A MIL-101 composite doped with porous carbon ... - ACS Publications

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A MIL-101 composite doped with porous carbon from tobacco stem for enhanced acetone uptake at normal temperature Denghui Li, Liqing Li, Ruofei Chen, Chunhao Wang, Hailong Li, and Haoyang Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00393 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Industrial & Engineering Chemistry Research

A MIL-101 composite doped with porous carbon from tobacco stem for enhanced acetone uptake at normal temperature Denghui Li a, Liqing Li a, *, Ruofei Chen a, Chunhao Wang a, Hailong Li a, Haoyang Lib

a

School of Energy Science and Engineering, Central South University,

Changsha 410083, Hunan, China

b

School of Materials Science and Engineering, Central South University,

Changsha 410083, Hunan, China

* Corresponding author. Tel: +86 13807483619, E-mail address: [email protected]

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Industrial & Engineering Chemistry Research 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

Abstract

A high efficiency adsorbent for acetone under normal temperature was prepared with metal-organic framework MIL-101 doped with porous carbon from tobacco stem (MIL-101/TC) by a solvothermal synthesis method. These synthesized composites were characterized and then investigated for acetone adsorption at 288 K and 298 K up to 20 kPa. The isosteric heat of adsorption was estimated from adsorption isotherms of acetone vapor. Temperature programmed desorption (TPD) was employed to calculate the activation energies of acetone desorption on MIL-101/TC and MIL-101. Results of characterization showed that MIL-101/TC possessed similar crystal structure and morphology to MIL-101. Despite smaller BET surface area, MIL-101/TC-40 and MIL-101/TC-30 composites had much higher acetone uptake of 1137 mg/g and 1123 mg/g at 18.9 kPa and 18.1 kPa in 288 K, respectively, which increased by 19.8% and 18.3% than that of MIL-101 composites. The increase in acetone uptakes was attributed to the associative effect of the enhancement of the surface dispersive forces and the activation of the unsaturated metal sites by TC loading. The isosteric heat of acetone adsorption on the MIL-101/TC was much higher than that on the MIL-101, and the maximum isosteric adsorption heat on the MIL-101/TC-40 was 52 kJ/mol. The activation energy of desorption obtained through TPD ranged from 24.5 kJ/mol to 48.5 kJ/mol. The acetone adsorption isotherms of the composites could be fitted favorably by the L-F equation.

Keywords: MOFs; Porous carbons; MIL-101; Adsorption; Acetone

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Introduction It is well known that volatile organic compounds (VOCs) are one of the most harmful air pollutants threatening human health because of their malodorous, toxic, mutagenic and carcinogenic nature.1, 2 Among these VOCs, acetone is a common and harmful air pollutant emitted from the medicine, pesticide and chemical industries.3 It is poisoning to humans by prolonged contact because of its high volatile at room temperature.4 Therefore, it is critical to control the concentration of acetone in the air,5 which means that finding optimized method to remove VOCs is extremely urgent. Several main methods for elimination of VOCs have been reported such as adsorption, catalytic combustion, membrane separation, etc.6, 7 Adsorption has been regarded as a safe, fast, simple and low-cost method to remove VOCs from contaminated air even if the concentration is not high enough.8, 9 And for the efficient adsorption of harmful VOCs, an excellent adsorbent with high adsorption capacity is the key point of adsorption technique. More recently, Metal-organic frameworks (MOFs) have drawn extensive interests owing to their high surface areas, porous structure, high micropore volume, well-defined framework structures and promising application in the fields of gas storage, separation and purification.10-12 Nevertheless, it is noticed that some weak points of MOFs hinder their potential applications. In particular, there are not enough dispersive forces to restrict small molecules due to the low density of atoms in MOFs and their open void space.13-15 Thus, the porosity of MOFs will be not fully used.13, 14 Building MOF-based composites is attracting attention as the adsorption properties of composites can be enhanced by introducing other materials such as silica,16 organic polymers,17 metal,18 carbon nanotubes,19, 20 graphite oxide21 and activated carbon.22, 23 Among these MOF-based composites, composites of 3



