Promotional Effects of Mesoporous Zeolites with Pt NanoparticleCatalysts during Reforming of MethylcyclopentaneKyungsu Na, Nathan Musselwhite, Xiaojun Cai, Selim Alayoglu,* and Gabor A. Somorjai*

Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley NationalLaboratory, Berkeley, California 94720, United States

*S Supporting Information

ABSTRACT: Selective C−C and C−H bond activations are an important catalyticprocess to produce various value-added hydrocarbons via reforming processes. Forproducing desired product with a high yield, control of reaction pathway through thedesign of catalyst and fundamental understanding and clarification of reactionmechanism are prerequisite. In this work, we designed heterogeneous catalysts bycombining Pt nanoparticles and two different mesoporous zeolites with microporousframeworks of BEA and MFI for the hydrogenative model reforming reaction ofhydrocarbon (i.e., methylcyclopentane). Depending on the catalyst combination, thereaction pathways of (i) dehydrogenation, (ii) ring-opening with isomerization, andring-enlargement with (iii) hydrogenation and (iv) dehydrogenation of C5-cyclicring to C6-cyclic ring (i.e., cyclohexane and benzene) can be controlled to producevarious products with high yields. Furthermore, we revealed a reaction intermediateformed at the interface of Pt and zeolite by real-time surface vibrational sum-frequency generation spectroscopic studies. This study would provide practical andfundamental insights for design of heterogeneous catalyst for controlling reaction pathways.

■ INTRODUCTION

Enhancement of catalytic activity and control of the reactionpathway to produce a high yield of desired product with closeto 100% selectivity are of paramount interest for thedevelopment of energy-saving and eco-friendly catalyticreactions.1−4 In order to simultaneously achieve high catalyticactivity with 100% product selectivity, catalytic systems shouldbe tailored delicately. Development of synthesis strategies formetal nanoparticles (NPs) with various sizes and shapes can beone of the great contributions toward a realization of thisgoal.3,4 Along with this, the development of various porousinorganic supports like mesoporous (2 < diameter < 50 nm)silica materials has allowed the metal NPs to be used as apractical catalyst.5−9 Such catalytic systems (i.e., metal NPssupported on porous support) have also enabled delicate andreliable investigation of catalytic function of the metal NPsaccording to the alteration of various parameters such as size,shape, and composition.8−11

In addition to the mesoporous silica materials, metal NPs canalso be supported on zeolites,12−14 mesoporous transition metaloxides,15−17 carbon,18,19 and polymers.20 These supportingmaterials are usually in porous structure having wide surfacearea and large pore volume. Among them, zeolites arecrystalline microporous (0 < diameter < 2 nm) aluminosilicatematerials that are one of the most widespread catalytic supportsin current chemical industries.12−14 Zeolites also have their owncatalytic activity due to the aluminosilicate framework that canbe the origin of strong acidity. Therefore, supporting a few

atom metal clusters within zeolite micropores can produce abifunctional catalyst.21−24

Conventional zeolite materials are solely microporouscrystals and, accordingly, possess very low molecular diffusionefficiency. Such a system can cause quick catalytic deactivationdue to a formation of a carbonaceous deposit inside themicropores.25 In addition, metal NPs can only be supportedinside the micropores as a form of small clusters, which limitedcatalytic studies.21−24 Instead, the ordered mesoporous silicamaterials with higher surface area and larger size of pores inmesopore regime (2 < diameter < 50 nm) can be used forstudying catalytic effects of metal NPs with various sizes,shapes, and compositions on many industrially importantcatalytic reactions.8−11 During the recent decades, there havebeen numerous attempts to synthesize hierarchically nano-porous zeolites having secondary mesopores in addition to theintrinsic micropores.25−30 The resultant materials are oftencalled mesoporous zeolites because they are constructed withnanocrystalline zeolite particles with inter- or intracrystallinemesoporosity. The presence of mesopores can improve themolecular diffusion efficiency that can enhance catalyticlifetime.25,30 In addition, bulky reactant molecules that cannotenter the micropores can be converted over the acid sites onthe mesopore wall.25,30

