Next Article in Journal
Flexible Demand Side Management in Smart Cities: Integrating Diverse User Profiles and Multiple Objectives
Previous Article in Journal
Hydrogen Cost and Carbon Analysis in Hollow Glass Manufacturing
Previous Article in Special Issue
European Green Deal: Substantiation of the Rational Configuration of the Bioenergy Production System from Organic Waste
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Energies
Volume 18
Issue 15
10.3390/en18154106
energies-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Tuning the Activity of NbOPO4 with NiO for the Selective Conversion of Cyclohexanone as a Model Intermediate of Lignin Pyrolysis Bio-Oils

by
Abarasi Hart
and
Jude A. Onwudili
*
Energy and Bioproducts Research Institute, College of Engineering and Physical Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK
*
Author to whom correspondence should be addressed.
Energies 2025, 18(15), 4106; https://doi.org/10.3390/en18154106
Submission received: 4 July 2025 / Revised: 29 July 2025 / Accepted: 30 July 2025 / Published: 2 August 2025
(This article belongs to the Special Issue Recent Advances in Biomass Energy Torrefaction, Pyrolysis and Gasification Technologies)
Browse Figures
Versions Notes

Abstract

Catalytic upgrading of pyrolysis oils is an important step for producing replacement hydrocarbon-rich liquid biofuels from biomass and can help to advance pyrolysis technology. Catalysts play a pivotal role in influencing the selectivity of chemical reactions leading to the formation of main compounds in the final upgraded liquid products. The present work involved a systematic study of solvent-free catalytic reactions of cyclohexanone in the presence of hydrogen gas at 160 °C for 3 h in a batch reactor. Cyclohexanone can be produced from biomass through the selective hydrogenation of lignin-derived phenolics. Three types of catalysts comprising undoped NbOPO4, 10 wt% NiO/NbOPO4, and 30 wt% NiO/NbOPO4 were studied. Undoped NbOPO4 promoted both aldol condensation and the dehydration of cyclohexanol, producing fused ring aromatic hydrocarbons and hard char. With 30 wt% NiO/NbOPO4, extensive competitive hydrogenation of cyclohexanone to cyclohexanol was observed, along with the formation of C6 cyclic hydrocarbons. When compared to NbOPO4 and 30 wt% NiO/NbOPO4, the use of 10 wt% NiO/NbOPO4 produced superior selectivity towards bi-cycloalkanones (i.e., C12) at cyclohexanone conversion of 66.8 ± 1.82%. Overall, the 10 wt% NiO/NbOPO4 catalyst exhibited the best performance towards the production of precursor compounds that can be further hydrodeoxygenated into energy-dense aviation fuel hydrocarbons. Hence, the presence and loading of NiO was able to tune the activity and selectivity of NbOPO4, thereby influencing the final products obtained from the same cyclohexanone feedstock. This study underscores the potential of lignin-derived pyrolysis oils as important renewable feedstocks for producing replacement hydrocarbon solvents or feedstocks and high-density sustainable liquid hydrocarbon fuels via sequential and selective catalytic upgrading.

1. Introduction

Pyrolysis technology is widely used to convert lignin-based biomass into biofuels and valuable chemicals. In addition to abundant lignin biomass, the pulping industry produces roughly 50 million tonnes of lignin per annum globally [1]. Pyrolysis bio-oil derived from lignin is primarily composed of phenolic compounds, such as phenol, guaiacol, and their derivatives [2,3]. These phenolic compounds can undergo hydrogenation over suitable heterogeneous catalysts, yielding over 85% cyclohexanone [4,5,6]. This derived cyclohexanone can serve as a feedstock for high-density bicycloalkanes and alkyl aromatics required in the transition to sustainable aviation fuels (SAFs). Producing SAFs from lignocellulosic biomass and biomass-derived feedstock offers a promising pathway for the decarbonisation of the aviation industry. In a recent Royal Society report, aviation fuels derived from biomass are low- or zero-carbon energy sources that might have a significant positive influence on the UK’s route to Net Zero and climate change mitigation [7]. This is in line with the United Nations’ Sustainable Development Goal (SDG) 13, which calls for swift action to alleviate the effects of climate change from fossil fuels. Based on a white paper published by the World Economic Forum [8], there is a huge mismatch between the predicted global demand for SAF and its production capacity. For example, global demand for SAF is expected to reach 17 million tonnes per year by 2030, and the current production capacity would need an additional 5.8 million tonnes to meet demand [8]. Hence, aviation industry operators and stakeholders are investing heavily in the development of various process routes for SAF production. While the hydroprocessed vegetable oil (HVO) route and other routes based on lipid feedstocks have seen consistent commercial growth, they mostly produce linear and branched-chain alkanes. Therefore, it is an operational necessity to limit the blending of SAF with fossil kerosene to 50%. Fossil kerosene provides the needed cycloalkanes for energy density [9] and aromatic compounds for better engine performance and efficient seal-swelling capacity [10]. Therefore, producing and using 100% sustainable aviation fuels containing high-energy-density components for current airplane engines is an important research area.
Given the limited supply of lipid-based feedstocks, conventional pyrolysis bio-oils offer opportunities for large-scale SAF production using appropriate chemistries [7,11]. However, the carbon chains of compounds in bio-oils are generally C6–C8 [12], which are much shorter than those of petroleum-based fuels such as aviation kerosene (C8–C16 hydrocarbons) and diesel. In contrast, lignin-derived bio-oils, consisting of a wide range of phenolic compounds, can be converted via selective hydrogenation to appropriate feedstocks such as cyclic ketones for the synthesis of high-energy-density bi-cycloalkanones via aldol condensation and C-C coupling. For instance, C-C coupling reactions of cyclic carboxylic acids, ketonisation [13,14], cyclic ketones, aldol condensation [15,16], alkylation, and hydrodeoxygenation [17], have been explored to elongate the carbon chains to match those in conventional fuels.
Carbon chains of monocyclic ketones commonly found in bio-oils derived from lignocellulosic biomass and lignin can serve as sustainable feedstocks for transformation into aviation fuel intermediate molecules such as bi-cycloalkanones via aldol condensation [18]. In the recent literature, cyclohexanone, a model cyclic ketone that can be produced through the hydrogenation of lignin-based phenolic compounds [19,20], has been converted and further hydrodeoxygenated to produce mono/bi-cycloalkanes, their alkylated derivatives, and alkyl monoaromatics, which exhibit high energy density, thermal stability, and volumetric heating values [21]. High-density bi-cycloalkane and cycloalkyl aromatic hydrocarbons suitable for SAF cannot be produced from certified methods based on lipids, sugars, and fatty acids because the chemical structures in these feedstocks lack the required moieties [22]. As a result, monocyclic ketones (e.g., C5/C6) found in lignin-based pyrolysis bio-oils with the moieties for high-density SAF hydrocarbon production are a potential feedstock. In addition to producing C-C coupled high-density SAF hydrocarbons, aldol condensation of cyclohexanone derived from lignin-based pyrolysis bio-oil provides a route to renewable chemicals for fuels and other applications and sustainable solvents. It has been reported that tuning catalytic sites through metal incorporation can maximise reaction intermediates in coupling reactions [23]. In literature, a NiO/NbOPO4 catalyst demonstrated significant activity in the hydrodeoxygenation (HDO) of palm oil into diesel [24]. NiO is known to provide active sites for hydrogen activation and C-O bond cleavage during HDO, promoting hydrogen activity [24] and moderating surface acidity, and its tuneable redox activity enables a change in oxidation state from Ni2+ to Ni3+. Therefore, by doping the synthesised NbOPO4 with NiO, the surface acidity can be optimised for C-C coupling to maximise selectivity towards key adduct intermediates due to the lower energy barrier and hydrogenation functionality of the catalyst. As reported in the literature, the NiO oxygen vacancies function as catalytically active sites, modulating the electronic structure of the catalyst and promoting efficient cyclohexanone and hydrogen adsorption and reaction pathways [25,26]. The importance of oxygen vacancies in catalytic processes has also been highlighted in a bimetallic MnCo2O4.5 sphere catalyst, which exhibited improved catalytic performance in methane catalytic oxidation due to oxygen activation and migration [27].
In contrast, about 95% selectivity towards the yield of single C-C condensation adducts of C12 has been produced via self-aldol condensation of cyclohexanone using aryl sulfonic acid supported on mesoporous silica (e.g., SO3H-SBA-15) at 100 °C for 2 h [19] and PdFe/hierarchical-ZSM catalyst at 240 °C [15]. However, the conversion of cyclohexanone was limited in the range of 20−55%. Additionally, self-aldol condensation of cyclohexanone has been explored using the superhydrophobic and superacid magnetic catalysts Fe3O4@SiO2@F-SO3H, HZSM-5, and Fe3O4@SiO2 at 4.0 g cyclohexanone, 0.4 g catalyst, and 150 °C for 48 h [15]. The results showed that the solid acid catalyst Fe3O4@SiO2@F-SO3H outperformed the other catalysts with cyclohexanone conversion of 78.1%, a bicyclic adduct yield of 73.6%, and bicyclic adduct selectivity of 94.2%. This came at a high catalyst-to-feedstock ratio of 0.1 (g/g) and an extensive reaction time of 48 h, which limits industrial applications. Furthermore, low conversion of cyclohexanone 29.1% was achieved with the TiO2/Al2O3 catalyst at a 120 °C reaction temperature, 8 wt% catalyst loading, 300 rpm stirring rate, and 120 min reaction time [28]. These catalysts are expensive and structurally complex and involve several synthesis steps and strategies, which limit scalability and industrial applications. Additionally, their activity declines due to in situ-produced water from the dehydration of aldol condensation intermediate products. As a result, a solid acid catalyst that is highly selective, water-resistant, and capable of maintaining high catalytic activity and favourable catalyst reusability is required. In comparison with acidic catalysts, alkaline catalysts, acid/base amphoteric catalysts, and ionic liquids, basic catalysts (e.g., NaOH) catalyse aldol condensation, resulting in high conversion and yield, but the recovery of liquid alkali from the product is costly and challenging [29].
Self-aldol condensation of cyclic ketone is likely to compete with side reactions like dehydration of the cyclic ketone to cycloalkene or hydrogenation into cyclic alcohol. Previous studies have shown that the NbOPO4 catalyst is hydrothermally stable and highly selective towards the production of the high-density aviation fuel precursor adducts such as 2-cyclohexylidenecyclohexanone, 2-(1-cyclohexen-l-yl) cyclohexanone, and [1,1′-bicyclohexyl]-2-one from cyclohexanone via aldol condensation [16,21]. One-pot two-stage sequential aldol condensation and hydrodeoxygenation would be economical and sustainable in the production of high-density aviation fuel-range hydrocarbons from biomass-derived cyclic ketones. A previous study showed that one-pot two-stage sequential self-aldol condensation of cyclohexanone at 160 °C followed by HDO at 300 °C produced high-density alkyl mono-/bi-cycloalkanes and alkyl monoaromatics (e.g., cyclohexyl benzene) using the NbOPO4 catalyst [21]. Despite the in situ water by-product, the catalyst exhibited excellent activity and selectivity for both aldol-condensation and HDO. However, the strong acidity of NbOPO4 led to excessive formation of undesirable polycyclic aromatic hydrocarbons (PAHs). Based on this consideration, this study aims to identify catalysts capable of moderating the formation of key C-C intermediates on their acidic sites to optimise the selectivity of aldol condensation and balance metallic site density to activate the HDO in a one-pot two-stage process. Thus, doping the synthesised NbOPO4 with NiO could optimise its acidic property for C-C coupling and provide the metallic site for the HDO stage in one-pot two-stage sequential aldol condensation and HDO. Therefore, tuning the acidity of NbOPO4 with NiO can influence its selectivity towards desirable bicyclic ketones, bicyclic aliphatic hydrocarbons, and monocyclic aliphatic hydrocarbons. Nickel-based catalysts are well known for their vast hydrogenation activity in fuel processing [30], and NbOPO4 catalysts doped with NiO may suppress the formation of PAHs. In the present study, the influence of doping NiO on the surface activity and selectivity of NbOPO4 during the catalytic conversion of cyclohexanone was investigated in a batch reactor without additional solvents. The aim of this study was to understand and evaluate the tuning effect of NiO on NbOPO4 towards the selective aldol condensation of cyclohexanone to C-C coupled bicyclic oxygenate adduct (C12) precursors or its direct hydrogenation to single-ring compounds. Results from the study could offer a novel combination of low-cost, scalable catalytic routes converting lignin-derived pyrolysis oils into high-density sustainable aviation fuels and/or sustainable hydrocarbon solvents and chemical feedstocks.

