11644
J. Phys. Chem. B 2000, 104, 11644-11649
In Situ XANES Study of Pt/Mordenite during Benzene Hydrogenation in the Presence of Thiophene L. Simon and J. G. van Ommen Catalytic Processes and Materials, Faculty of Chemical Technology, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
A. Jentys and J. A. Lercher* Lehrstuhl fu¨ r Technische Chemie II, Technische UniVersita¨ t Mu¨ nchen, Lichtenbergstrasse 4, D-85748 Garching, Germany ReceiVed: July 6, 2000; In Final Form: September 21, 2000
The influence of the Na+/H+ ratio on the benzene hydrogenation activity and the sulfur tolerance of Pt supported on mordenite (MOR) was studied by in situ XANES. An increase of the Pt white line was observed with increasing concentrations of thiophene up to 400 ppm, which indicates that an equilibrium exists between the thiophene concentration in the gas phase and the sulfur compounds present on the metal surface. Above a certain concentration of thiophene the Pt white line was constant, indicating a saturation of the metal surface with thiophene. The effect of sulfur poisoning on the metal surface was more effective for Pt supported on H-MOR compared to the more basic Pt/Na-MOR catalysts, which is attributed to the more pronounced decomposition of thiophene and the formation of H2S on Brønsted acid sites. During benzene hydrogenation, a decrease of the Pt white line resulting from the adsorption of benzene and other reaction intermediates on the Pt surface was observed. In the presence of sulfur, the increase of the Pt white line intensity during benzene hydrogenation suggests a higher sulfur coverage of the metal for Pt/H-MOR compared to Pt/NaHMOR. An optimum concentration of acid sites was found with respect to the benzene hydrogenation rate and sulfur tolerance of the catalysts, which is attributed to the presence of two reaction pathways for benzene hydrogenation, i.e., one which is catalyzed by Pt and another in which acid sites close to Pt particles are the catalitycally active sites.
Introduction Pt supported on basic zeolites has been shown to be highly active for hydrogenation and dehydrogenation1,2 as well as dehydrocyclization reactions.3,4 Using various Pt supported on LTL catalysts, Menacherry et al.5 showed that for n-hexane aromatization the benzene selectivity increases with increasing the electron density of the Pt particles, i.e., increasing the alkali content. For neopentane hydrogenolysis on Pt supported on LTL catalysts, TOF’s were shown to increase with increasing acidity of the support.6 The major drawback for the implementation of Pt supported on basic zeolites is, however, the high sulfur sensitivity of the noble metal.2,7,8 Compared to conventional Pt supported on Al2O3 catalysts, which can operate at sulfur levels up to 20 ppm9 and Pt/Pd supported on acidic zeolites, which can tolerate up to 1000 ppm sulfur,10 Pt supported on alkali zeolites is already poisoned by less than 1 ppm sulfur in the feed.3 The deactivation activity of Pt supported on K-LTL was attributed by Vaarkamp et al.7 to the loss of metal active sites as well as a high sintering and growth of metal particles outside the zeolite pores. For neopentane hydrogenolysis and isomerization, the presence of sulfur in the feed was shown to increase the coke formation and to decrease the interactions between the metal particles and the support.8 For benzene hydrogenation, sulfur poisoning of supported noble metals was studied on SiO2 and SiO2/Al2O3. Doping the metal surface with alkali metals strongly influences the electronic properties of the metal.11,12 Alkali promoters * Corresponding author. Fax: +49 (0)8928913544. E-mail: johannes.
[email protected].
