One of the most widely used chemical absorption techniques for carbon capture and storage/sequestration (CCS) is CO2 adsorption by ceramic materials. Once CO2 has been captured-fixed, it can be converted into value-added products such as precursors in chemical transformation reac­tions. CO2 is extensively used for enhanced oil recovery, as a monomer feedstock for urea and polymer synthesis, in the food and beverage indus­try as a propellant, and in production of chemicals. Therefore, the capture — fixation of CO2 would make a system suitable for accomplishing chemical transformation of CO2. The utilization of carbon dioxide is also very at­tractive because it is environmentally benign [105-115]. CO2 conversion to fuel and value-added products is an ideal route for CO2 utilization due to the simultaneous disposal of CO2 and the benefit that many products can be used as alternate transportation fuels [116]. CO2 chemical transfor­mation methods include (i) reverse water-gas shift, (ii) hydrogenation to hydrocarbons, alcohols, dimethyl ether and formic acid, (iii) reaction with hydrocarbons to syngas, (iv) photo — and electrochemical/catalytic conver­sion, and (v) thermo-chemical conversion [100-122].

CO2 can be catalyzed to valuable organic or inorganic compounds, where some basic catalytic materials (containing alkaline or alkaline-earth elements) are used. The activation of CO2 by alkali metals has received considerable attention in various surface science studies, which have demonstrated the formation of intermediate CO2, dissociation of CO2 and formation of oxalate and carbonate alkali compounds [118-121]. Carbon dioxide has been identified as one such potential vector molecule (through reduction to syngas, methanol, methane, formic acid, formaldehyde, di — methylether (DME) and short-chain olefins) [117-118, 120-122]. CO2 is a kinetically and thermodynamically stable molecule, so CO2 conversion reactions are endothermic and need efficient catalysts to obtain high yield. CO2 conversion to carbon monoxide (CO) looks like the simplest route for CO2 reduction [121]. CO is a feedstock or intermediate product for the production of methanol and hydrocarbon fuels via Fischer-Tropsch synthesis of CH4/CO2 reforming to form syngas (CO/H2) [122]. CO2 re­forming with CH4 is an example of CO2 being used as a soft oxidant, where the dioxide is dissociated into CO and surface oxygen, and oxygen abstracts hydrogen from methane to form water via the water-gas shift reaction (WGS) (Eqs. 11 and 12) [100-103, 121]. The catalytic chemis­try of the reverse water-gas shift reaction and the following transforma­tion to methanol/DME (or hydrocarbons via Fischer-Tropsch synthesis), and the subsequent production of gasoline (methanol-to-gasoline or diesel via hydrocracking of the alkanes produced in the Fisher-Tropsch process) are well established [102, 117-122]. On the other hand, methanol can be produced directly from carbon dioxide sources by catalytic hydrogena­tion and photo-assisted electrochemical reduction. A wide variety of CO2 photo-reduction methods have been performed to oxygenate products, in­cluding formic acid (HCOOH) and formaldehyde (HCHO). HCOOH and

HCHO are the simplest oxygenates produced from the reduction of CO2 with H2O (or proton solvents) [121]. Furthermore, CO2 can be utilized as a monomeric building block to synthesize various value-added oxygen-rich compounds and polymers under mild conditions. As an example, chemical conversion of CO2 through C-N bond formation can produce value-added chemicals such as oxazolidinones, quinazolines, carbamates, isocyanates and polyurethanes [105]. These commodity chemicals have been synthe­sized from green methods and have important applications in the pharma­ceutical and plastic industries. The chemisorption of CO2 based on C-N bond formation could be one of the most efficient strategies, utilizing liq­uid absorbents such as conventional aqueous amine solutions, chilled am­monia, amino-functionalized ionic liquids, and solid absorbents including amino-functionalized silica, carbon, polymers and resins. The processes by which chemicals for CO2 capture are manufactured should also be con­sidered in terms of their energy requirements, efficiencies, waste products, and CO2 emissions [105, 123]. In that sense, dimethyl carbonate (DMC) is a promising target molecule derived from CO2 catalyzed by inorganic dehydrating agents such as molecular sieves [107]. Dimethyl carbonate has received much attention as a safe, non-corrosive, and environmentally friendly building block for the production of polycarbonates and other chemicals, an additive to fuel oil owing to its high octane number and an electrolyte in lithium batteries due to its high dielectric constant. It can be synthesized through a two-step transesterification process utilizing CO2 as raw material [105, 107].

