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Multi-Metals CaMgAl Metal-Organic Framework as CaO-based Sorbent to Achieve Highly CO2 Capture Capacity and Cyclic Performance

In this study, Ca-based multi-metals metal-organic framework (CaMgAl-MOF) has been designed as precursor material for carbon dioxide (CO2) capture to enhance the CO2 capture capacity and stability during multiple carbonation-calcination cycles. The CaMgAl-MOFs were constructed from self-assembly of metal ions and organic ligands through hydrothermal process to make metal ions uniformly distributed through the whole structure. Upon heat treatment at 600 °C, the Ca-based multi-metals CaMgAl-MOF would gradually transform to CaO and MgO nanoparticles along with the amorphous aluminum oxide distributed in the CaO matrix. XRD, Fourier transform infrared (FTIR), and SEM were used to identify the structure and characterize the morphology. The CO2 capture capacity and multiple carbonation-calcination cyclic tests of calcined Ca-based metal-organic framework (MOF) (attached with O and indicated as Ca-MOF-O) were performed by thermal gravimetric analysis (TGA). The single metal component calcined Ca-MOF sorbent have the highest CO2 capture capacity up to 72 wt.%, but a lower stability of 61% due to severe particle aggregation. In contrast, a higher Ca-rich MOF oxide sorbent with tailoring the Mg/Al ratios, Ca0.97Mg0.025Al0.005-MOF-O, showed the best performance, not only having the high stability of ~97%, but also maintaining the highest capacity of 71 wt.%. The concept of using Ca-based MOF materials combined with mixed-metal ions for CO2 capture showed a potential route for achieving efficient multiple carbonation-calcination CO2 cycles.


Introduction
Carbon dioxide (CO 2 ) accumulation in atmosphere has been believed to be one of the main reasons for global warming [1,2]. Pathways for CO 2 emission mitigation are being developed, including the improvements of energy efficiency and carbon capture and storage (CCS) in order to limit this temperature increase in 2 • C. The implementation of the CCS technique to current power plants and industries is considered to be the most practical solution to the greenhouse gases (GHGs) problem. The main strategies of high-temperature CO 2 adsorption/separation have been focused on using inorganic solid sorbents, including calcium oxides [3][4][5][6][7][8][9][10]. However, most of the commonly studied adsorbents suffer from low capacity at elevated temperatures, because the CO 2 adsorption capacity of these materials decreases drastically with increasing temperature. Feasible improvements are being adopted by incorporating inert support material to act as a physical barrier to CaO particles, but the capture efficiency and capacity of CO 2 adsorption would be reduced [11][12][13][14].
In contrast, metal-organic frameworks (MOFs) are well recognized for their extraordinary surface areas and ultrahigh porosity applied for energy storage and environmental remediation [15,16]. The pore size and functionality of MOFs can be tailored by rational selection of the organic ligand, functional group, and metal ion. Several strategies have been adopted to improve the performance of MOFs in CO2 capture applications at low temperatures during the past several years [17,18]. However, little progress has been achieved to tune the MOF structures for enhancing CO2 uptake at intermediate-high temperatures. MOFs can be used as self-sacrificing template through calcination to generate the high-temperature sorbent and, meanwhile, create some microporous to enhance the adsorption and escape of CO2 from the structure. More importantly, using MOFs as a CO2 capture sorbent, anti-sintering material, such as Mg, Al components can be well distributed through the synthesis of mixed-metal MOF structure, which would effectively promote the CO2 capture performances, as shown in Scheme 1. In this study, porous multi-metals Ca-based metalorganic frameworks (Ca-MOFs), such as Ca-based multi-metals metal-organic frameworks (CaMgAl-MOFs), were synthesized by substituting different amounts of Ca 2+ for Mg 2+ and Al 3+ in the reaction solution to act as the precursor to generate CaO-based sorbent for high temperature CO2 capture in order to enhance both CO2 capacity and long-term stability during the multiple carbonationcalcination processes. The Ca-MOFs can be used as self-sacrificing templates through calcination to create porous structure, as the organic ligands were decomposed to small molecules and escaped from the oxides. In addition to creating a highly porous microporous structure, using MOFs as a CO2 capture media is to take advantage of the well-distributed anti-sintering material, such as Mg, Al components through the MOF structure, which would effectively promote the CO2 capture performances as shown in Scheme 1. The results indicated that partial Al substitution for Mg in the Ca-rich CaMg-MOF displayed significant enhancement on the CO2 capture with a high CO2 capture capacity approximately to pure CaO and maintained long-term stable carbonation-calcination cyclic performance. To our best knowledge, this is the first report to demonstrate that multi-metal-ions CaMgAl-MOF can present a much higher CO2 capture capacity and excellent cyclic carbonationcalcination performance. Scheme 1. Mechanism of calcined porous multi-metal-ions Ca-based multi-metals metal-organic framework (CaMgAl-MOF) with a high CO2 capture efficiency and long stable cyclic performance.

