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Hypothesis on Zeolites Physicochemical Properties
tribution to the actual mechanism of hydrogenation. Strictly, these conclusions apply only to the 11-pulse experiments at 0 and -31 °C on Pt/Si02 catalysts. The
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(8) G. Webb, Surf. Defect Prop. Solids, 3, 184 (1974); J. R. Anderson and B. G. Baker, “Chemisorption and Reactions on Metallic Films”, Vol. 2, J. R. Anderson, Ed., Academic Press, New York, 1971, p 81. For ethylene on Pt/Si02 see: R. Komers, Y. Amenomiya, and R. J. Cvetanovlc, J. Catal., 15, 293 (1969). (9) Y. Inoue, I. Kojlma, S. Morlkl, and I. Yasumorl, Proc. Int. Congr. Catal. 6th, 1, 139 (1976) (pub 1977). (10) N. C. Gardner and R. S. Hansen, J. Phys. Chem., 74, 3298 (1970). (11) K. Baron, D. W. Blakely, and G. A. Somorjai, Surf. Sci., 41, 45 (1974); G. A. Somorjai, Adv. Catal., 26, 1 (1977); W. H. Weinberg, . A. Deans, and R. P. Merrill, Surf. Sci., 41, 312 (1974). (12) S. J. Thomson and G. Webb, J. Chem. Soc., Chem. Commun., 526 (1976); A. S. AFAmmar and G. Webb, J. Chem. Soc., Faraday Trans.
agreement between turnover frequencies in pulse and flow experiments suggests that these conclusions can be extended to flow experiments protracted for many minutes. The fact that the apparent activation energy is independent of temperature13’18 suggests that the conclusions can be extended to temperatures higher than those of the present experiments. However, the present paper provides no evidence as to whether reactions at much higher temperatures involve or do not involve transfer of hydrogen via carbonaceous residues.
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of Physical Chemistry,
1, 74, 657 (1978), (13) J. C. Schlatter and M. Boudart, J. Catal., 24, 482 (1972). (14) This portion has been the subject of a preliminary communication: T. Hattorl and R. L. Burwell, Jr., J. Chem. Soc., Chem. Commun., 127 (1978). (15) T. Uchljlma, J. M. Herrmann, Y. Inoue, R. L. Burwell, Jr., J. B, Butt, and J. B. Cohen, J. Catal., 50, 464 (1977). (16) C. R. Mcllwrlck and C. S. G. Phillips, J. Phys. E, 6, 1208 (1973). (17) R. Moeseler, B. Horvath, D. Lindenau, E. G. Horvath, and H. L. Krauss, Z. Naturforsch., B, 31, 892 (1976). (18) P. H. Otero-Schipper, W. A. Wachter, J. B. Butt, R. L. Burwell, Jr., and J. B. Cohen, J. Catal., 50, 494 (1977). (19) Y. Inoue, J. M. Herrmann, H. Schmidt, R. L. Burwell, Jr., J. B. Butt, and J. B. Cohen, J. Catal., 53, 401 (1978). (20) T. Hattorl and Y. Murakami, J. Catal., 31, 127 (1973); Can. J. Chem. Eng., 52, 601 (1974). (21) . H. Kung, B. I. Brookes, and R. L. Burwell, Jr., J. Phys. Chem., 78, 875 (1974). (22) . M. Mestdagh, G. P. Lozos, and R. L. Burwell, Jr., J. Phys. Chem., 79, 1944 (1975).. (23) On platinum: G. Besenyel, D. Moger, and F. Nagy, Magy. Kern. Foly., 81, 313 (1975). (24) On nickel: W. K. Hall, F. E. Lutinski, and J. A. Hassell, Trans. Faraday Soc., 60, 1823 (1964); A. Leszczynski, A. Fracklewlcz, and W. Palczewska, Mechanisms of Hydrocarbon Reactions Symposium, 1973, F. Marta and D. Kallo, Eds., Elsevier, 1975, p 187. (25) M. Kobayashl, Ipatieff Laboratory, Northwestern University, unpublished. (26) N. R. Avery, J. Catal., 24, 92 (1972). However, our experiments in which the deposit is formed in the presence of hydrogen show no evidence for polymerization as noted here for palladium.
