Catalytic Upgrading of Vacuum Residue over Metal-Loaded Iraqi Kaolin Using a Fixed-Bed Reactor

Abstract

The catalytic upgrading of vacuum residue (VR) is constrained by the high cost, diffusional limitations, and rapid deactivation of conventional zeolite-based catalysts due to severe coking. Addressing this, we developed novel, low-cost, and coke-resistant catalysts utilizing naturally abundant Iraqi kaolin. A composite support comprising 80 wt.% Iraqi red kaolin and 20 wt.% white kaolin was synthesized via thermal activation at 800 °C and acid leaching. This support was subsequently impregnated with transition and rare-earth metals (Ni, Co, Ce) at 3–40 wt.% loadings, and comprehensively characterized using XRD, BET, SEM-EDX, and XPS. Catalytic performance was evaluated during VR upgrading in a fixed-bed batch reactor at 450 °C. Among the formulations, the 20 wt.% Ce-loaded catalyst (MKRW-800A@Ce20%) exhibited superior efficiency, achieving 80.15% VR conversion, 61.04% liquid yield, and minimal coke formation (3.81 g) compared to Ni and Co counterparts. This enhanced activity is attributed to synergistic effects of improved surface acidity, textural accessibility, and the Ce³⁺/Ce⁴⁺ redox couple, which promotes selective cracking while suppressing coke precursors. These findings provide new insights into the rational design of natural clay-based catalysts, establishing Ce-modified metakaolin as a viable, sustainable alternative to zeolites for industrial heavy-oil processing.

1. Introduction

Vacuum residue (VR) represents the heaviest fraction of crude oil and contains high concentrations of asphaltenes, resins, and metals such as nickel and vanadium. These complex, polyaromatic molecules are highly condensed, polar, and thermally resistant, making VR one of the most challenging feedstocks to upgrade [1,2]. Asphaltenes, in particular, are prone to aggregation and polymerization under thermal stress, leading to coke and tar formation, catalyst fouling, and a sharp decline in liquid yield. Therefore, the catalytic conversion of VR into lighter, more valuable products such as gasoline and diesel remains a critical but difficult task in modern refining [3].

Catalytic cracking enables the selective cleavage of C-C bonds and hydrogen transfer reactions that transform high-molecular-weight hydrocarbons into lighter fractions. The reaction pathway typically involves β-scission of large paraffins (C_nH_2n+2 → C_xH_2x+2 + C_γH_2γ), followed by secondary hydrogen transfer, cyclization, and isomerization reactions that stabilize olefinic intermediates and suppress aromatic condensation [4]. However, the success of these transformations depends heavily on the acidity of the catalyst, pore architecture, and resistance to deactivation by coke and metal deposition [5].

Zeolite-based materials, especially Y-type zeolites, have traditionally been used as the active component in fluid catalytic cracking (FCC) catalysts due to their strong Brønsted acidity and shape-selective microporous framework. While effective for lighter feeds, zeolites exhibit several drawbacks when applied to heavy residues. The micropore network restricts diffusion of bulky asphaltene molecules, leading to incomplete cracking and localized coke deposition inside the pores [6,7]. In addition, the high cost and limited thermal stability of zeolites under severe operating conditions make them less attractive for residue upgrading. Consequently, the industry has sought alternative catalyst supports that combine sufficient acidity, high thermal stability, and improved textural properties suitable for large hydrocarbon molecules [8].

Among these alternatives, acid-treated clays have received growing attention as low-cost, environmentally benign supports. Kaolin-derived metakaolinite (MKRW-800A) is particularly promising because of its tunable acidity, mesoporous texture, and structural stability at high temperatures [5,9,10]. Upon calcination at 700–900 °C and subsequent acid leaching, kaolin transforms into amorphous MKRW-800A with enhanced surface area and accessible acid sites, while retaining adequate mechanical integrity. These properties allow efficient processing of heavy feed molecules that cannot penetrate zeolitic micropores [11].

In this context, Iraqi kaolin represents a valuable yet underexplored raw material for catalyst production. Iraq possesses abundant deposits of both red and white kaolin, rich in aluminosilicate minerals and well suited for modification. When thermally activated at 800 °C and acid-leached with hydrochloric acid (HCl), Iraqi kaolin forms an amorphous, acid-active metakaolinite (MKRW-800A) characterized by strong acidity, high surface area, and a stable mesoporous framework [5,6]. These features make it a viable, low-cost substitute for conventional zeolites in heavy oil upgrading, especially for feeds rich in asphaltenes [6,9].

The efficiency of VR cracking depends on balancing acidity and porosity. Excess acidity can promote over-cracking and coke formation, while insufficient acidity or narrow pores limit accessibility and conversion [6]. Stratiev et al. [7] demonstrated that optimal catalytic performance arises from a synergy between moderate acidity and mesoporosity, which enhances the diffusion of heavy molecules and removal of coke precursors. For instance, La₂O₃-doped acid-treated clays with moderate acidity achieved higher residue conversion (~74%) than more acidic Cr₂O₃-doped catalysts (~66%), primarily due to their larger pore volume and better reactant accessibility. This highlights that mesoporosity can offset moderate acidity, resulting in superior cracking efficiency for asphaltenic feeds.

Beyond textural and acidic optimization, metal incorporation introduces hydrogenation-dehydrogenation functionality that significantly influences coke behavior and product selectivity. Transition metals such as cerium (Ce), nickel (Ni), and cobalt (Co) have been widely used as promoters for heavy oil cracking catalysts [8,9]. Cerium oxide (CeO₂) acts as a redox mediator through the reversible Ce³⁺/Ce⁴⁺ cycle, providing lattice oxygen to oxidize carbonaceous precursors and thus mitigating coke accumulation [10]. Nickel and cobalt, on the other hand, facilitate hydrogen transfer and olefin saturation, reducing polymerization pathways that lead to aromatic and coke formation [11]. Nevertheless, excessive metal loading may cause sintering, pore blockage, or even catalytic dehydrogenation that accelerates gas and coke formation [2,12].

