Further Search for Hydroxyl Nests in Acid Dealuminated Zeolite Y

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Further Search for Hydroxyl Nests in Acid Dealuminated Zeolite Y Istvan Halasz, Eric Senderov, David H. Olson, and Jian-Jie Liang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511082v • Publication Date (Web): 25 Mar 2015 Downloaded from http://pubs.acs.org on April 3, 2015

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The Journal of Physical Chemistry

Further Search for Hydroxyl Nests in Acid Dealuminated Zeolite Y Istvan Halasz*a, Eric Senderovb, David H. Olsonc, Jian-Jie Liangd a

PQ Corporation, Research & Development Center, 280 Cedar Grove Road, Conshohocken, PA

19428; [email protected] b

c

Rive Technology, Inc., Monmouth Junction, NJ 08852 (retired); [email protected];

Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road,

Piscataway, NJ 08854; [email protected]. d

Accelrys, Inc.,10188 Telesis Ct., San Diego, California 92121; [email protected]

*corresponding author

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Abstract The removal of Al with acid solution from a zeolite framework is customarily associated with formation of framework defects known as hydroxyl nests, but their existence has not been unambiguously confirmed thus far. In a recent study on acid dealuminated Y zeolite, preexchanged predominantly with Cs in order to ensure maximized elimination of molecular water that can conceal hydrogen bonded OH groups, no indication has been found for the presence of hydroxyl nests9.

We present here TPD (Temperature Programmed Desorption) and FTIR

(Fourier Transform Infra-Red) evidence that such hydroxyl nests cannot be identified in an approximately 20% acid dealuminated and solely sodium re-exchanged zeolite NaY(-Al) that has not been exposed to temperatures above 25 oC during

and after dealumination.

These

experimental conclusions were matched by results of combined DFT (Density Functional Theory)-based spectroscopic study and reactive-forcefield molecular dynamics calculations on full periodic model zeolites. They showed that: 1) in contrast to the general view, the four hydroxyls that would form a hydroxyl nest are energetically different from each other as attested by their computed vibrational spectra; 2) the most intense vibrations of hypothetical hydroxyl nests are missing from the experimental FTIR spectra of the dealuminated NaY zeolite; and 3) the Si-OH, O-H, and O-Na bonds dynamically break and interact with each other already at 25 o

C. Thus, we conclude that even if hydroxyl nest formation would follow the Al-removal from

the Y zeolite lattice by acid leaching, its existence may be ephemeral on a picosecond time scale. Keywords: hydroxyl nest, dealumination, silanol, zeolite Y, TPD, FTIR, DFT, molecular dynamics.


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1. Introduction Removal of aluminum from zeolite frameworks by leaching them with acid solutions has long been a frequent post-synthesis procedure1. It was demonstrated2 that sodium Y zeolite (NaY) could also be dealuminated with HCl solution without substantial loss in crystallinity when adjusted to the appropriate pH. The authors of this paper also showed that it takes 4 protons to extract each Al atom, which was in quantitative agreement with results reported a little earlier for the acid-dealumination of mordenite3. Thus both results point to the apparent formation of “hydroxyl nests”, initially proposed by Barrer and Makki4 who pioneered this zeolite post-treatment method:

This stable hydroxyl nest idea is based on structural similarity to their long known existence in hydrogarnet defects5. It ignores, however, that oxygen atoms of the OH-nest hydroxyls in a zeolite should be connected to silicon atoms (as shown in the scheme above), while in hydrogarnets the nest forming hydroxyl-oxygens are coordinated for example- by Ca2/3 and Al1/3 in the hydroglossular variety. Therefore, the structural entity of (SiOH)4 might be chemically quite different from (Ca2/3, Al1/3 –OH)4. Numerous research and review papers have referred to the presence of hydroxyl nests in a variety of zeolites ever since, even if the material was treated at several hundred oC6-8. This 3 ACS Paragon Plus Environment


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would suggest an enormous thermal stability for these entities of four vicinal OH groups. In our previous paper on the discussed issue [9[, we reviewed numerous attempts to prove the presence of hydroxyl nests in dealuminated zeolite Y, mordenite, ZSM5 and silicalite and did not find any arguments, which could unequivocally prove the existence of nests as zeolite framework defects. Their identification is extremely controversial even in zeolites treated only at lower temperatures owing to disagreements over the classification of signals from the applied analytical methods such as

