Researchers at The City College of New York (CCNY), New York City, have developed an approach that eliminates the need for compressed hydrogen gas in cylinders and the safety issues using them raises in hydrogenation applications.
“Hydrogenation … relies on the use of a finely divided metal such as palladium, often supported on charcoal,” says Mahesh K. Lakshman, who led the research. “It also needs a source of hydrogen gas… Compressed hydrogen gas cylinders are not only expensive but they pose an extreme explosion and fire hazard.”
Their research revealed that mixing two stable materials together in the presence of palladium on charcoal produces a mixture capable of hydrogenation, without requiring an external source of compressed hydrogen gas. An article in the journal Advanced Synthesis and Catalysis contains more detail.
While industrial-scale hydrogenations generally don’t rely on compressed hydrogen in small cylinders, bench-scale research and laboratory work usually do. So, among its applications, Lakshman believes this hydrogenation process could be useful in undergraduate chemistry teaching modules.
“In a laboratory setting, there are several factors to consider: (a) compressed gases are just generally unsafe as rapid depressurization of any compressed gas is physically dangerous, (b) specifically with hydrogen gas, there is always a worry about its innate hazardous nature, (c) a size 300 (Airgas) hydrogen tank will cost around $80 plus demurrage (if it is not all used up quickly) plus shipping. These would be primary drivers in considerations leading to whether or not one wants to have a gas tank sitting around in a lab,” explains Lakshman.
“We feel at the moment, the method is best for bench-scale research and instructional labs,” he adds. “This is because you avoid all of the problems indicated above and can conduct reduction reactions literally on demand. The reagents typically have 5-year shelf lives (or more, if stored with care). This eliminates the demurrage cost as most research labs operate solely on research grants and instructional labs are subject to fiscal controls often from outside the department. So, a one-time investment in materials can alleviate some of the issues for a prolonged period.”
The team also developed conditions for using deuterium in place of hydrogen for medicinal products.
“The fact that a C–D bond is broken with more difficulty than a comparable C–H bond, has been the basis of heavy drugs, which are essentially clinical pharmaceutical products that have one or more H replaced by D. For example, if enzymatic breakage of a C–H bond is detrimental to the effect of a drug, then replacement of the H with a D may provide an advantage. Other applications of the heavier isotope are as probes of chemical processes. For instance, one can gain an understanding of whether C–H bond breakage occurs in a ‘rate-limiting step’ of a chemical reaction, and so deuteriated compounds are important mechanistic probes. Deuteriated compounds are standards in NMR spectroscopy and mass spectrometry. In this context, I will note our method would be quite useful for the relatively small quantities of compounds generally needed as standards,” says Lakshman.
The primary challenge is the global availability of diboron reagents. “While diboron compounds are no longer esoteric molecules, much still needs to be learned about how they can be leveraged for novel chemistry. Secondly, as we demonstrate in the publication, availability of the deuteriated versions of tetrahydroxydiboron and the partially deuteriated 4-N,N-dimethylaminopyridine would make the applicability of aromatic deuteriations more accessible,” notes Lakshman.
The team now is working on a second-generation hydrogenation process that uses stable and greener materials, and would be amenable to much larger scales. “It would be terrific if the potential scalability of the chemistry to say 0.5 kg or 1 kg scales could be evaluated,” he adds.
“There is still some more work to be done and due to the shutdown, our work has been put on hold. This newer and greener procedure would also generally diversify the ability to incorporate deuterium into molecules, via easier processes,” he says.