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Sulfur-Tolerant Molybdenum Carbide Catalysts Enabling LowTemperature Stabilization of Fast Pyrolysis Bio-oil Zhenglong Li,† Jae-Soon Choi,*,† Huamin Wang,‡ Andrew W. Lepore,† R. Maggie Connatser,† Samuel A. Lewis,† Harry M. Meyer, III,† Daniel M. Santosa,‡ and Alan H. Zacher‡ †
Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37830, United States Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States
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S Supporting Information *
ABSTRACT: Low-temperature hydrogenation of carbonyl compounds can greatly improve the thermal stability of fast pyrolysis bio-oil, thereby enabling long-term operation of upgrading reactors which generally require high temperatures to achieve deep deoxygenation. The state-of-the-art hydrogenation catalysts, precious metals such as ruthenium, although effective in carbonyl hydrogenation, deactivate due to high sulfur sensitivity. In the present work, we showed that molybdenum carbides were active and sulfur-tolerant in low-temperature conversion of carbonyl compounds. Furthermore, due to surface bifunctionality (i.e., both metallic and acid sites present), carbides catalyzed both CO bond hydrogenation and C−C coupling reactions. Combined, these reactions transformed carbonyl compounds to more stable and higher molecular weight oligomeric compounds while consuming less hydrogen than pure hydrogenation. The carbides proved to be resistant to other deactivation mechanisms including hydrothermal aging, oxidation, coking, and leaching. These properties enabled carbides to achieve and maintain good catalytic performance in both aqueous-phase furfural conversion and real bio-oil stabilization in the presence of sulfur. This finding strongly suggests that molybdenum carbides can provide a catalyst solution necessary for the development of practical bio-oil stabilization technology.
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INTRODUCTION Fast pyrolysis is an economic and relatively simple process which can convert a variety of lignocellulosic biomass directly into a liquid product in high yield. Commonly known as bio-oil, the obtained liquid product is a mixture of oxygenated hydrocarbons whose main application has been largely limited to fuel oil substitution in boilers and furnaces for heat and electricity generation.1 For broader commercial adoption of the fast pyrolysis technology and resultant increase in biomass utilization, it is necessary to develop higher-value and largermarket applications for bio-oil (e.g., transportation fuels and chemicals), and accordingly, the necessary upgrading methodologies. In this regard, extensive research efforts have been made over the past several years, but achieving cost-effective bio-oil upgrading has proven elusive.2 The most difficult technical challenge facing bio-oil upgrading is ubiquitous coking and catalyst fouling. Bio-oil contains a significant amount (e.g., on the order of 4−5 mmol/g bio-oil) of carbonyl molecules such as sugars, aldehydes, and ketones.3,4 Under typical bio-oil upgrading conditions (e.g., >300 °C required for hydrodeoxygenation and zeolite cracking), these highly reactive carbonyl compounds tend to polymerize, leading to extensive and rapid coking, catalyst deactivation, and bed fouling/ plugging.5 Stabilization of bio-oil (i.e., conversion of reactive molecules into compounds less prone to thermally induced polymerization) is therefore a critical first step for successful upgrading (e.g., hydroprocessing) with long-term process operability.6,7 Of the numerous physical and chemical methods investigated for bio-oil stabilization, catalytic low-temperature hydrogenation of carbonyls appears particularly promising.6,7 Reduced © XXXX American Chemical Society
precious metal and other transition metal catalysts can catalyze hydrogenation of carbonyls into more stable molecules.8,9 Ruthenium (Ru) is especially active in aqueous-phase hydrogenation of carbonyls, generally outperforming other platinum group metals.10−13 The high oxophilicity of Ru is proposed to facilitate H2O chemisorption, which in turn enables an easier CO hydrogenation pathway.10 Excellent performance of Ru in reducing the carbonyl content of raw bio-oil has also been demonstrated in the temperature range of 80−160 °C.6,7 At such low temperatures, bio-oil, which contains 10−30% H2O, is in a condensed liquid phase, and therefore, the H2O is expected to play an important role in carbonyl hydrogenation. On the other hand, ruthenium and other metal-based catalysts are highly sensitive to sulfur impurities in the reactor feed, and deactivate quickly due to active-site poisoning.14 Fast pyrolysis bio-oil can contain sulfur at different levels depending on feedstock. For instance, woody biomass bio-oil typically has less than 100 ppm of sulfur. Even though this is a relatively small amount, it can cause significant catalyst deactivation. Wang et al.7 recently observed that Ru/TiO2 deactivated during stabilization of two different bio-oils (respectively, having 99 99 76 27 20 20 99 27 83 77 85 89 92 96 49 49 57 64 59 8
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