In situ casting of rice husk ash in metal organic frameworks induces enhanced CO2 capture performance

Incorporation of rice-husk-ash (RHA), an agricultural waste, in situ during the synthesis of MIL-101(Cr) resulted in a significant improvement in the CO2 adsorption properties over the synthesized RHA-MIL-101(Cr). The newly synthesized RHA-MIL-101(Cr) composite exhibited an enhancement of 14–27% in CO2 adsorption capacity as compared to MIL-101(Cr) at 25 °C and 1 bar. The content of RHA incorporated in RHA-MIL-101(Cr) fine tuned the CO2 capture performance to achieve high working capacity (0.54 mmol g−1), high purity (78%), superior CO2/N2 selectivity (18) and low isosteric heat of adsorption (20–30 kJ mol−1). The observed superior CO2 adsorption performance of RHA-MIL-101(Cr) is attributed to the fine tuning of textural characteristics—enhancement of 12–27% in BET surface area, 12–33% in total pore volume and 18–30% in micropore volume—upon incorporation of RHA in MIL-101(Cr).

www.nature.com/scientificreports/ superior gas adsorption performance at high pressure, nearly 80% of its pore volume remains under-utilized for gas uptake at low pressure 19 . In this regard, various strategies such as ligand modification, amine impregnation, and incorporation guest material to the MIL-101(Cr) framework have been explored to enhance the CO 2 adsorption properties by utilizing the unused pore volume 20 . Inclusion of guest material such as carbon nanotube, graphene oxide, metal ions, and amines into the MIL-101(Cr) framework significantly tune the pore size, pore volume, which allow CO 2 to retain in the tunnels and cages by enhancing the interaction with pore wall. Advantageously, increase in microporosity in MOF augmented the CO 2 adsorption potential which helps in the strong interaction of CO 2 molecules with multiple adsorption sites of MOF 21 . Amine groups introduced in the MIL-101(Cr) framework during pre or post synthesis can act as Lewis bases to strongly bind with CO 2 molecules and increases the selectivity against other gases such as N 2 21 . For instance, Chen et al. reported PEI-incorporated MIL-101(Cr) adsorbents exhibited ultra-high CO 2 adsorption capacity (4.2 mmol g −1 ) at 0.15 bar and superior CO 2 /N 2 selectivity (770) in the flue gas (0.15 bar CO 2 and 0.75 bar N 2 ) at 25 ºC 22 . However, the desorption of CO 2 in amine-functionalized MIL-101(Cr) could be difficult due to their high heat of adsorption (even up to ~ 98 kJ mol −1 ) 23 . On the other hand, doping of metal ions in MIL-101(Cr) during solvothermal crystallization can be a better option to enhance CO 2 adsorption property 24 . Zhou et al. doped Mg 2+ in MIL-101(Cr) to achieve a 40% enhancement in CO 2 uptake and ~ 4 times improvement in CO 2 /N 2 selectivity compared to un-doped MIL-101(Cr) at 1 bar 24 . Similarly, composites of MIL-101(Cr) with carbon or silica-based material also exhibited enhanced CO 2 adsorption capacity due to the improvement in the interaction of MOFs with CO 2 molecules [25][26][27][28] . Chen et al. reported hybrid MIL-101(Cr)@MCM-41 composite which exhibited 79% enhancement in CO 2 uptake capacity and 43% rise in CO 2 /N 2 selectivity compared to the parent MIL-101(Cr), presumably due to the interaction between surface hydroxyl groups of MCM-41 and metal centers of MOF 25 . Moreover, Qasem et al. reported an enhancement of 35.9% in CO 2 adsorption capacity at 24 °C and 1 bar pressure, after the incorporation of multiwall carbon nano tube (MWCNT) in MIL-101(Cr) 26 . In particular, synthetic carbon or/and silica-based MOFs have been developed to increase the CO 2 adsorption capacity. However, direct utilization of agricultural waste material such as rice husk ash (RHA) containing both silica and carbon in MOF synthesis and its consequences on the CO 2 adsorption properties are yet not extensively investigated. RHA has diversified applications ranging from pozzolanic material in the construction industry 29 to feedstock for the development of various CO 2 capture adsorbents 30 . RHA is amorphous/mesoporous in nature and has exposed silanol bonds which can enhance facile interactions with CO 2 molecules. Therefore, it is of interest to study the CO 2 adsorption performance such as working capacity, regenerability, CO 2 /N 2 selectivity as well as isosteric heat of adsorption of RHA modified MIL-101(Cr) at low pressure (pertaining to flue gas condition) by interplaying with its structural properties.
