Nanoporous polymers put a fresh face on old chemistry

A simple technique for fixing nanoscale rhodium catalysts inside porous organic polymers may be useful for manufacturers of products ranging from detergents to plastics, reports a new study published in the journal Petroleum Chemistry¹.

Readily available from petrochemical refineries, long-chain alkenes — hydrocarbons that contain a carbon–carbon double bond — are key ingredients in many synthetic processes, including transformations into industrially and commercially critical substances like alcohols and aldehydes.

Carrying out the ‘hydroformylation’ chemistry for such transformations at large scales requires metal catalysts such as rhodium that are dissolved in the same liquid as the other reagents. While this approach ensures speedy reaction rates, removing the catalyst from the final products is difficult and typically requires expensive extraction and distillation steps.

Dmitry Gorbunov and colleagues at Lomonosov Moscow State University are part of worldwide efforts to improve the hydroformylation process using heterogeneous approaches involving attaching catalyst metals to recyclable solid supports. In their latest work, the researchers demonstrate how polymers containing aromatic rings and phosphine (PH₃) ligands can repeatedly catalyse hydroformylation with minimal loss of active metal particles into the reaction liquid.

“Phosphine-based polymeric materials are promising supports for heterogeneous hydroformylation catalysts because they can be easily separated and reused, and they have a porous structure that promotes the effective transfer of the reagents to the active sites,” says Gorbunov. “Additionally, the phosphine fragments in the polymer could stabilize the catalyst and increase its selectivity.”

Using straightforward chemistry between aromatic precursors, the team’s experiments showed they could prepare phosphine-containing polymers with an average pore size of just 6 nanometres. They then infused the rhodium catalyst into the nanopores through a simple mixing process.

When the researchers tested their new catalyst, they found it had sustained activity through several hydroformylation cycles. Using a large suite of analytical techniques including electron microscopy, nuclear magnetic resonance and x-ray photoelectron spectroscopy, the team deduced that rhodium was firmly bound to the support, likely through favourable bonding interactions with aromatic rings. These characterizations also revealed that rhodium existed in two forms on the polymer, as oxidized complexes on the polymer surface and as neutral nanoparticles in the pores, and that both forms contributed to catalytic activity.

“Although the rhodium nanoparticles in the pores are less active for hydroformylation compared with the complexes on the polymer surface, the 2 to 6-nanometre particles could be less susceptible to loss,” says Gorbunov.