Zeolite Protonic Superacidity

Acid zeolites compared to their amorphous compositional counterparts often show several orders of magnitude enhanced intrinsic activity. Parameters invoked to rationalize this behavior are the higher concentration of Al in the respective matrices, the higher intrinsic turnovers of each of such sites, as well as the enhanced reactant concentration in the intracrystalline 

This is in line with electron withdrawal from the silanol O by EFai, leading to a decreased O-H bond strength and enhanced Br0nsted acidity [36]. An overview of potential EFai species as probed by different techniques is available [37]. However, superacidic sites in zeolites, viz. USY, compared to real superacids at low temperature are unable to activate a-bonds in alkanes. They have strengths only comparable to that of sulfuric acid [38, 39]. 

Br0nsted Acidity in Substituted Four-Coordinated Aluminophosphates 

In the compositional family of four-coordinated aluminophosphate materials, many structure types and topologies are available for the catalyst designer [2]. Among the substitution possibilities with ions of different valence, the use of Si to achieve thermally stable solids is obvious. Substitution with occurs mainly for A1 yielding neutral frameworks. With Si, isomorphic substitution 

Discrimination among rival substitution mechanisms has been attempted for several SAP04-n topologies. In SAPO-5 up to 25% of the T atoms was replaced by large siliceous patches [48], concentrating at the crystals surface [50]. During SAPO-11 crystalbzation, the ISp mechanism dominates with dipropylamine as template, while with diisopropylamine both ISp mechanisms occur [51]. 

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Figure 2 compares the results of theory and experiment for the specific case of p-fluoronitrobenzene. Inspection of the calculated structure shows that the proton is still on the zeolite, and the F shifts are more like chloroform solution than superacid solution. Furthermore, when the l F chemical shift was calculated for the theoretical structure, it was found to agree with the experimental result. 

General considerations on the mechanism of C3Hg reaction over HZSM-5 and Ga- HZSH-5. The products obtained from the reaction of C2 C5 alkanes over H-ZSM-5 zeolites were nicely interpreted (3-6) according to the classical carbenlum ion theory and the non-classical theory developed for reactions occurring in superacid media where an alkane is protonated to form the carbocation species. The general scheme proposed for propane reaction over H-ZSM-5 is ... 

Interconversion of isomeric xylenes is an important industrial process achieved by HF-BF3 or zeolite catalysts (see Section 4.5.2). Studies of xylenes and tri-and tetramethylbenzenes showed that the amount of catalyst used has a pronounced effect on the composition of isomeric mixtures.83 When treated with small amounts of HF-BF3, isomeric xylenes yield equilibrium mixtures (Table 4.2). Using a large excess of the superacid, however, o- and p-xylenes can be isomerized to m-xylene, which eventually becomes the only isomer. Methylbenzenes are well known to form stable a complexes (arenium ions) in superacids, such as HF-BF3. Since the most stable arenium ion formed in superacids is the 2,4-dimethylbenzenium ion (proto-nated m-xylene, 5), all other isomers rapidly rearrange into this ion. The equilibrium concentration of protonated m-xylene in the acidic phase, consequently, approaches 100%. 

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 order to protonate a molecule the acidic OH band has to be heterolytically broken. Not withstanding its acidic character, the energy cost to deprotonate a zeolite side is of the order of 1250 kj/gat. This is one of the main reasons why energetically protonation reactions in zeolites are quite different from reactions in superacids. 

This section describes elementary reaction steps and reaction chemistry of proton activated alkane reactions as understood mainly from studies in superacids. The reaction steps and reaction intermediates are also useful to consider in zeolite catalysis. However there is an important difference. Whereas carbonium-ion and carbenium-ion in superacids are usually stable intermediates, in zeolites they are highly activated states often corresponding to transition states [53]. 

The difference between liquid superacid catalysis and zeolite catalysis relates essentially to the fact that in liquids ionic protonated intermediates are formed, e.g. 

For skeletal rearrangements over zeolite, the nonclassical protonated cyclopropane intermediate could account for the experimental observations. Theoretical studies of the reaction mechanism indicated that protonated cyclopropane-type species do not appear as intermediates but rather as transition states. Considering all zeolite-catalyzed hydrocarbon reactions (hydride transfer, alkylation, disproportionation, dehydrogenation), only carbocations in which the positive charge is delocalized or sterically inaccessible to framework oxygens can exist as free reaction intermediates. In theoretical studies on the mechanism of the superacid-catalyzed isomerization of n-alkanes (ab initio and DFT calculations), protonated cyclopropanes were found to be transition states for the branching of both the 2-butyl cation and the 2-pentyl cation. ... 

Stabilization ( 900 kJ/mol) with the negative charge that develops on the zeolite framework when the proton is transferred. The major difference between superacids and zeolites is that in superacids cations and anions become solvated by the polar solvent molecules. No such stabilization occurs in zeolites. The chemistry in zeolites, when applied in hydrocarbon catalysis, is much closer to that in a vacuum (dielectric constant s 4) than to that in a polar medium such as a polar superacid (dielectric constant e 80-100). 

Inspired by the chemistry in superacidic media, it has been speculated that zeolites may be superacids and able to protonate even saturated hydrocarbon molecules to yield carbonium ions as a first step in catalytic cracking. Later, doubts have been raised as to whether carbenium ions obtained by protonation of unsatu-... 

An obvious candidate for a stable noncyclic carbenium ion is the tert-butyl cation observed in superacidic media. Even if the proton affinity of isobutene (Table 22.1) does not make it very likely that tert-butyl cations will exist in zeolites, several quantum chemical studies have localized stationary points for tert-butyl cations in zeolite and found that they are less stable than the adsorption complex, but are similar in stability to surface butoxides. Because of technical limitations vibrational analysis, which could prove that this cation is a local minimum on the potential energy surface, that is a metastable species, have only recently been made. Within a periodic DFT study of isobutene/H-FER a complete vibrational analysis for all atoms in the unit cell was made [48], and as part of a hybrid QM/MNDO study on an embedded cluster model of isobutene/H-MOR a vibrational analysis was made with a limited number of atoms [49]. Both reached the... 

Ihe activation of hydrocarbons over zeolites is widely held to result from direct protonation at C-C or at C-H bonds (16) (17) as preposed for reaction in superacid media (18) (19). Present results (14) are exettplified by Fig (11) and Table 2. Frctti the limiting slopes of plots of weight selectivity against conversion (20) the products at zero conversion may be estimated (Scheme 1). 

Zeolites.- Zeolites with high silicon to aluminium ratio such as H-mordenite or H-ZSM-5 are sometimes considered as superacids. The reason for such classification is that the BreSnsted centres of the zeolites act in a similar way to protons in superacid solution. It is however, necessary to point out that such centres, in spite of certain similarity to superacid protons, are less active. n-Alkane reaction takes place in the presence of zeolites at temperatures above 523K. Hydroxyl groups interacting with aluminium polymeric compounds (AlO) are responsible for... 

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. 

Aluminium ions dislodged by mild hydrothermal treatment could modify the acidity. This could lead to "superacidity". Recent proton NMR studies, however, show no enhanced Brdnsted acidity. On the other hand, IR shows new hydroxyl bands (which may be poisoned by Na) as clear evidence for very strong Brdnsted acidity. Whatever the interpretation, we know that mild hydrothermal treatment can increase acidity in several types of zeolites. 

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