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Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance sorbent for Hg0 removal in natural gas
Accepted Manuscript Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance sorbent for Hg0 removal in natural gas Huawei Zhang, Huamin Sun, Dingyuan Zhang, Wenrui Zhang, Shaojie Chen, Min Li, Peng Liang PII: DOI: Reference:
S1385-8947(18)32532-4 https://doi.org/10.1016/j.cej.2018.12.059 CEJ 20599
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
29 October 2018 10 December 2018 11 December 2018
Please cite this article as: H. Zhang, H. Sun, D. Zhang, W. Zhang, S. Chen, M. Li, P. Liang, Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance
sorbent for Hg0 removal in natural gas, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej. 2018.12.059
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Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance sorbent for Hg0 removal
in natural gas
Huawei Zhanga*,1Huamin Suna, Dingyuan Zhanga, Wenrui Zhanga, Shaojie Chenb, Min Lia, Peng Lianga* a
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590 PR China
b
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by
Shandong Province and the Ministry of Science and Technology, Qingdao 266590 PR China
Abstract A facile passivation treatment and amino functionalized method was developed for controllable synthesis of effective and water-resistant Ag/MCM-41 composite sorbent for Hg0 removal in natural gas. The Ag nanoparticles were successively confined inside the mesoporous channels of MCM-41 molecule sieve with an average diameter of 3 nm. The synthesized sorbent was found to achieve a complete removal of Hg0 at high space velocity of 50000 h-1 in ambient temperature. At 5% breakthrough, the Hg0 capture capacity of 1% (wt%) silver loaded Ag/MCM-41 was as high as 6.64 mg·g-1, which is much higher than the sample of Ag loaded outside of the pore channels (S-Ag/MCM-41) and other existing commercial sorbents. The H2O has *
Corresponding author. Tel: + 86 13806399945. E-mail address: [email protected] (H.W.Zhang) * Corresponding author. Tel:+86 13678890728. E-mail address:[email protected] (P.Liang)
negligible inhibitory effect on Hg0 removal performance of Ag/MCM-41. In addition, the spent Ag/MCM-41 could be easily regenerated without significant performance degradation over five cycles. We considered the high Hg0 capture efficiency, large adsorption capacity and excellent regeneration performance of Ag/MCM-41 are contributed to the nanoconfinement effect of mesoporous channels of MCM-41 molecule sieve which effectively prevented the growth and agglomeration of Ag nanoparticles. The good hydrophobicity of Ag/MCM-41 is beneficial for the high resistance to H2O. This work represents a giant step toward the preparation of effective sorbent for practical applications of Hg0 removal in natural gas. Keywords: Nanoconfinement; Ag nanoparticles; H2O resistance; regeneration; Hg0 removal 1. Instruction Mercury has become one of the most hazardous environmental pollutants due to its high toxicity, volatility, persistence and bio-accumulation. Its control technologies for releasing emission have caused considerable worldwide concerns in recent years [1-3]. As well known, coal fired flue gas is the primary anthropogenic sources of mercury emission and accounts for approximately 35% of the total emissions. Many effective sorbents such as Halide- and sulfur-impregnated activated carbon, transition metal oxides, metal sulfide, and fly ash have been extensively identified [4-7]. AkiraSano [8] found the activated carbon co-impregnated with both sulfur and chlorine was extremely effective for Hg removal from flue gas, the chlorine in outer layer is most effective for Hg adsorption with a high reactivity, and the sulfur of inner layer
indirectly contact with chlorine species of Hg to form sulfide and sulfate with high stability. Liu et al. [9] prepared Mn-Ce mixed oxides modified wheat straw chars by an ultrasonic-assisted impregnation method, the sorbent exhibited high mercury removal activity at 150°C, the presence of O2 and NO obviously promoted Hg0 removal. Wu et al. [10] developed a incipient wetness impregnation and sulfur-chemical vapor reaction method for the synthesis of CoMoS/γ-Al2O3, the Hg0 adsorption capacity of prepared material is as high as 18.95 mg·g-1, the good performance is attributed to the MoS2 nanosheets coated on surface of the maro- and meso-pores of γ-Al2O3. Duan et al. [11] revealed that NH4Br modification on the fly ash not only improves Hg0 oxidation, but also promotes Hg0 adsorption due to the generation of Br-containing functional groups, HgBr2 is the main adsorption form on the surface of the NH4Br-FA. Unfortunately, the removal of mercury in natural gas is generally ignored due to the lack of emission limitation for natural gas combustion flue gas. In recent years, the demand of natural gas as fuel for residential and transportation has increased significantly. Natural gas is considered a clean energy option for many large energy consumption countries such as Russia, America and China due to the lower contents of contaminant elements of sulfur and nitrogen. The mercury concentrations in natural gas obviously vary with the producing areas [12], and exist predominantly as elemental form. For example, the mercury concentrations in Middle East natural gas plant generally range from 0-50 µg·m-3, while in South East Asia are as high as 200-2000 µg·m-3. Due to the high annual output of natural gas, mercury in natural gas
requires to be removed before combustion to reduce emissions. Furthermore, in the process of natural gas low temperature treatment, the accumulation of mercury would erode aluminum heat exchangers and valves then led to serious accidents. The operation conditions of mercury removal in natural gas are distinct with those of in flue gas. For instance, the mercury in natural gas is typically removed at ambient temperature, which is much lower than that of in flue gas. Most of metal oxides based sorbents used for mercury removal in flue gas are not suitable for natural gas purification due to the reductive atmosphere. It is considered that low temperature activity, high adsorption capacity and regenerable ability are the essential properties for the mercury sorbents in natural gas. Noble metals including silver, gold and palladium can amalgamate with mercury to form amalgam alloys with low solubility at room temperature, the absorbed mercury can be easily decomposed through thermal treatment at higher temperatures, this provide a promising way to produce regenerable sorbents for mercury removal in natural gas. The mercury removal efficiency and capacity of Ag based sorbents is closely related to the valence state, particle size and disperity of Ag particles. Furthermore, in order to use silver effectively and economically, the Ag nanoparticles are generally loaded on different supporters, which showed tremendous superiority than silver films for higher reactivity and capacity. Xu et al. [13] reported an innovative method of depositing silver nanoparticles within the channels of SBA-15 matrix, the synthesized sorbent was found to achieve a complete capture of Hg0 at temperatures up to 200°C. At 1% Hg0 breakthrough, an Hg0 capture capacity as high as 13.2 mg·g-1 was achieved. Yan et al.
