Synthesis of Germanosilicate Molecular Sieves from Mono- and Di

Mar 18, 2016 - Stephen K. Brand , Joel E. Schmidt , Michael W. Deem , Frits Daeyaert , Yanhang Ma , Osamu Terasaki , Marat Orazov , Mark E. Davis...
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Synthesis of Germanosilicate Molecular Sieves from Mono- and DiQuaternary Ammonium OSDAs Constructed from Benzyl Imidazolium Derivatives: Stabilization of Large Micropore Volumes Including New Molecular Sieve CIT-13 Ben W. Boal,§ Michael W. Deem,‡ Dan Xie,† Jong Hun Kang,§ Mark E. Davis,§ and Stacey I. Zones*,† †

Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States Rice University, 6100 Main Street, MS 142, Houston, Texas 77005-1892, United States § Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ‡

ABSTRACT: A series of monoquaternary and diquarternary benzyl-imidazolium derivatives are prepared and used as organic structure direction agents (OSDAs) in germanosilicate syntheses. The goal of this work is to create new multidimensional large pore zeolites. For the OSDA made and tested, we looked for relationships based upon stereochemistry from the benzyl ring as part of each structure. Several known molecular sieves with the *BEA, BEC, IWS, or LTA topologies are obtained. Molecular modeling is carried out with the goal of understanding: (a) the product selectivity correlation with the OSDA and the zeolite obtained, and (b) why differential rates of crystallization are observed for isomers that lead to different zeolite products. Additionally, a new molecular sieve denoted CIT-13 is prepared and shown to possess intersecting 14- and 10-membered ring pores, which gives confidence to the soundness of this approach for OSDA construction to yield new multidimensional large pore zeolites. CIT-13 is the first molecular sieve to have this combination of pore sizes.

1. INTRODUCTION Zeolites play an important role as heterogeneous catalysts and are used in a variety of industrial settings. Initially, these materials were largely developed to support the petroleum industry in the quest to create more selective, robust catalysts for making gasoline and other fuels.1 Currently, these solids have emerged as specialty materials, with properties that are based upon structure and chemical composition able to handle specific large-scale applications. A notable current example is their use in the selective catalytic reduction (SCR) system that reduces nitrous oxide emissions from on-road combustion engines.2 While there is a considerable effort that must go into bringing a new material from the discovery phase into a commercially viable catalyst,3 there remains room for discovery of new structures with the hope that one might emerge as superior to the existing materials. Hence, interest remains in the discovery of new zeolitic phases. One goal toward finding new materials has been the hope that increasingly large pores that retain some catalytic properties in their interior surfaces can be capable of handling larger feed molecules in the oil upgrade arena.4 The research in discovering new structures is often a synergy of creating new microporosity within a crystalline lattice by finding new guest molecules that will occupy this space in the synthesis of the inorganic framework. Usually organocations, denoted OSDAs (Organic Structure-Directing Agents), are tested against a matrix of inorganic conditions of synthesis (hydrothermal, at © XXXX American Chemical Society

elevated temperatures). The choice of the inorganic components appears to provide some bias as to the type of atomic arrangements observed in a mostly silicate lattice.5 One of the most productive influences of this chemistry was the introduction of Ge for Si in the reaction mixtures, pioneered by the Corma lab.6−8 In these reactions (as is the case here), the reaction uses the OSDA as a hydroxide and is counterbalanced by equimolar amounts of HF. The incorporation of Ge gives a longer T−O bond length (T, tetrahedral atom) than Si, that in turn allows for different bond angles in the bonding into the silicate lattice. Quite typically, one observes the incorporation of Ge into subunits that are basically cubes of eight tetrahedral, four-coordinate oxides (termed double 4-rings or D4R). This invariably leads to materials with interconnecting channels, and thus larger void volumes once the OSDAs are removed.9,10 Our interest in developing large, open spaces in molecular sieves has led us to explore the synthesis of larger volume, multicharged OSDAs. In one study, using rigid skeleton trypticene derivatives, we showed, for the first time, an OSDA stabilization of VFI, an 18-ring sieve with a crosssection pore of 12 Å.11 Subsequently, borrowing on a facile one-step synthesis from the organic chemistry literature,12 we Received: January 4, 2016 Revised: March 3, 2016

