Paul et al. prepared an active nickel catalyst by reducing nickel salts such as nickel chloride or nickel acetate with sodium or potassium borohydride.17 The products thus obtained are neither magnetic nor pyrophoric and do not dissolve as quickly as Ra-ney Ni in hydrochloric acid or potassium triiodide, and showed an activity com-parable to or slightly inferior to Raney Ni, as examined in the hydrogenation of safrole, furfural, and benzonitrile at room temperature and atmospheric pressure.
Usually, the catalyst from nickel acetate was slightly more active than that from nickel chloride. In the hydrogenation of safrole, the catalysts exhibited greater re-sistance to fatigue than Raney Ni in a series of 29 hydrogenations. The average composition of the catalysts deviated very little from a content of 7–8% boron and 84–85% nickel, which corresponded to the formula of Ni2B. Hence, the catalysts have been denoted nickel borides. A more active catalyst was obtained by introduction of an alkali borohydride into the solution of the nickel salt, since the formation of nickel boride was always accompanied by decomposition of the alkali borohydride ac-cording to eq. 1.4. The overall reaction is formulated as in eq. 1.5, although the boron content of the products has been reported to vary with the ratio of reactants used in preparation.76,77
NaBH4 + 2H2O → NaBO2 + 4H2 (1.4)
2Ni(OAc)2 + 4NaBH4 + 9H2O → Ni2B + 4NaOAc + 3B(OH)3 + 12.5H2 (1.5) Later, Brown and Brown found that the nickel boride prepared by reaction of nickel acetate with sodium borohydride in an aqueous medium is a granular black material and differs in activity and selectivity from a nearly colloidal catalyst prepared in etha-nol.18,19 The boride catalyst prepared in aqueous medium, designated P-1 Ni, was more active than commercial Raney Ni toward less reactive olefins, and exhibited a markedly lower tendency to isomerize olefins in the course of the hydrogenation.
The boride catalyst prepared in ethanol, designated P-2 Ni, was highly sensitive to the structure of olefins, more selective for the hydrogenation of a diene or acety-lene, and for the selective hydrogenation of an internal acetylene to the cis olefin (see eq. 3.13; also eqs. 4.24 and 4.25).78,79 The high selectivity of the P-2 catalyst over the P-1 catalyst has been related to the surface layer of oxidized boron spe-cies, which is produced much more dominantly during the catalyst preparation in ethanol than in water.80 The reaction of sodium borohydride with nickel salts con-taining small quantities of other metal salts provides a simple technique for the preparation of promoted boride catalysts. The Ni–Mo, Ni–Cr, Ni–W, and Ni–V catalysts thus prepared were distinctly more active than the catalyst without a pro-moter in the hydrogenation of safrole. The Ni–Cr catalyst was almost twice as ac-tive as Raney Ni in the hydrogenation of furfural.17 The preparation of Ni boride catalyst in the presence of silica provides a supported boride catalyst with a highly active and stable activity.81
There appear to be known only few examples where Ni boride catalysts have been applied to the hydrogenation of the aromatic nucleus. Brown found no evidence for reduction of the aromatic ring. Benzene failed to reduce at all in 2 h at 25°C and at-mospheric pressure, although pyrocatechol was readily reduced to cyclohexanediol over P-1 Ni in an autoclave.77 Nishimura et al. studied the rates of hydrogenation of benzene, toluene, and o-xylene over Raney Ni and P-1 Ni as catalysts in methyl-cyclohexane (methyl-cyclohexane in the case of toluene) at 80°C (100°C for o-xylene) and the initial hydrogen pressure of 7.8 MPa.82 It is seen from the results in Table 1.8 that P-1 Ni is as active as or only slightly inferior to Raney Ni in the activity on the basis of unit weight of metal, but it is far more active than Raney Ni when the rates are compared on the basis of unit surface area. It is noted that the order in hy-drogen pressure for the rate of hyhy-drogenation of benzene is greater for P-1 Ni (1.04) than for Raney Ni (0.58). These results may be related to the fact that the Raney Ni retains a large amount of adsorbed hydrogen while the P-1 Ni practically no hydrogen.
Nakano and Fujishige prepared a colloidal nickel boride catalyst by reducing nickel chloride with sodium borohydride in ethanol in the presence of poly(vinylpyrroli-done) as a protective colloid.83 Catalytic activity of the colloidal catalyst was higher than P-2 Ni boride for the hydrogenation of acrylamide and markedly enhanced by the addition of sodium hydroxide in the hydrogenation of acetone.84
Ni Boride (by Paul et al.).17 In this procedure, 27 ml of a 10% aqueous solution of sodium borohydride is added with stirring, for about 20 min, to 121 ml of a 5%
aqueous solution of nickel chloride hexahydrate (equivalent to 1.5 g Ni). Hydrogen is liberated, while voluminous black precipitates appear; the temperature may rise to 40°C. When all the nickel has been precipitated, the supernatant liquid is colorless
TABLE 1.8 Rates of Hydrogenation of Benzene, Toluene, and o-Xylene over Raney Ni and P-1 Ni Catalystsa,b
Rate of Hydrogenation × 103 (mol ⋅ min–1⋅ g metal–1)
Rate of Hydrogenation × 105 [mol ⋅ min–1⋅ (m2)–1]c
Compound Raney Nid P-1 Nie Raney Nid P-1 Nie
Benzene 8.3 6.3 8.1 30.0
Toluene 3.3 2.7 3.2 12.9
o-Xylene 2.2 2.2 2.2 10.5
aNishimura, S.; Kawashima, M.; Onuki, A. Unpublished results; Onuki, A. Master’s thesis, Tokyo Univ.
