Construction of Ni/ZnCo₂O₄@ZnO composite metal oxide desulfurization agent and its reactive adsorption desulfurization-regeneration properties

SO_x released from the combustion of sulfur compounds in fuel oil has long been a serious environmental hazard, and there is an urgent need to limit the suifur content in gasoline to about 10×10⁻⁶ by using desuifurization technology to protect the environment. Reactive adsorption desuifurization (RADS) combines the advantages of hydrodesuifurization (HDS) and adsorption desuifurization (ADS), in which Ni/ZnO desulfurization agent has excellent RADS performance. Although Ni/ZnO desulfurization agent has been applied in large scale in industry, it still has the problems of insufficient desulfurization depth and poor regeneration performance. In this paper, metal Co was introduced into ZnO by co-precipitation-impregnation method to form composite metal oxides, and the composite metal oxide desulfurization agent with different Co contents was constructed, and its desulfurization activity and regeneration performance were investigated. The results of the desulfurization experiments show that the desulfurization performance of NZCo-x desulfurization agent after the introduction of metal Co is much better than that of NZ desulfurization agent, and its desulfurization performance shows a tendency of increasing and then decreasing with the increase of the Co introduction. Among them, NZCo-3 desuifurization agent has the most excellent desuifurization performance, and its desuifurization rate can reach 100%. The optimum operating conditions for NZCo-3 desuifurization agent were reaction temperature 300 ℃, total pressure 3 MPa, WHSV 2.2 h⁻¹, and H₂/Oil (v/v) 300, under which 100% desulfurization rate could be maintained. Systematic characterisation of the structure and properties of the desuifuriser using XRD, TEM, N₂ adsorption and desorption, XPS and H₂-TPR confirmed that a composite metal oxide desuifuriser with Ni/ZnCo₂O₄@ZnO structure was obtained. The formation of ZnCo₂O₄ in the composite metal oxide desuifuriser promotes the reduction of particle size, enhancement of dispersion and increase of specific surface area of the desuifuriser. The smaller NiO grains facilitated the reduction of NiO to Ni, generating more active sites for desuifurization. The smaller ZnO grains were favourable for adsorption of more H₂S. The XRD after the reaction showed that the formed ZnCo₂O₄ structure was able to adsorb the generated H₂S, which acted as a suifur adsorbent and improved the suifur adsorption capacity of the desuifurization agent, thus improving the desuifurization activity of the NZCo-x desuifurization agent. Finally, the evaluation results of the cyclic desulfurization experiment with NZCo-3 desulfurization agent showed that, under the reaction conditions of reaction temperature 275 ℃, reaction pressure 3 MPa, H₂/Oil (v/v) 300, and mass-air velocity of 2.2 h⁻¹, the desulfurization rate of NZCo-3 desulfurization agent could still be maintained at more than 77% after six reaction and regeneration cycles of the agent, which is only 16.51% lower than that of the fresh NZCo-3 desulfurization agent. The desulfurization rate of fresh NZ desulfurization agent was only 10.6%, which was much lower than that of NZCo-3 desulfurization agent after 6 regeneration cycles. In conclusion, the method of improving the desulfurization and regeneration performance of Ni/ZnO desulfurization agent by constructing a composite metal oxide structure in this paper provides a new idea for further designing high-performance Ni/ZnO desulfurisation adsorbents to meet the requirements of deep desulfurisation of catalytic cracking gasoline.