Microporous and Mesoporous Materials 119 (2009) 331–337

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Microwave synthesis of hybrid inorganic–organic materials including porousCu3(BTC)2 from Cu(II)-trimesate mixture

You-Kyong Seo a,b, Geeta Hundal a, In Tae Jang a, Young Kyu Hwang a,*, Chul-Ho Jun b, Jong-San Chang a,*

a Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Daejeon 305-600, Republic of Koreab Department of Chemistry, Center for Bioactive Molecular Hybrid, Yonsei University, Seodaemoonku, Seoul 120-749, South Korea

a r t i c l e i n f o

Article history:Received 21 August 2008Received in revised form 15 October 2008Accepted 31 October 2008Available online 9 November 2008

Keywords:MOFCuBTCMicrowave synthesisTemperature effectSolvent effect

1387-1811/$ - see front matter � 2008 Elsevier Inc. Adoi:10.1016/j.micromeso.2008.10.035

* Corresponding authors.E-mail addresses: ykhwang@krict.re.kr (Y.K.

(J.-S. Chang).

a b s t r a c t

Using a single, improved synthetic process, three different compounds comprising two known phasessuch as [Cu3(BTC)2(H2O)3] (1), [Cu2(OH)(BTC)(H2O)] � 2nH2O (2) and a new phase [Cu(BTC–H2)2-(H2O)2] � 3H2O (3) have been prepared by microwave-irradiation (MW) of identical reaction mixture ofCu(II) salt and trimesic acid, benzene-1,3,5-tricarboxylic acid (BTC–H3) at different temperatures. Themicrowave synthesis of (1) has been compared to its conventional hydrothermal synthesis. It has beenfound that by using MW synthesis (1) can be obtained in a much shorter synthesis time with improvedyield and physical properties. The effect of other synthetic parameters such as solvent, concentration ofreactant mixtures and reaction time on the product (1) has been also studied. At high temperature, thecompound (2) was obtained in the same solution. The new phase (3) has been characterized by elementalanalysis, thermogravimetric analysis, IR spectroscopy, scanning electron microscopy and powder XRDanalysis.

� 2008 Elsevier Inc. All rights reserved.

1. Introduction

The metal-organic frameworks (MOFs) have received consider-able attention in the recent years [1–5] for being excellent exam-ples of rational designing of porous solids, with specific surfaceareas and micropore volumes that surpass many of the zeolites,metal phosphates and activated carbons [6–8]. The pursuit is beingmade fervently because of their potential applications in gas stor-age [9–11], catalysis [12,13], separations [14], drug delivery [15]and molecular recognition [16]. Cu3(BTC)2(H2O)3 � xH2O (namedas HKUST-1) is one of the first robust metal-organic framework(MOF) materials with a microporous structure that is reminiscentof zeolite frameworks [17]. The HKUST-1 is one of the highly citedMOFs because it has a large surface area, high pore volume, highchemical stability, high Lewis acidity and lability of coordinatedwater molecules. These properties enable it to be a potential can-didate for adsorption, gas storage applications and catalysis [18–25].

Ever since the synthesis of Cu3(BTC)2(H2O)3 was reported [17]by Chui et al. the material has been synthesized a number of times[18–25] by conventional hydrothermal methods (henceforth CE) orelectrochemical method [26] so as to optimize the synthetic condi-tions and improve the purity of the product (Table 1). However,MOFs show considerable structural diversity and chemical trends

ll rights reserved.

Hwang), jschang@krict.re.kr

of their formation, which is dependent on synthetic conditionssuch as reaction temperature, pH, solvent, kinetic/thermodynamicfactors and reaction pathways [27]. We have, therefore, investi-gated the effects of temperature, solvent and starting precursormaterials and optimized synthesis conditions to get Cu3(BTC)2

(H2O)3 phase in high yields and purity by CE method. Two suchventures [28,29] have produced two entirely new coordinationpolymers having the molecular formula [Cu2(OH)(BTC)(H2O)] �2H2O and [Cu(BTC–H)(H2O)3] on changing the solvent system fromwater/ethanol to water in a reaction time of 24 h and 12 h, respec-tively, at 120 �C.

