3D-Printing of Pure MetalâOrganic Framework Monoliths | ACS

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3D-Printing of Pure Metal-Organic Framework Monoliths Gwendolyn J.H. Lim, Yue Wu, Bhuvan B. Shah, J. Justin Koh, Connie K. Liu, Dan Zhao, Anthony K. Cheetham, John Wang, and Jun Ding ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00069 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Materials Letters

3D-Printing of Pure Metal-Organic Framework Monoliths Gwendolyn J. H. Lim,ǂ1 Yue Wu,*ǂ1 Bhuvan B. Shah,ǂ2 J. Justin Koh,1,5 Connie K. Liu,3,4 Dan Zhao,2 Anthony K. Cheetham,1,6 John Wang,*1 and Jun Ding*1 1Department

of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, 117575,

Singapore 2Department

of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore 3Institute

of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, #08-03, 138634, Singapore 4Institute

of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, 627833, Singapore 5Singapore

Institute of Manufacturing Technology, Agency for Science, Technology and Research (A*STAR), 73 Nanyang Drive, 637662, Singapore 6Materials ǂThese

Research Laboratory, University of California, Santa Barbara, CA, 93106, USA

authors contributed equally to this work.

*E-mail: [email protected]; [email protected]; [email protected] ABSTRACT: Metal-organic frameworks (MOFs) are usually synthesized in powder form. For many practical applications, MOFs need to be shaped into monoliths that can be easily handled. However, conventional shaping methods such as pelletization often result in a decrease in functionality. Recently, MOF-containing monoliths have been made using direct ink writing (DIW; extrusion 3D printing), but to date high additive loadings have been required. In this work, we demonstrate that colloidal gels containing only ethanol and Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) (HKUST-1) nanoparticles can be used directly as an ink for the DIW of pure densely packed and self-standing MOF monoliths. The MOF gel shows ideal rheological properties for 3D extrusion-based printing, suggesting this method may be generalized to other MOF families that form gels. Importantly, the accessible porosity and surface area of the MOF is retained well after shaping. The 3D printed HKUST-1 monolith displays an exceptionally high BET surface area of 1134 m2/g, and a high mesopore volume. We demonstrate that for methane storage, a classical application of HKUST-1, the 3D printed monolith is comparable or superior to monoliths formed by other shaping methods.

Metal-organic frameworks (MOFs) are nanoporous functional materials that have seen great interest for applications such as catalysis, sensing, energy storage, and gas storage and separation.1-3 While a huge range of MOFs have been prepared on the laboratory scale, very few have made the leap to large scale applications and commercialization. For successful large-scale applications, MOFs must be able to retain their desirable properties (such as their extremely high porosities and surface areas) after processing. As MOFs are generally synthesized as bulk crystalline powders that are unsuitable for industrial applications, the shaping and consolidation of MOFs into monoliths is a crucial, but frequently overlooked aspect.4 To achieve this, MOFs are often pelletized and/or incorporated into polymer matrices before shaping into

usable structures (films, granules, spheres, filaments etc.).5 However, classical shaping and densification processes generally reduce the functionality (e.g. the level and distribution of porosity and accessible surface area) of the resulting monoliths for three main reasons: (i) loss of MOF crystallinity from mechanical forces during processing, (ii) reduction in MOF loading due to the polymer matrix, and (iii) blockage of access to MOF porosity by matrix materials.4 In addition, there are restrictions to the shapes of monoliths that can be produced by these methods. 3D printing has attracted a lot of recent attention as it allows the fabrication of multiple complex parts using the same machine without the need for expensive retooling, and with less waste than traditional subtractive

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manufacturing methods. Extrusion 3D printing or direct ink writing (DIW) is a simple, flexible, and low-cost printing process where a paste or “ink” is extruded layerby-layer through a fine nozzle.6 Importantly, extrusionbased 3D printing can operate at mild or ambient temperatures and thus enables the printing of materials that cannot be directly melted or sintered. An important step in the development of 3D printing has been the move from structural materials, such as inert polymers and simple ceramics, to materials with functional properties, such as electronic conductivity, ferroelectricity, or photonic bandgaps.6-8 In this paper, we focus on another important functional property: the nano/microporosity (and by extension, accessible host-guest chemistry) exemplified by MOFs.

monoliths to the triethylamine-induced HKUST-1 gels (HKUST-1gel-TEA) made by Chaudhari et al.13, which also have ideal rheological properties for printing but negligible accessible porosity.

