Coking and Deactivation of Catalyst Inhibited by Silanization Modification in Oxidation of Benzene to Phenol with Nitrous Oxide

The main cause to the deactivation of ZSM-5 catalyst, used for oxidation of benzene to phenol (BTOP) by nitrous oxide, is that the carbon deposition on the catalyst surface blocks the mouth of pores of the catalyst.In the experiments, ZSM-5 catalyst was modified by chemical surface deposition of silicon, and then the effect of modification condition on the catalyst activation was studied. The catalyst samples were characterized by XRF,EPS, XRD, TEM, N2 adsorption at low temperature, pyridine adsorption-infrared technique and etc. All the above results show that the uniform SiO2 membrane can be formed on ZSM-5 crystal surface. The SiO2 membrane covers the acid centers on ZSM-5 surface to inhibit surface coking, to avoid or decrease the possibility of ZSM-5 pore blockage so that the catalyst activity and stability can be improved efficiently. The optimum siliconiting conditions determined by the experiments are as follows: 4% load of silanizing agent, volume (ml)/mass (g) ratio of hexane/ZSM-5=15/1, and 16 h of modification time. Compared with the samples without siliconiting treatment,the samples treated under the above optimum condition can increase the productivity of phenol by 14% for 3 h reaction time and by 41% for 6 h reaction time respectively.

Pt/FeSnO(OH)_5： A Novel Supported Pt Catalyst for Catalytic Oxidation of Benzene

Pt/FeSnO（OH）_5 was synthesized as a novel catalyst for VOCs oxidation. Compared with Pt/γ-Al_2O_3 during catalytic oxidation of benzene, Pt/Fe Sn O（OH）5 showed better catalytic activity. After characterization of the catalysts by XRD, SEM, TEM, EDS, XPS, BET, TGA and DTA, we found most Pt could be reduced to metallic state when the hydroxyl catalyst was used as supporter, and the metallic Pt in Pt/Fe Sn O（OH）5 was more active than the oxidized Pt in Pt/γ-Al_2O_3 in catalytic oxidation of VOCs. Pt/FeSnO（OH）_5 shows both good catalytic activity and high stability, which may be a promising catalyst. This study may also be helpful for the design and fabrication of new catalysts.

Improving benzene catalytic oxidation on Ag/Co_(3)O_(4) by regulating the chemical states of Co and Ag

Silver(9 wt.%)was loaded on Co_(3)O_(4)-nanofiber using reduction and impregnation methods,respectively.Due to the stronger electronegativity of silver,the ratios of surface Co^(3+)/Co^(2+) on Ag/Co_(3)O_(4) were higher than on Co_(3)O_(4),which further led to more adsorbed oxygen species as a result of the charge compensation.Moreover,the introducing of silver also obviously improved the reducibility of Co_(3)O_(4).Hence the Ag/Co_(3)O_(4) showed better catalytic performance than Co_(3)O_(4) in benzene oxidation.Compared with the Ag/Co_(3)O_(4) synthesized via impregnation method,the one prepared using reduction method(named as Ag Co-R)exhibited higher contents of surface Co^(3+) and adsorbed oxygen species,stronger reducibility,as well as more active surface lattice oxygen species.Consequently,Ag Co-R showed lowest T_(90) value of 183℃,admirable catalytic stability,largest normalized reaction rate of1.36×10^(-4)mol/(h·m^(2))(150℃),and lowest apparent activation energy(E_(a))of 63.2 kJ/mol.The analyzing of in-situ DRIFTS indicated benzene molecules were successively oxidized to phenol,o-benzoquinone,small molecular intermediates,and finally to CO_(2) and water on the surface of Ag Co-R.At last,potential reaction pathways including five detailed steps were proposed.

