Xiaoxiao Zhang,Dangguo Cheng*,Fengqiu Chen,Xiaoli Zhan

Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

1.Introduction

Zeolite,crystalline microporous aluminosilicates,has been widely used as a heterogeneous catalyst in petrochemical process[1–4]such as catalytic cracking,hydrocracking,alkylation and isomerization of various hydrocarbon molecules[5–12].Itsvaluable success in industrial application can be ascribed to the combination of strong framework acidity and the uniform porosity[13–17].Despite its remarkable achievement,the catalytic reactivity and stability of the catalyst still need to further improve for industrial application.Composite zeolite with the binary structure would be a promising candidate,because it can not only keep the nature of individual zeolite but also give rise to new distinctive properties.Some zeolite composites,such as diatomite/MFI-type[18],CHA/AEI system[19],ZSM-5/SAPO-34[20–22],Y/MCM-41[23]and ZSM-5/Y[24],have been already synthesized.In this regard,composite ZSM-5/mordenite zeolites were developed and investigated in catalytic cracking of light hydrocarbons in our previous work[25].For this protonic acid catalyst,the active site is Br?nsted acid,created by replacing a Si with an Al atom and this substitution has to be compensated by a proton[26].However,these catalysts often lose their activity during the reactions because of the coke deposits on the surface.The common and effective method to prolong the durability of zeolite in application is to introduce water vapor in feedstock.Although the existence of watervaporcan maximize the contribution from monomolecular cracking and impede the is omerization of the primary products[27],the introduction of water vapor can negatively lead to the extraction of Al from the zeolite framework and the catalyst deactivation gradually.

Much effort has been devoted to better understand the zeolite dealumination process and its effect on acidity of zeolites[28,29]and reaction performance[30,31].For example,Almutairi et al.[32]found that severe steaming would result in a significant decrease in the framework Al,which led to an increased amount of methanol converted because of the lower coke formation rate.Other works have addressed the dealumination from a theoretical point of view by employing density functional theory(DFT)to model the kinetics of the dealumination process and determine the relevant hydrolysis reaction mechanism of a tetrahedral atom[26,33].Despite these efforts,various characterization methods have also been applied to investigate dealumination of zeolites by hydrothermal treatment experimentally[34–36].However,the change of zeolitic structure caused by the dealumination process and its correlation with deactivation of zeolites during the reaction have not been comprehensively described and discussed.A deeper comprehension of the dealumination process is highly desirable to determine better anti-deactivation reaction condition in industrial application.

Thus,in this work,we employed,the in-situ DRIFTS to study the mechanism and kinetics of the dealumination process on ZSM-5/MOR zeolite experimentally and choose the catalytic cracking of n-heptane with water vapor as a model reaction to investigate the correlation between the kinetics of the dealumination process and the structural deactivation of ZSM-5/MOR.Through the in-situ DRIFTS method,the variation of each hydroxyl group on zeolite can be observed and the amount of consumed and generated hydroxyl groups can be calculated by integrating area ofpeaks,thereby predicting dealumination rates as a function of temperature and pressure.A correlate mechanism for the dealumination process is also proposed.

2.Experimental

2.1.Catalysts and measurement

ZSM-5/MOR(Si/Al=20)was synthesized and provided by Shanghai Research Institute of Petrochemical Technology.The catalytic cracking of n-heptane was carried out in a fixed-bed reactor(9 mm i.d.)over 100 mg catalysts under a gas hourly space velocity of 1 h?1at 650°C.Argon,as a carrier gas,was mixed with n-heptane and water vapor with the molar ratio of n-heptane/vapor as 1/2,or without water vapor,atthe flow rate of20 ml·min?1,respectively.The products were analyzed on-line using a gas chromatograph equipped with a capillary column(50 m×0.32 mm,KB-Al2O3/Na2SO4)and flame ionization detector(FID).

2.2.In-situ DRIFTS study under hydrothermal treatment

A Nicolet 5700 FTIR spectrometer at a spectroscopic resolution of 8 cm?1with a diffuse reflectance attachment and a liquid N2-cooled MCT-A detector were used with an accumulation of 64 scans.Reactions were conducted in a controlled temperature and environment diffuse reflectance chamber(Spectra-Tech 0030-91)with ZnSe windows.30 mg zeolites were placed in a 2-mm-tall ceramic crucible with 8.5 mm OD and 7.2 mm ID placed in the environment chamber.To investigate the effect of vapor pressure on the dealumination process,the temperature was set as 650°C and helium,as a carrier gas,mixed with water vapor at flow of 15 ml·min?1is fed through the sample bed in the chamber under the vapor pressure of 10.1,20.0,29.9 and 40.3 kPa,respectively,generated by a water vapor saturator for 4 h.Afterwards,in order to explore temperature factor,the vapor pressure was kept at 29.9 kPa,and four different temperatures(600,650,700,and 750°C)were chosen.The structuralchanges ofthe sample were analyzed with IR.All spectra were taken with the use of KBr windows under helium purging as a background reference.

