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methane on zeolite-like molecular sieves
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Microporous and Mesoporous Materials 111 (2008) 627–631 www.elsevier.com/locate/micromeso
Short Communication
Separating nitrogen/methane on zeolite-like molecular sieves Martin J. Maple *, Craig D. Williams School of Applied Sciences, University of Wolverhampton, Wulfruna Street, Wolverhampton WV1 1SB, UK Received 12 May 2007; received in revised form 11 July 2007; accepted 12 July 2007 Available online 19 July 2007
Abstract Inverse gas chromatography has been used to determine rapidly the specific retention volumes and isosteric enthalpies of adsorption of N2 and CH4 on the silicoaluminophosphates SAPO-18 and SAPO-34, and the titanosilicate ETS-4. In the SAPOs studied, specific retention volumes and enthalpies of adsorption are found to be controlled by pore structure and framework composition, respectively. The SAPOs will separate nitrogen/methane mixtures, but their selectivity favours methane over nitrogen. The inclusion of extra-framework metal cations, as found in ETS-4 and the zeolite clinoptilolite, are likely to be important in obtaining the desired (reverse) selectivity. 2007 Elsevier Inc. All rights reserved. Keywords: Silicoaluminophosphate; ETS-4; Adsorption; Inverse gas chromatography; Natural gas purification
1. Introduction The separation of nitrogen from methane is a significant technical challenge, but one which is of increasing importance for natural gas production, many sources of which are contaminated with high levels of nitrogen (>20 wt%). Lowering N2-content is required to meet pipeline specifications for non-combustibles (<5 wt%), and this is typically achieved by cryogenic distillation. The combined costs of liquefaction and subsequent re-compression of the low pressure product make this an expensive process, economic only for large, highly contaminated fields. An alternative technology is pressure swing adsorption (PSA), which can operate at near ambient temperatures and maintain a high pressure product stream [1]. The commercialisation of PSA processes for N2/CH4 separation has been slow due to the difficulty in developing satisfactory sorbents. Yet the increased potential for recovering methane from small natural gas fields, for which cryogenic separation is uneconomic, make the development of such sorbents desirable. *
Corresponding author. Tel.: +44 (0) 1902 322737; fax: +44 (0) 1902 322714. E-mail address: [email protected] (M.J. Maple). 1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.07.014
Microporous materials, such as aluminosilicate zeolites, have been examined as potential adsorbents for N2/CH4 separation. Clinoptilolite (HEU) will achieve the separation efficiently, but only if its extra-framework cations are selected judiciously [2]. The synthetic form is difficult to prepare [3], and while the natural mineral is abundant, its commercial use has been hindered by the variability in the extra-framework cation composition with source deposit [4], and the tendency of impurities present to lead to its conversion to mordenite (MOR) during service. Alternative synthetic materials, which may be readily crystallised, are therefore required if a commercially viable adsorbent is to be obtained. Titanosilicate ETS-4 is one such synthetic material. It possesses a small pore network, the size of which may be tuned (reduced) by calcination to optimise its selectivity [5]. Differences in the equilibrium or kinetics of adsorption permit the separation of gases by PSA. The precise mechanism for N2/CH4 separation in zeolites is complex, being determined in part by the material’s framework composition, pore structure, and extra-framework cation types and locations. Modifying the extra-framework cations by ion exchange, for example, may effect either or both, the equilibrium and kinetic adsorption properties of the
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M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
Fig. 1. Illustrative diagrams of the cavities in (a) SAPO-18 and (b) SAPO34. Both frameworks are constructed from double 6-rings, in the former these are related by a mixture of translational and mirror symmetry, while in the latter they are related only by translational symmetry.
adsorbent. Guest molecules may be strongly attracted to extra-framework cations, yet predicting how this will occur (whether the quadrupole of N2, or the polarisability of CH4, will dominate) is not necessarily straightforward [6]. While the case of ion exchanged clinoptilolite, and that of ETS-4 modified by thermal treatment, illustrates the prospect of being able to engineer a particular desired pore size to accomplish a specific separation, it also exemplifies the complexity of the problem of trying to bring ‘design’ to adsorbent synthesis when so many parameters affect it. In this work, we explore the use of inverse gas chromatography (IGC) to study materials for N2/CH4 separation. We have selected silicoaluminophosphate materials of type AEI (SAPO-18) and CHA (SAPO-34) because they contain only protons balancing the charge on the framework, and because they are structurally related [7]. Two samples of SAPO-34 were prepared with different framework compositions. In this way we can examine the effect of cavity shape (Fig. 1) and framework composition on the separation, in the absence of large extra-framework cations. For comparison we have also considered the titanosilicate ETS-4, which is used commercially for this separation. 2. Experimental Hydrothermal syntheses were performed in a Baskerville stainless steel autoclave stirred at 400 rpm. SAPO-18 was prepared by a modified literature method [8], from a gel of composition 1.0 Al(OH)3:0.5 H3PO4:0.2 SiO2:0.7 R:20.0 H2O. 46.4 g of hydrated aluminium hydroxide (Aldrich) was added, with stirring, to a solution containing 34.8 g of orthophosphoric acid (85%, BDH) in 194.0 g of distilled water. Six grams of fumed silica (cabo-sil) was then added, followed by 51.0 g of N,N-diisopropylethylamine, R (99%, Aldrich). The resultant gel was heated hydrothermally at 190 C for 194 h.
