Gas-phase organometallic catalysis in MFM-300(Sc) provided by switchable dynamic metal sites†

Juan L. Obeso‡ ^ab, Alfredo López-Olvera‡ ^b, Catalina V. Flores ^a, Ricardo A. Peralta *^c, Ilich A. Ibarra *^b and Carolina Leyva *^a
aInstituto Politécnico Nacional, Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, LNAgua, Legaria 694, Col. Irrigación, Miguel Hidalgo, 11500, CDMX, Mexico. E-mail: zleyva@ipn.mx
^bLaboratorio de Fisicoquímica y Reactividad de Superficies (LaFReS), Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n, CU, Coyoacán, 04510, Ciudad de México, Mexico. E-mail: argel@unam.mx
^cDepartamento de Química, División de Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana Unidad Iztapalapa (UAM-I), 09340, Mexico. E-mail: rperalta@izt.uam.mx

First published on 17th February 2023

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Metal–organic frameworks (MOFs) have recently received considerable interest as promising catalysts.¹ MOFs feature high surface area, favorable pore size distribution, and high thermal and chemical stability.^1,2 The intrinsic properties of MOFs, such as porosity, well-defined structures, and robustness, make these materials superior to similar systems. Furthermore, using these porous materials in catalysis is useful due to the well-defined framework that allows rapid diffusion of large molecules (reactants and products), high catalytic selectivity, and recyclability that allows facile recovery from the reaction mixture.^3–10 Despite the exquisite catalytic properties of MOFs, most of this chemistry is performed in solution, thereby not taking advantage of the MOF's inherent properties. Gas-phase reactions are an excellent example of chemistry in which MOF characteristics can be exploited to fully realize their potential in commercial applications. Moreover, the extra steps associated with MOF synthesis make for prohibitively expensive materials for industrial applications. Open metal sites (OMS) are an intrinsic property of several MOFs,¹¹ typically generated by removing solvent molecules coordinated to the metal centers using heat treatment and vacuum.¹²

MOFs with OMS are one of the most extensively researched materials for catalysis since OMS are homologous to conventional catalysts.^13,14 These unsaturated sites are characterized by electron-deficient metal atoms which can act as Lewis Acid Sites (LAS). This property allows them to interact with other molecules accepting electron density and function as direct catalytic active sites.¹⁵ For example, LAS at HKUST-1 was used for hydrogenation reaction.¹⁶ MIL-101(Cr)-X (X: H, NO₂, SO₃H, Cl, CH₃, and NH₂) act as catalysts for epoxide ring opening, benzaldehyde acetalization, and coupling reaction by three Lewis acid-catalyzed reactions.¹⁷ Furthermore, different MOFs have been applied in a varied range of catalytic reactions, such as MIL-100 for the intermolecular carbonyl reaction,¹⁸ Cu-BTC for the cyanosilylation reaction,¹⁹ and UiO-66 for the CO₂ cycloaddition reaction.²⁰ Despite the advantages of MOFs possessing OMSs, some such materials are not sufficiently stable for catalysis conditions.²¹ Moreover, these catalysts can be deactivated affording complicated regeneration processes.²² The emergence of MOF catalysts that contain non-permanent rather than permanent OMS is a promising route toward industry-compatible catalysts.

Understanding the reversible dynamics of metal–ligand bonds in MOFs provides a new strategy for designing novel functional materials. A promising alternative in which hemilabile bonds lead to the facile formation of non-permanent OMS within the framework. This potentially allows these materials to be used in different catalytic routes without compromising the crystalline structure and increasing the life cycles. Morris et al. have already revealed the importance of lability and hemilability in MOFs.²³ We have recently demonstrated that NH₃ and H₂S adsorption in MFM-300(Sc) induced reversible labile scandium–carboxylate (Sc–O) bonds.^24,25 The non-permanent open metal sites in MFM-300(Sc) provided an outstanding catalyst for the classic Strecker hydrocyanation reaction.²⁶ MFM-300(Sc) exhibits a tetragonal crystal structure featuring a binuclear [Sc₂(μ₂-OH)] site (Fig. 1).²⁷

Buoyed by our previous findings, we explored the possibility of applying it in the gas-phase acetone (ACE) hydrogenation reaction. Generally, homogeneous systems showed conversions up to 100%; however, the separation, long-term stability, and strong acid conditions are significant conundrums for these systems.²⁸ Therefore, heterogeneous systems are more desirable and commonly used. Nevertheless, these catalysts have the operational disadvantage of working under harsh reaction conditions.²⁹ For example, a Ni–Al₂O₃ catalyst achieves high conversion but requires a harsh activation process.³⁰

Thus, ACE hydrogenation was selected as a model reaction to evaluate the catalytic activity in a gas-phase system using a continuous flow reactor. Hence, the dynamic Sc(III) bonding sites in MFM-300(Sc) can act as the active sites required for the reaction to proceed. To the best of our knowledge, this is the first time that the dynamic bonding phenomena have been exploited to perform a gas-phase reaction using a MOF. Our experimental studies confirmed the mechanical and chemical stability of the material are suitable for use in gas-phase catalysis. We employed a defined MOF particle size of 0.425–0.850 mm for the experiments. This work builds on our previous studies using this system to catalyze the Strecker reaction in solution, providing an important demonstration of effective gas phase catalysis using non-permanent open metal sites. The findings of this reaction expand our comprehension of using MOFs as platforms for gas-phase heterogeneous catalysis and build novel strategies for synthesizing materials with potential industrial applications.

