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ynthesis of isostructural metal–organic frameworks, CPO-27s, with ultrasound,icrowave, and conventional heating: Effect of synthesis methods and metal ions
namul Haque, Sung Hwa Jhung ∗
epartment of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea
r t i c l e i n f o
rticle history:eceived 3 May 2011eceived in revised form 27 July 2011ccepted 15 August 2011
eywords:
a b s t r a c t
Isostructural MOFs such as CPO-27-Zn, CPO-27-Co and CPO-27-Ni were synthesized with US (ultrasound),MW (microwave) and CE (conventional electric) heating from the very similar reactant mixtures to under-stand the effect of the synthesis methods and metal ions on the synthesis rates and on the physicochemicalproperties of the obtained MOFs. The porosities (surface area and micropore volume) and synthesis ratesboth for nucleation and crystal growth are on the order of US > MW > CE synthesis. The crystal sizes are
PO-27OFsltrasoundicrowave
inetics
on the order of US < MW <CE and this may be the reason of the low porosity of the CPO-27-Co obtainedby conventional electric heating. The synthesis kinetics of the three CPO-27s shows the importance ofthe lability of the metal ions in the synthesis rates (both in nucleation and crystal growth) of the MOFs.Therefore, it can be suggested that labile metal ions, compared with inert ions, may be treated mildly (alow temperature or a short reaction time) for the synthesis of isostructural MOFs with the same ligand
ability material.
. Introduction
Recently, remarkable progresses on porous materials have beenchieved because of developments in metal organic frameworkMOF) materials, crystalline inorganic-organic porous materials [1].he importance of MOF-type materials is due to the huge poros-ty and the easy tunability of their pore size and shape from a
icroporous to mesoporous scale [1]. Moreover, MOFs have lotsf potential applications including gas adsorption/storage [2], sep-ration [3], catalysis [4], adsorption of organic molecules [5], drugelivery [6], luminescence [7], electrode materials [8], carriers foranomaterials [9], magnetism [10], polymerization [11], imaging12] and removal of hazardous materials [13].
The majority of the researches on MOFs, so far, has been devotedainly to the syntheses, the structure analysis, and the potential
pplications for various fields [1]. Facile synthesis of MOFs is verymportant not only for fundamental understanding but also foriable applications in industry. MOFs have been mainly synthesizedith hydrothermal or solvothermal crystallization at relativelyigh temperatures using conventional electrical (CE) heating [1].
few new techniques have been tried in the synthesis of MOFs toecrease the reaction time or temperature and to find an alterna-
ive effective way. For example, ultrasound (US) has been applied tohe syntheses of Cu3(BTC)2 (Cu-BTC or HKUST-1) [14], Fe-BDC [15],n-BTCs [16], Zn3(BTC)2·12H2O [17], [Zn(BDC)(H2O)]n [18], MOF-5
[19]. BTC and BDC stand for 1,3,5-benzenetricarboxylate and 1,4-benezenedicarboxylate or terephthalate, respectively. Microwave(MW) has been used for the synthesis of porous materials becauseit has several advantages such as fast crystallization [20], phase-selectivity [21], diverse morphology/size [22], facile evaluation ofreaction parameters [23], and so on. This method has also been usedin the syntheses of MOFs, and similar favorable effects have beenobserved [24–26].
Moreover, detailed understanding on the synthesis includingkinetics and thermodynamics is very important for efficient andfacile syntheses. Recently, a few studies on the synthesis of MOFshave been emerging to understand the synthesis in more detail.For example, Forster et al. carried out the synthesis of hybrid mate-rials in wide reaction conditions [27] that lead to the conclusionthat thermodynamic stability, compared with kinetics, is impor-tant in the synthesis [27]. Cheetham et al. [28] and Mahata et al. [29]have also suggested that thermodynamics is more important thankinetics in the synthesis. However, Gándara and co-workers havefound that metastable lanthanide MOFs can be obtained due to thekinetic-controlled synthesis of MOFs [30]. Recently Cheetham et al.have shown the importance of thermodynamics and kinetics in thesyntheses of metal-tartrates and cyclohexane-dicarboxylates [31].Moreover, crystallization of MOFs has been studied both in situ andex-situ in various aspects [32].
