Î´-Al2O3 Catalysts for Isoprene

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Ind. Eng. Chem. Res. 1997, 36, 4094-4099

Catalytic Properties of Eggshell Pd/δ-Al2O3 Catalysts for Isoprene-Selective Hydrogenation: Effects of Water Poisoning Jen-Ray Chang* and Chil-Hung Cheng Department of Chemical Engineering, National Chung Cheng University, Chia-Yi, Taiwan, Republic of China

The influences of water poisoning on isoprene-selective hydrogenation over δ-alumina-supported eggshell Pd catalysts were investigated by test reactions at 40 °C and 420 psig of total pressure. Both fresh and water-poisoned catalysts were characterized by fast Fourier transform infrared (FFT-IR) spectroscopy and temperature-programmed desorption (TPD) of hydrogen. The catalytic performance tests showed that water poisoning suppresses the hydrogenation of isoprene and the double-bond migration of 2-methyl-1-butene and 3-methyl-1-butene to 2-methyl-2-butene, whereas it slightly facilitates the selectivity to the formation of isopentenes. FFT-IR spectroscopy characterizing CO adsorbed on the fresh and water-poisoned catalysts indicated that the bond strength between CO and palladium was weakened by the adsorption of H2O on Pd, suggesting that the adsorbed water abstracts electrons from Pd clusters. The TPD results further indicated that the formation of β-hydride was inhibited by this water adsorption. Together with the catalytic performance test, FFT-IR, and TPD results, the alteration of the catalytic properties by water poisoning is associated with the decrease of active sites for hydrogenation, the decrease of the electronic density of the Pd clusters, and the inhibition of β-hydride formation. Introduction A two-stage hydrogenation process is used to stabilize pyrolysis gasoline and to reduce its sulfur content (Griffiths et al., 1968; Lepage et al., 1987). The first stage of hydrogenation normally uses δ-alumina-supported eggshell Pd catalysts for partially saturating the unstable species such as isoprene, cyclopentadienes, alkenyl aromatics, and other conjugated diolefins to become relatively stable olefins. After hydrogenation, the C5 (hydrocarbon compounds with one molecule containing five carbon atoms) stream is separated from the pyrolysis gasoline in a depentanizer and used as the feedstock of the tert-amyl methyl ether (TAME) process (Jong et al., 1984; Lin and Chou, 1995). Since isopentenes are the desired active species in the C5 streams, it is important for the hydrogenation process to increase the conversion rate of isoprene into isopentenes while reducing the conversion of isopentenes into isopentanes. After the depentanizer, a deoctanizer is used to separate C9+ (hydrocarbon with one molecule containing nine carbon atoms or more) from pyrolysis gasoline, and then the second stage of hydrogenation is carried out over regenerable cobalt-molybdate catalysts to remove the sulfur-containing compound. Pyrolysis gasoline, a byproduct from naphtha cracker, is separated from C2 to C4 with a fractionator. The hot (110 °C) pyrolysis gasoline stream from the fractionator is cooled to about 87 °C with quench water (34 °C). A oil/water separator is then used before the pyrolysis hydrogenation process to separate the pyrolysis gasoline from the quench water. Foaming of the oil/water separator makes the separation difficult and leads to water contamination of the pyrolysis gasoline. The main goal of this research was to investigate the water-poisoning catalyst deactivation of δ-aluminasupported Pd catalysts for the selective hydrogenation of isoprene. The alteration of the catalytic properties induced by the water poisoning was also of interest. To implement our research goals, the Pd catalysts were tested with pure and water-containing isoprene in a * Author to whom correspondence should be addressed. S0888-5885(97)00256-X CCC: $14.00

