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W. Kladnig
Surface Acidity of Cation Exchanged Y-Zeolites W. Kladnig Centro de Petr6leo y Quimica, lnstituto Venezolanode lnvestigaclonesCientifcas, Apartado 1827, Caracas, Venezuela (Received June 24, 1975) lnstituto Venezolanode lnvestigaclones Cientificas
Measurements of the surface acidity of Y zeolites containing Na, K, Ca, Sr, La, and Gd ions have been carried out by means of amine titrations, observing the color change of adsorbed Hammett indicators in the Ho range +6.8 to -8.2 in benzene solution. A good correlation between the acid strength and physical parameters ( e / r ratios) of the cations in 86% exchanged Y zeolites was detected. While a completely K exchanged Y zeolite had no observable acidity, La and Gd exchanged species exhibited strong acidity even at lower degrees of exchange. Zeolites containing alkaline earth ions had the same acidity as NaY in the range 0-55% exchange, indicating that the alkaline earth cations were in inaccessible sites. Lanthanum ion exchanged forms showed a marked rise in acidity at higher degrees of exchange, evidently due to partial migration of La ions into SIsites. Acidic centers in the zeolite examined were presumed to be mainly Bronsted, but, on the basis of the results, Lewis and cationic centers also have to be considered.
Introduction It has become obvious in recent years that in many catalytic reactions acidic centers on the catalyst surface play an important role. Especially for cracking, isomerization, alkylation, alcohol dehydration, and polymerization considerable surface acidity is nece~sary.'-~ The great acidity and catalytic activity of some cationexchanged zeolites, mainly faujasites and mordenites, have made them of great importance to the petroleum industry. In a number of publications, and especially in those where infrared spectroscopy has been used, different types of acidic centers (Bronsted and Lewis) and total acidity have been correlated with the type of exchanged cation, the silica to alumina ratio, and different pretreatment condit i o n ~On . ~the ~ ~other hand, less attention has been paid to the acidity of zeolites by means of titration with a base in nonaqueous solutions. The aim of the present work was, therefore, to examine the influence of the type of exchanged cation, degree of exchange, and pretreatment temperatures on the surface acidity of a synthetic faujasite, type Y. For this purpose a SK-40 molecular sieve was exchanged with different charged cations of approximately the same cation radius, e.g., K (1.33 A), Ca (0.99 A), La (1.016 A), and Gd (0.938 A). The resulting changes in acidity were related to the original sodium containing material. Method. The acidity was determined by the microtitration method with n-butylamine as described by BenesP and Johnson7 using the color changes of adsorbed Hammett indicators.8 Surface acidity is thus expressed by the Hammett and Deyrup Ho function,s with
in this equation expresses the proton activity of the solid acid, and f B and fBHf give the activity coefficients of the basic and acidic form of the adsorbed indicators. As Wallingg has pointed out, color changes of adsorbed indicators are not and cannot be strictly assigned to a proton-donating process, since, depending on the material, Lewis (electron accepting) centers also play a role in formUHi
The Journal of Physical Chemistry, Vol. 80. No. 3, 1976
ing a colored complex with Hammett indicators. In this case the Hammett function is expressed by9
HO = -1%
[aA(fB/fAB)]
with U A being the activity of the Lewis acid, f B and fAB being the activity coefficients of the indicator base and the complex formed with the Lewis acid, respectively. It can be assumed, therefore, that with zeolites centers other than Bronsted sites, Lewis centers (tricoordinated aluminum) or the cations themselves can also take part in producing the acidic color of the indicator. This will be discussed later. Experimental Section
Material. The Y zeolite used was a SK-40 molecular sieve from Union Carbide. The material was sieved and washed thoroughly several times with deionized water until no more sodium ions could be detected in the washing water. Afterwards the zeolite was exchanged twice with 1 N NaCl solutions, washed free of chloride ions, and dried under vacuum ( Torr) at 200-220OC. The cake so obtained was ground and, after repeated drying under the same conditions, stored in a desiccator over saturated NH4C1 solution (79.4% relative humidity at 22-23'C). A constant moisture content of 25.9 wt % could be obtained after storage for 1 week. The chemical analysis of the pretreated material gave the following: Na (10.09%),A1 (11.84%), and Si (29.93%) or Na2O (13.60%), A1203 (22.37%), Si02 (64.03%), with SiOd A1203 = 4.84 or Si/Al = 2.43 according to formula Na56&6Sii360384. Ion Exchange. In order to exchange sodium the zeolites were placed in contact with the appropriate aqueous solutions of metal chlorides in polyethylene bottles in a thermostated bath at 25 -f 0.1'C for 24 h to obtain exchange equilibrium. The concentrations of the salt solutions were chosen in such a way that the normality of the solution was equal to the equivalent ratio of the exchanging ion to sodium in the zeolite. So 18.885 g of the water containing zeolite (25.9% H2O) was contacted with 66 ml of the 0.25, 0.5, 1.0,1.5, 2.0, 2.5, 3.0, 3.5,. , . 5.0 N solutions of the chlorides,
