MOLECULAR SEPARATION

Control of zeolite frameworkflexibility and pore topology forseparation of ethane and ethylenePablo J. Bereciartua,1 Ángel Cantín,1 Avelino Corma,1* José L. Jordá,1

Miguel Palomino,1 Fernando Rey,1* Susana Valencia,1 Edward W. Corcoran Jr.,2*Pavel Kortunov,2 Peter I. Ravikovitch,2* Allen Burton,2 Chris Yoon,2 Yu Wang,2

Charanjit Paur,2 Javier Guzman,3 Adeana R. Bishop,2 Gary L. Casty2

The discovery of new materials for separating ethylene from ethane by adsorption, insteadof using cryogenic distillation, is a key milestone for molecular separations because ofthe multiple and widely extended uses of these molecules in industry. This technique hasthe potential to provide tremendous energy savings when compared with the currentlyused cryogenic distillation process for ethylene produced through steam cracking. Herewe describe the synthesis and structural determination of a flexible pure silica zeolite(ITQ-55). This material can kinetically separate ethylene from ethane with an unprecedentedselectivity of ~100, owing to its distinctive pore topology with large heart-shaped cagesand framework flexibility. Control of such properties extends the boundaries forapplicability of zeolites to challenging separations.

Ethylene is a key feedstock for many chem-icals and polymers (1, 2), with a worldwideproduction exceeding 144 million metrictons in 2015 and being mostly producedvia steam cracking of ethane and liquefied

petroleum gas (3–5). However, steam crackersdo not produce neat ethylene; they also yield manyother hydrocarbons (mostly ethane) that mustbe separated for polymer production. Currently,ethylene is purified by cryogenic distillation, oneof the most energy-demanding processes in itsoverall production (6). The total energy used duringethylene and propylene separations accounts formore than 0.3% of the global energy consump-tion (7).Alternative technologies to cryogenic distilla-

tion processes rely on the development of newadsorptive separation processes, such as simu-lated moving bed (8) or vacuum pressure swingadsorption (9), as well as on the use of selectiveadsorbents that could result in substantial energysavings (7, 10, 11). In this regard, transitionmetal–containing materials have been disclosed as pos-sible selective adsorbents because they selectivelyinteract with ethylene through p complexation,which must be strong enough to provide the de-sired separation selectivity but not so strong thatit inhibits facile regeneration of the adsorbent(12–14). For separation of ethylene and ethane,Ag+-containing alumina (15), resins (15), clays (16),and zeolites (15, 17), as well as Cu+-containing

alumina (15), carbons (18), zeolites (19, 20), andmetal-organic frameworks (MOFs) (21–25), havebeen described as suitable adsorbents. The maindrawbacks of these types of selective adsorbentsare (i) the relatively low stability of the metal ad-sorption sites upon exposure to sulfur compoundsand/or moisture; (ii) the presence or formationof acid sites by partial reduction ofmetal cationsthat could drive oligomerization of olefins and,ultimately, block the pores; and (iii) the very dif-ficult regeneration of the adsorbent once poresare blocked, because thermal treatments wouldlead to metal clustering inside the pores.Other approaches for olefin separations were

based on preferential adsorption of olefin oncationic zeolites (26) and titanosilicates (6, 27)with selectivities of ~10. A reverse separationwith a preferential adsorption of paraffin overolefin has been proposed on high-silica zeo-lites (25) and zeolitic-imidazolate frameworks(ZIFs) (28, 29) but with low selectivities of onlyabout ~2.Siliceous zeolites exhibit structural micro-

porosity that allows for a very high selectivity ofolefins from paraffins. It is also possible to con-trol their physicochemical properties, thus increas-ing their affinity toward olefins, with respect toparaffins, while maintaining neutral frameworksand reducing deactivation by irreversible adsorp-tion of olefins. Thus, an ideal microporous ma-terial for olefin separation would be a pure silicazeolite, if the zeolite has the appropriate poreaperture for allowing entrance of the olefin buthindering paraffin diffusion. Pure silica zeolitesdo not contain any acid site and would be in-capable of oligomerizing olefins, even at hightemperatures. Additionally, these adsorbentsare thermally stable, permitting easy regener-ation when pore blocking does occur (30–32).Thus, a key challenge would be to identify and

synthesize pure silica zeolites for each particularseparation of olefins and paraffins.We found that ITQ-29 (the pure silica analog

