Recent Advances in Hydrotreating of Pyrolysis Bio-Oil and Its Oxygen-Containing Model Compounds

Abstract

Considerable worldwide interest exists in discovering renewable energy sources that can substitute for fossil fuels. Lignocellulosic biomass, the most abundant and inexpensive renewable feedstock on the planet, has a great potential for sustainable production of fuels, chemicals, and carbon-based materials. Fast pyrolysis integrated with hydrotreating, one of the simplest, most cost-effective, and most efficient processes to convert lignocellulosic biomass to liquid hydrocarbon fuels for transportation, has attracted significant attention in recent decades. However, effective hydrotreating of pyrolysis bio-oil presents a daunting challenge to the commercialization of biomass conversion via pyrolysis-hydrotreating. Specifically, the development of active, selective, and stable hydrotreating catalysts is problematic due to the poor quality of current pyrolysis bio-oil feedstock (i.e., high oxygen content, molecular complexity, coking propensity, and corrosiveness). Significant research has been conducted to address the practical issues and provide fundamental understanding of hydrotreating and hydrodeoxygenation (HDO) of bio-oils and their oxygen-containing model compounds, including phenolics, furans, and carboxylic acids. A wide range of catalysts have been studied, including conventional Mo-based sulfide catalysts and noble metal catalysts. Noble metal catalysts have been the primary focus of recent research because of their excellent catalytic performances and because they do not require the use of environmentally unfriendly sulfur. Recently, the reaction mechanisms of the HDO of model compounds on noble metal catalysts and their efficacy for hydrotreating or stabilization of bio-oil have been reported. This review provides a survey of relevant literature, published over the past decade, reporting advances in the understanding of the HDO chemistry of bio-oils and their model compounds, mainly on noble metal catalysts.

Conceptual Process Design and Techno-Economic Assessment of Ex Situ Catalytic Fast Pyrolysis of Biomass: A Fixed Bed Reactor Implementation Scenario for Future Feasibility

“…Accordingly, the extent of hydrogen incorporation in these fixed bed systems will be limited by thermodynamic equilibrium, with greater hydrogen incorporation achieved in fixed bed reactor #2 due to its lower operating temperature. The lower operating pressure of the ex situ upgrading reactors, chosen to maintain the pyrolysis products in the vapor phase, will also limit the extent of hydrogenation, as compared to typical hydrotreating conditions (50-100 bar) [17,18]. Deoxygenation can be achieved through decarbonylation (removal of oxygen as CO), decarboxylation (removal of oxygen as CO 2 ), hydrodeoxygenation (encompassing both direct hydrogenolysis and hydrogenation-dehydration; removal of oxygen as H 2 O), and certain C-C coupling reactions (discussed below) [12,17,18].…”

“…The lower operating pressure of the ex situ upgrading reactors, chosen to maintain the pyrolysis products in the vapor phase, will also limit the extent of hydrogenation, as compared to typical hydrotreating conditions (50-100 bar) [17,18]. Deoxygenation can be achieved through decarbonylation (removal of oxygen as CO), decarboxylation (removal of oxygen as CO 2 ), hydrodeoxygenation (encompassing both direct hydrogenolysis and hydrogenation-dehydration; removal of oxygen as H 2 O), and certain C-C coupling reactions (discussed below) [12,17,18]. Decarbonylation and decarboxylation proceed through cleavage of C-C bonds, thus reducing oxygen content at the expense of carbon efficiency, with decarboxylation being preferred as two oxygen atoms are removed per carbon atom.…”

“…Consequently, hydrodeoxygenation is the preferred deoxygenation route. The relative hydrodeoxygenation reactivity of various oxygenates present in biomass pyrolysis vapors is controlled, at least in part, by their C-O bond dissociation energies [4,17,19,20]. Under typical hydrotreating conditions, aldehydes and ketones can be hydrogenated to alcohols below 200°C, aliphatic ethers and alcohols as well as carboxylic groups can be deoxygenated at 200-300°C, and phenolics and aromatic ethers can be deoxygenated at temperatures greater than 300°C [4].…”

“…Research is ongoing to develop catalysts that can perform these transformations under the process conditions outlined in this study. Accordingly, a number of excellent reviews have been published in the last 3-4 years discussing potential catalysts for biomass pyrolysis vapor upgrading, highlighting the key advantages and disadvantages of the different materials, and identifying reaction mechanisms [12,17,18,48,49]. As the scope of this work is to evaluate a process design for ex situ catalytic fast pyrolysis incorporating hot gas filtration and fixed bed systems, a thorough discussion of catalyst types and functionalities is not included; however, a few catalyst types/formulations are highlighted as they are explicitly considered in the process design (Table 3).…”

Hydropyrolysis of Lignin Using Pd/HZSM-5

“…have differing rates, and different catalysts will promote DDO or HYD as a primary reaction depending on the nature of the metal and the support [10]. It has been observed that noble metals favor HYD whereas sulfided catalysts preferentially undergo DDO reactions [23], [24]. In addition to facilitating multiple reactions, an ideal bifunctional catalyst should still be able to exhibit a high turnover frequency (TOF) and the capability to maintain this activity after repeated trials.…”