Catalysis by Pincer-Iridium Complexes. Breaking C-H Bonds, Making C-C Bonds, and Various Combinations Thereof

Sep­tem­ber 2018

Pro­fes­sor Alan S. Gold­man
Depart­ment of Chem­istry and Chem­i­cal Biol­o­gy, Rut­gers — The State Uni­ver­si­ty of New Jer­sey
E-mail: alan.​goldman@​rutgers.​edu, Web: http://​ccb​.rut​gers​.edu/​g​o​l​d​m​a​n​-​a​lan
Abstract: Irid­i­um com­plex­es have played a lead­ing role in the organometal­lic chem­istry of
alka­nes and unre­ac­tive C-H bonds since the incep­tion of the field 30 years ago. We have found
that “PCP”-pincer-ligated irid­i­um com­plex­es are par­tic­u­lar­ly effec­tive for the dehy­dro­gena­tion of
alka­nes and have incor­po­rat­ed this reac­tion into tan­dem sys­tems for sev­er­al cat­alyt­ic
trans­for­ma­tions based on dehy­dro­gena­tion. A close­ly relat­ed class of reac­tions that we are
explor­ing is dehy­dro­gena­tive cou­pling. More recent­ly we have turned atten­tion to irid­i­um
Phe­box com­plex­es. Although the (PCP)Ir and (Phebox)Ir units are for­mal­ly iso­elec­tron­ic, the
for­mer oper­ates via C-H acti­va­tion by Ir(I) while the lat­ter effects dehy­dro­gena­tion via Ir(III) (as
an acetate com­plex) and pos­si­bly Ir(V) inter­me­di­ates. Such a high-oxi­da­tion-state cat­alyt­ic cycle
offers advan­tages for many poten­tial appli­ca­tions of dehy­dro­gena­tion. In par­al­lel, how­ev­er, we
find that the low-oxi­da­tion-state (+I) chem­istry of (Phebox)Ir offers its own nov­el hydro­car­bon
1. Gao, Y.; Guan, C.; Zhou, M.; Kumar, A.; Emge, T. J.; Wright, A. M.; Gold­berg, K. I.; Krogh-Jes­persen, K.; Gold­man, A. S. J. Am. Chem. Soc. 2017,
139, 6338–6350.
2. Wilk­low-Mar­nell, M.; Li, B.; Zhou, T.; Krogh-Jes­persen, K.; Bren­nes­sel, W. W.; Emge, T. J.; Gold­man, A. S.; Jones, W. D. J. Am. Chem. Soc. 2017,
139, 8977–8989.
3. Gold­berg, K. I.; Gold­man, A. S. Acc. Chem. Res. 2017, 50, 620–626.
4. Kumar, A.; Bhat­ti, T. M.; Gold­man, A. S. Chem. Rev. 2017, 117, 12357–12384.
5. Gao, Y.; Emge, T. J.; Krogh-Jes­persen, K.; Gold­man, A. S. J. Am. Chem. Soc. 2018, 140, 2260–2264.

Renewable Isoprene By Sequential Hydrogenation of Itaconic Acid and Dehydra-Decyclization of 3-Methyl-Tetrahydrofuran

2018 Spring Symposium

Omar Abdel­rah­man, Post-Doc, Paul Dauen­hauer Group, Depart­ment of Chem­i­cal Engi­neer­ing & Mate­r­i­al Sci­ence, Uni­ver­si­ty of Min­neso­ta, 421 Wash­ing­ton Ave. SE, Min­neapo­lis, MN 55455

