Adsorption-Based Synthesis of Environmentally Friendly

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Adsorption-Based Synthesis of Environmentally Friendly Heterogeneous Cr(III) Catalysts for Oxidation Reactions into Kaolinite, Saponite, and Their Amine-Modified Derivatives Carlos Alexandre Vieira, Breno F. Ferreira, Alexandre Fernando Silva, Miguel Angel Vicente, Raquel Trujillano, Vicente Rives, Katia Jorge Ciuffi, Eduardo José Nassar, and Emerson Henrique de Faria ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00643 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018

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ACS Applied Nano Materials

Adsorption-Based

Synthesis

of

Environmentally

Friendly

Heterogeneous Cr(III) Catalysts for Oxidation Reactions into Kaolinite, Saponite, and Their Amine-Modified Derivatives Carlos Alexandre Vieira†, Breno F. Ferreira†, Alexandre Fernando da Silva†, Miguel Angel Vicente‡, Raquel Trujillano‡, Vicente Rives‡, Katia J. Ciuffi†, Eduardo J. Nassar*,† and Emerson H. de Faria*,† † Universidade de Franca, Av. Dr. Armando Salles Oliveira, Parque Universitário, 201, 14404600, Franca, SP, Brazil ‡ GIR-QUESCAT-Dpto. Química Inorgánica – Universidad de Salamanca, 37008-Salamanca, Spain.

* Corresponding authors. E-mail address: [email protected] and [email protected]

______________________________________________________________________ ABSTRACT: Batch experiments were used to evaluate the Brazilian São Simão kaolinite (Kao), the Spanish Yuncillos saponite (Sa), and their derivatives organofunctionalized with 3-aminopropyltriethoxysilane (Kao-APTES and Sa-APTES) as Cr(III) adsorbents in aqueous medium. Cr(III) concentration and interaction time affected Cr(III) adsorption, which depended strongly on surface charge. The adsorption capacities of Kao and Sa increased after functionalization. Cr(III) adsorption reached equilibrium within 30 min for kaolinite solids and within 120 min for saponite solids. Kinetics tests showed that Cr(III)–adsorbent interactions did not follow a single model, and all the samples fitted the pseudo second-order model satisfactorily. Equilibrium studies showed that adsorption correctly fits the Langmuir models, but adsorption onto Kao and Sa-APTES fits the Freündlich model better. Desorption experiments showed that Kao-APTES desorbed only 9.7% of Cr(III) cations under different stressing

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conditions as compared to 47.8% desorption in the case of Sa-ATPES. The solids obtained after Cr(III) adsorption were used as heterogeneous catalysts for ciscyclooctene oxidation reaction, being promising catalysts (conversion higher than 50%) and showing not leaching of Cr(III).

KEYWORDS: saponite, kaolinite, grafting, heavy metals, adsorption, catalysis, epoxidation. ______________________________________________________________________ INTRODUCTION Several human activities release a wide variety of pollutants that contaminate the soil, water bodies, and the atmosphere. These pollutants disrupt the environment and cause significant damage to humankind, animals, and plants. Harmful pollutants involve both organic species like textile dyes and pesticides and inorganic species such as trace metals.1 Platinum, palladium, silver, copper, cadmium, lead, nickel, cobalt, zinc, and chromium, among other metals, are examples of trace metals that are natural constituents of the Earth’s crust and which may occur in the environment due to rock lixiviation. Besides their natural sources, trace metals may contaminate ecosystems through wastewater originating from anthropogenic sources like the weld and metal alloy, mining, steel and lamination, battery, fertilizer, pesticide, and leather industries. At certain concentrations, trace metals can (i) be carcinogenic, (ii) bioaccumulate in bones and soft tissues, and (iii) trigger a series of physiological disruptions.1−3 Leather production is among the sources of potentially toxic trace metals: leather processing industries use metallic chromium as raw material, which is partly discharged into solid and liquid waste. In particular, leather tanning preserves leather flexibility and provides the material with mechanical and thermal stability and adequate coloring.


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Unfortunately, leather tanning requires various mechanical and chemical steps that, in low-efficiency conditions, result in large amounts of complex and recalcitrant effluents containing chromium ions.4 Most leather tanning industry wastewater is treated by chromium ion precipitation in alkaline solution. Most often, medium saturation prevents chromium ion precipitation, which makes the treatment process inefficient.5 Among the various chromium products available for industrial application, basic chromium sulfate is the most recommended: water-soluble (Cr(OH)SO4) dissociates in aqueous medium, to release Cr(III) ions.4 Around 96% of raw leather tanning conducted in Brazil uses chromium ions because the process is less expensive. In 2015, the estimates were that leather tanning consumed 37,500 tons of chromium in Brazil.6 The most stable and hence the most common chromium forms are Cr(III) and Cr(VI). Trivalent chromium is one of the essential oligoelements of the mammalian biological system: Cr(III) ions are crucial to maintain glucose levels in the blood, to ensure lipid and carbohydrate metabolism, and to control serum cholesterol. Nevertheless, above tolerable limits, Cr(III) ions impair the organism’s physiological performance, whereas Cr(III) ion deficiency may culminate in complications like diabetes and cardiovascular problems.

