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Journal of Membrane Science 316 (2008) 3–17

Review

Microwave synthesis of zeolite membranes: A review

Yanshuo Li, Weishen Yang ∗State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

Received 2 July 2007; received in revised form 28 August 2007; accepted 31 August 2007Available online 14 September 2007

bstract

Significant progress has been achieved in the last years on microwave synthesis of zeolite membranes. In many cases, microwave synthesis hasroven to remarkably reduce the synthesis time. In addition, microwave synthesis could also result in different membrane morphology, orientation,omposition, and thus the different permeation characteristics as compared with those synthesized by conventional heating. This review attempts toummarize the obtained progress in microwave synthesis of zeolite membranes. Some topics are discussed, including: (1) case study of microwaveynthesis of zeolite membranes, e.g. LTA, MFI, AFI, and other types of zeolite membranes; (2) differences between conventional and microwave
ynthesis; (3) formation mechanism and the so called “specific microwave effect” in the case of microwave synthesis of zeolite membranes; (4)caling-up of zeolite membrane production by employing microwave heating. The latter three topics are mainly focused on LTA type zeoliteembranes. Concluding remarks and future perspective are also suggested in the end.2007 Elsevier B.V. All rights reserved.
eywords: LTA; MFI; AFI; Zeolite membrane; Microwave synthesis; Non-thermal effect

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. Brief introduction of microwave-assisted synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1. Basic physical principles of microwaves and microwave heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2. Advantages of microwave heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3. Specific microwave effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3. Microwave synthesis of zeolite membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1. LTA (NaA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. MFI (ZSM-5 and Silicalite-1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.3. AFI (AlPO4-5 and SAPO-5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4. Others (SOD, FAU and ETS-4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Comparison between microwave heating and conventional heating synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105. Formation mechanism of zeolite membranes under microwave irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116. Thermal and non-thermal effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127. Scaling-up of zeolite membrane production by adopting microwave heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138. Concluding remarks and future perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

∗ Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447.E-mail address: [email protected] (W. Yang).URL: http://yanggroup.dicp.ac.cn (W. Yang).

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376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2007.08.054

. Introduction

Since the mid of 1990s, owing to the potential molecular siev-ng action, controlled host–guest interactions and high thermalnd chemical stability, the preparations, characterizations and

mailto:[email protected]

dx.doi.org/10.1016/j.memsci.2007.08.054

4 Y. Li, W. Yang / Journal of Membrane Science 316 (2008) 3–17

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Fig. 1. The accumulated number of published articles on microwav

pplications of membranes, films and coatings of zeolite andeolite-like materials (in short called “zeolite membranes” inhis review) have been extensively investigated. A large numberf review articles [1–13] and several book chapters [14,15] haverovided extensive coverage of this subject.

In the last years, heating and driving chemical reactions byicrowave energy has been an increasingly popular theme in

he scientific community, and so in the fields of zeolite and zeo-ite membranes. The pioneer work on microwave synthesis ofeolite can be traced to 1988. In a US patent, Mobil researcherslaimed the microwave synthesis of microporous zeolites, suchs zeolite NaA and ZSM-5 [16]. The first published articlen microwave synthesis of zeolite appeared in 1993, in whichansen and co-workers reported that microwave-assisted crys-allization of Y-type and ZSM-5 zeolite could be finished in a

uch shorter synthesis time and free of undesired phase as com-ared with conventional heating [17]. Since then, the number ofublications began to increase year by year, especially after theid-1990s (Fig. 1). In 1995, Caro and co-workers reported the

ynthesis of large AlPO4-5 single crystals by microwave heating.y embedding these AlPO4-5 crystals in a Ni grid, a mem-rane with one-dimensional pore structure could be obtained18]. After then, there has been a steadily growing interest inicrowave synthesis of zeolite membranes (Fig. 1). In 1998,undy produced an excellent review on the microwave synthe-

is and modification of zeolites [19]. Recently, Tompsett et al.20] dedicated a comprehensive review on the microwave syn-hesis of nanoporous materials and summarized the preparationf zeolites, mixed oxides and mesoporous molecular sieves bymploying microwave energy.

This present review attempts to summarize the obtainedrogress in microwave synthesis of zeolite membranes. Begin-ing with a brief introduction of microwave and microwave-ssisted synthesis, the so far reported literatures on microwaveynthesis of zeolite membranes will then be summarized. Fur-hermore, the comparison between conventional and microwaveynthesis and the “specific microwave effect” will be discussed.
inally, an outlook on the future development of microwaveynthesis of zeolite membranes will be given. Based on theesults obtained in our laboratory, special attention is given tohe microwave synthesis of LTA type zeolite membranes.
eosi

isted synthesis of zeolites and zeolite membranes (Scopus search).

. Brief introduction of microwave-assisted synthesis

After several years of joint efforts of the chemists, materialcientists, and microwave engineers, microwave-assisted syn-hesis (especially microwave assistant organic synthesis) has

atured into a highly useful technique, and some review articlesere published which are well worth reading [19–27]. Mingos

nd co-workers [26] have given a thorough explanation of thenderlying theory of microwave dielectric heating. Nuchter etl. [22] have given a critical technology overview and focusedostly on reaction engineering in microwave field. In the follow-

ng section, we will briefly describe the basic physical principlesf microwave chemistry to the membranologists who are notamiliar with this subject. For more general information on theubject of microwave chemistry, the above-cited reviews areecommended.

