Bronsted acid precursors

A Practical Approach to Quantitative Metal Analysis of Organic Matrices The structures for iodonium salts and Bronsted acid precursors are ... 

A number of curing agents and catalysts used in epoxies are complex metal salts that are added to cure at room temperature or with heat. Curing agents or catalysts such as cationic dinonato (acetylacetone, etc.) complexes of Si, B, Ge, and P behave as hydrolytic activated Bronsted acid precursors, e.g. ... 

Complex hydrolytic instability and liberated Bronsted acid strength decrease in the order of P > B > Si > Ge for the cations and SbF > AsFe > PF > BF4 > CIO4 for the anions. Both aromatic and aliphatic epoxy resins can be used, with the aliphatic resins being the more reactive. Interestingly, the cures are only activated by the small amounts of water absorbed by a resin under ambient conditions. Addition of bulk water inhibits the resin cure, presumably by acting as a chain transfer agent. These Bronsted acid precursors can also be decomposed thermally and under UV irradiation. 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

In the mechanism illustrated in Figure 6, the combination of the redox and acid properties of the catalyst determines the relative contribution for the formation of MA and PA. It is generally accepted that the higher the crystallinity of the VPP, the more selective to PA is the catalyst (3,4,10-12,17,18). Poorly crystalline VPP, like that one formed after the thermal treatment of the precursor (especially when it is carried out under oxidizing conditions), is selective to MA, but non-selective to PA. On the contrary, a fully equilibrated catalyst, characterized by the presence of a well-crystallized VPP, yields PA with a good selectivity. The presence of dopants that alter the crystallinity of VPP may finally affect the MA/PA selectivity ratio (19). Moreover, the surface acidity also influences the distribution of products (17) an increase of Lewis acidity improves the selectivity to PA, while that to MA is positively affected by Bronsted acidity (2). 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

According to this scheme, the first step of the reaction is the formation of a hydrogen-bonded precursor FH B, followed by the protonation of the monomer, leading to the formation of FBH". Examples of this type are shown in Section IV.B, where the oligomerization of unsaturated molecules in protonie zeolites is discussed. It is important that this first step is common to other reactions catalyzed by Bronsted acid sites. For example, in Section IV.A, the formation of methyl-substituted benzene carbocations as intermediate species involved in the MTO process in Hp zeolite is diseussed. 

The sol-gel synthesis of a V205-Si02 catalyst and its application in the oxidative dehydrogenation of w-butane to give CO, CO2 or dehydrogenated compounds was described by Sham and coworkers using V(acac)3 and Si(OEt)4 (teos) as precursors. Calcination at 500 °C resulted in the formation of a solid with a high surface area, which allows a better dispersion of active species. Furthermore, a direct correlation between the catalytic activity and the Bronsted acidity was also observed. 

Despite the transformation of a certain number of amino groups into urea groups, the qualitative shape of the zeta-potential plots as a function of pH is not changed significantly. That indicates that PVFA-co-PVAm can be used as a precursor polymer for the gradual functionalization of silica particles and other Bronsted-acidic surfaces with amino functionality. Chemical reactions of PVFA-co-PVAm/silica particles with isocyanates or other electrophilic reagents offer a wide field of applications, because of the simple experimental procedure this material combination seems promising for the construction of tailor-made polyelectrolyte networks on Bronsted-acidic surfaces or for other multicomponent systems. 

Most strong Bronsted acids effectively facilitate the hydride transfer reaction required to make alkylate (4), but avoiding the formation of high molecular weight coke precursors has proven more difficult to achieve. This second hurdle is particularly important for the deactivation of solid acid catalysts and has proven to be a stumbling block for the technology. 

In the presence of H2, there is not much difference in the conversion levels between Pt-SAPO-31 and SAPO-31, but the yield of the isomerization products are more over Pt-SAPO-31 than over SAPO-31. This can be due to the activated hydrogen from metallic (Pt) sites reacting with the carbocation intermediates and decreasing the rate of the disproportionation reaction." Also the spilled over hydrogen could lower the coke formation on Lewis acid sites by controlling the concentration of benzylic carbocations (coke precursors) while it does not affect the activity of Bronsted acid sites in the isomerization reaction. ... 

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