Synthesis of Hierarchically Structured Hybrid Materials by Controlled

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Synthesis of hierarchically structured hybrid materials by controlled self-assembly of metal-organic framework with mesoporous silica for CO2 adsorption Chong Chen, Bingxue Li, Lijin Zhou, Zefeng Xia, Nengjie Feng, Jing Ding, Lei Wang, Hui Wan, and Guofeng Guan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08117 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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ACS Applied Materials & Interfaces

Synthesis of Hierarchically Structured Hybrid Materials by Controlled Self-Assembly of Metal-Organic Framework with Mesoporous Silica for CO2 Adsorption Chong Chen,a Bingxue Li,a Lijin Zhou,b,* Zefeng Xia,a Nengjie Feng,a Jing Ding,a Lei Wang,a Hui Wan,a and Guofeng Guana,* a

State Key Laboratory of Materials-Oriented Chemical Engineering, College of

Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, P. R. China b

Yangzi Petrochemical Company Ltd., Sinopec, Nanjing 210048, P. R. China

*Corresponding author Telephone: +86-25-8358 7198 E-mail: [email protected] (Lijin Zhou) E-mail: [email protected] (Guofeng Guan)

Keywords:

Metal-Organic

Framework;

Mesoporous

Silica;

Self-Assembly;

Hierarchical Structure; Composite; CO2 Adsorption

Abstract The HKUST-1@SBA-15 composites with hierarchical pore structure were constructed by in situ self-assembly of metal-organic framework (MOF) with mesoporous silica. The structure directing role of SBA-15 had an obvious impact on the growth of MOF crystals, which in turn affected the morphologies and structural properties of the composites. The pristine HKUST-1 and the composites with different contents of SBA-15 were characterized by XRD, N2 adsorption-desorption, SEM, TEM, FT-IR, TG, XPS, and CO2-TPD techniques. It was found that the composites were assembled by oriented growth of MOF nanocrystals on the surfaces of SBA-15 matrix. The interactions between surface silanol groups and metal centers induced structural changes and resulted in the increases in surface areas as well as micropore

ACS Paragon Plus Environment


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volumes of hybrid materials. Besides, the additional constraints from SBA-15 also restrained the expansion of HKUST-1, contributing to their smaller crystal sizes in the composites. The adsorption isotherms of CO2 on the materials were measured and applied to calculate the isosteric heats of adsorption. The HS-1 composite exhibited an increase of 15.9% in CO2 uptake capacity compared with that of HKUST-1. Moreover, its higher isosteric heats of CO2 adsorption indicated the stronger interactions between the surfaces and CO2 molecules. The adsorption rate of the composite was also improved due to the introduction of mesopores. Ten cycles of CO2 adsorption-desorption experiments implied that the HS-1 had excellent reversibility of CO2 adsorption. This study was intended to provide the possibility to assemble new composites with tailored properties based on MOF and mesoporous silica to satisfy the requirements of various applications.

1. Introduction Nowadays, the design and development of multifunctional materials with right properties for various applications is regarded as the main research focus in the field of materials science. In this context, the metal-organic frameworks (MOFs) have received broad attentions and research during the last decades not only due to their high surface areas and porosities, but especially for their tunable pore structures and functionalities.1,2 By selecting suitable metal ions/clusters and organic linkers, a wide range of MOFs have been constructed.3 Moreover, the properties of MOFs can be further modulated through post-synthetic modification (PSM) method,4 which provides more flexibility for them. These features give rise to the potential applications of MOFs in multiple areas.5,6 The excessive emission of greenhouse gases into the atmosphere, which has caused global climate change, is becoming one of the most serious environmental problems needed to be resolved urgently.7 For a long time, many efforts have been devoted to control this emission.8,9 However, the amount of atmospheric greenhouse gases keeps increasing as the large-scale burning of fossil fuels are still demanded for the rapid developments of economy and industry.10 Given that carbon dioxide (CO2)


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is the main cause of greenhouse effect,11 more attentions should be paid to reduce CO2 concentration. Among various technologies12,13 applied to capture CO2, the solid adsorption processes are more favorable for long-term use due to their lower loss and energy consumption in regeneration.14,15 Until now, the studies dedicated to finding solid materials as promising CO2 adsorbents are still in the period of developing. Many porous adsorbents such as activated carbon, zeolite, mesoporous silica, and MOFs have been widely reported in recent years.16-18 It is generally considered that the MOFs are more competitive with traditional adsorbents owing to their relatively high CO2 adsorption capacities. Despite this, the poor hydrothermal/water stabilities of MOFs and the weak interactions between CO2 molecules and the frameworks remain major challenges for practical applications.19,20 The chemical modification of MOFs, typically by introducing some basic species, can make improvements in CO2 capture.21,22 However, this process is often accompanied by the decreases in surface areas and pore volumes.23 The recent research is mainly concentrated on the fabrication of composites based on MOFs and other functional materials including polyoxometalates, carbon nanotubes, graphite oxides, polymers, metal nanoparticles, and mesoporous silicas.24-28 Such MOF-based composites can mitigate the weaknesses of pure MOFs and even exhibit better performances than both of the individual components. Liu et al.29 synthesized the composites of graphene oxide (GO) and Cu-BTC (BTC = benzene-1,3,5-tricarboxylate), and found that the materials containing GO had improved porosity and well-dispersed MOF nanocrystals, resulting in higher CO2 adsorption capacity than Cu-BTC. Besides, the incorporation of GO layers could also afford MOFs with higher surface areas and stronger dispersive forces to bind CO2 molecules.30 These studies present another alternative to construct new materials with enhanced CO2 capture capacities. To date, most of the MOFs reported are restricted within the microporous range. Although these micropores enable high adsorption capacities toward small molecule gases, the gas diffusions are limited as well.31 More recently, the MOF and mesoporous silica composites are beginning to emerge and arousing a lot of research interests due to their hierarchical micro- and mesoporous structure.32-34 To assemble



