The oversolubility of methane gas in nano-confined water in nanoporous silica materials

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The oversolubility of methane gas in nano-confined water in nanoporous silica materials Chih-Cheng Liu a, Hao-Ju Chou a, Chan-Yi Lin a, Damodar Janmanchi a, Po-Wen Chung a, Chung-Yuan Mou b, Steve S.-F. Yu a, Sunney I. Chan a, b, * a b

Institute of Chemistry, Academia Sinica, Nankang, Taipei, 11529, Taiwan Department of Chemistry, National Taiwan University, Taipei, 10617, Taiwan

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanoporous silica materials Mesoporous silica nanoparticles Methane sorption Nano-confined water Methane oversolubility

The oversolubility of the non-polar methane (CH4) gas in nano-confined liquid in nanoporous silica materials is investigated. A series of mesoporous silica materials with different pore sizes, pore volumes, and different amounts of nano-confined water (H2O) are prepared using a pore-expanding reagent and surfactants of different chain-lengths, and the CH4 absorption by the silica nano-materials is studied at 298 K under low CH4 gas pressures. Analysis of the CH4 absorption data reveals unequivocal evidence for oversolubility of CH4 in the nano-confined H2O of all the hydrated nano-materials. The solubility enhancements are several hundred fold relative to the CH4 solubility in bulk H2O. Interestingly, the enhancements are 25–30% higher when a tricopper cluster complex capable of efficient selective CH4 oxidation under ambient conditions is immobilized into the nano-channels of the silica materials. This dramatic enhancement of the CH4 solubility is attributed to specific CH4 binding to the tricopper cluster complexes embedded within the mesopores of the nanoporous materials.

1. Introduction Oversolubility, the large enhancement in the apparent gas solubility of inert gases in liquids confined in nanoporous solids, has been pro­ posed as a means to develop novel adsorption, phase separation, or catalytic processes [1–16]. The phenomenon can be exploited in liquid-phase catalysis involving a gas component to improve the local concentration of the dissolved gas at the site of catalysis. We recently immobilized the [(7-N-Etppz)CuICuICuI]1þ complex (Scheme 1) into mesoporous silica nanoparticles (MSN), where 7-N-Etppz refers to the ligand derived from 3,3’-(1,4-diazepane-1,4-diyl)bis[1-(4-ethyl­ piperazine-1-yl)propan-2-ol], and demonstrated that this methane (CH4) oxidation catalyst mediates the conversion of CH4 into methanol (CH3OH) with significantly greater catalytic efficiency compared to when the tricopper complex is solubilized and operated as a homoge­ nous catalyst in the bulk solvent [17–20]. The enhanced catalytic effi­ ciency was attributed to the oversolubility of CH4 in the liquid confined within the mesopores of the MSN [19]. Although this CH4 oversolubility has been estimated to be a factor of 10–100 from molecular dynamics simulations [12–14], there has been no direct measurement of this enhancement. Here, we present a study to fill in this paucity of data on

the CH4 oversolubility in MSN as well as MCM-41, both of which are under consideration to support the CH4 oxidation catalyst. The oversolubility of an inert gas like CH4 in the nano-confined liquid within a nanoporous material is a highly complex physiochemical problem. At least three factors are known to contribute to this oversolubility [14–16]. First, gas absorption to the pores inherent in these heterogeneous structural frameworks, including adsorption to the liquid solid interface. Second, the density of the nano-confined liquid is typically lower than that of the bulk solvent because of changes in the water structure. Third, CH4 can form the CH4 clathrate under suitable pressures, at least transiently. However, the relative importance of these contributions remains unclear at the moment. In this work, mesoporous silica (MS) materials are chosen to be the solvent-confining matrix for several reasons. First, MS materials possess controllable uniform pore sizes and large surface areas, and it is possible to prepare MS nanoporous materials of varying pore sizes from the microporous to the mesoporous size range. The outcome of the study would contribute data to allow better comparison with computer simulation studies in the future, where a single pore size is chosen by default. Second, the mesopore size range, 1.5 nm–8 nm, is the pore size range where the oversolubility phenomenon appears to be the most

* Corresponding author. Institute of Chemistry, Academia Sinica, Nankang, Taipei, 11529, Taiwan. E-mail address: [email protected] (S.I. Chan). https://doi.org/10.1016/j.micromeso.2019.109793 Received 2 August 2019; Received in revised form 2 October 2019; Accepted 3 October 2019 Available online 5 October 2019 1387-1811/© 2019 Published by Elsevier Inc.

Please cite this article as: Chih-Cheng Liu, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109793

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Before and after the mesoporous silica samples were employed to immobilize the tricopper complex, the zeta potentials of the samples were measured by a Malvern Nano-HT Zetasizer. Zeta-potential distri­ bution was obtained by the average of ten measurements. For these experiments, the samples were prepared at the concentration of 2 mg in 1 ml of deionized H2O. The pH of the solution was adjusted to different values by the addition of 0.02 M HCl(aq) or NaOH(aq). Before measure­ ment, each sample was ultra-sonicated for 5 min to prevent any aggregation. Thermogravimetric analysis (TGA) was carried out on a Dynamic TA 2950 thermogravimetric analyzer. About 5 mg material was treated isothermally for 15 min at 303 K under N2 (g) and then heated up to 873 K by using a gradient of 2 K/min. The carrier gas (air) flow rate was 60 ml/min. The CH4 sorption isotherms were obtained at 298 K on a Micro­ meritics 3-Flex gas analyzer with a temperature controller (Micro­ meritics ISO Controller) using pure CH4 (99.999%) as the gas source. Prior to the measurements, all the hydrated samples were outgassed at 298 K and 2 � 10 3 torr for 12 h.

Scheme 1. The chemical structure of [(7-N-Etppz)CuICuICuI]1þ.

prominent. For example, the solubility of H2 in confined solvents in the microporous solid zeolite (pore ~ 1 nm) is similar to its bulk counter­ part, while large oversolubilities are observed when mesoporous solids are considered [8]. Third, the contribution of surface adsorption versus enhanced solubility of the gas in the confined solvent can be separated as the amounts of confined liquid can be judiciously adjusted. Finally, we would like to demonstrate that the enhanced catalytic oxidation of CH4 to CH3OH by the [(7-N-Etppz)CuICuICuI]1þ catalyst embedded in the MSN is linked to the oversolubility of the reactant gases, CH4 and O2 in the mesopores.

