Exploring superiority of silatranyl moiety as anchoring unit over its trialkoxysilyl analogue for covalent grafting via fabrication of functionalized mesoporous silica possessing azomethinic pincers for dye adsorption

Accepted Manuscript Exploring superiority of silatranyl moiety as anchoring unit over its trialkoxysilyl analogue for covalent grafting via fabrication of functionalized mesoporous silica possessing azomethinic pincers for dye adsorption Ruchi Mutneja, Neha Srivastav, Raghubir Singh, Varinder Kaur, Nadine Bette, Volker Klemm, David Rafaja, Jörg Wagler, Edwin Kroke PII:

S1387-1811(18)30391-3

DOI:

10.1016/j.micromeso.2018.07.016

Reference:

MICMAT 9026

To appear in:

Microporous and Mesoporous Materials

Received Date: 3 May 2018 Revised Date:

7 July 2018

Accepted Date: 7 July 2018

Please cite this article as: R. Mutneja, N. Srivastav, R. Singh, V. Kaur, N. Bette, V. Klemm, D. Rafaja, Jö. Wagler, E. Kroke, Exploring superiority of silatranyl moiety as anchoring unit over its trialkoxysilyl analogue for covalent grafting via fabrication of functionalized mesoporous silica possessing azomethinic pincers for dye adsorption, Microporous and Mesoporous Materials (2018), doi: 10.1016/ j.micromeso.2018.07.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Exploring superiority of silatranyl moiety as anchoring unit over its trialkoxysilyl

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analogue for covalent grafting via fabrication of functionalized mesoporous silica

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possessing azomethinic pincers for dye adsorption

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Ruchi Mutneja,a Neha Srivastav,a Raghubir Singh,*b Varinder Kaur,*a Nadine Bette,c Volker

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Klemm,d David Rafaja,d Jörg Wagler,e Edwin Krokee a

Department of Chemistry, Panjab University, Chandigarh – 160 014, India, [email protected] b

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c

DAV College, Sector 10, Chandigarh – 160 011, India, [email protected]

Institut für Physikalische Chemie, Technische Universität Bergakademie, Freiberg – 09596 Freiberg, Germany

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Institute of Materials Science, Technische Universität Bergakademie, Freiberg – 09596 Freiberg, Germany

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Institut für Anorganische Chemie, Technische Universität Bergakademie, Freiberg – 09596 Freiberg, Germany

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Abstract:

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In the present work an azomethinic pincer (P) possessing an ONNNO donor set was prepared

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via condensation reaction of diethylenetriamine and o-hydroxyacetophenone followed by

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silyl-functionalization (introduction of a triethoxysilylpropyl- or silatranylpropyl group via

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urea linker) to afford P-Sil and P-Silt, respectively. The resulting products were characterized

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by various spectroscopic techniques, elemental analysis and single-crystal X-ray diffraction.

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They were further utilized for the fabrication of post-synthesis functionalized mesoporous

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silica nanoparticles (MSNs) using similar reaction conditions. The obtained hybrid materials

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(P-Sil@MSNs and P-Silt@MSNs) were characterized by scanning electron microscopy,

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transmission electron microscopy, thermogravimetry SEM, TEM, TGA, nitrogen gas

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adsorption-desorption measurements and C,H,N microanalysis. The corresponding results

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revealed particularly high loading of organic moieties for the P-Silt functionalized particles.

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In addition, adsorption behavior of both materials towards the anionic dye eriochrome black

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T (EBT) was evaluated by examining the effect of pH, solution temperature, time and dye

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ACCEPTED MANUSCRIPT concentration. The maximum adsorption capacity of P-Silt@MSNs and P-Sil@MSNs at pH

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= 3 was found to be 101 mg⋅g-1 and 69.9 mg⋅g-1, respectively (calculated on the basis of

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Langmuir model). The characterization and adsorption studies revealed better loading of

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organic moieties in P-Silt@MSNs with respect to P-Sil@MSNs. Furthermore, P-Silt@MSNs

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was used to adsorb the cationic dye methylene blue (MB) at pH = 8. Its maximum adsorption

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capacity reached 28 mg⋅g-1.

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Keywords: Silane, silatrane, Schiff base, mesoporous silica nanoparticles, dyes, adsorption.

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1. Introduction

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Functionalized silica materials have become ubiquitous in the fields of catalysis,

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separation, sensing, optoelectronics, environmental technology and biomedical applications

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due to their high specific surface area, thermal and mechanical stability, highly uniform pore

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distribution, tuneable pore size and structure [1–10]. High density of silanol groups on the

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silica surfaces facilitates covalent grafting of useful organic functionalities onto internal and

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external surface sites via condensation reactions (e.g., with Si-O bond formation) [11,12]. In

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the past, organosilanes with the general formula XnSiR4−n (n = 1,2,3; X = hydrolysable

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substituent, e.g., –Cl, -OR′, -C(O)R′, NR′2, H; R = non-hydrolysable substituent containing

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functional group) have been used as anchors for the covalent grafting of organic

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functionalities on silica surfaces either in post-synthesis steps or by co-condensation [13–16].

