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The catalytic condensation of furfural and 2-methylfuran was performed to produce C15 diesel precursors using sulfonated acidic carbon catalysts, including multi-walled carbon nanotubes, graphene oxide, carbon aerogel, activated carbon, and carbon black. Sulfonated multi-walled carbon nanotubes were prepared with mild sulfonate grafting procedure, and displayed negligible modification of their pore structures. They exhibited the highest turnover frequency, indicating that the acid strength, depicted by the pKa, determined the catalytic condensation activity. The pKa values of the catalysts needed to be less than 4 for enabling the condensation of furfural and 2-methylfuran, and catalysts with pKa values greater than 4 exhibited poor condensation. The pKa, or acid strength, was determined by the environment of the surface sulfonates, which were electronically modified to adjust the stability of the sulfonate anions to release protons.

Despite the recognized efficacy of rhenium (oxide) for promoting hydrogenolysis of the C O bond of biomass-derived oxygenates when combined with a hydrogenation metal such as Pd or Ru, the hydrogenation function of rhenium itself is relatively unexplored. Herein, we investigated the activity and selectivity of rhenium nanoparticles supported on various metal oxides for the hydrodeoxygenation of guaiacol, i.e., 2-methoxyphenol. Catalytic experiments performed at 280 °C and 20 bar H2 in a batch reactor revealed that rhenium itself adequately catalyzes the demethoxylation and dehydroxylation of guaiacol, producing completely deoxygenated products of cyclohexane/benzene. The reduced and unreduced rhenium catalysts (Re vs. ReOx) exhibited similar catalytic activity, and both bulk rhenium oxide (ReO3 and Re2O7) and supported rhenium oxide, i.e., ReOx/SiO2, were completely reduced to metallic rhenium after the reaction, suggesting that metallic rhenium is mainly responsible for the hydrogenolysis of guaiacol. Among the catalysts tested, Re/SiO2 exhibited the highest guaiacol conversion (98%) and the highest selectivity for fully deoxygenated cyclohexane (58%), followed by TiO2, Al2O3, ZrO2, and CeO2. The catalytic activity was generally correlated with the rhenium dispersion as measured by transmission electron microscopy and CO-chemisorption. Despite its high activity, sintering and leaching of rhenium occurred for SiO2 support, whereas the second most active TiO2-supported rhenium catalyst was leaching-resistant. Using Re/TiO2 as a benchmark catalyst, the effects of the rhenium loading, the mesoporosity of TiO2, and reaction conditions on the conversion of guaiacol were further explored.

The preparation of LaAlO3 perovskite catalysts for the oxidative coupling of methane (OCM) using a very simple solid-state method with good reproducibility is described. To obtain high performance LaAlO3 catalysts, preparation conditions such as time, temperature, and oxygen content, which have a strong effect on the catalytic activity, are controlled during the calcination process. As the calcination time and temperature increase, the catalytic activity of LaAlO3 improves due to the good homogeneity and high crystallinity of LaAlO3 perovskite. Especially, the LaAlO3 catalyst calcined at 1350 °C for 24 h shows the highest selectivity and yield of C2 hydrocarbons. LaAlO3 catalysts calcined at various oxygen fractions show a volcano-shaped curve with respect to oxygen content in calcination gas, and the LaAlO3 catalyst prepared with an oxygen content of 5 % exhibits the best catalytic performance in this reaction. The appropriate oxygen fraction provides the LaAlO3 catalyst with a large amount of electrophilic lattice oxygen species and a sufficient amount of adsorption oxygen species, which are known to play key roles in the OCM reaction. This study clearly reveals this simple, highly reproducible, and mass-productive solid-state method as a promising preparation method for high performance LaAlO3 perovskite catalysts for OCM.

Perovskite-supported alkaline-earth metal oxide catalysts were prepared for application in the oxidative coupling of methane (OCM). The selective production of C2+ compounds, including ethane, ethylene, acetylene, propane, and propylene, was observed at reaction temperatures

