The development of a metal–organic framework/α-alumina composite leads to a novel concept: efficient adsorption occurs within a plurality of radial micro-channels with no loss of the active adsorbents during the process. This composite can effectively remediate arsenic contaminated water producing potable water recovery, whereas the conventional fixed bed requires eight times the amount of active adsorbents to achieve a similar performance

Chen, JP

Lee, M

Li, K

Liu, X

Wang, B

Wang, CH

Chemical Communications

English

Spiral - Imperial College Digital Repository

This journal is©The Royal Society of Chemistry 2016 Chem. Commun., 2016, 52, 8869--8872 | 8869Cite this:Chem. Commun., 2016,52, 8869A metal–organic framework/a-alumina compositewith a novel geometry for enhanced adsorptiveseparation†Chenghong Wang,abc Melanie Lee,a Xinlei Liu,a Bo Wang,a J. Paul Chenbc andKang Li*aThe development of a metal–organic framework/a-alumina compositeleads to a novel concept: efficient adsorption occurs within a pluralityof radial micro-channels with no loss of the active adsorbents duringthe process. This composite can effectively remediate arsenic con-taminated water producing potable water recovery, whereas theconventional fixed bed requires eight times the amount of activeadsorbents to achieve a similar performance.Adsorptive separation is a process widely applied in most indus-trial sectors to achieve purification of liquid or gas mixtures.1In particular, it plays a significant role for water quality control,through the removal of impurities from wastewater streams, owingto its simplicity, efficiency, flexibility in design and low wasteproduction.1–3 Currently, in order to introduce an effective adsorp-tion process for wastewater remediation, various fixed or fluidizedbeds are employed in industry.4 These setups allow fluid streamsto contact porous adsorbent media and induce proper adsorptionprocesses along the way. Despite that, certain disadvantages ofusing adsorption beds are inevitable.4 For instance, the columnmay cause a large pressure drop because of too dense packing;also, channelling of fluid stream may occur, leading to nonidealflow in the adsorption media. In fluidized bed configuration, theproblem of channelling can be suppressed, but it is likely to resultin the attrition and break-up of adsorbent pellets; and theseunfavourable residuals may escape from the bed reactor, requiringadditional post-treatment towards the effluents. Hence, novelconcepts and designs on how to achieve more effective adsorptionare imperative.1Following the above remarks, the development of adsorptionprocesses cannot be considered separately from the developmentof adsorbent materials. Metal–organic frameworks (MOFs), a newtype of porous materials constructed by joining metal-containingunits with organic linkers through coordination bonds, hasattracted substantial attention in the scientific communities.5–7Owing to their unique properties like exceptionally high porosity,large surface area and customizable chemical functionality,MOF materials have exhibited a great potential in adsorptionapplications.8–10One of the representative examples is UiO-66,11–15 a prototypicalZr-MOF constructed with Zr6O4(OH)4 clusters and terephthalatelinkers (1,4-benzenedicarboxylate, BDC).11 Our previous studyunveiled that UiO-66 can effectively remove arsenic from waterwith the highest capacity to date (303 mg g1) and the widestpH working range (pH 1–10).16 Besides the outstanding thermo-dynamic performance, the UiO-66 adsorbent also exhibits arapid uptake profile for arsenic adsorption. Compared to othertypical arsenic adsorbents (Zr nanoparticle sorbent, Y–Mn binarycomposite, and Fe-exchanged zeolite) with particle sizes of severalhundred nanometres,17–19 as shown in Fig. 1, the UiO-66 adsorbentwith a similar particle size could provide a much faster adsorptionFig. 