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The resulting hybrid organic-inorganic silica frameworks exhibit enhanced hydrophobicity

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and thereby show hydrothermal stability with high adsorption capacity and better selectivity

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as compared to bare silica [17–19]. In various cases, the use of alkoxy- or chlorosilanes as

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anchoring units becomes inconvenient because of their high sensitivity towards hydrolysis

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and leads to the formation of aggregates [20,21]. Therefore, specific experimental conditions

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ACCEPTED MANUSCRIPT are required to control the hydrolysis of chlorosilanes (such as trapping of HCl released

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during hydrolysis). Although alkoxysilanes have been widely used for the functionalization

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of silica surfaces, a protection of hydrolysable groups (to some degree) is highly desirable to

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control its hydrolysis, thus damping reactivity and enhancing selectivity. A convenient way

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of protection is the transformation of silyls (such as trialkoxysilanes) to their triethanolamine

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derivatives, i.e. derivatization to silatranes.

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Earlier, we reported on organosilatranyls as new precursors for the modification of

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silica surfaces such as silica nanoparticles, magnetic nanoparticles and silica monoliths [22–

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25]. In addition to their enhanced stability towards moisture (with respect to corresponding

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triethoxysilanes), other advantages like easy preparation as well as isolation and purification

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by crystallization, aggregation-free fabrication of functionalized silica surfaces and uniform

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functionalization facilitated by the release of triethanolamine during hydrolysis have been

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observed. This was also validated by the utilization of fabricated materials for efficient

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adsorption of metal ions and nitrophenols. In some other reports, silatranes have been used

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for the immobilization of DNA on mica surfaces for atomic force microscopy applications

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[26,27]. In addition, they have been used as linkers for grafting sensitizing dyes onto

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transparent metal oxide layers in solar cells and for binding Au nanoparticles onto silicon

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substrates for the application as particle plasmon resonance sensor [28]. Recently, covalent

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linking of inorganic transition metal complexes with nano ITO (indium tin oxide) via

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silatrane functional groups have been reported and found to be stable in the range of pH 2 to

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11 in aqueous phosphate buffer [29]. Moreover, silatranes for the grafting of ligands to metal

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oxide surfaces provide better stability in aqueous media compared to their trialkoxysilane

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analogues [30,31]. With the aim of identifying the superior anchor (amongst triethoxysilane

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and its corresponding silatrane) for silica surface functionalization, we prospected their

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comparison in terms of synthesis of anchors, their utilization in grafting onto mesoporous

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ACCEPTED MANUSCRIPT silica nanoparticles (MSNs), and physicochemical studies of grafted MSNs such as surface

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morphology, nitrogen adsorption-desorption studies, metal loading, adsorption capacity, and

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recognition of dyes. Owing to the utilization of Schiff bases for recognition of some toxins

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such as metal ions and organic species [32–34], we derived an azomethinic pincer

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(possessing an ONNNO donor set) via condensation reaction of diethylenetriamine and

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acetophenone. This pincer was further functionalized with triethoxysilylpropylcarbamido-

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and silatranylpropylcarbamido groups for grafting onto mesoporous silica nanoparticles

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(MSNs). The functionalized MSNs were characterized and utilized for the adsorption of

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cationic and anionic dyes (hazardous water toxicants of textile industry).

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A variety of methods such as photo-degradation, electrochemical oxidation, filtration,

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sonication etc. has been employed for the degradation of dyes but they produce even more

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harmful degradation products. Hence, adsorption of dyes on solid surfaces is considered to be

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the best and effective method for their removal from aqueous solutions [35–38]. Although

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few methods have been reported in the past for the removal of EBT using solid sorbents such

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as biosorbents, clay, resins, activated carbon, polyurethane, polymers etc., they are associated

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with some serious problems such as high cost (associated with activated carbon); less

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sensitivity (of biosorbents), swelling (of polyurethane), poor mechanical stability (of resins)

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and poor efficiency (of clays), etc. With the development in nanoscience, nanometric solids

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such as NiFe2O3, iron oxide nanoparticles and modified magnetite nanoparticles have been

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investigated for the adsorption of EBT, however, the use of metallic oxides is not highly

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desirable due to their toxicity issues. Although hybrid organic-inorganic silica materials

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derived from triethoxysilanes possessing −NH2 and −CN like reactive moieties have been

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reported for the adsorption of dyes [39–44], pincer like Schiff base functionalized materials

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possessing active sites for the adsorption of dyes via non-covalent interactions have not been

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reported in the past. In the present study, EBT and MB are used as model dyes to investigate

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the adsorption properties of both P-Sil@MSNs and P-Silt@MSNs. The present study offers a

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comparison to select a better modifier as well as an efficient method for the adsorption of

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dyes from aqueous media using P-Silt@MSNs as sorbents.

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2. Experimental

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2.1. Materials and methods

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Synthesis of all the compounds was performed under a dry nitrogen atmosphere using

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Schlenk technique. Toluene, hexane and diethyl ether were refluxed over sodium

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benzophenone while chloroform over phosphorus pentaoxide and distilled prior to use. The

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chemicals

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triethoxysilane (Acros), 2-hydroxyacetophenone (Spectrochem), triethanolamine (Merck),

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sodium methoxide (Acros), Eriochrome Black-T (Merck), Methylene blue (Merck), sodium

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hydroxide (CDH), hydrochloric acid (National Chemicals), cetyl trimethylammonium

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bromide

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diethanolamine (Merck) were used as such without any purification.

