Influence of monovalent alkaline metal cations on binder-free nano-zeolite X in para-xylene separation

Milad Rasouli,Nakisa Yaghobi*,Seyedeh Zahra Movassaghi Gilani,Hossein Atashi,Majid Rasouli

1 Department of Chemical Engineering,Faculty of Engineering,University of Sistan and Baluchestan,Zahedan,Iran

2 Petrochemical Department,Iran Polymer and Petrochemical Institute,Tehran,Iran

3 Chemical Engineering Department,Amirkabir University of Technology,Tehran,Iran

Keywords:Nano-zeolite X Adsorption C8 aromatics Isotherm

ABSTRACT The adsorption process was studied for separating para-xylene from xylene mixture on modified nano-zeolite X in a breakthrough system.Nano-zeolitic adsorbent with different ratios of SiO2/Al2O3 was synthesized through hydrothermal process and ion-exchanged with alkaline metal cations like lithium,sodium and potassium.The product was characterized by X-ray diffraction,scanning electron microscopy(SEM),nitrogen adsorption,transform electron microscopy(TEM)and in situ Fourier transform infrared(FTIR)spectroscopy.The influence of nano-zeolite water content and desorbent type on the selectivity of para-xylene toward other C8 aromatic isomers was studied.The optimization of adsorption process was also investigated under variable operation conditions.The isotherm for each isomer of C8 aromatics and the desorbents possess the adsorption characteristics of Langmuir type.The selectivity factor of para-xylene relative to each of meta-xylene,ortho-xylene and ethylbenzene under the optimum conditions obtained to be 5.36,2.43 and 3.22,in the order given.

1.Introduction

Para-xylene(PX)is used primarily as a feedstock for the manufacture of purified terephthalic acid(PTA),which is an important chemical in the production of fiber and plastic bottles.The separation of PX is a key operation which requires high purity,high yield,and high capacity[1].The boiling points for these isomers are so similar that separating them via conventional distillation is not practical.Crystallization is a method widely used to perform the separation.In this method,PX can be readily separated from other xylene isomers taking advantage of its higher freezing point.In this way,separation processes based on crystallization and liquid/solid separation equipment are an alternative technology,allowing PX to be solidified and recovered in crystal form at a temperature at which the other isomers are present in liquid phase.

Swift et al.reported a crystallization process for the separation of PX.In this system 66,191 kg·h-1C8aromatic has been fed into the recovery crystallizer and approximately 9276 kg·h-1of PX crystal product was produced[2].Recently,adsorption processes because of its high efficiency are being applied worldwide,and the main licensors of these adsorption processes are the UOP,AXENS and TORAY companies.Selective adsorption by the use of a zeolitic adsorbent is generally considered to be the most economical among the industrial processes for the separation of C8aromatics.

The PAREX process,developed by Universal Oil Products(UOP)for the production of PX in 1969 was a pioneer in applying the principle of adsorption separation in industrial scale[3].The PAREX process is a continuous process based on simulated moving bed(SMB)technology.SMB helps to achieve the separation performance of a true moving bed while avoiding the difficulties in the movement of a solid phase.Separation is achieved by exploiting the differences in affinity of the solid zeolitic adsorbent(microcrystalline zeolite Y)that is selective for PX and is designed to recover＞97%(by mass)of the PX from the feed in a single pass while delivering a productpurity of≥99.9%(by mass)[4-8].

Smolin et al.applied microcrystalline Na-X and Li-X to separate PX from C8aromatics in batch system[9].He reported that the best selectivity of PX toward meta-xylene(MX),ortho-xylene(OX)and ethylbenzene(EB)was 5.60,1.68 and 3 for Na-X and 3.57,1.86 and 2.50 for Li-X adsorbent,in the order given.Roeseler et al.reported a process which preferably employed barium and potassium exchanged Faujasite(FAU)zeolite for the separation of PX[10].In this system,the influence of desorbent purity on PX recovery was investigated.Rasouli and coworkers[11]reported a process for the separation of PX from feed mixture containing all C8aromatic isomers using modified ZSM-5.This research was conducted on H/ZSM-5 adsorbentin a breakthrough system and the selectivity factor of PX toward OX,MX and EB was obtained as 16.78,24.98 and 6.76,respectively.

