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Journal of Natural Gas Science and Engineering 23 (2015) 184e194

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Journal of Natural Gas Science and Engineering

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

Catalytic conversion of methanol to propylene over high-silicamesoporous ZSM-5 zeolites prepared by different combinations ofmesogenous templates

Javad Ahmadpour, Majid Taghizadeh*

Chemical Engineering Department, Babol University of Technology, P. O. Box 484, 4714871167 Babol, Iran

a r t i c l e i n f o

Article history:Received 28 October 2014Received in revised form22 January 2015Accepted 23 January 2015Available online 29 January 2015

Keywords:Hierarchical porosityHigh silica ZSM-5Methanol to propylene (MTP)Mesogenous templates

* Corresponding author.E-mail address: m_taghizadehfr@yahoo.com (M. T

http://dx.doi.org/10.1016/j.jngse.2015.01.0351875-5100/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

The effects of mesoporosity on catalytic performance of methanol to propylene (MTP) reaction have beeninvestigated by using a series of high-silica nanosized hierarchical ZSM-5 zeolites with different degreesof mesoporosity, which were hydrothermally synthesized in the presence of different mesogenoustemplates, i.e., TPOAC, CTAB, and their combinations. The results were compared with a conventionalmicroporous ZSM-5 catalyst. The prepared catalysts were characterized by XRD, FE-SEM, BET, NH3-TPDand FT-IR techniques. Meanwhile, their catalytic activities were evaluated in a fixed-bed reactor underatmospheric pressure, 460 �C and WHSV of 1 h�1. Compared with conventional catalyst, the nanosizedhierarchical porous ZSM-5 templated with a mixture of 75% TPOAC and 25% CTAB displayed reliable MTPcatalytic lifetime (76 h) as well as sufficiently high propylene selectivity (43.65%) and propylene toethylene (P/E) ratio (4.05), which were predominately attributed to its optimal combination of acidityand porosity.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Propylene has been used as an essential feedstock for the pro-duction of polypropylene, acrylonitrile, propylene oxide, oxo alco-hols and cumene in the petrochemical industry. Due to theincreasing demand for propylene and rapid depletion of oil re-sources, new processes with high propylene yield are needed(Firoozi et al., 2009; Guo et al., 2013). The methanol to propylene(MTP) process is regarded as a promising alternative route for thehigh yield production of propylene, since methanol can be indus-trially produced in large scale from natural gas and coal usingproven technologies (Wu et al., 2011; Guo et al., 2013). The MTPtechnology was originally developed by Lurgi based on a fixed bedreactor with a ZSM-5 zeolite catalyst (Koempel and Liebner, 2007).Since the first industrial MTP plant was established in China in2010, much attention has been drawn to this process from bothscientific and commercial viewpoints, including enhancement ofthe MTP process (Wu et al., 2011; Guo et al., 2013; Hadi et al., 2014)and development of an effective catalyst with satisfactory stabilityand propylene selectivity (S. Hu et al., 2012; Z. Hu et al., 2014; Q.

aghizadeh).

Zhang et al., 2014).Currently, the high-silica ZSM-5 zeolite (usually with a Si/Al

molar ratio higher than 170) is a suitable catalyst for MTP reactiondue to its characteristic MFI topology resulting in comparativelygood propylene selectivity and catalytic stability (Chang et al., 1984;Mei et al., 2008; J. Liu et al., 2009; S. Hu et al., 2012). Despite thesesuperior results, the relatively small and sole micropores in ZSM-5zeolites significantly influence the mass transfer of the reactantsand products to/from the active sites, which would result in rela-tively easy coke formation and deactivation of catalysts (Choi et al.,2009; Kim et al., 2010; Z. Hu et al., 2014). Thus, it is still a challengeto further improve propylene selectivity and catalytic lifetime overthis catalyst for MTP reaction.

One strategy to overcome these hurdles is introducing sub-stantial intra- or intercrystalline mesoporous (i.e., pores with di-ameters ranging between 2 and 50 nm) besides the microporousstructure, leading to the creation of so-called hierarchical ZSM-5.The hierarchical ZSM-5 can possibly combine the advantages ofeach individual pore size regime to show superior catalytic per-formance. As a result, the highly mesoporous ZSM-5 exhibited highpropylene yield and long catalytic stability during the catalyticconversion of methanol to propylene due to its improved masstransfer properties and tolerance for a larger amount of coke (Sun

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194 185

et al., 2010; Hu et al., 2012; Z. Hu et al., 2014).Up to now, intensive efforts have been devoted to obtain mes-

oporous ZSM-5 in order to combine the advantage of both mesoand microporous materials. Moreover, a variety of synthesismethods including chemical leaching approaches (desilication(Behbahani and Soleimani Mehr, 2014) and dealumination(Triantafillidis et al., 2001)), the assembly of nanosized zeolitecrystals (Rownaghi et al., 2012), and synthesis in the presence ofvarious hard or soft templates (Schmidt et al., 2001; Sun et al., 2010)have been developed to prepare hierarchical mesoporous ZSM-5. Inrecent years, the dual templatingmethod by using both amoleculartemplate (i.e. TPAOH or TPABR) for microporosity and a supramo-lecular soft template such as cationic surfactant (Zhu et al., 2011),silylated polymer (Wang and Pinnavaia, 2006) and organosilane(Choi et al., 2006; Kim et al., 2010) for generation of mesoporosityhas been drawing increasing attention for its good compatibilitywith zeolite precursor and high efficiency to create mesoporosity.

Nonetheless, when conventional cationic surfactants (i.e. CTAB)were directly mixed with the synthetic mixture of a zeolite amicroporous template, the competition between these twodifferent templating systems led to phase separation of amorphousmesoporous material and crystalline microporous zeolite (Zhuet al., 2011). Although specially designed amphiphilic organo-silane surfactants (Choi et al., 2006; Kim et al., 2010) have cir-cumvented this drawback and resulted in synthesis of a crystallineZSM-5 with adjustable mesoporosity, they are difficult to synthe-size and are expensive. It is still desirable to use conventionalcationic surfactants to fabricate hierarchical mesoporous ZSM-5that reduce cost and are readily scalable (Zhu et al., 2011; Y.Zhang et al., 2014). However, synthetic strategy using conven-tional cationic surfactants still remains a great challenge in syn-thesizing mesoporous zeolites due to phase separation betweenmeso- and micro-phases. To solve this problem, hierarchical mes-oporous ZSM-5 have been successfully synthesized by using CTABsurfactant cooperated with ethanol (Zhu et al., 2011) or nonionicamphiphilic copolymer F127 (Zhu et al., 2013), sulfonic acid-endedorganosilane (Jin et al., 2011), and carboxyl-ended (Y. Zhang et al.,2014) as co-templates without the phase separation between themesoporous phase and zeolite crystal phase. In addition to these, Y.Wang et al. (2011) also prepared a hierarchical mesoporous ZSM-5using a dual surfactant system comprising an amphiphilic orga-nosilane (TPOAC) and an ordinary surfactant (CTAB), which wasintroduced as support for noble metal catalyst in the dibenzofuranhydrodeoxygenation reaction.

Catalytic performance of the low-silica mesoporous ZSM-5prepared by single TPOAC or CTAB as mesoporogen during thecatalytic conversion of methanol to olefin (MTO) has been inves-tigated in several open-literature publications (Y. Liu et al., 2010;Kim et al., 2010; Choi and Ryoo, 2010). More recently, Q.Y. Wanget al. (2014) reported that the mesoporous ZSM-5 templated withTPOAC exhibited an extremely long catalyst lifespan compared tothe conventional ZSM-5, while the mesoporous ZSM-5 templatedwith CTAB showed no advantage in prolonging the catalyst lifetimeduring the MTO reaction.

However, the effects of different combinations of two mesoge-neous templates (TPOAC/CTAB) on the mesoporosity and the cat-alytic performance of the high-silica mesoporous ZSM-5 zeolites inmethanol to propylene (MTP) reaction are still unclear, and has notbeen previously reported in detail.

