Single-Step Room-Temperature in Situ Syntheses of Sulfonic Acid

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Single-Step Room-Temperature in Situ Syntheses of Sulphonic Acid Functionalized SBA-16 with Ordered Large Pores: Potential Applications in Dye Adsorption and Heterogeneous Catalysis Haribandhu Chaudhuri, Subhajit Dash, and Ashis Sarkar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04162 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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Industrial & Engineering Chemistry Research

Single-Step Room-Temperature in Situ Syntheses of Sulphonic Acid Functionalized SBA-16 with Ordered Large Pores: Potential Applications in Dye Adsorption and Heterogeneous Catalysis

Haribandhu Chaudhuri*, Subhajit Dash, and Ashis Sarkar* Organic Materials Research Laboratory, Department of Applied Chemistry, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand-826004, India. *Corresponding author: Haribandhu Chaudhuri, E-mail: [email protected], Tel. +91 7549009253; Ashis Sarkar, E-mail: [email protected], Tel. +91 9430335255, Fax: +91 326-2307772. Abstract: A single-step in situ synthetic method of sulphonic acid functionalized SBA-16 material with ordered large pores has been fabricated involving ammonium fluoride (NH4F), tetraethylorthosilicate (TEOS) in heptane and 3-mercaptopropyltrimethoxysilane (MPTMS) in presence of hydrogen peroxide (H2O2) and Pluronic F127 (block copolymer) under mild acidic condition at room temperature. Due to the presence of numerous sulphonic acid groups on the surface, these materials show proficient adsorption capacities for dyes (qm: 365.46 mg g-1 for fuchsine basic, FB; qm: 217.87 mg g-1 for rhodamine 6G, R6G). Pseudo-second-order and intraparticle diffusion kinetics model as well as Langmuir isotherm model provided the best correlation with the obtained kinetic and isotherm data respectively. Beside this, these materials prove to be efficient heterogeneous catalyst and shows great catalytic activity towards Beckmann rearrangement of some oximes and esterification reaction. Moreover, these can be well regenerated in dilute acid solution and reused several times without causing any serious decrease of their adsorption capacity and catalytic activity. Finally, this work

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opens up a new vista of research in the direction of both water treatment applications and in one-step in situ generation of solid acid catalyst at room temperature. Keywords: Sulphonic acid functionalized SBA-16; single step in situ syntheses; ordered large pores; water treatment; catalysis. 1. Introduction: Functionalised mesoporous silica materials are very much important for adsorption, catalysis, sensing, and biological medicine [1-14]. Such materials have not only huge surface area but also ordered nano sized pores over the surface of the solid. Several workers have focused on the synthesis of active organic functional group containing mesoporous silicas with ordered pore size and high surface areas via direct synthesis or post-grafting processes [15-27]. Previous workers reported sulphonic acid functionalized mesoporous silicas using an organosilane, followed by thiol oxidation, acidification, washing, and drying [28]. But, pore sizes, pore volumes along with surface areas were decreased continuously [28, 29]. Moreover, it was observed that thiol groups were incompletely oxidised and formation of disulphide took place [30, 31]. On the otherhand, other workers reported more promising sulphonic acid functionalized mesoporous solids with uniform pore sizes, pore volumes, high surface areas along with high thermal stabilities after the removal of the surfactant with various solvents [26, 27]. But, considerable increment in pore sizes and pore volumes during single-step in situ syntheses were not observed. Recently, sulphonated mesoporous SBA-15 silica was prepared using three different methods grafting, in situ oxidation, and carbon infiltration [32]. The effects of catalyst matrix porosity composition on the catalytic efficiency have been examined using sulphated SBA-15. The study shows that for shorter carbon chain, the mass transport of the reagents through the porous structure is more important, while for long carbon chains, the effective wetting of the porous catalyst by the


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reactants is most important for the catalytic efficiency. In this context, different alkanes along with ammonium fluoride (NH4F) and heptane yield various morphologies of mesoporous silicas [33]. In the literature, mesoporous silicas were synthesised with changeable lengths, tunable large pores, and variable widths using heptane in presence of NH4F [34-36]. Furthermore, hollow silica SBA-16 (Santa Barbara Amorphous-16) spheres were synthesised by an emulsion templating method [37]. In this context, SBA-16 materials have an arrangement of a 3D mesopore and pore sizes of approximately 7–12 nm with cylindrical pores. So, to overcome those problems during the syntheses processes, it is important to increase the pore sizes along with high acid densities of the functionalized mesostructured silica materials without changing surface morphology for the sake of different suitable applications like adsorption [38-41] and catalysis [42-51]. Herein, for the first time we report a one-step in situ syntheses method of sulphonic acid functionalized SBA-16 material with enhanced ordered pores (Scheme 1). Heptane in NH4F acts as a pore expanding agent, resulting in ~11-17 nm sized ordered pores. Increasing heptane to Pluronic F127 molar ratio changes the size of the pores and the surface areas of these synthesised materials lies in the range of 521-765 m2/g with pore volumes of 1.11-1.97 cm3/g without changing the sheet like surface morphology. The acid concentration of these materials ranging from 2.26 mmol/g to 4.19 mmol/g with remarkable thermal stabilities. The synthesised materials were characterised by Fourier transform infrared (FTIR) spectroscopy, X-ray diffractograms (XRD), N2 sorption measurements, thermogravimetric analysis (TG/DTA), elemental analysis, Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), solid state

