and Fuels Changzhi Li,† Xiaochen Zhao,† Aiqin Wang,† George W. Huber,†,‡ and Tao Zhang*,† †State Key Laborotary of Catalysis, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States

Catalytic Transformation of Lignin for the Production of Chemicals and Fuels

As the oil reserves are depleting the need of an alternative fuel source is becoming increasingly apparent. One prospective method for producing fuels in the future is conversion of biomass into bio-oil and then upgrading the bio-oil over a catalyst, this method is the focus of this review article. Bio-oil production can be facilitated through flash pyrolysis, which has been identified as one of the most feasible routes. The bio-oil has a high oxygen content and therefore low stability over time and a low heating value. Upgrading is desirable to remove the oxygen and in this way make it resemble crude oil. Two general routes for bio-oil upgrading have been considered: hydrodeoxygenation (HDO) and zeolite cracking. HDO is a high pressure operation where hydrogen is used to exclude oxygen from the bio-oil, giving a high grade oil product equivalent to crude oil. Catalysts for the reaction are traditional hydrodesulphurization (HDS) catalysts, such as Co–MoS2/Al2O3, or metal catalysts, as for example Pd/C. However, catalyst lifetimes of much more than 200 h have not been achieved with any current catalyst due to carbon deposition. Zeolite cracking is an alternative path, where zeolites, e.g. HZSM-5, are used as catalysts for the deoxygenation reaction. In these systems hydrogen is not a requirement, so operation is performed at atmospheric pressure. However, extensive carbon deposition results in very short catalyst lifetimes. Furthermore a general restriction in the hydrogen content of the bio-oil results in a low H/C ratio of the oil product as no additional hydrogen is supplied. Overall, oil from zeolite cracking is of a low grade, with heating values approximately 25% lower than that of crude oil. Of the two mentioned routes, HDO appears to have the best potential, as zeolite cracking cannot produce fuels of acceptable grade for the current infrastructure. HDO is evaluated as being a path to fuels in a grade and at a price equivalent to present fossil fuels, but several tasks still have to be addressed within this process. Catalyst development, understanding of the carbon forming mechanisms, understanding of the kinetics, elucidation of sulphur as a source of deactivation, evaluation of the requirement for high pressure, and sustainable sources for hydrogen are all areas which have to be elucidated before commercialisation of the process.

A review of catalytic upgrading of bio-oil to engine fuels

In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis.

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the fu...

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

The gas phase hydrodeoxygenation (HDO) of guaiacol, as a model compound for pyrolysis oil, was tested on a series of novel hydroprocessing catalysts – transition metal phosphides which included Ni2P/SiO2, Fe2P/SiO2, MoP/SiO2, Co2P/SiO2 and WP/SiO2. The turnover frequency based on active sites titrated by the chemisorption of CO followed the order: Ni2P > Co2P > Fe2P, WP, MoP. The major products from hydrodeoxygenation of guaiacol for the most active phosphides were benzene and phenol, with a small amount of methoxybenzene formed. Kinetic studies revealed the formation of reaction intermediates such as catechol and cresol at short contact times. A commercial catalyst 5% Pd/Al2O3 was more active than the metal phosphides at lower contact time but produced only catechol. A commercial CoMoS/Al2O3 deactivated quickly and showed little activity for the HDO of guaiacol at these conditions. Thus, transition metal phosphides are promising materials for catalytic HDO of biofuels.

Hydrodeoxygenation of guaiacol as model compound for pyrolysis oil on transition metal phosphide hydroprocessing catalysts

Simultaneous hydrodesulfurization (HDS), hydrodeoxygenation (HDO), and hydrogenation (HYD) of model compounds were studied under high pressure conditions with a β-Mo2C catalyst. The β-Mo2C (hexagonal close packed) was synthesized by the temperature programmed reaction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO chemisorption, and BET surface area measurements. Simultaneous HDS, HDO, and HYD were observed with minimal deactivation of Mo2C for up to 30 ppm of sulfur, 2000 ppm of oxygen, and 5 wt % cumene for more than 80 h. A noble metal catalyst (Pt/γ-Al2O3) deactivated immediately upon addition of sulfur. The reactivity results on β-Mo2C indicate there are two types of sites on the surface when simultaneous HDS, HYD and HDO are occurring. The first type of site is a molybdenum carbide site which is active for HYD and HDO. The second site is a molybdenum carbosulfide site which is active for HYD, HDS, and HDO. The results of this study may be applied not only to the refining of petroleum feedstocks, but also to the upgrading of synthetic fuels such as coal-derived liquids.

