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Materials and Interfaces

N-doped carbon nanotube-reinforced N-doped mesoporous carbon for flue gas desulfurization Xinyu Song, Guoqing Ning, Xinlong Ma, Zhiqing Yu, and Gang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04858 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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Industrial & Engineering Chemistry Research

N-doped carbon nanotube-reinforced N-doped mesoporous carbon for flue gas desulfurization Xinyu Song†, Guoqing Ning*,†, Xinlong Ma, Zhiqing Yu, Gang Wang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

Abstract The carbon adsorbents used for flue gas desulfurization in a moving bed reactor often suffer from serious abrasion loss, which has significantly increased the operation cost and reduced the continuous operating period. Here, we present a novel approach to reinforcing N-doped mesoporous carbons (NMPCs) by adding a small amount of N-doped carbon nanotubes (NCNTs). Compared to the undoped carbon nanotubes, the NCNTs exhibit better dispersibility in solvent and enhanced interface combination with NMPCs, which has efficiently enhanced the mechanical strength of the NCNT-NMPC composite materials. The largest fracturing strength and the best attrition resistance are obtained at a NCNTs addition of 0.1 wt. %. Meanwhile, the NCNT-NMPC composite delivers an average SO2 adsorption capacity of 21.2 mg/g, which is 84 % higher than the undoped mesoporous carbon. Our results indicate that the defective sites introduced by N doping can help to improve the interface combination between different carbon species, and that the as-prepared NCNT-NMPC composite material is a promising high performance absorbent for flue gas desulfurization. Keywords: carbon nanotube, mechanical strength, compressive strength, attrition resistance, desulfurization performance

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

1. Introduction SO2 are known to be one of the pollutant emissions of chemical plants, also the major sources of acid rain, acid fog and haze. With the purpose of desulfurization and gas purification, carbonaceous sorbents have attracted much attention in recent years.1-4 The removal efficiency of SO2 was nearly 100% (80% for NOx) at 180 °C using active coke.5 Because the dry SO2 removal process in a moving bed requires carbon adsorbents with high adsorption capacity and high catalytic ability, the preparation and surface modification processes for carbon adsorbents were extensively studied by researchers.6-9 In our previous study,10 a N-doped mesoporous carbon (NMPC) derived from pitch powder has achieved a sulfur capacity 60% greater than the undoped mesoporous carbon (MPC). It indicates that N-doped porous carbons are promising to be used as high capacity absorbents in desulfurization processes. As the flue gas desulfurization process using carbon adsorbents in a moving bed reactor is considered, the serious abrasion loss of the carbon adsorbents has significantly increased the operation cost and reduced the continuous operating period, because of the high price and the low mechanical strength of the porous carbon adsorbents. Therefore, to enhance the mechanical strength for carbon absorbents is highly desirable for reducing their abrasion loss and consequently lowering the operation cost of desulfurization processes. CNTs have excellent electrical,11-13 thermal

14-16

and especially mechanical properties17-20 (1-2 TPa modulus, 100-200

GPa strength), and are envisaged to be promising additives in composite materials for conductive21-23 and mechanical enhancements.24-28 For example, Li et al.29 prepared a CNT film/epoxy composite with specific strength of 0.66 GPa/g/cm3, higher than the value of the chemical vapor deposition (CVD) grown film (0.35-0.44 GPa/g/cm3); Qian et al.30 added 1 wt% CNTs in a matrix material, and the stiffness of the resulting composite increased 36-42%; Schadeler et al.31 found a 40% stiffness increase of Epoxy resin with 5 wt% addition of CNTs. One can see that the addition of a small amount of CNTs is very effective for enhancing the mechanical properties for many 2


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composite materials.25 Good dispersion of CNTs in a composite and a high interface bonding strength between CNTs and the host material are crucial to a successful mechanical reinforcement by CNTs, but still remain great challenges.32 Here, N-doped CNTs (NCNTs) with good dispersibility are applied to reinforce NMPC. More defects introduced by N doping permit a tighter interface bonding between NCNTs and NMPC, thus leading to higher fracturing strength and attrition resistance for the NCNT-NMPC composite as compared to the CNT-MPC and CNT-NMPC composites. The SO2 adsorption capacity of the NCNT-NMPC composite is significantly superior to those of the undoped ones. Therefore, it is concluded that the novel NCNT-NMPC composite is capable of exhibiting both excellent mechanical strength and high adsorption capacity, which makes the material very promising to be a high quality adsorbent for flue gas desulphurization.

