synergistic effect between sn and k promoters on supported platinum catalyst for isobutane...

Journal of Natural Gas Chemistry 20(2011)639–646

Synergistic effect between Sn and K promoters on supported platinumcatalyst for isobutane dehydrogenation

Yiwei Zhang1, Yuming Zhou1∗, Lihui Wan1, Mengwei Xue1,2, Yongzheng Duan1, Xuan Liu11. School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, Jiangsu, China;

2. Biochemical and Environmental Engineering College, Nanjing Xiaozhuang University, Nanjing 211171, Jiangsu, China[Manuscript received March 28, 2011; revised September 23, 2011 ]

AbstractCatalytic dehydrogenation of isobutane has recently received considerable attention because of the increasing demand for isobutene. In thisstudy, the synergistic effect between Sn and K on PtSnK/γ-Al2O3 catalysts has been investigated by changing the content of Sn. It was foundthat with the presence of potassium, suitable addition of Sn could not only increase the metal dispersion, but also reduce the catalyst acidity.In these cases, the synergistic effect could also strengthen the interactions between the metal and support, which resulted in an increase inboth catalytic activity and stability. In our experiments, Pt-0.6SnK/Al catalyst exhibited the lowest deactivation rate (12.4%) and showed aselectivity to isobutene higher than 94% at the isobutane conversion of about 45.3% after running the reaction for 6 h. However, with theexcessive loading of Sn, surface property of active sites and the interactions between metal and support were changed. As a result, the initialoptimal ratio between the metallic function and acid function would be destroyed, which was disadvantageous to the reaction.

Key wordssynergistic effect; Pt-Sn; isobutane dehydrogenation

1. Introduction

The catalytic dehydrogenation of isobutane is of increas-ing importance because of the growing demand for isobutene[1−3]. Isobutene is used for the production of methyl tertiarybutyl ether (MTBE) and methacrylates, which are used as ad-ditive to gasoline to enhance the octane number. However, itis known that the reaction of isobutane dehydrogenation is anendothermic process, which requires a relatively high reactiontemperature to obtain a high yield of isobutene. Therefore, thedeactivation of the catalyst due to coke formation is inevitablebecause of the rigorous reaction conditions.

Platinum-based mono- and bimetallic catalysts supportedon γ-Al2O3 are broadly used in dehydrogenation processes.Compared to monometallic Pt/Al2O3 catalysts, supportedbimetallic platinum-tin catalysts show relatively high reactionactivity and selectivity [4,5]. Important progresses in the PtSncatalyst have been reported in a large number of papers [6−8].Generally, the catalytic properties of the PtSn bimetallic cat-alysts strongly depend on the interaction between Pt and Sn,and on the chemical state of Sn [6]. Nevertheless, during the

process of dehydrogenation, the acidity of the alumina usedas support can catalyze the undesirable cracking and isomer-ization reactions [9]. Therefore, it is necessary to further im-prove the catalytic performance of PtSn catalysts during lightparaffin conversion.

Recently, several studies have been carried out to evalu-ate the modification of the stabilities, selectivities and activ-ities of the PtSn/γ-Al2O3 catalysts by the addition of alkalimetal promoters [10−12]. The way in which these promot-ers are added could influence the final properties, not only ofthe support but also of the metallic phases [13]. Tasbihi etal. [3] systematically investigated the effect of potassium andlithium additions on catalytic performance of PtSn/Al2O3 cat-alyst for isobutane dehydrogenation. They found that the cat-alytic performances of the catalysts for the dehydrogenationof isobutane were mainly related to their structural character-istics and electronic properties. Compared with the Pt-Sn-Lisupported catalyst, Pt-Sn-K counter part showed high selec-tivity. Siri and co-workers [6] studied the modifying effectsof alkaline metals on the acid properties of the support and onthe behavior of PtSn/γ-Al2O3 catalytic systems for isobutane

∗ Corresponding author. Tel: +86-25-52090617; Fax: +86-25-52090618; E-mail: [email protected] work was supported by the National Natural Science Foundation of China (21106017 and 50873026), Specialized Research Fund for the Doctoral

Program of Higher Education of China (20100092120047) and Production and Research Prospective Joint Research Project of Jiangsu Province of China (GrantNo.BY2009153).

Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(10)60250-2

640 Yiwei Zhang et al./ Journal of Natural Gas Chemistry Vol. 20 No. 6 2011

dehydrogenation. They reported that during the reaction theisopropyl alcohol transformation rate decreased sharply withthe addition of alkaline metals according to the sequence ofLi<Na<K. Besides, the addition of K improved the stabilitylevel of the catalyst obviously.

Nevertheless, it should be noted that most of the publishedwork mainly focus on the effects of alkali metal promoterson the catalyst acidity and catalytic performance of PtSn/γ-Al2O3 catalyst. The interactions between Sn and the alkalimetal promoter have rarely been studied. Actually, the cata-lyst system with the coexistence of the alkali metal promoterand platinum-tin is quite different from that of the bimetal-lic Pt-Sn one. The effect of Sn on these alkali metal-dopedPt-Sn catalysts may result in different behaviors, which mayaffect the catalytic performance significantly. Therefore, theinvestigation of this effect is becoming necessary.

In the present study, PtSnK/γ-Al2O3 catalysts withdifferent contents of Sn were prepared for the dehydrogena-tion of isobutane. The catalysts were studied by several tech-niques, including XRD, nitrogen adsorption, H2 chemisorp-tion, TEM, NH3-TPD, TPR and TPO. Particular attentionwas focused on the distinctive effect between Sn and K onPtSnK/γ-Al2O3 catalyst. Their structural characteristics wererelated to their reaction performances. This can provide usimportant information to understand the synergistic effect be-tween Sn and K promoters on the tri-metallic catalyst.

2. Experimental

2.1. Catalyst preparation

PtSnK/γ-Al2O3 catalysts were prepared by sequential im-pregnation method in our laboratory. Before the impregnationstep, the γ-Al2O3 support was calcinated in air at 550 ◦C for4 h. Subsequently, the support was impregnated with an aque-ous solution of KCl. After that, these catalysts were then co-impregnated in hydrochloric acid solution of H2PtCl6·6H2Oand SnCl2·2H2O. After totally dried, the catalysts were cal-cined in air at 500 ◦C for 4 h, and dechlorinated in air contain-ing water vapor at 500 ◦C for 4 h.

In all cases, the loadings of Pt and K were 0.5 and0.6 wt%, respectively. The catalysts were labeled as fol-lows: PtK/Al for platinum-potassium supported on aluminaand PtSnK/Al for platinum-tin-potassium supported on alu-mina. Catalysts were marked as 0.3Sn, 0.6Sn, 0.9Sn, 1.2Snand 2.0Sn, and the numbers indicates the weight percentagesof Sn loaded on the catalyst.

2.2. Catalyst characterization

X-ray diffraction (XRD) patterns of different sampleswere recorded with a Siemens D5000 using nickel filteredCu Kα radiation. The X-ray tube was operated at 40 kV and40 mA, and an angular range 2θ from 10o to 80o was recorded.

Catalyst BET surface area was measured using a Mi-cromeritics ASAP 2020 adsorptive and desorptive apparatus.

The samples were evacuated under vacuum of 5×10−3 Torr at350 ◦C for 15 h. Total specific surface areas were calculatedusing the BET equation.

The platinum dispersion was determined from chemisorp-tion measurements. This experiment was carried out usingthe dynamic-pulse technique with an argon (99.99%) flow of50 mL/min and pulses of hydrogen. The experimental processwas the same as that reported by Dorado et al. [14], except thatthe sample reduction temperature was 500 ◦C and the temper-ature of the argon gas for removing the hydrogen was 40 ◦Chigher than the reduction temperature.

TEM studies were conducted using a JEM-2010 micro-scope operated at 200 kV. The samples were prepared bygrinding, suspending, and sonicating in ethanol. A drop ofthe suspension was placed on a carbon copper grid, where thesolvent was evaporated. Approximately one hundred Pt par-ticles were measured for determination of an average particlesize.

NH3-TPD experiment was measured with a conventionalTPD apparatus. About 15 mg of sample was placed in aquartz reactor and saturated with ammonia at room temper-ature. TPD was carried out from 100 ◦C to 500 ◦C with aheating rate of 10 ◦C/min and with helium (30 mL/min) as thecarrier gas.

Temperature-programmedreduction (TPR) was measuredwith a conventional TPR apparatus. Prior to the TPR exper-iments, the catalysts were dried in flowing N2 at 400 ◦C for1 h. 5% H2/N2 was used as the reducing gas at a flow rate of40 mL·min−1. The temperature ramp in the TPR experimentwas 10 ◦C·min−1 up to 800 ◦C.

