(476d) Ethanol Upgrading to Olefins over Metal-Containing Beta Zeolites: Characterization and Catalysis

We report the selective conversion of ethanol to butene-rich olefins over metal-containing (i.e., Cu, Zn, and Y) dealuminated Beta (deAlBeta) zeolite catalysts. CuZnY/deAlBeta was prepared by physically mixing metal salts with deAlBeta or by incipient wetness impregnation, followed by heating under flowing air (823 K). Catalysts were characterized by X-ray diffraction, nitrogen physisorption, X-ray adsorption spectroscopy, scanning transmission electron microscopy, and probe molecule chemisorption followed by collection of transmission FTIR spectra. Each metal plays a distinct role in the multistep pathway from ethanol to olefins. Ethanol dehydrogenation to acetaldehyde occurs on both Cu and Zn, followed by aldol condensation of acetaldehyde to form crotonaldehyde. With increasing Y weight loading (0-7 wt%), the C-C coupling rate increased from 0.3 to 7 x 10^-6 mol g_cat^-1s^-1 (<10% conversion, 543 K). FTIR analyses revealed that each metal binds pyridine as a Lewis acid site (LAS), and that the various LAS can be distinguished by the vibrational frequency of adsorbed pyridine. The amount of pyridine adsorbed to Lewis acidic Y sites (Y_LAS) increased linearly with increasing Y loading and in direct proportion to the C-C coupling rate, suggesting that Y_LAS are the predominant active sites for C-C coupling in CuZnY/deAlBeta. Titration with pyridine under static vacuum was used to quantify the integrated molar extinction coefficient for pyridine bound to Y_LAS, and to quantify the number of moles of Y_LAS by saturating Y/deAlBeta samples with pyridine. In situ pyridine titrations during conversion of ethanol and acetaldehyde to butadiene (503 K) were used to estimate the number of Lewis acidic active sites present during reaction, and these estimates agreed within experimental error to the number of Y_LAS quantified by pyridine FTIR (463 K). Thus, we propose that quantification of Y_LAS via ex situ pyridine FTIR reliably quantifies active sites for butadiene formation over Y/deAlBeta.