Continuous flow ZIF-8/NaA composite membrane microreactor for efficient Knoevenagel condensation

Article abstract of DOI:10.1016/j.catcom.2015.05.008

A ZIF-8/NaA composite membrane microreactor was fabricated on multi-channel stainless steel plate. NaA zeolite membrane was first introduced into the microchannel as a modification layer via a secondary growth. Subsequently, a layer of ZIF-8 membrane was

Full text of DOI:10.1016/j.catcom.2015.05.008

Catalysis Communications 68 (2015) 93–96
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Continuous flow ZIF-8/NaA composite membrane microreactor for
efficient Knoevenagel condensation
b
Guangcai Zhang a, Tong Zhang a, Xiongfu Zhang a, , King Lun Yeung
⁎
a
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A ZIF-8/NaA composite membrane microreactor was fabricated on multi-channel stainless steel plate. NaA
zeolite membrane was first introduced into the microchannel as a modification layer via a secondary growth.
Subsequently, a layer of ZIF-8 membrane was grown over the NaA zeolite layer as a catalytic layer by using
ZnO-induced synthesis method. Such ZIF-8/NaA composite membrane microreactor was applied in continuous
flow Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate. Nearly 100% product yield was
achieved in a short residence time under mild conditions. Importantly, no obvious deactivation was observed
even after 50 h, indicating its excellent stability.
Received 17 March 2015
Received in revised form 8 May 2015
Accepted 8 May 2015
Available online 9 May 2015
Keywords:
Microreactor
Metal organic frameworks
ZIF-8 membrane/film
Flow reaction
© 2015 Elsevier B.V. All rights reserved.
Knoevenagel condensation
1. Introduction
has been extensively studied and regarded as an efficient development
for the synthesis of continuous ZIF-8 membrane [9]. On the other hand,
Recently, continuous flow microreactor technology has attracted an
increasing interest both within academia and in industry [1,2]. This
is largely driven by the development of flow chemistry and the advan-
tages associated with microreactors compared with traditional batch
reactors based on protocols such as, improved efficiency, rapid mass
and heat transfer rates, use of supported reagents and reduced reaction
time [3–5]. Membrane microreactor (MMR), fabricated by assembling
membranes/films in a microreactor, significantly expands the applica-
tions of microreactor technology, especially in heterogeneous catalysis
[6,7]. The utilization of MMR in a reaction system can realize catalyst
immobilization, large contact area between reactants and catalyst,
optimal axial concentration profiles, supra-equilibrium conversions
and improved product purity. In some cases, it is also helpful to prevent
catalyst from poisoning and deactivation, leading to greatly intensified
production processes.
ZIF-8, emerging as an important subclass of Metal Organic Frame-
works (MOFs), has raised great interest in preparing films and mem-
branes due to its unique porosity as well as its thermal and chemical
stability. So far, conceptually different strategies for the fabrication of
ZIF-8 membrane have been explored and they are mainly classified
into two basic categories: in situ crystallization and seed induced
growth crystallization [8]. Very recently, motivated by its easy handling
and excellent induction effect, a so-called ZnO-induced growth method
when it comes to the applications of the ZIF-8 membrane, it is well
recognized that they are generally applied in gas separation and
chemical sensors [8], whereas few reports have been focused on their
catalysis applications. In fact, it has been found that ZIF-8 is an efficient
catalyst for Knoevenagel condensation reaction [10]. This work reported
a fabrication of ZIF-8/NaA composite MMR by ZnO-induced growth
strategy and its catalytic performance for continuous flow Knoevenagel
condensation between benzaldehyde (BA) and ethyl cyanoacetate
(ECA) to ethyl trans-α-cyanocinnamate.
2. Experimental
2.1. Fabrication and characterization of composite MMR
A multi-channel plate with ten microchannels, each 300 μm wide,
600 μm deep and 25 mm long, was used as microreactor. The synthesis
route of ZIF-8/NaA composite membrane was shown in Scheme 1.
According to our previous work [11], a layer of NaA zeolite membrane
was first prepared on the surface of the microchannels. Then, a ZnO
particles layer was deposited on the NaA zeolite layer by wetting
chemistry method. Finally, a layer of ZIF-8 membrane was grown on
the NaA zeolite layer in a ZIF-8 synthesis solution by ZnO-induced
growth. All the experimental details were described in the Supplemen-
tary Information.
The prepared samples were characterized by powder X-ray diffraction
(XRD) and scanning electron microscope (SEM, QUANTA 450). The XRD
⁎
Corresponding author.
E-mail address: xfzhang@dlut.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.catcom.2015.05.008
1566-7367/© 2015 Elsevier B.V. All rights reserved.