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2.2. Physical measurements

as

diethylenetriamine

(Merck),

tetraethylorthosilicate

(Acros),

absolute

3-(isocyanatopropyl)-

ethanol

(Merck),

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Infrared spectra were routinely obtained on Thermo scientific NICOLET IS50 FT-IR

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and Perkin Elmer RX-I FT-IR spectrophotometers. Mass spectral measurements (ESI source

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with capillary voltage 2500 V) were carried out on a VG Analytical (70-S) spectrometer. C,

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H, N elemental microanalyses were obtained on a FLASH-2000 organic elemental analyzer.

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The solution NMR spectra (1H, 13C) were recorded at 25 °C on Bruker Avance II FT NMR (AL

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400 MHz) spectrometer and on a JEOL (300 MHz) spectrometer. Chemical shifts (in ppm) are

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reported relative to tetramethylsilane (TMS). The thermo gravimetric analysis was investigated

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using an SDTQ-600 (TA instruments New Castle, DE). Sample measurements were carried out

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using alumina pans under nitrogen atmosphere. The samples were heated (ca. 10 mg) to the given

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temperature at a heating rate (β) of 20 °C min-1. Single-crystal X-ray structure analysis was carried out on a Stoe IPDS-2T

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diffractometer using Mo Kα radiation (λ= 0.71073 Å). The structure was solved by direct

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methods (SHELXS-97) and refined with full-matrix least-squares method (refinement of F2

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against all reflections with SHELXL-97). The morphology, size and shape of mesoporous

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silica nanoparticles were investigated by scanning electron microscopy (SEM) using a

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Hitachi SU8010 FESEM instrument, transmission electron microscopy (TEM) at 80 kV using

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a Hitachi H-7500 instrument and high-resolution transmission electron microscopy

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(HRTEM) using JEM 2200FS from Jeol that is equipped with a corrector for spherical

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aberration located in the primary beam and with an in-column Ω filter located in front of a

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CCD detector. Electronic spectral measurements were carried out on JASCO V-530 double

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beam spectrophotometer in the range 400-800 nm. Surface area and pore characteristics of

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parent and grafted mesoporous silica were characterized using Quanta Chrome Nova-1000

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surface analyzer instrument under liquid nitrogen temperature. Adsorption–desorption

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isotherm measurements were done in order to study the surface area and evolution of porosity

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and textural properties from BET method and BJH method, respectively.

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2.3. Synthetic details

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2.3.1. Azomethinic pincer (P)

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The azomethinic pincer (P) was derived as reported earlier by the condensation of

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diethylenetriamine (0.95 g, 9.20 mmol) with 2-hydroxyacetophenone (2.50 g, 18.36 mmol) in

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methanol [45]. The reaction mixture was heated to reflux overnight. After cooling, the

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solvent was removed under reduced pressure to give a yellow oil. The addition of diethyl

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ether (15 mL) to the oil afforded solid compound which was filtered and dried under vacuum.

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Yield: 2.15 g, 69.0%. M.p.: 70-75 °C. νmax/cm-1: 954 m (υ C–C), 1610 vs (υ C=N), 2833 s, 6

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2893 (υs CH), 3060 (υ −NH), 3366 s (υ −OH). 1H NMR (300 MHz, CDCl3): δ (ppm); 1.67

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(1H, NH), 2.17 (s, 6H7,12), 2.94 (t, 4H9,10, 3JHH = 6.0 Hz), 3.52 (t, 4H8,11, 3JHH = 6.0 Hz),

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6.60 (t, 2H5,17, 3JHH = 7.2 Hz), 6.74 (d, 2H6,18, 3JHH = 8.1 Hz), 7.12 (t, 2H4,16, 3JHH = 7.5

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Hz), 7.31 (d, 2H3,15, 3JHH = 7.2 Hz).

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49.8 (C9,10), 49.9 (C8,11), 117.1 (C3,15), 118.78 (C5,17), 119.4 (C1,13), 128.0 (C6,18), 132.4

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(C2,14), 163.8 (C4,16), 172.3(C=N). MS: m/z (relative abundance (%) assignment): 205

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{24.45, (L – C8H10O – CH3 + H)+}, 222 {100.0, (L – C8H10O + 2H)+}, 338 {38.89, (L – H)+},

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340 {33.52, (L + H)+}.

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2.3.2. Triethoxysilyl derivative of azomethinic pincer (P-Sil) the

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P

(2.00

g,

5.89

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C NMR (75 MHz, CDCl3): δ (ppm); 14.5 (C7,12),

mmol)

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in

dry

chloroform,

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(isocyanatopropyl)trimethoxysilane (1.45 g, 5.86 mmol) was added drop wise using a

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syringe. The resulting mixture was refluxed at 80 ⁰C for 5 h and the solvent was evaporated

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under vacuum. Hexane was added to the residue to get a yellow solid product, which was

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filtered and dried under vacuum. Yield: 2.2 g, 63.9%; mp: 67-70 °C. Anal. Calcd. for

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C30H46N4O6Si: C, 61.40; H, 7.90; N, 9.55. Found: C, 60.13; H, 7.89; N, 9.43. νmax/cm-1: 753

