Palladium Nanoparticles Immobilized on the Amine-Functionalized Lumen of Halloysite for Catalytic Hydrogenation Reactions

Abstract

Supported Pd-based catalysts have been widely applied in the hydrogenation of specific functional groups. Recent trends have focused on employing Pd-based heterogeneous catalysts supported on inorganic nanotubes, wherein inner surface functionalization modulates both palladium nanoparticle (Pd-NP) dispersion and the interaction between reactants and the catalyst surface, thereby influencing catalytic properties. This study aims to develop a catalytic system using amine-lumened halloysite nanotubes immobilizing Pd-NPs (Pd/HNTA) as catalysts for hydrogenation reactions. The formation of Pd-NPs within the organo-functionalized lumen—modified by 3-aminopropyltrimethoxysilane—is confirmed by transmission electron microscopy (TEM) imaging, which reveals a particle size of 2.2 ± 0.4 nm. For comparison, Pd-NPs supported on pristine halloysite (Pd/HNTP) were used as control catalysts, displaying a metal particle size of 2.8 ± 0.8 nm and thereby demonstrating the effect of organic functionalization on the halloysite nanotubes. Both catalysts were employed in the hydrogenation of furfural (FUR) and nitrobenzene (NB) as model reactions. Pd/HNTA demonstrated superior catalytic performance for both substrates, with TOF values of 880 h⁻¹ for FUR and 946 h⁻¹ for NB, and selectivities exceeding 98% for tetrahydrofurfuryl alcohol (THFOH) and aniline (AN), respectively. However, recyclability studies displayed that Pd/HNTA was deactivated at the 10 catalytic cycles during the hydrogenation of FUR, whereas, in the hydrogenation of NB, 5 catalytic cycles were achieved with maximum conversion and selectivity at 360 min. These results revealed that the liquid-phase environment plays a pivotal role in catalyst stability. In the hydrogenation of NB, the coproduction of H₂O adversely affects the interaction between the Pd particles and the inner amine-modified surface, increasing the deactivation of the catalyst with reuse. Thus, the Pd/HNTA catalyst holds significant promise for the development of noble-metal-based catalysts and their application in the transformation of other reducible organic functional groups via hydrogenation reaction.

1. Introduction

Heterogeneous noble-metal-based catalysts are formidable tools for greener chemistry, allowing for low-waste, energy-efficient, and selective reactions. They are currently considered the most active materials for a wide range of reactions, especially hydrogenation reactions [1,2]. Among these catalysts, Pd-based systems are highly efficient because of (i) the low activation energy for H₂ dissociative chemisorption on their surface and (ii) the high diffusion of hydrogen atoms into the bulk, leading to the formation of Pd hydride phases [3]. Despite these advantages, Pd nanoparticles (Pd-NPs) supported in carriers as heterogeneous catalysts present several challenges. The high cost and limited availability of this precious metal have prompted extensive research into alternative methods to optimize its use, with a strong emphasis on enhancing the performance and longevity of these catalysts while ensuring scalability and sustainability [4,5,6]. The performance of Pd is significantly affected by the carrier material. Thus, inorganic-based supports are the most used systems because they can enhance specific properties such as mechanical strength, distribution, stability, and catalytic reactivity [7,8]. Despite these advantages, the integration of Pd with inorganic supports to produce heterogeneous catalysts remains challenging, as issues related to nanoparticle agglomeration and the control of particle size and distribution can affect catalytic efficiency [7,9].

Over the past decade, nanotubes (NTs) have revolutionized almost all fields of science and technology. Due to their nanoscale size and morphology, NTs possess exceptional physicochemical and morphological properties related to their one-dimensional (1D) dimensions, hollow cylindrical form, porosity, and composition [10,11]. NTs are used as building blocks in numerous applications such as sensors, nanotransistors, energy storage, ceramic production, metallurgy, electronics, optical devices, catalysts, and biomedicine [12,13,14]. Among these, the use of inorganic NTs (INTs) varies depending on the desired application [15,16,17]. Carbon NTs (CNTs) have been predominantly studied in microelectronic technology, whereas INTs (especially metal sulfides or oxides) have been mostly fabricated for biomedical, photochemical, electrical, and environmental applications [10]. Among the various oxide INTs, special attention has been directed towards halloysite NTs (HNT, Al₂Si₂O₅(OH)₄ · nH₂O) because of their enhanced properties, natural abundance, and biocompatibility [10]. HNTs typically have an inner diameter of 10–20 nm, an external diameter of 40–60 nm, and a tube length of 0.5–1.5 µm [16,18]. They contain a gibbsite-like array of aluminol (Al–OH) groups within their lumen and siloxane (Si–O–Si) groups on their outer surfaces, both of which contribute to their reactivity and capacity for surface modification [18]. The development of HNT nanocomposites based on Pd-NPs attached to HNT holds great potential for catalytic applications. For instance, Melnikov et al. demonstrated the deposition of Pd-NPs onto HNT as catalysts for the efficient acetylene semi-hydrogenation to produce ethylene [19]. Asadi et al. reported that the hydrogenation of polyalphaolefins using a catalyst containing 1,3-diaminopropane immobilized on the outer surface of HNT exhibited high hydrogenation activity and elevated recyclability even after five consecutive catalytic cycles [20]. In addition, Zhang et al. prepared Pd/HNT using urea-modified supports as heterogeneous catalysts to explore an efficient non-mercuric catalyst for acetylene hydrochlorination, and their results demonstrated that the urea additive enhanced the catalytic performance of the Pd/HNT catalysts during the reaction [21]. Despite these reports, the organolumen-oriented preparation of HNT-supported Pd-NPs offers two major advantages: (i) grafting organic moieties provides an accumulation of the palladium precursors near the inner surface of the HNT, thus allowing for the control of the hydrogenation environment, and (ii) the presence of suitably selected modifiers can improve catalyst efficiency by promoting the accumulation of reagents on the modified HNT lumen. Wang et al. studied the liquid-phase oxidation of benzyl alcohol using hydrogen peroxide with the deposition of Pd-NPs supported in the lumens of HNT assisted by the ionic liquid (1-(2′-hydroxylethyl)-2,3-dimethylimidazolium chloride) as directing agents; this system displayed outstanding catalytic benzyl alcohol oxidation performance compared to other catalysts supported on lamellar clay (montmorillonite and kaolinite) [22]. Zheng et al. designed site-oriented loading-endowed catalysts with uniformly dispersed Pd-NPs by modifying HNT with cetyltrimethylammonium bromide to promote the inner deposition of metal precursors for methane combustion, which exhibited a remarkably higher activity compared to unmodified catalysts [23]. Based on this background, the use of HNT lumen-oriented loading of Pd-NPs appears to be a valuable strategy for improving the catalytic performance of this metal in hydrogenation reactions.

