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Examination of the adsorption of hydrocarbons at low coverage on faujasite zeolites
ELSF\rIER
Solid State Ionics 101-103
(1997) 799-805
Examination of the adsorption of hydrocarbons at low coverage on faujasite zeolites Chr. Papaioannoua’*,
G. Petroutsos”, W. GunBerb
“National Technical Universiry of Athens, Department of Chemical Engineering, Zografou Campus, 15773 Athens, Greece blJniversity of Hamburg, Institute of Physical Chemistry, Bundesstr. 45, 20146 Hamburg, Germany
Abstract The adsorption of benzene, cyclohexane, cyclohexene and methylcyclopentane on X and Y zeolites at 573 and 673 K was investigated by gas chromatography. The zeolites contained Naf and Ni*+ ions or nickel particles. The adsorption parameters were determined by fitting the Langmuir equation to the respective isotherms. Keywords:
Faujasite
zeolites; Adsorption;
Materials:
Benzene;
Cyclohexane;
Hydrocarbons;
Cyclohexene;
Nickel particles
Methylcyclopentane;
Zeolite X; Zeolite Y
1. Introduction In order to understand the catalytic properties of zeolites sufficient information about their adsorption activities with regard to the adsorbing species is required. It has been shown that the charge condensing cations constitute adsorption sites [ 1,2]. In reduced Ni zeolites, the surface of the metallic nickel particles contributes to the adsorption process. By adsorption of saturated and unsaturated hydrocarbons it can be distinguished between specific and nonspecific interactions and the adsorption centres [2,3]. It is interesting to compare the results of benzene, cyclohexane, cyclohexene and methylcyclopentane adsorption on faujasite zeolites, because these hydro-
carbons are interfered with in the catalytic hydrogenation of benzene. Gas-solid chromatographic technique was used for this examination, since it is a highly sensitive and practical method for adsorption measurements [4].
2. Experimental The chemical composition of the used zeolites is summarized in Table 1. N&,X and Ni,,Y zeolites
Table 1 Composition Chemical
of used zeolites
formula
Na,,(AlO,),,(Si0,),,;252H,O *Corresponding author. Present address: Joint Research Centre, European Institute for Transuranium Elements, PO. Box 2340, 76125 Karlsruhe, Germany; fax: +49 7247 951593. 0167-2738/97/$17.00 0 1997 Elsevier PII SO167-2738(97)00184-7
Science B.V. All rights reserved.
Ni,,,,Na,, ,(AlO,),,(SiO,),,;299H,O Na,,(AlO,),,(Si0,),,;234H,O Ni,,Na,,(A10,),,(Si0,),,;238H,O
Si/Al
Symbol
1.29 1.29 2.50 2.50
NaX N&X NaY Ni,,Y
C. Papaioannou et al. / Solid State Ionics 101-103
800
were prepared from NaX and NaY zeolites by ion exchange [5,6]. The measurements were carried out by means of a TCD equipped gas chromatograph. A 2 g zeolite sample with a diameter of 0.3-0.5 mm was placed in one of the chromatograph columns and dehydrated by a He stream with a flow rate of 24 ml/min, first at 723 K for 4 h and then at 673 K for 10 h. The N&,X and Ni,,Y zeolites were reduced by H, (flow rate 58 ml/min) for 2 h at 673 K. Benzene, cyclohexane, cyclohexene and methylcyclopentane pulses (max. 10 pl) were injected into the column at temperature 673 or 573 K using He as carrier gas (flow rate 24 ml/min) and the elution curves were recorded. Detailed procedure for the pulse technique has been given by Cremer et al. [4].
(1997) 799-805
separated peaks. For the simple peak elution curves the isotherms a =f(P) (where a is the adsorbed moles per gram of adsorbent and P the partial pressure in atm) has been derived using the relations corrected for diffusion from Cremer et al. [4]. It was not possible to calculate adsorption isotherms if the elution curves consisted of many peaks. These cases are shown in Table 2. 3.2. The adsorption
It has been attempted to fit the Freundlich and Langmuir equation to the adsorption isotherms. Since the Freundlich equation did not represent the isotherms satisfactorily, it may be concluded that the adsorption is not of this type. However, the Langmuir equation:
3. Results
a=G.b.Pl(l 3.1. The e&ion
+b*P),
(1)
curves
where G is the maximum adsorption capacity and b the adsorption constant, is easily fitted to the adsorption isotherms (Figs. l-4). In the initial region
The elution curves of the investigated systems consisted either of one peak or more not well
Table 2 The maximum
isotherms
adsorption
capacities
G and the adsorption
constants
b of the Langmuir
equation
and the Henry constants
K
Adsorbent zeolite X
G (mm01 g-‘)
b (atm-‘)
K (mm01 g-’ atm-‘)
Adsorbent zeolite Y
G (mm01 g-‘)
b (atm- ‘)
K (mm01 g-’ atm-‘)