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s (Si-O)s, 951 m (C–C), 1072 vs (Si-O)as, 1162 m (CH2O), 1610 vs (C=N), 1637 vs (C=O),

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2880 s, 2971 (CH)s, 3060 (NH), 3366 s (OH). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.52 (t,

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2H21), 1.12 (t, 9H23,25,27, 3JHH = 6.9 Hz), 1.51 (m, 2H20), 2.14 (s, 6H7,12), 3.09 (m, 2H19), 3.55

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(t, 4H9,10, 3JHH = 6.4 Hz), 3.69 (m, 10H8,11,22,24,26), 6.55 (td, 2H4,16, 3JHH = 8.0 Hz, 4JHH = 1.2

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Hz), 6.77 (dd, 2H6,18, 3JHH = 8.0 Hz, 4JHH = 1.2 Hz ), 7.14 (td, 2H5,17, 3JHH = 8.0 Hz, 4JHH =

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1.6 Hz), 7.27 (dd, 2H3,15, 3JHH = 8.0 Hz, 4JHH = 1.6 Hz), 15.7 (s, 2H, OH). 13C NMR (75.57

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MHz, CDCl3): δ (ppm); 14.47 (C21), 18.39 (C23,25,27), 23.40 (C20), 43.39 (C12,7), 48.47 (C19),

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49.97 (C9,10), 58.4 (C8,11,22,24,26), 111.27 (C3,15), 118.60 (C5,17), 119.31 (C1,13), 128.18 (C6,18),

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132.54 (C4,16), 157.97 (−C꞊O), 163.47 (C2,14), 173.20 (−C=N). MS: m/z (relative abundance

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(%) assignment): 340 {58, (L + H)+}, 469 {4.01, (M-Si(OEt)3 + 2Na)+}, 587 {100, (M +

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H)+}, 610 {3.05, (M +H+ Na)+}.

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2.3.3. Silatranyl derivative of azomethinic pincer (P-Silt) Triethanolamine (0.38 g, 2.54 mmol) and sodium methoxide (0.07 gm, 1.29 mmol)

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were added to the stirred solution of P-Sil (1.50 g, 2.55 mmol) in dry toluene at room

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temperature. The contents were stirred and heated to reflux for 6 h and the ethanol produced

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during the reaction was removed azeotropically. The contents were cooled to room

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temperature, filtered and the volatiles were evaporated in vacuum to afford the product as oil.

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Then, 10 mL of diethyl ether was added followed by stirring of the contents for 30 min. The

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resulting yellow precipitate was isolated by filtration and dried under reduced pressure.

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Yield: 0.997 g, 65.7%; Anal. Calcd. for C30H43N5O6Si: C, 60.28; H, 7.25; N, 11.72. Found:

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C, 57.45; H, 7.74; N, 11.27. νmax/cm-1: 583 m (Si←N), 718, 752 s (Si-O)s, 938 m (C–C),

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1084 vs (Si−O)as, 1164 m (CH2O), 1609 vs (C=N), 1641 vs (C=O), 2869 s, 2925 (CH2)s,

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2958 (NH), 3366 s (OH). 1H NMR (400 MHz, CDCl3): δ (ppm) 0.44 (t, 2H21,3JHH = 7.5 Hz),

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1.61 (m, 2H20), 2.28 (s, 6H7,12), 2.63 (t, 4H9,10, 3JHH = 4.0 Hz), 2.73 (t, 6H, NCH2, 3JHH = 6.0

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Hz), 3.22 (m, 2H19), 3.71 (t, 6H, OCH2, 3JHH = 6.0Hz), 3.77 (t, 4H8,11, 3JHH = 6.6 Hz), 6.75

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(td, 2H4,16, 3JHH = 8.0 Hz, 4JHH = 1.0 Hz,), 6.90 (dd, 2H6,18, 3JHH = 8.0 Hz , 4JHH = 1.0 Hz),

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7.27 (td, 2H5,17, 3JHH = 8.0 Hz, 4JHH = 1.6 Hz), 7.47 (dd, 2H3,15, 3JHH = 8.0 Hz, 4JHH = 1.5 Hz),

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16.1 (s, 2H, OH).

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(C7,12), 43.61 (C9,10), 48.87 (NCH2C), 51.9 (C19), 57.6 (OCH2C), 65.80 (C8,11), 117.31 (C3,15),

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118.36 (C5,17), 119.40 (C1,13), 128.30 (C6,18), 132.29 (C4,16), 157.97 (−C꞊O), 163.47 (C2,14),

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173.20 (−C=N). MS: m/z (relative abundance (%), fragment assigned): 174 {12.7,

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Si(OCH2CH2)3N)+}, 340 {70.1, (L + H)+}, 598 {100, (M + H)+}, 620 {15.4, (M + Na)+}, 636

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{6.8, (M + K)+}.

