- Email: [email protected]
Influence of the Exchangeable Cations of Montmorillonite on Gas Adsorptions
0957–5820/04/$30.00+0.00 # 2004 Institution of Chemical Engineers Trans IChemE, Part B, March 2004 Process Safety and Environmental Protection, 82(B2): 170–174
www.ingentaselect.com=titles=09575820.htm
INFLUENCE OF THE EXCHANGEABLE CATIONS OF MONTMORILLONITE ON GAS ADSORPTIONS C. VOLZONE* and J. ORTIGA Centro de Tecnologı´a de Recursos Minerales y Cera´mica (CETMIC), CONICET-CIC-UNLP, Gonnet, Argentina
ontmorillonites, before and after being saturated with Naþ , Kþ , Hþ , Ca2þ and Al3þ ions, were studied to analyse their O2, N2, CO, CH4, C2H2 and CO2 gas retentions at 25 C and 0.1 MPa, previously degassed at 100 C. The homoionic-montmorillonites were characterized by interlayer spacing measurements and BET surface area. The levels of gas adsorptions for C2H2 (0.184–0.218 mmol g1) and CO2 (0.170–0.208 mmol g1) by original and homoionic montmorillonites were higher than those for N2 (0.068–0.074 mmol g1), O2 (0.070–0.076 mmol g1), CO (0.070–0.080 mmol g1) and CH4 (0.082–0.086 mmol g1). The C2H2 and CO2 gas adsorptions by different cation-exchanged forms of montmorillonite follows the following order: H- > K- > Ca- > Al- > Na-montmorillonite. The ionic radius of the exchangeable cations, the interlayer separation and surface area of the montmorillonite influenced the gas adsorption.
M
Keywords: gas retention; O, N2, CO, CH4, C2H2 and CO2 gases; Na-, H-, K-, Ca- and Al-montmorillonite; homoionic-montmorillonites, bentonite.
INTRODUCTION
in making wine, beer, oils, molding sand, ore pelletization, petroleum, pesticides, catalysts, adsorbents, cosmetics, ceramics, paintings, etc. Many of the industrial uses of montmorillonite are related to its adsorptive capacity, which may be increased by different methods. The montmorillonite is 2:1 (or T-O-T) layer phyllosilicate clay formed by an octahedral sheet containing Al3þ or Mg2þ ions between two tetrahedral silica sheets (Brown, 1961; MacEwan and Wilson, 1980; Newman, 1987). The isomorphic substitution in octahedral and in tetrahedral sheets results in a deficit of surface charges which are balanced by exchangeable cations situated in the interlayer position, e.g. Naþ , Kþ , Ca2þ , etc. (Figure 1). Greeg and Sing (1991) mentioned that the surface area is one of the complementary part in adsorption of the gases or vapours. Thomas and Bohor (1968) analysed the influence of exchanged cations on textural characteristics (surface area, volume adsorbed) of the montmorillonites degassed at 110– 175 C. Additionally, Rutherford et al. (1997) found that the size of exchangeable cations influences the microporosity and surface area of the montmorillonites. Taking into account that the cation situated in interlayer position can have an effect on clay characterization (MacEwan and Wilson, 1980) and some industrial applications (molding sand, swelling, ceramic, etc.) it is interesting to evaluate the effect of some exchangeable cations of montmorillonite on retention of gases measured in standard conditions of temperature and pressure. In this paper we report the O2, N2, CO, CH4, C2H2 and CO2 gas retentions at 25 C and 0.1 MPa by untreated and Na-, H-, K-, Ca- and Al-montmorillonite.
The use of gas adsorption processes in the last three decades has become important for the chemical industry applied in gas separations, such as for example methane= carbon dioxide, methane=nitrogen, nitrogen=oxygen separations, etc. (Yang, 1987). Nowadays, in order to remedy environmental damage caused by the high concentrations of gases in the atmosphere, such as pollutants that affect people, animals and plants as well as different materials, gas adsorption processes are of great importance (e.g. removal of solvents, gasoline vapour, odours, air purification, etc.). As a result, scientists and engineers have developed different adsorbents for gas adsorption and=or separation, such as carbon and zeolites (Yang, 1987; Kappor and Yang, 1989). Yang and Baksh (1991) studied gas adsorption by clay minerals. Yang and Cheng (1995), Volzone and Ortiga (1988, 2000), Volzone et al. (1998, 1999, 2002), Melnitchenko et al. (2000), and Venaruzzo et al. (2002) reported on modified clay minerals (mainly montmorillonites and kaolinites) for gas adsorptions. Montmorillonite is a group of smectite clay. The smectite is the most important mineralogical component (80–90%) in bentonite. Bentonite is widely used in industrial processes
*Correspondence to: Dr C. Volzone, Centro de Tecnologı´a de Recursos Minerales y Cera´mica (CETMIC) (CONICET-CIC-UNLP), C.C. 49 Cno. Centenario y 506, (1897) M.B. Gonnet, Prov. Buenos Aires, Argentina. E-mail: [email protected]
170
EXCHANGEABLE CATIONS OF MONTMORILLONITE
Figure 1. Montmorillonite structure. T, tetrahedral sheet; O, octahedral sheet.
171
The interlayer spacing, d(001) spacing, of homoionicmontmorillonites was measured by X-ray diffraction, XRD (Brown and Brindley, 1980) at 25 C in two different conditions: (a) at relative humidity of 55% (RH); and (b) previously heated at 100 C for 12 h, stored in a desiccator and in random orientation (the same conditions required when gas adsorption was measured). The samples were scanned with a step of 0.02 2y using a Philips 3020 Goniometer equipment with PW 3710 Controller, Cu Ka ˚ ) and Ni filter at 40 kV and 20 mA, in radiation (l ¼ 1.5405 A the range of 3–15 2y. The accurate of the d(001) spacing ˚. values according our operative conditions was 0.01 A
EXPERIMENTAL A natural bentonite from San Juan province, Argentine was used. This material is around 90% montmorillonite, with quartz and feldspar as impurities, as determined by X-ray diffraction and chemical analysis (Volzone and Garrido, 1991). For experiments the bentonite was processed to <2 mm size fraction e.s.d. (equivalent spherical diameters) by sedimentation in deionized water and application of Stokes’ law of setting under gravity. The separation greatly decreased associated impurities. Montmorillonite saturated with different cations was prepared by suspension of 15 g of <2 mm fraction sample in 1000 ml of 2 N chloride solutions of Naþ , Kþ , Hþ , Ca2þ and Al3þ , for 24 h. The supernatant was separated and the treatments with salts were repeated three times. The excess of salts was removed by washing repeatedly with distilled water until a negative AgNO3 test was obtained (Brown, 1961). The samples were named: H-M, Na-M, K-M, Ca-M and Al-M (Table 1). Equilibrium adsorption was measured using standard volumetric apparatus which was connected to a gas flow system. Samples were outgassed at 100 C for 12 h prior to measurement. Adsorption was measured at 25 C and 0.1 MPa and the experimental absolute error was 0.0014 mmol g1. The gases tested for their adsorption behaviour were O2, N2, CO, CH4, C2H2 and CO2, which were dried by passing through presorbers before entering the system. BET (Brunauer, Emmett and Teller) surface area (Brunauer et al., 1938) was calculated from the first part of the N2 adsorption isotherm (P=Po < 0.3) obtained at liquid nitrogen temperature with N2 in Micromeritics Accusorb equipment previously degassed at 100 C for 12 h prior to measurement. The experimental absolute error of measurement for BET surface area was 1 m2 g1.