.1. Basic physical principles of microwaves andicrowave heating

Microwaves lie in the electromagnetic spectrum betweennfrared waves and radio waves. They have wavelengths between.01 and 1 m, and operate in a frequency range between.3 and 30 GHz. The typical bands for industrial applica-ions are 915 ± 15 and 2450 ± 50 MHz. To our knowledge,ll the reported microwave chemistry experiments are cur-ently conducted at 2450 MHz (the corresponding wavelengths 12.24 cm). One reason is that near to this frequency, the

icrowave energy absorption by liquid water is maximal.nother probable reason is that 2450 MHz magnetrons areostly often used in the available commercial microwave chem-

stry equipments.Interaction of dielectric materials with microwaves leads to

hat is generally described as dielectric heating due to a netolarization of the substance. There are several mechanismshich are responsible for this, including electronic, ionic, molec-lar (dipole), and interfacial (space-charge) polarization. For
asier understanding, it can be described that in the presencef an oscillating field, dipolar molecules try to orient them-elves or be in phase with the field. However, their motions restricted by resisting forces (inter-particle interaction and

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5Table 1LTA zeolite membranes from microwave synthesis

Substrate Synthesisstrategy

Synthesis Main results MembranePerformances

Microwaveoven type

Reference

Composition Preparation procedure

Copper andsilicon wafer

In situ 3:1:2:200, Na2O/Al2O3/SiO2/H2O 120 ◦C for 1 min followed by 100 ◦Cfor 30–240 min

On Cu substrates a dense film was formed;but on stainless steel no layer-like zeolite filmwas formed

N.R.a Goldstar Er-4010 [68]

Alumina Seeded 3:1:2:200, Na2O/Al2O3/SiO2/H2O Heated to 90 ◦C in 1 min; held for5–40 min

Four stage under microwave radiation:absorbing (0–15 min), nucleation,crystallizing (15–25 min) and dissolving(>25 min)

H2/C3H8 permselectivity was6.23 at 25 ◦C

Modified domesticmicrowave oven

[104]

�-Al2O3 modifiedSeeded

Gel: 3:1:2:200,Na2O/Al2O3/SiO2/H2O

Heated to 90 ◦C in1 min; held for5–45 min

The membrane formed on the�-Al2O3/�-Al2O3 substrate had a higher gaspermeance

HighestH2 permeance was2.64 × 10−6 mol s−1 m−2 Pa−1

Modifieddomesticmicrowave oven

[75]

�-Al2O3 and�-Al2O3

Sol: 5:1:50:1000,SiO2/Al2O3/Na2O/H2O

The membrane synthesized in sol had higherpermeance than that in gel

Alumina Seeded 3–6:2:1:150,Na2O/SiO2/Al2O3/H2O

Fluxed at 90 ◦C for 4–25 min Aging the gel favored the formation of NaAmembranes in short time; high alkalinity andsodium cation concentration disfavored theformation of membranes

N.R. NN-K580MFS P = 100 W [77]

Alumina Seeded 3:2:1:150, Na2O/SiO2/Al2O3/H2O Fluxed at 90 ◦C for 25 min H2/N2 selectivity increased with increasingsynthesis time.

H2/N2 selectivity was 3.97; H2

permeance was51.5 × 10−8 mol s−1 m−2 Pa−1

NN-K580MFS P = 100 W [79]

Alumina In situ 1:0.85:3.0:200,Al2O3/SiO2/Na2O/H2O

Synthesis in microwave oven at250 W for 15–20 min; dried inmicrowave oven at 120 W for 10 min

Microwave heating increased the reaction rategreatly; the thickness of the membranes couldbe controlled by varying the amount of thereaction mixture

N.R. N.R. [86]

Alumina In situ 5:1:50:1000,SiO2/Al2O3/Na2O/H2O

Aging at 50 ◦C for 7 h in air oven,and then microwave synthesis at90 ◦C for 25 min

In situ aging time, temperature andmicrowave heating time have influence on themembrane quality, in situ aging was necessaryfor microwave synthesis of LTA zeolitemembranes without seeding

H2/N2 and H2/C3H8

permselectivities were 5.60 and9.17, respectively.

HR-8801MHaier [66]


Aging at 50 ◦C for 7 h in oven, andthen microwave synthesis at 90 ◦Cfor 0–35 min

The formation process of LTA membranesunder microwave field was comprehensivelystudied; the formation mechanism of LTAzeolite membranes was proposed

N.R. HR-8801MHaier [64]


MH: aging at 50 ◦C for 7 h in oven,and then microwave synthesis at90 ◦C for 25 min; CH: aging at 50 ◦Cfor 7 h in oven, and thenconventional synthesis at 90 ◦C for3 h

Compared with conventional heating, LTAzeolite membranes with few non-zeolite porescan be synthesized by using microwaveheating

For H2O/IPA pervaporation,microwave synthesizedmembranes maintained highseparation factor even whenwater content was lower than1%

HR-8801M Haier [65]

Alumina Seeded 50:1:5:1000,Na2O/Al2O3/SiO2/H2O

Heated to 90 ◦C in 60 s; held for15 min

The synthesis time was 8–12 times shorterthan conventional heating; the permeance ofthe microwave synthesized zeolite membranewas increased by four times, while keepingcomparable permselectivity

H2/n-C4H10 permselectivitywas 11.8; H2 permeance was213 × 10−8 mol m−2 s−1 Pa−1


[84]



Microwave heating greatly accelerated thezeolite membrane formation; microwavesynthesized membrane was thinner than theone synthesized by conventional heating

Permeance of O2 and N2 were0.68 and0.66 × 10−6 mol m−2 s−1 Pa−1,respectively


[83]



The synthesis time was greatly reduced whilepermeance enhanced compared withconventional heating synthesis; multi-stagesynthesis resulted in the formation of moreimpurity phase

O2/N2 and H2/n-C4 H10

permselectivities were 1.02 and11.8, respectively; H2

permeance was21 × 10−7 mol m−2 s−1 Pa−1


[81]

a Not reported.

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Y. Li, W. Yang / Journal of Me

lectric resistance), which restrict their motion and generateeat.

