Combination of dimensional reduction and active site addition strategies for preparing unique {RE₉}-cluster-based MOFs: efficient CO₂ fixation and Knoevenagel condensation

Abstract

The current application of porous catalytic materials for organic synthesis is always confined to comparatively simple small substrates because of the diffusion barrier. Therefore, in this study, dimensional reduction and active site addition strategies were employed for preparing unique porous {RE₉}-cluster-based rare-earth metal–organic frameworks (MOFs) {[Me₂NH₂]₄[RE₉(pddb)₆(μ₃-O)₂(μ₃-OH)₁₂(H₂O)_1.5(HCO₂)₃]·6.5DMF·11H₂O}_n (MOF-RE, RE = Tb, Y, and Dy) with high-density multiple active sites. It was found that MOF-RE are rare {RE₉}-based two-dimensional (2D) networks including triangular-nanoporous (1.3 nm) and triangular-microporous (0.8 nm) channels decorated by abundant Lewis acid–base sites (open RE(III) sites and N_pyridine atoms) on the inner surface. As anticipated, due to the coexistence of Lewis acid–base sites, activated samples exhibited better catalytic activity (a yield of 96%, and a TON value of 768 for styrene oxide) than most previously reported 3D MOF materials for the cycloaddition of CO₂ and multifarious epoxides under moderate conditions. Moreover, as a heterogeneous catalyst, MOF-Tb, has excellent catalytic performance (with a TON value of 396 for benzaldehyde) for the Knoevenagel condensation reaction of malononitrile and aldehydes with high catalytic stability and recoverability. In addition, both reactions possessed high turnover numbers and frequencies. These dimensional reduction and active site addition tactics may permit the exploitation of new nanoporous MOF catalysts based on rare-earth clusters for useful and intricate organic conversions.

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Introduction

One of the most important concerns facing all nations on Earth is global climate change, as the main greenhouse gas, carbon dioxide (CO₂) has contributed various severe ecological issues, such as sea level rise and climate change, raising concerns for CO₂ capture and utilization technology.^1–4 The current efficient methods of using CO₂ to create valuable complexes may not only significantly reduce the amount of CO₂ in the environment but also help humanity economically, making them one of the most advantageous options.^5–7 Since cyclic carbonates are a type of extensively utilized chemical and chemical raw material, it has been discovered that producing cyclic carbonates synthetically from epoxides and CO₂ is among the most effective ways to solve environmental issues and realize resource utilization.^8–10 Nevertheless, CO₂ conversion typically involves synergistic catalysis with numerous active sites. Numerous efforts have been made to date to study promising heterogeneous catalysts. Although some are regarded as advantageous, including zeolites,¹¹ activated carbon,¹² metal oxides¹³ and organic polymers,¹⁴ they typically call for high catalytic loadings and harsh conditions, resulting in relatively low yields and conversions with poor recyclability due to the scarcity of catalytic sites.

Metal–organic frameworks (MOFs) containing different polynuclear metal-oxo clusters are unique porous crystalline materials with widespread promise for sensing, heterogeneous catalysis and gas storage/separation.^15–21 Recent studies have exhibited that polynuclear cluster-based rare-earth (RE) MOFs possess exceedingly high stability and abundant catalytic active sites,^22–24 in which the catalytic efficiency for CO₂ conversion and Knoevenagel condensation could be greatly accelerated by the synergistic effect of activated metal ions (Lewis acid sites, LASs) and nucleophilic groups (Lewis base sites, LBSs).^25,26 However, the accessibility of these RE-MOF materials to sterically demanding substrates is limited by expanding and stabilizing the active sites. Since the substrates cannot easily reach internal LASs, RE-MOFs perform poorly with larger substrates or complex reactions.^27,28 It is essential to design functional ligands and precisely build RE-microporous MOFs with a high specific surface area and access to multiple active sites via an in situ function-oriented synthesis strategy to extract the catalytic ability of polynuclear cluster-based RE-MOFs to the greatest extent.

