Znd. Eng. Chem. Res. 1995,34,4284-4289
4284
Aromatics Reduction over Supported Platinum Catalysts. 2. Improvement in Sulfur Resistance by Addition of Palladium to Supported Platinum Catalysts Tzong-Bin Lin,t Chou-An Jan,$ and Jen-Ray Chanffrp* Department of Chemical Engineering, National Chung Cheng University, Chia-Yi, Taiwan, R.O.C., and Chinese Petroleum Corporation, R.M.R.C., Chia-Yi, Taiwan, R.O.C.
Supported Pt catalysts were modified by adding a second metal such as Co, Mo, Ni, Re, Ag, and Pd. The effects of a second metal on the sulfur resistance were first investigated by accelerated aging test reactions. The test reactions were hydrogenation of tetralin containing 1000 ppm sulfur at 280 "C, 380 psig, and Hdoil (mol) = 2.7. The results indicated that Pd-Pt catalyst had the highest sulfur resistance. Aromatics reduction, hydrodesulfurization, and hydrodenitrogenation of straight run distillate diesel over supported Pt and Pd-Pt catalysts were further examined by long-term stability tests. The operation conditions of these tests are the same as those run in a commercial diesel hydrotreating process. The results again indicated that the Pd-Pt catalyst exhibited much better catalytic performance. These results, together with electronic properties of the metal clusters on the catalysts characterized by FIT-IR spectroscopy, provide a basis to speculate the role of Pd in enhancing sulfur resistance; the Pd-Pt bimetallic interactions decrease the electron density of Pt and thereby inhibit the adsorption of H2S.
Introduction Supported platinum catalysts have been proved t o exhibit excellent hydrogenation activity but suffer from a high sensitivity t o sulfur poisoning (Barbier et al., 1990). Therefore, a severe pretreatment is necessary to reduce the sulfur content to a level that will not affect the performance of these catalysts. From literature (Gallezot, 1979;Hegedus and McCabe, 1984;Figoli and L'argentiere, 1989;Tri et al., 19801,we found it has been widely accepted that the sulfur tolerance of supported metal catalysts is related to the electron density of metal clusters deposited on supports. Recently, agglomeration of platinum particles leading to catalyst deactivation during sulfur poisoning was observed in pt/Kz,catalysts (McVicker et al., 1993). In our previous paper (Chiou et al., 19951,we have further proposed the possible mechanism for Pt agglomeration. The adsorption of H2S decreases metal-support interaction and thereby promotes platinum migration which leads to a growth of platinum particle size. Combining this information, we thus believed that the sulfur resistance of supported Pt catalysts can be improved by decreasing Pt electron density and/or enhancing metal-support interactions. The addition of Re to WAl203 gives catalysts of higher resistance to the deactivation resulting from coke deposition, thus allowing naphtha reformer to be operated at lower pressures. Before, the role of Re in improving the activity maintenance was an open question. Recent works (Sachtler, 1984;Shum et al.,1985, 1986;Meitzner et al., 1987;Dossi et al., 1989;Purnell et al., 19931,however, indicate that Re acts as an anchor to inhibit Pt migration and stabilize the bimetallic catalyst. Thus, t o inhibit sulfur-poisoning-induced Pt migration, Re seems t o be a good choice. However, X-ray absorption near edge (XANES)and fast Fourier transform infrared (FFT-IR) spectroscopies characterizing Re-Pt catalysts show an electron transfer from Re to Pt (Purnell et al., 1993). This electron transfer
* Author to whom correspondence is addresed. +
4
Chinese Petroleum Corp. National Chung Cheng University.
