Acidic zeolite aniline alkylation

Recently, a novel reaction mechanism was proposed for the alkylation of aniline with methanol on acidic zeolite HY (243). By SF MAS NMR spectroscopy, the formation and decomposition of N, AA-trimethylanilinium ions on the working catalyst were observed. Figure 29 shows the MAS NMR spectra recorded under CF conditions during methylation of aniline by methanol at reaction temperatures of 473-523 K. In these experiments, a mixture of aniline and C-enrichcd methanol (WjF — 40gh/mol) in a molar ratio of 1 2 was injected into the MAS NMR rotor... 

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). 

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. 

In conclusion, potassium cation exchanged X- and Y-type zeolites have very weak acid and base sites, but they are nevertheless found to work as effective promoters in N-alkylation. of aniline derivatives in nonpolar organic solvent owing to the dual function of the acid and base. The highly selective N- wc n<9alkylation is ascribed to the reactions occurring inside the homogeneous narrow pores of zeolites. 

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