2.3.NH3-TPD analysis

The acidity of the samples was determined by ammonia temperature programmed desorption(TPD)in a Micromeritics chemisorption analyzer(AutoChem II 2920).The sample was activated by heating at 600 °C for 1 h,and after cooling to 100 °C,ammonia adsorption was carried out for 0.5 h.The physically adsorbed ammonia was removed under pure helium flow for 2 h.The chemisorbed ammonia was determined by increasing temperature with a linear program from 100°C to 650 °C at a heating rate of 10 °C·min?1under a helium flow of 50 ml·min?1.

2.4.Surface area and pore analyses

The N2adsorption–desorption tests were carried out at a temperature of 77 K using a Micromeritics ASAP 2020 analyzer.The surface areas of the samples were calculated using the Brunauer,Emmett and Teller(BET)equation.The mesopore size distributions were obtained from the desorption branch of the isotherms by the Barrett,Joyner and Halenda(BJH)method.The micropore size distributions were estimated according to the Horvath–Kawazoe(HK)model and the micropore surface areas were determined by the t-plot method.

3.Results and Discussion

3.1.Dealumination phenomenon

Our previous work[25]has investigated the catalytic cracking of nheptane with water vapor over ZSM-5/MOR zeolites by employing insitu DRIFTS to observe the changes of the location and strength of acidic sites on the zeolite framework.The results showed that the ZSM-5/MOR zeolites were deactivated quickly with catalytic steam cracking reaction proceeding,due to the facts that the bridging hydroxylb and in IRspectra decreased significantly and the peak corresponding to the extra frame work amorphous Al species appeared at the reaction time of 80 min.These suggested that the zeolite dealumination has gradually taken place during the reaction in the water vapor atmosphere.

To furtherconfirm this,two differentcatalytic cracking reaction conditions were carried out.One is in the presence of water vapor and the other only contains the reactant of n-heptane.In addition,the regeneration process is also conducted to observe if the loss of activity happens due to the dealumination from the framework.To regenerate the deactivated ZSM-5/MOR catalyst completely,the coke deposits are eliminated at 750°C under air atmosphere and then the activity of regenerated ones was also tested under the same condition.The results of activity tests on the fresh ZSM-5/MOR and the regenerated ones are exhibited in Fig.1.One can see that the activity of ZSM-5/MOR almost stays the same after regeneration,in the absence of water vapor in the catalytic cracking reaction.However,when the catalytic cracking reaction was accompanied with water vapor,the conversion of n-heptane decreases after the regeneration.This resultimplies that the steam in reactant leads to irreversible deactivation of ZSM-5/MOR zeolite at high temperature.

Fig.1.The activity test at 650°C on fresh and regenerated ZSM-5/MOR catalysts.

3.2.Characterization of catalysts

According to the data of NH3-TPD result listed in Table 1,the acid amount and the acid strength of strong acidity and weak acidity of ZSM-5/MOR zeolites are both decreased during the catalytic cracking reaction in the steam atmosphere and can hardly be recovered through regeneration process.For acidic zeolite,its acidity is composed of the Lewis acid and the Br?nsted acid.The former is created by extraframe work aluminum and the latter is the active site created by substituting a Si with Al atom and a proton balances the charge offramework.The decreased amount of acidity can indirectly verify that the irreversible structural change of ZSM-5/MOR has taken place.Moreover,from the results of pore structure parameters,the difference of textural properties between the fresh ZSM-5/MOR and regenerated ones is obvious.For the fresh ZSM-5/MOR sample,the micropore area is 225 m2·g?1and the volume of micropores is 0.115 ml·g?1[25].However,for the regenerated ZSM-5/MOR sample,the corresponding value decreases to 172 m2·g?1and 0.089 ml·g?1,respectively,as listed in Table 2.But the external area and the volume of mesopores of the regenerated ZSM-5/MOR noticeably increased to 132 m2·g?1and 0.142 ml·g?1,compared with the fresh ones.All these results can prove that the dealumination process indeed happens in the presence of water vapor at high temperature.

