Selective hydrogenation of amides and imides over heterogeneous Pt-based catalysts

Abstract

The hydrogenation of amides is an important but highly challenging reaction, which usually is applied under drastic conditions (temperature >160 °C). Here, we report a heterogeneous Pt–MoO_x/TiO₂ catalyst for amide and imide hydrogenation under milder conditions. The catalytic reactivity is proposed to originate from the synergistic effect between surface active species. Pt promotes formation of partially reduced MoO_x that is responsible for C=O activation, and MoO_x-modified Pt can more efficiently generate surface species from gas-phase H₂. This catalyst system enables the selective hydrogenation of several 2° and 3° amides as well as imides.

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1. Introduction

Amines constitute one of the most important classes of organic compounds. The majority of industrially applied amines are used for life science applications, specifically agrochemicals and pharmaceuticals. For the preparation of these products constantly new amine building blocks are needed. Among the various synthetic methodologies used for their preparation, reduction of amides is regarded as one of the most desired methodologies to produce amines since a plethora of starting materials is conveniently available.¹ However, this transformation also remains one of the most difficult and challenging reactions due to the low electrophilicity of C=O bond in amides,² which is less susceptible for nucleophilic attack. Thus, traditionally stoichiometric amounts (in reality in excess) of metal hydride reagents like LiAlH₄ or NaBH₄ are used for amide reduction, which inherently limit industrial-scale syntheses due to their price and safety aspects.³ In addition, considerable quantity of waste metal salts is generated during such processes. Obviously, catalytic reduction of amides to amines using molecular hydrogen offers a green solution since water is ideally the only by-product. Hence, the development of efficient and general catalysts for the hydrogenation of amides continues to be the research interest for many academic and industrial chemists.

In early studies, the hydrogenation of model amides, e.g., N-octanamide, acetanilide and N,N-dimethyldodecanamide, has been performed at rather harsh reaction conditions, i.e., at 200–350 °C and 200–370 bar H₂,⁴ using Ni-,⁵ Cu-⁶ and Re-based catalysts.⁷ In recent years, there is a growing interest in developing new catalysts for amide hydrogenation working at milder reaction conditions and significant progress has been made. From 2010, Whyman and co-workers developed a series of bimetallic materials (Rh/Mo,⁸ Ru/Mo,⁹ Rh/Re,¹⁰ and Ru/Re¹⁰ which are able to perform amide hydrogenation at milder conditions (130–160 °C and 50–100 bar H₂). Later on, Breit and co-workers¹¹ reported bimetallic Pd–Re/graphite and Pt–Re/graphite catalysts working at 120–160 °C and 5–70 bar H₂. Similarly, an effective Pt–Re/TiO₂ catalyst was reported by Thompson¹² and Cole-Hamilton¹³ as well as their co-workers. A solvent-free hydrogenation process was also achieved by Shimizu and co-workers¹⁴ using Lewis acidic Nb₂O₅ supported Pt catalyst at 180 °C and 50 bar H₂. In 2017, an interesting discovery was made by Kaneda and co-workers.¹⁵ They developed a Pt–V₂O₅ catalyst supported on hydroxyapatite (HAP) and obtained good yields of the corresponding amines. However, the toxicity of vanadium oxide limits its application potential. More recently, Wang and co-workers¹⁶ developed a bifunctional RuW/SiO₂ catalyst which afforded the hydrogenation of primary amides at 160 °C and 50 bar H₂. It is worth noting that molecularly-defined catalysts have been reported to be active for amide hydrogenations, too.^1,17 However, these homogeneous systems have several drawbacks from a practical point of view, like difficulties in catalyst recycling, requirement of acidic or basic additives, and complexity in ligand synthesis, restricting their application. In conclusion, the development of efficient and environmentally benign catalysts which allow for a practical and selective hydrogenation of amides is still highly desired and demanding.

Herein, we report a step towards this goal utilizing a Pt–MoO_x/TiO₂ heterogeneous catalyst, which permits the hydrogenation of several 2° and 3° amides as well as imides at comparably mild conditions (100–150 °C and 50 bar H₂). This bifunctional catalyst material was rationally designed to allow for both dissociation of H₂ and activation of C=O bond in an efficient manner. The synergistic effect of the key components Pt and MoO_x is revealed using various characterization techniques.

