Furfural Synthesis from d-Xylose in the Presence of Sodium Chloride: Microwave versus Conventional Heating

Towards efficient and greener processes for furfural production from biomass: A review of the recent trends

As mentioned in several recent reviews, biomass-based furfural is attracting increasing interest as a feasible alternative for the synthesis of a wide range of non-petroleum-derived compounds. However, the lack of environmentally friendly, cost-effective, and sustainable industrial procedures is still evident. This review describes the chemical and biological routes for furfural production. The mechanisms proposed for the chemical transformation of xylose to furfural are detailed, as are the current advances in the manufacture of furfural from biomass. The main goal is to overview the different ways of improving the furfural synthesis process. A pretreatment process, particularly chemical and physico-chemical, enhances the digestibility of biomass, leading to the production of >70 % of available sugars for the production of valuable products. The combination of heterogeneous (zeolite and polymeric solid) catalyst and biphasic solvent system (water/GVL and water/CPME) is regarded as an attractive approach, affording >75 % furfural yield for over 80 % of selectivity with the possibility of catalyst reuse. Microwave heating as an activation technique reduces reaction time at least tenfold, making the process more sustainable. The state of the art in industrial processes is also discussed. It shows that, when sulfuric acid is used, the furfural yields do not exceed 55 % for temperatures close to 180 °C. However, the MTC process recently achieved an 83 % yield by continuously removing furfural from the liquid phase. Finally, the CIMV process, using a formic acid/acetic acid mixture, has been developed. The economic aspects of furfural production are then addressed. Future research will be needed to investigate scaling-up and biological techniques that produce acceptable yields and productivities to become commercially viable and competitive in furfural production from biomass.

Kinetic model for the dehydration of xylose to furfural from a boronate diester precursor

A comprehensive kinetic model describes the dehydration of xylose starting from the boronate diester-protected xylose (PBA2X). The model incorporates (de)esterification of PBA2X, partitioning, and xylose dehydration, and aims to evaluate the effects of the solvent system on these steps. The model explores the effect of the water contents in monophasic solvent systems, and that of ionic strength and mixing in biphasic aqueous-organic systems. At low water content, hydrolysis of PBA2X is the rate-limiting step, while xylose dehydration is fast. Conversely, in a monophasic three-solvent system, where the water content is higher, complete hydrolysis of the diester is achieved quickly. Under biphasic conditions, xylose dehydration is fast at high ionic strengths, but the slower partitioning/hydrolysis of PBA2X results in an overall slower furfural production. Furthermore, the observed different but high, constant xylose-to-furfural selectivities observed experimentally are tentatively ascribed to a higher order of parallel side-product formation.

Continuous hydrothermal furfural production from xylose in a microreactor with dual-acid catalysts

An effective continuous furfural production from xylose in a microreactor over dual-acid catalysts was proposed. In this work, furfural was synthesized in an organic solvent-free system using formic acid and aluminum chloride as catalyst. The role of these catalysts in the consecutive reactions was examined and verified. The influence of operating conditions including xylose concentration, reaction temperature, residence time, total catalyst concentration, and catalyst ratio on the yield of furfural was investigated and optimized. The furfural yield of 92.2% was achieved at the reaction temperature of 180 °C, residence time of 15 min, catalyst molar ratio of 1 : 1, xylose concentration of 1 g L⁻¹, and total catalyst concentration of 16 mM. The superior production performance of our process was highlighted in terms of the low catalyst concentration and short residence time compared to those of other systems based on the literature. In addition, a continuous in situ catalyst removal (purification) was demonstrated, providing further insights into the practical development of continuous furfural production.

An effective continuous furfural production from xylose in a microreactor over dual-acid catalysts was proposed. In this work, furfural was synthesized in an organic solvent-free system using formic acid and aluminum chloride as catalyst.

pH-Controlled Efficient Conversion of Hemicellulose to Furfural Using Choline-Based Deep Eutectic Solvents as Catalysts

