修飾晶種法合成MFI型 分子篩膜包覆的活性炭顆粒

金煒陽 程黨國，* 陳豐秋 詹曉力

(1浙江大學(xué)化學(xué)工程與生物工程學(xué)系，杭州310027;2浙江大學(xué)化學(xué)工程與生物工程學(xué)系，生物質(zhì)化工教育部重點實驗室，杭州310027)

1 Introduction

Supported zeolite membrane has attracted increasing interest due to its potential application in separation，catalytic reactor，electrical insulator，etc.1-3Traditionally，zeolite membrane is prepared on surface of disk or tubular supports made of materials such as porous Al2O3，SiO2，and ceramic.4，5Recently，much effort has been devoted to coat a zeolite membrane shell on the surface of catalyst particles.Taking advantage of the molecular sieving ability of zeolite membrane shell，better selectivity and superior reusability of the catalysts can be achieved.6，7For example，MFI-type zeolite capsule catalysts were prepared for adjusting the products distribution of Fischer-Tropsch synthesis as well as realizing the combination of sequential reactions from syngas(CO/H2)to methanol and then to dimethyl ether.8-12

There are two main methods to synthesize zeolite membrane on the supports:in-situ hydrothermal synthesis and secondary growth on seeded supports.Compared to the oxides supports，due to the hydrophobic surface of activated carbon(AC)，it is more difficult to coat zeolite membranes on AC by using insitu hydrothermal process.Thus，methods like surface oxidation，impregnation with silicon atoms or adding hydrophilic montmorillonite clay were employed to increase the nucleation site carbon-based materials to facilitate zeolite synthesis on.13，14Also，concentrated nitric acid treatment was an efficient way to form membrane on carbon-based materials.15Generally，a long time is needed for in-situ zeolite membrane synthesis on the support.For instance，as reported in literature，16it takes 48 h to coat a zeolite membrane on AC particles.Moreover，the recipes used in a given support usually do not apply to the others due to their varying surface properties of different support materials.

Alternatively，the secondary growth can also form supported zeolite membrane.First the zeolite crystal seeds are deposited on the substrates，and upon sequential hydrothermal treatments under the appropriate conditions the seeds grow to form a continuous membrane，even when the crystallization conditions are not optimum.17The key factor of this method is to deposit and fix a continuous seed layer on the surface of supports.For this purpose，cation polymer has been used to modify the surface properties of oxide supports.18Considering the different shapes of carbon-based materials，seeded methods like slipcasting and electrophoretic deposition were employed.19，20

However，when using irregular catalyst particles as the support to coat zeolite membrane，there is another big challenge that the particle is always millimeter-sized and its surface is uneven，which makes the fixation and deposition of a seed layer become difficult.In this work，we present an efficient method by pre-coating activated carbon particles with a boehmite gel layer and then growing the membrane with seeds subsequently.

2 Experimental

2.1 Zeolite membrane preparation

2.1.1 Seed synthesis

Unless special statement，the chemicals used in this work were obtained from Sinopharm Chemical Reagent Co.，Ltd.All chemicals were used without further purification.Crystal seeds were prepared according to the method reported in literature.215.0 g of fumed silica(A200，Degussa，Germany)was added to the solution of 0.27 g of sodium hydroxide(NaOH，AR)and 20.0 mL of 1 mol·L-1tetrapropyl ammonium hydroxide solution(TPAOH，Wuhan Chi-Fei chemical Co.，Ltd.).The mixture was stirred for 24 h at room temperature and then was transferred to a 100 mL Teflon-lined autoclave and placed in an oven with static at 125°C for 8 h.After the treatments of cooling，washing，and filtering，the obtained powders were dried at 120°C for 10 h.

2.1.2 Seeded process

The irregular coconut shell based AC particles(20-40 mesh)were dried at 70°C overnight in a vacuum oven，followed by boehmite gel coating using the spray-coating method.Boehmite sol(0.36 mol·L-1)was prepared by the Yoldas process，22using aluminum isopropoxide(CP)as the precursor.Prior to coating，the pre-weighted AC particles were soaked with ethanol(the mass ratio of ethanol/AC is 0.3).Filling the pores of AC with ethanol would prevent boehmite gel from entering into the inner pores of AC particles.Then the boehmite sol was sprayed carefully on dispersed AC particles with a 100 mL plastic sprayer bottle，after then the AC particles were dried at 45°C for 1 h.This procedure was repeated several times until the mass ratio of AlOOH/AC reached 0.06.After that，0.1 g of seeds and 1.0 g of boehmite sol(0.36 mol·L-1)were dispersed in 9.0 g of ethanol and followed by ultrasonic treatment for 30 min.Then the mixture was sprayed on 5.0 g of boehmite gel coated AC particles with the aforementioned way.For comparison，bare AC particles were seeded with the similar way.

2.1.3 Hydrothermal synthesis

For membrane synthesis，a solution was made by mixing 2.26 g of TPAOH(25.0%(w)in water，Wuhan Chi-Fei Chemical Co.，Ltd.)and 0.44 g of NaOH in 28.0 g of de-ionized water.After magnetic stirring at room temperature for 30 min，3.47 g of tetraethyl orthosilicate(TEOS，AR)was added dropby-drop to the solution under stirring.The obtained solution has a mole ratio of SiO2:0.67NaOH:0.17TPAOH:100H2O:4EtOH.After stirring for 8.0 h，the clear solution was divided into two equal parts and mixed with 0.5 g of gel coated seeded AC and 0.5 g of no alumina sol modified AC particles，respectively.Then the mixtures were transferred to a Teflon-lined autoclave and kept under static condition for 0.5 h at room temperature.Crystallization was carried out in a rotation oven with a rotary speed of 2 r·min-1at 175 °C for 6 h.

