J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 387-393 Influence of Framework Substitution of A13+ by Fe3+on the Sorption Characteristics of p Zeolite Praphulla N. Joshi, Eapen M. Joseph and Vasudeo P. Shiralkar* Catalysis Division, National Chemical Laboratory, Pune 4 10 008, India The sorption characteristics of Al-B (SiO, : AI,O, = 39)and Fe-/I (SiO, : Fe,O, = 39-46) zeolites in both sodium and protonic forms have been compared. Sorption capacities at PIP, = 0.8 and 298 K for water, hexane, cyclo-hexane and butylamine (BA) yielded estimates of the void volume of the fi zeolites. Sorption kinetics with these probe molecules also confirmed the highly crystalline nature of the samples. BA sorption isotherms in both the zeolites in the temperature range 33H83 K have also been measured to characterize the acidic centres in the zeolites. Analysis of the BA sorption data in terms of different sorption models yielded useful information on the nature of sorbent surface and sorption centres. Sorption selectivities were also compared by evaluation of the chemical potential of the sorption at isotherm temperatures.lsosteric heats (QSt)of BA sorption were evalu-ated by the application of a thermodynamic approach to the sorption energetics. Over the entire coverage Q,, values varied within 40-60 kJ mol-' for HIA1-P and within 18-40 kJ mol-' for H/Fe-B zeolites. High-silica zeolites are important and attractive catalysts by virtue of their hydrophobicity and potential, thermal, hydro- thermal and acidic properties with resistance to coke forma- tion.' /? Zeolite is a wide pore, high-silica, crystalline aluminosilicate first synthesised by Wadlinger et al.' Although the synthesis, characterization, structure and cata- lytic properties of B zeolite are already documented,2-" the sorption properties have not been discussed in detail./3 Zeolite is the only high-silica zeolite to have a three-dimensional, 12-ring pore system. It also has a near-random degree of stacking faults whilst maintaining full sorption capacity. Sorption studies are often of primary importance in characterizing zeolite channels and pore openings. The extent of surface heterogeneity and changes in the physico-chemical properties are also studied by sorption measurements with probe molecules such as water, hexane and cyclohexane.The evaluation of thermodynamic parameters from sorption iso- therms of basic molecules such as butylamine (BA) helps to characterize the acidic nature of the zeolite catalysts. Varia- tions in the Si : A1 ratios12 and the extra-framework cationsI3 are reported to influence the sorption properties. In view of these aspects, studies on the sorption properties +of /Izeolite with framework A13 and Fe3 + have been carried out. Experimental Materials Al-B zeolite was synthesised following the method reported earlier3." and Fe-#l zeolite was prepared following a pro- cedure of Kumar et al." Both zeolites were calcined carefully around 773 K for 8 h to drive off templating species. The zeolites in the sodium form were then ion exchanged with 1 mol dm-3 ammonium nitrate (10 cm3 g-' of zeolite) solution at 368 K for 4 h.The ion-exchange procedure was repeated till the resultant solid contained <200 ppm sodium. The ammonium forms were then deammoniated at 723 K for 10 h to give the protonic form of the zeolites. AnalaR Grade (purity > 99.9%) butylamine, cyclohexane and hexane, dried over 3A molecular sieve extrudates, were used for the sorp- tion measurements. Doubly distilled water was used for the sorption measurements and for washing of ion-exchanged products. Methods Powder X-ray diffraction patterns were obtained using high- purity Si powder as an internal standard, on a Rigaku D Max/III VC diffractometer with nickel-filtered Cu-Ka radi- ation.The morphology and crystallite size were examined on a scanning electron microscope (model Cambridge, Stereoscan- 150 UK). The chemical analysis was performed by a combination of wet chemical and atomic absorption (Hitachi-2 8000, Japan) and inductively coupled plasma emission (Jobin Yuon-JY-38 VHR) spectroscopic methods. The zeolites were futher characterized by thermal analysis (NETZSCH, model STA-490), FTIR (Nicolet 60 SXB) and EPR (Brucker E 2000) spectroscopies. Solid-state magic-angle spinning (MAS) NMR for 29Si and 27Al were recorded at 