Coke was analyzed by thermogravimetric (TG) test andtemperature-programmed oxidation (TPO) experiment. Ther-mogravimetric analysis was measured in air flow (30 mL/min)with a LCT thermogravimetric analyzer (Beijing optical in-strument factory, China) from room temperature to 700 ◦C ata rate of 20 ◦C/min. 20 mg of catalyst was set in the analyzer.TPO was measured with the same apparatus as used for TPR.About 50 mg of sample was placed in a quartz reactor andthen heated up to 700 ◦C at a rate of 10 ◦C/min in a 5%O2/Hemixture (30 mL/min).

2.3. Catalytic reaction

The isobutane dehydrogenation was carried out in a con-ventional quartz tubular micro-reactor and all the catalystswere reduced in H2 at 500 ◦C for 8 h before catalytic eval-uation. Taking into account that the presence of hydrogenis very efficient in preventing coke formation on supportedPt catalyst [15], in our experiments, hydrogen is used as adiluent during the reaction. The reaction conditions wereas follows: 590 ◦C for reaction temperature, 0.1 MPa pres-sure, H2/iC4 = 1.0 (molar ratio) and gas hourly space velocity(GHSV) was 1500 h−1. The reaction products were analyzedwith an online GC-14C gas chromatography equipped with anactivated alumina packed column and a flame ionization de-tector (FID).

Journal of Natural Gas Chemistry Vol. 20 No. 6 2011 641

3. Results and discussion

3.1. Characterization of catalysts

The XRD patterns of the different samples are depictedin Figure 1. Among these patterns, the support shows the typ-ical hkl reflections characteristic of γ-Al2O3 phase, and theresult is comparable with that of the commercial γ-Al2O3 re-ported elsewhere [16]. Clearly, all the samples only exhibitthe characteristic peaks of γ-Al2O3, indicating that the origi-nal structure of the support was not destroyed during the pro-cess of impregnation. Furthermore, additional peaks corre-sponding to Pt, Sn and K are not observed, probably due tothe relatively low amounts of these metals present and/or be-cause of the high dispersion of them on the support. It is wellknown that the alumina phases are transformed accompaniedwith the increase of calcination temperature [17]. However,in our experiments the prepared catalysts were calcined at thesame temperature, thus it is unlikely for the alumina phasetransformation.

Figure 1. XRD patterns of different samples

Table 1 lists the basic characterization data of the differentcatalysts. Compared with PtK/Al catalyst, the presence of tinresults in the decrease in surface area. In general, the decreasein surface area of the alumina supports is mainly caused bythe pore blockages during impregnation process of the metalprecursors [18]. Thereby, with the increase of tin content,the possibility of Sn species plugging the pores of the sup-port increases. In addition, from Table 1, the values of metaldispersion (DH2) increase obviously when the content of Snincreases in the range of 0−0.6 wt%. However, the negativeeffect is found with the further addition of Sn, suggesting thatthe concentration of Sn has an obvious influence on the dis-persion of platinum particles. To explain these, it should benoted that the effects of K and Sn on surface metallic charac-ter are different on PtSnK/Al catalyst. As reported in literature[10], the presence of potassium can diminish the amount ofcoke formed on the catalyst (not only on the support, but alsoon the active metal sites), which in consequence increases thefraction of bare metallic Pt surface after carbon deposition. As

regards the role of Sn, it is more likely to prevent Pt agglom-eration since the increase in metal dispersion can reduce thenumber of large ensembles [19]. Therefore, when the loadingof K is held constant, the change of the metal dispersion can bemainly attributed to the dilution effect of Pt by Sn promoter.