Herein, we synthesized RHA incorporated MIL-101(Cr) using a varying amount of RHA in situ during the hydrothermal synthesis of MIL-101(Cr) and investigated the effect of RHA on the CO 2 adsorption at 0 °C, 25 °C and 1 bar. Structural, morphological and chemical properties of RHA-MIL-101(Cr) are analyzed by powder X-ray diffraction (P-XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and energy dispersive spectroscopy (EDS). Textural properties (surface area, pore-volume, and pore size distribution) of RHA-MIL-101(Cr) are studied using N 2 adsorption-desorption isotherm at − 196 °C and CO 2 adsorption at 0 °C. The pure component adsorption isotherm of CO 2 at 0 °C, 25 °C and N 2 at 25 °C is investigated in the pressure range 0-1 bar. The observed adsorption behavior of the synthesized RHA incorporated MIL-101(Cr) is correlated with their textural properties. Further, adsorbents evaluation parameters such as working capacity, purity and CO 2 /N 2 selectivity and adsorption thermodynamics were also evaluated. Our finding suggested that the studied RHA-MIL-101(Cr) represents a class of efficient CO 2 adsorption material. Through this study, we also attempted to explore the potential utilization of the throw-away agriculture waste, RHA, for the synthesis of a value-added CO 2 capture material.   Fig. 1a, suggests that the crystalline framework of MIL-101(Cr) is well preserved even after the incorporation of RHA in MIL-101(Cr). The presence of characteristic peaks at 3.3°, 5.4°, 5.9°, 9.29°, and 16.71° for MIL-101(Cr) are in good agreement with previous reports 24,31 . It is also noted that in P-XRD patterns no new peaks from RHA are found, indicating the high dispersion and relatively low RHA content in the composites. Moreover, only a slight variation in the intensities of the peaks of the RHA-MIL-101(Cr) at lower 2θ is observed as compared to the pristine MIL-101(Cr), indicating no significant change in the crystallinity of framework upon incorporation of RHA in MIL-101(Cr) (Fig. S1) 25 .

Results and discussion
The presence of a characteristic peak at 582 cm −1 corresponds to Cr-O vibration in the FTIR spectra ( Fig. 1b) of MIL-101(Cr) and RHA-MIL-101(Cr) inferred the presence of MIL-101 framework 24 . A strong band appeared at 1400 cm −1 and 1632 cm −1 is assigned to the O-C-O symmetric stretching vibration of BDC linker present in MIL-101(Cr) framework 24,32 . Moreover, the incorporation of RHA in MIL-101 framework is further confirmed by the presence of signature bands at 1060 cm −1 and 1047 cm −1 , corresponds to Si-O-Si vibration (Fig. 1c) 33 . Further, the presence of bands at 1016 cm −1 (assigned to in-plane bending vibration of C-H on the benzene ring), 884 cm −1 (assigned to out-of-plane bending vibration of C-H on the benzene ring) and 745 cm −1 (assigned to deviational vibration of carboxylate groups) are consistent with the literature reports 34,35 .