In light of the previous discussion on the standard of functional RE-MOFs as heterogeneous catalysts, this work thoroughly studied the effect of the topological structures of RE-MOFs on the catalytic activity and revealed dimensional reduction and active site addition methods to enhance catalytic reactivity through permitting unrestricted access to Lewis acid–base sites in two-dimensional (2D) MOFs (Scheme 1).^29–31 These strategies can accurately predict the structures of MOFs, so as to achieve the regulation of the catalytic performance. Hence, a series of unique porous 2D RE-MOFs {[Me₂NH₂]₄[RE₉(pddb)₆(μ₃-O)₂(μ₃-OH)₁₂(H₂O)_1.5(HCO₂)₃]·6.5DMF·11H₂O}_n (MOF-RE; RE = Tb, Y and Dy) with {RE₉} clusters as secondary building units (SBUs) are successfully prepared from C₂-symmetry V-shaped 2,6-bis(4′-carboxyl-phenyl)pyridine (H₂pddb) under solvothermal conditions. Notably, MOF-RE have high densities of quantified open metal sites (OMSs) acting as LASs and uncoordinated pyridines as LBSs, all of which are exposed in open channels. Benefiting from these active sites, MOF-RE exhibits excellent catalytic efficiencies for CO₂ transformation with high yields, turnover numbers (TONs), and turnover frequencies (TOFs) compared to those of many reported 3D MOF materials. Moreover, they have excellent catalytic properties for Knoevenagel condensations via the synergistic effects of the LASs and LBSs.

Scheme 1 Different polynuclear cluster-based rare-earth MOFs formed by the {RE₉} clusters and functional connectors.

Results and discussion

Description of the crystal structure

The reaction of rare-earth salts (RE = Tb, Dy, and Y) with H₂pddb in a mixed solution (DMF/H₂O) in the presence of HNO₃ and 2-fluorobenzoic acid (2-FBA) gave hexagonal crystals of MOF-RE. The presence of 2-FBA as a structure guiding agent is an essential condition for the assembly of the polynuclear cluster-based MOFs. The MOF-RE crystallize in a primitive hexagonal P6₃/mmc space group (Table 1) and are 2D porous networks. Remarkably, they possess unique polynuclear rare-earth-carboxylate clusters, i.e., a 12-c nonanuclear [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C-)₁₂(H₂O)_1.5(HCO₂)₃] core, in which formates are produced by the breakdown of DMF molecules.³²

Table 1 Crystal data and structure refinements for MOF-RE

a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Complex	MOF-Tb	MOF-Y	MOF-Dy
Empirical formula	C117H69N6O45.5Tb9	C117H69N6O45.5Y9	C117H69N6O45.5Dy9
Formula mass	3717.06	3086.97	3749.28
Crystal system	Hexagonal	Hexagonal	Hexagonal
Space group	P63/mmc	P63/mmc	P63/mmc
a [Å]	22.5962(4)	22.5233(11)	22.6331(9)
b [Å]	22.5962(4)	22.5233(11)	22.6331(9)
c [Å]	22.7452(5)	22.7551(15)	22.6392(11)
α [°]	90	90	90
β [°]	90	90	90
γ [°]	120	120	120
V [Å^3]	10 057.5(4)	9997.1(12)	10 043.4(9)
Z	2	2	2
D calcd. [g cm^−3]	1.227	1.026	1.240
μ [mm^−1]	3.173	2.634	3.357
F [000]	3524	3056	3542
θ [°]	2.071–25.349	2.072–25.385	2.078–25.380
Reflections collected	34 562/3409	64 794/3391	66 446/3411
GOOF	1.050	1.061	1.041
R ^, indices [I > 2σ(I)]	R 1 = 0.0261	R 1 = 0.0419	R 1 = 0.0270
	wR2 = 0.0648	wR2 = 0.1209	wR2 = 0.0730
R indices (all data)	R 1 = 0.0331	R 1 = 0.0590	R 1 = 0.0336
	wR2 = 0.0680	wR2 = 0.1303	wR2 = 0.0766

