Microporous and Mesoporous Materials 42 (2001) 157±175

www.elsevier.nl/locate/micromeso

Design and fabrication of zeolite-based microreactors and

membrane microseparators
Yu Shan Susanna Wan a, Joseph Lik Hang Chau a, Asterios Gavriilidis b,
King Lun Yeung a,*
a
Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
b
Department of Chemical Engineering, University College, London, Torrington Place, London WC1E 7JE, UK
Received 21 June 2000; received in revised form 6 September 2000; accepted 12 October 2000

Abstract
This paper demonstrates that zeolites (i.e., Sil-1, ZSM-5 and TS-1) can be employed as catalysts, membrane or
structural materials in miniature chemical devices. Traditional semiconductor fabrication technology was employed in
micromachining the device architecture. The fabricated miniature zeolite-based structures ®nd applications as catalytic
microreactors and membrane microseparators. Four strategies for the manufacture of zeolite catalytic microreactors
were discussed: zeolite powder coating, uniform zeolite ®lm growth, localized zeolite growth, and etching of zeolite±
silicon composite ®lm. These zeolites were deposited either as ®lm or discrete islands with controlled particle size,
crystal morphology, layer thickness (3±16 lm) and ®lm orientation (e.g., highly oriented (1 0 1) and (2 0 0) ®lms). Crystal
intergrowth was also manipulated through the use of growth inhibitors. Silicalite-1 was also prepared as free-standing
membrane for zeolite membrane microseparators. Ó 2001 Elsevier Science B.V. All rights reserved.

Keywords: Zeolite; Microreactor; Microseparator; MEMS; Membrane

1. Introduction micromixers, microseparators and microreactors

bring closer the realization of desktop miniature
Microelectromechanical systems (MEMS) rep- factories, micropharmacies and nanomills [1,2].
resent miniaturized versions of mechanical ma- It is recognized that microreactors can provide
chinery such as motors, turbines, pumps and better energy and material utilization leading to
actuators [1]. Nowadays, they also include sen- more ecient chemical production and less pol-
sors, heat exchangers and ¯uid handling devices. lution [3]. The large surface area-to-volume ratio
These microsystems provide opportunities to de- that can be attained in a microreactor enhances
sign more energy ecient chemical processes. Re- heat and mass transfer rates [4±9]. Higher chemical
cent advances in the design and fabrication of conversions were also obtained from such devices
[10]. In addition, the integration of microsensors
and actuators enables rapid and precise control
*
Corresponding author. Tel.: +852-2358-7123; fax: +852-
of the reactor operation [11,12]. Other advanta-
2358-0054. ges include simpler process optimization [13],
E-mail address: kekyeung@ust.hk (K.L. Yeung). rapid design implementation [14], easier scale-up

1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 3 3 2 - 2
158 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

through replication and better safety [15±17]. Most can be formed by anodization, and impregnated
separation processes can bene®t directly from with solution of the metal precursor [36,37].
the larger surface area-to-volume ratio that can However, the incorporation of zeolite catalysts
be obtained in a microseparator. In fact, a few requires a di€erent strategy. Several researchers
chemical engineering separation processes, such as have shown that zeolite layer can be grown onto
extraction and membrane separation have been silicon wafer using hydrothermal synthesis method
successfully miniaturized [5,18±22]. [38,39]. In a recent paper, den Exter and coworkers
Several fabrication technologies are currently [38] have grown free-standing silicalite-1 mem-
available for the manufacture of microreactors brane onto silicon microchannel. Rebrov et al. [40]
and microseparators. The most popular method have grown ZSM-5 zeolite on the walls of ma-
uses traditional semiconductor fabrication proce- chined stainless steel microchannels and studied its
dure [23,24]. It is a mature technology with exist- performance for catalytic reduction of NO with
ing production infrastructure in the form of ammonia.
semiconductor foundries. This fabrication method Zeolites are important catalysts in many
allows other MEMS devices to be easily incorpo- chemical and petrochemical processes, such as
rated in the system [11,25]. However, there is a production of fuels, pollution abatement and more
material restriction to silicon and design limitation recently, clean ®ne chemicals production [41±44].
to two dimensions. Another process, LIGA, which The aluminum containing MFI zeolite, Al-ZSM-5
is a German acronym for lithography, electro- is an important catalyst for many hydrogenation,
plating and molding, is used for high aspect ratio disproportionation, isomerization and alkylation
micromachining (HARM) [26,27]. The essential reactions [34,42,45]. It ®nds uses in petroleum re-
steps are X-ray lithography using synchrotron ra- ®ning, petrochemical production and more re-
diation, electroplating of metal to form the mold, cently, it is being considered as an alternative
followed by injection molding of plastics, and elec- catalyst for synthesis of ®ne chemicals [46,47].
troforming with the desired metal. This method Metal-exchanged ZSM-5 zeolites such as Cu-
has been used to produce micromixers, heat ex- ZSM-5 are potential environmental catalysts since
changers, microextraction systems, micropumps, they exhibit high activity for NOx reduction [48].
sensors and micro-optical components [5,28]. The The TS-1 zeolite is known to be an ecient cata-
process is relatively complex and requires sophis- lyst for selective oxidation of alcohols [49], epoxi-
ticated equipment. Complex microreactor design dation of alkenes [50] and hydroxylation of
can also be fabricated using a multi-lamination aromatics [51].
technique [29,30]. Metal layers are patterned sep- Zeolites with their molecular sieving properties
arately and assembled to form the desired reactor [52±54] and tunable pore structure [55] are strong
architecture. Micromachining of reactor parts and candidates for membrane separation in miniature
components using laser is becoming more popular devices. Separation eciency can be increased by
as the technology improves [31,32]. The direct- the large surface area-to-volume ratio that can
write ``dip pen'' nanolithography (DPN) method is be achieved in such microsystems. High perm-
also a potential tool for functionalizing micro de- selectivity has been observed in separation of
vices. The DPN employs scanning tunneling and azeotropic mixtures [56], permanent gases [57],
atomic force microprobes to write patterns with up close-boiling hydrocarbon compounds [58] and
to 30 nm linewidth resolution [33]. isomers [59,60] using zeolite membranes. The ze-
One of the important issues in catalytic mic- olite pores can host di€erent ions, atoms, mole-
roreactors is the proper incorporation of the active cules and clusters that can be used to modify its
catalyst within microchannels. For metal catalysts, structural, chemical, catalytic, separation, elec-
the most direct approach is to deposit a thin layer tronic and optical properties. Zeolite membranes
of active metal (e.g., Pd, Pt, Ag) using thermal can also be utilized in membrane reactors. Their
deposition, chemical vapor deposition or sputter excellent permselectivity and molecular sieving
coating [34,35]. Alternatively, a porous oxide layer properties can bene®t many reaction systems that
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 159

are constrained by unfavorable thermodynamics.