Herein, we have studied the effects of different types, amountsand combinations of two mesogenous templates, i.e. CTAB andTPOAC, on the catalytic performance of the high-silica mesoporousZSM-5 zeolites (Si/Al ¼ 175) in MTP reaction. To get a better un-derstanding of the relation between the mesoporosity and thecatalytic performance of the mesoporous ZSM-5, a conventional

microporous ZSM-5 was employed for comparison. One of theadvantages of the presented synthetic method is reducing the useof expensive organosilane (TPOAC) through regulating the molarratio between TPOAC and CTAB. Physicochemical properties ofprepared catalysts were characterized by XRD, FE-SEM, BET, NH3-TPD and FT-IR methods. Meanwhile, their catalytic performancewas also evaluated for MTP reaction using a fixed-bed flow reactorunder the same operating conditions. A number of findings thathave not been reported earlier are presented.

2. Experimental

2.1. Materials

Tetrapropylammonium hydroxide (TPAOH, 40 wt.% aqueoussolution), tetraethyl orthosilicate (TEOS, 98 wt.%), and methanol(99.5%) were purchased fromMerck while aluminum isopropoxide(AIP, 97 wt.%), 3-[(trimethoxysilyl) propyl]octyldimethyl-ammonium chloride (TPOAC, 72 wt.% in methanol), and cetyl-trimethylammonium bromide (CTAB, 98 wt.%) were purchasedfrom Aldrich. All chemical reagents were of analytical grade andused as received without further purification.

2.2. Catalyst preparation

The high-silica hierarchical mesoporous ZSM-5 zeolites wereprepared using the hydrothermal procedure involving differentamounts of single or mixture of the mesogenous templates, CTABand TPOAC. In a typical synthesis procedure, firstly, the calculatedamounts of aluminum isopropoxide (AIP) and TPAOH were mixedwith double distilled water in a polypropylene bottle under mag-netic stirring about 15 min until AIP was completely dissolved inthe solution. Next, single or mixture of mesopore generating tem-plates was added to this solution under vigorous stirring to obtainhomogeneous mixture, followed by dropwise addition of TEOS tothe resultant mixture to achieve a molar composition of SiO2: (1/350) Al2O3: 0.3 TPAOH: 15H2O: 0e0.05 CTAB: 0e0.05 TPOAC. Theobtainedmixturewas continuously stirred at room temperature for3 h to ensure complete hydrolysis of TEOS and AIP to ethanol andisopropyl alcohol, respectively. Then, the resulting mixture wastransferred into a home-made Teflon-lined stainless-steel auto-clave (100 mL, ca. 80% filled) for hydrothermal treatment at 180 �Cfor 72 h under autogenous pressure. Afterward, the zeolite sus-pension was immediately quenched to room temperature by im-mersion of the autoclave in an ice-water bath to terminate thecrystallization. Subsequently, the solid product was collected byfiltration, washed several times with double-distilled water untilpH neutral, dried in an oven at 110 �C overnight, and finally calcinedin a quartz tube furnace under a flow of air (25 mL/min) at 550 �Cfor 6 h to remove the organic templates completely. Several hier-archical mesoporous ZSM-5 samples prepared by adding differentamount of single or mixture of the mesogenous templates, CTAB (C)and TPOAC (T) were denoted as HMZ-xC-yT, in which x and yrepresent the molar ratio percent of CTAB to SiO2 and of TPOAC toSiO2, respectively. For comparison, conventional microporous ZSM-5 zeolite was synthesized under the same procedures in theabsence of mesopore-directing agent. This sample was denoted asCon-ZSM-5.

2.3. Characterization

X-ray diffraction (XRD) patterns of different samples were ob-tained with an X-pert Pro PW3064/60 diffractometer, using Cu Ka1radiation source (l ¼ 1.5406 A�) at room temperature withinstrumental settings of 40 kV and 40 mA. Data were recorded in

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194186

the 2q range from 5� to 50� with a step size of 0.025�.N2 adsorptionedesorption isotherms at 77 K were measured

using a NOVA 2200 instrument (Quantachrome, USA) in the relativepressure range from 0.05 to 0.99. Prior to N2-physisorption mea-surements, the samples were degassed at 473 K in a N2 flow for 16 hto remove the moisture adsorbed at surface and internal pores.Total specific surface areas (SBET) were calculated using the Bru-nauereEmmetteTeller (BET) method in the P/P0 range 0.05e0.25,and the total pore volume was estimated from the amount of ni-trogen adsorbed at a relative N2 pressure (P/P0) of 0.99. The t-plotmethod was employed to evaluate the micropore surface area(SMicro) and themicropore volume (VMicro) in the P/P0 range 0.1e0.4.The mesopore volume (VMeso) was calculated from the discrimi-nation between the VTot and VMicro. The mesopore size distributionsand the average diameter of mesopores were estimated by usingBarretteJoynereHalenda (BJH) method from the desorption branchof the isotherms.

The morphology and particle size were carried out using a fieldemission scanning electron microscopy (FE-SEM Model: MIRA3TESCAN, USA) operating at 15 kV. All samples were subsequentlysputter coated with a thin gold film to reduce charging effects.

The acidity of the samples was measured by temperature-programmed desorption of ammonia (NH3-TPD) using BELCAT-Ainstrument (Micromeritics, USA) with a conventional flow appa-ratus which included an on-line thermal conductivity detector(TCD). In a typical analysis, 35 mg sample was initially flushed withhelium at 300 �C for 2 h at a heating rate of 10 �C/min and thenwascooled down to 60 �C and further saturated with NH3. After NH3exposure, the samplewas purgedwith helium for 30min to removeweakly and physically adsorbed NH3 on the surface of the catalyst.After these operations, the sample was heated from 35 to 850 �C ata heating rate of 5 �C/min and the amount of ammonia in effluentwas measured via TCD and recorded as a function of temperature.

Fourier-transform infrared (FT-IR) analyses of the catalysts werecarried out for addressing surface functional groups. For FT-IRmeasurements, the powdered samples without KBr were placed

Fig. 1. A schematic view o

in a quartz cell. Spectra were collected on a Brucker FT-IR Vertex80spectrometer equipped with a MCTcryodetector working at 2 cm�1

resolution.

2.4. Catalytic performance

The performance of the catalysts for methanol to propylenereaction was investigated in a fixed-bed reactor under atmosphericpressure at 460 �C. A schematic view of the lab scale setup is shownin Fig. 1. The reactor was made of stainless-steel tube with an innerdiameter of 8 mm and a length of 70 cm, which was electricallyheated by a vertical three-zone tube furnace (PTF 12/75/750, Len-ton Ltd, UK). Before testing the catalytic activity, the catalystpowder was filled into a die and pelletized under pressure of 10tones for 10 min to obtain thin pellets. The pellets were thencrushed and sieved to obtain 18e25 mesh particles. For each test,1 g of mesh catalyst was loaded into the middle of the isothermalzone of the reactor. In addition, to prevent back-mixing and ho-mogenization of gas flow on the catalyst bed, inert quartz particles(12e16mesh) were filled upstream and downstream of the catalystpacking of the reactor.

A K-type thermocouple was positioned coaxially in the center ofthe catalyst bed in order to monitor the reaction temperature. Priorto the start of the MTP reaction, the sample was activated in-situ atheating rate of 3.5 �C/min under highly purified N2 flow (30 mL/min) and under atmospheric pressure. When the catalytic bedtemperature reached to 550 �C, it was maintained at that temper-ature for 2 h. After cooling to the reaction temperature in flowingnitrogen, a mixture of 50 wt.% methanol in water with methanolWHSV ¼ 1 h�1 was pumped through a HPLC pump (KnauerSmartline 1000; Germany) to an in-house built preheater operatingat 150 �C before being fed to the reactor. The reactor outlet streamwas cooled to 10 �C in a refrigeration bath and then the gas andliquid products were separated. To avoid possible condensation ofheavy compounds, the transfer line from the reactor outlet to therefrigeration bath was externally heated and maintained at 170 �C.

f the lab scale setup.

Table 1Structural properties of conventional and hierarchical ZSM-5 samples determinedfrom XRD.

Sample name Average crystal sizea (nm) Relative crystallinityb (%)

Con-ZSM-5 e 100HMZ-0C-5T 30.12 89.97HMZ-5C-0T 35.24 79.72HMZ-0C-2T 32.85 97.89HMZ-1C-1T 33.71 88.87HMZ-0.5C-1.5T 33.02 95.23

a Average crystal size estimated by Scherre's equation for the peaks between2q ¼ 7e10� .

b Relative crystallinity calculated based on the sum of peak areas between2q ¼ 22.5� and 25� from XRD pattern of hierarchical ZSM-5 samples compared tothat of conventional ZSM-5100% crystalline (ASTM D5758-01).