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The [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C-)₁₂(H₂O)_1.5(HCO₂)₃] cluster has a threefold symmetry and is composed of nine RE ions arrayed in a {RE₉} tricapped trigonal prism, according to detailed research of the nonanuclear cluster (Fig. S1a†). In contrast to the triangular planes of the tetragonal pyramids of the RE₉ tricapped trigonal prism, which are each capped by a μ₃-OH, the two triangular planes of the central RE₆ trigonal prism are each capped by a μ₃-O (Fig. S1b†). The six RE1 ions are each coordinated with eight O atoms: two from carboxylate moieties of two separate pddb²⁻ connecters and four from four μ₃-OH, and another two coordination sites are composed of O atoms from one HCO₂⁻ ligand and a μ₃-O (O1) (Fig. S2a†). The remaining three RE2 cations are each coordinated with nine O atoms: a terminal H₂O molecule, four μ₃-OH and four carboxylate O from four independent pddb²⁻ ligands (Fig. S2a†). The nonanuclear [RE₉(μ₃-O)₂(μ₃-OH)₁₂] cluster is created by twelve μ₃-OH and two μ₃-O connecting nine RE elements (Fig. 1a) and is terminated through twelve carboxylates from twelve independent pddb²⁻ linkers to generate a 12-connected [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C-)₁₂] SBU. As the extension point of the [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C-)₁₂] core, C atoms from the carboxylate groups of twelve independent pddb²⁻ linkers are arranged into a hexagonal prism, which conforms to the d6R vertex diagram of a 12-connected node. There are three H₂O molecules and three extra HCO₂⁻ bridging linkers coordinated with the RE ions to form a nonanuclear [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C-)₁₂(H₂O)_1.5(HCO₂)₃] cluster (Fig. 1a).

Fig. 1 (a and g) Types of {RE₉} cluster and ligand and their simplifications; (b and h) 2D structure and corresponding 12-connected net of MOF-RE; (c and d) different channels along the c axis; (e and f) 3D supramolecular arrangement with 1D hexagonal channels; (i) view of the porous structure of MOF-RE.

In the structure of MOF-RE, the pddb²⁻ ligands adopt a bidentate bridging mode to connect two adjacent nonanuclear clusters (Fig. S2b†) to form 2D anionic layers with triangular-microporous (∼0.8 nm) and triangular-nanoporous (∼1.3 nm) channels (Fig. 1b–d). The 2D infinite layers are stacked along the c axis with an average interlayer spacing of 11.37 Å (Fig. 1e and i). Notably, there are honeycomb-like hexagonal channels with a diameter of ∼1.3 nm along the c axis, which are full of protonated [Me₂NH₂]⁺ cations (Fig. 1f). The porosity computed by PLATON is ∼56.1% of the overall crystal volume after excluding free solvents. Topologically, the [RE₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C)₁₂(H₂O)_1.5(HCO₂)₃] clusters and ligands may act as 12-connected nodes and linear rods, respectively, and the whole structure can be represented as a 12-connected skeleton (Fig. 1g and h).

Gas adsorption studies

MOF-Tb was chosen as a representative for thorough studies due to its isomorphism. The freshly synthesized sample was vacuum-dried at 200 °C for 4 h prior to the sorption test to obtain the activated sample, MOF-Tba. The thermogravimetric analysis (TGA) curve was used to explore the activation temperature (Fig. S5†). Simultaneously, the framework integrality of activated MOF-Tba was verified via the powder X-ray diffraction (PXRD) pattern (Fig. S3†). A 77 K N₂ adsorption experiment was performed to confirm the pore properties of MOF-Tba (Fig. 2a), indicating that it possesses a traditional type-I adsorption isotherm as well as high Langmuir (825.33 m² g⁻¹) and Brunauer–Emmett–Teller (BET) surface areas (560.35 m² g⁻¹). The pore size distribution acquired through fitting the 77 K N₂-sorption isotherm using nonlocal density functional theory is consistent with the pore diameter provided by X-ray crystal data (Fig. 2a).

