Development of catalyst complexes for upgrading biomass into ester-based biolubricants for automotive applications: a review

Biomass-derived oils are recognised as the most promising renewable resources for the production of ester-based biolubricants due to their biodegradable, non-toxic and metal adhering properties. Homogeneous acid catalysts have been conventionally used in catalytic esterification and transesterification for the synthesis of ester-based biolubricants. Although homogeneous acid catalysts encounter difficulty during phase separation, they exhibit superior selectivity and good stereochemistry and regiochemistry control in the reaction. Consequently, transition metal complex catalysts (also known as homogeneous organometallic catalysts) are proposed for biolubricant synthesis in order to achieve a higher selectivity and conversion. Herein, the potential of both homogeneous transition metal complexes and heterogeneous supported metal complexes towards the synthesis of biolubricants, particularly, in esterification and transesterification, as well as the upgrading process, including hydrogenation and in situ hydrogenation–esterification, is critically reviewed.


Introduction
Automotive-type lubricants are utilised to reduce friction and to moderate the heat produced when two surfaces are in mutual contact. 1 The lubricants can form a thin lm between the two sliding parts, which generates a slippery surface for the metal parts to move easily. 2 Generally, lubrication can prolong the wear and tear duration of machinery and signicantly reduce the surface deformation of the parts. Besides acting as a coolant for the sliding surfaces, lubricants also act as a transport medium for foreign particles and forces on the parts of machinery.
Generally, petroleum-based oils or mineral oils have been used as lubricants in automobiles and machinery for several decades. The continuous depletion of mineral oils has raised the attention of scientists towards the utilisation of renewable biomass-based lubricants. 3 It has been widely reported that the petroleum-based oils can lead to serious groundwater pollution, 4 soil pollution, surface water contamination, air pollution, and agricultural material and food contamination. 5 Moreover, gaseous CO, CO 2 , NO x and SO x are liberated together with nanoparticle traced metals (Hg, Ca, P, Zn, Mg, and Fe) during the combustion of mineral oil, which results in a negative impact on the environment. 6 It is reported that mineral oil is carcinogenic as continuous inhalation of this lubricant emission may cause inammatory and analgesic effects on the human respiratory system. 7,8 In addition, by-products derived from the degradation of the lubricants are toxic and can lead to soil infertility.
Ester-based biolubricants are greener, renewable, non-toxic and emit zero greenhouse gas. 9 They can be used as additives for anti-oxidants, viscosity index promoters, pour point mitigation, detergents and emulsion stabilisers. For industrial application, these biolubricants are suitable to be applied for complete uid lubrication, boundary lubrication, and extreme pressure lubrication.
Notably, bioester-based lubricants are conventionally synthesised by reacting long-chain fatty acids (derived from hydrolysis of plant oil) with polyols (e.g. trimethylolpropane, neopentyl glycol, pentaerythritol) in the presence of liquid acid or Lewis acid catalysts by esterication. In contrast, the reaction between triglyceride-based feedstock (plant oil or animal fat) and alcohols/polyols occurs via the transesterication process in the presence of acid or base catalyst. [10][11][12] Recently, a new technology has been developed to upgrade the performance of ester-based biolubricants (especially in oxidation stability) via hydrogenation, and/or in situ hydrogenation-esterication in which molecular hydrogen is used at a high temperature and pressure in the presence of homogeneous or heterogeneous catalysts. These types of reactions aim to reduce the unsaturated bonding presence in the bioester into a saturated product for better stability of the product under extreme conditions and long-term storage.
Recent development focused on the production of various characteristics of biolubricant products has provided potential substitutes to mineral-based lubricants for automotive applications. 13,14 Nevertheless, to the best of our knowledge, there is a lack of comprehensive study on the usage of homogeneous transition complex catalysts in biolubricant synthesis. 15,16 As derived from the literature, a homogeneous-type transition metal complex catalyst is a potential catalyst for ester production, mainly due to its simple preparation step, large surface area, good molecular dispersion between reactants, and sufficient pore volume. 17 In contrast, the heterogeneous-type metal complex catalyst is scarce in the catalytic synthesis of biolubricants when long-chain/polyhydric alcohols are employed in the reaction due to the occurrence of undesirable side products, low selectivity and soap formation in the process. 18 Consequently, the catalytic activity of homogeneous transition metal complexes and the supported metal complexes (heterogeneous) towards the production of biolubricants is critically reviewed in this study. In addition, this review outlines the future prospects of and global demand for biolubricants.

Global demand for lubricant usage
The global demand for lubricants is expected to increase 2.0% per year to 45.40 million metric tons by 2020. 6 This growth is projected to increase as the demand for engine oils for new motor vehicles increases. The fastest demand is projected for the Asia and Pacic regions, where number of motor vehicles and industrialization are increasing. The demand for lubricants in Central and South America and the Middle East/Africa region is also expected to increase due to the increasing number of manufacturing units, rising motor vehicle manufacturing and proprietary rates. Fig. 1 shows the world demand of lubricant usage in 2016 (a) and world lubricant demand forecast for the year 2021 (b). 19,20 According to a published report by Kline & Company, the collected data in 2016 indicated that the demand for lubricants is increasing in Asia and South America (S. America), whereas, it is decreasing in North America (N. America) and Europe and is constant in Africa/Middle East (AME). The reasons are the strong economic growth and increase in vehicle ownership rate and industrialisation, which result in an increase in demand for lubricants. Besides, another analysis reported by RPS Energy (a multinational energy and environmental consultancy company) stated that the demand of lubricants is increasing continuously in the Asian region due to economic prospects, vehicle ownership and large-scale GDP growth. As GDP growth per capita income is rising in Asia and Middle East/Africa regions compared to that in North America and Europe, there is an increase in vehicle ownership rate, thus leading to an increase in lubricant demand (Table 1). 19,20

Type of automotive lubricants
Besides providing a slippery surface for the moving parts, a good automotive lubricant must be able to remain in the

