Constructing Pt/Hierarchical HY Bifunctional Catalysts for Selective Hydroisomerization of Phenanthrene to Alkyl-Adamantanes

Abstract

Designing bifunctional catalysts for efficient hydroisomerization of phenanthrene to alkyl-adamantane is a great challenge for producing high-density fuels. Herein, a bifunctional Pt catalyst was fabricated by developing hierarchical H-MSY-T zeolites with an NOA-co strategy. The influence of different mesopore template agents on the hierarchical structure of H-MSY-T zeolite was investigated. Effective regulation of pore structure and acid distribution of zeolites was achieved by adjusting the templating agents. The block copolymer P123 promoted the formation of mesoporous structures via self-assembly with a large mesopore centered at 8 nm. Large mesoporous structure and suitable distribution of Bronsted acid boosted the hydroisomerization of phenanthrene. The highest alkyl-adamantane yield of 45.9 wt% was achieved on the Pt/MSY-P1 catalyst and a reaction network of hydroisomerization was proposed. This work provides guidance to design highly selective bifunctional catalysts for the one-step hydroconversion of tricyclic aromatic hydrocarbons into high-density fuels.

1. Introduction

The rapid development of modern aviation technology puts forward higher requirements on the power propulsion capability of aircraft, in which high-density and high-volumetric-calorific-value aviation fuels are greatly desired [1,2,3,4]. High energy density fuels (HEDFs) have a higher density and volumetric calorific value than conventional liquid fuels and have been considered promising fuels to boost the flight performance of aerospace vehicles with limited size like missiles, helicopters, airplanes, etc. It is an important way to compress the size of aircraft fuel tanks, extend range, and increase speed using HEDFs [5,6,7,8]. Polycyclic aromatic hydrocarbons (PAHs), such as phenanthrene (PHE), fluorene, and acenaphthene, have a similar number of carbon atoms as aviation fuels. Therefore, it is a common method to prepare HEDFs by deep hydroconversion of PAHs [9,10,11]. However, these hydrogen-saturated hydrocarbons generally have poor low-temperature performance due to the high freezing temperature. An important way to enhance the low-temperature property is to convert the perhydrocarbons of PAHs into alkyl-adamantane, alkyl-decalin, and analogs via isomerization and/or selective ring-opening reactions [12,13]. Decalin and alkyl-decalin have a unique bicyclic structure to improve the volumetric calorific value and thermal stability of jet fuels [14,15]. Alkyl-adamantanes have shown better thermal stability, higher density, and larger volumetric net heat of combustion compared to alkyl-cycloalkanes [16]. However, the high resonance energies and spatial resistances of the aromatic rings make the hydroconversion of PAHs very difficult. In addition, the hydroisomerization and selective ring-opening mechanisms of tricyclic compounds remain unclear due to their more complex structure compared to bicyclic compounds [17]. Therefore, addressing these issues poses a great challenge.

Currently, it is common to carry out the hydroisomerization of polycyclic aromatic hydrocarbons (PAHs) by a two-step catalytic reaction [12,18,19,20,21]. In the first step, a hydrotreating catalyst like Pt/Al₂O₃ is employed to saturate the polycyclic aromatic hydrocarbons. Afterward, different zeolites like Beta, silica-alumina (ASA), USY, and HY are used as support for the hydroisomerization and selective ring-opening reactions of perhydroaromatics. It is found that USY-based catalysts are more active than ASA and Beta for hydroisomerization of perhydrophenanthrene due to their larger microporous structure and higher number of active acidic sites [12,18]. These two-step approaches are not only complex and time-consuming but also necessary to use distinct catalysts in different reaction phases, which leads to the increased complexity and cost of the overall process. In addition, the microporous structure restricted the accessibility and mass transfer, limiting the catalytic performance of catalysts. The introduction of mesopores in zeolite crystals is an effective way to overcome this problem [22,23,24,25,26]. Therefore, the one-step hydroconversion of PAHs using bifunctional catalysts with micro-mesoporous support is highly desired. Recently, Wang et al. prepared hierarchical USY zeolites by oxalic acid treatment and used them for the one-step hydroisomerization reaction of phenanthrene. However, only 2.3% isomerization product alkyl-adamantane was obtained due to the reduction of amounts of acidic sites and acidity of the support [27]. Li et al. used an NaOH-treated USY zeolite for hydroisomerization of perhydrophenanthrene (PHP) and the yield of alkyl-adamantane increased up to 32 wt% [21]. However, these micro-mesoporous zeolites prepared by post-treatment showed relatively poor catalytic performance due to the severe skeleton damage of the zeolite and the loss of active acidic sites in the micropores.

To solve these issues, direct synthesis of the hierarchical Y-type zeolites was developed by the soft template method [28]. Many efforts have been made to synthesize hierarchical Y zeolites by changing the mesoporous templates such as amphiphilic organosilanes (e.g., TPOAC) [29,30], block copolymer (e.g., P123) [28,31,32], and organosilanes (e.g., PHAPTMS, APTMS) [33,34]. For example, Juan et al. synthesized uniform mesoporous hierarchical nanosized zeolite Y by using TPOAC agents for the production of hydrocarbon-like biofuel and found that substantially enhanced the conversion of triolein and the selectivity of hydrocarbon [35]. Zhang et al. used PHAPTMS agents and similar templates to synthesize hierarchical beta zeolites for the catalytic cracking reaction of 1,3,5-triisopropylbenzene and found that mesoporous zeolites reduced coke fouling while retaining catalytic properties to a greater extent [36]. However, little work was carried out on the one-step hydroisomerization of PHAs using directly synthesized mesoporous Y zeolites. In addition, the conventional hierarchical Y zeolites possessed a low silica framework and exhibited poor (aqueous) thermal stability, which significantly restricted their industrial applicability. Recently, Liu et al. developed a simple organic silane surfactant-assisted NOA-co strategy to synthesize mesoporous high silicon Y zeolite (MSY). The synthesized material exhibited an octahedral morphology with abundant intragranular mesopores. The high silicon framework and mesoporous single-crystal properties endow MSY materials with excellent (hydrothermal) thermal stability and sufficient acidic sites [37,38]. Consequently, the micro-mesoporous high silica Y zeolites are expected to deliver outstanding activity in the one-step hydroisomerization of phenanthrene.

