Structured catalysts and reactors
Regular arrangement of catalysts in reactors decouples the scale dependent and independent phenomena, such as intrinsic kinetics, thermodynamics, mass and heat transport and hydrodynamics. This allows their independent optimization so that all rate processes in a reactor are in balance and the catalyst is used in the way it was designed for. Structuring ranges from the molecular to the reactor scale in a hierarchical way. Microscopically zeolites, MOFs and well defined clusters are used. Macroscopically one should think of monoliths, foams, corrugated packing etc. Combined with multifunctional operation this approach can give a large boost to process intensification. Radial heat transport in reactor packings is a big challenge and subject of study.
Multifunctional catalysis and reactors
Catalytic conversions are frequently performed in isolation. Often tedious and/or energy intensive separations are needed to purify products before further processing. Smart combination of catalytic reactions or reaction and separation may eliminate this need, allowing higher single pass conversions, reducing separation effort and energy consumption and increasing process efficiency. Examples of single reactor operations and reactive separations:
- Coupling endo- and exothermal reactions (dehydrogenation and oxidation)
- Selective removal of a product through a membrane in equilibrium limited reactions (water gas shift, dehydrogenation, esterification)
- Selective feeding of reactants
- Membrane reactors where combinations of the above occurs
- Dynamic kinetic resolution
This can be done on different scales, on the active site level, on the catalyst particle level and on a reactor level. Major challenge is to synthesize those combinations that result in overlapping of the operational regimes of the catalytic reactions and/or the separation processes. New membranes or catalysts are often needed.
Zeolite and metal-organic framework based membranes
Porous crystalline materials like zeolites and metal-organic frameworks (MOFs) posses uniform pores or windows of molecular size. Thin continuous layers on a support material offer a unique separation potential, namely based on molecular size or shape. We were among the first to develop silicalite-1 membranes. Their permeation and separation properties have been studied and modeled extensively.
Challenges are to synthesize membranes of other zeotypes and MOFs to be used in process intensification for energy reduction and multifunctional reactors, combining catalysis and separation. Clear examples are hydrogen and CO2 separation from various sources and the separation of propane from propene.
In a EU funded project (M4CO2) MOFs embedded in a polymeric matrix are developed as membranes for the separation of CO2 from stack gases and for the production of hydrogen. These MOF based Mixed Matrix Membranes are promising alternatives for the pure MOF or zeolite membranes.
Rate and transport processes
The overall performance of catalysts or sorbents in a process is the result of all contributing phenomena. With thermodynamics determining the driving forces for chemical conversion and transport phenomena, the kinetics of these processes determine the productivity and yields. Transient and steady state techniques are applied to determine catalytic reaction kinetics. Parallel reactors (‘six-flow’) were already applied long before commercial activities in this field developed. Sorption of single components and mixtures are essential for correct description of diffusion in zeolites and membranes and to determine the correct reactant concentrations at the active site of catalysts. Advanced techniques are used for the determination of the transport parameters, like TAP, TEOM, ZLC and breakthrough analysis.