The

recommended

daily

Cr(III)

ion

intake

is

around

50-200

mcg/individual.7,8 In contrast, Cr(VI) ions are toxic and potentially carcinogenic. Cr(VI) ions can be directly released by industrial processes or indirectly obtained by Cr(III) ion oxidation in aqueous medium. Owing to their well-known toxicity, Cr(VI) ions are one of the 129 major pollutants listed by the EPA (United States Environmental Protection Agency).9 In 2013, the European Union published the RoHS guideline 2 2011/65/EG (RoHS = Restriction of Certain Hazardous Substances), which restricted the use of certain dangerous substances, including Cr(VI) ions, in electric and electronic goods.10 In Brazil, there are no legislations restricting the use of chromium as raw material in the



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production of consumer goods, but the Brazilian Ministry of Health Ordinance number 2.914/2011 establishes that the maximum allowed limit is 0.05 mg L-1 chromium in potable water, without any mention of the valence.11 CONAMA (Brazilian Environmental

Council)

resolution

number

430/2011

limits

safe

chromium

concentrations for effluent discharge into water bodies to 0.1 and 1.0 mg L-1 for Cr(VI) and Cr(III) ions, respectively.12 The Brazilian leather tanning industry stands out worldwide. Brazil is the third world leather producer, after China and India. In the first quarter of 2015, Brazil produced approximately 8.11 million bovine leather parts. Given the Brazilian cattle herd size, there is potential for growth in the coming years.6 Southern and southeastern Brazil, especially the states of Rio Grande do Sul, Minas Gerais, and São Paulo, stands out in terms of leather production.13 Industrial effluents treated by Brazilian tanneries contain chromium concentrations above the limits allowed by the Brazilian legislation.14 Therefore, developing cost-effective processes to treat industrial waste containing potentially toxic trace metals is mandatory. Clay minerals have proven to be effective adsorbents of metal ions (Cr3+, Cr6+, Ni2+, Cu2+, Pb2+, Cd2+, Mn2+, Zn2+, Co2+, As3+, Mg2+, and Fe3+) from aqueous solutions. These minerals have adequate surface area, are inexpensive and widely available, and exhibit cation exchange capacity, among other advantages.15 Among clay minerals, kaolinites, saponites, halloysites, vermiculites, montmorillonites, and bentonite are worthy of note: they can be employed in natura and after purification or functionalization.16−22 Clay minerals can also be applied in catalysis, to accelerate the degradation of toxic compounds and pollutants like pesticides. Additionally, these minerals are often used to adsorb potentially toxic trace metal ions and dyes. Chemical elements (nitrogen and oxygen) present in the organic groups of hybrid materials such


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as functionalized clays provide the clays with the ability to act as Pearson bases, giving rise to good adsorbent–adsorbate interactions.16,23−28 Hybrid materials can also adsorb hydrophobic contaminants in aqueous solutions. Such materials display large specific surface area and have promising applications in systems that reduce herbicide lixiviation, photodegradation, and volatilization.27,28 Moreover, they can effectively adsorb p-nitrophenol and p-chlorophenol, which are largely employed in the pharmaceutical and petrochemical industries.29 Due to the growing need to develop new effective materials for use in remediation and environmental pollutant control practices, we have now purified and functionalized kaolinite and saponite with 3-aminopropryltriethoxysilane by following literature procedures (Figure 1).30 The solids have been characterized and then used as Cr(III) ion adsorbents. In an attempt to apply environmentally friendly techniques to dispose of and/or to reuse waste for other industrial purposes, the solids obtained at the end of the Cr(III) ion adsorption experiments have been used in catalytic reactions. Catalytic processes have significantly contributed to sustainable development and environmental protection within the scope of Green Chemistry. Catalysts with selectivity as close to 100% as possible are becoming increasingly relevant because they improve process efficiency and mitigate contaminant and byproduct generation. In this context, detailed knowledge of the catalytic phenomenon at the atomic-molecular level will be more and more necessary for reaction control.31



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Figure 1. Schematic representation of the grafting process of kaolinite and saponite.

The innovative character of the present investigation lies on (a) application of purified and functionalized kaolinite and saponite in Cr(III) ion adsorption assays in solution, based on kinetic and equilibrium experiments; (b) cation desorption study in different solutions of the solids obtained at the end of the Cr(III) adsorption tests; and (c) use of the solids obtained at the end of the Cr(III) adsorption tests as catalysts in ciscyclooctene oxidation.

EXPERIMENTAL SECTION Preparation of the materials. The mining company Darcy R.O. Silva e Cia provided kaolin, originated from deposits situated in the city of São Simão, state of São Paulo, Brazil. The mining company Tolsa (Madrid) supplied saponite, from Yuncillos deposit (Toledo, Spain). Purification followed the method described by Belver et al., which was successfully conducted in other works.32 Purified kaolinite and saponite (designated

