pervaporation separation and catalysis activity of novel zirconium silicalite-1 zeolite membrane

Chinese Journal of Chemistry, 2009, 27, 1692—1696 Full Paper

* E-mail: [email protected] (Pei Chen); Tel.: 086-029-85308081; Fax: 086-029-85308090 Received October 8, 2008; revised January 21, 2009; accepted March 27, 2009.

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Pervaporation Separation and Catalysis Activity of Novel Zirconium Silicalite-1 Zeolite Membrane

CHEN, Pei*,a,b(陈沛) CHEN, Xinbinga(陈新兵) CHEN, Xiangshuc(陈祥树) AN, Zhongweia(安忠维) KITA, Hidetoshib(喜多英敏)

a Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Materials Science, Shaanxi Normal University, Xi'an, Shannxi 710062, China

b Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Tokiwadai 2-16-1, Ube 755-8611, Japan

c College of Chemistry and Chemical Industry, Jiangxi Normal University, Nanchang, Jiangxi 330027, China

Novel zirconium silicalite-1 zeolite membrane was hydrothermally prepared on the mullite porous support at 150—185 ℃ for 40—72 h by an “in situ” method using tetraethyl orthosilicate (TEOS), zirconium butoxide (ZBOT) and tetrapropylammonium hydroxide (TPAOH) as silica source, zirconium source and organic structure directing agent, respectively. X-ray diffraction (XRD) patterns, fourier transformed infrared (FT-IR) spectra, and inductively coupled plasma-atomic emission spectrometry (ICP) of the accompanying zeolite powder confirmed that the zirconium was isomorphously incorporated into the zeolite framework. The surface chemical compositions of the obtained membrane were measured with an energy-dispersive X-ray spectral analyzer (EDS), and the mem-brane morphologies were observed by a scanning electron microscope (SEM). The results showed that the zeolite crystals growing on the support were zirconium silicalite-1 zeolites, and the dense membrane layer was composed of the well inter-growing zeolite crystals. The zirconium silicalite-1 zeolite membrane, which was derived from the synthesis solution having a molar ratio of 1.00SiO2∶0.01ZrO2∶0.17TPAOH∶120H2O, showed high ethanol permselectivity with a flux of 1.01 kg/(m2•h) accompanied with a separation factor of 73 for ethanol/water (5/95, w/w) system under a pervaporation condition at 60 ℃. Moreover, this membrane displayed pervaporation-aided catalysis activity for iso-propanol oxidation with hydrogen peroxide as oxidant, and the corresponding iso-propanol conversion was 35%.

Keywords zirconium silicalite-1, zeolite membrane, pervaporation, catalysis activity

Introduction

Zeolite membranes have received increasing atten-tion in recent years because of their great potential ap-plication to separation processes and chemical reactions. Up to now, most of the publications in this area are fo-cused on MFI type zeolite membrane1 due to its high chemical and thermal stability, and its channel opening size (0.51 nm×0.56 nm, 0.5l nm×0.54 nm) which are suitable for separation of industrially im- portant gas molecules. And the pure silica MFI zeolite (silicalite-1) membrane exhibits a potential application to recovering the bioethanol from biomass fermentation broth2,3 due to its high ethanol permselectivity.4 In order to improve further the separation performance of silicalite-1 zeolite membrane, many efforts have been made, such as, choosing suitable support, support pretreatment, im-proving membrane preparation technology, membrane post-treatment, etc. In addition, introducing some hybrid elements (Al, B, Ti, Ge, Fe, etc.) into the pure silica MFI structure is also an effective method. For example, Ge-substituted silicalite-1 zeolite membrane is

more effective than a silicalite-1 zeolite membrane in separating acetic acid5 or tetrahydrofuran (THF)6 from their aqueous solution by pervaporation (PV). The B, Al, Fe, and Ge substituted silicalite-1 zeolite membranes have higher separation selectivity than a silicalite-1 zeo-lite membrane for the separation of n-C4H10/i-C4H10 gas mixture7 and methanol/water mixture.8 Moreover, the MFI type zeolite is endowed with catalysis activity be-cause of the introduction of the catalytic heteroatom into the framework, and thus the corresponding membrane can be used as a catalytically active membrane reactor. It is reported that Al-substituted silicalite-1 (ZSM-5) zeolite membrane9 has been used in the methanol series reactions as a catalytically active membrane reactor. The results show that a high selectivity to olefin inter-mediates of 80%—90% is achieved accompanied by the methanol conversion of 60%—98%, which are higher than those of the traditional reactor. This is due to much proper contacting time of reactant molecules with active sites in catalytic ZSM-5 zeolite membrane layer.

