Precious Metals Catalyze the Saturated Hydrogenation of Polycyclic Aromatic Hydrocarbons in Coal Tar

Abstract

As a significant by-product of coal pyrolysis processes, coal tar is rich in polycyclic aromatic hydrocarbons (PAHs), garnering considerable attention for their potential conversion into high-value products through saturation hydrogenation. This paper presents a comprehensive review of recent advancements in two key areas: progress in high-activity saturated hydrogenation of PAHs catalyzed by precious metals and the regulation of cis–trans isomeric configuration of their hydrogenation products. Furthermore, the investigation addresses two critical challenges involved in the field: the susceptibility of precious metal catalysts to sulfur poisoning during the coal tar’s hydrogenation and the difficulty in controlling the stereo-isomerization of hydrogenation products. This review will advance fundamental understanding of PAHs hydrogenation mechanisms and provide critical technical guidance in coal tar utilization, supporting the sustainable development of clean energy technologies and high-value chemical production from coal by-products.

1. Introduction

As one of the three major fossil fuels in the world, the coal tar produced by coal pyrolysis has recently attracted widespread attention in the academic community. According to data released by Emergen Research, the global coal tar market size reached USD 14.38 billion in 2020 and is expected to reach further to USD 18.64 billion by 2028, with a CAGR of 3.2% during the forecast period. China is a big producer of coal tar and, according to the literature, China’s coal tar production will reach about 26 million tons in 2022 [1,2].The efficient utilization of this substantial coal tar resource has emerged as a pivotal research direction in energy science. Currently, three primary technological routes exist for coal tar processing: (i) fine chemical separation to obtain monomeric chemical products; (ii) delayed coking involving deep thermal cracking to produce coke, gaseous hydrocarbons, and light oils; and (iii) catalytic hydrogenation technology converting coal tar into high-value clean fuels [3,4]. The development of coal-based specialty fuels has become a national strategic priority under the “dual carbon” policy framework.

The complex composition of coal tar, rich in PAHs such as naphthalene, fluorene, anthracene, phenanthrene, and so on, presents both challenges and opportunities. These PAHs share carbon number (C9–C15) similarities with aviation fuels. Deep hydrogenation can convert PAHs into cycloalkanes (>90% purity) devoid of n-paraffins, yielding aviation fuels with exceptional density, calorific value, thermal stability, and low-temperature performance [5,6]. However, the high resonance energy and steric hindrance of PAHs make their hydrogenation saturation challenging [7,8,9]. Therefore, the development of efficient hydrogenation saturation catalysts has become a core issue in current research.

PAH hydrogenation catalysts are primarily classified into precious metal and non-precious metal categories [10]. Non-precious metal catalysts mainly consist of elements such as Co, Mo, Ni, and W [11,12,13]. However, current non-precious metal catalysts still face numerous challenges in achieving effective PAH hydrogenation saturation. For instance, Jing et al. [14] employed Ni₂P/Al₂O₃ catalysts for naphthalene hydrogenation in a fixed bed at 300 °C and 4 MPa, which required higher reaction temperatures compared to precious metal catalysts. Zhang et al. [15] synthesized Ni/S950, Cu/S950, and Co/S950 catalysts using the incipient wetness impregnation method, finding that 18 wt% Ni/S950 exhibited good catalytic performance with 95.6% tetralin selectivity under optimal conditions, though the non-precious metal loading was high. Yang et al. [16] investigated the hydrogenation of phenanthrene using CoMo and NiMo catalysts, demonstrating that CoMo catalysts could only partially hydrogenate phenanthrene, primarily producing dihydrophenanthrene, with limited deep hydrogenation products, indicating that non-precious metal catalysts exhibit low hydrogenation saturation. Wang et al. [17] successfully prepared supported and unsupported Ni₂P, Ni₃P, Co₂P, CoP, MoP, and WP catalysts using H₂ plasma in a dielectric barrier discharge reactor. While this method can effectively control the molar ratio of phosphorus to metal, it remains challenging to apply on an industrial scale.

Table 1. Conditions and results of hydrogenation of PAHs catalyzed by non-precious metals.

Catalyst	Reaction Feed	Reactor	T (°C)	H2/(MPa)	t(h) or WHSV 1	Con 2 (%)	SP.sele 3 (%)
Ni2P/Al2O3 [14]	Naphthalene–decane	Fixed bed	300	4	3/h	98	98
Ni2P/Al2O3 [18]	Phenanthrene–decalin		320	5	3.6/h	19.2	19.85
Ni2P/Al2O3-400 [19]	Naphthalene–decane		300	4	3	98	98
Ni2P/Al2O3-TiO2 [20]	Phenanthrene–decalin		320	5	12.6	95	68
MoS2/TiO2 [21]	Anthracene–tridecane	High-pressure reactor	350	8	4	99.7	0
Ni2P/Al2O3 [14]	Naphthalene–decane	Fixed bed	300	4	3/h	98	98
Ni-Mo/HY [22]	Phenanthrenen–dodecane	Fixed bed	290	5	2/h	99	5
β-Mo2C [23]	Naphthalene–dodecane	High-pressure	350	4	1	100	18

¹ WHSV: Weight Space Hourly Velocity. ² The abbreviation “Con” stands for “conversion”. ³ The abbreviation “SP.sele” stands for “saturated product selectivity”.

summarizes the conditions and results of several non-precious metal catalytic reactions, indicating that non-precious metal catalysts often require harsh conditions, such as high temperatures, prolonged reaction times, and low selectivity. These factors hinder the deep hydrogenation of PAHs and make it difficult to meet the requirements for industrial applications. In contrast, precious metal catalysts can overcome many of these thermodynamic limitations, making them ideal for deep hydrogenation under milder conditions.

There have been numerous experimental studies on PAHs, but a systematic review of the saturated hydrogenation of PAHs catalyzed by precious metals is still lacking. Moreover, there is no comprehensive review addressing the stereo-isomerization of hydrogenated PAH products under the influence of precious metal catalysts. Therefore, it is crucial to conduct a thorough and systematic review of the research on the saturated hydrogenation of PAHs catalyzed by precious metals. This would provide a knowledge framework that enhances our understanding of the hydrogenation reactions, sulfur resistance, and stereoisomerism in PAHs, holding both significant theoretical and practical value.

This article reviews the progress in research on the hydrogenation saturation of PAHs catalyzed by precious metals, with a particular emphasis on the thermodynamics, kinetics, and reaction networks of hydrogenation reactions for model compounds such as naphthalene, phenanthrene, and anthracene. It also explores the sulfur resistance of precious metal catalysts and the regulation of stereoisomers in hydrogenation products while suggesting potential directions for future research (as shown in Figure 1).

2. Characteristics of PAH Hydrogenation

In recent years, PAHs, a significant class of organic compounds, have garnered widespread attention due to their potential applications in the energy and chemical industries. Among these, compounds such as naphthalene, phenanthrene, and anthracene are commonly used as model substances for in-depth studies of their hydrogenation reaction characteristics. The hydrogenation of PAHs is a complex, multi-step process influenced by both thermodynamic and kinetic factors, rather than a simple one-step reaction (as shown in Figure 2). To gain a comprehensive understanding of the reaction mechanisms, researchers have developed detailed reaction networks, tracking the entire process from the initial reactants to the final products through experimental observations and analytical techniques. These studies not only provide a theoretical foundation for optimizing hydrogenation technologies but also offer valuable insights for expanding their industrial applications.