Interestingly, bimetallic formulations often exhibit synergistic effects. Istadi et al. [11] reported that a Ni-Co/HZSM-5 catalyst exhibited higher conversion and lower coke yield compared with monometallic Ni catalysts, owing to enhanced acidity and improved hydrogenation activity. Similarly, multimetallic Zr-Fe-Ni and Zr-Co-Ni catalysts have achieved 71–76% VR conversion, nearly double that of thermal cracking (~40%), demonstrating the benefits of combining acid and metal functionalities [7,8]. However, even advanced mesoporous zeolites still suffer from coke buildup during resid processing, emphasizing the need for alternative supports with better diffusion and redox properties [13,14]. In this context, metal-loaded metakaolin composites emerge as a highly promising alternative. Unlike conventional zeolitic systems that often suffer from diffusional constraints due to their predominant microporosity, the hierarchical mesoporous structure of acid-leached metakaolin allows bulky vacuum residue molecules (such as asphaltenes) to easily access the active metallic sites. This enhanced accessibility not only improves cracking efficiency but also mitigates rapid deactivation by preventing pore blockage.

To fully appreciate the benefits of these directed catalytic pathways, it is important to consider that thermal cracking of VR, by contrast, proceeds through non-selective free-radical mechanisms. These uncatalyzed reactions yield broad product distributions and substantial coke formation due to the uncontrolled polymerization of asphaltenes [15,16]. Without acid or hydrogenation sites, thermal cracking strongly favors the formation of polyaromatic coke and heavy residues. Conversely, catalytic systems based on metal-loaded, acid-treated clays provide a more selective pathway. By combining controlled bond scission and mild hydrogenation, they lead to higher yields of gasoline (C₅-C₁₀) and kerosene (C₁₀-C₁₆) fractions and lower coke production [17]. For example, Ni/Co-promoted metakaolinite catalysts have achieved total liquid yields exceeding 80% from VR feeds, with significantly reduced coke compared to non-catalytic processes [18].

Despite these encouraging results, systematic studies on Ce-, Ni-, and Co-loaded catalysts derived from naturally abundant Iraqi kaolin are still lacking. The specific roles of each metal in promoting VR conversion, hydrogen transfer, and coke suppression remain poorly understood. Moreover, the synergistic behavior between acidity, redox activity, and mesoporosity in kaolin-based systems has yet to be fully elucidated for asphaltene-rich residues.

Therefore, the primary aim of this work is to evaluate the catalytic upgrading of vacuum residue using novel metal-loaded (Ce, Ni, and Co) metakaolin catalysts derived from natural Iraqi kaolin. By assessing their performance in a continuous fixed-bed reactor, this study seeks to elucidate the specific roles of these metals in maximizing VR conversion, optimizing the distribution of light liquid products, and fundamentally suppressing coke formation. Ultimately, this research aims to provide new insights into the design of affordable, sustainable, and highly accessible catalysts as viable alternatives to conventional zeolite-based systems for heavy oil upgrading.

2. Materials and Methods

2.1. Raw Materials Collection and Preparation

Two types of kaolin, red (KR) and white (KW), were collected as natural rock samples from different locations in the western desert of Al-Anbar, Iraq, in collaboration with the Iraqi Geological Survey, Ministry of Industry and Minerals. The raw kaolin rocks were crushed, ground, and sieved to obtain powders with a particle size around 100 µm, ensuring homogeneity and facilitating subsequent calcination and chemical treatment, as reported in previous studies on clay activation [19,20,21]. Figure 1a,b illustrate the visual appearance of both raw and treated kaolin samples.

2.2. Thermal Activation and Acid Treatment of Kaolin

The textural and acidic properties of kaolin were enhanced by sequential calcination and acid leaching. The powdered kaolin samples (KR and KW; Figure 1 and Figure 2, left side) were calcined at 800 °C for 4 h in air to obtain metakaolin (MKR800 and MKW800; Figure 1 and Figure 2, right side), which exhibits higher reactivity toward acid treatment. The calcined samples were subsequently leached with 6 M hydrochloric acid (HCl, 37%, Sigma Aldrich) at 80 °C under continuous stirring for 5 h using a solid-to-liquid ratio of 1:20, which was employed to ensure sufficient acid penetration and effective extraction of exchangeable cations, iron species, and partially coordinated aluminum [22,23,24]. After leaching, the suspensions were filtered, thoroughly washed with deionized water until neutral pH and chloride-free filtrate were obtained, and finally dried at 120 °C for 24 h. The resulting materials are referred to as acid-treated metakaolin (MKRW-800A).

2.3. Preparation of the Catalysts

Nickel (Ni), cobalt (Co), and cerium (Ce) were selected as active metals for impregnation onto the acid-treated metakaolinite supports. The corresponding precursors—nickel(II) nitrate hexahydrate [Ni(NO₃)₂·6H₂O, 99%], cobalt(II) nitrate hexahydrate [Co(NO₃)₂·6H₂O, 98%], and cerium(III) nitrate hexahydrate [Ce(NO₃)₃·6H₂O, 98.5%]—were used as received from Sigma-Aldrich. Single-metal catalysts were prepared by the incipient wetness impregnation method at metal loadings of 3–8 wt.% for Ni and Co [25,26,27], to evaluate the effect of metal concentration on catalytic performance while minimizing the risk of sintering and pore blockage, and 10–40 wt.% for Ce, due to the structural and redox nature of cerium oxide and its ability to act as both a promoter and structural modifier [19,23]. This selection of distinct loading ranges was strategically based on established catalytic principles for heavy oil upgrading. Transition metals such as Ni and Co possess exceptionally high hydrogenation/dehydrogenation activity; thus, limiting their loading prevents severe metal agglomeration, excessive light gas production, and accelerated coke formation via carbon filament growth. Conversely, to form a robust CeO₂ phase capable of providing sufficient oxygen storage capacity (Ce³⁺/Ce⁴⁺) and mitigating rapid deactivation by bulky asphaltenes, significantly higher Ce loadings are fundamentally required.

