Highly Recyclable Fluoride for Enhanced Cascade Hydrosilylation

Sep 25, 2017 - State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, State Key Laboratory Breeding Base of Green Pesticid...
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Letter Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9640-9644

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Highly Recyclable Fluoride for Enhanced Cascade Hydrosilylation− Cyclization of Levulinates to γ‑Valerolactone at Low Temperatures Wenfeng Zhao,†,§ Tingting Yang,†,§ Hu Li,*,† Weibo Wu,† Zhongwei Wang,† Chengjiang Fang,† Shunmugavel Saravanamurugan,‡ and Song Yang*,† †

State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering (Ministry of Education), Center for R&D of Fine Chemicals, Guizhou University, Guiyang 550025, P.R. China ‡ Center of Innovative and Applied Bioprocessing (CIAB), Mohali 160071, Punjab, India S Supporting Information *

ABSTRACT: A facile and benign catalytic route has been developed to quantitatively yield γ-valerolactone (GVL; ca. 97%) from biomass-based levulinates at room temperature to 80 °C by using easily available polymethylhydrosiloxane (PMHS) and KF as the liquid H-donor and recyclable catalyst, respectively. No extra step was required to liberate GVL from the in situ-formed siloxane, and this catalytic system exhibited a lower activation energy (40.9 kJ/mol) compared to previously reported ones. The deuterium-labeled study further demonstrated the reaction proceeding through cascade hydrosilylation and cyclization with fluoride successively acting as the nucleophile and base. In addition, the PMHS-derived resin was extremely favorable to restrain the leaching of fluoride and maintain its constant activity for at least six cycles. KEYWORDS: Biomass conversion, Biofuel, Heterogeneous catalysis, Hydrogenation, Kinetic/mechanism study

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INTRODUCTION The hydrogenation of biomass-derived levulinic acid or alkyl levulinates to γ-valerolactone (GVL) has been deemed one of the key reactions in the development of biorenewable and atom-economical catalytic routes to liquid fuels and valuable chemicals.1 Noble metals (e.g., Au, Pd, and Ru) have been reported to be highly favorable for the hydrogenation step with molecular H2, but these catalytic systems are detrimental to the overall production cost for the synthesis of relatively low value biofuels by using precious catalysts and high-pressure H2.2,3 Alternatively, liquid hydrogen sources such as formic acid and alcohols (e.g., methanol, ethanol, 2-propanol, and 2-butanol) instead of H2 can be used for efficiently producing GVL over low-cost transition metal particles (e.g., Cu, Ni, and Co), oxides (e.g., ZrO2, ZrNi, and ZrAl), or hybrids (e.g., Zr-PhyA, mPhPZr, and ZrPN) via catalytic transfer hydrogenation (CTH), which are typically biomass-derivable, easy to transport and store, and avoidable to high initial pressure as H2 gas.4−6 However, almost all recent approaches face the significantly superfluous use of H-donor (relative to the substrate) and harsh reaction conditions (≥150 °C) for achieving the efficient transformation of levulinates to GVL, as representatively summarized in Table S1. As a byproduct of the silicone industry and an attractive liquid H-donor, polymethylhydrosiloxane (PMHS) is insensitive to water/air, nontoxic to organisms, and inexpensive, which © 2017 American Chemical Society

has drawn great attention in the ionic hydrogenation or hydrosilylation of unsaturated groups (e.g., CC, CO, and CN) catalyzed by metal hydrids, salts, or complexes.7−10 Unfortunatedly, some practical drawbacks like poor catalyst reusability and the requirement of proton species or extra hydrolysis steps for the dissociation of Si−X (X = O, N, and S) bonds to liberate products are always encountered, especially for the selective hydrogenation of carbonyl compounds.11,12 To the best of our knowledge, no effective approach has been reported to simplify the postprocess and further in situ couple the hydrosilylation with other reactions. To elimilate these obstacles that may obstruct the large-scale hydrogenation processes with PMHS, we herein report a facile and benign catalytic route to produce GVL directly from alkyl levulinates over KF and PMHS at room temperature to 100 °C, where fluoride (F−) acts as both nucleophile toward Si facilitating the hydrosilylation process and base to promote the subsequent cyclization, without observing competitive reactions (Scheme 1). Almost quantitative yields of GVL were obtained by using an extremely low dosage of PMHS, and no intermittent step was required to cleave the Si−O bond because of the intramolecular cyclization. The solid KF catalyst Received: August 10, 2017 Revised: September 15, 2017 Published: September 25, 2017 9640

DOI: 10.1021/acssuschemeng.7b02756 ACS Sustainable Chem. Eng. 2017, 5, 9640−9644

Letter

ACS Sustainable Chemistry & Engineering Scheme 1. Reaction Routes to GVL from Alkyl Levulinate over KF and PMHS via Key Intermediate Siloxane

was highly recyclable and kept constant activity in at least six consecutive cycles.

