TMA Offretite

Upon seeding, ZSM-20 appears better stabilized as no traces of zeolite Beta were found after 27 days heating. Our observation is in agreement with other recent works that showed the high efficiency of the addition of very small and freshly formed zeolite nuclei as seeds to batches giving zeolites TMA-Offretite 1481 or TMA-Omega 149). Both materials were formed more rapidly and selectively, free from stable side phases like Analcime or Mordenite that usually co-crystallize in absence of seeds. 

Thermograms of(Rb,TMA) offretite crystals in oxygen and nitrogen. Sample weight 15.15 mg. Heating rate 10 °C/min. Gas flow rate ... 

Source N. Y. Chen, J. L. Schlenker, W. E. Garwood, and G. T. Kokotailo, TMA-Offretite. Relationship Between Structural and Catalytic Properties, Journal of Catalysis 86 24-31 (1984). With permission. 

Tsitsishvili et al. have carried out experiments of methanol conversion on H-offretite and TMA-offretite. TMA-offretite zeolites were calcined at 200 and 450 °C. H-offretite zeolites were prepared by ammonium ion-exchange and then calcined at 300 and 450 C. TMA-offretite calcined at 200 C was inactive, probably because the channels are blocked by the large Me N ions so that the acid sites become inaccessible for methanol molecules. A hydrocarbon fraction containing principally propylene, propane, n-butane, and n-butene was obtained in the cases of TMA-offretite and H-offretite calcined at 450 "C. At reaction temperatures lower than 210 C only dimethyl ether was detected. H-offretite zeolites are active in the isomerization of xylenes, indicating that the removal of TMA-cations enlarged the pore opening. 

Givens et al. used erionite, TMA-offretite, zeolite T, and ZSM-34 as catalysts for methanol conversion. They claim that the use of steam as diluent enhanced the selectivity for ethylene. The results obtained after 2 hours on stream showed that methanol conversion was higher when a 30/70 wt% methanol-water mixture was fed (see Table 6). This may be due to the fact that deactivation by coke was more rapid when a nondiluted methanol feed was used. Hydrocarbon fractions with up to 90 wt% of C2-C4 olefins were attained for the case that ZSM-34 zeolite was used as catalyst. 

Synthesis methods are described below for four very different but important zeolite structures, A,2 Y,3 tetramethylammonium (TMA) offretite4 and tetra-propylammonium (TPA) ZSM-5.5,6 These four were selected because they span the composition range from 1 1 Si Al to a potentially aluminum-free zeolite structure (A to ZSM-5). In addition, these syntheses provide examples of fundamental concepts in crystallization such as templating (TMA offretite and ZSM-5), low-temperature nucleation (Y), and variable reactant (silica) sources. 

Givens et al. [150] used erionite, TMA-offretite, zeolite T, and ZSM-34 as catalysts for MeOH conversion. The best yield in C -C olefins at the highest conversion was obtained with ZSM-34 zeolite (Table 5). 

Fernandez et al. [133] adapted the combined technique consisting of hydro-thermal treatment and successive extraction with acids, already successfully used for the dealumination of Y zeoHte [86, 87], for the dealumination of K,TMA-offretite. With an increasing number of treatment cycles (up to 4), the aluminum content could be progressively decreased from 3.41 to 1.68 Al per u. c. The process was found to be associated with a significant increase in the thermal stability of the lattice, but was also accompanied by the formation of defects and holes in the crystal (mesopore system). 

---