Hongyou Cui

Upgrading bio-oil by esterification under supercritical CO2 conditions

Abstract Esterification of acetic acid (AC), propanoic acid (PA) and a mixed acid (AC + PA + acrylic acid) as well as real bio-oil with ethanol under supercritical CO 2 (scCO 2 ) conditions were investigated. It was found that these carboxylic acids could afford much higher conversions under scCO 2 condition in comparison with esterification at atmosphere pressure, revealing the promoting effect of scCO 2 as a result of that the formed esters are prone to be phase-transferred into the scCO 2 phase. Since raising pressure at constant temperature can enhance the dissolving power of scCO 2 , it was observed that esterification conversion increased with pressure. The results esterifying these acids separately showed significant difference in their conversions, whereas simultaneous esterification of the mixed acid mixture afforded very close conversions, suggesting that a transesterification mechanism might occur in the case of simultaneous esterification under scCO 2 . The appropriate operation conditions for upgrading bio-oil by scCO 2 extractive esterification were determined as at 80°C and 28.0 MPa for 3.0 h. Under this condition, the total acid conversion could reach up to 86.78%. pH value for the upgraded bio-oil increased from 3.78 to 5.11. The total volatile components in the upgraded bio-oil approached up to 100% at 140°C.

Conversion of glucose into 5-hydroxymethylfurfural catalyzed by acid-base bifunctional heteropolyacid-based ionic hybrids

The design of stable acid–base bifunctional heterogeneous catalysts has become crucial for efficient catalytic conversion of renewable biomass to high value-added chemicals. Heteropolyacids (HPAs) are of great interest due to their unique tunable structures and suitable acidity. In this study, a series of acid–base bifunctional ionic hybrid catalysts [MimAM]nH3−nPW12O40 (n = 1–3) were synthesized by the ion exchange method using an amino-functionalized imidazolium ionic liquid and H3PW12O40 as precursors. The introduction of the ionic liquid resulted in a varied Lewis–Bronsted acidity for [MimAM]nH3−nPW12O40. The synergistic effect of dual acidic properties endowed [MimAM]H2PW12O40 with more efficiency for glucose dehydration in THF/H2O–NaCl with 53.9% HMF yield. A unique sequential hydrogen transfer mechanism is proposed to understand the efficient heterogeneous catalysis. Among them, the absence of fructose and levoglucosenone in glucose dehydration may come from the formation of stabilized intermediates 1,2-enediol and anhydrosugars due to the existence of an effective hydrogen bond between O–H and OW on the heteropoly anion; the reversible process between glucose dehydration and anhydrosugar hydration is responsible for the absence of HMF during the early stage of reaction. This work provides a deep understanding of the HPA-catalyzed conversion of glucose to HMF.

Effect of the reactive compounds in bio-oils on esterification of the contained carboxylic acids in supercritical methanol

Abstract Using acetic acid (AC), acrylic acid (AR), acetyl acetone (AA), furfural, 2-methoxy phenol (MP) and water as representative compounds and p-toluene sulfonic acid as catalyst, the esterification upgrading was examined in supercritical methanol and under atmosphere pressure. The comparison results of esterification and simultaneous esterification of AC and AR as well as the effect of water show that supercritical esterification is superior to normal liquid-phase esterification in view of acid removal. Furthermore, transesterification reaction could take place when simultaneously esterifying mixed acids in supercritical methanol, leading to relatively higher conversion for the acid like AR which hardly reacts with methanol at normal condition. The reason for low acid conversion in the presence of water is attributed to the retarding effect of water on esterification, rather than a result of equilibrium shifting, since water tends to weaken the electrophilicity of positive carbonyl ion. However, such effect in supercritical methanol is much insignificant because supercritical reaction not only carries out at higher reaction temperature and consequently accelerates the reaction rate, but also provides a weaker hydrogen bond circumstance. Although AA and furfural have unnoticeable effect on the esterification of organic acids, AA itself can be converted into acetone and methyl acetate via a pyrohydrolysis and esterification mechanism. Acetalization reaction of aldehyde with methanol can occur when furfural is present in the esterification. It is found that phenols such as MP can obviously promote the esterification of AR with methanol and inhibit its polymerization, as a result, enhancing the conversion and selectivity of esterification reaction.