A key challenge of DIW is that the active material must be made into an “ink”, i.e. a slurry, with ideal rheological properties: it should flow continuously through a fine nozzle without clogging (shear thinning), and then solidify rapidly to retain its shape (controlled viscoelastic response8). Slurries are usually made by mixing fine particles of the active materials along with a solvent, dispersant and polymeric binders. The active materials are usually suspended in the solvent, before interparticle forces between the active materials are controlled by polymeric or polyelectrolyte dispersants to remove heterogeneities where weak agglomerates are broken down. Finally, a good binding of the materials to create a stable suspension can be achieved by using polymeric binders in the matrix. However, matrix materials can block access to the pores of MOFs.9 For example, Bible et al. combined acrylonitrile butadiene styrene (ABS) with MOFs by melt-blending, forming a filament for extrusion-based 3D printing.10 Their work demonstrated a decrease of more than 50% in N2 Brunauer–Emmett–Teller (BET) surface area after blending. Early work on the DIW of MOFcontaining inks involved the use of pre-made MOF with the addition of high proportions of additives (such as PVA polymer) to improve the flow characteristics of the ink and bulking agents (such as bentonite clay) to maintain the cohesion of the printed monolith. More recent work on the DIW of MOFs has used MOF mixed with photopolymers and removed the need for bulking agents11-12. The field is moving towards the 3D printing of completely additivefree active materials. This was recently reported for covalent-organic frameworks (COFs),9 and we demonstrate this with MOFs in the present study.

Synthesis of MOF Gel (HKUST-1gel): The synthesis process was modified from the literature14 to produce gels with a rheology suitable for extrusion printing. 1,3,5benzenetricarboxylic acid (H3BTC 0.103 mol) was dissolved by vigorous mixing in ethanol (2.7 mL). Copper(II) acetate (0.154 mol) was also dissolved in ethanol (10.0 mL). The H3BTC solution was then poured slowly into the copper(II) acetate/ethanol solution under vigorous stirring. The glass bottle containing the mixture was sealed and stirred overnight while held in a water bath at 65°C. The resulting mixture was centrifuged for 10 minutes at 10,000 rpm to remove excess solvent. The mixture was then washed one more time with 30 mL ethanol and centrifuged to obtain HKUST-1gel.

In this study, we have eliminated the need for any binders and use DIW to directly print colloidal gels of Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) (HKUST-1) nanoparticles (designated HKUST-1gel) into porous MOF monoliths. Inspired by the works of Chaudhari et al.13 and Qi et al.,14 we developed a synthesis route for a pure MOF gel that can be used as a DIW medium at room temperature, producing densely packed monoliths that maintain the important intrinsic properties (porosity, accessible surface area) of the MOF. We believe that this method should be generalizable to many other MOF types, given suitable control of particle size and surface chemistry. We compare the properties of our 3D printed

The easy shaping, porosity retention and dense loading of our MOF gel monoliths opens the door to many applications. HKUST-1 is well known as an exceptional sorbent for the low-pressure storage of natural gas (NG), a low-cost and relatively eco-friendly fuel source comprising primarily methane.15 We show below that our 3D printed MOF monoliths possess NG storage performance comparable or superior to most other shaped MOFs. EXPERIMENTAL SECTION