29

Si MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance), FTIR (Fourier

Transform Infra-Red), neutron diffraction, etc., as we reviewed in a recent paper9. For example, it was shown more than 50 years ago10 that the hydroxyl vibrations characterized by bands in the vicinity of 3500 cm-1 are often associated with internal (OH)x-groups of zeolites, but they should be assigned to H2O molecules interacting with the oxygen atoms of the framework. This assignment was supported by hydrogen bond length calculations derived from the stretching frequencies of OH vibrations of a natural zeolite, natrolite, which were in excellent agreement with X-ray derived distances between the oxygen atoms of H2O molecules and that of framework oxygen atoms10. We also presented recently TPD and FTIR evidence that an acid leached and Cs, Na exchanged Y zeolite9, which had not been exposed to higher than 80 oC post-treatment temperatures, did not have measurable hydroxyl nest content. In the present paper we show on a similar acid leached and then solely with Na cationreexchanged Y zeolite, which has never been exposed after the acid treatment/Al extraction step to temperatures above 25 oC, that it also lacks any hydroxyl nests measurable by TPD and FTIR. Full back-exchange with sodium cations into the acid treated material assured that Brønsted acidic hydroxyls were absent and did not interfere with our tests by undergoing thermal dehydroxylation. Moreover we demonstrate with quantum mechanics and reactive force field 4 ACS Paragon Plus Environment

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based calculations that even theory does not support the idea of stable hydroxyl nests in the FAU structure at ambient or higher temperatures.

2. Experimental and computational methods 2.1. Sample sources and preparation The Si/Al ~ 2.55 ratio NaY zeolite (Unit cell size = 24.66 Å; crystallinity = 100%; micropore volume 0.39 cm3/g) used in this study was an experimental product from our labs. The dealuminated NaHY(-Al) derivative was prepared at Rive Technology Company from commercial NaY (CBV 100) by a citric acid treatment procedure described in reference11. The dealuminated NaHY(-Al) was back-exchanged at room temperature with excess 10% NaCl solution at pH adjusted to 7.5 by drops of 10% Na2CO3 solution to produce fully sodium exchanged dealuminated NaY(-Al). Completion of sodium exchange was confirmed by XRF analysis, which indicated Na/Al = 1 atom ratio in the solid. The nominal composition of this dealuminated material is Na42Al42(OH)48Si138O336 based upon the conventional assumption that each removed Al atom would leave behind one OH-nest [(OH)4], i. e., twelve removed Al atoms form 12 defects all filled with (OH)4 hydroxyl nests. Its unit cell size was found to be 24.63 Å, it maintained about 75% crystallinity, and its micropore volume was 0.33 cm3/g.

2. 2. Analytical techniques The elemental composition of samples was calculated from X-ray fluorescence (XRF) analysis data obtained with an AXious analyzer from PANalytical. Unit cell size (UCS) and crystallinity were calculated from X-ray diffraction patterns recorded on a CubiXPRO



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diffractometer, also from PANalytical, using CuKα radiation at 45 KV. Porosity data were calculated by DFT (Density Functional Theory) method from Ar adsorption measurement data at 87 K on a Quadrasorb SI instrument from Quantachrome.

2. 2. 1. Temperature Programmed Desorption (TPD) TPD measurements were conducted on a Q50 thermal gravimetric analyzer from TA Instruments. In 100 cc/min of flowing dry nitrogen the sample was held at 30 °C for 600 minutes, followed by heating at 2 °C/min to 650 °C. 2.2.2. FTIR (Fourier Transform Infra-Red) FTIR analyses were carried out on a Nicolet 6700 spectrometer from Thermo Scientific. In transmission mode, self-supported material disks were evacuated at ~5 x 10-6 mbar to remove physisorbed water. Each measurement was performed at room temperature (estimated 23±2 °C) after keeping the sample at a desired pretreatment temperature overnight. Once the sample was in the sample holder and evacuated, it remained there under vacuum until the whole series of measurement was finished on it: for about one week.

2.3. Molecular Simulation 2.3.1. Virtual adsorption measurements These calculations were carried out using a statistical mechanics algorithm available through the Sorption module in Materials StudioTM from Accelrys (today Biovia) based on a Monte Carlo algorithm with Metropolis selection method in grand canonical ( VT) ensemble. Parameter assignments were made by the COMPASS II force field validated by Sun et al. 12-14.


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At least 106 equilibration steps were performed using Ewald15, 16 summation method with 1.0 x e4

kcal/mol accuracy.

