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.