7854 | Chem. Commun., 2016, 52, 7854--7857 This journal is©The Royal Society of Chemistry 2016

Cite this:Chem. Commun., 2016,

52, 7854

High-throughput synthesis and characterizationof nanocrystalline porphyrinic zirconiummetal–organic frameworks†

M. L. Kelty,‡a W. Morris,‡a A. T. Gallagher,a J. S. Anderson,a K. A. Brown,ab

C. A. Mirkin*abc and T. D. Harris*a

We describe and employ a high-throughput screening method to

accelerate the synthesis and identification of pure-phase, nanocrys-

talline metal–organic frameworks (MOFs). We demonstrate the effi-

cacy of this method through its application to a series of porphyrinic

zirconium MOFs, resulting in the isolation of MOF-525, MOF-545, and

PCN-223 on the nanoscale.

Metal–organic frameworks (MOFs) are a class of porous crystallinesolids formed through the self-assembly of metal ions or clustersand organic bridging ligands. Due to their large internal surfaceareas and design flexibility, these materials are promising can-didates for a variety of applications, including gas storage,1 gasseparations,2 sensing,3 and catalysis.4 Considerable research hasbeen directed towards the exploration of novel MOF structures,5

but their extension into the nanoscale size regime6,7 represents amuch less studied area of research. Indeed, nanoscale MOFsexhibit distinct chemical and physical properties compared totheir macroscale analogues, such as increased biodispersibilityand larger relative external surface areas. These materials aretherefore well suited for applications such as biological ima-ging,7c,8–10 drug delivery,7a,b,8–11 catalysis,7d and sensing.10,12

Despite the advantages associated with nanoscale MOFs, norapid method that allows for general control over the structure,size and shape of the nanocrystals has been developed.

A number of approaches have been applied to the synthesis ofnanoscale MOFs, including chemically-modulated synthesis,13

microwave-assisted synthesis,14 surfactant-mediated synthesis,15

and micro-emulsion methods.15 Analogous to the procedures forforming macroscopic MOF crystals, these processes are often time-intensive and require multiple rounds of condition optimization tocontrol crystal size and morphology. Moreover, translating theconditions used to synthesize macroscopic MOFs to the nanoscalefrequently results in the formation of amorphous and/or alterna-tive phases, as highlighted in a recently published paper wheresize control over nanoscale MOFs was achieved through dilution ofsingle crystal conditions.16 Alternatively, one can envision applica-tion of high-throughput techniques to the synthesis and rapididentification of nanoscale MOFs. While high-throughput synth-esis has been employed to accelerate the discovery of novel17 andisoreticular18 MOFs, these techniques have not been adapted totarget the synthesis of analogous nanoscale materials.

Porphyrinic zirconium MOFs represent an ideal family ofcompounds upon which to pursue high-throughput techniquesdue to the extensive range of structures that can result fromsimilar reaction conditions. Indeed, over ten distinct porphyriniczirconium MOFs have been reported,19,20 with six of them –MOF-525,19a MOF-545,19a–c,21 PCN-225,19d PCN-224,19e PCN-22119f

and PCN-223,19g – featuring a tetracarboxyphenyl porphyrin linker(H6TCPP) (see Fig. 1 and Fig. S1, ESI†). These solids typicallyexhibit structures supported by octahedral Zr6 clusters, with oneexception of a MOF that features square prismatic Zr8 clusters.19f

In general, the wide variation in topology of zirconium MOFsfeaturing H6TCPP linkers arises from the different coordination of

Fig. 1 Structures of porphyrinic zirconium MOFs involved in this investi-gation. Green octahedra represent zirconium atoms, while red, blue, andgrey spheres represent oxygen, nitrogen, and carbon atoms, respectively;hydrogen atoms are omitted for clarity.

a Department of Chemistry, Northwestern University, 2145 Sheridan Road,

Evanston, IL 60208, USA. E-mail: dharris@northwestern.edu,

chadnano@northwesern.edub International Institute for Nanotechnology, Northwestern University,

2145 Sheridan Road, Evanston, IL 60208, USAc Department of Materials Science and Engineering, Northwestern University,

2220 Campus Drive, Evanston, IL 60208, USA

† Electronic supplementary information (ESI) available: Detailed experimentaland analytical details, and MATLAB program for comparing of simulated andexperimental PXRD data. See DOI: 10.1039/c6cc03264h‡ These authors contributed equally to this work.