Surface Acidity of Carbon Exchanged Y-Zeolites whereby these concentrations equally expressed the equivalent ratio of the exchangeable ion relatively to the sodium in the zeolite (Me+/Na+ ratio). All solutions were checked for their salt concentrations by chloride titration. After equilibrium was reached, the phases were separated by filtration and the zeolite washed free of adsorbed chloride with deionized water. The extent of exchange was determined by atomic absorption of the sodium ions in the solution after equilibrium, considering the Na and moisture content of the starting material. Since the majority of cations do not enter the sodalite cages of the zeolite during exchange a t 25OC, some exchanges have been performed a t 6OoC with 2 N salt solutions, twice repeated (with exception of gadolinium; once only, 1 N solution at l0OOC). Zeolite Pretreatment. Before measuring the acid strength the zeolites were calcined under vacuum Torr) a t 25OOC and further treated in a muffle oven in the temperature range 350-500°C, raising the temperature every hour by 5OOC. At 5OOOC the temperature was held for 3 h. According to TGA measurements all physisorbed water was eliminated from the zeolite by this process. Acid Strength Measurement. The titration of acid strength was determined according to Benesi6 and John~ 0 n . 7Calcined zeolite (0.1 g) was weighed in test tubes (7-ml screw cap septum vials, Pierce Chem. Corp.), recalcined after the weighing process under the same conditions described above, and stored in a desiccator. To the cool probes 5 ml of dry benzene (analytical grade, redistilled and stored over Molecular Sieve 3A) was added. Afterwards 3 drops of 0.1% solutions of the indicators in dry benzene was added and the adsorption process accelerated by immersing the sealed test tubes in a water filled ultrasonic tank. This process was found to accelerate the adsorption process of the indicators considerably. Time of adsorption depended strongly on the indicator used as well as on the type of exchanged ion and the degree of exchange. In the case of strong acidic zeolites adsorption times of up to 3 days were necessary until equilibrium has been reached. The titration was carried out in two steps. First n-butylamine (0.1 N) was added in 0.1-ml amounts to a number of vials. The color changes were observed visually after the n-butylamine had come into equilibrium with the indicators. The approximate range of the color change could be determined in this way. The final point was reached by adding the butylamine titer in 0.01-ml steps to a new series of catalyst samples in the approximate range of the end point. Time of neutralization varied with the acidity of the zeolite and the indicator. The titrations were performed with a microburet (Buret Type A, Kimble Corp.), containing a needle point to pierce the sealed caps of the test tubes in order to avoid air moisture. With this procedure roughly 10-15 g of zeolite was necessary per indicator. Indicators. All indicators were Eastman Kodak products, used without further purification and dissolved in dry, redistilled benzene of analytical grade. In Table I the indicators used are listed, together with their color changes, pKa's, and corresponding sulfuric acid composition, as determined experimentally by Hammett and Deyrup.* In the cases of neutral red and 4-(p-ethoxyphenylazo)rn-phenylenediamine, which were only available in form of their hydrochloride salts, the indicator bases were obtained by precipitation of their aqueous solutions with 0.1 N
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NaOH, filtration, and careful drying under vacuum. The pK,'s were taken from the l i t e r a t ~ r e . ~ $ ~ Neutral red deserves to be mentioned here since the neutralization reaction using this indicator was problematic in cases of high acidic zeolites. End points could only be determined after a, neutralization time of up to l week, since the indicator was found to be strongly adsorbed. In contrast to zeolites, these difficulties were not observed with silica-alumina in our laboratories. With all other indicators titrations could be performed easily.