A

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2. EXPERIMENTAL SECTION

used a triquaternary OSDA where three equal groups (built from 1,2 dimethyl imidazole) are tethered to an aromatic center through benzylic linkages. The OSDA is shown in Figure 1 and

2.1. Synthesis of Organic Structure-Directing Agents. Unless stated otherwise, reactions were conducted in flame-dried glassware under an atmosphere of argon. All reagents were purchased from commercial sources and used as received. Liquid NMR spectra were recorded on Varian Mercury spectrometers. High-resolution mass spectra were obtained from the California Institute of Technology mass spectrometry facility. The OSDAs are described as the halide syntheses, then the halides are ion-exchanged on hydroxide resins in water to produce the hydroxide analogs that are then titrated before use in the zeolite syntheses. Benzyl-diimidazolium, 1. A 500 mL flask was charged with 1,2dimethyl imidazole (16.0 g, 166.7 mmol), α,α′-dichloro-o-xylene (20.0 g, 75.8 mmol) and ethanol (300 mL). The flask was fitted with a reflux condenser and heated to reflux for 15 h. The reaction was cooled to 0 °C and resulting solids were filtered and washed with ethyl acetate (3 × 50 mL) to give (27.10 g, 78% yield) a white solid. 1H NMR (500 MHz, CD3OD): δ 7.58 (d, J = 2.0, 2H), 7.45 (dd, J = 3.5, 2.5 2H), 7.43 (d, J = 2.0, 2H), 7.02 (dd, J = 3.5, 2.5 2H), 5.64 (s, 4H), 3.91 (s, 6H), 2.68 (s, 6H). 13C NMR (126 MHz, CD3OD): 147.04, 133.03, 130.76, 129.02, 124.21, 122.49, 50.19, 35.86, 10.36. Benzyl-diimidazolium, 2. The reaction was carried out as in 1, except the α,α dibromo-m-xylene was used (82% yield). H NMR (500 MHz,CD3OD) 7.53−7.47 (m, 5H), 7.44−7.42 (m, 1H), 7.35−7.33 (m, 2H), 5.45 (s, 4H), 3.86 (s, 6H), 2.65 (s, 6H). 13C NMR (126 MHz): 145.05, 135.08, 129.94, 127.95, 127.29, 122.57, 121.13, 50.82, 34.27, 8.70 Benzyl-diimidazolium, 3. This time the reaction as in 1 was repeated but using α,α dichloro-p-xylene. The product had a yield of 78%. H NMR (500 MHz, CD3OD) 7.52 (d, J = 2.5, 2H), 7.51 (d, J = 2.5 2H), 7.40 (s, 4H), 5.43 (s, 4H), 3.84 (s, 6H), 2.63 (s, 6H). 13C NMR (126 MHz, CD3OD): 146.44, 136.05, 129.88, 123.98, 122.56, 52.16, 35.62, 9.94. Benzyl-imidazolium, 4. A 500 mL flask was charged with 1,2dimethyl imidazole (7.73 g, 55.0 mmol), 2-methylbenzyl chloride (7.73 g, 60.5 mmol) and toluene (100 mL). The flask was fitted with a reflux condenser and heated to reflux for 15 h. Reaction was cooled to 25 °C and resulting solids were filtered and washed with ethyl acetate (3 × 20 mL) to give (11.84 g, 91% yield) a white solid. 1H NMR (500 MHz, CD3OD): δ 7.37−7.33 (m, 2H), 7.31 (d, J = 5.0 Hz, 1H), 7.30− 7.26 (m, 1H), 7.02 (d, J = 5.0 Hz, 1H) 5.45 (s, 2H), 3.92 (s, 3H), 2.67 (s, 3H), 2.38 (s, 3H). 13C NMR (126 MHz, CD3OD) δ 145.08, 136.40, 131.36, 130.69, 128.81, 127.56, 126.47, 122.44, 120.88, 49.59, 34.15, 17.66, 8.35. Benzyl-imidazolium, 5. A 500 mL flask was charged with 1,2dimethyl imidazole (11.60 g, 121.0 mmol), 3-methylbenzyl chloride (15.46 g, 110.0 mmol) and toluene (100 mL). The flask was fitted with a reflux condenser and heated to reflux for 15 h. Reaction was cooled to 25 °C and resulting solids were filtered and washed with ethyl acetate (3 × 50 mL) to give (22.65 g, 87% yield) a beige solid. 1 H NMR (500 MHz, CD3OD): δ 7.55 (s, 2H), 7.34 (dd, J = 10.0, 5.0, 1H), 7.25 (d, J = 10.0, 1H), 7.20 (s, 1H), 7.14 (d, J = 5.0, 1H), 5.40 (s, 2H), 3.88 (s, 3H), 2.67 (s, 3H), 2.39 (s, 3H). 13C NMR (126 MHz, CD3OD) δ 144.8, 139.1, 133.7, 129.3, 128.9, 128.1, 124.6, 122.4, 121.2, 51.3, 34.1, 19.9, 8.4. Benzyl-imidazolium, 6. A 500 mL flask was charged with 1,2dimethyl imidazole (11.60 g, 121.0 mmol), 4-methylbenzyl chloride (15.46 g, 110.0 mmol) and toluene (125 mL). The flask was fitted with a reflux condenser and heated to reflux for 15 h. Reaction was cooled to 25 °C and resulting solids were filtered and washed with ethyl acetate (3 × 50 mL) to give (24.10 g, 92% yield) a white solid. 1 H NMR (500 MHz, CD3OD): δ 7.54−7.53 (m, 2H), 7.28 (d, J = 5.0, 2H), 7.28 (d, J = 5.0, 2H), 5.34 (s, 2H), 3.87 (s, 3H), 2.67 (s, 3H), 2.39 (s, 3H). 13C NMR (126 MHz, CD3OD) δ 144.8, 138.8, 130.7, 129.5, 127.6, 122.4, 121.1, 51.2, 34.1, 19.7, 8.4. 2.2. Synthesis of Germanosilicates. All reactions were performed in 23 mL Teflon-lined stainless steel autoclaves (Parr Instruments). Reactions were performed statically or tumbled at approximately 40 rpm using spits built into convection ovens.