Hydrothermal Synthesis of CaMgAl Metal-Organic Framework Nanocrystals
The precursor of CaO-based sorbents, CaMgAl-MOFs, were synthesized through a hydrothermal method with various amounts of Al substituted into the CaMg-MOF structure. First, 6 mmole of H 2 BDC were dissolved in a pre-heated 40 mL DMA solution at 50 • C. After complete dissolution, a colorless solution was formed; a total amount of 6 mmole metal nitrates was then added into the container. After stirring for 10-15 min., the obtained colorless reaction solution was transferred into Teflon-lined stainless-steel 100 mL autoclave and then treated at 120 • C for 18 hour. White crystalline powders were washed with ethanol through centrifugation four times, and the collected solid was dried in a 60 • C oven 24 hour. The MOF-based materials were ground into fine powder by hand grinding through an agate mortar for 1-2 min. because of slight particle aggregation before use. Different reaction time, temperature, and relative metal ratio were synthesized while using the same procedure. The as-synthesized CaMg-MOFs were denoted as Ca x Mg 1−x -MOF, with x representing the Ca mole% of total metal component. On the other hand, the Ca-Mg-Al MOFs were labeled as Ca x Mg y Al 1−x−y -MOF, as shown in Table 1, where x, y, z represented the mole% of calcium, magnesium, and aluminum element, respectively. The raw powders were then calcined at 600 • C to convert MOF precursors to form CaO-based oxide sorbents, which was attached by "-O" and denoted as Ca x Mg 1−x -MOF-O or Ca x Mg y Al 1−x−y -MOF-O.

Characterization
XRD analysis of the crystallization and structures of powder samples was carried out on a Bruker D2 Advance X-ray Diffractometer (Bruker, Billerica, MA, USA) in reflection mode, while using Cu λ Kα = 1.5418 Å radiation. Field Emission Scanning Electron Microscope (FE-SEM) (JEOL, Tokyo, Japan) and Energy Dispersive X-ray Spectroscopy (EDXS) were performed on a JEOL JSM-6700F instrument (Abingdon, UK) with a field emitter as the electron source. The Fourier transform infrared (FTIR) measurements were collected on a PerkinElmer Spectrum 100 FTIR spectrometer (PerkinElmer, Waltham, MA, USA) while using the self-supporting KBr pallets in the wave number range from 450 to 4000 cm −1 . Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) measurements were collected on Jarrell-Ash, ICAP-9000 (Thermo Scientific, Waltham, MA, USA). The samples were digested with nitro-hydrochloric acid in order to determine Ca/Mg/Al metal ratio in as-synthesized powders. X-ray Photoelectron Spectroscopy (XPS) (Thermo Scientific, Waltham, MA, USA) measurements were carried out using spherical sector analyzer and Multi-channeltron. The source was monochromatic Al Kα source (1486.6 eV, 200 W) with a spot size of 650-120 µm. The binding energy scale of the system was calibrated using Au 4f 7/2 84.0 eV from freshly evaporated gold sample. Solid-state 27 Al NMR studies were performed on BRUKER AVIII 600 WB spectrometer (Bruker, Billerica, MA, USA) with a 14.1 Tesla magnetic field, in which these nuclei resonate at 600 MHz.

