| Hydrogenation | |
|---|---|
| Conditions | |
| Catalyst | Ni, Pd, Pt |
| Process type | Chemical |
|---|---|
| Industrial sector(s) | Food industry,petrochemical industry,pharmaceutical industry, agricultural industry |
| Main technologies or sub-processes | Various transition metal catalysts, high-pressure technology |
| Feedstock | Unsaturatedsubstrates andhydrogen or hydrogen donors |
| Product(s) | Saturated hydrocarbons and derivatives |
| Inventor | Paul Sabatier |
| Year of invention | 1897 |
Hydrogenation is achemical reaction between molecularhydrogen (H2) and another compound or element, usually in the presence of acatalyst such asnickel,palladium orplatinum. The process is commonly employed toreduce orsaturateorganic compounds. Hydrogenation typically constitutes the addition of pairs ofhydrogenatoms to a molecule, often analkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reducesdouble andtriple bonds inhydrocarbons.[1]
Hydrogenation has three components, theunsaturated substrate, the hydrogen (or hydrogen source) and, invariably, acatalyst. Thereduction reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst.
The same catalysts and conditions that are used for hydrogenation reactions can also lead toisomerization of thealkenes fromcis to trans. This process is of great interest because hydrogenation technology generates most of thetrans fat in foods. A reaction where bonds are broken while hydrogen is added is calledhydrogenolysis, a reaction that may occur to carbon-carbon and carbon-heteroatom (oxygen,nitrogen orhalogen) bonds. Some hydrogenations of polar bonds are accompanied by hydrogenolysis.
For hydrogenation, the obvious source of hydrogen isH2 gas itself, which is typically available commercially within the storage medium of a pressurized cylinder. The hydrogenation process often uses greater than 1 atmosphere ofH2, usually conveyed from the cylinders and sometimes augmented by "booster pumps". Gaseous hydrogen is produced industrially from hydrocarbons by the process known assteam reforming.[2] For many applications, hydrogen is transferred from donor molecules such asformic acid,isopropanol, anddihydroanthracene.[3] These hydrogen donors undergodehydrogenation to, respectively,carbon dioxide,acetone, andanthracene. These processes are calledtransfer hydrogenations.
An important characteristic of alkene andalkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with "syn addition", with hydrogen entering from the least hindered side.[4] This reaction can be performed on a variety of differentfunctional groups.
| Substrate | Product | Comments | Heat of hydrogenation (kJ/mol)[5] |
|---|---|---|---|
| R2C=CR'2 (alkene) | R2CHCHR'2 (alkane) | large application is production ofmargarine | −90 to −130 |
| RC≡CR' (alkyne) | RCH2CH2R' (alkane) | semihydrogenation givescis-RHC=CHR' | −300 (for full hydrogenation) |
| RCH=O (aldehyde) | RCH2OH (primary alcohol) | often employstransfer hydrogenation | −60 to −65 |
| R2CO (ketone) | R2CHOH (secondary alcohol) | often employstransfer hydrogenation | −60 to −65 |
| RCO2R' (ester) | RCH2OH + R'OH (two alcohols) | often applies to production offatty alcohols | −25 to −105 |
| RCO2H (carboxylic acid) | RCH2OH (primary alcohol) | applicable to fatty alcohols | −25 to −75 |
| RNO2 (nitro) | RNH2 (amine) | major application isaniline[6][7] | −550 |
With rare exceptions,H2 is unreactive toward organic compounds in the absence of metal catalysts. The unsaturated substrate ischemisorbed onto the catalyst, with most sites covered by the substrate. In heterogeneous catalysts, hydrogen forms surface hydrides (M-H) from which hydrogens can be transferred to the chemisorbed substrate.Platinum,palladium,rhodium, andruthenium form highly active catalysts, which operate at lower temperatures and lower pressures ofH2. Non-precious metal catalysts, especially those based onnickel (such asRaney nickel andUrushibara nickel) have also been developed as economical alternatives, but they are often slower or require higher temperatures. The trade-off is activity (speed of reaction) vs. cost of the catalyst and cost of the apparatus required for use of high pressures. Notice that the Raney-nickel catalysed hydrogenations require high pressures:[8][9]
Catalysts are usually classified into two broad classes:homogeneous andheterogeneous. Homogeneous catalysts dissolve in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are solids that are suspended in the same solvent with the substrate or are treated with gaseous substrate.
