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ZrO2-Al2O3 catalyst
Applied Catalysis A: General 276 (2004) 145–153 www.elsevier.com/locate/apcata
Simultaneous isomerization and desulfurization of sulfur-containing light naphtha over metal/SO42/ZrO2-Al2O3 catalyst Katsuya Watanabea,*, Takahito Kawakamia, Koji Babaa, Nobuyasu Oshioa, Takao Kimurab a
Research and Development Center, Cosmo Oil Co., Ltd., 1134-2, Gongendo, Satte, Saitama 340-0193, Japan b International Cooperation Center, Cosmo Oil Co., Ltd., Shibaura Square Bldg., 9-25, Shibaura 4-Chome, Minato-ku, Tokyo 180-8564, Japan Received in revised form 6 July 2004; accepted 30 July 2004 Available online 12 September 2004
Abstract The isomerization reaction of sulfur-containing light naphtha was studied. The Pd/SO42/ZrO2-Al2O3 catalyst in which the active metal was modified into Pd indicates the stable isomerization activity, although the isomerization activity of the conventional Pt/SO42/ZrO2-Al2O3 catalyst decreases due to the existence of sulfur in the feed. But the increase of sulfur content in feed caused a drop of the isomerization level over Pd/SO42/ZrO2-Al2O3 catalyst. The increase in the amount of sulfur supplied to the catalyst invites the increase of the amount of Pd required for a desulfurization reaction, and leads to the decrease in the relative amount of Pd that performs the homogeneous dissociation of gaseous hydrogen. By preparing the catalysts so that the methods of the Pd addition are different, one could study the correlation between the position of Pd in SO42/ZrO2-Al2O3 carrier and the isomerization activity. The catalyst, which forms Pd/Al2O3 shows the highest isomerization activity. This fact suggests that the optimal position of Pd in SO42/ZrO2-Al2O3 carrier is on Al2O3 and that the high desulfurization function is obtained by the formation of Pd/Al2O3. Furthermore, we researched about a catalyst that would work effectively in the isomerization of light naphtha, which contains sulfur in high concentrations. The catalyst where Pd is impregnated into Pt/SO42/ZrO2-Al2O3 can obtain the stably high activity in the isomerization of light naphtha that contains sulfur of 490 massppm at a reaction temperature of 200 8C. The results of EPMA analysis indicate that this catalyst forms a unique hybrid structure where Pt exists on zirconia and Pd exists on alumina, respectively. The high sulfur tolerance of hybridtype catalyst is brought about by the isomerization function of Pt/SO42/ZrO2 particles and the hydrodesulfurization function of Pd/Al2O3 particles. We propose a model of the optimal metal position for the isomerization of light naphtha, which contains sulfur in high concentration, and we consider more deeply about the catalytic action of the hybrid-type catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Pt/SO42/ZrO2-Al2O3; Pd catalyst; Sulfated zirconia catalyst; Skeletal isomerization; Research octane number
1. Introduction Sulfated zirconia (SO42/ZrO2) is well known as a super solid acid, which has strong acidity. Many researchers have tried this catalyst for applications to the various solid acid reactions [1–3]. Especially, SO42/ZrO2 catalyst shows the high catalytic activity in the isomerization reaction of low molecular weight paraffins such as n-pentane and n-hexane. * Corresponding author. Tel.: +81 480 42 2211; fax: +81 480 423 790. E-mail address: [email protected] (K. Watanabe). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.055
Further, we studied how to improve the catalysis of sulfuric acid-zirconia in our laboratory. We reported that the addition of Pt to SO42/ZrO2 increased the isomerization activity and reduced the amount of coke [4]. Although various oil products are obtained by the atmospheric distillation of the crude oil in oil refining, light naphtha is the hydrocarbon of the low boiling point fraction in their products and generally it is used as one of the gasoline blend stocks. However, the utilization of light naphtha tends to be restricted because of the octane requirements of motor gasoline because it contains much normal paraffin that has a low research octane
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number (RON). Then, the skeletal isomerization of normal paraffin to iso-paraffin with solid acid catalysts is one of the important technologies for RON improvement in oil refining [5,6]. Environmental concern about pollution and worldwide legislation about gasoline require companies to supply clean gasoline with high RON, low concentration of aromatics and low concentration of sulfur. Since the isomerate that is obtained by isomerization reaction of light naphtha is composed mainly of iso-paraffin, it has the potential to satisfy these requirements [7]. However, the raw light naphtha obtained by atmospheric distillation includes much sulfur, and the sulfur removal by hydrodesulfurization (HDS) of feed was necessary. That is because the sulfur accumulation over noble metal in isomerization catalysts causes the shortage of hydrogen in the catalyst surface and the carbon deposits on the solid acid sites make the isomerization activity disappear. Three kinds of solid acid catalysts; the SO42/ZrO2 catalyst, zeolite catalyst, and chlorided alumina catalyst are commercially used as isomerization catalysts of light naphtha [8]; the disappearance of the isomerization activity caused by the presence of sulfur in the feed is a phenomenon which is common to every catalyst. The activity of chlorided alumina catalyst is the highest among activity values for isomerization catalysts; such a catalyst enables operation at lower temperature. From the thermodynamic viewpoint, the low temperature reaction is profitable for the formation of the isomer with the high octane number because the isomerization reaction is slightly exothermic (DH = 4 to 20 kJ/mol) [6]. However, this chlorided alumina catalyst is very sensitive to poisons such as water and sulfur compounds. On the other hand, zeolite catalysts are less sensitive to sulfur and water, and it is reported that these catalysts show the catalytic activities in the isomerization reaction of sulfur-containing n-pentane fractions [9,10]. But these catalysts need a high temperature to form carbenium ion, which is the intermediate of the isomerization reaction, because of their low acidity. Such a high reaction
temperature is unfavorable for the formation of an isomer in thermodynamics. Recently, we reported that the Pd-loaded SO42/ZrO2/ Al2O3 catalyst exhibited excellent performance in both the isomerization reaction and HDS reaction [11,12]. This catalyst showed the isomerization activity at a lower temperature than zeolite catalyst did, and had the good sulfur tolerance in the isomerization reaction of sulfurcontaining light naphtha. However, the sulfur content of light naphtha fraction obtained by atmospheric distillation is higher than that of these materials. The content varies between 100 and 1000 massppm, though it depends on the crude oil. Therefore, it seems that the isomerization catalyst that has higher sulfur tolerance will be more effective in commercial utilization. The purpose of this study is to clarify the relations between the metal and the catalytic activity in the isomerization reactions of sulfur-containing light naphtha. Thus, we prepared the isomerization catalysts in which the kinds of metal were different, and we investigated the influence on the isomerization activity of the sulfur content in the feed in detail. Furthermore, when we prepared the catalysts, the methods of the Pd addition were different, and the positions of the metals in each case were specified by EPMA. Finally, we report a hybrid-type catalyst that works effectively in the isomerization reaction of light naphtha, which contains sulfur in high content.
2. Experimental 2.1. Catalyst preparation The catalyst preparation route is summarized in Fig. 1. Metal/SO42/ZrO2-Al2O3 catalysts for which the methods of adding the metal were different were used as isomerization catalysts used for this research. Zr(OH)4, used as a support, was of commercial grade (DAIICHI KIGENSOKAGAKU KOGYO). SO42/Zr(OH)4, which
Fig. 1. The summary of catalyst preparation.