SAPO-34a was prepared after the method of Prakash et al. [9], from a gel of composition 1.0 Al(OOH):0.6 H3PO4:0.3 SiO2:0.8 R:5.5 H2O. 57.5 g of pseudoboehmite (Versal 900, La Roche) was added to a solution containing 64.0 g of orthophosphoric acid (85%, BDH) in 41.7 g of distilled water, with stirring. In a separate container, 17.0 g of fumed silica (cab-o-sil) was stirred into a mixture containing 62.5 g of morpholine, R (99%, Aldrich) in 41.7 g of distilled water. A gel was formed by combining this mixture with the first. Once homogeneous, the gel was aged at room temperature for 24 h, before being transferred to the autoclave and heated hydrothermally at 190 C for a further 24 h. SAPO-34b was prepared after the method of Lok et al. [10], from a gel of composition 1.0 Al(iOPr)3:1.0 H3PO4:0.1 SiO2:1.0 R:33.7 H2O. 40.9 g of aluminium isopropoxide (98%, Aldrich) was added, with stirring, to a solution containing 23.1 g of orthophosphoric acid (85%, BDH) in 52.4 g of distilled water. To this 4.5 g of colloidal silica (Ludox AS-40, Aldrich) was added, followed by a further 4.0 g of distilled water and 73.6 g of aqueous tetraethylammonium hydroxide, R (40%, Fluka). The resultant gel was heated hydrothermally for 168 h at 190 C. Na-ETS-4 was prepared by a modified literature method [11], from a gel of composition 0.2 TiCl3:1.0 SiO2:0.5 NaF:1.8 NaOH:19.6 H2O. 9.5 g of sodium fluoride (99%, Fluka) was added, with stirring, to a solution containing 46.7 g of titanium chloride (30 wt% in HCl, BDH) in 49.2 g of distilled water. In a separate container, 36.4 g sodium hydroxide was added to a mixture containing 75.1 g colloidal silica (Ludox AS-40, Aldrich) in 49.2 g distilled water. To form the gel this mixture was combined with the first. Once homogeneous, the gel was heated hydrothermally at 190 C for 24 h. The crystalline products of each reaction were recovered by vacuum filtration, washed with distilled water and dried. Silicoaluminophosphates were calcined in air at 550 C to remove the included organic template. Sr-ETS-4 was obtained by ion exchange, in three steps, on the filtered, dried product of reaction, using aqueous solutions containing 5.0 wt%, 7.2 wt%, then 8.8 wt% strontium chloride [12]. Each gram of Na-ETS-4 was refluxed for 30 min with 30 cm3 of the solution; this provided essentially complete replacement of Na by Sr. The identity and purity of all solids was confirmed by powder XRD (Philips PW1710 diffractometer, Cu Ka Xrays). Inorganic elemental analysis was determined by XRF analysis (Spectro XEPOS spectrometer). Crystallite size was obtained by laser granulometry (Malvern Instruments Mastersizer) and micropore volumes from N2 adsorption isotherm measurements at 196 C (Coulter SA3100). Gas separation experiments were performed with an Ai Cambridge GC 94 M gas chromatograph, equipped with thermal conductivity detector (TCD) and integrator. For each experiment adsorbent samples were pressed into pellets, crushed and the 710–1000 lm fraction selected by siev-
M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
ing. These were loaded into 1 m · 6.35 mm o.d. stainless steel columns and activated in situ under flowing He at 190 C for SAPOs, and either 200 C or 270 C for ETS-4. After activation the temperature was reduced to 30 C and the He flow rate adjusted to 30 cm3 min1. To begin the experiment a pulse containing 25 wt% N2 in CH4 was introduced into the column via a gas syringe; gas retention times, relative to H2, were made at temperature steps of 20–110 C. To check reproducibility five measurements were taken at each temperature.