CO 2 Capture Capacity
Thermal gravimetric analysis (TGA) measurements were performed on a TA Instrument Q500 (TA Instrument, New Castle, DE, USA). Typically 5-8 mg powders were loaded into a platinum hanging plate to perform the tests. For the thermal property test, the samples were heated at a rate of 5 • C min −1 from room temperature up to 800 • C under continuous nitrogen gas flow. For the carbon dioxide multiple carbonation-clacination cyclic tests, the carbonation process was carried out at 700 • C for 25 min. in an atmosphere of carbon dioxide (N 2 balance, 40 mL min −1 ), while the clacination process was performed in nitrogen gas flow at 700 • C for 8 min. In this research, TGA were used to quantify the CO 2 capture capacity and stability of the sorbents according to the following Equations (1) and (2): where W is the weight of the sorbent after CO 2 adsorption, W 0 is the initial weight of the sorbent, and W f is the weight of the sorbent after multiple carbonation-calcination cycles.

Character of MOF with Different Ca/Mg Ratios
The powder X-ray diffraction results presented in Figure 1 showed that the characteristic diffraction peaks of CaMg-MOFs were in the range between 5-25 • . The characteristic diffraction 2θ peaks of pure calcium-MOF (denoted as Ca-MOF) were at 8.5 • , 12.7 • , 13.3 • , 14.3 • , 17.8 • , and 18.4 • (JCPDF No.46-1873). For pure magnesium-MOF, denoted as Mg-MOF, the characteristic diffraction 2θ peaks were at 9.8 • , 14.5 • , 18.5 • , and 19.6 • . When partial calcium was used to substitute for Mg, Ca/Mg mixed metal MOF could still be successfully synthesized via the same procedure. The PXRD patterns showed that with increasing Ca/Mg, the peak intensity of characteristic diffraction peaks of Ca-MOF phase increased, while those of Mg-MOF phase decreased and became amorphous, such as Ca 0.97 Mg 0.03 -MOF. Besides, as increasing Ca content in Ca x Mg-MOF (where x indicated relative mole ratio of Ca in Ca + Mg = 1), it was found that the peaks of Mg-MOF phase were also slightly shifted to smaller 2 theta, but peaks of Ca-MOF was slightly shifted to a higher 2 theta, indicating that both metal ions of Ca and Mg in the MOF structure may be partially accommodated in the MOF structure to cause the changes in d-spacing. Because CaMg-MOF was calcined at 600 • C, it would be completely decomposed and transformed to metal oxides, represented as CaMg-MOF-O, composed of calcium oxide, calcium carbonate (CaCO 3 ), and/or magnesium oxide (MgO).
FTIR were used to analyze the adsorption peaks of the organic functional group to assure the bonding between organic ligands and metal ions, as shown in Figure 2. During the synthesis process of CaMg-MOFs, the COOH in H 2 BDC will dissolve first, producing COO − to bond with dissolved metal ions Ca 2+ and Mg 2+ in the solution for forming M-O bonds. The adsorption peak at 1679 cm −1 represented the characteristic adsorption of C=O bonds of COOH in H 2 BDC, as shown in Figure 2a.
