Article pubs.acs.org/IECR
Mesoporous Carbon Beads Impregnated with Transition Metal Chlorides for Regenerative Removal of Ammonia in the Atmosphere Jitong Wang, Wuyou Jiang, Zixiao Zhang, and Donghui Long* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Mesoporous carbons have been widely investigated, but their real-world applications are still unfulfilled because of lacking costeffective material synthesis and application-specific structure and shapes. Herein, we designed an efficient adsorbent based upon mesoporous carbon beads (MCBs) for regenerative removal of NH3 in the atmosphere. By employing a suspension strategy, millimeter-sized MCBs are prepared which not only have well developed porous structure but also value-added properties such as regular spherical geometry, high mechanical properties, low dust, good wear-resistance, and good liquidity. While impregnated with transition metal chloride, the impregnated MCBs display outstanding breakthrough capacity of 111.4 mg/g for NH3 adsorption at 30 °C with a relative humidity of 80%. It is found that the type of transition metal chloride and their loading amount notably determine the adsorption performance. Moreover, the adsorption behavior is also determined by the process parameters such as humidity and NH3 concentration. The excellent adsorption performance is mainly due to the well-developed mesoporous structure of MCBs, which allow the uniform dispersion and immobilization of transition metal chlorides within their channels, while the residual mesoporous channel provides access to fast NH3 diffusion.
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livestock smells, and household air purification.12,13 The key to low-temperature adsorption of NH3 mainly depends on the high-performance adsorbents. Generally, the pore diameter, surface area, channel connectivity, and architecture of the adsorbents usually affect their action in the NH3 removal. Several adsorbents, such as activated carbons,9,11,14,15 alumina,16 zeolites,17,18 graphite oxide (GO),19−21 and MOFs22,23 have being extensively investigated. Among them, activated carbons exhibit certain advantages including large surface area, high pore volume, and controllable surface chemistry. These features could provide good adsorptive property, meanwhile the chemical stability make them easily regenerated for many practical applications.24,25 However, commercially available activated carbons have limited capacity of NH3 adsorption due to the weak physical forces. Moreover, the isosteric heat of ammonia adsorption on carbon is less than 30 kJ/mol which leads to ammonia easily being stripped from the surface when purged with air.26 Therefore, to overcome this problem, the adsorbents of ammonia must have a well-developed pore structure as well as an effective surface chemistry in order to improve their adsorption performance on NH3. Generally, surface chemistry is considered as one of the most critical parameters to determine the adsorption performance of activated carbons in previous studies. 27 So far, these
INTRODUCTION Ammonia (NH3) is a corrosive environmental pollutant and important health hazard, which is emitted from industrial waste gases, municipal solid odors, fertilizer manufacture, and livestock smells.1,2 The toxic effects on the human body are related to its high water solubility. As a strong base, ammonium could affect the eye, skin, and respiratory systems via exothermic reaction leading to severe burning. The American Conference Governmental Industrial Hygienists (ACGIH) has regulated the permissible NH3 concentration should be less than 25 ppm for time-weighted value and 35 ppm for shortterm value.3 Moreover, ammonia contributes to several environmental problems, including toxic effects on vegetation and air pollution, which leads to the eutrophication of ecosystems and causes secondary particulate matters in the surroundings.4,5 Therefore, removal of ammonia is necessary from the environmental and human health standpoints. To control the emissions of ammonia, several technologies such as solvent adsorption, biological treatment, thermal oxidation, ion exchange, and adsorption by porous materials have been developed.6−8 Many methods are available for dealing with high concentration odor gases; however, they are unfavorable for handling diluted pollutants from the capital point. Adsorption of ammonia by porous solids is a promising approach for dilute NH3-containing odor gases at ambient conditions, owing to its low cost, simplicity of design, and facile regeneration of the adsorbents.9−11 Therefore, adsorption technology has been extensively employed to control air pollutants in many fields such as industrial waste gases, © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
January 2, 2017 March 12, 2017 March 14, 2017 March 14, 2017 DOI: 10.1021/acs.iecr.7b00013 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
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modifications have included introduction of acidic groups and impregnation with metal oxides or metal chlorides.11,28 Oxidation of the carbon surface is a common way of surface modifications leading to the introduction of acidic groups. Some chemical interactions between ammonia and surface oxygen functional groups are formed. However, they are not strong enough to prevent slow desorption of NH3 from the carbon surface when the filter is purified with air, making this method impractical for NH3 removal. Another way of effective improvement of ammonia retention on the carbon surface is impregnation with metal chloride, which leads to the formation of complexes of ammonia with metal chlorides and water, as observed previously on porous alumina modified with chlorides of alkaline-earth metals. Sharanov and co-workers managed to improve ammonia removal by modifying porous alumina modified with chlorides of alkaline-earth metals.29 The salts used to impregnate alumina were BaCl2, CaCl2, and MgCl2. As a conclusion to their work, it was assumed that the excellent adsorption performance, compared to the results obtained with unmodified alumina, was due to salt−ammonia interactions that result in ammonia complexes. It was also noticed that the adsorption capacity depends on the metal, and their best results were obtained with alumina modified with MgCl2. Strong interactions of ammonia with Cu-ZSM-5 zeolites were studied by Valyon and co-workers.30 They detected the formation of copper(II)-diammine chloride complexes thermally stable up to 540 °C. Petit and co-workers studied the removal of ammonia by microporous carbons impregnated with different metal chlorides.31 The results demonstrate the metal chlorides as active centers could react with NH3 to form and water could improve the NH3 adsorption performance, which promotes its solution into the water film in the micropores to form NH4+ ions. However, the NH3 adsorption capacity of the materials developed is still limited at ambient conditions due to the low accessible adsorption sites and mass transfer rate. To further advance the adsorbents, the porous carbon materials, which allow more metal chlorides loading amount and have considerable accessibility to NH3, are highly desired in future work. For any material having practical application, however, it is necessary for processing and formulation into specific structure and shapes. This is especially true for porous carbons synthesized as irregular powders which are not appropriate for many industrial applications related to porous nature (e.g., adsorption, separation, catalysis), where regular shapes are often needed for ease of handling and recyclability. Recently, we combined the advantages of hard templating and the sol− gel method and developed a facile method to synthesize millimeter-scaled mesoporous carbon beads (MCBs),32,33 which could meet the strict requirements concerning mechanical strength and the hierarchically meso/microporous channel for practical applications. These MCBs had high specific surface area as well as large pore volume, which could increase the loading amount of active substance for NH3 adsorption as well as facilitate the mass transfer of the gas phase. On the basis of these MCBs as the supports, here we developed highly efficient and low-cost adsorbents for ammonia removal. The effect of the type of metal chlorides and the loading amount on the adsorption capacity were further investigated. The as-developed MCBs-supported adsorbents demonstrated a successful combination of low cost with high performance, which may well be the answer for the technical development of industrial NH3 removal.