Acknowledgment. Drs. Aidan Kennedy and Dr. Beverley I. Brookes in this laboratory conducted earlier work aimed at elucidating the role of carbonaceous residues in the hydrogenation of ethylene. The work was important in developing the techniques used in the present work and the results were consistent with those of the present work (see ref 21). However, at that time we did not achieve low enough values of ethylene/Pts per pulse to answer the basic questions critically.
References and Notes (1) On leave from the Department of Synthetic Chemistry, Nagoya University, Nagoya, Japan. (2) J. Horiutl and M. Polanyl, Trans. Faraday Soc., 30, 1164 (1934). (3) See, for example: S. Siegel, J. Catal., 3Ó, 139 (1973); “Catalysis Progress in Research”, F. Basolo and R. L. Burwell, Jr., Eds., Plenum Press, London, 1973. (4) 0. Beeck, Discuss. Faraday Soc., 8, 118 (1950). (5) K. C. Campbell and S. J. Thomson, Prog. Surf. Membr. Sci., 9, 163
(1975).
(6) J. Pritchard, Surf. Defect Prop. Solids, 1, 222 (1971); J. Erkelens, J. Catal., 37, 332 (1975). (7) R. Z. C. van Meerten, A. C. M. Verhaak, and J. W. E. Coenen, J. Catal., 44, 217 (1976).
A General Hypothesis on Zeolites Physicochemical Properties. Applications to Adsorption, Acidity, Catalysis, and Electrochemistry Denise Barthomeuf Laboratoire de Catalyse Organique, Laboratoire associé au CNRS No. 231, 69621, Vllleurbanne, France (Received February Revised Manuscript Received June 21, 1978)
17, 1978;
Publication costs assisted by CNRS
Based on a large number of zeolite properties a sharp comparison of zeolites with solutions is suggested. For example, it is proposed first that the increase in zeolite acid strength as the aluminum content is decreased may be explained on the basis of an analogy with inorganic acids in solution. Secondly the importance of interactions in zeolites imply the definition of activity coefficients for the ions or atoms of the framework, not only in ion exchange as postulated for a long time but also for other properties. The values of these activity coefficients will decrease as the interactions, i.e., the Al/Al + Si ratio, is increased. Such activity coefficients will have to be considered in studies on kinetics, adsorption, ion migration, etc. The properties of the adsorbed phase are also changed by activity coefficients as for a solute in a solvent. The behavior of transition metal cations in zeolites strongly suggests that electrode phenomena occur and that many of the reactions described are relevant to electrochemistry. In conclusion it is suggested that the zeolites’ properties are governed by the changes in the chemical potential of their framework ions and atoms. Their strong analogy with solutions suggests considering these solids as “crystalline liquids” where nearly all of the framework atoms belong to the surface.
Introduction
specific zeolites properties. During the past few years it has been shown that besides properties which may be observed with other porous oxide solids, some typical results place the zeolites at a boundary between the solid and liquid states. Hence it has been demonstrated that
The use of zeolites in many fields (catalysis, adsorption, ion exchange, etc.) has stimulated a wide variety of research. The interest in zeolites is now becoming more general and it tends to provide a better understanding of 0022-3654/79/2083-0249301.00/0
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they behave like ionizing solvents1"3 or electrolytes2,4 and that the atoms of the framework may also play the role of an anion or ligand in the formation of well-defined transition metal complexes.3 Considering further that all the concepts used in acidity characterization (nature, number and strength of sites, proton mobility) derive from the acidic properties in solution one can wonder whether it is not possible to proceed with the analogy of zeolite properties with those of solutions.