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2.3.3.1. X-ray crystal data and structure refinement of P-silt

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C NMR (100.62 MHz, CDCl3): δ (ppm) 14.29 (C21), 24.8 (C20), 41.90

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ACCEPTED MANUSCRIPT CCDC 1441990 contains the supplementary crystallographic data for P-Silt. Single-

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crystal X-ray diffraction analysis, selected parameters of data collection and structure

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refinement: empirical formula C30H43N5O6Si (FW 597.78), T 200(2) K, λ 0.71073 Å,

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monoclinic, P21/n, a 11.6488(3), b 14.378(5), c 18.2271(6) Å, α 90, β 91.765(2), γ 90 deg,

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V 3085.26(17) Å3, Z 4, ρcalc 1.287 Mg/m3, µ Mokµα 0.126 mm-1, F (000) 1280, Reflections

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collected 47976, Unique 7440 [R(int) 0.0332], θmax 28.0 deg, Completeness 99.7%,

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Data/restraint/parameters 7440/0/393, GoF on F2 1.104, R1 [I>2σ(I)] 0.0387, R1 (all data)

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0.0514, wR2 (all data) 0.1043.

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2.3.4. Mesoporous silica nanoparticles (MSNs)

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Mesoporous silica nanoparticles were prepared as described in literature [46]. In a

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typical preparation procedure, a mixture of CTAB (0.286 g, 8.9 mmol, dissolved in 7.2 mL

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distilled water), ethanol (2 mL), and diethanolamine (0.02 mL) was stirred for 30 min at 40

13

⁰C. To this solution tetraethoxysilane (0.73 mL) was added drop wise, and the solution was

14

kept stirring for 2 h. The product formed was collected through centrifugation and washed

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with ethanol. It was calcined for 8 h at 800 °C in a furnace to get mesoporous silica

16

nanoparticles (MSNs).

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Mesoporous silica nanoparticles functionalized via triethoxysilyl (P-Sil@MSNs) and

18

silatranyl anchors (P-Silt@MSNs). The post-synthesis grafting method was used for

19

functionalization of MSNs: 0.5 g of MSNs were suspended in ethanol (50 mL) and ethanolic

20

solution of P-Sil (0.23 g, 0.392 mmol) was added directly to the suspension. The mixture was

21

left to stir overnight under nitrogen atmosphere. The solid product was filtered off and

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washed several times with ethanol. The unreacted P-Sil was extracted with methanol using

23

Soxhlet apparatus. The obtained solid P-Sil@MSNs was oven dried at 80 °C for 3 h. An

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analogous procedure was adopted for the functionalization of mesoporous silica nanoparticles

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by adding P-Silt (0.23 g, 0.385 mmol) instead of P-Sil to obtain P-Silt@MSNs.

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2.3.5. Isothermal adsorption analysis details In the first step for the adsorption of anionic dye Eriochrome Black-T (EBT), 10 mg of

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adsorbent (P-Sil@MSNs and P-Silt@MSNs, respectively) was stirred with known

4

concentration of aqueous solution of dye (5 mL, ranging from 10-200 mg⋅L-1) at pH 3. The

5

same step for the adsorption of cationic dye Methylene blue (MB) was performed at pH 9 for

6

a known concentration of dye (in the range and 1-20 mg⋅L-1). The contents of first step were

7

centrifuged and the solution was decanted off. The supernatant liquid was used to determine

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the residual dye by recording its UV-Vis spectrum. Using this data, adsorption amount (qe,

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mmol⋅g-1) was calculated from the initial and residual concentration of dye by equation (3).

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V(Co − Ce) (3) M

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where Co is the initial concentration of azo dye (mmol⋅L-1), Ce is concentration of azo dye at

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adsorption equilibrium (mmol⋅L-1), V is the volume of dye solution (0.005 L), and M is the

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weight of the adsorbent (0.01 g). To optimize the pH for both anionic and cationic dye,

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experiments were performed in the range of pH 1-9 by stirring 150 and 20 ppm of EBT and

14

MB, respectively, with 0.01 g of P-Silt@MSNs. The pH of the solution was adjusted by

15

adding aqueous solutions of 1N HCl or 1N NaOH keeping the final volume same (5 mL).

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Secondly, effect of time and temperature for both the adsorbents P-Sil@MSNs and P-

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Silt@MSNs were studied in the range of 5-60 min and 30-60 °C, respectively for EBT (150

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ppm).

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3. Results and discussion

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3.1. Syntheses

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An azomethinic pincer (P) was prepared by the condensation reaction of diethylenetriamine

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and o-hydroxyacetophenone. It consists of two o-hydroxyazomethinic jaws with a secondary 10

ACCEPTED MANUSCRIPT amine group at the pivot position for further derivatization. The latter was utilized for

2

transforming P into its triethoxysilylpropylcarbamido derivative. The nucleophilic addition

3

reaction of triethoxypropylisocyanate with the secondary amide group facilitates the linkage

4

of P with triethoxysilyl moiety via ureido linkage to produce P-Sil. It was transformed into its

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silatranyl analogue (P-Silt) via transesterification with triethanolamine in the presence of sodium

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methoxide (Scheme 1).

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Scheme 1 Reaction pathway for the synthesis of P-Sil and P-Silt via ureido linkage Subsequently, mesoporous silica nanoparticles (MSNs) obtained via template synthesis

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were functionalized utilizing the pincer derivatives P-Sil and P-Silt for comparing their

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efficiency as anchoring unit. The hydrolysable units of both anchors (ethoxy group of P-Sil

12

and silatranyl cage of P-Silt) were subjected to hydrolysis resulting in the functionalization of

13

external and internal surface of mesoporous silica through Si-O covalent bond formation.