As a complementary technology to carbon sequestration and storage (CSS), the chemical recycling of carbon dioxide to fuels is an interesting opportunity. Chemical compounds such as alkane products (CnH(2n+2)) are un-branched hydrocarbons suitable for diesel fuel and jet fuel [121]. In this regard, biofuels or biodiesel, catalyzed using ceramic materials, can provide a significant contribution in energy independence and mitigation of climate change [109-127]. Today the main renewable biofuels are bio­ethanol and biodiesel. Biodiesel is a liquid fuel consisting of mono alkyl esters (methyl or ethyl) of long-chain fatty acids derived from vegetable oils, animal fats or micro and macro algal oils [127]. Biodiesel is a sustain­able, renewable, non-toxic, biodegradable diesel fuel substitute that can be employed in current diesel engines without major modification, offering an interesting alternative to petroleum-based diesel [106, 111-115, 124­128]. Besides this, it is free from sulfur and aromatic components, making it cleaner burning than petroleum diesel. Biodiesel has a high flash point, better viscosity and caloric power similar to fossil fuels. It can be mixed with petroleum fossil fuel at any weight ratio or percentage, and it can be used without blending with fossil fuel (B100) as a successful fuel [127, 128]. It has similar properties (physical and chemical) to petroleum die­sel fuel. Recently, transesterification (also called alcoholysis) has been re­ported as the most common way to produce biodiesel with lipid feedstock (such as vegetable oil or algal oil) and alcohol (usually methanol or etha­nol), in presence of an acid or base catalyst. Transesterification is the best method for producing higher-quality biodiesel and glycerol [108, 110-115, 124-132]. The reaction is facilitated with a suitable catalyst [129-131]. The catalyst presence is necessary to increase both, the reaction rate and the transesterification reaction conversion yield. The catalysts are clas­sified as homogeneous or heterogeneous. Homogeneous catalysts act in the same liquid phase as the reaction mixture. Conversely, if the catalyst remains in a different phase, the process is called heterogeneous catalytic transesterification [113, 127-131]. Heterogeneous catalysts are mostly ap­plied in transesterification reaction due to many advantages such as easy catalyst separation and reusability, improved selectivity, fewer process stages, no water formation or saponification reaction, including in green technology, and cost effectiveness [127, 132]. The heterogeneous catalysts increase the mass transfer rate during the transesterification reaction [127, 131]. Various ceramic materials have been investigated for the production of biodiesel [106, 109-115, 124-179]. Some ofthese solid catalysts include alkali and alkaline-earth metal carbonates and oxides such as magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium ox­ide (SrO) [124-131, 133-143]; lithium base ceramics (Li4SiO4 and Li2SiO3 [144-146]); sodium silicate (Na2SiO3 [147]); transition metal oxides and derivatives (titanium oxide, zinc oxide, mixed oxides catalysts [148-149]); ion exchange resin type acid heterogeneous catalysts [150]; MCM-metal impregnated materials [114]; layered double hydroxides (hydrotalcite-like hydroxides) [151-154]; hydrocalumite-like compounds [110,155]; sup­ported bases [156-163]; and zeolites [164-165].

FIGURE 6: Schematic representation of the membrane reactor concept using a CO2 permselective ceramic membrane: (a) CO2 dry methane reforming and (b) water-gas shift reaction with hydrogen purification wherein CO2 capture promotes the separation process.

Among the alkaline earth metal oxides, CaO is a promising basic het­erogeneous catalyst for synthesizing biodiesel at mild temperatures (be­low the boiling point of methanol, MeOH) and at atmospheric pressure due to its plentiful availability and low cost, but it is rapidly hydrated and carbonated upon contact with room temperature air. CaO is the most widely used catalyst for transesterification and produces a high yield of 98% of fatty acid methyl esters (FAME) during the first cycle of reaction [130]. Granados et al. [142] used activated CaO as a solid base catalyst in the transesterification of sunflower oil to investigate the role of water and carbon dioxide on the deterioration of the catalytic performance upon contact with air for different periods. The study showed that CaO was rapidly hydrated and carbonated in air. Consequently, the reusability of the catalyst for subsequent steps is a big question mark. Di Serio et al. [170] reported a 92% biodiesel yield with MgO catalyst, using 12:1 methanol to oil molar ratio with 5.0wt% of the catalyst at methanol supercritical con­dition for 1 h. Wen et al. [171] carried out transesterification from waste cooking oil with methanol at 170 °C for 6 h with 10wt% of MgO/TiO2 and 50:1 M ratio of MeOH and oil. Guo et al. [172] studied the methyl ester yield produced via transesterification of soybean oil using sodium silicate as a catalyst. Sodium silicate was an effective catalyst for the microwave — irradiated production of biodiesel and hydrothermal production of hydro­gen from by-product glycerol combined with Ni catalyst. The optimum reaction conditions obtained were 7.5:1 M ratio of alcohol/oil, 3wt% cata­lyst amount, 1 h reaction time and 60 °C reaction temperature. The FAME yield was ~100%. On the other hand, microwave-assisted transesterifica­tion of vegetable oil with sodium silicate is an effective and economical method for the rapid production of biodiesel. The reused catalyst after transesterification process for four cycles was recovered. Overall, sodium silicate was fully used in biodiesel production and glycerol gasification, and this co-production process provided a novel green method for bio­diesel production and glycerol utilization [172].