The mass of each metal precursor to achieve the target metal loading was calculated using the following mathematical formula:

$$m_{precursor}=\left({\displaystyle \frac{x·m_{support}}{100-x}}\right)·\left({\displaystyle \frac{{MW}_{precursor}}{{MW}_{metal}}}\right)$$

where x is the target metal loading (wt.%), m_support is the mass of the MKRW-800A support, MW_precursor is the molecular weight of the nitrate salt, and MW_metal is the atomic weight of the respective metal (Ni, Co, or Ce). The calculated amount of the precursor was dissolved in the minimum volume required of deionized water. This solution was then added drop by drop evenly over the entire support. The resulting mixture was continuously stirred at room temperature until a homogeneous slurry was obtained, to ensure a complete and uniform distribution of the metallic species within the matrix. The impregnated samples were aged at room temperature for 24 h, dried at 110 °C for another 24 h, and stored in a desiccator. Subsequent thermal activation was carried out in an inert atmosphere. Ni- and Co-loaded catalysts were heated at 400 °C, while Ce-loaded samples required 600 °C to ensure complete decomposition of nitrate precursors and formation of active oxide phases. The complete synthesis process is illustrated in Figure 1c. The resulting catalysts are denoted as MKRW-800A@Ni3%, MKRW-800A@Ni5%, MKRW-800A@Ni8%, MKRW-800A@Co3%, MKRW-800A@Co5%, MKRW-800A@Co8%, MKRW-800A@Ce10%, MKRW-800A@Ce20% and MKRW-800A@Ce40%. For clarity, the catalyst nomenclature follows a specific structure: for example, the code MKRW-800A@Ce20% denotes a mixed MetaKaolin support (Red and White), thermally activated at 800 °C, Acid-leached, and impregnated with 20 wt.% of Cerium.

2.4. Feedstock Description

The vacuum residue (VR) used as the feedstock was obtained from the Al-Durra Refinery in Baghdad, Iraq. The main physicochemical properties of the VR are summarized in

Table 1. Properties of VR used for catalytic cracking.

Property	Value
Viscosity @100 °C (cSt)	782
Pour Point (°C)	+39
Sulfur (wt.%)	6.55
Density @15 °C (g/cm3)	1.019
Flash Point (COC, °C)	314

. This heavy residue was employed for catalytic cracking tests to evaluate catalyst activity, selectivity toward lighter distillates, and coke formation

2.5. Reactor Setup and Catalytic Cracking Procedure

Catalytic cracking experiments were performed in a fixed-bed batch reactor equipped with temperature and pressure control systems (Figure 2). Prior to each run, the reactor was purged with nitrogen gas to eliminate residual oxygen and prevent unwanted oxidation reactions. Each experiment was carried out under atmospheric pressure, with the temperature gradually increasing to 450 °C using an electric heating system. The reaction was maintained for 1 h under constant stirring to ensure uniform heating and contact between the VR and catalyst. After each run, the reactor was cooled to room temperature under nitrogen flow, and the gaseous, liquid, and solid (coke) products were separated and quantified.

2.6. Characterization Techniques

The synthesized catalysts were extensively characterized to determine their physicochemical properties and establish structure–performance relationships. Crystalline phases were identified by X-ray diffraction (XRD) using a Bruker D8 diffractometer (Berlin, Germany) equipped with Cu Kα radiation (λ = 1.541 Å). Textural properties were evaluated from nitrogen adsorption–desorption isotherms measured at 77 K using a Micromeritics TRISTAR II PLUS 3030 analyzer (Norcross, GA, USA). The specific surface area (S_BET), micropore volume (W₀), and micropore width (L₀) of the samples were determined using the Brunauer–Emmett–Teller (BET) method and the Dubinin–Radushkevich (DR) equation, while the mesopore volume was calculated as the difference between the amount of N₂ adsorbed at a relative pressure of 0.95 (V_0.95) and the micropore volume according to the Gurvich rule. Surface morphology and particle dispersion were examined by scanning electron microscopy (SEM) using a Carl Zeiss GEMINI 1430-VP microscope (Oberkochen, Germany) (maximum magnification 500,000×), and the elemental composition was analyzed by coupled energy-dispersive X-ray spectroscopy (EDX). The surface chemical states and metal–support interactions were investigated by X-ray photoelectron spectroscopy (XPS) with a Kratos Axis Ultra-DLD spectrometer (Manchester, United Kingdom) equipped with heating (up to 600 °C) and cooling (down to −150 °C) capabilities, enabling detailed analysis of oxidation states and surface bonding environments. Finally, the composition of the liquid products from catalytic cracking was analyzed by gas chromatography-mass spectrometry (GC-MS) using a Shimadzu GC-2010 system (Kyoto, Japan) operated with helium as the carrier gas (100 kPa), injector temperature 280 °C, oven temperature 60 °C, and a total flow rate of 12.7 mL min⁻¹.

3. Results and Discussion

3.1. Chemical Composition of Iraqi Kaolin

Table 2. Chemical composition of Iraqi Kaolin (Red KR, White KW).

Sample	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	MgO (%)	CaO (%)	Na2O (%)	K2O (%)	TiO2 (%)	P2O5 (%)	LOI (%) (Loss on Ignition)
KR	50.22	21.51	14.74	1.55	0.43	0.09	1.47	1.62	0.06	8.30
KW	48.51	34.69	1.62	0.33	0.13	0.29	0.43	1.63	0.03	12.34

presents the chemical composition of the red (KR) and white (KW) kaolins as determined by X-ray fluorescence (XRF). Red kaolin (KR) contains higher amounts of Fe₂O₃ (14.74%) and SiO₂ (50.22%), reflecting the presence of iron and silica phases typical of less refined clays. The presence of iron oxides can influence thermal behavior during calcination and may contribute to intrinsic redox activity; however, excessive iron can also affect acidity and structural stability during acid treatment [28,29,30].