Ka

and

Sa,

respectively)

were


functionalized

with

3-

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aminopropyltriethoxysilane (APTES) according to a literature method,30,32 leading to the functionalized solids (Ka-APTES and Sa-APTES, respectively). Kinetic and Equilibrium Studies. To perform the kinetic tests, batch systems were set up by placing 0.050 g of Kao, Sa, Kao-APTES, or Sa-APTES in a test tube, which was followed by addition of 5 mL of chromium(III) chloride (CrCl3) 0.03 mol L-1 solutions, maintaining them for 5, 10, 20, 30, 60 120, 180, 240, and 1440 min under controlled pH (between 5.0 and 5.2). The kinetic studies revealed the ideal times for the equilibrium studies: 30 min for Ka and Ka-APTES and 120 min for Sa and Sa-APTES. The adsorption equilibrium studies were accomplished using CrCl3 solutions with concentrations 0.003, 0.008, 0.010, 0.030, 0.040, 0.080, and 0.100 mol L-1. Cr(III) concentrations were followed by means of UV/VIS spectrophotometry (HewlettPackard 8453, DiodeArray). Previous investigations have already employed this method for adsorption studies.23,33−35 Equation 1 helped to calculate qt (number of moles of adsorbed Cr(III) ions per mass unit of Kao, Sa, Kao-APTES, or Sa-APTES, in grams):

qt = �

(Ci−𝐶𝐶𝐶𝐶) m

�×V

(1)

where Ci is the initial Cr(III) ion concentration (mol L-1), Cf is the final Cr(III) ion concentration (mol L-1), m is the mass of adsorbent (gram) and V is the volume of the solution (L). Plots of qt versus time allow to observe the kinetic behavior of the adsorbents. 2.2.1. Mathematical treatments. The process was investigated by pseudo firstorder model, pseudo second-order model, and intraparticle diffusion kinetic models. This may allow to distinguish if the ion removal rate along time is directly proportional



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to the difference in saturation and in the number of active sites on the solid (pseudo first-order model), if the reaction rate depends on the amount of solute that adsorbs onto the adsorbent surface and on the amount of solute that is adsorbed at equilibrium (pseudo second-order model) or if intraparticle diffusion, evident in materials with highly porous structure, is present.34,36,37 The pseudo first-order kinetic model is described by Equation 2:

1

𝑞𝑞𝑡𝑡

𝑘𝑘

= 𝑞𝑞1 x 1

1

1

+ 𝑞𝑞 𝑡𝑡

1

(2)

where q1 and qt are the amounts of Cr(III) ions adsorbed onto the adsorbate at equilibrium (mol g-1), at different study times (t, min), and k1 is the adsorption constant (g mol-1 min-1) in the pseudo first-order model. The pseudo second-order kinetic model is described by Equation 3:

1

𝑞𝑞𝑡𝑡

= (𝑘𝑘

1

2 2 𝑞𝑞2 )

𝑡𝑡

+ 𝑞𝑞

2

(3)

where q2 is the maximum Cr(III) adsorption capacity (mol g-1) within a specific time (t, min), qt is the amount of Cr(III) adsorbed by the adsorbent at equilibrium (mol g-1), and k2 is the adsorption constant (g mol-1 min-1) in the pseudo second-order model. Equation 4 defines intraparticle diffusion:

𝑞𝑞𝑞𝑞 = 𝑘𝑘𝑑𝑑𝑑𝑑𝑑𝑑 √t + 𝐶𝐶

(4)

where qt is the amount of Cr(III) adsorbed (mol g-1) in time t (min), C is a constant related to diffusion resistance and Kdif is and adsorption constant (mol g-1 min-0.5).


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All mathematical treatments were accomplished by means of the graphic software Origin®. The Cr(III) cation removal rate was calculated on the basis of the highest concentration used during the equilibrium study (0.1 mol L-1); by means of Equation 5:

𝑟𝑟𝑎𝑎𝑡𝑡𝑡𝑡(%) =

Ci−𝐶𝐶𝐶𝐶 C𝑖𝑖

x 100

(5)

where Ci and Cf are the initial and final Cr(III) ion concentration (mol L-1). Langmuir and Freündlich isotherms (Equations 6 and 7) were applied to describe the nature of the equilibrium between the adsorbent and the adsorbate.38

1

𝑞𝑞𝑒𝑒

1

1

= 𝑞𝑞𝑞𝑞 + 𝑞𝑞𝑞𝑞+𝐾𝐾𝐾𝐾 x

1

𝐶𝐶𝐶𝐶

1

ln q e = ln 𝐾𝐾𝐾𝐾 + 𝑛𝑛 x ln 𝐶𝐶𝐶𝐶

(6)

(7)

The Chi-square (X2) test was applied to verify whether fitting of the Langmuir and Freündlich isotherms to the experimental data.39 X2 was determined by equation 8:

X2 =

(𝑞𝑞𝑞𝑞 𝑒𝑒𝑒𝑒𝑒𝑒.−𝑞𝑞𝑞𝑞 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐.)2 𝑞𝑞𝑞𝑞 𝑒𝑒𝑒𝑒𝑒𝑒.