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approximately 0.24 cm3 g1 was found for SAPO-18 and both SAPO-34 samples, as expected for materials containing 3 D-connected 8-ring pores. Specific retention volumes (Vg) were calculated from the IGC data according to Eq. (1), where tr (min) is the relative retention time, F (cm3 min1) is the carrier flow rate, w (g) is the mass of adsorbent in the column and T (K) is the column temperature [13] V g ¼ tr ðF =wÞð273=T Þ
ð1Þ 1
These data may be fitted to Eq. (2), where DH (kJ mol ) is the isosteric enthalpy of adsorption and R is the gas constant [13].
3. Results Powder XRD showed all materials to be phase pure and well crystallised (see Fig. 2). Table 1 gives selected compositional data and mean crystallite sizes. A pore volume of
ln V g ¼ ðDH =RT Þ þ C
ð2Þ
Plots of ln Vg as a function of 1/T (Fig. 3) permit calculation of DH from the slope of the straight line, and also
Fig. 2. Powder XRD patterns of (a) SAPO-18, (b) SAPO-34a, (c) SAPO-34b, and (d) ETS-4.
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Table 1 Comparison of adsorbent composition, crystallite sizes and pore volume Adsorbent
SAPO-18 SAPO-34a SAPO-34b ETS-4 Clinoptilolite (Beliplast) a
Framework Si/ (Si + Al + P) ratio
Principal extraframework species
Mean crystallite size (lm)
0.12 0.18 0.09 – –
H H H Sr K, Ca, Fe, Mga
3.94 5.26 3.01 3.54 –
Table 2 Specific retention volume (Vg) at 30 C and enthalpy of adsorption (DH) for methane and nitrogen on different adsorbents Adsorbent
Methane Vg (cm3 g1)
Nitrogen DH (kJ mol1)
Vg (cm3 g1)
Ref. DH (kJ mol1)
SAPO-18
8.1
26.1
2.6
24.7
SAPO-34a
11.4
25.6
3.6
22.5
SAPO-34b
11.6
27.3
3.8
25.6
See Ref. [4]. ETS-4a
0.03
27.7
0.7
41.5
ETS-4b
0.1
21.2
4.3
37.9
Clinoptilolite (Beliplast)
1.41
84.9
38.9
a b
Fig. 3. Retention of (a) methane and (b) nitrogen on adsorbents, (circle) SAPO-18, (square) SAPO-34a, (triangle) SAPO-34b, and (rhombus) ETS4.
confirms the validity of these data in the temperature range studied. Calculated values are shown in Table 2. 4. Discussion Nitrogen and methane elute as two peaks, in the IGC experiments, showing that all the adsorbents examined
6.62
This work This work This work This work This work [4]
Activated at 200 C. Activated at 270 C.
here will affect the separation in the temperature range studied. The specific retention volume of SAPO-18 is smaller than SAPO-34a, despite the pore windows (8-rings, diameter ca 0.43 nm) and pore volumes (ca. 0.24 cm3 g1) being similar. The cavity shapes in these solids (Fig. 1), their compositions and crystallite sizes are all different (Table 1), which makes elucidating the structure–property relationship difficult. Synthesising SAPOs of different framework-types with precisely the same composition and crystallite size, whilst also maintaining phase purity, is rarely possible. For this reason, SAPO-18 and SAPO-34a samples have not been obtained with identical Si loadings or mean crystallite sizes, which would enable direct comparisons to be made. So to disentangle the influences on specific retention volume a second sample, SAPO-34b, was synthesised (necessarily by a different route) which possessed a different composition and crystallite size (Table 1). Both SAPO-34 samples have broadly the same specific retention volumes (Table 2), despite these differences. This suggests pore structure (cavity shape) has a greater impact on specific retention volume than either composition or crystallite size, at least across families of materials with similar pore sizes and pore volumes and in the absence of large extra-framework cations, as is the case here. The cuboid cavity in SAPO-34 presumably allows CH4 and N2 to pack more efficiently than the pear-shaped cavity in SAPO-18, which leads to the greater specific retention volume. In all SAPO adsorbents the enthalpies of adsorption are greater for CH4 than N2, as would be expected [13] if the guest molecules were free to diffuse through the adsorbent (differences in crystallite size are therefore not significant). In that case the enthalpy of adsorption depends on the strength of the electrostatic interaction of the guest molecule with the framework adsorption sites. CH4 presumably interacts more strongly because of its greater polarizability ˚ 3 vs 1.40 A ˚ 3 for N2), despite the N2 quadrupole, and (2.60 A