Our group has devoted many efforts to preparing

Zeolite membrane Chin. J. Chem., 2009 Vol. 27 No. 9 1693


zeolite membrane for many years, and some thorough and meticulous work has been done on the silicliate-1 zeolite membrane.10-12 It has been found that Ti-substi-tuted silicalite-1 (TS-1) zeolite membrane13 exhibits much higher ethanol permselectivity than the silicalite-1 zeolite membrane prepared in a similar synthesis condi-tion. Moreover, TS-1 zeolite membrane shows high PV- aided catalysis activity for the oxidation of iso-propanol (IPA)14,15 and epoxidation of allyl chloride with hydrogen peroxide (H2O2) as oxidant.16 The new attempts are going on in our laboratory for the further application of TS-1 zeolite membrane as an active membrane reactor to other selective oxidation reactions, as well as exploring a new preparation method of zeolite membrane.17

Similar to titanium, zirconium is also in IVB race in the periodic table. Zirconium silicalite-1 (ZrS-1) zeolite powder has been prepared,18,19 which also shows inter-esting properties toward catalytic oxidation.20 How is it about the performance of the ZrS-1 zeolite membrane, especially the PV separation and catalysis activity per-formance? No literature upon this issue has been re-ported to date. In this work, the hydrothermal synthesis of ZrS-1 zeolite membrane on the porous support by an “in situ” crystallization method is presented, and its PV separation property for the ethanol/water system and catalysis activity for the oxidation of IPA are also inves-tigated.

Experimental

Reagents and materials

Tetraethyl orthosilicate (TEOS, 98 wt%), tetrapro-pylammonium hydroxide (TPAOH, 20.3 wt% in H2O), zirconium butoxide solution (ZBOT, 80 wt% in butanol), iso-propanol (IPA), and hydrogen peroxide solution (H2O2, 30 wt%) were purchased from Aldrich and used as received.

A porous mullite tube (Nikkato Corp., i.d.＝10 mm, o.d.＝12 mm, length＝10 cm) was used as the support, which has an average pore size of 1.3 µm in the outer surface. The surface of the mullite tube was very rough so that it needed to be polished with SiC paper. After polishment, it was cleaned successively in deionized water for 5 min under ultrasonication, and then dried at 100 ℃ in an oven overnight. The dried support was stored in a sealed container before use.

Preparation of synthesis gel

The preparation process was outlined here for 320 g of synthesis gel having the molar ratio of 1.00SiO2∶

0.01ZrO2∶0.17TPAOH∶120H2O. In a plastic bottle A, 3.68 g of TPAOH was mixed with 0.63 g of ZBOT, and a white precipitate immediately appeared due to the formation of ZrO2 resulting from the hydrolysis of ZBOT. In order to dissolve it, 3.12 g of 30 wt% H2O2 solution was added. This mixture was stirred until all the white precipitate dissolved and a clear solution was

observed. In a plastic bottle B, 18.22 g of TPAOH was mixed with 266.08 g of water. The mixture was stirred for 20 min, and then 28.27 g of TEOS was added. The stirring was continued until a clear solution was ob-tained. The two clear solutions in bottles A and B were mixed together, and the resultant mixture was stirred for 30 min until a homogeneous transparent synthesis gel was obtained.

In order to study the relationship between the syn-thesis gel composition and the membrane performance, a series of synthesis gels with different molar composi-tions were prepared.

Membrane preparation

The support and the synthesis gel were transferred into a 350 mL Teflon vessel in a stainless steel auto-clave. The support with two ends sealed by Teflon tape was held in a vertical position by a Teflon holder and completely immersed in the synthesis gel. The autoclave was then sealed, placed in a pre-heated convection oven, and heated at a synthesis temperature for a given time under a static condition. After crystallization, the auto-clave was cooled down to room temperature. The mul-lite tube supported membrane was taken out, and rinsed in deionized water at 100 ℃ for 1—2 h. After dried overnight in an oven at 80 ℃, the membrane was cal-cined in air to remove the organic templates at 500 ℃ for 30 h by using a muffle furnace with heating and cooling rates of 0.3 and 0.4 ℃/min, respectively. At last, a ZrS-1 zeolite membrane was prepared on the outer surface of the support.

