Silver-anchored porous aromatic framework for efficient conversion of propargylic alcohols with CO<sub>2</sub> at ambient pressure

Silver-anchored porous aromatic framework for efficient conversion of propargylic alcohols with CO₂ at ambient pressure

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Xiao-Xuan Guo^a^,¹, Zhao-Tian Cai^a^,¹, Yaseen Muhammad^b, Feng-Lei Zhang^c, Rui-Ping Wei^a, Li-Jing Gao^a, Guo-Min Xiao^a^,^*

Chinese Chemical Letters | 2023, 34(4) : 107740

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Chinese Chemical Letters | 2023, 34(4): 107740

• Communication •

Silver-anchored porous aromatic framework for efficient conversion of propargylic alcohols with CO₂ at ambient pressure

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Xiao-Xuan Guo^a^,¹, Zhao-Tian Cai^a^,¹, Yaseen Muhammad^b, Feng-Lei Zhang^c, Rui-Ping Wei^a, Li-Jing Gao^a, Guo-Min Xiao^a^,^*

Affiliations

^a School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

^b Institute of Chemical Sciences, University of Peshawar, 25120, KP, Pakistan

^c Intelligent Transportation System Research Center, Southeast University, Nanjing 211189, China

Published: 2023-04-15 doi: 10.1016/j.cclet.2022.08.020

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Abstract

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The conversion of propargylic alcohols and carbon dioxide (CO₂) into fine chemicals suffers from issues of harsh reaction conditions and difficult catalyst recovery. To achieve efficient CO₂ activation at low energy consumption, a silver-anchored porous aromatic framework catalyst Ag@PAF-DAB with high active phase density and CO₂ adsorption capacity was proposed. Since Ag@PAF-DAB has the dual functions of CO₂ capture and conversion, propargylic alcohols were completely converted into α-alkylidene cyclic carbonate or α‑hydroxy ketone as high value-added product under atmospheric pressure (CO₂, 0.1 MPa) and low silver equivalent (0.5 mol%). Notably, Ag@PAF-DAB exhibited broad substrate diversity, high stability, and excellent reusability. By applying FTIR and GC, the key to green synthetic route of α‑hydroxy ketone was confirmed to lie in the further hydration of α-alkylidene cyclic carbonate.

Key words

Porous aromatic framework / CO₂ capture / CO₂ conversion / α-Alkylidene cyclic carbonate / α-Hydroxy ketone

Cite this Article

Xiao-Xuan Guo, Zhao-Tian Cai, Yaseen Muhammad, Feng-Lei Zhang, Rui-Ping Wei, Li-Jing Gao, Guo-Min Xiao. Silver-anchored porous aromatic framework for efficient conversion of propargylic alcohols with CO₂ at ambient pressure[J]. Chinese Chemical Letters, 2023 , 34 (4) : 107740 - . DOI: 10.1016/j.cclet.2022.08.020

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The utilization of CO₂ as a raw material to produce high value-added chemicals is a profitable strategy while concurrently diminishes the obnoxious environmental impacts by decreasing CO₂ concentration in the atmosphere [1, 2]. However, CO₂ is an extremely stable molecule with a dissociation energy of 750 kJ/mol for the C=O bond, which is higher than other bonds such as C—H (430 kJ/mol) and C—C (336 kJ/mol) [3]. Therefore, the biggest obstacle to the large-scale production of important chemicals using CO₂ as a raw material is overcoming the high thermodynamic energy barriers and kinetic inertia to realize the conversion of CO₂. The synergistic activation of CO₂ with co-reagents (epoxides and nitrogen heterocyclic compounds etc.) is also crucial to achieve low energy consumption and high efficiency of CO₂ conversion. The intrinsic energy of these co-reagents provides the driving force that enhances the feasibility of CO₂ conversion [4].

