Catalyst Derived from Natural Pumice

Ind. Eng. Chem. Res. 2004, 43, 1659-1664

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High Surface Area Support/Catalyst Derived from Natural Pumice. Study of Pretreatment Variables A. Brito,* F. Garcı´a, C. A Ä lvarez, R. Arvelo, J. L. G. Fierro,† and C. Dı´az Department of Chemical Engineering and T. F. University of La Laguna, Avenue Astrofı´sico Francisco Sa´ nchez, s/n 38200, Canary Islands, Spain

The aim of this study was to obtain a catalyst or support material from a natural pumice that could then be used in the hydroisomerization of n-pentane. Acid treatment of the raw material with HCl was found to extract a larger amount of cations than NH4Ac (Ac ) acetate), yielding a product with a better developed texture and structure. The total number of protons present in the solution affects potassium extraction, while sodium is affected by both factors of concentration and volume of dissolution independently. The specific area of the material (meters squared per gram) obtained depends on the treatment conditions, and it value can be calculated by means of the treatment condition variables or by the total number of moles of the cations extracted. The treatments could be carried out by working at and above ambient temperatures and with and without fresh acid replacements. The optimum treatment for obtaining a catalytic support was 10 mL/g of pumice of 3 M HCl for 10 h with three replacements of fresh acid working at 70 °C. Introduction At present, bifunctional catalysts made of acidified zeolites with supported platinum are used in the isomerization of light paraffin produced from catalytic cracking and distillation to enhance the octane number. The high porosity of volcanic pumice rock and its richness in silica, alumina, and natural zeolites makes this material a promising candidate as a catalyst in reactions that require active centers or as a support for the metallic function required in isomerization or hydrogenation reactions. According to studies on the zeolite content of diverse pumice samples from several zones in the south of Tenerife, an appreciable concentration of phillipsite and other zeolites has been identified. This peculiarity makes this material a candidate for the preparation of catalysts and supports. In any case, the transformation of the raw material into a solid with fastored porous structure and surface acidity requires several treatments in aqueous media in order to remove impurities and contaminants. Natural zeolites are alkaline; therefore, to create acid centers, charge-balancing cations must be replaced by protons. This can be done by deliberately introducing protons, e.g., with mineral acids, although not all zeolites can exchange their charge-balancing cations for protons without losing their crystalline structure. For many zeolites, treatment with low pH solutions also removes aluminum ions from the crystal structure, which leads to a partial or complete collapse of the structure into an amorphous material. When this happens, the changes of cations could be done by a twostep process: the first step consists of the replacement of sodium by ammonium ions until all of the sodium is * To whom correspondence should be addressed. Tel.: +34 922 318 081. Fax: +34 922 318 004. E-mail: [email protected]. † Present address: Instituto de Cata ´ lisis y Petroleoquı´mica (CSIC), Camino de Valdelatas s/n, Campus University Auto´noma, 28049 Cantoblanco, Madrid, Spain.

removed, and the second consists of heating at 400500 °C to eliminate ammonia, leaving protons (H+) within the structure.1 It has been shown that the activity of a zeolite in hydrocarbon cracking/transformation is proportional to the number and strength of Bro¨nsted acid centers introduced through these treatments.2,3 In addition, it is well documented that the acid strength of each center increases when the number of structural aluminum atoms in a particular center decreases.4 The selection of a support substrate is of paramount importance in heterogeneous catalysis because it can influence greatly the properties of heterogeneous catalysts.5-7 The incorporation of a metallic funcion on these substrates results in a bifunctional catalyst. As a general rule, the development of small-size metal particles on the surface of these substrates offers economic advantages. However, the smaller they are, the more difficult it is to characterize the catalyst structure.8 Within this frame, pumice is an ideal support that presents low microporosity and minimizes the interference between the support and metal particles.9 Pumice could be of interest as a support in laboratoryscale reactions controlled by external diffusion and in industrial processes where mass and energy transfer is important. Palladium catalysts supported on pumice have been used in test reactions on a laboratory scale, in the hydrogenation of 1,3-cyclooctadiene10 and phenylacetylene,11,12 and also Pd-Ag catalysts have been used in the selective oxidation of benzyl alcohol13 and 1-butene also using platinum and rhodium particles supported on pumice.14 Similarly, Ni/pumice has been used on a laboratory scale to study their reactivity in carbon monoxide hydrogenation.15 Accordingly, this work was undertaken with the aim of investigating the possibility of using natural pumice from Tenerife as a catalyst or catalyst support for the hydroisomerization of n-pentane. Attention was particularly paid to finding an appropriate method of

