Facile Fabrication of Cuprous Oxide-based Adsorbents for Deep

Oct 27, 2015 - As-synthesized mesoporous silica SBA-15 was directly used as the support and the precursor Cu(NO3)2 was introduced to the confined spac...
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Research Article pubs.acs.org/journal/ascecg

Facile Fabrication of Cuprous Oxide-based Adsorbents for Deep Desulfurization Jiahui Kou,* Chunhua Lu, Weihua Sun, Ling Zhang, and Zhongzi Xu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China S Supporting Information *

ABSTRACT: Deep desulfurization via π-complexation adsorption is an effective approach for the selective capture of aromatic sulfur compounds. Among various π-complexation adsorbents, Cu(I)-containing materials attract great attention due to their low cost and high efficiency. In the present study, a one-pot thermal treatment strategy was developed to fabricate Cu2O-based adsorbents for the first time. As-synthesized mesoporous silica SBA-15 was directly used as the support and the precursor Cu(NO3)2 was introduced to the confined space between silica walls and template. The subsequent one-pot thermal treatment plays a triple role by decomposing Cu(NO3)2 to CuO, removing the template P123, and reducing CuO to Cu2O. In contrast to the traditional approach, our strategy provides a more convenient method for the preparation of Cu2O-based adsorbents. For a typical material CuAS-3 derived from as-synthesized SBA-15, the yield of Cu(I) is 73.3%, which is obviously higher than its counterpart CuCS-3 prepared from template-free SBA-15 (53.3%). We also demonstrate that the resultant materials are active in adsorptive desulfurization, and the amount of thiophene captured can reach 0.35 mmol·g−1 over CuAS-3, which is obviously better than that over CuCS-3 (0.27 mmol·g−1). Furthermore, the activity in adsorptive desulfurization can be well recovered with no apparent loss. The convenient preparation, high activity, and good reusability make the present materials highly promising for utilization as adsorbents in deep desulfurization. KEYWORDS: Adsorption, Cuprous oxide, As-synthesized mesoporous silica, Deep desulfurization, Thiophene

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INTRODUCTION Deep desulfurization of transportation fuels is of growing urgency due to increasingly stringent environmental regulations and the request for nearly sulfur-free hydrogen streams for fuel cells.1−8 The conventional technology for sulfur removal is hydrodesulfurization (HDS), which is performed at elevated temperatures (>300 °C) and high hydrogen pressures (>2 MPa).9−11 The HDS technique is efficient in eliminating thiols and sulfides; however, it is difficult to remove aromatic sulfur compounds (e.g., thiophene and its derivatives). In addition, the octane number of fuels is obviously decreased in the HDS process because of the hydrogenation of alkenes and arenes. Among various alternatives, deep desulfurization via πcomplexation adsorption attracts much attention, because it can capture aromatic sulfur compounds selectively at room temperature.12−15 It is known that adsorbents play a crucial role in the process of adsorptive desulfurization, and thus many attempts have been made toward the development of highperformance adsorbents based on various porous materials (e.g., inorganic mesoporous silicas,16 organic polymers,17 and metal−organic frameworks18). Since the discovery of mesoporous silica M41S, a series of mesoporous materials with various pore structures have been synthesized by using templating methods.19−25 These meso© XXXX American Chemical Society

porous silicas have high surface areas and large pore volumes and are of great interest for application as adsorbent supports.26−30 To introduce well-known active cuprous sites, for π-complexation adsorption, various approaches such as impregnation and spontaneous thermal dispersion have been employed.31−34 Take a typical adsorbent Cu2O/SBA-15 as an example: the preparation includes four steps as shown in Scheme 1A.35 As-synthesized mesoporous silica SBA-15 is first subjected to calcination, resulting in the generation of templatefree support with open pores. The cupric precursor Cu(NO3)2 is then introduced, followed by the second calcination to decompose the precursor to CuO. Finally, CuO is reduced to Cu2O through high-temperature treatment and the target adsorbent with cuprous sites is produced. Apparently, the traditional approach for the preparation of Cu2O-based adsorbents is rather complicated. Moreover, only about half of the CuO can be reduced to Cu2O [about 50% Cu(I) yield] in the process, which is far from being satisfactory.36 Hence, the development of a facile approach to prepare Cu2O-based adsorbents with a high Cu(I) yield is extremely desirable. Received: May 18, 2015 Revised: October 18, 2015