Article
EXPERIMENTAL METHODS
Preparation of Mesoporous Carbon Beads. All starting materials and solvents were purchased from Titanchem Co. and used without further purification. The mesoporous carbon beads (MCBs) were prepared via a combined sol−gel process and hard template method in water-in-oil emulsion with resorcinol and formaldehyde as carbon precursor and colloidal silica as hard templates, as we have previously reported.32 Typically, 11 g of resorcinol and 12 g of formaldehyde solutions (37 wt %) were poured into 100 mL of deionized water. Then 80 g of colloidal silica was added into the solution and stirred at 40 °C for 40 min. The mixture was further stirred at 80 °C for 40 min in a 500 mL container with 350 mL of paraffin oil at a speed of 250 rpm and aged at 80 °C for 1 day. The obtained hydrogel composite was dried at 80 °C followed by calcination at 800 °C for 3 h in N2 atmosphere. The MCBs were finally left by etching silica with 15 wt % NaOH solution at 80 °C, washed with distilled water, and dried at 100 °C. Preparation of Adsorbents. The metal chlorides impregnated MCBs adsorbents were prepared by a conventional wet impregnation method. In detail, the desired amounts of NiCl2· 6H2O, CuCl2·2H2O, ZnCl2, and MnCl2·4H2O powder were dissolved in 10 mL of deionized water. After adding 2.5 g of MCBs, the solution was treated in ultrasonic for 10 min. The mixture was finally dried at ambient temperature for 24 h and then at 120 °C for 12 h. The obtained materials are expressed as MCBs-y-x, where x represents the mass percent and y represents the type of metal salts in the adsorbents, respectively. Characterization. The morphologies and microstructures were observed under scanning electron microscopy (FEI, Qunta 300) and transmission electron microscopy (TEM, JEOL 2100F). All samples were adequately ground and dried before observation. The X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max 2550 diffractometer operating at 40 kV and 20 mA with Cu Kα radiation (λ = 1.5406 Å). Nitrogen adsorption/desorption isotherms were measured at 77 K with a Quadrasorb SI analyzer. Before the measurements, the samples were degassed in vacuum at 393 K. The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method and the total pore volume was obtained based on the adsorbed amount at P/P0 = 0.985. The mesoporous pore size distribution and pore volume were calculated using the Barrett−Joyner−Halenda (BJH) model. NH3−temperature-programmed desorption (NH3-TPD) data were acquired from a Chem BET Pulsar-1 with a thermal conductivity detector. Before the tests, all of the samples (∼30 mg) were purged with 90 mL min−1 of He for 30 min. For the NH3-TPD, the inlet gas was switched from He to 7.4 wt % NH3 under a flow of 90 mL min−1 at room temperature, and then the samples were heated from 30 to 800 °C at a heating rate of 10 °C min−1. Adsorption Performance Test. Dynamic tests at room temperature were performed in a fixed-bed flow adsorber to assess capacities of the samples for NH3 adsorption. About 0.3 g of the adsorbent was packed in the middle of the adsorber supported with quartz wool. For NH3 test, 200 ppm of NH3 and balanced N2 was used as inlet gas to go through the reactor containing adsorbents with the flow rate of 500 mL/min. The flow rate of the gas was controlled by electronic flow control instrument. The humidity of the stream was controlled by a humidifier. The reaction temperature was controlled by a B
DOI: 10.1021/acs.iecr.7b00013 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research thermocouple centered in the furnace. The influent and effluent gas was monitored by infrared gas analyzer (Beijing JFQ1150E) with a detection levels of 1 ppm. In this test, the NH3 breakthrough capacity (mg NH3/g adsorbent) was calculated from the NH3 concentration in the inlet and outlet gas, breakthrough time, flow rate, and the mass of material. Also, the breakthrough concentration of NH3 here was defined as 5 ppm.
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RESULTS AND DISCUSSION Characterization of MCBs and Adsorbents. The porosity and particle size of MCBs has been systematically studied in our previous work.32 Herein, the as-prepared MCBs (Figure 1a) have an average particle size of 0.8 mm and a high
Figure 2. N2 adsorption -desorption isotherms at 77 K (a) and BJH pore size distributions (b) of MCBs.
Table 1. Structural Parameters and NH3 Breakthrough Capacities of the Adsorbents ammonia capacity a
samples MCBs MCBNiCl2-30 MCBCuCl2-30 MCBMnCl230 MCBZnCl2-30 MCBNiCl2-10 MCBNiCl2-20 MCBNiCl2-40
Figure 1. Morphology of MCBs: (a) optical image, (b,c) SEM images, and (d) TEM image.