The calcined ZrS-1 zeolite membranes were charac-terized by X-ray diffraction (XRD, Shimadzu XRD-D1, Cu Kα radiation) and scanning electron microscopy (SEM, JEOL 6335F), and the membrane surface chemical compositions were measured with an energy-dispersive X-ray spectral analyzer (EDS, JED 2200). The accompanying powder collected from the bottom of the autoclave was also characterized by XRD (Shimadzu XRD-6100), fourier transformed infrared spectroscopy (JASCO FT/IR-610) using a KBr pellet technique, and inductively coupled plasma-atomic emission spec-trometry (ICP Optima 4300DV, Perkin-Elmer).

Evaluation of the ethanol permselectivity

The ethanol permselectivity of ZrS-1 zeolite mem-brane was evaluated by the separation of ethanol/water system under a PV condition at 60 ℃. The ethanol/ water (5/95, w/w) mixture was used as feed. The PV installation used in this work was similar to that de-scribed in literature.10 The permeated vapor was col-lected by a cold trap cooled with liquid nitrogen, and analyzed with gas chromatography (Shimadzu GC-8A). The total flux (Q) and the separation factor (α) were calculated according to the following equations:

Q＝W/(A×t) (1)

α＝(Ye/Yw)/(Xe/Xw)×100% (2)

1694 Chin. J. Chem., 2009, Vol. 27, No. 9 CHEN et al.


where W is the mass of the permeated (kg); A is the ef-fective area of the zeolite membrane in contact with the feed (m2); t is the permeation time (h); Ye, Xe and Yw, Xw are the mass factors of ethanol and water in the perme-ated and in the feed, respectively.

Evaluation of the catalysis activity

The catalysis activity of the obtained ZrS-1 zeolite membrane was tested by the probe reaction of IPA oxi-dation under the PV condition at 60 ℃. An aqueous solution with IPA and H2O2 concentrations of 1.67 mol•L－1 (IPA/H2O2 molar ratio of 1/1) was used as feed. The catalytic oxidation reaction took place in the ZrS-1 zeolite membrane layer during the permeation of IPA and H2O2 molecules from the feed side to the permeated side. The total flux (Q) was calculated with Eq. 1. The conversion of IPA (CIPA) was calculated according to the following equation:

CIPA＝ (Wacetone,p/58)/[(Wacetone,p/58)＋ (WIPA,p/60)]×

100% (3)

where Wacetone,p and WIPA,p are the mass concentrations of acetone and IPA in the permeated side, respectively.

The compositions of the feed and permeated in the IPA oxidation reaction system were measured by a gas chromatograph (Shimadzu GC-8A).

Results and discussion

For the preparation of ZrS-1 zeolite, the most diffi-cult point is how to avoid the formation of insoluble ZrO2 species. Once it is formed, the Zr species can not enter the MFI framework and exists in the form of ex-tra-frame zirconium, which will result in the failure of the ZrS-1 zeolite preparation. In the case of preparing TS-1 zeolite synthesis gel, the similar problem was en-countered, while it could be overcome by the hydrolysis of titanium butoxide in H2O2 solution.13 Based on this experience, we tried to dissolve ZBOT into H2O2 solu-tion, but failed with the appearance of a large quantity of ZrO2 precipitate, which was attributed to that ZrO2

precipitate derived from the hydrolysis of ZBOT in H2O2 solution could not dissolve in non-alkaline solu-tion. To increase the alkalinity of the system, ZBOT was mixed with a little amount of TPAOH solution. Then the H2O2 solution was added into the ZBOT/ TPAOH mixture slowly, although the ZrO2 precipitate was formed at the beginning, it dissolved slowly under stirring. At last, a transparent solution containing solu-ble zirconium peroxide coordination compound was obtained. The other preparation processes of ZrS-1 zeo-lite membrane were the same as those of silicalite-1 and TS-1 zeolite membrane.

The obtained ZrS-1 zeolite membranes were charac-terized by XRD and FTIR. Figure 1 shows the XRD patterns of ZrS-1 zeolite membrane (Z2 in Table 1), accompanying powder and the support. The typical peaks

Figure 1 XRD patterns of (a) Z2 zeolite membrane, (b) the accompanying powder, and (c) the support.

of MFI structure were observed from the XRD patterns of ZrS-1 zeolite membrane and powder, and the peak at 2θ＝24.4° in XRD pattern of the accompanying powder indicated the typical monoclinic symmetry of ZrS-1 zeolite,19 which confirmed that the crystals growing on the support were ZrS-1 zeolite. IR spectrum (Figure 2) shows a characteristic absorption peak of the transition metal substituted zeolite at 960 cm－1, which confirmed the incorporation of zirconium into the lattice frame-work.18 The surface chemical compositions of Z2 membrane and ICP results of the accompanying powder (Table 1) exhibited that zirconium had incorporated into the zeolite framework, and the molar ratio of Zr/Si in

Figure 2 FTIR spectrum of the accompanying powder of Z2 zeolite membrane.

