Selective Visible Light Aerobic Photocatalytic Oxygenation of Alkanes

6 days ago - The aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis offers the poten...
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Selective Visible Light Aerobic Photocatalytic Oxygenation of Alkanes to the Corresponding Carbonyl Compounds Miriam Somekh, Alexander M. Khenkin, Adi Herman, and Ronny Neumann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02999 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Catalysis

Selective Visible Light Aerobic Photocatalytic Oxygenation of Alkanes to the Corresponding Carbonyl Compounds Miriam Somekh, Alexander M. Khenkin, Adi Herman and Ronny Neumann* Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100 The aerobic, selective oxygenation of alkanes via C-H bond activation is an important research challenge. Photocatalysis offers the potential for the introduction of additional concepts for such reactions. Visible light photoactive semiconductors such as bismuth oxyhalides (BiOX, X= Cl, Br) used in this research, typically oxidize organic compounds through photocatalyzed formation of strongly oxidizing holes. The reactive oxygen species formed, react with organic compounds in one-electron processes, leading to radical intermediates and non-selective oxidation. Such oxidation reactions generally lead to total oxidation. Here, impregnation of BiOX with a polyoxometalate, H5PV2Mo10O40, as a strong electron acceptor changed the reactivity of BiOX, leading to Mars-van-Krevelen type reactivity, that is photoactivated oxygen donation from BiOX to the organic substrate followed by reoxidation by O2 and catalysis. This conclusion was supported by mechanistic studies involving isotope labelling studies. In this way ethane was selectively oxidized to acetaldehyde in a flow reactor with a turnover number (24 h) 415.

Keywords. Oxygen Transfer, Photocatalysis, Polyoxometalate, C-H bond activation, Bismuth Oxide, Semiconductor Introduction. Selective photocatalytic oxidation of alkanes, preferably with dioxygen and visible light, is a challenging research topic. Although oxygenations of alkanes with O2 are exergonic, room temperature photo-oxygenation reactions by commonly used photoactive semiconductors are not selective and tend to lead to total oxidation.1,2 Mechanistically speaking, photoexcitation of oxidizing semiconductors leads to the formation of strongly oxidizing holes that then form potent radical species such as OH• from H2O or alkyl radicals from hydrocarbons.3 Alkyl radicals from alkanes and alkenes and perhaps cation radicals from alkylarenes can be formed through C-H bond activation via hydrogen abstraction or electron transfer. In the presence of O2 further free-radical chain autooxidation reactions can occur leading to product formation whose selectivity is difficult to control.4-6 Photoactive semiconductors that have been used for alkane and alkylarene aerobic oxidation Include TiO2,7,8 ZnO,9 TiO2/MoO3,10 CuMoO411 V2O5,12 CdS,13 Fe2O314 and the decatungstate polyanion, a soluble semiconductor analog.15,16 Bismuth-based ternary metal oxides such as Bi2WO6 and BiVO4 are interesting photocatalysts due to

their narrower band-gap and high charge mobility.17-19 The advantageous properties of these compounds are attributed to hybridized band structures formed between the Bi 6s and the O 2p orbitals that raise the energy level of the valence band. The insertion of an oxyhalide component instead of a metal oxide led to the discovery of photoactive semiconductors such as BiOCl and BiOBr that showed remarkable photocatalytic activity in dye bleaching.20-25 It was further noticed that easily prepared bismuth mixed oxyhalides, especially BiOClxBr1-x where 0 RCH3 >> RCHO. For example, in the toluene series, that is R = Ph, the ionization

