Reasons for Choosing Palladium as the Catalyst in the Anthraquinone Process for Hydrogen Peroxide Production

1. What is a catalyst? (Simplest Explanation)

A catalyst is a substance that can speed up a chemical reaction without being consumed itself.Essentially, it does two things:

       1. Lowers the energy required for the reaction (activation energy) This allows the reaction to proceed faster.

       2. Guides the reaction along an easier pathway Making the reaction more efficient and selective.

However, a catalyst:

       · Does not change the types of reactants or products.

       · Remains unchanged in quantity before and after the reaction.

Hence, it is called a “catalyst”—it promotes the reaction without being part of the result.

2. Why is palladium necessary for hydrogenation in the anthraquinone process?

Hydrogenation reaction:

EAQ + H₂ → HEAQ

This is a typical selective hydrogenation of an aromatic ketone compound.

Such a reaction cannot occur spontaneously at normal temperature and pressure—it requires a catalyst to proceed.

In industrial practice, palladium (Pd) is the ideal and almost irreplaceable choice, for the following reasons:

✅ Reason 1: Palladium has a very strong ability to “adsorb and activate” H₂ 

Hydrogen (H₂) is a very stable molecule.

To make it react, it must first be “split” into active hydrogen atoms (H).

Palladium metal can:

       · Adsorb H₂

       · Dissociate the H—H bond → form active hydrogen atoms

       · Transfer these hydrogen atoms to the EAQ molecule

This step is called hydrogen activation.

Most other metals cannot perform this efficiently.

✅ Reason 2: Palladium provides the best selectivity for EAQ hydrogenation

Hydrogenating EAQ has two key requirements:

       1. Hydrogenation occurs only at the 9,10 positions of the anthraquinone backbone without damaging other structures.

       2. Avoid over-hydrogenation; do not fully hydrogenate the aromatic rings of EAQ.

Palladium achieves the best balance between activity and selectivity.

       · Nickel (Ni) → too active, may over-hydrogenate and damage the anthraquinone structure

       · Platinum (Pt) → even more aggressive, almost certainly destroys EAQ

       · Copper, Iron, etc. → too weak, cannot hydrogenate EAQ at all

Thus, palladium is one of the few metals capable of high-selectivity hydrogenation of EAQ.

✅ Reason 3: Palladium is active, stable, and safe under mild conditions

Palladium catalysts can hydrogenate EAQ at:

       · 40–50°C

       · 0.3–0.6 MPa

For industrial requirements of safety and stability, palladium meets all the criteria:

       · Resistant to poisoning

       · Easy to regenerate

       · Thermally stable

       · Suitable for long-term operation (1–2 years)

This makes Pd/Al₂O₃ the only mature industrial choice.

✅ Reason 4: Palladium hydrogenation of EAQ has optimal kinetics

Experimental and industrial data show:

       · High reaction rate

       · Selectivity > 95%

       · Little or no by-products

       · EAQ cycle remains intact

This allows the EAQ working solution to stably cycle dozens to hundreds of times.