Znd. Eng. Chem. Res. 1996,34,4277-4283
4277
Aromatics Reduction over Supported Platinum Catalysts. 1. Effect of Sulfur on the Catalyst Deactivation of Tetralin Hydrogenation Jaw-Fei ChiouJ Yi-Lin Huang,* Tzong-Bin LinP and Jen-Ray C h a n e t 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.
The effects of sulfur on the deactivation of y-alumina-supported platinum catalysts for aromatics reduction were investigated. The catalytic test reactions were carried out in a continuous fixedbed reactor a t 270 "C and 180-480 psig. Both fresh and used catalysts were characterized by elemental analysis, fast Fourier transform infrared (FFT-IR) spectroscopy, temperatureprogrammed reduction (TPR), and electron probe microanalysis (EPMA). A LangmuirHinshelwood reaction model, which is based on a chemisorption scheme with irreversible surface reaction control for tetralin hydrogenation and reversible surface reaction for sulfur poisoning, was proposed to describe the deactivation kinetics. The kinetic results show that the reactivation rate constant decreases with the severity of sulfur poisoning, while the deactivation rate constant is comparatively unrelated. The TPR results are consistent with the catalytic performance test; a severely sulfur-poisoned catalyst needs to be reduced at higher temperature. FFT-IR spectroscopy characterizing CO adsorbed on the fresh and sulfur-poisoned catalysts indicated that the bond strength between CO and platinum was weakened with an increase of sulfur poisoning. EPMA spectroscopy characterizing the metal profiles of the catalysts indicated a metal migration during sulfur poisoning.
Introduction The growing understanding of the effect of aromatics in diesel fuels on the formation of undesired emissions in exhaust gases and associated health hazards is leading to limitations on aromatics in many areas around the world (Ullman, 1989). Even though there are a number of differences between the specification of individual country or area (aromatic content for the US.currently is less than 35 vol % nationwide and 10% in California), it has been a general trend to reduce aromatics (Crow et al., 1989). In order t o meet the gradually strict diesel fie1 standards, refiners have been developing new catalyst systems and hydrotreating processes to meet the challenge. Two different routes, one with single-stageprocess and the other with a twostage process, are proposed to achieve the requirement. A single-stage system, which combines severe hydrotreating with hydrogenation using a single catalyst, can only achieve the necessary aromatics saturation (HDA) at pressures substantially higher (above 1200 psi) than that operated in current hydrodesulfurization units. A two-stage system, which uses a hydrotreating catalyst at the first reactor and a hydrogenation catalyst at the second, can produce low-aromatic diesel at a moderate partial pressure (about 700 psi) of hydrogen (Sprgarrd-Anderson et al., 1992;Cooper et al., 1992). Two-stage systems normally use supported platinum catalysts for aromatics reduction. These catalysts exhibit excellent hydrogenation activity but are very sensitive t o sulfur compounds in feedstock (Barbier et al., 1990);Pt4alumina catalysts normally need a severe hydrotreating pretreatment to reduce the sulfur concentration to below 2 ppm (wt) (Cooper et al., 1993).The application of these catalysts would thus be limited by such severe pretreatment conditions, unless the sulfur tolerance problems can be solved.
* Author to whom correspondence +
is addressed. National Chung Cheng University. Chinese Petroleum Corp.
It has been reported that the sulfur tolerance of a noble metal catalyst can be increased by using zeolite as a support (Barbier et al., 1990). A special carrier supported noble metal catalyst developed by Topsae can even tolerate sulfur up to 300 ppm (Sprgarrd-Andersen et al., 1992). However, notwithstanding a lot of progress in improving the sulfur tolerance of these catalysts,little is known about the fundamental catalytic chemistry involved in sulfur-poisoning catalyst deactivation. The results summarized in this paper were t o clarify the effects of sulfur concentration and operation conditions on Wy-alumina catalysts. To decouple the effects of sulfur and coke on the catalyst deactivation, tetralin hydrogenation reaction was chosen as a model reaction, because it can be carried out at low temperature (less than 300 "C), so that the catalyst deactivation caused by coke formation is negligible. Benzothiophene was chosen as the model compound for sulfur poisoning because (1)it is one of the sulfur compounds in diesel and (2)its catalytic chemistry was fully understood (Gates et al., 1979). A Langmuir-Hinshelwood model was formulated to summarize the reaction results. The effects of the severity of sulfur poisoning on the hydrogen reactivation were examined by temperature-programmed reduction (TPR). Electron properties of the sulfur-poisonedcatalysts were examined by fast Fourier transform infrared (FIT-IR)spectroscopy characterizing CO adsorbed on the fresh and used catalysts. Electron probe micro analysis (EPMA) was used to characterize catalyst metal concentration profiles to understand metal migration induced from sulfur poisoning.