The Diels-Alder reaction of biomass-derived dimethyl 2,5-furandicarboxylate (DMFDC) with ethylene followed by dehydrative aromatization is a sustainable route for the production of terephthalic acid, a monomer used for the synthesis of polyethylene terephthalate (PET). In this work, a series of heteropolyacid (HPA) catalysts with different supports, in this case SiO2, TiO2 and ZrO2, and hetero-addenda atoms, here phosphotungstic acid (HPW), silicotungstic acid (HSiW) and phosphomolybdic acid (HPMo), were used as catalysts, and their structure-activity correlation in the conversion of DMFDC to terephthalate was examined based on the density and strength of the acid sites. Characterization by XRD and Raman indicated that the Keggin structures of HPAs were well preserved on the SiO2 support compared to other supports, resulting in strong Bronsted acidity. The silica-supported HPW and HSiW catalysts exhibited significantly higher activity for terephthalate production than other supports and HPMo catalysts, but their activity were not correlated with the total acidity as measured by NH3-TPD. In contrast, the catalyst acid strength, as determined by the NH3 desorption temperature, decreased in the order HPW/SiO2 > HSiW/SiO2 >> HPW/ZrO2 > HPW/TiO2 > HPMo/SiO2 and was well correlated with the turnover rate for terephthalate production, suggesting that the higher activity of HPW/SiO2 is at least partly due to its stronger Bronsted acidity and that promoting the dehydration step of this reaction is a key to improving the selectivity to terephthalate. The effects of reaction conditions, in this case the acid loading amount, solvent used, temperature, and time, were also investigated with the most active silica-supported HPW catalysts to optimize the terephthalate yield. The 25%-HPW/SiO2 catalyst demonstrated a remarkable terephthalate yield of ∼60% with almost complete conversion under the optimum condition.

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na2WO4/Mn/SiO2. The reaction conditions, including the methane-to-oxygen ratio, ratio of feed gas dilution by N2, quantity of catalyst, and feed flow rate were adjusted for the continuous flow fixed bed reaction system. While the O2 gas initiated methyl radical formation from methane, the surface lattice oxygen atoms improved the dehydrogenation of paraffins to olefins without significant activation of methane. The addition of CO2 as a mild oxidizing agent was also tested and slightly improved OCM selectivity with slightly lower methane conversion were observed.

SiO2@MnOx@Na2WO4@SiO2 core–shell catalysts were prepared and their fabrication was confirmed using transmission electron microscopy. The formation of Mn-based nanosheets on the silica spheres is important for the deposition of nanoscopic Na2WO4. The SiO2@MnOx@Na2WO4@SiO2 core–shell catalysts were used for the oxidative coupling of methane at a temperature of 700–800 °C at which the nanostructures were completely destroyed. Although the core–shell structures did not survive the high-temperature oxidative coupling of methane, the selective production of olefins and paraffins can be attributed to highly dispersed Na2WO4 derived from confined core–shell structures.

Biogas is a promising renewable energy source, however, it needs to be upgraded to increase its low calorific value. In this study, oxidative coupling of methane (OCM) was selected to convert it to a higher fuel standard. Prior to establishing the scaled-up OCM process, the effect of organic/inorganic binders on catalytic activity was examined. The selection of the binders and composition of the catalyst pellet influenced the pore structure, fracture strength, and catalytic activity of the catalyst pellets. It was also observed that the O2 supply from the inorganic binder is a key factor in determining catalytic activity, based on which the composition of the catalyst pellets was optimized. The higher heating value increased from 39.9 (CH4, Wobbe index = 53.5 MJ/Nm3) to 41.0 MJ/Nm3 (OCM product mixture, Wobbe index = 54.2 MJ/Nm3), achieving the fuel standard prescribed in many countries (Wobbe index = 45.5–55.0 MJ/Nm3). The reaction parameters (temperature, gas hourly space velocity, size of the reaction system, and the CH4/O2 ratio) were also optimized, followed by a sensitivity analysis. Furthermore, the catalyst was stable for a long-term (100 h) operation under the optimized conditions.

Catalytic descriptors were studied to design optimum catalysts for the oxidative coupling of methane (OCM) by combining density functional theory (DFT) calculations and actual reaction experiments. SrTiO3 perovskite catalysts, selected for OCM, were modified using metal dopants, and their electronic structures were calculated using the DFT method. The CH3 adsorption energy Eads(CH3) and the oxygen vacancy formation energy Ef(vac) exhibited volcano-type correlations with the C2+ selectivity and O2-consumption for the formation of COx, respectively. The optimum catalytic activity, represented by the C2+ selectivity, was obtained for Eads(CH3) = −2.0 to −1.5 eV, indicating that overly strong adsorption of methyl radicals (or easily dissociated C H bonds of methane) and relatively insufficient oxygen supplementation to the catalyst surface improve deep oxidation to CO and CO2. Praseodymium (Pr)- and neodymium (Nd)-doped SrTiO3 catalysts confirm the DFT-predicted optimum electronic structure of the OCM catalysts.