Table 1 Surface chemical compositions of Z2 membrane and ICP results of the accompanying powder

Z2 compositiona/wt% The accompanying powder b/

(mg•L－1)

SiO2 98.22 Si 551.24

ZrO2 1.78 Zr 4.96

Al2O3 — Al 0.06

Zr/Si molar ratio 0.0088 Zr/Si molar ratio 0.009a EDS analysis of the surface section of Z2 membrane. b ICP analysis of the accompanying powder of Z2 membrane.

Zeolite membrane Chin. J. Chem., 2009 Vol. 27 No. 9 1695


the membrane layer was similar to that in the accompa-nying powder.

Figure 3 shows the SEM micrographs of the surface and cross section of Z2 membrane. It was observed that the support was fully covered by the well inter-growing crystals with an average size of above 10 µm, and the crystals randomly grew on the surface of the support. The thickness of the membrane layer was about 20 µm.

Figure 3 SEM images of Z2 zeolite membrane surface (a) and cross section (b).

A series of ZrS-1 zeolite membranes were prepared under different conditions. Their PV performances in the separation of ethanol/water (5/95, w/w) mixture are listed in Table 2. Compared with silicalite-1 zeolite membrane (S1), ZrS-1 zeolite membranes (Z1, Z2), prepared with the synthesis gel at the ZrO2/SiO2 molar ratio of 0.005/1 or 0.01/1, had higher fluxes and slightly lower separation factors. Further increasing the ZrO2/ SiO2 molar ratio to 0.02/1, the resulting zeolite mem-brane (Z3) was of leak, which means that the inter- growth of zeoltie crystals in the membrane layer was not good. The reason was that adding zirconium species into the synthesis gel of silicalite-1 zeolite inhibited the nucleation and lowered the crystallization rate15 due to the larger ionic radius of Zr4＋ (0.59 Å) than that of Si4＋

(0.26 Å), thus impacting the zeolite crystal inter-growth on the support surface. When the ZrO2/SiO2 molar ratio in the synthesis gel was increased to 0.02/1, this effect became so serious that no dense ZrS-1 zeolite mem-brane was obtained.

It is known that condensed synthesis gel is favorable for accelerating the nucleation and zeolite crystal growth. So the molar ratio of H2O/SiO2 in the synthesis gel was decreased from 120/1 to 30/1, and the obtained zeolite membrane (Z4) displayed much higher flux of

1.28 kg/(m2•h) but low separation factor of 55. Based on the experience in the preparation of silicalite-1 and TS-1 zeolite membranes, increasing the alkalinity of the syn-thesis gel or decreasing the crystallization temperature, was beneficial to the zeolite nucleation and/or crystalli-zation. Herein, the content of TPAOH in the synthesis gel was increased from 0.12 to 0.3, and the crystalliza-tion temperature was decreased to 150 ℃ at the same time. However, the obtained zeolite membrane (Z5) exhibited much higher flux of 1.41 kg/(m2•h) but much lower separation factor of only 8. The surface and cross section of Z5 membrane were scanned by SEM. As shown in Figure 4 (a) and (b), a very poor inter-growth between the small crystals with an average size of 1.5 µm was observed from surface view of the Z5 zeolite membrane, and the zeolite membrane layer was too thin to assign its thickness. This indicated that high concen-tration and high alkalinity in the synthesis gel extremely improved the nucleation rate, and the large quantity of nuclei consumed the nutriment in the synthesis gel, and thus no enough nutriment was provided for the little zeolite crystal further growing. In addition, the low crystallization temperature (150 ℃) was not beneficial to the crystal growth.

Figure 4 SEM images of Z5 zeolite membrane surface (a) and cross section (b).

