Cobalt is a magnetism metal used in different areas, like catalysts. When these cobalt catalysts are discarded, they must be appropriately disposed of in a suitable place. This work aims to recycle and apply circular economy with the synthesis of a magnetic composite through a spent catalyst (silica/cobalt). The catalyst is composed of 93 % Co3O4 and 7 % CoO. The best parameters to perform the thermal reduction of catalysts were using a CO-CO2 atmosphere were 1 h isothermal period at 350 °C. The single CoO structure was confirmed and encapsulated in silica by the hydrolytic sol–gel method. These composites were subjected to a thermal reduction. The magnetic properties increased from a range of 0.0008–0.001 emu g-1 to 15.86–30.96 emu g-1. Thus, this work presented a viable process for recycling a spent catalyst and synthesis of a composite likely to be applied to materials to improve or add magnetism.