GREEN CHEMISTRY
Crotonic acid hdrogenation
Nitrobenzene hdrogenation
Figure 2 – Effects of varying preparation parameters on hydrogenation activity
A
Figure 3 – SEM image of 2.5 wt % Pd / C carbon microspheres with precursors A ( left ) & B ( right )
metals as a direct replacement for powder-based catalysts . The catalysts presented here expand the boundaries of conventional precious metal-based catalysts and combine advantages of powder and shaped catalysts . Due to their small particle size compared to conventional shaped alumina or carbon catalysts , they can easily be deployed in smallscale set-ups and microreactors .
Moreover , their advantageous physical properties , such as narrow particle size distribution , high specific surface area and pore volume in combination with a high crushing strength , allow them to be used even under harsh reaction conditions in fixed bed- and batchoperated reactors .
Continuous flow hydrogenation reactions are of particular interest for many industrial players . Heraeus is actively developing and optimising catalysts for such processes , often in cooperation with partners and process owners . The company is also mapping out strategies
B
to obtain value-added products by using precious metal-based catalysts , while demonstrating cost attractiveness and sustainability responsibility by applying recycling strategies to keep the once-mined scarce precious metal ‘ in the loop ’.
Herein , we present newly developed platinum- and palladium-based catalysts on carbon microsphere supports . Figure 1 illustrates the core properties of carbon microspheres and the corresponding precious metal catalysts .
Results & discussion
The main goal of systematically optimising the preparation conditions was to achieve a homogeneous distribution of the precious metal on the catalyst support and ensure a high level of activity in hydrogenation test reactions .
Hydrogenation test reactions of nitrobenzene and crotonic acid were conducted to determine the catalytic activity , while scanning electron microscopy with energy
dispersive X-ray spectroscopy ( SEM / EDX ) analyses were used to assess the homogeneous distribution of the precious metal on the support .
The optimisation of the catalyst preparation procedure included three distinct stages . First , the impact of pre-treatment on the catalyst activity was investigated . Two different pretreatment procedures were applied to the support before loading the microspheres with precious metal .
By switching to more suitable conditions a significant increase in activity in the hydrogenation of nitrobenzene could be observed ( Figure 2 ). Subsequently , the variation of precious metal precursor solutions was studied , which led to a conversion increase of approximately 50 % in the hydrogenation of nitrobenzene .
The dispersion of the precious metal with precursor A resulted in a very non-homogeneous distribution of the active phase ( Figure 3 ). Significant improvement was achieved by using precursor B and a considerably more homogenous distribution of precious metal on the carrier could be achieved .
Quantitative determination of the palladium content of different catalysts particles by SEM-EDX analyses further supports the impression that palladium is homogeneously dispersed when precursor B is used . Palladium content prepared with precursor B was determined by SEM-EDX to be 2.33 %, 2.51 % and 2.44 % at three selected points .
MAY / JUN 2024 SPECCHEMONLINE . COM
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