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High-performance magnetic activated carbon from solid waste from lignin conversion processes. Part II: Their use as NiMo catalyst supports for lignin conversion. Mikel Oregui-Bengeochea, Nemanja Mileti#, Wenming Hao, Fredrik Björnerbäck, Mali Husby Rosnes, Jose Javier Saiz Garitaonandia, Niklas Hedin, Pedro L. Arias, and Tanja Barth ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02796 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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High-performance magnetic activated carbon from
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solid waste from lignin conversion processes. Part II:
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Their use as NiMo catalyst supports for lignin
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conversion.
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Mikel Oregui-Bengoechea*a (
[email protected]), Nemanja Miletićb,c
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(
[email protected]), Wenming Haod,e (
[email protected]), Fredrik Björnerbäckd,e
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(
[email protected]), Mali H. Rosnesa (
[email protected]), J. S. Garitaonandiaf
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(
[email protected]), Niklas Hedind,e (
[email protected]), Pedro L. Ariasb
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(
[email protected]) and Tanja Bartha (
[email protected])
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a
Department of Chemistry, University of Bergen, Allégaten 41, 5007 Bergen, Norway
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b
Department of Chemical and Environmental Engineering, School of Engineering, University of
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the Basque Country (EHU/UPV), C/Alameda Urquijo s/n,48013 Bilbao, Spain
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c
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34, 32000 Čačak, Serbia
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d
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University, SE-106 91 Stockholm, Sweden
Department of Food Technology, Faculty of Agronomy, University of Kragujevac, Cara Dušana
Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm
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e
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SE-106 91 Stockholm, Sweden
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f
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Keywords: Lignin catalytic conversion, formic acid, hydrodeoxygenation (HDO), magnetic activated carbons, NiFe
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Berzelii Center EXSELENT on Porous Materials, Arrhenius Laboratory, Stockholm University,
Fisika Aplikatua II Saila, Euskal Herriko Unibertsitatea, 644 P. K. 48080 Bilbao, Spain
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SYNOPSIS
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Large quantities of low-value hydrochar are typically produced in lignin conversion processes. In
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this paper, hydrochar is valorized into activated carbon and re-used as catalyst support in the
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same process.
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ABSTRACT
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Lignin conversion processes produce carbon rich residues1,2 that can be converted into valuable
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materials such as magnetic activated carbons (MACs). Such lignin-derived MACs can be further
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used as functional substrates for hydrotreating NiMo catalysts. In this work, we studied the
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activity of different NiMo-MACs for the catalytic conversion of lignin in a formic acid/ethanol
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media (LtL process). Two KOH-activated LtL hydrochars from Eucalyptus (MACE) and
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Norwegian Spruce (MACS) lignins were used as catalyst supports. In addition, the activity of the
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resulting NiMo-MACs, namely C-MACE and C-MACS, was compared with a NiMo catalyst
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supported on a commercial activated carbon (AC). At reaction conditions of 340 °C and 6 h, the
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best result was obtained for the NiMo-MACS with a yield of 72.2 wt.% of oil and 21.1 wt.% of
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organic solids. At 300 °C and 10 h, both NiMo-MAC catalysts displayed higher HDO activities
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than their commercial counterpart, yielding considerably higher oil yields. The higher HDO
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activities are tentatively assigned to the formation of NiFe species on the catalytic surfaces of the
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NiMo-MAC catalysts. In addition, the magnetism exhibited by the C-MACS made it easy to
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recover the catalyst. However, a considerable loss of activity was observed upon recycling due to
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a chemical modification of the catalyst surface.
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INTRODUCTION
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In a biorefinery, biomass conversion processes and technologies are integrated to produce fuels
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and value-added chemicals and products2,3. When developing sustainable and economically
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efficient bio-refinery concepts, it is especially interesting to study unexploited biomass resources
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and wastes, and their conversion into bio-oils and/or value-added products4. Lignin is a major
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byproduct from the pulp-and-paper industry and from biomass pretreatment processes for bio-
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ethanol production5,6. This natural amorphous polymer can be thermochemically converted into
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bio-oil and fine chemicals7,8, although in most cases there are side reactions that convert a
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considerable amount of the biopolymer into organic solids or hydrochar2,9–11.