673 673 673 573
NaX
N&X red.Ni,,X red.Ni,,X
0.506 0.199 0.164 0.326
18.46 13.09 13.92 47.43
9.33 2.62 2.28 15.46
NaY Ni,,Y red.Ni,,Y red.Ni ,4Y
0.492 0.265 0.237 0.608
4.21 7.69 1.24 16.85
2.07 2.04 1.71 10.24
Cyclohexane
673 673 673 573
NaX N&,X red.Ni,,X red.Ni,,X
0.219 0.189 0.84 0.199
5.06 3.62 8.83 13.01
1.11 0.68 0.74 2.60
NaY Ni,,Y red.Ni,,Y red.Ni,,Y
0.087 0.086 a
5.86 5.32
0.51 0.46
0.262
6.02
1.58
Methylcyclopentane
673 673 573 673 573
NaX N&,X
0.191 0.091
5.73 7.04
1.09 0.64
0.072
6.56
0.47
0.094
red.Ni,,X red.Ni,,X
0.087 0.164
8.27 15.42
0.72 2.53
NaY Ni,,Y Ni,,Y red.Ni 14Y red.Ni ,4Y
16.50 a
1.56 a
0.245
6.22
1.52
673 673 513 673 573
NaX N&,X N&X red.Ni,,X red.Ni,,X
0.447 r a a a
5.72 a
2 56
NaY Ni,,Y Ni,,Y red.Ni ,4Y red.Ni 14Y
0.126 a
6.95 a
0.87
Adsorbate
T WI
Benzene
Cyclohexene
a Evaluation
not possible
due to conversion
a
a
a
a
C. Papaioannou
et al. I Solid State Ionics 101-103
(1997) 799-805
0.20 G-
0 0.00
0.00 0.00
0.04
0.08
0.12
P /
0.16
0.20
atm
P /
Fig. 1. Adsorption isotherms of benzene (1, 5), cyclohexaae (2, 6), methylcyclopentane (3, 7) and cyclohexene (4, 8) on zeolites NaX and NaY respectively measured at 673 K.
atm
Fig. 3. Adsorption isotherms of cyclohexane on N&,X (I), on Ni,,Y (2) and on red.Ni,,X (3) measured at 673 K and on red.Ni,,X (4) and red.Ni,,Y (5) measured at 573 K.
0.15
7
cn
0
E E
0.10
\ 0
0.06
0.09
P /
0.12
0.
atm
P /
atm
Fig. 2. Adsorption isotherms of benzene on N&,X (l), on Ni,,Y (2), on red.Ni,,X (3) and on red.Ni,,Y (4) measured at 673 K and on red.Ni,,X (5) and red.Ni,,Y (6) measured at 573 K.
Fig. 4. Adsorption isotherms of methylcyclopentane on N&,X (1) and on red.Ni,,X (2) measured at 673 K and on red.Ni,,X (3), red.Ni,,Y (4) and Ni,,Y (5) measured at 573 K.
of the isotherms since b-P ++z1 the Langmuir tion is close to the Henry equation:
4. Discussion
a=G*b.P=K-P.
equa-
(2)
K is the Henry constant characterizing the adsorbate-adsorbent interaction. The adsorption becomes stronger as K increases [7,8]. The derived values of G, b and K are shown in Table 2.
4.1. Fitting the Langmuir adsorption isotherms
equation
to the
The good fitting of the Langmuir model to the isotherms was not expected, since zeolites have energetically an inhomogeneous surface. Neverthe-
802
C. Papaioannou et al. I Solid State Ionics 101-103
less, there have been reported cases of adsorption isotherms on zeolites satisfactorily described by the Langmuir equation [9- 111. The satisfactory description of adsorption isotherms by the Langmuir equation means that the molecules are adsorbed in energetically equivalent adsorption sites. Consequently, the adsorption energy is constant and the distances between the adsorbed molecules are sufficiently far apart so that no interaction takes place. Because of the size of the molecules, only the adsorption sites in the supercages are accessible to them. Expressed in molecules per supercage (Figs. l-4), the adsorbate loading was smaller than one molecule per supercage. The distances between the adsorbed molecules are therefore sufficiently far apart and their interaction can be neglected. However, while in Na zeolites this presupposition is satisfied (only Na+ in the supercages), in Ni zeolites there are more types of adsorption sites (Na+, Ni*+, protons). At 673 K the maximum adsorption capacities G are higher for Na zeolites, lower for Ni zeolites and the lowest for reduced Ni zeolites (Table 2). It should also be noted that the maximum adsorption capacities G for the reduced Ni zeolites are higher at 573 K than at 673 K. A basic assumption of the Langmuir model, however, is that the molecules are adsorbed at a fixed number of well defined localized sites and therefore the saturation adsorption capacity should be temperature independent [ 121. Therefore, adsorption in the systems examined does not strictly follow the Langmuir adsorption model. It should also be mentioned that in the case where the adsorbent has small pores which define an effective surface, further adsorption is prevented due to the filling of these pores and the adsorbate behaves in a manner resembling the Langmuir model [ 131. Borovkow et al. [ 141 found that in those zeolites where the micropores are channels between the supercages, the presence of as many as four adsorbed benzene molecules per supercages still does not prevent diffusion between neighbouring supercages. Since in our case there is less than one adsorbed molecule per supercage, the Langmuir behaviour of the examined adsobate-adsorbent systems cannot be due to this preventing of the adsorption to inner supercages.
