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Characterization of acidic properties of sulfated zeolite Beta
Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkovziand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.
1763
Characterization of acidic properties of sulfated zeolite Beta C. Woltz, A. Jentys, J.A. Lercher Lehrstuhl II f. Technische Chemie, Technische Universit~it Mtinchen, Lichtenbergstr.4, D-85747 Garching, Germany Treatment of Pt loaded zeolite Beta with H2S and subsequent oxidation led to the formation of additional Bronsted acidic sites, which were identified as surface sulfate species and were active for the pentane hydroisomerization. Temperature programmed desorption of ammonia and infrared studies during adsorption of Pyridine showed that acid strength and accessibility of the acid sites were not affected after sulfation. The concentration of Bronsted acidic sites for the untreated and samples sulfated at moderate temperature correlate with the isomerization activity, indicating that the reaction is limited by the acid sites and, therefore, can be enhanced by the introduction of Bronsted acidic sulfate groups. 1. INTRODUCTION Short chain branched isomers are important components of high octane motor fuels. Hydroisomerization of light n-paraffins is the most important catalytic route industrially implemented. The catalysts used are bifunctional, with a metal function (Pt, Pd) for the hydrogenation/dehydrogenation step and an acidic function for the rearrangement of the C-C bonds. In addition, the metal component is necessary for providing catalyst stability by facilitating the hydrogenation of coke precursors. Industrially (highly corrosive) chlorinated Pt/AI203 and Pt loaded zeolites are typically used. Assuming a sufficient concentration of metal surface atoms to be present in the catalysts (i.e. that the hydrogenation/dehydrogenation is in equilibrium) the rate-limiting step of the hydroisomerization for a bifunctional catalyst is the rearrangement (skeletal isomerization) of the carbenium ions. Therefore, concentration and strength of Bronsted acid sites, both controlled by the Si/Al-ratio of the zeolite, critically determine the activity by influencing the concentration and average lifetime of the carbenium ions [ 1]. Thus, the activity of the bifunctional catalysts for skeletal C-C isomerization or C-C bond breaking (cracking) reactions can be optimized by modifying the acidic and textural properties of the zeolites. While the presence of sulfur compounds, such as thiophene or HzS, usually reduces the catalytic activity for metal catalysed reactions, we have observed in preliminary experiments that reoxidation of such poisoned materials led to highly active and selective catalysts for isomerization.
2. EXPERIMENTAL 2.1. Catalyst preparation Zeolite H-BEA 25 (Si/AI=12.5) was obtained from Stid-Chemie AG and loaded with Pt by ion-exchange in aqueous Pt(NH3)4(OH)2 solution. The appropriate amount of
1764 Pt(NH3)4(OH)2 and of NH4OH was calculated based on the concentration of protons (0.28 mmol/g, determined. After the ion exchange the solid was centrifuged, washed and freeze dried. The samples were calcined in air 350~ for 16 h (heating rate 0.5~ and finally reduced at 300~ in H2 for 4 h. The sulfated samples were prepared by passing a mixture containing 1.8 vol% H2S in H2 over the Pt loaded zeolite at a WHSV of 2 [g(H2S)/(gcat*h)] for 3h. Subsequently the material was oxidized in air for another 2h at a WHSV of 2 [g(O2)/(gcat*h)] and reduced. The sulfation procedure was carried out at a temperature of 350~ 450~ and 550~
2.2. Catalyst characterization Temperature programmed desorption was performed in a 6-fold parallel TPD system. The catalysts were activated by heating in vacuum to 350~ (rate of 10~ for 2 h. Ammonia was adsorbed at a temperature of 150 ~ with a partial pressure of 1 mbar for 1 h and subsequently evacuated at 10.3 mbar for 2 h in order to remove physisorbed molecules. For the TPD experiments the samples were heated from 150~ to 800~ with a heating rate of 10~ and the species desorbing were monitored by mass spectrometer (Blazer QMG 420). In each set of experiments in the 6-fold parallel system a reference sample with known acidity site density was measured to calibrate the response of the MS. IR spectra during the adsorption of pyridine were measured using a Perkin Elmer 2000 spectrometer. The spectra were recorded in the region from 3800 to 1100 cm l at a resolution of 4 cm -1. A self supporting wafer was pressed and activated at 350~ (ramp rate 10 ~ for 90 min. After cooling the sample to 150~ the spectrum of the activated zeolite was recorded. Pyridine was adsorbed at 150~ with a partial pressure of 0.05 mbar for 30 min and the sample was subsequently evacuated until the IR spectra remained constant. To compare the spectra of the different samples all spectra were normalized by the intensity of the structural vibrations of the zeolite between 1750 and 2100 cm -~. Additionally the weight of the wafers used to determine the mass per surface area necessary for calculating the amount of Bronsted and Lewis acid sites according to the method published by Emeis [2]. In situ IR measurements during sulfur treatment and C5 isomerization were carried out using a flow cell equipped with ZnS windows and a resistance heated furnace for the sample holder. For in situ IR experiments the samples were pressed into self supported wafers (ca. 2 mg) and activated for 90 min at 350~ in hydrogen. For H2S and oxidation treatment a flow of 20 ml/min was used. The amount of sulfate species present on the samples was determined by liquid ionchromatography (Metrohm 690) using a anion column IC SUPER-SEP with phthalic acid (2.5mmol/1) and 5% acetonitrile as eluant. For the determination of the sulfate content 20mg of catalyst was dissolved in 100 ml sodium hydroxide (0.01 mol/1). Surface area and pore volume was determined by physisorption of nitrogen using a Sorptomatic 1990 Series sorptometer. About 500 mg of the sample was heated to 350~ and evacuated for l h before nitrogen adsorption was carried out at a temperature of-196~ The surface area and the pore size distribution were calculated according to the BET method.