In this context, a series of hierarchical Y zeolites with different micro-mesoporous structures and acidity were prepared by an NOA-co strategy using different mesoporous templates. The Pt/hierarchical zeolite bifunctional catalysts with strong metal-support interaction were fabricated by a strong electrostatic adsorption method. The influence of hierarchical pore structure and acid distribution on the one-step hydroconversion of phenanthrene into high-performance and high-density fuels was achieved. This would help to design highly selective bifunctional catalysts for the preparation of high-energy-density fuels from PHAs in one step.

2. Results and Discussion

2.1. Textural Structure and Morphology of As-Prepared H-MSY-T Zeolites

The topology structure of the synthesized H-MSY-T (T = TP, P1, PH, and AP) zeolites was investigated by X-ray diffraction. As illustrated in Figure 1a, the diffraction peaks at 2θ = 6.2°, 10.1°, 11.9°, 15.6°, 18.7°, 20.3°, 23.6°, 27.0°, and 31.4° were assigned to the (111), (220), (311), (331), (511), (440), (533), (642) and (555) facets of FAU topological structure (JCPDS PDF#39-1380) [39], implying that the as-synthesized zeolites had the characteristics of Y zeolite. The relative crystallinity (RC) was calculated compared with H-SY zeolite [40]. Apparently, all zeolites were well crystallized with no visible impurities. The calculated relative crystallinity decreases as H-MSY-AP (95%) > H-MSY-TP (92%) > H-MSY-P1 (90%) > H-MSY-PH (84%), suggesting that the mesoporous templating agent had an obvious influence on the crystallization of zeolite. The presence of amine groups and dipodal moieties in the organosilane APTMS increased its interaction with the protozeolitic nanounits in the gel, which led to the formation of hierarchical zeolites [41]. In addition, the organosilane reagent also acted as an additional source of silica that contributed to zeolite formation and enhanced the sample’s crystallinity [34]. Therefore, APTMS exhibited a good mesoporous structure with high crystallinity. In comparison, as for the H-MSY-PH zeolite, PHAPTMS was poorly bound to zeolite crystals and excessively interfered with the crystallization process due to the large size of the phenyl functional group [33]. TPOAC as a mesoporous template also interacted with zeolite crystals and thus affected the zeolite crystallinity of the H-MSY-TP zeolite because it was an amphiphilic surfactant molecule. The presence of methoxy silyl groups in the organosilane allowed it to be integrated into the resultant aluminosilicate framework through covalent linkages, while the long alkyl tail promoted the formation of intracrystal mesopores with relatively uniform size [29,30]. For the H-MSY-P1 zeolite, the RC value was 10% lower than that of the H-SY zeolite, probably due to the fact that pluronic P123 was a nonionic surfactant with excellent aggregation properties [31]. In the aqueous solution of the zeolite precursor, P123 performed as a space limitation agent and directed the zeolite precursor species to grow with mesoporosity, which decreased its zeolite crystallinity [31,32]. The FT-IR spectroscopy of the synthesized H-MSY-T zeolites also confirmed the XRD results. As shown in Figure 1b, the FT-IR spectra in the region of 400–2000 cm⁻¹ were collected to characterize the framework vibrations of Y zeolites. The band at 587 cm⁻¹ was attributed to the double-ring external linkage associated with the FAU structure. These results further confirmed the successful formation of Y zeolite. In addition, it was observed that all the characteristic bands of H-MSY-T zeolites were weakened compared to H-SY zeolite, indicating a decrease in crystallinity [31,42].

The surface morphologies of H-SY and H-MSY-T zeolites were investigated by SEM characterization. As shown in Figure 2, all the synthesized zeolites showed a typical octahedral morphology of Y zeolite [29]. The crystals of H-SY and H-MSY-T zeolites were well-distributed with a particle size of 50–250 nm. The mesoporous templating agents showed different effects on the morphology of H-MSY-T zeolites. For H-MSY-TP, H-MSY-P1, and H-MSY-PH zeolites, there were many small or amorphous silica-alumina particles to lower the crystallinity of zeolites, which was consistent with the XRD results. As for the H-MSY-P1 zeolite, the results showed that P123 had a slight effect on the morphology of the zeolite compared with H-SY zeolite. This was attributed to the fact that different templating agents filled the channels in different ways and the crystals grew in different directions, resulting in differences in morphology [43].

The pore size and distribution of the zeolite samples were further characterized by an N₂ adsorption-desorption technique. Compared with H-SY zeolites, all the H-MSY-T zeolites presented a type IV(a) isotherm with the hysteresis in the relative pressure range of P/P₀ = 0.4–0.9 as shown in Figure 3a, indicating the presence of mesoporous structures [33]. This demonstrated the successful synthesis of the hierarchical zeolites. The mesopore distribution was obtained based on the BJH method. As shown in Figure 3b, the mesopore sizes in these zeolites were centered at 3.7 nm for H-MSY-TP, 8.0 nm for H-MSY-P1, 3.6 nm for H-MSY-PH, and 3.2 nm for H-MSY-AP, which was related to the interaction of mesoporous templating agents with the zeolite crystals. The H-MSY-P1 zeolite exhibited the largest mesoporous structure due to the large hydrophobic components of P123, which served as a type of scaffold and offered a “confined space” to guide the aluminosilicate gel growing into zeolite crystals [28,32]. As a triblock copolymer, P123′s polypropylene oxide (PPO) fragments were significantly bulkier than the short siloxy-containing hydrophobic moieties in other templates, leading to the generation of the larger mesoporous structures. The large mesoporous structure could substantially reduce the diffusion resistance of bulky molecules. For the other hierarchical zeolites, the mesopore distribution was around 3.5 nm, probably due to the templating effect of agents containing siloxy groups, whose smaller hydrophobic units formed micellar aggregates of comparable size in zeolite crystallization to generate similar pore sizes.

The conventional H-SY zeolite showed a typical type I isotherm with a large specific surface area (S_BET) of 878 m²·g⁻¹ and microporous surface area (S_micro) of 851 m²·g⁻¹ as shown in

Table 1. Textural properties of the H-SY and H-MSY-T zeolites.

