Solvent-Free Selective Catalytic Oxidation of Benzyl Alcohol over Pd/g-C₃N₄: Exploring the Structural Impact of g-C₃N₄

Abstract

A series of Pd/g-C₃N₄ catalysts were synthesized using different graphitic carbon nitride precursors, and were found to exhibit significant variations in catalytic performance for solvent-free selective catalytic oxidation of benzyl alcohol. Through comprehensive characterization (XRD, N₂-BET, ICP-AES, TEM, and XPS), the experimental results found that the nitrogen chemical configuration and surface Pd²⁺concentration critically determined the catalytic efficiency. Among the various nitrogen species, N-(C)₃-type nitrogen demonstrated the strongest influence on catalytic activity, which was positively correlated with its abundance in g-C₃N₄ matrices. In particular, g-C₃N₄ derived from dicyandiamide contained the highest N-(C)₃ type nitrogen content. When serving as Pd nanoparticle support, this material simultaneously achieved the best Pd²⁺ surface concentration and catalytic performance compared to the other g-C₃N₄-supported catalysts.

1. Introduction

As a crucial intermediate in organic synthesis, benzaldehyde is widely used in pharmaceuticals, cosmetics, perfumes, etc. [1]. Currently, the primary method for producing benzaldehyde is through chlorobenzyl hydrolysis; however, residual chlorine impurities negatively impact the product quality and downstream applications [2,3,4]. The selective catalytic oxidation of benzyl alcohol (BA) by oxygen under solvent-free conditions has emerged as an environmentally benign alternative, eliminating hazardous solvents while reducing separation costs [5,6,7]. The catalyst design remains pivotal for this process. Noble metal catalysts (Au, Pd, Pt) have been extensively studied for BA oxidation [8,9,10,11,12], with Pd-based catalysts exhibiting exceptional activity due to their oxygen activation capabilities [11,13,14]. Support materials critically influence catalytic performance through metal–support interactions. While traditional supports like TiO₂, CeO₂, and zeolites have been explored [12,15,16,17,18,19,20], carbonaceous materials— N-doped supports in particular—enhance Pd dispersion and stability via strong electronic interactions [14,21,22]. This has motivated the investigation of g-C₃N₄ (gCN), a nitrogen-rich carbon matrix with unique electron-donating properties that can optimize Pd nanoparticle functionality [21,22]. Notably, the ability to tune gCN’s morphology and chemical structure through precursor engineering makes it an intriguing support candidate [23,24]. Recent work by Yi et al. demonstrated Pd/exfoliated-gCN’s efficacy in BA oxidation with toluene solvent. However, the potential of gCN-supported catalysts in solvent-free systems—a crucial advancement for industrial scalability and green chemistry—remains unexplored [22]. Therefore, in this research, three distinct gCN supports were synthesized, with the Pd/gCN catalysts characterized using XRD, N₂-BET, ICP-AES, TEM, and XPS to establish the structure–activity relationships.

2. Results and Discussion

2.1. Benzyl Alcohol Catalytic Oxidation

The catalytic performance of the Pd/gCN catalysts were evaluated using benzyl alcohol catalytic oxidation in a mechanically stirred reactor. Before the evaluation of the catalysts, the supports were tested under the same conditions, and no benzyl alcohol conversion was detected. As Figure 1a shows that the different gCN-supported Pd nanoparticles showed significant differences in catalytic performance. Pd supported on CND showed better catalytic performance than Pd supported on the other two gCN supports.

Table 1. Selective oxidation of benzyl alcohol over Pd/CNC, Pd/CND, and Pd/CNG catalysts.