2.2. Synthesis of the ligand 7-N-Etppz and preparation of the CuIICuIICuII(7-N-Etppz)4þ tricopper cluster complex The synthesis and characterization of the 7-N-Etppz ligand, as well as the preparation of the tricopper complex CuIICuIICuII(7-N-Etppz)4þ (CuEtp), have been described earlier [17,19]. We have repeated the chemical syntheses of these compounds and verified their structures according to the methods established earlier. The details are presented in the Supplementary data.

2. Experimental section 2.1. Materials and methods

2.3. Preparation of the pore-expanded nanoparticles (MSN-ex)

All chemicals were purchased from commercial suppliers (SigmaAldrich, ACROS) and used without further purification. TEM images were acquired on the Hitachi H-7100 at the acceleration voltage of 75 kV. The surface areas, pore sizes, and pore volumes of the nanoporous silica materials, including both MSN and MCM-41, were determined by nitrogen (N2) sorption isotherms obtained at 77 K on a Micromeritics ASAP 2010 apparatus. The silica samples were outgassed at 2 � 10 3 torr and 393 K up to 10–15 h prior to the N2 adsorption experiments. Specific surface areas were calculated by the BET (Bru­ nauer–Emmett–Teller) method over a relative pressure range of 0.05–0.30. The detailed range of the relative pressure as well as other information on the determination of the BET surface areas are given in Table S1. Plots of the pore size distributions for all the nanoporous materials were obtained from analysis of the adsorption isotherms using the BJH (Barrett–Joyner–Halenda) method. Pore sizes were determined by the peak positions of the plots, which were the common values with the highest frequency. The pore volumes were calculated from the BJH adsorption cumulative volume of the pores between 1.0 and 10 nm. 29 Si solid-state NMR experiments were carried out on a BRUKER AVIII 600 WB spectrometer equipped with a commercial 4 mm MAS NMR probe at a magnetic field of 14.1 T. All spectra were taken at room temperature. The magic-angle spinning (MAS) frequencies were set to 10 kHz for all experiments. The 29Si chemical shifts are referenced to tetramethylsilane. Quantitative determination of the copper contents of the tricopper complex immobilized in the silica samples was performed by inductively coupled plasma mass spectroscopy (ICP-MS) using the Agilent 7700 Series instrument. Samples were pretreated in 48% hydrofluoric acid and completely digested by the addition of concentrated nitric acid (69%) with vigorous stirring overnight. The digested solution was diluted with 2% nitric acid aqueous solution prior to the determinations. A calibration curve of diluted copper standard solutions was performed in the concentration range from 0 to 50 ppb. Elemental analyses for C, H, and N were carried out on a Heraeus vario III-NCH microanalyzer.

0.386 g of cetyltrimethylammonium bromide (C16TAB) was dis­ solved in 160 g of 0.30 M ammonia solution (NH3(aq)) at 323 K. The aqueous C16TAB solution was then mixed with a decane/EtOH solution prepared by dissolving 1.2 ml of decane in 15 ml of ethanol (EtOH) to form an oil-in-water emulsion by stirring. After the emulsion had been stirred at 323 K for 12 h, 3.33 ml of 0.88 M tetraethyl orthosilicate (TEOS) solution (in EtOH) was added under vigorous stirring at 323 K for 1 h. The resulting solution was then aged at 323 K for 24 h. The mixture contained the MSN-ex as the major product. The mesoporous thin film side-product was separated by filter paper (55 mm in diam­ eter). Hydrothermal treatment of the filtrate containing the assynthesized MSN-ex samples was then carried out at 353 K for 24 h. The final MSN-ex samples were collected by centrifugation at 14000 rpm for 30 min, washed and re-dispersed by EtOH several times. The sur­ factant templates were finally removed by extraction twice in ammo­ nium nitrate (NH4NO3)/EtOH solution (1 g of NH4NO3/50 ml of EtOH) at 333 K for 1 h. The final MSN-ex sample was collected by centrifuga­ tion, washed with EtOH several times, and dried at 333 K in air over­ night to obtain a white powder [(MSN-ex) (NH4)]. 2.4. Preparation of the MSN-TP nanoparticles As described previously [19–23], 0.58 g of C16TAB was dissolved in 300 g of 0.51 M NH3(aq) at 313 K, and 5 ml of 0.21 M dilute TEOS solu­ tion (in EtOH) was added to the mixture. After stirring at 313 K for 5 h, 250 μl of 3-(trihydroxysilyl)propyl methylphosphonate (TP) dissolved in 1 ml of de-ionized H2O and 5 ml of 0.88 M TEOS solution were added under vigorous stirring for another 1 h. The solution was then aged at 313 K for 24 h. The as-synthesized materials were collected by centri­ fugation at 12000 rpm for 25 min, washed and re-dispersed by EtOH several times. The surfactant templates were removed by extraction twice in NH4NO3/EtOH solution (1 g of NH4NO3/50 ml of EtOH) at 333 K for 1 h. Finally, the MSN-TP was collected by centrifugation, washed with EtOH several times, and dried at 333 K in air overnight to 2

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obtain a white powder [(MSN-TP)(NH4)].

2.8. Catalytic oxidation of CH4 mediated by the tricopper CuEtp complex immobilized in the mesoporous silica materials in the presence of H2O2 and trace amounts of O2

2.5. Preparation of Cn-MCM-41 materials (n ¼ 10, 12, 14, 16) with different pore sizes

35 mg of the partially hydrated CuEtp@MSN-ex (5.97 μmol), CuEtp@MSN-TP (3.97 μmol), or CuEtp@C16-MCM41 sample (3.00 μmol), containing CH4 equilibrated at the gas pressure of 1 atm, were well suspended in 2 ml of anhydrous MeCN in a 20 ml glass sample bottle sealed tightly with a rubber cap. Then sodium ascorbate (freshly prepared 1 M solution in deionized H2O) was added as a reducing agent (4 equiv. based on the amount of tricopper complex in the silica sample given above) under a N2 atmosphere. The heterogeneous mixture was stirred vigorously at room temperature to form the CuICuICuI(7-NEtppz)1þ in situ. An aliquot of H2O2 solution (diluted 1 M solution) containing 2 equiv. based on the total amount of adsorbed CH4 in the sample (the amounts of H2O2 used: CuEtp@MSN-ex, 15.17 μmol; CuEtp@MSN-TP, 12.30 μmol; and CuEtp@C16-MCM41, 11.28 μmol) was then added to the suspension. The reaction mixture was stirred continuously for 1 h at room temperature. After breaking the seal to the glass sample bottle, 3 μl of nitrobenzene was added to the solution to provide an internal standard (IS) for the quantitation of the products. The above experiment was repeated in the presence of excess CH4 gas. An additional 18 ml of CH4 at 1 atm, 303 K (724.10 μmol) was injected into the sealed glass bottle to obtain a total CH4 gas pressure of 1 atm in the catalytic system after the tricopper cluster complex was reduced by ascorbate. 50 equiv. of H2O2 (30%, based the amount of the CuEtp in the sample given above) was then added and the CH4 oxidation was followed as described above.