Fig. 2 (a) N₂ sorption isotherm at 77 K (inset displays the pore size distribution); (b) gas (C₂H_n, CO₂ and CH₄) adsorption heat Q_st for MOF-Tba. Adsorption selectivities of MOF-Tba at 273 K (c) and 298 K (d) calculated using IAST for equimolar mixtures of C₂H_n/CH₄, CO₂/CH₄, C₂H₂/C₂H_n and C₂H₂/CO₂.

The latent application of MOF-Tba for CO₂ and light hydrocarbon (CH₄ and C₂H_n) sorption (Fig. S6 and 7†)/separation (Fig. 2c and d) has been carefully examined because of its inherent perpetual porosity and channel circumstances. The results displayed that the loading capacity of C₂H₂ in MOF-Tba is higher than that of other gases, demonstrating the maximum interaction between C₂H₂ and the framework. The sorption enthalpies (Q_st) were determined using the virial approach to establish the adsorption affinity between the five gases and skeleton more accurately (Fig. 2b and Fig. S8†). The Q_st of MOF-Tba to C₂H₂ (37.6 kJ mol⁻¹) is higher than that of CO₂ (22.4 kJ mol⁻¹), C₂H₄ (27.2 kJ mol⁻¹), C₂H₆ (26.7 kJ mol⁻¹), and CH₄ (8.6 kJ mol⁻¹) under zero coverage, which agrees with the measured adsorption amount. Moreover, the possibility of separating CH₄ from light hydrocarbons was studied by ideal solution adsorbed theory (IAST) for binary equimolar mixtures (Fig. S9 and 10†).³³ The selectivities for CO₂, C₂H₂, C₂H₄ and C₂H₆ over CH₄ at 1 bar and 298 K are 5.9, 9.8, 6.8 and 7.0, respectively (Fig. 2c), making MOF-Tba an exceptional sorbent for effectively removing CO₂/C₂ light hydrocarbons from natural gas.

Catalytic performance for CO₂ conversion

Considering that MOF-Tba has the advantages of solvent-accessible nanoscale channels, high specific surface area and abundant coexisting Lewis acid–base sites ([Tb₉(μ₃-O)₂(μ₃-OH)₁₂(O₂C)₁₂(H₂O)_1.5(HCO₂)₃] clusters and N_pyridine atoms), it was employed as an effective heterogeneous catalyst for the cycloaddition of CO₂ and epoxy complexes (Scheme S1†). Our previous study³⁴ has shown that the catalysts were not recycled and exhibited poor activity due to the lack of high-density active sites when selecting the pddb ligand itself and physical mixture of the rare-earth metal-pddb ligand to catalyze this reaction. Therefore, this study conducted a range of control experiments based on styrene oxide, determined ideal reaction conditions, such as time, temperature, and catalyst dosage and identified products through ¹H NMR spectroscopy (Fig. S14–25†).

Entry 1 (Table 2) displays that when MOF-Tba (0.05 mol%) was added as a catalyst, only a small amount of product with a yield of 5% was detected within 12 h. In addition, only a slight conversion (10%) could be observed under the cocatalyst of n-Bu₄NBr (1 mol%) alone, as shown in entry 2. Nevertheless, the yield was tremendously improved to 29% (entry 3) when MOF-Tba (0.05 mol%) and n-Bu₄NBr (1 mol%) were concurrently introduced to the reaction, suggesting that MOF-Tba and n-Bu₄NBr synergistically activated the second-order reaction of CO₂ and epoxides. Increasing the temperature was used to demonstrate that one of the key factors was temperature for influencing the reaction outcome, as shown in entries 4 and 5. Furthermore, entries 6and 7 examined and listed the effect of the cocatalyst n-Bu₄NBr dosage, which demonstrated that the quantity of the cocatalyst had a clear impact on the reaction rate. The yield increased to 95% when 5 mol% n-Bu₄NBr cocatalyst was added. The amount of MOF-Tba was increased because the practical application of the catalyst will be severely hampered by the 12 h reaction time. Styrene oxide could be converted into 4-phenyl-1,3-dioxolan-2-one more rapidly, as described in entries 8–10. In conclusion, it was found that the ideal reaction conditions are 0.125 mol% MOF-Tba catalyst, 5 mol% n-Bu₄NBr cocatalyst, 60 °C and 6 h. In addition, we performed a detailed analysis of the ¹H NMR spectrum for entry 10, confirming that no by-products were generated during the reaction (Fig. S19†).

