Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Communication
Computationally-Guided Synthesis of SSZ-52, a Zeolite for Engine Exhaust Clean-up Tracy M Davis, Albert Tianxiang Liu, Christopher M. Lew, Dan Xie, Annabelle Benin, Saleh Elomari, Stacey I. Zones, and Michael W Deem Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04578 • Publication Date (Web): 13 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Computationally-Guided Synthesis of SSZ-52, a Zeolite for Engine Exhaust Clean-up Tracy M. Davisa, Albert Tianxiang Liua, Christopher M. Lewa, Dan Xiea, Annabelle Benina, Saleh Elomaria, Stacey I. Zonesa, Michael W. Deemb a. Chevron, Richmond, CA 94802, b. Rice University, Houston, TX 77005 ABSTRACT: Notable similarities between SSZ-52 (SFW) and the commercially important CHA structure indicate that this catalyst will likewise reduce NOx pollutants from mobile emission systems. However, the only organic compound known to direct SSZ-52 crystallization is cost prohibitive. Here, three alternative compounds have been identified through de novo design computations.
Small-pore cage-based zeolites such as SSZ-13 (CHA) serve as commercial catalysts in several important reactions including the selective catalytic reduction of NOxspecies [1, 2] and in the conversion of methanol to light olefins [3]. Structural similarities between SSZ-52 (SFW) and CHA that were recently published by Xie et al. [4] suggest SSZ-52 will be of similar utility in these applications. However, wide usage and manufacture of this unique material has been hindered to-date by the difficulty of its synthesis. Specifically, the only disclosed method [5] for making SSZ-52 requires use of an organic structure directing agent (SDA) that is highly time intensive to synthesize and therefore cost prohibitive. Now, through a combination of de novo design computations and experimental efforts, three alternative SDAs have been identified that are inexpensive and easily synthesized from commercially available reagents. Molecular sieves and zeolites in particular are used as heterogeneous catalysts for a number of chemical reactions relevant to oil refinement, petrochemical processes and noxious gas mitigation [6]. By definition, zeolites are microporous materials, whose crystalline frameworks are comprised of TO4 tetrahedra (where T = Si, Al, Ge, B). Unique combinations of pore openings (ranging from ~ 3 to 10 Å), pore shape and channel dimensionality lead to a wide variety of zeolite framework types (denoted by a 3letter code). Careful selection of the overall pore architecture along with zeolite composition, cation substitution, and in some cases, metal loading, allow for zeolites to be used over a broad range of applications. While only ~20 of the more than 200 known zeolite structures [7] have been utilized in commercial applications, the monetary value of these materials as catalysts is significant. In the emerging emissions control industry, the market value for zeolites is on the order of tens of billions dollars, and their market value is well in excess of 100 billion dollars in the production of chemicals and fuel from petroleum feedstocks.
