Separation of Mixtures of Zeolites and Amorphous Materials and Mixtures of Zeolites with Different Pore Sizes into Pure Phases with the Aid of Cationic Surfactants

Upon shaking for 3 min at the proper concentrations, surfactant solutions of n-alkylpyridinium iodide (C_nP⁺I^-, n = 16, 12, 10, and 8) readily develop very fine (<1 mm) and stable froth in the presence of zeolite crystals such as zeolite-A (A), mordenite (MOR), and zeolite-Y (Y) exchanged with Mⁿ⁺ (Mⁿ⁺ = Na⁺, K⁺, Ca²⁺, or NH₄⁺) as the charge-balancing cations. The frothy solution subsequently undergoes phase separation within 30 min into a top layer of thick froth and an underlying clear solution. Most of the zeolite particles are captured in the froth, and isolation of them is readily achieved by physical separation of the froth from the solution, followed by washing of the froth with ethanol. The recovered yield increases with increasing concentration of each surfactant and reaches a maximum (of greater than ∼90%) at a concentration that is slightly lower than the critical micelle concentration (CMC). The yield quickly decreases to a minimum (<20%) as the concentration passes the CMC. This phenomenon is attributed to the formation of hemimicelles and admicelles on the zeolite surfaces at concentrations below and above the CMC, respectively. In contrast, silica gel and alumina do not float under the same conditions. By using these two contrasting facts, the separation of mixtures of zeolites and silica or zeolites and alumina into each component is readily achieved. Interestingly, whereas the exchange of Mⁿ⁺ with C_nP⁺ preferentially occurs with ions residing on the external surfaces of MOR and Y, the exchange occurs selectively with the interior cations when H⁺ is the charge-balancing cation. As a result, HMOR and HY remain hydrophilic, and hence, they do not float even when the degree of exchange of H⁺ with C_nP⁺ reaches 13 and 15%, respectively. Consequently, the separation of mixtures of HMOR and HY is readily achieved by employing n-dodecylquinolinium iodide as the surfactant, which is size-excluded by HMOR but readily admitted into the interior of HY. The viability of the separation methods is demonstrated through the facile separations of autogenous mixtures of high-silica NaY and silica and zeolite-L and an unknown coproduced zeolite into each pure species.