of zeolite framework type LTA) was well suitedfor separating branched olefins from linear hydro-carbons (33). Additionally, ITQ-32 (IHW) couldbe used to kinetically separate propene from pro-pane and, depending on the temperature, 1-buteneand trans-2-butene from the C-4 raffinate (31).Other groups have also described the use of ITQ-12 (ITW) (30, 34), deca-dodecasil-3R (DDR), andsiliceous chabazite (Si-CHA) for selective separa-tion of C-3 fractions (32, 35). However, there areno reports of pure silica zeolites with the appro-priate selectivity for the separation of ethylenefrom ethane. We now describe the synthesis andstructural determination of a small-pore zeolite,ITQ-55, whose pore dimensions and shape ex-hibit a particular ability for separating smallgases—most notably ethylene from ethane.For the synthesis of the zeolite ITQ-55, we

used the N2,N2,N2,N5,N5,N5,3a6a-octamethyl-octahydropentalene-2,5-diammonium cation asthe organic structure-directing agent (OSDA),either in alkaline conditions (OH– media) orusing F– anions as mobilizer agents of the silica(F– media) (36). The synthesis procedure of theOSDA and examples of synthesis of pure silicazeolites ITQ-55 are described in the supplemen-tary materials (37). Physicochemical analyses ofthe as-made zeolites ITQ-55 [chemical analysesand 13C cross polarization/magic angle spinningnuclear magnetic resonance (CP/MAS NMR)]indicate that the occluded OSDA cation into theporous structure remained intact upon the for-mation of the zeolite (37). The 19F MAS NMR spec-trum of the pure silica zeolite ITQ-55 synthesizedin F–media shows a distinctive signal at –70 partsper million (ppm). We assigned this signal to F–

located in small cages but not at double 4-rings(D4Rs) that are often found in zeolites preparedin F– media that have a signal at ~–40 ppm (38).These findings suggest that zeolite ITQ-55 doesnot contain D4Rs in its structure (37).The zeolite ITQ-55 was thermally stable upon

OSDA removal by calcination at 1073 K (37).However, the calcined zeolite did not adsorbany N2 or Ar at 77 and 87 K, respectively, whichsuggests that its pore aperture is very small andseverely restricts the ability of these moleculesto access the porosity at cryogenic temperatures.Nevertheless, ITQ-55 is not a dense phase butrather a microporous material because it adsorbsother molecules in substantial amounts, whichindicates that the porosity of ITQ-55 is formedexclusively by ultramicropores, according to theIUPAC (International Union of Pure and AppliedChemistry) classification (39).A powder x-ray diffraction (PXRD) pattern was

obtained on a sample prepared in F– media thatwas calcined in situ at 1073 K for 5 hours under acontinuous flow of dry air to remove the occludedOSDA and ensure the complete evacuation of thesample. This PXRD pattern was tentatively in-dexed using the program TREOR. The resultssuggest amonoclinic unit cell with lattice param-eters a = 22.337 Å, b = 13.319 Å, c = 14.457 Å, and

RESEARCH

Bereciartua et al., Science 358, 1068–1071 (2017) 24 November 2017 1 of 4

1Instituto de Tecnología Química, Universitat Politècnica deValència–Consejo Superior de Investigaciones Científicas,Avenida de los Naranjos s/n, 46022 Valencia, Spain.2ExxonMobil Research and Engineering Company, 1545 Route22 East, Annandale, NJ 08801, USA. 3ExxonMobil ChemicalCompany, 4500 Bayway Drive, Baytown, TX 77520, USA.*Corresponding author. Email: acorma@itq.upv.es (A.C.);edward.w.corcoran@exxonmobil.com (E.W.C.); frey@itq.upv.es(F.R.); peter.ravikovitch@exxonmobil.com (P.I.R.)

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b = 92.618°. Analysis of the systematic extinctionsindicates that the most probable space groupsare C2, Cm, or C2/m. However, because of thelarge cell parameters and low symmetry, dataquality was reduced by severe peak overlapping,even at low 2q values, precluding the extractionof an appropriate set of intensities for structuraldetermination with powder data. Also, high-resolution PXRD patterns of calcined ITQ-55material collected with synchrotron radiationexhibited severe peak overlapping, and any at-tempt to solve the structure using conventionalpowder methods failed to provide a reasonablestructure.We took an alternative approach to solving the

structure by using electron diffraction tomogra-phy (EDT) (40–43). In this case, a modified EDTmethod was used, allowing ultrafast data collec-tion to reduce structural damage of the crystalunder the electron beam and to increase the