Biolubricants
Biolubricants are generally obtained from biomass or naturally occurring sources like soybean, palm, sunower, coconut, and rapeseed oils. 16 A biolubricant is an ester of glycerides, which is still limited for application as a lubricant as their freezing point is quite high, and they are easily oxidised. Biolubricants can be prepared using long-chain alcohols/polyols with fatty acids that full the toxicity and biodegradability criteria (refer to Table 4). Typically, biolubricants are biodegradable and release less carbon and greenhouse gases compared to petroleum-based lubricants. As a result, biolubricants evaporate slowly compared to petroleum oil and adhere to metal surfaces strongly even under extreme conditions for a long time. A biolubricant's adhering properties on metal surfaces and performance can also be increased by additives and can be used under abnormal conditions. For instance, fatty acid such as oleic acid, linoleic acid, butanoic acid, levulinic acid and palmitic acid react with long-chain/polyhydric alcohols forming mono-, di-, triester-based biolubricants. In general, conventional biolubricant oils found in the market are derived from biomass, such as castor oil, olive oil, coconut oil, sperm oil, rapeseed oil,  They are highly synthesized as nal reaction products, such as synthetic esters, silicones, and polyalphaolens On the basis of application Automotive oils/uids These oils are commonly used for automobile application and transportation sectors, e.g., engine oils, gear-box oils, transmission uids, and hydraulic brake uids Industrial instrument oils They are used for industrial goals such as compressor uids, machine oils, hydraulic uids, and metal-working oils Special uids These uids are used in special cases according to denite applications such as white uids, process uids, and instrumental uids rosin oil, palm oil, neatsfoot oil, and seal and whale oils. Commonly, mineral oil is blended with biolubricant base oil to increase the lube performance.

Synthesis of biolubricants
(I) Esterication. Generally, ester-based biolubricants can be prepared via esterication reactions between polyhydric alcohols with fatty acids in the presence of acid catalyst. The employed polyols such as neopentyl glycol (NPG), trimethylolpropane (TMP), pentaerythritol (PE) and glycerol (Gly) are able to render different characteristics to the biolubricant product (e.g. high viscosity, low freezing point, low pour point and high ash point). Meanwhile, a different carbon chain number of carboxylic acids, such as acetic acid, propionic acid, oleic acid, valeric acid, caprylic acid and levulinic acid, can be used for the esterication process. Generally, the length of the fatty acid chain for biolubricant synthesis are in the range of C 12 -C 24 . The two major factors of fatty acids that affect the properties of biolubricants are the carbon chain length and number of double bonds in the fatty chain. 31 A longer chain length of fatty acids produces biolubricants with a higher melting point and viscosity, whereas higher double bonds alter the property of biolubricants by lowering their melting points and viscosity and decreasing their oxidative stability. 1 Thus, different fatty acids and polyol feedstocks render different characteristics to biolubricant products for automotive and industrial usage. 32 (II) Transesterication. Transesterication reaction is another conventional process for synthesising ester-based biolubricants using triglyceride-based feedstock (plant-based or animal-based oil) and alcohols in the presence of catalyst. For the transesterication process, different types of triglyceridebased oils including soybean, palm, andiroba, piqui and cumaru oils are involved in the methanolysis in the presence of different Lewis acid catalysts. In this review, we tried to describe the Lewis acidity of catalysts, which inuences the trans-esterication reaction. For this reason, we included some metal salts, especially Sn(II) and Sn(IV) based complexes having Lewis acidity, with different ligands, which have been critically reviewed. Their catalytic activity was also compared with that of conventional mineral acid H 2 SO 4 catalyst. The main advantage of the Sn based-complex catalyst is that it is highly soluble in the oil phase and gives the maximum yield, but sometimes catalyst separation is difficult. The general transesterication reaction in the presence of catalyst is as follows: (III) Hydrogenation. Hydrogenation of esters is an important upgrading process for the synthesis of biolubricants, surfactants, plasticisers, and fatty alcohols, which have broad applications in agrochemicals, pharmaceuticals, and ne chemicals. The reduction of esters and ketones is by employing H 2 at higher temperature and pressure for producing ester-based biolubricants that have a signicant importance in our daily life. The normal esterbased lubricants have a high oxygen content and unsaturation level, which are not suitable for engine oils or ne chemicals, and are not biodegradable. On the other hand, ester-based lubricants aer hydrogenation process, increase the hydrogen content and saturated carbon chain can be used as engine oils, which might satisfy all types of lubrication. The high-potential catalysts for the reduction process of the carbonyl group are present in the ester including the precious Ru, Rh, Os, Pt, and Au metal-based complexes. However, the high price and limited availability of the valuable metals lead to the exploration of the highly abundant and less expensive active metal complexes, e.g., transition metal (Cu, Ni, Co and Fe) based complexes. The general ester hydrogenation in the presence of the catalyst is as follows: (IV) Hydrogenation-esterication. One-step hydrogenation-esterication is another upgrading process, in which bio-oils (derived from pyrolysis of biomass) are converted into biofuel or biochemicals (biolubricants). The nal products are suitable for combustion and become static oxygenous hydrocarbons, as they are hydrogenated esters in which esterication and hydrogenation are the fundamental reactions. The main constituents of bio oil are fatty acids, aldehydes and ketones, and phenols, which have a negative effect on the properties of bio-oils. Acetic acid, levulinic acid, furfural, hydroxyacetone, phenol, and ethane diol are considered as model compounds for producing biolubricants. To convert these mixtures into combustible and stable compounds (i.e. esters), a one-step hydrogenation-esterication (OHE) process is performed over different bifunctional, RANEY® Ni (RNs) or Lewis acid catalysts in methanol/ethanol.

Properties of synthetic biolubricants
Viscosity index. Viscosity index (VI) indicates the variation of viscosity with temperature. A high viscosity index reveals small changes in viscosity with temperature. Biomass ester-based lubricants have a higher VI than conventional petroleum oils, which ensure that biolubricants retain the same activity in a wide range of temperatures by maintaining the oil lm.
Pour point. It determines the temperature below which oils cannot be used as a lubricant in the engine. Pour point is an important factor for any lubricating oil. Biomass-derived esterbased lubricants have lower pour points than mineral oils, thus providing excellent lubrication in cold conditions.
Flash point. Lubricants have to tolerate high temperature when they are used. A ash point is the lowest temperature at which a lubricant must be heated when using before it vaporises. When mixed with air, a lubricant will ame up but will not burn. Flash points indicate lubricant volatility and reresistance properties, which are important factors for transportation. Biomass based-biolubricants have higher ash points than mineral oils.
Oxidation stability. Lubricants are generally oxidised to form various compounds. Oxidation stability of lubricants indicates the ability of resistance towards formation of oxides and sulphides, which increases when temperature rises. The most important things that lead to oxidation are temperature, pressure, metal surfaces, agitation, water, and contaminants. A low oxidative stability indicates that oil oxidises rapidly upon use if it is untreated. Biomass-derived ester-based lubricants have a higher oxidation stability than mineral oils.
Biodegradability. Biodegradability measures the ability of a material to be decomposed by microorganisms. A lubricant is classied as biodegradable if its degradation rate in a standard test exceeds a targeted level. Biomass-derived oils exhibit better biodegradability, of about 90-99%, 27 whereas mineral oil biodegradability is about 20-40%. Biodegradability of lubricants is mainly inuenced by the main component present in the base oil; hence, it depends on the structures of organic compounds.