In this study, we propose the use of Pd-NPs dispersed on an HNT lumen modified with 3-aminopropyltrimethoxysilane (APTMS) for catalytic hydrogenation reactions. The valorization of furfural (FUR) by the hydrogenation process produces different chemical substances, such as furfuryl alcohol (FOL), which is used in the production of resins and as an intermediate in the synthesis of pharmaceuticals (e.g., vitamin C and lysine) [24,25]; tetrahydrofurfuryl alcohol (THFOH), which serves as a green solvent and an intermediate for the preparation of C₄ or C₅ diols [26]; 2-methylfuran (2MF), which is considered a potential renewable fuel [27,28]; and 2-methyltetrahydrofuran (2MTHF), which is utilized in the synthesis of organometallic compounds and as a fuel additive [29,30,31]. Furthermore, nitroarene hydrogenation is an important reaction in organic chemistry, primarily used for producing anilines, which are vital intermediates in the synthesis of pharmaceuticals, agrochemicals, and dyes [32,33,34]. The hydrogenation of FUR and NB were used as a model reaction to evaluate the efficiency and recyclability of the proposed catalyst. To synthesize the catalytic system, HNT was modified with trimethylsilane moieties and subjected to pyrolytic treatment to block the outer surface, whereas the inner surface was modified with APTMS to promote metal deposition. Two Pd-supported systems were prepared using 0.5 wt.% metal loading, one employing the amine-lumened HNT and the other using pristine HNT as a control system. All systems exhibited high catalytic performance in both hydrogenation reactions.

2. Results and Discussion

2.1. Characterization of Supports and Catalysts

Table 1. Physicochemical properties of the synthesized supports and catalysts.

Material	Elemental Analysis			ICP-OES (Pd%)	SBET (m2 g−1)	dpore (nm)	Pore Volume (cm3 g−1)
	%N	%C	%H
HNTP	0.00	0.02	1.74	--	50	14.2	0.16
HNTA	0.13 (0.16)	0.86	1.55	--	46	13.5	0.15
Pd/HNTA	0.14 (0.15)	0.86	1.58	0.46 (0.50)	35	11.5	0.14
Pd/HNTP	0.00	0.04	1.78	0.41 (0.50)	42	12.8	0.15

The nominal content is in brackets.

summarizes the elemental analysis results for the supports. The total N content anchored on the surface was similar to the nominal content (0.14%), confirming that APTMS was successfully attached to the HNT support. In addition, after catalyst preparation, the N and C contents remained constant, indicating that the thermal reduction treatment did not volatilize organic components from the HNTA inner surface.

The characterization of the HNTP support was performed by N₂ adsorption isotherms at −196 °C, which showed a typical type IV profile according to the IUPAC classification. This behavior is characteristic of a mesoporous material with a hysteresis loop indicative of cylindrical pores [35], as displayed in Figure 1. In addition, after surface modification, a discrete decrease in both the maximum adsorption and the hysteresis loop was observed for HNTA, attributed to the combined modification effects of silylation–calcination processes on the outer surface and the anchoring of APTMS on the inner surface.

The Pd loading in both catalysts was estimated from ICP-MS analysis, revealing metal content of 0.44 wt% and 0.46 wt% for Pd/HNTA and Pd/HNTP, respectively (Table 1). This amount represents approximately 90% of the nominal Au content, which can be attributed to the leaching of metal nanoparticles during synthesis.

The specific area calculated from the BET method for HNTA was 50 m² g⁻¹, with an average pore diameter of 13.5 nm determined by the BJH method, and a total pore volume of 0.16 cm³ g⁻¹. The textural properties of Pd/HNTA were similar to those of the support, displaying a decrease in S_BET of 35 m² g⁻¹ and an average pore diameter of 11.5 nm, which is attributed to the incorporation of Pd-NPs on the inner surface of the support. A similar trend for Pd/HNT catalysts was detected (Figure S1 in Supplementary Materials).

