Catalytic cascade conversion of furfural to 1,4-pentanediol in a single reactor

Fei Liu a, Qiaoyun Liu ab, Jinming Xu a, Lei Li c, Yi-Tao Cui d, Rui Lang a, Lin Li a, Yang Su a, Shu Miao a, Hui Sun a, Botao Qiao *a, Aiqin Wang *a, Francois Jérôme *e and Tao Zhang a
aState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China. E-mail: bqiao@dicp.ac.cn; aqwang@dicp.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing, 100049, PR China
cSynchrotron Radiation Nanotechnology Center, University of Hyogo, Hyogo 679-5165, Japan
dInstitute for Solid State Physics, The University of Tokyo, Hyogo 679-5198, Japan
eInstitut de Chimie des Milieux et Materiaux de Poitiers, CNRS/University of Poitiers, Poitiers 86073, France. E-mail: francois.jerome@univ-poitiers.fr

Received 4th January 2018 , Accepted 6th March 2018

First published on 6th March 2018


The synthesis of bio-based linear diols is the subject of many research studies. However, one of the main obstacles in industrial development is the difficulty in controlling product selectivity. Here, we report the catalytic conversion of furfural to 1,4-pentanediol (PD) in the presence of Ru supported on an ordered mesoporous carbon (CMK-3) under pressure of H2 and CO2 in water. In contrast to previous catalytic pathways, this work is distinct in that it yields 1,4-PD as an exclusive product, instead of a mixture of 1,2- and 1,5-PD as usual. Under optimized conditions, 1,4-PD was obtained in 90% yield, and in a one-pot reaction, directly from furfural. We disclose that the conversion of furfural to 1,4-PD followed an unusual catalytic route. It implies a bifunctional catalytic pathway based on sequential catalytic hydrogenation reactions and an acid-catalyzed Piancatelli's rearrangement.


Introduction

Linear diols are of outmost importance in chemistry and nowadays find many applications in our society. In particular, diols have been widely employed as solvents in the cosmetics industry.1 They also serve as intermediates in the manufacture of fine/specialty chemicals, as monomers in the fabrication of polymers and as polar head groups in the synthesis of biosurfactants.2 To date, linear diols are being produced on an industrial scale from fossil-based resources. The rapid emergence of the plant-based industry is now opening opportunities to produce bio-based linear diols, often with a similar chemical structure to diols from a fossil origin, but with a much improved carbon footprint.3–7 Among the different strategies previously reported, the synthesis of bio-based pentanediol (PD) from industrially available furfural has attracted particular interest.8–10 To date, current catalytic strategies involve the hydrogenolysis of the C–O bond of the furanic ring in the presence of a metal-based catalyst.11–14 This catalytic pathway takes place at relatively high temperatures (100–170 °C) and pressures of H2 (3–6 MPa). Under these harsh reaction conditions, over-hydrogenation/hydrogenolyis reaction also occurs to a greater or lesser extent depending on the catalyst. On the other hand, these catalytic routes afford mainly 1,2 and 1,5-PD, often as a mixture of both, in a 50–80% yield (1,2 + 1,5-PD) (Fig. 1). As a general trend, metals on basic supports such as Pt/HT,12 Cu/LDO15 and Pt/CeO216 afford mainly 1,2-PD while metals on acidic supports such as Pd–Ir–ReOx/SiO214 and Rh–Ir–ReOx/SiO217 are more selective to 1,5-PD (Table S1). So far, none of these catalysts are able to selectively produce 1,4-PD. For instance, only a maximum yield of 30% of 1,4-PD was obtained from furfural over a Pd–Ir–ReOx/SiO2 bifunctional catalyst.14 In contrast to 1,2- and 1,5-PD, 1,4-PD is currently not produced (industrially speaking) from fossil oils which opens business opportunities in terms of product development. At the lab-scale, 1,4-PD and its precursor (3-acetyl-1-propanol, 3-AP) were reported as valuable organic building blocks, in particular in the synthesis of chloroquine, one of the most effective medicines for both prevention and treatment of malaria.18 Switching the selectivity of the current catalytic processes from 1,2/1,5-PD to 1,4-PD is not an easy task and it requires searching for novel catalytic pathways. 1,4-PD was previously synthesized through the catalytic hydrogenation of levulinic acid (LA) or γ-valerolactone at 80–200 °C and under 6–15 MPa of H2 in the presence of metal catalysts such as Ru, Pt/Mo, Rh/Mo and Cu, to mention a few.19–23 Although moderate to good yields (70–96%) were obtained, the availability and/or the current price of levulinic acid or γ-valerolactone are still important limitations for such application.
image file: c8gc00039e-f1.tif
Fig. 1 Originality of this work.