2.1. Thermodynamic Characteristics of PAH Hydrogenation

In the study of PAH hydrogenation, thermodynamic analysis is key to understanding reaction pathways and mechanisms. Thermodynamic calculations can significantly reduce the number of parameters that need to be estimated in kinetic studies, providing important references for optimizing reaction conditions. Among these, the group contribution method for calculating molecular thermodynamic properties has been widely applied [24]. This method can accurately calculate key data such as equilibrium constants, enthalpy changes, Gibbs free energy changes, and entropy changes at different temperatures, providing robust data support for the research system.

The hydrogenation saturation of PAHs is a reversible, strongly exothermic, and volume-reducing process. Higher hydrogen partial pressures and lower reaction temperatures favor the forward reaction. Cooper [8] studied the equilibrium concentrations of phenanthrene and naphthalene hydrogenation in relation to temperature and pressure. They found that the equilibrium concentration of aromatics increased with temperature, while at lower temperatures, the hydrogenation equilibrium concentration of the last ring was lower than that of the first ring. Additionally, increasing hydrogen pressure significantly reduces the equilibrium concentration of aromatics. Hou [25] further characterized the hydrogenation equilibrium constants of anthracene across various temperature ranges, revealing an inverse relationship between equilibrium constants and temperature. At the same time, temperature also affects product selectivity to a certain extent. Ma et al. [26] found that Ni catalyze naphthalene hydrogenation at 473–553 K is more conducive to the formation of cis-decalin.

2.2. Kinetic Characteristics of PAH Hydrogenation

The hydrogenation kinetics of PAHs exhibit significant complexity, primarily due to the multiple-step sequential reaction pathways and complex intermediates involved in the heterogeneous catalytic process. Research in this field has long focused on revealing the microscopic nature of the reaction mechanisms and constructing accurate kinetic models.

2.2.1. Adsorption Process of PAHs

Aromatic molecules tend to adsorb on catalyst surfaces in a planar manner, following the π-complexation adsorption mechanism (as shown in Figure 3) [27]. It is known that the mechanism of liquid-phase hydrogenation of naphthalene on Pt- and Ni-catalysts is quite similar and includes three adsorption modes (π-, π/σ-, and σ-adsorption), of which two are associative and one is dissociative [28]. On transition metal surfaces, the π-electrons of aromatic molecules form σ-bonds with the metal’s d-orbitals. Simultaneously, the metal’s d-electrons interact with the π-antibonding orbitals of the aromatic molecules, creating d-π back-bonds. These interactions strengthen the adsorption and bonding of the aromatic molecules to the metal surface. During the stepwise hydrogenation of PAHs, the increase in resonance energy and steric hindrance makes adsorption and hydrogenation more challenging. By modulating the electron density of the active metal, the adsorption capacity of aromatic molecules on the catalyst surface can be enhanced, driving the reaction toward deep hydrogenation.

2.2.2. Reaction Network of PAH Hydrogenation

The reaction network of PAH hydrogenation is intricate, involving multiple intermediate steps. Huang and Kang [29] investigated the hydrogenation kinetics of naphthalene on a Pt/Al₂O₃ catalyst, revealing that the hydrogenation pathway proceeds via naphthalene → tetralin → decalin (cis and trans). No ring-opening cracking or isomerization products were observed, indicating that the reaction follows a relatively simple pathway (as depicted in Figure 4) [30]. Song [31] examined the hydrogenation of naphthalene on a Pd-Pt catalyst and found that the conversion of naphthalene to tetralin is reversible. However, the dehydrogenation rate of tetralin is extremely low, resulting in a quasi-first-order reaction. Additionally, cis-decalin, being thermodynamically unstable, can convert to trans-decalin under specific conditions. The hydrogenation of naphthalene typically occurs at temperatures around 250 °C and pressures of 4–8 MPa, with the product distribution being influenced by the type of catalyst, the content of active components, and the synthesis method used.

The hydrogenation of anthracene and phenanthrene also follows a stepwise hydrogenation pathway, but the specific reaction pathways and product distributions vary depending on the catalyst and reaction conditions. Korre [32] studied the hydrogenation pathway of anthracene on a sulfided CoMo/Al₂O₃ catalyst, finding that the hydrogenation products of anthracene mainly include dihydroanthracene, tetrahydroanthracene, symmetric octahydroanthracene (sym-OHA), asymmetric octahydroanthracene, and perhydroanthracene (as shown in Figure 4). The reaction pathway for phenanthrene hydrogenation is controversial. One pathway, represented by Korre, involves hydrogenation of phenanthrene on a sulfided CoMo/Al₂O₃ catalyst at 350 °C and 68.1 atm hydrogen partial pressure (as shown in Figure 4) [32]. Another pathway, represented by Beltramone, involves hydrogenation on a NiMo/Al₂O₃ catalyst at 345 °C and 70 atm hydrogen partial pressure (as shown in Figure 4) [33].

In the hydrogenation reaction system of PAHs, adsorption and activation are the core steps for initiating the reaction. By employing lumped kinetic models, the reaction pathways can be optimized at a macroscopic level, enhancing the selectivity of the hydrogenation process and precisely obtaining the desired products. A thorough analysis of thermodynamic and kinetic factors helps to clearly identify the main challenges in the hydrogenation saturation process, providing a solid foundation for optimizing reaction conditions and improving process efficiency.

2.3. Challenges in the Hydrogenation Saturation of PAHs

The hydrogenation of PAHs is simultaneously controlled by kinetics and thermodynamics, making the reaction process highly complex. Through a systematic analysis of existing research, the main challenges in the hydrogenation saturation of PAHs can be identified, providing directional guidance for subsequent research and promoting breakthroughs in both fundamental studies and industrial applications.

2.3.1. Challenges in PAH Hydrogenation Reactions

The hydrogenation of PAHs faces significant challenges due to their unique molecular structure and electronic properties. The presence of a highly conjugated π-electron system in PAHs provides considerable aromaticity and stability to the molecules, which directly makes hydrogenation more difficult. Korre [34] identified two essential rules governing PAH hydrogenation through systematic simulations. (i) Sequential Hydrogenation Based on Ring Saturation: Hydrogenation progresses sequentially, focusing on ring saturation one after another. (ii) Faster Hydrogenation of Terminal Rings: The hydrogenation rate of terminal rings is considerably higher than that of middle rings. In addition, Zhao et al. [35] found that atoms in aromatic rings with larger Sr values are more likely to accept H atoms, which can be used to preliminarily judge the hydrogenation order of each ring.

Research has shown that the resonance energy of PAHs is directly related to their hydrogenation difficulty. The resonance energy order of common aromatics is phenanthrene > anthracene > naphthalene. This suggests that the higher the resonance energy, the greater the conjugation in the aromatic system, which enhances the stability of the molecule and, consequently, makes hydrogenation more challenging. Furthermore, as hydrogenation proceeds, the resonance energy of the unsaturated rings increases, eventually approaching that of benzene rings, making further hydrogenation even more difficult [33].

Dutta and Schobert [36] emphasized that an increase in the number of aromatic rings exacerbates thermodynamic limitations. As the number of rings increases, the hydrogenation equilibrium constant decreases, particularly at elevated temperatures. Furthermore, steric hindrance during hydrogenation impedes the adsorption and activation of reactants on the catalyst surface, which further complicates the reaction [37]. Peng et al. [38] used density functional theory (DFT) to simulate the hydrogenation reaction path of phenylene on a Ni-Mo/HY catalyst. It was found that when the hydrogenation reaction reached the octahydrophenanthrene stage, the cracking reaction began to occur, and this competitive side reaction significantly inhibited the deep hydrogenation process of phenanthrene.