13

C cross-

polarization (CP) magic angle spinning (MAS) nuclear magnetic spectroscopy (NMR), and solid state

29

Si NMR spectra. The active functional groups of the obtained materials show

proficient adsorption of cationic dyes. Moreover, Beckmann rearrangement of some oximes



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and esterification reaction were carried out to check the catalytic activity and reusability of the materials. 2. Experimental Section: Materials. TEOS (≥99 %), MPTMS (≥99 %), Pluronic F127 (≥98 %), benzophenone oxime (≥99 %), salicylaldoxime (≥99 %), and benzoic acid (≥99 %) were purchased from Sigma Aldrich. H2O2 (~30 %), NH4F (≥99 %), hydrochloric acid (HCl, ~37%), and heptane were procured from Merck India. FB and R6G (~ 98%) were supplied by Loba Chemie and used as received. The structures of these dyes are presented in the supporting information, SI (Figure S1). Distilled water was used for each experiment. Syntheses. In a typical procedure in which TEOS to MPTMS ratio was examined, 2 g of Pluronic F127 and 0.027 mg of NH4F were homogenised with stirring for 90 min in 1.8 (M) HCl at room temperature. Thereafter, TEOS along with 11 ml of heptane were added dropwise to the medium at room temperature and stirred for 45 min. After that, both MPTMS and H2O2 were added dropwise and the resulting mixture was stirred for 18 h at room temperature, after which the solution was aged at 90°C for 48 h. Then, the mixture was filtered and the as-synthesised materials were made template free by soxhletting with different solvents (distilled water; mixture of ethanol and distilled water; mixture of ethanol, distilled water, and dil. HCl) and dried under vacuum at 100°C. The final molar composition of each mixed solutions for 2 g of Pluronic F127 was XTEOS: (0.052-X) MPTMS: Y H2O2: 0.29 HCl: 6.23 H2O, where X =0.052 (0 %), 0.0508 (2 %), 0.0495 (5 %), 0.0467 (10 %), 0.0444 (15 %), 0.0412 (20 %) (where the numbers are indicating the percentages of silicon atoms in the initial mixed solution as MPTMS) and Y =0.0109 (2 % MPTMS), 0.0254 (5 % MPTMS), 0.0516 (10 % MPTMS), 0.0739 (15 % MPTMS), 0.0981 (20 % MPTMS). Yields for the respective ratios were found to be ~98%.


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In the other procedure in which heptane to F127 (H/F) ratio was investigated, 2 g of Pluronic F127 and 0.027 mg of NH4F were dissolved with stirring in 62 ml 1.8 (M) HCl at room temperature until the block copolymer dissolved completely. Thereafter, heptane was mixed with 4.55 ml of TEOS and the resulting mixture added dropwise to the medium at room temperature and stirred for 45 min. After that, both 1.75 ml of MPTMS and 2.15 ml of H2O2 were added dropwise and the mixture was stirred for 18 h at room temperature. The solution was aged at 90°C for 48 h. Thereafter, the mixture was filtered and the prepared materials were washed several times with washing solvents like mixture of ethanol, distilled water, and dil. HCl, mixture of ethanol and distilled water and only distilled water. Finally, the solids were dried under vacuum at 100°C. The final molar ratio for TEOS: MPTMS: H2O2: NH4F: HCl: F127: H2O: heptane was 1: 0.25: 2.36: 0.03: 3.32: 7.53: 201.42: X; X = 0, 4.5, 9.5, 14.5, 19.5, and 24.5 (assuming TEOS molar concentration 0.0416 (20 %)). Yields for the corresponding ratios were found to be ~98%. Adsorption measurements. For adsorption studies, pH of the medium was regulated using dilute HCl or NaOH solution to maintain a persistent reading. A stock solution of 500 mg L-1 dye was prepared in distilled water (for each dye). In all typical procedures, required amount of adsorbent was rigorously stirred with 25 mL of dye solution, whose concentration was already known. The dye adsorption study was performed using an orbital shaker (Rivotek, Kolkata, India) and the dye solution was isolated from the sorbent by centrifugation (Make: REMI; Model - R-24) at 3500 rpm for 15 min., filtered and the absorbance was determined using a UV-Vis spectrophotometer (Shimadzu, Japan; Model: UV 1800) at wavelength of 539 nm (FB), and 521 nm (R6G) respectively. The detailed procedure including the medium pH, medium temperature, contact time, dye concentration of the medium, sample dose, and stirring speed were carried out to determine the sorption conditions. Adsorption kinetics and isotherm studies were performed with different dye concentrations at different temperature



