Homogeneous Catalysis
Questions 1–4What is the key distinction between homogeneous and heterogeneous catalysis, and which statement best summarises their relative advantages?
In homogeneous catalysis the catalyst and reactants occupy the same phase (typically both in solution), which allows every catalytic centre to be accessible and often gives excellent selectivity and mild reaction conditions. The main disadvantage is that separating the dissolved catalyst from the product mixture can be difficult and expensive. Heterogeneous catalysts (solid catalyst, liquid or gas reactants) are easier to recover but may offer lower selectivity.
In the hydroformylation (oxo) process, an alkene reacts with CO and H2 in the presence of a cobalt or rhodium catalyst. What is the principal product when propene is used as the substrate?
Hydroformylation inserts CO into the alkene and then reduces the resulting acyl intermediate with H2, producing an aldehyde with one additional carbon atom. From propene (C3) the products are butanal isomers (C4): the linear n-butanal and the branched iso-butanal. Modern Rh-based catalysts with bulky phosphine ligands strongly favour the linear isomer, which is industrially more valuable.
The Wacker process converts ethylene to acetaldehyde using a PdCl2/CuCl2 catalyst system in aqueous solution. What roles do Pd and Cu play in this process?
In the Wacker process, Pd(II) coordinates ethylene and facilitates nucleophilic attack by water, ultimately producing acetaldehyde while Pd(II) is reduced to Pd(0). The co-catalyst Cu(II) reoxidises Pd(0) back to Pd(II), with Cu(I) being generated in the process. Molecular oxygen from air then reoxidises Cu(I) to Cu(II), completing the catalytic cycle and making the overall process catalytic in both metals.
Asymmetric catalysis produces one enantiomer preferentially. How did Noyori and Knowles achieve enantioselective hydrogenation, and why was this work significant?
Knowles developed the chiral phosphine ligand DIPAMP for Rh-catalysed hydrogenation of dehydroamino acids, enabling industrial production of L-DOPA for treating Parkinson’s disease. Noyori introduced BINAP-Ru catalysts that achieve exceptional enantioselectivity (>99% ee) across a range of substrates. Their work, along with Sharpless’s asymmetric oxidation, earned the 2001 Nobel Prize in Chemistry and transformed pharmaceutical synthesis.
Heterogeneous Catalysis
Questions 5–8
The Haber–Bosch process is one of the most important industrial reactions ever developed. Which metal catalyst is used and why is it effective for this reaction?
The Haber–Bosch process converts N2 + 3H2 → 2NH3 at ~400–500 °C and 150–300 atm over an iron catalyst promoted with K2O (electronic promoter) and Al2O3 (structural promoter). Iron sits near the top of the volcano plot for N2 dissociation: it binds nitrogen strongly enough to break the triple bond but not so strongly that the resulting surface nitrides cannot be hydrogenated and released as ammonia.
Heterogeneous catalysis proceeds through a sequence of elementary steps at the catalyst surface. Which sequence correctly describes these steps?
Heterogeneous catalysis involves five key steps: (1) reactant molecules diffuse to the catalyst surface; (2) they adsorb (typically by chemisorption, forming chemical bonds with surface atoms); (3) the adsorbed species undergo surface reaction, often involving bond breaking and formation between co-adsorbed molecules; (4) product molecules desorb from the surface; (5) products diffuse away. The rate-determining step varies by reaction — for example, N2 dissociation is rate-limiting in the Haber process.
The Fischer–Tropsch process uses a heterogeneous catalyst to convert synthesis gas (CO + H2) into useful products. What does this process primarily produce?
The Fischer–Tropsch (FT) process converts syngas (CO + H2) over Fe or Co catalysts at 150–350 °C into a distribution of hydrocarbons following the Anderson–Schulz–Flory distribution. Products include straight-chain alkanes, alkenes, alcohols, and waxes. The product selectivity depends on temperature, pressure, H2/CO ratio, and catalyst choice. FT synthesis is central to gas-to-liquids (GTL) technology and was historically used in coal-to-fuel conversion.
Automotive three-way catalytic converters use platinum-group metals to treat exhaust gases. Which combination of metals and reactions is correct?
A three-way catalytic converter simultaneously performs three reactions: (1) oxidation of CO to CO2, (2) oxidation of unburnt hydrocarbons to CO2 and H2O, and (3) reduction of nitrogen oxides (NOx) to N2. Pt and Pd primarily handle the oxidation reactions, while Rh is particularly effective for NOx reduction. The catalyst operates optimally at the stoichiometric air–fuel ratio, monitored by an oxygen sensor (lambda probe).
Catalyst Design & Green Chemistry
Questions 9–12Catalyst poisoning is a major concern in industrial processes. What is catalyst poisoning, and how does sulfur poison metal catalysts?
Catalyst poisoning occurs when a substance (the poison) binds strongly to the active sites of a catalyst, preventing reactant molecules from adsorbing and reacting. Sulfur compounds (H2S, thiophene, etc.) are notorious poisons for transition metal catalysts because sulfur forms very strong M–S bonds with metals like Pt, Ni, and Fe. Even parts-per-million concentrations of sulfur can deactivate a catalyst, which is why industrial feedstocks must be desulfurised (e.g. by hydrodesulfurisation) before entering catalytic reactors.
The Ziegler–Natta catalyst revolutionised polymer chemistry. How does the TiCl4/AlEt3 system polymerise olefins, and what are the roles of each component?
In the Ziegler–Natta system, AlEt3 (the co-catalyst) alkylates TiCl4 at the surface of TiCl3, generating a Ti–C bond at a coordinatively unsaturated Ti centre. The olefin coordinates to the vacant site on Ti and then inserts into the Ti–C bond (the Cossee–Arlman mechanism), extending the chain by two carbons per insertion. This coordination–insertion mechanism produces stereoregular (isotactic or syndiotactic) polyolefins, which was the breakthrough that earned Ziegler and Natta the 1963 Nobel Prize.
The 12 principles of green chemistry, formulated by Anastas and Warner, guide sustainable chemical practice. Which statement best describes how catalysis relates to these principles?
Principle 9 of green chemistry states: “Catalytic reagents are superior to stoichiometric reagents.” Catalysis is central to green chemistry because a catalyst is used in sub-stoichiometric amounts and regenerated, reducing waste (Principle 1). Catalysts enable milder conditions (lower temperature and pressure), saving energy (Principle 6). They improve selectivity and atom economy (Principle 2), and they often eliminate the need for protecting groups or harsh reagents, reducing the number of synthetic steps and overall environmental impact.
Metal–organic frameworks (MOFs) and zeolites are both used as catalyst supports or heterogeneous catalysts. Which comparison is most accurate?
Zeolites are crystalline microporous aluminosilicates (e.g. ZSM-5, faujasite) with well-defined pore sizes (3–10 Å) that enable shape-selective catalysis — only molecules that fit the pores can react. They are thermally robust (>500 °C) and widely used in petroleum cracking and petrochemistry. MOFs are built from metal ions or clusters linked by organic ligands, offering tuneable pore sizes (up to 50 Å) and surface areas exceeding 7000 m²/g, but they typically decompose at lower temperatures (200–400 °C), limiting their use in high-temperature catalysis.