I. Properties of Zeolites which Could Be Compared with Those of Solutions At first it may be considered that the typical structure of zeolites, widely open with nearly all the atoms of the framework located on the walls of the cavities, is quite different from that of other oxides. For zeolites with pores larger than 5-6 Á which allow molecules to enter the big cavity, the most of the structures’ atoms are accessible to the reactant and there is a close contact between the solid and the adsorbed phase as between molecules in solution. I. A. Cation and Proton Mobility. The ion mobility depends on the adsorbed phase. In hydrated zeolites cation mobility has been evidenced for a long time.5 Electrical conductivities68 and self-diffusion613 studies have evaluated the ion mobility in many hydrated zeolites. In dehydrated materials it has been shown that various molecules may induce cation migration (Cu, Ni) from inner cavities to accessible cages.7"9 Very stable complexes may be formed in the larger cavities with the transition metal cations attracted there.10 With various alkaline or monovalent cation dehydrated zeolites the electrical conductivity measurements have evidenced a high temperature (>350 °C) ionic conduction mechanism and a low temperature one (s(Si02)iS4.
TO„(OH)m form0 TO1.fl(OH)0i45
5
TO1.76(OH)0,34 * **1.8ot^*^0.20 TOli81(OH)0.19 TO1i83(OH)0.17 TO1,83(OH)0i17 TO1i87(OH)0.13
Hc(A102)6(Si03)3„ mordenite H8(AlO2)8(SiO3)40 Y DIId R»(A103)26(Si02)i66 ° Some of these zeolites do not exist in a fully cation free form. valent (Na+ or K+). d Aluminum-deficient samples. is well known that pyridine is more strongly linked to the acid sites than ammonia. The increase in the jump frequency of the proton from NMR studies of zeolites under vacuum and in a toluene or pyridine atmosphere15 could also be related to a “solvent” effect. The increase follows the basicity and the dielectric constant increase of the gas atmosphere. Of course no specific study has been performed on such a “solvent” effect and it is difficult to provide more examples. What seems important to consider in practice is that such a “solvent” effect should be important in catalysis due to the influence on the acid sites of at the same time (i) the basicity, (ii) the dielectric constant, and (iii) the ease of interactions with the ions, of the reactants, of the products, and of the coke deposit. II. A. 2. Structure Effect. II. A. 2a. Substituents Effect. Incorporating Cl or F has often been used to strengthen the acidity of oxides, for instance, A1203. The instability of zeolites in acidic media has prevented the extent of this process to these oxides. Nevertheless other zeolite properties are characteristics of substituent effects such as the known increase of acid strength upon introduction of polyvalent cations.30,31 The explanations proposed32,33 related to the field of the cation are quite relevant of substituents effects in solution. II. A. 2b. Oxygen Influence as in Inorganic Acids. It was pointed out that in solution significant regularities exist in the strength of the inorganic oxyacids though no general interpretation has been given.260’34 The acids have the general formula XO„(OH)m. Their strength increases with increasing n and does not depend significantly upon the m value. For example, the following series is observed for the strength of the oxyacids containing chloride: Cl(OH) < CIO(OH) < C102(0H) < C103(0H). The pK values decrease respectively from 7.2 to 2 to -1 and to -10. In order to anticipate the acid strength of zeolites on such a basis an attempt to write the zeolite formulas in the form of TO„(OH)m with T being Al + Si is presented. For instance an acidic H-Y zeolite has the general theoretical formula ^ ^^ ^ or (Al,Si)1920328(0H)56 which referring to (Al,Si) equals 1 gives TO171(OH)029 with T =
it
0.29
Al + 0.71 Si.