(1997) 799-805
In the adsorption of benzene, cyclohexane and methylcyclopentane on reduced Ni,,X and of benzene on reduced Ni,,Y, the lower maximum adsorption capacity G at 673 K compared to that at 573 K shows a decrease of the available adsorption sites with temperature. Since at its different maximum adsorption capacities, each system at the above two temperatures is equivalent with two different systems. Therefore the Clausius-Clapeyron equation Qiso = R . ln(P, lP,) . T, *T2 l(T, - T,)
(3)
may not give a real relationship between the isosteric heat of adsorption Qiso and the coverage a. The application of this equation gave an increase in the isosteric heat with coverage (Fig. 5). Of course, the physisorption Langmuir model is not sufficient to describe this behaviour. Such results can usually be seen in adsorbent-adsorbate systems where chemisorption appears [ 151. At temperatures of 573 and 673 K the adsorption in our systems is therefore probably chemical. In NaX and NaY zeolites the Na+ ions are adsorption sites. The decrease of the maximum adsorption capacities G follows the sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane, showing that there is an energetic scatter of the Na+ ions in the supercages.
0 /
mm01
g-’
Fig. 5. Isosteric adsorption heats of benzene (l), cyclohexane (2) and methylcyclopentane (3) on red.Ni,,X and of benzene on red.Ni,,Y (4) which result from the application of the ClausiusClapeyron equation.
C. Papaioannouet al. I Solid State Ionics 101-103
The adsorption sites in Ni zeolite, are Na+, Ni2+ and protons; in reduced Ni zeolites [ 16- 191 adsorption sites are additionally the surface molecules of the nickel particles. At 673 K the maximum adsorption capacity G is getting generally lower values for reduced Ni zeolites than for Ni zeolites (Table 2). This shows that at such a high temperature chemisorption takes place not only on Naf but also . Lower values of the maximum adon Ni” ions. sorption capacity in reduced Ni zeolites are obviously connected to the loss of energetically capable Ni*+ adsorption sites, because of the formation of metallic nickel particles, where adsorption is possible only on the surface Ni atoms, whereas the protons that have replaced the Ni*+ ions are probably not able to adsorb at these temperatures. At 673 K the maximum adsorption capacity G is lower for Ni zeolites than for Na zeolites. This is directly connected to the loss of energetically caions are pable adsorption sites, since two Naf replaced by one Ni2+ ion, and it also shows that adsorption on the protons probably does not take place at this temperature. At 573 K the proton ability to participate in the adsorption process can explain the considerable increase of the maximum adsorption capacity G on reduced Ni zeolites (Table 2). Choudhary et al. [20] report that at 523 K benzene interacts more strongly with Na’ ions than with protons. They attribute this to the fact that benzene is a weak base and therefore its interaction with protons is weaker compared with the interaction of the r benzene ring electrons with Na+ cations. Therefore, it is not unlikely that if the temperature increases, the interaction of benzene molecules with protons becomes essentially negligible, especially when the temperature exceeds the critical temperature of benzene (562 K) and the adsorption is not physical any longer. 4.2. Comparison
of the Henry constants K
A comparison of the Henry constants K shows that at 623 K benzene, cyclohexane and methylcyclopentane are more strongly adsorbed on NaX than on N&,X or on reduced Ni,,X. On NaX the adsorption strength decreases in the following sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane. The same adsorption strength order ap-
(1997)
799-805
803
peares in N&,X. In Nay, Ni,,Y and reduced Ni,,Y the adsorption strength of benzene is almost the same at 673 K. In NaY there is the same order as in NaX, namely: benzene > cyclohexene > cyclohexane = methylcyclopentane. The interaction between the adsorbed molecules on the charge condensing cations is of an electrostatic nature [ 131. The polar molecules are generally more strongly adsorbed than the non-polar ones. The adsorption strength of non-polar molecules is modified by their electric polarizability. It is expected that cyclohexene, with a permanent electric moment of 0.61 X lo-’ g”* cm5’* s-’ [21], is more strongly adsorbed on Na zeolites than cyclohexane or methylcyclopentane, which have no permanent electric moment. Benzene, however, although it has not got a permanent electric moment [24], does not follow this rule and is adsorbed more strongly than cyclohexene. This stronger adsorption of benzene molecules proves the contribution of the special interaction between the adsorption sites Na+ and the rr electrons of the benzene ring [7]. In Ni zeolites the Ni*+ ions, the Na+ ions and the acid sites are adsorption sites. If, however, the Ni*+ ions which are stronger adsorption sites than the Na+ ions, are ‘protected’ by OH- groups produced during hydrolysis [22,23], then they are not suitable for the adsorption process [24]. Since benzene is not adsorbed more strongly on Ni zeolites than on Na zeolites, taking into account that it is a weak base (and consequently its chemical interaction with protons is weaker than the interaction between its 7~ electrons and the Na+ cations [20]), it is concluded that the Ni*+ ions of the Ni zeolites are ‘protected’ by OH- groups. The higher constant K for the benzene-NaX system compared with the benzeneNi,,X system indicates weaker adsorption of the benzene molecules on the Na+ ions of the Ni,,X zeolite than on the Naf ions of the NaX zeolite. Benzene molecules are adsorbed in a similar way on the Na+ ions of the corresponding Y zeolites. Benzene is not adsorbed more strongly on reduced zeolites containing nickel metal particles than on the non-reduced ones. This means that the adsorption of benzene on the surface of the nickel metal particles is approximately as strong as on Na+ ions. Therefore, the specific interaction between ZF electrons of the benzene ring, ionic adsorption sites and surface
