J. Phys. Chem. C 2010, 114, 7767–7773
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Proton Transfer Mediated by Water: Experimental Evidence by Neutron Diffraction Alberto Alberti* and Annalisa Martucci Earth Sciences Department, UniVersity of Ferrara, Via G. Saragat 1, 44100 Ferrara, Italy ReceiVed: January 14, 2010; ReVised Manuscript ReceiVed: March 16, 2010
Neutron Rietveld refinement of a synthetic low silica ferrierite (Si/Al ) 8.5) in deuterium form rehydrated with D2O vapor (RD-FER) was performed in the Immm space group. Three Brønsted acid sites were identified. The first (D1) was on O4 framework oxygen, occupied to 18%; the second (D2) was on O6, occupied to 8%; the latter (D3) was on O1, occupied to 35%. This Brønsted site distribution strongly differs from that found in the nonrehydrated deuterium form (D-FER) in the same ferrierite sample (Martucci, A.; Alberti, A.; Cruciani, G.; Radaelli, P.; Ciambelli, P.; Rapacciuolo, M. Microporous Mesoporous Mater. 1999, 30, 95.), where D3 site is absent and D1 and D2 are occupied to 14 and 16%, respectively. The discrepancy between the Brønsted sites found in RD-FER and D-FER indicated remarkable acidic proton mobility and was explained by a proton transfer mechanism where water molecules or water clusters act as carriers of protons. Introduction Acid 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 The principal mechanism of acidity in these materials is the donation of Brønsted acid protons from bridging framework hydroxyls. The location and population of these hydroxyl groups in hydrogen zeolites provide a basis for the interpretation of their properties. For this reason, the protonated or deuterated forms of zeolites have long been the subject of research. Despite the large effort devoted to these studies, the knowledge of the interaction of Brønsted acid sites with polar molecules at an atomic level is still incomplete. An area of great practical interest is the mechanism of proton transfer inside microporous channels in zeolites. Extensive theoretical and experimental studies have been carried out to address this problem. The proton transfer from Brønsted acid sites to water molecules adsorbed in the channels of a microporous solid acid catalyst, which form hydroxonium ions (H3O)+ or charged water clusters (H3O)+(H2O)n, is now commonly accepted and has been demonstrated as possible, whereas the reaction pathway of the spontaneous proton transfer from a framework oxygen to another framework oxygen mediated by water is still being debated. In this research, we show that this latter proton transfer is possible and occurs in D-forms of ferrierite via neutron diffraction. To reach this objective, we need to know the location and population of Brønsted acid sites in this material before and after proton transfer. Up to now, a limited number of investigations on acid forms of zeolites have been carried out by X-ray or neutron diffraction to obtain the location and population of Brønsted sites. Neutron diffraction is the most direct experimental method used to probe Brønsted acid siting because neutrons interact strongly enough with hydrogen nuclei to have a significant effect on diffracted intensities. Deuterium is the isotope that is usually used in neutron diffraction experiments because of its large coherent scattering cross-section and tolerably small incoherent scattering cross-section. Neutron powder diffraction experiments were * To whom correspondence should be addressed. Tel: +39-532-974732. Fax: +39-532-974767. E-mail:
[email protected].