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was the catalyst precursor, was prepared by dipping Zr(OH)4 into the 1N H2SO4 for 3 h, filtering, evaporating for 3 h at 55 8C, and drying for one night at 110 8C. Catalyst-A (Pt/SO42/ZrO2-Al2O3): Pt/SO42/Zr(OH)4 was prepared by incipient wetness impregnation with an aqueous solution of H2PtCl6(Wako) over SO42/Zr(OH)4. The water was evaporated, and the powder was dried at 150 8C overnight. Subsequently, the mixture of dried Pt/ SO42/Zr(OH)4, alumina sol(AP-1, CCIC) and water was arranged in the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. Afterwards, the mixture was kneaded for 1 h and then extruded through the opening of a circular die of 1.6 mm diameter to form cylindrical pellets. Catalyst-A was obtained by calcining the extruded pellets in air at 600 8C for 3 h. Catalyst-B (Pd/SO42/ZrO2-Al2O3): catalyst-B was prepared in the same way as catalyst-A, except for changing the impregnation solution into a dilute HCl aqueous solution of PdCl2. Catalyst-C (Pd/SO42/ZrO2-Al2O3): a mixture of SO42/Zr(OH)4, alumina sol and a dilute HCl aqueous solution of PdCl2 was prepared for the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. The mixture was kneaded for 1 h and extruded through a circular opening of 1.6 mm diameter. The extruded pellets were dried at 120 8C overnight. CatalystC was obtained by calcining the dried pellets in air at 600 8C for 3 h. Catalyst-D (Pd/SO42/ZrO2-Al2O3): a mixture of SO42/ Zr(OH)4, alumina sol and water was prepared for the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. The mixture was kneaded and extruded through a circular opening. The extruded pellets were calcined at 600 8C for 3 h. The dilute HCl aqueous solution of PdCl2 was impregnated by incipient wetness over SO42/ZrO2-Al2O3 and dried afterwards at 120 8C for 3 h. Catalyst-D was obtained by the calcination of Pd-impregnated pellets in air at 600 8C for 3 h. Catalyst-E (Pd-impregnated Pt/SO42/ZrO2-Al2O3): a dilute HCl aqueous solution of PdCl2 was impregnated by incipient wetness over catalyst-A and dried afterwards at 120 8C for 3 h. Catalyst-E was obtained by calcining the dried pellets in air at 450 8C for 3 h. 2.2. Isomerization reaction
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Table 1 Feed properties Feed-1
Feed-2
Feed-3
Properties Density (at 15/4) Sulfur (massppm) Nitrogen (massppm) GC-RONa iso-C5 ratio (mass%) 22DMB selectivity (mass%)
0.6607 1 <1 64 39.1 1.0
0.6596 81 <1 66 40.0 1.1
0.6534 490 <1 67 40.3 0.9
Components (mass%) C3, C4 C5 C6 C7+ Benzene Naphthenes
3.3 38.6 40.2 7.3 2.6 8.0
2.4 42.5 38.0 6.5 2.6 8.0
a
1.3 52.0 36.6 1.0 1.7 7.5
GC-RON is the Research Octane Number calculated by GC.
2.9 MPa total pressure, H2/hydrocarbon molar ratio of 2, LHSV of 1.5 h1, and temperature in the 190– 240 8C range. Isomerization reactions were carried out using samples of light naphtha whose sulfur contents were different. Table 1 shows the main properties of these feeds. Feed-1 is sulfurfree light naphtha that was hydro-desulfurized, and feed-2 is Merox sweetening light naphtha that contains sulfur of 81 massppm. Feed-3 is the raw light naphtha obtained from the atmospheric distillation apparatus; it contains sulfur of 490 massppm. Liquid products were collected from a gas–liquid separator periodically and the components were analyzed by gas chromatography using an FID detector (GC-17A, Shimazu) with a fused silica capillary column of 50 m 0.21 mm ID (CP-Sil PONA CB, GL science). The sulfur content in liquid products was determined by an ultraviolet fluorescent method (TS-100, Mitsubishi Chemical). The detectable products in the isomerization reaction of light naphtha were isopentane and n-pentane in C5 fraction, and 2,2-demethybutane (22DMB), 2,3-demethylbutane (23DMB), 2-methylpentane (2MP), 3-methylpentane (3MP) and n-hexane in C6 fraction. The catalytic activities of sulfur-containing light naphtha over prepared catalysts were determined by the ratios of C5 and C6 isomers, respectively. iso-C5 ratio ðmass%Þ ¼
isopentane ðisopentane þ n-pentaneÞ 100 (1)
Isomerization reactions of light naphtha were carried out in a fixed-bed flow reactor. Before the isomerization reaction was started, catalysts were dried at 450 8C for 3 h in a muffle furnace; the catalysts were then reduced at 200 8C for 3 h with hydrogen flowing under atmospheric pressure. After reduction, the temperature was set to the reaction temperature and the isomerization reaction of light naphtha was started. The catalytic activities were performed at
22DMB selectivity ¼
22DMB 100 22DMB þ 23DMB þ 2MP þ 3MP þ n-hexane (2)
In addition, a hydrocracking reaction occurs during the isomerization reaction of light naphtha; then light hydrocarbons such as propane and butane are produced. The yields
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of isomerate were determined as the ratio of C5+ hydrocarbons in products to C5+ hydrocarbons in feeds. isomerate yield ð%Þ ¼
Cþ 5 hydrocarbon in products 100 Cþ 5 hydrocarbon in feeds (3)
2.3. The measurement of EPMA For the identification of the locations of the metal in the isomerization catalysts, some measurements using the electron probe microanalyzer (EPMA) were carried out. The mappings of metal concentration were analyzed with a JXM-8600-MX (JEOL). A catalyst pellet was picked out at random; it was embedded in the resin of the meta-methyl acrylic (MMA). After the embedded pellet was ground, the measurement samples were obtained by carrying out the vapor deposition of the carbon. The operating conditions of EPMA apparatus were carried out with the accelerating voltage of 20 kV and sample electric current of 1 107 A. In addition, the mappings were recorded for four elements: Zr, Al, Pt, and Pd.
3. Results and discussion 3.1. The performance of sulfur tolerant Pd/SO42/ZrO2-Al2O3 catalyst Fig. 2 shows the result of the isomerization test of feed-2 whose sulfur content is 81 massppm. Catalyst-A (Pt/SO42/ ZrO2-Al2O3) and catalyst-B (Pd/SO42/ZrO2-Al2O3) indicate a high iso-C5 ratio of 72%, which was close to the chemical equilibrium value during the early stage of a reaction [13]. However, the isomerization activity of catalyst-A declined linearly with the time on stream and the iso-C5 ratio in the product finally reached 40%. It seems that the isomerization activity disappeared completely,
Fig. 2. Isomerization reaction of sulfur-containing light naphtha over isomerization catalyst. (~): Catalyst-A; ( ) catalyst-B. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40%, sulfur = 80 massppm.