Finding innovative catalytic routes capable of selectively converting furfural to 1,4-PD in a one pot process would definitely increase the attractiveness of the furfural platform for the supply of bio-based PD. To reach this objective, two criteria should be fulfilled: (1) identifying a novel reaction pathway to switch the selectivity of the reaction from 1,2/1,5-PD to 1,4-PD and (2) designing solid catalysts able to promote this reaction under mild conditions to prevent over-hydrogenation/hydrogenolysis reactions of PD or furfural.

This work addresses this issue and we disclose here a catalytic pathway capable of selectively converting furfural to 1,4-PD in a one-pot process in the presence of Ru supported on an ordered mesoporous carbon, called CMK-3 (Fig. 1). This catalytic route involves a series of cascade hydrogenation and acid-catalyzed reactions. Hydrogenation reactions were catalyzed by Ru nanoparticles while acid sites stemmed either from the CMK-3 support or the acidification of water with CO2 as previously reported.24 Thanks to the mild reaction conditions (60–80 °C and 1 MPa H2), 1,4-PD is selectively obtained in up to 90% yield. To the best of our knowledge, this work constitutes an important advance in the field by opening the first selective access to 1,4-PD from furfural.

Results and discussion

The ordered mesoporous carbon (CMK-3) support was synthesized by a hard-templating method using 2D hexagonal SBA-15 as an inorganic template, as described previously.25 More information on the synthesis of CMK-3 is provided in the ESI. The Ru species were introduced on the CMK-3 by an impregnation method with an actual Ru loading of 2 wt%. The sample was treated thermally at 300 °C in a N2 flow for 3 h yielding the so-called Ru/CMK-3-unred catalyst. The Ru/CMK-3-unred catalyst was then subjected to a temperature programmed reduction (TPR) treatment under 10% H2/He gas from ambient temperature to 200 and 400 °C, and denoted hereafter as Ru/CMK-3-R200 and Ru/CMK-3-R400, respectively.

The X-ray diffraction (XRD) patterns of pure CMK-3 and Ru/CMK-3 catalysts showed that all samples presented only two broad peaks at around 23° and 43° which were ascribed to amorphous carbon (Fig. S1).26 It should be noted that no Ru diffraction peak was observed on all Ru/CMK-3 samples, suggesting that Ru species are highly dispersed on the support. To verify this, high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was further used to characterize the Ru species. Typical HAADF-STEM images show that Ru nanoparticles are uniformly dispersed on the CMK-3 support, and the particle size distribution depends on the pretreatment conditions (Fig. 2, and for more images see Fig. S2 and S3). For the un-reduced Ru/CMK-3 catalyst, the average particle size is 0.9 nm, while it is 1.0 nm for Ru/CMK-3-R200 and 3.8 nm for Ru/CMK-3-R400, indicating that the reduction under mild conditions (e.g., 200 °C) does not cause the obvious growing of Ru nanoparticles while a higher reduction temperature (e.g., 400 °C) leads to the agglomeration of Ru nanoparticles.


image file: c8gc00039e-f2.tif
Fig. 2 HAADF-STEM images of Ru/CMK-3-unred (A, A′), Ru/CMK-3-R200 (B, B′) and Ru/CMK-3-R400 (C, C′) and the corresponding size distributions.