1 Arsenic adsorption kinetics comparison: UiO-66 and other typicalsorbents with the same order-of-magnitude particle size.17–19a Department of Chemical Engineering, Imperial College London, SW7 2AZ, UK.E-mail: kang.li@imperial.ac.ukb NUS Graduate School for Integrative Sciences & Engineering (NGS),National University of Singapore, 28 Medical Drive, 117456, Singaporec Department of Civil and Environmental Engineering,National University of Singapore, 10 Kent Ridge, 117576, Singapore† Electronic supplementary information (ESI) available: Experimental and char-acterisation details. See DOI: 10.1039/c6cc02317gReceived 16th March 2016,Accepted 20th June 2016DOI: 10.1039/c6cc02317gwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 20 June 2016. Downloaded on 11/08/2016 10:15:42.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issue8870 | Chem. Commun., 2016, 52, 8869--8872 This journal is©The Royal Society of Chemistry 2016rate (at least fivefold faster, in terms of the initial reaction rateconstant). This fast kinetics allows an efficient adsorptive separa-tion process to be realized within a micro-space (i.e. very shortdistance).Despite all these preferable performances of UiO-66, in orderto be industrially applicable, the powder materials need to bespecifically shaped or supported.20,21 This is because in practicalapplications, dispersed particles could easily leak through theapplication compartment, resulting in the challenging spent-particles-separation issues and severe safety concerns.22Herein, we developed a MOF/a-alumina composite (composite-1)to take advantage of both the fast kinetics of UiO-66 adsorbentsand the novel hollow-fibre geometry of a-alumina for enhancedadsorptive separation, as shown in Fig. 2. The a-alumina wasdeliberately structured to work as a ceramic hollow fibre providinga plurality of micro-channels for efficient adsorptive separation aswell as a thin barrier layer to prevent any loss of active adsorbents.In comparison with traditional packed column beds, composite-1delivered a more effective adsorption process and consequently abetter water decontamination performance.Generally, a-alumina as a raw material is in abundant supplyand can provide great resistance to various chemical and thermalconditions. Through controllable spinning and sintering,23,24we specifically prepared a-alumina in a hollow fibre structure.Its novel geometry with two distinct layers is shown in Fig. 3: onevery thin barrier layer (approximately 20 mm of thickness, andwith an average pore size around 450 nm as shown in Fig. S1,ESI†) containing a 3D-pore network structure at the lumen, andalso one unique layer containing a plurality of conical micro-channels (approximately 500 mm in length, 25 mm in the openingdiameter) towards the shell side. These micro-channels not onlyreduce the mass transfer resistance, giving rise to competitivepermeation fluxes,23,25 but also form pockets within which activeadsorbents can be readily deposited. As the density of formedmicro-channels is quite high, the a-alumina hollow fibre offers aconsiderable amount of geometric surface area and accessiblevolume.25UiO-66 used in this study was synthesized via the typical solvo-thermal method with a minor modification.11,26,27 Its characteristicXRD pattern (see Fig. S2, ESI†) and N2 adsorption–desorptionisotherms (see Fig. S3, ESI†) confirm the crystal structure andporosity of this Zr-MOF, respectively.11 As shown in Fig. 3e and f,the UiO-66 crystals were octahedrally shaped with a particle sizeof around 600 nm.The formation of composite-1 was achieved via a facilevacuum filtration method (see the ESI†). As shown in Fig. S4 inthe ESI,† the vacuum condition was introduced into the lumen ofa-alumina hollow fibre, and the water solution carrying MOFsflowed at the shell side with the aid of stirring. Because of theestablished pressure difference, the water solution would firstaccess and fill the cavities of micro-channels; and then purewater permeated through the thin barrier layer entering the fibrelumen, whereas the MOF crystals could not escape through thebarrier layer and had to securely stay within the conical micro-channels owing to the size-exclusion effect (Fig. 2 and 3f). BothFig. 