Influence of Confining Environment Polarity on Ethanol Dehydration Catalysis by Lewis Acid Zeolites

2018 Spring Symposium

Jason S. Bates and Raja­mani Gounder, Charles D. David­son School of Chem­i­cal Engi­neer­ing, Pur­due Uni­ver­si­ty, West Lafayette, IN

Abstract — The dif­fer­ent reac­tiv­i­ty of Lewis acid sites (M) in zeo­lite frame­works, when con­fined with­in non-polar (hydropho­bic) or polar (hydrophilic) sec­ondary envi­ron­ments, can arise from dif­fer­ences in com­pet­i­tive inhi­bi­tion by sol­vents, sol­vent-medi­at­ed mech­a­nisms, and extend­ed sol­vent struc­tures.3 Frame­work Lewis acid cen­ters also adopt open ((HO)-M-(OSi≡)3) and closed (M-(OSi≡)4) con­fig­u­ra­tions that show dif­fer­ent reac­tiv­i­ty for Baey­er-Vil­liger oxi­da­tion, glu­cose iso­mer­iza­tion, and aldol con­den­sa­tion.5 Here, we inter­ro­gate the reac­tiv­i­ty of Sn cen­ters iso­lat­ed with­in Beta zeo­lites using bimol­e­c­u­lar ethanol dehy­dra­tion to diethyl ether (404 K). Sn sites in open and closed con­fig­u­ra­tions, quan­ti­fied from IR spec­tra of adsorbed CD3CN before and after reac­tion, con­vert to struc­tural­ly sim­i­lar inter­me­di­ates dur­ing ethanol dehy­dra­tion catal­y­sis (404 K) and revert to their ini­tial con­fig­u­ra­tions after regen­er­a­tive oxi­da­tion treat­ments (21% O2, 803 K). Dehy­dra­tion rates (404 K, 0.5–35 kPa C2H5OH, 0.1–50 kPa H2O) mea­sured on ten low-defect (Sn-Beta-F) and high-defect (Sn-Beta-OH) zeo­lites were described by a rate equa­tion derived from mech­a­nisms iden­ti­fied by DFT cal­cu­la­tions, and sim­pli­fied using micro­ki­net­ic mod­el­ing to iden­ti­fy kinet­i­cal­ly-rel­e­vant path­ways and inter­me­di­ates. Polar hydrox­yl defect groups locat­ed in con­fin­ing envi­ron­ments pref­er­en­tial­ly sta­bi­lize reac­tive (ethanol-ethanol) and inhibito­ry (ethanol-water) dimer­ic inter­me­di­ates over monomer­ic ethanol inter­me­di­ates. As a result, equi­lib­ri­um con­stants (404 K) for ethanol-water and ethanol-ethanol dimer for­ma­tion are 3–4× high­er on Sn-Beta-OH than on Sn-Beta-F, con­sis­tent with
insights from sin­gle-com­po­nent and two-com­po­nent adsorp­tion mea­sure­ments. Intrin­sic dehy­dra­tion rate con­stants (404 K) were iden­ti­cal among Sn-Beta-OH and Sn-Beta-F zeo­lites; thus, mea­sured dif­fer­ences in dehy­dra­tion turnover rates sole­ly reflect dif­fer­ences in preva­lent sur­face cov­er­ages of inhibito­ry and reac­tive dimer­ic inter­me­di­ates at active Sn sites. The con­fine­ment of Lewis acidic bind­ing sites with­in sec­ondary envi­ron­ments of dif­fer­ent defect den­si­ty con­fers the abil­i­ty to dis­crim­i­nate sur­face inter­me­di­ates on the basis of polar­i­ty, pro­vid­ing a design strat­e­gy to accel­er­ate turnover rates and sup­press inhi­bi­tion by water.

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