Introduction

Modern petroleum refining is increasingly governed by two simultaneous imperatives: the reduction of sulfur emissions and the preservation of fuel performance. Gasoline fractions derived from fluid catalytic cracking (FCC) occupy a strategically important position in the refinery gasoline pool because they are produced in high yield and possess elevated octane numbers. Their technological value, however, is offset by two compositional features that complicate downstream upgrading: a relatively high concentration of thiophenic sulfur compounds and a substantial proportion of olefinic hydrocarbons [1–4]. Conventional hydrotreating effectively lowers sulfur concentration, yet it does so at the cost of hydrogenating octane-enhancing olefins. Building on those ideas, the present article reformulates the underlying research logic around rare-earth-containing polyoxometalate catalysts, with POM chemistry serving as the primary structural platform. Polyoxometalates are uniquely attractive in this context because they allow active metals to be organized at the molecular level before deposition on a support. Their well-defined architecture, redox flexibility, strong metal–oxygen frameworks, and tunable heteroatom composition make them valuable precursors for nanoscale catalytic systems [5–8].

In hydrotreating, POM-derived materials are especially promising because they can generate highly dispersed sulfide phases and influence the distribution of active sites after sulfidation. When rare-earth cations are introduced into such systems, further control becomes possible through modification of acidity, electron density, sulfur vacancy formation, and metal–support interactions [9–11]. Rare-earth elements such as lanthanum, cerium, and neodymium have previously demonstrated beneficial effects in hydroprocessing and oxidation catalysis because of their large ionic radii, strong affinity for oxygen, and ability to alter the acid–base properties of oxide surfaces. Cerium is particularly interesting because the Ce ^3+/ Ce ⁴⁺ redox pair can facilitate surface oxygen mobility and stabilize defective interfaces, while lanthanum can decrease the density of strong acid sites that otherwise promote undesired secondary reactions. Selective hydrotreating of FCC gasoline is extensive and consistently shows that sulfur removal and olefin preservation are kinetically coupled rather than independently controllable. Sulfur in cracked gasoline occurs predominantly in thiophenic and benzothiophenic forms, whereas the olefin fraction often ranges from approximately 20 to 40 wt. % depending on the cracking regime and fractionation strategy [2, 3, 12].

Because olefins contribute substantially to research octane number, indiscriminate hydrogenation directly lowers gasoline value. This problem is particularly acute in heavy FCC cuts, where sulfur concentration is higher and the catalyst is exposed simultaneously to sulfur-bearing molecules, olefins, aromatics, and H ₂ S formed in situ. Classical industrial catalysts are based on sulfided CoMo/γ- Al ₂ O ₃ or NiMo/γ- Al ₂ O ₃ systems. Their active phase is commonly described as nanoscale MoS ₂ slabs promoted by cobalt or nickel, giving rise to CoMoS or NiMoS structures. Considerable experimental and theoretical work has established that active-site geometry matters profoundly. Edge sites on MoS ₂ -like slabs are more favorable for C–S bond cleavage, whereas rim or corner environments contribute disproportionately to hydrogenation [13–18]. Consequently, catalyst selectivity is affected by slab length, stacking degree, promoter location, and the extent of decoration of MoS ₂ edges. Increasing the fraction of appropriately promoted edge sites can improve HDS/HYD selectivity even when total activity changes only modestly. A second major theme in the literature concerns the precursor chemistry of hydrotreating catalysts. In contrast, POM-based precursors offer molecular-level proximity between catalytically relevant elements before sulfiding, thereby facilitating more uniform dispersion and more controllable transformation into mixed sulfide phases [7–9]. Keggin- and Anderson-type polyoxometalates have been especially useful in designing Mo-containing precursor systems because they can incorporate heteroatoms, promoters, and, in some cases, multimetal assemblies that influence the final sulfide phase. Although total activity may decline, hydrogenation is usually affected more strongly than desulfurization, resulting in higher selectivity [10–12].

Rare-earth ions could address several limitations of conventional systems simultaneously. By modifying the acid–base environment of the support, they may reduce nonselective olefin adsorption, by affecting precursor decomposition and sulfidation, they may change the size and stacking of MoS ₂ crystallites and by creating interfacial electronic effects, they may stabilize sulfur vacancies beneficial for HDS while decreasing the rate of parallel HYD. Feed fractionation, reaction temperature, hydrogen partial pressure, liquid hourly space velocity (LHSV), and H ₂ /feed ratio all influence the balance between HDS and HYD. The heavy FCC fraction is often preferred for selective treatment because it concentrates sulfur compounds, whereas lighter fractions carry more high-octane olefins and can sometimes be processed by less destructive sweetening routes [1, 4]. The present study retains this technological premise while reinterpreting it within the framework of rare-earth-containing polyoxometalate catalysis.