Nano-zeolites have higher external surface areas and reduced diffusion path lengths relative to conventional micrometer-sized zeolites,which have made them plausible catalytic materials and adsorbents.The liquid phase adsorption process is favored for its operational,maintenance and environmental advantages over the earlier technology[5].

In this work,for the first time binder-free nano-zeolite X modified with monovalent alkaline metal cations such as lithium,sodium and potassium has been introduced to the process for the separation of PX from xylene mixture.Modified nano-zeolite Xwith a molarratio of Si/Al=1.1 was used as the adsorbent.The preparation procedure of binder-free nano-zeolite X with a perfect structural framework was developed in our laboratory.The framework for nano-zeolite Xis built by linking sodalite cages through double six-rings.This creates a large cavity in nano zeolite X called the “supercage”(which should really be called a supercavity)accessible by a three-dimensional 12-ring pore system[12].PX can be adsorbed inside the channels and separated based upon their different affinities toward the framework structure.Adsorption process was carried out in breakthrough system at the desired temperature,pressure and feed flow rate.Process involved contacting the feed mixture with modified nano zeolite Xin the liquid phase.The degree of separation was characterized by the zeolite selectivity and capacity as well as the choice of desorbent and operating conditions such as temperature,feed and desorbent flow rates.

2.Material and Methods

2.1.Nano-zeolite X preparation

Nano-zeolite X was synthesized by hydrothermal crystallization.Colloidal crystals of nano-zeolite X were formed in a clear homogenized solution.In the preparation stage,aluminum hydroxide and tetramethylammonium bromide as a template were mixed in distilled water and stirred vigorously until the solution became clear.Then sodium silicate and tetraethylorthosilicate(TEOS)were slowly added to above mixture and stirred for 24-36 h at room temperature until they dissolved completely.The final molar composition was found to be:0.7(TMA)2Br:0.003 Na2O:1.94 Al2O3:2.16 SiO2:125 H2O.Zeolite crystallization was performed in a Teflon-lined stainless steel autoclave under hydrothermal conditions at 90-110°C for 36-48 h.Then,the resulting product was filtered and washed several times with distilled water until the pH was dropped to 9.Finally the product was dried in an oven at 110 °C overnight,and followed by calcinations in air at 550 °C for 12 h[13-16].Particles which passed through a 1.5 mm sieve mesh size and retained in 1 mm sieve mesh size were used in this work.

2.2.Ion-exchange method

Exchangeable ions of a zeolite are cations associated with the aluminum tetrahedra of the zeolite framework.These ions are available for ion-exchange with cations from the bulk solution[17].In this work,ion-exchange experiments were performed in 0.1 mol·L?1lithium chloride,sodium chloride and potassium chloride solutions for exchanging cationic sites with lithium,sodium and potassium,respectively.The exchange solutions with a pH of＜9 were prepared by dissolving 5 g of each lithium chloride,sodium chloride and potassium chloride in 250 ml de-ionized water.Five gram nano-zeolites were stirred in 250 ml0.1 mol·L?1of each prepared solution for exactly defined time periods.Ion-exchanges were performed for defined time periods between 6 and 12 h at temperature of 80-90°C.The final products were separated by filtration and washed with de-ionized water and eventually dried overnight at 80°C[5].

2.3.Characterization

Elemental chemical analysis of prepared nano-zeolites were performed by Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES)using a Horiba Jobin-Yvon model Ultima Spectrometer with SMEWW 3120 method.The X-ray(powder)diffraction(XRD)data were collected on a D-MAX/II A X-ray diffractometer.Operating conditions involved the use of CuKαradiation in the range of 5°＜ 2θ ＜ 50°with the scanning speed of 0.1(°)·min?1.Nitrogen physisorption measurements were performed at liquid nitrogen temperature in a Micromeritics ASAP 2010 apparatus.Before the measurements,all of the samples were degassed at 420 K overnight.The crystal size and morphology were monitored by scanning electron microscopy(SEM)using a Shimadzu S-520 microscope which was operated at 80 kV and equipped with a SIS Megaview III CCD camera.The average crystal size was estimated from the SEM images.Room temperature Fourier Transform Infrared(FTIR)spectra of the modified nano-zeolites in KBr pellets were measured using a Perkin Elmer spectrometer in the range 4000-2500 cm?1by averaging 20 scans using a resolution of 2 cm?1.