The oxidation states of Ru species were probed by X-ray absorption spectroscopy as it is sensitive to both the chemical state and the local structure of the measured metals. Considering that the reduced Ru species might be easily re-oxidized upon exposure to air, we performed an in situ measurement. The reduction procedure is slightly different from that of the TPR treatment. Under these conditions, 5% H2/He was used and the samples were reduced over 20 min at each temperature (for more details, see the ESI). The near edge (XANES) present in Fig. S4 suggests that the Ru species on the un-reduced Ru/CMK-3 sample exist in an oxidation state lower than +4 since the edge is lower than that of RuO2. After reduction at 200 °C, the Ru species were slightly reduced but they are still in a positively charged state. However, with reduction at 400 °C, Ru was almost completely reduced to the metallic state as its edge almost coincides with that of the Ru foil. A quick fitting of the spectra further confirmed this conclusion (Table S2). In the Ru/CMK-3-unred sample, besides the Ru–Ru bond, there are Ru–Cl and Ru–O bonds which should originate from the RuCl3 precursor and the interaction of Ru with the O-containing group on the CMK-3, respectively. In the Ru/CMK-3-R200 sample, Ru–Cl bonds are still observed while the Ru–O bond disappeared, which is consistent with a partial reduction of Ru. In the Ru/CMK-3-R400 catalyst, only the Ru–Ru bond was found, suggesting that Ru was fully reduced to form metallic Ru, in line with the near edge results. In addition, on the Ru/CMK-3-R400 sample, the coordination number of Ru–Ru was fitted to be 10.2. Considering that Ru has a very similar atomic radius to that of Pt, it could be roughly calculated, based on the coordination number according to a previous report,27 that the size of Ru NPs is about 3 nm. This value agrees well with the above described STEM results.

The acid sites of the catalysts were probed with NH3-TPD experiments and the results are shown in Fig. S5 and Table S3. Acid density varies with the reduction in temperature, and the Ru/CMK-3-R200 has the maximum acid density (425 μmol g−1), followed with Ru/CMK-3-unred (271 μmol g−1) and Ru/CMK-3-R400 (205 μmol g−1). In contrast, the blank CMK-3 support has the lowest acid density (61 μmol g−1), indicating that the acidity stems mainly from Ru species.28 Comparing the trend of acid density with that of the Ru valence state, we can conclude that acidity arises mainly from the positively charged Ru species, which is in accordance with the previous reports.28 The NH3-TPD profiles (Fig. S5) show that both Ru/CMK-3-unred and Ru/CMK-3-R200 contain primarily weak acid sites.

In the first set of experiments, furfural was hydrogenated in water in the presence of the Ru/CMK-3-R200 catalyst (0.57 mol% of Ru). The results are summarized in Table 1. At 80 °C and a pressure of H2 of 1 MPa, furfural was consumed completely after 20 h of reaction (entry 1). Unexpectedly, 1,4-PD was formed as the major product (61% yield), suggesting that this catalytic process did not occur through a classical hydrogenolysis mechanism. The only other detectable product was THFA (17% yield), resulting from the over-hydrogenation of furfural. The 22% unidentified carbon revealed that side reactions occurred concomitantly. When the H2 pressure was increased to 4 MPa, the yield of 1,4-PD remained unchanged. However, under these conditions, THFA was formed in 35% yield (entry 2), and only less than 5% of unknown products were formed. To our delight, when the reactor was pressurized with 2 MPa of CO2 and 2 MPa of H2 (total pressure of 4 MPa), the yield of 1,4-PD increased to 71% and further to 90% when the partial pressures of CO2 and H2 were adjusted to 3 MPa and 1 MPa, respectively (entries 3 and 4). These results suggest that the acidification of the reaction media (in situ formation of carbonic acid) had a positive effect on the selectivity to 1,4-PD. Interestingly, when the partial pressure of CO2 was further increased (CO2/H2: 3.5/0.5 MPa), the conversion of furfural decreased to 82% and the yield of 1,4-PD decreased to 57% (entry 5). Furthermore, without H2, no 1,4-PD was formed, indicating that the direct conversion of furfural to 1,4-PD presumably followed a bifunctional acid/hydrogenation catalytic route. More information on the reaction mechanism is provided below. It is worth noting that compared to the best results reported so far for the production of 1,2- and 1,5-PD from furfural (Table S1) and even 1,4-PD from LA (Table S4), the 1,4-PD yields obtained in this work are among the best ones, essentially thanks to the mildest conditions of pressures and temperatures.

