Characterization and activity of stannum modified Hβ catalysts for transesterification of dimethyl oxalate with phenol

The transesterification of dimethyl oxalate (DMO) with phenol over stannum modified Hβ was conducted to prepare diphenyl oxalate (DPO) used to produce diphenyl carbonate (DPC). The component, structure and phase of Hβ catalysts with various Sn loadings were investigated. The relationship between the catalytic properties and the amount of Sn loadings was discussed. The catalyst of Hβ with 2 wt.% Sn loading based on metal performed best, giving 75.3% conversion of DMO and 24.0% selectivity to DPO. At Sn loadings below 2 wt.%, Sn was highly dispersed as monolayer, but at higher loadings it was crystallized into bulk tin dioxide, and the catalytic activity decreased. The curve of XPS peak intensity ratio of Sn 3d/Si 2p versus Sn loadings estimated the dispersed capacity of Sn on Hβ to be 2.1% Sn loading, which was in good agreement with the value obtained from XRD analysis. NH3-TPD results showed that the amount of Sn loading had effect only on the strength of the weak surface acid sites on Hβ. And the decrease of selectivity to anisole was not due to the change of the weak acid sites. Maybe, SnO2 accelerated to the formation of MPO and DPO. FTIR analysis of adsorbed pyridine showed that many Lewis acid sites and few Brönsted acid sites were present on Hβ catalysts with or without Sn modified. The acid sites together with SnO2 active centers catalyzed the transesterification of DMO with phenol. Especially, SnO2 active centers were in favor of the disproportionation of MPO into DPO.