Benefiting from the rapid development of green chemistry, scientists have developed various strategies to convert CO₂ into high value-added chemicals [5, 6]. Among them, the carboxylative cyclization reaction of propargylic alcohols as a co-reagent and CO₂ to generate α-alkylidene cyclic carbonates has high atom economy with high market value of the final product [7, 8]. Over the past decade, great efforts have been devoted to developing suitable catalytic systems for carboxylative cyclization reaction. Metal catalytic systems mainly include Ru [9], Ag [10], Pb [11], Cu [12], Zn [13] and K₂CO₃/crown ethers [14], as well as organic catalytic systems like N-heterocyclic carbenes [15], N−heterocyclic olefins [16] and bicyclic guanidines [17] have been reported. Although significant progress has been made, there are widespread issues with these methodologies such as harsh reaction conditions, difficult catalyst recovery, and poor product selectivity. Therefore, the design of efficient and recyclable catalysts for CO₂ carboxylative cyclization remains a challenge.

At present, porous aromatic framework materials (PAFs), as a kind of important organic polymer materials, have attracted great interest of researchers due to their high CO₂ adsorption capacity, excellent chemical tunability and outstanding stability [18, 19]. PAFs not only serve as suitable carriers of metal active site but also effectively couple CO₂ capture and conversion. Inspired from these intriguing properties, herein, we report a silver based porous aromatic framework-3,3′-diaminobenzidine (Ag@PAF-DAB) catalyst with high active phase density and CO₂ adsorption capacity (Scheme 1a). Firstly, PAF-DAB was synthesized by efficient assembly of DAB (3,3′-diaminobenzidine) as building blocks through oxidative coupling reaction. Then, the azo functional groups of acted as stabilizers for the growth of Ag, ensuring that the Ag nanoparticles uniformly dispersed on PAF-DAB and maintained suitable sizes. Since the structure of PAF-DAB contains rigid aromatic group units connected by C—C bonds, thus it exhibits ultra-high specific surface area (SSA) and excellent stability [20]. The high active phase density and CO₂ adsorption capacity endows Ag@PAF-DAB with excellent catalytic activity and selectivity in the carboxylative cyclization of propargylic alcohols and CO₂ under ambient reaction conditions. Interestingly, a high-value-added product, i.e., α‑hydroxy ketone, was obtained in high yield at 80 ℃ and adding 2 equiv. H₂O in the original carboxylative cyclization reaction. The results show that Ag@PAF-DAB can be concomitantly and efficiently applied in the two reaction systems and hence enhances its broad-spectrum applications for large-scale applications (Scheme 1b).

Firstly, the PAF-DAB was synthesized by DAB monomer under the oxidative action of (diacetoxyiodo)benzene [21]. Then, Ag@PAF-DAB catalysts were obtained after introducing high catalytic activity phase "Ag" into the framework, which contains rich azo groups. The Ag@PAF-DAB catalyst could be stored in air for a long time and is insoluble in water or general organic solvents such as DCM and DMF, proving that the catalyst has a highly stable cross-linked structure. In the solid-state ¹³C NMR spectrum of PAF-DAB (Fig. S1 in Supporting information), a series of strong signals are observed at the chemical shifts of 110–155 ppm. Compared with the ¹³C NMR spectrum of DAB (Fig. S2 in Supporting information), it can be seen that these peaks are attributed to aromatic carbon of benzene ring, indicating that the raw material components DAB are successfully combined into polymerization network [21]. As shown in Fig. 1a, the stretching vibration peak (ν = 3380 cm⁻¹) and bending vibration peak (ν = 1637 cm⁻¹) are assigned to primary amine N−H [22]. These characteristic peaks disappeared in the aromatic skeleton of PAF-DAB, while a new absorption peak appeared at ν = 1410 cm⁻¹, which was attributed to the stretching vibration peak of −N=N−, demonstrating the successful coupling and polymerization of monomer through -NH₂ groups oxidation [23]. PAF-DAB has no diffraction peaks and presents an amorphous state as observed by PXRD pattern (Fig. 1b). However, there are obvious diffraction peaks at 38.1°, 44.2°, 64.5°, 77.5° in Ag@PAF-DAB after loading Ag, which correspond to the standard PDF card of Ag(0) (JCPDS: No. 89–3722) [24]. These results provide sufficient evidence that the active center Ag in the catalyst mainly exists in the form of elementary substance.