Several techniques have been investigated for the transesterification reaction using heterogeneous catalysts for biodiesel production, as fol­lows: transesterification via radio frequency microwaves, alcohol reflex temperature, alcohol supercritical temperature and ultrasonication [127, 173-177]. Recently, the use of ultrasonic irradiation has gained interest in biodiesel production [173-177]. Ultrasonic energy can emulsify the reactants to reduce the catalyst requirement, methanol-oil ratio, reaction time and reaction temperature and also provides the mechanical energy for mixing and the required activation energy for initiating the transesterifica­tion reaction [173-176]. The ultrasound phenomenon has its own physical and chemical effects on the liquid-liquid heterogeneous reaction system through cavitation bubbles, according to the following principles [175]:

(1) the chemical effect, in which radicals such as H+ and OH — are produced during a transient implosive collapse of bubbles (in a liquid irradiated with ultrasound), which accelerates chemical reaction in the bulk medium; and

(2) the physical effect of emulsification, in which the microturbulence generated due to radial motion of bubbles leads to intimate mixing (ho­mogenizing the mixture) of the immiscible reactants. Accordingly, the in­terfacial region between the oil and alcohol increases sharply, resulting in faster reaction kinetics and higher conversion of oil and biodiesel yield [127]. In 2000, the ultrasonication reactor was first introduced by Hiel — scher Ultrasonic GmbH for biodiesel production. Nishimura et al. [175] studied the transesterification of vegetable oil using low-frequency ultra­sound (28-40kHz). An excellent yield (~98%) was obtained at a 28 kHz ultrasound while a significant reduction of reaction time was obtained by using 40 kHz ultrasound. Salamatinia et al. [176] used ultrasonic assisted transesterification to improve the reaction rate. In this study, they used SrO and BaO as heterogeneous catalysts in the production of biodiesel from palm oil. The results showed that the basic properties of the catalyst were the main cause for their high activity. The low-frequency ultrasonic assisted transesterification process had no significant mechanical effects on SrO, but BaO catalyst study confirmed that the ultrasound treatment significantly improved the process by reducing the reaction time to less than 50 min at a catalyst loading of 2.8wt% to achieve biodiesel yield higher than 95%. Another study of alkali earth metals was carried out by Mootabadi et al. [177]. They reported the effect of ultrasonic waves at 20 kHz and 200W on the regenerated catalyst and compared mechanical stir­ring and ultrasonic irradiation. They investigated the optimum conditions, using palm oil for biodiesel production with catalysts such as CaO, SrO and BaO. They concluded that catalyst leaching was the main cause for the catalyst inactivity in the case of the re-used catalyst. BaO catalyst was found to be stable during the leaching. At the optimized condition, 95.2% yield was achieved with 60 min of reaction time for both BaO and SrO catalysts. For CaO catalyst, 77.3% yield was achieved with the same con­ditions. The use of ultrasound showed great enhancement of the reaction parameters in terms of the obtained yield and reaction time. The obtained yields were 30 to 40% higher in comparison to the corresponding results obtained using a conventional stirring reactor system without ultrasonica — tion. Deng et al. [178] prepared nano-sized mixed Mg/Al oxides. Due to their strong basicity, the nanoparticles were further used as catalyst for biodiesel production from jatropha oil. Experiments were conducted with the solid basic catalyst in an ultrasonic transesterification reaction. Un­der the optimum conditions, biodiesel yield was 95.2%. After removing the glycerol on the catalyst surface, the nano-sized mixed Mg/Al oxides were reused eight times. The authors concluded that calcination of hydro — talcite nanocatalyst under ultrasonic radiation is an effective method for the production of biodiesel from jatropha oil. The activity of base solid catalysts is associated to their basic strength, such that the most basic cata­lyst showed the highest conversion. In another work, Deng et al. [179] reported optimum conditions for biodiesel production in the presence of base solid catalysts. They studied BaO and Ca-Mg-Al hydrotalcite (the most effective). The 95% biodiesel yield from jatropha oils and Ca-Mg — Al hydrotalcite was established with 30 min of reaction time. Ca-Mg-Al hydrotalcite could be reused twelve times after washing of the adsorbed glycerol from the catalyst surface with ethanol. Other types of heteroge­neous catalysts under ultrasonic irradiation were used for transesterifica­tion by Georgogianni et al. [114]. They studied a wide range of catalysts including Mg-MCM-41, Mg-Al hydrotalcite and K+-impregnated zirco­nium oxide. They mixed frying oils, methanol and the catalyst in a batch reactor with mechanical stirring for 24 h and with ultrasonication for 5 h. The results suggested that the basic strength was the cause of the good activity of the catalysts. Mg-Al hydrotalcite achieved the highest reaction conversion of 87% at a reaction temperature of 60 °C. Overall, ultrasonic irradiation significantly enhanced the reaction rate, causing a reduction in reaction time, and the biodiesel yield increased [114]. Consequently, a better understanding of the use of ultrasonic sound waves to accelerate the transesterification process could lead to substantial future improvement of both batch and continuous production systems, to obtain a more sustain­able biodiesel production process [127].