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Characterization and ethylene adsorption of natural and modified clinoptilolites
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Applied Surface Science 254 (2008) 2450–2457 www.elsevier.com/locate/apsusc
Characterization and ethylene adsorption of natural and modified clinoptilolites Burcu Erdog˘an *, Meryem Sakızcı, Ertug˘rul Yo¨ru¨kog˘ulları Department of Physics, Science Faculty, Anadolu University, 26470 Eskis¸ehir, Turkey Received 12 March 2007; received in revised form 21 September 2007; accepted 21 September 2007 Available online 29 September 2007
Abstract The ethylene adsorption of Turkey clinoptilolite-rich tuff from Gordes and Bigadic region of western of Anatolia and their exchanged forms (K+, Na+ and Ca2+) were investigated. The clinoptilolite samples were characterized using XRD, TG–DTA and nitrogen adsorption methods. Adsorption isotherms for ethylene on natural and modified forms of both adsorbents at 277 K and 293 K were obtained at pressures up to 38 kPa. Uptake of ethylene increased as Na-CLN < Ca-CLN < K-CLN < Natural CLN for Gordes zeolite at 277 K, 293 K and for Bigadic zeolite at 277 K. For Bigadic zeolites at 293 K, uptake of ethylene increased in the order Ca-CLN < Na-CLN < K-CLN < Natural CLN. It was found that ethylene adsorption capacity of Bigadic clinoptilolite samples was much greater than Gordes clinoptilolite samples except K+ modified forms at both temperatures. These results show that both natural clinoptilolites have a considerable potential for the removal of ethylene. # 2007 Elsevier B.V. All rights reserved. PACS : 82.75.z; 68.43.h; 61.66.Hq; 67.80.Gb; 61.10.Nz Keywords: Zeolites; Adsorption; Ethylene; Thermal properties; XRD
1. Introduction Ethylene (C2H4) is a naturally occurred, simple two carbon gaseous plant growth regulator that has a number of effects on the growth, development and storage periods of many kinds of fruits, vegetables and ornamental crops [1]. Ethylene acts as a plant hormone. It accelerates respiration, leading to maturity and also softening and ripening of many kinds of fruits. Moreover, ethylene accumulation can result in yellowing of green vegetables and may be responsible for numerous types of specific postharvest defects in fresh fruits and vegetables. Although ethylene has some positive effects, it is often hazardous to the quality and shelf-life of fruits and vegetables. To extend shelf-life and keep an acceptable visual and organoleptic quality, accumulation of ethylene in the packaging should be prevented [2]. A lot of C2H4-adsorbing substances such as potassium permanganate (KMnO4), activated carbon and minerals (zeolites and clays) are characterized [3,4]. One of
* Corresponding author. Tel.: +90 222 3350580x5848; fax: +90 222 3204910. E-mail address: [email protected] (B. Erdog˘an). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.058
the most effective materials to remove ethylene is potassium permanganate and its commercial products are accessible. However, because of the chemical properties of potassium permanganate, precautions must be taken in order to prevent contamination of food products. The search for other materials with ethylene removal capability has been focused considerable attention on zeolites. Zeolites are hydrated aluminosilicates of alkali and alkaline earth elements with unique crystal structures consisting of a three-dimensional framework of SiO4 and AlO4 tetrahedral [5]. The isomorphic substitution of Si by Al leads to a negative charge density in the zeolite lattice. This charge is neutralized by introducing exchanged monovalent, divalent, or trivalent cations in the structural sites of the zeolite [6]. The mobile nonframework cations are placed in cavities in the channel walls and coordinated with the water molecules within the channel. Due to their chemical, physical and structural properties, zeolites act as a molecular sieve and also as an ionic exchanger [7]. Occurrences of sedimentary zeolites are widespread in central and western Anatolia, Turkey. They are associated with clay minerals, borates, carbonates and soda minerals similar to many other parts of the world [8].
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Tuffs of natural zeolites such as clinoptilolite generally rather pure and abundant, can be mined with simple techniques [9] and their adsorption characteristics can be tailored by ion exchange. Clinoptilolite is one of the most common natural zeolite minerals with a wide range of application. The basic structure of the clinoptilolite crystal contains three types of channels with the ˚ 4.4 A ˚ ), following approximate dimensions: channel A (7.2 A ˚ ˚ ˚ ˚ channel B (4.0 A 5.5 A) and channel C (4.1 A 4.0 A). Channels A and B, which are parallel to each other, intersected by channel C and exchangeable charge-balancing cations (Ca2+, Mg2+, Na+, K+, etc.) are located in these channels [10]. However, the adsorption of ethylene on zeolites has been searched by a number of experimental [11–19] and theoretical [20] methods, studies on adsorption of ethylene based on natural zeolites are very limited in the literature. The purpose of the current study is to investigate the efficiency of two natural zeolites and those of modified forms from Turkey, which mainly consist of clinoptilolite, in the removal of ethylene.
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Table 2 Chemical composition of the Bigadic zeolite Constituent
%
SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO P2O5 H2O
71.81 12.26 0.80 0.50 1.70 3.41 1.37 0.01 8.12
100 ml of 1 M HCl solution followed by a washing in deionized water. Second, the Na+, K+, and Ca2+ ionic forms of two natural minerals were prepared by the Batch method, using 1 M solutions in a shaker at 100 8C for 24 h. Finally, the treated samples were washed with distilled water until the filtrate was free from chloride ions and then dried at room temperature.