X-ray diffraction (XRD) studies provided crucial insights into the crystalline nature of the materials, as shown in Figure 2. Distinct diffraction peaks observed at specific 2θ angles (12.12°, 20.08°, 24.59°, and 35°) confirm the monoclinic crystalline structure of halloysite (JCPDS No. 29-1487) [36]. A comparison of the XRD patterns of the HNT precursor and the HNTA support confirms that the crystalline structure of the HNTs was maintained following APTMS immobilization on the inner surface. Additionally, diffraction peaks for metallic Pd were not detected, likely due to the formation of Pd-NPs with a mean crystal size lower than the detection limit (<5.0 nm).

The morphology of the HNTA nanomaterial was confirmed by TEM, which showed well-defined tubular structures with an average length of 130 ± 3 nm and an external diameter of 47 ± 1 nm, demonstrating that organo-functionalization did not change the original morphology of the nanomaterial (Figure S2 in Supplementary Materials). The TEM micrograph of the Pd/HNTP catalyst (Figure 3A) shows a spherical Pd-NP distribution within the tubular structure, that is, on both the inner and outer surfaces of the supports with a mean particle size of 2.8 ± 0.8 nm. However, the Pd/HNTA catalysts contained Pd-NPs dispersed mainly on the inner surface of the HNTA support with a mean particle size of 2.2 ± 0.4 nm. (Figure 3B). The average particle sizes were then determined, and a dispersion (D_Pd) of approximately 41% was obtained for Pd/HNTP and 32% for the Pd/HNTA catalytic systems. The inner surface functionalization of the support via the introduction of amine functional groups improves metal dispersion, thereby tuning the catalytic properties.

On the other hand, the TPD-H₂ profiles were obtained and analyzed to reveal the H₂ chemisorption behavior on the Pd/HNTP and Pd/HNTA catalysts, as displayed in Figure 4. The corresponding TPD-H₂ showed wide desorption peaks for both catalysts at temperatures ranging from 25 to 200 °C. For the Pd/HNTP system, the signal was deconvoluted in six contributions with the maximum rate of desorption at a temperature of about 79 °C. The presence of these peaks in the thermogram indicates that several adsorption states exist, which is the result of the occurrence of different adsorption centers with specific bond strengths of hydrogen (Figure 4A). The low temperature (<100 °C) of the maximum rate of desorption indicates that moderate bound hydrogen exists on the surface of the Pd/HNTP catalyst, while the Pd-NP size distribution is a consequence of the different deconvoluted contributions in the DTP-H₂ profile [37,38]. The addition of the amine-functional group on the HNT lumen changes hydrogen desorption for the Pd/HNTA catalyst (Figure 4B). This system displayed a displacement in the size of desorption peaks, deconvoluted signals, and temperatures, which means that the amount of sorbed hydrogen on the Pd/HNTA system increases in comparison with the Pd/HNTP catalyst. The peak of the maximum rate of desorption at the temperature of about 79 °C for the Pd/HNTP catalysts displaced to 84 °C in the case of Pd/HNTA, whereas the deconvoluted contributions in the TPD-H₂ profile decreased from six to four, respectively. This trend is attributed to the effect of organic functionalization, which generates a homogeneous metallic dispersion and narrow Pd-NP size distribution in the Pd/HNTA catalyst. In addition, the interaction of H₂ on the surface of Pd-NPs can be the reason for the different activity of the Pd/HNTP and Pd/HNTA catalyst in the hydrogenation reactions (vida infra).

XPS was performed to investigate the surface compositions and valence states of the prepared catalysts (Figure 4). Figure 5A displays the survey spectrum for the catalysts that exhibited peaks corresponding to the Al, Si, O, and Pd peaks for Pd/HNTP, whereas the coexistence of the N signal in the Pd/HNTA catalysts further confirmed the successful grafting of APTMS onto the surface of the HNTA. Figure 5B shows the existence of the N 1s peak at binding energy (BE) 398.8 eV, reflecting the presence of amino groups in the support matrix as a result of the covalent lumen functionalization of HNTA by APTMS [39,40]. The Pd/HNTP catalyst displayed main peaks that correspond to Pd 3d_5/2 and Pd 3d_3/2 of Pd⁰, respectively, as shown in Figure 5C and

Table 2. Pd XPS analysis of catalysts.

Sample	Pd5/2				Pd3/2				Atomic Ratio Pd2+/Pd0
	Pd2+		Pd0		Pd2+		Pd0
	BE (eV)	%	BE (eV)	%	BE (eV)	%	BE (eV)	%
Pd/HNTP	336.7	48.9	335.0	12.7	342.1	31.8	340.8	6.6	3.9
Pd/HNTA	336.2	22.6	334.6	26.4	341.3	24.9	339.4	26.1	0.9

[41]. The presence of Pd²⁺ species is attributed to the incomplete reduction in the PdL₄ or PdL_4−x(NH₂-HNT)_x precursor for Pd/HNTP or Pd/HNTA, respectively. Furthermore, Pd/HNTA catalysts displayed a decreased contribution of Pd²⁺ species in comparison with the Pd/HNTP system, as shown in Figure 5D and Table 2. The BE of Pd 3d in Pd/HNTA shifted slightly to lower values than those in Pd/HNTP; the decrease in Pd²⁺ species and the shift in the BE (ΔBE = 0.6 eV for Pd 3d_5/2) is attributed to the interaction between N atoms and Pd-NPs [39,42]. The modification of the inner surface with APTMS provides –NH₂ groups that facilitate the ligand exchange of the metal precursor PdL₄ (L: Cl or CH₃OH) to produce PdL_4−x(NH₂-HNTA)_x species, which are partially reduced under H₂ treatment at 200 °C for 1 h at 30 mL min⁻¹, producing a mixture of Pd²⁺ to Pd⁰ species.

