Flying MOFs: polyamine-containing fluidized MOF/SiO2 hybrid materials for CO2 capture from post-combustion flue gas† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sc05372j

Solid-state synthesis ensures a high loading and well dispersed growth of a large collection of metal–organic framework (MOF) nanostructures within a series of commercially available mesoporous silica allowing to render MOFs into fluidized solid sorbents for CO2 capture from post-combustion flue gas in a fluidized-bed reactor.

Introduction CO 2 capture from both coal-and natural gas-red power plants using solid sorbents has been proposed as the most promising alternative to current amine-solution scrubbing technologies because of their lower energy requirements for sorbent regeneration, as well as less amine evaporation and equipment corrosion. 1 The state-of-the-art for CO 2 solid sorbents from postcombustion ue gas is mainly based on polyamines containing more than two ethylamine monomers (i.e. diethylenediamine, tetraethylenepentamine or polyethyleneimine) either (1) conned or polymerized in situ from aziridine within mesoporous silica cavities 2 or (2) coordinated to the open metal sites of metal-organic frameworks (MOFs), 3 which are preferred among supports, including zeolites, polymers or activated carbons. 4 In these solid composites, polyamines selectively adsorb CO 2 via chemisorption at primary and secondary amines. MOFs also have the capability to absorb CO 2 via physisorption, either at Lewis-acid open metal sites 5 or via molecular size connement even in the presence of H 2 O, 6 which have both demonstrated excellent capacity and selectivity over N. However, CO 2 physisorption is reduced or limited by temperature, which leaves polyamine-impregnated MOFs as the most suitable strategy involving MOFs to prepare solid sorbents for CO 2 capture under ue gas conditions typically operating at temperatures 40 C and greater, although some promising results have been recently reported. 7 In the race for developing more energy-efficient and competitive processes involving amine-containing solid sorbents for CO 2 capture from post-combustion ue gas streams, uidized-bed reactors (FBRs) are proposed as a more energy-efficient CO 2 capture technology than packed-bed congurations due to their improved heat and mass transfer. 8 In this way, several polyamine-containing solid sorbents using mesoporous silica as uidized supports have been successfully tested in an FBR at pilot plant scale. 9 Nevertheless, the current uidized solid sorbents show capacities below 10 wt% of CO 2 capture and suffer long-term deactivation via leaching of the active adsorbent phase because of the weak interactions via hydrogen bonds between polyamines and the support.
To the best of our knowledge, MOF-based CO 2 solid sorbents have never been studied in a uidized-bed conguration because of the absence or limitation of some essential features required for operating under these conditions, such as uidizability and attrition resistance. Therefore, it is quite important to develop a general strategy for shaping MOFs into a uidized form, which may confer MOFs with the required physicochemical properties. Furthermore, additional synergistic features can originate from MOFs and support intimately combined at the nanoscale, such as hierarchical mesoporosity or microporosity, which may host and disperse more active polyamines while providing them with more long-term stability via either a coordination bond at open metal sites or a covalent bond at free functional groups. 10 Our group has recently developed an elegant approach to selectively conne MOF nanocrystals within mesoporous materials via novel solid-state synthesis, which allows the preparation of hybrid materials containing a high loading (up to 40 wt%) of the smallest nanocrystals yet reported for any MOF structure within a wide collection of mesoporous supports, also named HyperMOFs. 11 The resulting hybrid materials show improved features from the combination of a silica scaffold protecting and supporting the MOF nanocrystallites within its porous network. Some of these improved features include improved attrition resistance, excellent uidizability and handling, as well as the enhanced catalytic activity recently reported by our group. 12 This insight encouraged us to prepare a novel kind of "ying" MOF by using cheap, commercially available mesoporous silica as the uidizable carrier for MOFs.
Herein, a collection of polyamine-impregnated MOF/SiO 2 was tested for CO 2 capture under simulated ue gas conditions in a packed-bed reactor (PBR) and compared with the state-of-the-art sorbents composed of polyamines on both "uidized" silica and "non-uidized" bulk MOFs. This systematic study reveals the correlations between the features of MOF/SiO 2 , such as loading, composition, and functionality of the conned MOFs, and the performance of impregnated polyamines for CO 2 capture. To study their viability for a larger scale, the most promising materials have been evaluated for long-term stability in a PBR and a visual uidized-bed reactor (vFBR). In addition, a cost evaluation study was conducted by our group to compare with those of state-of-the-art MOF-based solid sorbents. Furthermore, a novel strategy for covalently bonding polyamines to the uidized MOF/SiO 2 hybrid materials was introduced during this work to precisely tailor the new generation of HyperMOF hybrid materials for specic applications.