MCM-41 materials of different pore sizes were prepared by hydro­ thermal synthesis using preformed β-zeolite seeds [24,25] as the silica source and organic long-chain quaternary ammonium salts of different hydrocarbon chain-lengths as the surfactant. The β-zeolite seeds (Si/Al ¼ 66) were prepared by mixing sodium aluminate (NaAlO2) (0.246 g), fumed silica (12.0 g), tetraethylammonium hydroxide (39 g), and sodium hydroxide (NaOH) (0.30 g) in H2O (32.4 g) under stirring at 323 K for 5 h. Then, the mixture was hydrothermally treated at 373 K in an autoclave for 18 h to obtain a colorless liquid. The procedure for the synthesis of the various MCM-41 samples is given as follows: 0.92 g of decyltrimethylammonium bromide (C10TAB, for synthesis of C10-MCM-41), 1.01 g of dodecyltrimethylammonium bromide (C12TAB, for synthesis of C12-MCM-41), 1.10 g of tetradecyl­ trimethylammonium bromide (C14TAB, for synthesis of C14-MCM-41), or 1.2 g of C16TAB (for synthesis of C16-MCM-41) was dissolved in 25 g of de-ionized H2O. 7 g of β-zeolite seeds were then added and the mixture was transferred into an autoclave for hydrothermal reaction at 393 K for 48 h. After cooling down to room temperature, the mixture was adjusted to pH ¼ 10 (initial pH: ~12.1) by 1.2 M sulfuric acid (H2SO4(aq)). Then the mixture was sealed into an autoclave at 373 K for another 48 h. The as-synthesized materials were collected by centrifu­ gation at 9000 rpm for 10 min, washed and re-dispersed by EtOH several times. The surfactant templates were removed by calcination at 813 K for 9 h to obtain a white powder.

3. Results 3.1. Physical characterization of the dry nanoporous silica samples with different pore sizes

2.6. Immobilization of the tricopper CuEtp complex in the mesoporous silica materials

The N2 sorption isotherms of the mesoporous silica samples are shown in Figs. 1 and 2 and Fig. S1 S2. Analysis of the data including

The CuEtp@MSN-ex, CuEtp@MSN-TP, and CuEtp@C16-MCM41 were prepared by the ion-exchange method via electrostatic attraction according to the procedure that we established earlier [18–20]. To begin, 150 mg of the as-synthesized silica was well dispersed in 25 ml of EtOH, and then 25 ml of a solution of 10 mM CuEtp in acetonitrile (MeCN) was added slowly and stirred at 298 K for 24 h to exchange out some of the TP. The solid was collected by centrifugation, washed with EtOH, and dried under vacuum. The loading of the CuEtp in the mes­ oporous silica material was determined by measuring the copper content by ICP-MS and the ligand content by C/N elemental analysis, and the results are given in Table S2. For all the silica samples, the copper-to-ligand molar ratios are very close to the stoichiometric values of the tricopper complexes, which indicate that the complex did not break apart during the immobilization on to the silica samples. 2.7. Hydration of the nanoporous silica materials To achieve full hydration of the mesopores of the silica materials, physical-absorption of H2O experiments were performed over several days. Dry silica materials were paved smoothly on to the surface of a small flat vessel. The vessel was then transferred to a closed container containing a beaker of H2O. The dry nanoporous silica samples were hydrated by exposure to the water vapor within the closed container at room temperature for 72 h. Subsequently, the hydrated silica materials were collected and quickly dried at room temperature by vacuum for about 1–3 min to remove the excess H2O that might be residing within the macropores of the mesoporous materials. The amounts of H2O physically absorbed inside the mesopores of the samples were then determined by mass balance, and the results were confirmed by TGA measurements.

Fig. 1. (a) N2 sorption isotherms of the MSN samples. (b) Plots of the pore size distributions. 3

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Fig. 2. (a–d) N2 sorption isotherms of the MCM41 samples. (e) Plots of the pore size distributions.

BET surface areas, BJH pore volumes, and pore diameter is summarized in Table 1. Indeed, the pore sizes and pore volumes of the silica materials can be controlled by using a pore expanding reagent and surfactants of different chain-lengths. In the case of the MSN samples, the pore di­ ameters are 4.77 nm for the pore-expanded MSN-ex sample, and 2.80 nm for MSN-TP sample; and the corresponding BJH pore volumes are 1.024 cm3/g for the MSN-ex sample and 0.713 cm3/g for the MSN-TP sample (MSN-ex > MSN-TP). In the case of the MCM41 samples, the pore diameter follows the order C16-MCM41 > C14-MCM41 > C12MCM41 > C10-MCM41; the BJH pore volumes exhibit the same trend. The C16-MCM41 sample has the largest pore diameter (2.40 nm) and BJH pore volumes (0.616 cm3/g) compared to the other MCM-41 sam­ ples. The pore sizes and volumes for the MCM-41 samples are all lower

than those of the MSN samples. Transmission electron microscopy (TEM) images of the mesoporous silica samples are shown in Fig. 3. The results for both MSN samples (Fig. 3a–b) demonstrate that the nanoparticles are well dispersed, with uniform sizes of about 70 nm and clear evidence of nano-channels. Statistical analysis (Fig. 3, inset) of the TEM images gives a particle size distribution of 68.3 � 7.0 nm for the MSN-ex sample and 71.6 � 8.0 nm for the MSN-TP sample. TEM images of the MCM41 samples reveal well-ordered nano-channel arrays (Fig. 3c). For clarifi­ cation, the MCM41 samples in this study have particle sizes in the micron range. MSN is the nano-particulate form of MCM-41 with much smaller particle size in the range of 50 nm–200 nm. The 29Si solid-state NMR of the mesoporous silica (Fig. 4) indicates

Table 1 Physical properties of the dry nanoporous silica samples and the corresponding changes after hydration.a Sample

b

MSN-ex MSN-TP C16-MCM41 C14-MCM41 C12-MCM41 C10-MCM41 CuEtp@MSN-ex CuEtp@MSN-TP CuEtp@C16-MCM41