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The gas phase was sent to gas flow meter (Ritter TG 05, Germany)and online gas chromatograph (GC-Varian 3800), equipped with aflame ionization detector (FID) and a 50 m HP-PONA capillarycolumn. The liquid products were collected after every 6 h reactionperiod, weighted, and analyzed off-line at the end of the experi-ments in the same gas chromatograph used for the on-line ana-lyses. They were injected by a microsyringe into the GC syringeinjection port for analysis.

Then the reaction performance results, including methanolconversion and hydrocarbon products selectivities were calculated.Due to very fast equilibrium reaction of methanol to DME, thosetwo species can be combined as one lumped reactant species (ox-ygenates) in the calculation of conversion and hydrocarbon prod-ucts selectivities. Hence, the conversion of methanol in the MTPreaction was calculated through the following equation:

Methanol conversion ¼ NiMeOH � �

NoMeOH þ 2No

DME

�

NiMeOH

� 100 (1)

The product selectivity was defined as the mole ratio of eachproduct (on CH2 basis referred to the moles of convertedmethanol):

Selectivity ¼xNo

CxHy

NiMeOH � �

NoMeOH þ 2No

DME

�� 100 (2)

Yield ¼ methanol conversion� selectivity (3)

where superscript i refers to the components at the inlet of reactorand superscript o refers to the components at the reactor outlet;subscript x refers to the number of carbon atoms.

3. Results and discussion

3.1. Physicochemical characterization of the ZSM-5 samples

Fig. 2 shows the XRD patterns of conventional microporousZSM-5 catalyst (Con-ZSM-5 sample) and the hierarchical meso-porous ZSM-5 catalysts (HMZ-xC-yT series samples), prepared bydifferent amounts of single or mixture of two mesoporogen tem-plates, i.e. CTAB and TPOAC.

Fig. 2. XRD patterns of conventional microporous

The XRD patterns of all samples exhibit a series of characteristicdiffraction peaks corresponding to the typical ZSM-5 frameworkstructure (Breck, 1974), and no other peaks that would indicate animpurity was observed. The average crystal size of hierarchicalmesoporous ZSM-5 samples were estimated by applying theScherrer equation (Eq. (4)) from XRD peak between 2q ¼ 7e10�:

L ¼ Klb cos q

(4)

where L is the average crystal size (approximation), K is the crystalshape factor (typically ~ 0.9) and b is the full width at half-maximum (FWHM) of a selected peak at 2q diffraction angle. Theaverage crystal sizes of nanosized samples are summarized inTable 1. Comparingwith the conventional ZSM-5 (sample Con.ZSM-5), the height of the diffraction lines of the mesoporous ZSM-5(HMZ-xC-yT series samples) is slightly lowered and broaden,which is due to the presence of nanometer-sized crystals based onthe Scherrer equation. However, as can be seen in Table 1, theaverage crystal size is not greatly affected and changes slowly inrang of nanometric scale.

The XRD technique is also used to assess the degree of ZSM-5crystallinity. In order to determine the effect of the mesogenoustemplates on the degree of ZSM-5 relative crystallinity, calculationswere developed according to procedure A described in ASTMD5758-01. This calculation is based on the ratio of the integrated

and hierarchical mesoporous ZSM-5 samples.

Fig. 4. N2 adsorptionedesorption isotherms of conventional and hierarchical ZSM-5samples.

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194188

peak areas between 2q ¼ 22.5� and 25� of the sample relative tothose of a highly crystalline reference material. With this goal inmind, the conventional ZSM-5 sample was considered as thereference sample (100% crystallinity) as it displayed the highestdiffraction intensities among all samples synthesized. This methodwas also employed by Al-Dughaither and de Lasa (2014) to estimatethe ZSM-5 crystallinity. Table 1 reports the relative crystallinity forall the ZSM-5 samples studied.

The relative crystallinity of the hierarchical mesoporous ZSM-5samples depends on both the type and the amount of mesogenoustemplate (Kim et al., 2010; Koekkoek et al., 2011a; Q.Y. Wang et al.,2014). At a fixed mesoporogen/SiO2 molar ratio, i.e., TPOAC/SiO2and CTAB/SiO2 equal to 0.05, the usage of different mesogenoustemplates during the synthesis procedure resulted in differences inthe relative crystallinity of the synthesized mesoporous ZSM-5catalysts. Hence, the relative crystallinity of HMZ-0C-5T (ca.89.97%) is a little lower than that of conventional ZSM-5, but higherthan that of HMZ-5C-0T (ca. 79.72%). Such samples contain someamorphous material consisting of the disordered mesoporousaluminosilica phase. However, CTAB has more retarding effect thanthat of TPOAC on the nucleation and crystallization of the MFIstructure (Q.Y. Wang et al., 2014). The very slow formation of theMFI phase in the presence of CTAB results in it being difficult totransform the generated mesoporous phase to crystallized ZSM-5.Therefore, more of the mesoporous phase remains, which resultsin the lowest relative crystallinity of HMZ-5C-0T among all thesamples (Choi et al., 2006).

Additionally, the relative crystallinity strongly depends on theamount of TPOAC used in the synthesis gel. The relative crystallinityof the hierarchical mesoporous ZSM-5 can be improved bydecreasing the amount of TPOAC in the synthesis gel (Kim et al.,2010; Koekkoek et al., 2011a). Noteworthy is also that a highTPOAC content completely prevents zeolite growth (Koekkoeket al., 2011a). At TPOAC/SiO2 ratio of 0.02 a nearly fully crystallinemesoporous ZSM-5 zeolite (97.89%), absence of amorphous phase,was obtained. Nevertheless, the high cost and complicated syn-thetic procedures of TPOAC prevent its wide acceptance in industry.However, due to lower cost of CTAB than that of TPOAC, preparationof mesoporous ZSM-5 catalyst using mixture of CTAB and TPOAC inan optimal mixing ratio (TPOAC/CTAB) is more cost-effective to

Fig. 3. FE-SEM images of conventional microporous and hierarchical m

achieve an appropriate degree of crystallinity and mesoporosity.Hence, two mesoporous ZSM-5 samples at (TPOAC þ CTAB)/SiO2molar ratio of 0.02 with different TPOAC/CTABmolar ratios (1:1 and3:1) were synthesized and named HMZ-1C-1T and HMZ-0.5C-1.5T,respectively. XRD results (Table 1) demonstrated that HMZ-0.5C-1.5T sample possessed higher relative crystallinity (95.23%) thanthat of HMZ-1C-1T sample (88.87%). Therefore, HMZ-0.5C-1.5T isthe best choice for obtaining a highly crystalline hierarchical ZSM-5through cost-effective synthesis procedure.

The addition of the mesogenous templates in the reaction gel,not only influences the products structure and their relative crys-tallinity, but also significantly alters both the morphology and thecrystal size of final products. Obviously, as shown in the FE-SEMimages of selected samples (Fig. 3), the morphology of hierarchi-cal mesoporous ZSM-5 (HMZ-0.5C-1.5T) is very different from thatof conventional microporous ZSM-5 (Con-ZSM-5), interestingly

esoporous ZSM-5 samples: (a) Con-ZSM-5 and (b) HMZ-0.5C-1.5T.

Table 2Textural properties of conventional and hierarchical ZSM-5 samples.

Sample name Surface area (m2/g) Pore volume (cm3/g) HFg

SBETa SMicrob SMeso

c VTotald Vmicro

e VMesof

Con-ZSM-5 345 310 35 0.178 0.136 0.042 0.077HMZ-0C-5T 395 174 221 0.300 0.108 0.192 0.201HMZ-5C-0T 430 150 280 0.666 0.087 0.579 0.085HMZ-0C-2T 378 205 173 0.272 0.115 0.157 0.194HMZ-1C-1T 410 169 241 0.337 0.101 0.236 0.176HMZ-0.5C-1.5T 384 201 183 0.278 0.114 0.164 0.195

a BET surface area.b Micropore surface area evaluated by t-plot method.c Mesopore surface area calculated using SBET � SMicro.d Total pore volume at P/P0¼ 0.99.e Micropore volume calculated by t-plot method.f Mesopore volume calculated using VTotal � VMicro.g The hierarchical factor.

