Green Synthesis of Cu and Pd Catalysts Using Mexican Oregano (Lippia graveolens) Extract and Their Application in the Conversion of a Biomass-Derived Molecule

Abstract

This work reports the synthesis of two monometallic catalysts, Cu/Al₂O_3, and Pd/Al₂O₃, using a green approach based on Mexican oregano (Lippia graveolens), a common food condiment. Its extract has been largely overlooked as a high-technology reactive for synthesizing catalysts, metallic or oxide nanoparticles, unlike other green leaf plants. The green synthesis was compared with a conventional catalyst synthesis methodology using commercial chemical reducing agents. Oregano extract shows promise for novel applications extending beyond its culinary use, valorizing it as a chemical reducer to produce catalysts. Thus, this kind of application could significantly elevate the value of oregano, empowering communities that rely on its cultivation for economic benefit and transforming the plant from a low-profit agro-industrial product to a high-added-value crop. The reduction kinetics involved in the formation of nanoparticles were monitored up to the first stage of nucleation and a first-order model adequately described the data. Activation energy analysis showed that the chemical reaction mechanism has a dominant role in controlling the reaction, compared to mass transfer effects. Notoriously, the Pd/Al₂O₃ green synthesis catalyst showed the smallest mean particle size (4.85 ± 1.30 nm). These findings underscore the potential of green synthesis as an economically viable and environmentally friendly alternative for producing catalysts. Concerning the 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) as a biomass-derived molecule, its oxidation with H₂O₂ using both Pd/Al₂O₃ catalysts (by green and chemical synthesis methods) exhibited significantly higher selectivity toward 2,5-diformylfuran (DFF) compared to Cu/Al₂O₃ catalysts, suggesting a possible inhibitory effect.

1. Introduction

Abundant biomass offers a renewable source of fuel and chemicals, tackling resource and environmental challenges. Based on the postulates of green chemistry [1,2], new technologies and methodologies have been developed, with the use of biomass (organic extracts, agro-industrial residues, etc.) being proposed as a renewable raw material to obtain valued added products and help solve the current pollution problems [3,4,5,6], also contributing to a circular economy. Valorizing industrial waste biomass offers a solution to waste and creates new, valuable products [7]. An example of this technology is phytonanotechnology, which is a green synthesis method and refers to obtaining metal nanoparticles from redox reactions between a metal salt and a biomass extract [8]. The use of this new route is currently being studied for the production of new catalysts [9,10]. In comparison, conventional catalyst synthesis methods such as wet impregnation, in which traditional chemical reducers are involved, become dangerous to the environment and require chemical stabilizers to prevent their degradation [11]. Green synthesis methods are performed using natural extracts from flowers, leaves, or roots, and are proposed as an alternative to conventional chemicals, offering not only metal salt reduction but also catalyst stabilization by phytochemical compounds [12,13]. At present, the use of natural extracts from leaves, plant roots, and even the shells of certain fruits offers a novel synthesis method for obtaining metallic nanoparticles such as palladium, silver, and gold, among others, which are mainly used in antimicrobial performance studies [13,14]. Although it is well known that precious metals tend to be expensive, metals with greater availability, such as copper, have been extensively studied for catalytic applications [13,15]. Observations indicate that scaling up catalyst synthesis often necessitates the utilization of alternative precursors, as small-scale precursors may not be commercially viable [16]. Consequently, the employment of biomass extracts as reducing agents can be considered a promising avenue for further investigation within the laboratory setting.

Green catalyst synthesis has been previously applied with different metals, generally being reported as a cost-effective and non-toxic procedure. For example, Mahmoudi et al. reported three-metallic oxide (Cu/Cr/Ni) nanoparticles prepared using Echinops persicus plant extract, evaluating their catalytic activity in the synthesis of biologically active quinoline and spirooxindole derivatives, describing good to excellent efficiencies [17]. Another example is that of Rafi Shaik et al., who synthesized unsupported palladium nanoparticles using Origanum vulgare L. extract, which were successfully applied as catalysts for the selective oxidation of alcohols [18]. However, Pd nanoparticles dispersed on alumina support could offer the advantage of potentially greater performance with lower metal loading (less than 10 wt.%). Furthermore, the addition of oregano extract could lead to the discovery of new high-technology applications for oregano, granting it greater value than its typical use as a food condiment. Additionally, it has the potential to empower communities currently lacking sufficient economic benefit from oregano cultivation. Oregano extract has the potential to be developed as a powerful bio-based reagent for the synthesis of valuable products (such as catalysts) in high-tech industries worldwide. Agroindustrial biomass waste extracts have also been used for catalyst synthesis. For instance, Rasool et al. synthesized Ag nanoparticles using Punica granatum (Pomegranate) fruit peel extract, which was used as a catalyst for the degradation of synthetic dyes [19]. Scientific works, such as the aforementioned, demonstrate the current scientific focus on the obtainment of catalysts while also contributing to a circular economy.