quality of the EDT data (44). During the anal-ysis of the collected EDT data of the calcinedzeolite ITQ-55, the Bragg reflections could be in-dexed with a monoclinic unit cell with param-eters a = 22.29 Å, b = 13.40 Å, c = 14.57 Å, andb = 93.13°, in acceptable agreement with thePXRD result. The reflection condition hkl: h +k = 2n (where h, k, and l are the Miller indicesand n is any integer number) confirmed the C-centered unit cell.The structure solution was achieved by classi-

cal direct methods, using the space group C2/m.The resulting partial structure with 10 Si and 21O atoms was refined against the ED data andwas completed by localizing the two missing Oatoms in the corresponding difference Fouriermap (37). Finally, the complete structure wasrefined using the Rietveld method (fig. S12; pro-jections of the structure along the main crystal-lographic axis are shown in fig. S13).

Zeolite ITQ-55 has a framework density of17.6 T atoms (Si or Al) per 1000 Å3. The mostnotable characteristic is the presence of twinedheart-shaped cages (Fig. 1A) that are intercon-nected by sharing a common 8-ring (8R) (ringaperture: 5.3 Å by 3.4 Å) (Fig. 1B). These cavitiesare accessible through two parallel systems of zig-zag 8R channels (ring aperture: 5.9 Å by 2.1 Å).Thus, this material can be described as a tortuousmonodirectional small-pore system with rela-tively large cavities (Fig. 1C).The complete construction of the ITQ-55 struc-

ture from its basic building units is shown in fig.S14. The structure was optimized by density func-tional theory (DFT) calculations (37). The calcu-lated unit cell parameters (a = 22.74 Å, b = 13.58 Å,c = 14.76 Å, and b = 93.13°) agree reasonably wellwith the experimentally determined values for thecalcined sample [a = 22.387(4) Å, b = 13.3405(18) Å,c = 14.499(2) Å, and b = 92.561(5)°; numbers inparentheses indicate the standard deviations]. Thesize of the smallest pore window was calculatedto be 2.33 Å by 5.71 Å, which is relatively com-parable to the experimental size (2.07 Å by 5.86 Å).On the basis of these values, no diffusion of anymolecules would be expected to occur if the struc-ture was rigid.To gain insight into the degree of flexibility

of the structure, we performed ab initio moleculardynamics (AIMD) simulations and calculated thedistributions of the minimal window size (Fig. 2).AIMD simulations strongly suggest that flexibilityof ITQ-55 is crucial for explaining its distinctivediffusion and separation properties. For the emptystructure, the mean window size is 2.38 Å, with astandard deviation of 0.17 Å. We also performedAIMD simulations in which C2H4 moleculeswere constrained to the center of the smallest8R window. Such a configuration is close to thetransition state and allowed us to calculate theextent of window flexibility during moleculardiffusion in ITQ-55. In this case, the mean win-dow size is 3.08 Å, with a standard deviation of0.16 Å.Numerous molecular simulation studies have

predicted that diffusion of tightly fitting mole-cules in small-pore zeolites should be stronglyinfluenced by framework flexibility (45–47). Frame-work flexibility generally accelerates the diffusionof tightly fitting molecules. Also, small-adsorbate–induced deformation during diffusion of hydro-carbons in small-pore zeolites has been predictedby simulations (46). However, experimental evi-dence was somewhat unclear because of the dif-ficulty to characterize flexibility directly andsensitivity of theoretical predictions to param-eters of intermolecular interactions (45, 48).Molecular diffusion in ITQ-55 presents the mostnotable example of framework flexibility in zeo-lites: The structure admits molecules that arealmost ~1 Å larger than the nominal crystallo-graphic pore aperture size. The ethylene mole-cule “braces the window open” during moleculardiffusion. Such large effects of framework flexi-bility have been recently reported for diffusionin much more flexible ZIF materials (49–51), butnot for zeolites. Thus, our simulations show that

Bereciartua et al., Science 358, 1068–1071 (2017) 24 November 2017 2 of 4

Fig. 1. Description of thezeolitic structure ofITQ-55. (A) A 48Theart-shaped cage.(B) Dimeric 48T cages.The yellow ring indicatesthe interconnecting8-ring (8R). (C) Projectionalong the b axis of theITQ-55 structure (oxygenatoms omitted for clarity;T atoms in gray; two of theheart-shaped cavitieshighlighted in blue and red,respectively, for clarity).