2.4.Experimental procedure

Breakthrough and reverse breakthrough experiments were performed using modified nano-zeolite X as adsorbent and toluene as desorbent.Breakthrough experiments with a xylene isomers mixture were conducted using a desired flow rate.Initially the column was fed with toluene;when thermal equilibrium is achieved,column feed is switched from toluene to xylene isomers mixture.At the same time,the fraction collector,which is programmed to collect samples during the experiments,is started.When the effluent reaches the feed concentration,reverse breakthrough is started.At this point,feed was switched from the xylene isomers solution to toluene.

About 2 g of 0.5-1 mm diameter of modified nano-zeolites were placed in a stainless steel column with an inner diameter of 0.5 cm.The feed rate was controlled using a syringe dosing system(Harvard Apparatus,0.00044-77 ml·min?1).The products were sampled and analyzed by GC-MS using capillary columns CP-Sil PONA CB.The experiments were carried out at the temperature of 120-160°C and pressure range of 0.4-0.6 MPa with a feed mixture containing 54%MX,24%PX,18%OX and 4%EB.Fig.1 shows the schematic of the breakthrough set up applied in the present work.Furthermore,to reduce the experimental errors,each experimental run was performed at least twice and,hence,the reported data are the average of at least two obtained data points.The relative errors for all our experiments were between 2%and 5%for the reported data.

2.5.Adsorption selectivity and capacity

In order to compare the performance of adsorbent and desorbent systems for the same feedstock it is best to use the selectivity beta ratio.The selectivity factor of PX over a particular xylene on the prepared nano-zeolites is depicted as follows:

where selectivity factor approaches 1.0,there is no preferential adsorption of PX by the nano-adsorbent with respect to other isomers;they are both adsorbed with about the same degree with respect to each other.As selectivity factor becomes less than or greater than 1.0,there is a preferential adsorption by the nano-adsorbent for PX component with respect to other C8aromatic isomers[18,3].

Adsorption capacity of component i is defined as the amount of component i which is adsorbed from the feed solution while it is exposed to the adsorbent.The adsorption capacity is calculated as follows:

where Aiis the adsorption capacity of component i(g·g?1);K0is the isooctane(IO)charge in the feed(g·g?1);Ci,0is the concentration of component i in the feed(g·g?1);Ciis the concentration of component i in the equilibrium solution(g·g?1);Cm,0is the concentration of IO in the feed(g·g?1);and Cmis the concentration of IO in the equilibrium solution(g·g?1)[5].

Fig.1.Schematic of breakthrough set up(MFC:mass flow controller,GC:gas chromatography).

3.Results and Discussion

3.1.Adsorbents structural characters

The elemental compositions of the ion-exchanged nano-zeolites are given in Table 1.In order to confirm the structure and crystallinity of nano-zeolite X,an XRD study was carried out.Fig.2 shows the XRD patterns of nano-zeolite X(A)and ion-exchanged nano-zeolites(B-D).XRD patterns indicate that nano-zeolites have crystallinity almost identical to the parents X[JCPDS card 28-1036],[19].The nano-zeolite Xused in this study constituted a specific nano-crystalline aluminosilicate cage structures in which the alumina and silica tetrahedral were intimately connected in an open three-dimensional network and were composed of nano-zeolite single crystals as shown in the SEM photo(Fig.3).An average crystal size around 100 nm was observed for nano-zeolite X.The textual properties of the nano-adsorbents such as surface area,pore size and the pore volume are given in Table 2.Results give indications to the incorporation of species into nanopore spaces of zeolite X.

