Synthesis of ITQ-2 Zeolites and Catalytic Performance in n-Dodecane Cracking

Jiangge Hao,Ying Wang,Guozhu Liu,Jingwen Zhang,Guozhu Li,*,Xuesong Ma2Key Laboratory ofGreen Chemical Technology of Ministry of Education,School of chemical engineering and Technology,Tianjin University,Tianjin 300072,China

2Science and Technology on Scramjet Laboratory,Beijing 100074,China

Synthesis of ITQ-2 Zeolites and Catalytic Performance in n-Dodecane Cracking

Jiangge Hao1,Ying Wang1,Guozhu Liu1,Jingwen Zhang1,Guozhu Li1,*,Xuesong Ma21Key Laboratory ofGreen Chemical Technology of Ministry of Education,School of chemical engineering and Technology,Tianjin University,Tianjin 300072,China

2Science and Technology on Scramjet Laboratory,Beijing 100074,China

A R T I C L E I N F o

Article history:

Received 24 December 2013

Received in revised form 3 January 2014

Accepted 17 January 2014

Available online 18 June 2014

MCM-22 zeolite

ITQ-2 zeolite

Swelling

Delamination

Catalytic cracking

n-Dodecane

ITQ-2 zeolites were prepared by sequentialalkali-swelling and ultrasonic-delamination of precursor MCM-22 and characterized by X-ray powder diffraction,scanning electron microscopy,nitrogen adsorption-desorption, ammonia temperature-programmed desorption and in-situ Fourier-transform infrared spectroscopy.The delamination induced a change in the morphology of ITQ-2 zeolites from aggregated thin platelets to scattered platelets,together with a significant increase in external specific surface area,which reached a plateau at the ultrasonic treatment time of 3 h.The catalytic cracking of n-dodecane over ITQ-2 zeolites was evaluated with ITQ-2 coated on the inside wall of a tubular reactor at 550°C and 4 MPa.The sample obtained by ultrasonic treatment of 3 h(ITQ-2-3)gave the highest initial conversion of n-dodecane,whereas those of 5 h and 1 h gave the conversion even lower than MCM-22,which was in agreement with the trend of the ratio of strong Lewis acid to the totalacid amount.Although the amount of cokes deposited on ITQ-2-3 was larger than that on MCM-22,the former deactivated slowly,suggesting that a large externalspeci fi c surface area benefits the stability of zeolite coatings.

?2014 The ChemicalIndustry and Engineering Society ofChina,and ChemicalIndustry Press.Allrights reserved.

1.Introduction

There has been a growing interest during the last years in the catalytic cracking of hydrocarbon fuels due to the potentialto enhance engine performance over the entire spectrum of fl ight regimes.For hypersonic fl ight,hydrocarbon fuels can serve as not only source of heat through combustion,but also coolant through the cracking reaction to remove waste heat from aircraft systems[1-3].Huang et al.[4] studied the catalytic cracking of fl ight fuels JP-7 and JP-8+100 in a zeolite-coated tubular reactor and found that hydrocarbon fuels could offer suf fi cient cooling capacity(heat sink)for supersonic aircrafts. Sobeland Spadaccini[5]performed a series ofcatalytic cracking tests over SAPO-34 and Y zeolite coatings at the conditions simulative of high-speed fl ight(650°C and 2.4 MPa)and concluded that the catalyst-coated surface was effective and essentialto the practical aircraft application of hydrocarbon fuels.Fan etal.[6]studied the catalytic cracking of China no.3 aviation kerosene over wall-coated HZSM-5 zeolite at conditions similar to the practicalscramjet applications(777°C and 7.0 MPa).Liu et al.[7,8]investigated the effects of the Si/Al ratio,crystal size(nano-and microscale)of ZSM-5 zeolite coated on the wallof a tubular reactor on the catalytic cracking of n-dodecane at 550°C and 4 MPa.The authors found that this wall-coated catalyst could sign ficantly reduce pressure drop and thermal resistance,and the conversion and product distribution were dependent upon the acidity of ZSM-5 and diffusivity of reactants and products.The ZSM-5 zeolite with a high Si/Al ratio which possessed the relatively large amount of Lewis acid sites and small amount of Br?nsted acid sites, and the nanoscale zeolite exhibited the higher catalytic cracking activity and stability.However,the sole presence of micropores in the zeolite inhibited facile mass transfer of bulky molecules to and from the active sites,limiting the catalytic performance.