Competitive adsorption plays a crucial role in hydrogenation efficiency. Liu investigated the competitive adsorption and hydrogenation of naphthalene and anthracene, revealing that naphthalene (a bicyclic aromatic compound) exhibits lower hydrogenation activity compared to anthracene (a tricyclic aromatic compound) [22]. Furthermore, these two compounds demonstrated mutual inhibition. In a mixed hydrogenation system involving tetralin, naphthalene, and phenanthrene on a Pd-Pt/Al₂O₃ catalyst, the conversion rate of phenanthrene decreased by 25%. In contrast, the conversion rates of naphthalene and tetralin experienced significant reductions, with naphthalene’s conversion rate declining markedly and tetralin’s conversion rate plummeting by as much as tenfold [39].

In summary, the primary challenges in the hydrogenation of PAHs include the following: (i) increased reaction difficulty due to changes in resonance energy; (ii) adsorption challenges arising from steric hindrance, which affects the activation of reactants on the catalyst; and (iii) inhibition of terminal ring hydrogenation caused by competitive adsorption among the products. These factors combine to make the hydrogenation of PAHs a complex and challenging process [33,40,41].

2.3.2. Challenges in PAH Hydrogenation Catalysts

The process of hydrogenating PAHs is crucial in various industrial applications such as refining, petrochemical production, and environmental protection. The development of efficient catalysts is essential for achieving optimal hydrogenation while ensuring both high activity and selectivity, which continues to pose a significant challenge. The primary factors contributing to catalyst deactivation include catalyst poisoning, carbon deposition, and physical deactivation.

Poisoning deactivation occurs when impurities in the feedstock, such as sulfur and nitrogen compounds, interact with the active sites of the catalyst, resulting in irreversible damage. Despite pre-treatment processes to remove these impurities, such as hydrodesulfurization and denitrification, residual traces can still negatively impact catalyst performance over time. Precious metals like Pt, Pd, and Rh, which are commonly used in PAH hydrogenation, are particularly vulnerable to poisoning by these impurities. Consequently, developing catalysts with enhanced resistance to poisoning or methods to neutralize impurities before they reach the catalyst is a critical area of research.

Coking is a prevalent issue encountered during the hydrogenation of PAHs. The unsaturated intermediates produced during this process can undergo polymerization, resulting in the formation of carbonaceous deposits (coke) on the catalyst surface. This buildup of coke can block the active sites, reducing the catalyst, diminishing its effectiveness and leading to decreased activity. While coke can be removed through techniques like combustion regeneration, this process is energy-intensive, costly, and may cause the catalyst to degrade with each regeneration cycle. Over time, the catalyst’s performance deteriorates due to coke accumulation, which can lead to irreversible damage. To address coking, it is necessary to design catalysts with enhanced resistance to carbon formation and to develop more efficient regeneration strategies.

Physical deactivation can occur due to various factors, including mechanical wear, sublimation, and changes in the physical properties of the catalyst during operation. For instance, when a catalyst is subjected to extreme temperatures or pressure fluctuations, it may undergo structural changes that adversely affect its performance. The loss of active components can occur through attrition (wear and tear) or sublimation (the vaporization of metal species at high temperatures). Additionally, the formation of catalyst fragments or the disarray of the catalyst bed can significantly impair its functionality. Therefore, optimizing reaction conditions—such as temperature and pressure—and enhancing catalyst stability and robustness are crucial for minimizing physical deactivation.

The development of novel catalysts that exhibit high activity, selectivity, and resistance to deactivation mechanisms is crucial for advancing the hydrogenation of PAHs. By tackling the challenges of poisoning, coking, and physical deactivation, researchers are striving to create more efficient, durable, and selective catalytic processes. Continued advances in catalyst design, particularly for precious metal-based catalysts, along with a deeper understanding of reaction mechanisms, are expected to yield more sustainable and economically viable solutions for PAH hydrogenation in industrial applications.

3. Precious Metal-Catalyzed Hydrogenation of PAHs

Precious metal catalysts are widely used in aromatic hydrogenation due to their efficient hydrogen dissociation capabilities at low temperatures, excellent stability, and low activation energy for aromatic adsorption. In recent years, many academic studies have focused on applying these catalysts to PAHs.

3.1. Hydrogenation of Naphthalene

Naphthalene, as a typical bicyclic PAHs, serves as a model compound for studying the hydrogenation of PAHs. The reaction conditions and the acidity of the carrier have important effects on the reaction.

Naranov [42] used Ru-based catalysts to screen the reaction conditions for naphthalene hydrogenation (hydrogen pressure of 3–7 MPa, temperature of 220–320 °C, reaction time of 0.5–3 h) that best meet the kinetic and thermodynamic requirements of the reaction. Experimental data showed that within the 3–7 MPa hydrogen pressure range, the selectivity for decalin significantly increased with the rise in hydrogen pressure. Temperature studies indicated that the reaction system reached the peak yield of decalin at 240 °C. Under this temperature condition, the catalyst both ensured sufficient hydrogenation activity and effectively suppressed the occurrence of side reactions. Liu [43] prepared a series of mesoporous ZSM-5-x (x = Si/Al molar ratio) supports with different Si/Al ratios without using auxiliary templates and studied the Pt/HZSM-5-x samples prepared by conventional wet impregnation for naphthalene hydrogenation. The results showed that lowering the Si/Al ratio of the mesoporous ZSM-5 support promoted the dispersion of the supported Pt species. The catalytic efficiency of the Pt/HZSM-5-x samples for naphthalene hydrogenation depended on the dispersion of Pt, the acidity of the ZSM-5 support, and the combined effect of the mesoporous structure. To investigate the reasons behind this phenomenon, scholars employed XANES (X-ray Absorption Near Edge Structure) characterization. They discovered that when Pt is supported on different SiO₂/Al₂O₃ silica–alumina supports, the decrease in Pt electron density enhances its interaction with the reactants, thereby improving catalytic activity. Consequently, the dispersion of active metal and the electron-deficient state of the precious metal catalysts are often key factors influencing their hydrogenation activity [44,45].

Other researchers have improved the catalytic effect of catalysts by the changing common support and incorporating other metals. Wang [46] prepared a 5% Pd/Al₂O₃ catalyst by dipping method. The conversion rate of naphthalene was 92.04% and the yield of decaline was 44.42%. In the study by Cuauhtemoc [47], the Rh/Fe₂O₃-TiO₂ catalyst was tested for the hydrogenation of naphthalene in a batch reactor with a hexadecane medium. Compared to Fe₂O₃-TiO₂, the catalyst increased the conversion rate from 80% to 100% within 1 h, indicating that the presence of Rh promoted the conversion of naphthalene. Tungsten trioxide (WO₃) is an n-type semiconductor with the unique advantage of maintaining a stable physical phase structure even in the presence of oxygen vacancies in its lattice. When used as a support for Pt, the oxygen vacancies promote the interaction between the support and the precious metal, as well as the rapid hydrogen desorption, resulting in high catalytic hydrogenation activity [48]. Zhao [49] found that WO₃-500, which contains a higher amount of W⁵⁺ species, exhibited stronger interaction with Pt. The interaction between W and Pt led to the formation of a catalyst with higher acidity and a greater number of divalent Pt²⁺ species on the surface of WO₃. The acidic sites and Pt²⁺ species have a strong adsorption effect on the electron-rich naphthalene ring, promoting excellent catalytic performance for naphthalene hydrogenation to decalin at low temperatures.

