Valorization of Waste Lipids through Hydrothermal Catalytic

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Valorization of Waste Lipids through Hydrothermal Catalytic Conversion to Liquid Hydrocarbon Fuels with In Situ Hydrogen Production Dongwook Kim, Derek R. Vardon, Dheeptha Murali, B. K. Sharma, and Timothy J. Strathmann ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01768 • Publication Date (Web): 15 Feb 2016 Downloaded from http://pubs.acs.org on February 18, 2016

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ACS Sustainable Chemistry & Engineering

Valorization of Waste Lipids through Hydrothermal Catalytic Conversion to Liquid Hydrocarbon Fuels with In Situ Hydrogen Production Dongwook Kim,1 Derek R. Vardon,2 Dheeptha Murali,3 Brajendra K. Sharma,3 Timothy J. Strathmann4,* 1. Department of Chemistry, Korean Military Academy, Seoul, Republic of Korea 2. National Bioenergy Center, National Renewable Energy Center, Golden, CO 80401, United States 3. Illinois Sustainable Technology Center, Champaign, IL 61821, United States 4. Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401, United States Corresponding Author: [email protected]

Abstract: We demonstrate hydrothermal (300ºC, 10 MPa) catalytic conversion of real waste lipids (e.g., waste vegetable oil, sewer trap grease) to liquid hydrocarbon fuels without net need for external chemical inputs (e.g., H2 gas, methanol). A supported bimetallic catalyst (Pt-Re/C; 5 wt% of each metal) previously shown to catalyze both aqueous phase reforming of glycerol (a triacylglyceride lipid hydrolysis co-product) to H2 gas and conversion of oleic and stearic acid, model unsaturated and saturated fatty acids, to linear alkanes was applied to process real waste lipid feedstocks in water. For reactions conducted with an initially inert headspace gas (N2), waste vegetable oil (WVO) was fully converted into linear hydrocarbons (C15-C17) and other hydrolyzed byproducts within 4.5 h, and H2 gas production was observed. Addition of H2 to the initial reactor headspace accelerated conversion, but net H2 production was still observed, in agreement with results obtained for aqueous mixtures containing model fatty acids and glycerol. Conversion to liquid hydrocarbons with net H2 production was also observed for a range of other waste lipid feedstocks (animal fat residuals, sewer trap grease, dry distiller’s grain oil, coffee oil residual). These findings demonstrate potential for valorization of waste lipids through conversion to hydrocarbons that are more compatible with current petroleumbased liquid fuels than the biodiesel and biogas products of conventional waste lipid processing technologies.

KEYWORDS: waste fats and grease, hydrogenation, decarboxylation, decarbonylation, deoxygenation, aqueous phase reforming, waste-to-energy 1

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INTRODUCTION Population growth and economic development are spurring increases in demand for new sources of energy and industrial chemical precursors while simultaneously producing growing quantities of waste. Concurrently, there is growing agreement that wastes should be viewed as valuable renewable resources,1–5 but new technologies are needed to recover energy and valorize waste materials. Waste lipid streams are an attractive target for valorization because of the high energy density and chemical similarity of fatty acid structures to petroleum-based diesel fuels.6 Reported estimates of the production of waste lipids, including waste vegetable oil (WVO), waste animal fat residuals from meat animal processing facilities, and sewer trap grease, in the U.S. exceed 10 billion pounds/year,7 providing a significant quantity of low-cost feedstocks for renewable fuel and chemicals production. Besides landfilling and other disposal practices, options for valorization of collected waste lipids have been limited mostly to anaerobic co-digestion with wastewater biosolids and biodiesel production.8–11 A growing number of wastewater treatment facilities now accept delivery fats, oils, and grease (collectively referred to as FOG) wastes from restaurants and food industries to co-digest with sludge generated during wastewater treatment to boost production of methane-containing biogas, which in turn is combusted to provide onsite electricity and heat.12 Although anaerobic digestion is a mature technology, methane has low value in comparison to longer chain liquid fuels and higher value organic chemicals that can be produced from the same feedstocks.13,14 Creating higher valorization potential streams could also reduce a widespread and unsafe black market practice in China of collecting waste cooking “gutter” oil and blending with fresh vegetable oil for resale.15 Waste lipids can also serve as a source for biodiesel production via transesterification processes, converting triacylglycerides (TAGs), the predominant component of neutral lipids, to fatty acid methyl esters (FAMEs).16,17 However, conventional transesterification processes require high quality TAG feedstocks free of moisture and free fatty acids (FFA) that are common in waste lipids that promote saponification and emulsification.17 The transesterification of lipids also requires large inputs of alcohol,16 and biodiesel has limited compatibility with conventional transportation fuels, requiring engine modifications if blended with diesel fuel at more than 20%.18 As an alternative, direct