Layered ITQ-2 zeolite combining the benefits of the large accessible external specific surface and the strong intrinsic acidity has been received growing attention in recentyears due to their potentialas catalysts in converting bulky molecules.Corma et al.[9-11]synthesized first time ITQ-2 zeolites by swelling of precursor MCM-22 with hexadecyltrimethyl ammonium bromide and tetrapropylammonium hydroxide followed by exfoliation of the resultant swollen sample at ultrasound conditions.The authors found this new layered material exhibited higher liquid product selectivity in cracking small molecule reactants such as n-decane and better catalytic activity in cracking larger molecule reactants such as 1,3-diisopropylbenzene(DIPB)and vacuum gasoilthan MWW-type zeolites.Other reactions catalyzed by ITQ-2 zeolites that have been reported include alkylation ofbiphenyl with propylene[12],isomerisation of m-xylene[13],hydroxyalkylation of 2-methoxynaphthalene and naphthalene with paraformaldehyde [14],and MTO reaction(methanol to olefins)[15].Additionally, supported NiMo/ITQ-2 and Pt/ITQ-2[16],Ga/ITQ-2[17]and heteroatom B-ITQ-2[18]were prepared and employed as catalysts for the mild hydrocracking(MHC)of vacuum gasoiland aromatic hydrogenation [16],and dehydrogenation of propane to propylene[17].

In this paper,ITQ-2 zeolite was synthesized from MCM-22 precursors by the sequentialswelling and delamination and characterized by X-ray powder diffraction,nitrogen adsorption-desorption measurements,scanning electron microscopy,ammonia temperatureprogrammed desorption and in-situ Fourier-transform infrared spectroscopy.The catalytic cracking of n-dodecane used as a model reaction to evaluate the catalytic performance of ITQ-2 zeolites was carried out in a tubular reactor with inside wallcoatings at 550°C and 4 MPa.The effects of the microstructure ofITQ-2 zeolites on the catalytic activity and coking were investigated.

2.Experimental

2.1.Materials

Hexamethyleneimine(HMI,99%)was purchased from Tokyo Kasei Co.(Tokyo,Japan).Cetyltrimethylammonium bromide(CTAB,99%) was purchased from Guangfu Chemical Reagent Co.(Tianjin,China). Tetrapropylammonium hydroxide solution(TPAOH,25%,by mass) was purchased from J&K Scienti fi c Co.(Shanghai,China).Sodium aluminate and sodium hydroxide were purchased from Jiangpu Chemical Reagent Plant(Shanghai,China).n-Dodecane(99.5%)was purchased from Sinopharm Chemical Reagent Co.(Shanghai,China). Ludox(AS-40,40%SiO2)was purchased from Sigma-Aldrich Co. (Shanghai,China).

2.2.Catalyst preparation

2.2.1.Synthesis ofprecursor MCM-22

Precursor MCM-22(MCM-22(P))was prepared according to the procedure described in the literature[19].HMI,sodium aluminates and sodium hydroxide were dissolved in deionized water and then Ludox(AS-40,40%SiO2)was added.The mixture(about 150 ml)with molar composition of 4 A12O3:15 NaOH:19 HMI:1500 H2O:100 SiO2was stirred for 1 h at room temperature and was transferred into a 200 ml Te fl on-lined steelautoclave.The hydrothermal crystallization was carried out at 427 K and autogenous pressure for 11 d.Finally,the products were recovered by centrifugation,washed with deionized water repeatedly and dried in air at 353 K overnight.

