TOWARD A SYSTEMATIC CHEMISTRY OF ORGANIC-REACTIONS IN ZEOLITES - IN-SITU NMR-STUDIES OF KETONES

The reactions of acetone and cyclopentanone on a variety of zeolites w ith different acidities and pore sizes were investigated in detail by both in situ C-13 solid-state NMR and ex situ methods following extrac tion. The overall reaction sequence was acid catalyzed aldol condensat ions followed by secondary reactions such as double bond migration, hy drogen transfer, and cracking, especially in the more acidic zeolites. The formation of reactive complexes between the ketones and the Brons ted acid sites of the zeolites is implicated as a precursor to condens ations at low temperature. The strength of complex formation was refle cted in the extent of proton transfer to the ketone, and this was mapp ed into significant C-13 isotropic chemical shift changes. These are i nterpreted quantitatively as a measure of the effective acidity of the zeolitic environment under actual reaction conditions using a procedu re proposed by Farcasiu and coworkers [J. Catal. 1992, 134, 118]. The order of activity for aldol condensations of acetone on the various ze olites (HZSM-5 > HY > HX > NaX, CsX > CsY, CsZSM-5) is in complete agr eement with this in situ measurement of acidity. The zeolites studied here were not superacids at 298 K. Free carbenium ions or hydroxycarbe nium ions did not form at 298 K as long-lived intermediates from the k etones studied, even at low loadings. The aldol products obtained from acetone and cyclopentanone were generally in agreement with solution chemistry with the exception that shape selectivity was evident in the formation of trindane from cyclopentanone on large pore but not on me dium pore zeolites. Cyclohexanone formed products analogous to those o f cyclopentanone. The in situ NMR experiments were effective guides to the design of zeolite-based syntheses of aldol products of cyclopenta none using either sealed glass tube or refluxing solvent protocols. Th e overall results of this investigation suggest the emergence of a phy sical organic pedagogy that will systematize synthetic reactions using zeolites. Such a methodology would be important in the development of zeolite-based procedures as alternatives to existing routes to fine c hemicals that also produce corrosive liquid or metal salt wastes.