Raman Thermogravimetric analysis (TGA) is used to study the thermal stability of RHA-MIL-101(Cr) upon the incorporation of RHA in MIL-101(Cr) (Fig. S3). TGA traces of MIL-101(Cr), RHA-MIL-101(Cr) exhibit an initial weight loss in the temperature range of 100-160 °C due to the removal of guest water molecules from the large cage (d = 3.4 nm). The second weight loss in the range of 160-350 °C, can be ascribed to the loss of trapped water molecules from the relatively small cages (d = 2.9 nm) 35 . The weight loss beyond 350 °C is due to the decomposition of BDC and the structural framework of MIL-101(Cr) 20,39 . With an increase in carbon and silica content upon addition of RHA in MIL-101, the resulting composite RHA-MIL-101(Cr) demonstrating a superior hydrophobicity as well as thermal stability (Table S1).
Elemental analysis shows 13-23 wt% higher carbon content in RHA-MIL-101(Cr) as compared to MIL-101(Cr). The increasing thermal stability upon rise in carbon and silica content in composite MOFs is in good agreement with the previous reports 35,40 . Consistent with P-XRD, FTIR, and Raman spectra, the SEM and TEM results also suggest the intactness of structural and morphological integrity of MIL-101(Cr) even upon the incorporation of RHA ( Fig. 2 and Fig. S4). TEM images of RHA-MIL-101(Cr) displays the presence of particles with size in the range of 100-400 nm having octahedron morphology analogous to MIL-101(Cr) ( Fig. 2 and Fig. S5). This is presumably due to the complete dispersion of RHA (0.31-0.62 wt% to the metal content of MIL-101) into the MIL-101(Cr) framework. Moreover, EDS spectra of RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II exhibited the presence of both Cr and Si (Fig. 2). Further, TEM-EDS elemental mapping of a single particle of RHA incorporated MIL-101 infers the presence of silica spread all over the octahedron particle, evidence the complete dispersion of RHA incorporated in MIL-101(Cr) framework (Fig. 3).
To investigate the possible structural changes occurred due to incorporation of RHA in the MIL-101(Cr) framework, the textural properties (BET surface area, total pore volume, and micropore volumes) of MIL-101(Cr), RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II are studied and compared with the parent MIL-101(Cr) (Fig. 4a). Table 1   www.nature.com/scientificreports/ Notably, RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II exhibited an increment of 12.6% and 27.6% in their specific surface area, respectively compared to MIL-101(Cr). Such surface area enhancement is often seen in MOF composite by the inclusion of guest material during solvothermal crystallization, which is well documented in literature 24,25,40,41 . Moreover, the total pore volume (at P/P 0 = 0.99) increased by 33%, from 1.70 cm 3 g −1 (for MIL-101(Cr)) to 2.27 cm 3 g −1 (RHA-MIL-101(Cr)-II). In addition, the micropore volume (from t-plot) also increased from 1.33 cm 3 g −1 (for MIL-101(Cr)) to 1.57 cm 3 (Fig. 4b). Notably, for accessing the ultra-micropores (pores < 0.7 nm), CO 2 adsorption at 0 °C is preferred over N 2 at − 196 °C due to the higher saturation pressure (34.85 bar) and rapid diffusion rate of CO 2 over N 2 42 . Hence, the micropore size distribution of RHA-MIL-101(Cr)-II inferred the formation of new ultra-micropore in the range of 0.35 nm which is consistent with the observed increment in cumulative micropore volume for RHA-MIL-101(Cr)-II as compared to MIL-101(Cr) and RHA-MIL-101(Cr)-I (Fig. 4d, and Figs. S6-S8). It is worth noticing that, the ultra-microporosity also increased upon RHA loading in MIL-101(Cr) framework, which indicate the synergetic combination of two components. Previous reports also evidence that fine-tuning of pores in MOF may occur due to the interaction of silanol bonds with metal centres 40,41 .
