FLUENT lets you describe a multiphase species transport and volumetric reaction (Section 23.8) in a fashion that is similar to setting up a single-phase chemical reaction using the Species Model panel (e.g., Figure 23.9.3).
Define Models Species Transport & Reaction...
In the Phase Properties panel, the material for each phase is listed in the Material drop-down list. From this list, you can choose the material that you want to use for a specific phase. The drop-down list contains all of the materials that have been defined for your simulation. If you want to inspect or edit any of the properties of any of the materials, then you need to open the Materials panel by clicking the Edit... button.
When modeling multiphase species transport, additional inputs may also be required depending on your modeling needs. See, for example, Section 23.9.6 for more information defining heterogeneous reactions, or Section 23.9.7 for more information on mass transfer effects.