2. Experimental
3. Instrumentation
2.1. Materials and reagents
Clinoptilolite-rich minerals from Bigadic and Gordes and those of modified forms were characterized by using XRD, TGA, DTA and nitrogen adsorption analysis techniques. XRD analyses of the samples were performed using Rigaku RINT2200 diffractometer in a scanning range of 5–408 (2u) at a rate of 28 (2u) min1. Cu Ka radiation was used. Monoclinic crystal system was used for the determination of unit-cell parameters (a, b, c, V and b). The values of the unit cell parameters were calculated from the characteristic diffraction reflections of clinoptilolite. TG–DTA curves were measured from 30 8C to 1000 8C at a rate of 10 8C min1 with Setsys Evolution Setaram thermal analysis apparatus. About 40 mg of the sample was used in each run. The water concentration in the samples was determined from the TG curve mass loss. The specific surface areas of the samples were evaluated by the nitrogen gas adsorption method at 196 8C, using a NOVA 2200 Instrument (Quantachrome Instruments, U.S.A.), employing multipoint BET isotherm adsorption data fitting. The adsorption isotherms of ethylene (C2H4) on natural and modified clinoptilolite-rich minerals were determined using automated volumetric equipment (Autosorb 1-Quantachrome Instruments, U.S.A.) at 277 K and 293 K. About 0.1 g of the sample was outgassed in vacuum at 200 8C for 7 h before ethylene adsorption.
Clinoptilolite-rich minerals used in their original and modified forms were obtained from Bigadic and Gordes Locations, in western Turkey. Bigadic and Gordes zeolites were obtained from Bigadic reserves and Incal Co., Izmir, Turkey, respectively. They were crushed into small pieces from rock forms and then powdered in a mortar. After sieved through <125 mm samples collected from under the sieve were used in the ethylene adsorption study. The mineralogical composition of both materials mainly consists of clinoptilolite. The associated minerals for Gordes and Bigadic are k-feldspar and quartz, respectively. The chemical compositions of both samples were determined by classical chemical analysis (Tables 1 and 2). Inorganic chemicals such as HCl, NaCl, KCl and CaCl2 were supplied by Merck (Darmstadt, Germany) and all solutions were prepared using de-ionized water. Ethylene was supplied with the purity of 99.9% by Gerling Holz Company, Germany. 2.2. Modification of original materials by cation exchange First, samples of 5.0 g of clinoptilolite-rich minerals from Gordes and Bigadic regions were activated by washing with Table 1 Chemical composition of the Gordes zeolite Constituent
%
SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO H2O
74.00 12.25 0.78 0.79 1.20 1.84 1.06 7.55
4. Results and discussion 4.1. X-ray analysis X-ray diffraction patterns of the clinoptilolite samples are shown in (Fig. 1) that modification of clinoptilolite with solutions did not lead to significant structural changes. The quantitative XRD analysis demonstrated that the naturalG.CLN mainly consisted of clinoptilolite with minor contents
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Fig. 1. X-ray powder diffraction patterns of Gordes and Bigadic clinoptilolite samples.
Table 3 XRD quantitative modal analysis of the natural-G.CLN and -B.CLN samples (wt.%) Samples
Clinoptilolite
Feldspar
Quartz
Opal-CT
Mica-illite
Opal-A + volcanic glass
Natural-G.CLN Natural-B.CLN
80 82
6–7 3–4
1–2 12–13
4–5 –
1–2 1–2
4–5 –
of feldspar, quartz, opal-CT, smectite, mica-illite and amorphous material (opal-A + volcanic glass) and the naturalB.CLN consisted of clinoptilolite with minor quantities of feldspar, quartz and mica-illite (Table 3). The method given by Esenli and Sirkeciog˘lu [21] was used to determine the ratio of clinoptilolite. The essential differences between both natural zeolites can be summarized as follows: natural-B.CLN hardly contains amorphous material. Although, natural-G.CLN involves both amorphous material which can be seen by means of hump and that of height between 2u is equal to 208 and 308 (Cu Ka) at XRD diagram and opal-CT that is observed by a ˚ and 4.10 A ˚ flat line with the d-spacing of between 4.05 A (cristobalite-tridimite transition). Natural-B.CLN includes quartz evidently, whereas natural-G.CLN hardly contains it. Amount of feldspar in samples of natural-G.CLN is much greater than that of in natural-B.CLN. The X-ray diffraction diagrams of the clinoptilolite samples given in Fig. 1 show characteristic clinoptilolite peaks [22] at 2u (Cu Ka) = 9.878, 22.48 and 308, respectively. In addition, the unit-cell parameters of the zeolitic samples are presented in Table 4. The calculated unit-cell parameters of natural and modified forms of the
samples do not show significant change. The dimension of c parameter is longer and therefore V is larger in the Bigadic clinoptilolite than that of the Go¨rdes clinoptilolite. On the other hand, a distinctive data are not found in between the parameters of cationic forms. The only following characteristics are defined: The effect of Na on the unit cell parameters of the Go¨rdes and Bigadic samples is different, K causes get lower the b value in Bigadic sample and Ca is ineffective both on two clinoptilolites. Table 4 Unit-cell parameters and volumes of the natural and those of modified forms of zeolites from Gordes and Bigadic Samples
a (nm)
b (nm)
c (nm)
b (8)
V (nm)3
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
1.7528 1.7870 1.7722 1.7709 1.7592 1.7250 1.7647 1.7672
1.7955 1.7900 1.7904 1.7904 1.7859 1.7904 1.7006 1.7932
0.7388 0.7416 0.7392 0.7319 0.7532 0.7229 0.7379 0.7392
115.6 116.5 115.5 114.7 114.0 113.4 116.5 116.1
2.096 2.122 2.116 2.108 2.161 2.049 2.098 2.103
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4.2. Thermal properties Thermal behavior of zeolites can be investigated by means of TG/DTA analysis. The existence of various exchangeable cations in zeolites leads to changes in temperature of zeolitic water elimination, which is held in a few stages [23]. TG/ DTA curves of all forms of clinoptilolite samples are shown in Fig. 2.
