Skeletal nickel

In comparison to skeletal nickel, skeletal copper has a significantly larger crystallite size of about 10-100 nm [32,46,92,96,100,101], Fasman and coworkers [46,100,101] examined the crystal structure more closely and found that it consisted of copper crystals that had agglomerated into granules or precipitated onto oxides. The copper crystal grains and subgrains were of about 10-13 nm in size, while the copper agglomerates were 50-80 nm. 

The catalytic hydrogenation of ethylene occurs on various metal catalysts, such as nickel, including active or skeletal forms produced by dissolving out... 

Tetrahydrofurfuryl alcohol reacts with ammonia to give a variety of nitrogen containing compounds depending on the conditions employed. Over a barium hydroxide-promoted skeletal nickel—aluminum catalyst, 2-tetrahydrofurfur5iarnine [4795-29-3] is produced (113—115). With paHadium on alumina catalyst in the vapor phase (250—300°C), pyridine [110-86-1] is the principal product (116—117) pyridine also is formed using Zn and Cr based catalysts (118,119). At low pressure and 200°C over a reduced nickel catalyst, piperidine is obtained in good yield (120,121). 

Sandwich complexes nickel. 5, 35 Sapphyrins, 2, 888 demetallation, 2, 891 metallation, 2, 891 reactions, 2, 891 synthesis, 2, 889 Sarcoplasmic reticulum calcium/magnesium ATPase, 6, 566 skeletal muscle... 

Skeletal metals are formed by leaching away one metal from an intimate alloy of two or more metals. The best example of this is Raney nickel. 

The most catalytically active metals are Ni, Pd, Pt, and Rh. Nickel is used extensively in hydrogenation. It is frequently used in skeletal form as Raney nickel (Ra-Ni or RNi). The hydrogenation of almost all hydrogenatable functional groups can be accomplished over some form of Ra-Ni. Ra-Ni is also useful for desulfurization of organic compounds, but this is a stoichiometric reaction, not a catalytic reaction. 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

In doses of 1.2 mg Ni/kg and up to 20 mg Ni/kg, nickel chloride caused increased resorption rates and a number of malformations in murine foetuses, specific to the foetal skeletal system, as shown by atomic absorption [425]. It was believed that nickel chloride might influence embryos during the passage through the oviduct, with subsequent effect on the development after implantation [426]. Preimplantation mouse embryos have also been used to investigate toxic effects of nickel chloride on early embryo development in vitro, and a dose-dependent effect has been found [427]. 

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. 

Skeletal copper is best made from the CuA12 intermetallic compound which has very close to 50 wt% aluminum in the alloy and gives an active and selective catalyst [27-29], Skeletal nickel is also best made from an alloy of about 50 wt% aluminum [25] however, in this case, the alloy consists of more than one intermetallic phase, the combination of which provides the best activity while maintaining adequate strength in the catalytic residue. The most active skeletal cobalt catalysts are made from an alloy of about 60-65 wt% aluminum, which consists of two intermetallic phases, Co2A19 + Co4A113 [30],... 

Ultrasonic agitation during leaching has recently been reported to increase the catalytic activity of skeletal nickel for the hydrogenation of benzene to cyclohexane, with the enhanced activity related to changes in the catalyst structure and surface species [47],... 

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],... 

Choudhary et al. [58] found reaction controlled kinetics with an activation energy of 56.6 kJ/mol for the leaching of skeletal nickel, similar to the leaching of skeletal copper. The kinetics did not fit Levenspiel s shrinking core model [57] but it should be noted that the leaching solution was agitated with a flat stirrer at 1500 rpm. 

Table 5.2 Effect of Promoters on the Activity of Skeletal Nickel Catalysts for Hydrogenation of Different Functional Groups...

Note Activity data reported as reaction rate relative to unpromoted skeletal nickel. aCo promoter = 2.5%. bMo promoter = 1.5%. 

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. 

Skeletal nickel consists of highly-dispersed nickel with a large surface area [68, 91-96], the structure often being likened to a sponge [51,74], The activity of the catalyst is proportional to the surface area and hence the degree of nickel crystallite dispersion [26,76,91], The nickel crystallites are about 1-20 nm in size [24,92,94-96], and decrease in size with decreasing temperature... 

Table 5.3 Summary of Crystallite Size, Surface Area, and Pore Volume of Skeletal Nickel Leached under Different Conditions...

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... 

Skeletal copper has lower activity toward hydrogenation compared with skeletal nickel, but it offers superior selectivity for certain reactions. Hydrolysis of acrylontrile over skeletal copper yields acrylamide, retaining the unsaturated bond [114,135] ... 

Raney-nickel was found to be selective in the hydrogenation of cyclopentadiene and cyclohexadiene and of their methyl and ethyl derivatives at 0-40 °C and 2-5 bar pressure137,138. The skeletal nickel proved to be selective in the semihydrogenation of conjugated polyenic compounds (equation 52)139. 

The spectra presented in Figures 1-3 demonstrate that high quality, transient resonance Raman spectra can be obtained for Ni(OEP) and Ni(PP) solutions using Soret excitation. These spectra can be interpreted on a molecular level by comparison with the extensive theoretical and experimental data base that exists for ground state nickel porphyrin species (8-16 and refs, therein). The coordination state of nickel porphyrins can easily be detetmined from the resonance Raman spectrum of the sample (10.12). Several modes in the Raman spectrum of porphyrins are quite sensitive to the state of axial ligation (10.12). In particular, the marker lines V4, 11 2> 3 10 (porphyrin skeletal mode designations follow those of Abe et al., (I2a). The designation... 

When the hydrochloride salt of 2,3,4,4a,5,6-hexahydro-l//-pyridazino [1,6-a]quinoline was subjected to catalytic hydrogenation in ethanol over Pt02, 3-[2-(l,2,3,4-tetrahydroquinolyl)]propylamine was obtained (66YZ608). Catalytic reduction of perhydropyrido[l,2-ft]pyridazine over a skeletal nickel catalyst in ethanol at 30 atm gave ring-opened 2-(3-aminopropyl)piperidine (66KGS91). 

Microscopic changes in skeletal muscle were not observed in rats or dogs fed nickel sulfate in the diet at doses up to 187.5 mg nickel/kg/day for rats and 62.5 mg nickel/kg/day for dogs (Ambrose et al. 1976). 

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