ABET [m2/g]

c

Vp [cm3/g]

d

e

1056 912 1046 1038 1066 1072

1.024 0.706 0.616 0.524 0.445 0.366

4.77 2.80 2.40 1.98 1.68 1.37

100.8 69.7 61.1 51.9 43.9 36.1

738 702 866

0.622 0.394 0.432

2.65 1.73 1.46

61.5 39.0 42.8

DBJH [nm]

ΔWH2O [%]

a

f

g

h

i

0.502 0.411 0.379 0.342 0.305 0.265

0.246 0.200 0.190 0.177 0.163 0.150

98.5 98.7 99.2 99.0 98.7 98.7

31.8 35.4 38.0 41.0 43.8 48.2

0.381 0.281 0.300

0.191 0.156 0.161

98.9 99.0 99.1

37.8 46.9 44.3

WH2O [g/g]

W’H2O [g/g]

OH2O [%]

O’H2O [%]

Experimental uncertainties estimated to be within �2% for the ABET, Vp, and DBJH; and �0.001 g/g for the WH2O and W’H2O; �0.1% for the ΔWH2O, OH2O, and O’H2O. ABET: BET surface area. c Vp: pore volume, deduced from the BJH adsorption cumulative volume of the pores between 1.0 nm and 10 nm (diameter). d DBJH: pore diameter. e ΔWH2O: percentage weight change of the silica sample after hydration. f WH2O: the gram-weight of absorbed H2O per gram of hydrated silica. g W’H2O: the gram-weight of absorbed H2O per gram of hydrated silica after degas pre-treatment. h OH2O: percentage of the pore volume in the dry silica sample occupied by absorbed H2O after physical H2O adsorption. i O’H2O: the percentage of pore volume in the dry silica occupied by the nano-confined H2O remaining after degas pre-treatment. b

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Fig. 4. 29Si solid-state NMR spectra of the mesoporous silica samples: (a) MSNex; (b) MSN-TP; and (c) C16-MCM41. Each spectrum displays a set of signals Q2, Q3, and Q4, attributed to the Si(OH)2(OSi)2, Si(OH)(OSi)3, and Si(OSi)4 sub­ structures, respectively. The peak percentages are obtained by de-convoluting each spectrum into their components and fitting the full-width at halfmaximum of each component spectrum.

that the silica samples are formed with a high degree of silica polycondensation with mainly Q3 (Si(OH)(OSi)3) and Q4 (Si(OSi)4) substructures. For comparison with the absorption of CH4 gas by the bare silica materials, the CuEtp complex is also immobilized in the nano-channels of the selected nanooporous silica samples with different pore diameters by the ion-exchange method via electrostatic attraction to obtain the CuEtp@MSN-ex, CuEtp@MSN-TP, and CuEtp@C16-MCM41 samples [19,21,22]. The loading amount of the CuEtp complex is 171 μmol/g for MSN-ex sample, 114 μmol/g for MSN-TP sample, and 86 μmol/g for C16-MCM41 sample (Table S3), as determined by measuring the copper content by ICP-MS and the quantity of the ligand from C, N elemental analysis. Analysis of the N2 sorption isotherms and the zeta-potentials of the nanoporous silica samples before and after immobilization of the CuEtp complex are given in Fig. S3 and Table S3. The surface area of the MSN-ex sample shows the largest reduction (from 1056 to 738 m2/g) compared to that of the MSN-TP sample (from 912 to 702 m2/g) and C16-MCM41 sample (from 1046 to 866 m2/g) after immobilization of the CuEtp; and the pore volume of the MSN-ex sample also shows the largest decrease (from 1.024 to 0.622 cm3/g) compared to that of the MSN-TP sample (from 0.706 to 0.394 cm3/g) and C16-MCM41 sample (from 0.616 to 0.432 cm3/g) (Table S3). The pore sizes have been reduced to 2.65 nm, 1.73 nm, and 1.46 nm for CuEtp@MSN-ex, CuEtp@MSN-TP,

Fig. 3. Transmission electron microscopy (TEM) images of several mesoporous silica samples with different pore diameters: (a) MSN-ex sample with 200 nm scale bar; (b) MSN-TP sample with 100 nm scale bar; and (c) C16-MCM41 sample with 100 nm scale bars. Statistical analysis of the TEM images gives a particle size distribution 68.3 � 7.0 nm for the MSN-ex sample (based on a patch size of 60 particles), as shown in the inset of (a); and 71.6 � 8.0 nm for the MSN-TP sample (based on a patch size of 30 particles), as shown in the inset of (b).

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and CuEtp@C16-MCM41, respectively. It is clear that the reduction of the surface area, pore volume, and pore diameter in these silica samples after the immobilization of the CuEtp complex is a direct consequence of loading of the CuEtp complex, indicating that the complex is immobi­ lized within the nano-channels of the silica samples and partially occupy the pore spaces. These results are also confirmed by the zeta-potential analysis (Table S3). 3.2. Physical characterization of the nanoporous silica samples after hydration To study the solubility of the nonpolar CH4 gas in a liquid occupying the pores of the nano-porous solids, H2O is employed as the liquid confined in the nano-channels of the mesoporous silica samples. The H2O absorption experiments are performed at room temperature for 3 days, and the hydration of the mesoporous silica materials are deter­ mined by mass balance and confirmed by TGA measurements (Fig. S4), as summarized in Table 1. As expected, the amounts of H2O absorbed vary with the pore volumes of the nanoporous silica materials. In the study, the amounts of absorbed H2O are controlled by using a pore expanding reagent or by varying the chain-length of the surfactant. Based on the estimated occupation percentage of the pore volume by H2O (OH2O) (Table 1), nearly full hydration (~99%) of the nanopore is observed in each case. Before CH4-absorption measurements on the hydrated nanoporous silica materials, the fully hydrated samples described above are sub­ jected to partial dehydration to remove a portion of the bound H2O by degassing under vacuum at room temperature (298 K and 2 � 10 3 torr for 12 h). This pretreatment minimizes the loss of the nano-confined H2O in the hydrated samples during the CH4 absorption experiments. If the nanoporous silica materials are fully hydrated, some loss of bound H2O might be expected due to expansion of the confined H2O upon CH4 gas absorption. When the degassing treatment is carried out under vacuum at high temperatures (393 K and 2 � 10 3 torr for 10–15 h), most of the absorbed H2O in the nanoporous silica materials is removed according to the TGA measurement (Fig. S4). However, controlled dehydration of the fully hydrated nanoporous silica materials can be accomplished by degassing treatment of the fully hydrated silica under vacuum at room temperature (see above). Results on the controlled dehydration of the nanoporous silica materials by this pretreatment as determined by mass balance are summarized in Table 1. The occupation percentage of the pore volume by H2O (O’H2O) is reduced to 30–50% for all the silica materials after the degassing treatment at 298 K. In the case of MSN-ex, even though the value of O’H2O (31.8%) is lower compared with the other samples, the amount of bound H2O remaining in the mesopores (W’H2O) is still the highest among all the silica materials (0.246 g absorbed H2O per gram of hydrated MSN-ex material). Since W’H2O is directly proportional to the pore volume of the silica sample, the amount of bound water in these nanoporous silica materials can be controlled by using a pore-expanding reagent or by longer-chain surfactants.