Zeolites with pores defined by 8 T-atoms that open into relatively large cages, and particularly those possessing double-six ring secondary building units, are known to perform well as catalysts in the selective catalytic reduction (SCR) of the nitrogen oxide species found in engine exhaust [8, 9]. This class of molecular sieves, described as cage-based small-pore zeolites includes CHA, AFX and AEI framework types. A less-studied member of this class of materials is SSZ-52, which has been assigned the framework code SFW by the Structure Commission of the International Zeolite Association [10]. Like CHA and AFX, SSZ-52 has a three-dimensional channel system whose pore openings are made up of 8 T-atoms, and it is a member of the ABC-6 family of zeolites. Due to the unique sequencing of its crystalline layers (AABBAABBCCBBCCAACC), the catalytically-active cages of SSZ-52 are significantly larger than those of CHA and AFX (Figure 1). Previous work by Fickel et al. [11] demonstrated the excellent performance of Cu-exchanged SSZ-13 (CHA) and SSZ-16 (AFX) in NH3-SCR. The hydrothermal stability and strong SCR activity of these zeolites was attributed to the architecture of their microporous frameworks. Specific studies [12, 13] aimed at understanding the hydrothermal stability of small-pore zeolites have shown that the restrictive pore apertures of these materials, which are just big enough to admit small gases, inhibit migration of Al- and Cu- species out of the zeolite. Considering the structural similarities between CHA, AFX and SFW, SSZ-52 is expected to facilitate NH3-SCR. Furthermore, it is anticipated that SSZ-52 will show unique performance properties as a result of its larger cage size impacting rates of reaction and the siting of Cu-ions. In general, the discovery of new zeolite structures is often driven by the utilization of novel organo-cations, which are commonly referred to as structure directing agents (SDAs). These organic compounds serve as a sort of scaffold around which smaller inorganic oxide units can organize, and also provide energetic stability to an otherwise thermodynamically unfavorable crystalline
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
phase. Thermal removal of the organic results in a microporous lattice, whose unique pore structure and acidity has been directed by the spatial conformation and charge of the SDA. To-date, commercial development of SSZ-52 has been hindered by the high cost of the N,Ndiethyl-5,8-dimethyl-azonium bicyclo[3.2.2.]nonane (shown in Table 1) SDA required for its synthesis [5]. Because of its complex synthesis scheme (as shown in Figure S1), there is significant incentive to identify alternative, lower cost organic molecules for the synthesis of SSZ-52.
Figure 1. Comparison of the cages present in a) CHA, b) AFX and c) SFW. The length of the cage, as denoted by the arrows, increases in size according to: CHA < AFX < SFW.
Historically, zeolite synthesis improvements and the discovery of new organic SDAs for a targeted zeolite structure have been made through trial and error experimentation. However, in a couple notable reports, Schmidt et al. [14, 15] recently demonstrated that computational methods could be used to predict novel SDAs for a given zeolite framework. More specifically, they were able to computationally “grow” chemically feasible organic SDAs inside a given microporous structure and then validate their predictions through laboratory experiments. Earlier methodologies, and specifically that pioneered by Lewis and co-workers [16], similarly grows candidate SDAs through the combination of smaller molecules. However, growth is controlled by a “van der Waals overlap function” that permits fragment addition by steps not necessarily true to synthetic chemical pathways. As such, the organic molecules grown by Lewis’ methodology may not be synthetically feasible. According to the approach developed at Rice University [17], the starting fragments are known commercial molecules and well-understood chemical reactions are used to computationally synthesize the candidate SDAs. In this manner, more realistic molecules are created with the goal of supplementing the use of expensive, novel organic compounds. In this report, two approaches were taken in order to identify alternative SDAs for SSZ-52. First, a large library of known quaternary nitrogen compounds was computationally screened, and the organic compounds were ranked according to their stabilization energy in the SFW structure. As a second approach, SDAs were “grown” using a genetic algorithm in the SFW framework. Highlyranked SDA candidates identified by these two approaches were then selected for laboratory experiments. In this way, three new SSZ-52 SDAs (shown in Table 1) that are easily synthesized from commercially available reagents have been identified. The genetic algorithm described in Ref. [17] was used in order to predict synthetically feasible organic SDAs likely to promote crystallization of SSZ-52. Initially, a subset of
Page 2 of 5
