Coke Formation on Pt–Sn-Al2O3 Catalyst in Propane Dehydrogenation

ORIGINAL PAPER

Coke Formation on Pt–Sn/Al 2O 3Catalyst in Propane

Dehydrogenation:Coke Characterization and Kinetic Study

Qing Li ?Zhijun Sui ?Xinggui Zhou ?Yian Zhu ?Jinghong Zhou ?De Chen

óSpringer Science+Business Media,LLC 2011

Abstract The in?uences of gas compositions on the rates of coke formation over a Pt–Sn/Al 2O 3catalyst are studied.The coke formed on the catalyst is characterized by ther-mal gravimetric analysis,IR spectroscopy,Raman spec-troscopy and elemental analysis.Two kinds of coke are identi?ed from the TPO pro?les and assigned to the coke on the metal and the coke on the support,respectively.The coke formed on the metal is softer (containing more hydrogen)than that formed on the support.The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen pressures but increasing with the propane pressure,while the rate of coke formation on the support is increasing with the propane and propylene pressures and decreasing with the hydrogen pressure.Based on the kinetic analysis,a mechanism for the coke formation on the Pt–Sn/Al 2O 3catalyst is proposed,and the dimerization of adsorbed C 3H 6is identi?ed to be the kinetic relevant step for coke formation on the metal.Keywords Coking mechanism áKinetics áPropane dehydrogenation áPt–Sn/Al 2O 3catalyst

1Introduction

Recently the rapidly rising needs for propylene [1–3]have driven searching for new propylene production techniques.Propane dehydrogenation,developed commercially in 1980s,is an on-purpose technique for propylene production and is now making big contributions to the world propyl-ene supply in recent years [4].

Pt based catalyst is the most popular catalyst for propane dehydrogenation and is used in commercial Ole?ex pro-cess.This catalyst,as well as other catalysts for alkane dehydrogenation,is quickly deactivated,mainly by coke formation,and the mechanism of coke formation on the Pt catalyst is still not clear.Understanding the mechanism of coke formation is of great signi?cance.On one hand,based on the mechanism of coke formation,one can optimize the reaction conditions to reduce the frequency of dehydroge-nation and regeneration cycle by controlling the rate of coke formation so as to increase the propylene productiv-ity.On the other hand,it can help the rational design of the catalyst to reduce the rate of coke formation while main-taining the activity and selectivity in dehydrogenation.Coke formation on Pt or modi?ed Pt catalysts during propane dehydrogenation has been studied by a number of researchers.Most of the studies were targeted to reduce the coking rate by modifying Pt,for example,with Sn or alkali metals,and/or using different catalyst supports.Praserthdam et al.[5]showed that alkali metals (such as Li,Na,and K)would help to reduce the coking rate by providing excess mobile electrons to the Pt catalyst.Another straightforward method to reduce the amount of coke formed is to use supports with weak acidity,e.g.SBA-15[6],which is effected by weakening propylene adsorption and suppressing propylene dehydrogenation or polymerization.

Q.Li áZ.Sui áX.Zhou (&)áY.Zhu áJ.Zhou

State Key Laboratory of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China e-mail:xgzhou@https://www.360docs.net/doc/c111392998.html,

D.Chen

Department of Chemical Engineering,Norwegian University of Science and Technology (NTNU),N-7491,Trondheim,Norway

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DOI 10.1007/s11244-011-9708-8

The rate of coke formation depends highly on the operating https://www.360docs.net/doc/c111392998.html,rsson et al.[7]studied the coke formation on Pt/Al2O3and Pt–Sn/Al2O3catalysts,and suggested that only a small part of the formed coke was responsible for catalyst deactivation,and that a major part of the coke was formed regardless of the gas composition but dependent on the temperature.They also concluded that hydrogen could reduce the rate of coke formation and correspondingly the rate of catalyst deactivation by sup-pressing coke precursor formation,but hydrogen could not remove the coke that had already been formed on the catalyst.Rebo et al.[8]studied the coke formation on Pt–

Sn/Al2O3catalyst using an oscillating microbalance reactor and concluded that coke formation was a structure sensitive reaction and hydrogen could decrease the rate of coke formation as well as the deactivating effect of the coke formed.

