How can we use catalysts

Manufacturers often create catalysts to speed processes in industry. One technology that needs a catalyst to work is a hydrogen fuel cell. In these devices, hydrogen gas H 2 reacts with oxygen gas O 2 to make water H 2 O and electricity. These systems can be found in a hydrogen vehicle where they create the electricity to power the engine. The fuel cell needs to separate the atoms in molecules of hydrogen and oxygen so that those atoms can reshuffle to create new molecules water.

Without some assistance, though, that reshuffling would take place very slowly. So the fuel cell uses a catalyst — platinum — to propel those reactions along. Platinum works well in fuel cells because it interacts just the right amount with each starting gas. In effect, it pulls them close together so that it encourages — speeds along — their reaction. Then it lets its handiwork float free. For years, other technologies have relied on platinum catalysts, too.

They turn milk into yogurt and petroleum into plastic milk jugs, CDs and bicycle helmets. Catalysts speed up a chemical reaction by lowering the amount of energy you need to get one going. Catalysis is the backbone of many industrial processes, which use chemical reactions to turn raw materials into useful products.

Catalysts are integral in making plastics and many other manufactured items. Even the human body runs on catalysts. Many proteins in your body are actually catalysts called enzymes, which do everything from creating signals that move your limbs to helping digest your food. They are truly a fundamental part of life. In most cases, you need just a tiny amount of a catalyst to make a difference.

Even the size of the catalyst particle can change the way a reaction runs. The catalyst turns propylene into propylene oxides, which is the first step in making antifreeze and other products.

Industrial manufacturing processes for plastic and other essential items often produce nasty by-products which can pose hazards to human health and the environment. Branched alkanes have a much higher octane rating than straight chain ones. Not only are the alkanes now branched, but cycloalkanes are also formed and, from them, aromatic hydrocarbons. All three classes of hydrocarbon have a higher octane rating than naphtha. Besides aluminium oxide and silicon dioxide, other oxides are important catalysts.

For example, in the Contact Process used to manufacture sulfuric acid, the catalyst for the oxidation of sulfur dioxide to sulfur trioxide is vanadium V oxide on the surface of silica:. Potassium sulfate is added as a promoter. Its mode of action is not absolutely clear but it appears to be because its presence lowers the melting point of the catalyst, and allows it to spread out as a very thin layer over the entire surface.

Several important industrial processes are catalysed by mixed metal oxides. The surfaces contain two or more different metal atoms, O 2- ions and -OH groups.

They are particularly useful in the oxidation of hydrocarbons, where selective oxidation is required. For example, propene can be oxidized to propenal acrolein using a mixture of bismuth III and molybdenum VI oxides. Without the catalyst, propene is oxidized to a large number of organic compounds, including methanal and ethanal, and eventually forming carbon dioxide.

The oxygen atoms on the surface of molydenum VI oxide are not very reactive, reacting selectively with propene and breaking the weakest bond in the alkene to form an allyl radical:. The allyl radical is then oxidized on the surface to yield propenal. It is postulated that the allyl radical is oxidized by an oxygen atom that is adsorbed at a molybdenum site. Another oxygen atom, adsorbed on a bismuth site, is then transported to the reduced molybdenum site to replace that oxygen.

There is a compensating transport of electrons to complete the cycle. The same catalyst is also used to manufacture propenonitrile :. Homogeneous catalysts are less frequently used in industry than heterogeneous catalysts as, on completion of the reaction, they have to be separated from the products, a process that can be very expensive. Table 3 Examples of industrial processes using homogeneous catalysis.

However, there are several important industrial processes that are catalysed homogeneously, often using an acid or base Table 3. One example is in the manufacture of ethane-1,2-diol from epoxyethane where the catalyst is a trace of acid:. Figure 12 A mechanism for the formation of ethane-1,2-diol from epoxyethane.

In the mechanism for this reaction a hydrogen ion is added at the start, and lost at the end. The hydrogen ion functions as a catalyst. Two other examples are concerned with the production of 2,2,4-trimethylpentane from 2-methylpropene, again using an acid as the catalyst. One uses 2-methylpropane Table 3 which yields the alkane directly. The other uses only 2-methylpropene.

The mechanism of the reaction also involves the addition of a hydrogen ion to a reactant Figure Figure 13 Part of a mechanism for the formation of 2,4,4-trimethylpentene from 2-methylpropene. The alkene is then hydrogenated, using nickel as the catalyst, to 2,2,4-trimethylpentane isooctane :. Ziegler-Natta catalysts are organometallic compounds which act as catalysts for the manufacture of poly ethene and poly propene. The catalysts are prepared from titanium compounds with an aluminium trialkyl which acts as a promoter:.

The alkene monomer, for example ethene or propene, attaches itself to an empty coordination site on the titanium atom and this alkene molecule then inserts itself into the carbon-titanium bond to extend the alkyl chain. This process then continues, thereby forming a linear polymer, poly ethene or poly propene. The polymer is precipitated when the catalyst is destroyed on addition of water.

