Preparation Of CoB Catalysts In Microemulsions And Their Performances For Selective Hydrogenation Of α, β-unsaturated Aldehydes

The selective catalytic hydrogenation ofα,β-unsaturated aldehydes is a green process,with good atom economy and application prospects. However,selective hydrogenation ofα,β-unsaturated aldehydes toα,β-unsaturated alcohols is a difficult task,since thermodynamics favors C=C bond hydrogenation over C=O bond.So the key work is to design highly active and selective metal catalysts.In this thesis,ultrafineamorphous CoB catalysts were prepared by chemical reduction of cobalt ion with borohydride in a water-in-oil microemulsion system comprising water,p-octyl polyethylene glycol phenyl ether(TritonX-100),n-hexanol and cyclohexane.Their catalytic performances for liquid phase selective hydrogenation ofα,β-unsaturated aldehydes were studied as well.The influences of microemulsion composition,reactants concentration,Co/B atom molor ratio,synthesis temperature and promoters on their properties were investigated.The catalysts were characterized by means of nitrogen adsorption-desorption (BET),X-ray diffraction(XRD),scanning electron microscopy(SEM), transmission electron microscopy(TEM),temperature programmed desorption of hydrogen(H₂-TPD),X-ray fluorescence spectrometry(XRF), and thermal gravimetry and differential scanning calorimetry(TG-DSC).The results show that the CoB catalyst prepared in microemulsion with composition(water/Triton X-100 + n-hexanol/cyclohexane) at 1/4.7/2.53,the mass ratio of surfactant to cosurfactant at 3:3,Co(NO₃)₂ concentration at 0.2 mol/L,KBH₄ concentration at 1.2 mol/L,the atom molor ratio of Co to B at 1:3,KBH₄ solution added in a single pouring exhibites excellent catalytic activity and selectivity for the hydrogenation of cinnamaldehyde.The conversion of cinnamaldehyde and the selectivity to cinnamyl alcohol is 78.1%and 89.7%,respectively,over this catalyst after reaction 30 min.Besides,the promoting effect of several metal promoters on amorphous CoB catalyst for the liquid phase hydrogenation of cinnamaldehyde has been investigated.Experimental results show that adding small amount of promoters can not change the amorphous structure of CoB catalyst,but can obviously effact the catalytic properties.CoB catalysts modified by Ce and La have better activity,but the selectivity of unsaturated alcohol decreases.Catalyst modified by Ru shows better catalytic activity on hydrogenation of C=C.Additon of promoter Sn and Au can improve the selectivity of unsaturated alcohols with the activity almost equal to that of nonpromoter CoB catalyst.In addition,the results show that microemulsion method has great advantages than general chemical reduction for preparation of nanocatalysts.The CoB catalyst prepared in microemulsion has smaller and uniform particle size,larger surface area,better thermal stability and moderate-intensity hydrogen absorption,and exhibits better catalytic performance for cinnamaldehyde hydrogenation.The reaction conditions of cinnamaldehyde hydrogenation were optimized as follows:CoB catalyst dosage 0.04 mmol/ml solvent, cinnamaldehyde concentration 0.06 g/ml solvent,reaction temperature 373K,hydrogen pressure 1.5 MPa,stirring speed 1000 rpm and reaction time 120 min.Under the optimal reaction conditions,the conversion of cinnamaldehyde and the selectivity to cinnamyl alcohol can reach 92.4% and 98.9%,respectively.The activity and selectivity of CoB catalyst remain basically unchanged after three runs.The catalyst also displays excellent catalytic performance for the liquid phase selective hydrogenation of otherα,β-unsaturated aldehydes such as citral.