[14] reported that graphene acted as a good supporter for Ag nanoparticles. Hg atoms transported on the surface of graphene and combined with Ag particles into Ag-Hg alloy, the acid gases had negligible effect on Hg0 removal efficiency. Carbon nanotube-silver composites also showed excellent removal performance of Hg0 at temperatures below 150°C [15]. In our previous work, an effective and regenerable Ag/4A zeolite nanocomposite for Hg0 removal from natural gas was successfully synthesized using chemical reduction methods, the Ag particles are highly dispersed on the surface of 4A zeolite with average diameter of 15 nm [16]. However, the H2O has an obviously inhibitory effect on Hg0 capture of Ag/4A sorbent. To date, most of the Ag based sorbents often suffered from the disadvantages of high amount of loaded silver, lower Ag atom utilization efficiency and deactivation of H2O. Hence, we developed a facile method for the synthesis of high efficiency and capacity, H2O resistance Hg0 sorbent by employing MCM-41 molecule sieve as the supporter, trimethylchlorosilane (TMCS) as external surface passivator, and 3-aminopropyl triethoxysilane (APTES) as modifier of internal channels. MCM-41 molecule sieve has the characteristics of hexagonal ordered mesopores with plentiful of Si-OH active bonds in the channels, large specific surface area, and the pore size ranged from 3-8 nm, which is a promising material as catalyst supporter and sorbents. For the sample of Ag/MCM-41, Ag nanoparticles were successfully incorporated into the channels of MCM-41 molecule sieve functionalized by amino and methyl groups. The content of loaded Ag is only 1 wt%, which is much lower than the reported Ag-SBA-15 (10 wt%) , indicating the prepared sorbent is much cheaper than other Ag
based sorbent in the industrial application. More importantly, the modification process significantly enhanced the hydrophobicity of the surface of MCM-41 molecule sieve, which is beneficial for the resistant to H2O in natural gas. Furthermore, the characterizations of X-ray diffraction (XRD), Fourier transformed infrared spectra (FTIR), UV-visible electronic absorption spectra (UV-vis), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) were applied to reveal the mechanism of mercury removal over Ag/MCM-41 sorbents. 2. Materials and methods 2.1. Samples preparation Our strategy for the synthesis of the Ag/MCM-41 and S-Ag/MCM-41 samples is illustrated in Fig. 1. For the preparation of Ag/MCM-41, it consists of several gradual functionalization steps. The passivation treatment was firstly carried out on the external surface of non-calcined MCM-41 molecule sieve (containing the cetyltrimethyl ammonium chloride cations of the tensioactive used as a template) by the reaction of TMCS with the silanols on the external surface of MCM-41, the reaction media in this step is n-hexane. The second step is the removal of template using multiple soxhlet extractions with an ethanol solution at 80°C, and the obtained sample is a mesoporous silica externally covered by trimethylsilyl groups. In the third step, the mesoporous MCM-41 molecule sieve was modified by the reaction of APTES with silicon hydroxyl groups in the channels of MCM-41 to introduce
anchoring amino groups. The reduction process of Ag+ to elemental Ag is reduced by hydrazine hydrate. The preparation procedures of S-Ag/MCM-41 are as follows: 1) the modification on the external surface of non-calcined MCM-41 molecule sieve by the reaction of APTES with the the silanols to form ammonia ligand; 2) the Ag+ was reduced to elemental Ag nanoparticles by hydrazine hydrate; 3) extraction of the template agent. All materials used in this study and the detailed preparation procedures were shown in SI. 2.2. Characterization of samples The physical and chemical properties of samples were characterized using XRD, FTIR, UV-vis, N2 adsorption-desorption isotherms, XPS and HRTEM. The detailed characterization methods can be referred in SI. 2.3. Experimental set-up and test procedures An experimental device for evaluating the mercury removal efficiency of the catalysts is shown in Fig. S1 [17]. Mercury concentrations were measured using the instrument (RA-915 Mercury Vapor Monitor, Lumex, Inc. Russia). In a typical test, 0.3 g of sorbent was placed in the quartz reactor (5 mm ID and 600 mm length), and the CH4 gas stream contained about 300 µg/m3 of gaseous Hg0 generated by a mercury permeation device (VICI Metronics) was introduced to the adsorption reactor. The inlet Hg0 concentration (Cin ) of the simulated natural gas stream was measured before the stream entered the reactor. The gas stream was then introduced into the reactor and passed through the sorbent bed to obtain the outlet Hg0 concentrations
( Cout). At the beginning of tests, the system firstly established stable and consistent Hg0 feedings, then performed a basic Hg0 mass balance experiment to ensure the error is less than 5%. The Hg0 removal efficiency was calculated according to the following equation:
Cin Cout Cin
(1)
The long-term evaluation of Hg0 capture was conducted to measure the Hg0 adsorption capacity of sorbents. The adsorption temperature maintained at 30°C and the space velocity was 50000 h-1. In this study, the capacity represents the mass of Hg0 absorbed by unit mass of sorbents once Hg0 removal efficiency reaches to 95%. It can be calculated by Equation (2). t
q
Cin Lt Cout Ldt 0
m
(2)
Where q represents the Hg0 adsorption capacity of sorbents, L is the gas flow rate, t is the adsorption time, and m is the weight of sorbent. 3. Results and discussion 3.1. characterization of sorbents The low-angle and wide-angle powder X-ray diffraction patterns of (a) MCM-41, (b) Ag/MCM-41 and (c) S-Ag/MCM-41 samples are shown in Fig. 2. It can be seen that the representative diffraction peaks of MCM-41 molecule sieve are located at 2.05º, 3.56º and 4.32º, respectively [18,19], which can be assigned to Miller indices (100), (110) and (200) planes, indicating the good crystallinity with the hexagonal
MCM-41 phase. Compared to the fresh MCM-41 molecule sieve, the intensity of XRD peaks in low-angle diffraction patterns of Ag/MCM-41 and S-Ag/MCM-41 decreased slightly, suggesting the inherent disorderness of mesoporous structure caused by the modification process. Furthermore, the four typical peaks at 38.12º, 44.08º, 65.76º and 77.38º appeared in wide-angle patterns of Ag/MCM-41 and S-Ag/MCM-41 samples, which corresponded to the (111), (200), (220) and (311) planes of face-centered cubic crystalline structure of silver, indicating the successful loading of Ag nanoparticles into the channels of mesopores or on the external surface of MCM-41 molecule sieve [20,21]. Considering the XRD spectra of S-Ag/MCM-41, it is clearly that the peaks in low-angle patterns are almost as same as the Ag/MCM-41, while the width of Ag characteristic peak of (111) plane decreased slightly, which implied the average Ag particle size of S-Ag/MCM-41 is larger than that of in Ag/MCM-41 sample. The FT-IR spectra of different samples are shown in Fig. 3a. It can be seen that all of the samples have three strong peaks at around 457 cm-1, 803 cm-1 and 1078 cm-1, which are assigned to the symmetric stretching vibration modes of Si-O-Si inorganic framework. The bands at 1633 and 3424 cm-1 are ascribed to the bending vibration and stretching modes of -OH group, respectively [22,23]. The absorption peak at 958 cm-1 in MCM-41 sample can be assigned to the characteristic mode of silanol groups. Compared to MCM-41 molecule sieve, the absorption peak at 960 cm-1 in Ag/MCM-41 sample almost disappeared, suggesting the silanol groups were replaced by other groups during modification process. In addition, the characteristic peaks of
