The Determination of Brønsted Acid Sites in Zeolite ERS-7 by Neutron and X-ray Powder Diffraction B. J. Campbell* and A. K. Cheetham Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106 T. Vogt Physics Department, BrookhaVen National Laboratory, Upton, New York 11973 L. Carluccio, W. O. Parker, Jr., C. Flego, and R. Millini EniTecnologie S.p.A., Via F. Maritano 26, I-20097 San Donato Milanese, MI-Italy ReceiVed: August 1, 2000; In Final Form: NoVember 20, 2000 The deuterated acid form of zeolite ERS-7 (Si/Al ) 8.4) has been prepared by repeated cycles of D 2 O exposure and calcination. 27 Al NMR data show that the relative proportions of tetrahedral and octahedral Al in the resulting material are 70 and 30%, respectively. Three Brønsted sites were identified by a combined Rietveld refinement that simultaneously employed synchrotron X-ray powder diffraction data and neutron powder diffraction data. Final lattice parameters were a ) 9.7843(1) Å, b ) 12.3676(1) Å, and c ) 22.8336(2) Å, the combined R wp factor was 6.3%, and l 2 (reduced) was 1.48. Acid deuterons were located approximately 1 Å from oxygens O3, O5, and O7, with occupancy factors of 0.16(2), 0.16(3), and 0.18(2), respectively. These correpsond to 1.28(16), 0.64(12), and 1.44(16) deuterons per unit cell, respectively, for a total of 3.36(26) acid sites per unit cell, which is consistent with the tetrahedral Al content. Only deuteron D5 lies on a special position. T-O bond lengths suggest a site preference for Al at T4, T1, and possibly T2, each of which is bonded to at least one of the observed acid-site oxygens. A bridging framework oxygen site was identified in both the PND and PXD difference Fourier maps that appears to be disordered over at least two distinct sites; this corresponds to different local arrangements of Al at the neighboring T-sites. The out-of-plane framework hydroxyl tilt angles at each acid site were comparable to those of H + SSZ-13 and D + RHO. Acid site D7 demonstrated a significant in-plane hydroxyl tilt angle (7.5°), consistent with ab initio quantum mechanical calculations. This tilt clearly indicates an Al preference for site T4 over T6. Introduction Acidic zeolite catalysts are widely used in the chemical and petroleum industries for their catalytic activity, their remarkable reaction selectivity, and their excellent chemical and thermal stability. 1 Important applications include the isomerization of xylenes, paraffin hydroisomerization, catalytic cracking, and heavy oil hydrocracking. 2,3,4 The principal mechanism of acidity in these materials is the donation of Brønsted acid protons from bridging framework hydroxyls. Acid protons balance the nega- tive charge that is acquired by the otherwise neutral framework when Al 3+ ions occupy some of the tetrahedrally coordinated Si 4+ sites. Both natural and synthetic aluminosilicate zeolites can be converted to the acid form from other cationated forms by ion exchange with an NH 4 salt and subsequent calcination to remove NH 3 . A Brønsted acid proton interacts with the framework in a very different way than do other cations; rather than ionically coordinating to multiple framework oxygens, it covalently bonds to one of the oxygens adjacent to a tetrahedral Al atom and significantly distorts the local framework bonds and angles. 5 Recent computational studies have shown that the geometry of the framework in the vicinity of an acid site can significantly influence the formation of the intermediate species that lead to the final reaction products. 6,7 Ab initio cluster calculations of ethylene adsorption on a high-silica acid zeolite, for example, suggest the formation of an intermediate ethoxide that forms a C-O bond to a nearby framework oxygen. 8,9 Because the local structure of an acid site controls acidity, accessibility by reactant molecules, intermediate state formation, and hence the reaction product, the knowledge of its locations and environments within the framework is critical to better understanding acid zeolite catalysis. Several diffraction-based approaches have been used to determine the locations of Brønsted acid sites in zeolites and other silicates. Early inferences were based on average Si-O bond lengths, as determined by X-ray diffraction, 10 which are expected to increase upon protonation in proportion to the acid site occupancy factor. In Ca 3 (SiO 3 OH) 2 (H 2 O) 2 (afwillite) and Na 2 H 2 (SiO 4 )(H 2 O) 8 , for example, certain Si-O bonds were observed to be 0.08 Å longer than the others, on average, and were attributed to the formation of an acidic hydroxyl. 11,12 Based on these observations, Olson and Dempsey investigated T-O (T ) Si, Al) distances in acidic zeolite Y (Si/Al ) 2.3) via single crystal X-ray diffraction, compared them to those of other acidic faujasite zeolites, and concluded that the O1 and O3 sites were preferentially protonated while the other framework oxygens were not. 13 * Corresponding author. Present address: Argonne National Laboratory, Materials Science Division, Argonne, IL 60439. E-mail: branton@anl.gov. 1947 J. Phys. Chem. B 2001, 105, 1947-1955 10.1021/jp002751t CCC: $20.00 © 2001 American Chemical Society Published on Web 02/15/2001