Catalysts pretreatment temperature

Selectivity of n-Hexane Transformations as a Function of Catalyst Pretreatment Temperature" (155d)... 

Catalyst Catalyst pretreatment temperature (K) Surface area (m /g) Reaction temp (K) Time (h) Yield (%)... 

One of the features commonly observed for single component oxides is strong dependency of the activity and selectivity on the catalyst pretreatment temperature. Once the surfaces are exposed to air, the surfaces are immediately covered with water and CO2, and lose the catalytic activities. To reveal the basic properties on the surfaces, the oxides need to be pretreated at a high temperature to remove strongly... 

Fig. 28. Effect of catalyst pretreatment temperature on initial rates of conversion of n-butene reactants over La203 at 0°C (Rosynek et al., 1981).

Catalyst Pretreatment temperature/K Reaction time/ min Conversion/ %... 

Fig. 3. compares the ammonia conversion for nanostructured vanadia/TiOa catalysts pretreated with O2 and 100 ppm O3/O2 gases. The reactions were conducted at 348 K for 3 h. No N2O and NO byproducts were detected in the reactor outlet. It is clear from the figure that higher vanadium content is beneficial to the reaction and ozone pretreatment yields a more active catalyst. Unlike the current catalysts, which require a reaction temperature of at least 473 K, the new catalyst is able to perform at much lower temperature. Also, unlike these catalysts, complete conversion to nitrogen was achieved with the new catalysts. Table 2 shows that the reaction rate of the new catalysts compared favorably with the established catalysts. 

Figure 4.21 The La/Ai intensity ratio from LEIS spectra for a series of alumina-supported lanthanum oxide catalysts of different loadings as a function of the pretreatment temperature indicates that the lanthanum oxide spreads over the alumina surface at high temperatures (from van Leerdam el al. [53]).

Peterson and Scarrah 165) reported the transesterification of rapeseed oil by methanol in the presence of alkaline earth metal oxides and alkali metal carbonates at 333-336 K. They found that although MgO was not active for the transesterification reaction, CaO showed activity, which was enhanced by the addition of MgO. In contrast, Leclercq et al. 166) showed that the methanolysis of rapeseed oil could be carried out with MgO, although its activity depends strongly on the pretreatment temperature of this oxide. Thus, with MgO pre-treated at 823 K and a methanol to oil molar ratio of 75 at methanol reflux, a conversion of 37% with 97% selectivity to methyl esters was achieved after 1 h in a batch reactor. The authors 166) showed that the order of activity was Ba(OH)2 > MgO > NaCsX zeolite >MgAl mixed oxide. With the most active catalyst (Ba(OH)2), 81% oil conversion, with 97% selectivity to methyl esters after 1 h in a batch reactor was achieved. Gryglewicz 167) also showed that the transesterification of rapeseed oil with methanol could be catalyzed effectively by basic alkaline earth metal compounds such as calcium oxide, calcium methoxide, and barium hydroxide. Barium hydroxide was the most active catalyst, giving conversions of 75% after 30 min in a batch reactor. Calcium methoxide showed an intermediate activity, and CaO was the least active catalyst nevertheless, 95% conversion could be achieved after 2.5 h in a batch reactor. MgO and Ca(OH)2 showed no catalytic activity for rapeseed oil methanolysis. However, the transesterification reaction rate could be enhanced by the use of ultrasound as well as by introduction of an appropriate co-solvent such as THF to increase methanol solubility in the phase containing the rapeseed oil. 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

The deoitygenation reaction of ethyl stearate was carried out for 4-6 h at reaction temperatures between 270-360°C and under reactor pressure of 0-7 bar. The effect of the catalyst pretreatment and the catalyst mass as well as the influence of reaction temperature were studied. The observed products in the ethyl stearate reaction are the deoxygenation products (desired), intermediate product (fatty acid), hydrogenation products (unsaturated Et-SA) and by-products (Figure 3). 

An ESR study by Yabrov et al. [355] revealed that, at least at low V205 content (0.05—5 wt. %), vanadium forms a solid solution of V4+ and V3+ in Ti02. The samples investigated were sealed in the reactor after steady state operation of the o-xylene oxidation at 350°C. The V4+ solid solution, which is considered the active phase, is not formed by the catalyst pretreatment at high temperature, but requires the interaction of the reaction mixture as was shown by the analysis of fresh catalysts. Solid state reactions between V2Os and Ti02 were also studied by Cole et al. [89]. 

Tanabe et al. (142, 143) find that silica-titania is highly acidic and has high catalytic activity for phenol amination with ammonia and for double-bond isomerization in butenes. Its acidity determined by n-butylamine titration varies with pretreating temperature and catalyst composition. The highest acidity per unit weight of catalyst was obtained when Ti02-Si02 (1 1 molar ratio) was heated at 500°C. 

A fixed bed reactor described by ASTM Method No. D3907 was employed for catalytic testing. A sour, imported heavy gas oil with properties described in Table II was used as the feedstock. Experiments were carried out at a reactor temperature of 800°K and catalyst residence time (9) of 30 seconds. Liquid and gaseous products were analyzed with gas chromatographs. Carbonaceous deposit on the catalyst was analyzed by Carbon Determinator WR-12 (Leco Corp., St. Joseph, MI). The Weight Hourly Space Velocity (WHSV) was varied at constant catalyst contact time to generate selectivity data of various products as a function of conversion. For certain experiments, conversion was also varied by varying the catalyst pretreatment conditions. 

---