Generally, materials can be classified into three categoriesased on their interaction with microwaves: (1) materials thateflect microwaves, typified by bulk metals and alloys, e.g. cop-er; (2) materials that are transparent to microwaves, typifiedy fused quartz, several glasses, ceramics, Teflon, etc.; and (3)aterials that absorb microwaves which constitute the most

mportant class of materials for microwave synthesis, e.g. aque-us solution, polar solvent, etc. Dissipation factor (often calledhe loss tangent, tan δ), a ratio of the dielectric loss (loss factor) tohe dielectric constant, is used to predict materials behavior in a

icrowave field. The microwave absorption ability of a materials directly proportional to its dissipation factor.

.2. Advantages of microwave heating

Conventional heating (i.e. conduction, convection, and heatadiation) has a heat source on the outside and relies onransferring the heat to the surface of the material and then con-ucting the heat to the middle of the material. Compared withonventional heating, microwave dielectric heating has the fol-owing advantages for chemical synthesis (thermal effects of

icrowave): (1) the introduction of microwave energy into ahemical reaction can lead to much higher heating rates thanhose which are achieved conventionally; (2) the microwavenergy is introduced into the chemical reactor remotely withoutirect contact between the energy source and the reacting chem-cals; (3) it is volumetric and instantaneous (or rapid) heatingith no wall or heat diffusion effects; (4) it can realize selec-

ive heating because chemicals and the containment materialsor chemical reactions do not interact equally with microwaves;5) “hot spots” yielded on local boundaries by reflections andefractions may result in a “super-heating” effect, which cane described best as local overheating and is comparable tohe delayed boiling of overheated liquids under conventionalonditions.

.3. Specific microwave effect

Firstly, it should be pointed out that the energy of microwavehoton (≈1 J/mol) is far too small to break typical chemi-al bands (usually >300 J/mol, even the energy of hydrogenond is several tens of Joules per mole). However, the numer-us experimental observations on reaction rate enhancementn microwave field, especially solid-state reactions, could note interpreted by thermal effects of microwave solely, whicheads to the concepts of non-thermal effects, also called “spe-ific microwave effect”. According to the explanation of Boosket al. [28], “thermal” refers to interactions resulting in increasedandom motion of particles (e.g. atoms, molecules, ions, or elec-rons) where the kinetic energy statistics of such fluctuations areepresented by a single thermodynamic equilibrium distribu-
ion (i.e. Maxwell–Boltzmann, Bose–Einstein, or Fermi–Dirac).Non-thermal” effects refer to interactions resulting in non-quilibrium energy fluctuation distributions or deterministic,ime-averaged drift motion of matter (or both). In a previ-
tr

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ne Science 316 (2008) 3–17

us review, Langa et al. [29] suggested the so-called “specificicrowave effect”. Afterward, Jacob et al. [30] published an

xcellent review on the synthetic results to which the microwaveffect has been attributed. Loupy and Perreux [31] publishedtentative rationalization of non-thermal effects in organic

ynthesis, and Kuhnert has given a highlight to discuss theon-thermal microwave effect in organic synthesis [32]. Kappe,owever, concluded that all the speculations on special and non-hermal effects in microwave heating has no basis when takingnto account the increased temperatures caused by super-heatingr concentration effects [21]. Nevertheless, this topic is still aontroversial matter and open to interpretation.

. Microwave synthesis of zeolite membranes

Commonly used strategies for zeolite membrane synthesisan be classified into three categories: in situ hydrothermal syn-hesis [33–45], secondary (seeded) growth synthesis [41,46–57],nd vapor phase transport synthesis [58–62]. Microwave tech-iques combining with these strategies could create some novelynthesis routes, among which, in situ microwave synthe-is [18,43,63–70] and microwave assistant secondary growth55,71–85] have been reported for membrane preparation. LTA,

FI, AFI, FAU, SOD, and ETS-4 types of zeolite membranesave been successfully synthesized by microwave heating. Thisection is intended to be a comprehensive collection of theseeports although accidental omission is inevitable.

.1. LTA (NaA)

Due to the relative simple synthesis gel/solution (contain-ng no templates) and low crystallization temperature (usually100 ◦C), LTA type zeolite membranes are most extensivelytudied in microwave synthesis of zeolite membrane research.able 1 summarizes the microwave synthesis conditions andain experimental results of LTA zeolite membranes.NaA zeolite membranes were microwave synthesized on alu-

ina support by Han et al. in 1999 [86]. Microwave heatingreatly accelerated the crystallization rate and reduced the syn-hesis time to 10 min. It was reported that the thickness of the

embranes could be controlled by varying the amount of theeaction mixture. This indicates that the membrane was formedy physical deposition of crystals from the bulk onto the support.o separation data was reported for this membrane.In our group, the efforts to synthesize of NaA zeolite mem-

ranes started in the late 1990s. Our initial idea is to take thedvantages of fast microwave synthesis to shorten the zeoliterystallization time so as to reduce the membrane thickness ando improve the flux, as illustrated in Fig. 2. Here, pre-seeding theupport with microwave synthesized NaA zeolite was needed tovercome the nucleation-related bottleneck [87,88]. Comparedith conventional heating, the synthesis time was shortened by–12 times by using microwave heating, and the permeance of
he membrane was increased by 4 times, while keeping compa-able permselectivity for H2/n-C4H10 [81,83,84].
In order to further improve the permeance and permselec-ivity of zeolite NaA membranes, we considered modifying the

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7

Table 2MFI zeolite membranes from microwave synthesis


Synthesis Main results Membraneperformances

Microwaveoven type

Reference


WO3/TiO2/Fe2O3/SiO2

coated glassIn situ 1:0.2:22 or 100,

TEOS/TPAOH/H2OHeated for 90 min at 150 ◦C;calcinated in air at 500 ◦C

The crystal exhibited the sameshape and orientation regardlessof the nature of the undercoatings; with the increment ofthe dielectric constants, thesurface coverage increased

