Exploring suitable ZSM-5/MCM-41 zeolites for catalytic cracking of n-dodecane:Effect of initial particle size and Si/Al ratio☆

Guozhu Li,Zhenheng Diao ,Jindan Na ,Li Wang ,*

1 Key Laboratory of Green Chemical Technology of Ministry of Education,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Collaborative Innovation Center of Chemical Science and Engineering,Tianjin University,Tianjin 300072,China

Keywords:ZSM-5/MCM-41 Zeolite Desilication Recrystallization Catalytic cracking

ABSTRACT In this study,various ZSM-5/MCM-41 micro/mesoporous zeolite composites have been prepared by alkalidesilication and surfactant-directed recrystallization of ZSM-5.The effects of particle size and Si/Al ratio of initial ZSM-5 zeolites on the structure and catalytic performance of ZSM-5/MCM-41 composites are studied.The results of XRD,TEM N2-adsorption-desorption,NH3-TPD and in situ FT-IR revealed that ordered hexagonal MCM-41 mesopores with 3-4 nm pore size were formed around ZSM-5 crystals,and the speci fic surface area and mesopore volume of composites increased with increasing the Si/Al ratio of initial ZSM-5.Catalytic cracking of n-dodecane(550°C,4 MPa)showed that the ZSM-5/MCM-41 composites obtained from the high Si/Al ratio and nano-sized initial ZSM-5 zeolites exhibited superior catalytic performance,with the improvement higher than 87%in the catalytic activities and 21%in the deactivation rate compared with untreated zeolites.This could be ascribed to their suitable pore structure,which enhanced the diffusion of reactant molecules in pores of catalysts.

1.Introduction

Catalytic cracking ofhydrocarbon fuels to selective molecules cannot only remove waste heat from aircraft system but also promote the subsequentcombustion by the obtained molecules with high heat[1,2].Because extreme working conditions including elevated pressure and temperature(3.4-6.0 MPa and above 400°C,i.e.the supercritical conditions)are used,HZSM-5 zeolites are ef ficient catalysts for the controllable cracking of fuel molecules[3,4].But their sole micropores severely hinder the mass transfer of reactants,especially under the supercritical conditions,which limits their applications.It is reported that the diffusion rate ofsupercriticalfuelmolecules signi ficantly decreases,associated with a liquid-like density of fuel under these conditions[5,6].To overcome this restriction,mesopores have been introduced to develop new zeolitic materials combining diffusional pathways on both microand meso-sizes[7-9].

?ejka et al.[10]and Proke?ová et al.[11]systematically summarized the developmentin the remarkably growing field ofthe synthesis,characterization and application of zeolite-based micro/mesoporous composites.The micro/mesoporous composite zeolites had been used as catalysts in acid-catalyzed reactions and oxidation reactions.M?ller et al.[12]reviewed recent development in the implementation of a mesopore system in zeolite,from top-down to bottom-up and template-assisted to template-free procedures.Verboekend et al.[13]also highlighted important criteria to ensure the ecological and economic viability of the manufacture of hierarchical zeolites.Research demonstrated that alkaline desilication is one of the most important strategies for the synthesis of hierarchical zeolites[14-16].But,its destructive nature usually causes a reduction of microporosity and acid sites.And then,their activity and stability will decrease when used in the catalytic reaction[17-19].Desilication in the presence ofpiperidine,long-chain alkylammonium surfactants and tetraalkylammonium hydroxides,or use of organic base as a desilication agent could fairly control the process,avoiding deep destruction of zeolite framework[20-22].

The approach of desilication and recrystallization is considered as the one of the mostadvanced method regarding scalability and applicability[23,24].Goto etal.[25]synthesized ZSM-5/MCM-41 composite zeolites using hexadecyltrimethylammonium chloride as templates in the presence of NaOH solution.The products showed strong acid sites like ZSM-5 and an increase in the cracking conversion of n-hexane.Na et al.observed that recrystallized zeolites in the presence of different chain lengths of alkyltrimethylammonium bromide possess narrow and ordered mesopores,and more acid sites which leads to the improvement in the catalytic cracking activity compared to desilicated ones[26].Moreover,some studies found that the physicochemical properties of the zeolite composites prepared through desilication and re-assembly processes depended on the Si/Al ratios,the hydroxide concentration and the presence of surfactants[27-29].Therefore,changes of zeolites in Si/Al ratio or particle size need to be carefully addressed.