Oregano (Origanum spp.) is a genus of aromatic plants native to the Mediterranean region, and northern Europe, and extending eastward to China. It has also been domesticated in other areas globally. Notably, several oregano species are found in the semi-arid regions of northern Mexico and the southern United States [20]. In Mexico, 40 species of oregano have been registered, and the Lippia variety is one of the most important due to its aromatic characteristics, reaching first place in world production of oregano by 2020 because of national programs focused on planting and harvesting of the plant for the obtention of diverse products such as medicines, flavorings, aromas, and extracts, due to the development of efficient production chains [21,22]. Given its widespread distribution as a native and cultivated plant throughout the northern regions of Mexico, oregano extract presents a potentially viable supply chain compared to other grassland and scrubland resources in these arid and semi-arid environments [23]. Given that Lippia graveolens is the most cultivated and marketed oregano species in northeast Mexico, with some export to North America, this study selected it to demonstrate the potential of the extract for high-tech applications like catalyst synthesis. The extracts of this plant can be used in the pharmaceutical, cosmetic, pesticide, and food industries, due to the chemical composition of its essential oil, which contains organic molecules such as thymol, carvacrol, and certain phenolic compounds with antioxidant properties and industrial interest [24,25]. In recent work, organic waste extracts were used as reducing agents to produce Ag nanoparticles and kinetic studies of formation were discussed, proposing three stages: nucleation, autocatalytic, and stabilization of nanoparticle growth [26]. Sitthisa et al. used Pd/SiO₂ and Pd-Cu/SiO₂ catalysts in the hydrogenation of furanic molecules such as furfural [27], while another work from Baruah et al. reported Cu nanoparticles deposited on cellulose for the oxidation of the furanic molecule HMF to 2,5-diformylfuran (DFF) [28]. A question that arises is whether Cu alone can serve as a competitive catalyst to Pd in the oxidation of another furanic molecule, such as hydroxymethyl-2-furancarboxylic acid (HMFCA). Exploring green synthesis methods for Cu and Pd catalysts could reveal their potential to contribute to conventional commercial catalytic processes such as hydrogenation of carbon–oxygen bonds [29] and cross-coupling reactions [30], respectively. It can be said this work aims to unite the green synthesis of high commercial value catalysts like Cu and Pd with their use in valorizing a molecule derived from biomass, which can come from agro-industrial waste. This contributes to core circular economy principles such as Waste as a Resource, Slowing the Loop, and Design for Circularity.

In another recent work from our research group, gobernadora (Larrea tridentata) extract was used to synthesize Pd–Fe bimetallic nanoparticles supported on γ-Al₂O₃ and applied as a catalyst for the hydrogenation of 2-methylfuran to form biorefinery products [9]. Motivated by our previous work on 5-hydroxymethylfurfural (HMF) oxidation [21,22,23], we will now focus on HMFCA as a key intermediate whose oxidation pathway needs further investigation. In the oxidation route of HMF, the production of 2,5-diformylfuran (DFF) can occur, and this can later be converted into a biofuel, such as a furoate ester biofuel produced in ethanol [31], or to produce 2,5-furandicarboxylic acid (FDCA), which is an important monomer for the production of poly(ethylene 2,5-furandicarboxylate) (PEF) as a technological material [32]. Also, it is important to denote that Pd has been reported as a catalyst in the oxidation of HMF [33,34] and other furan-derived molecules such as 2,5-bis(hydroxymethyl)furan (BHMF) [35].

The scientific innovation of this work relies on the study of the synthesis kinetics involved in the obtainment of monometallic Cu and Pd nanoparticles, using Mexican oregano (Lippia graveolens) extract, and later supported on γ-Al₂O₃, to determine if the extracted organic compounds present mass transfer effects. The catalysts will subsequently be used in the oxidation of HMFCA to report the obtained byproducts and contribute to the knowledge of the catalytic transformation of biomass-derived molecules. In this way, it demonstrates the oregano plant’s unexploited potential for technological innovation.

2. Materials and Methods

2.1. Oregano Extract

The methodology to obtain the oregano extract was based on the previous report by Elizondo et al. [36]. First, the size of the oregano leaf (Lippia graveolens) from northeastern Mexico was manually reduced using a mortar and sieving the powder to a size of 425–500 nm, employing a sieve with a mesh size of 35–40. Once this powder was obtained, warm ethanol (Sigma-Aldrich, USA ≥ 99.5%, ACS reagent) at 60 °C was added to the oregano powder with a 1:10 oregano/ethanol ratio, stirred for 30 min, and later centrifuged at 3500 rpm. The liquid extract was separated and stored at 4 °C before further use.

Since green extracts typically contain a complex mixture of compounds with varying polarities (e.g., flavonoids, terpenoids, alkaloids), high-performance liquid chromatography (HPLC), with its diverse stationary and mobile phases, can effectively separate and analyze a wide range of these compounds. Once these molecules are identified, it becomes possible to determine which specific functional groups within these molecules are responsible for the reduction of the metal ion and which can potentially stabilize the formed metallic nanoparticles through stabilizing interactions. The organic molecules were quantified using HPLC, with an instrument model YL9900 LC/ MS YL Instrument Co. LTD. A separation method was developed using a C-18 column (200 × 4.6 nm) and 1% acetonitrile in formic acid, along with 1% formic acid in water at 30 °C as mobile phases. The working pressure was 1680 psi, and the execution time of the method was 20 min, with a UV-Vis detector at 280 nm. To identify the phenolic compounds in extracted oregano oil, two reference standards were used: 2-Isopropyl-5-methylphenol known as thymol (≥98.5% of purity), and 5-isopropyl-2-methylphenol known as carvacrol (98% of purity), both obtained from Sigma-Aldrich. Both the standards and a 5% extract sample were diluted in ethanol before HPLC analysis.

2.2. Synthesis of Cu and Pd Nanoparticles by Conventional and Green Methods

To prepare the Cu and Pd nanoparticles, a green method (where the oregano extract was used as a metal-reducing agent) was used and compared with a conventional synthesis method, where NaBH₄ was applied.

2.3. Synthesis of Cu and Pd Nanoparticles and Catalysts by Conventional Methods

As a reference method for the extraction of oregano components, NaBH₄ and ascorbic acid solutions were selected due to their mild reaction conditions. This contrasts with other conventional methods of catalyst synthesis, which often require high temperatures, along with hazardous reducing agents, such as hydrogen gas [37]. Cu nanoparticles were synthesized following the methodology previously reported by Glavee et al. [38], where a 2000 mg/L copper solution was prepared using CuCl₂•2H₂O (Sigma Aldrich, USA ≥ 99.95%) as a precursor. A volume of 100 mL of the Cu solution was mixed with 100 mL of 62.94 mM NaBH₄ solution (Sigma Aldrich, USA ≥ 99.95%) as a chemical reducer. The solution was stirred constantly for 30 min and experienced a color change from blue to intense brown. The Cu/Al₂O₃ catalyst containing 5% wt. Cu was synthesized by adding commercial γ-alumina as a support (CATALOX, USA, SBa-200) immediately after the CuCl₂•2H₂O and stirring for 15 min. Then, 100 mL of 62.94 mM NaBH₄ solution was added and stirred for an additional 30 min. Finally, the solid was washed with ethanol and water to remove impurities and subsequently dried at 120 °C for 24 h.