In the full survey XPS spectra, it can be seen that PAF-DAB and Ag@PAF-DAB are consistent with the raw material components (Fig. S3 in Supporting information). Specifically, in the N 1s XPS spectra (Fig. 1c), the DAB monomer shows a signal at the binding energy (BE) of 398.4 eV, which is ascribed to N atom in −NH₂ [25]. This signal weakened significantly after the coupling, and a new signal appeared at the BE = 399.2 eV, which was attributed to N atom in −N=N−, confirming that –NH₂ was successfully bonded to form an azo group under oxidation environment [26]. After the coordination of Ag and azo group, the electron cloud density of N decreased and BE increased to 399.4 eV. Fig. 1d indicate that the BE positions of Ag 3d_5/2 and Ag 3d_3/2 in Ag@PAF-DAB appeared at 367.8 eV and 373.8 eV, which were consistent with the reported signal of Ag element, indicating the active elemental Ag center was successfully incorporated into the aromatic skeleton [27]. In addition, the content of Ag element in the catalyst was determined to be 7.26% by ICP-OES which is close to the theoretical value.

It is evident from SEM image (Fig. S4a in Supporting information) that Ag@PAF-DAB presents a micron-scale amorphous lumped structure, and the elemental distribution diagrams show that polar heteroatoms N and active center Ag are uniformly distributed on the aromatic framework (Figs. S4b and S5 in Supporting information). It was observed from TEM analysis that the loaded Ag nanoparticles on the aromatic framework were uniform in size and distribution, with a particle size of about 35 nm (Figs. 2a and b). Moreover, adsorption-desorption experiments were carried out with N₂ as the probe molecule at 77 K to conduct the SSA and pore size distribution of the aromatic framework materials. Fig. S6 (Supporting information) shows that their N₂ adsorption-desorption isotherms belong to typical Ⅰ-type curves [28], and the SSA of PAF-DAB is 753.4 m²/g. The SSA of Ag@PAF-DAB was slightly reduced to 561.9 m²/g after Ag loading, which is attributed to the coverage by metal [29], but the pore size remained unchanged at 0.59 nm (Fig. S7 in Supporting information). These results reveal that the aromatic skeleton catalyst exhibits high SSA and plentiful pore structure, which is beneficial to the efficient mass/heat transfer during the catalytic reaction. Fig. S8 (Supporting information) shows that the CO₂ adsorption amount is as high as 3.21 mmol/g at 273 K and 0.1 MPa, and the adsorption amount remained at 1.98 mmol/g at 298 K and 0.1 MPa. The CO₂ equivalent heat of adsorption of Ag@PAF-DAB was 36.2 kJ/mol under low CO₂ coverage according to the Clausius-Clapeyron equation [30]. This higher value demonstrates that the N-rich porous aromatic framework has a strong physical adsorption effect on CO₂, corresponding to the previous results of high CO₂ adsorption [31, 32]. Based on the above analysis, it can be predicted that the highly dispersed metal active centers and CO₂ adsorption will greatly intensify the subsequent CO₂ catalytic conversion.

After the synthesis and characterization of Ag@PAF-DAB catalyst, its catalytic performance in the carboxylative cyclization reaction of propargylic alcohols with CO₂ was investigated at room temperature and pressure. 2-Methyl-3-butyn-2-ol was selected as the model substrate and DBU (1,8-diazabicyclo[5.4.0]undec‑7-ene, 1 equiv.) was used as an effective base additive for the carboxylative cyclization reaction. The experiment was carried out according to the general procedure of the conversion reaction of propargylic alcohols and CO₂. The results indicated that in the absence of catalyst or 0.5 mol% PAF-DAB catalyst in the system, no product was generated after 36 h of reaction (Table S1 in Supporting information). When 0.5 mol% Ag@PAF-DAB was added, a nearly complete yield of α-alkylidene cyclic carbonate was obtained within 30 h (Fig. S9 in Supporting information). Compared with Ag-based catalysts previously reported, this amount of catalyst is much less [24, 33-38], which is also a rare CO₂ conversion reaction under these conditions, as shown in Table S2 (Supporting information). The high efficiency of Ag@PAF-DAB is attributed to the construction of the polymer framework that provides a microenvironment which is conducive for the reaction. At the same time, the high and uniform distribution of the Ag active center facilitates the proximity of the substrates, thus improving the reaction efficiency.