fixed-bed reaction system with operation conditions (40 °C and 30 atm) similar to those of commercial plants (Jong et al., 1984). The selectivity and activity of the catalysts were examined by these tests. The influence of water adsorption on the electronic properties of the catalysts was examined by FFT-IR spectroscopy, characterizing CO adsorbed on the catalysts. TPD of the hydrogen of the fresh and spent catalysts was used to characterize different types of hydrogen on the fresh and water-poisoned catalysts. Isoprene was chosen as the model compound for the selective hydrogenation of pyrolysis gasoline because (1) it is one of the unstable species of pyrolysis gasoline, (2) isoamylene, the selective hydrogenation product of isoprene, is the raw material for the TAME process, (3) the catalyst life of the TAME process is greatly influenced by the isoprene contained in the feed. Experimental Section Materials and Catalyst Preparation. The δ-Al2O3 support was prepared by calcining γ-Al2O3 with a particle size of about 2 mm (A2U, Osaka Yogyo) at 1000 °C for 6 h. The resulting material had a bulk density of 0.68 g/cm3. The BET surface area and pore volume measured with an Omnisop 360 analyzer were 82.4 m2/g and 0.570 cm3/g, respectively. The eggshell Pd catalysts were prepared by an impregnation technique with an excess of solution (Lin and Chou, 1994). The support was brought into contact with a solution of 0.1068 g of Pd(CH3COO)2 in 150 mL of toluene, followed by removing the solvent by filtration and then calcining at 350 °C for 6 h. The catalyst contained 0.2 wt % Pd, measured by inductively plasma optical emission spectroscopy using a Jarnell-Ash 1100 instrument. After reduction, the Pd dispersion measured with CO chemisorption was 8.64 µmol of CO uptake/g of catalysts. Isoprene Hydrogenation Reaction. The catalytic performance tests were carried out in a continuousdownflow fixed-bed reactor. The reactor was a vertical stainless steel tube with an inside diameter of 2.2 cm and an inside volume of 94.0 mL. It was heated by a © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4095

water-bath circulator, and the temperature was controlled by a PID temperature controller with a sensor in the center of the catalyst bed. The feed was prepared by mixing 10 wt % isoprene (Merck) in toluene (99.22% purity, No. 4 reforming plant, Chinese Petroleum Corp., Taiwan, ROC). For the water-poisoning tests, water with a flow rate of 5% reactants was pumped separately into the reaction system. The reactants and water mixture were preheated before entering the catalyst bed, and the reaction products were trapped by a condenser at -5 °C. The reactor was packed with 1.5 g of catalyst mixed with inert SiO2 (Merck) in a ratio of 1:20 by volume. A gradient packing method was used so that the catalyst bed would have a nearly uniform temperature and the wall and bypassing effects would be minimized. The catalyst-to-inert ratio increased from 0.0 at the inlet of the catalyst bed to 0.1 at the outlet. The ratio of bed length to catalyst particle diameter was approximately 50; the axial dispersion effects are inferred to have been negligible (Satterfield, 1975). The upstream part of the reactor was a preheater zone filled with SiO2. The catalyst samples were reduced with 12 L(NTP)/h hydrogen at 100 °C and 30 atm for 10 h. The hydrogenation reaction was then carried out with a weight hourly space velocity of 18 h-1 (g of feed/(h‚g of catalyst)), at 40 °C, 30 atm, and a H2/isoprene molar ratio ) 2.262. Up to 97% feed was recovered as reaction products in the material balance tests, indicating the reaction system was adequate for the performance test. Liquid products were collected periodically and analyzed by a gas chromatograph (Hewllett-Packard Model 5890 A, FID model) coupled with a data processor (SP4270). A capillary column (Petrocol DH 150, 150 m × 0.25 mm i.d., 1.0-µm phase film) was performed with He flowing at 20 cm/s, starting at 30 °C for 20 min and then increasing at a 10 °C/min temperature-programming rate until it reached 150 °C for 0.5 h (0.9-µL split 100:1). Temperature-Programmed Desorption (TPD) of H2. A quartz tube was packed with about 1.0 g of catalyst sample. The catalyst sample was then reduced under the same operation conditions as those used in the catalytic performance test except at 1 atm. After the reduction, the sample was cooled to room temperature. When system became steady (20 mL/min N2 flow rate and 40 °C), 1 mL of H2 was injected into the catalyst bed through N2 carrier gas. The injections were repeating until none of the H2 chemisorbed was detected by a thermal conductivity detector (TCD). The desorption experiments were then carried out with He flowing at 20 mL/min at 1 atm with a maximum temperature of 700 °C and a heating rate of 10 °C/min. Evolved H2 was monitored with a TCD. For the TPD measurement of the water-adsorbed catalyst sample, the procedure was the same as that for the fresh one except that the reduced catalyst sample was treated by flowing N2 gas with water saturated at room temperature for 0.5 h. Characterization of the Catalyst Samples by FFT-IR. Catalyst samples were loaded as a wafer into an IR cell. The cell was connected with a vacuum/gashandling manifold for in situ treatment. To characterize the CO adsorbed on the catalyst samples, before FFT-IR measurement, the samples were reduced at the same operation conditions as those of the stability test except with a H2 pressure of 1 atm. After the reduction, the sample was cooled to room temperature, and CO (flowing at 50-100 mL/min at 1 atm) was introduced