Catalyst	BA Conversion (%)	Product Selectivity (%)				TOF (h−1)
		Benzaldehyde	Toluene	Benzyl Benzoate	Others
Pd/CNC	5.82	82.5	11.4	3.6	2.5	741
Pd/CND	16.98	88.4	5.5	4.2	1.9	2164
Pd/CNG	9.97	74.2	7.3	10.3	8.2	1270

Reaction conditions: 15 mL BA, 30 mg catalyst, P_O2 = 0.3 MPa, T = 120 °C, t = 2 h, and 1000 rpm.

shows that the catalytic activity can be ranked as follows: Pd/CND > Pd/CNC > Pd/CNG. Pd/CND had the highest catalytic activity with a TOF value of 2164 h⁻¹. Notably, the Pd/CND catalyst exhibited the highest benzaldehyde selectivity throughout the reaction process (consistently > 70%), although it showed a significant time-dependent decline from 92.1% at 1 h to 71.1% after 6 h. In contrast, the Pd/CNC catalyst consistently displayed the lowest selectivity, with toluene and benzyl benzoate identified as its dominant byproducts. As shown in Table 1, the oxidation of benzyl alcohol generated not only benzaldehyde as the primary product, but also various byproducts including toluene, benzyl benzoate, benzene, benzoic acid, and diphenylmethane. Interestingly, the selectivity profile of Pd/CNG remained intermediate between Pd/CND and Pd/CNC across the entire reaction duration.

In [22], exfoliated gCN-supported Pd (the GCN was also prepared by heat treatment of dicyandiamide) showed a TOF value of 314 h⁻¹ at 90 °C while Pd/CND in the present work showed a superior catalytic activity. This indicates that gCN materials can used as a support for high-efficiency BA oxidation catalysts, and that the precursors of gCN influence the catalytic performance of Pd/gCN.

Furthermore, the Pd/CND, Pd/CNC, and Pd/CNG catalysts were reused three times under the same reaction conditions for the stability test. After each cycle, the used sample was centrifuged and then washed with acetone to clean the surface, followed by drying at 110 °C for 12 h. Figure 1b demonstrates that after three reaction cycles, the catalytic activities of these catalysts were reduced to different degrees, with Pd/CND decreasing by 20.1%, and Pd/CNC and Pd/CNG decreasing by 25.9% and 32.9%, respectively.

2.2. XRD, N₂-BET, and ICP-AES Results

The XRD patterns of the supports and catalysts are presented in Figure 2. It can be seen that the prepared gCN samples and commercial gCN exhibited similar diffraction features. The characteristic peaks appearing at 13.0°, 17.7°, 21.6°, and 27.4° can be attributed to the (100), (111), (210), and (002) lattice plane of gCN, respectively, confirming the dominant gCN structure across all the samples. These findings are consistent with prior reports by Fina et al. and Liao et al. [25,26]. After loading Pd, no peaks belonging to the Pd were detected, implying that the Pd nanoparticles on the catalysts were in a highly dispersed state or the Pd content was below the XRD detection limit. Additionally, the gCN peaks in the Pd/gCN catalysts remained nearly unchanged compared to the bare supports, which indicates that the gCN structure in the different samples had not been destroyed or significantly changed during the Pd-loading process.

The texture properties of the supports and catalysts were measured using the N₂-BET test, with the results summarized in

Table 2. Texture properties of CND, CNG, CNC, and Pd/CND, Pd/CNG, and Pd/CNC catalysts.

Sample	BET Surface Area (m2/g) a	Pore Volume (cc/g) b	Pore Size (nm) b
CND	10.9	0.130	14.75
Pd/CND	8.9	0.071	8.70
CNG	26.3	0.365	16.10
Pd/CNG	21.0	0.239	14.54
CNC	7.1	0.089	12.44
Pd/CNC	6.0	0.087	12.00

^a The surface area was calculated using the Brunauer–Emmett–Teller (BET) model. ^b The pore volume and pore size were calculated according to the data of the desorption branch.

. The results showed that the specific surface area of the CNG sample reached 26.3 m²/g, which was significantly larger than that of CNC (7.1 m²/g) and CND (10.9 m²/g), which may be due to the differences in their pore architectures. Upon loading Pd, the specific surface area of Pd/CND, Pd/CNG, and Pd/CNC all decreased. This reduction can be attributed to the deposition of Pd nanoparticles within the interlamellar spaces of layered g-C₃N₄, which restricts nitrogen adsorption pathways.