Samples	Specific Surface Area (m2·g−1)			Pore Volume (cm3·g−1)			Sext/Smicro
	SBET	Smicro	Sext	Vtotal	Vmicro	Vext
H-SY	878	851	27	0.3869	0.3297	0.0572	0.03
H-MSY-TP	865	619	246	0.5535	0.2416	0.3120	0.39
H-MSY-P1	802	614	188	0.4131	0.2356	0.1775	0.31
H-MSY-PH	798	623	175	0.4311	0.2386	0.1925	0.29
H-MSY-AP	928	791	137	0.4882	0.3002	0.1880	0.17

. The introduction of mesoporous templating agents decreased the S_micro value of H-MSY-T (T = TP, P1, PH, and AP) zeolites with an increased S_ext value, indicating the generation of mesopores. Different mesoporous templating agents produced different impacts on the generation of mesopore structures [43]. The H-MSY-TP, H-MSY-P1, and H-MSY-PH zeolites showed similar S_BET values (865 m²·g⁻¹, 802 m²·g⁻¹, and 798 m²·g⁻¹, respectively). In contrast, the H-MSY-AP zeolite exhibited a larger BET surface area of 928 m²·g⁻¹ because the organosilane APTMS may be able to better interact with the zeolite crystals, which can participate in the formation of zeolites as an additional source of silica. This interaction might reduce crystal volume, shorten pore size, and ultimately increase the surface area [33,34]. To quantify the microporous and mesoporous structures, the S_ext/S_micro ratio was further calculated. H-MSY-P1 zeolite showed a high S_ext/S_micro ratio of 0.31, suggesting that P123 could efficiently generate the mesopore. H-MSY-TP and H-MSY-PH zeolites also presented similar micro-mesopore distribution with H-MSY-P1. In contrast, H-MSY-AP had a much lower S_ext/S_micro ratio (0.17) than H-MSY-P1 zeolite, indicating a higher proportion of microporous structures formed during the crystallization process with APTMS.

Figure 4 depicted the morphology of zeolites and particle size distribution of loaded active metal Pt. As shown in Figure 4a, the H-SY zeolite was a classical octahedron morphology. The HRTEM image in Figure 4b manifested that the average size of Pt particles was about 2.12 nm. The hierarchical MSY-T zeolites showed a similar morphology with H-SY zeolite, while the Pt particles decreased significantly. The Pt/MSY-P1 catalyst exhibited a good Pt distribution with the smallest average particle size of 1.28 nm, which might contribute to the strong metal-support interactions. Analogously, the Pt metal particle sizes on H-MSY-TP, H-MSY-PH, and H-MSY-AP were centered at 1.30 nm, 1.51 nm, and 1.40 nm, respectively. This suggested that the mesoporous structure promoted the Pt dispersion [44]. The highly dispersed Pt particles with smaller sizes could expose more active sites, which was favorable for hydroisomerization reactions. Furthermore, the Si/Al ratio and actual Pt loadings of the prepared Pt/MSY-T catalysts were determined by ICP-OES. As shown in

Table 2. The Si/Al ratio and Pt loading of the Pt/SY and Pt/MSY catalysts.

Catalysts	Pt/SY	Pt/MSY-TP	Pt/MSY-P1	Pt/MSY-PH	Pt/MSY-AP
Si/Al ratio	6.65	6.78	6.37	6.33	6.84
Pt loading (wt%)	1.78	1.79	1.75	1.58	1.79

, the Pt loading of all the catalysts was similar at about 1.80 wt%, which was consistent with the theoretical loading. The Si/Al ratio of the Pt/MSY-P1 catalyst was 6.37, smaller than that of the Pt/SY catalyst. This result was attributed to the fact that the formation of the mesopores was probably accompanied by the enrichment of aluminum in the framework, which led to an increase in the Al content and the formation of aluminum acidic sites. When P123 acted as the scaffold to guide the growth of zeolite precursors in the form of mesopores, some new acidic sites were generated within the mesoporous channels [45], which increased the number of active acidic sites and the accessibility to active sites inside the micropores through fast mass transfer rates in the larger mesopores structure. In addition, it could be found that the Si/Al ratio of Pt/MSY-TP and Pt/MSY-AP catalysts was similar, slightly higher than that of the Pt/SY catalyst. This was most likely because these two templating agents also act as silicon sources involved in crystallization during the synthesis [29,34]. As for PHAPTMS, the excessive molecular size probably inhibited the formation of silica precursors, leading to structural defects in the silica-alumina framework. The incomplete structure reduced the coverage of Al sites by silanol groups, thereby potentially exposing more acidic Al sites [46].

2.2. The Acidity of As-Prepared H-MSY-T Zeolites

The concentration and distribution of acid sites in zeolites were investigated by NH₃-TPD and Py-IR methods. The NH₃-TPD profiles of the H-SY and H-MSY-T zeolites were shown in Figure 5a, and the calculated amount of the acid sites for each zeolite was listed in

Table 3. Summary of acidic properties of the H-SY and H-MSY-T zeolites from NH₃-TPD.

Samples	Peak Temperature (°C)		Acidic Sites (mmol·g−1)
	LTP	HTP	Weak	Strong	Total
H-SY	243	429	0.575	1.696	2.271
H-MSY-TP	242	427	0.451	1.703	2.154
H-MSY-P1	250	433	0.647	1.762	2.409
H-MSY-PH	255	443	0.822	1.888	2.710
H-MSY-AP	249	434	0.580	1.618	2.198

. Two well-resolved shoulder peaks were observed for all the zeolites. The low-temperature detachment (LTP) peaks at 242–250 °C were ascribed to the weak acid sites, while the high-temperature detachment (HTP) peaks at 427–443 °C were assigned to the strong acid sites [47]. The different desorption temperatures of LTP and HTP indicated the strength of the acidic sites in the catalyst. It could be found that the H-MSY-P1 zeolite showed higher LTP and HTP temperatures than H-SY zeolite, indicating that H-MSY-P1 had stronger acid sites. In addition, the acidity of H-MSY-P1 was also higher than that of the H-SY zeolite, suggesting that the zeolites after modification with P123 could expose more strong acidic sites (Table 2). Furthermore, the total acidity of zeolites decreased as H-MSY-PH > H-MSY-P1 > H-SY > H-MSY-AP > H-MSY-TP, consistent with the variation of Al content. As reported in the literature, more acid sites were beneficial to strengthen the hydroisomerization reactions for polyaromatics and increase the reaction rates, while too strong acidic sites would lead to excessive cleavage reactions [48,49].

