Bifunctional reactions

The finding of the low-temperature activation of propane on zeolite Ga/HZSM-5 indicates a bifunctional reaction mechanism 179,181. The highly dispersed gallium oxide species in close vicinity to the Bronsted acid sites promote the initial activation of propane. Derouane et al. 179,181 further showed that hydrogen inhibits the activation of propane. This effect was explained by a competitive adsorption of hydrogen on the gallium species or even by a reduction of Ga to Ga species. 

One would expect that a reaction with TCS at 623 K also would cause exclusively primary species, originating from either the main reaction (L) or the side reaction (N). Inspection of the NMR spectrum in figure 9.41 (b) shows that all silanols have disappeared. There is no band in the -100 ppm region, and this is consistent with the earlier calculated effectiveness factor of 1. The main feature still is the -36 ppm band, attributed to the primary species. However, a significant band is situated at -60 ppm, indicative for secondary species. The conclusion, based of the stoichiometry factor /, that no secondary species exist on a silica surface pretreated at 973 K, is therefore not entirely correct. Obviously, the bifunctional reaction (O) is highly improbable, so secondary reactions (M) must occur. This is only possible when a certain mobility of the surface species exists on the surface, since -on average- the distance between the surface species is too large to react with each other. 

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. 

Table IV shows a comparison of indexes derived from various test reactions. It is clear that Cl and Cl can not discriminate well between 12-membered ring molecular sieves while SI can. It is not clear at this time whether the SI will continue to increase in magnitude with larger pore materials. Also, as previously mentioned, the use of a bifunctional reaction may prove difficult for testing materials with low stability. This point may not be as important as we initially believed since we recently have shown that platinum can be impregnated into VPI-5 and n-hexane reacted at temperature as high as 800 K. The ratio 1,3,5/1,2,4-TIPB does increase with increasing pore/void size and does reveal a difference between 10-13 A sized microporous materials and the mesoporous SiO2-Al203. Thus, with further refinements this ratio may prove useful for characterizing larger pore molecular sieves.

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

The interaction of metal particles and protons can lead to electron-deficient adducts. These might catalyze classical bifunctional reactions in an unconventional manner, namely, replacing multistep, catalysis by reorganization of atoms in one step i.e., during one single residence of the reactant molecule on such a hybrid site. 

At 435°C the olefin concentration is found to be only about 0.02% at 30 atm. partial pressure of hydrogen. Thus, if we were to carry out paraffin isomerization by successive and separate steps of dehydrogenation of ri-paraffin to n-olefin, followed separately by skeletal isomerization of the n-olefin produced to iso-olefin (and subsequent rehydrogenation), the over-all conversion of such a scheme could be, at best, 0.02%. Thus, the paraffin isomerization, if accomplished in a bifunctional reaction system with a high conversion as might be described by formula (2), is an example of a nontrivial case as defined by (3) above. 

Figure 4 shows that the relative activity for n-pentane Isomerization drops linearly with the amount of carbon deposited on the acid function of the catalyst. This is so because isomerization of n-pentane is a typical bifunctional reaction controlled by the acid function of the catalyst. Hydrocracking to propane shows... 

On the other hand, there is the question of the close relationship between coke formation (catalyst stability) and aromatics cyclization (catalyst selectivity) over the acid sites present in the zeolites. On naphtha reforming, coke formation is a bifunctional reaction requiring the dehydrogenation capacity of the metallic function and the condensation capacity of the acidic function. Therefore, it is interesting to... 

Figure 2.45 Acid-base bifunctional reaction mechanism in the synthesis of cyclic carbonates over mixed oxides obtained from a Mg-Al hydrotalcite. Source Kaneda et al. [28].

The isomerization of n-alkanes over NiSMM is a bifunctional reaction,... 

Fig. 6.1 Modified bifunctional reaction scheme for reforming of C6 hydrocarbons.13 (Reprinted from J. Catalysis, 105, Liu et al., pp. 540-541, Copyright 1987 with permission from Elsevier.)...

Recently, some work has appeared which deals with reactions of heterocyclic /J-enamino esters with bifunctional reaction partners leading in two or more consecutive steps to polycyclic heterocyclic systems. Thus, allyl isothiocyanate... 

Coke Deposition on the Catalytic Functions and Their Deactivations. The acid and metal functions of the reforming catalysts are balanced to have the highest possible yield in the bifunctional reaction of paraffin dehydrocyclization. Coke is deposited on both acid and metal sites, decreasing their catalytic activities. It is important to know (under commercial conditions) the deactivation on both sites and which controls the reactions and fixes the catalyst s practical cycle. If the rate of reaction is several orders of magnitude higher on one site than on the other, the deactivation of the first site does not modify the rate of the bifunctional reaction and the deactivation of the second will control the whole reaction. If a reaction is so rapid on a catalytic site that it is under thermodynamic equilibrium, the deactivation of the site will not be noticed if the reaction is kinetically controlled, it is possible to follow the site deactivation by means of this reaction change. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

While the theory of Flory underestimates the gel point, the theory of Carothers overestimates it. For example, for the case of a trifunctional and bifunctional reaction system presented above, at equimolar reactive groups, while Flory s theory forecasts pg = 0.707, Carothers theory forecasts a value of pgei=0.833, while the experimental pgei is 0.765-0.775. Recently, however [18,19], thermodynamic reasons why the two formulas should be combined came to Kght. This led to the proposal of a very simple equation affording much higher precision than each theory alone. This simple equation also gave much better precision of all the very complex and difficult to use theories devised and presented since Flory s 1942 one ... 

Cations (alkali- or transition metals) may generate electrophile species that may effect nucleophilic attacks on molecules that are polariz(ed)/(able) in the zeolite cavities when subjected to the strong local electrostatic fields. We can encounter condensation reactions as well as substitution reactions where the active moiety is an electrophile. We may also encounter bifunctional reactions where the electrophile plays its part in a catalytic process. Usually the electrophile species is generated in association with the framework (a significantly electron rich framework such as X type zeolite and seldom highly siliceous frameworks). 

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