The microwave-assisted hydrothermal synthesis (henceforthMW) is known to be advantageous over CE synthesis because of ra-pid heating, faster kinetics, phase purity, higher yield and betterreliability and reproducibility [30–34]. In addition to this, it pro-vides an efficient way to control particle size distribution, phaseselectivity, and macroscopic morphology in the synthesis of nano-porous materials as well as inorganic solids [35–38]. Despite thesefacts, to the best of our knowledge, there are only a few reports onthe use of microwave for synthesis of hybrid materials [39–43].Here, we present the use of microwaves for the synthesis of[Cu3(BTC)2(H2O)3] (1) in a much shorter time and lower tempera-ture than reported for the original synthesis via CE [17]. Variousfactors which may affect the purity, yield, morphology and crystal-linity of (1) thus formed, have been investigated and the reactionconditions have been optimized. In addition to the above, we alsoreport here the synthesis of an already known MOF material[Cu2(OH)(BTC)(H2O)] � 2H2O (2), and another new material [Cu

Table 1A summary of various syntheses of Cu3(BTC)2 and their characteristics.

MOF SBET (m2/g) Reaction temperature (�C) Reaction time (h) Solvent used Method Reference

(1) 692 180 12 H2O, EtOH CE [17](1) 1500 110 18 H2O, EtOH CE [24](1) 964/1333 150 18 H2O, EtOH CE [24](1) * 120 12 H2O, EtOH CE [18](1) 1200–1400 110 15 H2O, EtOH CE [19](1) 1239 150 12 DMF APS [23](2) * 120 24 H2O CE [28](1) * 110 18 H2O, EtOH CE [20](1) * 130 12 H2O, EtOH CE [24]Cu(BTC)(H2O)3

* 120 12 H2O CE [29](1) 1392 140 0.5 H2O, EtOH MW This work(1) 1656a 140 1 EtOH MW This work(1) 1300–1500 20–23 4 MtOH EC [26]

a Surface area was obtained in high concentrated solution (3 times comparing to unit concentration).b EC denotes electrochemical method and APS denotes ambient pressure synthesis method, respectively.* Values are not available.

332 Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337

(BTC–H2)2(H2O)2] � 3H2O (3), synthesized from identical reactionmixtures by changing the reaction temperature only in a shortertime giving higher yields and phase purities.

2. Experimental

2.1. Synthesis of Cu–MOF compounds based on Cu(II) and BTC–H3

Various reaction conditions used for the syntheses and theiroutcomes are listed in Table 2. For having best yields and purityof the phases, the optimized synthetic conditions for small scalepreparation of compounds (1) and (2) under microwave-irradia-tion are as follows. Typically, an exact amount of H3BTC (2 mmol)and copper (II) nitrate trihydrate, Cu(NO3)2 � 3H2O (3.65 mmol)were dissolved in 24 mL of a 1:1 (w/w) mixture of water:ethanoland stirred magnetically for 10 min (Standard concentration). Theresulting mixture was then loaded into a Teflon autoclave, sealedand placed in a microwave oven (MARS-5, CEM). The autoclavewas heated for 60 min at 140 �C (1) and for 10 min at 170 �C for(2), respectively, in the microwave power of 300 W. After the com-pletion of the reaction, the mixtures were allowed to cool down toroom temperature, filtered and washed with water/ethanol mix-ture for several times and were dried at 100 �C overnight. For thesake of comparison, (1) was also prepared by CE following theknown method [17] using the above mixtures at 120 and 140 �C,respectively, for 12 h (Table 2).

2.2. Characterization

Powder X-ray diffraction patterns of all samples were obtainedby a Rigaku diffractometer (D/MAX IIIB, 2 kW) using Ni-filtered CuKa-radiation (40 kV, 30 mA, k = 1.5406 Å) and a graphite crystal

Table 2Temperature effect in synthesis of Cu–MOFs.