~56,000 commercially available compounds was selected as starting “reagents” for this computational chemistry. For each iteration of the algorithm, a set of 84 in silico reactions is used to create a new generation of compounds with improved stabilization energies in the target zeolite framework. The Dreiding forcefield is used in the GULP molecular modeling program to calculate the stabilization energies. Zeolite stabilization energy is defined as the difference between the energy of the occluded SDAs in one unit cell (with the IZA standard setting) and the energy of that same number of isolated SDAs and the empty zeolite. Through iterative reactions targeted at improving the stabilization energy of the product organic compound in the zeolite framework, the method predicts molecules with high propensities to promote the formation of the target material. Importantly, the simulation also provides a synthetic path for each of the predicted SDAs. Stabilization Energy, kJ/ mol Organic SDA
N,N-diethyl-5,8-dimethyl-azonium bicycle[3.2.2]nonane
SFW 2 SDA/ cage
CHA 1 SDA/ cage
-11.1
-13.2
-10.5
-12.6
-10.1
-11.9
-8.4
-13.3
N “SDA0” N-ethyl-N-(2,4,4-trimethylcyclopentyl) pyrrolidinium “SDA1”
N
N-ethyl-N-(3,3,5-trimethylcyclohexyl) pyrrolidinium “SDA2”
N N-cyclohexylmethyl-N-ethylpiperidinium “SDA3”
N
Table 1. Calculated stabilization energies for organic SDAs experimentally demonstrated to promote crystallization of SSZ-52. SDA0 is the original compound discovered experimentally; SDA1, SDA2 and SDA3 were identified through computational screening. Parameters of the genetic algorithm were customized here according to inputs from laboratory experience. Specifically, and as discussed by Xie et al. [4], the SFW cage is known to be stabilized by 2 molecules of the known SDA, and the GME cage that is present in the SFW framework is stabilized by sodium cations associated with water. For the calculations performed here, a loading of 2 SDA molecules per SFW cage was used and the GME cage was left unoccupied. Furthermore, the evolutionary search for SDAs was constrained to include only those molecules having one or zero torsional degrees of freedom. From the list of 84 possible in silico reactions, the Menshutkin type was selected so that predicted SDAs were more likely to
ACS Paragon Plus Environment
Page 3 of 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
be quaternized ammonium cations. In addition to growing novel SDAs through this “sprout and grow” methodology, a large set of in-house SDAs was screened and the molecules were ranked according to their stabilization energies in the SFW framework. For both approaches, the zeolite framework was assumed to be all-silica (zero charge) and electrostatic interactions between the SDA and zeolite were neglected, a simplification that was previously demonstrated [18] to accurately predict experimental observations. The original SSZ-52 SDA (referred to henceforth as “SDA0”) was calculated to have a stabilization energy of 11.1 kJ/ mol Si in the SFW structure at a loading of 2 SDA/cage. Results from screening an in-house library of 550 SDAs in the SFW structure yielded 61 molecules with stabilization energies less than -10 kJ/mol Si. From these 61 candidates, 10 organic compounds were selected for experimentation that met the following criteria: 1) 0 or 1 torsional degree of freedom to limit the number of possible conformations; 2) ease of synthesis to minimize cost; 3) stability under typical zeolite reaction conditions (e.g., high temperature and high pH); and 4) “poor” stabilization energy in competing zeolite phases (specifically, CHA). Expounding on this last criteria – SDA0 has a stabilization energy in CHA of -13.2 kJ/mol at a loading of 1 SDA/ cage. While this value is lower than its energy in SFW, comparisons between structure types are invalid. Rather, comparisons between organic molecules in a single framework type are relevant. In the case of CHA, the most selective SDA, trimethyladamantylammonium [19] provides a stabilization energy of -16.7 kJ/mol in CHA. Comparing this value to that of SDA0 in CHA (E = -13.2 kJ/mol), it is clear that the latter compound will not suitably stabilize the CHA structure. Experimentally, energy differences greater than 2 kJ/mol distinguish well-fitting SDA-zeolite pairs from those with poor stabilization energies. Of the 10 in-house compounds selected for experimentation (and identified by simulation), remarkably 2 were found to promote the crystallization of SSZ-52. The structures of these molecules, named SDA1 and SDA2, are provided in Table 1, and were calculated to have stabilization energies in SFW of -10.5 and -10.1 kJ/mol, respectively. Powder XRD of as-synthesized SSZ-52 made with SDA1 (zeolite product referred to as SFW_SDA1) confirmed pure SFW phase material; Figure 2 shows XRD and SEM of as-synthesized SSZ-52 made with SDA2 (zeolite product referred to as SFW_SDA2). The organic content for each of the as-made SSZ-52 materials was measured by TGA. When SDA0 was used, the organic content was measured to be 13.5 wt%; SFW_SDA1 and SFW_SDA2 respectively contained 11.6 and 12.2 wt% organic. For SFW_SDA1, the ratio of carbon to nitrogen in the as-made product was measured to be 12, a value that is in good agreement with the theoretical value of 14. Likewise, good agreement between experimental (C:N = 14) and theoretical (C:N = 15) values was observed for SDA2_SFW. These results confirm the presence of the predicted organic molecules inside the SFW structure.
