Improved template – ion exchange synthesis of Cu-nanostructured molecular sieves

Accepted Manuscript Improved template – Ion exchange synthesis of Cu-nanostructured molecular sieves Virginia M. Vaschetti, Belén M. Viola, Deicy Barrera, Karim Sapag, Griselda A. Eimer, Analía L. Cánepa, Sandra G. Casuscelli PII:

S1387-1811(19)30264-1

DOI:

https://doi.org/10.1016/j.micromeso.2019.04.037

Reference:

MICMAT 9441

To appear in:

Microporous and Mesoporous Materials

Received Date: 24 January 2019 Revised Date:

26 March 2019

Accepted Date: 18 April 2019

Please cite this article as: V.M. Vaschetti, Belé.M. Viola, D. Barrera, K. Sapag, G.A. Eimer, Analí.L. Cánepa, S.G. Casuscelli, Improved template – Ion exchange synthesis of Cu-nanostructured molecular sieves, Microporous and Mesoporous Materials (2019), doi: https://doi.org/10.1016/ j.micromeso.2019.04.037. 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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Improved template – ion exchange synthesis of Cu-nanostructured molecular sieves. ACCEPTED MANUSCRIPT

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Virginia M. Vaschettia, Belén M. Violaa, Deicy Barrerab, Karim Sapagb, Griselda A. Eimera, Analía L.

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Cánepaa, Sandra G. Casuscellia,*

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a

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López esq. Cruz Roja Argentina S/N, CP 5016, Córdoba, Argentina.

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de los Andes 950, CP 5700, San Luis, Argentina.

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* Corresponding author, E-mail address: [email protected] (Sandra G. Casuscelli).

Centro de Investigación y Tecnología Química, UTN-CONICET, Facultad Regional Córdoba, Maestro

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Laboratorio de Sólidos Porosos, INFAP-CONICET, Universidad Nacional de San Luis, Avenida Ejército

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A series of M41S type mesoporous molecular sieves modified with copper were synthesized through

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Template-Ion Exchange (TIE). The influence of hydrothermal treatment and mixing time at room

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temperature over the solids structural and chemical properties was evaluated in detail for a single Cu

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content. Characterization of the materials was carried out through various techniques: XRD, N2 adsorption-

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desorption, SEM, XPS, atomic absorption, UV-vis DR and TPR. It was found that different stirring times at

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room temperature had no significant influence over the physical and chemical characteristics of the final

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solid. However, hydrothermal treatment had a slight effect over the materials structure and metallic species

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distribution. In order to assess copper content influence, two other non-treated solids with different metallic

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contents were synthesized, characterized, and compared with hydrothermally treated samples. The catalytic

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performance of the hydrothermally treated and non-treated materials was tested in the liquid phase oxidation

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of limonene employing hydrogen peroxide (H2O2) as oxygen donor, an important fine chemical reaction.

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Only one of the evaluated Cu contents exhibited a minor difference in catalytic activity according to the

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applied synthesis conditions. Hence, a simple TIE procedure without hydrothermal treatment can be

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employed for the synthesis of Cu-MCM materials. By suppressing hydrothermal treatment, it is possible to

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save around 10% of the total energy requirement for the complete synthesis process.

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Keywords: Copper; Hydrothermal treatment; Limonene; MCM-41; TIE.

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1. Introduction

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In 1992, Beck et al. used the liquid crystal templating approach to synthesize a new family of

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mesoporous inorganic solids currently known as M41S. One of the first members of said family was a

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silicate molecular sieve designated as MCM-41 [1,2]. The MCM-41 presents a hexagonal arrangement of

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uniform cylindrical mesopores, which can be tailored between 2 nm and 50 nm. The adjustable pore system,

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added to an elevated specific area (up to 1500 m2/g) and high pore volume (up to 1.3 ml/g), have made this

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material one of the most extensible studied for adsorption, catalysis and sensing applications. The catalytic

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success of mesoporous materials is based on large pores that allow easy diffusion of bulkier molecules and

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their approach to well dispersed active sites located on the support vast surface [3].

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Considering that MCM-41 is an inert silicate material, the active species that grant it acidic, basic or

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redox properties, must be incorporated into the MCM-41 that serves as a support for the sites created.

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Multiple methods have been used to functionalize the MCM-41 surface. Different heteroatoms can be

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incorporated by direct hydrothermal synthesis into the tetravalent silicon network. However, one major

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drawback of this technique is that retention of mesoscopic order is sometimes difficult, and the material

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stability often depends on the dopant amount [4,5]. To overcome this issue, other surface functionalization

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techniques can be applied once the support is already synthetized. These techniques are grouped under the

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category of post-synthesis methods. Grafting, as well as impregnation, are two representative examples of

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those techniques. In general, post-synthesis methods imply bonding of organic ligands with surface silanol

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groups or anchoring of inorganic species on the solid surface [4]. However, post-synthesis procedures also

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present disadvantages like clogging of the pore system caused by extra framework species formed after a

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certain load of metal is incorporated. This produces a considerable loss of specific area, pore volume and

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pore diameter of the mesoporous material, and structural deterioration of the solid [6].

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In 1997, an interesting method for planting Mn(II) ions into the MCM-41 was developed by

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Yonemitsu et al. [7]. This surface functionalization method was called Template-Ion Exchange (TIE), and it

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is based on the principle that surfactants in the as-synthesized MCM-41 (before calcination) can be partially

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replaced by transition metal ions in a polar solvent solution. TIE is a simple and fast technique that allows

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incorporating high loads of metallic species finely dispersed on the support surface avoiding significant

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clogging of the pores and considerable distortion of the mesoporous structure [8].