A

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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dissolved in 75 g of aqueous HCl solution (1.6 M) with stirring at 40 °C. Then 4.25 g of tetraethylorthosilicate (TEOS) was added to the homogeneous solution, and the solution was stirred at this temperature for 24 h. Finally, the mixture was heated to 100 °C and held at this temperature for 24 h under static condition. The asprepared sample was recovered by filtration, washed with water, and air-dried at room temperature. Thermogravimetry (TG) analysis indicated a weight loss of 50% due to the decomposition of template. This weight loss is consistent with the reported value (52%),41 suggesting that the template is well maintained in the pores of SBA-15. The removal of template was carried out in an air flow at 500 °C for 5 h. The precursor Cu(NO3)2·3H2O was introduced to as-prepared SBA-15 by solid-state grinding at ambient conditions for 0.5 h according to the reported method.42,43 The homogeneous powder was thermally treated in a N2 flow at 700 °C for 12 h. The resultant material was denoted as CuAS-n, where n varies from 1 to 4, corresponding to the content of Cu ranged from 1.5 to 5.8 mmol·g−1 (Table 1). For comparison, Cu(NO3)2·3H2O was also introduced to template-free SBA-15 followed by calcination as described above. The obtained material was denoted as CuCS-n, where n varies from 1 to 4, corresponding to the content of Cu ranged from 1.5 to 5.8 mmol·g−1. Because all of the Cu species were recovered in the finial materials, the Cu amount in synthesized materials is identical to the amount of Cu introduced, which is also confirmed by induced coupled plasma-optical emission spectrometry (ICP-OES) analysis. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded using a Bruker D8 Advance diffractometer with Cu Kα radiation in the 2θ range from 0.5 to 5° and 5 to 70° at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Physical Electronic PHI-550 spectrometer equipped with an Al Kα X-ray source (hv = 1486.6 eV) operating at 10 kV and 35 mA. The N2 adsorption−desorption isotherms were measured using an ASAP2020 system at −196 °C. The samples were degassed at 200 °C for 4 h prior to analysis. The Brunauer−Emmett−Teller (BET) surface area was determined using the adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was calculated from the amount adsorbed at a relative pressure of about 0.99. The pore diameter was calculated from the adsorption branch by using the Barrett−Joyner−Halenda (BJH) method. Fourier transform infrared (IR) measurements were conducted on a Nicolet Nexus 470 spectrometer by means of the KBr pellet technique. The spectra were collected with a 2 cm−1 resolution. TG analysis was conducted on a thermobalance (STA-499C, NETZSCH). About 10 mg of sample was heated from 30 to 800 °C in a N2 flow (25 mL·min−1). The total copper content of samples was analyzed by J-A1100 ICP-OES. The content of Cu(I) was measured by titration in terms of the reported method.44 In a typical process, Cu2O in the sample was dissolved in 10 mL of a FeCl3 reagent and 50 mL of H2O. The FeCl3 reagent was prepared by dissolving 75 g of FeCl3 in 150 mL of HCl (37%), 400 mL of H2O, and 5 mL of H2O2 (30%), and the reagent was heated until boiling to remove excess H2O2. The reaction of Cu+ with Fe3+ leads to the formation of stable Cu2+ and Fe2+. The amount of Fe2+ was then determined by the titration with Ce(SO4)2 solution (0.0889 mol·L−1), where phenanthroline was employed as an indicator.38 Adsorptive Desulfurization Test. Thiophene was used as the representative of aromatic sulfur compounds. The model fuel

