The Catalyst Review July 2019 - 11

SPECIAL FEATURE
Another useful technique to characterize the immobilized catalyst is Nuclear Magnetic Resonance (NMR) spectroscopy. The
immobilization of homogeneous catalysts to a support can convert the NMR spectra observed from such species typical of a solution
to those of the solid state, which are much broader, and this may hinder unambiguous characterization of the surface sites. This
problem can be mitigated using magic angle spinning (MAS) solid state NMR. For example, solid state NMR has been used for ex-situ
measurements of immobilized providing considerable structural detail. For example, the use of a palladium-oranic complex adsorbed
on silica surface, with several NMR-active nuclei for study (1H, 13C, 31P, 11B, 19F, 29Si) demonstrated the integrity of the immobilized
complex on the silica surface (Wiench et al. 2009).
Design of Heterogenized Catalytic Processes Using In-Situ Characterization

While the techniques mentioned previously can give
information of the successful preparation and structure
of the immobilized homogeneous catalysts, open
questions on the behavior of the reactive fluid inside
such catalytic materials remain. For example, how does
diffusion affect the overall reaction rate? Can we probe
such effect close to reaction conditions? How about
the influence of the support surface on adsorbate/
adsorbent interactions? Figure 2 gives a schematic
idea of the phenomena encountered in such systems.

Figure 2. Schematic of the various physico-chemical phenomena involved in heterogenized
catalysts, which includes diffusion and adsorption of reactive species and products within
the porous matrix of the solid support and reaction over the active site immobilized over
the support.

These aspects, which play an important role in
heterogeneous catalysis, are likely to play a significant
Source: Author 2019
role also on heterogenized catalysts. Nonetheless, such
aspects have been scarcely investigated, mostly due to
challenges in measuring the behavior of reactive species inside the pore space of such materials. One tool to explore these aspects
is represented by NMR spectroscopy, diffusion and relaxation techniques. When exploiting 1H and 13C spectroscopy it is possible
to monitor kinetics (and in some cases product distribution) of chemical reactions occurring inside such materials to gain valuable
insights into the reaction kinetics as a pore scale level (D'Agostino et al. 2018); unlike traditional analysis of the bulk fluid, monitoring
of the reaction as it occurs inside the pore space can yield important information often missed by other techniques, for example,
the presence of species "trapped" inside the pore space, which may have detrimental effects on reaction rate, or the formation of
intermediates not detected in the bulk fluid surrounding the catalyst particle. The use of isotopically-labelled species, such as 2H and
13
C, can also be exploited to infer important information on reaction mechanisms and product distribution inside the pore space.
NMR relaxation data can assess the effect of pore structure and surface chemistry onto molecular dynamics and surface affinity
of the adsorbed species. The measurements of T1 and T2 NMR relaxation times, which are directly related to changes in molecular
dynamics of adsorbed surface species, can reveal important information on how the supports affect adsorption of reactants,
products and solvents, allowing studying, for example, competitive adsorption reactant/solvent and reactant/product. Such data can
then be used for a more rational selection of solid supports and solvents for a specific chemical reaction. In addition, using pulsedfield gradient (PFG) NMR it is possible to quantify diffusion rate of reactant, solvents and products inside heterogenized catalysts
and assess a variety of parameters, including pore size, type of solvents, nature of supports and structure of the immobilized
species. Combining such data with kinetics it is also possible to evaluate diffusion limitations using well-established tools in
chemical engineering such as the Thiele modulus analysis or Weisz-Prater criterion. In addition, measurements of diffusion of probe
molecules within pore structures of heterogenized catalysts may also be used to probe the pore network connectivity of different
material formulations by calculation of the so-called tortuosity of the pore network. It may hence become possible to investigate this
parameter to better design and optimize suitable porous structures to be used as support for homogeneous catalysts. Understanding
those physico-chemical aspects allows then to pinpoint the critical phenomena affecting the overall reactivity of these materials and
hence points toward a more rational material and process design.
The advantage of using the above-mentioned NMR techniques is in the novel hardware developments in the field. Traditionally, the
use of NMR, particularly from an industrial perspective, has been limited by the access, availability and cost of using large high-field
superconductive NMR spectrometers, which have high capital costs as well as operational and maintenance costs; in the past few
years, the development of high-quality, low-field, bench-top NMR spectrometers has been increasing significantly the availability
of NMR capabilities at a much more affordable cost. Such compact spectrometers cost 1/10 or less compared to superconducting
high-field spectrometers and can perform spectroscopy, diffusion and relaxation making such techniques more accessible also to
R&D labs. Indeed, leading catalyst companies such as Johnson Matthey have recently started using these instruments for their R&D
in catalysts and chemicals.
The Catalyst Review

July 2019

The Catalyst Review July 2019

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