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Since its development, the TIE method has been applied to several metal ions as well as different

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supports types. Moreover, a wide variety of synthesis methodologies based on this functionalization process

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has been reported [9-17]. Considering that the metal loading procedure may affect the material structure, the

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chemical environment of the active sites, and therefore its activity in catalytic applications, it is of great

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interest to analyze and contrast the effects that some of the applied synthesis conditions may have over the

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final solid. Some of the aspects discussed in this work include changes in the MCM-41 textural properties

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after copper modification, and chemical nature - localization of the metallic species on the support after

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desorption and calcination of the samples. Moreover, some of the synthetized solids were tested as catalysts

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in the liquid phase oxidation of limonene with hydrogen peroxide (H2O2), an important reaction in fine

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chemistry.

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Limonene is a low toxicity monoterpene that occurs naturally as a by-product in citrus processing,

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therefore it is considered a renewable raw material that constitutes a sustainable alternative to produce

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value-added oxygenated compounds. The latter are then used in the synthesis of fine chemicals employed to

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industrially produce resins and fragrances [18]. Often, liquid phase oxidation of limonene can be performed

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with a wide variety of oxidants; among the most used are hydrogen peroxide, t-butyl hydroperoxide and

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molecular oxygen [19-27]. Out of these three, the advantages offered by hydrogen peroxide such as low

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cost, ease of handling, innocuousness of byproducts formed in the reaction medium and high percentage of

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active oxygen, make it an outstanding oxygen donor [28,29]. However, H2O2-based oxidations usually need

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a catalyst to achieve good reaction rates and yields. Acceptable limonene conversions and selectivities to

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several interesting oxidation products have been obtained with copper modified materials, mainly supported

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complexes anchored on the surface of various supports. Modi et al. [30] synthetized Cu(II) Schiff base

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complexes entrapped into the supercages of a zeolite-Y. Then, the catalytic oxidation of limonene was

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carried out for 12 h using H2O2 as oxidant in a 1:1 molar ratio with the olefin. Similarly, Islam and others

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[31] found that a polymer supported Cu(II) Schiff base complex catalyzed limonene oxidation in 6 h

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employing a limonene/ H2O2 molar ratio of 1:2. However, the complex steps associated with the synthesis of

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the above mentioned materials imply a disadvantage for the limonene oxidation process in general and

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discourage the solids use as catalysts. Therefore, the straightforward modification of MCM-41 type supports

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by the TIE method represents an attractive synthesis methodology due to its simplicity and lower cost.

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Hence, in this particular work, we focus on analyzing the impact of: 1) stirring time between the as

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synthesized MCM-41 and the metal precursor solution and 2) the influence of hydrothermal treatment, over

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the physicochemical properties of Cu-MCM materials synthesized by TIE. The study of both variables is

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relevant since it could help to reduce energy requirement during catalyst synthesis. Furthermore, the use of

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copper to create active reaction sites in the final material is linked to high availability, low toxicity and

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inexpensiveness of the transition metal in question. Thus, the use of copper modified and TIE based MCM-

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41 catalysts presents practical advantages and therefore contributes to the development of a simple process

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for the heterogeneous catalytic oxidation of limonene.

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2. Experimental

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2.1 Synthesis

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The metal-free MCM-41 was synthesized employing the method previously reported by us elsewhere

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[32]. The pure siliceous matrix was modified with different copper contents (1, 2 and 5 wt.%) employing the

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template ion-exchange method. The support functionalization process consisted of stirring 3.16 g of as-

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synthesized MCM-41 with Cu(NO3)2.2.5H2O (J.T.Baker 99,7%) water solution at room temperature for 1 h,

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8 h or 20 h. Then the solids were filtered, dried at 333 K overnight and desorbed at 773 K in N2 atmosphere

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for 6 h. This procedure was followed by calcination in air for another 6 h at the same temperature. In order

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to evaluate the effects of hydrothermal treatment, the mixture stirred for 1 h at room temperature was later

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placed in a steel reactor internally lined with Teflon at 353 K for 20 h. After filtering the material, the same

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procedure carried out for the non-treated samples was followed. All solids were referred to as Cu-TIE(X)x-

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y, where “X” indicates the solids nominal metallic loading in wt.%, “x” denotes stirring time at room

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temperature (in hours) and “y” represents hydrothermal treatment time (in hours).

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2.2 Characterization

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The XRD (X-Ray Diffraction) patterns at low and high angles were recorded in a PANalytical X-Pert

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Pro Diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the range of 2° - 8° and 20° - 80°. The a0 lattice

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parameter was calculated employing the following equation: a0 = (2/√3)d100, where d100 indicates the

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distance between the structure (100) planes estimated employing the angle of the first signal observed in the

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diffraction pattern of the samples. To calculate the main crystallite size [D], a fitting was applied to both

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maxima found in the high angle diffraction pattern of each sample. Then using the peak width at half

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maximum after instrumental contribution subtraction (β), the x-ray wavelength (λ) and the diffraction angle

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in radians (θ), Scherrer equation was applied: [D] = 0.9α/βcosθ. All samples were degassed for 12 h at 573

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K and then N2 adsorption-desorption isotherms data was obtained with a Micromeritics ASAP 2000 at 77 K.