Table 1 Optimization of the reaction parameters over the Ru/CMK-3 catalysta

image file: c8gc00039e-u1.tif

Entry Catalyst CO2/H2 (MPa/MPa) Conv. (%) 1.4-PD yield (%) THFA yield (%) FA yield (%) 3-AP yield (%) Others yieldb (%) Productivityd (mol−1 gRu−1 h−1) × 102
a Reaction conditions: 2 wt% furfural in H2O, 5 mL H2O, 20 h and a metal/substrate ratio of 0.57 mol% at 80 °C. b Other products stem either from over-hydrogenation reactions (C–C bond cracking) or acid-catalyzed degradation of intermediates leading to the formation of oligomeric and tar-like products. c At 60 °C. d Productivities (mol product per g of Ru and per hour).
1 Ru/CMK-3-R200 0/1 100 61 17 0 0 22 5.3
2 Ru/CMK-3-R200 0/4 100 61 34 0 0 5 5.3
3 Ru/CMK-3-R200 2/2 100 71 18 4 0 7 6.2
4 Ru/CMK-3-R200 3/1 100 90 9 0 0 1 7.8
5 Ru/CMK-3-R200 3.5/0.5 82 57 8 0 15 2 4.1
6 Ru/C 3/1 67 0 11 26 10 20 0
7 Ru/CMK-3-unred 3/1 100 75 10 0 0 15 6.5
8 Ru/CMK-3-R400 3/1 100 35 15 0 0 50 3.0
9c Ru/CMK-3-R400 3/1 100 6 35 0 41 18 0.5


The mesopores play an important role in the activity and selectivity of Ru/CMK-3-R200. When SBA-15 (template) was removed during the preparation of the catalyst, Ru dispersed over microporous carbon was obtained and denoted hereafter as Ru/C. In the presence of this Ru/C, no 1,4-PD was obtained (Table 1, entry 6). Instead, FA (26% yield), THFA (11% yield), 3-AP (10% yield) and 20% of unidentified chemicals were obtained. The lower BET surface and the lower amount of acid sites (91 μmol g−1) on Ru/C may explain this difference of catalytic performances with Ru/CMK-3-R200 (see the ESI).

To collect more information on the role of Ru, the un-reduced Ru/CMK-3 and the Ru/CMK-3-R400 were also tested under the optimized conditions (entries 7 and 8). Evidently, the un-reduced Ru/CMK-3 negatively influences the rate of the hydrogenation step and in this case, an acid-catalyzed reaction occurred dominantly leading to the conversion of furfural to undesirable side products; the yield of 1,4-PD decreased from 90 to 75% (entry 7). As can be seen in Fig. S6, Ru/CMK-3-R200 and Ru/CMK-3-R400 exhibited a similar activity (97% conversion vs. 92% after 5 hours of reaction for Ru/CMK-3-R200 and Ru/CMK-3-R400, respectively). However, Ru/CMK-3-R200 was significantly more selective to 3-AP and 1,4-PD than in Ru/CMK-3-R400, the latter favoring over-hydrogenation/hydrogenolysis reactions. Lowering the reaction temperature to 60 °C partly inhibited over-hydrogenation/hydrogenolysis reactions with Ru/CMK-3-R400, but the yield of 1,4-PD remained as low as 6% (entry 8). This result suggested that the Ru/CMK-3-R400 catalyst has a higher H2 activation ability than Ru/CMK-3-R200 which is only partly reduced. In order to verify this claim, a microcalorimetric measurement of H2 adsorption was carried out.29 The amount of adsorbed H2 on both samples was small, i.e., ∼2 μmol g−1 on Ru/CMK-3-R200 and ∼10 μmol g−1 on Ru/CMK-3-R400, respectively. However, the adsorption strength was totally different: initial adsorption heat on Ru/CMK-3-R200 was about 30 kJ mol−1 whereas on Ru/CMK-3-R400 a value of about 70 kJ mol−1 was determined which is similar to the adsorption heat on the Pd/C catalyst.30 These results demonstrated that Ru/CMK-3-R200 is less active for H2 activation than Ru/CMK-3-R400 and this moderate H2 activation ability is crucial to achieve high selectivity to 1,4-PD. The difference in the capability for H2 activation is closely associated with the electronic properties of Ru. The valence state of Ru decreases in the order of Ru/CMK-3-unred > Ru/CMK-3-R200 > Ru/CMK-3-R400 (Fig. S4), which is in reverse order with the initial adsorption heat of H2, indicating that metallic Ru0 is responsible for hydrogenation.