Because it is linear, the polymer molecules are able to pack together closely, giving the polymer a higher melting point and density than poly ethene produced by radical initiation. Figure 14 Illustrating the role of a Ziegler-Natta catalyst. Not only do Ziegler-Natta catalysts allow for linear polymers to be produced but they can also give stereochemical control.

Propene, for example could polymerize, even if linear, in three ways, to produce either atactic, isotactic or syndiotactic poly propene. However, this catalyst only allows the propene to be inserted in one way and isotactic polypropene is produced. Even greater control of the polymerization is obtained using a new class of catalysts, the metallocenes. Many polymers are produced using radical initiators, which act as catalysts Table 4.

For example the polymerization of chloroethene is started by warming it with a minute trace of a peroxide R-O-O-R :. Figure 15 A mechanism for the free radical polymerization of chloroethene to poly chloroethene. In the case of ethene, by using the free radical process, the resulting polymer has a lower density and a lower softening point than the poly ethene produced using a Ziegler-Natta catalyst or a metallic oxide. The low density poly ethene , LDPE, has side chains because the radicals react not only with molecules of ethene, by addition, but also with polymer molecules, by a process known as hydrogen abstraction.

The polymer radical can also abstract a hydrogen atom from its own chain:. Both of these reactions lead to side chains so that the molecules of the polymer cannot pack together in a regular way.

The polymer thus has a lower melting point and lower density. Table 4 Examples of polymers produced using free radical polymerization. The researchers have applied the catalyst to coal gasification, he notes, as well as to a process for catalytic pyrolysis of waste tires for fuel production. For example, industrial processing, the use of consumer goods and medicines, and even the wearing away of jewelry leads to measurable amounts of catalyst metals such as gold, silver, and platinum accumulating at wastewater treatment plants.

One of the more prolific sources of these metals, though, is catalytic converters. Automobiles in the U. They do a good job on vehicle emissions by zapping pollutants such as unburned hydrocarbons, carbon monoxide, and nitrogen oxides and turning them into more benign products such as CO 2 , water, and nitrogen.

But as cars putt down the road, catalytic converters slowly disperse platinum, palladium, rhodium, and cerium into the environment. Researchers who have assessed the abundance of these dissipative metals think the concentrations are high enough in the environment, or will be over time, to make it worthwhile to recover them because of their high market values.

To assess the situation, environmental engineer Sebastien Rauch of Chalmers University of Technology and his colleagues measured platinum, palladium, and rhodium concentrations and fluxes in the environment using high-volume air particulate sampling. Concentration ratios of the metals and trace osmium isotope ratios allowed the team to peg catalytic converters as the source of the metals, rather than natural or industrial sources.

And harvesting the metals from the air could certainly have some limitations. It is of course possible to sample for longer periods, but platinum amounts would remain relatively small. Testing out the idea, geoscientist Hazel M. Prichard of Cardiff University, who passed away on Jan. Prichard had the idea that recovering the metals could be as simple as scooping up samples from the street or roadsides, storm drains, and wastewater treatment plants.

Prichard even investigated the collection bins in the bellies of street-sweeping machines. Several years ago, Prichard and her colleagues collected samples in Sheffield, England, finding gold primarily from jewelry, and platinum, palladium, and rhodium from catalytic converters. The platinum, palladium, and rhodium combined made up as much as 1.

They also found more than 3 ppm of gold, platinum, palladium, and rhodium concentrated in incinerated sewage sludge ash. By comparison, Prichard estimated that the minimum concentration needed to economically extract platinum-group metals from ore deposits is 2—4 ppm. As Rauch points out, mechanical sieving could potentially help concentrate the metals for use as catalysts.

As another option, Prichard and her team enlisted biochemist Lynne E. Macaskie of the University of Birmingham to help develop a fermentation process for metal-absorbing microbes to extract metals from the dust. Macaskie and her colleagues have tested palladium-containing bacterial biomass as a bioinorganic catalyst for cleaning up industrial waste and for hydrogenation reactions, finding that the material has potential for industrial applications, she says.

Although recovering metals could prove lucrative, Prichard was motivated by the fact that global supplies of precious metals are limited. Lipshutz of the University of California, Santa Barbara. Lipshutz and his group have taken a minimalist approach to organic synthesis , using the smallest possible amount of catalyst and organic solvent to see how green and efficient they can make everyday reactions.

In one case, they showed that parts-per-million traces of palladium impurity in iron chloride is enough to catalyze cross-coupling reactions. Finding creative ways to recycle such gifts from nature are clearly important challenges we face, but they are problems we can solve. Contact the reporter. Submit a Letter to the Editor for publication. Engage with us on Twitter. The power is now in your nitrile gloved hands Sign up for a free account to increase your articles. Or go unlimited with ACS membership.

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