2 Schematic diagram of adsorptive separation by composite-1: forarsenic contaminated water remediation. The inset demonstrates anenlarged cross-sectional view of composite-1. Blue molecule: water;green molecule: arsenic pollutant.Fig. 3 SEM and TEM images: (a) cross-section of a-alumina hollow fibre;the yellow dashed circle signifies two distinct layers. (b) Enlarged cross-sectional view showing open micro-channels; yellow lines highlight threeexamples of micro-channels. (c) Outer surface morphology of a-aluminahollow fibre, showing the opening of micro-channels at the shell side.(d) Inner surface of a-alumina hollow fibre. (e) UiO-66 crystals; the insetwith yellow dashed line border shows the corresponding TEM image.(f) UiO-66 crystals deposited within micro-channels; micro-channel wallsare formed by the packing of alumina particles; yellow shades indicate thedeposited octahedral UiO-66 crystals.Communication ChemCommOpen Access Article. Published on 20 June 2016. Downloaded on 11/08/2016 10:15:42.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2016 Chem. Commun., 2016, 52, 8869--8872 | 8871FTIR and TGA analyses towards the formed composite suggestedthat there was only physical attachment between the incorpo-rated MOF crystals and a-alumina matrix (Fig. S9 and S10, ESI†).Moreover, the critical parameters including the particle size ofMOFs, MOF concentration in the water solution, magnetic stirrerspeed for dispersing MOF crystals in the water solution, as well asduration of the vacuum filtration process were comprehensivelyinvestigated (listed in Table S1, ESI†), giving rise to an optimizedcomposite-1 (0.68 mg MOF per gram composite) for the arseniccontaminated water remediation.Since the arsenic concentration in most contaminated waste-water ranges from 0.1 to 1 ppm,28 the essential arsenic concen-tration of feed streams was set as 1 ppm. Both the developedcomposite-1 and equivalent packed columns were investigated interms of their respective kinetic breakthrough performance forwater decontamination.In the case of composite-1, the arsenic contaminated feedstream was introduced into the composite from the shell side tothe lumen. In such a manner, the barrier layer at the lumen sidecould well prevent any active MOF crystals from escaping thecomposite; and no UiO-66 was detected for the liquid outflow (seethe ESI†). When the wastewater stream was transported throughthe passages of micro-channels, it would contact with the UiO-66adsorbents; owing to the adsorbents’ fast uptake kinetics, selec-tive adsorption of arsenic from water occurred efficiently withinthis micro-space. With the optimized operating parameters (listedin Table S2, ESI†), composite-1 was capable of providing cleaneffluent recovery for 60 minutes, as shown in Fig. 4a. Water fromthis clean recovery meets the drinkable standards set by bothWHO and US-EPA (less than 10 ppb),28 and can be used as potablewater supply without any additional post-treatment. Also, theadsorption uptake with regard to the active adsorbent is calculatedas B23.4 mg g1 up to breakthrough. These results specified thatthe probability of feed stream leaking through the micro-channelwalls was negligible, as the mass transfer resistance of the wallsis orders-of-magnitude larger than that of the micro-channelpassages. Furthermore, to obtain an idea of the capability ofcomposite-1, we further increased the arsenic concentration offeed streams to 10 ppm and 20 ppm under the same operatingconditions. A much shorter breakthrough time (B15 min) wasnoticed for the 10 ppm case, while no clean recovery can becollected if the contaminated feed is as concentrated as 20 ppm.On the other hand, a comparative study under identicalexperimental conditions was carried out using packed columnbeds (see the ESI†). With an equal amount of UiO-66 adsorbentspacked into the column (same as the amount loaded in composite-1),the adsorption efficiency was found to be unsatisfactory as shownin Fig. 4b, while the cleanest recovery contained 40 ppb of arsenic.Even if we doubled the amount of