Journal of Industrial and Engineering Chemistry 16 (2010) 57–62

Effect of Si/Al ratio on performance of Pt–Sn-based catalyst supported on ZSM-5zeolite for n-butane conversion to light olefins

Zeeshan Nawaz *, Shu Qing, Gao Jixian, Xiaoping Tang, Fei Wei

Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology (FLOTU), Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China

A R T I C L E I N F O

Article history:

Received 4 May 2009

Accepted 11 August 2009

Keywords:

n-butane

Propylene

Selectivity

Cracking

Si/Al ratio

Dehydrogenation

Total olefins

A B S T R A C T

The performance of Pt–Sn-based catalyst, supported on ZSM-5 of different Si/Al ratios were investigated

for simultaneous dehydrogenation and cracking of n-butane to produce light olefins. The catalysts were

characterized by number of physio-chemical techniques including XRF, TEM, IR spectra, NH3-TPD and

O2-pulse analysis. Increase in Si/Al ratio of zeolite support ZSM-5 significantly increased light olefin’s

selectivity, while feed conversion decreases due to lower acidity of support. The results indicated that

both the n-butane cracking and dehydrogenation activity to light olefin’s over Pt–Sn/ZSM-5 samples

with increasing Si/Al ratios greatly enhanced catalytic performance. The catalysts were deactivated with

time-on-stream due to the formation of carbon-containing deposits. A coke deposition was significantly

related to catalyst activity, while at higher Si/Al ratio catalyst the coke precursors were depressed. These

results suggested that the Pt–Sn/ZSM-5 catalyst of Si/Al ratio 300 is superior in achieving high total

olefins selectivity (above 90 wt.%). The Pt–Sn/ZSM-5 also demonstrates resistance towards hydrother-

mal treatment, as analyzed through the three successive reaction-regeneration cycles.

� 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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1. Introduction

At present, both ethylene and propylene were produced as mainby-products in naphtha steam cracking units, this process,accounting for over 80% of total light olefins output [1]. In FCCunits light olefin’s yield is also limited, even with state of the artadvancements, like downer reactors and novel selective catalysts[2–4]. These conventional methods of producing ethylene andpropylene cannot meet the fast growing demand of light olefins.Moreover, alkanes are the world’s most abundant organic resourceand probably the versatile class of feedstock. Nowadays, the outputof butane fraction is about 150–200 million tons every year and avaluable raw material to produce light olefins [1,5]. The newmethods to produce light olefins particularly propylene andbutene includes butane dehydrogenation [6–10].

In the catalysts family, environmentally friendly and non-corrosive, solid acid catalysts (zeolites and zeolite-based catalysts)for the number of petrochemical processes have been underinterest over the past decade [11]. The mechanism of hydrocarbonconversion inside zeolites is quite complex, as a significant numberof different types of competing chemical reactions occur. Aluminasupported catalysts was observed to be an active catalyst in the

* Corresponding author. Tel.: +86 13 260276696; fax: +86 10 62772051.

E-mail address: engr.dr.zeeshan@gmail.com (Z. Nawaz).

1226-086X/$ – see front matter � 2010 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2010.01.021

dehydrogenation of light alkanes to alkenes, while it’s quickdeactivation affects its overall performance [3,12]. ZSM-5 zeolitehas been extensively investigated as an active catalyst and supportfor a variety of reactions owing to its high activity and stability[13–16]. Bocanegraa et al. [17] investigated the catalytic crackingand dehydrogenation of n-butane over Pt–Sn-based catalystsupported on MgAl2O4 and ZnAl2O4.

Previously, Pt–Sn/ZSM-5 was used for propane dehydrogena-tion to propylene [3,18]. While, no study has been focused on theinfluence of Si/Al ratio of support on simultaneous cracking anddehydrogenation of n-butane to produce light olefin’s. Also, muchattention has been given to intensify the metallic contents andcombinations. Role of the Pt–Sn as active metal and promoter,respectively, was characterized in the number of studies [3,19–21].In our previous experience, it was observed that the intensificationof support has more dominant influence on the performance ofdehydrogenation reactions [3,22]. It was also reported that themetallic doped catalysts pre-reduction with hydrogen decreasesthe coke formation and enhances dehydrogenation activitywithout influencing the secondary reaction of light olefin’sproduction [3,23]. The n-butane contains both primary andsecondary carbon atoms. The products obtained from crackingof n-butane were normally explained by penta-coordinatedcarbonium ion intermediates formed over Bronsted acid sites. Athigher conversions, olefin’s selectivity was declined due to hydridetransfer reaction activation.