Table 2. Conditions and results of hydrogenation of naphthalene catalyzed by precious metals.

Catalyst	Reaction Feed	Reactor	T (°C)	H2/(MPa)	t(h) or WHSV 1	Con 2 (%)	SP.sele 3 (%)
Pt-Pd/Al2O3-CeO2 [50]	cetane	High-pressure reactor	290	5.5	2	90.7	25.2
Pd/Al2O3 [46]	cetane		200	4	8	92.04	44.42
Pd/HY-a [51]	tridecane		200	4	1	99.13	38.08
0.5% Pd-α-MoC [52]	n-heptane	Fixed bed	100	4	2	100	63
Pt/WO3-α-Al2O3 [53]	n-acontane		70	1	3	100	100
1,0%Pd-0, 3%Pt/30%Al2O3 -70%SAPO-11 [54]	n-hexane		320	4	12	100	90
0,25%Pt/HZSM-5-25 [43]	cyclohexane	High-pressure reactor	250	5	1	99.7	65.6
Pt/Hβ-2 [55]	cyclohexane	Fixed bed	220	4	10	94.2	81.4
Pt/Hβ-75 [56]	cyclohexane	Fixed bed	220	4	10	96.7	76.7

¹ WHSV: Weight Space Hourly Velocity. ² The abbreviation “Con” stands for “conversion”. ³ The abbreviation “SP.sele” stands for “saturated product selectivity”.

summarizes the performance of different precious metal catalysts in naphthalene hydrogenation reactions.

3.2. Hydrogenation of Anthracene

Anthracene, as a typical polycyclic aromatic hydrocarbon, plays an important role in the hydrogenation of PAHs. To achieve high activity and high selectivity, researchers are primarily focusing on two aspects; one is the optimization of catalyst structure design and synthesis methods, the other is the regulation of the electronic structure and synergistic effects of the active components.

In terms of structure design and synthesis optimization, metal–support interface engineering and nanoparticle morphology control have shown significant advantages. Chen [57] developed a Pt-based catalyst supported on SiO₂, which significantly enhanced the hydrogenation reaction of anthracene by forming Pt₂Si and Pt-SiO₂ interface structures, achieving a high selectivity of 87% for sym-OHA. Kang [58] controlled the morphology of Rh nanoparticles (NPs) by altering the precursor type and preparation method. The results indicated that under the same reaction conditions, the conversion rates of tetrahedral Rh NPs catalyst, spherical Rh NPs catalyst, and conventional Rh/C catalyst were 100%, 99%, and 4.8%, respectively, while the selectivity for sym-OHA was 98%, 17%, and 18.8%, respectively. The tetrahedral Rh NPs catalyst exhibited a clear advantage. Jacinto [59] synthesized the Fe₃O₄@mSiO₂@Rh catalyst, which demonstrated excellent cyclic stability in the hydrogenation of anthracene. However, due to insufficient process parameter control, the catalyst prepared by this method primarily generates intermediate hydrogenated products during the reaction, leading to a limited saturation of the target product. The strong electrostatic adsorption (SEA) method is also an effective way to enhance activity. Compared to catalysts synthesized by traditional impregnation methods, Pt/Al₂O₃-SEA has smaller active metal particle sizes and better dispersion, which exposes more active sites and results in higher catalytic activity. Research has found that the Pt/Al₂O₃-SEA catalyst exhibits high selectivity for sym-O-OHA (93%), and the conversion rate of anthracene is close to 100% [60].

In terms of active component regulation, metal electronic state modification and multi-component synergistic effect have become the key breakthroughs. Wang [61] utilized HY as a support and loaded different amounts of Pt metal, systematically evaluating the anthracene hydrogenation reaction. The experimental results showed that when the Pt loading was 0.5 wt%, the yield of fully hydrogenated anthracene reached 73.86%. This finding strongly demonstrates that an appropriate Pt particle size in the catalyst has a high ability to activate hydrogen. Bai [62] introduced metals such as Zr, La, and Au into Pd/Al₂O₃. The study found that under the influence of La, the electron cloud density of Pd⁰ on the catalyst decreased, making it more electron deficient. This change in electronic structure effectively inhibited the detachment of intermediate products from the active metal surface, significantly improving the selectivity for perhydroanthracene. At 275 °C, the selectivity for perhydroanthracene reached 89%. Zhang [63] synthesized PdZn alloy catalysts using the strong interaction between the active metal Pd and the support (ZnO) for the hydrogenation of anthracene to symmetric sym-OHA. It was found that the PdZn alloy catalyst, reduced at 400 °C, exhibited 100% anthracene conversion and 84.77% selectivity under optimized conditions (200 °C, 1 MPa).

Table 3. Conditions and results of anthracene hydrogenation catalyzed by precious metals.

Catalyst	Reaction Feed	Reactor	T (°C)	H2/(MPa)	t(h) or WHSV 1	Con 2 (%)	SP.sele 3 (%)
Pt/SiO2 [57]	decalin	High-pressure reactor	250	5	10	100	22.4
Pt-Pd/AC [64]	cyclohexane		200	1	2	100	10.4
			320	1	3	100	100
Pd/Al2O3 [65]	tridecane		250	4	4	99.03	5.16
Pt/ASU-3.9 [66]	n-decane		240	4	4	100	98.9
Pd/MB-2000-LTA [67]	-	Reactor	300	3	1	99	8.3
Pt/HY-O3 [68]	n-decane	Reactor	130	3	1.5	100	99.6

¹ WHSV: Weight Space Hourly Velocity. ² The abbreviation “Con” stands for “conversion”. ³ The abbreviation “SP.sele” stands for “saturated product selectivity”.

summarizes the performance of different precious metal catalysts in anthracene hydrogenation reactions.

3.3. Hydrogenation of Phenanthrene

It is known that there are high levels of PAHs in coal tar, among which phenanthrene (PHE) accounts for about 4–6 wt% [69]. It is very important to study the hydrogenation of phenanthrene. Some scholars took phenanthrene as a model compound and investigated the thermodynamic equilibrium composition under different reaction conditions through HSC Chemistry software 4.0 [70]. It has been found that the hydrogenation saturation of phenanthrene to fully hydrogenated phenanthrene is thermodynamically feasible, with octahydrophenanthrene being the main by-product. Niu [71] prepared a Pt/HZ-SEA catalyst using the strong electrostatic adsorption method, achieving nearly 100% phenanthrene conversion and a combined selectivity of nearly 100% for octahydrophenanthrene and perhydrophenanthrene under conditions of 220 °C and 4.0 MPa. Niu [72] found that Pt/TiO₂ and Pt/CeO₂ catalysts exhibited enhanced hydrogen spillover due to strong metal–support interactions (EMSI), resulting in well-dispersed Pt^δ⁺ species. Among these, Pt/CeO₂ demonstrated the best performance. Liu [73] and colleagues investigated the effect of Pt loading (0%, 0.25%, 0.5%, 0.75% by mass) on the Ni/NiAlO_x catalyst during the hydrogenation of phenanthrene. The experiments revealed that at a Pt loading of 0.5%, the 0.5Pt/Ni/NiAlO_x catalyst demonstrated optimal performance, achieving a phenanthrene conversion rate of 96% and an increased selectivity for perhydrophenanthrene of 67%. Jing [74] prepared a catalyst in the patent (Ni/NiAl spinel catalyst: Palladium: boron = 100:0.003:1) and hydrogenated phenylene at 300 °C and 5.0 MPa of H₂. After 6 h, the conversion rate of phenanthrene reached 99.5%, with a selectivity for perhydrophenanthrene of 99.2%.