Some well known homogeneous catalysts are indicated below. These arecoordination complexes that activate both the unsaturated substrate and theH2. Most typically, these complexes contain platinum group metals, especially Rh and Ir.
ruthenium(II).png)
-iPr-PHOX.svg)
Homogeneous catalysts are also used in asymmetric synthesis by the hydrogenation of prochiral substrates. An early demonstration of this approach was the Rh-catalyzed hydrogenation of enamides as precursors to the drugL-DOPA.[10] To achieve asymmetric reduction, these catalyst are made chiral by use of chiral diphosphine ligands.[11] Rhodium catalyzed hydrogenation has also been used in the herbicide production of S-metolachlor, which uses a Josiphos type ligand (called Xyliphos).[12] In principle asymmetric hydrogenation can be catalyzed by chiral heterogeneous catalysts,[13] but this approach remains more of a curiosity than a useful technology.
Heterogeneous catalysts for hydrogenation are more common industrially. In industry, precious metal hydrogenation catalysts are deposited from solution as a fine powder on the support, which is a cheap, bulky, porous, usually granular material, such asactivated carbon,alumina,calcium carbonate orbarium sulfate.[14] For example, platinum on carbon is produced by reduction ofchloroplatinic acidin situ in carbon. Examples of these catalysts are 5%ruthenium on activated carbon, or 1%platinum on alumina. Base metal catalysts, such asRaney nickel, are typically much cheaper and do not need a support. Also, in the laboratory, unsupported (massive) precious metal catalysts such asplatinum black are still used, despite the cost.
As in homogeneous catalysts, the activity is adjusted through changes in the environment around the metal, i.e. thecoordination sphere. Differentfaces of a crystalline heterogeneous catalyst display distinct activities, for example. This can be modified by mixing metals or using different preparation techniques. Similarly, heterogeneous catalysts are affected by their supports.
In many cases, highly empirical modifications involve selective "poisons". Thus, a carefully chosen catalyst can be used to hydrogenate some functional groups without affecting others, such as the hydrogenation of alkenes without touching aromatic rings, or the selective hydrogenation of alkynes to alkenes usingLindlar's catalyst. For example, when the catalystpalladium is placed onbarium sulfate and then treated withquinoline, the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion ofphenylacetylene tostyrene.[15]
![Selective hydrogenation of the less hindered alkene group in carvone using a homogeneous catalyst (Wilkinson's catalyst).[16]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCarvoneH2.png)
![Hydrogenation of maleic acid to succinic acid.[17]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aSuccPdH2.png)
Transfer hydrogenation uses hydrogen-donor molecules other than molecularH2. These "sacrificial" hydrogen donors, which can also serve assolvents for the reaction, includehydrazine,formic acid, and alcohols such as isopropanol.[18]
Inorganic synthesis, transfer hydrogenation is useful for theasymmetric hydrogenation of polar unsaturated substrates, such asketones,aldehydes andimines, by employingchiral catalysts.
Polar substrates such asnitriles can be hydrogenatedelectrochemically, usingprotic solvents and reducing equivalents as the source of hydrogen.[19]
The addition of hydrogen to double or triple bonds inhydrocarbons is a type ofredox reaction that can be thermodynamically favorable. For example, the addition of hydrogen to ethene has aGibbs free energy change of -101 kJ·mol−1, which is highlyexothermic.[11] In the hydrogenation of vegetable oils and fatty acids, for example, the heat released, about 25 kcal per mole (105 kJ/mol), is sufficient to raise the temperature of the oil by 1.6–1.7 °C periodine number drop.
However, the reaction rate for most hydrogenation reactions is negligible in the absence of catalysts. Themechanism of metal-catalyzed hydrogenation of alkenes and alkynes has been extensively studied.[20] First of allisotope labeling usingdeuterium confirms theregiochemistry of the addition:
On solids, the accepted mechanism is the Horiuti-Polanyi mechanism:[21][22]
In the third step, the alkyl group can revert to alkene, which can detach from the catalyst. Consequently, contact with a hydrogenation catalyst allowscis-trans-isomerization. Thetrans-alkene can reassociate to the surface and undergo hydrogenation. These details are revealed in part using D2 (deuterium), because recovered alkenes often contain deuterium.