Membrane reactors can exhibit better yield and
higher selectivity than ®xed bed reactors [61±64].
The ability of a membrane reactor to control the
addition and mixing of reactants and the selective
removal of products result in better material uti-
lization, less waste and pollution, and safer oper-
ation. Other application includes miniature
electrochemical cells and sensors. In this case, ze-
olite ®lms can act as membrane barriers for cre-
ating microcompartments for storage of chemicals,
catalysts and enzymes.
The purpose of this paper is to investigate the
di€erent designs and fabrication strategies for
zeolite catalytic microreactors and membrane Fig. 1. A magni®ed picture of the T-shaped pattern on a
microseparators. MFI-type zeolites including sil- chromium glass mask used in the lithography process with the
icalite-1 (Sil-1), aluminum ZSM-5 and titanium dimensions in lm.
silicalite-1 (TS-1) were incorporated into the mic-
roreactor design as a structural material, catalyst
or membrane. The microstructure of these zeo- on reaction and operating conditions, but it is
lites was manipulated in terms of thickness, ori- generally small [65].
entation and intergrowth. These zeolite-based The schematic diagram shown in Fig. 1 displays
microdevices have potential applications as cata- the dimensions of the reactor pattern etched onto a
lytic reactors, separators, molecular ®lters, sen- chromium mask. Patterns with three di€erent
sors, membrane reactors and electrochemical channel widths of 200, 500 and 1000 lm were
cells. prepared. The microchannels were fabricated us-
ing a standard photolithography technique onto
silicon wafer [24]. The details of the fabrication
procedure are summarized in Fig. 2. A 4-in. di-
2. Experiment ameter p-type silicon wafer (1 0 0) was used as the
substrate (Fig. 2A). A thin layer (500 nm) of low
2.1. Microchannel design and fabrication stress silicon nitride was deposited onto the wafer
at 1113 K and 25 Pa (Fig. 2B). The nitride layer
In this paper, a T-shaped design pattern was was grown from the decomposition reaction of
chosen for the microchannel reactor and separa- dichlorosilane (64 sccm), ammonia (16 sccm) and
tor. The design is similar to that reported by nitrogen (100 sccm). A 1±3 lm thick photoresist
Srinivasan et al. [11]. It consists of two inlets and a (HPR 207) was subsequently deposited. The pat-
single outlet that can be connected to reactant terns on the mask were transferred onto the wafer
sources and analyzer, respectively, through the following exposure to UV light (Fig. 2C). After
round docking pads. This reactor design has the developing the transfer pattern (Fig. 2D), the ex-
advantage of independent control and monitoring posed silicon nitride layer was removed using dry
of the reactant and product streams. It also pro- plasma etching at a rate of 80 nm/min (Fig. 2E).
vides in situ mixing of reactants, thus avoiding The remaining layer of photoresist was stripped o€
some of the hazards associated with a premixed using oxygen plasma. The channels were etched
feed. The mixing eciency of the T-junction de- using 30 wt.% KOH solution at 343 K (Fig. 2F).
pends on ¯ow rates, nature of the reactant mole- The anisotropic etching rates along the silicon
cules and channel aspect ratio (width/height). This h1 1 1i and h1 0 0i directions result in a trapezoidal
means that the mixing length can vary depending channel cross-section with an angle of 54.7°. Once
160 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

Fig. 2. Process diagram for microreactor fabrication.

Fig. 3. Cross-sectional views of the T-reactor channel after wet etching for (a) 4 h, (b) 6 h, (c) 8 h and (d) 10 h in 30 wt.% KOH solution
at 353 K.

the desired channel dimension was obtained (e.g., hot H3 PO4 solution (Fig. 2G). The depth of the
Fig. 3a±d), the etching was terminated, and the microchannel used for zeolite growth was 250 lm
remaining silicon nitride layer was removed using (cf. Fig. 3b). The wafers containing the etched
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 161

microreactor patterns were cleaned thoroughly to lution. The mixture was stirred for 24 h at room
remove dirt and contaminants from the surface. temperature to produce a clear and homogeneous
The wafers were sonicated for 15 min each, in solution. This solution was then transferred to a
ethanol and deionized distilled water. They were 125 ml Te¯on autoclave (Parr Scienti®c Inc.) and
blown dry to remove excess water from rinsing, heated in an oven (Memmert GmbH Co.) to a
dried in an oven at 333 K for overnight, and stored temperature of 398 K. After 8 h of hydrothermal
in a container for later use. treatment, the colloidal zeolites were recovered
and puri®ed using a series of centrifugation and
rinsing steps. The transmission electron micro-
2.2. Zeolite synthesis and incorporation graph in Fig. 4a shows that the Sil-1 seeds have a
spherical shape and an average particle size of 100
2.2.1. Preparation of zeolite seeds and powders nm. X-ray di€raction analysis indicates that the
The colloidal Sil-1 seeds were prepared from a colloidal zeolites are crystalline and have a MFI-
synthesis mixture containing 15 g of fumed silica, type structure (Fig. 4b). The zeolite has a BET
0.85 g of sodium hydroxide, and 60 ml of 1 M surface area of 420 m2 /g as measured by N2
tetrapropyl ammonium hydroxide (TPAOH) so- physisorption (Coulter, SA3100).

Fig. 4. (a) Transmission electron micrograph, and (b) X-ray di€raction pattern of colloidal Sil-1 zeolite.
162 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