The aim of this paper is to demonstrate how the Si/Al ratio and particle size of ZSM-5 zeolites affect the pore structure of ZSM-5/MCM-41 composites,which will determine their catalytic activity and stability.Firstly,ZSM-5/MCM-41 zeolite composites were synthesized by sequential alkali-desilication and surfactant-directed recrystallization with cetyltrimethylammonium bromide as templates.The structures of the zeolite composites were manipulated by changing the SiO2/Al2O3ratio(50,80,and 140)and particle size(5 μm and 200 nm).Secondly,physicochemical properties of the ZSM-5/MCM-41 zeolite composites were characterized by X-ray powder diffraction,nitrogen adsorption-desorption measurements,transmission electron microscopy,pyridine-adsorbed Fourier Transform-infrared spectroscopy and ammonia temperature-programmed desorption.Finally,catalytic cracking of supercritical n-dodecane was carried out in a flowing tubular reactor with wall-coated ZSM-5/MCM-41 zeolite composites at 550°C and 4 MPa to evaluate their catalytic performance.

2.Experimental

2.1.Materials

The HZSM-5 zeolites(SiO2/Al2O3molar ratio of50,80 and 140)with the average crystal sizes of 2-5 μm and HZSM-5 zeolite with the SiO2/Al2O3ratio of 50 and crystal size of 200 nm were purchased from the catalystplantofNankaiUniversity(Tianjin,China).The physicalproperties ofthe virgin HZSM-5 zeolites(denoted as ZSM-5 series)are listed in Table 1.Cetyltrimethylammonium bromide(C16TAB)was purchased from J&K Chemical Ltd without further puri fication.n-Dodecane with 99.5%purity was obtained from Sinopharm Chemical Reagent Co.,Ltd(Shanghai,China).

2.2.Preparation of ZSM-5/MCM-41 zeolite composites

ZSM-5/MCM-41 zeolite composites were prepared via the surfactantdirected recrystallization process using cetyltrimethylammonium bromide as template.In a typical run,0.5 g virgin HZSM-5 was added to 1.5 mol·L-1sodium hydroxide aqueous solution(5 mL)and then the mixture was stirred for 30 min at room temperature.10 g aqueous solution of cetyltrimethylammonium bromide(10%,by mass)was added to the suspension,followed by an additional stirring for 30 min.Then the mixture was transferred into a Te flon-lined autoclave and hydrothermally treated at 120°C for 12 h.After cooling to room temperature,the pH value of the reaction mixture was adjusted to 8.5 by dropwise addition of 2.0 mol·L-1HCl under vigorous stirring.The mixture was transferred into the Te flon-lined autoclave and heated at 120°C for 24 h.Finally,the product was recovered by centrifugation,washed repeatedly with deionized water and dried at 100°C overnight.The resulting powder was calcined at 550°C for 6 h to remove the surfactant.

The protonated form of the samples was obtained by repeated ionexchange with 1.0 mol·L-1NH4NO3solution at 90 °C for 4 h and calcination at550°C for4 h.The samples synthesized frommicro-sized ZSM-5 with SiO2/Al2O3ratio of 50,80 and 140 are denoted as HZM-50,HZM-80 and HZM-140,respectively.The samples synthesized from nano ZSM-5 with SiO2/Al2O3ratio of 50 is named as HZM-50-N.