Table 1 The NH3-TPD result of the fresh and the regenerated ZSM-5/MOR catalysts

Table 2 Pore structure parameters of the fresh and the regenerated ZSM-5/MOR catalyst

3.3.In-situ DRIFTS study of structural evolution of ZSM-5/MOR during hydrothermal treatment

When zeolite catalysts are exposed to water vapor at high temperature,the number of Br?nsted acid sites,directly determined by the number of framework Al atoms,is reduced because of dealumination.The in-situ DRIFTS can monitor the evolution of each hydroxyl group bonding to Al or Si clearly.Hence,the study on the changes of the hydroxyl groups during the hydrothermal treatment can be used to analyze the dealumination process.In the IR spectrum,the bands related with hydroxyl groups often exist in the range of 3500–3800 cm?1.To find hidden peaks,deconvolution analysis is an effective way to distinguish different hydroxyl groups on the zeolite and evaluate the consumption and generation of hydroxyl groups quantitatively during the hydrothermal treatment[25].The deconvolved hydroxyl regions of the IR spectra with time to 4 h over the ZSM-5/MOR zeolite in the steam atmosphere at the temperature of 650°C and the vapor pressure of 10.1 kPa are shown in Fig.2.The components of hydroxyl groups are obtained by Gaussian fits.The peak at3597 cm?1assigned to the bridging hydroxylSi--O(H)--Alin the main pore channels[25]is rapidly decreased with time,corresponding to the process of extraction of framework Alatomsand thus leads to a decrease of the protonic acidity.The peak at 3652 cm?1,ascribed to the terminal Al--OH of the framework in the pore channels[25],is decreased slower than the bridging hydroxyl Si--O(H)--Al in the presence of high temperature steam treatment.The detailed description of the variation of each hydroxyl group during the hydrothermal treatment is demonstrated in Fig.3.The peaks at 3710 and 3735 cm?1are assigned to terminal Si--OH of the framework[37–39].The former is in the pore channels and the latter is on the external surface of the catalyst.In the beginning of the hydrothermal treatment,the amount of internal Si--OH increases quickly.This increase can be ascribed to the creation of four internal Si--OH groups as a result of the extraction of each framework Al atoms followed by the crystal lattice defect.Then,with the hydrothermal treatment proceeding,it causes a pronounced increase of the amount of Si--OH on the external surface and a small decrease of the amount of Si--OH in the internal surface.This results from the formation of the mesopores caused by the collapse of partial crystal lattices during the dealumination process and consequently generate new Si--OHon the externalsurface.Thus,the change of Si--OHon the lattice defect is determined by the rate of lattice collapse and the extraction of dealumination from framework.At the later stage of the hydrothermal treatment,more and more silicon atoms polymerize into amorphous structures due to the collapse of framework,thus the total amount of Si--OH decreased.The peak at 3772 cm?1is ascribed to hydroxyls on the amorphous Al species[40]which are non-acid sites or very weak acid sites,such as O=Al--OH.These hydroxyls increase rapidly at the beginning,then become invariable and decrease at the later period gradually.These results can be explained by the fact that,at an earlier stage,more aluminums from dealumination and removal process migrate onto the surfaces of catalysts so that the amount of thus-formed hydroxyls increased.However,with more and more aluminums extracted,polymerization of some aluminum[41]results in the decrease of hydroxyls on amorphous Al species and transformation of vibration positions.From the above analysis,the dealumination mechanism under the hydrothermal treatment is proposed in Scheme 1.When ZSM-5/MOR zeolites are exposed to steam at high temperature,the dealumination process could be generally described as a multiple-step process consisting of removal of Al from the framework,namely the hydrolysis of four Al--O bonds accompanied with the self-healing of Si--OH bonds.In addition,the obtained extra-framework aluminum would be partially condensed with water molecules to form the two kinds of structures,as illustrated in Scheme 1.

Fig.2.The deconvolution of the IR spectra of hydrothermal treatment on ZSM-5/MOR sample in the range of 3450–3850 cm?1(T=650 °C,vapor pressure P w=10.1 kPa,3%n-heptane in He, flow rate=15 ml·min?1).(—)single components,(--)computed spectra,(—)experimental spectra.

Fig.3.The amount of hydroxyls on ZSM-5/MOR during hydrothermal treatment with TOS at 650°C under vapor pressure of 10.1 kPa.