2.3.2. Deriving equilibrium structure containing OH nests Density functional theory17 implemented in the program CASTEP18 was used in arriving at equilibrium structures. Each structure was geometry optimized using the BFGS minimizer. Norm-conserving potentials were used. For H, O, and Si, the valence electrons included 1s1, 2s2 2p4, 3s2 3p2, respectively. The plane wave basis set cutoff was 750 eV. The k-point grid was kept to maintain a spacing of ca. 0.08 Å-1. The GGA functional of Perdew, Burke and Ernzerhof (PBE)19 was employed. The convergence criteria for total energy, max force, max stress, max displacement and SCF (Self Consistent Field) iterations were 2 x e−5 eV/atom, 0.05 eV/Å, 0.1GPa, 2 x e−3 Å and 2 x e-6 eV/atom, respectively.

2.3.3. Computing vibrational spectra The phonon density of states/vibrational spectra were calculated with CASTEP’s implementation of density functional perturbation theory (DFPT)20. A k-spacing of 0.0286 Å-1 was employed in sampling the vibrational density of states with sufficient resolution. The convergence criteria for SCF iterations of the ground state electronic structure was set to 5 x e-11 eV/atom to ensure convergence of DFPT wavefunctions. To see the deviation of computed spectra from the experimental ones we calculated the IR spectra of a periodic vacuum slab created from a Y zeolite structure and decorated with isolated terminal surface Si-OH groups. Such groups on various silica surfaces have long been known to give a characteristic O-H stretching vibration in roughly the 3740-3750 cm-1 range21. The computed results gave an



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average 3738 cm-1 with ±0.4% error margin depending on the accuracy of calculation. Since the calculation of bulk vibrations were generally found to be of similar accuracy22, 23 as these surface calculations, we present all computed IR spectrum without any modification.

2.3.4. Molecular dynamics modeling of bond dissociation in OH nest In modeling chemical reactions (bond dissociation in the OH nest), a reactive forcefield approach, ReaxFF24, as implemented in the software package GULP25, has been utilized. DFToptimized structure was used as starting model, for NVT dynamics, with a step width of 1 fs (for integration of the Newtonian equation of motion).

3. Results and Discussion 3. 1. TPD and FTIR Experiments TPD measurements of the parent NaY and dealuminated NaY(-Al) samples are presented in Figures 1 a and b, respectively. Both samples were first treated in the TGA unit for 10 hours at 30 oC under flow of dry nitrogen to remove the loosely bonded molecular H2O. At this condition the weight loss from NaY was much higher, around 170 mg/g, than from the dealuminated sample (~30 mg/g). However, this latter material released more water immediately after the heating began and the major weight loss occurred below 195 oC on both samples. The total weight loss was completed by 320 and 260 oC from samples NaY and NaY(-Al), respectively.

Since the total weight loss above ~200 oC is even bigger from the non-

dealuminated NaY than from the dealuminated NaY(-Al), which could contain (OH)4-nests, it is highly probable that this loss, assigned to loss of water bonded to Na+ ions in the sodalite cages, has nothing to do with nest-decomposition. Moreover, assuming that each Al-defect would 8 ACS Paragon Plus Environment

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correspond to one (OH)4-nest in NaY(-Al), the computed 37 mg weight loss is much bigger than the observed loss above 200 oC. However, one cannot exclude from these experiments that the nest-dehydration is overlapped with the big weight loss below 200 oC, presumably mostly associated with evaporation of molecular water3.

Fig. 1 Thermogravimetric (red) and differential weight change (green) data for (a) the Naexchanged, hydrated parent zeolite NaY and (b) the dealuminated, hydrated, Na-exchanged NaY(-Al) zeolites. Fig. 2a shows the FTIR spectra of molecular H2O in the parent and dealuminated samples, heated and evacuated at different temperatures. Water desorption practically completed after evacuating the dealuminated NaY(-Al) at 250 oC but the parent NaY had to be evacuated at 300 oC for full dehydration. The composition of the Na40Al40(OH)80Si132O304 model in Fig. 3 approximates the nominal composition of our experimental NaY(-Al) sample, but it contains eight more Al-defects 9 ACS Paragon Plus Environment


