Surface-interface Design Of FeO_x And Its Structure-activity Research In Selective Oxidation Of Cyclohexane

Partial oxidation of saturated C-H bonds of alkanes to high-value-added chemicals remains an important challenge for catalytic chemistry because the inertness of the C-H bonds presents difficulties for selectivity.It is of great theoretical and practical importance to develop high-efficiency catalyst for aerobic oxidation of cyclohexane to KA oil,which is an important feedstock of caprolactam,nylon-6 and nylon-66.The main disadvantage of this process is that a high selectivity to target products can only be reached at low C₆H₁₂conversion.There have been numerous fundamental studies about this oxidation process,however,the corresponding processes suffer from their major disadvantage of economic benefit.Because of durability,nontoxicity,copious supply and environmentally benign nature,FeO_xas catalyst is widely applied in energy conversion and storage,oil refining,and environmental decontamination.Furthermore,FeO_xis used to catalyze C-H bonds due to its abundance of highly active electrophilic oxygen species and high bulk oxygen mobility.First,three kinds of uniform?-Fe₂O₃nanocrystals,rhombohedral?-Fe₂O₃(Rh Fe)enclosed by{104}facets,pseudo-cubic?-Fe₂O₃(PCFe)enclosed by{012}facets,and hexagonal plate(He Fe)enclosed by{001}facets,were synthesized as model catalysts.It will be a very interesting work to engineer the crystal facets of?-Fe₂O₃by a selective decoration,so as to improve both the selectivity and conversion significantly.The better catalytic performance of new formed active centers is mainly derived from the charge difference between Cl and the neighbouring outmost O atoms,which is affected by the geometric and electronic structures of the original crystal facets.Cl-decorated{001}facets show the highest intrinsic activity,while Cl-decorated{012}facets show the best catalytic performance.Cl-decoration affects the catalytic reaction by accelerating the homolysis of O-O bond and C-H bond.On the other hand,Cl-decoration is conducive to improve the selectivity by promoting the desorption of KA oil.Herein,we reported a facile top-down approach to fabricate hierarchically durian-and clover-like?-Fe₂O₃microcrystals via selective oxalic acid digging and cutting,named DMe and CMe,respectively.They can effectively improve the C₆H₁₂conversion and KA oil selectivity in O₂atmosphere.CMe is composed of twelve smooth concave planes and the radially aligned opening macropores at the waist.The dissolution of iron oxides by means of oxalic acids is in fact a successive reduction process of lattice Fe^IIIcenters.Reduced Fe^IIcan be transferred to the solution more readily due to the kinetic instability of the Fe ^II-O bonds.For DMe,Fe^*is main sites for adsorption and activation of cyclohexane.For CMe,higher oxygen mobility could further accelerate the oxygen transfer to active oxygen,which would then be involved in the reaction.In addition,the Lewis acid-base interaction of Lewis basic group C=O and Lewis acid site Fe^*can effectively inhibit the further reaction of cyclohexanone.A hierarchical porous hybrid catalyst with a shell-core structure was obtained by coating FeO_xwith a carbon nanocages by chemical vapor deposition method or porous carbon film by Resorcinol-Formaldehyde methods respectively,and further etching was carried out in the form of self template.FeO_x@CNCs and FeO_x@PCMs were used to study the structure-activity of cyclohexane oxidation system.Due to the special preparation,the strong Fe-C bonds on FeO_x@CNCs can promote the surface electron conduction,and the hydrophobicity of carbon helps KA oil fall off the surface by molecular force.In the aspect of catalyst structure optimization,such as different thickness and types of carbon shells and different pore structures of FeO_xcore were obtained by changing the preparation parameters,which is helpful for us to establish more accurate structure-activity relationship.In addition,the strong magnetism of catalyst helps us to recover and reuse them.