Lewis acid of zeolites

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

A P- Al J-coupling constant for trimethylphosphine bound to the Lewis acid of Zeolite HY has been determined by Al to P INEPT methods,since coupling could not be resolved for this resonance (6= —49) in the P MAS NMR spectrum. The coupling was consistent with a five-coordinated aluminium Lewis acid/trimethyl phosphine complex, and thus a four-coordinated Lewis acid site. Bi- and tricyclic penta- and hexacoordinated-phosphoranes 25-28 have been... 

L Acidic properties are most important to obtain high activity and yield as, for example, dealuminated H-Faiyasite (Si/Al=15) and H-beta (Si/Al=25). Such a behaviour is in dose agreement with the literature concerning Hie increase in the acidity of zeolites through partial dealumination, i.e. the number of protonic species is decreased but their strength is reinforced. Characterization of BrOnsted and Lewis acidity of zeolites is relatively well documented (13), particularly in the gas phsise where it is possible to differentiate between the two types of acids from e analysis of products formed as, for example, in deamination of sec-butylamine... 

In general, it is accepted [140,141] that Lewis acidity of zeolites and related catalysts (sUica-alumina, y-alumina) is related to the presence of aluminium species on the smface. Therefore, two possibUities exist for the study of Lewis acidity by NMR (i) an analysis of highly resolved Al NMR spectra similar to the MAS NMR spectroscopy described in Sect. 2.1 and (ii) the use of probe molecules such as pyridine or others. 

Lewis acidity of zeolites to extraframework species, neglecting the evidence of their presence also at the external surface of the framework. 

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. 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

For faujasitic zeolites presenting a high A1 content, an easy stepwise decarbonyl-ation of [Mo(CO)6] occurs by heating under vacuum, the temperature of CO evolution depending on both the Lewis acidity of the cation and the Si/Al raho [39, 40,... 

The efficient and selective catalysis of some Diels-Alder reactions by lanthanide P-diketonate complexes has been known since 1975 [226, 227]. The fluorinated p-diketonate complexes Ln(fod)3 (cf. Scheme 12.5) selectively catalyze the Danishefsky transformation (Scheme 12.23) as a consequence of their mild Lewis acidity. Importantly, zeolites and Lewis acid modified silica or alumina also catalyze Diels-Alder reactions [228-232]. 

In order to study he Lewis acidity of the samples, the intensity of the 1450 cm pyridine band was also measured. Sample HYUS-8 shows a high amount of Lewis centers (Fig. 4d), relative thf HYD-400 sampl (Fig. 5c). This agrees with the absence of A1 as observed by A1 MAS-NMR for HYD samples. However, chemical analysis (Table I) indicates that there is more aluminum in this sample than in that from the unit cell constant m i urements. These differences cculd be explained considering that A1 MAS-NMR does not detect octahedral EFAL because of the low symmetry of its environment (i ). If this is so, it is remarkable that this EFAL does not show Lewis acidity as measured by pyridine ad y ption. On the other hand, if indeed thej is a small amount of A1, then the EFAL should be present as Al" outside the zeolite framework. In this case it should be present as amorphous silica-alumina. 

It is generally accepted that Lewis acidity in zeolites is due mainly to extraframework aluminum (16,17,18). Consequently, Lewis acid sites measured by pyridine adsorption must correlate with extra-framework aluminum. In Table I, the amount of pyridine coordinated to Lewis sites decreases for samples with the lowest Si/Al ratio, showing that, after thermal treatment, the amount of extraframework aluminum decreases with Si/Al ratio of the Beta zeolite. 

These results strongly pointed toward the involvement of the acidic hydroxyl groups in the catalytic reaction as suggested by Benesi (157), since the maximum activity was obtained when the zeolite was completely deammoniated. In addition, catalysts which had been dehydroxylated by high-temperature calcination demonstrated low activity. Thus, Benesi proposed that the Brtfnsted acid sites rather than the Lewis acids were the seat of activity for toluene disproportionation. This conclusion was supported by the enhancement in toluene disproportionation activity observed when the dehydroxylated (Lewis acid) Y zeolite was exposed to small quantities of water. As discussed previously, Ward s IR studies (156) indicated a substantial increase in Brdnsted acidity upon rehydration of dehydroxylated Y sieve. 

PCPs with well-defined pores and surface-isolated Lewis acid sites could potentially serve as size- or shape-selective heterogeneous catalysts, in a similar manner to zeolites.33 43 161-164 The two-dimensional PCP, [Cd(4,4 -bpy)2(H20)2] 2N03 4H20 , was the first example that showed catalytic properties for the cyanosilylation of aldehydes.33 Experimental data in the case of cyanosilylation of imines, which is also performed by the same compound, led to the conclusion that hydrophobic grid cavities bind to the substrate very efficiently to promote a rapid reaction, and that the heterogeneous reaction involves the selective activation of the imino nitrogen by the weak Lewis acid Cdn center.161 In this polymer, the NO3" anions exist in a coordination-free state. This situation contributes to increasing the Lewis acidity of the Cdn centers. 

TABLE 2. Comparison of Lewis acids and zeolites in catalytic bromination of toluene... 

The alpha values of synthetically prepared mordenltes are within the range of 10 -10°. There Ls only a small variation In a values of siliceous mordenlte as the aluminum content of the material varies. This Ls In contrast to dealumlnated mordenltes, which exhibit a much larger variation Ln alpha values as the aluminum content varies. These catalytic results on synthetic and acid dealumlnated mordenltes Indicate that factors other than the total aluminum content must contribute to the variation In activity of the catalysts. The linear correlation of alpha versus aluminum content reported for some zeolites (26) does not apply to the acid dealumlnated samples. Evidence has been presented for the presence of both Bronsted sites and Lewis sites enhancing the strong acidity of zeolite catalysts (27). The presence of extra-lattice aluminum Ln acid dealumlnated mordenlte samples was confirmed by 2 A1 NMR spectroscopy. The presence of both framework aluminum and extra-lattice aluminum Ln the acid treated materials may account for the wide variation Ln alpha values as a function of aluminum content. 

Even in the absence of Lewis acid functions, zeolites can accelerate gas phase Diels-Alder reactions. This rate enhancement, for instance in the butadiene cyclodimerization, is attributed to a concentration effect inside the zeolite pores. The effect is however not zeolite-specific any adsorbent with affinity for dienes, such as a carbon molecular sieve, displays similar effects (5). 

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

Sastre, G., Corma, A. Relation between structure and Lewis acidity of Ti- 3 and TS-1 zeolites A quantum-chemical study. Chem. Rhys. Lett. 1999, 302, 447 53. 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

However conventional HY, even those which have been submitted to an optimized acid leaching, possess less acid sites than the (IS) solids. This is not surprising since (HT) and (HTA1) HY are not purely zeolitic, they contain variable amounts of extra-framework material. This extra-framework material is mainly responsible for the Lewis acidity of (HT) and (HTA1) solids while the (IS) solids are nearly pure Bronsted acids. 

The synthesized zeolite ZSM-35 disc was calcinated at 873k for 12 hours to remove ethylenediamine and water. Because inorganic ions Na", was not used in synthesis process, the synthesized zeolite ZSM-35 disc was a zeolite H-ZSM-35. The surface acidity of zeolite H-ZSM-35 was tested by pyridine adsorption IR. From the data in fig. 4, the synthesized zeolite H-ZSM-35 had Bronsted acid site at 1549 cm and Lewis acid site at 1443 cm [24]. Certainly, zeolite ZSM-35 disc from Al203-Si02-EDA-H20 had good acidity and would be a good acid catalyst. 

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