A calcium carbonate (CaCO3)-supported Ru catalyst (Ru/CaCO3) prepared by a wet impregnation method was used for the hydrodeoxygenation (HDO) of eugenol, a lignin model compound representing coniferyl alcohols, and was shown to surpass Ru supported on neutral (ZrO2) and basic supports (MgAlOx, hydrotalcite, and MgO) in terms of deoxygenation activity. The production of deoxygenated compounds was observed during the batch reduction of eugenol with H2 (room-temperature-equivalent pressure = 50 bar) over Ru/CaCO3 at 250 °C, and the yields of compounds with no oxygen atoms (0-O) and one oxygen atom (1-O) were determined as 25.5 and 48.2%, respectively. X-ray photoelectron spectroscopy revealed that the improved hydrodeoxygenation activity could be ascribed to the increased electron density of catalytically active Ru species because of electron donation from the support.

Although chloride-ligated Pt compounds like (bpym)PtCl 2 , K 2 PtCl 4 , and (DMSO) 2 PtCl 2 has been reported to be highly active catalysts for the methane oxidation to methyl bisulfate (MBS) in oleum media, their applications is hampered by the catalyst deactivation to PtCl 2 . In this study, we investigated Pt black catalyzed methane oxidation, which has no ligand. A MBS yield of 82.1% with a selectivity of 96.5% was obtained at a catalyst loading of 1.6 mM at 180 °C, which proved the highest catalytic activity of Pt-black for this reaction. The reaction was thought to proceed by the dissolved Pt species, and no deactivation was observed during four consecutive experiments. However, at a concentration of over 30 mM, MBS yield fell due to the decomposition of MBS to CO 2 on the surface of heterogeneous Pt(0). Vacuum distillation experiments showed the potential for isolating MBS from the oxidation product mixture as a major component.

A novel bifunctional catalyst that enables an efficient one-pot conversion of furfural into γ-valerolactone (GVL) has been developed by anchoring heteropolyacid (HPA) on Zr-Beta zeolite. The catalysts were prepared by a post-synthesis procedure, which consists of the dealumination of Al-Beta, incorporation of Zr into the beta framework through solid-state ion-exchange and impregnation of the HPA. Zr-Beta is used as a Lewis acid catalyst to catalyze the transfer hydrogenation of furfural and levulinic acid/ester using 2-propanol as a hydrogen donor. To deal with the inability of Zr-Beta to catalyze the hydrolytic ring-opening of furans toward GVL, phosphotungstic acid (HPW) and silicotungstic acid (HSiW) were introduced to the Zr-Beta as BrOnsted acid sites. The characterization of the catalysts using XRD, UV–vis and XPS as well as TPD of ammonia and FT-IR spectroscopy of the adsorbed pyridine revealed that the HPA/Zr-Beta possesses both isolated Lewis and BrOnsted acid sites. When they were applied to the one-pot cascade conversion of furfural, the initial activity of the HPA/Zr-Beta toward GVL production were 2–3 times greater than that for Zr-Beta due to the enhanced hydrolytic ring-opening of the furans promoted by the added BrOnsted acidity. Especially, HPW loaded Zr-Beta demonstrated a remarkable GVL yield of ∼70% at 433 K after 24 h due to its high thermal stability and stronger BrOnsted acidity, and its activity far surpasses that of the conventional Sn-Al-Beta zeolite (∼40%). Overall, this study demonstrates that an incorporation of HPA into Lewis acid Sn- or Zr-Beta zeolites is an effective strategy to create isolated Lewis and BrOnsted acid sites within a single catalyst, thereby allowing the selective cascade catalysis for the cost-effective production of high-value chemicals.

The selective condensation of 2-methylfuran and furfural to C15 compounds is performed using supported phosphotungstic acid catalysts in a continuous-flow fixed-bed reactor. The formation of unwanted viscous tetramers (C20) is suppressed when the continuous-flow reaction system is used. Stronger acids, as measured by pKa, improved the condensation of furans, but the number of acid sites was rather weakly correlated with the catalytic activity. Suppressed catalytic activity in tiny pores smaller than 2.8–6.3 nm was also observed. Based on the observations of the acidity and pore structures, a strategy with which to design optimum catalysts for the condensation of furan compounds is suggested.