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and time, respectively. The % dye adsorption was evaluated using eq 1 [52-54]. % =

− × 100 (1)

The equilibrium efficacy was determined using eq 2 [52-54]. = ( − ) ×

(2)

Where qe represents equilibrium efficacy of dye (mg g-1) on sorbent, C0 and Ce are the initial and equilibrium concentration of sorbate solutions (mg L-1), respectively. V denotes the volume of the dye solution (L) used and W is the weight of the sorbent (g) used. Desorption measurements. The desorption studies were performed following the previous procedures [54]. First of all, 100 mg L-1 of dye solutions combined with sorbents (15 mg and 25 mg of SBA-16-SO3H for FB and MG respectively) and stirred for a specific time (40 min, and 50 min for FB, and MG respectively). Then, the solids were centrifuged at 3500 rpm for 15 min., ﬁltered and dried to isolate these sorbents. Afterwards, these sorbents were used for desorption experiments. Three different solutions having pH: 2, 7 and 10 were used to calculate the maximum % dye desorption from these sorbents. The % dye desorption was evaluated using eq. 3 [52-54]. % =

( /") × 100 (3)

( /")

Catalytic experiments. Beckmann rearrangement of some oximes and esterification reaction were carried out in a round-bottom (RB) flask fitted with a reflux condenser. For a typical experiment, the catalyst was added to a reaction mixture of oxime (benzophenone oxime /salicylaldoxime) in dry diethyl ether (5 ml) taken in a RB flask under room temperature and stirred for certain time. After that, the solvent was distilled off and 20 ml of distilled water added to RB. Then, the mixture was refluxed for 1 h. Finally, the supernatant liquid was decanted from the mixture and recrystallised, in the same RB from boiling ethanol (5 ml).


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The final molar ratios of benzophenone oxime: catalysts were 1: 0.102 (for 0.102 mole catalyst, Table S8, entry 1, SI) and 1: 0.146 (for 0.146 mole catalyst, Table S8, entry 2, SI). On the otherhand, the final molar ratios of salicylaldoxime: catalysts were 1: 0.048 (for 0.048 mole catalyst, Table S8, entry 3, SI), 1: 0.102 (for 0.102 mole catalyst, Table S8, entry 4, SI), and 1: 0.146 (for 0.146 mole catalyst, Table S8, entry 5, SI). For esterification reaction, a mixture of benzoic acid (1 mmol), catalyst, and dry ethanol (3 ml) as a solvent was put in a RB flask. Then, the mixture was refluxed and stirred for desired time. The final molar ratios of benzoic acid: catalysts were 1: 0.102 (for 0.102 mole catalyst, Table S8, entry 6, SI) and 1: 0.146 (for 0.146 mole catalyst, Table S8, entry 7, SI). All products were analysed by gas chromatograph (GC), gas chromatography-mass spectrometer (GC-MS) and 1H NMR spectrometer. After reaction, the synthesised catalysts were collected in dilute HCl and washed with distilled water. Then, the recovered solids were reused for next catalytic reactions. Characterizations. FTIR analyses were recorded using FTIR spectrometer (Model IR-Perkin Elmer, Spectrum 2000) using KBr pellet method. Powder XRD patterns were obtained using Thermal ARL X-ray diffractometer (Cu Kα radiation source and a graphite monochromator). N2 sorption measurements were carried out at 77 K with Nova 3200e (Quantachrome, USA). Pore size distribution was calculated from the adsorption isotherm using the KJS method [27] and the BET surface area from the relative pressure of 0.06–0.15. The total pore volume was estimated at P/P0=0.985. TG/DTA (NETZSCH STA-449f3, Jupiter) was recorded on a thermal analyser in the temperature range 30°C-800°C at a rate 5°C/min in nitrogen flow. Elemental analyses (CHNS) of all the samples were obtained using Elementar Vario EL III instrument having the digestion temperatures lying in the range 950°C-1200°C. The acidexchange capacity was determined by titration with NaOH solution. In a typical procedure, 0.1 g of solid was suspended in 20 g of 2 M NaCl solution. The resulting suspension was