2. Experimental

2.1 Catalyst preparation

The catalysts were prepared by a wet-impregnation method.¹⁸ In a typical synthesis procedure, a desired amount of aqueous H₂PtCl₆ solution (7.5 mg Pt per mL) and ammonium molybdate tetrahydrate was first dissolved in 16 mL deionized water at 60 °C with vigorous stirring at 800 rpm for 15 min, followed by the addition of a desired amount of TiO₂ support. The metal loading of Pt is set as 5 wt% and the molar ratio of Mo/Pt is 1. The mixture was stirred for 16 h with reflux. Then, the mixture was cooled to room temperature and water was removed in vacuo using a rotatory evaporator. The solid sample was dried in an oven at 110 °C over night, after which it was thoroughly grounded to fine powders. Finally, the as-prepared samples were calcined in static air at 450 °C for 4 h (heating rate: 10 °C min⁻¹) in a muffle furnace. Before reaction, the samples were reduced in 5% H₂/Ar at 250 °C for 1 h (heating rate: 5 °C min⁻¹) in a flow-tube furnace. The preparation of other catalysts followed the similar procedure, except different metal precursors were used. For the preparation of Pt–VO_x/TiO₂, Pt(acac)₂ and V(acac)₃ were dissolved in acetone.

2.2 Activity tests

The catalyst (2 mol% of the corresponding metal), substrate (0.5 mmol) and solvent (2 mL) were added in a 4 mL vial with magnetic stirring bar and septum cap. Then, a needle was inserted in the septum, allowing H₂ to enter the vial. The vials (up to eight) were set in an alloy plate and placed in a 300 mL steel Parr autoclave. The autoclave was flushed with H₂ for 3 times at 10 bar and finally pressurized to the desired value. Then, it was placed into an aluminum block and heated to the desired temperature. When the reaction is completed, the autoclave was quickly cooled down to room temperature with an ice bath and vented. Finally, the samples were removed from the autoclave with the addition of internal standard (n-hexadecane, 50 μL) and ethyl acetate to the crude mixture, followed by filtration to separate the solid catalysts using a celite pad. The organic layer was analyzed at the GC-FID (Agilent 7890A).

2.3 Characterizations

X-ray Photoelectron Spectroscopy (XPS). The XPS measurements were performed on an ESCALAB 220iXL (Thermo Fisher Scientific) with monochromated Al Kα radiation (E = 1486.6 eV). Samples are prepared on a stainless-steel holder with conductive double-sided adhesive carbon tape. The electron binding energies were obtained with charge compensation using a flood electron source and referenced to the C 1s core level of adventitious carbon at 284.8 eV (C–C and C–H bonds). For quantitative analysis the peaks were deconvoluted with Gaussian–Lorentzian curves using the software Unifit 2021. The peak areas were normalized by the transmission function of the spectrometer and the element specific sensitivity factor of Scofield.

In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). A commercial reaction cell (Harrick) with CaF₂ windows were implemented into a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific) equipped with a MCT detector. The catalyst samples (powder, 60–70 mg) were placed into the reaction cell, which practically acts as a fixed-bed flow reactor and spectra were collected at 4 cm⁻¹ resolution and 64 scans. To probe the hydrogenation process of 4-acetylmorpholine (AM), the sample was heated up in helium to 120 °C with a holding time of 30 min and then exposed to a gas mixture, containing helium and AM. The admixture of AM took place via a gas saturator in which the liquid was enriched in the carrier flow. The carrier gas helium was passed through at room temperature and with a flow rate of 25 mL min⁻¹, so that the AM flow rate was about 0.76 mL min⁻¹. After the AM adsorption the sample a and b were flushed with He (25 mL min⁻¹, 30 min) and treated with H₂ (10 mL min⁻¹) for 150 min at 120 °C.

H/D exchange. H/D exchange experiments were carried out in the temporal analysis of products (TAP-2) reactor. This reactor system was described in detail elsewhere.¹⁹ In the performed experiments each catalyst (Pt/TiO₂ or Pt–MoO_x/TiO₂, 8 mg, sieve fraction of 250–400 μm) was packed between two layers of quartz particles (sieve fraction of 250–355 μm) within the isothermal zone of the microreactor made of quartz. The reactor was initially heated from room temperature to 120 °C under vacuum conditions of ca. 10 Pa. Then the reactor was evacuated to about 10⁻⁵ Pa. Hereafter pulse experiments with a D₂ : H₂ : Ar = 1 : 1 : 1 mixture were performed at the same temperature. The overall pulse size in these experiments was between 9 × 10¹⁴–1.1 × 10¹⁵ molecules per pulse. Under such conditions, gas-phase reactions are strongly suppressed, and heterogeneously catalyzed reaction steps were studied.

