An Overview of Recent Research in the Conversion of Glycerol into Biofuels, Fuel Additives and other Bio-Based Chemicals

Upgrading Pyrolytic Residue from End-of-Life Tires to Efficient Heterogeneous Catalysts for the Conversion of Glycerol to Acetins

Recovered carbon blacks (rCBs) produced from end-of-life tires using pyrolysis were transformed into solid acid catalysts for the synthesis of acetins, i.e., products with a wide spectrum of practical applications. Tuning the chemical properties of the surface of samples and introducing specific functional groups on the rCBs were achieved through carbon functionalization with concentrated H2SO4. The initial and modified rCBs were thoroughly characterized using techniques such as elemental analysis, potentiometric back titration, thermogravimetric technique, scanning and transmission microscopy, X-ray photoelectron spectroscopy, etc. The catalytic activities of the samples were measured via batch mode glycerol acetylation performed at 110 °C and compared to the catalytic performance of the functionalized commercial carbon black. The modified rCBs were found to show a significant catalytic effect in the tested reaction, giving high glycerol conversions (above 95%) and satisfactory combined yields of diacetins and triacetin (~72%) within 4 h; this behavior was attributed to the presence of -SO3H moieties on the surface of functionalized rCBs. The reusability tests indicated that the modified samples were catalytically stable in subsequent acetylation runs. The obtained results evidenced the feasibility of using end-of-life tires for the production of effective acid catalysts for glycerol valorization processes.

MOF-808 as an Efficient Catalyst for Valorization of Biodiesel Waste Production: Glycerol Acetalization

Glycerol is the main residue in the biodiesel production industry; therefore, their valorization is crucial. The acetalization of glycerol toward fuel additives such as solketal (2,2-dimethyl-1,3-dioxolan-4-methanol) is of high interest, promoting circular economy since it can be added to biodiesel or even fossil diesel to improve their quality and efficiency. Straightforward-prepared metal-organic framework (MOF) materials of the MOF-808 family were applied to the valorization of glycerol for the first time. In particular, MOF-808(Hf) was revealed to be an effective heterogeneous catalyst to produce solketal under moderate conditions: a small amount of the MOF material (only 4 wt% of glycerol), a 1:6 ratio of glycerol/acetone, and a temperature of 333 K. The high efficiency of MOF-808(Hf) was associated with the high amount of acid centers present in its structure. Furthermore, its structural characteristics, such as window opening cavity size and pore diameters, were shown to be ideal for reusing this material for at least ten consecutive reaction cycles without losing activity (conversion > 90% and selectivity > 98%). Remarkably, it was not necessary to wash or activate the MOF-808(Hf) catalyst between cycles (no pore blockage occurred), and it maintained structural integrity after ten cycles, confirming its ability to be a sustainable heterogeneous catalyst for glycerol valorization.

Development and Digital Light Processing 3D Printing of a Vitrimer Composed of Glycerol 1,3-Diglycerolate Diacrylate and Tetrahydrofurfuryl Methacrylate

The development of biobased reshapable and repairable vitrimers has received extensive attention due to the growing focus on an environmentally friendly society. Therefore, the objective of this research was to synthesize sustainable polymers with an environmentally friendly strategy combining the benefits of renewable resources, UV curing, and vitrimers. Two biobased monomers, glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate, were chosen for the preparation of UV-curable resins and tested by real-time photorheometry and RT-FTIR spectroscopy to determine their suitability for digital light processing (DLP) 3D printing. DLP 3D-printed polymer showed shape memory, weldability, and repairability capabilities by triggering the dynamic transesterification process at high temperatures. The vitrimer with a weight ratio of 60:40 of glycerol 1,3-diglycerolate diacrylate and tetrahydrofurfuryl methacrylate showed shape memory properties with a recovery ratio of 100% and a 7-fold improved tensile strength compared to the original sample, confirming efficient weldability and repairability.

Glycerol Valorization—The Role of Biochar Catalysts

The conversion of renewable feedstocks into new added-value products is a current hot topic that includes the biodiesel industry. When converting vegetable oils into biodiesel, approximately 10% of glycerol byproduct is produced. Glycerol can be envisaged as a chemical platform due to its chemical versatility, as a scaffold or building block, in producing a wide range of added-value chemicals. Thus, the development of sustainable routes to obtain glycerol-based products is crucial and urgent. This certainly encompasses the use of raw carbonaceous materials from biomass as heterogeneous acid catalysts. Moreover, the integration of surface functional groups, such as sulfonic acid, in carbon-based solid materials, makes them low cost, exhibiting high catalytic activity with concomitant stability. This review summarizes the work developed by the scientific community, during the last 10 years, on the use of biochar catalysts for glycerol transformation.

Combined dehydrogenation of glycerol with catalytic transfer hydrogenation of H2 acceptors to chemicals: Opportunities and challenges

Catalytic transformation of low-cost glycerol to value-added lactic acid (LA) is considered as one of the most promising technologies for the upgradation of glycerol into renewable products. Currently, research studies reveal that anaerobic transformation of glycerol to LA could also obtain green H₂ with the same yield of LA. However, the combined value-added utilization of released H₂ with high selectivity of LA during glycerol conversion under mild conditions still remains a grand challenge. In this perspective, for the first time, we conducted a comprehensive and critical discussion on current strategies for combined one-pot/tandem dehydrogenation of glycerol to LA with catalytic transfer hydrogenation of H₂ acceptors (such as CO₂) to other chemicals. The aim of this overview was to provide a general guidance on the atomic economic reaction pathway for upgrading low-cost glycerol and CO₂ to LA as well as other chemicals.

