- Email: [email protected]
Nitric Oxide (NO) separation from flue gas by chemical modified mesoporous silica
Separation and Purification Technology 229 (2019) 115833
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Nitric Oxide (NO) separation from flue gas by chemical modified mesoporous silica
T
⁎
Yi Zhao , Han Wang MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NO separation Modified mesoporous silica Vinyltriethoxysilane Acryl amide Acrylic acid
Three kinds of chemically modified silica were prepared to adsorb NO by surface modifying with vinyltriethoxysilane, acrylic acid and acryl amide respectively, and characterized by N2 adsorption experiments, Fourier transform infrared spectroscopy, scanning electron microscopy and thermo-gravity analysis. The results showed that eCOOH and eCONH2 in the monomers had been grafted on the silica surface, and the pore volumes of S1, S2 and S3 decreased and exhibited the mesoporous structure; all the modified adsorbents could maintain steady under 300 °C. NO adsorption experiments were also carried out and the results suggested that the silica modified with acryl amide had the best NO adsorption capacity due to the relative large surface area and the stronger hydrogen bonds between NO molecules and the functional groups. The adsorbed NO molecules could be well desorbed by purging with N2 so that the adsorbents could be recycled and NO resource also had the chance to be reused.
1. Introduction Nitrogen oxides (NOx) resulting from the combustion of nitrogencontaining compounds is one of the conventional air pollutants, it is composed of nitrogen oxide (NO) and nitrogen dioxide (NO2). Among them, NO accounting for 95% [1] in the total NOx is a kind of very important molecule in the view of both environment [2,3] and biology [4–6]. In the perspective of environment, NO and NO2 are serious air pollutants which can cause the disease of respiratory and cardiac-cerebral vascular systems to humans; moreover, the acid rain caused by NO and NO2 can do harm to plants and buildings. In the perspective of biology, NO is an extremely important molecule as it plays role in regulation of diverse biological processes including vascular tone, neurotransmission, inflammatory cell responsiveness, defense against invading pathogens and wound healing [7]. Thus developing a feasible method of separating NO molecule from flue gas for resource utilization is meaningful. Coal-fired power plants are the main emission sources of NOx, and several techniques have been developed to remove NOx from flue gas, including selective catalytic reduction (SCR) [8–10] and selective non catalytic reduction (SNCR) [11–13], in which, SNCR alone cannot reach
the NOx emission limit in power plants of China. In order to meet the strict emission standard in China, increasing catalyst layers of SCR or using SNCR together with SCR can be an appropriate strategy. However, the applications of these technologies suffer from several problems of high equipment investment and operating costs, catalyst poisoning and ammonia escape, moreover, the spent catalyst is considered as a new hazardous solid waste. Hence, the oxidation methods [14] have widely been concerned, in which, NOx is oxidized to soluble nitrogenous species and absorbed by alkaline solutions. Although the desired NOx removals are obtained by using these methods, some studies suggest that the method have the disadvantages of higher reagent prices and releasing secondary environmental pollutants [15,16]. Adsorption can be a promising way to deal with NOx, because it has the obvious advantage of combining NOx removal with NOx resource utilization. Furthermore, adsorption process can overcome the disadvantages of SCR or SNCR and can operate at lower temperature (< 100 °C) [17]. Over this decade, activated carbon [18,19], zeolite [20,21], metal compound [22,23] and silica gel [24] were extensively used as adsorbents to remove NOx and other pollutants from flue gas. Among of them, silica gels with rich hydroxyl group had the proper pore size, providing the feasibility of grafting or impregnating the
⁎ Corresponding author at: MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China. E-mail address: [email protected] (Y. Zhao).
https://doi.org/10.1016/j.seppur.2019.115833 Received 4 October 2018; Received in revised form 16 July 2019; Accepted 19 July 2019 Available online 19 July 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
water to remove AN, and dried at 120 °C for 12 h. The synthetic process of S3 was the same as that of S2, except for using AAM instead of AA. For comparison, AA and AAM polymers, denoted as PAA and PAAM were also prepared separately. The preparation method of PAA was as follows: 30 mmol AA was added in 15 ml AN under stirring for 2 h, and then 30 mmol EGDMA and 0.3 mmol AIBN was added to the solution. The mixture was degassed by ultrasonic device for 15 min and blown by N2 for another 15 min to remove oxygen. After that, the mixture was sealed and reacted for 24 h at 60 °C, and the formed lumpy polymer was ground and screened to 50–150 μm. The particles were washed with HCl/methanol (1/9, v/v) solution for several times and then washed with high purity water to neutral. At last, the particles were dried overnight under vacuum at 120 °C. The synthetic process of PAAM was the same as that of PAA, except for using AAM instead of AA.
desired functional groups. By chemical modifying, the physical and chemical properties of silica gels could be evidently improved [25,26], which is important for adsorbing selectively a specific molecule. A well NOx adsorption and desorption process depends strongly on the high-efficiency adsorbent with a high adsorption capacity and low desorption temperature, thus the purpose of the work is making a kind of efficient NO adsorbent by modifying silica gels. Usually, the specific surface areas of mesoporous materials are larger than those of macroporous materials when the pore volumes are the same. Moreover, the inner structures of mesoporous materials are usually more organized than the macroporous materials, which can help the functional groups to be grafted more uniformly on the surface of the materials. Acrylic acid (AA) and acrylamide (AAM) are the common functional monomers which can provide acidic eCOOH and alkaline eCONH2 groups respectively [27]. In the adsorption of NO, the delocalized π-bonds of NO would be affected by the functional groups to form the relative stable intermediates (eNH2+NOe and eCOOH+NO−) which could be decomposed to release NO when the temperature was increased at purging with N2. In this way, the adsorbents could be reused, meanwhile, NO was recycled. The vinyltriethoxysilane (VTES) with eC]C was firstly used to modify the silica gels to carry out the connection between activated silica gels and the functional monomers (AA and AAM) by using the cross-linking agent (ethylene glycol dimethacrylate, EDGMA). Thus, three kinds of modified mesoporous silica were developed by different modification ways. To investigate the effect of modified adsorbents on the characteristics and NO adsorption/desorption behaviors of the adsorbents, the experiments of NO adsorption/desorption were also conducted
2.3. Characterization For BET experiments, N2 was as an adsorbate, the isothermal curve of adsorption/desorption for the adsorbents were measured by a surface area and pore size analyzer (Beckman Coulter SA 3100, USA) at different pressures, in which each sample was degassed under vacuum for 2 h at 150 °C before the measurement. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model of the adsorption isotherm were used to calculate the surface area, average pore size, and pore volume. Thermal stability of adsorbents was characterized by thermogravimetric analysis (TGA) (Netzch STA 449C, German), in which, adsorbents were pre-dried at 120 °C to remove moisture. The surface morphology of adsorbents was observed using a scanning electron microscope (SEM, JEOL JSM-7500F, Japan) and the Fourier transform infrared (FTIR) spectra of adsorbents were obtained by a FTIR spectrometer (Bruker Optics-Tensor II, German).