Fig. 2. ITQ-55 window sizes.Distributions of the minimal8R window size for the emptystructure of ITQ-55 (left) andthe ITQ-55 structure withethylene molecules con-strained to the center of the8R window (right), ascalculated from the DFTmolecular dynamics simula-tions at 300 K (37).

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diffusion kinetics is themechanismof separationin ITQ-55.Some T-site occupancies for ITQ-55 zeolites

were <1, and the extent of these vacancies variedfrom sample to sample. This observation is cor-roborated by 29Si MAS NMR measurements,which showed some variability in the Q3 signalfor different preparations (37). Initial examinationof this phenomenon revealed the most probablelocations of structural defects. Some of thesesites are located in the smallest pore windowand might possibly influence molecular diffusion.However, experimentally measured ethylene andethane selectivities for materials with differentlevels of Q3 defects appear to be relatively con-sistent, regardless of the presence or absence ofvacancies. Thus, we conclude that structural de-fects alone cannot explain molecular diffusionof ethylene in ITQ-55. Molecular simulations ofadsorption isotherms in the ideal ITQ-55 and“defective” ITQ-55 model structures showedgood agreement with experimental ethylene iso-therms (fig. S20), lending further support forthe validity of our structural solution and therelatively minor role of defects. We explored theability of the ITQ-55 zeolite to separate mixturesof ethane and ethylene gases, both with large(F– media) and small (OH– media) crystals (seesupplementary materials for complete discus-sion) (37). As expected, the severe diffusionallimitations for both molecules are an impedi-ment to reaching full equilibrium in the adsorp-tion data points of ITQ-55 synthesized in F–

media. Equilibrium was attained on small-crystal zeolites for ethylene only, but not forethane adsorption (fig. S20). This behavioris quite different from that of other zeolites inwhich both ethylene and ethane are fully equil-ibrated and adsorbed in very similar amounts(table S5). This difference strongly suggests apreferential diffusion of ethylene over ethaneon ITQ-55 material. Thus, we performed dif-

fusional studies of adsorption of both gases bymeasuring single-component gas adsorptionkinetics on small-crystal ITQ-55 (Fig. 3).The rate of ethylene adsorption on ITQ-55 is

much higher than the corresponding adsorptionrate of ethane (Fig. 3). The narrow pores of ITQ-55 reduce the molecular diffusion of ethylenerelative to other known narrow-pore zeolites,such as DDR (52). However, the molecular dif-fusion of the slightly larger ethane molecules isfurther hindered by approximately two ordersof magnitude, whereas in DDR it remains nearlyunchanged (fig. S21). The ability of ITQ-55 toseparate such closely sized molecules with mar-kedly different rates suggests various new ap-plications of ITQ-55 in kinetic gas separations.Overall, mass transport through the ITQ-55framework could be substantially increasedby heating (fig. S21). Alternatively, materialcooling reduced framework flexibility and fur-ther increased the difference in mass transportrates between ethane and ethylene.We estimate that kinetic (or membrane)

separation of C2H4 and C2H6 on practical scalescould be possible in smaller crystals (or thinnermembranes) of ITQ-55, at slightly elevated tem-peratures. In particular, at 112°C the moleculardiffusivity of C2H4 is already on a time scale ofseconds, even in existing ~0.5-mm crystals ofITQ-55, while maintaining unprecedented highC2H4/C2H6 selectivity of 50. We estimate thatthe diffusion flux rate for C2H4 could be com-parable with that of polymeric and carbon mo-lecular sieve membranes (53) and ~7.5 timesslower than that of CHA (SSZ-13) membranes(54). However, none of the above-mentionedcomparative materials exhibit selectivities higherthan ~10 to 13 (37) (table S6), including the poly-meric membranes with incorporated MOF crys-tals, which are limited to a selectivity of ~5 (55).Initial results indicate that the use of smallerAl-ITQ-55 crystals and the resulting faster trans-

port rates provide even greater selectivity of~300 (fig. S18). Efforts to fabricate such thininorganic membranes are ongoing. The factthat defect-free ~0.5-mm-thick inorganic mem-branes have already been reported with silicaMFI zeolites (56), as well as ~1-mm oriented mem-branes (57), offers further promise for preparingsuch membranes with ITQ-55.The distinctive property of ITQ-55 to sep-