Agric. Technol. (1992).
bThe compound (10 ml) was hydrogenated in 10 ml methylcyclohexane (cyclohexane for toluene) at 80°C (100°C for o-xylene) and the initial hydrogen pressure of 7.8 MPa over the catalyst containing 0.08 g of catalytic metal and prepared before use. The rates (at the initial stage) were obtained by an extrapolation method to get rid of an unstable hydrogen uptake at the initiation.
cThe surface areas were measured by means of Shimazu Flow Sorb II.
dA NiAl3 alloy was leached by the procedure for the N-4 catalyst to an 88% degree of development.
eThe catalyst was prepared by reduction of nickel acetate with NaBH4 in water according to the procedure of Brown, C. A. J. Org. Chem. 1970, 35, 1903.
1.1 NICKEL CATALYSTS 21
and has a pH approaching 10. The black precipitates are filtered and washed thoroughly, without exposure of the product to air. The catalyst can be kept in stock in absolute ethanol.
P-1 Ni Boride.18,77 Nickel acetate tetrahydrate (1.24 g, 5.0 mmol) in 50 ml distilled water is placed in a 125-ml Erlenmeyer flask connected to a mercury bubbler and flushed with nitrogen. To the magnetically stirred solution, 10 ml of a 1.0M solution of sodium borohydride in water is added over 30 s with a syringe. When gas evolution has ceased, a second portion of 5.0 ml of the borohydride solution is added. The aqueous phase is decanted from the granular black solid and the latter washed twice with 50 ml of ethanol, decanting the wash liquid each time.
P-2 Ni Boride.19,78 Nickel acetate tetrahydrate (1.24 g, 5.0 mmol) is dissolved in approximately 40 ml of 95% ethanol in a 125-ml Erlenmeyer flask. This flask is attached to a hydrogenator, which is then flashed with nitrogen. With vigorous stirring, 5.0 ml of 1M sodium borohydride solution in ethanol is injected. When gas evolution from the mixture has ceased, the catalyst is ready for use.
P-2 Ni Boride on SiO2.81 Finely powdered nickel acetate tetrahydrate (186.6 mg, 0.75 mmol) is placed in a flask, flushed with nitrogen, and to this 9 ml of degassed ethanol is added to dissolve the nickel salt by shaking under nitrogen (solution I). To 500 mg of finely powdered sodium borohydride is added 12.5 ml of ethanol and 0.5 ml of 2M aqueous sodium hydroxide, the mixture shaken for 1 min, the solution filtered, and the clear filtrate is immediately degassed and stored under nitrogen (solution II). In a flask is placed 500 mg silica gel [Merck, Artide 7729; φ ~0.08 (phase) mm], degassed for 15 min in vacuo, and flushed with nitrogen. To this 6 ml of solution I is added under a stream of nitrogen, evacuated, and flushed with nitrogen, and then 1 ml of solution II is added and shaken for 90 min under nitrogen. The P-2 Ni on SiO2 thus prepared contains 0.5 mmol of Ni (~5.5 wt% Ni). Unsaturated compounds are very rapidly hydrogenated with the P-2/SiO2 catalyst without solvent at 70–85°C and 10 MPa H2. A turnover number of 89,300 [mmol product ⋅ (mmol catalyst)–1] with an average catalyst activity of 124 [mmol product ⋅ (mmol catalyst)–1
⋅ min–1] was obtained in the hydrogenation of allyl alcohol (1025 mmol) over 0.01 mmol catalyst at 95°C and 1 MPa H2.
Colloidal Ni Boride.83 Nickel(II) chloride (NiCl2⋅6H2O, 0.020 mmol) and poly(vinylpyrrolidone) (2.0 mg) is dissolved in ethanol (18 ml) under hydrogen. To the solution, a solution of NaBH4 (0.040 mmol) in ethanol (1 ml) is added drop by drop with stirring. A clear dark brown solution containing colloidal particles of nickel boride results. Stirring is continued further for 15 min to complete the hydrolysis of NaBH4, which is accompanied by evolution of hydrogen. The colloidal nickel boride thus prepared is stable under hydrogen for more than several months, but decomposed immediately on exposure to air.
Besides Urushibara Ni and Ni boride catalysts, various finely divided nickel parti-cles have been prepared by reaction of nickel salts with other reducing agents, such as sodium phosphinate;20,85 alkali metal/liquid NH3;21 NaH-t-AmOH (designated Nic);22,86Na, Mg, and Zn in THF or Mg in EtOH;24 or C8K(potassium graphite)/THF–
HMPTA (designated Ni–Gr1).23,87 Some of these have been reported to compare with Raney Ni or Ni borides in their activity and/or selectivity.