Growth process and short chain alcohol separation performance of fluoride-containing NaY zeolite membrane

Xiaopan Chen,Meihua Zhu,*,Sitong Xiang,Tian Gui,Ting Wu,Yuqin Li,Na Hu,Izumi Kumakiri,Xiangshu Chen,,Hidetoshi Kita

1 State-Province Joint Engineering Laboratory of Zeolite Membrane Materials,Institute of Advanced Materials,College of Chemistry and Chemical Engineering,Jiangxi Normal University,Nanchang 330022,China

2 Graduate School of Science and Technology for Innovation,Graduate School Science and Engineering,Yamaguchi University,Ube 755-8611,Japan

Keywords:NaY zeolite membrane Growth process Fluoride-containing Short chain alcohols Pervaporation

ABSTRACT Growth process of the NaY zeolite membranes was investigated by fluoride-containing precursor synthesis gel.Compared with the fluoride-free precursor synthesis gel,the irregular NaY zeolite crystals were dissolved into amorphous by the fluoride-containing precursor synthesis gel initially,the amorphous contained the Y-type zeolite characteristic bands by the IR characterization.The fine square NaY zeolite crystals arose from the amorphous,which were accumulated and gradually grew into a dense NaY zeolite layer on the support surface after 6.5 h.Because the excessive NaY zeolites were dissolved by the strong alkaline and fluoride-containing precursor synthesis gel,there was plenty of amorphous on NaY zeolites layer for prolonging the crystallization time.The assynthesized NaY zeolite membranes had a good separation performance and repeatability for separation of 10 wt%methanol(MeOH)/methyl methacrylate(MMA)mixture by pervaporation,the flux and separation factor were(1.27±0.07)kg?m?2?h?1 and(4900±1500)at 323 K,respectively.Besides,the NaY zeolite membranes were applied to separate the other short chain alcohol from the various alcohol/organic ester and alcohol/organic ether mixtures,the NaY zeolite membranes showed high short chain alcohol perm-selectivity.

1.Introduction

Methyl methacrylate (MMA) is an environmentally friendly compound for the high value-added fine chemicals of polymerization and copolymerization,which has been used for artificial decorative materials,construction industry parts,optical information materials,and electrical information materials[1].Methylacrolein and methanol(MeOH)are the reaction materials for MMA in general,and the excessive MeOH prevent MMA hydrolysis and obtain a high MMA yield[2].Nevertheless,excessive MeOH gives rise to MeOH/MMA azeotrope mixture.Removing the excessive MeOH is carried out by the conventional distillation technology in general,which is identified as an energy-cost and expensive process[3].Besides,MMA has a low relative volatility and thermal sensitivity,and a high reflux distillation tower is necessary for separation of MeOH/MMA azeotrope[4].

Pervaporation(PV)has been proven to be an energy efficient and lowcost separation process,which is alternative candidate for conventional distillation[5].Separation of alcohol/organic mixtures by PV had been investigated in recent,which were based on polymeric membranes,i.e.cellulosic membranes,segmented copolymers[6].Because the swelling capacity of the polymeric membranes was poor,the stability of the polymeric membranes was short of separating alcohol/organic mixtures in industry[7].Zeolite membranes have attracted much interest in PV application,membrane reactor and gas separation processes by the chemical and thermodynamic stability[8].NaY zeolite membrane had large pore window size(0.74 nm),which was an alternative candidate for separation of short chain alcohol from the alcohol/organic mixtures[9].

