Abstract
A commercial Na-mordenite sample underwent ion exchange with HCl. The ion-exchanged sample was sequentially submitted to hydrothermal treatments at 823, 873 and 923 K, each followed by acid leaching of the extraframework alumina (EFAL) generated. Six mordenite samples, presenting different framework and extraframework compositions, were obtained. These samples were used to prepare bifunctional catalysts by mixing them with Pt/Al2O3 in different proportions. The generated samples presented distinct platinum contents and were tested in the hydroisomerization reaction of ethylbenzene. A maximum xylene selectivity at about 0.45 wt% of platinum was observed. Normally, the total activity increased as the platinum content increased; this effect was more pronounced in the samples which presented lower mesoporosity. The most dealuminated sample, which presented a high mesoporosity, did not show any change in activity with the increase in platinum content.
Hydroisomerization; ethylbenzene; xylenes; zeolite; mordenite; dealumination
Hydroisomerization of Ethylbenzene on Mordenite-Based Bifunctional Catalysts with Different Platinum Contents
L.D. Fernandes1,2** To whom correspondence should be addressed. To whom correspondence should be addressed., A. Corma3, A. Martinez3, E.F.S. Aguiar4,5 and J.L.F. Monteiro1
1NUCAT - Programa de Engenharia Química - PEQ/COPPE - UFRJ, Ilha do Fundão, CT,
Bloco G, CP68502, CEP 21945-970, Rio de Janeiro, Brazil; Fax: (55-21)290-6626
2DTQ/IT/UFRRJ, Antiga Estrada Rio São Paulo, km 47, CEP 23851-970, Seropédica,
Rio de Janeiro, Brazil; Fax: (55-21) 682-1865
3Instituto de Tecnología Química - ITQ/CSIC - UPV, Av. de los Naranjos, s/n. 46022,
Valencia, Spain
4CENPES/PETROBRÁS, Divisão de Catalisadores, Ilha do Fundão, Quadra 7,
CEP 21949-900, Rio de Janeiro, Brazil
5Escola de Química - UFRJ, Ilha do Fundão, CT. Bloco E, CEP 21499-900, Rio de Janeiro, Brazil
(Received: November 5, 1997; Accepted: March 9, 1998)
Abstract - A commercial Na-mordenite sample underwent ion exchange with HCl. The ion-exchanged sample was sequentially submitted to hydrothermal treatments at 823, 873 and 923 K, each followed by acid leaching of the extraframework alumina (EFAL) generated. Six mordenite samples, presenting different framework and extraframework compositions, were obtained. These samples were used to prepare bifunctional catalysts by mixing them with Pt/Al2O3 in different proportions. The generated samples presented distinct platinum contents and were tested in the hydroisomerization reaction of ethylbenzene.
A maximum xylene selectivity at about 0.45 wt% of platinum was observed. Normally, the total activity increased as the platinum content increased; this effect was more pronounced in the samples which presented lower mesoporosity. The most dealuminated sample, which presented a high mesoporosity, did not show any change in activity with the increase in platinum content.
Keywords: Hydroisomerization, ethylbenzene, xylenes, zeolite, mordenite, dealumination.
INTRODUCTION
Isomerization of the C8 aromatic fraction is a very important process in the petrochemical industry, due primarily to increasing demand for p-xylene for use in the production of polyester resins (Hsu, 1988). This fraction is usually obtained from naphtha reforming and it contains ethylbenzene (10-60%) and xylene isomers with a composition close to that of thermodynamic equilibrium (para/meta/ortho = 1/2/1). Since the boiling points of the C8 isomers are too close to make their separation by distillation feasible, p-xylene is usually separated from the other components by fractional crystallization or by selective adsorption using molecular sieves. The raffinate, now poor in p-xylene, is isomerized; in order to avoid ethylbenzene holding up in the process, it is most desirable to either isomerize it into xylenes or to dealkylate/transalkylate it into products which can be easily separated.
