Qineng Xia

Pd/NbOPO4 Multifunctional Catalyst for the Direct Production of Liquid Alkanes from Aldol Adducts of Furans

Great efforts have been made to convert renewable biomass into transportation fuels. Herein, we report the novel properties of NbO(x)-based catalysts in the hydrodeoxygenation of furan-derived adducts to liquid alkanes. Excellent activity and stability were observed with almost no decrease in octane yield (>90% throughout) in a 256 h time-on-stream test. Experimental and theoretical studies showed that NbO(x) species play the key role in C-O bond cleavage. As a multifunctional catalyst, Pd/NbOPO4 plays three roles in the conversion of aldol adducts into alkanes: 1) The noble metal (in this case Pd) is the active center for hydrogenation; 2) NbO(x) species help to cleave the C-O bond, especially of the tetrahydrofuran ring; and 3) a niobium-based solid acid catalyzes the dehydration, thus enabling the quantitative conversion of furan-derived adducts into alkanes under mild conditions.

Direct conversion of carbohydrates to 5-hydroxymethylfurfural using Sn-Mont catalyst

5-Hydroxymethylfurfural (HMF) is a very important intermediate in the fine chemical industry. This study aims to investigate the direct conversion of glucose or glucose-based carbohydrates, such as sucrose, cellobiose, inulin, starch and cellulose, to HMF by using a Sn-Mont catalyst. With the use of this catalyst, a HMF yield of 53.5% was achieved from glucose in a mono-phase medium of tetrahydrofuran (THF)/dimethylsulfoxide (DMSO) at 160 °C for 3 h. The success of one-step conversion of glucose to HMF is attributed to the Sn-Mont catalyst containing two types of acid sites, Lewis acid and Bronsted acid sites. The former one plays a crucial role in the isomerization of glucose to fructose and the latter one is active in the dehydration of generated fructose to HMF. Furthermore, Sn-Mont catalyst also demonstrated excellent activity in the conversion of disaccharides and polysaccharides and as high as 39.1% HMF was directly obtained from cellulose in a THF/H2O–NaCl bi-phasic system.

Direct hydrodeoxygenation of raw woody biomass into liquid alkanes

Being the only sustainable source of organic carbon, biomass is playing an ever-increasingly important role in our energy landscape. The conversion of renewable lignocellulosic biomass into liquid fuels is particularly attractive but extremely challenging due to the inertness and complexity of lignocellulose. Here we describe the direct hydrodeoxygenation of raw woods into liquid alkanes with mass yields up to 28.1 wt% over a multifunctional Pt/NbOPO4 catalyst in cyclohexane. The superior performance of this catalyst allows simultaneous conversion of cellulose, hemicellulose and, more significantly, lignin fractions in the wood sawdust into hexane, pentane and alkylcyclohexanes, respectively. Investigation on the molecular mechanism reveals that a synergistic effect between Pt, NbOx species and acidic sites promotes this highly efficient hydrodeoxygenation of bulk lignocellulose. No chemical pretreatment of the raw woody biomass or separation is required for this one-pot process, which opens a general and energy-efficient route for converting raw lignocellulose into valuable alkanes.

Effective Production of Octane from Biomass Derivatives under Mild Conditions

The diminishing reserves of fossil fuels have rendered the production of liquid fuels from renewable biomass resources particularly attractive, and worldwide many efforts have been devoted to the conversion of biomass into transportation fuels. 2] The overall goals when converting lignocellulosic biomass to hydrocarbon fuels are the removal of oxygen and the formation of C C bonds; the latter to control the molecular weight of the final hydrocarbons. So far, two integrated processes to produce liquid alkanes from biomass-derived carbohydrates exist, both developed by Dumesic’s group. In “aqueous-phase processing”, c] furfural or 5-hydroxymethylfurfural (HMF) first undergoes an aldol condensation with acetone in the presence of a basic catalyst, followed by hydrogenation of the condensation products (4– 5.5 MPa). Then, long-chain alkanes are formed by dehydration/ hydrogenation (5.2–6 MPa, 250–260 8C). The other process involves the integrated catalytic conversion of g-valerolactone (GVL), produced from levulinic acid by dehydration/hydrogenation (4 MPa, 200 8C), to liquid alkanes. In this process, the decarboxylation of GVL first occurs at elevated temperature and pressure (3.6 MPa, 375 8C) to produce a gas stream composed of butene and CO2. This stream is then fed directly into an oligomerization reactor to form condensable alkanes. Both strategies are creative, but must be operated at high pressure or temperature. Very recently, we developed a new Pt/Co2AlO4 catalyst to directly open furan rings and produce pentanediols from furfural under mild conditions (1–1.5 MPa, 130–150 8C). 4-(2-Furyl)-3buten-2-one 1 has a molecular structure that is similar to furfural. It is a single aldol adduct of furfural with acetone and also an important intermediate in aqueous phase processing for converting lignocellulosic biomass to octane. However, it must be hydrogenated over a Ru/C or Pd/Mgo ZrO2 catalyst at 4.5–5.5 MPa to prevent polymerization of C=C over a Pt/acid catalyst in the next step. If it could be hydrogenated over a Pt/ Co2AlO4 catalyst similar to furfural, 1,7and 2,5-octanediol would be obtained in mild conditions. The dehydration/hydrogenation of diols or polyols is known to be easier than for tetrahydrofuran derivatives. As a result, the reaction conditions of the process would be much milder than for the two processes mentioned above. Herein, we show how furfural (a biomass product, obtained from the dehydration of xylose) can be converted to octane under mild conditions with high yield. Scheme 1 shows the essential features of the reaction pathways for the production of octane from furfural and acetone, comprising the aldol condensation of furfural with acetone,

