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Effect of Synthesis Parameters on the Formation ofZeolitic Imidazolate Framework 8 (ZIF-8) Nanoparticlesfor CO2 AdsorptionLi Sze Lai a , Yin Fong Yeong a , Noraishah Che Ani a , Kok Keong Lau a & Azmi Mohd Shariff aa School of Chemical Engineering , Universiti Teknologi PETRONAS , Tronoh , Perak , MalaysiaAccepted author version posted online: 13 May 2014.Published online: 10 Jul 2014.

To cite this article: Li Sze Lai , Yin Fong Yeong , Noraishah Che Ani , Kok Keong Lau & Azmi Mohd Shariff (2014) Effectof Synthesis Parameters on the Formation of Zeolitic Imidazolate Framework 8 (ZIF-8) Nanoparticles for CO2 Adsorption,Particulate Science and Technology: An International Journal, 32:5, 520-528, DOI: 10.1080/02726351.2014.920445

To link to this article: http://dx.doi.org/10.1080/02726351.2014.920445

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Effect of Synthesis Parameters on the Formationof Zeolitic Imidazolate Framework 8 (ZIF-8)Nanoparticles for CO2 Adsorption

LI SZE LAI, YIN FONG YEONG, NORAISHAH CHE ANI, KOK KEONG LAU, and AZMI MOHD SHARIFF

School of Chemical Engineering, Universiti Teknologi PETRONAS, Tronoh, Perak, Malaysia

Zeolitic imidazole frameworks-8 (ZIF-8) is a subclass of metal-organic frameworks (MOFs) with the transition metal cations(Zn2þ) linked by imidazolate anions forming tetrahedral frameworks in zeolite-like topologies. This article reports on the synthesisof ZIF-8 nanoparticles by varying the synthesis parameters at room temperature. The crystallization duration, molar ratios, andpH of the mixture solution were varied in order to study the effects of these parameters on the formation of ZIF-8 nanoparticles.The structural and morphology transformation of the resultant particles were characterized using x-ray diffraction, field emissionscanning electron microscopy, and Brunauer–Emmett–Teller (BET) surface analysis. The CO2 adsorption characteristics of ZIF-8nanoparticles were tested using CO2 physisorption analysis. Mature structural evolution was observed for ZIF-8 synthesized at 60and 1440min, but insufficient crystallization was found for ZIF-8 synthesized at 5min. Meanwhile, ZIF-8 nanoparticles synthe-sized under lower amount of methanol resulted in larger particle size and higher crystallinity. Poorly intergrown ZIF-8 nanopar-ticles were observed for samples synthesized using a mixture solution with pH 8.2. Although different particle sizes and relativecrystallinities were obtained for the ZIF-8 samples, synthesis using different molar ratios of the mixture solution, insignificantvariations of BET surface areas, and CO2 adsorption capacities were found.

Keywords: CO2 adsorption capacity, crystallinity, particle size, surface area, ZIF-8 nanoparticles

1. Introduction

In industry, the gas sweetening purification processes ofnatural gas by removing the acidic contaminant are impor-tant prior to sale. Carbon dioxide (CO2) removal fromnatural gas is an essential process to protect pipelines andequipment from corrosion owing to its highly corrosivecharacteristics in the presence of water (Venna and Carreon2009). In recent decades, zeolitic imidazolate frameworks(ZIFs), a subcategory of metal-organic frameworks(MOF), has drawn attention of many researchers in CO2

removal process. In ZIFs, transition metals ions such asZn2þ or Co2þ were linked through N atoms by the imidazo-late linkers to form tetrahedral framework in zeolitetopology (Cravillon et al. 2009). With the combination ofthe attractive properties of MOF and zeolite, ZIFs showedplentiful framework diversity, tuneable pore apertures andorganic bridging ligands, high surface area, and thermal

