Solvation and acid strength effects on catalysis by faujasite zeolites Rajamani Gounder a , Andrew J. Jones a , Robert T. Carr a , Enrique Iglesia a,b,⇑ a Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720, United States b Division of Chemical Sciences, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States article info Article history: Received 1 July 2011 Revised 28 October 2011 Accepted 6 November 2011 Available online 14 December 2011 Keywords: Acid strength Faujasite Isobutane Sodium Solvation abstract Kinetic, spectroscopic, and chemical titration data indicate that differences in monomolecular isobutane cracking and dehydrogenation and methanol dehydration turnover rates (per H + ) among FAU zeolites treated thermally with steam (H-USY) and then chemically with ammonium hexafluorosilicate (CD- HUSY) predominantly reflect differences in the size and solvating properties of their supercage voids rather than differences in acid strength. The number of protons on a given sample was measured consis- tently by titrations with Na + , with CH 3 groups via reactions of dimethyl ether, and with 2,6-di-tert-butyl- pyridine during methanol dehydration catalysis; these titration values were also supported by commensurate changes in acidic OH infrared band areas upon exposure to titrant molecules. The number of protons, taken as the average of the three titration methods, was significantly smaller than the number of framework Al atoms (Al f ) obtained from X-ray diffraction and 27 Al magic angle spinning nuclear mag- netic resonance spectroscopy on H-USY (0.35 H + /Al f ) and CD-HUSY (0.69 H + /Al f ). These data demonstrate that the ubiquitous use of Al f sites as structural proxies for active H + sites in zeolites can be imprecise, apparently because distorted Al structures that are not associated with acidic protons are sometimes detected as Al f sites. Monomolecular isobutane cracking and dehydrogenation rate constants, normalized non-rigorously by the number of Al f species, decreased with increasing Na + content on both H-USY and CD-HUSY samples and became undetectable at sub-stoichiometric exchange levels (0.32 and 0.72 Na + /Al f ratios, respectively), an unexpected finding attributed incorrectly in previous studies to the presence of minority ‘‘super-acidic’’ sites. These rate constants, when normalized rigorously by the number of resid- ual H + sites were independent of Na + content on both H-USY and CD-HUSY samples, reflecting the stoi- chiometric replacement of protons that are uniform in reactivity by Na + cations. Monomolecular isobutane cracking and dehydrogenation rate constants (per H + ; 763 K), however, were higher on H-USY than CD-HUSY (by a factor of 1.4). Equilibrium constants for the formation of protonated methanol dimers via adsorption of gaseous methanol onto adsorbed methanol monomers, determined from kinetic studies of methanol dehydration to dimethyl ether (433 K), were also higher on H-USY than CD-HUSY (by a factor of 2.1). These larger constants predominantly reflect stronger dispersive interactions in H-USY, consistent with its smaller supercage voids that result from the occlusion of void space by extraframe- work Al (Al ex ) residues. These findings appear to clarify enduring controversies about the mechanistic interpretation of the effects of Na + and Al ex species on the catalytic reactivity of FAU zeolites. They also illustrate the need to normalize rates by the number of active sites instead of more convenient but less accurate structural proxies for such sites. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Faujasite (FAU, Y-zeolite) is used as a solid acid catalyst, often after thermal or chemical treatments render it more stable during catalysis [1–4]. Although FAU zeolites contain only one framework Al (Al f ) T-site, isolated Al f atoms give rise to OH groups with signif- icantly smaller deprotonation energies (DPE; 1161–1166 kJ mol 1 ) than OH groups on Al f atoms with next-nearest Al neighbors (1177–1247 kJ mol 1 ) [5]. As a result, isolated Brønsted acid sites in FAU zeolites should behave as uniform sites, equal in acid strength as well as solvating environment. This appears to be con- sistent with rates of alkane cracking (per g) that are proportional to the number of isolated Al f atoms on FAU zeolites treated by a given thermal or chemical protocol [6–10]. Yet, cracking rates (per Al f ) differ among FAU zeolites treated by different thermal and chem- ical methods [6–10], in apparent contradiction to the constant turnover rates expected from a single-site catalyst. Thermal treatments that convert Y-zeolite to its ultrastable form (USY) [1,4,11,12] create extraframework Al (Al ex ) moieties by the extraction of Al from framework sites and also increase 0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.11.002 ⇑ Corresponding author at: Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA 94720, United States. Fax: +1 510 642 4778. E-mail address: iglesia@berkeley.edu (E. Iglesia). Journal of Catalysis 286 (2012) 214–223 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat