biology

Homogeneous catalysis

Homogeneous catalysis is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Homogeneous catalysis rather than just read about it. In short: In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in distinct phases, typically solid and gas, respectively.

Key takeaways

Homogeneous catalysis belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Homogeneous catalysis to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Homogeneous catalysis from memory before moving on to harder problems.

Reference excerpt

In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in distinct phases, typically solid and gas, respectively. The term is used almost exclusively to describe solutions and implies catalysis by organometallic compounds. Homogeneous catalysis is an established technology that continues to evolve. An illustrative major application is the production of acetic acid. Enzymes are examples of homogeneous catalysts.

Examples

Acid catalyst

The proton is a pervasive homogeneous catalyst because water is the most common solvent. Water forms protons by the process of self-ionization of water. In an illustrative case, acids accelerate (catalyze) the hydrolysis of esters:

CH3CO2CH3 + H2O ⇌ CH3CO2H + CH3OH At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates.

Transition metal-catalysis

Hydrogenation and related reactions A prominent class of reductive transformations are hydrogenations. In this process, H2not added to unsaturated substrates. A related methodology, transfer hydrogenation, involves by transfer of hydrogen from one substrate (the hydrogen donor) to another (the hydrogen acceptor). Related reactions entail "HX additions" where X = silyl (hydrosilylation) and CN (hydrocyanation). Most large-scale industrial hydrogenations – margarine, ammonia, benzene-to-cyclohexane – are conducted with heterogeneous catalysts. Fine chemical syntheses, however, often rely on homogeneous catalysts.

Carbonylations Hydroformylation, a prominent form of carbonylation, involves the addition of H and "C(O)H" across a double bond. This process is almost exclusively conducted with soluble rhodium- and cobalt-containing complexes. A related carbonylation is the conversion of alcohols to carboxylic acids. MeOH and CO react in the presence of homogeneous catalysts to give acetic acid, as practiced in the Monsanto process and Cativa processes. Related reactions include hydrocarboxylation and hydroesterifications.

Polymerization and metathesis of alkenes A number of polyolefins, e.g. polyethylene and polypropylene, are produced from ethylene and propylene by Ziegler-Natta catalysis. Heterogeneous catalysts dominate, but many soluble catalysts are employed especially for stereospecific polymers. Olefin metathesis is usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.

Oxidations Homogeneous catalysts are also used in a variety of oxidations. In the Wacker process, acetaldehyde is produced from ethene and oxygen. Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of terephthalic acid from xylene. Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by the Halcon process and the Sharpless dihydroxylation.

Enzymes (including metalloenzymes) Enzymes are homogeneous catalysts that are essential for life but are also harnessed for industrial processes. A well-studied example is carbonic anhydrase, which catalyzes the release of CO2 into the lungs from the bloodstream. Enzymes possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst. Water is a common reagent in enzymatic catalysis. Esters and amides are slow to hydrolyze in neutral water, but the rates are sharply affected by metalloenzymes, which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis of acrylonitrile. US demand for acrylamide was 253,000,000 pounds (115,000,000 kg) as of 2007.

Advantages and disadvantages

Advantages Homogeneous catalysts are often more selective than heterogeneous catalysts. For exothermic processes, homogeneous catalysts dump heat into the solvent. Homogeneous catalysts are easier to characterize, making their reaction mechanisms amenable to rational manipulation.

Disadvantages The separation of homogeneous catalysts from products can be challenging. In some cases involving high activity catalysts, the catalyst is not removed from the product. In other cases, distillation can extract volatile organic products. Homogeneous catalysts have limited thermal stability compared to heterogeneous catalysts. Many organometallic complexes degrade below 100 °C. Some pincer-based catalysts, however, operate near 200 °C.

Illustrations

Worked examples

Example 1 — a first encounter with Homogeneous catalysis

Start with the simplest possible case. Write down what Homogeneous catalysis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Homogeneous catalysis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Homogeneous catalysis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Homogeneous catalysis

In research: Homogeneous catalysis appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Homogeneous catalysis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Homogeneous catalysis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Catalysis, Homogeneous catalysis, so understanding it makes those chapters shorter.
In everyday life: Look for Homogeneous catalysis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Homogeneous catalysis in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Homogeneous catalysis means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Homogeneous catalysis out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Homogeneous catalysis in simple terms?

Why does Homogeneous catalysis matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Homogeneous catalysis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Homogeneous catalysis.