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. Experimental setup for n-butane cracking and dehydrogenation.

Table 1XRF analysis.

Catalysts Si/Al

ratio

Pt content

(%, w/w)

Sn content

(%, w/w)

Pt(0.5 wt.%)–Sn(1 wt.%)/ZSM-5 50 0.46 0.89

140 0.50 0.86

300 0.49 0.91

Z. Nawaz et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 57–6258

In this paper n-butane was used as a feedstock to investigate itssimultaneous cracking and dehydrogenation over Pt–Sn/ZSM-5(having different Si/Al ratios of support) to produce light olefin’sand justified with proper characterization. Another aim in thestudy was to determine the effect of hydrothermal regenerationeffect and coke measurement. Carbon-containing deposits werethe inherent problem and formed during dehydrogenation thatdecrease the catalytic activity, necessitate periodic regeneration ofthe catalyst. Previously large emphasize has been given tocharacterize the catalysts by hiding actual facts; therefore, ourfocus is particularly on parametric characterization.

2. Experimental

2.1. Catalyst preparation

The bimetallic Pt–Sn/ZSM-5 catalyst was prepared by sequen-tial impregnation [3] with powder H-ZSM-5 (provided by NankaiCatalyst Company, Tianjin, China). At first, the powder H-ZSM-5zeolite of Si/Al ratio 50 was co-impregnated with 0.16 MSnCl2�2H2O at 80 8C to ensure 1 wt.% Sn in the catalyst and driedat 100 8C for 4 h. Then samples were calcined at 500 8C in a mufflefurnace for 4 h. The 0.03 M solution of H2PtCl6 was made for 0.5 wt.% Pt and co-impregnated with powdered Sn/ZSM-5 at 70 8C.Afterward prepared samples were dried at 100 8C for 4 h andcalcined at 500 8C for 4 h. The bimetallic catalyst of other Si/Alratios (SAR) of ZSM-5, 140 and 300 were prepared in a similarmanner.

2.2. Catalyst characterization

2.2.1. X-ray fluorescence analysis

The metallic contents were verified using X-ray fluorescence(XRF) on a Shimadzu XRF 1700 fluorimeter.

2.2.2. Temperature-programmed desorption of ammonia

The acid properties of Pt–Sn/ZSM-5 catalysts were determinedby NH3-TPD on an Autochem-II 2920 analyzer. About 0.2 g of eachcatalyst sample was placed into a U-type quartz reactor andtreated by flowing pure dry argon at 500 8C for 1 h prior toadsorption of ammonia. Then the samples were saturated withammonia at room temperature. Afterwards, the temperature waskept at 100 8C for 0.5 h to desorb the physically adsorbed ammonia.Subsequently, the temperature was raised to 600 8C with a heatingrate of 10 8C/min. The NH3 desorption profile was observed using athermal conductivity detector.

2.2.3. IR spectroscopy

IR spectra of adsorbed pyridine were recorded using a NEXUSapparatus (Nicolet, USA). The samples were dried, pressed into thinwafers and placed in a Pyrex glass cell equipped with CaF2 window.Then the samples were de-gassed at 350 8C for 30 min and thencooled to 25 8C. Afterward, ammonia was passed over the samplefor 30 min and pyridine adsorption spectra were recorded afterdesorption at 100 8C.

2.2.4. TEM

The Pt metal dispersion was verified by JEM-2010 high-resolution transmission electron microscope equipped withenergy dispersive spectroscopy at FLOTU, Tsinghua University.

2.2.5. O2-pulse coke analysis

The coke was determined by O2-pulse experiments, using a gaschromatograph flow system equipped with TCD. The experimentswere carried out at 700 8C by injecting pulses of pure oxygen(99.99%) until deposited coke was fully removed. The CO2 formed

was continuously measured from a TCD detector and the amountof coke deposited on each catalyst was calculated.

