Influence of N,N,N-trimethyl-1-adamantyl Ammonium Structure

Article Cite This: J. Phys. Chem. C 2019, 123, 17454−17458

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Influence of the N,N,N‑Trimethyl-1-adamantyl Ammonium Structure-Directing Agent on Al Substitution in SSZ-13 Zeolite Sichi Li,† Rajamani Gounder,‡ Anthony Debellis,§ Imke B. Müller,∥ Subramanian Prasad,⊥ Ahmad Moini,⊥ and William F. Schneider*,† †

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States § Quantum Chemistry and Hybrid Modeling Research, BASF Corporation, Tarrytown, New York 10591, United States ∥ Quantum Chemistry, BASF SE, Ludwigshafen 67056, Germany ⊥ BASF Corporation, 25 Middlesex Essex Turnpike, Iselin, New Jersey 08830, United States

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‡

S Supporting Information *

ABSTRACT: Structure-directing agents (SDAs) influence not only zeolite structures but also the distribution of Al on zeolite lattices. The origins of this Al-directing influence are not well understood. Here, we use first-principles methods to explore the interactions between an N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) OSDA and framework [AlO2]− in the dilute limit within an SSZ-13 lattice. Molecular dynamics and geometric and electronic structures illuminate the rotational freedom of TMAda+ OSDA within an SSZ-13 cage, the primarily electrostatic nature of its interactions with [AlO2]−, the effect of the solvent in screening the electrostatics, and the relative invariance of that interaction within a subset of Al locations within the same SSZ-13 major cavity. These results indicate that the Al distribution patterns observed in relevant SSZ-13 samples are not consequences of the location bias for Al substitution within a single TMAda+−cage complex but likely involve long-range electrostatics among multiple TMAda+−cage complexes. The computational protocol can be employed to examine other combinations of SDAs and zeotypes of interest. reported to have a negligible impact on relative ΔEstbl across different candidate OSDAs.9 A zeolite FW is constructed of charge-neutral SiO 2 tetrahedral (T-) sites, and in many materials of practical interest, some fraction of these SiO2 T-sites are substituted by anionic [AlO2]− tetrahedral (T-) sites. The net composition is characterized by a silicon-to-aluminum ratio (Si/Al). For Alcontaining zeolites, the distribution of Al is not random, as evidenced by the general adherence to the Löwenstein’s rule in laboratory-synthesized zeolites and the sensitivity of the observed Al distribution to the synthesis protocol.10 OSDAs typically contain functional groups that are positively charged under synthesis conditions, and thus their interactions with the [AlO2]− building units likely influence Al siting and distribution, as supported by experimentally observed differences in 27Al MAS NMR spectra11,12 or Rietveld refinement of X-ray diffraction patterns13 of samples with the same topology but synthesized with different OSDAs. Classical14 and supercell density functional theory (DFT)12 calculations reveal that the relative ΔEstbl across a set of symmetry-distinct T-sites does vary with choice and orientation of OSDA, highlighting the potential Al-directing effect of OSDAs.12,14

1. INTRODUCTION Zeolites are microporous, crystalline aluminosilicates of interest for separations and catalysis. Today, ∼230 zeotypes are known,1 over 2.6 million have been predicted to exist2 and this number does not even represent the upper limit. Common routes to synthesize a particular framework (FW) structure involve combining hydrated silica and alumina precursors with a mineralizing agent and an appropriate organic structuredirecting agent (OSDA). These OSDAs guide assembly of oxide precursors into a more ordered gel phase that crystallizes into the desired structure. This function of the OSDA has been exploited to identify OSDAs that yield a zeolite structure with the desired steric and chemical features3 or with particular chirality.4 Similarly, computer models have been used to predict new and simpler OSDAs that will promote the crystallization of a specific zeolite structure,5−8 through optimization of the stabilization energy (ΔEstbl) between the OSDA and the preformed, fully siliceous zeolite FW. ΔEstbl is computed as ΔEstbl = EFW + OSDA − E FW − EOSDA

