Lewis zeolites

Figure B3.3.14. Template molecule in a zeolite cage. The CFIA stmcture (periodic in the calculation but only a fragment shown here) is drawn by omitting the oxygens which are positioned approximately halfway along the lines shown coimecting the tetrahedral silicon atoms. The molecule shown is 4-piperidinopiperidine, which was generated from the dicyclohexane motif suggested by computer. Thanks are due to D W Lewis and C R A Catlow for this figure. For fiirther details see [225].

The next step is the ahstraction of a hydride ion hy a Lewis acid site from the zeolite surface to form the more stable allylic carhocation. This is again followed hy a proton elimination to form a cyclohexadiene intermediate. The same sequence is followed until the ring is completely aromatized. 

Silica gel [11] or alumina [11a, 12] alone, or silica and alumina together modified by Lewis-acid treatment [13] and zeolites [14], have been widely used as catalysts in Diels-Alder reactions, and these solids have also been tested as catalysts in asymmetric Diels-Alder reactions [12,13b,14]. Activated silica gel and alumina at 140 °C were used [15] to catalyze the asymmetric cycloaddition of (-)-menthyl-N-acetyl-a, S-dehydroalaninate (3) (R = NHCOMe) with cyclopentadiene in the key step for synthesizing optically active cycloaliphatic a-amino acids. When the reactions were carried out in the absence of solvent, a higher conversion was obtained. Some results are reported in Table 4.5 and compared with those obtained by using silica and alumina modified by treatment with Lewis acids. Silica gel gives a reasonable percentage of conversion after 24 h with complete diastereofacial selectivity in exo addition. 

Good yields and high diastereoselectivities were obtained by using zeolites in combination with Lewis-acid catalyst [21]. Table 4.7 illustrates some examples of Diels-Alder reactions of cyclopentadiene, cyclohexadiene and furan with methyl acrylate. Na-Y and Ce-Y zeolites gave excellent results for the cycloadditions of carbocyclic dienes, and combining these zeolites with anhydrous ZnBr2 further enhanced the endo diastereoselectivity of the reaction. An exception is the cycloaddition of furan that occurred considerably faster and with better yield, in comparison with the classic procedure [22], when performed in the presence of sole zeolites. 

Zeolite and Lewis-acid catalysis in Diels-Alder reactions of isoprene [20b]... 

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]. 

It must be noted that sometimes calcination is beneficial to create active species. Notable examples are the Sn-beta speciation [176] and generation of extra-framework Al-Lewis sites in beta zeolite for organic transformations... 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

The catalytic activity for NO oxidation [reaction(l)] was strongly inhibited by water vapor, because this reaction occurs on Lewis acid sites of zeolite as... 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

Another major cause of waste is the use of mineral acids (H2SO4, H3PO4, etc.) and Lewis acids (AICI3, ZnCL), often in stoichiometric amounts, which cannot be recovered and recycled. A typical example is the HNO3/H2SO4 mixture used in aromatic nitrations. Consequently, there is a discernible trend towards the use of solid, recyclable Brpnsted and Lewis acids, e.g. zeolites, acidic clays, etc. (see later) as alternatives to conventional mineral and Lewis acids. 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

Of course, diffusion limitations may affect the TPD pattern especially for microporous materials such as zeolites. Furthermore, no information is acquired on the nature of the acid sites. For instance, the technique does not discriminate between Lewis and Brbnsted sites. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

Figure 3.53. IR transmission absorption spectrum of pyridine adsorbed on partly dehydroxylated HY zeolite (Van Bekkum et al, 1991) B = Bronsted acid sites L = Lewis acid sites.

Elanany, M., Koyama, M., Kubo, M. el al. (2005) Periodic density functional investigation of Lewis acid sites in zeolites Relative strength order as revealed from NH3 adsorption, Appl. Surf. Sci., 246, 96. 

Traditionally, the production of LABs has been practiced commercially using either Lewis acid catalysts, or liquid hydrofluoric acid (HF).2 The HF catalysis typically gives 2-phenylalkane selectivities of only 17-18%. More recently, UOP/CEPSA have announced the DetalR process for LAB production that is reported to employ a solid acid catalyst.3 Within the same time frame, a number of papers and patents have been published describing LAB synthesis using a range of solid acid (sterically constrained) catalysts, including acidic clays,4 sulfated oxides,5 plus a variety of acidic zeolite structures.6"9 Many of these solid acids provide improved 2-phenylalkane selectivities. 

The amount of Lewis acid to be used is depicted as an effective amount and a minimum limit of 0.5 mole equivalent with respect to the sulfmated compound concentration was mentioned. A wide variety of Lewis acids was mentioned to be useful for the present invention in the patent document, but only copper (II) compounds were claimed. The way in which the Lewis acid is used (either as a homogeneous or a heterogeneous phase), was reported to be irrelevant. So, it could be employed in solution in the reaction medium or insoluble as powders or on a solid support, such as alumina or a zeolite. The Lewis acid is supposed to be acting as a catalyst in the desulfination process. The temperature and pressure conditions for this reaction are substantially higher than the microbial conditions. The temperature and pressure conditions did not form part of any claim, but the document stipulates values between 50°C and 100°C, and 10 and 15psi, respectively. The quantitative effectiveness or conversion values of this reaction were not given, but it looks like it would diminish the advantages of a biocatalytic process. 

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