mechanical strength of about 16 N. The SEM image of single MCBs is shown in Figure 1b, which show good sphericity and a smooth surface without obvious cracks or contaminations. This is desirable for application in the fixed-bed system due to the good abrasion resistance, high packing density and small fluid flow resistance. SEM and TEM images (Figure 1c and d) show MCBs are composed of three-dimensional (3-D) carbon frameworks with interconnected mesoporous and macroporous structure. The small mesopores are related to the single silica nanoparticle, while the large pores are formed from the aggregation effect of more colloidal silica nanoparticles. Besides, the porous structure of the MCBs was obtained by N2 adsorption/desorption at 77 K. As shown in Figure 2, MCBs exhibit a typical type IV adsorption−desorption isotherms with capillary condensation steps occurring over relative wide pressures of 0.6−0.9, suggesting the mesoporous characteristic of the materials. The calculated porosity parameters are summarized in Table 1. The pristine MCBs have a high BET surface area (SBET) of 1036 m2/g and total pore volume of 2.48 cm3/g (Vt) and a BJH average pore size (Dp) of 12 nm. The adsorbents were synthesized by loading metal chlorides onto the MCBs, which belongs to a kind of mild Lewis acid.31 For comparison, the loading amount of metal chlorides here was controlled to be 30 wt %. After the metal chloride impregnation, the porosity of the adsorbents obviously
b
c
SBET (m2/g)
Vt (cm3/g)
Dp (nm)
(mg NH3/g)d
(mg NH3/cm3)e
1036 533
2.48 1.49
12.5 12.1
6.1 111.4
2.7 70.9
467
1.46
11.9
75.8
52.3
446
1.36
12.0
45.4
34.7
434
1.44
12.0
70.3
49.8
751
2.04
12.5
61.6
30.2
630
1.78
12.5
81.1
45.5
479
1.25
12.0
79.8
60.5
a
BET specific surface area. bTotal pore volume (P/P0 = 0.985). cBJH desorption average pore size. dAmmonia breakthrough capacity per weight of the adsorbents. eAmmonia breakthrough capacity per bed volume of the adsorbents.
decreases as a result of a large number of metal chloride molecules filling. Surprisingly, it is noted in Figure 2a that all the samples still maintain available mesoporous structures. The porosity parameters summarized in Table 1 show that all the samples have similar pore volume of ∼1.4−1.5 cm3/g and BET surface areas of above 400 m2/g. The considerable residual channels and accessibility of interfacial area should effectively facilitate the NH3 molecular diffusion. NH3 Adsorption Performance of the Adsorbents Impregnated with Different Metal Chloride. The NH3C
DOI: 10.1021/acs.iecr.7b00013 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
adsorption behavior was further investigated. Herein, the structural parameters of NiCl2-impregnated MCBs with different loading amounts were characterized by N2 adsorption at 77 K and shown in Figure 4a. The total pore volume and
TPD experiments were used to investigate the strength and type of the acid sites for the four different metal chloride impregnants. As shown in Figure 3a, the original MCBs have no
Figure 3. NH3-TPD profiles (a) and NH3 breakthrough curves (b) of the adsorbents with different impregnants. The dynamic test were performed at 30 °C and 200 ppm of inlet NH3 concentration.
Figure 4. N2 adsorption−desorption isotherms (a) and XRD patterns (b) of the adsorbents with different NiCl2 loadings.
NH3 desorption peak, indicating only porous structure has negligible adsorption for NH3. The four metal chlorides impregnated samples present NH3 desorption peaks located at ∼150 °C, corresponding to NH3 desorption from the weak Lewis acid sites.34 From the peak area in the NH3-TPD profiles, MCB-NiCl2-30 should exhibit the highest NH3 desorption peak. In addition, it should be noticed that there is another desorption peak at a higher temperature of 290 °C for NiCl2impreganated adsorbent, indicating more acid sites exist in the material. The breakthrough adsorption performance was carried out in the fixed-bed system at 30 °C under the relative humidity of 80%, as presented in Figure 3b. The original MCBs exhibit only a low adsorption capacity (6.04 mg of NH3/g adsorbents), due to the limited physisorption of NH3 on the carbon framework. After impregnated with metal chlorides, the breakthrough time is prolonged to 90−220 min. Notably, the NH3 adsorption capacity is mainly determined by the type of the impregnants. Among these adsorbents, the NiCl2-impregnated MCBs exhibit much higher adsorption capacity than the other impregnated adsorbents, in good agreement with the TPD result. The adsorbent MCB-NiCl2-30 possesses a very impressive adsorption capacity of 111.4 mg-NH3/g-adsorbent, which is among the highest values reported in all commercial activated carbonbased adsorbents (