The calculated formulas are reported in the Table I. The zeolites are classified according decreasing Al/Al + Si ratios. This ratio is similar to a molar fraction in solution where Si is the diluent. Since the theoretical number of protons equals that of aluminum atoms the m values in TO„(OH)m are similar to these ratios. From what is known in solution,260 the n values are the more important in determining the acid strength. Since the aluminosilicates are built of T04 tetrahedra, for zeolites, + m = 2 and n is increasing from 1.55 to 1.87 while m is decreasing from 0.45 to 0.13. This suggests that the acid strength has to increase from the X to the more dealu-
6
T
=
nA\ + ng¡.
exchange degree,0 %
90-95 75 85 95 70
-100 95 0
desorpn ° temp C 350
350-400 -400 400 400 450-500 450
The remaining cations
ref 51
52 43 51 53
54,55 51
are mono-
minated Y zeolite. Table I reports an evaluation of the acid strength from the upper temperature of pyridine retention on protonic sites. No results are available on highly exchanged X zeolites which are thermally unstable. For the other zeolites with regards to the degree of exchange, the pyridine desorption temperature increases simultaneously with the n value. In fact it was already known that the highly siliceous zeolites have strong acid sites. What is deduced here is that zeolites behave like other inorganic oxyacids and that the theoretical TO„(OH)m formula rationalizes the phenomenum in terms of a
general approach. Another comment
on the n values in zeolites may be made. Bell260 considers that oxyacids with = 0 (Si(OH)4) = 1 they are weak. Their pKA are are very weak. When close to 2, for instance, 2 for CIO(OH), 2.1 for phosphoric
acid PO(OH)3, or 1.6 for IO(OH)5. Oxyacids with = 2 are strong, their pKA being generally less than 0, for instance, -1.4 for N02(OH), -1 for C102(OH), 0.8 for I02(OH), or Ca > Sr > Ba or Li > Na > K > Rb > Cs, it turns out that the hydrolysis phenomenum (eq 2) cannot explain all the published results and one can wonder whether no primary salt effect is involved. II. B. Electrode Potential. The importance of the electrode potential of bi- and trivalent cations inside a zeolite during the thermolysis of water has been pointed out.53
Studying the reducibility of transition metal cations in various zeolites, Uytterhoeven et al.54,55 reported that in a given zeolite the reducibility of the cation (evaluated from the reduction temperature) did not vary with the ionization potential of the metal but with the redox potential giving the following sequence for the nonnoble metals: Ag+ easier than Cu2+ » Ni2+ > Co2+ >>> Zn2+. They conclude that zeolite cations have the properties of the same cations in solution. Moreover for a given ion, Ni2+, they showed that the higher the Al/Al + Si ratio in various zeolites, the easier is the cation reducibility. The electrode potential x of an element is given by x
=
x° +
—¿7
nJ
In (Mrt+)
(4)
x° is the normal standard potential when the activity of the ion in solution is unity, nTf is the number of coulombs, and (Mn+) the activity of the cation. In zeolites, the dependence of ion reducibility on x° suggests to Uytterhoeven that eq 4 is valid. Considering the importance of the activity as defined before, one can then infer that the ions’ activity should play a role in the cations’ reducibility. This activity would depend on the activity coefficient arising from the Al/Al + Si ratio, on the ion concentration and nature, on the influence of other cations, on the various oxidation state of the cation, and possibly on the metal migration outside the zeolite crystallites. The important results of Uytterhoeven et al. are then more than a supplementary proof of the solvent and electrolytic properties of zeolites. If moreover one considers that the reversibility of the redox phenomenum has been observed under specified conditions with Cu, Ag, or Pd zeolites23"25 or that sunlight irradiation cleaves water on Ag-zeolit,es under Ag reduction-oxidation56 it is suggested that typical electrochemical effects occur in zeolites, the metal particles acting as electrodes with the ions being “dissolved” in the framework and the zeolite as a whole being a cell. The redox process should be governed by the AG changes in the system metal-cation with AG = ri3ir. Such an approach may open the field of electrochemistry to zeolites with a very large number of applications. At first an interpretation of some zeolites properties may be suggested in light of this view. For instance many of the ionization reactions described or reviewed by Rabo and Kasai2 may be considered as electrochemical reactions since they imply electron transfer between cations such as Ni, Cu, and other species such as NO or N02. As another example, the typical sulfur resistance of Pt57 or Ni54 zeolites could arise from the lower electrode potential