In contrast, for all of the Ca x Mg 1-x -MOFs, the C=O peak disappeared while strong C-O bands at 1600-1550 cm −1 and 1450-1380 cm −1 were presented, which can be assigned to the asymmetry and symmetry stretching modes of C-O bonds in coordinated COO − groups, respectively. The peak at 1505-1510 cm −1 was assigned to the vibration of the benzene ring. The above results indicated the coordination between COO − of BDC and metal ions in MOF structure, and they were consistent with literatures [19,20] In addition, the broad band at 3471 cm −1 indicated the adsorption of surface water on the Ca-MOF and Ca 0.85 Mg 0.15 -MOFs. The two extensive bands at 3515 cm −1 and 3302 cm −1 can be assigned to the asymmetric and symmetric stretching of coordinated water molecules, respectively. The same vibration and stretching band for water absorption at 3490 cm −1 and 3269 cm −1 in Mg-MOF was also observed and a slight red shift at 3417 cm −1 for coordinated water molecules indicated a lower bond strength in Mg-OH [21]. FTIR were used to analyze the adsorption peaks of the organic functional group to assure the bonding between organic ligands and metal ions, as shown in Figure 2. During the synthesis process of CaMg-MOFs, the COOH in H2BDC will dissolve first, producing COO − to bond with dissolved metal ions Ca 2+ and Mg 2+ in the solution for forming M-O bonds. The adsorption peak at 1679 cm −1 represented the characteristic adsorption of C=O bonds of COOH in H2BDC, as shown in Figure 2a.
In contrast, for all of the CaxMg1-x-MOFs, the C=O peak disappeared while strong C-O bands at 1600-1550 cm −1 and 1450-1380 cm −1 were presented, which can be assigned to the asymmetry and symmetry stretching modes of C-O bonds in coordinated COO − groups, respectively. The peak at 1505-1510 cm −1 was assigned to the vibration of the benzene ring. The above results indicated the coordination between COO − of BDC and metal ions in MOF structure, and they were consistent with literatures [19,20]     These results indicated that CaO-based MOF absorbents have a positive effect on both capture capacity and carbonation kinetics when compared to commercial CaO in Figure 3d, which displayed a slow adsorption kinetics of CO 2 capture as compared to other Ca-Mg-MOF adsorbents. Furthermore, a rapid cycle degradation was also observed for the commercial CaO, because it produced strong aggregation and particle growth during multiple carbonation-calcination cycle because the commercial CaO powder did not have any anti-sintering resistant, such MgO as in CaMg-MOF, powder systems.   component, the weight loss of Ca-MOF is 21 wt.% in the first step between 70 and 125 °C , the weight loss of Mg-MOF is 17% between 150 and 186 °C , mainly because of the removal of water molecules on the surface and coordinated ones. For multi-metal components, such as Ca0.97-Mg0.03-MOF and Ca0.97-Mg0.025Al0.005-MOF, the first part can be divided into two steps loss at 100 °C -120 °C and 125 °C -185 °C , indicating different coordination environment for water molecules to different metal ions. In the second part of weight loss step, Ca-MOF has 36.6 wt.% decreased between 510 °C to 600 °C , and Mg-MOF has 48.51 wt.% between 480 °C to 610 °C . For multi-metals Ca-Mg-MOFs,the weight losses are 43.6 wt.%-46.2 wt.%.  On the contrary, although Ca0.97Mg0.03-MOF-O with more CaO displayed the highest 72.82 wt.% capacity of CO2 capture, it can be maintained only up to the seventh cycle. After that, the capture decayed rapidly decayed to 43.89 wt.% on the 30 th cycle, resulting in only 60% stability due to vigorous sintering and the particle growth effect. Figure 5d summarizes the CO2 capture capacity of the CaMg-MOF-O sorbent dependent on the CaO content at the last (30 th ) capture cycle, which indicates that there is also a maximum weight fraction of CaO (between 85 and 90%) in the sorbent, beyond which the sorbent loses its sintering-resistant MgO is not enough for reducing the aggregation during the multiple carbonation-calcination cycles.