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na

SiO

The application of naturally abundant resources as alternative raw materials has been intensively studied because it is more economical and is also able to reduce solid waste generated by various industries. In this study, SiO2 was successfully extracted from sugarcane bagasse ash and pumice stone by using a low-temperature alkaline extraction method and acid precipitation process with high purity levels (98.4% and 96.3%, respectively). The extracted SiO2 was used as a catalyst support for Ag2O, where the two components were integrated with the impregnation method. The resulting catalysts were characterized to understand their structural features, as well as to see the catalytic performance. All samples of catalysts succeeded to reduce 4-nitrophenol, with differences in their reaction rates. The sample of Ag2O/SiO2, in which SiO2 was extracted from pumice stone, shows higher catalytic activity than that of sugarcane bagasse ash. From the analysis in this study, it is found that the aforementioned phenomenon is closely related to the structural properties of both Ag2O and SiO2 as the catalyst and its support.

The oxidative coupling of methane using silica-supported Na2WO4 (Na2WO4/SiO2) catalysts is observed by adjusting additives in order to understand the roles of additives and to design more efficient catalysts. By observing the effects of additives on the catalytic OCM activity of Na2WO4/SiO2 catalysts, the reducibility of metal additives, quantified by the standard reduction potentials, is observed to improve the methane conversion and olefin selectivity, indicating that the active sites of activating methane and dehydrogenating paraffin are identical. The supply of oxygen atoms from more easily reducible additives to tungsten oxide manipulate the OCM activity as described by the standard reduction potentials of the additives. The crystal and electronic structures of the catalysts depend on the additives, which can also be used to adjust the catalytic activity.

In this study, oxidative coupling of methane (OCM) was carried out to produce C2 and C3 hydrocarbons directly from methane in a plasma-catalyst hybrid system. Since catalyst only reaction requires high temperature above 700 °C, dielectric barrier discharge (DBD) plasma was applied as plasma source to lower the reaction temperature. Among various supports, only SiO2 showed the higher yield than plasma only reaction when combined with DBD plasma, which led us to focus on the SiO2-based catalyst. It was found that Ag/SiO2 demonstrated the highest C2+ hydrocarbon yield of about 9.7% at 385 °C. In this process, oxygen was proved to play an essential role in the coupling of methane to C2+ hydrocarbons over Ag/SiO2 catalyst. However, Ag/SiO2 catalyst under plasma condition became deactivated with time-on-stream because of coking. It was demonstrated that plasma regeneration at 378 °C gave rise to the full recovery of activity while thermal regeneration did not due to the partial removal of coke and the sintering of Ag.

We have previously shown that an activated carbon-supported ruthenium catalyst promoted with ReO x (RuRe/AC) is highly active for the hydrodeoxygenation (HDO) of lignin-derived phenols (e.g., guaiacol). In this work, we have investigated the effect of carbon supports on the structure and HDO activity of bimetallic RuRe particles using three different carbon supports, i.e., activated carbon (AC), carbon black (Vulcan carbon, VC), multi-walled carbon nanotube (MWCNT). The MWCNT- and VC-supported catalysts show remarkably enhanced activity and hydrocarbon selectivity for the HDO of a range of phenolic molecules (i.e., guaiacol, eugenol, benzyl phenyl ether) compared to RuRe/AC. STEM-EDS and XPS analyses reveal that bimetallic RuRe particles are more common than monometallic Ru or Re particles in the VC- and MWCNT-supported catalysts, and hexavalent rhenium species are more easily reduced to tetravalent rhenium during the HDO reactions in these catalysts, suggesting that Ru and Re in close proximity are required for the efficient hydrogenolysis of phenols. The formation of bimetallic particles on the AC surface is likely hindered by high microporosity and high surface oxygen functionalities, both of which restrict the mobility of Re and Ru for assembly.

The two-step hydrodeoxygenation of pine sawdust pyrolysis oil, or bio-oil, is performed using pairs of first-step hydrotreating and second-step hydrodeoxygenating catalysts. The reaction results demonstrate that the combination of hydrotreating carbon-supported 5 wt% Pd (5 wt% Pd/C) and hydrodeoxygenating tungstate-zirconia-supported 3 wt% Ru (3 wt% Ru/WZr) catalysts produced the highest yield of oil products and the lowest yield of cokes and tars. The hydrodeoxygenated liquid products are further analyzed using FT-IR, which indicates the removal of carbonyls and hydroxyls along with an increase of methyls. The roles of hydrotreating Pd/C are further studied using GC/MS results of the hydrotreated liquid products; these results indicate that the hydrogenation of carbonyls to alcohols and the saturation of furans occur during the first step of the hydrotreating process. The removal of carbonyls and unsaturated furans can suppress their strong adsorption to noble metal surfaces and then their carbonization to cokes.