The catalysis activity of the obtained ZrS-1 zeolite membrane was tested by the probe reaction of IPA oxi-dation under a PV condition at 60 ℃. In an active zeo-lite membrane reactor, the catalysis reaction occured when the reactants permeated the zeolite membrane layer from the feed side to the permeated side, and the product existed mainly in the permeated side. It is ob-vious that the chemical reaction and separation process

Table 2 The preparation condition and the ethanol permselectivities of the ZrS-1 zeolite membranes

No. Synthesis gel molar composition Temperature/℃ Time/h Q/(kg•m－2•h－1) α

Z1 1.00SiO2∶0.005ZrO2∶0.17TPAOH∶120H2O 185 40 0.74 79

Z2 1.00SiO2∶0.01ZrO2∶0.17TPAOH∶120H2O 185 40 1.01 73

Z3 1.00SiO2∶0.02ZrO2∶0.17TPAOH∶120H2O 185 40 Leaka

Z4 1.00SiO2∶0.02ZrO2∶0.17TPAOH∶30H2O 185 40 1.28 55

Z5 1.00SiO2∶0.02ZrO2∶0.30TPAOH∶30H2O 150 72 1.41 8

S1 1.00SiO2∶0.12TPAOH∶120H2O 185 40 0.70 84 a One end of the as-synthesized TS-1 zeolite membrane was blocked and the other end was connected to the vacuum pump through the silicone tube and vacuum line, the well-distributed water layer spreading on the membrane surface did not disappear in 2 min, and this membrane was evaluated as a good membrane, conversely, the membrane as a leaking one.

1696 Chin. J. Chem., 2009, Vol. 27, No. 9 CHEN et al.


are efficiently combined together by the active zeolite membrane layer.

The catalysis results of ZrS-1 zeolite membranes are listed in Table 3. Silicalite-1 zeolite membrane (S1) without zirconium had no catalysis activity. While the ZrS-1 zeolite membranes displayed activities for IPA oxidation in some extend with the CIPA of 0.6%—35% accompanied with Q of 0.08—0.75 kg/(m2•h). Among all the membranes, the Z2 membrane showed the high-est CIPA of 35%. It was noted that, with the increase of the ZrO2/SiO2 molar ratio in the synthesis gel from 0.005/1 to 0.1/1, the CIPA and Q increased from 28% to 36% and 0.08 to 0.16 kg/(m2•h), respectively. However, further increasing the ZrO2/SiO2 molar ratio to 0.02/1, the resulting membranes (Z4 and Z5) had low CIPA of only 0.6% and 3.4%. Although high Q was obtained from Z4 and Z5 membranes (0.43 and 0.72 kg/(m2•h), respectively), it was not our attentions for the active zeolite membrane reactor. It is known that the zirco-nium incorporated MFI framework is the catalysis ac-tive site. The above experimental results showed that, compared with Z1 and Z2 membranes, the effective content of zirconium in the zeolite framework decreased in Z4 and Z5 membranes although the molar ratio of ZrO2/SiO2 in the synthesis gel was increased to 0.02/1. This was confirmed by IR spectra of Z4 and Z5 mem-branes (Figure 5). The characteristic absorption peak at 960 cm－1, which was attributed to the asymmetric stretching vibration of Zr—O—Si bond in the ZrS-1 zeolite framework, became a weak shoulder peak for Z4 and Z5 membranes. Therefore, Z4 and Z5 membranes displayed much lower catalysis activity than Z1 and Z2

Table 3 Catalysis activities of the ZrS-1 zeolite membranes

No. WIPA,f

a/ wt%

Qb/ (kg•m－2•h－1)

WIPA,p/ wt%

WAcetone,p/ wt%

CIPA/ %

Z1 9.92 0.08 34.15 18.02 28

Z2 9.78 0.16 38.94 14.84 35

Z4 9.89 0.43 86.43 0.55 0.6

Z5 9.75 0.72 17.94 0.62 3.4

S1 9.65 0.43 86.43 0 0 a The mass concentration of IPA in the feed; b the total flux.

Figure 5 FTIR spectra of the accompanying powders of Z4 and Z5 zeolite membranes.

zeolite membranes. Further study is in progress. In conclusion, a series of novel ZrS-1 zeolite mem-

branes were hydrothermally prepared on the mullite porous support in 150—185 ℃ for 40—72 h by an “in situ” method. The obtained ZrS-1 zeolite membranes showed high fluxes and slightly lower ethanol/water separation factors for ethanol/water (5/95, w/w) system under a PV condition at 60 ℃. Moreover, the mem-brane displayed catalysis activity for the IPA oxidation with the IPA conversion of 0.6%—35 % accompanied with the flux of 0.08—0.75 kg/(m2•h).

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pervaporation separation and catalysis activity of novel zirconium silicalite-1 zeolite membrane

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