Compared to precious metal catalysts, non-precious metal catalysts are more commonly employed in the hydrogenation reactions of phenanthrene. In recent years, nickel-based catalysts have gained widespread use. Li et al., from Taiyuan University of Technology, investigated the application and reaction mechanisms of various nickel-based catalysts in phenanthrene hydrogenation [18,35,75,76]. The application of precious metal catalysts in phenanthrene hydrogenation needs to be strengthened.

Table 4. Conditions and results of phenanthrene hydrogenation catalyzed by precious metals.

Catalyst	Reaction Feed	Reactor	T (°C)	H2/(MPa)	t(h) or WHSV 1	Con 2 (%)	SP.sele 3 (%)
Pt/CeO2 [72]	decalin	Fixed bed	260	4	10.8	100	100
Pt/HZSM5 [71]	decalin		280	4	16.2	100	100
0.5Pt-Ni/NiAlOx [73]	decalin		300	5	52	96	67

¹ WHSV: Weight Space Hourly Velocity. ² The abbreviation “Con” stands for “conversion”. ³ The abbreviation “SP.sele” stands for “saturated product selectivity”.

summarizes the performance of different precious metal catalysts in phenanthrene hydrogenation reactions.

3.4. Hydrogenation of Other Polycyclic Aromatic Hydrocarbons

Methylnaphthalene is mainly used as an analytical reagent for determining the cetane number of fuels. It can also be used as a high-boiling solvent and reagent for the production of alpha-naphthylcarboxylic acid and other organic substances [77]. In the work of Masalska et al. [78], it was found that the conversion of 1-methylnaphthalene on Pt-containing catalysts was not affected by the aluminum precursor used in the preparation of Pt/AlSBA-15 catalysts. Aleksandra et al. [79] synthesized RuNi (8 wt.% NiO; 1.1 wt.% RuO₂) catalysts on ZSM-5 + Al₂O₃ and tested them in the hydrogenation reaction of 1-methylnaphthalene. The study found that the RuNi/D catalyst, using ammonium hexachlororuthenate as the precursor, achieved a 92.5% conversion of 1-methylnaphthalene at 200 °C and atmospheric pressure.

Methylnaphthalene has applications in medicine and materials science. For example, 2-methyl-1,4-naphthoquinone (vitamin K3) plays an important role in blood clotting due to its anti-bleeding properties [80]. Gonzalez et al. [81] investigated the use of supported Pt catalysts on mordenite (Pt/HMOR) in the 2-methylnaphthalene hydrogenation. It was found that mainly methyltetralins undergo hydrogenation, with 5-methyltetralin present in the largest amount among the products. The yield of 2-methyldecalins was very low (<1%).

Acenaphthene, derived from coal tar, has an average content of 1.2% to 1.5% in wash oil. Through hydrogenation, tetrahydroacenaphthene and perhydroacenaphthene can be saturated to obtain low-toxic products. These hydrogenation products have wide applications in fields such as polymers, pharmaceuticals, and fuels. Guo et al. [82] systematically investigated the key role of the support effect of ruthenium-based catalysts in the hydrogenation of anthracene. By comparing the performance differences of Ru/TiO₂, Ru/CeO₂, and Ru/SiO₂ catalytic systems, it was found that the catalyst supported on rutile-type TiO₂ exhibited significant advantages.

Fluorene mainly originates from high-temperature coal tar wash oil. Minabe et al. [83] used decalin as a solvent and studied the hydrogenation of fluorene in Pd/Al₂O₃, Rh/Al₂O₃, and Pt/Al₂O₃ catalytic systems under conditions of 200 °C and 7 MPa H₂. The results showed that in the Pd/Al₂O₃ system, fluorene was efficiently converted to hexahydrofluorene, with a conversion rate of 71%. In the Rh/Al₂O₃ system, the main product was dodecahydrofluorene, accounting for 72%. In contrast, when Pt/Al₂O₃ was used as the catalyst, the conversion of fluorene was very low (2.7%).

As a typical representative of four-ring PAHs, the catalytic hydrogenation of pyrene (Pyrene) shows significant differences from its homologs. Currently, catalytic hydrogenation systems for three-ring PAHs, represented by naphthalene, anthracene, and phenanthrene, have been relatively well-established. However, the catalytic hydrogenation of the four-ring polycyclic aromatic hydrocarbon pyrene is still in a relatively lagging state. Liang loaded Pt onto W-TiO₂ and SiO₂-Al₂O₃ supports and found that pyrene could adsorb onto the acidic sites of the support and undergo hydrogenation reactions with the hydrogen overflowed onto the support. The stronger the acidity of the support, the better the hydrogenation effect of pyrene [84]. Tang et al. [85] prepared a Pd/Beta-H catalyst and used it for the catalytic hydrogenation of pyrene. The experiments showed that the activity of the Pd/Beta-H catalyst was higher than that of Pd/Beta, Pd/Al-MCM-41, and Pd/γ-Al₂O₃ catalysts. The larger mesopore volume was considered to be beneficial for the adsorption and mass transfer of pyrene on the Pd/Beta-H catalyst.

The studies above clearly demonstrate that precious metal catalysts exhibit exceptional activity and selectivity in the hydrogenation of PAHs. However, further optimization is needed, particularly in terms of sulfur resistance and the control of product stereoisomerism. Future research should concentrate on the synergistic optimization of catalyst design, reaction mechanisms, and process conditions to facilitate the industrial-scale application of PAH hydrogenation technology.

4. Sulfur Resistance of Precious Metal Catalysts for PAHs in Coal Tar

Although precious metal catalysts exhibit high catalytic activity, they are highly sensitive to sulfur. Coal tar contains a significant amount of sulfur-containing compounds in PAHs, which presents a major challenge for the use of precious metal catalysts. Research has shown that when dibenzothiophene is used as an additive and doped into deep desulfurized oil at various concentrations, and catalytic hydrogenation is performed with both transition metal sulfide catalysts and precious metal catalysts, the precious metal catalysts experience poisoning and deactivation. This is because their surfaces interact with sulfur compounds, which have a strong adsorption capacity, leading to hydrodesulfurization reactions. Even at very low concentrations of dibenzothiophene (less than 1 ppm), poisoning and deactivation can still occur [86]. As a result, enhancing the sulfur resistance of precious metal catalysts has become a key research area.

4.1. Mechanism of Sulfur Poisoning

There are two main types of catalyst poisoning caused by sulfur-containing compounds. The first type is physical adsorption poisoning, where sulfur-containing compounds strongly adsorb onto the catalyst’s active sites, tightly binding with them and thereby blocking the active sites from performing their catalytic functions. The second type is chemical poisoning, where sulfur atoms react chemically with the metal on the catalyst surface, forming stable metal–sulfur bonds.

The alleviation of sulfur poisoning effects can be achieved by adjusting the physical and chemical environment of the catalyst surface. The mechanisms involved mainly include two approaches. The first is based on electronic structure modulation (such as the “electron-deficient” state theory) to change the chemical characteristics of the active sites [87]; the second is through physical isolation strategies to limit the contact between sulfur species and the active centers. The specific implementation paths include the following: (i) constructing sulfur-resistant alloy structures through electronic transfer effects, (ii) selecting carrier materials with suitable acidity and optimizing sulfur tolerance by adjusting the distribution of surface acidic sites, and (iii) utilizing the shape-selective effect of zeolite molecular sieves to effectively block the diffusion path of sulfides [88,89,90,91] (as shown in Figure 5).