Coke could be formed either on the support or on the metal,and the coke deposited on different sites is supposed to have different effects on catalyst deactivation.Studies on the in?uences of reaction conditions on the rate of coke formation and the nature of the coke deposited on different sites are essential for the understanding of the mechanism of coke formation as well as the mechanism of catalyst deactivation.However,studies on the in?uences of reac-tion conditions,especially gas compositions,during pro-pane dehydrogenation are seldom reported.

In this work,we study the in?uences of gas composi-tions on coke formation on the metal and support of a Pt–Sn/Al2O3catalyst during propane dehydrogenation.Based on these results,the kinetic relevant step in coke formation on the metal is identi?ed and a mechanism of coke for-mation is then proposed.

2Experimental

2.1Catalyst Preparation and Characterization

The Pt–Sn/Al2O3catalyst used for this study was prepared by incipient impregnation of alumina(Pural200)with H2PtCl6and SnCl4solutions,followed by calcination at 530°C and treatment in steam to remove chlorine.A Pt/ MgO catalyst was also prepared by impregnation method. The catalysts were characterized by N2physisorption on ASAP2010(Micromeritics,USA)at-196°C after out-gassing the samples for at least5h at190°C and1mm Hg vacuum.The speci?c surface areas were calculated with BET equation,and the pore volumes and pore size distri-butions were determined from the N2desorption isotherms by using the BJH method.Table1summarizes the char-acteristics of the catalysts.

Besides,alumina(Pural200),which was the support for the Pt–Sn catalyst,was also used as a blank catalyst for comparison.

2.2Coking Experiments

The catalysts were evaluated in a tubular stainless steel reactor with an inner diameter of6mm,the temperature of which was maintained by an electrical furnace jacketing outside of the reactor.Inserted inside the catalyst packing (100mg)was a thermocouple to indicate the real temper-ature of reaction.Silica particles as inert packing were used for uniform?ow distribution.An online gas chromatogra-phy(Agilent4890D)was used to measure the outlet gas concentrations.The catalyst(or the alumina),which had particle sizes between0.1and0.15mm,was?rstly reduced at500°C in?owing hydrogen(10ml/min)for100min and the temperature was then ramped to575°C in argon (40ml/min).The feed gas was then introduced into the reactor for coking experiment.In all the experiments,argon was used as a balance gas and the total?ow rate was maintained at80ml/min.After80min of reaction,the reactor was switched to pure argon?ow and then cooled down to room temperature.Table2shows the feed com-positions and averaged gas compositions in the reactor during the experiments.

2.3Coke Characterization

The amounts of coke formed on the spent catalysts(or coked alumina)were determined by TG(thermal gravi-metric)analysis(SDT Q600,TA Company,USA)in air. During the TG analysis,the temperature was increased from ambient temperature to600°C(sometimes higher)at a rate of10°C/min,during which the temperature pro-grammed oxidation(TPO)pro?les and heat?ow curves were recorded.Elemental analysis was carried out on a Vario EL III Elementar analyzer.The coked samples were ?rstly dissolved in a hydro?uoric acid solution(40%)at room temperature to liberate the coke from the support. Then the coke was dried at50°C and collected for testing. Table1Catalyst characteristics

Pt–Sn/Al2O3Pt/MgO Shape Pellet Pellet Diameter(mm) 1.00.13 Surface area(m2/g)56.647.7 Pore volume(m3/g)0.250.24 Average pore diameter(nm)18.519.2

Pt(%)0.500.75 Sn(%) 1.50–

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FTIR analysis was carried out on a Bruker Equinox-55 with a resolution of 4.0cm-1and the scanned wave number was ranged from4000to400cm-1.Raman analysis was performed at room temperature under ambient conditions on a Renishaw inVia?Re?ex Raman spec-trometer with a514.5nm Ar-ion laser beam.