No. Method Temperature (�C) Time (min)

1 MW 120 602 MW 130 603 MW 140 604 MW 150 105 MW 160 106 MW 170 107 CE 140 7608 CE 120 760

* Concentrations used in all cases are standard unit one (H3BTC 2 mmol, Cu(NO3)3�3H2Ob Indicates some impurity in the product formed.

monochromator. The crystal size and morphology were examinedusing a scanning electron microscope (SEM, JEOL JSM-840 A). Ther-mogravimetric analyses (TGA) were performed with TA instrumentmodel, TGA Q500 V6.7 with 10 �C/min in N2 atmosphere, on sam-ples which were placed in a chamber having saturated NH4Cl vaporfor 24 h. The BET analyses were performed with N2 adsorption–desorption isotherms at liquid nitrogen temperature (77 K) afterdehydration under vacuum at 423 K for 12 h using MicromeriticsTristar 3000. The specific surface areas were evaluated using theBrunauer-Emmett-Teller (BET) method in the p/p0 range of 0.05–0.2. IR spectra were recorded as KBr pellets on a Nicolet Magna-560 IR spectrophotometer. Elemental analyses were done on a CEinstruments EA-1110 elemental analyzer and on Jobin-Yvon Ulti-ma-C (ICP-AES) for Cu.

3. Results and discussion

3.1. Temperature-dependent synthesis of Cu–MOFs

Since the first synthetic report of a porous 3D coordinationpolymer Cu3(BTC)2 at 180 �C by Chui et al. [17], (1) has been pre-pared by many groups using CE at temperatures varying from110 to 180 �C (Table 1). The optimized reaction conditions were gi-ven by Schlichte et al. who showed that Cu2O free samples could beobtained after heating the synthesis mixture for 12 h at 120 �C. Thehigher temperature synthesis of the above materials is generallybeneficial for crystallographic investigations, but it is mostlyaccompanied by impurities of Cu2O. Since 140 �C proved to bethe best temperature in our case for MW synthesis of (1), we pre-pared the CE samples (for comparison) both at 120 and 140 �C. Asthe XRD patterns of both samples were exactly same (Fig. 1a–d),most characterization techniques were applied to 140 �C samples

Water/ethanol (w/w, g) Yields (%) Crystal structure

12:12 No product (1)12:12 19 (1)12:12 88 (1)12:12 84 (3)12:12 79 (2)b

12:12 92 (2)12:12 85 (1)12:12 95 (1)

3.65 mmol in 24 ml of solvents).

305 10 15 20 25

(h)

(g)

(f )

(e)

(d)

(b)

(a)

(c)

Inte

nsit

y / a

.u.

2 θ / degree

Fig. 1. XRD patterns of (a) simulated structure of Cu3(BTC)2(H2O)3 (1) and samplesprepared at (b) CE 120 �C, (c) CE 140 �C, (d) MW 140 �C, (e) MW 150 �C, (f) MW160 �C, (g) MW 170 �C and (h) simulated pattern of [Cu2(OH)(BTC)(H2O)] � 2H2O (2)for 1 h MW and for 12 h CE, respectively.

Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337 333

only, unless specified otherwise. No attempts were made to syn-thesize [Cu2(OH)(BTC)(H2O)] � 2H2O by the reported CE [28] and(2) has been characterized by comparison with its simulated XRDpattern and TGA analysis. Powder XRD patterns of typical samplesof (1) and (2) are given in Fig. 1a–d and are compared with thoserecorded for (1) prepared by CE and also with those calculatedfrom their known crystal structures [18,28]. The patterns showgood overall agreement with the simulated ones; however, somedeviations in relative intensities are expected because of variationsin the degree of hydration [18]. For example, the intensity of (111)reflection in (1) increases manifolds in the samples prepared at140 �C.

The above results indicated that the temperature plays a crucialrole in MW synthesis. There was no product obtained at 120 �C in

Fig. 2. Scanning electron micrographs of Cu3(BTC)2(H2O)3 (1) synthesized at (a) 14

