High energy demand from the market due to the rapid increment of the human population worldwide has urged society to explore alternatives to replace non-renewable energy. Renewable diesel produced from biomass could be the next potential energy source for its high stability, long-term storage, and comparable performance with diesel fuels. In producing renewable diesel, the application of catalyst is essential, and the catalyst support is synthesized with the catalyst to enhance the reaction rate and catalytic properties. The application of the supported catalyst in increasing the selectivity and yield of renewable diesel is significant, in which the catalytic properties depend on the interaction between catalyst and catalyst support. The supported catalyst as a favorable substance to assist in enhancing renewable diesel yield could lead to a sustainable and greener future for the biofuel industry in Malaysia.
Type of Support | ||||||
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Type of support | ||||||
Elemental Composition | ||||||
Type of catalyst | ||||||
Surface Area | ||||||
Composition of the active phase | ||||||
Pore Volume | ||||||
Surface area | ||||||
Remarks | ||||||
Pore volume | ||||||
Reference | ||||||
Remarks | ||||||
Reference | ||||||
AC | NiP | Ni: 5.14 wt% P: 2.23 wt% |
Micropore: 739 m2/g External: 15 m2/g |
Micro: 0.22 cm3/g Total: 0.25 cm3/g |
Charcoals from Iwasaki kiln | [33] |
AC | NiP | Ni: 4.66 wt% P: 2.24 wt% |
Micropore: 851 m2/g External: 16 m2/g |
Micro: 0.26 cm3/g Total: 0.31 cm3/g |
Charcoals from tube furnace | [33] |
AC | Ni2P | - | BET: 612 m2/g | - | Total acidity: 1.3 mmol/g | [42] |
AC | Ni | O (on the surface): 9.4% | BET: 807.26 cm2/g | Total: 0.185 cm3/g | - | |
AC | Mo2C | Mo(II): 52% Mo(IV): 8% Mo(VI): 40% |
Total: 417.02 m2/g | Total: 0.22 cm3/g | - | [43] |
AC | Mo2C | Mo2C (II): 52.17% MoO2 (IV): 8.2% MoO3 (VI): 39.63% |
BET: 322.20 m2/g | Total: 0.202 cm3/g | - | [39] |
AC | Co-Ag | C: 63.41 wt% O: 13.26 wt% P: 1.45 wt% Co: 9.57 wt% Ag: 12.31 wt% |
BET: 793 m2/g | Total: 1.67 cm3/g | Acidity: 8502.3 µmol/g Total basicity: 6220.2 µmol/g |
[40] |
AC | CoP | - | BET: 822.9 m2/g | Micro: 68.79% Meso: 31.21% Total 0.43 cm3/g |
Acidity: 52.5 µmol/g | [41] |
AC | C: 90.03 % H: 0.557% N: 0.367% S: 0.069% O: 8.98% C/H: 161.6 |
Micropore: 775 m2/g External: 15 m2/g |
Micro: 0.23 cm3/g Total: 0.26 cm3/g |
Charcoals from Iwasaki kiln | [37] | |
AC | C: 80.71 % H: 1.146% N: 1.094% S: 0.078% O: 16.97% C/H: 70.4 |
Micropore: 1202 m2/g External: 20 m2/g |
Micro: 0.39 cm3/g Total: 0.42 cm3/g |
Charcoals from tube furnace | [37] | |
AC | - | BET: 1484.33 cm2/g | Total: 1.038 cm3/g | Acid sites: 3.96 mmol NH3/g catalysts | [45] | |
AC | - | BET: 266.1 m2/g | Total: 0.17 cm3/g | Pre-treated with a nitric acid solution | [46] | |
AC | C: 88.57 wt% O: 8.01 wt% P: 3.42 wt% |
BET: 350 m2/g | Total: 1.88 cm3/g | Total acidity (144 °C): 1055.3 µmol/g Total acidity (852 °C): 2064.7 µmol/g Total basicity (902 °C): 1086.6 µmol/g |
[47] | |
AC | C: 79.1 w/w% H: 0.9 w/w% N: 0.9 w/w% O: 19.2 w/w% |
BET: 964 m2/g | Micro: 77.92% Meso: 22.08% Total: 0.57 cm3/g |
- | [48] | |
Type of support | Type of catalyst | Composition of the active phase | Surface area | Pore volume | Remarks | Reference |
AC | NiP | Ni: 5.14 wt% P: 2.23 wt% |
Micropore: 739 m2/g External: 15 m2/g |
Micro: 0.22 cm3/g Total: 0.25 cm3/g |
Charcoals from Iwasaki kiln | [37] |
AC | NiP | Ni: 4.66 wt% P: 2.24 wt% |
Micropore: 851 m2/g External: 16 m2/g |
Micro: 0.26 cm3/g Total: 0.31 cm3/g |
Charcoals from tube furnace | [37] |
AC | Ni2P | - | BET: 612 m2/g | - | Total acidity: 1.3 mmol/g | [49] |
AC | Ni | O (on the surface): 9.4% | BET: 807.26 cm2/g | Total: 0.185 cm3/g | - | [45] |
AC | Co-Fe | Co: 8.67 wt% Fe: 3.52 wt% |
Micropore: 459.91 m2/g | Micro: 0.22 cm3/g Total: 0.44 cm3/g |
- | [43] |
AC | Mo2C | Mo(II): 52% Mo(IV): 8% Mo(VI): 40% |
Total: 417.02 m2/g | Total: 0.22 cm3/g | - | [50] |
AC | Mo2C | Mo2C (II): 52.17% MoO2 (IV): 8.2% MoO3 (VI): 39.63% |