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such composites, SBA-15 and MCM-41 with highly ordered channels, adjustable pore sizes and high thermal stabilities are commonly applied as matrixes. The incorporation of mesoporous silica provides the composites with additional mesopores, which are beneficial for diffusion and mass transfer. On the other hand, the hydrothermal stabilities and mechanical properties of MOFs can also be improved.35 Chakraborty et al.36 reported the preparation of Mg-MOF-74@SBA-15 hybrids and proved that the MOF nanocrystals were present inside the mesopores of SBA-15. The integration of both components also gave rise to combined features of the composites. However, the composites showed lower CO2 uptake than that of Mg-MOF-74. Tari and co-workers37 synthesized the MCM-41@Cu(BDC) (BDC = benzene-1,4-dicarboxylate) composites by a microwave assisted approach. The formation of Cu(BDC) in MCM-41 mesopores led to the generation of new micropores and the increased surface area of MCM-41, and consequently resulted in an increase in CO2 adsorption capacity compared with MCM-41. The mesoporous silica-MOF composites are expected to make a breakthrough in CO2 capture. Nevertheless, the research of this field is still in its infancy. Many reported synthetic routes are complicated, which require the pre-functionalization of silica or a stepwise procedure.38 The uneven dispersion and loose attachment of MOF crystals on the surfaces of silica substrate may lead to the decreases in textural properties and CO2 adsorption capacities.39 Moreover, the potential role of mesoporous silica as the structure directing agent to modulate the growth of MOF crystals and the applications of such composites in adsorption have been rarely investigated. Thus, the controllable synthesis of mesoporous silica-MOF composites while ensuring their performances is the major objective to be achieved. Herein, we present a simple in situ method to fabricate the composites of HKUST-1 (HKUST = Hong Kong University of Science and Technology) and SBA-15, and investigate the roles of SBA-15 in the formation of HKUST-1. A small amount of SBA-15 without modification is directly mixed with MOF precursors and used for synthesis. Then, the composites can be obtained with ordered morphology and hierarchical structure. This implies that SBA-15 is not only an integral part of the


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composites, but also serves as a structure directing template that controls the oriented growth of MOF nanocrystals on its surfaces through the interactions between surface silanol groups and metal centers. The crystal sizes of HKUST-1 and the textural properties of composites can be tuned by controlling the contents of SBA-15. Moreover, the synergistic effect between HKUST-1 and SBA-15 can further induce the formation of new micropores. The as-synthesized HKUST-1@SBA-15 composites were characterized and applied for CO2 adsorption. The adsorption isotherms and kinetic data of CO2 on the materials were measured, and the isosteric heats of adsorption

were

also

estimated.

Finally,

consecutive

cycles

of

CO2

adsorption-desorption experiments were performed to evaluate the reversibility of CO2 adsorption on the composites.

2. Experimental 2.1 Materials Copper nitrate trihydrate (Cu(NO3)2·3H2O), benzene-1,3,5-tricarboxylic acid (H3BTC), pluronic P123 triblock copolymer (EO20PO70EO20, Mw=5800), tetraethyl orthosilicate (TEOS) and hydrochloric acid (HCl) were obtained from Sigma-Aldrich and used as received unless otherwise stated. Other solvents were also commercially available and all reagents were of analytical grade. 2.2 Adsorbent synthesis 2.2.1 Synthesis of HKUST-1 Pure HKUST-1 was prepared by a slightly modified solvothermal method according to our previous work.40 Generally, Cu(NO3)2·3H2O (2.174 g) and H3BTC (1.05 g) were respectively dissolved into deionized water (30 mL) and ethanol (30 mL) to form homogeneous solutions. After that, the two solutions were fully mixed under vigorous stirring for 30 min. The mixture was then placed into a 100 mL Teflon-lined autoclave and heated in an oven at 120 oC for 12 h. After cooling naturally, the supernatant was removed and the blue crystals were collected. The resultant was washed by ethanol for several times and dried under vacuum at 60 oC overnight to obtain the final product.



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2.2.2 Synthesis of SBA-15 SBA-15 was synthesized referring to a reported procedure with minor modifications.41 First, 4.0 g of P123 was added into 120 mL of hydrochloric acid aqueous solution (2.4 mol/L) and stirred for 4 h at 40 oC to obtain a clear and homogeneous solution. Second, 8.5 g of TEOS was added dropwise with stirring and the resulting mixture was allowed to react at 40 oC for another 24 h. Then the temperature was increased to 80 oC, and the abovementioned mixture was aged for 48 h under static conditions. The white precipitates were filtered, washed thoroughly with deionized water, and dried at 60 oC for 12 h. Finally, the P123 template was removed by calcining the SBA-15 precursor at 550 oC for 6 h. 2.2.3 Synthesis of HKUST-1@SBA-15 HKUST-1@SBA-15 composites were prepared via an in situ solvothermal method. Typically, a certain amount of SBA-15 was dispersed in copper nitrate aqueous solution and the turbid mixture was sonicated for 30 min. Subsequently, the well-dissolved ethanol solution of H3BTC was added, and the suspension was stirred violently for another 30 min before transferring into an autoclave. The synthesis of HKUST-1@SBA-15 composites followed the same steps as described for HKUST-1. To be specific, the SBA-15 amounts added during preparation of the composites were 0.5, 1, 1.5, 2, 2.5, and 3 wt.% (based on the mass of metal precursor) respectively, and the synthesized hybrid materials were correspondingly labeled as HS-0.5, HS-1, HS-1.5, HS-2, HS-2.5, and HS-3. 2.3 Adsorbent characterization Powder X-ray diffraction (XRD) patterns were recorded on a SmartLab diffractometer (Rigaku, Japan) using Cu Kα radiation and the data from 5o to 40o were collected with a step size of 0.02o in 2θ. N2 adsorption-desorption isotherms were measured on a Micromeritics ASAP 2020 instrument at -196 oC to obtain the Brunauer-Emmett-Teller (BET) surface areas, pore volumes, and pore size distributions of the samples. Before measurement, the samples were pretreated at 150 o