MW even after heating for 1 h. Microwave synthesis at both 130and 140 �C gives (1) but better yields are obtained at the latter(Table 2). Heating at 140 �C starts giving (1) after 5 min but maxi-mum yield is obtained after 60 min treatment. A 10 min heating at150 �C results in the new phase (3), which starts changing into yetanother phase [Cu2(OH)(BTC)(H2O)] � 2H2O (2) on heating at160 �C, completely becomes (2) at 170 �C and remains in thatphase on increasing the reaction time further (Fig. 1e–h). TheSEM photographs of the products formed at these four tempera-tures ( Fig. 2) clearly show the differences in the morphologies.Phase (3) consists of aggregates (�10 lm) of small particles withindefinite shapes. However, its XRD pattern is incomparable toboth (1) and (2) as well as to the other three known phases of Cu(II)with BTC obtained at ambient temperature [44,45]. Both (1) and(2) are crystalline with the former having octahedron-shaped sin-gle crystals of >1 lm in size and the latter having plate like crystalsof an average size 5 lm. For (1) both CE and MW form turquoiseblue crystals with different particle sizes (not shown). Under CEconditions, SEM image of Cu–BTC shows octahedron-shaped crys-tals with large particle size (P20 lm) whereas MW method yieldsa more irregular shaped product with the smaller particle size(610 lm).

The thermal stability was studied using TGA (Fig. 3), whichagain shows that (1) prepared by both CE and MW are similar.The first step in the curves is associated with the loss of physio-sorbed water and its exact height depends on the initial degreeof hydration of the material. Both the as-synthesized samples showa continuous weight loss (30%) up to a temperature of �110 �C cor-responding to a theoretical loss of (29.7%) for 15 water moleculesper Cu3 unit. The results are in accordance with those found bySchlichte et al. who have demonstrated a higher capacity for sol-vent molecules in the interior of the pores [18]. The second weightloss (40.0 %) starting at �280 �C disintegrates the metal-organicstructure (39.5% loss) with a complete transformation into Cu2Oand CuO. The TGA profile for (2) shows an initial weight loss of14.0%, in two steps corresponding to three water molecules(13.3%). The compound disintegrates at 282 �C with a weight loss

0 �C, (b) 150 �C, (c)160 �C and (d) 170 �C for 10 min under MW, respectively.

100 200 300 400 500 600

20

40

60

80

100

(d)

(c)(b)

(a)

Wei

ght

loss

/ %

Temperature / oC

Fig. 3. TGA curves of compound (1) synthesized at (a) 140 �C by CE, (b) 140 �C byMW, compound (2) at (c) 170 �C and compound (3) at (d) 150 �C by MW,respectively.

(e)

(d)

(c)

(b)

(a)

Inte

nsit

y / a

.u.

305 10 15 20 252 θ / degree

Fig. 4. XRD patterns of the products formed with change in water/ethanol ratio of(a) 24:0, (b) 20:4, (c) 16:8, (d) 8:16 and (e) 0:24.

334 Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337

of 37.8%, which is comparable with a theoretical loss of 36.7%accounting for decomposition into Cu2O and CuO. The chemicalformula [Cu(BTC–H2)2(H2O)2] � 3H2O of (3) was determined fromelemental and TGA analyses. The experimental and calculated datafor (3) (for C18H14O14Cu in parenthesis); C, 42.08 (41.8), H, 2.75(2.71), Cu, 12.9 (12.3) agree well with the molecular formulaCu(BTC–H2)2(H2O)2. The TGA curve of (3), however indicates threemolecules of water of crystallization. It shows an initial loss of 3.4%due to one water molecule (calc. 3.1%) at 84 �C, followed by a lossof four more water molecules up to 300 �C, in steps of two eachwith a loss of 7.3% both times (calc. 6.5% and 6.9%). The structuredisintegrates at 309 �C to leave Cu2O and CuO with 52.7% loss (calc.53.1%). The IR spectrum of (1) prepared by both methods are sim-ilar providing another evidence of them being the same material(not shown). The broad absorption band near 3500 cm�1 which isdue to water molecules (both coordinated and lattice) graduallydecreases in intensity due to dehydration upon heating the sample.

To confirm the permanent porosity, we obtained the N2 adsorp-tion–desorption isotherms of the samples obtained from CE andMW at �196 �C after evacuation at 150 �C for 12 h. The type I nitro-gen physiosorption isotherms of (1) prepared by MW at 140 �C for30 min give BET surface area of 1392 m2/g (total pore volume of0.56 cm3/g). As listed in Table 1, these values are much higher thanthe BET surface area of 974 m2/g for (1) prepared by CE by us andthose reported in literature [17,18,20] using similar concentra-tions. The values, however, look similar to those reported in the lit-erature [19,22–24]. However, in the synthesis of (1) they have useddifferent concentrations than used by us and thus are incompara-ble. Among results of similar concentrations as used in our case,the increase in surface area and small particle size (from SEM) ofour samples signify that the MW synthesis accelerates the nucle-ation and crystal growth steps and produces smaller particles withlarger surface areas.

