Comparative Study on the Adsorption of NO2 Using Different Clay

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Materials and Interfaces

Comparative study on the adsorption of NO2 using different clay/polyaniline composites Shiyang Zhang, Lin Gao, Liyuan Shan, Ruibin Wang, and Yonggang Min Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01072 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Industrial & Engineering Chemistry Research

Comparative study on the adsorption of NO2 using different clay/polyaniline composites

Shiyang Zhang, Yonggang Min †

†, ‡, Ж

†,

Lin Gao,

‡, Ж

Liyuan Shan,

‡

Ruibin Wang,

†,

*

*

School of Materials and Energy, Center of Emerging Material and Technology,

Guangdong University of Technology, Guangzhou 510006, People’s Republic of China ‡

Collage of Materials and Mineral Resources,

Xi'an University of Architecture and

Technology, Xi'an 710055, People’s Republic of China

Ж

These authors contributed equally to this work.

*

E-mail address: [email protected]; [email protected]. Phone number:

+86-15626490982; 18651590988. ACS Paragon Plus Environment

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: To directly denitrate the high-temperature flue gas, excellent thermal stability, along with good adsorption capacity and recyclability, is the key factor for practical application. Hence, polyaniline (PANI) seems a promising candidate for its well-known high adsorption capacity. Aims to further improve the PANI, in this study, aniline was in-situ polymerized with different clays to result in the clay/PANI composites. The results demonstrated the Attapulgite (ATP)/PANI composite (mass ratio of 2:1) had an adsorption capacity of 4.90 mmol·g-1, 59.1 % higher than that of pure PANI. Good recyclability (99.0% for ten cycles) and high decomposition temperature (over 500°C) were also observed for the ATP/PANI composite. Besides, as a byproduct, the washed-away NH4OH that was generated from the adsorbed NO2 is an agricultural fertilizer of wide application. Overall, this study shows a good prospect for direct denitrition of fresh glue gas over the clay/PANI composite adsorbents. Keywords: Polyaniline; clays; NO2 pollution reduction; wastes reclamation

1. INTRODUCTION In response to the global climate change, increasingly stringent laws and regulations have been issued in many countries. Especially against the emission of flue gases, which are from fossil fuel combustion and significantly contribute to the atmospheric pollution.1-3 It is known that nitrogen oxides (NOx) in flue gases have long caused serious environmental problems, including photochemical smog, acid rain, and haze, which have devastating effects on human health and ecosystem.4-7

ACS Paragon Plus Environment

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Over the past few decades, lots of literatures were reported on how to reduce the flue gas pollution with materials such as metal oxides, zeolite, ionic liquid, activated carbon, etc.8-15 However, rather than handling the freshly-emitted flue gas in ~230°C at the outlet, current techniques generally include an additional pre-heating or pre-cooling that is energy-intensive. Therefore, it is highly desired to find a suitable approach to deal with the high-temperature flue gas. In this situation, PANI seems an ideal candidate because of its high adsorption capacity, good stability, and ease of synthesis and treatment, etc.16-19 Currently, the PANI-based organic–inorganic hybrid adsorbents are intensively investigated, to further improve the performances of PANI. Bandosz, et al. prepared a graphite oxide−polyaniline (GO-PANI) composite, of which the activity for NO2 retention/reduction was evaluated, and this feature could be further increased by calcination at 350°C.16 A SnO2–ZnO/PANI composite thick film was fabricated by Cao, et al., it was found that this composite film has high selectivity, low optimum working temperature as 180°C and high stability to low concentration NO2 gas.17 In this study, aniline was hybridized with attapulgite, vermiculite, and diatomite, respectively, to result in a series of clay/PANI composites using the in-situ polymerization. The chemical functionality, thermal stability, microstructure, adsorption capacity and recyclability of the composites were all carefully addressed. The results demonstrated that the clay/PANI composites could efficiently adsorb NO2. After ten cycles, still 99% of the adsorption capacity for the first cycle was shown. It is anticipated that the clay/PANI composites is capable of directly dealing with the



high- temperature flue gas in a cost-effective manner.