2.3. Catalyst testing The catalytic activity for the pentane hydroisomerization was studied with a 20-fold parallel reactor system. Each plug flow reactor was controlled by an individual digital mass flow controller and pressure regulator. The liquid feed was adjusted and mixed with hydrogen by a digital Controlled-Evaporator-Mixer. For the analysis of the products a HP-MicroGC (GC M200), which is capable to separate the components C1 to C6 (including their isomers) in
1765 less then 2 min, was used. The complete computer control of the reaction system allowed to vary the total pressures, flow rates, temperatures and alkane concentrations automatically. All data recorded were stored in a MSAccess database to directly relate characterization and kinetic data. 3. RESULTS
3.1. Catalyst characterization In order to investigate the influence of the sulfation temperature during H2S and air treatment zeolite Beta with 1 wt% Pt was used. The total amount of acid sites of the zeolite samples was determined by NH3 TPD. The TPD profiles of NH3 from zeolite Beta (1 wt%-Pt) after sulfur treatment at different temperatures are compared to the untreated catalyst in Fig. 1. Two overlapping maxima were observed, indicating the presence of two acid sites with different strength. Although NH3-TPD can not be used to differentiate between Lewis and Bronsted acid sites, it can be clearly seen that with higher sulfation temperature the concentration of acid sites decreased, while for the catalyst sulfated below 450~ additional acidic sites were formed. Since the sulfur treatment did not markedly change the temperature of the maxima in the TPD, the strength of the acid sites seem to be not affected by the sulfur treatment. The IR spectra of zeolite Beta before and after pyridine adsorption and of the sulfur treated samples are shown in Fig. 2. Pyridine adsorption leads to a complete disappearance of the bands at 3608 cm -I and 3785 cm l , assigned to Bronsted acidic hydroxyl groups and A1OH groups, respectively, and to a decrease in the intensity of the (surface) silanol groups at 3743 cm -I indicating that all hydroxyl groups were accessible for pyridine and that the sulfur treatment did not lead to a blocking of the pores. The comparison of the IR spectra of the samples treated with sulfur at different temperatures reveals that with increasing temperature the intensity of the Bronsted acidic hydroxyl groups at 3608 cm 1 was reduced and additional silanol groups at (3733 cm~), assigned to SiOH groups at defects, were formed for temperatures above 350~ The concentration of Bronsted and Lewis acid sites, the sulfate concentration, the fraction of Pt surface atoms and the specific surface area are summarized in Table 1. Sulfur treatment at low temperatures led to an enhanced concentration of Bronsted acid sites, while the concentration of Lewis acid sites remained constant. With increasing temperature of the Sulfur treatment the concentration of Bronsted and Lewis acid sites decreased compared to the untreated Pt loaded zeolite Beta sample treated at 350 ~ The concentration of sulfate on the samples, which was measured using liquid ion-chromatography is decreasing with increasing temperature used for the sulfur treatment. It should be noted, however, that the concentration of sulfate on the sample is in the same order of magnitude than the number of acid sites, while it is exceeding the total number of Pt sites up to a factor of 9. Hydrogen chemisorption showed that the fraction of surface Pt atoms is reduced from 50% to below 5% after the sulfur treatment. In contrast the specific surface area was only slightly reduced after the sulfur treatment.
1766 2.5E-06
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450
600
750
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3500
3300
Wavenumber [cm 1]
Temperature ["C]
), Fig. 1. NH3-TPD for Pt/H-BEA ( • treated with H28/O2 at 350~ ( O ), at 450~ I-I )andat550~ /k )
i
3700
Fig. 2. FTIR for Pt/H-BEA (a) ( ), Pyridine adsorbed on Pt/H-BEA (b) ( .... ),Pt/H-BEA treated with H28/O2 (~) at 350~ (c), at 450~ (d) and at 550~ (e)
Table 1 Concentration of acid sites, sulfate concentration, fraction of Pt surface atoms and specific surface area of Pt/H-BEA after sulfur treatment at different temperatures Sulfate Ratio Bronsted Lewis Content S O 2_ / Surface BET Sample Pt[H/Pt] atom [m2/g] acid sites acid sites [mmol Pt [mmol/g] [mmol/g] S 0 2 - /g] [mol/mol] Pt/H-BEA Pt/H-BEA (sulf./oxid. at T=350~
0.28
0.28
-
0.52
478
0.39
0.29
0.44
8.7
0.11
446
Pt/H-BEA (sulf./oxid. at T=450~
0.32
0.27
0.14
2.6
0.06
458
Pt/H-BEA (sulf./oxid. at T-550~
0.24
0.21
0.06
1.2
n.d.