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of sulfur compared to that of the two metals. This restrains the oxidation of metal by sulfur. The increase in sulfur resistance of various Pd zeolites upon dealumination at constant Pd content or upon the increase in Pd content59,60 might also be related to the same effect. In both cases there would be an increase in Pd activity arising either from the activity coefficient increase (dealumination) or from the concentration increase. The third example is the spill-over effect which has been observed on zeolites.58 One can wonder whether it could come in some way from a hydrogen electrode effect, hydrogen atoms being transferred through a bridging compound to the solid where they are “dissolved” as H+ ions and where they remain adsorbed. If now one speculate on predictions based on the scale of electrode potential, it may be inferred for example that in the case of two systems metal-cation, the one with the lowest electrode potential will tend to give the cation and the other one the metal. For instance traces of iron as impurities in the sample may induce some redox reactions. Also, a large number of the electrochemical reactions described in solutions could occur in zeolites containing various cations or metals. Finally considering the electrode properties of very small particles occluded in the zeolite framework one can wonder whether it could be possible to build cells on an atomic scale in zeolite crystals, either concentration cells or cells with two different metals. II. C. Collective Properties. Interactions. Besides localized interactions between for instance adsorbed molecules and surface sites, several results have been published which are relevant of collective properties linked to the concentration of charges in the framework. Flanigen et al.61 reported regular changes in the wavenumber of zeolite framework bonds with the Al/Al + Si ratio. Recently62 Si ß X-ray emission band energies have been shown to increase approximately linearly with the same ratio indicating a regular significant destabilzation of silicon bonding orbitals. For the authors the results seem to depend mainly on the tetrahedral framework aluminum content and not on the crystalline structure. Field calculations in zeolites imply considering all the charge interactions as in any crystal. The results indicate a lower field in the cages of X than in those of Y zeolites.63 Statistical calculations on the interactions in zeolites allowed an evaluation of the value of 30 Al/unit cell as the aluminum content for faujasite zeolite type for which there is a maximum of aluminum atoms with no close neighbor in a square face of the sodalite cage, i.e., with a minimum of aluminum interactions.38 At higher aluminum content the interactions increase. The authors35,38 correlate the acid strength to the importance of the interactions between the negatively charged aluminum atoms. Another example concerns the IR wavenumber of the high-frequency band of acidic hydroxyls of some zeolites which decreases almost linearly as their Si/Al ratio increases. This has been related alternatively to an increase in acid strength64,653 or a decrease in the framework interactions.66 Both explanations seem to be consistent since the second hypothesis may involve the first. Figure 2 reports values taken from the literature for 14 zeolites in the protonic alkaline form as a function of the Al/Al + Si ratio. Six of the zeolites belong to the faujasite structure and eight to various structures. For five materials, wavenumber values obtained at various ion contents give a v range in which the higher value corresponds to a lower acid strength. These results show first that for similar rather weak acid strengths several zeolites do not show the
Denise Barthomeuf
Figure 2. Changes in the square wavenumber (P2) of acidic hydroxyl groups as a function of the Al/Al + Si ratio: (1) dealuminated mordenite (ref 73); (2) YDII (ref 70); (3) mordenite (ref 73 and 74); (4) clinoptllolite (ref 72); (5) YDI (ref 70); (6) offretite (ref 45); (7) (ref 75); (8) dealuminated Y (ref 64); (9) L (ref 71); (10) Y (ref 64); (11) chabazite (ref 76); (12) X (ref 64); (13) A (ref 77); (14) Ge zeolite (ref 64). same wavenumber and secondly that there is a characteristic P0H range for each zeolite type. It is then inferred that the Pqh value cannot characterize the acid strength alone. The correlation observed in Figure 2 between P2 and Al/Al + Si suggests that the increase in p2 at high Al/Al + Si values is connected to a large part to an increase in the force constant k of the hydroxyl bands. This might arise from an increase in the charge density (i.e., the number of framework aluminum atoms) and consequently from an increase in the interactions between the charges. This charge density seems then more important than the crystalline structure in determining the characteristic infrared wavenumber of each zeolite. It is noteworthy that in the range of each characteristic wavenumber cations (mono or polyvalent) may change the framework interactions and the acid strength, hence also the IR band
frequency.