because that value is equal to the iso-C5 ratio in feed-2. In addition, though the sulfur content in products was <1 massppm during the early stage of a reaction, it increased with the decline of the isomerization activity. Sulfur content in the time when the isomerization activity is not indicated was the same 81 massppm as in feed-1. This result indicates that the sulfur is accumulated in Pt with the time on stream, and that the isomerization reaction of catalyst-A does not progress completely when the amount of sulfur in Pt exceeds the capacity. Therefore, the changes of the isomerization activities of catalyst-A in Fig. 1 can be regarded as correlating with the amount of Pt, which is active for the isomerization reaction. On the other hand, catalyst-B in which the main active metal was changed from Pt to Pd reached the stable iso-C5 ratio in the isomerization reaction of sulfur-containing light naphtha after 50 h. Further, the iso-C5 ratio in the stationary state was very high. We found that catalyst-B has the skeletal isomerization function of normal paraffin even in the presence of sulfur. Moreover, the sulfur content in products was under 1 massppm all the time during the isomerization reaction of sulfur-containing light naphtha, so the organic sulfur compound in the feed was changed into the H2S completely. This result suggests that catalyst-B works on both the isomerization reaction of light naphtha and the HDS reaction of organic sulfur compound. Generally, when organic sulfur compounds are continuously supplied to a catalyst, the isomerization reaction does not advance because the metal is covered by sulfur and its dehydrogenation ability disappears. Therefore, the metal must be always kept in the activated state so that the isomerization activity of light naphtha may appear even in the presence of sulfur. Catalyst-B could isomerize sulfurcontaining light naphtha without the decline of catalytic activity. This result suggests that both the conversion into H2S on the catalyst surface and the desorption of H2S from the catalyst are carried out promptly, even if some organic sulfur compound adsorbs on Pd and some metal sulfide is formed on the catalyst. That is to say, it seems that the maintenance of dehydrogenation ability by prompt regeneration of Pd causes the stable isomerization activity even in the presence of sulfur [14]. On the other hand, catalyst-A cannot isomerize the sulfur-containing light naphtha because of the sulfur accumulation on the metal. Therefore, this result suggests that there are more amounts of sulfur supplied from feed than the amount of sulfur desorbed from the metal in catalyst-A. Fig. 3(a) shows the effect of sulfur content in the naphtha isomerization when using catalyst-B. The control of feed sulfur content was accomplished by the blending of feed-1, 2, and -3. The catalytic activities of isomerization reaction in each feed were compared with the average values for 150 h after start-up. The increase of sulfur content in feed caused declines of iso-C5 ratio and 22DMB selectivity, and the skeletal isomerization of normal paraffin was inhibited. Because the decline tendency with iso-C5 ratio and that with
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Fig. 3. The effect of sulfur content in feed over catalyst B: (a) isomerization activity, (b) isomerate yield. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40%, sulfur = 0, 30, 80, 160 massppm.
the 22DMB selectivity are about the same, it seems that the effect to the isomerization activity of the sulfur content in feed does not depend on carbon number. To complete the isomerization reaction of the sulfurcontaining light naphtha, three processes are necessary: HDS of organic sulfur compounds, the formation of carbenium ions by dehydrogenation of paraffin, and the supply of atomic hydrogen to carbenium ions [15,16]. It is assumed that these all processes advance on the same Pd in catalyst-B. The increase of sulfur content in the feed invites the increase of the amount of Pd required for the HDS reaction; this leads to the decrease of the relative amount of Pd that plays an important part for both the dehydrogenation of paraffin and the supply of atomic hydrogen. As a result, the isomerization activity of sulfur-containing light naphtha over catalyst-B seems to decrease. Fig. 3(b) shows the relationship between the sulfur content in feed and the yield of isomerate. The increase of sulfur content in feed improved the yield of isomerate, and
decreased the amount of gas component production caused by the hydrocracking reaction. Both the isomerization reaction and the hydrocracking reaction progress by way of the carbenium ion that is a reaction precursor [17,18]. It is clear that the increase of sulfur content in the feed inhibits the formation of carbenium ions. Thus, the existence of sulfur in light naphtha leads to the decline of formation ability of carbenium ion, since the amounts of Pd required for the HDS reaction increase. Consequently we conclude that sulfur reduces the progress of both reactions: the isomerization and the hydrocracking. 3.2. The effect of Pd addition to SO42/ZrO2-Al2O3 Fig. 4(a) shows the results of the isomerization activity of three kinds of Pd/SO42/ZrO2-Al2O3 catalysts (Pd preimpregnated catalyst (catalyst-B), Pd simultaneous-kneaded catalyst (catalyst-C) and Pd post-impregnated catalyst (catalyst-D)) for which the Pd loading methods are different.
Fig. 4. The isomerization activity of catalyst prepared by various types of Pd loading additions: (a) iso-C5 ratio, (b) 22DMB selectivity. (~): Catalyst-B, ( ): catalyst-C, (*): catalyst-D. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40.3%, sulfur =490 massppm.
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Feed-3, whose sulfur content was high, was used for these experiments. When the catalytic activities were compared in the iso-C5 ratio, the skeletal isomerizations of catalyst-C and -D were better than that of catalyst-B. When 22DMB selectivity was used as the index of the skeletal isomerization of C6 paraffin, the isomerization reaction of light naphtha over catalyst-D progressed considerably faster than for other catalysts, as shown in Fig. 4(b). These results indicate that the skeletal isomerization is very much influenced by the method of the Pd addition in the isomerization of high sulfur feed. In order to investigate the relations between the method of Pd addition and the composition of the catalyst surface, we identified the distribution of metal on catalyst-B, -C and D by EPMA. Fig. 5 shows the results of mappings of Pd and Al in the same view where aluminum particles were located for each catalyst. Since the intensity of Pd was very weak in the area where Al distribution in catalyst-B is strongly detected, the Pd in catalyst-B exists as the Pd/SO42/ZrO2 particles which were formed at the beginning of the catalyst preparation. On the other hand, though the correlation of the distribution of Al and Pd can be seen over catalyst-C in which Pd and alumina sol were kneaded simultaneously, it was confirmed that the Pd in catalyst-C existed in almost all the particles without any clear boundaries. However, as for catalyst-D in which the Pd was impregnated on the SO42/ ZrO2-Al2O3 carrier, the areas where Pd and Al were strongly detected corresponded very well. Pd exists as [PdCl4]2 in the impregnation solution used for the preparation of catalyst-D. Besides, SO42/ZrO2 particle is charged minus by the intense electron-attractive group of sulfuric acid. Therefore it is likely that [PdCl4]2 ions repelled each other with the surface charge of SO42/ZrO2 particle and adsorbed on the Al2O3 particles. Conversely, Pd in catalyst-B is not located on Al2O3 since catalyst-B was calcined at 450 8C in air after the formation of Pd/SO42/Zr(OH)4. The position of Pd element in catalyst-C is the intermediate state of catalyst-B and catalyst-D, and the simultaneous kneading of the impregnation solution including [PdCl4]2 and the alumina sol caused the uniform distribution of Pd on the surface of catalyst-C. These results clearly show that it is more advantageous for Pd to exist on Al2O3 rather than on ZrO2 during the simultaneous progress of the isomerization reaction and the HDS reaction, since catalyst-D, which forms Pd/Al2O3 possesses the high catalytic activity in the skeletal isomerization reaction under the presence of sulfur. Fujimoto et al. reported that the powdery mixture of Pt/ SiO2 and H-MFI zeolite possessed the high isomerization activity in the isomerization reaction of n-pentane [19]; they mentioned that hydrogen transfer between the particles occurs with the hybrid-type catalyst of powdery mixture. Our results also mean that the noble metal does not necessarily need to be loaded onto the solid acid and indicate that the isomerization reaction progresses by two active sites: with the acid sites and with the hydrogenation metal on
another particle which is close to an acid site. Namely, as for catalyst-D, since the atomic hydrogen formed by homogeneous dissociation of gaseous hydrogen over Pd/Al2O3 is supplied to SO42/ZrO2, the regeneration of acid sites and the stabilization of carbenium ion that is the isomerization precursor are promoted by spill-over of atomic hydrogen [20]. And as a result, we can infer that catalyst-D invites the high skeletal isomerization activity. Kabe and co-workers reported that a noble metalsupported Al2O3 catalyst shows the high catalytic activity to the HDS reaction of organic sulfur compounds [21]. Sulfur types in light naphtha are composed of thiol compounds (R–SH) and sulfide compounds (R–S–R) of low boiling point [22]; the HDS of these sulfur compounds are comparatively easier than those of the thiophene compounds [23]. Therefore, these results suggest that Pd/Al2O3 formed in catalyst-D can desulfurize the organic sulfur compounds in light naphtha to H2S immediately. The improvement of HDS activity leads the isomerization catalyst to the decrease of the amount of Pd that is necessary for the HDS reaction, and thus increases relatively the amount of Pd, which can supply atomic hydrogen. As a result, catalyst-D which formed Pd/Al2O3 shows the higher catalytic activity in the isomerization reaction for sulfur-containing light naphtha than Pd that is supported on SO42/ZrO2. 3.3. The development of hybrid-type isomerization catalyst for high sulfur feed From the study of the Pd loading methods, it is clear that the formation of Pd/Al2O3 in isomerization catalyst causes the high catalytic activity in the isomerization reaction of sulfur-containing light naphtha. Fig. 6 shows the result of the activity test of catalyst-E (Pd-impregnated Pt/SO42/ZrO2Al2O3), which is prepared by Pd impregnation over catalystA (Pt/SO42/ZrO2-Al2O3). Feed-3, which contained high sulfur content, was used as feed. The catalytic activities of catalyst-A and -B were also evaluated with the same conditions as comparative catalysts. The iso-C5 ratio of catalyst-A decreased rapidly from start-up due to the presence of high sulfur content, and the isomerization activity completely disappeared after 20 h. In addition, the decline of iso-C5 ratio with catalyst-B is moderate, and the iso-C5 ratio after 100 h was about 58%. On the other hand, catalyst-E, which was obtained by Pd impregnation over catalyst-A, did not show any decline of the isomerization activity even after 100 h; this catalyst kept the high iso-C5 ratio of 71% regardless of the reaction time. Moreover, sulfur content in products was less than 1 massppm during the isomerization reaction, and the sulfur of 490 massppm contained in feed was changed into the H2S completely. Since catalyst-A, which is Pt-promoted sulfate zirconia, did not show any catalytic activity at all for the isomerization reaction of sulfur-containing light naphtha, it is quite obvious that the catalytic performance of catalyst-E depends on the Pd that it is added over catalyst-A.