Encouraged by the successful experimental results, Ag@PAF-DAB was applied for the hydration reaction of propargylic alcohol. The experimental procedure was the same as that for carboxylative cyclization, except that additional H₂O (2 equiv.) and raising reaction temperature to 80 ℃. Similarly, no product was generated in the absence of active center Ag(0) or CO₂ in the blank experiment (Table S3 in Supporting information). However, under the catalysis of 0.5 mol% Ag@PAF-DAB, propargylic alcohol was completely converted within 24 h to generate another high value-added product i.e., α‑hydroxy ketone (Fig. S10 in Supporting information). In addition, the Ag@PAF-DAB system was still extended in the conversion of various types of substrates, including aliphatic chain-substituted, aliphatic-ring-substituted, and aromatic-ring-substituted substrates (Fig. 3). Apparently, the difficulty of catalytic conversion of substrates depends on steric hindrance effect of substrate substituents [39]. When the substrate substituent was methyl (1a and 2a), ethyl (1b and 2b), diethyl (1c and 2c) and other groups with less steric hindrance, the target product was obtained in high yield in a short time. When the substituents of the substrate were cyclopentyl (1d and 2d) and/or cyclohexyl (1e and 2e), the yield of the target product decreased significantly. In addition, the catalytic substrates were not limited to terminal propargylic alcohols, but also exhibited certain catalytic activity for endopropargylic alcohols (1f and 2f). These findings thus concluded that Ag@PAF-DAB exhibits catalytic diversity and remarkable functional group tolerance and hence has a wide application prospect.

The recycling performance of catalysts is an important index to evaluate the stability of catalysts. The reusability of Ag@PAF-DAB was separately investigated in carboxylative cyclization and hydration reactions. The catalyst was reused directly in the next cycle after simply filtering, washing and drying. It is worth noting that since the collection process caused a small loss of catalyst, fresh catalyst (< 15 wt%) was supplemented in the second and third cycle to ensure that the amount of catalyst was 0.5 mol%. According to Fig. S11 (Supporting information), Ag@PAF-DAB catalyzed the above two model reactions with high conversion (> 80%) for three successive reuses. Among them, the cycle performance decreased after the first cycle caused by Ag content loss (6.51%) in the long carboxylative cyclization reaction time. The ICP-OES analysis results showed that the Ag content of the reused Ag@PAF-DAB catalyst after three cycles in the two reactions was 6.23% and 6.84%, respectively, which was slightly lower than that of the fresh catalyst. Moreover, PXRD tests were performed on Ag@PAF-DAB(Ⅰ) and Ag@PAF-DAB(Ⅱ) and the results revealed that the PXRD pattern of reused catalyst realized minimal changes (Fig. S12 in Supporting information), thereby confirming the high stability of the catalyst. These results conclude that Ag@PAF-DAB could efficiently balance catalyst dose minimization and stability by its excellent recyclability and hence can be deemed of great promise for practical applications.

The reaction mechanism was further explored. Firstly, signal peak of hydroxyl group was detected by ¹H NMR spectroscopy (Fig. S13 in Supporting information). Propargylic alcohol showed a sharp single peak at δ = 5.28 ppm, which was attributed to the H atom signal peak in the alcoholic hydroxyl group. However, in the propargylic alcohol/DBU system, the signal peak of H changed to a wide peak, indicating that DBU could activate the hydroxyl group of propargylic alcohol to facilitate subsequent CO₂ insertion into −O···H. In addition, real time FT-IR and GC analyses were used to monitor the reaction process. The FR-IR spectra detected that the stretching vibration peaks of C=O (ν = 1825 cm⁻¹ and 1685 cm⁻¹) and C−O (ν = 1172 cm⁻¹ and 1034 cm⁻¹) got gradually clearer and sharper with the accumulation of time within 240 min of the reaction, confirming the process of CO₂ activation and insertion (Fig. 4a). It was further confirmed by the GC spectrum that propargylic alcohol was gradually converted into α-alkylidene cyclic carbonate. Moreover, the presence of DBU causes the chemical shift of the H atom in H₂O to the downfield by ¹H NMR spectroscopy, which is due to their hydrogen bond association [40]. It indicates that DBU can directly activate H₂O molecules (Fig. S14 in Supporting information). After adding water, the FT-IR spectrum monitored the stretching vibration peaks belonging to C=O and C−O gradually broadened and disappeared within 240–360 min, corresponding to the decarboxylation of α-alkylidene cyclic carbonate and the release of CO₂ (Fig. 4b). Meanwhile, the GC spectrum showed that α-alkylidene cyclic carbonate was converted into α‑hydroxy ketone as the final product within this time period (Fig. 4c). It can be seen that the key to the synthesis path of α‑hydroxy ketone lies in the further hydration of the intermediate (α-alkylidene cyclic carbonate). Based on the experimental results and previous literature reports [34, 41], we proposed the possible route for the carboxylative cyclization and hydration of propargylic alcohols with CO₂ catalyzed by Ag@PAF-DAB system as shown in Fig. S15 (Supporting information). Incipiently, the complex 1 is generated by the coaction of Ag@PAF-DAB, propargylic alcohol and DBU. The activated alcoholic hydroxyl group of the substrate further captures CO₂ with the assistance of organic framework to generate carbaminate adduct 2. Subsequently, the carbaminate rapidly attacks the activated alkynyl group, resulting in intramolecular cyclization to obtain the vinyl intermediate 3. The generated five-membered ring undergoes protonation to obtain the target product α-alkylidene cyclic carbonate, and dissociates Ag@PAF-DAB and DBU to participate in the next catalytic cycle. In addition, the α-alkylidene cyclic carbonate will further react in the presence of water. Water acts as a nucleophile in the presence of DBU to attach the carbonyl group, leading to the opening of the five-membered ring to generate intermediate 4. Finally, intermediate 4 is decarboxylated and tautomerized to obtain α‑hydroxy ketone.