Scheme 1. Preparation of Cu2O-based Adsorbents through (A) the Conventional Approach, (B) Thermal Treatment of Cu(NO3)2-Containing Template-Free SBA-15, and (C) Thermal Treatment of Cu(NO3)2-Containing AsSynthesized SBA-15, namely, One-Pot Thermal Treatment (TT) Strategy

It should be stated that mesoporous silicas after the removal of template are normally used as supports, and less attention has been given to the as-synthesized materials containing template. By incorporating amine into as-synthesized MCM-41 occluded with template, a new kind of adsorbent was produced and exhibited high activity in the capture of CO2.37 The precursor Cu(NO3)2 was introduced to as-synthesized SBA-15 by Sun’s group, which leads to the formation of CuO with high dispersion degree after calcination and the dispersed CuO can be converted to Cu2O efficiently by using formaldehyde vapor.38 Apparently, there is an extraordinary microenvironment between template and silica walls in as-synthesized mesoporous materials. In the present study, we report for the first time a one-pot thermal treatment strategy to fabricate Cu2O-based π-complexation adsorbents by using as-synthesized mesoporous silica SBA-15 as the support (Scheme 1C). The cupric precursor was directly introduced to the templatecontaining SBA-15, and Cu2O-based adsorbents were produced in a one-pot thermal treatment process. The one-pot thermal treatment plays a triple role by decomposing cupric precursor to CuO, removing the template, and reducing CuO to Cu2O. In comparison with the conventional approach, our strategy is much more convenient for the preparation of Cu2O-based adsorbents. More importantly, the yield of Cu(I) is enhanced obviously by utilizing our strategy. Our results also show that the obtained materials are active in adsorptive desulfurization, and that the activity can be well recovered.

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EXPERIMENTAL SECTION

Materials Synthesis. The support, mesoporous silica SBA-15, was prepared according to the procedure reported previously.39,40 In a typical synthesis, 2 g of triblock copolymer P123 (EO20PO70EO20) was

Table 1. Physicochemical Properties and Adsorption Desulfurization Performance of Different Samples Cu content (mmol·g−1) −1

−1

sample

SBET (m ·g )

Vp (cm ·g )

SBA-15 CuAS-1 CuAS-2 CuAS-3 CuAS-4 CuCS-3

695 519 404 327 222 279

0.921 0.756 0.663 0.536 0.386 0.495

2

3

Cu(I)+Cu(II)

Cu(I)

1.5 2.7 4.5 5.8 4.5

1.2 2.1 3.3 2.7 2.4 B

adsorption capacity (mmol S·g−1) Cu(I) yield (%)

breakthrough

saturation

80.0 77.8 73.3 46.6 53.3

0.09 0.13 0.16 0.25 0.22 0.19

0.14 0.21 0.24 0.35 0.31 0.27

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering containing 550 ppmw (parts per million by weight) sulfur was prepared by mixing thiophene with isooctane. The desulfurization capacity of materials was evaluated on the basis of breakthrough curves. Experiments were performed in a vertical quartz column with a supporting quartz grid. All adsorptive tests were conducted at room temperature. The testing fuel was pumped up with a mini creep pump. The feed was switched to the model fuel and the feed rate was kept at 10 mL·h−1. Effluent solutions were collected at regular intervals until saturation was reached. The sulfur content in effluent solutions was determined with a Varian 3800 gas chromatograpy (GC) instrument equipped with a pulsed flame photometric detector (PFPD). A calibration curve was prepared to correct the GC results. Breakthrough curves were generated by plotting the normalized sulfur concentration versus the cumulative fuel volume. The normalized concentration (c/ c0) was obtained from the detected content (c) divided by the initial content (c0), and the cumulative fuel volume was normalized by the adsorbent weight. The adsorption capacity was calculated by integral calculus. Regeneration of used adsorbents was carried out in the column in situ. A saturated adsorbent by sulfur was treated in flowing Ar at 700 °C for 12 h. The regenerated adsorbent was then cooled to room temperature for the desulfurization test again.

