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Tuneable photocatalytic activity of Palladium-decorated TiO2: non-hydrogen mediated hydrogenation or isomerization of benzyl-substituted alkenes Ayda Elhage, Anabel E. Lanterna, and Juan C. Scaiano ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02832 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016
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ACS Catalysis
Tuneable Tuneable photocatalytic activity of palladium palladiumalladium-decorated TiO2: nonnonhydrogen mediated hydrogenation or isomerization of benzylbenzylsubstituted alkenes Ayda Elhage, Anabel E. Lanterna* and Juan C. Scaiano* Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation (CCRI). University of Ottawa, 10 Marie Curie, Ottawa, ON K1N 6N5, Canada ABSTRACT: Palladium-decorated TiO2 is a moisture and air-tolerant versatile catalyst. Its photocatalytic activity can be tuned in favour of hydrogenation or isomerization of benzyl-substituted alkenes simply by changing the irradiation wavelength. Benzylsubstituted alkenes are selectively isomerized to phenyl-substituted alkenes (E-isomer) with complete conversion over Pd@TiO2 under H2-free conditions. The reaction can be thermally induced under air or driven by visible light irradiation at room temperature under Ar. UV irradiation in methanol solvent leads to efficient hydrogenation. The fine tunability of the catalyst can also be used for selective deuterium incorporation using deuterated solvents; here H/D exchange is used as a mechanistic tool but with clear potential for isotope substitution applications.
Keywords: photocatalysis, titanium dioxide, palladium, isomerization, hydrogenation.
Alkene isomerisation reactions have often been encountered as undesired side-reactions during asymmetric catalytic hydrogenation1 or metathesis reactions,2-3 that have the potential to compromise enantioselectivity and promote the formation of by-products. However, the deliberate and controlled isomerisation of a pre-existing alkene bond can afford positional and geometric isomers that are difficult to prepare using conventional methods. Established methods for carrying out these transformations include: treatment of an easily prepared alkene substrate with radicals4, boranes5, halogens,6 and strong acids7. However, there are many disadvantages associated with these methods due to functional group incompatibility with the harsh reaction conditions that are often required for isomerisation to occur. Photochemical methods have been applied to the (E)/(Z)-isomerisation of alkenes,8 but the success of this approach is substrate dependent, with mixtures of products controlled by the photostationary state often being produced.9 The use of transition metal-based catalysts offers several advantages over these more conventional methods, such as functional group tolerance, mild reaction conditions, product selectivity and reliability.1, 10-14 From an industrial perspective, catalytic processes are more desirable than those using stoichiometric reagents, as product separation is greatly simplified, particularly if heterogeneous catalysts are used. This is especially advantageous for the synthesis of pharmaceutical compounds intended for human consumption, which must adhere to strict purity requirements.15
The allyl phenyl ether group is important in the perfumery and food industries, where t-anethole and i-eugenol have high industrial demand as intermediates for the synthesis of various perfumery chemicals, pharmaceutical compounds, and beverages. t-Anethole is a naturally occurring product, extracted from anise and fennel oils, with variable proportion of its cis isomer as an impurity. However the increasing industrial demand has made it necessary to develop alternative synthetic routes for t-anethole, the only valuable product due to the toxicity of its cis isomer. From an academic point of view, several methodologies have been reported to isomerize the estragole to anethole either catalyzed by metal complexes of Pd, Ru and Rh,16-17 or carried out over catalysts belonging to the hydrotalcite family.18 Most of the selective bond isomerization of estragole studies has been done using homogeneous catalysis under a nitrogen atmosphere to maintain the catalyst stability. Nevertheless they are not commercially viable due to the high synthetic costs. The use of supported PdNPs has been explored in the last few years because of their potential to replace Pd complexes as catalysts.19-21 Imamura et al. have developed a solventmediated hydrogenation of alkenes using the photocatalytic ability of TiO2 combined with the well-known H-Pd surface interaction.22 On the other hand, Zhu and Shon have recently presented a mechanism for hydrogenation/isomerization reactions in the presence of H2 on PdNP surface.23-24 Here we present a water and air tolerant PdNP-decorated TiO2 (Pd@TiO2) catalyst that shows dual switchable catalytic activities changing from hydrogenation to isomerization on-
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demand, by minor variations of the reaction conditions. Thus, the ability of Pd@TiO2 to photo-assist the hydrogenation of alkenes upon UV excitation can be switched to lead isomerization simply by using visible light or thermal heating, without addition of H2. Like the hydrogenation reaction, the photoinduced isomerization is also performed under Argon, whilst the thermally induced reaction is air tolerant. The photocatalyst can be reused several times with excellent performance (vide infra). Both conditions selectively lead to the trans reaction product. The selective isomerization of estragol to tanethole has been studied using i-propanol and methanol as solvents and reducing agents under both reflux and visible light irradiation, respectively. Further, mechanistic studies demonstrate the reaction is solvent-mediated allowing selective deuterium incorporation. The scope of the catalytic activity has been explored with a group of valuable industrial compounds, including i-eugenol.
studied for the synthesis of t-anethole using estragol. Table 1 summarizes the conditions screened for the optimization of the thermally induced isomerization of estragol under air. The best conditions were found when estragol was mixed together with Pd@TiO2 catalyst in i-propanol and refluxed for 24 h. Changing the catalyst support for a known inert material such as crystal nanodiamonds (CND), the yield of the reaction decreases dramatically (cft. entries i-iii and iv-vii in table 1). These results show that the role of TiO2 is much more than simply anchoring the PdNP. Additionally, the reaction is highly favoured in alcohols, although small conversion and lower selectivity has been found using non-protic solvents (table 1, entries viii-ix). These two conditions might be explained because of the reduction of PdO over TiO2 by secondary alcohols, which is favoured at high temperature.27 Control reactions in the presence of TiO2 or in the absence of catalyst were also performed.