The feed mixture was prepared using D₂ (2.8, CK Special Gases Limited), H₂ (Air Liquide, 5.0) and Ar (Air Liquide, 5.0) without additional purification. Transient responses of the feed components and the reaction products were monitored at the reactor outlet using a quadrupole mass spectrometer (HAL RD 301 Hidden Analytical). The following m/z signals were recorded 4 (D₂), 3.0 (HD), 2.0 (H₂) and 40 (Ar). Ar was used as an inert standard. For each m/z, pulses were repeated 10 times and averaged to improve the signal-to-noise ratio. For a proper comparison of the appearance order and the shape of transient responses of substances differing in their molecular weights like D₂ and H₂, the data were transformed to a dimensionless form.¹⁹

3. Results and discussion

3.1 Reaction optimization

The general design strategy for the development of a more active amide hydrogenation catalyst is based on the idea of a bifunctional catalyst which combines specific metal sites for H₂ dissociation and an oxophilic promoter to adsorb and activate the carbonyl group. Following this concept, we initially screened diverse heterogeneous catalysts with different metal species (Table 1, entries 1–4), Lewis acidic promoters (Table 1, entries 1, 5–8) and supports (Table 1, entries 1, 9–11) for the hydrogenation of 4-acetylmorpholine to 4-ethylmorpholine as a model reaction. Here, Pt–MoO_x/TiO₂ catalyst showed a promising yield (80%) under comparably mild reaction conditions (120 °C, 50 bar H₂ and 16 h), outperforming other catalyst compositions, including previously developed as well as commercially available system (Table 1, entry 8). This may be because of the stronger Lewis acidic property of MoO_x and TiO₂ which could activate C=O group of amide more efficiently.¹⁴ Simply increasing the reaction time to 24 h resulted in an excellent yield (92%) of the desired product (Table 1, entry 12 and Fig. S1†). Stability test was performed at ca. 50% conversion, as shown in Fig. S2.† A slight decrease of activity was observed after each run. Inductively coupled plasma optical emission spectrometry (ICP-OES) test showed a slight Pt leaching after reaction (Table S2†), which could be the major reason for the decrease of activity. Although decreasing the reaction temperature or pressure led to a decline in the product yield (Table 1, entries 1, 13, 15–17), still 86% of N-ethyl morpholine was achieved at 100 °C (Table 1, entry 14). Notably, solvent screening showed that the best product yield was observed using the green solvent ethanol (Table S1†), which displayed in previous investigations with Pd/Re catalysts only a negligible activity.¹¹ To clarify this difference, we made a comparison using ethanol with different purities (>99.9% HPLC grade vs. 96% HPLC grade, Table S1,† entries 1 and 2). Only 15% yield of 4-ethylmorpholine was obtained using 96% ethanol as solvent. In addition, when 4% of water was added to the >99.9% ethanol a similar yield (22%) was detected (Table S1,† entry 3). These results clearly suggest that the presence of water in the reaction mixture plays a vital role in determining the reactivity. This is not surprising as water is one of the products for C–O cleavage reaction, the existence of which would shift the reaction equilibrium, inhibiting the reaction. In addition, the formed water (together with the amine product) would likely occupy a certain amount of catalytic active sites due to its incomplete desorption, resulting in a decreased reaction rate. This can be evidenced by the time-on-line plot (Table S1†), in which the reaction proceeds rapidly in the initial period, followed by being slowed down with increasing hours.