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2a

2b


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2c 2c

Fig. 2 Transmission FTIR spectra of the Na-exchanged, hydrated parent NaY and the dealuminated, hydrated, Na-exchanged NaY(-Al) zeolites after evacuation at different temperatures (but measured after cooling to room temperature): a) bending vibrations of molecular H2O; b) stretching OH vibrations, normalized to the Si-OH bands near 3740 cm-1; c) magnified terminal silanol vibration range (same as in Fig. 2b) i. e., twenty (OH)4 nests per unit cell versus twelve (OH)4 nests per unit cell in the experimental sample, assuming that 20 Al were removed from the framework of model zeolite. Fig. 3a illustrates that the density of remaining Na-ions and formed silanol groups is quite high. Fig. 3b shows the primitive cell of the same model after carrying out virtual sorption experiment at roughly the same conditions as those for the first FTIR measurement of H2O on NaY(-Al) in Fig. 2a. The red dots in Fig. 3b illustrate the statistical probability of arrangement of H2O molecules



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in the zeolite. The computed H2O coverage at such conditions was 63 mg/g, roughly the half of that what we saw in Fig. 1b after the dry nitrogen purge. Fig. 2b shows the FTIR spectra of our non-dealuminated and dealuminated samples in the OH stretching vibration range under the same treatment conditions as those in Fig. 2a. As also observed in our previous experiments with Cs,NaY samples9, these OH vibrations resemble each other on both samples and gradually decrease with the decreasing amount of adsorbed molecular water seen in Fig. 2a. Since the virtually defect-free NaY sample cannot contain substantial contributions from the hypothetical (OH)4 nests, which could in theory dehydrate along with the desorbing water from the NaY(-Al) sample, the similarity in their OH changes hints at the total absence of such nests as we have discussed in detail in our previous paper9. We also pointed out

Fig. 3 a) Channels of a dealuminated, sodium exchanged Y zeolite model with a nominal composition of Na40Al40(OH)80Si132O304, assuming that each of the removed 20 Al atoms from a defect-free unit cell with Na60Al60Si132O384 composition is substituted by an (OH)4 nest; b) the statistical probability for the position of H2O molecules (small red dots) adsorbed at 25 oC and 5 x 10-6 millibar in the primitive cell of the same model structure. Color code for atoms: Red = O, White = H, yellow = Si, Pink = Al, Purple = Na 12 ACS Paragon Plus Environment

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in this paper the exceptional behavior of ~3740 cm-1 band, which is also practically absent from our currently tested NaY material but appeared on the dealuminated NaY(-Al). This band shifts gradually from 3737 to 3745 cm-1, and is not affected by the increase of dehydration temperature in contrast to all other vanishing H-bonded hydroxyl groups represented by bands in the 3000 3700 cm-1 range.

This band has been assigned to isolated terminal silanols on the external

surface of silica derivatives, including Y zeolites21-23. The sharp band at 3695 cm-1 disappears after evacuating the samples at 50 oC. We are not sure at this time how to assign this band, but it is unlikely that it represents hydroxyl nests since it appears in both samples.

3. 2. Computer models To see what theory says about the FTIR spectra of the hypothetical (OH)4 nests and their stability at various temperatures, we performed DFT structure optimization and molecular dynamics (MD) using reactive forcefield model calculations.

The usual approach to

computational studies of vibrational spectra of hydroxyl nests in different crystals relies only on a cluster approximation, which lacks the specificity of crystal structure differences28-35. To be more realistic we maintained the full periodicity of the Y zeolite structure in our calculations. Fig. 4a shows a model of a dehydrated, Al-free FAU zeolite with one hydroxyl nest per primitive cell (4 nests per unit cell), after geometry optimization and energy minimization. The computed IR spectrum with about 50 cm-1 peak broadening, which is a common experimental value, is shown in Fig. 5a in comparison with the magnified FTIR spectrum of the 250 oC calcined dehydrated NaY(-Al) sample from Fig. 2b. The computed bands in the 3200-3600 cm-1 vibration range resemble the bands obtained by Bordiga et al. [35] for cluster models of hydrogen bonded SiO3(OH) units. These authors also show experimentally that the hydrogen



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Fig. 4 Primitive cell of a hypothetical Y zeolite containing 4 hydroxyls of a nest per unit cell; a) geometry optimized structure at 0 K and dynamically excited at 323 K after 169 ps (see Fig. 6a) and b) the same structure after 170 ps. The broken bond between the magnified oxygen and hydrogen atoms is clearly visible. Color codes are the same as in Fig. 3.

b

a

Fig. 5 IR spectra of a) the 4 OH-nest/Unit Cell model structure shown in Fig. 4a, the dealuminated model structure shown in Fig. 3 in comparison with the FTIR spectrum of the 250 o

C, in situ dehydrated, dealuminated NaY(-Al) sample; b) the Na-exchanged “parent” model

structure of NaY from which the dealuminated hydroxyl nest containing model structure was derived (the numerical values of vibrations in all model spectra are listed as Supporting Information).


