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Pt catalysts supported on β zeolite ion-exchanged with Cr(III) for hydroisomerization of n-heptane
Applied Catalysis A: General 371 (2009) 142–147
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Pt catalysts supported on b zeolite ion-exchanged with Cr(III) for hydroisomerization of n-heptane Ping Liu, Xingguang Zhang, Yue Yao, Jun Wang * State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing 210009, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 July 2009 Received in revised form 28 September 2009 Accepted 29 September 2009 Available online 7 October 2009
A series of Pt catalysts supported on Hb zeolite ion-exchanged with Cr(III) were prepared, characterized by ICP, XRD, BET, NH3-TPD, H2-TPR and H2-chemisorption techniques, and evaluated in hydroisomerization of n-heptane with an atmospheric fixed-bed flow reactor. Pt catalyst supported on Hb zeolite exhibits both lower selectivity to isomerized products and lower conversion of n-heptane than the counterparts supported on Cr(III)-exchanged Hb zeolite. The optimal composition for Cr-bearing catalysts is 0.4 wt.% of Pt loading and 0.7 wt.% of Cr loading. This catalyst proves to be highly efficient in the hydroisomerization of n-heptane, giving a very high selectivity to isomerized products: 95.4% coupled with a high conversion of n-heptane: 72.1%. The substantial promotion effect of ion-exchanged Cr(III) species is discussed in relation to catalyst physicochemical properties. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Hydroisomerization of n-heptane b Zeolite Bifunctional catalyst Pt Cr
1. Introduction Hydroisomerization of paraffins plays an increasingly important role in refineries to boost the octane number of the gasoline pool due to the stringent environmental requirements. Its influence exceeds those of other approaches such as blending with oxygenates and aromatics [1–3]. As one of the fractions in naphtha, n-heptane shows zero octane number, and its isomerization attracts great attention. However, Pt/mordenite commercially used as the catalyst for hydroisomerization of C5/C6 into light isoparaffins would readily cause cracking in hydroconversion of C7 [4,5]. The seeking of new catalysts for hydroisomerization of nheptane is therefore a requisite to enhance the selectivity to branched paraffins. Metal/acid bifunctional catalysts, especially the noble metal Pt- or Pd-bearing catalysts supported on SAPO molecular sieves [6,7], Y zeolite [8,9], b zeolite [10–13], SO42/ZrO2 [14,15] and heteropolyacid [16,17] have been extensively investigated. Among them, Pt/Hb is considered to be a promising candidate due to the favorable acidity and the unique structure of its three-dimensional interconnected channels of b zeolite [18– 21]. Nevertheless, the catalytic conversion is still not sufficiently high simultaneously with a high selectivity to isoheptanes [12,13]. In order to achieve higher activities, attempts are made to modify
* Corresponding author. Tel.: +86 25 83172264; fax: +86 25 83172261. E-mail address: [email protected] (J. Wang). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.045
the zeolite supported Pt catalysts [22–34]. In these studies, the introducing of a second metal as the promoter into supported noble metal catalysts was demonstrated to be an effective way to improve catalytic conversion, selectivity and stability for isomerization of n-paraffins [29–34]. Mao et al. [31] reported an enhanced yield of branched paraffins by the incorporation of a small amount of Al3+ species into HY supported Pt catalysts. Fu´nez et al. [34] observed a remarkable increase in conversion of n-heptane and selectivity to multi-branched isomers due to the addition of Ni into Pt/Hb catalyst. Recently, we prepared Pt-bearing catalysts supported on b or dealuminated b zeolite doped with a second metal of Cr by co-impregnation for hydroisomerization of nheptane, and found the enhanced catalytic activity [35,36]. However, in those previous reports, noble metal/zeolite catalysts promoted by a second metal were always prepared by impregnation. It is well known that ion-exchange is also an effective and commonly used approach to modify zeolite [37]. The presence of highly charged metal cations introduced into zeolite channels and/or cages through ion-exchange leads to the change of zeolite acidity [37,38]. Moreover, ion-exchanged metal cations favor the adsorption of alkanes on zeolite by the interaction between the adsorbed molecules and the ion-exchanged cations [39,40]. Based on the above considerations, it is reasonable to prepare and evaluate the noble metal/zeolite catalysts modified by ion-exchanged metal cations for hydroisomerization of n-heptane, but this has been overlooked. In this study, we prepare Cr(III) ionexchanged b zeolite supported Pt catalysts for hydroisomerization
P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147
of n-heptane, and reveal a highly improved isomerization selectivity for isoheptanes with a high catalytic conversion of nheptane due to the promotion effect of ion-exchanged Cr(III). 2. Experimental 2.1. Preparation of catalysts The hydrogen form of b zeolite (Hb) was prepared by the repeated ion-exchange of Nab zeolite (Fushun No. 3 Petrochemical Factory, CNPC, SiO2/Al2O3 molar ratio 29) with aqueous solution of NH4Cl (1.0 mol L1) at 90 8C, followed by washing with deionized water, drying at 100 8C for 12 h, and calcination at 550 8C for 3 h. nPt/mCr-Hb catalysts (n and m stand for Pt and Cr loading by weight percentage, respectively) were prepared via ion-exchange of Hb with chromium(III) nitrate and impregnation with chloroplatinic acid. In detail, Hb was ion-exchanged in aqueous solution of chromium(III) nitrate at 90 8C for 2 h with the mass ratio of solid to liquid 1:100, followed by washing with water, drying at 100 8C for 12 h and calcination at 400 8C for 3 h. Then, the obtained solid was immersed in aqueous solution of chloroplatinic acid, dried and calcinated once more. In this procedure, the concentration of Cr for the aqueous solution of chromium(III) nitrate was changed to adjust the Cr loadings of catalysts, which were determined using a Jarrell–Ash 1100 inductively coupled plasma (ICP) spectroscope. Totally six different Cr loadings by the weight percentage were obtained, i.e., 0.2%, 0.3%, 0.4%, 0.7%, 0.9%, and 1.3%, corresponding to the Cr/Al molar ratio of 0.04, 0.05, 0.07, 0.13, 0.16, and 0.23, respectively. Also, the concentration of Pt for aqueous solution of chloroplatinic acid was changed to make different loadings of Pt (0.05–1.0%) in catalysts, which were calculated directly according to the amount of Pt used in impregnation. 2.2. Characterization X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation at 40 kV and 30 mA with a scan rate of 28 min1 and a scanning range of 3–508. BET surface areas and pore volumes were measured using a Micromeritics ASAP2020 analyzer with nitrogen adsorption at 196 8C. The samples were pretreated at 200 8C in a vacuum of 0.01 kPa for 4 h. Temperature-programmed desorption of ammonia (NH3-TPD), temperature-programmed reduction of hydrogen (H2-TPR), and Pt dispersion measurements were conducted using a JAPAN BELCAT-Analyzer. For NH3-TPD, 200 mg of sample was pretreated at 500 8C for 2 h in a flow of helium, followed by adsorption of ammonia at 100 8C. NH3TPD was then performed at a heating rate of 10 8C min1. For H2TPR, the sample loaded in a quartz tube was pretreated at 400 8C for 1 h in a flow of oxygen, followed with cooling to 60 8C and sweeping with argon for 20 min. While the sample was heated at a mixture of hydrogen and argon at a heating rate of 10 8C min1 up to 700 8C, the temperature and consumption of hydrogen were recorded, respectively. The dispersion of Pt was determined using a pulsed technique of hydrogen chemisorption, based on 1:1 stoichiometry (H/Pt) according to the previous work [41]. Two hundred milligram of the catalyst was reduced in hydrogen flow at 400 8C for 2 h, and then swept under argon flow at 450 8C for 0.5 h. Afterwards, the catalyst was cooled down to 50 8C and the pulse of 1 mL of the H2/Ar mixture (10.2% H2) was repeatedly injected into the reactor via a six-way valve until the hydrogen signals from the thermal conductivity detector remained at a constant value. The volume of hydrogen chemisorbed was determined by summing the fractions of hydrogen consumed in each pulse.