and chemical stability (Park et al. 2006). Among the ZIFs,ZIF-8 received considerable interest by virtue of its excep-tionally high CO2 adsorption capacity (Venna and Carreon2009). ZIF-8 adsorbed CO2 up to 7.9mmol=g at 30 bar(Nune et al. 2010) and 1.02mmol=g at 1 bar (Yazaydınet al. 2009). ZIF-8 (Zn(mim)2) categorized as the I43m cubicspace groups and possessed large cavity of �11.6 A connec-ted by the six-membered ring windows with the apertureof �3.4 A (Bux et al. 2009). Figure 1 showed the atomicmolecules of ZIF-8 (Hu et al. 2012) and the three-dimensional illustration of ZIF-8 with four- andsix-membered rings (Phan et al. 2009). The six-memberedrings are the primary adsorption site for ZIF-8 (Hu et al.2012) while the four-membered rings did not contribute tothe mass transport of molecules through the cages ofZIF-8 due to the vertical concentration gradient from onecage to another below (Bux et al. 2011). In relation to theproperties of ZIF-8, great potential usage in different appli-cation areas were reported, such as gas storage and separ-ation, catalysis, chemical sensing, and construction ofadvanced nanotechnology devices (Zhou et al. 2008).

Formation of ZIF-8 involved the process of nucleationand crystallization. As shown in Figure 2, 2-methylimidazole(Hmim) dissolved in the methanol (MeOH) solvent by losinga proton ion (Hþ) and formed the imidazolium ion (meIm�),

Address correspondence to: Yin Fong Yeong, School ofChemical Engineering, Universiti Teknologi PETRONAS,Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia. E-mail:yinfong.yeong@petronas.com.myColor versions of one or more of the figures in the article can befound online at www.tandfonline.com/upst.

Particulate Science and Technology, 32: 520–528

Copyright # 2014 Taylor & Francis Group, LLC

ISSN: 0272-6351 print=1548-0046 online

DOI: 10.1080/02726351.2014.920445

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which was rich in electrons. Zn2þ from the zinc sourcereacted with the meIm� to form a building unit of ZIF-8.The Zn atom of a building unit bridged to the other buildingunits through the linkage with N atom formingsix-membered rings of ZIF-8 (Fri�sscic et al. 2013). Then,ZIF-8 continually grew through coalescence or particleaggregation process and particle-monomer addition mech-anism (Cravillon et al. 2011). The occurrence of this processowed to the differences in the chemical potential between thecrystallizing substance and the solution or liquid phase(Garside et al. 2002). Therefore, formation of ZIF-8 ceasedeventually after attaining equilibrium state with the mothersolution (Venna et al. 2010).

In review of the formation of ZIF-8 particles, severalresearchers synthesized ZIF-8 particles under varioussynthetic conditions, such as molar ratios of Zn2þ:Hmim:MeOH in the synthesis solution (Keser 2012), synthesis

duration (Venna et al. 2010) and solution pH (Cravillonet al. 2011). However, their findings do not account forthe effect of structure and morphology changes on theCO2 adsorption capacity. In this article, a preliminary studyon the effect of the structural evolution of ZIF-8 nanopar-ticles and their CO2 adsorption characteristics were carriedout. The structural and morphology transformation of theresultant nanoparticles were characterized using variety ofanalytical tools. CO2 adsorption capacities of the resultantZIF-8 samples were determined using CO2 physisorptionanalysis.

2. Experimental

2.1 Synthesis of ZIF-8

Zinc nitrate hexahyrate (Zn(NO3)2 � 6H20, 99%, Sigma-Aldrich, USA) were used as zinc source and 2-methylimida-zole (Hmim, 99%, Sigma-Aldrich) as organic ligand. Allchemicals were used as received without further purification.ZIF-8 were synthesized at room temperature following theprocedures reported by Cravillon et al. (2009). Three para-meters were varied in this work: synthesis duration, molarratio of Zn2þ:Hmim:MeOH, and pH of the synthesissolution. For instance, in synthesis of ZIF-8 using synthesissolution with molar ratio Zn2þ:Hmim:MeOH of 1:7.9:1002,Zn(NO3)2 � 6H2O (2.93 g) in 200mL of MeOH was rapidlymixed with Hmim (6.48 g) in 200mL of MeOH. The solutionwas stirred for 60min at room temperature. Then, the result-ing particles were separated from milky dispersion by centri-fugation at 7000 rpm for 15min. The precipitate was washedwith fresh MeOH and repeated for three times. Last, theresulting particles were dried in oven overnight at 353K.The synthesis duration was varied at 5, 60, and 1440minat constant molar ratio Zn2þ:Hmim:MeOH of 1:7.9:695and pH solution of 7.4. Then, the molar ratio Zn2þ:Hmim:MeOH were varied at 1:7.9:86.7, 1:7.9:695, and 1:7.9:1002under constant synthesis duration of 60min and pH solutionof 7.2–7.6. On the other hand, 2M of hydrochloric acid(HCl) and sodium formate (NaCOOH) were used to regulatethe pH of the synthesis solution to 6.8 and 8.2, respectivelyat constant synthesis duration of 60min and molar

Fig. 1. a) Atomic molecules of ZIF-8 (Hu et al. 2012) and b) three-dimensional illustration of ZIF-8 (Phan et al. 2009).

