Monolith Reactors for Intensified Processing in Green Chemistry - Process Intensification for Green Chemistry - page 175

Figure 6.1 Structured monoliths and their characteristics. Reprinted from Heibel et al
# 2001, with permission from Elsevier.
Figure 6.1. The central monolith in the figure illustrates the use of fins to increase the
available surface area for catalyst coating and improve the interfacial area for flowing films
that coat the surface during operation [25].
Having selected a suitable design for the support structure, the channels must be
washcoated with a thin layer of porous material, which will act as a high-surface-area
support for the catalyst metals. In preparing a washcoat, a powder such as g -alumina is
milled to the appropriate particle size, possibly mixed with a structure improving agent
such as dispersable boehmite and then mixed with water to form a slurry [26]. The pH of
the slurry is controlled using dilute HNO 3 or NaOH. Washcoating is achieved by dipping
the monolith into the slurry at a controlled rate, blowing excess slurry off the structure with
compressed air, then drying and calcining. Stutz and Poulikakos [22] studied the optimum
thickness of washcoat, for which it was noted that a thin washcoat gives a smaller, limiting
amount of catalyst, leading to a lowmethane reactant conversion in Syngas production. For
a thicker washcoat, the limiting effect is the reduced residence time, which occurs when the
channel cross-sectional area open to flow of fluid becomes narrower, resulting in a higher
flow velocity for the constant throughput of gas.
Mogalicherla and Kunzru [26] studied whether impregnating palladium on g -alumina and
using the resultant Pd/ g Al 2 O 3 powder to prepare the slurry for washcoating was more
effective than g -alumina washcoating followed by impregnation of palladium. Well-adhered
washcoats were observed using slurry prepared with g -alumina, but the washcoat prepared
fromPd/ g Al 2 O 3 powder was very poor and led to a lower rate of reaction in the hydrogenation
of a -methyl styrene than the former catalyst at low washcoat loadings. The addition of
binders could be used to improve the adhesion of the Pd/ g Al 2 O 3 powder washcoat.
Generally it is desirable to produce a uniform washcoat loading rather than an uneven
one. However, in contrast with uneven channels and washcoats, some novel designs have