10.1021/ie020442e CCC: $27.50 © 2004 American Chemical Society Published on Web 02/13/2004

1660 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 1. Chemical Composition of Pumice compound

%

compound

%

SiO2 Al2O3 Na2O K2O CaO

52.6 19.6 10.2 4.4 2.2

MgO Fe2O3 TiO2 H2O

3.6 3.7 0.7 3.0

extracting cations and characterizing the acidic materials obtained in terms of crystalline structure, Si/Al ratio, and degree of acidity. Finally, a study on the resultant surface area has also been carried out. Experimental Section The pumice used, extracted from a deposit in the south of Tenerife, was first sieved to remove basaltic material, then ground, and sieved again to a particle size of 0.5-1 mm. Its chemical composition is shown in Table 1. This material was then acidified by ion exchange using HCl and NH4Ac (Ac ) acetate). In both cases, a quantity of pumice was placed in contact with aqueous solutions of different concentrations of the above reagents in an Erlenmeyer flask under intermittent stirring. After a time interval found to be suitable for each treatment, it was filtered and then repeatedly washed with deionized water until the wash water was found to be neutral. The samples treated with HCl were then oven dried for 5 h at 110 °C, whereas those treated with NH4Ac were decomposed in air at 400 °C for 1 h to remove acetic acid and ammonia. The concentrations of Na+, K+, Ca2+, and Mg2+ cations in the liquid phase obtained after treating the samples with HCl or NH4Ac were determined by atomic absorption spectrometry in an acetylene-air flame. The acidity of the material was determined by infrared spectroscopy by looking at the vibrations of the O-H bond of the hydroxyl group present in pumice supports obtained through different treatments. The crystallinity of the acid materials was determined by X-ray diffraction, using Cu KR radiation (KR1 ) 1.540 56 Å and KR2 ) 1.544 39 Å). Prior to determining the chemical composition and Si/ Al ratio of several pumice samples, they were first dissolved with aqua reggia and hydrofluoric acid and then treated with a boric acid solution. The concentration of the different cations was measured by atomic absorption spectroscopy and the surface area of the materials by the Brunauer-Emmett-Teller (BET) method from the N-adsorption data at 77 K. Results and Discussion Experimental studies were carried out to determine which of the two pretreatments (HCl or NH4Ac) is the most appropriate. Once this pretreatment can be selected, the extraction capacity is investigated and the texture and structure of the pumice is examined. Comparison of HCl and NH4Ac Pretreatments. Following acidification treatments, the ion-exchange capacity of these two reagents were chosen and compared under the same experimental conditions. The amounts of sodium and potassium (the cations at the highest concentration in pumice) expressed as moles extracted from 100 g of material with HCl and NH4Ac were compared. The treatments were extended for 1 and 3 h, the ratios between the volume of solution/g of

Figure 1. Moles of Na+ and K+ eliminated from pumice treated with NH4Ac and HCl (15 mL g-1, 1 M, 3 h) per 100 g of pumice.