2. MATERIALS AND EXPERIMENTS 2.1. Chemicals. Attapulgite (ATP, one-dimensional nano-fiber materials in acicular, rodlike or fibroid shape), Vermiculite (VEM, in laminar structure), and Diatomite (DIM, in disc shape with porous structure) were all supplied by Sinopharm Chemical Reagent Co., Ltd, China. Aniline, ammonium persulphate, hydrochloric acid, and ammonia solution were purchased from Shanghai Lingfeng Chemical Reagent co. Ltd (China). All chemicals were of analytical grade. The gases used in the experiments were supplied by Xi'an Hengli Gas Co., Ltd. 2.2. Preparation of PANI and clay/PANI composites. PANI was prepared as follows: first, two solutions were formed by dissolving 0.93 g of aniline and 2.28 g of ammonium persulphate in 100 mL of hydrochloric acid (1 mol·L-1) under mild agitation, respectively. Then, to initiate the in-situ polymerization, the ammonium persulphate solution was slowly added into the aniline solution and stirring for 4 h at 0-5°C. The resulted suspension was filtrated and washed with deionized (DI) water repeatedly until the filtrate became colorless. Next, the filter cake was immersed in ammonia solution (1 mol·L-1) to remove unreacted materials, then filtrated and washed with DI water until neutral. In final, the solid was freeze-dried (DZF-6020, Shanghai Shenxian Thermostatic equipment, China) (-80°C and 30 Pa, 24 h) to receive the powdery PANI. Clay/PANI composites were prepared as follows: first, certain amount of ATP (VEM,


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or Dim), 0.93 g of aniline, and 300 ml of hydrochloric acid (1 mol·L-1) were mixed in a beaker together. The mass ratio of the other ingredient over aniline was controlled as 2:1, 1:1, 1:2, 1:5, and 1:10. The beaker was sonicated for 30 min to form a well dispersion, then moved to the ice bath. Next, 2.28 g of ammonium persulfate in 200 ml of hydrochloric acid (1 mol·L-1) was slowly added and mildly agitated for 2 h. Afterwards, excess ammonia solution (1 mol·L-1) was added and the stirring was remained for 30 min, followed by a vacuum filtration. The obtained filter cake was then washed by DI water until neutral, and conducted the freeze drying to receive clay/PANI composite composites in powder.20, 21 2.3. Adsorption behaviors of PANI and clay/PANI composites. As shown in Figure 1, one N2 airflow (flow rate of 0.12 m3·h-1) that contained freshly made water vapor joined with another mixed gas of NO2-N2 airflow (NO2/N2=0.1 v/v%, flow rate of 0.04 m3·h-1) in a three-necked flask. The flask was placed in a boiling water bath so avoided the water vapor cooling down that might affect the following adsorption. Then 0.20 g of adsorbent (PANI or clay/PANI composite) was placed in a self-made glass reactor, before pumping in the blended gas to start the adsorption batch of 40 min at room temperature. During this step, the output gas was continuously monitored by a flue gas analyzer. The adsorption capacity of NO2 by each absorbent was calculated using eq 1:

Ac =

mh - mq

(1)

M NO2 ×mq

Where Ac (mmol·g-1) is the calculated adsorption capacity of adsorbent on NO2, mq (g) and mh (g) are the mass of absorbent before and after the adsorption process,