Heterogeneous Catalysts for Conversion of Biodiesel-Waste Glycerol into High-Added-Value Chemicals

The valuable products produced from glycerol transformation have become a research route that attracted considerable benefits owing to their huge volumes in recent decades (as a result of biodiesel production as a byproduct) as well as a myriad of chemical and biological techniques for transforming glycerol into high-value compounds, such as fuel additives, biofuels, precursors and other useful chemicals, etc. Biodiesel has presented another challenge in the considerable increase in its byproduct (glycerol). This review provides a recent update on the transformation of glycerol with an exclusive focus on the various catalysts’ performance in designing reaction operation conditions. The different products observed and cataloged in this review involved hydrogen, acetol, acrolein, ethylene glycol, and propylene glycol (1,3-propanediol and 1,2-propanediol) from reforming and dehydration and hydrogenolysis reactions of glycerol conversions. The future prospects and critical challenges are finally presented.

Biodiesel Is Dead: Long Life to Advanced Biofuels—A Comprehensive Critical Review

Many countries are immersed in several strategies to reduce the carbon dioxide (CO2) emissions of internal combustion engines. One option is the substitution of these engines by electric and/or hydrogen engines. However, apart from the strategic and logistical difficulties associated with this change, the application of electric or hydrogen engines in heavy transport, e.g., trucks, shipping, and aircrafts, also presents technological difficulties in the short-medium term. In addition, the replacement of the current car fleet will take decades. This is why the use of biofuels is presented as the only viable alternative to diminishing CO2 emissions in the very near future. Nowadays, it is assumed that vegetable oils will be the main raw material for replacing fossil fuels in diesel engines. In this context, it has also been assumed that the reduction in the viscosity of straight vegetable oils (SVO) must be performed through a transesterification reaction with methanol in order to obtain the mixture of fatty acid methyl esters (FAMEs) that constitute biodiesel. Nevertheless, the complexity in the industrial production of this biofuel, mainly due to the costs of eliminating the glycerol produced, has caused a significant delay in the energy transition. For this reason, several advanced biofuels that avoid the glycerol production and exhibit similar properties to fossil diesel have been developed. In this way, “green diesels” have emerged as products of different processes, such as the cracking or pyrolysis of vegetable oil, as well as catalytic (hydro)cracking. In addition, some biodiesel-like biofuels, such as Gliperol (DMC-Biod) or Ecodiesel, as well as straight vegetable oils, in blends with plant-based sources with low viscosity have been described as renewable biofuels capable of performing in combustion ignition engines. After evaluating the research carried out in the last decades, it can be concluded that green diesel and biodiesel-like biofuels could constitute the main alternative to addressing the energy transition, although green diesel will be the principal option in aviation fuel.

Preparation of New Glycerol-Based Dendrimers and Studies on Their Behavior toward Essential Oil Encapsulation

Two new families of glycerol-based dendrimers (glyceroladendrimers (GADs) and glyceroclickdendrimers (GCDs)) have been synthesized. Three generations have been isolated for each family with good yields and were fully analyzed. The encapsulation of essential oils (citronella and cinnamon) in GADs, GCDs, and also in previously described glycerodendrimers GD-PAMAMs and GD-PPIs has been studied by dynamic-headspace gas chromatography coupled to mass spectrometry. The retention rates obtained were from -35.8 to 26.65% for citronella essential oil and from 2.14 to 38.84% for the cinnamon essential oil. In addition, the best results were obtained with GD-PAMAMs and GD-PPIs of higher generation. The interaction study between essential oils or more precisely their major components have been performed through NMR spectroscopy (¹H NMR and DOSY NMR). No direct interactions between dendrimers and essential oils have been observed, but a surprising behavior of compression of the dendrimer in stable emulsions was observed. Indeed, the hydrodynamic radius of GD-PPI-3 has been reduced in the presence of cinnamon essential oil.

Tuning the Catalytic Activity of Recyclable Heterogeneous Catalysts for the Direct Etherification Reaction of Glycerol Using Antagonistic Additives

Using zeolite as a heterogeneous catalyst, the reaction conditions were optimized to increase the yield and selectivity of diglycerol (DG) and triglycerol (TG) in the direct etherification reaction of glycerol. By the addition of weakly acidic alkali metal-based inorganic salts (NaHSO4 and KHSO4), the selectivities and yields of DG and TG increased. Although the conversion of glycerol was lowered due to the role of the additive as an inhibitor, the reaction conditions were optimized by controlling the amounts and reaction times of the additives to increase the yields of DG and TG. Under the optimized condition, the glycerol conversion was as high as 85.4%, and the highest yields of DG and TG were observed as 54.1% and 21.3%, respectively. The recyclability of the catalysts was much enhanced by the influence of the additives suppressing the formation of oligomers.

