Overview on catalytic deoxygenation for biofuel synthesis using metal oxide supported catalysts

https://doi.org/10.1016/j.rser.2019.06.031Get rights and content

Highlights

  • Metal oxide is promising support for biofuel synthesis via deoxygenation process.

  • High oxygen content of biofuel will lead to corrosion and instability.

  • Al2O3-TiO2 catalyst has drawn the attention in deoxygenation process for biofuel.

  • Controlled synthesis method can improve the effectiveness of Al2O3-TiO2.

Abstract

Catalytic deoxygenation is a biofuel upgrading process to eliminate the high oxygen content which will lead to corrosion, instability and lower heating value problems. Biofuel have a high oxygen content, which deteriorates the biofuel quality. Therefore, the upgrading of biofuels via catalytic deoxygenation is necessary. Metal oxide such as TiO2, Al2O3, SiO2, ZrO2 and CeO2 is known as a promising support for the production of hydrocarbon-graded biofuel via deoxygenation process. The choice of support is significant to provide the maximum acid strength for the hydrogenolysis of C-O bonds. Al2O3 supported catalyst has drawn attention due to the high acidity. However, the high acidity leads to coke deposition, unstable and deactivation of the catalyst. Thus, it is important to develop methods to reduce catalyst coking and enhance the lifetime of the catalyst. Recently, Al2O3-TiO2 supported catalyst has drawn increasing attention in deoxygenation process owing to its unique properties which can solve the issues from Al2O3. Controlled synthesis method is significant to improve the effectiveness of Al2O3-TiO2 in catalytic reaction since the physicochemical properties of the catalyst are co-related to the processing methodology. Hence, this review describes the use of selected metal oxide supported catalyst for biofuel conversion in deoxygenation process. Moreover, the synthesis method of Al2O3-TiO2 is comprehensively discussed. The physicochemical properties of Al2O3-TiO2, metals and metal oxides supported on Al2O3-TiO2 are further discussed. Finally, future prospective and challenges of deoxygenation process for biofuel synthesis are discussed in order to produce quality hydrocarbon like biofuel using metal oxide supported catalyst.

Introduction

Fossil fuel consumption and environmental issues are driving research in the area of renewable and environmentally sustainable alternatives [1]. Compared with fossil fuels, biodiesel is environmentally friendly, economical and technically feasible, and the synthetic sources of biodiesel are readily available [2]. As a result, biofuels are rapidly becoming one of the alternative renewable energy sources due to their pollution-free properties and cost competitiveness [3]. Meanwhile, bio oil as the main liquid product of pyrolysis, is a liquid mixture of organic compounds, high oxygen content. By using specific catalysts, hypoxic compounds can be reduced and hydrocarbons produced, which can be used as biofuels [4].Metal oxides are solid catalyst and extensively used as active phase or support for various organic reactions. Large category of heterogeneous catalyst is comprised of metal oxides owing to their acid-base as well as redox properties [5]. Deoxygenation is a process where oxygenated compounds are being eliminated from a molecule typically in the form of H2O, CO2 or CO. Deoxygenation reaction can be completed by using either hydrogen or ambient condition. The possible mechanisms of deoxygenation pathways are decarboxylation, decarbonylation, hydrodeoxygenation and hydrogenolysis as shown in Table 1. Decarboxylation/decarbonylation (deCOx) reaction is the typical deoxygenation (DO) process that removes oxygen in the form of CO2/CO through direct C–C bond cleavage under mild conditions [6]. Decarboxylation process targets carboxylic acids whereas decarbonylation process targets aldehydes, carboxylic acid and ketones for oxygen removal. On the other hand, hydrodeoxygenation (HDO) is also a cracking of C–C bond process which is carried out under high or low hydrogen pressure that forms water (H2O) as by-product. Moreover, hydrogenolysis where it involves cleavage of C–C or C–heteroatom bond when adding hydrogen [7]. Therefore, they are classified as deoxygenation process because the precedent reaction involves oxygen removal, just that the by-product and pathways differ from each other only.