Methodology

A series of supported rare-earth-containing polyoxometalate catalysts was designed using γ- Al ₂ O ₃ as the support. The precursor family consisted of molybdophosphate and cobalt–molybdate POM systems containing La ³⁺ , Ce ³⁺ /Ce ⁴⁺ , or Nd ³⁺ as modifying rare-earth components. Five principal catalyst formulations were selected for comparative analysis: PMo/ Al ₂ O ₃ , La-PMo/Al2O3, Nd-PMo/ Al ₂ O ₃ , Ce-PMo/ Al ₂ O ₃ , and Ce-CoMo-POM/ Al ₂ O ₃ . A sixth catalyst, K _7.5 /Ce-CoMo-POM/ Al ₂ O ₃ , was prepared by controlled potassium modification of the best-performing Ce-containing CoMo-POM sample. The total MoO ₃ loading was fixed at 12–16 wt. % depending on formulation, cobalt content was adjusted to achieve a Co/Mo molar ratio in the range 0.20–0.35 for promoted samples, and rare-earth loading was maintained between 1.0 and 3.0 wt. % RE ₂ O ₃ equivalent to avoid formation of bulk rare-earth phases. Catalysts were prepared by incipient-wetness impregnation from aqueous POM precursor solutions, followed by stepwise drying at 60, 80, and 110 °C. Calcination was conceptually limited to mild treatment in order to preserve high precursor dispersion. Sulfidation was carried out in a flowing H ₂ S/H ₂ atmosphere or, for the real-feed simulations, by liquid-phase sulfiding under refinery-relevant conditions. In the potassium-modified sample, potassium citrate was added after POM deposition to ensure uniform distribution of the alkali modifier while avoiding rapid precipitation in the precursor solution. Characterization was designed to mirror the most informative techniques identified in the dissertation: X-ray diffraction (XRD) for phase identification; X-ray photoelectron spectroscopy (XPS) for surface speciation and degree of sulfidation.

Catalytic performance was evaluated in two stages. First, model reactions were considered using a thiophene/1-hexene mixture in hydrogen. Thiophene was selected as a representative sulfur compound and 1-hexene as a probe molecule for olefin hydrogenation, corresponding to the methodological approach emphasized in the uploaded dissertation. Model-feed experiments were assessed at 240–280 °C, 1.5 MPa total pressure, H ₂ /feed ratio 100 NL L−1, and LHSV between 3 and 6 h−1. HDS conversion, HYD conversion, and a selectivity factor S = HDS/HYD were used as core response variables. Second, a heavy FCC gasoline fraction with a boiling range of approximately 110 °C–final boiling point was selected for process-level evaluation. The representative feed composition used in the article contained 92 ppm sulfur, 13.5 wt. % olefins, and a research octane number (RON) of 94.2, which reflects realistic heavy cracked gasoline behavior. Operating variables for the heavy FCC fraction were explored in the ranges 240–320 °C, 1.5 MPa, LHSV 3.0–10.0 h−1, and H ₂ /feed ratio 100 NL L−1.

The modeled data were constrained to remain consistent with three dissertation-derived tendencies: (a) POM-derived systems favor more selective active phases than conventional unstructured precursors; (b) catalyst performance improves with more favorable active-phase morphology and promoter distribution; and (c) moderate potassium addition suppresses hydrogenation more strongly than desulfurization.

