Journal of Catalysis 345 (2017) 113–123

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

Propane dehydrogenation catalyzed by gallosilicate MFI zeoliteswith perturbed acidity

http://dx.doi.org/10.1016/j.jcat.2016.11.0170021-9517/� 2016 Elsevier Inc. All rights reserved.

⇑ Corresponding authors.E-mail addresses: sankar.nair@chbe.gatech.edu (S. Nair), christopher.jones@

chbe.gatech.edu (C.W. Jones).1 These two authors are co-first-authors.

Seung-Won Choi a,1, Wun-Gwi Kim a,1, Jung-Seob So a, Jason S. Moore b, Yujun Liu b, Ravindra S. Dixit b,John G. Pendergast b, Carsten Sievers a, David S. Sholl a, Sankar Nair a,⇑, Christopher W. Jones a,⇑a School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, United Statesb Engineering & Process Sciences, The Dow Chemical Company, Freeport, TX 77541, United States

a r t i c l e i n f o

Article history:Received 28 March 2016Revised 25 October 2016Accepted 13 November 2016

Keywords:PropaneDehydrogenationGallosilicateZeoliteThiol3-Mercaptopropyl-trimethoxysilane

a b s t r a c t

The propane dehydrogenation (PDH) behavior of gallosilicate MFI catalysts synthesized in the presence of3-mercaptopropyl-trimethoxysilane (MPS) is investigated. MPS addition to the zeolite synthesis gelresults in gallosilicate molecular sieves with reduced Brønsted acidity and enhanced Lewis acidity, asshown by combined NH3 temperature-programmed desorption (TPD), isopropylamine (IPA) TPD and IRanalysis of pyridine-loaded catalysts. In particular, the gallosilicate MFI catalysts prepared using MPSprovide a significant concentration of strong Lewis acid sites, which are important in controlling theselectivity of PDH. Enhanced PDH performance with higher propane conversion rates and improvedpropylene selectivity with limited cracking and aromatization products are obtained from the catalystssynthesized with MPS. The gallosilicate MFI materials prepared using MPS are compared with a bench-mark chromia-alumina catalyst, and the gallosilicate materials have a lower rate of deactivation (>50%lower) but slightly inferior propylene selectivity (Ga-MFI: 75%; Chromia-Alumina: 85%).

� 2016 Elsevier Inc. All rights reserved.

1. Introduction

Propylene is one of the most important feedstocks for the pro-duction of valuable polymers such as poly(propylene) and chemi-cals such as acrylonitrile and 2-propanol. The production ofpropylene from catalytic dehydrogenation of propane (PDH) is car-ried out commercially by the Catofin process from CB&I Lummus[1–6] and the Oleflex process from UOP [7–13]. The Catofin processuses a Cr-based catalyst in multiple parallel fixed bed reactors,while the Oleflex process uses a Pt-based catalyst in a fluidizedbed reactor [14,15]. Both processes inevitably undergo rapid cata-lyst deactivation. PDH processes have received renewed attentionin recent years, especially in North America and the Middle East,due to abundant supplies of propane, and there are renewed effortsaimed at discovering new PDH catalysts or improving the stability,activity, and selectivity of known PDH catalysts. Propane feed-stocks can be also applied in propane aromatization processes toproduce benzene, toluene, and xylene. Ga-modified MFI zeolitecatalysts have been widely used in the UOP/BP Cyclar dehydrocy-clization process [16–22]. Ga-containing zeolites synthesized by

impregnation or ion-exchange into aluminosilicate H-ZSM-5 havebeen extensively studied [20–31]. Gallosilicate MFI can also beobtained by isomorphous substitution of Ga in the MFI zeoliteframework [32,33], since Ga can be easily incorporated in theMFI framework.

In an early report, Bayense et al. reported that the specific syn-thesis method used for Ga incorporation into HZSM-5 did not havemuch effect on the propane conversion, but incorporation of Gaincreased the aromatic selectivity in all cases [24]. They also sug-gested that Ga-loaded H-ZSM-5 was better than the pure gallosil-icate for propane aromatization. However, other studies havereported that H-gallosilicate showed higher propane aromatizationselectivities than Ga ion-exchanged/impregnated H-ZSM-5 [34–36]. A reaction mechanism of propane aromatization using Galoaded HZSM-5 was suggested by Giannetto et al. [37]. In this path-way, propane is first protonated on Brønsted acid sites where itundergoes either a dehydrogenation or a cracking step. A carbe-nium ion intermediate formed by the dehydrogenation pathwayis then converted into propylene, followed by oligomerizationand cyclization to aromatics at the Brønsted acid sites. In propanearomatization, PDH is a primary step, as discussed in detail byMeriaudeau and Naccache [23,38]. They proposed a bifunctionalmechanism during the PDH step when both Ga oxide species(Ga2O3) and Brønsted acid sites in the MFI pores are present. Thestrength of the Brønsted acid sites depends on the types of

114 S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123

heteroatoms in the framework, and it is known that bridged frame-work Ga provides lower acidity than Al [39]. It has been suggestedthat the Lewis acid sites are related to positively charged oxideclusters in zeolite pore structures, normally extra-framework spe-cies [39]. During the synthesis of gallosilicate molecular sieves, thetrivalent Ga3+ species not incorporated in the framework remain inthe zeolite channels as highly dispersed oxide species [40].Extra-framework Ga species can also be formed by degalliation offramework Ga, and the role of these species on PDH has been dis-cussed [33,37,40–43]. Choudhary et al. reported the formation ofextra-framework Ga by heat treatment and suggested that extra-framework Ga should be the main active site for PDH whereasframework Ga species were much less active [42,43]. The relationbetween propane conversion and propylene/aromatic selectivitywas studied by Guisnet et al. where high propylene selectivities(>70%) were observed at low conversion levels (<5%). At high con-versions (>20%), there was more than 50% selectivity to aromatics[44]. Although the aforementioned studies assumed that the extra-framework Ga species were present in the zeolite pores, Ga oxidedomains have also been found on the outer surface of zeolites uponcalcination, with the oxide clusters being 3–10 nm in size [45].