Fig. 2. Formation of ZIF-8 building units=cluster in MeOHsolvent.

Synthesis Parameters on the ZIF-8 Nanoparticles for CO2 Adsorption 521

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ratio Zn2þ:Hmim:MeOH of 1:7.9:695. Total seven sampleswere synthesized using different synthesis conditions assummarized in Table 1.

2.2 Characterization

2.2.1 X-Ray Diffraction

The x-ray diffraction (XRD) peaks were recorded at room-emperature using Bruker (USA) D8 Advance diffractometerequipped with reflectance Bragg–Brentano geometry. CuKaradiation was used at 40kV and 40mA. The 2h ranged from0� to 40� was scanned at a step size of 0.02�. The relativecrystallinity of the sample was determined based on ratio ofthe major peak of the samples at 2h �7.30� (110), relative tothose highly crystalline reference materials. The relative crys-tallinity is defined using Equation (1) as follows (Venna 2010):

Relative crystallinity

¼ Peak int ensities of the sample at ð110Þ planePeak int ensities of the references at ð110Þ plane ð1Þ

Table 1. Synthesis conditions for different samples

SampleZn2þ:Hmim:MeOH

molar ratiosSynthesis

duration (min)Solution

pH

A 1:7.9:695 5 7.4B 1:7.9:695 60 7.4C 1:7.9:695 1440 7.4D 1:7.9:86.7 60 7.6E 1:7.9:1002 60 7.2F 1:7.9:695 60 6.8G 1:7.9:695 60 8.2

Fig. 3. XRD patterns of ZIF-8 synthesised using different molar ratio of Zn2þ:Hmim:MeOH: a) sample E (1:7.9:1002); b) sample B(1:7.9:695); and c) sample D (1:7.9:86.7) at synthesis duration of 60min and pH 7.2 to 7.6.

Fig. 4. XRD patterns of ZIF-8 synthesized using different pH solution: a) sample G (pH¼ 8.2); b) sample F (pH¼ 6.8); andc) sample B (pH¼ 7.4) at synthesis duration of 60min and molar ratio Zn2þ:Hmim:MeOH of 1:7.9:695.

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2.2.2 Field Emission Scanning Electron Microscopy

The field emission scanning electron microscopy (FE-SEM)testing was carried out on Hitachi model SU8000 atmagnification 50K�. Samples were coated with platinumbefore the scanning due to the charging effect.

2.2.3 Nitrogen Adsorption-Desorption Measurement

The pore textural properties (surface area and pore volume)of ZIF-8 nanoparticles were measured using MicromeriticsASAP 2020 adsorption porosimeter equipped with liquidnitrogen as coolant at 77K. Samples were degassed at135�C for 3 h before the measurement to remove all thephysisorbed species from the surface of the particles.

2.3 CO2 Adsorption Measurement

CO2 adsorption isotherm was measured using CO2 physi-sorption analysis, BELSORP MINI II at 298K. The adsorp-tion measurement was performed under low pressure(ranged from 0 to 1 bar) using high purity CO2 gas (99.9%).

