Selective Hydrogenation of Polyunsaturated Fatty Acid Methyl Esters over Bifunctional Ligand-Modified Pd/MIL-101(Cr) Catalysts

Abstract

Highly dispersed palladium nanoparticles (Pd NPs) were incorporated into MIL-101­(Cr) frameworks functionalized with bifunctional ligands for the liquid-phase hydrogenation of polyunsaturated fatty acid methyl esters (FAMEs). A series of amino- and carboxylic-acid-containing ligandsethylenediamine (en), diethylenetriamine (DET), alanine (AN), 4-aminobutyric acid (ABA), 5-aminovaleric acid (AVA), glutamic acid (GA), and adipic acid (AA)were grafted onto MIL-101­(Cr), followed by Pd loading (0.5 wt %). Spectroscopic and structural analyses confirmed ligand coordination to both Cr nodes and Pd species. Catalysts bearing ABA, AVA, and GA exhibited Pd⁰ dispersion (<1 nm), yielding high turnover frequencies (up to ∼15,400  h^–1) and >94% selectivity for monounsaturated FAMEs. In contrast, strong Pd–N interactions in en- and DET-grafted materials suppressed Pd⁰ formation, reducing activity. Hot filtration and recyclability tests confirmed high catalyst stability and negligible Pd leaching. The bifunctional ligand architecture effectively tunes Pd speciation and activity, providing a robust platform for selective and reusable hydrogenation catalysts.

1. Introduction

Polyunsaturated fatty acid methyl esters (FAMEs), which are predominant components of biodiesels derived from soybean oil (64%), corn oil (61%), sunflower oil (66%), and used cooking oils (32%), , are known to negatively impact the cold-flow properties and oxidative stability of biodiesel. − Selective hydrogenation of polyunsaturated FAMEs to monounsaturated FAMEs (C16–C18) represents a critical process for improving biodiesel quality. − Furthermore, the resulting monounsaturated FAMEs serve as valuable precursors for the synthesis of α-olefins (C8–C14) and bifunctional chemicals (olefins and esters) through cross-metathesis with small olefins such as ethylene and propylene. , These intermediates are essential for producing polymers, lubricants, plasticizers, surfactants, and detergents. −

Conventional supported metal catalysts, including Cu, Ni, Co, and Pt, are unsuitable for the selective hydrogenation of polyunsaturated FAMEs due to their tendency to promote excessive hydrogenation, decarbonylation, and decarboxylation, leading to saturated FAMEs and hydrocarbons. ,, In contrast, highly dispersed Pd catalysts with particle sizes below 3 nm have shown promise for achieving selective hydrogenation. , However, maintaining Pd dispersion is challenging, as Pd nanoparticles (Pd NPs) often agglomerate or leach during liquid-phase reactions, even on high-surface-area supports like MCM-41, SBA-15 and ZSM-5, where solvent can disrupt silanol-Pd interactions. − Alternatively, metal–organic frameworks (MOFs), with their high surface areas and tunable pore structures, offer a promising alternative for stabilizing Pd NPs. MOFs such as MIL-53, UiO-66, and MIL-101­(Cr) can encapsulate Pd NPs within their cage- or channel-like structures through weak ionic interactions between Pd NPs and the framework nodes. − For instance, Pd NPs with sizes of 4.7 nm have been achieved on MIL-53 with 4.3 wt % Pd loading, while smaller Pd NPs (<3 nm) are typically obtained on MIL-101­(Cr) even at 4.9 wt % Pd loading. Despite these advances, Pd agglomeration remains an issue, particularly in MIL-101­(Cr), due to the relatively weak interaction between Pd NPs and the Cr nodes.

To enhance the interaction between Pd and MIL-101­(Cr), bifunctional ligands have been employed as linkers, anchoring to the Cr nodes on one end and coordinating with Pd on the other. For example, ethylenediamine (en) has been used to stabilize Pd²⁺ species on Pd-en-MIL-101­(Cr), achieving >99% dispersion under higher reduction temperature (150 °C) due to the formation of strong Pd–N coordination bonds. However, while such systems are highly effective for oxidation reactions, their performance in hydrogenation reactions, which require Pd⁰ as the active site, is limited. Thus, optimizing the Pd-ligand interaction is crucial to achieving highly dispersed Pd⁰ species that resist agglomeration and leaching while maintaining catalytic activity.

In this study, we investigate the role of bifunctional ligands in modulating metallic Pd dispersion, activity, selectivity, and stability for the selective hydrogenation of polyunsaturated FAMEs to monounsaturated FAMEs over Pd/MIL-101­(Cr) catalysts. A series of ligands (Scheme ), including ethylenediamine (en), diethylenetriamine (DET), amino acids (alanine (AN), 4-aminobutyric acid (ABA), 5-aminovaleric acid (AVA), glutamic acid (GA)), and dicarboxylic acids (adipic acid (AA)), were selected to systematically explore the effects of functional groups and chain length proximity on Pd-ligand interactions. The coordination of these ligands with Cr nodes and Pd NPs was characterized using FTIR and X-ray photoelectron spectroscopy (XPS), while Pd dispersion was confirmed by TEM. Additionally, the reaction pathway, catalyst stability, and recyclability were evaluated to demonstrate the potential of these systems for industrial applications.

