The Hazards of Hydrogen and Hydrogenation
Hydrogenation is a process used to reduce or saturate organic compounds by using hydrogen gas usually in the presence of a catalyst at ambient and/or elevated pressure. The process presents several safety concerns. The major being fire and explosion due to the pyrophoric nature of chemicals used (hydrogen, solvents, catalyst, instability of intermediates). There are steps that can be taken to minimize the risks once the hazards are known and appropriate safety procedures are followed.
Hydrogen
A primary hazard source in the process is hydrogen gas. It is a colorless odorless gas that burns with a non-luminous flame that can be invisible under bright light. It is a hazard as an asphyxiant and a flammable compound with a low ignition energy. It should be stored in a well-ventilated area away from oxidizers, open flames, sparks and other heat and ignitions sources. It must be 20 feet away from oxygen and flammable gases. Only regulators designed for use with hydrogen and non-sparking/intrinsically safe tools should be used when working with hydrogen.
Hydrogen gas is a small lighter than air molecule. This allows it to pass through porous materials and leak from apertures that other gases cannot pass through. Tubing approved for use with hydrogen should be used to keep the gas from entering the room atmosphere (check manufacture specs). Other possible sources of leaks are the regulators, joints, gaskets, seals and fittings. Color changing tape for detecting hydrogen leaks around fittings is highly recommended (http://www.detectape.com/), especially if a hydrogen monitor is not available.
Regulators or step-down pressure valves should be used to pressurize low-pressure equipment from a high-pressure hydrogen source. A flash arrestor between the equipment and the regulator will prevent a flashback from traveling through the lines to the regulator and turning it or the gas cylinder into a bomb.
Hydrogenation Process
Ambient hydrogenation is generally safer than high-pressure hydrogenation, but there are still dangers that need to be accounted for. All glassware should be crack free, intact, and contamination free. Existing cracks or chips are weak point and may give way during the process. This would expose both the hydrogen and pyrophoric chemicals to oxygen resulting in fire. The system should be set up in a fume hood. This provides both a well ventilated area to keep hydrogen from accumulating in the lab and offers a contained area if something were to go wrong. Sashes should be closed during the reaction process except during set-up, addition of chemicals to reaction vessel as necessary, and during work-up of the reaction mixture.
One of the most important precautions to take is the removal of as much oxygen from the system as possible before introducing the hydrogen. This can be done by using a vacuum or purging with an inert gas (nitrogen or argon). This will keep oxygen from reacting explosively with the process. Once the reaction is completed, the system should be repeatedly rinsed with inert gas to purge the hydrogen from the system to avoid producing hydrogen-oxygen mixtures in the presence of the catalyst.
Consideration should be given to the solvent used. Can a solvent with a lower flammability or aqueous solvent provide same or better results? If possible, the use of aqueous solvents (water and water:alcohol mixtures) are preferred over organic solvents for hydrogen bubbling in ambient hydrogenation due to their higher boiling point and low flammability. Never fill the vessel to capacity with the solution. Half full or less is safer. It provides room for the hydrogenation reaction to occur without stressing the system.
The catalyst is another hazard source. It should be considered potentially pyrophoric and be handled with caution. Use the minimum amount needed for the reaction if possible. The preferred transfer method is while the catalyst is wet as this reduces the risk of spontaneous ignition. The hydrogen saturated catalyst should be handled carefully and never allowed to become dry because it may ignite. The use of an inert gas during filtration is recommended to reduce the chance of oxygen exposure. Once filtered, the funnel and still moist catalyst filter cake should be put into a water bath immediately.
High-pressure hydrogenation reactions are more difficult to manage and require hands on experience with the reactor and detailed subject knowledge because of all the hazard variables. “…[I]t is important to consider not just the initial reaction conditions, but the kinetics and thermodynamics of the reaction as a whole” when designing a high-pressure hydrogenation reaction. (Prudent Practices in the Laboratory 4.E.3.) Each step of the process should be thought through and characteristics identified (temperature needed, pressure needed, compound characteristics, any exothermic reaction in the process, etc.) so the appropriate equipment can be selected for use.
The laboratory environment’s ability to handle catastrophic failure of the equipment must be considered as well. Adequate ventilation to handle discharge from a high pressure reaction is needed to prevent asphyxiation. Hearing protection may be required to guard against the sound of a rupture disc failure. Barricades are needed if injury or death of personnel can be caused by catastrophic equipment failure.
The most obvious hazard of a pressurized system is over pressurizing the pressure vessels which can cause fatal explosions and fire. It is important that the vessels be maintained and serviced per the vendor’s guidelines. Pressure of the system should be monitored so quick action can be taken if needed.
Heating method can be a spark source around highly flammable compounds. The use of a water bath, sand bath or heating block are the safer options for the procedure.
Never work alone. Place a sign on hood to alert other lab staff when a high-pressure hydrogenation reaction is in process and never leave it unattended.
Hydrogen and hydrogenation reactions are useful tools in the lab, but they can easily become a deadly force if the hazards are not fully understood and consciously countered.
For a more in-depth information about hydrogenation hazards see “Hazards associated with laboratory scale hydrogenations” by Talak Chandra and Jeffery P. Zebrowski in Journal of Chemical Health & Safety, July/August 2016.
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