Industrial application of biolubricants
Ester-based biolubricants have been used as alternative lubricants in industries and automotives as they reduce friction and wear and operating noise and improve heat transfer. They have a longer adherence period on metal surfaces, as they have polarbased chemical structures, whereas petroleum-based lubricants are non-polar hydrocarbons and do not exhibit adhering properties on metal surfaces (Fig. 2). The application of biolubricant products in automotive industry are including hydraulic uids, metal working oils, two-stroke and three stroke engine oils, 37 and chainsaw uids. 38 In addition, biolubricants are extensively used as engine oils, transmission uids, gear box oils, and brake and hydraulic uids and are able to provide a slippery surface, reduce metallic corrosion and enhance performance of the machinery (Table 5).

Advantages of biolubricants
The benets of biolubricants are that they produce a cleaner, safer, less toxic environment and cause fewer skin problems for those who are working with engines and hydraulic systems. The physical and chemical properties of biolubricants are safer owing to their higher ash point, low pour point, constant viscosity, and less vapor emissions and oil mist. 41 Moreover, they produce a low emission due to the high range of boiling temperature of esters. They can reduce pollution in stormwater from leaks in engines, hydraulic systems, and brake lines. These biolubricants are highly biodegradable 42 and their cost is less over the product's life-cycle, as less maintenance 43 and storage and disposal systems are needed. However, they are 3-5 times more expensive than petroleum oils (Table 6). 44

Catalytic reaction for synthesis of biolubricants
The homogeneous catalysts are molecularly dispersed in the reacting uids; hence, pore diffusion limitations are absent and bulk phase mass transfer limitations usually occur. Homogenous catalytic reactions are used not only for the biolubricant production 46,47 but also for many conventional organic transformation reactions ( Table 7).
The transition metal complexes (present in homogeneous form) are acidic and thus potentially catalyse many organic reactions (e.g. esterication, transesterication, and ester hydrolysis). Transition metal complexes are potentially applied as acid catalysts for ester-based lubricant synthesis. It exhibits properties of chemoselectivity, regioselectivity, 48 and enantioselectivity in which the reaction mechanism and kinetics are easy to understand. On the other hand, heterogenous catalyst are poorly dened in the reacting uids, and their synthesis problem and reaction mechanism are rarely understood. In case of heterogeneous catalysts, the reaction mixture is not well dispersed because of their larger particle, so a low yield with low selectivity is usually observed. In this context, as a homogeneous catalyst is well dispersed within the reacting uid, it is chosen for the production of biolubricants. In fact, transition metal complexes in the homogeneous form are reviewed for further study. Table 13 briey illustrates these catalysts.