Abstract — The cat­alyt­ic con­ver­sion of bio­mass-derived feed­stocks to val­ue added chem­i­cals is an impor­tant chal­lenge to alle­vi­ate the depen­dence on petro­le­um-based resources. To accom­plish this, the inher­ent­ly high oxy­gen con­tent of bio­mass com­pounds, such as that of lig­no­cel­lu­losic bio­mass, requires sig­nif­i­cant reduc­tion via hydrodeoxy­gena­tion strate­gies. The unsat­u­rat­ed car­boxylic acid ita­con­ic acid (IA) can be pro­duced from bio­mass via fer­men­ta­tion path­ways, for exam­ple. A path­way of inter­est is the con­ver­sion of IA to iso­prene, facil­i­tat­ing the renew­able pro­duc­tion of an indus­tri­al­ly rel­e­vant diolefin. IA can be suc­ces­sive­ly hydro­genat­ed to yield 3-methyl tetrahy­dro­fu­ran (3-MTHF), in a one-pot cas­cade reac­tion, where a Pd-Re bimetal­lic cat­a­lyst results in an 80% yield to 3-MTHF. The 3-MTHF can then be con­vert­ed to iso­prene, and oth­er pen­ta­di­enes, through an acid cat­alyzed vapor-phase dehy­dra-decy­cliza­tion. Mul­ti­ple sol­id acid cat­a­lysts, includ­ing alu­mi­nosil­i­cates, met­al oxides and phos­pho­rous mod­i­fied zeo­lites, were screened for the dehy­dra-decy­cliza­tion step. A new class of cat­alyt­ic mate­ri­als, all sil­i­con
phos­pho­rous con­tain­ing zeo­lites, were found to be the most selec­tive (70% iso­prene and 20% pen­ta­di­enes), where the major side reac­tion involved is a retro-prins con­den­sa­tion of 3-MTHF to butane and formalde­hyde. Through kinet­ic stud­ies, an inves­ti­ga­tion into the effect of Brøn­st­ed acid strength, pore size and oper­at­ing con­di­tions on the selec­tiv­i­ty to iso­prene are dis­cussed. The prospect of apply­ing this dehy­dra-decy­cliza­tion strat­e­gy to oth­er sat­u­rat­ed cyclic ethers will also be dis­cussed, which enables the pro­duc­tion of oth­er diolefin mol­e­cules of inter­est such as buta­di­ene and lin­ear pen­ta­di­enes.

[1] Abdel­rah­man, O. A.; Park, D. S.; Vin­ter, K. P.; Span­jers, C. S.; Ren, L.; Cho, H. J.; Zhang, K.; Fan, W.; Tsap­at­sis, M.; Dauen­hauer, P. J. ACS Catal. 2017, 7, 1428–1431.

Using Water as a Co-catalyst in Heterogeneous Catalysis to Improve Activity and Selectivity

2018 Spring Symposium

Lars C. Grabow, Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing, Uni­ver­si­ty of Hous­ton, Hous­ton, TX 77204–4004, USA

Abstract — “What hap­pens when you add water?” is pos­si­bly the most fre­quent­ly asked ques­tion after pre­sen­ta­tions in het­ero­ge­neous catal­y­sis. In this talk, I will demon­strate that this ques­tion is indeed para­mount and that the pres­ence of even minute amounts of water can dras­ti­cal­ly change reac­tion rates and prod­uct selec­tiv­i­ties. Exam­ples include water-medi­at­ed pro­ton hop­ping across a met­al-oxide sur­face, oxi­da­tion of car­bon monox­ide at the gold/titania inter­face, and hydrodeoxy­gena­tion of phe­no­lic com­pounds over tita­nia sup­port­ed ruthe­ni­um cat­a­lysts. Togeth­er, these exam­ples demon­strate that water can act as co-cat­a­lyst in a vari­ety of cat­alyt­ic reac­tions and by vary­ing the amount of water it may be pos­si­ble to tune reac­tion rates and prod­uct selec­tiv­i­ty.

Selective Catalytic Oxidation of Alcohols over Supported Metal Nanoparticles and Atomically-Dispersed Metal Cations

2018 Spring Symposium

Robert J. Davis, Depart­ment of Chem­i­cal Engi­neer­ing, Uni­ver­si­ty of Vir­ginia, Char­lottesville, VA, USA

Abstract — Selec­tive oxi­da­tion of alco­hols to car­bonyl com­pounds is an impor­tant reac­tion in organ­ic syn­the­sis and will like­ly play a sig­nif­i­cant role in the devel­op­ment of val­ue-added chem­i­cals from bio­mass. The indus­tri­al appli­ca­tion of a pre­cious met­al cat­a­lyst such as Pt, how­ev­er, can be hin­dered by deac­ti­va­tion and high price. We have there­fore explored the mode of deac­ti­va­tion dur­ing alco­hol oxi­da­tion on Pt by in-situ spec­troscopy and stud­ied the role of var­i­ous pro­mot­ers on cat­a­lyst per­for­mance. Results con­firm that slow decar­bony­la­tion of prod­uct alde­hyde deposit­ed unsat­u­rat­ed hydro­car­bon on the sur­face that blocked access to the active sites. Addi­tion of Bi as a pro­mot­er did not pre­vent the decar­bony­la­tion side reac­tion, but instead enhanced the acti­va­tion of dioxy­gen dur­ing the cat­alyt­ic cycle. In an effort to avoid the use of pre­cious met­als alto­geth­er, the oxi­da­tion of alco­hols over atom­i­cal­ly-dis­persed, non-pre­cious met­al cations (Fe, Cu, and Co) locat­ed in a nitro­gen-doped car­bon matrix was demon­strat­ed. Exten­sive char­ac­ter­i­za­tion of these non-pre­cious met­al cat­a­lysts revealed impor­tant insights into the oxi­da­tion mech­a­nism and sta­bil­i­ty of this new class of atom­i­cal­ly-dis­persed met­al cat­a­lyst.