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stirred at room temperature for 24 h until equilibrium was reached and potentiometrically titrated by dropwise addition of 0.1 M NaOH solution. For sorption experiments, the zeta potential was measured using a Zetasizer Nano-ZS90 (Malvern, UK). Raman spectra of the powdered samples were recorded using BRUKER RFS stand alone FT-RAMAN spectrometer. The laser source is Nd: YAG 1064 nm and spectra were recorded in the range 50-4000 cm-1. The morphologies of all the samples were characterised by using FESEM (Supra 55, Zeiss, Germany) and TEM (JEM-2100, JEOL, Japan). Solid state

13

C CP MAS

NMR spectra were obtained using JEOL ECA400 MHz instrument operated with 4 mm CD/MAS probe at room temperature. Solid state

29

Si NMR spectra were recorded using

JEOL 400 MHz high resolution multinuclear FT-NMR spectrometer. For catalytic reactions, the reaction products were collected and analysed using GC (GC 1000, with an SE-30 capillary column), GC-MS (Shimadzu-5050 spectrometer having a 30 m HP-30 capillary column) and 1H NMR spectrometer operating at 300.56 MHz with a MAS at 5 kHz. 3. Results and Discussion: Throughout different synthetic procedures, various amounts of TEOS were substituted by MPTMS with the increment in the molar ratios MPTMS/(TEOS+MPTMS) from 0.024 to 0.208. We have investigated the effect of MPTMS with the increment of its molar concentration in the initial mixture, on the final morphologies of the synthesised materials. Beside this, we have varied the H/F molar ratios to study the surface properties along with the change in total pore volume observed on the functionalized materials. 3.1. Characterizations of Synthesised Materials. From FTIR spectra (Figure 1), the presence of propyl sulphonic acid groups on the inner surface of the functionalized materials has been observed. With the increment in the molar ratios of MPTMS/(TEOS+MPTMS), the peak intensities of propyl sulphonic acid groups increase (Figure 1a) . For the variation in H/F molar ratios (0 to 24.5), the peak intensities regarding propyl sulphonic acid groups 8 ACS Paragon Plus Environment

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slightly increase. So, peak intensity appears much prominent for the molar ratios of MPTMS/(TEOS+MPTMS) compared to H/F molar ratios (Figure 1b) and pore morphologies retain nearly the same structures for H/F molar ratios. It has been noticed from Figure 1a that peak intensities increase near the range 2800-2900 cm-1 (corresponding to the –C-H stretching vibrations) with the increment in the molar ratios of MPTMS/(TEOS+MPTMS) [27]. Furthermore, the intensities of peaks between 1435 cm-1 to 1446 cm-1 increases prominently due to the presence of –S=O of propyl sulphonic acid group for MPTMS/(TEOS+MPTMS) molar ratios compared to for H/F molar ratios [36]. Moreover, bands between 1348 cm-1 to 1352 cm-1 are observed which indicate the presence of –SO3¯ group of the synthesised material [43]. The typical asymmetric Si-O-Si stretching band between 1076 cm-1 to 1082 cm-1, Si-O bending in Si-OH between 970 cm-1 to 980 cm-1 [54], symmetric stretching bands of Si-O-Si near 800 cm-1 associated with the siloxane moiety are present in all the cases. –OH bending vibration of Si-OH of adsorbed water molecules between 1635 cm-1 to 1645 cm-1 are present in all those synthesised solids [54]. The XRD patterns for the variations in MPTMS/(TEOS+MPTMS) molar ratios, H/F molar ratios, and washing solvents are presented in Figure 2. Figure 2a shows the patterns of solid prepared with various molar ratios of MPTMS/(TEOS+MPTMS), after soxhletting with the mixture of ethanol, HCl, and H2O. The peak intensities of (100), (110) and (200) have decreased gradually with the increment in molar ratios of MPTMS/(TEOS+MPTMS). So, amorphous nature of the material increases with the increment in higher loading of MPTMS. But, MPTMS loading may induce some innate chaos in the pores of those materials but not deformation of the structures. Figure 2b reveals XRD patterns for some solids with various H/F molar ratios. Here, F127 as well as NH4F were removed with H2O2. Functionalized materials show (100), (110), and (200) peaks confirming the ordered structures. With the increment in H/F molar ratios, amorphous natures of those materials increase along with the



