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ISSN: 2574 -1241
ISSN: 2574 -1241
Received: November 20, 2023; Published: November 29, 2023
*Corresponding author: Jiali Zhang and Yulong Zhang, College of Chemistry and Chemical Engineering, Henan Key Laboratory of Coal Green Conversion, Henan Polytechnic University, Jiaozuo, Henan 454000, PR China
DOI: 10.26717/BJSTR.2023.53.008474
Separation and purification of CO is an important chemical process. Adsorption is competitive due to its efficient operation, low energy and mature technology. For CO adsorption, the capacity and selectivity of the adsorbent is the key factor to determine the efficiency. In this paper, alcohol was used as the dispersing agent, CuCl2 and Cu(CH3COO)2 were used as the precursors mixing with HY molecular sieve, then followed by activation at different temperatures to prepare CO adsorbents. The maximum CO adsorption capacity was 42 mL , which was 28.4 % higher than that of the adsorbent prepared with water as the dispersing agent. The adsorbents were characterized by BET, XRD, XRF, XPS and TG-MS. It was revealed that CuII can be reduced to CuI at 290℃ and highly dispersed on surface of HY. Ion exchange occurred between HY molecular sieve and CuI salts, and CuIY came into being. Compared with CuII, Cu0 and even CuY/HY, CuIY had the best CO adsorption performance.
Keywords:CuIY; Binary CuII Precursors; Solvent; CO adsorption
Carbon monoxide (CO) is well known as an important chemical raw material.The synthesis of carbon-based chemicals such as acetic acid, dimethyl carbonate, acetic anhydride, methyl formate and isocyanate all require high-purity CO as the raw materials.1-3 CO is a toxic air pollutant that has brought up environmental problems such as acid rain, ozone depletion and climate change, which can lead to significant health risk.4,5 Moreover, slight CO is a harmful substance in some industrial gas, which can deactivate the active sites of catalyst, especially for some precious metal catalysts such as Pt et al.6,7 Therefore, the separation and purification of CO from gas mixtures is necessary due to its industrial benefits and environmental effects.There are many methods to separate CO from mixed gases such as cryogenic distillation, absorption and adsorption.8-10 In theory, separation is an energy-consuming process because the second law of thermodynamics does not favor it. Compared with the traditional technologies, adsorption is competitive due to its low energy cost, efficient operation and matured technology.11,12 In adsorption separation, the commonly used carriers are molecular sieve, activated carbon, synthetic resin, activated alumina and other porous carriers.13-15 Molecular sieve has been widely considered as a promising adsorbent material for CO separation and purification due to its good thermal stability, excellent adsorption selectivity, large specific surface area and narrow pore size distribution.16,17 For CO adsorption, the capacity and selectivity of the adsorbent is the key factor to determine the efficiency. Many adsorbents incorporated by CuІ or AgІ have received much attention because they can capture CO molecules through π-complexation.18,19 Compared with physical adsorption, the π-complexation bonds are generally stronger than electrostatic and Van der Waals interactions, so it shows higher selectivity. Compared with general chemisorption, its relative weak π-complexation force make it easy to regenerate.19 In particular, CuІ adsorbent, due to its cheap and easy availability, is regarded as a promising adsorbent, has been widely studied in recent years.
Traditionally, adsorbent was made by single copper precursors. As early as the 1970s, the United Carbon Corporation (UCC, US) had reported a patent about CO adsorbent prepared by ion exchange of Cu+ or Ag+, but its CO adsorption performance was poor.20 Based on a general principle that compounds tend to spontaneously disperse on the high surface supports to form a monolayer, Xie et al. prepared CuCl/Y adsorbent by a thermal treatment of a mixture of CuCl and Y molecular sieve.17, 21 The results showed that CuCl dispersed uniformly on Y molecular sieve at atomic level and the adsorbent was widely used in industry.22 Takahashi et al. exchanged excess Cu(NO3)2 solution with NaY for 24 hours at room temperature and reduced Cu2+ to Cu+ using different reducing agents such as ammonia or CO, followed by activation at 300-450 ℃ under He.23 Sun et al. used NaY molecular sieve as carrier and CuCl2 as precursor, and prepared CuCl/Y under mild reduction of glucose. It was found that the separation performance and the stability of the adsorbent were good.The application of single copper precursor was restricted due to their instability or insolubility. In order to introduce more CuI on molecular sieve, multiple attempts have been made. Using binary CuII precursors was a potential solution to solve the problems such as low-loading and easy oxidation. Some researchers had already made some explorations.5 A CuCl/AC adsorbent was made by a facile route of physically mixing CuCl2 and Cu(HCOO)2 powder with activated carbon (AC), followed by heating at 533 K under vacuum, and the result was good.It is well known that CuCl2 and Cu(CH3COO)2 are more stable and cheaper than CuCl and Cu(HCOO)2, and HY molecular sieve has higher thermal stability than active carbon. In this paper, CO adsorbent was prepared using binary CuII precursors and different dispersing agents, and the carrier was HY molecular sieve.