2.3.Characterization

X-ray powder diffraction(XRD)measurements were carried out using a Philips X'Pert MPD diffractometer equipped with Cu-Kα radiation(40 kV,200 mA for wide angle patterns and 100 mA for small angle patterns).Transmission electron microscope(TEM)images were obtained using a JEM-2100F microscope.Nitrogen adsorption-desorption measurements were carried out at 77 K using a Micomeritics ASAP 2020 volumetric adsorption analyzer.Samples were degassed at 300°C for 24 h prior to the exposure to nitrogen gas.The Brunauer-Emmett-Teller(BET)method was applied to estimate speci fic surface area.Micropore volumes(Vmicro)and external surface area were determined using the t-plot method.Mesopore volume(Vmeso)was calculated using Vmeso=Vt-Vmicro,and Vt(total pore volume)was calculated from N2uptake at a relative pressure(P/P0)of 0.99.The pore size distribution was calculated by the Horvath-Kawazoe(HK)model and Barrett-Joyner-Halenda(BJH)model using desorption branch of the isotherm.Ammonia temperature-programmed desorption(NH3-TPD)was measured on a Quantachrome CHEMBET 3000.The samples of 0.0500 g were charged in a quartz tubular reactor and pretreated at 600 °C with an Ar flow of 30 ml·min-1for 1 h and then cooled to 50°C.Ammonia(20%NH3in He)was introduced at a flow rate of 30 ml·min-1for 0.5 h at 50 °C and then a He stream was fed in until a constant TCD signal was obtained.The physisorbed ammonia was removed by flowing He for 60 min at 100°C.The chemically adsorbed ammonia was determined by rising the temperature up to 600°C with a heating rate of 10 °C·min-1.Pyridine-adsorbed infrared spectroscopy(Py-IR)was recorded on a FT-IR Bruker Equinox spectrometer.The powder samples were pressed to self-supported wafers(ca.10 mg·cm-2)and treated directly in an in situ IR cell connected to a vacuum adsorption apparatus allowing to obtain a residual pressure below 10-3Pa.Adsorption of pyridine proceeded at 60°C for 30 min,followed by desorption at 300°C for 20 min.Then,IR spectra were recorded with a resolution of 2 cm-1by collecting 45 scans for a single spectrum.

2.4.Catalytic cracking of n-dodecane

The catalytic cracking of n-dodecane was carried out in a flowing reactor with catalyst coatings consisting of the HZM series or ZSM-5 series,as described in our previous works[30].The reactor was 304stainless-steel tubes with 300 mm length,3 mm outside diameter and 0.5 mm wall thickness.A washcoating method was used for the preparation of the zeolite coatings on the inner surface of the reactor.The same compositions and conditions were used to obtain the close loading amounts of zeolites and thickness of coatings.The solid(zeolite:inert binder=1:1.5)loading amount was 4.200 ± 0.200 mg·cm-2and the coating thickness was 22.2 ± 0.3 μm.The reactor was heated by direct current power and its wall temperature was measured by K-type thermocouples.The pressure was maintained at 4 MPa by a backpressure valve.The flow rate of n-dodecane in inlet of the reactor was controlled by a high-pressure liquid chromatography pump and kept at 10 mL·min-1.The reaction products were cooled first by a condenser and then flowed into a gas-liquid separator.The liquid products were analyzed by a HP7890 gas chromatography with an FID and a PONA column(50 m×0.53 mm).Each sample was collected in 5 min to ensure enough weight of gas and liquid samples for the material balance.For each zeolite catalyst tested,two individual runs were performed to ensure the reproducibility of the experimental results.The conversion of n-dodecane(Xn-dodecane)was calculated as follows(Eq.(1)).

Table 1 Physical properties of virgin HZSM-5 zeolites

where Wn-dodecane,inand Wn-dodecane,outare the mass of n-dodecane in inlet and outlet of the reactor,respectively.

3.Results and Discussion

3.1.XRD

Fig.1 shows the XRD patterns of the various ZSM-5/MCM-41 zeolite composites(HZMs)obtained by alkali-desilication and surfactantdirected recrystallization.In the small angle diffraction range(Fig.1(a)),there are three diffraction peaks at 2θ =2.3°,4.1°and 4.7°,attributing to the(100),(110),and(210)crystal faces of longrange ordered hexagonal MCM-41 phase.The results indicate that mesoporous MCM-41 phase was formed during the surfactantdirected recrystallization after alkali treatment.Moreover,it can be found that the intensity of peaks at 2θ=2.3°for HZMs increases with increasing the Si/Al ratio of initial ZSM-5.It can be deduced that the ratio of mesoporous phase is higher in the composites obtained from high Si/Al ratio ZSM-5.It can be explained by the more severe desilication in the zeolites with higher Si/Al ratio,the chipped silica will form more MCM-41 during the surfactant-directed recrystallization.As seen in Fig.1(a),it is interesting that the composite obtained from nano-sized ZSM-5 has different small-angle XRD pattern in comparison to that from the micro-sized ZSM-5.Only strong diffraction peak at 2.3°without any other peak is displayed for HZM-50-N.It indicates that the obtained nano-sized ZSM-5/MCM-41 has relative worse long-range order,which may be due to its moderate desilication providing unsuf ficient source of silica for recrystallization.

