Pd/TS-1 catalyst

Pd and 0.02 wt% Pt, autoreduced under N2 flow at 150 °C. The increase in PO yield by the addition of minor amounts of Pt to a Pd/TS-1 catalyst was correlated with an increase in the fraction of Pd species, which play an important role in the reaction mechanism [26c]. 

Key Words Propylene, Propylene oxide. Liquid-phase epoxidation, Semibatch reactor. Supercritical CO2, Gas-phase epoxidation, Pd/TS-1 catalyst, Au/Ti02 catalyst, Au/Ti-Si02 catalyst, Ag catalyst, Mesoporous... 

Propylene epoxidation with O2/H2 mixture in MeOH or BuOH solvent can proceed with a selectivity above 88% over Pd/TS-1 catalyst in semibatch reactors. The addition of a small amoimt of Pt is effective to maintain cationic Pd and enhances the catalytic performance. 

On the other hand, reductive activation of O2 with H2 (in situ H2O2 production) in the liquid phase enables selective epoxidation of C3H6. Miyake [10] and Hoel-drich [11] reported that Pd/TS-1 and Pt-promoted Pd/TS-1 catalysts were active... 

FIGURE 11.10. A role of Pd in the in iitM generation of H2O2 from H2 +O2 over Pd(n)-TS-1 catalysts. S denotes the substrate and SO the oxygenated product. ... 

BASF has also claimed the use of metal-modified TS-1 catalysts [25]. Various catalyst compositions have been described, including (i) titanium or vanadium silicalites containing rare earth ions [25a] and (ii) titanium or vanadium silicalites containing noble metals (Ru, Rh, Pd, Os, Ir, Pt, Re, Au, Ag). In these systems the... 

Both Pd- and Pt-based systems are very active in propene and hydrogen conversion, but both catalyze the hydrogenation of propene. Nevertheless, an outstanding PO yield of 11.7% (selectivity 46%) vas obtained on a TS-1 catalyst loaded with 1 wt%... 

G. Jenzer, T. Mallat, M. Maciejewski, F. Eigenmann, A. Baiker, Continuous epoxidation of propylene with oxygen and hydrogen on a Pd-Pt/TS-1 catalyst, Appl. Catal. A Gen. 208 (2001) 125. 

Feasibility of propylene epoxidation has been proved with a Pd-Pt/TS-1 or TS-1 catalyst in a continuous fixed-bed reactor at high pressvure. Initial selectivity can reach 99%, but the catalyst deactivated rapidly and selectivity shifted towards methyl formate. Operation in SCCO2 had a beneficial effect on propylene oxide formation compared to N2 and other solvents, due to the improved removal of deposits from the catalyst surface. 

Titanosilicates molecular sieves, especially TS-1, have been widely studied for the selective oxidation of a variety of organic substrates, using aqueous H202. ° Recently, there have been attempts to substitute aqueous H2O2 by a mixture of H2 and O2 in the presence of metals such as Pd, Pt, Au, etc. Selectivities of 99% for propylene oxide formation from propylene were observed by Haruta and co-workers over Au-containing catalysts. We had also found that the epoxide selectivity in the epoxidation... 

TABLE 11.8. Epoxidation of allyl alcohol over TS-1 and Pd(n)-TS-l catalysts ... 

Several Technologies for In Situ HPPO with TS-1-Supported Pd Catalysts... 

ARCO Chem Tech (now Lyondell) has reported catalysts that show enhanced selectivity and yield for PO [24]. Again, the catalyst was made of Pd impregnated over TS-1, but the addition of either NH4OH, benzothiophene or triphenylphosphine in the reaction medium enabled an increase in the PO yield with respect to propene and H2. Selectivity was also improved due to the suppression of the propene hydrogenation reaction. 

The liquid-phase epoxidation of propylene with O2-H2 mixtures was first reported by Sato and Miyake of Tosoh Corp., who used BuOH as a solvent and Pd as the catalytic metal [10]. In liquid-phase epoxidation, TS-1 is usually used as a catalyst support for Pd because it exhibits the best performance for the epoxidation of propylene with H2O2 [13]. When the reaction is conducted at temperatures below 323 K, the selectivity to PO is above 90%, whereas the conversion of propylene is not sufficiently high and below 4%. 

The hydroxylation of benzene on TS-1 produces phenol as the primary product. Conversion is generally kept low, because introduction of a hydroxyl group activates the aromatic nucleus to further oxidation to hydroquinone, catechol, and eventually to tarry products (Eq. 2). Acetone, methanol, 2-butanone or just water are suitable reaction media [2,16,17]. In aqueous solution, benzoquinone was also found, in appreciable amounts, among the products. Hydroxylation of benzene with a mixture of hydrogen and oxygen, an in situ source of hydrogen peroxide, can be achieved on Pd-containing TS-1 [18]. This is, in principle, an easier route to phenol than that based on the preformed oxidant [19]. In practice, it proved less effective, because of faster catalyst decay (maximum TON... 

Wang et al. tested an MR in which titanium silicalite-1 (TS-1) was incorporated as catalyst in a Pd-based membrane. The direct hydroxylation of benzene to phenol was performed with hydrogen and oxygen as coreactants in the MR, and the effect of reactor configuration was examined. 

Such tandem approaches to selective epoxidation have been previously reported for TS-1 supported Au [144, 145] or Pd [146] nanoparticles as catalysts. These displayed good selectivity for propylene oxide formation in gas-phase reactions [147-150]. Less often, Ti-mesoporous silica materials such Ti-MCM-41 and -48 by Haruta [151] or Ti-SBA-15 by Nijhuis [152] were used. 

H2O2 on a TS-1 (titanium silicate) catalyst, which reduces this environmental impact. This requires the production of H2O2 on site, to avoid its storage and transport, which are hazardous activities. A possible route for the synthesis of H2O2 could he from H2 and O2 mixtures using Pd-hased catalysts. Proper reactor design to handle the explosive gas mixture has not been achieved so far. 

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