Mechanistic Kinetic Modelling Framework for the Conversion of Waste Crude Glycerol to Value-Added Hydrogen-Rich Gas

The kinetics for crude glycerol autothermal reforming was studied over S/C ratio of 2.6 and O2/C ratio of 0.125 using 5% Ni/CeZrCa catalyst. Both power law and mechanistic kinetic models were studied. The overall power law model for crude glycerol autothermal reforming was investigated with a pre-exponential factor of 4.3 × 1010 mol/gcat.min and activation energy of 8.78 × 104 J/mol. The reaction orders with respect to crude glycerol, water and oxygen are 1.04, 0.54 and 1.78 respectively. The power law model presented an absolute average deviation of 5.84%, which showed a good correlation between the predicted and experimental rate. Mechanistic models were developed for crude glycerol autothermal reforming. For steam reforming, the Eley–Rideal approach best described the reaction rate with the surface reaction being the rate-determining step (AAD < 10%). The kinetics of the total oxidation reaction was best described by the power law model with an AAD of less than 1%, whereas for the TOR process, the molecular adsorption of crude glycerol with an AAD of 14.6% via Langmuir Hinshelwood Hougen-Watson approach was best. CO2 methanation resulted in an AAD of 5.8% for the adsorption of carbon dioxide (CO2) by the Eley–Rideal mechanism.

Direct Etherification Reaction of Glycerol Using Alkali Metal Cation (Li+, Na+ and K+) Containing X-Type Zeolites as Heterogeneous Catalysts: Optimization of the Reaction Conditions

X-type zeolite (XZ-Na) containing Na+ as a cation was synthesized, and XZ-Li and XZ-K were prepared by exchanging the cations of XZ-Na with Li+ and K+, respectively. The specific surface areas, structures, and chemical compositions of the prepared zeolites were analyzed by BET, XRD, and SEM-EDX. The activity of the direct and selective etherifications of glycerol to diglycerol (DG) and triglycerol (TG) were investigated using each zeolite XZ-M (M = Li, Na or K) as a basic heterogeneous catalyst. The etherification reactions of glycerol were carried out at atmospheric pressure while controlling the reaction temperature, reaction time, and the amount of each zeolite. As the amount of each zeolite, reaction time, and reaction temperature increased, the conversion of glycerol also increased, but the selectivities of DG and TG decreased due to the increase in the production of oligomers. When each zeolite was used as a catalyst, the catalytic activity for the conversion of glycerol was observed as XZ-K > XZ-Li > XZ-Na, but the selectivities of DG and TG were observed as XZ-Li > XZ-Na > XZ-K. Especially, 3 wt.% of XZ-Li exhibited the excellent catalytic performance when the etherification of glycerol was optimized and carried out at 280 °C for 2 h: the conversion of glycerol was 89.6% and the yields of DG and TG were 61.2 and 21.2%, respectively.

A Short Review on Catalyst, Feedstock, Modernised Process, Current State and Challenges on Biodiesel Production

Biodiesel, comprising mono alkyl fatty acid esters or methyl ethyl esters, is an encouraging option to fossil fuels or diesel produced from petroleum; it has comparable characteristics and its use has the potential to diminish carbon dioxide production and greenhouse gas emissions. Manufactured from recyclable and sustainable feedstocks, e.g., oils originating from vegetation, biodiesel has biodegradable properties and has no toxic impact on ecosystems. The evolution of biodiesel has been precipitated by the continuing environmental damage created by the deployment of fossil fuels. Biodiesel is predominantly synthesised via transesterification and esterification procedures. These involve a number of key constituents, i.e., the feedstock and catalytic agent, the proportion of methanol to oil, the circumstances of the reaction and the product segregation and purification processes. Elements that influence the yield and standard of the obtained biodiesel encompass the form and quantity of the feedstock and reaction catalyst, the proportion of alcohol to feedstock, the temperature of the reaction, and its duration. Contemporary research has evaluated the output of biodiesel reactors in terms of energy production and timely biodiesel manufacture. In order to synthesise biodiesel for industrial use efficaciously, it is essential to acknowledge the technological advances that have significant potential in this sector. The current paper therefore offers a review of contemporary progress, feedstock categorisation, and catalytic agents for the manufacture of biodiesel and production reactors, together with modernised processing techniques. The production reactor, form of catalyst, methods of synthesis, and feedstock standards are additionally subjects of discourse so as to detail a comprehensive setting pertaining to the chemical process. Numerous studies are ongoing in order to develop increasingly efficacious techniques for biodiesel manufacture; these acknowledge the use of solid catalytic agents and non-catalytic supercritical events. This review appraises the contemporary situation with respect to biodiesel production in a range of contexts. The spectrum of techniques for the efficacious manufacture of biodiesel encompasses production catalysed by homogeneous or heterogeneous enzymes or promoted by microwave or ultrasonic technologies. A description of the difficulties to be surmounted going forward in the sector is presented.

Positive Effect of Antagonistic Additives on the Homogeneous Catalytic Etherification Reaction of Glycerol

Various compounds prepared using glycerol, diglycerol (DG), and triglycerol (TG) have been gaining increasing attention due to their wide range of applications. To increase the yield and selectivity of DG and TG syntheses, previous studies investigated a variety of catalysts with different basicity and variable reaction temperatures. In this study, we introduced additives that act as inhibitors to increase the selectivity of the etherification reaction for DG and TG production and depress the formation of higher oligomers by moderating the activity of the catalyst. By adding weakly acidic alkali metal-based inorganic salts (NaHSO4 and KHSO4), the selectivity of DG and TG formation could be enhanced, although the conversion of glycerol decreased due to the reduced activity of catalyst. We found that the decrease in the activity of the catalyst caused by the additives could be recovered and that side reactions were reduced if the reaction was carried out at an increased temperature of 280 °C and if the reaction time was shortened to 2 h to suppress the formation of oligomers. The dependence of the reaction on the amount of the additive, the reaction time, and the reaction temperature was investigated to elucidate the role of the additive.

