Conversion of Light Alkane to Value-Added Chemicals over ZSM-5/Metal Promoted CatalystsAnupam Samanta,† Xinwei Bai,† Brandon Robinson,† Hui Chen,*,‡ and Jianli Hu*,†

†Department of Chemical and Biomedical Engineering, Center for Innovation in Gas Research and Utilization, West VirginiaUniversity, Morgantown, West Virginia 26506, United States‡School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China

ABSTRACT: Gallium−platinum promoted HZSM-5 is found to be a promisingcatalyst for ethane aromatization reaction. The influence of Pt as a promoter onthe activity of Ga/HZSM-5 catalyst for ethane aromatization has been investigated.Comparative study was performed between bimetallic Ga−Pt based and Mo basedHZSM-5, where the GaPt/HZSM-5 showed better aromatic and hydrogenselectivity. Pt promoted Ga/HZSM-5 catalyst exhibited higher activity comparedto pure Ga/HZSM-5 catalyst. The presence of platinum in the gallium zeoliteconsiderably accelerated dehydrogenation step in ethane aromatization. Inaddition, GaPt/HSZM-5 deactivated significantly slower than Mo/HZSM-5 and Ga/HZSM-5. TPO study of spent catalystsrevealed that carbonaceous deposit on GaPt/HZSM-5 catalyst was burnt off at lower temperature compared to pure Ga/HZSM-5 catalyst, indicating the presence of Pt facilitated hydrogen spillover resulting in hydrogenolysis of coke precursors. The reactionmechanism associated with aromatic formation is postulated based on the correlation between catalytic performance and surfacecharacterization.

1. INTRODUCTION

From the past few decades, research on direct transformation oflower saturated hydrocarbons (alkanes) into aromatics hasgained considerable importance because of its potentialimplementation in the production of chemicals or fuels fromnatural gas. Ethane, one representative of lower alkanes, is animportant constituent of natural gas. Depending on its source,natural gas may contain ethane from traces to more than 10% .1

Usually, shale gases contain mainly methane but flaring gasfrom Bakken, North Dakota, contains over 20% ethane andpropane.2 Also the price of ethane is actually lower than naturalgas in some regions of the United States. Moreover, ethane isan important component in various refinery gases. Demand ofcrude oil in the United States could be brought down throughupgrading of these gases to value-added chemical. Nevertheless,the conversion of ethane to higher hydrocarbons is limited bythermodynamic barrier even at high temperature (700 K) andalso deposition of carbon residue is highly favored at such hightemeperature.3 Accumulation of carbon on the surface of thecatalyst blocks its active sites, resulting in reduction of catalyst’sactivity either partially or fully. Bragin and co-workers achievedhigher activity and effective selectivity for aromatic formationfrom ethane on platinum/palladium metal supported pentasil-like zeolites.4,5 ZSM-5 type zeolites are extensively explored forthe study of aromatization of short-chain hydrocarbons.6,7 Thistype of pentasil zeolites behaves uniquely in terms of catalyticactivity for the conversion of short-chain hydrocarbons toaromatic. This is attributed to their unique channel dimensionsand acidic properties.8

Propane to aromatic transformation using promoted galliumcatalyst to increase selectivity is well-known.9−12 This type of

catalyst is effective for the aromatization of ethane reaction also.Gallium exchanged H-ZSM-5 catalyst was utilized in ethaneconversion by Ono et al., and they reported aromatic yield of16.1% at 873 K.13 They have proposed that dehydrogenation ofethane is the first step to form ethylene on gallium activespecies and further ethylene undergoes oligomerization toproduce aromatic over zeolite acidic sites. Yakerson et al.conducted extensive IR spectroscopy and analytical electronmicroscopy studies to find out the active species for thetransformation of ethane to aromatic, and they have concludedthat gallium oxide situated on the external surface of the zeolitewould be active sites for the reaction.14 Platinum metal isshown to be effective species for ethane aromatization reactioneither as an active catalyst or as a promoter associated withother metal catalyst. Aromatization of ethane with high activityand selectivity was reported by Steinberg et al.1 However, theperformance of Pt catalyst’s activity diminished to a large extentwith time on stream because of high coking even usingplatinum at very low loading. Chetina et al. had shown thataddition of platinum to Ga-loaded zeolite resulted in substantialincrement of its activity in the aromatization of ethane, andthey have suggested that higher dehydrogenation activity ofplatinum improved aromatic yield and selectivity.15