Table 1 Hydrogenation of 4-acetylmorpholine over heterogeneous catalysts

Entry	Catalyst	T/°C	P/bar	t/h	Y/%
T: temperature; P: H2 pressure; t: reaction time; Y: yield. Reaction conditions: 4-acetylmorpholine (0.5 mmol), [M] catalyst (2 mol%), EtOH (2 mL). Yield was determined by GC-FID using n-hexadecane as an internal standard.
1	Pt–MoOx/TiO2	120	50	16	80
2	Pd–MoOx/TiO2	120	50	16	2
3	Ru–MoOx/TiO2	120	50	16	11
4	Ni–MoOx/TiO2	120	50	16	0
5	Pt–WOx/TiO2	120	50	16	15
6	Pt–NbOx/TiO2	120	50	16	8
7	Pt–VOx/TiO2	120	50	16	31
8	Pt–Re/TiO2	120	50	16	57
9	Pt–MoOx/ZrO2	120	50	16	2
10	Pt–MoOx/Al2O3	120	50	16	2
11	Pt–MoOx/C	120	50	16	1
12	Pt–MoOx/TiO2	120	50	24	92
13	Pt–MoOx/TiO2	100	50	16	37
14	Pt–MoOx/TiO2	100	50	48	86
15	Pt–MoOx/TiO2	80	50	16	13
16	Pt–MoOx/TiO2	120	30	16	38
17	Pt–MoOx/TiO2	120	10	16	6
18	Pt/TiO2	120	50	16	8
19	MoOx/TiO2	120	50	16	0
20	TiO2	120	50	16	0
21	Pt/TiO2 + MoOx/TiO2	120	50	16	9
22	Blank	120	50	16	0

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To understand the contribution of the individual components of the most active catalysts, Pt/TiO₂, MoO_x/TiO₂ and TiO₂ materials were also prepared and tested showing negligible activity (Table 1, entries 18–20). In addition, the physical mixture of Pt/TiO₂ and MoO_x/TiO₂ only showed a comparable yield with that of Pt/TiO₂ (Table 1, entry 21). These results, compared with the activity of the bifunctional Pt–MoO_x/TiO₂ catalyst, clearly indicate that a synergistic effect exists between Pt and MoO_x, which will be further discussed in the following section. As expected, no activity was observed without catalyst (Table 1, entry 21).

3.2 Catalyst characterizations

X-ray Photoelectron Spectroscopy (XPS) measurements of the Pt–MoO_x/TiO₂ catalyst with the MoO_x/TiO₂ were performed to determine the surface valence state of Mo, as shown in Fig. 1. The peak deconvolution of both materials revealed two doublets for each peak. The doublet with Mo 3d_5/2 at 231.9 eV and Mo 3d_3/2 at 235.0 eV can be assigned to Mo⁵⁺.²⁰ The other peaks including Mo 3d_3/2 around 236.2 eV and Mo 3d_5/2 at 233.0 eV are characteristic peaks for Mo⁶⁺.^20,21 Additionally, we calculated the ratio of Mo⁵⁺ and Mo⁶⁺ in both samples. As shown in Fig. 1, a higher Mo⁵⁺ ratio (27 at%) was observed over Pt–MoO_x/TiO₂ compared to MoO_x/TiO₂ (23 at%).

Fig. 1 XPS spectra of the Mo 3d region of Pt–MoO_x/TiO₂ (upper graph) and MoO_x/TiO₂ catalysts (lower graph).

These results suggest that the existence of Pt nanoparticles could reduce their neighboring Mo species to a certain extend. In other reported bifunctional catalysts (Ru–WO_x/SiO₂ ¹⁶ and Pt–V₂O₅/HAP¹⁵), the low-coordinated W⁵⁺ and V³⁺ were proposed to activate the C=O bond in amides. Hence, we would naturally consider that in our case, these coordinately unsaturated Mo species might also contribute to the adsorption and activation of carbonyl group in the amide. To verify the above-mentioned effect of Mo on C=O activation, we carried out the adsorption of 4-acetylmorpholine over four samples (TiO₂, MoO_x/TiO₂, Pt/TiO₂, Pt–MoO_x/TiO₂) using in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), as shown in Fig. 2. The peaks at 1589–1596 cm⁻¹ are ascribed to the stretching vibration of C=O group (ν_C=O) in the amide. The bands at 2841 cm⁻¹ and 2931 cm⁻¹ are attributed to the symmetric and asymmetric stretching vibration of cyclic –CH₂–, respectively. The ones at 2855 and 2960–2972 cm⁻¹ can be assigned to the symmetric and asymmetric stretching vibration of aliphatic –CH₃, respectively.²² A red shift of ν_C=O over Pt–MoO_x/TiO₂ (1589 cm⁻¹) is observed compared with the peaks of the other three samples (1596 cm⁻¹), suggesting that C=O moiety is activated most by Pt–MoO_x/TiO₂. This result confirms our previous hypothesis, i.e., the Mo species reduced by its neighboring Pt nanoparticles contributes to the activation of carbonyl group.