In addition to screening the in-house database of SDAs, several de novo computational design simulations of chemically synthesizable SDAs were performed as described above. These resulted in a variety of novel organic molecules with stabilization energy values comparable to that of SDA0. Molecules were selected for experimentation using the same set of criteria as previously defined. In some instances, minor modifications (specifically, quaternization of the nitrogen) were made to the resultant molecules. As an example, Syn005431, which showed promising stabilization energy in SFW (E = -10.7 kJ/ mol)
Figure 2. a) powder XRD pattern and b) SEM of the assynthesized zeolite product made with SDA2.
was modified in several ways as shown in Table 2. As was previously discussed by Schmidt et al. [14], minor modifications to the SDA, such as the addition of a single methyl group, significantly impacts the predicted stabilization energy as well as the experimental result. In this case, methylation of both nitrogen species (Syn005431_mod1) in the molecule reduces the stabilization energy by more than half. Organic SDA
Stabilization Energy, kJ/ mol
N Syn005431
Syn005431_mod1
Syn005431_mod2
NH
N
N
N
-10.7
-4.2
-6.4
N Syn005431_mod3
-8.4
Table 2. Syn005431 is a result from de novo design computations in SFW. As shown, quaternization of the nitrogen(s) leads to poorer stabilization energy, but in practice, is typically needed to balance the negative charge of the zeolite framework. Additional charge balance may be provided by alkali cations. Syn005431_mod3 (also referred to as “SDA3” in Table 1), which has a stabilization energy in SFW of -8.4 kJ/ mol, was one of the de novo designed molecules selected for experimentation. Initial zeolite reactions with this molecule resulted in a mixture of crystalline phases, including ABC-6 type small pore materials. Considering the higher (i.e., poorer) stabilization energy of SDA3 in SFW as compared with both SDA1 and SDA2, this result is not surpris-
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ing. The lowest energy configuration of SDA3 (shown in Figure 3) clearly shows that two of these molecules are not sufficient to fill the SFW-cage. As a refinement of this computationally-guided experimental work, a dualtemplating approach was investigated. Use of multiple SDAs in zeolite synthesis has been demonstrated previously [e.g., see Ref 20], and can provide a means for lowering the overall cost of a material by reducing the amount of the more expensive, but highly selective organic molecule. In the present case, combinations of SDA0 and SDA3 were used where SDA3 represented 20, 50 and 75 mol% of the total organic content in the synthesis gel. At all of these tested compositions, pure SSZ-52 crystallized. In conclusion, through the application of newly available computational methods, alternative cheaper SDAs for SSZ-52, an important pollution control zeolite, were identified according to their stabilization energy in the SFW structure. Moreover, the identified molecules were validated through laboratory experiments. In order to make certain that commercial scale-up of the SDA (and therefore, SSZ-52) was economical, identified molecules were required to meet the following criteria: 1) starting reagents are readily available; 2) number of synthetic steps is limited to 3, contrasted with the 9 steps needed to make the original SDA; and 3) yield is high at each synthetic step. With a new pathway opened for producing SSZ-52, sufficient quantities of material can be synthesized to allow for representative commercial testing.
Figure 3. Lowest energy configurations of a) SDA0, b) SDA1, c) SDA2 and d) SDA3 in the SFW-cage as predicted by molecular dynamics simulations.
