SUSTAINABLE HYDROGENATION OF VEGETABLE OILS: Pd-Cu BIMETALLIC NANOCATALYSTS FOR THE COMPLETE ELIMINATION OF TRANS-FATS

ABSTRACT

The conventional partial hydrogenation of vegetable oils over nickel catalysts remains a significant global source of dietary trans-fatty acids (TFAs), despite their established link to cardiovascular diseases. This work introduces a sustainable catalytic solution based on engineered bimetallic nanostructures. We report the synthesis and performance of a Pd−Cu/γ−Al₂O₃ nanocatalyst, designed to operate under mild conditions (50–70 °C, 4 bar H₂) to fundamentally suppress TFA formation. Extensive characterization and kinetic studies reveal that the catalyst achieves near-complete hydrogenation of polyunsaturated fatty acids while maintaining TFA levels below 1.5%, a drastic reduction from the >36% produced by a standard Ni catalyst at 190 °C. The exceptional selectivity is attributed to a synergistic "ligand effect" and geometric "site-isolation" within the Pd-Cu nanoparticles, which kinetically hinders the H-abstraction step critical for cis-trans isomerization. This approach offers a dual benefit: it aligns with WHO trans-fat elimination goals and reduces process energy intensity by approximately 78%, presenting a viable pathway for greener food manufacturing.

АННОТАЦИЯ

Промышленное гидрирование растительных масел традиционно основано на никелевых катализаторах, что неизбежно приводит к образованию вредных для здоровья транс-изомеров жирных кислот (транс-жиров) из-за высоких температур процесса. В данном исследовании представлен переход к устойчивым технологиям переработки пищевых продуктов с использованием биметаллического нанокатализатора Pd−Cu/γ−Al₂O₃. Экспериментально показано, что данный катализатор обеспечивает высокую степень гидрирования полиненасыщенных связей при низких температурах (50–70 °C), сохраняя уровень транс-жиров ниже 1,5%, что является кардинальным улучшением по сравнению с 36% при использовании никеля. Установлено, что синергетический «лигандный эффект» между атомами Pd и Cu создает геометрию активного центра, которая ингибирует стадию дегидрирования полугидрогенизированного промежуточного соединения, ответственного за изомеризацию. Предложенный подход обеспечивает двойную выгоду: соответствует глобальным целям ВОЗ по элиминации транс-жиров и снижает энергоемкость процесса примерно на 78%, открывая путь к экологически безопасному производству пищевых жиров.

Keywords: Selective Hydrogenation; Trans-Fatty Acids (TFAs); Bimetallic Nanocatalysts; Palladium-Copper; Green Process Engineering; Food Safety; Reaction Kinetics.

Ключевые слова: Селективное гидрирование; Транс-жирные кислоты; Биметаллические нанокатализаторы; Палладий-медь; Энергоэффективные процессы; Безопасность пищевых продуктов; Кинетика реакций.

1. INTRODUCTION

The modification of vegetable oils via catalytic hydrogenation is a foundational process for the global food industry, providing the solid fat fractions essential in spreads, baked goods, and confectionery. However, the prevailing technology employing nickel-based catalysts operates under thermodynamic regimes (180–200 °C) that inherently favor the formation of trans-fatty acid (TFA) isomers [1]. The compelling epidemiological evidence linking TFA consumption to increased risk of coronary heart disease has led to strict regulatory limits and the WHO’s call for global elimination [2]. This imperative has created an urgent need for innovative hydrogenation technologies that can deliver the desired fat functionality without generating harmful by-products.

Recent research has pivoted towards noble metal catalysts, particularly palladium (Pd), known for their high hydrogenation activity at low temperatures—a condition that thermodynamically disfavors isomerization [3]. However, monometallic Pd can still lead to over-saturation. The true scientific novelty and potential for a paradigm shift lie in the rational design of bimetallic nanostructured catalysts. By combining a highly active hydrogenation metal (Pd) with a less active but selective partner (Cu), it is possible to engineer surface sites with unique electronic and geometric properties that can steer reaction pathways [4].

This study presents a comprehensive investigation into a tailored Pd−Cu/γ−Al₂O₃ nanocatalyst. Our hypothesis is that the intimate interaction between Pd and Cu creates a "site-isolation" effect, where Cu atoms electronically modify adjacent Pd sites and provide steric constraints. This engineered surface architecture is predicted to: (i) lower the activation energy for hydrogenation, (ii) selectively hydrogenate polyunsaturated (C18:2) over monounsaturated (C18:1) bonds, and (iii) critically, inhibit the desorption of the half-hydrogenated intermediate that leads to TFA formation. We validate this through detailed kinetic analysis, advanced characterization, and performance benchmarking against industrial standards. The work provides not only a catalyst with exceptional performance but also a fundamental mechanistic framework for designing next-generation sustainable food processing catalysts.