PXRD pattern (Fig. S3, ESI†) of synthesized MFM-300(Sc) confirmed the phase purity of the material, showing the characteristic peaks of the crystalline structure.²⁷ Prior to the catalytic test, the crystalline structure of the defined particle size MFM-300(Sc) was verified by PXRD (Fig. S7, ESI†), and no substantial changes were observed. FTIR spectrum (Fig. S4, ESI†) exhibits the characteristic absorption bands assigned to the main functional groups of MFM-300(Sc). The band at 3621 cm⁻¹ is designated to μ₂-OH bridging the Sc atoms. The bands at 1675 and 1611 cm⁻¹ are assigned to the asymmetric and symmetric stretching vibrations for carboxylate groups coordinated.³¹ To verify the thermal stability of the material, TGA analysis was carried out (Fig. S5, ESI†), demonstrating the high thermal stability up to 450 °C. N₂ adsorption–desorption isotherm at 77 K (Fig. S6, ESI†) of MFM-300(Sc) shows a type-I isotherm, characteristic of microporous materials.³² The absence of hysteresis demonstrates complete reversibility. The calculated BET surface area is 1364 m² g⁻¹, with pore volumes of 0.56 cm³ g⁻¹, consistent with previously reported data.²⁷

MFM-300(Sc) was assessed as a catalyst for the ACE hydrogenation reaction leading to the formation of methyl isobutyl ketone (MIBK). Conventionally, MIBK is synthesized via a three-step process involving condensation (diacetone alcohol (DA) formation), dehydration (mesityl oxide (MO) formation), and hydrogenation (MIBK formation) steps (Scheme 1).

MIBK is a high-value chemical product.³³ The catalytic hydrogenation of ACE in both vapor and liquid phases for the synthesis of MIBK has been explored for several decades with different catalysts.^34,35 However, the highly efficient ACE conversion essentially relies on specific catalysts. Thus, the fascinating dynamic metal–ligand bonding presented in MFM-300(Sc) offers an excellent opportunity to initiate the study of different types of materials that may show this phenomenon.

For our case study, the main objective is to prove the catalytic activity of this MOF with non-permanent OMS in a gas-phase reaction, which is driven via the dynamic of the Sc(III) bonds. It is worth mentioning that the reaction conditions will be optimized for future work to increase ACE conversion (Table 1).

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First, a control experiment (without MFM-303(Sc)) was carried out in ACE/H₂. No acetone conversion was observed. Next, using MFM-300(Sc) the reaction was carried out at 180 °C and reached 27.7% conversion after 4 hours. Interestingly, MFM-300(Sc) shows comparable catalytic activity to benchmark catalysts (Tables S1 and S2, ESI†). For instance, 2.5% wt Ni supported in ceria (Ni/CeO₂) produces an ACE conversion of 26% at 150 °C³⁶ while Cu–Al mixed oxide materials reached 22.5% ACE conversion at 200 °C.³⁷ Also, Pd/(Nb₂O₅/SiO₂) achieves 16.6% ACE conversion at 160 °C³⁸ while Pd/Ce-MCF exhibits 31.6% ACE conversion at 300 °C.³⁹ MOF-based catalysts have been a recent entry into the field of ACE hydrogenation catalysis. He et al.,⁴⁰ reported palladium nanoparticles deposited on MIL-101(Cr) with an outstanding ACE conversion of 60.1% for the pristine MOF, and 81.0% for the MOF with 0.22% wt Pd. The high ACE conversion was attributed to the high density and accessibility of Cr(III) acid sites in the MOF. MFM-300(Sc) exhibits low conversion compared to MIL-101(Cr), possibly due to the presence of permanent OMS in MIL-101(Cr) compared to the non-permanent Lewis acid sites generated in MFM-300(Sc).⁴¹

To further investigate the role of the non-permanent LAS in MFM-300(Sc), the reaction was monitored for 4 h. Fig. 2 displays the catalytic performance with time-on-stream. Clearly, a variety of conversions are observed at different time intervals; we consider that this behavior occurs because the hemilabile sites in MFM-300(Sc) are not always available to interact with the reagents. During the reaction, a high conversion is maintained in the initial stages of the reaction (Sc(III) sites are continuously available), then the conversion decreases (the Sc(III) sites become less available or are generated at a lower rate), before recovering to a similar (slightly higher) value to that observed in the initial stages of the reaction (increased in conversion). Recently, Jeong et al.,¹⁶ reported that having permanent access to the LAS is essential for higher catalytic activity, which in our system is challenging, thereby corroborating the hypothesis that non-permanent access to the active sites is responsible for the observed behavior on stream. Generally, conventional catalysts show a conversion equilibrium over time-on-stream when the steady state is reached.⁴² Thus, this type of performance would be challenging to achieve with our material; however, catalytic activity is observed, and an acceptable conversion is obtained for this work. Moreover, PXRD analysis confirmed that the material remained intact, with no apparent changes in its structure (Fig. S7, ESI†), also, no significant changes in BET surface area were observed (Fig. S8, ESI†).