Catalytic activity of porous manganese oxides for benzene oxidation improved via citric acid solution combustion synthesis

Various manganese oxides(MnOx) prepared via citric acid solution combustion synthesis were applied for catalytic oxidation of benzene. The results showed the ratios of citric acid/manganese nitrate in synthesizing process positively affected the physicochemical properties of MnOx, e.g., BET(Brunauer-Emmett-Teller) surface area, porous structure, reducibility and so on, which were in close relationship with their catalytic performance. Of all the catalysts, the sample prepared at a citric acid/manganese nitrate ratio of 2:1(C2M1) displayed the best catalytic activity with T(90)(the temperature when 90% of benzene was catalytically oxidized) of 212 ℃. Further investigation showed that C2M1 was Mn2O3 with abundant nano-pores, the largest surface area and the proper ratio of surface Mn^4+/Mn^3+, resulting in preferable low-temperature reducibility and abundant surface active adsorbed oxygen species. The analysis results of the in-situ Fourier transform infrared spectroscopy(in-situ FTIR) revealed that the benzene was successively oxidized to phenolate, o-benzoquinone, small molecules(such as maleates, acetates, and vinyl), and finally transformed to CO2 and H2O.

Ionic-liquid-assisted synthesis of metal single-atom catalysts for benzene oxidation to phenol

Ionic liquids(ILs)have the advantages of low cost,eco-friendliness,abundant heteroatoms,excellent solubility,and coordinated ability with metal ions.These features make ILs a suitable precursor for fabricating metal singleatom catalysts(SACs).Herein,we prepared various metal single atoms anchored on ultrathin N-doped nanosheets(denoted as Cu_(1)/NC,Fe_(1)/NC,Co_(1)/NC,Ni_(1)/NC,and Pd_(1)/NC)by direct pyrolysis using ILs and g-C_(3)N_(4)nanosheets as templates.Taking benzene oxidation to phenol with H_(2)O_(2)as a model reaction to evaluate their catalytic performance and potential applications,Cu_(1)/NC calcined at 1000℃(denoted as Cu1/NC-1000)exhibits the highest activity with a turnover frequency of about 200 h^(-1)in the first 1 h at 60℃,which is better than that of most metal SACs reported in the literature.High benzene conversion of 82% with high phenol selectivity of 96% and excellent recyclability were achieved using the Cu_(1)/NC-1000 catalyst.This study provides an efficient general strategy for fabricating SACs using ILs for catalytic applications.

Fe modified mesoporous hollow carbon spheres for selective oxidation of ethylbenzene

Fe modified hollow carbon spheres with large cavity and mesoporous shell （Fe-MHCs） were successfully synthesized by a simple pyrolysis and simultaneous deposition method. The organic molecules gases （carbon species） from pyrolysis of polystyrene deposited in the hard template at the catalysis of Fe species existing in the sample during calcination at high temperature. The obtained Fe-MHCs showed uniform spherical morphology with large surface area （924 m^2 g^-1）, mesoporous structure and a certain amount of Fe loaded. The Fe species and the special structure endowed the materials excellent catalytic activity in the oxidation of ethylbenzene to acetophenone. The conversion of 94.5% and the high selectivity to targeted product （97.4%） could be achieved and the acceptable recycling stability was also exhibited.

Dynamic evolution of nitrogen and oxygen dual-coordinated single atomic copper catalyst during partial oxidation of benzene to phenol

Single atom catalysts(SACs)with metal_(1)-N_(x)sites have shown promising activity and selectivity in direct catalytic oxidation of benzene to phenol.The reaction pathway is considered to be involving two steps,including a H_(2)O_(2)molecule dissociated on the metal single site to form the(metal_(1)-N_(x))=O active site,and followed by the dissociation of another H_(2)O_(2)on the other side of metal atom to form O=(metal_(1)-N_(x))=O intermediate center,which is active for the adsorption of benzene molecule via the formation of a C-O bond to form phenol.In this manuscript,we report a Cu SAC with nitrogen and oxygen dual-coordination(Cu1-N3O1 moiety)that doesn’t need the first H_(2)O_(2)activation process,as verified by both experimental and density function theory(DFT)calculations results.Compared with the counterpart nitrogen-coordinated Cu SAC(denoted as Cu1/NC),Cu SAC with nitrogen and oxygen dual-coordination(denoted as Cu1/NOC)exhibits 2.5 times higher turnover frequency(TOF)and 1.6 times higher utilization efficiency of H_(2)O_(2).Particularly,the coordination number(CN)of Cu atom in Cu1/NOC maintains four even after H_(2)O_(2)treatment and reaction.Combining DFT calculations,the dynamic evolution of single atomic Cu with nitrogen and oxygen dualcoordination in hydroxylation of benzene is proposed.These findings provide an efficient route to improve the catalytic performance through regulating the coordination environments of SACs and demonstrate a new reaction mechanism in hydroxylation of benzene to phenol reaction.

