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来源:百通兼职_武汉兼职信息网      发布日期:2017-09-06

Mg-doped g-C3N4 composite catalysts were prepared using a facile thermal polymerization under air sphere using urea and magnesium  chloride as precursors. The obtained materials were employed as the solid base catalyst forinKnoevenagel condensations. The Mg-doped g-C3N4 composite catalyst exhibits high efficiency for promotingshows its great value to promotethe reactionsresponsesas a solid catalyst. The 5MgCN-U catalysts displayed superiorexceedingcatalytic activity to the pureabsoluteCN-U. The introductionimportof Mg speciessortsimproved the overallensemblebasic quantitypropertyof CN-U, which favored the Knoevenagel condensations, exhibiting high benzaldehyde conversion of 97.4%.

The Knoevenagel condensation is one of the most fundamentala greatest basiccondensation reactionsreaction of organic chemistryforformatingcarbon-carbon bond formation in organic chemistry. This carbon-carbon bond formation reaction is a practical path to accomplish fine chemical intermediates in the synthesis of fine chemicals, therapeutic drugs, natural products and functional polymers. Recently, many efforts in the development of catalysts for Knoevenagel condensation have been made. The most common homogenous bases such as primary and secondary amines, pyridine, ammonia, and ammonium salts are generally developed as the catalysts. Although the homogenous catalysts exhibit high activity for Knoevenagel condensation reaction, the separation and recovery of the catalysts discourage their extensive application. Owing to the increasing demand on greenneed of environmentaland the sustainablecontinuablechemical industry, much attention has been recently paid to efficient and economic heterogeneous solid catalysts which have some advantages such as high selectivity to target product, facilitated separation from the reactionreactivemixture and recycling of the catalyst and low operationrunningcost.

Therefore, a lot ofnumeroussolid catalysts, including hydrotalcites, metal oxide, alkali doped zeolites, metal-organic framework compounds, amines supported on the porous carbon and silica materials have been received great research interest recently. Because ofDue tothe unique physical and chemical propertiesqualitiesof inertness to various solvents and thermal stability, nanostructure carbon-based materialsstuffhave been extensivelywidelyused as a versatile catalyst forinKnoevenagel condensation reaction. Moreover, the introduction ofFurthermore,basic nitrogen groupswhich are introducedinto carbon can improvepromotethe catalytic activity of the materials. Bitter et al. found that the carbon nanotubes which contained the basic nitrogen groups exhibited good catalytic performance for the Knoevenagel condensations of benzaldehyde and ethylcyanoacetate.

Recently, carbon nitride is a well-knownfamousand fascinating materialattractive stuffwith two-dimensional (2D) layered structure, which has attractedfascinatedincreasing researchstudyinterest in the fields of in photocatalysis, fuel cells, gas storage and catalysis. The g-C3N4 planes consistingwhich made upof highly orderedof very regulartri-s-triazine (C6N7) units linked throughconnected withplanar tertiary amino groups contain many ideal coordinationlots of perfect balancesites called “nitrogen pots” in whichwherethe six nitrogen lone-pair electrons can interactmake interactionswith the metal ions. Generally, alkaline-Alkalineearth metal oxides, e.g. MgOwhich consists with with BaO, CaO and BaOMgO, are usually used as the “active site” to prepare supported solid basic catalysts, especially MgO, which can significantly improve the catalytic performance due to the increased basic species. Therefore, it is expected that the introductionimportationof Mg speciessortsinto layered g-C3N4 to form the compositewill show the high catalytic performance for Knoevenagel condensation reaction, because of the combining the advantages of both the unique chemical structure of g-C3N4 and the excellent basicity of Mg species.

Considering the advantage in catalytic separation and the high catalytic performance of g-C3N4, in this work, the Mg-doped g-C3N4 composite were synthesized based on 2D intercalated g-C3N4 synthesized by a simple thermalhotpolymerization of theureamixture of ureaand  MgCl2 underthe pressure ofair and ambient pressure, and the catalytic performance of Knoevenagel condensation reaction and the cycling stability were investigated.

