Chemisorption and Thermodynamic Data of the Interaction between a Chelate Free Acidic Center with Basic Groups Attached to Grafted Silicas

Journal of Colloid and Interface Science 249, 290–294 (2002) doi:10.1006/jcis.2002.8290, available online at http://www.idealibrary.com on

Chemisorption and Thermodynamic Data of the Interaction between a Chelate Free Acidic Center with Basic Groups Attached to Grafted Silicas Luiza N. H. Arakaki,∗,1 Antonio N. de Sousa,∗ Jos´e G. P. Esp´ınola,∗ Severino F. Oliveira,∗ and Claudio Airoldi† ∗ Departamento de Qu´ımica, CCEN, Universidade Federal da Para´ıba (UFPB), 58059-900 Jo˜ao Pessoa, Para´ıba, Brazil; and †Instituto de Qu´ımica, Universidade Estadual de Campinas, Caixa Postal 6154, 13081-970 Campinas, S˜ao Paulo, Brazil Received November 2, 2001; accepted February 7, 2002

The molecule 2-aminoethanethiol was added to the grafted silylant agent [(3-chlorosilylpropyl)trimethoxysilane] (cpts) onto silica gel(≡Sil–Cl), obtaining a surface (≡Sil–SNH2 ) and giving 0.70 mmol g−1 of nitrogen; the surface of silica was modified with [(3-mercaptosilylpropyl)trimethoxysilane] (mpts) with surface (≡Sil–SH), giving 0.78 mmol g−1 of sulphur. Both matrices, (≡Sil–SNH2 ) and (≡Sil–SH), adsorb copper and cobalt acetylacetonates from ethanolic solution. Adsorption, using a batchwise process, showed that copper chelate was the most adsorbed. The interactions between the basic centers attached to organic chains of these modified silicas with the cations in the chelates Me(acac)2 [M = Cu and Co] were followed through calorimetric titrations. Exothermic enthalpic results were obtained for the ≡Sil–SNH2 matrix. The spontaneity of these systems was reflected in negative free Gibbs energy and positive from entropic values. C 2002 Elsevier Science (USA) Key Words: silica gel; β-diketonates; chemisorption; calorimetry; thermodynamic data.

1. INTRODUCTION

The covalent attachment of an organic molecule on an inorganic matrix displays its ability to coordinate metal complexes, providing an interesting approach in the fields of immobilized catalysts (1–3) and high performance liquid chromatography (4–6). Silica gel is the most used inorganic support for that purpose because it is an abundant and inexpensive material, with good adsorbent proprieties due to its large surface areas (7). The presence of silanol groups distributed on the surface (8) increases the reactivity, due to an acidity behavior known as Br¨onsted acid (8–10), enabling easily the enhancement of active centers (11–13). The heterogeneous method (14) is the most used for attaching organic functionalities onto a silica surface. It is quite simple for

1 To whom correspondence should be addressed. E-mail: arakaki@ labpesq.quimica.ufpb.br.

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 C 2002 Elsevier Science (USA)