Experimental Section Materials and Catalyst Preparation. Supports of different pore diameters were prepared by using pseudoboehimitic alumina of different dispersibility indexes under controlled calcination temperature (Trimm and Stanislaus, 1986;Belding and Rigge, 1983). The y - A l 2 0 3 of 389 A average pore diameter and 64 m2/gsurface area was prepared from Conda-Pura1 200 alumina powder.
0888-5885/95/2634-4277$09.00/0 0 1995 American Chemical Society
4278 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
The powder was digested by a nitric solution to form a gel, then neutralized by aqueous ammonia, followed by kneading and extrudation, and finally calcined a t 600 "C for 6 h. The y-Al203 sample of 141 8, average pore diameter and 179 m2/g surface area was prepared from an equal ratio of Versa1 250 and Capital B powder with the same procedure as the preparation for the 400 8, sample except calcined at 700 "C. The catalyst samples were prepared by an impregnation technique. The y-Al2O3 samples (cylinder in shape, 1.1 mm diameter, and about 2-4 mm long) were brought in contact with a solution of [HzPtClsl (Strem) in doubly distilled deionized water, followed by evacuation at room temperature under vacuum, and then calcined at 450 "C for 4 h. Both samples contained 0.96 f 0.02 wt % Pt, as measured by inductively coupled plasma optical emission spectroscopywith a Jarnell-Ash 1100 instrument. Catalytic Performance. The catalytic performance tests were carried out in a continuous downflow k e d bed reactor. The reactor was a stainless steel tube with an inside diameter of 1.2 cm. It was heated electrically, and the temperature was controlled by a PID temperature controller with a sensor at the outer wall of the reactor. The temperature difference between the outer reactor wall and the center of the catalyst bed was about 15 "C. The upstream part of the reactor was filled with particles of a catalytically inactive ceramic material for preheating and the inhibition of channeling effects. The reaction system was first purged with dry nitrogen gas for 2 h. Catalyst samples (cylinder of 1.2 mm diameter, 2-4 mm long) of 1.5 g (about 2 cm3) were diluted with an inert ceramic in a ratio of 1x5 (particle size about 200 pm) and then reduced a t 450 "C under 480 psig of pure hydrogen for 4 h. After reduction of the catalyst, the catalytic reactions were carried out with a weight hourly space velocity (WHSV) ranging from 2.0 to 12.0 (g of feedhag of catalyst) with a hydrogen flow rate of 100 mumin, a t 270 "C, under 480 psig total pressure for the mass transfer limitation test. The pump used in this study is a high-performance liquid chromatography (HPLC) pump (GL Science Inc., Model 576). As the pump employs a small doubleplunger reciprocating pump which is controlled by a microprocessor, it can deliver a micro flow rate ranging from 0.001 to 9.99 mumin. Liquid products were trapped by a condenser at -5 "C. Samples were collected periodically and analyzed by gas chromatography. Before the reaction, tetralin (Merck) was dried with particles of an activated 4 A molecular sieve. For the sulfur-poisoning study, pretreated tetralin was mixed with a certain amount of benzothiophene to prepare a feed of 500-5000 ppm sulfur content. The operation conditions were the same as those for the mass transfer limitation test except the total pressure ranged from 180 to 480 psig and WHSV ranged from 2.0 to 4.8 h-l. The material balance is about 98% (&3%),indicating that the system is suitable for investigating catalyst deactivation caused by sulfur poisoning. For the elimination of sulfur contamination from the reaction apparatus, before each experiment, the system was purged with pure tetralin at 270 "C for 8 h and then with air at 600 "C for another 8 h. The GC system used for feed and product analysis was a Shimadzu Model GC-14B gas chromatograph equipped with a 10%OV-101 on a Chrom W-HP 80/100 mesh column of 18 m x 0.22 mm i.d. and a FID detector. The column was operated at 110 "C with a linear velocity of 20 c d m i n of dry nitrogen.
After each reaction experiment, the reactor was purged with dry nitrogen a t the reaction temperature for 2 h, and then the catalysts were removed and characterized by FFT-IR, TPR, EPMA, and elemental analysis.