(8)

Desorption assays. Desorption assays were carried out on Kao-APTES and SaAPTES, to determine their capacity to desorb Cr(III) ions. The tests were conducted adapting literature procedures.23 0.050 g of Kao-APTES or Sa-APTES was placed in



test tubes, followed by addition of 5 mL of 0.1 mol L-1 CrCl3 solution. Contact times were the same as the ones used in the equilibrium tests (30 and 120 min for KaoAPTES and Sa-APTES, respectively). The solids obtained were dried at ambient temperature, and then mixed with 5 mL of the desorbing solutions for 1 h, using the same procedures than for the kinetic and equilibrium studies were employed. The following solutions were used during the desorption tests: a – Distilled water; b – Acetic acid, 0.01 mol L-1; c – Phosphate buffer, pH 7.2, prepared with K2HPO4 and NaH2PO4, both 5 mmol L-1; d – Solution with pH 12, prepared with Na2CO3 0.28 mol L-1 and NaOH 0.5 mol L-1. Catalytic activity Cis-cyclooctene Epoxidation Reactions. The cis-cyclooctene oxidation reaction (i.e., epoxidation) was conducted in the presence of hydrogen peroxide (50% m/m) as oxidant and Kao-APTES or Sa-APTES containing adsorbed Cr(III) cations as catalysts. 10 mg of catalyst, 100 μL of oxidant (hydrogen peroxide), 150 μL of substrate (ciscyclooctene), 1044 μL of solvent (1,2-dichloroethane/acenotrile 1:1 v/v), and 6 μL of internal standard (n-dibutyl ether) were added to a 2-mL reaction flask with a screw cap and Teflon septum. The catalyst/substrate/oxidant molar ratio was 1:100:500. The products were analyzed by gas chromatography. The reaction kinetics was monitored by injection of the products formed at 2, 4, 24, and 48 h of reaction into the gas chromatograph, as recommended in the literature.40 For comparison, the reactions were also conducted in the presence of the hybrid solids (Kao-APTES or Sa-APTES) without Cr(III) ions, and in the presence of parent clays (Kao or Sa).


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The possible lixiviation of active species from solid matrixes was evaluated by UV-Vis absorption spectroscopy and Sheldon test was carried out using the reaction supernatant and adding and extra amount of oxidant.41 Characterization Techniques. X-ray diffractometry of the solids was conducted on an X-ray diffractometer Rigaku Miniflex II operating at 40 kV and 30 mA (1200 W) with filtered Kα Cu radiation. The scans were recorded for 2Ɵ angle 2 to 65º at a scan rate of 2º min-1. The infrared absorption spectra were acquired on a spectrometer Perkin Elmer Frontier operating from 350 to 4000 cm-1. The samples were analyzed by diffuse reflectance from a mixture of the sample with potassium bromide (KBr). The thermal analyses (TG/DTG) were conducted on a thermal analyzer TA Simultaneous DTA-TGA Instruments SDT Q600, from 25 to 1050 ºC, in synthetic air atmosphere at a flow rate of 100 mL min-1, at a heating rate of 20 ºC min-1. The microstructure of the solids was investigated with a scanning electron microscope Tescan Vega 3 SBH. The samples were sputtered with gold before analysis (15 kV), accomplished at magnification of 5,000 or 20,000 X. The chromatograms were registered on a gas chromatograph HP 6890 Series GC System equipped with a capillary column (length = 30 m; diameter = 0.2 μm), a manual injector, and an FID detector. H2 was used as carrier gas. A H2/synthetic air mixture helped to maintain the flame. The gas chromatograph was coupled to a microcomputer HP KAYAK-XA running the software that analyzed the products. RESULTS AND DISCUSSION Characterization of the solids. The preparation of these solids was reported by our groups a few years ago.30 So, the characterization for the solids was focused to determine if they were comparable to the previously reported solid, evidencing reproducible characteristics as adequate crystallinity, interlayer expansion, presence of



characteristic functional groups, thermal stability and textural alterations after insertion of organic groups. XRD patterns (Figure 2) revealed the presence of the typical kaolinite peak at 7.14 Å, which shifted to 8.21 Å for the kaolinite derivative Kao-; its intensity decreased after chromium adsorption, suggesting that coordination in aqueous medium could promote the reorganization of the stacking layers of kaolinite. The presence of an intense peak at 7.14 Å in all solids also confirmed the existence of a small fraction of non-intercalated kaolinite layers in the materials. In all cases, the intensities of the reflections corresponding to the basal spacing decreased along treatment (grafting with APTES and Cr3+ adsorption), suggesting that the adsorption process promoted delamination of the Kao-APTES particles. Other authors have described that the physical treatment in the aqueous medium enhanced disorder and induced drastic changes in the clay, as a result of decreased intermolecular interactions (hydrogen bonds) between the kaolinite layers.42-44 In the precise case of Kao-APTES, the APTES molecules functionalized in the Ka basal space led to fewer or no hydrogen bonds between the layers, which suggested that Kao-APTES was a delaminated material or a material with a large structural disorder. Further decrease and broadening of the other typical Kao reflections in the case of the Kao-APTES-Cr3+ sample reported herein confirmed that adsorption of cations promoted a greater disorder of the material. These results are similar to those achieved for iron(III)-picolinate complexes incorporated into natural kaolinite.45


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Page 13 of 34

Kao Kao-APTES Kao-APTES-Cr3+

Sap Sap-APTES Sap-APTES-Cr

200 c.p.s.

Intensity (c.p.s.)

200 c.p.s.

Intensity (c.p.s.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

30

40

50

60

10

20

30

40

50

60

2θ (Degrees)

2θ (Degrees)

Figure 2. X-ray powder diffraction patterns of the original clays, after functionalization with APTES, and after Cr3+adsorption for kaolinite and saponite series.