respectively, M NO 2 is the relative molecular mass of NO2. 2.4. Recycling of PANI and clay/PANI composites. The used adsorbent was put into overdosed ammonia solution (1 mol·L-1) and kept stirring for 30 min for desorption, in prior to the vacuum filtration. Then the obtained filter cake was washed with DI water until neutral and then freeze dried at -80°C for 24 h to receive the adsorbent in powder again. The regenerated PANI or clay/PANI composite was then subjected to repeated adsorption/desorption cycles as described in section 2.2 to investigate the recyclability of PANI and clay/PANI composites. 2.5. Characterization. UV–visible absorption spectra were recorded with a Shimadzu UV-240 spectrometer (Shimadzu Co., Kyoto, Japan). The morphologies of PANI and clay/PANI composites were characterized with a scanning electron microscope (SEM, Quanta 200, Hitachi company, Japan), and a transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, FEI Company, USA) that was operated at an accelerating voltage of 200 kV. For all samples, corresponding specific surface area was determined N2 gas adsorption measured with a surface area analyzer (Gemini VII2390, Micrometrics Instrument Corporation, US). X-ray diffraction (XRD, D/max-2200, Rigaku Corporation, Japan) patterns of all samples were conducted with an X-ray diffractometer with CuK a radiation of 1.54 Å at a generator voltage of 40 kV. Thermogravimetric analysis (TGA, TAQ50, Mettler Tolido company, Switzerland) was carried out on a thermal analyzer (±0.1 µg, ±1°C) at heating rate of 20°C·min-1 under N2 atmosphere, thus also generates the corresponding differential thermal gravity (DTG) curve. FTIR spectra of PANI and PANI composites samples were


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obtained by using a Fourier transformed spectrometer (FTIR, FTIR-850, Bruker company, Germany) in a range of 500–4000 cm−1. The concentration of NO2 was measured by Flue gas analyzer (MARIO Plus, York company, China).

Figure 1. A flow diagram illustrating how NO2 is adsorbed by PANI-based composites for combined air-pollution remediation and value-add transformation.

3. RESULTS AND DISCUSSION 3.1. Adsorption mechanism. The adsorption of NO2 onto pure PANI has been barely reported, so the related mechanism is not known yet. Figure 2 shows camera images of pure PANI before (labelled as PANI) and after the adsorption (labelled as PANI-ES). After the adsorption, the observed color prominently changes from bright blue to dark black. In Figure 2c-d, the initial rod-like PANI is thoroughly covered by flocs. To recognize it, further UV-visible absorption spectra of PANI and PANI-ES are introduced. In Figure 2e, PANI displays two peaks at 314 and 596 nm, implying the presence of the π-π transition of the benzoid ring and quinoid ring. However, for



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PANI-ES, the peak round 596 nm was red-shifted to 744 nm, which is attributed to the chemical reaction between PANI and NO2 that favors the formation of a conductive structure. Further, the absorbance gap between the two peaks of PANI is bridged for PANI-ES, suggesting that the adsorbed NO2 is anchored onto the quinoid ring.

22-25

The FTIR test (Figure 2f) also supports this, since the peaks assigned to the nitrogen quinone structure (1588 cm−1) and polarons in the nitrogen quinone (1146 cm−1) show significant intensity change after the NO2 adsorption.20 Based on above, a possible adsorption mechanism of this study is proposed (Figure 2g).16, 26, 27

Figure 2. Camera images of PANI (a) and PANI-ES (b) on the glass plates, which were subsequently magnified by SEM (c, d), respectively; (e) UV–visible absorption spectra of PANI and PANI-ES; (f) FTIR spectra of PANI and PANI-ES; (f) possible adsorption mechanism of NO2 onto pure PANI.


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3.2. Reserved functionalities. To enhance the performance of PANI, clays include ATP, VEM, and DIM are employed. FITR analysis was carried out to confirm the formation of PANI with the addition of clays in the composites (Figure 3a). To avoid a tedious description, only the composite has a clay/PANI mass ratio of 2:1 is addressed in the following if not specified. In general, the bands at 1588, 1498, 1306 and 1146 cm−1 correspond to the nitrogen quinone structure, benzenoid structure, C-N stretching vibration of an aromatic amine, and polarons in nitrogen quinone, respectively.17, 20, 21 Note, all clay/PANI composites exhibit almost the same FTIR spectra include these peaks, proving that PANI is decorated onto the clays in the in-situ polymerization process. Meanwhile, the chemical nonreactivity of clays to PANI is also confirmed, suggesting this study a feasible process to reserve the functionalities of PANI while playing a role of dispersant. XRD patterns of PANI, ATP, DIM, VEM and clay/PANI composites support this idea. Both the broad peaks locate in the range of 10–30° and center at 2θ = 23° represent the characteristic amorphous PANI (Figure 3b). As-prepared ATP/PANI composite yields characteristic diffraction peaks at 8.4, 19.7, 27.5, 34.6 and 42.5°, exactly the same as pure ATP, which indicates the ATP crystal structure does not change after PANI are successfully in-situ growth on ATP. For VEM/PANI composite, the characteristic diffraction peak of pure VEM at 7.3°(001) is absent, while the peak at 8.7°that is resulted from the impurities within VEM is present, suggesting that VEM is composited by PANI. The DIM/PANI composite shows the strong diffraction peaks at 20.8°and 26.6°, attributed to (100) and (101) faces of pure DIM, respectively, of. Shortly, all clay/PANI