The aged sample of MOF gel was obtained by sealing the washed MOF gel in a 50 mL plastic centrifuge tube and leaving it to stand for one month in ambient conditions before 3D printing (3DP-HKUST-1gel aged). Synthesis of MOF Gel – Triethylamine (HKUST-1gel-TEA): The synthesis method was adapted and modified from the literature.13 H3BTC (0.02 mol) was dissolved by vigorous mixing in methanol (10.0 mL) with TEA (0.06 mol) used to deprotonate the ligand. Copper nitrate hexahydrate (0.03 mmol) was dissolved in methanol (10.0 mL). The copper nitrate solution was added to the H3BTC solution under vigorous stirring, and within minutes it showed a gel-like rheology suitable for 3D printing. 3D printing (3DP-HKUST-1gel and 3DP-HKUST-1gel-TEA): HKUST-1gel-TEA gel was loaded into a 10 mL syringe with a Luer-lock stainless-steel blunt-end tip (300 μm, ABLE Industrial Engineering, Singapore) for printing. The MOF gel was printed using a single extruder, 3-axis micropositioning stage (Allevi 2 Bioprinter, Allevi) in a pattern designed using computer-aided design (CAD) software (SolidWorks Corporation, Repetier Host, Slic3r). A compressed air system was used to pressurize the syringe barrel (~30 PSI) and control the gel flow rate. The typical print speed for the MOF gel was 20 mms-1.


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Figure 1. a) HKUST-1gel loaded into a syringe and different 3D printed structures; b) SEM image of a single filament from a dried 3DP-HKUST-1gel monolith; c) SEM image of HKUST-1 nanoparticles within the dried 3DP-HKUST-1gel monolith; (d, e) side view of high-profile 3D-printed monoliths: (d) square-shaped monolith with 10 layers; (e) circular pellet monolith with 10 layers; (f) top view and side view of a high-profile mesh-like monolith; (g-i) rheological properties of HKUST-1gel showing that the gel is solid-like at rest, and exhibits shear-thinning behavior with increasing shear: d) Apparent viscosity as a function of shear rate. e,f) Small amplitude oscillatory shear results. e) The storage modulus G’ and the loss modulus G” obtained against the angular frequency. f) G’ and G” recorded against the shear stress-amplitude at a constant frequency of 6.283 rad/s. A circular pellet of diameter 15 mm and thickness 2 mm was printed for further characterisation. The height per printed layer was fixed at 0.41 mm and the infill density of the solid pellet printed was set at 75%. The filaments printed were rasterised in a rectilinear fashion to print the circular pellets. After 3D printing, the structures were left in a desiccator (to reduce exposure to humidity, known to degrade HKUST-1) for 24 h to allow excess ethanolic solvent to evaporate. Monoliths with superior shape retention were obtained by controlling the evaporation rate of ethanol, whereby the 3DP-HKUST-1gel monoliths were placed in a commercial refrigerator (~4°C) overnight before moving it into a desiccator for further evaporation. Characterization: Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance diffractometer (Cu Kα radiation). The morphology of the

synthesized samples was studied using a field emission scanning electron microscope (ZEISS Supra 40 SEM, accelerating voltage 10.0 kV) and transmission electron microscope (TEM, JEOL-2100F). The surface area of the MOF was measured by nitrogen adsorption–desorption isotherms (Micromeritics ASAP 2020/3Flex Physisorption at 77 K). Before each measurement, the sample was activated and outgassed under vacuum at 80 °C for 12 h. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method, as implemented in the Micromeritics 3Flex software. For high pressure CH4 adsorption, a volumetric Quantachrome iSorbHP1 instrument was used. ~300 mg of 3DP-HKUST-1gel was activated and degassed overnight at 80 °C. Excess methane adsorption isotherms at room temperature were then obtained. Small-angle X-ray scattering (SAXS) measurements were performed on a Xenocs Xeuss 2.0 instrument at the Institute of Chemical and Engineering Sciences, A*STAR. The MOF gel was loaded into the solid sample holder with kapton tape windows. The sample was



irradiated with CuKα radiation at a sample-to-detector distance of 1210 mm, which provided a q-range of 0.01 – 0.21 Å-1. Data were collected for 20 min at ambient temperature. 2D SAXS data was reduced and background corrected using Xenoc’s data processing and reduction program, Foxtrot, Synchrotron Soleil and Xenocs. The data were fitted in Igor Pro 6.37 using the fitting procedures provided by NIST. 16,17 Gel rheology measurements were conducted in both shear viscometry and oscillatory modes using the Discovery Hybrid Rheometer (TA Instruments) at 25°C using a 40 mm dual Peltier plate. The apparent viscosity was measured as a function of shear rate using logarithmically ascending series. Small amplitude oscillatory shear measurements were performed to determine the storage (G') and loss (G") moduli as functions of angular frequency. G’ and G” as functions of shear stress amplitude were measured at a constant frequency of 6.283 rad/s. Step-strain measurements were measured as a function of time under oscillatory mode to determine G’ and G” against time with alternating 0.1% and 100% strain. An angular frequency of 0.1 rad/s was used.

