Chapter 5 - Monolithic Catalysts and Reactors: High Precision with Low Energy Consumption

Abstract

Structured catalysts and reactors offer high precision in catalysis at all relevant scales of the catalytic process, from that of the catalytic species up to that of the reactor. Monoliths are the prime example of such catalysts because of their wide practical applications. Thus, monoliths are emphasized in this review, but most of the text is also relevant to all structured reactors, including microreactors. Conceptually, monoliths exhibit more degrees of freedom in design than conventional reactors, such as fixed-bed and slurry reactors. The flow in monoliths is laminar, and as a consequence, they are associated with high efficiency and minimum chaotic characteristics. The hydrodynamics of single-phase and multiphase flow reactors are remarkably simple. Under most conditions in multiphase systems, Taylor flow (segmented flow) prevails, associated with high rates of mass transfer notwithstanding low energy consumption, but under other conditions, the film flow regime can be realized either in cocurrent or in countercurrent flow of gas and liquid streams, making the monolith a good structure for novel technologies such as catalytic distillation. Monoliths offer freedom in the design of reactor configuration. Examples are loop reactors for strong exo- and endothermic reactions, which allow a combination with separate heat exchange without the penalty of a large energy consumption, which otherwise is usually unavoidable for the large recycle ratios needed. For applications in fine chemistry and in the laboratory, a convenient monolithic stirred reactor is presented. The principal bottleneck for practical application of monolith reactors is the synthesis rather than the design of the catalytic monolith. When a monolith reactor is considered as an alternative to a fixed-bed reactor packed with commercially available catalyst particles, the grim reality is that a development program is needed to producing the catalytic monolith. Therefore, preparation methods including synthesis of various coating layers and the deposition of active catalytic species are described in detail here. This chapter also includes an exhaustive review of practical applications of monolith reactors. In applications in which high gas flow rates have to be accommodated, monoliths monoliths are the state of the art in many cases, exemplified by automobile exhaust abatement reactors—because of the popularity of automobiles, more monolithic reactors are being used than fixed-bed reactors. Applications in processes with liquid-phase and gas–liquid-phase reactants are scarce, but one well-known commercial process (the reduction step in the production of hydrogen peroxide) shows the feasibility of monoliths. Several processes are in the development stage. Included in the review are an assessment of the impact of these reactors on process intensification and applications in biotechnology and photocatalysis.