While xylene isomerization can proceed on acid catalysts, ethylbenzene isomerization can only take place in the presence of bifunctional catalysts, presumably by a mechanism involving hydrogenated intermediates like alkylcyclohexenes (Röbschläger and Christoffel, 1979, 1980; Gnep and Guisnet, 1977a, 1977b). Currently, there is an increasing trend to use bifunctional catalysts in the presence of hydrogen co-feed to isomerize the C8 aromatic fraction. Besides enabling ethylbenzene conversion to be carried out, these catalysts are more stable to deactivation by coking than are the monofunctional ones.
Many bifunctional catalysts commercially used to isomerize the C8 aromatic fraction contain a zeolitic component (either mordenite or ZSM-5) in combination with a noble metal (platinum) supported on alumina or silica-alumina which provides the hydrogenation/dehydrogenation function. Although mordenite has the potential advantage of hydroisomerizing ethylbenzene into xylenes, the results obtained up to now show that there is some room for improvement (Ribeiro, 1989; Ribeiro et al., 1989, 1991; Travers et al., 1989; Basset et al., 1990; Benazzi et al., 1993; Silva et al., 1995a, 1995b). On the other hand, ethylbenzene is converted on ZSM-5 -based catalysts, mainly by dealkylation. This produces undesirably large amounts of benzene and light hydrocarbons (Haag and Olson, 1990), although there has been a recent claim (Brown and Huang, 1991) that the use of a high SAR ZSM-5 may suppress the dealkylation reaction.
This work was aimed at continuing the study of zeolite-based catalysts for ethylbenzene hydroisomerization (Fernandes et al., 1996). In this work, bifunctional catalysts were prepared by mixing mordenites, which were obtained by post-synthesis treatments and presented different framework and extraframework compositions, with Pt/Al2O3. The final platinum content was varied by using different zeolite - Pt/Al2O3 ratios to prepare the catalysts.
EXPERIMENTAL
Preparation of Catalysts
The parent mordenite sample was a commercial sodium mordenite (NaM) from PQ Corporation (VALFOR CP500-11) having a SiO2/Al2O3 ratio (SAR) of 10.7, as measured by X-ray fluorescence (XRF), and no EFAL species, as indicated by 27Al MAS NMR.
The NaM sample was converted into its acid form by ion exchange with a 0.5 M hydrochloric acid solution (H+/Na+ = 2.0) at 298 K for 1 h (sample HM000). The ion-exchanged sample (0.6 wt% of Na2O) was hydrothermally treated at 823 K in a tubular furnace with 100% steam for 2 h (sample HM201). Part of this sample was acid leached with a 4 M hydrochloric acid solution under reflux for 2 h using an acid to zeolite ratio of 30 cm3/g to produce sample HM211. The latter was submitted to two additional cycles of steaming and acid leaching to obtain a highly dealuminated mordenite (sample HM213). These cycles were similar to the previous one, except for the steaming temperature (873 K and 923 K for the 2nd and 3rd cycles, respectively). Sample HM000 was also acid washed with a 1.0 M HCl solution under reflux for 3 h (sample HM010).
The bifunctional catalysts were obtained by thoroughly mixing the zeolites with a Pt/Al2O3 sample in the proportions necessary to obtain the desired overall platinum content. The Pt/Al2O3 (0.6 wt% Pt) was prepared by impregnation of a g -Al2O3 (Merck) with the required amount of hexachloroplatinic acid dissolved in a 0.2 M hydrochloric acid solution. The slurry was dried in a rotavapor and calcined at 773 K for 3 h. The Pt/Al2O3 sample presented a BET area of 118 m2/g and a metallic area of 1.2 m2/g, as determined by N2 adsorption and H2 chemisorption, respectively.
A mordenite-based catalyst containing 1.0 wt% Pt was prepared in a similar way by mixing the zeolite with a Pt/Al2O3 (2.0 wt% Pt). To name the bifunctional catalysts, the platinum content was incorporated into the original name of the zeolite sample. Thus, sample HM000 (0.45%Pt) means the catalyst composed of the HM000 sample plus the Pt/Al2O3, presenting a 0.45 wt% global platinum content.