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indicating that the mesogenous templates played a dominant roleon the formation of the globular morphology and nano-structure ofHMZ-0.5C-1.5T sample. It can be seen that the conventional ZSM-5zeolite exhibits a typical MFI structure (Fig. 3a) ‘Boat-like’hexagonal-shaped crystal rods are quite uniform with particle sizeof 1e1.5 mm in length. Moreover, the surface of the conventionalZSM-5 zeolite is smooth. But after mesogenous templates wereintroduced into ZSM-5 initial gel, a completely differentmorphology was observed for the HMZ-0.5C-1.5T sample and theirparticle size decreased from micrometer into the nanometer scale.HMZ-0.5C-1.5T sample (Fig. 3b) is randomly aggregation of nano-sized zeolite crystals of around 30e40 nm with some degree oforientation, which are in good consistency with the size calculatedby Scherrer equation. This implies that in agreement with previ-ously reported mesoporous ZSM-5 aluminosilicate (Choi et al.,2006; Kim et al., 2010), in which the long alkyl tails of the surfac-tants with a hydrophobic nature linked on the surface of nano-crystals provide a capping effect, thus preventing the further crystalgrowth of the primary nanosized zeolite crystals.

N2 adsorptionedesorption isotherms and corresponding BJHpore size distribution (PSD) curve of all samples are illustrated inFigs. 4 and 5, respectively, and their textural properties are sum-marized in Table 2. Looking at the nitrogen isotherms, the con-ventional ZSM-5 exhibits only a representative type I isotherm,which is typical for microporous materials without significantmesoporosity. This is verified by its BJH pore size distribution curve(Fig. 5), where there is no obvious peak in the mesoporous range.

As listed in Table 2, the BET surface area and total pore volume ofsample Con-ZSM-5 are 345 m2/g and 0.178 cm3/g, while the mes-oporous surface area and pore volume are very low, only 35 m2/gand 0.042 cm3/g, respectively. On the other hand, HMZ-xC-yT seriessamples exhibited a type-IV isotherm with a hysteresis loop atrelative pressure higher than P/P0 ¼ 0.5, indicating the existence ofboth micropores and mesopores. These hysteresis loops are usuallyassociated with the capillary filling and condensation of N2 withinthe homogeneous slit-shaped intercrystalline mesopores formedby the aggregation of nanosized zeolite crystals. This was furtherverified by the corresponding pore size distributions (PSDs)calculated by BJH model based on desorption curves (Fig. 5).

Fig. 5. BJH pore size distribution curves of conventional and hierarchical ZSM-5samples.

As depicted in Figs. 4 and 5, different types, amounts andcombinations of two mesoporogens have a great effect on theporosity of prepared materials. The N2 adsorptionedesorptionisotherms recorded for HMZ-0C-5T shows a small hysteresis loopand a relatively uniform mesopore distribution centered at 3.6 nmand a tiny peak at ~15 nm. In contrast, HMZ-5C-0T exhibits a largehysteresis loop, and a bimodal pore size distribution with adiminished distribution centered at ~3.6 nm as well as a veryintense and broadened distribution centered at ~15 nm, respec-tively. This indicates the higher mesopores formation in HMZ-5C-0T than that of HMZ-0C-5T, and their difference in mesoporosity.As observed in Table 2, for HMZ-0C-5T, the mesoporous surfacearea is 221 m2/g, and the mesoporous volume is 0.192 cm3/g,whereas the values for HMZ-5C-0T are 280 m2/g and 0.579 cm3/g,respectively. However, the mesoporosity in HMZ-0C-5T and HMZ-5C-0T is developed at the expense of microporosity. Compared toconventional ZSM-5 with a microporous surface area of 310 m2/g,the microporous surface area decreases to 174 m2/g and 150 m2/gfor the samples HMZ-0C-5Tand HMZ-5C-0T, respectively. However,in the design of hierarchical zeolite, enhancing the mesoporoussurface area without severe loss of the micropore volume is abso-lutely necessary. The hierarchy factor (HF), defined as (Vmic/Vtotal) � (SExt/SBET), is one of the most important factors to describethe hierarchical zeolite (P�erez-Ramírez et al., 2009), whose maxi-mized value is highly desired. In the present work, Con-ZSM-5shows the lowest hierarchy factor of about 0.077 due to theabsence of mesostructures. Moreover, the HF of HMZ-5C-0T (0.085)is only slightly higher than that of Con-ZSM-5 since the relativeincrease in mesoporosity is higher than the relative decrease inmicroporosity. However, the HF of the HMZ-0C-5T (HF ¼ 0.201) issignificantly higher than that of HMZ-5C-0T, suggesting that HMZ-0C-5T owns well-spread hierarchical systems.

The difference in mesoporosity between HMZ-0C-5T and HMZ-5C-0T is more possibly due to the different mesogenous templatesusage in the synthesis gel. In HMZ-5C-0T the electrostatic inter-action between the positively charged CTAþ micelles and thenegatively charged zeolite subnanocrystallites to formation ofmesostructural ZSM-5 nanocrystallites is relatively weak. Conse-quently, the seeds could not effectively assemble around CTABmicelles (Kim et al., 2010). In contrast, in HMZ-0C-5T sample TPOACacts not only as a supramolecular template for the generation ofmesopore in the HMZ-0C-5T, but also as a part of the silica sourceand could be incorporated into MFI framework. In addition to thestrong interaction between the hydrolyzable methoxysilyl moietyof TPOAC and the growing crystal domains through the formationof covalent bonds, the positively charged quaternary ammoniumgroup of TPOAC will also have a higher flexibility to interact with

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the anionic aluminosilicate species. Therefore, the two cooperativefunctions working together lead to fabricate crystalline meso-porous ZSM-5.

The porosity also depends on the amount of TPOAC used in thesynthesis gel. As is clearly illustrated in Fig. 5, by decreasing theTPOAC/SiO2 molar ratio from 0.05 to 0.02, the PSD curves aretransformed from a relatively uniform type into a bimodal modecentered at ~3.6 nm and ~6.1 nm, respectively, although the narrowhysteresis loop is still observed. Moreover, the mesoporous surfacearea decreases from 221 m2/g for HMZ-0C-5T to 173 m2/g for HMZ-0C-2T, which counteracts an enhancement in microporous surfacearea and leads to a net decrease in BET surface area. Additionally,the mesopore volume also decreases steadily from 0.192 cm3/g forHMZ-0C-5T to 0.157 cm3/g for HMZ-0C-2T, whereas their hierarchyfactors remain almost constant. So, the addition of further TPOAC inthe synthesis gel is not more beneficial for the generation of mes-oporous ZSM-5, which also leads to decrease in the relative crys-tallinity. As mentioned before, to further decrease the consumptionof expensive TPOAC for preparing a highly crystalline mesoporousZSM-5, finding a rational combination of TPOAC and CTABwould beof interest.

Compared with HMZ-0C-2T, the sample prepared using equalmolar ratio of TPOAC and CTAB (HMZ-1C-1T) demonstrates a morepronounced hysteresis loop (Fig. 4) that results in a broader dis-tribution of sizes for the secondary porosity centered at ~15 nm.This is similar to that of HMZ-5C-0T (Fig. 5). This similarity is aconsequence of the dominant self-assembly between the sub-nanocrystal zeolite seeds and CTAB micelles during hydrothermalcrystallization (Zhu et al., 2011, 2013; Y. Zhang et al., 2014). Indeed,HMZ-1C-1T shows a higher mesoporous surface area of 241 m2/g,while the corresponding micropore volume slightly decreases to0.1 cm3/g. Nevertheless, although this modification of the meso-porosity increases the HF of HMZ-1C-1T to 0.176, it is still lower ascompared with HMZ-0C-2T (0.195). However, by the increase in themolar ratio of TPOAC/CTAB to 3:1, the role of TPOAC surpasses, andthe samples HMZ-0.5C-1.5T and HMZ-0C-2T show a very closemesoporosity (Figs. 4 and 5 and Table 2), leading to the same hi-erarchy factor of 0.195. This similarity clearly envisions the effectiveinteraction of two mesoporogens with zeolite precursor in 3:1 Mratio of TPOAC to CTAB. Based on this observation, it is concludedthat using 3:1 M ratio of TPOAC to CTAB at (TPOAC þ CTAB)/SiO2molar ratio of 0.02 is the proper amount to reach the maximum

Fig. 6. NH3-TPD profiles for the conventional and the hierarchical catalysts.

mesoporosity with minimum loss of microporosity.The acidity is viewed as a vital factor in determining the catalytic

performance in the MTP reaction. NH3-TPD measurements werecarried out to investigate the acidity of the selected samples, i.e.Con-ZSM-5 and HMZ-0.5C-1.5T and their NH3-TPD profiles areshown in Fig. 6.