C for at least 6 h under vacuum. Scanning electron microscopy (SEM) images of the

samples were observed using a Hitachi S-4800 field-emission scanning electron


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microscope after gold deposition. Transmission electron microscope (TEM) images were acquired with a JEOL JEM-2100 high-resolution transmission electron microscope. The samples were well dispersed in ethanol by sonication before daubing onto a copper-supported carbon film. Fourier transform infrared (FT-IR) spectra in the wavenumber range of 2000-500 cm-1 were obtained using a Thermo Nicolet 6700 spectrometer (United States). The specimens were fully dried and then mixed with anhydrous

potassium

bromide.

To

investigate

the

thermal

behavior,

the

thermogravimetry (TG) curves from 30 to 600 oC were carried out on a STA 409PC thermogravimetric analyzer under a continuous N2 flow with a heating rate of 10 o

C/min. X-ray photoelectron spectroscopy (XPS) technique was applied to verify the

interactions between surface silanol groups and metal centers, and the analyses were conducted on a PHI-5000 Versa Probe system equipped with a monochromatic Al Kɑ radiation source (1486.6 eV). The C 1s peak was used as internal reference to calibrate the binding energies. CO2 temperature programmed desorption (CO2-TPD) technique was applied to evaluate the activation energy of CO2 desorption from the samples.30 The experiments were performed on a Micromeritics Auto Chem II 2920 instrument installed with a thermal conductivity detector (TCD). Each time, about 30 mg of the sample was placed into a flow-through quartz tubular reactor, and then it was pretreated under a He atmosphere (30 mL/min) at 150 oC for 2 h. After cooling down to room temperature, pure CO2 at a flow rate of 30 mL/min was absorbed at 50 o

C for 30 min. The sample was then flushed with He flow (30 mL/min) for another 30

min to remove the excessive and weakly adsorbed CO2. Finally, the CO2 desorption measurement was carried out from 50 to 250 oC in He at a heating rate of 10 oC/min. The amounts of CO2 desorbed from the sample were recorded while the effluent curve of CO2 was obtained. 2.4 Adsorption and kinetics measurements of CO2 CO2 adsorption experiments were performed on a BELSORP-max instrument (MicrotracBEL, Japan) using a static volumetric method under low pressure (0-1.0 bar). Before each experiment, approximately 0.1 g of sample was outgassed at 150 oC for 6 h under vacuum. The CO2 adsorption isotherms at different temperatures (25-60



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o

C) were measured by setting the temperature of circulated water. The adsorption of CO2 on the sample at 25 oC was executed multiple times to

assess the reversibility of adsorption process. The sample was desorbed at 150 oC for 6 h prior to each cycle and the corresponding data were collected. The adsorption kinetic data of CO2 were obtained using an Intelligent Gravimetric Analyser (IGA-100, Hiden) at 25 oC and 1.0 bar. When testing, 0.05 g of pretreated sample (150 oC, 4 h) was used for pure CO2 adsorption and the weight changes of the sample over time were recorded until reaching equilibrium.

3. Results and discussion 3.1 Structure and morphology of the adsorbents Powder XRD patterns of HKUST-1 as well as the composites HS-0.5, HS-1, HS-1.5, HS-2, HS-2.5, and HS-3 are depicted in Figure 1. The main characteristic peaks of bare HKUST-1 matched well with those reported in literatures.40,42 In comparison, the composites exhibited the similar patterns, indicating that the introduction of SBA-15 did not prevent the growth of HKUST-1 component and the crystals were successfully formed on the surfaces of SBA-15. It was noticed that no new peaks from SBA-15 (Figure S1) were observed, which was attributed to the high dispersion and relatively low contents of SBA-15 in the composites. In addition, the intensities of peaks of the composites gradually decreased with increasing addition of SBA-15. The reason was that the insertion of high content of SBA-15 might cause the slight distortion of framework and decrease in crystallinity. The crystal sizes of HKUST-1 were estimated by Scherrer equation using the most prominent peak and the results are given in Table S1. The calculation revealed that the introduction of SBA-15 could reduce the average crystal size of HKUST-1 in the composites, and the value decreased while the SBA-15 contents increased.


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(g)

(f)

(e) Intensity (a.u.)

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(d)

(c)

(b) (a) 5

10

15

20

25

30

35

40

2 Theta (degree)

Figure 1. Powder XRD patterns of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, and (g) HS-3.

