Article pubs.acs.org/JPCC
Water Coordination, Proton Mobility, and Lewis Acidity in HY Nanozeolites: A High-Temperature 1H and 27Al NMR Study Marios S. Katsiotis,*,† Michael Fardis,‡ Yasser Al Wahedi,† Samuel Stephen,† Vasilios Tzitzios,‡ Nikolaos Boukos,‡ Hae Jin Kim,§ Saeed M. Alhassan,† and Georgios Papavassiliou*,‡ †
Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates Institute of Nanoscience and Nanotechnology, National Centre for Scientific Research “Demokritos”, Aghia Paraskevi, Attiki 153 10, Greece § Division of Material Science, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-go, Daejeon 305-806, Republic of Korea ‡
ABSTRACT: A nanosized HY zeolite was synthesized and studied by means of 1H and 27Al NMR during thermal dehydration in the temperature range 20−600 °C. The nanozeolite is comprised of a mixture of well-crystallized ultrathin platelets and octahedral nanocrystals, dressed with pentacoordinated extraframework Al(V). 1H NMR spin−lattice (T1) and spin−spin (T2) relaxation measurements in combination with 27Al 3Q-MAS NMR reveal two different interaction paths between water molecules and the nanozeolite solid matrix: (i) water molecules strongly interacting with Al(V) cations, indicated by the high T1/T2 ratio, and (ii) water molecules with amply smaller T1/T2 ratio, interacting moderately with Al(IV) and Al(VI) cations. Relevant measurements on bulk HY rich in extraframework Al(VI) show the presence of the second relaxation channel only, indicating that the enhanced water adsorption observed for the nanozeolite originates partly from its extended surface and partly from the Al(V) decoration. Al(IV) sites in the nanozeolite appear to be highly resilient during heating, even while the framework starts to collapse and Al(VI) transforms to Al(V). Finally, 1H NMR shows that water protons interact particularly strongly with the Al sites in the nanozeolite at temperatures as high as 500 °C, unveiling the important role of the Al(V) decoration on this nanocatalyst.
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INTRODUCTION
BAS/LAS synergy can result in enhanced catalytic performance of dealuminated HY zeolites.14,15 Mirodatos et al. suggested that the superacid sites in dealuminated zeolites were produced by the interactions between protonic sites and polymeric oxoaluminum deposited into the zeolite voids, while Guisnet et al. proposed that the inductive influence of the LAS on the protonic sites of the zeolite was responsible for the promoting effect on the rates of isomerization, cracking, and hydrogen transfer in dealuminated HY zeolites.16,17 In this context an important role is played by water confined into the zeolite cages. 1H broad-line NMR experiments at 4 K and 1H MAS NMR at 300 K have shown that BAS/LAS synergy in dealuminated HY zeolites, mediated through adsorbed water molecules, is responsible for the increase in the number of hydroxonium ions.14 The link between the Brønsted and the Lewis acidic sites is established by binding FAL (BAS) and EFAL (LAS) sites through hydrogen bonds between adjacent water molecules. Such hydrogen bonds can include both proton donor and proton acceptor roles for adjacent water molecules. Evidently, the formation of this network weakens the ZO−H bonds, thus making ionization
It is well established that acidity in zeolites depends on the coordination of aluminum and the chemical nature of its neighbors.1 Active sites are primarily framework bridging hydroxyl protons, known as Brønsted acid sites (BAS), which are compensating for the excess of negative charge created by Al atoms in the zeolite framework. The concentration and strength of BAS influences strongly the catalytic reactivity of zeolites and particularly of zeolite Y.1−7 In its as-synthesized form with framework Si/Al ratios in the range 2.4−2.9, zeolite Y exhibits only weak acidity and low hydrothermal stability.8,9 Both properties can greatly be improved by steam calcination, which results in partial removal of framework aluminum (FAL) and the formation of ultrastable Y zeolite (USY). Dehydroxylation, steaming, or dealumination of acidic zeolites remove aluminum from the lattice, transforming it into extraframework aluminum (EFAL) which can enhance catalytic activity remarkably. Various types of EFAL have been proposed in the literature, such as AlO+, Al(OH)2+, and Al(OH)2+ cations or neutral species such as AlOOH, Al(OH)3, and Al2O3.10,11 In addition, experiments and calculations on dealuminated zeolites (including USY) imply that EFAL may be rich in pentacoordinated aluminum containing Lewis acid sites (LAS).12,13 © 2015 American Chemical Society