Phase and Microstructure of Multi-Metals CaMgAl-MOFs
The abovementioned results showed that Ca0.85Mg0.15-MOF-O exhibited better cycling stability of CO2 capture when compared to Ca0.97Mg0.03-MOF-O but the latter has a higher CO2 capture capacity. Although MgO and Al2O3 have been known as anti-sintering agents against the On the contrary, although Ca 0.97 Mg 0.03 -MOF-O with more CaO displayed the highest 72.82 wt.% capacity of CO 2 capture, it can be maintained only up to the seventh cycle. After that, the capture decayed rapidly decayed to 43.89 wt.% on the 30th cycle, resulting in only 60% stability due to vigorous sintering and the particle growth effect. Figure 5d summarizes the CO 2 capture capacity of the CaMg-MOF-O sorbent dependent on the CaO content at the last (30th) capture cycle, which indicates that there is also a maximum weight fraction of CaO (between 85 and 90%) in the sorbent, beyond which the sorbent loses its sintering-resistant MgO is not enough for reducing the aggregation during the multiple carbonation-calcination cycles.

Phase and Microstructure of Multi-Metals CaMgAl-MOFs
The abovementioned results showed that Ca 0.85 Mg 0.15 -MOF-O exhibited better cycling stability of CO 2 capture when compared to Ca 0.97 Mg 0.03 -MOF-O but the latter has a higher CO 2 capture capacity. Although MgO and Al 2 O 3 have been known as anti-sintering agents against the carbonation-calcination cyclic degradation, the role of Mg-O and Al-O in CaMgAl MOFs on high temperature CO 2 sorption are significantly different. Liu et al. studied the CO 2 capture capacity of the CaO-MgO sorbents that were synthesized from calcium D-gluconate Monohydrate (CG) and magnesium D-gluconate hydrate (MG) and reported possible maximum weight fraction of CaO in the CaO-MgO sorbent in between 75 and 85% to maintain high stable performance over 24th capture cycle of utilization, which was attributed to MgO nanocrystalline that formed in the course of multistep decomposition of these precursors [22]. In our previous study, we found that the Al 3+ cation in the calcined Ca-Al layered double hydroxide (Ca-Al-CO 3 LDHs) can be incorporated into CaO matrix to form poor crystalline Ca-Al-O domains that can act as a physical barrier to separate the CaO nanoparticles. Therefore, in this study, multi-metal Ca 0.97 Mg 0.03 -MOF were selected for investigating the effect of partial Al substitution for Mg on the CO 2 capture performance effect by keeping 3 mol% of the total Mg + Al amounts, which was denoted as  Figure 6a presented Ca-MOF phase and a much broader and weaker peak at 2θ~10 • , where, without obvious phase difference, can be observed from the XRD as changing the substitution of Al for Mg in this composition. Therefore, a higher Mg content of Ca 0.85 (Mg/Al) 0.15 -MOF was used to understand the phase evolution of partial Al substitution for Mg. Figure 6b showed that, with a third metal (Al) incorporating into the CaMg-MOF, the characteristic peaks of Ca-MOF(8.5 • , 14.3 • ) and Mg-MOF phase (9.7 • , 14.7 • ) were shifted to a higher 2 theta for both Ca 0.85 Mg 0.15 -MOF and Ca 0.85 Mg 0.125 Al 0.025 -MOF, indicating Al ion possibly dissolved in the Ca-MOF and Mg-MOF, which was mainly attributed to the smaller ionic size of Al 3+ (~54 pm) than Ca 2+ (~114 pm) and Mg 2+ (~86 pm). Here, it was noted that a further increasing Al substitution, the peak intensity of Mg-MOF phase almost disappeared and transformed into an unknown broad phase around 10 • for Ca 0.85 Mg 0.07.5 Al 0.07.5 -MOF (Figure 6b-(iii)). In particular, when Mg was completely replaced by Al, Ca 0.85 Al 0.15 -MOF only displayed Ca-MOF phase with a weaker broad at 2 theta 8-12 • in Figure 6b-(iv), indicating that Al ion can not form Al-MOF, but it was probably distributed in the MOF structure or existed in Al-containing amorphous phase. However, we further analyzed the powders with 27 Al solid state NMR to assure the coordination environment of the Al in the structure since the PXRD results cannot provide the structure information about Al component. The results in Figure 5c shows that the signals located at −3.4 ppm indicated that Al element existed in an octahedral environment, six-coordinated with the COO − groups, and H 2 O molecules. However, the broad band centered at −25 ppm represented a typical character of amorphous phase, which implied it might only have short-range order bonding [23]. This indicates that the Al 3+ cations could be completely incorporated into the MOF framework, or may form an amorphous AlO x domain that was not detectable by using XRD. It was found that only Ca-MOF and Mg-MOF can be formed in CaMgAl-MOF samples, meaning that the addition of Al to CaMg-MOF system would affect the nucleation and growth of Ca-MOF and Mg-MOF, according to Figure 6b. According to literature reports [24,25], the CO2 capture amount and cycle stability of calcium-based CaMgAl-MOF sample are more important and strongly related to particle size and aggregation. The morphology of partial Al substitution for Mg in Ca0.97Mg0.03-MOF system was further examined while using SEM. Without Al substitution, Ca0.97Mg0.03-MOF system was mainly composed of well-defined crystalline Ca-MOF with a rectangular shape around 200-700 nm in length, as shown in Figure 7a. However, as Al substituted for Mg in both Ca0.97Mg0.03-MOF and Ca0.97Mg0.015Al0.015-MOF systems, an obvious change in morphology and crystal size can be seen for It was found that only Ca-MOF and Mg-MOF can be formed in CaMgAl-MOF samples, meaning that the addition of Al to CaMg-MOF system would affect the nucleation and growth of Ca-MOF and Mg-MOF, according to Figure 6b. According to literature reports [24,25], the CO 2 capture amount and cycle stability of calcium-based CaMgAl-MOF sample are more important and strongly related to particle size and aggregation. The morphology of partial Al substitution for Mg in Ca 0.97 Mg 0.03 -MOF system was further examined while using SEM. Without Al substitution, Ca 0.97 Mg 0.03 -MOF system was mainly composed of well-defined crystalline Ca-MOF with a rectangular shape around 200-700 nm in length, as shown in Figure 7a Figure 7d,e, respectively. However, Ca0.97Mg0.025Al0.005-MOF-O with minor Al substitution exhibited smaller particle with widely random distribution. On the contrary, Ca0.97Mg0.015Al0.015-MOF-O presented bigger irregular agglomerates and denser structure in Figure 7f, eventually resulting in a lower CO2 uptake.

