Dissolving homogeneous catalyst complexes or biocatalysts in thin films of liquid on porous supports can overcome the intrinsic separation problem associated with such liquid phase catalysts. High boiling organic liquids were intensively studied as supported liquid phase (SLP) catalysts in the 1970s, while thin water films were developed as supported aqueous phase (SAP) catalysts in the early 1980s. The advent of ionic liquids and molten salts has seen supported ionic liquid phase (SILP) materials being developed around 2000. The intrinsic property of having an extremely low vapor pressure makes SILP materials ideally suited for continuous gas-phase applications.