induced a strong decrease in activity of Pt for benzene hydrogenation, but also a decrease of the thiophene decomposition. For Pt supported on FAU an increase of benzene hydrogenation with decreasing the alkali exchange level of the catalyst was reported.13 A correlation between the activity and the increase of the metal electron deficiency was found and attributed to the decreasing alkali exchange level. In the present work the effects on the activity and sulfur resistance of partially Na+ exchanged Pt supported on mordenite (MOR) were studied by XAFS spectroscopy during benzene hydrogenation. Experimental Section Catalyst Preparation and Characterization. A series of Pt supported on MOR catalysts containing various amount of Na+ was prepared by liquid phase ion exchanged using Na-MOR (TOSOH, Si/Al ) 9.2). H-MOR was prepared by ion exchange of Na-MOR with a molar aqueous solution of NH4NO3 for three times at room temperature. Calcination in dry air with a flow of 100 mL‚min-1 per gram zeolite was performed at 723 K (heating rate of 5 K‚min-1) for 3 h. The required volume of Pt(NH3)4(OH)2 to achieve a 1 wt % Pt loading was used as precursor for Pt liquid phase ion exchange. The suspension containing the zeolite and the Pt precursor was stirred overnight. The sample was filtered, dried for 1 day at room temperature, calcined for 2 h in a dry air flow of 40 mL‚min-1 per gram catalyst at 493 K with a heating rate of 0.5 K‚min-1, and reduced for 1 h in a hydrogen flow of 35 mL‚min-1 per gram catalyst at 623 K with a heating rate of 0.5 K‚min-1. After reduction,
10.1021/jp002425d CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/2000
XANES Study of Pt/Mordenite
J. Phys. Chem. B, Vol. 104, No. 49, 2000 11645
Figure 1. Experimental setup for benzene hydrogenation followed by in situ XAFS.
Pt/Na-MOR was again ion exchanged with a solution of NaNO3 at 323 K, washed with deionized water and dried in air.14 The chemical composition of the catalysts was determined by X-ray fluorescence spectroscopy. The concentration of acid sites was measured by ammonia TPD. 90 mg of the sample was evacuated at 623 K for 1 h (heating rate of 10 K‚min-1). After cooling to room temperature ammonia was adsorbed for 1 h with a partial pressure of 10 mbar. Physisorbed ammonia was removed by heating the sample to 423 K for 1 h. TPD was performed up to 973 K with a heating rate of 10 K‚min-1. A mass spectrometer was used to detect the desorbing molecules. The number of accessible Pt atoms was determined by hydrogen chemisorption. The catalyst was reduced in flowing hydrogen (flow rate approximately 50 mL‚min-1) for 1 h at 623 K and subsequently evacuated (pressure Pt/NaH-MOR > Pt/Na-MOR remained unchanged. In Situ XANES at 623 K. In situ XANES spectra were collected during all steps of benzene hydrogenation in the absence and the presence of 50 ppm thiophene. The Pt white lines were integrated and the results are shown in Figure 6. During benzene hydrogenation the areas of the Pt white line decreased for all catalysts. The initial decrease observed was higher for Pt/H-MOR compared to Pt/NaH-MOR and Pt/NaMOR. However, after 1 h benzene hydrogenation and subsequently 10 min purging in H2 the areas under the white line reached their initial values, indicating that the adsorption of benzene and adsorbed reaction intermediates caused the changes in the white line intensity. During benzene hydrogenation in the presence of 50 ppm thiophene different trends were observed. While in the presence of benzene for all catalysts a decrease of the white line was observed, the Pt white line intensity initially decreased and then strongly increased for Pt/H-MOR, increased for Pt/NaH-MOR, and decreased for Pt/Na-MOR. The increase of the Pt white line areas observed during the thiophene poisoning experiments indicated a higher sulfur coverage of the metal for Pt/H-MOR and Pt/NaH-MOR compared to Pt/Na-MOR. After reaction and purging with hydrogen the areas of the white line remained higher than the initial values due to adsorbed sulfur species remaining on the Pt surface.