N2 adsorption-desorption measurements of HKUST-1, SBA-15, and the HKUST-1@SBA-15 composites were carried out to evaluate their textural properties, and the isotherms are displayed in Figure 2. HKUST-1 (Figure 2(a)) exhibited typical type I isotherms, which was an indication of microporous structure. The BET surface area of HKUST-1 was about 1466 m2/g, a moderate value in the range of the reported ones.43,44 The isotherms of SBA-15 (Figure 2(h)) were shown to be of type IV classification with an H1-type hysteresis loop, suggesting the mesoporous character of the material. Moreover, the SBA-15 sample possessed narrow pore size distribution with a peak around 3.55 nm (Figure S2). It could be seen that all the composites revealed the combination of type I and type IV isotherms as evidence for the existence of both micropores and mesopores. In addition, the hysteresis loops originated from the mesopores became more and more prominent as the contents of SBA-15 increased. To further investigate the roles of SBA-15 on the growth of HKUST-1, the mesopore and micropore size distributions of samples are plotted in Figure 3 and Figure 4 respectively. The mesopore volumes of hybrid materials were much lower than that of



the intact SBA-15 (0.87 cm3/g), which indicated that the mesopores were partially occupied by HKUST-1 crystals. It could therefore be inferred the formation of MOF occurred on both internal and external surfaces of SBA-15. Besides, the mesopore volumes of the composites increased in accordance with the amount of SBA-15, implying that higher concentrations of SBA-15 might lead to insufficient utilization of the volumes. As seen from Figure 4(a), the pure HKUST-1 had the structural model of cavities with window sizes of ~0.56 nm and cage sizes of ~0.94 nm. The integration of HKUST-1 with SBA-15 resulted in higher micropore volumes of samples, suggesting that there was a synergistic effect between them.30 Indeed, simple physical mixing of the two components could only produce macropores (Figure S3 and Figure S4). For this reason, it was speculated that SBA-15 acted as a structure directing agent during the growth of MOF crystals,45 while the interactions between surface silanol groups and metal centers were responsible for the formation of new micropores.

3

Volume adsorbed (cm /g, STP)

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0.0

(a) (b) (c) (d) (e) (f) (g) (h) 0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Figure 2. N2 adsorption-desorption isotherms of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, (g) HS-3, and (h) SBA-15.


(a) (b) (c) (d) (e) (f) (g)

-1 -1 3

5

10

15

20

Pore diameter (nm)

Figure 3. Mesopore size distributions of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, and (g) HS-3 obtained by BJH method.

(a) (b) (c) (d) (e) (f) (g)

3

-1

-1

Pore volume (cm g nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pore volume (cm g nm )

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0.5

0.6

0.7

0.8

0.9

1.0

Pore diameter (nm)

Figure 4. Micropore size distributions of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, and (g) HS-3 obtained by MP method.



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To monitor the structure changes arised from the different addition amount of SBA-15, the BET surface areas, total pore volumes, and micropore volumes of the samples and the physical mixture with the same composition as HS-1 were all calculated and listed in Table 1. It was obvious that the HS-1 had better structural properties than other composites, with a surface area of 1745 m2/g and a total pore volume of 0.74 cm3/g. Besides, the micropore volume of the physical mixture was much lower than that of HS-1, and this value was even lower than pure HKUST-1. Further increase in the content of SBA-15 was not favorable as the number of silanol groups exceeded the amount of metallic species that they could react with. On the other hand, excessive SBA-15 would possibly lead to the distorted crystal structure of MOF, confirmed by the decreases in surface areas and pore volumes. Table 1. Pore structure parameters of HKUST-1 and HKUST-1@SBA-15 composites. BET surface area (m2/g)

Total pore volume (m3/g)

Micropore volume (m3/g)

HKUST-1

1466

0.60

0.59

HS-0.5

1575

0.68

0.65

HS-1

1745

0.74

0.70

HS-1.5

1619

0.69

0.66

HS-2

1581

0.68

0.64

HS-2.5

1503

0.66

0.62

HS-3

1458

0.64

0.60

SBA-15

680

0.87

–

Mixture

1408

0.59

0.57

Sample

SEM and TEM images were applied to study the morphologies of HKUST-1, the HS-n composites, and SBA-15, which are presented in Figure 5. The as-synthesized HKUST-1 exhibited regular octahedral-shaped crystals with the particle size around 10 µm. As shown in Figure 5(f), the SBA-15 particles had a rod-like morphology, while the width and length of these nanorods ranged from 1-2 µm and 5-10 µm respectively. However, it was noticed that the composites showed a quite different


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morphology from both of them. This indicated that the incorporation of SBA-15 rods significantly affected the structure of the resultant composites. The hybrid materials were observed to have similar particle size to that of SBA-15, whereas the nanorods were simultaneously embedded with a series of lamellate grains. There was no doubt that these well-arranged platelets were HKUST-1 nanocrystals, as confirmed by the results of XRD. For HS-1 (Figure 5(b) and (c)), most of the platelets were rectangular in shape with the dimension of ~600 nm in width and ~250 nm in thickness. With the increasing content of SBA-15 (Figure 5(d) and (e)), the particle size of the nanocrystals further decreased, but the structure of composites tended to become disordered. According to the analysis above, it was concluded that the transition of morphology and reduction of particle size could be achieved by simply adjusting the concentration of SBA-15 in the composites. The coordination effect between the -OH groups of SBA-15 and the Cu(II) metal ions of HKUST-1 made for the oriented crystal growth of MOF, followed by the attachment of platelets at the silica surface. Afterwards, the composites were assembled with regular and ordered structure under the distortion force exerted by SBA-15. Moreover, the SBA-15 placed additional constraints on the degrees of freedom during the further expansion of HKUST-1 framework, resulting in smaller size of the crystals.45 The growth process of the hybrid materials was discussed in more detail below. The TEM images illustrated in Figure 5(g), (h), and (i) offer a more intuitive insight into the phase distributions of HKUST-1 crystals in SBA-15. The highly ordered mesoporous structure of original SBA-15 was clearly observed in Figure 5(i). By contrast, the HS-1 composite (Figure 5(g) and (h)) exhibited a feature of stratified microstructure, suggesting the regular arrangement of MOF platelets on SBA-15 substrates. It was obvious that the HKUST-1 nanocrystals grew along a certain direction under the influence of silica directing species, constructing a hybrid material with ordered hierarchical structure. In addition, the porous structure of SBA-15 was well conserved during the formation of composites. These findings were in line with the results of XRD and N2 adsorption-desorption measurements.



