Mesoporous silicas: improving the adsorption efficiency of phenolic compounds by the removal of amino group from functionalized silicas

Abstract

In this study, dimethyloctylamine-functionalized mesoporous silicas (MCM-41 and MCM-48 samples), which are designed to be anionic and cationic species sorbents, have been successfully deaminated by ethanolic extraction to obtain materials that would be able to remove neutral phenolic pollutants, namely, 4-nitrophenol and 2,4,6-trichlorophenol, from aqueous effluents. Solvent extraction was further applied to amine-functionalized MCM-41 and MCM-48 mesoporous materials with different chain lengths (N,N-dimethyldodecylamine, dodecylamine and hexadecylamine). The hexagonal mesoporous structure of MCM-41 and the three-dimensional cubic mesoporous structure of MCM-48 remain intact after dimethyloctylamine amination and template removal. The N₂ adsorption–desorption isotherms of the different mesoporous materials are of type IV according to the IUPAC classification. The alcoholic extraction of the dimethyloctylamine moiety led to the formation of materials with a wide open pore structure, which are particularly suitable for the rapid adsorption of hydrophobic molecules. Isotherm data at ambient temperature were well fitted with the Langmuir model. Solvent extraction was further applied to the amine-functionalized MCM-41 and MCM-48 mesoporous materials with different chain lengths (N,N-dimethyldodecylamine, dodecylamine and hexadecylamine) and a statistical analysis of the sorption capacities vs. structural properties was performed. ANOVA Kruskal–Wallis tests showed that the adsorption of pollutants was not dependent on the type of mesoporous silicas (MCM-41 vs. MCM-48), rather it was dependent on the phenolic structure (PNP vs. TCP). A (weak linear) correlation between the pore size and sorption capacities could be established.

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1. Introduction

The application of mesoporous silicas in adsorption and catalysis studies is the object of extensive research since the last few decades.^1–5 These materials are characterized by high surface areas, uniform and controllable pore sizes and the periodic order of their pore packing. The functionalization of the pore channels provides new opportunities for fine-tuning the properties of these materials, and amines are one of the most prominent chemical functionalities in this field.⁴

The adsorptions of ionic species (both cationic and anionic)^4,6–13 have been demonstrated with excellent sensitivities and high capacities using amine-functionalized mesoporous silicas with wide hydrophilic channels. Unfortunately, the adsorption of organic (uncharged) compounds is decreased due to this functionalization. In this case, to enhance the adsorption of organic pollutants, Sayari¹⁴ suggested the removal of the amine template to create hydrophobic surfaces and open-pore structures. Calcination,^15,16 oxidation^17–20 or solvent extraction^{14,15,21–23} have been suggested for their efficiency.

The focus of the present study was template removal by alcoholic extraction or calcination from dimethyloctylamine-MCM-41 and dimethyloctylamine-MCM-48, the two ordered mesoporous silicas from the Mobil M41S family of materials.^24–26 Solvent extraction was further applied to amine-functionalized MCM-41 and MCM-48 mesoporous materials with different chain lengths (N,N-dimethyldodecylamine, dodecylamine and hexadecylamine).^10,11 Phenol and its derivatives are considered as high priority and hazardous pollutants because of their high toxicity, carcinogenicity and resistance to biodegradation and also due their widespread use as reflected by their inclusion in the list of high volume production chemicals.²⁷ Different mesoporous materials were evaluated for the removal of p-nitrophenol (PNP) and 2,4,6-trichlorophenol (TCP), which were chosen as phenolic compounds.