94
G. Zhang et al. / Catalysis Communications 68 (2015) 93–96
Scheme 1. Synthesis route of ZIF-8/NaA composite membrane.
patterns were collected on a D/max-2400 diffractometer with a Cu target
(40 kV, 100 mA) from 5° to 50° at a scanning rate of 10°/min.
the metallic support and the desired layer, which could modify the
original surface properties of the support and therefore facilitate the
membrane growth [12]. NaA zeolite membrane has been reported as
an excellent modification layer for Pd membrane deposition on PSS
substrate for easy synthesis and high thermal and chemical stability
properties [13]. Therefore, in this context, NaA zeolite membrane was
chosen as the first modification layer to smooth the surface of the PSS
microchannel. Seeded growth method was adopted by using large
zeolite particles as seeds to prepare the NaA zeolite layer. XRD and
SEM analysis showed in Figs. S2 and S3 clearly revealed that the used
NaA zeolite seeds had a pure LTA-type structure and a mean particle
size of ~1 μm. After the secondary growth, the surface topography of
synthesized NaA zeolite membrane on PSS microchannel was shown
in Fig. 1(c) and (d). It could be observed that the PSS multichannel
surface was completely covered by a compact layer of NaA zeolite crys-
tals, and the macro pores that were originally visual on the PSS substrate
disappeared. The XRD pattern showed in Fig. S2 further confirmed the
typical NaA zeolite crystalline structure of the obtained membrane.
In the ZnO-induced method for ZIF-8 membrane synthesis, it is
crucial to form a continuous and well adherent ZnO particles layer on
the substrate. On one hand, the ZnO layer acted as a precursor seed
layer for continuous ZIF-8 membrane growth on different substrates
by reacted with the Hmim solution. On the other hand, the introduction
of ZnO layer prevented the delamination between ZIF-8 membrane and
the substrate, resulting in excellent structure stability [9]. After the NaA
zeolite modification layer was synthesized, a layer of ZnO particles
was introduced as the seed induction layer to facilitate the following
ZIF-8 membrane growth on the NaA zeolite membrane. Fig. 1(e) and
(f) showed the top view SEM images of ZnO particles grew on NaA
2.2. Catalytic performance tests
Knoevenagel condensation between BA and ECA was carried out in a
continuous flow MMR plant as showed in Fig. S1. The reactant mixture
with equimolar BA and ECA dissolved in dimethyl sulfoxide (DMSO) as
solvent was fed using a microflow pump (BT50-1J, Baoding Longer Pre-
cision Pump Co., Ltd.). The reaction mixture was collected and analyzed
by gas chromatography (GC7890F, Shanghai Techcomp Ltd.) using a
HP-5 column (30 m × 320 μm × 0.25 μm, Agilent) and flame ionization
detector.
3. Results and discussion
3.1. Characterization of composite MMR
Porous stainless steel (PSS, 316L) is an excellent substrate material
in membrane reactors for its potential advantages such as good corro-
sion resistance, excellent mechanical properties and low cost. However,
some characteristics related to the surface of this support, such as pore
size and roughness, have a crucial influence on the membrane growth
and make the generation of a totally continuous and defect-free thin
membrane difficult. Indeed, as evident from the SEM images (a) and
(b) in Fig. 1, there were only some ZIF-8 particles deposited and not
a continuous membrane layer generated in the unmodified PSS
microchannel by direct growth. To overcome this problem, an efficient
strategy was adopted to incorporate an intermediate layer between
Fig. 1. Top view SEM images of PSS microchannel after in situ ZIF-8 membrane growth (a) & (b), NaA zeolite membrane (c) & (d), ZnO particles layer (e) & (f), ZIF-8 membrane (g) & (h).