2.3. Reaction test

n-butane conversion to light olefin’s was investigated as afunction of time-on-stream at 585 8C for different Si/Al ratios ofZSM-5 supported Pt–Sn-based catalysts, in a steady plug-flowquartz micro-reactor at atmospheric pressure. The setup is shownin Fig. 1. The 99.9% n-butane was used as feed, provided by ZhongKe Hui Jie (HJAT), Beijing, China. The measured amount of sampleswas loaded to maintain desired WHSV (i.e. 3 h�1). The freshcatalyst sample was first reduced under flowing H2 (8 ml/min) at500 8C for 8 h prior to reaction test. The reaction mixture composedof H2/C4H10 (molar ratio 0.25) was charged to the reactor. Theproduct composition was analyzed through an on line gaschromatograph (GC 7890-II) equipped with FID detector. Hydro-thermal treatment of the prepared catalysts was performed in thesame reactor for three continues runs. The samples wereregenerated at a specified temperature 600 8C, under a flow of anitrogen-steam mixture at a flow rate of 40 ml/min and a steampartial pressure of 20 kPa for 4 h. After regeneration, the catalystswere again reduced with hydrogen before the reaction.

3. Results and discussion

3.1. Catalyst characterization

The corresponding metallic contents doped were confirmedfrom XRF analysis and the results were shown in Table 1. While it isobvious from our previous XRD analysis of Pt(0.5 wt.%)–Sn(1 wt.%)/ZSM-5 (SAR = 140), that doping of metals did destroythe original structural topology and texture of ZSM-5 [19]. It wasalso reported in the literature that the presence of Sn not onlyenhances Pt dispersion but also modifies electronic density of the

Fig. 2. TEM micrographs of Pt–Sn/ZSM-5 (Si/Al ratio = 140) before reaction.

Fig. 4. IR spectra of pyridine adsorption of Pt–Sn/ZSM-5 catalysts of different Si/Al

ratios.

Fig. 3. NH3-TPD profiles of Pt–Sn/ZSM-5 zeolites with different Si/Al ratios.

Z. Nawaz et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 57–62 59

catalyst [18]. H2-TPR results of Pt–Sn/ZSM-5 given by Yiwei et al.[18] shows the higher reduction peak at higher temperatureconfirms that, the Pt was well interacted with ZSM-5 zeolite in thepresence of Sn. This also verified from TEM image of the fresh Pt–Sn/ZSM-5 catalyst as shown in Fig. 2.

Fig. 3 represents the NH3-TPD profiles of Pt–Sn/ZSM-5 catalystswith different Si/Al ratios. There are two desorption peaks for eachcatalyst, one appears between 250 and 270 8C and the other inbetween 420 and 470 8C, corresponding to the weak acid sites andthe strong acid sites, respectively. Moreover, the desorption peaksshifted towards a lower temperature with the increase in Si/Alratio. The Bronsted and Lewis acidities of the samples were alsodetermined by pyridine adsorption using FT-IR spectroscopy, asshown in Fig. 4. The pyridine band at 1547 cm�1 can be assigned toBronsted acid and the band at 1454 cm�1 to Lewis acid sites. Theband at 1491 cm�1 demonstrates total acidity. With the increase inSi/Al ratio of ZSM-5, from 50 to 300, the intensity of both theBronsted acid sites and the Lewis acid sites were graduallydecreased [20]. That helps in enhancing light olefin’s selectivity.The formation of PtSn alloys or intermetallic compounds Pt0 andSn0 were affected the catalyst performance [3,18,20]. Some of theBronsted acid sites were consumed during the metallic interaction,

while it was known that Pt–Sn was well interacted with the ZSM-5[18]. As the reaction was competitive, therefore the weak acid siteswill favor the dehydrogenation over the cracking.

3.2. Catalyst performance analysis

The effect of Si/Al ratio on the performance of Pt–Sn-basedcatalyst supported on ZSM-5 was experimentally analyzed for lightolefin’s production from n-butane and the results are shown inFig. 5. It was clear from the experimental results that change in Si/Al ratio of support has a significant effect on reaction behavior.Under identical operating conditions, catalyst supported on ZSM-5of Si/Al ratio 300 shows superior selectivity for light olefin’s at thecost of lower conversion. The characterization and experimentaldata argues that lower acidity of the support decreases the n-butane conversion. While, higher acidity was due to lower Si/Alratio of the support, that promote surface reactions and decreasesoverall olefins selectivity. For the Pt–Sn/ZSM-5 zeolite with a Si/Alratio of 300, the butane selectivity was about 39–40% andpropylene selectivity was about 27–26% up to 8 h operations.The product selectivity was calculated using following relation-ship.