Sato et al.prepared NaY zeolite tubular membranes on asymmetric alumina supports,which had a good separation performance for separation of alcohol/organic ester mixture [10].Kita et al.reported the synthesis of NaY zeolite membranes by secondary growth that had a fine quality for separating alcohol/organic mixtures[11–13].NaY zeolite membranes showed a promising 1,3-propanediol selective permeation from glucose and glycerol[14].In our previous studies,the NaY membranes were prepared on mullite supports by microwave heating or traditional heating,which displayed good separation performance for 95 wt%n-butanol aqueous solution and 5 wt%–50 wt%methanol/MMA mixtures [15,16].Besides,fluoride-containing precursor synthesis gel was favor for preparing pure phase and high separation performance NaY zeolite membranes,either for dehydration of bio-alcohols(ethanol and butanol)aqueous or separation of ethanol/ethyl tert-butyl ether(ETBE) mixture in our previous studies [17,18].Herein,in order to improve the performance of NaY zeolite membrane,the formation process of the NaY zeolite membrane in fluoride-containing precursor synthesis gel was investigated in details.Furthermore,the perm-selectivity for separation of alcohol/ester and alcohol/ether mixtures were measured,such as MeOH/MMA,MeOH/methyl tert-butyl ether (MTBE),EtOH/ethyl tert-butyl ether(ETBE),MeOH/dimethyl carbonate(DMC),MeOH/methyl acetate(MA),and MeOH/methyl propionate(MP).

Fig.1.XRD patterns of fluoride-containing NaY zeolite membranes with different crystallization time.(a)seeded support,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h,(h)6.5 h,(i)7 h.

2.Experimental

2.1.Materials

NaY zeolite membranes were synthesized on porous mullite tubes by secondary growth method.Porous mullite supports(o.d.12 mm,i.d.9 mm,porosity 40%,pore size 1.0 μm,length 100 mm)were provided from Nikkato Corporation.Materials of the precursor synthesis gel were water glass(26.5 wt%SiO2,10.6 wt%Na2O,Aldrich),sodium aluminate(Al/NaOH=0.80,Wako),sodium hydroxide(96 wt%,NaOH,Tianjin flameclub Chemical Co.Ltd.),sodium fluoride (99 wt%,NaF,Wako)and deionized water.

2.2.Preparation of NaY zeolite membrane

Molar composition of the precursor synthesis gel was 25SiO2:1Al2O3:22Na2O:990H2O:7.5NaF,whose preparation procedure was identical with our previous study [18].The precursor synthesis gel was aged at 303 K for 14 h.The seeded supports and precursor synthesis gel were settled into an autoclave,and the hydrothermal synthesis was carried out at 373 K.In order to investigate the growth process of the NaY zeolite membrane,the hydrothermal synthesis time was adjusted from 1 to 6.5 h.The sample was washed with deionized water after hydrothermal synthesis.

2.3.Characterization and PV test

NaY zeolite membranes were characterized by X-ray diffraction(XRD,RIGAKU,Ultima IV) and the scanning electron microscopy(FE-SEM,HITACHI,SU 8020).Fluoride-free NaY crystals were observed by transmission electron microscope(TEM,JEOL,JEM2100).Si/Al ratios of the NaY crystals were analyzed by the inductively coupled plasmaatomic emission spectrometer (ICP-AES,Perkin-Elmer,4300DV).NaY crystals were analyzed by Fourier transform infrared spectrometer(FT-IR,Perkin-Elmer,Spectrum One)with samples pressed in KBr pellets.

PV performance of NaY zeolite membranes were evaluated by separation of short chain alcohol/ester and short chain alcohol/ether mixtures,and the PV installation was similar with our previous studies [19,20].The NaY zeolite membrane was contacted with the liquid feed,the compositions of the feed and the permeate were analyzed by a gas chromatograph equipped with TCD detector(GC-14C,SHIMADAU).PV performance of membrane was evaluated by total flux (Q,kg·m?2·h?1) and separation factor (αa/e).Total flux and separation factor were defined as following.

Fig.2.Surface SEM images of fluoride-containing NaY zeolite membranes with different crystallization time.(a)seeded support,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h,(h)6.5 h,(i)7 h.

Fig.3.Cross-sectional SEM images of fluoride-containing NaY zeolite membranes with different crystallization time.(a)seeded support,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h,(h)6.5 h,(i)7 h.

Where m was the mass of permeation,A(m2)was the effective surface area of membrane,t(h)was the operating time;Yaand Yewere the concentration of alcohol and ester/ether in permeation,Xaand Xewere the concentration of alcohol and ester/ether in the feed mixtures.