G. Zhang et al. / Catalysis Communications 68 (2015) 93–96
95
zeolite membrane. It revealed that a uniform and continuous ZnO parti-
cles layer covered the NaA zeolite membrane and the ZnO particles
glued the NaA zeolite layer compactly. The XRD pattern showed in
Fig. S4 confirmed the existence of the ZnO particles layer. After the
ZIF-8 membrane synthesis, the surface topography of ZIF-8 membrane
was shown in Fig. 1(g) and (h). It was clear that a continuous and
compact ZIF-8 membrane was formed with the assist of ZnO particles
covered on the NaA zeolite layer and the membrane layer exhibited
the typical surface morphology of ZIF-8 membrane. The XRD pattern
showed in Fig. S5 further proved the ZIF-8 crystalline structure.
The ZIF-8/NaA composite membrane structure was further confirmed
by the cross-sectional EDX mapping analysis shown in Fig. 2. The ele-
ments distribution in image (a) revealed that a continuous ZIF-8/NaA
composite membrane was formed along the PSS microchannel inner
wall. The detailed information in image (b) implied that the NaA zeolite
membrane (red area indicated by element Si) smoothed the PSS (green
area indicated by element Fe) microchannel surface by padding the
macropores. The ZIF-8 membrane (Blue area indicated by element Zn)
grew along with the NaA zeolite membrane surface, leading to the forma-
tion of the composite membrane structure.
Fig. 3. Reaction results at different reaction temperature: 30 °C (a), 40 °C (b), 50 °C (c).
Conditions: Residence time was 20 min; molar ratio of BA:ECA:DMSO was 1:1:1.
3.2. Catalytic performance
complex and regulate the reactant partitions at the solid–liquid inter-
face resulting in lower activation energy and faster reaction [14,15]. As
expected, more solvent usage exhibited higher product yield and better
catalytic stability. Nevertheless, from the green and economic chemistry
point of view, excess solvent dosage resulting in extra separation oper-
ating cost was not sustainable. On the premise of obtained high product
yield and good performance about catalyst stability, reaction carried out
with BA:ECA:1 DMSO was more rational than BA:ECA:5 DMSO in this
ZIF-8/NaA composite MMR.
The residence time dependence of the catalytic performance was
studied in this ZIF-8/NaA composite MMR at 5, 10 and 20 min. As a con-
trast, reactions in PSS microreactor and pure NaA zeolite MMR were also
investigated under the same conditions. In continuous flow heteroge-
neous catalysis reaction, the residence time represents the real contact
time of reactants and catalyst. The product yield mostly rises with the
residence time. Indeed, the results showed in Fig. 5 revealed that the
product yield increased from ~83% to ~99% with prolonging the
residence time from 5 to 20 min in the ZIF-8/NaA composite MMR. In
comparison with the results obtained in the PSS microreactor and
pure NaA zeolite MMR, ZIF-8 was further proved to be an efficient
catalyst for Knoevenagel condensation, which was in agreement with
the conclusion in the literature [10].
Continuous flow Knoevenagel condensation between BA and ECA
was conducted in this ZIF-8/NaA composite MMR. Three important pa-
rameters including reaction temperature, solvent usage and residence
time were investigated for catalytic performance test. The effect of
reaction temperature on the catalytic performance was studied at 30,
40 and 50 °C and the results were shown in Fig. 3. As an endothermic
reaction, it was expected that the Knoevenagel condensation reaction
rate increased quickly with the temperature. The product yield at
40 °C could reach a high level (N99%) within 20 min. Lower reaction
temperature resulting in uncompleted reactants conversion and higher
reaction temperature resulting in energy wastage were both unfavorable
for reactor performance.
The solvent usage dependence of the catalytic performance was in-
vestigated by performing the Knoevenagel condensation reaction with
different amounts of DMSO. The results in Fig. 4 showed that without
solvent, the catalyst suffered rapid deactivation with product yield
decreasing from ~88% to ~75% within 8 h. Reactions carried out with
solvent DMSO displayed high product yield (i.e., N98%), but the reaction
using less solvent of BA:ECA:0.5 DMSO resulted in a significant drop in
product yield to below ~90% within 8 h. Reactions carried out with
BA:ECA:1 DMSO and BA:ECA:5 DMSO displayed high product yield of
~99% and good performance about the stability of the catalyst within
8 h. It is reported that the solvent can stabilize the transition-state
It is well known that, for an equilibrium reaction constrained
by unfavorable thermodynamics, selective removal of one or more
products from the reaction can obtain higher conversion according
Fig. 2. Cross-sectional EDX mapping analysis (green area—Fe, red area—Si, blue area—Zn) of ZIF-8/NaA composite membrane structure.