MOFs doped with carbon-based materials (including different allotropes, such as activated carbon, nanotube, fullerene, graphite etc.) seem interesting, as MOFs are regarded as a kind of the state-of-the-art materials while carbon-based materials are identified as classical materials. And activated carbons can be used for gas storage by providing an appropriate medium due to its large specific surface area and an abundant porous structure.24 Besides, they have the advantages of low toxicity, low cost and easy availability. Thus, combined with the merit of activated carbon, integrating MOFs with carbon-based materials is not only meaningful to deal with the weakness of MOFs mentioned above, but also possibly brings about many novel functionalities like enhanced stabilities and adsorption capacities,25 improved electrical conductivities,26 etc. The purpose of this study is to synthesize the composites (MIL-101/TC) from MIL-101 and tobacco stem based carbons (TC) whose composition analysis is listed in Table S1, to explore their high adsorption capacities of acetone with properties characterization. And the reason for selecting acetone as the adsorbate is that acetone is representative for small molecules and widely used in many industries which may easily cause air pollution owing to its volatile property. MIL-101 and MIL-101/TC composites with different amounts of TC were synthesized through hydrothermal method. The characterization methods of the composites include X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, Scanning electron microscopy, Transmission electron microscopy and N2 adsorption isotherms. The adsorption isotherms of acetone are measured by Surface Analyzer. And their isosteric heat is calculated on the basis of their adsorption isotherm. Furthermore, temperature programmed desorption (TPD) is used to analyze the desorption behavior of acetone on MIL-101 and MIL-101/TC composites. 4


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Here, the adsorption performance of acetone on MIL-101 and MIL-101/TC materials is discussed and reported, hoping the study could be helpful for researchers to further research of MOF materials. Experimental Reagents and materials The chemical supplies including 1,4-benzenedicarboxylic acid (H2BDC, >99%, Sinopharm, China), chromium nitrate nonahydrate (Cr(NO3)3.9H2O, >99%, Sinopharm, China), hydrofluoric acid (HF, >40%, Sinopharm, China), N,N-dimethylformamide (DMF, 99.5%, Sinopharm, China), ethanol (>99.7%, Hengxing, China), ZnCl2 (>98.0%, Sinopharm, China), hydrochloride acid (HCl, 36-38%, XingKong, China), acetone (99.5%, Sinopharm, China) and N2 (99.999%, High-Tech Gas, China) were used. Tobacco stems were collected from the China Tobacco Hunan Industrial Co., Ltd. Preparation of microporous carbon from tobacco stems The microporous carbon from tobacco stems was prepared mainly following the reported procedure.27 In more detail, prior to use, the fresh tobacco stems were washed with distilled water and dried at 333 K, and then grounded by a high-speed rotary cutting mill and sieved into coarse granules (20 mesh). ZnCl2 was dissolved in distilled water (100 mL), and the tobacco stems of dried precursor were then added into the solution. The impregnation ratio which meant the mass ratio of activating agent to tobacco stems, was 8:1. After stirring for 0.5 h and aging at atmospheric temperature for 12 h, the impregnated sample was filtered and then dried at 333 K overnight. The sample was heated from room temperature to 773 K with a heating rate of 10 K/min and held for 60 minutes at the final temperature under N2 atmosphere (100 mL/min) in a 5



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horizontal tubular furnace, then cooled down to room temperature with N2 flush. The obtained solid was washed with HCl (0.5 M) and then distilled water to remove the remaining ZnCl2 until the filtrate was neutral, followed by drying at 378 K overnight, and then the product named TC was obtained. XPS of TC was shown in Figure S1 and Table S2. Preparation of MIL-101 The

synthesis

process

of

the

chromium

terephthalate

with

formula

Cr3F(H2O)2O[(O2C)-C6H4-(CO2)]3•nH2O (MIL-101) was performed following the reported procedure.28 The mixture of Cr(NO3)3•9H2O (4.00 g), H2BDC (1.64 g), distilled water (50 mL) and HF (0.5 mL, 40 wt%) was stirred for 30 minutes at room temperature, and then transferred into a 100 mL stainless steel reactor lined with polyphenylene for 8 h at 493 K. After the reactor cooled to room temperature, the solution was filtered, washed thoroughly with distilled water and dried at 423 K for 5 h. A green powder was obtained and amounts of non-reacted H2BDC were still presented as needle-shaped colorless crystals with the crude product. The green raw product was purified by two steps. The green powder was treated with 60 mL of DMF at 373 K for 8 h and then washed in 60 mL of ethanol for 8 h at 353 K, which was repeated for twice, filtered off, washed with distilled water and dried at 423 K for 5 h. Preparation of MIL-101/TC composite The synthesis of MIL-101/TC composite was carried out by adding TC to the reaction mixture of Cr(NO3)3.9H2O, H2BDC, distilled water and HF, and the procedure afterwards was the same as the synthesis of MIL-101, and the purification process was also carried out under the same conditions as MIL-101. The mass of TC was varied from 30 to 50 mg to prepare three different samples of MIL-101/TC composites, which were denoted as MIL-101/TC-X (X=30, 40, 50, respectively). In the 6