The Pd nanoparticles for the chemical method were synthesized following the method previously reported by Wojnick et al. [39], using ascorbic acid solution as a reference chemical agent. A 0.23 mmol PdCl₂ solution was prepared (Reagent Plus®, USA, 99%), then a solution of 0.47 mmol HCl (analytical grade, 36% v/v) was added until pH ≤ 3 was reached. Finally, a 0.31 mmol ascorbic acid solution (ACS reagent, ≥ 99%) was added as a chemical reducer, observing a gradual color change to gray. The solution was stirred constantly for 30 min.

The Pd/Al₂O₃ catalyst (5 wt.% Pd) was synthesized by adding the support (γ-alumina) and stirring for 15 min; afterward, a 0.31 mmol ascorbic acid solution was added and stirred for an additional 30 min. Finally, the solid was washed with ethanol and water to remove impurities and subsequently dried at 120 °C in a drying oven for 24 h.

2.4. Synthesis of Cu and Pd Nanoparticles and Catalysts Using a Green Method

The Cu nanoparticles were synthesized following the methodology previously reported by Nasrollahzadeh et al. [40], where a 2000 mg/L Cu solution was first prepared (using the same Cu precursor of conventional synthesis), later adding 80 mL of oregano extract (observing a gradual color change from blue to light brown) under constant stirring for 30 min. The preparation of the Cu/Al₂O₃ catalyst by the green method was identical to the conventional method, except the NaBH₄ solution was replaced with oregano extract.

To obtain the Pd nanoparticles by the green method, a 0.23 mmol PdCl₂ solution was prepared, and then a 0.47 mmol HCl solution was added until pH ≤ 3. Afterward, 15 mL of oregano extract (reducer) was added, observing a gradual color change to ocher. The obtained solution was stirred constantly for 30 min.

The green Pd/Al₂O₃ catalyst (5 wt.% Pd) was synthesized with the same methodology, adding the support (γ-alumina) and stirring for 15 min. Fifteen mL of oregano extract was added, observing a gradual color change from light blue to ocher in the first 30 min under constant stirring, finishing with an intense brown color after stirring for 24 h. Finally, the solid was washed with ethanol and water to remove impurities. All the catalysts were dried at 120 °C in a drying oven for 24 h using air or N₂ atmosphere.

Neither chemical nor green synthesis methods investigated the fate of reducing agents after the formation of Cu and Pd nanoparticles. Marslin et al. discussed the mechanism of secondary metabolites from plant extracts forming metallic nanoparticles [41]. It is logical to assume that the reducing agents in both methods are oxidized. Additionally, some of the capping agents in the plant extract, which aid in nanoparticle stabilization, might be damaged during the washing and drying process at 120 °C.

2.5. Kinetic Determination of Cu and Pd Nanoparticles Formation

Since both Cu²⁺ and Pd²⁺ ions exhibit electronic transitions within their d-orbitals, these transitions involve the absorption of energy in the UV-visible region of the electromagnetic spectrum, making UV-visible spectroscopy a suitable technique for their analysis. This analysis technique was chosen due to the availability of the pre-calibrated Hach Model DR/2010 Spectrophotometer, which accommodates over 120 colorimetric measurements, including Cu²⁺ and Pd²⁺. While other analytical techniques like Inductively Coupled Plasma Mass Spectrometry (ICP-MS) offer potentially higher accuracy, their unavailability and the high cost of argon consumption present disadvantages compared to the UV-visible spectroscopy utilized in this work.

To determine the kinetics of Cu²⁺ disappearance, a HACH^® methodology was used, using a HACH^® DR/2010 UV-visible spectrophotometer (HACH, USA). Samples of 1 mL were diluted in 100 mL of deionized water and the Hach Method 8143 was used. This method consists of a colorimetric procedure for quantifying copper in samples. This method utilizes the Porphyrin technique, employing a reagent set comprising powder pillows containing copper masking reagent, Porphyrin 1 reagent, and Porphyrin 2 reagent. The applicable copper concentration range for this method is 2 to 210 μg/L. To follow the kinetics of Pd²⁺ nanoparticles, a UV-visible instrument (Genesys 10S spectrophotometer, Thermo Scientific^®) was used, with a wavelength of 209 nm and 1 mL samples. The isotherms for kinetic determination were obtained at three different temperatures (10, 15, and 20 °C).

2.6. Characterization Techniques

The Fourier transform infrared spectroscopy (FTIR) technique is used for the identification of functional groups in green catalysts. The Cu and Pd green samples were dried in a static air oven at 120 °C for one hour. To evaluate the possibility of unwanted chemical reactions between the deposited metallic nanoparticles, the organic groups on the catalytic surface, and atmospheric air, a second group of samples were dried under a nitrogen environment. A constant flow of 60 mL/min N₂ was used to ensure the desorption of organic groups without undesirable reactions, protecting the formed nanoparticles. Gaseous nitrogen flowed into the oven for 30 min before the drying process to purge the air and create a protective atmosphere. The FT-IR spectra of the samples were then compared to evaluate the effects of the different drying methods. Previous studies have reported the use of N₂ to protect metallic green nanoparticles [42,43].

This analysis was performed in the mid-infrared range of 4000 to 600 cm⁻¹ (Perkin Elmer, Spectrum 100 Optica). An attenuated total reflectance (ATR) accessory was operated with 50 scans and a resolution of 4 cm⁻¹. For the analysis of the microtextural properties of the catalysts, nitrogen physisorption analysis was performed (Micromeritics, TriStar II Plus); the catalysts were degassed at 200 °C for 24 h. Nitrogen adsorption isotherms were obtained at 77 K and the Barrett–Joyner–Halenda (BJH) and Brunauer–Emmet–Teller (BET) methods were used to calculate the average pore diameter, pore volume, and surface area of each sample. Scanning electron microscopy (SEM) was used to analyze the morphological structure of the catalysts, as well as the average particle size presented by the samples. The analysis was carried out in the FEI Nova Nano SEM200 scanning electron microscope, which was operated at an acceleration voltage of up to 20 kV. The SEM instrument was coupled with energy-dispersive X-ray analysis (EDX, GENESIS XM4) to elucidate the elemental composition of the samples. Analysis by high-resolution transmission electron microscopy (HRTEM) was also performed (FEI Titan G2 80–30). For surface-sensitive quantitative analysis of Cu and Pd species on alumina, the X-ray photoelectron spectroscopy (XPS) technique was used with a Κ–Alpha Thermo Scientific equipment. The conditions of the test to measure the samples were the same as in our previous work [5].