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grafting a functional organosilane, as a silylating agent, synthesized normally as (RO)3 Si(CH2 )3 X (R = OMe, Oet and X is a functional group). In the silylating recovery process, it is necessary to carry out the activation of the silanol groups by heating under vacuum near 423 K to eliminate physically adsorbed water and to improve the immobilization of organosilane on the silica support. Immobilized silylating agents were easily modified to expand the previous bonded organic covalent chain introduced earlier, generating a new molecule with other basic center groups. The resultant modified surfaces can be applied to many areas, such as catalysis and chromatography, and it can serve as a sequestrant agent for cations (15–17), an ion-exchanger (18), as analytes for electroanalytical purposes (19), and in effluent waste removal from industrial processes (7). In the present work, we investigated the chemisorption and focused on thermodynamic determination upon and interaction of copper and cobalt acetylacetonate chelates with modified silica gel, after grafting with silylant agent [(3-mercaptopropyl) trimethoxysilane], (≡Sil–SH), and the matrix (≡Sil–Cl) modified with a 2-aminoethanethiol molecule, (≡Sil–SNH2 ).These modified silicas were previously investigated in an attempt to explore their capacities for adsorbing transitions metal cations, which have the counter anion chloride and nitrate, in a process performed in aqueous and nonaqueous media (14, 16, 20, 21). But, lack of thermodynamic data of these studied surfaces, concerning the β-diketonates complexes not yet reported in the literature, justifies such determinations in the present work. The acetylacetonate ligand was selected because of its large volume and a strong known tendency toward metal cation coordination (22). This kind of ligand forms stable complexes with a specified composition whose final compounds have well-known structures with high stability. Thus, these properties provide a substantial evaluation regarding the adsorption of the potencial free acidic centers on the complexes formed (23), which available sulphur from ≡Sil–SH, and sulphur and nitrogen from ≡Sil–SNH2 matrices can be bonded to them in a heterogeneous medium. Therefore, all experiments were carried out in

291

GRAFTED SILICAS

ethanolic medium, since the solute reagents are quite soluble in ethanol. 2. EXPERIMENTAL

2.1. Preparations (I) The propylthiol group was attached to silica gel as described elsewhere (20). Briefly, 40.0 g of activated silica gel was suspended in 150.0 cm3 of dry xylene and 15.0 cm3 (84.0 mmol) of mpts was slowly added to the suspension. The mixture was mechanically stirred under inert dry atmosphere of nitrogen for 72 h (21). The filtered solid was washed with xylene and acetone until thiol groups were no longer detected in the filtrate (8). The product obtained, ≡Sil–SH, was dried in vacuum for 8 h at 373 K. (II) The 2-aminoethanethiol molecule was incorporated in a previously modified silica through a heterogeneos route. In this procedure, 30.0 g of the activated silica was suspended in 100.0 cm3 of dry toluene to which 15.0 cm3 (75.0 mmol) of 3chloropropyltrimethoxysilane was dropwise added. The mixture was stirred mechanically under reflux of the solvent under inert dry atmosphere of nitrogen for 72 h (12). The resulting modified silica, 3-chloropropyl silica gel, was filtered and washed with toluene, bidistilled water, and ethanol. The yielded product (≡Sil–Cl) was dried in vacuum for 8 h at 393 K. Then, 10.0 g of this modified silica was suspended in 50.0 cm3 of dry toluene. This suspension was maintained under solvent reflux with mechanical stirring, followed by addition of 9.09 g (0.080 mol) of 2-aminoethanethiol hydrochloride and 15.0 cm3 of triethylamine under dry atmosphere of nitrogen. The mixture was allowed to stand for 24 h. The solid obtained (≡Sil–SNH2 ) by filtration was washed with toluene, water, and anhydrous ethanol and dried in vacuum at 0.10 mm Hg (13.3 Pa) for 8 h at 350 K. 2.2. Physical Measurements Activated and functionalized silicas were characterized by determination of the surface area using the BET method (25) in a Flowsorb II 2300 micromerics apparatus. Carbon, nitrogen, sulfur, and hydrogen contents were determined using a Perkin– Elmer Microelemental model PE-2400 analyzer. The degree of 2-aminoethanethiol attached to the silica surface was determined by analyzing the corresponding nitrogen content using the Kjeldhal method. Infrared spectra of the compounds were obtained in an MB-Bomem Ftir Spectrophotometer, using KBr pellets in region 4000 to 400 cm−3 , with a resolution of 4 cm−1 . Carbon-13 nuclear magnetic resonance spectra for solid samples were obtained in an AC/300p Bruker Spectrometer with cross polarization and magic angle spinning at a frequency of 75.47 MHz. 2.3. Adsorption Isotherms The adsorption isotherms were obtained using the batchwise method, consisting of suspending samples of 50.0 mg of mod-