Table 1. Basal spacing and BET surface area of the untreated and homoionic-montmorillonites. d(001) spacing dried at ˚ 100 C, A
BET surface area, m2 g1
Sample
d(001) spacing ˚ RH 55%, A
M
13.20
9.73
26
H-M Na-M K-M Ca-M Al-M
15.60 13.50 12.20 16.90 14.62
12.78 9.88 10.40 12.21 10.31
60 33 56 50 48
RESULTS AND ANALYSIS Characterization of Homoionic-montmorillonites The interlayer spacing, basal spacing or d(001) spacing of the montmorillonite is measured from the top of the Si tetrahedral silica sheet (T) to the top of the Si tetrahedral sheet of the following layer (Figure 1). The different basal spacing of the montmorillonite is a function of the saturated interlayer cation characteristics (MacEwan and Wilson, 1980). The d(001) spacing of the homoionic-montmorillonite ˚ (Table 1). measured at 55% RH is in the range 12.2–16.90 A Solvatation of Naþ ions occurs at low water content (Sposito ˚ et al., 1963) and the measured d(001) spacing of 13.50 A corresponds to a monolayer of water content. Ca2þ ions are coordinated with six to eight water molecules in the centre of the interlayer region (Keren and Shainberg, 1975; McBride ˚ for Ca-M may indicate et al., 1975) and the value of 16.90 A that a space able to accommodate two layers of water ˚ for Al-montmormolecule is available. The value of 14.62 A illonite is close in agreement with that reported for similar materials by Oades (1984), and Lahav and Shani (1978a). The Al3þ ions may be present mainly as Al(OH)2þ ions formed during montmorillonite treatment, while the pH increased from 1.70 to 4.81 from initial suspension (montmorillonite plus aluminium chloride solution) to final suspension, respectively. This assumption is based on the work of Lahav and Shani (1978b), in which final suspension pH was 4.86–4.52 and the proportion of such Al species could be found. The same authors also mentioned that some oligomerization of Al under these conditions might take place. The H-M sample showed interlayer spacing of ˚ (at RH 55%). At low water content protons (Hþ ) 15.60 A are present in montmorillonite exchange sites as H3Oþ ions (McBride et al. 1975), close to the silicate surfaces and only after heating at more than 200 C. When dehydration with the decomposition of hydronium ions takes place, the protons migrate through hexagonal holes (McBride et al., 1975). The hexagonal holes are formed by individual tetrahedrals (of the T layer) linked to neighbouring tetrahedrals by sharing basal oxygen (Figure 1). These cavities have diameters of about ˚ (Helsen et al., 1975). 2.6 A Lower d(001) spacing values and broad peaks were observed in the original and the Na- and Al-montmorillonites after drying at 100 C (Figure 2). Broad peaks suggest that the samples showed different size of interlayer spacing, smaller and larger than the spacing values indicated in Figure 2 and in Table 1. In dry state the Naþ ions are placed on the hexagonal cavities in the interlayer surface or
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
172
VOLZONE AND ORTIGA concluded that the differences among the surface areas of the different exchangeable montmorillonites reflected a certain degree of N2 penetration between silicate layers. The relatively higher surface area of the different homoionic montmorillonites may be due to accommodation of the cations in interlayer spacing of the montmorillonite that originate different d(001) values, as shown in Figure 2 and Table 1. Gas Adsorption
Figure 2. X-ray diffraction: d(001) spacing of the homoionic-montmorillonites dry at 100 C.
