Preparation of MoS₂@GQDs for Slurry-Phase Hydrotreating of Waste Oil to Produce Biohydrogenated Diesel

Abstract

Graphene quantum dots (GQDs) were prepared by acid oxidation with deoiled asphalt (DOA) as the precursor. MoS₂ was grown in situ on the surface of GQDs to obtain MoS₂@GQDs utilized for slurry-phase hydrotreating of palm acid oil (PAO) to produce biohydrogenated diesel (BHD). The confinement effect of GQDs is attributed to the functional groups, which effectively restricts the agglomeration of MoS₂ and contributes to its uniform distribution. There is a positive correlation between Mo loading and particle size of MoS₂@GQDs. MoS₂@GQDs with 10 wt % Mo loading (0.10MoS₂@GQDs) exhibited suitable an average particle size of 121.11 nm, resulting in the lower extent of agglomeration. The degree of sulfurization (the proportion of Mo⁴⁺ in the Mo metal) increased with elevating temperature, reached a maximum of 63.77% at 350 °C, and subsequently decreased upon further temperature increase. 0.10MoS₂@GQDs synthesized at 350 °C for 2 h exhibited the best catalytic performance. The BHD yield of 82.53 wt % and the PAO conversion of 102.38 wt % were obtained at a temperature of 380 °C, a residence time of 1 h, an initial H₂ pressure of 6 MPa, and 1 wt % 0.10MoS₂@GQDs.

1. Introduction

The component of traditional biodiesel is fatty acid methyl ester; it has low energy density and poor low-temperature fluidity due to the high oxygen content, limiting its blend ratio as an additive. , Conversely, biohydrogenated diesel (BHD) is the normal alkane obtained by hydrotreating of fats and oils with a composition similar to that of petrochemical diesel. Consequently, BHD exhibits a higher cetane number, stronger stability, and compatibility for blending with petrochemical diesel in any proportion. ,

Nonedible biomass is the main raw material for the production of BHD, including nonedible animal and vegetable oils, microbial oils, and waste oils. − Waste oils, with diverse sources and low prices, are favored as raw materials for production. Fixed-bed hydrotreating is a prevalent industrial process for producing BHD, and the commonly used commercial catalysts are NiMo/Al₂O₃ or CoMo/Al₂O₃. Although commercial catalysts have excellent catalytic performance, the high acid value and significant presence of impurities such as heteroatoms like Fe, Ca, and P in the waste oil result in the rapid deactivation of catalysts and catalyst bed plugging. − These factors lead to the poor adaptability of fixed-bed hydrotreating to feedstock. In contrast, the slurry-phase hydrotreating is more adaptable to feedstock. The catalyst comprises dispersed submicron or nanoscale particles suspended in the feedstock, which can effectively avoid the problem of catalyst bed plugging without considering the deactivation of catalysts.

MoS₂ is particularly noteworthy for its excellent hydrotreating effect, resilience against sulfide deactivation, and thermal stability, making it the preferred catalyst for slurry-phase hydrotreating. − The unique layered structure of MoS₂ renders its basal plane inert, concentrating the catalytic activity at edge sites and sulfur vacancies. The content of active edge sites directly correlates with the catalytic performance of MoS₂. , Sulfur vacancies serve as sites for oxygen adsorption to activate C–O bonds, while H₂ molecules adsorb on nearby Mo atoms for activation and dissociation to produce active hydrogen, which then attacks C–O bonds and leads to the deoxygenation of reactants. Luo successfully introduced nitrogen atoms into MoS₂ to disrupt the coordination environment of Mo and enhance sulfur vacancy density, demonstrating the catalytic benefits of sulfur vacancy formation in MoS₂ with a significant increase in BHD yield. Despite its high hydrotreating activity, MoS₂ tends to agglomerate during reactions, , reducing exposure of edge sites and sulfur vacancy density, consequently diminishing its catalytic performance. Therefore, it is a crucial issue how to effectively prevent the agglomeration of metal components.