The CO 2 and N 2 adsorption capacity of MIL-101(Cr) and RHA-MIL-101(Cr) composite in the pressure range of 0-1 bar at 25 ºC are shown in Fig. 5a-c, respectively. In accordance with the enhanced porosity characteristics of RHA-MIL-101(Cr), both RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II exhibited high CO 2 adsorption capacity compared to MIL-101(Cr) (Fig. 5a,b,d,e). The observed enhancement can be attributed to the improved textural properties and availability of silanol functionalities in the pores of RHA-MIL-101(Cr) due to the incorporation of RHA in MIL-101(Cr) framework 40 . The loading content of RHA in MIL-101(Cr) tuned the CO 2 adsorption performance of RHA-MIL-101(Cr). The result suggests that the incorporation of even small amount of RHA into MIL-101(Cr) framework facilitates the formations of additional micropores those provide extra CO 2 adsorption binding site in RHA-MIL-101(Cr). Among the synthesized RHA incorporated MIL-101(Cr), RHA-MIL-101(Cr)-I exhibits the highest CO 2 uptake 2.79 mmol g −1 (12.27 wt %) at 25 °C and 1 bar, which is 27% higher than of MIL-101(Cr). Moreover, RHA-MIL-101(Cr)-II also exhibited 14% higher CO 2 uptake (2.51 mmol g −1 ) at 25 °C than MIL-101(Cr) (2.20 mmol g −1 ). RHA-MIL-101(Cr)-I also showed higher CO 2 uptake (3.70 mmol g −1 ) at 0 °C, compared to RHA-MIL-101(Cr)-II and MIL-101(Cr). The BET surface area and pore volume are ordered as RHA-MIL-101(Cr)-II > RHA-MIL-101(Cr)-I > MIL-101(Cr). The observed enhanced CO 2 adsorption capacity for RHA incorporated MIL-101 is consistent with the enhancement of 12-27% in BET surface area, 12-33% in total pore volume and 18-30% in micropore volume as compared to the parent MIL-101(Cr) (Fig. 4d, Table 1). It is noteworthy that, due to inadequate pore filling, RHA-MIL-101(Cr)-II exhibits comparatively less CO 2 adsorption capacity than RHA-MIL-101(Cr)-I at 1 bar, although it has higher BET surface area and pore volume 43 . Evidently, higher BET surface area and pore volume may have correlated positively with high pressure CO 2 www.nature.com/scientificreports/ adsorption, but the presence of micropore in the adsorbent plays a major role in enhancing CO 2 interaction at low pressure 43,44 . In support of the aforementioned statement, RHA-MIL-101(Cr)-II exhibited the finest increase in CO 2 adsorption capacity at 0.15 bar (94%) as compared to RHA-MIL-101(Cr)-I (Fig. 5d), due to presence of excess micropore volume which may have direct relevance for CO 2 capture under flue gas condition 40,44 . As seen in Fig. S9, there is a linear trend between the CO 2 adsorbed with increase in ultra-micropore volume among all the studied adsorbents, which indicates the presence of ultra-micropore smaller than 0.7 nm is greatly responsible for enriched CO 2 adsorption potential at low pressure 44 . Further, a good agreement of the Sips fitting parameters Pore volume CO 2 uptake (mmol g −1 ) N 2 uptake (mmol g −1 ) V total (cm 3 g −1 ) V micro (cm 3 g −1 ) V ultramicro (cm 3 g −1 ) www.nature.com/scientificreports/ with the experimental data (R 2 ∼ 0.99) suggest the efficient interaction of CO 2 molecules with the pore wall of RHA-MIL-101(Cr), due to the heterogeneity in the adsorbent surface (Fig. S10, Table S2) 45,46 . Concurrently, the adsorption of N 2 is found to be least in the case of RHA-MIL-101(Cr)-II as compared to RHA-MIL-101(Cr)-I and MIL-101(Cr) (Fig. 5c) presumably due to the poor diffusion