2.2.2.Synthesis ofITQ-2

MCM-22(P)(1 g)was suspended in deionized water(4 g)and then CTAB solution(33%,by mass,8.4 g)and TPAOH solution(2.75 g)were added.The mixture obtained thus was re fl uxed for 20 h at 353 K. After cooling to room temperature,the mixture was treated in an ultrasonic bath(50 W,40 kHz)for 1 h.Afterwards,a few drops of concentrated hydrochloric acid(6 mol·L-1)were added untilthe pH of the slurry was slightly below2.Finally,the products were recovered by centrifugation,washed with deionized water repeatedly and dried at333 K overnight.The resultant powders were calcined at 813 K(heating rate of1 K·min-1)for 8 h,yielding ITQ-2.The ITQ-2 samples synthesized by ultrasonic treatment of 1 h,3 h and 5 h are assigned as ITQ-2-1, ITQ-2-3 and ITQ-2-5,respectively.For comparison,one portion of MCM-22(P)was calcined in air at 853 K for 4 h,giving MCM-22.

An ion-exchange/calcination procedure was applied for the preparation of H-form MCM-22 and ITQ-2 from Na-form MCM-22 and ITQ-2. The zeolite(1 g)was added to NH4NO3solution(1 mol·L-1,15 m L) and then the mixture was stirred at353 K for 6 h.This procedure was repeated twice.The solid phase was separated by fi ltration,dried at 353 K and calcined at 813 K for 6 h in air.

2.3.Characterization

X-ray powder diffraction(XRD)patterns were measured on a Bruker AXS D8-Focus diffractometer using Cu-Kαradiation(λ=0.15406 nm). The data was collected in the 2θrange from 1.5°to 32°with a step size of0.02°and a step time of6 s.N2adsorption-desorption measurements were carried out at 77 K using a CHEMBET-3000 instrument (Quantachrome,Boynton FL,USA).Scanning electron microscopy (SEM)images were obtained using a Hitachi S-4800 electron microscope(HitachiCo.,Japan).Ammonia temperature-programmed desorption(NH3-TPD)measurements were carried out on a Micromeritics 2910(TPD/TPR)(Micromeritics Instrument Co.,USA).In-situ Fourier transform infrared spectra with pyridine(Py-FTIR)and 2,6-ditertbutylpyridine(DTBPy-FTIR)as probe molecules were examined on a VERTE70 FTIR spectrometer(Bruker Co.,USA) with a resolution of 4 cm-1.The powder samples were pressed to self-supported wafers of ca.10 mg·cm-2and were pretreated at 400°C and vacuum of10-3Pa for 1 h and then cooled to room temperature.Adsorption ofpyridine(or DTBPy)proceeded at60°C for 30 min. The excess ofpyridine was removed in vacuum by outgassing for 0.5 h at 150°C and 300°C,respectively.After each heating period,the temperature was reduced to room temperature and an IR spectrum was recorded.

2.4.Catalytic test

The catalytic cracking of n-dodecane was carried out in a tubular reactor wall-coated with zeolites.The reactor was 304 stainless-steel tubes with 300 mm in length,3 mm in outside diameter and 0.5 mm in wallthickness.A washcoating method was used for the preparation of the zeolite coatings on the inner surface of the reactor[3,7,8]and the solid(the mass fraction of zeolite to inert binder is 1/2)loading amountwas(2.23±0.11)mg·cm-2.The tubular reactor was heated by direct current power,and its wall temperature was measured by K-type thermocouples and kept at 550°C.The reactor pressure was maintained at 4 MPa by a backpressure valve.The feed rate of n-dodecane was 10 ml·min-1.The reaction products were cooled first by a condenser and then fl owed into a gas-liquid separator. The liquid products collected were analyzed by a HP4890 gas chromatograph(Agilent Technologies,USA)with a flame ionization detector(FID)and a PONA column(50 m×0.53 mm).

3.Results and Discussion

3.1.Textural properties

3.1.1.XRD

Fig.1 shows the XRD patterns of MCM-22(P),MCM-22,the swollen sample and ITQ-2 series.The positions and relative intensities of all the diffraction peaks for MCM-22 are in good agreement with those reported in the literature[20,21].The swollen sample shows a strong peak at 2θ=1.6°,indicating an increase in layer spacing.For ITQ-2 samples, however,this peak at 2θ=1.6°disappears,together with the disappearance of the(001)and(002)peaks at 2θ=3°-7°and broadening of(100),(220)and(310)peaks at 2θ=7°-32°,suggesting the occurrence of delamination,thereby inducing the signi fi cant reduction in the long-range order of the structure.