(1)

where the individual terms are lattice energies often computed in periodic supercells using classical potentials that incorporate van der Waals (vdW) interactions between the OSDA and FW. Electrostatic interactions between OSDA and FW Al have been © 2019 American Chemical Society

Received: June 4, 2019 Revised: June 18, 2019 Published: June 24, 2019 17454

DOI: 10.1021/acs.jpcc.9b05334 J. Phys. Chem. C 2019, 123, 17454−17458

Article

The Journal of Physical Chemistry C The zeolite SSZ-13 has attracted considerable interest recently due to its structural simplicity and catalytic relevance to NOx selective catalytic reduction, methanol-to-olefin (MTO) conversion, and partial methane oxidation.15 SSZ-13 has the chabazite topology comprising one symmetry-distinct tetrahedral site that forms double-six-membered-ring secondary building units organized into a three-dimensional network of oblate cavities. The arrangement of FW Al in SSZ-13 is sensitive to the synthesis protocols used.10 SSZ-13 samples synthesized with N,N,N-trimethyl-1-adamantyl ammonium (TMAda+) OSDA are unable to exchange Co2+ ions, reflecting the absence of Al T-sites of appropriate proximity to host this divalent cation, for example paired Al sites in the 6-memberedring.16 In contrast, the SSZ-13 samples synthesized with Na+ as a co-SDA with TMAda+ have Co2+ uptake isotherms that reveal the presence of such proximal Al−Al configurations. These Al−Al configurations have consequences for the performance of SSZ-13 zeolites in acid-catalyzed methanol dehydration17 and MTO conversion,18 as well as to ion speciation and catalytic function in Cu- and Fe-exchanged SSZ-13.19−21 While the relative stabilization energies of various inorganic cations as a function of Al location have been probed using supercell DFT,22,23 little is known about the nature of the interaction between TMAda+ itself and FW Al. In this contribution, we address this gap using supercell DFT and ab initio molecular dynamics (AIMD) simulations, employing supercells selected to isolate the interaction of a single TMAda+ ion and AlO2− center.

Figure 1. Schematic illustration of six symmetry-distinct Al positions in the SSZ-13 main cage relative to TMAda+, highlighted by colors. TMAda+ element color coding: brown, C; blue, N; pink, H.

double-6-membered-ring units along the c axis. The long axis of the TMAda+ cation is defined by the adamantyl group at one end and the trimethylamine group at the other. Although SSZ-13 has only 1 type of crystallographically distinguishable T-site, occlusion of TMAda+ as-synthesized within the SSZ-13 cavity and orientation along the c-axis creates six symmetrydistinct sets of T-site positions relative to the OSDA. As mentioned above, the samples synthesized with solely TMAda+ as SDA do not exchange Co2+ and thus appear to lack proximal Al−Al, for example those within the same 6-memberedring.10,16 One previously proposed rationalization of this observation is that each TMAda+ directs Al towards position 1 only, and each 6-membered-ring touches only one CHA cavity, such that no 6-membered-ring can have two T-sites simultaneously substituted by Al atoms.10 To test the sensitivity of the stabilization energy to Al location, we constructed 36 T-site SSZ-13 supercells with Al in each of the six possible Al locations relative to TMAda+ and subjected the systems to AIMD at 433 K (selected arbitrarily to correspond with the temperature prevailing under hydrothermal synthesis conditions)10 and using the PBE exchange− correlation functional augmented with the D3 dispersion correction33 to capture electrostatic and nonbonded contributions to the FW−OSDA interaction, which we hypothesize to report on interactions governing Al siting during zeolite organization and crystallization. This supercell places 1 Al and 1 OSDA in every 3 cages and separates Al by at least 13.7 Å in any direction. We initially oriented TMAda+ at arbitrary angles relative to the long axis of the cages and observed in all cases a realignment along the axis. Energy differences between initial and reoriented structures were upwards of 200 kJ·mol−1 or more. These results both illustrate the mobility of the OSDA within the cage and its strong preference for orientation along the c axis. Details are reported in the Supporting Information Section S2. The reoriented structures were subjected to an additional 50 ps of AIMD to more extensively explore dynamics in the low energy orientation. Visual inspection of the AIMD trajectories reveals a rigid TMAda+ that rotates about its long axis and rocks relative to the long axis of the cage, as shown in Figure 2a. To quantify these motions, we computed the cumulative angle of rotation for a projection of one of the N−CH3 bonds onto the a−b plane along the c-axis and the angle between the N-adamantyl bond and the c axis (rock angle). The results versus trajectory time accumulated at Al position 3 are plotted in Figure 2b. The rotation is characterized by periods of