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Figure 5. SEM images of (a) HKUST-1, (b and c) HS-1, (d) HS-2, (e) HS-3, (f) SBA-15 and TEM images of (g and h) HS-1, and (i) SBA-15.

Figure 6 shows the FT-IR spectra of all the samples. For HKUST-1 (Figure 6(a)), the bands around 1647 and 1375 cm-1 were respectively attributed to the asymmetric and symmetric stretching vibrations of C=O existed in the BTC ligands.40 The band at 1449 cm-1 represented the C=C skeletal ring vibration of aromatic groups. In the case of SBA-15 (Figure 6(h)), the intense band observed at 1085 cm-1 was assigned to the asymmetric stretching in the Si-O-Si from silicon-oxygen tetrahedral,41 while the weak band corresponding to the symmetric stretching was found at 804 cm-1. The characteristic vibrational band related to the Si-OH groups was seen around 969 cm-1. As presented in Figure 6(b-g), it was found that the FT-IR spectra of all the in situ synthesized HS-n composites were similar to that of pure HKUST-1. Although the intensity of two adsorption bands associated with Si-O-Si was relatively low, the bands could still be detected (Figure S5) as evidence for the presence of SBA-15 in composites.


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(h)

969

1085

804

(g) Transmittance (a.u.)

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(f) (e) (d) (c) (b)

(a)

1647

2000

1449

1500

1375

1000 -1

500

Wavenumber (cm )

Figure 6. FT-IR spectra of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, (g) HS-3, and (h) SBA-15.

The TG curves of HKUST-1, HS-1, and SBA-15 are given in Figure 7. For pure SBA-15, the weight loss before 100 oC was related to the removal of adsorbed water. This was followed by a small weight loss at 100-600 oC, which could be ascribed to the dehydration of silanol groups. The TG profiles of HKUST-1 and the composite were very similar and both of them could be divided into three weight loss stages. The first one corresponding to the losses of adsorbed water and solvent molecules was observed at temperature lower than 100 oC. Then, the departure of coordinated water occurred, causing a weight loss between 100-250 oC. The final weight losses of ~34.50 wt.% for HKUST-1 and ~33.53 wt.% for HS-1 at the temperature range of 250-400 oC were attributed to the decomposition of BTC ligands as well as the collapse of MOF structure. Considering that the SBA-15 moiety showed almost no weight loss at this stage, the introduced SBA-15 contents in the composites could be calculated by comparing the percentages of weight losses with that of HKUST-1. Under this point, the content of SBA-15 in HS-1 was 2.81 wt.%, and the other results are displayed in Figure S6 and Table S2. It was found that the decomposition



temperature of HS-1 was slightly higher in comparison with that of HKUST-1, demonstrating a better thermal stability of the composite due to the incorporation of SBA-15. (c)

100

90

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

80

~34.50 wt.%

~33.53 wt.%

70

(b) 60

(a) 50

40 100

200

300

400

Temperature (

500

600

)

Figure 7. TG curves of (a) HKUST-1, (b) HS-1, and (c) SBA-15.

The XPS spectra of Cu 2p for HKUST-1 and HS-1 are given in Figure 8. As shown in Figure 8(a), two main peaks related to the Cu 2p3/2 and Cu 2p1/2 levels were observed at 933.9 eV and 953.8 eV respectively, indicating that the Cu element in HKUST-1 was mainly existed in the form of divalent Cu(II).46 Moreover, these Cu(II) species were further verified by the presence of “shake-up satellite” peaks.47 After the introduction of SBA-15, the binding energies of Cu 2p3/2 and Cu 2p1/2 in HS-1 composite shifted to lower values of 933.3 eV and 952.8 eV, respectively. This might be attributed to the change in coordination environment of Cu(II) centers.48 The surface silanol groups with lone-pair electrons coordinated to Cu(II) ions, which further led to an increase in electron density of the metal centers.


Page 17 of 38

933.3

952.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


933.9 953.8

(a) (b) 965

960

955

950

945

940

935

930

925

Binding Energy (eV)

Figure 8. The Cu 2p XPS spectra of (a) HKUST-1, and (b) HS-1.

To evaluate the interaction force between CO2 molecules and the adsorbents, the TPD measurements were performed and the results are shown in Figure 9. The CO2 desorption peaks of HUKST-1 and HS-1 were 391.1 K and 396.7 K, respectively. Clearly, the peak temperature (Tp) of HS-1 shifted slightly to a higher value, indicating that the desorption activation energy needed was higher. In other words, the interaction between CO2 and the surfaces of the composites were stronger than that of HKUST-1. Therefore, it could be inferred that the combination of HKUST-1 with SBA-15 was conductive to enhancing the binding capacity toward CO2.



(a) (b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

360

Page 18 of 38

Tp=396.7 K

Tp=391.1 K

370

380

390

400

410

420

430

Temperature (K)

Figure 9. CO2-TPD spectra of (a) HKUST-1, and (b) HS-1.