3.1.2.SEM

Fig.2 shows the SEMimages ofMCM-22 and ITQ-2 zeolites.Adrastic change in the morphology of ITQ-2 samples is observed in Fig.2.The MCM-22 zeolite is aggregated of thin,randomly connected platelets, whereas the ITQ-2 samples appear to be thin scattered platelets. Among the three ITQ-2 samples,ITQ-2-3 possesses the highest dispersity,suggesting a relatively sufficient exfoliation.But after a severe ultrasonic treatment of 5 h,the surface of particles becomes rough and some amorphous particles are found in the sample(ITQ-2-5). It may be ascribed to the deposition of SiO2formed by silicon dissolved from the zeolite framework during ultrasonic treatment at the alkaline conditions.

Fig.1.XRD patterns of MCM-22(P)and MCM-22(a),the swollen sample and ITQ-2(b)zeolites.

3.1.3.BET

Fig.3 presents the N2adsorption-desorption isotherms and the pore size distribution curves.The MCM-22 zeolite exhibits type Iadsorption isotherms typicalofmicroporous materials with a wide hysteresis loop at p/p0＞0.85 which is indicative of non-uniform accumulation pores. This is con firmed by Fig.3(b),where an extremely wide meso-pore size distribution ranging from10 to 26 nmfor MCM-22 can be observed. The ITQ-2 samples exhibittype IV adsorption isotherms with a hysteresis loop of type H3at p/p0＞0.4,indicating the presence of the slitshaped mesopores[22],which is consistent with the results reported by Corma et al.[9,10]and Yang et al.[23].The uniform mesoporosity of ITQ-2 zeolites is demonstrated by the pore size distribution curves depicted in Fig.3(b).The sharp pore size distributions centered on around 3.7 nm for ITQ-2 zeolites indicate the presence of regular mesopores.It can be seen in Table 1,where the values of totalspeci fi c surface area and volume,as well as that of external speci fi c area and mesoporous volume are summarized,total specific surface area and external specific surface area vary in the increasing sequence of ITQ-2-3＞ITQ-2-5＞ITQ-2-1＞＞MCM-22.The Si/Al ratio of MCM-22(P)is a crucialparameter affecting the exfoliation extent and it is con fi rmed by Schenkel et al.[24]and Frontera et al.[25]that the delamination process is favored by decrease of aluminum concentration of parent materials.For example,the total specific surface area of841 m2·g-1(external surface area of796 m2·g-1)was obtained from MCM-22(P)with the Si/Al molar ratio of 50[10],but that of 523 m2·g-1was obtained with that of 33[24].In this work, the Si/Alratio ofMCM-22(P)used is 15.5 and the largesttotal and external specific surface areas of ITQ-2 zeolites are 789 and 500 m2·g-1, respectively.Therefore,it could be assumed thata complete delamination was achieved at the ultrasonic treatment time of 3 h in this work.However,full extent of exfoliation in ITQ-2-1 does not seem to be achieved as indicated by its lower external specific area of337 m2·g-1.As seen in Table 1,the tendency towards mesoporous volume is not the same as that of totalvolume.ITQ-2-5 gives ahigher mesoporous volume than ITQ-2-3.It might be ascribed to the presence of accumulation pores in ITQ-2-5,which is formed by framework collapse caused by an excess delamination treatment extended up to 5 h.Frontera et al.[25]concluded that the time of ultrasound treatment is a very important parameter to obtain a final product with good adsorption properties and found that a long ultrasonic treatment promoted the undesired formation of MCM-41 mesoporous materialin their N2adsorption measurements.They explained that the reason for the MCM-41 formation atroomtemperature used for ultrasonic treatment on precursor MCM-22 was probably because the concentration of surfactants and the pH of the suspension favored the silica polymerization.In our work,however,no MCM-41 phase was found in ITQ-2-5 although desilication also occurred,which might be ascribed to the low surfactant concentration used.

Fig.2.SEMimages of MCM-22 and ITQ-2 zeolites.

Fig.3.Physicaladsorption characteristic curves of MCM-22 and ITQ-2 zeolites.