The pore structure information showed that the pore volume and pore diameter of the three gCN samples followed the order of CNG > CND > CNC. Furthermore, the introduction of Pd nanoparticles led to a reduction in both pore volume and pore diameter for the Pd/CND, Pd/CNG, and Pd/CNC catalysts. Considering the characteristics of the supported catalyst, it can be inferred that the pore volume and pore diameter reflect the lamella gap dimensions. The deposition of Pd nanoparticles effectively filled the lamella gaps, resulting in a slight decrease in the specific surface area, pore volume, and pore diameter of these three gCN materials.

The actual concentration of Pd on the Pd/CND, Pd/CNG, and Pd/CNC catalysts was determined by ICP-AES and the corresponding results are summarized in

Table 3. The Pd bulk composition on the Pd/CND, Pd/CNG, and Pd/CNC catalysts.

Catalysts	Pd (wt%)
	Nominal	Actual
Pd/CND	2.00	1.93
Pd/CNG	2.00	1.91
Pd/CNC	2.00	1.96
Used Pd/CND	-	1.42
Used Pd/CNG	-	1.35
Used Pd/CNC	-	1.26

. The results demonstrated that more than 96% of the Pd was successfully loaded on the catalyst surface. The difference between nominal and actual concentration further verifies the existence of metal loss during the preparation process, which might be related to the leaching of weakly adsorbed Pd during filtration or washing process.

Meanwhile, the ICP-AES results quantitatively demonstrated progressive Pd leaching during the recycling tests, with the residual Pd contents decreasing to 1.42% (Pd/CND), 1.35% (Pd/CNG), and 1.26% (Pd/CNC) after three cycles, corresponding to 26.4%, 29.3%, and 35.7% metal loss compared to the initial loading (2.0%), respectively. This leaching trend strongly correlated with the activity decay rates, confirming Pd loss as the dominant deactivation pathway.

2.3. TEM and HRTEM Results

TEM and HRTEM techniques were employed to observe the morphology of the support and catalyst samples and to measure the particle size of the supported Pd nanoparticles. The TEM and HRTEM characterization results are exhibited in Figure 3. Figure 3a,d,g show that there was a clear difference in the morphology of the CND, CNG, and CNC supports. Specifically, the morphology of CNC was closer to that of the particle sample, while CND and CNG were much more like the layered material. From the TEM images and the results of the N₂ absorption and desorption test that indicate that CNG had a larger specific surface area than CND, it can be concluded that the CNG sample had a more complete layer structure than the CND sample. After the deposition of Pd, the morphology of the supports remained unchanged, which is consistent with the XRD results that indicated no significant change in the crystal structure of gCN. Figure 3b,e,h demonstrate that the supported Pd nanoparticles were uniformly dispersed on the surface of each gCN material. The particle size statistics revealed that the diameters of the Pd particles in the three catalysts were similar, averaging around 3.3 nm. These results further confirmed that the morphology and the layered structure of the gCN samples were not significantly affected by the deposition of Pd nanoparticles.

The uniformity of the dispersion of Pd nanoparticles observed in the TEM images (Figure 3b,e,h) directly correlated with the catalytic performance. The narrow particle size distribution maximized the atomic utilization efficiency of Pd, as smaller nanoparticles generally exhibit higher surface-to-volume ratios and more accessible active sites. The preservation of the layered morphology in the Pd/-gCN catalysts suggests minimal blockage of the basal plane channels, which are critical for molecular diffusion in catalytic systems. This structural integrity ensures efficient mass transport while the metallic Pd provides abundant active sites for surface reactions, synergistically enhancing the catalytic activity.

2.4. XPS Results

Figure 4 shows the XPS spectra of the N1s region of the Pd/gCN catalysts. In the N1s region, there were three main chemical states of N in the Pd/gCN samples. The peak with a binding energy of around 398.6 eV can be attributed to the sp2-hybridized nitrogen in C–N=C, the peaks at about 399.2 eV were assigned to the tertiary nitrogen (N-(C)₃) groups, the peaks at about 400.7 were due to the amino groups carrying hydrogen (C-N-H), and the weak peak with a binding energy higher than 404 eV could be attributed to the charging effect [27]. The proportion of N-(C)₃ varied significantly between the different Pd/gCN samples. The proportion of N-(C)₃-type N in Pd/CND, Pd/CNG, and Pd/CNC accounted for 46.4%, 34.5% and 30.3% of the N content, respectively. Combined with the catalytic performance results, it can be inferred that N in the N-(C)₃ state may be the main site of gCN material binding with Pd, which plays a crucial role during the benzyl alcohol oxidation reaction.

