Modification and sequential treatment of EU-1 zeolite in mild alkali and alkaline-acid conditions

Xiaofeng Li*,Xiaotao Sun Yanting Zhang Junliang Zhang ,Pengchao Ren Xiaozhen Liu Tao Dou ,*

1 Research Institute of Special Chemicals,Taiyuan University of Technology,Taiyuan 030024,China

2 Key Laboratory of Catalysis of CNPC,China University of Petroleum-Beijing,Changping,Beijing 102249,China

1.Introduction

Acid and alkali post-treatments to zeolites can modify the compositions and produce mesopore,specifically the treatment methods contain alkali,acid and alkali-acid treatments[1–5].Both inorganic and organic acids and alkalies can be used in the treatment process.Alkali treatment is mainly used to create mesopores by dissolving part of silicon atoms in zeolite framework[6–8].Under optimal conditions,the microporous framework structure of the zeolite did not collapse,and the catalytic performance of zeolite can be improved,because the newly-created mesopores among microcrystallines and within microcrystallines can strengthen diffusion of reactant molecules.The acid treatment to zeolite can remove part of the aluminum atoms contained in zeolite framework,and improve the Si/Al ratio,acid strength and hydrothermal stability,and introduce some intercrystalline mesopores[9].The acid and alkaline treatment also affects the acidity of zeolite to some extent.Generally,the acid–alkali combined treatment is used to adjust acidity and Si/Al ratio of zeolites,and create mesopores in order to promote the catalytic performances of zeolites.

EU-1 zeolite is an important catalytic material,which possesses one dimensional topology structure[12].Due to its special structure,it has been demonstrated that EU-1 zeolite showed higher activity and selectivity in xylene isomerization reaction[13,14].According to the literature[10,11],the diffusion properties of EU-1 zeolite with mesopores which induced by sequential sodium hydroxide(NaOH)and nitric acid(HNO3)treatments was improved in dimethyl ether to propylene(DTP)reaction,and the treated sample exhibited a higher conversion of dimethyl ether.However,there is a great change in composition and acidity of treated samples,because the high-concentration NaOH was used to treat the parent sample.At present,our research group's main task is synthesizing of EU-1 zeolite used for xylene isomerization reaction[15–18].Although the EU-1 zeolite manufactured based on the route developed by our research group have been commercialized,the catalytic performance of EU-1 zeolite still needs to be further improved.In the industrial practice,it is found that the amorphous silicon and aluminum species deposited in the gap among microcrystallines cannot be removed thoroughly,and these species may cause diffusional limitation and side-reactions.Because the xylene isomerization reaction is a kind of diffusion controlled reaction,so improving EU-1 diffusion properties by sequential acid–alkali treatment is an effective means to upgrade its catalytic reaction performance.

In orderto enlarge intercrystalline mesoporous,improve the activity and selectivity of xylene isomerization reaction,retain the original EU-1 framework structure and its acid properties,we used the mild alkali(Na2CO3)and acid(low-concentration HCl)to remove the amorphous silicon and aluminum species.This process is supposed to ensure that the framework composition and acidity of the zeolite stay stable basically,to improve catalytic activity and selectivity by increasing mesopores.The reasons of the selection for the mild condition post-treatment as follow:(1)they must be effective for easy operation;(2)the degree of treatment cannot be too severe,otherwise,deep treatment may cause side effect in the catalytic reaction.In order to gain the optimal treatment conditions as well as the better catalytic performance of treated samples in xylene isomerization reaction,this study mainly explores the physical–chemical properties of samples treated by acid and alkali in mild conditions.

2.Experiment

2.1.Materials and reagents

The distilled water was made by ourselves.Hydrochloric acid(AR)was obtained from Taihua Group Chemical Pesticide Factory.Sodium carbonate(≥99.8%)was purchased from Tianjin Kemiou Chemical Reagent Co.,Ltd.The EU-1 zeolite(parent sample)was obtained from Taiyuan Da Cheng Huan Neng Chemical Technology Co.,Ltd.