mesoporous structure was well preserved after the introduction of Cu. In contrast to the support SBA-15, the Cu-containing samples exhibit weaker intensity of the diffraction peaks. This gives evidence of the incorporation of Cu species into the mesopores, which decreases the scatter contrast between pore walls and pore space. Figure 1B shows the wide-angle XRD patterns of different samples. The support SBA-15 gives a broad diffraction peak centered at 23°, which can be ascribed to amorphous silica walls. However, some diffraction peaks at 2θ of 36.4°, 42.3°, and 61.4° originated from Cu2O (JCPDS No. 65-3288) appear on the samples CuAS-1, CuAS-2, and CuAS-3 with a Cu content lower than 4.5 mmol·g−1,38 indicating the successful formation of Cu(I) sites by using the present strategy. Moreover, the diffraction peaks become intenser with the increase of Cu content. It is noticeable that on the sample CuAS-4 with a Cu content of 5.8 mmol·g−1, the peaks at 2θ of 35.6°, 38.7°, and 48.7° assigned to CuO (JCPDS No. 48-1548) are observed besides that of Cu2O.36 That means, only part of CuO can be reduced to Cu2O for the sample CuAS-4 with a high Cu loading. In the case of CuCS-3 prepared from template-free SBA-15, the diffraction peaks of CuO and Cu2O coexist. The presence of CuO suggests that the reduction of Cu(II) species to Cu(I) ones in CuCS-3 is more difficult than that in its counterpart CuAS-3 in spite of the same Cu content. In addition to CuCS-3, the XRD patterns of CuCS-1, CuCS-2, and CuCS-4 with different Cu contents were also recorded, and the results are shown in Figure S1. Regardless of the content of Cu, the diffraction peaks of Cu2O in CuAS samples are apparently weaker than that in CuCS counterparts, which implies that CuAS samples possess higher dispersion degree of Cu2O as compared with CuCS. This is due to the confined space between template and silica walls in as-synthesized SBA15, which is beneficial to the dispersion of guest oxides.45,46 The quantitative analysis of Cu(I) content was conducted, and the results are listed in Tables 1 and S1. The Cu(I) content is 1.2 mmol·g−1 in the sample CuAS-1. Taking into account that the total Cu content is 1.5 mmol·g−1, the yield of Cu(I) is as high as 80.0%. For the sample CuCS-1 with the same Cu content as CuAS-1, the Cu(I) content is only 0.5 mmol·g−1. This corresponds to a Cu(I) yield of 33.3%, which is apparently lower as compared with its counterpart CuAS-1. Similarly, the CuAS samples with various Cu contents show higher yield of Cu(I) than corresponding CuCS samples. In addition, the chemical states and quantitative analysis of Cu species in the samples were revealed by XPS. As shown in Figure 2, the peak fitting of Cu 2p3/2 spectra gives two peaks at 932.7 and 934.5

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RESULTS Structural and Surface Properties of Adsorbents. Figure 1A displays the low-angle XRD patterns of mesoporous

Figure 1. (A) Low-angle and (B) wide-angle XRD patterns of SBA-15, CuAS, and CuCS samples.

silica SBA-15 before and after the introduction of Cu. All samples possess an intense diffraction peak accompanied by two weak ones, which can be respectively indexed as (100), (110), and (200) reflections and corresponds to a 2D hexagonal pore symmetry. This means that the ordered

Figure 2. XPS peak fitting of Cu 2p3/2 spectra of (A) CuAS-3 and (B) CuCS-3 samples. C

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering eV, which are originated from Cu 2p3/2+ and Cu 2p3/22+, respectively. Further calculation shows that the material CuAS3 has 71.4% of Cu(I), which is obviously higher than that of CuCS-3 (35.2%). These results thus demonstrate the efficiency of the use of as-synthesized SBA-15 as the support. Figure 3 depicts N2 adsorption−desorption isotherms and pore size distributions of different samples. Corresponding