Figure 2. X-ray diffraction spectra for untreated and HCl-treated pumice (10 mL g-1, 1 M, 5 h).

pumice were 5, 10, and 15 mL g-1, and the concentrations were 0.5 and 1 M. The results show that, at low concentrations, the potassium ion is exchanged to a greater extent for ammonium ions than for protons. However, as seen in Figure 1, when the concentration of ions in solution is higher, the sodium and potassium ions are exchanged to a greater degree for protons than otherwise. Besides the degree of exchange obtained after treatment, the textural and structural characteristics of the resultant material are also measured in terms of crystallinity, surface area, and pore-size distribution. Crystallinity may be determined from the presence of natural zeolites in the material, so the level of phillipsite in raw pumice samples and those treated with HCl and NH4Ac was examined. Figure 2 shows the diffraction spectra for untreated and HCl-treated pumice. The phillipsite peak is lower in the latter, indicating a slight loss in crystallinity. A loss of the zeolite phase has been found after treatment of natural clinoptilolite with a HCl solution, but there are increases in the acidity and the effective diameter of the channels and pores of the zeolite.16 In the case of acetate treatment, the phillipsite disappears almost completely because of the high temperatures of the treatment, as was already observed by Brito et al.17 and Garcı´a et al.18 A shift of

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1661

Figure 3. Infrared spectra for pumice treated with HCl and NH4Ac (10 mL g-1, 1 M, 1 h) and untreated. Table 2. BET Surface Area and Micropore Area for Pumice Treated with HCl or NH4Ac

Figure 4. Extraction of cations with 1 M (10 mL g-1) HCl at several times.

conditions 10 mL g -1, 1M untreated HCl NH4Ac

t (h)

BET surface area (m2 g-1)

micropore area (m2 g-1) 6.9

1 3 1 3

28.2 63.8 119 17.8 19.8

87.9

Table 3. HCl Treatment (3 h of Reaction Time) moles extracted/100 g of pumice Va

Va ) 10

Va ) 15

concn (M)

K+

Na+

K+

Na+

K+

0.5 1 3 5

0.059 0.101

0.007 0.024

0.060 0.115 0.146 0.0160

0.022 0.051 0.059 0.058

0.110 0.156

0.042 0.065

1.9

the phillipsite peaks was also observed probably because of the contraction of the unit celt of phillipsite produced by aluminum elimination during acid treatment. Crystallinity can also be determined by infrared spectroscopy. Fierro et al.19 identified the frequencies corresponding to each vibration mode of the T-O bond, where “T” represents an atom of Si or Al situated at the center of the tetrahedral units forming the zeolite structure and “O” the oxygen atoms at the vertexes of these tetrahedra. The infrared spectra of the untreated and HCl- and NH4Ac-treated pumice are displayed in Figure 3. The intensity of the bands at frequencies at which the vibration of the T-O bond occurs is lower for the samples treated with HCl or NH4Ac with respect to the untreated pumice. Treatment with NH4Ac resulted in larger losses of crystallinity than with HCl treatment. This may be due to the heat treatment at 400 °C, which may cause the aluminum to migrate toward the surface, creating amorphous zones. The changes in the BET surface area during the acidification treatment are shown in Table 2. For all treatment conditions using HCl, there is an increase in the BET area compared to the value for untreated pumice, 28.2 m2 g-1, which may be justified by the formation of a microporous structure that becomes progressively more obvious as the structural cations are increasingly replaced by protons. This is deduced from the micropore area results. On the other hand, when the treatment is with NH4Ac, a reduction in the specific area occurs, possibly because of the sintering caused by the heat treatment at 400 °C. This causes surface area changes due solely to heating, independent of time or the reagent concentration. The differences in surface areas of the NH4Actreated samples are explained by the heterogeneous nature of the raw material. The disappearance of micropores by sintering of the support after acetate attack and subsequent heating is confirmed by comparing the micropore area (1.9 m2 g-1) with that for

)5

Na+

a

V: mL of HCl/g of pumice.