Table 1. Properties of the Pd Catalyst for the Pyrolysis Gasoline Hydrogenation Process Pd content, wt % shape surface area, m2/g total pore vol, cm3/g bulk density, g/cm3

0.3 1/ -in. × 3/ -in. sphere 16 16 60 0.60 0.66

Table 2. Operation Conditions of the Selective Hydrogenation of Pyrolysis Gasoline reactor temp, °C SORa EORb reactor pressure, kg/cm2 LHSV, h-1 H2 partial pressure, kg/cm2 SOR EOR min H2/C4+, molar ratio SOR EOR min liquid recovery, wt % SOR EOR fraction tower plates reflux/feedstock ratio tower bottom pressure, kg/cm2 abs tower bottom temp, °C product specifications: C5 fraction existing gums, mg/100 mL diene value C9-204.4 °C fraction final bp, °C existing gums, mg/100 mL diene value a

inlet 60 120

outlet 140 180 30

3.5 23 18.4 0.2 0.23 95.5 88.5 depentanizer 50 0.5 6.6

deoctanizer 35 0.4 0.9

rerun 20 0.33 0.38

178

180

169 4 max 3 max 204.4 max 4 max 1 max

Start of run. b End of run.

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 the IR spectra were recorded with a Shimadzu SSU-8000 instrument having a spectral resolution of 4 cm-1. For the wateradsorbed catalyst sample, before CO adsorption, the adsorption of water was carried out by flowing watersaturated N2 gas at room temperature for 0.5 h. Results and Discussion Performance of the Commercial Plant for Pure and Water-Containing Pyrolysis Gasoline. Gums formed from the polymerization of the highly reactive diolefins contained in pyrolysis gasoline will cause the difficulty in separation of C5 and C9+ from pyrolysis gasoline and deactivate the catalysts for the TAME process. The purpose of the first-stage pyrolysis gasoline hydrogenation process is to eliminate these unwanted diolefins. The process in the No. 4 reforming plant, Chinese Petroleum Corp., was developed by IFP (Institut Frana´ais du Pe`trole) technology (Derrien, 1986; Lepage et al., 1987). The catalyst for the process was commercially available supported Pd catalyst, and its properties are summarized in Table 1 (Lin and Chou, 1995; Cheng et al., 1986). The diene value (measured by the UOP 326/82 method) was used as an index to evaluate the extent of the hydrogenation reaction, and the value for the pyrolysis gasoline feed is 22 ( 1. The operation conditions of this process are shown in Table 2 (Jong et al., 1984). As shown in Figure 1, before 80 days on stream, the diene value of the C5 fraction of the reaction product was less than 3. However, after the feed had been contaminated by water (after 80 days

4096 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

Figure 1. Plot of the diene value of C5 fraction vs days on stream to demonstrate the effects of water poisoning on the performance of pyrolysis-gasoline-selective hydrogenation of a commercial plant (O, without water poisoning; 4, with water poisoning).

Figure 2. Effects of water poisoning on the isoprene conversion at T ) 40 °C, P ) 30 atm, WHSV ) 18 h-1, and H2/isoprene (mol) ) 2.262 catalyzed by eggshell Pd/δ-Al2O3 catalysts (O, without water poisoning; 4, with water poisoning).

