Unsupported catalysts, for

In order to reveal the nature of deactivation, the potential of the catalyst slurry was continuously measured during the partial oxidation of alcohols. Cyclic voltammetric measurements [16] were also performed in the same aqueous alkaline solution with model (unsupported) catalysts for the interpretation of the potential values. The experiments revealed that the oxidation of alcohols may be divided into three groups. The basis of classifying is the oxidation state of proroot-ed catalyst and its surface coverage with hydrogen or oxygen (OH) during reaction. 

Elemental copper can be used as an unsupported catalyst for the oxidative dehydrogenation of alcohols to their respective aldehydes. There are two main reaction paths partial oxidation to formaldehyde and total oxidation to carbon dioxide, which is thermodynamically favored. The... 

In the areas of fuel production and utilization the focus is in two directions the search for an unsupported catalyst for use in direct coal liquefaction processing, and the development of catalysts capable of generating specialty l roduct chemicals from coal and petroleum feedstocks. 

Little attentions are paid on the reaction transport in the catalysts unless the pore size of catalyst is too small to geometrically confine the diffusion of reactant molecules in the pore. However, if the catalytic support consisted of active species, rather than traditional support materials, the new catalytic support could also have chemical effect with reactants. The influence of chemical effect of such active support on the transport mechanism of reactants is unknown. In recent years, some investigators proposed the unsupported catalyst for designing the new catalyst. Instead of being loaded on the catalytic support, active species are made into the mesoporous material to increase the highly catalytic activity. Though it obtains some progress, the unit catalytic performance of unsupported catalyst cannot show a proportional improvement compared with that of supported catalysts (Eijsbouts et al, 2007). 

The most active unsupported catalysts for both model reactions are the permanganate-based samples Hn7, after calcination at 300 or 600"c these samples display the highest surface area and correspond to the highest oxidation number of manganese. On the contrary, the supported catalyst samples xMn7. prepared with the same precursor, exhibit a drastic drop in the catalytic activity... 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Catalysts. Nearly aU. of the industrially significant aromatic alkylation processes of the past have been carried out in the Hquid phase with unsupported acid catalysts. For example, AlCl HF have been used commercially for at least one of the benzene alkylation processes to produce ethylbenzene (104), cumene (105), and detergent alkylates (80). Exceptions to this historical trend have been the use of a supported boron trifluoride for the production of ethylbenzene and of a soHd phosphoric acid (SPA) catalyst for the production of cumene (59,106). 

Some of these same experiments have been done using 10% Fe/Al203 rather than the fused iron catalyst (53). Figure 22 shows the result of a switch from H2 to 10% CO in H2 over a freshly reduced catalyst. Here a large initial rate of methane formation is observed and water does not appear until most of the initial peak has passed. The probable explanation for the presence of the CHi peak is that water produced by methanation is adsorbed on the initially dry y-Al203 support (100 m2/g). Thus the iron remains briefly in a relatively reduced state. For the CCI catalyst the AI2O3 promoter is not sufficient to prevent the water from rising quickly as shown in Fig. 19. The H/0 ratio on the surface is reduced, and carburization occurs more rapidly than methanation, as for the unsupported catalyst. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Mossbauer Spectroscopy. Figure 1 shows room temperature Mossbauer emission spectra of two of the unsupported Co-Mo catalysts which we have studied in the present investigation. It is observed that the MES spectra of the two catalysts are quite different. For the catalyst with the low Co/Mo ratio (0.0625), a quadrupole doublet with an isomer shift of 6=0.33 mm/s and a quadrupole splitting of AE =1.12 mm/s are observed (spectrum a). These parameters are very similar to those observed previously for the Co-Mo-S phase in other catalysts (6-9). Furthermore, the spectrum of an unsupported catalyst with Co/Mo = 0.15 is found to be essentially identical to spectrum (a). The MES spectrum (b) of the catalyst with Co/Mo =... 

It has previously been found (3., 11, 18, 31-3 ) that unsupported catalysts exhibit a HDS activity behavior quite similar to that of supported catalysts. This suggests that although the support is of importance, it does not have an essential role for creation of the active phase. Thus, it is very relevant to study unsupported catalysts, both in their own right and also as models for the more e-lusive supported catalysts. Many different explanations have been proposed to explain the similarity in behavior of unsupported and supported catalysts ( 3, 31-3b). Recently, we have observed that for both types of catalysts the HDS activity behavior can be related to the fraction of cobalt atoms present as Co-Mo-S (9-11 35). 

From the Co EXAFS results alone one cannot conclude whether the Co atoms are located at edges or basal planes but a comparison of the Co EXAFS data with the above Mo EXAFS results indicates that the edge position is the most likely one. This Co location is illustrated in Figure T For the unsupported catalysts, many of these "surface" positions may be present at internal edges (i.e., at the "domain" boundaries). Recently, direct evidence confirming the edge position has been obtained by combining MES results (to ensure that Co is present as Co-Mo-S in the samples studied) with ir spectroscopy (lU) or with analytical electron microscopy (l ) ... 

Figure 5A shows that the selectivity towards butane formation (i.e. the rate of formation of butane relative to that of the butenes) decreases as the Co/Mo ratio increases in the unsupported catalysts. Similar results have previously been reported for alumina supported Co-Mo catalysts (37, 38) and this behavior does therefore appear to be a quite general feature of Co-Mo catalysts. The large change in the selectivity is observed (Figure 5B) to be related to a greater promotion of the HDS reaction rate compared... 

Mn-promoted Fe-based Fischer-Tropsch Catalysts. 4.1.1 Unsupported Fe-Mn Fischer-Tropsch Catalysts. Iron-based F-T catalysts possess both hydrogenation and WGS activity, imposing a flexible option as a working catalyst for typically coal-derived CO-rich syngas conversion. Iron-based catalysts often contain small amounts of K and some other metals/metal oxides as promoters to improve their activity and selectivity. Mn has been widely used as one of the promoters for unsuppported Fe-based F T catalysts, particularly in promoting the production of C2 C4 olefins. ... 

General Observations Based on the Literature Survey. From the above discussions it is evident that Mn promotion has been explored in more detail for unsupported than for supported F-T catalysts. Regardless of the type of support and the type of active F-T metal, similar trends are found regarding the promotion effect of Mn. The following conclusions can be drawn from this literature survey ... 

For unsupported catalysts, where particle sizes are typically an order of magnitude larger than those for supported catalysts, the mobility of various species in the bulk structure may be of interest when considering how the bulk structure and composition are reflected in the surface properties of the particle. In addition, bulk mobility is an important consideration in the understanding of solid state reactions and phenomena such as sintering. 

Conversions and enanhoselechvihes of CPG-immobilized Ti-TADDOLates match the results observed with unsupported catalysts under similar but homogeneous condihons, a finding also observed in other situations (for references, see the next section) ... 

Polymer-attached Cp2TiCl2 has been reduced by sodium naphthalide, and the resultant species, which may contain a mixture of Ti(IV), Ti(III), and Ti(II), are more active hydrogenation catalysts for olefins than is the unsupported Cp2TiCl2 (72). Although distinct Ti-H-containing species were not identified, it has been suggested that the complete reaction occurs at one metal center, in contrast to earlier suggestions that such reductions involve a bimolecular reaction of an intermediate titanocene alkyl and a titanocene hydride (30). 

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