Isostructural MOFs comprised of different metal ions and the
same ligands are interesting because the MOFs can be efficientlycompared to understand the effect of the metal ions on adsorption,synthesis, stability, and so on. CPO-27s, M2(DHTP)(H2O)2·8H2O(M is Co [33], Zn [34], Ni [35], Mn [36] or Mg [37]; DHTP: 2,5-
ihydroxyterephthalate), are typical isostructural MOFs havingigh concentrations of coordinatively unsaturated metal sites andD pore structure (pore diameter of about 1.1–1.2 nm). The CPO-7s have been deeply studied for the adsorption of gases such asydrogen [36,38,39], methane [40], and CO2 [41]. Interestingly, thedsorption enthalpy for hydrogen over CPO-27s is one of the high-st (13.4 kJ/mol) among the MOFs [38].
In this work, the reaction rates were quantitatively analyzed inhe synthesis of isostructural CPO-27s since the detailed synthesisf isostructural MOFs in wide variety of reaction conditions mayncrease the understanding on the synthesis of MOFs. The studyas been focused on the syntheses of isostructural MOFs (CPO-27-o, CPO-27-Ni and CPO-27-Zn) in order to understand the relativeates in both nucleation and crystal growth and to find any effectf the metal ion species on the syntheses kinetics. The CPO-27-Coas also synthesized with various methods like US, MW, and CEeating in order to find facile and efficient synthesis method forhe MOF preparation. Moreover, the physicochemical properties ofhe CPO-27-Co that was synthesized with the three methods haveeen compared.
. Materials and methods
.1. Materials and synthesis method
The metal CPO-27s were synthesized solvothermally at 70 ◦C
sing US, MW, and CE methods by modifying the synthesis con-itions from a procedure previously reported in the literature41]. To compare the relative synthesis kinetics of the CPO-7s, the US method was used to take advantage of the rapid
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ig. 1. Changes of XRD patterns of CPO-27-Co with synthesis time and synthesis methodiffractions of the simulated pattern were truncated to show small diffractions clearly.
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synthesis [15–19]. In a typical US synthesis procedure, a solidmixture of 0.5 mmol 2,5-dihydroxyterephthalic acid (H4DHTP,98%, Sigma-Aldrich) and 0.5 mmol metal nitrate hexahydrate(Zn(NO3)2·6H2O, Co(NO3)2·6H2O or Ni(NO3)2·6H2O) was added toa mixture of N,N-dimethylformamide (DMF, 20.0 mL) and water(1.25 mL). A glass vial (30 mL) containing the reaction mixture, afterstirring for 5 min, was set to the probe of an ultrasonic generator(VCX 750, Sonics & Materials, Inc). The temperature (70 ◦C) wascontrolled by circulating water at a constant temperature aroundthe vial using a thermostat. The reaction was continued for a fixedtime under ultrasound irradiation (35% of the maximum power ofthe generator was used).
To compare the relative synthesis kinetics of the three meth-ods (US, MW, and CE), CPO-27-Co was selected as a representativeCPO-27. For the MW and CE synthesis, 0.5 mmol H4DHTP wasmixed with 10.0 mL DMF and stirred for 5 min. Then another solu-tion mixture was prepared by mixing 0.5 mmol of cobalt nitratehexahydrate with 10.0 mL of ethanol. The two solution mixtureswere mixed under magnetic stirring for 5 min. For conventionalelectric crystallization, the reaction mixture was loaded in a Teflon-lined autoclave (100 mL) and put in a preheated electric oven at afixed temperature (70 ◦C) for a predetermined time. For microwavesynthesis, the reaction mixture was loaded in a 100 mL Teflon auto-clave, which was sealed and placed in a microwave oven (MARS-5,CEM, maximum power of 1200 W). The autoclave in the microwaveoven was heated to the reaction temperature (70 ◦C) in ∼0.5 min
and kept for a predetermined time. The microwave power was400 W throughout all of the synthesis steps including heating-upstage. Detailed procedures of the MW syntheses are described else-where [42]. After the synthesis, the vial or autoclaves were cooled
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8 gineering Journal 173 (2011) 866– 872
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68 E. Haque, S.H. Jhung / Chemical En
o room temperature and the solid products were recovered by fil-ration and washing with absolute ethanol and dried overnight at00 ◦C and stored in a glass vial for analysis.
