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Quantitative Evaluation of the Effect of the Hydrophobicity of the Environment Surrounding Brønsted Acid Sites on Their Catalytic Activity for the Hydrolysis of Organic Molecules Hiroki Miura, Shutaro Kameyama, Daiki Komori, and Tetsuya Shishido J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11471 • Publication Date (Web): 28 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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Journal of the American Chemical Society

Quantitative Evaluation of the Effect of the Hydrophobicity of the Environment Surrounding Brønsted Acid Sites on Their Catalytic Activity for the Hydrolysis of Organic Molecules

Hiroki Miuraa,b,d*, Shutaro Kameyamaa, Daiki Komoria, Tetsuya Shishidoa,b,c,d*

a

Department of Applied Chemistry for Environment, Graduate School of Urban Environmental

Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan b

Research Center for Hydrogen Energy-based Society, Tokyo Metropolitan University, 1-1

Minami-Osawa, Hachioji, Tokyo 192-0397, Japan c

Research Center for Gold Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa,

Hachioji, Tokyo 192-0397, Japan d

Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Katsura, Nishikyo-

ku, Kyoto 615-8520, Japan

* Corresponding authors: Tel: +81-42-677-2850, Fax: +81-42-677-2850 (T. Shishido) E-mail address: [email protected] (H. Miura) E-mail address: [email protected] (T. Shishido)

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Abstract Sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated to elucidate the effect of the environment surrounding the Brønsted acid site on their catalytic activity for the hydrolysis of organic molecules. A detailed structural analysis of these siloxane gels by elemental analysis, X-ray photoelectron spectroscopy, Fourier-transformed infrared (FT-IR) and 29Si MAS NMR revealed the formation of gel catalysts with a highly condensed siloxane network, which enabled us to quantitatively evaluate the hydrophobicity of the environment surrounding the catalytically active sulfo-functionality. A sulfo group in a highly hydrophobic environment exhibited excellent catalytic turnover frequency for the hydrolysis of acetate esters with a long alkyl chain, whereas not only conventional solid acid catalysts but also liquid acids showed quite low catalytic activity. Detailed kinetic studies corroborated that the adsorption of oleophilic esters at the Brønsted acid site was facilitated by the surrounding hydrophobic environment, thus significantly promoting hydrolysis under aqueous conditions. Furthermore, sulfo-functionalized siloxane gels with a highly hydrophobic surface showed excellent catalytic activity for the hydrolytic deprotection of silyl ethers.

Keywords Brønsted acid catalyst, siloxane gel, hydrophobic surface, hydrolysis of organic molecules


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1. Introduction Acid catalysis is the most fundamental tool for promoting chemical reactions in the petrochemical industry1–5 and fine-chemical synthesis.6–9 Recently, considerable attention has been paid to the up-grading of biomass-derived materials by Lewis and Brønsted acid catalysis in aqueous media.10–16 However the presence of water in acid-catalyzed reactions is generally considered to result in a decrease in efficiency, since water acts not only as a nucleophile in catalytic reactions but also as an inhibitor by adsorbing at the acid site due to its Lewis-basic nature. Hence, control of the adsorption of water and organic molecules at the acid site is anticipated to be one of the most important factors for taking full advantage of their potential for acid catalysis. Among various solutions for overcoming this obstacle, the introduction of a hydrophobic environment to the surroundings of the acid site is considered to be a promising strategy.17,18 In fact, proton-exchange organic resins19–21 and acidified-carbonaceous materials22–26 have been reported to function as water-tolerant solid acid catalysts for the hydrolysis of esters. The organic frameworks of these materials provide a hydrophobic environment around the acid site, which prevents deactivation of the catalytic active site caused by the strong coordination of water. High-silica zeolites with a highly condensed siloxane network in their microporous cages are also known as water-tolerant acid catalysts for promoting a series of reactions under aqueous conditions.27–32 As described above, the positive effect of a hydrophobic environment around acid sites on catalytic reactions under aqueous conditions is now widely accepted. However, solid catalysts generally have a variety of functional groups on their surface, which complicates the quantitative evaluation of the effect of the degree of hydrophobicity of the region surrounding the catalytically active acid site. A better understanding of this effect may lead to novel guidelines for designing highly active, durable and water-tolerant acid catalysts. In this context, we focused on the use of alkyl group-functionalized siloxane gels developed by Nakanishi and co-workers33 as acid catalysts. These materials exhibit a scaffold with highly



