PVDF mediated fabrication of freestanding AlF3 sub-microspheres: Facile and controllable synthesis of α, β and θ-AlF3

With Al(NO3)3·9H2O as the Al precursor and PVDF as the fluorine source, freestanding AlF3 sub-microspheres were fabricated following calcination, without the involvement of corrosive HF solution. With the varying of Al(NO3)3·9H2O to PVDF weight ratio, θ-AlF3 and β-AlF3, α-AlF3 were obtained respectively. The formation mechanism, morphology, surface properties and catalytic activities for the dehydrofluorination of 1,1,1,2-tetrafluoroethane to trifluoroethylene were investigated. During the fabrication process, Al(NO3)3 interacts with O=C bond in DMF. Meanwhile, DMF further interacts with CH2 species in PVDF. Following calcination, PVDF decomposes to CO2 and HF fluorinating Al species to AlF3. The growth of AlF3 crystal and particle are confined by PVDF. Consequently, θ-AlF3, β-AlF3 and α-AlF3 are achieved. At 450 °C, HFC-134a conversion of 23% and reaction rate of 1.9 mmol g−1 h−1 are approached over θ-AlF3 catalyst. The reaction rate is almost 3 times higher than that of β-AlF3 and is 40% higher than that of high surface area α-AlF3 prepared by sol-gel method. No significant deactivation is observed in time on stream of 40 h. The stability is attributed to the interaction of AlF3 with carbon frame work. The present study provides a scalable and facile synthesis route for AlF3 spheres with controllable phase structures.