1. Structure of Ligands.

2. Experimental Section

2.1. Materials

All chemicals were used as received without further purification. Chromium­(III) nitrate nonahydrate (Cr­(NO₃)₃·9H₂O, Acros, 99%), terephthalic acid (Acros, 99%), ethylenediamine (Aldrich, 99%), palladium­(II) acetate (Pd­(OAc)₂, Aldrich, 98%), diethylenetriamine (Thermo Scientific, 98%), l-alanine (Glentham, 98.5%), 4-aminobutyric acid (Aldrich, 99%), 5-aminovaleric acid (Thermo Scientific, 97%), adipic acid (Fluka, 99.5%), soybean oil (TVO-Thai, 60% polyunsaturated fatty acid methyl esters, FAMEs), potassium hydroxide (KOH, Carlo, 85%), acetone (Carlo, AR grade), methanol (Carlo, AR grade), absolute ethanol (Carlo, AR grade), N,N-dimethylformamide (Carlo, AR grade), dichloromethane (Carlo, AR grade), n-hexane (Carlo, AR grade), toluene (Carlo, AR grade), n-heptane (Carlo, 99%), n-octane (Carlo, AR grade), and n-dodecane (Thermo Scientific, 99%) were procured from commercial suppliers. High-purity nitrogen (N₂, 99.995%), hydrogen (H₂, 99.995%), and 10% H₂/N₂ gases were obtained from UIG Company.

2.2. Synthesis of MIL-101­(Cr)

MIL-101­(Cr) was synthesized according to a modified procedure reported by Hatton et al. Briefly, Cr­(NO₃)₃·9H₂O (2.0 g, 5 mmol) and terephthalic acid (0.83 g, 5 mmol) were dissolved in deionized water (20 mL) under sonication. The resulting mixture was transferred to a Teflon-lined autoclave and heated at 218 °C for 18 h. After cooling to room temperature, the green solid product was isolated by centrifugation (8,000 rpm, 10 min) and washed sequentially with deionized water, methanol, and acetone. The solid was then dispersed in N,N-dimethylformamide (DMF, 20 mL) under sonication for 10 min and stirred at 70 °C overnight. The product was recovered by centrifugation, washed repeatedly with methanol and acetone, and activated under a nitrogen atmosphere (60 mL/min) at 150 °C for 12 h.

2.3. Grafting of Ligands on MIL-101­(Cr)

Ligand functionalization of MIL-101­(Cr) was performed following the method described by Yan et al. Dehydrated MIL-101­(Cr) (1 g) was suspended in dry toluene (120 mL), and the system was purged with nitrogen (60 mL/min) for 30 min. A solution of the desired ligand (0.5 mmol) in toluene was added, and the mixture was refluxed under nitrogen for 12 h. After cooling to room temperature, the solid was separated by centrifugation (8,000 rpm, 10 min) and washed with dichloromethane. The product was then dispersed in ethanol under sonication for 1 h, centrifuged, and washed thrice to remove unreacted ligands. The final product denoted as L-MIL-101­(Cr), was dried under nitrogen (60 mL/min) at 150 °C. The grafting density was maintained at 0.5 mmol ligand per gram of MIL-101­(Cr), where L represents the specific ligand used.

2.4. Incorporation of Palladium (Pd)

Palladium incorporation (0.5 wt %) was achieved using the double solvent method reported by Wright et al. Dehydrated MIL-101­(Cr) or L-MIL-101­(Cr) (1 g) was suspended in dry n-hexane (120 mL) as a hydrophobic solvent. A solution of Pd­(OAc)₂ in acetone (1.1% w/v, 1 mL) was added dropwise under vigorous stirring. After 3 h, the solid was filtered, dried under nitrogen at 150 °C (60 mL/min) for 12 h, and reduced under a 10% H₂/N₂ (20 mL/min) atmosphere at 150 °C for 2 h.

2.5. Characterization

The elemental composition (C, H, N) of the samples was determined using a Thermo Flash 2000 elemental analyzer. Before analysis, the samples were dried in the oven at 60 °C overnight to ensure accurate measurements. Palladium (Pd) loading was quantified using inductively coupled plasma-optical emission spectrometry (ICP-OES) on a Varian VISTA-MPX system. Briefly, 0.05 g of each sample was digested in a 50 mL mix solution of HClO₄:H₂SO₄:H₂O (15:15:20 mL). The suspension was heated to 210 °C (2 °C/min) for 6 h using a graphite digestion instrument (ODLAB). After digestion, the solution was filtered and diluted with deionized water for analysis. The Pd quantity was calculated using the Pd standard curve.

The crystallinity of MIL-101­(Cr), ligand-modified MIL-101­(Cr), and Pd-L-MIL-101­(Cr) was assessed via powder X-ray diffraction (PXRD) using a Bruker D8 Discover diffractometer. The instrument was equipped with Cu Kα radiation (λ = 1.5406 Å) to ensure precise phase identification. Prior to analysis, the samples were finely ground to uniform powders. Diffraction patterns were recorded over a 2θ range of 1–90° with a step size of 0.01°, ensuring high-resolution structural characterization. Surface area and porosity of samples were determined using nitrogen adsorption–desorption isotherms measured at −196 °C on a Quantachrome gas analyzer. Before analysis, 0.05 g of each sample was degassed at 150 °C for 12 h under vacuum to remove adsorbed moisture and impurities. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, while the Barrett–Joyner–Halenda (BJH) method was employed to determine the pore size distribution and pore volume. Fourier-transform infrared (FTIR) spectroscopy was performed on a Nicolet NEXUS 670 spectrophotometer in transmission mode over 400–4,000 cm^–1. The samples were prepared by grinding them with potassium bromide (KBr) and pressing the mixture into pellets.