Fig. 5. EPMA images of various Pd/SO42/ZrO2-Al2O3 samples. (a) Al mapping on catalyst-B, (b) Al mapping on catalyst-C, (c) Al mapping on catalyst-D, (d) Pd mapping on catalyst-B, (e) Pd mapping on catalyst-C, (f) Pd mapping on catalyst-D
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Fig. 6. The results of isomerization reaction of high sulfur feed. (~): Catalyst-A, ( ): catalyst-B, (*): catalyst-E. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40.3%, sulfur =490 massppm.
In order to determine the positions of metals in catalyst-E, the concentration distribution was analyzed by EPMA for four elements: Zr, Al, Pt and Pd. Fig. 7 shows the results of mappings of each element in the same view. From the results of mapping analysis of Zr and Al elements, one finds that Al2O3 existed in the space between the ZrO2 particles, and one could confirm that Al2O3 is working as a binder. Moreover, elemental Pt was strongly observed in the positions where the ZrO2 particles existed, and so we concluded that Pt is located on the ZrO2 particles. On the contrary, little elemental Pd was detected on ZrO2 particles, and it corresponded with the concentration distribution of elemental Al. These results clearly indicate that catalyst-E is composed as a hybrid-type catalyst (Pt/SO42/ZrO2-Pd/ Al2O3) in which the Pt/SO42/ZrO2 particles and the Pd/ Al2O3 particles form a super-neighborhood.
From the results that the sulfur affects the catalytic activity in the isomerization reaction of light naphtha, we concluded that sulfur compounds in feed inhibit the formation of carbenium ions that are isomerization precursors. However, when the Pt/SO42/ZrO2 particles are covered with Pd/Al2O3 particles within the catalyst pellet like catalyst-E, the very high isomerization activity appears even in the presence of high sulfur content. This fact indicates that the formation of carbenium ion by Pt/SO42/ ZrO2 particle is not restricted because Pd/Al2O3 particles desulfurize most organic sulfur compounds in the reactant. Originally, when one considers that the Pt/SO42/ZrO2 catalyst is very sensitive to sulfur in feed, one can presume that Pd/Al2O3 plays the role of supplying the atomic hydrogen to other particles like catalyst-D, in addition to indicating the high HDS activity. By this function, the adsorption of sulfur compound to Pt in catalyst is inhibited or the desorption of sulfur adsorbed on Pt is accelerated. In either case, results suggest that Pd/Al2O3 controls both the dehydrogenation ability of paraffin with which Pt in Pt/ SO42/ZrO2 is originally equipped and the hydrogenation ability of isomerized carbenium ions, since Pd/Al2O3 protects the nearby Pt/SO42/ZrO2 particles. 3.4. The model of catalytic function in hybrid-type catalyst Fig. 8 illustrates a catalyst model of the hybrid-type catalyst. Catalyst-E, which indicates the high isomerization activity even in the presence of sulfur, is composed as a hybrid-type catalyst in which the Pt/SO42/ZrO2 particles are covered with alumina binder including Pd. When organic sulfur compounds contact the hybrid-type catalyst, most of those parts are desulfurized to H2S by Pd/Al2O3. And if some of the organic sulfur compounds and the produced H2S
Fig. 7. EPMA image of catalyst-E: (a) Zr mapping, (b) Al mapping, (c) Pt mapping, (d) Pd mapping.
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catalytic performance of the two types of particles promotes high sulfur tolerance.
Acknowledgement This work has been carried out as a research project of Petroleum Energy Center with a subsidy from the Ministry of Economy, Trade, and Industry, Japan. Fig. 8. The model of metal position in hybrid-type catalyst.
References touch the Pt/SO42/ZrO2 particles, it is suggested that they are desorbed promptly due to the function of atomic hydrogen supplied by Pd/Al2O3 [14]. Namely, Pd-loaded on Al2O3 has two functions: with the HDS of sulfur-containing feed and with the formation of atomic hydrogen by the activation of gaseous hydrogen [24]. On the other hand, Pt loaded on SO42/ZrO2 works on the formation of carbenium ion by the dehydrogenation of paraffin [25]. Furthermore, atomic hydrogen formed by Pd is moved to SO42/ZrO2, and it also plays a role in the stabilization of the isomerized carbenium ion [26]. Moreover, the supply of atomic hydrogen promotes the desorption of sulfur compounds adsorbed on Pt and the sulfur tolerance of Pt/SO42/ZrO2 particles improves substantially. Since the improvement of sulfur tolerance of Pt/SO42/ZrO2 particles prevents the decline of the production ability of the carbenium ion, the extremely high isomerization activity will appear even in the presence of sulfur over a hybrid-type catalyst.