Table 1. Characteristics of the catalysts

Atomic ratio BET surface DH2Catalyst Sn/Pt Sn/K area (m2/g) (%)PtK/Al 0 0 243 32

Pt-0.3SnK/Al 1.0 0.16 239 38Pt-0.6SnK/Al 2.0 0.33 236 43Pt-0.9SnK/Al 3.0 0.49 231 40Pt-1.2SnK/Al 4.0 0.66 229 36Pt-2.0SnK/Al 6.6 1.09 227 33

To further confirm the influence of Sn promoter on thedistribution of metallic particle, the TEM experiments ofthe corresponding catalysts are then investigated. As shownin Figure 2, it can be seen that on PtK/Al catalyst, metal-lic particles are not well distributed and the average size isaround 15 nm. In contrast, Pt-0.6SnK/Al sample containssmaller particles which are relatively homogeneously dis-tributed. Over this catalyst, the particle size distribution isnarrow and the average metallic particle size is determined tobe 10 nm, suggesting that the addition of suitable Sn is fa-vorable for the uniform distribution of metallic particles withthe presence of potassium. Clearly, these findings are in ac-cordance with the results of hydrogen chemisorption experi-ments. Generally, on bimetallic Pt-Sn catalyst, the long rangeorder of contiguous Pt atoms can be interrupted by Sn atoms[20]. In this way, the adjacent Pt atoms are separated by Snatoms, which results in the increased distance and decreasedparticle size. The particle size of Pt does not only influencethe catalytic activity, but also plays a crucial role in determin-ing selectivity and catalytic stability [20], which is expectedto have an obvious impact on catalytic performance.

The desorption of NH3 is used to characterize the acidicproperties of catalysts. Based on the NH3-TPD profilesof different samples (Figure 3), a semi-quantitative com-parison of the desorption strength distribution was achievedby deconvolution of the peaks using the Gaussian decon-volution method. The obtained results are listed in Ta-ble 2. It is obvious that all the samples exhibit threepeaks. The first peak around 190 ◦C (peak I) can be at-tributed to weak acid sites, the second centered at about250 ◦C (peak II) is assigned to medium acid sites, and thehighest temperature peak (III: 330–370 ◦C) is typically ofstrong acid sites [21]. Compared with the PtK/Al catalyst,a distinct decrease of ammonia desorption is observed onthe Sn-modified catalysts. The order of the acid amountis as follows: PtK/Al>Pt-0.3SnK/Al>Pt-0.6SnK/Al>Pt-0.9SnK/Al>Pt-1.2SnK/Al>Pt-2.0SnK/Al. Thus, it can beconsidered that the promoter of Sn can reduce the catalystacidity and the degree of this effect is related to the contentof Sn. On PtSnK/Al catalyst, the role of potassium is known


to reduce the support acidity due to its basic character [3,16].Therefore, the synergetic effect between potassium and tin topoison acidic sites on the alumina support might exist. How-ever, it should be pointed out that although the NH3-TPD re-sults show the acidity of the catalysts, the mass signal is verylow, and the effect of Sn to poison acidic sites is inferior tothat of K.

Figure 2. TEM micrographs of (a) PtK/Al and (b) Pt-0.6SnK/Al catalysts

Figure 3. NH3-TPD profiles of different samples

Table 2. NH3-TPD results over different catalysts

TM (◦C) Total area Peak fraction (%)CatalystI II III (a.u.) I II III

PtK/Al 193 250 359 9.52 17 35 48Pt-0.3SnK/Al 191 249 346 8.26 19 43 39Pt-0.6SnK/Al 195 245 340 7.05 18 23 59Pt-0.9SnK/Al 195 258 353 6.37 18 35 47Pt-1.2SnK/Al 195 256 353 6.18 15 32 53Pt-2.0SnK/Al 191 247 334 5.87 13 28 59