2. Experimental procedures

2.1. Materials and synthesis

All the chemicals used in the experiments were of analytical purity or highest purity available and used without any further treatment. MCM-41 and MCM-48 mesoporous silicas were prepared using cetyltrimethylammonium bromide, fumed silica for MCM-41 and TEOS for MCM-48 according to previously described methods.^10,11,28,29 The solutions were hydrothermally treated and the solids were dried in air. The functionalization was achieved post-synthesis. Mesoporous silicas (5 g) were added to an emulsion of N,N dimethyloctylamine (7 g) in distilled water (78 g) with magnetic stirring (30 min), and the mixture was sealed and heated in a Teflon autoclave for 72 h at 25 °C. The product was recovered by vacuum filtration, washed several times with Milli-Q water, and dried to obtain materials A (DMOA-41A and DMOA-48A). Solvent extraction was based on the work of Sayari¹⁴ and ethanol was used as the extracting solvent. The experiments were carried out in a Soxhlet apparatus and materials B were obtained (Table 1). Materials C were obtained by calcination at 550 °C for 6 hours.

Table 1 Mesoporous materials used in this study^a

Material	Mesoporous silica	Amine	Treatment
a The ending letter in the material name indicates: (A) aminated mesoporous silicas, (B) alcoholic extraction of the amine moieties, and (C) calcination of the aminated mesoporous silica (A).
MCM-41, MCM-48
DMOA-41A	MCM-41	Dimethyloctylamine	—
DMOA-41B	MCM-41	Dimethyloctylamine	Solvent extraction
DMOA-41C	MCM-41	Dimethyloctylamine	Calcination
DMOA-48A	MCM-48	Dimethyloctylamine	—
DMOA-48B	MCM-48	Dimethyloctylamine	Solvent extraction
DMOA-48C	MCM-48	Dimethyloctylamine	Calcination
DMDDA-41B	MCM-41	Dimethyldodecylamine	Solvent extraction
DDA-41B	MCM-41	Dodecylamine	Solvent extraction
HDA-41B	MCM-41	Hexadecylamine	Solvent extraction
DMDDA-48B	MCM-48	Dimethyldodecylamine	Solvent extraction
DDA-48B	MCM-48	Dodecylamine	Solvent extraction
HDA-48B	MCM-48	Hexadecylamine	Solvent extraction

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2.2. Characterization

Thermogravimetric curves were obtained on a TGA 2950 high-resolution thermogravimetric analyzer NETZSCH Iris TG 209C. FTIR spectra were recorded on a Bruker Vector 22 FTIR spectrometer in the 4000–400 cm⁻¹ region using the KBr method. Nitrogen physisorption was measured using a Micromeritics ASAP 2000 and ASAP 2010 at 77 K. Specific surface areas were calculated from the Brunauer–Emmett–Teller (BET) equation and pore size distribution was obtained using the Barrett–Joyner–Halenda method by the analysis of the desorption branch. The small-angle X-ray diffraction (XRD) patterns of the samples were collected with a Bruker AXS D-8 X-ray powder diffractometer using Cu Kα radiation (λ = 1.5418 Å). Zeta potential measurements were conducted in triplicate using a Zetaphoremeter IV (CAD instruments).

2.3. Adsorption studies

Stock solutions of nitrophenol (Sigma-Aldrich, 1000 mg L⁻¹) and 2,4,6-trichlorophenol (Sigma-Aldrich, 750 mg L⁻¹) were prepared in Milli-Q water. Adsorption kinetics and isotherms were performed at least in triplicate on an orbital shaker (200 rpm). Sorbent (100 mg) was thoroughly mixed with various concentrations of phenolic compounds (from 10 to 1000 mg L⁻¹ for 4-NP and from 10 to 750 mg L⁻¹ for 2,4,6-TCP) to obtain a final volume of 100 mL. The samples were filtered with a syringe filter (0.20 μm). Trichlorophenol (pK_a = 6.21) and nitrophenol (pK_a = 7.15) concentrations were determined using a Varian Cary UV 50 Probe spectrophotometer at 286 and 318 nm, respectively. Experimental curves were fitted using non-linear regression method and Statistica 6.0 software; their suitability was assessed on the basis of R².