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pore size. But, for all those cases, ordered characteristics remain almost the same. Figure 2c reveals XRD patterns for the variations in washing solvents [27]. Peak intensity appears much prominent for the mixture of ethanol: HCl: H2O in comparison to the mixture of ethanol: distilled water and only distilled water. It is clear that the mixture of ethanol: HCl: H2O can effectively remove surfactant from the so formed materials. So, all those spectra indicate that, in presence of TEOS, low organosilane molar concentrations are necessary to form ordered functionalized mesoporous silicas. This is attributed to the fact that in organosilane, surfactant moieties act as cosolvents which stop phase separation. After the extraction of structure directing agent, the weight changes were studied using TGDTA analyses for the sulphonic acid functionalized materials (Figure 3). For MPTMS/(TEOS+MPTMS)=0.024 (Figure 3a), a peak centered at 100°C is observed which is due to the decomposition of water molecules. Moreover, second weight loss at 252°C owing to the loss of surfactant F127 is also seen [27]. The peak at 434°C clearly indicates the decomposition

of

alkyl

sulphonic

acid

groups

[27].

On

the

otherhand,

for

MPTMS/(TEOS+MPTMS)=0.208 (Figure 3b), a peak at 100°C indicates that the removal of water molecules from sulphonic acid functionalized material. The peak at 251°C reveals the removal of the surfactant from the material. Beside this, the weight loss at 429°C implies that deformation of alkyl sulphonic acid groups of the solid. CHNS of the sulphonic acid functionalized materials were carried out and shown in Table 1. Carbon, hydrogen and sulphur contents in the synthesised materials were calculated from weight percentage method. Sulphur content increases from 9.27 % to 10.33 % as the molar composition of MPTMS/(TEOS+MPTMS) increase from 0.024 to 0.208 which is ascribed to the higher loading of MPTMS. The concentrations of sulphonic acid groups in synthesised solids were found to be much higher compared to the other sulphonic acid functionalized mesoporous silica materials, as reported earlier [26-27, 46-50]. Furthermore, Table 1


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summarises the number of active sites on the material surface. The highest acidity observed was 4.19 mmol/g for MPTMS/(TEOS+MPTMS)=0.208, indicating successful in situ oxidation of –SH groups in the framework. Nitrogen sorption isotherms were employed to investigate the changes that occurred in the morphologies of those materials after incorporation of MPTMS. Figure 4 shows nitrogen sorption isotherms and pore size distributions of those synthesised materials where surfactant was removed using H2O2. BET surface area, mean pore diameter, and total pore volume of all those solids are shown in Table 2. All those solids show a typical nitrogen IV type isotherm [27]. Figure 4a clearly shows that mercaptopropyl group loading may induce some inherent deformation in the pores of those materials without disturbing the overall morphology. Higher loading of mercaptopropyl groups introduce reduced surface areas in those solids and pore size (Figure 4b) remain nearly the same. For the variation in H/F molar ratio (Figure 4c), BET surface area decreases upto H/F=19.5, after which it increased. Some inherent deformation may be incorporated for H/F=24.5 so that surface area increased to 559 m2/g. On the otherhand, pore order increases upto 17.1 nm with the increment in H/F molar ratio (Figure 4d). At first, it increases steadily until H/F=19.5, after which it slightly increases to the maximum at H/F=24.5. It implies that the micelles are saturated at that point after which deformation occurred. In this respect, the mesopore volume reduces as the amount of mercaptopropyl group increases in mixture. Beside this, swelling effect of H/F molar ratio helps to increase mesopore volume to a large extent without affecting the surface characteristics. In Table 2, the unit cell parameters from all samples are also included. The unit cell reported is the average of the calculated unit cells from the (100), (110), and (200) diffraction peaks of XRD patterns. This is clearly indicates that the unit cell parameter is larger than the pore size for all samples.