295 K using a Brucker MSL-300 spectrometer. While acquiring 29Si spectra, a recycle time of 3 s was used with MAS at 3.5 KHz. TMS and aqueous AlC1, were used as references for 29Si and 7Al spectra.The magnetic susceptibility measure- ments, in the temperature range 94-297 K were carried out using a Faraday balance (Cahn-Ventron, USA). Sorption studies (both the kinetics and isotherms) were carried out in an all-glass, McBain-Baker type gravimetric vacuum unit described elsewhere. Results and Discussion Characterization Fig. 1 illustrates the powder X-ray diffraction (XRD) patterns of HIA1-P and H/Fe-P zeolites. The identification and the purity of these phases were examined using Si powder as an internal standard. The XRD pattern of the H/Al-#l sample closely matches the reported data.5 Although XRD patterns of both HIA1-B and H/Fe-fi are nearly identical, the intensity of the prominent peaks of H/Fe-P was found to be lower with a small shift of the peak towards lower 28 values.This may be due to the lattice expansion on account of the larger Fe3+ atoms in the framework positions. The absence of impurity peaks and amorphous material indicates the highly crys- talline nature of both the samples. Fig. 2 shows the frame- work IR spectra of H/Al-P and H/Fe-/I samples. The bands due to asymmetric stretching vibrations of Si-0-A1 at 1174 and 1074 cm-' in H/Al-B were found to be shifted to 1168 and 1059 cm- ' in the H/Fe-#l sample. The IR profiles of both the samples and the shift observed in the H/Fe-#l sample J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 9u .* 38 30 22 14 2tl/degrees Fig. 1 X-Ray powder diffraction patterns of (a) HIA1-B and (b) H/Fe-B zeolites (*Si as an internal standard) support the pure and crystalline nature of the A1 and Fe"' silicate /3 phases.Uniform ellipsoid-shaped crystallites of 0.4-0.6 pm and 0.5-0.75 pm is size were observed for HIA1-P and H/Fe-fl respectively. The SEM photographs (Fig. 3) also con- firmed the high purity and crystallinity of the /3 zeolite phases. A signal at g = 4.4in the EPR spectra and the data on magnetic susceptibility measurements support the pres- ence of Fe3+ in a tetrahedral coordination with no inter- action of Fe-0-Fe. Nuclear electron coupling of Si-0-Fe was revealed by signal broadening in the 29Si solid-state MASNMR spectra. On the basis of the above observations, it is clear" that the /3 zeolite samples are of high crystallinity without significant impurity or amorphous matter and that isomorphous replacement of Fe3+ in the framework has been achieved.The unit cell compositions cal- culated from the chemical analysis of the products are tabu- lated in Table 1. Table 1 Unit cell compositions of B zeolites ~ sample unit cell composition uc g-'/1020 n 1 I 1200 1000 800 600 ' wavenumber/cm -Framework IR spectra of (a) HIA1-b and (b)H/Fe-B zeolites Fig. 3 SEM photographs of (a) HIA1-B and (b)H/Fe-B zeolites Sorption Properties Equilibrium sorption capacities at PIP, = 0.8 at 298 K for different probe molecules in both H/Al-/3 and H/Fe-P are summarized in Table 2. The salient features of these results are as follows.Equilibrium sorption capacities for all four probe molecules in the protonic forms of the zeolites are mar- ginally higher than those in the sodium form, possibly because of the smaller size of the proton. However, the increase is larger in case of the Fe-p sample. During ammon- ium exchange, possibly Fe3 + species acting as charge bal- ancing extraframework cations were leached out and hence more void made available for sorption in H/Fe-/3. The enhanced Si : Fe for H/Fe-p (Table 1) supports these findings. Equilibrium sorption capacities usually depend on the size of the probe molecule, the zeolitic void volumes, the geometry of the cages and the probe molecule, the zeolitic void volumes, the geometry of the cages and the packing geometry/eficiency.Polar water molecules, being sufficiently smaller in size (kinetic diameter 2.65 A), penetrate almost all the cages in the zeolitic lattice and assume close packing in an attempt to interact with extra-framework cations which balance the charge on alumina tetrahedra. The equilibrium sorption capacity for water is usually regarded as an indica- Table 2 Sorption properties" of B zeolites Al-/!? Fe-#I sorbate Na form H form Na form H form hexane cyclohexane water BA 8.60 (0.28)b 10.31 (0.28) 59.51 (0.27) 11.09 (0.28) 8.61 (0.28) 10.34 (0.29) 59.83 (0.28) 11.12 (0.29) 8.30 (0.26) 8.83 (0.24) 51.90 (0.24) 9.94 (0.25) 8.82 (0.28) 10.60 (0.29) 61.36 (0.28) 10.33 (0.26) Expressed in molecules uc-',at 298 K, PIPo = 0.8.Values in par- entheses represent pore volume in cm3 g-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ,.