As seen, each of the profile can be deconvoluted to two Gaussiandistributions in the region below 500 �C. The low temperature peak(around 160 �C) is usually ascribed to ammonia desorbed fromweakly acid Lewis sites, which are catalytically inactive in MTPreaction. While the high temperature peak (around 340 �C) isattributed to desorption of ammonia from strong Br€onsted or Lewisacid sites related with the framework aluminum of the ZSM-5(Firoozi et al., 2009; J. Liu et al., 2009; Kim et al., 2010). The peakarea with the assumption of one NH3 molecule per acid sites givesthe amount of acid (S. Hu et al., 2012). Table 3 presents a compar-ison of the amount of NH3 desorbed in the low temperature andhigh-temperature regions for Con-ZSM-5 and HMZ-0.5C-1.5Tcatalysts.

As expected, the amount of acid sites for HMZ-0.5C-1.5T is lessthan that of the Con-ZSM-5, which is consistent with the XRD andporosity patterns. The probable reason is that the presence of CTAþ

and TPOAþ partially inhibits the aluminum from incorporating intothe framework of the zeolite (Feng et al., 2010). The results togethersummarize the role of mesogenous templates and a nearly crys-talline hierarchical ZSM-5 sample obtained in the presence ofmesogenous templates (HMZ-0.5C-1.5T) exhibit moderate acidityalong with a large number of mesopores. With the higher porevolume and larger space in the mesopores, the sample is expectedto exhibit potential catalytic applications in molecular reactionsinvolving bulky species, such as MTP.

The FT-IR spectra of the conventional ZSM-5 (Con-ZSM-5) andthe selected hierarchical ZSM-5 (HMZ-0.5C-1.5T) were recorded inthe range of 400e4000 cm�1. Using the FT-IR bands, useful infor-mation can be achieved on the framework vibrations of zeolites.Infrared spectra of these catalysts in the range of 400e1400 cm�1

are shown in Fig. 7a.It is seen that both conventional (Con-ZSM-5) and hierarchical

ZSM-5 samples (HMZ-0.5C-1.5T) reveal characteristic adsorptionbands of ZSM-5 at around 450, 550, 800, 1160 and 1225 cm�1. TheIR band at about 550 cm�1 is ascribed to the structure-sensitivevibration of double five-membered rings (D5R) in the fully peri-odic MFI lattice, which are connected with each other along the a-,b-, and c-axes or at the positions of crossing channels. While peakaround 450 cm�1 represents the TeO (T ¼ Si or Al) vibration of theSiO4 and AlO4 internal tetrahedral. The band near 800 cm�1 isassigned to the symmetric stretching of the external linkage andthe one near 1060 cm�1 is attributed to the internal asymmetricstretching vibration of SieOeT linkage. The external asymmetricstretching vibration near 1225 cm�1 is due to the presence ofstructures containing four chains of 5-member rings arrangedaround a twofold screw axis, as in the case of the ZSM-5 structure(Abrishamkar et al., 2011; Koekkoek et al., 2011b; Fathi et al., 2014).

Table 3The NH3-TPD data for the conventional and the hierarchical catalysts.

Catalyst Distribution and concentration ofacid sites (mmol NH3/g)

Peaktemperature(�C)

Region I Region II Total Td1 Td2

Weak Strong

Con-ZSM-5 0.151 0.048 0.199 154 338HMZ-0.5C-1.5T 0.114 0.033 0.147 161 342

Fig. 7. FT-IR spectra in the framework vibration region (a) and the OH region (b) of conventional and hierarchical ZSM-5 catalysts.

Fig. 8. Conversion of methanol as a function of time on stream over conventional andthe hierarchical catalysts.

Table 4Product distribution of MTP reaction over conventional and the hierarchical cata-lysts measured at steady state condition.

Catalyst Conversion(%)

Selectivity (Cemol.%) P/E

C1e4a C2H4 C3H6 C4H8 Cþ

5b C¼

2 � C¼4

Con-ZSM-5 99.9 9.53 13.18 35.72 20.29 21.28 69.19 2.71HMZ-0C-5T 99.8 5.13 9.20 45.64 25.65 14.38 80.49 4.96HMZ-5C-0T 99.7 3.23 8.35 48.83 28.71 10.88 85.89 5.85HMZ-0C-2T 99.9 6.51 11.61 40.07 22.30 19.51 73.98 3.45HMZ-1C-1T 99.7 4.19 10.43 44.85 25.24 15.29 80.52 4.30HMZ-0.5C-1.5T 99.8 5.30 10.78 43.65 24.31 15.96 78.74 4.05

a C1eC4 saturated hydrocarbons.b C5 and higher hydrocarbons.

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194 191

The ratio of the intensities of the peaks near 550 and 450 cm�1

provides an approximate estimate of the degree of crystallinity(Koekkoek et al., 2011b). The results showed that this ratio, andconsequently the relative crystallinity of HMZ-0.5C-1.5T is slightlyless than that of Con-ZSM-5 that is in close agreement with thatcalculated from the XRD (Table 1). Moreover, the peak at 3610 cm�1

(Fig. 7b) is ascribed to the OH stretching vibrations of the Si(OH)ALBrønsted acid sites (Al-Dughaither and de Lasa, 2014), which is ingood agreement with the NH3eTPD results.

3.2. Catalytic performance of the prepared catalysts in MTPreaction

The catalytic activities for MTP reaction over the conventionalmicroporous ZSM-5 (Con-ZSM-5) and the hierarchical mesoporousZSM-5 (HMZ-xC-yT) catalysts were evaluated in a continuous flowfixed bed reactor under atmospheric pressure and 460 �C using amixture of 50 wt.% methanol in water with methanol WHSV of1 h�1. The methanol conversion as a function of time on stream(TOS) over the various ZSM-5 catalysts is shown in Fig. 8.

As seen, owing to methanol's small size (Choi et al., 2009), all ofthe ZSM-5 catalysts exhibited nearly full methanol conversionduring the initial reaction period, indicating the high initial activityof all catalysts. However, with the increasing time on stream, all thetested catalysts are reducing their catalytic activity with differentdeactivation rates, which can be attributed to the coverage of acidsites and blockage of the pore mouth by carbon deposit, based onthe previous literature (S. Hu et al., 2012; Z. Hu et al., 2014).Hereafter, the ‘catalytic lifetime’ was defined as the time for whichthe conversion of oxygenates (methanol and dimethyl ether)exceeded 90%. As is clearly indicated in Fig. 8, themesoporous ZSM-5 samples templated with only CTAB (HMZ-5C-0T) and equal molarof TPOAC and CTAB (HMZ-1C-1T) exhibited shorter lifetime thanthat of the conventional microporous ZSM-5 (Con-ZSM-5). Inparticular, HMZ-5C-0T deactivated very quickly and presented theshortest lifetime in methanol conversion among all catalysts.

It is well believed that catalyst deactivation in MTP reaction isgenerally attributed to the coke deposition on the catalyst surface,which is caused by the side reactions that also take place on theacid sites. In any fixed bed catalyst operation, the portion of thecatalyst bed that is in the downstream of the reaction front isdeactivated by coking. As the reaction progresses with increasedtime, the deactivation front moves through the catalyst bed, untilmost of the catalyst bed is deactivated and the conversion dropsrapidly after breakthrough of methanol (and DME) (Mentzel et al.,2012). The MTP lifetime values thus obtained for Con-ZSM-5, HMZ-

5C-0T, and HMZ-1C-1T were 43, 18, and 26 h, respectively. For themesoporous ZSM-5 catalysts templated by TPOAC (HMZ-0C-5T andHMZ-0C-2T), not only the breakthrough of methanol is observedconsiderably later than for conventional ZSM-5, but also the declinein conversion is slower, indicating its higher overall methanolconversion capacity. A slower drop in conversion is also observed

Table 5Comparison of propylene and light olefins yields obtained from various MTPcatalysts.