Catalyst Characterization CO Chemisorption. A stainless steel tube was packed with 1g of catalyst sample. The catalyst sample was then reduced by the same operation conditions as those for the catalytic performance test. After reduction, the tube was then attached to a Shimadzu Model GC-14B gas chromatograph with a thermal conductivity detector (TCD). The tube for adsorption measurement was connected with a three-way ball valve, and the connection ports were carefully purged with He gas before adsorption experiments to prevent any air contamination. After the system became steady (20 m u min He flow rate and 35 "C), a 0.1 mL pulse of CO was repeatedly injected into the catalyst bed with 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. Characterization of Catalyst Samples by FFTIR. Infrared Fourier transform spectroscopy characterizing CO adsorbed on catalysts was recorded with a Shimadzu SSU-8000 instrument having a spectral resolution of 4 cm-l. The wafer samples were loaded into a IR cell. The cell was designed to allow it to connect t o a vacuudgas-handling manifold for in-situ treatment. The fresh catalyst samples were reduced at the same temperature and hydrogen flow rate as those for the catalytic performance test except at 1atm. After reduction, the sample was cooled 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 to a pressure of approximately 10-2-10-3 Torr, and IR spectra were recorded. As for used catalysts, the procedure was similar to that of a fresh one except that the sample was purged with He before CO adsorption. Temperature-Programmed Reduction (TPR). The apparatus used for the TPR studies was described by Jones and McNicol(1986). A gas stream of 10% H2 in argon passed through the fresh and used catalysts in a quartz reactor heated a t 10 "C/min to 550 "C with a temperature-programmed furnace. The water produced by reduction was trapped into a column of silica gel. The amounts of H2 consumed and H2S evolved in reduction were detected with a thermal conductivity detector (TCD). The reduction temperature was monitored by a thermocouple. Electron Probe Microanalysis (EPMA). The platinum concentration profiles of the catalysts were measured by EPMA (Jeol 840A link AN10). The catalyst samples were imbedded in a thermoplastic resin. After solidification, the catalyst particles were grinded off by a no. 1500 sand paper followed by no. 3000 sand paper for fine polishing. The samples were then coated with a layer of carbon by vacuum deposition. The acceleration voltage of the electron beam was 20 kV, and the sample current was 0.05 mA. The scanning speed was 0.02 m d m i n . Results and Discussion Examination of Internal Mass Transfer Limitation and Rate Expression for Tetralin Hydrogenation. Two catalyst samples of the same metal loading and dispersion but different average pore diameter (389
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4279 reported here, only a trace of styrene was detected. Moreover, elemental analysis indicated that the residue carbon on the used catalysts was less than 0.35 wt %. Accordingly, we proposed that the catalyst deactivation was caused by the adsorption of HzS and the formation of metal sulfides.
H,S
+ Pt
Pt-H,S
0.4
‘
0
I
1
I
4
0.1
0.2
0.3
0.4
I 0.5
0.6
Space time, h Figure 1. Tetralin conversion at 270 “C and 480 psig catalyzed by supported Wy-AlzOs catalysts: 0 , surface area = 64 m2/g; A, surface area = 179 m2/g.
PtS + H,
(3) Since the tetralin concentration is at least 3 orders higher than that of HzS, KTCTwas assumed to be much greater than K H ~ s C ~It ~ Swas . also assumed that the activity coefficient, 4, was proportional to the ratio of unpoisoned Pt to initial Pt, (Pto- PtS)/Pto, and catalyst deactivation was controlled by surface reaction after the induction period. The activity coefficient and tetralin conversion were thus formulated as follows: Q
0
IA 0
(2)
(4)
/&m#m..
5
10
15
20
25
Time on stream, h
Figure 2. Benzothiophene conversion a t 270 “C and 480 psig catalyzed by supported Wy-AlzO3 catalysts (surface area = 179 m2/g): 0 , sulfur = 500 ppm, WHSV = 2.0 h-l, 480 psig; 0, sulfur = 1000 ppm, WHSV = 2.0 h-l,480 psig; 0, sulfur = 1000 ppm, WHSV = 4.8 h-l, 480 psig; A, sulfur = 2500 ppm, WHSV = 4.8 h-l, 480 psig; W, sulfur = 1000 ppm, WHSV = 4.8h-l,380 psig.