2. Materials and Methods

2.1. Materials and Catalyst Preparation

We used 99+% purity cyclohexanone (Fisher Scientific, Leicester, UK) as a model cyclic ketone feedstock. The materials for catalyst preparation include niobium(v) oxalate hydrate, diammonium hydrogen phosphate, phosphoric acid, cetyltrimethyl ammonium bromide, CTAB (as a structural directing agent), and nickel(II) nitrate hexahydrate. All of these materials were purchased from Fisher Scientific, Leicester, UK. They were used as feedstock for experiments and NbOPO4 catalyst preparation without further treatment. Hydrogen gas (99+% purity) supplied by BOC Gas, UK, was used.
The NbOPO4 catalyst was synthesised by dissolving the precursor niobium(v) oxalate hydrate (28.1 g) in 100 mL of distilled water. It was mixed with a solution of 6.6 g diammonium hydrogen phosphate and 100 mL of distilled water. Both solutions were thoroughly mixed with a magnetic stirrer at ambient temperature, and a pH of 2 was maintained using phosphoric acid. A solution of cetyltrimethyl ammonium bromide (CTAB), 5.0 g in 75 mL of distilled water, was added, while the pH was maintained at 2. The mixture was stirred at 35 °C for 60 min. It was then transferred to a Teflon-lined autoclave for hydrothermal precipitation at 160 °C for 24 h. The recovered filtrate was washed, dried at 100 °C, and calcined at 500 °C for 4 h. The NiO-doped NbOPO4 was prepared via wet impregnation using the appropriate nickel precursor Ni(NO3)2·6H2O. The specific surface area, pore volume, and pore diameter of the synthesised catalysts were determined with the nitrogen physisorption technique using a Quantachrome Instruments NOVA 4200 (Quantachrome UK Limited, Leicester, UK) [21]. A Thermo Scientific Nicolet iS50 FTIR Spectrometer (Thermo Fisher Scientific, Cheshire, UK) for pyridine Fourier-transform infrared spectroscopy was used to determine the acid sites of the catalysts (i.e., Brønsted acid and Lewis acid sites). The deposited char on the catalyst after reactions was quantified using the thermogravimetric analyser (TGA) method (TGA/DSC 2 STARe System (Mettler-Toledo International, Leicester, UK), and the crystal structures of the synthesised catalysts were determined using X-ray diffraction (XRD) Bruker D8 Advance A25 (Bruker, Cambridge, UK) [16,21]. Detailed properties of the synthesised catalysts have been reported elsewhere [21]. Hence, the properties of the NbOPO4- and NiO-doped catalysts are summarised in Table 1. The specific surface area and total acidity of the NbOPO4 decreased as the NiO loading increased from 0 to 30 wt%. The Ni metal loading was determined using an iCAP 7000 series Inductively Coupled Plasma Optical Emission Spectroscopy device (ICP-OES Spectrometer-elemental analyser, Thermo Fisher Scientific, Cheshire, UK).