The Bronsted acid sites (BASs) and Lewis acid sites (LASs) on the surface of those zeolites were further measured by Py-IR. As shown in Figure 5b,c, the spectra in the range of 1400–1600 cm⁻¹ indicated the type and relative amount of acid sites on the surface of the zeolite. Two bands at 1452 and 1542 cm⁻¹ were assigned to the adsorption of pyridine by LASs and BASs, respectively, and the band at 1489 cm⁻¹ was formed by the combined action of the Bronsted and Lewis acid sites [50]. In HY zeolite, BASs were derived from the acidic Si-O(H)-Al groups. The dehydration of HY zeolite under high-temperature conditions led to the removal of oxygen atoms from the tetracoordinate aluminum atom to form a tricoordinate aluminum atom with an unoccupied electron orbital, namely the formation of LASs [46]. For H-MSY-T zeolites, different mesoporous templating agents also affected the acidity distribution and the ratios of the total BASs to LASs (B_T/L_T). As shown in

Table 4. Summary of acidic properties of the H-SY and H-MSY-T zeolites from Py-IR.

Catalysts	Bronsted Acid Sites (μmol·g−1)			Lewis Acid Sites (μmol·g−1)			BS/LS c	BT/LT c
	Total a	Weak	Strong b	Total a	Weak	Strong b
H-SY	522.29	128.38	393.91	327.95	185.00	142.95	2.76	1.59
H-MSY-TP	451.37	59.49	391.88	300.59	146.04	154.55	2.54	1.50
H-MSY-P1	614.45	108.65	505.80	291.27	141.13	150.14	3.37	2.11
H-MSY-PH	624.03	110.39	513.65	288.90	123.37	165.52	3.10	2.16
H-MSY-AP	514.71	131.41	383.30	202.78	116.54	86.24	4.44	2.54

^a The total amount sites of Bronsted acid and Lewis acid were calculated using bands at 1542 cm⁻¹ and 1452 cm⁻¹ respectively by Py-IR at 200 °C. ^b The strong amount sites of Bronsted acid and Lewis acid were calculated using bands at 1542 cm⁻¹ and 1452 cm⁻¹, respectively, by Py-IR at 350 °C. ^c S and T represent strong and total amount acid sites, respectively.

, TPOAC as mesoporous templating agents resulted in a severe decrease of BASs in the H-MSY-TP zeolite with a low B_T/L_T ratio at 1.50 due to the loss of weak BASs (about 50% compared to the SY zeolite) by the extensive destruction of the microporous structure, while P123 and PHAPTMS significantly boosted the formation of BASs, resulting in a higher B_T/L_T ratio than that of SY (2.11 vs. 1.59). However, APTMS hindered the crystal growth resulting in the decrease of LASs with the highest B_T/L_T ratio (2.54) among the as-synthesized catalysts. It was expected that the hierarchical H-MSY-T catalysts with high B_T/L_T ratios would accelerate the deep hydrotreating saturation of aromatic hydrocarbons and the rearrangement of saturated cycloalkanes [19,49,51].

2.3. Catalytic Activity Evaluation

The one-step hydroconversion of phenanthrene over the synthesized Pt/MSY-T and Pt/SY catalysts was carried out. From the GC-MS results, except for the target alkyl-adamantane products (PSA), the other five products were also obtained: (i) partial hydrogenation products of phenanthrene products (PTP) including dihydrophenanthrene and octahydrophenanthrene; (ii) perhydrophenanthrene products (PHP) [12]; (iii) skeletal isomerization of PHP products (PSI); (iv) selective ring-opening products (SRO); and (v) cracking products (CP) with the carbon atoms less than 14 as shown in Table S1 [18]. The product distribution over Pt/SY and Pt/MSY-T catalysts was summarized in Table S2 and Figure 6.

As shown in Figure 6, the as-prepared Pt catalysts showed excellent hydrogenation performance with a complete conversion of phenanthrene at 4 MPa, 180–250 °C. Six types of products were detected with different distributions as temperature. This indicated that similar one-step hydroconversion reaction pathways occurred over these catalysts just with different reaction rates. Over the Pt/SY catalyst, phenanthrene was mainly hydrogenated to PHP (62.1 wt%) and PTP (24.5 wt%) at 180 °C with the products of PSI and CP less than 5 wt%. Trace of PSA and SRO products was detected. This indicated that high-silica Pt/SY catalyst possessed good hydrogenation activity. As the temperature increased from 180 to 200 °C, it could be observed that PHP and PTP decreased significantly with a fast increase of PSI as well as a slight increase of SRO products to a maximum value of 11.2 wt% at 200 °C, while the yield of CP and PSA remained unchanged. This indicated that PHP tended to be converted into skeletal isomerization products, PSI, and further selective ring-opening products SRO. With the temperature further increased up to 220 °C, a sharp decrease of PHP and a significant increase of PSI to a maximum of 40.3 wt% were observed. In addition, skeletal rearrangement PSA products were detected from 210 °C and then quickly increased as well as cracking products with the temperature increasing, while the ring-opening products SRO decreased gradually. This further suggested that the PHP were mainly converted into more isomerization products and then into SRO and PSA products. The sharp increase in CP products might be due to the poor thermal stability of the SRO products. When the temperature further increased up to 230 °C, the PHP and SRO yield gradually decreased to zero and the PSI yield showed a notable decrease. Simultaneously, the PSA showed a rapid increase to a maximum of 40.3 wt%, which may be due to the high thermal stability of alkyl-adamantanes [52]. The conversion of PSI at high temperatures was prone to more thermally stable PSA products. Finally, when the temperature continued to increase much higher, the PSI products gradually disappeared at 240 °C and the PSA products began to decrease quickly. Meanwhile, the CP yield increased sharply and reached 93.1 wt% at 250 °C, revealing that the cracking was enhanced and all the isomerized products tended to crack at higher temperatures. Compared with previous literature [12,18,21,53], the Pt/SY catalyst achieved unprecedented catalytic performance in the hydrogenation isomerization of phenanthrene to alkyl-adamantane, which suggested that high silica Y zeolites had suitable acidic sites and stable structure for the enhancement of hydroisomerization reaction.