2.7. Catalytic Oxidation of HMFCA

To evaluate the activity of the Cu/Al₂O₃ and Pd/Al₂O₃ catalysts, the HMFCA oxidation reaction was performed in a batch reactor with a reaction volume of 100 mL, using 100 μL of H₂O₂ as an oxidizing agent. An initial concentration of 150 ppm HMFCA and 50 mg of catalyst were used, and the reaction was constantly stirred at 500 rpm for two hours at a controlled temperature. Samples (1 mL) were taken every 30 min, then centrifuged at 3200 rpm, and filtered at 45 µm. Later, 0.3 mL was taken for further analysis by HPLC, which was performed three times. A CAI 600 FTIR analyzer (California Analytical Instruments, Inc., USA), model 20, equipped with dual parallel cells, was employed to quantify CO₂ and CO production during HMFCA oxidation. Standard of CO₂ at 500 ppm and CO at 5000 ppm, both in N₂, along with ultrapure N₂, were utilized for calibration. A constant nitrogen flow of 60 mL/min was used as the carrier gas to transport the evolved gases from the liquid reaction system to the analyzer. To remove moisture before analysis, a silica gel trap was placed upstream of the analyzer cell. Only the Cu/Al₂O₃ and Pu/Al₂O₃ catalysts synthesized by the green method were evaluated because of HMFCA import limitations.

3. Results and Discussion

3.1. Identification of Phytochemical Agents in Oregano Extract

The phytochemical agents were identified on HPLC under different retention times (14.51 min for carvacrol and 14.36 min for thymol), and carvacrol showed an abundance nine times higher compared to thymol. Javanshir et al. studied the electrochemical behavior of carvacrol and found that its redox potential increased with increasing solvent polarity [44]. In this work, water is used as a solvent. Furthermore, from the thymol and carvacrol molecular structures (Figure 1), it is evident that the hydroxyl group in carvacrol is in the alpha position to the methyl group, which can be associated with a higher redox potential compared with thymol due to a lesser partially positive charge from the carbon in the methyl group compared with the C in the isopropyl group. This increases the stability of the negative charge of the oxygen atom (enhancing its reducing behavior) and, in turn, increases the acidity of carvacrol. This is confirmed by the lower value of pKa reported for carvacrol compared with thymol [45]. It would certainly be interesting to isolate and analyze the organic fraction after metal reduction. However, this work focused on the kinetic results of the reduction step for nanoparticle formation. Future studies can involve a more comprehensive extract analysis.

3.2. FT-IR of Catalysts Synthesized by the Green Method

The aim of FT-IR analysis of catalysts synthesized by the green method was to determine the extent of organic residue from the green synthesis method present on the synthesized catalysts. This characterization is crucial to ascertain if the metallic catalytic sites are effectively exposed and accessible for optimal catalytic activity. By detecting residual functional groups through FT-IR, the potential impact of organic remnants on the performance catalyst can be assessed. Figure 2 shows the FT-IR spectra of Cu/Al₂O₃ and Pd/Al₂O₃ catalysts synthesized using the green method. The Cu/Al₂O₃ catalyst was dried under two environments (air and N₂) at 120 °C, to evidence the best gas to clean undesirable organic compounds absorbed after washing. It was found that using air led to a significant decrease in the intensity of functional group signals, compared with the use of nitrogen. For both environments, the following signals were observed on Cu/Al₂O₃ catalysts: physisorbed water can be identified at 3694 cm⁻¹, which is very close to the range of O-H stretching bands observed in weakly hydrogen-bonded species [46]; the small band at 2079 cm⁻¹ can be related to CO adsorbed on the metallic nanoparticle [47], stretching of the C-H bond at 2970 and 2872 cm⁻¹, and its deformation at 1356 cm⁻¹, which is related to the organic matter, while the signal at 1050 cm⁻¹ can be related to CH₂-OH of an adsorbed primary alcohol, possibly from the ethanol used in the washing step. This means even after the cleaning steps (washing and drying), residual organic compounds were still detected in the oregano extract. Since it proved to be superior, only air drying was used for the Pd/Al₂O₃ catalyst. Signals related to adsorbed residual water were observed at 3500 and 1600 cm⁻¹; also, a band at 850 cm⁻¹ related to AlO₄ tetrahedra of alumina as support was found [48]. This band may have also appeared in the Cu/Al₂O₃ catalyst, but the intensity of the organic compound bands overshadows it. It can be discussed that the presence of copper on alumina is more favorable than Pd, bonding the remaining organic components from the oregano extract. One explanation can be the chelating capacity of copper toward organic extract components [49], and the stability of the complexes exposed to washing and drying.

3.3. Microstructural Characterization of Catalysts

Table 1. Textural properties of the catalysts.