Catalytic esterication reaction
Ester-based biolubricants are generally prepared by the esteri-cation reaction between long-chain carboxylic acids (C 4 -C 14 ) (e.g. butanoic acid, pentanoic acid, levulinic acid, oleic acid and linoleic acid) 49 with polyols including glycerol (Gly), neopentyl glycol (NPG), trimethylolpropane (TMP), pentaerythritol (PE)  Molecularly dispersed in the reacting uids, giving better yield with high product selectivity. Hence, reaction mechanism and kinetics are easy to understand Catalyst separation is difficult and requires high technology, which is sometimes expensive Base catalyst Schiff base complex (Lewis base), e.g. Co, Ni, Cd, Zn complexes Transesterication and hydrogenation reactions are inuenced by these catalysts As they are a little basic and soluble in reacting uids, they are used in the presence of another base and n-octanol, and n-butanol in the presence of liquid acid (e.g. H 2 SO 4 , HCl, HNO 3 ) or Lewis acid catalysts (e.g. metal salts or transition metal complexes). Basically, the liquid acid catalyst used for the esterication reaction causes major corrosion of the reacting system and produces a salt as a result of the neutralisation reaction with base chemicals. Besides, additional cost is required for product washing and water separation from the nal liquid ester product. 50 In this part, a comparative study on the homogeneous 51 and heterogeneous 52 catalysed esterication reactions are reviewed herein.
Noor et al. 53 studied the catalytic performance of perchloric acid, sulfuric acid, hydrochloric acid, and nitric acid (liquid acid) for biolubricant production. The process involved the esterication between Jatropha curcas oil with trimethylolpropane (TMP) at 150 C, reaction time of 3 h and molar ratio of FA : TMP was 4 : 1 with 2% w/w concentrated catalyst. They revealed a higher TMP ester yield, which was dependent on the esterication catalysts's acidity strength; here perchloric acid had the strongest acidity, giving a maximum of 70% TMP-ester. By contrast, sulfuric acid, p-toluenesulfonic acid, hydrochloric acid and nitric acid gave 46%, 42%, 41% and 37% ester yield, respectively.
Abiney et al. 54 assessed the inuence of SnCl 2 as the catalyst on the conversion of oleic acid to ethyl oleate in C 2 H 5 OH solution; the oleic acid : catalyst ratio was 100 : 1 (v/w), under 80 C of temperature for 6 h reux. The use of SnCl 2 has more advantages than the use of mineral acid catalysts, as the former is less corrosive, less expensive, shows Lewis acid characteristics, is required in low amounts, and is able to avoid unnecessary neutralisation of the products. The SnCl 2 has shown a better catalytic activity than mineral H 2 SO 4 acid and a better conversion rate with high selectivity (Fig. 3).
Ieda et al. 55 studied the catalytic performance of a series of metal salts for the esterication process of fatty acids with alcohols, as shown in Fig. 4. Metal salts have Lewis acid character and are highly soluble in reacting uids, and are used as the catalyst for the esterication reaction. Long-chain fatty acids including C 10 -C 18 and alcohols were examined in equimolar ratios in the catalysts, substrate/catalyst ¼ 200 (v/w) and reaction period ¼ 6 h in mesitylene solution. The salts including chlorides, sulfates, nitrates and acetates of Al 3+ , Fe 3+ , In 3+ , ZrO 2+ , HfO 2+ , Zn 2+ , Ni 2+ , Co 2+ , Cr 3+ , Cu 2+ , Mg 2+ and Mn 3+ were used. The conversion rate and selectivity depended on the amount of the catalyst used and the reaction time. Ferric salts, particularly, FeCl 3 $6H 2 O was the most active and 90% yield was attained in this case.
Shivankar et al. 56 synthesised the Co(II) and Ni(II) mixed ligand complexes using 8-hydroxyquinoline (Q) as the primary and amino acids (HA) as secondary ligands for ester hydrolysis. The hydrolysis of ethyl acetate and methyl acetate was studied using metal complexes as homogeneous catalysts. The amount of ester was produced under a reaction temperature of 40-50 C) and substrate/catalyst ratio ¼ 100-500 (v/w) to study the reaction mechanism and kinetics. The reaction mixture (5 mL) was withdrawn at regular intervals for titration against standard NaOH solution to measure the acid values. It was evident from their experiment that mixed ligand metal complexes are more efficient catalysts for ester hydrolysis. The rate constant (k) value increased (2) with an increase in the amount of the catalyst (0.05 g) at 40 C and ethyl acetate was 5.0 cm 3 as shown in Fig. 5.
The plot between k and the amount of catalyst (Fig. 5) used is a straight line, showing that the rate constant (k) is proportional to the amount of catalyst used for the hydrolysis process.  Shivankar et al. 57 in another report investigated the chiral mixed ligand complexes of 8-hydroxyquinoline as primary ligands, while dextrose, fructose, mannitol and tartaric acid were secondary ligands for ester hydrolysis. The hydrolysis of ethyl acetate and methyl acetate was studied using metal complexes as the homogeneous catalysts. The hydrolysis of ester is the opposite of the esterication reaction and is catalysed by transition metal complexes; researchers would like to determine the change in enthalpy (DH), entropy (DS) and Gibbs free energy (DG) in an activated state. In this case, the amount of ester at a temperature of 30-50 C was being constant when the amount of catalysts were varied, with substrate/catalyst ¼ 100-500 (v/w). Table 8 summarises that the rate constants are highly dependent on the type of catalyst at constant temperature (40 C), where changes in Gibb's free energy was almost similar, while the reaction entropy largely varied in an activated state. The value of rate constant (k) increased with the type of catalyst used; [Co(Q)(Val)]$H 2 O was found to have obtained the highest kinetic constant. The amount of catalyst was strongly dependent on the kinetic constant summarised in Table 8. For instance, [Co(Q)(Val)]$H 2 O consumed the least amount of catalyst as the rate constant was directly related to the amount of catalyst involved in the reaction.
Oliveira et al. 51 carried out an experiment using bivalent tin chelate of 3-hydroxy-2-methyl-4-pyrone (HMP) complex (Fig. 6) as a homogeneous catalyst for poly-esterication of neopentyl glycol (NPG), terephthalic acid (TFA) and trimethylolpropane (TMP) by using acid : alcohol ratio of 4.8 : 4.2 : 2.1 at 150-230 C for 5 h reux. The Sn(IV) with Lewis acid character together with its chelate complexes can enhance the esterication reaction by expanding the metal's d orbitals to increase the accessibility towards substract molecules via an associative exchange with ligands. By increasing the amount of catalyst loading, they observed a decrease in the reaction time, although the productivity was higher for reactions with lower catalyst loading. So, compared with other properties, they concluded that the behaviour was not linear (Table 9).
Meneghetti et al. 58 reported on Sn(IV)-based complexes as catalysts to produce alkyl esters from alcohol and carboxylic acid. The researchers proposed two types of reaction mechanisms; Lewis acid and exchange/insertion mechanism by introducing labile ligands, which can be exchanged between the feedstock and catalyst (Fig. 7). The Sn metal having vacant 5d orbitals could expand its coordination numbers by insertion with subtract molecules through non-bonding electron pairs and enhance the esterication reactions. First, Sn(IV) complexes bonded with alcohol to form an intermediate product. This intermediate product was then reacted with carboxylic acid, forming another transition compound with the ester product and water as by-products.  Zendehdel et al. 59 reported that the utilisation of NaY zeolite supported 2,6-diformyl-4-methylphenol (DFP) complexes (Fig. 8) as the heterogeneous organic catalyst for esterication of acetic acid with different alcohols (e.g. 2-pentanol, isoamyl alcohol). The results indicated that the Schiff based complexes 60 had a better catalytic activity towards esterication. The NaY zeolite catalyst supports the Schiff base complexes with available of Na ions on the zeolite Y matrix, which gives NaY-NH 2 a strong basic character even without organic bases. Hence, the xation of Schiff-based complexes over the NaY-zeolite surface could enhance the catalyst activity 61 towards the esterication reactions. The esterication was carried out at 70 C for 2 h with 50 mg of catalyst loading and a maximum of 90% conversion of acetic acid.
Oh et al. 62 reported on the preparation of biolubricants using long-chain alcohols (8 carbon atoms or more) with saturated and unsaturated fatty acids over sulphated zirconia as the potential heterogeneous catalyst. 63 Sulphated zirconia is a solid acid catalyst for developing environmentally benign and friendly processes for esterication, as they have acidic sites and the so-called solid superacid catalyst. Sulphated zirconia compounds consist of monoclinic and tetragonal zirconia phases, and the transition from the monoclinic phase to the tetragonal of zirconia is attributed to sulfate content in the Zr-O-S framework. The incorporation of SO 4 2À into the ZrO 2 matrix is achieved by the wet impregnation method by varying the SO 4 2À content. The alcohol structure is found to have greatly affected the conversion rate and the product yield. The yield of the product and rate of conversion for the esterication of stearic acid and unsaturated acids (e.g., oleic acid, linolenic acid and linoleic acid) with various alcohols using 6.25 mmol fatty acid, 7.5 mmol alcohol and 100 mg of catalyst at 140 C for 4 hours is shown in Table 10.
Adam et al. 64 described the kinetic study for the esterication of ethyl alcohols and acetic acid over the heterogeneous Lphenylalanine-Ru(III) complex immobilisation on silica. Initially, the reaction was performed with 0.10 g catalyst using ethyl alcohol to acetic acid ratio of 1 : 3 at 85 C and the conversion of ethyl alcohol in 1 h was 34% and the conversion reached 94% over the next 12 hours. As silica was graed on Ru(III) site, the complex catalyst rendered an acidic character capable of enhancing the esterication reaction. This catalyst could be regenerated by washing with ethanol and further be used without signicant loss of reactivity.
Kotwal et al. 18 reported that the biolubricants were synthesised using three-dimensional compounds of titanosilicates, Ti-SBA-12 and Ti-SBA-16. The incorporation of Ti in the silica framework produced Lewis acid sites that were active for the esterication reaction. The higher catalytic performances of these catalysts were due to Lewis acidic Ti sites and the mesoporous character of their structure. In a typical reaction containing 0.12 g catalyst, the oleic acid to polyol (NPG, TMP and PE) ratio was 1 : 1 and reaction temperature was 180 C with 1 h continuous reux condition; the following results of these complexes were found (Table 11).
Maki et al. 65 compared the catalytic activity of N-alkyl-4boronopyridinium halides and boric acid (H 3 BO 3 ) via esterication of a-hydroxycarboxylic acids with different alcohols (methanol, ethanol, propanol, butanol). The results suggested that N-methyl-4-boronopyridinium iodide showed a better reactivity than H 3 BO 3 catalyst with a maximum yield of 90%, which was attained for 6 h reux. However, boric acid catalyst was more effective for dehydrating esterication between an equimolar ratio of a-hydroxycarboxylic acids and alcohols. N-Polystyrene bound with 4-boronopyridinium chloride to form a heterogeneous catalyst was also effective and could be reused aer ltration.
Nandiwale et al. 66 reported the production of octyl levulinate biolubricants by the esterication process over the modied H-ZSM-5 (Meso-HZ-5) catalyst (where Si/Al ratio ¼ 37) of biomassderived levulinic acid with n-octanol and achieved a 99% yield at 100-120 C with 10-30 wt% (LA) catalyst. This was an efficient catalytic process for conversion of agricultural waste feedstock to valuable chemicals. The ZSM-5 zeolite is composed of several pentasil units linked together by oxygen bridges to form a pentasil chains and is a medium-pore zeolite with channels dened by ten-membered rings; H-ZSM-5 is in protonated form. The ZSM-5 zeolite catalyst exhibits Lewis acidity that enhances the esterication reaction (Table 12).

