1 Introduction
Formaldehyde is an important chemical intermediate and
product1,2, and the production of formaldehyde by
non-oxidative direct dehydrogenation of methanol has attracted
widespread attention by academia and industry3,4. No
greenhouse gas CO2 is produced during the process of
dehydrogenation of methanol with anhydrous formaldehyde obtained. In
general, the fixed bed reactors with catalyst particles stacked randomly
are used for dehydrogenation of methanol5. In this
case, internal and external diffusion effects cannot be avoided at the
scale of one catalyst particle owing to the limitation of mass transfer
and heat transfer6, which can weaken the catalytic
performance. Besides, the fluid channel between the catalyst particles
is uneven due to a large number of particles stacked randomly, leading
to the uneven flowing of the reactant across the catalyst particles. The
uneven flowing in the catalytic bed will cause non-uniform distribution
of residence time, temperature field and concentration field, which can
further weaken the catalytic performance7,8.
Especially, the local porosity of the catalytic bed is higher near the
wall of the reactor and a channeling phenomenon is prone to
existed9, which could make the non-uniform field
distribution more serious. The non-uniform distribution of the field and
the residence time would cause the formaldehyde converting into
by-products such as CO, HCOOH, and
HCOOCH34,10 .
Membranes with micro or nano scale pores are good carriers for catalyst
nanoparticles immobilization11. The catalytic membrane
nano reactor (CMNR) with deep-permeation nanocomposite structure (DPNS)
would be obtained if the nanoparticles are in situ immobilized in
membrane pores along the thickness direction of the membrane.
Multi-scale synergistic enhancement for catalysis can be achieved in
CMNR during the subsequent dehydrogenation of methanol. (1) At the
nanoparticles scale, the catalysts are immobilized in the membrane pores
without being shaped. In this case, the reactant fluid can contact with
all nanoparticles via flowing instead of diffusion12,
which could increase the effective contact area between catalysts and
reactant fluid, leading to the internal diffusion effect in conventional
fixed bed reactors being eliminated. (2) At the scale of membrane pores,
the catalytic reaction is confined within the membrane pores, which can
reduce the thickness of the mass transfer boundary
layer13, improve the mass transfer efficiency, and
eliminate the external diffusion effect in the conventional fixed bed
reactor14. (3) At the scale of the catalytic bed
layer, a sheet of membrane is designed as a catalytic bed layer, and
good flowing uniformity can be achieved owing to the uniform disperse of
the membrane pores. Therefore, the uniform distribution of residence
time, concentration field and temperature field will be
achieved15. Moreover, the multi catalytic bed layers
can be set up in series with each layer controlled independently.
Therefore, the precise control of temperature within the whole reactor
can be achieved during the catalysis process.
In our previous study, it has been found that the catalytic
nanoparticles can be in situ immobilized in membrane pores by
flowing synthesis16-20. During this process, the
precursor fluid for nanoparticles formation can permeate through the
membrane pores under the external driven force. Several advantages can
be realized by flowing synthesis (1) The catalytic nanoparticles can be
immobilized in each membrane pore and distributed evenly along the
thickness direction of the membrane. Under the external driven forces,
the precursor fluid can overcome the surface tension of the liquid and
flow in each membrane pores, which ensures the catalytic nanoparticles
immobilized in each membrane pore; (2) Rapid mixing of the precursor and
enhanced mass transfer and heat transfer can be achieved in the confined
space of the membrane pores with micro or nano scale, which could
promote the nanoparticles formation and structure
regulation21; (3) According to the principle of deep
filtration in porous media, the catalytic nanoparticles can be
immobilized stably in tortuous membrane pores during flowing synthesis
process under the effect of inertial force, Brownian motion and other
factors.
In the field of methanol dehydrogenation, Cu and ZnO nanoparticles were
performed high activity and remarkable selectivity to
formaldehyde4,22-24. It has been reported that the
yield of formaldehyde could be achieved at 60% under 650 ℃ with the
catalysts containing Cu and Zn. In this work, porous Titanium membrane
(Ti membrane) with good thermal conductivity, thermal stability and
mechanical properties was used as the substrate. In order to increase
the loading amount of Cu/ZnO nanoparticles immobilized in membrane
pores, the Ti membrane substrate was firstly treated by heat alkali and
silanization modified with amino groups grafted in membrane pores for
metal ion chelation before Cu/ZnO nanoparticles immobilized. After that,
the zeolitic imidazolate framework-8 (ZIF-8) was in situimmobilized in the treated Ti membrane substrate by flowing synthesis,
then the Cu2+ was introduced by ion-exchange between
Cu2+ and Zn2+ in ZIF-8. Finally,
Cu/ZnO nanoparticles were immobilized in the membrane pores by calcining
and reducing to fabricate the Cu/ZnO/Ti CMNR. During the experiments of
methanol dehydrogenation reaction, the effect of reaction temperature,
reactant gas flux and the number of membranes setting up in series has
been determined in CMNR.