Table 2 Cycloaddition of CO₂ with styrene oxide under different conditions^a

Entry	MOF (mol%)	n-Bu4NBr (mol%)	T (°C)	t (h)	Yield^b (%)
a Reaction conditions: solvent-free, styrene oxide (20 mmol), and CO2 (1 atm). b The product yield was determined by ^1H NMR.
1	0.05	0	25	12	5
2	0	1	25	12	10
3	0.05	1	25	12	32
4	0.05	1	40	12	54
5	0.05	1	60	12	78
6	0.05	3	60	12	86
7	0.05	5	60	12	95
8	0.075	5	60	10	94
9	0.1	5	60	8	95
10	0.125	5	60	6	96

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The catalytic universality of MOF-Tba was further evaluated utilizing a range of propylene oxide derivatives with distinct substituents and steric hindrance under determined ideal reaction circumstances (Table 3). The outcomes indicated that there was some regularity in how different substituents affected the yield. It can be seen from the comparison of entries 1 and 2 that the epoxy complexes with electron-withdrawing groups (–Br and –Cl) could improve the efficiency of the cycloaddition reaction, and the yield could reach more than 99%, the reason for which is that the electron-withdrawing group may decrease the electron density of ethylene oxide.^35,36 In contrast, electron-donating groups had a disadvantageous impact on this process, as in entry 3.^37,38 Furthermore, entries 4–6 exhibited a significant decrease in the conversion of epoxide with bulky substituents, confirming the idea that big substituents restrict the mobility of substrate molecules.^39–42 Table S1† lists information about the molecular sizes of all epoxide derivatives. Additionally, the TON of MOF-Tba for styrene oxide was notable compared to most reported TON values for MOF catalysts (Table S2†), which was likely attributable to the profitable contribution of {Tb₉} clusters and abundant N_pyridine groups in the channels.

Table 3 Cycloaddition of CO₂ with epoxides under optimal conditions^a

Entry	Epoxide	Product	Yield^b (%)	TON^c	TOF^d (h^−1)
a Reaction conditions: solvent-free, epoxides (20 mmol), n-Bu4NBr (5 mol%), Tb-MOF catalyst (0.125 mol%), CO2 (1 atm), 60 °C, and 6 h. b Yield was determined by ^1H NMR. c TON = [product (mmol)]/[catalyst (mmol)]. d TOF = TON/time.
1			>99	792	132
2			>99	792	132
3			96	768	128
4			96	768	128
5			96	768	128
6			95	760	127

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The actual organic synthesis industry depends heavily on the stability and recyclability of catalysts;⁴³ hence, additional tests were conducted about hot leaching, recovery, and recycling of MOF-Tba. First, the recycling stability of MOF-Tba for the cyclization reaction of CO₂ with styrene oxide was studied under the determined ideal reaction conditions. The conversion of styrene oxide was nearly unchanged for five cycles by the recovered catalyst MOF-Tba (Fig. S11†). Meanwhile, the PXRD pattern of the recovered MOF-Tba sample after five experiments was essentially matched with the newly formed one, showing that the MOF-Tba catalyst maintained the stability of the framework (Fig. S12†). Afterward, inductively coupled plasma (ICP) analysis was then used in leaching experiments. As a result, the recovered filtrate included just a little quantity of Tb(III) ions (0.015%), further demonstrating the stability of MOF-Tba in the organic reaction. Furthermore, a thermal filtration experiment was carried out, and the results showed that the reaction hardly happens when the catalyst is filtered out (Fig. S13†), meaning that MOF-Tba possessed a heterogeneous nature.

