The Catalyst Review July 2019 - 12

SPECIAL FEATURE
In conjunction with experiments, molecular dynamics simulation can also help rationalize the behavior of chemical reactions
inside heterogenized catalysts. For example, there have been studies on the asymmetric epoxidation reaction of cis- and transmethylstyrene on oxo-salen confined in the mesopore of MCM-41 using molecular dynamic simulations, which provided new insights
into the importance of electronic and steric effects of the salen ligand, substrate, immobilizing linker, and MCM-41 channel (Malek
et al. 2007). The calculations highlighted the influence of the confined framework, substrate (cis- vs trans-) and immobilizing linker
on the conformation of the immobilized Mn-salen complex (the actual homogeneous catalyst), which were able to explain the
enantioselectivity of the immobilized Mn-salen complex in MCM-41 in terms of the dynamics of the salen ligand.
Summary and Outlook
Immobilization of homogeneous catalysts onto solid supports represents a promising way to manufacture a variety of chemicals
by using materials with performances comparable to homogeneous catalysts as well as practical advantages in catalyst handling,
recycling and separation, including the possibility to run chemical processes using continuous fixed-bed reactors. Currently the
implementation of such processes on an industrial scale has not yet been achieved, mostly due to a series of challenges related to
catalyst performances, stability and poor understanding of the physical chemistry of such processes. To further develop this area
of catalysis major research efforts are needed; improving formulation and structure of such materials is essential and to do that, a
better understanding and quantification of physico-chemical aspects of such reactions, including transport and adsorption and how
these affects the overall reaction rate, is needed. In-situ spectroscopy such as X-ray adsorption spectroscopy, IR, NMR in conjunction
with molecular modelling can certainly lead to improve design and optimisation of such materials, which can then be engineered on
a larger scale both for batch and continuous processes.
Reactor design is another aspect to consider when making such processes commercially viable, with a emphasis on the design
and optimization of continuous processes, which can be in principle easily implemented using heterogenized catalytic materials.
An important aspect to consider from a process design perspective if that of productivity, which remains an issue. Wherein many
studies focus on measuring TOF of heterogenized catalysts (i.e., the number of substrate moles converted per unit time by a single
catalyst site immobilized over the support), a more industrially relevant quantity to measure is that of overall conversion per unit
time. A heterogenized catalyst may have a very large TOF but low overall conversion, and hence productivity, due, for example, to
a low density of immobilized catalyst sites. To improve productivity, higher loadings of immobilized homogeneous catalysts need
to be achieved, bearing in mind that a significant increase could lead to catalyst deactivation due to self-aggregation of a too much
"crowded" surface environment. Hence, optimization of homogeneous catalyst loading, which will also depend on the number of
anchoring sites of a specific support, is another important parameter to consider in designing these materials.
References

Adachi D, Koda R, Hama S, et al. (2013). "An Integrative Process Model of Enzymatic Biodiesel Production Through Ethanol
Fermentation of Brown Rice Followed by Lipase-Catalyzed Ethanolysis In A Water-Containing System." Enzyme and Microbial
Technology, 52: 118-122.
Barbaro P and Liguori F. (2010). Heterogenized Homogeneous Catalysts for Fine Chemicals Production. Springer.
Bek D, Balcar H, Žilková N, et al. (2011). "Grubbs Catalysts Immobilized on Mesoporous Molecular Sieves via Phosphine and Pyridine
Linkers." ACS Catalysis, 1: 709-718.
D'Agostino C, Mantle DM, Sederman AJ. (2018). "NMR Spectroscopy and Microscopy in Reaction Engineering and Catalysis," In
Handbook of Green Chemistry - Green Chemical Engineering, Anastas PT and Lapkin A (Eds.): Wiley-VCH: Weinheim, Vol. 12, pp
289-328.
Dhainaut J, Dacquin JP, Lee AF, Wilson K. (2010). "Hierarchical Macroporous-Mesoporous SBA-15 Sulfonic Acid Catalysts for Biodiesel
Synthesis." Green Chemistry, 12: 296-303.
Enders D and Gasperi T. (2007). "Proline Organocatalysis as a New Tool for the Asymmetric Synthesis of Ulosonic Acid Precursors."
Chemical Communications, 88-90.
Malek K, Li C, van Santen RA. (2007). "New Theoretical Insights into Epoxidation of Alkenes by Immobilized Mn-Salen Complexes in
Mesopores: Effects of Substrate, Linker and Confinement." Journal of Molecular Catalysis A: Chemical, 271: 98-104.
Motokura K, Maeda K, Chun WJ. (2009). "SiO2-Supported Rh Catalyst for Efficient Hydrosilylation of Olefins Improved by
Simultaneously Immobilized Tertiary Amines." ACS Catalysis, 7: 4637-4641.
Wiench JW, Michon C, Ellern A, et al. (2009). "Solid-State NMR Investigations of the Immobilization of a BF4−Salt of a Palladium(II)
Complex on Silica." Journal of the American Chemical Society, 131: 11801-11810.
Zamboulis A, Rahier NJ, Gehringer M, et al. (2009). "Silica-supported l-proline Organocatalysts for Asymmetric Aldolisation."
Tetrahedron: Asymmetry, 20: 2880-2885.
Zhao GL, Lin S, Korotvička A, et al. (2010). "Asymmetric Synthesis of Maraviroc (UK-427,857)." Advanced Synthesis & Catalysis, 352:
2291-2298.
Zhao XS, Bao XY, Guo W, Lee FY. (2006). "Immobilizing Catalysts on Porous Materials." Materials Today, 9: 32-39.
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The Catalyst Review

July 2019

The Catalyst Review July 2019

Table of Contents for the Digital Edition of The Catalyst Review July 2019