These examples show that structural properties such as bonds in the framework, OH vibration wavenumber, and acid strength may be linked to collective interactions. II. D. Solubility Product. In solution the solubility product gives the solubility of salts or hydroxides. The solubility depends on the ionic strength of the solution and on the nature of the added salt (if there is a common ion). The solubility product also governs the precipitation of salts or hydroxides formed from the cation implied in complexes. Considering zeolites as electrolytes the solubility product might be applied to various problems such as salt occlusion,2 or hydroxide or oxide precipitation.67 In this last case the authors mentioned some differences for the X and Y samples with Cu or Ni cations. One might also suggest that the formation and chemical stability of clusters (metallic or ionic) in zeolite cages could result from a right balance between the formation constant of the cluster and the solubility product of the salt resulting from the cation reaction with a reactant. As far as activity coefficients, i.e., solubility product, depend on the Al/Al + Si ratio all these reactions would give results dependent on the aluminum content.
III. Typical Properties in Zeolites III. A. Adsorption. Adsorption is the first step of any surface phenomena on solid materials and the fugacity of
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Hypothesis on Zeolites Physicochemical Properties
the adsorbed phase is usually employed in calculations. Using the analogy with solutions, Barrer et al.68 have evaluated the activity coefficient of various adsorbates in zeolites. At low coverage Henry’s law is applicable to the adsorption as for the dissolution of a gas in a liquid at low concentration. The adsorbed phase is then comparable to a solute dissolved in a liquid solvent giving a nonideal solution. The influence of the solid phase on the activity of a liquid or gaseous phase adsorbed will then lead (as in solutions) to the partition of an adsorbate between two zeolites following the ratios of its activities in the two adsorbents (Nernst’s distribution law). The same distribution law may also explain ion migration from one material to the other or from one part of a zeolite sample to the other. The analogies with solutions suggest that the zeolite itself acting as the solvent also has activity coefficients for adsorption sites. This was already suggested in part II.A.3a on the basis of chemical potential. In the process of adsorption of a given reactant, the smaller the activity coefficient of the adsorption site in a series of zeolites, the smaller would be the free enthalpy of adsorption and thus the amount adsorbed would also be smaller. Any reaction in the adsorbed phase would then be regulated at first by this process of decreasing the amount adsorbed Such an hypothesis might explain the differences observed in benzene or cyclohexane adsorption on X and Y zeolites for the enthalpy and entropy of adsorption per site and the lower amount adsorbed per cation in X type.69 III. B. Cage Effect. A solution is homogeneous on average while in zeolites the interactions differ from one cage type to the other and are reproduced on a periodic pattern in the crystal. In the range of activity coefficients deduced for each zeolite from the Al/Al + Si ratio and from the cation nature, there might be a specific influence of each cage type on the coefficients which may modify reaction rates, photochemical process, or cation distribution in the various zeolite cavities. In faujasite type zeolites, catalysis occurs in supercages and the results would not, for a given aluminum content, depend much on this effect. .