UiO-66 in the column media,none of the clean recovery could be collected. On further increas-ing the quantity of active adsorbents in the column, the columnsetup started to provide clean recovery; and when 8 times theamount of UiO-66 adsorbent was employed, it provided a similarperformance to that of composite-1.The inferior performance of the packed column beds is asso-ciated with the inherent drawbacks like inefficient packing andnon-ideal flow.4 Owing to the fact that the packing of adsorbentsinside a column is rather random, contaminated streams tend tobypass the adsorbents and directly join the effluent. This phenom-enon becomes even more serious when limited amount of adsor-bents are used.29 In contrast, composite-1 offers an alternativeconcept of introducing efficient adsorption within micro-channels.The micro-channels with the conical shape not only ensure a betterdistribution and more effective control of adsorbent materials, butalso form a transport network such that the mass transfer resistanceis greatly reduced. Hence, in comparison with the packed columnbed, a more ideal flow can be achieved in composite-1, as well as amore efficient contact between contaminants and active adsorbents.As a result, composite-1 provides a much more effective adsorptiveseparation process using a much smaller amount of active adsor-bents, which would lead to considerable cost saving.Fig. 4 Breakthrough studies: (a) using composite-1 for arsenic waterdecontamination (1 ppm, 10 ppm and 20 ppm as the arsenate concen-tration in the feed solution were investigated); (b) using equivalent packedcolumns for arsenic water decontamination (1 ppm as the arsenateconcentration in the feed solution was used for comparison). With refer-ence to the quantity of MOFs loaded in composite-1, the columns werepacked with: equal (1), twice (2), five times (5) and eight times (8) theamount of MOFs, respectively. The data in (b) are reported as the averageof duplicate experiments.ChemComm CommunicationOpen Access Article. Published on 20 June 2016. Downloaded on 11/08/2016 10:15:42.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online8872 | Chem. Commun., 2016, 52, 8869--8872 This journal is©The Royal Society of Chemistry 2016Nevertheless, the feasibility of composite-1 in repeated usewas restricted, as the arsenic adsorption by UiO-66 was found tobe an irreversible sorption process (see the ESI†). Desorption ofarsenic from the spent UiO-66 adsorbents was not pragmatic.A novel strategy for developing regenerable MOF adsorbents forarsenic adsorption is still under investigation. Future studies canincorporate these regenerable MOF adsorbents to form a morepowerful composite. However, composite-1 in this study resolvesa critical industrial problem: generally, for adsorbents to be usedin the industry, they must be formed into specific shapes orpellets by combining with a quantity of binder material,30 whilethe activities, functionalities and effectiveness of the adsorbentbecome reduced at the end; the incorporation of the activeadsorbent into the advanced a-alumina matrix provides a creativeapproach to use the adsorbent directly and more effectively.Besides, in practical water remediation applications, the barrierlayer of a-alumina hollow fibre could further serve as a separationmedium to reject suspended solids and micro-organisms.31Moreover, it is feasible to assemble such functional compositesas modules in different scales,23,32 ranging from a small portablewater purification unit for household use to a large hybridadsorption–filtration system in water decontamination plants.To conclude, a MOF/a-alumina composite with a novelgeometry was developed and effectively applied for water decon-tamination. The composite was formed by depositing UiO-66adsorbents into the unique micro-channels of a a-alumina hollowfibre through a facile vacuum filtration method. When it wasapplied for arsenic contaminated water remediation, composite-1produced the potable water recovery. To achieve a similar perfor-mance, the packed column bed required