Structural rectorite

Chemical analyses and structural formulas of rectorite-like clays... 

Hitherto the most successful pillaring has been carried out on smectite type clays. The original research on pillaring included several series of unsuccessful experiments on mica, vermiculite and the sheet silicic acid minerals magadiite, silhydrite and kenyaite. Recently renewed attention has focused on these and other alternative sheet structures. They include rectorite, zirconium phosphates, tetrasilicic micas, hydrotalcites and silicic acids, the general structural features of which are shown in Figure 4. 

Wang X., Du Y., Luo J., Lin B., Kennedy J.F., Chitosan/organic rectorite nanocomposite films structure, characteristic and drug delivery behavior, Carbohydrous Polymers 2007 69 41-49. 

Pillared rectorites are expanded clay minerals with a surface area in the 150-220 mVg range, and thermal and hydrothermal stability similar to that of zeolites with the faujasite structure (1-4). After steaming at 760°C/5h (100% steam, 1 atm), these materials retain their pillared structure, and at microactivity test conditions (MAT) they are as active as commercial fluid cracking catalysts (FCC) for gas oil conversion... 

It has been previously reported that the structure of interstratified clays (such as rectorite) consists of a combination of mica-(nonexpandable) and smectite- (expandable) type layers. The mica layers are more probably Na-paragonite while the smectite layers are of the montmorillonite or beidellite type (3, 11-13). For mica-like layers, the most typical interlayer spacing is equal to 9.6A while it is of 9.5A for the smectite-like layers, see Figure 6-2. However, these d-values may change depending on the nature of the interlayer cations... 

X-ray diffraction (XRD), pyridine chemisorption, and microactivity test (MAT) results have been used to characterize a sample of natural rectorite pillared with alumina clusters. After reaction with chlorhydrol, a pillared product was obtained that after drying at 100°C/10h had d(OOl) spacing of 28.7 A. The pillared rectorite retained its structure even after calcining in air at 800 C/5h or after steam aging at 760°C/5h with steam at 1 atm. Thus, pillared rectorites have thermal and hydrothermal stability comparable to that of zeolites with the Faujasite structure. 

Fig. 1. Schematic representation of the montmorilIonite (M) and rectorite (R) structure. The T-O-T 3-1ayers sequence (T=tetrahedra1, O=octahedral) is represented by trapezoids and rectangles. Exchangeable and non-exchangeable (fixed) charge compensating cations are represented by open and solid circles.

Pillared rectorites have thermal and hydrothermal stability much superior to that of similarly prepared montmorilIonites and hectorite catalysts. In fact, ACH-rectorites retain their pillared structure even after calcination in air at 800°C/5h or after steam-aging with 100% steam at 760°C/5h, Fig. 3. High temperature (800°C) calcination or steaming has little effect on the shape and intensity of the clay 001 and 002 reflections. Fig. 3. At 800°C the d(OOl)... 

Pillared montmorillonite (and hectorites) have thermal stability in air that does not exceed 800°C. In contrast, the ACH-rectorites under study after calcination at 800°C, retain their pillared structure (Fig. 3) and more than 902 of their initial surface area. As a result, after calcining at 800°C/5h, only minor changes in the cracking properties of the two ACH-rectorites are observed. Table 2. 

Typically, the pillared structure of montmorilIonites can be destroyed either by heating at 675°C for lOh or at 730°C for 2h in presence of steam (12). In addition to a high thermal stability in air, the ACH-rectorite under study retain most of their surface and cracking properties even after steamaging at 760°C/5h with 100 steam in a fluidized bed. Table 3. Steaming may have affected the iron distribution (migration) in the clay silicate layers. 

Natural rectorites pillared with alumina clusters have thermal as well as hydrothermal stability far superior to that of similarly prepared montmorilIonite catalysts. Their stability is comparable to that of zeolites with the Faujasite structure. Pyridine chemisorption experiments have indicated that these materials contain both B and L acid sites and that at cracking conditions acidity is essentially of the L-type. Steam-aged (760°C/5h) pillared rectorites at MAT conditions have cracking activity comparable to that of similarly steam-aged commercial FCC containing an estimated 35% CREY. Coke selectivity (as well as particle density) will have to be improved for ACH-rectorites to compete with zeolite-containing FCC. 

Henderson, G. V., and W. F. Bradley, 1970. Rectorite and the rectorite-like layer structure. Clays Clay Min. 18 115-119. 

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