The basal spacing increased from 14.84 Å in parent saponite to 15.54 Å and 16.41 Å for the Sap-APTES and Sap-APTES-Cr3+ solids, after functionalization and adsorption, respectively. The increase in the basal spacing during Cr3+ adsorption confirmed the presence of chromium complexes in the interlayer region, probably bonded to amine groups, promoting changes on the orientation on the molecular arrangement of APTES to coordinate Cr3+ cations via several amine groups. It may be remarked that chromium could be attached to clay layers in cationic form by electrostatic interaction. The small change in the basal spacing (0.9 Å) suggested also a possible hydration of the cationic species existing on the interlayer spacing of the saponite or also the exchange of these cations. The increasing of basal spacing resulted in a higher diffusivity of substrates between interlayer spaces and also suggested that these solids could be efficiently applied as heterogeneous catalyst.



A

E

B

F

Page 14 of 34

C

D

G

H

Figure 3. SEM micrographs of natural kaolinite (A and B) and kaolinite functionalized with APTES (C and D); natural saponite (E and F) and saponite functionalized with APTES (G and H). The morphological changes induced by grafting were clearly observed in SEM micrographs of the functionalized solids, which were compared to those of the original kaolinite and saponite clays (Figure 3). The shape of the kaolinite particles was typical for this material, showing a flake aspect, grouped forming plates and stacks with hexagonal habit. The functionalized solids maintained this habit, but the particles were clearly spongier, showing a partial separation and disintegration of the particles, as expected upon their interaction with the organic molecules. SEM micrographs of untreated saponite showed flakes, a characteristic morphology of this clay mineral; however, functionalized saponite also showed a clear swollen of the particles due to their interaction with the organic molecules. The amount of APTES involved in the intercalation/functionalization of Kao or Sa was determined from the thermal decomposition of the treated solid.46 The experimental ratio between the number of moles of APTES per mol of Kao or Sa, XT, was calculated in three stages, by assuming that water was lost by condensation in distinct steps along


Page 15 of 34

the functionalization process. On the basis of the ideal formula, XT was 0.084 mol of APTES per mol of Kao. The organic components of APTES decomposed above 280°C, which resembled literature results obtained for other Ka functionalized with organic molecules such as methanol, ethylene glycol, diols, polyols, aminoalcohols, and alkoxides.47,48 In the case of the saponite, XT was calculated considering the residue originating from thermal analysis, obtaining a value of 0.16 mol of APTES per mol of Sa. Figure 4 shows the infrared absorption spectra of chromium chloride hexahydrate (CrCl3.6H2O), and Kao-APTES or Sa-APTES solids, before and after Cr(III) adsorption. Cr(III) clearly coordinated with the adsorbents, as deduced from the band at 1404 cm-1 in the spectrum of Ka-APTES containing adsorbed Cr(III) ions. The presence of this band strongly suggests the coordination of Cr(III) to the amine groups in the functionalized solids. Kao-APTES-Cr Kao-APTES

20 %

Sap-APTES-Cr Sap-APTES

Transmittance (%)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 %

4000

3500

3000

2500

2000

1500

1000

500

4000

3500

Wavenumber (cm-1)

3000

2500

2000

1500

1000

Wavenumber (cm-1)

Figure 4. Infrared absorption spectra of the kaolinite and the saponite solids containing adsorbed Cr(III).


500


Sa-APTES containing adsorbed Cr(III) ions also exhibited a band at 1404 cm-1 (Figure 5) due to Cr(III) adsorption. The band between 3500 and 3000 cm-1 broadened as a result of Cr(III) ion hydration. Taking into account the characterization of the solids and the kinetic and equilibrium studies, the mechanism for Cr(III) adsorption onto Kao-APTES and Sa-APTES is proposed (Figure 5), based on the strong interaction between Cr(III) and O and N ligand atoms in APTES moieties and the cationic exchange between Cr(III) and the exchangeable cations in the case of the saponite. The most probable mode of coordination of Cr3+ in kaolinite and saponite may involve two or three NH2 groups from APTES grafted into the interlayer space for each Cr3+ cation, resulting in a lower leaching of the cation.

Figure 5. Schematic representation of the catalysts-based clays.

Kinetic and Equilibrium Studies. The kinetic studies indicated that 30 and 120 min were the ideal times to study the equilibrium in the case of Kao and Sa solids, respectively.


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The pseudo second-order model produced better results from a mathematical viewpoint. k1 and k2 values indicated the adsorption rates,33 whereas the correlation coefficient (R2) indicated that the experimental results obtained for the solids agreed with the kinetic linear models. C value different from zero when plotting qt versus t0.5 and lower R2 satisfactorily proved that intraparticle diffusion was not the main mechanism of Cr(III) adsorption. However, this mechanism may have occurred simultaneously with the pseudo second-order model once the C value (qt versus t0.5) was very close to the k2 value. Analysis of intraparticle diffusion plots revealed multilinearity, which again allowed to infer that this mechanism occurred simultaneously with other mechanisms.45 The results are summarized in Table 1. Results are asimilable to previous findings onto clay minerals like kaolinite, montmorillonite, and Akadama clay (its mineral composition was not reported),16,50,51 which evidence that the pseudo second-order kinetic mechanism prevails in most clay minerals used for adsorption purposes.33