Fluidized MOF/SiO 2 hybrid materials synthesis
The preparation and characterization steps of all the MOF/SiO 2 hybrid materials included in this work were conducted by following our recently reported procedure for the solid-state synthesis of MOFs within mesoporous materials. 11 Moderate concentrations of MOFs on SiO 2 were also achieved by incipient wetness impregnation of the MOF precursor in dimethylformamide (DMF) or MeOH, as described in the ESI. †
Polyamine impregnation on MOF/SiO 2 hybrid materials and bulk MOFs All the materials were evacuated at 120 C under vacuum for 1 h. Then, 35 wt% of polyamine-conned sorbents were prepared by impregnating a solution containing 0.54 g of polyamine in 2 mL of CHCl 3 on 1 g of the evacuated material and were subsequently dried at 120 C under vacuum for 15 min. Polyamine coordinated sorbents were prepared by impregnating a solution containing 0.54 g of polyamine in 2 mL of dry CHCl 3 on the evacuated material under N 2 . The mixture was le under N 2 for 2 h, subsequently washed with dry CHCl 3 and dried at 80 C under vacuum for 15 min.
Polyamines covalently bonded to (Zr)UiO-66(NH 2 )/SiO 2 via post-synthesis modication with tris(hydroxymethyl) phosphine In the rst step, 2 g of the previously evacuated sample of 4.2 wt% of (Zr)UiO-66(NH 2 )/SiO 2 was exposed overnight to the vapors of 200 mg of tris(hydroxymethyl)phosphine (THP) at 100 C inside a Schlenk ask under vacuum to enhance the degree of amine functionalization (because THP can be easily oxidized when O 2 is present). The material was thoroughly washed with dry CHCl 3 , and then evacuated at 120 C under vacuum. Finally, 35 wt% of polyethylenimine (PEI) was introduced into the modied material as previously described for preparing polyamine-conned sorbents.

CO 2 adsorption capacity and stability testing of solid sorbents in a PBR
During the next part of the study, 2 g of sorbent were loaded into a PBR and tested for CO 2 capture under simulated ue gas conditions. Specically, the conditions were as follows: CO 2 ¼ 15 v/v%, O 2 ¼ 4.5 v/v%, H 2 O ¼ 5.6 v/v% in balance with N 2 at 50 C during 45 min for the adsorption step, and H 2 O ¼ 5.6 v/v% in balance with N 2 at 120 C during 45 min for the regeneration step. To evaluate the stability of the sorbents in the presence of contaminants, controlled concentrations of SO 2 (50 or 200 ppm), NO x (20 ppm of NO and 180 ppm of NO 2 ), or H 2 S (1 v/v%) were added during the adsorption step.

Fluidization and stability test of solid sorbents in an FBR
During the next part of the study, 50 g of the sorbent were loaded into a vFBR. The adsorption was performed by exposing the sorbent to 2 SLPM of 15 v/v% of CO 2 , 8 v/v% of H 2 O in balance with N 2 at 40 C until CO 2 breakthrough was observed and the outlet CO 2 concentration was stable. Then, between the adsorption and regeneration steps, the reactor was purged with N 2 to remove the CO 2 in the gas-phase remaining in the system. Finally, the regeneration step was performed by treating the sorbent with 10 v/v% of H 2 O in N 2 at 100 C. This treatment was continued until the CO 2 was completely desorbed from the sorbent. To evaluate the uidizability, the sorbents were exposed to 80 v/v% of H 2 O in balance with N 2 at 100 C. The pressure drops across the FBR of each sorbent were also examined by varying the N 2 ow from 0.5 to 4.8 SLPM.