Fig. 5. Isotherms depicting the sorption of CH4 by (a) dry MSN-ex and C16MCM-41 samples; and (b) hydrated MSN-ex and C16-MCM-41 samples. Quan­ tity of CH4 absorbed is given in mg per gram of sample.

times higher than that by the dry sample without confined H2O (1.63 mg of CH4 absorbed per gram of sample at ~780 mm Hg) (Fig. 5a; Table S4). We surmise that there are three contributions to the amounts of CH4 absorbed by the mesoporous silica materials [14,15]: (1) CH4 gas occupying the void volume of the heterogeneous structural framework of the silica (n); (2) CH4 molecules physically adsorbed to the pore surface (or the walls) at the gas/liquid solid interface (physisorption of the Langmuir type) (N); and (3) CH4 gas dissolved in the nano-confined H2O (mH2O). Accordingly, the amount of CH4 absorbed by the silica material (Q) for a given CH4 gas pressure can be described by Q ¼ n þ N þ mH2O. The CH4 gas occupying the void volume can be estimated assuming ideal gas behavior at the low gas pressures employed in our study. The physisorption of CH4 to the pore surface at each CH4 pressure can be estimated from the amount of CH4 absorbed by the dry sample (Fig. S5 and Table S4), where Q ¼ n þ N, since mH2O ¼ 0 in the absence of nano-confined H2O. For the partially hydrated silica sample, we consider two scenarios to model the amount of physisorbed CH4 to the pore surface at the gas/liquid solid interface. In one scenario, N is esti­ mated assuming the physisorption of CH4 to the gas/liquid solid inter­ face is the same as in dry sample (Model A). In the other scenario, we assume that the CH4 molecules are physisorbed only to the surface accessible within the remaining void volume of the pore containing no confined H2O and we scale N according to the void volumes of the partially hydrated and dry samples, namely Vhydrated/Vdry (Model B). Within these limits, the amounts of CH4 solubilized in the nano-confined H2O (mH2O) can then be estimated and the oversolubility determined. The outcome of the above analysis for the MSN-ex and C16-MCM41 samples is summarized in Figs. 6–8 and Table 2. In Figs. 6 and 7, we summarize n, N, mH2O as deduced from the CH4 absorption (Q) for the hydrated MSN-ex and C16-MCM41 samples derived from their respective CH4 adsorption isotherms according to the two models (Model A and Model B) used to estimate the amounts of CH4 gas physisorbed to the pore surface of the mesoporous silica framework. In the case of the

3.3. CH4 sorption isotherms of the nanoporous silica materials We now describe the absorption of CH4 by the various hydrated silica samples as prepared according to the controlled dehydration of the fully hydrated samples. The amounts of CH4 absorbed by C16-MCM41 and MSN-ex samples with and without nano-confined H2O within the mes­ opores are compared over the pressure range from 0 to ~800 mm Hg in Fig. 5. The results show that the amount of CH4 gas absorbed by the partially hydrated C16-MCM41 (2.99 mg of CH4 absorbed per gram of sample at ~780 mm Hg) is only somewhat higher than that by the dry sample without confined H2O (2.17 mg of CH4 absorbed per gram of sample at ~780 mmHg) in these isotherms (Fig. 5b; Table S4). However, the amount of CH4 gas absorbed by the partially hydrated MSN-ex (4.02 mg of CH4 absorbed per gram of sample at ~780 mm Hg) is 2.5 6

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Fig. 7. Analysis of the CH4 absorption by the hydrated C16-MCM41 sample at 298 K as deduced from the CH4 adsorption isotherm in Fig. 5. Quantity of CH4 absorbed/adsorbed is given in mg per gram of sample. At each pressure of CH4 gas, Q ¼ n þ N þ mH2O. See text for details of the analysis.

Fig. 6. Analysis of the CH4 absorption by the hydrated MSN-ex sample at 298 K as deduced from the CH4 adsorption isotherm in Fig. 5. Quantity of CH4 absorbed/adsorbed is given in mg per gram of sample. At each pressure of CH4 gas, Q ¼ n þ N þ mH2O. See text for details of the analysis.

hydrated MSN-ex sample, the amount of CH4 solubilized in the nanoconfined water (mH2O) dominates for either model, accounting for 67–75% of the CH4 absorbed by the sample at the highest pressure in the study (825 mm Hg). A significantly smaller amount of CH4 is solubilized in the nano-confined H2O for the C16-MCM41 sample: 33–55% at 784 mm Hg depending on the model used to estimate the amount of CH4 physisorbed at the gas/liquid solid interface of the pores. Fig. 8 summarizes the solubility of CH4 in the nano-confined H2O at various CH4 gas pressures for both the MSN-ex and C16-MCM41 samples as well the corresponding solubility in bulk water. Henry’s law is used to obtain the CH4 solubility as a function of CH4 gas pressure over bulk water. From these results, it is clear that the amount of CH4 dissolved in the nano-confined H2O is substantial. The CH4 oversolublity at 1 atm is depicted in Table 2. The oversolubility of CH4 gas in the H2O confined within the mes­ opores of the hydrated MSN-ex and C16-MCM41 samples is given by the solubility enhancement factor f in Table 2. The CH4 oversolubility is about 440–490 fold for the hydrated MSN-ex sample. It is somewhat lower for the hydrated C16-MCM41 sample, about 200–350 fold relative to the solubility of CH4 gas in bulk H2O under the same conditions (0.0227 mg/g of H2O at 760 mm Hg and 298 K) [26,27]. With larger amounts of confined H2O in the mesopores of the MSN-ex sample, the amount of CH4 gas absorbed (Q) by the nano-material can be significant under these mild conditions (298 K, 760 mm Hg). For instance, the estimated amount of CH4 gas absorbed by fully hydrated MSN-ex at 760 mm Hg and 298 K (5.99 mg of CH4 absorbed per gram of sample) is 1.5 times higher than that by the partially hydrated MSN-ex (3.87 mg of CH4 absorbed per gram of sample). According to the results shown in Fig. 8, the amount of CH4 gas physisorbed to the surface of the pores approaches saturation at about 500–600 mm Hg for both hydrated silica samples. However, beyond this

Fig. 8. The solubility of CH4 gas in the nano-confined H2O within the nano­ porous silica samples at 298 K under various CH4 pressures. The solubility is expressed in mg of absorbed CH4 per gram of H2O. For comparison, the solu­ bility of CH4 in bulk H2O is also shown (dashed line). The experimental CH4 solubility in bulk H2O at 298 K is 0.0227 mg/g at 760 mm Hg; At other pres­ sures, Henry’s law is invoked.