2. Materials and methods 2.1. Materials Vinyltriethoxysilane (VTES, 97 vol%) was purchased from Macklin reagent (Shanghai, China) and ethylene glycol dimethacrylate (EGDMA, 98 vol%) was purchased from Aladdin reagent (Shanghai, China); acryl amide (AAM, 99 wt%), acrylic acid (AA, 99 vol%), azodiisobutyronitrile (AIBN, 98 wt%), acetonitrile (AN, 99 vol%), toluene (TOL, 99 vol%), ethanol (EtOH, 99.7 vol%), and hydrochloric acid (HCl, 36 wt%) were purchased from Kermel Chemical Reagent Ltd. (Tianjin). High purity water (> 18 MΩ) was produced by lab water purification system (Changfeng Co., Ltd., Beijing). NO and N2 used in experiments were supplied by North Special Gas Co., Ltd. Commercial silica gels (type 3, batch number 0170013) used with diameter of 0.15–0.75 mm were purchased from Qingdao Marine Chemical factory (Shandong, China).
2.4. Adsorption/desorption of NO NO adsorption/desorption property was assessed in a fixed-bed reactor as shown in Fig. 1. In an adsorption/desorption process, 1.5 g of the adsorbent was loaded in the reactor with a heating tape, and two thermal couples located at inlet and outlet of the reactor respectively were used to monitor the temperature during the operation, so that the temperature could be controlled with in ± 0.5 °C. The initial activation of adsorbents was carried out at 120 °C in a highly pure N2 stream at a flow rate of 80 ml/min for 3 h. After that the adsorbents were cooled to the desired adsorption temperature and at the same time the gas stream was quickly switched to 0.1% (volume fraction) NO balanced by N2. NO concentration at the outlet of the reactor was measured by a multi-functional flue gas analyzer (ecomJ2KNpro, German), and the NO breakthrough curve was drawn, from which NO adsorption capacity was calculated by integration of the breakthrough curve, as shown in Eq. (1). During the breakthrough experiments, the blank runs were conducted for three times firstly, and the average dead volume time was calculated as 26 s, as shown in Fig. 8. To truly reflect the adsorption capacity of adsorbents, 26 s were subtracted in the breakthrough runs.
2.2. Preparation In order to synthesize the modified silica adsorbents, the activated silica, denoted as S0 was firstly prepared by immersing silica gels in HCl solution of 8 mol/L for 14 h. Three modified silica adsorbents were obtained by grafting VTES, AA and AAM in the surface of S0, and denoted as S1, S2 and S3, respectively. For preparation of S1, 10 ml of VTES and 50 ml toluene were mixed and then stirred for 5 min, followed by adding 10 g S0. The mixture was refluxed under N2 atmosphere at 110 °C for 12 h with stirring. The remaining particles were washed with absolute ethyl alcohol for 10 h by Soxhlet extractor and then dried for 48 h at 50 °C under vacuum. For preparation of S2, 30 mmol AA was added in 15 ml AN under stirring for 2 h, and then 30 mmol EGDMA, 0.3 mmol AIBN and 1.5 g S1 were added to the solution. The mixture was degassed by ultrasonic device for 15 min and blown by N2 for another 15 min to remove oxygen. After that the supernatant was removed by suction filtration, and the remained particles were sealed and placed in water bath for 24 h at 60 °C. The resultant particles were washed with high purity
Qs =
F × ∫0t (C0 − C ) dt m
(1)
where Qs is the saturated adsorption capacity of NO over adsorbents, mg/g; F represents the influent velocity of NO, ml/min; m is the weight of the adsorbent, g. C0, C represent the concentration of influent and effluent NO respectively, mg/m3; t is the adsorption time, min. In order to investigate the adsorption stability of the prepared adsorbents, the experiments of NO adsorption-desorption cycle were performed by 10 times continuously for each adsorbent, and the desorption processes were conducted in N2 flow rate of 80 ml/min at 2
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 1. Diagram of experimental apparatus for NO adsorption: (1) NO cylinder; (2) N2 cylinder; (3, 4) pressure deducing valves; (5, 6) flow meters; (7–11) stop valves; (12) mixed gas cylinder; (13, 14) three-way valves; (15) heating box; (16) fixed-bed reactor; (17, 18) temperature controllers; (19) flue gas analyzer.
broken to bond with the cross linker (EGDMA) in the presence of initiator (AIBN), and then the functional groups such as eCOOH in AA and eCONH2 in AAM were grafted on the surface of S1 in the presence of cross linking.
120 °C.
3. Results and discussion 3.1. Preparation principle of adsorbents
3.2. Effect of chemical modification on characteristic of adsorbents The preparation principle of S1, S2 and S3 are shown in Fig. 2. Firstly, to prepare S1, the carbon-carbon double bonds (eC]C) are induced on the surface of S0 by the condensation reaction between hydroxyl groups on the surface of S0 and those in VTES. Furthermore, S2 and S3 are prepared by modifying the surface of S1 chemically through polymerization reaction, and in that reaction, the carboncarbon double bonds of AA, AAM and those on the surface of S1 are
Fig. 3 shows the desorption pore size distributions of the S0, S1, S2, and S3 (Fig. 3A), and the PAA and PAAM (Fig. 3B). It can be seen from Fig. 3A that the average diameter of the pores mainly distributes between 5 and 15 nm for activated and modified silica gels, and all of the silica gel materials have the relatively uniform pore sizes, besides, the pore curves show obvious decrease of the peak heights in order of S0, S1,
Fig. 2. Preparation principle of S1, S2 and S3. 3
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 3. Pore size distributions of S0, S1, S2, S3, PAA and PAAM.
Fig. 4. N2 adsorption isotherms of S0, S1, S2, S3, PAA and PAAM.