Catalytic fructose dehydration to 5-hydroxymethylfurfural over sulfonated carbons with hierarchically ordered pores

Abstract Sulfonated carbons with hierarchically ordered pores (SCHOP) were prepared by means of dual template self-assembly, carbonization, HF etching, and sulfonation. The effect of carbonization temperature on microstructure of the prepared carbons as well as the SCHOPs were surveyed at 500–700°C. Using fructose dehydration to 5-hydroxymethylfurfural as a probe reaction, catalytic performance of SCHOPs was examined. The results show that SCHOP-500 carbonized at 500°C exhibits the highest catalytic activity. SEM and TEM characterization as well as N2 adsorption desorption show that all of the SCHOPs possess hierarchically ordered macropores and mesopores, but high carbonization temperature damages the regularity of carbon materials slightly. FT-IR, EDS and –SO3H determination confirm that sulfonic acid group could be successfully introduced to carbon materials by sulfonation. However, high carbonization temperature lowers aromaticity of the carbon materials, and as a result leading to low sulfonation degree. Under the catalysis of SCHOP-500, as high as 93.4% of 5-HMF yield with 96.1% fructose conversion rate is achieved after reacting at 130°C for 20 min, indicating that SCHOP-500 is a highly effective and efficient solid acid catalyst.

Preparation of the Nb-P/SBA-15 catalyst and its performance in the dehydration of fructose to 5-hydroxymethylfurfural

Abstract Dehydration of fructose to 5-hydroxymethylfurfural (5-HMF) is one of the pivotal reactions in conversion of biomass towards valuable platform compounds. In this work, Nb/SBA-15 was prepared via incipient wetness impregnation with self-made SBA-15 as support; Nb/SBA-15 was further treated with phosphoric acid and calcined at 450°C, to obtain the Nb-P/SBA-15 catalyst. The Nb-P/SBA-15 catalyst was characterized by SEM, TEM, BET, XRD and NH 3 -TPD; its performance in the dehydration of fructose to 5-HMF was then investigated. The results indicate that the microscopic structure of SBA-15 is well preserved in Nb/SBA-15 with an internal channel diameter of about 10 nm and the niobic species are highly dispersed on the surface of the channels; the wall of channels became thinner after impregnation of Nb and treatment with phosphoric acid. After the treatment with phosphoric acid, the weak acid sites are increased, moreover, the medium and strong acidic sites are generated in Nb-P/SBA-15; as a result, for the dehydration of fructose in a water/MIBK biphasic system, Nb-P/SBA-15 exhibits higher catalytic activity and selectivity to 5-HMF. By reaction at 160°C for 1.5 h, with a water/MIBK volume ratio of ½, the conversion of fructose and the yield of 5-HMF reach 96.1% and 92.6%, respectively. Moreover, the Nb-P/SBA-15 catalyst also exhibits excellent stability in view of water tolerance; it still demonstrates high catalytic activity and selectivity to 5-HMF even after successive recycling for four times.

Supercritical CO2 Extraction of Corn Stalk Pyrolysis Oil

Supercritical CO 2 extraction was employed to separate simulated and real bio-oil. Effects of adsorbents on enrichment were investigated for a simulated bio-oil, which is composed of acetic acid (AC), propanoic acid (PA), furfural (FR), acetylacetone (AA) and 2-methoxyphenol (MP). Extraction of spiked bio-oil shows adsorbents are able to influence extraction efficiency and selectivity. Appropriate water content in the simulated bio-oil favors in improving extraction efficiency. Extraction with methanol (or ethanol)-modified CO 2 affords higher extraction efficiency than that with pure CO 2 , but might lead to lower extraction selectivities. Using 5A molecular sieve as adsorbent, the extraction efficiencies for MP, FR, PA, AA and AC under 45°C and 20.0 MPa respectively reach up to 100%, 84.9%, 57.8%, 57.1% and 22.8%. In addition, supercritical CO 2 extraction of a real bio-oil showed that water content and acidity of the bio-oil after extraction dropped significantly, suggesting remarkable upgrading in heat value and corrosiveness of the bio-oil.

Synergetic Effect of Brønsted/Lewis Acid Sites and Water on the Catalytic Dehydration of Glucose to 5-Hydroxymethylfurfural by Heteropolyacid-Based Ionic Hybrids

Abstract The effective dehydration of glucose to 5‐hydroxymethylfurfural (HMF) has attracted increasing attention. Herein, a series of sulfonic‐acid‐functionalized ionic liquid (IL)–heteropolyacid (HPA) hybrid catalysts are proposed for the conversion of glucose to HMF. A maximum total yield of HMF and levoglucosan (LGA; ≈71 %) was achieved in the presence of pyrazine IL‐HPA hybrid catalyst [PzS]H2PW in THF/H2O–NaCl (v/v 5:1). The mechanism of glucose dehydration was studied by tailoring the Brønsted/Lewis acid sites of the hybrid catalysts and altering the solvent composition. It was found that water and heteropolyanions have a significant effect on the reaction kinetics. Heteropolyanions are able to stabilize the intermediates and promote the direct dehydration of glucose and intermediate LGA to HMF. A small amount of water could facilitate the conversion of glucose to LGA and suppress the dehydration of LGA to levoglucosenone. In addition, the synergetic effect of Brønsted/Lewis acid sites and a little water was conducive to accelerated proton transfer, which improved the yield of HMF from glucose dehydration.