Zero-dimensional nanoconfinement materials, such as hollow nanoparticles , and nanocages, have been increasingly utilized in catalytic processes to prevent catalyst agglomeration. Graphene quantum dots (GQDs), a type of zero-dimensional carbon-based material, consist of one or more graphene layers with sp²-hybridized carbon atoms arranged in a honeycomb lattice. Additionally, functional groups are present at the edges, introducing structural defects while maintaining a stable graphene structure elsewhere. , These unique characteristics grant GQDs a large surface area, abundant active edges, defect sites, and an efficient electron transfer capacity. The quantum confinement effect of GQDs gives rise to distinctive fluorescence and luminescence properties, making them valuable for applications in biological and chemical sensors, bioluminescent probes, and various other fields. , Furthermore, GQDs have utility in chemical catalysis. Xia utilized GQDs as supports to synthesize single-atom catalysts with a loading of more than 40 wt %. The –NH₂ at the edge of GQDs provides anchor sites, and the high specific surface area of GQDs effectively improves the dispersion of metal atoms. Therefore, there is great potential to apply the confinement effect of GQDs to the field of chemical catalysis to avoid the agglomeration of catalysts.

Asphaltene, a complex nonhydrocarbon with the strongest polarity and heaviest component in petroleum, comprises a fused aromatic core surrounded by naphthenic and aromatic rings along with normal or isomerized alkyl side chains of different lengths and contains heteroatoms such as O, N, and S. , Similar in structure to GQDs, asphaltenes can be transformed into GQDs through the oxidation of their alkyl side chains. Vacuum residue (VR), a product left from the vacuum distillation of petroleum, contains substantial asphaltenes. Solvent deasphalting effectively separates asphaltenes from VR and produces deoiled asphalt (DOA), thereby reducing negative effects such as catalyst poisoning and equipment coking during VR processing. DOA, which is easily available and enriched with asphaltenes, can function as a precursor for the synthesis of GQDs. In this work, GQDs were synthesized through the oxidation of the DOA. Subsequently, a composite material consisting of MoS₂ grown in situ on GQDs, named MoS₂@GQDs, was prepared and utilized for the slurry-phase hydrotreatment of palm acid oil (PAO). The impact of the size constraints imposed on MoS₂ by the confinement effect of GQDs on the hydrotreating performance of MoS₂@GQDs was systematically examined.

2. Experimental Section

2.1. Materials

DOA was utilized in the synthesis of GQDs and for loading MoS₂. Its contents of C, H, S, and N are 88.11 wt %, 9.05 wt %, 0.42 wt %, and 1.35 wt %, respectively. Additionally, it has a H/C ratio of 1.23, softening point of 199 °C, and carbon residue of 41.12%. Activated carbon (AC) was employed for MoS₂ loading and verified confinement effects of GQDs in comparison with DOA. PAO as the waste oil was utilized in slurry-phase hydrotreating, with its properties, fatty acid composition, and elemental content listed in Table . The 400N base oil, sourced from Great Wall Lubricants Co. Ltd., functions as a solvent in the synthesis of catalysts. 65% nitric acid (AR grade), 98% sulfuric acid (AR grade), and sublimated sulfur (CP grade) were purchased from Sinopharm Group Chemical Reagent Co. Ltd. (Beijing, China). Homemade molybdenum naphthenate was employed in the process.

1. Properties, Fatty Acid Composition, and Elemental Content of PAO.

properties		fatty acid composition/wt %		content/%
specific gravity (20 °C)/g·cm–3	0.879	palmitic acid (C16:0)	42.31	C	76.46
acid value/mg KOH/g	183.9	palmitoleic acid (C16:1)	0.12	H	11.58
saponification value/KOH/g	203.1	stearic acid (C18:0)	3.57	S	0.24
iodine value/g I2/100 g	54.6	oleic acid (C18:1)	38.83	N	0.12
moisture/wt %	1.13	linoleic acid (C18:2)	8.61	O	11.23
impurities/wt %	0.38	linolenic acid (C18:3)	0.23
unsaponifiable matters/wt %	0.36	others	6.33

2.2. Preparation of GQDs and MoS₂@GQDs

2.2.1. Preparation of GQDs

GQDs were synthesized through the oxidation of DOA using nitric acid or mixed acid (V _nitric acid/V _{sulfuric acid} = 1:3). The synthesis involved adding DOA powder and acid (m _DOA/V _acid = 1:3 g/mL) to a round-bottom flask, followed by stirring at 350 rpm, heating at reflux for 4 to 10 h at temperatures ranging from 70 to 100 °C, and subsequently cooling the solution to room temperature postreaction. The resulting solution was diluted and filtered using a 0.2 μm microporous membrane, and the solid product was washed multiple times before being dried to obtain GQDs. GQDs synthesized via nitric acid oxidation were named N-GQDs, while those synthesized through mixed acid oxidation were named S-GQDs.

