Acid strength alkanes

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered. 

The variation of cracking selectivity in the conversion of alkanes over substituted H-Ga-MFI and H-Al-MFI zeolites has been correlated with the basicity of the C-C bond of the alkane, while the selectivity toward dehydrogenation was found to be related to the attenuation of the acid strength of the zeolite [251]. 

We conclude that there is no evidence for WZ catalysts having superacidic properties or sites with the acidic character that would be necessary for initiation of catalysis by alkane protonation. In as much as WZ catalysts are some four orders of magnitude more active than zeolites for alkane isomerization,26 it is clear that there is no one-to-one correlation between acid strength of WZ and its catalytic activity. We therefore infer that although the acidity of WZ catalysts is important in alkane conversion catalysis, the reaction is most likely initiated by a reaction other than protonation of the alkane by the catalyst or a species formed from it. 

In zeolites, this barrier is even higher. As discussed in Section II.B, the lower acid strength and the interaction between the zeolitic oxygen atoms and the hydrocarbon fragments lead to the formation of alkoxides rather than carbenium ions. Thus, extra energy is needed to transform these esters into carbonium ionlike transition states. Quantum-chemical calculations of hydride transfer between C2-C4 adsorbed alkenes and free alkanes on clusters representing zeolitic acid sites led to activation energies of approximately 200 kJ/mol for isobutane/tert-butoxide (29), 230-305 kJ/mol for propane/sec-propoxide, and 240 kJ/mol for isobutane/tert-butoxide (32), 130-150 kJ/mol for ethane/ethene (63), 95-105 kJ/mol for propane/propene, 88-109 kJ/mol for isobutane/isobutylene, and... 

Another important aspect of the problem, which can also be addressed using computer simulations, has to do with the distribution of the alkane molecules over the zeolite channels. If one takes into consideration the fact that a zeolite such as ZSM-5, for instance, has 48 different acidic sites, with distinct acidic strengths, the catalytic activity of the zeolite towards the different alkanes will be certainly related to the way the substrate molecules are distributed within the zeolite network. As mentioned in the last section, the previous simulations [24,26-29,31] predicted quite distinct distributions, but considering the variety of different simulation conditions employed, no clear conclusion could be reached. On the contrary, we have used exactly the same conditions (force fields, cluster size, loading, initial distribution of molecules, etc.) with the three methodologies, except in the case of the MM calculations with a single alkane molecule. 

For the linear alkanes studied (methane, ethane and propane), the fact that only one type of acid site can be represented with the 3T and 5T clusters should not be a major problem. As shown by the MD studies, because of their sizes, steric effects are of minor importance and these molecules have equal probability of visiting aU the distinct sites of the zeolite. In another words, for these molecules, as far as steric effects are concerned, the acid sites are all alike. Thus, the interaction between any of these molecules and the zeolite will depend mainly on the sites acidic strengths, which do not differ very much from each other. Therefore, for these molecules it is a reasonable approximation to treat aU the acid sites alike. However, for isobutane steric effects are more important and the molecule should be more sensitive to the type of the acid site. It will be easier for the isobutane molecule to approach the acid sites represented by 3T and 5T clusters than the one at the channels intersection, in the real zeolite, where it preferentially adsorbs. Therefore, for isobutane and other branched alkanes (and most probably for the large n-alkanes), the chemical reactions at the 3T and 5T clusters may take place artificially easier than in the real zeolite. 

In analogy to alkynes, alkenes, and alkanes, the three compound types differ in the hybridization of the nitrogen atom. Acid strengths vary in the same way ... 

The new catalysts possess both high acid strength, and the ability to cause low temperature n-alkane isomerization. Hence, they may be considered as superacids. The catalysts which have been... 

The highest proton-donor strengths are exhibited by zeolites with the lowest concentrations of AlOY tetrahedra such as H-ZSM-5 and the ultrastable zeolite HY. These are superacids, which at high temperatures (ca. 500 °C) can even protonate alkanes. It was foimd that the acid strength depends on the number of A1 atoms that are adjacent to a silanol group. Since the A1 distribution is nonuniform, a wide range of acid strengths results. 

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