N-H and C-H bands appeared at 3436 cm-1 and 2921 cm-1, respectively, which demonstrated the existence of -NH2 and -CH3 functional groups in Ag/MCM-41 sample [24,25]. For the FT-IR spectra of S-Ag/MCM-41, a broad band centered in 3434 cm-1 can be attributed to the N-H stretching modes, indicating the presence of amino-functionalized groups in S-Ag/MCM-41 sample. It can be seen from the UV-visible absorption spectra in Fig. 3b that the surface plasmon resonance of samples is quite different. Compared to MCM-41 molecule sieve, new sharp peaks at 391 nm and 400 nm appeared in the samples of Ag/MCM-41 and S-Ag/MCM-41, respectively, which can be attributed to the characteristic peaks of elemental silver [26], indicating the successful loading of Ag nanoparticles in the Ag/MCM-41 and S-Ag/MCM-41 samples. As shown in Fig. 4, the images of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples consisted of many worm-like domains with relatively uniform sizes, which are aggregated into ropelike macrostructures. The loading of silver nanaoparticles didn't change the morphology and structure of MCM-41 molecule sieve. Considering that the all samples have almost the same crystal sizes and morphology, it is proposed that the difference in their Hg0 removal performances might be resulted from their distinguishable Ag nanoparticles[27]. The N2 adsorption-desorption isotherm and pore size distribution of fresh MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples were shown in Fig. 5a, Fig. 5b and Fig. 5c. It can be seen that the N2 adsorption isotherms of three samples exhibited type IV isotherm with sharp capillary condensation steps at relative pressure (P/P0) ranged
with an obvious H1-type hysteresis loop, suggesting the mesoporous structure of samples. The pore size distributions ranged from 3 nm to 8 nm by the Barrett-Joyner-Halenda (BJH) calculation method. Compared with the MCM-41 and S-Ag/MCM-41 samples, the Ag/MCM-41 had an relative narrower hysteresis loop, this could be related to a blockage of the mesopores by Ag nanoparticles. The HRTEM images of fresh MCM-41 molecule sieve in Fig. 5d showed the regular and hexagonal array of uniform channels of mesopores, the pore diameter was around 4.4 nm, which is well agreement with the N2 adsorption-desorption analysis results. It can be seen in Fig. 5e that considerable spherical Ag nanoparticles dispersed on the outer surface of S-Ag/MCM-41 sample with an average diameter of 19 nm, there was almost no Ag particles entering into the pores of MCM-41, indicating the successful loading of Ag particles outside the pore channels of MCM-41. Fig. 5f illustrated the HRTEM image of Ag/MCM-41 sample, it is evidently that the Ag nanoparticles entered and highly dispersed in the pore channels of MCM-41 without obvious aggregation. The average Ag particle size was around 3 nm, which is much smaller than that of in S-Ag/MCM-41 sample. We considered the Ag nanoparticles were successively confined in the mesoporous channels of MCM-41 and then prevented from aggregation, which is beneficial to reduce the particle size of Ag particles. The HAADF-STEM and elemental mapping of Ag/MCM-41 sample in Fig. 5g further proved the nanoconfinement of Ag nanoparticles inner the pores of MCM-4 molecule sieve and the existence of Ag, Si, C and O elements. Fig. 6a showed the survey XPS spectrum of spent Ag/MCM-41 sample. As
anticipate, the peaks corresponded to O1s, N1s, Ag3d, C1s and Si2p appeared, which is consistent with the XRD, FT-IR and elemental mapping characterization results [28]. The narrow Ag3d spectra of fresh and spent Ag/MCM-41 samples were shown in Fig. 6b, two peaks with high intensity were observed at 368.23 eV and 374.33 eV, which were assigned to the photoelectron spectra of Ag0. This illustrated the existence of elemental Ag nanoparticles in Ag/MCM-41. After Hg0 adsorption, it is clearly that the peak strength decreasing and a slight shift of 0.3 eV to lower binding energy were observed for the spent Ag/MCM-41 sample, which indicated the formation of Ag-Hg amalgam [29,30]. The HRTEM images of Ag/MCM-41 sample before and after Hg0 adsorption at 30°C were shown in Fig. 7. In the previous work, we found the particle size of nano-scale silver on the surface of 4A zeolite increased dramatically after Hg0 adsorption due to the formation of Ag-Hg amalgam. However, the average silver particle size of Ag/MCM-41 is almost unchanged after Hg0 adsorption. We considered the mesoporous confinement of MCM-41 molecule sieve inhibited the growth of Ag-Hg amalgam nanoparticles. For the fresh Ag/MCM-41 sample in Fig. 7a, the distinct Ag lattice fringes corresponded to the identified (111) and (200) facets with an interplanar distance of 0.24 nm and 0.20 nm could be observed. After Hg0 adsorption for 8 days, it is clearly form Fig. 7b that the silver lattice fringes of spent sample disappeared completely, which is mainly due to the reaction of silver nano-particles with gaseous Hg0 then caused the transition of crystal Ag to amorphous Ag-Hg amalgam. The HRTEM results indicated the Hg0 firstly reacts with nano-scale silver
particles on its surface and then gradually penetrates into the interior of silver particles. The HAADF-STEM and elemental mapping of spent Ag/MCM-41 sample are shown in Fig. S2, which confirmed the adsorption of mercury in the Ag/MCM-41 sorbent. 3.2. Mercury removal performance of samples The Hg0 removal performances of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples at different temperatures and space velocities are shown in Fig. 8. It can be seen form Fig. 8a that the fresh MCM-41 molecule sieve has a poor Hg0 removal performance due to the absence of active sites despite its large but inert specific surface area. Loading of Ag nanoparticles on MCM-41 molecule sieve greatly improved Hg0 removal performance. Both Ag/MCM-41 and S-Ag/MCM-41 showed almost complete Hg0 capture at 30°C, the dramatic improvement can be attributed to the key role of Ag nanoparticles which can efficiently capture Hg0 by Ag-Hg amalgamation mechanism [31]. When the adsorption temperature increased from 30°C to 90°C, the mercury removal efficiency of Ag/MCM-41 sample almost remained stable, while that of S-Ag/MCM-41 sample decreased to 86.25%. Higher temperatures have adverse effect on Hg0 removal performance, approximately 92.32% and 65.95% of Hg0 breaks through the packed bed of Ag/MCM-41 and S-Ag/MCM-41 at 150°C, indicating the prepared material is not suitable for the processing of flue gas with higher temperatures. Generally, the Hg0 removal performance of Ag/MCM-41 is superior to S-Ag/MCM-41 due to the average particle size of nanoconfined Ag particles in the mesopores is much smaller than that of