Data of wide-angle XRD for ZSM-5/MCM-41 composites(Fig.1(b))show two diffraction peaks between 2θ =7 and 10°and three diffraction peaks between 2θ =22.5 and 25°,attributed to the(110),(020),(501),(151)and(303)crystalfaces,respectively,indicating the maintenance of the MFI structure.The relative crystallinity of ZSM-5/MCM-41 composites is in a reverse sequence with the peak intensity of the small angle diffraction range(Table 2).HZM-140 presents the lowest relative crystallinity,together with the largestloss of crystallinity in all the samples.Apparently,the destruction of crystalline structure caused by desilication and the formation of MCM-41 phase by recrystallization are responsible for the drop of the relative crystallinities.Shen et al.[31]compared the compositions and crystallinity of ZSM-5 zeolites with different SiO2/Al2O3ratios before and after alkali-treatment and found that the number of lost silicon species increases with the original SiO2/Al2O3ratio of the zeolite,but no obvious relationship appears to exist between change in the crystallinity and SiO2/Al2O3ratio.Herein,relatively suf ficient source of silica caused by severe desilication of high SiO2/Al2O3zeolite(ZSM-5-140)leads the larger formation of MCM-41 phase,which can explain the signi ficant decrease in the relative crystallinity of HZM-140.

Fig.1.(a)Small-angle and(b)wide-angle XRD patterns of the ZSM-5/MCM-41 composites.

3.2.N2 adsorption–desorption

Fig.2 illustrate the N2adsorption-desorption isotherms and the pore size distribution curves of various ZSM-5/MCM-41 composites.The corresponding textural properties are summarized in Table 2.As shown in Fig.2(a),all ZSM-5/MCM-41 composites show typical type I isotherms at the relative pressure below 0.2 and type IV isotherms with a capillary condensation loop at relative pressure higher than 0.4,indicating the presence of both micro-and mesoporous structures.In Fig.2(b),the obtained ZSM-5/MCM-41 composite exhibits mesopores with the pore size centered between 3 nm and 4 nm,due to the surfactant(hexadecyltrimethylammonium bromide)used in recrystallization.The existence of other sized mesopores(4.0-5.0 nm for microsized composites and 2.0-3.0 nm for nano-sized HZM-50-N)may be caused by piled pores associated with the voids between the individual nanounits.

For micro-sized ZSM-5/MCM-41 composites,the speci fic surface area becomes bigger with increasing Si/Al ratio as seen in Table 2.That attributes to the higher ratio of MCM-41 phase in composite,which has been proved previously by the XRD results.Similar speci fic surface area is found between untreated ZSM-5-50 and ZSM-5-50-N(Table 1).But after treatment,the as-synthesized HZM-50-N hasmuch smaller speci fic surface area together with a low volume of mesopores as compared to HZM-50.That is mainly because of the small formation of mesopore phase during recrystallization,which is in good agreement with the XRD results.However,it has a larger ratio of external to total speci fic surface area than ZSM-5-50,which might be ascribed to the decrease in degree of aggregation after treatment.

Table 2 Physical properties of ZSM-5/MCM-41 composites

Fig.2.N2 adsorption-desorption isotherms(a)and mesopore size distribution(b)of ZSM-5/MCM-41 composites.