Sample	Area (m2/g)	Pore Volume (cm3/g)	Average Pore Size (nm)
Al2O3	206	0.43	8.35
Cu/Al2O3 GS-A	184	0.34	7.42
Cu/Al2O3 GS-N	182	0.34	7.42
Cu/Al2O3 CS-A	190	0.39	6.18
Pd/Al2O3 GS-A	161	0.30	3.71
Pd/Al2O3 CS-A	210	0.41	5.82

GS = green synthesis, CS = chemical synthesis, A = dried with air, and N = dried with N_2.

summarizes the microstructural properties of the Cu/Al₂O₃ catalysts obtained with air and N₂ drying and synthesized by green and conventional methods, as well as Pd/Al₂O₃ catalysts synthesized by green and conventional methods with air drying. γ-Al₂O₃ is also presented for comparison. Notably, almost all metal-impregnated samples exhibit lower surface areas compared to pure alumina. This can be attributed to the presence of metals, which block some of the original pores in the support. Interestingly, the catalysts obtained with the green method exhibited lower surface areas and total pore volumes compared to the conventionally synthesized samples. Although the average pore size of the chemically synthesized Cu/Al₂O₃ catalyst was marginally smaller (1.24 nm) compared to Cu/Al₂O₃ GS-A, it has been reported that even a 1 nm pore size variation can have a profound influence on the catalytic behavior of the material, modifying the selectivity of the products [50]. On the other hand, concerning the results for Pd/Al₂O₃ catalysts, the conventionally synthesized catalyst showed an increased specific area and decreased total pore volume and average pore size, with the value of the latter being notoriously low. This can promote diffusion limitations for a molecule such as HMFA. Yashnik et al. reported that when using chloride salt precursors of Pd, the microstructural properties of the pure support were preserved [51]. Contrasting behavior between both methods of synthesis for Cu and the green method for the Pd upon deposition resulted in opposite effects on the specific area of the catalysts. Cu nanoparticles likely agglomerated, blocking mesopores [52], while Pd nanoparticles derived from green synthesis dispersed on alumina may partially occlude its micropores (< 2 nm), both contributing to surface area loss [50]. It is important to highlight that for both Pd/Al₂O₃ catalysts, sufficient hydrochloric acid (HCl) was used to achieve a pH of approximately 3. However, the green synthesis method, employing oregano extract, resulted in a 20 % decrease in the specific surface area of the alumina compared to the chemically synthesized catalyst (using ascorbic acid), which did not decrease its initial surface area. This discrepancy suggests that decomposition products or interacting components from the oregano extract might have decreased the textural properties of the Al₂O₃, provoking pore blocking by extract residues, even for the Pd nanoparticles lower than 4 nm shown in the next section.

3.4. Electronic Microscopy Analysis

Figure 3 shows the scanning electron microscopy (SEM) images for the Cu/Al₂O₃ catalyst synthesized by the green method, dried in air at 120 °C. In Figure 3a some nanoparticles with irregular morphology can be observed, which can be related to copper species over pieces of γ-alumina of greater size (around 7 μm). Figure 3b presents the nanoparticle size distribution; the range is from 50 to 180 nm with an average of 119.81 ± 24.47 nm, but 80 % of the nanoparticles show a smaller size between 60 and 140 nm. It is interesting how the reducing agents and other organic substances of the oregano extract promoted agglomeration of the copper species deposited on the γ-alumina as other chemical techniques such as using a flow of H₂ [52,53]. Chemical synthesis of Cu nanoparticles in aqueous media has been reported to predominantly result in the formation of CuO and CuO₂, which promote agglomeration [54]. The presence of such copper oxide species can be verified in the XPS results. It can be proposed that during the 24 h contact between the copper precursor, alumina, and oregano extract in the green method, the nanoparticles achieved the stabilization stage after nucleation (as will be shown in the next section) and autocatalytic stages of growth. Figure 3c shows the energy dispersive X-ray analysis (EDAX) on the same micrograph, observing a 1.94 %wt. copper content, lower than the 5 wt.% initial formulation. This suggests that a portion of the copper was not deposited during the green synthesis and likely remained in the solution or leached during subsequent washings. The kinetic results for Cu²⁺ (shown in the section) indicate that the concentration of this ionic species does not approach 0 ppm even at 100 min of analysis. It can also be suggested that the stabilization of nanoparticles might have prevented sufficient growth for precipitation on the surface [26]. This observation points toward a limited leaching effect associated with the green synthesis methodology. Additionally, a chloride content of 1wt.% was noticed, all of which may be related to the copper precursor salt. The high solubility of copper (II) chloride dihydrate on water (757 g/L) at room temperature suggests that some chloride ions likely reacted with specific organic components of the oregano extract, forming complexes that became firmly anchored on the γ-alumina surface during green synthesis and resisted subsequent washings. The observed O/Al atomic ratio of 1.8 in the Cu/Al₂O₃ for the green synthesis sample deviates from the expected 1.5 for pure Al₂O₃. This excess oxygen suggests the presence of AlO(OH) species on the alumina surface, as found in our previous work [48], or potentially other oxide species (including copper oxides) formed during the green synthesis process or during the 120 °C drying step in an air atmosphere, as presented in our previous work on alumina-supported Cu catalysts [52].

Figure 4 shows high-resolution transmission electron microscopy (HRTEM) analysis for the Pd/Al₂O₃ catalyst synthesized by the green method. It is important to note that the SEM technique was not enough to reveal the Pd nanoparticles on the support, while HRTEM is more convenient in order to obtain better resolution. As seen in Figure 4a, the Pd nanoparticles are well dispersed on the γ-alumina grain, which favors catalytic activity. Shaik et al. also obtained Pd nanoparticles as a catalyst (not supported) using Origanum vulgare L. extract, reporting an average nanoparticle size of 2.2 nm; however, 100 nm agglomerations (in the form of grains) were also observed [18]. Figure 4b displays the particle size distribution (PSD) obtained from the HRTEM micrograph. The data reveal an average particle size of 4.32 ± 1.30 nm and a near-symmetrical continuous curve around the mean, indicating a normal or Gaussian distribution. This suggests that the 30 min stirring time during green synthesis of Pd nanoparticles promoted good dispersion, resulting in a highly monodisperse PSD with most particles centered around the small size of 4.85 nm. This characteristically narrow size distribution could potentially contribute to enhanced catalytic activity.