Several catalytic data on ethane dehydroaromatization byusing gallium and platinum catalysts are available, but there islack of detailed discussion on the performance of catalysts

Received: May 22, 2017Revised: August 20, 2017Accepted: September 1, 2017Published: September 1, 2017

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© 2017 American Chemical Society 11006 DOI: 10.1021/acs.iecr.7b02095Ind. Eng. Chem. Res. 2017, 56, 11006−11012

correlating with their inherent property. There is growinginterest to understand intrinsic properties of the catalyst at newlevel to improve their catalytic activity. In the present work,bimetallic GaPt/HZSM-5 catalyst has been utilized for ethaneto aromatic (benzene and toluene) conversion as illustrated inFigure 1. The influence of platinum as promoter on the ethane

dehydroaromatization activity of Ga/ZSM-5 catalyst has beeninvestigated. Also comparison was performed between platinumpromoted gallium catalyst and pure molybdenum catalyst, andplatinum promoted catalyst had shown better aromatic andhydrogen selectivity than the molybdenum one under thereaction conditions used.

2. EXPERIMENTAL SECTION2.1. Catalyst Synthesis. The ammonium ZSM-5 zeolite

with SiO2/Al2O3 mole ratio (SAR) of 50 was supplied byZeolyst Inc. Ammonium heptamolybdate tetrahydrate((NH4)6Mo7O24·4H2O), gallium nitrate hydrate Ga(NO3)3·xH2O, and chloroplatinic acid hexahydrate H2PtCl6·6H2O werepurchased from Fischer Scientific.The NH4-ZSM-5 powder was calcined at 500 °C in air for 3

h to convert the powder from the ammonium form to itsprotonated form (HZSM-5). Conventional incipient wetnessimpregnation was used to prepare Mo, Ga, Pt, and GaPtcatalysts. Typically, gallium nitrate hydrate salt correspondingto 3 wt % Ga was dissolved in deionized water and was addeddropwise to HZSM-5 and the mixture was kept for 12 h fordrying at 100 °C. The powder was then calcined in air at 550°C. Molybdenum catalyst was also prepared following the sameway as gallium. Similarly Pt promoted gallium catalysts wereprepared by coimpregnation method. Details of all the catalystwith their metallic concentration are listed in Table 1.

2.2. Catalyst Characterization. Powder X-ray diffractionanalysis was performed on a PANalytical X’Pert Pro X-raydiffraction working under 45 kV and 40 mA using Cu Kαradiation. The 2θ angles were scanned from 5° to 50° (2θ). AnX’celerator solid-state detector with a scan speed of 4.8° min−1

was employed. Nitrogen adsorption/desorption isotherms wereacquired using a Micromeritics ASAP-2020 unit. The programconsisting of both adsorption and desorption branches typicallyran at −196 °C after the sample was degassed at 300 °C for 3 honce the final temperature had been maintained. The specific

surface area was calculated via the BET model. The H2-temperature-programmed reduction (H2-TPR) was carried outto study reducibility of the catalysts with MicromeriticsAutochem 2950. The catalyst (100 mg) was pretreated at100 °C under argon flow (50 mL/min) for 1 h and then cooledto 50 °C. TPR was performed from 50 to 900 °C with aramping rate of 10 °C/min under 10% H2 in argon (50 mL)flow. Temperature-programmed desorption of ammonia (NH3-TPD) was conducted using Micromeritics ASAP-2020 unit.Prior to each TPD run catalyst (200 mg) was dried at 500 °Cfor 30 min with pure He (50 mL/min). Then catalyst wasallowed to cool down at room temperature and final weight wastaken. Catalyst was heated up again to 150 °C at a ramp of 10°C/min and exposed to 30 mL/min of 15% NH3 in He for 35min. Then catalyst was purged with pure He for 30 min toremove excess NH3 before starting temperature ramp up to 750°C (5 °C/min) to get the NH3 desorption profile. Temper-ature-programmed oxidation (TPO) was performed with 30mg of spent catalyst under a flow of 50 mL (10% O2 in He)mixture by heating from room temperature to 750 °C with arate of 10 °C/min with a thermal conductivity detector inMicromeritics Autochem 2950. A JEOL TEM 2100 electronmicroscope operating at 200 kV was used for TEM sampleobservations. Samples for TEM were prepared by evaporating adroplet of dispersed catalyst in isopropanol onto a nickel mesh200 grid.

