The zeolite catalyst is an excellent material for catalyzing the conversion of biomass into valuable chemicals.zeolite catalyst The use of solid zeolite catalysts in place of homogeneous metal alkoxides reduces the formation of byproducts and waste during the reaction. Furthermore, zeolite catalysts can be separated and easily regenerated after use. These properties have led to the widespread use of zeolite in organic reactions.
Zeolites are crystalline microporous aluminosilicates with pore size and shape-selective properties that offer significant advantages in chemical processes. Zeolite catalysts are capable of enhancing the efficiency of many organic reactions and have gained great popularity in petrochemical and biochemical industries as they possess excellent physical properties, including high surface area, three-dimensional space availability and structural stability.
A zeolite's physicochemical properties are due to the presence of both Bronsted and Lewis acid sites as well as its tetrahedral framework structure with a Si/Al ratio varying between 0.7 and 2. This unique structure makes zeolites highly reactive and adsorbs small molecules on their surface, making them suitable for catalytic applications.
Zeolite catalysts are known to be able to promote the Meerwein-Ponndorf-Verley (MPV) reduction of aldehydes and ketones with secondary alcohols as hydrogen donors. They also work well for the Oppenauer oxidation of alcohols with carbonyl oxidants. Furthermore, zeolites are efficient for the transfer hydrogenation of primary alcohols to form cycloalkanes. These reactions can be carried out under mild conditions in contrast to the homogeneous aluminium alkoxides that require high hydrogen pressures.
However, the performance of zeolite catalysts depends on how uniformly they are distributed on the surface of the adsorbent and how stable they are under reaction conditions. Uniformity in the distribution of catalytic sites is essential to eradicate non-selective and hyperactive sites and improve the physicochemical stability of the catalyst.
The presence of Lewis acid and Bronsted acid sites in the tetrahedral pores of zeolites enables them to act as heterogeneous catalysts, providing a wide range of reactivity for acid-base reactions, redox reactions, and biomass valorization (i.e., converting platform molecules into value-added chemicals). This can be achieved by the introduction of heteroatoms into the zeolite framework via exchange with cations or protons.
Incorporation of heteroatoms into the zeolite structure is challenging and requires basic synthesis conditions that precipitate many metal ions as hydroxides, leading to long crystallization times. Moreover, the use of fluoride ions poses safety concerns for industrial scale synthesis.
A new synthesis method using mechanochemistry has been developed to introduce heteroatoms into zeolite frameworks. Hydrated sodium silicate and the relevant organic templates along with a source of heteroatoms in the form of simple salts or oxides were ground together and heated to induce crystallization. This process significantly reduced the crystallization time and yielded several types of zeolites with different structures (MFI, SOD, BEA, and FAU).
Mechanochemistry is a promising alternative to conventional synthesis methods that employ fluoride ions. Compared to conventional methods, it offers the following benefits: (i) smaller concentrations of organic SDA or even its elimination; (ii) solventless nature with intrinsic lower pressures during the crystallization process; and (iii) faster space-time yield. It is to be hoped that mechanochemistry will become the new standard for green and sustainable zeolite synthesis.