Table 1. Kinetics parameters for Cr(III) adsorption. Pseudo first-order model

Pseudo second-order model

Intraparticle diffusion

Solid R2

k1

R2

k2

R2

C

Kao

0.991

0.03647

1.0

0.03648

0.95

0.03608

Sa

0.856

0.16356

0.999

0.02777

0.13

0.02755

Kao– 0.949 0.09335 1.0 0.02762 0.41 0.02735 APTES Sa– 0.302 0.00627 1.0 0.02786 0.29 0.02762 APTES R2 = Correlation coefficient k1 = adsorption rate constant (pseudo first-order model) (g mol-1 min-1) k2 = adsorption rate constant (pseudo second-order model) (g mol-1 min-1)



0,005 0.005

0,005 0.005

a

b 0.004 0,004

0,003 0.003

0,003 0.003

qe (mol.g-1)

0.004 0,004

qe (mol.g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

0,002 0.002

0,001 0.001

0,002 0.002

0,001 0.001

Kao Kao-APTES

0,000 0.000 0,00 0.00

0,01 0.01

0,02 0.02

0,03 0.03

0,04 0.04

0,05 0.05

0,06 0.06

Sa Sa-APTES

0,000 0.000

0,07 0.07

0,00 0.00

0,01 0.01

Cr3+ (mol.L-1)

0,02 0.02

0,03 0.03

0,04 0.04

0,05 0.05

0,06 0.06

Cr3+ (mol.L-1)

Figure 6. Isotherms for Cr(III) ion adsorption onto the solids Kao and Kao-APTES (a), Sa, Sa-APTES (b). The isotherms obtained for Cr(III) adsorption onto Ka, Sa, Kao-APTES and Sa-APTES (Figure 6) were classified as type L subgroup 3 (Langmuir)on the basis of the initial slopes, while Kao-APTES showed an isotherm L subgroup 1. This evidenced that the surface of the solids was coated with Cr(III) ions; that is, a Cr(III) monolayer covered the adsorbent surface. Isotherm L subgroup 3 suggests that the first layer had already been arisen, while isotherm L, subgroup 1 represents a system with incomplete monolayer adsorption.52 This situation onto Ka-APTES solid was probably due to the exfoliated kaolinite structure, which provided more adsorption sites. Studies on the adsorption of metal ions (Cu2+, Zn2+, Pb2+, Cr6+, and Cr3+) onto clay minerals like saponite, motmorillonite and kaolinite, among others, described that isotherms were predominantly L (subgroups 3, 2, and 1),32,53 evidencing a classic behavior for clay minerals used in the adsorption of potentially toxic trace metals. Based on isotherm classification, Freündlich and Langmuir were applied to obtain R2 and the associated adsorption energies (KF and KL, respectively).33,34,54 The results better fitted the Freündlich model, as evidenced by the relatively high R2 values and the lower Chi-square values (Table 2).


0,07 0.07

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Table 2. Parameters from Langmuir and Freündlich equilibrium models.

Solid

qe

Langmuir R2 0.27243

KL 464.82

Freündlich X2 0.0030

R2 0.72980

KF 0.01984

Kao 0.003971 Kao0.004505 0.99093 18.18 0.0010 0.9945 0.07697 APTES Sa 0.004241 0.99527 50.99 0.0009 0.98425 0.03277 Sa0.003779 0.80604 741.29 0.0028 0.86766 0.00859 APTES qe = Maximum adsorption capacity (mol g-1) R2 = Correlation coefficient KL = adsorption energy associated with the Langmuir model (L mol-1) KF = adsorption energy associated with the Freündlich model (L mol-1) X2 = Chi-square test

X2 0.0012 0.0001 0.0001 0.0023

The Freündlich model indicated that Cr(III) ion adsorption onto the solids involved heterogeneous sites. Cr(III) adsorbed through cation exchange mechanism, very limited for kaolinite, and through coordination to the oxygen structure atoms (Ka and Sa) and to functionalization oxygen and nitrogen atoms (Ka-APTES and Sa-APTES), which also agrees with the high associated adsorption energies (KL) in the Langmuir model. As for the experimental adsorption capacity (qe) values at the highest Cr(III) ion concentration investigated herein (0.1 mol L-1), Ka-APTES had higher qe, 0.004505 mol g-1, followed by Sa, Ka and Sa-APTES. For comparison purposes, Table 3 depicts qe for Cr(III) ion adsorption onto the solids Kao, Sa, Kao-APTES, and Sa-APTES and onto other solids reported in the literature (converted to the same unit mol g-1), confirming the excellent behavior of the functionalized solids.

Table 3. Maximum adsorption capacity of Cr3+ onto different clay solids. Adsorbent

qe (mol g-1)

Kaolinite P

0.000037

Kaolinite a

0.000051

Reference

18



Page 20 of 34

Saponite P- Fe

0.001800

Chlorite P

0.000014

Illite P

0.000092

Kaolinite P

0.000037

Bentonite P

0.000957

21

Kaolinite P

0.000044

22

Bentonite P

0.002259

28

Kaolinite P Saponite P

16

0.003971 0.004241

b

0.004505

Saponite b

0.003779

Kaolinite

19

This work

P

Clay mineral purified by different methods; a Clay mineral functionalized with glycerol; b Clay mineral functionalized with 3-APTES.