4. Conclusion We studied the relations between the metal and the catalytic activity in the isomerization reactions of sulfurcontaining light naphtha. It became clear that the optimal position of Pd in SO42/ZrO2-Al2O3 carrier is on Al2O3. That is because the high HDS function is obtained by the formation of Pd/Al2O3. Besides, the catalyst which was prepared by Pd impregnation into Pt/SO42/ZrO2-Al2O3 has the hybrid structure where the Pt/SO42/ZrO2 particles and the Pd/Al2O3 particles adjoined each other, and this catalyst indicates the steadily high isomerization activity in the skeletal isomerization reaction of light naphtha which contains the sulfur of 490 massppm. This hybrid-type catalyst is divided into the HDS function due to Pd/Al2O3 particles and the isomerization function due to Pt/SO42/ ZrO2 particles. We concluded that cooperation of the
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Simultaneous isomerization and desulfurization of sulfur-containing light naphtha over metal/SO42/ZrO2-Al2O3 catalyst Katsuya Watanabea,*, Takahito Kawakamia, Koji Babaa, Nobuyasu Oshioa, Takao Kimurab a
Research and Development Center, Cosmo Oil Co., Ltd., 1134-2, Gongendo, Satte, Saitama 340-0193, Japan b International Cooperation Center, Cosmo Oil Co., Ltd., Shibaura Square Bldg., 9-25, Shibaura 4-Chome, Minato-ku, Tokyo 180-8564, Japan Received in revised form 6 July 2004; accepted 30 July 2004 Available online 12 September 2004
Abstract The isomerization reaction of sulfur-containing light naphtha was studied. The Pd/SO42/ZrO2-Al2O3 catalyst in which the active metal was modified into Pd indicates the stable isomerization activity, although the isomerization activity of the conventional Pt/SO42/ZrO2-Al2O3 catalyst decreases due to the existence of sulfur in the feed. But the increase of sulfur content in feed caused a drop of the isomerization level over Pd/SO42/ZrO2-Al2O3 catalyst. The increase in the amount of sulfur supplied to the catalyst invites the increase of the amount of Pd required for a desulfurization reaction, and leads to the decrease in the relative amount of Pd that performs the homogeneous dissociation of gaseous hydrogen. By preparing the catalysts so that the methods of the Pd addition are different, one could study the correlation between the position of Pd in SO42/ZrO2-Al2O3 carrier and the isomerization activity. The catalyst, which forms Pd/Al2O3 shows the highest isomerization activity. This fact suggests that the optimal position of Pd in SO42/ZrO2-Al2O3 carrier is on Al2O3 and that the high desulfurization function is obtained by the formation of Pd/Al2O3. Furthermore, we researched about a catalyst that would work effectively in the isomerization of light naphtha, which contains sulfur in high concentrations. The catalyst where Pd is impregnated into Pt/SO42/ZrO2-Al2O3 can obtain the stably high activity in the isomerization of light naphtha that contains sulfur of 490 massppm at a reaction temperature of 200 8C. The results of EPMA analysis indicate that this catalyst forms a unique hybrid structure where Pt exists on zirconia and Pd exists on alumina, respectively. The high sulfur tolerance of hybridtype catalyst is brought about by the isomerization function of Pt/SO42/ZrO2 particles and the hydrodesulfurization function of Pd/Al2O3 particles. We propose a model of the optimal metal position for the isomerization of light naphtha, which contains sulfur in high concentration, and we consider more deeply about the catalytic action of the hybrid-type catalyst. # 2004 Elsevier B.V. All rights reserved. Keywords: Pt/SO42/ZrO2-Al2O3; Pd catalyst; Sulfated zirconia catalyst; Skeletal isomerization; Research octane number
1. Introduction Sulfated zirconia (SO42/ZrO2) is well known as a super solid acid, which has strong acidity. Many researchers have tried this catalyst for applications to the various solid acid reactions [1–3]. Especially, SO42/ZrO2 catalyst shows the high catalytic activity in the isomerization reaction of low molecular weight paraffins such as n-pentane and n-hexane. * Corresponding author. Tel.: +81 480 42 2211; fax: +81 480 423 790. E-mail address: [email protected] (K. Watanabe). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.055
Further, we studied how to improve the catalysis of sulfuric acid-zirconia in our laboratory. We reported that the addition of Pt to SO42/ZrO2 increased the isomerization activity and reduced the amount of coke [4]. Although various oil products are obtained by the atmospheric distillation of the crude oil in oil refining, light naphtha is the hydrocarbon of the low boiling point fraction in their products and generally it is used as one of the gasoline blend stocks. However, the utilization of light naphtha tends to be restricted because of the octane requirements of motor gasoline because it contains much normal paraffin that has a low research octane
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number (RON). Then, the skeletal isomerization of normal paraffin to iso-paraffin with solid acid catalysts is one of the important technologies for RON improvement in oil refining [5,6]. Environmental concern about pollution and worldwide legislation about gasoline require companies to supply clean gasoline with high RON, low concentration of aromatics and low concentration of sulfur. Since the isomerate that is obtained by isomerization reaction of light naphtha is composed mainly of iso-paraffin, it has the potential to satisfy these requirements [7]. However, the raw light naphtha obtained by atmospheric distillation includes much sulfur, and the sulfur removal by hydrodesulfurization (HDS) of feed was necessary. That is because the sulfur accumulation over noble metal in isomerization catalysts causes the shortage of hydrogen in the catalyst surface and the carbon deposits on the solid acid sites make the isomerization activity disappear. Three kinds of solid acid catalysts; the SO42/ZrO2 catalyst, zeolite catalyst, and chlorided alumina catalyst are commercially used as isomerization catalysts of light naphtha [8]; the disappearance of the isomerization activity caused by the presence of sulfur in the feed is a phenomenon which is common to every catalyst. The activity of chlorided alumina catalyst is the highest among activity values for isomerization catalysts; such a catalyst enables operation at lower temperature. From the thermodynamic viewpoint, the low temperature reaction is profitable for the formation of the isomer with the high octane number because the isomerization reaction is slightly exothermic (DH = 4 to 20 kJ/mol) [6]. However, this chlorided alumina catalyst is very sensitive to poisons such as water and sulfur compounds. On the other hand, zeolite catalysts are less sensitive to sulfur and water, and it is reported that these catalysts show the catalytic activities in the isomerization reaction of sulfur-containing n-pentane fractions [9,10]. But these catalysts need a high temperature to form carbenium ion, which is the intermediate of the isomerization reaction, because of their low acidity. Such a high reaction
temperature is unfavorable for the formation of an isomer in thermodynamics. Recently, we reported that the Pd-loaded SO42/ZrO2/ Al2O3 catalyst exhibited excellent performance in both the isomerization reaction and HDS reaction [11,12]. This catalyst showed the isomerization activity at a lower temperature than zeolite catalyst did, and had the good sulfur tolerance in the isomerization reaction of sulfurcontaining light naphtha. However, the sulfur content of light naphtha fraction obtained by atmospheric distillation is higher than that of these materials. The content varies between 100 and 1000 massppm, though it depends on the crude oil. Therefore, it seems that the isomerization catalyst that has higher sulfur tolerance will be more effective in commercial utilization. The purpose of this study is to clarify the relations between the metal and the catalytic activity in the isomerization reactions of sulfur-containing light naphtha. Thus, we prepared the isomerization catalysts in which the kinds of metal were different, and we investigated the influence on the isomerization activity of the sulfur content in the feed in detail. Furthermore, when we prepared the catalysts, the methods of the Pd addition were different, and the positions of the metals in each case were specified by EPMA. Finally, we report a hybrid-type catalyst that works effectively in the isomerization reaction of light naphtha, which contains sulfur in high content.
2. Experimental 2.1. Catalyst preparation The catalyst preparation route is summarized in Fig. 1. Metal/SO42/ZrO2-Al2O3 catalysts for which the methods of adding the metal were different were used as isomerization catalysts used for this research. Zr(OH)4, used as a support, was of commercial grade (DAIICHI KIGENSOKAGAKU KOGYO). SO42/Zr(OH)4, which
Fig. 1. The summary of catalyst preparation.