N.A.a MARS-5 CEMP = 600 W

[113]

Silicon wafer In situ 6.5:1:675,SiO2/(TPA)2O/H2O

Rapid heating and cooling;temperature: 135–200 ◦C; duration:3–40 min

Microwave heating favored agreat control over thetemperature profile

N.A. SharpR-10R P = 50 W [93]

Cordierite honeycombsubstrates and aluminadisks

In situ 100:10:1800,SiO2/TPAOH/H2O

Heated at full power for 2–15 min,and then hydrothermal synthesis at150 ◦C for 3–24 h; calcinated at600 ◦C for 2 h

After 10 min microwaveheating and 6 h hydrothermalsynthesis, membrane with74 �m thickness was obtained;increasing the microwaveheating time produced denserand smaller zeolite crystals;increasing microwave powerproduced smaller numbers oflarger crystals

N.A. Domestic microwaveoven; P = 800 W

[73]

Alumina Seeded 25:3:1500:100,SiO2/TPAOH/H2O/EtOH

Crystallization temperature:120–180 ◦C, duration: 30–150 minCalcinated at 450 ◦C for 4 h

Microwave synthesis was apromising method for the rapidsynthesis of MFI membraneswith relatively high permeanceand selectivity. (1 0 1) orientedmembrane was synthesized at160 ◦C, while c-orientation at180 ◦C

Membrane with highern/i-butane selectivity(40–50) and mediumpermeance wassynthesized at highertemperature (160 ◦C)

ETHOS 1600,Milestone;Pmax = 400 W

[72]

Alumina Seeded 25:3:1500:100,SiO2/TPAOH/H2O/EtOH

Crystallization temperature: 160 ◦C;duration: 30–150 min; Calcinated at550 ◦C or ozone treated at 200 ◦C

MFI zeolite membranes with a(1 0 1) preferred orientationwere prepared at 160 ◦C for 2 h;three methods were used toeliminate the template: ozonetreatment at 160 ◦C for 1.5 h,calcination with a slow heatingrate (0.2 ◦C/min), andcalcination with a rapid heatingrate (5 ◦C/min)

The ideal selectivity forn/i-C4H10 lies in therange 47–50 at 20–50 ◦C;the same selectivity butlower permeance wasobtained for the ozoneactivated membrane

ETHOS 1600,Milestone;Pmax = 400 W

[71]

a Not reported.


icrow

mtoaH

sggtc

reaRiopwwwsigigqheboil

ma

3

mSMtzpmo

rScmesmeSttrma

Fig. 2. Comparative synthesis model of zeolite membrane by m

acroporous alumina support with a thin mesoporous top-layero prevent the penetration of the reagent into the support. Thebtained result coincided with our assumption. The H2 perme-nce was increased by 1.5–3.3 times, and the permselectivity for2/n-C4H10 was also increased to some extent [75].Cheng et al. also carried out the investigation on microwave

ynthesis of A-type zeolite membranes [77,79,89]. Secondaryrowth strategy was also adopted by them, but silica-aluminael instead of clear solution was used for zeolite crystalliza-ion. It was reported that the high alkalinity and sodium cationoncentration disfavored the growth of membranes [77].

In our previous investigation and most of the otheresearchers’ study, it was found that surface seeding is nec-ssary to promote the formation of NaA zeolite membranesnd suppress the formation of impurity phases [54,81,90–92].ecently, a new method called “in-situ aging-microwave heat-

ng” method was developed for zeolite membrane synthesis byur group. High quality NaA zeolite membranes with H2/N2ermselectivity of 5.6 were successfully microwave synthesizedithout seeding by this method [64,66]. LTA zeolite membranesith excellent pervaporation performance were also preparedith high reproducibility. This method decouples two succes-

ive steps in the formation of zeolite membranes. The first steps the rearrangement of synthesis mixture and the formation oferm nuclei on the support surface, which were obtained byn situ aging. The second step is the nucleation and crystalsrowth on the support, which were achieved by the conse-uential crystallization under fast and homogeneous microwaveeating. Because artificial seeding was replaced by in situ het-rogeneous nucleation, a much higher nucleation rate favored
y microwave heating greatly enhanced the number of nuclein the support surface. As a result, NaA zeolite crystals couldnter-grow together in a short time, and defect-free LTA zeo-ite membranes were thus formed. This convenient and effective
3

s

ave heating and conventional heating (redrawn from Ref. [83]).

ethod has made the mass production of LTA zeolite membranesreality. This aspect will be discussed in Section 7.

.2. MFI (ZSM-5 and Silicalite-1)

Although the largest amount of work has been done onembranes with the framework topology of MFI (ZSM-5 andilicalite-1), the articles published on microwave synthesis ofFI type zeolite membranes are limited. This is probable due

o the relative high temperature (usually >150 ◦C) for MFIeolite crystallization and the fast degradation of the TPA+ tem-late under microwave conditions [19]. Table 2 summarizes theicrowave synthesis conditions and main experimental results

f MFI type zeolite membranes.Microwave synthesis of Silicalite-1 zeolite membranes was

eported by Koegler et al. Films containing oriented 100 nmilicalite-1 crystals have been in situ synthesized on a sili-on wafer using rapid heating and cooling [93]. Combiningicrowave heating with secondary growth strategy, Motuzas

t al. synthesized Silicalite-1 zeolite membranes on aluminaupports with (1 0 1) and (0 0 1) preferred orientation [72]. Theorphology, thickness, homogeneity, crystal preferential ori-

ntation (CPO) and single gas permeation properties of theilicalite-1 membranes have been studied in relation to the syn-

hesis parameters. The ideal selectivity for n/i-C4H10 was inhe range 40–50. Recently, they developed an ultra-rapid andeproducible synthesis method for thin and good quality MFIembranes by coupling the microwave-assisted synthesis withrapid template removal method (ozone treatment) [71].

