Stabilization of fast pyrolysis liquids from biomass by mild catalytic hydrotreatment: Model compound study
Repolymerization is a huge problem in the storage and processing of biomass pyrolysis liquid (PL). Herein, to solve the problem of repolymerization, mild catalytic hydrotreatment of PL was conducted to convert unstable PL model compounds (hydroxyacetone, furfural, and phenol) into stable alcohols. An Ni/SiO 2 catalyst was synthesized by the deposition-precipitation method and used in a mild hydrotreatment process. The mild hydrotreatment of the single model compound was studied to determine the
... ed to determine the reaction pathways, which provided guidance for improving the selectivity of stable intermediate alcohols through the control of reaction conditions. More importantly, the mild hydrotreatment of mixed model compounds was evaluated to simulate the PL more factually. In addition, the effect of the interaction between hydroxyacetone, furfural, and phenol during the catalytic hydrotreatment was also explored. There was a strange phenomenon observed in that phenol was not converted in the initial stage of the hydrotreatment of mixed model compounds. Thermogravimetric analysis (TGA), Ultraviolet-Raman (UV-Raman), and Brunauer−Emmett−Teller (BET) characterization of catalysts used in the hydrotreatment of single and mixed model compounds demonstrated that this phenomenon did not mainly arise from the irreversible deactivation of catalysts caused by carbon deposition, but the competitive adsorption among hydroxyacetone, furfural, and phenol during the mild hydrotreatment of mixed model compounds. Catalysts 2020, 10, 402 2 of 20 Much effort has been devoted to increasing the stability of PL. The co-pyrolysis of biomass with other wastes, including plastic and tires, has been studied to obtain PL with a higher carbon and lower water content  . The catalytic upgrading of PL with an acid support like HZSM-5 and HY during pyrolysis has also been studied, in an attempt to increase the content of the aromatic hydrocarbon  . To reduce the total acid number, an alkaline catalyst like dolomite was used during the stabilization of PL  . Catalytic hydrogenation has been widely studied by researchers during the past 40 years [14, 15] and is considered a promising technology for improving the undesired properties of biomass PL. The studies essentially show that it is difficult to produce transportation fuel or gasoline fractions by the one-step catalytic hydrogenation of PL at a high temperature (>300 • C)  . A two-stage upgrading process, including mild hydrogenation and high-temperature hydrodeoxygenation (HDO), was proposed by Baker and Elliott  in 1988, which was later widely accepted and studied by other researchers [7, 10, 17, 18] . In the mild hydrogenation step, thermal liable components, especially aldehydes and ketones, are converted into corresponding alcohols and carbohydrates into sugar alcohols, which are more stable molecules and less prone to repolymerization. Additionally, the alcohols can be converted into hydrocarbons by HDO at a high temperature    . Generally, carbonyl-containing molecules and sugars are readily hydrogenated to corresponding alcohols, for example, glucose to sorbitol  and glycolaldehyde to ethylene glycol  . Meanwhile, the main parallel reactions, including acid-catalyzed reactions like sugar dehydration to furfural/5-HMF, further to humin-like molecules and condensation between aldehydes, ketones, and phenolics, predominate when poor mass transfer occurs due to viscous PL , especially when a continuous reactor configuration like a packed bed reactor with a large chunk of catalyst particles is applied . The latter case reactions are detrimental to hydrogenation catalysts as increased Mw promotes coke/char formation on the catalyst surface, further blocking the porous structure of the catalysts [18, 25] , so it is essential to know the reactivity of various groups of compounds in PL, especially versus reaction temperatures, so that those molecules can be converted into more stable ones before repolymerization. Therefore, repolymerization reactions are suppressed to a low extent during mild catalytic hydrotreatment. Examples of recent research on the hydrotreatment of PL model compounds are summarized in Table S1 . However, studies on the mild catalytic hydrotreatment of PL model compounds, especially mixed model compounds and the interactions among several models, are still scarce. Here, we report the catalytic hydrotreatment of model compounds from a PL study using a home-made Ni/SiO 2 catalyst, in order to understand the behavior of compounds in PL during mild catalytic hydrotreatment. Compared to Ni-based catalysts, noble metal catalysts are much more expensive and traditional presulfided catalysts require the introduction of S into the system to maintain the activity, which will contaminate the products . SiO 2 is a traditional inert material with a satisfactory thermal stability and has been commonly used as the support of catalysts in the hydrotreatment of PL and its model compounds  . Hydroxyacetone and furfural have previously been selected as model compounds because hydroxyacetone is an abundant molecule found in the aqueous phase of biomass fast pyrolysis liquids     , and furfural is the dehydration product of carbohydrates and hard to handle compared to the mother C5 sugar, and has thus been used to mimic intermediates from sugar dehydration [31, 32] . Phenol is also a component of PL  , and has often been selected as the model compound in PL hydrodeoxygenation studies as the Ph-OH bond possesses the highest dissociation energy among the relevant phenolic C-O      . The aromatic ring is a possible precursor for polyaromatic formation, and will result in coke formation on the catalysts. To obtain insights into the conversion of the aromatic ring during the hydrotreatment process, simple phenol is selected as the model compound from PL. The reaction pathways are studied first, and the effects of temperature and initial hydrogen pressure are taken into account for the selectivity of stable alcohols, and thus inhibit the repolymerization tendency of the PL. More importantly, to simulate the biomass PL more factually, hydrotreatments of mixed model compounds at different mixing conditions are studied in detail. The suppression to hydroxyacetone and phenol conversion are discovered in Catalysts 2020, 10, 402 3 of 20 this process and an explanation of this phenomenon is given according to the reaction results and the characterization of the used catalyst.