Special Issue: A. W. Castleman, Jr. Festschrift

Received: February 19, 2014Revised: April 11, 2014

Article

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In the present work, we designed heterogeneous catalystsbased on the combination of Pt NPs and two differentmesoporous zeolites with microporous frameworks of BEA andMFI for testing on a hydrogenative model reforming reaction ofhydrocarbon (i.e., methylcyclopentane, MCP). As drawn inScheme 1, reforming of MCP can be followed with five possiblereaction pathways. First, MCP can be converted into thedehydrogenated version of MCP through reaction pathway (1,green). Second, MCP can be ring-opened and subsequentlyisomerized into various branched isomers as well as the linearisomer (i.e., n-hexane) through reaction pathway (2, pink). TheC5-based cyclic ring of MCP can be further enlarged to C6-cyclic rings through ring-enlargement reaction pathways(black). In this reaction pathway, we are proposing thatcyclohexene is a strong candidate as an intermediate state. Ifcorrect, cyclohexene can be further converted into cyclohexanevia a hydrogenation pathway (3, blue) or into benzene via adehydrogenation pathway (4, red). The last reaction pathway iscracking (5, gray) to produce C1−C5-based hydrocarbons.Among these products, cyclohexane is desired for its highoctane rating among all the products, while benzene and n-hexane are unwanted for environmental concerns. In this work,these reaction pathways can be controlled by the design ofheterogeneous catalysts to facilitate the formation of cyclo-hexane. In particular, we also studied in situ sum-frequencygeneration (SFG) vibrational spectroscopic studies to monitorthe reaction dynamics under the real-time condition and to findan intermediate species. The present work may shed light on afundamental understanding of the reaction mechanism and apractical idea for design of high-performance heterogeneouscatalysts.

■ EXPERIMENTAL SECTION

Two mesoporous zeolites with framework types of BEA andMFI and mesoporous silica MCF-17 were synthesized byfollowing the literature reported elsewhere.17,31,32 Poly-(vinylpyrrolidone) (PVP)-capped Pt nanoparticles (NPs)with an average size of 2.5 nm were synthesized by followingthe literature reported elsewhere.17 The PVP-capped Pt NPswere used as the catalyst after a reductive treatment at 250 °C.

Thermal conditioning of the PVP-capped catalysts by this waygave rise to optimum activity and did not cause any measurableinterference to the product distributions.8,33,34 All chemicalreagents are purchased from Sigma-Aldrich and used withoutfurther purification (see Supporting Information for thedetails). All the prepared catalytic materials are summarizedin Table S1, Supporting Information. The two mesoporouszeolites without supporting Pt NPs were simply indicated asBEA and MFI, while the Pt NPs supported versions wereindicated as Pt/BEA and Pt/MFI, respectively. The pure silicaMCF-17 supporting Pt NPs was indicated as Pt/SiO2. Thecatalytic materials prepared in this manner were characterizedwith X-ray diffraction (XRD), transmission electron microscope(TEM), inductively coupled plasma atomic emission spectros-copy (ICP-AES), N2 physisorption analysis, and infrared (IR)analysis.The catalytic testing was performed using a lab-built plug-

flow reactor connected to a gas chromatograph (GC) equippedwith a flame ionization detector (FID). The catalytic testingwas performed using a lab-built plug-flow reactor connected toa Hewlett-Packard 5890 gas chromatograph (GC). A 10% SP-2100 on 100/120 Supelco port packed column in line with aFID was used to separate and analyze the C1−C6 hydro-carbons. Mass flow controllers were carefully calibrated using abubble flow meter and used to introduce the ultrahigh purity(99.9999% Praxair) H2 and He gases. Saturated vapor pressuresof methylcyclopentane (MCP) or cyclohexene (CHE) wereintroduced to the reactor using a bubbler. The reactant flowwas carefully calibrated at different temperatures and partialpressures of He carrier. A total flow of 40 mL/min was used.Partial pressure of reactants was calculated by using the knowntemperature vs saturated vapor pressure plots and was 50 Torrwith 5:1 H2 excess. The 50−100 mg charges of the catalystswere diluted by quartz sand with average granular size of 0.4mm and loaded in the reactor bed. The actual weight of catalystused was selected to give similar total conversions in each case.The catalysts were reduced at 250 °C for 2 h under a flow of210 Torr H2 in 550 Torr He prior to catalytic testing. Thecatalytic activity and selectivity were evaluated for total MCPconversions around 5%.