804
C. Papaioannou
et al. I Solid State Ionics 101-103
nickel atoms of the metal particles is almost the same. At 623 K the strength of adsorption of cyclohexane and methylcyclopentane on N&,X and reduced N&,X and of cyclohexane on Ni,,Y, is approximately the same as on the corresponding Na zeolites. This shows - taking into consideration the decrease in the adsorption ability of Ni2+ ions with the presence of OH- groups - that these molecules are adsorbed equally strongly on Na+ ions, protons and on the nickel metal particles surface of the reduced zeolites. The weaker adsorption of the investigated substances on the Y compared with the respective X zeolites, shows that the aluminium content of the zeolites modifies the energetic state of the adsorption sites. 4.3. The complicated
elution chromatograms
In Table 2 there are also shown the adsorbateadsorbent systems with elution chromatograms including complicated peaks. Such a chromatogram indicates that although only one substance is injected, several substances emerged from the zeolite column. These substances were formed by reactions, such as isomerization, cracking or oligomerization, catalyzed by the zeolite itself and favoured at higher temperatures. This phenomenon took place only in Ni zeolites. The divalent Ni2+ ions with a magnitude e/r-=2.56 (formal charge over radius in A) [25] generate a stronger electrostatic field in the zeolite system than the monovalent Na+ ions with a magnitude e/r= 1.03 [25], as well as a Bronsted acidity [22,26]. The strong electrostatic field or the Bronsted acidity seems to be necessary for the carbonionic reactions. Whereas this phenomenon takes place in the case of cyclohexene - an unsaturated substance sensitive to reactions - in NIX zeolites, in NiY zeolites where the electrostatic field is stronger and the Bronsted acidity higher [23,26] this phenomenon also takes place in the case of cyclohexane and methylcyclopentane.
5. Conclusion The adsorption of benzene, cyclohexene and methylcyclopentane
cyclohexane, on Na or Ni
(1997) 799-805
zeolites and on reduced Ni zeolites at 673 and 573 K is of a chemical nature (chemisorption). The adsorption strength on Na zeolites decreases according to the following sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane. The strong adsorption of benzene could be due to the special interaction between IT electrons of the benzene ring and Na+ adsorption sites. Cyclohexene as a polar molecule is more strongly adsorbed on NaC ions than cyclohexane or methylcyclopentane. The above adsorption strength sequence is also observed in Ni zeolites and reduced Ni zeolites. Because of the Ni2+ ions, the considerable acidity of the zeolite carrier favours reactions (oligomerization, cracking etc.) of the adsorbates, especially of the unsaturated ones. This phenomenon appears more often on Y zeolites where the acidity is higher and is not controlled by the adsorption strength which is greater on the X than on the Y zeolites.
Acknowledgements
Financial support by ‘Intemationales Btiro Jiilich’ (Germany) and by “General Secretariat of Research and Technology’ (Greece) is gratefully acknowledged. The authors thank Prof. F. Schmidt (Siid Chemie, Miinchen) for his assistance in preparing this manuscript.
References [l] M.M. Dubinin, VA. Astakhov, Adv. Chem. Ser. 102 (1971) 69. [2] T.A. Egerton, F.S. Stone, J. Coil. Inter. Sci. 38 (1972) 195. [3] B.G. Aristov,V Bosacek, A.V. Kiselev, Trans. Farad. Sot. 63 (1967) 2057. [4] E. Cremer, H. Huber, Angew. Chem. 73 (1961) 461. [5] R.A. Schoonheydt, L.J. Vardamme, P.A. Jacobs, J.B. Uytterhoeven, J. Catal. 43 (1976) 292. [6] S. Lomic, L.J. Gal, Croat. Chem. Acta 44 (1972) 403. [7] D. Barthomeuf, B.-H. Ha, J. Chem. Sot. Farad. Trans. I 69 (1973) 2147. [8] A.V. Kiselev, Adv. Chem. Ser. 102 (1971) 37. [9] V.R. Choudhary, K.R. Srinivasan, J. Catal. 102 (1986) 289. [lo] G. Peters, Dissertation, University of Hamburg, 1987. [ 1 l] W. Schweitzer, Dissertation, University of Hamburg, 1983.
C. Papaioannou et al. I Solid State Ionics 101-103 [12] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, 1984, pp. 49-50. [ 131 K. Hauffe, S.R. Morrison, Adsorption - Eine Einfiihrung in die Probleme der Adsorption, Berlin-New York, 1974. [14] VYu. Borovkow, W. Keith-Hall, VB. Kazansky, J. Catal. 51 (1978) 437. [15] A.A. Huang, J. Zwiebel, Adv. Chem. Ser. 121 (1973) 374. [ 161 M. Suzuki, K. Tsutsumi, H. Takahashi, Zeolites 2 ( 1982) 5 1. [17] P.A. Jacobs, St. Surf. Sci. Catal. 12 (1982) 71. [18] F. Schmidt, H. Kacirec, W. Gunsser, Ch. Minchev, V Kanazirev, L. Kosova, V Penchev, in: Proc. of the 4th Conf. on Catal., Varna, 1979, p. 331. [19] M. Kermarec, D. Olivier, M. Richard, M. Che, J. Phys. Chem. 86 (1982) 2818.