3. Result and Discussions

3.1 Crystallinity and Morphology Evolution by XRD andFE-SEM

3.1.1 X-Ray Diffraction

Figures 3, 4, and 6 show the XRD patterns of the ZIF-8 sam-ples synthesized at different synthesis durations, molar ratiosand pHs. All the samples synthesized in the present workmatched with the reported characteristic peaks of ZIF-8,at 2h values of 7.30�, 10.36�, 12.68�, 16.40�, and 17.98� (Parket al. 2006; Venna 2010; Cravillon et al. 2011; Keser 2012; Liuet al. 2013). The orientation and crystal face of the ZIF-8is indicated by the main peak of XRD at (110) (Lalena et al.2007). Cravillon et al. (2012) and Moh (2012) reportedthat, high intensity of peak (110) is attributed to the forma-tion of ZIF-8 with stable rhombic dodecahedron shape, whichresembled the final stage of the growth of ZIF-8 structure.Table 2 shows the relative crystallinity of the ZIF-8 samplessynthesized at different synthesis duration with constantmolar ratio Zn2þ:Hmim:MeOH of 1:7.9:695 and pH 7.4.It can be seen from Table 2 and Figure 6 that the relativecrystallinity of the samples gradually increases when thesynthesis duration increases from 5 to 60min. It was observedthat insignificant change in relative crystallinity was foundbetween the samples synthesized at 60 and 1440min, whichdemonstrated that optimum crystallinity was obtained at syn-thesis duration of 60min. This result was in good agreementwith the results reported by Venna and Carreon (2009), wherethe crystallinity increased with the synthesis duration dueto the Ostwald ripening effect. Besides, synthesis durationof 5min was insufficient for fully growth of ZIF-8 particles.Therefore, in the subsequent experiments, ZIF-8 samples weresynthesized using synthesis solution with different molarratios of Zn2þ:Hmim:MeOH at constant synthesis durationof 60min and pH 7.4.

Table 3 shows the relative crystallinity of the samplessynthesized using different solution molar ratio Zn2þ:Hmim:MeOH of 1:7.9:86.7, 1:7.9:695, and 1:7.9:1002. It

can be seen from Table 3 and Figure 3 that the crystallinityof ZIF-8 reduced gradually when the molar ratio of the syn-thesis solution changed from 1:7.9:86.7 to 1:7.9:695 and1:7.9:1002, which is consistent with the result reported byKeser (2012). This results show that highly crystallineZIF-8 particles were formed using lower amount of MeOH.This can be attributed to the presence of higher

Fig. 5. FE-SEM images of ZIF-8 particles synthesized at differentsynthesis duration: a) sample A (5min); b) sample B (60min); andc) sample C (1440min) at molar ratio Zn2þ:Hmim:MeOH of1:7.9:695 and pH 7.4.

Table 2. Relative crystallinity of ZIF-8 samples synthesized atmolar ratio Zn2þ:Hmim:MeOH of 1:7.9:695 with different syn-thesis duration and constant pH 7.4

Sample Synthesis duration (min) Relative crystallinity (%)

A 5 40B 60 97C 1440 100�

�The reference peak.

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concentration of Zn2þ and meIm� as the reactants in syn-thesis solution, which promoted the crystal growth of theparticles.

On the other hand, Table 4 shows the relative crystallinityof ZIF-8 samples synthesized using different solution pH.Referring to Table 4 and Figure 4, ZIF-8 sample obtainedfrom synthesis solution with pH 7.4 demonstrates highercrystallinity as compared to ZIF-8 sample obtained fromsynthesis solution with pH 8.2. These results were differentfrom the results reported by Cravillon et al. (2011), wheresimilar crystallinity was shown for ZIF-8 synthesized withand without NaCOOH. This might due to the turbulencecreated under stirring in our experiment causing all theZIF-8 particles aggregates and inter-grow with each other.

Furthermore, the role played by NaCOOH as a strongdeprotonator (Cravillon et al. 2011), might affect the growingof the particles into indistinguishable features. Therefore,lower crystallinity was observed. Although lower pH mightpromote the crystallization of the frameworks of ZIF-8(Venna et al. 2010), in the present work, the crystallinity ofthe particles were affected when HCl presented in the syn-thesis solution with pH 6.8. This might be due to the com-petence between the protons from HCl and the Zn2þ fromthe zinc source in the solution for the meIm- ion. The absenceof the deprotonated ligands limited the crystal growth of theparticles with the neutral charge on the surface of the parti-cles. Therefore, the formation of ZIF-8 was affected.

3.1.2 Field Emission Scanning Electron Microscopy(FE-SEM)

FE-SEM images of ZIF-8 samples synthesized at differentsynthesis durations are shown in Figure 5. The average par-ticle sizes for all the samples were summarized in Table 5.ZIF-8 synthesized at 5min shows spherical shape whereasthe other two samples which synthesized at 60 and1440min show rhombic dodecahedron shape. However,the size of the particles exhibited negligible changes for all

Fig. 6. XRD patterns of ZIF-8 synthesized at different duration: a) sample A (5min); b) sample B (60min); and c) sample C(1440min) at molar ratio Zn2þ:Hmim:MeOH of 1:7.9:695 and pH 7.4.