Transesterication reaction
Transesterication pathway is another conventional process to synthesise ester-based biolubricants using triglyceride-based feedstock and alcohol with the aid of a catalyst.  Transesterication is more advantageous compared to general acid-catalysed esterication processes from carboxylic acids and alcohols. For example, some carboxylic acids are sparingly soluble in organic solvents and homogenise esterication with great difficulty, while esters are generally soluble in most organic solvents. The ester-to-ester transformation is highly suitable when the parent carboxylic acids are changeable and difficult to isolate. The most frequently used acid catalysts are sulfuric, sulfonic, phosphoric, and hydrochloric acids. Basecatalysed transesterication is another conventional method, and a wide variety of metal alkoxides, acetates, oxides, and carbonates work in this method. The most popular bases are sodium and potassium alkoxides. Acid catalysed trans-esterication is much slower than alkali catalysed trans-esterication and typically requires a higher temperature. However, acid-catalysed transesterications are more favourable than base catalysed transesterications, since acid catalysis is not highly affected by the presence of free fatty acids in the feedstock. Practically, acid catalysts can simultaneously catalyse both the esterication and transesterication reactions. The most commonly used alcohol for this purpose is methanol and the process is sometimes called methanolysis. Here we intend to describe the Lewis acidity of catalysts that enhance the transesterication reaction. For this, we include some metal salts, especially Sn(II) and Sn(IV) based complexes with different ligands, which are reviewed. The main advantages of these catalysts are that they are highly soluble in the oil phase where the reaction occurs and give the maximum yield, but catalyst separation is quite difficult. Hao et al. 68 utilised Sn(IV), Hf(IV) and Yb(III) bis(per-uorooctanesulfonyl) amide complexes in the trans-esterication reaction between methyl butyrate (MB) and n-octanol (n-OcOH). The catalytic activity for transesterication was strongly affected by the Lewis acid character of the catalyst complexes. The ndings revealed that Sn[N(SO 2 C 8 F 17 ) 2 ] 4 catalyst exhibits an excellent yield of 89% with 99% selectivity for transesterication at an equimolar ratio of methyl butyrate with n-octanol in a uorous biphase system (FBS). All the catalytic activity of Sn(IV), Hf(IV) and Yb(III) was evaluated using a uorous solvent, which enhances the immobilisation of the catalyst and phase separation at the end of reaction (also known as FBS system). Table 13 shows the catalytic performances of metal complex catalysts, which were investigated in the FBS system under 80 C for 15 h. It was revealed that Sn[N(SO 2 C 8 F 17 ) 2 ] 4 was the outperformed catalyst due to the catalyst being completely immobilised in the uorous phase for the transesterication and the uorous solution being directly used in the subsequent reaction, i.e., it being recyclable.
Abreu et al. 69 Table 14 depicts the summary of catalytic performances of Sn-, Pb-and Zn-based complexes, and the H 2 SO 4 catalyst. It has been concluded in the study that catalytic activity decreases in the order Sn 2+ [ Zn 2+ [ Pb 2+ with decreasing Lewis acid character. Sn has a stronger Lewis acid character due to it having vacant 5d orbitals that can expand its coordination numbers by insertion with subtract molecules, which gives a higher yield.
Moreover, Serra et al. 71 also reported on the methanolysis of castor oil and soybean oil using homogeneous Sn(IV)-based catalysts. The Sn(IV)-based catalysts are dibutyltin diacetate ((C 4 H 9 ) 2 Sn(C 2 H 3 O 2 ) 2 ), di-n-butyl-oxo-stannane ((C 4 H 9 ) 2 SnO), butylstannoic acid ((C 4 H 9 )SnO(OH)) and dibutyltin dilaurate ((C 4 H 9 ) 2 Sn(C 12 H 23 O 2 ) 2 ), which were taken as Lewis acid catalysts for transesterication reactions. Two types of reactors were used, open glass reactor (OG), where the reactions were performed under methanol reux at atmospheric pressure (65 C), and closed steel reactor (CS), where the reactions were carried out at 80 C and 120 C. The reaction conditions for both reactors were MeOH : oil : catalyst ¼ 400 : 100 : 1 at constant magnetic stirring of 1000 rpm. The results implied that the methanolysis of the castor oil led to lower yields than soybean oil although Sn(IV) catalyst was employed due to the inuence of the chemical composition of the triglycerides on the activity of the catalysts based on the Lewis acid sites. Another important observation was the use of a CS reactor rather than an OG reactor, which increased the reaction temperature, leading to greater reaction yields for all Sn(IV) complexes. The FAMEs (% yield) by methanolysis of castor and soybean oil in the presence of Sn(IV) catalysts, using OG and CS reactors, are depicted in Table 15. The (C 4 H 9 ) 2 SnO catalyst gave the highest yield in the closed steel reactor because this was attained under high reaction temperature with high methanol concentration in the liquid phase, giving high reaction rates during methanolysis.
Ferreira et al. 72 investigated the methanolysis of soybean oil in the presence of tin(IV) complexes including FASCAT® 4100, 4201, and 4350 and LIOCAT® 118 catalysts (Table 16) under mild conditions at 80 C, methanol : oil : catalyst ratio of 400 : 100 : 1 (v/v/w), and reaction time of 10 h. Results indicated that the dibutyltin dilaurate catalyst offered the best reactivity in terms of reaction yield, approximately 43% of FAMEs aer a 10 h reaction period.
The tin(IV)-based compounds have the potential to act as heterogeneous or homogeneous precursors for esterication, transesterication and/or polycondensation reactions. Meneghetti et al. 58 proposed that the key role of organotin(IV) complexes for transesterication reactions was due to the Lewis acidity. Tin(IV) atoms can coordinate with many molecules in Table 13 Catalytic transesterification reaction between methyl butyrate and n-octanol in fluorous biphase system (FBS) 68  solution and associative exchanges in certain mobile ligands with other compounds. The proposed catalytic trans-esterication mechanisms involving organotin(IV) complexes including ligand association or exchange processes are shown in Fig. 9. In the case of exchange mechanism, Sn(IV) molecule were bonded with acid and alcohols to form an intermediate product. For coordination, Sn(IV) molecule was bonded with ester and alcohols to form a transition compound. In both cases, these products immediately decomposed, leading to ester and alcohols. The transesterication reactivity of Sn(IV) compounds including Bu 2 Sn(Lau) 2 , BuSn(O)OH, and Bu 2 SnO was investigated by Meneghetti et al. 58 In addition, the effect of the reaction parameters (reactor type, substrate to alcohol ratio, reaction time, temperature and catalyst loading) for the trans-esterication process is reported in Table 17. The nding reveals that the Bu 2 Sn(Lau) 2 catalyst exhibits an excellent yield (98%) at 150 C when MeOH : soybean oil : catalyst ¼ 400 : 100 : 1 (v/v/w) with constant stirring at 1000 rpm. Bu 2 -Sn(Lau) 2 is the best catalyst compared to Bu 2 SnO and BuSn(O) OH due to its higher degree of solubility and activation, which can be reached at higher temperatures.
Serio et al. 74 reported that the most effective catalysts (e.g., Cd, Mn, Zn, Pb carboxylic salts) have been individuated and a correlation of the activities with the cation acidity has been obtained. These catalysts are active in the presence of high FFA concentrations, whereas homogeneous alkaline catalysts pose great difficulties due to the presence of large amounts of free fatty acids (FFA). The study reveals that the catalysts activity depends on the metal acidity and the structures of ester and alcohols; hence, every ester-alcohol couple will have a specic metal choice that will give the maximum activity. 75 Besides,  these catalysts are more efficient in the presence of high free fatty acid concentrations with a 5 Â 10 À3 : 1 weight ratio of catalyst to oil at 200-250 C when 2.0 g (0.2% w/w of FFA) of soybean oil and 0.88 g of methanol is used (Table 18). The results show that the Cd(OAc) 2 catalyst successfully achieved a maximum of 89% conversion contrary to that of Ba(OAc) 2 , which generated a lower conversion under the same reaction conditions. Cd(OAc) 2 is the most promising catalyst in this study due to it's resistant to deactivation during by-product water formation from esterication of FFA process. It is possible to obtain high FAME yields (96%) and a low nal FFA concentration (<1%) in a relatively short reaction time (200 min) and low catalyst concentration (4 Â 10 À4 : 1 weight ratio of catalyst to oil). The most recommended and best catalyst for trans-esterication reaction from the above study with respect to economic prospects, reaction conditions and the conversion rate are Sn(II) and Sn(IV) complexes. Since Sn has a vacant 5d orbital, showing a Lewis acid character, it is the most promising catalyst in this study due to it being lowered by the water formation during transesterication of FFA using a low amount of catalyst. The main advantage of Sn-based complex catalyst is that it is highly soluble in the oil phase. Tin(IV) atoms can coordinate with many molecules in solution and be associatively exchanged in certain mobile ligands with other compounds. They are cheaper, easily available, form complexes with other electron donating groups, and are useful for biolubricant production through transesterication process. In fact, transesterication is ester-to-ester transformation, and the most commonly used alcohol is methanol and hence trans-esterication is sometimes called methanolysis (Table 19).