M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
the fact that it is expected to pack more efficiently within the cavities [6]. This is a limiting effect however, as can be seen if the change in enthalpy of adsorption of N2 and CH4 in SAPO-34a and SAPO-34b are compared (3.1 kJ mol1 to 1.7 kJ mol1 for N2 and CH4, respectively). The enthalpies of adsorption are inversely proportional to the amount of Si present in the framework, which is ascribed to the increase in the local electrostatic field gradient as Si-substitution decreases. This confirms that the separation is being achieved by different thermodynamic selectivities of the gas molecules in the SAPO materials. In contrast, for ETS-4 the specific retention volume of CH4 is very low, both absolutely and relative to N2. This is due to the reduction in pore window size, which was obtained by activating the sample at 270 C, and which greatly reduces the diffusion rate of the larger CH4 molecules relative to N2. A sample of ETS-4 activated at 200 C did not separate N2/CH4 so effectively. This reveals that kinetic separation is affected in ETS-4. This is similar to clinoptilolite (Table 2) where the extraframework cations are known to subtly control the pore window size. The enthalpies of adsorption follow the same trend as the specific retention volumes in both materials. ETS-4 is not as efficient, however, as natural clinoptilolite from Beliplast (Bulgaria), as can be seen for the first time from comparison of the N2/CH4 Vg ratios. 5. Conclusion Inverse gas chromatographic measurements enable specific retention volumes and isosteric enthalpies of adsorption to be readily calculated for adsorbent materials, greatly facilitating their characterisation. Specific retention volumes and enthalpies of adsorption have been found to be controlled by pore structure and framework composition, respectively, in the SAPO materials studied. Small pore SAPO materials have been shown to separate nitrogen/methane mixtures, but their selectivity favours methane over nitrogen. Extra-framework cations, as found in ETS-4 and clinoptilolite, are likely to be important in
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obtaining the desired (reverse) selectivity. We note that a range of small pore SAPO materials have recently been prepared with Ni-containing complexes [14]. On calcination these are found to contain extra-framework Ni2+ cations included within the pore cavities, thus offering the possibility of obtaining modified adsorbents without recourse to post-synthesis ion exchange. Acknowledgements We thank the Leverhulme Trust for funding this work. References [1] R.V. Jasra, N.V. Choudary, S.G.T. Bhat, Sep. Sci. Technol. 26 (1991) 885. [2] M.W. Ackley, R.F. Giese, R.T. Yang, Zeolites 12 (1992) 780. [3] C.D. Williams, Chem. Commun. (1997) 2113. [4] J.E. Guest, C.D. Williams, Chem. Commun. (2002) 2870. [5] S.M. Kuznicki, V.A. Bell, S. Nair, H.W. Hillhouse, R.M. Jacubinas, C.M. Braunbarth, B.H. Toby, M. Tsapatsis, Nature 412 (2001) 720. [6] T.J. Grey, K.P. Travis, J.D. Gale, D. Nicholson, Micropor. Mesopor. Mater. 48 (2001) 203. [7] K.P. Lillerud, D. Akporiaye, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. Ho¨lderich (Eds.), Proceedings of the 10th International Zeolite Conference: Zeolites, Related Microporous Materials: State of the Art 1994, Stud. Surf. Sci. Catal. 84 (1994) p. 543. [8] J. Chen, P.A. Wright, J.M. Thomas, S. Natarajan, L. Marchese, S.M. Bradley, G. Sankar, C.R.A. Catlow, J. Phys. Chem. 98 (1994) 10216. [9] A.M. Prakash, S. Unnikrishnan, J. Chem. Soc., Faraday Trans. 90 (1994) 2291. [10] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4,440,871 (Example 35), 1984. [11] A. Philippou, M.W. Anderson, Zeolites 16 (1996) 98. [12] C. Braunbarth, H.W. Hillhouse, S. Nair, M. Tsapatsis, A. Burton, R.F. Lobo, R.M. Jacubinas, S.M. Kuznicki, Chem. Mater. 12 (2000) 1857. [13] A. Arcoya, J.A. Gonzalez, G. Llabre, X.L. Seoane, N. Travieso, Micropor. Mater. 7 (1996) 1. [14] R. Garcia, T.D. Coombs, M.J. Maple, W. Zhou, I.J. Shannon, P.A. Cox, P.A. Wright, in: E. van Steen, L.H. Callanan, M. Claeys (Eds.), Proceedings of the 14th International Zeolite Conference: Recent Advances in the Science and Technology of Zeolites and Related Materials, Stud. Surf. Sci. Catal. 154 (2004) p. 993.