Table 1 The characteristics of adsorbents. Samples
SBET (m2/g)
Vp (cm3/g)
dp (nm)
NO capacity (mg/g)
S0 S1 S2 S3 PAA PAAM
284.1 264.8 204.4 215.1 163.2 186.5
0.773 0.688 0.581 0.587 0.605 0.577
8.847 8.185 8.131 8.201 31.65 35.05
0.078 0.101 0.108 0.125 0.065 0.060
materials are conformed to type Ⅳ according to IUPAC (International Union of Pure and Applied Chemistry) classification, which shows a feature of mesoporous material with uniform pore size, It can be seen from Fig. 4 that S0, S1, S2 and S3 all have the adsorption equilibrium isotherm of type Ⅳ, and the types of hysteresis are all H1, which indicates that the materials have the mesoporous structure with the narrow distribution of pore diameters. However for PAA and PAAM, the isotherms in Fig. 4 are of type Ⅱ characteristics and the hysteresis types of them are H3, suggesting that the adsorption processes are all multilayer reversible adsorption, so that the polymer materials have the features of unlimited pore size. Comparing the N2 adsorption isotherm of S1, S2 and S3 to that of S0, there is a significant decrease in N2 adsorbing capacity for the modified silica gels, which is consistent to the values of Vp as shown in Table 1. Fig. 5 depicts the FTIR spectra of the activated silica gels, chemical modified silica gels and polymers of AA and AAM. It can be found that the adsorption peaks of SieOeSi in the spectra of all the silica adsorbents are at about 1100 cm−1 and 802 cm−1, which may be
S3 and S2, but no obvious changes in the shape, indicating that although the pore volumes of the modified silica decrease after modification, the uniform pore size structures are retain. Table 1 summarizes the values of surface area (SBET), average pore diameter (dp) and the volume of pores (Vp) for all the materials, which shows that the SBET, dp and Vp of modified silica are all decrease compared with those of the activated silica gels, suggesting that the pore channels of silica materials have been occupied in the modification process. Moreover, the SBET and Vp of S1 are all larger than those of S2 and S3, suggesting that although adding cross linker (EGDMA) can increase the cross linking degree of S2 and S3, that is not favor of getting large SBET and Vp. However, there is no obvious distinction in the values of dp among S1, S2, and S3, which means that the modification do not change the shape of pores in the modified silica gels but blocks up some pores to make cross linking degree increase. It can be observed from Table 1 that the SBET values of PAA and PAAM are obviously smaller than those of S2 and S3, illustrating that the silica substrate makes polymers to be grafted on the surface of materials uniformly, and that is favorable to get the larger specific surface area. As seen in Fig. 3B, the pore sizes of the polymers are distributed at the range of 20–60 nm. Comparing to the pore sizes in Fig. 3A, the distributions of the pore size for PAA and PAAM are more dispersive. It is to note that SBET values of PAA and S2 are smaller than those of PAAM and S3, which can be speculated by that the interaction force of eCOOH is stronger than that of eCONH2 because electro-negativity of O in carboxyl is higher than that of N in amide, giving the higher electron cloud density to adsorbent, thus PAA and S2 have higher cross linking degree than PAAM and S3 respectively, which is not conducive to getting large specific surface area. Nitrogen adsorption isotherms of activated silica gels, chemical modified silica gels and polymers of AA and AAM are shown in Fig. 4. Clearly, the curves of all the silica materials in low Ps/P0 area are convex in shape and show an inflection point, indicating that the single molecule layer adsorption is saturated and the second adsorption layer starts with the increasing Ps/P0. In addition, all curves of silica
Fig. 5. FTIR spectra of S0, S1, S2, S3, PAA and PAAM. 4
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
thus affecting their thermo stability. A slightly decline of less than 5% can be observed in the curve of S0 during the thermal decomposition experiments, possibly because the condensation reaction of groups takes place. However for S1, when the temperature is higher than 500 °C, the proportion of weight loss is a little larger than that of S0, because C]C structures grafted by VTES begin to decompose on the surface of S1. The weight losses of functional groups in S2 and S3 were also determined as 14% for both of them, from which, the weight losses of functional group such as eCOOH and eCONH2 was calculated as 8.54 mg/g for S2 and 8.35 mg/g for S3. Fig. 7 shows the SEM pictures of all adsorbents, in which the SEM picture of S1 is smoother than that of S0, suggesting that after being grafted by VTES, the morphology of the adsorbents changes to more regular. While the SEM pictures of S2 and S3 are rougher than that of S1, suggesting that S2 and S3 are covered by the layer of AA and AAM polymers, respectively. This phenomenon can be explained by the SEM pictures of PAA and PAAM, in which, the pictures show a porous structure and are also consistent with the pore structure data.
resulting from the asymmetric stretch of SieOeSi and the symmetric stretch of SieOeSi, besides, the peaks at around 3430 cm−1 can be considered as OeH flexural vibrations of silanol groups. In the spectra of S1, a peak at 1640 cm−1 that is attributed to stretching vibration of C]C is found, showing that the addition of VTES induces double bonds to the silica during the preparation process. In the spectra of S2, the adsorption peaks at 1740 and 2970 cm−1 are observed, which may be resulting from the adsorption of C]O and stretching vibration of OeH, verifying that the structure of eCOOH is successfully grafted on the surface of S2 by the modification method. The spectra of S3 shows absorbance bands at 1740 and 1635 cm−1 on account of the adsorption peak of C]O and bending vibration of N–H, proving that the structure of eCONH2 is grafted after chemical modification. The FTIR spectra of PAA and PAAM displayed in Fig. 5 demonstrate that the peaks at 2960 cm−1, 1450 cm−1, and 1390 cm−1 are the saturated CeH stretching vibration, saturated CeH bending vibration, and rocking vibration of methyl and methyne for the polymers, while the adsorption peak of C]C vibration is not observed in the area of 1600–1660 cm−1, which may be because all C]C bonds of AA, AAM and EGDMA are broken to bond with cross linker. In the spectra of PAA, OeH stretching vibration at 3420 cm−1 and adsorption peak of C]O at 1720 cm−1 are observed, proving the existent of the framework of eCOOH. In the spectra of PAAM, there is a peak at 1660 cm−1 belonging to the NeH stretching vibration, which demonstrates that the amide group exists in the surface of PAAM after polymerization reaction yet. The thermal decomposition curves of the silica and polymer materials are illustrated in Fig. 6. Obviously, all the materials is stable below 300 °C except for PAA and PAAM, in which the curves of PAA and PAAM have a sharp decline when the temperature increases from 300 to 500 °C, suggesting that the structure of acryl amide and acrylic acid polymers may be destroyed quickly. When the temperature continuously increase from 500 to 700 °C, the rate of decomposition becomes slow. However, the weight of the adsorbents do not change while the temperature is above 700 °C, suggesting that the polymers have already been decomposed completely. For the curves of S2 and S3, when the temperature is higher than 350 °C, the weight of the adsorbents begins to loss, indicating that the decomposition of AA and AAM grafted on the surface of the silica happens, but the curves are maintain steady again at 750 °C, verifying that all polymers are decomposed completely at this temperature. It can be concluded that AA and AAM polymers grafted on the surface of S2 and S3 have a higher decomposed temperature compared to PAA and PAAM, confirming that the polymers in the surface of silica are more steady, and the reasons for this stability are speculated by that AA and AAM in the surface of the pore structures of silica may affect the length of the molecular chain of the polymers,
3.3. NO adsorption capacity and mechanism analysis The NO breakthrough curves of S0, S1, S2 and S3 obtained by the adsorption experiments are shown in Fig. 8 and the corresponding NO adsorption capacity values are summarized in Table 1, which are 0.078, 0.101, 0.108 and 0.125 mg/g for S0, S1, S2 and S3 respectively. Also, the NO adsorption capacity of PAA and PAAM are listed in Table 1 according to the experimental results in our previous study [27]. From Fig. 8 it can be observed obviously that chemically modified silica gels all have longer breakthrough time than that of the activated silica gel, which means that chemical modification can help to improve the surface structure of silica gels and to enhance the NO adsorption ability. The lower adsorption capacity of S0 can be explained as by that silanol groups in S0 appear a weaker intermolecular force for NO, thus decreasing NO adsorption activity. It is clearly from the data shown in Table 1, although the SBET and Vp of modified silica absorbents decrease after modification, the NO adsorption capacity increases, which shows that the effect of functional groups is more important than SBET and Vp in the NO adsorption. Meanwhile, It is found by comparing the adsorption capacity of S1 to those of S2 and S3 that adding polar functional groups benefits the adsorption of NO, because polar functional groups can enhance the hydrogen bond functions between NO and the surface of silica gels. In addition, compared the adsorption capacity of S2 to that of S3, although acrylic acid in S2 has the higher polarity than acrylamide in S3, the larger SBET helps S3 get better NO adsorption performance because more functional groups are exposed on the surface of S3. It can be also found in Table 1 that the adsorption capacity of PAA is better than that of PAAM, which may be resulting from the stronger hydrogen bond force of acrylic acid. By contrast, the NO adsorption capacities of PAA and PAAM are all smaller than those of the modified adsorbents, which can be interpreted by that the polymerization space of AA and AAM are not limited, making the molecular chain of the polymer twist and bind to be huge molecular cluster. However, for S2 and S3, the polymerization space of AA and AAM are limited in the pore channel, forming small molecular clusters, and compared with the small molecular cluster, functional groups of huge ones are harder to be exposed on the surface, using as the active points for NO adsorption, which is an important reason for lower adsorption capacity of PAA and PAAM. Additionally, the lower SBET and uneven pore structure of PAA and PAAM are also unfavorable for NO adsorption. 3.4. Regeneration of silica adsorbent In order to investigate the reuse characteristic of the modified adsorbents, 10 times successive adsorption-desorption cycle for each silica adsorbents were carried out. Fig. 9 shows that decrease rates of NO
Fig. 6. Thermal decomposition curves of S0, S1, S2, S3, PAA and PAAM. 5
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 7. SEM photograph of S0, S1, S2, S3, PAA and PAAM.