potentials as surrogates of the redox potential,31 correlate very well for an electron transfer initiated PCET oxygenation where PhCH2OH (8.27 eV) > PhCH3 (8.83 eV) >> PhCHO (9.50 eV).37 Reactivity did not correlate with the homolytic C-H bond disassociation energies (BDE).38 Notably, the aldehydic C-H is bond is the weakest, but the aldehyde formed is quite inert under reaction conditions.39 A PCET mechanism also explains the observation that alcohols are intermediate products that are typically more reactive than the initial alkane. The apparent PCET reaction is also associated with a fairly large kinetic isotope effect (KIE). Thus, a competitive oxidation of 1:1 ethylbenzene/ethylbenzene-d10 under conditions described in Table 2 yielded a KIE = 5.4 ± 0.2. This relatively high KIE also explains the preference of reaction at a secondary carbon atom versus a primary carbon atom. In addition, the relative reactivities, e.g. in the toluene series, combined with the relatively high KIE suggest a sequential PCET mechanism where the proton transfer may be rate limiting.40 The key further issue is the source of the oxygen atom in the product where, in principle there are three possibilities. (1) From O2, as is common, and as was observed in the past in BiOX photocatalyzed oxygenation in the presence of chloride;29 (2) from H5PV2Mo10O40 as an oxygen donor which has been documented for many reactions in the past,30-33 or (3) from BiOClxBr1-x, which as far as we know, has not been documented for any BiOX photocatalysts. Oxygen labelling can also come indirectly from residual H218O used in the preparation of H5PV2Mo1018O40, especially through exchange reactions once a carbonyl product has been formed. This was verified by reacting products with H218O in the presence of [BiOX-POM]. We have tested the source of oxygen using 18-O labelled O2 and H5PV2Mo10O40. The large amounts of water needed to synthesize BiOClxBr1-x made its 18-O labelling cost prohibitive. Both liquid phase reactions using neat ethylbenzene as substrate, where residual water is always present and gas phase reactions using n-butane as substrate were carried out. In the case of the n-butane to 2-butanone transformation only CH3CH2C(16O)CH3 was formed in the presence of 18O2 (>95% purity) or/and H5PV2Mo1018O40 (~85% purity). A largely similar result was observed in the liquid phase oxygenation of ethylbenzene, where oxidation yielded 1-phenyethanol and acetophenone with only ~5% 18-O labelled products. The results show that that the source of the O atom in the product in the gas phase reaction is neither from O2 nor from H5PV2Mo10O40 leaving BiOClxBr1-x as the only possible source of the oxygen atom in the product. This result additionally suggests that after formation of the caged as opposed to free alkyl/alkylarene radical an oxygen transfer reaction from BiOClxBr1-x to the substrate occurs. Further oxidation from the alcohol to the major

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ACS Catalysis carbonyl product occurs via an oxidative dehydrogenation reaction without O transfer. The small amounts of 18-O labelled products in the case of the liquid phase oxidation reaction are likely due to more minor secondary pathways: (1) reaction of the benzylic radical intermediate with 18O2; (2) oxygen transfer from H5PV2Mo1018O40 to the benzylic cation [PhCHCH3]+ after ET oxidation of the intermediate radical as shown in the past.31 Conclusions. The support of H5PV2Mo10O40 polyoxometalate as an electron acceptor onto a BiOClxBr1-x visible light photoactive semiconductor has enabled apparent improved charge separation upon photoactivation. The h+ species formed, initiated a PCET C-H bond activation of alkylarenes and alkanes as deduced from relative reactivity combined with the KIE = 5.4 observed in the oxidation of 1:1 ethylbenzene/ethylbenzene-d10. Instead of a radical coupling reaction between the alkyl radical formed and O2, which is typically considered a diffusioncontrolled reaction,41 reactions with 18O2 and H5PV2Mo1018O40 strongly suggest that oxygenation takes place through oxygen transfer from BiOClxBr1-x. This novel mechanistic change in reactivity leads to the conclusion that in the presence of H5PV2Mo10O40, BiOClxBr1-x acts as a photoactivated oxygen donor, in what can be generalized as a Mars-van Krevelen type oxygenation, where BiOClxBr1-x is re-oxidized with O2. The direct reaction of O2 with C-H bonds was not observed in the gas phase and was a minor pathway in the liquid phase. On the more practical side, gas phase reactions were particularly efficient with high TONs for selective oxygenation of ethane and propane to acetaldehyde and acetone, respectively. Methane, however did not react, possibly because ET or the sequential proton transfer from methane to the photoactivated BiOClxBr1-x was energetically unfavorable. Experimental Section. Preparation of BiOClxBr1-x: Synthesis of BiOClxBr1-x (x=~0.875) was carried out according to a literature method.26 Deionized water (85 mL), glacial acetic acid (45 mL), and bismuth nitrate (14.69 g, 30 mmol) were placed into a 250 mL flask and stirred at room temperature for 15 min until a clear, transparent solution is formed. Then hexadecyltrimethyl ammonium bromide (1.378 g dissolved in 10 mL of water, 3.5 mmol) and hexadecyltrimethyl ammonium chloride (2.12 g dissolved in 6.36 mL of water, 6.6 mmol) were added to the above solution in one batch, and the mixture is stirred for an additional 30 min at room temperature. The precipitate formed was filtered and washed five times with hot ethanol (300 mL) and five times with boiling water (1L) to remove the unreacted quaternary ammonium salts and then dried in air. Preparation of BiOClxBr1-x-2 % wt H5PV2Mo10O40: H5PV2Mo10O40•34H2O (20 mg) prepared by a literature procedure,42 was dissolved in 5 mL of CH3CN. BiOClxBr1-x (980 mg) was added as a fine powder; the slurry was stirred for 30