96
G. Zhang et al. / Catalysis Communications 68 (2015) 93–96
Long time stability was investigated for this ZIF-8/NaA composite
MMR at the optimized conditions of reaction temperature of 40 °C,
feed molar composition of BA:ECA:DMSO of 1:1:1 and residence time
of 20 min. The results in Fig. 5 demonstrated that no obvious deactiva-
tion was observed and at least 97% product yield could be maintained
over 50 h of reaction, showing an excellent catalytic stability. Compared
with the optimized conditions of reaction temperature of 90 °C, feed
molar composition of BA:ECA:DMSO of 1:1:5 and residence time of
30 min obtained in Cs ion exchanged of faujasite NaX zeolite MMR
in our previous work [18], this ZIF-8/NaA composite MMR exhibited
excellent catalytic performance under mild conditions.
4. Conclusions
This work reported a continuous flow ZIF-8/NaA composite MMR
fabricated on PSS plate by stepwise membrane growth strategy. NaA
zeolite membrane was introduced as a modification layer and ZIF-8
membrane was subsequently grown as a catalytic layer by using ZnO-
induced synthesis. The obtained ZIF-8/NaA composite MMR exhibited
high product yield and excellent reaction stability in liquid phase
Knoevenagel condensation between BA and ECA under mild conditions.
It is believed that this ZIF-8/NaA composite MMR with excellent perfor-
mance possesses potential and broad prospects in organic synthesis or
catalysis.
Fig. 4. Reaction results at different solvent usage of BA:ECA:DMSO = 1:1:x in which x = 0
(a), x = 0.5 (b), x = 1 (c), x = 5 (d). Conditions: Residence time was 20 min; reaction
temperature was 40 °C.
to the Le Chatelier's principle. In the previous report [6], the Knoevenagel
condensation between BA and ECA was conducted in ZSM-5 zeolite MMR.
Supra-equilibrium conversion with 85% product yield based on the
equilibrium yield of 60% was obtained through selective removal
of the water byproduct by pervaporation operation. In this work,
99% product yield could be achieved in residence time of 20 min, thus
demonstrating better performance with higher supra-equilibrium
conversion and product yield in this ZIF-8/NaA composite MMR.
Microreactors benefit reaction efficiency by improving mass and heat
transfer rates through increased interfacial area and shorter diffusion
path [16]. The performance of MMR is limited mainly by the kinetics
and a better catalyst is needed to obtain a higher product yield [17].
There is no doubt that this ZIF-8/NaA composite MMR exhibited
excellent Knoevenagel condensation performance by integrating the
advantages of microreactor and efficient ZIF-8 catalyst.
Acknowledgments
The authors gratefully acknowledge the Specialized Research Fund
for the Doctoral Program of Higher Education (20130041110022)
and the National Natural Science Foundation of China (21476039,
21173030 and 21076030).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.05.008.
References
[1] R.L. Hartman, K.F. Jensen, Lab Chip 9 (2009) 2495–2507.
[2] K.S. Elvira, X.C. i Solvas, R.C.R. Wootton, A.J. de Mello, Nat. Chem. 5 (2013) 905–915.
[3] L.K. Minsker, A. Renken, Catal. Today 110 (2005) 2–14.
[4] B.P. Mason, K.E. Price, J.L. Steinbacher, A.R. Bogdan, D.T. McQuade, Chem. Rev. 107
(2007) 2300–2318.
[5] C. Wiles, P. Watts, Green Chem. 14 (2012) 38–54.
[6] S.M. Lai, C.P. Ng, R.M. Aranda, K.L. Yeung, Microporous Mesoporous Mater. 66
(2003) 239–252.
[7] X.Y. Tan, K. Li, J. Chem. Technol. Biotechnol. 88 (2013).
[8] B.L. Chen, Z.X. Yang, Y.Q. Zhu, Y.D. Xia, J. Mater. Chem. A 2 (2014) 16811–16831.
[9] X.F. Zhang, Y.G. Liu, L.Y. Kong, H.O. Liu, J.S. Qiu, W. Han, L.T. Weng, K.L. Yeung, W.D.
Zhu, J. Mater. Chem. A 1 (2013) 10635–10638.
[10] U.P.N. Tran, K.K.A. Le, N.T.S. Phan, ACS Catal. 1 (2011) 120–127.
[11] G.H. Yang, X.F. Zhang, S.Q. Liu, K.L. Yeung, J.Q. Wang, J. Phys. Chem. Solids 68 (2007)
26–31.
[12] J.A. Calles, R. Sanz, D. Alique, L. Furones, Int. J. Hydrogen Energy 39 (2014)
1398–1409.
[13] M.L. Bosko, J.F. Múnera, E.A. Lombardo, L.M. Cornaglia, J. Membr. Sci. 364 (2010)
17–26.
[14] J. Gascon, U. Aktay, M.D. Hernandez-Alonso, Gerard P.M. van Klink, F. Kapteijn, J.
Catal. 261 (2009) 75–87.
[15] A. Corma, S. Iborra, I. Rodríguez, F. Sánchez, J. Catal. 211 (2002) 208–215.
[16] A. Gavriilidis, P. Angeli, E. Cao, K.K. Yeong, Y.S.S. Wan, Trans. IChemE 80 (2002)
3–30.
[17] S.M. Lai, R.M. Aranda, K.L. Yeung, Chem. Commun. (2003) 218–219.
[18] G.C. Zhang, X.F. Zhang, J. Lv, H.O. Liu, J.S. Qiu, K.L. Yeung, Catal. Today 193 (2012)
221–225.
Fig. 5. Reaction results at different residence time in PSS microreactor (a), pure NaA zeolite
MMR (b), ZIF-8/NaA composite MMR (c) and long-time continuous flow catalytic
performance in ZIF-8/NaA composite MMR. Conditions: Temperature was 40 °C; molar
ratio of BA:ECA:DMSO was 1:1:1.