Fig. 2 In situ DRIFTS for the adsorption of 4-acetylmorpholine over TiO₂, MoO_x/TiO₂, Pt/TiO₂, Pt–MoO_x/TiO₂ at 120 °C.

Besides C=O activation, the ability of the catalysts to activate gas-phase H₂, i.e., to provide surface hydrogen species, is also an important factor determining their overall performance in the hydrogenation of 4-acetylmorpholine. To check if the presence of MoO_x is decisive for this catalyst property, we carried out H/D exchange tests with Pt–MoO_x/TiO₂ and Pt/TiO₂ under transient conditions in the TAP reactor.^19,23 As evidenced by the formation of HD (Fig. 3), both catalysts are able to break H–H and D–D bonds and to form new H–D bonds. These reactions occur exclusively on the surface of catalysts. Although H₂ and D₂ are partially converted into HD, there is an intersection between their responses and the response of Ar. According to the theory of the TAP reactor,¹⁹ such intersection is a fingerprint of reversible adsorption of the reactants. Thus, the presence of MoO_x does not influence the mechanism of hydrogen activation. However, it influences the rate of H/D exchange. To illustrate this, we calculated the difference (Δt_max) between the dimensionless time of the maximal dimensionless flow of Ar (this time characterizes simple diffusion only) and that of HD (this time represents both diffusion and HD formation). The Δt_max values for Pt/TiO₂ and Pt–MoO_x/TiO₂ are 0.74 and 0.60, respectively. Thus, the MoO_x promoter enhances the rate of H/D exchange. This conclusion is also supported by the amount of HD formed in these experiments. The HD yield determined for Pt/TiO₂ is 35%, while in the presence of MoO_x it increases to 49%.

Fig. 3 Dimensionless responses of Ar, D₂, H₂ and HD recorded after pulsing of a D₂ : H₂ : Ar = 1 : 1 : 1 mixture in the TAP reactor over (a) Pt/TiO₂ and (b) Pt–MoO_x/TiO₂ at 120 °C.

To probe the reaction process, we carried out in situ DRIFTS measurements for the hydrogenation of 4-acetylmorpholine over Pt–MoO_x/TiO₂. The substrate was carried by He and introduced into the in situ cell, being adsorbed on the catalyst surface. Then, the gas was switched to H₂ for the hydrogenation reaction. The spectra were recorded as a function of time. As shown in Fig. 4, a peak around 2040 cm⁻¹, corresponding to Pt–H bond,²² immediately appeared after the dosage of H₂. This is easy to understand as Pt is very active for H₂ adsorption and dissociation. Meanwhile, the peak intensity of ν_C=O at 1589 cm⁻¹ quickly decreased, as also described in the inset of Fig. 4. This result indicates that the carbonyl group of the amide, which was activated by the coordinatively unsaturated Mo species, could readily react with H dissociated on Pt site. In addition, a control experiment was carried out using morpholine and ethanol solvent at 120 °C and 81% yield of N-ethylmorpholine was obtained (eqn (a)). This result suggests that the Pt–MoO_x/TiO₂ catalyst is also active and selective for alcohol amination reactions. It explains the high product selectivity when using ethanol as solvent. As shown in Table S1,† the reaction could also proceed with other solvents like i-PrOH, ethers, cyclohexane, and toluene. While the selectivity is not as high as that in ethanol, since C–N cleavage competitively took place which gives morpholine as the byproduct. In ethanol, it is also possible that C–N cleavage occurred. But the formed morpholine would readily react with ethanol to give N-ethylmorpholine. This result clearly shows that the desired secondary amine can be formed either directly or in a two-step process involving initial hydrogenolysis, followed by subsequent hydrogen borrowing alkylation. To understand which reaction pathway is dominating in the presence of our catalyst, the hydrogenation of N-acetylmorpholine was performed in 1-butanol as solvent (eqn (b)). The ratio of N-(n-butyl)morpholine and N-ethylmorpholine is indicative for the ratio of the two- or one-step hydrogenation process. As shown in eqn (b), 86% N-ethylmorpholine and 14% N-(n-butyl)morpholine are observed in the product mixture, suggesting that the one-step hydrodeoxygenation process dominates in the reaction pathway. Based on the above results, we proposed the reaction pathway as shown in Fig. 5.