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was the catalyst precursor, was prepared by dipping Zr(OH)4 into the 1N H2SO4 for 3 h, filtering, evaporating for 3 h at 55 8C, and drying for one night at 110 8C. Catalyst-A (Pt/SO42/ZrO2-Al2O3): Pt/SO42/Zr(OH)4 was prepared by incipient wetness impregnation with an aqueous solution of H2PtCl6(Wako) over SO42/Zr(OH)4. The water was evaporated, and the powder was dried at 150 8C overnight. Subsequently, the mixture of dried Pt/ SO42/Zr(OH)4, alumina sol(AP-1, CCIC) and water was arranged in the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. Afterwards, the mixture was kneaded for 1 h and then extruded through the opening of a circular die of 1.6 mm diameter to form cylindrical pellets. Catalyst-A was obtained by calcining the extruded pellets in air at 600 8C for 3 h. Catalyst-B (Pd/SO42/ZrO2-Al2O3): catalyst-B was prepared in the same way as catalyst-A, except for changing the impregnation solution into a dilute HCl aqueous solution of PdCl2. Catalyst-C (Pd/SO42/ZrO2-Al2O3): a mixture of SO42/Zr(OH)4, alumina sol and a dilute HCl aqueous solution of PdCl2 was prepared for the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. The mixture was kneaded for 1 h and extruded through a circular opening of 1.6 mm diameter. The extruded pellets were dried at 120 8C overnight. CatalystC was obtained by calcining the dried pellets in air at 600 8C for 3 h. Catalyst-D (Pd/SO42/ZrO2-Al2O3): a mixture of SO42/ Zr(OH)4, alumina sol and water was prepared for the kneader. The content of alumina in the mixture was set at 10 mass% as the oxide conversion. The mixture was kneaded and extruded through a circular opening. The extruded pellets were calcined at 600 8C for 3 h. The dilute HCl aqueous solution of PdCl2 was impregnated by incipient wetness over SO42/ZrO2-Al2O3 and dried afterwards at 120 8C for 3 h. Catalyst-D was obtained by the calcination of Pd-impregnated pellets in air at 600 8C for 3 h. Catalyst-E (Pd-impregnated Pt/SO42/ZrO2-Al2O3): a dilute HCl aqueous solution of PdCl2 was impregnated by incipient wetness over catalyst-A and dried afterwards at 120 8C for 3 h. Catalyst-E was obtained by calcining the dried pellets in air at 450 8C for 3 h. 2.2. Isomerization reaction
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Table 1 Feed properties Feed-1
Feed-2
Feed-3
Properties Density (at 15/4) Sulfur (massppm) Nitrogen (massppm) GC-RONa iso-C5 ratio (mass%) 22DMB selectivity (mass%)
0.6607 1 <1 64 39.1 1.0
0.6596 81 <1 66 40.0 1.1
0.6534 490 <1 67 40.3 0.9
Components (mass%) C3, C4 C5 C6 C7+ Benzene Naphthenes
3.3 38.6 40.2 7.3 2.6 8.0
2.4 42.5 38.0 6.5 2.6 8.0
a
1.3 52.0 36.6 1.0 1.7 7.5
GC-RON is the Research Octane Number calculated by GC.
2.9 MPa total pressure, H2/hydrocarbon molar ratio of 2, LHSV of 1.5 h1, and temperature in the 190– 240 8C range. Isomerization reactions were carried out using samples of light naphtha whose sulfur contents were different. Table 1 shows the main properties of these feeds. Feed-1 is sulfurfree light naphtha that was hydro-desulfurized, and feed-2 is Merox sweetening light naphtha that contains sulfur of 81 massppm. Feed-3 is the raw light naphtha obtained from the atmospheric distillation apparatus; it contains sulfur of 490 massppm. Liquid products were collected from a gas–liquid separator periodically and the components were analyzed by gas chromatography using an FID detector (GC-17A, Shimazu) with a fused silica capillary column of 50 m 0.21 mm ID (CP-Sil PONA CB, GL science). The sulfur content in liquid products was determined by an ultraviolet fluorescent method (TS-100, Mitsubishi Chemical). The detectable products in the isomerization reaction of light naphtha were isopentane and n-pentane in C5 fraction, and 2,2-demethybutane (22DMB), 2,3-demethylbutane (23DMB), 2-methylpentane (2MP), 3-methylpentane (3MP) and n-hexane in C6 fraction. The catalytic activities of sulfur-containing light naphtha over prepared catalysts were determined by the ratios of C5 and C6 isomers, respectively. iso-C5 ratio ðmass%Þ ¼
isopentane ðisopentane þ n-pentaneÞ 100 (1)
Isomerization reactions of light naphtha were carried out in a fixed-bed flow reactor. Before the isomerization reaction was started, catalysts were dried at 450 8C for 3 h in a muffle furnace; the catalysts were then reduced at 200 8C for 3 h with hydrogen flowing under atmospheric pressure. After reduction, the temperature was set to the reaction temperature and the isomerization reaction of light naphtha was started. The catalytic activities were performed at
22DMB selectivity ¼
22DMB 100 22DMB þ 23DMB þ 2MP þ 3MP þ n-hexane (2)
In addition, a hydrocracking reaction occurs during the isomerization reaction of light naphtha; then light hydrocarbons such as propane and butane are produced. The yields
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of isomerate were determined as the ratio of C5+ hydrocarbons in products to C5+ hydrocarbons in feeds. isomerate yield ð%Þ ¼
Cþ 5 hydrocarbon in products 100 Cþ 5 hydrocarbon in feeds (3)
2.3. The measurement of EPMA For the identification of the locations of the metal in the isomerization catalysts, some measurements using the electron probe microanalyzer (EPMA) were carried out. The mappings of metal concentration were analyzed with a JXM-8600-MX (JEOL). A catalyst pellet was picked out at random; it was embedded in the resin of the meta-methyl acrylic (MMA). After the embedded pellet was ground, the measurement samples were obtained by carrying out the vapor deposition of the carbon. The operating conditions of EPMA apparatus were carried out with the accelerating voltage of 20 kV and sample electric current of 1 107 A. In addition, the mappings were recorded for four elements: Zr, Al, Pt, and Pd.
3. Results and discussion 3.1. The performance of sulfur tolerant Pd/SO42/ZrO2-Al2O3 catalyst Fig. 2 shows the result of the isomerization test of feed-2 whose sulfur content is 81 massppm. Catalyst-A (Pt/SO42/ ZrO2-Al2O3) and catalyst-B (Pd/SO42/ZrO2-Al2O3) indicate a high iso-C5 ratio of 72%, which was close to the chemical equilibrium value during the early stage of a reaction [13]. However, the isomerization activity of catalyst-A declined linearly with the time on stream and the iso-C5 ratio in the product finally reached 40%. It seems that the isomerization activity disappeared completely,
Fig. 2. Isomerization reaction of sulfur-containing light naphtha over isomerization catalyst. (~): Catalyst-A; ( ) catalyst-B. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40%, sulfur = 80 massppm.