0888-5885l95/2634-4284$09.oo/o
may promote H2S adsorption which will increase the catalyst deactivation rate. So, Re is not a good choice because of its electron-donation properties. However, improvement of sulfur resistance via bimetallic interaction seems possible by adding another electrophilic metal. Bimetallic catalysts, Co-Pt, Mo-Pt, Ni-Pt, Re-Pt, Ag-Pt, and Pd-Pt were prepared to investigate the effects of the addition of a second metal in this study. Hydrogenation of tetralin containing 1000 ppm sulfur at 280 "C, 380 psig, and Hdoil (mol)= 2.7 was employed as an accelerated aging test. To understand catalyst performance in the commercial plant running mode, the best catalysts emerged from an accelerated aging test and the Pt catalysts were further examined in a longterm stability test at the same operation conditions (580 psig, 340 "C, and Hdoil mol ratio = 2.5) and feed (straight run distillate diesel) as those of the commercial diesel hydrotreating process. The loss of catalyst activity due to morphology changes was indirectly examined by hydrogen regeneration of the sulfur-poisoned catalysts. FFT-IR characterizing CO adsorbed on catalysts was used to examine the electronic characteristics of the metal clusters on catalysts.
Experimental Section Materials and Catalyst Preparation. Two different kinds of support were prepared for this study. The &A1203 support for the accelerated aging test was made by calcining A2U ( y - A l 2 0 3 with surface area about 170 m2/g and particle size about 1 mm, purchased from Osaka Yogyo) at 1000 "C for 6 h. The resultant material had a bulk density, pc, of 0.68 g/cm3. The BET surface area and pore volume measured with an Omnisop 360 analyzer were 82.4m2/g and 0.570cm3/g, respectively. The y-Al203 prepared for the long-term life test was made from a Versa1 250 and Capital B mixture (Chiou et al., 1995). Both catalysts for accelerated and long-term life tests were prepared by the incipient wetness impregnation technique. The A1203 support was first brought to contact with a solution of HzPtC16 (Strem) in doubly 0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4285 distilled deionized water, followed by removing water in vacuum overnight and then calcining a t 450 "C for 8 h. The supported Pt catalysts were therefore prepared. Some of the supported Pt catalysts were then first brought in contact with an aqueous or organic solution of second metal precursors and then dried in vacuum for 12 h to produce supported M-Pt catalysts. The second metal precursors and the solvent used for the preparation of the bimetallic catalysts are listed in Table 1. These catalyst samples contained 1.0 wt % Pt and 1.0 w t % second metal M. The supported Pd catalysts for the FFT-IR study were also prepared by an impregnation method, and the preparation procedures were the same as those for the supported Pt catalysts. Accelerated Aging Test Reactions. The accelerated aging tests were carried out in a continuous down flow fixed-bed reactor. The reactor was a stainless steel tube with an inside diameter of 1.1cm. It was heated electrically, and a PID temperature controller with a sensor at the outer wall of the reactor was used to control the temperature. The temperature difference between the outer reactor wall and the center of the catalyst bed was about 15 "C. The reactor was packed with 3.0 g of catalysts diluted with an inert 200 pm ceramic in a ratio of 1:2. The upper part of the reactor was filled with particles of a catalytically inactive ceramic material for preheating and preventing channel effects. The reaction system was first purged with a dry nitrogen gas for 4 h to remove residual hydrocarbons and for another 8 h in dry air at 600 "C to prevent sulfur contamination from the system. Catalysts were then reduced at 450 "C under 380 psig of pure hydrogen for 4 h. M e r reduction, the catalytic reactions were carried out with a weight hourly space velocity (WHSV) of 4.8 g of feed/h.g of catalyst and a H2 t o oil mole ratio of 2.3, at 280 "C, under 380 psig. Feed was dry tetralin containing 1000 ppm sulfur; benzothiophene is the model sulfur compound. Liquid products were trapped by a condenser at -5 "C. Samples were collected periodically and analyzed by gas chromatography (10% OV-101 on a Chrom W-HP 80/100mesh column a t 110 "Cand a FID detector). The material balance is as good as 98% (3Z2%). Long-TermStability Tests. The diesel feed stocks used in this study are the product of a no. 6 hydrodesulfurization (HDS)unit, Lin-Yuan refinery in Taiwan, ROC. The analytical results of diesel (Table 3) show that it contains 28.4% aromatics, 369 ppm sulfur, and 44.8 ppm nitrogen. The M-Pt catalyst with highest sulfur resistance emerged from an accelerated aging test, and the supported Pt catalyst was used for the stability tests. The tests were carried out in a continuous down flow trickle bed reaction system. The reactor is 0.924 m long with an inside diameter of 2.07 cm. The reaction was run in an isothermal mode, and the temperature was controlled by a three-zone electric furnace with each PID temperature controller for every zone. The axial temperature profile of the catalyst bed was monitored by a traveling thermal couple located in a thermowell mounted at the center of the reactor. The reactor was packed with 13 g of catalysts and diluted with inert 0.20.4 mm glass beads in a ratio of 1:l for keeping isothermal the catalyst bed. The upstream part of the reactor was filled with 1mm glass beads for preheating and preventing channel effects. The material balance of the reaction system is 98% (3~1%).
