FlowCAT // High-Pressure Flow-Chemistry in a compact unit
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Application Data

Hydrogenation of nitrobenzene

The first reaction presented is the hydrogenation of nitrobenzene to aniline using 1% Pd on carbon catalyst (around 0.1g) mixed with glass beads (around 400 micron diameter). Most of the experiments were done at 30C, using substrate/solvent flow rate of 0.5ml/minute and 20 litre/minute of hydrogen, corresponding approximately to 1.83 times stoichiometric of the gas. Taking into account the volume occupied by the inert beads, the void space available for reactant flow was around 1.2ml and therefore, the residence time is around 2.4 minutes.

The results are plotted in figure 6, where the amount of hydrogen consumed is plotted at different working pressures. Also shown is the solubility of hydrogen in the solvent at different pressures clearly, the consumption is not limited by solubility. The gas consumption values in fact correspond to substrate conversion of around 20% at the lowest pressure rising to around 60% at 90bar.

Sterioselective hydrogenation

Pfizer pharmaceutical company reported the benefits of using the FlowCAT to perform a hydrogenation where sterioselectivity control was important and they showed that control of the cis to trans ratio in the product could be tuned by adjusting the pressure. The chemistry in question is summarised on the right and the results they achieved which show the "tuning" capability with a flow reactor is plotted below.

Customer data

Pfizer data for a sterioselective hydrogenation in FlowCAT (J Hawkins, 2010)

Carbonylation of Iodotoluene

While hydrogenation reactors are very common in some industries other chemistries that involve gaseous reactants with solid catalysts, such as oxidation and polymerisation, can be  accommodated equally well. Another example is carbonylations, a reaction which is commercially important and quite difficult to perform. To demonstrate this on the FlowCAT, carbonylation of iodotoluene to tolualdehyde was attempted using 0.72 molar solution of the substrate in triethylsilane and triethyleamine using a Palladium catalyst supported on polymer, supplied by Johnson Matthey.

The chemistry is highly non-selective and results of some experiments in FlowCAT at 90C are summarised in figure 8.  The molar concentration in the product stream is plotted at different residences times (ie liquid flow rates) with the CO flow kept constant. This shows that at 25bar pressure, conversion is relatively low and the amount of desired product (toluadehyde) is much less than the amount of by-products. If the pressure is raised to 100bar, the situation is seen to improve considerably where the by-product amount is much reduced and the desired product increased.

Liquid-liquid reaction in Tubular reactor

It is also quite simple to perform reactions on the FlowCAT that do not involve a gaseous reactant and this was illustrated by studying the transfer hydrogenation of nitrobenzene to aniline using cyclohexene as a transfer molecule.

This is a much easier reaction than the carbonylation and selectivity is nearly 100%. With a back pressure of 2bar, temperatures of between 80 and 110C were studied at different residence times up to 10 minutes. The results (figure 10) show that in fact 100% conversion is achieved at these conditions.

Liquid-liquid reaction in coiled tube reactor

The standard tubular reactors used in the previous examples are limited when very fast reactions are performed as the only way to achieve a long residence time is slow down the feed rate and this is limited both by the pump and eventually poor mixing and heat transfer.

The alternative is to use 1/16" or 1/8" diameter piping which can be coiled to fit the heating zone. These can be easily fitted to replace the tubular reactors and hence common synthesis reactions at atmospheric or elevated pressure can be performed easily. Photographs of such a set up with 1/8" stainless steel tubing is shown below. This is held inside the same heating jacket as the tubular reactors previously and product leaving the pipe can be under pressure or simply by gravity.

The utility of this arrangement is illustrated by an interesting condensation reaction between diphenylacetone and benzyl in the a basic environment (typically NaOH).
The feed mixture is a very pale green and product becomes progressively darker with conversion, eventually becoming an intense purple. Several runs were performed at different flow rates with the temperature held at 90°C and the amount of caustic fixed and the product collected. The results are shown below with the fresh feed being on the right and then moving left the residence times are 2,4,6 and 12 minutes.