Comparative study of catalytic conversion of glycerol, a by-product of transesterification, to cyclic hydrocarbons using MCM-22, ZSM-5 and alumina

Recently chemical consumption has increased due to the growth of human population and industrialization. Depleting fuel reserves and increase in chemicals rise has led and researcher to focus on alternative bio based chemicals. Glycerol which is produced as a major byproduct from the trans-esterification reaction of fatty acids for producing biodiesel has been used in this work for conversion to value added products. Conversion of glycerol in presence of alumina, MCM-22 (pure silica based mesoporous catalyst) and ZSM-5 (Si-Al based catalyst) is investigated at different temperature and catalyst weight in a fixed bed reactor. The conversion of glycerol was found to be maximum in presence of alumina whereas maximum liquid products were obtained with ZSM-5. GC/MS analysis confirmed the production of Furan compounds in higher fraction with both alumina as well as ZSM-5 showing the importance of acid sites for the glycerol conversion to higher hydrocarbons. The GC/MS analysis of liquid product obtained in presence of catalyst was also observed with high area% of unconverted glycerol. The order is as follow 54% (MCM-22) > 44% (ZSM-5) > 42.2% (Alumina). For the investigation of the conversion for varying catalyst weight (0–3 g with 0.5 g weight difference), reaction temperature were varied between 450 and 550 °C. Different values of n = 0, 1, 2 etc. were used for the fitting of the respective plot. A change in reaction rate and the rate constant indicated that with the change of temperature, reaction rate was increased. The rate constant value obtained between 0.09 and 0.12 h−1. In all cases 450 °C and catalyst weight of 2.5 g was obtained as optimum for higher liquid yield. TGA analysis of spent catalyst also showed that alumina give high yield (∼50% by weight) of coke as compared to ZSM-5 and MCM-22.

Optimization of granular formulations of Metarhizium humberi microsclerotia with humectants

Granular microsclerotial formulations of entomopathogenic fungi deserve attention because of their post-application, in situ production of new conidia that enhance and prolong mycoinsecticidal efficacy against a target pest insect. Because high ambient moisture is a crucial condition to induce fungal development and conidiogenesis on granules, we tested the impacts of the additions of three humectants-glycerin, propylene glycol, and polyethylene glycol 400-on water absorption by pellets incorporating microsclerotia of Metarhizium humberi IP 46 with microcrystalline cellulose or vermiculite carriers, and on the production of infective conidia of IP 46 microsclerotia in ambient humidities suboptimal for routine conidiogenesis. Glycerin facilitated greater and faster absorption of water than the other humectants. Microcrystalline cellulose absorbed low quantities of water without any added humectant whereas vermiculite did not. IP 46 did not grow or sporulate on pellets prepared with or without glycerin at 86% relative humidity (RH) or on control pellets without glycerin at 91% RH; conidial production on pellets prepared with vermiculite or microcrystalline cellulose and 10% glycerin reached 1.1 × 10⁵  conidia/mg and 1 × 10⁵  conidia/mg, respectively, after 20 days of exposure at 91% RH. Hence, these results strongly support glycerin as a suitable humectant for granular microsclerotial formulations of this fungus.

Solketal Production via Solvent-Free Acetalization of Glycerol over Triphosphonic-Lanthanide Coordination Polymers

Biodiesel is one of the most significant and valuable alternatives to fossil fuels. In the process of transesterification to produce biodiesel from various feedstocks, glycerol is one of the side products obtained, in a high glycerol: biodiesel weight ratio (1:10). Therefore, the growing world demand for biodiesel prompted a glycerol surplus. It is, thus, of interest to find new and added-value paths for the transformation of this abundant chemical. One of the most auspicious glycerol applications is the production of fuel additives, namely cyclic acetals and ketals, from aldehydes and ketones, respectively. In this work, coordination polymers based on nitrile (trimethylphosphonic acid) and Ln3+/Eu3+ are used as catalysts for the acetalization of the bio-renewable glycerol into oxygenated fuel additives. Solketal is the major product obtained from the reaction of glycerol with acetone. This product improves the cold flow properties, lowering the viscosity of biodiesel, improving combustion, and boosting the octane number. The stability of the materials is studied as well as their recovery and reuse.

A combined periodic DFT and QM/MM approach to understand the radical mechanism of the catalytic production of methanol from glycerol

The production of methanol from glycerol over a basic oxide, such as MgO, using high reaction temperatures (320 °C) is a promising new approach to improving atom efficiency in the production of biofuels. The mechanism of this reaction involves the homolytic cleavage of the C₃ feedstock, or its dehydration product hydroxyacetone, to produce a hydroxymethyl radical species which can then abstract an H atom from other species. Obtaining a detailed reaction mechanism for this type of chemistry is difficult due to the large number of products present when the system is operated at high conversions. In this contribution we show how DFT based modelling studies can provide new insights into likely reaction pathways, in particular the source of H atoms for the final step of converting hydroxymethyl radicals to methanol. We show that water is unlikely to be important in this stage of the process, C-H bonds of C₂ and C₃ species can give an energetically favourable pathway and that the disproportionation of hydroxymethyl radicals to methanol and formaldehyde produces a very favourable route. Experimental analysis of reaction products confirms the presence of formaldehyde. The calculations presented in this work also provide new insight into the role of the catalyst surface in the reaction showing that the base sites of the MgO(100) are able to deprotonate hydroxymethyl radicals but not methanol itself. In carrying out the calculations we also show how periodic DFT and QM/MM approaches can be used together to obtain a rounded picture of molecular adsorption to surfaces and homolytic bond cleavage which are both central to the reactions studied.