.3. AFI (AlPO4-5 and SAPO-5)

In 1995, Caro et al. [18] reported the microwave synthe-is of large AlPO4-5 single crystals. These crystals could be

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9Table 3AFI zeolite membranes from microwave synthesis



Microwaveoven type

Reference


Ni grid Seeded 1:1.3:1.6:1.3:425:6,Al2O3/P2O5/TEA/HF/H2O/i-C3H7OH

Heated to 180 ◦C for 60 s Microwave greatly accelerated theformation of AlPO4-5 zeolitewithout any impurity; orientedmembrane showed excellentselectivity based on the molecularsize

Separation factor:n-butane/triisopropyl-benzene = 1220;n-heptane/triethylbeneze = 105;n-eptane/mesitylene = 1.7;n-heptane/toluene = 0.8

MLS1200;Pmax = 1200 W

[18]

Anodizedalumina

Seeded 1:1.3:2.4:1.3:425,Al2O3/P2O3/TEA/HF/H2O

Heated at 180 ◦C;ultrasonicated, washed anddried at 90 ◦C for 1 h

c-Oriented growth of AlPO4-5 wasobtained by microwave heating;microwave heating inducedheterogeneous nucleation in the poresystems of anodized aluminasubstrate

N.A.a MDS2000;CEM

[114]

AnodicAlumina

Seeded 1:1.3:0.1–0.45:2.4:1.3:425,Al2O3/P2O3/TEOS/TEA/HF/H2O

Heated at 185 ◦C for2.5–4.5 min; ultrasonicated,washed and dried at 90 ◦C for1 h

The degree of c-orientation andzeolite coverage varied withexperimental conditions

N.A. MDS2000;CEM

[85]

Gold coatedquartz crystalmicrobal-ances(QCM)

Seeded 1:1.28–1.32:0.7–1.0:50–900,Al2O3/P2O5/(TEA)2O/H2O

Reaction temperature:90–150 ◦C; duration: 1–10 min;microwave power:300–1000 W; calcinated inoxygen at 350 ◦C over night

Morphology, orientation, and size ofthe AlPO4-5 crystals could becontrolled by experimentalconditions; microwave heating led tohomogenous film growth andmicroporosity

N.A. Qwave3000;Questron

[69]

a Not reported.

Table 4SOD, FAU and ETS-4 zeolite membranes from microwave synthesis

Membranetype

Synthesisstrategy


Microwaveoven type

Reference


SOD In situ 50:1:5:1000,Na2O/Al2O3/SiO2/H2O

Heated to 90 ◦C in 60 s andheld for 15–45 min;multi-stage synthesis

Microwave heating not only promotedthe formation of SOD membrane but alsoinhibited the formation of impurity

H2/n-C4H10 permselectivity >1000;permeance of H2 was1.14 × 10−7 mol m−2 s−1 Pa−1


[76]

SOD In situ andseeded

1:5:50:1000 or 1:0.85:3:200or 1:3.5:50:1000 orAl2O3/SiO2/Na2O/H2O

Reaction temperature:160–200 ◦C; duration:15–180 min

The pre-seeding/secondary growthmethod, involving seeds prepared bymicrowave heating, was used tosynthesize larger area and homogeneousSOD membranes with goodreproducibility

N2 and He permeances were typicallyin the range0.5–2 × 10−8 mol m−2 s−1 Pa−1;highest selectivity was 6.2

ETHOS 1600Milestone P = 250 W

[67]

FAU Seeded 1:10:14:840,Al2O3/SiO2/Na2O/H2O

Reacted at 120 ◦C for 2 h;repeated twice

Dense inter-grown FAU layers with Si/Alratios of 1.3–1.8 and layer thicknesses inthe range of 0.8–6 �m were obtained;microwave heating resulted in a higherSi/Al ratios of the membrane

Permeance order:H2 > CH4 > N2 > O2 > CO2 > n-C4H10 > SF6 at 23 ◦C, N2/CO2

permselectivity was 8.4; CH4/CO2

permselectivity was 3.5

MLS 1200 MLSGmbH Leutkirch

[80]

ETS-4 Seeded 16:1:11.33:761.3:8.5,NaOH/TiO2/SiO2/H2O/H2O2

(pH 12.1)

Pulse laser deposition wasused for surface seeding;reacted at 235 ◦C for20–60 min

Microwave heating greatly acceleratedthe reaction rate; meanwhile it could curbthe formation of impurity phase

N.A.a MARSX-press CEM [74]

a Not reported.

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0 Y. Li, W. Yang / Journal of Me

ormed in as little time as 60 s, and were found to be ideal forne-dimensional molecular sieving. Mintova et al. [69] usedicrowave heating to prepare thin films of AlPO4-5 on gold

oated quartz crystal microbalances (QCMs). The temperature,icrowave heating time, power, and aging time affected the

hickness and orientation of the zeolite crystals. Films preparedy conventional heating showed no crystal orientation. Tsai et al.85] prepared well-aligned SAPO-5 membrane using microwaveeating on an anodized aluminum substrate. Various sets of sam-les with different silicon contents in reactant gels and differenteaction times in secondary microwave hydrothermal heatingere studied. The effects of various synthesis parameters on theegree of preferred orientation along the c-crystal axis of theFI structure and the zeolite coverage on alumina support wereiscussed.

Table 3 summarizes the microwave synthesis conditions andain experimental results of AFI type zeolite membranes.

.4. Others (SOD, FAU and ETS-4)

Some other types of zeolite membranes have also been syn-hesized by adopting microwave heating, as summarized inable 4.

Julbe et al. [67] have evaluated the potential of microwaveeating for synthesizing sodalite membranes on tubular sup-orts. They found that the dissolution of the �-alumina supportevealed a disturbing phenomenon for both the reproducibil-ty and scaling-up of the direct membrane synthesis. In ordero overcome this problem, the authors developed another indi-ect strategy, the pre-seeding/secondary growth method, whichllowed the synthesis of larger and homogeneous samples withgood reproducibility.

