Zeolite ZSM-5 (MFI framework) is used in the methanol-to-olefins (MTO) process. The selectivity toward light olefins (ethylene, propylene) rather than larger hydrocarbons is primarily due to:
AThe Bronsted acid sites in ZSM-5 are weaker than in other zeolites
BThe intersecting 10-ring channel system (5.1 x 5.5 and 5.3 x 5.6 Angstroms) allows only molecules up to about C10 to form in the pore intersections and only light olefins to exit through the channels
CThe low Si/Al ratio of ZSM-5 provides too few acid sites for larger molecules to form
DZSM-5 is always used at temperatures too low for larger hydrocarbons to form
ZSM-5's shape selectivity operates on both products and transition states. The 10-ring channels are large enough for small olefins to diffuse through but restrict the formation and escape of bulkier molecules. The pore intersections provide enough space for the hydrocarbon pool mechanism to operate (forming methylated aromatic intermediates), but product selectivity is governed by which molecules can physically escape the pore system. This is product shape selectivity — a purely geometric effect independent of acid site strength or number.
Question 2 True / False
Replacing Na+ with H+ in a zeolite (via NH4+ exchange followed by calcination) converts it from an ion exchanger to a solid acid catalyst.
TTrue
FFalse
Answer: True
Na-zeolites are excellent ion exchangers but poor catalysts because Na+ is not acidic. Exchanging Na+ for NH4+ (by treating with ammonium salt solution), then heating to decompose NH4+ into NH3 (which leaves) and H+ (which stays on the framework), creates Bronsted acid sites — bridging hydroxyl groups (Si-OH-Al) that donate protons to adsorbed molecules. The resulting H-form zeolite is a strong solid acid that catalyzes cracking, isomerization, alkylation, and many other acid-catalyzed reactions. The acid strength depends on the Si/Al ratio: higher Si/Al gives fewer but stronger acid sites because each Al is more isolated.
Question 3 Short Answer
Why do zeolites with higher Si/Al ratios show greater hydrothermal stability?
Think about your answer, then reveal below.
Model answer: Hydrothermal stability depends on the strength of framework bonds. Si-O bonds (bond energy ~452 kJ/mol) are stronger than Al-O bonds (~362 kJ/mol). Higher Si/Al ratios mean more Si-O-Si linkages and fewer Si-O-Al linkages in the framework, making the overall structure more resistant to hydrolysis by steam at high temperatures. Additionally, aluminum sites are the preferred points of hydrolytic attack — water molecules coordinate to Al and can extract it from the framework (dealumination). Fewer Al sites means fewer weak points. This is why high-silica zeolites like ZSM-5 (Si/Al = 15-300) survive the harsh conditions of fluid catalytic cracking units, while low-silica zeolites like type A (Si/Al = 1) collapse.
The tradeoff is that higher Si/Al means fewer ion-exchange sites and fewer acid sites per unit cell. Practical zeolite catalysts balance the need for sufficient active sites against the need for framework stability under operating conditions. Ultra-stable Y zeolite (USY), made by steam-treating NaY to raise the framework Si/Al from 2.5 to 5-10, is the workhorse catalyst in petroleum refining precisely because this balance has been optimized.