A similar preparation procedure was used for coarse powders were about 5 lm in size and ex-
making colloidal Al-ZSM-5 and TS-1 zeolite seeds. hibited a broad size distribution ranging from sub-
The synthesis solution for colloidal Al-ZSM-5 micrometers (0.1 lm) to tens of micrometers (100
contained 10SiO2 :0.2AlOOH:2.4TPAOH:1NaOH: lm). NaZSM-5 powder with narrow particle
110H2 O. The 120 nm Al-ZSM-5 zeolites (XPS size distribution was prepared by pre-seeding a
Si=Al ˆ 18, BET surface area ˆ 400 m2 /g) were synthesis solution containing 40SiO2 :0.8AlOOH:
obtained after 8 h of hydrothermal treatment at 0.4NaOH:10TPAOH:20,000H2 O with 20 mg of
398 K. The 160 nm TS-1 were prepared from a colloidal zeolite seeds. After the hydrothermal
solution with the composition of 20TEOS: treatment at 398 K and 48 h, NaZSM-5 powder
0.75TEOT:9TPAOH:404H2 O. Chemical analyses with a uniform crystal size of 0.5 lm and a Si/Al
using EDXS, XPS and micro-Raman indicated ratio of 16 (i.e., XPS analysis) was obtained.
that the colloidal TS-1 have a spatially uniform Si/
Ti content of 20, with most of the Ti ions incor- 2.2.2. Fabrication of zeolite-based microreactors
porated within the zeolite framework. The BET Four methods were examined for preparing
surface area of the TS-1 seeds (425 m2 /g) is com- zeolite-based microreactors. The ®rst three meth-
parable to that of Sil-1 and Al-ZSM-5. The Sil-1, ods were based on incorporating zeolites onto
Al-ZSM-5, TS-1 seeds were stored as 2 wt.% col- prefabricated microchannels (Fig. 5a±c), while in
loidal suspension in deionized, distilled water. Method 4, the microchannel was fabricated onto a
Commercial zeolite powders, NaA (Molecular zeolite±silicon composite wafer (Fig. 5d).
sieve 5A, Aldrich) and NH4 ZSM-5 (Si=Al ˆ 30,
Zeolyst) were used for zeolite incorporation within 2.2.2.1. Method 1. Zeolite powder coating simply
the reactor microchannel. The particles of the deposits the zeolite powder onto the microreactor

Fig. 5. Process diagram for zeolite incorporation using (a) Method 1, (b) Method 2, (c) Method 3 and (d) Method 4.
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 163

channel (see Fig. 5a). The zeolite was suspended in ature, the solution generates an autogenous pres-
liquid and introduced into the microreactor sure of about 230 kPa. It was ensured that the
channel using a micropipette. A light sonication liquid level was always more than 5 mm above the
ensures that the channel is evenly ®lled with zeolite sample surface. After the hydrothermal treatment,
slurry. The microreactor was subsequently dried at the reaction was quenched to room temperature,
333 K and calcined in air at 823 K. The amount of and the silicon sample was rinsed with deionized
zeolite added was determined from the weight gain distilled water and dried overnight at an oven
measured after the calcination. The desired zeolite temperature of 333 K.
loading was obtained by repeated coating with the
zeolite slurry. 2.2.2.3. Method 3. Localized zeolite growth was
achieved through selective seeding. In order to
2.2.2.2. Method 2. Uniform zeolite ®lm growth has con®ne zeolite growth only within the microchan-
been obtained by den Exter et al. [38] and Sch- nel, the microchannel was ®rst functionalized
oeman et al. [39] on plain and seeded silicon wafers, with mercapto-3-propyltrimethoxysilane and then
respectively. Fig. 5b illustrates the procedure used seeded with colloidal zeolite using a micropipette
for zeolite growth onto prestructured silicon wafer. (Fig. 5c). In some experiments, the microchannel
The zeolite was grown onto the silicon wafer by was seeded more than once to obtain the desired
immersing it in zeolite synthesis solution, either seed population. The wafer was calcined in air at
immediately after the fabrication of the microre- 823 K for 6 h. Sil-1 was then grown in the mi-
actor pattern or following pre-seeding with col- crochannel following the same hydrothermal syn-
loidal zeolites. The latter entailed functionalization thesis procedure described in Method 2. Al-ZSM-5
of the silicon wafer with mercapto-3-propyltri- and TS-1 zeolites were also deposited within the
methoxysilane (50 mM in ethanol), followed by microchannel. For the former, colloidal Al-ZSM-5
immersion in colloidal suspension of zeolite seeds was used for seeding the microchannel and the Al-
and ®nally drying at 333 K. The seeding procedure ZSM-5 layer was grown from a hydrothermal
was repeated three times to obtain the desired seed synthesis solution containing 40TEOS:4AlOOH:
population of (5  1013 seeds/m2 ). The seeds were 10TPAOH:4NaOH:20,000H2 O at a temperature
then calcined in air at 823 K for 6 h, prior to the of 398 K and for 24 h. TS-1 zeolite was grown
hydrothermal growth of the zeolite layer. within the con®nes of the microchannel by pre-
Sil-1 ®lm was grown from solution with the seeding with colloidal TS-1 followed by hydro-
composition of 40TEOS:10TPAOH:20,000H2 O thermal treatment in a solution with a composition
prepared by adding 1.65 ml of tetraethyl orthosi- of 40TEOS:1.6TEOT:10TPAOH:10,000H2 O at
licate (TEOS) drop-by-drop to 70.4 ml of 0.027 M 448 K and 24 h.
TPAOH solution. The mixture was stirred at room
temperature for 24 h to produce a clear and ho- 2.2.2.4. Method 4. Etching of zeolite-silicon com-
mogeneous solution. Aluminum, titanium and posite wafer is an alternative method for fabri-
other metal ions can be added to impart the zeolite cating zeolite-based microreactors. A uniform
with the desired catalytic activity. The wafer con- layer of zeolite ®lm was ®rst grown onto a 4-in.
taining the etched microreactor pattern was then diameter silicon wafer using hydrothermal syn-
placed in a Te¯on holder such that the pattern was thesis method. 1±20 lm thick MFI-type zeolite
facing downward. This was to prevent powders ®lms with (1 0 1), (0 0 2) and (2 0 0) orientations
and precipitates from being incorporated into the had been prepared. The details of their synthesis
growing zeolite ®lm. The wafer can also be placed were discussed in a separate paper [66]. In this
vertically in the Te¯on holder. The sample and study, a 5 lm thick, highly oriented (1 0 1) Sil-1 on
solution were transferred into a 125 ml Te¯on silicon composite wafer was used as the substrate
vessel and sealed within a stainless steel sleeve. The material for the T-microreactor. A 3 lm layer of
autoclave was placed in a preheated oven (T ˆ 398 HPR-207 photoresist was coated onto the 4-in.
K) and allowed to react for 24 h. At this temper- composite wafer, and the T-reactor pattern was
164 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