2.2. Catalytic Reactions of Cyclohexanone and Product Analysis

The solvent-free catalytic reactions of cyclohexanone were carried out in a 100 mL Parr non-stirred autoclave reactor (Parr Instrument Company, Moline, IL, USA) under a hydrogen atmosphere. Each experiment was conducted using 10 g cyclohexanone and 0.5 g catalyst loading suspended in the reactor with the aid of glass wool. Reactions were carried out at 160 °C and 10 bar initial hydrogen pressure. The reactions were expected to occur mostly in the vapour phase when the boiled-off cyclohexanone made contact with the catalyst suspended on the glass wool. This protocol, validated from previous studies [16,21,31], was found to significantly reduce the mass transport limitation and temperature gradient compared to mixing the catalyst with the feedstock.
The quantification and composition of the liquid products were carried out using gas chromatography–mass spectrometry, GC-MS (GCMS-QP2010 SE, Shimadzu, Milton Keynes, UK), based on previous work [16]. Briefly, the mass selective detector was operated using electron impact (EI) ionisation. For compound separation, a 30-meter-long SH-Rtx-5MS column with a 0.25 mm internal diameter, provided by Thames Restek (Saunderton, UK), was employed. The oven temperature was initially maintained at 40 °C for 6 min and then increased at a rate of 3 °C per minute until reaching 180 °C. After that, the temperature ramped up at 10 °C per minute to a final temperature of 280 °C, concluding the analysis. To estimate the amount of cyclohexanone converted, a calibration curve was prepared using 100, 250, 400, and 500 µL of cyclohexanone per 1 mL of acetone (vol/vol). The conversion of cyclohexanone was calculated based on the amount converted (i.e., the amount of cyclohexanone fed minus the amount remaining after the experiment) as shown in Equation (1), while selectivity was calculated as the peak area of a product divided by the total peak area of all compounds identified with GC-MS using Equation (2). Comparing this approach to internal or external standard techniques of quantification, it may not account for unknown substances. GC-MS peak areas, on the other hand, might serve as a reliable indicator of the different compound concentrations present in the liquid product after aldol condensation. In addition, a calibration curve of peak areas vs. mole of cyclohexanone was utilised to compute the conversion of cyclohexanone based on the amount fed into the reactor and converted via the aldol condensation reaction.
C o n v e r s i o n   % = m o l e s   o f   c y c l o h e x a n o n e   c o n v e r t e d m o l e   o f   c y c l o h e x a n o n e   f e d   i n t o   t h e   r e a c t o r × 100
S e l e c t i v i t y   % = P e a k   A r e a   o f   c o m p o n e n t i T o t a l   P e a k   A r e a   o f   a l l   C o m p o n e n t s   i n   l i q u i d   p r o d u c t × 100

3. Results and Discussion

3.1. X-Ray Diffraction (XRD)

Figure 1 shows the XRD patterns of the synthesised NbOPO4 and its NiO-doped versions. It reveals that NbOPO4 and 10 wt% NiO/NbOPO4 are amorphous in nature. The observed amorphousness of 10 wt% NiO/NbOPO4 can be attributed to homogeneity, with nanoscale-sized NiO approaching that of the amorphous substrate NbOPO4 support. Hence, there is no clear distinction between the NiO nanoparticles and the background structure size in Figure 1 [32]. This can be confirmed by the identical XRD profiles of NbOPO4 and 10 wt% NiO/NbOPO4, showing two broad peaks at 2θ = 25° and 53°, respectively. As the loading of NiO increased to 30 wt% on the surface of the NbOPO4, the size of the clusters increased, which is evident in the diffraction peaks identified at 2θ = 37°, 43°, 63°, 75°, and 79° in Figure 1. This formation of a large cluster of NiO at 30 wt% is the reason for its crystallinity compared to NbOPO4 and 10 wt% NiO/NbOPO4. The average diameter of NiO crystallite sizes based on the Scherrer equation is 41.4 ± 0.57 nm. This XRD pattern is consistent with that reported in the literature for NbOPO4 [33]. The nickel oxide (NiO) can be reduced to metallic nickel in the presence of a hydrogen atmosphere under the reaction conditions. However, as the temperature is low, there is partial reduction of NiO, resulting in incomplete conversion to form the Ni/NiO structure. This behaviour is in line with the hydrogen reduction of bulk NiO particles observed at low temperature using in situ hot-stage XRD [34].