413
The differences in the IR spectra before and after adsorption of pyridine on Pt loaded zeolite Beta after HzS treatment and subsequent oxidation at 350~ are compared in Fig. 3. After oxidation a characteristic band at 1385 cm -1 was observed, which can be assigned to the stretching frequency of a covalent S=O bond [3]. After adsorption of pyridine this band was removed and the typical bands of the pyridine ring vibrations at 1455, 1490, 1545, 1620 and 1635 cm ~ assigned to coordinatively adsorbed pyridine and protonated pyridinium ions were
1767
observed. In addition a band at 1335 cm -~, assigned to a S=O stretching vibration perturbed after adsorption of pyridine, was observed, indicating acidic ]aroperties of the sulfur-oxygen species. It was not possible to detect bands below 1300 cm- as the lattice vibration of the zeolite lead to a complete absorption in this energy range. To investigate the presence of sulfates species under reaction conditions the changes in intensity of the S=O band (1385 cm ~) were followed in situ on a sulfur treated Pt loaded zeolite Beta catalyst during the hydroisomerization reaction for a mixture of 3 mol% pentane in hydrogen at 300~ and 1 bar total pressure (see Fig. 4 ). The presences of hydrogen and pentane led to an immediate reduction of the intensity of the band at 1385 cm ~ which reached a constant level after a reaction time of 2 h. Besides the reduction of the band at 1385cm -~ the formation of water at was observed by the presence of the 6(OH) band at 1641 cm -1 during the hydroisomerization reaction. Under reaction conditions the intensity of the band at 1385 cm ~ decreases strongly, however, after heating the sample to 400~ in He for 2 h it was possible to regain almost 80% of the intensity compared to the sulfur treated sample before the reactions.
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0 t.~
<
(b)
(a) 1650
1500
1350
1200
1700
|
|
i
|
1600
1500
1400
1300
Wavenumber [cm -1]
Wavenu mber [cm-1]
Fig. 3. IR spectra for H2S/air treated Pt/H-BEA (a), after adsorption of pyridine (b) and difference spectra (b-a)
Fig. 4. Differences in the IR spectra after oxidation of H2S treated sample (- -); during hydroisomerization reaction ( ) and after reactivation in Helium at 400~
3.2. Isomerization activity The sulfated samples were investigated for the hydroisomerization reaction of pentane and compared to the parent zeolite. The catalytic activity of the parent Pt loaded zeolite Beta and the samples sulfated at different temperatures is given for a total pressure of 4 bar and a WHSV of 30 h -1 in a temperature range between 260 and 35()~ (in terms of plotting In k v s . l/T) in Fig. 5. The comparison of the catalyst in the kin,etic regime (linear part at low temperatures) shows that the lower the sulfation temperature the higher the rate for isomerization. For the catalyst treated with H2S and air below 550 ~ a higher activity than for the parent catalyst was observed. The apparent activation energies measure in the kinetic
1768
regime were in the range between 100-1 l0 kJ/mol. The flatting of the curves at higher temperatures can be explained by the approach towards the thermodynamic equilibrium. Additionally pore diffusion limitation might occur at the higher temperatures since the isomerization rate succeeds the rate of diffusion. The concentration of Brensted acid sites is correlated to the isomerization activity in Fig. 6. A direct linear correlation (proceeding to the origin) of the concentration of Brensted acid sites and isomerization activity was observed for the Pt loaded zeolite Beta catalyst and the sulfated samples treated below 550~ while the catalyst treated at 550~ shows a lower activity. 1.4 -2
"7,
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. _N
~ o.4 E
"5
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0.2 -6.5 0.0016
,
,
0.0017
0.0018
inverse Temperature [l/K]
Fig. 5. Isomerization activity between of Pt/H-BEA ( • ); treated with H2S/air at 350~ ( - O - ); at 450~ ( - O - ) and at 550~ ( --A- )
0.0 0.0019
0.0
i
i
i
i
0.1
0.2
0.3
0.4
concentration of Bronsted acid sites [mmol/g]
Fig. 6. Correlation of Bronsted acidity and isomerization activity ofPt/H-BEA ( X ) and H2S/air treated catalysts (/k).