Scheme 1 3M1B

IP

2M2B

IC5

2M1B

on stream), the diene value of the reaction products increased abruptly. These results suggest that the hydrogenation reaction of alkadienes was inhibited by water contamination. As indicated in the paper of Lin and Chou (1995), the total life of the palladium catalysts is about 3 years and the cycle life is about 8 month. For normal operation, the diene value of the reaction products should be less than 3. The abrupt increase of the diene value after 80 days on stream was caused by water contamination of the pyrolysis gasoline feed resulting from the foaming of the oil/water separator before the pyrolysis gasoline hydrogenation process. Once foaming happened, the amount of water contained in the pyrolysis gasoline is hard to control. The dispersion of the diene value shown in Figure 1 in the presence of water is due to the variation of water contained in the feed of the hydrogenation process. Effects of Water on the Isoprene Hydrogenation Reaction. The hydrogenation of isoprene is a typical consecutive reaction with isomerization of isopentenes, and those reactions can be depicted by the reaction scheme shown in Scheme 1 (Aduriz et al., 1991). By 1,2 addition of hydrogen, isoprene (IP) produces 3-methyl-1-butene (3M1B) and 2-methyl-1-butene (2M1B), whereas 2-methyl-2-butene (2M2B) is formed by 1,4 addition and the double-bond migration of 2M1B and 3M1B. Further hydrogenation of isopentenes, including 2M1B, 3M1B, and 2M2B, yields a undesired product, isopentane (IC5). Among these three species, 2M2B is the thermodynamically favorable product. The total conversion of isoprene is shown as a function of time on stream in the flow reactor for pure feed and water-containing feed (Figure 2). The initial activity of the catalyst sample for the pure feed was higher than that for the water-containing feed. The conversion for the two tests decreased with time on stream and became nearly time invariant after about 20 h for the pure feed, while it was about 50 h for the water-containing feed. The different induction periods observed for those two tests may be because of the difference in the catalyst

Figure 3. Effects of water poisoning on the isopentenes selectivity at T ) 40 °C, P ) 30 atm, WHSV ) 18 h-1, and H2/isoprene (mol) ) 2.262 catalyzed by eggshell Pd/δ-Al2O3 catalysts (O, without water poisoning; 4 with water poisoning).

deactivation mechanism; gum formation is the main factor of the catalyst deactivation for the pure feed, whereas both water poisoning and gum formation are important for the water-containing feed. Moreover, rapid catalyst deactivation resulting from water poisoning may greatly deteriorate the catalyst performance. In the near steady state, the conversion for the watercontaining feed is only about 25% of that for the pure feed. The lower hydrogenation activity for the catalyst with the presence of water is consistent with the results observed in the commercial plant. For the pure feed, gum buildup from the polymerization of diolefins has been suggested as the main reason for catalyst deactivation (Griffiths et al., 1968; Sze and Bauer, 1969; Trimm, 1983). For the watercontaining feed, it is inferred from the FFT-IR results that the very rapid catalyst deactivation is caused by the adsorption of water on Pd clusters. However, other possibilities such as the formation of palladium oxide cannot be rule out. In contrast to the decrease of isoprene conversion, the selectivity for isopentenes increased with the adsorption of water on Pd. As shown in Figure 3, in the nearly steady state, the selectivity to isopentenes [(isopentenes yield/isoprene converted)100%; the value has about a 2% deviation caused by the deviation of gas chromatography measurement] is about 97% for the watercontaining feed and is slightly higher than that for pure feed of 92% selectivity. The results are similar to those


Figure 4. Effects of water poisoning on the 2-methyl-2-butene selectivity at T ) 40 °C, P ) 30 atm, WHSV ) 18 h-1, and H2/ isoprene (mol) ) 2.262 catalyzed by eggshell Pd/δ-Al2O3 catalysts (O, without water poisoning; 4, with water poisoning).

