Selectivity and catalysts

Real reactors rarely run one reaction. In parallel reactions, A can turn into the wanted B or an unwanted C. In series reactions, A → B → C, and the wanted B goes on to something you don't want. Selectivity is how the reactor is steered toward B.

In parallel reactions, temperature favors the one with the higher activation energy. If making B has the higher Ea, run hotter; if C does, cooler. Concentration matters through order: if B's reaction is higher order in A than C's, keep the concentration of A high (batch or plug flow); if lower, keep it low (a CSTR, or dilute feed).

In series reactions, B builds up, peaks, then falls as it turns into C. Stop at the right time, or the right conversion, and you catch the peak. This is why partial oxidations (making a useful oxygenated chemical before it burns all the way to CO₂) are run at deliberately incomplete conversion, with the unreacted feed recycled.

A catalyst gives a new route with a lower activation energy and isn't used up. Most industrial ones are solids (heterogeneous): metals or metal oxides spread on porous pellets with huge internal area. The reaction happens on the surface: molecules diffuse in, stick (adsorb), react and come off. Good catalysts are also selective, speeding up the wanted reaction much more than the rest.

Catalysts wear out. They get poisoned (sulfur sticks to nickel and platinum), fouled by carbon deposits (coke), or sintered, when heat makes the tiny metal particles merge and lose area. Plants remove poisons upstream, burn coke off (regeneration), and change the catalyst every few months or years.