Structure And Performance Research On The Amorphous Ni-Based Catalysts For C₅/C₆ Alkane Isomerization

With the increasingly strict environmental protection requirements and increasing improvement for the performance of clean fuel,it is imperative to develop technologies for producing high octane gasoline blending components.The products produced by converting straight-chain C₅/C₆ into branched iso-alkanes,not only possess high octane number,but also contain little olefin and aromatic components which are harmful to the environment.However,the noble metal catalysts for C₅/C₆ isomerization which are now widely used have drawbacks of high cost and low tolerance to sulfur and other problems.The current non-noble metal catalysts are cheap,but their catalytic performance still needs to be improved.Therefore,in order to solve the problems existing in current catalysts for n-alkanes isomerization,novel amorphous Ni-based catalysts for n-alkanes isomerization were developed by combining the excellent catalytic performance of amorphous Ni-based active components with the appropriate acidity and pore structures of molecular sieve.And the structures,properties and mechanism of n-alkanes isomerization for Ni-based catalyst were studied.The effects of preparation conditions on the structure and catalytic performance of amorphous NiB/H?catalyst were studied.It was proved that with the increase of NiB loadings,the mesoporous structure of the catalyst was gradually destroyed and the amounts of strong Lewis acid gradually increased.And the catalyst with appropriate NiB loadings could have good dispersion of NiB on carrier.Increasing the calcination temperature within a certain range of temperature?100-400°C?has no effect on pore structures and acid properties of the catalyst,but has an influence on the amorphous structure of NiB,which affects the performance of the catalyst further.Meanwhile,the effects of other factors on catalytic performance of catalysts were also investigated.The water,olefins,aromatics and low concentrations of sulfides have little effect on catalytic performance.Nevertheless,high concentrations of sulfides could decrease the catalytic performance.Based on the study of catalyst performance,the role of hydrogen,acid site and the structure of active component,the mechanism of alkane isomerization catalyzed by amorphous NiB/H?catalyst based on H-NiB structure was proposed.The active component of the catalyst exists in the form of H-NiB with H-Ni bond,and the n-hexane isomerization is accomplished by the synergistic effect of the metal center provided by H-NiB structure and the acid site provided by carrier.Compared with traditional bifunctional catalysts,the amorphous NiB/H?catalyst has the H-NiB structure,which is favorable for enhancing the hydrogenation of the isomerized olefin produced in reaction.On the basis of amorphous NiB/H?catalyst,amorphous NiP/H?catalyst with better catalytic performance is further prepared by kneading method.The structure,surface properties and catalytic activity of amorphous NiP/H?catalyst were studied.It was found that the microporous structures of the catalyst were gradually destroyed with the increase of NiP loadings.With low NiP loadings,the amorphous phase of NiP was covered by the crystal phase of carrier.The amounts of strong Lewis acid of catalyst are mainly provided by unsaturated nickel atoms in NiP.The density of metal sites increases first and then decreases with the increase of NiP loadings.The pore structures and surface acid properties have little change when the calcination temperature was in a range of 100°C to 400°C.But when the temperature was above 300?,the crystalline phase transforms from amorphous NiP into the crystalline nickel structure,which affects the activity of the catalyst further.The effect of reaction conditions on catalytic performance of NiP/H?catalyst was investigated.The suitable reaction conditions for n-hexane isomerization were obtained as follows:reaction temperature is 300°C,reaction pressure is 2 MPa,H₂-to-n-hexane molar ratio is 4 and WHSV is 1 h^-1.It was proved that trace amounts of water,olefins and aromatics contained in the raw material had no significant effect on catalytic performance.Meanwhile NiP/H?catalyst showed good sulfur resistance.Rare earth metal modification could improve the selectivity to isohexane slightly.The catalyst prepared with carrier after sulfation showed a slight increase in n-hexane conversion,but the cracking performance of the catalytic reaction was also enhanced.Comparison results show that amorphous NiP/H?catalyst has a much higher catalytic performance in n-hexane isomerization than the crystalline Ni₂P/H?catalyst,and higher catalytic performance than the selected industrial isomerization catalyst.The isohexane yield for amorphous NiP/H?was about 20%higher than that for the crystalline Ni₂P/H?catalyst,and the total yield of i-C₅ and i-C₆ for amorphous NiP/H?was about 12%higher than that of the crystalline Ni₂P/H?.The isohexane yield for amorphous NiP/H?was 2%higher than the selected industrial catalyst.Meanwhile,i-C₆ and i-C₅+i-C₆ for amorphous NiP/H?catalyst were 20%and 11%higher than those for the crystalline Ni₂P/H?catalyst,respectively.And i-C₆ and i-C₅+i-C₆ for amorphous NiP/H?catalyst were 4%and 1%higher than those for selected industrial catalyst,respectively.The catalyst showed a good application prospect.The catalytic stability of the amorphous NiP/H?catalyst was investigated.And the results showed that the catalyst could maintain good catalytic stability within 600 h and the octane number of isomerization products could keep about 80.After the long period test,the pore structure,acid properties and dispersion of active components of the catalyst could keep good.Based on the study of initiator effect,hydrogen effect,pre-reduction effect and acid site effect,the mechanism of alkane isomerization for amorphous NiP/H?catalyst was proposed.