Information AboutHydrogenation |
| CATEGORIES ABOUT HYDROGENATION | |
| addition reactions | |
| homogeneous catalysis | |
| chemical engineering | |
| industrial processes | |
| hydrogen | |
| oil refineries | |
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The classical example of a hydrogenation is the attack of hydrogen on Unsaturated Bonds between Carbon Atom s, 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 cooking oils and fats. Complete hydrogenation converts unsaturated Fatty Acid s 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 Oil s 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 reasonably priced, are available in a wide range of consistencies and have other desirable characteristics such as greater 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 which has implications for human health is the Isomerization of the remaining unsaturated carbon bonds. The Cis configuration of these Double Bond s predominates in the unprocessed fats of most foods, but incomplete hydrogenation partially converts these molecules to Trans Isomer s, which, in fats, have been implicated in Heart Disease (see Trans Fat s). 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 ratio of trans/cis isomers is about 67/33. Food legislation in the US and codes of practice in EU, require the declaration of trans content on food labels in the retail trade. THE HYDROGENATION PROCESS The basic Chemical Reaction in the industrial hydrogenation processes involves the use of gaseous Hydrogen as a Reactant and a Catalyst to accelerate the reaction, with the reaction taking place at elevated temperature and pressure conditions. During most applications of hydrogenation, metal Catalysts pull gaseous hydrogen (H2) apart and stabilize the resulting hydrogen radicals, creating a chemical species with properties intermediate to 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 two new bonds between two hydrogen atoms.) The net result of hydrogenation can be either a reduction or an oxidation, depending on the chemicals being used, although the reaction itself may be best 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 are often 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 attacking 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, sufficient to raise the temperature of the oil by 1.6-1.7 deg 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. W. Normann of England was awarded the first patent in 1903 for the hydrogenation of liquid oils using hydrogen gas, which was the start of a huge industry world wide. SEE ALSO EXTERNAL LINKS
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