(596d) Novel “Sandwich Microreactor” for Heterogeneous Catalytic Processes:

one-step benzene to phenol with N₂O as a
case study

The manufacture of chemicals in catalytic
microreactors has become recently a new branch of chemical reaction engineering
focusing on process intensification and safety. Chemical microstructured
reactors (MSR) have multiple parallel channels with diameters between ten and
several hundred micrometers in which the chemical transformation occur. This
gives a high specific surface area in the range of 10000 to 50000 m²/m³
and allows an effective mass and heat transfer if compared to traditional
chemical reactors having usually ~100 m²/m³.

The important feature of MSR is
an integrated heat exchange, which makes the key difference
between MSR and other structured reactors, like honeycombs. MSR are
operated under laminar flow with the heat transfer coefficient for liquids
about 10 kW/(m²×K).
This is one order of magnitude higher than in the traditional heat exchangers
allowing to avoid hot-spots formation, to attain higher reaction temperatures
and to reduce reaction volumes. This in turn improves the energy efficiency and
reduces the operational cost.

One of the main problems in using
MSR for heterogeneously catalyzed gas-phase reactions is the introduction of
the catalyst in the reaction zone. The straight forward way is to fill microchannels
by catalyst powder, but this leads to high pressure drop. In addition, each
channel must be packed identically to avoid maldistribution, which is known to
broaden residence time distribution diminishing reactor performance.

Another approach
is MSR with catalytically active walls. The
specific surface area is increased by chemical treatment of the channel walls
or by their coating with a porous layer. The porous layer can serve as a
catalyst or a support for a catalytic phase. The main limitation for catalytic
wall MSR is the thickness of the porous layer. Since the majority of MSR are
used for fast highly exothermic reactions, the layer should be < 1-2 mm in order to avoid mass/heat transfer limitations.
Therefore, the total mass of the catalyst is too small for achieving process
intensification referred for a unit of the reactor volume.

In this paper we
present a new design of MSR: the ?Sandwich Microreactor?. It is constructed
from thin (~0.3mm) porous plates of sintered metal fibers (SMF) serving as
structured catalytic beds. SMF plates are sandwiched between regular metallic
plates, which provide integrated heat exchange. Each SMF plate has a porosity
of ~80% and an average diameter of individual fiber ~0.010 mm. The thin SMF plate possesses a 3-dimensional
regular microstructure, giving low pressure drop during the passage of reacting
gases. The SMF was coated by catalytically active Fe-ZSM5 thin film (< 0.002mm)
and the Sandwich MSR was tested for the benzene hydroxylation to phenol with N₂O.
The reaction temperature was controlled in a narrow range, leading to high
selectivity towards phenol formation. The results will be presented in detail.