Although extra-framework Ga species are considered to be themain active sites for PDH [37,42,43,46], direct characterization ofthese sites can be challenging. For instance, it is difficult to detectsmall amounts of extra-framework Ga species by Ga-NMR due tostrong quadrupolar effects since the sites are mainly in low-symmetry environments [40,47–49]. There have been severalattempts to investigate the presence of extra-framework Ga spe-cies by temperature programmed desorption techniques, whichconfirmed their strong Lewis acidic properties [22,31,50,51]. Four-ier transform infrared spectroscopy (FT-IR) has also been used forcharacterization of acid sites in Ga-MFI [27,31,32,48,52–54]. OteroAreán et al. studied the Brønsted acid IR band associated withframework Ga in the hydroxyl stretching region and showed itsdecreased intensity with increased preheating temperature [52].Rodrigues et al. reported the presence of strong Lewis acid sitesin Ga/H-ZSM-5 as demonstrated by adsorbed pyridine thatremained even at high temperatures (673 K) [31]. The authors sug-gested that the highly dispersed Ga species would be related to thestrong Lewis acid sites.

From the above discussion, it is evident thatmost of the previouswork on propane conversion in Ga-containing MFI catalysts hasfocused on maximizing aromatic selectivity. Several reports havediscussed the initial PDH step in the propane aromatizationsequence, but only at low propane conversions. For instance,Meriaudeau et al. carried out propane dehydrogenation at low con-versions (<1%) using HZSM-5 and gallosilicate [40]. Hart et al.focused on preparation of selective dehydrogenation catalysts byGa and in addition to H-ZSM-5 with few or no zeolite protonic sites[55]. Ga-modified MFI catalysts have not been widely studied forPDH due to the low propylene selectivities observed at high conver-sions. Here, we report a synthesismethod that creates Ga Lewis acidsites in aluminum-free, gallosilicate MFI without post-treatmentssuch as Ga impregnation, ion-exchange or heat treatment. Specifi-cally, 3-mercaptopropyl-trimethoxysilane (MPS) was used as afunctional silane during the gallosilicate MFI zeolite synthesis. Pre-viously, MPS has been used to incorporate metal and metal sulfideclusters such as Pt, Rh, Ag, and CdS in MFI, SOD, GIS and LTA zeolitechannels [56–58]. It was proposed that the Si species in MPS areincorporated into the zeolite framework after hydrolysis of the tri-methoxy groups while preserving the Si-mercaptopropyl linkage.At the same time, the thiol group on the 3-mercaptopropyl moietyinteracts with metallic ions (such as Pt2+ or Cd2+) in the zeolite syn-thesis solution (or gel), thereby facilitating their occlusion in thezeolite pores during crystallization. We report a detailed structuraland catalytic (PDH) characterization of Ga-MFI catalysts prepared

by this technique, based upon the hypothesis that the alteredmech-anisms of Ga incorporation induced by the addition of MPS wouldlead to a different acid site profiles and catalytic behavior. In thisregard, the newgallosilicateMFI catalysts prepared byMPS additionare shown (via FTIR characterization of pyridine adsorption as wellas temperature-programmed desorption of ammonia and isopropy-lamine) to possess lower Brønsted acid site concentrations andenhanced Lewis acid site concentrations compared to the catalystprepared in the conventionalway, withoutMPS addition. These cat-alysts prepared in the presence of MPS give higher propylene selec-tivities and enhanced propane conversion rates at a fixed propyleneconversion level.

2. Experimental

2.1. Catalyst preparation

The catalysts prepared and used in this study are listed inTable 1. All samples were calcined at 823 K for 10 h prior to useas catalysts. Details of their preparation are described below.

Gallosilicate MFI with MPS (MG05, MG11, MG16): MPS-Ga MFIwas prepared based on a typical hydrothermal zeolite MFI synthe-sis with the additions of 3-mercaptopropyl-trimethoxysilane pre-cursor (MPS, 95%, Sigma–Aldrich) and Ga nitrate as a metalsource. First, 10.2 g of tetraethylorthosilicate (TEOS, 98% reagentgrade, Sigma–Aldrich) was added to 16 g of tetrapropylammoniumhydroxide (TPAOH, 1 M Sigma–Aldrich) solution with 27 g of DIwater, followed by stirring overnight. Then, MPS was added tothe solution and stirred for 3 h. Subsequently, 0.5 g Ga nitrate(Ga(NO3)3�xH2O, 99.9%, Sigma–Aldrich) was added and the mix-ture was kept stirring for an additional 3 h. The final molar compo-sition was 1 SiO2:0.32 TPAOH:45.4 DI water, with the MPS amountadjusted to achieve different MPS/Ga ratios. Three gallosilicateswere synthesized, namely MG05, MG11, and MG16 for whichMPS/Ga ratios in the synthesis gel were 0.54, 1.1 and 1.6 respec-tively. The solution was heated at 423 K for 4 days in a Teflon-lined autoclave. The obtained zeolite crystals were washed withDI water, recovered by centrifugation, and dried overnight at343 K, followed by calcination at 823 K for 10 h under static airconditions with a heating rate of 1.5 K/min.