The phases of the products were identified with an X-ray diffrac-ometer (Mac Science, Model No. 1031, CuK� radiation). The XRDrystallinity was calculated by the summed intensity of the twoiffraction peaks ((2 −1 0) plane at 2� = 6.8 and (3 0 0) plane at� = 11.7) of the synthesized CPO-27s, compared with the fully crys-allized samples under a selected condition. The relative rates ofucleation and crystal growth were estimated by the reciprocal ofhe induction period and the slope of the crystallization curve (crys-allinity between 20% and 80%), respectively, similar to a reported
ethod [15]. The induction period or nucleation time is the timeequired to observe any crystallinity (XRD intensity of 0–5% to theully crystallized samples).
.2. Characterization
The morphology of the MOFs was examined using a field emis-ion scanning electron microscope (Hitachi, S-4300). The nitrogendsorption isotherms of the purified samples were obtainedt–196 ◦C with a surface area and porosity analyzer (Micromeritics,ristar II 3020) after evacuation at 150 ◦C for 12 h. The surface areand micropore volume were obtained using the BET equation and-plot, respectively.
. Results and discussion
.1. Effect of the heating methods on the synthesis of CPO-27-Co
In order to understand the effect of the heating methods on theynthesis of CPO-27s, the CPO-27-Co was synthesized at 70 ◦C withhree preparation methods: US, MW, and CE heating. Fig. 1 showshe XRD patterns of the synthesized CPO-27-Co from the variousynthesis methods and synthesis time. All of the XRD patterns agreeell with the calculated pattern of CPO-27-Co [33]. As the heating
ime increases, the XRD intensity increases for all of the synthe-is methods. However, there is little difference in the overall XRDatterns and the intensity saturates at a certain time.
The synthesis kinetics has been compared with the crystalliza-ion curves (Fig. 2), and the rates of syntheses are crudely in therder of US > MW > CE. The syntheses can be divided into two stagesf nucleation and crystal growth, and the rates of nucleation andrystal growth are summarized in Table 1. Compared to the conven-ional electric heating, the synthesis with US is accelerated by 19nd 29 times for the stages of nucleation and crystal growth, respec-ively. The MW synthesis is also accelerated; however, the degreesf acceleration are about 10 times both in the stage of nucleationnd crystal growth. Therefore, it can be concluded that the acceler-tion by US or MW is observed in both in the nucleation and crystalrowth stages, and the synthesis rates are US > MW > CE heating.
The acceleration with US in many synthesis reactions has beenxplained with ‘acoustic cavitation’ [43], the process comprisedf the formation, growth, and implosive collapse of micrometer-ized bubbles during ultrasound treatment. Local conditions under
able 1ucleation and crystal growth rates of CPO-27-Co synthesized at 70 ◦C by three different
Methods BET surfacearea (m2/g)
PVmicro (cm3/g) Nucleationtime (min)
US 1113 0.36 27
MW 1025 0.34 47
CE 910 0.31 510
a Calculated from the 1/(nucleation time).b Calculated from the slope of a crystallization curve (between 20% and 80% crystallinit
time (min)
Fig. 2. Crystallization curves of CPO-27-Co with synthesis methods.
bubble implosion lead to hot-spots having temperatures of∼5000 ◦C, pressures of ∼1000 atm, and extraordinary high heat-ing/cooling rates of ∼1010 ◦C/s [43]. Therefore, various reactionscan be accelerated even at room temperature under US becausethe instantaneous temperature and pressure may be very high. Theacceleration of CPO-27 syntheses, therefore, may be explained withhot-spots or transient temperatures and pressures. It has also beenreported that cavitation is more dependent on the physical char-acteristics (such as vapor pressure, viscosity and surface tension)than on the chemical properties (polarity, acidity, or basicity) [44].