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2.3. Catalytic test Hydroisomerization of n-heptane was carried out with an atmospheric fixed-bed flow reactor. A measured amount of the granular catalyst (0.565 g, 0.25–0.42 mm) was charged in the middle of the stainless steel tubular reactor. Before the starting of the reaction, the catalyst was reduced in situ in H2 flow at 300 8C for 3 h. The typical reaction conditions were as follows: reaction temperature 190–270 8C, weight hourly space velocity (WHSV) 2.7 h1, and molar ratio of H2 to n-heptane 7.9. The product mixture was quantitatively analyzed online with a gas chromatograph (Shimadzu GC-2014) equipped with a SE30 capillary column (50 m 0.25 mm 0.3 mm) and FID. The products were qualitatively confirmed by GC–MS (ThermoFinnigan). 3. Results and discussion 3.1. Characterization of various catalysts 3.1.1. XRD results XRD patterns of various catalysts are illustrated in Fig. 1. It shows that the diffraction curves of all Cr(III) exchanged Hb supported Pt catalysts were identical to that of their parent sample Hb, indicating a high comparative crystallinity of BEA zeolite phase after the loading of Cr and Pt. No diffraction peaks assigned to Pt and Cr species occur for any of the nPt/mCr-Hb catalysts, even at a high Pt loading of 0.8% or a high Cr loading of 1.3%. Such absence suggests a high dispersion of Pt on the surface of support, as well as a high dispersion of Cr ions in zeolite channels and cavities. 3.1.2. NH3-TPD NH3-TPD profiles of 0.4%Pt/mCr-Hb catalysts are shown in Fig. 2. Two peaks occurred for all samples, corresponding to desorption of ammonia on weak and strong acid sites. With the increase of the ion-exchanged Cr(III) in 0.4%Pt/mCr-Hb catalysts, the peak for strong acidity shifted gradually from 384 8C toward 408 8C. At the same time, the acid amount for strong acidity also increased. Based on the Plank–Hirschler mechanism [42,43], trivalent metal cations with high ionic potential can readily undergo hydrolysis to form new Bro¨nsted acid sites and acidic metal hydroxyl groups as follows: [Cr(H2O)n]3+ ! [Cr(OH)2]+ + 2H+. The generation of the stronger acid sites for the trivalent metal ion (M3+)-exchanged zeolites can be explained by different suppositions; for example, [M(OH)2]+
Fig. 1. XRD patterns of Hb and nPt/mCr-Hb catalysts. (a) Hb, (b) 0.4%Pt/Hb, (c) 0.4%Pt/0.3%Cr-Hb, (d) 0.4%Pt/0.7%Cr-Hb, (e) 0.4%Pt/1.3%Cr-Hb, (f) 0.6%Pt/0.7%CrHb, and (g) 0.8%Pt/0.7%Cr-Hb.
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P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147 Table 1 BET surface areas, pore volumes and Pt dispersions of selected catalysts. Catalyst
SBET/(m2 g1)
Pore volume/(cm3 g1)
Pt dispersion/%
Hb 0.4%Pt/Hb 0.4%Pt/0.3%Cr-Hb 0.4%Pt/0.7%Cr-Hb 0.4%Pt/1.3%Cr-Hb
578 564 568 543 524
0.2451 0.2357 0.2374 0.2265 0.2218
– 33 87 79 27
around 407 and 550 8C. It is clear that the curve for 0.4%Pt/0.7%CrHb was not a result of simply overlapping the profiles for the two monometallic samples. The occurrence of those peaks for 0.4%Pt/ 0.7%Cr-Hb is indicative of the strong interaction between Cr and Pt, i.e., the formation of Pt–Cr species, which are clearly in different status [50]. This implies that the reducibility of at least a part of Pt species in 0.4%Pt/0.7%Cr-Hb is dramatically improved by the presence of ion-exchanged Cr species.
Fig. 2. NH3-TPD profiles of 0.4%Pt/mCr-Hb catalysts. (a) 0.4%Pt/Hb, (b) 0.4%Pt/ 0.2%Cr-Hb, (c) 0.4%Pt/0.3%Cr-Hb, (d) 0.4%Pt/0.4%Cr-Hb, (e) 0.4%Pt/0.7%Cr-Hb, (f) 4%Pt/0.9%Cr-Hb, and (g) 0.4%Pt/1.3%Cr-Hb.