4.2. Electron Deficiency Theory

The theory of “electron-deficient” state plays an important guiding role in the research of sulfur-resistant materials. The application of this theory can be seen in two strategies. One is to adjust the electron density on the metal surface to form sulfur-resistant alloys and the other is to select suitable acidic support materials.

4.2.1. Electron Transfer Forms Alloys Resistant to Sulfur

Among the various strategies for enhancing the sulfur resistance of catalysts, alloying is an extremely effective and widely applied method. When suitable interactions are formed between metal components, it can significantly improve the catalyst’s sulfur resistance.

Pt-Pd alloys are widely used in the hydrogenation of PAHs. Early studies focused on the optimal ratio of Pt to Pd, with findings showing that the sulfur resistance of the catalyst reaches its maximum when the Pd/Pt ratio is 4 [92,93,94]. This discovery, from the perspective of different catalyst systems, provided new insights and data support for the study of the relationship between sulfur resistance and metal ratios. Nowadays, Pt-Pd bimetallic sulfur resistance is still widely used.

Lin [95] took a different approach by studying the CO absorption infrared spectra of Pt-Pd/γ-Al₂O₃ catalysts, where they found that Pt exhibited a positive ion character. Their research showed that by forming a Pt phase with a positive ionic character through Ptx^δ⁺-xPd^δ⁻, this unique structure could effectively prevent sulfur poisoning, revealing the intrinsic mechanism of sulfur resistance from the perspectives of microstructure and chemical properties. Shirokopoyas [96] studied the sulfur resistance of Pt-Pd/Al-SBA-15 catalyst, which converted up to 85% dimethylnaphthalene at 400 ppmDBT. Zhang [97] further expanded this research by preparing Pd₄Pt₁/SiO₂-Al₂O₃ catalysts for naphthalene hydrogenation. The experimental results demonstrated that, after the addition of 500 ppm dibenzothiophene, the catalyst’s conversion rate decreased by less than 10%, fully proving the excellent sulfur resistance of the catalyst and showcasing its tremendous potential for practical applications. Fierro et al. [98] prepared a γ-zirconium phosphate–SiO₂ (ZPS) support using the sol–gel method and compared the performance of PtPd/ZPS with Li-PtPd/ZPS in the catalytic hydrogenation of naphthalene. The results showed that in the presence of dibenzothiophene, Li-PtPd/ZPS exhibited better sulfur tolerance performance. This is because the addition of Li reduced the binding energy of Pt 4f₇/₂ and Pd 3d₅/₂, making it more difficult for the metal to coordinate with thiophene, thereby enhancing its sulfur resistance.

In recent years, many studies have been carried out on the Pt-Pd alloy catalyst system. For example, by introducing a third component (e.g., Pt-Pd-Ni ternary alloy) or constructing polymetallic high-entropy alloys (e.g., PtPdCoCuNi system), significant progress has been made in optimizing intermetallic electronic structures, regulating exposure to active sites, and enhancing sulfur tolerance [99,100]. Although bimetallic catalysts such as PdAu and PdRu show excellent catalytic activity in hydrogenation reactions [101,102], their application in sulfur-resistant catalytic systems has not been fully developed, and systematic research is needed to expand their application boundaries. In addition, research on the sulfur resistance of Pt-Pd alloys has not only made progress in the field of hydrogenation but also has a wide range of applications in combustion oxidation [103,104].

4.2.2. Choose a Suitable Acid Carrier to Resist Sulfur

Acidic supports can significantly enhance the sulfur resistance of catalysts. This phenomenon is mainly attributed to two key mechanisms. Firstly, the metal centers highly dispersed on the surface of the acidic support tend to have an electron-deficient state. This electron deficiency significantly weakens the metal–sulfur (M-S) bond strength, thereby inhibiting the adsorption of sulfur compounds on the catalyst surface and effectively improving the catalyst’s sulfur resistance. Secondly, the hydrogen spillover effect generates new hydrogenation active sites at the interface between the metal and acidic sites. The spilled hydrogen atoms react with the adsorbed aromatics, which significantly enhances the hydrogenation activity of the catalyst. The synergistic effect of these two mechanisms promotes the high efficiency and sulfur resistance stability of the catalyst in hydrogenation reactions.

Vannice [105] found that the hydrogenation desulfurization performance of Pd or Pt catalysts supported on acidic carriers is notably improved. Based on kinetic studies, they proposed that new hydrogenation active sites are formed in the interface region of the metal center. This discovery provides important insight into understanding the impact of acidic carriers on catalyst performance. Yasuda et al. [106] further prepared USY zeolites with different SiO₂/Al₂O₃ ratios (SiO₂/Al₂O₃ = 5.6–680) via an acid leaching method and studied the effect of carrier acidity on the desulfurization performance of precious metal catalysts using tetrahydronaphthalene hydrogenation as a model reaction. The study showed that the catalyst’s activity and desulfurization resistance are closely related to the SiO₂/Al₂O₃ ratio of the zeolite. When the SiO₂/Al₂O₃ ratio is between 15.0 and 40, the catalyst’s desulfurization performance reaches its best, and then it gradually decreases. Notably, stronger carrier acidity does not necessarily result in better catalyst activity and desulfurization resistance. Wang and Simon et al. [105,107] further pointed out that there is a subtle balance between the acidity site concentration of the carrier and the precious metal content. For a specific loading of precious metal catalysts, there exists an optimal acidity site concentration. When the acidity site concentration is too high, it promotes the formation of more cracking products during the catalytic reaction, leading to a decrease in the oil yield. This finding highlights the importance of optimizing the balance between carrier acidity and metal loading in catalyst design and provides theoretical guidance for the development of efficient desulfurization catalysts.

4.3. Catalyst Support Channel Shape Selection to Improve Sulfur Resistance

Zeolite micropores can selectively sieve molecules during the catalytic process, enabling shape-selective catalysis. Since metal nanoparticles are located within the zeolite framework, the zeolite micropores selectively allow molecules with diameters smaller than the pore size to diffuse while hindering larger molecules from approaching the metal sites. This is known as the shape-selective catalysis of zeolites [108]. Song et al. [109,110] proposed a novel catalyst design strategy based on the differences in shape-selective effects, hydrogen spillover effects, and sulfur poisoning in zeolites. Specifically, the precious metal particles exhibit a bimodal distribution within the zeolite pores, with different pore systems being either interconnected or evenly distributed. Thiophene organosulfides are unable to enter the smaller pores (6 Å) due to the shape-selective repulsion of the zeolite structure. Meanwhile, metal sites in the macropores, which are typically deactivated by sulfur poisoning, can be reactivated through the hydrogen spillover effect (as shown in Figure 6). Song et al. [111] continued the concept, designed a sulfur-resistant Pd-containing catalyst, and found that it has excellent sulfur resistance.

Fang et al. [112] constructed a zeolite molecular sieve with double micropores and mesoporous structures as the carrier of precious metal catalyst, and made full use of its shape selection effect and hydrogen overflow characteristics to further improve the sulfur resistance of the catalyst. They loaded Pt onto ZSM-5 with a mesoporous structure (Pt/MZ-5) and compared it with microporous Pt/ZSM-5 and Pt/Al₂O₃. They found that Pt/MZ-5 not only exhibited higher catalytic activity in the naphthalene hydrogenation reaction but also showed excellent sulfur resistance when dibenzothiophene was present as the organic sulfur source. This result is highly consistent with the dual-pore model catalyst design proposed by Song [110].