Pd/Nb2O5/SiO2 Catalyst for the Direct Hydrodeoxygenation of Biomass‐Related Compounds to Liquid Alkanes under Mild Conditions

A simple Pd-loaded Nb2 O5 /SiO2 catalyst was prepared for the hydrodeoxygenation of biomass-related compounds to alkanes under mild conditions. Niobium oxide dispersed in silica (Nb2 O5 /SiO2 ) as the support was prepared by the sol-gel method and characterized by various techniques, including N2 adsorption, XRD, NH3 temperature-programmed desorption (TPD), TEM, and energy-dispersive X-ray spectroscopy (EDAX) atomic mapping. The characterization results showed that the niobium oxide species were amorphous and well dispersed in silica. Compared to commercial Nb2 O5 , Nb2 O5 /SiO2 has significantly more active niobium oxide species exposed on the surface. Under mild conditions (170 °C, 2.5 MPa), Pd/10 %Nb2 O5 /SiO2 was effective for the hydrodeoxygenation reactions of 4-(2-furyl)-3-buten-2-one (aldol adduct of furfural with acetone), palmitic acid, tristearin, and diphenyl ether (model compounds of microalgae oils, vegetable oils, and lignin), which gave high yields (>94 %) of alkanes with little CC bond cleavage. More importantly, owing to the significant promotion effect of NbOx species on CO bond cleavage and the mild reaction conditions, the CC cleavage was considerably restrained, and the catalyst showed an excellent activity and stability for the hydrodeoxygenation of palmitic acid with almost no decrease in hexadecane yield (94-95 %) in a 150 h time-on-stream test.

Selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over MnOx–CeO2 composite catalysts

The selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) has attracted much attention recently, as FDCA could be used as a polymer precursor to produce recyclable bio-based polymers. But most studies focused on the design of noble metal catalysts for the efficient oxidation of HMF to FDCA, here, non-noble metal catalysts, MnOx–CeO2 mixed oxides, were prepared by a co-precipitation method and used in the direct aerobic oxidation of HMF to FDCA. As high as 91% yield of FDCA was achieved over the MC-6 (Mn/Ce = 6) catalyst and the catalyst can be reused five times without much loss of catalytic activity, this is the best result for non-noble metal catalyzed HMF oxidation to FDCA in the aqueous phase. Structural analysis revealed that surface Mn4+ ions are the active sites and play key roles for the oxidation of HMF in the absence of noble metals. In addition, the reaction mechanism over the MnOx–CeO2 catalyst was also proposed based on the synergistic interaction between Mn and Ce oxides.

Selective production of arenes via direct lignin upgrading over a niobium-based catalyst

Lignin is the only large-volume renewable source of aromatic chemicals. Efficient depolymerization and deoxygenation of lignin while retaining the aromatic functionality are attractive but extremely challenging. Here we report the selective production of arenes via direct hydrodeoxygenation of organosolv lignin over a porous Ru/Nb2O5 catalyst that enabled the complete removal of the oxygen content from lignin. The conversion of birch lignin to monomer C7–C9 hydrocarbons is nearly quantitative based on its monomer content, with a total mass yield of 35.5 wt% and an exceptional arene selectivity of 71 wt%. Inelastic neutron scattering and DFT calculations confirm that the Nb2O5 support is catalytically unique compared with other traditional oxide supports, and the disassociation energy of Caromatic–OH bonds in phenolics is significantly reduced upon adsorption on Nb2O5, resulting in its distinct selectivity to arenes. This one-pot process provides a promising approach for improved lignin valorization with general applicability.