outside the pore channels, which is beneficial for the capture of Hg0. The effects of different space velocities on Hg0 removal over the Ag/MCM-41 composite sorbent at 30°C are shown in Fig. 8b. The reaction time for each space velocity is about 2 h. It is clearly that the prepared Ag/MCM-41 sample showed extremely high resistance to space velocity, the Hg0 removal efficiency was almost 100% and kept constant when the space velocity was lower than 50,000 h-1, and then decreased slightly to about 98.01% at high space velocity of 70,000 h-1. The Ag/MCM-41 exhibited higher Hg0 removal efficiency than S-Ag/MCM-41 sample in the space velocity range of 30,000 h-1 to 70,000 h-1. We considered the excellent removal performance at high space velocity is associated with its high porosity with ordered mesopores, large surface area and the highly dispersed nanoscale active Ag particles. The Hg-TPD curve of absorbed Ag/MCM-41 sample at 30°C was shown in Fig. 9, only one peak centered at 215°C can be observed, which is corresponded to the desorption of formed Ag-Hg amalgam alloys. Evidently, the absorbed mercury starts to decompose at approximately 170°C and almost release completely over than 260°C, implying that temperatures at or slightly above 300°C could be enough for the regeneration of spent sorbent. Based on the Hg-TPD results, the spent samples were regenerated by thermal treatment at 300°C for 4 h in N2 atmosphere. As shown in Fig. 10, the adsorption performance of Ag/MCM-41 is still excellent after 5 cycles of regeneration. The Hg0 removal efficiency of regenerated Ag/MCM-41 sample is almost as same as the fresh sample, there is only a slight loss in Hg0 adsorption capacity, which decreases from
6.64 mg·g-1 for fresh sample to 6.20 mg·g-1 for the sample after 5 cycles of regeneration. In contrast, the regeneration process has a pronounced negative effect on Hg0 adsorption performance of S-Ag/MCM-41. After 5 cycles of regeneration, the Hg0 removal efficiency and maximum adsorption capacity decreased from 97.72% and 6.35 mg·g-1 to 94.12% and 4.55 mg·g-1, respectively. The decreasing extent of Hg0 removal efficiency and capacity are much higher than those of Ag/MCM-41 samples. It is well known that the Ag nanoparticles tended to agglomeration at high temperatures. We considered the regeneration process caused the agglomeration of Ag nanoparticles on the outside surface of MCM-41 molecule sieve, which led to the decreasing of Hg0 removal performance of regenerated S-Ag/MCM-41 samples. However, for the Ag/MCM-41 sample, the nanoconfinement effect of mesoporous channels effectively prevented the growth and agglomeration of Ag nanoparticles at high temperatures, which ensured its excellent regeneration performance. As an inevitable component of natural gas, H2O was generally considered has inhibitory influence for Hg0 capture over noble metal based sorbents [32]. The effects of H2O on Hg0 removal performance over Ag/MCM-41 and S-Ag/MCM-41 samples were investigated by introducing 3% to 7% of H2O in the gas stream. As shown in Fig. 11a, the Hg0 removal efficiency of Ag/MCM-41 decreased slightly from 99.9% to 97.07%, while the Hg0 removal efficiency of S-Ag/MCM-41 decreased dramatically from 97.72% to 90.87 % when 3% of H2O is introduced. Fig. 11b showed when the H2O concentrations increased from 3% to 7%, the Hg0 removal efficiencies of Ag/MCM-41 and S-Ag/MCM-41 decreased by 2.5% and 20.3%, respectively,
indicating the Ag/MCM-41 was relatively less affected by H2O. Generally speaking, the Ag/MCM-41 exhibited better resistance to H2O than S-Ag/MCM-41, which ensured the sorbent is potential suitable for treatment of high humidity natural gas. This excellent performance is mainly attributed to its high hydrophobicity caused by modification process. The contact angles of Ag/MCM-41 and S-Ag/MCM-41 samples were further measured to evaluate their surface wettability by H2O. Fig. 11a showed that the static water contact angles on the surface of Ag/MCM-41 and S-Ag/MCM-41 are about 110º and 28º, respectively. It can be concluded that the water can wet the surface of S-Ag/MCM-41 well, while the surface of Ag/MCM-41 is insufficiently wetted, which caused the better resistance to H2O of Ag/MCM-41. 4. Conclusions Two kinds of Ag loaded sorbents for Hg0 removal in natural gas were successfully synthesized by simple yet robust chemical reduction methods. For the Ag/MCM-41 sample, the Ag nanoparticles were confined by the mesopores of molecule sieve and highly dispersed inner the channels with average diameter of 3 nm. For the S-Ag/MCM-41 sample, the Ag nanoparticles were supported on the outside surface of molecule sieve with an average diameter of 19 nm. The Ag/MCM-41 exhibited a superior Hg0 removal performance than S-Ag/MCM-41. For instance, at 5% breakthrough, the Hg0 capture capacity of Ag/MCM-41 is 6.64 mg. The H2O in gas stream had little inhibitory effects on Hg0 removal performance of Ag/MCM-41 due to the high hydrophobicity on its surface caused by modification process, while significantly led to the deactivation of S-Ag/MCM-41. After 5 cycles of regeneration,
the Hg0 capture capacity of Ag/MCM-41 only slightly decreased by 6.20%, the excellent regeneration performance is mainly attributed to the confinement effect of mesoporous channels of Ag/MCM-41 sample effectively prevented the growth and agglomeration of Ag nanoparticles during regeneration process. Considering the high Hg0 capture capacity and Ag atom utilization ratio, strong tolerance to high space velocity and H2O, excellent activity in ambient temperature along with outstanding regeneration performance, the Ag/MCM-41 can be used as a very attractive option for Hg0 removal from natural gas. Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 51406107), the Key Research and Development Program of Shandong Province
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Fig. 1. Schematic illustration for the fabrication of Ag/MCM-41 (a) and S-Ag/MCM-41 (b) samples
Fig. 2. XRD spectra of different samples: MCM-41, Ag/MCM-41 and S-Ag/MCM-41
Fig. 3. FTIR and UV-vis spectra of different samples: MCM-41, Ag/MCM-41 and S-Ag/MCM-41
Fig. 4. The SEM image of fresh MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 5. Nitrogen adsorption-desorption isotherms, HRTEM and mapping images of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 6. The survey XPS spectrum of spent Ag/MCM-41 and narrow Ag3d spectra of fresh and spent Ag/MCM-41 samples
Fig. 7. The HRTEM images of fresh and spent Ag/MCM-41samples
Fig. 8. The effects of temperature and space velocity on Hg0 removal efficiency over MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 9. The Hg-TPD of the Ag /MCM-41 after Hg0 adsorption
Fig. 10. The effects of regeneration cycles on Hg0 removal performance over Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 11. The effects of water vapor on Hg0 removal efficiency over Ag/MCM-41 and S-Ag/MCM-41 samples
A facile method was developed to synthesize regenerable and H2O resistance sorbent for Hg0 removal.
The Ag nanoparticles were successively confined inside the mesoporous channels of MCM-41 molecule sieve.
The nanoconfinement effect is conducive to excellent Hg0 removal performance and regeneration ability.
The high hydrophobicity of Ag/MCM-41 is beneficial for the resistance to H2O.