-I, 0 2 4 6 8 10 12 14 16 2 4 6 8 10 12 14 16 Pporr Pporr Fig. 4 BA sorption isotherms for A, HIAI-0 and B, H/Fe-P zeolites at (a) 333, (b)363, (c) 393, (6)423, (e)453 and (f)483 K tion of the hydrophilic/hydrophobic character of the zeolite framework. Water uptake is ca. 59-61 molecules uc-' for H/Al-/? and H/Fe-/?. Uptake of BA, the next largest probe molecule, is ca. 10.33-11.12 molecules UC-I in the protonic forms of the zeolites. Cyclohexane (kinetic diameter = 6.0 A) being larger than BA, has a slightly lower uptake (10.3-10.6 molecules uc- ') than BA. The uptake of hexane (8.6-8.8 mol-ecules uc-') is rather lower than that of the BA, which may be due to the difference in the packing efficiency of the cylin- drical hexane (kinetic diameter = 4.3 A) and the almost spherical cyclohexane molecule.Table 2 shows that, with the exception of hexane, the uptake of probe molecules follow the order of the reciprocals of their molecular size in zeolites of comparable pore volumes. BA Sorption Isotherms Fig. 4 shows families of typical sorption isotherms in the tem- perature range 333-483 K for Al-/? and Fe-B zeolites. The shape of the isotherms in both the zeolites was found to be similar to Type I (Langmuir type) according to Ki~elev's'~ classification. It can be seen from the nature of the isotherms, that ca. 80% of the total sorption takes place over a low- pressure range (up to 4 Torr).The sorption isotherms clearly indicate that H/Fe-P has a lower equilibrium sorption capa- bility (molecules uc- ') than H/Al-/? over the entire isotherm temperature range. This may be due to the strong interaction between basic BA molecules with the sorption centres of higher acidic strength (usually hydroxyl) groups leading to solvation and volume-filling phenomena. The different extent of uptake may partly be explained on the basis of dif-ference in the charge on the framework oxygen i.e. by the basic character and by the degree of heterogeneity of the charge distribution. In addition to this, the distortion of the framework structure as indicated by the XRD and the com- paratively less non-specific orientation, may be hindering the rearrangement of BA molecules necessary to achieve satura- tion capacity at a particular temperature.This may also, in part, be responsible for the slower uptake and reduced equi- librium saturation capacities for BA sorption in the H/Fe-fi sample. Therefore, in spite of the nearly equal pore volume of the two samples, the saturation capacity of BA varies con- siderably. Applications of Isotherm Equations Analysis of the sorption data in terms of different isotherm models usually yields useful information about the nature of the sorption centres. It was therefore thought appropriate to investigate the influence of the nature of the framework cations on the applicability of various isotherm equations to the BA sorption data obtained in the present study on HIA1-P and H/Fe-/? zeolites.Langmuir Isotherm Equation The Langmuir isotherm model describes sorption equilibrium in a system wherein all the sorption centres are of the same energy and the sorbate molecule is localized on a sorption centre. Fig. 5 shows typical Langmuir plots for BA sorption in HIA1-B and H/Fe-P zeolites. It can be seen that both the zeolites exhibit excellent linear plots with different intercepts. Thus it is evident that the Langmuir sorption model is applicable to sorption in these samples. Similarly the Lang- muir sorption equation was found to represent satisfacto-rily BA sorption data in EU-1 zeolites. Accordingly all the sorption centres appear to have the same sorption potential. The monolayer capacities obtained from the reciprocals of the slopes of these linear plots are tabulated in Table 3.These values of the saturation capacities are in good agreement Table 3 Comparision of saturation capacities of /3 zeolites saturation capacities (molecules uc- ') temperat ure/K method HIA1-B H1Fe-B 333 experimental