Heterogeneous+Catalysis+

Heterogeneous catalysis has the catalyst in a different phase to the reactants, usually a solid catalyst with either liquid or gaseous reactants; this makes it easier to recover and regenerate the catalyst at the end of the reaction. There are other benefits to using heterogeneous catalysts, a key one being the maximisation of surface area. The surface sites of a material, eg ZnO or NiO are the catalytically active sites.¹⁰

As the materials studied in the thesis work are all heterogeneous catalysts, they will be discussed in more detail. Some of the most important groups of heterogeneous catalysts will be discussed, showing the uses of these materials and their development in the literature.

1.5.1 Zeolite+Based+Catalysts++

Zeolites are crystalline materials formed from corner-linked SiO4 and AlO4 tetrahedra, with the separation between Al and Si is governed by Löwenstein’s rule; essentially this means that Al-O-Al linkages are forbidden.¹³ Ground-breaking work by Barrer and Milton in 1962 led to the creation and use of synthetic Faujasites (zeolites X and Y) for the cracking of petrochemicals.¹⁴ One of the unique features of zeolites is their uniform pore diameters and molecular dimensions, these contribute to their suitability for catalytic applications.¹⁴ The International Zeolite Association (IZA), composed of sixteen experienced crystallographers is responsible for approving new zeolite structures and assigning three letter codes, for example FAU for Faujasite.¹⁴

The main use for zeolite catalysts has been in the petrochemical industry, though zeolites have also been used as catalysts for organic reactions since the 1960’s. ^14,15 There have also been increasing investigations in using zeolite catalysts for the production of fine chemicals.¹⁵ In the late 1990’s there was a shift in trend for zeolite catalysis with the main emphasis being

on, low temperature, liquid-phase reactions, utilising smaller scale processes for reactants and products containing heteroatoms with a view to increase in the variety of chemical reactions that could be catalysed.¹⁵ A number of chemical species have been successfully encapsulated in zeolite structures, such as metal particles,^16,17 metal oxide clusters (for base and oxidation catalysis)^18,19 and organometallic complexes (for asymmetric catalysis).²⁰

Catalysis can proceed from the acid sites, which can be modified by substituting metal ions into framework sites.^15,17,21 This requires the replacement of any tetravalent framework Si⁴⁺

site with a non-tetravalent ion, e.g. Ga³⁺, Fe³⁺, CO²⁺, Zn²⁺ thus creating lattice charge.¹⁴ The change in charge balance results in a change in strength of the acid site, with the strength of the acid being related to the nature of the substituting element.¹⁵

One substituting metal of note is titanium, which replaces Si⁴⁺ with Ti⁴⁺ ions. Whilst there is no change in charge caused by the substitution the Ti⁴⁺ centre can be utilised as an active site for oxidation catalysis.^15,16 The key material in this area is TS-1, with an example of it’s use as an oxidation catalyst being the ammoxdation of 4-hydroxyacetophenone to the oxime which has a yield of near 100%.¹⁵ The oxime formed is an important component of Tylenol and paracetamol, which can be formed through a Beckman rearrangement. Other substituting elements such as Sn⁴⁺ and Zn⁴⁺ have been reported, providing zeolite based catalysts for oxidation and Lewis acid-mediated catalysis reactions.¹⁵ For substituting elements other that Ti⁴⁺, there is evidence of leaching suggesting that the ions may not in fact be incorporated into the framework, but residing in extra framework locations.¹⁵

Another important feature of zeolite catalysis is their ability to control access for solvents and reactants to the active sites through steric hindrance caused by the size of the zeolite pores.¹⁵ Larger molecules and branched systems can be excluded from entering the structure, improving the selectivity for the final product. The main limitation to the use of zeolites is the size of the pores, many fine chemicals and intermediates are too large to enter the structure.¹⁵ By the mid 1980’s there were a number of solutions for large pore systems such as phosphate-based molecular sieves, e.g. cacoxenite with a pore diameter of 15 Å. The first extra-large pore material VPI-5 was synthesized and it is aluminophosphate based. Following on from this discovery there were a number of other zeo-type structures reported, such as GaPO₄, FePO4 and AlPO4.^15,22 There are also mesoporous materials such as MCM-41, which is an aluminophosphate with a pore size that can vary from 15-100 Å.²³

There are three main categories of shape selectivity that can be applied to zeolite cataysts;¹⁴

I. Reactant shape selectivity – reactants with different molecular dimensions, for example branched hydrocarbons or straight chain hydrocarbons, the bulkier molecules are hindered and the smaller molecules pass through and react preferentially.²⁴

II. Product shape selectivity – for at least two products with different molecular dimensions, if the diffusion of the larger product is hindered by the pores then the less bulky molecule will be formed preferentially.²⁵

III. Restricted transition state shape selectivity – in this instance it is the intermediate transition state that is hindered by the pores; with the reactant and product molecules free to move through the structure. Out of two or more possible reaction path ways, the one with the less bulky and hindered transition state will be favoured. This can result in completely suppressing the other reaction pathway.²⁶

These are some of the main features for zeolite-based catalysts. They can be used for a variety of different of applications thanks to their structures, solid-acid catalyst nature and their ability to contain other species, such as metal particles, for catalysis.