transferred onto the wafer using standard photo- treatment in a synthesis solution containing
lithography (Fig. 5d). The reactor channel was 40TEOS:10TPAOH:20,000H2 O. The remaining 50
then etched by BOE solution (1HF:6NH4 F, Olin) lm thick silicon was then etched using TMAOH
and the remaining photoresist was stripped using solution to expose the zeolite membrane.
acetone (Lab-scan).
2.3. Characterization
2.2.3. Fabrication of zeolite membrane microsepar-
ators The fabricated units were analyzed by optical
The membrane microseparator consists of a T- microscope (Olympus BH-2) and scanning elec-
shaped channel on the front and a rectangular tron microscope (SEM, JEOL JSM 6300) before
recess at the back. The fabrication procedure used and after the incorporation of the zeolites. The
is illustrated in Fig. 6. Using standard microfab- optical microscope was used mainly for quality
rication technology, the T-microchannel and the control at each stage of the fabrication. The SEM
rectangular recess were etched onto the silicon was employed to determine the exact shape and
wafer (Fig. 6, steps A±E). The etching rate and dimensions of the microchannels, and to measure
time were used to control the depths of the mi- the thickness of the zeolite layer deposited onto the
crochannel and the recess. Etching was allowed channel. The zeolite crystal size, shape and inter-
to continue until less than 50 lm of silicon sepa- growth were also determined from analysis of the
rates the microchannel and the recess. The Sil-1 SEM micrographs. The structure, crystallinity and
membrane was then deposited onto either the orientation of the deposited zeolites were analyzed
microchannel (Fig. 6, steps F and G) or the hol- by an X-ray di€ractometer (XRD, Philip PW
lowed out recess (Fig. 6, steps H and I) following 1030) equipped with CuKa radiation source
seeding with colloidal Sil-1 and hydrothermal and graphite monochromator. Bulk and surface

Fig. 6. Process diagram for the fabrication of zeolite membrane microseparator.

 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 165

compositions of the incorporated zeolites were Method 1. Commercial zeolite powders including
examined by energy dispersive X-ray spectroscopy NaA and NH4 ZSM-5 have been successfully
(EDXS, Philips PW 1830) and X-ray photoelec- coated onto the T-microchannel as shown in Fig.
tron spectroscopy (XPS, Physical Electronics PHI 7a±d, respectively. The cubic NaA crystals have a
5600). broad particle size distribution (1±5 lm) with most
of crystals agglomerated into large clusters (Fig.
2.4. Materials 7a). This resulted in uneven surface coating (Fig.
7a) with portions of the microchannel devoid of
The potassium hydroxide (85%, BDH Lab.), zeolite particles (Fig. 7b). Fig. 7c and d display the
tetramethylammonium hydroxide (25% Moses NH4 ZSM-5 coated microchannel. The prismatic
Lake Ind.) and phosphoric acid (85%, Olin) were zeolite crystals are evident in Fig. 7c, and like
used to wet-etch the miniature reactor and mem- NaA, the zeolite coating was non-uniform (Fig.
brane patterns onto the silicon wafer and to strip 7d). The size of the zeolite powder must be uni-
the protective silicon nitride layer. Ethanol (99.9%, form and smaller than the channel width in order
Fisher) was used in degreasing and cleaning the to prevent blockage and to ensure a uniform de-
wafer at various stages of the device fabrication position. Fig. 7e shows a microchannel coated
and zeolite incorporation. Mercapto-3-propyltri- with monodispersed NaZSM-5 crystals (0:5  0:05
methoxysilane (99%, Aldrich) was employed in lm) grown from seeded synthesis solution. The
grafting the zeolite seeds onto the microchannels. zeolite crystals uniformly coated the surface of the
For the zeolite synthesis, the chemicals used microchannel forming a zeolite layer 7  1:2 lm
were fumed silica (0.01 lm, 99.8%, Aldrich), thick and a loading of 0.3 mg (Fig. 7f).
TEOS (98%, Aldrich), alumina sol (Vista), tetra- This technique is relatively simple and
ethyl orthotitanate (95%, MERCK-Schuchardt), straightforward, and can be easily used to incor-
TPAOH (1 M, Aldrich), triethoxymethylsilane porate other types of catalyst in addition to zeo-
(TEMS) (99%, Aldrich) and sodium hydroxide lites. The zeolite loading can be controlled by
(98%, BDH). After hydrothermal synthesis, the changing the concentration of zeolite in the slurry
Te¯on containers and sample holders were cleaned and through repeated coating of the microchannel.
using 1 M NaOH solution under hydrothermal However, the powder adhesion was poor and the
conditions at 423 K overnight. These accessories zeolites were easily removed. Using chemical
were then carefully rinsed in distilled water. This grafting and polymer adhesives can signi®cantly
cleaning procedure was necessary to remove zeo- improve powder adhesion, but they can also in-
lite particles that may have formed on the walls of terfere with the function of the catalyst and the
the containers and holders. operation of the reactor. An alternative method is
to grow the zeolite layer onto the microchannel
using the other methods described in this paper
3. Results and discussion (i.e., Methods 2 and 3).

The main goal of this paper is to illustrate dif- 3.1.2. Method 2: uniform zeolite ®lm growth
ferent strategies for incorporating zeolites as cat- In this method, a uniform zeolite ®lm was
alyst, membrane or structural material into the grown onto the etched T-microreactor pattern as
architecture of miniature chemical devices such as shown in Fig. 8a. The ®gure shows that the MFI
microreactor and microseparator. zeolite ®lm uniformly covers the wafer (Fig. 8b),
including the etched microchannel (Fig. 8c). The
3.1. Zeolite-based microreactors ®lm was made of well-intergrown zeolite crystals
that were oriented with their h1 0 1i crystallo-
3.1.1. Method 1: Zeolite powder coating graphic axis perpendicular to the silicon surface.
Zeolite powder can be incorporated directly From the channel cross-section, the zeolite ®lm
onto the prefabricated microreactor channel using was measured to be about 3 lm thick along the
166 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

Fig. 7. Scanning electron micrographs of (a) and (b) NaA, (c) and (d) NH4 ZSM-5, and (e) and (f) NaZSM-5 zeolite powders deposited
onto the T-reactor microchannel using Method 1.

wafer surface but was slightly thinner (2.5 lm) at onto the etched microreactor pattern as shown in
the ¯oor of the channel (Fig. 8d). This indicates Fig. 8e. The ®gure shows a 3:8  0:6 lm thick
that mass transfer resistance is present during highly oriented (2 0 0) zeolite ®lm made of inter-
the zeolite growth, or di€erent nucleation/growth grown MFI crystals that have an average size of
mechanism may be dominant on the wafer surface 2.5 lm (Fig. 8e, inset).
and the microchannel ¯oor. Fig. 8d inset shows A rougher deposit is often desirable, since it
that the ®lm is smooth with a surface roughness increases the area of the catalyst in direct contact
(peak-to-valley) less than 0.3 lm. Other zeolite with the reactant ¯uid. This can be accomplished
orientations besides (1 0 1) can also be deposited by growing the zeolite on the wafer without prior
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 167