After the initial experiment, two-cycle tests were performed, and the spent MFM-300(Sc) was reused to investigate the cyclability of MFM-300(Sc). The ACE conversion in the runs was approximately the same, highlighting the material's recyclability which is essential for real-world application. One more test was carried out with a fresh sample of MFM-300(Sc) for the repeatability catalytic activity. The ACE conversion was 26.6%, which is in good agreement with the initial test, corroborating the reproducibility of the obtained results. Additionally, the reaction was performed at a low temperature (80 °C). The obtained ACE conversion was 17.2%, a lower conversion than the one at 180 °C.

Considering the dynamics of Sc–O bonds present in MFM-300(Sc) and established literature, we have proposed a reaction mechanism for the catalytic hydrogenation of ACE to IMBK (Fig. 3).^43–45 We hypothesized that the condensation, dehydration, and hydrogenation processes occur in different steps at the Sc(III) hemilabile sites: (i) first, two ACE molecules are coordinated to the Sc(III) metal center. This coordination of ACE molecules to metal centers was reported.⁴⁶ This stage, leads to the formation of the hemilabile Sc–O bond. This hemilabile bond reflects the electron-deficient character of the Sc(III) in MFM-300(Sc), a characteristic feature of LAS in MOFs.⁴⁷ (ii) Following the DA formation, where a proton from one molecule of ACE is attracted by the open carboxylate group to compensate for the charge, a CC bond is generated forming an enolate. The enolate rapidly attacks the carbon of the carbonyl group of another ACE molecule.^48,49 Subsequently, the proton is recovered and one of the Sc(III) metal sites returns to its original state. (iii) The hydroxyl radical of the DA attacked a proton, and MO is formed liberating a water molecule. (iv) Then, the hydrogenation step, leads to the cleavage of H₂ molecule, where is dissociated leading to the breakage of the CC bond. This mechanism has been previously observed for different palladium and platinum-based catalysts.⁵⁰ For example, the hydrogenation of CO₂ towards methanol,⁵¹ and nitrobenzene hydrogenation to aniline.⁵² (v) Finally, the IMBK formation and release occur, bringing the other Sc(III) center back to the initial state.

Herein, we have reported the catalytic activity of MFM-300(Sc) in a gas phase system for ACE hydrogenation employing a continuous flow reactor. The catalyst performance is related to the dynamics of Sc–O bonds capable of generating non-permanent LAS. The reaction was successfully catalyzed using a defined particle size (0.425–0.850 mm). PXRD analysis and N₂ adsorption isotherms of the catalyst after the reaction confirmed the high mechanical and chemical stability of the material under catalysis conditions. This confirms MFM-300(Sc) to be a platform for reactions that fully exploit the intrinsic characteristics of MOFs such as high surface area, crystallinity, and permanent porosity. Furthermore, ACE conversion was 27.7% at 180 °C and atmospheric pressure, comparable to benchmark catalysts, with the advantage of maintaining catalytic activity over multiple cycles. The time-on-stream performance proved the existence of dynamic, non-permanent reaction sites. Additionally, based on the reported literature and the experimental data obtained, a catalytic reaction mechanism was proposed that implicates hemilabile Sc–O bonds in the formation of Sc(III) Lewis acid sites. This work opens new opportunities to explore a broad array of MOF materials, understand and employ hemilabile coordination bonds, and assess their capacity to catalyze gas-phase reactions via non-permanent Lewis acid site formation.

J. L. O. and A. L.-O. thanks to CONACYT for Ph.D. fellowship (1003953 and 766200). We thank U. Winnberg (Pharma View Consulting) for scientific discussions and G. Ibarra-Winnberg for scientific encouragement. I. A. I. thanks PAPIIT-UNAM (IN202820), Mexico for financial support. C. L. thanks to the LNAgua (CONACYT Project 315880) and SIP-Innovation-IPN Projects (20210779, 20212053, 20220433, and 20221097).

Conflicts of interest

There are no conflicts to declare.

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Footnotes

† Electronic supplementary information (ESI) available: Instrumental techniques, characterization, catalysis. See DOI: https://doi.org/10.1039/d2cc06935k

‡ These authors contributed equally to this manuscript.

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