arate two similarly sized molecules was con-firmed with multicomponent gas breakthroughexperiments (Fig. 4; see fig. S22 for experimen-tal setup). When a gas mixture containing 50%ethane and 50% ethylene was fed to the ad-sorption column with helium-filled ITQ-55 crys-tals, ethane initially displaced helium fromthe intercrystalline volume and broke throughthe column. The outlet concentration of ethanenearly reached 100% because of the adsorptionof ethylene on ITQ-55 crystals. As pores of ITQ-55 were gradually saturated with adsorbed mol-ecules, ethylene began to break through thecolumn and diluted the ethane. The describedadsorptive gas separation was achieved by se-lective adsorption of ethylene on ITQ-55 (withtrace amount of ethane) and the resulting ethaneenrichment in the gas phase. Adsorbed ethyl-ene was then recovered from the pores of ITQ-55 by either decreasing the gas pressure or heat-ing the ITQ-55 column.Trace amounts of slowly diffusing ethane

molecules in the zeolite pores did not blockthe faster-diffusing ethylene molecules be-cause of large heart-shaped cages of ITQ-55that allowed molecules to pass each other. Suchdynamic properties of ITQ-55 are highly bene-ficial in terms of gas separation processes.The pure silica ITQ-55 zeolite was also ex-

tremely stable against pore blocking due toirreversible adsorption of hydrocarbons. Theadsorption properties of the studied samplesremained unaltered upon exposure to olefins

Bereciartua et al., Science 358, 1068–1071 (2017) 24 November 2017 3 of 4

Fig. 3. Adsorption kinetics. Adsorption of ethylene (red) andethane (blue) on small-crystal ITQ-55, as measured with theHIDEN-IMI manometric adsorption system at 30°C and 0.45 barand 0.6 bar, respectively.

Fig. 4. Breakthrough studies. Outlet gas composition of an adsorptioncolumn packed with 1100 mg of ITQ-55 small crystals at 50°C fora feed mixture containing 50:50 ethylene:ethane at 8.5 bar flowing at10 standard cubic centimeters per minute.

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for as long as 3 months. This stability is attrib-uted to the lack of acidity in the zeolite.

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ACKNOWLEDGMENTS

We gratefully acknowledge financial support from ExxonMobilResearch and Engineering Company. Instituto de TecnologíaQuímica researchers also thank the European Research Council(grant ERC-2014-AdG-671093 “MATching zeolite SYNthesis withCATalytic activity”) and the Spanish government (grants MAT2015-71842-P MINECO/FEDER and Severo Ochoa SEV-2012-0267 andSEV-2016-0683) for economic support. Synchrotron powder x-raydiffraction experiments were performed at MSPD beamline atALBA Synchrotron with the collaboration of ALBA staff. We alsothank R. Blanco Gutierrez, H. Deckman, L. Koziol, D. Leta, andK. Strohmaier for useful discussions. Finally, we specially thank theElectron Microscopy Service of the Universitat Politècnica deValència and, in particular, M. J. Planes and J. L. Moya for theirinvaluable support with EDT data acquisition. All data are reportedin the main text and supplementary materials. Relevant patents:ES2554648 (B1) (2015), US2016009618 (A1) (2016),US2016008756 (A1) (2016), US2016008753 (A1) (2016), andUS2016009563 (A1) (2016).

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/358/6366/1068/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S25Tables S1 to S6References (58–75)

5 June 2017; accepted 12 October 201710.1126/science.aao0092

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ethyleneControl of zeolite framework flexibility and pore topology for separation of ethane and

Adeana R. Bishop and Gary L. CastyW. Corcoran Jr., Pavel Kortunov, Peter I. Ravikovitch, Allen Burton, Chris Yoon, Yu Wang, Charanjit Paur, Javier Guzman, Pablo J. Bereciartua, Ángel Cantín, Avelino Corma, José L. Jordá, Miguel Palomino, Fernando Rey, Susana Valencia, Edward

DOI: 10.1126/science.aao0092 (6366), 1068-1071.358Science 

, this issue p. 1068Scienceadsorption experiments, the zeolite preferentially adsorbed ethylene from a mixed stream of ethylene and ethane.hydrocarbons. However, molecular dynamics suggested that the pores should be flexible. Indeed, in competitive

synthesized a pure silica zeolite with very small pores, which, if static, would not adsorb either of theseet al.Bereciartua from ethane, an energy-consuming step. In theory, pure silica zeolites are well suited to separate olefins from paraffins.

Ethylene is a key feedstock for many chemicals and polymers, but its production requires cryogenic separationPurifying ethylene with flexible zeolites

ARTICLE TOOLS http://science.sciencemag.org/content/358/6366/1068

MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/11/21/358.6366.1068.DC1

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