Langmuir 9.58 9.61 8.47 8.80 9.60 8.75 363 experimental LangmuiriBET 8.80 9.09 8.98 7.55 7.89 7.85 393 experimental Langmuir 8.07 8.33 8.22 6.66 7.06 6.97 423 experiment a1 Langmuir 7.08 7.40 5.85 6.27 7.30 6.23 453 experimental Langmuir 6.45 6.89 5.20 5.57 6.79 5.54 483 experimental Langmuir 6.04 6.45 6.33 4.68 5.28 5.03 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I 1 L I 0 5 10 15 PfTorr P/Torr Fig. 5 Langmuir plots for BA sorption in A, H/Al-P and B, H/Fe-B zeolites at (a) 333, (b)363, (c) 393, (6)423, (e)453 and (f) 483 K. a is the amount sorbed in molecules (uc)- '. with those obtained experimentally. These data also show the higher sorption capacity of H/Al-fi than that of H/Fe-fl. It was also suggested16 that BA sorption is localized and a basic molecule such as BA shows strong interaction with acidic protons and the other acidic species. In the present studies, because of localized sorption of BA and interaction with acidic protons in p zeolites, the Langmuir isotherm equation yields linear plots and represents BA sorption data satisfactorily.The lower saturation capacity in H/Fe-p indi- cates the moderate interaction of BA with the acidic protons suggesting the lower acidity of protons bridged to Fe3+ through oxygen. Another salient feature of these Langmuir plots is the extent of decrease in the value of the intercept on the y-axis with temperature. It was observed that the inter- cept on the y-axis of HIAl-fl is lower than that of H/Fe-p for A -120 IIIIIII all temperatures. The intercept on the y-axis is usually related to the strength of sorption and as the intercept decreases, stronger interaction is involved in the sorption process. This is reflected in the higher acidic strength of H/AI-@ than that of HIFe-B. BET Isotherm Equation Linear plots were obtained by applying the BET equation to the sorption data of BA in HIA1-P and H/Fe-P zeolites, Fig.6. BET plots of H/Fe-/l yield higher values of the intercept on the y-axis indicating a lower value of C, i.e. the heat of sorp- tion of the first layer of sorbate on the sorbent surface. Monolayer capacities calculated from the slopes and the intercepts of these plots (shown in Table 3) are in good agree- ment with those obtained experimentally. The linearity of the r B 40 30 c-I pp 20 v (D 10 0 4 8 12 16 20 (pipo)x 103 Fig. 6 BET plots for BA sorption in A, H/Al-8 and B, H/Fe-/3 zeolites at (a) 333, (b)363, (c)393, (d)423, (e) 453 and (f)483 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 plots indicates that, the BA sorption data for B zeolite can be described by the BET sorption model.The lower value of heat of BA sorption in H/Fe-/? also suggests moderate inter- action of acid centres with BA molecules. Dubinin Isotherm Equation An attempt is made here to apply Polanyi’s potential theory modified by Dubinin and Radushkevich17 for BA sorption in HIA1-P and H/Fe-B in the temperature range 333-483 K. The Dubinin-Rudushkevich equation is expressed as, B log w = log w,--CT 1% P/P,lZ2.303 /?’ where W is the amount sorbed at equilibrium pressure P, W, is the total sorption capacity, B is the constant independent of temperature and characteristic of sorbent pore structure and fi is the affinity coefficient. When BA sorption data were fitted to the above equation, reasonably linear plots were obtained at all temperatures. Typical Dubinin plots are shown in Fig.7 and indicate that the BA sorption data in P zeolite could be satisfactorily expressed by the Dubinin- Rudushkevich equation. The slopes of these plots were found to increase with increase in temperature. The values of satu- ration capacities and B//?’ obtained from the intercepts on the y-axis and the slopes respectively are summarized in 0 1 2 3 4 5 6 (log 7 1.0 I B I .. 