The probable catalytic mechanism can be deduced from prior MOF-related literature,^44–47 and the distinctive structural characteristics of MOF-Tba, including high specific surface area, functional channel, and plentiful {Tb₉} clusters (Fig. 3). First, the epoxide rapidly diffuses into the MOF-Tba catalyst and makes weak contact with its exposed metal sites in a confined environment. Then, the nucleophilic attack of less-obstructed carbon atoms in the epoxide by the Br⁻ anion released by n-Bu₄NBr promotes the formation of the alkylcarbonate anion. Subsequently, polarized CO₂ molecules tend to undergo nucleophilic addition reactions with alkylcarbonate anions to generate alkylcarbonate salt. Finally, the ring closure behavior results in the production of cyclic carbonate and liberation of catalysts.

Fig. 3 Proposed mechanism of CO₂ conversion catalyzed by MOF-Tba.

Catalytic performances for Knoevenagel condensation

Knoevenagel condensation is a classical C–C bond coupling reaction, and its reaction mechanism involves the covalent C=C bond formed by the carbonyl group combining with the methylene group activated via both electron-withdrawing moieties.^40–42,48 Pharmaceuticals and fine compounds are frequently synthesized using this process. Recent research has proven that active metal centers (LASs) and nucleophilic moieties (LBSs) in porous MOFs could work together to significantly speed up the Knoevenagel reaction.^49–52 Therefore, more research was done on active porous MOF-Tba to catalyze the Knoevenagel condensation (Scheme S2†).

Initially, perfect reaction conditions were studied with benzaldehyde and malononitrile as substrates in the presence of desolvated MOF-Tba as a heterogeneous catalyst, as seen in Table 4 and Fig. S29–31.† Entry 1 shows that only trace 2-benzylidenemalono-nitrile (3%) was produced at 25 °C without the catalyst MOF-Tba, whereas when MOF-Tba (0.1 mol%) was introduced, a yield of 38% was generated within 2 h (entry 2), indicating that the reaction could scarcely be carried out without the catalyst. The conversion of the substrate increased progressively when all other factors governing the reaction were held constant, and only the catalyst dosage was increased (entries 3 and 4). At 25 °C, the yield was 85% when the MOF-Tba dosage was raised to 0.25 mol% (entry 5). The reaction substrate was virtually entirely converted as the temperature rose from 25 °C to 60 °C in parallel investigations using 0.25 mol% MOF-Tba (entries 6 and 7), demonstrating that temperature was one of the crucial elements in the condensation reaction. Additionally, the connection between the conversion with reaction time was examined under the reaction environments of 0.25 mol% MOF-Tba at 60 °C (entries 8–10). It can be seen from the above results that the ideal reaction conditions were 0.25 mol% MOF-Tba, 60 °C, and 2 h when the ratio of malononitrile to aldehyde was 2 : 1.

Table 4 Knoevenagel condensation from substrates of benzaldehyde and malononitrile^a

Entry	MOF-Tba (mol%)	Time (h)	T (°C)	Yield^b (%)
a Reaction conditions: malononitrile (20 mmol), benzaldehyde (10 mmol). b The product yield was determined by ^1H NMR.
1	0	2	25	3
2	0.1	2	25	38
3	0.15	2	25	58
4	0.2	2	25	76
5	0.25	2	25	85
6	0.25	2	40	93
7	0.25	2	60	99
8	0.25	1.5	60	95
9	0.25	1	60	88
10	0.25	0.5	60	65

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Based on aforementioned discoveries, we chose several aldehyde derivatives with various substituents and steric hindrance (Table S3†) to confirm the MOF-Tba catalyst's suitability for the Knoevenagel condensation reaction, and the outcomes are displayed in Table 5 and Fig. S32–38.†