because that value is equal to the iso-C5 ratio in feed-2. In addition, though the sulfur content in products was <1 massppm during the early stage of a reaction, it increased with the decline of the isomerization activity. Sulfur content in the time when the isomerization activity is not indicated was the same 81 massppm as in feed-1. This result indicates that the sulfur is accumulated in Pt with the time on stream, and that the isomerization reaction of catalyst-A does not progress completely when the amount of sulfur in Pt exceeds the capacity. Therefore, the changes of the isomerization activities of catalyst-A in Fig. 1 can be regarded as correlating with the amount of Pt, which is active for the isomerization reaction. On the other hand, catalyst-B in which the main active metal was changed from Pt to Pd reached the stable iso-C5 ratio in the isomerization reaction of sulfur-containing light naphtha after 50 h. Further, the iso-C5 ratio in the stationary state was very high. We found that catalyst-B has the skeletal isomerization function of normal paraffin even in the presence of sulfur. Moreover, the sulfur content in products was under 1 massppm all the time during the isomerization reaction of sulfur-containing light naphtha, so the organic sulfur compound in the feed was changed into the H2S completely. This result suggests that catalyst-B works on both the isomerization reaction of light naphtha and the HDS reaction of organic sulfur compound. Generally, when organic sulfur compounds are continuously supplied to a catalyst, the isomerization reaction does not advance because the metal is covered by sulfur and its dehydrogenation ability disappears. Therefore, the metal must be always kept in the activated state so that the isomerization activity of light naphtha may appear even in the presence of sulfur. Catalyst-B could isomerize sulfurcontaining light naphtha without the decline of catalytic activity. This result suggests that both the conversion into H2S on the catalyst surface and the desorption of H2S from the catalyst are carried out promptly, even if some organic sulfur compound adsorbs on Pd and some metal sulfide is formed on the catalyst. That is to say, it seems that the maintenance of dehydrogenation ability by prompt regeneration of Pd causes the stable isomerization activity even in the presence of sulfur [14]. On the other hand, catalyst-A cannot isomerize the sulfur-containing light naphtha because of the sulfur accumulation on the metal. Therefore, this result suggests that there are more amounts of sulfur supplied from feed than the amount of sulfur desorbed from the metal in catalyst-A. Fig. 3(a) shows the effect of sulfur content in the naphtha isomerization when using catalyst-B. The control of feed sulfur content was accomplished by the blending of feed-1, 2, and -3. The catalytic activities of isomerization reaction in each feed were compared with the average values for 150 h after start-up. The increase of sulfur content in feed caused declines of iso-C5 ratio and 22DMB selectivity, and the skeletal isomerization of normal paraffin was inhibited. Because the decline tendency with iso-C5 ratio and that with
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Fig. 3. The effect of sulfur content in feed over catalyst B: (a) isomerization activity, (b) isomerate yield. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40%, sulfur = 0, 30, 80, 160 massppm.
the 22DMB selectivity are about the same, it seems that the effect to the isomerization activity of the sulfur content in feed does not depend on carbon number. To complete the isomerization reaction of the sulfurcontaining light naphtha, three processes are necessary: HDS of organic sulfur compounds, the formation of carbenium ions by dehydrogenation of paraffin, and the supply of atomic hydrogen to carbenium ions [15,16]. It is assumed that these all processes advance on the same Pd in catalyst-B. The increase of sulfur content in the feed invites the increase of the amount of Pd required for the HDS reaction; this leads to the decrease of the relative amount of Pd that plays an important part for both the dehydrogenation of paraffin and the supply of atomic hydrogen. As a result, the isomerization activity of sulfur-containing light naphtha over catalyst-B seems to decrease. Fig. 3(b) shows the relationship between the sulfur content in feed and the yield of isomerate. The increase of sulfur content in feed improved the yield of isomerate, and
decreased the amount of gas component production caused by the hydrocracking reaction. Both the isomerization reaction and the hydrocracking reaction progress by way of the carbenium ion that is a reaction precursor [17,18]. It is clear that the increase of sulfur content in the feed inhibits the formation of carbenium ions. Thus, the existence of sulfur in light naphtha leads to the decline of formation ability of carbenium ion, since the amounts of Pd required for the HDS reaction increase. Consequently we conclude that sulfur reduces the progress of both reactions: the isomerization and the hydrocracking. 3.2. The effect of Pd addition to SO42/ZrO2-Al2O3 Fig. 4(a) shows the results of the isomerization activity of three kinds of Pd/SO42/ZrO2-Al2O3 catalysts (Pd preimpregnated catalyst (catalyst-B), Pd simultaneous-kneaded catalyst (catalyst-C) and Pd post-impregnated catalyst (catalyst-D)) for which the Pd loading methods are different.
Fig. 4. The isomerization activity of catalyst prepared by various types of Pd loading additions: (a) iso-C5 ratio, (b) 22DMB selectivity. (~): Catalyst-B, ( ): catalyst-C, (*): catalyst-D. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40.3%, sulfur =490 massppm.
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Feed-3, whose sulfur content was high, was used for these experiments. When the catalytic activities were compared in the iso-C5 ratio, the skeletal isomerizations of catalyst-C and -D were better than that of catalyst-B. When 22DMB selectivity was used as the index of the skeletal isomerization of C6 paraffin, the isomerization reaction of light naphtha over catalyst-D progressed considerably faster than for other catalysts, as shown in Fig. 4(b). These results indicate that the skeletal isomerization is very much influenced by the method of the Pd addition in the isomerization of high sulfur feed. In order to investigate the relations between the method of Pd addition and the composition of the catalyst surface, we identified the distribution of metal on catalyst-B, -C and D by EPMA. Fig. 5 shows the results of mappings of Pd and Al in the same view where aluminum particles were located for each catalyst. Since the intensity of Pd was very weak in the area where Al distribution in catalyst-B is strongly detected, the Pd in catalyst-B exists as the Pd/SO42/ZrO2 particles which were formed at the beginning of the catalyst preparation. On the other hand, though the correlation of the distribution of Al and Pd can be seen over catalyst-C in which Pd and alumina sol were kneaded simultaneously, it was confirmed that the Pd in catalyst-C existed in almost all the particles without any clear boundaries. However, as for catalyst-D in which the Pd was impregnated on the SO42/ ZrO2-Al2O3 carrier, the areas where Pd and Al were strongly detected corresponded very well. Pd exists as [PdCl4]2 in the impregnation solution used for the preparation of catalyst-D. Besides, SO42/ZrO2 particle is charged minus by the intense electron-attractive group of sulfuric acid. Therefore it is likely that [PdCl4]2 ions repelled each other with the surface charge of SO42/ZrO2 particle and adsorbed on the Al2O3 particles. Conversely, Pd in catalyst-B is not located on Al2O3 since catalyst-B was calcined at 450 8C in air after the formation of Pd/SO42/Zr(OH)4. The position of Pd element in catalyst-C is the intermediate state of catalyst-B and catalyst-D, and the simultaneous kneading of the impregnation solution including [PdCl4]2 and the alumina sol caused the uniform distribution of Pd on the surface of catalyst-C. These results clearly show that it is more advantageous for Pd to exist on Al2O3 rather than on ZrO2 during the simultaneous progress of the isomerization reaction and the HDS reaction, since catalyst-D, which forms Pd/Al2O3 possesses the high catalytic activity in the skeletal isomerization reaction under the presence of sulfur. Fujimoto et al. reported that the powdery mixture of Pt/ SiO2 and H-MFI zeolite possessed the high isomerization activity in the isomerization reaction of n-pentane [19]; they mentioned that hydrogen transfer between the particles occurs with the hybrid-type catalyst of powdery mixture. Our results also mean that the noble metal does not necessarily need to be loaded onto the solid acid and indicate that the isomerization reaction progresses by two active sites: with the acid sites and with the hydrogenation metal on
another particle which is close to an acid site. Namely, as for catalyst-D, since the atomic hydrogen formed by homogeneous dissociation of gaseous hydrogen over Pd/Al2O3 is supplied to SO42/ZrO2, the regeneration of acid sites and the stabilization of carbenium ion that is the isomerization precursor are promoted by spill-over of atomic hydrogen [20]. And as a result, we can infer that catalyst-D invites the high skeletal isomerization activity. Kabe and co-workers reported that a noble metalsupported Al2O3 catalyst shows the high catalytic activity to the HDS reaction of organic sulfur compounds [21]. Sulfur types in light naphtha are composed of thiol compounds (R–SH) and sulfide compounds (R–S–R) of low boiling point [22]; the HDS of these sulfur compounds are comparatively easier than those of the thiophene compounds [23]. Therefore, these results suggest that Pd/Al2O3 formed in catalyst-D can desulfurize the organic sulfur compounds in light naphtha to H2S immediately. The improvement of HDS activity leads the isomerization catalyst to the decrease of the amount of Pd that is necessary for the HDS reaction, and thus increases relatively the amount of Pd, which can supply atomic hydrogen. As a result, catalyst-D which formed Pd/Al2O3 shows the higher catalytic activity in the isomerization reaction for sulfur-containing light naphtha than Pd that is supported on SO42/ZrO2. 3.3. The development of hybrid-type isomerization catalyst for high sulfur feed From the study of the Pd loading methods, it is clear that the formation of Pd/Al2O3 in isomerization catalyst causes the high catalytic activity in the isomerization reaction of sulfur-containing light naphtha. Fig. 6 shows the result of the activity test of catalyst-E (Pd-impregnated Pt/SO42/ZrO2Al2O3), which is prepared by Pd impregnation over catalystA (Pt/SO42/ZrO2-Al2O3). Feed-3, which contained high sulfur content, was used as feed. The catalytic activities of catalyst-A and -B were also evaluated with the same conditions as comparative catalysts. The iso-C5 ratio of catalyst-A decreased rapidly from start-up due to the presence of high sulfur content, and the isomerization activity completely disappeared after 20 h. In addition, the decline of iso-C5 ratio with catalyst-B is moderate, and the iso-C5 ratio after 100 h was about 58%. On the other hand, catalyst-E, which was obtained by Pd impregnation over catalyst-A, did not show any decline of the isomerization activity even after 100 h; this catalyst kept the high iso-C5 ratio of 71% regardless of the reaction time. Moreover, sulfur content in products was less than 1 massppm during the isomerization reaction, and the sulfur of 490 massppm contained in feed was changed into the H2S completely. Since catalyst-A, which is Pt-promoted sulfate zirconia, did not show any catalytic activity at all for the isomerization reaction of sulfur-containing light naphtha, it is quite obvious that the catalytic performance of catalyst-E depends on the Pd that it is added over catalyst-A.