Figure 1. General reaction scheme used to produce triquaternary OSDAs, NR3 = tertiary amine.

Figure 2. Calculated conformation of the triquat in LTA.

the folding into the guest lattice LTA is shown in Figure 2. This molecule with the surprising folding configuration has a strong specificity for the LTA lattice that contains a large cage and small, but usable, pore sizes for small molecule chemistry.13 In this contribution, we focus on smaller components from within the triquat OSDA (Figure 1), used in the Si/Ge/F synthetic regime, and report on the molecular sieve synthesis selectivity for three monoquaternary and three diquaternary OSDAs with changes in symmetry relative to the aromatic ring substitution. The six OSDAs are shown in Figure 3.

Figure 3. Organic structure-directing agents (OSDAs).

We investigate phase selectivity and kinetic behavior of the six OSDAs as the synthesis conditions are varied. Emerging from these studies, a new chemical composition for the LTA topology was found,14 and another new phase discovered. The latter has been termed CIT-13, and has been shown to possess an intersecting 14-ring and 10-ring pore system.15 It is the first known molecular sieve with this architecture. B

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Table 1. Synthesis Resultsa with OSDAs 1−6; Si/Ge is Shown for Reactants

Syntheses were performed at 140, 150, 160, or 175 °C. Silicon source was tetraethyl orthosilicate (99.9% Si(OCH2CH3)4, Strem). The germanium source was germanium oxide (99.99% GeO2, Strem). Gels for germanosilicate reactions were prepared by adding germanium oxide to a solution of the organic structure directing agent in water directly into the 23 mL Teflon liner. This mixture was stirred at 25 °C for 5 min, or until germanium oxide had dissolved into the solution. Tetraethyl orthosilicate was then added, reaction vessel capped, and stirred for an additional 12 h to hydrolyze the tetraethyl orthosilicate. The reaction vessel was then uncapped and a stream of air blown over the gel while it was mechanically stirred until the required excess of water and hydrolyzed ethanol had been evaporated. In certain cases, the gel was put under vacuum to remove small amounts of residual water when evaporation failed to remove the required amount of water. Hydrofluoric acid was then added in a dropwise fashion to the gel and the Teflon liner was sealed into the stainless steel autoclave and put into the oven and run static. Runs were carried out at 160 °C. The reactors were cooled and opened every 6−7 days to assess reaction progress. When settled solids were observed, after homogenizing, a small sample was successively washed with D.I. H2O (2 × 10 mL) and acetone: methanol (1:1, 3 × 10 mL) and the XRD pattern was inspected. All reactions were monitored for at least 1 month. The variable ratios used for the reactions were 1.0 SiO2/xGeO2 where x = 0.50 and lower/0.50 OSDA as OH−/0.50 HF/ 5 H2O. In the products there is a slight enrichment of Si/Ge over the starting ratio and in our CIT-13 product where reaction Si/Ge = 4, the product has a value closer to 5 (by EDX measurements). 2.3. Characterization. Powder X-ray diffraction (PXRD) patterns were collected using a Rigaku Miniflex II diffractometer and Cu K αradiation. Scanning electron micrographs were collected using a Hitachi S-570 instrument. 