untreated pumice (6.9 m2 g-1). These results are in agreement with those18 for the heat treatment of pumice. This comparative study demonstrates that treatment with HCl solutions is the most appropriate for the preparation of either support or supported catalyst based on natural pumice. A thorough experimental study of HCl treatments was therefore carried out under different conditions, and their influence on the textural parameters of these modified pumice samples was determined. HCl Treatment. Some relevant variables affecting the degree of extraction of the different cations were studied using various volumes of acid per g of pumice (between 2.5 and 15 mL g-1), solution concentrations (between 0.5 and 6 M), and treatment times (from 15 min to 12 h). Experiments were also done at temperatures above ambient and with acid renewed with equal volumes at regular time intervals to shift the equilibrium toward the replacement of structural cations by protons. An example of the influence of time is given in Figure 4 when using a 1 M HCl solution. Most of the amounts of extracted cations occurred during the first hour of treatment. One possible explanation of this behavior is that the equilibrium of extraction is almost reached under the experimental conditions used here or all of the pores in the medium are filled. Then, the rate becomes much slower and tends to be stabilized. These effects are clear for all of the extracted cations, although they are more clearly identified in the Na+ extraction. For the two cations extracted, the extent of extraction increases gradually with the HCl concentration if the other variables remain constant. While potassium extraction rises with the volume of solution used, the sodium extraction only increases significantly for volumes above 10 mL g-1, as is seen in Table 3, when some

1662 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 Table 4. Cation Extraction with HCl (10 mL g-1 Pumice) moles extracted/100 g of material Na+ concn (M)

t (h)

Na

0.5

1 5 1 5 1 5 5 5 1 5

0 0 0 0 0 0 1 3 0 0

1 3

5 Figure 5. Experimental and calculated values by correlation (1) of moles of extracted Na+.

a

K+

Tambient

70 °C

Tambient

70 °C

0.055 0.064 0.079 0.115 0.106 0.144

0.082 0.092 0.115 0.130 0.144 0.146 0.182 0.224 0.134 0.155

0.016 0.025 0.040 0.057 0.052 0.059

0.024 0.026 0.044 0.061 0.066 0.069 0.090 0.097 0.068 0.068

0.112 0.136

0.059 0.061

N: number of acid replacements per raw material.

Table 5. Al/Si: Treatment Conditions HCl (10 mL g-1, 70 °C)

Figure 6. Experimental and calculated values by correlation (2) of moles of extracted K+.

support

concn (M)

pumice A pumice B pumice C pumice D pumice E pumice F pumice G H-ZSM5 H-mordenite

0.5 1 3 3 3 5 3 3

a

t (h) untreated 5 5 5 5 10 5 5 5

Na 0 0 0 1 3 0 0 0

N: number of acid replacements per raw material.

of the results obtained for the contentration of the main extracted ions are presented as an example. These effects can be quantified when the moles of Na+ and K+ extracted from 100 g of pumice are correlated with the different extraction variables (molarity, treatment time, and solution volume per g pumice), obtaining the following empirical correlations:

[Na+] ) 1.3V0.2t0.1(0.61M0.023 - 0.57)

(1)

[K+] ) (2.33t0.004 - 2.28)(1 - e-0.12VM)

(2)

where t is time in hours, V is the volume of HCl in mL g-1, and M is molarity. The total number of protons present in the solution (VM) appears to influence the potassium extraction, while the sodium extraction is governed by a combined effect of the concentration and volume of solution independently. Figures 5 and 6 show the experimentally extracted amounts (moles) from 100 g of pumice versus the values derived from correlations (1) and (2). In both cases, the points are clustered around the diagonal with a mean dispersion that seems acceptable considering the heterogeneous nature of the material, which makes it difficult to have identical samples for comparison of the ion-exchange or extraction capacity. The effect of temperature on the degree of extraction of Na+ and K+ is shown in Table 4. From the moles extracted per 100 g of material under different conditions, at ambient temperature and 70 °C, for a volume of acid of 10 mL of HCl per gram of pumice, it is clear that the degree of extraction of both Na+ and K+ increases when the ionexchange treatments are performed at 70 °C. In all of the experiments, an increase in the treatment temperature leads to an increase of sodium extraction, which, in turn, is more marked than that for potassium.

Figure 7. Ratio SiO2/Al2O3 for pumice and zeolite H-ZSM-5 and mordenite supports. Conditions are given in Table 4.

