Selective Hydrogenation of Benzene to Cyclohexene on a Ru/Al₂O₃/Cordierite Monolithic Catalyst: Effect of Mass Transfer on the Catalytic Performance

Abstract

A Ru/Al2O3/cordierite monolithic catalyst was prepared, characterized, and examined in selective hydrogenation of benzene to cyclohexene in a monolithic fixed-bed reactor with an aqueous solution of ZnSO4. The Carberry number and Wheeler−Weisz group were calculated to analyze the effects of external and internal diffusions of H2, benzene, and cyclohexene. According to the results of calculations, the water film, solubility, and diffusion coefficients of the three reactants (H2, benzene, and cyclohexene) play important roles in the mass-transfer rate. Under proper reaction conditions, the effects of the external mass transfer of H2 and benzene on the reaction rate are negligible. For the hydrogenation of cyclohexene, the diffusion of cyclohexene from the organic phase to the catalyst is the limiting step in the presence of water, which is the most important factor for obtaining high cyclohexene selectivity. The absence of pore diffusion of the three reactants, which is attributed to the thin eggshell distribution of in the catalyst, is another important factor for the higher cyclohexene selectivity. In addition, the optimum reaction conditions were found to be 413−423 K and 5 MPa.

Preparation and Characterization of Ru/Al2O3/Cordierite Monolithic Catalysts for Selective Hydrogenation of Benzene to Cyclohexene

“…When alumina loading reaches 19 wt%, the value of g/ 2 exceeds 0.1, which indicates the diffusion resistance of cyclohexene in the catalyst pores was higher enough to give a negative influence on the selectivity. As reported in our previous paper [21], the absence of the pore diffusion resistance of cyclohexene will favor the cyclohexene selectivity due to its prompt mass-transfer rate from the inner pore of the catalyst to the surface. For the monolithic catalysts with higher alumina loadings, consecutive hydrogenation of cyclohexene was possible and relatively lower selectivity to cyclohexene was obtained.…”

“…The values of L p , r obs , D eff and C i * were calculated by the method reported in the previous paper [21]. The calculation of g/ 2 for the catalyst with alumina loading of 3.0 wt% was not considered because the mass-transfer route (L p ) was rather difficult to be evaluated due to the penetration of alumina into the cordierite substrate.…”

“…A series of Ru/Al 2 O 3 /cordierite monolithic catalysts were prepared by the method mentioned in the earlier paper [21]. Firstly, an alumina slurry was prepared by wet-milling the mixture of Al 2 O 3 ÁH 2 O, c-Al 2 O 3 , Al 2 O 3 Á3H 2 O and Al(NO 3 ) 3 Á9H 2 O in HNO 3 aqueous solution with a planetary mixer (BM-BP, Nanjing University Instrument Plant) at room temperature for 18 h. The pH of the slurries was adjusted to 4.0 with HNO 3 .…”

“…The egg-shell distribution of active component and large pores in the walls of cordierite monolith were brought forth as the important reasons for its better performances. The mass transfer in monolithic catalyst during the reaction was investigated and it was found that the external mass transfers of H 2 and benzene were significantly high because of the Taylor flow formed in the monolith channels [21]. It was also deduced that the absence of pore diffusion limitation of cyclohexene played an important role to achieve high cyclohexene selectivity.…”

Effect of method of preparation on activity of Pd/Al2O3 monolith catalysts

“…First, the support is deposited on the monolith by washcoating using a slurry of the support material with or without a binder. In the second step, the active metal is impregnated on the washcoated support (Kreutzer et al, 2001;Boger et al, 2004;Naitividad et al, 2004;Zhao et al, 2008). In multiphase reactions, the reactor performance is a combined effect of both internal and external mass transfer resistances.…”