Characterization of Catalysts
The chemical composition of the samples was measured by X-ray fluorescence (XRF) using a Phillips PW1407 spectrometer. X-ray diffraction analyses were performed with a Phillips PW1729 diffractometer, using monochromated CuKa radiation at 40 kV and 40 mA and a scanning rate of 0.1° (2q )/s. Crystallinity was calculated by comparing the areas under 2q = 5-31.6° peaks of the diffractograms to the corresponding areas of NaM, taken as a 100% crystalline reference.
A Nicolet 710 FTIR spectrometer was used to measure the acidity of the samples. Self-supported wafers with 10 mg/cm2 were pretreated overnight under vacuum at 673 K and then cooled to room temperature. After acquisition of the spectrum in the OH stretching region, pyridine (6.6 kPa) was added to the cell until equilibrium was reached. Next, the samples were heated consecutively under vacuum to 523, 673 and 723 K with spectra acquisition at room temperature after each desorption treatment.
Textural properties were determined from N2 physisorption at 77 K in a Micromeritics ASAP 2400 apparatus. Before measurement, the samples were pretreated in a vacuum at 573 K for 3 h.
Solid-state 29Si and 27Al NMR spectra were collected in a Varian VXR-300 FT NMR spectrometer at 7.05 T, equipped with a Varian CP-MAS probe. The 29Si MAS NMR spectra were obtained at 59.8 MHz using 8.2 µs (p /2) pulses and a 20 s delay, with a total accumulation of 500 pulses. The magic angle spinning speed was 3 kHz. The 27Al MAS NMR spectra were obtained at 78.4 MHz using 0.7 µs (p /12) pulses and 0.2 s delay, with a total accumulation of 3000 pulses. The magic angle spinning speed was 7 kHz. The 27Al NMR spectra were recorded before and after washing the samples with a 38% ethanolic solution of acetylacetone (ACAC). This treatment was carried out to try to account for all the Al species, including the so-called NMR "invisible" ones.
The surface zeolite composition was measured by X-ray photoelectron spectroscopy (XPS). The XPS spectra were recorded at room temperature using a Perkin Elmer model 1257 spectrometer with a hemispherical analyzer operated with a MgKa X-ray source (hn = 1253.6 eV).
Catalytic Evaluation
The bifunctional catalysts, formed by the Pt/Al2O3-zeolite mixture, were pelletized, crushed and sieved to 0.59-0.84 mm particles. Before starting the reaction, the catalysts were submitted to an in situ calcination at 773 K under nitrogen flow, followed by reduction at 723 K under hydrogen flow. The ethylbenzene isomerization reaction was performed in a stainless steel tubular reactor at a total pressure of 15 bar, a hydrogen/ethylbenzene ratio of 5.0 (mol/mol), and a temperature in the 653-693 K range. Space velocity (WHSV, based on the weight of zeolite) was varied between 5 and 100 h-1 in order to obtain different ethylbenzene conversions. Products were analyzed on line by gas chromatography with a 60 m length CP-WAX capillary column and a FID. Since the catalysts were considered to be very stable, the experimental data were obtained in steady state after 4 hours of TOS, with sampling conducted every 30 minutes.
RESULTS AND DISCUSSION
Characterization
Table 1 presents a description of the treatments used to obtain the different zeolite samples and their nomenclature; composition, measured by X-ray fluorescence, 29Si and 27Al solid-state NMR and XPS; and crystallinity, measured by X-ray diffraction.
It can be seen from this table that the framework SAR of HM201, as measured by 29Si NMR, is higher than the value measured by 27Al NMR. This indicates that EFAL species that were NMR invisible (Bodart et al., 1986) were present. The leached samples (HM211 and HM213) presented values of framework SAR, as measured by both these two techniques, that did not differ appreciably, indicating that those so-called NMR invisible EFAL species were extracted preferentially by acid leaching. Solid-state NMR showed that the acid leaching removed practically all EFAL species that were generated during the previous hydrothermal treatment (compare framework and global SAR values for the HM211 sample). However, an extraction of framework aluminum could also be observed.
From XPS analyses, it could be observed that the hydrothermally treated samples presented an aluminum-rich outer shell, suggesting that the EFAL species migrated from the crystal center during steaming. When acid leaching of these samples was carried out, a preferential aluminum removal from the outer shell could be observed. Both EFAL migration and preferential extraction of Al from the outer shell have been reported in the literature (Scherzer, 1984), including previous works from our own group (Bartl, 1990; Fernandes, 1992; Fernandes et al., 1994).