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The characteristics of mercaptopropyl groups formed over mesostructured SBA-16 silica materials were studied using Raman spectroscopy (Figure 5). The presence of sharp peaks near 2900 cm-1 indicates the presence of associated organic functional groups arising from the alkyl chain of the organic linkers [43]. The bands near 1425 cm-1 are because of the presence of SO3¯ [43]. The wagging mode vibrational bands of -CH2-Si are observed near 1290 cm-1 [27]. The bands near 1000 cm-1 are attributed to the existence of O3Si-OH and siloxane bridges. Beside this, the bands near 560 cm-1 are related to the S-C stretching vibrational modes [45]. Moreover, with the increment in MPTMS/(TEOS+MPTMS) molar ratios, all those peak intensities increase sharply, which indirectly implies the higher loading of MPTMS onto the mesostructured SBA-16 materials. Thus, all those discussions convey the successful incorporation of propyl chain sulphonic acid groups within those support solids. FESEM (Figure 6) was carried out to examine the changes that happened in the surface characteristics for all those propyl sulphonic acid functionalized SBA-16 in accordance with various molar ratios of MPTMS/(TEOS+MPTMS) and H/F. With the increment in the molar ratios of MPTMS/(TEOS+MPTMS), initially the ordered nature of the synthesised particles were retained upto certain molar concentration (Figure 6 a-d). After that, it shows to slightly agglomerated morphology (Figure 6e) and finally converted to distorted structure (Figure 6f). A variation in H/F molar ratios was also studied. A perusal of Figure 6 a’-d’ shows that the ordered morphology of synthesised particles increase to a certain limit, after which it shows slightly agglomerated character (Figure 6e’) and finally converted to distorted structure (Figure 6f’). Figure 7 reveals TEM micrographs of sulphonic acid functionalized SBA-16 materials with respect to different molar ratios of MPTMS/(TEOS+MPTMS) and H/F. The F127 was mostly removed by H2O2 during one step synthesis procedure for all the materials. By increasing the


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molar ratios of MPTMS/(TEOS+MPTMS), the pore morphology of the material moved from ordered (Im3m), Figure 7 (a), (b), (c), (d), and (e) to slightly aligned trend (Figure 7f). With the incremental amount of heptane in the synthesis, the pore morphology of functionalized SBA-16 material shifted from a little arranged, Figure 7 (a’) and (b’), to completely ordered, Figure (c’), (d’), and (e’), to slightly aligned (f’). Thus, exact molar ratios of MPTMS/(TEOS+MPTMS) with optimum amount of H/F molar ratio are required to form sulphonic acid functionalized mesostructured silicas with ordered large pores. The incremental loading of alkyl sulphonic acid groups on SBA-16 material during single step in-situ synthetic procedure was confirmed by

13

C CP MAS NMR spectra, as shown in

Figure 8. With the increment in the molar ratios of MPTMS/(TEOS+MPTMS), the peak intensities increase in accordance with the loading of propyl sulphonic acid groups. It can be seen that the sulphonic acid functionalized SBA-16 material shows the presence of three distinct C3, C2, and C1 carbon peaks at 54.6 ppm, 18.7 ppm, and 12.3 ppm, respectively confirming the presence of alkyl sulphonic acid groups anchored to the inner pores of the material [43]. The peaks near 71.2 ppm, 65.2 ppm, 39.7 ppm, and 23.9 ppm are because of the presence of –RCH2O- and C-C in the surfactant F127 and propyl sulphonic acid group, respectively. There are no evidence of the presence of some other peaks near 22 ppm and 29 ppm, which are assigned to the carbon atom adjacent to the –SH moiety and the central carbon atom of the alkyl linker in the propyl sulphonic acid moiety, respectively, indicating that thiol groups are oxidised during synthetic procedure [55]. There are no signals due to the existence of some other sulphur compounds such as disulphide species (near 36 ppm) and thiosulphonate species (near 39 ppm) which is attributed to the incomplete oxidation of the mercaptopropyl groups during synthesis [43]. Moreover, absence of any signal near 52 ppm corresponding to the carbon atom of methoxy groups (Si-OCH3), reveals that both MPTMS and TEOS are completely hydrolysed in the synthetic procedure [56].



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Furthermore, the incorporation of alkyl sulphonic acid groups into mesostructured silicas was also monitored by means of solid state 29Si NMR (Figure 9). Different distinct peaks can be observed for organosiloxane [Tx= RSi(OSi)x -(OH)3-x; R=alkyl group or chain, x=1-3, T2 at 54 ppm and T3 at 62 ppm] and siloxane [Qy=Si(OSi)y-(OH)4-y; y=2-4, Q3 at 99 ppm and Q4 at 108 ppm] species [27]. From the above mentioned data and Figure 9, it is assumed that the peak intensities increase with an increase of MPTMS concentration in the synthesised solids. The relative integrated intensities of organosiloxane (Tx) and siloxane (Qy) solid state NMR resonances (Tx/Qy) provide the quantitative assessment of incorporated organic moiety [27]. There are two reasons for the formation of large pores in those synthesised materials compared to the other in-situ one step synthetic procedures [35]. These are effective swelling of the micelles in presence of heptane with NH4F and the absence of reduction from calcination. It is known that large pores can be formed in presence of short hydrocarbon chains [35]. At room temperature, the methyl groups of polypropylene oxide in F127 are hydrated and critical micelle concentration is reached [57]. By suppressing the hydration, alkanes make ordered structures at room temperature. Not only F ¯ ions in NH4F cause dehydration of the polyethylene oxide chains in F127 but also polypropylene oxide chains in presence of heptane increase. So, core radius of micelle increases as polyethylene oxide chain adjacent to the polypropylene oxide in F127 becomes hydrophobic [58]. As a result, with the addition of heptane and NH4F, the critical packing parameter persists in the range between 1/3 and 1/2 which is suitable for the formation of hexagonal cylindrical pores at room temperature, and it is viable to get bigger size further compared to the theoretical value. Initially, the pore size remains the same at low concentration of heptane [35]. By increasing the amount of heptane, pore size increases (16.8 nm) until H/F=19.5 without effecting the ordered porous morphology of the synthesised material. That means the micelles are not completely filled with heptane at this point. Beyond this concentration, pore size increases