Table 1 Elemental mass composition of ion-exchanged nano-zeolites

A further evidence for the FAU structure of the nano-zeolites is provided by IR spectroscopic examinations.As shown in Fig.4,for the adsorbents on nano-zeolite X an intense band was observed around 3420 cm?1which could be assigned to hydroxyl groups of zeolite.

3.2.Adsorption mechanism

Key zeolitic adsorbent characteristics include framework structure,zeolite particle sizes,chemical composition,binder,counter exchange ion and water content.These variables are carefully modified to selectively adsorb one particular component over others.The adsorbed component can then be removed or desorbed from the adsorbent using a suitable solvent behaving as a desorbent.To optimize separation by liquid phase adsorption,two opposing forces must be balanced:the adsorptive force of the adsorbent to the adsorbate and the desorptive force of the desorbent to the adsorbate.Ideally,the adsorbent should exhibit lower selectivity toward the desorbent than the adsorbed component[11].

Fig.2.XRD patterns of(A)nano-zeolite H-X,(B)nano-zeolite Li-X,(C)nano-zeolite Na-X and(D)nano-zeolite K-X.

Fig.3.SEM of nano-zeolite X particles.

Table 2 Textual properties of synthesized nano-zeolites

The mechanism present in this study for PX separation is equilibrium selective adsorption.The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent.While all the adsorbates have access to the adsorbent sites,the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction.This in turn results in higher adsorbent selectivity for one component than the others[5].Interaction between the acidic sites of the zeolite and basic sites of the adsorbate is one of the important parameter that affects the equilibrium selective adsorption mechanism.Specific physical properties of zeolites,such as framework structure,choice of exchanged metal cations,SiO2/Al2O3ratio and zeolite water content can be manipulated to influence the acidity of zeolites,which in turn affects separation performance.The degree of separation is characterized by the nano-zeolite selectivity and capacity as well as the operating conditions such as temperature,pressure and feed flow rate.

3.2.1.Adsorbent framework structure

One of the most significant variables affecting zeolite adsorption properties is the framework structure.Each framework type(e.g.,FAU,LTA,MOR)has its own unique topology,cage type(alpha,beta),channel system(one-,two-,three-dimensional),free apertures,preferred cation locations,preferred water adsorption sites and kinetic pore diameter[20].

Fig.4.FT-IR spectra of(A)nano-zeolite H-X,(B)nano-zeolite Li-X,(C)nano-zeolite Na-X and(D)nano-zeolite K-X,after post-treatment.

According to the critical molecular dimensions of C8aromatic isomers,large pore size zeolites are more suitable for separating PX from other isomers.Three-dimensional channel zeolites because of the expected higher adsorption and desorption rates are considered as the preferred mass separating agent of choice compared to one-or two-dimensional channels for the liquid adsorption separation.The FAU-type zeolite possesses a straight channel system with a window size of 0.74 nm×0.74 nm[21,22].As mentioned above,nano-zeolite X from FAU family was used as the adsorbent for separation of PX from its isomers.

3.2.2.The influence of ion-exchange on PX adsorption

The influence of lithium,sodium and potassium cations on the selectivity factor of modified nano-zeolite X was investigated.Feed mixture containing 54%MX,24%PX,18%OX and 4%EB which was diluted with the inert component IO,was used to study the PX adsorption separation.The experimental results at the pressure of 0.7 MPa,temperature of 150 °C and feed rate of 2 ml·min?1are given in Table 3 which shows the selectivity factor of the PX to MX,OX and EB on modified nanozeolite X.It can be seen that adsorption of PX from the feed mixture highly depend on the exchanged cationic sites.Exchanged cations with lower ionic radii have higher zeolite acidity.Zeolite acidity increases for monovalent exchanged cations from K＜Na＜Li.In the presence of strong acids,xylene isomers have varying basicity with MX being themost basic and PX the least basic among the C8aromatics[23].Based on the basicity of the xylene isomers,the acidity of the prepared nanozeolite can be properly adjusted to selectively adsorb PX.Accordingly,the weaker acidic zeolite such as K-X will selectively adsorb PX from other C8aromatics.The selectivity factor of PX on nano-zeolite K-X was calculated as 5.36 for PX/MX,2.43 for PX/OX and 3.22 for PX/EB,respectively.This indicated the excellent characteristic of the nanozeolite K-X for the separation of PX from its isomers.