Acetalization of glycerol and benzaldehyde to synthesize biofuel additives using SO4 2-/CeO2-ZrO2 catalyst

Synthesis of 1,3- dioxane and 1,3-dioxolane, using sulfated CeO₂–ZrO₂ catalyst for acetalization of glycerol with benzaldehyde, is the focus of present work. SO₄^2-/CeO₂–ZrO₂ catalyst was synthesized using combustion method. Experiments were carried out to analyze the effect of various solvents (n-hexane, toluene, tert-butyl alcohol, pentanol), molar ratios (1:3, 1:5, 1:7), catalyst loadings (3 wt%, 5 wt%, 9 wt %) and temperatures (80 °C, 90 °C, 100 °C) on glycerol conversion and selectivity of the products. Selectivity of 87.20% dioxolane and 12.80% dioxane was obtained at molar ratio of 1:3, 9 wt% catalyst loading and temperature of 100 °C.Strong NH₃ desorption peak from NH₃-TPD study indicated the high acidic strength of sulphated catalyst. Strong surface acidity and surface porosity (observed from TEM and SEM analysis) contributed to an enhanced activity of the catalyst for glycerol acetalization reaction. The kinetics of the reaction was studied using an elementary kinetic law. A correlation coefficient of 0.98 from the selected kinetic model proved that the rate of acetalization reaction was dependent on glycerol concentration and acetal formation was instantaneous. The study demonstrated the application of an environmentally benign, inexpensive, thermally stable, active SO₄^2-/CeO₂–ZrO₂ catalyst for glycerol acetalization reaction to synthesize 1,3-dioxolane as the desired product.

Recent Advances in Glycerol Catalytic Valorization: A Review

Once a biorefinery is ready to operate, the main processed materials need to be completely evaluated in terms of many different factors, including disposal regulations, technological limitations of installation, the market, and other societal considerations. In biorefinery, glycerol is the main by-product, representing around 10% of biodiesel production. In the last few decades, the large-scale production of biodiesel and glycerol has promoted research on a wide range of strategies in an attempt to valorize this by-product, with its transformation into added value chemicals being the strategy that exhibits the most promising route. Among them, C3 compounds obtained from routes such as hydrogenation, oxidation, esterification, etc. represent an alternative to petroleum-based routes for chemicals such as acrolein, propanediols, or carboxylic acids of interest for the polymer industry. Another widely studied and developed strategy includes processes such as reforming or pyrolysis for energy, clean fuels, and materials such as activated carbon. This review covers recent advances in catalysts used in the most promising strategies considering both chemicals and energy or fuel obtention. Due to the large variety in biorefinery industries, several potential emergent valorization routes are briefly summarized.

Process Strategies for the Transition of 1G to Advanced Bioethanol Production

Nowadays, the transport sector is one of the main sources of greenhouse gas (GHG) emissions and air pollution in cities. The use of renewable energies is therefore imperative to improve the environmental sustainability of this sector. In this regard, biofuels play an important role as they can be blended directly with fossil fuels and used in traditional vehicles’ engines. Bioethanol is the most used biofuel worldwide and can replace gasoline or form different gasoline-ethanol blends. Additionally, it is an important building block to obtain different high added-value compounds (e.g., acetaldehyde, ethylene, 1,3-butadiene, ethyl acetate). Today, bioethanol is mainly produced from food crops (first-generation (1G) biofuels), and a transition to the production of the so-called advanced ethanol (obtained from lignocellulosic feedstocks, non-food crops, or industrial waste and residue streams) is needed to meet sustainability criteria and to have a better GHG balance. This work gives an overview of the current production, use, and regulation rules of bioethanol as a fuel, as well as the advanced processes and the co-products that can be produced together with bioethanol in a biorefinery context. Special attention is given to the opportunities for making a sustainable transition from bioethanol 1G to advanced bioethanol.

A review on recent trends in reactor systems and azeotrope separation strategies for catalytic conversion of biodiesel-derived glycerol

The increasing demand for biodiesel (BD) as a renewable and sustainable energy source has impelled the generation of abundant and low-cost byproduct glycerol, which accounts for 10 wt% of total BD production and requires urgent utilization. The transesterification reaction, which utilizes glycerol and dimethyl carbonate (DMC) to synthesize valuable glycerol carbonate (GC) is an established reaction pathway to valorize oversupplied glycerol. Commercialization of inexpensive GC is constrained by the nature, stability, and basicity of applied catalyst, reaction conditions, types of the reactor system and separation methods of reaction products. This study presents a review and diversity of recent reports on reactor systems and DMC-methanol azeotrope separation strategies explored in GC synthesis from biodiesel-derived glycerol. Also, recent trends on heterogeneous catalysts, their performance, and the effects of reaction conditions were presented. Conducted studies revealed that the choice for reactor systems is constrained by factors such as energy consumption and operational safety and a significant mild reaction conditions could be realized using a microwave reactor. Furthermore, the reactive-extractive distillation and pervaporation processes showed high energy-efficiency and appreciable separation of DMC-methanol azeotrope. Thus, the development of stable catalyst and process intensification to fabricate an integrated reactor-separation system with high energy efficiency are fundamental and must be explored. This study portrays the recent research effort made in this direction and the limitations that require urgent attention.