Extractions were also carried out by replacing the solutions in contact with the solid, taking into account that the exchange of charge-balancing cations in the pumice for protons (from HCl) is an equilibrium process. The removal of cations that have already been exchanged from the solution displaces the equilibrium toward an in-depth exchange of cations for protons. This procedure results in an increase of the degree of exchange, as seen in Table 4, which is greater for potassium than for sodium (33% vs 25%). It is emphasized here that the activity, selectivity, and stability of acid zeolites depend on the Si/Al structural ratio and therefore on their density of acid centers. To see how the Si/Al ratio varies in materials obtained by different acid treatments, this parameter has been determined in both modified pumices and commercial zeolites used as hydroisomerization catalysts (Mordenite and ZSM5). The treatment conditions for each sample are shown in Table 5. The results obtained for the Si/ Al ratio (Figure 7) demonstrate a progressive dealumi-

Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004 1663

Figure 8. Change in curvature at 980 cm-1 in infrared spectra for pumice treated with HCl (10 mL g-1, 1 h).

nization of the material as molarity of the acid increases, with more acute changes at low concentrations. From acid concentrations higher than 3 M, the Si/Al ratio remains practically constant because a more aggressive treatment might dissolve part of the material, with Si and Al being simultaneously removed. When the equivalent materials H-ZSM5, H-mordenite, and pumice are compared, the latter presents Si/Al ratios of about half of those of commercial preparations but within the range of 10-50 proposed by Voorhies and Bryant20 as optimal in catalysts used to obtain maximum activity and selectivity for the n-pentane isomerization reaction. Structural and Textural Characteristics of the Materials. Textural and structural properties of the acid-modified samples were studied with the aim of identifying the most suitable for use as catalysts or supports. Crystallinity, acidity, pore-size distribution, and specific surface area were measured. The X-ray diffraction patterns of the original untreated pumice and others treated with HCl (10 mL g-1, 5 h, 1 M) are shown in Figure 2. They show that the phillipsite peak observed at 27.5° in the untreated sample is lower for the 1 M acid-treated sample and practically disappears for the sample treated with 3 M acid, which indicates a progressive loss of crystallinity with an increase in the severity of the treatment. These changes in crystallinity can also be studied by infrared spectroscopy. If a comparison is made between the spectra of materials treated with different volumes of acid, a decrease in the intensity of the frequency bands corresponding to the T-O bond is clearly observed. This means that crystallinity has been reduced depending on the acid volume/g of pumice ratio employed. The same result is obtained when the concentration of the solution increases. The comparison of the infrared spectra also gives a qualitative idea of the acidity changes brought about by the pretreatments. Thus, in Figure 8, a turning point appears at 980 cm-1 and the curvature increases for more severe acidification treatments. This change is due to the vibration mode of the Si-O bond of the silanol groups Si-O-H,21 indicating an increase in the proportion of Bro¨nsted acid centers when the HCl concentration is raised. The turning point and curvature increase also appear in the spectra of the samples pretreated with larger acid volume/g of pumice ratios for longer times. In the spectra, the 790 cm-1 band is more intense with a higher HCl concentration in the pumice treatment, which corresponds to progressive dealuminization in

Figure 9. BET surface area obtained from eq 3 compared to that obtained by N2 adsorption. Table 6. BET Surface Area for Natural and Modified Pumice HCl treatment concn (M) t (h) mL g-1

BET surface area (m2 g-1)

micropore area (m2 g-1)

untreated 5 1 3 5 5 1 1

28.2 68.4 63.8 119.0 139.9 73.8 123.4 132.1

6.9

0.5 1 1 1 1 3 6

10 10 10 10 5 10 10

87.9 91.1

accordance with the postulates of Fejes et al.22 This frequency band is also more intense when the acidity is increased by raising the acid volume/pumice weight ratio or increasing the treatment time. Characterization studies with dealuminized mordenite have shown that the peak corresponding to the T-O band rises while the aluminum content decreases,23,24 with linear trends being found in both cases. In this study, the shift observed in the band at 790 cm-1 has been attributed to the degree of Na+ and K+ replacement by protons, which is related to the Si/Al ratio. A linear tendency was found. The specific surface area, As (m2 g-1), determined by nitrogen adsorption isotherms, was determined for the natural pumice and the modified counterparts by different treatments. The results are summarized in Table 6. These values have been correlated with the extraction variables (time, HCl volume, and concentration), arriving at the empirical equation

