Propylene oxide (PO) is an important industrial chemical used to produce high value-added materials such as polyurethane. However, current major PO production routes involve multiple stages and require additional separation and/or purification units that increase the cost. Thus, a single-step, direct catalytic partial oxidation of propylene to PO using molecular oxygen has long been desired. The discovery by Haruta  and coworkers that nanoscale gold particles on titania supports provide a highly selective (~ 99%) route to vapor phase PO production using a mixture of propylene, oxygen, and hydrogen under ambient pressure has opened a new page in the production of PO. Early work in our laboratory demonstrated that the supports with high Ti dispersion benefit PO catalytic performance and has led us to use TS-1 (titanium silicate -1) as the support for nano-gold particles to enhance the PO production rate up to ~100 gPO/h/kgCat at 80% selectivity. However, PO catalytic performance is still short of the goal of PO selectivity of 90% at a propylene conversion higher than 10% and hydrogen selectivity greater than 50%. Furthermore, the literature shows that control of gold atom efficiency (gPO/h/gAu) in Au/TS-1 has not yet been achieved. Therefore, this work has begun with a systematic study of the effects of specific preparation conditions – the pH of the synthesis slurry (gold solution + TS-1) and the mixing time. Then the effect of the gold loading on PO reaction was investigated.
TS-1 was synthesized by using the method developed by Khomane et al. . In order to isolate the effects of gold deposition conditions on the catalytic performance of Au/TS-1, the same source of TS-1 with identical chemical/physical properties should be used. Since only small amounts (~8g) of TS-1 could be obtained for each synthesis batch, 10 batches of TS-1 were prepared, calcined to remove the template, and then mixed. Gold was deposited using the DP method as outlined by Tsubota et al. .
The bulk structure of the TS-1 was confirmed by X-ray diffraction, and the local environment of the Ti in the TS-1 was evaluated using diffuse reﬂectance ultraviolet–visible spectroscopy. Metal loadings were determined using atomic absorption spectroscopy. Catalytic activity was measured at 200 ℃ in a 10 mm diameter Pyrex reactor using 0.15 g catalyst sieved to 60-80 mesh and diluted with ~1g of 70-80 mesh quartz sand. The reactant mixture consisted of 10/10/10/70 vol % of hydrogen, oxygen, propylene and nitrogen with a total ﬂow of 35 sccm, resulting in a space velocity of 14000 mL/h/gcat.
Effects of pH – Samples of Au/TS-1 prepared at the different pHs showed, in agreement with the work of Oyama et al. , that the gold loading increases as the pH of the gold slurry decreases. In addition, we found that the PO production rate (space time yield) generally tracks the gold loading. Since catalysts prepared at pH~7-8 showed the highest PO production rate, those were chosen for the further studies of mixing time.
Effect of mixing time - When the mixing time increased from 5 h to 9.5 h, the PO rate also increased from an average value of 136 to 154 gPO/h/KgCat at 200 ºC. It should be noted that this catalyst (prepared at pH~7.5, mixed for 9.5h) shows the highest PO rate yet reported. The reproducibility of four catalysts prepared at the mixing time of 9.5h and pH ~7.5 was better than +/- 10%.
Effect of gold loading – The PO production rate (gPO/h/kgCat) as a function of gold loading (wt%) for the samples prepared at the same conditions (pH~7.5) but with different initial gold precursor concentrations showed a linear dependence up to a loading of ~0.08%, where it reached a plateau. This level of control of the gold atom efficiency for the PO reaction (PO rate / mole of gold) is unprecedented.
Gold addition by the deposition precipitation method presumes that the chlorine in chloroauric acid is sequentially replaced by OH and that the AuOH bond reacts with surface OH to anchor the gold. If we assume that the hydroxyl replacement and subsequent deposition are kinetically controlled, we can understand the beneficial effect of increased mixing time. Raising the pH has two competing effects. The higher OH concentration increases the rate of Cl replacement, but at the same time makes the TS-1 surface, which has point of zero charge in the pH 2-3 region, more negative and less amenable to adsorption of anions. The lower Au coverage at high pH suggests that the surface charge is the dominant effect.
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