3.2 Growth mechanism of HKUST-1@SBA-15 with a hierarchical structure In order to explore the possible formation mechanism of HKUST-1@SBA-15 composites, the HS-1 samples were synthesized at different times and the TEM images are collected in Figure 10. The corresponding theoretical models were also proposed based on the experimental results and all the analyses above. Briefly, the mechanism for the growth of MOF nanocrystals in the presence of SBA-15 might be associated with the coordination-driven self-assembly, which mainly involved three stages: coordination, oriented nucleation and growth, controlled assembly and attachment. First, when SBA-15 was dispersed into the metallic precursor solution, the metal ions diffused rapidly into the mesopores and then coordinated with the surface silanol groups (Figure 10(a)). As a result, a large number of metal seeds generated. On the second stage, these metal seeds further reacted with the BTC linkers, forming crystal nuclei on the substrates. During this process, the active silanol groups of SBA-15 acted as nucleation sites, which controlled the oriented nucleation of crystals. Then, the grains grew along certain directions and gradually connected together to form MOF platelets, as shown in Figure 10(b). In the final stage, the


Page 19 of 38

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controlled assembly and attachment of platelets occurred, contributing to the regular arrangement of MOF nanocrystals on SBA-15 matrix (Figure 10(c)). The introduction of mesoporous silica imposed additional constraints and forces on the expansion of the framework, and thus resulted in reduced crystal size of MOF in the composite. Consequently, the HS-1 composite with ordered morphology and hierarchical structure was constructed under the structure directing effects of SBA-15.

Figure 10. TEM images and the proposed theoretical models of HS-1 synthesized at different reaction times: (a) 0 h, (b) 3 h, and (c) 12 h.

3.3 Adsorption isotherms of CO2 The adsorption isotherms of CO2 on HKUST-1, the as-synthesized composites, and SBA-15 are described in Figure 11. It could be seen from Figure 11(a) that the CO2 adsorption capacity of pristine HKUST-1 at 25 oC and 1.0 bar reached 93.2 cm3/g, which was higher than most of MOFs under the same conditions. On the contrary, the SBA-15 weakly adsorbed CO2, with an uptake capacity of 12.8 cm3/g only. When HKUST-1 was integrated with SBA-15 matrix, the CO2 adsorption capacities of the composites with SBA-15 content up to 2 wt.% (HS-0.5, HS-1, HS-1.5, and HS-2) were enhanced. Clearly, the HS-1 sample exhibited the highest uptake value (108.0 cm3/g), increased by 15.9% than that of HKUST-1. Note that, this value was around 8.4 times as much as that of SBA-15. The CO2 uptake of all the samples were also calculated based on the mass of HKUST-1 and the results are listed in Table S2. It was observed that the CO2 adsorption capacity of HS-1 achieved 111.1 cm3/g HKUST-1, showing an increase of 19.2%. The enhancement in adsorption performance could be mainly attributed to the increased BET surface areas and the generated new



micropores of the composites, as confirmed by the data in Table 1. The hierarchical structure along with ordered nanocrystal arrays of the hybrid materials also had positive effects on improving the adsorption performance. On one hand, the mesopores introduced by SBA-15 were more conducive to the diffusion of CO2 molecules, providing better access to deep inside of the pores. On the other hand, the well-dispersed MOF platelets with smaller sizes had shorter diffusion path for CO2 than the bulk crystals, which further enhanced the mass transfer.45 Thus, the synergistic effects between HKUST-1 and SBA-15 were responsible for the improved capacities. Increasing the amount of SBA-15 to a higher value (HS-3) led to a decrease in CO2 adsorption capacity. This was ascribed to the lower HKUST-1 content and distorted crystal structure of the material. 100

80

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

60

(a) (b) (c) (d) (e) (f) (g) (h)

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Figure 11. Isotherms of CO2 on (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, (g) HS-3, and (h) SBA-15 at 25 oC.

The adsorption capacities of CO2 on some different porous materials49-62 are also summarized in Table 2. As expected, the CO2 uptake capacity of HS-1 composite was higher than those of traditional adsorbents such as porous carbon materials and molecular sieves. Besides, this value was comparable to some of the MOF-based composites containing porous carbons, ionic liquids, or polymers, inferring that the incorporation of SBA-15 could also form new hybrids with improved characteristics. Furthermore, such mesoporous silica-MOF composite materials combined the merits


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of both micro and mesopores, making them promising candidates for CO2 capture. Table 2. Adsorption capacities of CO2 on different porous materials. Adsorption Capacity

Temperature

Pressure

(cm3/g)

(oC)

(bar)

HKUST-1

93.2

25

1

This work

SBA-15

12.8

25

1

This work

HS-1

108.0

25

1

This work

HKUST-1

93.7

25

1

49

MOF-5

43.3

23

1

50

MIL-101(Cr)

21.4

46

1

51

Mg-MOF-74

160

20

1

36

Mg-MOF-74@SBA-15

88

20

1

36

MCM-41

38.1

30

4

37

MCM-41/Cu(BDC)

44.8

30

4

37

HKUST-1/MCFs

37.6

30

0.1

52

MSS-NH2MIL-53(Al)

44.8

0

1

34

49

0

1

53

UiO-66/GO-5

75.5

25

1

54

MOF/GO-U3

107.1

22

1

44

MC-60

35.2

25

1

55

HCM-Cu3(BTC)2-3

61.6

25

1

56

11.2

25

1

57

100.0

25

35

58

PEI/MIL-101(Cr)

80.6

25

0.15

59

30PEI@ZIF-8

30.2

25

1

60

OMC

33.4

25

1

61

AC

65.4

25

1

62

Adsorbent

10GO-ZIF

Reference

TSIL@NH2-MIL-101(Cr) (5 wt.%) [BMIM][BF4]/CuBTC (30 wt.%)