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bonded internal Si-OH associated FTIR bands in this vibrational range are thermally stable up to about 600 oC in the ZSM5 structured silicalite. Nothing proves however indisputably that some of these bands could represent isolated (OH)4 hydroxyl nests. We certainly can claim that this high temperature stability of hydrogen bonded internal silanols is not characteristic on our dealuminated Y zeolite, which loses all of them at around 250 oC [Fig. 2b]. Moreover, our periodic crystal based hydroxyl nest model spectra in Fig. 5a clearly indicate two characteristic and intense IR bands at 2945 and 3085 cm-1, which do not show up in our experimental spectra. We are not sure what caused the small disturbance near 3085 cm-1 in the strongly magnified experimental spectrum, but it is also visible in the similarly magnified FTIR spectrum of the nondealuminated NaY, so it has nothing to do with the dealumination process. As Bordiga et al. [35] [SiO3(OH)]4 cluster calculations also indicated, our computed spectra revealed that the OH groups of a hydroxyl nest can have substantial energetic difference. This observation is in contrast to the general view that the OH-groups of the hydroxyl nests are roughly equivalent with each other, but their energetic differences are clear from the numerical values of their IR vibrations listed as Supporting Information. Fig. 5a also compares the computed IR spectrum of the more complicated Na40Al40(OH)80Si132O304 model, shown in Fig. 3, the composition of which closely resembles our experimental NaY(-Al) sample. As the numerical data in Supplemental Information indicate, in this case we also obtained different vibrational energies for almost every nest-hydroxyl within a primitive cell. Fig. 5a illustrates that that the most intense peaks of the more complicated O-H vibrational pattern of this complex structure are still absent from the spectrum of the experimental sample. Consequently hydroxyl nests do not exist above 250 oC. Since these peaks are also absent from the spectrum of the fully hydrated sample, treated only at 25 oC (Fig. 2b), it is also unlikely that durable hydroxyl nests form at all during dealumination.



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Fig. 5b illustrates that the computed IR spectra of the “parent” Na-exchanged Y zeolite and its dealuminated derivative show similar shifts in the ~1000 cm-1 region, representing the asymmetric Si-O stretch, to that observed experimentally on similar alkaline-exchanged Y zeolites9.

Such an effect of Al-deficiency has been known for a long time35,

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.

These

calculations also illustrate that the selected DFT method with the applied parameters provides realistic IR spectra. We have found earlier that the same is valid for the >2600 cm-1 range, in which the computed hydroxyl vibrations were reproduced within less than 0.5% deviation from the experimental values22, 23. To probe the stability of potentially formed hydroxyl nests, we carried out subnanosecond MD simulation using a reactive forcefield approach, ReaxFF, that is capable of simulating bond forming/breaking processes24. Calculations on the more simple structure from Fig. 4a were carried out at 25, 50, and 100 oC, respectively. At 25 oC we have not observed any bond breakage, but at 50 and 100 oC we did at this time scale. Fig. 6a illustrates the total potential energy evolution at 50 oC, where an energy jump near 170 ps is evident. This jump, when examining the MD trajectory, turned out to be associated with a bond breakage event (Fig. 3b). Similar results were obtained at 100 oC. Thus, we can conclude that hydroxyl nests cannot be stable at or above 50 oC. Next we carried out dynamic experiments with the more realistic model in Fig. 3, also at 25, 50 and 100 oC. For this structure we have found bond breakage at all three temperatures. Fig. 6b illustrates such a bond breaking event(s) reflected by the total potential energy evolution at 25 oC. Fig. 7b illustrates two such events from the MD trajectory when two Si-OH bonds broke near 14 ps on the MD simulation scale. The corresponding video clip showing bond


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Fig. 6 Time dependence of energy change a) at 50 oC of the minimized structure shown in Fig. 4/a; and b) that of the structure in Fig. 3 at 25 oC Breaking Si-OH bonds