Fig. 8. The NO adsorption breakthrough curves of S0, S1, S2 and S3 at 40 °C. Fig. 9. NO adsorption capacity of S0, S1, S2 and S3 during 10 cycles of NO adsorption at 40 °C and desorption under N2 stream at 120 °C.
adsorption capacity for S0, S1, S2 and S3 are 13.7%, 12.9%, 20.2% and 22.3% in 10 times adsorption-desorption cycle, respectively. It can be observed in Fig. 9, all of the modified adsorbents do not have severe decrease of NO adsorption capacity, suggesting that the modified modified adsorbents have better reproducible ability, and the reaction force is a kind of physical interaction, thus NO can be desorbed.
Comparing the decrease rate of the modified adsorbents, the smaller decrease rate of adsorption capacity for S0 and S1 may be because that the silanol and C]C groups on the surface of S0 and S1 respectively has weaker interaction with NO molecule, thus the NO desorption process is 6
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Universities of China (No. 2015ZZD07, No. 2019MS105 and 2016XS109); the National Science-technology Support Plan of China (No. 2014BAC23B04-06); the Special Scientific Research Fund of Public Welfare Profession of China (No. 201309018); the Beijing Major Scientific Technological Achievement Transformation Project of China (No. Z151100002815012) and Technology development projects of Sanhe power plant of China (SH [2015]-QT44).
more easily. For S2 and S3, the larger decrease rate of NO adsorption capacity can be attributed to the larger interaction function between the silica gel surfaces and NO molecules. It is speculated that with the increase of recycle numbers, the adsorbed NO molecules in the adsorbent may combine with functional groups by stronger chemical bonds, and it is difficult to be desorbed, from which, the adsorption capacity of the adsorbents will sharply decrease because the adsorbing sites of the adsorbents are occupied by the generated molecules.
References 4. Conclusion [1] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369–379. [2] J. Xiang, L. Wang, F. Cao, K. Qian, S. Su, S. Hui, Y. Wang, L. Liu, Chem. Eng. J. 302 (2016) 570–576. [3] Y. Guo, Y. Li, T. Zhu, M. Ye, Fuel 143 (2015) 536–542. [4] P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Ferey, R. E. Morris, C. Serre, 112 (2011) 1232–1268. [5] B. Xiao, P.S. Wheatley, X. Zhao, A.J. Fletcher, S. Fox, A.G. Rossi, I.L. Megson, S. Bordiga, L. Regli, K.M. Thomas, R.E. Morris, J. Am. Chem. Soc. 129 (2007) 1203–1209. [6] B. Supronowicz, A. Mavrandonakis, T. Heine, J. Phys. Chem. C 117 (2013) 14570–14578. [7] G. Narin, S. Ülkü, Micropor. Mesopor. Mat. 234 (2016) 120–129. [8] X. Cheng, X.T. Bi, Particuology 16 (2014) 1–18. [9] X. Lou, P. Liu, J. Li, Z. Li, K. He, Appl. Surf. Sci. 307 (2014) 382–387. [10] M. Karthik, H. Bai, Appl. Catal. B-Environ. 144 (2014) 809–815. [11] N. Modlinski, Energy 29 (2015) 67–76. [12] J. Hao, W. Yu, P. Lu, Y. Zhang, X. Zhu, 122 (2015) 213–218. [13] S. Fu, Q. Song, Q. Yao, Chem. Eng. J. 285 (2016) 137–143. [14] Y. Zhao, Y. Han, T. Ma, T. Guo, Environ. Sci. Technol. 45 (2011) 4060–4065. [15] Y. Zhao, T. Guo, Z. Chen, Y. Du, Chem. Eng. J. 160 (2010) 42–47. [16] Y. Zhao, R. Hao, T. Wang, C. Yang, Chem. Eng. J. 273 (2015) 55–65. [17] J.F. Brilhac, A. Sultana, P. Gilot, J.A. Martens, Environ. Sci. Technol. 36 (2002) 1136–1140. [18] R. Murillo, T. García, E. Aylón, M.V. Navarro, J.M. López, A.M. Mastral, Carbon 42 (2004) 2009–2017. [19] R. Chauveau, G. Grévillot, S. Marsteau, C. Vallières, Chem. Eng. Res. Des. 91 (2013) 955–962. [20] Deng, H.H. Yi, X. Tang, Q. Yu, P. Ning, L. Yang, Chem. Eng. J. 188 (2012) 77–85. [21] H. Yi, H. Deng, X. Tang, Q. Yu, X. Zhou, H. Liu, J. Hazard. Mater. 203 (2012) 111–117. [22] W.B. Li, X.F. Yang, L.F. Chen, J.A. Wang, Catal. Today 148 (2009) 75–80. [23] G. Meng, X. Song, M. Ji, J. Hao, Y. Shi, S. Ren, J. Qiu, C. Hao, Appl. Phys. 15 (2015) 1070–1074. [24] N.N. Linneen, R. Pfeffer, Y.S. Lin, Chem. Eng. J. 254 (2014) 190–197. [25] R. Qu, Y. Niu, J. Liu, C. Sun, Y. Zhang, H. Chen, C. Ji, 68 (2008) 1272–1280. [26] Y. Zhao, Y. Shen, L. Bai, S. Ni, Appl. Surf. Sci. 261 (2012) 708–716. [27] Y. Zhao, H. Wang, T. Wang, Funct. Mater. Lett. 10 (2017) 53–56.
The effects of the mesoporous silica of chemical modification on NO adsorption were investigated by modifying silica gels with vinyltriethoxysilane, acrylic acid and acryl amide polymer, respectively. It was concluded from the characterizations of modified adsorbents that the functional groups (eCOOH and eCONH2) had successfully been grafted at the surface of S2 and S3 respectively, and all of the modified adsorbents had perfect thermal stability. Compared to the acrylic acid and acryl amide polymers in our early works, the silica grafted by these polymer showed the larger SBET and higher NO adsorption capacity. It was interesting to note that though the SBET values of S2 and S3 were lower than that of S1, the NO adsorption capacities of S2 and S3 were larger than that of S1, due to the functional groups on the surface of them. The flow in our experimental system is considered as the nonideal flow, and it is inevitable to have the situations of channeling, backmixing and the uneven distribution of flow rate because of the uneven diameter distribution of adsorbents, bending of fluid pipe and so on, and those all can cause the axial dispersion, which may exist effect the flow rates in our adsorption column. Due to limited space of the manuscript, the influence of axial dispersion on flow rates will be investigated in future by using the axial dispersion model. Acknowledgements The authors appreciate the financial support by a grant from the key project of the National major research and development Program of China (No. 2016YFC0203701, No. 2016YFC0203705 and No. 2017YFC0210603); the Fundamental Research Funds for the Central
7
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Nitric Oxide (NO) separation from flue gas by chemical modified mesoporous silica
T
⁎
Yi Zhao , Han Wang MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: NO separation Modified mesoporous silica Vinyltriethoxysilane Acryl amide Acrylic acid
Three kinds of chemically modified silica were prepared to adsorb NO by surface modifying with vinyltriethoxysilane, acrylic acid and acryl amide respectively, and characterized by N2 adsorption experiments, Fourier transform infrared spectroscopy, scanning electron microscopy and thermo-gravity analysis. The results showed that eCOOH and eCONH2 in the monomers had been grafted on the silica surface, and the pore volumes of S1, S2 and S3 decreased and exhibited the mesoporous structure; all the modified adsorbents could maintain steady under 300 °C. NO adsorption experiments were also carried out and the results suggested that the silica modified with acryl amide had the best NO adsorption capacity due to the relative large surface area and the stronger hydrogen bonds between NO molecules and the functional groups. The adsorbed NO molecules could be well desorbed by purging with N2 so that the adsorbents could be recycled and NO resource also had the chance to be reused.