Table 3. Relative crystallinity of ZIF-8 samples synthesized atsynthesis duration of 60min with different molar ratioZn2þ:Hmim:MeOH and pH 7.2–7.6

SamplesZn2þ:Hmim:MeOH

molar ratiosRelative

crystallinity (%)

E 1:7.9:1002 16B 1:7.9:695 47D 1:7.9:86.7 100�

�The reference peak.

Table 4. Relative crystallinity of ZIF-8 samples synthesized atsynthesis duration of 60min and molar ratio Zn2þ:Hmim:MeOH of 1:7.9:695 with pH variations

Samples Solution pH Relative crystallinity (%)

G 8.2 25F 6.8 62B 7.4 100�

�The reference peak.

Table 5. Average particle size for ZIF-8 samples synthesized atdifferent synthesis conditions

Samples Parameter variations Average particle size (nm)

A 5mina �77B 60mina �80C 1440mina �80D 1:7.9:86.7b �330E 1:7.9:1002b �40F pH 6.8c �70G pH 8.2c N=AaVariation of synthesis duration.bVariation of molar ratios Zn2þ:Hmim:MeOH.cVariation of solution pH.

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three samples and average particle size of �80 nm wasobtained (Table 5). These results were inconsistent with theresults reported by Venna et al. (2010), where the particlesize of ZIF-8 were increased from �50 nm, �230 nm, and�500 nm at synthesis duration of 10, 30, and 720min,respectively, at room temperature using solution with molarratio Zn2þ:Hmim:MeOH of 1:8:705. Larger sizes of ZIF-8particles could not be obtained in the present work althoughthe synthesis duration was increased from 5 to 60min and1440min. This might be due to the limited solubility of reac-tants in MeOH that restricted the particles growth. Figure 7shows the FE-SEM images of ZIF-8 samples synthesized atdifferent molar ratios Zn2þ:Hmim:MeOH of 1:7.9:86.7 and1:7.9:1002. The particle size of the samples decreases from�330 nm to �40 nm (Table 5). Larger particles with therhombic dodecahedron shape were apparently observed forsample D (Figure 7a), indicates the production of highlycrystalline particles using molar ratio Zn2þ:Hmim:MeOHof 1:7.9:86.7. Smaller particle sizes of �80 nm and �40 nmwere observed for samples synthesized at molar ratioZn2þ:Hmim:MeOH of 1:7.9:695 (Figure 5b) and 1:7.9:1002(Figure 7b), respectively. These results were correlated wellwith the XRD patterns (Figure 3), where high relative crys-tallinity resulted in larger particle size. Besides, the resultswere consistent with the results reported by Keser (2012),

whereby the particle size increases when the amount ofMeOH presented in the synthesis solution decreases(890 nm and 138 nm based on synthetic molar ratioZn2þ:Hmim:MeOH of 1:7.9:86.9 and 1:7.9:1042.7, respect-ively). This was due to the fact that, lower amount of MeOHreduced the number of nuclei formed and stimulated the for-mation of larger crystal, which caused by higher concentra-tions of Zn2þ and meIm-. In contrast, higher amount ofMeOH present in the synthesis solution resulted in the for-mation of smaller crystals attributed to relatively largernucleation rate than the crystal growth rate at lower concen-trations of Zn2þ and meIm�(Keser 2012).

Figure 8 shows the FE-SEM images of ZIF-8 particlesformed using the synthesis solutions of pH 6.8 and pH 8.2.It was found that spherical ZIF-8 particles with smaller size(�70 nm) were formed using synthesis solution of pH 6.8with the addition of HCl. On the other hand, ZIF-8 particlessynthesized in the presence of NaCOOH were unaccountabledue to particles aggregation. The results were consistent withXRD pattern (Figure 4), where ZIF-8 obtained from lowersolution pH exhibiting spherical features and ZIF-8 synthe-sized using higher solution pH causing indistinguishable fea-tures that resulted in lower crystallinity. Although theformation of well-defined and distinguishable rhombicdodecahedron ZIF-8 microcrystals (�1 mm) was reportedby Cravillon et al. (2011) using synthesis solution in the

Fig. 7. FE-SEM image of ZIF-8 particles synthesized at differ-ent molar ratio Zn2þ:Hmim:MeOH: a) sample D (1:7.9:86.7)and b) sample E (1:7.9:1002) at synthesis duration of 60minand pH 7.2 to 7.6.