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A promising and relatively new approach to convert lignin, known as lignin-to-liquid
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(LtL), involves the use of formic acid (FA) together with a solvent, either ethanol or water. High
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oil yields, with high H/C and low O/C ratios, are obtained, whilst retaining the phenol-type
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structure of the bio-oil components. Nevertheless, high temperatures (typically 350-400 °C) and
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long reactions times (typically 8-16 h) are needed to convert the lignin feedstock into bio-oil in
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high yields12,13. Thus, in order to produce fuels and chemicals that can compete with traditional
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petroleum based products, two main issues need to be addressed: lowering the process severity
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(temperature and reaction time), and valorizing the low-value side streams. Such side streams
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include solid residues/hydrochars that can be further processed into value-added products.
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Two different strategies can be used to enhance the competitiveness of the LtL process.
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The reaction rates can be increased by using a catalyst and/or the hydrochar byproducts can be
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further processed into valuable products. With a suitable catalyst, the lignin de-polymerization
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and hydrodeoxygenation (HDO) rates for producing the target aromatic compounds could be
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improved. In this context, several combinations of traditional hydrotreating catalysts have been 4 ACS Paragon Plus Environment
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extensively studied, and their activities for thermo-catalytic conversion of lignin into bio-oil have
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been proven. Typical examples of such catalysts are NiMo-based sulfided catalysts and noble
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metals (Rh, Ru, Pd, Pt) over a wide variety of supports (i.e. alumina, silica, zeolites, activated
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carbon and zirconia)2,14–16. However, sulfided catalysts require a continuous addition of H2S to
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remain stable and active, making them less suited for lignin HDO due to the risk of sulfur
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incorporation in the bio-oil products17. Noble metals are not the preferred choice as their large
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ring hydrogenation selectivity decreases the content of aromatic products of the bio-oil increasing
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the amount of saturated alcohols instead. Furthermore, their cost may make them less relevant
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for industrial production of lignin-derived aromatics18. Regarding the support, HDO catalyst
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supported on activated carbons (ACs) are promising alternatives to traditional metal oxide
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supported catalysts due to (i) less coking19,20 (ii) low cost and (iii) the possibility of recovering
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the active metals from spent catalysts by burning off the carbon. Moreover, the textural and
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surface chemical properties of ACs might be tailored by varying the activation procedure21.
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The second approach to increase the competitiveness of the LtL process is to convert the
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solid organic residues to functional carbon materials. Several groups have studied the conversion
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of lignin and other biomass sources into renewable ACs for various purposes7,22–28. To our
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knowledge, except for a previous joint study29, no previous research been performed to convert
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solid organic residues of LtL processes (or other lignin processing related solid residues) into
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functional carbon materials such as ACs. In that study, we activated such residues into magnetic
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activated carbons (MACs) with high surface areas and studied their potential use in CO2
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separation.
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Here, we studied these lignin-derived and iron-containing MACs as catalyst supports for non-sulfided NiMo catalysts in the LtL conversion process. The activity of these NiMo-MACs 5 ACS Paragon Plus Environment
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was studied and compared with a NiMo catalyst supported on a conventional AC. The observed
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increases in the oil yields and decreases in the solid yields were compared to the non-catalyzed
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system (NC) and used as the primary evaluation parameters. The catalytic effect of the supports
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alone and the influence of pre-reducing the catalysts on the LtL conversion yields were also
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investigated. The promoting effects of Fe on the NiMo/MACs catalysts were also evaluated and
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discussed. Finally, the stability of the C-MACS catalyst upon recycling was determined.
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EXPERIMENTAL SECTION
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Chemicals
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Formic acid (>98%), tetrahydrofuran (THF) (>99.9%), ethyl acetate (EtAc) (99.8%), hexadecane (>99.8
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%) and anhydrous sodium sulphate (>99.0 %) were purchased from Sigma Aldrich and used as supplied.
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Nickel(II) nitrate hexahydrate (99.9+% Ni), ammonium molybdate tetrahydrate (99.98% Mo) and a
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commercial AC were purchased from Strem Chemicals Inc.
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Rice straw lignin from strong acid carbohydrate dissolution pre-treatment was received from
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Technical College of Bergen. The lignin was ground, and sieved (