2.2.Post-treatment of samples

The parent EU-1 zeolite(5 g)was added to the 70 ml Na2CO3solution with different concentration.The concentration of Na2CO3was 0.03 mol·L-1,0.05 mol·L-1,0.10 mol·L-1,1.0 mol·L-1,respectively.The suspension was kept 80°C for 24 h while stirring.The slurry was then suction- filtered and washed thoroughly with distilled water until the pHvalue of filtrate reach 7.0.And the products were designated S-0.03,S-0.05,S-0.10,S-1.0 according to the concentration of Na2CO3solutions.

HCl was selected for the dealumination treatment.5 g of the alkali treated sample was added to the 0.1 mol·L-1HCl solution.The suspension was kept at room temperature for 24 h while stirring.The slurry was then suction- filtered and washed thoroughly with distilled water until the pH value of filtrate reach 7.0.The treated samples were designated S-0.03-0.1,S-0.05-0.1,S-0.1-0.1,S-1.0-0.1 according to the concentration of Na2CO3solutions.

2.3.Characterization of samples

The structural characterization of the parent and treated samples was carried out by X-ray powder diffraction(XRD).The XRD patterns were obtained on a(Rigaku D)max-2500 X-ray diffractometer.The relative crystallinity of the samples was calculated by the sum of five peaks(2θ =19°,20.8°,22.2°,23.2°,23.9°).Scanning electron microscopy(SEM)was performed on a EOL/JSM-6700F microscope which was made in Japan.The temperature programmed desorption(NH3-TPD)of the samples was analyzed in TL5000-II multi-purpose adsorption apparatus.Nitrogen adsorption/desorption measurements were carried out on a Micromeritics/ASAP2000 nitrogen physical absorption device.The27Al of the samples was measured with Bruker Advance 400 MAS NMR.

2.4.Preparation and evaluation of catalyst

The treated EU-1 zeolite was mixed with high-purity aluminum hydroxide evenly.Then,the mixture was added with aqueous nitric acid solution(3 wt%),and was made into the carrier by mixing,squeezing,drying,dicing and calcinating.The carrier was converted into the H-form through NH4+exchange(with NH4Cl,C=1 mol·L-1for 2 h,at 80–90 °C),and the carrier was then washed thoroughly with distilled water.After that,it was dipped into chloroplatinic acid and sorbent—trichloroacetic acid(TCA).Finally,the catalyst of EU-1 was made after activation and reduction[19].

A small-sized fixed-bed reactor was used to evaluate catalytic performance ofEU-1 catalyst in xylene isomerization reaction.The raw material for reaction was prepared with ethylbenzene and m-xylene at a molar ratio of 3:17,and the filling amount of catalyst was 1 g.At first,the catalyst was activated for 2 h by pumping in H2at 400°C,and then the temperature was reduced to 360°C for reaction.The H2pressure was 0.5 MPa,the H2flow rate was 4.2 L·h-1,the material feeding rate for reaction was 5.4 ml·h-1,and the weight hourly space velocity(GHSV)was 4.5 h-1.The sample was taken once every 3 h after the material was charged,the gas chromatography with a FID detector was used for analyzing the products.The column furnace temperature was 60 °C,and the detector temperature was 250 °C.The Agilent capillary column chromatography(60 m×0.320 mm×0.25 μm)was used.

The balanced mass fraction ofp-xylene(PX)was used as activity index,and the selectivity of C8aromatic hydrocarbon was used as selectivity index.According to the literature[20],both reaction activity and selectivity are defined as follows:

Catalyst activity=WPX/WPX+MX+OX×100%

Selectivity of C8aromatic hydrocarbon=WPX+MX+OX+EB×100%

The yield ofp-xylene（ PX）=WPX×100%

where,Wis the mass fraction of components in the product,for example,WPXrepresents the mass fraction of PX in the product.

Fig.1.XRD patterns of samples by alkaline treatment in different concentrations.

Fig.2.XRD patterns of samples modified by sequential alkaline-acid treatment.