2.3. Catalytic Study. The reaction was carried out in acontinuous-flow reaction system at atmospheric pressure inMicromeritics Autochem 2950 unit. A quartz tubular reactorwith an inner diameter of 8 mm was packed with 100 mg ofpowder catalyst for testing. Before the reaction, the catalyst washeated in helium flow (50 mL/min) to 650 °C with a heatingrate of 10 °C/min and kept at this temperature for 90 min.Then the feed consisting of 30 vol % C2H6 + 70 vol % He wasintroduced with a flow rate of 50 mL/min. Helium was used asan internal standard to account for the changes of ethane flowrate due to the reaction.

2.4. Product Analysis. Benzene, toluene, and hydrogenwere analyzed using Pfeiffer Omnistar mass spectrometer (MS)connected with the reactor. Amount of naphthalene producedin this reaction was not accounted. Mass spectrometer wascalibrated with the appropriate standard gas mixtures.Conversion of ethane was calculated based on hydrogenbalance of the reaction. Also conversion of ethane andselectivity of products are calculated on mol % and wt %(hydrocarbon) basis, respectively.

3. RESULTS AND DISCUSSIONFigure 2 represents X-ray diffraction pattern for all freshlyprepared catalysts together with HZSM-5. High intensity ofdiffraction peak at 2θ = 22−25° reveals that the zeolitecrystallinity remains unaltered for all the catalysts even aftermetal loading. XRD patterns with the absence of any metaloxide peak indicate good metal dispersion with small particlesize on the surface of the zeolite.16,17

BET surface areas measured for HZSM-5, Mo/HZSM-5, Ga/HZSM-5, GaPt/HZSM-5, and Pt/HZSM-5 catalysts are 380,360, 360, 370, and 380 m2/g, respectively.H2-TPR profiles of Mo/HZSM-5, Ga/HZSM-5, and GaPt/

HZSM-5 are presented in Figure 3. The reduction profile ofMo/H-ZSM-5 catalyst exhibits two distinct reduction peaks.The first peak at 470 °C corresponds to the reduction of MoO3to MoO2, and the second peak at 630 °C is due to the

Figure 1. Ethane to aromatic conversion on GaPt/HZSM-5 catalyst.

Table 1. Composition of the Catalysts Used in This Study

catalyst Ga, wt % Pt, wt % Mo, wt %

Mo/HZSM-5 0 0 3Ga/HZSM-5 3 0 0GaPt/HZSM-5 2.5 0.5 0Pt/HZSM-5 0 0.5 0

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reduction of MoO2 to metallic Mo.18,19 The addition ofplatinum onto the Ga/H-ZSM-5 catalyst decreased thereduction temperature of gallium from 480 to 425 °C. Thepresence of platinum increased the reducibility of gallium oxideand resulted in a shift in the reduction temperature of thegallium oxide peak.Probably a hydrogen spillover mechanism is responsible for

this type enhancement of reducibility of gallium oxide. Inhydrogen spillover mechanism, first hydrogen moleculedissociates into hydrogen atom through its adsorption onplatinum active sites, and then these dissociated hydrogenatoms migrate to the gallium oxide interface and readily helpreduction of gallium oxides even at lower temperature. Thistype of observation was reported earlier in the case of platinumpromoted molybdenum catalyst.20

The total number of acid sites on the catalysts was measuredby ammonia-temperature-programmed desorption (NH3-TPD)analysis. From this characterization, information about thedifferent types of acid sites and their strength can be obtainedon a regular basis. Figure 4 represents NH3-TPD profiles ofHZSM-5 before and after loading with different metals. PureHZSM-5 exhibited two NH3 desorption peaks at 213 °C (T1)and 406 °C (T2) from weak acid sites (mostly Lewis acid) andstrong acid sites (mostly Bronsted acid), respectively.17

Quantitative analysis results of different acid sites and their

NH3 desorption temperature corresponding to acid strengthare given in Table 2.