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The specific surface was calculated employing the BET method, VTP (total pore volume) was estimated at

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0.98 relative pressure (Gervich rule), and VPMP (primary mesoporous volume) was calculated by the αS-plot

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method using as reference isotherm the standard LiChrospher for silicas. The PSD (pore size distributions)

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were determined by Non-Local Density Functional Theory (NLDFT) (adsorption branch, N2 kernel at 77 K on

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silica for cylindrical pores). The metal content in the final solids was estimated by AA (Atomic Absorption

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Spectroscopy) using a Shimadzu Model AA7000. SEM (Scanning Electron Microscopy) images were

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obtained in a Hitachi TM-1000 with a 15 kV acceleration voltage. XPS (X-Ray Photoelectron Spectroscopy)

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experiments were performed on a Multi-technique SPECS dual X-Ray source Mg/Al spectrometer model

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XR50 equipped with a hemispherical fixed transmission mode analyzer (FAT) 150 PHOIBOS. Before

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analyzing, all solids were submitted to 473 K and vacuum (10-2 mbar) for 10 minutes then ultra-high

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vacuum was applied for two hours. Spectra were gathered with a 30 eV power passage and a Mg anode

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operated at 200 W under pressure lower than 1.10-9 mbar. UV-vis DR (UV-vis Diffuse Reflectance Spectra)

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were recorded in air at room temperature using a Jasco V-650 spectrometer with an integrated sphere in the

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range of 200 nm to 900 nm. TPR (Temperature Programmed Reduction) profiles were collected in a

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ChemiSorb 2720. All materials were heated in N2 atmosphere at 423 K for 30 minutes before the analysis. In

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these experiments, the samples temperature raised from 298 K up to 973 K at a 10 K/min rate, using a

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mixture of 5% H2/N2 as carrier gas (20 ml/min).

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2.3. Catalytic experiments

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The catalytic experiments of limonene oxidation with hydrogen peroxide (H2O2) were performed in a

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batch system. A typical reaction consisted of adding acetonitrile (AcN, Sintorgan 99.5%) (solvent),

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limonene (Lim, R(+)-Fluka 98%) (substrate), H2O2 (Cicarelli 30% in water) (oxidant), and the Cu-TIE(X)x-

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y (catalyst) into a glass round bottom flask connected to a reflux condenser. Standard reaction conditions

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were: Lim/H2O2 molar ratio = 2:1, AcN/Lim molar ratio = 15:1, total reaction volume = 7 mL, catalyst mass

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= 14.3 g/L, temperature = 343 K and total reaction time = 5 h. At different time intervals, 0,1 mL aliquots of

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reaction medium were extracted through a reactor lateral tabulation. All samples were filtered to eliminate

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the catalyst and immediately analyzed by gas chromatography (GC). The chromatographic analyses were

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carried out employing an Agilent chromatograph model 7820 with two FID detectors and a HP-1 capillary

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column. The area normalization method, employing response factors estimated from calibration curves, was

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used to calculate each component percentage in the reaction sample. H2O2 consumption was determined by

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iodometric titration. Conversion of limonene and oxidant were expressed as the ratio of converted species to

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initial concentration. Selectivity to products and catalyst turnover number (TON) were calculated as (moles

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of product/converted limonene) x 100 and (moles of converted limonene/moles of metal in the catalyst),

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respectively. H2O2 efficiency was calculated as the percentage of converted oxidant to total oxidized

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products.

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3.1 Material characterization

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Figure 1A shows the low angle XRD patterns for the calcined support and the rest of the synthesized

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materials modified with a nominal copper content of 2 wt.%. All samples exhibit the typical diffraction

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pattern of an MCM-41 material. The intense signal at 2θ between 2.33º and 2.55º for each sample can be

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assigned to the (100) reflection. Meanwhile the other two low intensity signals between 4º and 5º can be

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indexed to the reflections of the planes (110) and (200) respectively. The reflections observed for all the

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solids are indicative of a long-range well-ordered MCM-41 type structure, which is not strongly affected by

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copper incorporation into the matrix [33]. Nevertheless, it is worth mentioning a slight widening of the main 6

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peak for the Cu-TIE(2%)1-20 sample which could give account for a certain loss of structural order,

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probably due to the thermal treatment carried out during synthesis [34]. The high angle XRD patterns of the

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same samples are shown in Figure 1B. At 2θ ~ 25°, all solids show the typical wide signal corresponding to

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amorphous silica [35]. Moreover, for all Cu-TIE materials two peaks at 2θ ≅ 35.4º and 38.5º can be

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observed, which indicate the presence of copper(II) oxide (CuO) nanoparticles formed on the external

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surface of the support [36]. The mean CuO crystallite size for all the copper modified samples was 16 ± 3

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nm, estimated by the Scherrer equation. Although these oxide nanoparticles exist on the solid surface, it is

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not possible to exclude the presence of other types of metallic species such as finely dispersed nanoclusters

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or isolated cations linked to network oxygens. Signals corresponding to copper(I) oxide (Cu2O)

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nanoparticles were not detected for any of the synthesized solids.

The N2 adsorption-desorption isotherms at 77 K of the calcined materials and their corresponding

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PSD are shown in Figure 2A and 2B, respectively. For comparison of the different materials, the isotherm

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labeled as “e” has been vertically offset and all the PSD curves have been shifted in the y-axis. In addition,

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Table 1 collects the chemical composition as well as textural and structural parameters of each sample. All

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solids exhibit type IV isotherms according to IUPAC which are characteristic of defined mesoporous

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structures, with a pronounced adsorption increase at relative pressures around 0.2-0.35, typical of materials

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with capillary condensation inside the mesopores [37]. Such condensation steps are in agreement with the

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narrow unimodal pore size distributions observed in Figure 2B, which correspond to well-ordered solids

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with uniform pore arrangement [38]. As it can be noted, the position of the isotherm increase for the

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hydrothermally treated sample is slightly displaced towards higher p/p0 values, indicating a slight increase in

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the pore size (Table 1), also depicted in Figure 2B. In addition, all materials present isotherms with

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hysteresis loops type H4 formed by parallel and almost horizontal adsorption and desorption branches. This

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feature is frequently related with the presence of slit type pores [39]. Moreover, the quasi plateau and the

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small hysteresis loop observed for the support and the non-treated samples, after the capillary condensation

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step, suggests that the presence of secondary mesoporosity is low for these samples. On the other hand, Cu-

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TIE(2%)1-20 isotherm exhibits a pronounced hysteresis loop with a sharp decrease in the desorption branch

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at p/p0 between 0.45 and 0.5 related to the existence of ink-bottle pores [40]. This type of porosity might be

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originated through the rupture of thin pore walls during hydrothermal treatment, generating slightly larger

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cavities eventually interconnected through necks [10]. Nevertheless, a percolation effect associated with

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pore blocking cannot be discarded. The sharp increase observed in the adsorption branch at high relative

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pressures may be associated with the presence of structural defects favored by the synthesis conditions, or a

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certain degree of secondary mesoporosity generated also by hydrothermal treatment in the same way as

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explained for ink-bottle pores [34]. This is also in accordance with that observed in the low angle XRD

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pattern of the treated solid.

