3.1 CO adsorption of prepared adsorbents
Figure 2 showed CO adsorption capacities of the prepared adsorbents. According to the literature17, 21, 24-26, the dispersion of copper in molecular sieve was a key factor affecting CO adsorption. Figure 2a summarized the effect of solvent in solvothermal process on adsorbent performance. When using water as the solvent, the CO adsorption capacity of the prepared adsorbents were 1.14 mL/g, 9.87 mL/g and 4.79 mL/g respectively. However, when using C2H5OH as the solvent, the adsorption capacity of the prepared adsorbents significantly improved, which were 1.35 mL/g, 10.00mL/g and 12.67 mL/g respectively. The CO adsorption capacity of the adsorbent prepared with C2H5OH as solvent was 28.4 % higher than that of the adsorbent prepared with water as solvent. It indicated that, in solvothermal process, the solvent played an important role in ion exchange and ion distribution. Different solvent had different solubility, polarity, surface tension and volatility, which resulted in a different presence state and mobility of copper salts in solvothermal process. C2H5OH is a good dispersing agent in chemical industry. When C2H5OH was used as the solvent, it would enhance the mass transfer and distribution of the copper salts in HY molecular sieve. So, C2H5OH was chosen as the solvent in solvothermal process in this paper.
Figure 2b showed the effect of solvothermal temperature on adsorbent performance. In solvothermal process, with increasing solvothermal temperature, the thermal motion of the ions in solution became more active, which enhance the mass transfer and ion exchange in solvothermal process. Therefore, high solvothermal temperature was beneficial to improve the CO adsorption performance of the adsorbent. Considering the volatility of the solvent, 70 ℃ was chosen as the solvothermal temperature.
Figure 2c showed the effect of activation temperature on adsorbent performance. Before activation, the adsorbent had a very low CO adsorption capacity, as listed in Figure 2c. According to the literature, Cu(II) almost had no contribution to CO adsorption, while Cu(І) was recognized as the preferred adsorbent form, which had strong affinity to CO in CO adsorption27-29. In order to change the valence state of copper from Cu2+ to Cu+, it was necessary to activate the sediments. The activation temperature directly affected the adsorbent performance because it deternined the amount of Cu(І) active sites generated. With the increase of activation temperature, the CO adsorption capacity of the prepared adsorbents increased obviously. When the activation temperature was 290 ℃, the CO adsorption capacity of the adsorbent reached the highest, indicating more Cu(I) active sites generated. When the activation temperature increased further, the CO adsorption capacity of the adsorbent did not increase, but decreased gradually. The reason may exist in two aspects: one was that higher temperature turned Cu(I) into Cu, which had low CO adsorption ability30-32; The other was that higher temperature may cause the aggregation of the active sites. So, the activation temperature was chosen as 290 ℃.
It was important to choose an appropriate mole ratio of CuCl2·2H2O/Cu(CH3COO)2·H2O. Figure 2d showed the effect of Cu(CH3COO)2·H2O/CuCl2·2H2O mole ratio on adsorbent performance. Even if the total amount of copper was constant, the CO adsorption capacity of the adsorbents were quite different. Whether water or C2H5OH as the solvent, it was obvious that the prepared adsorbent had a very low CO adsorption capacity when using a single Cu(CH3COO)2 or CuCl2 as the copper precursor. The reasons were as follows: 1) Very little Cu2+ loading onto HY molecular sieve through ion exchange; 2) Very little amount of Cu2+ converted into Cu+ during activation process. More explore was needed.
With binary copper salts as the precursors, the experimental results were interesting. When using water as the solvent, the optimal Cu (CH3COO) 2 ·H2O / CuCl2·2H2O mole ratio was 1/1, as listed in Figure 2a. Xue had revealed the mechanism of equal mole of Cu(HCOO)2 and CuCl2 impregnating on active carbon5:
Cu(HCOO)2 + CuCl2 = 2CuCl + H2O + CO2 + CO (1)
The divalent copper was reduced to monovalent copper via self-redox reactions, and the monovalent copper was highly dispersed on the surface of AC. When Cu (HCOO) 2 / CuCl2 mole ratio was small, the decomposition product Cu (HCOO) 2 was not enough to reduce the CuCl2 to monovalent copper, resulting in less active sites on the adsorbent, so the CO adsorption capacity was small; When Cu (HCOO) 2 / CuCl2 mole ratio was 1, Cu (HCOO) 2was appropriate to reduce the CuCl2 to monovalent copper, resulting in much active sites on the adsorbent, so the CO adsorption capacity was large; When the molar ratio of Cu (HCOO) 2 / CuCl2 was greater than 1, a part of Cu (HCOO)2 reduced CuCl2 to monovalent copper, producing the active site, and the excess Cu (HCOO)2 played a small role in CO adsorption, so the adsorption capacity of CO was reduced, as shown in (1).