The valorization of hemicellulose isolated from lignocellulosic biomass (wheat straw, rice husk, and bagasse) to furfural was achieved by pH-controlled acid catalysis using choline-based Brønsted acidic (BA) and natural acidic (NA) deep eutectic solvents (DES) serving both as catalyst and solvent. The effect of pH variation on the catalytic activity of various BADES and NADES prepared in 1 : 1 molar ratio was observed, and choline chloride/p-toluene sulfonic acid (ChCl/p-TSA) was found to be the best with lower pH value of 1.0. The yield of furfural decreased from 85 to 51 % with increase in pH from 1.0 to 3.0. The molar ratio of hydrogen bond donor to acceptor components was varied from 1 : 1 to 1 : 9 to achieve the lowest possible pH values of the DESs and to increase the furfural yield. Further optimization of reaction conditions was also done in terms of DES loading, time of reaction, and temperature using the model DES to achieve higher furfural yield. The best results were obtained using 5 mmol DES at pH 1.0 in 1.5 h at 120 °C. ChCl/p-TSA and ChCl/oxalic acid among BADES and ChCl/levulinic acid among NADES investigated in this work yielding 85 % furfural were found to be most efficient. The reported methodology is advantageous in terms of using bio-based green solvents, mild reaction conditions, and efficient scale-up of the reaction. The DESs were found to be efficiently recyclable up to five consecutive runs for the process.

Metabolic Engineering of E. coli for Xylose Production from Glucose as the Sole Carbon Source

Xylose is the raw material for the synthesis of many important platform compounds. At present, xylose is commercially produced by chemical extraction. However, there are still some bottlenecks in the extraction of xylose, including complicated operation processes and the chemical substances introduced, leading to the high cost of xylose and of synthesizing the downstream compounds of xylose. The current market price of xylose is 8× that of glucose, so using low-cost glucose as the substrate to produce the downstream compounds of xylose can theoretically reduce the cost by 70%. Here, we designed a pathway for the biosynthesis of xylose from glucose in Escherichia coli. This biosynthetic pathway was achieved by overexpressing five genes, namely, zwf, pgl, gnd, rpe, and xylA, while replacing the native xylulose kinase gene xylB with araL from B. subtilis, which displays phosphatase activity toward d-xylulose 5-phosphate. The yield of xylose was increased to 3.3 g/L by optimizing the metabolic pathway. Furthermore, xylitol was successfully synthesized by introducing the xyl1 gene, which suggested that the biosynthetic pathway of xylose from glucose is universally applicable for the synthesis of xylose downstream compounds. This is the first study to synthesize xylose and its downstream compounds by using glucose as a substrate, which not only reduces the cost of raw materials, but also alleviates carbon catabolite repression (CCR), providing a new idea for the synthesis of downstream compounds of xylose.

Magnesium Impregnated on NaX Zeolite Synthesized from Cogon Grass Silica for Fast Production of Fructose via Microwave-Assisted Catalytic Glucose Isomerization

Fructose is a crucial intermediate in the production of several chemical platforms. Fructose is mainly produced from glucose isomerization either through immobilized enzymes or heterogeneous catalysts using a conventional heating source, and this is time-consuming. Thus, this work discloses a fast production of fructose via microwave-assisted catalytic glucose isomerization using Mg catalysts supported on NaX zeolite from cogon grass silica. The catalysts were prepared by the impregnation of magnesium nitrate solution and subsequently transformed into MgO on NaX by calcination. The effect of 3, 6 and 9 wt.% Mg content on NaX on the performance of glucose isomerized to fructose was tested at 90 °C for 15 min. The best catalyst was selected for studying the effect of reaction times of 5, 15, 30 and 60 min. Results from X-ray diffraction (XRD), N2 sorption and CO2 temperature-programmed desorption (CO2-TPD) suggested that crystallinity, surface area and micropore volume decrease but basicity increases with Mg content. The X-ray photoelectron spectroscopy (XPS) result confirmed the presence of mixed phases of MgO and Mg2CO3 in all catalysts. The glucose conversion enhanced with the Mg loading but the fructose yield gave the highest value with Mg of 6 wt.%, probably due to the tuning of high active sites and surface area. The greatest fructose selectivity and yield (71.9% and 25.8%) were obtained within 15 min by microwave-assisted catalytic reaction, shorter than the reported value in the literature, indicating a suitable reaction time. Mg (6 wt.%)/NaX catalyst preserves the original catalytic performance up to three cycles, indicating that it is a promising catalyst for fructose production.

Optimization with Response Surface Methodology of Microwave-Assisted Conversion of Xylose to Furfural

The production of furfural from renewable sources, such as lignocellulosic biomass, has gained great interest within the concept of biorefineries. In lignocellulosic materials, xylose is the most abundant pentose, which forms the hemicellulosic part. One of the key steps in the production of furfural from biomass is the dehydration reaction of the pentoses. The objective of this work was to assess the conditions under which the concentration of furfural is maximized from a synthetic, monophasic, and homogeneous xylose medium. The experiments were carried out in a microwave reactor. FeCl3 in different proportions and sulfuric acid were used as catalysts. A two-level, three-factor experimental design was developed for this purpose. The results were further analyzed through a second experimental design and optimization was performed by response surface methodology. The best operational conditions for the highest furfural yield (57%) turned out to be 210 °C, 0.5 min, and 0.05 M FeCl3.