Mo/HZSM-5 has shown higher amount of weak acid sites(0.30 mmol/gcat) compared to the parent HZSM-5 (0.15mmol/g) due to the presence of highly dispersed MoO3 thatgenerates extra Lewis acid sites. Mo/HZSM-5 and Ga-HZSM-5have exhibited lower amount of strong acid sites as a result ofexchange of zeolite proton by Mo and Ga metal ions. For Mo/HZSM-5, the total amount of acid sites increases (0.52 mmol/gcat) in comparison with HZSM-5 (0.44 mmol/gcat) but thestrength of strong acid sites is decreased significantly, reflectedby NH3 desorption temperature (386 °C) while HZSM-5 has406 °C. Pt/HZSM-5 has shown that NH3 desorptiontemperature shifted to higher values (221 °C, 429 °C) forweak and strong acid sites respectively but the total amount ofacid sites remains almost unchanged (0.44 mmol/gcat). GaPt/HZSM-5 behaves almost similar to that of Pt/HZSM-5 catalyst.The effect of catalyst composition on the catalyst activity and

stability in ethane dehydroaromatization reaction was inves-tigated at 650 °C for 5 h on-stream. Before observation ofprogress of the reaction with a mass spectrometer, the catalystswere first exposed to helium gas for 1.5 h. Benzene, toluene,ethylene, and hydrogen are the main products detected inethane dehydroaromatization reaction. Product composition ofthe reaction stream at 50 min time is given in Table 3. Totalhydrocarbon (benzene, toluene, and ethylene) is considered as100 wt %. Hydrogen is given as vol % directly from the outletstream of the reaction.The results obtained during the aromatization of ethane over

Mo, Ga, GaPt, and Pt loaded zeolite are presented in Figures5−7 for ethane conversion, aromatic (benzene + toluene)

Figure 2. X-ray diffraction patterns for (a) HZSM-5, (b) Mo/HZSM-5, (c) Ga/HZSM-5, (d) GaPt/HZSM-5, and (e) Pt/HZSM-5 freshlyprepared catalyst.

Figure 3. Hydrogen TPR profiles of (a) Ga/HZSM-5, (b) Mo/HZSM-5, and (c) GaPt/HZSM-5 catalysts.

Figure 4. NH3-TPD profiles of HZSM-5 and different metal loadedcatalysts.

Table 2. Quantitative Analysis of NH3-TPD Profiles ofDifferent Metal/HZSM-5 Catalysts

desorption temp(°C)

MNH3(mmol NH3/gcat)

catalyst T1 T2 T1 T2 MNH3 (total)

HZSM-5 213 406 0.15 0.29 0.44Mo/HZSM-5 217 386 0.30 0.22 0.52Ga/HZSM-5 218 407 0.17 0.25 0.42GaPt/HZSM-5 221 415 0.17 0.28 0.45Pt/HZSM-5 221 429 0.16 0.28 0.44

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selectivity, and ethylene selectivity, respectively. Figure 8presents concentration of hydrogen in the reaction stream forethane dehydrogenation reaction by various catalysts. Figure 5illustrates that there is remarkable increment of ethaneconversion (80%) by GaPt/HZSM-5 among the four catalystsin the initial period of reaction (100 min). Ethane conversionby Ga catalyst is accelerated from 30% to 80% due to smallamount of (0.5 wt %) Pt addition. Upon Pt addition similartype of enhancement was accounted for in the case of aromaticselectivity also. As shown in Figure 6, GaPt/HZSM-5 exhibits a