3.Results and Discussion

3.1.Effect of sequential treatment on EU-1 crystallinity

The XRD results as shown in Figs.1 and 2 elucidate the preservation of crystallinities of EU-1 zeolites after the sequential alkaline and acid treatment.It can be known that the treated samples still have the characteristic diffraction peaks of EU-1 zeolite,and the post-treatment does notchange the topological structure of the EU-1 zeolite.The crystallinity of samples change between 85%and 105%,and its lattice structure is stillstable.It can be found in Table 1 that the crystallinity change of samples treated by different concentrations of Na2CO3has certain regularity,with the increase of sodium carbonate concentration,the crystallinity increases first and then decreases.

Table 1Relative crystallinity and SiO2/Al2O3 ratios of parent EU-1 zeolite and sequentially treated samples

The crystallinity of S-0.05 goes up to 106.5%,while the S-0.1's goes down to 94%.This is because the amorphous material is removed,which is equivalent to the purification for the zeolite,and resulting in increasing first in crystallinity.The crystallinity decreases with the concentration ofNa2CO3solution increased,indicating thatNa2CO3solution with high alkalinity destroyed part of EU-1 zeolite framework.In the sequential alkaline-acid treatment,the crystallinity of the samples changes less than the single alkaline treatment due to the continuous application of low-concentration HCl solution.

3.2.Effect of sequential treatment on EU-1 SiO2/Al2O3 ratio

It is known that alkaline solution can selectively dissolve the framework and silica–alumina amorphous species of zeolite.Desilicication can affect the SiO2/Al2O3ratio of EU-1 zeolite to some extent.The changes of SiO2/Al2O3ratio of EU-1zeolite were listed in Table 1.It is found that the SiO2/Al2O3ratio decreases continuously with the increasing of Na2CO3concentration,the SiO2/Al2O3ratio of S(parent sample),S-0.05,S-0.10 is 38,30 and 28.1 respectively,indicating that the alkaline treatment has different desilicication rates to EU-1 zeolite,the desilicication rate is greater than the dealumination rate,which agrees with the regularity reported in literature[10].

Fig.3.The SEM images of sequentially treated samEU-1 zeolites.

Fig.4.Mechanism diagram of EU-1 zeolite in sequential alkaline-acid treatment.

After further acid treatment,it is found that the SiO2/Al2O3ratio of alkali-acid treated sample is lower than that of the samples treated only by alkali.It is also found based on comprehensive analysis that a certain amount of Al species is still deposited on the EU-1 surface.At this time,the SiO2/Al2O3ratio of EU-1 zeolite is really not the true framework SiO2/Al2O3ratio.The HCl solution can remove this part of non-framework aluminum,resulting in the increase of SiO2/Al2O3ratio[21].It is significant to suppress the rate of the side reactions by removing the non-framework aluminum.It is stated in details in the following sections.

3.3.Effect of sequential treatment on EU-1 morphology

Fig.3 shows the SEM images of sequentially treated samples.It can be seen from SEM images that there are big particles piled up densely by small particles on the surface of parent EU-1 sample(S),and these big particles are different in size.The surface of treated samples is irregular and deformed a little.This phenomenon becomes quite obvious with the increase of Na2CO3concentration.

The morphology of S-0.05 and S-0.03 changed little,and some particles of S-0.1 have been dissolved and damaged,which are notevident.It is thus seen that the Na2CO3treatment is a more mild alkaline treatment method,and has a sight effect on EU-1 morphology relative to that of caused by the strong alkali—NaOH[10].

The SEM images of S,S-0.05 and S-0.05-0.1 are different to some extent.Relative to the parentsample(S),the particles in S-0.05 do notpile up tightly with a greater gap.It is found that S-0.05 and S-0.05-0.1 change less in morphology,indicating the mild acid treatment can't change the EU-1 particle pile-up state.