As for the hierarchical Pt/MSY-T catalysts, the yield of partial hydrogenation products PTP was significantly reduced to nearly zero with the increase of perhydrocarbon PHP and the subsequent products SRO, PSA, PSI, and CP, especially for Pt/MSY-P1, Pt/MSY-PH, and Pt/MSY-AP catalysts at 180–250 °C. This indicated that the introduction of mesopores in Y zeolite enhanced hydrogenation efficiency probably due to the enhanced Pt dispersion. Comparing Pt/MSY-TP with Pt/MSY-P1, Pt/MSY-PH, and Pt/MSY-AP, poorer hydrogenation saturation performance was obtained although it had a smaller Pt nanoparticle size, suggesting that the mesopore structure derived from P123, PHAPTMS, and APTMS improved mass transport to promote hydrogenation. The Pt/MSY-T catalysts exhibited a different catalytic hydroconversion of PHP. The Pt/MSY-TP and Pt/MSY-P1 catalysts showed a nearly linear decrease of PHP with increasing temperature and to zero at 220 °C. While Pt/MSY-PH presented a similar PHP conversion with Pt/SY, and Pt/MSY-AP showed two distinct stages: a slight decrease at 180–200 °C and a sharp decrease to zero at 200–230 °C. These results suggested that different mesoporous structures affected PHP diffusion rates, thereby influencing conversion kinetics. As in the conversion of PHP, all the Pt/MSY-T catalysts showed a similar volcano-type variation of PSI and PSA products. The optimum skeletal isomerization (PSI) selectivity was obtained at a lower temperature of 210 °C over these catalysts than that Pt/SY catalyst. This indicated that mesopores facilitated faster access of reactants to active sites, boosting isomerization rates. The optimum rearrangement reaction (of PSA products) was achieved at 220 °C over Pt/MSY-P1 and Pt/MSY-TP lower than that (230 °C) over Pt/MSY-PH, Pt/MSY-AP, and Pt/SY catalysts, indicating that the mesoporous structure derived from the template of P123 and TPOAC significantly enhanced the skeletal rearrangement rate by improving intracrystalline diffusion. In contrast, PHAPTMS and APTMS as mesopore templates had no obvious effect on rearrangement rate. In addition, all the prepared hierarchical Pt/MSY-T catalysts enhanced the yield of target products PSA, i.e., the maximum yields of 42.7 wt% (Pt/MSY-TP), 41.6 wt% (Pt/MSY-PH), and 43.3 wt% (Pt/MSY-AP) were obtained. The highest PSA yield of 45.9 wt% was achieved over the Pt/MSY-P1 catalyst. These results indicated that the hierarchical mesoporous structure in MSY-P1, particularly its larger mesopores, enhanced the selectivity of PSA products by facilitating efficient mass transport of reactants to active sites. Since the kinetic diameter of linear adamantane (0.74 nm) and PHE (0.7 nm), PHP was similar to the pore size of HY zeolite, the generation of micro-mesoporous HY zeolite by in situ hydrothermal method could reduce the mass transfer impact, thereby improving its catalytic activity [24,26]. According to the BET and Py-IR results, the Pt/MSY-P1 catalyst had the largest mesoporous pore size of around 8 nm compared with other catalysts, which ensured faster passage of larger molecules such as PHP, and PSI and reduced mass transfer resistance. Moreover, its well-structured microporous structure provided a large number of acidic sites for hydrogenation isomerization reactions and most of its extra Bronsted acid sites active sites may be located in the mesoporous channel, so that the intermediates could arrive at the active sites for hydroisomerization. Therefore, the Pt/MSY-P1 catalyst exhibited remarkable performance in the one-step hydroconversion of PHE to the target products, PSA, at low temperatures. In addition, a high SRO yield about 10 wt% over Pt/MSY-P1, Pt/MSY-PH, and Pt/MSY-AP catalysts was achieved at 180 °C. This indicated that the hierarchical structure enhanced the ring-opening reaction of PSI and selectivity at low temperatures. The hierarchical Pt/MSY-T catalysts showed similar cracking activity with Pt/SY, suggesting that the slight variation of acid distribution and acidity of hierarchical catalysts had hardly affected the cracking reaction, which was mainly dependent on the reaction temperature.

To further understand the catalytic performance of the Pt/MSY-P1 catalyst, the one-step hydroconversion of PHE was also carried out at 220 °C with different weight hourly space velocity (WHSV) from 2.1 to 29.4 h⁻¹. Figure 6f exhibited the yields of liquid products and alkyl-adamantanes at different WHSVs over the Pt/MSY-P1 catalyst. At a WHSV not more than 8.4 h⁻¹, the products were dominated by PSA (ca.46 wt%) and CP (38~42 wt%) with a small amount of PSI (8~13 wt%), and the product distribution tended to be stable. This indicated that the reactions were prone to skeletal rearrangement and cracking at enough retention time and reached a steady state with an WHSV less than 8.4 h⁻¹. With the WHSV increased to 16.8 h⁻¹, the yield of PSA and CP products significantly decreased with a sharp increase of PSI to a maximum of 61.9 wt%. This confirmed that the PSA and CP products were mainly formed from the further conversion of PSI. As the WHSV further increased, the yield of PHP and SRO products demonstrated an apparent linear increase with a slow decrease in PSI. This demonstrated that the PHP tended to convert into PSI and further ring-opening products SRO in short contact with the metal and acid sites. In addition, the yield of PSA products reached the maximal value of 46.6 wt% at 2.1 h⁻¹, showing that PSA was the further transformation of the products.

2.4. Possible Reaction Pathways

To further reveal the reaction process of the one-step hydroisomerization of phenanthrene to alkyl-adamantane, the reaction pathways were investigated. Based on the hydroisomerization network of phenanthrene reported in the literature [12,18,53] and the above experimental results, a cascaded hydroconversion pathway of PHE to PSA was proposed as Scheme 1. From the results in Figure 6, it could be found that PHE was first fast converted into the hydrogenation saturation products PTP and PHP over the as-prepared hierarchical Pt/MSY-T catalysts. Then PHP were isomerized into PSI, the important intermediates. Simultaneously, the selective ring-opening of partial PSI into SRO and further into cracking products CP occurred, indicating that PSI was easily ring-opened over these hierarchical catalysts but with a low selectivity probably due to the still unreasonable structure or acid distribution. In addition, the low reaction rate favored the ring-opening of PSI. Under high reaction rates (high temperature or low WHSV), the PSI products were prone to convert into the skeletal rearrangement PSA and incidental CP products.