The oxidative coupling of methane (OCM) using hybrid catalysts containing BaTiO3 perovskite and Mn-Na2WO4 exhibited high activity and high selectivity at low temperature: 66.3 % C2+ (olefins and paraffins) selectivity and 25.9 % C2+ yield at 700 °C which is 100 °C lower than that used for Mn-Na2WO4 catalysts (800 °C). Upon the preparation of complex catalysts, the insertion of Mn into BaTiO3 (BaTi(Mn)O3) and the formation of NaxMn(Ti)O2 improved the oxygen-supplying ability of the catalysts. Additionally, strong interactions between the WO42− anions and BaTi(Mn)O3 or NaxMn(Ti)O2 stabilized the WO42− anions on the surface and improved the methane activation ability of the catalysts for the favorable production of hydrocarbons. The nanoscopic modification of catalysts was confirmed using H2–temperature-programmed reduction, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy results.

As part of an effort to prepare a refrigerant with zero ozone depletion potential (ODP) and with a low global warming potential (GWP), the catalytic dehydrofluorination of 1,1,1,2,3-tetrafluoropropane (HFC-245eb) is performed to produce 2,3,3,3-tetrafluoropropene (HFO-1234yf). Cr2O3 prepared by the sol-gel method (Cr2O3(SG)) exhibits good catalytic activity attributable to the formation of surface chromium oxyfluoride, which exhibits better catalytic activity with high-valent chromium and a larger fluorine content. The formation of chromium oxyfluoride from chromium oxide is achieved by in-situ fluorination in the early stage of dehydrofluorination, avoiding the conventional fluorination method using toxic HF.

The Diels-Alder cycloaddition of biomass-derived furans and subsequent dehydration are promising routes for the sustainable production of commodity chemicals such as p-xylene (PX). In this paper, we have investigated the catalytic performances of a range of phosphotungstic acid (HPW) and silicotungstic acid (HSiW) catalysts supported on various oxides, i.e., SiO2, Al2O3, TiO2 and ZrO2 and their structure-activity correlation in the conversion of 2,5-dimethylfuran (DMF) and ethylene to PX. The characterization studies of the catalysts using XRD, BET, Raman and 31P MAS-NMR spectroscopy reveal that all of the supported heteropolyacid (HPA) catalysts (except HPW/ZrO2) retain their Keggin structure on the surface of oxide supports. Results from ammonia- and n-propylamine-TPD studies show that all of the supported HPA catalysts possess well-defined Bronsted acid sites with the total acidity decreasing in the following order: HPA/SiO2 > HPA/Al2O3 > HPA/ZrO2 > HPA/TiO2. The conversion of DMF and the initial rate of PX production generally increase with an increase in the total acidity, with HPA/SiO2 being the most active catalyst. The turnover frequency of PX production for HPA/SiO2 is also considerably greater than those for the HPAs supported on Al2O3, ZrO2, and TiO2, which suggests that the higher activity of HPA/SiO2 is at least partly due to the enhanced strength of Bronsted acid sites. Both the silica-supported HSiW and HPW catalysts demonstrate remarkably high PX selectivity (82–85%) at high DMF conversion (91–94%) at 250 °C after 6 h reaction. The effects of reaction conditions such as acid loading, reaction temperature, and reaction time have also been investigated with the most active silica-supported HSiW catalysts to optimize the PX yield.

The improved catalytic activity of Rh nanoparticles deposited on the swollen and pillared zeolites was observed in the hydrodeoxygenation (HDO) reactions of 1,3,5-trimethoxybenzene (1,3,5-TMB), a bulky lignin model compound, and guaiacol, one of the most frequently used lignin model compounds. As the high dispersion of metal nanoparticles increases HDO activity, the swelling/calcination and pillaring of crystalline MCM-22 zeolites increased the dispersion of Rh metal nanoparticles on the external surface area, and thus the corresponding HDO activity with respect to 1,3,5-TMB. On the contrary, although the mesopores in the amorphous MCM-41 and silica-alumina aerogel (SAA) supports accommodated higher Rh dispersion, overall, the resulting catalysts suffered from mass transfer limitation, and thus showed poor HDO reaction activity for 1,3,5-TMB. Finally, the Rh nanoparticles supported on the pillared zeolite showed the highest HDO activity for guaiacol, mainly due to the higher Rh dispersion and acid sites on the external surface among the zeolite-supported Rh catalysts.