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conversion of triacylglycerides to diesel-grade hydrocarbons (so-called green diesel) by catalytic hydrodeoxygenation (HDO) has attracted growing interest, and commercialization efforts have targeted conversion of lipids extracted from oil-rich vegetable crops.19 Like the methanol requirements for biodiesel, the HDO process requires large stoichiometric inputs of H2(g) to catalytically reduce oxygenated lipid functional groups.20 For example, vegetable oil HDO through UOP/Eni’s Ecofining process requires a continuous 1.5-3.8 wt% H2(g) in feedstream.6 The moisture content and heterogeneity of many waste lipid sources presents additional barriers to processing waste lipids through HDO with typical refinery hydrotreatment catalysts (e.g., CoMo/Al2O3, NiMo/Al2O3) that are deactivated by water; high costs and energy requirements for pretreatment are prohibitive.21,22 As an alternative to conventional refinery processing, there is growing interest from industry and the U.S. Department of Energy in the application of hydrothermal processing for valorization of lipidcontaining biomass and waste materials, especially feedstocks with high moisture content.23–26 Direct feedstock conversion in condensed water phase eliminates the need to dry feedstocks or extract lipids prior to processing. Heating high moisture content feedstocks (e.g., harvested algae biomass typically >80% water) under pressure to typical hydrothermal processing conditions (e.g., 250-350°C, 4-16 MPa) requires markedly less heat input than that required to vaporize water,26 and efficient heat recovery operations in modern refineries minimizes net input energy requirements.23 Recent studies demonstrate lipid hydrolysis and catalytic deoxygenation of fatty acids in hydrothermal environments (250 – 380ºC).27–32 Fu et al. reported conversion of model saturated fatty acids (palmitic, stearic, and lauric acids) with high selectivity (>90%) to the n-alkane decarboxylation/decarbonylation products using carbon-supported Pt and Pd catalysts (Pt/C, Pd/C) with no added H2(g).27,28 However, the same conditions were ineffective in decarboxylation/decarbonylation of unsaturated fatty acids (oleic and linoleic acids), instead hydrogenating these compounds to stearic acid without further decarboxylation/decarbonylation. Facile conversion of the unsaturated fatty acids to n-alkanes can be accomplished by adding H2(g),33 but introduction from an external sources (typically steam reforming of natural gas) increases the cost and carbon footprint of fuels produced from lipids. Recently, Vardon et al. showed that oleic acid, a monounsaturated fatty acid, could be stoichiometrically converted to 3



