Acid zeolite catalysts Lewis type

In addition to the Bronsted acidity in zeolites, these materials also have Lewis acidity. According to Lewis, an acid is an electron pair acceptor, a broader definition than that given by Bronsted since a proton is a particular case of an electron pair acceptor, then the definition of Lewis covers practically all acid-base processes, whereas the definition of Bronsted represents only particular types [25], 

In zeolite-based catalysts, the Lewis acidity is related to the existence of extra-framework A1 (EFAL) species formed during the zeolite dealumination process [18], It occurs frequently in zeolite activation, for example, during the calcination process 

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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). 

Summary Meerwein-Ponndorf-Verley and Oppenauer reactions (MPVO) are catalysed by metal oxides which possess surface basicity or Lewis acidity. Recent developments include the application of basic alkali or alkaline earth exchanged X-type zeolites and the Lewis-acid zeolites BEA and [Ti]-BEA. The BEA catalysts show high stereoselectivity, as a result of restricted transition state selectivity, in the MPV reduction of substituted alkylcyclohexanones with i-PrOH. 

Heterogeneous catalysts which are active for the catalysis of the MPVO reactions include amorphous metal oxides and zeolites. Their activity is related to their surface basicity or Lewis acidity. Zeolites are only recently being developed as catalysts in the MPVO reactions. Their potential is related to the possibility of shape-selectivity as illustrated by an example showing absolute stereoselectivity as a result of restricted transition-state selectivity. In case of alkali or alkaline earth exchanged zeolites with a high aluminium content (X-type) the catalytic activity is most likely related to basic properties. For zeolite BEA (Si/Al=12), however, the dynamic character of those aluminium atoms which are only partially connected to the framework appear to play a role in the catalytic activity. Similarly, the Lewis acid character of the titanium atoms in aluminium free [Ti]-BEA explains its activity in the MPVO reactions. 

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because... 

Because acidified titanium oxide is the catalyst usually employed commercially for the transformation of 1 into 2 [8] there has been much investigation of this catalytic system [9]. A 1995 paper by Stefanis et al. [10] reported an investigation of the reaction of 1 in several alumina-pillared clays (PILCs montmorillonite- and beidellite-based, and their and Ca" -exchanged congeners) under Lewis acid conditions (solid is activated by heat to remove all water). The results were compared with those obtained by use of medium-pore zeolites USY, NH4+-ZSM-5, and H-mordenite. Conversion to 2 > 50% was always observed. The aim of the work was to clarify differences between site availability and acidity for the two types of solid. 

In this chapter, the use of solid acids as heterogeneous catalysts for the Friedel-Crafts acylahon reaction is described. Our review is split up into seven sechons, describing the application of zeolites, clays, metal oxides, sulfated zirconia, heteropoly acids. Nation, and other less-utilized solid catalysts (i.e., graphite). When possible, the relationship between the acid properhes of the solids (namely, Bronsted and Lewis types) and the catalytic efficiency is shown, as well as the role of the active site location on the catalyst surface. ... 

Hydroxyalkylations are catalyzed by Lewis-type acids, like AICI3, and by mineral Bronsted acids. Some papers and patents have appeared in recent years, where zeolitic materials are described as catalysts for this reaction [1-4,7]. Solid acid materials are highly desirable catalysts, since the environmental impact of the process benefits fi-om easier separation of the catalyst, the absence of liquid wastes containing inorganic salts, and less severe corrosion problems. 

Acidity has been ascribed to various combinations of Bronsted, Lewis or defect-type sites. Some people emphasize ordering as a critical property, but on the other hand, a good case can also be made for the importance of lattice defects in providing acidic hydroxyl groups. It can be argued that silica-alumina was active because of partial ordering of a disordered structure, while zeolite catalysts were active because of partial disordering of an ordered structure 41). 

Fan et al. (312) report the alkylation on Y-type zeolite catalysts of two olefin/paraffin systems (a) isobutene with isopentane and (b) isobutene with isobutane. The isopentane and isobutane, respectively, were also used as the solvents in this work, and results were compared for gas-, liquid-, and SCF-phase conditions. Alkylations conducted in the SCF phase were reported to exhibit higher activity and longer catalyst lifetimes in comparison with reactions conducted in the gas and liquid phases. Catalyst deactivation observed in the gas-and liquid-phase results was attributed to deposition of high-molecular-weight olefinic oligomers on the Lewis acidic sites that were determined to be active for the alkylation reactions. Operation under SCF-phase conditions resulted in successful extraction of these oligomers in situ and extended the catalyst life. Additional aspects of this report are discussed in a subsequent commentary (313) and rebuttal (314). 

Alkylations are Friedel-Crafts-type electrophilic reactions requiring the participation of a strong Lewis catalyst. Obviously they were carried out in the presence of hazardous Lewis acid catalysts as metal halides. Switching from these molecular compounds to solid embedded triflates led to catalysts on which such reactions occurred with turnover frequency (TOFs) having at least one order of magnitude higher than on classic acid solid catalysts as zeolites [100,101]. Examples of such reactions are alkylations of phenols or naphthols occurring on immobilized La, Ag, or r rr-butyldimethylsilyl-trifluoromethane-sulfonate catalysts. 

Besides the classical Si and Al-containing zeolites, there is an ongoing search toward other Lewis acidic zeolites and other microporous materials suitable for this reaction. Taaming et al. made a comparison between beta zeolites substituted with Al, Sn, Zr, and Tl. As with the homogeneous catalysts, this comparison pointed to Sn-based catalysts as extremely selective for this reaction. The authors obtained lactic acid and lactate yields of 90% and >99% at 125 and 80°C, respectively, with an Si Sn ratio of 125 [67]. Since then, numerous reports were published including Sn-montmorillonite [68], mesoporous Sn-MCM-41 (Mobil Composition of Matter) [69,70], Sn-MFI (Mordenite Framework Inverted) [70,71], Sn-deAl-beta [72], Sn-SBA-15 (Santa Barbara Amorphous type material) [70], Sn-MWW (Zeolite Framework Type M-22)... 

Zeolites are known to be important catalysts for a number of industrially important reactions. A question of basic interest, which provides opportunity for development of catalyst with suitable and tailored characteristics, is to determine the correlation between number, strength and strength distribution of active sites and the promotion of catalytic activity. Therefore, the investigation of acid sites, both Lewis and Bron-sted type, is very important subject. Properties of zeolites as catalysts will depend on many factors the adsorption or desorption temperature of the probe, pretreatment of the sample, proton exchange level, influence of coking as well as Si/Al ratio and dealumination and influence of exchanged cations [47]. 

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. 

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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