Morphological analysis was conducted using scanning electron microscopy (SEM) on a FEI Quanta 250 instrument. The samples were coated with a thin layer of gold to enhance conductivity before imaging. Transmission electron microscopy (TEM) was performed on a JEM-2010 instrument with energy-dispersive X-ray spectroscopy (EDX) for elemental mapping. Approximately 10 mg of the catalyst sample was dispersed in ethanol. The dispersion was sonicated for 1 min to achieve a uniform suspension of fine particles. After sonication, 50 μL of the sample solution was carefully deposited onto a nickel grid supported by a carbon film. The sample was allowed to dry under ambient conditions before imaging. XPS was carried out on a PHI 5000 Versa Probe II, ULVAC-PHI, Japan at the SUT-NANOTEC-SLRI joint research facility, Synchrotron Light Research Institute (Thailand). Monochromatic AlKα X-ray (1,486.6 eV) was used as an excitation source. The core-level spectra of C 1s, O 1s, N 1s, and Cr 2p were analyzed to determine the chemical states and interactions of the elements. The binding energies were calibrated using the adventitious carbon peak (C 1s) at 286.12 eV.

2.6. Catalytic Testing

2.6.1. Hydrogenation of Polyunsaturated FAMEs

The catalytic activity of the synthesized materials was evaluated in the hydrogenation of polyunsaturated FAMEs. A reduced catalyst (50 mg) was placed in a three-necked round-bottom flask and subjected to three cycles of vacuum purging with hydrogen. The catalyst was then reduced at 100 °C under a hydrogen flow (50 mL/min) for 1 h. Subsequently, a 50% FAMEs solution (10 g) was injected, and the reaction was carried out at 100 °C under vigorous hydrogen bubbling (150 mL/min) for the specified time. After completion, the catalyst was separated by vacuum filtration, and the filtrate was analyzed using a gas chromatograph (HP-7890B) equipped with an HP-88 capillary column (100 m × 250 μm × 0.2 μm) and a flame ionization detector. Conversion and yield were quantified using n-octane as an internal standard.

2.6.2. Catalyst Recycling

The spent catalyst was recovered by filtration, washed with n-hexane, and dried at 60 °C before reuse in subsequent cycles. Reaction parameters were calculated as follows:

$$\mathrm{Conversion}(\%)=\frac{({\mathrm{mol\; feed}}_0-{\mathrm{mol\; feed}}_t)}{{\mathrm{mol\; feed}}_0}\times 100$$

$$\mathrm{Yield}(\%)=\frac{({\mathrm{mol\; product\; A}}_t-{\mathrm{mol\; product\; A}}_0)}{{\mathrm{mol\; feed}}_0}\times 100$$

$$\mathrm{Selectivity}=\frac{{\mathrm{yield\; A}}_t}{\mathrm{conversion}}\times 100$$

$$\mathrm{Hydrogenation\; rate}\left(\frac{\mathrm{mmol}}{{\mathrm{g}}_{\mathrm{cat.}}·\mathrm{h}}\right)=\frac{\mathrm{conversion}\times {\mathrm{g}}_{\mathrm{feed}}\times 1000}{{\mathrm{MW}}_{\mathrm{feed}}\times {\mathrm{g}}_{\mathrm{cat.}}\times \mathrm{time}(\mathrm{h})\times 100}$$

Where, feed = polyunsaturated FAMEs and A₀ = existing C18:1 or C18:0 in the feed.

3. Results and Discussion

3.1. Synthesis and Characterization

The Pd-L-MIL-101­(Cr) materials were synthesized via a two-step process: (i) grafting bifunctional ligands onto MIL-101­(Cr) (0.5 mmol ligand per gram of MIL-101­(Cr)) and (ii) introducing palladium (Pd) using Pd­(OAc)₂ as a precursor (∼0.05 mmol Pd per gram of L-MIL-101­(Cr)) via an adsorption method. , The crystallinity of all modified samples was retained, as confirmed by powder X-ray diffraction (PXRD, Figure ). Scanning electron microscopy (SEM, Figure ) further revealed that the crystal size and morphology remained consistent across all Pd-L-MIL-101­(Cr) samples. Fourier-transform infrared (FTIR) spectroscopy further confirmed the structural integrity of MIL-101­(Cr) upon functionalization (Figure c). The characteristic vibrational bands of MIL-101­(Cr) linkers were retained across all Pd-L-MIL-101­(Cr) samples, including: 748, 1,041, and 1,172 cm^–1 (C–H stretching of benzene rings), 1,400 and 1,623 cm^–1 (asymmetric and symmetric COO^– stretching modes). Notably, no distinct bands, corresponding to the bifunctional ligands, were observed, likely due to their low loadings (∼1–2 wt %, Table S1).

1.

(a) PXRD patterns of ligand-functionalized MIL-101­(Cr) (L-MIL-101­(Cr)), (b) PXRD patterns of Pd-loaded L-MIL-101­(Cr), and (c) FTIR spectra of Pd-L-MIL-101­(Cr).

2.

SEM images of MIL-101­(Cr) and Pd-L-MIL-101­(Cr) samples: (a) MIL-101­(Cr), (b) Pd-MIL-101­(Cr), (c) Pd-en-MIL-101­(Cr), (d) Pd-DET-MIL-101­(Cr), (e) Pd-AN-MIL-101­(Cr), (f) Pd-ABA-MIL-101­(Cr), (g) Pd-AVA-MIL-101­(Cr), (h) Pd-GA-MIL-101­(Cr), and (i) Pd-AA-MIL-101­(Cr).

