Zeolite organic bases

The s)mthesis of zeolites is traditionally performed by crystallisation from a sol-gel mixture comprising reagents such as silica, sodium aluminate, sodium hydroxide and water. Another key component of the sol-gel mixture is a base whose main role is to regulate the pH of the mixture. If an organic base is used then a templating effect may also be observed... 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

Specific structure-directing effects of some organic bases or cations When in the procedure BT Pr N is replaced by other organics, various pentasil-type zeolitic precursors are formed. It appears that specific zeolites are formed only when quaternary ammonium salts are used, their nature (structure) being essentially dependent on the length of the alkyl chains pure ZSM-8, ZSM-5 and ZSM-11 are obtained respectively with Et N+, Pr N+ and Bu N cations. TG data indicate that the latter fill nearly completely the zeolitic channel system (Table VIII). 

Table VIII. Characteristics of some pentasil zeolites obtained from synthesis B using various organic bases or cations (adapted from ref (25), by permission).

It is proposed that in mixed organic base-alkali systems, the presence of the organic base changes the solid-liquid equilibrium and stabilizes larger sol-like aluminosilicate species ( 25 m/ ). The alkali ion affects agglomeration of the sol particles to larger amorphous precipitate particles from 100 to 500 min size which subsequently crystallize to zeolite. 

In order to compare the catalytic behaviour of zeolites with organic bases in homogeneous media, the reactions have also been carried out on two nitrogen bases currently used for catalyzing this type of reactions pyridine (pKb = 8.8) and piperidine (pKb= 11.12). The amount of these liquid bases used was 1.6 meq, which is the same as the number of cations present in the zeolite in similar experiments. 

A recent interesting application of 13C n.m.r. has been in the investigation of the role of organic base in the preparation of ZSM-type catalysts. Tetra-propylammonium ions, for example, are very effective in the synthesis of crystalline ZSM-5. The 13C n.m.r. spectra of these occluded ions have been found to be sensitive to different environments within the zeolite,122-124 and hence can serve as useful probes in understanding the role of these template-like molecules in the synthesis mechanism and, as such, provide excellent complementary information to that obtained from Si and A1... 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

ZSM-34 preparation. Preparation of ZSM-34 zeolite was based on the patent literature (j>). However choline was used instead of choline chloride as the organic template, on the basis that zeolites prepared by using choline have better performance for olefin synthesis from methanol than zeolites prepared by using choline chloride (7). 

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

Recently, several selective bromination reagents for reactive aromatic amines have been developed, for example, 2,4,4,6-tetrabromocyclohexa-2,5-dienone (35), iV-bromosuccinimide-dimethylformamide (36), and hexabro-mocyclopentadiene (37). Although molecular bromine is too reactive to perform selective bromination (mono- versus polybromination), the combined used of bromine and zeolites X and Y has been reported to be applicable to the selective bromination of halobenzenes and alkylbenzenes (38). This zeolite method, however, was not successful in the selective bromination of highly active aromatic compounds. Bromine preadsorbed on zeolite 5A (Ca type) was found to monobrominate aniline in carbon tetrachloride with excellent regioselectivity (91-93% para selectivity) in the presence of organic base, pyridine or 2,6-lutidine (Table XII) (39). The preadsorption of bromine on zeolite 5A is necessary for selective bromination, because the inverse procedure of adding bromine to aniline that had been adsorbed on zeolite beforehand caused a nonselective reaction. 

Such high selectivity induced by bromine on zeolite 5 A may be explainable by the idea that bromine is first activated to form Br with a OH site on zeolite SA, and thus the most active and less hindered para position of the aniline nucleus has dominant access to the Br that is located near a pore window of the zeolite, as an aniline molecule is too large to enter the pores of zeolite SA. It is interesting that the presence of organic bases such as pyridine or 2,6-lutidine not only improved the conversion, owing to neutralization of the generated HBr, but also increased the para-bromination selectivity. 

The present method, which requires only readily available reagents (bromine, zeolite SA, and organic base) and can be operated under mild reaction conditions, appears to be useful for the selective bromination of various aromatic amines in organic synthesis [Eq. (10)]. 

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