Femtosecond laser assisted synthesis of silicate-1 zeolite

Zeolites are microporous (pore sizes < 2 nm) inorganic aluminosilicate materials with well-defined molecular pores and high surface areas used widely for various chemical processes, primarily as catalysts, sorbents, and ion exchangers. Aside from 40 types of natural zeolite, 253 different synthetic zeolitic framework types are synthesized and recognized by the International Zeolite Association (IZA). Zeolite synthesis requires moderate temperatures between 50°C - 270°C and high pressures (up to 120 bar). A fundamental challenge in zeolite synthesis is to elucidate and control the nucleation and growth of the zeolite crystals. The main reason for this is the fast kinetics of zeolite synthesis and rapid conformational transitions between quasi-equilibrium phases. Zeolite synthesis is a complex process because more than 40 different types of silica polymerization and depolymerization reactions occur simultaneously in a reaction mixture (i.e., precursor suspension). Reaction time scales of the silica polymerization are within the range of picoseconds and femtoseconds. Using the traditional hydrothermal synthesis method, which is occurring near thermal equilibrium, it is impossible to control the system at the time scale of these simultaneous polymerization reactions due to the slow energy deposition, which can be from 24 hours to several days. Other types of zeolite synthesis methods such as microwave synthesis are capable of depositing high energies in short time scales. However, the synthesis method is lacking the control of the excess heating of the full volume of the precursor suspension. During microwave heating, several hot spots form inside the precursor suspension, causing the boiling of the liquid. Growth by inhomogeneous heating leads to the formation of fused (i.e., interconnect) crystals instead of the discrete ones that dominate the end product in microwave-assisted synthesis method of zeolites. Here, we introduce a novel femtosecond laser-assisted synthesis method for the synthesis of Silicalite-1 zeolites. Femtosecond laser pulses ensure the delivery of a precise amount of energy per area within a given time interval, and therefore the spatiotemporal control over the energy delivered to the precursor suspension could be done on the time scale of the polymerization reactions of the zeolite synthesis. Thanks to the femtosecond laser pulses, the appropriate environment for zeolite synthesis, such as local high temperature and local high pressure (shock waves), has been created. In the laser-assisted synthesis method, the time required for zeolite synthesis decreased drastically compared to the hydrothermal method, overall control and product quality increased compared to the microwave synthesis method of zeolites. Unlike other rapid synthesis methods such as microwave synthesis, the uncontrollable temperature rise over the full volume of precursor suspension was not observed, resulting in 'discrete' crystals in the final product. Energy intake of the transparent precursor suspension was achieved through multiphoton absorption of the femtosecond laser pulses inducing steep spatiotemporal thermal gradients. Since surface tension of fluid is a function of temperature, surface tension gradients form as well, causing Marangoni flow. The ‘stirring effect’ of these flows leads to the distribution of the formed clusters evenly to the system, which is not attained by static hydrothermal synthesis of zeolites. It is proposed that vigorous flow induced in the laser-assisted synthesis assembles nuclei/polymerized clusters much faster than the other synthesis methods, which may be the reason for the drastically reduced reaction times compared to hydrothermal synthesis (i.e., 30 - 48h for hydrothermal vs. 3h - 5h for laser-assisted syntheses). Growth kinetics of the Silicalite-1 zeolite was examined in detail. In addition, templatefree nanosized microporous Zeolite Y, and mesoporogen-free hierarchical ZSM-5 zeolites with micro and meso-porosity were synthesized with reduced reaction times through laser-assisted synthesis method (i.e., 24 - 45h for hydrothermal vs. 1 - 5h for laser-assisted syntheses), which is important in terms of green synthesis approaches drawing attention in recent years.