Selectivity of alkenesð%Þ

¼ ð% content of alkenes in productÞ � 100

Conversionð%Þ (1)

Using ZSM-5 as a metallic support, it has been observed thatwith the increase in the Si/Al ratio, stability of the catalyst wasstrongly affected and results quick deactivation. Table 2 shows theactivity decay with respect to time-on-stream of different catalystssupported. The deactivation rate was calculated by using therelationship Dr = [(X0�Xf)/X0 � 100], where X0 is the initial n-butane conversion at 1 min and Xf is the final conversion at 8 h. Itwas observed that the amount of coke formed over the catalystsurface that leads to deactivation was directly related to catalystactivity. Therefore, the decreased in coke amount over higher Si/Alratio was due to lower activity. The deposition of coke as a functionof time-on-stream was investigated after hydrogen pre-reductionas there is no carbon-containing material was formed during thereduction. The amount of coke and the selectivity of total olefinswere tabulated in Table 2. The amount of coke increased linearlywith time-on-stream and the dehydrogenation activity decreasedwith an increase in the selectivity to isobutene.

Fig. 5. Light olefin’s selectivity and feed conversion over Pt–Sn/ZSM-5 of different SAR at 585 8C and WHSV 3 h�1.

Table 2Deactivation as a function of different Pt–Sn metallic content loaded on ZSM-5. The deactivation parameter was defined as Dr = [(X0�Xf)/X0�100]; where X0 is the initial

conversion at 1 min, Xf is the final conversion and Sf is the final total olefin selectivity at 8 h. The reaction temperature was 585 8C.

Pt(0.5 wt.%)–Sn(1 wt.%)/ZSM-5

catalyst of different Si/Al ratio

X0 Xf DraCoke Sf

50 78.1 64.2 13.8 0.41 47.2

140 75.5 58.3 17.2 0.46 52.3

300 38.7 26.6 12.1 0.29 86.1

a O2-pulse coke analysis

Z. Nawaz et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 57–6260

The complete comparison of total olefins and total paraffin’sproduced were shown in Fig. 6. It was observed that the balancebetween acidity and activity of catalyst in order to obtain robustcontrol of stereo-chemistry is needed. Activation energyrequirements for dehydrogenation decreases with the increasein carbon chain [24], and it can be observed from productdistribution shown in Fig. 7. It was noted that all by-productswere remained stable with the time-on-stream. n-butane cracks

Fig. 6. Total olefins and paraffin selectivity of Pt–Sn-based catalyst supported on

ZSM-5 of different Si/Al ratio with time-on-stream, at temperature 585 8C and

WHSV 3 h�1.

quickly over bimetallic zeolite supported catalyst and thephenomenon was more enhanced with the increase in TOS.Initial experimental data shows that most of the butaneconverts to ethane due to faster cracking rate compared todehydrogenation. Sudden after an hour, product distributionproceeds towards better olefin’s selectivity with the increase indehydrogenation selectivity. The propane selectivity was ob-served to be constant with TOS.

Fig. 7. Other by-products over Pt(0.5 wt.%)–Sn(1 wt.%)/ZSM-5(SAR = 300) during

n-butane conversion to light olefins, at temperature 585 8C and WHSV 3 h�1.

Fig. 8. Influence of temperature on total olefins selectivity and yield over Pt–Sn-based catalyst supported on ZSM-5 of different Si/Al ratios, at TOS = 1 min and WHSV 3 h�1.

Table 3Total olefins selectivity (T.O.C.) and feed conversion at WHSV = 3 h�1, 585 8C and TOS = 8 h.