As ) V0.92(-0.57t1.25M1.97 + 5.4t0.74M + 3.4) (3) The calculated specific surface area values are plotted in Figure 9 versus the experimental results. It is clear that the experimental values are fitted well. In the ideal plot, all of the points should be placed in the diagonal. The error interval of (5% obtained is good. The specific area was found to be greater after a longer treatment time with the same acid concentration,

1664 Ind. Eng. Chem. Res., Vol. 43, No. 7, 2004

Figure 10. BET surface area as a function of moles of chargebalancing cations extracted [experimental values: (b, 9) (5% error; (s) eqs 5 and 6].

reaching values of up to 139.9 m2 g-1. Upon an increase of this concentration, the specific area increases to a greater extent after longer times, with a higher specific area being attained with a HCl treatment in a 1 M solution for 5 h than in a 6 M solution for 1 h. After treatments, the resultant specific areas of the different materials should be related in some way to the cation extraction rate. This variable has therefore been correlated with the total amounts extracted (moles) from all cations (mt) that are present in appreciable concentrations (Na+, K+, Ca2+, and Mg2+). The following expression was derived:

As ) 26.5e8mt

(4)

Taking into account that the most easily determined cations extracted from pumice are Na+ and K+, to facilitate calculation of the surface area by analysis of the least number of cations, only these most abundant cations (Na+ and K+) were considered for the sake of calculations. Thus, eq 4 is slightly modified:

As ) 27e9mNa+K

(5)

Correlations (4) and (5) are plotted in Figure 10 together with the experimental results, with the data fitting within a (5% error. From the results of acidity, crystallinity, structural cation extraction, and surface area, the most suitable treatments can be estimated to obtain an acid catalyst or a support to incorporate a metallic function. An above ambient temperature was selected because it allows a higher degree of cation extraction. Only treatments below a 5 M concentration and 5 h duration should be considered. Other more severe conditions did not result in a further improvement. Finally, the treatments selected for obtaining supports to be used in the next step of incorporation of a metal phase, which defines a bifunctional catalyst, are the labels C-F in Table 5. At first sight, the designated sample with label F appears as the most appropriate for both acid catalysts and supports with a specific surface area of 527 m2 g-1. Literature Cited (1) Wojciechowski, B. W.; Corma, A. Catalytic Cracking. Catalysis. Chemistry and Kinetics; Marcel Dekker: New York, 1986. (2) Shertukde, P. V.; Hall, W. K.; Dereppe, J. M.; Marcelin, G. Acidity of H-Y Zeolites: Role of Extralattice Aluminum. J. Catal. 1993, 139, 468. (3) Boudart, M.; Djega-Mariadassou, G. Reaction Kinetics in Heterogeneous Catalysis; Fundan University Press: Shangai, China, 1988; p 199.