Vegetable oil-based feeds, containing C8–C24 fatty acids are regarded as an alternative source for hydrocarbon fuel production [9]. In general, biodiesel feedstock can be divided into four main categories such as edible vegetable oil, non-edible vegetable oil, waste or recycled oil and animal fats [10]. Vegetable oil is extracted from triglyceride-based plant and further processed to biofuel. However, the biodiesel enrichment caused an increase in viscosity and reduces the volatility of the blend [11]. Besides, severe engine problems such as carbon deposit, lubricant thickening and etc are produced owing to the high oxygen contents, low volatility, high unsaturated acid contents and high viscosity of the extracted bio-oil [12,13]. The large consumption of raw material and high production cost of plant oil lead to less cost-effective when compared to the petrochemical industry [14]. Many attempts are made to find new and cost-effective technology for better, cleaner, and renewable fuel [15]. Biodiesel, as a substitute for traditional petroleum liquid fuel, has been widely used in recent years [16]. However, the first generation of hyperoxic biodiesel (fatty acid methyl ester) is not suitable for large-scale use because it is incompatible with fossil fuels [17,18]. Therefore, green biodiesel (second generation biodiesel) prepared from the catalytic HDO of the first generation of biodiesel has been gradually developed [19]. Biodiesel has many advantages after deoxygenation, and its composition is similar to that of petroleum fuel, which can be directly used in the fuel industry. In order to obtain the expected product, an accessible production process and a suitable catalyst system are required [20]. Biodiesel has many advantages after deoxygenation, and its composition is similar to that of petroleum fuel, which can be directly used in the fuel industry. In order to obtain the expected product, an accessible production process and a suitable catalyst system are required [21]. Transesterification of the triglycerides in vegetable oils is the conventional technology to produce renewable fuel. The biodiesel also known as fatty acid methyl ester (FAME) produced from transesterification has high O2 contents, causing the engine compatibility problems, instability and less effective cold flow properties [22]. Besides, high O2 content in biodiesel blends will increase the NOx emissions [23]. Thus, the oxygen content in the vegetable oil can be eradicated either as H2O or COx via deoxygenation to produce hydrocarbon biofuel [24]. The properties of biofuel produced via catalytic deoxygenation of vegetable oil-based feeds possess similarity to petroleum fuel allowed them to replace transesterification technology [25]. The vegetable oil-based model compounds such as fatty acids, fatty acid esters and specific triglycerides are used to investigate the fundamentals of deoxygenation process owing to their structural similarity to vegetable oils [9].

In the first generation feedstock, edible oil crops such as sunflower, coconut, palm, rapeseed etc are used to produce biofuel [26]. However, the first-generation biofuel leads to food competition and ecosystem imbalance owing to the demand of large arable land to cultivate these crops [27]. Thus, second generation feedstock, non-edible feedstock such as agricultural waste and waste cooking oils are used to convert into biofuel [28]. However, jatropha, rapeseed, rubber trees and other non-edible feedstock are usually have higher free fatty acids and need for pretreatment process, thus, non-edible feedstock are hard to meet global energy demands [29]. Microalgae are currently being promoted as third generation biofuel feedstock [30]. Microalgae have high cell lipid content and high photosynthetic rate, and are ideal raw materials for the production of biofuels [31]. Algae biomass can capture about 3–8% of the incident solar energy, and terrestrial plants can only convert 0.5%. The oil content of 3–5 parts of microalgae dry biomass was greater than 60%, and the average oil content was about 20–50% wt.%. The amount of Nissan oil per unit volume of microalgae broth is related to the growth rate of algae and the oil content of biomass. Compared with traditional oil-producing terrestrial plants such as rapeseed and sunflowers, microalgae grow 12 times faster and yield 30 times higher per hectare [32]. In addition, it does not require arable land, as microalgae cultivation will compete with food production, which can use wastewater to provide nitrogen sources and industrial carbon dioxide (flue gas) to promote the formation of algal biomass [33]. Therefore, the cultivation of microalgae has a synergistic effect with the recycling of biological wastewater, and it is an active CO2 reservoir. These characteristics make microalgae a promising green energy biological resource [34]. However, the third generation has drawback which is low lipid content due to high growth rate. For instance, the lipid content of most of the microalgae are around 20–50%, (w/w), thus the efficiency is lower compared to other sources [35]. Therefore, the engineered microalgae (so called fourth generation) is emerged to solve that particular problem [36]. Microalgae contain high-value biological products such as pigments, vitamins and antioxidants that can be extracted and utilized. In order to maintain the sustainable development of microalgae based biodiesel production, the cultivation of biomass should be induced by lipid culture, efficient harvesting, as well as the combination of lipid extraction and ester exchange reaction of biodiesel synthesis [37]. Palmitic acid, oleic acid, palmitoleic acid, lionoleic acid and stearic acid are common and general fatty acids found in microalgae cell (Table 2). These fatty acids can be used as model compound in deoxygenation reaction to study the conversion of green hydrocarbon owing to their long carbon chain (C > 12). From Table 2, it can be seen that palmitic acid is the main component of microalgae–derived bio-crude [38]. Therefore, catalytic deoxygenation is required to thoroughly study the conversion process of vegetable oil model compounds which are fatty acid, fatty acid ester or specific triglyceride before implementing at commercial scale.