3.2. C-C Coupled Adducts in Relation to NiO Loading on NbOPO4

This study aims to provide insights into how the NiO loading affected the catalytic activity and selectivity of NbOPO4 with respect to bicyclic C12 oxygenate adducts formation. Figure 2 presents the reaction pathways leading to the formation of products, based on the identified compounds from the GC-MS analysis of the liquid product (Supplementary Information Tables S1–S3). These pathways also agree with those reported in the literature using sulfonic acid supported on mesoporous silica [19], PdFe/hierarchical-ZSM catalysts [15], ion exchange resin [35], and hydrotalcite [36] catalysts. The identified products (Tables S1–S3) indicated that dehydration of cyclohexanone into cyclohexene and/or its hydrogenation into cyclohexanol were notable side reactions that competed with the self-aldol condensation pathway.
As shown in Figure 2, the aldol condensation of cyclohexanone would mechanistically involve two major steps: aldol addition, followed by dehydration into C12 C-C coupled bi-cycloalkanones. Firstly, cyclohexanone adsorbed onto the acid sites of the NbOPO4 catalyst to form an enol, which dimerised with a co-adsorbed cyclohexanone molecule. The polarity of the carbonyl group would activate its α-hydrogen, resulting in the formation of bicyclic β-hydroxyketone intermediate 1′-hydroxy-[1,1′-bicyclohexyl]-2-one [37]. Considering the acidity of NbOPO4, rapid dehydration and dehydrogenation of the intermediate adduct would follow to give [1,1′-bicyclohexyl]-2-one, 2-cyclohexylidenecyclohexanone, and 2-(1-cyclohexen-l-yl) cyclohexanone. The structures of these bicyclic compounds make them suitable as energy-dense jet biofuel precursors [20]. Figure 3 shows the influence of NiO loading on the NbOPO4-catalyzed solvent-free self-aldol condensation of cyclohexanone. The conversion of cyclohexanone increased in this order: 30wt% NiO/NbOPO4 (78 ± 3.23%) > NbOPO4 (67.7 ± 2.14%) > 10 wt% NiO/NbOPO4 (66.8 ± 1.82%) (Figure 3), whereas the selectivity towards dimeric adducts can be summarised in the following order: 10 wt% NiO/NbOPO4 (96.8 ± 2.82%) > NbOPO4 (66 ± 2.81%) > 30wt% NiO/NbOPO4 (55.4 ± 3.5%). The selectivity of 10 wt% NiO/NbOPO4 towards the bicyclic compounds represented about 31% and 41% over those obtained when just NbOPO4 and 30 wt% NiO/NbOPO4 catalysts were used. The phosphate and niobium components contribute to Brønsted and Lewis acid sites of NbOPO4, which are known to activate C-C coupling, leading to the formation of bi-cycloalkanone intermediates [24]. However, tuning NbOPO4 through NiO impregnation appeared to have moderated the catalyst surface acidity (Table 1), which optimised the selectivity towards C-C condensed oxygenate adducts (i.e., C12). The higher acidity of just NbOPO4 promoted bicyclic C12 oxygenate adducts’ overreaction via dehydration to cyclohexylbenzene (Figure 2, Table 1 and Table S1), and bicyclic ketones condensation into 2+ C-C multiple ring C18 adducts (Table S1). The multiple-ring compounds included polycyclic aromatic hydrocarbons (PAHs), which are not suitable for jet fuel. The presence of undesired polycyclic adducts (e.g., 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydro-triphenylene) was significant with NbOPO4, explaining the higher conversion but low selectivity towards bicyclic C12 precursors observed.
In contrast, the hydrogenation pathway in Figure 2 would begin with the conversion of cyclohexanone to cyclohexanol and then to cyclohexene via dehydration, and finally to cyclohexane via hydrogenation. Hence, higher loading of the NbOPO4 with 30 wt% NiO increased hydrogenation of the cyclohexanone side reaction, resulting in the formation of cyclohexanol (Figure 2), which lowered the selectivity to bicyclic C12 adducts. GC-MS results for the liquid product when 30wt% NiO/NbOPO4 was used revealed a significant presence of cyclohexene, cyclohexane, and cyclohexylbenzene (Tables S1–S3), thus indicating the competing conversion of cyclohexanone into cyclohexene and dehydration of single C-C coupled oxygenate C12 adducts into cyclohexyl benzene (Figure 2). The formation of dehydration products was higher with NbOPO4 than with its NiO-doped versions (Tables S1–S3), due to its stronger acid sites (Table 1). Notably, as the NiO loading on NbOPO4 increased from 0 to 30%, the selectivity towards the formation of multiple-ring products (2+ C-C coupled adducts) decreased from 32.9% to 1.3% (Figure 3). Evidently, as the NiO content increased, the hydrogenation functionality of the catalyst increased, while the total acidity of the NbOPO4 acid sites responsible for the C-C coupling reaction decreased (Table 1) [26].
On the flipside, the 30 wt%NiO/NbOPO4 catalyst also exhibited considerable selective activity towards the production of cyclohexanol (i.e., without C-C coupling condensation) (Figure 2; Table S3). This activated side reaction, when the 30 wt%NiO/NbOPO4 catalyst was used, significantly competed with self-aldol condensation, thereby decreasing the formation of single C-C coupled bicyclic adducts (Figure 2). Moreover, the side reaction leading to the formation of polycyclic adduct products, such as 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydro-triphenylene, significantly decreased as NiO loading increased from 0 to 30 wt% (Supplementary Information Tables S1–S3). Again, mechanistically, the Brønsted and Lewis acid sites of the NbOPO4 would activate protonation of the cyclohexanone and subsequent enolate intermediate formation, a nucleophile attacking another carbonyl carbon of the cyclohexanone molecule to form the C-C bond [38]. Consequently, the acid sites would facilitate spontaneous dehydration. Notably, as the surface acidity of the catalyst decreased, the extent of dehydration and PAH formation decreased (Table 1). As reported in the literature, the oxygen vacancies of NiO function as catalytically active sites, modulating the electronic structure of the catalyst and promoting efficient cyclohexanone and hydrogen adsorption reaction pathways [25,26]. Therefore, compared to NbOPO4 and 30 wt% NiO/NbOPO4 catalysts, the 10 wt% NiO/NbOPO4 catalyst greatly suppressed the formation of by-products resulting from side reactions (Figure 3). Thus, due to the trade-off between surface acidity and hydrogenation activity exhibited by 10 wt% NiO, the reaction pathway favoured the C-C coupling reaction into C12 cycloalkanone adducts. Similarly, the balance of acid sites via Ni loading in the Ni/HZSM-5 catalyst for aldol condensation and hydrogenation was reported to enhance the yield of the desired product [38]. It has also been reported that the surface properties of a catalyst, mostly its acidity and basicity, significantly influence the rate and selectivity of aldol condensation reactions [39].
Figure 4 shows the selectivity of products in terms of oxygenate adducts and hydrocarbons formed from cyclohexanone during these catalytic reactions. All of the liquid products indicated that the major products from the NbOPO4 catalysed solvent-free self-aldol condensation were the desired single C-C coupled bicyclic C12 oxygenate adducts, such as [1,1’-bicyclohexyl]-2-one, 2-cyclohexylidenecyclohexanone, and 2-(1-cyclohexen-l-yl) cyclohexanone). High-density bi-cycloalkanes and alkyl monoaromatic aviation fuel-range hydrocarbons with chains in the range of C11-C12 can be formed from these precursors [16,19]. The acidity of the catalysts is one of the critical factors influencing their activity in the formation of these products, especially the dehydration step.
The acidic and metallic sites of the bifunctional NiO supported on NbOPO4 catalysts played a synergistic role in facilitating hydrodeoxygenation (HDO) of the cyclohexanone and bicyclic oxygenate adducts, leading to the formation of non-aromatic (alicyclic) hydrocarbons. The selectivity to these hydrocarbons via HDO followed the same trend as the formation of bicyclic adducts: 10 wt% NiO/NbOPO4 > NbOPO4 > 30 wt% NiO/NbOPO4 (Figure 4). It has been reported that in a bifunctional catalyst, the metallic sites activate hydrogen-donating and hydrogenation reactions, while the principal acidic sites activate C-O bonds [40]. Furthermore, Ni metal is oxophilic (i.e., propensity to form oxides through hydrolysis or abstraction of oxygen), which allows oxygenated compounds (e.g., C=O bond) to be readily absorbed. This property enables the activation of hydrogen molecules and facilitates hydrogenation/dehydrogenation reactions [41]. In the present study, NiO dopant provided active sites for hydrogen-based activity, moderating surface acidity for C-C coupling and suppressing condensation into PAHs [24]. From Figure 4, the moderate acidity of 10 wt% NiO/NbOPO4 further improved its selectivity towards single C-C coupled oxygenate adducts. Additionally, the interface between the NbOPO4 support and the 10 wt% NiO could have promoted effective cyclohexanone and hydrogen adsorption, dissociation, and synergistic effects, improving selectivity towards C12 bicyclic ketone intermediates [26]. In contrast, 30 wt% NiO/NbOPO4 produced more C6 oxygenate precursors via the hydrogenation of cyclohexanone into cyclohexanol. Therefore, for one-pot two-stage aldol condensation and then HDO, the following reactions are likely to compete: dehydration, hydrogenation/dehydrogenation, polymerisation, and aromatisation [42]. The occurrence of these reactions depended on the bifunctionality (i.e., acidic and metallic sites) properties of the NbOPO4 catalyst (Table 1). For instance, in a one-pot two-stage reaction process, it was found that a 10 wt% NiO/NbOPO4 catalyst produced the highest levels of bi-cycloalkanes, alkyl monoaromatic (C7-C12). It also produced partially hydrogenated polyaromatic hydrocarbons (C18) but significantly suppressed PAHs formation [21]. Therefore, it is evident from the present study that the acidity of the NbOPO4 baseline material was tuned via NiO loading. As a result, with the 10 wt% NiO/NbOPO4 catalyst, significant production of SAF-relevant energy-dense precursors such as bi-cycloalkanones was achieved. Thus, the 10 wt% NiO/NbOPO4 catalyst exhibited balanced acidity, promoted mild hydrogenation, optimised the C12 adduct, and enhanced the yields of SAF precursors.
Table 2 compares the performance of 10 wt% NiO/NbOPO4 to that of other catalysts reported in the literature in terms of conversion and selectivity for cyclohexanone self-aldol condensation. Most of the catalysts showed low conversion and high selectivity towards bicyclic C12 oxygenate adducts. It can be observed that zeolite-based catalysts such as MCM-41 and HZSM-5 have low conversion and low selectivity, suggesting that they are unsuitable for aldol condensation reactions despite their strong acid sites [43]. For instance, the literature shows that the PdFe/hierarchical-ZSM catalyst achieved cyclohexanone conversion of 54.1% and selectivity to bicyclic adducts C12 of 97.4% at a reaction temperature of 240 °C, pressure of 4 bar, and reaction time of 8 h [15]. From Table 2, the results demonstrate that 10 wt% NiO/NbOPO4 exhibited superior activity and selectivity towards bicyclic C12 oxygenate adduct precursors over other catalysts. In a previous study [16], higher cyclohexanone conversion was observed for the NbOPO4 catalyst under nitrogen atmosphere. Such a reaction condition was deemed to have facilitated the dehydration and dehydrogenation of primary bicyclic oxygenate adducts into cyclohexylbenzene as well as the formation of undesired polycyclic adduct products [16]. Additionally, the superhydrophobic and superacid magnetic catalyst Fe3O4@SiO2@F-SO3H catalyst achieved cyclohexanone conversion of 78.1% and bicyclic adduct selectivity of 94.2% at 150 °C for 48 h [20]. Clearly, 10 wt% NiO/NbOPO4 accomplished higher conversion and comparable selectivity at a lower reaction temperature of 160 °C and a shorter reaction time of 3 h. Consequently, the 10 wt% NiO/NbOPO4 catalyst is cost-effective, simple to prepare, and scalable compared to these catalysts. The 10 wt% NiO/NbOPO4 catalyst can be prepared via established hydrothermal precipitation and wet impregnation methods, simplifying its scalability and industrial-scale applications.