Materials and preparations
CuCl2·2H2O was obtained from Tianjin Fengchuan Chemical Co., Ltd.( Tianjin city, China ), Cu(CH3COO)2·H2O and C2H5OH were purchased from Tianjin Hongyan Chemical Co., Ltd.( Tianjin city, China ), and they were all AR grade without further purification. HY molecular sieve（SiO2/A12O3 = 5.4）was purchased from Tianjin Nankai Catalyst Factory( Tianjin city, China ), which was pretreated before use. CO and Ar ( 99.999 % ) were obtained from Henan Yuanzheng Special Gas Ltd.( Xinxiang city, China ), N2 ( 99.999 % ) was purchased from Henan Ruian Keji Co. Ltd.( Zhengzhou city, China ), as listed in Table 1.CuI/Y were prepared from binary CuII precursors using different dispersing agents. Firstly, different mole ratio of Cu (CH3COO) 2·H2O/CuCl2·2H2O ( the total amount of Cu2+ was certain）were dissolved in a solvent (deionized water or C2H5OH). Secondly, the pretreated HY was added to the above solution, and the hydrothermal ion exchange was performed at different temperature for 12 h. Thirdly, The sediment was separated via centrifugation. Finally, the sediment was dried and activated in a tube furnace under Ar flow. The experiments were conducted by heating the sediments from room temperature to the final temperatures at a rate of 2 ºC min-1, followed by an isothermal treatment for 4 h and finally cooled down to room temperature.
Figure 1
CO adsorption/regeneration measurments
Dynamic adsorption of CO was evaluated with a fixed bed, as shown in Figure 1( Jiaozuo city, China ). 1 g tablet sample was put into a quartz tube with a diameter of 10 mm and a length of 38 mm. The adsorption temperature was maintained at 30 ºC by a electric furnace temperature controller and a cryogenic coolant circulation tank. The N2 flow containing 1 % CO passed through the fixed bed at a flow rate of 150 mL min-1. The adsorbed gas was detected by KE200-U multi-component gas analyzer (Xi'an Kepeng Electrical Equipment Co., Ltd. Xian city, China ) with an accuracy of 1 ppm and a range of 0-1000 ppm. In the breakthrough experiment, the zero-time was determined by the blank test. The breakthrough point was set at the concentration of each adsorbent in outlet gas reached 5 % of the component in the feed. The tail gas was vented to the atmosphere after absorption. Regeneration was performed with N2 as the regeneration gas. The relative experimental error of the estimated amounts was lower than ± 5 %.
CO adsorption isothermal
The adsorption isotherms of CO were measured by a corrosive gas adsorption analyzer BSD-PMC ( Beishide Instrument, Beijing city, China ). Approximately 0.1 g of the sample was degassed at 523 K for 3 h in vacuum. The adsorption capacity of the adsorbent were obtained by a static volumetric method at 30 ℃ with a error of ± 1 %.
Characterization
The crystalline structures of CuІY adsorbents were examined by powder X-ray diffractometer ( EMPYREAN, Holland ). The X-ray diffractometer used Cu Ka radiation. The tube voltage and current were 40 kV and 30 mA, respectively. For all samples, the 2q angle was scanned from 5° to 60° with a scanning speed of 5° min-1 and 0.02° per step. The XRD patterns were analyzed using MDI Jade software package. Nitrogen adsorption-desorption isotherms ( BET ) were obtained at -196 K using ASAP-2460 instrument. The samples were outgassed at 250 ℃ for 3 h in a dynamic vacuum before the adsorption isotherm was determined. The PSD was obtained from density functional theory( DFT ) method. The total pore volume ( Vt ) was generated according to the N2 adsorption at the relative pressure ( P/P0 ) of 0.99. Micropore volume ( Vmic ), Micropore volume ( Vmic ), micropore ratio surface ( Smic ) and external surface area ( Sex ) were obtained by the t-plot method.