Similarly, rapid heating and the creation of hot spots areimportant factors associated with an increase in synthesis ratesunder MW irradiation [20,45]. Similar to US synthesis, hot spotsare important in the accelerated synthesis under MW conditions[20,45]. In this study, little difference, excluding the degree ofacceleration, is found between the US and MW methods in theaccelerated syntheses. Therefore, the accelerations in the synthesisof CPO-27-Co under US and MW, similar to previous works [15,44],may be due to physical processes such as hot spots rather thanchemical effects.
Figs. 3 and 4 compare the morphology and nitrogen adsorptionof the fully crystallized CPO-27-Co samples obtained with the threesynthesis methods. The XRD patterns of the fully crystallized MOFs(supporting Fig. 1) confirm they have the same structure irrespec-tive of the synthesis methods. The crystal sizes are in the order ofCE > MW > US. The adsorbed nitrogen, surface area, and microporevolume (as shown in Table 1) increase in the order of CE < MW < US.In other words, the CPO-27 synthesized with US shows the highestporosity and smallest size. On the contrary, the material obtainedwith CE heating has the largest sized crystals with the lowest poros-
ity.
It has been understood that crystal size depends on a balancebetween the nucleation rate and the crystal growth rate [46,47].The crystal size is small when the nucleation rate is larger than
methods.
Relativenucleationratea
Crystal growthrateb (min−1)
Relative crystalgrowth rate
19 2.11 × 10 −2 2911 6.96 × 10 −3 10
1.0 7.33 × 10 −4 1.0
y).
E. Haque, S.H. Jhung / Chemical Engineering Journal 173 (2011) 866– 872 869
Fig. 3. SEM images of fully crystallized CPO-27-Co MOFs. (a) Synthesized at 70 ◦Cff
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that CPO-27-Zn shows much higher reactivity in both nucleiand crystals formation compared with CPO-27-Co and CPO-27-Ni,whereas CPO-27-Co shows higher reactivity than CPO-27-Ni. In the
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or 75 min with US (inset shows the magnified SEM image), (b) Synthesized at 70 ◦Cor 3 h with MW and (c) Synthesized at 70 ◦C for 24 h with CE heating.
he crystal growth rate [46,47]. A high nucleation rate is attainedy increased supersaturation because the nucleation rate, com-ared to the growth rate, rises more sharply (or exponentially) withupersaturation [46]. Therefore, small particles are readily obtainedith US or MW irradiation probably because of the high reaction
ate or decreased synthesis time. Increased nuclei concentration,ue to rapid nucleation, may be another reason for decreased crys-al size from ultrasound or microwave irradiation.
Fig. 4. Nitrogen adsorption isotherms of fully crystallized CPO-27-Co MOFs. (a) Syn-thesized at 70 ◦C for 75 min with US, (b) Synthesized at 70 ◦C for 3 h with MW and(c) Synthesized at 70 ◦C for 24 h with CE heating.
The high surface area and micropore volume of the US or MWsample are probably due to the small crystal size of the MOFsbecause small crystals may have high external surface. Moreover,the pore structure of big crystals (obtained with CE heating) mayeasily clog. Actually, the steady increase of adsorbed nitrogen withrelative pressure over the CE sample (Fig. 4(c)) may suggest asteady increase of the adsorption on the pores that cannot be easilyapproached. Therefore, it can be suggested that US or MW irradia-tion is helpful to get highly porous (or easily purified/opened) MOFseven though the improved porosity cannot be explained explicitly.
3.2. Synthesis of the three CPO-27s with ultrasound
Supporting Fig. 2 shows the change in XRD patterns of the threeCPO-27 s (CPO-27-Zn, CPO-27-Co and CPO-27-Ni) with synthesistime under US irradiation. As summarized in the crystallizationcurves (Fig. 5), the rate of the synthesis is CPO-27-Zn > CPO-27-Co > CPO-27-Ni. The crystallization curves and Table 2 show
time (min)
Fig. 5. Crystallization curves of CPO-27-Zn, CPO-27-Co and CPO-27-Ni MOFs syn-thesized with US irradiation at 70 ◦C.