[20] [21] [22] [23]
(1997) 799-805
80.5
VR. Choudhary, K.R. Srinivasan, J. Catal. 102 (1986) 328. Landolt-Bornstein, 6. Auf. Bd. B/9, Springer-Verlag, 1970. J.W. Ward, J. Catal. 10 (1968) 34. J.A. Rabo (Ed.), Zeolite Chemistry and Catalysis, Washington, 1976. [24] G. Petroutsos, Catalysis on Metal Oxides, National Technical University of Athens, Dept. of Chem. Engineering, 1982. [25] H. Noller, Acta Chim. (Academiae Scientiarum Hungaricae) 109 (1982) 429. [26] J.A. Rabo, C.L. Angel], P.H. Kasai, V. Schoemaker. Disc. Farad. Sot. 41 (1966) 328.
Solid State Ionics 101-103
(1997) 799-805
Examination of the adsorption of hydrocarbons at low coverage on faujasite zeolites Chr. Papaioannoua’*,
G. Petroutsos”, W. GunBerb
“National Technical Universiry of Athens, Department of Chemical Engineering, Zografou Campus, 15773 Athens, Greece blJniversity of Hamburg, Institute of Physical Chemistry, Bundesstr. 45, 20146 Hamburg, Germany
Abstract The adsorption of benzene, cyclohexane, cyclohexene and methylcyclopentane on X and Y zeolites at 573 and 673 K was investigated by gas chromatography. The zeolites contained Naf and Ni*+ ions or nickel particles. The adsorption parameters were determined by fitting the Langmuir equation to the respective isotherms. Keywords:
Faujasite
zeolites; Adsorption;
Materials:
Benzene;
Cyclohexane;
Hydrocarbons;
Cyclohexene;
Nickel particles
Methylcyclopentane;
Zeolite X; Zeolite Y
1. Introduction In order to understand the catalytic properties of zeolites sufficient information about their adsorption activities with regard to the adsorbing species is required. It has been shown that the charge condensing cations constitute adsorption sites [ 1,2]. In reduced Ni zeolites, the surface of the metallic nickel particles contributes to the adsorption process. By adsorption of saturated and unsaturated hydrocarbons it can be distinguished between specific and nonspecific interactions and the adsorption centres [2,3]. It is interesting to compare the results of benzene, cyclohexane, cyclohexene and methylcyclopentane adsorption on faujasite zeolites, because these hydro-
carbons are interfered with in the catalytic hydrogenation of benzene. Gas-solid chromatographic technique was used for this examination, since it is a highly sensitive and practical method for adsorption measurements [4].
2. Experimental The chemical composition of the used zeolites is summarized in Table 1. N&,X and Ni,,Y zeolites
Table 1 Composition Chemical
of used zeolites
formula
Na,,(AlO,),,(Si0,),,;252H,O *Corresponding author. Present address: Joint Research Centre, European Institute for Transuranium Elements, PO. Box 2340, 76125 Karlsruhe, Germany; fax: +49 7247 951593. 0167-2738/97/$17.00 0 1997 Elsevier PII SO167-2738(97)00184-7
Science B.V. All rights reserved.
Ni,,,,Na,, ,(AlO,),,(SiO,),,;299H,O Na,,(AlO,),,(Si0,),,;234H,O Ni,,Na,,(A10,),,(Si0,),,;238H,O
Si/Al
Symbol
1.29 1.29 2.50 2.50
NaX N&X NaY Ni,,Y
C. Papaioannou et al. / Solid State Ionics 101-103
800
were prepared from NaX and NaY zeolites by ion exchange [5,6]. The measurements were carried out by means of a TCD equipped gas chromatograph. A 2 g zeolite sample with a diameter of 0.3-0.5 mm was placed in one of the chromatograph columns and dehydrated by a He stream with a flow rate of 24 ml/min, first at 723 K for 4 h and then at 673 K for 10 h. The N&,X and Ni,,Y zeolites were reduced by H, (flow rate 58 ml/min) for 2 h at 673 K. Benzene, cyclohexane, cyclohexene and methylcyclopentane pulses (max. 10 pl) were injected into the column at temperature 673 or 573 K using He as carrier gas (flow rate 24 ml/min) and the elution curves were recorded. Detailed procedure for the pulse technique has been given by Cremer et al. [4].
