Covalently tethering disulfonic acid moieties onto polyoxometalate boosts acid strength and catalytic performance for hydroxyalkylation/alkylation reaction

Solid acid catalysts are widely used in the production of various high-value added and industrially important chemicals.Although the use of organosilicon compounds to modify the vacancy site has been extensively studied,the covalent tethering-SO_(3)H functionalized organosilicon modified polyoxometalates(POMs)has been rarely reported.In this work,two catalysts(TBA_(4)[SiW_(11)O_(39)(O(SiC_(3)H_(6)SO_(3)H)_(2))](compound 2)and TBA_(4)[SiW_(11)O_(39)(O(SiC_(8)H_(8)SO_(3)H)_(2))](compound 3))were synthesized successfully through covalently grafting different sulfonic acid(-SO_(3)H)groups onto[SiW_(11)O_(39)]^(8−)cluster,respectively.Compound 2 was achieved by surface grafting and in situ oxidation(3-mercaptopropyl)-trimethoxysilane,while compound 3 was achieved by surface grafting of 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane.Strong Brønsted acid strength of compounds 2 and 3 can be demonstrated by different methods including potentiometric titration,pyridine adsorption studies,and the ^(31)P trimethylphosphine oxides(TMPO)nuclear magnetic resonance(NMR).The systematic investigation of the relationship among POM structures,acid strength,and electron density was carried out based on density functional theory(DFT)calculation and experimental results,which revealed that covalent modification of compounds 2 and 3 decreased the electron density of the O-H bond,and promoted the release of H^(+).When applied in hydroxyalkylation/alkylation(HAA)reaction of 2-methylfuran(2-MF)with cyclohexanone,compound 2 exhibited better catalytic performance with conversion of ~93%,monocyclic fuel precursors(1a)yield of 79.9% and selectivity of 85.7% than compound 3,which can be attributed to strong Brønsted acid and the intramolecular hydrogen-bonding interactions between the neighboring -SO_(3)H moieties in compound 2.Finally,compound 2 also showed excellent catalytic activity in the HAA reaction of 2-MF with several different aldehydes and ketones(e.g.,furfuraldehyde,5-methylfurfuraldehyde,acetone,butyraldehyde and 4-methoxybenzaldehyde).This result opens a new pathway for design and fabrication of novel solid acid catalysts.