1. Introduction Nitrogen oxides (NOx) resulting from the combustion of nitrogencontaining compounds is one of the conventional air pollutants, it is composed of nitrogen oxide (NO) and nitrogen dioxide (NO2). Among them, NO accounting for 95% [1] in the total NOx is a kind of very important molecule in the view of both environment [2,3] and biology [4–6]. In the perspective of environment, NO and NO2 are serious air pollutants which can cause the disease of respiratory and cardiac-cerebral vascular systems to humans; moreover, the acid rain caused by NO and NO2 can do harm to plants and buildings. In the perspective of biology, NO is an extremely important molecule as it plays role in regulation of diverse biological processes including vascular tone, neurotransmission, inflammatory cell responsiveness, defense against invading pathogens and wound healing [7]. Thus developing a feasible method of separating NO molecule from flue gas for resource utilization is meaningful. Coal-fired power plants are the main emission sources of NOx, and several techniques have been developed to remove NOx from flue gas, including selective catalytic reduction (SCR) [8–10] and selective non catalytic reduction (SNCR) [11–13], in which, SNCR alone cannot reach
the NOx emission limit in power plants of China. In order to meet the strict emission standard in China, increasing catalyst layers of SCR or using SNCR together with SCR can be an appropriate strategy. However, the applications of these technologies suffer from several problems of high equipment investment and operating costs, catalyst poisoning and ammonia escape, moreover, the spent catalyst is considered as a new hazardous solid waste. Hence, the oxidation methods [14] have widely been concerned, in which, NOx is oxidized to soluble nitrogenous species and absorbed by alkaline solutions. Although the desired NOx removals are obtained by using these methods, some studies suggest that the method have the disadvantages of higher reagent prices and releasing secondary environmental pollutants [15,16]. Adsorption can be a promising way to deal with NOx, because it has the obvious advantage of combining NOx removal with NOx resource utilization. Furthermore, adsorption process can overcome the disadvantages of SCR or SNCR and can operate at lower temperature (< 100 °C) [17]. Over this decade, activated carbon [18,19], zeolite [20,21], metal compound [22,23] and silica gel [24] were extensively used as adsorbents to remove NOx and other pollutants from flue gas. Among of them, silica gels with rich hydroxyl group had the proper pore size, providing the feasibility of grafting or impregnating the
⁎ Corresponding author at: MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China. E-mail address: [email protected] (Y. Zhao).
https://doi.org/10.1016/j.seppur.2019.115833 Received 4 October 2018; Received in revised form 16 July 2019; Accepted 19 July 2019 Available online 19 July 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
water to remove AN, and dried at 120 °C for 12 h. The synthetic process of S3 was the same as that of S2, except for using AAM instead of AA. For comparison, AA and AAM polymers, denoted as PAA and PAAM were also prepared separately. The preparation method of PAA was as follows: 30 mmol AA was added in 15 ml AN under stirring for 2 h, and then 30 mmol EGDMA and 0.3 mmol AIBN was added to the solution. The mixture was degassed by ultrasonic device for 15 min and blown by N2 for another 15 min to remove oxygen. After that, the mixture was sealed and reacted for 24 h at 60 °C, and the formed lumpy polymer was ground and screened to 50–150 μm. The particles were washed with HCl/methanol (1/9, v/v) solution for several times and then washed with high purity water to neutral. At last, the particles were dried overnight under vacuum at 120 °C. The synthetic process of PAAM was the same as that of PAA, except for using AAM instead of AA.
desired functional groups. By chemical modifying, the physical and chemical properties of silica gels could be evidently improved [25,26], which is important for adsorbing selectively a specific molecule. A well NOx adsorption and desorption process depends strongly on the high-efficiency adsorbent with a high adsorption capacity and low desorption temperature, thus the purpose of the work is making a kind of efficient NO adsorbent by modifying silica gels. Usually, the specific surface areas of mesoporous materials are larger than those of macroporous materials when the pore volumes are the same. Moreover, the inner structures of mesoporous materials are usually more organized than the macroporous materials, which can help the functional groups to be grafted more uniformly on the surface of the materials. Acrylic acid (AA) and acrylamide (AAM) are the common functional monomers which can provide acidic eCOOH and alkaline eCONH2 groups respectively [27]. In the adsorption of NO, the delocalized π-bonds of NO would be affected by the functional groups to form the relative stable intermediates (eNH2+NOe and eCOOH+NO−) which could be decomposed to release NO when the temperature was increased at purging with N2. In this way, the adsorbents could be reused, meanwhile, NO was recycled. The vinyltriethoxysilane (VTES) with eC]C was firstly used to modify the silica gels to carry out the connection between activated silica gels and the functional monomers (AA and AAM) by using the cross-linking agent (ethylene glycol dimethacrylate, EDGMA). Thus, three kinds of modified mesoporous silica were developed by different modification ways. To investigate the effect of modified adsorbents on the characteristics and NO adsorption/desorption behaviors of the adsorbents, the experiments of NO adsorption/desorption were also conducted
2.3. Characterization For BET experiments, N2 was as an adsorbate, the isothermal curve of adsorption/desorption for the adsorbents were measured by a surface area and pore size analyzer (Beckman Coulter SA 3100, USA) at different pressures, in which each sample was degassed under vacuum for 2 h at 150 °C before the measurement. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model of the adsorption isotherm were used to calculate the surface area, average pore size, and pore volume. Thermal stability of adsorbents was characterized by thermogravimetric analysis (TGA) (Netzch STA 449C, German), in which, adsorbents were pre-dried at 120 °C to remove moisture. The surface morphology of adsorbents was observed using a scanning electron microscope (SEM, JEOL JSM-7500F, Japan) and the Fourier transform infrared (FTIR) spectra of adsorbents were obtained by a FTIR spectrometer (Bruker Optics-Tensor II, German).
2. Materials and methods 2.1. Materials Vinyltriethoxysilane (VTES, 97 vol%) was purchased from Macklin reagent (Shanghai, China) and ethylene glycol dimethacrylate (EGDMA, 98 vol%) was purchased from Aladdin reagent (Shanghai, China); acryl amide (AAM, 99 wt%), acrylic acid (AA, 99 vol%), azodiisobutyronitrile (AIBN, 98 wt%), acetonitrile (AN, 99 vol%), toluene (TOL, 99 vol%), ethanol (EtOH, 99.7 vol%), and hydrochloric acid (HCl, 36 wt%) were purchased from Kermel Chemical Reagent Ltd. (Tianjin). High purity water (> 18 MΩ) was produced by lab water purification system (Changfeng Co., Ltd., Beijing). NO and N2 used in experiments were supplied by North Special Gas Co., Ltd. Commercial silica gels (type 3, batch number 0170013) used with diameter of 0.15–0.75 mm were purchased from Qingdao Marine Chemical factory (Shandong, China).