The recyclability of the Ru-CMK-3-R200 did not reveal any decrease of its catalytic activity cycle after cycle. However, the selectivity to 1.4-PD started decreasing after 3 catalytic cycles (Fig. S7), suggesting a change in the catalyst chemical composition. A hot filtration test revealed that it was a truly heterogeneously-catalyzed process which means that a proper reactivation of the Ru-CMK-3-R200 will be thus required for long term recycling.

To elucidate this unexpected reaction pathway observed with Ru/CMK-3-R200, the temperature of the reaction was varied (Fig. 3). In agreement with the Arrhenius law, the temperature has an important effect on the reaction progress. At 100 °C (CO2/H2: 3/1 MPa), THFA was formed in a higher proportion due to the enhanced hydrogenation ability of Ru/CMK-3-R200 at this temperature. In contrast, at 60 °C, 3-AP was formed as a dominant product (73% yield, and even 84% at a prolonged reaction time) while at 40 °C, FA was observed as the main product (70%). Note that the 3-AP yield obtained at 60 °C is comparable to or even better than those reported from previous studies involving bacteria or homogeneous catalysts (Table S5) using LA, alkynes and even 1,4-PD as a starting material.31–36 Altogether, these results suggest that the catalytic conversion of furfural to 1,4-PD involves a cascade of reactions with FA and 3-AP being two important intermediates.


image file: c8gc00039e-f3.tif
Fig. 3 Effects of reaction temperature on the conversion of furfural and the yields of products in the presence of Ru/CMK-3-R200. a[thin space (1/6-em)]Reaction conditions: 2 wt% furfural in H2O, 5 mL H2O, 20 h and a metal/substrate ratio of 0.57 mol%. b[thin space (1/6-em)]Reaction time: 30 h.

To support this claim, we plotted the yield of FA, 3-AP and 1,4-PD versus time at both 80 °C and 60 °C (Fig. 4). At 60 °C, FA was selectively formed with 82% yield up to 8 hours of the reaction. At a prolonged reaction time, the FA yield decreased gradually with a concomitant increase of the 3-AP yield up to 84% at 30 hours accompanied by the formation of a small amount of THFA and a negligible 1,4-PD generation. At 80 °C, the conversion of furfural reached 90% after 5 hours of reaction. In this case the main products were FA and 3-AP, accompanied by a small amount of THFA and 1,4-PD (Fig. 3b). At a prolonged reaction time, both FA and 3-AP were consumed and the yield of 1,4-PD increased gradually to reach a maximum of 90% after 20 h of reaction. These kinetic profiles clearly revealed that the catalytic process involves the following sequence (1) the hydrogenation of furfural to FA followed by (2) the conversion of FA to 3-AP and then (3) the hydrogenation of 3-AP to 1,4-PD.


image file: c8gc00039e-f4.tif
Fig. 4 Kinetic time course of the furfural transformation in the presence of Ru/CMK-3-R200 at 60 °C (A) and 80 °C (B). Reaction conditions: Furfural (0.1 g), H2O (5 mL), CO2/H2: 3 MPa/1 MPa, a metal/substrate ratio of 0.57 mol%.

In aqueous acidic media, it is generally believed that FA can react with water to form LA.37 Therefore, one possible route is the conversion of FA to LA, LA to 3-AP and finally 3-AP to 1,4-PD. However, in the kinetic profiles presented in Fig. 3 and 4, no LA was detected, suggesting that in our reaction conditions either LA was not an intermediate or its in situ hydrogenation to 3-AP was quasi instantaneous. To check this possibility, we then performed the reduction of LA under identical optimized conditions reported in Table 1, entry 4. To our surprise, neither 3-AP nor 1,4-PD were generated, and instead LA was converted to γ-valerolactone in 95% yield (Fig. S8A). A control experiment under identical experimental conditions but with γ-valerolactone as the starting material was further carried out and the result showed also that no conversion occurred at all within a 20-hour test (Fig. S8B). This result is consistent with the published reports which have indicated that the hydrogenation of γ-valerolactone over Ru-based solid catalysts required harsher conditions of pressure and temperature than ours.38