S1385-8947(18)32532-4 https://doi.org/10.1016/j.cej.2018.12.059 CEJ 20599
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
29 October 2018 10 December 2018 11 December 2018
Please cite this article as: H. Zhang, H. Sun, D. Zhang, W. Zhang, S. Chen, M. Li, P. Liang, Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance
sorbent for Hg0 removal in natural gas, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej. 2018.12.059
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Nanoconfinement of Ag nanoparticles inside mesoporous channels of MCM-41 molecule sieve as a regenerable and H2O resistance sorbent for Hg0 removal
in natural gas
Huawei Zhanga*,1Huamin Suna, Dingyuan Zhanga, Wenrui Zhanga, Shaojie Chenb, Min Lia, Peng Lianga* a
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590 PR China
b
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by
Shandong Province and the Ministry of Science and Technology, Qingdao 266590 PR China
Abstract A facile passivation treatment and amino functionalized method was developed for controllable synthesis of effective and water-resistant Ag/MCM-41 composite sorbent for Hg0 removal in natural gas. The Ag nanoparticles were successively confined inside the mesoporous channels of MCM-41 molecule sieve with an average diameter of 3 nm. The synthesized sorbent was found to achieve a complete removal of Hg0 at high space velocity of 50000 h-1 in ambient temperature. At 5% breakthrough, the Hg0 capture capacity of 1% (wt%) silver loaded Ag/MCM-41 was as high as 6.64 mg·g-1, which is much higher than the sample of Ag loaded outside of the pore channels (S-Ag/MCM-41) and other existing commercial sorbents. The H2O has *
Corresponding author. Tel: + 86 13806399945. E-mail address: [email protected] (H.W.Zhang) * Corresponding author. Tel:+86 13678890728. E-mail address:[email protected] (P.Liang)
negligible inhibitory effect on Hg0 removal performance of Ag/MCM-41. In addition, the spent Ag/MCM-41 could be easily regenerated without significant performance degradation over five cycles. We considered the high Hg0 capture efficiency, large adsorption capacity and excellent regeneration performance of Ag/MCM-41 are contributed to the nanoconfinement effect of mesoporous channels of MCM-41 molecule sieve which effectively prevented the growth and agglomeration of Ag nanoparticles. The good hydrophobicity of Ag/MCM-41 is beneficial for the high resistance to H2O. This work represents a giant step toward the preparation of effective sorbent for practical applications of Hg0 removal in natural gas. Keywords: Nanoconfinement; Ag nanoparticles; H2O resistance; regeneration; Hg0 removal 1. Instruction Mercury has become one of the most hazardous environmental pollutants due to its high toxicity, volatility, persistence and bio-accumulation. Its control technologies for releasing emission have caused considerable worldwide concerns in recent years [1-3]. As well known, coal fired flue gas is the primary anthropogenic sources of mercury emission and accounts for approximately 35% of the total emissions. Many effective sorbents such as Halide- and sulfur-impregnated activated carbon, transition metal oxides, metal sulfide, and fly ash have been extensively identified [4-7]. AkiraSano [8] found the activated carbon co-impregnated with both sulfur and chlorine was extremely effective for Hg removal from flue gas, the chlorine in outer layer is most effective for Hg adsorption with a high reactivity, and the sulfur of inner layer
indirectly contact with chlorine species of Hg to form sulfide and sulfate with high stability. Liu et al. [9] prepared Mn-Ce mixed oxides modified wheat straw chars by an ultrasonic-assisted impregnation method, the sorbent exhibited high mercury removal activity at 150°C, the presence of O2 and NO obviously promoted Hg0 removal. Wu et al. [10] developed a incipient wetness impregnation and sulfur-chemical vapor reaction method for the synthesis of CoMoS/γ-Al2O3, the Hg0 adsorption capacity of prepared material is as high as 18.95 mg·g-1, the good performance is attributed to the MoS2 nanosheets coated on surface of the maro- and meso-pores of γ-Al2O3. Duan et al. [11] revealed that NH4Br modification on the fly ash not only improves Hg0 oxidation, but also promotes Hg0 adsorption due to the generation of Br-containing functional groups, HgBr2 is the main adsorption form on the surface of the NH4Br-FA. Unfortunately, the removal of mercury in natural gas is generally ignored due to the lack of emission limitation for natural gas combustion flue gas. In recent years, the demand of natural gas as fuel for residential and transportation has increased significantly. Natural gas is considered a clean energy option for many large energy consumption countries such as Russia, America and China due to the lower contents of contaminant elements of sulfur and nitrogen. The mercury concentrations in natural gas obviously vary with the producing areas [12], and exist predominantly as elemental form. For example, the mercury concentrations in Middle East natural gas plant generally range from 0-50 µg·m-3, while in South East Asia are as high as 200-2000 µg·m-3. Due to the high annual output of natural gas, mercury in natural gas
requires to be removed before combustion to reduce emissions. Furthermore, in the process of natural gas low temperature treatment, the accumulation of mercury would erode aluminum heat exchangers and valves then led to serious accidents. The operation conditions of mercury removal in natural gas are distinct with those of in flue gas. For instance, the mercury in natural gas is typically removed at ambient temperature, which is much lower than that of in flue gas. Most of metal oxides based sorbents used for mercury removal in flue gas are not suitable for natural gas purification due to the reductive atmosphere. It is considered that low temperature activity, high adsorption capacity and regenerable ability are the essential properties for the mercury sorbents in natural gas. Noble metals including silver, gold and palladium can amalgamate with mercury to form amalgam alloys with low solubility at room temperature, the absorbed mercury can be easily decomposed through thermal treatment at higher temperatures, this provide a promising way to produce regenerable sorbents for mercury removal in natural gas. The mercury removal efficiency and capacity of Ag based sorbents is closely related to the valence state, particle size and disperity of Ag particles. Furthermore, in order to use silver effectively and economically, the Ag nanoparticles are generally loaded on different supporters, which showed tremendous superiority than silver films for higher reactivity and capacity. Xu et al. [13] reported an innovative method of depositing silver nanoparticles within the channels of SBA-15 matrix, the synthesized sorbent was found to achieve a complete capture of Hg0 at temperatures up to 200°C. At 1% Hg0 breakthrough, an Hg0 capture capacity as high as 13.2 mg·g-1 was achieved. Yan et al.
