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UJ researchers have taken new steps to transform hydrogenation into a safe, low-energy process. They use a very stable three-phase emulsion to convert toxic waste into valuable raw materials. The process does not require flammable compressed hydrogen.
Emulsion catalyzes efficient hydrogenation of nitrobenzene to aniline at room temperature. Aniline is widely used in the pharmaceutical industry. After that, the bimetal hydrogenation catalyst is completely recovered.
Without hydrogenation, it would be impossible to manufacture many of today's drugs. It is the backbone of the pharmaceutical and chemical industries. But hydrogen is expensive. Safety measures to prevent explosions in factories and laboratories are also expensive.
However, if you don't need to compress hydrogen at all, you can save a lot of money. This also means that many chemical processes can be safer and easier to use.
Chemists at the University of Johannesburg proved this in a study published in Colloids and Surfaces.
They used a catalytic hydrogenation process in Pickering emulsion to convert nitrobenzene to aniline.
The emulsion process may become a safer industrial hydrogenation process than currently used.
“Pickering emulsions have been around for 150 years. But the use of them for catalysis did not appear until 2014,” said Professor Reinout Meijboom. Meijboom is a researcher in the Department of Chemical Science.
Yogurt is an example of Pickering emulsion. This emulsion is a mixture of water-soluble particles and oil-soluble particles. What makes yogurt a Pickering emulsion is that it also contains enzymes, which are insoluble solid particles.
Nitrobenzene is produced in large quantities worldwide as a waste of chemical manufacturing. It is a highly toxic, persistent organic pollutant described by institutions such as the World Health Organization, the Environmental Protection Agency, and the Centers for Disease Control and Prevention.
The manufacture of polyurethane uses nitrobenzene as an intermediate. It is also used as a solvent for petroleum refining. Nitrobenzene is often contained in wastewater from dye production enterprises. It is an oily liquid and has a fire hazard.
Aniline is a commodity with important industrial significance. It is the raw material for a large number of chemical products, including many drugs.
The process designed by the researchers uses toluene to dissolve nitrobenzene. This forms the first phase, organic phase, or toluene phase of the process. For the second water phase, they dissolved sodium borohydride in water.
The catalyst is the third stage of the process. It is composed of modified silica microspheres and palladium. They also used bimetallic catalysts in which palladium is combined with cobalt or nickel.
Meijboom said that if the three phases are added together, but not mixed into an emulsion, the combination can be stored for days or weeks. A small amount of hydrogenation will occur, but the process will not actually proceed until a proper emulsion is formed.
The catalyst also acts as a stabilizing emulsifier.
When the three phases are mixed into an emulsion, the catalyst starts the hydrogenation process. The formation of the emulsion takes a few seconds. On a laboratory scale, the reaction takes about two hours.
The hydrogen required for hydrogenation is provided by dissolved sodium borohydride. Hydrogenation is carried out efficiently at room temperature, thereby saving energy.
No need to store or transport hydrogen. This eliminates most of the explosion risk in the process.
Meijboom said that the three-phase process in Pickering emulsion has the advantage of a larger catalytic surface compared to single-phase or two-phase processes.
The catalytic efficiency can be adjusted by adjusting the volume ratio of toluene and water phase in the Pickering emulsion system.
"Each drop of toluene and nitrobenzene solution in the emulsion effectively becomes a microreactor. This is how to adjust the process to make it effective at room temperature," he added.
After the hydrogenation is complete, the resulting emulsion is stable enough to be stored for several days before the aniline is isolated.
Mr. Peter Dele Fapojuwo said that this study is the first time that a bimetallic palladium catalyst has been effectively used to hydrogenate aromatic compounds in an aqueous Pickering emulsion system. He is a graduate researcher in the department.
"By adding nickel or cobalt to the catalyst, we improved the dispersion of palladium on the surface of the emulsifier," he added.
Palladium is much more expensive than nickel or cobalt, so the use of bimetallic catalysts further reduces costs.
"Compared with traditional surfactants, the use of solid particles as catalysts and emulsifiers or stabilizers poses less threat to the environment. Their ingredients are less toxic," Fapojuwo said.
He added that when sodium borohydride instead of hydrogen is used as the reducing agent, the reaction platform is much safer.
“Using petroleum-derived hydrogen for hydrogenation is neither completely environmentally friendly nor economical. It requires high-pressure hydrogen, which requires expensive reactor equipment. This increases the cost of the process,” he added.
"Theoretically, this process can be applied to maintain one phase in a fixed bed reactor and perform flow synthesis. The result will be a continuous process of catalytic reaction between two immiscible phases," Meijboom said.
"This is proof of the principle stage. We are working hard to promote this process," he said.
"We designed a process that can be extended to a series of important industrial reactions.
"By using emulsion chemistry, we have a system where the catalyst, emulsifier, water phase, and organic phase are all mixed into an extremely stable system," Meijboom said. Further explore new technologies to complete important industrial reactions five times faster. More information: Dele Peter Fapojuwo et al. Bimetallic PdM (M = Co, Ni) catalyzes nitrate at the water/oil interface in Pickering emulsion, colloid and surface A Hydrogenation of Benzene: Physical Chemistry and Engineering (2021). DOI: 10.1016/j.colsurfa.2021.126513 Citation provided by the University of Johannesburg: Hydrogenation without Hydrogen: Efficient catalysis in a Stable Emulsion Gel (2021, June 23), retrieved on November 25, 2021 from https://phys .org/news/2021-06-hydrogenation-hydrogen-efficient-catalysis-stable.html This document is protected by copyright. Except for any fair transaction for private learning or research purposes, no part may be copied without written permission. The content is for reference only.
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