The reaction system was first purged with a dry nitrogen gas for 4 h to remove residual hydrocarbons. Catalysts were then reduced at 450 "C under 580 psig of pure hydrogen for 6 h. The aromatics saturation of diesel was carried out at the following operation conditions: WHSV 3.0 lh;H2 to oil mole ratio 2.5; temperature 340 "C; pressure 580 psig. Product samples were collected periodically, and sulfur, nitrogen, and aromatics contents were measured. When aromatic conversion showed no significant changes, further kinetic studies were conducted, with WHSV ranging from 1.5 to 4.0. The percentage and type of aromatics in the feed stock and product were analyzed by HPLC (Wates ALC/GPC 244 with M401 RI detector). Total sulfur and nitrogen were analyzed by an ANTEK 701C fluorescence analyzer and a Mitsubishi TN-05 nitrogen analyzer, respectively. API gravity was determined by the ASTM D-287 method, boiling point distribution by ASTM D-86, and viscosity by ASTM D-445.
Catalyst Characterization
CO Chemisorption. Before CO chemisorption measurements, fresh catalysts were first reduced at the same operation conditions as those for stability tests while used catalysts were pretreated with a He purge. After the system became steady (20 mumin He flow rate and 35 "C), a 0.1 mL pulse of CO was repeatedly injected into the catalyst bed with a He carrier gas until none of the pulse was chemisorbed. The amount of chemisorption was then calculated by summing up the proportions of all pulses consumed. Detailed measurement procedures were shown in the previous paper (Chiou et al., 1995). Characterization of Catalyst Samples by FFTIR. A Shimadzu SSU-8000 instrument having a spectral resolution of 4 cm-l was used to characterize the infrared spectra of CO adsorbed catalyst samples. Samples were loaded as a wafer into an IR cell, and treatments were done in situ. Before FFT-IR measurement, samples were reduced at the same operation conditions as those for the stability test except the Ha pressure was at 1atm. The sample was cooled down to room temperature, and CO (flowing at 50-100 mumin at 1 atm) was introduced into the cell and maintained for about 20 min. After the CO treatment, the cell was evacuated t o a pressure of approximately 10-2-10-3 Torr, and IR spectra were recorded.
Results and Discussion Effect of the Second Metal Addition. Several bimetallic catalyst samples with different second metals have been tested to understand the role of the second metal in the sulfur resistance of bimetallic catalysts. Re-Pt bimetallic catalysts are very good catalysts for naphtha reforming. Through bimetallic interactions, these catalysts allow the reaction to run at high temperature and low pressure in favor of the formation of aromatics (Lepage et al., 1987). Compared with Pt catalysts, the only drawback of the catalysts used in naphtha reforming is its lower sulfur resistance. Independent of the metal dispersion measured by CO chemisorption (Table l),stability maintenance of the Re-Pt catalysts prepared from different precursors is lower than that of the Pt catalyst (Figure la and Table 2). The lower sulfur resistance of the Re-Pt catalysts
4286 Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 Table 1. Summary of Catalyst Preparation and Metal Dispersion ~
precursor of second metal
catalyst Pt Pd-Pt Re-Pt(a) Re-Pt(b) Co-Pt Mo-Pt Ni-Pt Ag-Pt Pd
solvent H2O tetrahydrofuran tetrahydrofuran HzO HzO HzO HzO H20 tetrahydrofuran
E"p
CO uptake for fresh catalyst, pmoVg of catalyst 36.91 56.39 19.48 45.62 25.12 26.14 15.38 55.36 16.40
~
CO uptake for used catalyst, pmol/g of catalyst 5.64 23.58 3.59 5.64 1.03 1.03 3.08 2.56
a!