Hydrogenation
The hydrogenation of esters is crucial to the chemical industry 76,77 for the synthesis of biolubricants or surfactants that have broad applications in agrochemicals, pharmaceuticals, ne chemicals and consumer goods. The reduction of aldehydes, esters and ketones by employing molecular H 2 at higher temperatures and pressures for producing ester-based lubricants is important to our daily life. The normal ester content of high oxygen is not suitable for them to be used as ne chemicals and engine oils as they are not biodegradable. Moreover, aer hydrogenation of ester-based lubricants, the oxygen content decreases and the hydrogen content increases; hence, these lubricants can be potentially used as engine oils, which satises all types of lubrication.
The potential catalysts that inuence the reduction process of carbonyl group present in ester products include Ru, Rh, Os, or other valuable metal based complexes, like Pt, Au, Pd, Ag, Ir, In. 78,79 However, the high price and limited availability of the precious metal-based catalysts has led to the development of less expensive active metals. Transition metal (Cu, Ni, Co, Cr and Fe) based complexes 80 have gained recent interest. To date, organometallic complexes as catalysts for hydrogenation of esters are broadly used for creating a homogeneous phase. The ester product becomes highly hydrogenated, but catalyst separation is quite difficult. The main challenges of ligand-based catalysts are the stability of the catalysts, 81 catalytic reactivity, and limitation for overcoming electron transfer routes that can be found in transition metal complexes. 82 Valencia et al. 83 investigated the hydrogenation of b-enamino esters, which was catalysed by cobalt complexes with a chiral (R-BINAP) ligand, bidentate phosphine, or achiral (PPh 3 ) ligand,  Table 20. Fig. 10 implies how one molecule of hydrogen reacted with b-enamino ester, forming a hydrogenated ester in the presence of the Co 2 (CO) 8 catalyst and racemic ligand at 450 psi. Chakraborty et al. 84 studied iron-based compounds as catalysts for the hydrogenation of esters to corresponding ester based-lubricants, surfactants or plasticisers bearing a PNPpincer ligand. The PNP-pincer ligand contains a pyridine type molecule having donor atoms P, N, and P, which form complexes with iron acting as a catalyst for the hydrogenation of esters to ester-based lubricants at 115 C under 63 psi per g of H 2 pressure in toluene. In this study, the hydrogenation of CE-1270, derived from coconut oil, containing methyl laurate (C 12 , 73%), methyl myristate (C 14 , 26%), and a small amount of C 10 and C 16 methyl esters, was performed at 135 C under 750 psig of H 2 pressure and the iron complex was used as the catalyst (1 mol%). CE-1270 (typically 1.6-1.7 g) was fully converted in 3 h, producing ester-based oil that was conned by GC (yield; 98.6%).
Karamé et al. 85 carried out an experiment on the asymmetric hydrogenation of acetophenone using N 4 -Schiff based chiral complexes containing sulfonamide or amine functionalities of ruthenium. The asymmetric hydrogenation (AH) was conducted at room temperature at 30 bar hydrogen pressure in the presence of a chiral catalyst (whereby the catalyst was synthesised by introducing the chiral ligand to the metallic precursor in absolute media). The asymmetric hydrogenation reaction of acetophenone with some metals of Rh, In, and Ru was also performed under the same conditions. The hydrogenation with the Ru species was usually carried out under solvent conditions (e.g. tert-butanol, isopropanol), and the mixture was stirred with H 2 for 24 hours (Table 21). Fig. 11 indicates that in the presence of metal complexes and i-PrOH and t-BuOH, the ester was converted to the corresponding hydrogenated ester (lubricant) under the mentioned reaction conditions. Prabhu et al. 86 described the transfer hydrogenation (TH) process of ketones using Ru(II) carbonyl complexes bearing benzoylhydrazone ligands. The TH process of aromatic, aliphatic, heterocyclic, and cyclic ketones 87 was studied by using Ru(III) complex and iso-PrOH/KOH (base) at 82 C and the results are summarized in Table 22. The following reaction shows the reaction scheme of the Ru(III) complex catalysed transfer hydrogenation process of ketones to the corresponding hydrogenated ester. In this case, 4-nitro ketone and 4-cyano    Hectorites could be incorporated into metal complexes as well as many organic compounds. Since tempered clays have certain interlayer sites, the interaction of chiral ligands with the substrates could occur. Such potential interaction may increase the selectivity in the asymmetric reactions. The proposed mechanism for hydrogenation is shown in Fig. 13. Itaconate has two forms: trans isomer of itaconate is known as mesaconate and cis-isomer is known as citraconate. cis-Isomer (i.e., citraconate on hydrogenation) gives the corresponding saturated hydrogenated ester. The high selectivity was found for the hectorite catalyst in all the solvents compared to that for the homogeneous catalysts, since hectorite still maintained its layer structure even aer being swollen with the solvents. The most recommended operating parameters for the mentioned hydrogenation process in the suggested catalyst were hydrogen pressure (P H 2 ) ¼ 1.01 Â 10 5 Pa; temperature (T) ¼ 30 C; substrate concentration ¼ 6.25 Â 10 À4 mol; and substrate/Rh ¼ 100 (Table 23). Table 24 summarises different catalysts used for the hydrogenation process in order to produce hydrogenated esters. The potential catalysts for this purpose are valuable metals like Ru, Rh, Os, Pt, Au, Pd, and Ir based complexes. In fact, the high price and limited availability of these metalbased catalysts leads to the development of highly abundant and less expensive active metals, i.e., transition metal (e.g., Cu, Ni, Co, Mn and Fe) based complexes are gaining recent interest. Nowadays, transition metal complexes as catalysts for hydrogenation of esters are widely used for producing ester-based lubricants.