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The DTA curves of natural and Na+ forms of both sorbents display a single endotherm at temperatures ranging from 125 8C to 135 8C as a result of a single-step dehydration process. Endotherm minima temperatures increase in the sequence: Natural-G:CLNð125 CÞ ! Natural-B:CLNð128 CÞ ! Na-G:CLNð133 CÞ ! Na-B:CLNð135 CÞ
Fig. 2. TG/DTA curves of all forms of Gordes and Bigadic clinoptilolite samples.
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The DTA curves of Ca-G.CLN, K-G.CLN and K-B.CLN are characterized by two endotherms, related to a two-step dehydration process. The endotherms range between 118 8C and 164 8C (first reaction). Endotherm minima temperatures increase in the sequence: K-B:CLNð118 CÞ ! K-G:CLNð129 CÞ ! Ca-G:CLNð164 CÞ: The second endotherm, occurring at 424 8C (Ca-G.CLN), 442 8C (K-B.CLN) and 455 8C (K-G.CLN), increases in temperature following a different sequence. The DTA curve of Ca-B.CLN gives three peaks at 121 8C, 161 8C and 409 8C, as a result of a three-step dehydration process. A small endothermic peak appears at 774 8C, 881 8C and 725 8C, respectively, in the DTA curves of K-B.CLN, NaB.CLN and Ca-G.CLN, accompanying the weight loss shown on the DTA curve. Collapse of the zeolite structure happens at this temperature. As follows from the DTA curves (Fig. 2) in the case of zeolite samples, endothermic peaks connected with elimination of physically adsorbed water are observed up to 200 8C. Table 5 reports the total weight loss for all cation forms determined by TG analysis. The zeolite water loss is higher in the zeolites exchanged with bivalent ion. In addition, in the monovalent ions, the larger cations have less zeolite water (Table 5). Water loss continued smoothly for all samples from 30 8C to 500 8C. Most of the physisorbed water is lost between 30 8C and 200 8C and in the broad interval between 200 8C and 500 8C more strongly associated water is lost. Dehydroxylation is observed with further water loss at the higher temperatures.
Table 6 Calculated surface areas of natural and modified natural zeolite samples Sample
BET surface area (m2/g)
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
60 54 96 29 47 51 73 28
Fig. 3. N2 adsorption isotherms of natural and modified Bigadic clinoptilolite samples.
4.3. Specific surface area Zeolite samples were outgassed at 180 8C for 4 h under vacuum (103 Torr) before N2 adsorption. The surface areas of the samples are listed in Table 6. Adsorption isotherms of the samples are of type II in the IUPAC classification [24] (Figs. 3 and 4). 4.4. Adsorption Adsorption isotherms for ethylene on natural and modified forms of both adsorbents at 277 K and 293 K were obtained at pressures up to 38 kPa. The adsorption isotherms (absolute amount adsorbed per gram of adsorbent) of C2H4 on all the samples are shown in Figs. 5–12. The curves are similar in
Fig. 4. N2 adsorption isotherms of natural and modified Gordes clinoptilolite samples.
Table 5 The results of thermal analysis of natural and modified natural zeolite samples Sample
Weight loss (wt.%)
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
11.98 14.76 14.42 18.68 11.72 12.94 12.75 24.87
Fig. 5. Adsorption of ethylene on Gordes zeolite at 277 K.
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457
Fig. 6. Adsorption of ethylene on Gordes zeolite at 293 K.
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Fig. 10. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite at 277 K and 293 K.
Fig. 7. Adsorption of ethylene on Bigadic zeolite at 277 K. Fig. 11. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite and Bigadic zeolite at 277 K.
Fig. 8. Adsorption of ethylene on Bigadic zeolite at 293 K.
shape and have a classic isotherm form. As the temperature increases, the total amount adsorbed decreases as expected (Figs. 9 and 10). Reproducibility of the experiments was checked by repeating some experiments in identical conditions
Fig. 9. Adsorption of ethylene on natural Na, K, Ca cationic forms of Bigadic zeolite at 277 K and 293 K.
and confirming that a good replication was obtained. The absolute amounts adsorbed per gram of Gordes and Bigadic zeolites are given in Table 7. Natural forms of both clinoptilolite zeolites showed much higher ethylene adsorption amount compared to that of ˚ ) can pass modified forms. Ethylene (kinetic diameter 3.9 A through both clinoptilolite zeolite pore openings. It is observed that uptake of ethylene on Gordes zeolite at 277 K and 293 K increases in the following sequence (Figs. 5 and 6): Na-G:CLN < Ca-G:CLN < K-G:CLN < Natural-G:CLN Although, the sequence at 277 K for Bigadic zeolite is the same as determined above, adsorption amount of ethylene on
Fig. 12. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite and Bigadic zeolite at 293 K.
