Learn what catalysts are and how they affect the activation energy and reaction rate of a chemical reaction.
A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by the reaction and it may participate in multiple reactions at a time. The only difference between a catalyzed reaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.
Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.
A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.
Catalysts permit an alternate mechanism for the reactants to become products, with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectivity. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.
Regulations controlling diesel exhaust become more exacting with each passing year. Accordingly, diesel fuel properties are constantly being analyzed in an attempt to further reduce fuel emissions. There are many options, most often refinement processes or improving the cetane number. Essentially, short and branched ethers (used in gasoline) have a good octane number but poor cetane number, while those ethers used in diesel are linear and have a comparatively long chain (ideally 9 or more carbons). Di-n pentyl ether (DNPE) has shown most effective in reducing emissions, and is also relatively simple to synthesize via the bimolecular dehydration of 1-pentanol on acid catalysts, as seen below.
However, the dehydration reaction results in quite a lot of byproducts, including other ethers. As such, a selective catalyst is required to favor production of DNPE by reducing the amount of alkenes. Increased selectivity can be accomplished via gel-type acidic resins at a reaction temperature of 150°C. The article I looked at analyzed the selectivity and reaction rate of the dehydration of 1-pentanol to DNPE using a gel-type resin at various temperatures and alcohol flow rates.
Fischer esterification can be a time-consuming process, requiring days for a reaction to reach equilibrium. In this article, researchers developed a way to hasten this process by using a specially designed microwave to heat the reaction quickly and evenly and at an increased pressure. In order to test the efficiency of the device, they synthesized 2-ethylhexyl benzoate from benzoic acid and 2-ethylhexanol as shown below.
Sulfuric acid as well as para-toluene sulfonic acid (PTSA) were used to catalyze the reaction. In order to shift the reaction towards the products, a large excess of 2-ethylhexanol was used and the water produced was constantly removed. One of the disadvantages of Fischer esterification is that dehydration can also occur, resulting in unwanted ether and alkene products. Because of this, the temperature and catalyst concentration must be carefully monitored. The researchers were able to show that microwave heating causes no ill effects on the reaction and reduces the time required to a matter of minutes while still producing a high level of the desired product.
This reaction is an example of the catalytic hydrogenation of an acid in an ionic liquid similar to the reagents sodium metal/liquid ammonia discussed in lecture. This particular reaction has sorbic acid and hydrogen gas reacting with a ruthenium catalyst and a biphasic 1-butyl-3-methyl imidazolium hexafluorophosphate (bmimPF6)/methyl tert-butyl ether (MTBE) system to create cis-hex-3-enoic acid. The above reaction occurs with ~85% selectivity. The author of this paper was examining enantioselective hydrogenation in ionic liquids because this mechanism could provide a means for facile recycling of metal complexes of expensive chiral ligands. The author also studies some hydrogenation reactions that lead to the precursor of the antiinflammatory drug ibuprofen, the active ingredient in Advil.
Along with the growing interest in Fuel Cell powered cars comes the need for higher production methods of Hydrogen, both in bulk form and in-car conversions (for fuels such as methanol and ethanol to be converted to hydrogen on board). Previous methods of converting Ethanol to hydrogen was by means of high-temperature steam reformation (at temperatures in excess of 600° C) to produce Hydrogen gas and CO.
This journal describes a special method of low-temperature dehydrogenation of ethanol over special Raney catalyst with Cu added to it. The first step produces one mole each of hydrogen gas and acetaldehyde (per mole of ethanol). This is followed by the decarbonylation of acetaldehyde to form methanol and CO. The whole reaction undergoes a water-gas shift to net one mole each of Methane and Carbon dioxide and two moles of Hydrogen.
Compared to high temperature reformation methods, which produces 6 moles of hydrogen per mole of ethanol, this reaction doesn’t seem as fuel efficient, though the authors were confident, that with an internal combustion engine on-board that uses the methane produced as fuel, the total output energy would be equal.