Elemental analysis (CHN, Table ) indicated increased carbon content (wt % C) upon ligand incorporation. However, nitrogen content (wt % N) varied depending on the ligand composition. For example, nitrogen-rich ligands such as ethylenediamine (en) and diethylenetriamine (DET) resulted in a higher wt % N in Pd-en-MIL-101­(Cr) and Pd-DET-MIL-101­(Cr) than in pristine MIL-101­(Cr). In contrast, ligands with lower nitrogen content (e.g., AN, ABA, AVA, GA) or nitrogen-free ligands (e.g., AA) led to a lower nitrogen content than pristine MIL-101­(Cr), suggesting that the ligand may replace the DMF that previously coordinated with Cr nodes. The ligand loading was estimated to be ∼0.5 mmol/g, based on CHN analysis correlated with N 1s XPS data (see Figures S1 and S2 and Table S1 ).

1. Elemental Composition and Physical Properties of the Samples.

		Elemental Loading (wt %)				Content(mmol/g)		N 2 -adsorption
Entry	Catalysts	Pd	C	H	N	Pd	Ligand	S BET (m 2 /g)	V pore (cm 3 /g)	D pore (nm)
1	MIL-101(Cr)	-	18.83	4.67	1.84	-	-	2,161	1.03	2.86
2	Pd-MIL(Cr)	0.45	18.78	6.03	0.76	0.04	-	1,764	0.58	2.91
3	Pd-en-MIL101(Cr)	0.51	32.07	3.10	2.31	0.05	0.58	1,610	0.46	2.76
4	Pd-DET-MIL101(Cr)	0.50	23.69	4.69	2.18	0.05	0.39	1,053	0.32	2.88
5	Pd-AN-MIL-101(Cr)	0.46	25.34	3.76	1.07	0.04	0.49	1,539	0.61	2.28
6	Pd-ABA-MIL101(Cr)	0.33	23.39	3.62	1.29	0.03	0.56	1,592	0.46	2.89
7	Pd-AVA-MIL101(Cr)	0.32	28.38	4.43	1.04	0.03	0.48	1,027	0.49	2.48
8	Pd-GA-MIL101(Cr)	0.35	29.49	5.92	1.14	0.03	0.49	1,633	0.51	2.79
9	Pd-AA-MIL101(Cr)	0.45	27.47	2.32	1.22	0.04	0.41	1,208	0.59	2.45

^aDetermined by ICP-OES.

^bDetermined by CHN analysis.

^cCalculated from the N 1s XPS peak area and elemental analysis (Figures S1 and S2 and Table S1).

^dEstimated by the BET method.

^eEstimated by the BJH method (desorption pore diameter and cumulative desorption pore volume).

To evaluate the effects of ligand grafting and Pd incorporation, the porosity of MIL-101­(Cr) and Pd-L-MIL-101­(Cr) was examined by N₂ adsorption–desorption measurements using the Brunauer–Emmett–Teller (BET) method. As shown in Table and Figure , pristine MIL-101­(Cr) possessed an exceptionally high BET surface area of 2,161 m² g^–1 and a pore volume of 1.03 cm³ g^–1, consistent with its large cage-type architecture and wide pore windows (∼28 Å and 36 Å). Ligand modification induced only a slight decrease in surface area and pore volume, giving values of ∼1,401–1,700 m² g^–1 and ∼0.8–0.9 cm³ g^–1, respectively (Table S2 and Figure S3), indicating that the grafted ligands were accommodated within the intrinsic pore cavities without significantly restricting accessibility. Importantly, the ligand-modified MIL-101­(Cr) samples retained higher surface areas and pore volumes than their Pd-loaded analogues, suggesting sufficient residual porosity to host Pd species. Subsequent Pd incorporation resulted in a more pronounced reduction, with surface areas of ∼1,100–1,600 m² g^–1 and pore volumes of ∼0.4–0.6 cm³ g^–1, consistent with Pd species occupying the same pore cavities as the ligands. Across all samples, the pore size distribution remained essentially unchanged (∼2.3–2.9 nm), confirming preservation of the framework structure and supporting the conclusion that Pd deposition occurred primarily within the internal cavities rather than on the external surface. Considering the large pore windows of MIL-101­(Cr), all ligands employed in this study (kinetic diameters 4–7 Å) are readily able to diffuse into and reside within the porous framework.

3.

(a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution of Pd-L-MIL-101­(Cr) samples.

The incorporation of ligands significantly enhanced the dispersion of Pd within the MIL-101­(Cr) framework compared to samples without ligands (Figure ). The absence of Pd nanoparticle peaks in PXRD (Figure ) also suggests that the occluded Pd is well-dispersed. At comparable Pd loadings, the Pd-L-MIL-101­(Cr) samples exhibited smaller Pd nanoparticle sizes and a narrower size distribution than Pd-MIL-101­(Cr). Energy-dispersive X-ray (EDX) mapping of the Pd-L-MIL-101­(Cr) samples further confirmed the uniform distribution of Pd across the MIL-101­(Cr) support, in contrast to the more aggregated Pd NPs observed in Pd-MIL-101­(Cr). This improved dispersion can be attributed to the interaction between the ligands and Pd²⁺ species, likely through coordinated covalent bonding, as previously described by Makmeesub et al. Such interactions may also inhibit the agglomeration of Pd nanoparticles during the reduction process, leading to enhanced dispersion. Notably, the majority of Pd nanoparticles in the Pd-L-MIL-101­(Cr) samples were less than 1 nm in size.