In contrast, white kaolin (KW) shows higher Al₂O₃ content (34.69%). This composition indicates a higher kaolinite purity and fewer iron impurities, which is consistent with the high crystallinity observed in its XRD pattern (Section 3.3). This is desirable for generating uniform acid sites after calcination and leaching. The higher loss on ignition (LOI ≈ 12.3 wt.%) observed for KW is consistent with a higher hydroxyl content, confirming a more ordered kaolinite structure prior to thermal activation [31,32]. Both clays contain minor amounts of TiO₂ and alkali metals, with very low transition metal content; these compositional differences will influence the behavior during calcination, dealumination and, ultimately, the catalytic potential and thermal stability of the derived metakaolinite materials.

3.2. Catalytic Performance: GC-MS Analysis

The catalytic activity of the MKRW-800 A-based materials was evaluated for vacuum residue (VR) cracking in a fixed-bed reactor.

Table 3. Gas chromatography-mass spectroscopy results.

Catalyst Kinds	Liquid (g)	Coke (g)	Gas (g)	% L.N 1	% H.N 2	% Kerosene	% A G.O 3	% L.V.G.O 4	% H.V.G.O 5	% V.R.C 6
MKRW-800A	9.58	4.37	5.23	38.6	27.14	8.22	14.1	5	5.5	1.4
MKRW-800A@Ni3%	10.68	6.11	2.4	4.67	10.84	34.66	18.07	15.49	16.27	0
MKRW-800A@Ni5%	7.22	5.61	6.35	7.65	11.98	53.42	3.97	4.41	18.57	0
MKRW-800A@Ni8%	6.86	5.3	7.02	9.63	0.1198	42.15	15.51	12.88	10.96	8.87
MKRW-800A@Co3%	7.75	4.88	6.55	7.4	0	37.39	16.32	5	22.14	0
MKRW-800A@Co5%	9.636	5.1	4.34	0	28.75	25.39	16.03	6.54	12.4	10.89
MKRW-800A@Co8%	9.52	5.31	4.6	0	38.82	41.35	8.33	7.4	4.1	0
MKRW-800A@Ce10%	9.2208	3.96	6.06	12.5	25.97	12.37	15.93	15.4	14.87	2.95
MKRW-800A@Ce20%	11.715	3.81	3.66	19.5	17.9	24.72	19.82	12.1	4.12	4.12
MKRW-800A@Ce40%	10.548	4.35	4.29	19.7	18.97	19.3	17.46	4.65	12.08	0
Thermal cracking	12.648	5.02	1.52	0	0	2.36	4.69	7.43	85.52	0

¹ L.N = light Naphtha, ² H.N = heavy Naphtha, ³ A.G.O = atmospheric Gas Oil, ⁴ L.V.G.O = Light Vacuum Gas Oil, ⁵ H.V.G.O = Heavy Vacuum Gas Oil, ⁶ V.R.C = Vacuum Reduced Crude.

and Figure 3, Figure 4 and Figure 5 summarize the yields of liquid, gas, and coke, as well as the distribution of liquid fractions into light naphtha, heavy naphtha, and kerosene.

Regarding the best performing catalyst, MKRW-800A@Ce20% exhibited the highest catalytic efficiency, producing 11.72 g of liquid (≈61% of total product) and the lowest coke formation (3.81 g, ≈20%) [2,7,8]. This superiority is fundamentally driven by a bifunctional synergistic mechanism between the acidic support and the redox-active ceria. Specifically, the cerium oxide does not primarily participate in the direct C-C bond cleavage; the direct cracking of bulky VR molecules into lighter fractions is executed by the Brønsted and Lewis acid sites of the metakaolinite via carbocation intermediates. However, these acid-catalyzed reactions inherently generate polyaromatic radicals that quickly polymerize into coke, deactivating the sites. At this critical stage, the Ce³⁺/Ce⁴⁺ redox couple acts as a protective mediator. By providing high oxygen mobility (lattice oxygen), ceria continuously oxidizes these carbonaceous precursors before they can condense into hard coke, effectively keeping the adjacent acid sites clean and accessible. Additionally, this redox cycle facilitates mild hydrogen transfer, which stabilizes the cracked olefinic intermediates and prevents their recombination into heavier tars [8,10,33,34,35]. This direct synergy, where the support cracks the molecules and the metal preserves the active sites, explains the sustained liquid yield and minimal coke formation. Moreover, MKRW-800A@Ce20% also enhanced selectivity toward light fractions, yielding 19.76% light naphtha, 18.97% heavy naphtha, and 24.72% kerosene, demonstrating effective secondary cracking and hydrogenation [7,13,14,34].

Ni-doped samples (especially 5–8 wt.% Ni) produced lower liquid yields (6.86–7.22 g) and higher coke (6.35–7.03 g), suggesting that at these loadings nickel promotes excessive cracking/dehydrogenation and polymerization under these conditions, increasing carbonaceous deposit formation. This behavior aligns with the known sensitivity of residue cracking to the balance between hydrogen transfer and coke-forming condensation, particularly when metal sites become overly dominant or poorly dispersed [3,7,8,35].

Co-modified samples presented intermediate behavior (liquid yields 7.75–9.63 g; coke 4.3–5.3 g), indicating moderate activity and selectivity; cobalt provides some hydrogen-transfer/redox functions but did not outperform the Ce-promoted system under the tested conditions. Thermal runs produced 12.648 g liquid but of very poor quality (0% naphtha fractions, only 2.36% kerosene) and left ≈ 85.5% of the feed as unconverted VR, highlighting the importance of catalytic assistance for upgrading VR [35,36,37].