Figure 4. IR spectra of different samples of (A) before and (B) after thermal treatment in N2 at 700 °C.

demonstrating that the thermal treatment in N2 at 700 °C can remove the template efficiently. In the case of CuAS-3, the vibration bands at 1763, 1383, and 822 cm−1 originated from nitrate are observed besides the bands from P123.52−54 After thermal treatment, all of these bands become invisible, which reflects that that both the decomposition of template and the conversion of Cu(NO3)2 can be completed in a one-pot process. Moreover, the band at 961 cm−1 disappears, suggesting the consumption of silanol groups. By comparing CuCS-3 before and after thermal treatment, it can also be found that Cu(NO3)2 is converted on template-free SBA-15. On the basis of the results above, it is conclusive that thermal treatment in N2 is efficient in removing the template and/or converting the cupric precursor. The Cu(I)-based adsorbents can be produced in a one-pot thermal treatment process, which is much more convenient in contrast to the conventional approach. The resultant Cu-containing samples exhibit ordered mesostructure comparable to the support SBA-15. For the CuAS samples prepared from as-synthesized SBA-15, the yield of Cu(I) is obviously higher than that for the CuCS sample prepared from template-free SBA-15. Moreover, the dispersion of Cu(I) species in CuAS is better as compared with that in CuCS. Proposed Pathway for the Formation of Adsorbents. The formation pathway for the adsorbents was first explored by TG. As presented in Figure 5, the weight loss below 200 °C is negligible for as-synthesized SBA-15, which indicates a trace amount of physisorbed water owing to the presence of organic

Figure 3. (A) N2 adsorption−desorption isotherms and (B) pore size distributions of SBA-15, CuAS, and CuCS samples. Curves are plotted offset for clarity.

surface areas and pore volumes are listed in Table 1. The isotherm shape of Cu-containing samples is similar to the support SBA-15, namely, a type IV isotherm with an H1 hysteresis loop. This is characteristic of materials with cylindrical mesopores.47 It is worth noting that for the Cucontaining samples prepared from as-synthesized SBA-15, there is a “tail” in the isotherms and the desorption is delayed. Furthermore, the phenomena become more obvious with the increase of Cu content. Correspondingly, the pore size distributions become broader and shift to small pore diameters to some extent. Interestingly, both the isotherm and the pore size distribution of CuCS-3 are quite similar to the support SBA-15. On the basis of these results, it is safe to say that Cu species should locate inside the mesopores in the CuAS samples, while outside the mesopores in the CuCS sample. As shown in Table 1, the increase of Cu content leads to the decrease of surface area and pore volume. For the sample CuAS-3 prepared from as-synthesized SBA-15, both surface and pore volume are larger than that of CuCS-3 prepared from template-free SBA-15. It should be noted that the pore diameters of Cu-containing samples are close to that of the support SBA-15. Actually, the shrinkage of mesoporous frameworks takes place in the process of thermal treatment for template removal. Nevertheless, the incorporation of Cu species avoids the shrinkage of mesoporous frameworks during thermal treatment, and subsequently eludes the reduction of pore diameters.36 Figure 4 gives IR spectra of the support SBA-15 and Cucontaining samples before and after thermal treatment. Assynthesized SBA-15 presents IR bands at about 2850−3000 and 1350−1500 cm−1, which can be respectively attributed to the C−H stretching and bending vibrations of the template P123.48 The band at 961 cm−1 is caused by the bending vibration of silanol groups (Si−OH).49−51 After thermal treatment, the bands of P123 disappear and the band of silanol groups becomes weak. This is caused by the decomposition of template and the condensation of silanol groups, thus

Figure 5. (A) TG and (B) DTG curves of SBA-15, CuAS-3, and CuCS-3 before thermal treatment. D