3.Results and Discussion

3.1.Growth process of NaY zeolite membrane

NaY zeolite membranes have been successfully prepared by the fluoride-containing precursor synthesis gel in our previous studies,and effects of fluoride on pervaporation performance of the NaY membranes was investigated[18].However,formation process of the NaY zeolite membrane was not clear,which was important for improving performance of zeolite membrane.In this work,growth process of the NaY zeolite membranes from the fluoride-containing precursor synthesis gel was investigated in details.

Fig.1 shows XRD patterns of NaY zeolite membranes with different crystallization time.When the synthesis time was from 1 h to 3 h,the intensity of FAU zeolite diffraction peaks of the membranes was quite weak.Pure FAU zeolite diffraction peaks of the membrane could be obviously detected after 4 h,and the intensity of FAU zeolite diffraction peaks(2θ=15.6°,18.6°and 27°)enhanced with hydrothermal synthesis time.

Figs.2 and 3 were the surface and cross-sectional SEM images of NaY zeolite membranes in the fluoride-containing synthesis gel with different crystallization time.The seeded support was fully covered with ca.1.2 μm zeolite crystals in initial(Fig.2(a)),and the seeded crystals were dissolved partially in the precursor synthesis gel after 1 h(Fig.2(b)).Fig.2(c) presented that a few NaY zeolite crystals were growing on the mullite support after 2 h,and the fine NaY zeolites accumulated and aggregated at 3 h(Fig.2(d)),and a thin film was observed(Fig.3(d)).Fig.2(e)exhibited that all the crystals gathered together after 4 h,while there were amorphous fully packed and covered on the NaY zeolite crystals.The NaY zeolites became larger and the octahedral crystals began to appear with prolonging synthesis time(Fig.2(f) and (g)).When the synthesis time was 6.5 h,amorphous was disappeared and a dense NaY zeolites layer was formed on the support surface(Fig.2(h)),the thickness of the zeolite layer was about 7 μm(Fig.3(h)).Because the NaY zeolites could be corroded and dissolved by the fluoride-containing and high-alkalinity precursor synthesis gel,the edge and corner of zeolite crystals were not obvious at 7 h and many voids were found between the zeolites layer(Fig.2(i)).

Table 1 displays the PV performance of NaY zeolite membranes with different crystallization time(4 h–6.5 h)for 10 wt%MeOH/MMA mixture at 323 K,when the crystallization time is 6.5 h,the M-7 and M-8 NaY zeolite membranes have the best perm-selectivity.Besides,the fluoride-containing NaY zeolites corresponding with NaY zeolite membranes were prepared with different synthesis time in this work,whose preparation procedure was identical with the NaY zeolite membranes.Fig.4 exhibits the SEM images of fluoride-containing NaY crystals with different crystallization time.Amount of amorphous were covered on the crystals surface after 1 h(Fig.4(b)),indicating the NaY seeds was dissolved partially.Thereafter,the number of amorphous was decreased with the synthesis time (2–6.5 h),and morphology of NaY crystals became more and more evident,which was accumulated and formed typical octahedral structure gradually(Fig.4(g)).

Table 1 PV performances of NaY zeolite membranes with different crystallization time for 10 wt%MeOH/MMA mixture at 323 K

Fig.4.SEM images of NaY crystals top view with different crystallization time.(a)seed,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h.