maximum selectivity of 40 wt % aromatic at 25 min of reactionduring time-on-stream. It takes 100 min for GaPt/HZSM-5catalyst to drop aromatic selectivity up to 25%. In comparisonwith GaPt/HZSM-5, Mo/HZSM-5 catalyst exhibited lowethane conversion (40%) and aromatic selectivity (35%)under similar reaction conditions. As illustrated in Figure 7,initially, ethylene selectivity decreases due to aromaticformation but increases with time-on-stream because of gradualreduction of aromatic production. Aromatic production isdecreased as a result of coke formation. It was observed that, in

terms of hydrogen generation, GaPt/HZSM-5 exhibits the bestresult compared to the other catalysts (Figure 8).

The very high activity of GaPt/HZSM-5 catalyst indehydroaromatization of ethane can be explained based onthe function of Pt as promoter on Ga catalyst performance. Fewstudies are published on lower alkane conversion to aromaticby applying Ga catalyst along with Pt as promoter. Sattler et al.had shown that Pt promoted Ga/Al2O3 material is a highlyactive, selective, and stable catalyst for dehydrogenation ofpropane.21 They have suggested a bifunctional active phaseconsisting of coordinately unsaturated Ga3+ species and Ptwhere the Ga3+ is the main active species and Pt behaves as apromoter for dehydrogenation reaction. Mikhailov et al.performed an investigation to deduce the structure of activesites in a mixed Pt/GaZSM-5 catalyst for aromatization reactionby applying the DFT cluster method.22 For modeling of theactive sites of Pt/GaZSM-5 catalyst, Ga2Pt4Al2Si4O6H12 clusterwas employed. From the above literature it was pointed outthat the formation of alloy bimetallic particles is highly favoredowing to the introduction of the platinum atoms into thegallium zeolite. It was assumed that the presence of Ga atomsinside the zeolite structure stabilizes the bimetallic particles inthe zeolite framework. As a consequence, active sites for alkanedehydrogenation are generated. Also theoretical investigationrevealed that the activation energy for dehydrogenation (C−Hbond activation) is minimized because of Pt incorporation andthis makes the dihydrogen desorption step easier also.Zaikovskii et al. prepared platinum and palladium incorporatedgalloaluminosilicate (GAS) to study ethane to aromatichydrocarbons conversion.23 They had shown that activity and

Table 3. Product Composition of the Reaction Stream at 50min

catalystbenzne(wt %)

tolune(wt %)

ethylene(wt %)

hydrogen(vol %)

Mo/HZSM-5 17.08 4.86 78.06 9.10Ga/HZSM-5 15.06 8.20 76.74 10.26GaPt/HZSM-5 18.97 12.14 68.89 22.55Pt/HZSM-5 1.19 0.95 97.86 3.59

Figure 5. Conversion of ethane by different catalysts with time-on-stream.

Figure 6. Selectivity of aromatics by different catalysts with time-on-stream.

Figure 7. Selectivity of ethylene by different catalysts with time-on-stream.

Figure 8. Hydrogen concentration in reaction exit for ethanedehydroaromatization reaction by different catalysts.

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selectivity for the conversion of ethane to aromatic hydro-carbons are enhanced due to the modification of GAS withplatinum and palladium. They have concluded that the primarystep for ethane conversion, i.e., ethane dehydrogenation, isenhanced by the incorporation of Pt and Pd into the GAS. Thismakes the desorption of hydrogen and ethylene easier. Thecombination of active metal component with moderate acidsites of zeolite ensures that the activity of the catalyst is high inthe ethane aromatization reaction. Dehydroaromatization isprocessed through two important steps: the primary step is thedehydrogenation of hydrogen assisted by metal active sites toproduce ethylene, and the second one is the oligomerization ofethylene in the presence of moderate acid sites, mainlyBronsted acid sites. Metals like Mo and Ga can replaceBronsted proton, resulting in change of acidity of thezeolite.15,17 Our NH3-TPD analysis has reflected thisphenomenon (Table 2). Since Pt does not substitute Bronstedproton at all,15 Pt has little influence on the acid sites of zeolite.NH3-TPD analysis demonstrates that addition of Pt metal onparent zeolite does not change the amount of strong acid sites(0.29−0.28 mmol/gcat), but strength of strong acid sites isincreased, reflected by NH3 desorption temperature incrementfrom 406 to 429 °C. GaPt/HZSM-5 catalysts have shown littlemore Bronsted acid sites (0.28 mmol/gcat) and strength (T2 =415 °C) in comparison with Ga/HZSM-5 (0.25 mmol/gcat andT2 = 407 °C). Here we have observed significant improvementin the activity of Ga/HZSM-5 catalyst promoted by Pt metal.This improvement is expected as a result of Ga−Pt intrinsicmetal interaction along with influence of zeolite acidity sinceaddition of Pt has slightly increased the number of acid sitesand strength (Bronsted acid) compared to that of Ga/HZSM-5.Therefore, literature provides evidence regarding the