2.4. Adsorption/desorption of NO NO adsorption/desorption property was assessed in a fixed-bed reactor as shown in Fig. 1. In an adsorption/desorption process, 1.5 g of the adsorbent was loaded in the reactor with a heating tape, and two thermal couples located at inlet and outlet of the reactor respectively were used to monitor the temperature during the operation, so that the temperature could be controlled with in ± 0.5 °C. The initial activation of adsorbents was carried out at 120 °C in a highly pure N2 stream at a flow rate of 80 ml/min for 3 h. After that the adsorbents were cooled to the desired adsorption temperature and at the same time the gas stream was quickly switched to 0.1% (volume fraction) NO balanced by N2. NO concentration at the outlet of the reactor was measured by a multi-functional flue gas analyzer (ecomJ2KNpro, German), and the NO breakthrough curve was drawn, from which NO adsorption capacity was calculated by integration of the breakthrough curve, as shown in Eq. (1). During the breakthrough experiments, the blank runs were conducted for three times firstly, and the average dead volume time was calculated as 26 s, as shown in Fig. 8. To truly reflect the adsorption capacity of adsorbents, 26 s were subtracted in the breakthrough runs.
2.2. Preparation In order to synthesize the modified silica adsorbents, the activated silica, denoted as S0 was firstly prepared by immersing silica gels in HCl solution of 8 mol/L for 14 h. Three modified silica adsorbents were obtained by grafting VTES, AA and AAM in the surface of S0, and denoted as S1, S2 and S3, respectively. For preparation of S1, 10 ml of VTES and 50 ml toluene were mixed and then stirred for 5 min, followed by adding 10 g S0. The mixture was refluxed under N2 atmosphere at 110 °C for 12 h with stirring. The remaining particles were washed with absolute ethyl alcohol for 10 h by Soxhlet extractor and then dried for 48 h at 50 °C under vacuum. For preparation of S2, 30 mmol AA was added in 15 ml AN under stirring for 2 h, and then 30 mmol EGDMA, 0.3 mmol AIBN and 1.5 g S1 were added to the solution. The mixture was degassed by ultrasonic device for 15 min and blown by N2 for another 15 min to remove oxygen. After that the supernatant was removed by suction filtration, and the remained particles were sealed and placed in water bath for 24 h at 60 °C. The resultant particles were washed with high purity
Qs =
F × ∫0t (C0 − C ) dt m
(1)
where Qs is the saturated adsorption capacity of NO over adsorbents, mg/g; F represents the influent velocity of NO, ml/min; m is the weight of the adsorbent, g. C0, C represent the concentration of influent and effluent NO respectively, mg/m3; t is the adsorption time, min. In order to investigate the adsorption stability of the prepared adsorbents, the experiments of NO adsorption-desorption cycle were performed by 10 times continuously for each adsorbent, and the desorption processes were conducted in N2 flow rate of 80 ml/min at 2
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 1. Diagram of experimental apparatus for NO adsorption: (1) NO cylinder; (2) N2 cylinder; (3, 4) pressure deducing valves; (5, 6) flow meters; (7–11) stop valves; (12) mixed gas cylinder; (13, 14) three-way valves; (15) heating box; (16) fixed-bed reactor; (17, 18) temperature controllers; (19) flue gas analyzer.
broken to bond with the cross linker (EGDMA) in the presence of initiator (AIBN), and then the functional groups such as eCOOH in AA and eCONH2 in AAM were grafted on the surface of S1 in the presence of cross linking.
120 °C.
3. Results and discussion 3.1. Preparation principle of adsorbents
3.2. Effect of chemical modification on characteristic of adsorbents The preparation principle of S1, S2 and S3 are shown in Fig. 2. Firstly, to prepare S1, the carbon-carbon double bonds (eC]C) are induced on the surface of S0 by the condensation reaction between hydroxyl groups on the surface of S0 and those in VTES. Furthermore, S2 and S3 are prepared by modifying the surface of S1 chemically through polymerization reaction, and in that reaction, the carboncarbon double bonds of AA, AAM and those on the surface of S1 are
Fig. 3 shows the desorption pore size distributions of the S0, S1, S2, and S3 (Fig. 3A), and the PAA and PAAM (Fig. 3B). It can be seen from Fig. 3A that the average diameter of the pores mainly distributes between 5 and 15 nm for activated and modified silica gels, and all of the silica gel materials have the relatively uniform pore sizes, besides, the pore curves show obvious decrease of the peak heights in order of S0, S1,
Fig. 2. Preparation principle of S1, S2 and S3. 3
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 3. Pore size distributions of S0, S1, S2, S3, PAA and PAAM.
Fig. 4. N2 adsorption isotherms of S0, S1, S2, S3, PAA and PAAM.