3.3. Coke Formation in Relation to NiO Loading on NbOPO4

The acid sites of the NbOPO4 catalyst can activate further structural condensation, leading to the formation of 2+ C-C adducts and polycyclic compounds (e.g., 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydro-triphenylene). Subsequent dehydrogenation of the naphthenic ring can lead to the formation of coke. The thermogravimetric (TG) and differential TG (DTG) profiles of the spent catalysts after aldol condensation at 160 °C for 3 h were carried out in an air atmosphere using a temperature programme from 40 to 900 °C at 10 °C/min and 900 °C for 10 min. The DTG curves in Figure 3 show two regions: the first region (loss of absorbed cyclohexanone, C-C coupled bicyclic and polycyclic adduct products) and the second region (loss of formed coke) [46,47,48,49]. The resulting coke from the aldol condensation of cyclohexanone can be classified as follows: (1) soft coke, the early formation of hydrogen-rich carbonaceous material, or (2) hard coke, an unsaturated carbonaceous deposit formed by the further dehydrogenation of earlier formed soft coke. Essentially, carbonaceous materials formed on spent catalysts, which burn off in the temperature region of 400 °C, are classified as soft coke [50].
Figure 5 shows the TG and DTG curves of coke formed on NbOPO4 and its NiO-doped catalysts during the aldol condensation of cyclohexanone. The absorbed light organics on the catalyst, such as cyclohexanone, C-C coupled adducts, and other side products, volatilised and decomposed in the temperature range of 100–368 °C (NbOPO4), 100–336 °C (10 wt% NiO/NbOPO4), and 100–292 °C (30 wt% NiO/NbOPO4). The formed coke burn-off regions for the different catalysts were 368–600 °C (NbOPO4), 336–550 °C (10 wt% NiO/NbOPO4), and 292–417 °C (30 wt% NiO/NbOPO4), indicating a significant shift in burn-off temperature. Thus, the coke content of the spent catalysts was as follows: 12.2% (NbOPO4), 11.86% (10 wt% NiO/NbOPO4), and 11.4% (30 wt% NiO/NbOPO4).
The differential thermogravimetric (DTG) curves in Figure 3 indicate that the coke formed when a 10 wt% NiO/NbOPO4 catalyst was completely burnt off at 550 °C, which is 50 °C less than that produced with NbOPO4. A further increase in NiO loading to 30 wt% resulted in a 133 °C decrease in the burn-off temperature of the formed coke compared to that deposited on NbOPO4 (Figure 5). This difference affirmed the increasing hydrogenation functionality of the NiO as it increased from 10 to 30 wt% on NbOPO4. Hence, in the presence of NiO, the coke formed on the catalysts was less condensed and softer than that produced on the undoped NbOPO4. This is consistent with the trend in 2+ C-C adduct condensate compound formation, which follows the pattern NbOPO4 > 10 wt% NiO/NbOPO4 > 30 wt% NiO/NbOPO4 (Figure 3). The dehydrogenation of the naphthenic rings led to the formation of aromatic compounds, which are common coke precursors. Catalysts with strong acid sites, such as NbOPO4, are known to promote the formation of reactive intermediates that lead to the formation of coke precursors. Further condensation reactions between unsaturated molecules and dehydrogenation result in the formation of larger, more complex PAHs and eventually less volatile carbonaceous deposits, called coke [51], whereas the hydrogenation activity of NiO-doped NbOPO4 greatly stabilises the reactive intermediates and the catalysts, suppressing coke formation. Therefore, increasing NiO loading suppressed coke-forming reactions such as the formation of 2+ C-C adducts, further structural condensation, and aromatisation. Additionally, the formation of large molecular weight adduct compounds also conforms to the acidity of the catalysts, which follows the ranking of total acidity: NbOPO4 > 10 wt% NiO/NbOPO4 > 30 wt% NiO/NbOPO4 (Table 1). In essence, the combination of hydrogen atmosphere and NiO played a critical role in limiting the formation of aromatic and polyaromatic compounds. Possibly, the reduced amount of coke formed and its softness in the presence of NiO metal on the NbOPO4 could further prolong the lifetime of the catalyst relative to the undoped NbOPO4.

3.4. Gas, Coke, and Liquid Products Distribution

Based on the quantified coke formation, the mass balance of the gas, liquid, and solid product distribution is shown in Table 3. The balance closures are within approximately 96% accuracy, with any losses attributable to the condensation of water in the gas outlet line and other volatile losses upon opening the reactor. Except for the gas products, the yields of the coke and liquid products are within the margin of standard deviation error of triplicate experiments in Table 3. The gas product is composed of C1-C4 alkanes and olefin gases, H2, CO2, and CO, indicating the occurrence of ring-opening reactions, cleavage of the C-C bond, and dehydrogenation reactions. The typical compositions of these gases have been well documented elsewhere [16].
As mentioned above, the presence of NiO significantly suppressed undesirable side reactions that could form 2+ C-C coupled adduct products (Figure 3 and Figure 4). Otherwise, these large molecular weight polycyclic compounds (e.g., 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydro-triphenylene) could absorb on the catalyst surface and consequently result in coke formation due to the higher acidity of just the NbOPO4 (Figure 5). Thus, the catalyst lifespan would be greatly shortened with just NbOPO4 relative to NiO-doped versions. Evidence from these experimental results demonstrates that about 10 wt% NiO loading on NbOPO4 could effectively catalyze the sequential solvent-free aldol condensation of cyclohexanone. The higher selectivity of the 10 wt% NiO/NbOPO4 catalyst towards single C-C bicyclic C12 adducts, and low yield of coke maximised the yield of high-density aviation fuel hydrocarbons. This was also observed in the preliminary results of one-pot and two-stage aldol condensation and HDO reported in the literature [21]. An approach such as this appeared to be effective for producing bicyclic aliphatic compounds from lignin-derived cyclic ketones via aldol condensation. The conversion of these intermediates to high-density aviation fuel-range hydrocarbons in a subsequent catalytic HDO process could be both sustainable and economically viable, for which further studies would be required.
A preliminary study on the effect of 10 wt% bio-oils on cyclohexanone conversion at 160 °C for 3 h with the same catalysts showed a significant reduction in conversion and selectivity, possibly due to poor volatilisation of condensation products to reach the suspended catalyst bed [21]. This observation suggests the need for further studies on the influence of reaction conditions and catalyst metal-support optimisation. Moreover, the NbOPO4 catalyst demonstrated stability and reusability in three cycles of repeated aldol condensation reaction with negligible changes in the conversion of cyclohexanone and selectivity towards single C-C condensation adducts (i.e., C12), constantly within 93% [16]. In this present study, coke formation on the catalysts was observed to decrease as the NiO loading increased from 0 to 30 wt%, which may imply improvement in the catalysts’ stability. However, further tests on the stability of the NiO-doped NbOPO4 catalysts will be explored for confirmation. In addition, techno-economic analysis and life-cycle assessment of the process would be carried out to evaluate its scalability, viability, and sustainability.

4. Conclusions

The Brønsted and Lewis acid sites of NbOPO4 effectively catalyzed the self-aldol condensation of cyclohexanone, a model compound representative of lignin pyrolysis bio-oil, into C12 coupled C-C adducts. The biofuel intermediates produced were within the range of the high-density SAF. The surface acidity of NbOPO4 was tuned via doping with NiO, which decreased the total acidity as the dopant’s content increased from 0 to 30 wt%. The results showed that the moderated surface acidity of 10 wt% NiO/NbOPO4 maximised the selectivity towards bi-cycloalkanones (i.e., C12) and suppressed condensation into polycyclic aromatic hydrocarbons. Hence, tuning the surface acidity and hydrogenation functionality of NbOPO4 could facilitate the rational design and development of cost-effective, stable, and selective catalysts for SAF production and scale-up for industrial applications. However, as the NiO loading increased to 30 wt%, the hydrogenation of cyclohexanone into cyclohexanol became a strongly competitive reaction against aldol condensation into single C-C coupled adduct precursors (C12). This was attributed to the reduced acidity of the NbOPO4 with increasing NiO loading. Moreover, using undoped NbOPO4 alone as a catalyst produced hard coke, while NiO-doped NbOPO4 catalysts produced soft coke. Preferably, successful conversion of the C12 precursors to high-density hydrocarbon components could represent an important approach towards producing a bio-based SAF suitable for modern aircraft engines, without the need for blending with fossil kerosene. Moreover, the prospect of producing hydrocarbon solvents (cyclohexane) and chemical feedstocks (cyclohexane) is also interesting. Overall, optimizing the catalytic conversion of lignin-derived pyrolysis bio-oils to produce sustainable liquid hydrocarbons can advance pyrolysis technology towards decarbonisation of the global chemicals and energy sectors.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/en18154106/s1. Tables S1–S3 show the GC-MS results of liquid products from the catalytic aldol condensation of cyclohexanone using NbOPO4, 10 wt% NiO/NbOPO4, and 30 wt% NiO/NbOPO4 catalysts at 160 °C temperature, 3 h reaction time, 0.5 g loading of catalyst, and 10 bar initial hydrogen pressure.