Microporous and Mesoporous Materials 111 (2008) 627–631 www.elsevier.com/locate/micromeso
Short Communication
Separating nitrogen/methane on zeolite-like molecular sieves Martin J. Maple *, Craig D. Williams School of Applied Sciences, University of Wolverhampton, Wulfruna Street, Wolverhampton WV1 1SB, UK Received 12 May 2007; received in revised form 11 July 2007; accepted 12 July 2007 Available online 19 July 2007
Abstract Inverse gas chromatography has been used to determine rapidly the specific retention volumes and isosteric enthalpies of adsorption of N2 and CH4 on the silicoaluminophosphates SAPO-18 and SAPO-34, and the titanosilicate ETS-4. In the SAPOs studied, specific retention volumes and enthalpies of adsorption are found to be controlled by pore structure and framework composition, respectively. The SAPOs will separate nitrogen/methane mixtures, but their selectivity favours methane over nitrogen. The inclusion of extra-framework metal cations, as found in ETS-4 and the zeolite clinoptilolite, are likely to be important in obtaining the desired (reverse) selectivity. 2007 Elsevier Inc. All rights reserved. Keywords: Silicoaluminophosphate; ETS-4; Adsorption; Inverse gas chromatography; Natural gas purification
1. Introduction The separation of nitrogen from methane is a significant technical challenge, but one which is of increasing importance for natural gas production, many sources of which are contaminated with high levels of nitrogen (>20 wt%). Lowering N2-content is required to meet pipeline specifications for non-combustibles (<5 wt%), and this is typically achieved by cryogenic distillation. The combined costs of liquefaction and subsequent re-compression of the low pressure product make this an expensive process, economic only for large, highly contaminated fields. An alternative technology is pressure swing adsorption (PSA), which can operate at near ambient temperatures and maintain a high pressure product stream [1]. The commercialisation of PSA processes for N2/CH4 separation has been slow due to the difficulty in developing satisfactory sorbents. Yet the increased potential for recovering methane from small natural gas fields, for which cryogenic separation is uneconomic, make the development of such sorbents desirable. *
Corresponding author. Tel.: +44 (0) 1902 322737; fax: +44 (0) 1902 322714. E-mail address: [email protected] (M.J. Maple). 1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.07.014
Microporous materials, such as aluminosilicate zeolites, have been examined as potential adsorbents for N2/CH4 separation. Clinoptilolite (HEU) will achieve the separation efficiently, but only if its extra-framework cations are selected judiciously [2]. The synthetic form is difficult to prepare [3], and while the natural mineral is abundant, its commercial use has been hindered by the variability in the extra-framework cation composition with source deposit [4], and the tendency of impurities present to lead to its conversion to mordenite (MOR) during service. Alternative synthetic materials, which may be readily crystallised, are therefore required if a commercially viable adsorbent is to be obtained. Titanosilicate ETS-4 is one such synthetic material. It possesses a small pore network, the size of which may be tuned (reduced) by calcination to optimise its selectivity [5]. Differences in the equilibrium or kinetics of adsorption permit the separation of gases by PSA. The precise mechanism for N2/CH4 separation in zeolites is complex, being determined in part by the material’s framework composition, pore structure, and extra-framework cation types and locations. Modifying the extra-framework cations by ion exchange, for example, may effect either or both, the equilibrium and kinetic adsorption properties of the
628
M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
Fig. 1. Illustrative diagrams of the cavities in (a) SAPO-18 and (b) SAPO34. Both frameworks are constructed from double 6-rings, in the former these are related by a mixture of translational and mirror symmetry, while in the latter they are related only by translational symmetry.
adsorbent. Guest molecules may be strongly attracted to extra-framework cations, yet predicting how this will occur (whether the quadrupole of N2, or the polarisability of CH4, will dominate) is not necessarily straightforward [6]. While the case of ion exchanged clinoptilolite, and that of ETS-4 modified by thermal treatment, illustrates the prospect of being able to engineer a particular desired pore size to accomplish a specific separation, it also exemplifies the complexity of the problem of trying to bring ‘design’ to adsorbent synthesis when so many parameters affect it. In this work, we explore the use of inverse gas chromatography (IGC) to study materials for N2/CH4 separation. We have selected silicoaluminophosphate materials of type AEI (SAPO-18) and CHA (SAPO-34) because they contain only protons balancing the charge on the framework, and because they are structurally related [7]. Two samples of SAPO-34 were prepared with different framework compositions. In this way we can examine the effect of cavity shape (Fig. 1) and framework composition on the separation, in the absence of large extra-framework cations. For comparison we have also considered the titanosilicate ETS-4, which is used commercially for this separation. 2. Experimental Hydrothermal syntheses were performed in a Baskerville stainless steel autoclave stirred at 400 rpm. SAPO-18 was prepared by a modified literature method [8], from a gel of composition 1.0 Al(OH)3:0.5 H3PO4:0.2 SiO2:0.7 R:20.0 H2O. 46.4 g of hydrated aluminium hydroxide (Aldrich) was added, with stirring, to a solution containing 34.8 g of orthophosphoric acid (85%, BDH) in 194.0 g of distilled water. Six grams of fumed silica (cabo-sil) was then added, followed by 51.0 g of N,N-diisopropylethylamine, R (99%, Aldrich). The resultant gel was heated hydrothermally at 190 C for 194 h.