Complete descriptions of the PBR and vFBR, as well as the experimental procedures are included in the ESI † and in our previous work. 18

Results and discussion
The CO 2 adsorption performance of polyamine-containing MOF/SiO 2 under simulated ue gas conditions: initial study for (Cr)MIL-101(SO 3 H)/SiO 2 (Cr)MIL-101(SO 3 H) has been selectively grown in a commercially uidized mesoporous silica (silica[A]) at various loadings, such as 5, 19 and 40 wt%, by following our recently reported procedure. 11 This MOF was selected as an initial candidate because of its well-known chemical and thermal stabilities and because of the presence of functional groups and available open metal sites for coordinating polyamines, which has been recently reported as an approach for preparing polyaminecontaining MOFs for CO 2 capture. 19 As shown in Fig. 1, the resulting uidized MOF/SiO 2 hybrids exhibited similar physicochemical properties to their bulk counterparts because N 2 and CO 2 adsorption was proportional to the MOF loading (measured by X-ray uorescence [XRF]), thus conrming that these are preserved aer integrating MOF nanocrystals within the mesoporous cavities. The presence, loading, and homogeneity of MOF nanocrystals conned on these MOF/SiO 2 hybrid materials were further conrmed by XRF, X-ray diffraction (XRD), Fourier transform infrared (FTIR), confocal microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM), as described in Fig. 1 and in the ESI. † Two congurations of polyamine-containing uidized MOF hybrid sorbents were prepared by using (Cr)MIL-101(SO 3 H)/ SiO 2 hybrid materials: anchored and conned polyamines. The rst approach consisted of treating the hybrid materials with linear, but not highly volatile, polyamines, such as tetraethylenepentamine (TEPA) and diethylenetriamine (DETA), in noncoordinating solvents (chloroform) to anchor them via both coordinative bonds through the open metal site at the chromium oxocluster and the ionic bond through the free sulfonic acid group at the ligand, as shown in Fig. 2. Aer washing with chloroform, the resulting composites exhibited a full degree of functionalization for both open metal sites and sulfonic groups, as suggested by the nitrogen to chromium ratios (N : Cr) of ca. 10 measured by using a combination of elemental analysis and XRF (see Tables S1 and S2 in the ESI †). TEPA approximately corresponds to one TEPA molecule coordinated to one chromium atom and another reacted with one sulfonic acid group  This nding may be attributed to the constrained diffusion of TEPA molecules through the pore system of the bulk material, especially aer the partial blockage of MOF pore apertures by TEPA molecules already anchored. Therefore, MOF nanocrystals dispersed on mesoporous silica exhibiting a 100-times smaller crystal size than their bulk counterparts allow for better polyamine diffusion and coordination along the MOF crystalline domain.
Next, 2 g of these composites were tested in a PBR under simulated ue gas conditions for 10 cycles to evaluate CO 2 adsorption capacity and stability. As shown in Fig. 2, TEPA coordinated on both 19 and 40 wt% MOF/SiO 2 exhibited better performance and amine use for CO 2 capture compared with pure bulk MOFs. 43% of amine utilization was calculated for 19 wt%, 26%, and for 40 wt% and only 14% for bulk MOFs under these experimental conditions, that is, considering that four of the TEPA molecules may coordinate to CO 2 while one is coordinated to the MOF (Table S2 in the ESI †). Lower CO 2 adsorption capacity was measured for materials fully functionalized by shorter polyamines because of their lower number of amines per molecule, such as DETA or ethylenediamine ( Table   S2 in the ESI †). This evidence shows the benecial effects of having a moderate concentration of well-dispersed MOF nanocrystals anchoring polyamines to enhance the accessibility and efficiency of the amine adsorption sites under simulated ue gas conditions. However, the second approach via connement within the pores leads to a higher loading of active polyamines, and thereby higher CO 2 capacities because the hierarchical mesoporosity and microporosity of the MOF/SiO 2 hybrid materials can be impregnated with up to 35 wt% of branched polyamines (PEI, M w ¼ approximately 800) without revealing signs of stickiness, which would avoid their application in uidized beds. 18 The inltration of PEI on MOF/SiO 2 microporosity and mesoporosity, as compared in Fig. 2b for 19 wt% of (Cr)MIL-101(SO 3 H)/SiO 2 containing anchored TEPA (occupying microporosity) and conned 35 wt% of PEI (occupying both microporosity and mesoporosity), has been conrmed by using N 2 sorption isotherms. Thus, CO 2 adsorption capacity of conned PEI is also highly dependent on the MOF loading because the presence of 19 and 40 wt% of MOF within the mesoporous silica reduces the CO 2 adsorption capacity of the amines compared with that of bare silica. This inhibition phenomena may be attributed to the interaction of PEI amines with both open metal sites and free functional groups at the conned MOF and the defective sites located at the MOF boundary inherent to the nanocrystalline domains. Therefore, the CO 2 adsorption capacity drops proportionally with the MOF loading because a lower capacity was found for 40 wt% MOF loading. Surprisingly, PEI inltrated on a HyperMOF containing only 5 wt% of MOF leads to a slight enhancement (ca. 10 wt%) of the CO 2 adsorption capacity (Fig. 2c). However, additional experiments conning 35 wt% of TEPA on MOF/SiO 2 materials revealed a high deactivation because of the leaching of noncoordinated TEPA molecules during the regeneration step at 120 C in the presence of wet nitrogen. This nding was also observed for 35 wt% of TEPA impregnated on the same bulk MOF and as described later in this manuscript for other impregnated short polyamines (see Fig. 3 and Table 1). Therefore, the use of TEPA as a polyamine was discarded for this application. Branched PEI was found to be more stable for uidized MOF hybrid sorbents under simulated ue gas conditions.