2.4. Computational Work. The computations were carried out as in ref 16. The stabilization energy is defined as the difference in energy between the material with the OSDA occluded per unit cell and the energy of the isolated OSDAs and the empty zeolite host framework. The stabilization energy was optimized for different loadings, and the results are reported for optimal loadings. In all cases the predicted loadings match those measured experimentally. The DREIDING interatomic potential17 was used in the molecular dynamics program GULP to calculate these energies.18−21 After inserting the templates within the zeolite structures, four rounds of energy minimization were carried out. Two different minimizers, BFGS and a conjugate gradient, were used for the minimization. The interaction energy for the OSDAs in the zeolite were calculated by molecular dynamics (MD) simulations. Three different time steps and three different length runs were used to ensure equilibration: 0.1, 1.0, and 30.0 ps. The reported energy values are taken as the average energy calculated from the last 5.0 ps of a 30.0 ps molecular dynamics calculation at 343 K. The results thus take into account thermal motion at synthesis temperatures and are not simple energy minimizations.

OSDA

Si/Ge = 2

Si/Ge = 4

Si/Ge = 8

Si/Ge = 16

1 2 3 4 5 6

layered IWS BEC IWS CIT-13 BEC/LTA

layered IWS/*BEA BEC/*BEA CIT-13 CIT-13 BEC/LTA

*BEA *BEA *BEA/BEC LTA/Amorphous CIT-13 LTA

*BEA *BEA *BEA Amorphous Amorphous LTA

The reactions were run static at 160 °C and sampled as described in the Experimental Section. Representative length of time to achieve products is provided in Figures 7 and 8.

a

increase of selectivity to make *BEA with all three diquaternary OSDAs. In retrospect, this is not surprising given the very large diquaternary OSDA (often having para substitution relative to a central ring component) used by Ogino to also obtain very high silica *BEA phases.24 The *BEA is a multidimensional (3D) large pore zeolite but not so rich in 4-rings, indicative of a minor contribution of Ge. 3.2. Products from Diquaternary OSDAs (1−3). As the Ge content is increased, products with D4Rs like BEC (ITQ17) and IWS (ITQ-26) are obtained as the primary crystallization products. Both have large 3D pores. The structures for multidimensional large pore zeolites *BEA, BEC, IWS are shown in Figure 4, along with LTA. Interestingly,

Figure 4. Framework topology of molecular sieves BEC, IWS, *BEA, and LTA.

1, with an ortho ring substitution, does not make a 3D zeolite structure at the high Ge end, but simply stops at the stage of a layered product.25 On the other hand, the decrease of the addition of Ge moves the products to converge on the high silica *BEA, where there is less selectivity attributable to the OSDA isomers. This happens as the ratio is fixed at 16/1, for example. Individually, we can see the transition for 2 and 3 from 3D structures with Ge in double 4-rings, to the *BEA product, as we reduce the Ge contribution in the synthesis (Figures 5 and 6). Given the inability of 1 to produce a material such as BEC or IWS, molecular modeling was carried out to provide further insights. The energies for placement of the three OSDAs into three host lattices are shown in Table 2. Stabilization energies at 373 K were computed using prior methods.16 The work was done on all-silica materials for the structures and so numbers computed are expected to changes lightly if the introduction of Ge is made into the lattice.