0.5L I 1 I 1 I 0 1 2 3 4 5 6 (log p,lp)2 Fig. 7 Dubinin plots for BA sorption in A, HJAl-8 and B, HJFe-fl zeolites at (a)333, (b)363, (c)393, (6)423, (e)453 and (f)483 K 39 1 Table 4. These saturation capacities are in close agreement with those obtained experimentally.This shows that the BA sorption in fl zeolites nearly follows the Polanyi potential theory of volume filling. Since B is independent of tem-perature, the fact that B/B2 increases with increase in tem- perature and at constant temperature is higher for H/Fe-P than HIA1-B confirms the high affinity coefficient, B, for HIA1-B for a particular set of temperatures. Thus, the varia- tion in the affinity coeficient at constant temperature can be used as a means of confirming the isomorphous framework substitution and strength of acidic sites. Sips Isotherm Equation Sips’8 suggested a new theoretical absorption model to cal- culate the distribution of adsorption energies of the sites of a sorbent surface, when sorption isotherms were known and sorption was localized without sorbate-sorbate interaction. The original Sips eq~ation’~ on linearization takes the form: 0log -= log A + c log P1-8 where A and c are constants and P is the equilibrium pres- sure at coverage 8.For calculating 8, the saturation capacities were obtained from Langmuir plots. In order to check the applicability of Sips equation, the experimental values of 8 and P were substituted in the above equation. The salient feature of the Sips plots obtained in the present study is that BA sorption data only yielded linear plots for H/Fe-B, as shown in Fig. 8. The magnitude of c (obtained from the slopes) deviates from unity in the lower temperature region up to 393 K, suggesting deviation from the Langmuir approach in the same region.However, the value c was nearly constant, 1.0 0.04, in the temperature 1.4 1.2 1.o 0.8 m I 0.6 &-a-0 0.4 0.2 0.0 -0.2 I L I I , I -1.0 -0.8 -0.6 -0.4 4.2 0 0.2 log P Fig. 8 Sips plot for BA sorption in H/Fe-8 zeolite at (a) 333, (b)363, (c)393, (6)423, (e)453 and (f)483 K Table 4 Saturation capacities and BIB2 obtained from Dubinin plots for B-zeolites HjAl-f3 H/Fe-f3 temperature saturation capacity saturation capacity /K B/,P x 107 /molecules uc- BJ~x 107 /molecules uc - 333 1.13 9.60 2.34 9.12 363 1.19 9.22 2.50 8.12 393 1.28 8.17 2.67 7.48 423 1.61 7.20 2.84 6.76 453 2.10 6.64 3.10 6.23 483 3.39 6.20 3.81 6.02 392 J.CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 equation to the BA sorption data. Deviation from linearity was observed at higher pressures and at higher temperatures for both the samples. The extent of deviation from linearity for HIA1-/3 was less than for H/Fe-fl at the same temperature. BA sorption data in Fe3+-exchanged type Y16 and, in a dif- ferent cationic form, LTL' zeolites were also satisfactorily represented by linear plots, but the Freundlich sorption model failed to represent BA sorption in EU-1" and in titan- V-0.4L ' ' ' ' ' ' ' ' ' 1 osilicates with MFI structure. 2o 0.1 0.3 0.5 0.7 0.9 1.1 log P Chemical Affinity and the Selectivity of the Sorbed Phase 1.0 -B A reversible and isothermal transformation of a gas, at a standard pressure Po (760 Torr) into an infinite amount of 7h -*_--u--/-sorbent-sorbate mixture under equilibrium pressure, P, decreases the chemical potential.The chemical affinity, when the non-ideality of the sorbate is neglected may be expressed2' as : Ap = RT ln(P/P,) The value of -Ap may be taken as the quantitative measure of the chemical affinity of the sorbate for the sorbent. The ,/f 1 I 1-range 423-483 K. Therefore, complicating factors such as irreversibility in the BA sorption, sorbate-sorbate inter-actions etc., may be operative in the low temperature region in H/Fe-P. However, Sips equation was not found applicable to the case of HIAl-/3 suggesting stronger sorbate-sorbate interactions through localized sorption.Freundlich Isotherm Equation Analysis of BA sorption data in H/Al-fl and HIFe-8 zeolites in terms of the Freundlich isotherm model yielded linear plots in the higher-pressure region in the temperature range 333-483 K. Typical Freundlich plots for BA sorption in both samples are shown in Fig. 9. The excellent linearity of these plots confirms the applicability of the Freundlich isotherm 6.5 I* BA(rnolecu1es uc-') plots of -Ap against the amount sorbed also serves as useful criteria for the comparison of the sorption affinities of a probe molecule in the lattices of zeolites with different tetra- hedral cations. Typical chemical affinity plots for BA sorp-tion in H/Al-P and H/Fe-#l are shown in Fig.10. It can be clearly seen from the figure, that the decrease in -Ap is sharper with the increase in sorption temperature. H/Al-P shows higher sorption affinity than H/Fe-B and the decrease in the sorption affinity is rather gradual in HIA1-P while it is comparatively rapid in HIFe-P. In other words, the chemical affinity for BA sorption in HIA1-B is higher than that in H/Fe-/3 over the entire coverage over the isotherm tem-perature range 333-483 K. Isosteric Heats of Sorption (Q,,) The isosteric heat of sorption is derived from the sorption isosters by applying the Clausius-Clapeyron equation at con- stant sorbate loading using the equation: If Q,, is independent of temperature, the plots of In (P)vs. 1/T 7.3, 1 IB 6.51 I 5.7 .