Author Contributions

A.H.: investigation; methodology; data curation; formal analysis; validation; visualisation; and writing—original draft and editing. J.A.O.: conceptualisation, methodology; project administration; resources; supervision; investigation; validation; visualisation; writing—original draft; and writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Innovate UK Energy Catalyst Round 8: Clean Energy—Experimental Development (Project Number 75521).

Data Availability Statement

All data generated from this study are included in this paper and the Supplementary Information.

Acknowledgments

The authors would like to thank Innovate UK for funding this research and also thank all our PyroPower Africa 2 project partners for all their support. In addition, the authors are grateful to Energy & Bioproducts Research Institute (EBRI) and Aston University, UK, for all the support received.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SAFsSustainable Aviation Fuels
SDGSustainable Development Goal
HVOHydroprocessed Vegetable Oil
CTABCetyltrimethyl Ammonium Bromide
TGAThermogravimetric Analyser
XRDX-ray diffraction

References

  1. Elumalai, S.; Arumugam, B.; Kundu, P.; Kumar, S. Phenol derivatives of lignin monomers for aromatic compounds and cycloalkane fuels. In Biomass, Biofuels, Biochemicals; Elsevier: Amsterdam, The Netherlands, 2020; pp. 459–483. [Google Scholar] [CrossRef]
  2. Lu, X.; Gu, X. A review on lignin pyrolysis: Pyrolytic behavior, mechanism, and relevant upgrading for improving process efficiency. Biotechnol. Biofuels Bioprod. 2022, 15, 106. [Google Scholar] [CrossRef]
  3. Zhong, D.; Zeng, K.; Li, J.; Qiu, Y.; Flamant, G.; Nzihou, A.; Vladimirovich, V.S.; Yang, H.; Chen, H. Characteristics and evolution of heavy components in bio-oil from the pyrolysis of cellulose, hemicellulose and lignin. Renew. Sustain. Energy Rev. 2022, 157, 111989. [Google Scholar] [CrossRef]
  4. Shafaghat, H.; Rezaei, P.S.; Daud, W.M.A.W. Catalytic hydrodeoxygenation of simulated phenolic bio-oil to cycloalkanes and aromatic hydrocarbons over bifunctional metal/acid catalysts of Ni/HBeta, Fe/HBeta and NiFe/HBeta. J. Ind. Eng. Chem. 2016, 35, 268–276. [Google Scholar] [CrossRef]
  5. Xue, G.; Yin, L.; Shao, S.; Li, G. Recent progress on selective hydrogenation of phenol toward cyclohexanone or cyclohexanol. Nanotechnology 2022, 33, 072003. [Google Scholar] [CrossRef] [PubMed]
  6. Xu, K.; Weng, L.; Wu, L.; Tan, L.; Tang, Y. Hydrogenation of phenol to cyclohexanone catalyzed by isolated Pd cations in the micropores of zeolite. Appl. Catal. O Open 2024, 193, 206977. [Google Scholar] [CrossRef]
  7. Society, T.R. Net Zero Aviation Fuels: Resource Requirements and Environmental Impacts; Royal Society: London, UK, 2023; Available online: https://royalsociety.org/-/media/policy/projects/net-zero-aviation/net-zero-aviation-fuels-policy-briefing.pdf (accessed on 17 October 2024).
  8. World Economic Forum. Financing Sustainable Aviation Fuels: Case Studies and Implications for Investment; World Economic Forum: Geneva, Switzerland, 2025. [Google Scholar]
  9. Holladay, J.; Abdullah, Z.; Heyne, J. Sustainable Aviation Fuel: Review of Technical Pathways Report; U.S. Department of Energy: Washington, DC, USA, 2020. [Google Scholar]
  10. Gundekari, S.; Manupathi, A.; Chandu, S.; Varkolu, M.; Kumar, P.; Karmee, S.K. Catalytic hydroconversion of lignin-based aromatics to aviation fuels—A review. Biomass Convers. Biorefinery 2025, 15, 6557–6583. [Google Scholar] [CrossRef]
  11. Griffin, M.B.; Iisa, K.; Dutta, A.; Chen, X.; Wrasman, C.J.; Mukarakate, C.; Yung, M.M.; Nimlos, M.R.; Tuxworth, L.; Baucherel, X.; et al. Opening pathways for the conversion of woody biomass into sustainable aviation fuel via catalytic fast pyrolysis and hydrotreating. Green Chem. 2024, 26, 9768–9781. [Google Scholar] [CrossRef]
  12. Yevdokimova, O.; Shcherban, N.; Martinez-Klimov, M.; Weydisch, R.; Cueto, J.; Giner, E.A.; Eränen, K.; Peuronen, A.; Lastusaari, M.; Russo, V.; et al. Synthesis of jet-fuel precursors from renewable biomass through aldol condensation of cyclopentanone and furfural on base catalysts. Catal. Today 2025, 443, 114962. [Google Scholar] [CrossRef]
  13. Hart, A.; Patel, H.; Yildirir, E.; Onwudili, J.A. Influence of surface acidity/basicity of selected metal oxide catalysts and reaction atmospheres on the ketonisation of propionic acid to produce 3-pentanone as a liquid biofuel precursor. Renew. Energy 2025, 250, 123386. [Google Scholar] [CrossRef]
  14. Miller, J.H.; Hafenstine, G.R.; Nguyen, H.H.; Vardon, D.R. Kinetics and Reactor Design Principles of Volatile Fatty Acid Ketonization for Sustainable Aviation Fuel Production. Ind. Eng. Chem. Res. 2022, 61, 2997–3010. [Google Scholar] [CrossRef]
  15. Zhang, J.; Kuang, X.; Xiao, X.; Zhu, L.; Zhou, Z.; Qi, F. Efficient Synthesis C12 Biojet Fuel Precursors from Cyclohexanone via Self-Condensation. Energy Fuels 2025, 39, 8339–8346. [Google Scholar] [CrossRef]
  16. Hart, A.; Patel, H.; Onwudili, J.A. Sustainable Aviation Fuel-range Intermediates from Self-aldol Condensation of Cyclohexanone Using Low-cost Niobium Phosphate Catalyst. J. Clean. Prod. 2025, 508, 145597. [Google Scholar] [CrossRef]
  17. Chang, C.-F.; Paragian, K.; Sadula, S.; Rangarajan, S.; Vlachos, D.G. Sustainable Aviation Fuel Molecules from (Hemi)Cellulose: Computational Insights into Synthesis Routes, Fuel Properties, and Process Chemistry Metrics. ACS Sustain. Chem. Eng. 2024, 12, 12927–12937. [Google Scholar] [CrossRef] [PubMed]
  18. Li, X.; Sun, J.; Shao, S.; Hu, X.; Cai, Y. Aldol condensation/hydrogenation for jet fuel from biomass-derived ketone platform compound in one pot. Fuel Process. Technol. 2021, 215, 106768. [Google Scholar] [CrossRef]
  19. Martín, A.; Arribas-Yuste, E.; Paniagua, M.; Morales, G.; Melero, J.A. Efficient Self-Condensation of Cyclohexanone into Biojet Fuel Precursors over Sulfonic Acid-Modified Silicas: Insights on the Effect of Pore Size and Structure. ACS Sustain. Chem. Eng. 2024, 12, 10175–10185. [Google Scholar] [CrossRef]