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457
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Table 7 The absolute amounts adsorbed per gram of adsorbent Sample
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
Amount adsorbed (mmol/g zeolite) 277 (K)
293 (K)
0.966 0.109 0.758 0.422 1.492 0.370 0.685 0.611
0.956 0.069 0.719 0.226 1.233 0.291 0.482 0.259
Bigadic zeolite at 293 K increases in the following order (Figs. 7 and 8): Ca-B:CLN < Na-B:CLN < K-B:CLN < Natural-B:CLN Among all the modified zeolites, it was found that the K+ forms of both clinoptilolite samples had the most ethylene adsorption capacity. The difference in adsorption capacities is most probably due to the strength of adsorption sites. Electronegativity value of potassium is lower than those of sodium and calcium. In addition, potassium has the biggest atomic diameter compared to sodium and calcium [25]. Hence, interaction between K+ form and ethylene should be much more regarding the other forms of zeolite. It was figured out that uptake of ethylene was generally greater for the exchanged zeolite containing Ca2+ ion than for Na+ ion. For both zeolites exchanged with divalent cation like Ca2+, a higher adsorption capacity could be expected than that with Na+ because two monovalent cations are replaced by a single divalent cation and the ionic radii of calcium are not greater than that of sodium. Thus, the net volume occupancy by these divalent cations would be less than one half as with sodium. Divalent cations in zeolites are known to be strong adsorptive centers, and ethylene has a strong quadropole moment. The interaction of the divalent Ca2+ ions with ethylene double bond (p-bond) is likely to be the reason for the strong adsorption [15] on Gordes and Bigadic clinoptilolite zeolites. Various cation-exchanged forms of clinoptilolite samples may lead to significant differences in the adsorption of ethylene due to both the location and size of the interchangeable cations which affect the local electrostatic field, and the polarization of the adsorbates. As seen in Figs. 5–12 ethylene adsorption exhibits the isotherm of type I. 5. Conclusion In this study, characterization of clinoptilolite samples (XRD, TG–DTA and N2 adsorption methods) and the efficiency of those of samples in the removal of ethylene have been investigated. X-ray analysis of the zeolite samples have shown that the modification of clinoptilolites with solution does not lead to significant structural changes. The thermal behavior of clinoptilolite samples is influenced by clinoptilolite cationic form. Some differences attributed to the different structure of the two clinoptilolite minerals are
observed between the temperature of dehydration of the Gordes and Bigadic zeolites. Isotherms for the adsorption of ethylene on clinoptilolite samples have been obtained at 277 K (refrigerator temperature) and 293 K (room temperature) in the pressure range 0.004– 38 kPa. It is found that ethylene-adsorption capacity of Bigadic clinoptilolite samples is much greater than Gordes clinoptilolite samples except K+ modified forms at both temperatures (Figs. 11 and 12). Among all the modified zeolites, it is found that the K+ forms of both clinoptilolite samples have the most ethylene adsorption capacity. It is shown that the uptake of ethylene is generally greater for the exchanged zeolite containing Ca2+ ion than for Na+ ion. In addition, it is found that the affinity of the natural Gordes and Bigadic zeolite surfaces to ethylene is greater than those of modified forms. In conclusion, compared with other ethylene-adsorbing substances (potassium permanganate, activated carbon and synthetic zeolites), the natural zeolites are much cheaper, more abundant and more available. Removal of ethylene via natural zeolites retards decaying. For that reason, storage life of fruit and vegetables could be prolonged. Acknowledgments Financial support by Eskisehir-Anadolu University via Scientific Research Projects (No. AUAF 041040) is acknowledged. Special thanks to Fahri Esenli and Matthias Thommes for their helpful suggestions and corrections. References [1] M.E. Salveit, Effect of ethylene on quality of fresh fruits and vegetables, Postharvest Biol. Tech. 15 (1999) 279–292. [2] L. Vermeiren, F. Devlieghere, M. Van Beest, N. de Kruijf, J. Debevere, Developments in the active packaging of foods, Trends Food Sci. Technol. 10 (3) (1999) 77–86. [3] K. Abe, A.E. Watada, Ethylene adsorbent to maintain quality of lightly processed fruits and vegetables, J. Food Sci. 56 (6) (1991) 1589–1592. [4] T. Suslow, Performance of zeolite based products in ethylene removal, Perishables Handling Quarterly 92 (1997) 32–33. [5] A. Dyer, An Introduction to Zeolite Molecular Sieves, John Willey and Sons Press, 1988. [6] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1984. [7] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978. [8] R. Birsoy, Activity diagrams of zeolites: implications for the occurrences of zeolites in Turkey and of Erionite worldwide, Clay Clay Miner. 50 (2002) 136–144. [9] T. Armbruster, Clinoptilolite–heulandite: applications and basic research, Stud. Surf. Sci. Catal. 135 (2001) 13–27. [10] M.W. Ackley, Clinoptilolite: untapped potential for kinetic gas separations, Zeolites 12 (1992) 780–788. [11] S. Hyun, R.P. Danner, Equilibrium adsorption of ethane, ethylene, isobutene, carbon dioxide and their binary mixtures on 13X molecular sieves, J. Chem. Eng. Data 27 (2) (1982) 196–200. [12] E. Costa, G. Calleja, A. Jimenez, J. Pau, Adsorption equilibrium of ethylene, propane, propylene, carbon dioxide and their mixtures on 13X zeolite, J. Chem. Eng. Data 36 (1991) 218–224. [13] G. Calleja, P.L. Dominguez, P. Perez, Multicomponent adsorption equilibrium of ethylene, propane, propylene and CO2 on 13X zeolite, Gas Sep. Purif. 8 (4) (1994) 247–256.
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457 [14] K. Berlier, M.G. Olivier, R. Jadot, Adsorption of methane, ethane and ethylene on zeolite, J. Chem. Eng. Data 40 (1995) 1206–1208. [15] R.W. Triebe, F.H. Tezel, K.C. Knulbe, Adsorption of methane, ethane and ethylene on molecular sieve zeolites, Gas Sep. Purif. 10 (1) (1996) 81–84. [16] S.B. Jang, M. Jeong, Y. Kim, Crystal structures of the ethylene and acetylene sorption complexes of fully Ca2+-exchanged zeolite X, J. Phys. Chem. 101 (1997) 3091–3096. [17] J. Limtrakul, T. Nanok, S. Jungsuttiwong, P. Khongpracha, T.N. Truong, Adsorption of unsaturated hydrocarbons on zeolites: the effects of the zeolite framework on adsorption properties of ethylene, Chem. Phys. Lett. 349 (2001) 161–166. [18] S.I. Kim, T. Aida, H. Niiyama, Binary adsorption of very low concentration ethylene on water vapor on mordenites and desorption by microwave heating, Sep. Purif. Technol. 45 (2005) 174–182. [19] N.A. Al-Baghli, K.F. Loughlin, Binary and ternary adsorption of methane, ethane, and ethylene on titanosilicate ETS-10 zeolite, J. Chem. Eng. Data 51 (2006) 248–254.