2. Experimental materials and procedures 2.1. Raw Materials. Pitch powder supplied by Liaohe Refinery, China, was used as carbon source. The pitch powder is composed of 48.08% C, 1.80% S and 1.83% N (weight percent) with a hydrogen to carbon ratio (H/C) of 1.18. Light MgO and melamine were purchased from Sigma-Aldrich Corporation (Shanghai, China) in analytical purity. Water-soluble phenolic resin was purchased from Zhicheng company, China. CNTs were prepared by a CVD process using a FeMo/Al2O3 catalyst and a reactant gas of C2H4.33, 34 NCNTs were prepared by a CVD process using C2H4 (500 mL/min) as carbon source and NH3 (50 mL/min) as nitrogen source. The optimized reaction temperature was 700 °C. Porous MgO templates were prepared by a boiling treatment of light MgO, as previously reported.10 2.2. Synthesis Process. Two types of materials were prepared: cylindrical monoliths for fracturing tests, and powders for attrition tests and SO2 adsorption tests. In a typical synthesis of NCNT-NMPC composite cylindrical monoliths, a certain amount of NCNTs (0.05-0.2 wt%, based on the mass of pitch powder) were dispersed in a small amount of ethanol with the help of ultrasonification for 30 minutes. The NCNTs prepared by a CVD process were grinded in a mortar, and then was directly 3



used without further treatment. After that, 10 g pitch powder, 10 g melamine, 10 g porous MgO and 5 g phenolic resin (curing agent to help maintain the shape) were added into the obtained NCNTs ink with the consecutive stirring to help homogeneous mixing. The slurry was then evaporated at 70 °C with stirring. The obtained grey powder was dried in an oven at 70 °C overnight to remove the solvent thoroughly, and then smashed to a fine powder using a grinder. The fine powder was briquetted to carbon cylindrical monoliths with a diameter of 10 mm and a height of 10 mm at a moulding pressure of 2 to 8 MPa. The final N-doped carbon composite material obtained after calcination at 700 °C for 30 min were named as xNCNT-NMPC (x is the weight percent of NCNTs). As comparison samples, NCNT-MPC and CNT-NMPC monoliths were prepared by the similar processes without using melamine or replacing NCNTs by the undoped CNTs. Meanwhile, MPC and NMPC monoliths without carbon nanotubes were also prepared for comparison. The as-obtained monoliths were directly used as samples in the fracturing tests, and further crush was performed on the monoliths to obtain powdery samples (see below). 2.3. Characterization. The as-prepared materials were characterized by scanning electron microscopy (SEM, Quanta 200F, FEI, Holand), transmission electron microscopy (TEM, F20), Brunauer–Emmett–Teller (BET) SSA measurements by N2 adsorption-desorption (JW-BK222, JWGB Sci & Tech) and thermogravimetric analysis (TGA, STA409MPC, Net-zsch, Germany) under oxygen flow at a temperature ramp rate of 5 °C/min. Pore size distribution was derived from the adsorption branch of the isotherms, calculated using the Barrett-Joyner-Halenda (BJH) method. The pressure testing machine is ZQ-PT880D manufactured by Zhiqu Precision Instruments co. LTD. 2.4. Fracturing tests. Firstly, the tested cylindrical monolith was placed in the center of the stainless steel tray of the pressure testing machine. Once all the parameters had been set (such as press direction, moving speed, et al.), the pressure testing instrument started to slowly squeeze the monolith to broken. The instrument recorded the biggest pressure at the breaking point. The fracturing strength (Fs) is 4