in close packing with the oxygen of the tetrahedral silicon (McBride et al., 1975), this may cause the low d(001) ˚ . The peaks of the H-, K- and Caspacing of 9.88 A montmorillonite were better defined. Potassium ions are placed into the holes and tend to be fixed. Mamy and Gaultier (1975) demonstrated that in the K-montmorillonite, when heated at 80 C, the basal spacing diminishes irrever˚ and the layer becomes more ordered sibly to around 10A (more defined peak). Ca2þ ions are not in the hexagonal cavities of the silicate surface at 100 C (Keren and ˚ Shainberg, 1975; McBride et al., 1975) requiring 12.20 A ˚ interlayer spacing value space. The H-M showed an 12.78 A and indicated that not all H3Oþ ions were dehydrated at 100 C as mentioned above. After saturating the montmorillonites with different cation, the BET surface areas (obtained by N2 adsorption isotherms at 77 K) were: 26, 33, 48, 50, 56 and 60 m2 g1 for original, Na-, Al-, Ca-, K- and H-montmorillonite, respectively (Table 1). Rutherford et al. (1997), who characterized Na-, Ca- and K-montmorillonites, found a similar tendency. These authors also mentioned that the microporous surface can differ depending on the exchangeable cations of montmorillonites. Thomas and Bohor (1968)
The retentions of gases by the homoionic montmorillonites were in the order of N2 O2 4 CO 4 CH4 CO2 < C2H2 (Table 2). A similar tendency was found using pillared clays, acid-clays and organo-clays as adsorbents (Yang and Baksh, 1991; Volzone and Ortiga, 2000; Volzone et al., 2002). Baksh and Yang (1992) showed that the amount of gas adsorption depend on the size of the gas molecule and the physicochemical characteristics of the adsorbents such as pillared clays, zeolites, carbon, etc. The levels of N2 (0.068–0.074 mmol g1), O2 (0.070– 0.076 mmol g1), CO (0.070–0.080 mmol g1) and CH4 (0.082–0.086 mmol g1) gas adsorptions by the original and homoionic montmorillonites were low, whereas the C2H2 (0.184–0.218 mmol g1) and CO2 (0.170–0.208 mmol g1) retentions were higher. The adsorption for C2H2 and CO2 gases by homoionic montmorillonites were duplicate whereas for the N2, O2, CO and CH4 gases were triplicate. The H- and K-montmorillonites were better adsorbents for retentions of the CO2 (0.208 and 0.193 mmol g1, respectively) and for C2H2 (0.218 and 0.211 mmol g1, respectively) gases compared with (Ca-M, Al-M and Na-M samples Table 2). CO2 gas adsorptions by homoionicmontmorillonites (0.170–0.208 mmol g1) were similar to those by Al-, Zr-, Cr- and Ti-pillared clays (0.210, 0.185, 0.100 and 0.141 mmol g1) but were smaller than those by metal exchanged amorphous kaolinite (0.284 mmol g1), modified Al-PILCs (0.300 mmol g1), montmorillonite treated with organic compounds (0.480 mmol g1) and acid fine bentonite (0.586 mmol g1), in all cases measured under the same experimental adsorption conditions, which were reported in previous papers (Volzone and Ortiga, 1998, 2002; Volzone et al., 1998, 1999, 2000). It is important to keep in mind that the modified clays, which were mentioned previously, are clays treated in different conditions, where textural and structural changes were obtained. All those samples are different from to the samples of the present paper because in this case, only exchangeable cations were modified but not the original montmorillonite structure (T-O-
Table 2. Gas adsorption and gas adsorption ratio by untreated and monoionic-montmorillonites at 25 C and 0.1 MPa. Gas adsorption mmol g1 Sample
N2
Gas adsorption ratio O2
CO
CH4
CO2
C2H2
CO2=N2
C2H2=N2
M
0.071
0.070
0.070
0.083
0.162
0.174
2.28
2.45
H-M Na-M K-M Ca-M Al-M
0.068 0.071 0.068 0.078 0.074
0.076 0.070 0.073 0.076 0.075
0.080 0.077 0.078 0.077 0.075
0.085 0.086 0.082 0.083 0.084
0.208 0.170 0.193 0.181 0.174
0.218 0.186 0.211 0.194 0.191
3.06 2.39 2.84 2.32 2.35
3.20 2.61 3.10 2.49 2.58
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
EXCHANGEABLE CATIONS OF MONTMORILLONITE
173
Figure 3. CO2 and C2H2 retentions by homoionic-montmorillonites vs. BET surface area of the adsorbents.
Figure 4. CO2 and C2H2 retentions vs. ionic radius of the exchangeable cations.
T layer). The levels of adsorption for N2, O2, CH4 and CO by the homoionic montmorillonites are similar to those reported in previous papers and near to 1.5–2 times larger than the kaolin amorphous derivatives (Volzone et al., 1999).
Cheng (1995) reported 0.060 and 0.049 mmol N2 g1 for alkali metals Na- and K- exchanged ZrO2-pillared montmorillonite, respectively, where the adsorbent was a montmorillonite treated with Zr and thermal treatment. Our Na-M and K-M samples adsorbed 0.071 and 0.068 mmol N2 g1 of homoionic montmorillonite (Table 2). The kinetic diameters of the N2, O2 and CH4 molecules are ˚ , respectively, and their retentions by 3.64, 3.46 and 3.8 A homoionic montmorillonites were smaller than CO2, which ˚ ). These different has a similar kinetic diameter (3.3 A tendencies could be attributed to the different quadruple moments (1.5 1026 and 4.3 1026 Erg1=2 cm5=2 for N2 and CO2, respectively). The gas retention ratio is defined as the ratio of retention of a pure gas at equilibrium measured at one defined temperature and pressure, to that of the other pure gas under the same conditions. Yang and Baksh (1991) reported that an equilibrium gas retention ratio value higher than 3.00 by one adsorbent is considered to be viable for commercial gas separation. The H- and K-montmorillonites of the present paper could be proposed as potential materials for C2H2=N2 separation because the ratios were 3.20 and 3.10 respectively, and H-M for CO2=N2 separation (Table 2). The results of this study showed that the CO2 and C2H2 retentions by one montmorillonite could be improved by exchange cation treatment, such as H3Oþ or Kþ . However, higher gas adsorption values by montmorillonites could be obtained by acid, inorganic or organic treatments.