(1997) 799-805
separated peaks. For the simple peak elution curves the isotherms a =f(P) (where a is the adsorbed moles per gram of adsorbent and P the partial pressure in atm) has been derived using the relations corrected for diffusion from Cremer et al. [4]. It was not possible to calculate adsorption isotherms if the elution curves consisted of many peaks. These cases are shown in Table 2. 3.2. The adsorption
It has been attempted to fit the Freundlich and Langmuir equation to the adsorption isotherms. Since the Freundlich equation did not represent the isotherms satisfactorily, it may be concluded that the adsorption is not of this type. However, the Langmuir equation:
3. Results
a=G.b.Pl(l 3.1. The e&ion
+b*P),
(1)
curves
where G is the maximum adsorption capacity and b the adsorption constant, is easily fitted to the adsorption isotherms (Figs. l-4). In the initial region
The elution curves of the investigated systems consisted either of one peak or more not well
Table 2 The maximum
isotherms
adsorption
capacities
G and the adsorption
constants
b of the Langmuir
equation
and the Henry constants
K
Adsorbent zeolite X
G (mm01 g-‘)
b (atm-‘)
K (mm01 g-’ atm-‘)
Adsorbent zeolite Y
G (mm01 g-‘)
b (atm- ‘)
K (mm01 g-’ atm-‘)
673 673 673 573
NaX
N&X red.Ni,,X red.Ni,,X
0.506 0.199 0.164 0.326
18.46 13.09 13.92 47.43
9.33 2.62 2.28 15.46
NaY Ni,,Y red.Ni,,Y red.Ni ,4Y
0.492 0.265 0.237 0.608
4.21 7.69 1.24 16.85
2.07 2.04 1.71 10.24
Cyclohexane
673 673 673 573
NaX N&,X red.Ni,,X red.Ni,,X
0.219 0.189 0.84 0.199
5.06 3.62 8.83 13.01
1.11 0.68 0.74 2.60
NaY Ni,,Y red.Ni,,Y red.Ni,,Y
0.087 0.086 a
5.86 5.32
0.51 0.46
0.262
6.02
1.58
Methylcyclopentane
673 673 573 673 573
NaX N&,X
0.191 0.091
5.73 7.04
1.09 0.64
0.072
6.56
0.47
0.094
red.Ni,,X red.Ni,,X
0.087 0.164
8.27 15.42
0.72 2.53
NaY Ni,,Y Ni,,Y red.Ni 14Y red.Ni ,4Y
16.50 a
1.56 a
0.245
6.22
1.52
673 673 513 673 573
NaX N&,X N&X red.Ni,,X red.Ni,,X
0.447 r a a a
5.72 a
2 56
NaY Ni,,Y Ni,,Y red.Ni ,4Y red.Ni 14Y
0.126 a
6.95 a
0.87
Adsorbate
T WI
Benzene
Cyclohexene
a Evaluation
not possible
due to conversion
a
a
a
a
C. Papaioannou
et al. I Solid State Ionics 101-103
(1997) 799-805
0.20 G-
0 0.00
0.00 0.00
0.04
0.08
0.12
P /
0.16
0.20
atm
P /
Fig. 1. Adsorption isotherms of benzene (1, 5), cyclohexaae (2, 6), methylcyclopentane (3, 7) and cyclohexene (4, 8) on zeolites NaX and NaY respectively measured at 673 K.
atm
Fig. 3. Adsorption isotherms of cyclohexane on N&,X (I), on Ni,,Y (2) and on red.Ni,,X (3) measured at 673 K and on red.Ni,,X (4) and red.Ni,,Y (5) measured at 573 K.
0.15
7
cn
0
E E
0.10
\ 0
0.06
0.09
P /
0.12
0.
atm
P /
atm
Fig. 2. Adsorption isotherms of benzene on N&,X (l), on Ni,,Y (2), on red.Ni,,X (3) and on red.Ni,,Y (4) measured at 673 K and on red.Ni,,X (5) and red.Ni,,Y (6) measured at 573 K.
Fig. 4. Adsorption isotherms of methylcyclopentane on N&,X (1) and on red.Ni,,X (2) measured at 673 K and on red.Ni,,X (3), red.Ni,,Y (4) and Ni,,Y (5) measured at 573 K.
of the isotherms since b-P ++z1 the Langmuir tion is close to the Henry equation:
4. Discussion
a=G*b.P=K-P.
equa-
(2)
K is the Henry constant characterizing the adsorbate-adsorbent interaction. The adsorption becomes stronger as K increases [7,8]. The derived values of G, b and K are shown in Table 2.
4.1. Fitting the Langmuir adsorption isotherms
equation
to the
The good fitting of the Langmuir model to the isotherms was not expected, since zeolites have energetically an inhomogeneous surface. Neverthe-
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less, there have been reported cases of adsorption isotherms on zeolites satisfactorily described by the Langmuir equation [9- 111. The satisfactory description of adsorption isotherms by the Langmuir equation means that the molecules are adsorbed in energetically equivalent adsorption sites. Consequently, the adsorption energy is constant and the distances between the adsorbed molecules are sufficiently far apart so that no interaction takes place. Because of the size of the molecules, only the adsorption sites in the supercages are accessible to them. Expressed in molecules per supercage (Figs. l-4), the adsorbate loading was smaller than one molecule per supercage. The distances between the adsorbed molecules are therefore sufficiently far apart and their interaction can be neglected. However, while in Na zeolites this presupposition is satisfied (only Na+ in the supercages), in Ni zeolites there are more types of adsorption sites (Na+, Ni*+, protons). At 673 K the maximum adsorption capacities G are higher for Na zeolites, lower for Ni zeolites and the lowest for reduced Ni zeolites (Table 2). It should also be noted that the maximum adsorption capacities G for the reduced Ni zeolites are higher at 573 K than at 673 K. A basic assumption of the Langmuir model, however, is that the molecules are adsorbed at a fixed number of well defined localized sites and therefore the saturation adsorption capacity should be temperature independent [ 121. Therefore, adsorption in the systems examined does not strictly follow the Langmuir adsorption model. It should also be mentioned that in the case where the adsorbent has small pores which define an effective surface, further adsorption is prevented due to the filling of these pores and the adsorbate behaves in a manner resembling the Langmuir model [ 131. Borovkow et al. [ 141 found that in those zeolites where the micropores are channels between the supercages, the presence of as many as four adsorbed benzene molecules per supercages still does not prevent diffusion between neighbouring supercages. Since in our case there is less than one adsorbed molecule per supercage, the Langmuir behaviour of the examined adsobate-adsorbent systems cannot be due to this preventing of the adsorption to inner supercages.