No. Yield of propylene Yield of light olefins (C¼2 � C¼

4 ) References

1 37.35 67.86 Yaripour et al. (2015)2 44.87 76.42 Jiao et al. (2013)3 50.99 76.62 S. Hu et al. (2012).4 42.03 67.51 Mei et al. (2008)5 53.1 74.5 Wen et al. (2015)6 41.96 71.63 S. Zhang et al. (2015)7 44.04 75.59 H.R. Zhang et al. (2014)8 44.89 75.21 Wei et al. (2011)9 43.56 78.58 This work

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194192

for HMZ-0.5C-1.5T. The catalytic lifetime decreased in the orderHMZ-0C-2T (83 h) > HMZ-0.5C-1.5T (76 h) > HMZ-0C-5T(66 h) > Con-ZSM-5 (43 h).

Table 4 lists the product distribution measured at steady statewhen the catalysts yielded nearly full conversion and were pre-sumably free from coke. The products obtained are classified intolight hydrocarbons (C1eC4), light olefins (C¼

2 � C¼4 ) and C5

þ hydro-carbons. The catalytic performance of conventional microporousZSM-5 is ordinary. The Con-ZSM-5 with Si/Al ratio of 175 delivers apropylene selectivity of 35.72% with a C¼

2 � C¼4 olefins selectivity of

69.19%, which is associated with the formation of a lot of C1eC4alkanes (9.53%) and Cþ

5 products (21.28%). This is possibly due to thehigher Si/Al ratio of ZSM-5 zeolite employed in the present study.Earlier studies indicated that high Si/Al ratio of ZSM-5 was adetermining factor in the MTP process since it largely influencedthe catalytic lifetime and the product distribution especially forpropylene and aromatics productions, owing to the lower numberof acid site literature (J. Liu et al., 2009; Sun et al., 2010).

However, for this acid-catalyzed reaction (MTP), microporousZSM-5 catalyst still tends to undergo rapid coking reactions, leadingto blocking of micropore channels and catalyst deactivation (Z. Huet al., 2014). Such contradiction would be well solved by the mes-opore containing ZSM-5 catalyst catalysts due to their improvedmass transfer properties and tolerance for a larger amount of coke.In this way, even with a lower Si/Al ratio, the mesoporous ZSM-5catalyst is active for a longer period of time than the conventionalcatalyst with a higher Si/Al ratio (Sun et al., 2010; Z. Hu et al., 2014).

It can be clearly seen (Table 4) that compared with conventionalZSM-5, the mesoporous ZSM-5 catalysts templated with TPOAC(HMZ-0C-5T) or CTAB (HMZ-5C-0T), especially with CTAB, shownoticeably higher selectivities to propylene and butylenes, whiletheir selectivities toward ethylene, C1eC4 alkanes, Cþ

5 hydrocarbonsare relatively lower. Detailed results show that when the reaction isperformed over HMZ-5C-0T, a much prominently larger amount ofpropylene (ca. 48.83%) and a high P/E ratio (ca. 5.85) were observed.While for HMZ-0C-5T, 45.64% of propylene selectivity as well as4.96 of P/E ratio were obtained. Moreover, in comparison with themesoporous ZSM-5 using TPOAC as the mesogeneous template, themesoporous ZSM-5 templated with CTAB displayed fairly lowerselectivities to Cþ

5 hydrocarbons (10.88% vs.14.38%) and C1eC4 al-kanes (5.13% vs. 3.23%). Thus, a considerably high selectivity towardC¼2 � C¼

4 olefins (ca. 85.89% at nearly full methanol conversion) wasobtained over HMZ-5C-0T catalyst, which was much higher thanthose on HMZ-0C-5T (80.49%), and Con-ZSM-5 (69.19%) catalysts.However, its catalytic lifetime was substantially shorter ascompared with HMZ-0C-5T and ConZSM-5.

In addition, the MTP lifetime of mesoporous ZSM-5 increasedwith the decrease of TPOAC addition. As shown in Fig. 8, HMZ-0C-2T with a TPOAC/SiO2 ¼ 0.02 exhibited a catalyst lifetime of 83 hwhich is substantially longer as compared with HMZ-0C-5T (66 h);while its selectivity toward propylene as well as P/E ratio arerelatively lower. Therefore, it appeared that the mesoporous ZSM-5templated with appropriate amount of TPOAC showed a significantimprovement in catalyst lifetime and a relatively increase in pro-pylene selectivity in MTP reaction, while mesoporous ZSM-5 tem-plated with CTAB, although showed no advantage in prolonging thecatalyst lifetime, gave the remarkable improvement in propyleneselectivity.

Since the aim of this studywas developing a highly efficientMTPcatalyst, more catalytic tests were carried out using the meso-porous ZSM-5 catalysts templated with the different combinationsof TPOAC and CTAB to find their optimal mixing ratio (TPOAC/CTAB)in order to balance propylene selectivity and catalyst stability. Themolar ratio between TPOAC and CTAB in zeolite synthesis mixturehas a large influence on the catalytic stability and propylene

selectivity in MTP reaction. As mentioned before, the catalyticlifetime of the mesoporous ZSM-5 templated with equal molarratio of TPOAC to CTAB at (TPOAC þ CTAB)/SiO2 molar ratio of 0.02(HMZ-1C-1T), similar to that of the mesoporous material preparedby using only CTAB at CTAB/SiO2 molar ratio of 0.05 (HMZ-5C-0T),was shorter than those of conventional microporous ZSM-5 (Con-ZSM-5) and HMZ-0C-2T, while its propylene selectivity (ca. 44.85%)as well as P/E ratio (ca. 4.30) were relatively higher than those oflatter materials. This indicates the dominant role of CTAB in thegeneration of mesopores in HMZ-1C-1T.

However, compared with conventional catalyst, the nanosizedhierarchical porous ZSM-5 templated with a mixture of 75% TPOACand 25% CTAB at (TPOACþ CTAB)/SiO2 molar ratio of 0.02 displayedreliable MTP catalytic lifetime (76 h) as well as sufficiently highpropylene selectivity (43.65%) and propylene to ethylene (P/E) ratio(4.05), which were predominately attributed to optimal acidity(Fig. 8) and the enhancement in diffusion efficiency (Table 4). Thecomparison between the yields of propylene as well as light olefinsobtained from the present study and those reported in the litera-ture for MTP reaction are presented in Table 5. It can be seen that,with our efficient HMZ-0.5C-1.5T catalyst, the yields of the pro-pylene and light olefins are both reasonably improved.

The prominent difference in the catalytic performance of hier-archical porous ZSM-5 and conventional microporous ZSM-5 cat-alysts can be explained by the difference in the level of porosity,acidity, as well as crystallite size. In the first place, with the additionof CTAB or TPOAC, especially with CTAB addition, hierarchical ZSM-5 samples reveal the reduction in overall acidity which can alleviatesecondary reaction on strong acid sites, such as hydrogen transfer,cyclization and aromatization and so on. As a result, the tendencyfor competitive polycyclic aromatization of the multi-methyl ben-zene intermediate, which will eventually lead to coking, is largelydepressed. Secondly, in the case of nanosized hierarchical porousZSM-5 catalysts, coke is formed exclusively at the external surfaceand/or mesopores, while for the solely microporous zeolites, thecoke is more heavily deposited inside the micropores. Considerablyhigh mesoporosity and shorter diffusion path length in nanosizedhierarchical ZSM-5 facilitate the transfer of the coke precursor tooutside of micropores. As mentioned earlier coke deposited withinmicropores more effectively causes catalyst deactivation because itcan cover the catalytically active acid sites and block microporeseven at low coking levels; in contrast, external coke causes rela-tively little hindrance to diffusion unless it covers the entireexternal catalyst surface. Consequently, slight coke deposition andless susceptible to the more coke deposits are contributing to theslowing down of the hierarchical porous ZSM-5 deactivation, whichensures the operation of the long-termmethanol conversion (S. Huet al., 2012; Q. Zhang et al., 2014; Q.Y. Wang et al., 2014).

Unfortunately, for the case of HMZ-5C-0T, even mesopores canbe formed, the low relative crystallinity and low level of the hier-archy factor (0.085), indicate only a portion of this material

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194 193

possesses microporous structure which contain acid sites. Inaddition, because of the relatively low amount of acid sites in ZSM-5 with a high Si/Al ratio, the amount of the acid sites in HMZ-5C-0Tis too low, which are covered by coke deposits and subsequentlysharp deactivation is observed (J. Liu et al., 2009). It is interesting tonote that for the case of HMZ-1C-1T, similar to the case of HMZ-5C-0T, lower catalytic lifetime than that of conventional microporousZSM-5 was also attained due to its lower micropore surface area(Table 2) and consequently poorer acid property as well. Thus,appropriate amount of acid sites is crucial for the catalytic con-version of methanol to propylene.

