Ion conductivity of nano-scaled Al-rich ZSM-5 synthesized in the pores of carbon black

Abstract

A novel way to obtain nano-sized Al-rich ZSM-5 crystallites by constrained growth in the pores of a carbon black, followed by removal of the carbon matrix by oxidation is reported. Tailoring the size of the formed zeolite particles by selecting a suitable templating carbon material with requested porosity and the easy separation of the synthesized product by simple filtration instead of centrifugation, are the two main advantages of this method. The carbon black is infiltrated with the synthesis solutions in two steps: First an ethanolic solution containing the Al-source is used, followed by a second infiltration with a tetraethylorthosilicate (TEOS) solution after evaporation of the alcohol. During a subsequent hydrothermal treatment (48 h, 180 °C) the zeolite nanocrystals grow inside the pores. Because of the separate addition of the aluminium and silicon sources, this procedure enables to vary the silica to alumina ratio in a wide range. For reaching high alumina contents, decreasing of the silicon concentration in the synthesis mixture is more suitable than increasing the amount of infiltrated aluminium. About 50 nm ZSM-5 particles with Si/Al ratios down to 10 could be obtained with acceptable crystallinity. According to impedance spectroscopy measurements the nano-ZSM-5 shows higher ion conductivity than μm-sized ZSM-5 and small ZSM-5 particles obtained by ball-milling since the latter procedure damages the crystal zeolite structure. This finding demonstrates that small particle sizes as well as a good crystallinity are important to reach high ion conductivities. This statement is in all likelihood transferable to other zeolites.