2. COMPUTATIONAL METHODS Plane-wave, periodic supercell DFT calculations were performed with the Vienna Ab initio Simulation Package (VASP), version 5.4.1,24 using projector augmented wave treatment of core−valence interactions25,26 and Brillouin zone sampling at the Γ point. Two different SSZ-13 supercells were used in this work, which are the 36 T-site supercell obtained from the IZA database1 and the 1 × 1 × 2 repetition of it, shown in the Supporting Information Section S1. NVT AIMD were performed in a 36 T-site supercell spin-unpolarized with a time step of 0.5 fs and within the Perdew−Becke−Ernzerhof (PBE) generalized gradient approximation (GGA)27 with D3 dispersion correction, a plane wave cutoff of 300 eV, and selfconsistent field (SCF) electronic energies converged to 10−4 eV in each MD step. The temperature was held at 433 K using a Nosé−Hoover thermostat. Geometry relaxations were performed in a 36 or 72 T-site supercell spin-polarized within the GGA and with or without D3 correction. The plane wave cutoff was set to be 400 eV, SCF energies converged to 10−6 eV and atomic forces to less than 0.03 eV/Å. Bader charge analyses of the relaxed structures were performed using the algorithm developed by Henkelman et al.28−30 The electron localization function (ELF)31 of each structure was computed based on the preoptimized wavefunction. The implicit solvent model was implemented by VASPsol package.32 The plane wave cutoff was increased to 500 eV to better converge the cavitation energy. Only single point energies for PBE-D3optimized structures were computed. 3. RESULTS AND DISCUSSION Figure 1 shows a schematic representation of a TMAda+ cation within the major SSZ-13 cavity, which is connected by two 17455

DOI: 10.1021/acs.jpcc.9b05334 J. Phys. Chem. C 2019, 123, 17454−17458

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The Journal of Physical Chemistry C

interaction, we extracted the lowest-energy structural snapshots visited during the AIMD trajectories, transferred to a supercell doubled in the c direction (72 T-site), and quenched to 0 K. TMAda+ maintains its c-axis orientation and the methyl groups relax towards the FW Al, decreasing the N−Al distance. The relaxed structures are shown in the Supporting Information Section S4. Figures 3 and S7 report relaxed stabilization

Figure 3. PBE-, PBE + D3-, BEEF-vdW-, RPBE + D3-computed stabilization energies of relaxed TMAda+ in the 72 T-site SSZ-13 supercell vs Al position and referenced to position 1. Corresponding Al−N distances in Å are shown.

