Propylene epoxidation defects

Based on Au/TS-1, Delgass envisioned that both the simultaneous mechanism and sequential mechanism operated in parallel [152]. For the combination of Au sites and bare Ti-defect sites, propylene epoxidation proceeded by the sequential mechanism described above. However, the simultaneous mechanism dominated on Au-Ti interface sites (at least on the Au/TS-1 with a low Au loading of 0.01-0.06 wt%), where propylene epoxidation was accomplished by the attack of propylene adsorbed on H-Au-OOH species [153]. 

Even well-made TS-1 contains a small fraction of Si-vacancy defects [73,74]. Consistent with FTIR results on H2O2/TS-I [75], previous DPT calculations on nondefect (tetrapodal) and metal-vacancy defect (tripodal) Ti sites in TS-1 suggested that H2O2 attack on Ti-defect sites leads to Ti-OOH species (and water), while H2O2 attack on Ti-nondefect sites is kinetically and thermodynamically less favorable [76]. Moreover, Ti-OOH species can catalyze propylene epoxidation to PO [76-78]. Recent QM/MM calculations on adsorption of Aui 5 clusters inside the TS-1 pores suggest that the Ti-defect site is also the most favorable binding site for small Au clusters [66]. Therefore, defects in TS-1 are likely to stabilize adsorbed Au clusters and prevent sintering. 

Recently, we developed a simple method, namely solid grinding, to deposit gold clusters (l-2nm) on alkali-treated TS-1 on which many surface defects were created after KOH (or NaOH) treatment of TS-1. Gold clusters with diameters of 1-2 nm could be stabilized by these surface defects even after reaction at 200 °C for 36 h. These 1-2 nm gold clusters could catalyze the epoxidation of propylene with molecular oxygen if a small amount of water ( 2.0% in volume) was introduced to the feed gas. A PO selectivity of 52% and a C3H5 conversion of 0.88% were achieved [15]. Once water was removed from the feed gas, PO could not be produced again. 

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