4. DISSCUSION Temperature programmed desorption of NH3 showed that the concentration of acid sites was increased after treating the Pt loaded zeolite Beta catalysts with H2S followed by a subsequent oxidation. Moreover, it was shown that it is essential to perform the sulfur treatment at moderate temperature in order to maximize the acid site concentration, which results most probably from the higher concentration of sulfate adsorbed on the sample at lower temperatures. The desorption profiles of NH3 remained similar after the sulfur treatment indicating that significant changes in acid site strength were not obtained. Furthermore the apparent activation energies which were obtained for the pentane hydroisomerization reaction showed similar values after the sulfation treatment. It therefore can be assumed that no changes in the enthalpy of adsorption occur which is another indication that the sulfated samples possess similar acid strength. The sulfur treatment at higher temperature led to the
1769 reduction in the concentration of the Bransted acidic bridging SiA1OH groups, while sulfur treatment at low temperatures led to an increase of Bronsted acid sites. The concentration of Lewis acid sites was slightly decreasing after the sulfur treatment. The sulfur treatment followed by a subsequent oxidation resulted in the appearance of a band assigned to a S=O double bond (characteristic band at 1385 cm-~), which indicates that the increase in the concentration of the Bronsted acid sites results from the formation of sulfate species. The interaction of these species with pyridine reveals that the sites are accessible surface species and have acidic character. Moreover, the sulfur content of the samples correlates with their Bronsted site concentration confirming the formation of Bronsted acidic sulfate species. In situ IR measurements during the oxidation of the HzS treated sample showed a strong increase of sulfate species. A band near 1380 cm -1 was reported in ref. [4] after sulfating A1203, TiO2 or ZrO2. All samples possessed Bronsted acidity, assumed to be generated by hydrogenosulfate ions. Under anhydrous conditions bands at 1380 cm ~ and 1040 cm -I were observed after oxidation of HzS or SO2 treated A1203 and TiO2 [5]. In presence of H20 vapor the high-frequency band disappearec and new bands between 1300 and 1150cm -1 were created. On the Pt loaded zeolite Beta we expect that the Bronsted acidic sulfate groups were formed in a similar way as on y-A1203. In [6] a band at 1388cm -1 was observed after sulfur poisoning of y-alumina supported Pt c~ttalysts and as well assigned to sulfate species. The intensity of the peak decreased to about 20% after reactivation of the sample at 550~ in hydrogen. Furthermore sulfation of zeolite modernite with (NH4)2SO4and subsequent oxidation lead to a band at 1351 cm -~ [7]. As the content of SO42- species on the sample exceeds the total amount of Pt atoms the major part of the $042" species should be located on the extra-framework A1 species of the zeolite. The (A10)3S=O groups, proposed for H2S/O2 treated y-A1203 in ref. [5], are transformed to a Bronsted acidic (A10)2SOO-H + group in the presence of water vapor. As the lattice vibrations of the zeolite led to a complete absorption in the energy range below 1300 cm -I it was not possible to detect the shift of the S=O bond. However, the disappearance of the S=O vibration during the hydroisomerization reaction, which can be regained by heating up the sample in He, confirms that the sulfate species are converted to ionic species than reduced to e.g. sulfdes. For the kinetic experiments external diffusion limitations were experimentally excluded. An enhanced pentane hydroisomerization activity was observed fi)r the catalyst sulfated at 350~ and 450~ while a decrease in the catalytic activity was observed after sulfation at 550~ As the activity correlates with the Bronsted acid site concentral:ion it can be assumed that the kinetic relevant step of the reaction pathway is the acid catalyzed rearrangement of the carbenium ions. A direct linear correlation of the acid site cor~centration with the activity was observed for the parent and the catalysts treated at 350~ and 450 ~ although the metal surface is reduced, while the lower rate for the sample sulfated at 550~ can be explained by a shift in the rate determining step towards the dehydrog.enation step resulting from an excessive reduction of metal surface due to the sintering and metal poisoning at the elevated temperatures. Since the dehydrogenation step of that sample is not equilibrated the acid catalyzed step is no longer the kinetic relevant step and hence the isomerization rate does not correlate with the number of Bronsted acid site. 5. C O N C L U S I O N S Gas phase treatment of Pt loaded zeolite Beta with H2S and air enhances the activity for the npentane hydroisomerization. The enhancement of the activity results from the formation of Bronsted acidic surface sulfate species that are formed during oxidation of the H2S treated
1770 samples. The sulfation procedure did not lead to a change in the strength of the acid sites. Although catalyst sulfation leads to an enhancement in acidity it reduces the accessible metal surface. It was shown that it is advantageous to use low temperatures for the H2S and oxidation treatment in order to maximize the hydroisomerization activity as the concentration of sulfur species and the fraction of the metal surface atoms is higher. Enhancement of isomerization activity correlated with concentration of Bronsted acid site indicating that the rearrangement of the carbenium ion is the kinetic relevant step for the catalyst treated with HzS/air at low temperature. ACKNOWLEDGEMENT The work is financially supported by the Bundesministerium ffir Bildung und Forschung Project (BMBF:03C0307D). REFERENCES
[1 ] K.-J. Chao, H.-C. Wu and L.-J. Leu, Appl. Catal. A. 143 (1996) 223. [2] C.A. Emeis, J. Catal. 141 (347-354) (1993) [3] C.P. Hubbard, K. Otto, H.S. Gandhi and K.Y.S. Ng, J. Catal. 144 (1993) 484. [4] M. Waqif, J. Bachelier, O. Saur and J.-C. Lavalley, J. Mol. Catal. 72 (1992) 127. [5] O. Saur, M. Bensitel, A.B. Mohammed Saad, J.C. Lavalley, Carl P. Tripp and B.A. Morrow, J. Catal. 99 (1985) 104. [6] J.-R. Chang and S.-L. Chang, J. Catal. 176, (1998) 42. [7] T. Lei, J.S. Xu, Z. Gao, Mater. Chem. Phys. 60 (1999) 177.