CH4 gas pressure, the amounts of CH4 adsorbed by the samples continue to increase. A similar saturation behavior is noted for the dry samples (Fig. 5a). These observations are consistent with physisorption of the CH4 molecules to the surface of the pores. Apparently, a primary monolayer of adsorbed CH4 is first formed (adsorbate-adsorbent and adsorbate-absorbate interactions) at the lower CH4 gas pressures, fol­ lowed by formation of second secondary layer (adsorbate-adsorbate interactions) at the higher CH4 gas pressures. Note that the amount of physisorbed CH4 is much higher for the hydrated C16-MCM41 sample 7

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Table 2 The amounts of CH4 absorbed by the hydrated MSN-ex and C16-MCM41 samples and the oversolublity at 298 K and 760 mm Hg.a Sample

Model

b

MSN-ex

A B A B

3.868 3.868 2.856 2.856

C16-MCM41

Q (mg/g)

c

d

0.457 0.457 0.250 0.250

0.968 0.660 1.741 1.079

n (mg/g)

N (mg/g)

e

f

2.443 2.751 0.865 1.527

9.94 11.19 4.56 8.05

mH2O (mg/g)

SCH4 (mg/g of H2O)

g

f

438 493 201 354

a Experimental uncertainties estimated to be within �0.001 mg/g for the amount of adsorbed CH4 (Q, n, N, mH2O), �0.01 mg/g of H2O for the CH4 solubility SCH4, and �0.5% for the solubility enhancement f. b Q: the amount of CH4 absorbed by the silica material at 760 mm Hg (mg per gram of silica material). c n: the amount of CH4 occupying the void volume of the silica material at 760 mm Hg (mg per gram of silica material). d N: the amount of CH4 physisorbed to the pore surface (or the walls) at the gas/liquid solid interface at 760 mm Hg (mg per gram of silica material). e mH2O ¼ Q – n – N, the amount of CH4 dissolved in the confined H2O of the silica materials at 760 mm Hg (mg per gram of silica material). f SCH4: the solubility of CH4 in the confined H2O in the silica materials at 760 mm Hg (mg per gram of bound H2O). g f: the solubility enhancement, defined by the ratio of the CH4 solubility in the nano-confined H2O in the mesopores of the MSN-ex and C16-MCM41 materials to the solubility in bulk H2O at 760 mm Hg.

(N ¼ 1.079–1.741 mg/g at 760 mm Hg depending on the model used to estimate this quantity, or 38–61% of the total amount of absorbed CH4 by the nano-material, Q) compared to that for the MSN-ex sample (N ¼ 0.660–0.968 mg/g at 760 mm Hg or 17–25% of the total amount of absorbed CH4, Q). The absorption of CH4 to dry C16-MCM41 is also significantly larger compared with the dry MSN-ex. For comparison, the CH4 physisorbed to the pore surface of the silica framework accounts for 59% and 81% of the total CH4 absorbed by the dry MSN-ex and C16MCM41 samples, respectively (Table S4). Although these samples possess almost the same surface areas (Table 1), a substantial portion of the area is external for the MSN-ex sample, while for MCM41, the in­ ternal surface dominates. This may explain the difference in the CH4 adsorption behavior. The amount of CH4 gas solubilized in the nano-confined H2O is 2–3 times higher for the MSN-ex sample compared with C16-MCM41. This quantity increases monotonically with the CH4 gas pressure for both hydrated silica samples. This is the anticipated result, as the solubility of the CH4 gas in the confined H2O is expected to increase with gas pressure

(Henry’s law). Fig. 9 summarizes the CH4 sorption isotherms for all the partially hydrated silica samples over the pressure range from 0 to ~800 mm Hg at 298 K. All the CH4 sorption isotherms exhibit hysteresis. For all the partially hydrated samples, the absorbed CH4 cannot be fully released (at ~0 mm Hg) during the desorption process, indicating that a part of absorbed CH4 remains dissolved in the nano-confined H2O due to the strongly confinement effect or physically adsorbed to the walls of the pores. These observations indicate that the kinetics of CH4 absorption is significantly faster than desorption. As shown in Fig. 9, the absorption amounts of CH4 for the partially hydrated samples follow the order MSN-ex > MSN-TP > C16-MCM41 > C14-MCM41 > C12-MCM41 > C10MCM41. Thus, the performance of CH4 absorption is much better for the MSN samples than for the MCM41 samples. The CH4 sorption isotherms of the partially hydrated silica samples before and after immobilization of CuEtp tricopper complex are compared in Fig. 10. The amount of bound nano-confined H2O is sub­ stantially reduced upon the immobilization of the CuEtp complex,

Fig. 9. The CH4 sorption isotherms for all the hydrated silica samples. The quantity of CH4 is given in mg per gram of sample. 8

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Table 3 Comparison of the CH4 oversolubility for the various hydrated nanoporous silica materials at 298 K and 760 mm Hg.a Sample

b

MSN-ex MSN-TP C16-MCM41 C14-MCM41 C12-MCM41 C10-MCM41 CuEtp@MSN-ex CuEtp@MSN-TP CuEtp@C16-MCM41

438 436 201 187 154 110 589 576 270

fA

b

fB

493 501 354 364 364 362 648 661 446

c

fQ

693 686 663 668 678 670 804 796 709

a Experimental uncertainties estimated to be within �0.5% for the solubility enhancement fA, fB, and fQ. b fA and fB: the solubility enhancement for the silica samples estimated by the two models (A and B) used to estimate the amounts of the physisorbed CH4 noted earlier. c fQ: the solubility enhancement for the silica samples estimated by the total quantity of CH4 absorbed (Q).

