Synthesis,Characterization And Application Of Hybrid Nano Gold-Polyoxometalate Supported Catalytic Materials For Green Epoxidation Of Olefins By Molecular Oxygen

Even though olefin epoxidation is an important industrial reaction from many years,the research into new routes towards epoxides continues,because of their incorporation into a wide variety of applications in the modern life as the products of epoxidation are valuable intermediates for production of pharmaceuticals,fine and heavy chemical industry.Traditionally,epoxidation is achieved through chlorohydrin process,organic peroxides or peracids.These processes are not favorable both economically and environmentally because of high cost,using harmful oxidants and release of organic waste.On the contrary,catalytic epoxidation with oxidants such as H₂O₂ and O₂ is rewarding as these oxidants have a high content of active oxygen species,produce water as the by-product and are cost effective as compared to organic peracids or peroxides.Indeed,O₂ would be the ideal oxidant for all kinds of oxidation reactions in view of green chemistry if such catalytic systems can be developed which can activate O₂ successfully as the sole oxidant leaving scientists inquisitive about developing new catalytic materials to date.Molecular O₂ is quite unreactive toward olefins epoxidation with the catalysts reported especially under mild reaction conditions.Therefore,green catalytic epoxidation of olefins by molecular O₂ is a great challenge.Since the high activity of supported nano gold has come to light for CO oxidation at low temperature,gold nanoparticles have been used extensively as catalytic materials.Catalytic oxidation using nano gold catalysts is significant for the development of green processes using eco-friendly oxidants such as pure O₂ activated by stable,selective and non-toxic heterogeneous catalysts.The ability of nano gold to act as an oxidation catalyst is seen to be depending on the size where the catalytic activity weakens with an increase in size.The sintering of nano gold is a key issue and many supports have been studied to be able to keep nanoparticles from agglomerating.Polyoxometalates?POMs?which are metal oxygen anion clusters where Keggin type lacunary POMs {XM₁₁O₃₉^?n+4?-,XM₁₀O₃₂^?n+5?-,where X=Si,P;n=4,3;M =Mo???,W???} have continued to attract attention in particular as a result of their unique catalytic abilities,show potential in being a stabilizer for nano gold particles?Au NPs?.Their anionic charge repulsion from each other prevents NPs from sintering when used as a stabilizer for nano gold.Apart from this ability,their own catalytic ability for oxidation reactions is interesting too.The integration of different functional units into one hybrid system may lead to desired catalytic materials.Hence,in this thesis,we have focused on the development of such catalytic systems where the catalytic abilities of nano gold and POMs can be realized along with the stabilizing effect of POMs towards Au NPs on different supports called as hybrid nano gold-polyoxometalate supported catalytic materials.It includes syntheses,characterization and application of these catalytic materials for green epoxidation of olefins using molecular O₂ as an oxidant.These catalytic systems and their catalytic perfomances and properties are as follows:In the first project,a heterogeneous POM-gold catalyst named as Au/BW₁₁/Al₂O₃ was developed.EDS analysis and ICP-OES were carried out to determine the elemental composition of BW₁₁ and the spherical morphology of the support after dispersion of BW₁₁ and Au NPs was confirmed by SEM.XRD patterns brought out the crystalline nature of the materials and the characteristic diffraction peaks were indexed as the?1 11?,?2 2 0?and?3 11?planes of Au.The FT-IR spectrum displayed characteristic peaks at 706 cm^-1,892 cm^-1 attributed to stretching vibrational bands of W-O-W and W=O stretching and supported the retention of Keggin type crystal structure of BW₁₁ after immobilization on an Al₂O₃ surface.In the BET studies,a Type-?isotherm was seen where the catalyst was found to have a better adsorption capacity at high pressure.The reaction was looked into at different calcination temperatures?200 to 500??to optimize the catalyst preparation conditions where increasing the calcination temperature improved the conversion and selectivity.The catalyst was tested for solvent-free epoxidation of cyclooctene under mild reaction conditions using molecular O₂ as an oxidant and TBHP as a reaction initiator.A temperature of 80?was considered as the optimum