therefore performed in a number of dehydrated acid zeolites2-8 to localize acid 2H and determine their occupancy. The same experimental method has been used by Martucci et al.9 to determine the population and location of hydroxyl groups in calcined D-ferrierite. Ferrierite is a zeolite that is well-known as both a natural and a synthetic material. It is a medium pore material described as an excellent shape-selective catalyst for skeletal isomerization of n-butene to isobutene,10 which is an important feedstock for the production of methyl tert-butyl ether (MTBE), a commercial oxygenate additive in unleaded motor fuel. Its framework is characterized by two systems of mutually perpendicular onedimensional channels delimited by 10- and 8-membered rings. The crystal structure of natural Mg-rich ferrierite has been solved in the orthorhombic Immm space group,11 which is also the maximum topological symmetry of the framework, with four symmetrically independent tetrahedral sites (T1, ..., T4). Mgrich ferrierites are characterized by the presence of an Mg(H2O)62+ octahedron at the center of the so-called “ferrieritecage”,12 namely, a [82626458] cage. A monoclinic symmetry (with a metrically orthorhombic unit cell), space group P21/n, has been reported for a natural Mg-poor, Na- and K-rich ferrierite13 and for an as-synthesized K-rich, Si-poor synthetic ferrierite.14 On the contrary, ferrierites that have been cation exchanged with NH4+14 as well as in their H-form14 display an Immm symmetry. This symmetry reduction has mainly been ascribed to the movement of one bridging oxygen away from the inversion center at 1/4,1/4,1/4 site to avoid 180° T-O-T bond angles.12 Although the Immm space group has been successfully used in structure refinements in Mg-rich ferrierites, a lowering of the real symmetry to Pnnm (unit cell choice: a > b > c) has been suggested as due to static disorder in the Mg(H2O)62+ octahedron.12 The reduction of symmetry from Immm to Pmnn (the same unit cell choice as above) has been found in all-silica ferrierite.15 Immm symmetry is restored in synthetic all-silica ferrierites above 400 K.15 Neutron Rietveld refinements of dehydrated synthetic low silica ferrierite in deuterium form9 was also performed in the Immm space group. Unfortunately, the structural information about the (Si, Al) distribution available for ferrierite is very scarce. To date, only five single-crystal structure refinements, on natural samples, have
10.1021/jp100370u 2010 American Chemical Society Published on Web 04/02/2010
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TABLE 1: Lattice Parameters and Refinement Details for RD-FERa space group Immm a (Å) b (Å) c (Å) V (Å)3 refined pattern min/max 2θ (°) Rwp (%) Rp (%) RF2 (%) Nobs Nvar
18.968(2) 14.114(1) 7.445(1) 1993.1(4) 7.7-137° 3.08 2.30 8.6 2568 79
a Neutron radiation, λ )1.59943 (1) Å. Rp ) Σ[Yio - Yic]/ΣYio; Rwp ) [Σwi(Yio - Yic)2/ΣwiYio2]0.5. RF2 ) Σ|Fo2 - Fc2|/Σ|Fo2|. Estimated standard deviations in parentheses refer to the last digit.
been performed. The results16,17 seem to indicate disorder, or partial order, in the (Si, Al) distribution, with an Al preference in the order of T2 > T1 > T3 > T4. Experimental Methods A sample of a synthetic low-silica ferrierite (Engelhardferrierite EZ-500), with the chemical composition K2.7Na1.1Al3.8Si32.2O72 · 12H2O, was used in this study. The ND4-form was obtained by the ion exchange of the K,Na-form with 1 M ND4NO3 aqueous solution for 139 h at room temperature. The residual K and Na contents after exchange were determined via atomic absorption spectroscopy and proved to be T3 > T4. This discussion has schematized the process, which results in the different distributions of deuterons in D-FER and RDFER as well as possible. As a matter of fact, if we compare the populations of the D sites, instead of their occupancies, in D-FER and RD-FER, we have D1 D2 D3 total
D-FER 1.12 deuterons per unit cell 1.28 2.40
RD-FER 1.46 0.64 1.40 3.50
Therefore, it appears that all D sites, and not only D2 and D3, are involved in the redistribution of deuterons. It is evident that whatever this complex mechanism is, strong proton migration occurs in the framework of the ferrierite rehydration process, and in this process, a decisive factor is played by proton transfer mediated by water. However, it should be noted that the relative stability of the Brønsted sites not only depends on the Si, Al distribution in the different tetrahedral sites but also on the Si/ Al ratio of the zeolite.52
W11
a Estimated standard deviations in parentheses refer to the last digit.
