MCM-41 Catalysts on the

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The Influence of Tin Loading and Pore Size of Sn/ MCM-41 Catalysts on the Synthesis of Nopol Daniel Casas-Orozco, Edwin Alarcón, Carlos A. Carrero, Juan M. Venegas, William P. McDermott, Ellen Klosterman, Ive Hermans, and Aída-Luz Villa Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Industrial & Engineering Chemistry Research

The Influence of Tin Loading and Pore Size of Sn/MCM-41 Catalysts on the Synthesis of Nopol Daniel Casas-Orozco,a Edwin Alarcón,a Carlos A. Carrero,b Juan. M. Venegas,c William McDermott,b Ellen Klosterman,b Ive Hermans,b,c and Aída-Luz Villaa*.

a. Universidad de Antioquia, Chemical Engineering Department, Environmental Catalysis Research Group, Calle 70 No. 52-21, Medellín 050010, Colombia b. University of Wisconsin—Madison, Department of Chemistry, 1101 University Avenue, Madison, WI 53706, USA c. University of Wisconsin—Madison, Department of Chemical and Biological Engineering, 1415 Engineering Drive, Madison, WI 53706, USA

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ABSTRACT

In this work, we investigate the influence of the Sn-loading and the pore size of MCM-41 materials on the catalytic nopol production. Sn(IV) was anchored onto MCM-41 by incipient wetness impregnation with metal coverages within 0.01 and 0.5 Sn nm-2 (i.e., below the monolayer content). We provide evidence that at coverages below 0.06 Sn nm-2, Sn(IV) is predominantly present as isolated centers, whereas at higher coverages octahedral and/or oligomeric species are formed which exhibit lower catalytic activity. The rate of nopol production was ten times higher over Sn/MCM-41 than that of analogous Sn silica gel materials. The Turnover Frequency (TOF) features a maximum as a function of Sn coverage between 0.03 and 0.05 Sn nm-2 for Sn/MCM-41 catalysts and 0.15 Sn nm-2 for Sn silica gel materials. These results show that both the metal content and pore size can be tuned to enhance catalytic performance of Sn/MCM-41 materials.

1. INTRODUCTION Sn-containing micro- and mesoporous materials have gained significant attention in the scientific literature because of their catalytic activity in a variety of organic reactions.1,2 It is well-established that the Sn(IV) sites possess Lewis acidic character and can initiate the reaction by polarizing electron-rich groups such as carbonyls and alcohols. These catalysts have been successfully tested in several chemical transformations, such as the Baeyer-Villager oxidation of ketones to lactones

3–5

, and the Meerwein-Ponndorf reduction of aldehydes and ketones.4–6 Also

in biomass upgrading, these catalysts have attained a lot of attention, e.g. for sugars isomerization

7–10

, and the cyclization of citronellal

11,12

. Sn-containing catalysts have been also

used in the cosmetic, fragrance and flavour industry as in the production of the flavour melonal


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through the aldol condensation of 6-methyl-5-hepten-2-one and acetaldehyde to citral, which is subsequently oxidized to melonal.5 Carbon-carbon coupling via Prins condensation has also been carried out with both micro and mesostructured Sn-based materials

13,14

. In particular, the synthesis of nopol, has been

extensively studied with Sn-containing mesoporous catalysts 15–18. This primary terpenic alcohol, obtained by reacting β-pinene - a renewable feedstock from pine trees - and paraformaldehyde (Scheme 1), is used in the synthesis of agrochemicals, fragrances, pharmaceuticals, and in the formulation of household products

19–21

. Traditional methods to synthesize nopol include

autoclaving the reactants between 180 and 200 °C using zinc chloride as a catalyst; another method is refluxing the reactants at 120 °C in the presence of acetic acid to produce nopyl acetate, which can be further saponified to nopol. Nonetheless, these processes are either energy or time-consuming, use large amounts of corrosive chemicals which are hard to separate from the final reaction mixture, or offer low nopol selectivity.22 For these reasons, catalytic routes have been thoroughly investigated, aiming to develop selective and reusable catalysts that facilitate the reaction under mild reaction conditions23,24.

Scheme 1. Reaction scheme of nopol synthesis. Both the amount, as well as the method of tin-incorporation (hydrothermal or post-synthetic) controls the Sn coordination and its dispersion in the framework or on the surface. Several studies have claimed high catalytic activity of isolated Sn(IV) centres, whereas formation of


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stable octahedral Sn sites, resembling the crystalline structure of SnO2, have been recognized as low-activity Sn species

4,25

. This is in line with other investigations on supported metal oxides,

that suggest that the proportion of active and inactive species is a strong function of metal loading 26. Not only the metal content, but also the pore size, exerting both size-exclusion and confinement effects, controls the catalytic activity. The former effect was observed by Corma et al 14, who found an increase in the rate of formation of nopol by 30% when using Sn-MCM-41 instead of Sn-beta catalyst; this difference was attributed to lower steric impediments on Sn/MCM-41 material. On the other hand, when the steric impediments are negligible, pore sizes become important since they allow confinement effects that enhance catalytic activity via Van der Waals interactions with the chemisorbed molecules

6,27,28

. This effect has been observed in

the cyclization of citronellal with Sn-beta and Sn/SBA-15 catalysts 12. Analogously, the pore size played an important role in the Fischer-Tropsch reaction with Ru-based catalysts 29.