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[20] S. Ketrat, J. Limtrakul, Theoretical study of the adsorption of ethylene on alkali-exchanged zeolites, Int. J. Quantum Chem. 94 (2003) 333–340. [21] F. Esenli, A. Sirkeciog˘lu, The relationship between zeolite (heulandite– clinoptilolite) content and the ammonium-exchange capacity of pyrodastic rocks in Gordes, Turkey, Clay Miner. 40 (4) (2005) 557–564. [22] G. Gottardi, E. Gali, Natural Zeolites, Springer-Verlag, Berlin, 1985. [23] M.W. Kasture, P.N. Joshi, H.S. Soni, V. Joshi, V.P. Choudhari, V.P. Shiralkar, Sorption properties of the natural, K and partially deammoniated (H/NH4) forms of clinoptilolite, Adsorpt. Sci. Technol. 16 (1998) 135–151. [24] K.S.W. Sing, et al., Reporting Physisorption data for gas/solid system with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (4) (1985) 603–619. [25] W.F. Smith, Principles of Materials Science and Engineering, 2nd ed., McGraw Hill Inc.,, New York, 1990.
Applied Surface Science 254 (2008) 2450–2457 www.elsevier.com/locate/apsusc
Characterization and ethylene adsorption of natural and modified clinoptilolites Burcu Erdog˘an *, Meryem Sakızcı, Ertug˘rul Yo¨ru¨kog˘ulları Department of Physics, Science Faculty, Anadolu University, 26470 Eskis¸ehir, Turkey Received 12 March 2007; received in revised form 21 September 2007; accepted 21 September 2007 Available online 29 September 2007
Abstract The ethylene adsorption of Turkey clinoptilolite-rich tuff from Gordes and Bigadic region of western of Anatolia and their exchanged forms (K+, Na+ and Ca2+) were investigated. The clinoptilolite samples were characterized using XRD, TG–DTA and nitrogen adsorption methods. Adsorption isotherms for ethylene on natural and modified forms of both adsorbents at 277 K and 293 K were obtained at pressures up to 38 kPa. Uptake of ethylene increased as Na-CLN < Ca-CLN < K-CLN < Natural CLN for Gordes zeolite at 277 K, 293 K and for Bigadic zeolite at 277 K. For Bigadic zeolites at 293 K, uptake of ethylene increased in the order Ca-CLN < Na-CLN < K-CLN < Natural CLN. It was found that ethylene adsorption capacity of Bigadic clinoptilolite samples was much greater than Gordes clinoptilolite samples except K+ modified forms at both temperatures. These results show that both natural clinoptilolites have a considerable potential for the removal of ethylene. # 2007 Elsevier B.V. All rights reserved. PACS : 82.75.z; 68.43.h; 61.66.Hq; 67.80.Gb; 61.10.Nz Keywords: Zeolites; Adsorption; Ethylene; Thermal properties; XRD
1. Introduction Ethylene (C2H4) is a naturally occurred, simple two carbon gaseous plant growth regulator that has a number of effects on the growth, development and storage periods of many kinds of fruits, vegetables and ornamental crops [1]. Ethylene acts as a plant hormone. It accelerates respiration, leading to maturity and also softening and ripening of many kinds of fruits. Moreover, ethylene accumulation can result in yellowing of green vegetables and may be responsible for numerous types of specific postharvest defects in fresh fruits and vegetables. Although ethylene has some positive effects, it is often hazardous to the quality and shelf-life of fruits and vegetables. To extend shelf-life and keep an acceptable visual and organoleptic quality, accumulation of ethylene in the packaging should be prevented [2]. A lot of C2H4-adsorbing substances such as potassium permanganate (KMnO4), activated carbon and minerals (zeolites and clays) are characterized [3,4]. One of
* Corresponding author. Tel.: +90 222 3350580x5848; fax: +90 222 3204910. E-mail address: [email protected] (B. Erdog˘an). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.058
the most effective materials to remove ethylene is potassium permanganate and its commercial products are accessible. However, because of the chemical properties of potassium permanganate, precautions must be taken in order to prevent contamination of food products. The search for other materials with ethylene removal capability has been focused considerable attention on zeolites. Zeolites are hydrated aluminosilicates of alkali and alkaline earth elements with unique crystal structures consisting of a three-dimensional framework of SiO4 and AlO4 tetrahedral [5]. The isomorphic substitution of Si by Al leads to a negative charge density in the zeolite lattice. This charge is neutralized by introducing exchanged monovalent, divalent, or trivalent cations in the structural sites of the zeolite [6]. The mobile nonframework cations are placed in cavities in the channel walls and coordinated with the water molecules within the channel. Due to their chemical, physical and structural properties, zeolites act as a molecular sieve and also as an ionic exchanger [7]. Occurrences of sedimentary zeolites are widespread in central and western Anatolia, Turkey. They are associated with clay minerals, borates, carbonates and soda minerals similar to many other parts of the world [8].
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457
Tuffs of natural zeolites such as clinoptilolite generally rather pure and abundant, can be mined with simple techniques [9] and their adsorption characteristics can be tailored by ion exchange. Clinoptilolite is one of the most common natural zeolite minerals with a wide range of application. The basic structure of the clinoptilolite crystal contains three types of channels with the ˚ 4.4 A ˚ ), following approximate dimensions: channel A (7.2 A ˚ ˚ ˚ ˚ channel B (4.0 A 5.5 A) and channel C (4.1 A 4.0 A). Channels A and B, which are parallel to each other, intersected by channel C and exchangeable charge-balancing cations (Ca2+, Mg2+, Na+, K+, etc.) are located in these channels [10]. However, the adsorption of ethylene on zeolites has been searched by a number of experimental [11–19] and theoretical [20] methods, studies on adsorption of ethylene based on natural zeolites are very limited in the literature. The purpose of the current study is to investigate the efficiency of two natural zeolites and those of modified forms from Turkey, which mainly consist of clinoptilolite, in the removal of ethylene.
