Study On The Structure And Catalytic Performance Of Triiron(Ⅲ)-substituted Keggin-type Silicotungstates

Oxidation of alcohols to the corresponding carbonyl compounds is one of themost important organic synthesis reaction of functional group transformation, becausethey have widespread applications,such as,spices, dye and pesticidechemicalindustries. N-hexanal can be used for the organic synthesis of plasticizers, rubber, andpesticides.Conversion of n-hexanol to hexanal can produce a higher economic value,therefore the study of oxidation of hexanol is of great significance. Many traditionalinorganic oxidants and several transition metals, such as, palladium, ruthenium andmolybdenum, have been used for alcohols oxidation which inevitably bring aboutenvironmental pollution and lager economic costs,etc. H2O2is used as oxygen donorin the oxidation reaction widely because it has low prices, environment-friendlyandmild reagent with high active oxygen content and reaction byproduct is water.Recently, as the catalysts in oxidation of the alcohol, heteropoly compoundshavereceived constant attention because they show fairly high thermal stability, highsolubility in polar solvents, high oxidation potential, electrons and protonsmove/storage capacity. Meanwhile, heteropoly compounds have well defined frontierorbital HOMO-LUMO gap and the the electron of heteropoly compounds would beactive state under light-induced conditions,so heteropoly compounds could be used intreatment of waste water as effective photocatalysts. In this paper, heteropolycompounds were modified by using different counterions and applied to the catalyticoxidation reaction.The influence of different counterions on catalytic properties,structural characteristic and physicochemical propertiesof heteropoly compoundswere studied systematically.1. By adjusting the experimental conditions,Triiron(III)-substituted Keggin-typesilicotungstates[(C2H5)4N]3.25H3.75[α-SiW9{Fe(OH2)}3O37],[(C4H9)4N]3.25H3.75[α-SiW9{Fe(OH2)}3O37],[C19H42N]3.25H3.75[α-SiW9{Fe(OH2)}3O37] were synthesized withdifferent carbon chain length of the quaternary ammonium salt as counterions.UsingFT-IR, UV-vis, XRD, SEM, DTA-TG to study the structure and physicochemical properties of catalystsand the catalytic activities ofcatalystswere tested in theoxidation of n-hexanol. By introducing different counterions into catalysts had noeffect on α-Keggin-type structure, but it changed the secondary structure ofcatalysts so that the catalysts had different crystallinity and presence morphology.The catalysts with the best crystallinity and the smallest particle packing density, hadmaximum solubility during the reaction and showed the highest activity when thecounterion was (C4H9)4N+and maked sure the optimum reaction condition.2. Triiron(III)-substituted Keggin-type silicotungstates Cs7[α-SiW9{Fe(OH2)}3O37]and Cs3.25H3.75[α-SiW9{Fe(OH2)}3O37] were synthesized by using different content ofCs+as counterion. FT-IR, UV-vis, XRD, SEM were used to study the structure andphysicochemical properties.The results showed the different contents of Cs+had noeffect on structure and stability of catalysts which remained α-Keggin structure duringthe reaction.Cs3.25H3.75[α-SiW9{Fe(OH2)}3O37] has the smallest particle packing densityand high solubility so that it could contacted with hexanol more fully and have bettercatalytic activity.The mechanism of Triiron(III)-substituted Keggin-type silicotungstatescatalyzed oxidation of n-hexanol was studyed.The results demonstated that the oxidationreaction of hexanol was radical reaction mechanism and the redox centers Fe of thecatalyst participated in the oxidative dehydrogenation reaction of n-hexanol.3.Triiron(III)-substituted Keggin-type silicotungstates [TMGDH]3.25H3.75[α-SiW9{Fe(OH2)}3O37] were synthesized by using cation of a guanidinium-based ionic liquidcompound as counterion. FT-IR、XRD、BET、SEM were used to study the structureand morphology state of [TMGDH]SiW9Fe3.The catalyst with themost probable poresize of44.2nm was a water-insoluble solid particles that can adsorped cationic dyerhodamine B and methylene blue.The catalyst in the photocatalytic degradation ofmethylene blue, rhodamine B and methyl orange showed excellent catalyticperformance.The photocatalytic activities of catalyst in the degradation of differentconcentrations rhodamine solution were investigated. The results proved that thedegradation rate would gradually reduced with the increase of the initialconcentrations of the rhodamine solution.