Scope for 35 wt% of PEI conned HyperMOFs
We considered to extend this study to other MOF nanocrystals grown within mesoporous silica to evaluate the effects that the MOF composition and loading have on the activity for CO 2 capture of a dened amount of impregnated PEI. Furthermore, a list of hybrid solid sorbents containing various loadings of a collection of chemically stable MOFs was selected to provide a better understanding about the boost in CO 2 adsorption capacity observed for moderate MOF loadings. In doing so, MOFs having different metals at the nodes (with both coordinatively unsaturated and saturated sites) and ligands (with and without free functionality) were prepared at various MOF loadings for this scope. To compare our uidized hybrid materials with the most promising polyamine-containing MOFbased CO 2 sorbents, bulk Mg 2 (dobpdc) decorated with up to 32 wt% of N,N 0 -dimethylethylenediamine (mmen) was prepared according to the literature 16 and tested under dry (no H 2 O) and simulated ue gas conditions. A similar characterization was conducted for all resulting hybrid solid sorbents listed in Table  1 (also, see the ESI †).
As shown in Table 1, a similar tendency was observed for 35 wt% of PEI impregnated on HyperMOF hybrid materials at various loadings compared with the results presented in Fig. 2c. In all of the cases, a boost in the CO 2 adsorption capacity was found for ca. 5 wt% of MOF loading compared with just PEI impregnated on pure silica, whereas lower CO 2 adsorption capacity and higher deactivation were measured for higher MOF loadings. Therefore, the initial CO 2 sorption capacity of PEI amines is improved by the presence of MOF nanocrystals, but at moderate loadings, because a concomitant inhibition mechanism is signicantly affecting the adsorption performance for excessive presence of MOFs. The irreversible inhibition pathways may be one or a combination of three scenarios. The rst  scenario involves neutralization of the PEI amines by defective ligands and/or coordinatively unsaturated metal sites. The second scenario involves oxidation of the PEI amines by MOF metal components at the nodes at high temperatures. The last scenario involves hindrance or aggregation of the PEI amines because of connement within microporous cavities. By comparing the results obtained for (Zn)ZIF-8 (entries 12 through 16) or (Zn)ZIF-7 (entry 17), which consists of imidazolate-type ligands, with those of the remainder of the carboxylate-type MOFs, such as (Cr)MIL-101(SO 3 H) (entries 2 through 5) or (Zr)UiO-66(NH 2 ) (entries 7 through 11), the initial CO 2 adsorption capacity and the deactivation rate are less affected by the MOF loading. Thus, boundary crystal defects in the MOF nanocrystals containing carboxylate-type ligands may neutralize more numbers of PEI amines than MOFs consisting of N-type ligands. Therefore, better long-term stability was observed for (Zn)ZIFs compared with that of carboxylatecontaining MOFs that exhibit higher deactivation in all cases. However, materials containing Co (entry 22) or Fe (entry 25) at the metal nodes show the highest deactivation rates compared with their isostructural counterpart containing Zn (entry 12) or Cr (entry 24) for the same MOF loadings, respectively. This nding can be attributed to the superior activity of these metals for catalytic amine oxidation.