Fig. 8. (a) Scanning electron micrograph of the zeolite-based miniature T-reactor prepared by Method 2. (b) Top-view of a well-
oriented (1 0 1) zeolite ®lm grown on the surface of the silicon wafer. (c) Top-view of the well-oriented (1 0 1) zeolite ®lm grown within
the reactor microchannel. (d) A cross-sectional view of the well-oriented (1 0 1) zeolite ®lm within the reactor microchannel. (The ®gure
inset is at 2 higher magni®cation.) (e) A cross-sectional view of a well-oriented (2 0 0) zeolite ®lm within the reactor microchannel.
(The ®gure inset is at 2:5 higher magni®cation.) (f) A cross-sectional view of a randomly oriented zeolite ®lm deposited onto a reactor
microchannel. (The ®gure inset is at 5 higher magni®cation.)

seeding. In this case, as shown in Fig. 8f, a layer of good intergrowth between them. Fig. 8d±f show
randomly oriented zeolite crystal was obtained. that the zeolites grew directly from the surface of
The layer has a average thickness of 16 lm and a the wafer and thus have an excellent adhesion.
surface roughness of about 8 lm. It can be seen From the above results, it is evident that seeding
from the ®gure that the layer consists of both induces preferential orientation of the zeolite
prismatic and twinned zeolite crystals, and there is crystals, and resulted in more compact zeolite ®lm
168 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

(cf. Fig. 8d and e versus f). Method 2 may present cipitated from the solution decorates the surface of
some diculty in reactor sealing if smooth surfaces the wafer (Fig. 9b).
are required, as in the case of thermal and anodic Fig. 10a displays a T-microreactor (1000 lm)
bonding [24]. Although adhesives and cements may with a deposited layer of ZSM-5 catalyst within its
be employed for sealing, they can introduce other channel. The intergrown zeolite ®lm (Fig. 10b) was
complication such as low thermal resistance, con- 4.7 lm thick which gave a catalyst loading of about
tamination and incompatibility with reactants. 0.5 mg. From XPS analysis, the Si/Al ratio of the
deposited zeolite catalyst was determined to be 27.
3.1.3. Method 3: localized zeolite growth The ZSM-5 can be readily transformed into a
Method 3 di€ers from Method 2 in that the protonated HZSM-5 acid catalyst using well-es-
zeolite is incorporated only within the con®nes of tablished treatment procedure [53,67]. Method 3
the microchannel. This was achieved through was also used to prepare the TS-1 zeolite microre-
surface modi®cation using chemical functional- actor shown in Fig. 10c and d. In this case, the TS-1
ization and seeding to locally promote the growth zeolites were grown on a 200 lm T-microreactor
of the zeolite layer. A zeolite-based T-microreactor (Fig. 10c). Using isomorphous substitution of tita-
prepared by Method 3 is shown in Fig. 9a. Fig. 9b nium ions into the MFI zeolite framework [68,69], a
and c show that the zeolite growth is con®ned Si/Ti ratio of 17 was obtained for the zeolite ®lm
within the microchannel and that the Sil-1 ®lm is shown in Fig. 10d. The ®lm is well intergrown but
well-intergrown and uniform along the length of exhibits a smaller crystal size than the ZSM-5.
the channel (Fig. 9c). Outside the microchannel, a Fig. 11 displays the di€erent zeolite ®lm mi-
few zeolite crystals (one zeolite per 100 lm2 ) pre- crostructures that had been successfully grown

Fig. 9. (a) Scanning electron micrograph of the zeolite-based miniature T-reactor prepared by Method 3. (b) Image of the wafer
surface outside of the T-reactor channel. (c) Microstructure of the zeolite layer grown within the con®nes of the microchannel.
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 169

Fig. 10. SEM images of the zeolite-based microreactor and zeolite catalyst layer prepared by Method 3. (a) and (b) Al-ZSM-5 with Si/
Al ratio of 27, (c) and (d) TS-1 with Si/Ti ratio of 17.

Fig. 11. Cross-sectional images of (a) highly oriented (1 0 1) zeolite ®lm, (b) well-oriented (2 0 0) zeolite ®lm and (c) zeolite coating with
controlled intercrystalline porosity.
170 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

Fig. 12. X-ray di€raction patterns of zeolite ®lms shown in (a) Fig. 11a, (b) Fig. 11b and (c) Fig. 11c.

within the con®nes of the microreactor channels neighboring crystals resulting in a more open mi-
using Method 3. The 2.5 lm zeolite ®lm shown in crostructure. It is worth noting that the crystal
Fig. 11a is oriented along the h1 0 1i direction, orientation was not a€ected and remains h2 0 0i as
which exposes the zigzag pores towards the sur- shown in Fig. 12c. The individual crystals have an
face. The X-ray di€raction pattern in Fig. 12a in- average height of 6.9 lm and a diameter of about
dicates that the ®lm is highly oriented. The second 10 lm. The open microstructure obtained can fa-
®lm shown in Fig. 11b is 3.1 lm thick and was cilitate ¯uid through the ®lm ensuring an intimate
deposited with a preferred orientation of (2 0 0) contact between the zeolites and reactant mole-
(Fig. 12b). In this case, molecules can access the cules. The localized zeolite growth that can be
zeolite interior through a di€erent set of pore achieved using Method 3 permits the precise in-
channels. Both ®lms have low surface roughness corporation of zeolites within the microreactor.
and completely cover the surface of the micro- The ability to engineer the microstructure of the
channel. They also display excellent intergrowth zeolite ®lm and to tailor its chemical and catalytic
and uniformity along the channel length. properties is important for achieving optimum
Fig. 11c shows a zeolite coating made of upright microreactor performance. By con®ning the zeo-
twinned crystals prepared with the addition of lite growth within the microchannel, the microre-
TEMS in the synthesis solution used for ®lm actor can be sealed by either thermal or anodic
growth. TEMS inhibits the intergrowth between bonding.
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 171