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indicating gentle deformation does not cause microstructural disruption (i.e. the irreversible destruction of IM bonds) and is therefore reversible.20 G’ has a value approximately an order-of-magnitude higher than G” (Figure 1h, i), consistent with a gel exhibits elastic solid behavior; that is, the physical connections between the nanoparticles (e.g. van der Waals and hydrogen bonds) are strong enough to hold the entire gel network together to form a self-standing structure. However, when the applied shear exceeds the yield stress of the interactions in the gel (σy), the gel can then flow. σy is proportional to the total interaction energy between the nanoparticles, and it is roughly 1100 Pa for this system. The gel will immediately start retaining its shape once the imposed shear falls below σy. The applied pressures during the extrusionbased 3D printing are much larger than σy, allowing the printing of the gel through fine nozzles. In addition, the shape-retention ability of the MOF gel after extrusion was studied using a rheological step-strain measurement (Figure S2). The results indicate that the HKUST-1gel has self-healing capabilities as the gel is able to recover almost immediately after the strain is released, which is an important property of a colloid system for DIW.

Morphology and Structural Characterizations Recent studies have shown that certain MOFs can be formed into free-standing gels that can be easily shaped, and then dried to form very dense monoliths.18 In the present work, we initially discovered that certain known MOF gels (in particular HKUST-1gel-TEA13 and ZIF-819) had ideal characteristics as DIW inks (SI section 1), enabling us to 3D print freely shaped MOF monoliths. However, the synthesis of these gels required hazardous chemicals (dimethylformamide, DMF, and triethylamine, NEt3) that were retained in the monoliths. Other reports14, 18 showed that non-hazardous reagents could be used to prepare nanoparticles of HKUST-1. Inspired by these important findings, we developed a printable colloidal gel of HKUST-1 nanoparticles synthesized using only copper(II) acetate, H3BTC, and ethanol as the solvent (Figure 1a-c); the acetic acid produced during reaction is easily washed away. The printability of the gel is shown in Figure 1d, e and f. 10 layers of 0.41 mm height were printed in a stack. This demonstrates that the MOF gel is self-standing and is able to retain its integrity after the gel is deposited to a high profile. As seen in Figure 1d and e, bulk structures of square and circular monoliths were printed to 5 mm in height respectively. The MOF gel can be also printed into a high-profile mesh-like structure (Figure 1e). The gel exhibits shear thinning, a rheological property necessary for extrusion-based 3D printing. It has very high viscosity at low shear rates, but the viscosity drops rapidly as shear increases, allowing it to flow through a fine nozzle on the application of pressure (Figure 1g). HKUST-1gel shows the required viscoelasticity mentioned above, where the gel behaves as a rigid body at low stresses but flows as a viscous fluid above the yield stress. This is also supported by the oscillatory frequency sweep results (Figure 1h). The storage modulus G’ is angular frequency independent below the yield stress and strain rate,

Figure 2. a) XRD patterns of 3DP-HKUST-1gel-activated, 3DPHKUST-1gel-dried, HKUST-1gel, 3DP-HKUST-1gel-TEA and