14

However, in case of P-Silt, the triethanolamine released during hydrolysis helped in the

15

dispersion of particles reducing their aggregation. The mesoporous silica nanoparticles

16

obtained via triethoxysilyl (P-Sil@MSNs) and silatranyl anchors (P-Silt@MSNs) were

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ACCEPTED MANUSCRIPT further utilized for the pH specific adsorption of cationic and anionic dyes from aqueous

2

media (Figure 1).

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Figure 1 An illustration representing the complete procedure of functionalization of

5

mesoporous silica with P-Sil and P-Silt, and their utilization for the adsorption of EBT and

6

MB dyes, A, B, C represent the color change of adsorbent (A) P-Sil@MSN and P-Silt@MSN

7

(B)

8

3.2. Characterization

the

adsorption

of

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EBT

and

(C)

after

the

adsorption

of

MB.

The appearance of vibrational bands at 3366 cm-1 and 1610 cm-1 in the IR spectrum of

10

P can be ascribed to –OH and C=N groups present in its jaw like fragments. The origin of a

11

new vibrational band at 1637 and 1641 cm-1 in P-Sil and P-Silt, respectively (in addition to -

12

OH and C=N) may be attributed to C=O stretching. In addition, P-Silt exhibited a band at

13

610 cm-1 corresponding to Si←N vibration indicating the protection of P-Sil via

14

triethanolamine. A comparison of 1H NMR spectra of P, P-Sil and P-Silt at room temperature

15

supported the derivatization of P into P-Sil and P-Silt in terms of appearance of signals

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pertaining to OCH2 (3.55 ppm) and OCCH3 (1.12 ppm) in P-Sil; to OCH2 (3.71 ppm) and

2

NCH2 (2.73 ppm) in P-Silt. Mass spectra of both P-Sil and P-Silt showed their characteristic

3

molecular ion peak at m/e = 587 and 598, respectively. The molecular structure of P-Silt was elucidated by single-crystal X-ray diffraction

5

(Figure 2). The compound crystallizes in the monoclinic crystal system in space group P21/n.

6

The two phenyl rings were almost perpendicular to each other (dihedral angle 80.23(5)°). In

7

the crystal, the silatrane moiety is projected away from the ONNNO pincer like claw. The

8

structure features some intra-molecular H-bond interactions; interaction between phenolic

9

hydroxyl (H6O) and azomethinic N atom (N5) (1.520 Å), and interaction between O from

10

silatranyl cage and NH of ureido group (2.80 Å). The X-ray diffraction parameters are

11

summarized in the experimental part. Synthesis details of P-Silt and selected bond

12

parameters are given in Table S1.

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Figure 2 Molecular structure of P-silt, [Ellipsoids are shown at the 40 % (C, N, O and

15

Si-atoms) and 10 % probability level (H-atoms)]. Selected bond lengths [Å] and

16

angles [deg] are summarized in Table S1.

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The N2 adsorption-desorption isotherms for MSNs, P-Sil@MSNs and P-Silt@MSNs

18

(Figure 3) showed typical type-IV isotherms with a H3 hysteresis loop, thus indicating the

19

presence of mesopores. They clearly show the shifting of capillary condensation towards

20

lower pressures along with decrease in the volume of N2 adsorbed in P-Sil@MSNs and P-

21

Silt@MSNs suggesting decrease in pore size due to functionalization of MSNs. The relative 13

ACCEPTED MANUSCRIPT shift of capillary condensation was more pronounced in P-Silt@MSNs indicating enhanced

2

functionalization of the pores with respect to P-Sil@MSNs. The same conclusion can be

3

drawn by comparing the values of surface area, pore volume and pore diameter of MSN, P-

4

Sil@MSNs and P-Silt@MSNs. The inner diameter decreases from 3.1 nm (MSNs) to 2.9 nm

5

(P-Sil@MSNs) and finally to 2.7 nm (P-Silt@MSNs) revealing functionalization on the inner

6

wall (Figure 3, bottom).

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Figure 3 N2 adsorption/desorption isotherms of MSNs, P-Sil@MSNs, P-Silt@MSNs and

9

pore-size distribution (Bottom: Structure parameter for silica and adsorbent).

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TGA analysis was performed under nitrogen atmosphere to determine the approximate

11

amount of functional groups on the silica surface. The data suggested initial loss of physically

12

adsorbed molecules below 200 °C (10%, 7% and 8.2% in bare MSNs, P-Sil@MSNs and P-

13

Silt@MSNs, respectively). The total weight loss up to 800 °C was 15.15% and 18.58% for P-

14

Sil@MSNs and P-Silt@MSNs respectively, relative to weight loss of 13.81% in bare silica

15

(MSNs). The highest weight loss was observed for P-Silt@MSNs, which indicated an

16

additional (2.23%) grafting of organic moieties compared to P-Sil@MSNs (Figure 4(i)).