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process of crystallization, ions in solutions formed crystals on the surface of carbon particles and kept repeating the process on a nearby carbon surface. So it formed the composite of MIl-101/TC. The obtained final weight of the MIL-101, MIL-101/TC-30, MIL-101/TC-40 and MIL-101/TC-50 was 1.099 g, 1.139 g, 1.174 g and 1.238 g, respectively. The contents of TC in the MIL-101/TC-30, MIL-101/TC-40 and MIL-101/TC-50 composite could be calculated to be 2.63 wt%, 3.41 wt% and 4.03 wt%, respectively. Characterization The features of the samples were identified by X-ray diffraction (XRD, X’ PertPro MPD, PANalytical B.V., NED) with a PANalytical powder diffractometer operated at 40 mA and 40 kV using Cu/Kα as the radiation source. The scanning range was from 1.5° to 40° with a step size of 0.02° per second. The microstructures of the adsorbents were observed through a scanning electron microscope (SEM, Helios NanoLab 600i, FEI Co., USA) with an accelerating voltage of 5.0 kV and Transmission electron microscopy (TEM, Tecnai G2 F20, FEI Co., USA) after the samples were dispersed in ethanol by sonication. Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700 FT-IR, Thermo Fisher Scientific Inc., USA) was used to qualitatively evaluate the surface chemical properties of MIL-101 and TC-incorporated MIL-101 samples. The spectrum was obtained over the frequency range of 400-4000 cm−1. X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi, Thermo Fisher Scientific, Inc., USA) was employed to investigate the selected element content. The thermal characteristics of MIL-101 and MIL-101/TC were determined by thermogravimetric analysis (TGA, SDT Q600, Waters Co., USA) from room temperature to 873 K under argon atmosphere, with the heating rate of 10 K/min. The specific surface area, pore volume, and pore diameter of the synthesized MIL materials were determined by nitrogen 7



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adsorption at 77 K with a BET Surface Area Micropore Size Vapor Analyzer (JW-BK132Z, Beijing JWGB Sci. & Tech. Co., Ltd., China). The BET surface area, micropore volume, micropore and mesopore

size

distribution

were

estimated

from

the

adsorption

data

by

the

Brunauer–Emmett–Teller (BET) equation, t-plot method, Barrett-Joyner-Halenda (BJH) method and Horvath-Kawazoe (HK) method, respectively. Acetone adsorption measurements To study adsorption capacities of the synthetic materials for acetone, a BET Surface Area Micropore Size Vapor Analyzer (JW-BK132Z, Beijing JWGB Sci. & Tech. Co., Ltd., China) was employed to carry out the acetone adsorption experiments. Adsorption isotherms of acetone vapor on MIL-101 samples were obtained at 288 K and 298 K with the adsorption pressure ranging from 0 to 20 kPa. Before each test, the sample was degassed at 423 K for 3 hours under vacuum. Acetone desorption measurements The temperature programmed desorption (TPD) tests were performed on MIL-101 samples (about 8 mg) saturated with acetone at 298 K using a thermogravimetric analyzer (TGA, SDT Q600, Waters Co., USA) under Ar flow (50 mL/min) at different heating rates from 10 to 40 K/min, and the outlet concentration of desorbed acetone (m/z=58) was detected by a mass spectrometer (MS, OmniStar, Pfeiffer Vacuum GmbH, Germany), meanwhile the TPD curves were obtained. Results and discussion Characterization analysis Figure 1 displays XRD patterns of virgin MIL-101 and MIL-101/TC in this work. The 8


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characteristic peaks of MIL-101 appeared at 2.95, 3.40, 5.25, 8.55 and 9.15 degrees, which were in accordance with the earlier data on MIl-101.29 The major peak positions in the XRD pattern of MIL-101/TC composites materials were nearly the same as that of the pure MIL-101, confirming that TC incorporation did not disturb or destroy its crystal structure. It was noticed that the peak intensity of MIL-101+TC sample was obvious weaker than that of MIL-101 and MIL-101/TC, probably because TC sheltered part of MIL-101 crystal in the mixture.