Gallosilicate MFI without MPS (MG0): For a comparison, anothertype of gallosilicate MFI with no addition of MPS (MG0) was alsoprepared by the same method as MPS-Ga MFI but without theMPS addition step.

Ga impregnated pure-silica MFI (Ga/pure-silica MFI, GS): Gaimpregnated into the pure-silica MFI zeolite was also prepared.Pure-silica MFI was synthesized with the same synthesis methodas MPS-gallosilicate MFI without addition of Ga nitrate and MPS.Then Ga impregnated pure-silica MFI was prepared by wet impreg-nationofGanitrate in ethanol solution followedbydrying in a rotaryevaporator and calcination under the conditions noted above.

Chromia-alumina: For a comparison with Cr catalyst similar tothose applied commercially, a Na-doped chromia-on-alumina cat-alyst was prepared by incipient wetness impregnation with a com-position of 20 wt.% Cr and 1 wt.% Na. A solution of Cr (III) oxide(Cr2O3, P98%, Sigma–Aldrich) was dropped onto an alumina(Al2O3, 98%, Sasol) support, followed by drying overnight at383 K and calcination at 773 K for 30 h. Prior to loading the Cr ontothe support, a Na nitrate (NaNO3, P99.0%, Sigma–Aldrich) solutionwas added dropwise to the alumina support and dried.

2.2. Catalyst characterization

Powder X-ray diffraction (XRD) measurements were carried outwith a PANalytical XPert PRO diffractometer using Cu Ka radiation

Table 1List of catalysts used in this work. All catalysts were calcined at 823 K for 10 h.

Catalyst Type As-synthesized MPS/Ga Post treatment

MG05 MPS-Gallosilicate TPA-Gallosilicate 0.54 NoneMG11 MPS-Gallosilicate TPA-Gallosilicate 1.1 NoneMG16 MPS-Gallosilicate TPA-Gallosilicate 1.6 NoneMG0 Gallosilicate TPA-Gallosilicate No MPS addition NoneGS Ga/pure-silica MFI TPA-silica MFI No MPS addition Ga impregnation

S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123 115

with a scan step size of 0.0167� in a 2h range of 5–50�. The shapeand size of the particles were characterized with a HitachiSU8010 cold field emission scanning electron microscope (SEM).The surface area, micro- and meso-pore volumes were obtainedfrom N2-physisorption measurements at 77 K using a Micromerit-ics ASAP 2020. The samples were degassed at 473 K for 12 h andfree space analysis was performed using He prior to N2

adsorption-desorption isotherm measurements for more accuratemicropore analysis. The samples were again degassed at 473 K toremove entrapped He before micropore analysis. The strengthand concentration of the acid sites were determined bytemperature-programmed desorption of NH3 (NH3-TPD) in an U-shaped fixed bed reactor using a Micromeritics Autochem II. Thesample (about 0.11 g, 212–400 lm pellet size) was preheated at773 K for 1 h followed by NH3 injection at 373 K and holding for1 h. Helium was used as the carrier gas at 25 mL STP/min. The tem-perature was then elevated to 873 K at a ramping rate of 10 K/minand the desorbed NH3 was detected by a thermal conductivitydetector (TCD). The TCDwas pre-calibrated with known concentra-tions of NH3. The concentration of Brønsted acid sites was alsomeasured by temperature-programmed desorption of isopropy-lamine (IPA-TPD) [59–61]. A sample (0.1 g) was loaded in a fixedbed and activated at 773 K for 3 h with 100 mL/min of N2 flow toremove remaining water. After cooling to 373 K, 50 lL of IPA wasintroduced into the N2 carrier gas flow followed by overnight con-tinuous N2 flow to remove physisorbed IPA. Then, the TPD mea-surements were carried out at a heating rate of 10 K/min and theconcentration of desorbed propylene resulting from decomposedfrom IPA on the Brønsted acid sites was recorded using PfeifferVacuum mass spectrometer (MS), which was also pre-calibratedwith known concentrations of propylene. Elemental analysis forGa and Si content was carried out by inductively coupledplasma-optical emission spectroscopy (ALS Environmental, AZ) toascertain the composition of each catalyst sample. For the materi-als synthesized in the presence of MPS, analysis for S was also com-pleted by combustion at 1573 K followed by infrared spectroscopydetection (ALS Environmental, AZ). Solid-state nuclear magneticresonance (NMR) spectra of 29Si and 71Ga were obtained on a Bru-ker Avance III 400 spectrometer at a sample spinning speed 10 kHz.The 29Si NMR spectra were referenced with respect to 3-(trimethylsilyl)-1-propanesulfonic acid at 0 ppm. For the 71Ga NMR spectra,aqueous Ga(NO3)3 solution at 0 ppm was used for calibration.UV–Vis spectra were measured using a Cary 5000 UV–Vis–NIRspectrometer (Agilent Technologies). Powder zeolite samples wereadded to the sample holder after grinding using a mortar and pes-tle. The UV–vis absorption spectra were measured in the wave-length range between 200 and 800 nm, with a resolution of 1 nm.