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ACCEPTED MANUSCRIPT Moreover, elemental analysis of P-Sil@MSNs and P-Silt@MSNs also indicated the

2

high loading of organic groups in P-Silt@MSNs (C = 6.171, H = 1.001, N = 1.232) compared

3

to P-Sil@MSNs (C = 4.312, H = 0.638, N = 0.793). In FT-IR spectra of MSNs, P-Sil@MSNs

4

and P-Silt@MSNs (Figure S1), peaks observed at 807, 981 and 1050 (or 1098 or 1104) cm−1

5

correspond to Si-O-Si stretching and bending modes and a broad band in the region 3413-

6

3430 cm−1 corresponds to –OH stretching. An absorption band at 1637 cm−1 in P-Sil@MSNs

7

and P-Silt@MSNs was attributed to C=N confirming the grafting of P. Moreover, this signal

8

is relatively intense in P-Silt@MSNs indicating the presence of more organic functionalities

9

as compared to P-Sil@MSNs.

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The SEM investigations (Figures 4iiA-C) showed a spherical and smooth morphology

11

of bare MSNs. However, agglomeration of particles was observed in both silane and

12

silatranyl grafted MSNs, but the SEM micrographs depicted low aggregation in P-Silt-MSNs,

13

which might be due the slow hydrolysis of silatrane moiety. The relatively low agglomeration

14

in case of Silt-MSNs was also supported by dynamic light scattering in which particle size of

15

MSNs was increased from diameter ca. 438 nm to 1373 nm and 1174 nm in P-Sil-MSNs and

16

P-Silt-MSNs, respectively (Figure S2). The functionalization of particles resulted in the

17

increase in the size of P-Sil@MSNs and P-Silt@MSNs. The transmission electron

18

micrograph taken in the defocused Fresnel contrast mode (Figures 4D-4F) revealed a similar

19

enhancement in the size of functionalized MSNs (MSN 45.6, P-Sil@MSNs 104 nm, P-

20

Silt@MSNs 106 nm) indicating the presence of a thin layer on the surface of the

21

functionalized MSNs. The presence of this layer was confirmed by energy filtered TEM

22

(EFTEM) imaging that was done in front and behind the carbon K-edge (284 eV). The

23

HRTEM pictures (Figures 4D and 4F) evidence retention of the morphology of MSNs even

24

after the functionalization.

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Figure 4 (i) TGA curve for MSNs, P-Sil@MSNs and P-Silt@MSNs (samples were heated

3

(ca. 10 mg) at a heating rate of 20 °C min-1 under nitrogen atmosphere) (ii) SEM images of

4

MSNs, P-Sil@MSNs and P-Silt@MSNs (A, B and C); TEM image of MSNs (D), HRTEM

5

images of P-Sil@MSNs and P-Silt@MSNs (E and F)

6

3.3. Application to adsorption of dyes

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To apply the azomethinic pincer decorated MSNs for the removal of dyes, we selected

8

a cationic dye (MB) and an anionic dye (EBT) as target. All the parameters such as pH,

9

contact time and temperature were optimized to achieve efficient adsorption of dye from the

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aqueous solution onto the functionalized silica surface.

11

3.4. Effect of pH

EP

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Solution pH is one of the most important factors, as it influences the overall adsorption

13

process by changing net charge of the adsorbent and/or adsorbate. The effect of pH (in the

14

range 2-9) on the removal of Eriochrome black–T (EBT) and Methylene blue (MB) was

15

studied using P-Silt@MSNs keeping the initial dye concentration (150 mg⋅L-1), temperature

16

(30 °C) and time (15 min) constant. The adsorption of EBT on P-Silt@MSNs at different pH

17

values clearly indicated maximum adsorption at pH 3 for this anionic dye (Figure S3 (A and

18

B)). It is expected that protonation of hydroxyl and/or urea moiety at lower pH generated

19

overall positive charge on the surface, consequently, electrostatic interaction between

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ACCEPTED MANUSCRIPT positively charged adsorbent and negatively charged dye resulted in the adsorption of EBT.

2

In contrast, low solution pH was unfavourable for the adsorption of MB (cationic dye). The

3

tendency of adsorbent to adsorb MB increased with increasing the solution pH, reaching its

4

maximum at pH 9. This suggested electrostatic interaction between negatively charged

5

adsorbent (generated in the presence of base) and positively charged dye. Hence, pH 3 and

6

pH 9 was selected for the adsorption of EBT and MB, respectively.

7

3.5. Effect of temperature

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The studies were performed by stirring 10 mg adsorbent (P-Sil@MSNs and P-

9

Silt@MSNs) in 5 mL of dye solution (150 and 200 ppm) for 25 min at different temperatures

10

(Figure S3 (C and D)). With increase in temperature from 30 °C to 40 °C, the adsorption

11

capacity gradually increased but declined beyond 40 °C. This might be due to an increased

12

solubility of dye in aqueous solution at the temperatures up to 40 °C, and due to a decrease in

13

adsorbate-adsorbent interaction at higher temperatures. Thus, further adsorption experiments

14

were carried out at 40 °C.

15

3.6. Effect of contact time

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To understand the optimum time required to attain equilibrium, EBT was adsorbed for

17

various time intervals (between 0 and 60 min) using both P-Silt@MSNs and P-Sil@MSNs.

18

The study was carried out by stirring 10 mg of each adsorbent in 150 ppm dye solution. The

19

increase in contact time up to 25 min of stirring improves the removal of dye and acquired a

20

constant plateau afterwards (Figure S3E and S3F). Therefore, further adsorption studies

21

were performed by stirring the contents for 25 min.