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simulated HKUST-1.21 b) Thermogravimetric analysis (TGA) under simulated ambient conditions (SI section 5), showing desolvation followed by oxidation of 3DP-HKUST1gel to CuO. Scanning electron microscopy (SEM) images show that the dried 3DP-HKUST-1gel is densely packed (Figure 1b and c), suggesting that the printed structure is composed of nanoparticles of HKUST-1, around 20-50 nm in size. TEM images, however, are unclear in determining the size of the particles. (Figure S3a and 3b). Therefore, to further verify the size of the nanoparticles, small-angle X-ray scattering (SAXS) measurements were carried out on the assynthesized HKUST-1gel. The average particle size fitted by SAXS was ca. 19 nm (SI section 4), consistent with the SEM images. The primary nanoparticles seen by SEM in 3DP-HKUST1gel-TEA (Figure S5a and S5b) are less uniform and the sample appears to contain significant amounts of non-MOF material. PXRD show the broadest peaks in HKUST-1gel-TEA (Figure 2a), but still align with the simulated HKUST-1 peaks due to the crystallites (Figure S5c) that are formed in the gelation process. On the other hand, the PXRD results for 3DP-HKUST-1gel-activated, 3DP-HKUST-1gel-dried, HKUST-1gel show strong but broad peaks for all three systems, consistent with crystalline nanoparticles. Significant Scherrer broadening is observed, and this may be due to the small particle size. The significant background at lower angles is consistent with a smallangle scattering contribution from nanoparticles or aggregates. To determine the composition of the MOF gels, we performed TGA under N2/O2 (3:2) (Figure 2b). Following smooth weight loss plateauing at 100-300 °C (consistent with the desolvation of a near-pure MOF), we see the full decomposition of the MOF to CuO above ~320 °C. Backcalculating, we see a value within 2% of the expected mass of Cu3(BTC)2 by Cu stoichiometry (full calculations in SI section 6). Together with our gas-sorption results discussed below, this provides strong evidence that there is a very high proportion of MOF within HKUST-1gel. Gas Adsorption of 3D-Printed MOF Monoliths The BET surface areas obtained from the nitrogen adsorption isotherms are summarized in Table 1. Importantly, two similar systems (extrusion-based) are studied in comparison to this work. Bible and co-workers10 developed a 3DP ABS-HKUST-1 (ABS = acrylonitrile butadiene styrene) system, whereby a composite was made into a filament and extruded through a fuseddeposition modelling 3D printing system. However, a large component of the matrix was made up of the polymer ABS. The printed monolith exhibits 18% of the BET surface area of the pure powder (329 m2/g). Küsgens et al., extruded HKUST-1 with Silres MSE 100 (CH3Si(O)1.1(OCH3)0.8 as a binder and methyl hydroxyl propyl cellulose MHPC 2000P as a plasticizer.22 The BET surface area of the product was 484 m2/g, i.e. 26% that of the powder. Table 1 also consolidates results from the pelletization and

compression of powders for applications. Peterson et al. used powdered CuBTC pressed at 1000 and 10,000 psi each at 1 min to demonstrate that pelletization does not affect the crystal structure of HKUST-1, but pelletization results in a decrease in BET surface area due to localized pore structure collapse.23 Kim and co-workers also demonstrated the reduction of water sorption capacity of HKUST-1 after pelletization, indicating the reduction in porosity during pelletization. They also prepared a compressed sample with PVA binder, which also displayed a reduction in porosity and water sorption capacity.24 It is then further concluded that by shaping the MOFs by compression or pelletization, the addition of additives and binders decreases the overall surface areas of the particles due to partial blockage of the pores and inherent collapsing of the pores during shaping processes. For successful large-scale applications, MOFs must be able to retain their desirable properties (high porosity and surface area) after processing or shaping. Here we report the retention of high porosity by 3DP-HKUST-1gel after shaping. Initially, HKUST-1gel was used directly for DIW after centrifugation, without further washing. The printed samples exhibited a BET surface area of 730.2 m2/g, already high compared to other shaped MOFs in the literature (Table 1). Subsequently, the MOF gels were dispersed once in ethanol and re-centrifuged, giving an exceptional BET surface area of 1134 m2/g (Figure 3a). This is attributed to the removal of unreacted precursors and acetic acid that is generated during the reaction. The N2 sorption experiments were done on activated 3DPHKUST-1gel monoliths (80°C, 12h) as mentioned in the experimental section. The monoliths turned dark purple, characteristic of fully activated HKUST-1 (Figure 3b).25 Also, 3DP-HKUST-1gel is able to retain its structural integrity after activation. Thus, our 3D printing approach allows the extrusion of MOF into complex shapes and structures while retaining its mechanical integrity and high porosity, which is desirable for gas storage and other applications which require a combination of pore accessibility and ease of bulk handling. 3DP-HKUST-1gel-TEA was also tested with porosimetry, given that its rheological behavior and XRD results were similar to HKUST-1gel. However, the surface area of 3DP-HKUST-1gel-TEA was only 0.7 m2/g, showing no microporosity. This indicates that the pores of HKUST-1 in HKUST-1gel-TEA are inaccessible (Figure 3a) and so even though it can be shaped ideally, HKUST-1gel-TEA is unusable for gas storage and other hostguest applications. The stability of HKUST-1gel as a 3D printing ink was also studied the manufacture, transport and storage of inks on a large scale will be an important step in the development of MOF 3D printing. The integrity of the gel can be easily maintained by keeping the gel in a sealed container to prevent ethanol evaporation. Assynthesized gel was aged for one month in a sealed plastic tube, then washed and centrifuged before being used for DIW. The gel showed no apparent changes in printing quality compared to freshly prepared gels. The BET surface area of the 3DP monoliths