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RESULTS AND DISCUSSION In the preliminary studies, the catalytic performance of different alkali metal salts (i.e., KF, CsF, LiF, NaF, KCl, and KBr) was tested for the conversion of EL to GVL by using 2.5 equiv PMHS as H-donor in an aprotic solvent DMF (N,Ndimethylformamide) at a low reaction temperature of 80 °C within 7 h. KF and CsF were found to show the highest activity, being capable of almost completely converting EL to GVL with 91% and 90% yield, as well as 97% and 98% selectivity, respectively (Figure 1A). In contrast, nearly no reaction took place by using LiF, NaF, KCl, and KBr as catalyst. These results indicated that F− might be the active species, while its activity is tightly dependent on the type of metal cation, which is in accordance with previous reports using F− as the nucleophile,13,14 implying that the reaction may proceed via SN2 other than SN1 nucleophilic substitution. It is worth noting that the reatively higher hygroscopicity of CsF than that of KF may partially lower its nucleophilicity and reactivity in the synthesis of GVL from EL.15 Further, a much lower price of KF prompts us to use it for succeeding studies. To balance the basic and nucleophilic properties of KF, different solvents were screened for the conversion of EL to GVL (Figure 1B). It was observed that KF was unable to be dissolved into aprotic solvents (e.g., 0.008 wt % solubility in DMSO), implying higher nucleophilicity and less basicity of KF in these reaction media.15 Among them, the relatively polar solvents such as DMF and DMSO exhibited comparable activity and were most favorable for both EL conversion (94%) and GVL formation (91% yield), but less polar or nonpolar solvents (e.g., THF and n-hexane) were inactive for the reaction (80% yield of GVL could be achieved at room temperature while requiring a quite long reaction time of 120 h (Figure 2D), and the coproduct was found to be siloxane. These results clearly indicated that the in situ-formed siloxane was thermally sensitive to yield GVL, which could also be supported by the apparently increased reaction rate constant (k) of 0.001, 0.002, and 0.006 min−1 at 40, 60, and 80 °C, respectively (Figure S1A), and the relatively lower activation energy (Ea = 40.9 kJ/mol) of KF (Figure S1B) in comparison with most previously reported catalysts such as Zr-Beta (44.8 kJ/mol) by using 2-butanol as H-donor17 and Shvo-Ru (69 kJ/ mol) with formic acid,18 as well as Ru/C (48 kJ/mol)19 and Rutris(m-sulfonatophenyl)phosphine (61 kJ/mol)20 under H2 atmosphere. Moreover, methyl, n-propyl, and n-butyl levuli9641

DOI: 10.1021/acssuschemeng.7b02756 ACS Sustainable Chem. Eng. 2017, 5, 9640−9644

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ACS Sustainable Chemistry & Engineering

Figure 2. Effect of reaction temperature and time on the conversion of EL to GVL at 40−100 °C (A−C) and room temperature (D). Reaction conditions: 0.5 mmol EL, 2.5 equiv PMHS, 15 mg KF, and 2.0 mL DMF.

and EL conversion (46−50%) at 80 °C after 1 h (Figure 4A). The filtration experiments further indicated the heterogeneous catalytic behavior of KF in the reaction system (Figure 4B). It was worth noting that the weight of the recovered solid catalyst was increased to about 90 mg in the sixth run due to the formation of insoluble PMHS-derived resin.21 Figure S4 presents the photos of the KF-catalyzed reaction mixtures before and after reactions as well as the recovered silicone resin containing F species. The increased catalyst weight, surface area, and pore size could be ascribed to the porous resin in situ formed by encapsulation of the fluoride species with PMHS, which are in agreement with previous reports that claimed the cross-linking of the silicon−polymer to soak up homogeneous species.21−23 In addition, the presence of Si and F species in the recovered catalyst was clearly illustrated by XPS spectra (Figure S5), and EDX spectra show the nearly equal amount of F species in the fresh and recovered catalysts (Figure S6). 1H and 19 F NMR spectra of PhSiH3 in DMSO-d6 without or with KF stirring for 10 min confirm the formation of an F−Si bond (Figures S7−S8). On the other hand, the increased surface area (125 vs 37 m2/g) and pore size (8.4 vs 4.7 nm) may enhance the capability of physical adsorption toward F (Figures S9−S10, Table S3), which possibly also promote the reaction with more accessible active sites. All these chemical and physical effects render the F species to be little leachable and highly recylable for multiple cycles.