formation of bimetallic active sites carrying both metals: Gaas active catalyst and Pt as promoter. Pt promoter makes theprocess easier in dehydrogenation of ethane which is theprimary step for aromatic formation. Also the presence of twodifferent metals like Ga and Pt improves the acidic property ofzeolite observed through NH3-TPD analysis, resulting in higheraromatic selectivity through controlled ethylene oligomeriza-tion. In this scenario our Pt promoted Ga/HZSM-5 catalystmanifested higher ethane conversion and aromatic andhydrogen selectivity because of the formation of complexbimetallic catalyst containing Ga−Pt active sites.Carbon accumulation on the surface of metal/zeolite

catalysts is inevitable in a reaction at such high temperatures,like 650 °C. Color of the catalyst changing toward black duringreaction specifies that coke formation on the catalyst is one ofthe primary reasons for catalyst deactivation. Reactants areprevented from access to the active sites of metal surface andacidic sites of zeolite since coke makes a protective layer onthese and also coke blocks the channel mouth of zeolite thatcould cause loss of activity and shape selectivity. Due to thissevere coke accumulation around the active metal sites andBronsted acid sites, activity of the catalyst deteriorates rapidlyunder time-on-stream. As a result, conversion (Figure 5) andselectivity (Figure 6) declined very quickly for all the catalysts.Though at the beginning activity of GaPt/HZSM-5 wassignificantly higher compared to other catalysts, under time-on-stream actives sites are completely blocked by carbondeposition and promoter action of Pt was unable to rescuecatalyst’s activity from deterioration. To further inquire intocoke deposition on the catalysts, TPO measurements wereperformed, and the results are shown in Figure 9. TPO profile

of Mo/HZSM-5 showed two peaks at 460 and 560 °C, whichcorrespond to the soft or low temperature burnoff cokeassociated with Mo2C species and the hard or high temperatureburnoff aromatic type coke accumulated on the acidic sites ofthe zeolites, respectively.24 TPO measurements of spent Mocatalyst in the case of dehydroaromatization reaction arestudied extensively by several research groups.25,26 Mo catalystsexhibited three TPO peaks at 460, 510, and 560 °C,respectively. In the beginning TPO peak at 460 °C correspondsto the oxidation of carbonaceous material deposited on Mo2Clocating at external surface of zeolite, whereas the peak at 510°C is most likely considered to indicate the presence of cokeassociated with Mo2C inside zeolite channel. The last TPOpeak at high temperature (560 °C) is generated as a result ofoxidation of inert and irreversible coke deposited at Bronstedacid sites of zeolite. In our TPO study Mo/Mo/HZSM-5showed only two peaks at 460 and 560 °C, respectively.Presence of peak at 460 °C indicates the existence of soft cokeassociated with Mo2C at the external surface of zeolite. Also theoxidation peak at 460 °C suggests that soft coke is amorphouspolyolefinic in nature, and the high temperature peak at 560 °Ccorresponds to the hard coke which is polyaromatic type.24,27

The other three catalyst Ga/HZSM-5, GaPt/HZSM-5, and Pt/HZSM-5 exhibited only a high-temperature single peak at 580,554, and 606 °C, respectively. Temperature range indicatesthese hard cokes are polyaromatic in nature. Typically soft cokeor low temperature burnoff coke is formed inside the zeolitechannel due to oligomerization or cracking of ethylene andhard coke appears as a result of polycondensation of aromaticsover the Bronsted acid sites of the external surface of zeolite.28

