Shape Selectivity Revisited: Higher Catalytic Rates in Smaller Zeolite Channels

2011 Spring Symposium

Aditya Bhan
Depart­ment of Chem­i­cal Engi­neer­ing and Mate­ri­als Sci­ence
Uni­ver­si­ty of Min­neso­ta
Twin Cities

Abstract — Zeo­lites are crys­talline inor­gan­ic frame­work oxides with chan­nel and pock­et dimen­sions typ­i­cal­ly small­er than 1 nanome­ter. Their con­strained envi­ron­ments are well known to select for chem­i­cal reac­tions via steric mech­a­nisms, typ­i­cal­ly, by exclu­sion of mol­e­cules or tran­si­tion states based on size. The strong effects of pore size and shape as they become com­men­su­rate with those of reac­tant species and the con­comi­tant effects on the enthalpy and entropy of adsorp­tion have also been broad­ly and con­vinc­ing­ly not­ed. We inquire instead, what are the effects of con­fine­ment in small chan­nels? In this talk, I will present three exam­ples where reac­tiv­i­ty in small 8-mem­bered ring pock­ets of H-MOR dif­fers from that in larg­er 12-mem­bered ring chan­nels of MOR.

(i) We show that the appar­ent effects of pro­ton den­si­ty and of hydrox­yl group envi­ron­ment on DME car­bony­la­tion turnover rates reflect instead the remark­able speci­fici­ty of eight-mem­bered ring zeo­lite chan­nels in accel­er­at­ing kinet­i­cal­ly rel­e­vant *CH3-CO reac­tion steps.

(ii) In zeo­lite pores large enough to accom­mo­date ethanol dimers, ethanol pref­er­en­tial­ly dehy­drates via a bimol­e­c­u­lar path­way to gen­er­ate diethyl ether since the for­ma­tion of ethanol dimer­ic species is ener­get­i­cal­ly more favor­able than the for­ma­tion of ethanol monomers. In zeo­lite chan­nels too small to accom­mo­date ethanol dimers, ethanol is selec­tive­ly dehy­drat­ed via a uni­mol­e­c­u­lar reac­tion path­way to gen­er­ate eth­yl­ene.

(iii) For iso­mer­iza­tion reac­tions of n-hexa­ne, 8-MR chan­nels of H-MOR min­i­mize the free ener­gy of required car­bo­ca­tion­ic tran­si­tion states, pos­si­bly via par­tial con­fine­ment effects that increase the entropy of the tran­si­tion state at the expense of the reac­tion enthalpy. These find­ings show that con­fine­ment in zeo­lite chan­nels influ­ences rate and selec­tiv­i­ty of hydro­car­bon reac­tions more fun­da­men­tal­ly than sim­ple con­sid­er­a­tions of size and shape.

Speaker’s Biog­ra­phy – Aditya Bhan received his Bach­e­lor of Tech­nol­o­gy (B. Tech.) in Chem­i­cal Engi­neer­ing from IIT Kan­pur in 2000. Sub­se­quent­ly, he moved to West Lafayette, Indi­ana and joined the group of Nick Del­gass at Pur­due, where he devel­oped micro­ki­net­ic mod­els to describe propane arom­a­ti­za­tion on pro­ton- and gal­li­um- form ZSM-5 mate­ri­als for his PhD. In 2005, he moved to the Uni­ver­si­ty of Cal­i­for­nia at Berke­ley to pur­sue post-doc­tor­al stud­ies in Pro­fes­sor Enrique Iglesia’s group to study the kinet­ics, mech­a­nism, and site require­ments of dimethyl ether car­bony­la­tion. In Sep­tem­ber 2007, Dr. Bhan took up his present posi­tion as an Assis­tant Pro­fes­sor in the Depart­ment of Chem­i­cal Engi­neer­ing and Mate­ri­als Sci­ence at the Uni­ver­si­ty of Min­neso­ta. Dr. Bhan leads a research group that focus­es on the struc­tur­al and mech­a­nis­tic char­ac­ter­i­za­tion of inor­gan­ic mol­e­c­u­lar sieve cat­a­lysts use­ful in ener­gy con­ver­sion and petro­chem­i­cal syn­the­sis. His research at Min­neso­ta has been rec­og­nized with the McK­night Land Grant Pro­fes­sor and 3M Non-tenured Fac­ul­ty awards.