3.3.TEM

TypicalTEMimages ofvarious ZSM-5/MCM-41 composites are summarized in Fig.3.For micro-sized ZSM-5/MCM-41 composites,the ordered hexagonal mesopore shell with a size between 3.0 nm and 4.0 nm could be observed around the cof fin shape crystal typical of micropore MFI zeolite.Moreover,the shell thickness increases with the increase in Si/Al ratio of the initial zeolite.For HZM-50-N,uniform and ordered hexagonal mesopore phase could also be observed.However,the mesopore phase is separated with the micropore phase in HZM-50-N.In addition,there is another difference between micro-and nano-sized ZSM-5/MCM-41 composites.As shown in inserts of Fig.3,the randomly distributed irregular voids could be observed in the interior of the crystals of micro-size composites,indicating the existence of irregular mesopores in micro-sized ZSM-5/MCM-41 crystals(intracrystal mesopores)and their amountincreases as the Si/Alratio ofthe initialzeolite increased.However,HZM-50-N appears to have less intracrystal mesopores,which provide direct evidence for the speculation of moderate desilication of the raw nano-sized zeolite.

Fig.3.TEM images of HZM-50(a),HZM-80(b),HZM-140(c)and HZM-50-N(d).

3.4.NH3-TPD

Fig.4 presents NH3-TPD results of ZSM-5/MCM-41 composites with various Si/Al ratios and particle sizes.The corresponding raw ZSM-5 zeolites were also characterized by NH3-TPD and results are summarized in Fig.4.For all samples,there are two desorption peaks at 190-220 °C and 380-420 °C,corresponding to the weak and strong acid sites,respectively.The area of both peaks decreases as the Si/Al ratio increases.It can be concluded that all samples have both acid sites(weak and strong acid sites),and the density and intensity of acid sites will decrease with the increase of Si/Al ratio(Table 3).It is noticed that the intensity and amount of acid sites are similar for both ZSM-5-50 and ZSM-5-50-N in spite of their remarkably different particle sizes.After treatment,the obtained HZM-50 and HZM-50-N have diverse weak acid sites.HZM-50-N has lower amount of weak acid sites as compared to HZM-50.Overall,the differences of acid sites among the ZSM-5/MCM-41 composites are smaller compared to that of ZSM-5 zeolites.

Fig.4.NH3-TPD pro files of ZSM-5 zeolites(A)and ZSM-5/MCM-41 composites(B).

Table 3 Acid properties,average conversion and deactivation rate of the catalysts

3.5.Pyridine-adsorbed FT-IR

Pyridine-adsorbed FT-IR spectra for the ZSM-5/MCM-41 composites and initial ZSM-5 zeolites were collected at 150°C.Fig.5 shows characteristic bands at around 1450 cm-1,1540 cm-1and 1490 cm-1,assigning to the interactions of pyridine with Lewis(L),Br?nsted(B),and B+L acid sites,respectively.As shown in Fig.5(a)and Table 3,with the increase of Si/Al ratio,the amount of Br?nsted acid sites decreases.As indicated by Fig.5(b)and Table 3,HZM-50-Nhas the highest Br?nsted acid sites among all the ZSM-5/MCM-41 composites,due to the low Si/Al ratio and higher crystallinity of its parent zeolite.The amounts of L acid sites in the samples are without any order,indicating the randomicity for the formation of L acid sites during the desilication and recrystallization.

Fig.5.Pyridine-absorbed FTIR spectra of ZSM-5 zeolites(a)and ZSM-5/MCM-41 composites(b).

3.6.Catalytic cracking test

The original ZSM-5 zeolites and corresponding ZSM-5/MCM-41 composites were coated inside stainless-steel tubes to get a comparable loading amount of 4.2± 0.2 mg·cm-3.The obtained tubes were used for the continuous catalytic cracking of n-dodecane.Fig.6 shows the conversion of n-dodecane as a function of time on stream(TOS)catalyzed by ZSM-5 zeolite and ZSM-5/MCM-41 composite coatings,respectively.The conversion of n-dodecane over various ZSM-5 coatings decreases quickly with the going on ofreaction,indicating the quick deactivation of catalysts.Their initial conversions of n-dodecane are 27.5%-35%,which drops to 15%-20%when TOS is 37.5 min.The trend of deactivation is especially obvious at the initial period(0-12.5 min).

Fig.6.Conversion versus time-on-stream(TOS)on ZSM-5 zeolites and ZSM-5/MCM-41 composite coatings.