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For all samples, the calculated specific surface and total pore volume values are in the characteristic

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range for mesoporous materials [41]. All copper-modified solids have a very similar metallic content, which

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may be slightly raised by the increasing mixing time at room temperature. The materials without

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hydrothermal treatment present area values close to that of the matrix. Meanwhile, Cu-TIE(2%)1-20 exhibits

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a minor decrease in its specific area which may be related to the lower structural order evidenced for this

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sample. Additionally, the wall thickness of all solids remains constant even after copper incorporation into

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the matrix, this would account for the dispersion of metal species within the pores of the mesoporous

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structure.

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Figure 3 shows the SEM images for the Cu-TIE(2%)1-0 material (selected as representative of the

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non-treated solids) and the Cu-TIE(2%)1-20 sample. In both cases, the presence of smaller particles

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clustered to form bigger aggregates of several sizes can be observed. This results in a sponge-like

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morphology in the micrometers range with a high proportion of segregated fragments [33,42].

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Information about the oxidation state of copper in the synthesized solids was obtained employing

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XPS. Figure 4 shows the experimental x-ray photoelectron spectrum of Cu-TIE(2%)1-0 sample, taken as

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representative of the non-hydrothermally treated solids. As it can be observed, two spin-orbit components

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are present, Cu2p3/2 and Cu2p1/2, with maxima at binding energy values of 933.6 eV and 953.4 eV,

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respectively. Both signals are symmetrical and exhibit satellites depicted in the same figure. The binding

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energy value found for Cu2p3/2 and the presence of the shake-up peak are characteristic of Cu2+ species.

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Furthermore, the corresponding Auger spectrum (presented as an inset in Figure 4) shows a CuLMM signal

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centered at about 917.7 eV. This value corresponds to the Auger kinetic peak associated with copper in a +2

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oxidation state [43,44]. These results show that copper is mainly present as Cu2+ species at the surface of the

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non-treated materials. A similar analysis was previously performed by us for the hydrothermally treated

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sample [32].

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Figure 5 shows the UV-vis DR spectra of the synthesized materials, which were recorded to

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understand the chemical environment of the developed copper species as a function of the synthesis

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conditions. Intensities have been normalized in order to compare the samples absorption profiles. As

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discussed by us elsewhere [10,32], the corresponding spectra show different absorption regions that

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originate due to the presence of different metallic species. From smaller to longer wavelengths it is possible

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to assign three absorption bands which correspond to mononuclear cations Cu2+ in coordination with oxygen

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from the network, small copper oxides species such as nanoclusters (CuO)n probably located inside the

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mesoporous channels and CuO nanoparticles segregated from the siliceous structure (as the ones detected by

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XRD), respectively. As it can be observed, the Cu-TIE(2%)1-20 sample exhibits increased UV absorption in

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the second and third region of its spectrum. This fact is consistent with the presence of metal oxide species

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in higher proportion. Therefore, although the synthesized materials have a similar final metallic content,

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hydrothermal treatment could favor the formation of nanoclusters or nanoparticles on the support surface.

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Moreover, different mixing times at room temperature did not influence on the nature of copper species

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developed on the siliceous structure.

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To evaluate the reducibility of the different metallic species present in the materials, the samples

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were characterized by TPR. The corresponding profiles are shown in Figure 6. In order to analyze these

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profiles, it is important to take into account the general dependence of the reduction temperature on the

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particle size, as well as the degree of interaction that these species have with the siliceous structure.

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Depending on the TPR experimental parameters, which have shown to heavily influence reduction

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temperature, size or interaction can have a greater influence on the obtained results [45]. It is known that

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bulk CuO is often directly reduced to metallic Cu without going through an intermediate phase such as Cu2O

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or Cu4O3. Under the employed reduction conditions all copper-modified materials presented a single H2

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consumption peak at low temperature, which is consistent with reduction of oligonuclear clusters and

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nanoparticles from Cu2+ to Cu0 [46,47]. It is important to highlight that the reduction temperature of the

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oxide species present in all Cu-TIE samples is lower than that of pure CuO (≅ 640 K). This effect is related

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to the small size of CuO nanoparticles and nanoclusters, which makes them more easily reducible than bulk

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copper(II) oxide [48]. Additionally, Cu-TIE(2%)1-20 solid reduction signal presents a small shift of the

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maximum to higher temperature values. This could give account for metallic species of slightly larger size

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present in the sample as seen by UV-vis [49]. It should be clarified that a small contribution of the isolated

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ions reduction (Cu2+

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symmetry and the maximum reduction temperature close to that of the supported CuO/SiO2, allow us to

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infer that the obtained H2-consumption signals correspond mainly to reduction of Cu2+ species to Cu0 easily

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available on the support surface like nanoclusters and nanoparticles.

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Cu+) to the samples TPR profiles cannot be discarded [50-52]. However, the peaks

Table 1 shows the results of integrating the area under the TPR signals (in arbitrary units) for each

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sample. This allows us to compare roughly H2 consumption employed in the reduction of the same mass of

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different materials. For the hydrothermally treated solid, H2 consumption is markedly higher. This effect

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may indicate a higher proportion of nanoclusters and/or nanoparticles present in the sample, which is in

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agreement with the observations made by UV-vis DR analysis. Taking into account that the samples copper

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content is similar; the variation in the observed signal intensity would be indicating that the Cu2+ isolated

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cations are not contributing to the reduction signal. Therefore, the isolated ions would be strongly stabilized

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by network oxygen atoms, which makes them resistant to reduction under the applied conditions [42].

