Facile synthesis of hydrothermal stable hierarchically macro-mesoporous hollow microspheres γ-Al2O3-graphene oxide composite: As a new efficient acid-base catalyst for transesterification reaction for biodiesel production

[Display omitted] •Alumina-hollow-microspheres- graphene oxide (GO) composites had been synthesized.•GO/alumina composites exhibited unique acid/base properties and macro-porosity.•5 wt.% GO/alumina exhibited transesterification of sunflower oil to esters of 97%.•Same catalyst showed water tolerance up to 3.0 wt.% and reusability for six times.•GO coatings improved hydrothermal stability of alumina at 200 °C for 12 h. Alumina-hollow-microspheres- Graphene oxide (GO) composites with hierarchical macro- mesoporous structure and controllable GO loadings from 2 to 20 wt% had been synthesized using a self-limiting hydrogel method. The composite with 5 wt% GO loading exhibited the best catalytic activity in sunflower oil transesterification, giving an oil conversion of 85.2% by using only 1.0 wt% catalyst to oil at 60 °C in traditional batch reactor. Furthermore, the same composite (GO(5 wt%)-alumina) exhibited higher oil conversion (97%) to methyl esters in autoclave reactor at 120 °C. The significant high activity of this composite toward oil conversion was attributed to that its macroporous structure facilitated the diffusion of products, and it has higher amounts of both acidic and basic oxygen functional groups than either the neat GO or alumina. In addition, GO(5 wt%)-alumina composite, also showed good water tolerance, as the oil conversion was varied from ca. 85.2 to 79.2% in 10 h when the water content in reaction mixture was increased from 0.0 to 3.0 wt%. Furthermore, the catalyst could be easily reused for at least six reaction cycles with loss in catalytic activity by factor of 3% and 14% for fresh and waste cooking oil respectively. The stability of this composite in liquid water at 200 °C for 12 h was also examined using variety of physicochemical techniques. It is shown that the presence of GO (5 wt%) inhibited the hydration of γ-alumina to form boehmite.