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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before the decomposition of Cu(NO3)2, the confined space no longer exists in the process of Cu(NO3)2 decomposition. In this case, the dispersion degree of Cu species cannot be improved, which is actually analogous to the use of template-free SBA-15 as the support. As a result, the decomposition sequence of Cu precursor and template is of great significance. Wide-angle XRD patterns were employed to characterize the status of Cu species at different stages. Only the diffraction peaks of CuO are observed on the samples treated at temperatures lower than 600 °C (Figure 7). The weak intensity

template. The removal of template P123 occurs between 200 and 600 °C, which corresponds to a DTG peak at 360 °C. The weight loss is 50% and includes the decomposition of P123, the elimination of residual carbonaceous species, and the dihydroxylation of silanol groups. The temperature for the decomposition of P123 in N2 atmosphere (centered at 360 °C) is obviously higher than that in an air atmosphere (centered at 168 °C).36 The conversion of Cu(NO3)2·3H2O on templatefree SBA-15 initiates at the removal of water followed by the decomposition of Cu(NO3)2, corresponding to two main DTG peaks at 130 and 280 °C. An interesting decomposition behavior of Cu(NO3)2·3H2O on as-synthesized SBA-15 is observed. The weight loss below 190 °C can be attributed to the desorption of water. The subsequent weight loss between 190 and 270 °C is derived from the conversion of Cu(NO3)2 as confirmed by the IR results below. The conversion of Cu(NO3)2 displays two sharp peaks at 200 and 230 °C in the DTG curve. Afterward, the decomposition of P123 takes place and persists to 600 °C, which gives a DTG peak at 430 °C. This decomposition temperature is apparently higher than that of P123 in SBA-15 without Cu (360 °C). It is reported that silica walls can catalyze the decomposition of P123, so that the decomposition of P123 in SBA-15 takes place at the temperature lower than that of pure P123.55 When Cu species are introduced between the template and silica walls, however, silica walls can no longer catalyze the decomposition of template. As a result, the decomposition of P123 takes place at higher temperatures. These results also confirm the location of Cu species in the confined space between silica walls and template. Figure 6 presents the IR spectra of CuAS-3 thermally treated in a N2 atmosphere at different temperatures. As described

Figure 7. Wide-angle XRD patterns of the sample CuAS-3 thermally treated in a N2 atmosphere at different temperatures.

of diffraction peaks indicates the high dispersion degree of CuO in mesoporous silica SBA-15. For the sample treated at 700 °C, the diffraction peaks of CuO disappear, while those of Cu2O are detected. This indicates that the temperature of 700 °C is sufficient for the formation of Cu(I) sites for π-complexation adsorption. This process called autoreduction is widely used in the preparation of Cu(I)-based adsorbents at high temperatures in an inert atmosphere, in which Cu(II) can be selectively converted to Cu(I).35,56,57 Further increasing the temperature to 800 °C, Cu2O can also be obtained, whereas aggregation of Cu2O takes place as the diffraction peaks become intense. On the basis of the aforementioned results, it is clear that the formation of adsorbents includes three main steps, namely the decomposition of Cu(NO3)2 to CuO, the degradation of template, and the autoreduction of CuO to Cu2O. It is worthy of note that all of the three steps are completed in a one-pot thermal treatment process, which is apparently superior to the conventional approach that requires repeated calcination. Adsorption Desulfurization Performance. Figure 8 shows the breakthrough curves of a typical aromatic sulfur compound, thiophene, in a fixed-bed adsorber with different samples. The support SBA-15 displays a poor performance and is capable of removing only 0.14 mmol·g−1 at saturation. The introduction of Cu improves the desulfurization capacity remarkably. Even for the sample containing a small amount of Cu, CuAS-1, the adsorption capacity increases to 0.21 mmol· g−1. It is worth noting that the amount of thiophene captured can reach 0.35 mmol·g−1 over CuAS-3, which is evidently higher than that over CuCS-3 (0.27 mmol·g−1) with the same Cu content of 4.5 mmol·g−1 that prepared from template-free SBA-15. In addition to CuCS-3, the adsorptive desulfurization performance of CuCS samples with other Cu contents was also examined. Figure S2 presents the breakthrough curves of thiophene. Table S1 listed the adsorption capacity and compared with that of CuAS samples. It is clear that the adsorption capacity of all CuAS samples is higher than that of