[14] reported that graphene acted as a good supporter for Ag nanoparticles. Hg atoms transported on the surface of graphene and combined with Ag particles into Ag-Hg alloy, the acid gases had negligible effect on Hg0 removal efficiency. Carbon nanotube-silver composites also showed excellent removal performance of Hg0 at temperatures below 150°C [15]. In our previous work, an effective and regenerable Ag/4A zeolite nanocomposite for Hg0 removal from natural gas was successfully synthesized using chemical reduction methods, the Ag particles are highly dispersed on the surface of 4A zeolite with average diameter of 15 nm [16]. However, the H2O has an obviously inhibitory effect on Hg0 capture of Ag/4A sorbent. To date, most of the Ag based sorbents often suffered from the disadvantages of high amount of loaded silver, lower Ag atom utilization efficiency and deactivation of H2O. Hence, we developed a facile method for the synthesis of high efficiency and capacity, H2O resistance Hg0 sorbent by employing MCM-41 molecule sieve as the supporter, trimethylchlorosilane (TMCS) as external surface passivator, and 3-aminopropyl triethoxysilane (APTES) as modifier of internal channels. MCM-41 molecule sieve has the characteristics of hexagonal ordered mesopores with plentiful of Si-OH active bonds in the channels, large specific surface area, and the pore size ranged from 3-8 nm, which is a promising material as catalyst supporter and sorbents. For the sample of Ag/MCM-41, Ag nanoparticles were successfully incorporated into the channels of MCM-41 molecule sieve functionalized by amino and methyl groups. The content of loaded Ag is only 1 wt%, which is much lower than the reported Ag-SBA-15 (10 wt%) , indicating the prepared sorbent is much cheaper than other Ag
based sorbent in the industrial application. More importantly, the modification process significantly enhanced the hydrophobicity of the surface of MCM-41 molecule sieve, which is beneficial for the resistant to H2O in natural gas. Furthermore, the characterizations of X-ray diffraction (XRD), Fourier transformed infrared spectra (FTIR), UV-visible electronic absorption spectra (UV-vis), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) were applied to reveal the mechanism of mercury removal over Ag/MCM-41 sorbents. 2. Materials and methods 2.1. Samples preparation Our strategy for the synthesis of the Ag/MCM-41 and S-Ag/MCM-41 samples is illustrated in Fig. 1. For the preparation of Ag/MCM-41, it consists of several gradual functionalization steps. The passivation treatment was firstly carried out on the external surface of non-calcined MCM-41 molecule sieve (containing the cetyltrimethyl ammonium chloride cations of the tensioactive used as a template) by the reaction of TMCS with the silanols on the external surface of MCM-41, the reaction media in this step is n-hexane. The second step is the removal of template using multiple soxhlet extractions with an ethanol solution at 80°C, and the obtained sample is a mesoporous silica externally covered by trimethylsilyl groups. In the third step, the mesoporous MCM-41 molecule sieve was modified by the reaction of APTES with silicon hydroxyl groups in the channels of MCM-41 to introduce
anchoring amino groups. The reduction process of Ag+ to elemental Ag is reduced by hydrazine hydrate. The preparation procedures of S-Ag/MCM-41 are as follows: 1) the modification on the external surface of non-calcined MCM-41 molecule sieve by the reaction of APTES with the the silanols to form ammonia ligand; 2) the Ag+ was reduced to elemental Ag nanoparticles by hydrazine hydrate; 3) extraction of the template agent. All materials used in this study and the detailed preparation procedures were shown in SI. 2.2. Characterization of samples The physical and chemical properties of samples were characterized using XRD, FTIR, UV-vis, N2 adsorption-desorption isotherms, XPS and HRTEM. The detailed characterization methods can be referred in SI. 2.3. Experimental set-up and test procedures An experimental device for evaluating the mercury removal efficiency of the catalysts is shown in Fig. S1 [17]. Mercury concentrations were measured using the instrument (RA-915 Mercury Vapor Monitor, Lumex, Inc. Russia). In a typical test, 0.3 g of sorbent was placed in the quartz reactor (5 mm ID and 600 mm length), and the CH4 gas stream contained about 300 µg/m3 of gaseous Hg0 generated by a mercury permeation device (VICI Metronics) was introduced to the adsorption reactor. The inlet Hg0 concentration (Cin ) of the simulated natural gas stream was measured before the stream entered the reactor. The gas stream was then introduced into the reactor and passed through the sorbent bed to obtain the outlet Hg0 concentrations
( Cout). At the beginning of tests, the system firstly established stable and consistent Hg0 feedings, then performed a basic Hg0 mass balance experiment to ensure the error is less than 5%. The Hg0 removal efficiency was calculated according to the following equation:
Cin Cout Cin
(1)
The long-term evaluation of Hg0 capture was conducted to measure the Hg0 adsorption capacity of sorbents. The adsorption temperature maintained at 30°C and the space velocity was 50000 h-1. In this study, the capacity represents the mass of Hg0 absorbed by unit mass of sorbents once Hg0 removal efficiency reaches to 95%. It can be calculated by Equation (2). t
q
Cin Lt Cout Ldt 0
m
(2)
Where q represents the Hg0 adsorption capacity of sorbents, L is the gas flow rate, t is the adsorption time, and m is the weight of sorbent. 3. Results and discussion 3.1. characterization of sorbents The low-angle and wide-angle powder X-ray diffraction patterns of (a) MCM-41, (b) Ag/MCM-41 and (c) S-Ag/MCM-41 samples are shown in Fig. 2. It can be seen that the representative diffraction peaks of MCM-41 molecule sieve are located at 2.05º, 3.56º and 4.32º, respectively [18,19], which can be assigned to Miller indices (100), (110) and (200) planes, indicating the good crystallinity with the hexagonal
MCM-41 phase. Compared to the fresh MCM-41 molecule sieve, the intensity of XRD peaks in low-angle diffraction patterns of Ag/MCM-41 and S-Ag/MCM-41 decreased slightly, suggesting the inherent disorderness of mesoporous structure caused by the modification process. Furthermore, the four typical peaks at 38.12º, 44.08º, 65.76º and 77.38º appeared in wide-angle patterns of Ag/MCM-41 and S-Ag/MCM-41 samples, which corresponded to the (111), (200), (220) and (311) planes of face-centered cubic crystalline structure of silver, indicating the successful loading of Ag nanoparticles into the channels of mesopores or on the external surface of MCM-41 molecule sieve [20,21]. Considering the XRD spectra of S-Ag/MCM-41, it is clearly that the peaks in low-angle patterns are almost as same as the Ag/MCM-41, while the width of Ag characteristic peak of (111) plane decreased slightly, which implied the average Ag particle size of S-Ag/MCM-41 is larger than that of in Ag/MCM-41 sample. The FT-IR spectra of different samples are shown in Fig. 3a. It can be seen that all of the samples have three strong peaks at around 457 cm-1, 803 cm-1 and 1078 cm-1, which are assigned to the symmetric stretching vibration modes of Si-O-Si inorganic framework. The bands at 1633 and 3424 cm-1 are ascribed to the bending vibration and stretching modes of -OH group, respectively [22,23]. The absorption peak at 958 cm-1 in MCM-41 sample can be assigned to the characteristic mode of silanol groups. Compared to MCM-41 molecule sieve, the absorption peak at 960 cm-1 in Ag/MCM-41 sample almost disappeared, suggesting the silanol groups were replaced by other groups during modification process. In addition, the characteristic peaks of