2.2. Catalytic Activity

2.2.1. Hydrogenation of Furfural

The performance of the catalysts for FUR hydrogenation was evaluated. Control experiments conducted without the catalyst and with HNTP and HNTA supports demonstrated that the kinetic barrier for the H₂ hydrogenation of FUR requires the Pd metallic center (Figure S3 in the Supplementary Materials). Figure 6A shows the FUR conversion achieved with the Pd/HNTP and Pd/HNTA catalysts. The catalytic data were fitted to the pseudo-first-order kinetic model, revealing an induction time of 10 min. This behavior can be attributed to the incomplete presence of partially oxidized Pd²⁺ species in both catalysts. Under the reaction conditions (120 °C and 30 bar of H₂ pressure), the catalysts chemosorbed H₂, thereby enabling the catalytic action of Pd on the surface of the catalyst. The Pd/HNTP catalyst converted 94.3% of FUR after 300 min of the reaction, whereas the Pd/HNTA catalyst reached 99.0% conversion at the same time. However, a different value for the kinetic pseudo-constant indicates that the order of k_app is Pd/HNTA (0.0149 min⁻¹) > Pd/HNTP (0.0109 min⁻¹). Furthermore, TOF values were 880 h⁻¹ for Pd/HNTA and 192 h⁻¹ for Pd/HNTP, respectively. This indicates that the catalytic efficiency for FUR hydrogenation is higher for the Pd/HNTA catalysts. These results are consistent with the physicochemical properties of the catalysts. In the case of Pd/HNTA, the Pd-NPs were deposited in the amine-functionalized lumen of the HNT support, which provided the uniform deposition of the active phase, as observed by TEM measurements, and the nanotubular environment promoted a confinement effect during the hydrogenation process. However, Pd/HNTPs displayed random deposition of Pd-NPs on the support surface with larger distribution size compared to the Pd/HNTA system. In addition, selectivity showed that only a small quantity of FOL was detected as a product of FUR hydrogenation; this intermediate was consumed to produce tetrahydrofurfuryl alcohol (THFOH), as shown in Supplementary Figure S4. As shown in Scheme 1, the hydrogenation of FUR yields multiple products, mainly via C=O hydrogenation to form FOL on Pd-based catalysts [43]. Subsequently, the accumulation of FOL produces secondary C–O hydrogenolysis to 2MF or furan ring hydrogenation to produce THFOH. In our case, the main product was THFOH (S_THFOH > 98%), and neither 2MF nor ring-opening products (e.g., butanols and pentanols) were detected with either catalyst. These results are consistent with data reported by Mironenko et al. for the hydrogenation of FUR using a Pd-based catalyst [44]. The hydrogenation capacity of Pd-based catalysts depends on the size distribution of the metallic NPs, which demonstrates that Pd/HNTA mainly facilitates the hydrogenation process while avoiding hydrodeoxygenation or ring-opening reaction pathways.

Due to its superior catalytic performance, Pd/HNTA was selected for further specific kinetic studies. Temperature-dependent studies were performed to determine the apparent activation energy (E_aa) of FUR hydrogenation using the Arrhenius equation. For this purpose, four temperatures between 100 °C and 160 °C were used. Figure 6B shows a direct temperature dependence, with k_{app,160 °C} > k_{app,140 °C} > k_{app,120 °C} > k_{app,100 °C}. Based on the calculated rate constants, E_aa was determined to be 43.0 kJ mol⁻¹. In addition, the product distribution displayed the same tendency at all evaluated temperatures (Figure S5 in the Supplementary Materials). The increase in temperature from 120 to 160 °C does not provide sufficient energy to initiate the hydrodeoxygenation of the C–OH bond in THFOH. Shanmugaraj et al. reported that the use of Pd-NPs supported on TiO₂ nanosheets synthesized via a similar amine–silane-assisted method as a catalyst for FUR hydrogenation to THFOH under 20 bar of H₂ pressure and 1.0 wt% of Pd metal loading, finding E_aa = 55.6 kJ mol⁻¹ [45]. In our study, the Pd/HNTA showed superior or comparable activity by achieving higher conversion (100%) and selectivity (>99%) for the hydrogenation of FUR to THFOH under moderate reaction conditions such as temperature (120 °C), H₂ pressure (30 bar), and low Pd content (0.5 wt%).

Figure 6C shows the influence of the H₂ pressure on the catalytic hydrogenation of FUR to THFOH. An increase in H₂ gas pressure from 10 bar to 40 bar significantly increased the hydrogenation rate of furfural to THFOH and decreased the induction time. FUR was completely hydrogenated and selectively converted into THFOH at all evaluated pressures, even at 40 bar of H₂ pressure, and there was no change in the product distribution during hydrogenation, indicating that the H₂ pressure influences the reaction rate but not the product distribution. This result is consistent with the reports by Byun et al. on the hydrogenation of FUR using different Pd-based catalysts [46]. They proposed that an increase in H₂ pressure increases its dissolution in the liquid phase, thereby activating and stabilizing the Pd catalyst surface; this is attributed to the increased formation of THFOH and the reduction in the induction time.

