Abstract

Methyl tertiary butyl ether (MTBE) an antiknock additive presently used in gasoline blends as a safer alternative to lead compounds, is currently synthesized industrially from methanol and isobutene over an acidic ion-exchange resin Amberlyst-15. This commercial catalyst, although very active, still suffers from several drawbacks such as thermal instability, acid leaching, and need of a high methanol over isobutene ratio. Zeolites for their higher thermal stability and shape selectivity, were thus proposed to remedy to such disadvantages. However, they did not show a surface acidity which is as high as that of the organic resin. In gas phase synthesis of MTBE, the incorporation of an organic superacid trifluoromethanesulfonic acid onto the zeolite surface have shown an important enhancement in catalytic activity and product selectivity for the large pore sized Y-type zeolite. However this method did not increase significantly the catalytic activity of the medium pore sized ZSM-5 zeolite, because of limited diffusion phenomena. ZSM-5 zeolite, when doped with fluorine species in relatively low concentration using ammonium fluoride as precursor, and subsequently activated stepwise at high temperatures, showed two maxima for the MTBE yield. The first maximum occurring at the final activation temperature 325C̕, corresponded to the positive effect of chemisorbed HF acidic species onto the zeolite surface. The second maximum occurring at 450C̕, provided the highest MTBE yield and is attributed to the increased Brønsted acid sites density and strength resulting from the reaction of zeolite surface with the proton from the ±proton-fluoride» ions pair. Data obtained from different characterization techniques such as FTIR, 27 Al MAS-NMR, XRD, NH 3 -TPD, BET, MeOH adsorption tests, Fluoride-SIE and Pyridine adsorption tests supported the proposed mechanism for the modification of the fluorine treated ZSM-5 surface as a function of final activation temperatures. Controlled desilication of ZSM-5 zeolite by treatment with an aqueous solution of sodium carbonate under mild conditions has provided materials having much lower Si/Al ratio with an unchanged zeolite structure. The desilicated ZSMS zeolite exhibited a higher Bronsted acid sites density compared to the parent zeolite and showed a significant increase of MTBE conversion. The combination of these two modification techniques, i.e. desilication followed by incorporation of fluorine/thermal treatment produced a highly efficient catalyst: the yield of MTBE obtained was higher than that of the commercial catalyst and no isobutene dimer by-products were formed owing to the shape selectivity of ZSM-5 structure. With respect to the parent zeolite, the total acid density increased by almost 80% and the formation of some stronger acid sites were observed. This new inorganic catalyst can be advantageously applied to a wide range of acido-catalysed reactions up to 450C̕ which require a high surface acidity, a pronounced shape-selectivity and a strong chemical resistance.