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1.5.2 MetalHOxide+Based+Catalysts++

One of the most commonly used metal oxide based catalysts is titanium dioxide, TiO2. Titanium dioxide has found increasing use in environmental photocatalysis, including areas such as self-cleaning surfaces, and photo-induced hydrophilicity which includes self-cleaning and anti-fogging applications.²⁷ TiO2 has been widely studied for it’s potential as a photocatalyst for the water splitting reaction.²⁸ In the original investigation, the authors (Akira Fujishima), showed that water could be decomposed using UV-light and platinum and TiO2

electrodes.²⁹ the properties of titanium dioxide, such as high chemical stability, low cost and highly oxidising photogenerated holes make it an ideal photocatalyst.²⁸ One of the main limitations of the materials however is the dependency on UV light. There have been several attempts to achieve photocatalysis with visible light through doping with other metal ions, such as Cr³⁺ and Fe³⁺,^30,31 or rare earth metals,³² in order to shift the band gap of the material so that absorption bands are accessible in the visible spectral range.

TiO2 has also shown potential for antibacterial applications under weak UV illumination. . TiO2 based antibacterial products have been commercialized in Japan, with products such as tiles, fibres and sprays and have been used in hospital operating rooms to maintain sterile

conditions.²⁸ Commercialisation of titanium dioxide photocatalytic products commenced in the 1990s in Japan, and has grown rapidly.²⁸

ZnO has a variety of different applications, including catalysis, this will be discussed in more detail in chapter six.³³ Nickel oxides are well known catalysts for industrial processes such as the reforming of methane,³⁴ and as hydrogenation catalysts.¹²

Another important metal oxide is vanadium oxide, it is used as an additive in steel and for forming alloys for aerospace applications; here we are interested in it’s use as a catalyst.³⁵ The use of vanadium oxides in catalysis is linked to its chemical properties such as variable oxidation states and geometries.³⁵ There are also important variations between supported vanadium oxides and unsupported species, as supported species can end up in a variety of forms such as isolated clusters or long chains.³⁵

1.5.3 Supported+Catalysts++

As mentioned previously loading a catalyst on a support can lead to changes in its structure, which can be of major benefit.³⁵ By utilising a support material there is less expensive catalytic material required, and the number of catalytic active sites can be maximised.

Support materials for catalysts are usually chosen for their large surface areas (pore volumes and uniform pore-size distributions), chemical stability and in some cases the interaction between the support and the catalyst.^36–39 This interaction, known as the metal support interaction (MSI), can have some benefits for the material and is widely studied in the literature.^39,40 There are two main distinctions that can be applied to support materials, inert or reactive.

1.5.3.1 Inert+Supports+

Inert supports are materials that are chemically stable and undergo very few changes over the course of the catalysis. A good example of an inert support material is silica SiO2, which can be used to support a host of catalytically active species such as metal nanoparticles⁴¹; although Pt/SiO2 materials have also been shown to exhibit metal support interactions indicating that the SiO2 was reduced in the reaction.⁴² Another material that can be considered as an unreactive or inert support is alumina, which is also used to support a variety of metal nanoparticles.^38,43 It was found that for supported palladium nanoparticles the sintering mechanisms were dependant as much on the location of the Pd on the surface as the Pd particle size.³⁸

1.5.3.2 Reactive+Supports+

The second class of support materials are reactive supports, also known as reducible supports.^42,44 These are support materials which whilst chemically stable can take an active part in the reaction as well, through having the surface partially reduced.²¹ A good example of a group of reactive supports is zeolites and other related porous frameworks. As mentioned previously zeolites can be used to contain metal nanoparticles for catalysis inside the pores, maximising the surface area of the metallic component and taking advantage of the zeolite structure.^21,45

Transition-metal oxides also make very good catalyst supports, Cu/ZnO2 catalysts are widely known for there use in methanol synthesis, methanol oxidation, and methanol steam reforming.⁴⁶ Titania has also been widely reported for its use a catalyst support, and was one of the first materials for which a metal support interaction (MSI) was reported.⁴⁷ The term strong metal-support interactions (SMSI) was coined for group VIII noble metals supported on titania and the vast change in chemisorption properties observed.⁴⁷ Titania itself can also be supported for photochemical applications.⁴⁸

Another important reactive support is CeO2, which is most widely known for its use as a support for three-way catalysts (TWCs).^40,49 Ceria plays an important role in TWCs thanks to it’s oxygen storage capacity (OSC), which means it can supply oxygen under fuel rich conditions to convert CO and store it under fuel lean conditions.⁴⁹

The use of a support for catalytically active materials has greatly increased both the potential and complexity of catalyst design. The type of support used and its impact on the catalysis should always be taken into account, in addition to any potential benefits and synergistic effects that can arise from material combinations.