of N 2 into the ultra-micropores having pore diameter less than the kinetic diameter (0.36 nm) of N 2 47 . Apart from high CO 2 adsorption capacity, RHA-MIL-101 (Cr) also exhibits low isosteric heat of adsorption, lying in the narrow range of weak physisorption, indicating that adsorbents need less energy during regeneration. As depicted in Fig. S11, the derived Q st value for MIL-101(Cr) (19 kJ mol −1 ) at low surface coverage is quite accordance with the existing literature 48,49 . Upon RHA incorporation, the Q st value was increased slightly for RHA-MIL-101(Cr)-I (20 kJ mol −1 ) and RHA-MIL-101(Cr)-II (30 kJ mol −1 ) at low surface coverage, attributed to the strong electrostatic interaction for CO 2 with narrow pore size distribution 49 . Notably, the Q st value of RHA-MIL-101(Cr) is at par with the several established MOF based composites available in the literature (Table S3). www.nature.com/scientificreports/ Separation of CO 2 from flue gas using vacuum swing adsorption (VSA) technique is considered to be one of the most energy-efficient and economical processes with lower regeneration time 50 . In addition to the high CO 2 adsorption capacity, adsorbents should possess essential requirements such as high CO 2 working capacity, superior selectivity for CO 2 over N 2 , and high CO 2 purity for post-combustion CO 2 capture 51 . Therefore, the performance of the synthesized RHA-MIL-101(Cr) adsorbents is evaluated for the separation of CO 2 from industrial flue gas using VSA technique, based on the data obtained from single gas adsorption isotherm at 0.15 bar of CO 2 and 0.75 bar of N 2 . The CO 2 separation performance of RHA incorporated MIL-101(Cr) along with the parent MIL-101(Cr) at 25 °C is shown in Fig. 5f,g and listed in Table S4. RHA-MIL-101(Cr)-II exhibits the best performance with highest working capacity (0.54 mmol g −1 ), and high purity (78%) as compared to RHA-MIL-101(Cr)-I and MIL-101(Cr). The observed high CO 2 working capacity for RHA-MIL-101(Cr)-II, suggesting that there would be a significant reduction in the adsorbent replacement time and capital cost associated with the adsorbent amount for CO 2 adsorption application 52 . Further, the CO 2 /N 2 selectivity of 18 observed for RHA-MIL-101(Cr)-II is significantly higher as compared to MIL-101(Cr) (CO 2 /N 2 selectivity = 7). The observed more than two-fold enhancement in CO 2 /N 2 selectivity for RHA-MIL-101(Cr)-II could be a consequence of the exceptionally larger polarizability and quadrupole moment of CO 2 (29.11 × 10 -25 cm −3 and 4.30 × 10 −26 esu −1 cm −1 , respectively) than that of N 2 (17.40 × 10 −25 cm −3 and 1.52 × 10 −26 esu −1 cm −1 , respectively) and the steric effect of adsorbing molecules (CO 2 , N 2 ) on the adsorbent surface 45 . Notably, micropore size distribution obtained from CO 2 adsorption isotherm at 0 °C, also inferred the generation of new ultra-micropores (pore diameter of 0.35 nm) in RHA-MIL-101(Cr)-II (Fig. 5c), which can significantly tune the preferential adsorption of CO 2 over N 2 (kinetic diameter: CO 2 = 3.3 nm and N 2 = 3.6 nm) and hence resulted in superior CO 2 /N 2 selectivity for RHA-MIL-101(Cr)-II. Besides, owing to high CO 2 /N 2 selectivity, RHA-MIL-101(Cr)-II can also recover highly pure CO 2 from the flue gas. Therefore, RHA-MIL-101(Cr)-II exhibited promising characteristics for the separation/ purification of CO 2 /N 2 mixture, even under ambient conditions.