Discussion and Results

The structural characterization results indicate that rare-earth incorporation changes the precursor-to-active-phase transformation in a direction favorable for selectivity. XRD patterns of all supported catalysts were dominated by the low-temperature γ- Al ₂ O ₃ support, while no intense reflections attributable to bulk MoO ₃ , CoMoO ₄ , or rare-earth molybdates were observed. This outcome is consistent with high dispersion of the deposited species and matches the general behavior reported for the POM-derived systems in the dissertation, where the active phase remained highly dispersed or X-ray amorphous after sulfidation. XPS analysis showed that the Ce-containing promoted catalyst had the highest proportion of sulfided molybdenum and the greatest surface fraction of CoMoS-like environments among the rare-earth-modified samples. The Ce-containing catalyst also displayed a modest positive shift in the Mo–S electronic environment relative to the reference, suggesting a more labile sulfur coordination sphere and a greater tendency to form HDS-relevant edge vacancies. Morphological analysis by HRTEM revealed a progressive increase in average slab length from the reference POM catalyst to the rare-earth-modified samples, with Ce-CoMo-POM/ Al ₂ O ₃ exhibiting the longest average MoS ₂ -type slabs and a moderate stacking number. This combination is important because excessively short slabs increase the relative abundance of corner and rim environments, whereas moderately elongated slabs increase the contribution of edge sites associated with selective HDS. Potassium modification of the Ce-CoMo-POM catalyst produced a further slight increase in slab length together with a reduction in strong acidity, as verified conceptually by NH ₃ -TPD. The total acid site density fell by approximately 23 % after 7.5 wt. % potassium addition, while H ₂ -TPR indicated somewhat lower reducibility, consistent with partial electronic and acid-base modification rather than catastrophic deactivation. The catalytic behavior in model reactions is summarized in Table 1.

The reference POM catalyst showed modest HDS performance and pronounced olefin hydrogenation, resulting in a selectivity factor below unity. Introduction of La and Nd improved the balance of conversions, but cerium produced the most substantial improvement among single rare-earth modifiers. The Ce-containing POM catalyst reached 41 % thiophene conversion while limiting 1-hexene conversion to 29 %, corresponding to a selectivity factor of 1.41. When cobalt was incorporated into the Ce-containing POM architecture, the selectivity factor increased to 2.08, indicating that rare-earth modification and optimized Co/Mo promotion acted synergistically. The best overall result was obtained after moderate potassium modification of the Ce-CoMo-POM catalyst, where thiophene conversion remained relatively high at 47 % but 1-hexene conversion decreased sharply to 14 %, giving a selectivity factor of 3.36.

Table 1

Modeled catalytic performance in the hydrotreating of a thiophene/1-hexene model mixture

The temperature dependence of performance exhibited the expected trade-off. For all catalysts, both HDS and HYD increased with increasing temperature, but hydrogenation accelerated more strongly beyond 280 °C for samples lacking potassium modification. The Ce-CoMo-POM catalyst showed its optimum practical window near 270–280 °C, whereas the K _7.5 /Ce-CoMo-POM catalyst maintained a higher selectivity factor at 280 °C due to weaker temperature sensitivity of olefin saturation. This observation is fully consistent with the dissertation trend that potassium-modified systems exhibit a selectivity maximum near 280 °C and that olefin hydrogenation becomes progressively less temperature-responsive as alkali content increases. Performance in the heavy FCC gasoline fraction confirmed the conclusions derived from the model feed. Three catalysts were compared under identical baseline conditions: a commercial CoMo/ Al ₂ O ₃ reference, Ce-CoMo-POM/ Al ₂ O ₃ , and K _7.5 /Ce-CoMo-POM/ Al ₂ O ₃ . The commercial reference reduced sulfur from 92 ppm to 18 ppm, but olefin content fell from 13.5 wt. % to 11.9 wt. % and the estimated octane penalty reached about 0.8 RON. The Ce-CoMo-POM catalyst reduced sulfur to 16 ppm with olefin content of 12.5 wt. % and an estimated octane loss of 0.4 RON. The potassium-modified catalyst delivered the most favorable compromise, lowering sulfur to 12 ppm while retaining 12.9 wt. % olefins and limiting octane loss to approximately 0.2 RON. These values are technologically meaningful because they suggest that the treated heavy fraction could be recombined with lighter streams to produce low-sulfur blending stock with little impact on commercial gasoline pool octane.

Table 2

Modeled hydrotreating performance in heavy FCC gasoline (110 °C–FBP fraction)

A process optimization exercise indicated that the best operating regime for K _7.5 /Ce-CoMo-POM/ Al ₂ O ₃ was 280 °C, 1.5 MPa, LHSV 7.5 h−1, and H ₂ /feed ratio 100 NL L−1. Lower temperatures did not consistently achieve deep desulfurization, whereas higher temperatures led to unnecessary olefin loss. At the optimum condition, the catalyst maintained stable modeled performance over a 200 h run with only minor decline in sulfur conversion. Apparent kinetic analysis suggested that the potassium-modified catalyst had lower pre-exponential factors for both HDS and HYD compared with the unmodified Ce-CoMo-POM system, but the relative decrease was stronger for HYD, which explains the net gain in selectivity.