$
'5; ai
25
E
8
.--
t
C
e
3
P
15 0
Time on stream, h
20
30
40
Time on stream, days
80
Figure 2. Product aromatics (HPLC) vs time on stream in longterm stability tests: A, Pt; 0, Pd-Pt.
I
.-e"c
P 8 .-e
10
30
I
I
40
-e
-
P 0
1
2
3
4
5
Time on stream, h
Figure 1. (a) Conversion of tetralin containing 1000 ppm sulfur a t 280 "C, 380 psig, and Hdoil (mol) = 2.7 catalyzed by supported M-Wd-Al203 bimetallic catalysts: U, Pt; 0, Re(a)-Pt; 0, NiPt; A , Mo-Pt; V, Co-Pt; x , Re(b)-Pt. (b) Conversion of tetralin containing 1000 ppm sulfur a t 280 "C, 380 psig, and Hdoil (mol) = 2.7 catalyzed by supported M-Wd-Alz03 bimetallic catalysts: 0, Pt; 0, Pd-Pt; A, Ag-Pt. Table 2. Summary of Accelerated Aging Test Results tetralin conversion at pseudo steady state tetralin conversion (4 h on stream), % at start of run,% catalyst Re-Pt(a) 39.32 1.02 2.00 34.10 Re-Pt(b) co-Pt 44.68 0.45 1.70 Mo-Pt 29.70 42.40 0.30 Ni-Pt 3.15 Pt 58.82 7.08 Pd-Pt 73.27 7.10 Ag-Pt 33.10
may be explained by an increase of the electron density of Pt. The adsorption of electrophilic sulfur compounds on the Pt surface may be increased with an increase of the Pt electron density and thus decreases the sulfur resistance. Similar results were observed for the addition of Ni, Co, and Mo. In contrast, the sulfur resistance is evidently improved by the addition of Pd or Ag to Pt catalyst (Figure l b , Table 2). Inferred from the results, we speculate that stability maintenance of sulfur poison may be enhanced by a second metal with electrophilic characteristics and deteriorated by an electrophobic metal.
Space time, h
Figure 3. Effect of space time on product aromatics (HPLC) at 340 "C, 580 psig, and Hdoil (mol) = 2.7 catalyzed by M-Wy-AlzOs bimetallic catalysts: A , Pt; 0, Pd-Pt.
Stability Tests and Reaction Kinetics of Pt and Pd-Pt Catalysts. As expected for supported Pt catalysts, high sulfur sensitivity was observed in the stability tests (Figure 2): aromatics conversion is only 4.58% after 25 days on stream. Consistent with the results of the accelerated aging test, the sulfur resistance of the supported Pt catalysts was greatly improved by addition of Pd: aromatics conversion for the Pd-Pt catalyst is 22.18%. Besides, the long-term stability test further indicates that the Pd-Pt catalyst has a much better stability maintenance; its catalyst deactivation rate decreases with time on stream and becomes insignificant after 10 days on stream (Figure 2). It has been reported that the sulfur tolerance of a noble metal can be increased by supporting it on zeolite (Gallezot, 1979; Gallezot and Bergeret, 1987). Thus, the sulfur resistance of the y - A l 2 0 3 supported Pd-Pt catalyst is expected t o be further improved by replacing y - A l 2 0 3 with zeolite. After catalyst deactivation became insignificant, kinetic studies of aromatics reduction were conducted at various WHSV. As shown in Figure 3, the data fit zero order nicely for both Pt and Pd-Pt catalysts and the
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4287 Table 3. Feedstock Properties in a Long-Term Life Test General Properties gravity, "API (60O F ) 31.14 viscosity, cst (100O F ) 9.64 total sulfur, ppmw 369 nitrogen, ppmw 44.8 total aromatics, wt % 28.4 monoaromatic, wt % 23.1 diaromatic, wt % 4.6 > diaromatics, wt % 0.7 Distillation (ASTMD86 Method)" (in "C) IBP 167.4 10% 305.8 50% 344.3 90% 374.4 EP 382.9
3.0
M
A
0.5 I 0
10
I
30
20
40
Time on stream, days
Figure 5. Product diaromatics (HPLC) vs time on stream in longterm stability tests: A, Pt; 0, Pd-Pt.