3.1.4. Method 4: etching of zeolite-silicon composite catalysts can be precisely engineered to optimize
wafer the microreactor performance. Miniature electro-
In Method 4, the T-reactor pattern was etched chemical cells and sensors, in which membrane
onto a highly oriented (1 0 1) zeolite ®lm grown barriers are an integral part of the design, can also
onto 4-in. silicon wafer (Fig. 13a). This zeolite- be made using this procedure.
silicon composite wafer was obtained by seeding
the wafer ®ve times with colloidal zeolites followed 3.2. Zeolite membrane microseparator
by 48 h hydrothermal synthesis at 398 K in a so-
lution containing 40TEOS:10TPAOH:20,000H2 O. For commercial application, a large membrane
A clean pattern was obtained (Fig. 13a and b) area is usually needed to provide the required
where the 5 lm thick interlocking Sil-1 crystals separation or production throughput. However,
form the wall that de®nes the reactor channel (Fig. the scale-up of zeolite membranes is dicult.
13c). The good precision that can be attained using Made of inorganic crystals, the zeolite membrane
this method is apparent in Fig. 13b. The channel accumulates stress during thermal treatment and
was etched all the way through the zeolite layer to high-temperature operations that can lead to me-
expose the silicon underneath which forms the chanical failure. The mechanical stress increases in
¯oor of the channel (Fig. 13d). There are several proportion to the membrane area, making large
advantages in using this fabrication methodology. membranes more susceptible to cracks and defects.
Microfabricated catalysts can also be realized us- Using miniaturization technology developed for
ing this method. Here, the shape, morphology, microchemical devices, one can achieve membrane
quantity and individual locations of the zeolite scale-up through replication while maintaining the

Fig. 13. (a) Scanning electron micrograph of the zeolite-based miniature T-reactor prepared by Method 4. (b) Top-view of the reactor
microchannel etched through the zeolite ®lm layer. (c) Microstructure of the deposited zeolite ®lm layer that made up the wall of the
reactor channel. (d) Microstructure of the etched silicon wafer, that formed the base of the microchannel.
172 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

individual membrane area small to prevent stress- formly deposited along the length and width of the
related failure. In addition, the large surface area- microchannel. A portion of this ®lm formed the
to-volume ratio that can be obtained in these free-standing zeolite membrane as shown in Fig.
microsystems is expected to enhance the mem- 14a. XRD analysis indicated that the ®lm has
brane separation performance. a (2 0 0) orientation and SEM characterization
Following the procedure described earlier in the showed that the ®lm was made of intergrown Sil-1
experimental section (Fig. 6), two di€erent zeolite crystals that have an average size of about 7 lm
membrane microseparator designs were fabri- (Fig. 14c). An image of the recess etched onto the
cated. The free-standing zeolite membrane was back of the silicon wafer is shown in Fig. 14d. The
deposited either on the microchannel (Fig. 14) or zeolite membrane is clearly evident in the ®gure. It
the recess located on the back of the wafer (Fig. has dimensions of 0:7  4:8 mm2 as de®ned by the
15). The picture shown in Fig. 14a was taken with width of the microchannel and the length of the
a light shining from the back of the fabricated recess.
pattern. The free-standing membrane is easily Instead of depositing the zeolite layer onto the
distinguished by its lighter color. Fig. 14b is a microchannel, the ®lm can be grown on the rect-
higher magni®cation image of the deposited zeolite angular recess as shown in Fig. 15. This con®gu-
membrane. It is clear from the ®gure that the ze- ration frees most of the microreactor channel for
olite growth was con®ned within the microreactor catalyst incorporation allowing the design of hy-
channel. The 16 lm thick zeolite ®lm was uni- brid membrane microreactors. Catalysts can be

Fig. 14. (a) A SEM picture of the T-microchannel coated with a layer of Sil-1 membrane (the light rectangular area is the free-standing
zeolite membrane). (b) A higher magni®cation image showing that the zeolite membrane was con®ned within the microchannel. (c) A
higher magni®cation image of the zeolite membrane grown within T-reactor channel. (d) A picture of the recess fabricated at the back
of wafer with the exposed zeolite membrane.
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 173

Fig. 15. (a) A SEM picture of the T-microchannel etched onto the silicon wafer. (b) A higher magni®cation image of the microchannel
showing both zeolite and silicon surfaces. (The ®gure inset is a 1:3 higher magni®cation image of the zeolite surface.) (c) A picture of
the recess fabricated at the back of wafer coated with a layer of zeolite membrane. (d) A higher magni®cation image of the zeolite
membrane deposited within the recess.

incorporated using Method 1 or alternatively the 4. Concluding remarks

silicon surface can be oxidized to form silicon
oxides to which active catalyst components can be The ability to engineer the zeolite ®lm micro-
chemically grafted. Fig. 15a shows the T-shaped structure and tailor its chemical and catalytic
channel etched onto the silicon wafer. A section of properties is important for the successful incorpo-
the microchannel (Fig. 15b) shows both zeolite ration of zeolites as catalyst, membrane or struc-
membrane (Fig. 15b, inset) and silicon substrate. tural materials in microchemical devices that
The chemically roughened silicon substrate within include microreactors and microseparators. Dif-
the channel can provide better anchorage for the ferent fabrication strategies were adopted to in-
catalyst. The Sil-1 membrane was deposited onto corporate zeolites as powder catalysts (Method 1)
the 5  6 mm2 rectangular recess (Fig. 15c). The or ®lm within the reactor microchannel (Method
membrane surface was rough due to growth im- 3). Alternatively, a uniform zeolite ®lm was de-
perfections caused by adhesions of zeolite precip- posited onto a prefabricated silicon wafer (Method
itates during the hydrothermal synthesis. The 2) or plain silicon to form zeolite±silicon composite
zeolite crystals formed a continuous ®lm with onto which the reactor pattern was etched using
(1 0 1) orientation. A higher magni®cation image traditional semiconductor fabrication techniques
in Fig. 15d shows that the membrane was well- (Method 4). Using these methods, Sil-1, ZSM-5
intergrown. Depositing the zeolite onto the recess and TS-1 zeolites were successfully integrated
provides a larger area for membrane adhesion re- into microreactors either as catalyst or part of the
sulting in greater stability. reactorÕs architecture. Di€erent ®lm thickness,
174 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175