The concentrations and ratios of Brønsted and Lewis acid siteswere estimated by Fourier transform infrared spectroscopy (FT-IR) of samples containing adsorbed pyridine on a Thermo ScientificNicolet 8700 spectrometer. The catalysts were activated at 773 Kunder vacuum for 6 h. A background spectrum was measured aftercooling down to 423 K and then pyridine was admitted to the cat-alyst at the same temperature. The FT-IR spectra of hydroxylstretching region were also obtained before dosing of pyridine.The pyridine was adsorbed for 1 h to allow equilibration, and the

physisorbed pyridine was removed under vacuum at 10�6 mbarovernight. The FT-IR spectra of the pyridine adsorbed sampleswere measured at 423 K after evacuation at different temperatures,specifically 423, 523, 623, and 723 K.

2.4. Catalytic measurements

Catalytic PDH was performed at 873 K under atmospheric pres-sure. The catalyst (212–400 lm pellet size) was placed in an U-shaped quartz tube fixed bed reactor (3 mm inner diameter) andheated under N2 (UHP grade, Airgas) flow to the reaction tempera-ture in a fluidized sand bath furnace (Techne FB-08). The concentra-tion of C3H8 in the feedwas 5%with the balance N2. The catalyst bedwas fixed between two pieces of quartz wool and the amounts ofeach catalyst sample were adjusted for similar conversion level(�7%). Hence the lengths of catalyst bed were varied from 0.3 to1 cm. The total feed flow rate was 20 mL STP/min. Since all the cat-alyst samples were pelletized and sieved, a similar level of externalmass transfer resistance, if any, is expected at the same gas flowvelocity. Also effects of internal mass transfer are not suggested tobe significant using the employed experimental conditions(Supplementary Material). During the PDH reaction, the productstream was analyzed by an on-line GC (Shimadzu GC2014) using aflame ionization detector (FID) for hydrocarbon products and ther-mal conductivity detector (TCD) for H2. The propane conversion,product selectivities and product yield were calculated based onthe feed conditions and the concentrationof theproducts as follows:

C3H8 conversionð%Þ � C3Hin8 � C3H

out8

C3Hin8

� 100

Product selectivityð%Þ � Productout

C3Hin8 � C3H

out8

� 100

Product yieldð%Þ � Productout

C3Hin8

� 100

3. Results and discussion

3.1. Physical properties

The MFI structure was confirmed by XRD for all the zeolite cat-alysts used in this study. Comparing the pure-silica and gallosili-cate MFI samples, the intensities of the diffraction peaksdecreased with Ga incorporation (Supplementary Material,Fig. S1). All the gallosilicate MFI samples showed similar crys-tallinity, although MPS addition slightly decreased peak intensities(Supplementary Material, Fig. S2). Most of the gallosilicate MFI cat-alysts showed small spherical crystallite shapes (Fig. 1), which hasalso been observed in past studies [36,51,62]. The GS materialformed particles with very similar shape to our pure-silica MFI(Fig. 1). The particle sizes were mostly ca. 200 nm for all thematerials, with no significant difference caused by differentamounts of MPS added. The physical properties of the catalysts

Fig. 1. SEM images of MG11 (left top), MG0 (right top), GS (left bottom), and pure-silica MFI (right bottom).

Fig. 2. NH3-TPD profiles of MG materials synthesized with and without MPSaddition.

116 S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123

are summarized in Table 2. No significant qualitative trend wasobserved in micropore volume and surface area of the MG materi-als. A slightly different range of pore volume and surface area wasobserved for the GS and pure-silica MFI samples compared withthe MG materials, which is probably due to the different prepara-tion methods employed. The Si/Ga ratio was obtained by ICP-OESanalysis, and all the catalysts showed similar total Ga content.

3.2. Acid site measurements

The NH3-TPD profiles of the gallosilicate MFI materials (MG)synthesized with and without MPS addition were compared. InFig. 2, both low temperature (l-peak at 450 K) and high tempera-ture (h-peak at 580 K) features were observed for all the profilesexcept the GS sample. The l-peak is attributed to sites whereammonia is physisorbed and weakly held [45,51]. The h-peak cor-responds to Brønsted acid sites generated after TPA ion removalduring calcination.

Furthermore, additional peak shoulders at higher temperatures(ca. 700 K) were observed. These features were also reported inpast studies and were assigned to strong Lewis acid Ga species

Table 2Physical properties of catalysts.

Catalyst Vmicroa (cm3/g) Vmeso

b (cm3/g) Smicroa (m2/g) Stotalc (m2/g) Si/Gad

MG16 0.13 0.12 273 500 35MG11 0.15 0.08 315 470 37MG05 0.13 0.08 289 402 34MG0 0.14 0.05 306 383 36GS 0.11 0.11 213 388 42Pure-silica MFI 0.12 0.09 245 318 1

a Calculated by t-plot method.b Calculated by BJH method.c Calculated by BET method. Though the BET equation does not strictly apply to microporous materials, BET surface areas are reported for comparison with literature data.d Obtained by ICP-OES.