DISCUSSION AND CONCLUSIONS The ionic radius of the exchangeable cations, situated in interlayer position of the montmorillonite, influenced the CO2 and C2H2 gas retentions (also for CO gas with smaller values, Table 2) by montmorillonites as shown in Figure 3. The different size of the ionic radius originated different distance between layers as d(001) spacing of the solids shown in Figure 2 and Table 1. H-, K- and Ca-montmorillonites showed better defined peak of d(001) and better gas retentions (Table 2, Figure 2). The d(001) spacing indicates the distance between layers, however, it does not show if it was more or less free for the entrance of gas molecules. The gas retentions also increased with interlayer spacing of the studied adsorbents, except for Kmontmorillonite, which may be explained by fixed spacing after treatment at 100 C (Table 2). According to a previous paper (Volzone et al., 2002), where the d(001) spacing of intercalated montmorillonite with hexadecylpyridinium ˚ , low CO2 gas retention resulted were in the range 18–26 A because this type of cation showed less free space available for adsorption capacity. Then, the d(001) spacing is an important characteristic but it is also necessary to determine the free space available for adsorption, which can be obtained for example by surface area measurement. The surface areas of different homoionic montmorillonites showed the location at which adsorption can take place. In Figure 4 is shown the influence of surface area of homoionic montmorillonites on CO2 and C2H2 gas retention. Then, the size ionic exchangeable inorganic cation situated in the interlayer position of the montmorillonite influences the available free siloxane mineral surface accessible for CO2 and C2H2 retentions. The levels of N2, O2, CO and CH4 gas adsorptions by homoionic-montmorillonites were low and showed different retention tendencies with respect to CO2 and C2H2 (Tables 1 and 2). Molinard and Vansant (1994) reported no adsorption of N2 at room temperature by clays, and Yang and
REFERENCES Baksh, M.S.A. and Yang, R.T., 1992, Unique adsorption properties and potential energy profiles of microporous pillared clays, AIChE J, 38: 1357–1368. Brown, G., 1961, The X-ray Identification and Crystal Structure of Clay Minerals (Mineralogical Society, London), p 167. Brown, G. and Brindley, G.W., 1980, X-ray diffraction procedures for clay mineral identification, in Crystal Structures of Clay Minerals and their X-ray Identification, Brindley, G. and Brown, G. (eds) (Mineralogical Society, London), pp 305–356. Brunauer, S., Emmett, P.H. and Teller, E., 1938, Adsorption of gases in multimolecular layers, J Am Chem Soc, 60: 309–319. Greeg, S.J. and Sing, K.S.W., 1991, Adsorption, Surface Area and Porosity (Academic Press, London), p 303.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
174
VOLZONE AND ORTIGA
Helsen, J.A., Drieskens, R. and Chaussidon, J., 1975, Position of exchangeable cations in montmorillonites, Clays Clay Miner, 23: 334–335. Kapoor, A. and Yang, R.T., 1989, Kinetic separation of methane-carbon dioxide mixture by adsorption on molecular sieve carbon, Chem Eng Sci, 44: 1723–1733. Keren, R. and Shainberg, I., 1975, Water vapor isotherms and heat of immersion of Na=Ca-montmorillonite systems—I: homoionic clays, Clays Clay Miner, 23: 193–200. Lahav, N. and Shani, U., 1978a, Cross-linked smectites. I. Synthesis and properties of hydroxy-aluminum-montmorillonite, Clays Clay Miner, 26: 107–115. Lahav, N. and Shani, U., 1978b, Cross-linked smectites. II. Flocculation and microfabric characteristics of hydroxy-aluminum-montmorillonite, Clays Clay Miner, 26: 115–124. MacEwan, D.M.C. and Wilson, M.J., 1980, Interlayer and intercalation complexes of clay minerals, in Crystal Structures of Clay Minerals and their X-ray Identification, Brindley, G. and Brown, G. (eds) (Mineralogical Society, London), pp 197–248. Mamy, J. and Gaultier, J.P., 1975, Evolution de l’ordre cristallin dans la montmorillonite en relation avec la diminution d’e´changebilite´ du potassium, Proceedings of International Clay Conference (Applied Publishing, Wilmette, IL), pp 149–155. McBride, M.B., Mortland, M.M. and Pinnavaia, T.J., 1975, Exchange ion positions in smectite. Effects on electron spin resonance of structural iron, Clays Clay Miner, 23: 162–163. Melnitchenko, S., Thompson, J.G., Volzone, C. and Ortiga, J., 2000, Selective gas adsorption by metal exchanged amorphous kaolinite derivative, Appl Clay Sci, 17: 35–53. Molinard, A. and Vansant, E.F., 1994, Gas adsorption properties of cation modified alumina pillared montmorillonite, in Separation Technology, Vasant, E.F. (ed) (Elsevier, Amsterdam). Newman, A.C.D., 1987, Chemistry of Clays and Clay Minerals (Longman Scientific & Technical=Mineralogical Society, London), p 480. Oades, J.M., 1984, Interaction of polycation of aluminum and iron with clay, Clays Clay Miner, 32: 49–57. Rutherford, D.W., Chiou, C.T. and Eberl, D.D., 1997, Effects of exchanged cation on the microporosity of montmorillonite, Clays Clay Miner, 45: 534–543. Sposito, G., Prost, R. and Gaulttier, J.P., 1963, Infrared spectroscopic study of adsorbed water on reduced-charge Na=Li-montmorillonites, Clays Clay Miner, 31: 9–16.
Thomas, J. Jr and Bohor, B.F., 1968, Surface area of montmorillonite from the dynamic sorption of nitrogen and carbon dioxide, Clays Clay Miner, 16: 83–91. Venaruzzo, J.L., Volzone, C., Rueda, M.L. and Ortiga, J., 2002, Modified bentonitic clay minerals for CO, CO2 and SO2 adsorption, Microporous Mesoporous Mater, 56: 73–80. Volzone, C, and Garrido, L.B., 1991, The effect of some physicochemical and mineralogical properties on the Na2CO3 activation of Argentine bentonites, Appl Clay Sci, 6: 143–154. Volzone, C. and Ortiga, J., 1988, O2, CH4 and CO2 gas adsorption by acid di- and tri-octahedral smectites, International Workshop of Activated Clays, 6–8 December, La Plata, Argentina, pp 45–54. Volzone, C., and Ortiga, J., 2000, Retention gas O2, CH4 and CO2 by acid smectites with and without thermal treatment, J Mater Sci, 35: 5291–5294. Volzone, C., Cavalieri, A.L. and Porto Lo´pez, J.M., 1988, Desorption of ethylene glycol from activated bentonites, Mater Chem Phy, 20: 359–369. Volzone, C., Garrido, L.B., Ortiga, J. and Pereira, E., 1998, Sı´ntesis de arcillas pilareadas para su utilizacio´n como adsorbentes, in Desarrollo de adsorbentes para la separacio´n de gases, Programa Iberoamericano de Ciencia y Tecnologı´a (CYTED) Subprograma V. Cata´lisis y Adsorbentes. Proyecto V.3., Rodrı´guez-Reinoso, F. and Andreu, P. (eds). (CYTED, Espan˜a), pp 97–119. Volzone, C., Thompson, J.G., Melnitchenko, A., Ortiga, J. and Palethorpe, R.S., 1999, Selective gas adsorption by amorphous clay mineral derivative, Clays Clay Miner, 47: 647–657. Volzone, C., Rinaldi, J.O. and Ortiga, J., 2002, N2 and CO2 adsorptions by TMA- and HDP-montmorillonite, Mater Res, 5: 475–479. Yang, R.T., 1987, Adsorption and adsorption isotherms, in Gas Separation of Adsorption Processes (Butterworth, Boston, MA), pp 9–39. Yang, R.T. and Baksh, M.S.A., 1991, Pillared clays as a new class of sorbents for gas separation, AIChE J, 37: 679–686. Yang, R.T. and Cheng, L.S., 1995, Pillared clays and ion-exchanged pillared clays as gas adsorbents and catalysts for selective catalytic reduction of NO, in Access in Nanoporous Materials, Pinnavaia, T.J. and Thorpe, M.F. (eds) (Plenum Press, New York), pp 73–92.
