On the Synergistic Catalytic Properties of Bimetallic Mesoporous Materials Containing Aluminum and Zirconium: The Prins Cyclisation of Citronellal

Abstract

Bimetallic three-dimensional amorphous mesoporous materials, Al-Zr-TUD-1 materials, were synthesised by using a surfactant-free, one-pot procedure employing triethanolamine (TEA) as a complexing reagent. The amount of aluminium and zirconium was varied in order to study the effect of these metals on the Brønsted and Lewis acidity, as well as on the resulting catalytic activity of the material. The materials were characterised by various techniques, including elemental analysis, X-ray diffraction, high-resolution TEM, N2 physisorption, temperature-programmed desorption (TPD) of NH3, and 27Al MAS NMR, XPS and FT-IR spectroscopy using pyridine and CO as probe molecules. Al-Zr-TUD-1 materials are mesoporous with surface areas ranging from 700–900 m2 g−1, an average pore size of around 4 nm and a pore volume of around 0.70 cm3 g−1. The synthesised Al-Zr-TUD-1 materials were tested as catalyst materials in the Lewis acid catalysed Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone, the intermolecular Prins synthesis of nopol and in the intramolecular Prins cyclisation of citronellal. Although Al-Zr-TUD-1 catalysts possess a lower amount of acid sites than their monometallic counterparts, according to TPD of NH3, these materials outperformed those of the monometallic Al-TUD-1 as well as Zr-TUD-1 in the Prins cyclisation of citronellal. This proves the existence of synergistic properties of Al-Zr-TUD-1. Due to the intramolecular nature of the Prins cyclisation of citronellal, the hydrophilic surface of the catalyst as well as the presence of both Brønsted and Lewis acid sites synergy could be obtained with bimetallic Al-Zr-TUD-1. Besides spectroscopic investigation of the active sites of the catalyst material a thorough testing of the catalyst in different types of reactions is crucial in identifying its specific active sites.

Contribution to elucidation of the mechanism of preparation of 2-isobutyl-4-methyltetrahydro-2H-pyran-4-ol

“…These two types of product may both be formed during the reaction and the selectivity depends on the reaction conditions, especially the type of catalyst [3]. Lewis [4][5][6][7] or Brønsted acids (inorganic [1,8], organic [9], or ion exchangers [10,11]) may be used as catalysts; occasionally, heteropolyacids [12], organometallic complexes [13] or heterogeneous aluminosilicates [14] are used. The accepted mechanism [15] involves hemiacetal formation in the first step followed by cyclization to form the substituted tetrahydropyranol or dihydropyrans (Fig.…”

Zr-SBA-15 Lewis Acid Catalyst: Activity in Meerwein Ponndorf Verley Reduction

“…Within this context, the incorporation of zirconium species onto mesoporous supports, most of them silicas, has revealed to be an excellent alternative to both hydrous zirconia and zirconium-functionalized zeolites. Thus, zirconium species have been incorporated onto the surface of several mesoporous silicas like MCM-41 [20,21], SBA 15 [22] and TUD-1 [23,24]. These materials display an open porous structure with pore sizes in the range of mesopores, which facilitates the diffusion of bulk molecules inside the material, minimizing mass transfer hindrances.…”

Potential applications of Zr-KIT-5: Hantzsch reaction, Meerwein–Ponndorf–Verley (MPV) reduction of 4-tert-butylcyclohexanone, and Prins reaction of citronellal

“…Because of their demonstrated strong Lewis acidity, the Zr-KIT-5 catalysts are excellent candidates for reactions such as the Prins cyclization or the MPV reaction, as also shown previously with other mesoporous Zr silicates, Zr-TUD-1 [17]. These two reactions may lead to interesting applications such as the synthesis of menthol (in which the cyclization of citronellal is a first step) or the MPV reduction of steroids such as cholesterol [18,19].…”

“…For the Prins cyclisation of citronellal, the following general protocol was applied (method reported earlier [2,17]). In a Schlenk flask of 50 mL (placed in an oven at 70°C overnight), 50 mg of catalyst (activated in an oven overnight at 100°C) were introduced.…”

“…For the Meerwein-Ponndorf-Verley reduction of 4-tert-butylcyclohexanone, the following general protocol was applied (method reported earlier [2,17]). In a Schlenk flask of 50 mL (placed in an oven at 70°C overnight), 50 mg of catalyst (freshly calcined) was introduced followed by 2 mmol of 4-tert-butylcyclohexanone (308.5 mg).…”