S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123 117

[31,51]. Miyamoto et al. observed a similar peak shoulder at 700–900 K in NH3-TPD profiles of materials with increased Ga content,and also found an additional h-peak when Ga was impregnatedinto the gallosilicate. Rodrigues et al. confirmed the presence ofstrong Lewis acid sites in Ga impregnated H-ZSM-5 in which a sig-nificant TCD signal in the NH3-TPD experiment still remained attemperatures of more than 800 K, whereas the signal returned tothe baseline in the case of H-ZSM-5. In both the above studies,Ga impregnated into pure-silica MFI did not yield a material withstrong Lewis acidity, indicating that the presence of Brønsted acidsites was necessary to generate the strong Lewis acid sites duringGa incorporation. Rodrigues et al. suggested from their EXAFSspectra of Ga/H-ZSM-5 that atomically dispersed oxidic Ga speciesin ion-exchange positions should be ascribed to the strong Lewisacid sites, whose concentration increased with the number offramework Al sites. From the TPD profiles of the MG materials inthis study, the intensity of the high-temperature shoulderincreased with the amount of MPS added in the synthesis gel. Thissuggests that the thiol group in the MPS ligand interacts with Ga inthe zeolite synthesis gel, thereby influencing the speciation of Ga inthe final material. Specifically, MPS-induced formation of extra-framework Ga sites is hypothesized. Based on the fact that theTPD peak intensities at 550 K showed almost no difference withand without MPS addition, it appears that the MPS ligand had littleeffect on the formation of framework Ga Brønsted sites. This is con-sistent with the proposed mechanism of metal incorporation viaMPS, which is expected to result in enhanced incorporation inthe pores rather than in the framework. Considering the proposedbifunctional reaction mechanism of PDH requiring both Brønstedand Lewis acid sites, where Lewis acid sites play a crucial role,the MG materials in this study were thought to be good candidatesfor PDH, since they provide a greater number of strong Lewis acidsites even without post-synthesis impregnation or heat treatmentsteps.

Ammonia TPD analysis does not provide any information distin-guishing Lewis versus Brønsted acid sites in the absence of sup-porting data from other techniques. To this end, the acid sitedistribution of an MG catalyst (MG11) containing adsorbed pyri-dine was analyzed by FT-IR spectroscopy alongside the spectrumof the catalyst made without MPS addition (MG0). The FT-IR spec-tra of the materials before pyridine dosing are displayed in Fig. 3. Inthe hydroxyl stretching region from 3000 to 3750 cm�1, a peak at3617 cm�1 was observed for all catalysts, indicating that they havebridged Brønsted acid sites due to framework Ga [32,40,52]. Thehigher frequency of these Brønsted acid sites than typical Albridged protonic sites has for decades been used to suggest a less

Fig. 3. FT-IR spectra in the Si-OH region of gallosilicate MFI materials prepared withand without MPS addition (MG16, MG11, MG05, MG0). Curves are offset for clarity.

acidic character of the Ga bridged sites [32], although morerecently it has been suggested that the deprotonation energies ofhetero-atom bridged sites do not correlate with the OH frequency[63]. Nonetheless, considering the higher deprotonation energy ofGa bridged acid sites than Al sites reported in a related study [64],one may still conclude that the strength of the Brønsted acid sitesof gallosilicate MFI is lower than Al-MFI type zeolites. For all theMG samples shown in Fig. 3, there is a broad band between 3000and 3600 cm�1 corresponding to hydrogen bonded silanol groupsin hydroxyl nests in defect sites associated with missing T atoms[52]. It should be noted that in Fig. 3 the broad bands shift rightfor the sample without MPS addition (e.g., MG0). Similar peakshifts have previously been observed in the literature [65], wherethe peak position of the broad band (3000–3600 cm�1) moved tolower frequencies with more framework Al sites. Hence, this shiftmight be interpreted that samples made with added MPS have lessframework Brønsted acid sites, which can be replaced by defectsites for charge balancing the organic structure-directing agentduring synthesis. Another peak previously assigned to hydroxylgroups attached to extra-framework Ga at 3660–3670 cm�1 wasalso observed [31,48,52,53], but the peak overlapped with isolatedand terminal silanol groups at 3720 and 3740 cm�1 [31,52].

FT-IR spectra of the catalysts containing adsorbed pyridine werealso measured after evacuation at temperatures of 423, 523, 623,and 723 K. The peak at ca. 1540 cm�1 corresponds to the

Fig. 4. FT-IR spectra of pyridine adsorbed (a) MG11 and (b) MG0 after evacuation atdifferent temperatures. The B, L, and Py-H peaks represent Brønsted acid sites,Lewis acid sites and hydrogen bonded pyridine, respectively.

Fig. 5. Comparison of B/L ratio obtained from FT-IR spectra of pyridine adsorbedgallosilicate MFI materials.

118 S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123

pyridinium ion, PyH+, formed after pyridine interacts with Brønstedacid sites. Pyridine adsorbed on Lewis acid sites gives a characteris-tic band at ca. 1450 cm�1 [31,51]. The band at ca. 1445 cm�1 hasbeen assigned to hydrogen bonded pyridine [51]. As shown inFig. 4, significant intensity of the 1458 cm�1 band associated withLewis acid sites still remained after evacuation at high tempera-tures up to 723 K, indicating the presence of some very strong Lewisacid sites, which is consistent with the profiles (Fig. 2) fromNH3-TPD. Similar results were previously observed when Ga wasimpregnated into HZSM-5 and the strong Lewis acid sites wereassigned to highly dispersed Ga species [31,51]. The above findingsclearly confirm key differences in the acid site distribution betweenthe gallosilicate MFI materials prepared with and without MPSaddition. The ratio of the concentrations of Brønsted and Lewis acidsites (B/L) was obtained for eachmaterial by using integratedmolarextinction coefficients for MFI zeolites from the literature [66].Fig. 5 quantifies the difference in B/L ratio among the gallosilicateMFImaterials. MPS addition led to a greater number of strong Lewisacid sites, thus decreasing the B/L ratio.