energies computed with the PBE functional with and without dispersion corrections and referenced to Al position 1. The results in the larger supercell are distinctly different from the 36 T-site model, revealing a considerably stronger preference for Al to site near the N end of TMAda+. We also re-optimized the systems at each Al position with the BEEF-vdW and RPBE + D3 functionals. The structures are nearly invariant to functional and stabilization energies are also the same within several kJ·mol−1 across functionals, as shown in Figure 3. Energies vary by more than 40 kJ·mol−1 and the overall trend across Al positions is insensitive to the inclusion of dispersion or the functional used in the energy model, consistent with a primarily electrostatic contribution to the Al site dependence. Sites 2, 3, and 4, around the “midriff” of the cage, are close in energy (within 5 kJ·mol−1) and share similar separations of about 5 Å between formally anionic Al and the formally cationic N center on TMAda+. Al sites 5 and 6, near the uncharged end of TMAda+, are considerably higher in energy, while position 1, at the base of the cage, is approximately 10 kJ· mol−1 higher in energy. All Al sites near the charged TMAda+ end are thus similar in stabilization energy. We next turn to electronic structure analyses to gain insights into the association between TMAda+ and FW Al. We performed Bader charge analysis on the relaxed 72 T-site supercells. The full results are in the Supporting Information Section S6. In all cases, TMAda+ carries nearly a complete single positive charge that is localized primarily on the three methyl groups attached to N and, to a lesser extent, on the adamantyl group. To visualize the charge transfer between OSDA and FW, we computed the charge density difference (Δρzeolite−TMAda) between the TMAda+-compensated lattice and the uncompensated lattice and TMAda+, each treated as a neutral species, following

Figure 2. (a) Dominant motions observed for TMAda+ confined within an SSZ-13 cavity at 433 K. (b) Time evolution of the cumulative angle of rotation, rock angle for TMAda+, and N−Al distance along 50 ps 433 K AIMD trajectory.

relative invariance concluded by a full rotation in 50 ps. The rock angle oscillates more regularly and maximizes at approximately 30° with respect to c. The Al−N distance fluctuates in concert with this rock, also shown in Figure 2b. Similar results are obtained for all Al locations, as detailed in the Supporting Information Section S5. While supercell sizes are too small and simulations too short to draw quantitative conclusions, the trajectories highlight the loose fit between TMAda+ and SSZ-13 cage and the absence of specific OSDA− FW interactions that orient TMAda+ toward a specific symmetry-distinct set of Al positions. The stabilization energies extracted from the 36 T-site supercell model (Figure S7) are similar to those reported previously.18 As shown in Supporting Information Section S3, the 36 T-site supercells are potentially contaminated by periodic image interactions between TMAda+ and Al along the c direction, as Al, when positioned close to the supercell boundary, comes in close contact with TMAda+ in the next periodic image. In an attempt to isolate the TMAda+−AlO2−

Δρzeolite ‐ TMAda = ρzeolite ‐ TMAda − ρzeolite − ρTMAda 17456

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DOI: 10.1021/acs.jpcc.9b05334 J. Phys. Chem. C 2019, 123, 17454−17458

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The Journal of Physical Chemistry C Charge transfer is primarily from the formally cationic TMAda+, polarized toward the FW O atoms adjacent to Al. These results are consistent with a largely electrostatic interaction between TMAda+ and AlO2− centers. The ELF, plotted on the bottom right in Figure 4, similarly reports the

Figure 5. PBE + D3-computed energy of relaxed TMAda+ in 72 Tsite SSZ-13 supercell vs Al substitution position, referenced to Al position 1 and incorporating implicit solvent effects with a continuum dielectric ranging from vacuum to H2O. Corresponding Al−N distances of PBE + D3-optimized structures shown in Å.

Figure 4. Calculated charge density difference across a full 72 T-site supercell (left) and close-up of the TMAda+−FW Al region (top right). Yellow and cyan isosurfaces correspond to electron accumulation and depletion at values of 1 × 10−3 and 5 × 10−3 e Å−3, respectively. Corresponding ELF localization domains (bottom right) shown at an iso-surface value of 0.8. Element color coding: Si, yellow; O, red; C, brown; N, blue; H, pink.