Fig.5 presents the FT-IR spectra of the fluoride-containing NaY zeolite crystals with different crystallization time.The spectra contains the Y-type zeolite characteristic bands,which are the bending vibrations of T-O (T=Si or Al) (ca.468 cm?1),hexagonal double-ring (ca.569 cm?1),symmetric stretching(ca.689 cm?1and 766 cm?1)and asymmetry stretching(ca.999 cm?1).The band at ca.1640 cm?1is originated from H2O deformation mode by the incomplete dehydration of the sample.Moreover,the band at ca.3460 cm?1is originated from the asymmetry stretching of inner tetrahedral and water hydroxyl bond[21].When the synthesis time is extended from 1 h to 2 h,the symmetric stretching vibration band(ca.766 cm?1)becomes weak,which shows the graduate dissolution of the SiO2part from NaY zeolites in the fluoride-containing gel.Intensity of the hexagonal double-ring bending vibrations(ca.569 cm?1)decreases in initial,illustrating NaY crystals are dissolved.Besides,the T-O stretching band shifts at ca.1045 cm?1appears,which implies a tetrahedral incorporation of Al into the gel framework at the beginning(3 h)[22].Si-OH bending vibration is assigned at ca.850 cm?1,the intensity of the structure sensitive bands is decreased gradually,suggesting that the number of framework atoms connected via oxygen bridges increases during hydrothermal process [23].Hence,the NaY zeolites are dissolved in the fluoridecontaining precursor synthesis gel at 1–2 h,and the dissolved NaY zeolites still contain FAU typical structure,which is associated with the XRD pattern (Fig.1).For the sake of investigating the relation between growth process and Si/Al ratio of zeolite crystals,the Si/Al ratio of the accompanying NaY zeolites of NaY zeolite membranes were characterized by ICP,which was summarized in the Table S1(Supplementary Information).The results indicated that aluminum of the synthesis solution was consumed for the crystal growth,and a tetrahedral incorporation of Al into the gel at the beginning(1–3 h),which was in accordance with the FT-IR spectra(Fig.5).

Fig.5.FT-IR spectra of fluoride-containing NaY crystals with different crystallization time.(a)seed,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h,(h)6.5 h.The synthesis condition of NaY crystals was similar with NaY zeolite membrane,the seed content was 1%.

In order to investigate the effect of fluoride source on the growth process of the NaY zeolite membrane,the NaY zeolite membranes were prepared in fluoride-free synthesis gel with different hydrothermal crystallization time in this study.Fig.6,Figs.S1 and S2(Supplementary Information)showed surface SEM images,XRD patterns and cross-sectional SEM images of the NaY zeolite membranes without fluoride source.Besides,FT-IR spectra and TEM images (Figs.S3 and S4,Supplementary Information) were in good agreement with the SEM images,the NaY zeolites could not be dissolved in the fluoride-free precursor synthesis gel,the seed crystals gradually grew into large and inter-grown NaY zeolites,and formed the compact zeolite layer (Fig.S5,Supplementary Information).Table S2 and S3 displayed the Si/Al ratios of NaY crystals and NaY zeolite membranes from fluoride-free synthesis gel respectively.The pervaporation performances of these membranes were also listed in Table S4.The as-synthesized NaY zeolite membranes from the fluoride-free synthesis gel had poor perm-selectivity.

Because the fluorine anions could dissolve the silicon precursor to bespecies undergo a hydrolysis in the basic gel and the fluorine anion is released,resulting in accelerating the dissolution of NaY seeds[17].Thereafter,Fig.7 illustrates the growth process of NaY zeolite membrane with fluoride source by secondary growth in this work.The seed crystals were partially dissolved into the NaY zeolites precursor in the fluoride-containing synthesis gels in initial.The fine NaY zeolite crystals formed the crystal clustered,which gradually grew into a dense layer on the support surface.While the fluoride-containing precursor synthesis gel could corrode the NaY zeolite layer with the excessive synthesis time.

3.2.PV performance of NaY zeolite membrane for MeOH/MMA mixture

The NaY zeolite membrane was applied to separation of different concentration MeOH/MMA mixtures by PV,and the influence of MeOH content(10 wt%–40 wt%MeOH/MMA mixtures)on PV performance of the as-synthesized NaY zeolite membrane(M-7)was investigated at 323 K(Fig.8).When MeOH concentration was 10 wt%,the NaY zeolite membrane displayed the highest separation performance in this study,the total flux and separation factor were 1.34 kg·m?2·h?1and 5900.When MeOH concentration was increased,both the total flux and separation factor were decreased rapidly.Flux of NaY zeolite membrane was from 1.34 kg·m?2·h?1(10 wt%)to 0.71 kg·m?2·h?1(40 wt%),and the accompanying separation factor was from 5900(10 wt%) to 80 (40 wt%).The kinetic diameter of MMA and MeOH molecules were ca.0.61 nm×0.91 nm and 0.38 nm,the separation mechanism of NaY zeolite membrane for MeOH/MMA mixture was molecular-sieving [13].MMA could be self-polymerization during PV process,both the separation factor and flux decreased with increasing MeOH concentration in the mixtures.