3. Results and discussion

3.1. Dimethyloctylamine mesoporous silicas: characterization, ethanolic extraction vs. calcination

FTIR peaks at 2925 and 2855 cm⁻¹ were attributed to the CH₂ stretching vibrations originating from the amino template (Fig. 1A). These peaks disappeared after calcination and less than 10% of DMOA (estimated by thin layer chromatography) was observed after ethanolic treatment. Further analyses were conducted using thermogravimetric analyses (Fig. 1B). Three major mass losses vs. temperature were observed (Table 2). A first loss between 25 °C and 150 °C was assigned to adsorbed water and solvent molecules. The template loss started at nearly 150 °C and occurred in several steps, which may be related to different interactions between the organic templates. Above 600 °C, samples exhibited a weight loss due to the dehydroxylation of the silicate networks.

Fig. 1 Dimethyloctylamine mesoporous MCM-48 silica (DMOA-48A), mesoporous silica obtained after alcoholic extraction of the amine moiety in DMOA-48A (DMOA-48B) and calcined DMOA-48A (DMOA-48C). (A) FTIR spectra. Circles show the stretching vibrations originating from the amino template. (B) Thermogravimetric curves. (C) Low-angle XRD diffractograms.

Table 2 Thermogravimetric curves: mass loss vs. temperature

Sample	Weight loss (%) 25–150 °C	Weight loss (%) 150–450 °C	Total weight loss (%)
DMOA-41A	12	64	76
DMOA-41B	25	21	46
DMOA-41C	7	3	10
DMOA-48A	17	59	76
DMOA-48B	25	29	54
DMOA-48C	7	2	9

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The X-ray diffraction patterns of the mesoporous materials were compared. In the pattern of MCM-41, a dominant (100) peak reflection is attributed to the 2D-hexagonal symmetry (p6mm).²⁴ The dimethyloctylamine grafting to MCM-41 caused a considerable decrease in the XRD intensity. The peaks (211), (220), (321), (420) and (332) were associated with Ia3d cubic symmetry in the pattern of MCM-48 (Fig. 1C). The highly efficient mesostructures of the MCM-41 and MCM-48 mesoporous silicas were not affected by the treatment. XRD patterns showed several scattering peaks in the low 2-theta region corresponding to the 〈100〉 and 〈211 〉 Bragg reflections for MCM-41 and MCM-48, further confirming that the hexagonal mesoporous structure of MCM-41 and the three-dimensional cubic mesoporous structure of MCM-48 remain intact after amination and template removal. A shift of the unit cell parameter was observed after solvent-template removal, while this parameter decreased after calcination for MCM-41. In contrast, a₀ increased with calcination and decreased with solvent-extraction for MCM-48. The N₂ adsorption–desorption isotherms of the mesoporous materials are of type IV according to the IUPAC classification. In accordance with the XRD patterns presented above, isotherm curves were similar after modifications of the MCM-41 and MCM-48 mesoporous silicas. Structural and textural parameters are summarized in Table 3. The nonexpanded calcined (MCM-41) parent sample exhibited a reversible adsorption–desorption isotherm with the characteristic nitrogen condensation evaporation step at a relative pressure of 0.3 and exhibited a H1 hysteresis loop, which is a characteristic of periodic mesoporous materials.^24,25 Low pore volume, low surface areas and broad PSD were observed for aminated materials (DMOA-41A and 48A) containing both CTMA and DMOA surfactants. The alcoholic extraction of the amine moiety (DMOA-41B and 48B), which contained about 50 wt% of cetyltrimethylammonium (CTMA) surfactant, leads to the formation of materials with a wide-open pore structure, which are particularly suitable for the rapid adsorption of hydrophobic molecules. As expected, the calcined material (DMOA-41C and 48C) exhibited highly enlarged pores with relatively narrow size distributions. It is well-known that functionalization decreases the surface area and pore volume because the terminal organic groups are tethered inside the pores of the mesoporous silicas. The removal of the template by both ethanolic extraction (materials B) and calcination (materials C) sharply increased the surface area and pore volume.