The catalytic recyclability of both the Pd/HNTP and Pd/HNTA catalysts was investigated using FUR hydrogenation. The procedure consisted of consecutive cycles under the same reaction conditions. After each batch reaction, the catalysts were recovered by filtration, and their catalytic performance, chemical composition, and physicochemical properties were evaluated. FUR conversion and THFOH selectivity as functions of the number of experimental runs are shown in Figure 7A. The Pd/HNTA catalyst showed moderate stability and remained unchanged over 10 cycles, with catalytic activity exceeding 84% FUR conversion and selectivity over 99% for THFOH after 360 min. However, the continuous decrease in the k_app during the catalytic cycle is 38% less than the k_app in the first catalytic cycle. In the case of the Pd/HNTP catalyst, there was significantly lower stability at 360 min of reaction in comparison with Pd/HNTA catalysts, reaching six continuous cycles with a maximum of 50% FUR conversion and 100% selectivity for THFOH formation (Figure 7B). Furthermore, a drastic decrease in k_app was observed, reaching 17% in the sixth cycle with respect to the value observed in the first cycle. These findings confirm the deactivation of the continuous catalysts during the recyclability studies.

Metal leaching was confirmed by a hot-filtration test conducted after the third cycle (Figure S6 in Supplementary Materials). After separation, both catalysts exhibited increased conversion levels, with the increase being more pronounced for Pd/HNTP than for Pd/HNTA. Inductively coupled plasma (ICP) analysis was performed on the catalysts recovered in the last cycle to determine their post-reaction metal content. The metal content decreased from 0.41 wt% to 0.19 wt% for Pd/HNTP and from 0.46 wt% to 0.35 wt% for Pd/HNTA, respectively, indicating the leaching of the metal into the liquid phase at higher cycles. The TEM characterization of the spent catalysts showed evidence of the sintering of the Pd-NPs after the final cycle (Figure 7C,D), displaying a Pd-NP size distribution of 5.4 ± 1.8 nm for Pd/HNTP and 3.4 ± 1.2 nm for Pd/HNTA, respectively (Figure S7 in Supplementary Materials). These results suggest that, during consecutive reaction cycles, Pd/HNTP experienced the substantial sintering and leaching of Pd, which decreased its activity, whereas Pd/HNTA displayed moderate deactivation due to metal leaching accompanied by more pronounced Pd-NP sintering.

A possible mechanism of catalyst deactivation could be attributed to the presence of THFOH in both catalytic systems because, in all runs, FUR hydrogenation displayed maximum selectivity for this product. Under the reaction conditions −120 °C and 30 bar of H₂ pressure, the presence of nucleophilic THFOH could promote solvothermal reactions, which decreased Pd-NP stabilization via the HNTP and HNTA supports. Nevertheless, the improved recyclability of Pd/HNTA could be attributed to the strong metal–amine interaction of the organo-functionalized lumen as well as the confinement effects of the Pd-NPs deposited on the inner surface of the HNTA support.

To evaluate catalytic efficiency and recyclability, the Pd/HNTA catalyst was compared with earlier-reported Pd-based catalysts, as shown in

Table 3. Comparison of the catalytic activities of heterogeneous Pd-based catalysts for FUR hydrogenation.

Entry	Catalyst	Reaction Conditions	Recycles	TOF (h−1)	Ref.
1	1.1%Pd/Al2O3	30 °C, 5 bar, 0.5 h, water	--	204	[48]
2	1.0%Pd/CNT	150 °C, 30 bar, 1 h, water	--	14,040	[47]
3	1.0%Pd/UiO-66	110 °C, 20 bar, 2 h, isopropanol	3	206	[49]
4	2.0%Pd/Al2O3	25 °C, 60 bar, 0.5 h, isopropanol	5	1155	[50]
5	1.0%Pd/TiO2 nanosheets	140 °C, 20 bar, 4 h, n-dodecane	5	2898	[7]
6	Pd/HNTA	120 °C, 30 bar, 6 h, n-dodecane	10	880	This work

. The catalysts in entries 1 and 2 were carried out in water as the solvent showed higher TOF values with higher metal loading compared with our catalyst, but the authors did not provide evidence related to the recycling studies. Among them, 1.1%Pd/CNT showed the highest TOF, which was attributed to the harsh reaction conditions and the hydrophobic nature of the CNT support [47]. Entries 3 and 4 displayed the FUR hydrogenation in isopropanol as the solvent, achieving a similar TOF value for the 1.0%Pd/UiO-66 catalyst with a formulation four times higher than the Pd/HNTA catalyst, while the reuses reached five continuous catalytic cycles. The 1.0%Pd/TiO₂ nanosheets catalysts (entry 5) displayed a TOF value three times higher than the Pd/HNTA at similar reaction conditions, which is attributed to the physicochemical nature of the nanosupport as well as its higher metal dispersion [45]. However, the catalyst’s stability upon running is lower than the Pd/HNTA catalysts, achieving only five consecutive catalytic cycles. These results demonstrate that the amine-functionalized lumen of the HNT support provided a Pd-based catalyst with superior stability and recyclability than other reported systems for FUR hydrogenation.