Received 18th April 2016,Accepted 12th May 2016

DOI: 10.1039/c6cc03264h

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aqua, hydroxo, and porphyrin based carboxy ligands at the Zr4+

ions. These differences lead to broad structural diversity, with fiveunique net topologies of ftw, csq, sqc, she, and shp-a having beenreported for TCPP-based zirconium MOFs. The wide range ofpotential morphologies in this family of compounds renders thedirected formation and isolation of analogous nanocrystallineforms challenging, in contrast to other zirconium MOFs whereinone topology dominates.22,23 Consequently, only three examplesof nanocrystalline porphyrinic zirconium MOFs have beenreported.7d,16,24,25 Herein, we report a high-throughput screeningmethod that allows for the synthesis and rapid identification ofpure-phase, size-controlled nanocrystals of the known porphyriniczirconium compounds MOF-525, MOF-545, and PCN-223. To ourknowledge, this represents the first example of a high-throughputmethod for the synthesis and rapid identification of nanoscaleMOFs, and the generality of this approach could enable itsextension to a broad range of other structures.

In designing a high-throughput protocol, we targeted a methodthat would enable a large number of reactions with varyingconditions to be carried out in parallel and the rapid assessmentof crystallinity and phase purity of the reaction products (seeFig. 2). Toward this end, we employed a programmable liquidhandler equipped with an automated syringe to accurately andsimultaneously prepare up to 96 reactions. The resulting reactionproducts were then analyzed by high-throughput powder X-raydiffraction (PXRD), and conditions yielding crystalline productswere repeated on the preparative scale. Products from preparative-scale syntheses were analyzed using PXRD and scanning (SEM) ortransmission electron microscopy (TEM).

Initial reaction conditions were selected based on previousreports of nanoscale MOFs26 as well as preparative scalesyntheses reported for TCPP-based zirconium MOFs.19a,b,d,e,27

Reaction variables including stoichiometry, zirconium source(ZrCl4 or ZrOCl2�8H2O), acid modulator (acetic acid, benzoicacid, formic acid or dichloroacetic acid), temperature (90 1C,120 1C, 130 1C), reaction time (18 or 72 hours), and solvent(dimethylformamide (DMF) or diethylformamide (DEF)) weresystematically varied over a 96-well plate such that a majorityof adjacent reaction conditions differed by only one variable.In total, 12 plate runs were programmed, and 1027 unique

sets of reaction conditions, out of 1036 total reactions, werecarried out. Note that while the precision of the automatedhigh-throughput liquid handler allows for microscale reactionsand the conservation of reactants, a similar screening of reactionconditions could be achieved using adjustable volume pipettesand stock solutions.

To accelerate the identification of crystalline products, PXRDexperiments utilizing a synchrotron light source were carried out atthe Advanced Photon Source of Argonne National Laboratory. Due tothe difficulties associated with characterizing such a large number ofreactions, and owing to the systemic nature of the reactions carriedout, PXRD patterns of one in six reactions were obtained for eachrun to identify conditions that gave rise to crystalline products (seeFig. 2a). In order to pinpoint the optimal reaction conditions forpure-phase crystalline product, reactions surrounding samples iden-tified to give crystalline product were also characterized in a secondround of pattern collection (see Fig. 2b).

The powder patterns were analyzed using a custom-designedMATLAB program to compare the experimental PXRD patternswith simulated patterns of published porphyrinic zirconium MOFs(available for download in the ESI†). In total, 254 unique PXRDpatterns were obtained and analyzed, and this analysis led to theidentification of the compounds MOF-525, MOF-545, and PCN-223as cubic (edges of ca. 880 nm), rod-shaped (ca. 300 � 40 nm), andellipsoidal (ca. 30 � 13) nanocrystals, respectively (see Fig. S3–S5,ESI†). As noted above, the syntheses of nanocrystalline MOF-525(edge of 14025 and 500–1000 nm24), MOF-545 (length range of350–900 nm7d) and of PCN-224 (diameter range of 30–190 nm)16

have very recently been reported. To our knowledge, this reportprovides the first synthesis of nanocrystalline PCN-223.