Fig. 8. FE-SEM image of ZIF-8 particles synthesized at differ-ent pH: a) sample F (pH¼ 6.8) and b) sample G (pH¼ 8.2) atsynthesis duration of 60min and molar ratio Zn2þ:Hmim:MeOH of 1:7.9:695.

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presence of the NaCOOH, the result obtained in the presentwork is different with their finding. This might due to theturbulence effect caused by the stirring process in our experi-ments that affected the aging process for the particles.

3.2 Nitrogen Physisorption

The nitrogen adsorption-desorption isotherms for the ZIF-8particles synthesized using different molar ratios are shownin Figure 9, which displayed type I isotherms. The increasein the volume adsorbed at low relative pressure indicatingthe presence of micropores while second adsorption at highrelative pressure indicating the presence of meso- andmacroporosity by the packing of the particles (Liu et al.2013). The results were in good agreement with reported iso-therm for ZIF-8 (Pan et al. 2011; Venna 2010). However,hysteresis loop of type H2 was observed for samples B andE. This result shows that there was a difference between con-densation and evaporation processes occurring in the poresand the networks (Sing et al. 1985). Therefore, ZIF-8 parti-cles synthesized at the molar ratio Zn2þ:Hmim:MeOH of1:7.9:695 and 1:7.9:1002 might consist both micro- andmesoporosity (Cravillon et al. 2011). Table 6 shows theBrunauer–Emmett–Teller (BET) surface area for ZIF-8

samples synthesized at different molar ratios (1344m2=g,1386m2=g and 1266m2=g for samples D, B, and E, respect-ively). The results were not in the same trend as reported byKeser (2012), which were 1143m2=g, 1192m2=g, and1309m2=g for synthetic molar ratios of 1:7.9:86.9, 695.1,and 1042.7, respectively. Nevertheless, BET surface areasolely cannot be represented for true surface area of a micro-porous adsorbent since it does not consider the possibilityfor the filling of the adsorbate into the micropores or thecavities of the samples (Sing et al. 1985). Therefore, externalsurface areas were measured with the slope of the isotherm.The external surface areas obtained were in trend as reportedby Keser (2012), which were 35.43, 128.54, and 131.75m2=gfor samples D, B, and E (46m2=g, 130m2=g, 213m2=g forsamples obtained from solution molar ratios of 1:7.9:86.9,1:7.9:695.1, and 1:7.9:1042.7, respectively). The increase inexternal surface areas was in compatible with the decreasein the particle sizes of the samples. Besides, the pore volumesfor the ZIF-8 samples were higher than the results reportedby Zhang et al. (2011) (0.45 cm3=g for micropore volume and0.54 cm3=g for total pore volume) and were near to the idealvalues reported by Park et al. (2006) (0.663 cm3=g for micro-pore volume). In comparison among the samples, there wasan increase in total pore volume at a relative pressure of 0.99with the increase in external surface areas. These results sug-gested that the interparticle porosity between the ZIF-8nanoparticles were increased with the decrease of the particlesizes (Kida et al. 2013), and, thus, decrease the microporevolumes of the samples. This might be due to the presenceof amorphous materials for the ZIF-8 samples synthesizedat molar ratio Zn2þ:Hmim:MeOH 1:7.9:1002, which wereconsistent with the relative crystallinity as shown inFigure 3. Amorphous materials refer to some residual spe-cies presented in the cavities, such as unreacted Hmim, thatcaused by the low synthesis temperature or solvent system(Pan et al. 2011). Therefore, reduction in particle size didnot increase the BET surface area of the resultant particlesmost probably due to the decrease in the crystallinity ofthe samples.

Fig. 9. Nitrogen adsorption-desorption isotherms for samples B, D, and E.