The influence of temperature on CO2 adsorption capacity was discussed and the isotherms of HKUST-1 and HS-1 at five temperatures (25, 30, 40, 50, and 60 oC) are displayed in Figure 12 and Figure 13 respectively. It was found that the adsorbed amounts of CO2 on both samples decreased with temperature increasing. This indicated that the adsorption between CO2 molecules and the surfaces of samples was physical adsorption. To gain a clearer insight into the adsorption behavior of CO2, the single-site Langmuir-Freundlich (LF) equation (1) was chosen to fit the CO2 isotherms of HKUST-1. For adsorption of CO2 on HS-1, given the heterogeneity of its pore

surfaces,63

the

experimental

data

were

fitted

using

the

dual-site

Langmuir-Freundlich (DSLF) equation (2). The two adsorption models could be expressed as follows: ⁄

= ⁄ ⁄

= ,

(1)

⁄

⁄

+ ,

(2)

⁄

where q (mmol/g) and p (kPa) were the equilibrium uptake of adsorbent and the corresponding pressure of bulk gas, respectively; qmax,i (mmol/g) was the saturation capacity of sites i (i=1,2); bi (1/kPa) was the affinity coefficient related to sites i; and ni was the deviation from an ideal homogeneous surface. The fitting parameters of LF and DSLF equations were separately given in Table 3 and Table 4. (a) (b) (c) (d) (e)

4

Quantity adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

3

2

1

0

0

20

40

60

80

100

Bulk pressure (kPa)

Figure 12. LF equation fitting of CO2 adsorption on HKUST-1 at different


Page 23 of 38

temperatures: (a) 25 oC, (b) 30 oC, (c) 40 oC, (d) 50 oC, and (e) 60 oC (points: experimental data; lines: fitting curves). 5

Quantity adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) (b) (c) (d) (e)

4

3

2

1

0 0

20

40

60

80

100

Bulk pressure (kPa)

Figure 13. DSLF equation fitting of CO2 adsorption on HS-1 at different temperatures: (a) 25 oC, (b) 30 oC, (c) 40 oC, (d) 50 oC, and (e) 60 oC (points: experimental data; lines: fitting curves).

Table 3. Fitting parameters of LF equation for CO2 adsorption on HKUST-1. Temperature (oC)

qmax (mmol/g)

b (1/kPa)

n

R2

25

16.67

6.31*10-3

1.161

0.997

30

15.22

5.13*10-3

1.108

0.998

40

13.29

4.50*10-3

1.105

0.997

50

11.44

4.24*10-3

1.113

0.995

60

9.51

2.62*10-3

1.005

0.997

Table 4. Fitting parameters of DSLF equation for CO2 adsorption on HS-1. Temperature

qmax,1

b1

(oC)

(mmol/g)

(1/kPa)

25

17.04

9.96*10-3

30

15.99

40

14.40

n1

n2

R2

3.01*10-5

0.417

0.999

1.89

5.56*10-6

0.353

0.999

1.74

4.95*10-6

0.361

0.999

qmax,2

b2

(mmol/g)

(1/kPa)

1.415

2.09

7.48*10-3

1.374

6.25*10-3

1.366



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

50

12.73

5.74*10-3

1.375

1.49

1.91*10-6

0.339

0.999

60

11.39

5.39*10-3

1.379

1.25

5.00*10-7

0.307

0.999

As shown in Figure 12 and Figure 13, almost all the experimental data were closely in line with the fitting curves, indicating that the selected adsorption models were appropriate to describe the adsorption of CO2 on HKUST-1 and HS-1 at different temperatures. This was further evidenced by the correlation coefficients (R2) listed in Table 3 and Table 4, as the R2 values were greater than 0.995 in all cases. 3.4 Estimation of the isosteric heat of adsorption The enthalpy of adsorption, which was usually expressed as the isosteric heat of adsorption (Qst), was an important parameter to estimate the strength of interaction between adsorbent and adsorbate.64 Therefore, the Qst value was essential for CO2 capture applications. Here, the fitting adsorption isotherms at 25, 30, and 40 oC were applied to calculate the Qst of CO2 adsorption on HKUST-1 as well as HS-1 composite based on the Clausius-Clapeyron equation (3).

ln = − ⁄ 1⁄ +

(3)

where P (kPa), T (K), R (kJ/(mol·K)), and C were the pressure, temperature, gas constant, and integration constant, respectively; and Qst (kJ/mol) was the isostetric heat of adsorption at a specific CO2 loading (mmol/g). The Qst could be calculated from the slopes (-Qst/R) of the straight lines obtained by fitting the plots of (1/T) versus (ln P). The plots of Qst as a function of adsorbed amounts of CO2 for the two samples are illustrated in Figure 14. For bare HKUST-1, the isosteric heat of CO2 adsorption was about 31 kJ/mol at low surface coverage, while the value slightly decreased to 25 kJ/mol at high surface coverage. The minor change in Qst suggested the relatively homogeneous surfaces of the sample.65 However, the isosteric heats of CO2 adsorption on HS-1 dropped from 46 kJ/mol to 28 kJ/mol with the increase of surface coverage, which was due to the structural heterogeneity of the composite. It should be noticed that the Qst of CO2 adsorption on HS-1 were higher than on HKUST-1 especially at low CO2 loadings, indicating the stronger affinity of the composite


Page 25 of 38

toward CO2 molecules.66 The synergistic effects between SBA-15 and HKUST-1 endowed the hybrid material with higher volume of micropores and more open metal sites, which in turn provided more adsorption sites and enhanced electrostatic interactions for CO2 uptake. The Qst also had an impact on the regeneration of adsorbents. For comparison, the Qst of CO2 adsorption on different materials determined at low coverage are listed in Table S3. As the Qst values of the samples were relatively low (in the range of 30-50 kJ/mol),67 they could be easily regenerated after the adsorption of CO2. (a) (b)

45

40

-Qst (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35

30

25 0.0

0.5

1.0

1.5

2.0

2.5

3.0

CO2 adsorbed (mmol/g)

Figure 14. Isosteric heat of CO2 adsorption on (a) HKUST-1, and (b) HS-1.