SAPO-34a was prepared after the method of Prakash et al. [9], from a gel of composition 1.0 Al(OOH):0.6 H3PO4:0.3 SiO2:0.8 R:5.5 H2O. 57.5 g of pseudoboehmite (Versal 900, La Roche) was added to a solution containing 64.0 g of orthophosphoric acid (85%, BDH) in 41.7 g of distilled water, with stirring. In a separate container, 17.0 g of fumed silica (cab-o-sil) was stirred into a mixture containing 62.5 g of morpholine, R (99%, Aldrich) in 41.7 g of distilled water. A gel was formed by combining this mixture with the first. Once homogeneous, the gel was aged at room temperature for 24 h, before being transferred to the autoclave and heated hydrothermally at 190 C for a further 24 h. SAPO-34b was prepared after the method of Lok et al. [10], from a gel of composition 1.0 Al(iOPr)3:1.0 H3PO4:0.1 SiO2:1.0 R:33.7 H2O. 40.9 g of aluminium isopropoxide (98%, Aldrich) was added, with stirring, to a solution containing 23.1 g of orthophosphoric acid (85%, BDH) in 52.4 g of distilled water. To this 4.5 g of colloidal silica (Ludox AS-40, Aldrich) was added, followed by a further 4.0 g of distilled water and 73.6 g of aqueous tetraethylammonium hydroxide, R (40%, Fluka). The resultant gel was heated hydrothermally for 168 h at 190 C. Na-ETS-4 was prepared by a modified literature method [11], from a gel of composition 0.2 TiCl3:1.0 SiO2:0.5 NaF:1.8 NaOH:19.6 H2O. 9.5 g of sodium fluoride (99%, Fluka) was added, with stirring, to a solution containing 46.7 g of titanium chloride (30 wt% in HCl, BDH) in 49.2 g of distilled water. In a separate container, 36.4 g sodium hydroxide was added to a mixture containing 75.1 g colloidal silica (Ludox AS-40, Aldrich) in 49.2 g distilled water. To form the gel this mixture was combined with the first. Once homogeneous, the gel was heated hydrothermally at 190 C for 24 h. The crystalline products of each reaction were recovered by vacuum filtration, washed with distilled water and dried. Silicoaluminophosphates were calcined in air at 550 C to remove the included organic template. Sr-ETS-4 was obtained by ion exchange, in three steps, on the filtered, dried product of reaction, using aqueous solutions containing 5.0 wt%, 7.2 wt%, then 8.8 wt% strontium chloride [12]. Each gram of Na-ETS-4 was refluxed for 30 min with 30 cm3 of the solution; this provided essentially complete replacement of Na by Sr. The identity and purity of all solids was confirmed by powder XRD (Philips PW1710 diffractometer, Cu Ka Xrays). Inorganic elemental analysis was determined by XRF analysis (Spectro XEPOS spectrometer). Crystallite size was obtained by laser granulometry (Malvern Instruments Mastersizer) and micropore volumes from N2 adsorption isotherm measurements at 196 C (Coulter SA3100). Gas separation experiments were performed with an Ai Cambridge GC 94 M gas chromatograph, equipped with thermal conductivity detector (TCD) and integrator. For each experiment adsorbent samples were pressed into pellets, crushed and the 710–1000 lm fraction selected by siev-
M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
ing. These were loaded into 1 m · 6.35 mm o.d. stainless steel columns and activated in situ under flowing He at 190 C for SAPOs, and either 200 C or 270 C for ETS-4. After activation the temperature was reduced to 30 C and the He flow rate adjusted to 30 cm3 min1. To begin the experiment a pulse containing 25 wt% N2 in CH4 was introduced into the column via a gas syringe; gas retention times, relative to H2, were made at temperature steps of 20–110 C. To check reproducibility five measurements were taken at each temperature.