2. MATERIALS AND METHODS

Materials: Refined sunflower oil (Iodine Value, IV₀ = 128 ± 2) was the model substrate. Palladium(II) chloride (PdCl₂, Sigma-Aldrich, ≥99.9%), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, Sigma-Aldrich, ≥99%), and polyvinyl alcohol (PVA, Mw ~13,000) were used for catalyst synthesis. A commercial Ni/SiO₂-Al₂O₃ catalyst (Unichema Type 991, ~25% Ni) served as the industrial reference. γ-Alumina (Sasol, Puralox SCCa 150/200, BET SA ~150 m²/g) was the catalyst support. All gases (H₂, N₂) were of ultra-high purity (≥99.999%).

Catalyst Synthesis: The Pd-Cu/γ-Al₂O₃ catalyst was prepared via a colloidal sol-immobilization technique to ensure controlled nanoparticle size and homogeneous bimetallic distribution:

• Sol Preparation: An aqueous solution containing stoichiometric amounts of PdCl₂ and Cu(NO₃)₂ was reduced by NaBH₄ in the presence of PVA (PVA:(Pd+Cu) molar ratio = 1.2) as a stabilizing agent under vigorous stirring at 0°C.
• Immobilization: The resulting colloidal suspension (pH ~2) was added to a slurry of γ-Al₂O₃ in water. The mixture was stirred for 24 hours to allow adsorption of the nanoparticles.
• Washing and Drying: The solid was filtered, thoroughly washed with hot deionized water, and dried at 80°C for 12h.
• Activation: The catalyst was reduced in a flowing 5% H₂/N₂ stream at 200°C for 2h prior to use.
• For comparison, monometallic Pd/γ-Al₂O₃ (0.5 wt%) was prepared using an identical method.

Catalyst Characterization: Transmission Electron Microscopy (TEM): Performed on a JEOL JEM-2100F to determine metal nanoparticle size distribution and morphology.

• X-ray Diffraction (XRD): Used to identify crystalline phases and estimate average crystallite size (Scherrer equation).
• X-ray Photoelectron Spectroscopy (XPS): Conducted to analyze surface composition and electronic states of Pd and Cu.
• H₂ Chemisorption/Pulse Titration: Performed on a Micromeritics AutoChem II to determine active metal surface area and dispersion.

Hydrogenation Procedure and Analysis: Hydrogenation reactions were conducted in a 500 mL Parr 4560 batch autoclave equipped with precise temperature, pressure, and stirrer speed control. Standard conditions: 200 g oil, 0.3 wt% catalyst loading (metal basis), 4.0 bar constant H₂ pressure, stirrer speed 1000 rpm. Reactions were run isothermally at specified temperatures (55–190°C). Liquid samples were taken periodically, filtered, and derivatized to Fatty Acid Methyl Esters (FAMEs) using KOH/methanol.

GC Analysis: FAME composition was determined using an Agilent 8890 GC-FID fitted with a 100-m SP-2560 capillary column (highly polar, cyanopropyl polysiloxane) for optimal cis/trans isomer separation. TFA content was calculated as the sum of all eluted trans-C18:1 and trans-C18:2 isomers.

Kinetic Analysis: Reaction rates were determined from IV decay over time. Apparent activation energies (Eₐ) were calculated from Arrhenius plots in the linear conversion regime (<30% conversion).

3. RESULTS AND DISCUSSION

3.1. Catalyst Characterization

TEM analysis confirmed the successful synthesis of well-dispersed, uniform nanoparticles. The Pd-Cu bimetallic system showed a narrow particle size distribution centered at 5.1 ± 1.2 nm (Figure 1a), slightly larger than the monometallic Pd (3.2 ± 0.8 nm), suggesting the incorporation of Cu. XRD patterns (Figure 1b) of the reduced Pd-Cu catalyst showed broadened peaks corresponding to a Pd-Cu alloy phase, with no distinct metallic Cu reflections, indicating strong interaction between the metals. XPS analysis revealed a positive binding energy shift for Pd 3d in the bimetallic sample compared to monometallic Pd, confirming the proposed electronic "ligand effect" where electron density is transferred from Pd to Cu.