-I-z 4.9 Y,63 I 4.1 3.3 2 4 6 8 BA(rnolecu1esuc-') Fig.10 Chemical affinity curves for BA sorption in A, HIA1-P and B, H/Fe-fi zeolites at (a)333,(6)363,(c) 393,(d)423,(e)453 and (f)483 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -32 -56 c -I -28-52 3 -'wam 24 -48 -20 -44 -16 -40 123456 BA(molecu1es uc-') Fig. 11 Profiles of isosteric heats of the BA sorption for A, H/Al-P and 0,H/Fe-/3 zeolites should be linear. In the present study, isosters for BA sorp- tion in H/Al-p and H/Fe-P were found to be linear. The iso- steric heat was calculated using the slopes of these isosters. Fig. 11 depicts the isosteric heats of the BA sorption in H/Al-p and H/Fe-P zeolites. H/Al-fl zeolite exhibits Q,, values ranging from 40-63 kJ mol-' which is significantly higher than that (18-32 kJ mol-') for H/Fe-/?.For HIA1-P in the lower-coverage region Q,, increases slowly, passes through a maximum, decreases rather rapidly and then exhibits humps in the higher-coverage region. In the lower-coverage region, sorbate-sorbent interactions are usually predominant fol-lowed by both the sorbate-sorbent and sorbate-sorbate interactions in the mid-coverage region. In the higher-coverage region only sorbate-sorbate interactions are oper- ative. For H/Fe-P Q,, also increases initially and then decreases slowly in the mid-coverage region, passes through a minimum and then increases considerably in the higher-coverage region. Both the zeolitic surfaces display heter-ogeneous character towards BA sorption.In the lower-coverage region, HIA1-P exhibits higher values of Q,, than those of H/Fe-P, suggesting the higher acid strength of H/Al-P zeolite than H/Fe-p zeolite. Conclusions Sorption kinetics and equilibrium sorption capacities were found to be in accordance with the molecular sizes of the probe molecules. The void volume, evaluated from equi- librium sorption capacities of different probe molecules, was found to be almost identical for Al-P (0.27-0.29 cm3 g-') and for Fe-p (0.24-0.26 cm3 g- ') in sodium form. Void volumes increased only marginally for /?-A1 but increased notably for p-Fe on their conversion from the sodium to the protonic form. Of the total Fe3+ in the solid, a small fraction, present as charge-balancing cations, seemed to be leached out during conversion from sodium to protonic form via ammonium exchange.Sorption capacitites also revealed a difference in packing efficiency in zeolite voids of hexane and cyclohexane because of their shape differences. BA sorption isotherm data were satisfactorily represented by BET, Dubinin and Langmuir approaches. The Sips equation could represent BA sorption in H/Fe-P zeolite whereas, the Freundlich sorption model was found to be applicable at higher pressures only. H/Al-P exhibits higher sorption selectivity than H/Fe-b for BA sorption. The isosteric heat ((Is,)variation with the cover- age revealed the heterogeneous character of both the sorbent surfaces.Higher isosteric heat for HIA1-p than H/Fe-P is an indication of the comparatively higher strength of acid centres in the former. References 1 J. Schemer, Catalytic Materials: Relationship between Structure and Reactivity, ed. R. T. K. Baker, E. G. Derouane, R. A. Dalla Betta and T. E. Whyte, jun., American Chemical Society, Wash- ington, DC, 1984, p. 157. 2 R. L. Wadlinger, G. T. Kerr and E. J. Rosinski, US Pat. 3 308 069, 1967. 3 M. M. J. Tracy and J. M. Newsam, Nature (London), 1988, 352, 249. 4 J. B. Higgins, R. B. La-Bierre, J. L. Schlenker, A. C. Rohrman, J. D. Wood, G. T. Kerr and W. J. Rohrbaugh, Zeolites, 1988, 8, 446. 5 J. 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Larkin, Proc. R. Irish Akad. B, Centen-ary Issue, 1977,77, 383. 19 R. Sips, J. Chem. Phys., 1948, 16,491. 20 S. P. Mirajkar, A. Thangraj and V. P. Shiralkar, J. Phys. Chern., 1992,%, 3073. 21 V. P. Shiralkar and S. B. Kulkarni, Zeolites, 1985,5, 37. Paper 3/03635I; Received 24th June, 1993