Nonuniform Chain-Length-Dependent Diffusion of Short 1‑Alcohols in SAPO-34 in Liquid Phase

Liquid-phase diffusion of 1-alcohols in SAPO-34 was explored by batch experimentation. The uptake of pure and binary mixtures of 1-alcohols, dissolved in <i>tert</i>-butanol, was obtained for C1–C8 1-alcohols at temperatures between 25 and 80 °C, concentrations varying between 0.5 and 10 wt %, and crystal sizes between 7.5 and 20 μm. The experimental uptake data were fitted with an intracrystalline diffusion model and a linear driving force model. The intracrystalline diffusion coefficient showed a nonuniform stepwise decrease with chain length, ranging from 10^–12 m²/s for methanol to 10^–20 m²/s for 1-pentanol. No effect of the external concentration on the intracrystalline diffusion coefficient was observed. Variation of the crystal size showed that the intracrystalline diffusion is the rate-limiting step. On the basis of the Arrhenius equation, the activation energies of diffusion of ethanol, 1-propanol, and 1-butanol were determined, being, respectively, 27.8, 47.8, and 47.2 kJ/mol. Co-diffusion occurred in the uptake of binary mixtures of methanol/ethanol, methanol/1-propanol, and ethanol/1-propanol, where mutual effects could be noticed. From this experimental work, it could be concluded that the small dimensions of the SAPO-34 framework generate a very sterically hindered diffusion of 1-alcohols into the crystals, resulting in a chain-length-dependent behavior, interesting to obtain efficient kinetic-based separations

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