Pillared preparation

Figure 5.9. SEM images of (a) Si pillars prepared by microsphere lithography (b) ZnO nanorods grown on Si pillars.

Preparation of Pillared Clay Catalysts. PAG products are used for the preparation of zeolite-like catalysts by intercalation, the insertion of Al polycations molecules between the alurninosiHcate sheets of clay (3,33). Aqueous clay suspensions are slowly added to vigorously stirred PAG solutions, and the reaction mixture is aged for several hours. The clay is separated from the PAG solution and washed free of chloride ion. The treated clay is first dried at low temperature and then calcined in air at 450—500°G, producing a high surface area material having a regular-sized pore opening of about 0.6 to... 

Nowadays biocatalysis is a well-assessed methodology that has moved from the original status of academic curiosity to become a widely exploited technique for preparative-scale reactions, up to the point that the so-called industrial biotechnology (to which biocatalysis contributes to the most extent) is one of the three pillars of the modern sustainable chemistry. 

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). 

The original epoxidation with titanium-tartrate is homogeneous, but it can be carried out heterogeneously without diminishing enantioselectivity by using titanium-pillared montmorillonite catalyst (Ti-PILC) prepared from titanium isopropoxide, (+)-DAT, and Na+-montmorillonite.38 Due to the limited spacing of Ti-PILC, the epoxidation becomes slower as the allylic alcohol gets bulkier. 

The intercalant solution was prepared by titration of an Al3+/ Fe3+cationic solution with 0.2 molL"1 NaOH. The cationic solution contained 0.18 and 0.02 molL"1 of A1C13 and FeCl3, respectively. The NaOH solution was slowly added to the cationic solution at 70°C until the OH/cation molar ratio was equal to 1.9. The intercalant solution was added to the clay suspension under stirring. The final ( Al+Fe) /clay ratio was equal to 3.8 mmol/g of dry clay. After aging for 24h, the pillared clay precursor was washed until total elimination of chloride ions, dried at 60°C and finally calcined at 500°C for 5h. The resulting material is (Al-Fe)PILC. 

The alkylation of phenol investigated over H-MCM-22, H-ITQ-2 and H-MCM-36 showed that the delamelation and pillaring did not improve the catalytic activity and this was explained on the secondary processes taking place during the preparation of the corresponding materials, and which strongly affect the total acidity and the acidity on the external surface. Also, the composition of the reaction products is not influenced to a considerable extent by product shape selectivity effects. This seems to show that the tert-butylation reaction preferentially proceed at (or close to) the external surface of the zeolite layers. 

Microwave irradiation has also been applied to the preparation of Fe203/S04-superacid [20, 21] and high-surface aluminum pillared montmorillonites [20],... 

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). 

Influence of Preparation Conditions on the Catalytic Properties of Al-Pillared Montmorillonites... 

Aging of hydroxyalumlnum polycation solutions used to prepare pillared clays affects the properties. Including surface area and cracking activity, of these pillared clays. The effects were correlated with the state of hydrolysis of the aged solutions. Dilution of the pillaring reagent, Chlorhydrol, causes depolymerlzatlon of polycations present In this reagent and the formation of new polycations which react to form the pillared clay. 

Lower dilution levels did not allow sufficient depolymerlzatlon and higher dilution caused excessive depolymerlzatlon In the aged solutions. Pillared clays prepared from aged dilute solutions had an enhanced microstructure which showed an Increased activity for selectively cracking large molecules to the light cycle oil range. This microstructure Is lost In the presence of steam which also reduces the formation of catalytic coke. Addition of rare earth zeolite to pillared clay can partially overcome the effects of this loss of microstructure. 

The surface areas of pillared clays prepared from dilute Chlorhyd-rol solution depend on the extent of dilution and age of the dilute solution. Dilution produces polycations favorable to the production of pillared clays by depolymerizing larger polycations present in the Chlorhydrol solution. Aging is the reequilibration of these depolymerized solutions. 

---