observed for the catalytic properties of Pd catalysts influenced by sulfur compounds (Oudar, 1980; Deng et al., 1993; Hegedus and McCabe, 1981; Lepage et al., 1987) and by CO adsorption (Jong et al., 1984). Adsorption of these compounds on the catalyst retards the hydrogenation activity, whereas it promotes the selectivity to olefins. The consensus regarding the role of CO and sulfur compounds in changing selectivity to olefins is that the interaction between those compounds and the Pd clusters decreases the electron density of Pd and, thus, strengthens the adsorption between the electron donor (diolefins) and the electron-deficient Pd clusters. The decrease of isoprene conversion with the presence of water was inferred to be caused by the adsorption of water on the Pd clusters, leading to a decrease of the active sites available for the hydrogenation reaction. The increase of the selectivity to isopentenes, however, was inferred to be caused by the increasing affinity between Pd clusters and isoprene. The adsorbed H2O, which abstracts electrons from Pd clusters, promotes the adsorption of isoprene. The adsorption of isoprene displaces isopentenes from the surface and thus inhibits the subsequent hydrogenation of isopentenes. The increase of the selectivity to isopentenes can also be explained by the decrease of the possibility of overhydrogenation. For the reaction with water present, water competes with hydrogen for adsorption sites, hence retarding the hydrogenation of isopentenes. A comparison of the selectivity to methylbutene isomers between the test with pure feed and watercontaining feed is shown in Figures 4-6. In the nearly steady state, the selectivity to 2M2B [(2M2B yield/ isoprene converted)100%] for the test with watercontaining feed was lower than that for the test with pure feed; on the contrary, a higher selectivity to 2M1B and to 3M1B was observed for the water-containing feed. Among the isomers of isopentenes, only 2M2B and 2M1B are valuable for the TAME process and 2M2B is the major product of the hydrogenation of isoprene. Thus, besides the lower conversion, the water-poisoninginduced selectivity change is not favorable for the TAME process as well. The isomerization of 2M1B and 3M1B was suppressed by the adsorption of water. As reported by Germain (1969), on metal catalysts, at high temperatures, the isomerization of alkenes can be carried out without hydrogen. However, at low temperatures, free hydrogen is necessary for the reaction. As indicated in the results



of hydrogen TPD, the water adsorption markedly inhibits the formation of β-hydride. Since the test reactions were performed at 40 °C, for the test with water present, the lower selectivity to 2M2B and higher selectivity to 2M1B and to 3M1B might be regarded as the decrease of the isomerization rate of 2M1B and 3M1B caused by the shortage of free hydrogen. Characterization of CO Adsorbed on the Fresh and Water-Poisoned Catalysts. When CO is adsorbed on the Pd clusters, electron transfer occurs from the d orbitals of Pd to the π* (antibonding) orbital of CO (Cotton and Wilkinson, 1988). A sample with greater electron density on the palladium causes more backbonding from the metal to the CO π* orbital. This backbonding strengthens the Pd-C bond and weakens the C-O bond. As shown in Figure 7a, for the supported palladium catalyst, two CO adsorption bands located at 2066 and 1911 cm-1 were observed and were assigned as the terminal and bridging CO ligand, respectively (Little, 1966). Two main peaks were also observed for the water-poisoned Pd catalyst, while the peak for the terminal ligand shifted to 2082 cm-1 (Figure 7b). Besides, an additional shoulder was observed at 1960 cm-1. The shoulder may be due to the interactions of the adsorbed water and/or the OH groups on the surface with bridging CO ligands. The shift of the νCO absorption band for both terminal and bridging ligands to higher frequency suggests that the adsorbed water abstracts electron from Pd clusters, thereby

4098 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997

Figure 7. Infrared spectra in the νCO stretching region characterizing the CO adsorbed on the Pd catalysts: (a) fresh catalyst; (b) water-poisoned catalysts.

on stream may be interpreted in terms of the relatively rapid decrease of the Pd sites for the hydrogenation of isopentenes, compared with that for the hydrogenation of isoprene. Therefore, the possibility of overhydrogenation is decreased with the reduction of active sites. For the test with water-containing feed, at the start of the run, both water adsorption and gum formation reduce the possibility of overhydrogenation; hence, the elevation rate of isopentenes selectivity is higher than that for the test without water present. However, after the selectivity to isopentenes increases up to about 97%, the tendency for the catalysts to bind water may be decreased with increasing gum formation on the catalysts; the contact angle between the catalysts and water may be increased concomitantly with gum formation. The decline of the hydrophilic property of the catalysts lessens the mass-transfer limitation of hydrogen to the catalyst surface, thereby slightly increasing the possibility of overhydrogenation and decreasing the selectivity to isopentenes. The selectivity change for 2M2B, 2M2B, and 3M1B is mainly influenced by the formation of isopentenes. Therefore, similar phenomena are observed in Figures 4-6. Conclusions