composite present similar pattern as pure clays (ATP, VEM, or DIM), rather than PANI.28-31 Based on above, it is concluded that the active amino groups of PANI that can result in high adsorption capacity were preserved well in all composites.

Figure 3. FTIR spectra (a) and XRD patterns of the PANI-based composites.

3.3. Improved thermal stability. As the direct absorbent of NO2 contained in the fresh flue gases, excellent thermal stability is highly required. Thus, TGA was performed to investigate this feature and exhibited in Figure 4. The total weight losses (~15%) of all composites are much lower than PANI (~35%), along with the first decompositions take place at 500°C or above, rather than 250°C of PANI. As well-known, the thermal degradation of pure PANI consisted of two distinct stages, from 100 to 300°C relates to desorption of moisture and oligomers, and from 350 to 800°C caused by the chemical desorption and decomposition of PANI.32 However, once the thermally resistant clays (elucidated in Figure S1) were incorporated, the thermal motion of polymer chains within PANI was consequently impeded, thereby enhancing the thermal stabilities of composites.18


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Figure 4. TGA (a) and DTG (b) comparisons among different clay/PANI composites.

3.4. Upgraded microstructure. To understand the performance difference between the as-prepared composites presented here, the morphology needs to be carefully analyzed. The microstructure of clay/PANI composites was probed by SEM as displayed in Figure 5. Pure PANI is rod-like clustered and in diameter of 5–15 nm, while the PANI clusters those grow on different clays in the composites present various morphologies. PANI networks in ATP/PANI composite are compressed as film, indicating that ATP enables PANI a uniform spreading, which is consistent with the TEM photographs shown in Figure S2. Meanwhile, VEM/PANI composite presents a PANI-like morphology, and PANI in DIM/ATP composite is lumped to seal most cavities of DIM. This is in agreement with the surface area data, as ATP/PANI composite exhibits the highest value of 21.65 m2·g-1, 47 and 12 % higher than that of VEM/PANI composite and DIM/PANI composite. 3.5. Enhanced adsorption and recyclability. The adsorption performance is the most important aspect for the successful application of an adsorbent on flue gas



Figure 5. SEM images of (a, e) PANI, (b, f) ATP/PANI composite, (c, g) VEM/PANI composite, and (d, h) DIM/PANI composite, (scale bar: 5 μm for upper row; 2 μm for lower row).

treatment. The Ac of different clay/PANI composites prepared at various mass ratios is depicted in Figure 6a. ATP/PANI composite, VEM/PANI composite and DIM/PANI composite, at the same clay/PANI mass ratio of 2:1, have the maximum absorption capacity as 4.90, 4.55 and 3.45 mmol·g-1, 59.1, 47.7 and 12.0 % higher than that of pure PANI, respectively (Table 1). This result suggests that the incorporation of clays into PANI successfully enhances the adsorption of NO2, which may be through similar mechanism. ATP/PANI composite outperforms the others because the fibrous ATP can act as a hard template in the coating of PANI to prevent its aggregation, however, VEM sacrifices its lamellar structure in dispersing PANI, and DIM in DIM/PANI needs higher temperature of over 400°C to function better.33-35 Meanwhile, the microstructures of clays may also play a role here. As mentioned above, in


