•The synthesis of a basic solid catalytic material with strong basic sites is described.•The catalyst is prepared by a simple and low-cost method using biomass waste as the starting material.•The catalytic material is quite promising for being used as heterogeneous catalysts. The synthesis procedure of activated carbons with basic properties is optimized in order to prepare a basic solid with a high number of strong basic active sites on the surface of the carbon material. The basic solid is obtained from biomass waste (olive stone) and present appropriate porous structure for being used as heterogeneous catalyst. The acid/basic properties of the surface of the carbons are studied by pH titrations. These carbon materials are used for the isomerization of lactose to lactulose, which finds many applications in both pharmaceutical and food industries. This reaction is a route for revalorization of whey (which contains lactose). The results show that these carbon materials are quite promising for being used as heterogeneous catalysts for this reaction, avoiding the use of corrosive homogeneous catalysts and revalorizing two different biomass wastes (olive stone and whey).