condensed siloxane networks. Thus, their surface is expected to consist only of alkyl groups for hydrophobic sites and some other functionality for hydrophilic acid sites. The presence of limited functional groups around acid sites should simplify the evaluation of the effect of hydrophilicity on their catalytic activity. Furthermore, with microporous zeolite catalysts, the heterogeneity of the acid site complicates the effect of hydrophobicity on their catalytic activity. However, this concern can be avoided with siloxane gels due to their non-porous structure. In this paper, sulfo-functionalized siloxane gels with a variety of surface hydrophobicities were fabricated to elucidate the effect of the environment surrounding the Brønsted acid site on their catalytic activities for the hydrolysis of organic molecules. A detailed analysis of the relationship between structure and acid catalysis revealed that the hydrophobicity of the environment surrounding the acid sites significantly dominated their catalytic turnover frequency. While siloxane gel catalysts with hydrophobic surface properties exhibited excellent activity for the hydrolysis of acetate esters with a long alkyl chain, not only conventional solid acid catalysts but also liquid acids showed quite low catalytic activity. Furthermore, the present hydrophobic gel catalysts also showed high activity for the hydrolytic deprotection of silyl ethers.


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2. Experimental Section 2.1. Materials Acetone, methanol, ethyl acetate, acetic acid, urea, aqueous H2O2 30wt%, and H2SO4 were purchased

from

FUJIFILM

Hexadecyltrimethylammonium (MPTMS),

Wako chloride

trimethoxymethylsilane

Pure

Chemical

(CTAC),

(TMMS),

Co.

Ltd.

(Japan).

n-

3-mercaptopropyltrimethoxysilane

3-mercaptopropyldimethoxymethylsilane

(MPDMS), and dimethoxydimethylsilane (DMDMS) were obtained from Tokyo Chemical Industry, Ltd. (Japan). Amyl acetate, octyl acetate, and dodecyl acetate were purchased from Tokyo Chemical Industry, Ltd. Amberlyst-15 and Nafion-NR50 were obtained from Tokyo Chemical Industry, Ltd and FUJIFILM Wako Pure Chemical Co. Ltd., respectively. H-ZSM-5 (JRC-Z5-90H) and H-Beta (JRC-HB-150) were kindly supplied by the Catalysis Society of Japan. All of the chemical regents were used as received.

2.2. Preparation of siloxane gel catalysts Sulfo-functionalized siloxane gels were prepared through a three-step sequence: synthesis of a thiol-functionalized siloxane gel, oxidation and protonation of thiol to sulfo. Thiolfunctionalized siloxane gels were prepared as reported by Nakanishi and co-workers.33 CTAC (0.80 g) and urea (5.0 g) were dissolved in 5 mM aqueous acetic acid (15 mL). To the solution were added alkyl trimethoxysilane (MPTMS and TMMS, 21 mmol) and dialkyl dimethoxysilane (MPDMS and DMDMS, 14 mmol) at the same time under vigorous stirring at ambient temperature, and stirring was continued to give a clear solution. Gelation and aging of the obtained sol were performed in an oven at 90 °C for 6 h. After the formed siloxane gel was washed with methanol and distilled water to remove the residual surfactant and other chemicals, the obtained powder was dried in an oven at 80 °C. Subsequently, 2.1 g of the obtained siloxane gel with thiol and methyl groups was treated with 12 mL of aqueous H2O2 30wt% in 18 mL of methanol at ambient temperature. After 24 h, the resulting powder was



washed with methanol and water. Finally, the powder was stirred in a mixture of H2O and methanol (v/v = 1/1) containing 1.8 mL of conc. H2SO4 at ambient temperature for 4 h. The thus-obtained gel was washed with methanol and water, and then dried in an oven at 80 °C to form sulfo-functionalized siloxane gel catalysts. The contents of surface functional groups on the gel were controlled by changing the molar ratio of S and Me (S/Me) in the starting silicon materials,34 and gel catalysts with S/Me=XX were designated as Gel cat.(XX). In the present study, S/Me was varied from 0.17 to 2.50.

2.3. Physical and analytical measurements The products of the catalytic runs were analyzed by GC-MS (Shimadzu GCMS-QP2010, CBP-10 capillary column, i.d. 0.25 mm, length 30 m, 50250 °C) and gas chromatography (Shimadzu GC-2014, CBP-20 capillary column, i.d. 0.25 mm, length 30 m, 50250 °C). Liquid NMR spectra were recorded on a JMN-ECS400 instrument (FT, 400 MHz (1H), 100 MHz (13C)). Chemical shifts () of 1H and