Table 3 Textural and structural parameters

Material	Surface area (m^2 g^−1)	Pore size Dp (nm)	Pore volume (cm^3 g^−1)	2θ	d(100) (nm)	d(211) (nm)	a0 (nm)
a From ref. 10. The materials used in the adsorption studies are underlined.
DMOA-41A	219	1.39	0.31	2.13	4.14	—	4.78
DMOA-41B	538	4.96	1.14	2.04	4.32	—	4.98
DMOA-41C	1284	9.66	1.59	2.19	4.05	—	4.67
DMOA-48A	176	1.28	0.23	2.42	—	3.65	8.94
DMOA-48B	458	4.36	0.91	2.50	—	3.53	8.64
DMOA-48C	1027	9.17	1.26	2.36	—	3.74	9.17
DDA-41B^a	439	4.85	0.92
DMDDA-41B^a	373	5.84	0.97
HDA-41B	284	5.31	0.98
DDA-48B^a	433	4.46	0.85
DMDDA-48B^a	381	4.71	0.82
HDA-48B	316	5.64	0.93

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Furthermore, the average pore volume observed for MCM-48 materials are slightly lower than that of MCM-41 materials. Ethanolic extraction and/or calcination modified the surface charge properties of the materials. Isoelectric points (IEP) shifted to higher values after amination, which provided an indirect evidence of the existence of cationic groups in the structure. IEP point decreased with ethanolic extraction and calcination (Table 4). It has been suggested that the removal of the template by solvent-extraction better preserves the crystallinity and pore structure of the material, whereas calcination increases the condensation degree of the silica and leads to a shrinkage of the MCM framework.³⁰

Table 4 Isoelectric points^a

	DMOA-41A	DMOA-41B	DMOA-41C	DDA-41B	DMDDA-41B	HDA-41B
a The mesoporous silicas are positively charged when pH < pHiep and negatively charged when pH > pHiep. The materials used in the adsorption studies are underlined.
pHiep	8.5	5.3	3.7	3.9	3.8	5.1

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	DMOA-48A	DMOA-48B	DMOA-48C	DDA-48B	DMDDA-48B	HDA-48B
pHiep	9.1	4.3	3.5	4.5	3.9	4.9

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3.2. Adsorption of p-nitrophenol and 2,4,6-trichlorophenol

The ethanolic extraction of the DMOA amine moiety significantly improved the sorption capacities (Table 5) toward phenolic compounds, while calcination led to weak sorption properties. More than 70% of the phenols were removed by materials B (conditions are given in Table 5), while less than 30% of heavy metals were removed by materials B in the same conditions (these results could be compared with those of previous studies^10,11 on (aminated) mesoporous silicas). Materials C obtained by calcination were consequently not studied further for the adsorption of phenolic pollutants. Inumaru^31–33 suggested that both hydrophilic and hydrophobic groups of adsorbates interact with hydroxyl groups and organic moieties on the pore surface of organic–inorganic silica, respectively. Phenolic compounds have both hydrophobic phenyl groups and hydrophilic hydroxyl groups. Materials B after ethanolic extraction had hydrophobic methyl and hydrophilic hydroxyl groups, and phenolic compounds were expected to have two types of adsorbent–adsorbate interactions. One interaction is hydrogen bonding between the hydrophilic hydroxyl groups of MCM-41 and MCM-48B and adsorbates. The other is π–π interaction between the benzene rings and organic moieties of MCM-41B and MCM-48B. The initial amount of adsorbed 4-NP or 2,4,6-TCP increased gradually up to 50 min and slowly reached the maximum concentration within 60 min. Pseudo-second order model (eqn (1)) fitted well with the kinetic data obtained for material B (Table 6), while a first-order model failed to reproduce the data.