3. RESULTS AND DISCUSSION 3.1. Summary of Synthesis Results. Table 1 shows the ratios explored for Si and Ge in the initial molecular sieve synthesis, and the resulting materials obtained when the 6 OSDAs were each used in a synthesis. A standard set of conditions that one uses in this chemistry is to vary the Si and Ge while keeping a 1:1 ratio of the OSDA (in the OH form) to HF, and under relatively concentrated run conditions. Earlier studies have demonstrated that the product selectivity can change for reactions with F as a contributor, with the variation of concentration for the overall reaction. More concentrated systems with less water experienced the greatest contribution of F influencing molecular sieve products with higher contents of four-rings and five-coordinate Si−F sites.22,23 Both contribute to greater channel branching in the resulting products.5 One correlation that can be observed, in terms of the effect of the Ge contribution, is that as it diminishes there is an C

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Figure 5. Effects on the products obtained by altering the Si:Ge ratio with syntheses containing 2, top to bottom Si/Ge = 16, 10, 8, 6, 4, 2.

Figure 7. Kinetics of BEC formation with 3.

Figure 8. Kinetics of IWS formation with 2. Figure 6. Effects of the crystalline products obtained by altering the Si:Ge ratio with syntheses containing 3, top to bottom Si/Ge = 20, 16, 12, 8, 4, 2.

are quite different. BEC forms in less than a week, whereas IWS requires ca. 2 weeks to form. This is likely an example demonstrating that the thermodynamic end product, although calculably stable and favorable, is sometimes not achieved rapidly (if at all) if there are barriers to the correct orientation in creating stable nuclei.26 3.3. Products from the Monoquaternary OSDAs (4−6). Figure 9 shows three products from OSDAs 4−6 run at Si/Ge

Table 2. Stabilization Energy Calculationsa

a

Figure 9. Si:Ge synthesis (at Si/Ge = 2) with monocharged OSDAs 4−6.

Stabilization energies in kJ (mol Si)−1.

= 2. One of the interesting surprise results was the crystallization of LTA from some of the monoquaternary OSDAs (see Table 1 at higher Si/Ge ratios), particularly 6. These OSDAs can be thought of as fragments of the triquaternary OSDA that was explored by Schmidt et al.13 to make LTA under some compositions. The monoquaternary OSDAs make an elusive composition of LTA, where the material has a high silica-to-alumina ratio. This success is important for the potential of use in catalytic applications. The

Here, it does not seem to show an obvious reason why 1 is not producing a 3D zeolite product. Following the modeling, if 2 and 3 make these products and have similar energies, is there any competition to produce these phases we find? Figures 7 and 8 show the time profiles for producing BEC from 3, and then IWS from 2. Although each has unique product selectivity, given the small differences in the energy of OSDA packing into the respective molecular sieve host, it is surprising that the rates D

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Chemistry of Materials use in MTO (methanol-to-olefin) reactions has recently been shown in a related study.14 Additionally, 13C MASNMR showed that the OSDA used in making the various products remained intact within the host lattice. 3.4. Characterization of the Guest−Host Products. Five products are obtained in this work. Some of the OSDAs, under certain reaction conditions, are capable of making multidimensional large pore zeolites, IWS, *BEA and BEC. For the smaller OSDAs, we observe LTA under some conditions. Finally, an unknown material is reproducibly obtained under some conditions, and is termed CIT-13 (structure presented below). This material can be calcined, and was found to be microporous. For the other four materials, where the structures of the host lattice are known and the internal void volume computed from the structure, we find that the amount of OSDA released in a TGA experiment is consistent with the expected void space. 3.5. Determining the Crystallization Field for CIT-13. 3.5.1. Summary of Key Factors in the CIT-13 Chemistry. Here, we are juxtaposing the crystallization selectivity for six OSDAs. The molecules differ in both the ring substitutions of benzene or toluene, both in terms of mono- and diquaternary units, and then the positioning via ring placement. That is, the syntheses will exploit differences in the favorable fits of the OSDAs and the possibility of frequent double 4-ring units appearing in the host lattice. We differentially biased that possibility by varying the Si/Ge ratio. When the Ge has a good contribution, the products BEC, IWS and now CIT-13 (from 5) all have a better chance to form. It follows that with this key building unit as part of the structure, we will get multidimensional pore systems in the crystallizing products. In a single instance the stereochemistry of 5 seems sufficient to redirect (as it were) how the double 4-ring building block is being used, and we get a 14-ring channel intersected by a 10-ring, instead of the 12 into 12 seen in most of the other products here. Similar to the diquaternary OSDA studies above, once we decrease the Ge content, the ability to make CIT-13 disappears. It is also interesting that the smaller monoquaternary OSDAs 4−6 do not default to making *BEA as the Ge content diminishes (as was seen for OSDAs 1−3). In two instances no product is found, and in the case of 6, the para isomer, a good selectivity to LTA occurs. 3.5.2. Proposed Structure for CIT-13. Using a sample created from 5 and the Si/Ge ratio shown Table 1, a product was made that gave an unknown PXRD pattern (see Figure 10), but seemed somewhat homogeneous from the SEM (Figure 11). This sample was then used to undertake 3dimensional Rotating Electron Diffraction (RED) analysis, a