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Table 1. BET surface areas and porosity data of Cu3(BTC)2 / HKUST-1 shaped by extrusion and compression. Sample

BET surface area (m2/g)

Description of Sample

Ref.

HKUST-1 powder

1850

Loose powder has the highest surface area

Peng et al.26

3DP-HKUST-1gel (Washed)

1134

This work

3DP-HKUST-1gel (Unwashed)

730

This work

3DP-HKUST-1gel (Aged Sample)

1132

This work

3DP-HKUST-1gel-TEA

0.73

This work

CuBTC-P1000 (68 atm)

1045

CuBTC-P10000 (680 atm)

892

Peterson et al.23

ABS-HKUST-1

329

Bible et al.10

Extruded HKUST-1 with MCHP and Silres MSE 100

484

Küsgens et al. 22

HKUST-1 pellet (PVA binder)

963

Kim et al.24

HKUST-1 pellet (50 atm):

1007


953

Kim et al.24


529

Kim et al.24


71

Kim et al.24

HKUST-1 pellet: 0.05g powder/5kN compression force

453

Bazer-Bachi et al. 27

Easily broken apart by mortar and pestle to form loose powder

Lowest mechanical strength and durability

made using the aged gel was also similar to the asprepared gel/ink (1132 m2/g, Figure 3a) and the nitrogen adsorption isotherms are almost indistinguishable. Therefore, our preparation of HKUST-1gel opens a promising direction for shaping MOFs with relatively high crystallinity and active surface area for large scale manufacturing, as HKUST-1gel without binders or additives can be prepared in large amounts and stored without material degradation until required. Methane Adsorption of 3D-Printed MOF Monoliths Methane storage performance of the 3DP-MOF monoliths was also tested using high pressure methane adsorption up to 90 bar at room temperature. The absolute volumetric uptake isotherm of the 3DP-MOF monolith was compared with that of powdered HKUST-1 (data provided by Peng et al.26 as a benchmark comparison), as shown in Figure 4a.

Peterson et al.23

Kim et al.24

The absolute uptake values, Nabs, were calculated from the experimentally obtained excess uptake values, Nexc, using the following equation: Nabs = Nexc + ρVpore where ρ is the density of methane in the gas phase at corresponding pressures and 298 K, and Vpore is the total volume of pores in the adsorbent.28 Note that the calculation was done based on the assumption that the volume of the adsorbed phase is equal to the total pore volume, which remains constant over the complete pressure range. Although there is an increase in density from 0.43 g/cc (tapped density) for the powder sample to 0.8 g/cc for the monolith (minimum density, estimated by calculation of volume from the external dimensions of a densely-packed round 3DP monolith), there is a substantial loss of c.a. 22 % reduction in total pore volume (0.78 cc/g to 0.61 cc/g).


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Figure 3. a) N2 isotherms of the washed, unwashed, and aged samples of 3DP-HKUST-1gel, in comparison with the 3DPHKUST-1gel-TEA b) Photos of 3DP-HKUST-1gel at different stages of processing. monoliths, demonstrating the promise of incorporating functional properties into 3D printed monoliths. The absolute uptake capacities of 131 cm3 (STP)/cm3 and 271 cm3 (STP)/cm3 were obtained for 3DP-HKUST-1gel and HKUST-1 powder by Peng et al., respectively, at 65 bar saturation pressure. In terms of the volumetric deliverable capacity, which is the difference in absolute uptake at 65 bar adsorption pressure and 5.8 bar desorption pressure, a value of 64 cm3 (STP)/cm3 was obtained, which is lower than the value of 185 cm3 (STP)/cm3 obtained by Peng et al.,26 assuming that the packing efficiency loss is ignored. The corresponding gravimetric methane uptake and deliverable capacity is shown in Figure 4b. Hypothetically for large scale use, HKUST-1gel could be 3D-printed into any shape, including close-packed solids such as cubes, pellets, or other suitable shaped solids. This can be done by designing a suitable 3D model using a CAD program, before using another slicing program to 3D print the shaped MOF. This will ultimately give rise to negligible packing losses.

