Synthesis and Immobilization of Quarternary Ammonium Ions in Zeolites

Synthesis and Immobilization of Quarternary Ammonium Ions in Zeolites 

C MAS NMR spectra recorded after the reaction of C-enriched methanol with TEA on zeolite HY (nsi/ Ai = 2.7) at 493 K (a), with 1-MPi on zeolite HZSM-5 ( si/ Ai = 22) at 493 K (b), and with 1-methylpyrrolidine (1-MPy) on zeolite HZSM-5 ( si/ Ai = 22) at 483 K (c). The assignment of the signal at 52.5 ppm to A, A-dimethylpyrrolidinium ions was confirmed by C MAS NMR spectroscopy of N, A-dimethylpyrrolidinium iodide (d). Asterisks denote spinning sidebands. Reproduced with permission from (257). Copyright 2003 The Royal Society of Chemistry. 

When the above-mentioned approach was used, a variety of quaternary ammonium ions were synthesized in high yields under both batch and flow conditions on calcined acidic zeolites HY and HZSM-5 (257). The reaction parameters were optimized by investigation of the conversion with MAS NMR spectroscopy. The reaction temperatures and the NMR shifts of the cationic species formed are summarized in Table 4. 

The above-mentioned route to the preparation of quarternary ammonium ions in acidic zeolites is remarkable for the following reasons an immobilization of quaternary ammonium ions in a well-controlled concentration is an approach to modification of zeolite catalysts. Furthermore, the synthesis of isotopically labeled compounds is of importance in organic, pharmaceutical, and agricultural chemistry. This method is an approach to the synthesis of C-labeled (or C, N-labeled) tertiary amines via a thermal decomposition (243,251) of the corresponding quaternary ammonium ions in zeolites. 

Investigations of template-related issues by solid-state NMR spectroscopy have suffered from the low NMR sensitivity associated with the low natural abundance of C-isotopes. The formation of selectively C-labeled (or N-labeled) quaternary ammonium ions offers possibilities to overcome this limitation. 

The main reactions of the MTG/MTO process can be summarized as follows the first is the dehydration of methanol to DME on acidic zeolite catalysts. The equilibrium mixture of methanol, DME, and water is then converted to light alkenes, which react further to form higher alkenes, n- and Ao-alkanes, aromatics, and naphthenes by hydrogen transfer, alkylation, polycondensation, isomerization, and other secondary reactions. 

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