Solid phase catalysis in continuous flow

What is catalysis?

Catalysis is the increase in the rate of reaction of a chemical reaction by use of a catalyst^[1].

What is a catalyst?

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the reaction. This enables it to be recycled during the reaction.

A catalyst can be thought of as a facilitator – it makes the process of getting from point A to point B much less energy intensive, so the reaction proceeds faster. It does this by opening a different reaction pathway with a lower activation energy; this is different from the overall reaction energy (ΔG) – which does not change.

How does catalysis work?

Consider the reaction:

X + Y → Z   (equation 1)

This reaction without the presence of a catalyst has a high activation energy (E_a).

If we introduce a catalyst, we provide a different pathway for the reaction to proceed:

X + Y → X-Catalyst-Y → Z + Catalyst   (equation 2)

In this example, this reaction is activated by the presence of a catalyst to form an intermediate. The energy for the formation of the intermediate from the starting materials and the formation of the product from the intermediate has a lower activation energy.

This activation usually occurs by the formation of an intermediate, or a series of intermediates.

Note: The overall Activation Energy (Ea) of the reaction is the energy required to overcome the biggest step

Additionally, we can see at the end of the reaction our catalyst is regenerated and can be used further in subsequent reactions.

Why would you want to incorporate a catalyst into your chemistry?

Catalysts allow for much more efficient reactions:

• When we want to speed up our rate of reaction
• Recycling of the catalyst allows us to improve the yield of the reaction over the same amount of time as an uncatalyzed reaction
• Lower activation energy means that lower temperatures/conditions can be used
• Catalysts can also increase the selectivity of certain reactions

What types of catalysis can you do using flow chemistry?

An example of a solid phase catalyst in a glass column for use in flow chemistry

Homogeneous catalysis

Homogeneous catalysis is catalysis where the catalyst and the starting materials are in the same phase (gaseous, liquid, solid). Most of the time, this refers to catalysis that happens in solution where the catalyst and starting materials are miscible.

The nature of flow means that any liquid flow system can be used to run homogenous reactions – if, of course, the chemistry is well defined (solvent solubility, catalyst efficacy etc.).

Heterogeneous catalysis

Heterogeneous catalysis refers to catalysis which occurs at the interface of two phases. Typically, this refers to gas-solid or liquid-solid reactions and either phase can be the starting material or the catalyst. These surfaces are a solid, typically in the form of fine particles.

In these instances, the starting materials are adsorbed onto the surface where the intermediate is formed.

To perform heterogeneous catalysis in flow chemistry, packed bed reactors are a common route.

Here you would fill a column with particles of solid catalyst – like packing sand in a bucket on the beach – then flow through your reagents. The size of the particles can be altered depending on the chemistry required.

Figure 4 – Asia Heater allows for incorporation of Solid Phase Reactor to ensure reactions are temperature controlled

The nature of flow chemistry means you can adjust the residence time and ultimately the amount of time your reagents are exposed to the catalyst surface.

As catalysts are recycled and not used up, these can be left to run for hours enabling a large throughput of product.

References

[1] – http://goldbook.iupac.org/html/C/C00876.html