Therefore, in the present TPO profile a single peak could beassigned to the carbonaceous deposits associated with the metalspecies located at the external surface of the HZSM-5 zeolite.Also absence of signal peak corresponding to the coke on theinternal surface of the metal species suggests that metal oxidesare prevented from diffusing into the channel under reactionconditions. For GaPt/HZSM-5 catalyst it was observed thatcoke burnoff occurs at lower temperature (550 °C) comparedto Ga/HZSM-5 (580 °C). This temperature shift can beascribed to the hydrogenolysis of coke due to spillover ofhydrogen atom on it in the presence of Pt on the catalyst, andthis process helps partial removal of coke from the surface ofthe catalyst.20 As a result, coke deposited on GaPt/HZSM-5 iscomparatively less hard than on Ga/HZSM-5, evidenced byshift of coke burnoff temperature (30 °C). Thus, Pt helps as apromoter to maintain stability of GaPt/HZSM-5 catalyst for a

Figure 9. TPO profiles for spent catalysts: (a) Mo/HZSM-5, (b)GaPt/HZSM-5, (c) Ga/HZSM-5, and (d) Pt/HZSM-5.

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longer time (100 min) and prevents its deactivation undertime-on-stream.To further investigate the deactivation of catalyst’s activity

under time-on-stream, TEM measurement was performed forGaPt/HZSM-5 catalyst. Fresh catalyst does not show anypresence of metal particle, and this proves active metals arehighly dispersed in nature (Figure 10a). During TEM

measurement, several regions on grid are scanned thoroughlyat high resolution, but typical lattice fringes of ZSM-5 wereobserved only (inset Figure 10a). Spent catalyst showed thepresence of metal particles of 5−8 nm in size (Figure 10b).Also EDS analysis has confirmed that these particles consist ofGa and Pt metals. During reaction at high temperature, 650 °Cdispersed metal particles are agglomerated and become biggerin size compared to fresh ones. Because increasing size, metalparticles of spent catalyst are visible at TEM analysis.Confinement of metal nanoparticles inside the channels ofzeolite is crucial for getting higher aromatic selectivity sincethese channels provide both acidity and shape selectiveenvironment for producing aromatics. Average pore diameterof ZSM-5 is ∼5.5 Å. During reaction at high temperature undertime-on-stream particles size gets bigger (5−8 nm), resulting inremoval of them from shape selective zeolite channelenvironment. This decreases aromatic selectivity along withactivity remarkably (Figures 5 and 6). Also selected are EDSanalysis (Figure 10b) of the spent catalyst that exhibited higheramount of Pt (3 wt %) and lower amount of Ga (1.8 wt %),although the fresh catalyst contains lower amount Pt (0.5 wt %)and higher amount of Ga (3 wt %). This EDS analysis provesthat particles observed in TEM image of spent catalyst aremostly Pt in nature. This is clearly evidence that structures ofmetal nanoparticles are reorganized under reaction conditionsat high temperature (650 °C). Pt is a noble metal, and it hasless interaction with the zeolite support. Due to this, Ptnanoparticles are mobile in nature on the zeolite support athigh temperature, causing agglomeration to form bigger particlein size. But Ga in oxide form has more interaction with zeolitesupport and is prevented from agglomeration and keepsdispersion intact. One of the essential requirements for gettingbetter activity by GaPt/HZSM-5 catalyst is the homogeneousdistribution of Ga and Pt metals which paves the way to makeGa−Pt association more effective through alloy or bimetallicnanoparticles formation. This intense association providesoptimum promoter action of Pt to the Ga catalyst. But thishomogeneous distribution of Ga and Pt metal gets disturbedunder time-on-stream at high temperature revealed by theformation of bigger Pt nanoparticles (Figure 10b). From

homogeneous distribution to heterogeneous Ga, Pt associationdeteriorates the promoter action of Pt on Ga metal described inFigure 11. This causes rapid deactivation of GaPt/HZSM-5