2.2.2. Preparation of MoS₂@GQDs

GQDs and molybdenum naphthenate were weighed in a mass ratio of GQDs/Mo = 10:0.5–2, while sulfur powder was weighed in a molar ratio of Mo/S = 1:2. These raw materials were combined in 400N base oil, stirred at room temperature for 10 min, sonicated for 30 min, and then transferred to a 500 mL autoclave reactor (Dalian Tong Chan High-pressure Reactors Manufacture Co. Ltd., China). The reaction took place at 250–400 °C for 1–4 h. Following the reaction, the solution was cooled to room temperature, filtered to collect solids, washed thrice with deionized water and anhydrous ethanol, and subsequently dried to obtain MoS₂@GQDs.

GQDs loaded with Mo in mass ratios of 10:0.5, 10:1.0, 10:1.5, and 10:2.0 were named 0.05MoS₂@GQDs, 0.10MoS₂@GQDs, 0.15MoS₂@GQDs, and 0.20MoS₂@GQDs, respectively. MoS₂@GQDs synthesized at different temperatures and times were respectively named temperature-MoS₂@GQDs and time-MoS₂@GQDs; e.g., 350-MoS₂@GQDs represented the catalyst prepared at 350 °C and 2-MoS₂@GQDs represented the catalyst prepared at 2 h.

2.3. The Characterization of GQDs and MoS₂@GQDs

Fluorescence properties of GQDs were examined using a fluorescence spectrometer (Fluoromax-4). Infrared spectra of DOA, GQDs, PAO, and diesel products were acquired using Fourier transform infrared spectroscopy (FT-IR, NEXUSFT). The defect density at the edges of GQDs was assessed through Raman spectroscopy (Raman, GL-MRS-532). X-ray photoelectron spectroscopy (XPS, ESCALab 250Xi) was employed to investigate the bonding environments of C and O elements in GQDs and the valence states of Mo in MoS₂@GQDs. X-ray diffraction (XRD, Bruker D8 X’pert PRO MPD) was utilized to characterize characteristic diffraction peaks of GQDs and MoS₂@GQDs synthesized under different conditions. Transmission electron microscopy (TEM, JEM-210UHR) was utilized for microstructural observations of MoS₂@GQDs.

2.4. Hydrotreating of PAO

PAO and MoS₂@GQDs were weighed based on 1 wt % MoS₂@GQDs and introduced into a 500 mL autoclave reactor (Dalian Tongchan Autoclave Manufacturing Co. Ltd., China) containing 6 MPa of H₂. The mixture underwent a reaction at 380 °C for 1 h, followed by cooling to room temperature postreaction. Subsequently, Engler distillation was performed on the resulting product, and the mass of each fraction was determined. The products were categorized as gas, water, gasoline (<180 °C), diesel (180–360 °C), and bottom (>360 °C).

Table exhibited the mass balance obtained by product and gas analysis. Based on the mass balance, the product yield (Y) and the conversion of PAO (C _PAO) are defined as

$$Y_{\mathrm{i}}=\frac{m_{\mathrm{i}}}{m_{\mathrm{P}\mathrm{A}\mathrm{O}}}\times 100\%$$ 1

$$C_{\mathrm{P}\mathrm{A}\mathrm{O}}=\frac{m_{\mathrm{g}\mathrm{a}\mathrm{s}}+m_{\mathrm{w}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}}+m_{\mathrm{g}\mathrm{a}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}}+m_{\mathrm{d}\mathrm{i}\mathrm{e}\mathrm{s}\mathrm{e}\mathrm{l}}}{m_{\mathrm{P}\mathrm{A}\mathrm{O}}}\times 100\%$$ 2

where m _i (i represents gas, water, gasoline, diesel, or bottom) denotes the mass of each fraction in Engler distillation (g) and m _PAO is the mass of PAO involved in the reaction (g).

2. Material Balance Obtained by Slurry-Phase Hydrotreating of PAO at 380 °C, 1 h, 6 MPa H₂ Pressure, and 1 wt % 0.10MoS₂@GQDs.

input component/wt %		output component/wt %
PAO	100	gas	6.42
H2	3.60	water	10.87
		gasoline	2.56
		diesel	82.53
		bottom	1.22
total	103.60	total	103.60

The turnover frequency (TOF) reflects the intrinsic activity of the catalyst, and it can be used to evaluate the hydrotreating performance of MoS₂, which is defined as

$$\mathrm{T}\mathrm{O}\mathrm{F}=\frac{r_{{\mathrm{H}}_2}}{n_{\mathrm{M}\mathrm{o}}}$$ 3

where r _H2 represents the consumption rate of H₂ during the hydrotreating of PAO (mol/s) and n _Mo is the molar number of Mo⁴⁺ in the MoS₂@GQDs (mol).