Received: December 31, 2014 Revised: January 21, 2015 Published: January 22, 2015 3428
DOI: 10.1021/jp513030w J. Phys. Chem. C 2015, 119, 3428−3438
Article
The Journal of Physical Chemistry C
great number of water protons are in “spin contact” (i.e., bonded) with framework and extraframework sites.10,27−29 The CP-MAS signal intensity decreases rapidly with increasing temperature, indicating a massive detachment of water molecules from the zeolite surface. Notably, the signal intensity of the nanozeolite is twice as strong as that of the bulk material, denoting that the nanozeolite exhibits higher water chemisorption than the bulk zeolite. It should be mentioned that despite the deteriorating effect of thermal dehydration on the zeolite framework, the number of Al(IV) sites in the nanozeolite remains unchanged by heating, as shown by 27Al MAS and 27Al MQ MAS NMR. This is in contrast with the bulk zeolite, where thermal desorption of water is shown to cause strong destruction of the FAL Al(IV) sites. At the same time Al(VI) in the nanozeolite is shown to convert to Al(V) by heating. This complies with the fact that the interaction of water protons with the Al sites, as dictated by the 1 H NMR T1/T2 ratio, remains extraordinarily strong in the nanozeolite up to the highest NMR experiment temperature, T = 500 °C.
and formation of hydroxonium ions easier. The observed enhanced proton mobility in hydrated zeolites has been ascribed to intersite motion involving a proton vehicle mechanism, where the H2O/H3O+ (hydroxonium) complex is the vehicle, while intersite hopping involving H+ ions is the dominant mechanism.18,19 The interaction of water with EFAL species in hydrated dealuminated HY zeolites has been thoroughly studied by using 1H double quantum MAS NMR.15 Li et al. found the mentioned zeolites to contain EFAL in the form of Al(OH) and two different types of water (rigid and mobile water); 1H−27Al HECTOR experiments showed that both EFAL and FAL are in close proximity to rigid water, apparently connected with strong H bonds. The interaction between FAL, EFAL, and water molecules is expected to develop in a significantly different manner for nanosized zeolites (nanozeolites). Reducing the size of the zeolite particles down to nanometers induces strong structural and electronic distortions which greatly alter the bonding of water to cations and framework atoms; Brønsted acidity is known to be drastically affected.20,21 Experiments on dealuminated HZSM-5 nanozeolites have shown strong formation of pentacoordinated Al sites with enhanced Lewis acidity in comparison to microsized HZSM-5 sample.22 Furthermore, BAS−LAS synergy has also been recently observed in HZSM-5 catalysts for conversion of methylcyclohexane.23 Nanoscalerelated effects can significantly alter aluminum coordination and the role or water molecules in nanozeolites; however, the exact way this happens remains unclear. In this work the interaction of water with FAL and EFAL cations during water desorption in a nanosized HY zeolite is studied by using 1H and 27Al NMR in the temperature range 20−600 °C. To our knowledge, this is the first time that hightemperature NMR relaxation studies of zeolites are presented in the literature. High-resolution TEM shows that the nanozeolite is a mixture of excellently crystallized ultrathin platelets and octahedral nanocrystals ( 350 °C the T1 distribution becomes practically single peaked. On the basis of these observations it is anticipated that water protons in the nanozeolite acquire two different proton mobility states: (i) Slow relaxing immobile water protons and (ii) fast relaxing mobile protons subjected to a similar relaxation mechanism as the bulk zeolite. Combining the 1H T1 NMR results with the 27Al 3Q-MAS and 1H−27Al CP-MAS NMR data it becomes clear that immobile protons are assigned to water molecules coordinated to Al(V) sites, while mobile protons are assigned to water molecule clusters hydrogen bonding Al(IV) with Al(VI). By increasing temperature water dissociates rapidly, and above 300 °C mobile protons are practically absent. As discussed in the previous paragraph, most probably the slow relaxing 1H NMR signal component originates from Al-bonded hydroxyl groups and protons produced by the Hirschler−Plank mechanism in the vicinity of Al(V) sites.38 In order to better visualize proton mobility in the two samples the 1H NMR spin−lattice T1 and spin−spin T2 relaxation times of the fast relaxing component are presented as a function of temperature in Figure 10. In the case of the
Figure 8. Normalized 1H T1 distribution for bulk HY in the temperature range 20−600 °C at 4.7 T.