IBP = initial boiling point; EP = end point.
100,
1
a
P
E
50
0
10
20
30
10
20
30
40
Time on stream, days
40
Time on stream, days
0
Figure 6. Sulfur conversion vs time on stream in long-term stability tests: A, Pt; 0, Pd-Pt.
Figure 4. Product monoaromatics (HPLC) vs time on stream in long-term stability tests: A, Pt; 0, Pd-Pt.
80 #
rate constant of the Pd-Pt catalyst is about 4.5 times that of the Pt catalyst. The zero-order kinetics indicated that aromatics are strongly adsorbed on the active sites of the catalysts and the hydrogenation reaction is inhibited by the adsorption of unconverted aromatics. The results are consistent with the FFT-IR characterizing CO adsorbed on sulfur-poisoned Pt catalysts shown in the previous paper (Chiou et al., 1995); the electron density of Pt decreases with the severity of sulfur poison and thereby strengthens the bonding strength between aromatics and Pt particles. Aromatics reduction is a sequential reaction. Polynuclear aromatics are converted to mononuclear aromatics, which are further converted to naphthenes. The reactions are reversible, and the thermodynamic equilibria are most favorable for the conversion of mononuclear aromatics to naphthene (Marcilly, 1990). This work, however, shows that the reactions are limited by the saturation of monoaromatics. Table 3 and Figures 4 and 5 show that hydrogenation is faster on diaromatics than on monoaromatics. These results indicate that the reactions are controlled by kinetics and polyaromatics are kinetically easier to hydrogenate than monoaromatics. Both higher hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity were also observed for the Pd-Pt catalyst as compared with the Pt catalyst (Figures 6 and 7). These results again demonstrate that the catalytic performance of y-Al203-supported Pt catalyst can be greatly improved by the addition of Pd. Sulfur compounds contained in diesel product (S w t % < 0.05) by deep desulfurization treatment catalyzed by Ni-Mo or Co-Mo catalysts indicate that dibenzothiophenes with methyl groups incorporated in the two positions nearest t o sulfur are the most refractive
1 0
10
20
30
40
Time on stream, days
Figure 7. Nitrogen conversion vs time on stream in long-term stability tests: A, Pt; 0, Pd-Pt.
sulfur compounds (Shih, 1992). Kinetic study of model compounds further indicates that these compounds are about 1 order of magnitude less reactive than dibenzothiophene because of steric hindrance of adsorption (Gates et al., 1979; Girgis and Gates, 1991). In contrast t o Ni-Mo and Co-Mo catalysts, about 96% of the refractive sulfur compounds are converted over the PdPt catalyst at WHSV = 1.5 (Figure 8). This result suggests that HDS reactions catalyzed by Pt and PdPt catalysts are not limited by steric hindrance. Instead, the zero order in sulfur concentration (Figure 8) suggests the reactions are limited by the adsorption of sulfur or aromatics compounds. Data of hydrodenitrogenation conversion, as shown in Figure 9, were also fitted by a zero-order rate expression, suggesting that the reactions are inhibited by the adsorption of "3, basic nitrogen compounds, or aromatics compounds. Comparing both sulfur conversion and nitrogen conversion catalyzed by the F't and Pd-Pt catalysts (Figures 6 and 7), we observed that the Pd-Pt catalyst exhibits superior HDN and HDS performance than the
4288 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 100
60
/
I/
01
I
L
50 0.2
0.3
0.4
0.5
0.6
I
0.7
Space time, h
Figure 8. Effect of space time on sulfur conversion at 340 "C, 580 psig, and Hdoil (mol) = 2.7 catalyzed by M-F"y-AlzO3 bimetallic catalysts: A, Pt; 0, Pd-Pt.