Figure 1. XRD patterns of MIL-101, MIL-101/TC-X Figure 2 shows FTIR spectra of the MIL-101 and MIL-101/TC. It was observed that the spectrum for the MIL-101 and the composites were nearly similar. The sharp peaks with high intensity in 3443 cm−1 indicated the stretching of hydroxyl groups (O−H).30 The peak in 1624 cm−1 indicated the presence of adsorbed water. The peak in 1510 cm−1 was due to the (C=C) stretching vibration31 and the deformation vibration (C−H) at 1163, 1018, and 748 cm−1. The band at 1401 cm−1 corresponded to the symmetric vibration (O−C−O), inferring the presence of dicarboxylate in the MIL-101.32 The moderate intensity peak in 587 cm−1 indicates Cr−O vibration, implying that the chromium-based metal-organic framework indeed has been formed. 9



Figure 2. FTIR spectra of TC, the MIL-101and the composites Figure 3 shows the TGA and DTG curves of MIL-101 and the composites for evaluating the thermal stabilities. The trend of curves were well matched with the literature published.25, 32 It could be seen that the TGA curves showed two distinct weight loss steps. The first peak in the range of 303-373 K corresponded to desorption of physically adsorbed water. And the peak from 373-573 K meant the loss of the coordinated and anion crystalline water molecules.33 With hydrophobic nature of TC, the water content of the composites incorporated TC were less than the initial MIL-101. Then, the sharp peaks in the DTG curves were about 633 K, might come from the elimination of OH/F groups,29 indicating the decomposition of the MIL-101 units. Subsequently, the sharp peaks were about 698 K, followed by the sharp peaks occurring at about 753 K, which might be related to the collapse of MIL-101 structure owing to the decomposition of structural terephthalic acid linked to the framework. In short, the MIL-101 and composites samples had similar thermal stability, indicating the introduction of a little TC did not disturb the thermal stability of the MIL-101 units.

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Figure 3. TGA (a) and DTG (b) curves of the five samples Figure 4a-h shows the SEM and TEM images of the materials prepared. Figure 4b made clear that the sample of MIL-101 presented clear polyhedral crystals. The composites with doped porous carbon also had polyhedral morphology similar to that of the MIL-101 (Figure 4c-e), but the crystal morphology of MIL-101/TC-50 had a noticeable change (Figure 4e). The particles became more irregular with increased amount of doped TC in MIL-101/TC composites. This may be ascribed to the excess TC doped in MIL-101, which destroyed the crystallization of MIL-101. And the TEM images in Figure 4f-h revealed that the TC was a thin layer of carbon (Figure 4f) and MIL-101 presented the crystal symmetry in nature. In addition, some crystals of MIL-101 were embedded in the doped TC (Figure 4h), indicating the TC was indeed incorporated into MIL-101 crystallites. There were some deformation in the crystal of MIL-101/TC, which meant that the crystal was defective. 34

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Figure 4. SEM images of TC (a), MIL-101 (b), MIL-101/TC-30 (c), MIL-101/TC-40 (d), and MIL-101/TC-50 (e); TEM images of TC (f), MIL-101 (g) and MIL-101/TC-40 (h). 12


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Figure 5 and Figure 6 show the N2 adsorption-desorption isotherms and pore size distribution curves of the five samples. It could be found that the samples of MIL-101 and MIL-101/TC had a Type-I isotherm that was the feature of microporous materials (Figure 5), while the isotherm for TC was a typical Type-IV character. And the structural properties of the samples are listed in Table 1. The BET surface area, total pore volume and average pore width of MIL-101 were 3974 m2/g, 2.3 cm3/g, 2.26 nm, respectively, which were consistent with the reported value.28 However, after TC incorporation, the BET values and pore volume have presented a decreasing trend with increased TC content, and the reason may be that the excess amount of TC blocked the existing adsorptive sites.

Figure 5. N2 adsorption-desorption isotherms of the five samples

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Figure 6. Pore size distribution of the five samples.

Table 1. N2 adsorption properties of MIL-101 and MIL-101/TC at 77 K

Sample

MIL-101 MIL-101/TC-30 MIL-101/TC-40 MIL-101/TC-50 TC

Carbon content (%)

BET surface area (m2/g)

Average Pore width (nm)

Total pore volume (cm3/g)

t-Plot micropore volume (cm3/g)

Micropore percentage (%)