The manuscript was received 2 September 2002 and accepted for publication after revision 29 September 2003.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
www.ingentaselect.com=titles=09575820.htm
INFLUENCE OF THE EXCHANGEABLE CATIONS OF MONTMORILLONITE ON GAS ADSORPTIONS C. VOLZONE* and J. ORTIGA Centro de Tecnologı´a de Recursos Minerales y Cera´mica (CETMIC), CONICET-CIC-UNLP, Gonnet, Argentina
ontmorillonites, before and after being saturated with Naþ , Kþ , Hþ , Ca2þ and Al3þ ions, were studied to analyse their O2, N2, CO, CH4, C2H2 and CO2 gas retentions at 25 C and 0.1 MPa, previously degassed at 100 C. The homoionic-montmorillonites were characterized by interlayer spacing measurements and BET surface area. The levels of gas adsorptions for C2H2 (0.184–0.218 mmol g1) and CO2 (0.170–0.208 mmol g1) by original and homoionic montmorillonites were higher than those for N2 (0.068–0.074 mmol g1), O2 (0.070–0.076 mmol g1), CO (0.070–0.080 mmol g1) and CH4 (0.082–0.086 mmol g1). The C2H2 and CO2 gas adsorptions by different cation-exchanged forms of montmorillonite follows the following order: H- > K- > Ca- > Al- > Na-montmorillonite. The ionic radius of the exchangeable cations, the interlayer separation and surface area of the montmorillonite influenced the gas adsorption.
M
Keywords: gas retention; O, N2, CO, CH4, C2H2 and CO2 gases; Na-, H-, K-, Ca- and Al-montmorillonite; homoionic-montmorillonites, bentonite.
INTRODUCTION
in making wine, beer, oils, molding sand, ore pelletization, petroleum, pesticides, catalysts, adsorbents, cosmetics, ceramics, paintings, etc. Many of the industrial uses of montmorillonite are related to its adsorptive capacity, which may be increased by different methods. The montmorillonite is 2:1 (or T-O-T) layer phyllosilicate clay formed by an octahedral sheet containing Al3þ or Mg2þ ions between two tetrahedral silica sheets (Brown, 1961; MacEwan and Wilson, 1980; Newman, 1987). The isomorphic substitution in octahedral and in tetrahedral sheets results in a deficit of surface charges which are balanced by exchangeable cations situated in the interlayer position, e.g. Naþ , Kþ , Ca2þ , etc. (Figure 1). Greeg and Sing (1991) mentioned that the surface area is one of the complementary part in adsorption of the gases or vapours. Thomas and Bohor (1968) analysed the influence of exchanged cations on textural characteristics (surface area, volume adsorbed) of the montmorillonites degassed at 110– 175 C. Additionally, Rutherford et al. (1997) found that the size of exchangeable cations influences the microporosity and surface area of the montmorillonites. Taking into account that the cation situated in interlayer position can have an effect on clay characterization (MacEwan and Wilson, 1980) and some industrial applications (molding sand, swelling, ceramic, etc.) it is interesting to evaluate the effect of some exchangeable cations of montmorillonite on retention of gases measured in standard conditions of temperature and pressure. In this paper we report the O2, N2, CO, CH4, C2H2 and CO2 gas retentions at 25 C and 0.1 MPa by untreated and Na-, H-, K-, Ca- and Al-montmorillonite.
The use of gas adsorption processes in the last three decades has become important for the chemical industry applied in gas separations, such as for example methane= carbon dioxide, methane=nitrogen, nitrogen=oxygen separations, etc. (Yang, 1987). Nowadays, in order to remedy environmental damage caused by the high concentrations of gases in the atmosphere, such as pollutants that affect people, animals and plants as well as different materials, gas adsorption processes are of great importance (e.g. removal of solvents, gasoline vapour, odours, air purification, etc.). As a result, scientists and engineers have developed different adsorbents for gas adsorption and=or separation, such as carbon and zeolites (Yang, 1987; Kappor and Yang, 1989). Yang and Baksh (1991) studied gas adsorption by clay minerals. Yang and Cheng (1995), Volzone and Ortiga (1988, 2000), Volzone et al. (1998, 1999, 2002), Melnitchenko et al. (2000), and Venaruzzo et al. (2002) reported on modified clay minerals (mainly montmorillonites and kaolinites) for gas adsorptions. Montmorillonite is a group of smectite clay. The smectite is the most important mineralogical component (80–90%) in bentonite. Bentonite is widely used in industrial processes
*Correspondence to: Dr C. Volzone, Centro de Tecnologı´a de Recursos Minerales y Cera´mica (CETMIC) (CONICET-CIC-UNLP), C.C. 49 Cno. Centenario y 506, (1897) M.B. Gonnet, Prov. Buenos Aires, Argentina. E-mail: [email protected]
170
EXCHANGEABLE CATIONS OF MONTMORILLONITE
Figure 1. Montmorillonite structure. T, tetrahedral sheet; O, octahedral sheet.
171
The interlayer spacing, d(001) spacing, of homoionicmontmorillonites was measured by X-ray diffraction, XRD (Brown and Brindley, 1980) at 25 C in two different conditions: (a) at relative humidity of 55% (RH); and (b) previously heated at 100 C for 12 h, stored in a desiccator and in random orientation (the same conditions required when gas adsorption was measured). The samples were scanned with a step of 0.02 2y using a Philips 3020 Goniometer equipment with PW 3710 Controller, Cu Ka ˚ ) and Ni filter at 40 kV and 20 mA, in radiation (l ¼ 1.5405 A the range of 3–15 2y. The accurate of the d(001) spacing ˚. values according our operative conditions was 0.01 A
EXPERIMENTAL A natural bentonite from San Juan province, Argentine was used. This material is around 90% montmorillonite, with quartz and feldspar as impurities, as determined by X-ray diffraction and chemical analysis (Volzone and Garrido, 1991). For experiments the bentonite was processed to <2 mm size fraction e.s.d. (equivalent spherical diameters) by sedimentation in deionized water and application of Stokes’ law of setting under gravity. The separation greatly decreased associated impurities. Montmorillonite saturated with different cations was prepared by suspension of 15 g of <2 mm fraction sample in 1000 ml of 2 N chloride solutions of Naþ , Kþ , Hþ , Ca2þ and Al3þ , for 24 h. The supernatant was separated and the treatments with salts were repeated three times. The excess of salts was removed by washing repeatedly with distilled water until a negative AgNO3 test was obtained (Brown, 1961). The samples were named: H-M, Na-M, K-M, Ca-M and Al-M (Table 1). Equilibrium adsorption was measured using standard volumetric apparatus which was connected to a gas flow system. Samples were outgassed at 100 C for 12 h prior to measurement. Adsorption was measured at 25 C and 0.1 MPa and the experimental absolute error was 0.0014 mmol g1. The gases tested for their adsorption behaviour were O2, N2, CO, CH4, C2H2 and CO2, which were dried by passing through presorbers before entering the system. BET (Brunauer, Emmett and Teller) surface area (Brunauer et al., 1938) was calculated from the first part of the N2 adsorption isotherm (P=Po < 0.3) obtained at liquid nitrogen temperature with N2 in Micromeritics Accusorb equipment previously degassed at 100 C for 12 h prior to measurement. The experimental absolute error of measurement for BET surface area was 1 m2 g1.