4.

TEM images of (a) Pd-MIL-101­(Cr), (b) Pd-en-MIL-101­(Cr), (c) Pd-DET-MIL-101­(Cr), (d) Pd-AN-MIL-101­(Cr), (e) Pd-ABA-MIL-101­(Cr), (f) Pd-AVA-MIL-101­(Cr), (g) Pd-GA-MIL-101­(Cr), and (h) Pd-AA-MIL-101­(Cr).

3.2. Catalytic Reaction

3.2.1. Effect of Bridging Ligands

Despite the higher Pd dispersion observed in TEM (Figure ) for Pd-en-MIL-101­(Cr) and Pd-DET-MIL-101­(Cr), these materials exhibited significantly lower hydrogenation activityapproximately 4-fold and 2-fold lower, respectivelycompared to ligand-free Pd-MIL-101­(Cr) (Table ). This suggests that the presence of ethylenediamine (en) and diethylenetriamine (DET) ligands induces strong interactions between the ligands and Pd nanoparticles, which reduce the electron density at the Pd surface, thereby weakening H₂ dissociation and diminishing catalytic efficiency. These interactions are likely mediated via Pd–N coordination. ,,

2. Selective Hydrogenation Activity of Pd-L-MIL-101­(Cr) Compared with Pd-l-MIL­(Cr) .

				Yield (%)		Selectivity (%)
Entry	Catalyst	TOF (h –1 )	Conversion (%)	C18:1	C18:0	C18:1	C18:0
1	Pd-MIL-101(Cr)	8,600	38	35.7	2.3	94	6
2	Pd-en-MIL-101(Cr)	1,828	10	8.6	0.8	88	9
3	Pd-DET-MIL-101(Cr)	3,881	18.6	17.9	0.6	96	3
4	Pd-ABA-MIL-101(Cr)	15,210	48	46.3	1.6	96	3
5	Pd-AVA-MIL-101(Cr)	15,414	47	45.5	1.7	96	4
6	Pd-GA-MIL-101(Cr)	15,456	50	46.8	3.0	96	4
7	Pd-AN-MIL-101(Cr)	8,897	39	37.9	1.4	96	4
8	Pd-AA-MIL-101(Cr)	8,384	37	34.8	2.1	94	6
9	MIL-101(Cr)	-	0.0	-	-	-	-

^aReaction conditions: 50%w/w FAMEs in dodecane (10 g), catalyst (0.05 g) at 100 °C for 20 min under 150 mL/min H₂ flow.

The strong interactions between Pd⁰/Pd²⁺ and N donor ligand were deduced from XPS, as shown in Figure . The incorporation of Pd⁰/Pd²⁺ into bare MIL-101­(Cr) induced blue shifts in the binding energies of key elements, indicating electronic perturbation within the framework. Specifically, the Cr 2p_3/2 binding energy shifted from 577.32 to 577.82 eV (∼0.5 eV, Figure a), while the O 1s binding energy associated with C–O–Cr bonds increased from 530.52 to 530.92 eV (∼0.4 eV, Figure b). Consistent with these observations, shifts were also detected in the ligand environment, with the N 1s binding energy of NO₃ ^– increasing by 0.3 eV (Figure c) and the C 1s binding energy of C = O shifting by 0.2 eV (Figure d). These spectral shifts suggest that Pd²⁺ interacts with O^2– species within the Cr nodes, leading to polarization of electron density at, likely through ionic Pd^2+···O^2– interactions, as illustrated in Scheme a.

5.

XPS spectra of Pd-MIL-101­(Cr), Pd-en-MIL-101­(Cr), Pd-DET-MIL-101­(Cr), MIL-101­(Cr), en-MIL-101­(Cr), and DET-MIL-101­(Cr) showing (a,a′) Cr 2p_3/2, (b,b′) O 1s, (c,c′) N 1s, and (d,d′) C 1s regions.

2. Proposed the Interaction of the Ligand with Pd NPs within the (a) MIL-101­(Cr) Frameworks, (b) En, and (c) DET .

^a Pd species in this scheme could be either Pd or Pd²⁺.

In different manner to the ligand-free Pd-MIL-101­(Cr) system, the XPS spectra of Cr 2p_3/2, O 1s, N 1s, and C 1s exhibited no significant shifts upon Pd incorporation into en-MIL-101­(Cr) and DET-MIL-101­(Cr) (Figure ), suggesting that Pd⁰/Pd²⁺ preferentially coordinates with the nitrogen-donor ligands rather than interacting directly with the Cr nodes (Scheme ). The presence of additional N-donor sites in ethylenediamine (en) and diethylenetriamine (DET) facilitates the formation of coordinate covalent Pd–N bonds. This enables the ligands to function as bridges between the Cr nodes and Pd species (Scheme b,c), anchoring Pd²⁺/Pd⁰ within the framework. It is noted that the ligand–node interactions can be evidenced by the observed red shift in Cr 2p_3/2, O 1s, N 1s, and C 1s binding energies relative to bare MIL-101­(Cr), as shown in Figure a–d, indicating electron density redistribution upon ligand grafting.