From Table 1 it can be seen that crystallinity was preserved, even for the most dealuminated sample. On the contrary, an increase in crystallinity values can be observed as the degree of dealumination increases. This could be due to the increase in the homogeneity of the samples caused by the framework aluminum removal. The lower crystallinity value of HM000 ought to be due to the fact that this sample was submitted to an acid treatment that removed framework aluminum, leaving structural defects (silanol sites) that were not healed by thermal treatments. The HM211 and HM213 samples were also submitted to acid treatments; however, they had suffered previous hydrothermal treatments that had already removed most of their framework aluminum without leaving behind silanol groups, which are condensed under these conditions. Thus, the main effect of the acid treatments on these samples was EFAL extraction.
The textural characteristics, determined by nitrogen physisorption, and the acidity, measured by IR spectroscopy of adsorbed pyridine, are presented in Table 2.
From N2 adsorption/desorption isotherms, a hysteresis loop was observed for relative pressures higher than 0.4, which increased with an increase in degree of dealumination. This phenomenon indicates a progressive mesopore formation as the degree of dealumination increases. This observation may be confirmed in Table 2, where higher mesopore volumes can be observed for the more dealuminated samples. TEM micrographs confirmed these data, showing the appearance of lighter regions presenting a diameter of about 50 - in the more dealuminated samples (Fernandes, 1996).
Chemical composition, XRD crystallinity and treatments performed to obtain the different samples
Textural properties and acidity of the mordenite samples as measured by IR with adsorption of pyridine and dessorption at different temperatures
It can be observed that the micropore volume of the most dealuminated sample (HM213) corresponds to ca. 75% of the initial value (sample HM000), indicating that a significant drop in crystallinity did not take place. The micropore volume decrease was probably associated with amorphous debris deposited in the zeolite channels. These debris originated from parts of the framework which partially collapsed, giving rise to the mesopores. It can be seen that the HM010 sample had a micropore volume close to that of the HM000 sample. This indicated that this sample presented good crystallinity despite the lower value from X-ray diffraction.
These results are in agreement with previous work by our group on zeolites dealumination (Fernandes et al., 1994), in which a more detailed discussion about the influence of dealumination on the physicochemical and textural properties of mordenite was made.
From the IR spectra, it could be observed that only a fraction of the hydroxyls vibrating at ca. 3610 cm-1 (acidic OH) interacted with pyridine at 523 K for the HM000, HM010 and HM201 samples, suggesting that some of the Brønsted acid sites in these samples were inaccessible to the pyridine molecules. Theoretically, the mordenite structure does not present cavities with openings smaller than the kinetic diameter of the pyridine molecules. Thus, the presence of hindered acid sites in the HM000 and HM010 samples having low mesoporosity (Table 2), might possibly arise from the presence of stacking faults that may reduce the effective diameter of the mordenite channels.
These hindered acid sites could explain why the HM000, HM010 and HM201 samples presented low values of Brønsted acid site concentration (Table 2), as determined by IR spectroscopy of adsorbed pyridine. For example, sample HM000 presented a structural aluminum content five times that of sample HM211. However, it presented only twice as many Brønsted acid sites as sample HM211. For HM211, the presence of a mesoporous system connecting the main channels should have made all acid sites accessible to the pyridine molecules.
The HM201 sample presented low Brønsted and Lewis acidities indicating that there were acid sites that were blocked by the EFAL generated during the steam treatment. The acid treatment of HM000, producing HM010, caused an increase in the number of Brønsted acid sites accessible to the pyridine molecules. However, from the IR spectra, the presence of hindered acid sites could still be observed. The most dealuminated sample (HM213) presented low acidity due to the very low framework aluminum content.
Catalytic Evaluation
Some experiments were initially carried out using the zeolite and Pt/Al2O3 samples separately. On the zeolite samples, disproportionation and dealkylation reactions occurred and only traces of isomerization products (0.2% xylene selectivity) were obtained. In addition, the catalyst deactivated rapidly due to coke formation.