Simultaneous hydrodesulfurization, hydrodeoxygenation, and hydrogenation with molybdenum carbide

Unsupported molybdenum nitride (Mo2N) and molybdenum carbide supported on alumina ( Mo 2 C Al 2 O 3 ) were compared against commercial sulfided MoS 2 Al 2 O 3 > and NiMoS Al 2 O 3 for hydrotreating coal-derived gas oil and residuum at 633 K (360°C) and 13.7 MPa (2000 psig). When the catalytic rates were compared on the basis of active sites measured by chemisorption, the nitrides and carbides were estimated to have activities as much as five times that of NiMoS Al 2 O 3 and MoS 2 Al 2 O 3 . The comparison was based on sites titrated by CO on the carbide and nitride and by O2 on the sulfided catalysts. The gas oil and resid product quality from the carbide and nitride catalysts was significantly better than the thermal blank, indicating that the materials were active under practical hydrotreating conditions. X-ray photoelectron spectroscopy analysis after reaction of the Mo2N and Mo 2 C Al 2 O 3 catalysts indicated that surface sulfiding was not extensive.

Catalytic hydrotreating by molybdenum carbide and nitride: unsupported Mo2N and Mo2CAl2O3

Abstract The major obstacle in thermochemical biomass conversion to hydrocarbon fuels using pyrolysis has been the high oxygen content and the poor stability of the product oils, which cause them to solidify during secondary processing. We have developed a fractional catalytic pyrolysis process to convert biomass feedstocks into a product termed “biocrude oils” (stable biomass pyrolysis oils) which are distinct from unstable conventional pyrolysis oils. The biocrude oils are stable, low viscosity liquids that are storable at ambient conditions without any significant increases in viscosity; distillable at both atmospheric pressure and under vacuum without char or solid formation. About 15 wt% biocrude oils containing 20–25% oxygen were blended with 85 wt% standard gas oil and co-cracked in an Advanced Catalyst Evaluation (ACE™) unit using fluid catalytic cracking (FCC) catalysts to produce hydrocarbon fuels that contain negligible amount of oxygen. For the same conversion of 70% for both the standard gas oil and the biocrude oil/gas oil blends, the product gasoline yield was 44 wt%, light cycle oil (LCO) 17 wt%, heavy cycle oil (HCO) 13 wt%, and liquefied petroleum gas (LPG) 16 wt%. However, the coke yield for the standard gas oil was 7.06 wt% compared to 6.64–6.81 wt% for the blends. There appeared to be hydrogen transfer from the cracking of the standard gas oil to the biocrude oil which subsequently eliminated the oxygen in the fuel without external hydrogen addition. We have demonstrated for the first time that biomass pyrolysis oils can be successfully converted into hydrocarbons without hydrogenation pretreatment.

Co-Processing of standard gas oil and biocrude oil to hydrocarbon fuels. Biomass and Bioenergy

Molybdenum carbide on alumina was studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed surface reaction (TPSR) with hydrogen. The molybdenum carbide was prepared by nitridation of 12.5 wt% MoO3/Al2O3 in a flow of NH3 at 700°C, followed by carburization in a flow of 20% CH4/H2 also at 700°C for 3 h. The sample was compared to an unsupported material prepared from MoO3 in the same manner. The CH4 formation profile during TPSR was deconvoluted into three peaks with maxima at 490°C, 690°C, and 810°C. The peak at 490°C was assigned to reduction of carbidic carbon, and the peaks at 690°C and 810°C were assigned to the removal of pyrolytic and graphitic carbon, respectively, as indicated by XPS and XRD measurements. Overall, the results suggest that molybdenum carbide was formed on the alumina supported by the carburization treatment at 700°C, in the same manner as with an unsupported reference sample. Prenitridation before carburization resulted in the formation of a carbide with a larger surface area and less free carbon, compared to the carbide formed by direct carburization.

S.T. Oyama

Papers

Hydrodeoxygenation of guaiacol as model compound for pyrolysis oil on transition metal phosphide hydroprocessing catalysts

Simultaneous hydrodesulfurization, hydrodeoxygenation, and hydrogenation with molybdenum carbide

Catalytic hydrotreating by molybdenum carbide and nitride: unsupported Mo2N and Mo2CAl2O3

Co-Processing of standard gas oil and biocrude oil to hydrocarbon fuels. Biomass and Bioenergy

Preparation and characterization of alumina-supported molybdenum carbide