quantity (N þ mH2O) determined from the total quantity of CH4 absorbed (Q) by the sample after correcting only for the CH4 gas occupying the void volume. For the MSN samples, fA and fB are rather similar, reflecting the significantly smaller amount of physisorbed CH4 relative to the amount solubilized in the nano-confined H2O. Interestingly, the enhancement factors are 25–30% higher with the CuEtp complex immobilized within the nano-channels. Presumably CH4 binding to the tricopper cluster complex (hydrophobic pocket) contributes further to the CH4 oversolubility. However, we cannot rule out that there is further lowering of the density of the liquid H2O confined in the nano-channels from additional disruption of the water structure by the CuEtp complex in the nano-channel. The enhancement factor f is consistently lower for MCM41 sample, and the difference between fA and fB can be quite large, reflecting the greater amounts of CH4 physisorbed to the walls of the pores compared to the quantities solubilized in the nano-confined H2O. This is the expected outcome given the smaller pore sizes associated with samples of the MCM41 series. The surface to volume ratio of the pore is typically a factor of 2 greater for the MCM41 samples compared to the MSN samples. The amount of bound H2O can be as much as 40% lower in the MCM-41 samples (0.150 g/g in the case of C10-MCM41 versus 0.246 g/g for MSN-ex). The pore volume Vp for C10-MCM41 is only 35% of that for MSN-ex (0.366 cm3/g versus 1.024 cm3/g).

Fig. 10. Isotherms depicting the sorption of CH4 by (a) hydrated MSN-ex, (b) hydrated MSN-TP, and (c) hydrated C16-MCM-41 samples, before and after immobilization of the tricopper complex CuEtp (adsorption: filled circles; desorption: unfilled circles).

reflecting the significantly decreased ABET, Vp, and DBJH relative to the parent bare silica samples (Table 1). The absorption of CH4 to the partially hydrated CuEtp-immobilized silica samples varies approxi­ mately linearly with CH4 gas pressure from 0 to ~800 mm Hg at 298 K, with a similar slope compared to that of the bare dry and partially hy­ drated silica samples. The kinetics of CH4 desorption for the CuEtp complex immobilized silica samples, however, is significantly slower than that for the bare silica samples. The difference in the hysteresis in the CH4 sorption isotherms is dramatic. These observations suggest a much stronger nano-confinement of the absorbed CH4 when the CuEtp complex is immobilized in the silica materials. We surmise that binding of the CH4 to the hydrophobic pocket at the base of the CuEtp complex contributes to this hysteresis. It is also possible that micropores are induced in the nanoporous samples upon immobilization of the CuEtp complex into the nanochannels [23]. Since there are no micropores without the copper complex, they are most likely associated with some neck structure formed between the copper complex and the walls of the silica framework, which would account for the strong hysteresis in the CH4 sorption.

3.5. Catalytic CH4 oxidation by the CuEtp tricopper complex immobilized in the mesoporous silica materials The CuEtp tricopper complex immobilized in the three mesoporous silica samples, CuEtp@MSN-ex, CuEtp@MSN-TP, and CuEtp@C16MCM41, supports the conversion of CH4 to CH3OH, as expected. We have studied this CH4 oxidation under two conditions: (1) with the limiting amount of CH4 taken up by the mesopores during equilibration of the sample with CH4 gas at 1 atm; and (2) in presence of a large excess of CH4 gas added externally to the catalytic system at 1 atm. The turn­ over numbers (TONs) obtained after 1 h of incubation are summarized in Table 4. In the first set of experiments, the observed TONs are limited by the 2 equiv. of H2O2 added to initiate the turnover. The amounts of the CuEtp catalyst and CH4 associated with the mesopores are rather similar in these samples: (CuEtp: 5.97, 3.97 and 3.00 μmol; CH4: 7.58, 6.15 and 5.64 μmol; for CuEtp@MSN-ex, CuEtp@MSN-TP, and CuEtp@C16MCM41, respectively.). In this scenario, not all of the CuEtp complexes residing in the nano-channels would be activated and turning over by the 2 equiv. of H2O2 added to the system, so it is not possible to convert all the CH4 in the mesopores into CH3OH. Nevertheless, it is evident that all three samples can support CH4 oxidation, a finding that underscores the efficacy of the CuEtp tricopper cluster for CH4 oxidation. In the presence of excess CH4 gas, these samples can sustain multiple turnovers

3.4. Comparison of the CH4 over-solubility for the hydrated silica materials In Table 3, we summarize the CH4 oversolubility or solubility enhancement in the nano-confined H2O at 298 K and 1 atm CH4 gas pressure for all the partially hydrated MSN and MCM41 samples that we have examined, including the three samples with the CuEtp tricopper complex immobilized within the nano-channels. The oversolubility is given by fA and fB for the two models (Model A and Model B) used to estimate the amounts of physisorbed CH4 noted earlier (see Table S5 for details of the analysis). We also give the apparent oversolubility fQ, as the oversolubility is frequently estimated in the literature by the 9

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Microporous and Mesoporous Materials xxx (xxxx) xxx

1 mol of CH4 gas in the nano-confined H2O to give a 1 molal CH4 solu­ tion in the MSN-ex and C16-MCM41 samples under standard conditions of temperature and pressure (298 K and 1 atm) are given in Supple­ mentary data. Since the solubility of CH4 in the nano-confined water is lower than 1.0 molal (0.6–0.7 molal in the case of MSN-ex and 0.3–0.5 for C16-MCM41), the ΔG0s are positive, of the order of several kJ/mole for both samples. Nevertheless, these are remarkably high concentra­ tions of CH4 in H2O under ambient solutions. The standard free energy changes associated with physisorption of 1 mol of CH4 to the surface of the pores in the hydrated MSN-ex and C16MCM41 samples under standard temperature and pressure are also given in Supplementary data. Here the ΔG0s are negative. It takes CH4 pressures lower than 1 atm to achieve total coverage of the binding sites. In the previous work [19,20] we have developed a heterogeneous catalyst capable of efficient selective conversion of CH4 into CH3OH with multiple turnovers under ambient conditions. This heterogeneous catalyst is assembled by immobilization of the tricopper complex CuEtp into the nano-channels of the mesoporous silica nanoparticles. The catalytic TON of CH3OH catalyzed by the heterogeneous catalyst (like the CuEtp@MSN-TP sample) is ~25 fold higher than that catalyzed by the homogeneous CuEtp catalyst without immobilization of the tri­ copper cluster in the mesopores of the silica sample [17,18]. The dra­ matic enhancement of the catalytic performance was attributed to the oversolubility of the CH4 gas in the liquid confined within the meso­ porous solids compared to the low solubility of CH4 in the bulk solvent. In bulk water or a typical organic solvent, the CH4 solubility is too low to sustain productive catalytic turnover by the tricopper catalyst and the turnover is limited by abortive cycling. It suffices to say, the nano-confined water should also give rise to a large oversolubility of O2, which would help to stabilize and retain the dioxygen intermediate for the CH4 oxidation.

