L.H. Staal

Effects of zinc addition to silica supported copper catalysts for the hydrogenolysis of esters

Abstract When using silica supported copper-zinc oxide catalysts in the hydrogenolysis of methyl acetate, a profound effect of zinc on the activity and selectivity of the catalysts has been observed. The activity and selectivity improve with increasing zinc content. The increase in activity is likely to be an effect that originates either from finely dispersed copper particles in close contact with the zinc oxide phase or from the promotion of copper metal crystallites by zinc oxide particles. A decrease in ethane formation with higher zinc content is attributed to the elimination of dehydrating surface sites. This lowering of ethane selectivity goes hand in hand with a higher production of diethyl ether, which is explained in terms of a less severe dehydration on acid and basic surface sites. Catalysts supported on aerosil are more selective than catalysts supported on sol-gel silica, probably because of differences in the contents of impurities and surface density of hydroxyl groups. In addition to this, a hysteresis has been observed in the conversion as a function of the reaction temperature. The effects have been studied in relation to the role of adsorbed hydrogen and the nature of the adsorbed phase. The increase in the hysteresis with higher zinc content provides indirect evidence for the participation of the zinc oxide component in the activation of the reactants.

Binuclear metal carbonyl DAB complexes. II. The syntheses and coordination properties of Mn(CO)5M'(CO)3DAB (M' = Mn, Re; DAB = 1,4-diazabutadiene)

Abstract Mn(CO) 5 M′(CO) 3 DAB complexes (M′ = Mn, Re; DAB = R 1 N=C(R 2 )-C(R′ 2 )=NR 1 ) can be easily obtained from the reaction between Mn(CO) 5 − and M′(CO) 3 X(DAB) (M′ = Mn, Re; X = Cl, Br, I). The complexes are formed by a nucleophilic mechanism, while a redistribution is responsible for the formation of a small amount of Mn 2 (CO) 10 . A diastereotopic effect can be observed in the 1 H and 13 C NMR spectra of complexes having isopropyl groups attached to the DAB ligand skeleton. A comparison is made with mononuclear complexes of the same symmetry, and the chemical shift differences for the methyl groups strongly depend on the substituent on the central metal responsible for the asymmetry. The low temperature enhancement of the σ → σ ★ transition localised on the metal—metal bond, which is normally observed for this type of compounds, was not observed for the Mn(CO) 5 M′(CO) 3 (DAB) complexes. The metal—metal bond can be activated by irradiating at the wave lengths associated with the CT transitions between the metal and the DAB ligand. Metal—metal bond cleavage occurs and Mn 2 (CO) 10 is formed.

New aspects of the reaction between tBuNCHCHNtBu and Fe2(CO)9, solving the controversy about the coordination mode of the DAB ligand in Fe(CO)3 (tBuNCHCHNtBu) (DAB = 1,4-diazabutadiene)

Fe2(CO)9 and tBuNCHCHNtBu react to form Fe(CO)3(tBuNCHCHNtBu), tBuNCHCHN(tBu)CO and Fe(CO)5 in equimolar amounts. The Fe(CO)3(tBuCHCHNtBu) complex contains the DAB (1,4-diazabutadiene) ligand in the σ,σ-coordination mode, which is in contradiction with the earlier assumption that the DAB ligand is σ, π-coordinated. tBuNCHCHN(tBu)CO can be prepared by reacting DAB with CO in a pressure vessel, using Fe2(CO)9 as a catalyst. Only three catalytic cycles could be obtained using CO pressures ranging from 2.5 to 60 atm. A possible reaction mechanism is discussed.

Field desorption mass spectrometry of a series of various metal carbonyl 1,4-diazabutadiene compounds

It is shown that field desorption mass spectrometry is a useful method of characterizing the thermally labile and involatile organometallic complexes mentioned in the title. In most cases only molecular ions are generated, but in some cases also (M@?CO)}+{}.{ fragment ions are observed.

A study of the electronic properties of M(CO)4DAB (M = Cr, Mo, W; DAB = diazabutadiene). II. Conformational change below 200 K as evidenced from absorption and resonance raman spectra

Abstract The resonance Raman and electronic absorption spectra of M(CO) 4 DAB (M = Cr, Mo, W; DAB = 1,4-Diazabutadiene) in alkane solutions or glasses at temperatures below 200 K indicate the existence of a second isomer in which the M(CO) 4 moiety has a pseudo C 4ν symmetry. In polar solvents a continuous shift of the CT band is found when the temperature is lowered. This thermochromism is caused by an increase of the solvatochromism at low temperature.

Structure-activity relation and ethane formation in the hydrogenolysis of methyl acetate on silica supported copper catalysts.