The global product distribution (Figure 3) further confirms that MKRW-800A@Ce20% offered the best overall balance, with 61% liquid, 19% gas, and 20% coke. Lower Ce loadings (10%) and higher ones (40%) both showed inferior results. The macroscopic drop in performance at 40 wt.% Ce strongly suggests that such excessive loading leads to severe metal agglomeration and pore blockage, restricting the diffusion of bulky VR molecules to the active sites. This demonstrates the critical importance of optimizing metal concentration to achieve high dispersion and stable redox behavior [17]. The combined GC–MS and product distribution data clearly establish MKRW-800A@Ce20% as the optimal composition, offering high liquid yield, enhanced light-fraction selectivity, and superior coke resistance [2,8,38].

Although multi-cycle reusability tests fall outside the scope of this initial proof-of-concept study, the remarkably low coke deposition observed for the optimal MKRW-800A@Ce20% catalyst strongly suggests a high resistance to deactivation. This indicates excellent potential for long-term stability and reusability, which will be the primary focus of our forthcoming investigations.

3.3. X-Ray Diffraction (XRD)

The XRD patterns (Figure 6) reveal the structural transformation of the kaolin-based materials. The raw samples (KR and KW) exhibit the characteristic reflections of crystalline kaolinite at 2θ = 12.3° and 24.8°. Notably, the KW sample displays sharper and more intense kaolinite reflections compared to KR, which perfectly corroborates its higher structural purity and superior Al₂O₃ content previously determined by XRF. After calcination at 800 °C, these specific kaolinite peaks disappear, confirming the dehydroxylation into metakaolinite. Such calcination-driven structural collapse and loss of long-range order are consistent with thermal transformation behavior reported for kaolinite-rich clays. However, a prominent sharp reflection remains at 2θ = 26.6° across all samples, indexed to quartz (SIO₂) present as a persistent crystalline impurity in the precursor clay. Regarding the metal-modified samples (Ni 3% and Co 3%), no significant diffraction peaks corresponding to NiO or Co₃O₄ phases were detected [39,40]. This absence suggests that these metals are either present as clusters below the detection limit of the instrument or are highly dispersed within the amorphous metakaolinite matrix, a feature often beneficial for catalytic performance.

Even at a higher loading of Ce (20%), the characteristic fluorite-type CeO₂ reflections [41,42] are remarkably weak or absent. Given that 20 wt.% is a substantially high loading, this lack of distinct peaks is a strong indicator of excellent metal dispersion. It indicates that the cerium species are either highly amorphous or present as extremely small nanoclusters (<3–4 nm) falling below the XRD detection limit, being successfully incorporated into the porous structure of the acid-leached support (MKRW-800A) without forming long-range crystalline domains [43,44,45,46].

3.4. Surface and Pore Structure

Surface area and porosity data (

Table 4. Surface area and pore structure of the material.

Sample	SBET (m2/g)	W0 (N2) (cm3/g)	L0 (N2) (nm)	V0.95 (cm3/g)	Vmeso (cm3/g)
KR	63	0.026	1.59	0.097	0.071
KW	23	0.009	1.56	0.063	0.059
MKR-800	29	0.012	1.73	0.072	0.060
MKW-800	29	0.013	1.81	0.093	0.080
MKRW-800A	190	0.079	1.49	0.166	0.087
MKRW-800A@Ni3%	19	0.008	2.10	0.041	0.033
MKRW-800A@Co8%	56	0.024	1.82	0.067	0.043
MKRW-800A@Ce20%	36	0.015	1.86	0.061	0.046

S_BET: BET surface area, W₀: micropore volume, L₀: micropore width, V_0.95: adsorbed volume at a relative pressure of 0.95, V_meso: mesoporous volume.

) demonstrate that both structural and chemical treatments significantly influence the textural characteristics of kaolin. Acid activation of metakaolinite (MKRW-800A) markedly increased its surface area to 190 m²/g and pore volume to 0.166 cm³/g, primarily due to dealumination and iron removal, which open the matrix and enhance accessibility to reactants [24,40]. However, subsequent metal loading reduced surface area as metal oxides partially blocked pores and covered active sites, yielding 19, 56, and 36 m²/g for the optimal Ni-, Co-, and Ce-loaded samples (MKRW-800A@Ni3%, MKRW-800A@Co8%, and MKRW-800A@Ce20%), respectively [47]. Despite this reduction, the micropore width (L₀) remained between 1.49 and 2.10 nm. More importantly, the catalysts successfully preserved an adequate mesoporous volume (V_meso = 0.033–0.087 cm³/g), which provides the essential wider channels (>2 nm) that facilitate diffusion and suppresses coke formation [47,48,49]. It is well established that pores between 2 and 10 nm are optimal for upgrading heavy oils [50]. In summary, the combination of moderate surface area, stable mesoporosity, and dispersed redox-active species such as Ce⁴⁺/Ce³⁺ and Co₃O₄ enhances acidity, oxygen mobility, and long-term catalytic stability [51,52].

The N₂ adsorption–desorption isotherms (Figure 7) for both the raw and modified materials are characteristic of Type IV isotherms according to the IUPAC classification, which is typical of mesoporous structures. At low relative pressures (P/P₀ < 0.1), the curves show a slight initial uptake related to monolayer adsorption in micropores, particularly evident in the acid-activated sample (MKRW-800A), which aligns with its significantly higher S_BET. All samples exhibit a distinct H3-type hysteresis loop in the P/P₀ range of 0.4–1.0. This specific hysteresis type is associated with non-rigid aggregates of plate-like particles, such as those found in clay minerals, forming slit-shaped pores. Notably, the acid treatment (MKRW-800A) enhances the adsorption capacity across the entire pressure range, reflecting the development of a more open porous network. Conversely, after metal loading (Ni, Co, and Ce), the isotherms maintain their Type IV shape and H3 hysteresis but show a marked decrease in the total volume of adsorbed N₂. This reduction is mainly associated with partial pore filling and surface coverage by metal oxide species, a phenomenon widely reported for impregnated clay and oxide supports [53,54]. Despite this decrease, the catalysts retain a significant mesoporous volume (V_meso = 0.033 − 0.046 cm³ g⁻¹), which remains within the optimal range for heavy oil upgrading [55]. Therefore, whilst the fundamental mesoporous architecture of the metakaolinite-quartz matrix is preserved, the impregnated metal species partially occupy or block the pore channels, consistent with the reduction in pore volume (V_0.95) and surface area observed in Table 4.