The Mg-doped g-C3N4 composite catalysts for Knoevenagel reaction with different amount of MgCl2 were prepared via a thermal polymerization method. 10 g of urea was ground togetherprocessedwith calculated amountnumberof MgCl2 6H2O for 30 minhalf an hour. The obtained powder was took into ancovered-alumina cruciblewith a cover, and then calcined atunder550 °C for 2 hhours. The content of Mg was 0, 3, 5 and 10 wt%, the corresponding catalysts were marked as xMgCN-U (x=0, 3, 5 and 10). 10 g of dicyandiamide and melamine powder were also ground together with calculated amount of MgCl2 6H2O for 30 minhalf an hour, and thencalcined atunder550 °C for 2 hhours. The obtained catalysts were denoted 5MgCN-D and 5MgCN-M.

X-ray diffraction (XRD) curves were recorded on a Bruker D8 Advance diffractometer, using Cu Kα radiation at 40 kV and 50 mA in a scanning rangescanned areaof 3-80° (2θ). N2 adsorption-desorption isotherms were obtained using a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature. TheAfter multi-point BET (Brunauer-Emmett-Teller) procedure,  SBET(specific surface areas(SBET) of the samples were calculatedfollowing the multi-point BET (Brunauer-Emmett-Teller) procedure. The pore size distributions were collected from the adsorption branch of the isotherms using the NLDFT (non-local density functional theory) models. Before carrying out the measurement, each sample wasAfterdegassed at 200 °C for more thanover6 hhours, each sample will be measured. The thermo-gravimetric (TG) patterns were measured on a TA SDT Q600 analyzer under a constant air flow rate of 100 mL/min and a heating rate of 10 °C /min. Fourier transform infrared spectra (FT-IR) were determined on a PE spectrum spectrometer, with the KBr pellet method and the ranges of spectrograms were 4000-400 cm-1. Scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) was observed by Rigaku S-3400 microscope. Transmission electron microscopy (TEM) images were obtained on a Rigaku H-7650 microscope, operating at 200 kV. The sample was dispersed in ethanol under ultrasound for 5 min, and then deposited on a copper grid coated with preformed holey carbon film. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGA ESCALAB 250 spectrometer (Thermo Electron, UK) equipped with a non-monochromatic Al Ka X-ray source (1486 eV). CO2-TPD experiments were carried out using a U-shaped quartz micro-reactor on Quantachrome CHEMBET 3000. About 0.2 g of sample was heated to 500 °C under the flow of pure He gas with a rate of 10 °C/min, and kept at this temperature for 1 h. After cooled to 30 °C, adsorb CO2 with 5%CO2/He at the temperature, the gas was switched to pure He gas until the baseline was stable. Then the CO2-TPD pattern was recorded with a thermal conductivity detector at a heating ratetemperature lossof 10 °C/min.

The as-prepared Mg-doped C3N4 composite were measured as catalysts forinKnoevenagel condensation reaction.as catalysts.Benzaldehyde (10.0 mmol), malononitrile (10.0 mmol) and toluene (10.0 ml) were mixedfixedin a three-necked bottom flask, and the mixture were heated to 70 °Cunder stirringin a water bath under stirring, then 50 mg of catalyst was addedappendedinto the flask. The reactants and products were analyzed bySP-6800A6 gas chromatographequipped, loadedwith aFID(flame ionization detector(FID)., is used to analyze the reactants and products.

Fig.1 showsdisplaysthe XRD patternsimagesof CN-U and xMgCN-U catalysts with differentdistinctMg content. For the CN-U, the diffraction peak around 27.5 °are clearly observed, corresponding to the d-spacing of 0.343 nm, which can be indexedsearchedthe (002) crystal plane of carbon nitride, indicating the presence of a typical interplanar stacking peak in conjugated aromatic systems. In contrastAs opposedto the CN-U, the (002) diffraction peak of xMgCN-U shift toward a higher 2θ value compared to the pure CN, which could be attribute to the crystal lattice distortion due tobecause ofthe Mg species doping in g-C3N4.

The X-ray diffraction (XRD) patternsimagesof the 5MgCN-Y with different carbon nitride precursor are shown in Fig.2. The 5MgCN-D and 5MgCN-M exhibit twodisplay 2peaks at 13.1° and 27.3°, corresponding tofitting in with(100) and (002) crystal planes of g-C3N4, respectively. Additionally, compared with g-C3N4, nowithoutpeak shift in the patternimageof 5MgCN-D and 5MgCN-M is observed. NoteMarkthat the (002) diffraction peaks of 5MgCN-U are rather broadvery wide, which might be attributedprobably owingto theheterogeneous intercalationinterlayerof Mg ion.