eight times the amountof active UiO-66 adsorbents. Therefore, as a proof of concept,composite-1 has exhibited a promising potential to be appliedfor industrial water decontamination. Looking forward, basedon the specific adsorptive separation applications, various func-tional composites could be formed. A wide range of adsorbentscan be selected and loaded into the a-alumina hollow fibre, ofwhich the micro-channel sizes can be well controlled duringfabrication.This work was supported by the research funding providedby Engineering and Physical Sciences Research Council in theUnited Kingdom (Grant no. EPSRC, EP/J014974/1). C. Wanggratefully acknowledges the National University of SingaporeGraduate School for Integrative Sciences and Engineeringscholarship.Notes and references1 I. Ali, Chem. Rev., 2012, 112, 5073.2 Y. Yu, C. Wang, X. Guo and J. Paul Chen, J. Colloid Interface Sci.,2015, 441, 113.3 S.-W. Nam, C. Jung, H. Li, M. Yu, J. R. V. Flora, L. K. Boateng, N. Her,K.-D. Zoh and Y. Yoon, Chemosphere, 2015, 136, 20.4 A. Dąbrowski, Adv. Colloid Interface Sci., 2001, 93, 135.5 H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science,2013, 341, 1230444.6 H. C. Zhou and S. Kitagawa, Chem. Soc. Rev., 2014, 43, 5415.7 Y.-S. Li, H. Bux, A. Feldhoff, G.-L. Li, W.-S. Yang and J. Caro,Adv. Mater., 2010, 22, 3322.8 N. A. Khan, Z. Hasan and S. H. Jhung, J. Hazard. Mater., 2013,244–245, 444.9 Y.-Y. Fu, C.-X. Yang and X.-P. Yan, Chem. Commun., 2013, 49, 7162.10 H. Furukawa, F. Gandara, Y. B. Zhang, J. Jiang, W. L. Queen,M. R. Hudson and O. M. Yaghi, J. Am. Chem. Soc., 2014, 136, 4369.11 J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti,S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850.12 K. K. Yee, N. Reimer, J. Liu, S. Y. Cheng, S. M. Yiu, J. Weber, N. Stockand Z. Xu, J. Am. Chem. Soc., 2013, 135, 7795.13 H. B. Shang, C. X. Yang and X. P. Yan, J. Chromatogr. A, 2014,1357, 165.14 Z. Hu and D. Zhao, Dalton Trans., 2015, 44, 19018.15 X. Liu, N. K. Demir, Z. Wu and K. Li, J. Am. Chem. Soc., 2015,137, 6999.16 C. Wang, X. Liu, J. P. Chen and K. Li, Sci. Rep., 2015, 5, 16613.17 Y. Yu, L. Yu and J. P. Chen, Ind. Eng. Chem. Res., 2015, 54, 3000.18 Y. Ma, Y.-M. Zheng and J. P. Chen, J. Colloid Interface Sci., 2011, 354, 785.19 Z. Li, J.-S. Jean, W.-T. Jiang, P.-H. Chang, C.-J. Chen and L. Liao,J. Hazard. Mater., 2011, 187, 318.20 D. Wisser, F. M. Wisser, S. Raschke, N. Klein, M. Leistner, J. Grothe,E. Brunner and S. Kaskel, Angew. Chem., Int. Ed., 2015, 54, 12588.21 S. J. Tesh and T. B. Scott, Adv. Mater., 2014, 26, 6056.22 J. He, T. S. Siah and J. Paul Chen, Water Res., 2014, 56, 88.23 M. Lee, Z. Wu, R. Wang and K. Li, J. Membr. Sci., 2014, 461, 39.24 M. Lee, B. Wang and K. Li, J. Membr. Sci., 2016, 503, 48.25 M. Lee, B. Wang, Z. Wu and K. Li, J. Membr. Sci., 2015, 483, 1.26 A. Schaate, P. Roy, A. Godt, J. Lippke, F. Waltz, M. Wiebcke andP. Behrens, Chem. – Eur. J., 2011, 17, 6643.27 G. Lu, C. Cui, W. Zhang, Y. Liu and F. Huo, Chem. – Asian J., 2013, 8, 69.28 D. Mohan and C. U. Pittman, Jr., J. Hazard. Mater., 2007, 142, 1.29 S. Kundu, Water Res., 2004, 38, 3780.30 C. A. Jeffs, M. W. Smith, C. A. Stone, C. G. Bezzu, K. J. Msayib, N. B.McKeown and S. P. Perera, Microporous Mesoporous Mater., 2013,170, 105.31 J. He, T. Matsuura and J. P. Chen, J. Membr. Sci., 2014, 452, 433.32 K. Li, Ceramic membranes for separation and reaction, Wiley, 2007.Communication ChemCommOpen Access Article. Published on 20 June 2016. Downloaded on 11/08/2016 10:15:42.  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A metal–organic framework/α-alumina composite with a novel geometry for enhanced adsorptive separation

10.1039/c6cc02317gChemical Communications52578869-887

Wang, C

Paul Chen, J

ScholarBank@NUS

A metal-organic framework/?-alumina composite with a novel geometry for enhanced adsorptive separation

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A metal–organic framework/α-alumina composite with a novel geometry for enhanced adsorptive separation

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