Microwave-Assisted Furfural Production Using Hectorites and Fluorohectorites as Catalysts

It has previously been reported that the use of microwave heating, together with the presence of co-solvents, improves the efficiency of furfural production from biomass. Solid acid catalysts can be a good alternative to mineral acids, since they can prevent corrosion and can be reused. However, the formation of humines should be minimized. Several delaminated and fluorinated hectorites, with different types and strengths of acid sites, were synthesized and tested as catalysts for the production of furfural from commercial xylose and from an acid biomass extract of almond shells. A new methodology was developed to prepare crystalline fluorohectorite at 800 °C in just 3 h. The presence of F significantly increased the acidity strength in the protonated fluorohectorite (H-FH) taking into account its high ammonia desorption temperature (721 °C). Additionally, this sample had fourteen times higher total acidity by m2 than the reference H-βeta acid catalyst. H-FH was the most efficient catalyst at short reaction times (1 h) for the transformation of xylose to furfural under microwaves using toluene as co-solvent, regardless of whether the xylose was commercial (20% furfural yield) or an extract of almond shells (60% furfural yield). However, the acidity of the extract affected the fluorohectorite structure and composition.

Microwaves in the Catalytic Valorisation of Biomass Derivatives

The application of microwave irradiation in the transformation of biomass has been receiving particular interest in recent years due to the use of polar media in such processes and it is now well-known that for biomass conversion, and particularly for lignocellulose hydrolysis, microwave irradiation can dramatically increase reaction rates with no negative consequences on product selectivity. However, it is only in the last ten years that the utilisation of microwaves has been coupled with catalysis aiming towards valorising biomass components or their derivatives via a range of reactions where high selectivity is required in addition to enhanced conversions. The reduced reaction times and superior yields are particularly attractive as they might facilitate the transition towards flow reactors and intensified production. As a consequence, several reports now describe the catalytic transformation of biomass derivatives via hydrogenation, oxidation, dehydration, esterification and transesterification using microwaves. Clearly, this technology has a huge potential for biomass conversion towards chemicals and fuels and will be an important tool within the biorefinery toolkit. The aim of this chapter is to give the reader an overview of the exciting scientific work carried out to date where microwave reactors and catalysis are combined in the transformation of biomass and its derivatives to higher value molecules and products.

Hydrolysis of Hemicellulose and Derivatives-A Review of Recent Advances in the Production of Furfural

Biobased production of furfural has been known for decades. Nevertheless, bioeconomy and circular economy concepts is much more recent and has motivated a regain of interest of dedicated research to improve production modes and expand potential uses. Accordingly, this review paper aims essentially at outlining recent breakthroughs obtained in the field of furfural production from sugars and polysaccharides feedstocks. The review discusses advances obtained in major production pathways recently explored splitting in the following categories: (i) non-catalytic routes like use of critical solvents or hot water pretreatment, (ii) use of various homogeneous catalysts like mineral or organic acids, metal salts or ionic liquids, (iii) feedstock dehydration making use of various solid acid catalysts; (iv) feedstock dehydration making use of supported catalysts, (v) other heterogeneous catalytic routes. The paper also briefly overviews current understanding of furfural chemical synthesis and its underpinning mechanism as well as safety issues pertaining to the substance. Eventually, some remaining research topics are put in perspective for further optimization of biobased furfural production.

Furfural Production from d-Xylose and Xylan by Using Stable Nafion NR50 and NaCl in a Microwave-Assisted Biphasic Reaction

Pentose dehydration and direct transformation of xylan into furfural were performed in a water-cyclopentyl methyl ether (CPME) biphasic system under microwave irradiation. Heated up between 170 and 190 °C in the presence of Nafion NR50 and NaCl, d-xylose, l-arabinose and xylan gave furfural with maximum yields of 80%, 42% and 55%, respectively. The influence of temperature and reaction time on the reaction kinetics was discussed. This study was also completed by the survey of different reactant ratios, such as organic layer-water or catalyst-inorganic salt ratios. The exchange between proton and cation induced by an excess of NaCl was monitored, and a synergetic effect between the remaining protons and the released HCl was also discovered.