Although, the addition of TPOAC in the synthesis gel does notdecrease the zeolite micropores surface area as much as CTAB doesin the synthesizing procedure, higher relative crystallinity andmicropore surface area are observed by lowering TPOAC/SiO2 molarratio from 0.05 to 0.02 without a considerable reduction in hier-archy factor and porosity (Table 2). Moreover, due to the fact thatthe Brønsted acid sites are only located in the micropores of theZSM-5 (Kim and Ryoo, 2014), the mesoporous sample preparedfrom a TPOAC/SiO2 ratio of 0.02 has more value of acid sites thanthat of the sample synthesized from a TPOAC/SiO2 ratio of 0.05,which together with its relatively good mesoporosity are possiblythe explanations for the higher longevity of HMZ-0C-2T comparedto HMZ-0C-5T.

The outstanding diffusivity of the hierarchical porous ZSM-5catalysts has a great effect on their product distribution in MTPreaction, which facilitates the removal of the intermediate prod-ucts, particularly propylene and butylene with larger molecularsizes, from the reactive acid sites of the catalysts. As a result, thereaction equilibrium shifts to the formation of propylene andbutylene, and also the probabilities that these olefins further formhigher olefins, paraffins, aromatics and naphthenes via varioussecondary reactions on the acid sites of the catalysts are reduced (S.Hu et al., 2012; Kim and Ryoo, 2014). Thus, selectivities to propyl-ene and butylene as well as P/E ratio over hierarchical porous ZSM-5 catalysts are enhanced as compared with the conventionalmicroporous ZSM-5 catalyst (see Table 4), while selectivities toother products including C1eC4 alkanes, ethylene and Cþ

5 hydro-carbons (including aromatics) are decreased, which correspondswith the hydrocarbon-pool mechanism. According to this mecha-nism, there are two possible hydrocarbon pool cycles on ZSM-5catalyst, Cþ

3 alkenes methylation and cracking cycle and (poly)methylbenzene methylation and dealkylation cycle (Svelle et al.,2006). Propylene and higher alkenes are mainly formed throughalkenes methylation and cracking cycle, whereas ethylene is pre-dominantly formed from the (poly)methylbenzene methylationand dealkylation cycle. According to this inference, with theimprovement of diffusivity, the residence time of the (poly)meth-ylbenzene intermediates in the microporous reaction zone isshorter in the nanosized samples, leading to less secondary deal-kylation reactions, then the formation of ethylene is diminishedand Cþ

3 alkenes cycle plays a major role in the MTP reaction (S. Huet al., 2012; Kim and Ryoo, 2014). Therefore a noticeably higherpropylene selectivity and higher P/E ratio were achieved on thehierarchical porous ZSM-5 catalysts.

4. Conclusions

In this study, the effects of different types, amounts and com-binations of two mesogenous templates, i.e. CTAB and TPOAC, onthe textural properties and MTP catalytic performance of the high-silica mesoporous ZSM-5 (Si/Al ¼ 175) catalysts were investigated,and the results were compared with a conventional microporousZSM-5 catalyst. The characterization results demonstrated thatCTAB has more retarding effect than that of TPOAC on the

nucleation and crystallization of the MFI structure particularly athigh addition content. On the other hand, the higher cost of TPOACthan that of CTAB, prevents its wide acceptance in industry. How-ever, the preparation of mesoporous ZSM-5 with appropriate de-gree of crystallinity, mesoporosity and acidity by using mixture ofTPOAC and CTAB in an optimal mixing ratio is more cost-effective.

Furthermore, MTP reaction results demonstrated that comparedwith conventional catalyst, the mesoporous ZSM-5 templated withappropriate amount of TPOAC showed a significant improvement incatalyst lifetime and a relative increase in propylene selectivity,while the mesoporous ZSM-5 templated with CTAB, althoughshowed no advantage in prolonging the catalyst lifetime, gave theremarkable improvement in propylene selectivity. However, thenanosized hierarchical porous ZSM-5 templated with a mixture of75% TPOAC and 25% CTAB displayed reliable MTP catalytic lifetime(76 h) as well as sufficiently high propylene selectivity (43.65%) andpropylene to ethylene (P/E) ratio (4.05) compared to conventionalZSM-5. This promising catalytic performance could be attributed toits optimal combination of acidity and porosity. Notably, the pre-sent conclusions could be generalized for other acid-catalyzed re-actions and for other hierarchically porous zeolites. Further studiesare currently under way in our laboratory.

Acknowledgment

This workwas financially supported by the Iran National ScienceFoundation (INSF).

References

Abrishamkar, M., Azizi, S.N., Kazemian, H., 2011. Using Taguchi robust designmethod to develop an optimized synthesis procedure for nanocrystals of ZSM-5zeolite. Z. Anorg. Allg. Chem. 637, 154e159.

Al-Dughaither, A.S., de Lasa, H., 2014. HZSM-5 zeolites with different SiO2/Al2O3ratios. characterization and NH3 desorption kinetics. Ind. Eng. Chem. Res. 53,15303e15316.

ASTM Standard Test Method D5758-01, 2011. Standard Test Method for Determi-nation of Relative Crystallinity of Zeolite ZSM-5 by X-ray Diffraction. WestConshohocken, PA. United States.

Behbahani, R.M., Soleimani Mehr, R., 2014. Studying activity, product distributionand lifetime of Sr promoted alkali modified low Si ZSM-5 catalyst in MTOprocess. J. Nat. Gas. Sci. Eng. 18, 433e438.

Breck, D.W., 1974. Zeolite Molecular Sieves. Willey, New York.Chang, C.D., Chu, C.T.W., Socha, R.F., 1984. Methanol conversion to olefins over ZSM-

5: I. effect of temperature and zeolite SiO2/Al2O3. J. Catal. 86, 289e296.Choi, M., Cho, H.S., Srivastava, R., Venkatesan, C., Choi, D.H., Ryoo, R., 2006.

Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunablemesoporosity. Nat. Mater. 5, 718e723.

Choi, M., Na, K., Kim, J., Sakamoto, Y., Terasaki, O., Ryoo, R., 2009. Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts. Nature 461,246e249.

Choi, M., Ryoo, R., 2010. Method of the preparation of microporous crystallinemolecular sieve possessing mesoporous frameworks. US Patent 7785563.

Fathi, S., Sohrabi, M., Falamaki, C., 2014. Improvement of HZSM-5 performance byalkaline treatments: comparative catalytic study in the MTG reactions. Fuel 116,529e537.

Feng, H., Chen, X., Shan, H., Schwank, J.W., 2010. Solid-state transformation ofhollow silica microspheres into hierarchical ZSM-5 having tunable mesopores.Catal. Commun. 11, 700e704.

Firoozi, M., Baghalha, M., Asadi, M., 2009. The effect of micro and nano particle sizesof H-ZSM-5 on the selectivity of MTP reaction. Catal. Commun. 10, 1582e1585.

Guo, W.Y., Wu, W.Z., Luo, M., Xiao, W.D., 2013. Modeling of diffusion and reaction inmonolithic catalysts for the methanol-to-propylene process. Fuel Process.Technol. 108, 133e138.

Hadi, N., Niaei, A., Nabavi, S.R., Farzi, A., Shirazia, M.N., 2014. Development of a newkinetic model for methanol to propylene process on Mn/H-ZSM-5 catalyst.Chem. Biochem. Eng. Q. 28, 53e63.

Hu, S., Shan, J., Zhang, Q., Wang, Y., Liu, Y.S., Gong, Y.J., Wu, Z.J., Dou, T., 2012. Se-lective formation of propylene from methanol over high-silica nanosheets ofMFI zeolite. Appl. Catal. A Gen. 445, 215e220.

Hu, Z., Zhang, H., Wang, L., Zhang, H., Zhang, Y., Xu, H., Shen, W., Tang, Y., 2014.Highly stable boron-modified hierarchical nanocrystalline ZSM-5 zeolite for themethanol to propylene reaction. Catal. Sci. Technol. 4, 2891e2895.