TPR profiles of the different samples are shown in Fig-ure 4. As can be seen, γ-Al2O3 support exhibits a relativelysharp reduction peak at about 750 ◦C and it is speculated thatit is linked with the reduction of OH groups or alumina ionson the surface of support [22]. When Pt was present, this peakshifts to a low temperature and another two reduction peaksare present: one with the maximum temperature at 300 ◦C andthe other at 480 ◦C. This is because two types of Pt oxidesmay exist on PtK/Al catalyst: one in a weak interaction withthe support and the other in a strong interaction with the sup-port [23,24]. When the three components of Pt, Sn (0.3 wt%)and K coexist, the profile presents a peak at about 280 ◦C,and also two other peaks at about 410 ◦C and 710 ◦C. Forthis complicated profile, the peak at 710 ◦C may contain thereduction of the corresponding component and also the char-acteristic reduction of the support. In general, the signal at280 ◦C is ascribed to reduction of Pt oxide species, whereasthe two high-temperature peaks represent the partial reductionof Sn4+ to Sn2+ and Sn2+ to Sn0 [25−27]. Moreover, mostof platinum species can be reduced to Pt0 during the courseof TPR experiment, whereas the consumed hydrogen can notreduce all Sn species [3,22]. When 0.6 wt% Sn is introduced,increasing amounts of hydrogen consumption for Pt specieswere found, which implies that more amounts of Pt particlesmay be reduced and then locate on the external surface of thesupport, thus increasing the metal dispersion. With the furtheraddition of Sn (0.9 and 1.2 wt%), it is interesting to note thatthe reduction temperatures of Pt oxides and the Sn speciesat high reduction temperatures shift towards the higher re-gion. These observations indicate that the interactions amongsupported Pt-oxide species, tin promoter and support becomestrengthened. However, when the concentration of Sn is ex-cessive (2.0 wt%), the reduction temperature for Pt speciesshifts towards the lower temperature and the intensity of thispeak decreases relatively sharply. Moreover, in this case, thepeak area corresponding to the reduction of Sn species at hightemperature increases effectively, meaning that more amountsof Sn0 species have been produced. Generally, on bimetallicPt-Sn catalyst, the state of Sn species has an obvious influenceon the catalytic properties. When Sn exists in a metallic state(Sn0), it may be a poison; when it exists in a nonmetallic state(Sn4+ or Sn2+), it acts as a promoter [22,23,27,28]. In thisway, the formation of PtSn alloy is inevitable, which resultedin the change of the surface metallic character and reducedmetal dispersion as revealed in Table 1. These results arein agreement with those reported by Cortright and Dumesic[29], who reported that a proportion of Sn interacted with Pt


to form Pt/Sn alloy particles in reduced Pt/Sn/K-L catalystswith an excess amount of Sn. On the other hand, since thesecond reduction peak of Sn species attributed to the com-plete reduction is located at higher temperature (about 700 ◦C)than the studied temperature (590 ◦C), the possibility of thistransformation under the reaction conditions is relatively lit-tle. Therefore, the influence of excessive amount of Sn oncatalytic performance can be mainly related to the changeof metallic character and the interactions between metaland support.

Figure 4. TPR profiles of different samples

3.2. Catalytic performances

Figure 5 presents the catalytic performances of thedifferent catalysts. It can be seen that the PtK/Al catalystshows the poorest reaction activity and stability. Meanwhile,the selectivity to isobutene increases progressively with thereaction time. Clearly, this phenomenon may be interpretedin terms of the effect of produced coke. In this case, the car-bon depositions may act as a promoter to improve the selec-tivity by deactivating the active sites. In contrast, the addi-tion of tin can influence the catalytic performances remark-ably. Moreover, the content of tin also affects the catalyticactivity. The initial conversions of isobutane catalyzed byPtSn(0.3)K/Al, PtSn(0.6)K/Al, PtSn(0.9)K/Al, PtSn(1.2)K/Aland PtSn(2.0)K/Al are 50.9%, 51.7%, 53.8%, 49.0% and46.4%, respectively; after the reaction for 6 h, the conversionsdecrease to 41.9%, 45.3%, 43.9%, 38.5% and 32.5%, respec-tively. Meanwhile, the positive effects of Sn on isobutene se-lectivity are also observed. The selectivity to isobutene in-creases progressively with increasing Sn loadings in the rangeof 0.3−2.0 wt%. Obviously, the higher Sn content, the higherisobutene selectivity.

To further assess the performances of the catalysts, thedeactivation rate with the time on stream is presented (Fig-ure 6). The deactivation of the catalysts during the reactionmay result from the formation of carbon deposits and thechanges of the metallic character. The deactivation parame-ter (D) along the reaction time is defined as D = 100×(X0–Xf)/X0, where X0 is the initial conversion (at the beginning

of reaction time) and Xf is the final conversion (at 6 h of thereaction time) [27,30]. As expected, with the absence of Sn,the catalyst posses the highest deactivation rate during thefirst hour and then deactivates relatively slowly. The presenceof Sn on PtSnK/Al catalysts results in the lower deactiva-tion rate, suggesting that the catalyst deactivation dependson the Sn promoter and the synergistic effect between potas-sium and tin is beneficial for the catalyst stability. In thiscase, significant differences are also found among these cat-alysts as a function of Sn content. After the reaction for 6 h,the deactivation rate decreases in the following order: PtK/Al(41.6%)>PtSn(2.0)K/Al (29.9%)>PtSn (1.2)K/Al (21.3%)>PtSn (0.3)K/Al (19.3%)>PtSn(0.9)K/Al(18.4%)>PtSn(0.6)K/Al(12.4%), where the number in the brackets represents thedeactivation percentage of the corresponding catalysts.

