The Trials and Tribulations of Zeolite Structure Analysis

2011 Spring Symposium

Lynne B. McCusker and Chris­t­ian Baer­locher
Lab­o­ra­to­ry of Crys­tal­log­ra­phy
ETH Zurich

Abstract — Struc­tur­al infor­ma­tion is essen­tial to the under­stand­ing of zeo­lite chem­istry. Whether it is the syn­the­sis process, the adsorp­tion prop­er­ties, the ion-exchange selec­tiv­i­ty, or the cat­alyt­ic activ­i­ty that is of inter­est, the key lies in the struc­ture. How­ev­er, zeolitic mate­ri­als tend to be poly­crys­talline, so stan­dard sin­gle-crys­tal meth­ods of struc­ture analy­sis can­not be applied. For­tu­nate­ly, pow­der dif­frac­tion meth­ods have devel­oped enor­mous­ly in the last 20 years, and as a result, struc­ture analy­sis using pow­der dif­frac­tion data has become almost rou­tine. Nev­er­the­less, zeo­lite struc­tures often pose a chal­lenge to these meth­ods.

For struc­ture solu­tion, i.e. when the zeo­lite frame­work struc­ture is not known, the prob­lems to be over­come are twofold: (1) the basic phase prob­lem, which is cen­tral to all crys­tal­lo­graph­ic struc­ture analy­ses (includ­ing sin­gle crys­tals), and (2) the reflec­tion over­lap prob­lem, which is spe­cif­ic to pow­der dif­frac­tion data. Over the years, our group has devel­oped sev­er­al dif­fer­ent approach­es to these prob­lems, and a few of these (the use of chem­i­cal infor­ma­tion to sup­ple­ment the pow­der dif­frac­tion data, the adap­ta­tion of the charge-flip­ping algo­rithm to accom­mo­date pow­der dif­frac­tion data, and the use of elec­tron microscopy data to com­ple­ment the pow­der dif­frac­tion data) will be described.

Once the basic frame­work struc­ture is known, inves­ti­ga­tion of the details of the struc­ture can begin. Where is the struc­ture direct­ing agent? Where are the iso­mor­phous­ly sub­sti­tut­ed atoms in the frame­work? What hap­pens upon cal­ci­na­tion? Where are the ions before and after ion exchange? Where are the sorbed mol­e­cules? Under favor­able cir­cum­stances, these ques­tions can be addressed by gen­er­at­ing dif­fer­ence elec­tron den­si­ty maps using the mea­sured pow­der dif­frac­tion pat­tern and the frame­work struc­ture mod­el. How­ev­er, not all prob­lems are amenable to this tech­nique and even when they are, the inter­pre­ta­tion of the maps still requires con­sid­er­able patience and per­se­ver­ance. The pos­si­bil­i­ties and lim­i­ta­tions will be dis­cussed.

Speaker’s Biog­ra­phy — Lynne McCusker has been study­ing zeo­lite crys­tal struc­tures since she start­ed her doc­tor­al research with Karl Seff at the Uni­ver­si­ty of Hawaii in 1976. After spend­ing three years as a post­doc in the group of Wal­ter Meier at the ETH in Zurich, Switzer­land learn­ing pow­der dif­frac­tion tech­niques from Chris­t­ian Baer­locher, she moved on to Texas A&M Uni­ver­si­ty to put these new­ly acquired skills into prac­tice. In 1985, she moved back across the Atlantic for anoth­er post­doc, this time at Oxford Uni­ver­si­ty in Mike Glazer’s group. There she learned the val­ue of syn­chro­tron radi­a­tion and solved her first nov­el zeo­lite frame­work struc­ture (Sig­ma 2). For this work, she received the Bar­rer Award from the British Zeo­lite Asso­ci­a­tion in 1987 and the Phys­i­cal Crys­tal­log­ra­phy Award from the British Crys­tal­lo­graph­ic Asso­ci­a­tion in 1989. In 1988, she returned to the ETH and for the last 20 years has head­ed a group, togeth­er with Chris­t­ian Baer­locher, devot­ed to the devel­op­ment of method­ol­o­gy for solv­ing zeo­lite frame­work struc­tures from pow­der dif­frac­tion data. In 2007, they received the Breck Award of the IZA (togeth­er with Osamu Terasa­ki) for their zeo­lite struc­tur­al work. In 2010, she received the IZA award for the research she has done with Chris­t­ian and for her ser­vice to the IZA com­mu­ni­ty.