3.5. Scanning Electron Microscopy (SEM)

SEM micrographs reveal a direct relationship between morphological evolution and catalytic performance. The raw kaolins KR and KW (Figure 8) exhibit smooth, compact lamellar surfaces with limited porosity, restricting diffusion and catalytic accessibility [56].

After calcination to 800 °C, metakaolinite (MKR800, MKW800) shows partial amorphization and dehydroxylation, increasing reactivity slightly but still limiting diffusion (Figure 9) [57,58]. Acid treatment (MKRW-800A) drastically alters the morphology, producing a fractured, porous texture (Figure 9, left). This transformation, driven by dealumination and iron extraction, enhances surface area, introduces Brønsted acidity, and creates open diffusion channels essential for VR cracking [18,59].

Figure 10 and Figure 11 show SEM images of the acid-leached metakaolinite (MKRW-800A) before and after being loaded with metals like Ni, Co, and Ce. Overall, the typical layered or plate-like structure of the metakaolin is still visible in all the samples. There are no drastic changes in the surface shape or noticeable collapse of the structure after metal loading. Although we cannot directly see or identify the metal oxide particles (like NiO, Co₃O₄, or CeO₂) from these SEM images alone, we can assume that the metals are present based on how the catalysts behave during cracking. For a clearer picture of where the metals are and how well they are spread, we refer to the EDX results [60,61,62].

3.6. SEM–EDX Analysis

SEM–EDX characterization of MKRW-800A@Ni3%, MKRW-800A@Co8%, and MKRW-800A@Ce20% catalysts confirms the elemental composition and metal dispersion patterns. The acid-leached support (MKRW-800A) provides a porous, high-area scaffold suitable for anchoring metal oxides. In MKRW-800A@Ni3% (Figure 12), NiO nanoparticles are evenly distributed across the porous surface, and EDX spectra confirm the presence of Ni, Si, Al, and O, indicating good dispersion and minimal sintering, features consistent with Ni/kaolin systems in steam reforming [63,64,65]. MKRW-800A@Co8% (Figure 13) shows Co clusters and rough morphology, while EDX verifies Co incorporation into the aluminosilicate matrix, forming redox-active Co₃O₄ domains [66]. In both Ni- and Co-loaded metakaolinite materials, the atomic percentage of Ni and Co fits fairly well with that theoretically incorporated in certain regions of the sample. The MKRW-800A@Ce20% catalyst (Figure 14) exhibits partial agglomeration of CeO₂ with slightly reduced porosity; however, its strong Ce⁴⁺/Ce³⁺ redox cycle and oxygen storage capacity impart excellent coke resistance, aligning with previous findings for CeO₂-supported Ni systems [67]. The detected cerium percentage being slightly below the nominal 20 wt.% is attributable to the inherent semi-quantitative and highly localized nature of the EDX analysis. Because the EDX interaction volume samples a specific, micron-scale region, slight deviations from the overall nominal loading are typically a reflection of microscopic heterogeneity in the metal distribution across the individual metakaolinite particles.

3.7. XPS Analysis

XPS spectra (Figure 15, Figure 16, Figure 17, Figure 18, Figure 19 and Figure 20) and elemental compositions (

Table 5. Atomic percentage by XPS survey analysis.

Sample	C (wt.%)	O (wt.%)	Al (wt.%)	Si (wt.%)	Fe (wt.%)	N (wt.%)	Na (wt.%)	Cl (wt.%)	F (wt.%)	Ni (wt.%)	Co (wt.%)	Ce (wt.%)
KR	13.7	56.4	10.6	17.7	0.74	0.56	0.10	-	-	-	-	-
KW	22.8	50.2	11.3	15.0	-	-	0.48	-	-	-	-	-
MKR-800	13.7	55.4	11.3	18.8	0.65	-	-	-	-	-	-	-
MKW-800	19.1	49.9	13.7	16.9	-	-	0.13	-	-	-	-	-
MKRW-800A	3.48	61.8	4.34	30.3	-	-	-	-	-	-	-	-
MKRW-800A@Ni3%	13.8	51.9	7.30	25.3	-	-	-	1.15	-	0.46	-	-
MKRW-800A@Co8%	11.7	56.0	4.18	27.2	-	-	-	-	-	-	0.78	-
MKRW-800A@Ce20%	12.9	51.7	4.39	26.3	-	-	-	1.22	-	-	-	3.34

) confirm significant chemical and electronic transformations throughout the catalyst synthesis process. The high-resolution spectra of the raw kaolin samples display the typical core-level emissions corresponding to the aluminosilicate framework. The Al 2p spectrum (Figure 15) shows a main peak at 73.6 eV attributed to Al–O bonds in octahedral Al³⁺ sites within the kaolinite lattice, accompanied by a secondary component at 74.5 eV assigned to Al(OH)₃, indicating the presence of surface hydroxylated aluminum species. The Si 2p peak is centered at 102.8 eV, corresponding to Si–O–Si and Si–O–Al linkages, typical of layered kaolinite structures. The O 1s region exhibits two distinct contributions: a dominant peak at 532.8 eV due to lattice oxygen (Si–O–Al/Si–O–Si) and a weaker one at 531.2 eV attributed to hydroxyl oxygen or adsorbed species. The C 1s spectrum is deconvoluted into four components: C–C/C=C (284.6 eV), C–O (286.3 eV), C=O (287.6 eV), and CO₂ or carbonate species (288.6 eV), reflecting minor surface contamination and atmospheric adsorption [68]. These deconvoluted features establish a reference for the subsequent modifications induced by calcination, acid leaching, and metal impregnation.