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In the adsorption of benzene, cyclohexane and methylcyclopentane on reduced Ni,,X and of benzene on reduced Ni,,Y, the lower maximum adsorption capacity G at 673 K compared to that at 573 K shows a decrease of the available adsorption sites with temperature. Since at its different maximum adsorption capacities, each system at the above two temperatures is equivalent with two different systems. Therefore the Clausius-Clapeyron equation Qiso = R . ln(P, lP,) . T, *T2 l(T, - T,)
(3)
may not give a real relationship between the isosteric heat of adsorption Qiso and the coverage a. The application of this equation gave an increase in the isosteric heat with coverage (Fig. 5). Of course, the physisorption Langmuir model is not sufficient to describe this behaviour. Such results can usually be seen in adsorbent-adsorbate systems where chemisorption appears [ 151. At temperatures of 573 and 673 K the adsorption in our systems is therefore probably chemical. In NaX and NaY zeolites the Na+ ions are adsorption sites. The decrease of the maximum adsorption capacities G follows the sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane, showing that there is an energetic scatter of the Na+ ions in the supercages.
0 /
mm01
g-’
Fig. 5. Isosteric adsorption heats of benzene (l), cyclohexane (2) and methylcyclopentane (3) on red.Ni,,X and of benzene on red.Ni,,Y (4) which result from the application of the ClausiusClapeyron equation.
C. Papaioannouet al. I Solid State Ionics 101-103
The adsorption sites in Ni zeolite, are Na+, Ni2+ and protons; in reduced Ni zeolites [ 16- 191 adsorption sites are additionally the surface molecules of the nickel particles. At 673 K the maximum adsorption capacity G is getting generally lower values for reduced Ni zeolites than for Ni zeolites (Table 2). This shows that at such a high temperature chemisorption takes place not only on Naf but also . Lower values of the maximum adon Ni” ions. sorption capacity in reduced Ni zeolites are obviously connected to the loss of energetically capable Ni*+ adsorption sites, because of the formation of metallic nickel particles, where adsorption is possible only on the surface Ni atoms, whereas the protons that have replaced the Ni*+ ions are probably not able to adsorb at these temperatures. At 673 K the maximum adsorption capacity G is lower for Ni zeolites than for Na zeolites. This is directly connected to the loss of energetically caions are pable adsorption sites, since two Naf replaced by one Ni2+ ion, and it also shows that adsorption on the protons probably does not take place at this temperature. At 573 K the proton ability to participate in the adsorption process can explain the considerable increase of the maximum adsorption capacity G on reduced Ni zeolites (Table 2). Choudhary et al. [20] report that at 523 K benzene interacts more strongly with Na’ ions than with protons. They attribute this to the fact that benzene is a weak base and therefore its interaction with protons is weaker compared with the interaction of the r benzene ring electrons with Na+ cations. Therefore, it is not unlikely that if the temperature increases, the interaction of benzene molecules with protons becomes essentially negligible, especially when the temperature exceeds the critical temperature of benzene (562 K) and the adsorption is not physical any longer. 4.2. Comparison
of the Henry constants K
A comparison of the Henry constants K shows that at 623 K benzene, cyclohexane and methylcyclopentane are more strongly adsorbed on NaX than on N&,X or on reduced Ni,,X. On NaX the adsorption strength decreases in the following sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane. The same adsorption strength order ap-
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peares in N&,X. In Nay, Ni,,Y and reduced Ni,,Y the adsorption strength of benzene is almost the same at 673 K. In NaY there is the same order as in NaX, namely: benzene > cyclohexene > cyclohexane = methylcyclopentane. The interaction between the adsorbed molecules on the charge condensing cations is of an electrostatic nature [ 131. The polar molecules are generally more strongly adsorbed than the non-polar ones. The adsorption strength of non-polar molecules is modified by their electric polarizability. It is expected that cyclohexene, with a permanent electric moment of 0.61 X lo-’ g”* cm5’* s-’ [21], is more strongly adsorbed on Na zeolites than cyclohexane or methylcyclopentane, which have no permanent electric moment. Benzene, however, although it has not got a permanent electric moment [24], does not follow this rule and is adsorbed more strongly than cyclohexene. This stronger adsorption of benzene molecules proves the contribution of the special interaction between the adsorption sites Na+ and the rr electrons of the benzene ring [7]. In Ni zeolites the Ni*+ ions, the Na+ ions and the acid sites are adsorption sites. If, however, the Ni*+ ions which are stronger adsorption sites than the Na+ ions, are ‘protected’ by OH- groups produced during hydrolysis [22,23], then they are not suitable for the adsorption process [24]. Since benzene