Catalysts, general pretreatment

Molybdena catalysts generally need to be activated by reduction or sulfidation in order to obtain an active catalyst for most reactions in which they are employed (except for oxidation-type reactions). Therefore, it is important to determine what changes occur in the state of the oxidized catalyst when it is subjected to these activation pretreatments. 

Before use the catalyst is generally pretreated in a stream of air containing a low concentration of sulfur dioxide. This sulfates the vanadium compounds and avoids an undesirable exothermic reaction when operation begins. However, final sulfation in the reactor does assist in start-up by increasing catalyst temperature faster than using heated feed gas and heat exchange between the beds. 

As shown in Table 3, after a pretreatment performed at 333 K, the activity of the K3P sample increased with time on stream (TOS), giving rise to a high production of dimethylhexanes (DMH) and of olefins (Cg" ). After a dehydratation performed at 423 K, the conversion of C4= and the selectivities towards TMP were initially high. As generally observed in the aliphatic alkylation reaction with solid acids, the decrease of the catalyst activity was accompanied by a concomitant decrease of the selectivity in TMP and an increase of the selectivities in DMH and olefins (C4 dimerization) indicating... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

Low temperature isomerization catalysts are of the Friedel Crafts type, such as AICI3 and AlBr3, activated with HX, and dissolved in a suitable solvent such as SbCl3. Application of these extremely acidic and corrosive systems requires special handling and disposal of the catalyst and careful pretreatment of the feed-stock to remove contaminating materials. Low temperature isomerization (< 100° C) is used mainly for isomerization of w-butane, which is generally available in sufficient purity by normal refinery processes. 

Pretreatment and activation. Following the preparation of a catalyst or following its insertion into a catalytic reactor, a catalyst is often subjected to various treatments before the start of a catalytic run. The term pretreatment may, in general, be applied to this set of treatments. In some cases the word activation is used. It implies that the material is converted into a catalyst or... 

In general, utilization of high-sulfur char as a boiler fuel does not appear to be economically attractive. Char may be utilized as a gasifier feedstock this possibility is particularly attractive when the char contains gasification catalyst used as a coal pretreatment prior to hydrocarbonization. Finally, as noted above, it is possible to optimize hydrocarbonization processes for the production of no net char. 

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