For aromatic substrates, the first hydrogenation is slowest. The product of this step is a cyclohexadiene, which hydrogenate rapidly and are rarely detected. Similarly, thecyclohexene is ordinarily reduced to cyclohexane.
In many homogeneous hydrogenation processes,[23] the metal binds to both components to give an intermediate alkene-metal(H)2 complex. The general sequence of reactions is assumed to be as follows or a related sequence of steps:
Alkene isomerization often accompanies hydrogenation. This important side reaction proceeds bybeta-hydride elimination of the alkyl hydride intermediate:[24]
Often the released olefin is trans.
The hydrogenation of nitrogen to give ammonia is conducted on a vast scale by theHaber–Bosch process,[25] consuming an estimated 1% of theworld's energy supply.
Oxygen can be partially hydrogenated to givehydrogen peroxide, although this process has not been commercialized. One difficulty is preventing the catalysts from triggering decomposition of the hydrogen peroxide to form water.[26][27]
Catalytic hydrogenation has diverse industrial uses. Most frequently, industrial hydrogenation relies on heterogeneous catalysts.[2]
The food industry hydrogenates vegetable oils to convert them into solid or semi-solid fats that can be used in spreads, candies, baked goods, and other products likemargarine. Vegetable oils are made from polyunsaturated fatty acids (having more than one carbon-carbon double bond). Hydrogenation eliminates some of these double bonds.[28]
In petrochemical processes, hydrogenation is used to convert alkenes and aromatics into saturated alkanes (paraffins) and cycloalkanes (naphthenes), which are less toxic and less reactive. Relevant to liquid fuels that are stored sometimes for long periods in air, saturated hydrocarbons exhibit superior storage properties. On the other hand, alkenes tend to formhydroperoxides, which can form gums that interfere with fuel handling equipment. For example,mineral turpentine is usually hydrogenated.Hydrocracking of heavy residues into diesel is another application. Inisomerization andcatalytic reforming processes, some hydrogen pressure is maintained tohydrogenolyzecoke formed on the catalyst and prevent its accumulation.
Hydrogenation is a useful means for converting unsaturated compounds into saturated derivatives. Substrates include not only alkenes and alkynes, but also aldehydes, imines, and nitriles,[29] which are converted into the corresponding saturated compounds, i.e. alcohols and amines. Thus, alkyl aldehydes, which can be synthesized with theoxo process fromcarbon monoxide and an alkene, can be converted to alcohols. E.g.1-propanol is produced from propionaldehyde, produced from ethene and carbon monoxide.Xylitol, apolyol, is produced by hydrogenation of the sugarxylose, an aldehyde. Primary amines can be synthesized byhydrogenation of nitriles, while nitriles are readily synthesized from cyanide and a suitable electrophile. For example, isophorone diamine, a precursor to thepolyurethane monomerisophorone diisocyanate, is produced from isophorone nitrile by a tandem nitrile hydrogenation/reductive amination by ammonia, wherein hydrogenation converts both the nitrile into an amine and the imine formed from the aldehyde and ammonia into another amine.
The earliest hydrogenation was that of the platinum-catalyzed addition of hydrogen to oxygen in theDöbereiner's lamp, a device commercialized as early as 1823. The French chemistPaul Sabatier is considered the father of the hydrogenation process. In 1897, building on the earlier work ofJames Boyce, an American chemist working in the manufacture of soap products, he discovered that traces of nickel catalyzed the addition of hydrogen to molecules of gaseous hydrocarbons in what is now known as theSabatier process. For this work, Sabatier shared the 1912Nobel Prize in Chemistry.Wilhelm Normann was awarded a patent in Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid oils, which was the beginning of what is now a worldwide industry. The commercially importantHaber–Bosch process, first described in 1905, involves hydrogenation of nitrogen. In theFischer–Tropsch process, reported in 1922 carbon monoxide, which is easily derived from coal, is hydrogenated to liquid fuels.