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calculated by the formula as follows: ‫= ݏܨ‬

ܲ ܵ

where Fs is the intensity pressure (MPa) of the fracturing strength, P is the max pressure (N) recorded by the instrument, and S is the stressed area (0.785 cm2 for the monolith with a diameter of 1 cm). The sample number for each material is not less than 5. The fracturing strength is the average value for all parallel samples. 2.5. Attrition tests. The test method is based on the most commonly used fluidized-bed test standard D5757-95 with some modifications to adapt to the materials. The as-prepared carbon cylindrical monoliths were slightly ground and sieved to particles within the size range of 0.1-0.8 mm. In a typical run, 5 g sieved particles were directly used without purification, and were put into the tubular fluidized-bed with an updraft air flow of 6 L/min, which is supplied by an air compressor. A cylindrical bag of filtration paper was used to collect fine particles that had been carried out by the air flow. The attrition process was kept for 5 hours in total. The collected fines at the first 1 hour are regarded as the initial ultrafine powder contained in the pristine sieved particles. The Attrition Index (AI) is used to evaluate the attrition resistance, and defined as a unitless value numerically equal to the percent attrition loss in the last four hours, and the calculation method is based on the following equation: AI, % =

݉ସ ݉ସ + ݉ோ

where m4 = the mass of fines collected at the last four hours, g; mR = the mass of particles remained in the attrition tube after the test, g. All the masses recorded are accurate to the nearest 0.01 g. 2.6. Desulfurization tests. The sieved particles were purified by 1 M hydrochloric acid to remove MgO. Desulfurization study was carried out in a tubular fixed-bed reactor (40 mm in diameter), using 1.5 g of the purified carbon material. The height of the fixed bed is around 8 mm. The tested carbon materials were tiled with a very flat 5



surface to ensure the uniform contact for gas-solid, and no channeling in column was observed. The simulated flue gas containing 400 ppm SO2, 5% O2 (v/v) and balanced N2 was introduced from the top of the tubular reactor, with a total gas flow rate of 2 L/min. Considering the better adsorptive effect can be achieved at a relatively lower temperature and 308 K is the environment temperature, the adsorption test was performed at 308 K. The outlet SO2 concentration is recorded by a flue gas analyzer (Ecom-EN2) continuously every 5 seconds. Breakthrough time (min) and breakthrough sulfur capacity (mgSO2/gcarbon) were engaged to evaluate the desulfurization performance.

3. Results and discussion

Figure 1. Synthesis process of the NCNT-NMPC composite.

3.1. Characterization of the NCNT-NMPC composite As shown in Figure 1, the NCNT-NMPC composite is prepared by briquetting the mixture of NCNTs, porous MgO (templates), pitch powder (carbon source) and melamine (N source) into a cylinder, followed by carbonization in N2 atmosphere. The CNTs prepared by CVD processes have diameters in the range of 10-15 nm (Figure 2a). The bamboo-like structure of NCNTs (Figure 2b and Figure S1 in Supporting Information) forms a sharp contrast with the undoped CNTs, which is a typical characteristic for N-doped CNTs. The diameters of the NCNTs are centered at ~20 nm, obviously larger than the undoped CNTs. In order to compare the dispersibility of the two kinds of CNTs, the NCNTs and the undoped CNTs were respectively dispersed in ethanol with the help of ultrasonication for 30 min. As shown in Figure 2c, the NCNTs suspension kept uniform appearance without obvious 6