ified silicas in 20.0 cm3 of an ethanolic solution, containing the chelate at several concentrations, varying from 4.0 × 10−3 to 5.0 × 10−2 mol dm3 , and mechanically stirring for 2 h at 298 ± 1 K . The solid was then separated by centrifugation. The adsorbed cations was determined by sampling the supernatant, which was complexometrically titrated with EDTA solution, using the corresponding indicator (26) for each metal. The adsorption capacity (mmol g−1 ) was calculated through the expression: n f = (n i − n s )/m, where n f is the number of moles adsorbed on silica surface, n i and n s are the initial and supernatant after the equilibrium, and m the mass of modified silica. An identical procedure was applied to the untreated silica and did not detect any adsorption. 2.4. Calorimetry The thermal effect evolving from cation-basic center interaction on anchored pendant groups at the solid/liquid interface was calorimetrically measured by a Hard Scientific isoperibolic calorimeter, model 4285. For each operation, a sample of functionalized silica, varying from 0.1 to 0.2 g, were suspended in 25.0 cm3 of ethanol under stirring at 298.15 ± 0.02 K. The thermostated 2.00 cm3 cation solution in the 0.015 to 0.03 mol dm−3 concentration range was incrementally added into the calorimetric vessel and the titration thermal effect (Qt) was determined. Under the same experimental conditions, the corresponding cation thermal effect of dilution was obtained in the absence of the support (Qd). Under such experimental  conditions the net thermal effect ( Qr ), was  of adsorption  obtained through the equation: Qr = Qt − Qd. 3. RESULTS AND DISCUSSION

The elemental analyses of the silica modified with [(3mercaptopropyl) trimethoxysilane], ≡Sil–SH, gave 2.5% of sulfur, which corresponds to 0.78 mmol of thiol groups per gram of support (20). Identically, ≡Sil–SNH2 matrix 0.98% of nitrogen, obtained through the kjeldhal method, was found to correspond to 0.70 mmol of these groups per gram of silica (14). Activated silica gel characterization based on infrared spectroscopy showed a large and broad band near 3600 cm−1 , which corresponds to O–H stetching vibrations. Some additional bands are incorporated after anchoring the mpts, generating surface, and ≡Sil–SH; a caracteristic band at 2564 cm−1 is related to S–H stretching vibration frequency. The compounds ≡Sil–SH and ≡Sil–SNH2 showed two well-defined bands at 2949 and 2840 cm−1 , which was attributed to C–H stretching vibration modes (14, 21). In the carbon-13 nuclear magnetic resonance, the ≡Sil–SH, spectrum showed a series of peaks (Scheme A, below). The weak signal at 49.6 ppm was due to the C atom of the methoxygroups of the attached organosilane. The sharp peak at 27.4 ppm was assigned to the 2 and 3 methylene C atoms, and the peak at 11.3 ppm is due to the 1 methylene C atom.

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TABLE 1 Number of Moles Adsorbed (n f ), Maximum Adsorption Value (ns ), Coefficients of Correlation (r), and Constant (b) for M(acac)2 Chelates on ≡Sil–SH and ≡Sil–SNH2 Surfaces, at 298 ± 1 K Surface ≡Sil–SH ≡Sil–SNH2

The peaks related to the bonded organomolecule on matrix ≡Sil–SNH2 , which at 50.8 ppm showed the methoxy carbon groups, and at 11.2 and 23.3 ppm the peaks were attributed to carbons 1 and 2, respectively. The peak at 33.3 ppm corresponded to carbons 3 and 4, and finally the peak at 40.0 ppm was assigned to carbon 5, (Scheme B, below).