Table 5 Sorption capacities^a

qe (mg g^−1)	MCM-41	DMOA-41A	DMOA-41B	DMOA-41C
a [PNP] = [TCP] = 100 mg L^−1, m = 100 mg, pH = 5.5 (TCP) and 6.0 (PNP), ambient temperature, contact time = 24 hours, shaking rate = 200 rpm.
PNP	48	41	83	31
TCP	33	25	70	19

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qe (mg g^−1)	MCM-48	DMOA-48A	DMOA-48B	DMOA-48C
PNP	44	39	81	28
TCP	29	27	69	16

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Table 6 Kinetic parameters

Material	Pollutant	Second-order			Weber–Morris
		k2 (g min^−1 mg^−1)	qe (mg g^−1)	R^2	kid (mg g^−1 min^−1/2)	L (mg g^−1)	R^2
DMOA-41B	4-NP	3.7 × 10^−3	88	0.995	4.0	40	0.963
DMOA-48B		1.5 × 10^−3	82	0.998	3.0	28	0.948
DMOA-41B	2,4,6-TCP	4 × 10^−3	79	0.988	4.7	47	0.943
DMOA-48B		1.2 × 10^−3	74	0.991	3.9	35	0.951

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The possibility of intraparticle diffusion (Table 6) was explored by the Weber–Morris intraparticle diffusion model (eqn (2))

q_t = k_dt^1/2 + L (2)

where k_d (mg g⁻¹ min^−0.5) is the intraparticle diffusion rate constant and L is the intercept representative of the boundary layer thickness. The first sharp portion of the Weber–Morris plots (not shown) indicates that the sorption of NP and TCP is controlled by external mass transfer, and the plots do not pass through the origin or have intercept (Table 6) characteristics of the boundary layer thickness.

The equilibrium adsorption isotherm is the basic requirement in the design of adsorption systems. Isotherm data at ambient temperature (Fig. 2) for DMOA-41B and DMOA-48B were well fitted with the Langmuir model (eqn (3)) based on assumptions that the monolayer coverage of adsorbate occurs over homogeneous sites and that a saturation point is reached where no further adsorption can act.

where q_e is the adsorption capacity of the adsorbent (mg g⁻¹), C_e is the equilibrium concentration of adsorbate in solution (mg L⁻¹), q_max is the maximum monolayer adsorption capacity (mg g⁻¹), and k_L is the adsorption equilibrium constant related to the affinity of the binding sites and energy of adsorption (L mg⁻¹). Table 7 summarizes the results.

Fig. 2 Adsorption isotherms. The solid lines represent the fits with the Langmuir isotherms. [NP] = 10–1000 mg L⁻¹, [TCP] = 10–750 mg L⁻¹, m_sorbent = 100 mg, pH = 5.5 (TCP) and 6.0 (NP), ambient temperature, contact time = 24 hours, shaking rate = 200 rpm. Experimental points were obtained respectively from four replicates for 4-nitrophenol and three replicates for 2,4,6-trichlorophenol. (A) DMOA-41B. (B) DDA-41B. (C) DMDDA-41B. (D) HDA-41B.

Table 7 Langmuir parameters

4-NP	DMOA-41B	DDA-41B	DMDDA-41B	HDA-41B
qmax (mg g^−1)	201 ± 4	222 ± 5	240 ± 4	247 ± 5
kL (L mg^−1)	(2.55 ± 0.23) × 10^−2	(2.30 ± 0.20) × 10^−2	(4.84 ± 0.42) × 10^−2	(5.78 ± 0.57) × 10^−2
R^2	0.992	0.994	0.994	0.992

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	DMOA-48B	DDA-48B	DMDDA-48B	HDA-48B
qmax (mg g^−1)	189 ± 4	210 ± 5	225 ± 5	241 ± 4
kL (L mg^−1)	(2.07 ± 0.16) × 10^−2	(1.57 ± 0.13) × 10^−2	(4.02 ± 0.38) × 10^−2	(5.22 ± 0.36) × 10^−2
R^2	0.994	0.994	0.992	0.996

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	Carbonaceous materials	Modified clay	Zeolite	Synthetic resins
qmax (mg g^−1)	179,^34 313,^35 333,^36 526,^37 268,^42 376–542 ^49	436 ^41	330,^43 55–91,^50 19 ^51	11–21,^46 16,^47 931 ^48