Ru nanoparticles supported on tungstated zirconia (WZr) were prepared and used for the catalytic hydrodeoxygenation of guaiacol to deoxygenated hydrocarbon fuels. The tuning of tungsten, from 5 to 20 wt%, adjusted the number of acid sites on the tungstated zirconia and the size of the Ru nanoparticles. The optimum fraction of tungsten was determined for the optimum hydrodeoxygenation activity. The conversion of guaiacol reached a maximum of 96.8% for 10 wt% of tungsten. Increasing the tungsten fraction increased the number of acid sites and the dispersion of Ru, but the formation of tungsten oxide (WO x ) particles with too large fraction of tungsten inhibited the formation of smaller Ru nanoparticles, thus reducing the catalytic activity.

Catalytic depolymerization of empty fruit bunch (EFB)-derived organosolv lignin, which is free of ash and catalyst-poisoning sulfur, is performed using supported metal catalysts. Without the improved reaction activity in the presence of supercritical solvents, water and ethanol in this study, at the subcritical conditions, Hβ-supported Ru catalyst achieved 16.5% yield of small molecule phenolic hydrocarbons, including guaiacol, 4-methylguaiacol, 4-ethylguaiacol, cerulignol, and iso-eugenol, and produced degraded lignin polymer with 63% decreased weight-average molecular weight based on polystyrene-calibrated gel permeation chromatography results. The catalytic depolymerization activity was determined by the quantity of acid sites on the catalysts; however, the presence of metal nanoparticles is required to supply hydrogen atoms to the reaction system, which particles improve the catalytic depolymerization activity. The structure of organosolv lignin prepared by extraction using a mixture of ethanol and water was also studied using GPC and NMR results, which indicated that a possibly linear polymer composed of phenolic monomers with two or fewer linking functionalities.

A biphasic process for the conversion of the xylose that remains as a by-product after biomass fermentation into furfural was considered to maximize the economics of biomass-based fuel production. Widely used solvents such as toluene and methyl isobutyl ketone are toxic and environmentally unfriendly; therefore, 2-pentanone, which is one of the intermediates of the modeled process in this work, is suggested as an alternative. The amounts of the solvent that allowed 95% of the produced furfural to be recovered were determined, and the required numbers of trays in the solvent recovery column were calculated. In the production of nearly pure furfural, the process with 2-pentanone required a smaller number of trays than that with the other solvents, resulting in the lowest equipment costs and energy usage. In addition, because 2-pentanone can be converted into hydrocarbons during the downstream processes for biofuel production, the required purity for the furfural product in the separation column could be decreased, whereas the other solvents should be fully recovered so that they can be kept within the process. As the purity was decreased, the number of trays needed also decreased, indicating that the economics of the entire process could be improved through lower capital costs.

The increase in energy demands and problems related to fossil fuels have encouraged the development of suitable renewable energies as petroleum alternatives. Fast pyrolysis is a thermochemical process that can produce liquid bio-oil, which has the potential to alter transportation fuel. However, liquid bio-oil has disadvantages such as instability and low calorific value. Hence, various upgrading methods have been proposed to enhance its quality and stability. One such method is the transfer hydrogenation of bio-oil in supercritical fluids, which could convert it into a stable and low acidic fuel with notably reduced oxygen content even without external hydrogen. However, the bio-oil thus derived consists of a number of compounds, the composition and reaction mechanisms of which have not been accurately determined. Some researchers have suggested a mechanism using model compounds, but this is insufficient to commercialize the upgrading process. Therefore, this study reviews the transfer hydrogenation method to upgrade bio-oil, focusing on the major factors affecting the process. It concludes by identifying future challenges and the requirements to make the transfer hydrogenation upgrading process commercially available.

The hydrodeoxygenation of lignin monomer guaiacol via catalytic transfer hydrogenation was studied with 2-propanol as a hydrogen donor and Ru-based catalysts. Guaiacol was mainly converted into a partially deoxygenated product, cyclohexanol (> 70% selectivity), over Ru/C catalysts with the hydrogen produced in-situ from 2-propanol. An addition of Re to Ru/C catalysts significantly enhanced the rate of C—O hydrogenolysis, resulting in the complete deoxygenation to cyclohexane (~ 60% selectivity). The remarkable deoxygenation ability of the bimetallic RuRe/C catalyst is attributed to its bifunctional characteristic, by which Ru catalyzes the hydrogenation-hydrogenolysis of guaiacol and Re provides acid sites to promote cyclohexane production via hydrogenolysis.