Table 1. Basal spacing and BET surface area of the untreated and homoionic-montmorillonites. d(001) spacing dried at ˚ 100 C, A
BET surface area, m2 g1
Sample
d(001) spacing ˚ RH 55%, A
M
13.20
9.73
26
H-M Na-M K-M Ca-M Al-M
15.60 13.50 12.20 16.90 14.62
12.78 9.88 10.40 12.21 10.31
60 33 56 50 48
RESULTS AND ANALYSIS Characterization of Homoionic-montmorillonites The interlayer spacing, basal spacing or d(001) spacing of the montmorillonite is measured from the top of the Si tetrahedral silica sheet (T) to the top of the Si tetrahedral sheet of the following layer (Figure 1). The different basal spacing of the montmorillonite is a function of the saturated interlayer cation characteristics (MacEwan and Wilson, 1980). The d(001) spacing of the homoionic-montmorillonite ˚ (Table 1). measured at 55% RH is in the range 12.2–16.90 A Solvatation of Naþ ions occurs at low water content (Sposito ˚ et al., 1963) and the measured d(001) spacing of 13.50 A corresponds to a monolayer of water content. Ca2þ ions are coordinated with six to eight water molecules in the centre of the interlayer region (Keren and Shainberg, 1975; McBride ˚ for Ca-M may indicate et al., 1975) and the value of 16.90 A that a space able to accommodate two layers of water ˚ for Al-montmormolecule is available. The value of 14.62 A illonite is close in agreement with that reported for similar materials by Oades (1984), and Lahav and Shani (1978a). The Al3þ ions may be present mainly as Al(OH)2þ ions formed during montmorillonite treatment, while the pH increased from 1.70 to 4.81 from initial suspension (montmorillonite plus aluminium chloride solution) to final suspension, respectively. This assumption is based on the work of Lahav and Shani (1978b), in which final suspension pH was 4.86–4.52 and the proportion of such Al species could be found. The same authors also mentioned that some oligomerization of Al under these conditions might take place. The H-M sample showed interlayer spacing of ˚ (at RH 55%). At low water content protons (Hþ ) 15.60 A are present in montmorillonite exchange sites as H3Oþ ions (McBride et al. 1975), close to the silicate surfaces and only after heating at more than 200 C. When dehydration with the decomposition of hydronium ions takes place, the protons migrate through hexagonal holes (McBride et al., 1975). The hexagonal holes are formed by individual tetrahedrals (of the T layer) linked to neighbouring tetrahedrals by sharing basal oxygen (Figure 1). These cavities have diameters of about ˚ (Helsen et al., 1975). 2.6 A Lower d(001) spacing values and broad peaks were observed in the original and the Na- and Al-montmorillonites after drying at 100 C (Figure 2). Broad peaks suggest that the samples showed different size of interlayer spacing, smaller and larger than the spacing values indicated in Figure 2 and in Table 1. In dry state the Naþ ions are placed on the hexagonal cavities in the interlayer surface or
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
172
VOLZONE AND ORTIGA concluded that the differences among the surface areas of the different exchangeable montmorillonites reflected a certain degree of N2 penetration between silicate layers. The relatively higher surface area of the different homoionic montmorillonites may be due to accommodation of the cations in interlayer spacing of the montmorillonite that originate different d(001) values, as shown in Figure 2 and Table 1. Gas Adsorption
Figure 2. X-ray diffraction: d(001) spacing of the homoionic-montmorillonites dry at 100 C.
in close packing with the oxygen of the tetrahedral silicon (McBride et al., 1975), this may cause the low d(001) ˚ . The peaks of the H-, K- and Caspacing of 9.88 A montmorillonite were better defined. Potassium ions are placed into the holes and tend to be fixed. Mamy and Gaultier (1975) demonstrated that in the K-montmorillonite, when heated at 80 C, the basal spacing diminishes irrever˚ and the layer becomes more ordered sibly to around 10A (more defined peak). Ca2þ ions are not in the hexagonal cavities of the silicate surface at 100 C (Keren and ˚ Shainberg, 1975; McBride et al., 1975) requiring 12.20 A ˚ interlayer spacing value space. The H-M showed an 12.78 A and indicated that not all H3Oþ ions were dehydrated at 100 C as mentioned above. After saturating the montmorillonites with different cation, the BET surface areas (obtained by N2 adsorption isotherms at 77 K) were: 26, 33, 48, 50, 56 and 60 m2 g1 for original, Na-, Al-, Ca-, K- and H-montmorillonite, respectively (Table 1). Rutherford et al. (1997), who characterized Na-, Ca- and K-montmorillonites, found a similar tendency. These authors also mentioned that the microporous surface can differ depending on the exchangeable cations of montmorillonites. Thomas and Bohor (1968)
The retentions of gases by the homoionic montmorillonites were in the order of N2 O2 4 CO 4 CH4 CO2 < C2H2 (Table 2). A similar tendency was found using pillared clays, acid-clays and organo-clays as adsorbents (Yang and Baksh, 1991; Volzone and Ortiga, 2000; Volzone et al., 2002). Baksh and Yang (1992) showed that the amount of gas adsorption depend on the size of the gas molecule and the physicochemical characteristics of the adsorbents such as pillared clays, zeolites, carbon, etc. The levels of N2 (0.068–0.074 mmol g1), O2 (0.070– 0.076 mmol g1), CO (0.070–0.080 mmol g1) and CH4 (0.082–0.086 mmol g1) gas adsorptions by the original and homoionic montmorillonites were low, whereas the C2H2 (0.184–0.218 mmol g1) and CO2 (0.170–0.208 mmol g1) retentions were higher. The adsorption for C2H2 and CO2 gases by homoionic montmorillonites were duplicate whereas for the N2, O2, CO