Obviously,the effects of post treatment under mild condition with lower concentration of Na2CO3and HCl solutions to the morphology of EU-1 zeolite are different from those caused by strong alkali and acid[11].Fig.4 gives the mechanism diagram in alkali–acid conditions.In this diagram,low-concentration Na2CO3alkaline solution can remove amorphous silica–alumina species.In addition,it can enlarge mesopore volume and specific surface area without causing great damage of EU-1 framework.But part of the non-framework aluminum exposed by desilicication,and it can be formed Al(OH)3with theanion.The deposited Al species can be moved by further acid treatment,so the mesoporous volume can be increased.

3.4.Effect of sequential treatment on EU-1 speci fi c surface area and pore volume

3.4.1.Sequential alkaline treatment

Fig.5 shows the N2adsorption/desorption isotherms and the pore diameter distribution of alkaline treatment samples.It is seen from Fig.5 that adsorption/desorption curves of all samples are IV-type isotherm,where there is a hysteresis loop at the highP/P0areas,indicating the samples have mesoporous structure.

The finding shows that total BET surface area is in a downtrend after alkaline treatment,and goes down continuously with increasing of Na2CO3concentration.Both external specific surface area and mesopore volume firstly increase,then decrease(Table 2).

The external specific area and mesopore volume of sample treated by 0.05 mol·L-1Na2CO3is 76.3m2·g-1and 0.155cm3·g-1separately,increasing 9.8%and 6.9%respectively compared with the parent sample(S).It can be seen that the low concentration of Na2CO3treatment can remove the amorphous silicon aluminum species,dredge the occluded channel,increase the surface of the mesoporous surface and the volume of the mesopore.It is also found that total specific surface area and total pore volume decrease in the study which is similar to the desilicication of one-dimensional ZSM-22[21].

3.4.2.Sequential alkali-acid treatment

Fig.5.Isotherms of N2 adsorption/desorption and pore size distribution of parent sample and alkali treated samples.

Table 2N2 adsorption data of parent sample and treated samples with alkaline

Table 3N2 adsorption data of sequentially treated alkaline-acid samples and parent sample

Table 3 shows the N2adsorption data of sequentially alkali-acid treated samples and parent sample.As compared with those of the parent sample,the BET surface area and micropore surface area of S-0.05–0.1 decrease little,but the micropore volume increases and mesopore volume decreases.Moreover,the mesopore volume of S-0.05–0.1 increase 11.7%compared to the parent sample.The mesopore volume increases because the HCl can remove Al species inside of pores.

Fig.6 gives pore diameter distribution data of sequentially treated samples and parent sample.The pore diameter distribution of S-0.05–0.1 is consistent with that of S-0.05 basically,indicating a less effect of sequential alkali–acid treatment on pore diameter distribution.

3.5.Effect of sequential treatment on the coordination of Al

Fig.7 shows the27Al NMR spectra of parent and treated EU-1 samples.It is known from the spectra that no peak occurs at 0 of parent sample(S),indicating there is no non-framework Al in parent sample.Butthere is peak at0 in the sample ofS-0.05,indicating that the removal of part of framework silicon causes the Si–O–Si breakage,and part of Al atoms are exposed out of the framework.They are reacted with theanion,producing Al(OH)3which are deposited on the EU-1 zeolite surface and block the pores of the zeolite.The peak at 0 of S-0.05–0.1 disappears,demonstrating the non-framework Alis removed by HCl solution.More importantly,this proves the hypothesis of the sequential alkali–acid treatment mechanism.

Fig.7.27Al NMR spectra of the parent sample and treated EU-1 samples.

Fig.6.Isotherms plot for N2 adsorption/desorption and pore size distribution of parent sample and alkali treated samples.