Jiao, Y.L., Jiang, C.H., Yang, Z.M., Liu, J., Zhang, J.S., 2013. Synthesis of highly acces-sible ZSM-5 coatings on SiC foam support for MTP reaction. Microporous

J. Ahmadpour, M. Taghizadeh / Journal of Natural Gas Science and Engineering 23 (2015) 184e194194

Mesoporous Mater. 181, 201e207.Jin, H., Ansari, M.B., Park, S.E., 2011. Mesoporous MFI zeolites by microwave induced

assembly between sulfonic acid functionalized MFI zeolite nanoparticles andalkyltrimethylammonium cationic surfactants. Chem. Commun. 47, 7482e7484.

Kim, J., Choi, M., Ryoo, R., 2010. Effect of mesoporosity against the deactivation ofMFI zeolite catalyst during the methanol-to-hydrocarbon conversion process.J. Catal. 269, 219e228.

Kim, W., Ryoo, R., 2014. Probing the catalytic function of external acid sites locatedon the MFI nanosheet for conversion of methanol to hydrocarbons. Catal. Lett.144, 1164e1169.

Koekkoek, A.J.J., Templeman, C.H.L., Degirmenci, V., Guo, M., Feng, Z., Li, C.,Hensen, E.J.M., 2011a. Hierarchical zeolites prepared by organosilane templat-ing: a study of the synthesis mechanism and catalytic activity. Catal. Today 168,96e111.

Koekkoek, A.J.J., Xin, H., Yang, Q., Li, C., Hensen, E.J.M., 2011b. Hierarchically struc-tured Fe/ZSM-5 as catalysts for the oxidation of benzene to phenol. Micropo-rous Mesoporous Mater. 145, 172e181.

Koempel, H., Liebner, W., 2007. Lurgi's methanol to propylene (MTP®) report on asuccessful commercialisation. Stud. Surf. Sci. Catal. 167, 261e267.

Liu, J., Zhang, C., Shen, Z., Hua, W., Tang, Y., Shen, W., Yue, Y., Xu, H., 2009. Methanolto propylene: effect of phosphorus on a high silica HZSM-5 catalyst. Catal.Commun. 10, 1506e1509.

Liu, Y., Yu, X., Liao, Z., Wang, J., Yang, Y., 2010. Effect of long-chain cationic surfactanton ZSM-5 synthesis and methanol to olefins reaction. Acta. Pet. Sin. Pet. Proc.Sect. 26, 767e772.

Mei, C., Wen, P., Liu, Z., Liu, H., Wang, Y., Yang, W., Xie, Z., Hua, W., Gao, Z., 2008.Selective production of propylene from methanol: mesoporosity developmentin high silica HZSM-5. J. Catal. 258, 243e249.

Mentzel, U.V., Højholt, K.T., Holm, M.S., Fehrmann, R., Beato, P., 2012. Conversion ofmethanol to hydrocarbons over conventional and mesoporous H-ZSM-5 and H-Ga-MFI: major differences in deactivation behavior. Appl. Catal. A Gen. 417e418,290e297.

P�erez-Ramírez, J., Verboekend, D., Bonilla, A., Abell�o, S., 2009. Zeolite catalysts withtunable hierarchy factor by pore-growth moderators. Adv. Funct. Mater. 19,3972e3979.

Rownaghi, A.A., Rezaei, F., Hedlund, J., 2012. Uniform mesoporous ZSM-5 singlecrystals catalyst with high resistance to coke formation for methanol deoxy-genation. Microporous Mesoporous Mater. 151, 26e33.

Schmidt, I., Boisen, A., Gustavsson, E., Ståhl, K., Pehrson, S., Dahl, S., Carlsson, A.,Jacobsen, C.J.H., 2001. Carbon nanotube templated growth of mesoporouszeolite single crystals. Chem. Mater. 13, 4416e4418.

Sun, C., Du, J., Liu, J., Yang, Y., Ren, N., Shen, W., Xu, H., Tang, Y., 2010. A facile route tosynthesize endurable mesopore containing ZSM-5 catalyst for methanol topropylene reaction. Chem. Commun. 46, 2671e2673.

Svelle, S., Joensen, F., Nerlov, J., Olsbye, U., Lillerud, K.P., Kolboe, S., Bjørgen, M., 2006.Conversion of methanol into hydrocarbons over zeolite H-ZSM-5: ethene for-mation is mechanistically separated from the formation of higher alkenes.

J. Am. Chem. Soc. 128, 14770e14771.Triantafillidis, C.S., Vlessidis, A.G., Nalbandian, L., Evmiridis, N.P., 2001. Effect of the

degree and type of the dealumination method on the structural, compositionaland acidic characteristics of H-ZSM-5 zeolites. Microporous Mesoporous Mater.47, 369e388.

Wang, H., Pinnavaia, T.J., 2006. MFI zeolite with small and uniform intracrystalmesopores. Angew. Chem. Int. Ed. 45, 7603e7606.

Wang, Q.Y., Xu, S.T., Chen, J.R., Wei, Y.X., Li, J.Z., Fan, D., Yu, Z.X., Qi, Y., He, Y.L.,Xu, S.L., Yuan, C.Y., Zhou, Y., Wang, J.B., Zhang, M.Z., Su, B.L., Liu, Z.M., 2014.Synthesis of mesoporous ZSM-5 catalysts using different mesogenous tem-plates and their application in methanol conversion for enhanced catalystlifespan. RSC Adv. 4, 21479e21491.

Wang, Y., Fang, Y., He, T., Hu, H., Wu, J., 2011. Hydrodeoxygenation of dibenzofuranover noble metal supported on mesoporous zeolite. Catal. Commun. 12,1201e1205.

Wei, R.H., Li, C.Y., Yang, C.H., Shan, H.H., 2011. Effects of ammonium exchange andSi/Al ratio on the conversion of methanol to propylene over a novel and largeparticle size ZSM-5. J. Nat. Gas. Chem. 20, 261e265.

Wen, M., Wang, X., Han, L., Ding, J., Sun, Y., Liu, Y., Lu, Y., 2015. Monolithic metal-fiber@HZSM-5 coreeshell catalysts for methanol-to-propylene. MicroporousMesoporous Mater. 206, 8e16.

Wu, W.Z., Guo, W.Y., Xiao, W.D., Luo, M., 2011. Dominant reaction pathway formethanol conversion to propene over high silicon H-ZSM-5. Chem. Eng. Sci. 66,4722e4732.

Yaripour, F., Shariatinia, Z., Sahebdelfar, S., Irandoukht, A., 2015. Conventional hy-drothermal synthesis of nanostructured H-ZSM-5 catalysts using various tem-plates for light olefins production from methanol. J. Nat. Gas. Sci. Eng. 22,260e269.

Zhang, H.R., Liu, H.Y., Jiang, Y., Chang, X.H., Yuan, K., Wang, B., Meng, S.M., 2014.Methanol conversion to propylene over Mo-HZSM-5 zeolite. Adv. Mater. Res.834, 476e480.

Q. Zhang, Hu, S., Zhang, L.L., Wu, Z.J., Gong, Y.J., Dou, T., 2014. Facile fabrication ofmesopore-containing ZSM-5 zeolite from spent zeolite catalyst for methanol topropylene reaction. Green. Chem. 16, 77e81.

Zhang, S., Gong, Y., Zhang, L., Liu, Y., Dou, T., Xu, J., Deng, F., 2015. Hydrothermaltreatment on ZSM-5 extrudates catalyst for methanol to propylene reaction:finely tuning the acidic property. Fuel Process Technol. 129, 130e138.

Zhang, Y., Zhu, K., Duan, X., Li, P., Zhou, X., Yuan, W., 2014. Synthesis of hierarchicalZSM-5 zeolite using CTAB interacting with carboxyl-ended organosilane as amesotemplate. RSC Adv. 4, 14471e14474.

Zhu, Y., Hua, Z., Song, Y., Wu, W., Zhou, X., Zhou, J., 2013. Highly chemoselectiveesterification for the synthesis of monobutyl itaconate catalyzed by hierarchicalporous zeolites. J. Shi. J. Catal. 299, 20e29.

Zhu, Y., Hua, Z., Zhou, J., Wang, L., Zhao, J., Gong, Y., Wu, W., Ruan, M., Shi, J., 2011.Hierarchical mesoporous zeolites: direct self-assembly synthesis in a conven-tional surfactant solution by kinetic control over the zeolite seed formation.Chem. Eur. J. 17, 14618e14627.