Effect of the simultaneous presence of sodium and potassium cations on the hydrothermal synthesis of MCM-22 zeolite

Abstract

The hydrothermal synthesis of MCM-22 zeolite was investigated in reaction systems with different proportions of sodium and potassium cations. The potassium content R, defined as the molar ratio between potassium and the total inorganic cations amounts in the synthesis mixture, varied from 0 to 0.9, keeping constant the cationic concentration and the alkalinity of the system. The materials were characterized by X-ray diffraction (XRD), N2 adsorption/desorption and scanning electron microscopy (SEM). The K+ ions favored the formation of MCM-22 when 45% of sodium was replaced by potassium, reducing the time required to synthesize the MCM-22(P) precursor and producing more crystalline samples. Furthermore, the relative amounts of Na+ and K+ ions remarkably affected the morphology and particle size of the samples. The use of higher potassium contents (R = 0.68 – 0.9) hindered the crystallization of MCM-22 zeolite. Thus, the use of reaction mixtures with adequate proportions of Na+ and K+ can be an effective strategy to produce highly crystalline samples in shorter times, reducing the cost of synthesis of such zeolite

Different approaches to the synthesis of ZSM-22 zeolite with application in n-heptane cracking

“…All samples provided two main peaks of ammonia desorption, one approximately between 100 and 320 °C and the other in the range of 320 and 550 °C, indicating the presence of Lewis and Brønsted acidity, respectively (Li, et al, 2017;Silva, et al, 2020). The first low temperature desorption event represents the weak acidic sites and the second high temperature event is attributed to the strong acidity sites (Gao, et al, 2020;Quintela, et al, 2021). The exceptions were the Z22-Vynil and Z22-CaCO3-HCl samples, which exhibited a third less intense peak between 550 and 620 °C.…”