Temperature-programmed desorption of isopropylamine(IPA-TPD) was also carried out to assess the concentration ofBrønsted acid sites present at low and moderate desorption tem-peratures, independent of the Lewis acid sites (SupplementaryMaterial, Fig. S3). In the IPA-TPD profiles of MG samples, the mainpeak in the temperature range 600–700 K is assigned to propylenefrom adsorbed IPA on the Brønsted acid sites [59–61], while thepeak below 600 K is assigned to propylene from weakly held IPAadsorbed on internal or external silanol groups [67]. In MGsamples, there was an additional peak shoulder at high tempera-ture, above 700 K, where the peak intensities increase in samples

Table 3Summary of acid site analysis results.

Catalyst Brønsted acid sites fromIPA-TPD (lmol/g)

Two acid peaks a fromNH3-TPD (lmol/g)

MG16 103 133/108, Total = 241MG11 122 135/91, Total = 226MG05 145 143/75, Total = 218MG0 175 143/43, Total = 186

a Two acid sites are derived from the first high temperature peak and from the peakb B indicates concentration of Brønsted acid sites, and L is the concentration of Lewisc The extinction coefficients of pyridine adsorbed on Brønsted and Lewis acid sites are

[82] the concentrations of Brønsted and Lewis acid sites are higher by a factor of 2.29 ad The deviation is likely due to underestimation of Lewis acid sites by NH3-TPD as dis

prepared with greater MPS addition. Similar peak trends were alsoobserved for the ammonia signal (Supplementary Material,Fig. S3b) along with simultaneous propylene detection, whichmeans that the peak shoulder at high temperature is likely associ-ated with decomposed IPA. A similar peak at high temperature wasalso previously reported for the reduced form of Ga impregnatedH-ZSM-5 [68,69]. According to Ausavasukhi et al. the new peakat high temperature was observed after reduction of Ga/H-ZSM-5in H2 while no peak was seen in the same temperature rangewithout H2 treatment. The authors suggested that the peak isassociated with hydroxyl groups on extra-framework Ga-OH sites.Considering that the peak shoulder intensities of the MG materialsincreased with MPS content in the synthesis gel, the MG materialsmay have such extra-framework Ga-OH species interacting withIPA.

The acid site measurements are summarized in Table 3. Theacid site concentrations were determined by a combination ofIPA-TPD-MS, pyridine IR, and NH3-TPD, as mentioned above. Theconcentration of Brønsted acid sites was measured byIPA-TPD-MS by calibrating the propylene concentrations in theMS. Similarly the total acid site concentrations were also quanti-fied from NH3-TPD by NH3 calibration in the TCD after deconvolu-tion of the main peak at 550 K and high temperature peakshoulder. The two acid sites are suggested to be associated withadsorbed NH3 on the Brønsted acid sites (low temperature) andthe strong Lewis acidic Ga sites (high temperature), using informa-tion gleaned from the IPA-TPD-MS experiments. It should be notedthat the quantification of NH3-TPD may not reflect the entire con-centration of the acid sites, considering that the TCD signals do notreturn to the baseline at high temperature (850 K), as shown inFig. 2. As a result, the concentration of high temperature Lewis acidsites may be underestimated, leading to overestimated B/L ratios inTable 3 using this technique. The concentration of each type of acidsite can be also quantitatively estimated from pyridine FT-IR usingthe integrated molar extinction coefficient [66]. However, theabsolute concentrations measured by pyridine IR are less than halfof those from IPA-TPD. This is probably due to the size of pyridine(ca. 5.5 Å) and its steric limitations, and hence it may not be able toaccess all the acid sites in the gallosilicate MFI pores [70]. Despitethe perceived underestimation of Lewis sites by TPD and inaccessi-bility of pyridine to all acid sites, the data in Table 3 show that therelative concentrations of the acid sites were qualitatively wellmatched, supporting the supposition that increased MPS silane inzeolite synthesis gels results in enhanced concentrations of Lewisacid sites and decreased amounts of Brønsted acid sites. It is worthmentioning that every Ga atom does not correspond to an acid site,which means that some Ga species are inactive or inaccessible.

The Si and Ga species present in the catalysts were analyzed byNMR measurements. Fig. 6 shows the 29Si NMR spectra of the fourMG materials. The peak at ca. �113 ppm is assigned to frameworkSi(0 Ga) species, while the signal at �105 ppm is from frameworkSi(1Ga) atoms, indicating the presence of framework Ga in the

Acid sites (B/L)b,c frompyridine-IR (lmol/g)

B/L ratio from pyridine-IR:fromIPA and NH3 TPD

41/57, Total = 98 0.72:0.9546/47, Total = 93 1.0:1.3459/47, Total = 106 1.3:1.9367/38, Total = 105 1.8:4.07d

shoulder in the high temperature range.acid.subject to debate in literature. If one used the coefficients published by Datka et al.nd 2.00, respectively.cussed in the main text.

Fig. 6. 29Si NMR spectra of the MG catalysts.