min, the solvent was removed by rotovapor and the BiOClxBr1-x2 % wt H5PV2Mo10O40 was dried at RT overnight under high vacuum. 18O-labeled H PV Mo O . H PV Mo 18O 5 2 10 40 5 2 10 40 was prepared by drying H5PV2Mo10O40•34H2O at 120 °C for 24 h and then adding 50 equiv of H218O (98%) and mixing for 5 h. The dryingexchange cycle was repeated 3 times. The 18O enrichment was estimated by deconvolution of the IR spectra assuming a theoretical 40 cm-1 isotope shift and expected equal peak intensities for labeled and unlabeled M-O bonds. Total enrichment excluding the four internal oxygen atoms was ~ 95%, ~85% overall. Liquid phase photocatalytic oxidation reactions. Substrate and catalysts in the amounts noted in the Tables were placed in 15 mL Pyrex pressure tube under 2 bar O2. The slurry obtained was stirred for 24 h under irradiation with four cold-white LED lamps (GU10 LED, 10W, 5500-6500K, 730 lumens, 120⁰ beam spread, produced by “Spectrum LED”; see Figure S6 for spectrum) placed on four sides of the pressure tube at a distance of 4 cm. After reaction the slurry was filtered and the liquid analyzed by GC-MS for qualitative analysis of products and, GC-FID for quantitative measurements. A 30 m, 0.32 mm ID 5% crosslinked phenylmethyl siloxane capillary column with a 0.25 m film thickness and a split injection mode was used with He as carrier gas. Liquid phase isotope labelling experiments were carried out in the same way and analysed for relative m/z by GC-MS. Gas phase photocatalytic reactions. Reactions were carried out in a sapphire tube reactor (5 mm ID, 85 mm long) rated at 200 bar and loaded with 250 mg BiOClxBr1-x-2 % wt H5PV2Mo10O40 specific mass flow controllers and a backpressure regulator. 1:1 Light alkanes (99.99%) and O2 were reacted under a total flow of 3.0 - standard cm3/min at Ptotal = 5 bar. The tube was irradiated by eight cold-white LED lamps as described above, Figure S7. Products were quantified by on-line sampling using a GC-TCD. The columns used for separation were two 1/8” ID, 6 foot columns in series packed with molecular sieves 13X and Hayesep D using He as eluant. Photocatalytic oxidation of n-butane with 18O2. The sapphire tube used in the flow system described above was loaded with 250 mg BiOClxBr1-x-2 % wt H5PV2Mo10O40 with 1 bar n-butane and 3 bar O2, capped and then irradiated for 24 h as described above. The reaction mixture was sampled by injection in GCMS. Other measurements. Powder XRD Diffraction measurements were carried out in reflection geometry using an Ultima III diffractometer equipped with a sealed Cu anode X-ray tube operating at 40 kV and 40 mA. A bent graphite monochromator and a scintillation detector were aligned in the diffracted beam. θ/2θ scans were performed under specular conditions in the Bragg-Brentano mode with variable slits. The samples were scanned from 2 to 90 degrees with step size of 0.025 degrees and scan speed of 1 degree per minute. EM analysis was performed using a Gemini SEM 500 operating at 15 kV. EDX measurements were performed using the same instrument, equipped with an EDX detector from Bruker (60mm), at 20 kV. A sample of BiOClxBr1-x-2 % wt H5PV2Mo10O40 was irradiated under Ar for 30 min in an EPR capillary tube and analyzed at RT by a X-band CW EPR using a Bruker ELEXSYS 500 spectrometer equipped with a Bruker ER4102ST resonator at 20 K with microwave power of 20 mW, 0.1 mT modulation amplitude and

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100 kHz modulation frequency. 31P NMR were measured on a 400 MHz instrument using 85% H3PO4 as external standard.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]

ORCID Ronny Neumann: 0000-0002-5530-1287 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors note no competing financial interest

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Electron microscope images, powder XRD measurements, and EPR and 31P NMR spectra, and other data (PDF).

ACKNOWLEDGMENT This research was supported by the Israel Science Foundation grants 2046/14 and 1237/18. Srinivasa Rao Amanchi is thanked for his input on this topic. Tong Bian is thanked for his help in EM measurements. R.N. is the Rebecca and Israel Sieff Professor of Organic Chemistry.

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