Hydrothermal stability, pillared clays

Fast deactivation rates due to coking and the limited hydrothermal stability of pillared clays have probably retarded the commercial development of these new type of catalysts and prevented (to date) their acceptance by chemical producers and refiners. However, there is a large economic incentive justifying efforts to convert inexpensive (i.e. 40-100/ton) smectites into commercially viable (pillared clay) catalysts (56). Therefore, it is believed that work on the chemical modification of natural (and synthetic) clays, and work on the preparation and characterization of new pillared clays with improved hydrothermal stability are, and will remain, areas of interest to the academic community, as well as to researchers in industrial laboratories (56). 

Hydrothermal Stability and Catafytic Cracking Performance of Some Pillared Clays... 

The acidities of cliys and pillared clays are between those of amorphous aluminosilicates and zeolites. Pillared clays can provide large-pore two-dimensional networks. Hectorite, montmorillonite, saponite and beidellite are the clays most often used to make pillared clays. Most pillared clays coke and deactivate st. This, and low thermal and hydrothermal stabilities have so far limited catalytic applications. Al, Ti, Zr, Cr, Si, and Fe and their mixtures give more stable pillars than those tried in the past. Occelli and Robson reviewed pillrued clays [52]. 

Pillared clays with high hydrothermal stability. 

Pinnavaia has reviewed the fundamentals, syntheses, and properties of pillared clays. The layers of these clays may be "floppy" or rigid (i.e., they may or may not be distorted by the pillars). Four types of layer-arrangement defects may be identified. Supergalleries 10-20 A high may be made. Whereas high coking and poor thermal and hydrothermal stabilities characterized practically all early pillared... 

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... 

At microactivity test (MAT) conditions, the calcined (500 C/2h) expanded clay exhibits high cracking activity and selectivities typical of pillared clays. In contrast to pillared smectites, these materials have some hydrothermal stability and, can be... 

Pillared clay development started in the mid 1950 s by Barrer and co-workers (Baiter, 1978). They synthesized high-surface-area materials by pillaring mont-morillonite clay with cations of N(CH3)4+ and N(C2H5)4+. However, such materials have low thermal and hydrothermal stabilities and therefore have limited use as adsorbents and catalysts. Mnch interest and research have been directed toward the synthesis of pillared clays with high thermal and hydrothermal stabilities. The... 

Although pillared clays could generate low cost fluidized cracking catalysts (FCC) with unique selectivity properties, they have not yet been accepted by the petroleum industry. In fact, refiners (to date) have been reluctant to field test these new catalysts because, in addition to a high tendency for coke generation, they exhibit hydrothermal stability inferior to that of those zeolites used in hydrocarbon conversion processes. The physicochemical properties of pillared clays have been reviewed elsewhere (1,2). 

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)... 

In recent years great interest has been focussed on the prepciration and the characterization of different typ>es of pillared clays and also on possible applications for these materials primarily as catalysts or adsorbents. One of the most interesting potential applications for pillared clays is as active components in cracking catalyst formulations designed for cracking of heavy oil fractions. The commercial use of pillared clays for this application has, however, been limited by their lack of thermal and hydrothermal stability. 

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