The catalyst formed only by Pt/Al2O3 produced mainly C8 naphthenes and only traces of others products when the reaction was carried out at 653 K. At 693 K low amounts of cracking (or hydrogenolysis) products, benzene and toluene were also observed.
For the bifunctional catalysts, formed by mixing the zeolite and Pt/Al2O3, the formation of isomerization products (xylenes), along with those originating from secondary reactions, was observed. It was also observed that all hybrid catalysts were very stable, with no changes in activity and selectivity throughout the runs. However, after the experiments, the samples presented a gray color, indicating that some coking took place.
Conversion at a constant space velocity and selectivities at isoconversion (ca. 60%) for the mordenite-based catalysts containing different amounts of platinum are presented in Table 3.
A maximum yield to xylenes at a Pt content of about 0.45 wt% can be observed. The total activity showed a similar behavior or hardly changed at higher Pt loadings. Moreover, the yield to disproportionation (benzene + diethylbenzene) and dealkylation (benzene + light compounds) products decreased when the Pt content increased.
In Table 4, the catalytic properties of the mordenite-based catalysts containing 0.45 wt% and 0.30 wt% of platinum are compared.
The HM211 sample was the most active among the catalysts tested for both platinum contents, in spite of the higher acidity of sample HM000. This indicated that there were others factors besides acidity that were influencing catalyst activity. One possibility is the presence of diffusional restraints in the pores of the samples with low mesoporosities. The values for mesopore area and volume of the HM000 sample are low (see Table 2). In addition, the mordenite structure is formed by a monodimensional microporous system. These facts suggest that the HM000 sample should have presented high diffusional restraints to the reactant molecules, which could explain the low activity of this sample in relation to its acidity. From acidity determination by IR spectroscopy of adsorbed pyridine, the existence of acid sites that were inaccessible to the probe molecules had already been evidenced. Thus, the reaction should be even more strongly affected than pyridine adsorption because the ethylbenzene molecule is larger than that of pyridine.
-1 and 693 K) and product selectivities obtained at ca. 60% conversion with Pt/Al2O3-mordenite catalysts with different platinum contents-1.
The HM211 sample presented fewer diffusional restraints, probably due to the mesopore system connecting its main channels, thus compensating for its lower acidity, resulting in a more active catalyst.
The HM000 and HM010 samples were those which presented the best performance in terms of xylene selectivity. The HM211 sample, despite being the most active, presented low yields of xylenes, producing mainly gas and benzene.
Comparing samples HM211, HM000 and HM010, it can be observed that HM211 was the most active because it promoted the dealkylation and the disproportionation reactions, while the other two samples were more active for the isomerization reaction. This suggests that for the samples presenting diffusional restraints (indicated by IR spectroscopy of adsorbed pyridine), the reaction occurred close to the external surface of the zeolite, increasing interaction with the neighboring platinum, and thereby favoring bifunctional reactions.
It could also be observed that the difference in activity between sample HM000 and the more dealuminated ones becomes smaller for the catalysts presenting 0.45 wt% platinum. The HM000-based catalyst with 0.30 wt% Pt was less active than the corresponding HM010-based one, while for 0.45 wt% platinum both catalysts presented similar activities. When HM213- and HM000-based catalysts are compared, this trend is even more pronounced, with the latter being more active for the larger platinum content. This behavior seems to be associated with mesoporosity. Thus, for samples presenting high mesoporosity (HM211 and HM213), activity hardly changed when going from the 0.30 to 0.45 wt% Pt catalysts, while a strong increase in activity can be observed for the low mesoporosity samples (HM000 and HM010). A possible explanation could be the coke formation during the first minutes of TOS. Although no catalytic deactivation could be observed for all the catalysts tested, there was evidence that some coking took place. Over the 0.45 wt% Pt catalysts coke formation should have be more strongly inhibited than over the 0.30 wt% Pt ones, thus explaining the higher activity of the former catalysts. Samples presenting low mesopore volumes should be more sensitive to coking than those presenting high mesopore volumes.
As far as the yields at 60% conversion are concerned, there was a clear increase in the selectivity for xylenes and naphthenes as Pt content was increased, the reverse being observed for benzene and gas. This points to a competition between those reactions demanding acidic sites (dealkylation) and those involving metallic sites (hydrogenation and isomerization), the latter being favored at higher Pt contents.