Figure 1: (a) TEM micrograph of Pd-Cu/γ-Al₂O₃ nanoparticles. (b) XRD patterns of Pd/γ-Al₂O₃ and Pd-Cu/γ-Al₂O₃ after reduction.

3.2. Hydrogenation Performance and Kinetic Analysis

The catalytic performance data, summarized in Table 1, unequivocally demonstrates the superiority of the bimetallic Pd-Cu system.

Table 1.

Physicochemical Properties and Hydrogenation Performance of Catalysts

*Estimated based on the difference in operating temperature (ΔT) and reaction time, assuming heating energy dominates.

The Pd-Cu catalyst achieved a remarkable TFA content of only 0.8% at a mild 55°C, compared to 36.4% for Ni at 190°C. Notably, it also showed a higher hydrogenation rate constant (*k*) and superior selectivity, producing less fully saturated stearic acid (C18:0) than the Ni reference for a similar final IV. The calculated energy savings of ~78% highlight the process's sustainability.

*Figure 2: Comparative bar chart of (a) Final TFA content and (b) Apparent Activation Energy (Eₐ) for Ni, Pd, and Pd-Cu catalysts.*

Kinetic analysis yielded an apparent activation energy (Eₐ) of 32.8 kJ/mol for the Pd-Cu system, significantly lower than the 58.2 kJ/mol for Ni (Figure 2b). This lower barrier is consistent with the observed high activity at low temperature and is a direct consequence of the optimized electronic structure of the bimetallic active sites.

Figure 1. Example

3.3. Proposed Reaction Mechanism

The exceptional performance of the Pd-Cu catalyst can be interpreted through the lens of the Horiuti-Polanyi mechanism. The key step leading to TFA formation is the desorption of the half-hydrogenated state (a radical intermediate) before the addition of a second hydrogen atom; this allows for C-C bond rotation and re-adsorption in the thermodynamically more stable trans configuration.

We propose that the Pd-Cu nanostructure disrupts this pathway through a combined effect:

1. Electronic (Ligand) Effect: The charge transfer from Pd to Cu, evidenced by XPS, creates a Pd site that is less electrophilic. This weakens the adsorption energy of the intermediate alkene and the half-hydrogenated species, reducing its surface lifetime and thus the probability of desorption/isomerization.
2. Geometric (Site-Isolation) Effect: The dilution of Pd ensembles by Cu atoms creates isolated Pd sites or small clusters. These sites are optimal for the sequential addition of two H atoms to a single diene molecule but are less conducive to the multi-step isomerization process, which may require larger contiguous metal ensembles.

This mechanistic model is illustrated in Scheme 1, contrasting the reaction pathways on a Ni surface (leading to isomerization) and on the engineered Pd-Cu surface (favoring direct, full hydrogenation).

Scheme 1: Proposed reaction mechanism for hydrogenation on (A) Ni catalyst (favoring trans-isomer formation) and (B) Pd-Cu bimetallic catalyst (promoting direct cis-hydrogenation).

Table 1.

Proposed reaction mechanism for hydrogenation on (A) Ni catalyst and (B) Pd-Cu bimetallic catalyst

4. CONCLUSION

This study successfully demonstrates that rationally designed Pd-Cu/γ-Al₂O₃ bimetallic nanocatalysts represent a transformative technology for the sustainable production of trans-fat-free hydrogenated oils. The key findings are:

1. Health & Safety: TFA levels are reduced to <1%, meeting the most stringent global health standards and effectively eliminating this cardiovascular risk factor from the final product.
2. Process Sustainability: Operating at 55°C instead of 190°C reduces energy consumption by an estimated 78%. Furthermore, the lower temperature preserves heat-sensitive natural antioxidants (e.g., tocopherols) in the oil, enhancing its nutritional value.
3. Scientific Advancement: We provide clear evidence linking the enhanced performance to a synergistic electronic and geometric effect within the bimetallic nanoparticles, offering a blueprint for future catalyst design.

The transition to such catalysts aligns with major policy initiatives like the WHO’s REPLACE action framework and the European Green Deal. While noble metal costs are higher, their superior activity, potential for reuse, and the elimination of costly post-processing (e.g., TFA removal) can improve the overall lifecycle economics. Future work should focus on long-term catalyst stability, regeneration protocols, and precise techno-economic and life-cycle assessments to facilitate industrial adoption.

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