Although several studies on Sn-based materials for nopol production have been published, it is still unknown how the metal content in materials prepared by a post-synthetic procedure affects the catalytic activity, especially at low Sn loadings (below 0.12 wt%) and how confinement affects the behaviour of the system with different mesoporous catalysts. We previously reported the influence of the method of Sn incorporation and Sn precursor on MCM-4117,30, the metal used in the catalytic material,17 and the effect of solvent on catalytic activity and selectivity30,31 in the synthesis of nopol. In this work, we aim to elucidate both pore size and metal loading effects on the catalytic synthesis of nopol, by using a series of Sn-based materials synthesized by incipient wetness impregnation. To assess the effect of metal loading, the Sn content was varied


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between 0.12 and 8 wt%; the effect of pore size and pore arrangement was determined by comparing the activity of nopol production over MCM-41 synthesized with two different organic templates, and on amorphous silica.

2. EXPERIMENTAL 2.1. Synthesis of MCM-41 MCM-41 support was synthesized at room temperature following the method reported by Grun et al

32

. Aiming to assess the influence of pore size on catalytic activity, two different organic

templates were used to tune the porous size in the final MCM-41 materials: myristyl trimethyl ammonium bromide (CH3(CH2)13N+(CH3)3 Br–, 99 wt%, Sigma-Aldrich) and hexadecyl trimethyl ammonium bromide (CH3(CH2)15N+(CH3)3 Br–, 99 wt%, Sigma-Aldrich). The template was added to deionized water under vigorous stirring, to yield a 0.055 mol L-1 solution. Subsequently, the pH was adjusted by adding aqueous ammonia (28 - 30 wt. %, EM Science). Afterwards, tetraethyl orthosilicate (TEOS 98 wt. %, Sigma Aldrich) was added drop-wise at 1 mL min-1, using a peristaltic pump. The molar ratio of the final mixture was 1 TEOS: x template: 1.63 NH4OH: 146.64 H2O, where x was 0.138 or 0.127 for C17H38BrN and C19H42BrN templates, respectively. The obtained white suspension was allowed to crystallize for one hour under stirring, filtered and dried overnight at 100 °C. Finally, the material was placed in a muffle furnace, calcined at 1 °C min-1 to 550 °C for 5 h under static air. The synthesized supports were coded as M14 and M16 when C17H38BrN or C19H42BrN were used as templates, respectively. M16 materials were calcined twice to ensure a complete removal of the organic template. For comparison, pristine silica gel (Merck, density: 0.79 mL g-1, surface area: 510 m2 g-1) was used as a support with no further treatment.


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2.2. Synthesis of Sn/MCM-41 and Sn/SiO2 catalysts. Sn was anchored onto MCM-41 and SiO2 via incipient wetness impregnation. In order to determine the wetness point of the supports, ethyl acetate was slowly added to previously dried materials, until the solid was saturated with solvent. The obtained values were 1.6 mL g-1 for silica gel, 1.5 mL g-1 for M14 and 1.8 mL g-1 for M16. Depending on the desired metal loading, a clear solution of SnCl2.2H2O (AlfaAesar) in ethyl acetate (Aldrich, 99.8%) – ranging from 1.55 to 141 mg mL-1 - was added drop-wise to a sample of previously dried support, under nitrogen and with manual homogenization. SnCl4 was not used as a tin precursor due to its instability and hygroscopicity.17 The solution was not completely clear for 3.7 and 7.8 wt% Sn loadings. The obtained materials were dried overnight at room temperature and then calcined as described above. Water was discarded as a solvent for impregnation to prevent it from interacting with silanol groups on the surface of the support, as well as with the tin chloride precursor. Silica catalysts were labeled as Sn/SiO2(x) and MCM-41 catalysts as Sn/M14(x) and Sn/M16(x); where x represents the metal weight percentage, determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).

2.3. Characterization Powder X-Ray Diffraction patterns (XRD) were recorded in a Bruker D8 Advance Difractometer, equipped with a nickel-filtered, monochromatic CuKα radiation source (λ = 1.5406 Å). Surface areas were determined using the Brunauer-Emmett-Teller model (BET) model, and pore sizes were calculated with the Barrett-Joyner-Halenda (BJH) equations, with data from nitrogen physisorption experiments run on a Micromeritics 3-Flex instrument. Samples


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were outgassed in vacuum at 100 °C for 6 hours prior to measurements. Raman measurements were carried out with a Renishaw InVia Raman Spectrometer, equipped with a 785 nm excitation laser. All measurements used a 1200 L mm−1 grating and were taken within 100−1200 cm−1 and a dispersion of 1.36565 cm−1 pixel−1. The experiments were performed in a high-temperature Linkam CCR1000 cell. Samples were dehydrated by heating to 450 °C (10 °C min−1 ramp) under 16 mL min−1 He for 2 h before measurement. UV-vis Diffuse Reflectance Spectroscopy (UVvis-DRS) was performed in a Maya 200 spectrometer (Ocean Optics) equipped with a UV−vis deuterium/halogen light source (DH-2000-BAL from Mikropack) using BaSO4 as a background. All the analyses were carried out inside a glovebox (

MCM-41 Catalysts on the

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