59.85 60.06 60.58 60.87 −

3974 3508 3522 3202 2182

2.26 2.34 2.20 2.32 2.87

2.30 2.05 1.94 1.68 1.57

1.59 1.42 1.37 0.96 0.24

69.1 69.3 70.6 57.1 15.3

Note：Determined by XPS, the content of the carbon was from both TC and MIL-101. Acetone adsorption evaluation Figure 7 presents the acetone adsorption isotherms. Acetone adsorption studies were performed in MIL-101 and MIL-101/TC samples at 288 K and 298 K up to 20 kPa. Note that the adsorption of acetone declined with the rising temperature, implying physically adsorbing on the samples. The proper doping TC had a significant enhancement of acetone adsorption capacity compared with the material MIL-101. At 288 K, the composites MIL-101/TC-40 showed higher acetone sorption capacity compared to bare MIL-101, and the maximum amount of acetone 14


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adsorbed was 1137 mg/g at 18.9 kPa on the MIL-101/TC-40. The adsorption quantity had an increase of 19.8% compared with the MIL-101 whose acetone adsorption quantity was 949 mg/g at 18.1 kPa. It might possibly have two reasons for the increase of acetone adsorption capacity. One of the reasons was attributed to an increase in the surface dispersive forces of the MIL-101/TC. And it could be confirmed with the acetone adsorption isotherms based on per unit surface area of the adsorbent shown in Figure 8. By comparison, there was much higher acetone quantity adsorbed per unit surface area on MIL-101/TC in Figure 8. It meant that the introduction of TC resulted in the enhancement of the surface dispersive forces and then the improvement of acetone uptakes per unit surface area on the MIL-101/TC. The other reason was due to the increase of the Cr3+ metal sites. The unreacted H2BDC remaining inside the pores poisoned the chromium metal sites in bare MIL-101. The TC particles present inside the MIL-101 prevented the coordination of the chromium metal sites with BDC ligand. Thus it might disturb the crystals’ surface and defects in MIL-101 units, causing an exposure of more unsaturated Cr (III) metal sites in the composites.35

Figure 7. Acetone adsorption isotherms at 288 K and 298 K. 15



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Figure 8. Acetone adsorption isotherms based on the unit surface area at 288 K and 298 K For purpose of describing the adsorption behavior of acetone on the samples quantitatively, the Langmuir-Freundlich (L-F) and the dual-site Langmuir-Freundlich (DSLF) equations were separately applied to fit the experimental values. The equations of two models can be expressed as follows: Langmuir-Freundlich equation

Q = Qmax

n

KL P

1+ KL P

(1)

n

where Q (mg/g) refers to the amount of adsorbate on per unit weight of adsorbent, (mg/g) is the saturated amount uptake of acetone, is a constant related to adsorption rate, P (kPa) is the equilibrium pressure in the gas phase and n is the Langmuir-Freundlich coefficient. Dual-site Langmuir-Freundlich (L-F) equation 1  n  b1 P 1 Q = Qmax,1  1  1 + b P n1 1 

1   n   b2 P 2 1  + Qmax,2    1 + b P n2 2  

    

16


(2)

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Where Qmax,1 and Qmax,2 are the saturated amount uptake of sites 1 and 2, b1 and b2 are the affinity coefficients to sites 1 and 2, 1/n1 and 1/n2 are a measure of the heterogeneity of the adsorbent.

Figure 9. Adsorption isotherms of Acetone on the MIL-101 and MIL-101/TC at 288 K and 298 K. (a) L-F equation fitting of acetone adsorption on the MIL-101 and MIL-101/TC, (b) DSLF equation fitting of acetone adsorption on the MIL-101 and MIL-101/TC (points: experiments data; lines: fitting curves).

17



Figure 10. Adsorption isotherms of Acetone on the MIL-101 and MIL-101/TC based on the unit surface area at 288 K and 298 K. (a) L-F equation fitting of acetone adsorption on the MIL-101 and MIL-101/TC, (b) DSLF equation fitting of acetone adsorption on the MIL-101 and MIL-101/TC (points: experiments data; lines: fitting curves). Figure 9 and Figure 10 show the fitting results of the experimental values. And the fitting parameters of the two models are listed in Table S3. As Table S3 shows, the correlation coefficients (R2) of the L-F and Langmuir which represent the coincidence degree between the isotherm equation fits and experimental values are up to 0.99 for the samples, indicating that the experimental data fit well with the L-F and Langmuir model. The parameter of n represents the deviation from an ideal homogeneous surface (i. e. heterogeneity factor).33 For a homogeneous material, the value of n=1, and thus the L-F equation became the Langmuir equation. If the parameter n was deviated from 1, the material was heterogeneous, and when n>1, adsorption cooperativity was positive, while when 0

A MIL-101 composite doped with porous carbon ... - ACS Publications

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