Table 1 The characteristics of adsorbents. Samples
SBET (m2/g)
Vp (cm3/g)
dp (nm)
NO capacity (mg/g)
S0 S1 S2 S3 PAA PAAM
284.1 264.8 204.4 215.1 163.2 186.5
0.773 0.688 0.581 0.587 0.605 0.577
8.847 8.185 8.131 8.201 31.65 35.05
0.078 0.101 0.108 0.125 0.065 0.060
materials are conformed to type Ⅳ according to IUPAC (International Union of Pure and Applied Chemistry) classification, which shows a feature of mesoporous material with uniform pore size, It can be seen from Fig. 4 that S0, S1, S2 and S3 all have the adsorption equilibrium isotherm of type Ⅳ, and the types of hysteresis are all H1, which indicates that the materials have the mesoporous structure with the narrow distribution of pore diameters. However for PAA and PAAM, the isotherms in Fig. 4 are of type Ⅱ characteristics and the hysteresis types of them are H3, suggesting that the adsorption processes are all multilayer reversible adsorption, so that the polymer materials have the features of unlimited pore size. Comparing the N2 adsorption isotherm of S1, S2 and S3 to that of S0, there is a significant decrease in N2 adsorbing capacity for the modified silica gels, which is consistent to the values of Vp as shown in Table 1. Fig. 5 depicts the FTIR spectra of the activated silica gels, chemical modified silica gels and polymers of AA and AAM. It can be found that the adsorption peaks of SieOeSi in the spectra of all the silica adsorbents are at about 1100 cm−1 and 802 cm−1, which may be
S3 and S2, but no obvious changes in the shape, indicating that although the pore volumes of the modified silica decrease after modification, the uniform pore size structures are retain. Table 1 summarizes the values of surface area (SBET), average pore diameter (dp) and the volume of pores (Vp) for all the materials, which shows that the SBET, dp and Vp of modified silica are all decrease compared with those of the activated silica gels, suggesting that the pore channels of silica materials have been occupied in the modification process. Moreover, the SBET and Vp of S1 are all larger than those of S2 and S3, suggesting that although adding cross linker (EGDMA) can increase the cross linking degree of S2 and S3, that is not favor of getting large SBET and Vp. However, there is no obvious distinction in the values of dp among S1, S2, and S3, which means that the modification do not change the shape of pores in the modified silica gels but blocks up some pores to make cross linking degree increase. It can be observed from Table 1 that the SBET values of PAA and PAAM are obviously smaller than those of S2 and S3, illustrating that the silica substrate makes polymers to be grafted on the surface of materials uniformly, and that is favorable to get the larger specific surface area. As seen in Fig. 3B, the pore sizes of the polymers are distributed at the range of 20–60 nm. Comparing to the pore sizes in Fig. 3A, the distributions of the pore size for PAA and PAAM are more dispersive. It is to note that SBET values of PAA and S2 are smaller than those of PAAM and S3, which can be speculated by that the interaction force of eCOOH is stronger than that of eCONH2 because electro-negativity of O in carboxyl is higher than that of N in amide, giving the higher electron cloud density to adsorbent, thus PAA and S2 have higher cross linking degree than PAAM and S3 respectively, which is not conducive to getting large specific surface area. Nitrogen adsorption isotherms of activated silica gels, chemical modified silica gels and polymers of AA and AAM are shown in Fig. 4. Clearly, the curves of all the silica materials in low Ps/P0 area are convex in shape and show an inflection point, indicating that the single molecule layer adsorption is saturated and the second adsorption layer starts with the increasing Ps/P0. In addition, all curves of silica
Fig. 5. FTIR spectra of S0, S1, S2, S3, PAA and PAAM. 4
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
thus affecting their thermo stability. A slightly decline of less than 5% can be observed in the curve of S0 during the thermal decomposition experiments, possibly because the condensation reaction of groups takes place. However for S1, when the temperature is higher than 500 °C, the proportion of weight loss is a little larger than that of S0, because C]C structures grafted by VTES begin to decompose on the surface of S1. The weight losses of functional groups in S2 and S3 were also determined as 14% for both of them, from which, the weight losses of functional group such as eCOOH and eCONH2 was calculated as 8.54 mg/g for S2 and 8.35 mg/g for S3. Fig. 7 shows the SEM pictures of all adsorbents, in which the SEM picture of S1 is smoother than that of S0, suggesting that after being grafted by VTES, the morphology of the adsorbents changes to more regular. While the SEM pictures of S2 and S3 are rougher than that of S1, suggesting that S2 and S3 are covered by the layer of AA and AAM polymers, respectively. This phenomenon can be explained by the SEM pictures of PAA and PAAM, in which, the pictures show a porous structure and are also consistent with the pore structure data.
resulting from the asymmetric stretch of SieOeSi and the symmetric stretch of SieOeSi, besides, the peaks at around 3430 cm−1 can be considered as OeH flexural vibrations of silanol groups. In the spectra of S1, a peak at 1640 cm−1 that is attributed to stretching vibration of C]C is found, showing that the addition of VTES induces double bonds to the silica during the preparation process. In the spectra of S2, the adsorption peaks at 1740 and 2970 cm−1 are observed, which may be resulting from the adsorption of C]O and stretching vibration of OeH, verifying that the structure of eCOOH is successfully grafted on the surface of S2 by the modification method. The spectra of S3 shows absorbance bands at 1740 and 1635 cm−1 on account of the adsorption peak of C]O and bending vibration of N–H, proving that the structure of eCONH2 is grafted after chemical modification. The FTIR spectra of PAA and PAAM displayed in Fig. 5 demonstrate that the peaks at 2960 cm−1, 1450 cm−1, and 1390 cm−1 are the saturated CeH stretching vibration, saturated CeH bending vibration, and rocking vibration of methyl and methyne for the polymers, while the adsorption peak of C]C vibration is not observed in the area of 1600–1660 cm−1, which may be because all C]C bonds of AA, AAM and EGDMA are broken to bond with cross linker. In the spectra of PAA, OeH stretching vibration at 3420 cm−1 and adsorption peak of C]O at 1720 cm−1 are observed, proving the existent of the framework of eCOOH. In the spectra of PAAM, there is a peak at 1660 cm−1 belonging to the NeH stretching vibration, which demonstrates that the amide group exists in the surface of PAAM after polymerization reaction yet. The thermal decomposition curves of the silica and polymer materials are illustrated in Fig. 6. Obviously, all the materials is stable below 300 °C except for PAA and PAAM, in which the curves of PAA and PAAM have a sharp decline when the temperature increases from 300 to 500 °C, suggesting that the structure of acryl amide and acrylic acid polymers may be destroyed quickly. When the temperature continuously increase from 500 to 700 °C, the rate of decomposition becomes slow. However, the weight of the adsorbents do not change while the temperature is above 700 °C, suggesting that the polymers have already been decomposed completely. For the curves of S2 and S3, when the temperature is higher than 350 °C, the weight of the adsorbents begins to loss, indicating that the decomposition of AA and AAM grafted on the surface of the silica happens, but the curves are maintain steady again at 750 °C, verifying that all polymers are decomposed completely at this temperature. It can be concluded that AA and AAM polymers grafted on the surface of S2 and S3 have a higher decomposed temperature compared to PAA and PAAM, confirming that the polymers in the surface of silica are more steady, and the reasons for this stability are speculated by that AA and AAM in the surface of the pore structures of silica may affect the length of the molecular chain of the polymers,
3.3. NO adsorption capacity and mechanism analysis The NO breakthrough curves of S0, S1, S2 and S3 obtained by the adsorption experiments are shown in Fig. 8 and the corresponding NO adsorption capacity values are summarized in Table 1, which are 0.078, 0.101, 0.108 and 0.125 mg/g for S0, S1, S2 and S3 respectively. Also, the NO adsorption capacity of PAA and PAAM are listed in Table 1 according to the experimental results in our previous study [27]. From Fig. 8 it can be observed obviously that chemically modified silica gels all have longer breakthrough time than that of the activated silica gel, which means that chemical modification can help to improve the surface structure of silica gels and to enhance the NO adsorption ability. The lower adsorption capacity of S0 can be explained as by that silanol groups in S0 appear a weaker intermolecular force for NO, thus decreasing NO adsorption activity. It is clearly from the data shown in Table 1, although the SBET and Vp of modified silica absorbents decrease after modification, the NO adsorption capacity increases, which shows that the effect of functional groups is more important than SBET and Vp in the NO adsorption. Meanwhile, It is found by comparing the adsorption capacity of S1 to those of S2 and S3 that adding polar functional groups benefits the adsorption of NO, because polar functional groups can enhance the hydrogen bond functions between NO and the surface of silica gels. In addition, compared the adsorption capacity of S2 to that of S3, although acrylic acid in S2 has the higher polarity than acrylamide in S3, the larger SBET helps S3 get better NO adsorption performance because more functional groups are exposed on the surface of S3. It can be also found in Table 1 that the adsorption capacity of PAA is better than that of PAAM, which may be resulting from the stronger hydrogen bond force of acrylic acid. By contrast, the NO adsorption capacities of PAA and PAAM are all smaller than those of the modified adsorbents, which can be interpreted by that the polymerization space of AA and AAM are not limited, making the molecular chain of the polymer twist and bind to be huge molecular cluster. However, for S2 and S3, the polymerization space of AA and AAM are limited in the pore channel, forming small molecular clusters, and compared with the small molecular cluster, functional groups of huge ones are harder to be exposed on the surface, using as the active points for NO adsorption, which is an important reason for lower adsorption capacity of PAA and PAAM. Additionally, the lower SBET and uneven pore structure of PAA and PAAM are also unfavorable for NO adsorption. 3.4. Regeneration of silica adsorbent In order to investigate the reuse characteristic of the modified adsorbents, 10 times successive adsorption-desorption cycle for each silica adsorbents were carried out. Fig. 9 shows that decrease rates of NO
Fig. 6. Thermal decomposition curves of S0, S1, S2, S3, PAA and PAAM. 5
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Fig. 7. SEM photograph of S0, S1, S2, S3, PAA and PAAM.