Valorization of Biodiesel Byproduct Crude Glycerol for the Production of Bioenergy and Biochemicals

The rapid growth of global biodiesel production requires simultaneous effective utilization of glycerol obtained as a by-product of the transesterification process. Accumulation of the byproduct glycerol from biodiesel industries can lead to considerable environment issues. Hence, there is extensive research focus on the transformation of crude glycerol into value-added products. This paper makes an overview of the nature of crude glycerol and ongoing research on its conversion to value-added products. Both chemical and biological routes of glycerol valorization will be presented. Details of crude glycerol conversion into microbial lipid and subsequent products will also be highlighted.

Bifunctional Heterogeneous Catalyst for Biodiesel Production from Waste Vegetable Oil

Bifunctional solid catalysts facilitate the esterification of free fatty acids (FFA) into alkyl esters alongside the transesterification reaction, which allows for the use of waste vegetable oils with high water and FFA contents for biodiesel production. This makes the process economically viable and greener, as the waste fats and oils are readily available. The concurrent esterification and transesterification of waste palm oil (WPO) and waste sunflower oil (WSO) with methanol was investigated in the presence of calcium oxide on alumina catalyst in a conventional batch process. The catalyst characterization showed the existence of calcium oxide aluminates (calcined at 750 °C), which exhibited crystalline phases with porous/spongy-like particles. The high concentration of CaO in CaO/Al2O3 was a favorable support material in the heterogeneously-catalyzed transesterification reactions. The optimum catalyst parameters for the production of fatty acid methyl esters (FAMEs) were observed at 65 °C for 4 h with a methanol-to-oil ratio of 9:1, 60% (waste palm oil, or WPO) and 80% (waste sunflower oil, or WSO), CaO/Al2O3 (% wt/wt) catalyst ratio as well as 4% CaO/Al2O3 concentration (% wt.) for WSO and WPO. The simultaneous esterification/transesterification reactions at optimum conditions on WPO and WSO led to high yield of FAMEs of 89, 61 and 55% for WPO and 54, 75 and 98% for WSO at catalyst ratios (wt %) of 60, 70 and 80% respectively. The use of bifunctional heterogeneous catalyst (CaO/Al2O3) with waste vegetable oil can result in high performance and the upscaling of biodiesel production.

Enhancing the understanding of the glycerol to lactic acid reaction mechanism over AuPt/TiO2 under alkaline conditions

The oxidation of glycerol under alkaline conditions in the presence of a heterogeneous catalyst can be tailored to the formation of lactic acid, an important commodity chemical. Despite recent advances in this area, the mechanism for its formation is still a subject of contention. In this study, we use a model 1 wt. % AuPt/TiO₂ catalyst to probe this mechanism by conducting a series of isotopic labeling experiments with 1,3-¹³C glycerol. Optimization of the reaction conditions was first conducted to ensure high selectivity to lactic acid in the isotopic labeling experiments. Selectivity to lactic acid increased with temperature and concentration of NaOH, but increasing the O₂ pressure appeared to influence only the rate of reaction. Using 1,3-¹³C glycerol, we demonstrate that conversion of pyruvaldehyde to lactic acid proceeds via a base-promoted 1,2-hydride shift. There was no evidence to suggest that this occurs via a 2,1-methide shift under the conditions used in this study.

Contribution to the production and use of biomass-derived solvents – a review

In this review key processes for the synthesis of greener or more sustainable solvents derived from renewable sources (saccharides, lignocellulose and triglycerides) are discussed. It is shown that a series of platform chemicals such as glycerol, levulinic acid and furans can be converted into a variety of solvents through catalytic transformations that include hydrolysis, esterification, reduction and etherification reactions. It was also considered several aspects of each class of solvent regarding performance within the context of the reactions or extractions for which it is employed.

Biodiesel Production Using Solid Acid Catalysts Based on Metal Oxides

The development of solid acid catalysts, especially based on metal oxides and different magnetic nanoparticles, gained much awareness recently as a result of the development of different nano-based materials. Solid acid catalysts based on metal oxides are promising for the (trans)esterification reactions of different oils and waste materials for biodiesel production. This review gives a brief overview of recent developments in various solid acid catalysts based on different metal oxides, such as zirconia, zinc, titanium, iron, tungsten, and magnetic materials, where the catalysts are optimized for various reaction parameters, such as the amount of catalyst, molar ratio of oil to alcohol, reaction time, and temperature. Furthermore, yields and conversions for biodiesel production are compared. Such metal-oxide-based solid acid catalysts provide more sustainable, green, and easy-separation synthesis routes with high catalytic activity and reusability than traditionally used catalysts.

Progress of Catalytic Valorization of Bio-Glycerol with Urea into Glycerol Carbonate as a Monomer for Polymeric Materials

Versatile polymers with highly adjustable characteristics and a broad range of applications are possibly developed owing to the contemporary industrial polymerization techniques. However, industrial production of large amounts of chemicals and polymers heavily depends on petroleum resources which are dwindling and unsustainable. Of particular interest is to utilize sustainable and green resources for the manufacture of polymeric materials. The efficient transformation of bio-glycerol to the relevant functional derivatives are being widely investigated owing to the increasing demand for enhancing the value of glycerol manufactured by biodiesel and oleochemical industries. With respect to glycerol-based polymer chemistry and technology, considering the economy and environmental benefits, using effective catalysts for the selective transformation of bio-glycerol and urea into glycerol carbonate (GC) as a polymer monomer is of great significance. In this review, recent studies on GC synthesis involving the catalysts such as zinc, magnesium, tungsten, ionic liquid-based catalysts, reaction conditions, and possible pathways are primarily described. Some critical issues and challenges with respect to the rational development of heterogeneous catalytic materials like well-balanced acid-base sites are also illustrated.