870 E. Haque, S.H. Jhung / Chemical Engineering Journal 173 (2011) 866– 872
Table 2Nucleation and crystal growth rates of CPO-27 s synthesized at 70 ◦C by US irradiation.
a Calculated from the 1/(nucleation time).b Calculated from the slope of a crystallization curve (between 20% and 80% cryst
ynthesis of CPO-27-Zn, the nucleation rate is 9.0 and 4.1 times,nd the crystal growth rate is 15 and 4.1 times, faster than those ofhe synthesis of CPO-27-Ni and CPO-27-Co, respectively. The nucle-tion and crystal growth rates of the synthesis of CPO-27-Co are.2 and 3.7 times faster than those of the synthesis of CPO-27-i. The kinetics of the syntheses depending on the metal ions is
ummarized in Scheme 1.From this result, it can be assumed that the rate of deprotona-
ion of H4DHTP to DHTP is faster than the complexation processo form CPO-27s because the lability of the metal ions controls theeaction rate. If the deprotonation is slow and is the rate determin-ng step, the lability of the metal ions does not have a noticeableffect on the synthesis rate. A similar result of rapid deprotonationas also observed in the synthesis of metal-carboxylates [16,48].enerally, compared with CoII and NiII, ZnII is considered to be
labile ion in kinetics due to a small ligand field stabilizationnergy [49]. The rate constants for the water exchange reactions also ZnII > CoII > NiII [49]. The synthesis rate of the three CPO-27s,herefore, correlates well with the lability of metal ions, showinghe importance of lability or inertness of metal ions in the syn-hesis rates of MOFs due to the simple process of complexationr ligand exchange in MOFs synthesis. The importance of labil-ty/inertness of metal ions in the synthesis rate is similar to theesults in the syntheses of metal-BDCs [48] and lanthanide-BTCs16]. Therefore, it may be concluded that a mild reaction conditionlow temperature and short reaction time) is sufficient for the crys-allization of MOFs when the materials are synthesized from a labileation
Supporting Figs. 3 and 4 show the typical morphologies anditrogen adsorption isotherms, respectively, of the fully crystallizedPO-27s. Irrespective of the metal ions, the crystals are very homo-eneous and are highly porous. Moreover, the nitrogen adsorptionsotherms are typical type-I, showing the microporosities and per-
ect crystals.
COOH
COO-
Fast step
Isostructural MOFs, CPO-27sCPO-27-Z n CPO-27- Co CP O-27-Ni
Nuclei
Nuclei
Nuclei+
k NU = 1
COOH
COO-
Ni(II )
Co(II)
Zn(II )
k NU = 2.2
k NU =9.0
k XG = 1k XG = 3.7
k XG = 1 5
OH
HO
OH
HO
cheme 1. Relative synthesis kinetics of CPO-27s depending on the metal ions.
y).
4. Conclusions
Isostructural MOFs, CPO-27s, have been synthesized solvother-mally from the very similar reaction compositions using threesynthesis methods (US, MW, and CE). Three metal (Zn, Co, and Ni)nitrates and 2,5-dihydroxyterephthalic acid have been used in thesynthesis. The reaction rates of both nucleation and crystal growthare on the order of US > MW > CE because of physical effects like hotspots under US or MW irradiation. The CPO-27-Co obtained withUS shows the highest porosity with the smallest sized crystals;however the CE-synthesized CPO-27-Co has the lowest porositywith the largest sized crystals. The low porosity of CPO-27-Cosynthesized with CE heating may be due to the large crystal size(having readily clogged pore structures). The synthesis of the threeisostructural CPO-27s with US irradiation shows that the synthe-sis kinetics for both the nucleation and crystal growth increaseswith increasing the lability of the metal ions, and the deprotona-tion of the acid into carboxylate is relatively fast compared withthe complexation to form MOFs. Therefore, relatively mild reactionconditions (a low temperature and a short reaction time) may besufficient for the crystallization of isostructural MOFs from a labilecation.
Acknowledgement
This work was supported by Mid-career Researcher Programthrough NRF grant funded by the MEST (R01-2007-0055718, 2008-0055718, 2009-0083696, 2010-0028783).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cej.2011.08.037.
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