Russian chemists have developed membranes based on a silicon-containing siloxane compound to release aldehydes, organic compounds used in pharmaceuticals, medicine, food and cosmetics. The authors added hydroxyl groups to the structure of the starting material, due to which the membrane began to highly selectively pass carbon dioxide and aldehydes. The development of scientists will reduce the consumption of resources in the production of aldehydes by reducing the cost of separating gas-liquid mixtures. The results of the work, supported by a grant from the Russian Science Foundation (RSF), were published in the journal Polymers.
Membranes – materials of various nature and chemical composition that separate liquids and gases – help chemists to obtain in pure form the necessary substances, such as aldehydes, to purify the air or separate petroleum gases. Nowadays, silicon-containing compounds, such as siloxanes, are increasingly used as materials for such membranes. The results of these works are already applied in the industry: for natural gas conditioning, in wastewater treatment and in the food industry for the extraction of aromatic components from juices.
Russian chemists from the Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences (Moscow) have created a membrane for the separation of gases and liquids based on polydecylmethylsiloxane (PDecMS). This is a siloxane, in the chain of which there are “tails” of hydrocarbons, called decyls. Various chemical groups can be attached to these “tails”, thereby changing the properties of the original substance. So, scientists added hydroxyl OH groups to the original molecules and investigated the new substance as the basis for the manufacture of a membrane that would allow aldehydes to be isolated from a mixture of gases and liquids. They are used in the production of preservatives, leather and perfumes, organic acids, alcohols are obtained from them. polymers and plastics.
Scientists introduced OH groups into tailed silicone during the hydrosilylation reaction – the addition of monomers or polymers containing an Si-H bond to unsaturated compounds, then with double and triple intercarbon bonds. On the basis of the obtained polymers, membranes of complex composition were developed. The thickness of such a membrane with a microfiltration substrate is about 250 microns (for comparison, the thickness of a human hair is from 40 to 120 microns). The working – selective – layer of the membrane is only from 5 (if it is silicone without an OH group) to 15 microns (in the case of the addition of an OH group).
Chemists have tested membranes and their transport (the rate of penetration into the membrane) and separating (the difference between the rates of penetration of different substances) properties. To do this, they used gases – nitrogen, oxygen, carbon dioxide – and liquids – hydrocarbons with one double intercarbon bond (1-hexene, 1-heptene, 1-octene, 1-nonene) and aldehydes (heptanal and decanal). Moreover, scientists conducted experiments both with each substance separately and with their mixtures. In particular, the researchers measured the uptake of substances by the membrane to explain differences in how the membrane passes these substances through itself.
It turned out that tailed siloxane membranes become less permeable when OH groups are introduced into their structure. The presence of these fragments led to an increase in the selectivity of the membrane with respect to substances interacting with it. For example, the introduction of 7% OH groups into polysiloxane made it possible to increase the concentration of heptanal (aldehyde) by an eight times compared to 1-hexene (olefin) after the substance passed through the membrane. “The polysiloxane membranes developed at the Institute of Chemistry of the Russian Academy of Sciences have shown their potential for the separation of aldehydes from mixtures with olefins. The introduction of OH-groups into the polymer membrane made it possible to increase its ability to selectively transfer CO2 and aldehydes, ”says Evgenia Grushevenko, head of the project, supported by a grant from the Russian Science Foundation, Ph.D.
The scientists also noted that modified materials with OH groups require lower reaction activation energies (and therefore lower temperature and pressure) for transport and separation of aldehydes from olefins. The developed composite membranes will make it possible to switch to membrane reactors, which will reduce the cost of separating the mixture and protect the target product from side reactions, such as oxidation.