11648 J. Phys. Chem. B, Vol. 104, No. 49, 2000 The Pt white line extrapolated to 50 ppm thiophene (Figure 5) was lower for Pt/H-MOR (28 instead of 28.6) and higher for Pt/NaH-MOR and Pt/Na-MOR (28.1 and 27.7 instead of 27.1 and 26.0, respectively) than the experimental values shown in Figure 6. This indicates that the presence of benzene in the feed decreased the level of sulfur poisoning of the Pt surface for the basic support and enhanced it on more acidic supports. Discussion For all catalysts the concentration of acid sites determined by ammonia TPD was lower than the theoretical acid site concentration based on the aluminum content of the zeolite. An average Pt particle size of 1.5 nm (consisting of around 250 Pt atoms) can be estimated from the Pt-Pt coordination numbers in Table 4. Assuming that one Pt particle shields one proton located either inside or at the entrance of the side pocket, 1 × 10-4 mol of protons will be shielded. Therefore, the concentration of acid sites per unit cell size estimated will be close to the theoretical one, i.e., 0.32, 1.03 and 4.02 for Pt/Na-MOR, Pt/NaH-MOR and Pt/H-MOR, respectively. The increase of the Pt white line intensity shown in Figures 5 and 6 with increasing alkali content of the zeolite is in good agreement with previous studies showing an increase of the metal electron density with increasing the alkali content11,25 and with decreasing the Si/Al ratio of the zeolite.26-28 The increase of the white line in the presence of increasing concentrations of thiophene (Figures 5 and 6) is still controversially discussed and can be explained by two mechanisms: (i) an increase of the electron deficiency of the metal surface due to the attraction of electrons by the sulfur species on the Pt surface or (ii) a change in the electron orbital energy of the metal due to the formation of Pt-S bonds. In both cases, the increase of the white line intensity should be proportional to the concentration of sulfur adsorbed on the metal particles. The constant white line intensity for thiophene concentrations above 400 ppm and the constant white line intensity as a function of reaction time indicate that at lower thiophene concentration sulfur species did not cover the complete metal surface. Consequently, an equilibrium between the gas phase thiophene and the adsorbed sulfur containing species is concluded to exist. After thiophene poisoning, the Pt white line intensities remained constants, indicating that most of the adsorbed sulfur species are irreversibly adsorbed. Thiophene adsorption and decomposition was studied on Na+, K+, and H+-ZSM5 zeolite.29 The decomposition and hydrogenolysis of thiophene was observed on Brønsted acid sites of H+-ZSM5 but not on Na+ and K+-ZSM5. The authors emphasized the need of two thiophene molecules for the decomposition to take place and the reported a relative slow process of thiophene decomposition. In agreement, our results30 show that for Pt supported on alkali or acidic MOR and LTL zeolites the formation of H2S by decomposition of thiophene was observed only for acidic catalysts. The higher increase of the white line intensity observed for Pt/H-MOR was attributed to the enhanced decomposition of thiophene on the acid sites of the zeolite, which resulted in a higher H2S formation and, thus, a stronger poisoning of the Pt sites. For all catalysts a decrease of the Pt white line during benzene hydrogenation compared to the reduced samples was observed. This decrease can be attributed to the adsorption of benzene and the adsorbed reaction intermediates on the metal surface during benzene hydrogenation. The adsorption of benzene on Pt (111) has been shown to be a flat adsorption via the π orbitals31 with Kekule distortions,32 which results in an electron transfer from the aromatic ring to the unoccupied d orbitals of
Simon et al. Pt.33-35 The increase of the electron density induced by the chemisorption of benzene via π electrons on the Pt particles is clearly reflected in the decrease of the white line intensity during benzene hydrogenation (Figure 6a). In a previous contribution36 it has been shown that the catalytic properties of Pt supported on MOR cannot be explained by the metal only catalyzed pathway generally described in the literature.37-39 In the conventional model the increase of benzene conversion is explained by the increase of the Pt electron deficiency with decreasing alkali content.25,40,41 In contrast to this but in agreement with Vannice et al.,42 we propose that benzene hydrogenation occurs on two catalytically active sites, (i) on the metal and (ii) on the acid sites close to the metal surface. We showed that an optimum acid site concentration for Pt/NaH-MOR for the benzene hydrogenation activity and the Pt sulfur tolerance exists at various pressures, which is explained by the two benzene hydrogenation pathways on Pt supported MOR. The kinetic results obtained in the XAFS setup (Tables 2 and 3) fully confirmed these results.36 The low benzene conversion for Pt/Na-MOR (Table 2) can be explained by the low concentration of acid sites, while the low benzene hydrogenation activity on Pt/H-MOR resulted from the higher deactivation by coke formation. In both cases, the acidic pathway of benzene hydrogenation was strongly affected. The higher white line intensity during benzene hydrogenation in the presence of 50 ppm thiophene compared to benzene hydrogenation under standard conditions indicates that already low thiophene concentrations overcompensate the metal electronic potential changes in the presence of benzene. With respect to the conversion of thiophene, two models are proposed in the literature. Stanislaus et al.44 suggested that thiophene hydrogenolysis and benzene hydrogenation occurs on different catalytically active sites. Other authors suggested that these two reactions occur on identical sites.45,46 In the latter case the authors explained the marked decrease in benzene hydrogenation activity by the competitive adsorption of benzene and thiophene on the metal. We would like to speculate that the lower value of the white line intensities found at the beginning of the reaction is due to the coverage of the metal surface by benzene and thiophene and the interactions of their π-orbitals with the metal surface.32,47 The aromatic character of the two molecules29 suggested that the competitive adsorption of benzene and thiophene occurs on both Pt and Brønsted acid sites. In ref 47 the authors showed that the presence of aromatic compounds had a strong inhibiting effect on C-S bond hydrogenolysis on metal surface. Therefore, the decomposition of thiophene was reduced on the metal surface and the sulfur poisoning of the Pt lowered. On acid sites, in contrast, two molecules are needed to initiate the decomposition of thiophene.29 We speculate, thus, that on the acid sites (and due to the similarity of the aromatic ring) one benzene molecule reacts with one molecule of thiophene and, thus, increases the rate of thiophene decomposition. This behavior led to a higher poisoning of the Pt surface by H2S formed in that reaction and to an increase of cracking products (see Table 3) and coke formation with increasing the acid site concentration. During benzene hydrogenation in the presence of 50 ppm thiophene, the surface coverage with sulfur species formed by thiophene decomposition increased and, thus, the Pt electron deficiency decreased. Consequently, the higher initial activity in the presence of thiophene is explained by an increase of the electron deficiency of the metal surface caused by the metal sulfur bonds. The rapid poisoning of the metal hydrogenation sites by sulfur species led to a strong initial decrease of activity.
