Sol-gel synthesis of zeolite coatings and their application in catalytic microstructured reactors

Sol-gel hydrothermal synthesis is one of the most promising methods for the obtaining of zeolitic coatings (films, membranes) on the internal surface of channels of catalytic microstructured reactors. In this review, we discuss the basic methods for the synthesis of zeolite coatings, the processes that influence the rate of crystallization and crystal growth on a substrate, and the methods for modification of the substrate surface before the hydrothermal synthesis. By the example of the synthesis of β, A, and ZSM-5 zeolite coatings, it is shown that the hydrophilic behavior of the substrate and the presence of nano- and microroughness on it have a significant effect on the rate of nucleation of zeolite crystals and the homogeneity of obtained zeolite films. Depending on zeolite type and desired Si/Al ratio in the coating, by several examples. There exists a sufficiently narrow range of conditions (temperature, mixture heating rate, and ionic strength of solution) leading to zeolite coating formation on the substrate rather than to homogeneous crystallization in the authoclave volume. The fundamental mechanisms mechanisms responsible for the formation of zeolite coatings are presented. The acceleration of the hydrothermal synthesis under the action of microwave radiation is shown. The influence of different factors that should be taken into account to scale-up the hydrothermal synthesis is presented. Potential applications fields of microreactors and microadsorbers with zeolite coatings are discussed. Most industrial companies assign microtechnologies to the “high risk-high impact” group. The high risk is attributed, first of all, to the necessity of a cardinal change in the procedure sheet and to the application of new catalysts that allow an increasing rate of processes. Meanwhile, advantages of introduction of the new technologies—the basic ones being the reduction of energy consumption and significant decrease in the formation of by-products—allow companies to reduce operation costs.