Bronsted acids properties

The previous sections have shown that desihcation of ZSM-5 zeohtes results in combined micro- and mesoporous materials with a high degree of tunable porosity and fuUy preserved Bronsted acidic properties. In contrast, dealumination hardly induces any mesoporosityin ZSM-5 zeolites, due to the relatively low concentration of framework aluminum that can be extracted, but obviously impacts on the acidic properties. Combination of both treatments enables an independent tailoring of the porous and acidic properties providing a refined flexibility in zeolite catalyst design. Indeed, desihcation followed by a steam treatment to induce dealumination creates mesoporous zeolites with extra-framework aluminum species providing Lewis acidic functions [56]. 

Recently, advantage was taken of the fact that several Ti-molecular sieves contain both Lewis as well as Bronsted acidic properties. For example, Corma et al showed that Ti-MCM-41 and Ti-Al-Beta are able to catalyze the epoxidation of linalool in the presence of TBHP followed by ring-closure to cyclic hydroxy ethers in a one-pot reaction. Neri et al found that TS-1 catalyzes the epoxidation of styrene, with aqueous H2O2 in MeOH, with in-situ rearrangement to the corresponding aldehyde in 75% yield within 4h. ... 

Furfural is formed by dehydration of pentose. Xylose is a major aldopentose and is involved as a form of xylan in hemicelluloses. Unlike glucose, furfural can be formed from xylose by Bronsted acids alone at high temperature, although the furfural selectivity is low. A variety of Bronsted acid catalysts have been examined for furfural synthesis and they are H-type zeolites such as H-mordenite and H-Y faujasite [183], delaminated zeolite [184], H-MCM-22 [185], ion-exchange resins [186], sulfonated porous silicas [187-189], porous niobium silicate [190], metal oxide nanosheets [51], and sulfated zirconia [191]. Sulfated tin oxide (S04 /Sn02) is an effective catalyst for furfural formation [192] because of the combination of Lewis acid and Bronsted acid properties, as well as HMF synthesis. 

N. Feng, A. Zheng, S.J. Huang, H. Zhang, N. Yu, C.Y. Yang, S.B. Liu, F. Deng, Combined solid-state NMR and theoretical calculation studies of Bronsted acid properties in anhydrous 12-molybdophosphoric acid, J. Phys. Chem. C 114 (2010) 15464-15472. 

In the following sections the properties of photogenerators of strong Bronsted acids and their use in microlithography are summarized. 

Catalyst acid properties depend on several parameters, including method of preparation, dehydration temperature, silica-to-alumina ratio, and the ratio of Bronsted to Lewis acid sites. 

The pillaring process also affected the concentration and the strength of acid sites, as confirmed by NH3-TPD (Table 1). Also, the ion exchange with Ni2+ cations modified the acid properties of surfaces new Lewis (nickel cations) and Bronsted (H+) acid sites have been created during the ion exchange and thermal activation (eq. 1), respectively. 

The LLB catalyst system needs a rather long reaction time and the presence of excess ketone to get a reasonable yield. Yamada and Shibasaki63 found that another complex, BaBM (91), was a far superior catalyst. Complex 91 also contains a Lewis acidic center to activate and control the orientation of the aldehyde, but it has stronger Bronsted basic properties than LLB. The preparation of BaBM is shown in Scheme 3-35. 

Acidic properties of zeolitic materials have widely been investigated in the past fifteen years ( 1), particularly for Y-type zeolites. The presence of strong acid sites, as it is usually the case for acid zeolites, results in the presence of only weak basic sites, if any. Therefore, up to now, majority of the studies has dealt with the characterization of acid+rather than basic properties. The acid sites (Bronsted s H, Lewis Al) and basic sites (0, OH ) may be characterized directly by using physical... 

One of the important properties that make zeolites useful for catalysis is their acidity-particularly, their Bronsted acidity. The strength, location and accessibility of these sites will change depending on the structure of the zeoHte and its com-posihon (Si/Al). Therefore, it is important to be able to measure these differences... 

Hence, water molecules enhance the acidic properties of the zeolite s Bronsted acids. Adsorbate-adsorbent interactions and, therefore, adsorbent selectivity and adsorbate mass transfer rates are altered due to water polarization. When developing an adsorbent to be used in a commercial adsorptive separation process, the water content of the adsorbent is adjusted to balance adsorbent selectivity and component mass transfer rate. 

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