3Results

3.1Thermal Gravimetric Analysis

Figures1–3show the TPO pro?les of the spent Pt–Sn/ Al2O3catalysts coked with different gas compositions at 575°C for80min.Two peaks,locating in the interval of 150–280°C(Peak I)and in the interval of380–430°C (Peak II),respectively,are present in each pro?le.How-ever,for coked alumina(Sample S9)and Pt/MgO catalyst

(Sample S10),only one peak,at around470and310°C, respectively,is present in the TPO pro?les(not shown here).To determine the amount of coke losses,the TPO pro?les are deconvoluted by multiple Gaussian functions using a non-linear least-squared optimization procedure based on Levenberg–Marquardt algorithm.This method ?ts the TPO pro?les quite well and the results are sum-marized in Table3.

Figure1shows that the area of Peak II increases while that of Peak I is almost unchanged when the partial pres-sure of propylene is increased(Samples S1–S3).Consequently the total amount of coke increases with the partial pressure of propylene.Figure2indicates that the addition of hydrogen in the feed has greatly suppressed coke formation:if there is only1.9%hydrogen(Sample S4)in the gas,the total amount of coke is as high as 3.80wt%,while if the hydrogen content in the gas is increased to13.6wt%(Sample S5),the total coke amount is decreased to1.44wt%.It is also noted that for sample S4 or S5,the decrease in the total amount of coke,denoted by the areas of Peak I and Peak II,is mainly due to the decrease of the area of Peak II,as seen from Table3.

Table2Summary of the experimental conditions(temperature, 575°C)

Samples Feed

compositions d(%)Averaged gas compositions e(%)

C3H8C3H6H2C3H8C3H6H2

S1a34.30.08.330.0 2.911.9 S2a35.6 3.0 6.631.2 5.610.5 S3a37.09.59.134.511.110.1 S4a35.07.80.032.49.4 1.9 S5a36.07.312.034.08.513.6 S6a20.00.00.015.9 4.0 5.3 S7a34.90.00.030.0 3.3 4.0 S8a49.10.00.042.7 4.3 4.5 S9b0.010.00.00.09.90.0 S10c35.30.00.035.00.20.2 a Pt–Sn/Al

2

O3

b Al

2

O3

c Pt/MgO

d Gas compositions at th

e inlet o

f the reactor

e Average compositions o

f gas between the inlet and outlet of the reactor

Table3Characterization results of coked samples

Samples AI AII Total AI/AII QI/QII I D/I G H/C

S1 1.100.33 1.43 3.33–– 1.89 S20.960.49 1.45 1.96 3.02– 1.83 S3 1.16 1.20 2.360.97 2.48– 1.78 S40.82 2.98 3.800.28–– 1.18 S50.780.66 1.44 1.18 2.45– 2.00 S60.310.390.700.79–0.66–

S70.700.54 1.24 1.30–0.68 1.81 S8 1.18 1.19 2.370.99–0.60 1.79 S9n.d. 3.21 3.21/–0.64 1.17 S10 1.21n.d. 1.21/–0.80 2.66

AI and AII:peak areas of TPO Peak I and Peak II in Figs.1,2and3 AI/AII:ratio of area of Peak I to area of Peak II

QI/QII:ratio of released heats during the oxidation of coke repre-sented by Peak I and Peak II

I D/I G:intensity ratio of D band to G band in the Raman spectrum H/C:ratio of hydrogen atoms to carbon atoms of the coke

n.d.not detected,–not characterized,/not calculated

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Figure3shows that the areas of both Peak I and Peak II increase with the partial pressure of propane.

Moreover,Peak II shifts to higher temperature,from 385.3to389.6and then to398.0°C,as the partial pressure of propylene increases(Fig.1)and shifts to lower tem-perature as the partial pressure of hydrogen increases (Fig.2).Peak I also shifts to higher temperature as the partial pressure of propane increases(Fig.3).