2.2.2. Hydrogenation of Nitrobenzene

Aromatic amines are crucial products in the pharmaceutical industry because they are used as building blocks for a wide range of drugs [51]. The hydrogenation of nitrobenzene is a test reaction to explore the ability of the catalysts to produce the desired aromatic amines. Control experiments carried out in the absence of the catalyst displayed behavior similar to that observed in FUR hydrogenation. The NB molecule was not hydrogenated in the absence of catalysts or when employing the HNTP and HNTA supports (Figure S3). Figure 8A shows the conversion of NB using the Pd/HNTP and Pd/HNTA catalysts. The NB consumption as a function of time was fitted to a pseudo-first-order kinetic model with an induction time of 30 min for Pd/HNTP and 10 min for Pd/HNTA. This behavior can be ascribed to the presence of Pd²⁺ in the Pd/HNTP and Pd/HNTA catalysts, as detected by XPS. However, in comparison to FUR hydrogenation, the reaction conditions used are 30 °C and 10 bar of H₂ pressure. In this environment, the Pd²⁺ on the surface of the Pd/HNTP catalyst appears to retard its complete reduction and initiation of the hydrogenation process, whereas Pd/HNTA reduces the palladium oxide species faster under the same reaction conditions. The conversion consumption vs. time curves displayed a different trend for both catalysts, with the order of k_app being Pd/HNTA (0.0262 min⁻¹) > Pd/HNTP (0.0118 min⁻¹). The TOF values were 946 h⁻¹ for Pd/HNTA and 518 h⁻¹ for Pd/HNTP, indicating higher catalytic efficiency for the hydrogenation of NB with the Pd/HNTA catalysts. These results are consistent with those observed for FUR hydrogenation. In addition, the selectivity showed that the main product was aniline (AN), with only a small quantity of nitrosobenzene (NBO) and N-phenyl-hydroxylamine (NBOH) as intermediates (Figure S8 in Supplementary Materials). The catalytic hydrogenation of NB is mediated by the nucleophilic attack on the -NO₂ group produced by the chemisorbed hydrogen at the metallic center [21]. The hydrogenation of nitrobenzene follows two possible routes, as shown in Scheme 2. The first route is a direct pathway involving the consecutive formation of NBO and NBOH intermediates, leading to AN as the final product. The second route, called the condensation route, involves the recombination of NBO and NBOH intermediates to form side products such as azonitrobenzene, azoxynitrobenzene, and/or hydrazonitrobenzene derivatives. In this context, the challenge in catalytic hydrogenation has been to develop catalytic systems that operate exclusively through a direct reaction route, thereby accelerating the hydrogenation of NBO and NBOH to produce the target product with the highest selectivity. According to our results, both catalysts showed a direct route with the negligible accumulation of intermediates. These results are consistent with the ability of Pd-NPs to produce the corresponding aromatic amines by hydrogenation under mild reaction conditions.

Pd/HNTA was also selected for more specific kinetic studies because of its enhanced catalytic performance compared to that of the Pd/HNTP system. The temperature effects were studied at 30 °C, 50 °C, 70 °C, and 90 °C, respectively. Figure 8B shows the direct temperature dependence with an Arrhenius model fit with E_aa = 40.5 kJ mol⁻¹. This calculated energy is lower than that of the Pd-NPs supported on titanate nanotubes modified with APTMS catalysts reported by Shanmugaraj et al., who obtained an E_aa of 46.3 kJ mol⁻¹ [52]. In addition, the product distribution displayed the same tendency at all evaluated temperatures (Figure S9 in the Supplementary Materials), suggesting that the production of secondary products is independent of temperature.

Figure 8C shows the influence of H₂ pressure on the catalytic hydrogenation of NB. Increasing the H₂ gas pressure from 5 bar to 20 bar significantly increased the hydrogenation rate and decreased the induction time. NB was completely hydrogenated and selectively converted into AN at all the H₂ pressures, similar to the hydrogenation of FUR. This evidence suggests that a similar H₂ effect on solubility in the liquid phase improves both the catalytic activity and the reduction rate of the residual Pd²⁺ species in the Pd/HNTA system.

The recyclability of both the Pd/HNTP and Pd/HNTA catalysts for the hydrogenation of AN was investigated. NB conversion and AN selectivity as functions of the number of experimental runs are shown in Figure 9. The Pd/HNTA catalyst showed moderate stability, remaining unchanged after five cycles, with >99% NB conversion and >99% AN selectivity, after 360 min. However, the continuous and substantial decrease in k_app during the catalytic cycles reached 14% less than the k_app in the first catalytic cycle (Figure 9A). In the case of the Pd/HNTP catalyst, a significantly lower stability at 360 min of reaction in comparison with Pd/HNTA catalysts was found, reaching four continuous cycles with a maximum of 52% NB conversion and >97% AN selectivity (Figure 8B). Furthermore, a drastic decrease in k_app was observed, reaching 14% of the fourth cycle with respect to the value observed in the first cycle. This confirms the occurrence of continuous catalyst deactivation during the recyclability studies, which was more pronounced than that observed during FUR hydrogenation.

Metal leaching was confirmed by a hot-filtration test after the third cycle (Figure S10 in Supplementary Materials); both catalysts increased their conversion level after separation in the tendency of Pd/HNTP >> Pd/HNTA. ICP analysis was performed on the catalysts recovered in the last cycle to determine their post-reaction metal content. The metal content decreased from 0.41 wt% to 0.12 wt% for Pd/HNTPs and from 0.46% to 0.20% for Pd/HNTA, respectively, confirming significant Pd leaching into the liquid phase. The TEM characterization of the spent catalysts showed the evident sintering of Pd-NPs after the last cycle (Figure 9C,D), displaying a Pd-NP size distribution of 8.2 ± 4.2 nm for Pd/HNTP and 4.5 ± 2.1 nm for Pd/HNTA, respectively (Figure S11 in Supplementary Materials).

