Doped chromia catalysts

Fundamental Properties of Fluorinated Chromia 198 Attempts to Describe Active Sites on Fluorinated Chromia 199 Synthesis of 1,1,1,2-Tetrafluoroethane Over Fluorinated Chromia 200 Oxidation States of Chromium 200 Doped Chromia Catalysts 201... 

The drop-in replacement selected for the most widely used CFC refrigerant, CCljFj, was 1,1,1,2-tetrafluoroethane, CHjFCFj (Figure 7.2). One of the routes to this compound used the existing technology for the fluorinated chromia-catalyzed route to CCljFCFj followed by a hydrodechlorination step the other used a new zinc-doped chromia catalyst precursor in a two-step process to convert CHQ=CCl2 to CH2FCF3 directly [16]. The doped chromia developed is described later in this chapter. 

The use of a fluorinated zinc-doped chromia catalyst for a large-scale synthesis of CH2FCF3 [28], referred to above, was undoubtedly a factor in generating widespread interest for studying the behavior of doped chromia and supported chromia in the fluorination of... 

Similarly 5% chromia was doped on Zirconia and Thoria. After doping chromia it was found that the catalyst activity has been enhanced. The Fig.2 shows the results effect of addition of chromia doping on all the carriers viz. Alumina, Zirconia and Thoria. 

Andreev et al. [12] synthesized first row transition metal doped Fe-Cr catalysts. They synthesized 5% Zn, Mn, Cu and Co Fe-Cr catalysts by co-precipitation method. Cu- and Co-oxides proved to be catalytic promoters of interesting relevance. In the late 1980s Sud-Chemie developed Fe-Cr-Cu catalysts for the low steam to CO ratio applications [11]. Operating iron-chromia catalysts at lower steam to CO ratios leads to the formation of iron carbides and iron carbides are very active catalysts for Fisher-Tropsch reactions. Figure 2.1 compares the methane formation across the cOTiventional Fe-Cr catalysts and Cu-promoted Fe-Cr catalysts. It is seen that Cu promotion suppresses the methane formatiOTi. They proposed that Cu suppresses the C-O cleavage and prevents the formation of iron carbides. 

Idakiev et al. [15] also explained the promotional effect of copper. Initially, they investigated the effect of temperature on the activity of 5% Cu and 15% Cu-doped Fe-Cr catalysts. The CO cmiversion increases with increasing Cu loading. Also, the degree of conversimi of carbon monoxide on the unpromoted iron-chromia catalyst sharply decreases with the increase of the space velocity. On the copper oxide promoted samples, it remains almost unchanged. However,... 

Carbonates, diaryl, reactions with cyclohepta-amylose, 23 240 Carbon dioxide adsorption, 21 44 on chromia, 20 27 on gallium-doped NiO, 22 247-251 on nickel catalysts, 22 87-96 dissociative, 22 93-96... 

Other transition M(II) dopants do not produce promotional effect to match that of Zn(II), although nickel(II) nor Mg(II) doped catalysts have characteristics in common with the Zn(II) analogues. These results are in accordance with other studies of Ni(II) [88] and Mg(II) [89] as dopants for chromia. Although lightly doped Mg(II) chromias had inferior conversion of CF3CH2C1, these catalysts had longer lifetimes than chromias which were undoped. 

XPS and TEM measurements indicate that after fluorination, zinc and nickel, on the surface of these catalysts is present as ZnF2 or NiF2, respectively and these findings support the idea that the Zn(II) and Ni(II) species on doped fluorinated chromias do not become integrated into the surface structure but are, in fact, distinct phases on the surface. 

It has long been known that it is difficult to wash chromia gel free of ammonium nitrate when it is prepared by methods similar to ours (2, S). A sample of our gel was analyzed in duplicate by the Micro-Tech Laboratories, Skokie, Illinois and reported to contain carbon, 1.26, 1.45% nitrogen, 2.60, 2.68% hydrogen, 3.12, 3.23%. Distillation from a solution of sodium hydroxide liberated ammonia equivalent to 1.60% nitrogen. Distillation from 3% sulfuric acid liberated carbon dioxide equivalent to 0.28% carbon. The nature of the remaining carbon is unknown. Infrared absorption of the pellets described in Section XI,A exhibited a sharp line at 1384 cm i characteristic of both NO3 and COg. Analysis of catalysts deliberately doped with urea established that urea, if any, corresponded to less than 0.15% carbon and 0.35% nitrogen. 

Conversion of Isophorone to Metaxylenol is a well documented reaction in the patent literature. Activity and life-time of three eatalytic systems Alumina, Zirconia and Thoria with or without variable level of Chromia doping have been studied in the present work. For Zirconia and Thoria doping with Chromia results in remarkable enhancements in catalytic activities, but for alumina, the effect is less pronounced. In all these cases, there is cin optimum level of Chromia doping theat results in maximum efficiency. Although high initial activities are obsereved with all these catalysts, the rates of deactivation are fast in all cases. Spectroscopic and other physico-chemical measurements have been carried out on these catlysts. A nationalisation of the observed activities is offered on the basis of such physico-chemical data... 

Here we describe the effect of chromia doping on three different catalysts viz. alumina, zirconia and thoria. The basis for choosing these catalysts and using chromia as a doping are two fold. First literature references clearly indicate that the first two materials when doped with chromia are active for the above mentioned reaction. Thoria whose thermal stability, high selectivity in dehydration reactions are well documented, was tried to compare the activities of acidic catalysts with that of an amphoteric one.(7 8)... 

As alumina shows considerably higher initial activity (more than 95%) than Zirconia (-60%) and Thoria (-75%), attempts were made to increase the life-time of the catalyst by doping it with chromia. As already mentioned chromia was chosen on the basis of literature reports. 

The literature contains many other studies of supported oxides by adsorption microcalorimetry, and in particular oxides used for propane or isobutane dehydrogenation such as chromia supported on Z1O2 [121] or Y-AI2O3 [122], or Ca-doped chromium oxide catalysts supported on Y-AI2O3 [123]. 

---