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The different characterization techniques show that the time of mixing at room temperature has no

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significant influence over the structural characteristics of the final solid or the nature of the copper species

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developed on the siliceous structure. On the other hand, hydrothermal treatment produces a certain degree of

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structural distortion and species sintering, generating nanoclusters and nanoparticles. Moreover, a study on

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energy consumption involved in the different stages of the Cu-TIE(X)1-20 materials synthesis showed that

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by suppressing hydrothermal treatment, it is possible to save an estimate 10% of the total energy

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requirement for the synthesis process. At the same time, this results in saving around 2.5 dollars per 100 g of

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produced SiO2 and it leads to a 30% reduction in total synthesis time (for calculations details, see Appendix

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A). These findings represent significant improvements on the synthesis of copper nanostructured materials

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modified by template-ion exchange. Thus, it is interesting to evaluate the effect of metallic content over the

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physicochemical properties and catalytic activity of non-treated materials. Therefore, two other copper

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modified solids were synthesized without hydrothermal treatment and employing one mixing hour at room

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temperature. The samples were then characterized and catalytically tested in the limonene oxidation

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reaction.

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Figure 7 shows the XRD patterns at low and high angles for the samples Cu-TIE(1%)1-0 and Cu-

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TIE(5%)1-0. Both solids exhibit a highly ordered mesoporous structure with hexagonal pore array,

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evidenced by the well-defined signals corresponding to the planes (100), (110) and (200). The high angle

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XRD patterns (inset in Figure 7) show the main peaks related to the presence of CuO particles. The relative

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intensities of these peaks indicate that a higher copper loading favors agglomeration of nanoclusters and

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smaller particles into bigger ones, even though the samples have been synthesized without hydrothermal

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treatment. The presence of Cu2O in the solids has been discarded based on the results obtained from

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previous analyses presented in this work.

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N2 adsorption-desorption isotherms and the pore size distribution of Cu-TIE(1%)1-0 and Cu-

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TIE(5%)1-0 samples are collected in Figure 8A and 8B, respectively. Table 1 summarizes the textural

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properties and chemical composition of these samples, as well as the support used as a reference. The shape

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of both isotherms indicates the presence of ordered mesoporous structure. For Cu-TIE(1%)1-0, the

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corresponding isotherm presents similar characteristics to those of the non-treated Cu-TIE(2%) materials.

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However, for the sample Cu-TIE(5%)1-0 less amount of nitrogen is adsorbed (Table 1), which may be

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related to a decrease in the solid structural order or the presence of bigger oxide nanoparticles that may be

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blocking some of the structure mesopores. Moreover, the adsorption branch increase at high p/p0 observed

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for this material could be associated with secondary mesoporosity generated by agglomeration of the CuO

22

nanoparticles on the support surface due to high metal content [53]. This effect is also depicted in the

23

corresponding PSD. Likewise, Table 1 also shows that high metal content may result in wall thickness

24

increase.

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UV–Vis DR spectra of the Cu-TIE(1%)1-0 and Cu-TIE(5%)1-0 materials are shown in Figure 9.

26

The spectrum corresponding to Cu-TIE(2%)1-0 (previously shown) has been also included in this figure as

27

reference. The three solids exhibit an intense absorption band in the UV region, which can be divided into 11

1

three regions assigned to Cu2+ isolated cations, (CuO)n nanoclusters and CuO nanoparticles, as mentioned

2

above. As it can be seen, the spectrum intensity increases with increasing Cu loading due to a higher total

3

absorption in the UV range. When comparing the UV-vis spectra of Cu-TIE(1%)1-0 and Cu-TIE(5%)1-0

4

samples with those of the corresponding solids synthesized employing hydrothermal treatment (inset in

5

Figure 9), opposed to Cu-TIE(2%)1-0 and Cu-TIE(2%)1-20, it is found that the spectra are very similar to

6

each other in both cases. For Cu-TIE(1%)1-0 and 1-20, this fact can be attributed to the low metallic content

7

in the final solid which causes high dispersion of Cu species on the support surface, despite hydrothermal

8

treatment application. On the other hand, for Cu-TIE(5%)1-0 and 1-20, since the copper content in the final

9

solid is high, the support surface becomes saturated and species agglomeration occurs even when no

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hydrothermal treatment is applied.

Figure 10 shows the TPR profiles for Cu-TIE(1%)1-0 and Cu-TIE(5%)1-0. Once more, Cu-

12

TIE(2%)1-0 is included in the same figure as reference. The highest Cu loading sample presents the same

13

reduction profile that Cu-TIE(2%)1-0 which presents one signal corresponding to (CuO)n/CuO

14

process at 549 K. Additionally, H2 consumption for this sample is notably higher, which is related to the

15

metallic content present in the final solid. On the other hand, although Cu-TIE(1%)1-0 exhibits a similar

16

reduction temperature profile to that of the other samples, two characteristics must be highlighted: (1) a

17

shifting of the maximum to higher temperatures values, which would give account for a hindered reduction

18

of smaller size nanoclusters interacting more strongly with the support, and (2) the presence of a shoulder at

19

550 K, which indicates the existence of a smaller amount of nanoclusters and nanoparticles whose size is

20

similar to that of the other two samples. This behavior is also in accordance with the low copper content of

21

this sample, which reflects a decrease in amount and reducibility of copper oxides.