The released heats by oxidation of the cokes on Samples S2,S3,and S5at the two peak temperatures are calculated from the integration of the heat?ow curve and have already been listed in Table3in terms of the heat ratios.It is observed that the heat ratio is always larger than the area ratio.3.2Elemental Analysis

The results of elemental analysis of the coke are listed in Table3in terms of H/C ratio.For most samples(except for Samples S4,S9,and S10),the H/C ratios are in the range of 1.7–2.0,which is an indication of the relatively high hydrogen content of the coke formed on Pt–Sn/Al2O3 catalysts.However,the H/C ratio of S4is much lower than those of other coked Pt–Sn samples,which is because of the very low hydrogen concentration in the feed?ow.The coked alumina(S9)has the lowest while the coked Pt/MgO (S10)has the highest H/C ratio,which indicates that the coke formed on the alumina is relatively de?cient in hydrogen while the coke formed on the Pt surfaces is rich in hydrogen.

3.3IR Characterization

Figures4and5show the IR spectra of the Pt–Sn/Al2O3 catalysts coked with different gas compositions.Several absorption bands appear between600and3000cm-1,from which different characteristics of the cokes can be identi?ed. Generally,the bands at1350–1470cm-1re?ect the bend-ing vibrations of C–H in CH2and CH3groups[9],while the stronger absorption at2850–2960cm-1re?ects the stretching vibrations of C–H in CH,CH2and CH3groups [9–12].The vibration bands of C–H in alkenes(between675 and1000cm-1)[13]overlap the deformation vibration bands of C–H in aromatics(between680and880cm-1) [14],while the bands between1640and1680cm-1(rep-resenting the stretching vibrations of C=C in alkenes[13]) overlap the bands1660–2000cm-1(representing the external-plane-bending vibration of C–H in aromatic rings [15]).The bands between1450and1600cm-1are origi-nated from the skeleton vibration of C=C in aromatic rings [12,16].The absorption at3000–3020cm-1is the charac-teristic of C=C bonds in aliphatic hydrocarbon,while the absorption at3020–3200cm-1is the characteristic of C=C bonds in aromatics[9,17].Based on this information,the strongest absorption at about2920cm-1is chosen to rep-resent the aliphatic nature of the coke and the absorption at around3060cm-1is chosen as the sign of the aromatic nature of the coke.The involved bands are shown in Figs.4 and5,in which,the absorption intensities at2920cm-1are scaled to the same magnitude for convenience of comparison.

As shown in Figs.4and5,the intensity at3060cm-1 increases with the partial pressure of propylene but decreases with the partial pressure of hydrogen.Propylene promotes while hydrogen inhibits the formation of aro-matic coke.These results are in good agreement with the shift of Peak II in Figs.1and2.

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3.4Raman Spectral Analysis

Figure6shows the Raman spectra of the coked catalysts (Samples S6,S7,S8,S9,and S10).Five peaks are identi-?ed at1336,1602,2700,2920,and3200cm-1,respec-tively.The bands at1336and1602cm-1are generally considered as the D band and G band[18,19]and attrib-uted to the ring stretching in polyaromatic compounds, which are considered to be the graphite-like carbon species [20,21].The region of2700–3000cm-1is supposed to be the C–H stretching in alkyl hydrocarbon[19,21,22]while the band at3200cm-1is regarded as the C–H stretching in aromatics[21,22].The intensity ratio of D band to G band is always used as an important measure of the degree of graphitization of carbon materials[23,24].A high ratio indicates a low degree of graphitization.Table3lists the I D/I G ratios of the coked catalysts.

As shown in Table3,the I D/I G ratios of Samples S6–S9 are very close to each other.However,the I D/I G ratio of the coked Pt/MgO catalyst(Sample S10)is large indicating the coke on the Pt/MgO catalyst has a low degree of graphi-tization.This is consistent with the results of elemental analysis and indicates that the coke on the alumina and Pt–Sn catalysts has a H/C ratio lower than that on the Pt/MgO catalyst.Furthermore,as shown in Fig.6,the lower intensity of C–H vibration in the aromatics(3200cm-1)on coked Pt/MgO catalyst were observed comparing to other samples.It indicates that the degree of graphitization of the coke on the Pt surfaces is lower,and coke molecules are more like ole?n/paraf?n.