Based on Song’s idea of designing catalysts, mesoporous composites such as MCM-41, Y, Beta, and ZSM-5 have also been designed [113,114,115]. Yang et al. [116] encapsulated Pt in KA zeolite, and after treatment with 5% H₂S, the hydrogen adsorption capacity remained at 60%, proving that the encapsulation structure effectively protected Pt from sulfur poisoning [117]. Gao et al. prepared a Pt/SOD-H catalyst that maintained a stable benzene conversion rate of 68.2% in the presence of 5.02% H₂S. In the sulfur resistance test of Pt/MOR-PMOs-100@MSNs synthesized by Wang et al. [118] at 300 °C for 9 h, the hydrogenation conversion of naphthalene was 84.9% and the desulfurization rate of DBT was 91.5%. Ju et al. [119] successfully synthesized three different types of catalysts by differentiating the metal state: Pt/Meso-KA-i (metal encapsulated inside), Pt/Meso-KA-i+s (50% encapsulated, 50% impregnated), and Pt/Meso-KA-s (impregnated). Among these, Pt/Meso-KA-i+s showed the best performance, with naphthalene conversion dropping from 90% to 80% under 3% H₂S and 3000 ppm DBT conditions.

Figure 6. Shape selection of support channels. (I) Insulate active metals and sulfides from contact with sulfur resistance [120]. (II,III) Metal double modes are distributed in zeolite channels [110,119].

Figure 6. Shape selection of support channels. (I) Insulate active metals and sulfides from contact with sulfur resistance [120]. (II,III) Metal double modes are distributed in zeolite channels [110,119].

In summary, the sulfur sensitivity of precious metal catalysts significantly limits their application in processing sulfur-rich coal tar. However, through in-depth research on the sulfur poisoning mechanism and the development of corresponding countermeasures, substantial progress has been made. Alloying, by carefully adjusting the metal ratios in alloys, optimizes the interaction between metal components, thereby enhancing the catalyst’s resistance to sulfur. The selection of an appropriate acidic support not only weakens the metal–sulfur bond but also fosters the creation of new hydrogenation active sites, achieving a balance between catalytic activity and sulfur tolerance. Moreover, the construction of encapsulation structures offers a physical isolation approach, protecting precious metal particles from sulfur-containing compounds and preserving their catalytic performance. These strategies, while differing in their methods, all contribute to the overarching goal of improving the sulfur resistance of precious metal catalysts. Looking ahead, ongoing research in these areas, along with the exploration of novel techniques, holds great potential for the development of highly efficient, sulfur-resistant precious metal catalysts. Such catalysts would be crucial for the clean and efficient utilization of sulfur-rich resources. They would drive the advancement of industries like coal chemical engineering and refining in a more sustainable and environmentally friendly direction.

5. Cis–Trans Isomerization in Precious Metal-Catalyzed Hydrogenation of PAHs

Converting coal tar into high-value clean fuel is the main utilization method of coal tar. The products of saturated hydrogenation of PAHs in coal tar have the characteristics of high density, high calorific value, excellent thermal oxidation stability and low temperature performance [5,6]. However, the stereoisomerism of the product has a significant effect on the fuel properties. Among these, the cis-isomers, due to their higher density and volumetric calorific value, are ideal components for high-energy-density aviation fuels. For example, the calorific value of cis-decalin (38.3 MJ/m³), a hydrogenation product of naphthalene, is significantly higher than that of trans-decalin (37.2 MJ/m³) [121]. The importance of controlling the stereoisomerism of polycyclic aromatic hydrocarbon hydrogenation products for obtaining high-quality clean energy is self-evident. The stereo-isomerization of the fully hydrogenated products of PAHs hydrogenation is shown in Figure 7.

Rautanen [122] performed thermodynamic Gibbs free energy calculations using the ad FLOWBAT program and found that the trans isomer of decalin accounts for more than 90% in the equilibrium system. However, in practical hydrogenation reactions, the amount of cis-decalin produced often exceeds the thermodynamically predicted value of 10%. Based on this significant discrepancy between theory and experiment, the researchers proposed the hypothesis that the cis–trans isomer ratio is primarily determined by the kinetic pathway, rather than being constrained by thermodynamic equilibrium. This hypothesis was validated by Schmitz [123], whose research showed that the isomer ratio is strongly correlated with the type of metal and the properties of the support in the catalytic system.

Schmitz [123] has systematically studied the configurational selectivity of different precious metal catalysts; Pt-based catalysts have high selectivity for cis-decalin, while Pd-based catalysts tend to produce trans-decalin. This finding provides an important theoretical basis for the directional control of isomer selectivity in catalysts.

Huang et al. [124] studied the catalytic performance of Pt loaded on alumino-phosphate molecular sieves for hydrogenation reactions. They found that when the Al/P ratio was between 6 and 10, the catalyst exhibited higher hydrogenation activity and selectivity for the trans-decalin product. This was attributed to the fact that the acidity of the alumino-phosphate molecular sieve was stronger than that of the γ-Al₂O₃ support, which enhanced the electron deficiency of the active component, Pt, thereby improving its catalytic activity. Niu [125] demonstrated that a 0.5 wt% Ru/C catalyst achieved 83.6% selectivity for cis-decalin during naphthalene hydrogenation at 180 °C. Guo found that a Ru/TiO₂ catalysts exhibited 100% selectivity for cis-decalin, highlighting the importance of the metal–support interaction, which contributes to Ru’s exceptional cis-selectivity [82]. Additionally, Norihito [126] achieved 81% selectivity for cis-decalin using a Rh/C catalyst in a supercritical CO₂ medium.

Some scholars have found that the selectivity of PAHs hydrogenation products is also affected by the size of active metal particles. The Ni/Al catalyst prepared by nickel carbonate has a larger particle size than that prepared by nickel nitrate and has a higher selectivity to cis-decalin [127]. The smaller the mean particle size of the Ni/Si MCM-41 catalyst, the higher the reaction rate constant to decahydride naphthalene [128]. More importantly, the selectivity of NiO with a particle size of 3.5 nm and Ni with a particle size of 4.2 nm to cis-decalin is 62.1% and 61.1%, respectively, while the selectivity of the Pt catalyst with a particle size of 5.5 nm (smaller than the above Ni/NiO particles) to cis-decalin is only 40% [129,130,131]. However, Liang et al. [132] found that the size-dependent inverse relationship of Pt particles between 1 nm and 3.5 nm can be well maintained, which proves that the variation in the size of Pt nanoparticles does not affect the selectivity of decahydronaphthalene. Therefore, the size of the particle has no decisive effect on cis-trans isomerization.

Table 5. Study on stereo-isomerization of PAHs products catalyzed by precious metals.

Catalyst	Reaction Feed	Reactor	T (°C)	H2/(MPa)	t(h) or WHSV 1	Con 2 (%)	SP.sele 3 (%)	Cis–Trans
Pd-0.3P/HY [133]	Naphthalene–tridecane	High-pressure reactor	200	4	1	99.58	94.17	0.49
Pd (OAc)2 [134]			200	4	1	100	78	0.393
Ru/TiO2 [82]	Acenaphthene–decalin		200	4	1	100	100	0.637
0.8 wt % Pt/NA-HSN-4 [135]	Naphthalene– cyclohexane	Fixed bed	260	4	10/h	100	96.6	≈0.55
Pt-Ni/AC [136]	Tetralin	High-pressure reactor	100	6	0.5	99.1	98.6	2.9
Pt/AC [136]			100	6	0.5	88.9	93.5	3.2

¹ WHSV: Weight Space Hourly Velocity. ² The abbreviation “Con” stands for “conversion”. ³ The abbreviation “SP.sele” stands for “saturated product selectivity”.

summarizes the performance of different precious metal catalysts in phenanthrene hydrogenation reactions. Table 5 summarizes the studies on the stereo-isomerization of hydrogenation products of PAHs catalyzed by different precious metal catalysts.