CONCLUSION
The use of mordenite-based bifunctional catalysts, presenting different amounts of platinum for the ethylbenzene hydroisomerization reaction, showed that in addition to zeolite acidity, this reaction was also affected by reactant diffusion inside the zeolite pores. Therefore, samples presenting low diffusional restraints, due to their high mesoporosity, were more active than those presenting greater acidity but high diffusional restraints. Platinum content affected xylene selectivity, since the metal favored the formation of ethylbenzene isomerization hydrogenated intermediates (naphthenes).
ACKNOWLEDGEMENTS
The authors thank CNPq for supporting this work.
REFERENCES
Bartl, P.E., M.Sc. thesis, PEQ/COPPE/UFRJ (1990).
Basset, J.M., Choplin, A., Raatz, F., Theolier, A. and Travers, C., World Patent WO 90/09845 (1990).
Benazzi, E., Tavernier, S., Beccat, P., Joly, J.F., Nedez, Ch., Besset, J.M. and Choplin, A., Prep. Div. Petr. Chem., 206th National Meeting, American Chemical Society, Chicago, 561 (1993).
Brown, L.M. and Huang, T.J., U.S. Pat. 5028573 (1991).
Bodart, P., Nagy, J.B., Debras, G., Gabelica, Z. and Jacobs, P.A., J. Phys. Chem., 90, 5183 (1986).
Fernandes, L.D., M.Sc. thesis, PEQ/COPPE/UFRJ (1992).
Fernandes, L.D., Monteiro, J.L.F, Bartl, P.E., Gusmão, J., Menezes, S.C. and Cardoso, M.J.B., Zeolites 14, 533 (1994).
Fernandes, L.D., Corma, A., Martinez, A., Sousa-Aguiar, E.F. and Monteiro, J.L.F., Anais do XIV Simposio Iberoamericano de Catálise, Vol. 1, p. 311, Cordoba, Argentina (1996).
Fernandes, L.D., D.Sc. dissertation, PEQ/COPPE/UFRJ (1996).
Gnep, N.S. and Guisnet, M., Bull. Soc. Chim. Fr. 5-6, 429 (1977a).
Gnep, N.S. and Guisnet, M., Bull. Soc. Chim. Fr. 5-6, 435 (1977b).
Haag, W.O. and Olson, D.H., U.S. Pat. 0434347 (1990).
Hsu, Y.S., Lee, T.Y. and Hu, H.C., Ind. Eng. Chem. Res. 27, No. 6, 942 (1988).
Ribeiro, M.F.G., D.Sc. dissertation, Universidade Técnica de Lisboa (1989).
Ribeiro, M.F.G., Travers, Ch., Raatz, F., Marcilly, Ch. and Ribeiro, F.R., Isomerization of C8 Aromatics Cuts on Modified Zeolite Catalysts with Different Structures, In Zeolites: Facts, Figures, Future, Jacobs, P.A. and van Santen (Eds.), Elsevier Science Pub., Amsterdam, p. 1349 (1989).
Ribeiro, M.F., Lemos, F., Ribeiro, F.R, Marcilly, Ch., Travers, Ch. and Raatz, F., Proc. Div. Petr. Chem., American Chemical Society, New York City Meeting, p. 872 (1991).
Röbschläger, K.H. and Christoffel, E.G., Ind. Eng. Chem. Prod. Res. Dev. 18, 347 (1979).
Röbschläger, K.H. and Christoffel, E.G., Can. J. Chem. Eng. 58, 517 (1980).
Scherzer, J., The Preparation and Characterization of Aluminum-Deficient Zeolites, In: Catalytic Materials: Relationship between Structure and Reactivity, Whyte Jr., T.E., Dalla Betta, R.A., Derouane, E.G. & Baker, R.T.K. (eds.), ACS Symp. Ser. 248, p. 157 (1984).
Silva, J.M., Ribeiro, M.F., Ramôa Ribeiro, F., Benazzi, E. and Guisnet, M., Appl. Catalysis 125, 1 (1995).
