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Propylene oxide (PO) is an important industrial chemical used to produce high value-added materials such as polyurethane.  However, current major PO production routes, the chlorohydrin and hydroperoxide processes, 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 [1] 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 has opened a new page in PO production. Early work in our laboratory demonstrated that supports with high Ti dispersion benefit catalyst stability and has led us to use TS-1 (titanium silicate -1) as the support for nano-gold particles to enhance the propylene conversion to around 8% at 80% selectivity and provide stability for at least 40 hours [2]. 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%.   This current work started with a study of the effects of pH of the synthesis slurry solution and the mixing time during catalyst preparation.  Then the effect of the gold loading on PO activity was investigated.  Other strategies of improving the PO catalytic performance are ongoing.

TS-1 was synthesized by using the method developed by Khomane et al. [3].  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. [4].  Hydrogen tetrachloroaurate(III) trihydrate was dissolved in deionized water, followed by addition of  TS-1.  Then the pH was adjusted to the desired value with 1N Na₂CO_3(aq) and/or 0.3N HNO₃.  The slurry was then allowed to mix for 5 h at R.T..  For the effect of mixing time, 9.5 h was chosen for comparison.  After mixing, the solid was separated via centrifugation and washed with ~50 mL of deionized water. Finally, the catalyst was separated by centrifugation and dried in a vacuum oven at room temperature overnight.

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¨Cvisible spectroscopy. Metal loadings were determined using atomic absorption spectroscopy.  Catalytic activity was measured at 200 °æ in a 10 mm diameter Pyrex reactor using 0.15 g of 60-80 mesh catalyst diluted with  ~1g 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. [5], that the gold loading increased as the pH of the gold slurry decreased.  In addition, we found that the PO production rate (space time yield) generally tracked the gold loading.  The stability of the catalysts with lower gold loading (prepared at higher pH) was better than those with higher loading (prepared at lower pH), however. Since catalysts prepared at pH~7 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 C.    It should be noted that this catalyst (prepared at pH~7, 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 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) but with different initial gold precursor concentrations showed a linear dependence up to a loading of ~0.08wt%, where it reached a plateau. We note that this level of control of the gold atom efficiency for the PO reaction (PO rate / mole of gold) is unprecedented in the literature.

Gold addition by the deposition precipitation method presumes that the chlorine in chloroauric acid is sequentially replaced by OH and that the gold hydroxyl species react with the surface 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 can have multiple 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.  It is well established in the literature that Au agglomerate size is a factor in determining its PO activity.  The stability of the Au efficiency at controlled loadings below 0.1 wt% suggests that Au is being added in the optimal cluster size.  Higher loadings promote particle growth, decreasing both the initial specific rate and promoting sintering during reaction, which we have found to be the primary source of deactivation.

References

1. T. Hayashi, K. Tanaka, M. Haruta, J. Catal. 178 (1998) 566.

2. Taylor B, Lauterbach J, Delgass WN Appl Catal A General 291 (2005) 188.

3. R.B. Khomane, B.D. Kulkarni, A. Paraskar, S.R. Sainkar, Mater.Chem. Phys. 76 (2002) 99.

5. S. Tsubota, D.A.H. Cunningham, Y. Bando, M. Haruta, Stud. Surf. Sci.. Catal. 91 (1995) 227.

6. Lu J, Zhang X, Bravo-Sua´rez JJ, Fujitani T, Oyama ST Catal Today 147 (2009) 186.