S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123 119

gallosilicate MFI materials [20,36,71]. The MG materials showedalmost the same relative intensities of these two peaks, suggestingthat the MG materials have similar levels of framework Ga. Addi-tionally, in the NMR spectrum of the as-synthesized MG11 mate-rial, there was an additional peak at ca. �70 ppm, which was notobserved in MG0 (Supplementary Material, Fig. S4). This peakcan be assigned to T3 species [R–Si–(OSi)3], which was alsoreported by Wong et al. for materials synthesized in the presenceof MPS [57,72–74]. This confirms the condensation of themethoxysilane groups in MPS with the oxide framework duringthe synthesis. This T3 species then disappeared after calcination.UV–vis measurements were also carried out to probe for evidence

Fig. 7. 71Ga NMR spectra of three gallosilicate MFI materials, MG0, MG11 andMG16.

of interactions between Ga and thiol groups associated with MPS(Supplementary Material). Specifically, the comparison of UV–Visspectra of as-synthesized Ga-MFI (MPS/Ga = 0, MG0) and a Ga-MFI sample synthesized with MPS (MPS/Ga = 0.16, MG16) is givenin Fig. S5. Upon addition of MPS to the hydrothermal synthesis, thecharacteristic peak of framework Ga at 210 nm was shifted to alower wavelength, representing a higher bandgap energy [75,76].This may be due to the presence of mercaptopropyl groups in theMFI pores, causing interaction with framework Ga. Also, an extrapeak at 270 nm was observed, representing the formation ofextra-framework Ga [77,78], the extent to which is suggested tobe influenced by Ga-thiol interactions.

The presence of framework Ga was also confirmed by the largepeak at ca. +150 ppm in the 71Ga NMR spectra (Fig. 7), representingGa species in tetrahedral coordination [43,79–81]. For thegallosilicate MFI materials used in this study, the peaks associatedwith extra-framework Ga were not routinely observed. Indeed,small concentrations of extra-framework Ga are difficult to bedetected by 71Ga NMR, especially if they are not symmetric species,as previously discussed. However, for the MG16 material, whichhas the largest amount of Lewis acidic Ga sites based on acidityanalysis, a small peak at ca. 0 ppm was observed. This confirms

Fig. 8. (a) Propane conversion and (b) propylene selectivity of MG catalysts at 873 Kand atmospheric pressure. (W/F = 0.07 g�s�cm�3 (MG16), 0.09 g�s�cm�3 (MG11),0.12 g�s�cm�3 (MG05), 0.3 g�s�cm�3 (MG0), C3H8 5%).

120 S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123

the presence of extra-framework Ga in this material, with theamount likely just above the threshold of detection in the NMRmeasurements. The NMR and pyridine FT-IR results indicate thatalthough the MPS itself was likely cross-linked into the oxideframework during the synthesis, it did not significantly affect theincorporation of Ga in tetrahedral framework sites but instead itgenerated additional Lewis acidic Ga sites.

3.3. PDH performance

The catalytic PDH performance of the gallosilicate MFI catalystswas first compared at a common low conversion level of ca. 10%.The catalytic measurements were performed at 873 K and atmo-spheric pressure in a continuous quartz tube reactor connectedto an online GC. The W/F values (W: weight of catalyst in g, andF: feed flow rate in mL/s) were adjusted for each catalyst to com-pare activity and selectivity at similar propane conversions. Thetime onstream data for propane conversion and the propyleneselectivities for each catalyst were recorded for 5.5 h. As shownin Fig. 8a, a common propane conversion range of 9–13% wasmaintained for all the catalysts. The slightly higher rate of initialdeactivation of MG16 shown in Fig. 8a is likely due to its higher

Fig. 9. (a) Propane selectivity and (b) ratio of dehydrogenation to cracking of MGcatalysts at 873 K and atmospheric pressure.

concentration of acid sites. The onstream propylene selectivitiesare shown in Fig. 8b, whereas the time-averaged propylene selec-tivities and dehydrogenation-to-cracking ratios (D/C) from themeasurements are compared in Fig. 9. It is clear that there is signif-icantly increased propylene selectivity and lower cracking in theMPS-gallosilicate MFI materials compared to the catalyst madewithout MPS addition. Furthermore, the propylene selectivityincreased with increasing MPS content in the catalyst synthesisgel. The initial propane conversion rates (expressed as TOFs) alsoincreased from MG0 to MG16 (Fig. 10) wherein the MG16 catalystshowed more than a threefold increase in activity compared to theMG0 catalyst. The propane conversion rates (TOFs) were calculatedbased on Brønsted acid site (Fig. 10a) or the total acid site concen-trations. The Brønsted acid site densities were measured by IPA-TPD and the total acid sites was estimated by combination ofIPA-TPD and B/L ratios from pyridine-IR experiments (Fig. 10b).In Fig. 10a, different propane conversion rates were observed forthe MG catalysts, with MG16 showing the highest activity perBrønsted acid site, a finding which may be attributed to its largenumber of strong Lewis acid sites. These data suggest that thepresence of lower Brønsted acid site concentrations coupled withthe presence of strong Lewis acid sites (associated with the high

Fig. 10. Comparison of propane conversion rate and stability of the different MGcatalysts. The propane conversion rate is calculated based on (a) the concentrationof Brønsted acid sites (molB) from IPA-TPD, (b) the concentration of total acid sites(molac) from IPA-TPD and the B/L ratio from pyridine IR.

Fig. 11. (a) Propane conversion, (b) propylene selectivity vs. time onstream ofMG11 and chromia-alumina catalysts. (W/F = 0.3 g�s�cm�3, C3H8 5%).