Long-term CO2 Carbonation-Calcination Cyclic Performances of Al-substitution Ca-Mg-MOF
The Ca0.97Mg0.03-MOF-O with a higher CaO content presented a larger CO2 capture capacity when compared to the Ca0.85Mg0.15-MOF-O since the CaO adsorbent plays a more significant role in CO2 capture at high temperature. However, it was noted that the Ca0.97Mg0.03-MOF-O still displayed a cycle-degradation behavior. Long-term CO2 carbonation-calcination cyclic tests were performed for 30 cycles in order to further investigate the effect of partial Al substitution for Mg in the Ca0.97Mg0.03-MOF system on the CO2 capture character and cycle stability. As shown in Figure 8a, Ca0.97Mg0.03-MOF-O has the highest CO2 capacity about 74 wt.% at first cycle. However, there was a rapid decay after the fifth cycle and finally reduced to 40 wt.% with an overall 54.6% stability. With 0.5 mol% Al substituted for Mg in the Ca0.97Mg0.03-MOF-O structure in Figure 8b, Ca0.97Mg0.025Al0.005-MOF-O displayed the highest capacity about 70 wt.% in the first 10 cycles and maintained the CO2 capacity to 65 wt.% after 30 cycles with a well stability of 92% when compared to Ca0.97Mg0.03-MOF-O. Further, although further increasing Al substitution to 1.5 mole % still kept a high CO2 capture capacity (68 wt.%) with well stable up to 85% in the 30 cycles, the overall CO2 capture performance was affected, as shown in Figure 8c. The results in Figure 8b have demonstrated the excellent performance of Ca0.97Mg0.025Al0.005-MOF-O in the repeated carbonation-calcination cyclic test. The number of cycles was prolonged to one hundred cycles in order to explore the limit of the sorbent, as shown in Figure  8d. The highest capacity can be reached at the first cycle of 72 wt.% and, after the 30 th cycle, the capacity was held at 65 wt.%, reaching a relatively stable structure to maintain 93% stability of original CO2 capture capacity. Subsequently, the capacity was again slowly decayed but at last it still has 55 wt.% capacity, resulting in an overall 77% stability at 100 cycle test. The SEM image presented in Figure 9 showed that after 100-cycle carbonation-calcination at temperature from RT to 700 °C was