3.5. Kinetics of Cu²⁺ and Pd²⁺ Reduction

Figure 5 shows the concentration of metal reduction as a function of time and their fitting to a first-order kinetic model, at 10, 15, and 20 °C. For the synthesis of copper catalysts with the conventional method (Figure 5a), all concentrations at three temperatures showed good fitting to first-order kinetics until around 20 min. Although it is known that both cation and electron-donating reducing agents participate in the reduction reaction, a first-order dependence of the kinetic model on the cation concentration implies an excess of the reducing agent. As observed from the figure, the concentration of copper ions at 20 °C remains constant for at least 25 min, after which deviates from a first-order kinetic model. This suggests the nucleation stage for the Cu nanoparticle growth may have already occurred, after the nucleation and autocatalytic stages [55]. In the case of the green synthesis of copper catalyst (Figure 6b), the Cu²⁺ concentrations at the three temperatures follow a first-order kinetic model, but with less pronounced slopes than the conventional synthesis. The slower process may have influenced the formation of large, agglomerated nanoparticles, which were observed in the SEM micrographs (Figure 3). Notably, this first-order behavior prevails for longer times (approximately 40 min), after which the experimental concentrations begin to increase and deviate from the predicted behavior of the kinetic model, particularly at 10 and 20 °C. This behavior may be related to the complex interplay of reducing and stabilizing agents within the green extracts. Quin et al. reported Ag nanoparticles synthesized using waste tea extract and proposed that the phenolic compounds initially promote reduction, while polysaccharides, caffeine, and tannic acids may subsequently influence particle aggregation [56]. The synthesis of Pd by the conventional method (Figure 5c) is notoriously faster than Cu at all temperatures; the Pd²⁺ within the aqueous phase was depleted in less than 15 min. Pd green synthesis results are not presented due to inconsistent Pd²⁺ measurements using UV-visible spectroscopy, with some values exceeding the initial concentration. This suggests potential interference with the complex organic compounds of the oregano extract. Future work will explore alternative analytical methods to obtain reliable Pd²⁺ data. A potential solution involves conducting the experiment at a temperature lower than 10 °C to promote slower kinetics or exploring the ICP-MS technique, despite its higher cost, as mentioned in the Methodology section.

Table 2. Kinetic parameters for cations reduction processes and coefficient of determination (r²) of the linear form of the Arrhenius equation.

Kinetic Data	Activation Energy (J/mol)	Pre-Exponential Constant (min−1)	r2
Cu2+ over Al2O3 CS	84,019	1.0397 × 1013	0.9996
Cu2+ over Al2O3 GS	71, 646	1.2192 × 1010	0.9130
Pd2+ over Al2O3 CS	83,493	8.9367 × 1014	0.9932

CS = conventional synthesis, GS = green synthesis, and r² = determination coefficient related to the Arrhenius equation.

shows the kinetic parameters obtained from the analysis of the first-order kinetic constants. The linearized form of the Arrhenius equation was fitted to the first-grade kinetic constants of reaction rate at different temperatures using the Microsoft 365 Excel^® functions SLOPE and INTERCEPT, which do not provide error accuracy. Additionally, the R-SQUARED (r²) function was used to determine the fitting’s goodness-of-fit. It is important to note that r² indicates the correlation between the data and the fitted line. The values of activation energy are similar for all three cases, with the green synthesis of Cu/Al₂O₃ presenting the lowest value but far from the criteria of the mass transfer regime (above 41,840 J/mol) [57]. This suggests that all mechanisms are controlled by chemical reactions, and diffusion limitations do not play a significant role in the green synthesis of Cu nanoparticles despite the presence of diverse organic species. For the pre-exponential constant, the value for chemical synthesis of Pd nanoparticles on γ-alumina is 85 times larger than conventional Cu synthesis and 7 × 10⁴ times larger than the Cu green synthesis values. Although the reaction mechanism can be similar due to the values of activation energies, the rapid reaction between Pd and ascorbic acid in the green synthesis might impede the autocatalytic and stabilization phases, which could consequently limit the achievable size of deposited nanoparticles on γ-alumina. Finally, high determination coefficients (r² > 0.9) indicate that the Arrhenius model accurately describes the temperature dependence of reaction rate constants. The green synthesis method, while requiring more water to extract secondary metabolites from oregano compared to the direct water solubility of ascorbic acid in the metal reduction step, offers a more economically advantageous route by utilizing extracted substances with inherent reduction and stability capabilities instead of pure ascorbic acid. Although the extraction process generated biodegradable solid oregano residues suitable for municipal waste treatment, these residues were correctly managed as hazardous waste within the laboratory before proper disposal via an authorized service.

3.6. XPS of Cu and Pd on Catalysts

Figure 6 presents the XPS spectra for Cu 2p. The chemical states of copper species on the Cu/Al₂O₃ catalyst obtained by green synthesis and Cu/Al₂O₃ catalyst by conventional method are shown in Figure 6a,b, respectively. A single peak around 932–933 eV was found, which was associated with metallic copper (Cu⁰), Cu 2p_3/2 [53,58]. Additional signals associated with Cu 2p_3/2 and Cu 2p_1/2 (related to copper cation, Cu²⁺) were also found, between 935 and 942 eV and 953, 957, 961 eV, respectively [59,60,61]. The XPS spectra concerning Pd3d for Pd/Al₂O₃ catalyst by green synthesis and the Pd/Al₂O₃ for conventional method are presented in Figure 6c,d, respectively, where four signals are observed. Two signals correspond to the spin–orbit of Pd 3d_5/2 and 3d_3/2 related to elemental palladium (Pd⁰), occurring between 334 and 340 eV, respectively [62,63]. Additional satellite signals were also found between 336 and 342 eV, which can be attributed to Pd 3d_5/2 and 3d_3/2 of palladium oxide (Pd²⁺) [50,64].

The results confirm the presence of Cu⁰ and Pd⁰ in both alumina monometallic catalysts. Actually, for Cu/Al₂O₃, the elemental copper content (

Table 3. XPS data of catalysts Cu-Al₂O₃ and Pd-Al₂O₃ catalysts. For Cu catalysts, the green synthesis method yielded results comparable to the chemical method. However, for Pd catalysts, the green synthesis method resulted in a lower degree of metal reduction compared to the chemical method.