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sedimentation for even 30 minutes, while obvious sedimentation of CNTs was observed for the CNTs suspension after 5 min standing. It indicates that stably dispersing NCNTs in ethanol is much easier than the undoped CNTs. The better dispersibility of NCNTs is ascribed to the N-containing functional groups, which improve the hydrophily of NCNTs’ sidewalls.21 The stable dispersion of NCNTs is helpful to construct an evenly-distributed network of nanotubes in the NCNT-NMPC composite. In order to evenly mixing the solid materials, powdery MgO templates, pitch and melamine were used. As shown in Figure 2d, the MgO powder is composed of porous hexagonal MgO sheets with abundant mesopores. The pitch powder (Figure 2e) demonstrates irregular morphology (Figure 2f). TEM images of the NCNT-NMPC composite are shown in Figure 2g & h. Even distribution of NCNTs in the composite is found in a wide view field of TEM observation. Disordered carbon layers with many breakage structures can be observed in both NCNTs and NMPC (Figure 2h). A NCNT to pitch ratio of 0.1% was used in the synthesis of the typical NCNT-NMPC composite. Considering that the carbon yield of pitch is usually around 50%, the NCNT content in the composite is estimated to be ~ 0.2 wt. %.

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Figure 2. TEM of (a) CNTs and (b) NCNTs; (c) the dispersion of NCNTs and CNTs in ethanol (left: NCNTs, right: CNTs, and the concentration of nanotubes is approximately 0.5 mg/ml); (d) SEM of MgO after 700 °C calcination (the inset is a corresponding TEM image); (e) photo and (f) SEM picture of pitch powder; (g & h) TEM images of the NCNT-NMPC composite.

XPS spectra of NCNTs, NMPC and 0.1NCNT-NMPC are shown in Figure 3a. XPS analysis is carried out to further investigate the complex manner between 8


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NCNTs and NMPC. The N content of NCNTs detected by XPS analysis is 2.8 atom%. The evaporated melamine may be enclosed and adsorbed by pitch and porous MgO, which makes it hardly run away into gas phase. The N content for NMPC is 9.9 atom% with the addition of melamine, much higher than that for MPC (4.82 atom%) (Figure S2). It indicates that the addition of melamine has effectively led to N doping of the carbon material. N doping of carbon materials using melamine as N source was also reported in many previous works.35, 36 The N doping level of 0.1NCNT-NMPC (6.1 atom%) is lower than that of NMPC (9.9 atom%). Adding NCNTs in 0.1NCNT-NMPC has likely led to a looser stacking manner of the porous carbon sheets, as compared to that without NCNTs (NMPC). Therefore, more melamine might volatilize during the calcination with the presence of NCNTs, thus resulting in a lower N doping level. The high resolution of N1s of NCNTs can be deconvoluted into three peaks (Figure 3b), corresponding to pyridine N (398.75 eV), graphitic N (401.45 eV) and N-oxides (405.21 eV), respectively.37 N1s spectra of NMPC and 0.1NCNT-NMPC composite exhibit the similar fitting configuration and are composed of pyridine N and graphitic N (Figure 3c and d). Melamine can release highly active N atoms in the high temperature decomposition, and the active N atoms are inserted into the carbon frameworks during the polymerization of pitch molecules in the form of pyridine N or graphitic N. The reason that no N-oxides peak is observed for the 0.1NCNT-NMPC composite is ascribed to the low concentration of NCNTs and the possible reduction of the O-containing groups during the carbonization of pitch. The Raman spectra of the as-prepared materials are shown in Figure 3e. The D band to G band intensity ratio (ID/IG) of the NCNT-NMPC composite (0.899) is obviously lower than those for NMPC (0.945) and NCNTs (0.948). It means that the amount of defects in the NCNT-NMPC composite is smaller than the single NMPC or NCNTs, which can be attributed to the covalent combination of the active carbon atoms on the defective sites during the formation of the NCNT-NMPC composite. XPS analysis coupled with Raman spectra indicates that there is a strong interaction between NCNTs and NMPC in the composite. 9



Figure 3. (a) XPS survey patterns and high resolution N 1s spectra of (b) NCNTs, (c) NMPC and (d) 0.1NCNT-NMPC; (e) Raman spectra of NCNTs, NMPC and 0.1NCNT-NMPC.