Rational design and precise manipulation of nano‐catalysts

Nano‐catalysis plays a vital role in the chemical transformations and significantly impacts the booming modern chemical industry.The rapid technological enhancements have resulted in serious energy and environmental issues,which are currently spurring the exploration of the novel nano‐catalysts in diverse fields.In order to develop the efficient nano‐catalysts,it is essential to understand their fundamental physicochemical properties,including the coordination structures of the active centers and substrate‐adsorbate interactions.Subsequently,the nano‐catalyst design with precise manipulation at the atomic level can be attained.In this account,we have summarized our extensive investigation of the factors impacting nano‐catalysis,along with the synthetic strategies developed to prepare the nano‐catalysts for applications in electrocatalysis,photocatalysis and thermocatalysis.Finally,a brief conclusion and future research directions on nano‐catalysis have also been presented.

Alkali ion-promoted palladium subnanoclusters stabilized on porous alumina nanosheets with enhanced catalytic activity for benzene oxidation

Catalytic C−H bond activation is one of the backbones of the chemical industry.Supported metal subnanoclusters consisting of a few atoms have shown attractive properties for heterogeneous catalysis.However,the creation of such catalyst systems with high activity and excellent anti-sintering ability remains a grand challenge.Here,we report on alkali ion-promoted Pd subnanoclusters supported over defectiveγ-Al_(2)O_(3) nanosheets,which display exceptional catalytic activity for C−H bond activation in the benzene oxidation reaction.The presence of Pd subnanoclusters is verified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy,X-ray absorption spectroscopy,and X-ray photoelectron spectroscopy.This catalyst shows excellent catalytic activity,with a turnover frequency of 280 h^(−1) and yield of 98%,in benzene oxidation reaction to give phenol under mild conditions.Moreover,the introduction of alkali ion greatly retards the diffusion and migration of metal atoms when tested under high-temperature sintering conditions.Density functional theory(DFT)calculations reveal that the addition of alkali ion to Pd nanoclusters can significantly impact the catalyst’s structure and electronic properties,and eventually promote its activity and stability.This work sheds light on the facile and scalable synthesis of highly active and stable catalyst systems with alkali additives for industrially important reactions.

Large-scale synthesis of hierarchical MnO_2 for benzene catalytic oxidation

Hierarchical sea-urchin-shaped manganese oxide microspheres were synthesized via a facile method based on the reaction between KMnO4 and MnSO4 in HNO3 solution at 50℃. The average diameter of the microspheres is -850 nm. The microspheres consist of a core of diameter of -800 nm and nanorods of width -50 nm. The nanorods exist at the edge of the core, The Brunauer-Emmett-Teller surface area of the sea-urchin-shaped microspheres is 259.4 m^2/g. A possible formation mechanism of the hierarchical sea-urchin-shaped microspheres is proposed. The temperature for 90% conversion of benzene （T90%） on the hierarchical urchin-shaped MnO2 microspheres is about 218 ℃.

Promoted catalytic performance of Ag-Mn bimetal catalysts synthesized through reduction route

VOCs can exert great harm to both human and environment,and catalytic oxidation is believed to be an effective technique to eliminate these pollutants.In this paper,Ag-Mn bimetal catalysts with 10 wt.%of silver were synthesized using doping,impregnation,and reduction methods respectively,and then they were applied to the catalytic oxidation of benzene.Through series of characterizations it showed that the loading of silver using reduction method significantly resulted in improved physico-chemical properties of manganese oxides,such as larger surface area and pore volume,higher proportion of surface Mn~(3+)and Mn~(4+),stronger reducibility and more active of surface oxygen species,which were all beneficial to its catalytic activity.As a result,the Ag-Mn catalysts synthesized by reduction method showed a lower T_(90)value(equals to the temperature at which 90%of initial benzene was removed)of 203℃.Besides,both the used and fresh Ag-Mn catalysts synthesized by reduction method showed preferable stability in this research.