	Cu/Al2O3-GS		Cu/Al2O3-CS			Pd/Al2O3-GS		Pd/Al2O3-CS
Phase	B.E. (eV)	% Area	B.E. (eV)	% Area	Phase	B.E. (eV)	% Area	B.E. (eV)	% Area
Cu0	933.15	29.02	932.54	31.75	Pd0	334.85	22.3	334.76	38.3
Cu+2	935.48	15.01	935.13	11.01	Pd0	340.07	8.1	340.12	38.5
Cu+2	942.00	29.25	942.37	25.04	Pd+2	336.40	33.7	336.67	15.0
Cu+2	953.61	21.38	953.04	27.05	Pd+2	341.13	35.9	342.62	8.1
Cu+2	961.92	5.34	957.18	5.15

Where B.E. = Binding energy, GS = green synthesis, and CS = conventional synthesis.

) is very similar for both types of synthesized catalysts, which means the reduction of copper salt (Cu²⁺) has similar results both using the synthetic reduction agent NaBH₄ or the oregano extract. On the other hand, although the use of oregano extract for synthesis of Pd/Al₂O₃ catalysts presents activity as a reducing agent (Pd²⁺ to Pd⁰), the elemental palladium content on Pd/Al₂O₃ synthesized by the green method is 60% (calculated from Pd⁰ areas in Table 3) below the results obtained when using ascorbic acid as a reducing agent for the Pd²⁺ reduction on the surface of the alumina support. Both synthesis methods resulted in an incomplete reduction of the Cu and Pd precursors. As observed for the Cu catalysts, the activation energy values appear similar for both methods, suggesting that the Cu²⁺ depletion mechanisms in the solution might not be significantly different. However, if the green synthesis method led to the reoxidation of metal nanoparticles, this could have altered the reaction mechanism, potentially impacting the kinetic data.

In anticipation of the catalytic behavior of the mixed metal phases, it is important to consider the potential role of the different metal species. Pd–PdO interface can be a very selective active site for the dehydrogenation of organic molecules at room temperature [65]. The presence of different electronic states of the metal species can significantly impact its activity. For example, Sun et al. reported that Cu⁺ exhibited the highest activity for H₂O₂ activation and hydroxyl radical generation, followed by Cu²⁺ and Cu⁰ [66]. In our case, there is a presence of Cu⁰ and Cu²⁺. On the other hand, metal oxides can promote the adsorption of furan-type organic molecules [67].

3.7. Catalytic Oxidation of HMFCA

Figure 7 presents the HMFCA oxidation conversion with H₂O₂ using green and conventionally synthesized Cu and Pd catalysts at 25 °C. Equation (1) shows how the conversion was calculated.

$$\% \mathrm{C}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}=100\times \left({\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}\mathrm{M}\mathrm{F}\mathrm{C}\mathrm{A}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}-\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}\mathrm{M}\mathrm{F}\mathrm{C}\mathrm{A}}_{\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e} \mathrm{t}}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}\mathrm{M}\mathrm{F}\mathrm{A}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}}\right)$$

(1)

Both Cu catalysts exhibit exceptionally fast and nearly complete conversion of HMFCA. In less than one minute of reaction, Cu/Al₂O₃ (conventional synthesis) reached a 44.24% HMFCA conversion, while Cu/Al₂O₃ obtained by green synthesis reached a 60% conversion. At eight minutes, both catalysts reached 99.60% conversion, and after 114 min, both reached 100% conversion. It is important to remark that both catalysts presented almost the same composition of Cu⁰ and Cu²⁺ species on the surface (Table 3). Also, both Pd catalysts initially show rapid conversion, reaching 51.98% and 30.92% for conventionally synthesized and green catalysts, respectively, within the first minute. After 40 min, both catalysts reached a maximum between 94 and 97% conversion before declining, with a more pronounced slope for the conventionally synthesized Pd catalyst. This maximum conversion could be attributed to product inhibition, where HMFCA molecules adsorbed on the active sites block access to fresh reactants [68]. A possible Langmuir–Hinshelwood (LH) mechanism of fresh reactant adsorption (in this case, HMFCA) over a layer of the same reactant on the active sites of the catalyst [69] could explain this behavior. Sitthisa et al. reported that furfural exhibits two distinct adsorption modes on Cu/SiO₂ and Pd-Cu/SiO₂ catalysts: planar binding through the π-electrons of the furan ring, and vertical coordination with the C-O-C group oxygen atom. These binding modes likely influence the subsequent catalytic pathway [27]. HMFCA is a furanic molecule that features a hydroxyethyl group at position 5 and a carboxylic acid at position 2, providing diversity in the adsorption on the metallic catalytic sites. Leveraging our previous work, a mechanism of 2-methylfuran adsorption over the Pd-Fe catalyst surface was proposed to occur at the flat position of the structure of the furan molecule, exposing the C=C groups of the furan and facilitating the reaction with a second adsorbed reactant, such as H₂ [70]. This work suggests that the HMFCA could be adsorbed in the flat position, reacting rapidly with H₂O₂.

Figure 8 shows the selectivity of the analyzed substances considered as byproducts. Equation (2) shows how selectivity was calculated, where X is one of the substances explained in the next paragraph.

$$\% \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=100\times \left({\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{X}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{e}\mathrm{d} \mathrm{a}\mathrm{t} \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e} \mathrm{t}}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}\mathrm{M}\mathrm{F}\mathrm{C}\mathrm{A}}_{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}-\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{s} \mathrm{o}\mathrm{f} {\mathrm{H}\mathrm{M}\mathrm{F}\mathrm{C}\mathrm{A}}_{\mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e} \mathrm{t}}}}\right)$$

(2)

