Bronsted protonated

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

Another example is the extensive study by Haase and Sauer, whose calculations were carried out for a methanol molecule with a number of cluster models, the most sophisticated of which was sufficiently large to represent a portion of the faujasite lattice formed by three 4-R rings and one 6-R. All structures were fully optimized at the SCF and/or MP2 level. The authors found that, for all cases studied, the IP complex was a transition state, whereas the HB complex corresponded to a minimum on the PES of the system. However, the adsorption energies for both structures were very close to one other, with a barrier for proton transfer of only a few kilojoules per mole. A strong H-bond between the Bronsted proton and the O atoms of methanol, and a long and weak H-bond between the H atom of the molecule and the O atom of the zeolite... 

Figure 30 Proton-exchange reaction between methanol molecule and the Bronsted proton of sodalite lattice. Solid and dotted lines show distances between the methanol oxygen and the methanol and zeolite protons and respectively.

This work will be continued by examining the X-ray patterns of materials that had been subjected to the heat treatment. Undoubtedly the Lewis acid form (decationated zeolite) has a different X-ray pattern than the Bronsted (protonated zeolite) because of the charge separation... 

The nature of this catalytically active zeolite can be either Bronsted (proton donor) or Lewis (electron-pair acceptor) acidic, with protons attached to the framework tetrahedra or not (Figure 7.19). 

Acetic acid sorbed on H+ZSM-5 held at 150°C was studied by tga, td/tas and FTIR. It was found that one molecule of acetic acid was sorbed per zeolite acid site with only a partial transfer of the Bronsted proton to the acetic acid. 

The Bronsted proton had been transferred to the acetic acid with two broad bands centred at -2870 and 2480 cm-1 observed. Similar bands have been observed for HgO sorbed at 80°C [9,10], but not for sorbed NH3. In the case of ammonia sorption the proton is completely transfered giving NH4+ and no broad bands are observed. The broad bands probably result from protons with a broad range of energies shared between the zeolite and the acetic acid. 

Acetic acid is strongly sorbed on to H+ZSM-5 held at 150°C. Tga showed a one-to-one association with the zeolite acid sites. The acetic acid molecule remained intact with very little or no further reaction at this temperature. FTIR spectra showed that the Bronsted protons cure only partially transfered to the acetic acid molecules and that they occupy a wide range of energy states. No behaviour was observed that could indicate the presence of basic sites. 

The effective activation energies, heff, thus obtained are shown in Fig. 3 to be functions of both the exchanged ion and the degree of substitution, even at high silica/alumlnium ratios. The latter result is not expected if the relaxation is associated with the exchanged Bronsted proton. Furthermore... 

In contrast to the loss of the Bronsted proton at 3610 cm 1, the silanol absorption at 3750 cm-1 remains unchanged, though new absorptions appear at 3790 cm-1 and 3670 cm-1. The kinetics, as shown in Fig. 5, is second-order, consistent with the requirement for pairwise dehydroxylation in order to generate H20 molecules. Such an interpretation requires mobility of the framework aluminiums. 

Fig. 5 The fraction, a, of Bronsted protons remaining as a function of anneal time at 750°C. Plotted as a-1 vs t the linearity implies second order kinetics. The same data are also plotted as In a vs t to illustrate that the kinetics is not first order.

Figure 22.5 CPMD simulation (PW 91 functional) of two HjO molecules per two Bronsted sites in H-SAPO (the other cell contains only one Bronsted site and one HjO molecule) [7], One of the Bronsted protons is residing on the SAPO-framework all the time, the distance of the other Bronsted proton to O of the nearest HjO molecule shows large variations (upper curve). The lower curve...

Cooperativity in the conduction of protons in hydrated PEMs encompasses effects including the mobility of protons via a flux of water molecules, the amphotericity (i.e. the ability to act as both a Lowry-Bronsted proton donor and acceptor) of the protogenic groups, and the motion of either the proto-genic group or side chain that facilitates the hand off or net transport of a proton. In this section we exclude our discussion to only the latter flexibility of the side chains of PFSA membranes. 

Alkanes are extremely weak bases. Nevertheless interactions with acids of both the Bronsted (proton donor) and Lewis (electron acceptor) types have been widely studied. Acid-catalysed reactions of alkanes, particularly cracking and isomerization to optimize the number of branched isomers in the Cg-Cg range, are important industrially in the upgrading of saturated hydrocarbons for fuels. 

Because strong Bronsted (proton) acids and Lewis acids can initiate styrene polymerization, other cationically polymerizable monomers can be added to the styrene-based copolymer list. Due to the facile occurrence of chain transfer processes of polymer chains with impurities, cationically prepared polystyrene-based polymers are low molecular weight materials. Nevertheless, low molecular weight polystyrenes still find important applications as additives, as tackifiers for pressure sensitive adhesives, and in hot melt adhesives. However, the market for low molecular weight polystyrene is small. 

Superacids encompass both Bronsted (proton donor) and Lewis (electron acceptor) acids as well as their conjugate pairs. The concept of acidity and acid strength can be defined only in relation to a reference base. According to an arbitrary but widely accepted suggestion... 

Solid acids Over Bronsted (protonic) solid acids, the reaction intermediate is a sec-butyl cations formed by the addition of a proton from the solid surface to butene. Hence, when solid acids are deuterated the deuterium of the catalyst is incorporated into both reactant and isomerized butenes. The same intermediate is often involved in the case of Lewis solid acids. In the latter case, protonic acid is induced by the reaction of butene on the Lewis acid site. For example, CH3CH = CH2 + L CH3CH — GHz — L (L Lewis site), where H at CH3 or CH2L acts as acidic proton. The intermediate is illustrated in Fig. 4.1 A. Fig. 4. IB shows the stereochemical reaction scheme for the case in which the deuterium atom (Da) is attached to cw-2-butene from below. If the Hb atom is removed downward from 1 or 2, the intermediate is transformed to (ranf-2-butene. The removal of D can produce tracer experiments based on this model reveals detailed information on the dynamic behavior of the intermediate. ... 

All Bronsted proton transfer reactions fit into this type. 

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