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heptadecane under hydrothermal conditions using a bimetallic Pt-Re/C catalyst without external H2 addition (Figure 1).29 The catalyst displayed bifunctional activity, catalyzing hydrogenation, decarboxylation/decarbonylation, and deoxygenation reactions of the fatty acid while also promoting in situ generation of H2(g) by aqueous phase reforming (APR) of glycerol, a TAG hydrolysis coproduct of the FFAs. Conversion was accelerated when external H2(g) was initially added to the reactor, but the net H2(g) balance was positive under most conditions, indicating that in situ generation from APR exceeded process requirements. At elevated initial H2(g) pressures (≥3.4 MPa before heatup), octadecane was observed as a minor product and the H2(g) balance was slightly negative, indicating a switch from the H2-neutral decarboxylation/decarbonylation pathways to a H2-consuming carboxyl reduction pathway (Figure 1).34,35 Studies to date focusing on elucidating hydrothermal catalytic conversion mechanisms, catalyst design, and process optimization have studied purified model fatty acids and triacylglycerides. Further advancement of this valorization platform necessitates further demonstration with less refined lipid feedstocks.36 Herein, we demonstrate subcritical hydrothermal catalytic conversion of real waste lipids (e.g., WVO collected from a university dining hall, animal fat residuals, sanitary sewer trap grease, distiller’s dried grains with solubles oil, and oil derived from waste coffee grounds) to liquid hydrocarbon fuels without need for external chemical inputs (e.g., H2 gas, methanol). As a demonstration, we processed the waste lipids using a supported bimetallic catalyst (Pt-Re/C; 5 wt% of each metal) previously shown by our team to promote in situ H2(g) production from glycerol reforming and hydrogenation and deoxygenation of oleic acid, a model unsaturated fatty acid.29 Constituent fatty acids and hydrocarbon products were tracked and H2(g) partial pressure in the reactor headspace was quantified to assess in situ production. The effect of supplemental H2 overhead gas on lipid conversion rates, in situ H2 production/consumption, and final conversion product distribution are discussed in this context. To our knowledge, this contribution is the first to document hydrothermal catalytic conversion of real waste lipid feedstocks derived from vegetable and animal fat sources to liquid hydrocarbon fuels, demonstrating promise for application this high-valorization strategy for waste carbon. 4


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Figure 1. Hydrothermal conversion of triacylglyceride (TAG) lipids to linear alkanes through hydrolysis to free fatty acids (FFAs) and catalytic hydrogenation, decarboxylation/decarbonylation, and deoxygenation reactions of the FFAs, where net H2 process needs are met by catalytic aqueous phase reforming of the glycerol, coproduct of TAG hydrolysis. Adapted from Vardon et al.29

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EXPERIMENTAL Reagents. Hydrocarbon analytical reference standards (tetradecane, pentadecane, hexadecane, heptadecane, and octadecane), carbon-supported platinum catalyst (Pt/C; nominal 5 wt% Pt), and ammonium perrhenate (NH4ReO4) were purchased from Sigma-Aldrich. Stearic acid and palmitic acid reference standards were obtained from TCI Chemicals, and oleic acid reference standard was acquired from Alfa Aesar. Dichloromethane (DCM) and methanol solvents were purchased from Fisher. Waste Lipids. WVO (soybean-based) was collected from a student dining hall on the campus of the University of Illinois at Urbana-Champaign. Prior to hydrothermal processing, the WVO was heated to 80ºC to melt solidified lipids and the upper portion was collected to minimize gross solid impurities. Animal fat residuals were obtained from local grocery market and phase separated by heating on stir plate to separate oil from solid particulates. Distiller’s dry grains with solubles (DDGS) was obtained from Atkins Energy LLC (Lena, IL) and oil was extracted at Viobin (Monticello, IL) using a combination of acetone and ethyl acetate. Sewer trap grease was obtained from the UrbanaChampaign Sanitary District (UCSD). Crude lipids were extracted from spent coffee grounds (16.2 wt% of the spent coffee grounds) collected from local coffee shops as described previously.37 Catalyst. A Pt-Re/C catalyst (5 wt.% Pt, 5 wt.% Re nominal loadings) was used for conversion studies. The bimetallic formulation was selected to meet the dual goals of fatty acid conversion and in situ H2 generation. Catalyst screening experiments previously showed that Pt was most effective in promoting hydrothermal decarboxylation of saturated fatty acids,38 and separate reports have documented the effectiveness of Re as a promoter metal for APR processes that generate H2 from low molecular weight oxygenates like glycerol.39–41 The bimetallic catalyst was prepared following a procedure described previously.29 Briefly, a fresh batch of catalyst was prepared for each reaction by aqueous adsorption of NH4ReO4 onto Pt/C in a 450-mL Parr 4562 stainless steel reactor pressurized with H2(g) to 200 psi and vented three times before heating under continuous mixing for 2 h at 200ºC. The constituent metals are well dispersed throughout the support with metal crystallites

Valorization of Waste Lipids through Hydrothermal Catalytic

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