Scheme 1. Five Reaction Pathways of Catalytic Reforming of MCP, Indicated by Different Numbers and Colorsa

aFirst, MCP can be converted into the dehydrogenated version of MCP (1, green). Second, MCP can be ring-opened and subsequently isomerizedinto branched isomers as well as the linear isomer (2, pink). The C5-based cyclic ring of MCP can be further enlarged to C6-cyclic rings throughhydrogenation (3, blue) or dehydrogenation (4, red) of reactive intermediate species that can be proposed as cyclohexene. The last reaction pathwayis cracking (5, gray) to produce C1−C5-based hydrocarbons.

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The SFG experiments were conducted using a mode-lockedNd:YAG dye laser (Continuum D-20) with a fundamentaloutput at 1064 nm and a pulse width of 20 ps (see SupportingInformation for the details).

■ RESULTS AND DISCUSSIONThe Pt NPs with average size of 2.5 nm and two mesoporouszeolites with micropore frameworks of BEA and MFI were

synthesized by following literature methods (see SupportingInformation for synthesis details and materials character-izations).17,31,32 Then, the mesoporous zeolites were used aspure acid catalysts or as supporting materials for Pt NPs. As areference catalyst, Pt NPs supported on mesoporous silicaMCF-17 was tested for the same catalytic reaction. All theprepared catalysts and their physicochemical properties aresummarized in Table S1, Supporting Information. Because ofthe nanocrystalline morphology, the mesoporous BEA and MFIzeolites exhibited much broader X-ray diffraction patterns thanconventional zeolites in a bulk crystal (Figure S1 in SupportingInformation). The mesoporous BEA zeolite was obtained as

very small nanocrystals with an average diameter of 10 nm thatare aggregated as having intercrystalline mesopores, and the

Figure 1. Product selectivity for various catalysts in hydrogenativeMCP reforming. Selectivities to reaction pathways are indicated withdifferent colors following those in Scheme 1. Pure mesoporous zeolites(BEA and MFI) without Pt NPs can do both dehydrogenation andring-opening with isomerization. Among the two zeolites, MFI diddehydrogenation more dominantly (∼80%) whereas BEA did slightlymore ring-opening with isomerization (∼55%). Pt NPs supported onmesoporous silica MCF-17 (Pt/SiO2) showed the highest selectivitytoward ring-opened isomers (∼75%). Pt NPs supported on BEA (Pt/BEA) produced the largest amount of cyclohexane (∼60%), whereasPt NPs supported on MFI (Pt/MFI) produced the largest amount ofbenzene (∼80%).

Table 1. Activity and Product−Time−Yield Data

product−time−yieldb

catalyst mass activitya (1)c (2)d (3)e (4)f

BEA 0.32 0.14 0.18 0 0MFI 0.21 0.17 0.04 0 0Pt/SiO2 0.42 0.10 0.32 0 0Pt/BEA 2.00 0.11 0.11 1.17 0.62Pt/MFI 2.65 0.15 0.08 0.31 2.11

aIn mol/h/g (×103). Mass activity calculated from the mole amount ofMCP consumed (mol) per given reaction time (h) and catalystamount (g). bIn mol/h/g (×103). Product−time−yield calculated bymultiplying overall activity with product selectivity in Figure 1. c(1)Dehydrogenation. d(2) Ring-opening with isomerization. e(3) Ring-enlargement with hydrogenation. f(4) Ring-enlargement withdehydrogenation.

Figure 2. Comparison of (A) mass activity and (B) product selectivityover MFI, Pt/SiO2, physical mixture of MFI and Pt/SiO2, and Pt/MFI.Selectivities to reaction pathways are indicated with different colors byfollowing Scheme 1. MFI shows an activity of 0.21, while Pt/SiO2shows 0.42. When these two catalysts are physically mixed, an activityvalue of 0.38 is obtained. However, when Pt NPs are supported onMFI (Pt/MFI), an activity is significantly enhanced to 2.65. In the caseof product selectivity, the physically mixed catalyst exhibits averageranges of product selectivities to dehydrogenation (green) and ring-opened isomers (pink), whereas Pt/MFI can produce benzene (red)with ∼80% selectivity.