Abstract The activity and selectivity of silica-supported copper catalysts in the hydrogenolysis of methyl acetate were studied in relation to the structure of the active phase. The activity was found independent of the copper loading, when the weight of copper in the reactor was kept constant. Copper catalysts were prepared by homogeneous precipitation. In the range of reduction temperatures between 543 K and 743 K, the activity was constant, in spite of considerably different copper metal surface areas. The absence of a correlation between activity and copper surface area is assigned to the formation of highly dispersed copper not active in methyl acetate hydrogenolysis. In contrast to the equal activity, catalysts of low copper loading produced more ethane than the higher loaded catalysts, based on the weight of copper. A remarkable parallel between the amount of ammonia chemisorbed on the reduced catalyst and the ethane selectivity suggests that a dehydration reaction to ethene on acid sites located at the silica support, followed by hydrogenation on the copper metal phase, is responsible for the formation of ethane.

The effect of free fatty acid on the reactivity of copper based catalysts for the hydrogenolysis of fatty acid methyl esters

In the synthesis of fatty alcohols by hydrogenolysis of fatty acid methyl esters, small amounts of free fatty acids in the feed negatively affect the reactivity of copper-silica based catalysts. The effect of the acid was investigated in relation to the production of water, the nature of the inhibiting species, and the degree of reduction of the catalyst. Inhibition is reversible and not due to catalyst deactivation. Water is not the inhibiting species. Furthermore, formation of copper and zinc soaps was excluded. Lauric acid in the methyl ester feed reacts preferentially, but at a lower rate than the ester. Inhibition most likely stems from a preferential adsorption of the acid at the active sites of the catalyst. The consequences for practical applications are discussed.

Monitoring of the formation of binuclear ruthenium carbonyl complexes during reactions by means of high performance liquid chromatography

Abstract Reversed-phase high performance liquid chromatography has been used to monitor the reactions of Ru 3 (CO) 12 with 1,4-diazabutadiene (DAB) and of Ru 2 (CO) 6 (DAB) with DAB. The kinetic data show that the formation of an intermediate in the former reaction is the rate determining step, which is first order in Ru 3 (CO) 12 as well as in DAB. The reactivity depends strongly on the type of substituent on DAB. Exchange of free and coordinated ligands (isopropyl DAB and tert-butyl DAB) is demonstrated in the reaction of Ru 2 (CO) 6 (DAB) with DAB. A reversible reaction is proposed to account for this exchange.

The eight electron donor mode of 1,4-diazabutadiene ligands as revealed by the crystal structures of Ru2(CO)4(η-2-acetylene)[σ,σ,η2,η2-glyoxalbis(isopropylimine)] and Ru4(CO)8[σ,σ,η2,η2-glyoxalbis(isopropylimine)]

Abstract The crystal structures of Ru 2 (CO) 4 (μ 2 -acetylene){glyoxalbis(isopropylimine)} and Ru 4 (CO) 8 {glyoxalbis(isopropylimine)} 2 have been determined, and provide the first examples of the σ- N , σ- N ′, η 2 -CN, η 2 CN′-coordination mode of the DAB ligands. The chemical shift values of the imine hydrogen atoms are 6.17 and 6.56 ppm, respectively, and confirm the bonding mode of the 1,4-diazabutadiene (DAB).

Binuclear metal carbonyl DAB complexes. V. The syntheses, structural and chemical properties of Co2(CO)6(DAB) (DAB = 1, 4-diazabutadiene)

Complexes of the position Co2(CO)6(DAB) (DAB = R R = C(R′)C(″) = N R; R = isopropyl, 2, 6-xyly, 2, 4, 6-mesityl, p-methoxyphenyl; R′ = H, R″ = H, CH3) have been obtained by a substitution reaction involving Co2(CO)8 and DAB. Refluxing of Co2(CO)6(DAB) in n-hexane yields Co4(CO)8(DAB)2, while reactions with DAB lead to the formation of Co2(CO)4(DAB)2. The complexes have been characterized by means of mass spectrometry, IR, UV, Raman and NMR spectroscopy. It si shown that Co2(CO)6(DAB) contains a σ, σ-coordinated 1, 4-diazabutadiene, linked to one of the Co-atoms and four terminal and two bridging carbonyl groups. Both 1H and 13C NMR show high field chemical shifts for the imine H and imine C atoms respectively, indicating strong π-backbonding to the diimine ligand. Indeed it was found tahat in a series of zerovalent d6, d7, d8, d9 metal carbonyl DAB complexes, in which the DAB ligand acts a σ-N, σ-N′-donor, the chemical shifts of the imine hydrogen atoms depend on the number of d-electrons, i.e. there is an increasing π-backbonding in teh order d6 < d7 < d8 ,d9. Finally, it is argued that for binuclear complexes teh relative stability of teh 4e-σ, σ-donor mode versues the 6e-σ-N, μ2-N′, η2-C = N ′ coordination mode, in which the diimine bridges two metals (e.g. in Ru2(CO)6(DAB)), is strongly dependent on a number of factors.