Basic zeolites aniline alkylation

The alkylation of aniline by methanol is carried out at 623-723K over LiY and NaY non-protonic and non-basic zeolites to study the effect of reaction temperature on the catalytic activity and the selectivity. The reaction temperature influences strongly the aniline conversion, the selectivity and the catalyst life. Two types of products are observed, C- or N- alkylates whether the alkylation takes place on the ring or on the N atom. A general trend is that as the reaction temperature increases the catalytic activity and selectivity of N-alkylation increase while the selectivity of C-alkylation decreases. The IR results of benzene adsorption have been used to try to explain the catalytic properties of these two catalysts. 

C MAS NMR has been performed in-situ under batch conditions to investigate the mechanism of aniline alkylation with methanol on acidic H-Y and basic CsNa-Y impregnated with cesium hydroxide. On acidic zeolite H-Y, methanol reacts with zeolitic hydroxyl groups to give surface methoxy groups which, in turn, play a role as alkylating species in aniline methylation. On the basic zeolite Y, meth ol is converted into formaldehyde, which is responsible for the N-alkylation. Both on acidic and basic zeolites, N-methylaniline is the primary alkylation product. Toluidines and N-methyltoluidines are formed only on the acidic zeolite at elevated temperatures after complete conversion of methanol into N-methylaniline. 

Some zeolitic and non-zeolitic molecular sieve catalysts are claimed to be capable for ortho- and para-selective alkylation using olefin as alkylating agent (refs. 1,2). Zeolite catalysts are less active and selective in the methylation of aniline by methanol (refs. 3,4). Reaction is usually carried out with a large excess of methanol since a large fraction of the alcohol decomposes without participating in the alkylation. Numerous N- and C-alkylated aniline derivatives appear in the reaction product. It was found that N-alkylation requires basic sites while C-alkylation occurs mainly on acidic sites (refs. 5-7). 

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

Recently basic sites in ALPOS and SAPOs have been detected by IR spectra [13] of chemisorbed pyrolle and it has been reported that small amounts of basic sites in zeolite exhibit more activity in the methylation of aniline. However an excessive amount covers the active sites and deactivates the catalyst. ALPO and its derivatives contain both acidic and basic sites. The basic sites are due to high aluminium content but small amount of protons, resulting in a highly negative charge on frame work oxygen. Introduction of Mg increases total basicity and decreases total acidity of the material. Due to this the successive alkylation of NMA to NNDMA is suppressed. AEL type materials have steady activity in this reaction. 

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