Mechanisms, active intermediates, and descriptors for epoxidation rates and selectivities on dispersed early transition metals

2018 Spring Symposium

Daniel Bre­gante, Alay­na John­son, Ami Patel, David Fla­her­ty, Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing, Uni­ver­si­ty of Illi­nois, Urbana-Cham­paign

Abstract — Ear­ly tran­si­tion met­al atoms (groups IV-VI) dis­persed on sil­i­ca and sub­sti­tut­ed into zeo­lites effec­tive­ly cat­alyze the epox­i­da­tion of alkenes with hydro­gen per­ox­ide or alkyl per­ox­ide reac­tants, yet the under­ly­ing prop­er­ties that deter­mine the selec­tiv­i­ties and turnover rates of these cat­a­lysts are unclear. Here, a com­bi­na­tion of kinet­ic, ther­mo­dy­nam­ic, and in situ spec­tro­scop­ic mea­sure­ments show that when group IV — VI tran­si­tion met­als are dis­persed on sil­i­ca or sub­sti­tut­ed into zeo­lite *BEA, the met­als that form the most elec­trophilic sites give greater selec­tiv­i­ties and rates for the desired epox­i­da­tion path­way and present small­er enthalpic bar­ri­ers for both epox­i­da­tion and H2O2 decom­po­si­tion reac­tions.

In situ UV–vis spec­troscopy shows that these group IV and V mate­ri­als acti­vate H2O2 to form pools of hydroper­ox­ide and per­ox­ide inter­me­di­ates. Time-resolved UV–vis mea­sure­ments and the iso­mer­ic dis­tri­b­u­tions of cis-stil­bene epox­i­da­tion prod­ucts sug­gest that the active species for epox­i­da­tions on group IV and V tran­si­tion met­als are only M-OOH and M-(O2)2– species, respec­tive­ly. Mech­a­nis­tic inter­pre­ta­tions of turnover rates show that these group IV and V mate­ri­als cat­alyze epox­i­da­tions (e.g., of cyclo­hex­ene, styrene, and 1-octene) and H2O2 decom­po­si­tion through sim­i­lar mech­a­nisms that involve the irre­versible acti­va­tion of coor­di­nat­ed H2O2 fol­lowed by reac­tion with an olefin or H2O2. Epox­i­da­tion rates and selec­tiv­i­ties vary over five- and two-orders of mag­ni­tude, respec­tive­ly, among these cat­a­lysts and depend expo­nen­tial­ly on both the ener­gy for lig­and-to-met­al charge trans­fer (LMCT) and chem­i­cal probes of the dif­fer­ence in Lewis acid strength between met­al cen­ters. Togeth­er, these obser­va­tions show that more elec­trophilic active-oxy­gen species (i.e., low­er-ener­gy LMCT) are more reac­tive and selec­tive for epox­i­da­tions of elec­tron-rich olefins. The micro­p­ores of zeo­lites about active sites can serve to pref­er­en­tial­ly sta­bi­lize reac­tive states that lead to epox­i­da­tions by chang­ing the mean diam­e­ter of the pore or the den­si­ty of near­by silanol groups. Con­se­quent­ly, these prop­er­ties pro­vide oppor­tu­ni­ties to increase rates of epox­ide for­ma­tion over that with­in meso­porous sil­i­cas. Con­sis­tent­ly, H2O2 decom­po­si­tion rates pos­sess a weak­er depen­dence on the elec­trophilic­i­ty of the active sites and the sur­round­ing pore envi­ron­ment, which indi­cates that cat­a­lysts with both greater rates and selec­tiv­i­ties may be designed fol­low­ing these struc­ture-func­tion rela­tion­ships.