Bulk versus hybrid MOFs
To the best of our knowledge, literature describing the performance and stability of polyamines conned within bulk MOFs for CO 2 capture under simulated ue gas conditions in a PBR is very limited. In this current work, such study was included for selected bulk MOFs (Table 1, see entries 5, 6, 11, 15, 16, 18, 19, 20, and 21) to compare the performance with the stability observed for our uidized MOF/SiO 2 hybrid sorbents under the same conditions (see Fig. 3). A clear inhibition of the amine capacity was observed for mmen-Mg 2 (dobpdc) when simulated ue gas conditions were used. The presence of 5.6 wt% of H 2 O in the stream leads to a reduction in CO 2 adsorption capacity from an excellent capacity of ca. 14 wt% (entry 19) measured under dry conditions in both thermogravimetric analysis (TGA) and a PBR (conrming reported value of 15.6 wt%) to 6.2 wt% (entry 18) under simulated ue gas conditions. According to elemental analysis, the N contained is also notably reduced from 10.1 wt% (conrming a reported value of 10.3 wt%) to 6.9 wt% (31% N loss) aer 10 cycles, probably because of a progressive displacement by the H 2 O molecules of volatile diamines, which are evaporated during the regeneration stage at 120 C, as also recently described in the literature. 7 In the same way, deactivation was observed for bulk (Zn)ZIF-8 (entries 15 and 16) and (Cr)MIL-101(SO 3 H) (entries 5 and 6) in the presence of H 2 O, although CO 2 adsorption capacity is quite similar under both "wet" and dry conditions for (Zn)ZIF-8, but is lower for dry (Cr)MIL-101(SO 3 H) compared with "wet" conditions. In the same way, very low deactivation (i.e. <1%) was measured for all polyamine-containing bulk MOFs under dry conditions. Moreover, an FTIR analysis of fresh and used TEPA impregnated in (Cr)MIL-101(SO 3 H) and (Zn)ZIF-8 revealed some differences that may be attributed to the degradation of MOF structures in the presence of polyamines promoted by either O 2 , H 2 O, and temperature or a combination of them (Fig. S6 in the ESI †). In the same way, similar stability issues under simulated ue gas conditions were reported in the literature 20 for TEPA-impregnated bulk MOFs and were criticized by researchers 21 for bulk MOFs adsorbing CO 2 by physisorption, such as (Mg)MOF-74 and SIFSIX-3-Cu, which show a drastic drop in CO 2 adsorption capacity in a few cycles compared with highly stable copper silicalite. Nevertheless, NbOFFIVE-1-Ni overcame these observed stability issues because of its hydrophobicity (entries 20 and 21), although the CO 2 adsorption capacity was lower and reversibly dropped by 28% in the presence of 5.6 v/v% of H 2 O (Fig. S8 in the ESI †).
Another hypothetical reason for this inherent drop in CO 2 adsorption capacity could be caused by the loose polyamine dispersion and subsequent mutual aggregation that leads to an auto-inhibiting effect of the amine sorption sites. This effect was observed for our hybrid materials containing more than 40 wt% of PEI and for PEI conned on pure mesoporous silicas exhibiting both smaller and larger pore sizes (e.g. 9.5 nm in SBA-15 or 42 nm in silica[D], respectively, see Fig. S9 in the ESI †) than mesoporous silica(A). In comparison, silica(A) exhibited a optimal polyamine dispersion and was also enhanced by the presence of MOF nanocrystals. In addition, this effect may also be attributed to a hygroscopic inhibition that leads to a reduction of the CO 2 adsorption capacity because of the accumulation of H 2 O around the highly hydroscopic polyamineimpregnated solids. Therefore, these major limitations for the direct application of polyamine-containing bulk MOFs under simulated ue conditions are efficiently overcome by using a moderate concentration of MOF nanocrystals dispersed within mesoporous silica. Doing so will raise the adsorption efficiency of the impregnated amines while avoiding the inhibition caused by polyamine aggregation and hydroscopic hindrance, which further encourage the use of our MOF/SiO 2 hybrid solid sorbents for this application.

Boost in capacity
Several compounds have been used as additives for improving the CO 2 adsorption capacity of PEI conned on mesoporous silica, such as surfactants, 22 which enhance the PEI dispersion along the mesoporous cavities, thus active PEI amines are better exposed at the gas-solid interface. Otherwise, a poor dispersion of polyamines commonly involves that a signicant number of amines can be sterically hindered by their mutual aggregation by intermolecular H bonds, which inherently leads to the inhibition of their CO 2 adsorption capacity and the limited CO 2 diffusion through the amine-impregnated pores. In the same way, well-dispersed MOF nanocrystals may also act as a dispersing agent (see Fig. 4), thus providing a more active surface area to stabilize PEI molecules. MOF nanocrystals within the silica reduce the mesoporosity while those cavities are lled with microporous crystallites, which can interact with PEI molecules by bonding or conning them. Nevertheless, we have determined that the excessive loadings of MOFs lead to the proportional inhibition of the PEI amines. Therefore, an optimal compromise between polyamine dispersion and inhibition was found for ca. 5 wt% of MOF loading.