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slightly (17.1 nm) along with a little wide size distribution implying all the pores are saturated. The pore size can’t be affected with further addition of heptane [35]. At this point, a little morphological deformation occurred even though the heptane entered the micellar cores which are energetically favourable. Here, the sheet like morphology always is facing heptane and heptane enters and expands those pores. Instead the excess heptane forms droplets outside the crystallites that increase in size [35]. Herein, hexagonal p6mm structure ordered pores have been formed which differs from the pore arrangement reported in other literature [37]. One possible reason might be that the packing factor plays a crucial role with the loading of mercaptopropyl group in presence of NH4F and heptane. That’s why, hexagonal ordered pores have been formed. XRD patterns along with the nitrogen sorption isotherms indicate cylindrical pore structures, rather than spherical pores. 3.2. Adsorption of Dyes. Characterizations of Adsorped Material. The FTIR spectroscopic studies were carried out for pre and post adsorped solids and presented in Figure S2 (SI). As shown in Figure S2a (SI), bands at 2935 cm-1 and 2855 cm-1 are found because of the presence of asymmetric and symmetric stretching mode of -CH2 group in the synthesised material. -OH bending mode is observed at 1643 cm-1. The band at 1442 cm-1 is found due to the presence of –S=O of propyl sulphonic acid group. Beside this, the band at 1349 cm-1 reveals the existence of -SO3¯ group in the synthesised solid. Asymmetric Si-O-Si stretching mode is found at 1087 cm-1 and Si-O bending in Si-OH is noticed at 964 cm-1 [54]. Furthermore, symmetric stretching of Si-O-Si is noticed at 803 cm-1. Some sort of peak shifting is noticed for these dye adsorped solids (Figure S2, SI). For FB adsorped solid (Figure S2b, SI), the asymmetric and symmetric stretching modes of -CH2 group are found at 2930 cm-1 and 2851 cm-1 respectively. -OH bending vibrational mode of Si-OH shifted to 1634 cm-1. The bands due to -S=O group and SO3¯ group of alkyl sulphonic acid group are moved to 1440 cm-1 and 1344 cm-1. Moreover,



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Si-O bending in Si-OH is moved to 956 cm-1 and symmetric stretching of Si-O-Si is shifted to 799 cm-1. Asymmetric Si-O-Si stretching mode is shifted to 1082 cm-1 [54]. On the otherhand, as shown in Figure S2c, SI, asymmetric and symmetric stretching bands of -CH2 group are shifted to 2931 cm-1 and 2847 cm-1 respectively and the bending vibrational mode of -OH in Si-OH shifted to 1637 cm-1. The bands corresponding to the -S=O group and -SO3¯ of propyl sulphonic acid group are shifted to 1431 cm-1 and 1345 cm-1. Moreover, Si-O bending in Si-OH is shifted to 953 cm-1 and symmetric stretching of Si-O-Si is moved to 796 cm-1 [54]. Moreover, asymmetric Si-O-Si stretching mode is shifted to 1081 cm-1. So, it may be assumed that hydrogen bonding interaction may play a key factor during the sorption process and for which the shifting of bands took place. The XRD patterns were determined for pre and post dye adsorped sulphonic acid functionalized materials and summarised in Figure S3 (SI). The sulphonic acid functionalized material with intense (100), (110), and (200) peaks imply that the synthesised material have ordered morphology. But, for dye adsorped materials (SBA-16-SO3H/FB and SBA-16SO3H/R6G), the ordered morphology have fully disappeared, which results in proficient sorption of dyes. The nitrogen adsorption-desorption isotherms were recorded for all the used solids and shown in Figure S4 (SI). It has been noticed that sulphonic acid functionalized material shows sorption isotherm of type IV [27]. Moreover, it can be seen from Table S1 (SI) that synthesised material has a high surface area (696 m2/g), mean pore diameter (2.8 nm) along with overall pore volume (0.84 cm3/g). But, after dye sorption, SBA-16-SO3H/FB and SBA16-SO3H/R6G exhibits a massive loss in surface area (70 m2/g and 58 m2/g respectively) and overall pore volume (0.11 cm3/g and 0.10 cm3/g respectively) and the capillary condensation (Figure S3, SI) found missing in these isotherms. The reason behind the fact that adsorbate molecules may move into the pores of the material for which pores become unreachable to