Kinetic Modeling of Dihydroxyacetone Production from Glycerol by Gluconobacter oxydans ATCC 621 Resting Cells: Effect of Fluid Dynamics Conditions

Dihydroxyacetone production from glycerol has been studied. Cultures of Gluconobacter oxydans ATCC 621, a promising microorganism that is able to convert glycerol into dihydroxyacetone, has been employed. In this work, the influence of oxygen transport rate and the fluid dynamic conditions have been studied working with resting cells cultures. Several experiments were carried out at two different scales: 250 mL Erlenmeyer flasks and a 2 L stirred tank bioreactor, varying the agitation speed. Product and substrate concentration were determined employing high-performance liquid chromatography. Additionally, oxygen concentration was measured in the runs carried out in stirred tank reactors. Taking into account the results obtained in these experiments, three different behaviors were observed, depending on the mass transfer and chemical reactions rates. For experiments with low stirring speed (below 200 rpm for shake flasks and 300 rpm for reactors), the oxygen transport rate is the controlling step, while at high stirring speed (over 300 rpm in shake flasks and 560 rpm in the bioreactor), the chemical reaction is controlling the overall process rate. In some runs conducted at medium agitation, a mix control was found. All the kinetic models were able to reproduce experimental data and fulfill thermodynamic and statistical criteria, highlighting the importance of the mass transfer rate upon this system.

Microfluidic processing of HZSM-5 films in a capillary microreactor for the continuous acetalisation reaction of glycerol with acetone

This work presented microfluidic processing of zeolite films in a capillary microreactor and its potential application for biomass conversion.

Catalytic Activity and Kinetic Modeling of Various Modules HZMS-5 and Treated MCM-41 Catalysts, for the Liquid-Phase Ketalization of Glycerol With Acetone

Studies of the Ketalization reaction using trivalent alcohol glycerol in combination with acetone and their kinetics modeling are still limited. The focus of this current study is an investigation into HZSM-5 with various silica to alumina molar ratios (M = 35, 90, and 160) for the reaction between glycerol and acetone. In addition, the influence of reaction temperatures (25, 50, and 60°C) on the rate of the reaction have also been considered. Additionally, this investigation established the rate law for all HZMS-5 models (M = 35, 90, and 160) which showed “n” order equals half while the activation energy was found to be 164.34 kJ mol⁻¹ with a constant reaction rate of k₀ = 5.2678^*10²⁸ (Concentration^1/2. min-¹). Furthermore, MCM-41 pure mesoporous materials were separately treated using various methods. The first involved treatment using Dichlorodimethylsilane MCM 41(TD) and later treatment of a pure sample with sulfuric acid MCM-41. The sulfated MCM-41 sample (MCM41-SU) showed that reaction order equals n = −1 and a rate constant of (k) = 3.9 × 10² (Concentration⁻². min⁻¹). A close correlation and agreement was found between the experimental modeling and the theory. Additionally, this current kinetic study showed that water production has no effect on the conversion activity within 10 min from the start of reaction. Besides, further kinetics investigations were performed to ascertain the estimated time for water production based on the conditions applied during the reaction system. It resulted in an average time of 3 min for equilibrium to be reached in the reaction system. It was found that the estimated reaction equilibrium time (t_eq) is within the range from zero to 10 min in agreement with the proposed kinetic model in this work. Finally, it was also observed that a low equilibrium conversion (X_Aeq) had been obtained in the present work about 0.42 (42%). At a reaction temperature of 60°C (333.15 K) and at one atmosphere, the acetone was shown to exert a vapor pressure of about 113.737 mm Hg. Hence, the overall order of the reaction was determined by the method of initial rates. Similarly, in order to ascertain the dispersion of aluminum, together with its distribution on the surface of a catalyst for a zeolite that has varying molar ratios of silica to alumina as is the case for example with ZSM-5 (35), a mathematical approach is proposed in this study for its calculation.

Glycerol Acetylation Mediated by Thermally Hydrolysed Biosolids-Based Material

Crude glycerol is the main by-product of many renewable diesel production platforms. However, the process of refining glycerol from this crude by-product stream is very expensive, and thus does not currently compete with alternative processes. The acetylation of glycerol provides an intriguing strategy to recover value-added products that are employable as fuel additives. In this work, the conversion of glycerol to acetyl derivatives was facilitated by a heterogeneous catalyst generated from the thermal hydrolysis of biosolids obtained from a municipal wastewater treatment facility. The reaction was studied using several conditions including temperature, catalyst loading, acetic acid:glycerol molar ratio, and reaction time. The data demonstrate the potential for using two distinct by-product streams to generate fuel additives that can help improve the process economics of renewable diesel production.