the most reasonable explanation for this result is proton migration from the O6-D2 hydroxyl to the new D3 position bonded to the O1 framework oxygen. In the previous sections,
Conclusions Neutron Rietveld structure refinements of a synthetic low silica ferrierite in its rehydrated deuterium form (RD-FER) revealed strong differences in the location and amount of Brønsted acid sites with respect to its calcined deuterium form (D-FER). These differences involved both the number and the occupancies of the hydroxyl groups. In fact, only two hydroxyl groups were localized in D-FER on O4 and O6 framework oxygen atoms, with an occupancy of 0.14 and 0.16, respectively, whereas three hydroxyl groups were localized in RD-FER on
Proton Transfer Mediated by Water O4, O6, and O1 framework oxygen atoms and their occupancies were 0.18, 0.08, and 0.35, respectively. As a result, 2.4 and 3.5 Brønsted sites were located in D-FER and RD-FER, respectively, in comparison with the expected 3.8 Brønsted sites on the basis of the aluminum content. Therefore, these results indicate the remarkable mobility of acidic protons in ferrierite. In particular, the discrepancy between the Brønsted sites found in D-FER and their theoretical value was attributed to a proton transfer to reabsorbed H2O molecules, forming either hydroxonium ions or water charged clusters because of the presence in D-FER in a small amount of water molecules. The mobility of protons evidenced by the differences in location and occupancies of Brønsted deuterons between D-FER and RD-FER was explained by a proton transfer mechanism where water molecules or water clusters act as carriers of protons. These results are of paramount importance as the most important catalytic reactions in zeolites are catalyzed by acid sites, and consequently, location and occupancy of Brønsted sites play a fundamental role. It is therefore evident that the presence of water molecules, even in undetectable amounts, is important in the zeolitic catalyst in catalytic processes. References and Notes (1) Corma, A. Chem. Rew. 1995, 95, 559. (2) Czjzek, M.; Jobic, H.; Fitch, A. N.; Vogt, T. J. Phys. Chem. 1992, 96, 1535. (3) Fischer, R. X.; Baur, W. H.; Shannon, R. D.; Staley, R. H.; Abrams, L.; Vega, A. J.; Jorgensen, J. D. Acta Crystallogr. 1987, 179, 281. (4) Smith, L. J.; Davidson, A.; Cheetham, A. K. Catal. Lett. 1997, 49, 143. (5) Smith, L. J.; Cheetham, A. K.; Morris, R. E.; Marchese, L.; Thomas, J. M.; Wright, P. A.; Chen, J. Science 1996, 271, 799. (6) Martucci, A.; Cruciani, G.; Alberti, A.; Ritter, C.; Ciambelli, P.; Rapacciuolo, M. Microporous Mesoporous Mater. 2000, 35-36, 405. (7) Campbell, B. J.; Cheetham, A. K.; Vogt, T.; Carluccio, L.; Parker, W. O., Jr.; Flego, C.; Millini, R. J. Phys. Chem. B 2001, 105, 1947. (8) Bull, L. M.; Cheetham, A. K.; Hopkins, P. D.; Powell, B. M. J. Chem. Soc., Chem. Commun. 1993, 1196. (9) Martucci, A.; Alberti, A.; Cruciani, G.; Radaelli, P.; Ciambelli, P.; Rapacciuolo, M. Microporous Mesoporous Mater. 1999, 30, 95. (10) Xu, W.-Q.; Yiu, Y.-G.; Suib, S. C.; Edwards, J. C.; O’Young, C. L. J. Phys. Chem. 1995, 99, 9443. (11) Vaughan, P. A. Acta Crystallogr. 1966, 21, 983. (12) Alberti, A.; Sabelli, C. Z. Kristallogr. 1987, 178, 249. (13) Gramlich-Meier, R.; Gramlich, V.; Meier, W. M. Am. Mineral. 1985, 70, 619. (14) Cruciani, G.; Alberti, A.; Martucci, A.; Knudsen, K. D.; Ciambelli, P.; Rapacciuolo, M. Proceedings 12th International Zeolite Conference, Baltimore, MD, July 5-10, 1998; Treacy, M. M. J., Markus, B. K., Bisher, M. E., Higgins, J. B., Eds.; Materials Research Society: Warrandale, PA, 1998; p 2361. (15) Bull, I.; Lightfoot, P.; Villaescusa, L. A.; Bull, L. M.; Gover, R. K. B.; Evans, J. S. O.; Morris, R. E. J. Am. Chem. Soc. 2003, 125, 4342. (16) Alberti, A.; Gottardi, G.; Lai, T. (1989) Guidelines for Mastering the Properties of Molecular SieVes. Relationship between the Physicochemical Properties of Zeolitic Systems and Their Low Dimensionality; Barthomeuf, D., Derouane, E. G., Holderich, W., Eds.; NATO ASI Series B: Physics; Plenum Press: New York, 1989; Vol. 221, p 145. (17) Yokomori, Y.; Wachsmuth, J.; Nishi, K. Microporous Mesoporous Mater. 2001, 50, 137.
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JP100370U