6. Future Perspectives

In the context of global energy transition and the “dual carbon” strategy, the efficient and clean utilization of coal resources has become a key issue. The hydrogenation of PAHs rich in coal tar to prepare high-value-added products provides an important approach for the deep development of coal resources. Research in this field has significant practical significance and strategic value. Although certain achievements have been made in the research on hydrogenation of coal-based PAHs using precious metal catalysts, clarifying the thermodynamic and kinetic characteristics of the reactions, constructing typical reaction networks, and making breakthroughs in sulfur resistance and cis–trans isomer studies, there are still many challenges. The hydrogenation reaction of PAHs is inherently constrained by thermodynamic and kinetic factors, and its complex electronic structure, resonance energy effects, and steric hindrance significantly increase the difficulty of hydrogenation. At the same time, precious metal catalysts are easily affected by sulfur poisoning, and it is challenging to achieve an ideal balance between activity and selectivity, as well as to control the cis–trans isomer products. Therefore, future research should continue to explore the reaction mechanisms of PAH hydrogenation, the development of new catalysts, and the optimization of reaction processes to promote the further development and application of PAH hydrogenation. To enhance precious metal-catalyzed hydrogenation of PAHs in the future, we propose the following research ideas and prospects.

The research and application prospects of precious metal-catalyzed hydrogenation of PAHs in coal tar are entering a critical stage of development. With its unique advantages in heavy oil upgrading, pollutant reduction, and the synthesis of high-value-added products, this technology has become a frontier focus in the energy and chemical industries. This paper systematically outlines the future development pathways of this technology from three dimensions: technological breakthroughs, process innovations, and industrial collaboration (as shown in Figure 8).

6.1. Technological Breakthroughs: Multi-Scale Innovation of Catalyst Systems

Future research should focus on developing new catalytic systems to address the limitations of existing ones. A promising direction is to explore the synergistic effects of non-traditional precious metals, such as iridium, ruthenium, and their alloys. This approach can be combined with acidic supports to enhance the hydrogenation capability, sulfur resistance, and selectivity of PAH hydrogenation. Additionally, the development of multifunctional supports, such as metal–organic frameworks (MOFs) or gradient pore materials, could optimize the hydrogenation process by improving the dispersion of active sites, enhancing sulfur tolerance, and promoting the hydrogen spillover effect. Moreover, drawing from the “lock-and-key” recognition mechanism in enzyme catalysis, researchers have designed gradient acid sites and hydrophobic microregions on catalyst surfaces, achieving directional adsorption of PAH molecules and precise control over hydrogenation pathways.

6.2. Exploring Reaction Mechanisms and Process Optimization Through Artificial Intelligence Applications

Artificial intelligence (AI), as a data-driven innovative tool, is opening new paradigms for the optimization of PAH hydrogenation reactions. Based on experimental data and employing multi-scale and multi-technology integration methods, combining machine learning, density functional theory (DFT) calculations, and chemical process simulations, this line of research systematically analyzes the micro-mechanisms and macro-process optimization paths of catalytic hydrogenation of PAHs in real complex systems. By simulating catalyst structures through DFT calculations, comparing the adsorption energies, reaction pathways, and reaction barriers of coal tar components, and leveraging machine learning to rapidly process and analyze large amounts of data, predictions can be made regarding catalyst performance, reaction pathways, and conditions, providing theoretical guidance for catalyst design, process optimization, and enhancing the saturation of PAHs. By integrating AI technologies, traditional trial-and-error limitations can be transcended, reducing development cycles for catalysts, and providing innovative solutions for the high-value utilization of PAHs in coal tar.

6.3. Industrial Application and Market Expansion

Precious metal catalysts used for the hydrogenation of PAHs in coal tar have transformative potential for industrial applications and market expansion. As the international community increases its demand for emission reductions and sustainable aviation fuels, the potential market for synthetic aviation fuel is enormous. This high-value aviation fuel can significantly reduce the carbon footprint of air transportation, aligning with global environmental protection and sustainable development trends. Through the development of innovative technologies and methods, coal tar can be converted into low-freezing-point synthetic aviation fuels (SAF), ultra-pure cycloalkanes, liquid organic hydrogen carriers (LOHC), and other high-value products. This helps to capture a share of the trillion-dollar market for aviation fuels and specialty chemicals. Ultimately, the successful implementation of this technology will facilitate the transition of coal resources from traditional fuels to high-value chemicals. Future research and development can then establish an industrial chain for processing products.

7. Conclusions

This paper systematically outlines the study of the characteristics, reactions of PAHs, sulfur resistance, and the regulation of stereo-isomerization. Based on the reaction characteristics, the challenges associated with the saturated hydrogenation of PAHs are as follows. (i) The difficulty of the reaction increases due to changes in resonance energy. (ii) Steric hindrance affects the activation of reactants on the catalyst, thereby posing a challenge to adsorption. (iii) Competitive adsorption between products inhibits end-ring hydrogenation. Additionally, it has been observed that precious metal catalysts can become deactivated due to factors such as poisoning, carbon accumulation, and physical reasons. The challenges of the hydrogenation saturation reaction of thickened aromatic hydrocarbons are discussed from two perspectives. At present, many researchers have conducted a variety of experiments on this reaction and have classified typical thick cyclic aromatic hydrocarbons, such as naphthalene, anthracene, phenanthrene, and others. From the summary of the four parts, it is evident that precious metal catalysts are more commonly used in the hydrogenation reaction of naphthalene; however, the application of anthracene, phenanthrene, and other tricyclic aromatic hydrocarbons is less prevalent. In the future, research should place greater emphasis on tricyclic aromatic hydrocarbons.

In light of the characteristics of precious metal catalysts in environments with extremely low sulfur content, there are two primary methods for enhancing the sulfur resistance of precious metals: electron deficiency theory and physical isolation. The electron deficiency theory encompasses two main approaches: the formation of sulfur-resistant alloys and the selection of carriers with appropriate acidity. Physical isolation primarily involves the strategic selection of the carrier’s shape. Currently, these three approaches have yielded significant results. The saturated hydrogenation products of PAHs exhibit various stereoisomers. Cis-stereoisomerism is a favorable option for high-density aviation fuel. This paper summarizes the roles of several metals in cis–trans isomerism. While there is a substantial body of research on the stereoisomerism of naphthalene, there are comparatively few studies on the stereoisomerism of anthracene and phenanthrene, which have the potential to become a research hot spot.

Although catalytic performance has significantly improved through strategies such as nano-structure regulation and carrier modification, precious metal catalysts still face the core challenge of the current technical bottlenecks, we propose future research directions. Future studies should focus on three key areas for breakthroughs: (i) technological innovation (multi-scale advancements in catalyst systems); (ii) exploration of reaction mechanisms and process optimization through the application of artificial intelligence; and (iii) industrial applications and market expansion. These research directions hold significant strategic importance for achieving the high-value utilization of coal tar and developing new energy materials.

The review will enhance the fundamental understanding of the mechanism of PAH hydrogenation and offer essential technical guidance for coal tar utilization. This will support the sustainable development of clean energy technologies and the high-value chemical production of coal by-products.