Bio-oil can be produced via pyrolysis process of lignocellulosic biomass such as lignin, cellulose and hemicellulose [41]. Types of lignocellulosic biomass are shown in Fig. 1. Lignin is the hardest component to be converted into high quality liquid fuels due to its easy coking property in biomass pyrolysis process [42]. The liquid fuels can be obtained by lignin biomass pyrolysis. However, the liquid fuels obtained from lignin pyrolysis are bio-oil with rich oxygen contents (10–45 wt%) due to the depolymerisation and fragmentation reactions of ligno-cellulosic biomass that eventually leads to issues such as high viscosity, low heating value, low stability and cannot mix with conventional hydrocarbon fuels [43]. Thus, these bio-oils obtained from pyrolysis are needed to undergo chemical upgrading process which is deoxygenation to lower the oxygen content and produce high quality liquid fuel [44]. The pyrolysis oils obtain from wooden-based raw are complex chemical polymeric mixtures consisting of different molecules with high oxygen contents such as carboxylic acids, phenols, alcohols, ketones, furfurals, aldehydes or carbohydrates [45]. Different pyrolysis conditions such as pyrolysis temperature, heating rate, residence time, low-temperature pretreatment of biomass and reaction time will determine the product yields of that particular model compound in the pyrolysis bio-oil [46,47]. This complex nature of pyrolysis oil will cause difficulty in deoxygenation upgrading process because detrimental side reactions such as thermal pathways may happen if the parameter conditions are not taken into account for some high reactive oxygen-rich molecules [43]. Pyrolysis is a promising option for sustainable development for bio-oil. However, pyrolysis technology is also faced with a lot of critical challenges such as the development of new cost-effective and efficient reactors as well as novel catalyst and post-pyrolysis treatment is needed to improve the process and oil properties. Thus, a model can be established in order to predict the condition of pyrolysis reaction [48]. Therefore, the study of model compounds is the first and foremost step rather than the bio-crude oils for catalytic screening and simplify the complex issues in pyrolysis oil caused from oxygenated compounds. Bio-oil has a complex nature and therefore, independent study on each model compound presents in the bio-oil is useful owing to the kinetics and the reaction mechanisms can be studied in detail for that particular functionality. The whole real feed stock study is also significant because it makes up for the synergistic of all model compounds from the complex mixtures [49].

This review aims to summarize and discuss the catalytic deoxygenation for hydrocarbon biofuel production using selected metal oxide (TiO2, Al2O3, SiO2, ZrO2, CeO2 and Al2O3-TiO2) supported catalysts. The synthesis process and physicochemical properties of Al2O3-TiO2 are reviewed. Metals and metal oxides supported on Al2O3-TiO2 are further discussed. Then, future prospective and challenges for biofuel synthesis by catalytic deoxygenation process are discussed. This provides appropriate catalyst recommendations for the production of hydrocarbon biofuels by catalytic deoxygenation process [51,52] to obtain better quality of biofuels. Finally, to meet the goal of utilizing renewable biofuels to replace traditional fuels and meet the growing demand for fossil fuels as well as addressing the related environmental problems [53].

Section snippets

Deoxygenation process using metal oxide supported catalyst

This section summarized and discussed the deoxygenation process of the selected metal oxide such as TiO2, Al2O3, SiO2, ZrO2, CeO2 and Al2O3-TiO2 supported catalysts to produce hydrocarbon-graded biofuel. Deoxygenation of biologically derived molecules has evolved into a promising synthetic tool for the synthesis of fuels and chemicals [44]. The development of homogeneous and heterogeneous catalysts with good chemical stability and high activity are the main support of deoxygenation reaction.

Synthesis of Al2O3-TiO2

Al2O3-TiO2 supported catalyst exhibits high surface area, excellent reducibility and metal dispersion, superior catalytic activity and thermal stability [69,143,144]. It is an established fact in the previous studies that active metals dispersed on Al2O3-TiO2 produced higher catalytic activities compared to the one supported on single Al2O3 or TiO2 [126,127,143]. These results confirm that Al2O3-TiO2 is a promising support material for catalyst as in it can solve the coking problem due to high

Al2O3-TiO2

As depicted by previous literature, the physicochemical properties of Al2O3-TiO2 and metal or metal-oxide supported on Al2O3-TiO2 were characterized by various techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), temperature programmed desorption/reduction (TPD/TPR), N2 adsorption–desorption isotherms, transmission electron microscopy (), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Raman spectroscopy

Future prospective and challenges

The research progress of biomass catalytic deoxygenation in recent years is reviewed. Catalytic cracking bio-oil is a promising method to improve the quality of biological oil [52]. Microporous Molecular Sieve, mesoporous molecular sieve and metal oxide catalyst for biomass catalytic cracking were studied. On the other hand, it is of great significance to develop a method to reduce the coking of catalyst and improve the service life of catalyst [4]. In addition, it is very important to

Conclusion

Biomass can be converted to biofuels with a range of production method, such as bio-oils and biodiesel. However, the high oxygen content of biofuels can reduce the quality of biofuels. Therefore, it is necessary to promote biofuels by catalysing deoxygenation process. Metal oxide such as TiO2, Al2O3, SiO2, ZrO2 and CeO2 supported catalysts are used to produce hydrocarbon-graded biofuel via deoxygenation process. This will meet the current social demand for alternative clean energy sources. Al2O3

Acknowledgements

This study is supported by the Ministry of Education, Malaysia and University of Malaya, Malaysia under the FRGS-MRSA (grant no:MO014-2016) and RU grant (grant no: ST009-2017).

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