Synthesis and catalytic performance of Linde-type A zeolite (LTA) from coal fly ash utilizing microwave and ultrasound collaborative activation method

Abstract Pure LTA zeolites of high crystallinity have been synthesized from coal fly ash (CFA) by utilizing a method of microwave and ultrasound collaborative activation (MUCA) in situ for the first time. Compared to the activation method of only microwave or ultrasound, the MUCA have more efficient ability to dissolve silica-aluminum species from CFA while avoiding the digestion of other impurities, greatly reduced the energy consumption. While the liquid-solid ratio (LSR) of sodium hydroxide solution to CFA was 10 mL/g, the molar conversion rates of silicon and aluminum were 70.26% and 68.03%, respectively. Furthermore, with different LSR, pure LTA zeolite could be synthesized by adjusting the ratio of Si/Al. The CFA used in this paper contains Fe2O3 of about 12 wt percentage, and there have found almost no element of iron existing in the LTA zeolite products, indicating that the method of MUCA is suitable for the preparation of high purity zeolite. Finally, according to the mechanism of crystallization-dissolution-recrystallization, the morphology of LTA products could be well tuned by adjusting parameters such as LSR or crystallization time. The maximum BET surface area was 474 m2/g without additional calcium ion exchange steps by direct using of municipal water instead of deionized water, and the maximum CO2 adsorption capacity was 1.96 mol/g under 0 ℃. The mesoporous LTA zeolite also have been prepared just through the adjustment of crystallization time without addition of any other templates, the intramesopores of about 17 nm made greatly improvement in catalytic cracking of 1,3,5-tri-isopropylbenzene which is inaccessible to the conventional LTA micropores, and the conversion rate of 1,3,5-tri-isopropylbenzene reached to 25.40% as well as 11.05% of the selectivity of secondary cracking product benzene.