3.5Reaction Orders to Propane,Propylene

and Hydrogen

Kinetic experiments were carried out under different gas compositions as summarized in Table2,where the aver-aged compositions of gas between inlet and outlet of the reactor are also listed.During the course of reaction for 80min,we observed that the amounts of the two types of coke were both increasing approximately linearly with time on stream(not shown here).The linear build-up of coke with time was also reported by Kumar et al.[25].There-fore,the average coking rates were estimated in the present

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work based on the measured coke contents in80min of time on stream by assuming a constant coking rate.

The coking rates r1and r2were determined based on Peak I and II from the TPO spectra.The logarithms of the rates were then plotted against the logarithms of the partial pressures of propane,propylene and hydrogen,respec-tively,as shown in Figs.7,8and9.

Figure7shows that r1is independent of the propylene pressure.As a result,Samples S1–S5and Sample S7are all used to?nd the in?uences of the hydrogen pressure on r1, because these samples were obtained under similar partial pressures of propane.Figure8shows that r1is also inde-pendent of the partial pressure of hydrogen.Samples S1–S8are then used to?nd the in?uences of the propane pressure on r1.Figure9shows that r1is dependent on the partial pressure of propane with an apparent reaction order of1.7.Summarizing Figs.7,8and9we see that r2is dependent on the partial pressures of propane,propylene and hydrogen and the reaction orders are1.4,1.0and-0.7, respectively.

4Discussions

4.1Assignment of Peaks in the TPO Pro?les

TG is a very useful method to identify and quantify the coke on the catalysts[26–28].The peaks in the TPO pro-?les can be attributed to the loss of carbonaceous deposits with different compositions and structures and/or the car-bonaceous deposits located at different sites on the catalyst. The two peaks identi?ed in the TPO pro?les of coked catalysts shown above indicate that two types of coke are formed.

The two types of coke are usually considered to be formed on the metal and support,respectively[29,30].To con?rm the locations of the coke,comparative experiments were carried out on pure alumina,which was used as the support for the Pt–Sn catalyst,and the Pt/MgO catalyst, which had no acid sites on the support(Table1).For coked alumina,a single peak emerges at around470°C in the TPO pro?le,while for coked Pt/MgO catalyst the peak appears at around310°C.This information con?rms that Peak I and Peak II in the TPO pro?les of Samples S1–S8 are corresponding to the coke formed on the metal and support,respectively.As a result,in this study,we assign Peak I to the coke deposited on the metal and Peak II to the coke deposited on the support.

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4.2The Nature of Coke

It is interesting to note from Table3that the ratios of the combustion heats of the two types of coke are much higher than the ratios of the masses determined by TPO.This fact clearly indicates that the coke on the metal has a higher H/C ratio than that on the support,which is consistent with the results of elemental analysis(Samples S9and S10). Based on the results of TPO,TG,and DTA of the Pt/Al2O3 and Pt–Sn/Al2O3catalysts coked in n-butane dehydroge-nation,Zhang et al.[31]also suggested that the coke formed on the metal had a higher hydrogen content.The same conclusion was obtained by Srihiranpullop et al.[32] when studying the coke formation on Pt,Pt–Sn,and Pt–Sn–K catalysts during n-hexane dehydrogenation.In addition,we note that Peak I shifts to higher temperature when the propane pressure is increased.This is because the coke covers the metal surface and reduces the rate of coke combustion by increasing the resistance of oxygen diffu-sion[29].

The Raman spectral analysis of Samples S9and S10 shows that the I D/I G ratio of Sample S9is much lower than that of Sample S10,which indicates that the coke formed on the support is more graphitized.The very weak intensity of the band at3200cm-1in the Raman spectrum of coked Pt/MgO catalyst also con?rms that the coke on the metal is less dehydrogenated.

Based on this analysis,the catalyst samples containing more coke on the support should have lower I D/I G ratios. Hence the I D/I G ratios of Samples S6,S7,S8,S9,and S10 should have been in the decreasing order of S10[S7[S8[S6[S9.However,the I D/I G ratio of Sample S8is the lowest,and the coke is hard to burn,as indicated by the rightward shift of the position of peak II of Sample S8.This is because the high propane concentration greatly increases the concentration of coke precursor on the support,which leads to an increased degree of polymeri-zation and aromatization of the coke.