Silva, J.M., Ribeiro, M.F., Ramôa Ribeiro, F., Benazzi, E. and Guisnet, M., Appl. Catalysis 125, 15 (1995).
Travers, Ch., Raatz, F., Marcilly, Ch., Ribeiro, F.R. and Ribeiro, M.F.G., EP 0363253 (1989).
- Bartl, P.E., M.Sc. thesis, PEQ/COPPE/UFRJ (1990).
- Benazzi, E., Tavernier, S., Beccat, P., Joly, J.F., Nedez, Ch., Besset, J.M. and Choplin, A., Prep. Div. Petr. Chem., 206th National Meeting, American Chemical Society, Chicago, 561 (1993).
- Brown, L.M. and Huang, T.J., U.S. Pat. 5028573 (1991).
- Bodart, P., Nagy, J.B., Debras, G., Gabelica, Z. and Jacobs, P.A., J. Phys. Chem., 90, 5183 (1986).
- Fernandes, L.D., M.Sc. thesis, PEQ/COPPE/UFRJ (1992).
- Fernandes, L.D., Monteiro, J.L.F, Bartl, P.E., Gusmăo, J., Menezes, S.C. and Cardoso, M.J.B., Zeolites 14, 533 (1994).
- Fernandes, L.D., Corma, A., Martinez, A., Sousa-Aguiar, E.F. and Monteiro, J.L.F., Anais do XIV Simposio Iberoamericano de Catálise, Vol. 1, p. 311, Cordoba, Argentina (1996).
- Fernandes, L.D., D.Sc. dissertation, PEQ/COPPE/UFRJ (1996).
- Gnep, N.S. and Guisnet, M., Bull. Soc. Chim. Fr. 5-6, 429 (1977a).
- Gnep, N.S. and Guisnet, M., Bull. Soc. Chim. Fr. 5-6, 435 (1977b).
- Haag, W.O. and Olson, D.H., U.S. Pat. 0434347 (1990).
- Hsu, Y.S., Lee, T.Y. and Hu, H.C., Ind. Eng. Chem. Res. 27, No. 6, 942 (1988).
- Ribeiro, M.F.G., D.Sc. dissertation, Universidade Técnica de Lisboa (1989).
- Ribeiro, M.F.G., Travers, Ch., Raatz, F., Marcilly, Ch. and Ribeiro, F.R., Isomerization of C8 Aromatics Cuts on Modified Zeolite Catalysts with Different Structures, In Zeolites: Facts, Figures, Future, Jacobs, P.A. and van Santen (Eds.), Elsevier Science Pub., Amsterdam, p. 1349 (1989).
- Ribeiro, M.F., Lemos, F., Ribeiro, F.R, Marcilly, Ch., Travers, Ch. and Raatz, F., Proc. Div. Petr. Chem., American Chemical Society, New York City Meeting, p. 872 (1991).
- Röbschläger, K.H. and Christoffel, E.G., Ind. Eng. Chem. Prod. Res. Dev. 18, 347 (1979).
- Röbschläger, K.H. and Christoffel, E.G., Can. J. Chem. Eng. 58, 517 (1980).
- Scherzer, J., The Preparation and Characterization of Aluminum-Deficient Zeolites, In: Catalytic Materials: Relationship between Structure and Reactivity, Whyte Jr., T.E., Dalla Betta, R.A., Derouane, E.G. & Baker, R.T.K. (eds.), ACS Symp. Ser. 248, p. 157 (1984).
- Silva, J.M., Ribeiro, M.F., Ramôa Ribeiro, F., Benazzi, E. and Guisnet, M., Appl. Catalysis 125, 1 (1995).
- Silva, J.M., Ribeiro, M.F., Ramôa Ribeiro, F., Benazzi, E. and Guisnet, M., Appl. Catalysis 125, 15 (1995).
- Travers, Ch., Raatz, F., Marcilly, Ch., Ribeiro, F.R. and Ribeiro, M.F.G., EP 0363253 (1989).
Publication Dates
-
Publication in this collection
09 Oct 1998 -
Date of issue
June 1998
History
-
Accepted
09 Mar 1998 -
Received
05 Nov 1997