Mo-modified Pd/Al_2O_3 catalysts for benzene catalytic combustion

Mo-modified Pd/Al2O3catalysts were prepared by an impregnation method and tested for the catalytic combustion of benzene. The catalysts were characterized by N2 isothermal adsorption, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, temperatureprogrammed desorption of NH3（NH3-TPD）, H2temperature-programmed reduction（H2-TPR）, and high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）. The results showed that the addition of Mo effectively improved the activity and stability of the Pd/Al2O3catalyst by increasing the dispersion of Pd active components, changing the partial oxidation state of palladium and increasing the oxygen species concentration on the surface of catalyst. In the case of the Pd-Mo/Al2O3catalyst,benzene conversion of 90% was obtained at temperatures as low as 190°C, which was 45°C lower than that for similar performance with the Pd/Al2O3catalyst. Moreover, the 1.0% Pd-5% Mo/Al2O3catalyst was more active than the 2.0% Pd/Al2O3catalyst. It was concluded that Pd and Mo have a synergistic effect in benzene catalytic combustion.

Constructing the separation pathway for photo-generated carriers by diatomic sites decorated on MIL-53-NH2(Al)for enhanced photocatalytic performance

High yield production of phenol from hydroxylation of benzene with low energy consumption is of paramount importance,but still challenging.Herein,a new strategy,consisting of using diatomic synergistic modulation(DSM)to effectively control the separation of photo-generated carriers for an enhanced production of phenol is reported.The atomic level dispersion of Fe and Cr respectively decorated on Al based MIL-53-NH_(2)photocatalyst(Fe1/Cr:MIL-53-NH_(2))is designed,in which Cr single atoms are substituted for Al3+while Fe single atoms are coordinated by N.Notably,the Fe1/Cr:MIL-53-NH_(2)significantly boosts the photooxidation of benzene to phenol under visible light irradiation,which is much higher than those of MIL-53-NH_(2),Cr:MIL-53-NH_(2),Fe1/MIL-53-NH_(2),and Fe nanoparticles/Cr:MIL-53-NH_(2)catalysts.Theoretical and experimental results reveal that the Cr single atoms and Fe single atoms can act as electron acceptor and electron donor,respectively,during photocatalytic reaction,exhibiting a synergistic effect on the separation of the photo-generated carriers and thereby causing great enhancement on the benzene oxidation.This strategy provides new insights for rational design of advanced photocatalysts at the atomic level.

Synthesis of Ferrocene-Modified Carbon Nitride Photocatalysts by Surface Amidation Reaction for Phenol Synthesis

A polymeric Fc-CO-NH-C_(3)N_(4)(Fc-CN)material was synthesized by amidation reaction of ferrocenecarboxylic acid(Fc-COOH)with NH_(2)groups on the surface of mesoporous graphitic carbon nitride(MCN).The properties of the as-synthesized samples were characterized by X-ray diffraction,Fourier transform infrared spectroscopy,UV-Vis diffuse reflectance spectra,N2 adsorption-desorption isotherm,photoluminescence spectroscopy,transmis-sion electron microscopy,electron paramagnetic resonance,(photo)electrochemical measurement and X-ray photo-electron spectroscopy.The resultant catalysts were investigated as heterogeneous catalysts for the selective oxida-tion of benzene to phenol using H_(2)O_(2)as a green oxidant under visible light irradiation.The results reveal that Fc-modified samples can not only extend the visible light absorption,but also accelerate the bulk-to-surface charge transfer and separation via surface dyadic structures,both of which are favorable for phenol production from ben-zene photocatalytic hydroxylation with H_(2)O_(2).Under the optimal conditions,up to 16.9%phenol yield(based on benzene)is obtained by Fc-CN/1.5-5 sample,and the corresponding Fe content is about 0.64 wt%.Furthermore,after the second run,no significant decrease of the activity(in term of TOF)and the selectivity is found in Fc-CN/_(1.0)-5 sample.Combined with the experimental results and Fenton-chemistry,a possible photocatalytic reaction mecha-nism on the hydroxylation of benzene to phenol at neutral medium with visible light is proposed.