Increasing the propylene concentration will increase the degree of polymerization of the polymers[33],which will be further dehydrogenated to aromatics.The degree of the aromatization of the coke is enhanced as a result of the increased propylene concentration and weakened as a result of the increased hydrogen concentration,as hydrogen will reduce the molecular weight of the polymer during propylene polymerization[34].This fact also explains the shift of Peak II in Figs.1and2.

In general,the H/C ratios of the cokes are very high (Table1),especially on the metal surfaces,indicating that the coke contains mainly aliphatic hydrocarbons.The high H/C ratios on metal surfaces are consistent with the relatively large peak of2920cm-1in IR spectra and the relatively large peak of2700–3000cm-1in Raman spectra.

4.3Dependence of Coking Rate on Gas Concentration The rate of coke formation on the metal is found dependent on the propane pressure(Fig.9).The apparent reaction order is about1.7with respect to propane and zero reaction order with respect to hydrogen and propylene.Figures7 and9also show that the rate of coke formation on the support increases with the propylene or propane pressures, while Fig.8shows that it decreases with the hydrogen pressure.The apparent reaction orders of coking on the support are about?rst order with respect to propylene (Fig.7),-0.7order with respect to hydrogen(Fig.8)and 1.4order with respect to propane(Fig.9).It suggests that coke can be formed both from propane and propylene,but with different reaction mechanisms.An apparent reaction order of1.4with respect to propane can not explain that propylene is the main precursor for coke formation,since the reaction order to propane is about one for propylene formation,and the reaction order to propylene is about one for coke formation.

4.4Coke Formation Mechanism

4.4.1Coke Formation on the Metal

Many reaction mechanisms have been tested,and only the following mechanism of coke formation on the metal can describe the experimental observation:

C3H8eg)t2?)C3H7?tH?e1TC3H7?t?)C3H6?tH?e2TC3H6?)C3H6eg)t?e3T2C3H6?)C6H12?t?e4TH?tH?,H2t2?e5TThe apparent reaction order of propane of about1.7 indicates that two C3intermediates are involved in the carbon formation.Furthermore,the H/C ratios of the coked Pt–Sn catalysts of1.7–2.0indicate a high hydrogen content of the coke.Therefore,the formation of C6H12*from two C3H6*intermediates is proposed as the kinetic relevant step for coke formation(Step4).

It is assumed that the dehydrogenation steps(Steps1 and2)are both irreversible and far from equilibrium [35–37],and the desorption of propylene(Step3)is also considered irreversible.Step4is assumed to be the kinetic relevant step and the rate of Step4is considered to be much slower than those of Steps2and3.

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By assuming the steady-state of the intermediates, C3H7*and C3H6*,we obtain,

d h C3H7

dt

?k1P C3H8h2?àk2h C3H7h??0e6T

d h C

3

H6 d t ?k2h C

3

H7

h?àk3h C

3

H6

?0e7T

Then the site coverages of C3H7*and C3H6*are determined by,

h C

3H7

?

k1P C

3

H8

h

?

k2

e8T

h C

3H6

?

k1P C

3

H8

h2

?

k3

e9T

The coke formation rate on the metal is given by Eq.

(10)by assuming C3H7*as the most abundant surface specie:

r c?k4h2C

3H6

?

k c P2

C3H8

e1tK I P C

3

H8

T4

e10T

where,

k c?k4

k1

k3

2

e11T

K I?k1

k2

e12T

Increasing propane concentration in the gas phase concurrently increases the concentration of adsorbed propylene(C3H6*)and hence the amount of coke on the metal.A relatively low value of K I in Eq.(10)can lead to the apparent order of1.7with respect to propane.The irreversible desorption of propylene subsequently leads to the zero order to propylene.The irreversible reaction of steps in propane dehydrogenation and relatively weak adsorption of hydrogen comparing to the C3surface intermediate result in the weak effect of hydrogen. By?tting the rates of coking on the metal at different gas compositions,k c and K I are estimated to be 4.15910-5mg coke/(mg cat.s)and7.18910-1, respectively.Moreover,one can see that the model?ts the experimental data quite well(Fig.10),which implies the proposed kinetic model for coking on the metal is reasonable.