Results in Table 3 correspond to works reporting experimental conditions that resembled our conditions. Koppelman et al.20 studied Cr(III) ion adsorption onto purified kaolinite, finding lower qe values even though kaolinite was in contact with the Cr(III) solution for six days. Other studies reported even smaller qe values.18,23 GhorbelAbid et al. investigated Cr(III) adsorption onto a natural bentonite with high kaolinite content, obtaining qe values of 0.0023 and 0.0012 mol g-1 for the natural clay and for the clay washed with 1 mol L-1 NaCl.15 Turan et al.22 tested Cr(III) adsorption onto kaolinites, achieving lower qe values than ours; their kaolinite had different chemical composition than our, with distinct iron, manganese, and titanium contents. This suggests that the adsorption capacity may be influenced by the impurities and the minority elements, and in this sense São Simão kaolinite shows a particular composition.


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Kaolinites intercalated and/or functionalized with dimethylsulfoxide, iodomethane, carboxylic acids, and alcohols, for instance, have been evaluated in metal adsorption experiments,54 but none of these materials presented higher adsorption capacity than Ka-APTES under similar conditions. Affinity between the metal cation and the functional groups in the functionalizing agent may be a key factor for adsorption. Indeed, amine groups (-NH2) in APTES could provide extra adsorption sites (Pearson principle),17,26,55 in addition to the oxygen atoms on the clay structure. Matuzik et al. used kaolinite functionalized with iodomethane to adsorb Cr(III), but obtaining 25 times lower qe.22 Hence, both São Simão kaolinite peculiarities and the alkoxide used during clay functionalization may influence the very high qe of the solid (0.004505 mol g-1). In the case of Sa, qe slightly decreased after functionalization, Sa-APTES adsorb about 10% less than the parent clay. Incorporation of APTES decreases the CEC of the clay, high in the saponite, and at the same time decreases the free space in the saponite interlayer. Thus, the adsorption in the parent Sa may be mainly due to a cation exchange mechanism, and is not completely compensated by the binding to APTES groups in the functionalized solid. The kinetic studies pointed out that Sa-APTES established a more stable interaction with Cr(III) during the studied periods (similar as observed for KaAPTES). Brigatti et al.18 used high iron content saponite for Cr(III) adsorption, varying the pH and the source of Cr(III) (nitrate, chloride, and acetate salts). Chromium chloride solutions at their original pH provided the best results, but the qe values were about 2.5 times lower (0.0018 mol g-1) than the ones reported herein even though the authors allowed the materials to remain in contact with Cr(III) ions for four days. This confirms that, as observed for kaolinite, the origin of the clay mineral and the experimental conditions affected adsorption.



Page 22 of 34

Desorption Assays. Table 4 summarizes the results of Cr(III) desorption tests conducted with Kao-APTES and Sa-APTES solids. The complex Kao-APTES - Cr(III) was highly stable; desortion in acetic acid was negligible, and only the test carried out at buffered pH 12 showed a significant Cr(III) desortion, namely, the 5.4% of the adsorbed amount of the cation. In the case of Sa-APTES, the desorption of the cation was higher, mainly in the buffered pH 7, in which 33.7% of the adsorbed Cr(III) was desorbed. This confirms that bonding of Cr(III) on Sa-APTES is weaker than in Kao-APTES, strongly supporting that adsorption may be controlled by cation echange in the first case and by coordination to the active groups of APTES in the second one. To our best knowledge, there are no literature studies on Cr(III) ion desorption from APTES functionalized clays. Kaolinite functionalized with iodomethane was used to adsorb Cr(VI) ions, finding slightly higher desorption rates as compared to our results.23,53

Table 4. Desorption of Cr(III) from the Cr(III)-containing solids. % of Cr(III) ion Desorbed Cr(III) ion Solid

Medium

desorption in the concentration (mg/L) solutions

Kao-APTES

Sa-APTES

H2O

62.7

3.2

Acetic acid

--

--

buffer pH 7

16.7

0.9

buffer pH 12

102.3

5.4

Total

181.7

9.5

H2O

68.7

4.4


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acetic acid

--

--

buffer pH 7

535.4

33.7

buffer pH 12

101.6

9.7

Total

705.7

47.8

The results pointed out that Sa-APTES reuse after the adsorption and desorption experiments was feasible, while in the case of Kao-APTES, the strong interaction between the cations and the solid favour the removal of the cations, but difficults their desorption and the reuse of the solid.

Catalytic Studies. The catalytic activity of Kao, Kao-APTES, Sa, and Sa-APTES containing adsorbed Cr(III) was evaluated in the oxidation of cis-cyclooctene (diagnostic substrate) by hydrogen peroxide (50% m/v) at 25°C and ambient pressure. This substrate was chosen because the resulting epoxide (cyclooctene oxide) is highly stable and is usually the major product, facilitating the catalyst efficiency assessment. Furthermore, epoxides are extremely useful in the chemical industry and are the starting compounds to prepare a large array of products. All the solids led to high ciscyclooctene conversion to the epoxide (Table 5). In all the cases, the product yield increased along time, reaching excellent conversion after 24 and, mainly, 48 hours of reaction, reaching 55.1% conversion. Selectivity for cyclooctene oxide was 100% for all the catalysts. Probably, high-valent oxo-metal species participated in the oxidation of the substrate. As reported by similar systems.32 Control reactions were conducted by using as catalysts the parent solids, that is, not submitted to Cr(III) adsorption, and in the absence of the oxidant (Table 5). These tests only yielded the epoxide after 24 h of reaction and with low conversion values. This