Tuning the Electrocatalytic Oxygen Reduction Reaction Activity of PtCo Nanocrystals by Cobalt Concentration and Phase Transformation Methods

2018 Spring Symposium

Jen­nifer D. Lee, Ph.D. Can­di­date, Christo­pher B. Mur­ray Group, Depart­ment of Chem­istry, Uni­ver­si­ty of Penn­syl­va­nia

Abstract — The pro­ton exchange mem­brane fuel cell (PEMFC) is a crit­i­cal tech­nol­o­gy to enhance the clean, sus­tain­able pro­duc­tion and usage of ener­gy, but prac­ti­cal appli­ca­tion remains chal­leng­ing because of the high cost and low dura­bil­i­ty of the cath­ode cat­a­lysts that per­form oxy­gen reduc­tion reac­tion (ORR). Efforts have been placed on the study of intro­duc­ing first-row tran­si­tion met­als in Pt-M alloys to reduce the Pt load­ing and mod­u­late geo­met­ric, struc­tur­al and elec­tron­ic effects. To fur­ther improve the ORR reac­tion rate and cat­a­lysts sta­bil­i­ty, alloys that adopt an inter­metal­lic struc­ture, espe­cial­ly the tetrag­o­nal L10-PtM phase, has been one of the most promis­ing mate­ri­als. In this con­tri­bu­tion, monodis­perse PtCo nanocrys­tals (NCs) with well-defined size and Co com­po­si­tion are syn­the­sized via solvother­mal meth­ods. The trans­for­ma­tion from face-cen­tered cubic (fcc) to ordered face-cen­tered tetrag­o­nal (fct) struc­ture was achieved via ther­mal anneal­ing. Depend­ing on the selec­tion of trans­for­ma­tion meth­ods, dif­fer­ent degrees of order­ing were intro­duced and fur­ther cor­re­lat­ed with their ORR per­for­mance. A detailed study of the anneal­ing tem­per­a­ture and com­po­si­tion depen­dent degree of order­ing is also high­light­ed. This work pro­vides the insight of dis­cov­er­ing the opti­mal spa­tial dis­tri­b­u­tions of the ele­ments at the atom­ic lev­el to achieve enhanced ORR activ­i­ty and sta­bil­i­ty.

Commercial Perspective of Alternative Routes to Acrylic Acid Monomer

2018 Spring Symposium

Jin­suo Xu, The Dow Chem­i­cal Com­pa­ny, 400 Arco­la Rd, Col­legeville, PA 19426

Abstract — Acrylic acid and cor­re­spond­ing acry­lates are major monomers for a vari­ety of func­tion­al poly­mers used broad­ly in our dai­ly life such as coat­ing, sealant, and per­son­al care. The two-stage selec­tive oxi­da­tion of propy­lene to acrolein and then to acrylic acid was first com­mer­cial­ized in ear­ly 70s and quick­ly became the dom­i­nant route to acrylic acid. Dri­ven by feed­stock cost or avail­abil­i­ty or sus­tain­abil­i­ty, sig­nif­i­cant efforts from both indus­try and acad­e­mia were devot­ed to devel­op­ing alter­na­tive routes to acrylic acid. Cat­a­lyst plays crit­i­cal role in the key step of trans­form­ing dif­fer­ent raw mate­r­i­al into prod­uct effec­tive­ly, for exam­ple, mixed met­al oxides MoVTeN­bOx in propane selec­tive oxi­da­tion, sol­id acids in dehy­dra­tion of glyc­erin or 3-HP, and var­i­ous oxides in aldol con­den­sa­tion of acetic acid and formalde­hyde. This pre­sen­ta­tion will dis­cuss the progress of these major routes, the chal­lenges towards com­mer­cial­iza­tion, and poten­tial solu­tions.

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.