Figure 6. IR spectra of the sample CuAS-3 thermally treated in a N2 atmosphere at different temperatures.

above, the bands at 2850−3000 and 1383 cm−1 can be used to monitor the decomposition of P123 and Cu(NO3)2. It can be seen that the decomposition of Cu(NO3)2 initiates at 200 °C; the IR band at 1383 cm−1 declines obviously at 250 °C and disappears at 300 °C. This is in good agreement with the TG results. It is noticeable that no obvious decomposition of P123 is observed even after the complete decomposition of Cu(NO3)2. For the characteristic bands of P123 at 2850− 3000 cm−1, the intensity becomes apparently weak at 400 °C and almost invisible at 500 °C, which is also in line with the TG results. These results give evidence of the prior decomposition of Cu(NO3)2, which plays an important role in the dispersion of Cu species. If the decomposition of template was completed E

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 9. Adsorbent recycling studies in capture of thiophene using CuAS-3.

Figure 8. Breakthrough curves of thiophene in a fixed-bed adsorber with different samples.

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DISCUSSION Despite great efforts have been devoted, the development of a facile method to prepare Cu2O-based adsorbents is still an open question. In the present study, a one-pot thermal treatment strategy was designed to fabricate Cu2O-based adsorbents using as-synthesized mesoporous silica as the support. The one-pot thermal treatment strategy plays a triple role including conversion of cupric precursor to CuO, degradation of template, and reduction of CuO to Cu2O. On the basis of the results of TG, IR, and XRD, the pathway for the formation of adsorbents can be proposed. As shown in Scheme 2,

their CuCS analogues, thus evidencing the better performance of CuAS samples in adsorptive desulfurization. The obtained materials were also compared with that reported by Sun’s group.38,41 In their studies, two steps are required for the generation of Cu2O/SBA-15 from Cu(NO3)2containing as-synthesized SBA-15, namely calcination in air and autoreduction in Ar.41 In the present study, however, a one-pot treatment strategy was used, that is, direct treatment of Cu(NO3)2-containing as-synthesized SBA-15 in Ar. We postulate that the behavior of Cu species is different in each process. The present one-pot treatment strategy leads to a Cu(I) content of 73.3% for CuAS-3, which is obviously higher than that reported by Sun’s group (62.5%).41 In addition to autoreduction, formaldehyde was also employed for the reduction of Cu(II) by Sun’s group.38 Their results show that formaldehyde is quite efficient in converting Cu(II) to Cu(I) selectively, and the yield of Cu(I) is higher than 99%. Despite the high yield of Cu(I), operations other than thermal treatment (e.g., treatment with formaldehyde) should be included. However, only one single thermal treatment operation was demanded for the present strategy. Moreover, the present material CuAS-3 is capable of removing 0.35 mmol· g−1 of thiophene, and such capacity is much better than that reported by Sun’s group (0.20 mmol·g−1).41 Because the total Cu content is comparable and the precursor is identical, the great difference in Cu(I) yield and adsorption capacity should be attributed to the different synthetic method. Taking account of the importance of reusability in practical applications, regeneration of the spent adsorbent CuAS-3 was conducted after saturation by thiophene. As shown in Figure 9, the breakthrough curves of thiophene with the regenerated adsorbents are quite close to that with the fresh one. Further calculation shows that the adsorbent after three time regeneration can still remove 0.32 mmol·g−1 thiophene at saturation, which is similar to the fresh adsorbent (0.35 mmol· g−1) and means that more than 90% of adsorption capacity of the spent adsorbent can be recovered. He et al.44 reported the use of CuCl2-modified Al-SBA-15 for adsorptive desulfurization. Their results showed that about 80% of adsorption capacity was recovered after three times regeneration. Through the investigation on the regeneration of PdCl2/SBA-15, Wang et al.58 found that only 48% of adsorption capacity could be recovered. The comparison indicates the good reusability of our materials, which may offer a potential candidate for applications in adsorptive desulfurization.