While using C2H5OH as the solvent, the experiment results were different. When Cu(CH3COO)2·H2O/CuCl2·2H2O mole ratio was 1/3, the CO adsorption capacity of the prepared adsorbent was only 1.35 mL/g. When Cu(CH3COO)2·H2O/CuCl2·2H2O mole ratio was 1/1, the CO adsorption capacity of the prepared adsorbent increased significantly, which was similar to that of the adsorbent prepared with water as the solvent. When Cu(CH3COO)2·H2O/CuCl2·2H2O mole ratio was 3/1, the CO adsorption capacity of the prepared adsorbent reached a maximum, which was 12.67 mL/g. It was strange that the optimal Cu (CH3COO) 2 ·H2O / CuCl2·2H2O mole ratio was no longer 1/1, but 3/1. The experimental results indicated that, more complicated mechanism perhaps occurred. C2H5OH probably acted not only as a dispersion agent, but also as a reducing agent in the interaction. In the similar self-redox reactions described above, C2H5OH also played a role, resulting in the presence of more active Cu(I) in the adsorbent,and further exploration was needed. In summary, the optimal Cu (CH3COO) 2 ·H2O / CuCl2·2H2O mole ratio was chosen as 3/1.
The total copper amount was also important because it decided the Cu(I) active site number in HY molecular sieve. When Cu (CH3COO) 2 ·H2O / CuCl2·2H2O mole ratio remained 3/1, the effect of total copper amount on adsorbent performance was illustrated in Figure 2e. The results showed that as the total copper amount increased from 1.75 mmol Cu2+/gHY to 7.0 mmol Cu2+/gHY, the CO adsorption capacity increased from 6.70 mL/g to 15.15mL/g. However, it gradually decreased with the further increase of the total copper amount. Adsorption occurs at the active sites of an adsorbent. At low copper loading, optimal CO adsorption capacity may not be reached due to lack of active sites, while excess copper loading may block the pore entrances of the Y molecular sieve to decrease the surface area and CO adsorption capacity. Therefore, the optimal copper loading was 7.00 mmol Cu2+/gHY.
An adsorption isotherm can provide information on the adsorption capacity of an adsorbents. Figure 2f illustrated the CO adsorption/desorption isotherms. It was a typical type I isotherm, suggesting the strong interaction between CO and the adsorbent via a π-complexation. In Figure 2f, the plateau for the adsorbent (7.0 mmol Cu2+/gHY) was reached very near the value of 42 mL/g, while for the adsorbent (3.5 mmol Cu2+/gHY) , the plateau was reached at about 30 mL/g. That was, the adsorbent with 7.0 mmol Cu2+/gHY contributed more active sites than the adsorbent with 3.5 mmol Cu2+/gHY.
The stability of CuY was tested in the fixed bed. The regeneration was performed at 100 ℃ at atmospheric pressure at N2 flow rate of 100 mL/min. After 5 times of adsorption and regeneration, the adsorption capacity was almost unchanged, indicating that the character of the adsorbents were stable.