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Table 2 Chemical composition of the Bigadic zeolite Constituent
%
SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO P2O5 H2O
71.81 12.26 0.80 0.50 1.70 3.41 1.37 0.01 8.12
100 ml of 1 M HCl solution followed by a washing in deionized water. Second, the Na+, K+, and Ca2+ ionic forms of two natural minerals were prepared by the Batch method, using 1 M solutions in a shaker at 100 8C for 24 h. Finally, the treated samples were washed with distilled water until the filtrate was free from chloride ions and then dried at room temperature.
2. Experimental
3. Instrumentation
2.1. Materials and reagents
Clinoptilolite-rich minerals from Bigadic and Gordes and those of modified forms were characterized by using XRD, TGA, DTA and nitrogen adsorption analysis techniques. XRD analyses of the samples were performed using Rigaku RINT2200 diffractometer in a scanning range of 5–408 (2u) at a rate of 28 (2u) min1. Cu Ka radiation was used. Monoclinic crystal system was used for the determination of unit-cell parameters (a, b, c, V and b). The values of the unit cell parameters were calculated from the characteristic diffraction reflections of clinoptilolite. TG–DTA curves were measured from 30 8C to 1000 8C at a rate of 10 8C min1 with Setsys Evolution Setaram thermal analysis apparatus. About 40 mg of the sample was used in each run. The water concentration in the samples was determined from the TG curve mass loss. The specific surface areas of the samples were evaluated by the nitrogen gas adsorption method at 196 8C, using a NOVA 2200 Instrument (Quantachrome Instruments, U.S.A.), employing multipoint BET isotherm adsorption data fitting. The adsorption isotherms of ethylene (C2H4) on natural and modified clinoptilolite-rich minerals were determined using automated volumetric equipment (Autosorb 1-Quantachrome Instruments, U.S.A.) at 277 K and 293 K. About 0.1 g of the sample was outgassed in vacuum at 200 8C for 7 h before ethylene adsorption.
Clinoptilolite-rich minerals used in their original and modified forms were obtained from Bigadic and Gordes Locations, in western Turkey. Bigadic and Gordes zeolites were obtained from Bigadic reserves and Incal Co., Izmir, Turkey, respectively. They were crushed into small pieces from rock forms and then powdered in a mortar. After sieved through <125 mm samples collected from under the sieve were used in the ethylene adsorption study. The mineralogical composition of both materials mainly consists of clinoptilolite. The associated minerals for Gordes and Bigadic are k-feldspar and quartz, respectively. The chemical compositions of both samples were determined by classical chemical analysis (Tables 1 and 2). Inorganic chemicals such as HCl, NaCl, KCl and CaCl2 were supplied by Merck (Darmstadt, Germany) and all solutions were prepared using de-ionized water. Ethylene was supplied with the purity of 99.9% by Gerling Holz Company, Germany. 2.2. Modification of original materials by cation exchange First, samples of 5.0 g of clinoptilolite-rich minerals from Gordes and Bigadic regions were activated by washing with Table 1 Chemical composition of the Gordes zeolite Constituent
%
SiO2 Al2O3 Fe2O3 Na2O K2O CaO MgO H2O
74.00 12.25 0.78 0.79 1.20 1.84 1.06 7.55
4. Results and discussion 4.1. X-ray analysis X-ray diffraction patterns of the clinoptilolite samples are shown in (Fig. 1) that modification of clinoptilolite with solutions did not lead to significant structural changes. The quantitative XRD analysis demonstrated that the naturalG.CLN mainly consisted of clinoptilolite with minor contents
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Fig. 1. X-ray powder diffraction patterns of Gordes and Bigadic clinoptilolite samples.
Table 3 XRD quantitative modal analysis of the natural-G.CLN and -B.CLN samples (wt.%) Samples
Clinoptilolite
Feldspar
Quartz
Opal-CT
Mica-illite
Opal-A + volcanic glass
Natural-G.CLN Natural-B.CLN
80 82
6–7 3–4
1–2 12–13
4–5 –
1–2 1–2
4–5 –
of feldspar, quartz, opal-CT, smectite, mica-illite and amorphous material (opal-A + volcanic glass) and the naturalB.CLN consisted of clinoptilolite with minor quantities of feldspar, quartz and mica-illite (Table 3). The method given by Esenli and Sirkeciog˘lu [21] was used to determine the ratio of clinoptilolite. The essential differences between both natural zeolites can be summarized as follows: natural-B.CLN hardly contains amorphous material. Although, natural-G.CLN involves both amorphous material which can be seen by means of hump and that of height between 2u is equal to 208 and 308 (Cu Ka) at XRD diagram and opal-CT that is observed by a ˚ and 4.10 A ˚ flat line with the d-spacing of between 4.05 A (cristobalite-tridimite transition). Natural-B.CLN includes quartz evidently, whereas natural-G.CLN hardly contains it. Amount of feldspar in samples of natural-G.CLN is much greater than that of in natural-B.CLN. The X-ray diffraction diagrams of the clinoptilolite samples given in Fig. 1 show characteristic clinoptilolite peaks [22] at 2u (Cu Ka) = 9.878, 22.48 and 308, respectively. In addition, the unit-cell parameters of the zeolitic samples are presented in Table 4. The calculated unit-cell parameters of natural and modified forms of the
samples do not show significant change. The dimension of c parameter is longer and therefore V is larger in the Bigadic clinoptilolite than that of the Go¨rdes clinoptilolite. On the other hand, a distinctive data are not found in between the parameters of cationic forms. The only following characteristics are defined: The effect of Na on the unit cell parameters of the Go¨rdes and Bigadic samples is different, K causes get lower the b value in Bigadic sample and Ca is ineffective both on two clinoptilolites. Table 4 Unit-cell parameters and volumes of the natural and those of modified forms of zeolites from Gordes and Bigadic Samples
a (nm)
b (nm)
c (nm)
b (8)
V (nm)3
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
1.7528 1.7870 1.7722 1.7709 1.7592 1.7250 1.7647 1.7672
1.7955 1.7900 1.7904 1.7904 1.7859 1.7904 1.7006 1.7932
0.7388 0.7416 0.7392 0.7319 0.7532 0.7229 0.7379 0.7392
115.6 116.5 115.5 114.7 114.0 113.4 116.5 116.1
2.096 2.122 2.116 2.108 2.161 2.049 2.098 2.103
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4.2. Thermal properties Thermal behavior of zeolites can be investigated by means of TG/DTA analysis. The existence of various exchangeable cations in zeolites leads to changes in temperature of zeolitic water elimination, which is held in a few stages [23]. TG/ DTA curves of all forms of clinoptilolite samples are shown in Fig. 2.