To provide insight into the inuence the presence of MOF nanocrystals has on the CO 2 dynamics of impregnated PEI, the adsorption-desorption proles for 35 wt% of PEI conned on 4.6 wt% of (Zn)ZIF-8/SiO 2 and bare SiO 2 at the tenth cycle are compared in Fig. 5. The adsorption breakthrough prole for the MOF-containing solid sorbent shows a superior CO 2 adsorption to PEI/SiO 2 , as shown in Table 1. The characteristic adsorptionregeneration proles for the PBR lled with inert SiC 4 are represented by the red line, which determines the dead volume of the PBR. However, the regeneration prole shows an interesting difference in terms of CO 2 desorption for the composite incorporating MOF nanocrystals. Larger concentrations of the early released adsorbate are measured for the MOF/SiO 2 hybrid sorbent, which is attributed to weakly adsorbed CO 2 via physisorption because the temperature required to release them is lower than 80 C.
PEI/SiO 2 also exhibited an early release, but it is mainly attributed to CO 2 trapped in the dead volume of the PBR, as suggested by comparison with the prole for the inert SiC 4 . In addition, PEI/MOF/SiO 2 exhibited higher CO 2 desorption between 80 and 100 C, which suggests the slightly better use of the PEI amines for CO 2 chemisorption as well. This result highlights the unusual dual adsorption performance of our hybrid sorbents containing MOF nanocrystals compared with their pure silica counterpart.

Long-term stability in the presence of SO 2 and NO x contaminants in a PBR
Fluidized hybrid sorbents containing zinc imidazolates, such as (Zn)ZIF-8 and (Zn)ZIF-7, have demonstrated good CO 2 adsorption capacity under simulated ue gas conditions because they exhibited 140% higher CO 2 adsorption capacity than the reference PEI impregnated on bare mesoporous silica (Table 1, entry Fig. 4 Overview of our novel hybrid CO 2 solid sorbents.  Table 1) and 35 wt% of PEI/SiO 2 (entry 1, Table 1). 1) and the lowest 10-cycle deactivation ( Table 1, entries 12 and 17) compared with other MOFs. This nding encourages us to analyse these two sorbents for long-term stability in a PBR. Both sorbents exhibited excellent stability for 250 cycles under simulated ue gas conditions, as shown in Fig. 6 and resumed in Table  S3 in the ESI. † Flue gas from coal-red power plants typically contains other acid-gas impurities, such as SO 2 and NO x , that can dramatically inuence the CO 2 capture efficiency. 23,24 Therefore, during this current work, the performance of our MOF-based sorbents in the presence of these contaminants was evaluated. The results show a clear deactivation of the CO 2 adsorption capacity of the sorbents in the presence of SO 2 . This deactivation is because of the irreversible reaction occurring during the adsorption step between SO 2 and PEI amines, which are not further active for the CO 2 capture (Fig. 6).
Amine deactivation was proportional to the SO 2 concentration, as shown by the S content determined by CHNS elemental analysis aer 100 cycles (see Table S4 in the ESI †). Also, FTIR spectra of these samples measured aer 100 cycles revealed the clear, irreversible binding of SO 2 to the hybrid sorbent (Fig. S7 in the ESI †). However, excellent stability at higher NO x concentrations was observed. Therefore, MOF nanocrystals within the hybrid solid sorbent did not reduce the tendency of PEI amines to be deactivated by irreversible binding with SO 2 , as a similar deactivation for PEI/SiO 2 was observed by our group. 18 Therefore, another unit should be added upstream for scrubbing the SO 2 levels in the ue gas down to a single-digit parts per million level before reaching the FBR to elongate the life of the hybrid solid sorbents and reduce the make-up rate. Good stability was measured in the presence of H 2 S, typically found in sour gas streams, although an initial drop in capacity was observed because the study was performed under dry conditions and without O 2 because H 2 was in the H 2 S cylinder.