Long-term CO 2 Carbonation-Calcination Cyclic Performances of Al-Substitution Ca-Mg-MOF
The Ca 0.97 Mg 0.03 -MOF-O with a higher CaO content presented a larger CO 2 capture capacity when compared to the Ca 0.85 Mg 0.15 -MOF-O since the CaO adsorbent plays a more significant role in CO 2 capture at high temperature. However, it was noted that the Ca 0.97 Mg 0.03 -MOF-O still displayed a cycle-degradation behavior. Long-term CO 2 carbonation-calcination cyclic tests were performed for 30 cycles in order to further investigate the effect of partial Al substitution for Mg in the Ca 0.97 Mg 0.03 -MOF system on the CO 2 capture character and cycle stability. As shown in Figure 8a, Ca 0.97 Mg 0.03 -MOF-O has the highest CO 2 capacity about 74 wt.% at first cycle. However, there was a rapid decay after the fifth cycle and finally reduced to 40 wt.% with an overall 54.6% stability. With 0.5 mol% Al substituted for Mg in the Ca 0.97 Mg 0.03 -MOF-O structure in Figure 8b, Ca 0.97 Mg 0.025 Al 0.005 -MOF-O displayed the highest capacity about 70 wt.% in the first 10 cycles and maintained the CO 2 capacity to 65 wt.% after 30 cycles with a well stability of 92% when compared to Ca 0.97 Mg 0.03 -MOF-O. Further, although further increasing Al substitution to 1.5 mole % still kept a high CO 2 capture capacity (68 wt.%) with well stable up to 85% in the 30 cycles, the overall CO 2 capture performance was affected, as shown in Figure 8c. The results in Figure 8b have demonstrated the excellent performance of Ca 0.97 Mg 0.025 Al 0.005 -MOF-O in the repeated carbonation-calcination cyclic test. The number of cycles was prolonged to one hundred cycles in order to explore the limit of the sorbent, as shown in Figure 8d. The highest capacity can be reached at the first cycle of 72 wt.% and, after the 30th cycle, the capacity was held at 65 wt.%, reaching a relatively stable structure to maintain 93% stability of original CO 2 capture capacity. Subsequently, the capacity was again slowly decayed but at last it still has 55 wt.% capacity, resulting in an overall 77% stability at 100 cycle test. The SEM image presented in Figure 9 showed that after 100-cycle carbonation-calcination at temperature from RT to 700 • C was applied to the Ca 0.97 Mg 0.025 Al 0.005 -MOF-O sample, we found that particle aggregation was formed. However, the particle morphology and size of Ca 0.97 Mg 0.025 Al 0.005 -MOF-O sample were still unchanged with uniform distribution. These results demonstrated that, by using multi-metals CaMgAl-MOF as the precursor for CaO-based sorbent, and tuning the amount of Ca, Mg, and Al compositions, the Ca 0.97 Mg 0.025 Al 0.005 -MOF-O sorbent presented a significant enhancement in CO 2 capture efficiency and cycle stability suitable for high temperature utilization.
Materials 2020, 13, x FOR PEER REVIEW 12 of 14 applied to the Ca0.97Mg0.025Al0.005-MOF-O sample, we found that particle aggregation was formed. However, the particle morphology and size of Ca0.97Mg0.025Al0.005-MOF-O sample were still unchanged with uniform distribution. These results demonstrated that, by using multi-metals CaMgAl-MOF as the precursor for CaO-based sorbent, and tuning the amount of Ca, Mg, and Al compositions, the Ca0.97Mg0.025Al0.005-MOF-O sorbent presented a significant enhancement in CO2 capture efficiency and cycle stability suitable for high temperature utilization.

Conclusion
In summary, we have successfully synthesized multi-metal MOFs, CaMgAl-MOFs as precursors to fabricate CaO-based sorbents for CO2 capture. The sorbents showed excellent CO2 capture performances and high resistance toward thermal sintering under severe conditions in the high temperature condition. By partially substituting Al for Mg, the CaMgAl-MOF-O series showed better applied to the Ca0.97Mg0.025Al0.005-MOF-O sample, we found that particle aggregation was formed. However, the particle morphology and size of Ca0.97Mg0.025Al0.005-MOF-O sample were still unchanged with uniform distribution. These results demonstrated that, by using multi-metals CaMgAl-MOF as the precursor for CaO-based sorbent, and tuning the amount of Ca, Mg, and Al compositions, the Ca0.97Mg0.025Al0.005-MOF-O sorbent presented a significant enhancement in CO2 capture efficiency and cycle stability suitable for high temperature utilization.

Conclusion
In summary, we have successfully synthesized multi-metal MOFs, CaMgAl-MOFs as precursors to fabricate CaO-based sorbents for CO2 capture. The sorbents showed excellent CO2 capture performances and high resistance toward thermal sintering under severe conditions in the high temperature condition. By partially substituting Al for Mg, the CaMgAl-MOF-O series showed better

Conclusions
In summary, we have successfully synthesized multi-metal MOFs, CaMgAl-MOFs as precursors to fabricate CaO-based sorbents for CO 2 capture. The sorbents showed excellent CO 2 capture performances and high resistance toward thermal sintering under severe conditions in the high temperature condition. By partially substituting Al for Mg, the CaMgAl-MOF-O series showed better stability than CaMg-MOF-O sorbents because the supported amorphous Al-O phase was formed in between. Furthermore, the Ca 0.97 Mg 0.025 Al 0.005 -MOF-O showed best optimization in CO 2 capture (CaO) and cycle stability (due to the anti-sintering of MgO and Al 2 O 3 ). An outstanding performance in 100 cyclic tests with the highest CO 2 capacity of 71.8 wt.% and excellent stability of 77% can be achieved. The results may help future research of multi-metal MOFs and hopefully it can be further applied for the mitigation of CO 2 emission.