Catalysts and Catalyst Supports
One of the key areas of interest for the research and development (R&D) department of Kisuma Chemicals is the use of hydrotalcites and other layered double hydroxides (LDH), including non-magnesium containing materials, in catalysis.
The drive in academia and industry to apply Layered Double Hydroxide (LDH) type materials in catalyst systems, apart from their reactivity, is threefold. Generally speaking LDHs are environmentally more benign than alternative catalyst systems (heavy metals), and potentially cheaper. The final reason is that LDHs are heterogeneous catalysts, greatly improving the ease of catalysts separation after reaction. Sometimes this heterogeneity is also cause for significant increases in reaction rate, due to the enzyme-like pre-adsorption of substrates on the catalyst surface.
Hydrotalcites may be used as such or they may be calcined to form mixed oxides that are useful catalysts. The materials are mainly applied because of their base properties, or as redox catalysts. Moreover, in the uncalcined form, hydrotalcites can be used as catalyst supports, and this confers to these materials an even wider spectrum of activities and selectivities.
HTs have anion-exchange capacity. A second characteristic of HTs is that they behave as solid bases. Whereas for the hydrated material, the active base sites are mainly structural hydroxyl anions, strong Lewis basic O2-–Mn+ pairs are present in completely water-free calcined materials. The basicity is affected by the calcination procedure, typically at 400-500 °C, and by structural and compositional parameters. Cations like Zn or Ni give less basicity than Mg; less basic catalysts are also obtained from Cl- or SO42- precursors than from CO32- - or OH--containing materials. The basicity also depends on the Mg/Al ratio. It has been reported that the total number of basic sites increases by decreasing the Mg/Al ratio, but the portion of strong basic sites decreases. The correlation of the HT basic properties with the Mg/Al ratio, however, is not always straightforward.
As mentioned before not all LDHs are suitable for all reaction types. Broadly speaking there are three types of catalyst system for which LDHs can be used:
- Redox Catalyst: Various LDHs have been reported to be active as oxidation or reduction catalysts. These types are generally types including copper or a heavy metal.
- Acid/Base catalyst: The unique structure of the Mg/Al HTs allows for a highly tunable structure with both acidic and basic properties. This bi-functionality allows for the use of these materials as catalysts for various organic conversions.
- Catalyst support: LDHs of various types, with a majority being Mg/Al types, have been reported as support for catalytic species, such as transition metals, alkali metals and even various anions.
A quick inspection of the search results found when queried for “Hydrotalcite AND catalyst” reveals a plethora of applications for a wide variety and often very specific processes. Often, specific types of hydrotalcites are described as best suited for a respective process. Kisuma Chemicals has developed a range of materials with interesting properties. We are greatly interested to start collaborations with partners who want to develop catalytic systems based on (custom) hydrotalcites.
Contact our R&D
For more questions, or to explore common grounds to start a co-operation revolving around hydrotalcites for catalyst applications, contact our R&D department today!