Table 3 Influence of monovalent cations on selectivity factor of PX toward other isomers at temperature of 150°C and the pressure of 0.7 MPa

3.2.3.The influence of SiO2/Al2O3 molar ratio

A high density of AlO4?in the zeolite framework could actually lower the acid strength of the adsorbent.The reversing trend in acid strength can be explained by the dipolar repulsion of the AlO4?groups outweighing the increase in polarizability[24,7].Zeolite acidity increases in strength as the molar ratio of SiO2/Al2O3decreases due to the increase in AlO4?sites,which strengthen the electrostatic field in the zeolite and increases the number of acidic sites[25].

To investigate the influence of SiO2/Al2O3molar ratio on relative selectivity,nano-zeolite K-X was synthesized at the SiO2/Al2O3molar ratios of 1.1,1.4,1.8 and 2.2 by adjusting the amount of sodium aluminate.The results are given in Table 4 and point to the criticality of the silica to alumina ratio for the nano-zeolite K-X as being more effective in PX separation.The increase in SiO2/Al2O3molar ratio causes a decrease in PX selectivity.

Table 4 Influence of SiO2/Al2O3 ratio in PX separation at temperature of150°C and the pressure of 0.7 MPa

3.2.4.The influence of moisture content

It is generally known that zeolites have strong adsorption behavior for moisture.The moisture content of zeolites is even over 20%of its own mass.Water molecules enhance the acidic properties of the zeolite's Br?nsted acids.Adsorbate-adsorbent interactions and,therefore,adsorbent selectivity and adsorbate mass transfer rates are altered due to water polarization.So the effect of moisture content is an important indicator on the adsorption separation performance of zeolites and must be investigated.

Series of tests were performed to investigate the effects of adsorbent water content as measured by the loss of ignition(LOI)at 550°C on PX selectivity factor and results are listed in Table 5.The moisture in the adsorbent had little influence on the selectivity factor of the separation.For instance,the selectivity of PX/MXwas 5.2+0.16 when the moisture of the nano-zeolite K-X was in the range of 0.16%-1.75%(by mass).Themaximum water content of the dry sample of the nano-zeolite K-X was 0.16%(by mass),in which no chemically adsorbed water was measured with the mass loss at temperature of 550°C in TGA.The remaining water was referred to as physically sorbed water.The water just remained on the external surface of the zeolite crystallites in the adsorbent,when the moisture content of the nano-zeolite K-X was increased.Thus,this part of physically adsorbed water had no influence on the adsorption behavior of the adsorbent.

Table 5 Influence of water content of nano-zeolite K-X on selectivity of PX at temperature of 150°C and the pressure of 0.7 MPa

3.2.5.The influence of desorbent type

The function of the desorbent is to desorb and recover the extracted feed components from the adsorbent.In order for the desorbent to perform well in the process,a suitable interactive force between the desorbent and the extracted components to the adsorbent is required.Desorption studies were conducted to select the optimum desorbing solution to be employed in successive regeneration cycles.Using a desorbent that has a higher boiling point than the feed stock typically results in the most economical process.Desorption of adsorbed xylene isomers from modified nano-zeolite X was examined for two different desorbents;toluene and indan.The data of desorbent screening are listed in Table 6.Results indicated these desorbents were sufficient to separate adsorbed species and toluene could achieve more effective desorption of xylene isomers adsorbed onto nano-zeolite K-X,showing that toluene can replace the adsorbed xylene isomer from nano-zeolite K-X.

Table 6 Influence of desorbent on PX separation at temperature of 150°C and the pressure of 0.7 MPa

3.2.6.Adsorption isotherms

A series of batch tests were performed to determine the adsorption isotherms of xylene isomers on nano-zeolites K-X.In each experimental run,about 1 g of feed solution was put in contact with about 1 g of the adsorbent for 1-2 h at desired adsorption condition.Then,the raffinate solution was analyzed with GC for determining the adsorption capacities.

