Conversion of renewable feedstocks to aviation fuels is paramount to reducing the carbon footprint of the aviation industry. Aviation fuel is a mixture of C9+ olefins, paraffins, and cycloalkanes. Current routes for production of C9+ olefins involve endothermic ethanol dehydration followed by ethylene oligomerization to higher olefins. The carbon emissions and capital costs for this two-step process could both be reduced via a single-step process that directly upgrades ethanol to C4+ olefins. Copper-, zinc-, and yttrium-containing dealuminated Beta molecular sieves (CuZnY/Beta) catalyze ethanol upgrading to C3+ olefins in 88% yield (623 K), with Cu performing vital ethanol dehydrogenation and intermediate hydrogenations in the presence of co-fed H2 (94.3 kPa). The initial rate of C-C bond formation from ethanol, acetaldehyde, and H2 containing feeds (543 K) correlates with the peak area for pyridine bound to Lewis acidic Y sites (YLAS) in ex situ FTIR spectra (463 K) for a series of CuZnY/Beta catalysts of varied Y loading (Si/Y 19-150). We quantified the molar extinction coefficient for pyridine bound to YLAS in order to quantify YLAS via saturation with pyridine and measurement of transmission IR spectra. Active sites were quantified during reaction via in situ pyridine titration during reactions of ethanol and acetaldehyde that form butadiene (BD; 423-623 K). These independently measured amounts of YLAS and active sites for BD formation are identical within experimental error across a series of Y/Beta catalysts (Si/Y 13-300). The fraction of reactive Y ranges between 10-50% of the total Y regardless of the synthetic protocol used to produce Y/Beta. These results enable quantification of per site BD formation turnover frequencies (TOFs) by measurement of BD site time yields per mol Y and ex situ transmission IR spectra of pyridine-saturated Y-containing molecular sieves. Per-active-site TOFs will be compared with those measured over La-, Zn- and Zr-containing molecular sieves. Y-containing sites that form crotonaldehyde and crotyl alcohol are distinct from those that catalyze crotyl alcohol dehydration, based on in situ site titrations and time-on-stream C4 product selectivities. A distribution of Y sites of currently unknown local structure(s) must exist on these materials. These findings demonstrate that further refinements in synthesis of Y-containing molecular sieves are possible to further improve their performance in ethanol upgrading catalysis.

Quantification of Lewis acidic yttrium sites in Y-containing molecular sieves during ethanol conversion to butadiene

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