3. Results and Discussion

3.1. Characterization of GQDs

Figure reveals two distinct emission peaks. The first peak, observed in the range at 450–550 nm, exhibits the independent fluorescence attributed to the eigenstate emission of GQDs. This emission is a result of photoluminescence influenced by both the quantum confinement effect and the conjugated π electron system. The emission wavelength is determined by the bond gap of subdomains in GQDs and the energy gap of the π–π* transition, leading to its independent fluorescence. The second peak, situated in the range of 550–650 nm, displays a red-shifted peak position that becomes more pronounced with increasing excitation wavelength. This emission peak arises from defect state emission in GQDs, characterized by dependent fluorescence, which can be explained by the different surface states of GQDs. Liu identified dependent fluorescence in carbon quantum dots as stemming from distinct emission sites at different energy levels. Lau proposed that these emission sites consisted of oxygen-containing functional groups such as O–H, CO, and C–O–C, which dominated at specific excitation wavelengths. Consequently, the dependent fluorescence of GQDs is attributed to distinct surface states arising from different oxygen-containing functional groups. These different surface states form multiple emission sites where the dominant site changes with excitation wavelength. The observation of dual fluorescence characteristics indicates that the luminescence mechanism of pitch-based GQDs in this work is rooted in the coordinated modulation of a π-conjugated system, quantum confinement effects, and surface states formed by distinct oxygen-containing functional groups. Furthermore, it suggests that the fluorescence properties of GQDs are minimally impacted by the choice of acid.

1.

Fluorescence emission spectra of GQDs prepared at 90 °C for 8 h: (a) S-GQDs and (b) N-GQDs.

As shown in Figure a, the characteristic peaks observed in the 2800–3000 cm^–1 range arise from the stretching vibration of C–H, with a weaker peak in S-GQDs and a stronger peak in DOA. This saturated C–H bond originates from the alkane side chains and naphthenic rings in asphaltenes. Both S-GQDs and N-GQDs exhibit a broad absorption peak attributed to O–H in the 3000–3400 cm^–1 range and a prominent absorption peak of CO around 1720 cm^–1, but DOA lacked these two absorption peaks. These results suggest that asphaltenes undergo oxidation to form –COOH, O–H, and CO. A comparison between N-GQDs and S-GQDs reveals a higher intensity of CO absorption peak and O–H absorption peak in N-GQDs, indicating the content of CO and O–H in N-GQDs is higher than that in S-GQDs. N-GQDs display absorption peaks related to asymmetric and symmetric stretching vibrations of –NO₂ at 1546 cm^–1 and 1348 cm^–1, respectively, which are absent in S-GQDs and DOA.

2.

(a) FT-IR spectrum of DOA and GQDs prepared at 90 °C for 8 h and (b) XRD pattern of N-GQDs and S-GQDs prepared at 90 °C for 8 h.

Figure b reveals two diffraction peaks at 2θ of 23° and 43°, corresponding to (002) and (100) crystal planes, respectively. The characteristic peak (002) of graphene is observed around 2θ = 26°, while the corresponding peak of GQDs shifts to the left, suggesting a larger interlayer spacing in GQDs compared with graphene. , This result is attributed to the abundance of oxygen-containing and nitrogen-containing functional groups in GQDs generated during the acid oxidation process. The steric hindrance posed by these functional groups leads to an increase in interlayer spacing. Due to the higher disorder and smaller size, N-GQDs exhibit lower (002) peak intensity and greater broadening compared with S-GQDs, weakening the coherent scattering of the crystal plane. This finding indicates that the oxidation effect of nitric acid is more pronounced than that of mixed acid, reducing sizes of asphaltenes and generating a greater number of functional groups.

In Figure a, peaks are observed at 285.7, 399.2, and 530.1 eV, corresponding to C 1s, N 1s, and O 1s, respectively, indicating the presence of carbon, nitrogen, and oxygen elements in N-GQDs. Figure b reveals peaks at 284.6, 285.7, 287.4, and 288.8 eV, representing CC, C–O, CO, and –COOH, respectively. The CC peak originates from the graphene-like planar carbon structure consisting of sp² hybridized carbon within the core of GQDs, while C–O, CO, and –COOH correspond to oxygen-functional groups located at the edges of GQDs. As shown in Table , in comparison to S-GQDs, N-GQDs exhibit a lower peak area ratio of CC and a higher peak area ratio of CO and –COOH, suggesting that the carbon framework of asphaltene is more deeply broken following nitric acid than mixed acid and most edge groups are oxidized into oxygen-containing functional groups, which is consistent with results in Figure . The confinement effect of GQDs is attributed to functional groups, and the high content of these groups facilitates the improvement of the confinement effect. Therefore, the nitric acid is optimal for the synthesis of GQDs, and N-GQDs have a better confinement effect.