Fig.4.NH3-TPD pro fi les ofMCM-22 and ITQ-2 zeolites.Acid density/mmolNH3·g-1:for weak acid:MCM-22(3.66);ITQ-2-1(3.43);ITQ-2-3(4.73);ITQ-2-5(5.93).For strong acid:MCM-22(1.94);ITQ-2-1(2.29);ITQ-2-3(2.86);ITQ-2-5(3.57).

3.2.Acidity properties

3.2.1.NH3-TPD

Fig.4 shows the NH3-TPDresults of MCM-22 and ITQ-2 series.There are two desorption peaks at ca.200°C and 400°C corresponding to weak acid sites and strong acid sites,respectively,on curves ofallsamples.The intensity ofboth peaks for ITQ-2-3 and ITQ-2-5 is much higher than that for MCM-22,together with the almost same maximum peak temperature(Tmax),which reveals that the amount of acid sites on ITQ-2-3 and ITQ-2-5 is much larger than that on MCM-22 and their acid strength is close.However,ITQ-2-1 gives the acid properties, including the amount ofweak and strong acid sites and their strengths close to MCM-22.

3.2.2.Pyridine-FTIR

Fig.5 presents pyridine-adsorbed FTIR spectra ofMCM-22 and ITQ-2 samples at150°C and 300°C.The vibration bands atca.1540,1450 and 1490 cm-1are generally assigned to vibrations of pyridine bounded coordinately to Br?nsted(B),Lewis(L)and B+L acid sites[26].As seen in Fig.5,the amount of both L and B acid sites for ITQ-2-1 and MCM-22 is close,together with an almostsame ratio of L to B acid site (L/B).With any extension of the exfoliation time longer than 1 h,the amount of both L and B acid sites significantly increases,together with a plateau of L/B at the ultrasonic treatment time of 3 h and the smallest value of L/B at the ultrasonic treatment time of 5 h.But, the ratio of strong L to total acid site(Lstrong/L+B)is found to be in the decreasing order:ITQ-2-3＞MCM-22＞ITQ-2-5＞ITQ-2-1,which is not in agreementwith the trend of L/B.This result that the delamination of layered precursor MCM-22 induces the increase in L/(L+B), especially(Lstrong/L+B)is also reported by Corma et al.[10]and Antunes et al.[26].

3.3.Catalytic performance

Fig.6 shows the conversion of the n-dodecane cracking as a function of time-on-stream(TOS)over ITQ-2s and MCM-22 at550°C and 4 MPa.Three ITQ-2 zeolites exhibit different catalytic behaviors from the MCM-22 zeolite.ITQ-2-3 gives a higher conversion of n-dodecane than MCM-22.The conversion over ITQ-2-1 is lower than that over ITQ-2-5,which is in its turn lower than that over MCM-22 at the beginning of the cracking reaction but close to that over MCM-22 at the reaction time longer than 20 min.The tendency of the initialconversion of n-dodecane over four catalysts is in agreementwith the order of their Lstrong/(L+B).van Bokhoven et al.[27]studied the n-hexane cracking over different zeolites,including HZSM-5,HY and HMOR,and pointed out that the enhanced adsorption of n-hexane on L acid sites promotes the reaction rate by a factor of 2-5,although the amount of B acid sites decreased.Siddiqui et al.[28]compared the catalytic performance of ZSM-5 and SSZ-33 as FCC catalyst additives for the catalytic cracking of Arabian Light VGO and found the conversion of the catalytic cracking was higher over SSZ-33 with the higher L/B than over ZSM-5.Qu et al.[8]studied the n-dodecane cracking over HZSM-5 zeolite coatings prepared by the washcoating method at 550°C and 4 MPa,and found that catalytic cracking activities and stabilities of the HZSM-5 coatings increased with the Si/Alratio of parent zeolites,which was in well accordance with the increase in the L acid amount and the decrease in the B acid amount of the parent HZSM-5.Therefore,the strong L acid sites are responsible for improving the conversion of the n-dodecane cracking.

Table 1 Specific surface area and pore volume of ITQ-2 and MCM-22 zeolites

Fig.5.Pyridine-adsorbed FTIR spectra of MCM-22 and ITQ-2 zeolites.

