Catalytic cracking bifunctional catalysts

We attempted to compare our results to earlier reports for catalytically active Si-VPI-5 samples, however, relevant results for a direct comparizon were not found. Martens et al. [6] and Kraushaar-Czametzki etal. [13] reported their results for Pt/Si-VPI-5 bifunctional catalysts in the isomerisation of n-decane [6], and in the hydroconversion of n-heptane, respectively. Davis et al. also used metals supported on VPI-5 to test catalytic reactions [12]. Our main interest were the cracking capability and the selectivity to useful fuel fractions, using the Si-VPI-5 catalysts in reactions of trans-decalin (model compound) or atmospheric residue. 

The objectives of the catalytic reforming of naphtha are to increase the naphtha octane number (petroleum refination) or to produce aromatic hydrocarbons (petrochemistry). Bifunctional catalysts that promote hydrocarbon dehydrogenation, isomerization, cracking and dehydrocyclization are used to accomplish such purposes. Together with these reactions, a carbon deposition which deactivates the catalyst takes place. This deactivation limits the industrial operation to a time which depends on the operational conditions. As this time may be very long, to study catalyst stability in laboratory, accelerated deactivation tests are required. The knowledge of the influence of operational conditions on coke deposition and on its nature, may help in the efforts to avoid its formation. 

These could refer to the main and coking reactions in catalytic cracking, or to hydrogenation and isomerization reactions in bifunctional catalysts. 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

Hydrocracking is a process that accomplishes the same goals as catalytic cracking but the presence of hydrogen and more specific catalyst often allows a much better control of the reaction and therefore results in a better distribution of products. The hydrocracker is operated at elevated pressures (several thousand psi in the case of the heavier feedstocks) and employs a bifunctional catalyst that has sites capable of promoting the hydrogenation reactions as well as the cracking reactions. 

The acidic function of bifunctional catalysts (Ft - Cu alloys in KEY and USY zeoUtes) has also been studied by microcalorimetry, using ammonia as the probe, in relation to the catalytic conversion of n-alkanes to isoalkanes or cracked products. Although the acidity and the bifunctional catalytic performances of the copper-exchanged Pt-REY are improved, these materials are much less efficient than the corresponding USY samples. The activation of USY and the reduction of the Pt - Cu-USY catalysts generate in all cases the same number of protons [259]. 

These mechanistic features were elucidated in detail in the 1960s. Based on the pioneering work of Mills et al. and Weisz ", a carbenium ion mechanism was proposed, similar to catalytic cracking plus additional hydrogenation and skeletal isomerization. More recent studies of paraffin hydrocracking over noble metal-loaded, zeolite based catalysts have concluded that the reaction mechanism is similar to that proposed earlier for amorphous, bifunctional hydrocracking catalysts. ... 

A problem with monofunctional reactions, e.g., cracking, alkylation, etc. is that they have a tendency to quickly deactivate because of coke deposition. This problem is usually not of concern with bifunctional reactions, e.g., those that employ a metal function in addition to the acid sites. However, we avoided the use of metal function because of the possible unknown modifications that could be introduced to a given sample by the metal deposition procedure. This is especially important when dealing with samples like VPI-5. Thus, to minimize the rate of deactivation, the alkylation experiments were conducted at 463 K. This low temperature introduces another problem, namely, the adsorption of reactants and products. At the experimental conditions employed here, the catalyst bed becomes saturated at time of 10 minutes or less (depending on sample). From this point onward, deactivation is clearly observable via the decrease in conversion with time. The data reported here were obtained at 11-13 minutes on-line. Since meta-diisopropylbenzene proceeds through several reaction pathways that lead to a number of products, it is most appropriate to compare the catalytic data at the constant level of conversion. Here we report selectivities at approximately 25 % conversion. For each catalyst, the results near 25 % conversion were repeated three times to ensure reproducibility. 

A completely different catalytic system for PE conversion in a fixed bed reactor has been studied by Uemichi et al.2X The catalyst consisted of activated carbon impregnated with different transition metals (Pt, Fe, Mo, Zn, Co, Ni and Cu), which led to a bifunctional material with both cracking and dehydrogena-tion/hydrogenation activity. In all cases, high conversions were obtained and the main products were linear alkanes and aromatics, with little formation of... 

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