Hydrogenation-esterication reaction
In the one-step hydrogenation-esterication (OHE) process, biooils are converted into biofuels (biolubricants) simultaneously in the same reaction vessels employing the same catalyst and reaction conditions. The nal products become highly suitable for combustible hydrocarbons as they are hydrogenated to form esters via basic reactions such as esterication 89,90 and hydrogenation. 91,92 The main constituents of biooils normally employed for the one-step hydrogenation-esterication process are fatty acids, aldehydes and ketones, and phenols, which affect the properties of biooils negatively. For instance, acetic acid, levulinic acid, furfural, hydroxyacetone, phenol, and ethanediol are considered as the recommended raw materials 93 for production of biolubricants. The OHE process utilises different bifunctional metals-based catalysts, 94 such as RANEY® Ni (RN) or Lewis acid catalyst under solvent conditions (methanol/ethanol) in order to generate combustible and stable compounds, i.e. hydrogenated esters. Xu et al. 95 investigated a method to convert biooils into the hydrogenated ester using RN catalyst for the hydrogenation-esterication reaction. Acetic acid, furfural and hydroxyacetone were used as model compounds and 100% conversion of hydroxyacetone and furfural were attained over Ni catalysts modied with Mo, Sn, Fe, and Cu in the presence of methanol. The rate of conversion of CH 3 COOH was 35.1% when methanol was not added, but when 6 g/8 g methanol/biooil was added, the rate of conversion of acetic acid increased to 81.1%. Table 25 indicates that when Mo-RN was modied by 5% Fe, the conversion of methanol decreased from 35.6% to 28.6%. The degree of decrease was not as signicant as the one of Fe-RN. However, the conversion of phenol decreased signicantly by addition of Fe as compared to Mo-RN.
Yu et al. 96 described the bio-oil upgrading by the hydrogenation-esterication process using acetic acid and furfural as model compounds over bifunctional Pd. The OHE was performed in a 100 mL stainless steel autoclave with an equimolar ratio of furfural and acetic acid, with 0.40 g catalyst (0.40 Pd/C + 0.40 g Al 2 (SiO 3 ) 3 for mixed bifunctional catalyst) by adding toluene solvent. Reaction time of 4 h at 1.0-4.0 MPa of H 2 and 80-200 C with a stirring speed of 800 rpm and a particle size of 400 meshes were adopted to evaluate the catalytic performances of the tested catalysts. The catalytic performances of the catalysts used for OHE reaction of FAL and HAc is listed; among the tested catalysts, 5% (v/w) Pd/Al 2 (SiO 3 ) 3 showed the best catalytic activity. The better OHE activities over the composite bifunctional catalysts may be attributed to their better cooperative effects among metal sites and acid sites.
Yu et al. 97 studied Al-SBA-15 supported Pd bifunctional catalysts (Pd/C, Pd, etc.) to upgrade bio-oils using acetic acid and furfural as the model compounds via the OHE process. The OHE experiments were conducted with an equimolar mixture of 0.10 mol furfural and 0.10 mol acetic acid, which were dissolved in 10.0 mL toluene and 0.40 g of Pd/Al-SBA-15 (5%) or 0.40 g Pd/C (5%) + 0.40 g Al-SBA-15 catalyst was added to the reactor. The reaction was performed under optimum condition of 2.0 MPa H 2 pressure, 150 C of reaction temperature, stirring speed of 800 rpm for 4 h of reaction time. It was important that the synergistic effect between the metal sites and the acid sites on bifunctional Pd/Al-SBA-15 (5%) favoured the OHE reaction, and the following results were obtained. Table 26 summarises the effects of the acidity on the support materials, as the reaction activity was studied over 5% Pd/ Al-SBA-15(X) with different Si/Al ratios and the catalytic results are  also shown. It is illustrated that with increasing Si/Al of Al-SBA-15 (decrease in acidity of the supports), Z (FAL) (conversion of furfural) decreases but X (D) (yield) increases. Tang et al. 98 investigated the catalytic activity of bifunctional Pd and Pt catalyst loaded with acidic supports by upgrading the bio-oils through the OHE route using fatty acid and aldehyde as the starting material. Acetic acid and butyl aldehyde were selected as model compounds for the OHE process using Pt catalysts with supported HZSM-5 and/or amorphous aluminium silicate as the bifunctional catalysts, which means they exhibited the properties of hydrogenation and esterication. The catalysts with a large surface area, high pore volume, small particle size, and strong acidic nature may be favourable for OHE reaction. The reaction was performed at 150 C with hydrogen pressure of 15 atm, 0.2 g of catalyst, and reaction time of 4 h with a stirring speed of 750 rpm.
From the above study, the most recommended catalyst for the hydrogenation-esterication process of bio-oil upgrading are Pt catalysts with acidic supports e.g., HZSM-5 or amorphous aluminium silicate. These catalysts are bifunctional, have a large surface area, high pore volume, small particle size, and strong acidic nature and are the most favourable for the onestep hydrogenation-esterication (OHE) reaction. The better OHE activities of the bifunctional catalysts may be attributed to its better cooperative effect with metal sites and acid sites. The OHE reaction between aldehyde and acid to ester is feasible over a bifunctional catalyst under high temperature and pressure for producing biolubricants (Table 27).