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approximately 0.24 cm3 g1 was found for SAPO-18 and both SAPO-34 samples, as expected for materials containing 3 D-connected 8-ring pores. Specific retention volumes (Vg) were calculated from the IGC data according to Eq. (1), where tr (min) is the relative retention time, F (cm3 min1) is the carrier flow rate, w (g) is the mass of adsorbent in the column and T (K) is the column temperature [13] V g ¼ tr ðF =wÞð273=T Þ
ð1Þ 1
These data may be fitted to Eq. (2), where DH (kJ mol ) is the isosteric enthalpy of adsorption and R is the gas constant [13].
3. Results Powder XRD showed all materials to be phase pure and well crystallised (see Fig. 2). Table 1 gives selected compositional data and mean crystallite sizes. A pore volume of
ln V g ¼ ðDH =RT Þ þ C
ð2Þ
Plots of ln Vg as a function of 1/T (Fig. 3) permit calculation of DH from the slope of the straight line, and also
Fig. 2. Powder XRD patterns of (a) SAPO-18, (b) SAPO-34a, (c) SAPO-34b, and (d) ETS-4.
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Table 1 Comparison of adsorbent composition, crystallite sizes and pore volume Adsorbent
SAPO-18 SAPO-34a SAPO-34b ETS-4 Clinoptilolite (Beliplast) a
Framework Si/ (Si + Al + P) ratio
Principal extraframework species
Mean crystallite size (lm)
0.12 0.18 0.09 – –
H H H Sr K, Ca, Fe, Mga
3.94 5.26 3.01 3.54 –
Table 2 Specific retention volume (Vg) at 30 C and enthalpy of adsorption (DH) for methane and nitrogen on different adsorbents Adsorbent
Methane Vg (cm3 g1)
Nitrogen DH (kJ mol1)
Vg (cm3 g1)
Ref. DH (kJ mol1)
SAPO-18
8.1
26.1
2.6
24.7
SAPO-34a
11.4
25.6
3.6
22.5
SAPO-34b
11.6
27.3
3.8
25.6
See Ref. [4]. ETS-4a
0.03
27.7
0.7
41.5
ETS-4b
0.1
21.2
4.3
37.9
Clinoptilolite (Beliplast)
1.41
84.9
38.9
a b
Fig. 3. Retention of (a) methane and (b) nitrogen on adsorbents, (circle) SAPO-18, (square) SAPO-34a, (triangle) SAPO-34b, and (rhombus) ETS4.
confirms the validity of these data in the temperature range studied. Calculated values are shown in Table 2. 4. Discussion Nitrogen and methane elute as two peaks, in the IGC experiments, showing that all the adsorbents examined
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This work This work This work This work This work [4]
Activated at 200 C. Activated at 270 C.
here will affect the separation in the temperature range studied. The specific retention volume of SAPO-18 is smaller than SAPO-34a, despite the pore windows (8-rings, diameter ca 0.43 nm) and pore volumes (ca. 0.24 cm3 g1) being similar. The cavity shapes in these solids (Fig. 1), their compositions and crystallite sizes are all different (Table 1), which makes elucidating the structure–property relationship difficult. Synthesising SAPOs of different framework-types with precisely the same composition and crystallite size, whilst also maintaining phase purity, is rarely possible. For this reason, SAPO-18 and SAPO-34a samples have not been obtained with identical Si loadings or mean crystallite sizes, which would enable direct comparisons to be made. So to disentangle the influences on specific retention volume a second sample, SAPO-34b, was synthesised (necessarily by a different route) which possessed a different composition and crystallite size (Table 1). Both SAPO-34 samples have broadly the same specific retention volumes (Table 2), despite these differences. This suggests pore structure (cavity shape) has a greater impact on specific retention volume than either composition or crystallite size, at least across families of materials with similar pore sizes and pore volumes and in the absence of large extra-framework cations, as is the case here. The cuboid cavity in SAPO-34 presumably allows CH4 and N2 to pack more efficiently than the pear-shaped cavity in SAPO-18, which leads to the greater specific retention volume. In all SAPO adsorbents the enthalpies of adsorption are greater for CH4 than N2, as would be expected [13] if the guest molecules were free to diffuse through the adsorbent (differences in crystallite size are therefore not significant). In that case the enthalpy of adsorption depends on the strength of the electrostatic interaction of the guest molecule with the framework adsorption sites. CH4 presumably interacts more strongly because of its greater polarizability ˚ 3 vs 1.40 A ˚ 3 for N2), despite the N2 quadrupole, and (2.60 A