XRF experiments
The main elemental components of the prepared adsorbents was tested by X-ray fluorescence spectroscopy (XRF). In this paper, Thermo Scientific ARL Perform’X X-ray fluorescence spectrometer was used to test the powder adsorbents.
XPS experiments
The chemical state of copper in CO adsorbents were investigated by X-ray photoelectron spectrometer ( XPS ). XPS analysis were performed by XSAM800 instrument from KRATOS, UK. With Al Kα monochromatic radiation ( 1486.6 eV ) and a high-vacuum chamber, the anode was operated at 120 W, and the analyzer was operated at a constant narrow scan pass energy of 30 eV. The XPS analysis was calibrated by the C1s peak at 284.6 eV. Due to the coexistence of CuІ and CuII, the XPS spectra were deconvoluted using the casa XPS software and the peak position and peak region were determined.
TG-MS experiments
In order to explore the mechanism of adsorbent in preparation, the thermogravimetric and mass spectrometric experiments ( TG-MS ) were conducted on NETZSCH STA449F3-QMS403 Aeolos Quadro analyzer coupled with a mass spectrometer. About 14 mg of un-activated sample were placed in a ceramic crucible. The samples were heated from room temperature to 290 ℃ with a heating rate of 5 ℃ min-1 followed by an isothermal period for 1 h. The experiments used high purity Ar as carrier gas at a constant flow rate of 50 mL min-1.
CO adsorption
Figure 2 showed CO adsorption breakthrough capacities of the prepared adsorbents. According to the literature,17, 21, 24-26 the dispersion of copper in HY 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 breakthrough capacity of the prepared adsorbents were 1.14 mL g-1, 9.87 mL g-1 and 4.79 mL g-1 respectively. However, when using C2H5OH as the solvent, the CO adsorption breakthrough capacity of the prepared adsorbents significantly improved, which were 1.35 mL g-1, 10.00 mL g-1 and 12.67 mL g-1 respectively. The CO adsorption breakthrough capacity of the adsorbent prepared with C2H5OH as solvent was 28.4 % higher than that of the adsorbent prepared with water as the 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.
Figure 2
It was important to choose an appropriate CuCl2·2H2O/Cu(CH3COO)2·H2O mole ratio. 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 explorations were needed. With binary copper salts as the precursors, the experimental results were interesting. As listed in Figure 2a, when using water as the solvent, the optimal mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O was 1/1. Xue had revealed the mechanism of equal mole of Cu(HCOO)2 and CuCl2 impregnating on the surface of the active carbon:5
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 the mole ratio of Cu(HCOO)2 /CuCl2 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 the mole ratio of Cu(HCOO)2/CuCl2 was 1, Cu(HCOO)2 was appropriate to reduce 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 another pare of Cu (HCOO)2 played a small role in CO adsorption, so the CO adsorption capacity was reduced, as shown in (1).
While using C2H5OH as the solvent, the experiment results were different. As listed in Figure 2a, when the mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O was 1/3, the CO adsorption capacity was only 1.35 mL g-1. When the mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O 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 the mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O was 3/1, the CO adsorption breakthrough capacity of the prepared adsorbent reached a maximum, which was 12.67 mL g-1. It was strange that the optimal mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O 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 CuI in the adsorbent，and further exploration was needed. In summary, the optimal mole ratio of Cu(CH3COO)2·H2O/CuCl2·2H2O was chosen as 3/1.
The total copper amount was also important because it decided the number of CuI active sites in HY molecular sieve. The Cu(CH3COO)2·H2O/CuCl2·2H2O mole ratio was fixed at 3/1, and 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)-1 to 7.0 mmol Cu2+(gHY)-1, the CO breakthrough adsorption capacity increased from 6.70 mL g-1 to 15.15 mL g-1. However, it gradually decreased with the further increase of the total copper amount. Adsorption occurs at the active sites of the 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 HY molecular sieve to decrease the surface area and CO adsorption capacity. Therefore, the optimal copper loading was 7.00 mmol Cu2+(gHY)-1.
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)-1) was reached at 42 mL g-1. While for the adsorbent (3.5 mmol Cu2+(gHY)-1) , the plateau was reached at about 30 mL g-1. That was, the adsorbent with 7.0 mmol Cu2+(gHY)-1 contributed more active sites than the adsorbent with 3.5 mmol Cu2+(gHY)-1. 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-1. After 5 times of adsorption and regeneration, the adsorption capacity was almost unchanged, indicating that the character of the adsorbents were stable.