It is noted here that although the hydrogen pressure has little effect on the rate of coking on the metal,it changes signi?cantly the hydrogen content in the coke.At higher hydrogen partial pressure,the coke is less dense and compact.In addition,the coke formed on the metal can be further softened through hydrogenation[38].4.4.2Coke Formation on the Support

Coke formation on the support mainly involves poly-merization/oligomerization,condensation,cyclization, hydride transfer,etc.Referring to the mechanism proposed by Caeiro et al.[39],coke formation on the support can be divided into two stages,i.e.,the conversion of propane to ole?ns(propylene and ethylene)by protolysis and trans-formation of these ole?ns to aromatic hydrocarbons.The second stage involved a number of reactions,such as oligomerization–cracking reactions(producing C4–C10 ole?ns),hydrogen transfer(producing dienes),cyclization (generating cycloalkenes),and further hydrogen transfer (producing cyclic diole?ns and?nally aromatics).Based on this mechanism,increasing the propylene partial pressures will certainly increase the amount of coke on the support.

The coke precursor formed on the metal may migrate to the support[32]and then undergoes subsequent poly-merization/oligomerization,condensation and so on.Thus, increasing the partial pressure of propane would increase the rate of coke formation on the support.Sn in the Pt catalyst will weaken the binding of hydrocarbon to the metal[30,40],and promote the migration of the coke precursor from the metal to the support.The presence of hydrogen will weaken the acidity of the support by con-verting Br?nsted acid sites to Lewis acid sites and thus reduce the coke formation rate.Based on the above dis-cussions,a coke formation mechanism is proposed as in Fig.11.Propane is?rstly dissociated on the metal and the coke precursor is formed through dehydrogenation;then the‘‘soft coke’’is generated on the metal from the coke precursor.The coke precursor generated on the metal will also migrate to the acid sites.On these acid sites,the coke precursor and the adsorbed propylene undergo poly-merization/oligomerization,condensation,cyclization and

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hydride transfer,etc.,resulting in the formation of ‘‘hard coke’’.

5Conclusion

Two types of coke are identi?ed on coked Pt–Sn/Al 2O 3catalyst and assigned to coke on the metal and the support,respectively.Coke formed on the metal has aliphatic hydrocarbon characteristic,containing more hydrogen than that formed on the support.Coke formed on the support has an aromatic characteristic.The rate of coke formation on the metal is weakly dependent on the propylene and hydrogen pressures but increases concurrently with the propane pressure while the rate of coke formation on the support increases concurrently with the propane and pro-pylene pressures and decreases with the hydrogen pressure.A mechanism for coke formation has been proposed based on the kinetic analysis.The reaction between the two strong adsorbed C 3H 6*,which was formed by dehydroge-nation of propane,was identi?ed as the kinetic relevant step for the coke formation on the Pt surfaces.A portion of the precursor migrates to the acid site and is involved in the coke formation on the support.In addition,the propylene in the gas phase can also adsorb on the support and form coke through reactions such as polymerization/oligomerization,condensation and so on.

Acknowledgment This work is supported by Natural Science Foundation of China (No.20736011)

References

1.Galvita V,Siddiqi G,Sun P,Bell AT (2010)J Catal 271:209

2.Liu H,Zhang L,Li X,Huang S,Liu S,Xin W,Xie S,Xu L (2009)J Nat Gas Chem 18:331

3.Sun K (2004)Petrochem Des 21:25

4.Cosyns J,Chodorge J,Commereuc D,Torck B (1998)Hydrocarb Process 77:61

5.Praserthdam P,Mongkhonsi T,Kunatippapong S,Jaikaew B,Lim N (1997)Stud Surf Sci Catal 111:153