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contrast to VEM and DIM, the unique configuration of ATP is easier to lead a better dispersion of PANI, rendering more active adsorption sites, =N– and –NH– groups, to be exposed from PANI.16, 19, 33 Nevertheless, for all composites, further increasing the clay/PANI mass ratio from 2:1 to 1:10 results in a drop of Ac without obvious rules. That is, although PANI is the decision factor of adsorbing, greater amount of PANI in composites will cause clays an overload that against the adsorption. Also, excessive PANI in composites may agglomerate together, which offsets the dispersion of PANI promoted by clays thus reduces the effective absorption sites of the composite. In addition, the absorption capacity of NO2 by ATP/PANI composite is even higher than other reported adsorbents, such as wood-based activated carbon (3.04 mmol·g-1), amine-functionalized mesoporous silica (2.69 mmol·g-1), and activated carbon fibers (1.20 mmol·g-1).9,

11, 12

Thus, the comparison of adsorption capacities shows that

ATP/PANI is an efficient adsorbent for the uptake of NO2. As a crucial factor of commercialization, the outstanding recyclability of composites were carefully tested and graphed in Figure 6b. All samples demonstrated no significant change through ten adsorption-regeneration cycles, indicating good recyclability of them. Particularly, over 99.0 % of the original adsorption capacity was viewed here not only from pure PANI, but also from the composites. That is, the recyclability of PANI was perfectly inherited to the composites, endowing them with great applicable potentials. Meanwhile, the decisive role of PANI in the composites is also ascertained. This excellent recyclability may be owing to the chemical adsorption of NO2 by PANI is recoverable as long as it is post-treated by enough ammonia



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solution.36 In addition, the physical adsorption of clays should also make a positive difference, owing to the pore size (0.77 nm of ATP/PANI composite, 0.79 nm of VEM/PANI composite, 0.78 nm of DIM/PANI composite) is wide enough to allow the NH4OH molecule to enter and exit. Overall, the enhanced adsorption and recyclability of different composites might be ascribed to the unchanged functionalities, improved thermal stability, and modified microstructure.

Figure 6. (a) Bar plot showing the Ac of different clay/PANI composites prepared at various mass ratios; (b) recyclability of different clay/PANI composites tested after various cycles.

Table 1. Adsorption capacity of different clay/PANI composites. Mass ratio

2: 1

1: 1

1: 2

1: 5

1: 10

Adsorption

ATP/PANI composite

4.90

3.43

3.40

2.33

3.32

capacity

VEM/PANI composite

4.55

3.09

2.58

3.11

2.85


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/mmol·g-1

DIM/PANI composite

3.45

2.93

2.95

2.88

2.86

4. CONCLUSION A series of clay/PANI composites with different clays of various mass ratios were synthesized, characterized, and employed in the adsorption of NO2, the major composition of flue gases, for the first time. These composites not only showed similar functionalities as pure PANI, but also enhanced thermal stability and microstructure compared with pure PANI. Particularly, the ATP/PANI composite in mass ratio of 2:1 exhibited a NO2 adsorption capacity of 4.90 mmol·g-1, 59.1 % higher than that of pure PANI. Moreover, 99.0% of this value was remained even after ten cycles, suggesting that ATP/PANI composite was endowed with great recyclability. Besides, during the desorption process, the adsorbed NO2 was rinsed by NH4OH to form NH4NO3, a general fertilizer in agriculture. Therefore, this work is expected to not only offer a possible method to directly denitrate the flue gases from coal-fired power plants, but also chemically establish a new type of nitrogen cycle.

ACKNOWLEDGEMENTS This work was supported by the Hundred Talent Program of Guangdong University of Technology (220418095), the Bring-in Innovation and Entrepreneurship Team of Guangdong “Zhujiang Talents Plan” (2016ZT06C412), and the Xi'an optical machinery institute of Chinese academy of sciences.



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Supporting Information for publication: (1) the abbreviations appear in the manuscript are listed with original words in the supporting information, which avoids taking too much space in the text; (2) aims to compare the thermal stability of pure ATP, VEM, and DIM with corresponding clay/PANI composites, of whom the TGA and DTG curves are combined with Figure 4 to give Figure S1. (3) Figure S2 displays the TEM photographs of ATP, PANI, and ATP/PANI.



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Comparative Study on the Adsorption of NO2 Using Different Clay

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