Figure 3. In situ SFG spectra for (A) Pt/BEA and (B) Pt/MFI duringthe MCP reforming at 150 °C. For both spectra, a peak at ∼3035 cm−1

indicates the existence of CH; peaks at 2850, 2905, and 2920 cm−1

are CH symmetric stretch (sym), asymmetric stretch (asym), andFermi resonance (FR) mode for CH2 functional group, respectively,and a peak at ∼3080 cm−1 is the asymmetric stretch of the terminalolefin functional group (CH2). In panel A, the symmetric stretch ofthe terminal olefin functional group appeared, whereas no obviousasymmetric stretch of the CH2 is shown in panel B. SFG spectrumfor Pt/MFI (B) shows a peak at ∼3065 cm−1, which can be assigned asthe physisorbed benzene.

Table 2. Comparison of Benzene and CyclohexaneSelectivity According to the Reactant (i.e., MCP and CHE)for Pt/BEA and Pt/MFI Catalysts

Pt/BEA Pt/MFI

reactant BENa CHAb B/Cc BENa CHAb B/Cc

MCPe 30.8% 58.6% 0.53 79.4% 11.8% 6.73CHEd 18.9% 39.4% 0.48 65.4% 10.2% 6.41

aBEN, benzene. bCHA, cyclohexane. cProduct selectivity ratio ofbenzene (B) to cyclohexane (C). dCHE, cyclohexene. eMethylcyclo-pentane, MCP.

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mesoporous MFI zeolite was obtained as thin nanosheetmorphology (Figure S2 in Supporting Information). Theaverage mesopore diameters are more than 10 nm, which is asufficient size that can support Pt NPs with average diameter of2.5 nm. Therefore, on these zeolite samples, Pt NPs weresupported with good dispersion (Figure S2 in SupportingInformation). The average loading of Pt NPs on the supportingmaterials used in this work is between 0.4 and 0.5 wt % that areanalyzed by ICP-AES (Table S1 in Supporting Information).Figure 1 shows the product selectivity trend versus the tested

catalysts at 150 °C. All the catalytic studies in this work wereperformed at a low conversion level (∼5% conversion of MCP)where the catalytic deactivation was negligible and transientcatalytic studies were possible. The results show a remarkablechange in product selectivity depending on the catalyst. Thetwo mesoporous zeolites evaluated without supporting Pt NPscould catalyze dehydrogenation (green) and ring-opening withisomerization (pink) more dominantly than any other reactionpathways. No other reaction pathways were observed. Amongthem, MFI catalyzed the dehydrogenation pathway (∼80%)more than ring-opening with isomerization (∼20%), whereasBEA catalyzed a little bit more ring-opening with isomerization(∼55%) than dehydrogenation (∼45%). From this result, wecan confirm that the pure acidic zeolites without supporting PtNPs could do both ring-opening with isomerization anddehydrogenation for MCP.One possible reason for such different catalytic phenomenon

between BEA and MFI zeolites is a difference in Bronstedacidic properties. The Bronsted acid sites are known to beinvolved in the C−C bond breaking and protolytic dehydrogen-ation of hydrocarbon.35−38 According to the quantification of

Bronsted acid sites by FT-IR (Figure S3A in SupportingInformation), BEA has about three times more Bronsted acidsites (1.13 mmol/g) than MFI (0.36 mmol/g). The strength ofBronsted acid sites was also analyzed by monitoring desorptiontendency of pyridine from zeolites with increasing thedesorption temperature, showing a steeper decrease in BEAthan in MFI (Figure S3B in Supporting Information). Thismeans that the average strength of Bronsted acid sites on BEAis weaker than MFI. Since dissociation of C−H bond (∼98kcal/mol) requires more energy than that of C−C bond (83−85 kcal/mol), MFI having stronger Bronsted acid sites coulddehydrogenation via protolytic C−H bond dissociation moreselectively than BEA.Pt/SiO2 catalyst can demonstrate a sole role of Pt NPs on the