and 141 A) were used t o examine the internal mass transfer of tetralin hydrogenation over supported platinum catalysts. As shown in Figure 1, the tetralin conversion has no significant difference between these two samples, indicating that the internal mass transfer limitation is negligible. According to Boudart (1962), the reaction is a simple irreversible first-order surface reaction. The rate expression was thus formulated as k,CHzCT/(l KTCT),where CT is the concentration of tetralin, C His ~ the hydrogenation concentration in liquid phase, KT is the tetralin adsorption equilibrium constant, and k, is the product of the surface reaction rate constant and the total active sites. By using leastsquares criteria and the gradient method (Seinfeld and Lapidus, 19741,k$H, and KTwere estimated to be 15.62 h-l and 0.0912 mol-l, respectively. Model for Catalyst Deactivation by Sulfur Poisoning. Conversion of benzothiophene over Co-Mo/ A 1 2 0 3 catalyst containing 2.4 wt % Co and 6.7 wt % Mo at 400 “C and 1 atm indicates that about 96% of benzothiophene is converted to ethylbenzene, styrene, and hydrogen sulfide (Gates et al., 1979; Kilanowski et al., 1978).
+
benzothiophene
Pt-H,S
”
100
60
-
+ H,-
+
In the above equations, t is time on stream, t(l/ WHSV) is space time, K H ~isSthe adsorption equilibrium constant of HzS, kdea is the deactivation coefficient, krea is the reactivation coefficient, and (3-is 1, the total hydrogen at both the gas and liquid phase. By eq 4 and krCHz and KT estimated from Figure 1,the time-dependent activity coefficients for each run of the experiments were calculated. To estimate deactivation and reactivation Constants in eq 5, we define kdeaCHzS = k’dea, kreaCH2 = k’,a, a@/ak’dea = y and a@/ak’,,, = z. By the steepest descent method (Seinfeld and Lapidus, 1974), the differential equations for parameter estimation were formulated as:
(7)
dz dt = -(1 - 9)
(8)
The initial conditions of the above equations are defined as @(to)= 4O,y(0) = 0, and z(0) = 0, where t o is the time right aRer the induction period (about 2 h after the start of the run) and is the corresponding activity coefficient. By least-squares criteria (the minimized sum of the squares of the difference between the observed and predicted activity coefficient), the cost function was defined as: J = x ( 9 i - &I2 (9) where & is noted as experimental data. The sensitivity equations were thus formulated as:
+
ethylbenzene styrene H,S (1) As shown in Figure 2, insensitive to time on stream, benzothiophene concentration, and operation conditions, about 90% benzothiophene was converted t o ethylbenzene after the induction period; in the experiments
(11) Initial guesses for k’dea and k’re, were obtained from plotting ln(1 - q5/@eq) against time on stream by assum-
4280 Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 16
60 50
c -
-&=-
14.
- 90
2
12.
*
L
30
c
e " ,
u
40
.-B
20
-80
10.
= C
10
E
-70
8.
0
3
6
9
1 2 1 5
Time on stream, h
Figure 5. Effect of operation pressure on the catalyst deactivation for the catalyst of surface area 179 m2/g at 270 "C, WHSV = 4.8 h-l, H2/oil= 4.5, and sulfur = 1000 ppm: 0,480 psig; 0,380 psig; 0, 280 psig; A , 180 psig.
?T--
g
.-g
I
Y
75
E P
8
50
.-e I
c'
25
0
8
4
12
16
20
24
Time on stream, h
Figure 4. Effect of sulfur concentration on the catalyst deactivation for the catalyst of surface area 179 m2/g at 270 "C, WHSV = 2.0 h-l, H2/oil= 10.8, and 480 psig: 0 ,sulfur = 500 ppm; 0, sulfur = 1000 ppm; 0 , sulfur = 2500 ppm; A, sulfur = 5000 ppm; solid line, data calculated from model. Table 1. Summary of Parameter Estimation Results for Sulfur-PoisoningCatalyst Deactivation sulfur content, ppm
WHSV, h-l
pressure, psig
Hdoil, moUmol
kdeaCH&
kreaGH,,
h-'
h-l
500 1000 2500 5000 1000 1000 1000 1000
2 2 2 2 4.8 4.8 4.8 4.8
480 480 480 480 480 380 280 180
10.8 10.8 10.8 10.8 4.5 4.5 4.5 4.5
0.121 0.456 0.763 1.250 0.527 0.570 0.548 0.524
0.042 0.045 0.010 0.009 0.024 0.013 0.007 0.010
+
ing KTCT