BET: 322.20 m2/g | Total: 0.202 cm3/g | - | [46] |
AC | Co-Ag | C: 63.41 wt% O: 13.26 wt% P: 1.45 wt% Co: 9.57 wt% Ag: 12.31 wt% |
BET: 793 m2/g | Total: 1.67 cm3/g | Acidity: 8502.3 µmol/g Total basicity: 6220.2 µmol/g |
[47] |
AC | CoP | - | BET: 822.9 m2/g | Micro: 68.79% Meso: 31.21% Total 0.43 cm3/g |
Acidity: 52.5 µmol/g | [48] |
Metal oxides are well known as catalyst supports for industrial catalysts, for example, alumina, zirconia and silica [44]. They often consist of a metal cation and oxide anion, which form bases through reaction with water and form salts through reaction with acids. The nature of metal oxides varied with the oxidation states, where the metal oxides are more stable and more acidic with higher oxidation states as compared to lower oxidation states. The properties of metal oxides such as stability, strength and chemical nature are influenced by the type of metal/element as shown in the periodic table, with increasing basic nature and solubility of metal oxides observed down the column [45]. The metallic elements also dominate their wide range of attractive physicochemical properties, from the aspects of morphological, electronic, textural, structural, and redox [46].
The catalyst support, alumina (Al2O3) also known as the aluminium oxides, are often found compatible with a mixture of catalysts (containing two or more than two types of catalysts). It was reported that the catalyst, 4Pt-8MOx (where M includes Sn, W, Mo and Re) has exhibited improved catalytic activity on hydrodeoxygenation compared to “neat” Pt supported on alumina [47]. These metal oxides impregnated with Pt have affected the electronic and textural properties of Pt, resulting in hydrodeoxygenation reaction rather than decarboxylation or decarbonylation reaction during deoxygenation. The metal oxides such as MoOx and ReOx have higher weak (100-200 oC) and moderate (200-350 oC) acid sites than “neat” Pt; while the SnOx and WOx have less weak acid sites and higher strong (>350 oC) acid sites than the “neat” Pt. The good compatibility of NiMo with Al2O3 was proved in another study as high product yield (80 wt%), with relatively stable activity if NiMo is sulphided even after 5 cycles of re-use [48]. The sulphided NiMo has a better performance compared to sulphided CoMo due to the amount of stronger sulphided sites and higher metal dispersion (75% higher than sulphided CoMo), while other characteristics such as pore volume, pore diameter, surface area and reaction temperature at intermediate temperature did not vary significantly. The favourable reaction of these catalysts supported on γ-alumina was decarbonylation/decarboxylation, where carbon atom was released in the form of CO2 or CO. Parameters such as reaction pressure and time possess a positive effect on the yield of the product while the negative effect was observed in temperature [48]. Other good matches of Pt-MoOx and Pt-WOx with Al2O3 as catalyst support also outstands Pt-ReOx with similar support, where the selectivity of C18 exceeds 50 wt% after 3 cycles without significant decrement [47]. In the study of Sotelo-Boyás et al., the highest yield of renewable diesel was obtained using NiMo/γ-Al2O3 as compared to Pt/H-Y and Pt/H-ZSM-5, constitute of mostly n-paraffins of C15 to C18, which contributed to poor cold flow properties [49]. The relationship between the yield of renewable/ isomerization degree and acid sites’ activity of catalysts was directly influenced the obtained cold flow properties of the product. It was reported that the bimetallic (NiCu) with ZrO2 as support is more effective than Ni alone as a catalyst because the reduction of NiO was facilitated by the Cu during hydrodeoxygenation [50]. Other advantages to utilising NiCu over Ni as catalyst supported with ZrO2 were low choking and sintering observed after the reaction. The increasing Cu loading also enhanced the hydrodeoxygenation activity to 82% efficiency with a higher yield of crude bio-oil, using Cu loading up to 6.29 wt% and decreased yield observed thereafter.