Biodiesel at the Crossroads: A Critical Review

The delay in the energy transition, focused in the replacement of fossil diesel with biodiesel, is mainly caused by the need of reducing the costs associated to the transesterification reaction of vegetable oils with methanol. This reaction, on an industrial scale, presents several problems associated with the glycerol generated during the process. The costs to eliminate this glycerol have to be added to the implicit cost of using seed oil as raw material. Recently, several alternative methods to convert vegetable oils into high quality diesel fuels, which avoid the glycerol generation, are being under development, such as Gliperol, DMC-Biod, or Ecodiesel. Besides, there are renewable diesel fuels known as “green diesel”, obtained by several catalytic processes (cracking or pyrolysis, hydrodeoxygenation and hydrotreating) of vegetable oils and which exhibit a lot of similarities with fossil fuels. Likewise, it has also been addressed as a novel strategy, the use of straight vegetable oils in blends with various plant-based sources such as alcohols, vegetable oils, and several organic compounds that are renewable and biodegradable. These plant-based sources are capable of achieving the effective reduction of the viscosity of the blends, allowing their use in combustion ignition engines. The aim of this review is to evaluate the real possibilities that conventional biodiesel has in order to success as the main biofuel for the energy transition, as well as the use of alternative biofuels that can take part in the energy transition in a successful way.

Controlled Synthesis of Pt Doped SnO2 Mesoporous Hollow Nanospheres for Highly Selective and Rapidly Detection of 3-Hydroxy-2-Butanone Biomarker

Listeria monocytogenes (L. monocytogenes) has been recognized as one of the extremely hazardous and potentially life-threatening food-borne pathogens, its real-time monitoring is of great importance to human health. Herein, a simple and effective method based on platinum sensitized tin dioxide semiconductor gas sensors has been proposed for selective and rapid detection of L. monocytogenes. Pt doped SnO2 nanospheres with particular mesoporous hollow structure have been synthesized successfully through a robust and template-free approach and used for the detection of 3-hydroxy-2-butanone biomarker of L. monocytogenes. The steady crystal structure, unique micromorphology, good monodispersit, and large specific surface area of the obtained materials have been confirmed by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and Photoluminescence spectra (PL). Pt doped SnO2 mesoporous hollow nanosphere sensors reach the maximum response of 3-hydroxy-2-butanone at 250°C. Remarkably, sensors based on SnO2 mesoporous hollow nanospheres with 0.16 wt% Pt dopant exhibit excellent sensitivity (Rair/Rgas = 48.69) and short response/recovery time (11/20 s, respectively) to 10 ppm 3-hydroxy-2-butanone at the optimum working temperature. Moreover, 0.16 wt% Pt doped SnO2 gas sensors also present particularly low limit of detection (LOD = 0.5 ppm), superb long-term stability and prominent selectivity to 3-hydroxy-2-butanone. Such a gas sensor with high sensing performance foresees its tremendous application prospects for accurate and efficient detection of foodborne pathogens for the food security and public health.

Continuous-Flow Process for Glycerol Conversion to Solketal Using a Brönsted Acid Functionalized Carbon-Based Catalyst

The acetalization of glycerol with acetone represents a strategy for its valorization into solketal as a fuel additive component. Thus, acid carbon-based structured catalyst (SO3H-C) has been prepared, characterized and tested in this reaction. The structured catalyst (L = 5 cm, d = 1 cm) showed a high surface density of acidic sites (2.9 mmol H+ g−1) and a high surface area. This catalyst is highly active and stable in the solketal reaction production in a batch reactor system and in a continuous downflow reactor, where several parameters were studied such as the variation of time of reaction, temperature, acetone/glycerol molar ratio (A/G) and weight hourly space velocity (WHSV). A complete glycerol conversion and 100% of solketal selectivity were achieved working in the continuous flow reactor equipped with distillation equipment when WHSV is 2.9 h−1, A/G = 8 at 57 °C in a co-solvent free operation. The catalyst maintained its activity under continuous flow even after 300 min of reaction.

Recent Development of Heterogeneous Catalysis in the Transesterification of Glycerol to Glycerol Carbonate

Glycerol is one of the most crucial by-products in the production of biodiesel, and owing to its oversaturation in the market, several synthetic strategies have been developed to transform it into other higher value-added products such as glycerol carbonate, epichlorohydrin, 1,3-propanediol, etc. Amongst them, glycerol carbonate is considered to be the most valuable product. Considering the facile separation and reusability of catalyst, heterogeneous base catalysts have attracted considerable attention due to the obvious advantages over Brϕnsted acid and homogeneous base catalysts in the transesterification of glycerol. Herein, we will give a short overview on the recent development of the heterogeneous catalysis in the transesterification of glycerol with dialkyl carbonate. Focus will be concentrated on the heterogeneous base catalysts including alkaline-earth metal oxides (MgO, CaO, and mixed oxides), hydrotalcites, zeolites, clinoptilolites, organic bases, etc. Their catalytic mechanisms during the heterogeneous process will be elucidated in detail.

An Overview of the Production of Oxygenated Fuel Additives by Glycerol Etherification, Either with Isobutene or tert-Butyl Alcohol, over Heterogeneous Catalysts

Biodiesel production has considerably increased in recent decades, generating a surplus of crude glycerol, which is the main drawback for the economy of the process. To overcome this, many scientists have directed their efforts to transform glycerol, which has great potential as a platform molecule, into value-added products. A promising option is the preparation of oxygenate additives for fuel, in particular those obtained by the etherification reaction of glycerol with alcohols or olefins, mainly using heterogeneous catalysis. This review collects up-to-date research findings in the etherification of glycerol, either with isobutene (IB) or tert-Butyl alcohol (TBA), highlighting the best catalytic performances reported. Furthermore, the experimental sets employed for these reactions have been included in the present manuscript. Likewise, the characteristics of the glycerol ethers–(bio)fuel blends as well as their performances (e.g., quality of emissions, technical advantages or disadvantages, etc.) have been also compiled and discussed.