activity in comparison with Ga/HZSM-5 catalyst and othercatalysts (Figures 5 and 6). So this TEM EDS analysis suggeststhat restructuring of Ga, Pt metal nanoparticles along withsintering and coke formation could be an important reason forexhibiting different deactivation rate among the catalysts.There are three important reasons for catalyst’s deactivation

during reaction; (1) metal nanoparticle sintering, (2)heterogeneous distribution of Ga, Pt metals caused byrestructuring of metal nanoparticle, and (3) coke depositionon active sites discussed throughout the paper. Severe cokedeposition observed for dehydroaromatization reaction is asignificant cause for catalyst’s deactivation that we haveobserved in our ethane dehydroaromatization reaction.Decreasing dispersion of metal particles due to agglomerationat high reaction temperature ascertained by TEM measurementcould be another reason for catalyst’s activity deterioration.Also deviation from homogeneous distribution of Ga and Ptmetal ions to heterogeneous distribution could be one of thesignificant causes for rapid deactivation under time-on-stream.Few studies are reported by previous researchers with Ga/

HZSM-5 catalyst for ethane dehydroaromatization reaction.Here we have attempted to compare the activity and selectivityexhibited by our catalytic study with the previous report.Chetina et al.15 studied aromatization of ethane over GaPt/HZSM-5 catalyst using 400 cm3 g−1 h−1 gas hour space velocity(GHSV). They improved aromatic yield and selectivity up to61 wt % and 78%, respectively, using ZrFe2 intermetallichydrogen acceptor to shift the equilibrium of the dehydroar-omatization reaction to the right side. Schulz et al. reported28% ethane conversion and 57% aromatic selectivity with Ga/HZSM-5 catalyst at W/F = 19.2 g h mol−1 (W/F = catalystmass used/reactant gas flow rate).29 For our catalytic studyusing GaPt/HZSM-5 catalyst, we have achieved maximum 80%ethane conversion and 40% aromatic selectivity using ethaneGHSV 9000 cm3 g−1 h−1 (W/F = 2.8 g h mol−1) . Incomparison with previous report, ethane conversion andselectivity that we have achieved are noticeable.

4. CONCLUSION

On the basis of the experimental data obtained in this work andfrom previous literature, it can be concluded that theintroduction of Pt platinum to Ga-loaded zeolite results inessential increase of its activity in the aromatization of ethane.Pt increases reducibility of gallium oxide and probably helps itsdehydrogenation activity to produce ethylene from ethane. Pt

Figure 10. TEM images of (a) fresh GaPt/HZSM-5, scale bar = 50nm, with lattice fringes in inset, scale bar = 5 nm, and (b) spent GaPt/HZSM-5, scale bar = 20 nm, with encircled area in inset, scale bar = 10nm, where selected area EDS analysis for encircled portion shows Ga =1.8 wt % and Pt = 3 wt %.

Figure 11. Homogeneous distribution of Ga and Pt metals (freshcatalyst) is destroyed due to the increment of Pt nanoparticle (spentcatalyst) size under time-on-stream, and this causes deterioration of Ptpromoter action on Ga metal and resulted in rapid deactivation.

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also improves the stability of Ga catalyst during time-on-stream.TPO experiment revealed that coke formed on GaPt/HZSM-5is less hard compared to Ga/HZSM-5. In comparison withconventional Mo/HZSM-5, GaPt/HZSM-5 catalyst performedbetter in ethane dehydroaromatization reaction. Due topossessing better activity and stability, GaPt/HZSM-5 catalystcan be regenerated for real industrial application on aromaticsproduction.

■ AUTHOR INFORMATIONCorresponding Authors*H.C.: e-mail, Huichen_410@163.com.*J.H.: e-mail, john.hu@mail.wvu.edu.ORCIDJianli Hu: 0000-0003-3857-861XAuthor ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge financial support from WestVirginian University Research Corporation and West VirginianUniversity Benjamin M. Statler College of Engineering andMineral Resources.

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Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.7b02095Ind. Eng. Chem. Res. 2017, 56, 11006−11012

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