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 77 K. 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 surface.33,34 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 HY 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-1 to 15.15 mL g-1 with the increase of copper loading amount from 0 to 7.0 mmol Cu2+(gHY)-1, which was attributed to the π complexation between CO and CuІ 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+(gHY)-1 to 10.5 mmol Cu2+(gHY)-1, the CO adsorption capacity decreased slightly because much copper active site reduced the specific area of the Y molecular sieve.
Figure 3
Figure 4
Table 1: CAS registry number, purity, and analysis method of the chemicals.
Table 2: Physical adsorption parameters of adsorbents at different activation temperatures.
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 temperatures.35 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 Xue, and the possible reason was that the loading methods were different in adsorbent preparation.5 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 28.5o, implying more CuCl come into being after 170 ℃ activation.19,36 At the same time, some CuCl perhaps changed into CuІY 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 CuІY, the following TG-MS experiments confirmed this opinion too. Much CuІY in adsorbent was benefit to CO adsorption, which was in agreement with the CO adsorption experiment results. At same time, some Cu0 came into being from CuCl or CuІY. According to theory, experiments and literature, it was supposed that the mechanism of the copper in preparing process was below:37-39
Cu2+ ⇄ Cu+ ⇄ Cu(І)Y ⇄ Cu (2)
CuCl + HY ⇄ Cu(І)Y + HCl (3)
Cu2+ ⇄ Cu+ ⇄ Cu (4)
It was verified that Cu2+ and Cu0 had almost no contribution to the CO adsorption of the adsorbents. Compared with Cu+, CuІY 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)-1, 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)-1, the diffraction peaks of Cu0 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)-1, not only the diffraction peaks of Cu0 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+(gHY)-1, and the conclusion was in accord with experimental results.
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+(gHY)-1 to 10.5 mmol Cu2+(gHY)-1, 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+(gHY)-1, 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+(gHY)-1, 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+(gHY)-1, the mole ratio of Cu/Cl recovered a little. This may be because more Cu was generated, and Figure 4b verified this conjecture.
Table 3: Main chemical elements in the prepared adsorbents.
XPS analysis
Figure 5
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 et.al, and it indicated that there was Cu+ coming into being in solvothermal process.19,40,41 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+ and Cu+ 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 CuІY. The XPS result showed a positive correlation with the CO adsorption performance of the adsorbent, which was in agreement with the above mechanism.
Figure 6
Figure 7
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-1 and then maintained for 1 h under Ar and the flow rate is 50 mL min-1. 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 too, some CuCl exchanged with HY molecular sieve, and a small amount of gas such as HCl were generated correspondingly, as proposed in (3).41,42.
Figure 8
It was a pity that the mechanism was still not completely clear and further exploration were required. The question were below:
1. The role of C2H5OH in adsorbent preparing;
2. The transform of Cu(CH3COO)2;
3. How to enhance the ion exchange in the activation process.
In this paper, C2H5OH or water was used as the dispersing agent, binary CuII precursors CuCl2 and Cu (CH3COO) 2·H2O were used to prepare CuІY, and the CO adsorption performance was evaluated. The conclusions were as bellow:
1. The CO adsorption breakthrough capacity prepared with C2H5OH as dispersing agent increased by 28.4% compared to water as dispersing agent.
2. C2H5OH played an important role in adsorbent preparation. In the preparation of the adsorbent, C2H5OH acted as a dispersing agent, which enhanced the mass transfer and made copper salts distribute on HY molecular sieve more evenly. In addition, C2H5OH maybe also act as a reducing agent in Cu2+ transforming to Cu+.
3. Binary CuII precursors CuCl2 and Cu (CH3COO) 2H2O introduced more copper active sites to molecular sieve, which improved the CO adsorption capacity of the adsorbent.
4. In adsorbent preparation, copper often existed in multiple forms simultaneously.
5. Compared with CuII, Cu0 and even CuCl/HY, CuІY played a more important role in CO adsorption. The mechanism of the adsorbent preparation was proposed in this paper, and further exploration was required.
This work is performed by School of Chemistry and Chemical Engineering, Henan Key Laboratory of Green Coal Conversion, Distinguished Foreign Scientist Workshop on Coal Green Conversion (GZS20200012), Henan Polytechnic University for AramcoTech, under the framework of a contract of AramcoTech 2019-017/3.
The authors declare no competing financial interest.
Abstract
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