Strong surface acidity, excellent thermal stability, and a large number of micropores extend the applicability of pillared interlayered clays (PILCs) as catalyst supports and shape-selective adsorbents. Microporosity is one of the most significant characteristics of PILCs.

Numerous researchers have been looking for ideal pillars for nearly four decades. Inorganic cations with large particle sizes as pillars were reported, including positively charged polymeric hydroxy-cations of Al, Ti, Fe, or their composites such as Al/Ce, Al/Fe, Al/Co, etc.

However, few of these cations are big enough to bring attractive porosity. Although aluminum pillaring agents are well understood, structures of many other metal polymeric hydroxy-cations need identification and their hydrolysis behaviors require a comprehensive investigation.

Recently, Dr. ZHU Jianxi's group from Guangzhou Institute of Geochemistry of Chinese Academy of Sciences proposed a convenient method to synthesis Keggin-Al₃₀ pillared interlayered montmorillonite (Mt) with Mt and a base-hydrolyzed solution of Al (III) chloride. The study was published in Microporous and Mesoporous Materials.

With vigorous stirring under an oil bath, NaOH solutions are addded into AlCl₃ solutions at a certain rate. New polycation of aluminum with the Keggin-type structure (Keggin-Al₃₀ ([Al₃₀O₈(OH)₅₆(H₂O)₂₄]¹⁸⁺) can be formed in this hydrolysis solutions of aluminum. Ratio of OH/Al, temperature, and the rate of adding NaOH solutions are three key factors in this process.

Compared with Keggin-Al₁₃, Keggin-Al₃₀ has bigger particle size, higher ion charge density and more OH groups. By the exchange reaction taken place between Keggin-Al₃₀ and the interlayer cations, Keggin-Al₃₀ could move to the interlayer spacing of Mt. In this way, Keggin-Al₃₀ intercalated Mt is formed. 

After the calcination at 300°C, the Keggin-Al₃₀ pillared Mt (transformed from the Keggin-Al₃₀ intercalated Mt) could be got. It is the first time that porous pillared interlayered Mt with large Keggin-Al₃₀ pillars has been successfully prepared, which may open up entirely new avenues for developing novel PILCs as heterogeneous catalysts and porous adsorbents.

Figure: Structural representation of (A) Keggin-Al₁₃ and (B) Keggin-Al₃₀ (Image by ZHU Jianxi)