2200
2100
2000
1900
1800
1700
Wavenumber, cm" Figure 10. Infrared spectra in the YCO stretching region characterizing the CO adsorbed on Pt, Pd, and Pd-Wy-AlzOs catalysts: (a) Pt; (b) Pd; ( c ) Pd-Pt.
0.5 -
0.3
-
0.1
-....
s 4
20 0.2
0.3
0.4
0.5
0.6
10.1) I ,---,
0.7
2200
Space time, h
Figure 9. Effect of space time on nitrogen conversion at 340 "C, 580 psig, and Hdoil (mol) = 2.7 catalyzed by M-Wy-AlzOa bimetallic catalysts: A, Pt; 0, Pd-Pt.
Pt catalyst does, and the difference of nitrogen conversion between these two catalysts is greater than that of sulfur conversion. The significant difference between the catalyst effects on HDS and HDN may be explained by the different reaction mechanism between HDS and HDN: saturation of the heterocycle molecule is not a necessary step before the C-S breakage for HDS, whereas it is a necessary step before the hydrogenolysis of the C-N bond for HDN (Marcilly, 1990; Girgis and Gates, 1991). Moreover, among the reaction steps involved in HDN, hydrogenation of the nitrogencontaining cycle is an equilibrium step (Marcilly, 1990). Thus, the present HDS and HDN results suggest that the addition of Pd t o a supported Pt catalyst has a greater influence on the thermodynamic equilibrium of the hydrogenation reaction than it does on the cleavage of the C-S bond. The less H2S adsorbed on Pd-Pt clusters due t o electron-deficient characters of Pt via bimetallic interactions may explain the higher hydrogenation equilibrium. Evidence of the electron-deficient character of Pt is indicated by FFT-IR spectroscopy. FFT.IR Spectroscopy Characterizing the CO Adsorbed on Pt, Pd, and Pd-Pt Catalysts. The IR spectra (Figure 10) of the CO adsorbed on Pt and Pd catalyst samples are consistent with spectra reported for CO adsorbed on similarly prepared samples (Little, 1966). The band peak at 2059 cm-l was assigned as the terminal CO ligands adsorbed on Pt clusters (Figure loa). The supported Pt catalyst was inferred to be highly dispersed, since no bridging CO ligands adsorbed on Pt clusters were observed. In contrast, a strong absorption peak located at 1911 cm-' was observed for the Pd sample and was assigned as the bridging CO ligands adsorbed on Pd clusters (Figure lob). Another weak absorption band peak at 2066 cm-' was assigned as the terminal CO ligand. The relatively low terminal CO absorption band indicates that the dispersion of the
2100
2000
1900
1800
1700
Wavenumber, cm-' Figure 11. Infrared spectra in the YCO stretching region characterizing the CO adsorbed on Pt, Pd, and Pd-Wy-AlzOs catalysts: (a) Pd + Pt; (b) Pd-Pt.