3.

XPS spectra of N-GQDs synthesized at 90 °C for 8 h: (a) survey spectrum and (b) C 1s high-resolution spectrum.

3. XPS Fitting Analysis of GQDs.

GQDs	functional group
	C–C (%)	C–O (%)	CO (%)	COOH (%)
N-GQDs	48.13	19.86	14.42	17.59
S-GQDs	52.38	21.41	13.47	12.74

In Figure a,b, both spectra show characteristic absorption peaks corresponding to –COOH, CO, bending vibrations of C–H, and asymmetric and symmetric stretching vibrations of –NO₂. Figure a illustrates that the peak intensity at 70 °C is the lowest, while the absorption peak intensity of –COOH at temperatures ranging from 80 to 100 °C is similar. Moreover, the absorption peak intensity of CO at 90 °C is the most pronounced. These results indicate that the deepest oxidation of DOA was observed at 90 °C. In Figure b, the absorption peak intensities at different oxidation times exhibit similarity.

4.

FT-IR and Raman spectra of N-GQDs synthesized under different conditions: (a,c) oxidation temperature and (b,d) oxidation time.

In Figure c,d, Raman spectra of N-GQDs prepared under different conditions reveal absorption peaks at 1585 cm^–1 and 1354 cm^–1. The peak at 1585 cm^–1 corresponds to the G band, representing in-plane vibrations of ordered sp² hybridized carbon in GQDs. The peak at 1354 cm^–1 corresponds to the D band, arising from disordered sp² hybridized carbon vibrations at the edge of GQDs. The disordered carbon breaks the original graphene lattice by engaging in functional group formation and leads to the edge defects, forming the confinement effect of GQDs. The degree of edge defect has an influence on the intensity of the confinement effect. Consequently, the ratio of intensities (I _D/I _G) between the D and G bands serves as a metric for assessing the degree of edge defect to compare the intensity of confinement effect of GQDs prepared under different conditions. The I _D/I _G values in Figure c are 0.80, 0.82, 0.88, and 0.85, respectively, with the peak temperature corresponding to 90 °C. This result indicates that elevating the temperature enhances the edge defects, but excessive temperature prevents the formation of edge defects, with the highest degree of edge defects occurring at 90 °C. Therefore, 90 °C was determined as an optimal oxidation temperature. Corresponding I _D/I _G values for different oxidation times in Figure d were 0.87, 0.89, 0.88, and 0.89, suggesting minimal impact of oxidation time on the degree of edge defect. Additionally, contents of functional groups in GQDs synthesized at different oxidation times are similar, as shown in Figure b. To determine the optimal oxidation time, the yield of GQDs synthesized at different oxidation times was assessed, and the highest yield was obtained at 8 h.

3.2. Characterization of MoS₂@GQDs

As mentioned before, N-GQDs prepared at 90 °C for 8 h have the best confinement effect, and thus they were used to synthesize MoS₂@GQDs for characterization. Figure shows XRD patterns of catalysts, with a broad peak at 2θ between 22° and 28°, corresponding to the DOA, AC, and GQDs. In Figure a, DOA and AC loaded with MoS₂ were named MoS₂@DOA and MoS₂@AC, respectively. These three catalysts were prepared at 350 °C for 2 h and had a 10 wt % Mo loading. The characteristic diffraction peaks at 2θ of 33.7° and 58.5° are observed, corresponding to the (101) and (110) crystal planes of 2H–MoS₂, respectively. Figure b is the XRD pattern of MoS₂@GQDs synthesized at 350 °C for 2 h. Additional characteristic diffraction peaks at 2θ of 14.4° are present, corresponding to the (002) crystal plane. These characteristic peaks confirm the formation and deposition of MoS₂ on supports within the system.

5.

XRD patterns of catalysts: (a) support, (b) Mo loading, (c) sulfurization temperature and (d) sulfurization time.