Fig.6.Surface SEM images of fluoride-free NaY zeolite membranes with different crystallization time.(a)seeded support,(b)1 h,(c)2 h,(d)3 h,(e)4 h,(f)5 h,(g)6 h,(h)6.5 h,(i)7 h.

Fig.7.Growth process of NaY zeolite membrane in fluoride-containing precursor synthesis gel.

Fig.8.PV performance of NaY zeolite membrane(M-7)for MeOH/MMA mixtures as a function of MeOH concentration at 323 K.

Besides,preparation reproducibility of zeolite membranes was critical for the large scale preparation and industrial application,8 pieces of tubular NaY zeolite membrane(10 cm length)were prepared in this work.As shown in Table 2,all NaY zeolite membranes had similar PV performance for separation of 10 wt%MeOH/MMA mixture at 323 K,both flux and separation factor were about 1.25 kg·m?2·h?1and 4000,which suggested that the fluoride-containing precursor synthesis gel was favorable for preparing good reproducible NaY zeolite membranes.

3.3.Separation properties of the NaY zeolite membranes in binary mixtures

Since the pore size of NaY zeolite(0.74 nm)is larger than the kinetic diameter of methanol(0.38 nm)and ethanol(0.43 nm)molecules,theNaY zeolite membrane preferentially permeated alcohol from all alcohol/organic mixtures in our previous work[11,18].Wang et al.revealed the adsorption capacities on the NaY crystal powders were in order:methanol>ethanol>DMC>MMA>ETBE[5].Besides,the dynamic diameter of MA,MP is smaller than that of MMA,MTBE,ETBE and DMC,the MA,MP molecules can enter the NaY zeolite channels,methanol adsorption capacity of NaY zeolites is to be reduced by the competitive adsorption with MA and MP[13].

Table 2 PV performances of NaY zeolite membranes for 10 wt%MeOH/MMA mixture at 323 K

Table 3 summarizes PV performance of the NaY zeolite membrane toward the short chain alcohol/ether and short chain alcohol/ester mixtures in this study.Therefore,the NaY zeolite membrane could absorb preferentially alcohols in this work,the membrane had a good separation performance for 10 wt%methanol/MTBE,methanol/MMA,methanol/DMC,and methanol/ETBE mixtures.The flux and separation factor of NaY zeolite membrane were 0.30 kg·m?2·h?1and 34 for separating methanol/MA(10 wt%/90 wt%)mixture at 323 K[13].In this work,fluxes and separation factor of the NaY zeolite membranes were 0.80 kg·m?2·h?1,0.80 kg·m?2·h?1and 70 wt%,100 wt%for 10 wt%methanol/MA and 10 wt%methanol/MP mixtures.Apparently,the fluoride-containing NaY zeolite membranes had a good separation performance for the short chain alcohol/ether and short chain alcohol/ester mixtures by PV in this study.

4.Conclusions

The NaY zeolite crystals could be dissolved into the amorphous by the fluoride-containing precursor synthesis gel.The amorphous contains the Y-type zeolite characteristic bands by the IR characterization,which could be active in inducing growth into fine NaY zeolite crystals,crystals clustered and gradually grew into a dense NaY zeolite layer on the support surface.The NaY zeolite membranes showed good PV performances for separation of various short chain alcohol/organic mixtures,which had a good reproduction quality.The remarkably high perm-selectivity of NaY zeolite membranes can be attractive for practical applications.

Table 3 Comparison of PV performances for short chain alcohol/organic mixtures by NaY zeolite membranes

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.21868012 and 21968009),Jiangxi Provincial Department of Science and Technology (20171BCB24005,20181ACH80003,20192ACB80003 and 20192BBH80024).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.005.