Brpnsted acid sites in zeolites

The initial adsorption of the oxime in zeolites was studied through a combination of solid-state NMR spectroscopy and theoretical calculations ". The calculated adsorption complexes formed over silanol groups and complexes over Brpnsted acid sites in zeolites are depicted. This study suggests that the A-protonated oxime is formed over Brpnsted acid centers, but not over weakly acidic silanol groups. It has been also suggested that weakly acidic or neutral silanol groups or silanol nests are active catalysts of the rearrangement reaction ... 

Makarova M A, Zholobenko V L, Al-Ghefalli K M, Thompson N E, Dewing J and Dwyer J (1994), Brpnsted acid sites in zeolites , J Chem Soc, Faraday Trans, 90, 1047. 

Fig. 1 Generation of Lewis and Brpnsted acid sites in zeolites.

Kochar NK, Merims R, Padia AS (1981) Ethylene from Ethanol Chem Eng Prog 77 66-70 Kondo JN, Ito K, Yoda E (2005) An ethoxy intermediate in ethanol dehydration on Brpnsted acid sites in zeolite. J Phys Chem B 109 10969-10972 Lee RL, Mark SL, Charies EW et al (2008) How biotech can transform biofuels. Nat Biotechnol... 

Ganapathy, S., Kumar, R., Delevoye, L., and Amoureux, J.P. Identification of distinct Brpnsted acidic sites in zeolite mordenite by proton localization and [ A1]- H READPOR NMR spectroscopy. Chem. Commun. 2003, 2076-2077. 

Hunger, M. Brpnsted acid sites in zeolites eharacterized by multi-nuclear solid-state NMR spectroscopy. Catal Rev. Sci. Eng. 1997,39, 345-393. 

Xu, B., et al., 2006. Catalytic activity of Brpnsted acid sites in zeolites intrinsic activity, rate-limiting step, and influence of the local structure of the acid sites. Journal of Catalysis 244 (2), 163-168. 

H. Soscnn, O. Castellano, J. Hernandez and A. Hinchliffe. Theoretical study of the structural, vibrational, and topologic, properties of the charge distribution of the molecular complexes between thiophene and Brpnsted acid sites in zeolites. Int. J. Quantum Chem. 87, 2002, 240-253. 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

FIGURE 13.2 Example of Brpnsted and Lewis acid sites in zeolites. (From Humphries, A., Harris, D. H., and O Connor, R, Stud. Surf. Sci. Catal., 76, 41-82, 1993.)... 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

The theoretical modeling of the activation and reaction of methanol by Brpnsted acid sites within zeolites has attracted a wide interest. This is in part a consequence of the industrial importance of the interaction—as the first step of the conversion of methanol into gasoline in the MTG process (213). However, a great deal of the theoretical interest has arisen because of the possibility of proton transfer from the zeolite lattice to methanol. An early investigation was that of Vetrivel et al. (214), who employed ab... 

The role of Brpnsted acid sites in the oligomerization of ethylene over HZSM-5 has been studied.[30,31] Amin and Anggoro1311 concluded that dealumination of HZSM-5 led to higher ethylene conversion, but the gasoline selectivity was reduced compared with a nondealuminated HZSM-5 (Si/Al = 15) zeolite sample. 

For ferromagnetic cobalt particles in zeolite X, spin-echo ferromagnetic resonance has been used to obtain unique structural information (S6). In addition, study of the catalytic signature of metal/zeolite catalysts has been used by the groups of Jacobs (87), Lunsford (88), and Sachtler (47,73,89). Brpnsted acid protons are identified by their O—H vibration (90,91) in FTIR or indirectly, by using guest molecules such as pyridine or trimethylphosphine (92,93). An ingenious method to characterize acid sites in zeolites was introduced by Kazansky et al., who showed by diffuse reflection IR spectroscopy that physisorbed H2 clearly discerns different types of acid sites (94). Also, the weak adsorption of CO on Brpnsted and Lewis acid sites has been used for their identification by FTIR (95). The characterization of the acid sites was achieved also by proton NMR (96). 

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). 

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... 

Figure 10.6 A Brpnsted acid site in a partial zeolite framework.

Whatever the operating conditions, in the presence or absence of NH3, the CO yield increased with the reaction temperature and a 100% yield was obtained at 400°C (Fig. 5.13b). However, at 300°C, while 1-MN was fully transformed in the presence of NH3, only 10% carbon dioxide was produced, mainly because of the formation of oxygenated compounds retained in the zeolite pores ( coke )7 which was favoured by NH3 as revealed by elemental analysis showing a larger amount of carbon after the reaction. As for the coke oxidation reaction, it was shown that 1-MN oxidation into carbon dioxide required strong Brpnsted acid sites. In our case, the basic character of NH3 favours its adsorption at low temperatures on strong acid sites, which are able to transform 1-MN into carbon dioxide. Indeed, at 300°C the carbon dioxide yield was close to 20% in the absence of NH3 as opposed to 10% in the presence of NH3. When the reaction was carried out with NH3, only the weaker sites were able to work, but these sites seemed to be active only in the conversion of 1-MN into intermediate oxygenated compounds that remained adsorbed on the solid surface. 

Nitration of the toluene appears to be a three-step process, as demonstrated with DFT calculations of benzene nitration in the gas phase by Olah and coworkers." The acetyl nitrate readily transfers a nitronium -like moiety to the Jt-system of the toluene close to the para or ortho sites. This is a jt-complex with no real bonding interaction. An sp to sp hybridization of the para or ortho carbon must then occur to create a o-complex (aka Wheland intermediate or arenium cation). Finally, this 0-complex must transfer the proton from the para or ortho site to a Brpnsted acid site in the zeolite wall. Upon optimizing the H-beta structure with the acetyl nitrate site and toluene, it became clear that the most likely acid transfer site after creation of the o-complex is the All-02 site in our model, which, as we already... 

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

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. 

It has been proposed that hydride transfer in zeolites requires the presence of two adjacent Brpnsted acid sites (69). In light of the above-mentioned theoretical examinations and also adsorption isotherms of 1-butene and n-butane on USY zeolites with various aluminum content (70), this proposition seems unlikely. 

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

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