is not adsorbed more strongly on Ni zeolites than on Na zeolites, taking into account that it is a weak base (and consequently its chemical interaction with protons is weaker than the interaction between its 7~ electrons and the Na+ cations [20]), it is concluded that the Ni*+ ions of the Ni zeolites are ‘protected’ by OH- groups. The higher constant K for the benzene-NaX system compared with the benzeneNi,,X system indicates weaker adsorption of the benzene molecules on the Na+ ions of the Ni,,X zeolite than on the Naf ions of the NaX zeolite. Benzene molecules are adsorbed in a similar way on the Na+ ions of the corresponding Y zeolites. Benzene is not adsorbed more strongly on reduced zeolites containing nickel metal particles than on the non-reduced ones. This means that the adsorption of benzene on the surface of the nickel metal particles is approximately as strong as on Na+ ions. Therefore, the specific interaction between ZF electrons of the benzene ring, ionic adsorption sites and surface
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et al. I Solid State Ionics 101-103
nickel atoms of the metal particles is almost the same. At 623 K the strength of adsorption of cyclohexane and methylcyclopentane on N&,X and reduced N&,X and of cyclohexane on Ni,,Y, is approximately the same as on the corresponding Na zeolites. This shows - taking into consideration the decrease in the adsorption ability of Ni2+ ions with the presence of OH- groups - that these molecules are adsorbed equally strongly on Na+ ions, protons and on the nickel metal particles surface of the reduced zeolites. The weaker adsorption of the investigated substances on the Y compared with the respective X zeolites, shows that the aluminium content of the zeolites modifies the energetic state of the adsorption sites. 4.3. The complicated
elution chromatograms
In Table 2 there are also shown the adsorbateadsorbent systems with elution chromatograms including complicated peaks. Such a chromatogram indicates that although only one substance is injected, several substances emerged from the zeolite column. These substances were formed by reactions, such as isomerization, cracking or oligomerization, catalyzed by the zeolite itself and favoured at higher temperatures. This phenomenon took place only in Ni zeolites. The divalent Ni2+ ions with a magnitude e/r-=2.56 (formal charge over radius in A) [25] generate a stronger electrostatic field in the zeolite system than the monovalent Na+ ions with a magnitude e/r= 1.03 [25], as well as a Bronsted acidity [22,26]. The strong electrostatic field or the Bronsted acidity seems to be necessary for the carbonionic reactions. Whereas this phenomenon takes place in the case of cyclohexene - an unsaturated substance sensitive to reactions - in NIX zeolites, in NiY zeolites where the electrostatic field is stronger and the Bronsted acidity higher [23,26] this phenomenon also takes place in the case of cyclohexane and methylcyclopentane.
5. Conclusion The adsorption of benzene, cyclohexene and methylcyclopentane
cyclohexane, on Na or Ni
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zeolites and on reduced Ni zeolites at 673 and 573 K is of a chemical nature (chemisorption). The adsorption strength on Na zeolites decreases according to the following sequence: benzene > cyclohexene > cyclohexane = methylcyclopentane. The strong adsorption of benzene could be due to the special interaction between IT electrons of the benzene ring and Na+ adsorption sites. Cyclohexene as a polar molecule is more strongly adsorbed on NaC ions than cyclohexane or methylcyclopentane. The above adsorption strength sequence is also observed in Ni zeolites and reduced Ni zeolites. Because of the Ni2+ ions, the considerable acidity of the zeolite carrier favours reactions (oligomerization, cracking etc.) of the adsorbates, especially of the unsaturated ones. This phenomenon appears more often on Y zeolites where the acidity is higher and is not controlled by the adsorption strength which is greater on the X than on the Y zeolites.
Acknowledgements
Financial support by ‘Intemationales Btiro Jiilich’ (Germany) and by “General Secretariat of Research and Technology’ (Greece) is gratefully acknowledged. The authors thank Prof. F. Schmidt (Siid Chemie, Miinchen) for his assistance in preparing this manuscript.
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C. Papaioannou et al. I Solid State Ionics 101-103 [12] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, 1984, pp. 49-50. [ 131 K. Hauffe, S.R. Morrison, Adsorption - Eine Einfiihrung in die Probleme der Adsorption, Berlin-New York, 1974. [14] VYu. Borovkow, W. Keith-Hall, VB. Kazansky, J. Catal. 51 (1978) 437. [15] A.A. Huang, J. Zwiebel, Adv. Chem. Ser. 121 (1973) 374. [ 161 M. Suzuki, K. Tsutsumi, H. Takahashi, Zeolites 2 ( 1982) 5 1. [17] P.A. Jacobs, St. Surf. Sci. Catal. 12 (1982) 71. [18] F. Schmidt, H. Kacirec, W. Gunsser, Ch. Minchev, V Kanazirev, L. Kosova, V Penchev, in: Proc. of the 4th Conf. on Catal., Varna, 1979, p. 331. [19] M. Kermarec, D. Olivier, M. Richard, M. Che, J. Phys. Chem. 86 (1982) 2818.
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