Scheme 2. Pathway for the Formation of Adsorbents by using One-Pot Thermal Treatment Strategy

physically adsorbed water is first desorbed at the temperatures lower than 200 °C. At temperatures between 200 and 300 °C, the decomposition of Cu(NO3)2 takes place in the confined space between silica walls and template, leading to the formation of CuO with high dispersion degree. Afterward, the template is degraded and open mesopores are obtained at 300−600 °C. Further enhancing the temperature above 600 °C, the autoreduction of CuO to Cu2O occurs, and the πcomplexation adsorbents Cu2O-modified SBA-15 are produced. Autoreduction is widely utilized for the selective conversion of Cu2+ to Cu+, in which the formation of Cu+ is achieved thermally, without a reductant such as H2 and CO, in vacuo or under an inert gas flow at high temperatures. The autoreduction of Cu2+ to Cu+ has been realized on various supports such as silica,59 alumina,56 and zeolites.60−62 It is generally accepted that the autoreduction of Cu2+ ions proceeds as follows. The Cu2+(OH−) species are condensed by dehydration followed by the formation of Cu2+−O2−−Cu2+ dimer or oligomer species, and the bridging extra-lattice oxygen F

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

ACS Sustainable Chemistry & Engineering atoms are desorbed as oxygen molecules accompanied by the reduction of Cu2+ to Cu+.63,64 It has been reported that the dispersed Cu2+ species are easily reduced to Cu+ ones in autoreduction for the Al2O3-supported catalyst, whereas the less dispersed Cu2+ species cannot be fully reduced to Cu+ ions under the same conditions.56 On the basis of the report and present investigations, it is postulated that the dispersion degree of CuO has a crucial effect on the yield of Cu(I) during autoreduction. As stated above, the confined space in assynthesized mesoporous silica benefits the dispersion of CuO.45,46 As a result, higher CuO dispersion degree of CuAS samples corresponds to higher Cu(I) yield as compared with that of their CuCS analogues. It is known that the active sites for thiophene capture are Cu(I) species, and the adsorption proceeds through the πcomplexation mechanism.65−67 In the process of adsorption, Cu(I) can form the usual σ bonds with their empty s-orbitals and, in addition, their d-orbitals can back-donate electron density to the antibonding π-orbital (π*) of the sulfur rings. A π-complex is thus formed between adsorbate and adsorbent, and desulfurization by adsorption is realized. Therefore, the content of Cu(I) is essential to the formation of π-complexes and subsequently to the capture of thiophene. By using assynthesized SBA-15 as the support, a high Cu(I) yield can be obtained in the one-pot thermal treatment process. The sample CuAS-3 has the highest Cu(I) content among the samples studied, which should be responsible for the best capacity for thiophene adsorption. The adsorption performance of other samples also correlates with the content of Cu(I).

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS A one-pot thermal treatment strategy was developed to prepare Cu2O-based adsorbents for adsorptive desulfurization. The precursor Cu(NO3)2 was directly introduced to the confined space between silica walls and template in as-synthesized SBA15. The subsequent thermal treatment allows the decomposition of Cu(NO3)2 to CuO, the removal of template, and the reduction of CuO to Cu2O in a one-pot process. In contrast to the conventional approach, our strategy provides a more convenient method for the preparation of Cu2O-based adsorbents. More importantly, the yield of Cu(I) is enhanced evidently by use of our strategy. We also demonstrate that the resultant materials are active in adsorptive desulfurization, and that the activity can be well recovered with no obvious loss. Our strategy may open up an avenue for the design and fabrication of new functional materials by taking advantage of confined spaces. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01051. Additional data for CuCS samples including XRD, Cu(I) contents, and adsorption desulfurization performance (PDF).

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Financial support from the National Natural Science Foundation of China (No. 51303079) is gratefully acknowledged.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*J. Kou. E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.5b01051 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX