ABSTRACT
As much discussed in other parts of this book, honeycomb monoliths have become the standard catalyst shape in most applications of environmental catalysis. However, the adoption of monolithic catalysts in other areas of heterogeneous catalysis started to be explored only at the beginning of the 1980s, after their successful commercial application to the control of automotive exhausts and to the reduction of nitrogen oxides. Particularly attractive were the expectedly low pressure drop and the potentially smaller size of the reactor as compared to conventional pelletized catalysts in gas-phase processes; early studies in this field, using methanation and hydrogenation as model reactions, pointed out additional prospective benefits. For example, in a pioneering piece of work, Tucci and Thomson carried out a comparative study of methanation over ruthenium catalysts both in pellet and in honeycomb form [1]. In addition to pressure drops lower by two orders of magnitude they found also significantly higher selectivities (97% versus 83%) over the monolith catalyst, likely resulting from lower internal diffusional resistances. Parmaliana and co-workers [2-5] investigated the hydrogenation of benzene and dehydrogenation of cyclohexane in ceramic monoliths washcoated with alumina impregnated with either Ni or Pt. Again, the low diffusion resistance of monolith catalysts allowed the authors to determine intrinsic kinetic expressions based on an Eley-Rideal mechanism.
