Selective Separation of Alcohols on Chabazite and SAPO-34

Adsorption and separation of 1-alcohols were studied on the
SAPO-34 molecular sieve, which is the catalyst of choice for the methanol-toolefins
(MTO) process. Vapor phase adsorption isotherms of methanol and
ethanol were measured at 343 K using the gravimetric technique. Liquid phase
isotherms of pure 1-alcohols and mixtures were obtained by batch adsorption
measurements at room temperature. These experiments highlighted the occurrence
of a window effect, giving rise to a strongly chain length-dependent adsorption and
diffusion. Lower alcohols such as methanol and ethanol were able to fill the entire
pore volume. Alcohols larger than 1-butanol only adsorb in very small amounts after
3 h, as their adsorption in the SAPO-34 pores is sterically hindered, which gives rise
to diffusional limitations. Binary batch experiments showed preferential adsorption
of short chain alcohols (i.e., ethanol) from longer chain molecules. Breakthrough
separation experimentswith several alcohol mixtures were performed at 298473K
and varying flow rates from 0.1 to 4.0 mL/min. Separation of ethanol from hexanol
could be achieved at room temperature. SAPO-34 is also able to selectively remove ethanol from 1-propanol at room temperature. An
increase in temperature to 348 K, however, improves the separation as a result of the improved ethanol diffusivity. The observed effect can
be used to separate short chain molecules from longer ones.