Fig. 7 a) Geometry optimized and energy minimized unit cell, viewed from the channel entrance, of the dealuminated, dehydrated model structure, Na40Al40(OH)80Si132O304, a) before the molecular dynamics experiments (also shown in Fig. 3); b) after dynamic agitation at 14 ps (note the break-away OH-groups); and c) at the end of the 1000 ps run (note many random “free” OH, H, and Na atoms). Color codes are the same as in Fig. 3. Original Si-OH “bonds” were retained in the graphics to illustrate the bond-breakings and where those free OH groups came from. changes in the first 20 ps is attached as Supporting Information. Such events continued during the whole run, including intense dynamic movements of H, OH, and Na ions in the structure, ending with the obviously not settled structure in Fig. 7c. This intense dynamical move of



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exchange ions fits quite well, for example, experimental MAS NMR observations of Hunger et al.37. Conclusions Our experimental TPD and FTIR measurements on a Si/Al ~ 2.55 ratio NaY zeolite and its Si/Al ~ 3.3 ratio dealuminated derivative, NaY(-Al), which has not been exposed to temperatures above 25 oC, have not revealed any evidence for the presence of (OH)4 hydroxyl nests despite the long standing literature assumption that such entities would form in the place of the removed Al atoms and would be stable and detectable even at elevated temperatures. DFTbased computed IR spectra of relevant periodic models with assumed hydroxyl nests indicate that some of their most intense O-H stretching vibrations were in the 2900 to 3200 cm-1 range, which are absent from the FTIR spectra of the dehydrated experimental zeolite samples. These model calculations also revealed that, in contrast to the general assumption, the energies (vibration modes) of the individual hydroxyl groups forming the (OH)4 nests could not be totally identical, if such entities even exist. Reactive force field based molecular dynamics calculations evidenced ephemeral existence for hydroxyl nests even at ambient temperature. They must form as intermediates upon Al removal from the zeolite lattice to obey the stoichiometry of the dealumination reaction (see in Introduction), but they would dehydroxylate spontaneously and very quickly, on the time scale of picoseconds. This instability of hydroxyl nests makes one wonder how the remaining atoms are arranged around the Al-vacancy. To our knowledge this issue has not been experimentally explored. Hence one can only set up logical hypotheses for now. One plausible assumption is that the extraction of Al followed by the dehydroxylation results in formation of strained 18 ACS Paragon Plus Environment

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siloxane bridges, which require the neighboring atoms to undergo some position rearrangement in the vicinity of the dealuminated site resulting in loss of long-range periodicity. Judged from the ~3740 cm-1 range FTIR band appearing on the dealuminated NaY(-Al) sample (Fig. 2c), these bridges could be interrupted with formation of terminal silanols. The described defective regions must be extremely reactive during any subsequent chemical modification of the zeolite. Under action of various mineralizers or at elevated temperatures they might “heal” via a wider ranged annealing process of the zeolite lattice. Finally we have to emphasize that these detailed studies have focused on the effect of acid leaching of Y zeolites. To prove if hydroxyl nest could form on other zeolites and, if they can, see if their thermal stability could be significantly higher than that on the Y zeolite, one has to do specific experimental and computational work on the other zeolites as well.

Acknowledgements Authors appreciate support and permission for publication from Zeolyst International and PQ Corporation (IH), Rive Technology Company (ES),. The authors thank Mr. Mukesh Agarwal of PQ Corporation for carrying out the FTIR measurements, Mr. Ibrahim Qureshi of Rive for sample preparation. DHO is pleased to acknowledge the general encouragement of Professor Jing Li and her group members for their assistance.

Supporting Information Available 1) Computed vibrations of models structures. 2) Bond Changes in Na40Al40(OH)80Si132O304 during 20 ps (movie) This information is available free of charge via the Internet at http://pubs.acs.org. 19 ACS Paragon Plus Environment


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[36] G. W. Skeels and D. W. Breck, Zeolite Chemistry V – Substitution of Silicon for Aluminum in Zeolite Via Reaction With Aqueous Fluorosilicate. Proc. 6th IZC, UK, 1983, ed. by Olson, D. and Bisio, A. 1984, 87-96. [37] Hunger, M.; Schenk, U.; Buchholz, A. Mobility of Cations and Guest Compounds in Cesium-Exchanged and Impregnated Zeolites Y and X Investigated by High-Temperature MAS NMR Spectroscopy. J. Phys. Chem. B 2000, 104, 12230-12236.


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Further Search for Hydroxyl Nests in Acid Dealuminated Zeolite Y

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