1763
Characterization of acidic properties of sulfated zeolite Beta C. Woltz, A. Jentys, J.A. Lercher Lehrstuhl II f. Technische Chemie, Technische Universit~it Mtinchen, Lichtenbergstr.4, D-85747 Garching, Germany Treatment of Pt loaded zeolite Beta with H2S and subsequent oxidation led to the formation of additional Bronsted acidic sites, which were identified as surface sulfate species and were active for the pentane hydroisomerization. Temperature programmed desorption of ammonia and infrared studies during adsorption of Pyridine showed that acid strength and accessibility of the acid sites were not affected after sulfation. The concentration of Bronsted acidic sites for the untreated and samples sulfated at moderate temperature correlate with the isomerization activity, indicating that the reaction is limited by the acid sites and, therefore, can be enhanced by the introduction of Bronsted acidic sulfate groups. 1. INTRODUCTION Short chain branched isomers are important components of high octane motor fuels. Hydroisomerization of light n-paraffins is the most important catalytic route industrially implemented. The catalysts used are bifunctional, with a metal function (Pt, Pd) for the hydrogenation/dehydrogenation step and an acidic function for the rearrangement of the C-C bonds. In addition, the metal component is necessary for providing catalyst stability by facilitating the hydrogenation of coke precursors. Industrially (highly corrosive) chlorinated Pt/AI203 and Pt loaded zeolites are typically used. Assuming a sufficient concentration of metal surface atoms to be present in the catalysts (i.e. that the hydrogenation/dehydrogenation is in equilibrium) the rate-limiting step of the hydroisomerization for a bifunctional catalyst is the rearrangement (skeletal isomerization) of the carbenium ions. Therefore, concentration and strength of Bronsted acid sites, both controlled by the Si/Al-ratio of the zeolite, critically determine the activity by influencing the concentration and average lifetime of the carbenium ions [ 1]. Thus, the activity of the bifunctional catalysts for skeletal C-C isomerization or C-C bond breaking (cracking) reactions can be optimized by modifying the acidic and textural properties of the zeolites. While the presence of sulfur compounds, such as thiophene or HzS, usually reduces the catalytic activity for metal catalysed reactions, we have observed in preliminary experiments that reoxidation of such poisoned materials led to highly active and selective catalysts for isomerization.
2. EXPERIMENTAL 2.1. Catalyst preparation Zeolite H-BEA 25 (Si/AI=12.5) was obtained from Stid-Chemie AG and loaded with Pt by ion-exchange in aqueous Pt(NH3)4(OH)2 solution. The appropriate amount of
1764 Pt(NH3)4(OH)2 and of NH4OH was calculated based on the concentration of protons (0.28 mmol/g, determined. After the ion exchange the solid was centrifuged, washed and freeze dried. The samples were calcined in air 350~ for 16 h (heating rate 0.5~ and finally reduced at 300~ in H2 for 4 h. The sulfated samples were prepared by passing a mixture containing 1.8 vol% H2S in H2 over the Pt loaded zeolite at a WHSV of 2 [g(H2S)/(gcat*h)] for 3h. Subsequently the material was oxidized in air for another 2h at a WHSV of 2 [g(O2)/(gcat*h)] and reduced. The sulfation procedure was carried out at a temperature of 350~ 450~ and 550~
2.2. Catalyst characterization Temperature programmed desorption was performed in a 6-fold parallel TPD system. The catalysts were activated by heating in vacuum to 350~ (rate of 10~ for 2 h. Ammonia was adsorbed at a temperature of 150 ~ with a partial pressure of 1 mbar for 1 h and subsequently evacuated at 10.3 mbar for 2 h in order to remove physisorbed molecules. For the TPD experiments the samples were heated from 150~ to 800~ with a heating rate of 10~ and the species desorbing were monitored by mass spectrometer (Blazer QMG 420). In each set of experiments in the 6-fold parallel system a reference sample with known acidity site density was measured to calibrate the response of the MS. IR spectra during the adsorption of pyridine were measured using a Perkin Elmer 2000 spectrometer. The spectra were recorded in the region from 3800 to 1100 cm l at a resolution of 4 cm -1. A self supporting wafer was pressed and activated at 350~ (ramp rate 10 ~ for 90 min. After cooling the sample to 150~ the spectrum of the activated zeolite was recorded. Pyridine was adsorbed at 150~ with a partial pressure of 0.05 mbar for 30 min and the sample was subsequently evacuated until the IR spectra remained constant. To compare the spectra of the different samples all spectra were normalized by the intensity of the structural vibrations of the zeolite between 1750 and 2100 cm -~. Additionally the weight of the wafers used to determine the mass per surface area necessary for calculating the amount of Bronsted and Lewis acid sites according to the method published by Emeis [2]. In situ IR measurements during sulfur treatment and C5 isomerization were carried out using a flow cell equipped with ZnS windows and a resistance heated furnace for the sample holder. For in situ IR experiments the samples were pressed into self supported wafers (ca. 2 mg) and activated for 90 min at 350~ in hydrogen. For H2S and oxidation treatment a flow of 20 ml/min was used. The amount of sulfate species present on the samples was determined by liquid ionchromatography (Metrohm 690) using a anion column IC SUPER-SEP with phthalic acid (2.5mmol/1) and 5% acetonitrile as eluant. For the determination of the sulfate content 20mg of catalyst was dissolved in 100 ml sodium hydroxide (0.01 mol/1). Surface area and pore volume was determined by physisorption of nitrogen using a Sorptomatic 1990 Series sorptometer. About 500 mg of the sample was heated to 350~ and evacuated for l h before nitrogen adsorption was carried out at a temperature of-196~ The surface area and the pore size distribution were calculated according to the BET method.