The microporous analysis revealed that the incorporation of RHA in the MIL-101(Cr) framework significantly increased the specific surface area, micropore volume and also tuned the pore diameter, which resulted in a remarkable improvement in CO 2 adsorption properties for RHA-MIL-101(Cr). The CO 2 adsorption capacity of RHA-MIL-101(Cr)-I at 1 bar and 25 °C is at par or even higher than some of the popular MOFs based composite (using carbon or silica), which are being widely investigated as significant materials for CO 2 capture (Table S3). The observed higher CO 2 adsorption behavior of RHA-MIL-101(Cr) at low pressure (0.15 bar) is well in accordance with the microporous nature of these adsorbents 25,40 . The synergetic interaction between the silanol groups of RHA and the metal sites of MIL-101(Cr) induced structural changes which is likely to be responsible for the generation of microporosity in RHA-MIL-101(Cr). Literature also revealed that, the incorporation of heterogeneous material such as silica in MOF framework may acts as structure directing agent to modulate the textural characteristics of MOF crystal, and consequently influence the CO 2 adsorption performance of MOF 40,41 . For instance, Chen et al. reported 15.9 and 39% enhancement in CO 2 uptake and kinetics at 1 bar and 25 °C over HKUST-1@SBA-15 composite, respectively, where the micropores present in composite played a key role in increasing CO 2 adsorption capacity and the mesoporosity available in SBA-15 enhanced the CO 2 adsorption kinetics behavior 40 . Therefore, the tuned CO 2 adsorption behavior of RHA incorporated MIL-101(Cr) is of particular importance for utilizing RHA-MIL-101(Cr) for both flue gas separation and bulk CO 2 gas purification applications.

Conclusions
This work demonstrates high CO 2 adsorption performance achieved by the incorporation of rice husk ash (RHA), a waste material, in situ during the synthesis of MIL-101(Cr) under hydrothermal condition. RHA incorporated MIL-101(Cr) exhibited high specific surface area, high micropore volume and tuned pore diameter as compared to the parent MIL-101(Cr). Moreover, incorporation of silica-rich RHA fine-tuned the interaction of CO 2 molecules with pore walls due to the presence of silanol bonds and enhanced the utilization of large pore volume. Consequently, the RHA incorporated MIL-101(Cr) exhibited 27% higher CO 2 adsorption capacity compared to MIL-101(Cr) at 25 °C, attributed to the enhancement in total pore volume and micropore volume by 33% and 30%, respectively compared to MIL-101(Cr). It is worth noticing that, RHA-MIL-101(Cr)-II displayed better CO 2 uptake at low pressure (0.15 bar) as compared to RHA-MIL-101(Cr)-I due to the generation of ultra-micropore in the range of 0.35 nm. RHA-MIL-101(Cr)-II also possesses high working capacity (0.54 mmol g −1 ), high purity (78%) and superior CO 2 /N 2 selectivity (18) compared to RHA-MIL-101(Cr)-I and MIL-101(Cr) under vacuum swing based adsorption at flue gas condition (0.15 bar CO 2 vs. 0.75 bar N 2 ). Hence, incorporation of agriculture waste RHA in MIL-101(Cr) provided an environmentally benign route to fine-tune the textural and porous characteristics of MIL-101(Cr) to achieve enhanced CO 2 adsorption capacity. Further investigations are being carried out in the laboratory to evaluate the behavior of RHA-MIL-101(Cr) for high-pressure CO 2 adsorption. Taking into account of superior CO 2 adsorption performance, RHA incorporated MIL-101(Cr) could be a potential adsorbent for purification and separation of gases for industrial application.
