Microporous materials synthetic clays

The use of clays as supports for hydroprocessing has been reported and summarized [9-11], Dibenzothiophene (DBT) diluted with hexadecane (0.75 wt% S) was the liquid feed for HDS tests. The pore diameter of the MSC catalysts is seen to have a strong effect on both the HDS activity and selectivity (Figure 4). A commercial catalyst (Crosfield 465, Co/Mo alumina) was also measured under these conditions where it gave 77% DBT conversion and 61% BP selectivity. In a previous study [12], other synthetic hectorites were compared using these conditions except that a 1 wt% S feed was utilized. One sample was a control made without template that consisted of only micropores. The DBT conversion and BP selectivity were very low for this microporous material. The Crosfield material has significant macroporosity (42% of the pore volume) in addition to a broad distribution of mesoporosity, and has clearly been optimized to perform well under these HDS conditions. 

The pillared clays (PlLCs) are microporous materials obtained by intercalation of inorganic species in natural or synthetic expandable clays. Currently the PlLCs are considered promising... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

K.A. Carrado and L.Q. Xu, Materials with controlled mesoporosity derived from synthetic polyvinylpyrrolidone-clay composites, Micropor. Mesopor. Mater. 27 87 (1999). 

The high-specific-surface-area supports (10 to 100 m /g or more) are natural or manufactured materials that normally are handled as fine powders. When processed into the finished catalyst pellet, these materials often give rise to pore size distributions of the macro-micro type mentioned previously. The micropores exist within the powder itself, and the macropores are created between the fine particles when they are compressed together in a pellet press. Diatomaceous earth and pumice (or cellular lava) are naturally occurring low-cost materials that are representative of this class of catalyst support. Among the synthetic carriers that can be created by modem technology are those derived from clays, bauxite, activated carbon, and xerogels of silica gel and alumina gel. 

Over the past ten years, a new family of microporous crytals, referred to as pillared clays , have attracted considerable interest owing to their wide range of applications from catalysts to molecular devices (Mitchell 1990). The number of materials has increased further with the use of specific clays, such as rectorite (Jie, Ze, and Zhiquing 1986) and synthetic fluor-tetrasilicic mica (TSM) (Urabe, Sakurai, and Izumi 1986 Johnson and Brody 1988). 

---