2.2 Characterization

X-ray diffraction(XRD)patterns were determined on an X-ray diffractometer(XRD-6000，Shimadzu，Japan)，using Cu Kαradiation，with a working voltage of 40 kV and a current of 40 mA.Apart from the scanning electron microscopy(SEM)of seeds obtained from Carl Zeiss Ultra55(Germany)，the morphologies of the other samples were examined by Hitachi TM-1000(Japan).

3 Results and discussion

The synthesized crystal seeds were confirmed with XRD and SEM，as shown in Fig.1.XRD pattern of the sample has a good agreement with the characteristic diffraction peaks of silicalite-1(PDF card 45-0737).Elliptic and twin crystals with a size about 500 nm are observed from the inserted micrograph.Combined with the XRD results，it suggests that uniform silicalite-1 seeds are synthesized successfully，which are often used for the preparation of MFI-type(TS-1，ZSM-5，Silicalite-1)membrane.

The irregular AC particle and boehmite gel coated AC particle are shown in Fig.2A and Fig.2B，respectively.The irregular AC particle has an uneven surface and possesses some visible pores with sizes up to ten microns.A smoother surface is obtained after coating a boehmite gel layer onto AC particles.And the majority of visible pores are covered by boehmite gel.For AC particles，it is difficult to directly deposit a dense seed layer on this uneven surface.After the seeded process，most of the seeds are located onto the low-lying place of surface(see Fig.2C).Instead，a uniform seeded surface is obtained on gel pre-coated AC surface(see Fig.2D).It seems that the gel precoated layer provides a flat surface for seed dispersion.

Fig.1 XRD pattern and SEM image of crystal seeds

Representative results after hydrothermal treatment are shown in Fig.3.Only sporadic crystals attached onto the AC surface are observed on boehmite gel free seeded AC particles.In addition，some zeolite powders mixed with AC particles are obtained in final products，implying that the seeds on AC surface are not firm and would shed during the rotary hydrothermal treatment.In comparison，as shown in Fig.3B，the boehmite gel pre-coated AC particles are wrapped by a continuous zeolite membrane.No zeolite powders mixed with the final particles are found with naked eyes.With the boehmite sol mixed in seeded solution，it seems that all seeds are firmly attached onto the surface of particles.Owing to the firm seed layer，the nucleation in the synthesis solution is totally suppressed，so nearly all precursors in zeolite solution preferentially participate in seed growth for membrane formation.

Fig.2 Surface SEM images of(A)AC particle;(B)boehmite gel coatedAC particle;(C)AC surface after seeded operation;(D)gel coatedAC surface after seeded operation

Fig.3 Overall surface SEM images of(A)AC and(B)boehmite gel pre-coatedAC after hydrothermal synthesis for 6 h

X-ray diffraction was used to identify the membrane and its evolution from boehmite gel coated AC particles.As shown in Fig.4，broad peaks at 24.2°and 43.5°are generated by the AC particles，suggesting that some graphitized micro-crystal carbon exists in AC.23The peak at 48.9°in the diffraction patterns is assigned to the gel layer(alumina oxide hydroxide)onto AC surface.24For seeded gel pre-coated particles，besides the diffraction peaks derived from AC particles，characteristic peaks of silicalite-1 at 2θ=23.2°，23.8°，24.4°are also observed.After hydrothermal treatment for 6 h，the composite presents some much stronger characteristic peaks of MFI-type zeolite.Note that the diffraction peak at 48.9°for AlOOH shown in Fig.4c is not obvious as shown in Fig.4(a，b)，this phenomenon is possibly because the boehmite gel layer is covered by the membrane.The other possibility is that the gel layer may be partial dissolution in alkaline solution during hydrothermal synthesis.

Fig.4 XRD patterns of(a)boehmite gel coatedAC particles，(b)seeded on gel pre-coatedAC particles，and(c)seeded particles after hydrothermal treatment for 6 h

Fig.5 SEM images of(A)top view and(B)cross view for zeolite membrane wrapped particles after hydrothermal treatment

The top-view and corresponding cross-section view of the membrane are shown in Fig.5.As shown in Fig.5A，after hydrothermal treatment，the seed layer has grown into a continuous membrane.Some crystals grow epitaxially out of the underlying compact membrane.This phenomenon is most likely caused by the disorder arrangement of the seeds，which is in agreement with the morphology of seed layer.From the crosssectional view，a membrane with a thickness of about 5 μm is observed(see Fig.5B)，which is without giving any consideration of the irregular epitaxial growth of partial crystals.Obviously，a continuous membrane is formed by using the boehmite gel modified seeded method.Based on the above characterization，the function of beomite gel layer can be illustrated in Fig.6.One is offering a flat surface on the particles，the other one is a binder for fixing seeds.The interaction between AC surface and gel layer mainly involves physical bonding force，including the wedge force generated due to the rough AC surface.25

Fig.6 Schematic illustration of the boehmite gel modified secondary synthesis procedure

4 Conclusions

MFI-type zeolite membrane enwrapped AC particles were successfully fabricated using a boehmite gel modified seeded method.The pre-coated gel layer gives a more flat surface than that ofAC，with which the seeds can be well dispersed.Boehmite gel is also used for seeds anchoring in the synthesis.This method provides an efficient way for coating zeolite membrane onto the carbon materials or the other inert supports，which can extend the promising application of the membranes for gas separation and catalytic processes.

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