We have also successfully synthesized high quality pureydroxy-sodalite zeolite membranes on �-Al2O3 support bymicrowave-assisted in situ crystallization method [76]. Only5 min was needed by this method, more than eight times fasterhan by the conventional hydrothermal synthesis method. A pureydroxy-sodalite zeolite membrane was easily synthesized byicrowave heating method, while a zeolite membrane, which

onsisted of NaX zeolite, NaA zeolite and hydroxy-sodalite zeo-ite, was usually synthesized by conventional heating method.he pure hydroxy-sodalite zeolite membrane synthesized byicrowave heating method was found to be well inter-grown.he gas permeation results showed that the H2/N2 permselec-

ivity of the hydroxy-sodalite zeolite membrane was larger than000.

Weh et al. synthesized composite membranes that consistedf thin faujasite layers on the surfaces of porous �-Al2O3 supportisks by using a two-step synthesis (seeded growth) method [80].wo different kinds of heating were used in the secondary growthtep, either conventional heating or microwave heating.

Engelhard titanosilicate (ETS-4) has octahedral coordina-ion in addition to tetrahedral coordination, and recently it was
uccessfully synthesized as a membrane form by pulsed lasereposition (for seeding) and microwave treatment (for secondaryrowth) [74]. Microwave irradiation shortened the synthesisime considerably as compared to conventional heating, which
5atc

ne Science 316 (2008) 3–17

esulted in the rapid synthesis of ETS-4 in less than 1 h comparedo 36–48 h for the traditional synthesis.

. Comparison between microwave heating andonventional heating synthesis

In the above section, it has been shown that compared withonventional heating, microwave heating can remarkably reduceynthesis time. This is one of the most frequently mentioneddvantages of microwave synthesis. In this section, we willainly discuss the differences in membrane characteristic and

erformance of the membranes synthesized by microwave heat-ng and conventional heating.

In the synthesis of porous materials, it has been reportedhat the microwave synthesis method could provide an efficientay to control particle size distribution [94–97], phase selec-

ivity [98], and macroscopic morphology [96,99–102]. In thease of zeolite membranes, microwave synthesis also leads toifferent membrane characteristics from those synthesized byonventional heating [64–66,80–82,84].

Our recent studies found that microwave assistant hydrother-al synthesis can be an efficient tool for the morphology control

f LTA type zeolite membranes. As shown in Fig. 3, a LTA zeo-ite membrane synthesized by conventional heating is composedf well-shaped cubic LTA crystals, while the membrane synthe-ized under pulsing microwave irradiation consists of sphererains without well-developed crystal faces. Under continu-us microwave irradiation, however, bundles of layered crystalsorm the tightly packed membrane.

Besides this morphological difference, microwave synthesisan also lead to a compositional difference of the synthesizedeolite membranes. For examples, Weh et al. [80] reported thathen using microwave heating instead of conventional heating,

he as-synthesized FAU type zeolite membrane had a slightlyncreased Si/Al ratio (from 1.4 to 1.8). In our study, we have alsoound that microwave heating is favorable for the synthesis of theigh silica analogue of FAU type zeolite membranes, i.e. Y-typeeolite membranes. In our researches on microwave synthesis ofTA type zeolite membranes, it was interesting to find that thes-synthesized NaA zeolite membrane had a Si/Al ratio of 1.43s determined by XPS characterization, which is higher than theommon value of NaA zeolite [64–66].

Our initial researches on microwave synthesis of zeoliteembranes mainly focused on how to reduce synthesis time and

mprove the membrane permeance by introducing microwaveeating technique. Although NaA zeolite membranes synthe-ized by microwave heating in short time usually had highermeance, the permselectivity of the membranes was not veryigh (permselectivity for H2/N2 was just around the Kundseniffusion ratio), as shown in Table 1. Our recent progress inhe in situ microwave synthesis of zeolite membranes (“in-situging-microwave heating” method [66]) improved the mem-rane selectivity remarkable (permselectivity for H2/N2 was
.60) while maintained the membrane permeance high. Wettributed this to the distinctive morphology of microwave syn-hesized LTA zeolite membranes (sphere grains with undefinedrystal facets) from that of conventional heating synthesized

Y. Li, W. Yang / Journal of Membrane Science 316 (2008) 3–17 11

pulsed and continuous) heating synthesized NaA zeolite membranes.

obgstw[

catt01itb1pthlitamhdh

tmt6

5m

b

Fig. 3. SEM images of conventional heating and microwave (both

ne (well-shaped cubic crystals) (see Fig. 3). As pointed outy Mcleary and Jansen, the ever-occurring triangularly shapedap between growing crystals on the support would lead to theystematic imperfections of the synthesized membranes, andhe formation of sphere grains with undefined crystal facetsas necessary to achieve a pinhole-free continuous zeolite layer

103].The compactness difference between microwave heating and

onventional heating synthesized NaA zeolite membranes waslso confirmed by pervaporation separation test [65]. As forhe membrane synthesized by microwave heating (Fig. 4), ashe water concentration in feed (WF) decreased from 9.6 to.2 wt.%, the water concentration in permeate (WP) varied from00.0 to 90.0 wt.%, with the permeation flux decreasing approx-mately linearly from 0.86 to 0.08 kg m−2 h−1. However, forhe membrane synthesized by conventional heating, when WFroke through 2.0 wt.%, WP decreased rapidly. When WF was.0 wt.%, WP was only 18.0 wt.%, and at the same point, theermeate flux abnormally increased dramatically. This indicatedhat the membrane synthesized by conventional heating mightave quite a few non-zeolitic pores (in nano-size). When WF wasow, the adsorbed water were not enough to block the defects tonhibit isopropanol permeation, thus much isopropanol begano pass through the pores, which leaded to a decrease in WPnd increase in permeate flux. The quality difference betweenicrowave heating synthesized membranes and conventional

eating synthesized ones through the same strategy should beue to the different heating mechanism between microwaveeating and conventional heating.