nates could also be converted to GVL in yields of 97%, 94%, and 91% over KF under identical conditions, respectively. To explicitly elucidate the reaction pathway for converting EL to GVL, the ex situ 1H NMR spectra were conducted at 80 °C by varying the reaction time from 1 to 7 h (Figure 3). Although the peaks of DMSO-d6 (2.5 ppm), HDO (3.3 ppm), and the dissolved or reacted PMHS might affect the observation of the NMR signals of each compound, EL could be still distinctly detected to be transformed into GVL with the extension of reaction duration. In addition, the disapperance of protons in methine of GVL (1H NMR) by using deuterium Ph2SiD2 as H-donor while the presence of characteristic peaks of GVL in 13C NMR (Figure S2) verified the occurrence of hydrosilylation for producing GVL from EL. This could also be supported by the absence of GVL formed when using either αangelica lactone or 2(5H)-furanone as substrate. Notably, the band belonging to Si−O−CH2 species at around 3.6 ppm was also observed and markedly enlarged by prolonging the reaction time (Figure 3), which further demonstrated that during the cyclization process the breakage of the Si−O bond preformed via hydrosilylation with EL and the formation of new Si−O species with alkoxide moiety of EL to give GVL and another siloxane took place simultaneously (Scheme 1). Hence, no extra postprocess was needed to release the product after hydrosilylation, which was otherwise coupled with cyclization. For practical application, catalyst recyclability is a crucial issue to clarify. The choice of 15 mg of KF and 2.5 equiv of PMHS was found to be optimal for the recycling study (Figure S3). Furthermore, the easily available and inexpensive PMHS displayed superior activity to other commercial silanes for this reaction (Table S2). In six consecutive catalytic cycles, KF afforded almost constant GVL yield/selectivity (ca. 45%/95%)

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CONCLUSION In conclusion, a facile and benign catalytic strategy has been developed for the efficient conversion of levulinates to GVL in 91−97% yields at room temperature to 80 °C by using easily available KF and a slight excess of PMHS as the catalyst and 9642

DOI: 10.1021/acssuschemeng.7b02756 ACS Sustainable Chem. Eng. 2017, 5, 9640−9644

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cascade hydrosilylation−cyclization for transforming EL into GVL, where KF acted as both the nucleophile and base. The PMHS-derived resin was favorable for restraining fluoride leaching and maintaining its constant activity for at least six cycles.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02756. Experimental section; tables; kinetic study; 1H, 13C, and 19 F NMR spectra; effect of catalyst and PMHS dosage; XPS spectra; EDX spectrum; N2 adsorption−desorption isotherms; and pore size distribution. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.L.). Tel: (+86)851 8292171. Fax: (+86)851 8292170. *E-mail: [email protected] (S.Y.). Tel: (+86)851 8292171. Fax: (+86)851 8292170. ORCID

Shunmugavel Saravanamurugan: 0000-0002-3980-5020 Song Yang: 0000-0003-1301-3030 Author Contributions §

W.Z. and T.Y. contributed equally to this work.

Author Contributions

W.Z. and T.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

■ Figure 3. Ex situ 1H NMR spectra of EL-to-GVL conversion at 80 °C for 1−7 h. Reaction conditions: 0.5 mmol EL, 2.5 equiv PMHS, 15 mg KF, and 2.0 mL DMSO-d6.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21576059 and 21666008) and the Key Technologies R&D Program of China (2014BAD23B01). S.S. thanks the Department of Biotechnology (Government of India), New Delhi, India.

liquid H-donor, respectively. This catalytic system had a lower activation energy compared to previously reported ones. The deuterium-labeled study verified the reaction proceeding via

(1) Zhang, Z. Synthesis of γ-valerolactone from carbohydrates and its applications. ChemSusChem 2016, 9, 156−171.

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REFERENCES

Figure 4. Catalyst recycling study (A) and catalytic behavior (B) of KF in the conversion of EL-to-GVL. Reaction conditions: 0.5 mmol EL, 2.5 equiv PMHS, 15 mg KF, 2.0 mL DMF, 80 °C for 1 h (or 0.5−24 h). 9643

DOI: 10.1021/acssuschemeng.7b02756 ACS Sustainable Chem. Eng. 2017, 5, 9640−9644

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DOI: 10.1021/acssuschemeng.7b02756 ACS Sustainable Chem. Eng. 2017, 5, 9640−9644