Conversion of raw lignocellulosic biomass into branched long-chain alkanes through three tandem steps.

Synthesis of branched long-chain alkanes from renewable biomass has attracted intensive interest in recent years, but the feedstock for this synthesis is restricted to platform chemicals. Here, we develop an effective and energy-efficient process to convert raw lignocellulosic biomass (e.g., corncob) into branched diesel-range alkanes through three tandem steps for the first time. Furfural and isopropyl levulinate (LA ester) were prepared from hemicellulose and cellulose fractions of corncob in toluene/water biphasic system with added isopropanol, which was followed by double aldol condensation of furfural with LA ester into C15 oxygenates and the final hydrodeoxygenation of C15 oxygenates into branched long-chain alkanes. The core point of this tandem process is the addition of isopropanol in the first step, which enables the spontaneous transfer of levulinic acid (LA) into the toluene phase in the form of LA ester through esterification, resulting in LA ester co-existing with furfural in the same phase, which is the basis for double aldol condensation in the toluene phase. Moreover, the acidic aqueous phase and toluene can be reused and the residues, including lignin and humins in aqueous phase, can be separated and carbonized to porous carbon materials.

High yield production of HMF from carbohydrates over silica–alumina composite catalysts

An efficient and selective production of 5-hydroxymethylfurfural (HMF) from carbohydrates is achieved in the presence of mesoporous AlSiO catalysts in a THF/H2O–NaCl biphasic system. These mesoporous AlSiO catalysts are prepared by a facile sol–gel method and have tunable acidity. Their acidic sites are characterized and quantified by NH3-TPD, microcalorimetry of NH3 adsorption and Py-FTIR, then correlated with the catalytic isomerization and dehydration of glucose to HMF. The detailed studies show that the AlSiO-20 catalyst with a Si/Al ratio of 18 is favorable for HMF production due to its inherently high surface area, high amounts of acid sites and a suitable Bronsted/Lewis acid ratio. Over the AlSiO-20 catalyst, an HMF yield of 63.1% is obtained at 160 °C for 1.5 h in the biphasic THF/H2O–NaCl medium with 10 wt% glucose in water. Further conducting the amplification experiment 30 times, the HMF yield still reaches 60.2% and the yield has no obvious decline after four catalytic cycles; this is the best result for an amplification experiment of HMF from glucose over a heterogeneous catalyst so far. After separation, HMF can be used for the production of 2,5-furandicarboxylic acid (FDCA) and as high as 95% FDCA yield is obtained over the Pt/C catalyst. Furthermore, the AlSiO catalyst demonstrates excellent activity in the conversion of disaccharides, polysaccharides and even lignocellulosic biomass, indicating it would be a promising catalyst for the conversion of glucose and glucose-based carbohydrates to HMF in industry applications.

Comprehensive Understanding of the Role of Brønsted and Lewis Acid Sites in Glucose Conversion into 5-Hydromethylfurfural

The conversion of glucose and selectivity into 5‐hydromethylfurfural (HMF) were investigated over various silica–alumina composite (AlSiO) catalysts. The type, amount, and strength of the acidic sites were characterized by using NH3 temperature‐programmed desorption and FTIR spectroscopy and then correlated to the catalytic conversion of glucose into HMF to provide a quantitative relationship between the acidity and product selectivity. Lewis acid sites played an important role in glucose conversion, which can enhance the isomerization of glucose to fructose, whereas Brønsted acid sites had a detrimental effect. HMF selectivity had an almost linear relationship with the weak/total Lewis acid ratio (L*/L), indicating that weak Lewis acids could promote formation of HMF. The medium‐to‐strong Lewis acid sites can enhance the formation of undesired byproducts (levulinic acid, humins). The Brønsted to Lewis acid ratio (B/L) had an influence on the HMF selectivity; at similar L*/L ratios, volcano curves were obtained with the increase of the B/L ratio, but the influence was not as great as that of the L*/L ratio. Nb‐doped AlSiO catalysts were prepared and used in the conversion of glucose into HMF, which also confirmed the above findings. Under the optimized conditions, the HMF selectivity can reach 71 % at 92.6 % conversion of glucose with no clear decline after four catalytic cycles.