Hydrogenation

Hydrogenation is a class of reductive chemical reactions in which the net result is an addition of hydrogen (H₂). Targets of hydrogenation are often alkenes or imine. Most hydrogenations involve the direct addition of diatomic hydrogen under pressure, often in practice with catalysts. Some hydrogenations involve the indirect addition of hydrogen, these are called transfer hydrogenations.

The classical example of a hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms, converting alkenes to alkanes. Numerous important applications are found in the pharmaceutical and petrochemical industries.

Hydrogenation reactions also include the reaction between hydrogen and organic sulfur compounds to form gaseous hydrogen sulfide (H₂S). Hydrogenation of that type is very widely used in the petroleum refining and petrochemical industries to desulfurize various final products, intermediate products and process feedstocks by converting sulfur compounds to gaseous hydrogen sulfide which is then easily removed by simple distillation. In those industries, desulfurization process units are often referred to as hydrodesulfurizers (HDS) or hydrotreaters.

Processes accomplishing the reverse are called "dehydrogenation"s.

Hydrogenation in food

Hydrogenation is widely applied to the processing of vegetable oils and fats. Complete hydrogenation converts unsaturated fatty acids to saturated ones. In practice the process is not usually carried to completion. Since the original oils usually contain more than one double bond per molecule (that is, they are poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil; that is some, but usually not all, of the double bonds in each molecule have been reduced.

Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats, such as those present in margarine. Changing the degree of saturation of the fat changes some important physical properties such as the melting point, which is why liquid oils become semi-solid. Semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Since partially hydrogenated vegetable oils are more reasonably priced than animal source fats, are available in a wide range of consistencies, and have other desirable characteristics (eg, increased oxidative stability (longer shelf life)), they are the predominant fats used in most commercial baked goods. Fat blends formulated for this purpose are called shortenings.

Health implications

A side effect of incomplete hydrogenation having implications for human health is the isomerization of the remaining unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats in most edible fat sources, but incomplete hydrogenation partially converts these molecules to trans isomers, which have been implicated (for poorly understood causal reasons) in circulatory diseases including heart disease (see trans fats). The catalytic hydrogenation process favors the conversion from cis to trans bonds because the trans configuration has lower energy than the natural cis one. At equilibrium, the trans/cis isomer ratio is about 67/33. Food legislation in the US and codes of practice in EU has long required labels declaring the fat content of foods in retail trade, and more recently, have also required declaration of the trans fat content.

The hydrogenation process

The basic chemical reaction in industrial hydrogenation processes involves the use of gaseous hydrogen as a reactant and a catalyst to accelerate the reaction, typically under elevated temperature and pressure conditions. Most hydrogenation processes use metal catalysts to pull gaseous hydrogen (H₂) apart and temporarily stabilize the resulting hydrogen radicals, creating a chemical species with properties intermediate between H₂ and •H (radical.) These 'activated' hydrogen atoms are highly reactive and will try to regain a full s orbital (containing two electrons) by grabbing an electron from other chemicals (classically, by breaking the double bond of an alkene, eventually replacing one of the alkenyl bonds between the two carbons with a new bond to a hydrogen atom.) The net result of hydrogenation can be either a reduction, or an oxidation, depending on the chemicals being used, although the reaction may best be thought of as 'hydrogen oxidation'.

A wide range of hydrogenation catalysts are available, with varying levels of activity. The platinum group metals (platinum, palladium, rhodium, ruthenium) are particularly effective as hydrogenation catalysts, although non-precious metal hydrogenation catalysts (such as Raney nickel and Urushibara nickel) have also been developed and can be much more economical. Their activity can be further adjusted by combining such metals with various other elements and compounds. The value of varying levels of catalytic activity is that a carefully chosen catalyst can be used to hydrogenate some functional groups without interfering with others, such as the hydrogenation of alkenes without breaking aromatic rings, or the selective hydrogenation of alkynes to alkenes using Lindlar's catalyst.

In the food industry, an undissolved (or "heterogeneous") metal catalyst is used , such as nickel (often in the form of Raney nickel), or in some very rare cases palladium and platinum.

In the pharmaceutical industry, and for special chemical applications, palladium and platinum, or in "homogeneous" catalyst such as the rhodium-based compound known as Wilkinson's catalyst, or the iridium-based Crabtree's catalyst are often used.

In the petroleum refining and petrochemical industries, cobalt-molybdenum or nickel-molybdenum catalysts are the most commonly used hydrogenation catalysts.

Temperatures

The reaction is carried out at different temperatures and pressures depending upon the substrate. Hydrogenation is a strongly exothermic reaction. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released is about 25 kcal per mole (105 kJ/mol), sufficient to raise the temperature of the oil by 1.6-1.7 °C per iodine number drop.

The inventor

The French chemist Paul Sabatier is considered the father of the hydrogenation process. In 1897 he discovered that the introduction of a trace of nickel as a catalyst facilitated the addition of hydrogen to molecules of gaseous carbon compounds. Wilhelm Normann was awarded a patent in Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid oils using hydrogen gas, which was the beginning of what is now a very large industry world wide.

External links

Hydrodesulfurizer Technologies - an excellent review of the hydrotreating technologies, suppliers and costs
UOP Company - engineering design and construction of large-scale, industrial hydrogenation plants
Axens Company - engineering design of large-scale, industrial hydrogenation plants and catalyst supplier
Parr Instrument Company - laboratory-scale hydrogenation apparatus

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