diminish energy differences associated with the Al location. At the highest dielectric constant considered, positions 1−4 become essentially isoenergetic, while biases against positions 5 and 6 are diminished. Solvent corrections thus do have a quantitative impact on computed TMAda+−[AlO2]− interactions, and the potential modulating role of the solvent will be an important consideration in modeling OSDA−[AlO2]− chemistry, in particular in cases in which OSDAs fit loosely in zeolite cavities or channels.

expected covalent character within TMAda+ and the absence of any ELF density indicative of covalency between TMAda+ and FW. Again, the results are invariant to the Al location, as detailed in the Supporting Information Section S6. Identifying and quantifying the dominant role of electrostatics in TMAda+−[AlO2]− interactions can potentially guide parameterization of models appropriate for larger-scale simulations relevant to synthesized materials. Under conditions of zeolite synthesis, interactions between OSDA and the organizing FW will likely be screened by solvent. To estimate the magnitude of these effects, we explored the influence of explicit and implicit H2O on computed results. In a first set of calculations, we inserted a single H2O molecule into the cage with TMAda+ in a 72 T-site supercell, using again Al in position 3 and relaxed the structure. H2O remains in the cage, in between NTMAda+ and Al, and displaces the N 1.3 Å further from the Al center. The relaxed structure is shown in the Supporting Information Section S7. The H2O insertion energy is −35 kJ·mol−1 relative to H2O vapor and the relaxed, H2O-free cage. Consistent with the dynamics results above, even when fully formed, the SSZ-13 cage can accommodate both TMAda+ and H2O. We next recomputed the stabilization energies, again on the 72 T-site supercells relaxed with PBE + D3 with solvent corrections using an implicit-only continuum solvation model, implemented with VASPsol.32 The model treats solvent as a dielectric continuum and thus simulates the screening of TMAda+ by solvent without capturing size exclusion effects or orientation-specific hydrogen bonding between the solvent, FW and TMAda+. As we expected these latter effects to be roughly constant across Al sites, the implicit model should reasonably estimate the influence of the solvent on relative stabilization energy across sites. The dielectric constant εr of the continuum was set to span a range up to that appropriate for bulk H2O. Figure 5 reports results versus Al substitution positions, referenced to position 1. Consistent with the electrostatic picture that emerges from the charge analysis above, the effect of the solvent is to

4. CONCLUSIONS Collectively, these results indicate that a TMAda+ cation within the SSZ-13 cage fits loosely, is rigid but dynamic at experimentally relevant conditions, and has little energetic preference for the Al location within a cage except to bias against Al locations close to the adamantyl group. Electronic structure analysis attributes these observations to TMAda+− FW interactions that have a large electrostatic component and are screened by the solvent. The observation that Al atoms do not occupy two T-sites in the same 6-membered-ring in the SSZ-13 samples prepared using TMAda+ as the lone SDA10 is thus not evidently the result of specific location biases imposed by a single TMAda+−cage complex in an as-crystallized lattice, but rather reflect the collective interactions between TMAda+ and Al across cages, and/or kinetics or thermodynamics during transient phases, which are not captured in this model and needs to be determined in order to ultimately predict mesoscale Al distribution patterns. The reported computational protocol can be extended to crystallographically distinguishable T sites in lower symmetry FWs, such as MFI, as well as to the influence of various OSDAs on the ordering of Al in zeolites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b05334. Relaxed structures, effect of supercell size on computed energetics, AIMD analyses of TMAda+, and orientation and mobility and analyses of electronic structures (PDF) 17457

DOI: 10.1021/acs.jpcc.9b05334 J. Phys. Chem. C 2019, 123, 17454−17458

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sichi Li: 0000-0002-2565-5906 Rajamani Gounder: 0000-0003-1347-534X William F. Schneider: 0000-0003-0664-2138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed with the financial support of the BASF Corporation. We thank the Center for Research Computing at Notre Dame for support of computational resources.

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DOI: 10.1021/acs.jpcc.9b05334 J. Phys. Chem. C 2019, 123, 17454−17458