  20. Zhang, J.; Wang, X.; Yang, B.; Zeng, X.; Zhang, Q.; Zou, J.-J.; Xie, J. Superhydrophobic and superacid magnetic catalyst induced highly selective aldol condensation and alkylation for high-density biofuels. Fuel 2024, 378, 132930. [Google Scholar] [CrossRef]
  21. Hart, A.; Onwudili, J.A.; Yildirir, E.; Hashemnezhad, S.E. Energy-dense sustainable aviation fuel-range hydrocarbons from cyclohexanone as a biomass-derived feedstock via sequential catalytic aldol condensation and hydrodeoxygenation. Chem. Eng. J. 2025, 509, 161494. [Google Scholar] [CrossRef]
  22. Zahid, I.; Nazir, M.H.; Chiang, K.; Christo, F.; Ameen, M. Current outlook on sustainable feedstocks and processes for sustainable aviation fuel production. Curr. Opin. Green Sustain. Chem. 2024, 49, 100959. [Google Scholar] [CrossRef]
  23. Wang, Y.; Xia, S.; Chen, K.; Zhang, J.; Yu, C.; Wu, J.; Wang, P.; Zhang, W.; Wu, Y. Balancing Intermediates Formation on Atomically Pd-Bridged Cu/Cu2O Interfaces for Kinetics-Matching Electrocatalytic C─N Coupling Reaction. Angew. Chem. Int. Ed. Engl. 2025, 64, e202503011. [Google Scholar] [CrossRef]
  24. Yani, F.T.; Husin, H.; Muhammad, S.; Abnisa, F.; Nasution, F. Palm oil hydrodeoxygenation into green diesel over NiO/NbOPO4 catalyst: A novel approach of synthesizing NbOPO4 from NbCl5. J. Clean. Prod. 2022, 354, 131704. [Google Scholar] [CrossRef]
  25. Zhou, J.; Yang, S.; Wan, W.; Chen, L.; Chen, J. Synergistic catalysis of mesoporous Cu/Co3O4 and surface oxygen vacancy for CO2 fixation to carbamates. J. Catal. 2023, 418, 178–189. [Google Scholar] [CrossRef]
  26. Puigdollers, A.R.; Schlexer, P.; Tosoni, S.; Pacchioni, G. Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies. ACS Catal. 2017, 7, 6493–6513. [Google Scholar] [CrossRef]
  27. Yang, K.; Li, C.; Zhu, Q.; Wang, H.; Qi, J. Rich Oxygen Vacancies in Bimetallic MnCo2O4.5 Spheres for Enhancing Lean Methane Catalytic Oxidation. Nanomaterials 2025, 15, 524. [Google Scholar] [CrossRef]
  28. Shi, Y.; Ren, X.; Liu, B.; Xu, S.; Zhang, H.; Gu, Y.; Xu, Y.; Lu, L.; Xu, W.; Liang, J. Ultrahigh selectivity self-condensation of cyclohexanone over TiO2/Al2O3 catalysts and kinetics study. React. Kinet. Mech. Catal. 2023, 136, 2071–2087. [Google Scholar] [CrossRef]
  29. Zhang, X.; Li, Y.; Qian, C.; An, L.; Wang, W.; Li, X.; Shao, X.; Li, Z. Research progress of catalysts for aldol condensation of biomass based compounds. RSC Adv. 2023, 13, 9466–9478. [Google Scholar] [CrossRef] [PubMed]
  30. Mudi, I.; Hart, A.; Ingram, A.; Wood, J. Catalytic Hydrodeoxygenation of Vanillin, a Bio-Oil Model Compound over Renewable Ni/Biochar Catalyst. Catalysts 2023, 13, 171. [Google Scholar] [CrossRef]
  31. Onwudili, J.A.; Scaldaferri, C.A. Catalytic upgrading of intermediate pyrolysis bio-oil to hydrocarbon-rich liquid biofuel via a novel two-stage solvent-assisted process. Fuel 2023, 352, 129015. [Google Scholar] [CrossRef]
  32. Heinemann, K.; Soria, F. On the detection and size classification of nanometer-size metal particles on amorphous substrates. Ultramicroscopy 1986, 20, 1–14. [Google Scholar] [CrossRef]
  33. Zhang, Y.; Wang, J.; Ren, J.; Liu, X.; Li, X.; Xia, Y.; Lu, G.; Wang, Y. Mesoporous niobium phosphate: An excellent solid acid for the dehydration of fructose to 5-hydroxymethylfurfural in water. Catal. Sci. Technol. 2012, 2, 2485–2491. [Google Scholar] [CrossRef]
  34. Richardson, J.T.; Scates, R.; Twigg, M.V. X-ray diffraction study of nickel oxide reduction by hydrogen. Appl. Catal. A Gen. 2003, 246, 137–150. [Google Scholar] [CrossRef]
  35. Peng, X.; Zeb, S.; Zhao, J.; Zhang, M.; Cui, Y.; Sun, G. Highly selective self-condensation of cyclohexanone: The distinct catalytic behaviour of HRF5015. R. Soc. Open Sci. 2020, 7, 200123. [Google Scholar] [CrossRef]
  36. Kikhtyanin, O.; Kadlec, D.; Velvarská, R.; Kubička, D. Using Mg-Al Mixed Oxide and Reconstructed Hydrotalcite as Basic Catalysts for Aldol Condensation of Furfural and Cyclohexanone. ChemCatChem 2018, 10, 1464–1475. [Google Scholar] [CrossRef]
  37. Chen, Y.; Yuan, S.; Yin, H.; Chen, Z.; Mao, C. Kinetics of the reversible dimerization reaction of cyclohexanone over γ-alumina catalyst. React. Kinet. Mech. Catal. 2011, 102, 183–194. [Google Scholar] [CrossRef]
  38. Shao, S.; Xia, X.; Li, X.; Zhang, H.; Xiao, R. Aldol condensation of biomass-derived aldehydes and ketones followed by hydrogenation over Ni/HZSM-5 to produce aviation fuel: Role of acid sites. Fuel Process. Technol. 2023, 250, 107904. [Google Scholar] [CrossRef]
  39. Li, Y.; Ge, T.; Jing, Z.; Guo, H.; Zheng, Y.; Li, Y.; Guo, X.; Huang, H.; Zuo, C. Enhancing aldol condensation with coupling oxygen storage materials and VPO catalysts: Impact of oxygen, kinetics and mechanism. Appl. Catal. A Gen. 2025, 693, 120136. [Google Scholar] [CrossRef]
  40. Wang, X.; Zhang, Z.; Yan, Z.; Li, Q.; Zhang, Y. Catalysts with metal-acid dual sites for selective hydrodeoxygenation of lignin derivatives: Progress in regulation strategies and applications. Appl. Catal. A Gen. 2023, 662, 119266. [Google Scholar] [CrossRef]
  41. Teles, C.A.; Rabelo-Neto, R.C.; de Lima, J.R.; Mattos, L.V.; Resasco, D.E.; Noronha, F.B. The Effect of Metal Type on Hydrodeoxygenation of Phenol Over Silica Supported Catalysts. Catal. Lett. 2016, 146, 1848–1857. [Google Scholar] [CrossRef]
  42. Zeng, Y.; Zhang, T.; Xu, Y.; Ye, T.; Wang, R.; Yang, Z.; Jia, Z.; Ju, S. Cu/Mg/Al hydrotalcite-like hydroxide catalysts for o-phenylphenol synthesis. Appl. Clay Sci. 2016, 126, 207–214. [Google Scholar] [CrossRef]
  43. Deng, Q.; Nie, G.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Highly selective self-condensation of cyclic ketones using MOF-encapsulating phosphotungstic acid for renewable high-density fuel. Green Chem. 2015, 17, 4473–4481. [Google Scholar] [CrossRef]
  44. Mahajan, Y.S.; Kamath, R.S.; Kumbhar, P.S.; Mahajani, S.M. Self-Condensation of Cyclohexanone over Ion Exchange Resin Catalysts: Kinetics and Selectivity Aspects. Ind. Eng. Chem. Res. 2008, 47, 25–33. [Google Scholar] [CrossRef]
  45. Lorenzo, D.; Santos, A.; Simón, E.; Romero, A. Kinetic of Alkali Catalyzed Self-Condensation of Cyclohexanone. Ind. Eng. Chem. Res. 2013, 52, 2257–2265. [Google Scholar] [CrossRef]