Figure 5. (a) Selectivity and (b) conversion as a function of time fordifferent catalysts. Reaction conditions: 590 ◦C, H2/iC4 = 1.0 (molar ratio),GHSV = 1500 h−1

To explain this, it should be noted that the dehydrogena-tion reactions proceed on small ensembles of surface platinumatoms, and on PtSn/Al2O3 catalyst there are two active cen-ters (metal and acid sites). Usually, platinum is the only ac-tive metal and the isobutene is only formed on the metal bydehydrogenation. The main cracking products (C1–C3 hy-drocarbons) are mainly formed from cracking on the carrier,while the hydrogenolysis reaction products are mainly formedon the metal [15]. Furthermore, it is well-known that the de-


hydrogenation and cracking of alkane are assumed to pro-ceed through carbonium-ion intermediates [31]. The higheracid sites generally promote the subsequent cracking reactionof the initially formed C+

3 carbenium ions. Therefore, thechanges of Pt active sites and support acidity may influencethe catalytic performances effectively. There should be an op-timal ratio between the number of active metal sites and acidsites on the catalyst. As illustrased above, with the presenceof potassium, suitable addition of Sn can not only increasethe metal dispersion, which is favorable for the distribution ofmetallic particles, but also strengthen the interactions amongsupported Pt-oxide species, tin promoter and the support.

Moreover, in these cases, the decrease of the catalyst acid-ity is inevitable due to the synergetic effect between potassiumand tin. Therefore, the matching between the Pt active sitesand the acid function can be improved, which is advantageousto the dehydrogenation of isobutane. In our experiments, thebehavior of Pt-0.6SnK/Al catalyst merits to be highlighted, itexhibits the lowest deactivation rate (12.4%) and shows a se-lectivity to isobutene higher than 94% at the isobutane conver-sion of about 45.3% after running the reaction for 6 h. How-ever, when the amount of Sn is excessive, the surface characterof active sites and the interactions between metal and supporthave been changed effectively. In this circumstance, the re-duction of Sn species at high temperature becomes relativelyeasy. Therefore, the initial matching between the metallic ac-tive center and acid center on Pt-0.6SnK/Al catalyst would bedestroyed significantly, which is disadvantageous to the reac-tion activity and stability.

Figure 6. Deactivation rate as a function of the time on stream overdifferent catalysts. Reaction conditions: 590 ◦C, H2/iC4 = 1.0 (molar ratio),GHSV = 1500 h−1

Since Pt-0.6SnK/Al catalyst shows almost the highest re-action activity and stability, this catalyst is chosen for the sub-sequent tests. Figure 7 shows the stability test of this catalyst.It can be seen that the selectivity to isobutene increases rel-atively quickly in the initial period and then becomes stable,and a high value (96.2%) is obtained in the overall process.On the other hand, only a relatively low deactivation rate isobserved in the overall process. The mean yield of isobuteneis about 40.2% during the course of the reaction for 60 h.

Figure 7. Stability test of Pt-0.6SnK/Al catalyst in the dehydrogenation ofisobutane at 590 ◦C

3.3. Coke analysis

As mentioned before, in our experiments platinum parti-cles were highly dispersed on the external surface of the cat-alyst, and the carbon deposits produced during the reactioncan physically block the active metal, which is the main rea-son for the catalyst deactivation. The amount of coke overeach used catalyst is shown in Table 3. Figure 8 shows thetemperature-programmed oxidation (TPO) profiles of the cor-responding catalysts. It can be seen that two successive peaksrepresenting two different carbon deposits are displayed inTPO profiles. Generally, the carbon deposits correspondingto the first peak at low temperature are mainly the ones thatcover the active metal, while the second peak at high temper-ature represents the ones that located on the external surfaceof γ-Al2O3 carrier [32].

Figure 8. TPO profiles of different samples

From Figure 8, it is clear that for PtK/Al catalyst, mostof the carbon deposits cover the active metal and in this casethe temperature is the lowest, meaning that the deposited car-bon is the most reactive. After the addition of 0.3 wt% Sn,sharply decreased amount of the coke is observed, which sug-gests that the promoter of Sn can reduce the amount of coke.