and CH4 gases were triplicate. The H- and K-montmorillonites were better adsorbents for retentions of the CO2 (0.208 and 0.193 mmol g1, respectively) and for C2H2 (0.218 and 0.211 mmol g1, respectively) gases compared with (Ca-M, Al-M and Na-M samples Table 2). CO2 gas adsorptions by homoionicmontmorillonites (0.170–0.208 mmol g1) were similar to those by Al-, Zr-, Cr- and Ti-pillared clays (0.210, 0.185, 0.100 and 0.141 mmol g1) but were smaller than those by metal exchanged amorphous kaolinite (0.284 mmol g1), modified Al-PILCs (0.300 mmol g1), montmorillonite treated with organic compounds (0.480 mmol g1) and acid fine bentonite (0.586 mmol g1), in all cases measured under the same experimental adsorption conditions, which were reported in previous papers (Volzone and Ortiga, 1998, 2002; Volzone et al., 1998, 1999, 2000). It is important to keep in mind that the modified clays, which were mentioned previously, are clays treated in different conditions, where textural and structural changes were obtained. All those samples are different from to the samples of the present paper because in this case, only exchangeable cations were modified but not the original montmorillonite structure (T-O-
Table 2. Gas adsorption and gas adsorption ratio by untreated and monoionic-montmorillonites at 25 C and 0.1 MPa. Gas adsorption mmol g1 Sample
N2
Gas adsorption ratio O2
CO
CH4
CO2
C2H2
CO2=N2
C2H2=N2
M
0.071
0.070
0.070
0.083
0.162
0.174
2.28
2.45
H-M Na-M K-M Ca-M Al-M
0.068 0.071 0.068 0.078 0.074
0.076 0.070 0.073 0.076 0.075
0.080 0.077 0.078 0.077 0.075
0.085 0.086 0.082 0.083 0.084
0.208 0.170 0.193 0.181 0.174
0.218 0.186 0.211 0.194 0.191
3.06 2.39 2.84 2.32 2.35
3.20 2.61 3.10 2.49 2.58
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
EXCHANGEABLE CATIONS OF MONTMORILLONITE
173
Figure 3. CO2 and C2H2 retentions by homoionic-montmorillonites vs. BET surface area of the adsorbents.
Figure 4. CO2 and C2H2 retentions vs. ionic radius of the exchangeable cations.
T layer). The levels of adsorption for N2, O2, CH4 and CO by the homoionic montmorillonites are similar to those reported in previous papers and near to 1.5–2 times larger than the kaolin amorphous derivatives (Volzone et al., 1999).
Cheng (1995) reported 0.060 and 0.049 mmol N2 g1 for alkali metals Na- and K- exchanged ZrO2-pillared montmorillonite, respectively, where the adsorbent was a montmorillonite treated with Zr and thermal treatment. Our Na-M and K-M samples adsorbed 0.071 and 0.068 mmol N2 g1 of homoionic montmorillonite (Table 2). The kinetic diameters of the N2, O2 and CH4 molecules are ˚ , respectively, and their retentions by 3.64, 3.46 and 3.8 A homoionic montmorillonites were smaller than CO2, which ˚ ). These different has a similar kinetic diameter (3.3 A tendencies could be attributed to the different quadruple moments (1.5 1026 and 4.3 1026 Erg1=2 cm5=2 for N2 and CO2, respectively). The gas retention ratio is defined as the ratio of retention of a pure gas at equilibrium measured at one defined temperature and pressure, to that of the other pure gas under the same conditions. Yang and Baksh (1991) reported that an equilibrium gas retention ratio value higher than 3.00 by one adsorbent is considered to be viable for commercial gas separation. The H- and K-montmorillonites of the present paper could be proposed as potential materials for C2H2=N2 separation because the ratios were 3.20 and 3.10 respectively, and H-M for CO2=N2 separation (Table 2). The results of this study showed that the CO2 and C2H2 retentions by one montmorillonite could be improved by exchange cation treatment, such as H3Oþ or Kþ . However, higher gas adsorption values by montmorillonites could be obtained by acid, inorganic or organic treatments.
DISCUSSION AND CONCLUSIONS The ionic radius of the exchangeable cations, situated in interlayer position of the montmorillonite, influenced the CO2 and C2H2 gas retentions (also for CO gas with smaller values, Table 2) by montmorillonites as shown in Figure 3. The different size of the ionic radius originated different distance between layers as d(001) spacing of the solids shown in Figure 2 and Table 1. H-, K- and Ca-montmorillonites showed better defined peak of d(001) and better gas retentions (Table 2, Figure 2). The d(001) spacing indicates the distance between layers, however, it does not show if it was more or less free for the entrance of gas molecules. The gas retentions also increased with interlayer spacing of the studied adsorbents, except for Kmontmorillonite, which may be explained by fixed spacing after treatment at 100 C (Table 2). According to a previous paper (Volzone et al., 2002), where the d(001) spacing of intercalated montmorillonite with hexadecylpyridinium ˚ , low CO2 gas retention resulted were in the range 18–26 A because this type of cation showed less free space available for adsorption capacity. Then, the d(001) spacing is an important characteristic but it is also necessary to determine the free space available for adsorption, which can be obtained for example by surface area measurement. The surface areas of different homoionic montmorillonites showed the location at which adsorption can take place. In Figure 4 is shown the influence of surface area of homoionic montmorillonites on CO2 and C2H2 gas retention. Then, the size ionic exchangeable inorganic cation situated in the interlayer position of the montmorillonite influences the available free siloxane mineral surface accessible for CO2 and C2H2 retentions. The levels of N2, O2, CO and CH4 gas adsorptions by homoionic-montmorillonites were low and showed different retention tendencies with respect to CO2 and C2H2 (Tables 1 and 2). Molinard and Vansant (1994) reported no adsorption of N2 at room temperature by clays, and Yang and