3.6.Effect of sequential treatment on EU-1 acidity

3.6.1.Alkaline treatment

Fig.8 shows NH3-TPD of EU-1 zeolite treated by Na2CO3solution.The NH3desorption peak centered at200°C is corresponding to the weak acid site,and the NH3desorption peak at390°C is corresponding to the strong acid site of EU-1 zeolite[22].The acid properties of treated samples and parent sample were summarized in Table 4.It is seen that the strong acid amount of the samples treated by different concentrations Na2CO3decreases,and the weak acid strength is unchanged nearly.The peak positions of S-0.05 is close to that of original sample(S).Weak acid amount,strong acid amount and total acid amount decrease with the increase of Na2CO3concentration.In the mild condition(0.05 mol·L-1Na2CO3),the peak positon and acid amount decrease slightly,but these change greatly in rigorous condition(1.0 mol·L-1Na2CO3).

Fig.8.NH3-TPD profiles of parent sample and alkali treated EU-1 samples.

3.6.2.Alkaline-acid treatment

Fig.9 shows NH3-TPD patterns of sequentially treated samples and parent sample.It is known from the figure that acid peak position of the three samples is unchanged,but the acid amount of samples changes significantly.Strong and weak acid amount of S-0.05-0.1 decrease,because non-framework Al and part framework Al are removed by HCl solution and this lead to that total acid amount of EU-1 zeolite decreases.

3.7.Effect of post-treatment EU-1 zeolite in xylene isomerization

Fig.9.NH3-TPD patterns of sequentially treated samples and parent sample.

Table 5 gives catalytic data of EU-1 catalyst in xylene isomerization reaction.It is seen from Table 5 that the activity of the samples by sequential treatment becomes higher than that of original sample,it increased by 0.67 at most.Activity and selectivity of S-0.05 are improved greater,but with the further increasing of Na2CO3concentration,the catalytic performance of treated samples is becoming worse.

The selectivity of S-0.05-0.1 has a significant improvement compared with thatofS-0.05.The improvement of selectivity is because the EU-1 acidity is changed and the rate of side reaction was reduced after acid treatment.In addition,it is also because the aluminum hydroxide deposited is removed by acid-treatment[11].

Alkali and sequential alkaline-acid treatment can increase mesopores of EU-1zeolite and improve diffusion properties under mild condition.More importantly,xylene isomerization is a kind of diffusion controlled reaction,the mesopore volume of treated samples increase compared with the parent sample.So the sequential alkali–acid treatment is an effective way to improve catalytic activity.

4.Conclusions

(1)The EU-1 zeolite sequentially treated with different concentrations of alkaline solutions is studied in this paper.The treatment ofNa2CO3solution can adjustEU-1 pore structure and acid property in a mild condition.The EU-1 zeolite with larger mesopore volume can be created in alkaline treatment in the proper concentration;

(2)The EU-1 topological structure is not changed in mild alkali and alkaline-acid treatment;the crystallinity of samples ranges between 85%and 105%.The rate of desilicication and dealumination is different in the alkali treatment,the desilicication rate is greater than dealumination rate,and the SiO2/Al2O3ratio of samples can decreased continuously with alkaline concentration increasing;

(3)The silicon speciesis dissolved greatly in alkaline treatment.The Al species deposited on the surface of EU-1 zeolite is a primary cause to severe side reaction.The acid treatment can further clean theEU-1 surface to improve selectivity of catalytic reaction and remove non-framework Al.This is of importance to reduce catalytic side reaction;

Table 4Acid properties of different EU-1 samples

Table 5Catalytic data of xylene isomerization reaction of catalyst for sequentially treated samples

(4)The BET specific surface area is in a downtrend after alkaline treatment,and with the increase of Na2CO3concentration,the BET specific surface area of samples decreases continuously.The external specific surface area and mesopore volume both firstly increase,then decrease;

(5)The weak acid amount,strong acid amount and total acid amount decrease with the increase of Na2CO3concentration;the acidity changes less in mild condition(0.05 mol·L-1Na2CO3);

(6)The catalytic performance of Na2CO3treated samples improve greatly.However,the selectivity ofEU-1 zeolite treated by sequential alkali-acid can be improved significantly.The activity and selectivity of sample treated by sequential 0.05 mol·L-1Na2CO3and 0.1 mol·L-1HClcan be improved effectively in xylene isomerization reaction.

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