MCP reforming because purely siliceous framework is catalyti-cally inert. As shown in Figure 1, the Pt/SiO2 catalyst did thering-opening with isomerization reaction pathway with thehighest selectivity (∼75%) among the tested catalysts. This isalready widespread knowledge due to the catalytic function ofPt NPs.39 However, the product selectivity was totally changedafter supporting Pt NPs on the mesoporous zeolites (i.e., Pt/BEA and Pt/MFI). When Pt NPs were supported on bothmesoporous zeolites, ring-enlargement reaction pathway wasmore preferable, by which cyclohexane and benzene wereproduced more selectively as compared to the other products.Since pure acidic zeolites and pure Pt NPs cannot producebenzene and cyclohexane under the same reaction conditions,such different product selectivities can be explained bysynergistic catalysis between Pt NPs and acid sites on thezeolites. Between the two catalysts, Pt/BEA zeolite producedcyclohexane more exclusively with ∼60% selectivity, while the

Scheme 2. Schematic Representation for Proposed Reaction Mechanism of MCP Reforminga

aFirst, MCP was dehydrogenated on the surface of Pt NPs and subsequently migrated to acidic sites on the zeolite to make a ring-openedunsaturated carbocation. This was converted to cyclohexene intermediate by ring-closure, which could be further hydrogenated to cyclohexane ordehydrogenated to benzene depending on the acidic properties of the zeolites. The double bond in molecules was designated as green forclarification, and the hydrogen feed in the reaction was omitted on the scheme for simplicity.

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selectivity for benzene was ∼30%. In the case of Pt/MFIzeolite, benzene was produced with ∼80% selectivity.The combination of Pt NPs with mesoporous zeolites could

also enhance the catalytic activity very significantly. As Table 1summarizes (see also Figure 2A), after supporting Pt NPs onthe mesoporous zeolites, the mass activity was enhanced from0.32 to 2.00 for BEA and from 0.21 to 2.65 for MFI. Note thatthe mass activity was defined as the mole of MCP consumed(mol) during the reaction per given time (h) and catalystweight (g). Since two catalytic functions from Pt NPs and acidsites on zeolites were combined together, we just compared theactivity based on a catalyst mass. In addition, since the averageloadings of Pt NPs on each catalyst are in a similar range (TableS1 in Supporting Information), it is highly reasonable forcomparison of catalyst activity based on mass activity.Considering individual catalytic activities of Pt NPs and puremesoporous zeolite, the degree of activity enhancement isdramatic.We assumed that such a big enhancement of catalytic activity

and different product selectivities might be originated from aclose proximity between Pt and acid sites on zeolites thatformed catalytic interfaces. Testing the same reaction with aphysical mixture of Pt NPs and mesoporous zeolite coulddirectly evidence the importance of close proximity between PtNPs and acid sites. To prove this hypothesis, we mixed MFIand Pt/SiO2 catalysts physically using a mortar and pestle, thentested them in the MCP reforming reaction. The activity forthis physical mixture catalyst (shown in Figure 2A) was foundto be 0.38, which was close to the average activity values of pureMFI catalyst and Pt/SiO2. In addition, the physical mixturecatalyst produced almost equal amounts of dehydrogenationand ring-opening with isomerization products (Figure 2B),which were very close to the average values of productselectivities from pure MFI and Pt/SiO2. Most importantly,cyclohexane and benzene were not produced over the physicalmixture catalyst. Through this reaction study, it can beconcluded that the close proximity of the two catalyticfunctionalities of Pt NPs and zeolite acid sites is very importantfor the activity enhancement as well as selectivity control.However, it was still unclear to explain the remarkable