RESULTS AND DISCUSSION DISCUSSION
Table 1: Effect of solvents and catalyst supports on the thermal isomerization of estragol
PdNPs were synthesized by reduction of Pd2+ under UVA irradiation in the presence of TiO225 (band gap energy ~3.2 eV26), Scheme 1. The major advantage of this synthetic route is that it does not require an organic ligand/reductant that could arrest particle growth; it also maximizes the available nanoparticle surface. Pd@TiO2 were characterized by inductively coupled plasma optical emission spectrometry (ICP-OES), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) to determine the total Pd loading, the particle size and the oxidation state of the Pd species, respectively. The synthesis described in the SI leads to 2 % Pd loading with average particle size of (1.6 ± 0.9) nm (Scheme 1 and Figure S1). The XPS analysis of the binding energies of Pd was calculated on the basis of the binding energy (284.8 eV) of C 1s and using reference values from the literature.27 Some differences are expected on the binding energy of an element in the nanoscale and in bulk. A high-resolution scan over the range 327-347 eV reveals several peaks associated to different Pd species present in the sample (Figure S6). Two pairs of doublet signals can be recognized after deconvolution: a double of a Pd (3d5/2) peak at around 334.7 eV and a Pd (3d3/2) peak at around 339.9 eV which can be assigned to Pd(0);28 the second pair located in a higher region (337.2 and 342.3 eV) in comparison with the Pd(0) indicates the presence of PdO.29-30
Scheme 1. Proposed mechanism for the photochemical synthesis of supported PdNPs The direct, selective, and non-hydrogen mediated isomerization of benzyl-substituted alkenes over Pd@TiO2 catalyst was
Solvent
Catalyst
% Conv
% A Yield
i
i-propanol
Pd@CND
20
18 (≥90)
ii
Toluene
Pd@CND
35
32 (71)
iii
Acetonitrile
Pd@CND
ND
ND
iv
i-propanol
Pd@TiO2
100
>99 (>90)
i-propanol
Pd@TiO2
ND
ND
a
Pd@TiO2
70
69 (86)
b
b
v vi
Methanol
vii
Methanol
Pd@TiO2
11
6 (>90)
viii
Toluene
Pd@TiO2
45
33 (60)
ix
Acetonitrile
Pd@TiO2
15
8 (53)
x
i-propanol
--
ND
ND
i-propanol
TiO2
ND
ND
xi a
b
Conversion yields drop to ca. 39% in deuterated methanol. Reaction time: 6h. Solvent boiling point: i-propanol = 85 °C, Acetonitrile = 85 °C, Toluene = 111 °C, Methanol = 65 °C. % Yields 1 and conversions were calculated by GC-FID and/or H NMR using t-butylbenzene and dimethyl sulfone as external standards, respectively. Percent A Yield correspond to the total isomerization products, values between brackets show the selectivity towards the trans isomer. No hydrogenation products have been detected under these conditions. Similar yields were found under an inert atmosphere.
Considering that upon UV light irradiation Pd@TiO2 can catalyze the hydrogenation of alkenes22 (tested here for estragol (table 2, entry vi)), and that it has an important absorption in the visible region, probably due to the presence of Pd or PdO NPs (See Figure S2); the same reaction was tested upon visible irradiation at room temperature. Unexpectedly, only isomerization products are found after 4 h of irradiation, as can be noticed in table 2, entry i. The reaction works well in methanol and goes to completion in 24 h (with traces of hy-
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drogenation product, probably due to some UV light contamination on the LED emission, see Figure S3) under Ar atmosphere. The temperature of the mixture rises up to ca. 50 ºC upon excitation at 465 nm. Notice that the total irradiance used under visible light is about 3 times higher compared to UV light. Neither the hydrogenation nor the isomerization work under air, suggesting that a reducing environment is required for both reactions. Different catalyst concentrations were tested for both isomerization and hydrogenation (Table S1). Overall, different products can be obtained under essentially the same conditions, simply by switching the excitation wavelength. Further, when TiO2 is replaced by CND (with same NPs size distribution, cft. Figure S1 c and d), the isomerization products are also formed, albeit in lower yields. However, for Pd@CND, hydrogenation products could not be detected, even upon 368 nm irradiation. Finally, comparing the isomerization yields obtained after conventional heating (entry vii, table 1), microwave irradiation (entry x, table 2) and light irradiation (entry i, table 2) there is clear evidence that the catalytic activity of the Pd@TiO2 is dominated by a photoinduced process even if a minor thermal contribution may be involved in the isomerization process. This is reminiscent of what has been recently described for Pt NPs (