reaction temperature and 24 hours as the optimum reaction time where the conversion of cyclooctene enhanced fairly from 33.42%to 42.01%when the catalyst amount was increased up to 0.2g.Conversion of cyclooctene also improved significantly by increasing the gold content from 0.2%to 0.7%.It was evidenced while testing the effect of other oxidants on the catalytic performance that TBHP was the best and NaIO4 the most undesirable oxidant in terms of high conversion and selectivity.The conversion and selectivity profile was substantial when observed in the absence of any solvent.The catalyst was recyclable up to three cycles with no substantial loss in selectivity for epoxide.The catalyst showed a good conversion of 41.36%and 87.02%selectivity for epoxide in the absence of any organic solvents under the reaction conditions;0.15g of catalyst calcinated at 500?,5g cyclooctene,0.01g TBHP,a temperature of 80?,reaction time of 24 hours and an O₂ pressure of 0.4 MPa.Secondly,two heterogeneous catalysts named as GaW₁₁-APTES@SiO₂ and Au/GaW₁₁-APTES@SiO₂ were fabricated by means of lacunary Keggin-type Na9GaW₁₁O39?GaWi11?as the primary POM.A heterogeneous hybrid catalyst GaW₁₁-APTES@SiO₂ comprising of GaWi11 supported on nano-sized silica through an organic linker?3-Aminopropyl?triethoxysilane?APTES?was prepared.In addition,Au/GaW₁₁-APTES@SiO₂,was prepared by the wet deposition of nano gold on GaW₁₁-APTES@SiO₂.The POM moiety was confirmed by EDS while SEM showed a support comprising of uniform nano-sized silica particles.The TEM images of Au/GaW₁₁-APTES@SiO₂ manifested the successful dispersion of Au NPs on the surface of GaW₁₁-APTES@SiO₂.Comparison of XRD pattern of GaW₁₁-APTES@SiO₂ with Au/GaW₁₁-APTES@SiO₂ affirmed it further.The FT-IR spectra of the catalysts GaW₁₁-APTES@SiO₂ and Au/GaW₁₁-APTES@SiO₂ before and after the catalytic were also provided.XPS spectra were also displayed to support the successful formation of Au/GaW₁₁-APTES@SiO₂.The porous textures of the catalysts were inquired by adsorption-desorption isotherms.The effect of different reaction parameters such as the amount of catalyst,reaction temperature and reaction time on the conversion of cyclohexene was examined.Conversion of cyclohexene improved significantly by increasing the gold content of Au/GaW₁₁-APTES@SiO₂ from 0.5%to 1%.Both these catalysts worked well with H₂O₂,tert-BuOOH or molecular O₂ as oxidants with significant variation towards oxidized products selectivity.GaW₁₁-APTES@SiO₂ showed a high conversion of up to 62.02%with a selectivity of 59.13%towards epoxide at a mild temperature of 50??a catalyst amount of 0.06g,cyclohexene 2g,reaction time 24 hours,and O₂ pressure 0.4 MPa?.At 80?,Au/GaW₁₁-APTES@SiO₂ showed a conversion of 69.32%with 57.34%selectivity to 2-cyclohexene-l-ol?0.02g catalyst,2g cyclohexene,reaction time of 24 hours and oxygen pressure of 0.4 MPa?.The heterogeneous catalysts were recyclable with no significant loss in conversion or selectivity for the oxidized products.In the third project,a hybrid catalyst containing a POM,Au NPs and SB A-15 was prepared namely Au/PMo₁₁/SBA-15.Before immobilization of the POM,SBA-15 support was functionalized by chloropropyl-triethoxysilane?CPTES?.The functionalized support material was utilized to intercalate both undecamolybdophosphate(PMo₁₁)and Au NPs.Also,PMo₁₁ and Au NPs were immobilized on the support separately so as to have a comparison between their catalytic performance for mild cyclohexene epoxidation using molecular O₂ as an oxidant.The fibrous morphology of SBA support was confirmed by SEM where it persisted after functionalization,and POM and Au NPs immobilization.The hexagonal ordered channel structure of parent SBA-15 support was maintained in the designated hybrid materials and Au NPS were clearly visible in the obtained TEM.XRD patterns were obtained for SBA-15,PMo₁₁/SBA-15,Au/SBA-15 and Au/PMo₁₁/SBA-15 as well.Likewise,the FT-IR spectra of Au/PMo₁₁/SBA-15 presented strong absorbed bands in the region of 2000-400 cm^-1,attributed to the characteristic vibrations of Si-O-Si bonds.For SBA-15,the peaks at?800?and?1090?cm^-1 corresponded to the rocking,bending?or symmetric stretching?and asymmetric stretching modes,respectively,of the inter tetrahedral oxygen atoms in the SiO₂ structure.Nitrogen gas porosimetry was performed to analyze the mesoporosity of the tested materials.All the samples delivered type IV nitrogen sorption isotherms with type H1 hysteresis loops which are typical for mesoporous materials having cylindrical porous channels with an average pore diameter of 5.602 nm for Au/PMo₁₁/SBA-15.All the prepared catalytic materials were active in cyclohexene epoxidation with variation in their catalytic activity.Reaction optimization was done for Au/PMo₁₁/SBA catalyst with a number of parameters such as nano gold content,effect of reaction temperature,reaction time and catalyst weight.At a mild reaction temperature of 50?,1%Au/PMo₁₁/SBA gave a conversion of up to 48.17%with a selectivity of 36.17%for epoxide?0.01 g catalyst,2g cyclohexene,reaction time of 24 hours and pressure of oxygen 0.4 MPa?.The heterogeneous catalysts were recyclable free of any loss of activity and selectivity over a course of three catalytic cycles.In the fourth project,different types of heterogeneous hybrid catalytic systems were developed for aerobic epoxidation of cyclohexene by the immobilization of BW₁₁,GaW₁₁ or PMo₁₁ together with Au NPs alternately on Al₂O₃ or HAP support namely 1%Au/Al₂O₃,1%Au/HAP,2%Au/Al₂O₃,2%Au/HAP,BW₁₁/Al₂O₃,GaW₁₁/Al₂O₃,PMo₁₁/Al₂O₃,BW₁₁/HAP,GaW₁₁/HAP,PMo₁₁/HAP,1%Au/BW₁₁/Al₂O₃,2%Au/BW₁₁/Al₂O₃,1%Au/GaW₁₁/Al₂O₃,2%Au/GaW₁₁/Al₂O₃,1%Au/PMo₁₁/Al₂O₃,2%Au/PMo₁₁/Al₂O₃,1%Au/BW₁₁/HAP,2%Au/BW₁₁/HAP,1%Au/GaW₁₁/HAP,2%Au/GaW₁₁/HAP,1%Au/PMo₁₁/HAP and 2%Au/PMo₁₁/HAP.TEM images of four hybrid catalytic materials chosen as samples i.e.,1%Au/Al₂O₃,1%Au/HAP,1%Au/BW₁₁/Al₂O₃ and 1%Au/BW₁₁/HAP,were established for analyzing the size distribution and morphology.The FT-IR spectra and XRD patterns of fourteen catalytic materials were presented with the aim of determining the crystalline nature of the hybrid catalytic materials.These were 1%Au/Al₂O₃,1%Au/HAP,BW₁₁/Al₂O₃,GaW₁₁/Al₂O₃,PMo₁₁/Al₂O₃,BW₁₁/HAP,GaW₁₁/HAP,PMo₁₁/HAP,1%Au/BW₁₁/Al₂O₃,1%Au/GaW₁₁/Al₂O₃,1%Au/PMo₁₁/Al₂O₃,1%Au/BW₁₁/HAP,1%Au/GaW₁₁/HAP and 1%Au/PMo₁₁/HAP?mild temperature,absence of solvents,molecular O₂ as the oxidant?respectively.The results of the catalytic performance of all of these hybrid materials was displayed after carrying out the olefins epoxidation?cyclohexene and cyclooctene?reaction under similar reaction conditions.All the catalytic materials behaved differently for the epoxidation reaction keeping all the parameters constant and same.The variances between the catalytic activity of 1 and 2%Au/Al₂O₃,and 1 and 2%Au/HAP validated that a slight change in the nano gold species content in the catalytic cycle could alter the catalytic performance.Most of the catalysts contributed to a good conversion with substantial epoxide selectivity,for instance 1%Au/GaW₁₁/Al₂O₃ and 1%Au/GaW₁₁/HAP.Quite the reverse,both the Au/BW₁₁/Al₂O₃ catalysts achieved a very high conversion,but with reduced epoxide selectivity.When supported on HAP,the same materials(Au/BW₁₁/HAP)furnished a sound conversion accompanied by a marked increase in epoxide selectivity contrary to their Al₂O₃ supported counter parts.The catalytic performance was optimized by changing reaction parameters of time and temperature.1%Au/GaW₁₁/Al₂O₃ gave very good results for cyclohexene conversion up to 91.31%and selectivity for epoxide 28.52%?0.05g catalyst,2g cyclohexene,pressure of oxygen 0.4 MPa,80?temperature and 24 hours of reaction time?.The same catalyst also gave a cyclohexene epoxide selectivity as high as 35.25%for 28.41%conversion under given conditions;0.05g catalyst,2g cyclohexene,pressure of oxygen 0.4 MPa,at 50?temperature and reaction time of 6 hours.Also for cyclooctene the same catalyst gave an excellent epoxide selectivity of 91.64%with a good conversion of 47.25%?with 0.05g catalyst,1.6g cyclooctene,O.Olg TBHP,pressure of oxygen 0.4 MPa,at 80?temperature and reaction time of 24 hours?.Under the same reaction conditions,2%Au/BW₁₁/Al₂O₃ also gave a cyclooctene conversion of 40.23%with an epoxide selectivity 86.65%.The proposed mechanistic pathways for the catalysts studied are also presented.In conclusion,the catalytic activity of heterogeneous POMs and Au NPs,POMs'stabilizing effect on the Au NPs in conjunction with high surface area influence of the supports on the catalytic performance was observed for all these hybrid catalytic materials.The ability of these hybrid materials towards molecular O₂ activation is analyzed.These catalytic materials performed very well towards the activation of olefins for oxidation as well as molecular O₂ as an oxidant under mild reaction conditions following all the principles of green chemistry.The catalytic mechanisms of the new catalytic materials might be explained using catalysis theory.Many of these catalysts prove to be alternatives to traditionally used catalysts and employing molecular O₂ as the ideal oxidant in the industry hopefully in future.