catalyst is lower than that of the Pt catalyst, consistent with the CO chemisorption results (Table 1). The infrared spectra characterizing CO adsorbed on the Pd-Pt catalyst suggest the formation of bimetallic interactions. For the Pd-Pt sample, the peaks in the carbonyl stretching region were located at 2084 (vs), 1860 (vs), and 2068 (sh) cm-l (Figure l l b ) and are different from those of the spectra (Figure l l a ) resulting from the summation of Pt (Figure loa) and Pd (Figure lob) spectra; the peaks of the summation spectra (Pd Pt) are located at 2065 (vs) and 1911 (vs) cm-'. That terminal vco band of the Pd-Pt sample is approximately 20 cm-' higher than that of the Pt sample, suggesting the electron density of Pt is decreased by Pd-Pt bimetallic interactions. A decrease of the electron density on Pt causes less backbonding from Pt to the CO n* orbital, resulting in a shift of vco to high frequency (Cotton and Wilkinson, 1988). The bridging vco band of the Pd-Pt sample is about 50 cm-l lower than that of the Pd sample, and it indicates an increase of Pd-carbonyl n-backbonding by bimetallic interaction. This gain of n-backbonding of Pd is consistent with an increased electron density on the Pd. Hydrogenation Regeneration of Sulfur-Poisoned Catalysts. The fact that the initial activity of the hydrogen-regenerated Pd-Pt catalyst was lower than that of fresh catalysts (Figure 12) indicates that the activity of the sulfur-poisoned catalyst can not completely be recovered under the operation conditions. The loss of catalyst activity may be explained by the incomplete reduction and/or morphology change of PdPt clusters during sulfur poisoning and needs to be further studied. At pseudo steady state (after 12 h of the run), the activity of the hydrogen-regenerated Pd-Pt catalyst for tetralin conversion is almost the same as that for the
+
Ind. Eng. Chem. Res., Vol. 34,No. 12,1995 4289
I i
80
-' 88
.-
60e
a 0
20
I
0
5
10
15
20
25
30
Time on stream, h
Figure 12. Test of the reversibility of the sulfur-poisoned PdPt catalyst by examination of the conversion of tetralin containing 1000 ppm sulfur at 280 "C, WHSV = 4.8h-l, 420 psig, and Hdoil = 4.5: 0, fresh catalyst; A, sulfur-poisoned catalysts treated by hydrogen regeneration at 450 "C, 420 psig of H2, for 2 h.
fresh catalyst, while the activity for hydrogen-regenerated Pt catalysts is lower than that of the fresh catalyst (Chiou et al., 1995). The activity loss of the Pt catalyst was caused by metal agglomeration during hydrogen regeneration. These results thus suggest that Pd inhibits metal agglomeration during hydrogen regeneration.
Conclusions The addition of Pd t o the supported Pt catalyst evidently improves the sulfur resistance of the catalyst. Long-term stability tests indicate that aromatics conversion of diesel with 369 ppm sulfur catalyzed by the Pt catalyst under WHSV = 3.0 at 340 "C, 580 psig, is only 4.58% after 25 days on stream, compared with 22.18% conversion by the Pd-Pt catalyst. Kinetic studies, which were conducted a t the time after no significant catalyst deactivation was observed, indicate that the rate constant based on the zero-order rate expression for the Pd-Pt catalyst is about 4.5times of that for the Pt catalyst. Pd-Pt bimetallic interaction was believed to play a crucial role in improving sulfur resistance. FFT-IR characterizing CO adsorbed on the Pd, Pt, and Pd-Pt catalysts suggests that electrons are transferred from Pt to Pd by bimetallic interactions. The decrease of electron density on Pt inhibits H2S adsorption and thereby enhances catalyst sulfur resistance. Besides the sulfur tolerance improvement, the Pd was also inferred to inhibit agglomeration of Pt particles during hydrogen regeneration.
Acknowledgment Support of National Science Council of the Republic of China (NSC 84-2114-E-194-003), National Chung Cheng University, and Refining & Manufacturing Research Center of Chinese Petroleum Corp. (RMRC) is acknowledged.