Following high-throughput synthesis and rapid PXRD analy-sis, preparative-scale syntheses of the nanocrystalline solids werecarried out to enable size control through subtle acid variationacross a small series of reactions (see Fig. 2c). In general, eachsmall-scale, high-throughput reaction that yielded a pure-phaseproduct was scaled up by approximately one order of magnitude(see ESI†). In the case of MOF-525, MOF-545, and PCN-223,scale-up of the exact reaction conditions ascertained from high-throughput experiments gave rise to nanocrystals of the respec-tive solids, albeit with slightly different sizes. Next, using these

Fig. 2 Schematic outline of high-throughput screening method. Each oval represents one set of reaction conditions in a 96-well plate. (a) 96 uniquesolvothermal reactions are prepared per run, and a PXRD pattern is collected for 1 in 6 reactions. (b) When a reaction is determined by rapid PXRD analysisto be crystalline, PXRD data are collected for surrounding reactions. (c) Reaction conditions determined by PXRD analysis to give pure-phase crystallineproducts are used in preparative-scale reactions. (d) Preparative-scale reactions that are determined by PXRD analysis to give pure-phase crystallineproducts are characterized by electron microscopy to determine size scale and dispersity.

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conditions, several preparative-scale reactions were set up inparallel, where acid concentration was subtly varied across theseries to yield differently sized particles (see ESI†). These reac-tions were then canvassed using PXRD (see Fig. 3), and the pure-phase crystalline products were then analyzed by SEM/TEM todetermine the average size and polydispersity of the nanocrystals(see Fig. 2d).

In total, this tandem high-throughput and scale-up approachyielded nanocrystals of MOF-525, MOF-545, and PCN-223, com-prising three distinct size ranges for each compound. In the caseof MOF-525, cubic crystals with edges of 122(20), 256(39), and341(62) nm were obtained (see Fig. 4a–c and ESI† for discussionof standard deviation estimation). Additionally, reactions to synthe-size nanocrystalline MOF-545 gave rod-shaped crystals of dimen-sions 117(22) � 34(7), 203(35) � 52(11), and 502(95) � 122(30) nm(see Fig. 4d–f). Finally, in the case of PCN-223, ellipsoidal crystals ofdimensions 535(51) � 331(32), 679(159) � 383(74), 804(133) �400(59) nm were isolated (see Fig. 4g–i).

Analysis of the high-throughput PXRD results allows us toestablish a set of qualitative design principles for morphologicalcontrol of porphyrinic zirconium MOFs. Using the pattern-matchingMATLAB program, 30% of the 254 PXRD analyzed were assignedas ‘‘No Match,’’ and 33% of PXRD contained no peaks at all (seeTable S2, ESI†). Reactions in DMF at 90 1C favored MOF-525 (25%)with little formation of other structures, while reactions carried outat 130 1C yielded MOF-525 (12%), MOF-545 (10%), PCN-223 (15%)and PCN-225 (3%) (see Table S3, ESI†). The preferential formationof MOF-525 at lower temperatures corresponds to a lower activationenergy relative to other structures and suggests MOF-525 to be thekinetically favored phase. Reactions run in DEF at 120 1C exclusivelyresulted in lower-symmetry structures, namely PCN-223 (20%),MOF-545 (20%), and PCN-225 (7%).

Reactions containing acetic acid as modulator preferentiallyformed MOF-525 (25%) and PCN-223 (11%), whose 12-connectedZr6 nodes do not feature defects (see Fig. S1 and Table S4, ESI†).The relatively high pKa of 4.76 for acetic acid allows for a greaterrate of exchange between modulator and linker during crystalformation, possibly facilitating the correction of defects duringcrystal growth. Concordant with the hypothesis, benzoic acid,with a slightly lower pKa of 4.20, also showed preferentialformation of MOF-525 (16%) and PCN-223 (16%), but to a lesserextent than acetic acid. Furthermore, formic acid, pKa = 3.77,yielded equal amounts of MOF-525 (5%), MOF-545 (5%), and

PCN-223 (5%). Finally, dichloroacetic acid, pKa = 1.29, gaveexclusive formation of MOF-545 (25%) and PCN-223 (10%).In all of the aforementioned categories, ‘‘No Match’’ and ‘‘NoPeaks’’ assignments account for the remaining percentages.

