A Study On The Modification Of HTiNbO₅ Nanosheets And Their Catalytic Performances In Photodegradation And Alcoholysis Of Styrene Oxide

Photocatalytic degradation of organic contaminants is an advanced water treatment technology.Since this method possesses many advantages such as environmentally-friendly,low energy consumption and low-cost,it has attracted increasing attention in recent years.To develop novel and efficient photocatalysts is one of important ways to solve the increasingly serious water pollution.Epoxide ring-opening reaction can generate specific aminolysis,alcoholysis and hydrolysis products as important pharmaceutical intermediates.The traditional homogeneous acid catalysts have many drawbacks including difficulty to separate the catalyst from the product and unpurity of the target product.Therefore,it is meaningful to develop efficient heterogeneous acid catalysts and improve product selectivity.Layered transition metal oxides have aroused widespread concern because of their unique structures and properties.Among them,layered HTiNb05 is made up of NbO6 and TiO6 octahedra,and the band gap is 2.9 eV.Thus it has potential photocatalytic applications.In addition,[TiNbO5]-nanosheets from the delamination of HTiNbO5 could be reassembled with specific positively charged ions/nanoparticles,leading to the formation of nanocomposite materials with a large specific surface area and pore volume.This is helpful for the traditional heterogeneous catalytic reactions.In view of these facts,HTiNbO5 and exfoliated-HTiNbO5 doped with metal and nonmetal were successfully prepared and the resulted novel nanocomposite catalysts are evaluated in photodegradation of organic pollutants under visible light irradiation and styrene epoxide ring-opening reaction under mild conditions.The nanocomposite catalysts were charaeterized in detail by X-raydiffraetion(XRD),scanning electron microscopy(SEM).transmission electron microscopy(TEM),N2 adsorption-desorption measurements,Ultraviolet-visible diffuse reflectance spectra(UV-DRS),X-ray photoelectron spectroscopy(XPS),pyridine adsorption,microcalorimetric adsorption for ammonia,Fourier transform infrared(FT-IR)spectroscopy.In addition,the possible photocatalystic and ring-openging reaction mechanisms were disussed.The main contents are as follows:(1)S-doped HTiNbO5 nanosheets were prepared via an exfoliation-flocculation-calcination process by using layered HTiNbO5 as the starting material and thiourea as sulphur resource.It was found that the resulted catalyst has both increased surface area and obvious visible light absorption compared with unexfoliated HTiNbO5 and HTiNbO5 nanosheets.The adsorption and photodegradation of rhodamine B(RhB)under visible light irradiation on the obtained catalysts were evaluated.The results showed that both HTiNbO5 nanosheets and S-doped HTiNbO5 nanosheets have a good adsorption capacity and a high visible-light photodegradation rate.By comparison,a relatively high mineralization degree of 41%was achieved for S-doped HTiNbO5 nanosheets while almost zero for HTiNbO5 nanosheets,indicating that S doping can effectively improve the photocatalytic activity of nanosheets.The possible photodegradation mechanisms were discussed.(2)Iron-doped HTiNbO5 nanosheets have been prepared via the assembly of exfoliated-HTiNbO5 nanosheets and iron sols.The obtained aggregates were calcinated at a range of 200-500 ?,resulting in a series of Fe oxides/HTiNbO5 nanosheets catalysts for the alcoholysis of styrene epoxide with different alcohols.Compared with undoped HTiNbO5 nanosheets,Iron-doped HTiNbO5 nanosheets exhibited an excellent catalytic performance with various alcohols.Besides,iron sols with different hydrolysis time of 1 h,24 h and 48 h assembled with HTiNbO5 nanosheets were also evaluated.It was found that iron sols with hydrolysis time of 1 h and the calciantion temperature of 400 ? displayed a relatively high catalytic performance.(3)Sulfur and iron co-doped HTiNbO5 nanosheets were successfully prepared by first exfoliating layered HTiNbO5 in tetrabutylammonium hydroxide(TBAOH)to obtain HTiNbO5 nanosheets,then reassembled with Fe sol and finally calcinated with thioure in air.And the resulted samples were evaluated as a novel solid acid catalyst in the alcoholysis of styrene epoxide at room temperature.Different alcohols were used as nucleophilic reagent and the results indicated that methanol was incredibly easy to accomplish.Compared with undoped,iron-doped and sulfur-doped titanoniobate nanosheets,the resulted sulfur and iron co-doped catalyst exhibited not only an excellent catalytic performance(conversion of 99%and selectivity of 100%in 1 h)but also a superior thermostability.The catalytic activities were still maintained after recycling for 3 times.