change in product selectivity. The two mesoporous zeolitessupporting Pt NPs (i.e., Pt/BEA and Pt/MFI) can commonlyproduce C6-cyclic ring products (i.e., cyclohexane andbenzene) most dominantly through the ring-enlargement ofthe C5-cyclic ring of MCP. The product selectivity toward C6-cyclic ring products over both catalysts is more than 90%. Webelieved that a specific intermediate state must be formed,which can further be converted to C6-cyclic ring products withhigh product selectivity. If this is correct, it is highly probablethat there is a specific and reactive intermediate state. For sucha bifunctional catalytic process due to the Pt NPs and acid siteson the zeolites, the rate-determining step is assumed to theskeletal rearrangement of carbenium ion with or without beta-scission on the acid sites while the Pt NPs establishes only arapid equilibrium between paraffin and olefin. Therefore, theproduct distribution should be strongly influenced by theintermediate species of high reactivity. On the basis of thisphenomenon, in order to detect such an intermediate species,we did reaction studies using an in situ SFG spectroscopic toolunder the reaction conditions. SFG spectroscopy is a surface/interface specific nonlinear technique that can providemolecular vibrational information under reaction condi-tions.40,41

Figure 3 shows the in situ SFG spectra obtained at theinterface of Pt/BEA (Figure 3A) and Pt/MFI (Figure 3B)during the MCP reforming reaction at 150 °C. The appearanceof a peak at ∼3035 cm−1 in both spectra demonstrates theexistence of CH.42−45 Broad peaks between 2850 and 2920cm−1 in Figure 3A can be deconvoluted into three peaks with2850, 2905, and 2920 cm−1, which corresponds to symmetric(sym), asymmetric (asym), and Fermi resonance (FR) of CH2,respectively. These peaks are also present in Figure 3B, but onlya peak corresponding to FR of CH2 is significant, indicatingthat either the molecules with CH2 are ordered or theconcentration of the CH2 on the surface is high, or both. Sinceno cracking was observed at this reaction condition, these peakscan be attributed to the C6-based unsaturated hydrocarbonspecies such as cyclohexene or cyclohexadiene.A peak at ∼3095 cm−1 can be assigned as an asymmetric

stretch of the terminal olefin functional group (i.e., CH2),and its symmetric stretch is shown at 2995 cm−1 in Figure 3A,which indicates the presence of the ring-opened olefinintermediate such as 1-hexene. No obvious symmetric stretchmode for CH2 was observed in Figure 3B. However, Pt/MFIsample shows a much broader peak at ∼3065 cm−1 that isoriginated from a physisorbed benzene species (Figure 3B).46

As already evidenced above, Pt/MFI produced more benzenethan Pt/BEA, which is confirmed again by SFG. There is ashoulder peak at ∼2780 cm−1, which can be assigned by theCH2 stretch in unsaturated C6-based cyclic intermediates.47

According to the SFG study data, the strongest possiblecandidate for an intermediate species can be unsaturatedhydrocarbon with CC double bond. Considering the generalcatalytic functions of Pt NPs and acid sites of zeolite, we couldsuggest cyclohexene (CHE) as a strong candidate for anintermediate species. If this is true, CHE can be furtherhydrogenated to cyclohexane (reaction pathway (3)) ordehydrogenated to benzene (reaction pathway (4)) dependingon the zeolite types. As the product selectivity data shows, Pt/BEA produced more cyclohexane, whereas Pt/MFI yieldedmuch more benzene. This means that Pt/MFI did moredehydrogenation on the assumption that cyclohexene was theintermediate state. This tendency was also highly consistentwith the fact that the pure MFI zeolite without supporting PtNPs did dehydrogenation much more predominantly than BEAdid (Figure 1).For the justification of our hypothesis as well as supporting

the SFG data, we tested the reaction under identical conditionswith CHE instead of MCP. Figure S4, Supporting Information,shows the GC FID signal data obtained from the CHEhydrogenation over four catalysts, i.e., BEA, MFI, Pt/BEA, andPt/MFI from bottom to top. In this signal data, the peaks ofbenzene and cyclohexane were highlighted with pink and bluecolors, respectively. As summarized in Table 2, the selectivityratio for benzene to cyclohexane (i.e., B/C ratio) from CHEhydrogenation over Pt/BEA was 0.48, which was a very closevalue to the B/C ratio from MCP reforming over the samecatalyst (0.53 in Table 2). In addition, the B/C ratio from CHEreforming over Pt/MFI was 6.41, which was also similar to thevalue from MCP reforming (6.73 in Table 2). It is alsonoteworthy that CHE hydrogenation yielded >80% conversionover the Pt/zeolite catalysts due to the higher reactivity of CHEthan MCP. From the comparison reaction results, it can beconcluded that CHE is the strongest candidate as anintermediate state. This intermediate state with high reactionprobability was formed at the catalytic interface between Pt