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The DTA curves of natural and Na+ forms of both sorbents display a single endotherm at temperatures ranging from 125 8C to 135 8C as a result of a single-step dehydration process. Endotherm minima temperatures increase in the sequence: Natural-G:CLNð125 CÞ ! Natural-B:CLNð128 CÞ ! Na-G:CLNð133 CÞ ! Na-B:CLNð135 CÞ
Fig. 2. TG/DTA curves of all forms of Gordes and Bigadic clinoptilolite samples.
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The DTA curves of Ca-G.CLN, K-G.CLN and K-B.CLN are characterized by two endotherms, related to a two-step dehydration process. The endotherms range between 118 8C and 164 8C (first reaction). Endotherm minima temperatures increase in the sequence: K-B:CLNð118 CÞ ! K-G:CLNð129 CÞ ! Ca-G:CLNð164 CÞ: The second endotherm, occurring at 424 8C (Ca-G.CLN), 442 8C (K-B.CLN) and 455 8C (K-G.CLN), increases in temperature following a different sequence. The DTA curve of Ca-B.CLN gives three peaks at 121 8C, 161 8C and 409 8C, as a result of a three-step dehydration process. A small endothermic peak appears at 774 8C, 881 8C and 725 8C, respectively, in the DTA curves of K-B.CLN, NaB.CLN and Ca-G.CLN, accompanying the weight loss shown on the DTA curve. Collapse of the zeolite structure happens at this temperature. As follows from the DTA curves (Fig. 2) in the case of zeolite samples, endothermic peaks connected with elimination of physically adsorbed water are observed up to 200 8C. Table 5 reports the total weight loss for all cation forms determined by TG analysis. The zeolite water loss is higher in the zeolites exchanged with bivalent ion. In addition, in the monovalent ions, the larger cations have less zeolite water (Table 5). Water loss continued smoothly for all samples from 30 8C to 500 8C. Most of the physisorbed water is lost between 30 8C and 200 8C and in the broad interval between 200 8C and 500 8C more strongly associated water is lost. Dehydroxylation is observed with further water loss at the higher temperatures.
Table 6 Calculated surface areas of natural and modified natural zeolite samples Sample
BET surface area (m2/g)
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
60 54 96 29 47 51 73 28
Fig. 3. N2 adsorption isotherms of natural and modified Bigadic clinoptilolite samples.
4.3. Specific surface area Zeolite samples were outgassed at 180 8C for 4 h under vacuum (103 Torr) before N2 adsorption. The surface areas of the samples are listed in Table 6. Adsorption isotherms of the samples are of type II in the IUPAC classification [24] (Figs. 3 and 4). 4.4. Adsorption Adsorption isotherms for ethylene on natural and modified forms of both adsorbents at 277 K and 293 K were obtained at pressures up to 38 kPa. The adsorption isotherms (absolute amount adsorbed per gram of adsorbent) of C2H4 on all the samples are shown in Figs. 5–12. The curves are similar in
Fig. 4. N2 adsorption isotherms of natural and modified Gordes clinoptilolite samples.
Table 5 The results of thermal analysis of natural and modified natural zeolite samples Sample
Weight loss (wt.%)
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
11.98 14.76 14.42 18.68 11.72 12.94 12.75 24.87
Fig. 5. Adsorption of ethylene on Gordes zeolite at 277 K.
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457
Fig. 6. Adsorption of ethylene on Gordes zeolite at 293 K.
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Fig. 10. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite at 277 K and 293 K.
Fig. 7. Adsorption of ethylene on Bigadic zeolite at 277 K. Fig. 11. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite and Bigadic zeolite at 277 K.
Fig. 8. Adsorption of ethylene on Bigadic zeolite at 293 K.
shape and have a classic isotherm form. As the temperature increases, the total amount adsorbed decreases as expected (Figs. 9 and 10). Reproducibility of the experiments was checked by repeating some experiments in identical conditions
Fig. 9. Adsorption of ethylene on natural Na, K, Ca cationic forms of Bigadic zeolite at 277 K and 293 K.
and confirming that a good replication was obtained. The absolute amounts adsorbed per gram of Gordes and Bigadic zeolites are given in Table 7. Natural forms of both clinoptilolite zeolites showed much higher ethylene adsorption amount compared to that of ˚ ) can pass modified forms. Ethylene (kinetic diameter 3.9 A through both clinoptilolite zeolite pore openings. It is observed that uptake of ethylene on Gordes zeolite at 277 K and 293 K increases in the following sequence (Figs. 5 and 6): Na-G:CLN < Ca-G:CLN < K-G:CLN < Natural-G:CLN Although, the sequence at 277 K for Bigadic zeolite is the same as determined above, adsorption amount of ethylene on
Fig. 12. Adsorption of ethylene on natural Na, K, Ca cationic forms of Gordes zeolite and Bigadic zeolite at 293 K.