species are the source of the stronger acidity [44], or alternatively, the polarizing and inductive effects of the introduced metal cations (M3+ and [M(OH)]2+) would weaken the OH bonds of the bridging hydroxyl groups and make them more acidic [45–47]. This may account for the enhanced strong acidity in both acid strength and acid number for Cr(III)exchanged zeolitic catalysts. 3.1.3. H2-TPR Fig. 3 shows H2-TPR profiles for 0.4%Pt/Hb, 0.7%Cr-Hb and 0.4%Pt/0.7%Cr-Hb catalysts. 0.4%Pt/Hb displayed a hydrogen consumption peak at around 450 8C, indicating the strong interaction between Pt2+ and b zeolite with the formation of PtðOSiBBÞ2y species [48,49]. 0.7%Cr-Hb presented a large peak y at 393 8C, which is assigned to the reduction of Cr(III) species [50,51]. The bimetallic catalyst 0.4%Pt/0.7%Cr-Hb exhibited four reduction peaks: two large at around 161 and 271 8C; two small at
3.1.4. Surface area and pore volume Table 1 lists the BET surface areas and pore volumes for selected catalysts. It can be seen that Hb exhibited a high surface area and pore volume. Upon the loading of Pt and Cr to Hb zeolite, surface areas and pore volumes decreased only marginally. On the other hand, with the increase of Cr loading, surface areas and pore volumes decreased slowly; at the high Cr loading of 1.3%, they reduced significantly to 524 m2 g1 and 0.2218 cm3 g1, respectively. 3.1.5. Pt dispersion The Pt dispersion on the Cr-free support and the Cr(III)exchanged counterparts are also compared in Table 1. It can be seen that 0.4%Pt/Hb showed the Pt dispersion of 33%, and in contrast, after b zeolite was ion-exchanged by Cr(III), with loadings of 0.3% and 0.7%, remarkable increases of Pt dispersions were observed (87% and 79%). It is noticeable that the interaction between the supported Pt species and acid sites has been claimed by previous reports [52,53]. Also, the H2-TPR results in Fig. 3 have indicated the presence of the new Pt–Cr species. One thus can suggest that for the b zeolite ion-exchanged by Cr(III), it is the reaction of the Cr-related acid sites such as [Cr(OH)2]+ [42,43] with Pt species that causes the formation of the Pt–Cr species. The existence of this Pt–Cr species may greatly hinder the Pt active sites from sintering during the high temperature treatment for the catalyst, which would account for the high Pt dispersions for the Cr-containing samples. However, by introducing excessive Cr, the Pt dispersion decreased seriously: a low Pt dispersion of 27% was found for the sample with a high Cr loading of 1.3%. This result is still not understandable. 3.2. Catalytic stability
Fig. 3. H2-TPR profiles of (a) 0.4%Pt/Hb, (b) 0.7%Cr-Hb, and (c) 0.4%Pt/0.7%Cr-Hb.
Conversions of n-heptane in the hydroisomerization of nheptane over various catalysts at 230 8C as a function of reaction time on stream are displayed in Fig. 4. It can be seen that the reaction reached a steady stage at 120 min of reaction time on stream over all catalysts, although some of them deactivated more or less at the very beginning of the reaction. For 0.4%Pt/Hb, the conversion decreased considerably within 80 min. Comparatively, Cr(III)-exchanged b zeolitic catalysts (Cr loading: 0.2–0.7%) showed much less deactivation, indicating that the introduction of Cr improves the catalytic stability. However, for 0.4%Pt/1.3%CrHb with high Cr loading, the conversion of n-heptane decreased drastically to the lowest stable value within 40 min, even though it gave a very high initial activity. These results prompt us to compare their catalytic activity at 120 min of TOS in the following measurements.
P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147
Fig. 4. Conversion of n-heptane over catalysts with different Cr loadings at 230 8C as a function of reaction time on stream (TOS). (&) 0.4%Pt/Hb; ($) 0.4%Pt/0.2%Cr-Hb; (*) 0.4%Pt/0.3%Cr-Hb; (5) 0.4%Pt/0.4%Cr-Hb; (~) 0.4%Pt/0.7%Cr-Hb; (!) 0.4%Pt/ 0.9%Cr-Hb; (^) 0.4%Pt/1.3%Cr-Hb.
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Fig. 6. Conversion of n-heptane and isomerization selectivity over nPt/0.7%Cr-Hb catalysts with different Pt loadings. (&) Conversion of n-heptane; (!) selectivity to isomerization.
Fig. 5 shows the conversion of n-heptane and selectivity to isomerization as a function of Cr loadings for 0.4%Pt/mCr-Hb (m = 0–1.3%) at 230 8C and 120 min. For 0.4%Pt/Hb, considerable conversion of n-heptane of 64.3% was obtained, but with a low selectivity to isomerization of 79.1%. When Cr was introduced into 0.4%Pt/Hb, generally speaking, both conversion and selectivity increased a lot. In detail, in the low Cr loading range, the conversion increased quickly and reached the highest value of 82.5% at the Cr loading of 0.3%; in the high Cr loading range, it decreased slowly. Meanwhile, the selectivity to isomerized products always increased with the increase of Cr loading. Compromising conversion and selectivity, one can conclude that 0.4%Pt/0.7%Cr-Hb and 0.4%Pt/0.9%Cr-Hb are preferred catalysts. Over 0.4%Pt/0.7%Cr-Hb, both high conversion of n-heptane 72.1% and high selectivity to isoheptanes 95.4% were obtained. On the other hand, for 0.7%CrHb, the conversion and isomerization selectivity gave very low level of 4.4% and 43.5%, respectively, demonstrating that Cr species
themselves do not serve as the metallic active centers in hydroisomerization of n-heptane. For hydroisomerization of paraffins over metal/acid bifunctional catalysts, the paraffin transformation involves hydrogenation– dehydrogenation on metal sites, isomerization and/or cracking on acid sites, and diffusion of the olefinic intermediates from acid to metal sites and inversely [54]. When acid sites are not sufficient, the reaction on acid sites is the rate-determining step. On the contrary, in cases of scarcity of the available metal sites, the hydrogenation– dehydrogenation becomes the rate-determining step. Based on the above proposal, 0.4%Pt/Hb most possibly provides large amounts of acid sites and is short of metal sites because of the low Pt dispersion. It is thus understandable that the increase of conversion and isomerization selectivity at the low Cr loadings 0.3% is due to the remarkable increase of the Pt dispersion (Table 1). With the further increase of the Cr loading to 0.7%, acid strength and amount also increase (Fig. 2), but the Pt dispersion no longer increases simultaneously (Table 1), which may be the reason for the high activity. In this case, we think that the metal and acid sites match well to each other. For 0.4%Pt/1.3%Cr-Hb, the lowest conversion of nheptane might be caused by the very low Pt dispersion of 27%, as well
Fig. 5. Conversion of n-heptane and isomerization selectivity over the 0.4%Pt/mCrHb catalysts with different Cr loadings. (&) Conversion of n-heptane for 0.4%Pt/ mCr-Hb; (!) selectivity to isomerization for 0.4%Pt/mCr-Hb; (§) conversion of nheptane for 0.7%Cr-Hb; ($) selectivity to isomerization for 0.7%Cr-Hb.
Fig. 7. Influence of WHSV on catalytic performance of 0.4%Pt/0.7%Cr-Hb in the hydroisomerization of n-heptane. (&) Conversion of n-heptane; (*) multibranched yield; (~) mono-branched yield; (!) isomerization selectivity; ($) cracking selectivity; (&) cyclization selectivity.
