Adsorption and Separation of C1-C8 Alcohols on 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-to-olefins (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 experiments with several alcohol mixtures were performed at 298-473 K 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.