Figure 5 shows the chemical state of Pd in the Pd/CND, Pd/CNC and Pd/CNG catalysts. There were four peaks at around 335.5 eV, 340.8 eV, 336.7 eV, and 342.4 eV in these catalysts. According to a previous report [22], it can be concluded that the peaks at about 335.5 eV and 340.8 eV are the 3d split signal of Pd⁰, and the peaks at around 336.7 eV and 342.4 eV are the 3d electron orbit split signal of Pd²⁺. However, the electronic binding energy of Pd⁰ and Pd²⁺ on these three catalysts was a little different: the chemical states of Pd supported on CNC and CNG were closer to each other, while the metal binding energy of Pd⁰ supported on CND was 335.5 ± 0.1 eV and 340.8 ± 0.1 eV, but the binding energy of Pd²⁺ was 342.4 ± 0.3 eV and 336.7 ± 0.3 eV, respectively. The shift in the Pd⁰ peak can be attributed to initial and final state effects. Meanwhile, the comparison of the three catalysts showed that the Pd²⁺ supported by CND has a lower electron-binding energy, indicating that it had better electronic interactions with the support. In literature reports [14,22], CND-supported Pd exhibited the highest Pd²⁺ content of 35.4%, which was higher than that of both Pd/CNC (27.8%) and Pd/CNG (24.7%). Based on the catalytic activity evaluation results, we propose that the content of Pd²⁺ on the catalyst surface plays a pivotal role in the catalytic oxidation of BA under solvent-free conditions. Based on the chemical state analysis results for N, the supported Pd on CND is likely to transfer electrons to the support through the connection with N, thereby increasing the ratio of N-(C)₃ in CND. The enhanced catalytic activity of Pd/CND likely originates from the synergistic effects of the higher N-(C)₃ site density facilitating strong electronic interactions between Pd nanoparticles and the g-C₃N₄ support.

3. Materials and Methods

3.1. Materials and Reagents

Benzyl alcohol (99.8%, purity), PdCl₂ (99.999% metals basis), guanidine hydrochloride (99.5%, purity), dicyandiamide (99.0%, purity), PVA (Mw ≈ 8000), and sodium borohydride (98.0%, purity) were purchased from the Aladdin company (Shanghai, China) and used without any further purification. The commercial gCN was supplied by Xi’an Qiyue Chemical Technology Co., Ltd., Xi’an, China. H₂ (>99.999%) and O₂ (>99.999%) were supplied by the Taiyuan Iron and Steel Corporation (Taiyuan, China).

3.2. Catalyst Preparation

All the gCN supports were prepared using the thermal polymerization method. Guanidine hydrochloride and dicyandiamide were applied as the precursors. In the typical preparation process, the precursor was placed into a crucible with a cover and then heated to 550 °C at a heating rate of 3 °C/min and maintained for 5 h. The obtained yellow powder was directly used as the catalyst support. For simplicity, gCN prepared using dicyandiamide and guanidine hydrochloride was denoted as CND and CNG, and the commercial gCN was abbreviated as CNC.

The sol-immobilization method was used to prepare the Pd/gCN catalysts [16]. A 2.5 mL volume of a PdCl₂ solution (4 mg Pd/mL) was introduced into 300 mL of deionized water under continuous stirring at ambient temperature for 15 min. Subsequently, polyvinyl alcohol (PVA) was incorporated into the system at a controlled PVA:Pd mass ratio of 0.8. Immediately following this addition, a stoichiometrically calculated volume of a freshly prepared NaBH₄ solution (NaBH₄:Pd molar ratio = 5.0) was introduced, inducing a rapid color transition to a dark-brown colloidal suspension. After 30 min of vigorous agitation, 0.49 g of the synthesized gCN was added and the pH of the mixed solution was adjusted to 1 using sulfuric acid under vigorous stirring. After 1 h, the products were filtered, washed with deionized water, and dried at 105 °C for 12 h in an oven.