Future prospects and conclusion
The increasing demand towards renewable and sustainable energy has *attracted the interest of researchers to develop sustainable biolubricants for automotive and machinery applications. Studies revealed that biolubricants exhibit a better price-performance ratio compared to conventional mineral oils, as they can adhere on metal surfaces for a longer time. This review demonstrates that biolubricants are potentially utilised as alternative lubricants for automobile applications such as engine oils, hydraulic and metal working uids, and grease and chainsaw oils. Our review indicates that metal complex catalysts are potentially used to catalytically synthesise biolubricants as high Lewis catalytic activity of metal complex catalysts were observed in the reactions. For esterication and trans-esterication, tin complexes of Sn(II) and Sn(IV) catalysts were found to have enhanced the reaction signicantly, as they exhibited Lewis acid character and the vacant 5d orbital was well coordinated with subtract molecules in the reaction mixture. Meanwhile, the bifunctional catalyst of 5% (w/w) Pt/ Al 2 (SiO 3 ) 3 had the highest activity for ester formation via in situ hydrogenation-esterication reaction due to its large surface area, high pore volume and strong acidic nature, supplemented with 91% yield. In addition, research ndings showed that the transition metal catalysts could maximise the selectivity of biolubricants and minimise the formation of undesirable products.   T ¼ 150 C, P H 2 ¼ 2.0 MPa, rpm ¼ 800, time ¼ 4 h, 9.60 g FAL + 6.00 g HAc + 10.0 mL toluene Furfural and acetic acid 70 97 To date, obtaining suitable and well-performed lubricants for specic engines operation is challenging in the present lubricant industry. Therefore, it is essential to develop new-generation lubricants for industries to meet the demand of lubricated automotives. Biodegradable and non-toxic lubricants can reduce environmental pollution by lowering carbon emission. In addition, the good lubricating properties, high load-carrying capacity, longer service life, and rapid biodegradability of biolubricants have enhanced the current eld of interest.
Furthermore, the development of new lubricant additives, which give a better lubrication under extreme conditions, is another important challenge in the biolubricant eld. The new transition metal complexes act as a homogeneous catalyst for upgrading biomass and non-edible oil based lubricants for automotive applications. Nevertheless, considerable works is yet to be accomplished in this eld, especially in the development of a new and economic method to develop transition metal-complex catalysts. Moreover, the application of potential metal complex catalysts by upgrading biomass in ester-based lubricants is necessary. The development of effective catalysts for various types of biolubricants are vital and have been recognised to perform better than conventional petroleum oils in modern automotive applications.

Conflicts of interest
There are no conicts to declare.