Pore Size Effect of Ru-Zn/ZrO₂ Catalyst on Partial Hydrogenation of Benzene to Cyclohexene

Abstract

Partial hydrogenation of benzene to cyclohexene is an important industrial process and features exceptional superiority to processes such as dehydration of cyclohexanol, dehydrogenation of cyclohexane, and the Birch reduction in terms of inexpensive feedstock, succinct reaction route and consequently, improved operational simplicity. In this work, the pore size effect on the partial hydrogenation of benzene to cyclohexene over the Ru-Zn/ZrO 2 catalysts was studied for the first time. Three ZrO 2 supports with the same tetragonal crystallographic form (t-ZrO 2 ) but different pore sizes were synthesized by the precipitation and the solvothermal methods. Using these ZrO 2 samples, the Ru-Zn/ZrO 2 catalysts were prepared by the deposition-precipitation method followed by reduction in ZnSO 4 •7H 2 O aqueous solution. The supports and catalysts were characterized by powder X-ray diffraction (XRD), N 2 physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), CO chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), temperature-programmed reduction of H 2 (H 2 -TPR), and transmission electron microscopy (TEM). It is identified that the Ru nanoparticles (NPs) on these catalysts had similar size and chemical state. In the partial hydrogenation of benzene to cyclohexene, a pronounced pore size effect of the catalyst was identified. With the increase in the pore size, while the turnover frequency (TOF) of benzene was essentially unchanged, the initial selectivity (S 0 ) to cyclohexene increased steadily. The Ru-Zn/ZrO 2 (11.7) catalyst with the ZrO 2 support having the pore size of 11.7 nm exhibited the highest S 0 (88%) and yield (54%) of cyclohexene. On the basis of the characterization results, the similarity in the TOFs of benzene on the

Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalysts: Mechanism Investigation on NaOH as a Reaction Additive

“…This is in a good agreement with the discussion before. Additionally, it was also reported by Zhou et al that the increased pore diameter might benefit the escape of cyclohexene from the Ru pores, thus improving its selectivity [30]. Figure 5 shows XPS and AES spectra of Ru-Zn (0.27) catalyst before and after hydrogenation with the addition of 0.600 mol L −1 NaOH as a reaction additive.…”

“…This is in a good agreement with the discussion before. Additionally, it was also reported by Zhou et al that the increased pore diameter might benefit the escape of cyclohexene from the Ru pores, thus improving its selectivity [30]. Table 2.…”

“…Three main roles of highly dispersed ZnO are addressed, as follows: (1) parts of electrons of Ru could be transferred to ZnO, causing the generation of Ru δ+ species of electron deficiency. It was suggested by Mazzieri et al [38] and Zhou et al [30] that Ru δ+ has weaker capability of cyclohexene adsorption in comparison to Ru, which accelerates the desorption of cyclohexene; (2) the atomic radius of Zn 2+ from highly dispersed ZnO is similar to that of Ru, which could firstly adsorbed on Ru surface and cover part of the Ru active sites that unsuitable for cyclohexene formation [39]; and, (3) a stable complex could be formed between the Zn 2+ of highly dispersed ZnO and cyclohexene, which could stabilize the synthesized cyclohexene and prohibit its further hydrogenation to cyclohexane, and thus improve the cyclohexene selectivity [40,41]. When combined with characterization as well as the hydrogenation results, it is obvious that NaOH as a reaction additive has no effect on improving the cyclohexene selectivity over monometallic Ru catalyst, and promoter ZnO with a specific amount can only feebly increase the catalytic selectivity.…”

“…Figure 1a indicates that Ru and Zn exist as metallic Ru and ZnO, respectively. It was suggested by Zhou et al that Zn exists as ZnO for Ru-Zn/ZrO 2 [30]. Yan et al also reported that Zn exists as Zn(OH) 2 for Ru-Zn/ZrO 2 [19].…”

Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalysts: Investigations on the Effect of Zn Content and ZrO2 as the Support and Dispersant

“…Commonly, ZrO 2 is selected as a proper support. For instance, Zhou et al [9] prepared Ru-Zn/ZrO 2 catalyst via the deposition-precipitation method, from which a 54% cyclohexene yield was obtained. Furthermore, Ru-Zn/ZrO 2 catalyst was prepared via a two-step impregnation method by Yan et al [1], which gave a 48.5% cyclohexene yield.…”