3.5 Kinetics of CO2 adsorption Figure 15 gives the adsorption kinetic curves of CO2 on HKUST-1 and HS-1. It was observed that the HS-1 with higher CO2 uptake achieved the adsorption equilibrium in a shorter time than HKUST-1, suggesting a higher adsorption rate of CO2 on HS-1. To make a more intuitive comparison, the kinetic rate constants of CO2 adsorption were estimated by fitting the experimental results with the linear driving force (LDF) model (equation (4)).68

= ∗ −

(4)

The integral form of equation (4) could be written as shown below.

ln "1 − ∗ # = − % ACS Paragon Plus Environment

(5)


Here, q (mmol/g) was the amount of CO2 adsorbed on the adsorbent at time t (s); q* (mmol/g) was the equilibrium uptake at 25 oC and 1 bar; and k (1/s) was the kinetic rate constant. On the basis of equation (5), a line was fit to a plot of -ln(1-q/q*) versus t, and then the rate constant k could be obtained from the slope of the line (Table 5). It was clear that the kinetic rate constant of HS-1 was larger than that of HKUST-1. This might be ascribed to the well-arranged MOF nanocrystals and the additional mesopores introduced by SBA-15, which reduced the mass transfer resistance at crystal surfaces.69 (a) (b)

5

Amount adsorbed (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4

3

2

1

0 0

500

1000

1500

2000

Time (s)

Figure 15. Adsorption kinetic curves of CO2 on (a) HKUST-1, and (b) HS-1.

Table 5. LDF kinetic rate constants k of CO2 on HKUST-1 and HS-1. Sample

k (1/s)

HKUST-1

1.59*10-3

HS-1

2.21*10-3

3.6 Multiple cycles of CO2 adsorption-desorption To investigate the regenerability of the HS-1 sample and the reversibility of CO2 adsorption on this composite, the CO2 adsorption-desorption experiment was performed for multiple cycles at 25 oC and 1.0 bar. Between each cycle, the adsorbent was heated at 150 oC for 6 h in vacuum. As observed in Figure 16, all the adsorption


Page 26 of 38

Page 27 of 38

isotherms of CO2 were very similar and there was no obvious change in capacity during ten cycles. Thus, it was concluded that the composite could maintain the stable performance during repeated use, showing excellent reversibility of CO2 adsorption. Moreover, no residual CO2 was discovered after desorption process, which suggested that the HS-1 could be effectively regenerated by thermal and vacuum treatment. 100

80

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 1

2

3

4

5

6

7

8

9

10

40

20

0

Cycles

Figure 16. Ten consecutive cycles of CO2 adsorption-desorption on HS-1 at 25 oC.

4. Conclusion In summary, a novel composite HKUST-1@SBA-15 was assembled by a simple in situ hydrothermal method. The SBA-15 matrix not only served as a structure directing agent during the oriented growth of HKUST-1 nanocrystals, but also provided the composite with ordered morphology and hierarchical structure. By adjusting the concentration of SBA-15 in materials, the particle sizes of HKUST-1 as well as the textural properties of composites could be controlled. Combining the merits of both components endowed the hybrid material with enhanced adsorption performance and higher adsorption rate toward CO2. The adsorption capacity of HS-1 composite at 25 oC and 1.0 bar was 108 cm3/g, which was much higher in comparison with that of pure HKUST-1. The improvement in CO2 uptake capacity was confirmed to be driven not only by the smaller particle sizes of MOF crystals, but also by the increased BET surface areas and the generated new micropores of the composites. In addition, the synergistic effect between SBA-15 and HKUST-1 also gave rise to stronger affinity of the composite toward CO2 molecules. Taking into account its



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

excellent regenerability, the HKUST-1@SBA-15 composite could act as a promising candidate for CO2 capture. Moreover, such mesoporous silica-MOF composites with additional mesopores might open up a new route to design multifunctional materials for widespread applications.


Page 28 of 38

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Associated content Supporting information Small-angle XRD pattern of SBA-15 (Figure S1); Crystal sizes of HKUST-1 in the composites estimated by Scherrer equation (Table S1); Pore size distribution of SBA-15 obtained by BJH method (Figure S2). N2 adsorption-desorption isotherms of HKUST-1, HS-1, and the physical mixture (with the same composition as HS-1) (Figure S3); Micropore size distributions of HKUST-1, HS-1, and the physical mixture (with the same composition as HS-1) (Figure S4); Expanded FT-IR spectra of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, and (g) HS-3 (Figure S5); TG curves of (a) HKUST-1, (b) HS-0.5, (c) HS-1, (d) HS-1.5, (e) HS-2, (f) HS-2.5, (g) HS-3, and (h) SBA-15 (Figure S6); The SBA-15 contents and CO2 adsorption capacities of HKUST-1 and HKUST-1@SBA-15 composites (Table S2); Summary of isosteric heats of CO2 adsorption on different materials (Table S3). These materials are available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding authors *E-mail: [email protected] (Lijin Zhou) *E-mail: [email protected] (Guofeng Guan)

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21476110), the Natural Science Foundation of Jiangsu Province of China (No. BK20151531), the Key Project for University Natural Science Foundation of Jiangsu Province (No. 14KJA530001), the Prospective Joint Research Program of Jiangsu Province (BY2015005-09), and the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University (No. ZK201305).



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Table of Contents (TOC)


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Synthesis of Hierarchically Structured Hybrid Materials by Controlled

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