22

3.7. Calculation of adsorption parameters

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Langmuir and Freundlich models were used to calculate adsorption parameters using Langmuir isotherm represented below: 1 Ce Ce = + qe qm (KLqm)

(1) 17

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Figure 5 (A) Langmuir and (B) Freundlich isotherms for the adsorption of EBT on P-

2

Sil@MSNs and P-Silt@MSNs; (Bottom: Adsorption isotherm parameters determined from

3

experimental data.)

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In Eq. (1), Ce (mmol⋅L-1) and qe (mmol⋅g-1) are concentration and adsorption amount of

5

dyes, respectively, at the adsorption equilibrium, qm (mmol⋅g-1) and KL (L⋅mmol-1) are the

6

theoretical maximum adsorption capacity and the Langmuir equilibrium constant related to

7

theoretical maximum adsorption capacity and energy of adsorption, respectively. The

8

experimental data was plotted as Ce/qe versus Ce according to equation (1) (Figure 5).

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Similarly, Freundlich model can be written as follows: lnqe = lnKf +

1 lnCe (2) n

10

where Freundlich constant Kf (L1/nmg(1-1/n)/g) and Freundlich exponent 1/n represent

11

adsorption capacity and adsorption intensity, respectively. The constants were calculated

12

from intercept and slope of the plot lnqe versus lnCe (Figure 6). The regression coffecients of

13

the linear dependence were found to be close to unity revealing the well fitting of adsoption 18

ACCEPTED MANUSCRIPT data in Freundlich adsorption isotherm and n value more than 1 indicating the favourable

2

process.

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Figure 6 Langmuir (A) and Freundlich (B) isotherm MB on P-Silt@MSNs

5

The adsorption parameters calculated from Langmuir isotherm model revealed maximum

6

adsorption capacity for EBT in acidic medium of 69.9 mg⋅g-1 and 101 mg⋅g-1 for P-

7

Sil@MSNs and P-Silt@MSNs, respectively, suggesting better adsorption capacity of the

8

latter. This is indicative of higher loading of the organic moieties (P) in P-Silt@MSNs

9

relative to P-Sil@MSNs. Moreover, P-Silt@MSNs was further used for the adsorption of

10

cationic dye (MB). The adsorbent showed maximum adsorption capacity of 28.5 mg⋅g-1 (in

11

basic media), which is less than for the adsorption of EBT (in acidic media). Moreover, the

12

adsorption capacity of functionalized MSNs was found high as compared to bare MSNs due

13

to involvement of coordinating sites in the adsorption (i.e for EBT = 13.15 mg g-1 and MB =

14

16.78 mg g-1; see details in ESI Figure S4). The better adsorption of anionic dye (EBT) as

15

compared to cationic dye (MB) on P-Silt@MSNs can be rationalized on the basis of both

16

hydrogen and electrostatic interactions involved during adsorption. The surface of P-

17

Silt@MSNs is equipped with hydroxyl, imine and ureido like active groups, which may

18

interact with hydroxyl groups and sulfonic groups present in the EBT via hydrogen bond

19

formation in acidic medium. At the same time, positive charge generated on the surface of

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ACCEPTED MANUSCRIPT adsorbent may interact with the anionic dye via electrostatic interactions. Hence, both the H-

2

bond formation and electrostatic interaction contributed to the adsorption of EBT on to

3

functionalized MSNs. In contrast, a negative charge generated on the surface of adsorbent in

4

basic medium interacted with cationic MB via electrostatic interactions only. Notably, change

5

in the basic conditions imposed a significant effect. At the pH 8, adsorption of MB was

6

observed without any destruction of the material. However, with the increase in the basic

7

strength, dissolution of silica released the dye adsorbed on the surface of P-Silt@MSNs.

8

Therefore, the adsorbed dyes can be recovered by treating the material in highly basic

9

conditions (1N NaOH).

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4. Conclusions

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Silatranyl derivatives displayed high silylation efficiency compared to similar

12

triethoxysilane derivatives. Both the derivatives were successfully combined with an

13

ONNNO Schiff base moiety linked via formation of an ureido bridge and used for the surface

14

modification of MSNs. Detailed characterization confirmed the superior silylation behavior

15

of silatranyl derivatives. Further, functionalized mesoporous silica surfaces were used as

16

adsorbent for the removal of EBT from aqueous solution. Compared with P-Sil@MSNs, P-

17

Silt@MSNs showed higher adsorption capacity for the anionic dye. An overall comparison of

18

silane and silatranes as coupling agents demonstrated the reaction of silatranyl cage with

19

silanol groups produce surfaces with high surface coverage, thereby, providing an alternative

20

to the chemical functionalization of metal oxides or similar OH-terminated surfaces.

21

Acknowledgement

22

Authors are thankful to CSIR, New Delhi [01(2909)/17/EMR-II dated 03-05-2017] for

23

providing financial support. Ms Jasleen Kaur is acknowledged for supporting experimental

24

part.

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Highlights •

Derivatization of a coordinating donor set P to silyl and silatranyl derivative via silylfunctionalization Utilization of derivatives for the fabrication of post-synthesis functionalized

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mesoporous silica nanoparticles (MSNs) i.e. P-Sil and P-Silt Comparative studies of P-Sil and P-Silt as linkers

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Evaluation of adsorption behavior of both materials towards the anionic dye

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eriochrome black T (EBT)