crystal morphology and intergrowth were obtained [5] W. Ehrfeld, Cl. G artner, K. Golbig, V. Hessel, R. Konrad,
by controlling the seed population, synthesis H. L owe, Th. Richter, Ch. Schulz, Microreaction Tech-
nology: Proceedings of the First International Conference
chemistry and hydrothermal treatment conditions. on Microreaction Technology, in: W. Ehrfeld (Ed.),
Both randomly oriented deposits and highly ori- Springer, Berlin, 1998, p. 72.
ented ®lms (i.e., (1 0 1) and (2 0 0)) were prepared. [6] A.L.Y. Tonkovich, S.P. Fitzgerald, J.L. Zilka, M.J.
Two zeolite membrane microseparators were LaMont, Y. Wang, D.P. VanderWiel, R.S. Wengeng,
made using a new fabrication technology. The Microreaction technology: industrial prospects, in: W.
Ehrfeld (Ed.), Proceedings of the Third International
zeolite membrane layer was deposited either within Conference on Microreaction Technology, Springer, Ber-
the T-microchannel or the rectangular recess at the lin, 2000, p. 364.
back of the wafer. Both designs promise great [7] W. Ehrfeld, V. Hessel, H. L owe, Proceedings of the Fourth
numbers of potential applications as membrane International Conference on Microreaction Technology,
devices, membrane microreactor, electrochemical Atlanta, USA, 2000, p. 3.
[8] J. Brandner, M. Fichtner, U. Schygulla, K. Schubert,
cells and sensors. Two di€erent membrane ori- Proceedings of the Fourth International Conference on
entations (i.e., (1 0 1) and (2 0 0)) as well as two Microreaction Technology, Atlanta, USA, 2000, p. 244.
di€erent membrane thicknesses were used to il- [9] M.W. Losey, S. Isogai, M.A. Schmidt, K.F. Jensen,
lustrate the fabrication methodology. Studies are Proceedings of the Fourth International Conference on
now underway to test the performance of zeolite Microreaction Technology, Atlanta, USA, 2000, p. 416.
[10] E. Dietzsch, D. H onicke, M. Fichtner, K. Schubert, G.
catalytic microreactors and membrane microse- Wieûmeier, Proceedings of the Fourth International Con-
parators for chemical reactions and separations. ference on Microreaction Technology, Atlanta, USA,
2000, p. 89.
[11] R. Srinivasan, I.M. Hsing, P.E. Berger, K.F. Jensen, S.L.
Acknowledgements Firebaugh, M.A. Schmidt, M.P. Harold, J.J. Lerou, J.F.
Ryley, AIChE J. 43 (1997) 3059.
[12] D.J. Quiram, K.F. Jensen, M.A. Schmidt, J.F. Ryley, P.L.
We gratefully acknowledge the British Council
Mills, M.D. Wetzel, J.W. Ashmead, R.D. Bryson, T.M.
Joint Research Scheme (grant UK/HK JRS98/32) Delaney, D.J. Kraus, J.S. McCracken, Proceedings of the
for providing a travel fund that enabled the success Fourth International Conference on Microreaction Tech-
of this collaborative work. One of the authors also nology, Atlanta, USA, 2000, p. 55.
thanks the Croucher Foundation for her scholar- [13] O. W orz, K.P. J
ackel, Th. Richter, A. Wolf, Proceedings of
the Second International Conference on Microreaction
ship. We also acknowledge the technical support
Technology, New Orleans, USA, 1998, p. 183.
provided by the Microelectronics Fabrication Fa- [14] H. L owe, W. Ehrfeld, K. Gebauer, K. Golbig, O. Hausner,
cility (MFF) and Materials Characterization and V. Haverkamp, V. Hessel, Th. Richter, Proceedings of the
Preparation Facility (MCPF) of the Hong Kong Second International Conference on Microreaction Tech-
University of Science and Technology (HKUST). nology, New Orleans, USA, 1998, p. 63.
[15] J.J. Lerou, K.M. Ng, Chem. Engng. Sci. 51 (1996) 1595.
[16] K.F. Jensen, AIChE J. 45 (1999) 2051.
[17] R. Besser, M. Prevot, Proceedings of the Fourth Interna-
References tional Conference on Microreaction Technology, Atlanta,
USA, 2000, p. 278.
[1] K. Dutta, P. Dev, P. Dewilde, B. Sharma, R. Newcomb, [18] I. Robins, J. Shaw, B. Miller, C. Turner, M. Harper, in:
Proceedings International Conference on Microelectronics, W. Ehrfeld (Ed.), Microreaction Technology: Proceedings
Circuits and System Theory, Sydney, Australia, 1970, p. of the First International Conference on Microreaction
36. Technology, Springer, Berlin, 1998, p. 35.
[2] N. Kawahara, T. Suto, T. Hirano, Y. Ishikawa, T. [19] W. Pompe, M. Mertig, R. Kirsch, H. Engelhardt, T.
Kitahara, N. Ooyama, T. Ataka, Microsystems Technol. Kronbach, in: W. Ehrfeld (Ed.), Microreaction Technol-
3 (1997) 37. ogy: Proceedings of the First International Conference on
[3] R.S. Wegeng, M.K. Drost, D.L. Brenchley, Microreaction Microreaction Technology, Springer, Berlin, 1998, p. 104.
technology: industrial prospects, in: W. Ehrfeld (Ed.), [20] W.E. TeGrotenuis, R.J. Cameron, M.G. Butcher, P.M.
Proceedings of the Third International Conference on Martin, R.S. Wegeng, Sep. Sci. Technol. 34 (1999) 951.
Microreaction Technology, Springer, Berlin, 2000, p. 2. [21] C. Turner, J. Shaw, B. Miller, V. Bains, Proceedings of the
[4] M.K. Drost, C.J. Call, J.M. Cuta, R.S. Wegeng, J. Fourth International Conference on Microreaction Tech-
Microscale Thermophys. Engng. 1 (1997) 321. nology, Atlanta, USA, 2000, p. 334.
 Y.S.S. Wan et al. / Microporous and Mesoporous Materials 42 (2001) 157±175 175