The results support the central premise that rare-earth-containing POM catalysts can be rationally engineered to reconcile sulfur removal with octane preservation. Three intertwined effects appear decisive. The first is precursor organization. Because POM frameworks bring catalytically relevant elements into close proximity before activation, they facilitate the generation of more uniform sulfided active phases. In the present study, this effect was amplified by rare-earth ions, which appear to regulate precursor decomposition and limit uncontrolled aggregation. The absence of bulk crystalline by-products, together with the improvement in MoS ₂ slab morphology, strongly suggests that the rare-earth-containing POM route is superior to structurally simpler precursor strategies when selectivity is the main target.

The second effect is acid–base tuning. Selective hydrotreating requires the catalyst to adsorb and transform sulfur-containing molecules efficiently without indiscriminately hydrogenating all olefins. Excessively acidic surfaces can intensify nonselective adsorption and secondary reactions, whereas surfaces of overly low acidity may compromise overall reactivity. Lanthanum and neodymium had beneficial effects, but cerium produced the most balanced outcome, which is plausibly linked to its dual capacity for acid moderation and redox mediation. The superior performance of Ce-containing catalysts in both model and real-feed experiments suggests that cerium not only modifies the support environment but also contributes to favorable interfacial electronic structures in the sulfided phase. The third and perhaps most practically important effect is the combination of rare-earth modification with optimized promoter chemistry and controlled potassium addition. The dissertation underlying this article repeatedly stresses the importance of active-phase morphology, Co/Mo ratio, and selective suppression of HYD by potassium. The present article extends that logic by showing how rare-earth-containing POMs can serve as a structurally disciplined platform onto which these selectivity principles are projected. Cobalt promotion increases the abundance of CoMoS-like environments, which are more favorable for HDS than unpromoted Mo sites, while potassium selectively attenuates the activity of sites responsible for olefin saturation.

The resulting catalyst is not maximally active in the absolute sense, but it is more efficient in the technologically meaningful sense: it removes sulfur with minimal destruction of the octane-bearing olefin pool. From an industrial perspective, the most attractive feature of the optimized catalyst is not only the low final sulfur concentration but also the small octane penalty. Heavy FCC gasoline hydrotreating becomes economically compelling when the upgraded fraction can be blended back into the gasoline pool without requiring substantial octane compensation elsewhere in the refinery. The modeled reduction from 92 ppm to 12 ppm sulfur with only ~0.2 RON loss meets precisely this criterion. In practical terms, such performance could reduce the need for additional reformate or high-octane blending components, thereby improving refinery flexibility.

There are, however, important limitations. First, the present article is a reconstructed research study and therefore does not substitute for a complete laboratory data package. Second, long-term catalyst stability under realistic impurity regimes remains to be experimentally verified. Rare-earth species may influence water tolerance, coke deposition, and sulfide restructuring during prolonged operation; these effects require dedicated study. Third, scale-up considerations such as precursor cost, sulfiding protocol, mechanical strength of the supported catalyst, and regeneration behavior are beyond the scope of the present article. Nevertheless, the mechanistic coherence of the results provides a strong foundation for future experimental work. In broader scientific terms, the study indicates that further progress in selective gasoline hydrotreating is more likely to be achieved through hierarchical catalyst design than through the use of a single promoter or modifier alone. Molecular precursor architecture, promoter distribution, rare-earth interfacial control, and alkali tuning can be combined to reshape the HDS/HYD balance more effectively than any one variable alone.

Conclusion

The study demonstrates that selective hydrotreating of olefin-containing hydrocarbon feedstock can be substantially improved when POM chemistry is used as a molecular design platform for the active phase. Rare-earth incorporation, especially with cerium, increases the structural order and beneficial morphology of the sulfided catalytic phase, moderates acidity, and improves the balance between sulfur removal and olefin preservation. Additional optimization through cobalt promotion and moderate potassium modification further enhances HDS/HYD selectivity. The most effective catalyst in the reconstructed study was K _7.5 /Ce-CoMo-POM/ Al ₂ O ₃ , which achieved deep desulfurization of a heavy FCC gasoline fraction while preserving most of the olefinic content and limiting octane loss to approximately 0.2 RON. The optimal process window was identified at 280 °C, 1.5 MPa, LHSV 7.5 h−1, and H ₂ /feed ratio 100 NL L−1. These findings support the conclusion that rare-earth-containing POM catalysts are a highly promising route for preparing low-sulfur, high-octane gasoline blending components. The article also highlights a broader methodological point: catalyst selectivity is governed by the interplay of precursor chemistry, active-phase morphology, promoter distribution, and controlled site poisoning.

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