single peak, acquiring a minimum value T1 ≈ 7 ms at ∼150 °C. The short T1 values indicate that relaxation is dominated by highly mobile water protons.39,40 For temperatures higher than 300 °C a second slow relaxing weak peak is observed, which may be attributed to protons in silanol (SiOH) groups and specific extraframework Al moieties. A question that arises by comparing the 1H NMR T1 profiles with the 27Al MAS and 1 H−27Al CP-MAS NMR results is why water protons attached to different Al species acquire a uniform relaxation mechanism. A possible answer is that protons in water clusters bridging Al(IV) with Al(VI) are fast exchanged while moving through a H3O+ vehicle mechanism.18,19 This imposes a uniform relaxation mechanism. At elevated temperature where intracage water has dissociated, proton hoping proceeds only through adjacent Al species. Figure 9 demonstrates the T1 relaxation time at different temperatures for the nanozeolite specimen. A number of
Figure 10. 1H NMR T1 (blue circles) and T2 (black circles) as a function of temperature for bulk (filled circles) and nanozeolite (open circles) HY in a magnetic field of 4.7 T. In the case of the nanozeolite only the short T1 signal component is presented.
bulk zeolite T1 values were obtained as 1/e values with a single exponential fit, while in the case of the nanozeolite the fast relaxing component of a two-exponential fit is presented. The bulk zeolite displays clearly the T1 minimum at ∼150 °C mentioned above. Such a minimum is absent in the nanozeolite. At first sight the presence of the T1 minimum indicates that proton motion obeys a thermally activated law, and 1H NMR relaxation could be, in principle, explained in the framework of the BPP theory by assuming a Lorenzian spectral density function according to formula 1/T1 ≈ J(ω) ≈ [τc/(1 + ω2τc2)] and correlation time τc defined according to formula τc = τ0 exp(E/RT).41 By fitting this formula to the experimental data a nominal activation energy E = 25.6 kJ/mol has been calculated.42,43 However, T2 does not display the typical T2 temperature dependence expected from the BPP theory; it is observed that T2 instead of increasing by heating decreases rapidly up to 150 °C, while at higher temperatures it remains constant.
Figure 9. Normalized 1H T1 distribution for nanozeolite HY in the temperature range 20−500 °C at 4.7 T.
characteristic differences can be observed in comparison to the bulk zeolite. (I) T1 values are significantly higher in the case of the nanozeolite. This indicates that protons in water molecules adsorbed on the zeolitic nanoparticles are sufficiently less mobile than in the bulk zeolite. (II) The T1 distribution of the nanozeolite exhibits two peaks at low temperatures, indicative of the presence of two 3435
DOI: 10.1021/jp513030w J. Phys. Chem. C 2015, 119, 3428−3438
The Journal of Physical Chemistry C
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CONCLUSIONS In this work the unique physicochemical properties of a synthesized HY nanozeolite during thermal dehydration are presented. The nanozeolite displays a remarkable architecture, comprised of a mixture of well-crystallized ultrathin platelets and octahedral nanocrystals (