2.3. Catalyst testing The catalytic activity for the pentane hydroisomerization was studied with a 20-fold parallel reactor system. Each plug flow reactor was controlled by an individual digital mass flow controller and pressure regulator. The liquid feed was adjusted and mixed with hydrogen by a digital Controlled-Evaporator-Mixer. For the analysis of the products a HP-MicroGC (GC M200), which is capable to separate the components C1 to C6 (including their isomers) in
1765 less then 2 min, was used. The complete computer control of the reaction system allowed to vary the total pressures, flow rates, temperatures and alkane concentrations automatically. All data recorded were stored in a MSAccess database to directly relate characterization and kinetic data. 3. RESULTS
3.1. Catalyst characterization In order to investigate the influence of the sulfation temperature during H2S and air treatment zeolite Beta with 1 wt% Pt was used. The total amount of acid sites of the zeolite samples was determined by NH3 TPD. The TPD profiles of NH3 from zeolite Beta (1 wt%-Pt) after sulfur treatment at different temperatures are compared to the untreated catalyst in Fig. 1. Two overlapping maxima were observed, indicating the presence of two acid sites with different strength. Although NH3-TPD can not be used to differentiate between Lewis and Bronsted acid sites, it can be clearly seen that with higher sulfation temperature the concentration of acid sites decreased, while for the catalyst sulfated below 450~ additional acidic sites were formed. Since the sulfur treatment did not markedly change the temperature of the maxima in the TPD, the strength of the acid sites seem to be not affected by the sulfur treatment. The IR spectra of zeolite Beta before and after pyridine adsorption and of the sulfur treated samples are shown in Fig. 2. Pyridine adsorption leads to a complete disappearance of the bands at 3608 cm -I and 3785 cm l , assigned to Bronsted acidic hydroxyl groups and A1OH groups, respectively, and to a decrease in the intensity of the (surface) silanol groups at 3743 cm -I indicating that all hydroxyl groups were accessible for pyridine and that the sulfur treatment did not lead to a blocking of the pores. The comparison of the IR spectra of the samples treated with sulfur at different temperatures reveals that with increasing temperature the intensity of the Bronsted acidic hydroxyl groups at 3608 cm 1 was reduced and additional silanol groups at (3733 cm~), assigned to SiOH groups at defects, were formed for temperatures above 350~ The concentration of Bronsted and Lewis acid sites, the sulfate concentration, the fraction of Pt surface atoms and the specific surface area are summarized in Table 1. Sulfur treatment at low temperatures led to an enhanced concentration of Bronsted acid sites, while the concentration of Lewis acid sites remained constant. With increasing temperature of the Sulfur treatment the concentration of Bronsted and Lewis acid sites decreased compared to the untreated Pt loaded zeolite Beta sample treated at 350 ~ The concentration of sulfate on the samples, which was measured using liquid ion-chromatography is decreasing with increasing temperature used for the sulfur treatment. It should be noted, however, that the concentration of sulfate on the sample is in the same order of magnitude than the number of acid sites, while it is exceeding the total number of Pt sites up to a factor of 9. Hydrogen chemisorption showed that the fraction of surface Pt atoms is reduced from 50% to below 5% after the sulfur treatment. In contrast the specific surface area was only slightly reduced after the sulfur treatment.
1766 2.5E-06
ii i
1
co
i
,~ 2.0E-06 t.m
O9
~ 1.5E-06 o {-.
,
,
rr 1.0E-06
~
(c)
cO .m ,,i-,
o CO
121
5.0E-07
J
O.OE+O0 150
,t,k
(b) 3900 300
450
600
750
i
3500
3300
Wavenumber [cm 1]
Temperature ["C]
), Fig. 1. NH3-TPD for Pt/H-BEA ( • treated with H28/O2 at 350~ ( O ), at 450~ I-I )andat550~ /k )
i
3700
Fig. 2. FTIR for Pt/H-BEA (a) ( ), Pyridine adsorbed on Pt/H-BEA (b) ( .... ),Pt/H-BEA treated with H28/O2 (~) at 350~ (c), at 450~ (d) and at 550~ (e)
Table 1 Concentration of acid sites, sulfate concentration, fraction of Pt surface atoms and specific surface area of Pt/H-BEA after sulfur treatment at different temperatures Sulfate Ratio Bronsted Lewis Content S O 2_ / Surface BET Sample Pt[H/Pt] atom [m2/g] acid sites acid sites [mmol Pt [mmol/g] [mmol/g] S 0 2 - /g] [mol/mol] Pt/H-BEA Pt/H-BEA (sulf./oxid. at T=350~
0.28
0.28
-
0.52
478
0.39
0.29
0.44
8.7
0.11
446
Pt/H-BEA (sulf./oxid. at T=450~
0.32
0.27
0.14
2.6
0.06
458
Pt/H-BEA (sulf./oxid. at T-550~
0.24
0.21
0.06
1.2
n.d.
