Gold-titania catalysts are attractive to produce propene oxide directly. Very low gold-loaded catalysts have a remarkably high epoxidation activity, not only per amount of gold, but also per total catalyst mass. The small gold nanoparticles, however, have a significant propene hydrogenation activity as an undesirable side reaction. In this work, we are presenting an effective way to prevent the propene hydrogenation: co-feeding a small amount of CO. Even though CO is not significantly converted, this completely blocks the propene hydrogenation reaction while it does not affect the epoxidation. An added advantage is that also the direct water formation is reduced. We will be presenting the latest results on the development of highly active low gold loaded catalysts, reaction kinetics, including operation with explosive mixtures, and improvements that can be obtained by co-feeding CO.
Propene oxide, one of the mayor commodity chemicals used in chemical industry, is in need of a new process for its production because of the disadvantages encountered with the currently available processes. These processes, most importantly the chlorohydrin and the hydroperoxide processes, will be discussed. New processes and catalysts under development for both the direct and indirect epoxidation of propene are discussed as well as the challenges that are still limiting the applications of some of those. The most important new processes and catalysts discussed are the hydrogen peroxide combination process, the high-loaded silver catalysts, the molten salt systems, and the gold-titania catalysts systems.
propene Introduction Propene oxide is a very important chemical intermediate, produced at about 6 million tons per year (2006) with demand still growing by 5 % annually. One of the important new developments for the production of propene oxide are the gold titania based epoxidation catalysts. Gold-titania catalysts can very selectively epoxidize propene at mild conditions using molecular oxygen in the presence of hydrogen as sacrificial reductant. Because of the use of sacrificial hydrogen, this reaction is best called a hydro-oxidation rather than an oxidation reaction. Gold-titania based catalyst systems have a number of disadvantages which need to be improved: the conversion levels remain low, often the catalyst stability is insufficient, and the hydrogen efficiency is low. The hydrogen efficiency (defined as the amount of propene oxide produced divided by the hydrogen consumed) is dominated by the water produced by the direct hydrogen oxidation. The low hydrogen efficiency is the most important problem that needs to be solved before this catalyst system can be applied commercially. This paper presents a kinetic study into the hydrogen oxidation. Both the behavior of gold on titania, the ‘standard’ catalysts of choice for the propene hydro-epoxidation, and the behavior of gold on silica are investigated. Gold on silica does not have any significant propene epoxidation activity and is studied to investigate the possible role of the support and as a way do de-couple the epoxidation from the water formation. For the hydro-oxidation of propene, it is commonly known that the support plays a crucial role in the reaction. Figure 2. Water production during hydrogen oxidation over 1 wt% Au/SiO2 catalyst. In one cycle propene is co-fed to the reaction mixture for 5 hours. (353 K, GHSV=9000 h) Figure 1. Propene oxide and water formation over a 1 wt% Au/TiO2 catalyst. Activity during epoxidation and hydrogen oxidation without propene. (325 K, GHSV=9000 h)
Au-Ti-SBA-15 catalysts have been synthesized using different preparation routes. Ti-SBA-15 supports with Si/Ti ratio ranging from 10 to 80 have been prepared by grafting and direct synthesis. X-ray fluorescence (XRF), X-ray diffraction (XRD), diffuse reflectance UV-visible spectroscopy (DRS), transmission electron microscopy (TEM) and nitrogen sorption were applied for the characterization of the prepared catalysts. The catalytic activities of the Au/Ti-SBA-15 materials have been investigated in the direct epoxidation of propene by using a mixture of H2 and O2 and its performance was compared with an Au/TiO2 catalyst. The activity results were then interpreted with reference to the characterization.
Recently, there has been a marked increase in the interest shown in catalysis by gold. It is now recognised that gold has unique properties as a catalyst for many reactions with pre-eminence in the oxidation of carbon monoxide. However, it is also known that supported gold catalysts can be used for other reactions involving carbon monoxide, for example the water gas shift reaction. Supported gold catalysts have also been shown to be effective for hydrogenation reactions. This paper reports the possible use of gold as a catalyst for the hydrogenation of carbon monoxide. In particular, we describe the preparation and characterisation of Au/ZnO and Au/Fe2O3 as catalysts for CO hydrogenation and for the synthesis of alcohols in particular. Alcohols including methanol, ethanol, 1-propanol, 2-propanol and 1-butanol have been successfully synthesised at 300 degrees C at a pressure of 25 bar over supported Au catalysts.
Grain size, pore content, and arrangement of pore constituents have a profound effect on acoustic and strength properties of sediments. We tested specimens containing gas hydrate, methane, and water in the pore space of coarse- and fine-grained sediments to simulate the marine environment and of frozen coarse-grained sediment to simulate permafrost conditions. The measured compressional wave velocity (Vp) changes with the extent to which the pore material cements sediment grains. Hence, for equal effective stresses, Vp is lowest in gas-charged sediments, increases for water-saturated sediments, then increases significantly for hydrate-bearing sediments because of sediment cementation provided by hydrate. Frozen sediment, effectively fully saturated and fully cemented sediment, exhibits the highest Vp. Sediment strength follows the same pattern but also shows a strong dependence on sediment grain size. For consolidation stresses associated with the upper several hundred meters of subbottom depth, pore pressures decreased during shear in coarse-grained sediments containing gas hydrate, thereby increasing strength, whereas pore pressure in fine-grained sediments typically increased during shear, which decreased strength. The presence of free gas in pore space damped the pore-pressure response during shear and reduced the strengthening effect of gas hydrate in sands.