Fig. 5. EPMA images of various Pd/SO42/ZrO2-Al2O3 samples. (a) Al mapping on catalyst-B, (b) Al mapping on catalyst-C, (c) Al mapping on catalyst-D, (d) Pd mapping on catalyst-B, (e) Pd mapping on catalyst-C, (f) Pd mapping on catalyst-D
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Fig. 6. The results of isomerization reaction of high sulfur feed. (~): Catalyst-A, ( ): catalyst-B, (*): catalyst-E. Reaction conditions: pressure = 3.1 MPa, H2/HC = 2 mol/mol, LHSV = 1.5 h1, temperature = 195 8C, catalyst volume = 7 cm3; feed properties: iso-C5 ratio = 40.3%, sulfur =490 massppm.
In order to determine the positions of metals in catalyst-E, the concentration distribution was analyzed by EPMA for four elements: Zr, Al, Pt and Pd. Fig. 7 shows the results of mappings of each element in the same view. From the results of mapping analysis of Zr and Al elements, one finds that Al2O3 existed in the space between the ZrO2 particles, and one could confirm that Al2O3 is working as a binder. Moreover, elemental Pt was strongly observed in the positions where the ZrO2 particles existed, and so we concluded that Pt is located on the ZrO2 particles. On the contrary, little elemental Pd was detected on ZrO2 particles, and it corresponded with the concentration distribution of elemental Al. These results clearly indicate that catalyst-E is composed as a hybrid-type catalyst (Pt/SO42/ZrO2-Pd/ Al2O3) in which the Pt/SO42/ZrO2 particles and the Pd/ Al2O3 particles form a super-neighborhood.
From the results that the sulfur affects the catalytic activity in the isomerization reaction of light naphtha, we concluded that sulfur compounds in feed inhibit the formation of carbenium ions that are isomerization precursors. However, when the Pt/SO42/ZrO2 particles are covered with Pd/Al2O3 particles within the catalyst pellet like catalyst-E, the very high isomerization activity appears even in the presence of high sulfur content. This fact indicates that the formation of carbenium ion by Pt/SO42/ ZrO2 particle is not restricted because Pd/Al2O3 particles desulfurize most organic sulfur compounds in the reactant. Originally, when one considers that the Pt/SO42/ZrO2 catalyst is very sensitive to sulfur in feed, one can presume that Pd/Al2O3 plays the role of supplying the atomic hydrogen to other particles like catalyst-D, in addition to indicating the high HDS activity. By this function, the adsorption of sulfur compound to Pt in catalyst is inhibited or the desorption of sulfur adsorbed on Pt is accelerated. In either case, results suggest that Pd/Al2O3 controls both the dehydrogenation ability of paraffin with which Pt in Pt/ SO42/ZrO2 is originally equipped and the hydrogenation ability of isomerized carbenium ions, since Pd/Al2O3 protects the nearby Pt/SO42/ZrO2 particles. 3.4. The model of catalytic function in hybrid-type catalyst Fig. 8 illustrates a catalyst model of the hybrid-type catalyst. Catalyst-E, which indicates the high isomerization activity even in the presence of sulfur, is composed as a hybrid-type catalyst in which the Pt/SO42/ZrO2 particles are covered with alumina binder including Pd. When organic sulfur compounds contact the hybrid-type catalyst, most of those parts are desulfurized to H2S by Pd/Al2O3. And if some of the organic sulfur compounds and the produced H2S
Fig. 7. EPMA image of catalyst-E: (a) Zr mapping, (b) Al mapping, (c) Pt mapping, (d) Pd mapping.
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catalytic performance of the two types of particles promotes high sulfur tolerance.
Acknowledgement This work has been carried out as a research project of Petroleum Energy Center with a subsidy from the Ministry of Economy, Trade, and Industry, Japan. Fig. 8. The model of metal position in hybrid-type catalyst.
References touch the Pt/SO42/ZrO2 particles, it is suggested that they are desorbed promptly due to the function of atomic hydrogen supplied by Pd/Al2O3 [14]. Namely, Pd-loaded on Al2O3 has two functions: with the HDS of sulfur-containing feed and with the formation of atomic hydrogen by the activation of gaseous hydrogen [24]. On the other hand, Pt loaded on SO42/ZrO2 works on the formation of carbenium ion by the dehydrogenation of paraffin [25]. Furthermore, atomic hydrogen formed by Pd is moved to SO42/ZrO2, and it also plays a role in the stabilization of the isomerized carbenium ion [26]. Moreover, the supply of atomic hydrogen promotes the desorption of sulfur compounds adsorbed on Pt and the sulfur tolerance of Pt/SO42/ZrO2 particles improves substantially. Since the improvement of sulfur tolerance of Pt/SO42/ZrO2 particles prevents the decline of the production ability of the carbenium ion, the extremely high isomerization activity will appear even in the presence of sulfur over a hybrid-type catalyst.
4. Conclusion We studied the relations between the metal and the catalytic activity in the isomerization reactions of sulfurcontaining light naphtha. It became clear that the optimal position of Pd in SO42/ZrO2-Al2O3 carrier is on Al2O3. That is because the high HDS function is obtained by the formation of Pd/Al2O3. Besides, the catalyst which was prepared by Pd impregnation into Pt/SO42/ZrO2-Al2O3 has the hybrid structure where the Pt/SO42/ZrO2 particles and the Pd/Al2O3 particles adjoined each other, and this catalyst indicates the steadily high isomerization activity in the skeletal isomerization reaction of light naphtha which contains the sulfur of 490 massppm. This hybrid-type catalyst is divided into the HDS function due to Pd/Al2O3 particles and the isomerization function due to Pt/SO42/ ZrO2 particles. We concluded that cooperation of the
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