413
The differences in the IR spectra before and after adsorption of pyridine on Pt loaded zeolite Beta after HzS treatment and subsequent oxidation at 350~ are compared in Fig. 3. After oxidation a characteristic band at 1385 cm -1 was observed, which can be assigned to the stretching frequency of a covalent S=O bond [3]. After adsorption of pyridine this band was removed and the typical bands of the pyridine ring vibrations at 1455, 1490, 1545, 1620 and 1635 cm ~ assigned to coordinatively adsorbed pyridine and protonated pyridinium ions were
1767
observed. In addition a band at 1335 cm -~, assigned to a S=O stretching vibration perturbed after adsorption of pyridine, was observed, indicating acidic ]aroperties of the sulfur-oxygen species. It was not possible to detect bands below 1300 cm- as the lattice vibration of the zeolite lead to a complete absorption in this energy range. To investigate the presence of sulfates species under reaction conditions the changes in intensity of the S=O band (1385 cm ~) were followed in situ on a sulfur treated Pt loaded zeolite Beta catalyst during the hydroisomerization reaction for a mixture of 3 mol% pentane in hydrogen at 300~ and 1 bar total pressure (see Fig. 4 ). The presences of hydrogen and pentane led to an immediate reduction of the intensity of the band at 1385 cm ~ which reached a constant level after a reaction time of 2 h. Besides the reduction of the band at 1385cm -~ the formation of water at was observed by the presence of the 6(OH) band at 1641 cm -1 during the hydroisomerization reaction. Under reaction conditions the intensity of the band at 1385 cm ~ decreases strongly, however, after heating the sample to 400~ in He for 2 h it was possible to regain almost 80% of the intensity compared to the sulfur treated sample before the reactions.
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(b)
(a) 1650
1500
1350
1200
1700
|
|
i
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1600
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1400
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Wavenumber [cm -1]
Wavenu mber [cm-1]
Fig. 3. IR spectra for H2S/air treated Pt/H-BEA (a), after adsorption of pyridine (b) and difference spectra (b-a)
Fig. 4. Differences in the IR spectra after oxidation of H2S treated sample (- -); during hydroisomerization reaction ( ) and after reactivation in Helium at 400~
3.2. Isomerization activity The sulfated samples were investigated for the hydroisomerization reaction of pentane and compared to the parent zeolite. The catalytic activity of the parent Pt loaded zeolite Beta and the samples sulfated at different temperatures is given for a total pressure of 4 bar and a WHSV of 30 h -1 in a temperature range between 260 and 35()~ (in terms of plotting In k v s . l/T) in Fig. 5. The comparison of the catalyst in the kin,etic regime (linear part at low temperatures) shows that the lower the sulfation temperature the higher the rate for isomerization. For the catalyst treated with H2S and air below 550 ~ a higher activity than for the parent catalyst was observed. The apparent activation energies measure in the kinetic
1768
regime were in the range between 100-1 l0 kJ/mol. The flatting of the curves at higher temperatures can be explained by the approach towards the thermodynamic equilibrium. Additionally pore diffusion limitation might occur at the higher temperatures since the isomerization rate succeeds the rate of diffusion. The concentration of Brensted acid sites is correlated to the isomerization activity in Fig. 6. A direct linear correlation (proceeding to the origin) of the concentration of Brensted acid sites and isomerization activity was observed for the Pt loaded zeolite Beta catalyst and the sulfated samples treated below 550~ while the catalyst treated at 550~ shows a lower activity. 1.4 -2
"7,
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~
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Fig. 5. Isomerization activity between of Pt/H-BEA ( • ); treated with H2S/air at 350~ ( - O - ); at 450~ ( - O - ) and at 550~ ( --A- )
0.0 0.0019
0.0
i
i
i
i
0.1
0.2
0.3
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concentration of Bronsted acid sites [mmol/g]
Fig. 6. Correlation of Bronsted acidity and isomerization activity ofPt/H-BEA ( X ) and H2S/air treated catalysts (/k).