[22] C. Turner, J. Shaw, B. Miller, V. Bains, Proceedings of the Surface Science and Catalysis, vol. 85, Elsevier, Amster-
Fourth International Conference on Microreaction Tech- dam, 1994, p. 509.
nology, Atlanta, USA, 2000, p. 106. [44] J.N. Armor, Chem. Engng. 106 (1999) 70.
[23] K.D. Wise, K. Naja®, Science 254 (1991) 1335. [45] P.A. Jacobs, J.A. Martens, Introduction to zeolite science
[24] M. Madou, Fundamentals of Microfabrication, CRC and practice, in: H. van Bekkum, E.M. Flanigen, J.C.
Press, New York, 1997, pp. 145±215. Jansen (Eds.), Studies in Surface Science and Catalysis, vol.
[25] I.M. Hsing, R. Srinivasan, M.P. Harold, K.F. Jensen, 58, Elsevier, Amsterdam, 1991, p. 445.
M.A. Schmidt, Chem. Engng. Sci. 55 (2000) 3. [46] S. Feast, J.A. Lercher, Recent advances and new hori-
[26] H. Lehr, W. Ehrfeld, J. de Physique IV 4 (1994) 229. zons in zeolite science and technology, in: H. Chon,
[27] W. Ehrfeld, V. Hessel, H. L owe, Ch. Schulz, L. Weber, S.I. Woo, S.E. Parks (Eds.), Studies in Surface Sci-
Microsystem Technol. 5 (1999) 105. ence and Catalysis, vol. 102, Elsevier, Amsterdam, 1996,
[28] W. Ehrfeld, K. Golbig, V. Hessel, H. L ow, T. Richter, Ind. p. 363.
Eng. Chem. Res. 38 (1999) 1075. [47] H. van Bekkum, Proceedings of 1998 Catalysis and Zeolite
[29] P.M. Martin, D.W. Matson, W.D. Bennett, Proceedings of Workshop, Hsinchu, Taiwan, 1998, p. 15.
the Second International Conference on Microreaction [48] M.D. Amiridis, T. Zhang, R.J. Farrauto, Appl. Catal. B 10
Technology, New Orleans, USA, 1998, p. 75. (1996) 203.
[30] D.W. Matson, P.M. Martin, D.C. Stewart, A.L.Y. Ton- [49] F. Maspero, U. Romano, J. Catal. 146 (1994) 476.
kovich, M. White, J.L. Zilka, G.L. Roberts, Microreaction [50] M.G. Clerici, G. Bellussi, U. Romano, J. Catal. 129 (1991)
technology: industrial prospects, in: W. Ehrfeld (Ed.), 159.
Proceedings of the Third International Conference on [51] A.J.H.P. van der Pol, A.J. Verduyn, J.H.C. van Hoo€,
Microreaction Technology, Springer, Berlin, 2000, p. 62. Appl. Catal. A 92 (1992) 113.
[31] S. Johansson, J.A. Schweitz, H. Westberg, M. Boman, J. [52] J.C. Jansen, D. Kashchiev, A. Erdem-Senatalar, Advanced
Appl. Phys. 72 (1992) 5956. zeolite science and applications, in: J.C. Jansen, M.
[32] J. Arnold, U. Dasbach, W. Ehrfeld, K. Hesch, H. L owe, Stocker, H.G. Karge, J. Weitkamp (Eds.), Studies in
Appl. Surf. Sci. 86 (1995) 251. Surface Science and Catalysis, vol. 85, Elsevier, Amster-
[33] R.D. Piner, J. Zhu, F. Xu, S. Hong, C.A. Mirkin, Science dam, 1994, p. 215.
283 (1999) 661. [53] H.G. Karge, J. Weitkamp (Eds.), Molecular Sieves ±
[34] J.M. Thomas, W.J. Thomas, Principles and Practice of Science and Technology, Springer, Berlin, 1998.
Heterogeneous Catalysis, VCH Publishers, Weinheim, [54] J. Coronas, J. Santamaria, Separat. Puri®cat. Meth. 28
Germany, 1997, pp. 1±60. (1999) 127.
[35] A. Kursawe, D. H onicke, Proceedings of the Fourth [55] Y. Yan, T. Bein, J. Amer. Chem. Soc. 117 (1995) 9990.
International Conference on Microreaction Technology, [56] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami, K.
Atlanta, USA, 2000, p. 153. Haraya, J. Membr. Sci. 95 (1994) 221.
[36] G. Wieûmeier, D. H onicke, Ind. Engng. Chem. Res. 35 [57] K. Kusakabe, T. Kuroda, S. Morooka, J. Membr. Sci. 148
(1996) 4412. (1998) 13.
[37] G. Wieûmeier, D. H onicke, J. Micromech. Microengng. 6 [58] H.H. Funke, A.M. Argo, C.D. Baertsch, J.L. Falconer,
(1996) 285. R.D. Noble, J. Chem. Soc. ± Farad. Trans. 92 (1996) 2499.
[38] M.J. den Exter, H. van Bekkum, C.J.M. Rijn, F. Kapteijn, [59] H.H. Funke, M.G. Kovalchick, J.L. Falconer, R.D.
J.A. Moulijn, H. Schellevis, C.I.N. Beenakker, Zeolites 19 Noble, Ind. Engng. Chem. Res. 35 (1996) 1575.
(1997) 13. [60] G. Xomeritakis, A. Gouzinis, S. Nair, T. Okubo, M. He,
[39] B.J. Schoeman, A. Erdem-Senatalar, J. Hedlund, J. Sterte, R.M. Overney, M. Tsapatsis, Chem. Engng. Sci. 54 (1999)
Zeolites 19 (1997) 21. 3521.
[40] E.V. Rebrov, G.B.F. Seijger, H.P.A. Calis, M.H.J.M. de [61] J.N. Armor, Appl. Catal. 49 (1989) 1.
Croon, C.M. van den Bleek, J.C. Schouten, Proceedings of [62] H.P. Hsieh, Catal. Rev. ± Sci. Engng. 33 (1991) 1.
the Fourth International Conference on Microreaction [63] J. Zaman, A. Chakma, J. Membr. Sci. 92 (1994) 1.
Technology, Atlanta, USA, 2000, p. 250. [64] G. Saracco, V. Specchia, Catal. Rev. ± Sci. Engng. 36
[41] W.F. H olderich, H. van Bekkum, Introduction to zeolite (1994) 305.
science and practice, in: H. van Bekkum, E.M. Flanigen, [65] K.K. Yeong, D. Gobby, P. Angeli, A. Gavriilidis, Z. Cui,
J.C. Jansen (Eds.), Studies in Surface Science and Catal- Proceedings of the Fourth International Conference on
ysis, vol. 58, Elsevier, Amsterdam, 1991, p. 631. Microreaction Technology, Atlanta, USA, 2000, p. 270.
[42] I.E. Maxwell, W.H.J. Stork, Introduction to zeolite science [66] J.L.H. Chau, Y.S.S. Wan, K.L. Yeung, in preparation.
and practice, in: H. van Bekkum, E.M. Flanigen, J.C. [67] J.A. Biscardi, G.D. Meitzner, E. Iglesia, J. Catal. 179
Jansen (Eds.), Studies in Surface Science and Catalysis, vol. (1998) 192.
58, Elsevier, Amsterdam, 1991, p. 571. [68] M. Taramasso, G. Perego, B. Notari, US Patent no.
[43] H. van Bekkum, E.R. Geus, H.W. Kouwenhoven, Ad- 4410501, 1983.
vanced zeolite science and applications, in: J.C. Jansen, M. [69] A. Tuel, J. Diab, P. Gelin, M. Dufaux, J.F. Dutel, Y.B.
St
ocker, H.G. Karge, J. Weitkamp (Eds.), Studies in Taarit, J. Mol. Catal. 63 (1990) 95.