3.3. Effect of Cr and Pt loadings for Pt/Cr-Hb catalysts
P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147
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Table 2 Comparison of reactivity of hydroisomerization of n-heptane over 0.4%Pt/Hb and over 0.4%Pt/0.7%Cr-Hb catalysts at different reaction temperatures. Catalyst
T/8C
Conversion/%
Isomerization yield/%
YMulti/YMonoa
Isomerization
Cracking
0.4%Pt/Hb
190 210 220 230 240 250 270
9.8 21.5 37.7 64.3 81.5 90.7 96.2
8.9 20.2 33.5 50.9 45.0 24.9 4.5
0.08 0.18 0.25 0.32 0.41 0.55 0.62
91.2 94.0 88.8 79.1 55.2 27.4 4.7
0.8 3.9 9.7 19.8 43.9 71.7 94.3
8.0 2.1 1.5 1.1 0.9 0.9 1.0
0.4%Pt/0.7%Cr-Hb
190 210 220 230 240 250 270
6.5 25.1 45.4 72.1 85.7 89.9 96.5
5.2 24.0 43.9 68.8 75.6 65.3 23.6
0.02 0.07 0.14 0.31 0.47 0.56 0.64
80.1 95.4 96.6 95.4 88.2 72.6 24.5
0.8 0.8 1.1 3.6 10.9 26.5 74.4
19.1 3.8 2.3 1.0 0.9 0.9 1.1
Selectivity/% Cyclization
a Ratio of the yield of multi-isoheptanes to that of mono-isoheptanes. Multi-isoheptanes include 2,2-dimethylpentane (98), 2,3-dimethylpentane (91), 2,4dimethylpentane (83), 3,3-dimethylpentane (81) and 2,2,3-trimethylbutane (112), and mono-isoheptanes include 2-methylhexane (42), 3-methylhexane (52) and 3ethylpentane (65). The number in parentheses is the research octane number.
as by the decrease of the surface area and pore volume (Table 1) that would hinder the access of reactant to acid sites [34]. Moreover, the promotion effect of Cr is suggested to associate with the improved reducibility of Pt species (Fig. 3) and enhanced Pt dispersion (Table 1), which would be favorable to the promotion of hydrogenation–dehydrogenation activity at metal sites. Fig. 6 gives the influence of Pt loadings for nPt/0.7%Cr-Hb catalysts on the catalytic conversion and selectivity at 230 8C and 120 min. At low Pt loadings, the conversion of n-heptane increased quickly with the increase of supported Pt; it reached a plateau value of around 73% when the Pt loading was up to 0.4%. As the Pt loading exceeded 0.2%, the isomerization selectivity amounted to a high level, around 95.0%. This result is in agreement with the expectation of the classical bifunctional reaction mechanism for the hydroisomerization of paraffins [55,56]. Owing to the rather high conversion and the highest isomerization selectivity obtained over Pt/0.7%Cr-Hb, 0.4% is considered to be the preferred loading of Pt. 3.4. Effect of WHSV and reaction temperature Fig. 7 presents the influence of WHSV on the catalytic behavior of 0.4%Pt/0.7%Cr-Hb catalyst in hydroisomerization of n-heptane at 230 8C and 120 min. It can be seen that, with the increase of WHSV, the conversion of n-heptane, the yield of multi-branched products and the selectivity to cracking decreased gradually, whereas the yield of mono-branched products reached a maximum value at a medium WHSV. As for selectivity to isomerized products, it increased at low WHSV and then stayed at a high level of 95% when WHSV was beyond 2.7 h1. Only a trace amount of cyclized products was observed at those conditions. Therefore, one can conclude that 2.7 h1 is the suitable WHSV for this catalyst. Table 2 compares the catalytic activity of 0.4%Pt/0.7%Cr-Hb with that of 0.4%Pt/Hb at different reaction temperatures. It can be seen that, with the increase of reaction temperature, conversion of n-heptane rose very quickly for both catalysts. Similar conversions were observed over the two catalysts when the reaction temperature was very low or very high; however, at middle temperatures of 220–240 8C, 0.4%Pt/0.7%Cr-Hb showed an obviously higher conversion. At low temperatures, selectivity to isomerization increased slowly due to the gradual disappearance of cyclized products, but at high temperatures it decreased dramatically because of the creation of more and more cracking products. Compared to 0.4%Pt/Hb, 0.4%Pt/0.7%Cr-Hb generally exhibited a much higher selectivity to isomerization at any
temperature. 230 8C would be the optimal reaction temperature for 0.4%Pt/0.7%Cr-Hb catalyst, at which the very high isomerization selectivity of 95.4% coupled with the high conversion of 72.1% and high yield of isomerization products of 68.8% was achieved. In addition, it seems that the high temperature was more beneficial for the creation of multi-branched heptanes proposed from the gradual increase of the ratio YMulti/YMono. 4. Conclusions In hydroisomerization of n-heptane, Pt catalysts supported on Cr(III)-exchanged Hb zeolite are revealed to exhibit both a much higher selectivity to isomerized products and a significantly higher catalytic conversion than the parent catalyst 0.4%Pt/Hb. Cr(III) species that are introduced into 0.4%Pt/Hb zeolite by ion-exchange are able to increase the number and the strength of acid sites, to enhance the reducibility of supported Pt, and to improve Pt dispersion, which leads to a better balance between metal and acid sites in the bifunctional catalyst and thus promotes the catalytic performance. The optimal Pt and Cr loadings are 0.4% and 0.7%, respectively. This composite catalyst turns out to be a highly efficient catalyst for hydroisomerization of n-heptane at a reaction temperature of 230 8C, WHSV of 2.7 h1, and molar ratio of H2 to nheptane of 7.9, presenting high conversion of n-heptane (72.1%) and very high selectivity to isomerized heptanes (95.4%). Acknowledgments The authors thank Jiangsu Provincial Key Natural Science Foundation for Universities (06KJA53012), and National Natural Science Foundation of China (Nos. 20476046 and 20776069). References [1] A. Corma, Catal. Lett. 22 (1993) 33–52. [2] C. Marcilly, Stud. Surf. Sci. Catal. 135 (2001) 37–60. [3] X.-T. Ren, N. Li, J.-Q. Cao, Z.-Y. Wang, S.-Y. Liu, S.-H. Xiang, Appl. Catal. A: Gen. 298 (2006) 144–151. [4] A. Chica, A. Corma, J. Catal. 187 (1999) 167–176. [5] M.A. Arribas, A. Martinez, Catal. Today 65 (2001) 117–122. [6] B. Parlitz, E. Schreier, H.-L. Zubowa, R. Eckelt, E. Lieske, G. Lischke, R. Fricke, J. Catal. 155 (1995) 1–11. [7] H.-L. Zubowa, G. Lischke, B. Parlitz, E. Schreier, R. Eckelt, G. Schulz, R. Fricke, Appl. Catal. A 110 (1994) 27–38. [8] T.D. Pope, J.F. Kriz, M. Stanciulescu, J. Monnier, Appl. Catal. A: Gen. 233 (2002) 45– 62. [9] A. Patrigeon, E. Benazzi, C. Travers, J.Y. Bernhard, Catal. Today 65 (2001) 149–155.