Figure c shows the XRD pattern of 0.10MoS₂@GQDs synthesized at 2 h. Characteristic peaks were observed in both 350-MoS₂@GQDs and 400-MoS₂@GQDs, but they were absent in 250-MoS₂@GQDs and 300-MoS₂@GQDs. The crystal growth process involves the initial formation of crystal nuclei followed by crystal growth. Nucleation formation encounters an energy barrier that can be overcome by elevated temperature. Under the conditions of 350 and 400 °C, the system’s energy exceeds this barrier during nucleation, leading to the formation of MoS₂. However, the peak intensity in 400-MoS₂@GQDs was lower than that in 350-MoS₂@GQDs, indicating that the content of MoS₂ synthesized at 400 °C was less than that synthesized at 350 °C. These results demonstrate that elevating temperature contributes to the sulfurization of Mo, with the highest content of MoS₂ obtained at 350 °C. There is an adverse effect on the sulfurization of Mo when the temperature exceeds 350 °C, leading to a decrease in the content of MoS₂. In Figure d, 0.10MoS₂@GQDs were synthesized at 350 °C. The diffraction peak intensity of 1-MoS₂@GQDs is the lowest, while that of the other MoS₂@GQDs is similar. This result indicates the completion of the Mo sulfurization within the initial 2 h, establishing the stability of the MoS₂ phase.

TEM images in Figure a–c show the surface morphologies of three catalysts, among which the dark spots are MoS₂. In Figure a, the layered structure is DOA, and most MoS₂ with different sizes are not supported on it. During the sulfurization of Mo, MoS₂ particles generated with a small size have a higher surface Gibbs free energy. To achieve a steady state, MoS₂ particles will spontaneously aggregate to reduce the surface Gibbs free energy, leading to an increase in their particle sizes. The lamellar structure shown in Figure b corresponds to the AC. Due to its porous nature, MoS₂ can be effectively dispersed, but a certain extent of agglomeration occurs. In Figure c, compared to MoS₂@DOA and MoS₂@AC, MoS₂ particles are similar in size and uniformly distributed, indicating that GQDs effectively restrict the agglomeration of MoS₂.

6.

TEM images of catalysts synthesized at 350 °C for 2 h: (a) 0.05MoS₂@DOA, (b) 0.05MoS₂@AC, and (c) 0.05MoS₂@GQDs.

As shown in Figure , the uniform distribution of MoS₂ particles is evident in 0.05MoS₂@GQDs and 0.10MoS₂@GQDs, but the agglomeration of MoS₂ particles is observed in 0.15MoS₂@GQDs and 0.20MoS₂@GQDs, with the most serious situation being that of 0.20MoS₂@GQDs. The average particle size of MoS₂@GQDs increases from 99.93 to 155.82 nm, demonstrating a positive correlation between the Mo loading and the particle size of MoS₂@GQDs. Luo investigated the impact of MoS₂ size on H₂ adsorption, activation, and dissociation in the slurry-phase hydrocracking of residue, demonstrating that enlarging the size of MoS₂ inhibited the activation of H₂ and the dissociation of active hydrogen, which had an adverse effect on subsequent reactions. As shown in Figure c,d, excessive Mo addition intensifies the agglomeration of MoS₂ and enlarges the particle size, leading to a poor catalytic performance in 0.15MoS₂@GQDs and 0.20MoS₂@GQDs.

7.

TEM images and average particle sizes of MoS₂@GQDs synthesized at 350 °C for 2 h with different Mo loadings: (a) 5 wt %, (b) 10 wt %, (c) 15 wt %, and (d) 20 wt %.

In Figure a, peaks at 229.28 and 232.61 eV correspond to MoS₂ (Mo⁴⁺), while peaks at 233.37 and 235.63 eV correspond to MoO _x S _y (Mo⁵⁺) and MoO₃ (Mo⁶⁺), respectively. In Figure c, Mo⁴⁺ peaks are observed at 229.29 and 232.57 eV, while Mo⁵⁺ and Mo⁶⁺ peaks are at 233.31 and 236.07 eV, respectively. Figure b,d reveals that the bonding environment of the S element is metal–sulfur bonds. As shown in Table , the peak area ratio of Mo⁴⁺ represents the degree of sulfurization, and a maximum degree of sulfurization was observed at 350 °C compared with 400 °C. The MoS₂ phase was generated in 350-MoS₂@GQDs and 400-MoS₂@GQDs in Figure c. During the sulfurization of Mo, H₂S generated from the sulfur source reduces Mo⁶⁺ from the Mo source to generate MoS₂. The results indicate that elevating temperature contributes to the sulfurization of Mo, but subsequently elevating temperature oxidizes part of Mo⁴⁺ into Mo⁶⁺ when the temperature exceeds 350 °C, which leads to a decrease in the degree of sulfurization. Consequently, the sulfurization temperature of 350 °C was determined. Furthermore, the peak area ratio of Mo⁴⁺, Mo⁵⁺, and Mo⁶⁺ in MoS₂@GQDs synthesized for 2–4 h exhibited marginal changes, with the degree of sulfurization remaining stable at around 63.70%, exceeding the 54.28% observed at 1 h, suggesting that the sulfurization time of 2 h is most favorable.

