Spectroscopic Technique Development for Understanding Solvent Effects in Liquid Phase Reactions

Meeting Program — April 2018

Nicholas Gould — Stu­dent Speak­er

Advi­sor: Bingjun Xu
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Uni­ver­si­ty of Delaware

Abstract — Bio­mass con­ver­sion reac­tions are fre­quent­ly con­duct­ed in a sol­vent, due to the high­ly oxy­genat­ed nature of the feed­stock.1,2 Thus, het­ero­ge­neous cat­alyt­ic active sites exist at a sol­id-liq­uid inter­face, where the sol­vent can mod­i­fy sur­face and adsor­bate ener­get­ics. Even when the sol­vent does not play a direct role in the reac­tion mech­a­nism, it can sta­bi­lize or desta­bi­lize adsor­bates, inter­me­di­ates, and tran­si­tion states, often lead­ing to marked­ly dif­fer­ent rates and selec­tiv­i­ties between sol­vent choic­es.3–5 How­ev­er, sol­vent effects are poor­ly under­stood because cat­a­lyst char­ac­ter­i­za­tion tech­niques, such as probe mol­e­cule adsorp­tion in FTIR, are most often con­duct­ed under vac­u­um or in vapor phase.6,7 Fur­ther, most stud­ies on sol­vent effects focus on screen­ing sol­vents via cat­alyt­ic activ­i­ty test­ing, where mul­ti­ple fac­tors that can influ­ence reac­tiv­i­ty exist simul­ta­ne­ous­ly: com­pet­i­tive adsorp­tion, sta­bi­liza­tion of reac­tants and tran­si­tion states, and phase equi­lib­ria dif­fer­ences. Thus, there is cur­rent­ly a need for exper­i­men­tal tech­niques capa­ble of extract­ing fun­da­men­tal ther­mo­dy­nam­ic prop­er­ties of sol­vents in sim­ple sys­tems, with the end goal of decou­pling the effects of sol­vent in cat­alyt­ic activ­i­ty tests.8

Atten­u­at­ed total reflec­tion (ATR) fouri­er trans­form infrared spec­troscopy (FTIR) was used to char­ac­ter­ize zeo­lites with probe mol­e­cules in the pres­ence of sol­vent. The ATR-FTIR was fur­ther devel­oped into a quan­ti­ta­tive tech­nique, with a pro­ce­dure for deter­min­ing extinc­tion coef­fi­cients for adsorbed pyri­dine on zeo­lites in the pres­ence of sol­vent.9 This allowed for quan­ti­ta­tive com­par­isons of the effect of sol­vent on probe mol­e­cule uptake and pro­to­na­tion in zeo­lite pores. Ongo­ing appli­ca­tions of the ATR-FTIR cell include adsorp­tion isotherms, dif­fu­sion mea­sure­ments, and tem­per­a­ture pro­grammed des­orp­tion (TPD) in porous mate­ri­als in liq­uid phase. Fur­ther, the effect of sol­vent on charge sta­bi­liza­tion in zeo­lite pores was stud­ied using a home­made TPD set up under back pres­sur­ized, flow­ing sol­vent. Pre­lim­i­nary pyri­dine des­orp­tion tem­per­a­tures from an H/ZSM-5 sam­ple reveal that the abil­i­ty of a sol­vent to sta­bi­lize pyri­dini­um ions decreas­es in the order: water > ace­toni­trile > alka­ne ≈ vac­u­um.


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