Al 2 O 3 -cogelled Ru nanoparticle (Ru@Al) catalyst was prepared by a one-pot in-situ alumina gelation method using a PVP-stabilized Ru colloid solution. The Ru@Al catalyst exhibited excellent catalytic activity during the liquid-phase hydrodeoxygenation of vanillin, demonstrating 100% conversion, as well as significantly higher yields of fully deoxygenated compounds compared to other conventional alumina-supported Ru catalysts. We also observed better selectivity to deoxygenated dimers with the Ru@Al catalyst. The improved catalytic selectivity was attributed to the hypothesized three-dimensional structures of Al 2 O 3 surrounding the Ru nanoparticles, which improved the two-step reaction, containing the dimerization of the phenolic compounds and the hydrodeoxygenation of phenolic dimers to produce deoxygenated high-carbon-number hydrocarbons.

Five carbon materials, including multi-walled carbon nanotubes (MWCNT), carbon aerogel (CARF), carbon black (Vulcan carbon), activated carbon (AC), and graphite, were used as supports of the carbon-supported Ru catalysts, and the hydrodeoxygenation of lignin-derived monomers and lignocellulose pyrolysis oil was performed. Ru/MWCNT exhibited the highest deoxygenation activity, and the origin of the improved catalytic activity was studied. The metal dispersion, the acidity as measured by means of temperature-programmed desorption, the pore structure, and the surface area were investigated in an effort to understand the catalysis results. We observed that the quantity of accessible Ru nanoparticles on the mesopores determined the hydrodeoxygenation activity.

In this study, red mud (RM), a highly alkaline waste generated from alumina production industries, was used as a catalytic material for both fast copyrolysis of organosolv lignin (OL) and polypropylene (PP) and toluene removal under ozone at room temperature. The RM was pretreated with HCl to investigate the effect of alkalinity. In the catalytic fast copyrolysis of the OL and PP, the acid-treated RM (HRM) produced more aromatics, phenolics, and light olefins (C3 to C5) but less oxygenates and heavy olefins (C6 to C46) than the RM. The difference in pyrolytic performance between the RM and HRM was likely attributed to the concentrated Fe2O3 species in the HRM catalyst. In addition, more efficient toluene removal was observed over MnOx/HRM than over MnOx/RM owing to the large Brunauer–Emmett–Teller surface area, high amounts of Al and Fe, and optimal Mn3+/Mn4+ ratio. This study demonstrates that the RM, an industrial waste, can be reused as an effective catalytic material for not only biofuel production but also pollutant removal.

The efficient depolymerization of technical lignin to produce high-quality and phenolic monomer-rich bio-oil is challenging owing to the inherent recalcitrant structure of lignin. In addition, the phenolic fragments formed during lignin depolymerization are highly reactive and easily undergo rapid repolymerization to yield undesired heavy oils or char. In the present work, we report a new hybrid catalyst of Ru/C combined with MgO/ZrO2 that is highly effective at depolymerizing lignin to high-quality bio-oil by a cooperative catalysis of Ru-catalyzed reductive depolymerization and MgO/ZrO2-catalyzed stabilization of the reactive phenolic monomers through in-situ capping of the phenol repolymerization initiator, i.e., formaldehyde, by reaction with ethanol. A screening study of various catalyst combinations, including transition metal catalysts such as Pd, Ni, and Ru supported on carbon or Al2O3 as well as basic oxide catalysts such as MgO, MgO/ZrO2, ZrO2, and Mg-Al mixed oxides, revealed that Ru/C–MgO/ZrO2 was the only combination that had a significant positive synergistic catalytic effect on lignin depolymerization, thereby highlighting its unique and exceptional activity. To elucidate the synergistic roles of Ru and MgO/ZrO2 in lignin depolymerization, their individual and integrated catalysis as well as the effect of the Mg/Zr ratio were systematically investigated through a comprehensive analysis of the bio-oil products using several techniques, including 2D heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy (HSQC-NMR). A high bio-oil yield of 76.2 wt% with a low molecular weight of 401 g/mol, large HHV of 34.47 MJ/kg, and high phenolic monomer yield of 31.44 wt% was successfully obtained over the integrated catalyst of Ru/C and MgO/ZrO2 (Mg/Zr = 3) at 350 °C and 30 bar-H2 after 4 h.

