Metal supported molybdenum catalysts

Propane Dehydrogenation over Supported Molybdenum Catalysts. The combined energy-dispersive (ED)-XAFS, UV-Vis, and Raman represents a powerful device that couples three spectroscopic techniques in one reactor, which probes the same part of a metal oxide catalyst under true reaction conditions and is capable of delivering subsecond time resolution. A scheme of the setup is given in Figure 32. 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Supported Rhodium Catalysts Alkali Promoters on Metal Surfaces Cobalt-Molybdenum Sulfide Hydrodesulfurization Catalysts Chromium Oxide Polymerization Catalysts... 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

TOF-SIMS images (Figs. 13.5 and 13.6) illustrate the ability to detect changes in the dispersion (uniform or presence of metal clusters) of the active phase in supported-oxide catalysts. Figure 13.5 shows nearly uniform distribution of molybdenum. The surface contamination with NH4+ ions coming from a precursor, which were not removed during the catalyst preparation process, is also observed. Cobalt clusters in the range of several micrometers are clearly visible in Fig. 13.6. 

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

The M(C0)6 (M = Cr, Mo, W) stable carbonyls have been used to prepare metal supported catalysts of elements of group 6 that have been used as catalysts in several reactions, such as metathesis, water-gas shift, CO hydrogenation and olefin hydrogenation and polymerization [15-24]. Table 8.2 compiles several examples in which M(CO)s (M = Cr, Mo, W) compounds are used as an alternative for preparing chromium-molybdenum or tungsten-based catalysts. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

This assumption stems from the fact that under the same conditions adopted to heterogenize conventional molybdenum catalysts using differently functionalized polymeric supports such as surface boronic (ref. 3) or phosphonic (ref. 4) acid groups, the Mo(VI)-fixation on a support such as (EGDA) invariably leads to a metal to acid group ratio equal to 2.0 rather than 1.0. 

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

Hydrotreating catalysts are usually alumina supported molybdenum based catalysts with cobalt or nickel promotors. By 1990, the demand for hydrotreating catalysts is expected to reach 80,000,000 pounds annually (1). The increased demand for these catalysts and the limitations on the availability and supply of the active metals increase the urgency to develop effective catalyst regeneration techniques. 

Molybdenum oxide catalysts are widely employed in oxidation, hydrodesulfur-izahon and hydrodenitrogenation reachons. The achve phase is most commonly supported on alumina, although mixed-metal oxides are also used. As for supported vanadium catalysts, both stahc and MAS NMR techniques were widely employed in early studies of such systems [125-129]. Like and Al, Mo is a... 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

As the edge sites of sulfide slabs are generally known as active sites in hydrotreating reactions [1], it seemed thus possible to us that reactions take place preferentially between sulfide edge sites and organometallic complexes, if placed in contact each other. Selective modification of edge sites could then be made by different metals like cobalt or tin. In this study, we applied SOMC to the modification of the conventional hydrotreating catalysts alumina-supported molybdenum sulfides with or without cobalt promoter. The Co and Sn... 

---