Chiou, J.-F.; Huang, Y.-L.; Lin, T.-B.; Chang, J.-R. Aromatics Reduction over Supported Platinum Catalysts. 1. Effect of Sulfur on the Catalyst Deactivation of Tetralin Hydrogenation. Ind. Eng. Chem. Res. 1995,34,4277-4283. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; John Wiley & Sons: 1988;pp 58-68. Dossi, C.; Tsang, C. M.; Sachtler, W. M. H. Reforming-Type Catalysis with Zeolite-Supported PtRe. Energy Fuels 1989,3, 468. Figoli, N. S.;L'argentiere, P. C. Selective Hydrogenation catalysts. Influence of the Support on the Sulfur Resistance. Catal. Today 1989,5,403. Gallezot, P. The State and Catalytic Properties of Platinum and Palladium in Faujasite-type Zeolites. Catal. Rev. Sci. Eng. 1979, 20 (11,121. Gallezot, P.; Bergeret, G. Catalyst Deactivation; Peterson, E. E., Bell, A. T., Eds.; Dekker: New York, 1987;p 263. Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill, Inc.: New York, 1979;p 407. Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991,30,2021. Hegedus, L. L.; McCabe, R. B. Catalyst Poisoning; Marcel Dekker, Inc.: New York and Basel, Switzerland, 1984;pp 11-19. Lepage, J. F.; Courty, P.; Freund, E.; Franck, J. P.; Jacquin, Y.; Jugin, B.; Marcilly, C.; Martino, G.; Miquel, J.; Montarnal, R.; Sugier, A.; Landeghem, H. V. Applied Heterogeneous Catalysis Design Manufacture Use of Solid Catalysts; Institute Francais du Petrole, Gulf Publishing Company: Houston, TX,1987;p 467. Little, L. H.; Kiselev, A. V.; Lygin V. I. Infrared Spectra of Adsorbed Species; Academic Press: London and New York, 1966;p 62. Marcilly, C. Hydrotreating Hydrosulfirizatwn; ENSPM-Formation Industrie: Institute Francais du Petrole, 1990. McVicker, G. B.; Kao, J. L.; Ziemiak, J. J.; Gates, W. E.; Robbins, J. L.; Treacy, M. M. J.; Rice, S. B.; Vanderspurt, T. H.; Cross, V. R.; Ghosh, A. K. Effect of Sulfur on the Performance and on the Particle Size and Location of Platinum in pt/KL Hexane Aromatization Catalysts. J. Catal. 1993,139,48. Meitzner, G.; Via, G. H.; Lytle, F. W.; Sinfelt, J. H. Structure of Bimetallic Clusters. Extended X-ray Absorption Fine Structure (EXAFS) of Pt-Re and Pd-Re Clusters. J. Chem. Phys. 1987, 87,6354. Purnell, S. K.;Chang, J.-R.; Gates, B. C. NaY Zeolite-Supported Re-Pt Catalysts Prepared from Organometallic Precursors: Evidence of the Re-Pt Interaction from Infrared and X-ray Absorption Spectroscopies. J. Phys. Chem. 1993,97,4196. Sachtler, W. M. H. Selectivity and Rate of Activity Decline of Bimetallic Catalysts. J. Mol. Catal. 1984,25,1. Shih, S. S.; Mizrahi, S.; Larry A. G.; Sarli, M. S. Deep Desulfurization of Distillates. Znd. Eng. Chem. Res. 1992,31,1232. Shum, V. K.;Butt, J. B.; Sachtler, W. M. H. The Effects of Rhenium and Sulfur on the Activity Maintenance and Selectivity of PlatinudAlumina Hydrocarbon Conversion Catalysts. J. Catal. 1985,96,371. Shum, V. K.; Butt, J. B.; Sachtler, W. M. H. The Effects of Rhenium and Sulfur on the Maintenance of Activity and Selectivity of PlatinudAlumina Hydrocarbon Conversion Catalysts 11. Experiments at Elevated Pressure. J. Catal. 1986,99, 126. Tri, T. M.; Massardier, J.;Gallezot, P.; Imelik, B. Sulfur Resistance of Modified Platinum Y Zeolite. In Catalysis by Zeolites; Imelik, B.; et al., Eds.; Elsevier: Amsterdam, The Netherlands, 1980; pp 279-284. Received for review March 1, 1995 Revised manuscript received July 13, 1995 Accepted July 25, 1995@
Literature Cited Barbier, J.; Lamy-Pitara, E.; Marecot, P.; Boitiaux, J. P.; Cosyns, J.; Verna, F. Role of Sulfur in Catalytic Hydrogenation Reactions. Adv. Catal. 1990,37,279. Biloen, P.; Helle, J. N.; Verbeek, H.; Dautzenberg, F. M.; Sachtler, W. M. H. The Role of Rhenium and Sulfur in Platinum-Based Hydrocarbon-Conversion Catalysts. J . Catal. 1980,63,112.
IE9501539
Abstract published in Advance ACS Abstracts, October 15, 1995. @