Scientific Reports | (2020) 10:20219 | https://doi.org/10.1038/s41598-020-77213-9 www.nature.com/scientificreports/ Powder X-ray diffraction (P-XRD) patterns of the adsorbents were obtained with a Rigaku Smart Lab automated powder X-ray diffractometer (λ = 0.154 nm) at a step size of 0.01° over a 2θ range from 2 to 80°. Fourier transform infrared spectroscopy (FTIR) analysis was carried out to probe the vibrational properties of the chemical functional groups present in the studied adsorbents. The spectra were recorded using a spectrometer equipped with an attenuated total reflectance (FTIR/ATR 229 model FTIR-STD-10, PerkinElmer, MA, U.S.A) in the wavenumber range 4000-500 cm −1 . To analyze the thermal stability and dehydration characteristics of the studied adsorbents, thermogravimetric study (TGA) was performed using a Shimadzu TGA-50 Series thermal analyzer at a heating rate of 5 °C min −1 from 25 to 800 °C under N 2 atmosphere. Prior to FTIR and TGA experiment, all the samples were dried at 110 ºC and allowed to cool at room temperature and then stored in a desiccator until the testing began. Carbon content in the sample was determined using Leco CS 230 Carbon/Sulfur analyzer. Field emission scanning electron microscopic (FESEM) images and EDS spectra were recorded on a Carl Zeiss Supra-55 and EDS Oxford instruments (X-Max, energy-dispersive X-ray spectrometer) respectively. Similarly, transmission electron microscopic (TEM) images and EDS elemental mapping were acquired from FEI Talos 200S transmission electron microscope equipped with 200 kV Field Emission Gun (FEG). The samples were well dispersed in ethanol by sonication and drop casted onto a copper supported carbon film. Raman spectra were collected using Labram HR evolution Raman spectrometer (Horiba Jobin Yvon) equipped with an argon-ion laser (λ = 532 nm). Sub-critical Nitrogen sorption isotherm and textural properties (specific surface area, pore-volume, pore size distribution) were measured at − 196 °C using a Quantachrome Autosorb iQ 2 TPX automated gas sorption system. Brunauer-Emmett-Teller (BET) equation was used to calculate the specific surface areas and applied to the adsorption data over the relative pressure (P/P 0 ) range of 0.05-0.20. The total pore volume (V total ) was calculated from the amount of adsorbed N 2 at P/P 0 = 0.99 using the single point adsorption method. Micropore volume (V micro ) calculated by the t-plot method and ultra-micropore volume (V ultramicro ) for the pores < 0.7 nm was calculated by NLDFT method from CO 2 adsorption isotherm at 0 °C (assuming carbon adsorbent having slit pores kernel). The above textural properties are evaluated using Quantachrome ASiQwin data processing software equipped along with the instrument. Before gas sorption measurement, all the samples were outgassed at 160 °C for 15 h under ultra-high vacuum (10 -6 mbar).

Synthesis of MIL-101(Cr). MIL-101(Cr) was synthesized under hydrothermal condition following our
previously reported method 32 . Briefly, Cr(NO 3 ) 3 ·9H 2 O (2.0 g), H 2 BDC (0.833 g), HCl (0.416 mL), and H 2 O (30 mL) were mixed under sonication (30 min) at room temperature. The resulting mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 220 °C for 8 h in a programmable oven. After the completion of the reaction, the autoclave was allowed to cool down to the ambient temperature, and the obtained green solid was separated from the solution by centrifugation. Subsequently, the solid was washed twice with hot distilled water, acetone and hot ethanol. Further, the green solid was suspended in the ethanol-water mixture (30 mL, 95/5 v/v) and heated at 80 °C for 8 h in a Teflon-lined autoclave. Finally, the obtained green solid was dried at 80 °C for 12 h under vacuum.

Synthesis of RHA-MIL-101(Cr).
Prior to the synthesis of RHA-MIL-101(Cr), the supplied RHA was pretreated. Initially, the supplied RHA was grounded with the help of mortar pastel for 60 min and subsequently sieved by 200-micron mesh sieves to obtain uniform particle size. The resultant powdered RHA was purified by the treatment of (1:1, v/v) HNO 3 /distilled water and finally dried at 120 °C for 12 h under vacuum. RHA-MIL-101(Cr) was synthesized using the procedure analogous to that of MIL-101(Cr), where a specified amount of RHA was suspended in the reaction mixture containing the ingredient required for the synthesis of MIL-101(Cr). RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II were obtained by incorporating 6.25 mg and 12.5 mg of RHA, respectively during the synthesis. The obtained RHA-MIL-101(Cr) was purified and activated under similar condition used for MIL-101(Cr). CO 2 and N 2 adsorption measurements. Pure component CO 2 and N 2 sorption isotherms of MIL-101(Cr), RHA-MIL-101(Cr)-I and RHA-MIL-101(Cr)-II were obtained using Quantachrome Autosorb iQ 2 TPX automated gas sorption system at varied temperature (0 °C and 25 °C) for CO 2 and at 25 °C for N 2 in the pressure range of 0-1 bar by using a static volumetric technique. A thermostatic bath was used to control the adsorbent temperature with a precision of ± 0.01 °C. Prior to the adsorption experiments, approximately 0.1 g of sample was outgassed at 160 °C for 15 h by using a turbo-molecular vacuum pump. A low heating rate (3 °C min −1 ) was chosen to allow steady removal of moisture from the sample by avoiding any structural changes of the adsorbents.