Interestingly, it was found that the highest yield of SRO products was obtained at 190 °C over Pt/MSY-T catalysts. To find out the relationship between the active acidic sites and the SRO products, the B_T/L_T ratio was correlated with the optimal yield of SRO products. As shown in Figure 7a, the selective ring-opening yield of SRO at 190 °C increased with the increase of the B_T/L_T ratio. Thus, compared to Lewis acid sites, Bronsted acid sites played an important role in driving the selective ring-opening reaction [6,54]. Based on the previous discussion and hydroconversion reaction pathways of PHE, the variable values (Δ) of PSA, PHP, PSI, and CP were quantified, and an apparent average reaction rate (k_6′) of PSA was calculated as shown in

Table 5. The product yield variation value of PSA, PSI, PHP, and CP in 210~230 °C.

Catalysts	ΔPSA	ΔPSI	ΔPHP	ΔCP	k6′ a
Pt/SY	19.65	−32.42	−8.28	24.39	0.48
Pt/MSY-TP	20.88	−30.36	−8.74	26.57	0.53
Pt/MSY-P1	26.24	−32.20	−10.84	19.13	0.61
Pt/MSY-PH	34.18	−34.19	−32.88	39.79	0.51
Pt/MSY-AP	35.16	−31.53	−30.84	34.33	0.56

^a ${\mathrm{k}}_{6^{\prime }}={\displaystyle \frac{\mathsf{\Delta }\mathrm{P}\mathrm{S}\mathrm{A}}{\mathsf{\Delta }\mathrm{P}\mathrm{S}\mathrm{I}+\mathsf{\Delta }\mathrm{P}\mathrm{H}\mathrm{P}}}$.

. Here, ΔPSA, ΔPHP, ΔPSI, and ΔCP denoted the yield variations (where Δ = Y_t − Y₀) of the corresponding components (PSA, PHP, PSI, and CP) during the reaction process in the range of 210~230 °C, representing the differences between final (Y_t) and initial (Y₀) yields. It was found that the as-prepared Pt catalysts with the micro-mesoporous hierarchical structure substantially increased the value of k_6′. The maximum yield of PSA products in relation to k_6′ was plotted in Figure 7b. There was a positive correlation between k_6′ and PSA, suggesting that k_6′ was the most critical for the selectivity of PSA. As for the Pt/MSY-P1 catalyst, it had the largest k_6′, which was attributed to its large mesoporous structure and acid accessibility [55,56]. The BET and Py-IR results indicated that the Pt/MSY-P1 catalyst had the largest mesoporous pore size of around 8.0 nm compared with other catalysts, ensuring faster diffusion of larger molecules such as PHP and PSI, and reducing the mass transfer resistance. In addition, most of the active acidic sites promote the hydroisomerization reaction, resulting in the highest k_6′, 0.61. Thus, the mesoporous pores and the acid sites played a synergetic effect on enhancing k_6′ to improve the hydroisomerization selectivity of PSA products. On the contrary, for the Pt/MSY-TP catalyst, the addition of TPOAC had the smallest total amount of Bronsted acid sites, as its quaternary ammonium groups exerted strong electrostatic interactions with the negatively charged zeolite framework during crystallization. This severely damaged part of the microporous structure, resulting in a smaller microporous surface area and acidic sites in micropores than H-SY zeolite [57,58,59]. However, its k_6′ value was increased by 0.05 compared to the Pt/SY due to the inhibited intermediates’ mass transfer resistance with the mesoporous channels centered at 3.7 nm. The Pt/MSY-PH catalyst generated a small portion of 3.6 nm mesopore but too many acidic sites causing serious cleavage of the intermediates. Thus, its cracking selectivity was increased significantly by 39.79 wt% in 210~230 °C. The Pt/MSY-TP and Pt/MSY-PH catalysts did not achieve better pore-acid synergism, resulting in a small increase in k_6′, 0.53, and 0.51. Furthermore, the Pt/MSY-AP catalyst presented a small mesopore about 3.2 nm and similar Bronsted acid sites with H-SY zeolite, facilitating the mass transfer diffusion of intermediates and ultimately leading to an increase in k_6′. These results also suggested that mesoporous structures and reasonable pore-acid sites enhanced PSA selectivity.

The influence of pore structure and acidic sites on the target products was further investigated. As shown in Figure 8, the selectivity of PSA products showed a nearly linear increase with the amount of Bronsted acidic sites (<620 μmol·g⁻¹) and mesopore size (>3.5 nm). Compared to Pt/SY, the presence of a mesoporous structure resulted in an increased yield of PSA products despite the presence of more or less active acidic sites in Pt/MSY-T, suggesting that the mesoporous structure played a greater role than the Bronsted acid site, and a larger mesopore size (MSY-P1 zeolite) was more favorable for hydroisomerization of PHE.

2.5. Distribution of PSA Products

A large number of alkyl-adamantanes (PSA) were produced in the hydroisomerization reaction of the PHE and the products were further analyzed by GC-MS, which can be mainly classified into four types of adamantanes as shown in Table S2: 1,3,5,6-tetramethyladamantane (1,3,5,6-TMA), 1,3,5,7-tetramethyladamantane (1,3,5,7-TMA), 1,3-dimethyl-5-ethyladamantane (1,3-D-5-EA), and 1,3-diethyladamantane (1,3-DEA). Figure 9 illustrated the distribution of PSA products at different temperatures over the Pt/SY and Pt/MSY-T (T = TP, P1, PH, and AP) catalysts. The 1,3,5,6-TMA and 1,3-D-5-EA were the major components in PSA products followed by 1,3,5,7-TMA and 1,3-DEA, indicating the better selectivity for 1,3,5,6-TMA and 1,3-D-5-EA on Pt/SY and Pt/MSY-T catalysts. With the increase in temperature, the yield of four alkyl-adamantanes increased first and then decreased. For Pt/SY catalyst, the maximum yields of 1,3,5,6-TMA, 1,3,5,7-TMA, 1,3-D-5-EA, and 1,3-DEA was obtained at 230 °C with 10.4 wt%, 2.9 wt%, 25.7 wt%, and 1.4 wt%, respectively, while the Pt/MSY-P1 catalyst achieved the optimal yields at 220 °C with 12.2 wt% (1,3,5,6-TMA), 4.1 wt% (1,3,5,7-TMA), 27.5 wt% (1,3-D-5-EA), and 2.1 wt% (1,3-DEA). The other three Pt/MSY-T catalysts also showed a similar distribution of PSA products at 220 °C or 230 °C with the selectivity of 1,3-D-5-EA >> 1,3,5,6-TMA >> 1,3,5,7-TMA > 1,3-DEA. In addition, it could be observed that the selectivity of 1,3-D-5-EA was more significantly enhanced than 1,3,5,6-TMA as the temperature increased to 220 or 230 °C and all the catalysts exhibited the highest selectivity of 1,3-D-5-EA at the optimal conditions. This indicated that the selectivity of different alkyl-adamantanes mainly depended on the active sites rather than the pore structure. Combining the results in Figure 6, it could be inferred that the enhancement of hierarchical structure on the selectivity of PSA products was mainly attributed to the acceleration of the formation rate.

