Propylene oxide (PO) is an important bulk chemical for the production of a wide variety of derivatives, among which polyether polyols and propylene glycol are the main end products.[1] The world PO production of about 5 million tons per year comes from two current industrial processes: the chlorohydrin process and the organic hydroperoxide process (Halcon method). These two methods consist of two reaction stages and produce large amounts of by-products and coproducts, which makes optimization difficult. [2] Although bulk gold was for a long time regarded as a poor catalyst, gold nanoparticles (2-5 nm) deposited on a variety of metal oxides are surprisingly active for many reactions.[3] An alternative route for synthesizing PO by reductive activation of O 2 by H 2 over nanoparticulate gold catalysts has been under investigation for the past few years. [4][5][6][7] We have estimated that the requirements for a viable industrial process are a propylene conversion of 10 %, a PO selectivity of 90 %, and a H 2 efficiency of 50 % for the H 2 O 2 formed in situ for epoxidation purposes. Recently, we reported a trimethylsilylated Ba(NO 3 ) 2 -Au/titanosilicate (Ti/Si 3:100) catalyst that thus far exhibits the highest PO yield among the gold catalysts.[8] The main hurdles for the industrial application of this catalyst are its fast deactivation caused by the accumulation of oligomerized and oxidized PO by-products around the gold nanoparticles, the lower activity of the successively regenerated catalyst, and its low H 2 efficiency. Nowadays, the development of catalysts for use on a commercial scale seeks high atom efficiencies of chemical reactions to reduce manufacturing costs and to minimize burdens on the environment. A comparison of different processes shows that the epoxidation with O 2 and H 2 at 90 % PO selectivity and 50 % H 2 efficiency gives an atom efficiency of 50 %, which is close to the 76 % PO selectivity for the epoxidation with O 2 alone (Supporting Information). This clearly demonstrates that, at present, a more feasible and rational path towards higher atom efficiency is the direct epoxidation of propylene with O 2 and H 2 .We now report that the presence of trimethylamine (TMA), a strong Lewis base with a pK a value of 9.9, [9] at extremely low concentrations (10-20 ppm) in the reactant gas mixture can remarkably improve the catalytic performance of gold nanoparticles deposited on titanosilicate in terms of catalyst lifetime, catalyst regeneration, PO selectivity, and H 2 efficiency to a level where commercial requirements are almost fulfilled. We have previously shown that a small amount of H 2 O can act as a gaseous promoter by drastically altering the catalytic activity of supported gold nanoparticles for low-temperature CO oxidation.[10] The concept behind this investigation was that gaseous trimethylamine should have a positive effect on the overall PO catalyst efficiency. One of the assumptions is that TMA can poison the Lewis acidic sites of the support, mainly isolated Ti 4+ centers. By oligome...

In situ ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy was used in combination with in situ Ti K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo species during the gas-phase epoxidation of propylene with H 2 and O 2 at reaction conditions over a Au-Ba/Ti-SiO 2 (Ti-TUD) catalyst. The in situ UV-vis measurements showed growth of a signal due to Ti-hydroperoxo species when the catalyst was put in contact with H 2 /O 2 /Ar (1/1/8) and C 3 H 6 /H 2 /O 2 /Ar (1/1/1/7) gas mixtures at 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the Ti K-edge XANES spectra of the catalyst were used to estimate the Ti-hydroperoxo species coverages (θ) under operating conditions. Transient Ti K-edge XANES experiments with H 2 /O 2 /Ar (1/1/8) and C 3 H 6 /H 2 /O 2 /Ar (1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving the determination of dθ/dt. It is shown that the Ti-hydroperoxo species are true intermediates because their initial rate of reaction measured from the in situ transient XANES data (3.4 × 10 -4 s -1 ) has the same order of magnitude as the steady-state turnover frequency for propylene epoxidation based on the total Ti (2.5 × 10 -4 s -1 ) measured in a catalytic flow reactor. This is the first use of XANES to measure the turnover rate of a catalyzed reaction.

In recent years, there have been great experimental and theoretical advances in the understanding of the epoxidation of propylene by O(2) and H(2) over Au supported on titanium-containing oxidic supports; however, thus far spectroscopic evidence of reacting species for proposed mechanisms has been lacking. Hydroperoxide species have been postulated as an intermediate responsible for the epoxidation of propylene with O(2) and H(2). In order to obtain direct evidence for the different type of active oxygen species, in situ UV-vis and EPR measurements were carried out during the epoxidation of propylene with O(2) and H(2) over a Au/Ti-SiO(2) (Ti/Si = 3:100) catalyst. It was determined that the adsorbed species of oxygen (O(2)(-)) resided on Au, more likely at a perimeter site, and it led to the formation of titanium hydroperoxo species. These results support the possible mechanism of formation of these hydroperoxo species via H(2)O(2) produced from O(2) and H(2) adsorbed on the Au surfaces.

Recebido em 29/4/03; aceito em 3/11/03; publicado na web em 27/05/04The synthesis of layered double hydroxides (LDHs) by hydrothermal-LDH reconstruction and coprecipitation methods is reviewed using a thermodynamic approach. A mixture model was used for the estimation of the thermodynamics of formation of LDHs. The synthesis and solubility of LDHs are discussed in terms of standard molar Gibbs free energy change of reaction. Data for numerous divalent and trivalent metals as well as for some monovalent and tetravalent metals that may be part of the LDH structure have been compiled. Good agreement is found between theoretical and experimental data. Diagrams and tables for the prediction of possible new LDH materials are provided.

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