FBR
The uidizability of the PEI-impregnated MOF/SiO 2 solid sorbents and their performance under realistic conditions are of high importance to minimize unexpected operational issues to further scale-up this technology. In this work, we report the rst example of CO 2 capture from simulated ue gas in a uidized bed conguration by using a MOF-based CO 2 solid sorbent. Application of these materials in this conguration has never been proposed because of poor attrition and handling, as well as a lack of uidizability of bulk MOFs. In doing so, we have developed an experimental setup consisting of a glass-type column. The column works as an absorber and as a regenerator, where the uidization of the sorbent is carried out with a gas stream 25 (see the ESI †), which permits the visual evaluation of the sorbent uidizability.
During this study, the PEI/SiO 2 sorbent and the most promising PEI/MOF/SiO 2 sorbent (i.e. 4.6 wt% of [Zn]ZIF-8/SiO 2 ) were exposed to a series of testing conditions to evaluate their uidizability. First, the sorbents were subjected to an adsorption-regeneration cycle to determine their CO 2 adsorption breakthrough curves. Fig. 7b shows the CO 2 breakthrough curve for both sorbents during the adsorption step compared with that of bare SiO 2 , which allows the dead volume of the system to be determined.
The breakthrough for the bare SiO 2 occurred aer approximately 30 s, whereas the use of 50 g of the solid sorbent containing PEI resulted in a signicantly larger lag-time in breakthrough (ca. 6 to 7 min), suggesting that both sorbents are capable of deep CO 2 scrubbing, close to 100% efficiency, before saturation. As observed in the results obtained in the PBR (Table 1), the sorbent containing MOF nanocrystals also exhibited higher adsorption capacity than PEI/SiO 2 in the vFBR.
Aerwards, both sorbents were exposed to a stream containing 80 v/v% of H 2 O balanced with N 2 at 100 C for 1 h. These very aggressive conditions in the regeneration step were used because the stability under these conditions is crucial for further application, as the CO 2 adsorption capacity of the sorbent could signicantly drop because of the amine leaching under steam stripping. It is important to note that the extremely long exposure time of 1 h used for this experiment does not represent the actual residence times of the uidized sorbent in the regenerator, which is typically 2 to 5 min. Fig. 7c is a photograph of the sorbent bed during the initial CO 2 loading test and aer exposure to steam stripping for 1 h. Surprisingly, PEI/SiO 2 showed a more severe amine leaching in comparison with the material containing MOF nanocrystals under these conditions. Moreover, the PEI/SiO 2 sorbent became yellow and wet and stuck to the internal reactor wall, completely losing its uidizability. In contrast, the MOF-containing sorbent appeared to be more resilient to the steam stripping because the material maintained its uidizability over 1 h of testing. These results are consistent with the breakthrough adsorption curves measured for these two sorbents aer the steam exposure.
Aer a drying treatment with 1 SLPM of N 2 at 100 C for 2 h, CO 2 adsorption capacity was practically maintained for PEI/ MOF/SiO 2 , whereas a signicant drop was observed for the MOF-free sorbent (see Fig. 7b). This improved steam stability of the sorbents containing ZIF-8 nanocrystals can be attributed to the well-known hydrophobicity of the MOFs, which can be enhanced further by using longer aliphatic substituents of the imidazole ring. 26 Currently, the inuence on the CO 2 capture performance of other substituents at the imidazole ring of PEI/ MOF/SiO 2 sorbents is under investigation by our group.
Finally, the pressure drops across the FBR reactor bed for all the materials was also examined by varying the N 2 ow from 0.5 to 4.8 L min À1 (SLPM). The sorbent containing MOF nanocrystals exhibited a slightly higher pressure drop than PEI/SiO 2 , which can be attributed to the higher density because of the presence of the MOF (0.75 kg m À3 for PEI/MOF/SiO 2 versus 0.68 kg m À3 for PEI/SiO 2 ). The main benets of using denser uidized sorbents are the reduction of the column size, the increment of the gas velocity and the lower sorbent carried over by ue gas. Nevertheless, an excessive pressure drop in the system can lead to a higher energy requirement to drive the uidized sorbent through the bed. Fortunately, the sorbents containing moderate MOF loadings demonstrate an equilibrated combination of opposite features in terms of uidizability when operating at the optimal gas velocity range (0.1-0.2 m s À1 (ref. 27)) (i.e. denser particles, but not involving drastic pressure drops).

Scale-up and cost analysis study
During this work, we also demonstrated the scale-up of the approach for preparing our novel uidized hybrid sorbents because up to 1 kg of material was prepared by following our recent published solid-state synthesis approach 11 (photographs of the big batch are in Fig. S10 in the ESI †). In addition, the use of inexpensive chemicals makes the synthesis very cost effective compared with the costly bulk MOF consisting of expensive ligands and polyamines, such as mmen used for Mg 2 (dobpdc).