3.2. Factors affecting the formation of phases

From the above discussion it has been established that MW canbe used to produce (1) and (2) in a very short time. Since our mainaim is to find an economical method for the preparation of (1), wetherefore examined systematically the factors which could affectthe yield, morphology, particle size, surface area and porosity of(1) being formed by MW. The factors like solvent, reaction time

and concentration of synthesis mixture have been compared vis-à-vis CE as well as MW methods to obtain (1) in high yield andphase purity.

3.2.1. Role of solventThe nature of the phase formed is very much dependent on the

(water:ethanol) solvent ratio used. In the reference synthesis of (1)reported by Chui et al. a 1:1 (w/w) mixture of water/ethanol wasused and later this method of preparation was optimized bySchlichte et al. [4] with the same solvent system at lower tempera-ture of 120 �C. Chen et al. [28] produced [Cu2(OH)(BTC)(H2O)] �2H2O (2) on changing the solvent system from water/ethanol towater alone in 24 h, whereas Gascon et al. [10] have reported theformation of a completely nonporous material, Cu(BTC–H)(H2O)3

[44] in 12 h, with all other conditions being same as used bySchlichte et al. The use of a different solvent i.e. DMF has been re-ported by Krawiec et al. [23] in their ambient pressure synthesisof (1) involving refluxing and repeated heating at various tempera-tures to get (1). To the best of our knowledge, there has not been getany report of a systematic variation of water/ethanol ratio in thesynthesis of (1). Since it is now known that a 1:1 water/ethanolmixture gives (1) and water only yields (2) in a CE synthesis, wetherefore decided to study the effect of varying the water/ethanolratios in the MW synthesis and observe the resulting changes inthe physicochemical properties in the synthesis of (1).

A complete deprotonation of H3–BTC is required for its mult-identicity to produce three dimensional, rigid and thermally stablesystems. H3BTC is less soluble in water than in water/ethanol sys-tem, leading to different products depending on the amount of dis-solved acid and degree of its deprotonation. Therefore the ratio ofwater/ethanol used in the reaction turned out to be critical. Thesynthesis yields of (1) in the pure form when the water:ethanol ra-tios were varied from 1:5 to 1:1 (Fig. 4). On increasing the amountof water to 2:1 gives impurity in the phase which is evident in theform of an extra peak at 2h = 10.4 and a shoulder at 13.8�. The reac-tion in pure ethanol also yields the product (1) but impurity peak isseen at 2h = 12.9�. Increasing the ratio of water/ethanol 20:4 (5:1)gives the phase Cu2(OH)(BTC)(H2O) (2). Further increase inwater:ethanol ratio to 24:0 changes it into the new phase (3). FromFigs. 1 and 4, it is quite evident that in MW synthesis, the phase (2)may be prepared either by increasing the reaction temperature to170 �C or by just changing the solvent mixture to 20:4 water:eth-anol instead of a 1:1 mixture in MW at 140 �C synthesis. Similarly(3) could be obtained by changing the temperature to 150 �C or

(f )

(e)

(d)

(c)

Inte

nsit

y / a

.u.

0 20 40 60 100 110 1200

20

40

60

80

1000 20 40 60 100 110 120

(a)

(b)

Reaction time / h

Yie

ld /

%

Reaction time / min

305 10 15 20 25

2 θ / degree

Fig. 6. Yields of MOF according to the reaction time under (a) MW and (b) CE. XRDpatterns of Cu3(BTC)2(H2O)3 prepared by MW at 140 �C for (c) 60 min, (d) 30 min,(e) 10 min and (f) 5 min.

Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337 335

running the reaction in a water only system at 140 �C. However,Gascon et al. [29] and Chen et al. [28] have reported the synthesisof one dimensional phase Cu(BTC–H)(H2O)3 and (2), respectively,in similar CE hydrothermal syntheses at 120 �C, in the absence ofethanol (Table 1). The phase (3) is nevertheless a new phase (cf.XRD patterns) can also be verified from a comparison of the IRspectra and TGA curves for these samples. The IR spectra of MWat 140 �C (water:ethanol 24:0) sample is identical to that obtainedfor MW at 150 �C (water:ethanol 1:1), i.e. (3) (Fig. 5) whereas thoseof MW at 140 �C (1) and MW at 170 �C (2), both (water:ethanol1:1) are very much different to each other. A comparison of (1)and (3) reveals that IR spectrum of the latter shows additionalpeaks than those present in (1). The most significant things of thesepeaks are the stretching VC@O, VC–O and bending O–H vibrational fre-quencies seen at 1710, 1193 and 1232 cm�1, respectively, indicat-ing the presence of a carboxylic acid group [46,47]. The VO–H bandis much broader than (1) and shows a low energy component at2550 cm�1, which is characteristic of a strongly H-bonded O–H ofthe carboxylic acid [46]. From these bands, it may be concludedthat BTC group in phase (3) is only partially deprotonated. Simul-taneously, there are more peaks seen in the 1300–1600 cm�1 re-gion than in (1). As these peaks stand for bridging bidentatecoordination of carboxylate group, therefore a greater number ofthese peaks indicate a rather more unsymmetrical kind of coordi-nation by the different carboxylic acid/carboxylate groups to dif-ferent metal ions, in comparison with a symmetrical dimericcopper (II) carboxylate type of structural units found in (1). TheTGA curves for MW at 160 �C, MW at 170 �C and MW at 140 �C(water:ethanol 20:4) samples are identical resulting all to be thephase (2). Similarly, TGA curve of MW at 140 �C (water:ethanol24:0) sample is identical to that obtained for MW at 150 �C(water:ethanol 1:1) sample demonstrating these to be the samephase (3).

3.2.2. Effect of reaction timeWith the optimum temperature being 140 �C for the MW syn-

thesis of (1) in the pure form, we further studied the effect of reac-tion time on the products being formed. Fig. 6 shows that MW theyield of (1) increases linearly with increase in crystallisation timefrom 5 to 30 min, with a larger increase (�50%) between 10 and30 min. From 30 to 60 min the increase is there but to a relativelysmaller extent (�20%). It is worth noting that the yields are incred-ibly low when the crystallization time is of the order of 5 min.However, in CE the yield decreases with an increase in reaction

3000 2500 2000 1500 1000 500

(b)

(a)

Tra

nsm

itta

nce

/ a.u

.

Wavenumber / cm-1

Fig. 5. FT–IR spectrum of (a) compound (3) and (b) compound (1), the arrows pointtowards the significant additional peaks in (3).

time, although the total decrease is only �30%. The SEM image ofMW products at various reaction times shows that the dimensionsof the crystals formed increase (from 10 to 20 lm) with increasingcrystallization times (Fig. 7a and b), whereas reverse is true in thecase of high temperature (HT) synthesis. In CE (Fig. 7c and d), thecrystal size is decreasing from �25 lm (for crystallization time12 h) to a constant value of �10 lm (for 24 h and 48 h). Both,the decrease in yield and particle size may be explained by consid-ering that in CE synthesis the nucleation and crystal growth areslow processes and thus it takes much longer time. Due to longcrystallization times (12–48 h), the crystals formed remain in con-tact with the solvent for a much longer period. This may result increating equilibrium of the kind, Solute(crystallized) M Solute(dissolved),i.e. between the amount of solute crystallized and that remainingin the solution. Under the effect of heating in CE conditions, espe-cially at low pH as in the existing case (<2), the reaction may shiftlargely towards right depending on time duration, before it reachesequilibrium. In MW, however the heating is fast and uniform, cre-ating nuclei throughout the solution which quickly grow to crys-tals. Initially, with increasing time the number of nuclei formedincreases to a greater extent and hence there is a big jump in yieldfrom 5 to 30 min and as both the nucleation and crystallizationsteps are accelerated [35–43] by MW effect and therefore all thenuclei once formed grow more with longer crystallization time.

Fig. 7. SEM images of Cu3(BTC)2(H2O)3 at 140 �C for (a) 5 min, (b) 30 min by MW, (c) 12 h, and (d) 24 h by CE, respectively. Concentration of solution is (e) 3 times and (f) 5times by MW at 140 �C, for 1 h.