In our studies on FAU type zeolite membranes, we also foundhat microwave heating could be an effective tool to improve

embrane compactness, e.g. the pervaporation selectivity of Y-ype zeolite membranes for ethanol/water were increased from0 to more than 300 by using microwave heating.

. Formation mechanism of zeolite membranes under
icrowave irradiation
Up to now, the study on microwave synthesis of zeolite mem-ranes is still at the initial stage, and the optimization of zeolite

Fig. 4. Comparison of pervaporation performances between NaA zeolite mem-branes synthesized by microwave heating and conventional heating. Wp: thewater concentration in permeate.


F pe ze(

mldmtlslaaoanitnttaapip

co

mtwvfztfie

6

“b

ig. 5. Schematic illustration of the proposed formation mechanism of NaA tyafter Ref. [63]).

embrane synthesis is still a trial-and-error procedure, which isow effective and difficult to be generalized. Therefore, a fun-amental understanding of the formation mechanisms of zeoliteembranes is of great significance, but the relevant reports on

he formation mechanism rather rare. We have once tried to pre-iminarily characterize the formation mechanisms of microwaveynthesized LTA zeolite membranes under the existence of zeo-ite seeds (secondary growth method) [81,104]. Recently, wedopted gravimetric analysis, XRD, SEM, XPS, ATR/FTIR,nd gas permeation to characterize the whole formation processf LTA zeolite membranes which were synthesized by “in-situging-microwave heating” method, and the formation mecha-ism of LTA zeolite membranes was proposed [64], as illustratedn Fig. 5. With this mechanism, a gel layer is first formed onhe support after in situ aging, which contains plenty of pre-uclei. During the following microwave assistant crystallization,hese pre-nuclei rapidly and simultaneously develop into crys-al nuclei, and then crystal growth by propagation through themorphous primary particles (with size of ca. 50 nm) goes on,
nd finally, the amorphous particles transform into LTA crystalarticles with the same size. This propagation growth processs accompanied by the agglomeration and densification of therimary particles. In this way, compact LTA zeolite membranes
fpif

olite membrane synthesized by an “in-situ aging-microwave heating” method

onsisting of spherical grains with undefined crystal facets arebtained.

Based on the above results and referred to the reported for-ation mechanism of LTA zeolite [105,106], it is believed

hat the essential mechanism of zeolite formation is similarhat ever in powder form or membrane form. This obser-ation is in accordance with Zah et al.’s latest report on theormation mechanism of conventional heating synthesized LTAeolite membranes [107]. Therefore, it can be concluded thathe function of microwave irradiation during zeolite membraneormation is mainly derived from its thermal effect. However,t is still suspected that there will be some specific microwaveffects, and this will be discussed in the following section.

. Thermal and non-thermal effects

In order to distinguish between the“thermal effect” and thenon-thermal effect” in the synthesis of the NaA zeolite mem-ranes by microwave heating, in our previous study [81], the
ollowing experiment was designed. A seeded support waslaced in the synthesis mixture and heated up quickly to 90 ◦Cn a microwave oven, and then the synthesis system was trans-erred to a conventional oven, which was preheated to 90 ◦C.

Y. Li, W. Yang / Journal of Membrane Science 316 (2008) 3–17 13

tering

Acczsotoz“

cthbzcdtoiftbitr

tqes

ttr

7a

ofiMInt

Fig. 6. Pilot scale vapor permeation dewa

fter 15 min of synthesis, the as-synthesized membrane washaracterized by XRD. It was observed that there is no signifi-ant difference in the intensity of the diffraction patterns of NaAeolite between the as-synthesized membrane and the seededupport. The intensity of the diffraction patterns of NaA zeolitef the as-synthesized membrane is much smaller than that ofhe NaA zeolite membrane synthesized by microwave heatingf 15 min. Thus, we concluded that the fast formation of NaAeolite membranes by microwave was mainly promoted by thenon-thermal effect” [81].

Recently, a more elaborately designed experiment wasarried out to further investigate the microwave effect in the syn-hesis of NaA zeolite membranes by “in-situ aging-microwaveeating” method [108]. Different from what we have concludedefore [80], it was found that the linear growth rate of NaAeolite under microwave field was at the same level as underonventional heating conditions. This indicates that microwaveoes not enhance the crystallization kinetics of NaA zeolite, andhe acceleration effect of microwave synthesis is mainly a resultf fast and simultaneous nucleation. On the other hand, we havenvestigated the preparation of NaA zeolite membranes with dif-erent pulse lengths of microwave irradiation. It was found thathe optimal synthesis time for compact membranes (evaluatedy pervaporation tests) was determined by the total microwaverradiation time and not by the total heating time. This indicateshat the existence of non-thermal effects can not be completelyuled out.

Based on the comparison of the conventional heating syn-hesis and microwave heating synthesis, combined with someualitative theoretical analysis, the thermal and non-thermalffects of microwaves during “in-situ aging-microwave heating”
ynthesis are speculated as bellow:
Thermal effects: (1) selective heating, i.e. microwaves selec-tively couple with the gel layer because the gel phase has a

mdfim

installation with 0.8 m2 membrane area.

higher dissipation factor (tan �); (2) volumetric heating, i.e. thegel layer is heated quickly and entirely and homogeneously,which results in a simultaneous nucleation and synchronouspropagation of crystallization.Non-thermal effects: (1) electric double layer structures ofthe primary particles are destroyed through Maxwell–Wagnerpolarization mechanism, which enhances the agglomeration ofthese particles; (2) the H-bonds are destroyed under microwaveirradiation, which increases the reaction rate in the gel phase.This coincides with the “active water” concept proposed byJansen et al. [109]; (3) the Si O (Al O ) bonds on thesurface of the primary particles are selectively activated bymicrowaves, which hastens the densification between agglom-erated particles. This proposal is referred to the reports ofHwang et al. [100] and Blanco et al. [110,111].