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2,4,6-TCP	DMOA-41B	DDA-41B	DMDDA-41B	HDA-41B
qmax (mg g^−1)	156 ± 3	174 ± 3	180 ± 3	179 ± 4
kL (L mg^−1)	(4.97 ± 0.39) × 10^−2	(5.62 ± 0.33) × 10^−2	(6.98 ± 0.40) × 10^−2	(9.30 ± 0.82) × 10^−2
R^2	0.992	0.996	0.996	0.991

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	DMOA-48B	DDA-48B	DMDDA-48B	HDA-48B
qmax (mg g^−1)	143 ± 2	151 ± 2	155 ± 3	162 ± 4
kL (L mg^−1)	(7.83 ± 0.70) × 10^−2	(15.3 ± 1.46) × 10^−2	(10.7 ± 1.02) × 10^−2	162 ± 4
R^2	0.990	0.990	0.990	0.976

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	Carbonaceous materials	Modified clay	Zeolite	Other mesoporous silicas
qmax (mg g^−1)	479,^35 476,^38 794,^39 247 ^40	123 ^45	1.5 ^51	500 ^44

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Solvent extraction was further applied to amine-functionalized MCM-41 and MCM-48 mesoporous materials with different chain lengths (N,N-dimethyldodecylamine, dodecylamine and hexadecylamine). Fig. 2 shows 4-NP and 2,4,6-TCP adsorption isotherms on materials B. The adsorption closely follows a Langmuir isotherm (Fig. 2) in accordance with the results obtained for dimethyloctylamine mesoporous silicas, and the Langmuir parameters are given in Table 7. The sorption performance of the materials B was compared to other sorbents^34–51 (Table 7).

A statistical analysis of the sorption capacities (Table 7) vs. textural and structural properties of materials B (Tables 3 and 4) was performed. The normality of variable distribution was checked by applying the Shapiro–Wilk statistical test and in all the cases, the distribution was far from normal. Non-parametric tests were applied, and ANOVA Kruskal–Wallis tests showed that pollutant adsorption was not dependent on the type of mesoporous silica (MCM-41 vs. MCM-48) (Fig. 3A). In contrast, the adsorption of pollutant was dependent on the phenolic structure (PNP vs. TCP – Fig. 3B). Data were analysed using factor analysis (Fig. 3C) to highlight the relation between the elements. Only the factors with loading greater than 0.7 were considered important. Two factors described 69% of the total variability contained in the raw data set. The eigen values were 2.29 (F1) and 1.82 (F2), and a statistical estimate revealed a (weak) correlation between the pore size and q_max (F1). Plotting q_max vs. pore size (Fig. 3D) confirms the weak relationship and a linear function could be established.

Fig. 3 Statistical analysis of the sorption capacities vs. structural properties of the mesoporous silicas. (A) q_max vs. mesoporous silicas structure (p = 0.34, Z = 0.94). (B) q_max vs. pollutant structure (p < 0.01, Z = 3.36). (C) q_max vs. pore size (n = 16). (D) Factor analysis (n = 42, 6 variables).

4. Conclusion

The experimental analysis indicate that the removal of the amine moiety by ethanolic extraction from aminated-mesoporous silicas leads to the formation of materials that are able to remove phenolic pollutants from aqueous media, which were 4-nitrophenol and 2,4,6-trichlorophenol in this study. The calcination of aminated-mesoporous silicas failed to improve the sorption capacities. From the present and previous results, the evaluation of the partial removal of the amine moiety from modified mesoporous silicas would be interesting, which could offer the simultaneous removal of charged (both cationic and anionic) and neutral organic pollutants from aqueous media.

Acknowledgements

A. Benhamou gratefully acknowledges support from collaborative Project no. 87/ENS/FR/2007-2009 between Limoges University and Oran Science and Technology University.

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