Figure 11. Scanning electron micrograph of CIT-13.

powerful new approach in the electron microscope and pioneered by Zou and Hovmuller in Stockholm.15 By combining PXRD data and RED data a C-centered orthorhombic unit cell (a = 13.77A and b = 27.32A and c = 10.38A) was identified. The RED data was then further used in the FOCUS structure solution process,27 and a unique framework topology with 7 crystallographic T atoms was repeatedly found among all 5000 trials carried out. This was the only solution proposed by FOCUS. The structure reveals the assembly of repeat building units and the generation of the 10ring portals and the perpendicular 14-rings in the structure. The third axis shows no portals, and thus this framework has a 2dimensional pore structure. The various features are illustrated in Figure 12. It nonetheless is the first reported material with

Figure 12. Schematic illustrations of the CIT-13 structure.

intersecting 14 and 10 ring portals. The constriction of the 14ring portal is similar to what has been seen for the UTL materials made.9 Consistent with the expectations of a product that is 14 by 10 ring (but no portal in the third dimension), the TGA loss of 17 wt % OSDA mass sits between the known 3D large pore (12-rings) value of 21% and a value of 15 wt % for a 1D 14-ring product.28 More detailed discussion of the structure solution and Rietveld refinement, along with the detailed crystallographic information, will be published separately.

4. CONCLUSIONS In this work, we have investigated the behavior of six OSDAs in Si/Ge molecular sieve syntheses. Each of the OSDAs shares the

Figure 10. Powder X-ray diffraction patterns of two preparations of CIT-13 with Si/Ge = 4. a, batch 1; b, batch 2. E

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common feature of the 2-methyl imidazolium ring attached to a benzyl group; in some cases, there are two, and in others the second position for the imidazolium group was simply taken up by a methyl substituent. Aside from the changes in the size and shape of the OSDA, the principal variable in the study was the contribution by Ge atoms (into the lattice in tetrahedral positions). Once there was sufficient Ge content, the likelihood of the presence of double 4-rings (D4R) in the structure of the resulting product was high. This translated into products that were known, and one new structure, all of which featured the D4R component. Past studies have shown that D4Rs were good sites in a host lattice for the Ge atoms to reside. Overall, the inorganic chemistry was dominating the determination of the eventual lattice details when Ge was involved. Once the Ge was removed with the same set of six OSDAs, only *BEA was produced if any crystalline material was obtained. Although emphasis was placed on the importance of Ge in these reactions, there was also a good amount of fluoride present operating under higher solution concentrations. Past experience has shown that this type of composition also leads to creating 4DR, with F in the center of such cubes, or Si with expanded coordination valences. The latter can lead to different bond angles making the intersection of void spaces in a growing structure more likely. The product distribution in Table 1 shows that the three isomeric OSDAs do not make the same product. Hence, packing into a microporous host must still remain important even when the subconstruction is dominated by the presence of Ge and F. When the ring substitution was para (for example), whether there is a second charged group (imidazolium) or simply a methyl substituent, there was also an impact on the product formed. Interestingly, 5, a singly charged and ortho substituted benzyl imidazolium not only gave a new structure, but one with a 14ring portal as part of the structure. In fact, it is the first known example of a 10-ring, 14-ring system. This result shows that there remain opportunities to create new frameworks by employing the methods we describe here in going from the triquaternary OSDAs to analogous diquaternary and monoquaternary OSDAs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Chevron Energy and Technology Company for support of this research. In particular, Dr. Robert Saxton (Chevron) is thanked for his support of this research collaboration. We thank Dr. Joel Schmidt for useful comments throughout the research study. The computational method described here was developed by DOE Grant No. DE-FG0203ER15456.

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REFERENCES

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DOI: 10.1021/acs.chemmater.6b00031 Chem. Mater. XXXX, XXX, XXX−XXX