  46. Hart, A.; Leeke, G.; Greaves, M.; Wood, J. Down-hole heavy crude oil upgrading by CAPRI: Effect of hydrogen and methane gases upon upgrading and coke formation. Fuel 2014, 119, 226–235. [Google Scholar] [CrossRef]
  47. Feng, S.; Zhang, X.; Zhang, Q.; Liang, Y.; Zhao, X.; Wang, C.; Ma, L. Synthesis of branched alkane fuel from biomass-derived methyl-ketones via self-aldol condensation and hydrodeoxygenation. Fuel 2021, 299, 120889. [Google Scholar] [CrossRef]
  48. Kikhtyanin, O.; Chlubná, P.; Jindrová, T.; Kubička, D. Peculiar behavior of MWW materials in aldol condensation of furfural and acetone. Dalton Trans. 2014, 43, 10628–10641. [Google Scholar] [CrossRef] [PubMed]
  49. Barman, B.N.; Skarlos, L.; Kushner, D.J. Simultaneous Determination of Oil and Coke Contents in Spent Hydroprocessing Catalyst by Thermogravimetry. Energy Fuel 1997, 11, 593–595. [Google Scholar] [CrossRef]
  50. Díaz, M.; Epelde, E.; Valecillos, J.; Izaddoust, S.; Aguayo, A.T.; Bilbao, J. Coke deactivation and regeneration of HZSM-5 zeolite catalysts in the oligomerization of 1-butene. Appl. Catal. B Environ. 2021, 291, 120076. [Google Scholar] [CrossRef]
  51. Guisnet, M.; Magnoux, P. Organic chemistry of coke formation. Appl. Catal. A Gen. 2001, 212, 83–96. [Google Scholar] [CrossRef]
Energies 18 04106 g001
Figure 1. XRD diffraction patterns of the synthesised catalysts.
Figure 1. XRD diffraction patterns of the synthesised catalysts.
Energies 18 04106 g001
Energies 18 04106 g002
Figure 2. Reaction pathway deduced from GC-MS-identified products.
Figure 2. Reaction pathway deduced from GC-MS-identified products.
Energies 18 04106 g002
Energies 18 04106 g003
Figure 3. Effect of NiO loading on cyclohexanone conversion and selectivity towards C-C condensed adducts at a temperature of 160 °C, 10 g cyclohexanone, 0.5 g catalyst loading, and 10 bar initial hydrogen pressure. [Note: The error bars are the standard deviation of triplicate experiments].
Figure 3. Effect of NiO loading on cyclohexanone conversion and selectivity towards C-C condensed adducts at a temperature of 160 °C, 10 g cyclohexanone, 0.5 g catalyst loading, and 10 bar initial hydrogen pressure. [Note: The error bars are the standard deviation of triplicate experiments].
Energies 18 04106 g003
Energies 18 04106 g004
Figure 4. Effect of NiO loading on selectivity towards hydrocarbon formation and C-C condensed adducts at a temperature of 160 °C, 10 g cyclohexanone, 0.5 g catalyst loading, and 10 bar initial hydrogen pressure. [Note: The error bars are the standard deviation of triplicate experiments].
Figure 4. Effect of NiO loading on selectivity towards hydrocarbon formation and C-C condensed adducts at a temperature of 160 °C, 10 g cyclohexanone, 0.5 g catalyst loading, and 10 bar initial hydrogen pressure. [Note: The error bars are the standard deviation of triplicate experiments].
Energies 18 04106 g004
Energies 18 04106 g005
Figure 5. Thermogravimetric and differential profiles of the spent catalysts as a function of NiO loading on NbOPO4 after the aldol condensation of cyclohexanone at 160 °C temperature, initial hydrogen pressure 10 bar, 0.5 g catalyst loading, and 3 h reaction time.
Figure 5. Thermogravimetric and differential profiles of the spent catalysts as a function of NiO loading on NbOPO4 after the aldol condensation of cyclohexanone at 160 °C temperature, initial hydrogen pressure 10 bar, 0.5 g catalyst loading, and 3 h reaction time.
Energies 18 04106 g005
Table 1. Textural and acidic properties of the catalysts.
Table 1. Textural and acidic properties of the catalysts.
ParameterNbOPO410wt% NiO/NbOPO430wt% NiO/NbOPO4
Specific surface area (m2·g−1)411272125
Pore volume (cm3·g−1)0.470.430.35
Average pore diameter (nm)3.433.413.41
Lewis acid (mmol)39.433.026.6
Brønsted acid (mmol)23.519.716.0
Brønsted + Lewis acid (mmol)68.258.047.9
NiO loading (wt%)NA9.228.5
Table 2. Aldol condensation of cyclohexanone over different catalysts reported in the literature and this study: conversion and selectivity.
Table 2. Aldol condensation of cyclohexanone over different catalysts reported in the literature and this study: conversion and selectivity.
CatalystConversion (%)Selectivity Towards C12 (%)Reference
Amberlyst-15 and Amberlyst-70 resins, SO3H-SBA-3, SO3H-SBA-16, and SO3H-FDU-1225–4095Ref. [19]
SO3H-SBA-15, SO3H-LP-SBA-15, and SO3H–SiNF 20–2295Ref. [19]
Ion exchange resin and T-63 28–4090–92Ref. [44]
NbOPO468–9653–70Ref. [16]
ZSM, Pd/ZSM, and PdFe/hierarchical-ZSM8.3–5552–97Ref. [15]
Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@F-SO3H4.2–7822–94.2Ref. [20]
Amberlyst 155090Ref. [45]
Phosphotungstic acid (PTA), MIL-100, PTA@MIL-1008–4810–97Ref. [43]
TiO2/Al2O329.11100Ref. [28]
10 wt% NiO/NbOPO466.896.8This study
Table 3. Liquid, gas, and coke products balance based on 10 g cyclohexanone feedstock used.
Table 3. Liquid, gas, and coke products balance based on 10 g cyclohexanone feedstock used.
CatalystGas (wt%)Coke (wt%)Liquid (wt%)Balance (wt%)
NbOPO45.22 ± 0.120.72 ± 0.0889.71 ± 1.2495.65 ± 0.62
10 wt% NiO/NbOPO46.50 ± 0.020.66 ± 0.1189.52 ± 0.9696.68 ± 0.43
30 wt% NiO/NbOPO46.26 ± 0.070.64 ± 0.0489.71 ± 1.0596.61 ± 0.27
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Hart, A.; Onwudili, J.A. Tuning the Activity of NbOPO4 with NiO for the Selective Conversion of Cyclohexanone as a Model Intermediate of Lignin Pyrolysis Bio-Oils. Energies 2025, 18, 4106. https://doi.org/10.3390/en18154106

AMA Style

Hart A, Onwudili JA. Tuning the Activity of NbOPO4 with NiO for the Selective Conversion of Cyclohexanone as a Model Intermediate of Lignin Pyrolysis Bio-Oils. Energies. 2025; 18(15):4106. https://doi.org/10.3390/en18154106

Chicago/Turabian Style

Hart, Abarasi, and Jude A. Onwudili. 2025. "Tuning the Activity of NbOPO4 with NiO for the Selective Conversion of Cyclohexanone as a Model Intermediate of Lignin Pyrolysis Bio-Oils" Energies 18, no. 15: 4106. https://doi.org/10.3390/en18154106

APA Style

Hart, A., & Onwudili, J. A. (2025). Tuning the Activity of NbOPO4 with NiO for the Selective Conversion of Cyclohexanone as a Model Intermediate of Lignin Pyrolysis Bio-Oils. Energies, 18(15), 4106. https://doi.org/10.3390/en18154106

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop