Bio-oils Upgrading for Second Generation Biofuels

Abstract

The envisaged upgrading of lignocellulosic biomass derived feedstocks (bio-oils) in dedicated units or by coprocessing in existing units of the refinery, to partially replace crude oil in the production of transportation fuels, is a topic that has been receiving much attention from both industry and academia in recent years. Regardless of lignocellulosic biomass origin, these feedstocks are complex mixtures of many oxygenated hydrocarbons. Therefore, their upgrading toward liquid fuels must include oxygen removal. So far, two main routes have been proposed, considering many studies at laboratory scale and others from industry: catalytic hydrotreatment (HDT), mainly hydrodeoxygenation (HDO), and catalytic cracking, technologies that are already present in today's refineries configuration. HDO has been performed at high hydrogen pressure, using catalysts based on those typically applied in conventional hydrotreating, as well as a new type of supported noble metal and transition metal catalysts. Catalytic cracking occurs at atmospheric pressure, using acid catalysts, mainly the active phases of fluid catalytic cracking (FCC) catalysts (HY and HZSM-5 zeolites). The present review is then focused on the upgrading possibilities of renewable nonedible feedstocks, obtained from biomass fast pyrolysis or liquefaction, in petroleum refineries, toward the production of second generation biofuels. It includes the recent studies concerning the alternative of bio-oils coprocessing together with crude oil feedstocks. In fact, although all these feedstocks have the potential to be directly converted into transportation fuels in dedicated units, it seems more attractive to upgrade them in combination with conventional feedstocks.

Activated Carbon, Carbon Nanofiber and Carbon Nanotube Supported Molybdenum Carbide Catalysts for the Hydrodeoxygenation of Guaiacol

“…The most abundant reaction products were typically catechol and phenol, albeit in some instances significant amounts of cyclohexane were produced (a more comprehensive account of the results of product analysis is given in Table S2 of the Supplementary Materials accompanying this article). The fact that catechol was observed in all reaction mixtures suggests that guaiacol is converted into phenol via sequential demethylation and HDO [7], which contrasts with the direct demethoxylation pathway proposed by Jongerius et al [2], albeit the possibility for these two pathways to be operating in parallel cannot be discounted.…”

“…The fact that the performance of both CNF-supported catalysts was almost identical at 350 °C indicates that at this temperature the reaction is fast enough as to be driven to completion by the lower number of active sites present in the 7.5% Am/CNF catalyst. Similarly, the fact that no catechol was detected in the products of the reactions catalyzed at 350 °C suggests that if the conversion of guaiacol into phenol is intermediated by catechol, this reaction was also driven to completion (since phenol presents the most difficult C-O bond to cleave, it is particularly resistant to HDO [7]). Notably, the conversion and selectivity to phenol values obtained at 350 °C over the CNF-supported Mo2C catalysts are among the best in the literature for carbide catalysts, being comparable to those reported by Jongerius et al [2].…”

“…However, bio-oil also contains other families of oxygenated compounds stemming from the holocellulosic fraction of biomass, which must be taken into account in bio-oil upgrading studies due to the fact that the reactivity of individual compounds and families of compounds can change when mixed due to synergistic [7] and/or inhibitory [27] effects. With this in mind, a guaiacol upgrading experiment over 20% Am/CNF was performed in the presence of acetic acid and furfural, which are used as model compounds to represent the fractions within bio-oil stemming from hemicellulose and cellulose, respectively [3].…”

“…Guaiacol possesses both phenolic and methoxy moieties which are present throughout lignin, and affords the opportunity to examine the selectivity toward hydrogenation/hydrogenolysis of the -OH and -OCH3 functionalities versus the aromatic ring [4][5][6]. Guaiacol is also more representative of actual lignin product streams than other simple model compounds due to its greater propensity for coking and its tendency to be more refractory towards hydrodeoxygenation (HDO) than other common lignin models [7].…”

Hydrodeoxygenation of acetone–furfural condensation adducts over alumina-supported noble metal catalysts

“…Hydrodeoxygenation (HDO) of the condensation adducts leads to drop-in fuels, (from C8 to C15 lineal alkanes, a diesel-quality fraction) [5,6]. Gasoline-range hydrocarbons can also be obtainedfrom alcohols, aldehydes or ketones, products of the fast pyrolysis of the lignocellulosic biomassthrough condensationhydrogenation sequences [7,8].…”