8.

XPS spectra of Mo 3d and S 2p for 0.10MoS₂@GQDs synthesized for 2 h: (a,b) 350 °C and (c,d) 400 °C.

4. XPS Fitting Analysis of 0.10MoS₂@GQDs Synthesized under Different Conditions.

peak type	relative peak area (%)
	350 °C-2 h	400 °C-2 h	350 °C-1 h	350 °C-2 h	350 °C-3 h	350 °C-4 h
Mo3d	63.77 ± 0.22	57.26 ± 0.25	54.28 ± 0.30	63.77 ± 0.22	63.65 ± 0.23	63.71 ± 0.24
Mo3d	15.11 ± 0.18	15.08 ± 0.19	18.47 ± 0.25	15.11 ± 0.18	15.46 ± 0.20	14.98 ± 0.19
Mo3d	21.12 ± 0.20	27.66 ± 0.28	27.25 ± 0.27	21.12 ± 0.20	20.89 ± 0.21	21.31 ± 0.22

3.3. Slurry-Phase Hydrotreating of PAO

In Figure a–c, catalysts were prepared at 350 °C for 2 h with 10 wt % Mo loading. As shown in Figure a, MoS₂@GQDs exhibit higher Y _diesel and C _PAO than those of MoS₂@DOA and MoS₂@AC, which indicates the excellent catalytic performance of MoS₂@GQDs, demonstrating that the confinement effect of GQDs effectively restricts the agglomeration of MoS₂. In Figure , the peak TOF value was observed at MoS₂@GQDs, suggesting an improvement in the intrinsic catalytic activity of MoS₂. Luo showed that both the average slab length and average stacking number increase with the particle size of MoS₂. Furthermore, the shorter the slab length and the fewer the stacking numbers, the more exposed the edge sites of MoS₂ and the better the catalytic performance. , MoS₂@GQDs have fewer stacking numbers and shorter slab length due to effective restriction of particle size through the confinement effect, which leads to more exposed catalytic sites and the greatest intrinsic catalytic activity.

9.

Product yields and C _PAO during slurry-phase hydrotreating of PAO at 380 °C, 1 h, and 6 MPa H₂ pressure with 1 wt % catalysts: (a) support, (b) oxidation temperatures of GQDs, (c) oxidation times of GQDs, (d) Mo loading, (e) sulfurization temperatures of MoS₂, and (f) sulfurization times of MoS₂.

10.

TOF value of catalysts prepared with different supports.

In Figure a, MoS₂@N-GQDs exhibited superior catalytic performance with a Y _diesel of 82.53 wt % and C _PAO of 102.38 wt %, which are higher than those of MoS₂@S-GQDs, suggesting that N-GQDs obtained by nitric acid oxidation can more effectively restrict the agglomeration of MoS₂, resulting in great catalytic performance. As shown in Figures and , higher content of functional groups is observed in N-GQDs, indicating a more pronounced confinement effect associated with increased content of functional groups. This result may be attributed to the steric hindrance by the functional groups. In Figure b, the difference of catalysts is the oxidation temperatures of N-GQDs. As the temperature increases, both Y _diesel and C _PAO exhibit an upward trend, peaking at 90 °C before subsequently decreasing, which indicates that 90 °C is the optimal oxidation temperature. Figure c shows a similar variation trend between the degree of edge defects and temperature. This further supports that a higher content of functional groups contributes to the improvement of the confinement effect. The difference of catalysts shown in Figure c is the oxidation times of N-GQDs. The Y _diesel are 82.43 wt %, 82.48 wt %, 82.53 wt %, and 82.49 wt %, respectively. Although the Y _diesel peaks at 8 h, the overall change in Y _diesel is minimally affected by time extension. The results for C _PAO exhibit a similar trend. Furthermore, the yield of GQDs also reaches its maximum at 8 h. Consequently, 8 h is the optimal oxidation time.

