Study On Isobutanol Synthesis From Syngas Over CuZnAl Composites

Isobutanol is not only used as industrial chemical to synthesize medicines,paints and rubber but also have high octane value and good anti-explosiveness,so it can be used as a new substitute for inflammable and volatile additive of vehicle fuels（methyl tert-butyl ether）.Until now,isobutanol is mainly obtained by propylene carbonylation,but the method is too inefficient to meet the requirement for isobutanol production,which leads to import of a large amount of isobutanol.Therefore,synthesis of isobutanol from syngas derived from coal accords with characteristic of energy resources in our country.At present,there is much shortage for the catalysts used to synthesize isobutanol from syngas,such as weak catalytic activity,low selectivity for isobutanol,poor stability,high inactivity.Owing to the uniform distribution of metal elements in hydrotalcite-like compounds（HTLCs）and mild working conditions of CuZnAl catalysts,CuZnAl hydrotalcite-like compounds（CuZnAl HTLCs）was used as precursors to prepare Cu ZnAl catalysts with high catalytic activity and mild working conditions.In this dissertation,CuZnAl catalysts and carbon materials combined to promote dispersion of active components and the sintering resistance,and potassium promoter and complexants were used to modify the composites in order to improve synergistic effect of modifiers and carbon materials and to obtain the modified composites with higher catalytic activity.By investigating the effect of carbon materials and modifiers on composite structure,we revealed the relation between catalytic performance and catalyst structure and determined the best composition and working conditions of the composites.Furthermore,we preliminarily knew the catalytic mechanisms of the composites,and the major conclusions were drawn as follows:（1）On the surface of carbon fibers,hydroxyl and carboxyl groups facilitate growth of CuZnAl-HTLCs.With the increasing content of carbon fibers,the specific surface area of the composites increased and then decreased,but their average pore diameters decreased and then increased.The size of CuO crystal gradually decreased with increase in content of carbon fibers,and the active components was uniformly dispersed on the surface of carbon fibers.（2）After activity evaluation of 480 h,CO conversion and selectivity for isobutanol maintained around 45%and 17%,respectively.The composite catalysts hardly showed inactivity for isobutanol synthesis,but the selectivity for methanol started to decrease.The decline in the selectivity related to decrease in specific surface area and pore diameter resulting from increase in particle size of the catalysts.（3）The best working conditions and composition of CuZnAl/carbon fibers composite catalysts（CuZnAl/ACFs）were listed as follows:reaction temperature is 320°C;pressure is 4 MPa;H₂/CO ratio of syngas is 2;mass fraction of carbon fibers is 30%;calcination temperature of precursors is 400°C.（4）The catalytic mechanism of CuZnAl/ACFs for isobutanol synthesis from syngas mainly related to CO insertion:（a）carbon fibers in CuZnAl/ACFs can enhance electron transition,which facilitates intermediate formation during alcohol synthesis;（b）carbon fibers can promote growth of carbon chains by improving medium CO adsorption;（c）carbon fibers can improve dispersion of active components and thus promote formation of CuO/ZnO solid solution.The Cu O/Zn O solid solution resulted in Cu/ZnO interface during H₂ reduction.During isobutanol synthesis,Cu⁰ activate CO molecules,and ZnO reserve hydrogen and activate H₂ molecules.On the Cu/ZnO interface,their synergistic effects promote the isobutanol synthesis.（5）Potassium promoter or carbon fibers facilitate formation of CuO/ZnO solid solution and medium CO adsorption.However,the simultaneous addition of potassium promoter and carbon fibers leads to shedding of CuZn Al nanoflakes,and thus the content of active components decreases on the surface of carbon fibers.（6）Triethanolamine（TEA）or elhylene diamine tetraacetic acid（EDTA）leads to the loose flocculent morphology on the surface of CuZn Al catalyst,which is beneficial to mass transfer of products and reactants.The steric hindrance of TEA or EDTA restrain H₂ desorption and thus increase contact time of H₂ and Cu O,which improve reduction of CuO.The synergistic effects of TEA and carbon fibers promote CuO reduction and medium CO adsorption,and thus the CuZnAl/ACFs modified by TEA showed higher activity for isobutanol synthesis.