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600(b)

(c)

Vol

ume

adso

rbed

/ m

l g-1

P/P0

(a)

Fig. 8. N2 isotherm of Cu3(BTC)2(H2O)3 prepared by MW at 140 �C for 1 h withdifferent solute concentration. Concentration of solution is a (a) standard, (b) 3times and (c) 5 times.

336 Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337

From 30 to 60 min, an increase in nucleation is there but is rela-tively less, resulting in a lesser but definite increase in the yieldaccompanied by larger crystals as well. Since the maximum reac-tion time given is 1 h only, MW method does not suffer from backof dissolution of crystals in the solution (see Fig. 8).

3.2.3. Effect of concentration change in reactantAlthough the MW method usually does not yield crystals with

size adequate enough for single crystal studies, it produces uni-form and faster nucleation [13] leading to faster crystal growthand therefore the crystal sizes may be varied by changing the reac-tion concentrations. Thus smaller particles may be produced byreducing the concentration of the starting solution and vice versa.Gascon et al. [29] have reported changes in yield and size of thecrystals with concentration of the reactants in a typical CE synthe-sis of (1). They observed that the yield of the product formed wasindependent of the concentration of the reactants (close to 98%),however the crystal size was highly dependent on it. More concen-trated reaction mixtures produced larger crystals (Fig. 7c and d).The yield becomes quantitative with increase in concentration ofthe reactants by 3 times (�98%), subsequently becoming invarianton a further increase up to 5 times (�99%) of the initial concentra-tion (�88%). The SEM images of the products (Fig. 7e and f) clearly

Y.-K. Seo et al. / Microporous and Mesoporous Materials 119 (2009) 331–337 337

show an increase in crystal size from �20 lm to 50 lm on increas-ing the concentrations from 3 to 5 times, and this when comparedto Fig. 7a and b gives a linear relationship between concentrationand crystal size. The present results are consistent with those ob-tained by high-throughput screening in synthesis of HKUST-1[48]. The BET surface areas measured by N2 adsorption isothermshowever showed a non-linear increase with concentration. TheBET surface areas (SBET) and pore volume of (1) were found to be1304, 1656, 1577 m2/g and 0.56, 0.81, 0.74 ml/g for concentrationsof the reactants 1, 3 and 5 times of the original concentration,respectively. The increase in size from 610 to 20 lm on changingfrom 1 to 3 times concentration is consistent with an increase inthe SBET from 1304 to 1656 m2/g. The subsequent increase in sur-face area may be attributed to good crystallinity in 3 times en-hanced concentration. The surface area in the sample with 5times concentration decreased because of small sized sphericalimpurities of Cu2O (5–10 lm), which are conspicuous in Fig. 7f(see the arrow points).

In spite of the formation of metal-organic frameworks, whichcould not be established, acceleration in nucleation and crystalliza-tion steps are very much noticeable in MW synthesis because ofthe fast dissolution and deprotonation of the BTC–H3 and enhancedcondensation of metal-oxygen networks under microwave-irradia-tion condition [38,39] by hot spots and superheating effects.

4. Conclusions

We have successfully synthesized three different phases ofCu(II)-(BTC–H3) system, starting from identical reaction mixturesof Cu(NO3)2 and BTC–H3 by irradiating them in MW for differentreaction temperatures from 140 to 170 �C, in a 1:1 water:ethanolmixture. The products (1–3) could also be obtained by runningthe reaction at a fixed temperature of 140 �C in MW and varyingthe water:ethanol ratio only. The procedure gives a highly usefulMOF material Cu3(BTC)2 (1) in its dehydrated form, in a consider-ably lesser time in comparison to CE. Various factors which influ-ence the yield, quality and morphology of the product (1) havealso been investigated and optimized. Efforts are currently beingmade to adapt the procedure of this batch method for a continuoussynthesis of (1) Cu3(BTC)2 with a kilogram scale in an hour.

Acknowledgments

This work was supported by MKE through the Research Centerfor Nanocatalysts and Institutional Research Program. Dr. GeetaHundal gratefully acknowledges KOFST for awarding the Brain Poolfellowship at KRICT, Korea.

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