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3.2. Catalytic evaluation

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In order to analyze the catalytic activity of the synthesized materials, Cu-TIE(1%)1-0, Cu-TIE(2%)1-

25

0 and Cu-TIE(5%)1-0 were applied in the limonene oxidation reaction employing hydrogen peroxide (H2O2)

26

as oxidant. This reaction has been previously reported by us using Cu-TIE(1%)1-20, Cu-TIE(2%)1-20 and

27

Cu-TIE(5%)1-20 as catalysts [32]. However, based on the material characterization presented in this paper, 12

1

it is of interest to study the performance of the non-treated solids in the oxidation of limonene, and to

2

compare the obtained results with those found for treated samples.

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Before analyzing the catalytic reactions results, we must consider all processes occurring in the

4

liquid-phase oxidation of limonene employing H2O2 as oxygen donor and copper modified MCM-41

5

materials as catalysts. The products obtained from limonene oxidation are shown in Scheme 1. At the

6

beginning of the reaction, Cu-TIE materials activate the oxidant generating free radicals by means of a redox

7

process, where the oxidation state of copper varies between +2 and +1. Product (1) can be generated through

8

direct oxygen transfer and a radical pathway. Species (2) and (3) are formed by over-oxidation and

9

hydrolysis of (1), respectively. Additionally, products (4) to (7) are formed through radical pathways.

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A blank experiment was performed in order to assess reaction progress in absence of catalyst. In this

11

case, limonene conversion was low (1 mol%), being limonene oxide (1) and carveol (4) the main reaction

12

products. Hence, the presence of the Cu-TIE solid is essential to start the reaction. Figure 11 shows TON

13

evolution over reaction time employing Cu-TIE(1%)1-0, Cu-TIE(2%)1-0 and Cu-TIE(5%)1-0, under

14

standard reaction conditions. The highest activity of the sample with lower Cu content relates to a good

15

dispersion of the metal species on the support [54]. Table 2 presents limonene conversion and TON values

16

at 5 hours of reaction for Cu-TIE(1%), Cu-TIE(2%) and Cu-TIE(5%), with and without hydrothermal

17

treatment. The molecular sieves modified with the lowest and highest metallic content exhibit a very similar

18

activity, which is in accordance to the characterization results. Cu-TIE(2%)1-0 sample shows a somewhat

19

higher TON value to that observed for Cu-TIE(2%)1-20 at 5 hours. This fact may be related to a better

20

dispersion of the active sites on the material support as seen from UV-vis DR. Moreover, considering that a

21

2:1 limonene/H2O2 molar ratio results in a maximum olefin conversion of 50 mol%, substrate conversion

22

values obtained with Cu-TIE(X)x-y solids under standard conditions are comparable with those achieved by

23

Modi et al. [30]. However, in their work, a greater amount of H2O2 was used (limonene/H2O2 1:1) and

24

reaction time was almost twice as long. In the same way, Islam [31] reached conversion values around 56

25

mol% after 6 hours, meanwhile the amount of oxidant involved doubled that of the olefin.

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Another interesting result is related to values of oxidant conversion and efficiency at 5 h of reaction,

27

presented also in Table 2. It is a known fact that H2O2 decomposes into water and molecular oxygen in the 13

1

reaction medium. This behavior is promoted by the presence of nanoclusters and oxide species deposited on

2

the support surface [55]. For both families of Cu-TIE samples (treated and non-treated), oxidant conversion

3

increases with copper content, whereas efficiency follows an opposite trend. On one hand, treated and non-

4

treated Cu-TIE(1%) exhibit similar values of oxidant conversion and efficiency, due to their similar species

5

distribution and comparable limonene conversion. Same is the case for treated and non-treated Cu-TIE(5%).

6

On the other hand, non-treated Cu-TIE(2%) presents a slightly higher H2O2 conversion than treated Cu-

7

TIE(2%) related to a higher limonene conversion. Furthermore, oxidant efficiency is also increased for this

8

sample, which is in agreement with a lower proportion of nanoclusters and oxide particles. Therefore, less

9

unproductive oxidant decomposition is observed.

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From Scheme 1, species (1), (2) and (3) are considered epoxidation products, meanwhile species (4)

11

to (7) are classified as allylic products. Epoxidation and allylic oxidation are competitive processes in olefins

12

oxidation and often both occur simultaneously giving a mixture of reaction products [56]. In our previous

13

work [32] it was found that the nature and distribution of the species present in the catalyst influence

14

reaction selectivity. Isolated Cu2+ cations favor epoxide selectivity, whereas the presence of nanoclusters

15

and bigger copper oxide species tend to increase selectivity to allylic products. Table 2 gathers reaction

16

selectivity results at 5 h, obtained from application of the Cu-TIE materials as catalysts. As expected from

17

the UV-Vis analyses performed, the similar species distribution found for the treated and non-treated Cu-

18

TIE(1%) samples, as well as for treated and non-treated Cu-TIE(5%) solids, result in a very similar catalytic

19

behavior with almost the same percentages of epoxide and allylic products in each case. Cu-TIE(2%)1-0

20

sample allows obtaining a slightly higher percentage of epoxide products than Cu-TIE(2%)1-20, which

21

would be related to the lower proportion of nanoclusters and bigger oxide particles found in the non-

22

hydrothermally treated sample.

