Chromia with Alumina

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

Eischens and Selwood 176) have made a study of the activity of reduced chromia-on-alumina catalysts for the dehydrocyclization of n-heptane. The activity per unit weight of chromium was found to increase sharply at concentrations below about 5 wt. % Cr activities were measured down to 1.9 wt. % Cr concentration at which point the highest activity was observed. Selwood and Eischens concluded that this effect is due to the fact that the chromia is most dispersed at these low concentrations, in agreement with the present EPR data. However, if a two-site mechanism 177) is necessary for dehydrocyclization, the activity may drop at even lower chromium concentrations due to isolation of individual chiomium spins. 

The reactor equipment used for solution polymerizations is typically glass-lined stainless steel. An example of solution polymerization is the reaction of ethylene in isooctane with a chromia silica alumina catalyst initiator (see Figure 3.23) to form polyethylene. Typical reaction conditions for this polymerization are 150-180°C and 2.1-4.8 MPa (300-700 psi). 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

Similar reduction experiments were performed with alumina-supported chromia (Kanervo and Krause, 2001, 2002), and several complementary techniques were employed, including DRIFT, Raman, and EXAFS spectroscopies (Airaksinen et al., 2003). 

Although the first technical plants for CFC manufacturing used the Swarts catalyst exclusively, heterogeneously catalysed processes are competitive in the situations described above. Metal(III) oxides, especially chromia and alumina, are frequently used as solid catalysts. Moreover, they have often been used mixed with traces of other, usually metal(II), oxides, to prepare catalysts that have perceived advantages. 

A comparison of the UV Raman spectrum measured for coke deposited during the MTH reaction with that deposited during butane dehydrogenation catalyzed by chromia on alumina (66) shows clear differences in the spectral intensity distribution (Fig. 11). In particular, the intensity of the features in the regions 1340-1440cm and 1560 1630 cm are nearly equal for the MTH reaction. 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

Comparison of the d-values of the observed lines with those of chromia bayerite, alumina bayerite, and alumina boehmite showed that the bayerite phase in the 28.4-mol. % Cr20s gel is nearly pure alumina-hydrate, whereas the boehmite phase in the 18.4-mol. % Cr203 contained chromia in solid solution. 

Chromia-alumina catalysts exhibit certain properties which would be qualitatively predicted from a consideration of the independent properties of the parent oxides, chromia and alumina, but one also observes phenomena which can only be attributed to a synergistic interaction of the two phases. Therefore, although the principal concern of the present review is the chromia-alumina catalyst system, it will obviously be necessary to include in this discussion some mention of the intrinsic properties of alumina and chromia themselves. In addition, comparisons will be made with various other chromia catalysts such as, for example, chromia supported on silica-alumina. 

The 8-phase chromium resonance in chromia-alumina has been observed by several workers. It is dependent upon the alumina support since it does not appear in samples of chromia-silica, although it is observed in the case of chromia-silica-alumina. The isolated Cr + ions could conceivably be situated either in the bulk of the support, or on its surface. There is some evidence that both situations prevail. Nuclear magnetic resonance studies to be discussed presently (121) indicate that for impregnated chromia-alumina catalysts calcined at 500 the majority of these ions are on the surface. However, when such catalysts are heated above 600°, the 8 phase is greatly enhanced because of diffusion of chromium ions from the -phase into interior sites in the alumina lattice. The same situation arises with coprecipitated chromia-alumina catalysts (34). The S-phase resonance intensity of coprecipitated chromia-alumina was found to increase with calcination temperature, indicating an increasing three-dimensional dispersion of Cr + ions. In general, the S-phase resonance dominates the ESR spectra of chromia-alumina catalysts at low chromium concentrations, and therefore it... 

Of course, if the protective scale of chromia or alumina is not penetrated by SO2, sulphide cannot form at the scale-metal interface. This was found for Ni-20 wt% Cr, Co-35 wt% Cr and Fe-35 wt% Cr alloys exposed to pure SO2 at 900 °C and emphasizes the resistance of a chromia scale to permeation. On the other hand, alloys in the Fe-Cr-Al, Ni-Cr-Al and Co-Cr-Al systems were exposed to atmospheres in the H2-H2S-H2O system. These atmospheres had compositions that supported the formation of chromia or alumina together with the sulphides of Fe, Ni and Co at the scale-metal interface. In these cases, a protective layer of chromia or alumina that formed initially was penetrated by sulphur to form iron, nickel, and cobalt sulphides at the scale-metal interface. Furthermore, iron, nickel, and cobalt ions apparently diffused through the oxide layer to form their sulphides on the outside of the protective scale. Thus the original protective scale was sandwiched between base-metal sulphides. 

Fe-25Cr alloy was relatively free from iron. For both alloys, however, breakdown of the pre-formed Cr203 scales eventually occurred. Stott etal. (1985) showed that breakdown of pre-formed chromia and alumina scales in high-Ps, H2-H2O-H2S atmospheres is associated with the development of sulfide channels through the scales. Pre-formed alumina scales were found to provide significantly longer protection than the chromia scales. 

Soon after the introduction of the catalyst stabilized with alumina, others, containing chromia, were also being produced. Processes based on both of the new catalysts were operating from 1971 and many patents were pubUshed as shown in Table 10.12. 

While the spatial resolution of AES, XPS and SIMS continues to improve, atomic scale analysis can only be obtained by transmission electron microscopy (TEM), combined with energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS). EDX detects X-rays characteristic of the elements present and EELS probes electrons which lose energy due to their interaction with the specimen. The energy losses are characteristic of both the elements present and their chemistry. Reflection high-energy electron diffraction (RHEED) provides information on surface slmcture and crystallinity. Further details of the principles of AES, XPS, SIMS and other techniques can be found in a recent publication [1]. This chapter includes the use of AES, XPS, SIMS, RHEED and TEM to study the composition of oxides on nickel, chromia and alumina formers, silicon, gallium arsenide, indium phosphide and indium aluminum phosphide. Details of the instrumentation can be found in previous reviews [2-4]. 

As addressed above, a coating with excellent oxidation performance should exhibit a short initial and transient oxidation stage in which continuous chromia- or alumina-TGO scale can form. However, whether the initial and transient stage is short depends on the critical content of chromium or aluminium in the coating, below which less protective oxide scale forms. According to the classical Wagner theory, external scale of chromia or alumina can be exclusively formed if the content of chromium or aluminium of an alloy reaches a critical value, N, ... 

Properties Kaolin-base dRCF High purity RCF RCF with 2irconia RCF with chromia Poly-crystalline alumina... 

Finally, the reaction of vinyl chloride with hydrogen fluoride [7664-39-3], HF, over a chromia [1308-38-9], on-alumina [1344-28-1], Al O, ... 

Chromia—alumina catalysts are prepared by impregnating T-alumina shapes with a solution of chromic acid, ammonium dichromate, or chromic nitrate, followed by gentie calciaation. Ziac and copper chromites are prepared by coprecipitation and ignition, or by thermal decomposition of ziac or copper chromates, or organic amine complexes thereof. Many catalysts have spiael-like stmctures (239—242). 

---