The ketonization of hexanoic acid, which can be prepared by the fermentation of sugars, was performed in an effort to obtain the diesel-blendable high-carbon-number ketone 6-undecanone in this study. Based on observations of the leaching of highly active base catalysts, magnesium oxide and manganese oxide, the stable amphoteric solid catalyst zirconia was selected for the acid-rich reaction environment. A high-surface-area zirconia aerogel was prepared, which successfully ketonized hexanoic acid to 6-undecanone with high conversion, 72.3%, and high selectivity, 92.6%. The further optimization of the porous structures of zirconia led to possibly improved mass transfer with approximately 35 nm of pores, which improved the catalytic activity of zirconia.

Pt nanoparticles stabilized with alumina nanorods (Pt@Al2O3) were synthesized by a simple preparation method using a mixture of Pt nanoparticles, an alumina precursor, an organic surfactant, and a reducing agent. Because the alumina nanorods stabilized Pt nanoparticles, the sintering of Pt nanoparticles was significantly suppressed during a high temperature water gas shift reaction, demonstrating 1.7–13.6 times higher CO conversions or 1.0–8.0 times higher TOF compared to other alumina-supported Pt catalysts. In addition, the increased metal–support interface for Pt@Al2O3 significantly improved the water gas shift reaction activity.

The catalytic oxidative coupling of methane (OCM) on transition metal-doped TiO2 nanowire catalysts was performed and the effects of metal dopants were studied. With transition metal doping, the electric and optical properties of nanowires were adjusted, which seemed to improve the catalytic activity and selectivity of the OCM reaction. A Mn-doped TiO2 nanowire catalyst exhibited the highest C2 yield with the highest (ethylene)/(ethane) ratio because of its moderate oxidation activity, while a highly active Rh-doped TiO2 nanowire catalyst converted methane into fully oxidized CO and CO2. The electric conductivity assessed by UV–vis absorption represented the oxidation activity of the nanowire catalysts.

Two-step hydrodeoxygenation of furan trimers using supported Ni catalysts was successfully performed by substituting expensive noble metal catalysts for hydrodeoxygenation, which can contribute to the development of feasible hydrodeoxygenation processes with which to prepare high-carbon-number hydrocarbon fuels. Although more cracking of hydrocarbon products was observed for the Ni catalysts, the substitution of noble metal catalysts with Ni catalysts was successful, leading to good HDO activity. The combination of the first step of Ni/CeO2 (hydrogenation) and the second step of Ni/tungstated zirconia (hydrodeoxygenation) led to complete deoxygenation, achieving 87.8% of the theoretical maximum yield of the oil phase product, which is slightly lower than the value of 91.1% for the combination of the first step of Pd/C (hydrogenation) and Ru/tungstated zirconia (hydrodeoxygenation). The roles of first-step hydrogenation were revealed as the hydrogenation of unsaturated furan rings and the hydrolysis of saturated furan rings.

In the pyrolysis of lignin, lignin tends to agglomerate during the feeding stage, and the char produced can expand easily to form foam that prevents continuous operation. To solve this problem, a new bench-scale rotary kiln reactor with alumina balls was used for the pyrolysis of organosolv lignin. Also, a bench-scale fixed-bed reactor was used as a reference reactor to gain an insight on the pyrolysis phenomena from rotary kiln reactor. Compared to the fixed bed reactor, the rotary kiln reactor resulted in a higher yield of organic phase oil with high selectivity to monomeric phenolic compounds. Moreover, a larger amount of CO2 was obtained from the rotary kiln reactor, which strongly suggests that decarboxylation is main deoxygenation path. Furthermore, a char without agglomeration and foaming inside the reactor was generated from the rotary kiln reactor. The catalytic properties of the chars produced from both reactors with nickel impregnation were tested for the gasification of high-density polyethylene to produce H2. Among the four different catalysts (Ni/Rotary Kiln char, Ni/Fixed Bed char, Ni/Al2O3, Ni/Activated carbon) tested, the Ni/Fixed Bed char exhibited the highest yield of H2 (~two times higher H2 generation) from the gasification process of high-density polyethylene. X-ray photoelectron spectroscopy and scanning electron microscopy offers that the high dispersion/distribution of Ni occurred when the fixed bed char was used as a support. All experimental findings from this study significantly suggest that Ni supported on char could be an economically viable catalyst for producing H2 from gasification of high-density polyethylene.