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NPs and acid sites on zeolites, which can migrate between twocatalytic sites to be converted further into cyclohexane orbenzene. As proved above, the distance between Pt NPs andacid sites should be sufficiently close for migration of CHEintermediate before its extinction.From the controlled experiments with CHE and SFG studies

with MCP, we proposed a reaction mechanism of MCPreforming to produce cyclohexane or benzene selectively(Scheme 2). The MCP was first dehydrogenated on thesurface of Pt NPs and subsequently migrated to the acidic siteson the zeolite to produce a ring-opened unsaturatedcarbocation. This short-lived reactive intermediate speciescould be converted further to form cyclohexene by ring-closure. Depending on the acidic properties of zeolite, thecyclohexene can be further hydrogenated to cyclohexane ordehydrogenated to benzene.Effect of reaction temperature was also investigated. As the

temperature increased from 150 to 250 °C, the overall activityof all the catalysts increased as expected (Figure S5, SupportingInformation). However, the increasing tendency was quitedifferent. The overall activity of Pt/SiO2 catalyst was not sosignificantly enhanced, whereas other zeolite-based catalystsexhibited huge enhancement of catalytic activity. This isbecause the catalytic contribution of zeolite becomes muchmore significant than that of Pt NPs as the temperatureincreases. However, the dominant products were C1−C5cracked hydrocarbons as the reaction temperature increaseddue to the enhancement of cracking tendency by acidic sites onzeolites.

■ CONCLUSIONSIn conclusion, we discovered highly active and selective, towardthe ring-enlargement of MCP molecule, catalytic interfacesformed when colloidal Pt NPs were supported on mesoporouszeolites of BEA- and MFI-type. We also identified CHEmolecule as a reactive intermediate within the time scale ofreaction turnovers and as playing an important role for thecontrol of reaction pathways. This CHE was converted tocyclohexane and/or benzene depending on the specific zeolitetype. Furthermore, the CHE reaction intermediate formed atthe interface of Pt and zeolite was also revealed by real-timeSFG spectroscopic study. In a practical point of view, thepresent work would provide how to control the reactionpathway via the rational design of heterogeneous catalyst basedon mesoporous zeolites. In the case of scientific viewpoint,fundamental understanding of a catalytic reaction withclarification of the intermediate state would be given forselective C−C and C−H bond activations and their subsequentreassembly. Further works on the effects of acidic properties,size, and composition of metal NPs on MCP reforming andalso SFG studies on Pt/BEA and Pt/MFI are underway.

■ ASSOCIATED CONTENT*S Supporting InformationSynthesis of catalytic materials, their characterization, andsupplementary catalytic reaction results. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*(G.A.S.) E-mail: somorjai@berkeley.edu. Phone: +1-510-486-4831.*(S.A.) E-mail: salayoglu@lbl.gov.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is funded by The Chevron Energy TechnologyCompany. We acknowledge support from the Director, Officeof Science, Office of Basic Energy Sciences, Division ofChemical Sciences, Geological and Biosciences of the US DOE,under contract DE-AC02-05CH11231. The authors acknowl-edge support of the National Center for Electron Microscopy,Lawrence Berkeley Lab, which is supported by the U.S.Department of Energy under Contract No. DE-AC02-05CH11231. Work at the Molecular Foundry was supportedby the Director, Office of Science, Office of Basic EnergySciences, Division of Material Sciences and Engineering, of theU.S. Department of Energy under Contract No. DE-AC02-05CH11231. K.N. is thankful for the financial support fromBasic Science Research Program through the National ResearchFoundation of Korea (NRF) funded by the Ministry ofEducation (2012R1A6A3A03039602). We thank Prof. PeidongYang and Prof. Omar M. Yaghi for use of the TEM and XRDinstruments, respectively.

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The Journal of Physical Chemistry A Article

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The Journal of Physical Chemistry A Article

dx.doi.org/10.1021/jp501775q | J. Phys. Chem. A XXXX, XXX, XXX−XXXG