The other type of metal oxides, silicon oxide is also known as silica (SiO2), is utilized as catalyst support due to the uniform distribution of pores and pore sizes as well as their good chemical and thermal stability [51]. These features are important as catalyst support is utilised aiming to improve the catalytic activity of reactions [52]. It was reported that the high activity of Ir-ReOx/SiO2 was elucidated by the synergy effect between partially reduced ReOx sites and Ir, to achieve such yield using a Re/Ir molar ratio of 3 [53]. The comparable catalytic activity was also observed for such combination after regenerated with calcination. For the combination of Ni-P/SiO2, the variance of metal composition has led to varied competitive pathways in producing C15 and C16 from palmitic acid, either decarbonylation with/without water formation, or the dehydration of hexadecanol intermediate pathway [54]. Besides common metal catalyst, SiO2 as support is suitable to be utilised with coal fly ash as a heterogeneous catalyst in renewable diesel production. The study of Pelemo et al. showed the mentioned catalyst capable to tolerate high temperatures up to 950 oC with a thermal equilibrium of 700 oC. As compared to pure coal fly ash, the reinforcement of SiO2 as its support has enhanced the pore and micropore volume as well as BET surface area of coal fly ash [55]. As for the surface structure, the catalyst with SiO2 reinforced was found to be having hexagonal-shaped particles, regular surface structure, as well as more pores, compared to the catalyst without SiO2, representing the high amount of SiO2 that benefits the catalysis efficiency. The combination showed good thermal stability as no further weight loss was observed at 700 oC and the thermal degradation was reported to be minimal due to the low composition of calcite (an unstable compound). Other types of metal oxides such as TiO2, ZnO, or CaO are potential to be utilized as they are widely used as part of the catalyst in other applications even biodiesel production. However, these metal oxides are often utilised with other types of components and act as catalyst support in a combination of two or more components.The up-gradation of biodiesel to renewable diesel can be performed using zeolites supported catalysts. The study of Fani et al. concluded that Ni catalysts supported on activated natural MOR led to the production of 25 wt% renewable diesel using the highest loading of Ni as catalyst [58]. The deposition-precipitation method is more favourable compared to other synthesis methods (incipient wetness impregnation, wet impregnation and infiltration) for Ni/natural MOR due to the resulting better catalyst dispersion, specific surface area as well as a lower amount of carbon deposits on the catalyst surface. The highest loading of Ni leads to the balanced amount of weak and strong acid sites along with the highest Ni surface area, to produce the highest efficiency for renewable diesel production [58]. Another study reported on the application of Fe/natural zeolite (NZ) showed enhancement on catalytic properties and selectivity towards hydrocarbons with straight-chain alkanes (C15-C18), and the presence of Fe did not change the morphology and crystal structure of zeolites supports [59]. The Fe particles were found to be well-dispersed on the natural zeolite support although the Fe/NZ tends to be agglomerated. It was observed that the impregnation of Fe on NZ led to reduced BET surface area and pore volumes due to the micropore blocking, which was in agreement with the study of Rostamizadeh et al. (2016) [59][60]. The mentioned study has achieved 89% conversion of palm oil into renewable diesel using Fe/natural zeolites as compared to pure natural zeolites as catalysts (58%), showing the maleficent results of natural zeolites as catalyst support.
Synthetic zeolites with optimized structure and surface characteristics can be produced via a thermal process by controlling the composition of materials and process temperature during synthesis. The synthesis of synthetic zeolites can be completed using natural raw materials via microwave-assisted synthesis, dialysis, fusion method, ultrasonic method, molten salt method, alkali activation and hydrothermal synthesis [61][62][63]. However, the synthesis of synthetic zeolites from the chemical source of alumina and silica is expensive. Therefore, the raw material namely kaolin was studied as a precursor of zeolite due to their silica and alumina ratio.
Short synthesizing time, the versatility of catalytic properties in synthetic zeolites and the possibility to generate desired zeolite structures are the advantages of synthetic zeolites over natural zeolites as catalyst support. Ni was found a versatile catalyst working well with synthetic zeolites as catalyst support. The compatibility of Ni with zeolites is found in agreement with the study of Li et al., where 79% to 90% of diesel range alkanes were produced using both H-ZSM-5 and H-MOR as catalyst support [64]. The desired properties of Ni/H-ZSM-5 to produce such as a high yield of diesel range alkanes (90%) are moderate Ni dispersion (9%) with the acid amount of 1.15 mmol/g with a SiO2/Al2O3 molar ratio of 25. In this case, the catalyst, Ni/H-MOR with a higher amount of acid sites, slightly lower value of SiO2/Al2O3 molar ratio as well as low metal dispersion produced a lower yield of diesel range alkanes as compared to Ni/H-ZSM-5 [106]. For Pt/H-ZSM-5 catalyst, hydrotreating of rapeseed oil leads to the production of denoting quantities of green gasoline and renewable diesel, favouring iso-paraffins rather than n-paraffins [49]. It was reported that the performance of a similar combination of catalyst and catalyst support was affected by the synthesis method. Other types of zeolite such as zeolite beta have good catalytic performance with the catalyst, La2O3, producing diesel-ranged (C13-C18) products with high selectivity of 83% and excellent conversion rate up to 99% from the hydrodeoxygenation of oleic acid. It was contributed by the textural properties of a catalyst that has large pore diameter, an appropriate amount of weak and medium acid sites as well as the synergistic interaction between La-Si-Al [65]. Other types of metals such as MgO, MnO, Fe2O3, ZnO, Co3O4 also favours the production of diesel-like hydrocarbon, but with a lower conversion percentage (71%-98%) and selectivity compared to La2O3.