3.2. Mechanical property of the NCNT-NMPC composite

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Figure 4. Photos of carbon cylindrical monoliths (a) before and (b, c) after crushed; (d) fracturing strength of the NCNT-MPC composites with different NCNT content; (e) fracturing strength of the as-prepared materials at different CNT or NCNT content; (f, g) SEM of the NCNTs interdigitate junction in the fracture surface of 0.1NCNT-NMPC.

The pictures of carbon cylindrical monoliths before and after crushed are presented in Figure 4a-c. The effect of NCNT content for the NCNT-MPC composites has been investigated in a NCNT content range from 0 to 2 % (Figure 4d). The NCNT-MPC composite with a NCNT content of 0.1 wt. % exhibits the highest fracturing strength (5.65 MPa), 21 % higher than that for MPC (4.68 MPa). At higher 11



NCNTs content (0.2-2 wt. %), the fracturing strength begins to drop considerably, probably because that the excess addition of fluffy NCNTs has aroused a loose pack of the composite materials, and that agglomeration of NCNTs has increased at a higher NCNT concentration (Figure S3). As shown in Figure 4e, the initial fracturing strength of NMPC (5.26 MPa at the CNT content of 0 %) is slightly higher than that of MPC (4.68 MPa). SEM and TEM observation of the MPC (Figure S4) and NMPC (Figure S5) shows that these materials are composed of mesoporous carbon sheets with an appearance similar to the porous MgO templates. The higher fracturing strength of NMPC is likely because of the existence of hard carbon derived from melamine10 and the increased adhesion stress between defective N-doped carbon segments. Replacing CNTs by NCNTs (NCNT-MPC) or replacing MPC by NMPC (CNT-NMPC) has obviously enhanced the fracturing strength of the composite materials. As indicated by Figure 4e and Figure 5d, the NMPC composites exhibit higher mechanical strength as compared to the MPC composites. These results can be attributed to the enhanced interaction between NMPC sheets due to N doping. The fracturing strength of the CNT-NMPC composite is much higher than that of the NCNT-MPC composite (Figure 4e), implying that N doping of MPC is more efficient than N doping of CNTs for improving the mechanical strength of the composites. The phenomenon can be ascribed to the fact that the contents of CNTs in the composites are much less than the contents of MPC, and therefore the amount of N atoms in NMPC is much more than that in NCNTs. It indicates that the N doping is efficient to increase the binding force between CNTs and porous carbons. The NCNT-NMPC composite with the NCNTs content of 0.1 wt% exhibits the highest fracturing strength (7.35 MPa), which is 57% higher than that of MPC. The fracture surface of the 0.1NCNT-NMPC composite was observed by SEM. In Figure 4f, a single NCNT pulled out of the composite is observed, indicating that the nanotube has contributed to resist the breaking of the monolith and thus has enhanced the mechanical strength of the monolith. In Figure 4g, a curly NCNT is connected with two carbon blocks, helping to construct an elastic structure that can resist continuous disturbance. It has 12


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been reported that the addition of CNTs less than 2% is capable of improving the interlaminar shear strength of composite materials.25 Our results indicate that the N doping is an efficient approach to further enhancing the combination between nanotubes and porous carbons, which consequently leads to a much enhanced mechanical property for the composite material.

Figure 5. SEM images of (a) the original 0.1NCNT-NMPC particles, (b) the 0.1NCNT-NMPC particles after attrition test and (c) the surface of 0.1NCNT-NMPC particles after attrition; (d) the relationship between AI and NCNT concentration.

