Acidity, beta zeolite

The cinnamyl ester can be prepared from an activated carboxylic acid derivative and cinnamyl alcohol or by transesterification with cinnamyl alcohol in the presence of the H-Beta Zeolite (toluene, reflux, 8 h, 59-96% yield). It is cleaved under nearly neutral conditions [Hg(OAc)2, MeOH, 23°, 2-A h KSCN, H2O, 23°, 12-16 h, 90% yield]or by treatment with Sulfated-Sn02, toluene, anisole, reflux. The latter conditions also cleave crotyl and prenyl esters. 

The NH4-Y (CBV712, ao = 24.35 A), H-Beta (CP811E-75), NH4-Beta (CP814E) zeolites were obtained from Zeolyst International. The NH4-Y and Beta zeolites were transformed to proton forms through step calcination procedure in a muffle oven. Zeolites containing 1 wt-% platinum were prepared by wet-impregnation method using hexachloroplatinic acid as the Pt-source. 

The highly oxygenated bio oil can be de-oxygenated, and thereby upgraded, over acidic zeolite catalysts through the formation of mainly water at low temperatures and C02 and CO at higher temperatures [1-3], Successful catalytic pyrolysis of woody biomass over Beta zeolites has been performed in a fluidized bed reactor in [4]. A drawback in the use of pure zeolitic materials has been the mechanical strength of the pelletized zeolite particles in the fluidized bed. 

The chemical composition of the bio oil was changed depending on the catalyst used. The more acidic ZSM-5 catalysts produced more polyaromatics and less aldehydes than the less acidic Beta catalysts. Interestingly, levoglucosan was only found in the non-catalytic and in the pure ZSM-5 zeolite oil. 

Table 2. Relative acidity of ITQ-21 and BETA zeolites as determined by pyridine adsorption-...

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

The wide-pore H-Beta zeolite has strong Bronstcd acid hydroxyl groups and other advantage chemical environment which govern the adsorption and consecutive conversion of methanol to dimethyl ether and further to hydrocarbons, mostly isobutane. This character can be modified by Fe ion-exchange. 

In this work the methanol and methyl iodide conversion and their co-reaction are investigated on Fe-Beta zeolite without any oxygen. Partly Fe-ion-exchanged Beta-300 i.e. Fe-H-Beta-300 (shortly Fe-Beta-300) zeolite keeps the light acidity to a certain extent, however the presence of Fe ions (as transition metal, Fe is an excellent Lewis acid) can modify the reaction pathway. This Fe-Beta-300 has been tested already by low temperature peat pyrolysis [6], At present, the adsorption as well as desorption of methanol are followed-up by radiodetectors using ( -radioisotopic labeling [4, 7]. The... 

Corma and Renz6 developed an effective heterogeneous catalyst system. Incorporation of tin into a beta zeolite network (Sn-Beta) gave a catalyst that was used to convert citronellal 5 to racemic isopulegol (6, Equation (3)) with 85% diastereoselectivity. It was calculated that each metal site performed 11,500 reaction cycles. No leaching of the tin was detected. This catalyst system is advantageous over normal Lewis acids, since precautions against humidity are not required, and it is suitable for use in a fixed bed continuous reactor. 

The catalyst with lower acid strength (beta zeolite) presents the higher selectivity to (TMP) at high reaction temperatures, while the sulfated zirconia presents an opposite trend the lower the reaction temperature, the higher the selectivity to TMP is. In the case of sulfated zirconia catalysts, cracking rather than alkylation is favored at high reaction temperatures, while oligomerization rather than alkylation is favored on the beta zeolite at low reaction temperature. 

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catal54ic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-sihca presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative. ... 

Figure 13.8 Catalyst decay during the isobutane alkylation on nafion/Si02, sulfated zirconia, beta-zeolite, and MCM-41-supported 12-tungstophosphoric acid.

A series of Beta zeolites have been synthesized in the presence of tetraethylammonium hydroxide (TEA). Samples with Si/Al ratio in the 7-100 range have been characterized by X-ray powder diffraction, I.R. spectroscopy, and pyridine adsorption. The fraction of TEA which is compensating the charge of the framework aluminum is removed at temperatures higher than those required to remove "occluded" TEA. Three hydroxyl bands are observed at 3740 cm l (silanol groups), 3680 cm" (extraframework Al) and 3615 cm 1 (acid hydroxyl groups interacting with pyridine). 

Acidity. Pyridine adsorption on samples of Beta zeolite with Si/Al ratio between 7 and 40 allow us to determine the acidic nature of the hydroxyl groups and their evolution as a function of its aluminum content. 

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. 

Studies with unpromoted and Pt-promoted solid acids, specifically, H morde-nite,308 Pt Beta-zeolite,313 and acidic polyoxometalate cesium salts314,315 applied in the presence of hydrogen, showed that the isomerization of n-butane follows a monomolecular pathway. This consists of dehydrogenation on the metal site and isomerization on acidic sites. A new study using isotope labeling provided direct... 

In acylation with acidic ammonium and cesium salts of phosphotungstic acid, direct correlation was found between activity and the number of accessable Br0nsted acid sites.69,70 Moreover, cesium salts were found to be more active, than HY and H-Beta zeolites.70... 

Aromatics. The application of solid acid catalysts provides excellent possibilities to carry out aromatic electrophilic substitutions in an environmentally friendly way. Various zeolites were found by Smith and coworkers to exhibit high activities and selectivities.250 Acetyl nitrate generated in situ from acetic anhydride and HNO3 transforms alkylbenzenes to the corresponding para-nitro derivatives in high yield (92-99%) and with excellent selectivity (79-92%) when applied in the presence of large-pore H-Beta zeolites.251 Lattice flexibility and the coordination of acetyl... 

The activation of carbonyl compounds by Lewis acid sites in the tin species has been proposed to play an important role in the oxidation reaction (Figure 5.11). The pore size of beta zeolite ( 6.5 A x 7.5 A) may pose some restrictions on the substrate size. For much bulkier substrates, a new tin-MCM-41 (pore size around 20 A) catalyst has been developed by the same research group [64]. 

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