XANES Study of Pt/Mordenite A stable activity for benzene hydrogenation is subsequently observed (Figure 4) when the equilibrium between the thiophene in the gas phase, thiophene decomposing on the acid sites and the sulfur species on the metal surface resulting from thiophene decomposition and H2S poisoning has been reached. The bonding of sulfur species on the Pt surface is concluded to have changed the electronic properties of Pt. Therefore, the Pt white line intensity stayed higher after reaction and flushing the catalysts with hydrogen compared to the reduced samples. This also confirmed that a large amount of the sulfur species that was adsorbed irreversibly on the Pt particles during benzene hydrogenation at 623 K. Conclusions Increasing thiophene concentrations increased the Pt white line intensity up to a maximum, which was the highest for Pt/ H-MOR. This indicates a higher sulfur poisoning of Pt supported on acidic MOR compared to Pt/NaH/MOR and Pt/Na-MOR and was explained by the higher extent of thiophene decomposition leading to a subsequent H2S formation on the Brønsted acid sites of MOR, which increased the sulfur poisoning of the Pt sites. An equilibrium between the adsorbed sulfur species on the metal surface and the gas phase thiophene was observed for increasing concentrations of thiophene. Most of the sulfur species adsorbed during sulfur poisoning and benzene hydrogenation in the presence of thiophene are irreversibly adsorbed on the Pt surface. The in situ benzene hydrogenation in the absence and the presence of 50 ppm thiophene studied by in situ XANES revealed a decrease of the white line intensity, which resulted from the electron donor character of benzene to the metal surface. Alternatively, the increase of the white line during benzene hydrogenation in the presence of sulfur-bonded species on the Pt surface can be explained by an electron acceptor character of the adsorbed sulfur species or by the change of the Pt electronic orbital energy due to the formation of Pt-S bonds. The highest increase of Pt white line intensity was observed for Pt/ H-MOR, indicating a higher sulfur poisoning of the metal surface on this catalysts compared to Pt/NaH-MOR and Pt/NaMOR. The highest activity for benzene conversion was observed for Pt supported on partially sodium exchanged MOR. The decrease of conversion at higher sodium content is explained by the decrease of the Pt electron deficiency with increasing alkali content and the decrease of the acidic pathway for the benzene conversion. The lower activity of Pt supported on acidic MOR is attributed to the higher coke formation on this latter support compared to Pt/NaH-MOR and Pt/Na-MOR. The competitive adsorption of benzene and thiophene on the metal surface and the acid sites leads to a lower decomposition of thiophene on the metal surface and an increase of the thiophene decomposition on the acid sites. As a consequence of the more significant H2S formation, an increase of sulfur poisoning of the metal surface by H2S with increasing the acid site concentration is observed. Acknowledgment. This work has been performed under the auspices of NIOK, The Netherlands Institute of Catalysis Research. It was supported by STW/NWO, The Netherlands, under the project number 349-3787. XAFS experiment was supported by the TMR-Contract ERBFMGECT950059 of the European Community on the beamline X1 at HASYLAB, DESY, Hamburg, Germany. References and Notes (1) Law, D. V.; Tamm, P. W.; Detz, C. M. Energy Prog. 1987, 7 (4), 215.
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