Results and Discussion A. Ion Exchange. Figures 1 and 2 show the ion exchange isotherms as determined for the exchange Na --* K, Ca, Sr, La, Gd in the range of 0-5 equiv Me*+/Na+. Table I1 gives the relative selectivity coefficients for the exchange of a given ion pair as calculated by the formula K, = (XACB)/(XBCA) from results obtained experimentally. X A and X B are the ion fractions of the cations in the crystal (XX 100 = percent exchange) a t equilibrium and CA/CB gives the equivalent ratio of [NaIzeolite: [ion],,l, before ion exchange. The constants are salt concentration dependent. The ion exchange selectivity in the SK-40 zeolite is therefore Gd > Ca, Sr > La > Na > K. This sequence is, with the exception of La, in accordance with the exchange selectivity expected on a thermodynamic basislo in the range of low concentrations with nearly ideal exchange behavior. Figures l and 2 further show that the exchange possibilities for Sr, Ca, La, and Gd a t 25OC have a limit even when the salt concentrations of exchangeable ions become very high. Table I11 gives the values obtained for maximum exchanges a t different temperatures. The values for the exchanges a t 25°C are in very good agreement with the results obtained by Sherryll for Ca and Sr and for La.12 Exchanges with gadolinium have not been reported in literature. Due to its lower cation radius a higher degree of exchange should result as in the case of La. This was not observed, probably due to hydrolysis of GdCl3 at higher salt concentrations. At a 25°C exchange the solvated Ca, Sr, La, and Gd ions therefore cannot enter the small cages, which are equivalent to the SI sites as defined by Breck.13 The explanation of this phenomenon as given by SherrylO lies in a stripping effect of those solvated ions whereby large hydration radii as well as high hydration enthalpies prohibit entrance through the 2.2-A windows of the sodalite cages. Raising the temperature of the exchange system allows an easy 100% exchange with K, in accordance with the literature,14 while the higher charged ions show difficulties in entering the small pores. As reported by Sherry,12 100%exchange with La was only possible after a 47-day treatment a t 100OC. B. Acidity. Table IV gives the zeolites used for acidity measurements after a calcination temperature of 500OC. The table includes total acidity as approximately calculated from the results obtained. Acidity of K-Y. The acidity distribution of different K exchanged Y zeolites is shown in Figure 3. It is clear from this distribution that with rising K exchange the surface acidity becomes lower until at a 100% exchange no more acidic function of the zeolite is observable. In Figure 4 the correlation between degree of ion exchange and butylamine titers at Ho +6.8 and +4.0 are shown. The curve a t Ho +6.8 bends sharply down a t about 85% K exchange, which could be explained by the complete filling of SI and SII sites The Journal of Physical Chemistry, Vol. BO, No. 3, 1976
W. Kladnig
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TABLE I : Indicators for Determining Acid Strength Color Indicator
Basic
Neutral red 4-(pEthoxyphenylazo)m -phenylenediamine 4-Phenylazo-lnaphthylamine 4-o-Tolylazo-o-toluidin 4-Phenylazodiphenylamine 2-Nitrodiphenylamine Dicinnamalacetone Chalcone Anthraquinone a Reference 3.
Acidic
PK,
Yellow
Red
+6.8
Yellow
Red
+5.0
Yellow Yellow
Red Red
+4.0
Yellow Orange Yellow Colorless Colorless
Purple Purple Red Yellow Yellow
+1.5 -2.1 -3.0 -5.6 -8.2
Wt % H,SO,' 8X
5 x 10-5 5 x 10-3
+2.0
2 x 1048 71 90
TABLE 11: Selectivity of Exchange K ,
"1
$/
Zeol te Y ( S K Q Sodiun ExcPange 2N-normal 5°C 5olLtiOnS
=
(XACB)/(XBCA)
CA
B = Na A=K
B = Na A=Ca,Sr
B = Na A=La
B = Na A = Gd
0.125 0.25 0.3 0.5
0.6014 0.5747 0.6164
1.0600 1.0709 1.157 1.3915
0.8739 0.9424 1.2415
1.2018 1.4632
TABLE 111: Maximum Ion Exchange in SK-40 Molecular Sieve Exchange
oiy 0
1
I
1
2
I
3
5
4
Menf No
Equivolenls ( ~ ) ( n o r m a l r l yof solution)
Flgure 1. Sodium exchange in SK-40 molecular sieve with N normal solutions of KCI, SrCI2,and CaC12 at 25OC (24 h).
Zeolite Y iSK401 Scciun E 2 5 'C
N-norwal s o ~ t i o n s 20
4
Equrvoients
(4) No
inorrnoirty of solution)
Figure 2. Sodium exchange in SK-40 molecular sieve with N normal solutions of Lac13 and GdCI3 at 25OC (24 h). The Journal of Physical Chemistry, Vol. 80.No. 3, 1976
Ion
T,"C
CA,N
Time, h
%
K Ca Sr La Gd
25 25 25 25 25
5.0 3.0 5.0 3.0 2.0
24 24 24 24 24
78.64 70.05 70.05 68.73 61.64
K Ca Sr La Gd
60 60 60 60 100
2.0 2.0 2.0 2.0 1.0
24 repeated 24 repeated 24 repeated 24 repeated 24
100.00 86.14 86.14 76.90 75.00
(85.8% theoretically). Completing the exchange (at 100%) eliminates all detectable acidic functions in the zeolite. K therefore can be considered as a typical catalyst poison as has been described by Danforth.15 Catalytic reactions with K exchanged zeolites thus cannot include carbonium ion mechanisms in which an acidic surface is considered to be necessary. In investigating the condensation reactions of toluene and methanol at about 50Ooc, Sidorenko et a1.16 describe preliminary styrene and ethylbenzene formation with K exchanged X and Y zeolites that is typical of a radical process.17 A c i d i t y of Sr-Y. In changing sodium against strontium no change in acidity occurs until the degree of exchange reaches approximately 55%. Figure 5 shows the acidity distributions in Ho range from +6.8 to -5.6 for various Sr exchanged Y zeolites. With rising degree of exchange, acidity becomes considerably higher than that of Nay. Figure 6 gives the correlation between the percentage of ion exchange and butylamine titers. It can be clearly seen that the acidity of SrNaY is equal to that of NaY until 55% ex-
Surface Acidity of Carbon Exchanged Y-Zeolites
285
TABLE IV: Accumulated Acidities in Ion Exchanged SK-40 Molecular Sieves after Calcination at 500"C
- - ~ _ _ _ _ _
Butylamine titer, mmol/g, in Horange +6.8 to +4.0 t o +1.5 to +4.0 +1.5 -5.6