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4. Conclusions

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This work evaluated the influence of mixing time at room temperature and the effect of hydrothermal

26

treatment in the synthesis of copper modified mesoporous materials employing template-ion exchange. On

27

one hand, the obtained results indicate that different mixing times at room temperature had no significant 14

1

impact on the final solid structure, nor on the chemical nature of the metallic species formed. On the other

2

hand, x-ray diffraction and N2 adsorption-desorption isotherms of the Cu-TIE(2%) samples showed that

3

hydrothermal treatment produces a certain degree of structural distortion in the final solid. Moreover, UV-

4

vis and TPR studies indicate that a greater amount of oxide species (nanoclusters and nanoparticles) seem to

5

be favor by hydrothermal treatment application during the synthesis process. The presence of isolated Cu2+

6

cations in coordination with network oxygen was detected for both, the non-treated and the hydrothermally

7

treated solids. Other materials, modified with copper nominal contents of 1% and 5% were synthetized

8

employing 1 h mixing time at room temperature without hydrothermal treatment. These two solids were

9

compared structurally and chemically with hydrothermally treated materials of the same nominal metallic

10

content. For the lowest and highest copper contents it was found that the species distribution on the support

11

surface was the same. Cu-TIE(1%)1-0, Cu-TIE(2%)1-0 and Cu-TIE(5%)1-0 were evaluated in the liquid

12

phase oxidation reaction of limonene with H2O2 at 343 K. The results were compared with those of the

13

treated materials. Treated and non-treated solids for 1% and 5% contents exhibited no significant differences

14

in their catalytic behavior. Cu-TIE(2%)1-0 presented a slightly higher limonene and oxidant conversion than

15

CuTIE (2%)1-20. The same was observed for H2O2 efficiency and selectivity to epoxidation products. This

16

is probably related to an improved distribution of the metallic species present in the non-treated material.

17

Therefore, we propose a simple TIE procedure by which highly ordered copper modified mesoporous

18

materials can be synthesized. These solids present comparable catalytic activity to that of samples

19

synthetized by a TIE method employing hydrothermal treatment. We have demonstrated also that by

20

suppressing hydrothermal treatment it is possible to save almost 10% of the total energy requirement for the

21

synthesis process. At the same time, this results in saving around 2.5 dollars per 100 g of produced SiO2 and

22

it leads to a 30% reduction in total synthesis time.

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5. Appendix A

25

The stages of higher energy requirement involved in the synthesis process of Cu-TIE(X)1-20

26

materials are listed in Table A.1. The cost associated with each stage was estimated considering the

27

corresponding electric company fee. 15

Table A.1. Stages of higher energy requirement involved in the synthesis process of Cu-TIE(X)1-20 ACCEPTED MANUSCRIPT materials. Stage a MCM-41 synthesis

1- Mixing: TEOS + CTAB (400 rpm) at room temperature (4 h) 2- Mixing TEOS + CTAB (400 rpm) at 343 K (3 h) 3- Heating at 343 K (3 h) Cu-TIE(X)1- 4- Mixing: MCM-41 + Cu(NO3)2 (400 20 synthesis rpm) at room temperature (1 h) 5- Hydrothermal treatment at 353 K (20 h) 6- Calcination and desorption at 773 K (12 h)

Magnetic Stirrer

0.04

0.0024

Digital Controller

0.65

0.0445

Magnetic Stirrer

0.01

0.0007

Electric Furnace Electric Furnace

0.53

0.0368

4.03

0.2780

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Energy, cost and time savings were calculated according to Eq. (A.1), (A.2) and (A.3): ∑

Total energy

Total energy

Total cost $ )= ∗ 100 g SiO( Mass SiO(

× 100 × 100

(*1) A typical batch produces 1.5 g of SiO2. Time saving % =

Time × 100 Total synthesis time ∗(

(A.1) (A.2)

(A.3)

(*2) Stages 1 - 6 of the synthesis process and 24 additional hours of overnight drying.

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Overnight drying for MCM-41 and Cu-TIE(X)1-20 (as synthetized) are not considered as their energy requirement is negligible compared to that of the other stages. b Measurement results. c Total Cost ($) = Total energy (kWh) x Electric Company Fee ($/kWh).

Cost saving $

9

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a

Energy saving % =

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Total cost ($) c

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Total energy (kWh) b

Equipment

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Information about the measuring equipment used is detailed below. Additionally, a diagram of the process for data acquisition is presented in Scheme A.1.

12

Measuring equipment: Ecamec Power Quality Analyzer model ECA-PQ4. Experimental data was processed

13

employing Ecamec 2,0 system. In each case, the obtained reports meet the standards requested by EN50160

14

y IEEE1459-2000.

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Scheme A.1. Diagram of the process for data acquisition.

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2 3 6. Acknowledgements

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The authors would like to thank Consejo Nacional de Investigaciones Científicas y Técnicas

6

(CONICET) and Universidad Tecnológica Nacional (Facultad Regional Córdoba) – Argentina, for their

7

financial support, as well as Instituto de Catálisis y Petroleoquímica (ICP), Consejo Superior de

8

Investigaciones Científicas (CSIC) – España, for the SEM images. The authors would also like to extend

9

their gratitude to Dr. Vaschetti Jorge C. for the energy requirement measurements and to Dra. Carraro Paola M. and Dra. Elías Verónica R. for their valuable contributions.

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[49] S. Velu, L. Wang, M. Okazaki, K. Suzuki, S. Tomura, Micropor. Mesopor. Mat. 54 (2002) 113–126.

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[50] S. Kieger, G. Delahay, B. Coq, B. Neveu, J. Catal. 183 (1999) 267–280.

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https://doi.org/10.1006/jcat.1999.2398.

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Chem. Mater. 17 (2005) 6117–6127. https://doi.org/10.1021/cm051120e.

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Perspectives, Wiley-VCH, USA, 2016.

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Tables

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Table 1: Textural properties and chemical composition of the MCM-41 and Cu-TIE(X)x-y materials.

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MCM-41 Cu-TIE(2%)1-0 Cu-TIE(2%)8-0 Cu-TIE(2%)20-0 Cu-TIE(2%)1-20 Cu-TIE(1%)1-0 Cu-TIE(5%)1-0

Cu (wt.%) a 1.52 1.58 1.64 1.45 0.83 3.59

SBET (m2/g) b 1190 1170 1160 1140 1030 1180 1020

VTP (cm3/g) c 0.57 0.83 0.68 0.82 0.95 0.89 0.95

VPMP (cm3/g) d 0.54 0.75 0.62 0.72 0.75 0.79 0.63

wp (nm)

e

3.5 3.5 3.5 3.5 3.8 3.6 3.7

ao (nm) f

tw (nm) g

4.1 4.2 4.1 4.2 4.4 4.2 4.6

0.6 0.7 0.6 0.7 0.6 0.6 0.9

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ATCD (u.a.) h 0.7 0.7 0.8 2.4 -

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a

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Table 2: Catalytic performance of Cu-TIE(X)x-y samples in Limonene oxidation with H2O2 under standard conditions.