Figure 8. H2 TPD profiles for (a) fresh catalyst and (b) waterpoisoned catalysts.

decreasing the electronic density of the Pd clusters. A decrease of the electron density on the Pd clusters lessens backbonding from Pd to the CO π* orbital, resulting in a shift of νCO to high frequency. Hydrogen Temperature-Programmed Desorption. For the Pd catalyst, four hydrogen desorption peaks appearing at 203, 414, 502, and 555 °C were observed (Figure 8). Those four peaks are assigned as the desorption of four different types of hydrogen (Paa´l, and Menon, 1983; Konvalinka and Scholten, 1977; Chang et al., 1985): β-hydride (203 °C), chemisorbed hydrogen (414 °C), spillover hydrogen (502 °C), and hydride (555 °C). The presence of water on the Pd clusters markedly inhibits the formation of β-hydride and the adsorption of hydrogen, evidenced by the decrease in the peak intensity at 414 °C and the disappearance of the peak at 203 °C. The TPD results further confirm that water interferes with hydrogen adsorption, resulting in an increase in the selectivity to isopentenes. Catalyst-Deactivation-Induced Selectivity Change. As shown in Figures 2 and 3, isopentenes selectivity increased with decreasing catalyst activity for the test with pure feed and then became time invariant after 20 h. The selectivity to isopentenes also increases with decreasing catalyst activity for the test with the presence of water, whereas at about 20 h on stream, the selectivity reached a maximum and then declined with decreasing catalyst activity and became time invariant at about 50 h. Before 20 h on stream, for the test with pure feed, the increase of the selectivity to isopentenes with time

The change in the catalytic properties caused by water poisoning is related to the nature of the interactions between the adsorbed water and Pd clusters. The results of the test reactions indicate that isoprene conversion and 2-methyl-2-butene selectivity decrease with the presence of water, whereas the selectivity to isopentenes slightly increases. Inferred from the FFTIR characterizing CO adsorbed on the fresh and waterpoisoned catalysts, we conclude that the adsorption of water on the Pd clusters reduces the active sites available for isoprene hydrogenation and induces the decrease of electronic density of Pd clusters, leading to a decrease of isoprene conversion and a slight increase of the selectivity to isopentenes. The decrease of the selectivity to 2-methyl-2-butene results from the decrease of the isomerization rate of 2M1B and 3M1B caused by the decrease of free hydrogen, inferred from the TPD of hydrogen. The results of the present investigation also indicate that water poisoning of the catalysts decreases the hydrogenation activity and the selectivity to 2M2B formation. As a consequence, the gum formed from the polymerization of the unreacted diolefins contained in pyrolysis gasoline causes several disadvantages: (1) it declines the efficiency of the depentanizer and deoctanizer and makes the separation of C5 and C9+ more difficult; (2) it raises the pressure drop of the reactor for second-stage hydrogenation; (3) it deactivates the catalysts for the TAME process; and (4) it deteriorates the quality of gasoline. Moreover, the efficiency of the TAME process is reduced by the decrease of 2M2B yield. Acknowledgment The data of the commercial plant were provided by Dr. Tzong-Bin Lin. This research were supported by the National Science Council of the Republic of China (Contract No. NSC 85-2214-E-194-002), National Chung Cheng University, and the Refining & Manufacturing Research Center of the Chinese Petroleum Corp. (RMRC).


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Received for review April 1, 1997 Revised manuscript received July 9, 1997 Accepted July 12, 1997X IE970256M

X Abstract published in Advance ACS Abstracts, September 1, 1997.

Î´-Al2O3 Catalysts for Isoprene

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