Fig. 8. The NO adsorption breakthrough curves of S0, S1, S2 and S3 at 40 °C. Fig. 9. NO adsorption capacity of S0, S1, S2 and S3 during 10 cycles of NO adsorption at 40 °C and desorption under N2 stream at 120 °C.
adsorption capacity for S0, S1, S2 and S3 are 13.7%, 12.9%, 20.2% and 22.3% in 10 times adsorption-desorption cycle, respectively. It can be observed in Fig. 9, all of the modified adsorbents do not have severe decrease of NO adsorption capacity, suggesting that the modified modified adsorbents have better reproducible ability, and the reaction force is a kind of physical interaction, thus NO can be desorbed.
Comparing the decrease rate of the modified adsorbents, the smaller decrease rate of adsorption capacity for S0 and S1 may be because that the silanol and C]C groups on the surface of S0 and S1 respectively has weaker interaction with NO molecule, thus the NO desorption process is 6
Separation and Purification Technology 229 (2019) 115833
Y. Zhao and H. Wang
Universities of China (No. 2015ZZD07, No. 2019MS105 and 2016XS109); the National Science-technology Support Plan of China (No. 2014BAC23B04-06); the Special Scientific Research Fund of Public Welfare Profession of China (No. 201309018); the Beijing Major Scientific Technological Achievement Transformation Project of China (No. Z151100002815012) and Technology development projects of Sanhe power plant of China (SH [2015]-QT44).
more easily. For S2 and S3, the larger decrease rate of NO adsorption capacity can be attributed to the larger interaction function between the silica gel surfaces and NO molecules. It is speculated that with the increase of recycle numbers, the adsorbed NO molecules in the adsorbent may combine with functional groups by stronger chemical bonds, and it is difficult to be desorbed, from which, the adsorption capacity of the adsorbents will sharply decrease because the adsorbing sites of the adsorbents are occupied by the generated molecules.
References 4. Conclusion [1] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369–379. [2] J. Xiang, L. Wang, F. Cao, K. Qian, S. Su, S. Hui, Y. Wang, L. Liu, Chem. Eng. J. 302 (2016) 570–576. [3] Y. Guo, Y. Li, T. Zhu, M. Ye, Fuel 143 (2015) 536–542. [4] P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, P. Couvreur, G. Ferey, R. E. Morris, C. Serre, 112 (2011) 1232–1268. [5] B. Xiao, P.S. Wheatley, X. Zhao, A.J. Fletcher, S. Fox, A.G. Rossi, I.L. Megson, S. Bordiga, L. Regli, K.M. Thomas, R.E. Morris, J. Am. Chem. Soc. 129 (2007) 1203–1209. [6] B. Supronowicz, A. Mavrandonakis, T. Heine, J. Phys. Chem. C 117 (2013) 14570–14578. [7] G. Narin, S. Ülkü, Micropor. Mesopor. Mat. 234 (2016) 120–129. [8] X. Cheng, X.T. Bi, Particuology 16 (2014) 1–18. [9] X. Lou, P. Liu, J. Li, Z. Li, K. He, Appl. Surf. Sci. 307 (2014) 382–387. [10] M. Karthik, H. Bai, Appl. Catal. B-Environ. 144 (2014) 809–815. [11] N. Modlinski, Energy 29 (2015) 67–76. [12] J. Hao, W. Yu, P. Lu, Y. Zhang, X. Zhu, 122 (2015) 213–218. [13] S. Fu, Q. Song, Q. Yao, Chem. Eng. J. 285 (2016) 137–143. [14] Y. Zhao, Y. Han, T. Ma, T. Guo, Environ. Sci. Technol. 45 (2011) 4060–4065. [15] Y. Zhao, T. Guo, Z. Chen, Y. Du, Chem. Eng. J. 160 (2010) 42–47. [16] Y. Zhao, R. Hao, T. Wang, C. Yang, Chem. Eng. J. 273 (2015) 55–65. [17] J.F. Brilhac, A. Sultana, P. Gilot, J.A. Martens, Environ. Sci. Technol. 36 (2002) 1136–1140. [18] R. Murillo, T. García, E. Aylón, M.V. Navarro, J.M. López, A.M. Mastral, Carbon 42 (2004) 2009–2017. [19] R. Chauveau, G. Grévillot, S. Marsteau, C. Vallières, Chem. Eng. Res. Des. 91 (2013) 955–962. [20] Deng, H.H. Yi, X. Tang, Q. Yu, P. Ning, L. Yang, Chem. Eng. J. 188 (2012) 77–85. [21] H. Yi, H. Deng, X. Tang, Q. Yu, X. Zhou, H. Liu, J. Hazard. Mater. 203 (2012) 111–117. [22] W.B. Li, X.F. Yang, L.F. Chen, J.A. Wang, Catal. Today 148 (2009) 75–80. [23] G. Meng, X. Song, M. Ji, J. Hao, Y. Shi, S. Ren, J. Qiu, C. Hao, Appl. Phys. 15 (2015) 1070–1074. [24] N.N. Linneen, R. Pfeffer, Y.S. Lin, Chem. Eng. J. 254 (2014) 190–197. [25] R. Qu, Y. Niu, J. Liu, C. Sun, Y. Zhang, H. Chen, C. Ji, 68 (2008) 1272–1280. [26] Y. Zhao, Y. Shen, L. Bai, S. Ni, Appl. Surf. Sci. 261 (2012) 708–716. [27] Y. Zhao, H. Wang, T. Wang, Funct. Mater. Lett. 10 (2017) 53–56.
The effects of the mesoporous silica of chemical modification on NO adsorption were investigated by modifying silica gels with vinyltriethoxysilane, acrylic acid and acryl amide polymer, respectively. It was concluded from the characterizations of modified adsorbents that the functional groups (eCOOH and eCONH2) had successfully been grafted at the surface of S2 and S3 respectively, and all of the modified adsorbents had perfect thermal stability. Compared to the acrylic acid and acryl amide polymers in our early works, the silica grafted by these polymer showed the larger SBET and higher NO adsorption capacity. It was interesting to note that though the SBET values of S2 and S3 were lower than that of S1, the NO adsorption capacities of S2 and S3 were larger than that of S1, due to the functional groups on the surface of them. The flow in our experimental system is considered as the nonideal flow, and it is inevitable to have the situations of channeling, backmixing and the uneven distribution of flow rate because of the uneven diameter distribution of adsorbents, bending of fluid pipe and so on, and those all can cause the axial dispersion, which may exist effect the flow rates in our adsorption column. Due to limited space of the manuscript, the influence of axial dispersion on flow rates will be investigated in future by using the axial dispersion model. Acknowledgements The authors appreciate the financial support by a grant from the key project of the National major research and development Program of China (No. 2016YFC0203701, No. 2016YFC0203705 and No. 2017YFC0210603); the Fundamental Research Funds for the Central
7