Introduction

One of the most important challenges of future research is the optimization of processes by reducing environmental and production costs. In this contest Process Intensification (PI) plays a crucial role^[1], as strategy to reduce the sizes of chemical plants, integrating multiple processing steps into a single unit operation, and optimizing critical process parameters such as heat and mass transfer. The most used reactors in heterogeneous catalyzed reactions are the fixed bed one, while the most used catalysts are pellets, spheres and in general non-thermally conductive materials^[2]. Despite the wide diffusion, this type of catalysts represents a serious limit to the efficiency of catalytic processes of primary importance, such as those based on exothermic reactions, for example CO-Water Gas Shift (WGS). To overcome the limitations due to the increasing of the temperature on the catalytic bed, in adiabatic conditions, caused by the heat of the reaction, this kind of processes is commonly  performed into multiple processing steps, in which to each step, a cooling follows. A multistage process allows high reaction rates to be exploited in high temperature stages, and to achieve high conversions of the reagents in the low temperature stages. This type of reactor configuration is certainly effective, but not at all efficient, being energetically wasteful and not suitable for small-scale production, for example in the distributed production of hydrogen. Despite the huge number of works published, much space has been reserved for the study of new catalytic formulations, much less to innovative reactor systems. In this article we present a summary of our studies on the intensification of WGS process by the use of highly thermal conductive structured catalysts, obtained by coupling highly active catalytic formulations with conductive aluminum foam structures.

Materials

The structured catalysts are characterized by a structure with a prefixed geometry, they can be ceramic or metal in nature and usually contain the active components as a dispersed phase^[3]. The structures are generally characterized by low surface areas and sometimes are incompatible with the active phases, making it necessary to use high surface area compatibilizers that act as a primers^[4]. The most suitable primers are for sure the alumina-based materials; alumina is considered a key component in the development of structured catalysts^[5], as a main component of washcoating slurries, widely used in the preparation of these catalysts^[6]. The alumina guarantees a good adhesion to the structured carriers, a high surface area and the possibility to load the active components with easy methods, however it is not inert with respect the most of heterogeneous catalytic reactions therefore, in the design of the catalyst, the effect on the process must be evaluated^[7].

The traditionally used catalysts for water gas shift reaction, are based on iron/chromium for high temperature shift (HTS) and copper/zinc for low temperature shift (LTS) however, more recently the attention has been focused on the use of noble metals supported on reducible oxides, such as ceria and ceria/zirconia^[8]. Noble metals based catalysts present numerous advantages, between which, high activity in a wide range of temperatures and safety are of primary importance. As already mentioned the structures can be ceramic or metal in nature; the choice of the type cannot disregard the objectives, so that the thermal conductivity and the geometry have been evaluated with the intent to guarantee both an optimal heat distribution on the catalytic bed, and an effective interaction between the reactants in the gas phase and the surface of the catalyst.

In our preliminary studies we demonstrated that aluminum foams show both excellent heat exchange with the gaseous phase, promoting a flattening of the profile, and irrelevant pressure losses even with a typical flow rate of pilot plants^[9]. In the last years our group has tuned the preparation of structured catalysts by washcoating of aluminum foams and aluminum monoliths with alumina-based colloidal suspensions, followed by impregnation with the precursor salts of ceria and zirconia supports and with the precursor salts of platinum. The resulting structured catalysts were tested in WGS reaction, the performance were evaluated with respect those of the corresponding powder catalysts^[10], the results related to the textural properties^[11] and the type of carrier^[12] and validated by CFD modeling studies^[13].

Results

The