Nevertheless, the Pd⁰/Pd²⁺–N coordination interactions facilitated by ethylenediamine (en) and diethylenetriamine (DET) bridging ligands are proposed to stabilize palladium predominantly in its oxidized Pd²⁺ state. This stabilization suppresses the formation of catalytically active Pd⁰ species and consequently reduces the number of accessible active sites. As a result, both Pd-en-MIL-101­(Cr) and Pd-DET-MIL-101­(Cr) exhibited comparatively lower catalytic conversion. Notably, DET possesses a longer chain length (∼6.5 Å) relative to en (∼4.5 Å), introducing a greater occupancy within the framework. This increased steric bulk significantly reduces the surface area (from 2,161 to 1,053 m²/g; Table ) and limits Pd incorporation. Such spatial constraints are believed to attenuate the Pd⁰/Pd²⁺–N interaction, thereby enhancing the availability of metallic Pd⁰ species in Pd-DET-MIL-101­(Cr). Consequently, Pd-DET-MIL-101­(Cr) exhibits approximately double the catalytic activity of Pd-en-MIL-101­(Cr) (Table ). It is worth noting that all catalysts maintained high selectivity toward methyl oleate (C18:1), exceeding 88%.

A significant enhancement in catalytic activity was observed upon employing bifunctional ligands containing both carboxylic acid (α–COOH) and amine (ω–NH₂) functionalitiesspecifically ABA, AVA, and GA (Scheme )as bridging ligands (as deduced from the XPS in Figure S4). These systems exhibited approximately a 1.8-fold increase in turnover frequency (TOF, from 8,600 h^–1 to ∼15,400 h^–1; Table ) compared to the ligand-free Pd-MIL-101­(Cr). This improvement is likely due to Pd²⁺–COO^– interactions in the amino acid-derived ligands (Scheme ), which leads the higher fraction of ultrafine Pd nanoparticles (as evidenced by TEM analysis, Figure e–g) relative to the ligand-free Pd-MIL-101­(Cr) (Figure a). Moreover, catalysts incorporating ABA, AVA, and GA exhibited an 8.5-fold higher activity than Pd-en-MIL-101­(Cr), which can be attributed to the relatively weaker Pd²⁺–COO^– coordination (Scheme ) compared to the stronger Pd²⁺–N interactions in the en and DET systems (Scheme ). This weaker coordination environment likely facilitates the more efficient in situ reduction of Pd²⁺ to Pd⁰, resulting in a higher fraction of catalytically active Pd⁰ species. Additionally, the enhanced accessibility and electronic environment of the Pd⁰ surface in these COO^– -anchored systems favor more effective H₂ dissociation. This leads to an increased surface hydrogen coverage (H-coverage), which is critical for sustaining a high rate of hydrogenation, especially for polyunsaturated substrates like FAMEs. In contrast, in the en- and DET-modified systems, the stronger Pd–N coordination can hinder H₂ activation, resulting in insufficient surface H-coverage and thus limited catalytic turnover. Therefore, the combination of abundant Pd⁰ sites and favorable hydrogen surface dynamics in ABA-, AVA-, and GA-functionalized catalysts contributes to their superior hydrogenation activity and excellent methyl oleate selectivity (>94%).

3. Proposed the Interaction of the Ligand with Pd NPs within the MIL-101­(Cr) Frameworks: (a) ABA, (b) AVA, and (c) GA.

When alanine was incorporated as a bridging ligand, Pd-AN-MIL-101­(Cr) (TOF ∼ 8,896.6 h^–1) exhibited significantly lower catalytic activity compared to systems utilizing AVA, ABA, and GA (TOF ∼ 15,400 h^–1). Although alanine possesses both α-carboxylic acid and β-amino groups (Scheme ), the short molecular structure results in close spatial proximity between these functionalities. This constrained geometry likely limits the ability of alanine to act as an effective bridging ligand between Pd and Cr nodes, thereby preventing the stabilization of Pd⁰/Pd²⁺ species observed in AVA-, ABA-, and GA-based systems (Scheme ). Consequently, the catalytic performance of Pd-AN-MIL-101­(Cr) closely resembles that of ligand-free Pd-MIL-101­(Cr). Supporting this interpretation, TEM analysis reveals comparable Pd nanoparticle morphologies for both Pd-AN-MIL-101­(Cr) and the ligand-free counterpart. These findings underscore the critical role of ligand geometry and functional group positioning in dictating the coordination environment and resulting catalytic performance of Pd-L-MIL-101­(Cr) systems.

Notably, the amino group appears to play a critical role in serving as a bridging ligand for anchoring to the Cr nodes. This inference is supported by the limited catalytic enhancement observed when α,ω-dicarboxylic acid (AA) was employed as the ligand (Table ). Despite the appropriate spatial arrangement of its two carboxylic acid groups, the catalytic performance of Pd-AA-MIL-101­(Cr) remains comparable to that of ligand-free Pd-MIL-101­(Cr) (TOF 8,383 vs 8,600 h^–1, Table ). This suggests that the interaction between carboxylate groups and the Cr nodes is relatively weak and insufficient to form effective coordination bonds. This is in contrast to the stronger anchoring capability of amino groups. This behavior aligns with the position of −COOH as a weaker field ligand compared to −NH₂ in the spectrochemical series. As AA fails to act as a bridging and stabilizing ligand, the dispersion of Pd nanoparticles in Pd-AA-MIL-101­(Cr) closely resembles that observed in the ligand-free system, as confirmed by TEM analysis (Figure ).