Upon thermal activation at 800 °C (Figure 16), the kaolinite structure transforms into amorphous metakaolinite (MKRW800). The Al 2p peak slightly shifts to ~74.2 eV and broadens, consistent with dehydroxylation and the collapse of the ordered layers, generating unsaturated Al³⁺ centers that serve as Lewis acid sites. The Si 2p peak remains at ~103.0 eV but narrows, reflecting the predominance of Si–O–Si linkages after partial removal of structural hydroxyls. The O 1s band also becomes broader, dominated by lattice oxygen (531.4 eV), indicative of the increased defect density and amorphous character of MKRW800. These spectral changes confirm the destruction of long-range kaolinite order and the formation of a reactive, defect-rich aluminosilicate surface suitable for catalytic modification [69].

After acid leaching (MKRW-800A, Figure 17), strong dealumination occurs, as evidenced by the marked reduction in Al 2p intensity (from 11.3 to 4.34 wt.%) and an enhancement of the Si 2p signal (up to 30.3 wt.%). The O 1s peak sharpens and shifts slightly toward higher binding energy, dominated by Si–O–Si and Si–O–Al contributions, confirming the formation of a silica-enriched surface. This modification increases both Lewis acidity (due to exposed Al³⁺ in distorted tetrahedral coordination) and Brønsted acidity (from residual surface hydroxyls), which are key to catalytic cracking activity [62,70]. The persistence of the C 1s components (C–C, C=O, CO₂) indicates minor carbonaceous residues or re-adsorbed CO₂ but does not affect the surface chemistry significantly.

Upon metal impregnation, the general structure of the aluminosilicate matrix remains intact, with new electronic states appearing from the incorporated metals. In Ni-modified samples (Figure 18), the Ni 2p_3/2 signal appears at 855.0 eV, with a satellite peak near 861 eV, confirming the presence of Ni²⁺ in NiO, while a faint shoulder at 852.8 eV may indicate traces of metallic Ni⁰ formed during calcination. The coexistence of Ni²⁺/Ni⁰ enhances hydrogen transfer and dehydrogenation–hydrogenation balance during vacuum residue (VR) cracking, though it may promote coke at higher loadings [71].

For Co-doped catalysts (Figure 19), the Co 2p_3/2 peak at 778.5 eV, together with a satellite near 787 eV, is characteristic of Co₃O₄, containing both Co²⁺ and Co³⁺ oxidation states [72]. This redox duality facilitates oxygen transfer reactions that assist in partial oxidation of coke precursors, although the strong interaction between Co³⁺ and the aluminosilicate matrix can induce local aggregation, consistent with SEM–EDX data [66].

In the Ce-modified catalyst (MKRW-800A@Ce20%, Figure 20), the Al 2p and Si 2p peaks remain largely unchanged, confirming the structural stability of the kaolinite-derived framework after metal incorporation. The O 1s spectrum shows a main component at 532.6 eV (Si–O–Al/Si–O–Si) and a new shoulder at 530.4 eV assigned to Ce–O bonds, evidencing intimate metal–support interaction. The Ce 3d spectrum exhibits a multiplet pattern typical of CeO₂, with intense peaks at 882.3, 888.6, 898.0, 900.8, and 914.0 eV corresponding to Ce⁴⁺, and weaker features at 885.1 and 903.3 eV attributed to Ce³⁺ [72,73]. The strong Ce⁴⁺ signal around 914 eV indicates that Ce predominantly exists as CeO₂, while the presence of Ce³⁺ species confirms oxygen vacancies and reversible redox cycling (Ce⁴⁺ ⇌ Ce³⁺). This redox flexibility enhances oxygen mobility, promoting the oxidation of carbonaceous intermediates and reducing coke formation during VR cracking. The coexistence of acidic (Si–O–Al) and redox-active (Ce⁴⁺/Ce³⁺) sites generates a synergistic acid–redox environment, responsible for the improved catalytic activity and stability of Ce-loaded MKRW-800A [74,75].

This transformation pathway agrees well with previous reports. The incorporation of redox-active metals, particularly cerium, aligns with studies describing the strong redox flexibility and oxygen-storage capacity of CeO₂ [76,77]. The shift from crystalline kaolinite to amorphous metakaolinite, followed by dealumination and silica enrichment after acid leaching, is also consistent with findings on thermochemically treated clay materials [78]. Together, these studies explain why the Ce-modified catalyst exhibits more balanced surface chemistry and, consequently, higher VR conversion with lower coke formation.

3.8. Comparative Performance with Literature

The catalytic cracking of vacuum residue (VR) aims to convert heavy hydrocarbons into valuable light fractions such as gasoline, kerosene, and diesel. While zeolite-based catalysts are widely used, acidified metakaolinite (MKRW-800A) represents a low-cost and effective alternative. Comparative data (

Table 6. Summary of the previous work concerning VR cracking.

