Novel Heterogeneous Catalysts with Nano-Engineered Porosity

Roughly 90% of all industrial chemical processes are using a catalyst. In the industry, a heterogeneous catalyst is often chosen rather than a homogeneous catalyst because of the easy separation and recycling of heterogeneous catalysts. However, it is often more difficult to achieve a high catalytic activity and selectivity by using a heterogeneous catalyst.

A major challenge in the development of robust heterogeneous catalysts is to design a catalyst with high catalytic activity and selectivity while ensuring sufficient catalyst stability. As the reaction conditions in industrial processes are often detrimental to the catalyst, the catalyst has to be highly thermally stable without leaching of the active metals.

Today it is possible to prepare advanced nanostructured materials with pore systems designed with high precision. Encapsulation of catalytically active metals in nanostructured porous materials has proven to hold great potential for the development of size-selective and stable metal catalysts. The highly controlled encapsulation of catalytically active metals opens up the possibility to design robust heterogeneous catalysts with improved thermal stabilities and no leaching while maintaining the catalytic activities.

In this dissertation, the aim was to prepare novel heterogeneous catalysts with low metal sintering and leaching. The prepared catalysts were tested in different existing catalytic processes. The catalysts were prepared by encapsulation of the active metals in nano-engineered porous materials by the use of sacrificial metal precursors or by metal impregnation.

Chapter 3 describes how the metal-organic frameworks, Co-based ZIF-67 and Zn-based ZIF-8, were used as sacrificial metal precursors to create Co nanoparticles encapsulated in porous nitrogen-doped carbon matrices. Different thermally stable catalysts were prepared by varying the synthetic parameters and the metal ion composition (ZIF-67/ZIF-8).

The catalyst prepared from ZIF-67/8 crystal precursors at a low temperature was found to have higher stability and activity in the hydrolytic dehydrogenation of ammonia borane. In Chapter 4, the concept of using metal-organic frameworks as sacrificial metal precursors was further investigated to design nanorattle catalysts consisting of Co3O4 nanoparticles in mesoporous SiO2 shells.

The thermal treatments (carbonization and/or calcination) and the Co/Zn ratios of the ZIF-67/8 precursors were optimized to give low-sintering catalytically active catalysts. These nanorattle catalysts were thermally stable and active when tested in a CO oxidation reaction. Chapter 5 describes how the concept of encapsulation was further extended to encapsulate iron molybdate catalysts in modified porous SiO2 shells and zeolites.

The encapsulated iron molybdate catalysts were prepared by vacuum impregnation of the modified porous supports. Here, the pore size of the supports was varied in order to ensure complete confinement of the active metals. The iron molybdate catalyst encapsulated in desilicated silicalite-1 zeolites was found to maintain a high catalytic activity with minimized metal leaching when tested for the selective oxidation of methanol to formaldehyde.

However, the metal impregnation procedure was challenging to reproduce. In Chapter 6, the encapsulation of heteropolyoxometalates in modified zeolites is described. The encapsulation was performed by vacuum impregnation of the zeolite support. Here, the pore size of the zeolite support was designed to limit the leaching of the active metals in polar media.

The encapsulation of W-based heteropolyoxometalates in recrystallized silicalite-1 zeolites was found to prevent metal leaching in the tested phenyl acetate Fries rearrangement reaction. However, it was found that encapsulation of the more unstable V substituted Mo-based heteropolyoxometalates was more challenging and the catalyst suffered from metal leaching when tested for the selective oxidation of biomass to formic acid process in water.

All prepared catalysts were characterized by several analytical techniques, which are described in Chapter 2. In this dissertation, ZIF-67/8 crystals were successfully used as sacrificial metal precursors in the design of thermally stable heterogeneous catalysts with maintained high catalytic activities when tested in catalytic processes.

In addition, it was found, that the encapsulation of iron molybdate catalysts and heteropolyoxometalates, by metal impregnation of nano engineered porous materials, could be used to design nonleaching heterogeneous catalysts with maintained catalytic activities. However, especially when encapsulating heteropolyoxometalates, the catalytic activities were decreased due to a limited mass diffusion.

The introduction of active metals into porous supports by metal impregnation was often found to be challenging in terms of achieving high metal loadings or reproducible metal loadings. It was found, that these challenges could be overcome, by using a sacrificial metal precursor to introduce the active metals.