Table 4 Conversion of CH4 to CH3OH catalyzed by the CuEtp tricopper complex immobilized in mesoporous silica materials. Sample

CuEtp@MSN-ex CuEtp@MSN-TP CuEtp@C16MCM41

TON a

CH4 in the mesopores of the hydrated sample

b

0.65 0.59 0.39

31.4 27.8 4.6

Excess CH4 gas at 1 atm

a Oxidation of the CH4 in the mesopores of the sample driven by 2 equiv. of H2O2. b Excess CH4 gas (1 atm) is added to the sample and the CH4 oxidation is initiated by the addition of 50 equiv. of H2O2.

of the tricopper catalysts, as expected. In the case of CuEtp@MSN-ex and CuEtp@MSN-TP, the observed TONs and catalytic efficiencies are similar to the results reported earlier for CuEtp@AlMSN30-ex and CuEtp@MSN-TP under similar conditions [19,20]. Interestingly, the TON is very low for the CuEtp@C16-MCM41 sample. Thus, the choice of nanomaterial to support the CuEtp catalyst is paramount for optimal CH4 oxidation. 4. Discussion In this study, we have determined the oversolubility of CH4 gas in the nano-confined H2O in MSN-ex, MSN-TP, C16-MCM41, C14-MCM41, C12MCM41 and C10-MCM41, as well as in three samples with the CuEtp tricopper cluster complex immobilized within the pores of the nano­ porous silica materials (CuEtp@MSN-ex, CuEtp@MSN-TP, and CuEtp@C16-MCM41), relative to the CH4 solubility in bulk H2O. The solubility enhancement is about 440–500 for the MSN-ex and MSM-TP samples, and 200–300 or lower for the MCM41 samples. If the total amount of CH4 absorbed by the sample is used to estimate this oversolubility, the apparent enhancement factor is ~690 fold and ~660 fold for the hydrated MSN-ex and C16-MCM41 samples, respectively at 298 K and CH4 gas pressure of ~1 atm. Given the large amount of CH4 physically adsorbed to the walls of the pores at the gas/liquid solid interface in the case of the C16-MCM41 sample, the CH4 oversolubility would be grossly over-estimated by fQ. In any case, our findings are in general overall agreement with the results reported earlier by Ho et al. [14] (25 fold for ZSM-5 and ~450 fold for MCM-41). These workers based their estimates using the total amount of absorbed CH4, Q, including the CH4 physically adsorbed to the walls of the pores. Thus, it seems the CH4 oversolubility in H2O for MCM-41 might be over-estimated. Regardless, the outcome of this study provide strong support for the ideas advocated by molecular dynamics simulations of Ho and coworkers for CH4 and other non-polar gases such as H2 and CO2 [7,10,14,15]. In the molecular dynamics simulation study, it is shown that the layered liquid water structure in the nanopores is the main reason for lowering of the water density and enhancing the gas solubility. If so, there should be a pore-size dependence of this phenomenon. For microporous materials such as zeolites (for example, ZSM-5 in Ho’s work [14]), the pores are probably too small (<1 nm) to support a density layering. Thus, ZSM-5 is expected to show a lower solubility enhancement (f) than mesoporous MCM-41. On the other hand, for macroporous pores in the range >1000 nm, the solubility behavior in the confined liquid should approach that of the bulk solvent. This reasoning would predict a pore-size dependence for the CH4 over­ solubility and the solubility enhancement factor f should follow the trend: macroporous < MSN > MCM-41 > ZSM-5. In the present study, we find that f decreases as we go from MSN to C16-MCM41 to C10-MCM41 samples (Table 3). Estimates of the standard free energy changes (ΔG0) for solubilizing

5. Conclusions In this study, the CH4 oversolubility in nano-confined H2O in nano­ porous silica materials is measured. The nanomaterials studied include MSN and MCM-41 of varying pore sizes and volumes. The observed CH4 solubility enhancements in MSN are ~500 fold relative to bulk H2O, and ~300 fold for MCM-41. Immobilization of the tricopper catalyst in MSN enhances the CH4 oversolubility by 30%; and the CH4 oxidation activity is significantly higher for MSN than for MCM-41. It is clear from these results that a catalyst embedded in a liquid confined within the nanochannels of a nanoporous material can be deployed to sustain efficient catalysis of many reactions involving nonpolar gases such as CH4, O2, CO2, H2, etc., as we have already demonstrated in our earlier studies on efficient CH4 oxidation by the tricopper cluster complex supported on MSN. However, the selection of nanomaterial to host the catalyst is important. Based on the performance of the various nanoporous mate­ rials studied, MSN is clearly the support system of choice to encapsulate the tricopper catalyst for the conversion of CH4 to CH3OH. Author contributions CCL designed the study, prepared the materials, performed the ex­ periments, and analyzed the data; HJC and CYL carried out the CH4 absorption measurements; DJ synthesized the 7-N-Etppz ligand; PWC provided the facilities for the measurements of CH4 absorption; CYM provided the facilities for the preparation of silica samples and mea­ surement of the N2 adsorption; SIC interpreted the results and wrote the manuscript with CCL; and SSFY provided the funding for the research from his research grants. Notes The authors declare no competing financial interest.

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Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Academia Sinica, the Research Program on Nanoscience and Nanotechnology (2393-105-0200) and grants (to SSFY) from the “Taiwan’s Deep Decarbonization Pathways towards a Sustainable Society” (AS-KPQ-106-DDPP). CCL acknowledges a post­ doctoral fellowship from Academia Sinica, Taiwan. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109793. References [1] X.C. Xu, C.S. Song, J.M. Andresen, B.G. Miller, A.W. Scaroni, Energy Fuels 16 (2002) 1463–1469. [2] A. Luzar, D. Bratko, J. Phys. Chem. B 109 (2005) 22545–22552. [3] D. Bratko, A. Luzar, Langmuir 24 (2008) 1247–1253. [4] S. Miachon, V.V. Syakaev, A. Rakhmatullin, M. Pera-Titus, S. Caldarelli, J. A. Dalmon, ChemPhysChem 9 (2008) 78–82. [5] M. Pera-Titus, S. Miachon, J.A. Dalmon, AIChE J. 55 (2009) 434–441. [6] V. Rakotovao, R. Ammar, S. Miachon, M. Pera-Titus, Chem. Phys. Lett. 485 (2010) 299–303.

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