Proton exchange, acid- base

Shen J, Xi J, Zhu W, Chen L, Qiu X (2006) A nanocomposite proton exchange membrane based on PVDF, poly(2-acrylamido-2-methyl propylene sulfonic acid), and nano-Al203 for direct methanol fuel cells. J Power Sources 159 894—899... 

Guan YS, PuHT, JinM, Chang ZH, WanDC (2010) Preparation and characterisation of proton exchange membranes based on crosslinked polybenzimidazole and phosphoric acid . Fuel Cells 10 973. 

Lou, L.D., Pu, H.T. (2011) Preparation and properties of proton exchange membranes based on Nafion and phosphonic acid-functionahzed hollow sihca spheres. International Journal of Hydrogen Energy, 36, 3123-3130. 

W.H. Choi, W.H. Jo, Preparation of new proton exchange membrane based on self-assembly of poly(styrene-co-styrene sulfonic acid)-b-poly(methyl methacrylate)/ poly(vinylidene fluoride) blend, J. Power Sources, 188 (2009) 127-131. 

Che, Q. He, R. Yang, J. Feng, L. Savinell, R. F. (2010). Phosphoric acid doped high temperature proton exchange membranes based on sulfonated... 

Leikin, A. Yu. Bulycheva, E. G. Rusanov, A. L. and Likhachev, D. Yu., High-temperature proton-exchange membranes based on polymer-acid complexes, Vysokomol. Soed., B, 48(6), 1031-1040 (2006). 

Deuteration of C-methyl protons in simple methylpyrimidines and their amino and hydroxy derivatives has been studied under acidic and basic conditions. The exchange is acid/base catalyzed with, for example, a minimal rate at pH 4 for 1,4,6-trimethylpyrimidin-2(lH)-imine (67JCS(B)171). 

Consider a nucleus that can partition between two magnetically nonequivalent sites. Examples would be protons or carbon atoms involved in cis-trans isomerization, rotation about the carbon—nitrogen atom in amides, proton exchange between solute and solvent or between two conjugate acid-base pairs, or molecular complex formation. In the NMR context the nucleus is said to undergo chemical exchange between the sites. Chemical exchange is a relaxation mechanism, because it is a means by which the nucleus in one site (state) is enabled to leave that state. 

Of course, such exchange is most successful for relatively acidic protons, such as those a to a carbonyl group, but even weakly acidic protons can exchange with bases if the bases are strong enough (see p. 228). 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

C) The Bronsted-Lowry or proton theory interprets the acid-base reaction as a mere proton exchange between the acid (proton donor) and the base (proton acceptor) however, the Lewis theory or electron theory interprets the reaction as a donation and acceptance of a lone pair of electrons, where the... 

The generalization was based on the introduction of the concept of donor-acceptor pairs into the theory of acids and bases this is a fundamental concept in the general interpretation of chemical reactivity. In the same way as a redox reaction depends on the exchange of electrons between the two species forming the redox system, reactions in an acid-base system also depend on the exchange of a chemically simple species—hydrogen cations, i.e. protons. Such a reaction is thus termed proto lytic. This approach leads to the following definitions ... 

As free proton cannot exist alone in solution, reactions in which a proton is split off from an acid to form a conjugate base cannot occur in an isolated system (in a homogeneous solution although this is possible in electrolysis (Section 5.7.1)). The homogeneous solution must contain another base Bn that accepts a proton from the acid HAr (acid HA is, of course, not conjugate with base Bn). It will be seen that this second base can even be the solvent molecule, provided it has protophilic properties. Acid-base reactions thus depend on the exchange of a proton between an acid and a base that are not mutually conjugate ... 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

This section deals with the quantitative description of the proton transfer processes (denoted by Eqs. (4) and (6) in Scheme 1), identified by the qualitative NMR experiments on the acid/base behavior of the Mo(IV), W(IV), Re(V), Tc(V), and Os(VI) systems as described in Section II. The data obtained on the signal behavior from these similar complexes were used to simulate spectra and model the proton exchange processes to finally obtain rate constants associated therewith. 

Proton transfer reactions on the aqua oxo complex are described by Eq. (8) (acid catalysis or protolysis), Eq. (9) (base catalysis or hydrolysis), and Eq. (10) (direct proton exchange). 

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