IV. Conclusions Anhydrous zeolites have been viewed as expanded ionic crystals able to ionize adsorbed molecules in order to improve the Madelung energy of the system. Then they act as powerful “solid state ionizing solvents”.1 These results indicate that zeolites do not behave completely as usual solid materials. In this scope this paper tries an approach to correlate some zeolite properties to those of the liq uid state. Of course, all the zeolite chemistry related to the specific importance of the cages (sieving, ions location, catalysis, etc.) is not forgotten. The attempt tries to give an overall view of the general trend of the physical chemistry of zeolites. Some of the properties presented are related to nonelectrolyte solutions, others to electrolytes. As to the first class one may note the comparison of adsorption on zeolites with nonideal solutions which introduce activity coefficients for the adsorbed phase and for the adsorption site and hence the possibility of partition of molecules according to the Nernst distribution law. Coordination complex formation and the mobility of framework atoms is also comparable to what occurs in solution. Of course the zeolite atoms mobility is several orders of magnitude lower than in solutions. Due to the ionic character of the framework bonds the larger part of the other zeolite properties considered in this paper are connected to electrolytes solutions. Then collective interactions between
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the charges, acid-base properties, activity coefficients of the ions, salt hydrolysis, solubility product, electrode potential, conductivity, etc. are typical of electrolytes. Such comparisons with solutions allowed three main points to be emphasized. First the TO„(OH)m formula parallels the increase in acid strength with the number of oxygen as in other inorganic acids. Secondly the comparison of metal loaded zeolites with electrochemical systems offers a large field of research. Thirdly the comparison with solutions suggests defining activity coefficients for the ions and atoms of the zeolite framework. They might be very important in many stages of zeolite chemistry. Consequently they should be considered in the explanation of experimental results and in the description of the properties of variously treated zeolites or of new zeolites. When based on thermodynamics the activity coefficient concept might be connected to the overall hypothesis that the capability of a framework atom or species i to react is governed by the changes in its chemical potential: µ = µ° + RT In a¡ a¡
being its activity. As usual the rules of thermodynamics
will orientate the changes in the zeolite-reactant system.
This approach can be related to adsorption, acidity, and catalytic activity results. It is also substantiated by the correlation between reducibility of cations and their redox potential, i.e., the AG changes. Such a concept of activity coefficients would of course need the definition of a reference state on a thermodynamical basis. Besides the various extrapolations which have been made in the text from comparison with solutions, one can also speculate whether some other solutions properties could be clearly evidenced in zeolites. So are for example the changes in the boiling and freezing point of the adsorbed molecules (Raoult’s law) or the definition of an ionic strength related to the chemical composition. To conclude, the strong analogies of zeolites and solutions properties are very probably due to the typical network of these solids where a large part of the atoms are on the surface. It is then suggested that zeolites could be considered as “crystalline liquids”. This implies that zeolites will show at the same time many solutions properties independent of the solid state properties. In that sense they constitute a unique class of materials. Other materials could, to a quite lesser extent and only on their surface, show some of the analogies studied (for instance interactions leading to less than 1 mol adsorbed per site on highly exchanged surfaces with the correlative decrease in titrable acidity and catalytic properties). Note Added in Proof: Very recently (J. Catal., 52, 321
1978)) Poncelet et al. reported no water dissociation on CaGe-X zeolites. In the scope of section II.A.5 on salt hydrolysis, this is quite consistent with the observed weak
acidity of this zeolite type.
Acknowledgment. Helpful discussions with Professor J. E. Germain and Doctor Y. Trambouze are acknowledged. The authors thank the referee for his very stim-
ulating remarks.
References and Notes (1) (2) (3) (4)
P. H. Kasai and R. J. Bishop, J. Phys. Chem., 77, 2308 (1973). J. A. Rabo and P. H. Kasai, Prog. Solid State Chem., 9, 1 (1975). J. H. Lunsford, Catal. Rev., 12, 137 (1975). J. A. Rabo, “Zeolite Chemistry and Catalysis", J. A. Rabo, Ed.,
American Chemical Society, Washington, D.C., 1976, p 332.
(5) W. H. Baur, Am. Mineral., 49, 697 (1964). (6) D. W. Breck, “Zeolite Molecular Sieves", Breck, Ed., Wiley, London, 1974: (a) p 397; (b) p 571; (c) 517; (d) p 529.
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