The molecules chosen for this study were based on our previous experiences with HMF oxidation, which yielded HMFCA, 2,5-diformylfuran (DFF), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furandicarboxylic acid (FDCA) [71,72]. Interestingly, only DFF and FFCA were detected as byproducts for all four catalysts in this work. Recent studies on HMF oxidation pathways propose that FFCA directly arises from HMFCA oxidation, while HMF oxidation proceeds in parallel to yield DFF and HMFCA [72,73,74]. Therefore, DFF is not a product of HMFCA oxidation. Our findings would contradict this note and support a parallel production of DFF and FFCA from HMFCA oxidation with H₂O₂. In our last work [72], a catalytic molecular pumping analysis was performed using the Brønsted–Evans–Polanyi (BEP) relationship [75], finding that the photocatalytic HMF oxidation yields HMFCA as intermediate and later FFCA. Furthermore, FFCA formation was the rate-limiting step. However, our current findings present a potential discrepancy with our previous work [19]. For Cu and Pd catalysts by green and chemical methods, the DFF/FFCA selectivity values were from 10, 25, 213, and 87, respectively, significantly higher than expected if FFCA formation were the rate-limiting step. The transformation of HMFCA to DFF necessitates the removal of both the OH from the carboxylic acid group and a hydrogen atom from the hydroxymethyl moiety of the HMFCA molecule. Instead of forming the FFCA molecule, a lower grade of oxidation is necessary; only one H from the carboxylic acid group of HMFCA is removed. As discussed in Section 3.6, the Pd–PdO interface has been reported as a selective active site for room-temperature dehydrogenation reactions [65]. Furthermore, it has been suggested that this interface promotes synergistic C–C bond activation [76], which may contribute to the observed higher selectivity toward DFF formation compared to FFCA. For all experiments, the variation was was less than 5%, and this error margin was not represented in the results in Figure 7 and Figure 8.

The evolution of CO with time from the Cu/Al₂O₃ catalyst synthesized via the green method (Figure 9a) exhibits at least five different peaks, potentially corresponding to a sequential reaction pathway [77], reaching the highest concentration of 35 ppm in the peak at three minutes. The end time of the fifth CO peak corresponds with the eight minutes of the fast rise in the conversion (Figure 7). Additionally, some CO₂ is produced in some ascending steps and later descends, but with a concentration of only 3 ppm at its highest. The presence of CO and some CO₂ can be attributed to the decarboxylation of HMFCA or decarbonization due to furan ring rupture. Additionally, the very low production of DFF and FFCA species at eight minutes (Figure 8) supports the idea that Cu/Al₂O₃ synthesized by the green method promotes the degradation of the HMFCA organic molecule. This suggests a planar adsorption mode likely involving the C=C groups of the furan ring [27].

Figure 9b presents the outlet CO and CO₂ concentrations for the Pd/Al₂O₃ catalyst synthesized by the green method. During the initial four minutes, CO production is observed at around 1 ppm, while at longer times the concentration is close to zero. CO₂ production starts at minute three, reaching 1 ppm. Notably, a pronounced increase in CO₂ follows, reaching a maximum of 12 ppm at minute six. Subsequently, a decrease with a less pronounced negative slope is observed, interrupted by a plateau at 6 ppm from minutes nine to fourteen. Finally, CO₂ production increases moderately, reaching only 7 ppm before the test concludes at minute fifteen. These observations suggest a possible shift in the CO₂ production mechanism. The first pronounced peak at minute three may represent one mechanism, while the plateau observed from minutes nine to fourteen could indicate another. It is noteworthy that the maximum CO₂ concentration of 12 ppm is nearly three times lower compared to the maximum CO concentration observed for the Cu/Al₂O₃ catalyst (Figure 9a). This difference, along with the potential change in the CO₂ production mechanism, may be connected to the inhibition effect seen in HMFCA conversion over the Pd/Al₂O₃ catalyst (Figure 7). The preferential formation of CO₂ over CO suggests a more extensive oxidation pathway for the carbon atoms within the HMFCA molecule. This oxidation can target various functional groups, including carboxylic acid, the hydroxyethyl chain, or the furan ring. Furthermore, the choice of metal catalyst supported on alumina significantly impacts the oxidation process. From the experimental results, it is evident that Pd exhibits a stronger oxidation tendency compared to Cu. However, the inhibition phenomenon observed during HMFCA conversion on the Pd/Al₂O₃ catalyst (Figure 7) might suppress the reactivity of H₂O₂ on Pd, thereby moderating the production of CO₂, but there may be a risk of catalyst deactivation [78]. It can be said that the statistical validation of the CO and CO₂ measurement was supported by adhering to the calibration procedures outlined in the manufacturer instructions, which guarantees a ± 1 ppm measurement accuracy. Conversely, evaluating the reusability of the green synthesized catalyst is desirable and can be addressed in future studies.

4. Conclusions

Cu/Al₂O₃ and Pd/Al₂O₃ catalysts were successfully synthesized using Mexican oregano extract (Lippia graveolens) as green synthesis methodology and compared with the same catalysts using conventional reducing agents, demonstrating the plant’s untapped potential for technological applications. Additionally, the lower processing temperatures contribute to the enhanced sustainability of this alternative methodology. Carvacrol and thymol were identified in the oregano extract, which can be considered to have significant redox potential to reduce the Cu and Pd cations. While both methods yielded catalysts with the desired metal species deposited on γ-alumina, Cu nanoparticles by green synthesis exhibited significant agglomeration compared to their Pd counterparts, which maintained a well-defined nanometric size of 4.85 nm. The reduction kinetics of Cu²⁺ and Pd²⁺ to nanoparticles for both green and chemical synthesis methods were elucidated, demonstrating a first-order reaction model in most cases. Notably, the activation energy analysis revealed the dominant role of the chemical reaction mechanism compared to mass transfer effects, even in green synthesis. To assess the feasibility of adapting this green synthesis method for industrial-scale applications, further investigation into the autocatalytic and stabilization stages of nanoparticle growth is crucial. Key challenges include mitigating the leaching effect of the metals and minimizing residual organic material on the supports. Subsequently, all catalysts exhibited high conversions of HMFCA’s oxidation. The Cu/Al₂O₃ catalyst obtained by the green method presented high production of CO in five steps, with negligible formation of furanic compounds. Both Pd/Al₂O₃ catalysts favored the formation of DFF over FFCA, and the green catalyst formed some CO₂ in two different modes. An inhibitory phenomenon might be directing the reaction pathway on Pd/Al₂O₃ catalysts toward DFF formation.