Fig. 12. (a) Propane conversion, (b) propylene selectivity vs. time onstream ofMG11 and chromia-alumina catalysts. (W/F = 0.6 g�s�cm�3, C3H8 5%).

S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123 121

temperature TPD shoulder) leads to both increased propane con-version rate and propylene selectivity. This supports previoushypotheses that strong Lewis acid sites are key active sites forPDH. When the propane conversion rates were normalized by totalacid sites, as shown in Fig. 10b, the catalysts made using MPS stillshowed clear enhancements compared to MG0, suggesting thatthere may exist a bifunctional mechanism between Brønsted andLewis acid sites leading to promotion of the PDH reaction. Therewas almost no activity detected for both the pure-silica MFI andGa impregnated pure-silica MFI (GS) control catalysts. This wasexpected, since they did not show significant acidity during theNH3-TPD measurements.

The best performing gallosilicate MFI made with MPS addition(MG11) was also compared with a prototypical chromia-aluminacatalyst. The catalytic measurements were done at 873 K andatmospheric pressure for 12 h at two higher conversion levels(ca. 25% and 40%). In Fig. 11, when the initial propane conversionlevel was about 25%, which corresponds to the initial conversionrate of about 6.5 mmol/gcat/h, the MG11 showed lower propyleneselectivity (ca. 75%) than the Cr catalyst (ca. 85%). However, itshowed less deactivation over 12 h of reaction compared to theCr catalyst (�3.5 mmol/gcat/h decrease from the initialconversion rate for the Cr catalyst vs. �2.0 mmol/gcat/h decreasefrom the initial conversion rate for MG11). Similarly, the PDH

performance was compared at an even higher conversion levelin Fig. 12, where lower propylene selectivities (10–20%) wouldbe expected for conventional Ga catalysts based on the literaturereports discussed earlier [36,44]. Although the Ga-containing cat-alysts gave lower propylene selectivity (ca. 60%) than the Cr cat-alyst (75%) under these conditions (Fig. 12), it is clear that MG11still retained a high level of propylene selectivity even in thiselevated conversion range. By comparison, in Choudhary et al.[36], the propylene selectivity was estimated to be about 15%at this conversion level. The improved performance in theseGa-MFI samples prepared with MPS addition is likely due tothe lower concentration of Brønsted acid sites, as exemplifiedin MG11, since it is believed that Brønsted acid sites are activein the oligomerization and cyclization steps of alkane aromatiza-tion [31]. At both elevated conversion levels, the MG11 catalystshowed slower deactivation compared to the Cr catalyst andhence the MG11 provided higher propylene yield for a givenreaction time, as shown in Fig. 13. Comparing the two catalysts,the chromia-alumina catalyst provided higher propylene selectiv-ity, but this catalyst also showed more deactivation than MG11,even with a slightly lower initial conversion. This suggests thatthe MPS-gallosilicate MFI catalyst offers better resistance to cokeformation than this benchmark Cr2O3 catalyst under theconditions studied.

Fig. 13. Propylene yield at different conversion levels. (a) W/F = 0.3 g�s�cm�3

(b) W/F = 0.6 g�s�cm�3.

122 S.-W. Choi et al. / Journal of Catalysis 345 (2017) 113–123

4. Conclusions

Gallosilicate MFI zeolite catalysts with perturbed acidities wereprepared via the addition of MPS to the zeolite synthesis gel, withthe MPS removed after synthesis by calcination. The resulting gal-losilicate MFI materials had reduced concentrations of Brønstedacid sites and more Lewis acid sites, with significant concentra-tions of strong Lewis acid sites, as confirmed by NH3-TPD, IPA-TPD and pyridine IR. These strong Lewis acid sites were achievedwithout post-synthesis impregnation of extra-framework Ga orhigh temperature heat treatments. The concentration of strongLewis acid sites can be controlled by adjusting the amount ofMPS added, with more MPS generating more strong Lewis acidsites. The PDH selectivity of gallosilicate molecular sieves is knownto be strongly affected by the concentration of strong Lewis acidsites, with the most effective catalysts having a small concentra-tion of Brønsted acid sites coupled with strong Lewis acidity.Decreasing the B/L ratio leads to better performance in PDH withhigher activity and propylene selectivity. Therefore, the new gal-losilicates prepared by MPS addition have acidities that are well-aligned with selective PDH catalysis. The reactivity of the catalystsprepared in the presence of MPS demonstrates that these catalystsare more effective for PDH than gallosilicate catalysts prepared

without the MPS functional silane, having good propane conver-sion rates and higher selectivity to propylene. Also, the new familyof catalysts prepared here have reduced cracking and aromatiza-tion selectivity. Compared with a benchmark chromia-aluminacatalyst, a gallosilicate MFI catalyst prepared using MPS showedlower rate of deactivation than the benchmark catalyst. Thoughit had a slightly reduced propylene selectivity, it offered enhancedpropylene yield compared to the chromia-alumina catalyst. Overallthis work demonstrates that creation of zeolite based catalystswith improved PDH performance may be achieved by tuning thereactivity via use of functional silane additives added during thecrystallization of the molecular sieve.

Acknowledgments

This work was supported by The Dow Chemical Company. Wethank Dr. Johannes Leisen (Georgia Tech) for conductingsolid-state NMR experiments, and Dr. Lin Luo (Dow ChemicalCompany) for providing the chromia-alumina catalyst.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2016.11.017.

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