Table 6. Surface area and porosity characteristics of ZIF-8samples

Samples

Zn2þ:Hmim:MeOH

molar ratios

BETsurfacearea

(m2=g)

Externalsurfaceareaa

(m2=g)

Microporevolumea

(cm3=g)

Totalpore

volumeb

(cm3=g)

D 1:7.9:86.7 1344 35.43 0.635 0.68B 1:7.9:695 1386 128.54 0.614 1.07E 1:7.9:1002 1266 131.75 0.556 1.07at plot method.bAt P=P0� 0.99.

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3.3 CO2 Adsorption Measurement

CO2 physisorption analysis was used to measure the physi-sorbed CO2 on the surface of the ZIF-8 nanoparticles atroom temperature at low pressure (0–1 bar). Table 7 showsthe amount of CO2 adsorbed among ZIF-8 samples synthe-sized at different molar ratios Zn2þ:Hmim:MeOH. CO2

adsorbed amount of 0.53mmol=g, 0.55mmol=g and0.47mmol=g were obtained for sample D, sample B andsample E, respectively. The results obtained were lower thanthe reported result (1.02mmol=g at 1 bar (Yazaydın et al.2009)). However, ZIF-8 sample with smaller particle size(sample B) has slightly higher CO2 uptake as compared tothe ZIF-8 with larger particle size (sample D). This resultis consistent with the BET and external surface areas ofthe particles, except for sample E with relatively low crystal-linity. Hence, sample E exhibited lowest CO2 uptakealthough it possessed smallest particle size and high externalsurface areas. This was mainly due to its insufficientstructural evolution, which can be observed from its XRDpatterns (Figure 3a).

The CO2 adsorption mechanism on ZIF-8 can beexplained by the grand canonical Monte Carlo (GCMC)simulations reported by Liu et al. (2013). CO2 gas moleculeswere absorbed on ZIF-8 through the three methyl rings andthe six imidazole rings. Therefore, electron-deficient CO2

molecules tend to interact with the p-electrons on the imi-dazole ring of ZIF-8. Furthermore, Amrouche et al. (2011)reported that, CO2 possessed quadrupolar moment thatcan be easily attracted by ZIF-8, which attributed to thedispersion-repulsion and multipolar interaction betweenZIF-8 and CO2 molecules.

4. Conclusions

In summary, the XRD pattern showed that all the samplessynthesized in the present work demonstrated ZIF-8 struc-ture regardless to the synthesis duration, molar ratio ofZn2þ:Himim:MeOH in the synthesis solution and solutionpH. Referring to XRD patterns, all major peaks obtainedwere matching well with the XRD patterns of ZIF-8reported by Park et al. (2006).

FE-SEM images showed that ZIF-8 particles synthesizedusing lower amount of MeOH produced larger particle sizeof ZIF-8. Besides, ZIF-8 particles synthesized at longer dur-ation (60 and 1440min) exhibit higher crystallinity withrhombic dodecahedron structure while particles synthesizedat very short duration (5min) showed spherical features.

Presence of NaCOOH in the synthesis solution caused theparticles to agglomerate and resulted in difficulty in particlesize measurement. Addition of HCl in the synthesis solutionresulted in smaller particles size which was particularlyinteresting for further investigation.

CO2 adsorption results showed that particles with variesparticle sizes do not have significant effect on the CO2

adsorption and this result was consistent with the BET sur-face area results. This was mainly attributed to the crystal-linity of the samples. Although the particle sizes reduced,the crystallinity of the resultant ZIF-8 was also reduced.The surface areas might be off-set by the decrease in thecrystallinity of the sample and, thus, reduction in particlesize could not increase the BET surface area of the resultantparticles. Therefore, the CO2 adsorption for the samples didnot change much although the particle size reduced.

As a conclusion, ZIF-8 is a potential material in CO2

removal due to its high CO2 adsorption capability. Thus, itcan be used as an adsorbent in many technological applica-tions, such as pre-combustion and post-combustionprocesses, in order to reduce greenhouse effect. Besides, itcan be used for the removal of bulk CO2 from natural gasto produce high quality fuel gas.

Funding

The financial and technical supports provided by CO2

Management (MOR) research group, Universiti TeknologiPETRONAS and Ministry of Education (Higher EducationDepartment) under MyRA Incentive Grant for CO2 RichNatural Gas Value Chain Program are duly acknowledged.

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