Fine-Tuning of Acidity in Hierarchical Zeolites for the Efficient Prins Cyclization Yielding Florol

Abstract

In this contribution, several hierarchical zeolites comprising mainly Brønsted acid sites with different morphologies (nanosheets, nanoparticles, nanorods) were synthesized and applied to the efficient synthesis of the perfume Florol (2-isobutyl-4-methyl-tetrahydro-2H-pyran-4-ol) via Prins cyclization of isoprenol and isovaraldehyde. Tuning the acidity in a zeolite was done by either incorporating metal centers or changing the initial composition and crystallization time. Furthermore, the interplay among the pore structure, specific mesoporosity, and accessibility of acidic sites is crucial for optimizing Florol synthesis. Strong Brønsted acid sites enhance the formation of the dehydration products, while mild-acidic Brønsted acid sites with a medium strength of accessible acid sites improve the formation of Florol. A maximum Florol yield of ca. 80% was achieved with Al-MFI and Al-FER (only 30 min) with typical nanosheet (nsh) morphology; in addition, values of TOF in the order of 1121 and 2655 h^–1, respectively, were also achieved. Al-FER-nsh showed itself to be robust, because it can be reused several times without any apparent loss of its catalytic activity. Finally, different green chemistry parameters were also applied to the studied zeolites in the synthesis of Florol, suggesting the high efficiency of the best catalyst (Al-MFI-nsh).