The acceleration effect of microwave synthesis is mainly dueo its thermal effects, whereas, the extraordinary morphology ofhe microwave synthesized LTA zeolite membranes is mainly aesult of the non-thermal effects of microwave synthesis.

. Scaling-up of zeolite membrane production bydopting microwave heating

To overcome the thermal gradients and density gradients thatccurred in large-scale membrane production, temperature pro-les and control within the synthesis autoclave are necessary.icrowave heating is a promising solution to solve this problem.

n microwave processing, energy is supplied by an electromag-etic field directly to the material. This results in a rapid heatinghroughout the material with reduced thermal gradients. Volu-

etric heating can also reduce processing times, and thus theensity gradients caused by sedimentation under the gravityeld will not be significant any longer. In addition, the unifor-ity of the microwave field (multi-mode applicators) can be

1 mbra

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mproved by increasing the size of the oven cavity. In terms ofndustrial production, fast and energy-saving microwave heat-ng can lower the fabrication cost and make the continuousroduction possible. Therefore, microwave synthesis of zeoliteembranes is especially promising for industrial production.ombining hydrothermal synthesis and microwave heating tech-ique, scaling-up of NaA zeolite membrane has been achievedn our group. A pilot scale vapor permeation dewatering installa-ion based on NaA zeolite membranes has been set up recently, ashown in Fig. 6. This installation produces 225 L/D fuel ethanolontaining less than 0.3 wt.% water from the bio-ethanol with0 wt.% water. The vapor permeation process is operated at10 ◦C and 0.2 MPa. The separation unit consists of four mod-les with a total permeation area of about 0.8 m2; each modules made up of 7 NaA zeolite membrane tubes. The tubular zeo-ite membranes are 85 cm long and 12 mm in outer diameter.ased on the data obtained on this installation, it is calcu-

ated that the energy consumption of zeolite membrane basedapor permeation dewatering process is much smaller than thatf azeotropic distillation, pressure swing adsorption and poly-eric membranes. Due to the very high water selectivity oficrowave synthesized NaA zeolite membranes, ethanol recov-

ry of >99.8% was obtained. Long term stability test (>1500 h,till carrying on now) showed that microwave synthesized NaAeolite membranes provide very good stability in vapor perme-tion.

Now, a 5000 tonnes per year organic dewatering plant and aeaction-separation integrated unit based on NaA zeolite mem-rane (microwave synthesized) are in construction.

. Concluding remarks and future perspective

Microwave synthesis is a promising method for the fast syn-hesis of zeolite membranes. LTA, MFI, AFI, SOD, FAU andTS-4 types of zeolite membranes have been reported to beuccessfully synthesized under the irradiation of microwaves.esides remarkably accelerating the zeolite membrane for-ation, inhibiting the formation of impurity is another main

dvantage of microwave synthesis, which has been observed byany researches in numerous cases.Due to the unique features of microwave heating, e.g. vol-

metric and instantaneous (or rapid) heating, specific andelective heating, reaction acceleration effect, and the so-calledpecific microwave effect (although there is still controversy),icrowave synthesis is especially suitable for mass production

f large-area zeolite membranes. This has already been realizedn the case of NaA zeolite membranes in our group, which can beeen as a milestone in the development of microwave synthesisf zeolite membranes. Scaling-up microwave synthesis of FAUype zeolite membranes is ongoing in our group now. Bundledynthesis of hollow fiber supported zeolite membranes, takinghe advantage of microwave volumetric heating, are planned toe carried out in the near future.

Comparison between microwave heating and conventionaleating has been performed preliminarily, especially in the casef LTA type zeolite membranes. It was found that besides theften mentioned microwave accelerating effect, microwave syn-

ne Science 316 (2008) 3–17

hesis could also result in different membrane morphology,rientation, composition, and thus in different permeation char-cteristics as compared with conventional heating synthesizednes.

Attempts have been made to understand the formation mech-nism of zeolite membranes under microwave irradiation. As forTA zeolite membranes, it was found that the essential mecha-ism of zeolite formation is similar, whether under microwaver conventional heating conditions. Therefore, it was concludedhat the function of microwave irradiation during zeolite mem-rane formation is mainly derived from its thermal effect.owever, it is still suspected that there will be some specificicrowave effects, such as “active water”, “selective activation”,

tc.Significant progress has been achieved in the past several

ears on microwave synthesis of zeolite membranes. Neverthe-ess, it is still in the early stages of development, and furtheresearch is required to expand the research area, to improvehe membrane performance, to increase the synthesis repro-ucibility, and to scale-up membrane production. This needs thenterdisciplinary cooperation among material science, physicalhemistry and microwave engineering. First of all, the reactorngineering must be considered in order to understand and con-rol microwave synthesis, as pointed by Conner et al. [112]. Inddition, in order to understand the formation mechanism ando provide generalized guidance for the microwave synthesisf zeolite membranes, in situ characterization of the changesn chemistry and physics (energy transfer) are required, such aseutron and X-ray scattering, and vibrational spectroscopy (refero Tompsett et al.’s review [20]). Comprehensive characteriza-ion of the chemical and physical changes during microwaveynthesis and in-depth theoretical analysis, e.g. non-equilibriumhermodynamics and molecular dynamics, are the basis fornderstanding the real function of microwave irradiation in syn-hesis, including thermal and non-thermal effects.

cknowledgements

This work was supported by the Ministry of Science andechnology of China (Grant No. 2003CB615802), BP and Youthcience Fund of DICP (No. S200609).

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microwave synthesis of zeolite membranes a review

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