evidenced the important role that the adsorbed cations play in oxygen transfer to the substrate. Ciuffi et al.40 studied the catalytic activity of a takovite nanocomposite containing adsorbed Cr(III) cations. The experiments were similar to those now conducted, finding that cis-cyclooctene conversion to cyclooctene oxide ranged from 70 to 90%, higher than for the solids now prepared. On the contrary, Lu et al.56 used chromyl chloride (CrO2Cl2) as catalyst and oxidant in cis-cyclooctene oxidation reactions, obtaining low epoxide yields (~ 3%) as well as undesired byproducts; so the conversion and selectivity showed by our catalyst are really attractive. Ciuffi et al.40 carried out a control reaction using the nanocomposite without Cr(III) ions, affording only 20% conversion, showing that the presence of Cr(III) was essential to generate the catalytically active species that transferred oxygen from the oxidant to the substrate. One of the main difficulties concerning the use of heterogeneous transition metal catalysts, with the metals immobilized onto solid supports, is the lixiviation of the active species to the reaction medium. Therefore, the possible lixiviation of Cr(III) from the solids was evaluated by UV-Vis spectroscopy of the supernatants. The spectra of the supernatants collected after 72 h of reaction did not evidence Cr(III) lixiviation from any of the studied systems. To prove that Cr(III) was immobilized and that reaction was truly heterogeneous, the catalysts were filtrated from the reaction medium, extra oxidant (the same amount used at the beginning of the reaction) was added to the filtered supernatant, and the reaction was again conducted under the same initial conditions until 72 h (Sheldon test). After this period, the amount of epoxide increased only a very low amount (Table 5), which confirmed the heterogeneous catalysis. However, the lixiviation of catalytically inactive species, although not detected by the techniques used herein, cannot be ruled out.


Page 24 of 34

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The catalysts remained stable after three times of reuse and the yield of epoxide was maintained, confirming that this series of catalysts were truly heterogeneous in agreement to Sheldon test. Probably the coordination bonds between NH2 groups and Cr3+ ions resulted on major stability of the metal on the active sites. However, for the purified clays, in which the metals were bonded by ionic strengths, the epoxide yield decreased after the first reuse to 50.2 and 48.0 % for saponite and kaolinite, respectively, and a gradual decrese was observed after each cycle.

Table 5. Cis-cyclooctene conversion to cyclooctene oxide (%) during oxidation by hydrogen peroxide. Catalysts Sa-APTES-Cr (without H2O2) Sa-APTES-Cr Sa-APTES Sa-Cr Sa Kao-APTES-Cr (without H2O2) Kao-APTES-Cr Kao-APTES Kao-Cr Kao

2

Time (h) 4 24

48

Sheldon test (72h)

1st reuse

2nd reuse

3rd reuse

-

-

-

-

-

-

-

-

3.7 3.3 -

4.7 3.4 -

50.0 9.0 42.3 10.0

52.0 11.0 39.0 17.0

54.0 51.3 -

54 50.2

54 40.5

54 35.2

-

-

-

-

-

0 0 -

4.5 4.5 -

20.9 2.0 42.7 3.0

20.2 5.0 55.1 7.0

26.6 50.0 -

25.5

25.4

25.0

48.0

37.0

30

CONCLUSIONS Kaolinite and saponite, purified (Kao and Sa) and functionalized with 3aminopropyltriethoxysilane (Kao-APTES and Sa-APTES), were used as adsorbents of Cr(III) cations. The stability of the solids containing Cr(III) was evaluated by desorption tests. Kao-APTES showed the highest adsorption capacity, followed by Sa, Kao, and Sa-APTES. Functionalization significantly increased the adsorption capacity of the clay minerals and provided more stable adsorbent–adsorbate complexes. The pseudo second-



order mechanism along with intraparticle diffusion governed most of the adsorption process. The isotherms adjusted better to the Freündlich model. Adsorption seems to be dominated by cation exchange in the case of saponite solids, and by coordination to amine groups in the functionalized solids. Desorption tests showed that Sa-APTES is potentially recyclable, as Cr(III) desorption from it took a relatively short time. These solids were used as catalysts of cis-cyclooctene oxidation. Conversions higher than 50% were found, observing that the presence of Cr(III) is essential for the reaction (blank tests gave low conversions), the reaction is truly heterogeneous and Cr(III) does not lixiviate to the reaction medium. The active agents are probably high-valent oxometal species.

ACKNOWLEDGMENTS The authors thank a Cooperation Grant jointly financed by Universidad de Salamanca (Spain) and FAPESP (Brasil, 2016/50322-2), and a Spain–Brazil Interuniversity Cooperation Grant, jointly funded by the Spanish Ministry of Education, Science and Sports (PHBP14/00003) and CAPES (317/15).The Brazilian group acknowledges support from Brazilian research funding agencies Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (311767/2015–0, 2012/08618-0, 2013/19523-3 and 2016/01501–1), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

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Adsorption-Based Synthesis of Environmentally Friendly

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