Study On The Reaction And Mechanism Of Photocatalytic Oxidation Of Alcohols

As important chemical raw materials and chemicals,alcohols can be oxidized to produce a number of chemicals,such as aldehydes,ketones,acids,esters and acetals.Therefore,it is very important for the rational use of alcohols in the future field of energy and chemical industry.At the same time,the green and efficient chemical conversion process of alcohols occupies an important position in the modern chemical industry.In this paper,photocatalytic oxidation of ethanol and isopropanol were studied respectively.Specific work and conclusions are as follows:(1)According to the literature,ethanol could exhibit dehydrogenation C-C coupling behavior in ethanol-H2O under Pt/TiO2 photocatalytic condition,howerer,our research results showed that ethanol could be converted into 1,1-diethoxyethane(DEE)via an acceptorless dehydrogenated C-O coupling(ADC)reaction in organic or neat ethanol(sel.99%).The reaction medium is extremely sensitive to the photocatalytic dehydrogenated coupling reaction of alcohols.Therefore,the electron paramagnetic resonance spectrum(EPR)was employed for an in situ analysis of radical species captured during the photocatalytic processes.The EPR spectrum showed the main ethyoxyl CH3CH2O radical is approximately 70%and hydroxyl ethyl CH3CH(OH)radical is 28%in the sole ethanol system.While,in ethanol-H2O system contains 78%hydroxyl ethyl CH3CH(OH)radical,12%OH radical and the nearly disappearance of CH3CH2O radical.It is clear to understand that the effect of the reaction media to the photocatalytic reaction,which has a very important value of the reaction condition optimization and new photocatalytic acceptorless dehydrogenation coupling in the future.(2)This paper systematically studied the effects of different TiO2 crystalline structure(anatase,rutile and brookite)to photocatalytic direct conversion of ethanol to acetal.We found that the reaction rate and selectivity is strongly dependent on the composition and structure of the photocatalysts.To further clarify the effect of hydroxyl group,80%surface OH groups on P25-TiO2 were replaced by fluorine(F)via a simple ligand exchange reaction between F and surface hydroxyl groups on P25 in water.It is reported for the first time that TiO2 surface hydroxyl dominates the reaction process of the photocatalytic C-O coupling of ethanol in neat ethanol.More hydroxyl group resulted in higher activity,but hydroxyl doesn't involve in catalytic cyclic process,which made high selectivity of DEE.Here,hydroxyl plays an indirect site-holding role.In ethanol-H2O,the strong oxidant·OH radical was involved in the photocatalytic process,which made poor selectivity of 2,3-BD.This finding gives fundamental insight to the role of TiO2 surface hydroxyl during photocatalytic dehydrogenation process of alcohols,which disclose the nature role of surface function group of catalyst.The exchange between reantant/solvent and surface group of catalyst plays an important role in activity and selectivity of photocatalytic reaction.(3)Since the solar spectrum consists of 5%ultraviolet light,45%visible light and 50%infrared light,and our study of photocatalytic acceptorless dehydrogenation coupling of primary alkyl alcohols into acetals is carried out under UV irradiation.To take full advantage of solar energy,it is of great significance to explore the photocatalytic acceptorless dehydrogenation coupling of primary alkyl alcohols into acetals driven by visible light.Because it possesses excellent characteristics for photocatalysis,such as harvesting sunlight,chemical and photocatalytic stability,g-C3N4 has attracted increasing research interests.The results showed that with the amount of g-C3N4 and noble metal cocatalyst Pt increasing within a certain range,the conversion of ethanol and the reaction rate increased.Further findings indicated the H+ concentration may be beneficial to the reaction activity.Thus,we modulated the H+ concentration of reaction system by using H2SO4,which can be found the conversion of ethanol is proportional to the H+ concentration.(4)Further,we studied the photocatalytic oxidation behavior of isopropanol.To improve the efficiency of the photocatalytic process,we want to use the photo-generated holes and electron at the same time.Thus,we designed the catalyst system of isopropanol dehydrogenation-levulinic acid hydrogenative cyclization,which can be realized intermolecular hydrogen transfer efficiently.An intermolecular hydrogen transfer from isopropanol to LA can be realized efficiently under photocatalytic conditions over gold-loaded TiO2 catalysts.In this manner,isopropanol is dehydrogenated asacetone and pinacol with the total selectivity of>99%,whereas LA is hydrogenated and cyclized as GVL with the selectivity of up to 85%and the hydrogen transfer efficiency(?)up to 74%.This hydrogenation-dehydrog-enation coupling process provides an atom-economical green way for the conversion of LA into GVL.To understand the photocatalytic reaction mechanism,we monitored the radicals formed during photocatalytic processes via the electron paramagnetic resonance spectrum(EPR).On the basis of these observations and deductions,we propose that the photocatalytic hydrogenative cyclization of LA is mediated with the formation of acetylpropionyl radicals,which is further hydrogenated and cyclized as GVL.