Reference	Catalyst and Feedstock	Conditions	Key Findings
Ulfiati et al., 2022 [5]	ZSM-5 vs. Ni–Mo	350 °C, 1 MPa	Increases C3–C5 production Outperformed non-catalytic runs
Al-Karim et al., 2022 [13].	ZnFeNi and ZnCoNi on VR	400–450 °C, 2–3 h	Maximum activity with ZnCoNi 76.3% VR conversion
Kohli et al., 2020 [38].	NiMo/SBA-15, NiMo/AC, NiMo/Al2O3 on VR	410 °C, high-pressure H2	Best VR upgrading with NiMo/Al2O3 Highest removal of impurities
Dokoutchaiev et al., 2024 [67].	Spent Al–Co–Mo on VR	440–460 °C	39.8% light fraction at 460 °C
Almeida et al., 2023 [79].	Mg, Cu, Ni on used motor oil	380–390 °C	85% conversion Low coke formation
Ahmad et al., 2023 [81].	USY zeolite + kaolin + VGO	550 °C	High yield Low coke formation vs. pure zeolite
Naranov, E. R. et al., 2019 [82]	H-ZSM-5 + chlorinated oil	450–550 °C	47.3 wt.% gasoline yield 20.7 wt.% olefins yield
Kumar et al., 2024 [83].	Kaolin + LDPE (10–20%)	550–850 °C	Maximum 24.8% VR conversion, 10% coke formation at low temp.
Al-Attas et al., 2019 [84]	Dispersed Ni, Mo, Fe, Co catalysts on heavy oil/VR	400–500 °C, high pressure (batch & slurry reactors)	Higher VR conversion and liquid yield Lower coke formation than conventional fixed-bed catalysts
Jermy, B. Rabindran, et al. 2023 [85]	ZSM-5 + alumina, SiO2 + pyrolysis oil	675 °C	47% liquid yield 14.7% aromatics yield
Liu et al., 2018 [86]	Four zeolites + VR	700 °C	BTEX yield: 3.63 wt.% (NaY) and 5.31 wt.% (HY)
Trisunaryanti, Wega et al. 2020 [87].	ZSM-5 + hexadecane	475 °C, 1.2 h−1	93% conversion highC7–C9 selectivity
Attique et al., 2020 [88]	Alumina-substituted Keggin tungstoborate/Kaolin, LDPE	500 °C	Low coke formation High hydrocarbon selectivity
This Work	MKRW-800A@Ce20% on VR	450 °C, 0.5 h−1	Highest liquid yield (61.04%) 80.15% VR conversion Minimal coke formation (3.81 g) Synergistic acid-redox mechanism

) demonstrate that MKRW-800A@Ce20% achieves superior conversion (≈80.15%) and high liquid yield (≈61%), with minimal coke formation (≈3.81 g). This performance surpasses several benchmark catalysts reported in recent studies, including metal-modified zeolites and mixed oxides [33,79]. The improved behavior is mainly due to the redox functionality of cerium, which enhances hydrogen transfer, oxygen mobility, and coke oxidation. Ni and Co loadings at lower levels also promote activity but tend to deactivate faster at higher concentrations due to sintering or excessive polymerization. Therefore, the synergy between acidified metakaolinite and an optimal Ce loading offers a promising balance between cost, efficiency, and stability [80].

3.9. Environmental Implications

From an environmental perspective, metal-loaded MKRW-800A catalysts, particularly those containing Ce, offer cleaner and more sustainable routes for VR upgrading. Reduced coke generation minimizes reactor regeneration frequency and energy consumption [89]. Moreover, the use of natural kaolin as a precursor aligns with green chemistry principles, providing a non-toxic, abundant, and recyclable support material [34,90]. The redox properties of Ce accelerate coke oxidation, extend catalyst life, and reduce waste, while improved selectivity for light fractions diminishes dependence on hydrotreatment and lowers CO₂ emissions [76]. Upgrading heavy residues to light fuels supports decarbonization goals by improving resource efficiency and decreasing sulfur emissions [73]. Additionally, these findings may extend to related processes, including plastic and PET waste upcycling, offering both economic and environmental benefits [91,92,93,94].

4. Conclusions

Natural kaolin-derived materials represent a sustainable and low-cost alternative to conventional zeolitic catalysts for heavy oil upgrading. Their abundance, structural stability, and tunable acidity make them excellent supports for developing active, thermally robust catalysts. Through controlled thermal activation and acid leaching, kaolinite transforms into highly porous, silica-enriched metakaolinite (MKRW-800A), which provides accessible active sites and a suitable framework for metal dispersion. This approach enables the design of efficient catalytic systems using locally available minerals, reducing dependence on synthetic zeolites and promoting cleaner refining technologies. This study evaluated acid-treated metakaolinite (MKRW-800A) catalysts loaded with Ni, Co, and Ce for vacuum residue (VR) cracking in a fixed-bed batch reactor. Among all catalysts, the Ce-loaded MKRW-800A@Ce20% exhibited the highest performance, achieving 80.15% VR conversion, 61.04% liquid yield, and only 3.81 g coke. Such improvement stems from the Ce⁴⁺/Ce³⁺ redox couple, which provides oxygen storage capacity and promotes oxidative removal of coke precursors while maintaining strong surface acidity for effective cracking reactions. Ni- and Co-modified catalysts displayed moderate activity at low loadings (3–5 wt.%), but their performance deteriorated at higher concentrations due to agglomeration, pore blockage, and increased coke formation arising from metallic sintering. These findings confirm that catalytic efficiency is not governed solely by surface area or crystallinity, as indicated by XRD and BET, but rather by the balance between acidity, redox behavior, and metal dispersion. In particular, Ce-modified samples, despite showing only moderate BET surface area, delivered superior selectivity and coke resistance due to their dynamic redox cycling and enhanced oxygen mobility. Complementary surface analyses further elucidated this behavior. EDX revealed partial heterogeneity in the distribution of Ce at the micrometer scale, while XPS, being more surface-sensitive, confirmed a dominant presence of Ce⁴⁺/Ce³⁺ species. This complementarity indicates that catalytic performance is governed primarily by surface-active cerium species rather than bulk uniformity, consistent with the improved stability and lower coke yield of Ce-loaded MKRW-800A. Overall, all metal-loaded catalysts outperformed thermal cracking in conversion, product selectivity, and coke suppression. The combination of acid activation and metal incorporation into the kaolinite matrix yields a thermally stable, low-cost, and environmentally benign catalyst. Among them, Ce-modified MKRW-800A exhibits the most balanced interplay between surface acidity, redox capacity, and mesoporosity, key attributes for industrial-scale VR upgrading, cleaner fuel production, and sustainable refinery operations.