Based on the above analysis, a porous aromatic framework catalyst Ag@PAF-DAB was designed and synthesized through oxidative coupling reaction. The flexible framework and abundant polar nitrogen atoms endowed Ag@PAF-DAB with strong CO₂ affinity and high CO₂ adsorption capacity (3.21 mmol/g, 273 K, 0.1 MPa). Due to the synergistic properties of CO₂ capture and activation, Ag@PAF-DAB achieves efficient catalytic carboxylative cyclization and hydration of propargylic alcohols under atmospheric conditions. In addition, Ag@PAF-DAB as a heterogeneous catalyst also exhibited excellent reusability, high catalytic diversity, and outstanding substrate universality. Overall, this work provides an important reference for the further development of a highly integrated chemical process that combine CO₂ capture and utilization.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

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This study was supported by the National Key Research and Development Program of China (No. 2019YFB1504003).

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.08.020.

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Year 2023 volume 34 Issue 4

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doi: 10.1016/j.cclet.2022.08.020

Receive Date：2022-04-19
Online Date：2025-11-21
Published：2023-04-15

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Received：2022-04-19
Revised：2022-07-26
Accepted：2022-08-09

Affiliations

^a School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

^b Institute of Chemical Sciences, University of Peshawar, 25120, KP, Pakistan

^c Intelligent Transportation System Research Center, Southeast University, Nanjing 211189, China

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Scheme 1 (a) Schematic illustration for the synthesis of Ag@PAF-DAB. (b) Transformation of propargylic alcohols into α-alkylidene cyclic carbonate and α‑hydroxy ketone.

Fig. 1. (a) FT-IR spectra of DAB and PAF-DAB. (b) XRD patterns of PAF-DAB and Ag@PAF-DAB. (c) N 1s and (d) Ag 3d XPS spectra of Ag@PAF-DAB.

Fig. 2. (a) TEM image of Ag@PAF-DAB. (b) Particle size distribution of Ag.

Fig. 3. Results of the carboxylative cyclization and hydration of other propargylic alcohols with CO₂ over Ag@PAF-DAB. Reaction: substrate (2 mmol), catalyst amount equal to the amount of Ag, DBU (1 equiv.), MeCN (4 mL). Determined by GC using naphthalene as an internal standard.

Fig. 4. (a) FT-IR spectra of the carboxylative cyclization reaction system. Reaction: 2-methyl-3-butyn-2-ol (6 mmol), Ag@PAF-DAB (4 mol%), DBU (1 equiv.), MeCN (12 mL), CO₂ (0.1 MPa), 40 ℃, 1 mL reaction mixture was collected for detection every time. (b) FT-IR spectra of the hydration reaction system. Reaction: 80 ℃, H₂O (2 equiv.). (c) GC patterns of the carboxylative cyclization and hydration reaction system.

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