Sites In ZSM-5 Research Articles

Promotion and characterization of zeolitic catalysts used in the synthesis of hydrocarbons from syngas

Bifunctional metal-zeolite catalysts cobalt-ZSM-5 and cobalt-thoria-ZSM-5 have been examined for the conversion of synthesis gas (H 2/CO = 1) at 280 °C, 21 bar and WHSV = 0.77 g(g cat) −1 h −1 Thoria in amounts as low as 0.4 wt.% added to a cobalt-ZSM-5 with 9.0 wt.% cobalt caused a considerable increase in synthesis gas conversion and selectivity to gasolinerange hydrocarbons. Addition of thoria to the catalyst also resulted in a decrease of the aromatic fraction in the hydrocarbon product. Some Co-ThO 2-ZSM-B catalysts provided nearly 65 wt.% selectivity to gasolinerange product that contained 26 vol.% aromatics (alkylbenzenes). The selectivity to gasoline-range product exceeded the limit (48 wt.%) set by Schulz-Flory kinetics. Chemisorption and magnetization studies have been performed to determine metal particle size. Addition of thoria resulted in a decrease of the cobalt particle diameter from about 40 nm to 10 nm and caused an increase in CO/H adsorbed. The action of thoria as a promoter is possibly related to the presence of basic sites on its surface. Infrared studies have demonstrated a relative decrease in the number of Brönsted acid sites in ZSM-5 upon the addition of thoria, thus accounting for the decrease of the aromatic fraction in the liquid hydrocarbon product.

Oxygen-18 exchange between zeolite ZSM-5 and water

Porotektosilicate frameworks of zeolites, including high silica materials, are considerably more reactive than we tend to think. This has been demonstrated by studies of /sup 18/O exchange between high-purity samples of sysnthetic zeolite ZSM-5 in the ammonium form and liquid water at 95/sup 0/C. The investigation of the /sup 18/O-exchange kinetics revealed a two-step reaction which can be interpreted in terms of hydroxyl groups exchanging relatively rapidly (primary exchange) and oxygen atoms from T-O-T bridges exchanging at a rate about 40 times slower (secondary exchange). Since the T sites in ZSM-5 contain mostly Si and only minor amounts of Al, the reported findings imply that not only Si-O-Al but also Si-O-Si bridges are cleaved under relatively mild conditions in the presence of water. 2 figures, 1 table.