Fig. 4 In situ DRIFTS for the hydrogenation of 4-acetylmorpholine over Pt–MoO_x/TiO₂ at 120 °C. Inset: integrated area of the bands at 1589 cm⁻¹vs. time.

Fig. 5 Proposed mechanism for the hydrogenation of 4-acetylmorpholine over Pt–MoO_x/TiO₂.

3.3 Substrate scope

With an optimized amide hydrogenation catalyst and suitable reaction conditions in hand, we then investigated the reactivity of selected other substrates utilizing this Pt–MoO_x/TiO₂ catalyst (Table 2). Moderate to good yields were observed for the hydrogenation of tertiary amides at 120–150 °C (Table 2, entries 1–8). In case of a strongly electron-withdrawing group (–CF₃) – presented next to the C=O bond, C–N cleavage readily took place (Table 2, entry 6). With respect to the hydrogenation of a secondary amide (ε-caprolactam), 66% yield of the secondary amine (azepan, Table 2, entry 10) was obtained using 1,4-dioxane as solvent to prevent further alcohol amination. Applying a primary amide, no desired product was observed with this Pt–MoO_x/TiO₂ catalyst (Table S2,† entry 1), similar with the results reported using Pt-based catalysts.¹⁵ Interestingly, this catalyst also works well for the selective hydrogenation of imides (Table 2, entries 11–13) and esters (Table S2,† entry 4), giving >90% yield of corresponding amides and 87% of benzyl alcohol, respectively.

Table 2 Hydrogenation of amides and imides over Pt–MoO_x/TiO₂ catalyst

Entry	Substrate	Product	Temperature/°C	Time/h	Selectivity/%	Yield/%
Reaction conditions: substrate: 0.5 mmol, [Pt] catalyst (2 mol%), H2 (50 bar), EtOH (2 mL). Yield was determined by GC-FID using n-hexadecane as an internal standard.a 1,4-Dioxane as solvent.b C–N cleavage mainly accounts for the byproducts.
1			120	24	99	92
2			120	24	99	35
3			140	48	99	54
4			120	24	98	56^a
5			150	48	97	97^a
6			120	24	99	98^a
7			120	24	60^b	53
8			120	24	75^b	74
9			120	24	70^b	37^a
10			140	48	67^b	66^a
11			120	24	98	96
12			120	24	90	75^a
13			140	24	95	90^a

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4. Conclusions

We presented a Pt–MoO_x/TiO₂ catalyst for the hydrogenation of amides to amines. Compared to most known catalysts for this challenging transformation, this novel material allows for selective reductions under comparably mild conditions. Characterizations revealed the synergistic effect between Pt and MoO_x. In more detail, the latter species which is reduced by neighboring Pt nanoparticles, is responsible for amide adsorption and activation of its C=O bond. On the other side, modifying Pt by MoO_x enhances the capability of the former to dissociate H₂. The bifunction of Pt and MoO_x contributes to the good catalytic performances at a temperature of 120 °C. Control experiments revealed a one/two-step hydrogenation pathway as the main mechanism. Apart from amide reductions, this catalyst system also offers interesting possibilities for the green and sustainable alkylation of amines by alcohols.

Author contributions

R. Q. conceived the idea and designed the experiments; R. Q. prepared the catalysts; R. Q., S. M. and H. Q. performed the experiments and analyzed the data; J. W. performed in situ DRIFTS measurements; V. K., and E. K. were responsible for TAP experiments; S. B. carried out XPS experiments; A.-E. S. worked on the heat treatment of the catalysts; K. J. and M. B. supervised the research activities and supported the project with funding acquisition; R. Q., K. J. and M. B. co-wrote the paper; all the authors contributed to editing the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the staff in the analytical department of the Leibniz Institute for Catalysis for their excellent service. R. Q. and H. Q. gratefully acknowledges the financial support from the Alexander von Humboldt Foundation (CHN 1212814 HFST-P and CHN 1220532 HFST-P).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01274c

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