Fluid catalytic cracking (FCC) — catalysts used in the riser/reactor Which of the following materials is used as the active cracking catalyst in modern fluidised-bed catalytic cracking?

Introduction / Context:
Fluid catalytic cracking converts heavy gas oils to gasoline and lighter products in a circulating fluidised bed. The choice of catalyst determines activity, selectivity, and stability under severe regeneration conditions.

Given Data / Assumptions:

• Classical FCC catalysts are based on silica–alumina matrices, with modern catalysts incorporating zeolites (e.g., Y-zeolite) dispersed in a silica–alumina matrix.
• Other materials may serve as supports, binders, or historical curiosities but are not the primary active cracking component in contemporary FCC.

Concept / Approach:
Silica–alumina provides the acidic sites required for catalytic cracking reactions. Zeolitic catalysts significantly enhanced activity and selectivity but are still carried in a silica–alumina matrix. Clays like bentonite act mainly as binders or for physical properties, not as the active catalyst.

Step-by-Step Solution:
Identify the active acidic framework → silica–alumina (with zeolite in practice).Silica–magnesia was used in other cracking/polymerisation contexts, not standard modern FCC.Bentonite clays are not the active cracking catalyst.

Verification / Alternative check:
Refinery references describe FCC equilibrium catalyst (E-cat) as zeolite on silica–alumina matrix.

Why Other Options Are Wrong:

• Silica–magnesia: not the mainstream FCC active catalyst today.
• Bentonite: binder/physical aid rather than active cracking catalyst.
• “All of the above” overstates applicability.

Common Pitfalls:

• Ignoring the matrix when focusing on zeolite; both work together, but the matrix is silica–alumina.

Final Answer:
Silica–alumina