13

C{1H} NMR spectra are referenced to SiMe4. The

catalysts were analyzed by nitrogen gas and water vapor adsorption, 13C CP/MAS NMR, 29Si MAS NMR, elemental analysis, XPS, FT-IR, water contact angle and acid-base titration. The Brunauer–Emmett–Teller (BET) specific surface area was estimated from N2 isotherms obtained using a BELSORP-mini II (MicrotracBEL) at 77 K. The analyzed samples were evacuated at 423 K for 15 h prior to measurement. Water vapor adsorption was evaluated using BELSORP-max at 25 °C. The analyzed samples were evacuated at 353 K for 15 h prior to measurement. Solid NMR spectra were recorded on a JMN-ECA500 instrument (FT, 100 MHz (13C), 98.37 MHz (29Si)). The rate of sample-spinning was set at 6 kHz, and the number of scans was 3000. Elemental analyses were performed using an EAI CE-440 CHN/S Elemental Analyzer (Exeter Analytical, Inc.). XP spectra were recorded using a JPS-9010 MX instrument. The spectra were measured using MgKα radiation (15 kV, 400 W) in a chamber at a base pressure of ~10-7 Pa. All spectra were calibrated using O1s (532.0 eV) as a reference. FT-IR


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spectra were recorded with FT/IR-4200 (JASCO) by a transmission method using a KBr disc containing the sample at 1 wt%. Water contact angles were measured with a Drop Master DM300 (Kyowa Interface Science Co., Ltd., Japan). The volume of a water droplet was fixed at 3.0 μL and the contact angle was determined at 30 seconds after attachment to the gel surface. The proton conductivity in the gel catalysts was measured by electrochemical impedance spectroscopy over the frequency range from 50 Hz to 50 kHz (Hioki 3532-50, Tokyo, Japan) under dry conditions. A pellet of the gel catalyst (0.5cm in diameter and 0.1 cm in thickness) and two blackened platinum plate electrodes were placed in a Teflon cell. The titration technique was used to determine the acid amount of the siloxane gel catalysts by using a mixture of aqueous saturated sodium chloride and methanol as exchange agents.35

2.4 General procedure for the catalytic hydrolysis of esters A typical reaction procedure was as follows: 0.5 mL of ethyl esters and 2.5 mL of H2O were added to a Schlenk tube containing solid acid catalyst (100 mg) under an Ar atmosphere. The reaction was carried out at 60 °C (in the case of ethyl acetate) or 90 °C (in the case of other esters) for 3 h with a fixed stirring speed of 500 rpm. After the reaction solution was cooled to room temperature, to the solution were added decane (1 mmol) as an internal standard and 9 mL of acetone to homogenize the organic and aqueous phases of the solution. The activity of the catalysts was evaluated in terms of the initial formation rate of alcohols as determined by gas chromatography.

2.5 General procedure for the hydrolytic desilylation of silyl ether A typical reaction procedure was as follows: 0.25 mmol of silyl ether and 4.0 mL of H2O were added to a Schlenk tube containing solid acid catalyst (50 mg) under an Ar atmosphere. The reaction was carried out at 100 °C for 24 h under a fixed stirring speed of 500 rpm. After the reaction solution was cooled to room temperature, to the solution were added decane (0.5



mmol) as an internal standard and 20 mL of acetone to homogenize the organic and aqueous phases of the solution. The activity of catalysts was evaluated in terms of the amount of formed alcohols as determined by gas chromatography.


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3. Results and discussion Characterization of siloxane gel catalysts The amounts of functional groups actually incorporated in the synthesized siloxane gel catalysts are summarized in Table 1. The total amount of carbon, hydrogen and sulfur atoms was estimated by elemental analysis, and the sum weight of these elements is about from 35 to 40 wt% regardless of S/Me ratio. TG analysis of Gel cat.(0.26) under air flow condition (Figure S3 in supporting information) shows about 40% weight loss by increasing the temperature, which suggests that the parts of siloxane gel catalysts other than C, H and S were composed by Si-O-Si unit. XP spectra show the presence of S6+ and S2- species which originated on the surface sulfo and thiol groups on siloxane gels, respectively (Figure S1).36 The molar ratio of S6+ and S2- species was calculated from the intensity ratio of these peaks, which quantified the amount of sulfo groups on the siloxane gel catalysts. Furthermore, the acid amount on siloxane gels estimated by acid-base titration technique was also summarized in Table 1. Notably, the S6+ ratio and acid amount decreased with a decrease in the S/Me ratio. This is probably due to the difficultly of hydrophilic H2O2 accessing a thiol group tethered on the hydrophobic surface of siloxane gel, which contributed to why the S/Me ratio did not directly reflect the ratio of S6+/Me actually incorporated in the synthesized siloxane gel catalysts. On the other hand, the present gel catalysts possess very small BET surface area (

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