In Figure d–f, N-GQDs synthesized at 90 °C for 8 h were chosen to prepare MoS₂@GQDs. In Figure d, MoS₂@GQDs were synthesized at 350 °C for 2 h. The Y _diesel and C _PAO obtained from 0.05MoS₂@GQDs and 0.10MoS₂@GQDs are higher compared to those from 0.15MoS₂@GQDs and 0.20MoS₂@GQDs, which is attributed to the result that excessive Mo loading leads to agglomeration of MoS₂ and results in a poor catalytic performance as shown in Figure . The excellent catalytic performance of 0.10MoS₂@GQDs over 0.05MoS₂@GQDs can be attributed to the higher MoS₂ content in 0.10MoS₂@GQDs, resulting in a greater number of exposed catalytic sites. The Y _diesel and C _PAO of 0.10MoS₂@GQDs synthesized at different sulfurization temperatures and times are shown in Figure e,f. In Figure e, the Y _diesel and C _PAO of 250-MoS₂@GQDs and 300-MoS₂@GQDs are similar, but both are lower than those of 350-MoS₂@GQDs and 400-MoS₂@GQDs, which is attributed to the result that the MoS₂ phase was not generated at temperatures below 350 °C, as seen in Figure c. The Y _diesel and C _PAO of 350-MoS₂@GQDs are higher than those of 400-MoS₂@GQDs due to the maximum degree of sulfurization at 350 °C. As seen in Table , subsequently elevating temperature decreased the degree of sulfurization, leading to a poor catalytic performance. In Figure f, the Y _diesel and C _PAO of 2-MoS₂@GQDs, 3-MoS₂@GQDs, and 4-MoS₂@GQDs are similar, but all exceed those of 1-MoS₂@GQDs, which is attributed to the fact that the sulfurization degree of Mo shows little change within 2 to 4 h of sulfurization time. In conclusion, 0.10MoS₂@GQDs synthesized at 350 °C for 2 h exhibited the best catalytic performance. The BHD yield of 82.53 wt % and the C _PAO of 102.38 wt % were obtained at a temperature of 380 °C, a residence time of 1 h, an initial H₂ pressure of 6 MPa, and 1 wt % 0.10MoS₂@GQDs.

As shown in Figure , a pronounced absorption peak corresponding to the CO bond was observed at 1720 cm^–1 in PAO, but it completely disappeared in diesel. The absorption peaks within the range of 1100–1300 cm^–1 in PAO were attributed to the stretching vibrations of the C–O bond, whereas these peaks were absent in diesel. These results suggest that no unconverted triglycerides are present in the diesel.

11.

FT-IR spectrum of PAO and diesel.

4. Conclusion

In this work, GQDs were prepared by acid oxidation using DOA as a precursor, and MoS₂ was grown in situ on the surface of GQDs to obtain MoS₂@GQDs utilized for slurry-phase hydrotreating of PAO to produce biohydrogenated diesel. The confinement effect of the GQDs formed by functional groups effectively restricts the agglomeration of MoS₂ and facilitates uniform distribution of MoS₂. In comparison to mixed acid, nitric acid deeply oxidized DOA, resulting in an increased content of functional groups. Elevating the oxidation temperature improved the oxidation of DOA, which introduced more defects at the edges of GQDs. At a temperature of 90 °C, the degree of edge defects reached its maximum. Mo loading, sulfurization temperature, and time emerged as factors in the synthesis of MoS₂@GQDs. The 10 wt % Mo loading was determined because excessive Mo loading like 15 and 20 wt % led to the agglomeration of MoS₂ and lower Mo loading like 5 wt % resulted in poor catalytic performance due to less Mo content. Temperatures of 350 °C and above led to the formation of the MoS₂ phase, but excessively high temperatures inhibited the sulfurization of Mo. The degree of sulfurization decreased from 63.77% to 57.26% as the temperature increased from 350 to 400 °C. The sulfurization of Mo was completed within 2 h, with a marginal impact from extended sulfurization time. MoS₂@GQDs with 10 wt % Mo loading (0.10MoS₂@GQDs) synthesized at 350 °C for 2 h exhibited the best catalytic performance. At a temperature of 380 °C, a residence time of 1 h, an initial H₂ pressure of 6 MPa, and 1 wt % 0.10MoS₂@GQDs, the BHD yield reached 82.53 wt %, and the PAO conversion attained 102.38 wt %.

The authors declare no competing financial interest.