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Pt catalysts supported on b zeolite ion-exchanged with Cr(III) for hydroisomerization of n-heptane Ping Liu, Xingguang Zhang, Yue Yao, Jun Wang * State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing 210009, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 July 2009 Received in revised form 28 September 2009 Accepted 29 September 2009 Available online 7 October 2009
A series of Pt catalysts supported on Hb zeolite ion-exchanged with Cr(III) were prepared, characterized by ICP, XRD, BET, NH3-TPD, H2-TPR and H2-chemisorption techniques, and evaluated in hydroisomerization of n-heptane with an atmospheric fixed-bed flow reactor. Pt catalyst supported on Hb zeolite exhibits both lower selectivity to isomerized products and lower conversion of n-heptane than the counterparts supported on Cr(III)-exchanged Hb zeolite. The optimal composition for Cr-bearing catalysts is 0.4 wt.% of Pt loading and 0.7 wt.% of Cr loading. This catalyst proves to be highly efficient in the hydroisomerization of n-heptane, giving a very high selectivity to isomerized products: 95.4% coupled with a high conversion of n-heptane: 72.1%. The substantial promotion effect of ion-exchanged Cr(III) species is discussed in relation to catalyst physicochemical properties. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Hydroisomerization of n-heptane b Zeolite Bifunctional catalyst Pt Cr
1. Introduction Hydroisomerization of paraffins plays an increasingly important role in refineries to boost the octane number of the gasoline pool due to the stringent environmental requirements. Its influence exceeds those of other approaches such as blending with oxygenates and aromatics [1–3]. As one of the fractions in naphtha, n-heptane shows zero octane number, and its isomerization attracts great attention. However, Pt/mordenite commercially used as the catalyst for hydroisomerization of C5/C6 into light isoparaffins would readily cause cracking in hydroconversion of C7 [4,5]. The seeking of new catalysts for hydroisomerization of nheptane is therefore a requisite to enhance the selectivity to branched paraffins. Metal/acid bifunctional catalysts, especially the noble metal Pt- or Pd-bearing catalysts supported on SAPO molecular sieves [6,7], Y zeolite [8,9], b zeolite [10–13], SO42/ZrO2 [14,15] and heteropolyacid [16,17] have been extensively investigated. Among them, Pt/Hb is considered to be a promising candidate due to the favorable acidity and the unique structure of its three-dimensional interconnected channels of b zeolite [18– 21]. Nevertheless, the catalytic conversion is still not sufficiently high simultaneously with a high selectivity to isoheptanes [12,13]. In order to achieve higher activities, attempts are made to modify
* Corresponding author. Tel.: +86 25 83172264; fax: +86 25 83172261. E-mail address: [email protected] (J. Wang). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.045
the zeolite supported Pt catalysts [22–34]. In these studies, the introducing of a second metal as the promoter into supported noble metal catalysts was demonstrated to be an effective way to improve catalytic conversion, selectivity and stability for isomerization of n-paraffins [29–34]. Mao et al. [31] reported an enhanced yield of branched paraffins by the incorporation of a small amount of Al3+ species into HY supported Pt catalysts. Fu´nez et al. [34] observed a remarkable increase in conversion of n-heptane and selectivity to multi-branched isomers due to the addition of Ni into Pt/Hb catalyst. Recently, we prepared Pt-bearing catalysts supported on b or dealuminated b zeolite doped with a second metal of Cr by co-impregnation for hydroisomerization of nheptane, and found the enhanced catalytic activity [35,36]. However, in those previous reports, noble metal/zeolite catalysts promoted by a second metal were always prepared by impregnation. It is well known that ion-exchange is also an effective and commonly used approach to modify zeolite [37]. The presence of highly charged metal cations introduced into zeolite channels and/or cages through ion-exchange leads to the change of zeolite acidity [37,38]. Moreover, ion-exchanged metal cations favor the adsorption of alkanes on zeolite by the interaction between the adsorbed molecules and the ion-exchanged cations [39,40]. Based on the above considerations, it is reasonable to prepare and evaluate the noble metal/zeolite catalysts modified by ion-exchanged metal cations for hydroisomerization of n-heptane, but this has been overlooked. In this study, we prepare Cr(III) ionexchanged b zeolite supported Pt catalysts for hydroisomerization
P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147
of n-heptane, and reveal a highly improved isomerization selectivity for isoheptanes with a high catalytic conversion of nheptane due to the promotion effect of ion-exchanged Cr(III). 2. Experimental 2.1. Preparation of catalysts The hydrogen form of b zeolite (Hb) was prepared by the repeated ion-exchange of Nab zeolite (Fushun No. 3 Petrochemical Factory, CNPC, SiO2/Al2O3 molar ratio 29) with aqueous solution of NH4Cl (1.0 mol L1) at 90 8C, followed by washing with deionized water, drying at 100 8C for 12 h, and calcination at 550 8C for 3 h. nPt/mCr-Hb catalysts (n and m stand for Pt and Cr loading by weight percentage, respectively) were prepared via ion-exchange of Hb with chromium(III) nitrate and impregnation with chloroplatinic acid. In detail, Hb was ion-exchanged in aqueous solution of chromium(III) nitrate at 90 8C for 2 h with the mass ratio of solid to liquid 1:100, followed by washing with water, drying at 100 8C for 12 h and calcination at 400 8C for 3 h. Then, the obtained solid was immersed in aqueous solution of chloroplatinic acid, dried and calcinated once more. In this procedure, the concentration of Cr for the aqueous solution of chromium(III) nitrate was changed to adjust the Cr loadings of catalysts, which were determined using a Jarrell–Ash 1100 inductively coupled plasma (ICP) spectroscope. Totally six different Cr loadings by the weight percentage were obtained, i.e., 0.2%, 0.3%, 0.4%, 0.7%, 0.9%, and 1.3%, corresponding to the Cr/Al molar ratio of 0.04, 0.05, 0.07, 0.13, 0.16, and 0.23, respectively. Also, the concentration of Pt for aqueous solution of chloroplatinic acid was changed to make different loadings of Pt (0.05–1.0%) in catalysts, which were calculated directly according to the amount of Pt used in impregnation. 2.2. Characterization X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation at 40 kV and 30 mA with a scan rate of 28 min1 and a scanning range of 3–508. BET surface areas and pore volumes were measured using a Micromeritics ASAP2020 analyzer with nitrogen adsorption at 196 8C. The samples were pretreated at 200 8C in a vacuum of 0.01 kPa for 4 h. Temperature-programmed desorption of ammonia (NH3-TPD), temperature-programmed reduction of hydrogen (H2-TPR), and Pt dispersion measurements were conducted using a JAPAN BELCAT-Analyzer. For NH3-TPD, 200 mg of sample was pretreated at 500 8C for 2 h in a flow of helium, followed by adsorption of ammonia at 100 8C. NH3TPD was then performed at a heating rate of 10 8C min1. For H2TPR, the sample loaded in a quartz tube was pretreated at 400 8C for 1 h in a flow of oxygen, followed with cooling to 60 8C and sweeping with argon for 20 min. While the sample was heated at a mixture of hydrogen and argon at a heating rate of 10 8C min1 up to 700 8C, the temperature and consumption of hydrogen were recorded, respectively. The dispersion of Pt was determined using a pulsed technique of hydrogen chemisorption, based on 1:1 stoichiometry (H/Pt) according to the previous work [41]. Two hundred milligram of the catalyst was reduced in hydrogen flow at 400 8C for 2 h, and then swept under argon flow at 450 8C for 0.5 h. Afterwards, the catalyst was cooled down to 50 8C and the pulse of 1 mL of the H2/Ar mixture (10.2% H2) was repeatedly injected into the reactor via a six-way valve until the hydrogen signals from the thermal conductivity detector remained at a constant value. The volume of hydrogen chemisorbed was determined by summing the fractions of hydrogen consumed in each pulse.
