310.

for oxygen and nitrogen. However, Sr190 (Sr-ETS-4 sample regenerated at 190 °C) shows a considerably higher capacity for methane as compared to the assynthesized Na form (parts a and b of Figure 9). This increase in capacity might be due to cation relocation in the ETS-4 framework making available more active sites than in Na-ETS-4. In Figures 7-9, we also observe the effect of regeneration temperature on the capacity of Sr-ETS-4 for the three adsorbate gases. It is clear from the figures that, in the experimental pressure range, the isotherms exhibit a drop with increasing regeneration temperature. The capacity for oxygen and nitrogen shows a progressive decrease from Sr190 to Sr270 to Sr310, as seen in Figures 7b-d and 8b-d. This

can be attributed to the shrinkage of the structure owing to water loss on heating. Methane isotherms shown in Figure 9b-d also show a drop from Sr190 to Sr270 and then to Sr310. However, the drop in capacity is higher than in the case of oxygen and nitrogen. In fact, the methane isotherms become lower than those for oxygen and nitrogen on Sr270 and Sr310. The effect of pore shrinkage on pore potential for adsorption also depends on the size of the adsorbate molecule. A methane molecule, being somewhat larger in size than the other two, appears to experience a stronger repulsive force in the shrunken pores. The effect of adsorbate size on the uptake rates of the diffusing gases, however, is even more dramatic, as discussed in the next section.

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Whereas the isotherms of the three gases exhibit the expected temperature dependence for adsorption on NaETS-4 (dehydrated at 175 °C) and Sr190 samples, the isotherms appear to become practically temperature independent in the case of Sr310. These results seem to suggest that, as the pores continue to shrink, the potential profile along the pore diameter gives a sharp minimum close to zero. D. Uptake Measurements. Uptake measurements were also carried out on the four adsorbent samples over the same temperature range as in the equilibrium measurements. Figure 10 shows representative uptake results for oxygen, nitrogen, and methane on Na-ETS4, Sr190, Sr270, and Sr310. The uptake results shown were measured in the linear range where the surface coverage was low (θ f 0). Surface coverage is defined as θ ) qc/qs, where qc and qs are the adsorbate concentration in the solid phase and the saturation capacity for the adsorbate, respectively. From Figure 10, it can be seen that, in all four adsorbents, the order of the uptake rates is oxygen > nitrogen > methane. Figure 10a shows, that in Na-ETS4, nitrogen and methane have comparable rates of uptake whereas oxygen diffusion is somewhat faster. The difference is, however, not large enough for their mixture to be separated by a kinetic process. As seen in Figure 10b, in Sr190, the uptake rates follow the same order as in Na-ETS-4, but their relative rates are significantly affected. Upon exchange with strontium, the uptakes of both oxygen and nitrogen become significantly faster, whereas the uptake of CH4 remains considerably slower. One possible explanation might be that replacing two Na+ ions by one Sr2+ ion widens the pore mouth, thereby providing less hindrance to the relatively smaller oxygen and nitrogen molecules. Dehydration at a higher temperature also has an influence on the diffusion of adsorbate molecules, as is evident from Figure 10c and d. Sr270 also exhibits the same trend as in Sr190, but all adsorbates diffuse more slowly than in Sr190. Nitrogen uptake is affected to a larger extent than that of oxygen. Further, dehydration at 310 °C shows an even slower kinetics of all three adsorbates. The reduction in the uptake rate is clear evidence of a pore size reduction due to the structural shrinkage resulting from water loss. In a recent study, Jepps et al.26 showed that micropore diffusivity has a strong inverse relationship with the kinetic diameter of the diffusing molecule. Given that the Lennard-Jones kinetic diameter of methane is 3.82 Å compared to 3.68 Å for nitrogen and 3.43 Å for oxygen,27 the uptake rates are expected to fall in the order oxygen > nitrogen > methane. This is consistent with the observations in the present study, as shown in Figure 10. 4. Conclusions ETS-4 has been synthesized in the sodium form and then exchanged with strontium. Good control over the crystallization temperature and initial gel pH are vital in obtaining a product with high crystallinity. The physical characterization of the product by SEM, XRD, and TGA provides insight into the morphology and structural stability of ETS-4. Adsorption equilibrium and kinetic studies show that strontium exchange significantly affects the uptake kinetics of the gases studied, whereas its effect on the equilibrium capacity is not as strong. Given that the capacities for the three gases in Na-ETS-4 are not significantly different, the

equilibrium selectivity (ratio of Henry’s constants) between oxygen/nitrogen and nitrogen/methane cannot be exploited. The capacity is not significantly altered for oxygen and nitrogen upon strontium exchange (Sr190), but that for methane is higher than in Na-ETS4. However, the difference is not sufficient for useful equilibrium-based separation of nitrogen/methane mixtures. The same applies for the equilibrium selectivity of the gases on Sr-ETS-4 dehydrated at higher temperatures (Sr270 and Sr310). The difference in uptake rates between nitrogen and methane, on the other hand, is markedly improved for nitrogen/methane separation in Sr190 compared to Na-ETS-4. At higher dehydration temperatures (i.e., in Sr270 and Sr310), the difference in the uptake rates of these two gases gradually drops, but at the same time, the equilibrium selectivity begins to favor nitrogen. Hence, it is not be clear, in the absence of further systematic modeling, whether the kinetic selectivity (ratio of Henry’s constants multiplied by the square root of the ratio of the diffusivities) is higher in Sr190 or Sr270. Drops in equilibrium isotherms and uptake rates are both consequences of shrinking pore size. Thus, among the four adsorbents, Sr190 and Sr270 represent promising potential candidates for the much desired separation of N2/CH4 mixtures in relation to natural gas upgrading. This is particularly important because the current technology, cryogenic distillation, is highly cost-intensive. The potential cost savings provide further incentive for research and development of ETS-4 to make a breakthrough in this vital application. Acknowledgment R.P.M. gratefully acknowledges the Research Scholarship awarded by NUS. Notation c ) gas-phase concentration cp ) gas concentration in the macropores j ) jth step of equilibrium measurement K ) Henry’s constant mt ) mass of adsorbate adsorbed by adsorbent up to time t m∞ ) mass of adsorbate adsorbed by adsorbent at equilibrium n ) total number of moles of adsorbate adsorbed by adsorbent nv ) number of gas moles flowing through the valve n ) number of moles adsorbed by adsorbent at step j P ) pressure Pd ) pressure in the dose chamber Pu ) pressure in the test (uptake) chamber P0+ d ) initial pressure in the dose chamber P∞d ) final equilibrium pressure in the dose chamber P0u ) initial pressure in the test chamber P∞u ) final equilibrium pressure in the test chamber P∞ ) final pressure in the constant volumetric system () P∞d ) P∞u ) q, qc ) adsorbed concentration based on microparticle volume qp ) adsorbed concentration based on macroparticle volume Rg ) universal gas constant t ) time Ts ) system temperature Va ) volume of adsorbent Vd ) volume of dose cell Vu ) volume of uptake cell

5290 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Greek Letters θ ) fractional coverage of the adsorption sites  ) bed voidiage p ) particle void fraction

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Received for review March 5, 2004 Revised manuscript received June 3, 2004 Accepted June 7, 2004 IE049818+