ASSOCIATED CONTENT Synthesis methods and characterization details are provided in the Supplementary Information. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Authors * Tracy M. Davis:
[email protected], Michael Deem:
[email protected] Funding Sources The computational method described here was developed by DOE grant number DE-FG02-03ER15456. The work was supported by Chevron Research and Technology.
for use as commercial diesel engine deNOx catalysts. Top. Catal. 2013, 56, 1441-1459. (2) Jacoby, M. Ever-cleaner auto exhaust. Chem. Engr. News 2012, 90, 10-16. (3) Olsbye, U., Svelle, S., Bjorgen, M., Beato, P., Janssens, T.V.W., Joensen, F., Bordiga, S., Lillerud, K.P. Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity. Angew. Chem. 2012, 51, 5810-5831. (4) Xie, D., McCusker, L.B., Baerlocher, C. Zones, S.I., Wan, W., Zou, X. SSZ-52, a zeolite with an 18-layer aluminosilicate framework structure related to that of the deNOx catalyst Cu-SSZ-13. JACS 2013, 135, 10519-10524. (5) Lee, G.S., Zones, S.I. US Patent 6,254,849 2001. (6) Maesen, T., Zeolite scene. Studies in Surface Science & Catalysis 2007, 168, 1-2. (7) Zones, S.I., Translating new materials discoveries in zeolite research to commercial manufacture. Micro. Meso. Mater. 2011, 144, 1-8. (8) Cavataio, G., Girard, J., Lambert, C., Cu/zeolite SCR on high porosity filters: laboratory and engine performance evaluations SAE Technical Paper 2009-01-0897. (9) Brandenberger, S., Krocher, O., Tissler, A., Althoff, R., The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catalysis Reviews 2008, 50, 492-531. (10) Baerlocher, C., McCusker, L.B., Database of Zeolite Structures, http://www.iza-structure.org/databases/. (11) Fickel, D.W., D’Addio, E., Lauterbach, J.A., Lobo, R.F., The ammonia selective catalytic reduction activity of copperexchanged small-pore zeolites. Appl. Catal. B 2011, 102, 441-448. (12) Andersen, P.J., Bailie, J.E., Casci, J.L., Chen, H-Y, Fedeyko, J.M., Foo, R.K.S., Rajaram, R.R. WO 2008/132452 2008. (13) Blakeman, P.G., Burkholder, E.M., Chen, H-Y, Collier, J.E., Fedeyko, J.M., Jobson, H., Rajaram, R.R. The role of pore size on the thermal stability of zeolite supported Cu SCR catalysts. Catalysis Today 2014, 231, 56-63. (14) Schmidt, J. E., Deem, M.W., Davis, M.E. Synthesis of a specified, silica molecular sieve using computationally predicted organic structure directing agents. Angew. Chem. Int. Ed. 2014, 53, 8372-8374. (15) Schmidt, J.E., Deem, M.W., Lew, C., Davis, T.M. Computationally-guided synthesis of the 8-ring zeolite AEI. Topics in Catalysis 2015, 58, 410-415. (16) Lewis, D.W., Willock, D.J., Catlow, C.R.A., Thomas, J.M., Hutchings, G.J. De novo design of structure-directing agents for the synthesis of microporous solids. Nature 1996, 382, 604-606. (17) Pophale, R., Daeyaert, F., Deem, M.W. Computational prediction of chemically-synthesizable organic structure directing agents for zeolites. J. Mater. Chem. A 2013, 1, 6750-6760. (18) Burton, A.W., Lee, G.S., Zones, S.I. Phase selectivity in the syntheses of cage-based zeolite structures: An investigation of thermodynamic interactions between zeolite hosts and structure directing agents by molecular modeling. Microporous Mesoporous Mater. 2006, 90, 129-144. (19) Harris, T., Zones, S.I., A study of guest/host energetics for the synthesis of cage structures NON and CHA. Stud Surf Science & Catal. 1994, 84, 29-36. (20) Zones, S.I. US Patent 8,298,511 B2 2012.
REFERENCES (1)
Page 4 of 5
Gao, F., Kwak, J.H., Szanyi, J., Peden, C.H.F. Current understanding of Cu-exchanged chabazite molecular sieves
ACS Paragon Plus Environment
Page 5 of 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
TOC Graphic
ACS Paragon Plus Environment
5