Pt–Sn/ZSM-5 Catalyst of SAR Run I Run II Run III

T.O.C. (%) Conv. (%) T.O.C. (%) Conv. (%) T.O.C. (%) Conv. (%)

50 47.2 64.2 47.6 64.1 47.9 64.0

140 52.3 58.3 52.7 58.2 53.2 58.0

300 86.1 26.6 85.9 26.4 86.5 26.1

Z. Nawaz et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 57–62 61

It can be seen from Fig. 8, that the selectivity of total olefins andyields rapidly increased with the increasing of reaction tempera-ture. The selectivity was reached the maximum value of 90% at585 8C at Pt–Sn/ZSM-5 catalyst Si/Al ratio (SAR) 300. The similartrend was observed for SAR 50 and 140. The yield of total olefinswas continuously increased with the rise in temperature, owing todistinct increase in conversion. The maximum value of the yield oftotal olefins was 41.01% at 615 8C at SAR 140. The reactionmechanism could be speculated from the product distribution inFigs. 5 and 7. The n-butane undertook catalytic cracking anddehydrogenation rather than pyrolysis over Pt–Sn/ZSM-5 molecu-lar sieve catalyst, when reaction temperature was lower than585 8C. When the reaction temperatures were higher than 585 8C,the olefins were also cracked to paraffin’s. It can also be observedfrom product distribution.

3.3. Catalysis performance in a continuous mode

Table 3 shows catalytic activities of the Pt–Sn/ZSM-5 zeolite ofdifferent Si/Al ratios for n-butane conversion in a continuous modeup to three runs at 585 8C and 3 h�1 WHSV. The fresh sampleexhibits the highest initial reaction activities for each Si/Al ratiosimilar to Figs. 5 and 6. Before the second run, the used catalystwas regenerated in nitrogen mixed seam (mild steaming) for 4 hand the reduced in hydrogenation environment. It is observed thatthe catalyst performance was remains unaffected after regenera-tion. Similarly, the third experimental run was conducted oversame catalyst used in previous runs. The result shows that therewas a slight decrease in conversion of three catalysts of differentSi/Al ratio, while a slight increase in total olefins selectivity wasnoted. This increase in total olefins conversion was not due to

catalyst activity enhancement but due to decrease in n-butaneconversion.

The hydrothermal treatment has an obvious effect on thecatalytic activity and is the necessary requirement of the processfor regeneration. The chances of n-butane to enter the ZSM-5channels were little, because the active metal was located on theexternal surface of the catalyst. Therefore, reactant gas firstinteracted with the active metal over the surface of support. Thesefindings suggest that the steam treatment was strongly influencedthe catalytic properties. Over the bifunctional catalyst Pt–Sn/ZSM-5, both metal particles and acid sites of support, may workcollaboratively [25]. Therefore, the balance between the number ofactive sites and the number of acid sites may exist. Moreover, thePt particles were sintered, so that the metallic function of thecatalysts was destroyed. Furthermore, the surface area of thecatalyst was also decreased after hydrothermal treatment thataffects catalysts performance. The experimental results argue thatat mild treatment the catalyst activity was stable. Before steamingFAI (framework Al) are mostly in pair those contributed to reactionbut steaming converts FAI to EFAL (ex-framework Al) and in overallactive acid sites decreased [19].

4. Conclusion

The influence of Si/Al ratio (SAR) of the ZSM-5 zeolite support ofPt–Sn-based catalyst was experimentally studied for n-butaneconversion to light olefins. It was noted that the higher SAR favorslight olefin’s selectivity due to lower acidity. On the other handlower SAR of support, significantly increased conversion. Thishigher conversion was due to higher cracking and dehydrogena-tion rate. The coke formation was an inherent problem and catalyst

Z. Nawaz et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 57–6262

deactivation rate was determined. It was observed that the cokeformation was related to the catalyst activity. Above 90% totalolefin selectivity was obtained at Si/Al ratio 300. The productdistribution at SAR 300 shows that both cracking and dehydroge-nation reactions were simultaneously exists. The higher selectivityof total olefins with optimum yield was obtained at 585 8C. Athigher temperatures cracking and pyrolysis reactions werebecome dominant, those affects olefin’s selectivity. Hydrothermaltreatment has an obvious influence on catalytic properties of Pt–Sn/ZSM-5 catalyst. Under the mild treatment the catalyst wastested for three consecutive runs, and it was found that the catalystactivity was stable.

Acknowledgments

This work was financially supported by the Higher EducationCommission, Islamabad, Pakistan and Beijing Key Laboratory forGreen Chemical Reaction Engineering and Technology, TsinghuaUniversity, Beijing, China.

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