To clarify the difference among catalysts,the average conversion and deactivation rate(rd)of the catalysts were calculated and summarized in Table 3.Although the amount of acid sites is smallest than the others,ZSM-5-140 exhibits the highest activity(average conversion of 26.8%)and good stability(rdof 40.9%)among four ZSM-5 zeolites.Qu et al.[33]detailedly studied the effects of Si/Al ratio of ZSM-5 zeolites on catalytic cracking of n-dodecane and found thatcatalytic cracking activities and stabilities increased with the Si/Alratio ofzeolites,and one with the highest Si/Al ratio gave the highest activity and stability.Authors explained that the main reason for this is weakened deactivation effect from coke in the reaction because of the low amount of acid sites.The other possible reason is the relatively large speci fic surface area and mesopore volume from ZSM-5-140,which provides the rapid diffusion for reactant molecules.It is noted that for nano-sized ZSM-5-50-N,its coating has slightly higher activity and worse stability than microsized ZSM-5-50.They have similar BET speci fic area,amount of acid sites and intensity of acid sites.Therefore,the higher activity of nanosized zeolite than micro-sized one might be ascribed to its short path length of diffusion.It is well known that the consecutive reactions leading to the deactivation of catalyst,such as successive hydrogen transfer,grow exponentially with the level of conversion[34].Therefore,the slightly worse stability of nano-sized zeolite than micro-sized one is mainly because of the slightly higher conversion of n-dodecane over it.

In comparison,ZSM-5/MCM-41 zeolite composites exhibit superior activity to their corresponding parent ZSM-5 zeolites.This is mainly due to their larger speci fic surface area and mesopore volume than untreated zeolites,which provide the channels forthe rapid diffusion ofreactantmolecules.The pore diffusion was a rate-limited step for catalytic cracking of n-dodecane,which would become worse under supercritical conditions.Itis reported that the diffusion rate of supercritical fuel molecules in channels of catalysts signi ficantly decreases[5].For example,the diffusion rate ofsupercritical n-heptane in the micropores ofzeolites is about one quarter of that under subcritical condition[6].Under such situations,apparently,the improvementin effective diffusivity ofthe reactant molecules in zeolite-based structured catalysts would bring bene fits for catalytic cracking.

Among all the catalysts,HZM-140 has the highest activity with the 87%improvement compared to the untreated ZSM-5-140.Although the much higher conversion of n-dodecane was achieved,HZM-140 exhibits the good catalytic stability with rdof 32.1%.This could be ascribed to more mesopores and the smaller amount of acid sites in it.The introduction of mesopores into zeolite is helpful for the diffusion of reactant molecules,which increases its activity and delays deactivation.Additionally,the smaller amount of acid sites could avoid an abrupt loss of activity,which bene fits the stability of catalyst.Therefore,the optimal initialSiO2/Al2O3ratio is around 140 to obtain ZSM-5/MCM-41 composite zeolites with high catalytic activity.

Itcan be found thatreaction catalyzed by nano-sized HZM-50-Nalso shows much higher conversion of n-dodecane and better stability compared to the micro-sized HZM-50.This veri fies the superior performance of nano-sized composite combining the bene fits of short path length of diffusion from the nanosize together with the decrease in the degree of aggregation of crystals and mesopores giving the effective diffusivity of the reactant molecules in the zeolite pores.The introduction ofmesopores into nano-sized could delay the pore-mouth plugging of zeolite,which bene fits the activity and stability of zeolite.

4.Conclusions

ZSM-5/MCM-41 zeolite composites with the dual-model structures of micropores and mesopores were prepared by desilication and surfactant-directed recrystallization process.The recrystallization is bene ficialto formregularmesopores in the resulting zeolite composites,which has been controlled by varying Si/Al ratio or particle size of the initialZSM-5 zeolite.Because ofthe presence ofmesopores,the diffusivity of ZSM-5/MCM-41 composite zeolites is improved appreciably.As a result,the optimized ZSM-5/MCM-41 zeolite composites exhibitsuperior catalytic activity in n-dodecane cracking than the virgin ZSM-5.The conversion of n-dodecane has been increased by about 25%without any obvious deactivation.Moreover,the nano-sized composite also shows superior catalytic activity and stability to the corresponding micro-size one because of the combination of short diffusion path length,the presence of mesopores and low degree of aggregation.