(4) Beaumont, R.; Barthomeuf, D. X, Y Aluminum-Deficient and Ultrastable Faujasite-Type Zeolites. I. Acidic and Structural Properties. J. Catal. 1972, 26, 218. (5) Boitiaux, J. P.; Cosyns, J.; Vasudevan, S. Hydrogenation of Highly Unsaturated Hydrocarbons over Highly Dispersed Palladium Catalyst. Part I: Behavior of Small Metal Particles. Appl. Catal. 1983, 6, 4. (6) Boitiaux, J. P.; Cosyns, J.; Vasudevan, S. In Preparation of Catalysts; Poncelet, G., Grange, Jacobs, P. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1983; p 123. (7) Aduriz, H. R.; Bodnariuk, P.; Dennehy, M.; Gigola, C. E. Activity and Selectivity of Pd/R-Al2O3 for Ethyne Hydrogenation in a Large Excess of Ethene and Hydrogen. Appl. Catal. 1990, 58, 227. (8) Fagherazzi, G.; Benedetti, A.; Deganello, G.; Duca, D.; Martorana, A.; Spoto, G. Pumice-Supported Palladium Catalysts. J. Catal. 1994, 150, 117. (9) Venezia, A. M.; Floriano, M. A.; Deganello, G.; Rossi, A. Study of Pumice Supported Palladium and Platinum Catalysts. Surf. Interface Anal. 1992, 18, 532. (10) Deganello, G.; Duca, D.; Liotta, L. F.; Martorana, A.; Venezia, A. M.; Benedetti, A.; Fagherazzi, G. Pumice-Support PdPt Bimetallic Catalysts: Synthesis, Structural Characterization, and Liquid-Phase Hydrogenation of 1,3-Cyclooctadiene. J. Catal. 1995, 151, 125. (11) Duca, D.; Liotta, L. F.; Deganello, G. Selective Hydrogenation of Phenylacetylene on Pumice-Supported Palladium Catalysts. J. Catal. 1995, 154, 69. (12) Duca, D.; Liotta, L. F.; Deganello, G. Liquid-Phase Hydrogenation of Phenylacetylene on Pumice Supported Palladium Catalysts. Catal. Today 1995, 24, 15. (13) Liotta, L. F.; Venezia, M.; Deganello, M.; Longo, A.; Martorana, A.; Schay, Z.; Guczi, L. Liquid phase selective oxidation of benzyl alcohol over Pd-Ag catalysts supported on pumice. Catal. Today 2001, 66, 271. (14) Boutonnet, M.; Kizling, J.; Mintsa-Eya, V.; Choplin, A.; Touroud, R.; Maire, G.; Stenius, P. Monodisperse Colloidal Metal Particles from Non-Aqueous Solution. Catalytic Behaviour in Hydrogenation of But-1-ene of Platinum Palladium, and Rhodium Particles Supported on Pumice. J. Catal. 1987, 103, 95. (15) Venezia, A. M.; Parmaliana, A.; Mezzapica, A.; Deganello, G. Pumice-Supported Nickel Catalysts. Structural and Reactivity Study in the Hydrogenation of CO. J. Catal. 1997, 172, 463. (16) Rodrı´guez-Iznaga, I.; Go´mez, A.; Rodrı´guez-Fuentes, G.; Benı´tez-Aguilar, A.; Serrano-Ballan, J. Natural clinoptilolite as an exchanger. Microporous Mesoporous Mater. 2002, 53, 71. (17) Brito, A.; Garcı´a, F. J.; Borges, M. E.; Gonza´lez, A. R.; A Ä lvarez, M. C. Uso de la Pumita como Catalizador o Soporte Catalı´tico. Congreso de la Sociedad Espan˜ola de Cata´lisis SECAT’97, 1997. (18) Garcı´a, J.; Gonza´lez, M.; Ca´ceres, J.; Notario, J. Structural Modifications in Phillipsite-Rich Tuff Induced by Termal Treatment. Zeolites 1992, 12. (19) Fierro, J. L. G. Spectroscopic Characterization of Heterogeneous Catalyst: Methods of surfaces analysis. Stud. Surf. Sci. Catal. 1990, 57. (20) Voorhies, A.; Bryant, P. A. Hydroisomerization of Normal Pentane over a Zeolite Catalyst. AIChE J. 1968, 14, 852. (21) Fierro, J. L. G. Spectroscopic Characterization of Heterogeneous Catalysts. Chemisorption of Probe Molecules. Stud. Surf. Sci. Catal. 1990, 57. (22) Fejes, P.; Hannus, I.; Kiricsi, I. Dealumination of Zeolites with Phosgene. Zeolites 1984, 4, 73. (23) Musa, M.; Tarina, V.; Stoica, A. D.; Ivanov, E.; Postinaru, D.; Pop, E.; Pop, Gr.; Ganea, R.; Birjega, R.; Musca, G.; Paukshtis, E. A. Some Structural Characteristics of Dealuminated Synthetic Mordenites. Zeolites 1987, 7, 427. (24) Van Niekerk, M. J.; Fletcher, J. C. Q.; O’Connor, C. T. Characterization of Dealuminated Large-Port Mordenites. J. Catal. 1992, 138, 150.

Received for review June 18, 2002 Revised manuscript received October 22, 2002 Accepted December 13, 2002 IE020442E