These results suggest that both catalysts displayed severe deactivation due to metal leaching. A plausible explanation for this phenomenon is the production of H₂O as a co-product during the hydrogenation process. Despite the mild temperature of 30 °C and moderate 10 bar of H₂ pressure, the adsorption of H₂O on the HNT surface [53] may induce the leaching and sintering of the Pd-NPs on both the HNTP and HNTA supports. However, the moderate recyclability of the Pd/HNTA is ascribed to the metal–amine interaction within the organo-functionalized lumen and the confinement effects experienced by the Pd-NPs deposited on the inner surface of the HNTA support.

Table 4. Comparison of the catalytic activities of heterogeneous Pd-based catalysts for NB hydrogenation.

Entry	Catalyst	Reaction Conditions	Recycles	TOF (h−1)	Ref.
1	0.5%Pd/γ-Al2O3	25 °C, 1 bar, 3 h, THF	3	583	[54]
2	2.0%Pd/meso-γ-Fe2O3	50 °C, 1 bar, 2 h, ethanol	4	626	[55]
3	3.75%Pd-NPs stabilized PEG-4000	25 °C, 1 bar, 3 h, ethanol	10	126	[56]
4	1.0%Pd/TiO2 nanotubes	25 °C, 20 bar, 6 h, ethanol	7	342	[52]
5	Pd@SiO2 core-shell	45 °C, 1 bar, 2 h, ethanol	10	6335	[57]
6	Pd/HNTA	30 °C, 10 bar, 6 h, ethanol	10	946	This work

presents the results of previous studies of the catalytic performances of Pd-based catalysts for the hydrogenation of NB using ethanol as the green solvent. Entries 1–4 display catalysts with lower TOF values and recovery ability than the Pd/HNTA catalyst. The optimal catalyst is Pd@SiO₂ core-shell (entry 5), showing TOF values six times higher than our catalyst and similar recyclability, reaching 10 continuous catalytic cycles. Despite the Pd@SiO₂ core-shell catalyst showing improved performance and recyclability for the NB hydrogenation, this kind of catalyst possesses drawbacks such as complexity in large-scale production and cost-effectiveness. In this way, Pd/HNTA catalysts displayed several advantages in catalytic NB hydrogenation, where the support can provide stability to the Pd-NPs, and the selective encapsulation on the HNT lumen offers optimal catalytic activity. Additionally, core-shell catalysts can improve selectivity and activity, and their unique structure can also enhance chemical/physical stability.

3. Experimental Section

3.1. Preparation of Catalysts

Five grams of commercial HNT support (Sigma-Aldrich, St. Louis, MO, USA) was pyrolyzed in an N₂ atmosphere (Linde Chile, Santiago, Chile) for 360 min at 400 °C, with a heating rate of 10 °C min⁻¹. The resulting solid material was then deposited into 50 mL of dry toluene, and triethylamine (1.0 mL, Sigma-Aldrich, St. Louis, MO, USA) was added. Subsequently, chlorotrimethylsilane (300 μL, Merck, Darmstadt, Germany) was added dropwise and the mixture was stirred for 2 h at 20 °C. The obtained solid was filtered, washed seven times with absolute ethanol (Merck, Darmstadt, Germany), and once with acetone (Merck, Darmstadt, Germany) and dried in an electric oven at 50 °C for 12 h. Support modification was performed following a modified methodology reported by Campos et al. [35]. Four grams of the modified HNT was ultrasonically dispersed in 200 mL of toluene (Merck, Darmstadt, Germany), and a solution of APTMS (60 μL, Merck, Darmstadt, Germany) was added; the mixture was then heated at 110 °C for 12 h. The amine-functionalized HNT support was collected by centrifugation, washed with an acetone:toluene (3:1) mixture, and dried at 50 °C for 12 h. The obtained solid was dispersed in 100 mL of absolute ethanol and sonicated for 10 min. An appropriate volume of a methanolic (Merck, Darmstadt, Germany) PdCl₂ (Merck, Darmstadt, Germany) solution (8.0 × 10⁻⁴ mol·L⁻¹) was added and stirred for 4 h, and then dried in a rotary evaporator under vacuum at room temperature to produce 0.50 wt.% of metal loading. Subsequently, the impregnated material was reduced at 200 °C with a heating rate of 5 °C min⁻¹ for 2 h under H₂ (Linde Chile, Santiago, Chile). The resulting catalyst was labeled as Pd/HNTA. A control system was prepared for comparison following the same procedure using pristine HNT support instead of modified HNT, and it was labeled as Pd/HNTP.

