Bronsted catalyst design

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]. 

UOP developed two varieties of zeolite catalyst designated as QZ-2000 and QZ-2001 (47,48). Its research indicated that the catalysts have Bronsted acidity values. As a result, oligomerization of propylene is essentially eliminated. Accelerated stability tests indicated that the percentage of the bed of catalyst employed for desired reactions increases with time of operation. QZ-2001 demonstrates improved stability and operation at benzene/propylene ratios of 2.0 in its liquid-phase process. Unreacted benzene and polyisopropylbenzenes are recovered and recycled so that cumene jdelds of about 99% are obtained. Four catalyst beds in series are employed. The catalysts need to be reactivated after about 2-5 years. UOP have licensed several units. 

One possible way to take advantage of such abilities may be to apply a combined acids system [2] to the catalyst design. The concept of combined acids, which can be classified into Bronsted acid-assisted Lewis acid (BLA), Lewis acid-assisted Lewis acid (LLA), Lewis acid-assisted Br0nsted acid (LBA), and Bronsted acid-assisted Bronsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more organized structure, which will allow an effective asymmetric environment to be secured. 

Climent, M. J., Corma, A., Ihorra, S. and Velty, A. Designing the adequate base solid catalyst with Lewis or Bronsted basic sites or with acid-base pairs, J. Mol. Catal., A, 2002, 182-183, 327-342. 

The allylation of carbonyl compounds with allyltin reagents is still an active area of organotin chemistry from the methodological point of view, and also for synthetic applications. For completeness we should add several alternative techniques, such as the development of trifluoromethanesulphonic acid as a Bronsted acid catalyst for the allylation of aldehydes in water123, or the design of fluorous allyltin reagents for the platinum-catalysed allylation of aldehydes124. 

Sc(() l f) ( is an effective catalyst of the Mukaiyama aldol reaction in both aqueous and non-aqueous media (vide supra). Kobayashi et al. have reported that aqueous aldehydes as well as conventional aliphatic and aromatic aldehydes are directly and efficiently converted into aldols by the scandium catalyst [69]. In the presence of a surfactant, for example sodium dodecylsulfate (SDS) or Triton X-100, the Sc(OTf)3-catalyzed aldol reactions of SEE, KSA, and ketene silyl thioacetals can be performed successfully in water wifhout using any organic solvent (Sclieme 10.23) [72]. They also designed and prepared a new type of Lewis acid catalyst, scandium trisdodecylsulfate (STDS), for use instead of bofh Sc(OTf) and SDS [73]. The Lewis acid-surfactant combined catalyst (LASC) forms stable dispersion systems wifh organic substrates in water and accelerates fhe aldol reactions much more effectively in water fhan in organic solvents. Addition of a Bronsted acid such as HCl to fhe STDS-catalyzed system dramatically increases the reaction rate [74]. 

Bronsted acid assisted chiral Lewis acid catalysts (BLA) are designed for the asymmetric Diels-Alder reaction [11]. Both BLA (17) [11] and (18) [12] were employed in... 

For aldol reactions, typically proline-type catalysts were used. However, Palomo and coworkers reported cross-aldol reactions between unmodified aldehydes and ynals. This transformation was enabled by the cooperative action of newly designed catalyst C7, copper iodide and a Bronsted acid. In this way, remarkably high levels of diastereo- and enantioselectivily were achieved (Scheme 8.32). 

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