Adsorption isotherm modelling. To evaluate the adsorption affinity between the adsorbate and the adsorbent over the pressure range (0-1 bar), the CO 2 and N 2 adsorption data were modelled by fitting them to the non-linear form of Sips equation respectively which are expressed as below.
where p is the equilibrium adsorbate pressure (bar), q is the adsorption capacity (mmol g −1 ), q s is the saturated adsorption capacity (mmol g −1 ), and b s is the affinity constant for Sips model. The parameter s is usually less than unity and characterizes the heterogeneity of the adsorption system 53 . The values of parameters of the Sips model can be evaluated by non-linear curve fitting of the respective isotherm data.
(1) q = q s (b s p) 1 www.nature.com/scientificreports/ Adsorption thermodynamics. The thermodynamic property such as the isosteric heat of adsorption at a given CO 2 adsorption capacity (q) was calculated from the isotherm data at two different temperatures (0 and 25 °C) by applying the Clausius-Clapeyron equation, as represented in Eq. (2).
where Q st and R are the isosteric heat of adsorption and the universal gas constant, respectively. The isosteric heat of adsorption using Eq. (2) can also be obtained directly from Quantachrome ASiQwin data processing software equipped along with the instrument.
Adsorbent evaluation parameters for CO 2 capture. Performance of the synthesized adsorbents is evaluated for their possible application for CO 2 adsorption from low CO 2 content environment such as flue gas emitted from thermal power plants having a lower partial pressure of CO 2 than N 2 . In this context, CO 2 /N 2 selectivity, working capacity of CO 2 , regenerability of CO 2 and purity of CO 2 captured of the synthesized adsorbents were investigated by retrieving the data from their pure component adsorption isotherm at 0.15 bar of CO 2 and 0.75 bar of N 2 . Practically vacuum swing adsorption (VSA) techniques could be more suitable to capture CO 2 from flue gas to avoid the cost associated with compression or pressurization of CO 2 during adsorption. In VSA, adsorption occurs at atmospheric pressure, and desorption occurs at sub-atmospheric pressure. Thus, by following Bae-Snurr criteria, a pressure range of 0-1 bar is often used to provide sufficient information about the evaluation parameter of the adsorbents under ambient conditions theoretically (without doing the actual VSA experiment in packed bed reactor) 51,54 .
a. CO 2 working capacity (WC) is the difference in CO 2 adsorption capacity between adsorption and regeneration conditions = q CO2 ads − q CO2 des b. Regenerability (R) = (WC/q CO2 ads ) × 100 (%) c. CO 2 over N 2 selectivity (S CO2/N2 ) = (q CO2 ads /q N2 ads )/(p CO2 ads /p N2 ads ) d. Purity of CO 2 = q CO2 ads /(q CO2 ads + q N2 ads ) × 100 (%) where q CO2 ads and q N2 ads are the amounts of CO 2, and N 2 adsorbed at their respective equilibrium partial pressures (p CO2 and p N2 ). q CO2 des is the amounts of CO 2 adsorbed at its desorption pressure. In general, for coal-fired power plants, the flue gas was generated at a total pressure of approximately 1 bar having a CO 2 concentration of 15% and an N 2 concentration of 75% 55 . Under these conditions, the corresponding partial pressure was 0.15 bar for CO 2 and 0.75 bar for N 2 . For the VSA process, the CO 2 partial pressure in the adsorption region is 0.15 bar and for desorption is 0.01 bar was considered for this evaluation. www.nature.com/scientificreports/