Abstract:

Selective hydrogenation is an essential catalytic reaction in modern industrial chemistry. For instance, butene can be used to produce many important organic chemical products, but the catalytic cracking of naphtha to produce olefins also produces some diolefins, which contain approximately 0.2%–2.0% 1,3-butadiene. The selective hydrogenation of 1,3-butadiene is a crucial step in purifying single olefins and prevents poisoning of the catalysts used in polymerization. Currently, the most common industrially employed catalysts in the reaction are palladium-based catalysts, but drawbacks associated with these include high cost and low abundance. Transition metal Ni-based catalysts have the advantages of being low cost and having high hydrogenation activity, but they are prone to excessive hydrogenation in butadiene hydrogenation reactions. This leads to reduced selectivity and the loss of monoolefins in the feed gas. In addition, Ni-based catalysts tend to accumulate carbon on the surface, which results in catalyst deactivation. Therefore, designing Ni-based catalysts with excellent catalytic performance has been an industrial research priority. Herein, we synthesized Ni₃Zn/Al₂O₃ catalysts by impregnation and achieved the alumina-supported Ni₃ZnC_0.7 structure by acetylene atmosphere treatment. Interstitial sites of the Ni₃Zn intermetallic catalyst were modified by introducing interstitial carbon atoms. This enhances the catalytic performance of the 1,3-butadiene hydrogenation reaction. X-ray diffraction and transmission electron microscopy revealed that the catalyst presents a typical Ni₃ZnC_0.7 phase. The interstitial carbon structure can suppress excessive hydrogenation, exhibiting up to 93% butene selectivity at a 98% conversion of 1,3-butadiene, which renders it superior to the Ni₃Zn/Al₂O₃ catalyst. More importantly, the selectivity to 1-butene is improved by approximately 40% compared to the Ni₃Zn/Al₂O₃ intermetallic catalyst. In addition, the Ni₃ZnC_0.7/Al₂O₃ catalyst exhibits superior and stable selectivity within a wide H₂/1,3-butadiene ratio range and can operate reliably under fluctuating conditions. CO-diffuse reflectance infrared Fourier transformed spectroscopy (CO-DRIFTS) demonstrated that coordinating the carbon atom in the interstitial site with the neighboring Ni atoms alters the electron structure of the Ni sites in the Ni₃ZnC_0.7 structure. The electrons at the surface Ni sites are transferred to the carbon atoms at the interstitial sites rendering Ni more electron-deficient and decreasing the adsorption strength of 1-butene, which inhibits the excessive hydrogenation reaction pathway. It is also noteworthy that the interstitial carbon structure can inhibit carbonaceous species formation and accumulation significantly improving the Ni₃ZnC_0.7/Al₂O₃ catalyst's stability. This work is significant for understanding the structure-performance relationship at the interstitial sites in transition metal catalysts. Furthermore, it provides new insights into the design of hydrogenation catalysts.

Key words: Ni-based catalyst, Interstitial site, Ni₃ZnC_0.7, Selective hydrogenation, Subsurface carbon