3.2. Characterization

N₂ adsorption–desorption isotherms at −196 °C were measured using a Micromeritics^® Tristar II 3020 surface area and porosity analyzer (Micromeritics Instrument Co., Norcross, GA, USA). The specific surface areas were calculated using the Brunauer–Emmett–Teller (SBET) method. The organic matter deposition (C, N and H) during HNT modification was verified by elemental analyses (EA) using a Fisons^® EA1108CHN-S (Fisons Instruments, Rodano, Italy). The Pd content in both fresh and used catalysts was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Varian^® 715-ES (Agilent instruments, Santa Clara, CA, USA). Powder X-ray diffraction patterns were acquired using Cu Kα1 radiation (λ = 1.5418 Å) on a Rigaku^® diffractometer (Rigaku Holdings Corp, Tokyo, Japan) over a range of angles from 2θ = 20° to 80°. Transmission electron microscopy (TEM) micrographs were obtained with a JEOL^® model JEM-1200 EX II microscope (JEOL Instruments, Tokyo, Japan). The metal dispersion (D_Pd) was estimated using the method reported by Leal et al. [58]. A JEOL JEM-2200FS microscope (JEOL Instruments, Mitaka, Japan) was used to obtain high-resolution transmission electron microscopy (HRTEM) images. The temperature-programmed desorption of H₂ (TPD-H₂ was carried out in a Autochem II chemosorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA) equipped with a TCD detector following the protocol reported by Guzman et al. with some modifications [59]. The Pd-impregnated pre-catalyst (0.01 g) was reduced in the equipment in an H₂ flow of 30 mL min⁻¹ at 200 °C for 2 h. Then, the sample was cooled at room temperature. Afterwards, the sample was treated with 5%H₂/Ar until saturation at room temperature; then, the sample was purged with Ar for 15 min. Finally, the TPD spectra were recorded by heating the catalyst from room temperature up to 300 °C with an Ar flow of 50 mL⁻¹ min by using a program rate of 10 °C min⁻¹. X-ray photoelectron spectroscopy (XPS) spectra of the catalysts were recorded on a SPECS^® spectrometer (SPECS, Berlin, Germany) equipped with a PHOIBOS^® 150 WAL hemispherical energy analyzer (angular resolution < 0.5°) and fitted with an XR 50 X-Ray Al-X-ray source and a μ-FOCUS 500 X-ray monochromator (Al excitation line). The catalysts were reduced in situ in the XPS prechamber prior to analysis.

3.3. Catalytic Tests

The catalytic activity of the catalysts for FUR and NB hydrogenation was evaluated using 0.025 g of the catalyst sample and the amount of substrate sufficient to achieve a mol_substrate:mol_Pd ratio of 250 for FUR and 1000 for NB. All activity measurements were performed in triplicate in a Parr^®-type reactor with magnetic agitation at 800 rpm containing 25 mL of solvent (n-dodecane for FUR or absolute ethanol for NB) under different H₂ pressures and temperatures. The progress of the reaction was monitored by gas chromatography using a Shimadzu GC Nexis GC-2030 (Shimadzu Corp, Kyoto, Japan) with an Rtx-5 Amine capillary column (30 m, 0.53 mm ID, 3.00 µm). Variations in the concentration of reactants and products were determined from calibration curves prepared with standard compounds. The conversion (X_substrate) of NB or FUR and the selectivity (S_product) were calculated as follows.

$$X_{Substrate}={\displaystyle \frac{{\left[Substrate\right]}_i-{\left[Substrate\right]}_t}{{\left[Substrate\right]}_i}}\times 100$$

(1)

$$S_{Product}={\displaystyle \frac{{\left[Product\right]}_t}{{\left[Substrate\right]}_i-{\left[Substrate\right]}_t}}\times 100$$

(2)

The turnover frequency (TOF) of the catalysts was calculated as follows.

$$TOF\left(h^{-1}\right)={\displaystyle \frac{mol of hydrogenated substrate }{ mol of metal on surface\times time \left(h\right)}}$$

(3)

For TOF calculations, a 10% isoconversion was used for the corresponding substrate. The number of moles of reduced substrate on the catalyst surface was estimated using the conversion level and real metal loading, as determined by ICP-OES and D_Pd measurements. Additionally, an Arrhenius plot was constructed using a pseudo-first-order relationship (see below) for the best catalyst to determine the apparent activation energy (E_aa) for the tandem synthesis of 3-methyl indole, as expressed below:

$$\ln k_{app}=\ln A-{\displaystyle \frac{E_{app}}{R}}\times {\displaystyle \frac{1}{T}} ,$$

(4)

where k_app is the pseudo-first-order kinetics constant for NB hydrogenation, A is the frequency factor, and T is the reaction temperature (K).

Recycling experiments for the hydrogenation reactions were conducted under identical conditions to those for the conversion experiments using the recovered catalyst. The catalyst was recovered from the reaction mixture by centrifugation, washed three times with the reaction solvent, and dried in an oven at 50 °C overnight. The recovered catalyst was then used for another cycle without reactivation or purification.

4. Conclusions

Supported Pd nanoparticle catalysts prepared on the amine-functionalized lumen of halloysite efficiently promoted hydrogenation reactions. Compared to the catalyst prepared on pristine support, Pd-NPs on the proposed catalysts showed increased catalytic activity and improved recyclability in the hydrogenation of FUR and NB, respectively. Additionally, the catalytic system was reusable for up to 10 cycles, with a moderate loss in activity after 360 min of reaction in FUR hydrogenation, whereas NB hydrogenation achieved 5 continuous hydrogenation cycles with a substantial decrease in catalyst stability, suggesting that H₂O as a co-product played a pivotal role in catalyst deactivation. This study provided a method for the preparation of noble metal nanoparticles on a halloysite support for hydrogenation applications.