3.3. Catalyst Characterization

XRD was carried out with a X’Pert3 Powder diffractometer (PANalytical B.V., Almelo, The Netherlands) equipped with a Cu Kα irradiation source (λ  =  1.5418 Å), operating at 40 kV and 40 mA and a scan speed of 0.02 °/s. The morphology and particle size of the samples were determined by transmission electron microscopy (TEM) on a JEOL JEM-2100F field emission electron microscope working at 200 kV (JEOL Ltd., Tokyo, Japan). For the calculation of particle size, at least 200 particles were randomly measured for the determination of the mean particle diameter. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB Model 250Xi (Thermo Fisher, Waltham, MA, USA) with a monochromatized Al Kα source (hν = 1486.6 eV) to characterize the chemical speciation of the samples. All the XPS spectra were calibrated using the C 1s peak at 284.8 eV as the energy reference, and Shirley-type background subtraction was uniformly applied to all the spectra. ICP-AES was performed to quantitatively analyze the Pd content of the prepared Pd/gCN catalysts using an Agilent 735-ES instrument. The texture structure was measured using an MFA-140 (Beijing Builder Electronic Technology Co., Ltd., Beijing, China) at −196 °C. The samples were pre-treated at 200 °C for 2 h before the measurements. The surface area was calculated using the Brunauer–Emmett–Teller (BET) model, and the pore volume and pore diameter were calculated according to the data of the desorption branch.

3.4. Catalytic Test

The BA catalytic oxidation was performed in a mechanically stirred reactor using 50 mL glass-lined min calves (Anhui Kemi machinery Technology Co., Ltd., Hefei, China). The autoclave was charged with 15 mL of benzyl alcohol (BA) and 30 mg of catalyst, followed by five consecutive oxygen-purging cycles. The reaction system was pressurized to 0.3 MPa O₂ under ambient conditions. The reaction mixture was heated to the target temperature with continuous stirring at 1000 rpm, while the reactor maintained the oxygen supply via an interconnected reservoir to compensate for consumption during the process. The reaction progress was monitored via gas chromatographic analysis (Fuli GC 9790, Wenling, China) employing a DM-5 capillary column (30 m × 0.25 mm × 0.25 μm) coupled with flame ionization detection.

The BA conversion X, the product selectivity S, and the TOF values were calculated as follows:

$$\begin{array}{l}\mathrm{X}={\displaystyle \frac{({\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{i}\mathrm{n}}-{\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{o}\mathrm{u}\mathrm{t}})}{{\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{i}\mathrm{n}}}}\times 100\%;\\ \mathrm{S}={\displaystyle \frac{{\mathrm{n}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}}{({\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{i}\mathrm{n}}-{\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{o}\mathrm{u}\mathrm{t}})}}\times 100\%;\\ \mathrm{T}\mathrm{O}\mathrm{F}={\displaystyle \frac{{\mathrm{n}}_{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{l}\mathrm{c}\mathrm{o}\mathrm{h}\mathrm{o}\mathrm{l},\mathrm{i}\mathrm{n}}\times \mathrm{X}}{{\mathrm{n}}_{\mathrm{P}\mathrm{d}}\times \mathrm{t}}};\end{array}$$

where ‘n’ is the molar mass and ‘t’ is the reaction time.

4. Conclusions

A series of Pd/gCN catalysts were prepared using different gCN materials as supports. The XRD and TEM analysis results indicated that the gCN samples possessed the same crystal phase structure and the Pd particle sizes in the various catalysts were uniform and close to 3.3 nm in size. However, the performance of these Pd/gCN catalysts were quite different from each other in the catalytic oxidation of BA under solvent-free conditions. While the TEM, XPS, and N₂-BET tests results showed that the texture characteristics and morphology of the support had a limited impact on the supported Pd particles and their catalytic performance, the chemical state of N within the gCN structure and the concentration of surface Pd²⁺ played crucial roles in determining the catalytic performance. By combining the various characterization results, it was evident that a higher proportion of N-(C)₃-type N in the gCN material and a greater concentration of Pd²⁺ in the Pd/gCN catalysts correlated with superior catalytic performance.