3.2 Adsorbents characterization
Figure 3 illustrated the N2 adsorption-desorption equilibrium isotherms ( BET ) of HY molecular sieve and the adsorbents prepared with different amount of copper loading at 77K. According to IUPAC, the adsorption-desorption isotherms of N2 all exhibited the typical type І isotherm that was obtained on micro-porous materials, where mono-layers adsorption were formed on the surface33,34. From Figure 3a, it was found that, with the increase of copper loading on the adsorbents, their N2 adsorption amounts apparently decreased. The BET surface area, pore volume and average pore diameters of HY molecular sieve and the adsorbents prepared with different amount of copper loading were summarized in Table 2. With the copper loading increasing, the specific surface area and pore volume of the molecular sieve decreased gradually because the copper loaded on the cage of the Y molecular sieve. For the typical type І isotherm, the surface area of the sample was much smaller than the surface area of the hole volume, and the adsorption capacity was controlled by the volume of the hole. As for the CO adsorption, the adsorption of CO was not only closely related to its specific surface area, but also to the amount of active sites. For the copper loading on the molecular sieve, on the one hand, it increased the number of CO adsorption active sites, but on the other hand, it reduced the specific surface area of the molecular sieve. This conclusion was consistent with the experimental results, as shown in Table 1. It could be observed that the CO adsorption capacity increased from 0.12 mL/g to 15.15 mL/g with the increase of copper loading amount from 0 to 7.0 mmol Cu2+/gHF, which was attributed to the π complexation between CO and Cu(І) active sites of the adsorbents. The CO adsorption/desorption isotherms of the adsorbents were in good agreement with the figure of N2 adsorption-desorption isotherms, indicating that all the micropores were all accessible to CO. When the copper loading amount increased from 7.0 mmol Cu2+/gHF to 10.5 mmol Cu2+/gHF, the CO adsorption capacity decreased slightly because much copper active site reduced the specific area of the Y molecular sieve.
Figure 4a illustrated the XRD patterns of HY molecular sieve and the adsorbents activated at different temperatures. Compared with HY molecular sieve, the crystalline structure of the adsorbents were well maintained after activated at different temperatures35. Before activation, though there were Cu (CH3COO)2 ·H2O and CuCl2·2H2O loading on HY molecular sieve through solvothermal process, XRD did not show any information about Cu (CH3COO)2·H2O, CuCl2 ·2H2O or CuCl. The result was different from Xue5, and the possible reason was that the loading methods were different in adsorbent preparation. Compared with the dipping method by Xue et al., the solvothermal method made the copper salts disperse on HY molecular sieve more evenly. After activation at 170 ℃, the diffraction peaks of CuCl appeared, which displayed a weak diffraction peak at 28.5o19,36, implying more CuCl come into being after 170 ℃ activation. At the same time, some CuCl perhaps changed into Cu(І)Y through ion exchange, the following TG-MS experiments confirmed this opinion. After activation at 290 ℃, the diffraction peaks of CuCl disappeared and only a little of Cu displayed a very weak diffraction peak at 43.3o, indicating that higher activation temperature perhaps promoted the ion exchange between CuCl and HY to form Cu(І)Y, the following TG-MS experiments confirmed this opinion too. Much Cu(І)Y in adsorbent was benefit to CO adsorption, which was in agreement with the CO adsorption experiment results. At same time, some Cu came into being from CuCl or Cu(І)Y.
According to theory, experiments and literature37-39, it was supposed that the mechanism of the copper in preparing process was below:
Cu2+ ⇄ Cu+ ⇄ Cu(І)Y ⇄ Cu (2)
CuCl + HY ⇄ Cu(І)Y + HCl (3)
Cu2+ ⇄ Cu+ ⇄ Cu (4)
It was verified that Cu2+ and Cu had almost no contribution to the CO adsorption of the adsorbents. Compared with Cu+, Cu(І)Y play a more important role in CO adsorption. XRD results indicated that the multiple copper valences maybe coexistence together in the adsorbents, and more explore was needed.
Figure 4b illustrated the XRD patterns of HY molecular sieve and the adsorbents prepared with different amounts of copper salt. When the copper loading was 3.5 mmol Cu2+/gHY, after activation at 290 ℃, only a little of Cu displayed a very weak diffraction peak at 43.3o. When the copper loading was 7.0 mmol Cu2+/gHY, the diffraction peaks of Cu increased at 2θ values of 43.3o and 50.4o; At the same time, the diffraction peaks of CuCl appeared. When the copper loading was 10.5 mmol Cu2+/gHY, not only the diffraction peaks of Cu and CuCl further increased, but also the diffraction peak of CuCl2 appeared, indicating that copper salts overloaded on the molecular sieve. The overloaded copper salts were stacked on the molecular sieve, which had no benefit to CO adsorption. So, the optimum copper loading amount was 7.0 mmol Cu2+/gHF, and the conclusion was in accord with experimental results.