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2.3. Catalytic test Hydroisomerization of n-heptane was carried out with an atmospheric fixed-bed flow reactor. A measured amount of the granular catalyst (0.565 g, 0.25–0.42 mm) was charged in the middle of the stainless steel tubular reactor. Before the starting of the reaction, the catalyst was reduced in situ in H2 flow at 300 8C for 3 h. The typical reaction conditions were as follows: reaction temperature 190–270 8C, weight hourly space velocity (WHSV) 2.7 h1, and molar ratio of H2 to n-heptane 7.9. The product mixture was quantitatively analyzed online with a gas chromatograph (Shimadzu GC-2014) equipped with a SE30 capillary column (50 m 0.25 mm 0.3 mm) and FID. The products were qualitatively confirmed by GC–MS (ThermoFinnigan). 3. Results and discussion 3.1. Characterization of various catalysts 3.1.1. XRD results XRD patterns of various catalysts are illustrated in Fig. 1. It shows that the diffraction curves of all Cr(III) exchanged Hb supported Pt catalysts were identical to that of their parent sample Hb, indicating a high comparative crystallinity of BEA zeolite phase after the loading of Cr and Pt. No diffraction peaks assigned to Pt and Cr species occur for any of the nPt/mCr-Hb catalysts, even at a high Pt loading of 0.8% or a high Cr loading of 1.3%. Such absence suggests a high dispersion of Pt on the surface of support, as well as a high dispersion of Cr ions in zeolite channels and cavities. 3.1.2. NH3-TPD NH3-TPD profiles of 0.4%Pt/mCr-Hb catalysts are shown in Fig. 2. Two peaks occurred for all samples, corresponding to desorption of ammonia on weak and strong acid sites. With the increase of the ion-exchanged Cr(III) in 0.4%Pt/mCr-Hb catalysts, the peak for strong acidity shifted gradually from 384 8C toward 408 8C. At the same time, the acid amount for strong acidity also increased. Based on the Plank–Hirschler mechanism [42,43], trivalent metal cations with high ionic potential can readily undergo hydrolysis to form new Bro¨nsted acid sites and acidic metal hydroxyl groups as follows: [Cr(H2O)n]3+ ! [Cr(OH)2]+ + 2H+. The generation of the stronger acid sites for the trivalent metal ion (M3+)-exchanged zeolites can be explained by different suppositions; for example, [M(OH)2]+
Fig. 1. XRD patterns of Hb and nPt/mCr-Hb catalysts. (a) Hb, (b) 0.4%Pt/Hb, (c) 0.4%Pt/0.3%Cr-Hb, (d) 0.4%Pt/0.7%Cr-Hb, (e) 0.4%Pt/1.3%Cr-Hb, (f) 0.6%Pt/0.7%CrHb, and (g) 0.8%Pt/0.7%Cr-Hb.
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P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147 Table 1 BET surface areas, pore volumes and Pt dispersions of selected catalysts. Catalyst
SBET/(m2 g1)
Pore volume/(cm3 g1)
Pt dispersion/%
Hb 0.4%Pt/Hb 0.4%Pt/0.3%Cr-Hb 0.4%Pt/0.7%Cr-Hb 0.4%Pt/1.3%Cr-Hb
578 564 568 543 524
0.2451 0.2357 0.2374 0.2265 0.2218
– 33 87 79 27
around 407 and 550 8C. It is clear that the curve for 0.4%Pt/0.7%CrHb was not a result of simply overlapping the profiles for the two monometallic samples. The occurrence of those peaks for 0.4%Pt/ 0.7%Cr-Hb is indicative of the strong interaction between Cr and Pt, i.e., the formation of Pt–Cr species, which are clearly in different status [50]. This implies that the reducibility of at least a part of Pt species in 0.4%Pt/0.7%Cr-Hb is dramatically improved by the presence of ion-exchanged Cr species.
Fig. 2. NH3-TPD profiles of 0.4%Pt/mCr-Hb catalysts. (a) 0.4%Pt/Hb, (b) 0.4%Pt/ 0.2%Cr-Hb, (c) 0.4%Pt/0.3%Cr-Hb, (d) 0.4%Pt/0.4%Cr-Hb, (e) 0.4%Pt/0.7%Cr-Hb, (f) 4%Pt/0.9%Cr-Hb, and (g) 0.4%Pt/1.3%Cr-Hb.
