The Catalyst Review July 2019 - 9

SPECIAL FEATURE
× 10 over 24 h (Motokura et al. 2009). Such materials also offer
the ability to tune the solid support with additional functions,
with synergistic effects. Application of such materials have
also been done in organic chemistry in the for asymmetric
aldolisation on silica-supported L-proline organocatalysts
(Zamboulis et al. 2009), for example.
However, despite the recognized potential of heterogenized
catalysts, relatively few processes exploiting such materials
have been implemented in industry, mostly in biocatalysis,
and practical applications of these materials remain a
challenge. For example, supported homogeneous catalysts
can be handled like heterogeneous catalysts and used in
simple flow reactors. Despite this idea being simple in
principle, no process exploiting this idea has been so far
commercialized (Barbaro & Liguori 2010).

Figure 1. Difference between (a) traditional homogeneous catalysts, i.e., the
catalytic function (X) is in solution and (b) heterogenized catalysts, i.e., the same
catalytic function (X) is immobilized over a solid support, which makes the
catalyst insoluble and hence easy to separate and recycle.

Source: Author 2019

Challenges in Heterogenized Catalysis
Where does the issue lie with heterogenized catalysts then? There are multiple reasons as to why the application of these materials
has so far been limited, which can be summarized in the following key aspects.
Intrinsic Reaction Rate and Influence of the Solid Support

One issue reported for homogeneous catalysts heterogenized over solid supports is the generally lower reaction rate of the
heterogenized material compared to the homogeneous counterpart. For example, when studying chiral immobilized iridium
diphosphine complex for the asymmetric hydrogenation of an imine to (S)-metolachlor, the turnover frequency (TOF) of the
homogeneous catalysts immobilized over silica gel was found to be 8-to-10 times lower than that of the homogeneous catalyst
(Barbaro & Liguori 2010). The reasons for such a drop-in activity are to a large extent unclear. A key role could be played by selfaggregation of the catalytic functions when those are immobilized over the surface, leading to loss of catalytic sites available to react
with the substrate. In addition, the role of the support is still unclear. It has been reported that surface chemistry of support surfaces
can significantly influence adsorption of reactive species, which in turn affect surface concentration and rate of reaction. To address
such issues, an in-depth investigation to underpin key parameters affecting the rate of reaction is needed.
Stability and Deactivation of the Catalyst

Another issue related to immobilized homogeneous catalysts is related to their stability. For such materials to be commercialized,
recycle and reuse-as well as stability of catalytic performances of the reused materials-is an essential feature that needs to be
addressed. Ideally, the homogeneous catalyst should be immobilized in such a way that it remains tightly bound to the surface. A
major cause of instability and deactivation of heterogenized catalysts is often associated with leaching of the catalytic functions in
solution, the extent of which depends on the type of immobilization. One would expect leaching to be a more severe problem for
materials whereby the catalytic function is immobilized through physical interactions whereas materials prepared by either covalent
or ionic bonds should be more resistant to leaching (Zhao et al. 2006). To suppress or mitigate leaching problems, a key point to
address is how to make the bonding between the homogeneous catalytic function and the support stronger. Another cause of
deactivation can be due to self-aggregation of the immobilized catalyst molecules, which leads to a decrease in available catalytic
sites over the surface and could in theory affect also reaction mechanisms, hence activity and selectivity. In the case of immobilized
metal nanoparticles, aggregation can also lead to the formation of larger, less active agglomerates (Barbaro & Liguori 2010). Thermal
stability can also be an issue as it may affect leaching as well, however in the case of many liquid-phase catalytic processes (e.g., fine
chemical production) processes can be run at relatively mild conditions hence avoiding problems such as thermal decomposition of
the catalytic material.
Currently, there is little and scattered information on fundamental understanding of stability and deactivation of heterogenized
catalysts and to achieve further developments that can lead to heterogenized catalysts with improved stability systematic research
efforts are needed.
Mass Transport Effects

A major issue arising in heterogenized catalysts is that of mass transport, particularly that of intra-particle diffusion. Homogeneous
catalysts in solutions are generally free from any mass transport limitation; however, when such catalysts are immobilized inside
The Catalyst Review

July 2019

The Catalyst Review July 2019

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