B. Erdog˘an et al. / Applied Surface Science 254 (2008) 2450–2457
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Table 7 The absolute amounts adsorbed per gram of adsorbent Sample
Natural-G.CLN Na-G.CLN K-G.CLN Ca-G.CLN Natural-B.CLN Na-B.CLN K-B.CLN Ca-B.CLN
Amount adsorbed (mmol/g zeolite) 277 (K)
293 (K)
0.966 0.109 0.758 0.422 1.492 0.370 0.685 0.611
0.956 0.069 0.719 0.226 1.233 0.291 0.482 0.259
Bigadic zeolite at 293 K increases in the following order (Figs. 7 and 8): Ca-B:CLN < Na-B:CLN < K-B:CLN < Natural-B:CLN Among all the modified zeolites, it was found that the K+ forms of both clinoptilolite samples had the most ethylene adsorption capacity. The difference in adsorption capacities is most probably due to the strength of adsorption sites. Electronegativity value of potassium is lower than those of sodium and calcium. In addition, potassium has the biggest atomic diameter compared to sodium and calcium [25]. Hence, interaction between K+ form and ethylene should be much more regarding the other forms of zeolite. It was figured out that uptake of ethylene was generally greater for the exchanged zeolite containing Ca2+ ion than for Na+ ion. For both zeolites exchanged with divalent cation like Ca2+, a higher adsorption capacity could be expected than that with Na+ because two monovalent cations are replaced by a single divalent cation and the ionic radii of calcium are not greater than that of sodium. Thus, the net volume occupancy by these divalent cations would be less than one half as with sodium. Divalent cations in zeolites are known to be strong adsorptive centers, and ethylene has a strong quadropole moment. The interaction of the divalent Ca2+ ions with ethylene double bond (p-bond) is likely to be the reason for the strong adsorption [15] on Gordes and Bigadic clinoptilolite zeolites. Various cation-exchanged forms of clinoptilolite samples may lead to significant differences in the adsorption of ethylene due to both the location and size of the interchangeable cations which affect the local electrostatic field, and the polarization of the adsorbates. As seen in Figs. 5–12 ethylene adsorption exhibits the isotherm of type I. 5. Conclusion In this study, characterization of clinoptilolite samples (XRD, TG–DTA and N2 adsorption methods) and the efficiency of those of samples in the removal of ethylene have been investigated. X-ray analysis of the zeolite samples have shown that the modification of clinoptilolites with solution does not lead to significant structural changes. The thermal behavior of clinoptilolite samples is influenced by clinoptilolite cationic form. Some differences attributed to the different structure of the two clinoptilolite minerals are
observed between the temperature of dehydration of the Gordes and Bigadic zeolites. Isotherms for the adsorption of ethylene on clinoptilolite samples have been obtained at 277 K (refrigerator temperature) and 293 K (room temperature) in the pressure range 0.004– 38 kPa. It is found that ethylene-adsorption capacity of Bigadic clinoptilolite samples is much greater than Gordes clinoptilolite samples except K+ modified forms at both temperatures (Figs. 11 and 12). Among all the modified zeolites, it is found that the K+ forms of both clinoptilolite samples have the most ethylene adsorption capacity. It is shown that the uptake of ethylene is generally greater for the exchanged zeolite containing Ca2+ ion than for Na+ ion. In addition, it is found that the affinity of the natural Gordes and Bigadic zeolite surfaces to ethylene is greater than those of modified forms. In conclusion, compared with other ethylene-adsorbing substances (potassium permanganate, activated carbon and synthetic zeolites), the natural zeolites are much cheaper, more abundant and more available. Removal of ethylene via natural zeolites retards decaying. For that reason, storage life of fruit and vegetables could be prolonged. Acknowledgments Financial support by Eskisehir-Anadolu University via Scientific Research Projects (No. AUAF 041040) is acknowledged. Special thanks to Fahri Esenli and Matthias Thommes for their helpful suggestions and corrections. References [1] M.E. Salveit, Effect of ethylene on quality of fresh fruits and vegetables, Postharvest Biol. Tech. 15 (1999) 279–292. [2] L. Vermeiren, F. Devlieghere, M. Van Beest, N. de Kruijf, J. Debevere, Developments in the active packaging of foods, Trends Food Sci. Technol. 10 (3) (1999) 77–86. [3] K. Abe, A.E. Watada, Ethylene adsorbent to maintain quality of lightly processed fruits and vegetables, J. Food Sci. 56 (6) (1991) 1589–1592. [4] T. Suslow, Performance of zeolite based products in ethylene removal, Perishables Handling Quarterly 92 (1997) 32–33. [5] A. Dyer, An Introduction to Zeolite Molecular Sieves, John Willey and Sons Press, 1988. [6] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1984. [7] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978. [8] R. Birsoy, Activity diagrams of zeolites: implications for the occurrences of zeolites in Turkey and of Erionite worldwide, Clay Clay Miner. 50 (2002) 136–144. [9] T. Armbruster, Clinoptilolite–heulandite: applications and basic research, Stud. Surf. Sci. Catal. 135 (2001) 13–27. [10] M.W. Ackley, Clinoptilolite: untapped potential for kinetic gas separations, Zeolites 12 (1992) 780–788. [11] S. Hyun, R.P. Danner, Equilibrium adsorption of ethane, ethylene, isobutene, carbon dioxide and their binary mixtures on 13X molecular sieves, J. Chem. Eng. Data 27 (2) (1982) 196–200. [12] E. Costa, G. Calleja, A. Jimenez, J. Pau, Adsorption equilibrium of ethylene, propane, propylene, carbon dioxide and their mixtures on 13X zeolite, J. Chem. Eng. Data 36 (1991) 218–224. [13] G. Calleja, P.L. Dominguez, P. Perez, Multicomponent adsorption equilibrium of ethylene, propane, propylene and CO2 on 13X zeolite, Gas Sep. Purif. 8 (4) (1994) 247–256.
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