species are the source of the stronger acidity [44], or alternatively, the polarizing and inductive effects of the introduced metal cations (M3+ and [M(OH)]2+) would weaken the OH bonds of the bridging hydroxyl groups and make them more acidic [45–47]. This may account for the enhanced strong acidity in both acid strength and acid number for Cr(III)exchanged zeolitic catalysts. 3.1.3. H2-TPR Fig. 3 shows H2-TPR profiles for 0.4%Pt/Hb, 0.7%Cr-Hb and 0.4%Pt/0.7%Cr-Hb catalysts. 0.4%Pt/Hb displayed a hydrogen consumption peak at around 450 8C, indicating the strong interaction between Pt2+ and b zeolite with the formation of PtðOSiBBÞ2y species [48,49]. 0.7%Cr-Hb presented a large peak y at 393 8C, which is assigned to the reduction of Cr(III) species [50,51]. The bimetallic catalyst 0.4%Pt/0.7%Cr-Hb exhibited four reduction peaks: two large at around 161 and 271 8C; two small at
3.1.4. Surface area and pore volume Table 1 lists the BET surface areas and pore volumes for selected catalysts. It can be seen that Hb exhibited a high surface area and pore volume. Upon the loading of Pt and Cr to Hb zeolite, surface areas and pore volumes decreased only marginally. On the other hand, with the increase of Cr loading, surface areas and pore volumes decreased slowly; at the high Cr loading of 1.3%, they reduced significantly to 524 m2 g1 and 0.2218 cm3 g1, respectively. 3.1.5. Pt dispersion The Pt dispersion on the Cr-free support and the Cr(III)exchanged counterparts are also compared in Table 1. It can be seen that 0.4%Pt/Hb showed the Pt dispersion of 33%, and in contrast, after b zeolite was ion-exchanged by Cr(III), with loadings of 0.3% and 0.7%, remarkable increases of Pt dispersions were observed (87% and 79%). It is noticeable that the interaction between the supported Pt species and acid sites has been claimed by previous reports [52,53]. Also, the H2-TPR results in Fig. 3 have indicated the presence of the new Pt–Cr species. One thus can suggest that for the b zeolite ion-exchanged by Cr(III), it is the reaction of the Cr-related acid sites such as [Cr(OH)2]+ [42,43] with Pt species that causes the formation of the Pt–Cr species. The existence of this Pt–Cr species may greatly hinder the Pt active sites from sintering during the high temperature treatment for the catalyst, which would account for the high Pt dispersions for the Cr-containing samples. However, by introducing excessive Cr, the Pt dispersion decreased seriously: a low Pt dispersion of 27% was found for the sample with a high Cr loading of 1.3%. This result is still not understandable. 3.2. Catalytic stability
Fig. 3. H2-TPR profiles of (a) 0.4%Pt/Hb, (b) 0.7%Cr-Hb, and (c) 0.4%Pt/0.7%Cr-Hb.
Conversions of n-heptane in the hydroisomerization of nheptane over various catalysts at 230 8C as a function of reaction time on stream are displayed in Fig. 4. It can be seen that the reaction reached a steady stage at 120 min of reaction time on stream over all catalysts, although some of them deactivated more or less at the very beginning of the reaction. For 0.4%Pt/Hb, the conversion decreased considerably within 80 min. Comparatively, Cr(III)-exchanged b zeolitic catalysts (Cr loading: 0.2–0.7%) showed much less deactivation, indicating that the introduction of Cr improves the catalytic stability. However, for 0.4%Pt/1.3%CrHb with high Cr loading, the conversion of n-heptane decreased drastically to the lowest stable value within 40 min, even though it gave a very high initial activity. These results prompt us to compare their catalytic activity at 120 min of TOS in the following measurements.
P. Liu et al. / Applied Catalysis A: General 371 (2009) 142–147
Fig. 4. Conversion of n-heptane over catalysts with different Cr loadings at 230 8C as a function of reaction time on stream (TOS). (&) 0.4%Pt/Hb; ($) 0.4%Pt/0.2%Cr-Hb; (*) 0.4%Pt/0.3%Cr-Hb; (5) 0.4%Pt/0.4%Cr-Hb; (~) 0.4%Pt/0.7%Cr-Hb; (!) 0.4%Pt/ 0.9%Cr-Hb; (^) 0.4%Pt/1.3%Cr-Hb.
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Fig. 6. Conversion of n-heptane and isomerization selectivity over nPt/0.7%Cr-Hb catalysts with different Pt loadings. (&) Conversion of n-heptane; (!) selectivity to isomerization.
Fig. 5 shows the conversion of n-heptane and selectivity to isomerization as a function of Cr loadings for 0.4%Pt/mCr-Hb (m = 0–1.3%) at 230 8C and 120 min. For 0.4%Pt/Hb, considerable conversion of n-heptane of 64.3% was obtained, but with a low selectivity to isomerization of 79.1%. When Cr was introduced into 0.4%Pt/Hb, generally speaking, both conversion and selectivity increased a lot. In detail, in the low Cr loading range, the conversion increased quickly and reached the highest value of 82.5% at the Cr loading of 0.3%; in the high Cr loading range, it decreased slowly. Meanwhile, the selectivity to isomerized products always increased with the increase of Cr loading. Compromising conversion and selectivity, one can conclude that 0.4%Pt/0.7%Cr-Hb and 0.4%Pt/0.9%Cr-Hb are preferred catalysts. Over 0.4%Pt/0.7%Cr-Hb, both high conversion of n-heptane 72.1% and high selectivity to isoheptanes 95.4% were obtained. On the other hand, for 0.7%CrHb, the conversion and isomerization selectivity gave very low level of 4.4% and 43.5%, respectively, demonstrating that Cr species
themselves do not serve as the metallic active centers in hydroisomerization of n-heptane. For hydroisomerization of paraffins over metal/acid bifunctional catalysts, the paraffin transformation involves hydrogenation– dehydrogenation on metal sites, isomerization and/or cracking on acid sites, and diffusion of the olefinic intermediates from acid to metal sites and inversely [54]. When acid sites are not sufficient, the reaction on acid sites is the rate-determining step. On the contrary, in cases of scarcity of the available metal sites, the hydrogenation– dehydrogenation becomes the rate-determining step. Based on the above proposal, 0.4%Pt/Hb most possibly provides large amounts of acid sites and is short of metal sites because of the low Pt dispersion. It is thus understandable that the increase of conversion and isomerization selectivity at the low Cr loadings 0.3% is due to the remarkable increase of the Pt dispersion (Table 1). With the further increase of the Cr loading to 0.7%, acid strength and amount also increase (Fig. 2), but the Pt dispersion no longer increases simultaneously (Table 1), which may be the reason for the high activity. In this case, we think that the metal and acid sites match well to each other. For 0.4%Pt/1.3%Cr-Hb, the lowest conversion of nheptane might be caused by the very low Pt dispersion of 27%, as well
Fig. 5. Conversion of n-heptane and isomerization selectivity over the 0.4%Pt/mCrHb catalysts with different Cr loadings. (&) Conversion of n-heptane for 0.4%Pt/ mCr-Hb; (!) selectivity to isomerization for 0.4%Pt/mCr-Hb; (§) conversion of nheptane for 0.7%Cr-Hb; ($) selectivity to isomerization for 0.7%Cr-Hb.
Fig. 7. Influence of WHSV on catalytic performance of 0.4%Pt/0.7%Cr-Hb in the hydroisomerization of n-heptane. (&) Conversion of n-heptane; (*) multibranched yield; (~) mono-branched yield; (!) isomerization selectivity; ($) cracking selectivity; (&) cyclization selectivity.