REFERENCES Baksh, M.S.A. and Yang, R.T., 1992, Unique adsorption properties and potential energy profiles of microporous pillared clays, AIChE J, 38: 1357–1368. Brown, G., 1961, The X-ray Identification and Crystal Structure of Clay Minerals (Mineralogical Society, London), p 167. Brown, G. and Brindley, G.W., 1980, X-ray diffraction procedures for clay mineral identification, in Crystal Structures of Clay Minerals and their X-ray Identification, Brindley, G. and Brown, G. (eds) (Mineralogical Society, London), pp 305–356. Brunauer, S., Emmett, P.H. and Teller, E., 1938, Adsorption of gases in multimolecular layers, J Am Chem Soc, 60: 309–319. Greeg, S.J. and Sing, K.S.W., 1991, Adsorption, Surface Area and Porosity (Academic Press, London), p 303.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174
174
VOLZONE AND ORTIGA
Helsen, J.A., Drieskens, R. and Chaussidon, J., 1975, Position of exchangeable cations in montmorillonites, Clays Clay Miner, 23: 334–335. Kapoor, A. and Yang, R.T., 1989, Kinetic separation of methane-carbon dioxide mixture by adsorption on molecular sieve carbon, Chem Eng Sci, 44: 1723–1733. Keren, R. and Shainberg, I., 1975, Water vapor isotherms and heat of immersion of Na=Ca-montmorillonite systems—I: homoionic clays, Clays Clay Miner, 23: 193–200. Lahav, N. and Shani, U., 1978a, Cross-linked smectites. I. Synthesis and properties of hydroxy-aluminum-montmorillonite, Clays Clay Miner, 26: 107–115. Lahav, N. and Shani, U., 1978b, Cross-linked smectites. II. Flocculation and microfabric characteristics of hydroxy-aluminum-montmorillonite, Clays Clay Miner, 26: 115–124. MacEwan, D.M.C. and Wilson, M.J., 1980, Interlayer and intercalation complexes of clay minerals, in Crystal Structures of Clay Minerals and their X-ray Identification, Brindley, G. and Brown, G. (eds) (Mineralogical Society, London), pp 197–248. Mamy, J. and Gaultier, J.P., 1975, Evolution de l’ordre cristallin dans la montmorillonite en relation avec la diminution d’e´changebilite´ du potassium, Proceedings of International Clay Conference (Applied Publishing, Wilmette, IL), pp 149–155. McBride, M.B., Mortland, M.M. and Pinnavaia, T.J., 1975, Exchange ion positions in smectite. Effects on electron spin resonance of structural iron, Clays Clay Miner, 23: 162–163. Melnitchenko, S., Thompson, J.G., Volzone, C. and Ortiga, J., 2000, Selective gas adsorption by metal exchanged amorphous kaolinite derivative, Appl Clay Sci, 17: 35–53. Molinard, A. and Vansant, E.F., 1994, Gas adsorption properties of cation modified alumina pillared montmorillonite, in Separation Technology, Vasant, E.F. (ed) (Elsevier, Amsterdam). Newman, A.C.D., 1987, Chemistry of Clays and Clay Minerals (Longman Scientific & Technical=Mineralogical Society, London), p 480. Oades, J.M., 1984, Interaction of polycation of aluminum and iron with clay, Clays Clay Miner, 32: 49–57. Rutherford, D.W., Chiou, C.T. and Eberl, D.D., 1997, Effects of exchanged cation on the microporosity of montmorillonite, Clays Clay Miner, 45: 534–543. Sposito, G., Prost, R. and Gaulttier, J.P., 1963, Infrared spectroscopic study of adsorbed water on reduced-charge Na=Li-montmorillonites, Clays Clay Miner, 31: 9–16.
Thomas, J. Jr and Bohor, B.F., 1968, Surface area of montmorillonite from the dynamic sorption of nitrogen and carbon dioxide, Clays Clay Miner, 16: 83–91. Venaruzzo, J.L., Volzone, C., Rueda, M.L. and Ortiga, J., 2002, Modified bentonitic clay minerals for CO, CO2 and SO2 adsorption, Microporous Mesoporous Mater, 56: 73–80. Volzone, C, and Garrido, L.B., 1991, The effect of some physicochemical and mineralogical properties on the Na2CO3 activation of Argentine bentonites, Appl Clay Sci, 6: 143–154. Volzone, C. and Ortiga, J., 1988, O2, CH4 and CO2 gas adsorption by acid di- and tri-octahedral smectites, International Workshop of Activated Clays, 6–8 December, La Plata, Argentina, pp 45–54. Volzone, C., and Ortiga, J., 2000, Retention gas O2, CH4 and CO2 by acid smectites with and without thermal treatment, J Mater Sci, 35: 5291–5294. Volzone, C., Cavalieri, A.L. and Porto Lo´pez, J.M., 1988, Desorption of ethylene glycol from activated bentonites, Mater Chem Phy, 20: 359–369. Volzone, C., Garrido, L.B., Ortiga, J. and Pereira, E., 1998, Sı´ntesis de arcillas pilareadas para su utilizacio´n como adsorbentes, in Desarrollo de adsorbentes para la separacio´n de gases, Programa Iberoamericano de Ciencia y Tecnologı´a (CYTED) Subprograma V. Cata´lisis y Adsorbentes. Proyecto V.3., Rodrı´guez-Reinoso, F. and Andreu, P. (eds). (CYTED, Espan˜a), pp 97–119. Volzone, C., Thompson, J.G., Melnitchenko, A., Ortiga, J. and Palethorpe, R.S., 1999, Selective gas adsorption by amorphous clay mineral derivative, Clays Clay Miner, 47: 647–657. Volzone, C., Rinaldi, J.O. and Ortiga, J., 2002, N2 and CO2 adsorptions by TMA- and HDP-montmorillonite, Mater Res, 5: 475–479. Yang, R.T., 1987, Adsorption and adsorption isotherms, in Gas Separation of Adsorption Processes (Butterworth, Boston, MA), pp 9–39. Yang, R.T. and Baksh, M.S.A., 1991, Pillared clays as a new class of sorbents for gas separation, AIChE J, 37: 679–686. Yang, R.T. and Cheng, L.S., 1995, Pillared clays and ion-exchanged pillared clays as gas adsorbents and catalysts for selective catalytic reduction of NO, in Access in Nanoporous Materials, Pinnavaia, T.J. and Thorpe, M.F. (eds) (Plenum Press, New York), pp 73–92.
The manuscript was received 2 September 2002 and accepted for publication after revision 29 September 2003.
Trans IChemE, Part B, Process Safety and Environmental Protection, 2004, 82(B2): 170–174