In order to investigateFor investigatingthe mechanism of the formation of urea-derived g-C3N4, the thermal analysis was carried outopennedby TG-DSC. And the TG-DSC results of the uncalcined CN-U and 5MgCN-U catalyst are shown in Fig. 3. The weight loss of 71% occurred at the range of 125-260 °C, according to the two endothermic peaks at 135 °C and 214 °C in DSC, respectively. The lower temperature may be caused byresulted fromthe decomposition of urea to produce isocyanic acid and biuret by eliminating NH3, another temperature may be attributed to continuation of urea and biuret decomposition, producing some intermediates including cyanuric acid, ammelide and ammeliline. Another endothermic peak is observedwatchedat 350°C. This endothermic peak at the temperature range from 260-360 °C with the totaloverallmass loss of 24% is due to the thermal decomposition of the intermediates into melamine. The mass loss of 5% in the temperature rangeareaof 360-520 °Crefers to the formation of tris-s-triazine via melamine rearrangement and condensation of this unit to polymers, networks and potentially the final polymeric C3N4 by takingconsideringDSC curveinto account which produce an endothermic peak at 410 °C. The CN-U sample only offers a very low yield of 5% whereas the 5MgCN-U affords a productivity of 10%. The gap confirms that the introduction of Mg, consistent with the Mg loading amount.

Fig. 4 present the N2 adsorption-desorption isotherms and the correspondinghomologouspore size distribution curves of CN-U and 5MgCN-U catalyst. The textural propertiesqualitiesare providedsuppliedin Table 1. All the prepared samples present classical type III isotherms, which is due tobecause ofthe weakfaintadsorbent–adsorbent interaction. The correspondingrelevantpore size distribution curves of the CN-U and 5MgCN-U catalysts with different Mg contents, calculated by the NLDFT method, further confirmedprovedthe formation of mesopores. One centered atfocus on1.7 nm can be associated with the mesopores in the layers; the others in the rangearea4-20 nm were attributed to the accumulation of g-C3N4 layers. For CN-U, the BET surface arearangeand total pore volume were 43 m2/g and 0.187 cm3/g,respectively.separately.While the 5MgCN-U exhibited a relative lower special surface arearangeof 38 m2/g and the totaloverallpore volume of 0.185 cm3/g, compared to those of the CN-U. The mesopore size for 5MgCN-U catalyst has scarcely changed after Mg doping, suggestingadvisingthat the mesostructure is well-preserved after introducing with Mg on CN-U.

The N2 adsorption-desorption isotherms and the correspondinghomologouspore size distribution curves of 5MgCN-Y catalyst with differentdistinctcarbon nitride precursor indicate that 5MgCN-U, 5MgCN-D and 5MgCN-M exhibit type III behavior (Fig.5). The BET surface area of 5MgCN-U (38 m2/g) is much larger than that of 5MgCN-D (5 m2/g) and 5MgCN-M (4 m2/g). These data illustrate that using urea as a precursor could significantly introduce porous structure with increased surface area, enlarged pore volume and narrow pore size distribution, which was the resultcauseof the drastic decomposition or volatilization of urea at higher temperature, amino groups decomposed into NH3, thus large amountstherefore, lotsof bubbles were formed, which acted as the templates for the mesopores, leading toresulting inthe formation of mesoporous. The large surface arearangecould supplyprovidemore active speciessorts, which might promote its catalytic activity.

Fig. 6 showsdisplaysthe SEM and TEM imagespatternsof the CN-U and 5MgCN-U. Both of the materials exhibitdisplaya layered structure similarquality closingto that of theirtheanalogue graphite (as shown in Fig. 6a, b). No obviouscleardifference was observedwatchedbetween CN-U and 5MgCN-U, indicating that loading Mg did not influence the morphology of CN-U. The TEM images (Fig. 6c, d) showsdisplaysa two2-dimensional structure consistingproperties made upof small flat sheets with wrinklesfoldingand irregular shape.strange pattern.A typical porous morphology of CN-Y and 5MgCN-Y powders is exhibited, which is in agreement with the resultsapproveof the nitrogen adsorption analysis.