M.J. Maple, C.D. Williams / Microporous and Mesoporous Materials 111 (2008) 627–631
the fact that it is expected to pack more efficiently within the cavities [6]. This is a limiting effect however, as can be seen if the change in enthalpy of adsorption of N2 and CH4 in SAPO-34a and SAPO-34b are compared (3.1 kJ mol1 to 1.7 kJ mol1 for N2 and CH4, respectively). The enthalpies of adsorption are inversely proportional to the amount of Si present in the framework, which is ascribed to the increase in the local electrostatic field gradient as Si-substitution decreases. This confirms that the separation is being achieved by different thermodynamic selectivities of the gas molecules in the SAPO materials. In contrast, for ETS-4 the specific retention volume of CH4 is very low, both absolutely and relative to N2. This is due to the reduction in pore window size, which was obtained by activating the sample at 270 C, and which greatly reduces the diffusion rate of the larger CH4 molecules relative to N2. A sample of ETS-4 activated at 200 C did not separate N2/CH4 so effectively. This reveals that kinetic separation is affected in ETS-4. This is similar to clinoptilolite (Table 2) where the extraframework cations are known to subtly control the pore window size. The enthalpies of adsorption follow the same trend as the specific retention volumes in both materials. ETS-4 is not as efficient, however, as natural clinoptilolite from Beliplast (Bulgaria), as can be seen for the first time from comparison of the N2/CH4 Vg ratios. 5. Conclusion Inverse gas chromatographic measurements enable specific retention volumes and isosteric enthalpies of adsorption to be readily calculated for adsorbent materials, greatly facilitating their characterisation. Specific retention volumes and enthalpies of adsorption have been found to be controlled by pore structure and framework composition, respectively, in the SAPO materials studied. Small pore SAPO materials have been shown to separate nitrogen/methane mixtures, but their selectivity favours methane over nitrogen. Extra-framework cations, as found in ETS-4 and clinoptilolite, are likely to be important in
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obtaining the desired (reverse) selectivity. We note that a range of small pore SAPO materials have recently been prepared with Ni-containing complexes [14]. On calcination these are found to contain extra-framework Ni2+ cations included within the pore cavities, thus offering the possibility of obtaining modified adsorbents without recourse to post-synthesis ion exchange. Acknowledgements We thank the Leverhulme Trust for funding this work. References [1] R.V. Jasra, N.V. Choudary, S.G.T. Bhat, Sep. Sci. Technol. 26 (1991) 885. [2] M.W. Ackley, R.F. Giese, R.T. Yang, Zeolites 12 (1992) 780. [3] C.D. Williams, Chem. Commun. (1997) 2113. [4] J.E. Guest, C.D. Williams, Chem. Commun. (2002) 2870. [5] S.M. Kuznicki, V.A. Bell, S. Nair, H.W. Hillhouse, R.M. Jacubinas, C.M. Braunbarth, B.H. Toby, M. Tsapatsis, Nature 412 (2001) 720. [6] T.J. Grey, K.P. Travis, J.D. Gale, D. Nicholson, Micropor. Mesopor. Mater. 48 (2001) 203. [7] K.P. Lillerud, D. Akporiaye, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. Ho¨lderich (Eds.), Proceedings of the 10th International Zeolite Conference: Zeolites, Related Microporous Materials: State of the Art 1994, Stud. Surf. Sci. Catal. 84 (1994) p. 543. [8] J. Chen, P.A. Wright, J.M. Thomas, S. Natarajan, L. Marchese, S.M. Bradley, G. Sankar, C.R.A. Catlow, J. Phys. Chem. 98 (1994) 10216. [9] A.M. Prakash, S. Unnikrishnan, J. Chem. Soc., Faraday Trans. 90 (1994) 2291. [10] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan, E.M. Flanigen, US Patent 4,440,871 (Example 35), 1984. [11] A. Philippou, M.W. Anderson, Zeolites 16 (1996) 98. [12] C. Braunbarth, H.W. Hillhouse, S. Nair, M. Tsapatsis, A. Burton, R.F. Lobo, R.M. Jacubinas, S.M. Kuznicki, Chem. Mater. 12 (2000) 1857. [13] A. Arcoya, J.A. Gonzalez, G. Llabre, X.L. Seoane, N. Travieso, Micropor. Mater. 7 (1996) 1. [14] R. Garcia, T.D. Coombs, M.J. Maple, W. Zhou, I.J. Shannon, P.A. Cox, P.A. Wright, in: E. van Steen, L.H. Callanan, M. Claeys (Eds.), Proceedings of the 14th International Zeolite Conference: Recent Advances in the Science and Technology of Zeolites and Related Materials, Stud. Surf. Sci. Catal. 154 (2004) p. 993.