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TON 43.7 25.9 12.8 43.4 31.8 12.3

H 2O 2 Conversion (mol%) 50.4 77.5 100.0 48.9 88.0 94.8

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Cu-TIE(1%)1-20 Cu-TIE(2%)1-20 Cu-TIE(5%)1-20 Cu-TIE(1%)1-0 Cu-TIE(2%)1-0 Cu-TIE(5%)1-0

Limonene Conversion (mol%) 8.9 8.5 10.3 8.5 10.8 10.0

H 2O 2 Efficiency

Epoxidation Selectivity

Allylic Selectivity

17.6 10.9 10.1 17.5 12.3 10.5

55.6 58.6 47.6 55.8 63.2 46.1

44.4 41.4 52.4 44.2 36.8 53.9

Reaction conditions: Lim/H2O2 molar ratio = 2, AcN/Lim molar ratio = 15, Cu-TIE(X)x-y = 100 mg, temperature = 343 K, solvent: AcN, reaction time = 5 h.

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In the final solid, determined by AA. Determined by BET. c Total pore volume obtained at p/p0 = 0,98. d Primary mesopores volume calculated by the αS-plot method. e wp: pore width. f a0 = (2/√3)d100 g Wall thickness: tw = a0 - wp h Area under the curve of the H2-TPR signals. b

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Figure captions

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Figure 1. X-Ray Diffraction patterns at (A) low angle, (B) high angle. (a) MCM-41, (b) Cu-TIE(2%)1-0, (c)

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Cu-TIE(2%)8-0, (d) Cu-TIE(2%)20-0, (e) Cu-TIE(2%)1-20, (*) CuO.

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Figure 2. (A) N2 adsorption-desorption isotherms at 77 K, (B) Pore size distribution. (a) MCM-41, (b) Cu-

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TIE(2%)1-0, (c) Cu-TIE(2%)8-0, (d) Cu-TIE(2%)20-0, (e) Cu-TIE(2%)1-20 (“e” vertically offset by 50.6

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cm3.g-1).

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Figure 3. Scanning Electron Microscopy of: (a,b) Cu-TIE(2%)1-0 and (c,d) Cu-TIE(2%)1-20.

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Figure 4. Cu 2p core level photoelectron profile of Cu-TIE(2%)1-0. Inset: Auger kinetic energy of Cu-

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TIE(2%)1-0.

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Figure 5. UV-Vis DR spectra of the materials: (a) MCM-41, (b) Cu-TIE(2%)1-0, (c) Cu-TIE(2%)8-0, (d)

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Cu-TIE(2%)20-0, (e) Cu-TIE(2%)1-20.

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Figure 6. TPR profiles of the samples (a) MCM-41, (b) Cu-TIE(2%)1-0, (c) Cu-TIE(2%)8-0, (d) Cu-

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TIE(2%)20-0, (e) Cu-TIE(2%)1-20. Inset: TPR profile of CuO.

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Figure 7. X-Ray Diffraction patterns at low angle and high angle (Inset) of the samples: (a) Cu-TIE(1%)1-0

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and (b) Cu-TIE(5%)1-0, (*) CuO.

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Figure 8. (A) N2 adsorption-desorption isotherms, (B) Pore size distribution. (a) Cu-TIE(1%)1-0 and (b)

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Cu-TIE(5%)1-0.

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Figure 9. UV-Vis spectrum of the samples: (a) Cu-TIE(1%)1-0, (b) Cu-TIE(2%)1-0 and (c) Cu-TIE(5%)1-

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0. Inset: UV-Vis spectrum comparison for Cu-TIE(1%) and Cu-TIE(5%), with and without hydrothermal

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treatment.

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Figure 10. TPR profiles of the samples: (a) Cu-TIE(1%)1-0, (b) Cu-TIE(2%)1-0 and (c) Cu-TIE(5%)1-0.

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Scheme 1. Products obtained from Limonene oxidation with H2O2 catalyzed by Cu-TIE(X)x-y.

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Figure 11. TON vs. reaction time: (♦) Cu-TIE(1%)1-0, (■) Cu-TIE(2%)1-0, (▲) Cu-TIE(5%)1-0. Reaction

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conditions: Lim/H2O2 molar ratio = 2, AcN/Lim molar ratio = 15, Cu-TIE(X)x-y = 100 mg, temperature =

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343 K, solvent: AcN, reaction time = 5 h.

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Figure 1.

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Cu LMM

Cusat 2p1/2

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Kinetic Energy (eV)

Cu 2p3/2

Cu 2p1/2 Cusat 2p3/2

960

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Binding Energy (eV) 4 25

930

Figure 5.

(nanoparticles)

510

Bigger Cu oxides

Isolated Cu

Kubelka-Munk (a.u)

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Figure 7.

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Figure 9.

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Kubelka-Munk (a.u.)

Kubelka-Munk (a.u.)

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Cu-TIE(5%)1-20 Cu-TIE(1%)1-0

Cu-TIE(1%)1-20

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Scheme 1.

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ACCEPTED MANUSCRIPT Highlights Cu-MCM-41 solids were successfully synthesized by template-ion exchange.

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Structural/chemical effects of hydrothermal treatment and mixing time were studied.

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Oxidation of limonene with H2O2 was employed to test the selected catalysts.

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Hydrothermal treatment has little influence over the solids catalytic performance.

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A low energy consumption TIE method for Cu-MCM synthesis was proposed.

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