Acidity of pillared clay

The acidity of pillared clays has been characterized by both microcalorimetric measurements of the adsorphon of aromatic molecules and pyridine and the catalytic ethylbenzene test reaction [111]. The aromatic probe molecules used were a reactant and a product of the catalytic reaction ethylbenzene and m-diethylben-zene, respectively. In this way, only the strongest of the accessible acid sites were htrated. The heats of adsorphon of these molecules indicate that a zirconium oxide pillared clay had stronger acidity than an aluminum oxide pillared clay, whereas the pyridine results were equal for both samples. 

It has been reported(refs. 12-14) that pillars as well as clay sheets are responsible for acidity of pillared clays. The results obtained in the present study also indicate that acid sites exist on the pillars. The pore opening and the pore volume are reduced by introduction of excess number of pillars. These results lead us to conclude that the observed change in the catalytic activity is related to the shape selective property of pillared montmorillonite diffusion of 1,2,4-TrMB or formation of the intermediate for disproportionation is restricted by the limited interlayer space. 

Bentonite, whose main ingredient is montmorillonite, is one kind of layer structure clay mineral. It is an ideal material for preparation of pillared clays. If metal cations with high catalytic activity were intercalated in the interlayers of montmorillonite, a new type of solid acid catalyst can be obtained. The catalytic activity of the catalyst is closely associated with its porosity, specific surface area and surface total acidity. One of the effective ways to enhance catalytic activity is to prepare catalyst with appropriate metal cations and structure. 

Ming-Yuan, H., Zhonghui, L. and Enze, M., 1988. Acidic and Hydrocarbon Catalytic Properties of Pillared Clays. Catalysis Today, 2 321. 

Sakurai, H., K. Urabe, and Y. Izumi. 1989. Acidity enhanced pillared clay catalysts. Modification of exchangeable sites on fluor-tetrasilicic mica by the fixed interlayer cations. Bull. Chem. Soc. Jpn. 62 (10) 3221-8. 

For industrial production of pillared clays, the pillaring process has to be optimized and scaled up. Sonication of the clay suspension, acid and base treatments, and competitive exchange are some of the techniques found in the literature (9). In laboratory preparations, large amounts of water are needed. Thus pillaring of concentrated aqueous smectite suspensions has been tried in order to find better conditions for scale-up (10). 

H. Ming-Yuan, L. Zhonghui, and M. Enze, Acidic and hydrocarbon catalytic properties of pillared clay. Catalysis Today, 2 (1988), 321-38. 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

K. Lourvanij and G. L. Rorrer, Dehydration of glucose to organic acids in microporous pillared clay catalysts, Appl. Catal. A Gen., 109 (1994) 147-165. 

Auroux, A. (2002) Microcalorimetry methods to study the acidity and reactivity of zeolites. Pillared clays and mesoporous materials. Top. Catal., 19, 205-213. 

---