N-H and C-H bands appeared at 3436 cm-1 and 2921 cm-1, respectively, which demonstrated the existence of -NH2 and -CH3 functional groups in Ag/MCM-41 sample [24,25]. For the FT-IR spectra of S-Ag/MCM-41, a broad band centered in 3434 cm-1 can be attributed to the N-H stretching modes, indicating the presence of amino-functionalized groups in S-Ag/MCM-41 sample. It can be seen from the UV-visible absorption spectra in Fig. 3b that the surface plasmon resonance of samples is quite different. Compared to MCM-41 molecule sieve, new sharp peaks at 391 nm and 400 nm appeared in the samples of Ag/MCM-41 and S-Ag/MCM-41, respectively, which can be attributed to the characteristic peaks of elemental silver [26], indicating the successful loading of Ag nanoparticles in the Ag/MCM-41 and S-Ag/MCM-41 samples. As shown in Fig. 4, the images of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples consisted of many worm-like domains with relatively uniform sizes, which are aggregated into ropelike macrostructures. The loading of silver nanaoparticles didn't change the morphology and structure of MCM-41 molecule sieve. Considering that the all samples have almost the same crystal sizes and morphology, it is proposed that the difference in their Hg0 removal performances might be resulted from their distinguishable Ag nanoparticles[27]. The N2 adsorption-desorption isotherm and pore size distribution of fresh MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples were shown in Fig. 5a, Fig. 5b and Fig. 5c. It can be seen that the N2 adsorption isotherms of three samples exhibited type IV isotherm with sharp capillary condensation steps at relative pressure (P/P0) ranged
with an obvious H1-type hysteresis loop, suggesting the mesoporous structure of samples. The pore size distributions ranged from 3 nm to 8 nm by the Barrett-Joyner-Halenda (BJH) calculation method. Compared with the MCM-41 and S-Ag/MCM-41 samples, the Ag/MCM-41 had an relative narrower hysteresis loop, this could be related to a blockage of the mesopores by Ag nanoparticles. The HRTEM images of fresh MCM-41 molecule sieve in Fig. 5d showed the regular and hexagonal array of uniform channels of mesopores, the pore diameter was around 4.4 nm, which is well agreement with the N2 adsorption-desorption analysis results. It can be seen in Fig. 5e that considerable spherical Ag nanoparticles dispersed on the outer surface of S-Ag/MCM-41 sample with an average diameter of 19 nm, there was almost no Ag particles entering into the pores of MCM-41, indicating the successful loading of Ag particles outside the pore channels of MCM-41. Fig. 5f illustrated the HRTEM image of Ag/MCM-41 sample, it is evidently that the Ag nanoparticles entered and highly dispersed in the pore channels of MCM-41 without obvious aggregation. The average Ag particle size was around 3 nm, which is much smaller than that of in S-Ag/MCM-41 sample. We considered the Ag nanoparticles were successively confined in the mesoporous channels of MCM-41 and then prevented from aggregation, which is beneficial to reduce the particle size of Ag particles. The HAADF-STEM and elemental mapping of Ag/MCM-41 sample in Fig. 5g further proved the nanoconfinement of Ag nanoparticles inner the pores of MCM-4 molecule sieve and the existence of Ag, Si, C and O elements. Fig. 6a showed the survey XPS spectrum of spent Ag/MCM-41 sample. As
anticipate, the peaks corresponded to O1s, N1s, Ag3d, C1s and Si2p appeared, which is consistent with the XRD, FT-IR and elemental mapping characterization results [28]. The narrow Ag3d spectra of fresh and spent Ag/MCM-41 samples were shown in Fig. 6b, two peaks with high intensity were observed at 368.23 eV and 374.33 eV, which were assigned to the photoelectron spectra of Ag0. This illustrated the existence of elemental Ag nanoparticles in Ag/MCM-41. After Hg0 adsorption, it is clearly that the peak strength decreasing and a slight shift of 0.3 eV to lower binding energy were observed for the spent Ag/MCM-41 sample, which indicated the formation of Ag-Hg amalgam [29,30]. The HRTEM images of Ag/MCM-41 sample before and after Hg0 adsorption at 30°C were shown in Fig. 7. In the previous work, we found the particle size of nano-scale silver on the surface of 4A zeolite increased dramatically after Hg0 adsorption due to the formation of Ag-Hg amalgam. However, the average silver particle size of Ag/MCM-41 is almost unchanged after Hg0 adsorption. We considered the mesoporous confinement of MCM-41 molecule sieve inhibited the growth of Ag-Hg amalgam nanoparticles. For the fresh Ag/MCM-41 sample in Fig. 7a, the distinct Ag lattice fringes corresponded to the identified (111) and (200) facets with an interplanar distance of 0.24 nm and 0.20 nm could be observed. After Hg0 adsorption for 8 days, it is clearly form Fig. 7b that the silver lattice fringes of spent sample disappeared completely, which is mainly due to the reaction of silver nano-particles with gaseous Hg0 then caused the transition of crystal Ag to amorphous Ag-Hg amalgam. The HRTEM results indicated the Hg0 firstly reacts with nano-scale silver
particles on its surface and then gradually penetrates into the interior of silver particles. The HAADF-STEM and elemental mapping of spent Ag/MCM-41 sample are shown in Fig. S2, which confirmed the adsorption of mercury in the Ag/MCM-41 sorbent. 3.2. Mercury removal performance of samples The Hg0 removal performances of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples at different temperatures and space velocities are shown in Fig. 8. It can be seen form Fig. 8a that the fresh MCM-41 molecule sieve has a poor Hg0 removal performance due to the absence of active sites despite its large but inert specific surface area. Loading of Ag nanoparticles on MCM-41 molecule sieve greatly improved Hg0 removal performance. Both Ag/MCM-41 and S-Ag/MCM-41 showed almost complete Hg0 capture at 30°C, the dramatic improvement can be attributed to the key role of Ag nanoparticles which can efficiently capture Hg0 by Ag-Hg amalgamation mechanism [31]. When the adsorption temperature increased from 30°C to 90°C, the mercury removal efficiency of Ag/MCM-41 sample almost remained stable, while that of S-Ag/MCM-41 sample decreased to 86.25%. Higher temperatures have adverse effect on Hg0 removal performance, approximately 92.32% and 65.95% of Hg0 breaks through the packed bed of Ag/MCM-41 and S-Ag/MCM-41 at 150°C, indicating the prepared material is not suitable for the processing of flue gas with higher temperatures. Generally, the Hg0 removal performance of Ag/MCM-41 is superior to S-Ag/MCM-41 due to the average particle size of nanoconfined Ag particles in the mesopores is much smaller than that of