3.3 XRF analysis
Table 3 listed the main chemical elements in the prepared adsorbents, and the mole ratio of Cu (CH3COO)2·H2O/CuCl2 ·2H2O was 3. It was found that the main elements in the adsorbents were Si, Al, Cu, Cl and Na elements. With the increase of copper loading from 1.75 mmol Cu2+/gHF to 10.5 mmol Cu2+/gHF, the weight concentration of copper element and chlorine element increased obviously, while the mole ratio of Cu/Cl was neither 2/1 nor 1/1. It was verified that the presence of copper on the HY molecular sieve was very complex. When the copper loading amount was 1.75 mmol Cu2+/gHF, the mole ratio of Cu/Cl was 6.92, implying that the majority of the chlorine element changed into HCl via ion exchange with HY molecular sieve and evaporated from the molecular sieve, as shown in (3). The results also indicated that, in the course of the morphological transition of copper, the molecular sieve also played a role. When the copper loading amount was 7.00 mmol Cu2+/gHF, the mole ratio of Cu/Cl decreased to 2.15. Although more ion exchange occurred on the molecular sieve, it was unquestionable that more chlorine stayed on the molecular sieve. When the copper loading amount was 10.00 mmol Cu2+/gHF, the mole ratio of Cu/Cl recovered a little. This may be because more Cu was generated, and Figure 4b verified this conjecture.
3.4 XPS analysis
In order to investigate the relationship between the valence states of the copper and the CO adsorption performance of the prepared adsorbents, XPS experiment was carried out. Figure 5a-d showed the survey XPS results of the prepared adsorbents. From Figure 5, Si, Al, Cu, Cl, C and O were all existed in the adsorbents. After activation at 170 ℃, 290 ℃ and 380 ℃, there was Si, Al, Cu, C and O still existed in survey spectra, while the characteristic peak of Cl gradually weakened and disappeared. From Figure 5, it was found that copper often existed in multiple forms simultaneously, and chlorine may be removed from the molecular sieve because of ion exchange.
Figure 6 showed the XPS spectra of Cu element in the adsorbents prepared at different conditions. Before activation, the adsorbent showed two intense peaks at 935.6 eV and 954.7 eV, accompanied with the Cu2+ satellite peak at 940-947 eV, which could be attributed to the binding energy of Cu2+ 2p3/2 and Cu2+ 2p1/2 respectively, as shown in Figure 6a. Figure 6a also showed two intense peaks at 932.6 eV and 952.5 eV, which could be attributed to the binding energy of Cu+ 2p3/2 and Cu+ 2p1/2 respectively. The result was not in agreement with Gao’s, and it indicated that there was Cu+ coming into being in solvothermal process19,40,41. After activation at 170 ℃, 290 ℃ and 380 ℃, Cu2+ 2p3/2 peak, Cu2+ 2p1/2 peak, Cu+ 2p3/2 peak and Cu+ 2p1/2 peak still existed while the height of the peaks changed, which indicated the amount of Cu2+ andCu+ changed. The fitted XPS spectra of the adsorbents activated at different temperatures was listed in Figure 7. From Figure 7 it was found that the amount of Cu+ was the highest after activation at 290 ℃ while the amount of Cu+ was the lowest before activation. Combining XRD and XRF analysis, it should be emphasized that Cu+ existed not only as CuCl, but more as Cu(І)Y. The XPS result showed a positive correlation with the CO adsorption performance of the adsorbent, which was in agreement with the above mechanism.
3.5 TG-MS analysis
TG-MS experiment was conducted to investigate the mechanism of the adsorbent prepared at different stages, as shown in Figure 8. The experiment was carried out from room temperature to 290 ℃ at a rate of 5 ℃/min and then maintained for 1 h under Ar and the flow rate is 50 mL/min. From Figure 8a, the first step weight loss was at about 50-110℃, which was attributed to the evaporation of water. The second weight loss step was at 150-290℃, which was attributed to the activation of the adsorbent. In the activation process, some Cu(CH3COO)2 and CuCl2 was reduced to CuCl, and a small amount of gases such as H2O and CO were generated correspondingly, as listed in Figure 8b. In activation process, as listed in Figure 8b too41,42, some CuCl exchanged with HY molecular sieve, and a small amount of gas such as HCl were generated correspondingly, as proposed in (3).
It was a pity that the mechanism was still not completely clear and further exploration were required. The question were below:
- The role of C2H5OH in adsorbent preparing;
- The transform of Cu(CH3COO)2;
How to enhance the ion exchange in the activation process