In order to confirmFor provingthe structurepropertyof the preparedfixedcatalysts and tofurther identifyconfirmingthe chemical statequalityof Mg, the natureessentialityand coordinationbalanceof the carbon and nitrogen atoms in CN-U and 5MgCN-U were investigated by FT-IR and XPS techniques. Fig. 7 shows the FT-IR spectra of CN-U and 5MgCN-U. In CN-U, a seriesamountsof peaks rangingareabetween 1200 and 1600 cm-1 are assigned to the typical stretching modes of CN heterocycles, whereas the sharp peak located at 810 cm-1 is assigned to the bending vibration of heptazine rings, indicating that the synthesized g-C3N4 is composed of heptazine units. In addition, the absence of the band around 2200 cm-1 confirms that there are nowithoutC≡N bonds in the samples, indicatingspecimens, notingthe presence of 1,3,5-triazine ring. A broad band centered around 3200 cm-1 can be ascribed to the stretching vibrations of the N-H groups in the aromatic ring. The characteristic peaks of g-C3N4 do not move in 5MgCN-U curves, indicating that Mg loading did not change the structural features of CN-U.

XPS technique was conducted to get insight into the chemical composition and chemical status of the elements in the synthesized samples. Fig. 8 shows the XPS spectra of CN-U and 5MgCN-U. It should be mentioned that CN-U and MgCN-U samples showspecimens displayssimilar C 1s and N 1s spectra, indicative of similar surface compositions, indicating that the overalltotalchemical compositions on the surface of CN-U material had not changedwas the same as before. The C1s spectrum was deconvoluted into two peaks with binding energies of 284.7 and 288.1 eV, in which the peak centered at 284.7 eV is assigned to the sp2 C-C bonds, whereas the peak at 288.1 eV is attributed to the N-C=N coordination. In the N1s spectra, three peaks at aboutapproximately398.8, 400.6 and 404.3 eV can be observedwatched. The lower binding energy peak (398.8 eV) is attributed to nitrogen atoms bonded to the C atoms in aromatic rings, i.e. pyridine nitrogen. While the higher peak at 400.6 eV is assigned to N atoms trigonally bonded to sp2 carbons or bonded with the sp2 carbon atoms and hydrogen atoms, viz. tricoordinated nitrogen species and/or amino groups. Moreover, the last minute peak yet with the highest binding energy value (404.3 eV) was due to the quaternary N species and/or charging effect. MoreoverFuthermore, the last minutefinal momentpeak yet with the highestgreatestbinding energy valueimportance(404.3 eV) was due to the quaternary N species and/or charging effect. The Mg 1s peak centered at 1304.5 eV is ascribed to the presence of Mg2+.

Generally, CO2-TPD measurements are usedemployedto identifyprovethe basicity and provideofferinformation about the kindspecieand distribution of basic sites presentexsistin a material. CO2 adsorption is related toassociated withthe heterogeneity of the basic sites on the CN-U and MgCN-U. The CO2-TPD patterns of the CN-U and MgCN-U at indicatedobvioustemperatures are givendisplayedin Fig. 9. CN-U showeddisplayeda broad desorption peak in the rangeareaof 110 °C; such peak was attributedowingto the basic sites associated withrelated tochemical or even physical adsorption of acidic CO2 molecules. The CO2-TPD profile of MgCN-U consist of two overlapping CO2 desorption reflections, reachinggettingthe maximum desorption rates at aboutalmost117 °C (weak basic sites) and 309 °C (medium basic sites). Weak basic sites form on weak basic nitrogen species and mediumsecondarybasic sites adsorb on Mg-O site pairs, namely the desorption reflection to CO2 species adsorbed on nitrogen species (the lower temperature peak) and Mg-O ion pairs (the higher temperature peak).

Knoevenagel condensation reaction is one of the reactions used fora reaction employed to formatecarbon-carbon bondformation and,what,smore, ithas been widely used forgenerally employed tothe commercialbusinesspreparation of fine chemistry intermediates.This reaction is also extensively employedIt is widely applicatedas model reactiontestfor the evaluation ofestimatingsolid base catalystsas well. The catalytic properties of xMgCN-U were investigated by usingemployingKnoevenagel condensations of benzaldehyde and malononitrile as model reactions under mild conditions and the obtainedsuppliedcatalytic results are showndisplayedin Fig. 10. The catalytic activity of the pure CN-U and the blank test are also included inconstanted ofthe same figure for comparison. The results showedexhibits all catalysts havea similar behavioractionthat the benzaldehyde conversion increasedincreaseswith the increaselastingof the catalytic reaction timefor all the catalysts..It should beis worthy topoint that the blankout atest (without catalyst) gave a quite lowis called blank test which has a very smallbenzaldehyde conversion ofabout20.6%. A slight improvement in the conversion of benzaldehyde (31.7%) was obtainedofferedby using pureabsoluteCN-U as the catalyst, which may be due to the weak basic property of CN-U. After the addition of xMgCN-U catalyst, the catalytic activity was enhanced significantly. As to 3MgCN-U, the benzaldehyde conversion is 72.7%, 5MgCN-U exhibits higher catalytic activity than 3MgCN-U, giving the benzaldehyde conversion of 97.4%. However, a further increaseimprovein Mg loading amount resulted inled toa decrease of the benzaldehyde conversion. NonethelessHowever, the 10MgCN-U still showed highgoodcatalytic activityqualitywith the benzaldehyde conversion of 90.6%. This showsdisplaysthat all the xMgCN-U samplesspecimenscan effectivelyusefullycatalyze the Knoevenagel condensation reaction. And the product (benzylidene malononitrile) selectivities in the condensations are closenearto 100% for all the xMgCN-U samplesspecimens.

Knoevenagel condensation reaction is an effectiveusefuland simpler routeeasy wayto evaluateestimatethe basicityalkalinityof a material. By comparing the catalytic performance of CN-U and 5MgCN-U, it can be found that xMgCN-U showed higher catalytic activity than pure CN-U, which is attributedowingto the highersuperiorbasicity. The highersuperiorbasicity gives rise to a superior catalytic activity for 5MgCN-U catalysts, which attributedowingto the abundantrichbase sites. This can be confirmed by the obvious desorption peakDesorption peakin the correspondingresults of homologousCO2-TPDresults(Fig. 9).) proves this activity.The 5MgCN-U sample show thedisplay that the desorption peak islarger desorption peakthan CN-U, indicatingfiguring outthe more basic sites in 5MgCN-U due tobecause ofthe introduction of Mg species. Additionally, the higher catalytic activity of 5MgCN-U dependentrelingon the amountlotsofsuperficialavailable basic siteson the surface, but also relatedhave somethingtodo withthe typeskindsof nitrogen species. Indeed, it was found that 5MgCN-U has a highersuperiorcontent of tricoordinated nitrogen and/or amino groups than CN-U (XPS results in Table 2). ThusTherefore, the highersuperiorcatalytic activityqualityof 5MgCN-U than CN-U may be also attributed to its higher content of the tricoordinated nitrogen and/or amino groups on the surfaceseemingly, which is consistentin agreementwith the previous result of the reaction of benzaldehyde and malononitrile.

Fig.11 gave the benzaldehyde conversions over Mg-doped g-C3N4 composite catalysts with different carbon nitride precursor. The results clearly illustrated that the nature of precursor played an important role in determining the activity of composite catalysts. After 4 h, benzaldehyde conversions over 5MgCN-D and 5MgCN-M catalysts were only 85.7% and 77.3%, respectively, while benzaldehyde conversion over 5MgCN-U catalyst was up to 97.4%. The catalytic activities over Mg-doped g-C3N4 composite catalysts can be ranked as 5MgCN-U > 5MgCN-D> 5MgCN-M, signifying that urea was the suitable precursor for Knoevenagel condensation reaction. ItAllcan beseen that the higher the surface arearangeof the carbon supports, the higher catalytic activity for condensation, signifying that the surface area was an important factor for the activityqualityof the Mg-doped g-C3N4 composite catalysts.

For practical applications,operatingin addition toreal situation, as well asa high catalytic activity, the stability is another very important featurevaluable qualityof a good catalyst. As seen in Fig. 12, the benzaldehyde conversions of 5MgCN-U for the four cycles are all higher thanover87% with benzylidene malononitrile selectivity of about 100%, suggestingindicatinga good cyclingvery continuablestability of the materialstuffas a basic catalyst and can be reused forsustainable inKnoevenagel condensation reactions.

Novel types of solid basic catalysts based on carbon nitride doped with MgCl2 have been synthesized by using a facile thermal polymerization method. Combining the advantage of the unique chemical structure of C3N4 with the good basic property of MgCl2, the MgCN-U catalysts exhibit high catalytic performance forinKnoevenagel condensations, givingprovidingbenzaldehyde conversions of 97.4%. The catalytic activity of MgCN-U is not only related todecide byits basicitybut also dependent on,the pore structure and the diversities of the surface nitrogen species.




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