Support properties, precursor choice and synthesis all influence catalytic performance, says Dr Artur Gantarev of Heraeus Precious Metals

Heterogeneously catalysed hydrogenations account for a significant share of fine chemical and pharmaceutical synthesis steps, with Pd / C and Pt / C catalysts widely used for this. Some key advantages of these catalysts are their robustness, atom economy and well-established processing behaviour, which leads to easy separation from the product and reusability.

Despite this critical role, these catalysts are still too often regarded as commodity materials and assumed to be interchangeable based solely on metal loading or a generic description such as‘ 5 wt % Pd / C’. Additionally, the form and appearance of typical metal-containing activated carbon catalysts— often described simply as black powders— offer little differentiation.

In practice, however, catalysts with identical nominal specifications can differ substantially in catalytic activity and overall process performance. Depending on the specific application, distinct conversion profiles, selectivities, and catalyst lifetimes or numbers of reaction cycles are observed. Figure 1 illustrates this, using the example of the hydrogenation of crotonic acid and nitrobenzene using various Heraeus catalysts, all of which have a Pd loading of 5 wt %.

Based on this, choosing the most suitable individual catalyst for a certain process is crucial to boost the efficiency and the economics of the whole production process. Selecting from tailored catalyst options, such as Heraeus’ HeraSelect * portfolio, can support this process-specific optimisation. To evaluate this choice, a certain understanding of the two key ingredients of heterogeneous carbon-supported catalysts – the carbon carrier and precious metal precursor- is needed.

Carbon supports

One of these differences lies in the complexity of activated carbon as a support material, which represents the first key ingredient of a heterogeneous precious metal carbon catalyst. Activated carbons vary widely depending on their raw feedstock, each providing distinct porosity, specific BET surface area, particle size distribution and adsorption properties. These structural parameters govern adsorption behaviour, metal anchoring, and the potential for achieving uniform or intentionally tailored metal distributions.

The porous architecture and chemical functionality of carbon supports determine the dispersion and accessibility of precious metal nanoparticles, thereby strongly influencing catalytic activity and stability in hydrogenation reactions. Additional variations in activation method( steam v. chemical), posttreatment( acid-washed v. non-acidwashed) and ash content further contribute to the diversity of available carbon supports.

Figure 1- Hydrogenation activity of Pd / C catalysts

10 % Pd / C

Crotonic acid( CA) hydrogenation

4 % Pd – 1 % Pt / C

5 % Pd / C

CA NB

Nitrobenzene( NB) hydrogenation

3 % Pd / C

68 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981