Table 5 Knoevenagel condensation reaction of aldehyde derivatives under optimal conditions^a

Entry	Substrate	Product	Yield^b (%)	TON^c	TOF^d (h^−1)
a Reaction conditions: aldehyde derivatives (10 mmol), malononitrile (20 mmol), catalyst MOF-Tba (0.25 mol%), 2 h, 60 °C. b The yield was calculated by ^1H NMR. c TON = [product (mmol)]/[catalyst (mmol)]. d TOF = TON/time.
1			>99	396	198
2			>99	396	198
3			>99	396	198
4			>99	396	198
5			98	384	192
6			94	372	186
7			87	348	174

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Entries 2–4 show that the conversion efficiency of benzaldehyde with electron-drawing groups (–F, –Br, and –NO₂) surpassed 99%, whereas the catalytic yield was slightly decreased due to the presence of the electron-donating groups (–CH₃ and –CH₂CH₃) (entries 5 and 6), indicating that the electron-donating moieties greatly inhibit the Knoevenagel condensation reaction. Additionally, the conversion efficiency clearly reduced as the molecular size and steric hindrance of the substrate increased (entry 7). Remarkably, benzaldehyde had a TON value of 396, which was much higher than the majority of previously documented MOF catalysts (Table S4†).

The best experimental conditions were used to study the stability and recyclability of MOF-Tba. The utilized MOF-Tba catalyst was recovered and repeatedly cleaned with DMF after each reaction. Over 97% of 2-benzylidenemalono-nitrile was produced after five repetitions of the process using MOF-Tba, which retained excellent catalytic activity (Fig. S26†). Furthermore, the PXRD peaks of gathered MOF-Tba demonstrated that the host framework remained unchanged, suggesting the great stability of the microporous heterogeneous catalyst (Fig. S27†). Following the catalytic recycling experiment, leached homogenous Tb(III) was monitored by ICP analysis as well. The probability of leaching metal ions from the MOF-Tba network during Knoevenagel condensation was ruled out when trace Tb(III) of 0.018% was found in the filtrate. Under optimal reaction circumstances, a heat filtration test was conducted to confirm the heterogeneous nature of MOF-Tba. The solid catalyst was filtered out after the reaction had been going on for 0.5 hours. Since the conversion rate barely altered (Fig. S28†), the heterogeneous nature of MOF-Tba was further supported.

Fig. 4 implies a likely catalytic reaction mechanism based on relevant published studies^53–59 and structural characteristics of MOF-Tba. First, the carbonyl oxygen of the aldehyde group made a weak interaction with the exposed Tb(III) sites of MOF-Tba, converting its carbon atom into a positive carbon center and initiating the reaction. Simultaneously, N_pyridine atoms as the LBSs caused the carbonyl carbon of malononitrile to polarize into a negative center. Second, a covalent bond was formed between two carbon atoms with opposing electric charges, resulting in an imine intermediate. Finally, the recombination of intramolecular electrons combined with the release of H₂O molecules and the used catalyst resulted in the formation of the product benzylidenemalononitrile.

Fig. 4 Proposed mechanism for Knoevenagel condensation by MOF-Tba.

Conclusions

A series of 2D {RE₉}-cluster-based rare-earth MOF-RE with Lewis acid–base dual functional sites were designed and synthesized by dimensional reduction and active site addition strategies. As expected, they benefited from high-density active sites and had exceptional catalytic properties for the chemical fixation of CO₂ with epoxides under moderate conditions, together with satisfactory catalytic efficiencies for Knoevenagel condensation. These strategies proposed in this work not only provide a new method for the preparation of nanoporous cluster-based RE-MOFs with various catalytic activities but also lay a foundation for the research of the catalytic mechanism.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are thankful for the financial support from the NSFC (22071194) and Natural Science Foundation of Henan Province (232300421232).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and supporting figures. CCDC 2216930–2216932. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi02527f

‡ These authors contributed equally to this work.

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