6.Santhosh Kumar M,Holmen A,Chen D (2009)Microporous Mesoporous Mater 126:152

https://www.360docs.net/doc/c111392998.html,rsson M,Hulte

′n M,Blekkan EA,Andersson B (1996)J Catal 164:44

8.Rebo HP,Chen D,Blekkan EA,Holmen A (1998)Stud Surf Sci Catal 119:617

9.Yang JL,Stansberry PG,Zondlo JW,Stiller AH (2002)Fuel Process Technol 79:207

10.Eisenbach D,Gallei E (1979)J Catal 56:377

11.Van Doorn J,Moulijn JA (1990)Fuel Process Technol 26:3912.Cerqueira HS,Sievers C,Joly G,Magnoux P,Lercher JA (2005)

Ind Eng Chem Res 44:2069

13.Geach A (1996)Wear Check Afr Tech Bull 2:1

14.Wolthuis E,Bossenbroek B,DeWall G,Geels E,Leegwater A

(1963)J Org Chem 28:148

15.Sato K,Ikeda S,Iida M,Oshima A,Tabata Y,Washio M (2003)

Nucl Instrum Methods Phys Res Sect B 208:424

16.Matsushita K,Hauser A,Mara?A,Koide R,Stanislaus A (2004)

Fuel 83:1031

17.Jong SJ,Pradhan AR,Wu JF,Tsai TC,Liu SB (1998)J Catal

174:210

18.Guichard B,Roy-Auberger M,Devers E,Rebours B,Quoineaud

AA,Digne M (2009)Appl Catal A 367:1

19.Korhonen ST,Airaksinen SMK,Ban

?ares MA,Krause AOI (2007)Appl Catal A 333:30

20.Zeng Z,Natesan K (2003)Chem Mater 15:872

21.Airaksinen SMK,Ban

?ares MA,Krause AOI (2005)J Catal 230:507

22.Chua YT,Stair PC (2003)J Catal 213:39

23.Dumont M,Chollon G,Dourges M,Pailler R,Bourrat X,Naslain

R,Bruneel JL,Couzi M (2002)Carbon 40:1475

24.Li J,Naga K,Ohzawa Y,Nakajima T,Shames AP,Panich AI

(2005)J Fluor Chem 126:265

25.Santhosh Kumar M,Chen D,Holmen A,Walmsley JC (2009)

Catal Today 142:17

26.Yang Z,Zhang Y,Wang X,Zhang Y,Lu X,Ding W (2010)

Energy Fuels 24:785

27.Sun L,Guo X,Liu M,Wang X (2010)Ind Eng Chem Res 49:50628.Li X,Zhang W,Li X,Liu S,Huang H,Han X,Xu L,Bao X

(2009)J Phys Chem C 113:8228

29.Duprez D,Hadj-Aissa M,Barbier J (1989)Appl Catal 49:6730.Liwu L,Tao Z,Jingling Z,Zhusheng X (1990)Appl Catal 67:1131.Tao Z,Jingling Z,Liwu L (1991)Stud Surf Sci Catal 68:14332.Srihiranpullop S,Praserthdam P,Mongkhonsi T (2000)Korean J

Chem Eng 17:548

33.Novokshonova LA,Tsvetkova VI,Chirkov NM (1963)Russ

Chem Bull 12:1077

34.Soga K,Siono T (1982)Polym Bull 8:261

35.Li Q,Sui Z,Zhou X,Chen D (2011)Appl Catal A 398:18

36.Azzam KG,Jacobs G,Shafer WD,Davis BH (2010)Appl Catal

A 390:264

37.Biloen P,Dautzenberg FM,Sachtler WMH (1977)J Catal 50:7738.Liu K,Fung SC,Ho TC,Rumschitzki TC (2003)Ind Eng Chem

Res 42:1543

39.Caeiro G,Carvalho RH,Wang X,Lemos MAND,Guisnet M,

Ramo

?a Ribeiro F (2006)J Mol Catal A 255:13140.Lieske H,Sa

′rka ′ny A,Vo ¨lter J (1987)Appl Catal 30:69Propane

Al 2O 3

Pt Top Catal

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