3.2.2. Reaction Pathway and Stability

To gain insight into the reaction mechanism, Pd-GA-MIL-101­(Cr) was employed, and the time-dependent product distribution is presented in Figure . At the initial reaction stage (10 min), cis-C18:1 was the predominant product, whereas trans-C18:1 and C18:0 were detected in minor amounts. After 30 min, the concentration of cis-C18:1 significantly decreased, concomitant with a sharp increase in trans-C18:1. This observation indicates that cis-C18:1 serves as the primary product, while trans-C18:1 is formed via isomerization of both the feedstock and hydrogenated cis-C18:1, thus representing a secondary product. Once conversion surpasses approximately 60%, the rate of isomerization exceeds that of hydrogenation, likely due to the depletion of polyunsaturated FAMEs in the reaction mixture. The gradual increase in C18:0, a fully hydrogenated product, albeit at a relatively lower rate, suggests nonselective hydrogenation of C18:1 derivatives once the polyunsaturated species are largely consumed. These findings support a stepwise reaction mechanism involving initial formation of cis-C18:1, its subsequent isomerization to trans-C18:1, and eventual hydrogenation to C18:0. The proposed catalytic pathway facilitated by the bifunctional ligand-modified Pd-MIL-101­(Cr) system is illustrated in Scheme .

6.

Time-dependent product distribution during the hydrogenation of polyunsaturated FAMEs over Pd-GA-MIL-101­(Cr). (Reaction conditions: 50 wt.% FAMEs in dodecane (10 g), catalyst (0.05 g), preactivated under H₂ (150 mL/min) at 100 °C for 1 h; hydrogenation performed at 100 °C under continuous H₂ flow (150 mL/min) for various durations.)

4. Proposed Reaction Mechanism of Selective Hydrogenation of Polyunsaturated FAMEs over the Bifunctional Ligand Modified Pd-L-MIL-101­(Cr) Catalysts.

The Pd–COO^– interaction in bifunctional ligand-modified Pd-MIL-101­(Cr) (using ABA, AVA, and GA) effectively suppressed Pd leaching, consistent with literature reports for Pd-en-MIL-101­(Cr)²⁹, as demonstrated by the hot filtration test shown in Figure . Initially, the hydrogenation reaction was carried out for 30 min at 100 °C under a continuous flow of H₂ (150 mL/min), employing 0.05 g of catalyst and a substrate composed of 50 wt % FAMEs in dodecane. The catalyst was then rapidly filtered at the reaction temperature to minimize the adsorption of reactants and products. The reaction was repeated using the filtrate under identical conditions to evaluate the presence of leached Pd species. As seen in Figure , no further conversion was observed for any of the Pd-L-MIL-101­(Cr) catalysts (L = ABA, AVA, GA), indicating that no Pd species had leached into the reaction medium. These results highlight the strong anchoring effect provided by the incorporated α-carboxylate and ω-amino functional groups, which not only improve Pd dispersion and hydrogenation activity but also significantly enhance the stability of the catalytically active Pd species.

7.

Hot filtration tests for (a) Pd-ABA-MIL-101­(Cr), (b) Pd-AVA-MIL-101­(Cr), and (c) Pd-GA-MIL-101­(Cr). (Reactions were performed with 50 wt % FAMEs in dodecane (10 g), catalyst (0.05 g) preactivated under H₂ (150 mL/min) at 100 °C for 1 h, followed by hydrogenation at 100 °C for 20 min. Filtration was conducted at reaction temperature to assess Pd leaching.)

A recyclability test was conducted over four consecutive runs without additional activation between cycles. The catalysts maintained a similar catalytic activity (∼50% conversion) after the fourth run (Figure ), with only a marginal decrease observed from the first to the second run. While Pd NP agglomeration during the initial cycles could be considered, TEM analysis of the spent catalysts after four cycles (Figure ) revealed Pd particle sizes comparable to those in the fresh catalysts (Figure ). This confirms that no significant aggregation occurred and suggests that dissolution–reduction of Pd species did not take place during the reaction. The slight initial drop in activity is more likely due to adsorption of feed and product molecules on the catalyst surface, particularly in the first two cycles. Given the inherently high surface area of MIL-101­(Cr) (∼2,100 m²/g) and the relatively low combined ligand and Pd loading (∼2 wt %), a large number of vacant sites remain in the porous framework that can nonselectively trap reactants or products, temporarily reducing the number of accessible Pd⁰ active sites. Once adsorption sites within the MOF become saturated, the catalytic performance proceeds steadily. Supporting this interpretation, N₂ sorption analysis of the spent catalyst showed a decrease in surface area, pore volume, and pore size compared with the fresh material (Table S4 and Figure S5a,b), consistent with partial pore blockage rather than framework collapse. XRD patterns before and after recycling (Figure S5c) confirmed that the MIL-101­(Cr) framework retained its crystalline integrity. These results, together with the strong anchoring effect of ABA, AVA, and GA ligands, confirm the high structural and morphological stability of the catalyst system under the applied reaction conditions, highlighting its potential for industrial hydrogenation applications.

8.

Catalyst recyclability test of Pd-ABA-MIL-101­(Cr) over four consecutive hydrogenation cycles. (Reaction conditions: 50 wt % FAMEs in dodecane (10 g), catalyst (0.05 g) preactivated under H₂ (150 mL/min) at 100 °C for 1 h; each run performed at 100 °C for 20 min under continuous H₂ flow (150 mL/min).)

9.

TEM image of spent Pd-ABA-MIL-101­(Cr) catalyst after four consecutive hydrogenation cycles.