outside the pore channels, which is beneficial for the capture of Hg0. The effects of different space velocities on Hg0 removal over the Ag/MCM-41 composite sorbent at 30°C are shown in Fig. 8b. The reaction time for each space velocity is about 2 h. It is clearly that the prepared Ag/MCM-41 sample showed extremely high resistance to space velocity, the Hg0 removal efficiency was almost 100% and kept constant when the space velocity was lower than 50,000 h-1, and then decreased slightly to about 98.01% at high space velocity of 70,000 h-1. The Ag/MCM-41 exhibited higher Hg0 removal efficiency than S-Ag/MCM-41 sample in the space velocity range of 30,000 h-1 to 70,000 h-1. We considered the excellent removal performance at high space velocity is associated with its high porosity with ordered mesopores, large surface area and the highly dispersed nanoscale active Ag particles. The Hg-TPD curve of absorbed Ag/MCM-41 sample at 30°C was shown in Fig. 9, only one peak centered at 215°C can be observed, which is corresponded to the desorption of formed Ag-Hg amalgam alloys. Evidently, the absorbed mercury starts to decompose at approximately 170°C and almost release completely over than 260°C, implying that temperatures at or slightly above 300°C could be enough for the regeneration of spent sorbent. Based on the Hg-TPD results, the spent samples were regenerated by thermal treatment at 300°C for 4 h in N2 atmosphere. As shown in Fig. 10, the adsorption performance of Ag/MCM-41 is still excellent after 5 cycles of regeneration. The Hg0 removal efficiency of regenerated Ag/MCM-41 sample is almost as same as the fresh sample, there is only a slight loss in Hg0 adsorption capacity, which decreases from
6.64 mg·g-1 for fresh sample to 6.20 mg·g-1 for the sample after 5 cycles of regeneration. In contrast, the regeneration process has a pronounced negative effect on Hg0 adsorption performance of S-Ag/MCM-41. After 5 cycles of regeneration, the Hg0 removal efficiency and maximum adsorption capacity decreased from 97.72% and 6.35 mg·g-1 to 94.12% and 4.55 mg·g-1, respectively. The decreasing extent of Hg0 removal efficiency and capacity are much higher than those of Ag/MCM-41 samples. It is well known that the Ag nanoparticles tended to agglomeration at high temperatures. We considered the regeneration process caused the agglomeration of Ag nanoparticles on the outside surface of MCM-41 molecule sieve, which led to the decreasing of Hg0 removal performance of regenerated S-Ag/MCM-41 samples. However, for the Ag/MCM-41 sample, the nanoconfinement effect of mesoporous channels effectively prevented the growth and agglomeration of Ag nanoparticles at high temperatures, which ensured its excellent regeneration performance. As an inevitable component of natural gas, H2O was generally considered has inhibitory influence for Hg0 capture over noble metal based sorbents [32]. The effects of H2O on Hg0 removal performance over Ag/MCM-41 and S-Ag/MCM-41 samples were investigated by introducing 3% to 7% of H2O in the gas stream. As shown in Fig. 11a, the Hg0 removal efficiency of Ag/MCM-41 decreased slightly from 99.9% to 97.07%, while the Hg0 removal efficiency of S-Ag/MCM-41 decreased dramatically from 97.72% to 90.87 % when 3% of H2O is introduced. Fig. 11b showed when the H2O concentrations increased from 3% to 7%, the Hg0 removal efficiencies of Ag/MCM-41 and S-Ag/MCM-41 decreased by 2.5% and 20.3%, respectively,
indicating the Ag/MCM-41 was relatively less affected by H2O. Generally speaking, the Ag/MCM-41 exhibited better resistance to H2O than S-Ag/MCM-41, which ensured the sorbent is potential suitable for treatment of high humidity natural gas. This excellent performance is mainly attributed to its high hydrophobicity caused by modification process. The contact angles of Ag/MCM-41 and S-Ag/MCM-41 samples were further measured to evaluate their surface wettability by H2O. Fig. 11a showed that the static water contact angles on the surface of Ag/MCM-41 and S-Ag/MCM-41 are about 110º and 28º, respectively. It can be concluded that the water can wet the surface of S-Ag/MCM-41 well, while the surface of Ag/MCM-41 is insufficiently wetted, which caused the better resistance to H2O of Ag/MCM-41. 4. Conclusions Two kinds of Ag loaded sorbents for Hg0 removal in natural gas were successfully synthesized by simple yet robust chemical reduction methods. For the Ag/MCM-41 sample, the Ag nanoparticles were confined by the mesopores of molecule sieve and highly dispersed inner the channels with average diameter of 3 nm. For the S-Ag/MCM-41 sample, the Ag nanoparticles were supported on the outside surface of molecule sieve with an average diameter of 19 nm. The Ag/MCM-41 exhibited a superior Hg0 removal performance than S-Ag/MCM-41. For instance, at 5% breakthrough, the Hg0 capture capacity of Ag/MCM-41 is 6.64 mg. The H2O in gas stream had little inhibitory effects on Hg0 removal performance of Ag/MCM-41 due to the high hydrophobicity on its surface caused by modification process, while significantly led to the deactivation of S-Ag/MCM-41. After 5 cycles of regeneration,
the Hg0 capture capacity of Ag/MCM-41 only slightly decreased by 6.20%, the excellent regeneration performance is mainly attributed to the confinement effect of mesoporous channels of Ag/MCM-41 sample effectively prevented the growth and agglomeration of Ag nanoparticles during regeneration process. Considering the high Hg0 capture capacity and Ag atom utilization ratio, strong tolerance to high space velocity and H2O, excellent activity in ambient temperature along with outstanding regeneration performance, the Ag/MCM-41 can be used as a very attractive option for Hg0 removal from natural gas. Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 51406107), the Key Research and Development Program of Shandong Province
(Grant 2017GSF17101), the Project of Shandong Province Higher
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Fig. 1. Schematic illustration for the fabrication of Ag/MCM-41 (a) and S-Ag/MCM-41 (b) samples
Fig. 2. XRD spectra of different samples: MCM-41, Ag/MCM-41 and S-Ag/MCM-41
Fig. 3. FTIR and UV-vis spectra of different samples: MCM-41, Ag/MCM-41 and S-Ag/MCM-41
Fig. 4. The SEM image of fresh MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 5. Nitrogen adsorption-desorption isotherms, HRTEM and mapping images of MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 6. The survey XPS spectrum of spent Ag/MCM-41 and narrow Ag3d spectra of fresh and spent Ag/MCM-41 samples
Fig. 7. The HRTEM images of fresh and spent Ag/MCM-41samples
Fig. 8. The effects of temperature and space velocity on Hg0 removal efficiency over MCM-41, Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 9. The Hg-TPD of the Ag /MCM-41 after Hg0 adsorption
Fig. 10. The effects of regeneration cycles on Hg0 removal performance over Ag/MCM-41 and S-Ag/MCM-41 samples
Fig. 11. The effects of water vapor on Hg0 removal efficiency over Ag/MCM-41 and S-Ag/MCM-41 samples
A facile method was developed to synthesize regenerable and H2O resistance sorbent for Hg0 removal.
The Ag nanoparticles were successively confined inside the mesoporous channels of MCM-41 molecule sieve.
The nanoconfinement effect is conducive to excellent Hg0 removal performance and regeneration ability.
The high hydrophobicity of Ag/MCM-41 is beneficial for the resistance to H2O.