Catalyst skeletal

Skeletal catalysts were first discovered in the 1920s by Murray Raney [1,2], In recognition of their inventor, the catalysts are often referred to as Raney catalysts, although this trademark is now owned by the Davison division of W.R. Grace Co., who supply a range of catalysts for industrial use. Another common name is metal sponge, which refers to the porous structure of the catalysts. 

Raney predicted that many other metal catalysts could be prepared with this technique, but he did not investigate them [8], Copper and cobalt catalysts were soon reported by others [4,5], These catalysts were not nearly as active as Raney s nickel catalyst and therefore have not been as popular industrially however they offer some advantages such as improved selectivity for some reactions. Skeletal iron, ruthenium and others have also been prepared [9-13], Wainwright [14,15] provides two brief overviews of skeletal catalysts, in particular skeletal copper, for heterogeneous reactions. Table 5.1 presents a list of different skeletal metal catalysts and some of the reactions that are catalyzed by them. 

The activity and stability of skeletal catalysts can be improved with the use of additives, often referred to as promoters. These can be added to the alloy before leaching, or alternatively can be added to the leaching solution [16-19], An example is the use of zinc to promote skeletal copper for the catalytic synthesis of methanol from synthesis gas [20-22], Mary other promoters have been considered, both inorganic and organic in nature. 

Table 5.1 Summary of Skeletal Catalysts and the Types of Reactions Catalyzed by Them (not exhaustive)...

As an alternative to crashing an alloy into small particles, Ostgard et al. [43] first proposed the manufacture of hollow skeletal catalyst spheres. Precursor alloy is deposited on an organic polymer sphere that is later oxidized completely by heating in air. The hollow alloy spheres that remain are then leached as usual to give the catalyst. 

Instead of using high-temperature melting to make the precursor alloys, an alternative wet chemistry technique has been proposed where nickel(O) and aluminum coordination compounds are blended together and treated to give nanocrystalline NiAlx alloys with 1 < x < 3 [48], The alloys are leached in the same way as standard skeletal catalysts. Catalysts with higher activity than commercially available Raney nickel have been prepared by this technique, with the activity attributed to the finer structure and homogeneity of the alloys [48,49],... 

Promoter species have been mentioned previously. These are additional metals or organic compounds present in either the original alloy or in the lixiviant. They are more than just a second catalytic metal, although bi-metal skeletal catalysts are possible. Promoter species increase the activity of the... 

Surface modification of skeletal nickel with tartaric acid produced catalysts capable of enantiose-lective hydrogenation [85-89], The modification was carried out after the formation of the skeletal nickel catalyst and involved adsorption of tartaric acid on the surface of the nickel. Reaction conditions strongly influenced the enantioselectivity of the catalyst. Both Ni° and Ni2+ have been detected on the modified surface [89]. This technique has already been expanded to other modified skeletal catalysts for example, modification with oxazaborolidine compounds for reduction of ketones to chiral alcohols [90],... 

Most research on the structure of skeletal catalysts has focused on nickel and involved methods such as x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), electron diffraction, Auger spectroscopy, and x-ray photoelectron spectroscopy (XPS), in addition to pore size and surface area measurements. Direct imaging of skeletal catalyst structures was not possible for a long while, and so was inferred from indirect methods such as carbon replicas of surfaces [54], The problem is that the materials are often pyrophoric and require storage under water. On drying, they oxidize rapidly and can generate sufficient heat to cause ignition. 

The structure of skeletal catalysts is so fine that electron microscopes are required for sufficient resolution. The use of a focussed ion beam (FIB) miller has enabled a skeletal copper catalyst to be sliced open under vacuum and the internal structure to be imaged directly [61], Slicing the catalyst enabled viewing beyond the obscuring oxide layer on the surface. A uniform, three-dimensional structure of fine copper ligaments was observed [61], which differed from the leading inferred structure at the time of parallel curved rods [54],... 

Skeletal catalysts can lose activity over time. This phenomena has been attributed to several causes depending on the application and that include surface fouling with by-products, surface oxidation, and structure rearrangement. 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

Skeletal catalysts are primarily used for hydrogenation and dehydrogenation reactions. The first application of skeletal nickel was hydrogenation of cottonseed oil [1], Skeletal catalysts have since... 

Enantioselective organic synthesis using modified skeletal catalysts has wide application in areas such as pharmaceutical production for example, synthesis of chiral alcohols from ketones [90], which is described in detail elsewhere in this book. 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

What considerations are required when deciding which composition to make a precursor alloy for a skeletal catalyst If the alloy is to contain more than one catalytically active metal, does the percentage of aluminum change significantly in proportion ... 

Can skeletal catalysts be prepared by acid leaching instead of alkali leaching Why is alkali leaching preferred ... 

What are the three mechanisms leading to deactivation of skeletal catalysts during use Which of these can cause deactivation while the catalyst is still leaching ... 

What are the common types of reactions where skeletal catalysts are employed ... 

What are the main advantages and disadvantages of skeletal catalysts compared to other types of catalysts ... 

The shrinking core models described by Levenspiel cater for both reaction- and diffusion-controlled systems. Referring to the literature, how do these systems differ and which of these models do skeletal catalysts fit during their preparation by leaching ... 

A thin layer of skeletal catalyst precursor alloy can be coated on a supporting material prior to leaching. Referring to the literature, discuss (1) why is a coating used, and (2) what processes are used to achieve this coating, both for a flat surface and for a highly porous support ... 

Impregnation with the solution of the compound of a poisoning element [Sn, Pb, Bi, P etc.) is considered as the most general way to poison nickel catalysts. However, upon using conventional impregnation techniques the optimal selective poisoning of the supported or skeletal catalysts cannot be guarantecu[4]. 

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