In 1922, Voorhees and Adams described an apparatus for performing hydrogenation under pressures above one atmosphere.[30] The Parr shaker, the first product to allow hydrogenation using elevated pressures and temperatures, was commercialized in 1926 based on Voorhees and Adams' research and remains in widespread use. In 1924Murray Raney developed a finely powdered form of nickel, which is widely used to catalyze hydrogenation reactions such as conversion of nitriles to amines or the production of margarine.
In the 1930s, Calvin discovered that copper(II) complexes oxidized H2. The 1960s witnessed the development of well defined homogeneous catalysts using transition metal complexes, e.g.,Wilkinson's catalyst (RhCl(PPh3)3). Soon thereafter cationic Rh and Ir were found to catalyze the hydrogenation of alkenes and carbonyls.[31] In the 1970s, asymmetric hydrogenation was demonstrated in the synthesis ofL-DOPA, and the 1990s saw the invention ofNoyori asymmetric hydrogenation.[32] The development of homogeneous hydrogenation was influenced by work started in the 1930s and 1940s on theoxo process andZiegler–Natta polymerization.
For most practical purposes, hydrogenation requires a metal catalyst. Hydrogenation can, however, proceed from some hydrogen donors without catalysts. Illustrative hydrogen donors includediimide andaluminium isopropoxide, the latter illustrated by theMeerwein–Ponndorf–Verley reduction. Some metal-free catalytic systems have been investigated. One such system for reduction of ketones consists oftert-butanol andpotassium tert-butoxide and very high temperatures.[33] The reaction depicted below describes the hydrogenation ofbenzophenone:
Achemical kinetics study[34] found this reaction isfirst-order in all three reactants suggesting a cyclic 6-memberedtransition state.
Another system for metal-free hydrogenation is based on thephosphine-borane, compound1, which has been called afrustrated Lewis pair. It reversibly accepts dihydrogen at relatively low temperatures to form thephosphoniumborate2 which can reduce simple hinderedimines.[35]
The reduction ofnitrobenzene toaniline has been reported to be catalysed byfullerene, its mono-anion, atmospheric hydrogen and UV light.[36]
Today's bench chemist has three main choices of hydrogenation equipment:
The original and still a commonly practised form of hydrogenation in teaching laboratories, this process is usually effected by adding solid catalyst to around bottom flask of dissolved reactant which has been evacuated usingnitrogen orargon gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then supplied from a H2-filledballoon. The resulting three phase mixture is agitated to promote mixing. Hydrogen uptake can be monitored, which can be useful for monitoring progress of a hydrogenation. This is achieved by either using a graduated tube containing a coloured liquid, usually aqueouscopper sulfate or withgauges for each reaction vessel.
Since many hydrogenation reactions such ashydrogenolysis ofprotecting groups and the reduction ofaromatic systems proceed extremely sluggishly at atmospheric temperature and pressure, pressurised systems are popular. In these cases, catalyst is added to a solution of reactant under an inert atmosphere in apressure vessel. Hydrogen is added directly from a cylinder or built in laboratory hydrogen source, and the pressurized slurry is mechanically rocked to provide agitation, or a spinning basket is used.[37] Recent advances inelectrolysis technology have led to the development ofhigh pressure hydrogen generators, which generate hydrogen up to 1,400psi (100 bar) from water. Heat may also be used, as the pressure compensates for the associated reduction in gas solubility.
Flow hydrogenation has become a popular technique at the bench and increasingly the process scale.[citation needed] This technique involves continuously flowing a dilute stream of dissolved reactant over a fixed bed catalyst in the presence of hydrogen. Using establishedhigh-performance liquid chromatography technology, this technique allows the application of pressures from atmospheric to 1,450 psi (100 bar). Elevated temperatures may also be used. At the bench scale, systems use a range of pre-packed catalysts which eliminates the need for weighing and filteringpyrophoric catalysts.
Catalytic hydrogenation is done in atubular plug-flow reactor packed with a supported catalyst. The pressures and temperatures are typically high, although this depends on the catalyst. Catalyst loading is typically much lower than in laboratory batch hydrogenation, and various promoters are added to the metal, or mixed metals are used, to improve activity, selectivity and catalyst stability. The use of nickel is common despite its low activity, due to its low cost compared to precious metals.
Gas liquid induction reactors (hydrogenator) are also used for carrying out catalytic hydrogenation.[38]
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