M(acac)2

n f /mmol g−1

n s /mmol g−1

b

r

Cu Co Cu Co

0.346 0.244 0.336 0.326

0.361 0.246 0.506 0.356

2.770 4.063 1.974 2.810

0.9999 0.9991 0.9990 0.9996

therms shown in Fig. 1. Each isotherm obtained from chemisorption experiments resulting from the plot of the number of moles of solute adsorbed versus its number of moles at equilibrium per volume of solution, provided the maximum adsorption values, which are distinguishable for all these systems. For the series of isotherms, the data revealed that the adsorption process conforms to the modified langmuir model, as proposed for a series of systems, as expressed by the Cs Cs 1 = s + s , nf n n b

The expected distribution of pendant groups incorporating basic centers on modified surfaces, are potentially available as complexing moieties to adsorb metallic chelates, in which the acidic centers are located on divalent copper or cobalt in the form of bis-acetylacetonates (acac) in ethanolic solution (11–16). Results of the adsorption of those cations on modified surfaces are presented in the Table 1, which were quantified through the isos-

where Cs is the concentration of the supernatant cations (mol dm−1 ) in the equilibrium, n s is the maximum amount of solute adsorbed per gram of the silica surface, and b is a constant, obtained from the angular and linear coefficients based on the linearized form of the adsorption isotherms by considering Cs /n f to be related to Cs , as shown as an example in

0.35

0.35

0.30

Cu

0.30

(A)

0.25

0.25

0.20

0.20

nf / mmol g

-1

Co 0.15

0.15

0.10

0.10

0.05

0.05

0.00 0.00 0.35

10.00

20.00

30.00

0.00 0.00

40.00

Co

0.25

0.20

0.20

(C)

0.15

0.20

0.30

0.40

0.10

0.05

0.05

1.00

2.00

3.00

4.00

5.00

Cu

(D)

0.15

0.10

6.00

7.00

0.00 0.00

Cs / mmol dm

FIG. 1.

0.10

0.30

0.25

0.00 0.00

(B)

0.35

0.30

[1]

0.10

0.20

0.30

0.40

-3

Chemisorption of Me(acac)2 , (Me = Co and Cu), for ≡Sil–SH (A, B) and ≡Sil–SNH2 (C, D) surfaces from ethanolic solution at 298 ± 1 K.

293

GRAFTED SILICAS

Fig. 2 (14, 21). The constant b, listed in Table 1, is related to the “bond-term” values (27). These values are larger for cobalt chelate for both matrices. These values suggest there is a high thermodynamic stability for matrices Cu(acac)2 chelate on these surfaces. These interactive processes effected a maximum retention capacity (n s ) for copper chelate, when compared to the correspondent cobalt chelate on both surfaces, ≡Sil–SH and ≡Sil–SNH2 . The adsorption capacity variations indicate a considerable influence of the matrix used here (Table 1). The surface encapped with thiol groups, ≡Sil–SH, adsorbs less than the surface containning amino groups, whose behavior can be analized in terms of n s values. This fact could be related to the presence of two basic centers, sulphur and nitrogen, on pendant groups on ≡Sil–SNH2 surface, permeating the diffusion of the chelate, and interact with these complexing groups anchored on surfaces. The stoichiometry for metal-basic center interactions on the ≡Sil–SNH2 surface was previously established as 1 : 1 for copper and cobalt in aqueous solutions of divalent cations in form of nitrates salts (14). In the present case, for these acetyacetonates complexes, Me(acac)2 , the molar ratio chelate molecule : basic center, presented a different behavior, varying from 1 : 1.3 to 1 : 3. This fact reflects the difficulty of the bulky chelate molecules to reach the basic centers of the pendant groups, due to its own size or geometry. Despite this, the intensisty of the colors acquired by the treated silica samples just after contact with the acetyacetonate chelate solution is strong evidence that those bulky molecules are adsorbed on the modified surfaces. Thermodymanic data were obtained from calorimetric titration. The collected thermal effect values for chelate-basic center interactions, due to the presence of the available groups disposed on modified surface, were adjusted to the Langmuir modified equations, as represented by X 1 X + = , H Hm Hm (k − 1)

[2]

