Ruiyang
Qu
,
Shuxin
Mao
,
Jana
Weiß
,
Vita A.
Kondratenko
,
Evgenii V.
Kondratenko
,
Stephan
Bartling
,
Haifeng
Qi
,
Annette-Enrica
Surkus
,
Kathrin
Junge
* and
Matthias
Beller
*
Leibniz-Institut für Katalyse e.V., Albert-Einstein-Straße 29a, 18059 Rostock, Germany. E-mail: Kathrin.Junge@catalysis.de; Matthias.Beller@catalysis.de
First published on 4th December 2023
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–MoOx/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 MoOx that is responsible for CO activation, and MoOx-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.
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–MoOx/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 CO bond in an efficient manner. The synergistic effect of the key components Pt and MoOx is revealed using various characterization techniques.
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
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 |
To understand the contribution of the individual components of the most active catalysts, Pt/TiO2, MoOx/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 MoOx/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–MoOx/TiO2 catalyst, clearly indicate that a synergistic effect exists between Pt and MoOx, which will be further discussed in the following section. As expected, no activity was observed without catalyst (Table 1, entry 21).
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Fig. 1 XPS spectra of the Mo 3d region of Pt–MoOx/TiO2 (upper graph) and MoOx/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–WOx/SiO216 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, MoOx/TiO2, Pt/TiO2, Pt–MoOx/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 (νC
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 νC
O over Pt–MoOx/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–MoOx/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.
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Fig. 2 In situ DRIFTS for the adsorption of 4-acetylmorpholine over TiO2, MoOx/TiO2, Pt/TiO2, Pt–MoOx/TiO2 at 120 °C. |
Besides CO 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 MoOx is decisive for this catalyst property, we carried out H/D exchange tests with Pt–MoOx/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 MoOx does not influence the mechanism of hydrogen activation. However, it influences the rate of H/D exchange. To illustrate this, we calculated the difference (Δtmax) 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 Δtmax values for Pt/TiO2 and Pt–MoOx/TiO2 are 0.74 and 0.60, respectively. Thus, the MoOx 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 MoOx it increases to 49%.
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Fig. 3 Dimensionless responses of Ar, D2, H2 and HD recorded after pulsing of a D2![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
To probe the reaction process, we carried out in situ DRIFTS measurements for the hydrogenation of 4-acetylmorpholine over Pt–MoOx/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 νCO 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–MoOx/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.
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Fig. 4 In situ DRIFTS for the hydrogenation of 4-acetylmorpholine over Pt–MoOx/TiO2 at 120 °C. Inset: integrated area of the bands at 1589 cm−1vs. time. |
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 |
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120 | 24 | 99 | 92 |
2 |
![]() |
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120 | 24 | 99 | 35 |
3 | 140 | 48 | 99 | 54 | ||
4 |
![]() |
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120 | 24 | 98 | 56a |
5 | 150 | 48 | 97 | 97a | ||
6 |
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120 | 24 | 99 | 98a |
7 |
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120 | 24 | 60b | 53 |
8 |
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120 | 24 | 75b | 74 |
9 |
![]() |
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120 | 24 | 70b | 37a |
10 | 140 | 48 | 67b | 66a | ||
11 |
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120 | 24 | 98 | 96 |
12 |
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120 | 24 | 90 | 75a |
13 | 140 | 24 | 95 | 90a |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cy01274c |
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