The price determined in our group for the kilogram-scale preparation of these sorbents is approximately $20 per kg according to the cost evaluation study we recently conducted (Fig. S11 in the ESI †). This cost can be reduced to $15 per kg by implementing some cost-saving measures such as recycling solvents and nding alternative mesoporous silica and PEI vendors. That reduced cost is in the same range as the production cost for some limited bulk MOFs via liquid assisted grinding (LAG), as recently reported in the literature. 28 Future directions toward more stable uidized solid sorbents: polyamines covalently bonded to MOF/SiO 2 hybrid materials Using pure steam for the regeneration stage is desirable for intensifying the CO 2 capture process from post-combustion ue gas because H 2 O can be easily condensed, leading to a pure CO 2 stream. Nevertheless, steam can progressively displace or wash out active polyamines from current solid sorbents based on PEIimpregnated uidized sorbents. This limitation may also affect the long-term stability of our novel hybrid solid sorbents during the regeneration stage in an FBR using pure steam because of the relatively weak interactions between MOF nanocrystals and polyamines because H 2 O molecules can displace coordinated amines as previously discussed for mmen-Mg 2 (dobpdc). Therefore, an efficient approach for covalent bonding of polyamines to uidized solids is a high priority.
During the past decade, a wide collection of strategies for post-synthesis modication have been developed for MOFs containing free functional groups at the organic ligand, especially for amino functionality at the aminoterephthalate ligand. 24 However, to the best of our knowledge, an efficient, one-pot, environmentally friendly and general approach for covalently bridging amine to amine is still missing in MOF chemistry. However, the use of THP as a coupling reagent for the immobilization of enzymes onto chitosan lms via Mannich-type condensation resulted in greater enzyme performance than many conventional coupling routes involving glutaraldehyde. 23,29 Hence, we applied this efficient approach of creating strong covalent bonds amine-to-amine via a methylene phosphine bridge or bridges (N-P-N) to link PEI to (Zr)UiO-66(NH 2 ) nanocrystals while keeping all of the resulting amines still active for CO 2 adsorption. This approach is in contrast to conventional glutaraldehyde that forms weak imine bonds, which do not exhibit CO 2 chemisorption (see Fig. 8).
An optimal phosphine graing (99% by XRF) was achieved via gas-phase phosphine functionalization and the material retained most of the PEI impregnated (95% by elemental analysis) aer thoroughly washing the material with methanol aer 10 cycles, as shown in Fig. 8. This approach provides a powerful synthetic tool to bind amine-containing organic or organometallic compounds to amine-containing MOFs or other solid supports, preserving their activity to be subsequently used for a variety of applications. The applications range from gas adsorption or separation to heterogeneous catalysis to bioimaging or drug delivery.

Conclusions
MOFs have been successfully engineered into a uidized form via solid-state synthesis within mesoporous silica, demonstrating excellent uidizability and handling and improved attrition resistance. MOF/SiO 2 hybrid materials containing a moderate loading of Zn imidazolate MOFs (ca. 5 wt%) have been found to be optimal to boost the performance of the 35 wt%impregnated polyamines for CO 2 capture under simulated ue gas conditions because CO 2 adsorption capacity and stability are progressively inhibited by increasing the concentration of MOF nanocrystals.
Long-term stability tests in the presence of contaminants and experiments on a visual FBR showed their excellent stability, uidizability, and outstanding performance under steam regeneration because of the hydrophobicity conferred by the presence of MOF nanocrystals.

Conflicts of interest
There are no conicts to declare. Fig. 8 Results of the CO 2 performance and stability test of 35 wt% of PEI/4.6 wt% of (Zr)UiO-66(NH 2 )/SiO 2 (green) and 35 wt% of PEI/THP/ 4.6 wt% of (Zr)UiO-66(NH 2 )/SiO 2 (orange) in a packed-bed reactor (PBR) under simulated flue gas conditions. Sorbents were thoroughly washed ex situ with MeOH after the tenth cycle and were re-loaded into the PBR. The scheme shows the post-synthesis modification of (Zr)UiO-66(NH 2 ) nanocrystals confined on SiO 2 via two steps: gasphase treatment with THP followed by PEI impregnation.