We note that both MCM-22 and ITQ-2-3 which have the relatively high initial catalytic activities exhibit the poor stabilities.However, their deactivation characters may not be the same.To further understand the decay behavior of the n-dodecane cracking over MCM-22 and ITQ-2-3,TPO measurements oftwo catalysts used in the cracking reaction were performed according to the method described by Meng etal.[3],and the results are presented in Fig.7.As seen,an interesting fact that the amount of cokes formed is larger in ITQ-2-3 than in MCM-22 during the reaction buta higher catalytic activity is maintained over ITQ-2-3 can be found.Corma et al.[29]compared the cracking kinetics and decay behavior of the n-heptane cracking on MCM-22, ZSM-5 and beta zeolites.They found that in the case of MCM-22 cracking occurred in the 10-member ring channelsystem(0.41×0.51 nm), as well as in the large cavities(0.71×0.71×1.82 nm)formed by 12-member rings could be accessible only through 10-member ring windows(0.4×0.55 nm)[11,20,29].Therefore,it appears that poremouth plugging of MCM-22 occurs during the n-dodecane cracking because of the diffusion limitation,resulting in a rapid decay of the catalyst activity at a relatively low levelof the n-dodecane conversion.

Prokes?váet al.[30]studied the effects of the particle size of zeolite beta on the toluene alkylation and found the toluene conversion increased with the decrease in the particle size of the catalysts possessing similar acidic properties.Therefore,it was pointed out that appropriate acid sites together with the decrease in the transport limitations were important to the enhancement of the catalyst activity.A similar conclusion that the decrease in the diffusion limitation brought about the increase in the catalytic activity of the n-dodecane cracking catalyzed by the zeolite catalyst was reported by Ishihara et al.[31],Meng et al. [32],and Liu et al.[7].ITQ-2-3 possesses the dual-model structures of micropores and mesopores and the large external speci fi c surface, which benefit the diffusion of reactant and product molecules to and from active sites,thus exhibiting the higher catalytic activity although it reveals a relatively large amount of cokes in TPO measurement.

Generally,it could be expected that enlargement of the external surface favors the stability of catalysts because low amount of cokes resulted from the enhancement in the diffusion of the primary products. However,there is a relatively large amount of cokes on ITQ-2-3.Corma et al.[11],Schenkel et al.[24],and Antunes et al.[26]observed that delamination of the layered precursor led to an increase in the number of external B acid sites in their FTIR spectroscopy measurement with 2,6-ditertbutylpyridine or 2,4,6-trimethylpyridine as probe molecules. A similar result that a much larger number of externalacid sites are presented in ITQ-2-3 than in MCM-22 was also observed in our in-situ FIIR measurements with 2,6-ditertbutylpyridine(kinetic diameter 1.05 nm)as probe molecules(as shown in Fig.8).Some reports point out that the rate of coke formation which implies successive bimolecularhydrogen transfer is sensitive to the number of accessible B acid sites [15,29,31,33].Therefore,the reason for it may be ascribed to the large density of external B acid sites on the ITQ-2-3 because the external surface is hardly shape-selective.

Fig.6.Conversion of n-dodecane on ITQ-2 and MCM-22 zeolites.

Fig.7.TPO pro fi les ofcokes over MCM-22 and ITQ-2-3.

Fig.8.2,6-Ditertbutylpyridine-adsorbed FTIR spectra of MCM-22 and ITQ-2-3.

4.Conclusions

The ITQ-2 zeolites were synthesized by ultrasound delamination of a MCM-22 precursor swollen first with tetrapropylammoniumhydroxide and cetyltrimethylammonium bromide.Externalspeci fi c surface area and the amount ofacid sites of ITQ-2 zeolites could be affected greatly by the ultrasonic treatment time.The sample obtained by ultrasonic treatmentof3 h(ITQ-2-3)exhibited the largest external specific surface area and the Lstrong/(L+B)ratio.As a result,ITQ-2-3 gave the high catalytic activity in the cracking of n-dodecane at 4 MPa and 550°C. It was indicated that the relatively higher Lstrong/(L+B)ratio with a large external specific surface area was beneficial to the improvement of the catalytic activity.

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*Corresponding author.

E-mailaddress:gzli@tju.edu.cn(G.Li).