1.4

Cs/nf / g dm

-3

1.2 1.0

TABLE 2 Themodynamic Data for Me(acac)2 Modified ≡Sil–SH and ≡Sil–SNH2 Surfaces, at 298.15 ± 0.02 K Surface ≡Sil–SH ≡Sil–SNH2

M(acac)2

H /kJ mol−1

Cu Co Cu Co

8.59 ± 0.36 4, 41 ± 0.05 −21.54 ± 0.45 −7.13 ± 0.11

−G/kJ mol−1 S/J mol−1 K−1 27.31 ± 0.13 27.74 ± 0.12 27.23 ± 0.12 27.13 ± 0.11

120 ± 1 240 ± 1 19 ± 1 67 ± 1

where the X is the mole fraction of the each cation in solution, H is the integral enthalpy of adsorption, k is a proportionality factor which includes the equilibrium constant, and Hm is the integral heat of adsorption for formation of a monolayer of the unitary mass of immobilized material. From these calorimetric titrations, the results obtained were calculated and are listed in Table 2. The molar fraction X of the acidic metal center in each chelate in equilibrium was calculated with the aid of the n s values of the calorimetric titration process, which also included n s values for the solvent used. These values associated with the calorimetric titration data permit one to obtain the enthalpy Hm of a monolayer formed on the surface and simultaneously k values as explained above. The H values, representing the entalpy of adsorption, were calculated by considering the number of moles of the cations adsorbed, using the expression H = Hm /n s . From these values other thermodynamic data, such as G and S were also calculated from those values (14, 16, 17, 26, 27) based on the expressions G = −RT ln k and S = (H − S)T −1 , and the results are listed in Table 2. The acidic center on chelate-basic center interaction on both surfaces showed, for copper and cobalt chelates for surface ≡Sil–SH, enthalpic values of 8.59 ± 0.36 and 4.41 ± 0.05 kJ mol−1 , respectively, while for the matrix ≡Sil–SNH2 enthalpic values of −21.54 ± 0.45 and −7.13 ± 0.11 kJ mol−1 were found. These results demostrate that the enthalpic values are exothermic for ≡Sil–SNH2 matrix on both chelates, reflecting the interaction of a more effective interactive process with the basic centres of this pendant groups. For all determinations the metallic center on chelates-basic center interactions on the surface demonstrates a spontaneity of the proposed reactions, as shown by the negative free Gibbs energy values, whose results are corroborated with the favorable positive entropic data for all studied systems.

0.8

4. CONCLUSIONS 0.6 0.4 0.00

0.10

0.20

0.30

Cs / mmol dm

0.40

0.50

-3

FIG. 2. Linearization of chemisorption data on ≡Sil–SNH2 surface, for Cu(acac)2 from ethanolic solution at 298 ± 1 K.

Both organofunctionalized surfaces are able to extract copper (II) and cobalt (II) bis-acetylacetonate chelates from ethanolic solution, as demonstrated through batchwise method. This behavior is attenuated with the anchored surface containning two basic centers, sulphur and nitrogen, where copper chelate gave a molar ratio (basic centers : metal) of 1.3 : 1, and cobalt chelate gave a molar ratio of 2 : 1. The matrix contanning only a basic

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sulphur center showed a molar ratio of 2 : 1 for copper and 3 : 1 for cobalt. In all systems highly significant coefficient correlations from each linearization were obtained. This clearly shows a good fit of the data obtained with these surfaces to the adsorption model of Langmuir. Comparing the thermodynamic data for both surfaces demonstrated they have a similar behavior. Therefore, from the results obtained with the ≡Sil–SNH2 matrix, the exothermic enthalpic values demonstrated a better interaction with both chelates. The free Gibbs energy for all systems was calculated and the values obtained show evidence of the spontaneity in the occurrence of favorable adsorptions in all interactive processes, supported by the positive entropic values obtained here. ACKNOWLEDGMENTS The authors (L.N.H.A. and C.A.) are indebted to CNPq for fellowships and UFPB for financial support.

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