Next, the reaction was examined from FA in the presence of the Ru/CMK-3-R200 catalyst under the same reaction conditions but in the absence of H2. As shown in the kinetic profile presented in Fig. 5, under these conditions, 4-hydroxy-2-cyclopentenone (4-HCP) was formed with ∼25% yield after 1 h of reaction. LA was concomitantly formed but at a much lower rate. Other products were soluble and insoluble tar-like materials such as humins and/or oligomers of FA. 4-HCP can be obtained through the Piancatelli rearrangement of FA. In contrast to the conversion of FA to LA that typically occurs in the presence of strong acid sites, the Piancatelli rearrangement of FA is generally favored by the presence of weak acid sites.39–41 In good agreement, when the reaction was conducted under CO2/H2O in the absence of the Ru/CMK-3-R200 catalyst, 4-HCP was still produced as the major product, although at a different rate. From a catalytic point of view, this result indicates that Ru/CMK-3-R200 exhibits weak acid sites, otherwise LA would have been produced as the major product. In our case, the weak acid sites required for the reaction are thus provided by both the positively charged Ru species and the water-dissolved CO2.


image file: c8gc00039e-f5.tif
Fig. 5 Kinetic time course of the FA conversion in the CO2/H2O system at 80 °C in the absence of hydrogen.

From all these results, a plausible reaction pathway may be proposed to explain this unexpected and selective conversion of furfural to 1,4-PD instead of 1,2 or 1,5-PD as usual. The first step is the hydrogenation of furfural to FA. Then, FA underwent a Piancatelli rearrangement catalyzed by weak acids present on Ru/CMK-3-R200 or generated by dissolving CO2 in water (in situ production of carbonic acid). The overall Piancatelli transformation is believed to proceed through a cascade sequence that terminates with a 4π electrocyclic ring closure of a pentadienyl cation, as shown in Fig. S9. Under our reaction conditions, we propose that the pentadienyl cation was trapped in situ by hydrogenation over Ru/CMK-3, yielding 3-AP and then 1,4-PD. This reaction pathway is displayed in Fig. 6.


image file: c8gc00039e-f6.tif
Fig. 6 Proposed mechanism for 1,4-PD synthesis in the dual acid/hydrogenation catalytic system.

Conclusions

We demonstrate that the hydrogenation of furfural at 80 °C over Ru/CMK-3-R200 in water and under a CO2/H2 pressure permitted an access to 1.4-PD in up to 90% yield. This catalytic route is distinct from previous strategies in that it exclusively affords 1,4-PD instead of a mixture of 1,2- and 1,5-PD as usual. In addition, the mild conditions of pressure and temperature required in our strategy suppresses unwanted over-hydrogenation/hydrogenolysis reactions which are often an important limitation of the current catalytic processes converting furfural to 1,2/1,5-PD. We discovered that the conversion of furfural to 1,4-PD followed a catalytic pathway unreported to date. It implies a bifunctional catalytic pathway: (1) the catalytic hydrogenation of furfural to FA follows (2) an acid-catalyzed Piancatelli-type rearrangement yielding 3-AP, after in situ hydrogenation of the pentadienyl cation intermediate, and finally (3) the hydrogenation of 3-AP to 1,4-PD. Under our experimental conditions, the Piancatelli rearrangement of FA was favored by the presence of weak acid sites which are provided by the positively charged Ru species and the water-dissolved pressurized CO2. Importantly, by lowering the temperature of the reaction to 60 °C, it was even possible to selectively stop the reaction at the 3-AP product, another valuable chemical. It is noteworthy that a further drop of the temperature to 40 °C may also be a selective means to synthesize furfuryl alcohol, highlighting that the selectivity of this reaction can be easily tuned. To the best of our knowledge, this work constitutes an important advance in the field by opening a one-pot and selective access to 1,4-PD (or 3-AP) from furfural, thus definitely increasing the attractiveness of the furfural platform for the supply of bio-based PD.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21606227, 21690080–21690084, 21673228, 21776270), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020100), National Key Projects for Fundamental Research and Development of China (2016YFA0202801) and Department of science and technology of Liaoning province (2015020086-101). The synchrotron radiation experiment was performed at the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) operated at 200 mA and 2.2 GeV, and the BL08B2 of SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (Proposal No. 2017A3302).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8gc00039e
These authors contributed equally.

This journal is © The Royal Society of Chemistry 2018