In order to investigate the actual application performance for carbon particles in a moving bed reactor, attrition tests were carried out on the as-prepared materials. As shown in Figure5a and b, the 0.1NCNT-NMPC particles after an attrition test become smoother and the particle size is smaller (almost a half) than the original particles. NCNTs can be clearly observed on the surface of a composite particle (Figure 5c). As shown in Figure 5d, the AI value of xNCNT-MPC significantly decreases from 14.64% to 9.44% as the NCNT content increases from 0 to 0.1 wt. %. xNCNT-NMPC 13



composites show the similar trend, well consistent with the above results for the fracturing tests. The lowest AI value (2.9%), corresponding to the best attrition-resisting performance, is achieved in 0.1NCNT-NMPC. It indicates that the samples containing 0.1 wt. % NCNTs exhibit the largest mechanical strength.

3.3. Desulfurization Performance. To evaluate the desulfurization performances of the composite carbon materials, three typical samples, MPC, NMPC and 0.1NCNT-NMPC, were used in the desulfurization tests, after acid washing to remove the MgO templates. The N2 adsorption-desorption isotherms (Figure 6a) and pore distributions (Figure 6b) of NMPC and 0.1NCNT-NMPC are similar, and the surface specific area (SSA) of 0.1NCNT-NMPC (223.45 m2/g) is higher than NMPC (202.94 m2/g). It shows that the addition of NCNTs has a positive effect on maintaining the porous structure of NMPC. Among the three samples, 0.1NCNT-NMPC delivers the best desulfurization performance, with the longest breakthrough time (42 min, Figure 6c) and the largest breakthrough capacity (21.2 mg/g in average for the 2nd to 9th cycles, Figure 6d). The average SO2 capacity by 0.1NCNT-NMPC (21.2 mg/g) is 84 % higher than that by MPC (11.5 mg/g in average for the 2nd to 9th cycles). The enhanced SO2 capacity for 0.1NCNT-NMPC is attributed to the essentially high SO2 capacity of N-doped carbon species10 and the homogeneous distribution of NCNTs between NMPC porous sheets to inhibit their agglomeration. Our results indicate that N doping is capable of enhancing the mechanical performance and the SO2 adsorption capacity for the CNT-MPC composites at the same time. The as-obtained NCNT-NMPC composite material is promising to be used as a superior absorbent in the flue gas desulfurization processes.

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Figure 6. (a) N2 adsorption-desorption isotherms and (b) mesopore size distribution of NMPC and 0.1NCNT-NMPC; (c) desulfurization curve and (d) breakthrough SO2 capacity of MPC, NMPC and 0.1NCNT-NMPC.

4. Conclusions A NCNT-reinforced NMPC composite (NCNT-NMPC) has been successfully fabricated with an even dispersion of nanotubes and much enhanced interface combination, which is realized by introducing N doping to both the nanotube and the porous

carbon.

markedly-enhanced

As

the

results,

mechanical

the

strength

NCNT-NMPC (fracturing

composite

strength

and

exhibits anti-wear

performance) and higher SO2 adsorbability at the same time. The strong interaction between NCNTs and NMPC in the composite is attributed to the covalent combination of the active carbon atoms on defective sites. With an optimized NCNT content of 0.1 wt. %, the best attrition-resisting performance with the lowest AI value of 2.9 % and the largest SO2 capacity of 21.2 mg/g is achieved. The as-obtained 15



NCNT-NMPC composite material is promising to be used as a superior absorbent in the flue gas desulfurization processes. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. EDS mapping picture of NCNTs, XPS survey of MPC and the high resolution of N1s, SEM and TEM pictures of MPC, NMPC and 2NCNT-NMPC(PDF)

Author Contributions X.Y.S and G.Q.N. contribute equally to this manuscript. X.Y.S did the experiments. Z.Q.Y did the characterization works. X.Y.S, X.L.M. and G.Q.N wrote the paper. G.Q.N. supervised the project. G.W. provided pitch powers. All authors discussed the results. Corresponding Author *E-mail: [email protected]. ORCID Guoqing Ning: 0000-0002-6711-8201 Notes The authors declare no competing financial interests.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21706283), Science Foundation of China University of Petroleum, Beijing (No. 2462015YQ0314 and No. 2462017YJRC003).

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