3.4.The kinetics of dealumination

Br?nsted acid sites play an important role in the catalytic cracking reaction,and the dealumination process would cause the loss of catalytic activity.So,it is significantly necessary to figure out the kinetics of dealumination and hence the prediction of structural deactivation of ZSM-5/MOR zeolites.This can help determine a better anti-deactivation reaction condition in practical application.According to the investigation of dealumination mechanism and the rate of decreased hydroxyls,it is obvious that the extraction of framework Al atoms is irreversible.To investigate the kinetics of dealumination,some prerequisites should be determined.Due to the small size of catalyst particles(0.03–0.05 mm),the influence of internal and external diffusion of water vapor can be ignored under the high gas velocity.Hence,the kinetic equation is illustrated as follows:

where NAlis the number of Al from framework per unit mass of catalyst,kDis the apparent rate constant of dealumination,Pwis the partial pressure of water vapor,n is the reaction order of concentration of tetrahedral Al atom and m is the reaction order of partial pressure of water vapor.

The number of Alfrom framework perunitmass isproportionalto AOHwhich is the area of the peak at 3597 cm?1,so AOHcan take the place of NAl.To obtain the reaction order n of the concentration of tetrahedral aluminum atom,the experiment of dealumination under the different temperatures and vapor pressures has been investigated.The result is depicted in Fig.4,thus,the reaction order n is approximately equal to 1.It accords with the result of NMR in previous publications[36,42–44].Also,to determine the reaction order m,more experiments are conducted.Fig.5 shows the variation of the dealumination amount from the framework in the presence of different vapor pressures at the temperature of 650°C.Besides,Fig.6 demonstrates the effect of different vapor pressures on the dealumination rate at 650°C.From the above results,the reaction order m can be acquired,m≈0.6.Accordingly,the equation can be simplified as follows:

where AOHis the integral area of hydroxyl groups at 3597 cm?1.

Fig.4.The exponential fit of the integrated areas of the bridging hydroxyl with TOS under different hydrothermal conditions.

Scheme 1.The mechanism of hydrothermal dealumination of ZSM-5/MOR catalysts.

Fig.5.The exponential fit of the integrated areas of the bridging hydroxyl with TOS under different vapor pressures at 650°C.

The parameter E,the activation energy,can be calculated by the change of reaction rate constant under different reaction temperatures and the activation energy meets the Arrhenius equation:

where k0is the pre-exponential factor,R is the gas constant,E is the activation energy and T is reaction temperature.

The value of activation energy E and pre-exponential factor k0can be obtained from the slope and intercept of the straight line which is fitted by the reaction rate constant and reciprocal of temperature.From Figs.7 and 8,the value of parameter E and k0is 21.1 kJ·mol?1and 0.70,respectively.The kinetic equation of dealumination can be written as follows:

Fig.7.The exponential fit of the integrated areas of the bridging hydroxyl with TOS under vapor pressures of 29.9 kPa at different temperatures.

Fig.8.Effect of the temperature on the dealumination rate under vapor pressure P w of 29.9 kPa.

where Pwis the vapor pressure.We argue that the kinetic dealumination equation mainly states the self-deactivation kinetics of catalysts with the prerequisite of no coke deposition in the reaction.This is due to the fact that from the IR spectra,the bridging hydroxyls of Si--O(H)--Al,which are the catalytic active sites,decrease rapidly and this phenomenon is consistent with the dealumination process,namely the structural deactivation during the reaction.Moreover,from the obtained kinetics of dealumination,it can be used to determine a more effective antideactivation reaction condition by properly adjusting the reaction temperature and pressure to prolong the catalytic lifetime and maintain the high catalytic activity of zeolites.And it can also be used to predict the dealumination rate at other pressures or temperatures than already tested in practical application.

4.Conclusions

In this work,the dealumination deactivation has been observed during the catalytic cracking reaction due to the water vapor in feedstock.To determine a better anti-deactivation reaction condition towards steaming,in-situ DRIFTS has been employed to investigate the mechanism of structural deactivation under hydrothermal treatment.Through the deconvolution analysis,the hydroxyl groups located in different positions on the catalyst have been distinguished.Also,the consumption and the generation of hydroxyl groups can be quantitatively evaluated.The results show that the initial consumption rates of bridging hydroxyl groups,corresponding to the decrease of Br?nsted acid sites,are remarkably faster than that during later stage of the hydrothermal treatment.This explains the fast initial deactivation of the catalysts during the catalytic steam cracking reaction.Furthermore,based on the deconvolution analysis,the mechanism of the dealumination process has been successfully obtained,which consists of the hydrolysis of four Al--O bonds and the self-healing of Si--OH bonds accompanied with partial condensation of the extra-framework Al species.In addition,the kinetics of dealumination has also been obtained and it is in excellent agreement with the kinetics of deactivation in the presence of no coke deposition.With a better explanation of the dealumination process,the relationship between the catalytic performance and the acidic properties of the zeolites can be deeper understood.The obtained dealumination kinetics during the catalytic cracking reaction is also of directive significance to determine better anti-deactivation reaction condition in industrial application.

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