3. Experiments

3.1. Materials

All the chemical reagents were used as received. Aluminum isopropoxide (≥98 wt%), sodium hydroxide (NaOH, 96 wt%), tetraethyl orthosilicate (TEOS, >99 wt%), and sodium aluminate (54 wt% Al₂O₃, 41 wt% Na₂O) were purchased from Shanghai Aladdin Industrial Co., Ltd., Shanghai, China. Tetraethylammonium hydroxide (TEAOH, 35 wt% in H₂O), tetrabutylammonium hydroxide (TBAOH, 40 wt% in H₂O), and silica gel (200~300 mesh) were obtained from Shanghai Annaiji Chemical Reagent Co., Ltd., Shanghai, China. [3-(trimethoxysilyl)propyl] octadecyldimethylammonium chloride (TPOAC, 42 wt% in methanol) was purchased from Sigma-Aldrich Co., Ltd., Shanghai, China. P123 (P123, ≥99 wt%), phenylaminopropyltrimethoxysilane (PHAPTMS, 96 wt%), and 3-aminopropyl trimethoxysilane (APTMS, 97 wt%) were provided by Tianjin Xiensi Aude Technology Co., Ltd., Tianjin, China. Tetraammine platinum (II) nitrate (Pt(NH₃)₄(NO₃)₂, Pt ≥ 50.0 wt%) was purchased from Macklin Co., Ltd. Phenanthrene (C₁₄H₁₀, 97 wt%) was purchased from TCI Co., Ltd., Shanghai, China. Octane (C₈H₁₈, 98 wt%) was purchased from Meryer Co., Ltd., Shanghai, China.

3.2. Preparation of Hierarchical MSY Zeolites

The hierarchical MSY zeolites were hydrothermally synthesized using the NOA-co strategy by a modified mothed reported in ref [38]. The nuclei solution was first prepared with a molar composition of 1SiO₂:0.1Al₂O₃:0.01Na₂O:0.6(TEA)₂O:18.3H₂O. For details, the aluminum isopropoxide was added into the TEAOH solution under stirring to form a clear solution. NaOH was added and then TEOS was dropwise into the above solution under vigorous stirring for 4 h. The obtained solution was transferred into a Teflon-lined autoclave and heated at 50 °C for 12 h and then at 100 °C for 48 h in an oven. The low-alkalinity gel solution was prepared with a molar ratio of 1SiO₂:0.05Al₂O₃:0.12Na₂O:0.2TBAOH:20H₂O. Sodium aluminate was added to the NaOH solution. Then TBAOH was added to the solution under stirring. Subsequently, the mesoporous template agents (1.5 mol% in regards to the SiO₂ in the gel) and silica gel were added to the solution. Finally, a desired amount of nuclei solution was dropped into the low-alkalinity gel solution and aged for 12 h under strong agitation at room temperature to form the final gel. The final gel was transferred into a Teflon liner of 50 mL autoclave. The crystallization was conducted at 120 °C under rotation (30 r/min) for 3.5 d. The residue of the uncrystallized precursor solution was removed by centrifugation in ethanol and deionized water. Finally, the sample was dried overnight at 120 °C and calcined at 550 °C for 6 h in air.

The obtained samples using TPOAC, P123, PHAPTMS, and APTMS as mesoporous template agents were denoted as MSY-TP, MSY-P1, MSY-PH, and MSY-AP zeolites, respectively. The structure of the four mesoporous template agents was shown in Figure S1. In contrast, a reference zeolite was synthesized under the same conditions but without the mesopore templates. The sample was denoted as SY zeolite. Finally, those MSY-T (T = TP, P1, PH, and AP) and SY powders were ion-exchanged with 1M NH₄Cl aqueous solution three times at 80 °C, centrifugated, and calcined at 550 °C for 4 h with a heating rate of 5 °C min⁻¹, named H-MSY-T and H-SY, respectively.

3.3. Preparation of Pt/Zeolite Bifunctional Catalysts

Pt/MSY-T and Pt/SY catalysts were prepared by a strong electrostatic adsorption method with a Pt loading of 1.8 wt% [60,61]. Firstly, 1 g of H-MSY-T or H-SY particles was suspended in 200 mL of water at room temperature. Ammonia solution was dropped to keep the solution at pH = 11 and stirred vigorously for 30 min. An aqueous solution of the desired amount of metal precursor Pt(NH₃)₄(NO₃)₂ was added to the suspension and stirred for 2 h. The suspension was filtered, washed with 300 mL of water, and dried overnight under vacuum at 80 °C. Finally, the dried catalyst was calcined at 450 °C at a ramp of 1 °C min⁻¹. The obtained bifunctional catalysts were named Pt/MSY-T (T = TP, P1, PH, and AP) and Pt/SY, respectively. The prepared Pt catalysts were crushed and sieved into the particles with a diameter of 20~40 mesh for catalytic activity evaluation.