The conditions identified for nanocrystalline MOF-525, MOF-545, and PCN-223 all vary significantly from the published synthesesfor macroscale analogues, which demonstrates the utility of scan-ning a wide range of chemical space to synthesize nanoscaleporphyrinic zirconium MOFs over multiple size regimes. Addition-ally, size control over small ranges can be obtained while maintain-ing phase purity by using incremental acid variation (0.05–0.1 mLincrements). In accordance with previous reports,23,28 reactionswith increased acid concentration yielded larger particle sizes.However, large changes in acid concentration, both with thesyntheses reported here for nanocrystalline PCN-223 and MOF-525and with the syntheses reported previously for macroscale MOFs

Fig. 3 PXRD patterns of preparative-scale MOF-525 (left), MOF-545 (center), and PCN-223 (right). The asterisk denotes an unidentified impurity.

Fig. 4 TEM/SEM images of nanocrystalline MOFs synthesized in thisstudy. Compound and average size are reported with standard deviationsin parentheses. (a) MOF-525: 122(20) nm, (b) MOF-525: 256(39) nm,(c) MOF-525: 341(62) nm, (d) MOF-545: 117(22) � 34(7) nm, (e) MOF-545:203(35) � 52(11) nm, (f) MOF-545: 502(95) � 122(30) nm, (g) PCN-223:535(51) � 331(32) nm, (h) PCN-223: 679(159) � 383(74) nm, (i) PCN-223:804(133) � 400(59) nm.

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(see Fig. S2, ESI†), resulted in mixed-phase products. Theseobservations suggest that subtle acid variation may providea useful route to adjust crystallite size over small size ranges(i.e. hundreds of nanometers) but not over large size ranges(i.e. micrometers to tens of micrometers).

The foregoing results demonstrate the utility of a high-throughput approach to mitigate the challenges associatedwith nanocrystalline porphyrinic zirconium MOF synthesis byenabling the rapid screening of a wide range of chemical space.By combining high-throughput synthesis with rapid PXRDanalysis and subsequent scale-up, pure-phase nanocrystals ofMOF-525, MOF-545, and PCN-223 were obtained, with eachcompound being isolated in three distinct size ranges. The abilityto control the size and morphology of porphyrinic zirconiumMOFs can potentially couple the advantages of nanoscale MOFs,such as increased pore access and external surface area, with thefunctionality and applications of metalloporphyrins includingcatalysis,19a light harvesting,29 photodynamic therapy16,30 andchemical sensing.30 Current work is geared toward exploringthe size-dependent properties of these nanocrystalline MOFs.

This research was supported by AFOSR (FA9550-14-1-0274,FA9550-11-1-0275, and FA9550-12-1-0280) and DARPA (HR0011-13-2-0018). MLK is supported by Northwestern University’sChemistry of Life Processes Institute Lambert Fellowship Award.ATG is supported by the National Science Foundation throughthe Graduate Research Fellowship Program. KAB gratefullyacknowledges support from the International Institute for Nano-technology (IIN) Postdoctoral Fellowship. We thank S. Wang forexperimental assistance. The content of the information does notnecessarily reflect the position or the policy of the Government,and no official endorsement should be inferred. The content issolely the responsibility of the authors and does not necessarilyrepresent the official views of Northwestern University.

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6 In this report the term nanoscale is used to describe particles up toseveral hundred nanometers in the shortest dimension and notexceeding 1000 nm in the longest dimension. This definition is useddue to the practical applications and interesting properties of particlesbetween 100 and 1000 nm in size (see ref. 12b and c) and previouslabelling of such particles as nanoparticles (see ref. 12a and d).

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21 This compound was also reported as PCN-222 and MMPF-6, given inref. 12b and c, respectively.

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