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nitrogen during analysis [54]. Figure S5 (SI) shows the surface morphology of pre and post dye adsorped functionalized material determined by FESEM and TEM analyses. A perusal of Figure S5 discloses that the particles show amorphous character and the pores are filled with dyes due to sorption of adsorbate molecules. It is quite well known that sorption of dye from aqueous solution using sorbent depends upon various points like solution pH, equilibrium (contact) time, solution temperature, dye concentration of the medium, sorbent dosage, and stirring speed of the mixture [52-54]. The details have been explained in SI (Adsorption Optimization, Figure S7). Sorption Kinetics. Adsorption kinetics assists in interpreting not only the rate but also the mass transfer pathway from liquid (sorbate) to solid (sorbent) phase during adsorption process. Many kinetics models have been employed to examine the mechanism of adsorption. Herein, pseudo-first-order [59], pseudo-second-order [60], second order [61], and intraparticle diffusion [62] kinetics models were incorporated to get a concrete idea about the rate as well as the mechanism of sorption. Pseudo-first-order and second order kinetics models were elaborated in SI (Figure S8, Table S2-S3). The linear form of the pseudo-second-order equation is expressed as: t 1 t = + (4) ( q& k ( q) q)

Where, rate constant of the pseudo-second-order kinetics model is expressed as k ( (g mg-1 min-1). The parameters k2, qe, R2 and χ2 have been shown in Table S2-S3 (SI) and Figure 10a, d shows t/, vs. t plot. From all the evaluated data and the plot, it may be assumed that kinetics models are linear with lower χ2 values and higher regression coefficients (R2), indicating the perfect fit with pseudo-second-order kinetics model. Beside this, the intraparticle diffusion kinetics model has been expressed in linear form as: .

q& = k - t ( + c (5) 17 ACS Paragon Plus Environment


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Where, rate constant of the intraparticle diffusion kinetics model is represented as k - (mg g-1 min-1/2), boundary layer width is expressed as c, and the parameters k - , c, R2 and χ2 have 1

been shown in Table S2-S3 (SI) and Figure 10b, e shows q& vs. t 2 plot. The plots have two parts which indicates that double steps have been involved in the process of adsorption. Boundary layer diffusion i.e. movement of mass from liquid to solid phase has been indicated by the first segment. The intraparticle diffusion i.e. a constant sorption on the surface of synthesised material has been shown by the second part [63]. Furthermore, if the plots qt vs. t1/2 passes through the origin, then intraparticle diffusion claims to be the rate limiting step [52]. But, if the plot does not pass through the origin, then the key factor is the thickness of boundary layer. So, from all above statements, it is observed that intraparticle diffusion may play a key part in the mechanism of adsorption. So, it is assumed that intraparticle diffusion as well as the surface sorption may occur jointly during dye sorption. Sorption Isotherms. In adsorption isotherm, distribution of dye molecules between the liquid and solid phase can be demonstrated by employing some renowned isotherm models like Langmuir [64], Freundlich [65], and Temkin [66]. Freundlich and Temkin isotherm models have been illustrated in SI (Figure S9, Table S4-S5). Langmuir isotherm model reveals that the monolayer adsorption of adsorbate molecules occurs at homogeneous active sites of sorbent. Langmuir isotherm models have been shown in Figure 10c, f and linear equation is: c) 1 c) = + (6) q) q3 b q3 Where, c) (mg L-1) is the concentration of dye at equilibrium, q) (mg g-1) is the amount of adsorbate adsorbed by the synthesised material at equilibrium after sorption process, the efficiency of maximum adsorption of dye molecules by the synthesised material is expressed as q3 (mg g-1) and b implies the required energy during sorption. From all the evaluated data


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and the plot, it may be assumed that Langmuir isotherm model accurately fitted with lower χ2 values and higher R2 values, compared to Freundlich and Temkin isotherm models. In this context, one parameter is separation factor (R 7 ), which is very important from the view point of sorption isotherm and can be denoted as: R7 = 8

1 : (7) 1 + bC

Where, C is the concentration of adsorbate solution and Langmuir constant is denoted as b. From R 7 value, one can get the idea regarding the Langmuir model. If Langmuir model is linear, then R 7 =1; if it is favourable, then 0

Single-Step Room-Temperature in Situ Syntheses of Sulfonic Acid

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