3.4. Catalyst Characterization

The X-ray diffraction (XRD) patterns of the zeolite samples were obtained using a Rigaku D/MAX-2500 (40 kV, 150 mA) with Ni-filtered Cu Kα (λ = 1.5406 Å) radiation source over the 2θ range of 5–90°. The FT-IR spectra of the zeolites were performed on Bruker VERTEX 70 spectrometer in the wavenumber range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹. The samples were mixed with KBr (spectroscopy grade). The morphologies of the samples were characterized by field emission scanning electron microscopy (SEM, Regulus 8100). Field-emission transmission electron microscope (FETEM), high-resolution transmission electron microscope (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and elemental mapping were carried out on a JEOL JEM-F200 equipment outfitted with an energy disperse spectroscopy at an accelerating voltage of 200 kV. The actual loading of metals and Si/Al ratios were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES) on Agilent 5110 equipment (Santa Clara, CA, USA). A nitrogen adsorption-desorption test was carried out on a Micromeritics ASAP 2460 model system. The textural properties of the samples were measured by nitrogen (N₂) adsorption-desorption isotherms at −196 °C using a Micromeritics ASAP 2460 model system. All samples were degassed under vacuum at 300 °C for 12 h before the measurements. The total surface area was calculated using the Brunauer–Emmett–Teller (BET) equation, and the microporous specific surface area and volume were calculated by the t-plot method. Barret–Joyner–Halenda (BJH) method was used to evaluate the specific surface area mesopore size distribution.

The acidic properties of the samples were determined by temperature-programmed desorption of NH₃ (NH₃-TPD) using a Micromeritics AutoChem II 2920 device. First, 100 mg of the sample was evacuated at 300 °C for 120 min in the He atmosphere with a flow of 30 mL·min⁻¹. Then the sample was exposed to 10% NH₃/He (30 mL·min⁻¹) at 100 °C for 60 min. The weakly adsorbed ammonia was removed by flowing He (30 mL·min⁻¹) for 120 min, and the chemically adsorbed ammonia was determined by heating to 650 °C (10 °C·min⁻¹) with a TCD detector in He. Infrared spectroscopy of adsorbed pyridine (Py-IR) was conducted on a Tensor 27 spectrometer. Typically, 10 mg of the sample was crushed into a self-supported wafer. The cell containing the sample wafer was kept at 400 °C for 60 min under vacuum status to remove impurities adsorbed on the sample and then cooled to 350 °C and 200 °C to record the background spectra. After injecting pyridine/Ar at 50 °C for 30 min, the IR spectra were measured in the range from 400 to 4000 cm⁻¹ at 200 °C and 350 °C. The corresponding spectra were recorded to determine Bronsted and Lewis acids. The densities of Bronsted and Lewis acid were calculated based on the following formulas [62].

$${\mathrm{C}}_{\mathrm{B}}={\displaystyle \frac{1.88\mathrm{I}{\mathrm{A}}_{\mathrm{B}}{\mathrm{R}}^2}{\mathrm{W}}}$$

$${\mathrm{C}}_{\mathrm{L}}={\displaystyle \frac{1.42\mathrm{I}{\mathrm{A}}_{\mathrm{L}}{\mathrm{R}}^2}{\mathrm{W}}}$$

where IA_B and IA_L are the integral absorbance of Bronsted acid and Lewis acid, respectively. R is the radius of the catalyst disk, and 1.88 and 1.42 are the integral extinction coefficients of Bronsted acid and Lewis acid, respectively. W is the weight of the catalysts.

3.5. Hydroisomerization Activity of Different Catalysts

The hydroisomerization and selective ring-opening reactions of phenanthrene were carried out in a downflow tubular fixed-bed reactor (Shanghai Yanzheng Experimental Instrument Co., Ltd., Shanghai, China.) with a length of 600 mm and an inner diameter of 10 mm. In a typical run, 1.0 g of synthesized catalyst (20~40 mesh) was diluted to 4 mL with SiC, and placed in the middle of the tubular reactor. Three thermocouples in the furnace were used to regulate the heating temperature and the pressure was stabilized by a counterbalance valve at the outlet. Subsequently, the catalyst was heated to 450 °C and reduced in situ for 210 min under 4 MPa of hydrogen pressure at a flow rate of 200 mL·min⁻¹. The reactor was then cooled to the desired reaction temperature (180–250 °C) and the flowing rate of hydrogen was changed to 100 mL·min⁻¹. The feedstock (2.0 wt% of PHE in n-octane) was injected into the reactor using a high-pressure constant-flux pump with a weight hourly space velocity (WHSV) of 8.4 h⁻¹.

After the reaction reached a steady state, the hydrogenation products were collected and qualitatively identified via using a Shimadzu GCMS-QP2020 (GC/MS, Shanghai, China) instrument equipped with a Rtx-5MS column, while the quantitative analysis was conducted on a Shimadzu 2010plus gas chromatography. The conversion of PHE and selectivity of hydrogenation products were calculated as follows:

$$\mathrm{Conversion}(\%)={\displaystyle \frac{{\mathrm{m}}_{\mathrm{P}\mathrm{H}\mathrm{E},\mathrm{i}\mathrm{n}}-{\mathrm{m}}_{\mathrm{P}\mathrm{H}\mathrm{E},\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathrm{m}}_{\mathrm{P}\mathrm{H}\mathrm{E}\mathrm{,}\mathrm{i}\mathrm{n}}}}\times 100\%$$

$$\mathrm{Selectivity}(\%)={\displaystyle \frac{{\mathrm{m}}_{\mathrm{products}\_\mathrm{i}}}{\mathsf{\Sigma }{\mathrm{m}}_{\mathrm{products}\_\mathrm{i}}}}\times 100\%$$

where m_PHE,in is the mass of PHE in the feedstock, m_PHE,out is the mass of PHE in the liquid product, and m_{products_i} is a specific compound in the hydrogenation product.

4. Conclusions

In summary, a series of hierarchical H-MSY-T zeolites with different pore structures were synthesized using the NOA-co strategy in combination with different templating agents. The Pt was loaded by electrostatic adsorption to prepare a metal acid bifunctional catalyst. The introduction of mesopore in Y zeolite significantly promoted the hydrogenation ability and isomerization rate. The mesopore structure derived from P123, PHAPTMS, and APTMS was more beneficial for hydrogenation. The hierarchical structure enhanced the ring-opening reaction of PSI and SRO selectivity at low temperatures. The synergetic effect of the hierarchical structure and the acid sites improved the hydroisomerization selectivity of PSA products and the mesoporous structure played a greater role than the Bronsted acid site. The hydroconversion of PHE over hierarchical Pt/MSY-T followed a cascade reaction pathway. All the catalysts exhibited the highest selectivity of 1,3-dimethyl-5-ethyladamantane. This work provided a new insight on designing highly selective bifunctional catalysts for hydroconversion of polycyclic aromatic hydrocarbons for the preparation of high-energy-density fuels.