Even though the values obtained are lower than the US DOE target and ARPA-E targets, generally, the results show that the 3DP monoliths can provide a promising avenue for shaping MOFs in applications. The upturn and hysteresis in the isotherms of the 3DP-HKUST-1gel sample is consistent with capillary condensation in mesopores, suggesting that colloidal MOF gels may be a way to generate monoliths with hierarchical micro- and mesoporosity. This is the first study of the natural gas storage performance of 3DP MOF monoliths, demonstrating the promise of incorporating functional properties into 3D printed monoliths.

A micro-indentation test for the structural hardness of the dried and activated 3DP-HKUST-1gel monolith was also carried out (full calculation in SI Section 8). The hardness obtained was 17.5 kPa. Although the hardness is relatively small, the mechanical strength of the 3DP-HKUST-1gel monolith is reasonable and robust enough for applications. After solvent evaporation and activation, the monolith will only be held by the interparticle forces (van der Waals, hydrogen bonds) between nanoparticles while packing closely to each other without compression. Therefore, the overall structure can maintain its integrity with a certain level of mechanical strength for gas storage applications.



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This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. ǂThese authors contributed equally to this work. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT J. Ding acknowledges the financial support from NUS Strategic Research Fund R-261-509-001-646 and R-261-509-001-733, Singapore Maritime Institute and the Advanced Materials & Manufacturing R&D Program SMI-2016-OF-04 (R261502032592) and NRF-CRP16-2015-01 (R-284-000-159281). J. Wang and Y. Wu acknowledge the financial support from Singapore MOE Tier 2 (MOE2016-T2-2-138), conducted at the National University of Singapore (NUS). C. K. Liu thanks Liangfeng Guo for technical assistance (ICES, A*STAR).

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. Figure 4. Absolute methane uptake isotherms for 3DPHKUST-1gel and powdered HKUST-1; a) absolute volumetric methane uptake and b) absolute gravimetric methane uptake. We have discovered that certain types of binder-free MOF nanoparticle dispersions form colloidal gels that have ideal rheological characteristics as a medium for extrusion 3D printing, finally enabling the creation of freely-shaped MOF monoliths with extremely high MOF loadings. The existence of colloidal gels of a number of archetypal MOFs such as UiO-66 and ZIF-8 suggests that this method may be broadly applied, giving access to diverse chemical and physical functionalities. These monoliths retain an excellent level of porosity and crystallinity, making them ideal for applications in areas such as such as energy storage and conversion, gas storage, separations, and heterogeneous catalysis. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Rheological properties of MOF-gel-TEA, TEM analysis, Small angle x-ray scattering (SAXS) of MOF gel, SEM of MOF-gel-TEA and TGA analysis.

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(18) Tian, T.; Zeng, Z.; Vulpe, D.; Casco, M. E.; Divitini, G.; Midgley, P. A.; Silvestre-Albero, J.; Tan, J.-C.; Moghadam, P. Z.; Fairen-Jimenez, D. A sol–gel monolithic metal–organic framework with enhanced methane uptake. Nature materials 2018, 17, 174. (19) Chaudhari, A. K.; Tan, J.-C. A mechano-responsive supramolecular metal–organic framework (supraMOF) gel material rich in ZIF-8 nanoplates. Chemical Communications 2017, 53, 8502-8505. (20) Lacey, S. D.; Kirsch, D. J.; Li, Y.; Morgenstern, J. T.; Zarket, B. C.; Yao, Y.; Dai, J.; Garcia, L. Q.; Liu, B.; Gao, T. Extrusion-Based 3D Printing of Hierarchically Porous



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