The adsorption isotherms of PX,MX,OX,and EB in a liquid phase of IO on nano-zeolite K-X are shown in Fig.5.Most of the sorption of all adsorbates took place at low concentrations,which indicates the presence of micropore filling.According to the experimental adsorption isotherms obtained in this work,the xylene isotherms show a very steep trend.As it is shown in Fig.5,the isotherms of all components could be fitted by the Langmuir isotherm model.The adsorption capacities of PX,MX,OX,and EB were obtained to be 101,18,42,and 32 mg·g?1,in the order given.Based on the basicity of the xylene isomers and the acidity of the prepared nano-zeolite,it can be observed that the low acidic K-X zeolite can adsorb PX more than other isomers.With respect to the result for zeolite adsorbents[26,18],it is evident that the nano-zeolite K-X with a perfect framework used in ourexperiments plays a key role in the course of adsorptive process.The adsorption isotherms of toluene and indan as desorbents on nano-zeolite K-X are shown in Fig.6,which indicated the typical type of Langmuir adsorption.The adsorption capacity of toluene in nano-zeolite was more than that of PX and this implied that the affinity of these compounds with the nano-zeolite was stronger than that with PX.

Fig.5.Adsorption isotherms of xylene isomers on nano-zeolite K-X at temperature of 150°C and the pressure of 0.7 MPa.

Fig.6.Adsorption isotherms of desorbents on nano-zeolite K-X at temperature of 150°C and the pressure of 0.7 MPa.

3.3.Optimization of adsorption conditions

It is well known that temperature is one of the most important parameters that dominate the physicochemical behaviors of adsorption process.Also,sufficient pressure and feed mixture flow rate must be applied for maintaining the system in the liquid phase during the entire process[3].Adsorption process yields good separation when the diffusion rates of the feed components through the permeable barrier differ by a wide margin[11].The process has been conducted at the temperature of 120-160°C,pressure range of 0.4-0.8 MPa and also feed flow rate of 2 ml·min?1.The LOI of the used nano-zeolite was obtained to be 0.16 at 500°C.Toluene as a desorbent stream was used for the desorption section.According to the principle described in the section of experimental procedure,the selectivity factor of PX toward other isomers was calculated.The experimental results are given in Table 7.It can be seen that the highest selectivity of PX toward other isomers was achieved at the temperature of 150°C,pressure of 0.7 MPa.

Table 7 Selectivity factor for PX toward other isomers

4.Conclusions

Binder free nano-zeolite X was hydrothermally synthesized,characterized,ion-exchanged with alkaline metalcations and finally tested for separation of PX from xylene mixture in the liquid phase adsorption.The selectivity of PX toward other isomers was calculated for all modified nano-zeolites.The results indicated that nano-zeolite K-X was significantly effective in PX adsorption separation.The experimental results exhibit that PX selectivity decreased by increasing the SiO2/Al2O3ratio from 1.1 to 2.2.Nano-zeolite K-Xwith SiO2/Al2O3ratio of1.1 indicated the best performance.The influence of zeolite moisture content was also investigated.The results illustrated a little difference for the selectivity factor of PX toward other isomers and that increasing water content could keep the selectivity factor low.The influence of desorbent type on selectivity of PX to other isomers shows that toluene is more effective than indan to desorb an adsorbed species.All isotherms of xylene isomers and desorbents showed the typical Langmuir type of adsorption.The saturation capacities of the adsorption on nano zeolite K-X were about 42 mg·g?1for OX,18 mg·g?1for MX,101 mg·g?1for PX,32 mg·g?1for EB,161 mg·g?1for toluene and 131 mg·g?1for indan respectively.The operation conditions were optimized and it was observed that the temperature of 150°C,pressure of 0.7 MPa in a feed flow rate of 2 ml·min?1were the best operation conditions for separating PX.The best selectivity factor of potassium ion-exchanged nanozeolite X for PX/MX,PX/OX and PX/EB was found to be 5.36,2.43 and 3.22,respectively.