4. DISSCUSION Temperature programmed desorption of NH3 showed that the concentration of acid sites was increased after treating the Pt loaded zeolite Beta catalysts with H2S followed by a subsequent oxidation. Moreover, it was shown that it is essential to perform the sulfur treatment at moderate temperature in order to maximize the acid site concentration, which results most probably from the higher concentration of sulfate adsorbed on the sample at lower temperatures. The desorption profiles of NH3 remained similar after the sulfur treatment indicating that significant changes in acid site strength were not obtained. Furthermore the apparent activation energies which were obtained for the pentane hydroisomerization reaction showed similar values after the sulfation treatment. It therefore can be assumed that no changes in the enthalpy of adsorption occur which is another indication that the sulfated samples possess similar acid strength. The sulfur treatment at higher temperature led to the
1769 reduction in the concentration of the Bransted acidic bridging SiA1OH groups, while sulfur treatment at low temperatures led to an increase of Bronsted acid sites. The concentration of Lewis acid sites was slightly decreasing after the sulfur treatment. The sulfur treatment followed by a subsequent oxidation resulted in the appearance of a band assigned to a S=O double bond (characteristic band at 1385 cm-~), which indicates that the increase in the concentration of the Bronsted acid sites results from the formation of sulfate species. The interaction of these species with pyridine reveals that the sites are accessible surface species and have acidic character. Moreover, the sulfur content of the samples correlates with their Bronsted site concentration confirming the formation of Bronsted acidic sulfate species. In situ IR measurements during the oxidation of the HzS treated sample showed a strong increase of sulfate species. A band near 1380 cm -1 was reported in ref. [4] after sulfating A1203, TiO2 or ZrO2. All samples possessed Bronsted acidity, assumed to be generated by hydrogenosulfate ions. Under anhydrous conditions bands at 1380 cm ~ and 1040 cm -I were observed after oxidation of HzS or SO2 treated A1203 and TiO2 [5]. In presence of H20 vapor the high-frequency band disappearec and new bands between 1300 and 1150cm -1 were created. On the Pt loaded zeolite Beta we expect that the Bronsted acidic sulfate groups were formed in a similar way as on y-A1203. In [6] a band at 1388cm -1 was observed after sulfur poisoning of y-alumina supported Pt c~ttalysts and as well assigned to sulfate species. The intensity of the peak decreased to about 20% after reactivation of the sample at 550~ in hydrogen. Furthermore sulfation of zeolite modernite with (NH4)2SO4and subsequent oxidation lead to a band at 1351 cm -~ [7]. As the content of SO42- species on the sample exceeds the total amount of Pt atoms the major part of the $042" species should be located on the extra-framework A1 species of the zeolite. The (A10)3S=O groups, proposed for H2S/O2 treated y-A1203 in ref. [5], are transformed to a Bronsted acidic (A10)2SOO-H + group in the presence of water vapor. As the lattice vibrations of the zeolite led to a complete absorption in the energy range below 1300 cm -I it was not possible to detect the shift of the S=O bond. However, the disappearance of the S=O vibration during the hydroisomerization reaction, which can be regained by heating up the sample in He, confirms that the sulfate species are converted to ionic species than reduced to e.g. sulfdes. For the kinetic experiments external diffusion limitations were experimentally excluded. An enhanced pentane hydroisomerization activity was observed fi)r the catalyst sulfated at 350~ and 450~ while a decrease in the catalytic activity was observed after sulfation at 550~ As the activity correlates with the Bronsted acid site concentral:ion it can be assumed that the kinetic relevant step of the reaction pathway is the acid catalyzed rearrangement of the carbenium ions. A direct linear correlation of the acid site cor~centration with the activity was observed for the parent and the catalysts treated at 350~ and 450 ~ although the metal surface is reduced, while the lower rate for the sample sulfated at 550~ can be explained by a shift in the rate determining step towards the dehydrog.enation step resulting from an excessive reduction of metal surface due to the sintering and metal poisoning at the elevated temperatures. Since the dehydrogenation step of that sample is not equilibrated the acid catalyzed step is no longer the kinetic relevant step and hence the isomerization rate does not correlate with the number of Bronsted acid site. 5. C O N C L U S I O N S Gas phase treatment of Pt loaded zeolite Beta with H2S and air enhances the activity for the npentane hydroisomerization. The enhancement of the activity results from the formation of Bronsted acidic surface sulfate species that are formed during oxidation of the H2S treated
1770 samples. The sulfation procedure did not lead to a change in the strength of the acid sites. Although catalyst sulfation leads to an enhancement in acidity it reduces the accessible metal surface. It was shown that it is advantageous to use low temperatures for the H2S and oxidation treatment in order to maximize the hydroisomerization activity as the concentration of sulfur species and the fraction of the metal surface atoms is higher. Enhancement of isomerization activity correlated with concentration of Bronsted acid site indicating that the rearrangement of the carbenium ion is the kinetic relevant step for the catalyst treated with HzS/air at low temperature. ACKNOWLEDGEMENT The work is financially supported by the Bundesministerium ffir Bildung und Forschung Project (BMBF:03C0307D). REFERENCES
[1 ] K.-J. Chao, H.-C. Wu and L.-J. Leu, Appl. Catal. A. 143 (1996) 223. [2] C.A. Emeis, J. Catal. 141 (347-354) (1993) [3] C.P. Hubbard, K. Otto, H.S. Gandhi and K.Y.S. Ng, J. Catal. 144 (1993) 484. [4] M. Waqif, J. Bachelier, O. Saur and J.-C. Lavalley, J. Mol. Catal. 72 (1992) 127. [5] O. Saur, M. Bensitel, A.B. Mohammed Saad, J.C. Lavalley, Carl P. Tripp and B.A. Morrow, J. Catal. 99 (1985) 104. [6] J.-R. Chang and S.-L. Chang, J. Catal. 176, (1998) 42. [7] T. Lei, J.S. Xu, Z. Gao, Mater. Chem. Phys. 60 (1999) 177.