Coke quantitative analysis (Table 3) clearly shows that theamount of coke over Pt-0.3SnK/Al catalyst (4.1%) is muchlower than that over PtK/Al catalyst (6.7%). Moreover, theamount of coke over tin-doped PtK catalyst is determined bythe Sn content. The lowest amount of coke (1.6%) is observedover the Pt-2.0SnK/Al catalyst. Meanwhile, as revealed inFigure 8, it is worth noting that the presence of Sn can alsofacilitate the transfer of the carbon deposits from the metal tothe support as evidenced from the change in the ratio of thepeak heights in the TPO profiles. Obviously, this effect mustbe related with the adsorption behavior of the dehydrogenatedspecies on the active sites of the catalyst. To explain these, itshould be noted that the coke formation on the catalyst usuallyinvolves several processes [33]: (1) successive dehydrogena-tion/cyclization of alkyl chains; (2) n-alkane oligomerization;(3) Diels-Alder type reactions. Olefins are primary precursorsof the mechanism of coke formation and the intrinsic acidityof the support can promote the undesirable reactions such ascracking/isomerization, thus increasing the carbon deposits.According to this mechanism, it is proposed that the changeof Pt active sites and catalyst acidity may influence the cokeformation obviously. As discussed before, suitable additionof tin can increase the metallic dispersion and strengthen theinteractions among supported Pt-oxide species, tin promoterand support, thus decreasing the amounts of carbon precursorsduring the reaction. Moreover, the presence of Sn can alsodecrease the catalyst acidity, which in consequence decreasesthe amounts of coke and improves the reaction stability. Whenthe content of Sn is relatively high (1.2 wt% and 2.0 wt%), thedecreased amounts of carbon depositions are inevitable due tothe decreased amounts of produced carbon precursors. In ad-dition, it should be noted that with the presence of potassium,the addition of Sn can decrease the size of the surface Pt en-sembles so that the hydrocarbon can not readily formmultiplecarbon-metal bonds [34], and the adsorbed species are moremobile and can more easily migrate to the support.

Table 3. Amount of coke on the catalysts after isobutanedehydrogenation at 590 ◦C for 6 h

Catalysts Coke amount (%) a

PtK/Al 6.7Pt-0.3SnK/Al 4.1Pt-0.6SnK/Al 3.2Pt-0.9SnK/Al 2.5Pt-1.2SnK/Al 2.0Pt-2.0SnK/Al 1.6

a Experimental value calculated from thermogravimetric (TG) analysis

4. Conclusions

In summary, different contents of tin have obvious im-pacts on the catalytic performances of PtSnK/γ-Al2O3 cat-alysts for isobutane dehydrogenation. With the presence ofpotassium, suitable addition of tin (0.3 and 0.6 wt%) resultedin an increase in the platinum dispersion and a decrease in thecatalyst acidity due to the synergistic effect between potas-sium and tin. Moreover, this synergetic effect can decrease

the amount of coke that covered the active metal and facili-tate the transfer of the carbon deposits from the metal to thesupport. In these cases, the existence of Sn can strengthen theinteractions among supported Pt-oxide species, promoter andsupport, which is beneficial to inhibit the side reactions, thusincreasing the catalytic reaction activity and stability. In ourexperiments, Pt-0.6SnK/Al catalyst exhibited the lowest de-activation rate (12.4%) and showed a selectivity to isobutenehigher than 94% at the isobutane conversion of about 45.3%after running the reaction for 6 h. Furthermore, the meanisobutene yield of about 40.2%was obtained during the courseof the stability reaction for 60 h.

However, when the concentration of Sn is excessive, thesurface character of active sites and the interactions betweenmetal and support have been changed. Therefore, the initialoptimal ratio between the metallic function and acid functionwould be destroyed, which is disadvantageous to the reaction,and so a decrease in the catalyst acidity is inevitable.

AcknowledgementsThe authors are grateful to the National Natural Science Foun-

dation of China (21106017 and 50873026), Specialized ResearchFund for the Doctoral Program of Higher Education of China(20100092120047) and Production and Research Prospective JointResearch Project of Jiangsu Province of China (BY2009153) for fi-nancial supports.

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