3.3. Effect of Cr and Pt loadings for Pt/Cr-Hb catalysts
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Table 2 Comparison of reactivity of hydroisomerization of n-heptane over 0.4%Pt/Hb and over 0.4%Pt/0.7%Cr-Hb catalysts at different reaction temperatures. Catalyst
T/8C
Conversion/%
Isomerization yield/%
YMulti/YMonoa
Isomerization
Cracking
0.4%Pt/Hb
190 210 220 230 240 250 270
9.8 21.5 37.7 64.3 81.5 90.7 96.2
8.9 20.2 33.5 50.9 45.0 24.9 4.5
0.08 0.18 0.25 0.32 0.41 0.55 0.62
91.2 94.0 88.8 79.1 55.2 27.4 4.7
0.8 3.9 9.7 19.8 43.9 71.7 94.3
8.0 2.1 1.5 1.1 0.9 0.9 1.0
0.4%Pt/0.7%Cr-Hb
190 210 220 230 240 250 270
6.5 25.1 45.4 72.1 85.7 89.9 96.5
5.2 24.0 43.9 68.8 75.6 65.3 23.6
0.02 0.07 0.14 0.31 0.47 0.56 0.64
80.1 95.4 96.6 95.4 88.2 72.6 24.5
0.8 0.8 1.1 3.6 10.9 26.5 74.4
19.1 3.8 2.3 1.0 0.9 0.9 1.1
Selectivity/% Cyclization
a Ratio of the yield of multi-isoheptanes to that of mono-isoheptanes. Multi-isoheptanes include 2,2-dimethylpentane (98), 2,3-dimethylpentane (91), 2,4dimethylpentane (83), 3,3-dimethylpentane (81) and 2,2,3-trimethylbutane (112), and mono-isoheptanes include 2-methylhexane (42), 3-methylhexane (52) and 3ethylpentane (65). The number in parentheses is the research octane number.
as by the decrease of the surface area and pore volume (Table 1) that would hinder the access of reactant to acid sites [34]. Moreover, the promotion effect of Cr is suggested to associate with the improved reducibility of Pt species (Fig. 3) and enhanced Pt dispersion (Table 1), which would be favorable to the promotion of hydrogenation–dehydrogenation activity at metal sites. Fig. 6 gives the influence of Pt loadings for nPt/0.7%Cr-Hb catalysts on the catalytic conversion and selectivity at 230 8C and 120 min. At low Pt loadings, the conversion of n-heptane increased quickly with the increase of supported Pt; it reached a plateau value of around 73% when the Pt loading was up to 0.4%. As the Pt loading exceeded 0.2%, the isomerization selectivity amounted to a high level, around 95.0%. This result is in agreement with the expectation of the classical bifunctional reaction mechanism for the hydroisomerization of paraffins [55,56]. Owing to the rather high conversion and the highest isomerization selectivity obtained over Pt/0.7%Cr-Hb, 0.4% is considered to be the preferred loading of Pt. 3.4. Effect of WHSV and reaction temperature Fig. 7 presents the influence of WHSV on the catalytic behavior of 0.4%Pt/0.7%Cr-Hb catalyst in hydroisomerization of n-heptane at 230 8C and 120 min. It can be seen that, with the increase of WHSV, the conversion of n-heptane, the yield of multi-branched products and the selectivity to cracking decreased gradually, whereas the yield of mono-branched products reached a maximum value at a medium WHSV. As for selectivity to isomerized products, it increased at low WHSV and then stayed at a high level of 95% when WHSV was beyond 2.7 h1. Only a trace amount of cyclized products was observed at those conditions. Therefore, one can conclude that 2.7 h1 is the suitable WHSV for this catalyst. Table 2 compares the catalytic activity of 0.4%Pt/0.7%Cr-Hb with that of 0.4%Pt/Hb at different reaction temperatures. It can be seen that, with the increase of reaction temperature, conversion of n-heptane rose very quickly for both catalysts. Similar conversions were observed over the two catalysts when the reaction temperature was very low or very high; however, at middle temperatures of 220–240 8C, 0.4%Pt/0.7%Cr-Hb showed an obviously higher conversion. At low temperatures, selectivity to isomerization increased slowly due to the gradual disappearance of cyclized products, but at high temperatures it decreased dramatically because of the creation of more and more cracking products. Compared to 0.4%Pt/Hb, 0.4%Pt/0.7%Cr-Hb generally exhibited a much higher selectivity to isomerization at any
temperature. 230 8C would be the optimal reaction temperature for 0.4%Pt/0.7%Cr-Hb catalyst, at which the very high isomerization selectivity of 95.4% coupled with the high conversion of 72.1% and high yield of isomerization products of 68.8% was achieved. In addition, it seems that the high temperature was more beneficial for the creation of multi-branched heptanes proposed from the gradual increase of the ratio YMulti/YMono. 4. Conclusions In hydroisomerization of n-heptane, Pt catalysts supported on Cr(III)-exchanged Hb zeolite are revealed to exhibit both a much higher selectivity to isomerized products and a significantly higher catalytic conversion than the parent catalyst 0.4%Pt/Hb. Cr(III) species that are introduced into 0.4%Pt/Hb zeolite by ion-exchange are able to increase the number and the strength of acid sites, to enhance the reducibility of supported Pt, and to improve Pt dispersion, which leads to a better balance between metal and acid sites in the bifunctional catalyst and thus promotes the catalytic performance. The optimal Pt and Cr loadings are 0.4% and 0.7%, respectively. This composite catalyst turns out to be a highly efficient catalyst for hydroisomerization of n-heptane at a reaction temperature of 230 8C, WHSV of 2.7 h1, and molar ratio of H2 to nheptane of 7.9, presenting high conversion of n-heptane (72.1%) and very high selectivity to isomerized heptanes (95.4%). Acknowledgments The authors thank Jiangsu Provincial Key Natural Science Foundation for Universities (06KJA53012), and National Natural Science Foundation of China (Nos. 20476046 and 20776069). References [1] A. Corma, Catal. Lett. 22 (1993) 33–52. [2] C. Marcilly, Stud. Surf. Sci. Catal. 135 (2001) 37–60. [3] X.-T. Ren, N. Li, J.-Q. Cao, Z.-Y. Wang, S.-Y. Liu, S.-H. Xiang, Appl. Catal. A: Gen. 298 (2006) 144–151. [4] A. Chica, A. Corma, J. Catal. 187 (1999) 167–176. [5] M.A. Arribas, A. Martinez, Catal. Today 65 (2001) 117–122. [6] B. Parlitz, E. Schreier, H.-L. Zubowa, R. Eckelt, E. Lieske, G. Lischke, R. Fricke, J. Catal. 155 (1995) 1–11. [7] H.-L. Zubowa, G. Lischke, B. Parlitz, E. Schreier, R. Eckelt, G. Schulz, R. Fricke, Appl. Catal. A 110 (1994) 27–38. [8] T.D. Pope, J.F. Kriz, M. Stanciulescu, J. Monnier, Appl. Catal. A: Gen. 233 (2002) 45– 62. [9] A. Patrigeon, E. Benazzi, C. Travers, J.Y. Bernhard, Catal. Today 65 (2001) 149–155.
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