Compared to previously reported catalysts (Table ), the ligand-modified Pd-MIL-101­(Cr) systems (ABA, AVA, and GA) demonstrate superior performance, exhibiting a high turnover frequency (TOF > 15,400) while maintaining high selectivity. Our catalyst design incorporates a tailored bifunctional ligand (e.g., α-COOH and ω-NH₂) that optimizes Pd NPs dispersion and stability. Furthermore, the ligand effectively mitigates Pd NP agglomeration, enabling excellent recyclability with consistent performance for at least 4 cyclesreliable with literature benchmarks. These results also highlight the advantageous role of MIL-101­(Cr) as a support material compared to activated carbon, SiO₂, SBA-15, Al₂O₃, MCM-41, and polyethylene glycol (PEG) owing to its structural stability and favorable metal–support interactions. ,,,−

3. Comparison of the Catalytic Activity of Pd-L-MIL-101­(Cr) with Other Catalysts Reported in the Literature for Selective Hydrogenation.

Entry	Catalyst	Pd loading (wt %)	Temp. (°C)	Pressure (bar H2)	Feed	Solvent	TOF (h–1)	Selectivity (%)	Recyclability (cycle)	Ref.
1	Pd/γ-Al2O3	1.00	80	5	Palm-BDF	solvent-free	0.48	53	n/a
2	Pd/MCM-41	1.00	100	4	Soybean-FAMEs	solvent-free	36	88	n/a
3	Pd/C	0.50	50	1	Com-FAMEs	n-hexane	100	98	n/a
4	Pd-pol	2.50	rt	10	WCO	MeOH	160	54	n/a
5	Pd/PEG4000	0.50	75	1	Methyl linoleate	solvent-free	265	92	5
6	Pd/NPC-H	4.84	100	4	Palm-BDF	solvent-free	1,140	84	n/a
7	Pd/SBA-15	1.00	100	4	Soybean-FAMEs	solvent-free	5,590	89	n/a
8	Pd/SiO2-Q30	1.00	80	3	Rapeseed-BDF	solvent-free	8,400	-	n/a
9	Pd-MIL-101(Cr)	0.45	100	1	Soybean-FAMEs	dodecane	8,600	94	n/a	This work
10	Pd/SiO2	1.00	120	4	Soybean-FAMEs	solvent-free	10,380	93	n/a
11	Pd-ABA-MIL-101(Cr)	0.33	100	1	Soybean-FAMEs	dodecane	15,209	96	4	This work
12	Pd-AVA-MIL-101(Cr)	0.32	100	1	Soybean-FAMEs	dodecane	15,414	96	n/a	This work
13	Pd-GA-MIL-101(Cr)	0.35	100	1	Soybean-FAMEs	dodecane	15,455	94	n/a	This work

^aBiodiesel fuel.

^bCommercial.

^cRecyclable polymer = 2-(acetoacetoxy)­ethyl methacrylate.

^dWaste cooking oil.

4. Conclusions

The incorporation of bifunctional ligands into MIL-101­(Cr) has proven highly effective in modulating palladium dispersion, speciation, and catalytic performance for the chemoselective hydrogenation of polyunsaturated fatty acid methyl esters (FAMEs). Ligands bearing both – NH₂ and – COOH functionalities, such as ABA, AVA, and GA, facilitated the formation of highly dispersed Pd nanoparticles (< 1 nm) through weak Pd⁰/Pd²⁺–ligand interactions, as confirmed by TEM and XPS analyses. These interactions mitigate Pd aggregation and leaching under liquid-phase hydrogenation conditions. The molecular architectures of ABA, AVA, and GA ligands, characterized by their appropriate chain lengths and spatially separated amino and carboxylate groups, provide a geometrically flexible and sterically accessible environment for Pd coordination. This configuration facilitates the formation of well-dispersed Pd⁰ active sites while minimizing electronic deactivation. Therefore, significantly enhanced turnover frequencies (TOF up to ∼15,400 h^–1) were achieved, greater than those of ligand-free systems which suffer from Pd agglomeration. In contrast, strong Pd–N coordination observed in ethylenediamine (en) and diethylenetriamine (DET) functionalized systems led to Pd²⁺ stabilization but significantly diminished hydrogenation activity, due to the hindered reduction to catalytically active Pd⁰.

Catalysts maintained structural integrity and activity over multiple cycles, with no detectable Pd leaching, highlighting the robustness of ligand–Pd–MOF interactions. Notably, Pd-ABA-MIL-101­(Cr), Pd-AVA-MIL-101­(Cr), and Pd-GA-MIL-101­(Cr) catalysts demonstrated exceptional selectivity (>94%) toward monounsaturated FAMEs (C18:1). The hydrogenation pathway proceeded via initial formation of cis-C18:1, followed by partial isomerization to trans-C18:1. Overhydrogenation to fully saturated C18:0 occurred only after near-complete consumption of the unsaturated feedstock, indicating effective suppression of excessive hydrogenation under optimized conditions. Overall, this work demonstrates that precise control of ligand proximity and functionality in MOF-based systems enables the fine-tuning of electronic and steric environments around Pd centers. These findings open new avenues for rational design of selective, stable, and reusable Pd catalysts for FAMEs upgrading and other challenging liquid-phase transformations.

Supporting Information: (PDF) The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c02268.

• Additional characterization data (ligand content calculation, N₂ adsorption, XPS, XRD), and catalytic performance table with standard deviation (PDF)

The authors declare no competing financial interest.