Research Article pubs.acs.org/journal/ascecg
Ni/Co-Natural Clay as Green Catalysts for Microalgae Oil to DieselGrade Hydrocarbons Conversion Vineet Kumar Soni, Pragati R. Sharma, Ganpat Choudhary, Shubham Pandey, and Rakesh K. Sharma* Department of Chemistry, Indian Institute of Technology Jodhpur, Ratanada, Jodhpur, Rajasthan, India 342011 S Supporting Information *
ABSTRACT: The Ni/Co-natural clay catalysts have been prepared for the conversion of algae oil into diesel-grade hydrocarbons. Methyl oleate was used as a model compound for the present study. Ni/clay catalyst promotes decarboxylation/decarbonylation, whereas remarkable selectivity in hydrodeoxygenation (HDO) is achieved with Co/clay catalysts. Powder XRD and DRS studies of substrate mixed catalysts reveal a more prominent adsorption of substrate molecules over the Ni surface, which results in low HDO selectivity of nickel catalysts by surpassing the essential contribution of acidic sites. The HDO process provides higher carbon atom economy and energy value over decarboxylation/decarbonylation, while further reducing the formation of greenhouse gases such as CO2 and CH4. Total yield of saturated hydrocarbons from algae oil was 84−86 wt % with similar selectivity. The HDO rates of different fatty acids present in algae oil were independent of the fatty acid chain length. The catalysts are cost-effective and recyclable, and metal leaching during hydroprocessing is less than 1 ppm in all cases. This process is advantageous in terms of metal-to-substrate ratio, use of solvents and their concentration, and comparable HDO selectivity over the previously reported catalysts. A hydroprocessing reaction was also performed under solvent free conditions, which could be useful in industrial applications of this approach. KEYWORDS: Natural clay, Non-noble metals, Heterogeneous catalysis, Deoxygenation, Green process
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INTRODUCTION While addressing the limited availability and increasing demand of liquid fuels, efforts have been made toward developing efficient catalytic systems for the production of fuel grade hydrocarbons from biomass feedstocks.1−9 The utilization of materials with high oxygen content such as lignin and carbohydrate based substrates is less attractive for such transformations. Alternatively, triglycerides and fatty acid/esters are suitable precursors. Therefore, the use of edible/nonedible oils is considered more appropriate, and several processes such as pyrolysis, transesterification (first generation of biodiesel), blending with diesel, and microemulsification with alcohols have been developed.1 However, the presence of oxygen content in these processing products delimits their use in diesel engines. To overcome these limitations, the hydrotreating process is investigated for converting triglycerides and free fatty acids derived from oils into straight chain diesel-grade hydrocarbons (second generation biodiesel).10−14 In this perspective, microalgae have the potential to act as a renewable energy resource due to a noncompetitive high growth rate with respect to the plants. The presence of high mono-, di-, and triglyceride in algae oil is responsible for relatively low oxygen content.15−18 Deoxygenation of long chain fatty acids (C12− C22) produces straight chain hydrocarbons with high cetane number, and thus useful for diesel engines. Deoxygenation of biomass-derived fatty acid/esters and glycerides follows decarbonylation and decarboxylation pathways to yield CN−1 hydrocarbons, whereas hydrodeoxygenation (HDO) results in © 2017 American Chemical Society
the formation of CN hydrocarbons. The process of HDO is advantageous over decarboxylation/decarbonylation in terms of carbon atom economy and high energy value. Although the HDO process requires more hydrogen, the byproduct is H2O instead of CO/CO2, which is environmentally friendly and obviates undesired hydrogenation side reactions to yield another greenhouse gas, i.e., methane. Sulfided metal catalysts are generally used for deoxygenation of microalgae oil and model compounds.19−21 However, the oxygenates and moisture cause deactivation of sulfided NiMo and CoMo catalysts and require addition of a sulfur agent during the reaction. This process is unwelcome for both practical and environmental reasons. The effectiveness of rather expensive noble metal (such as Pd, Pt, Ru, Rh, and Ir) in this process has been substantially explored.21−26 In most of the cases, these heterogeneous catalysts provide low to moderate selectivity toward hydrodeoxygenation (HDO) over decarboxylation/ decarbonylation. Recent reports on sulfur-free catalytic transformation of microalgae and vegetable oil into hydrocarbons showed good to high selectivity toward CN alkanes.27−39 Supported Pt based catalysts29,30 and molybdenum carbides31 on carbon showed high activity and selectivity toward HDO of fatty acids/esters. Bitter and co-workers32−35 substantially explored the catalytic conversion of vegetable oil and related Received: March 2, 2017 Revised: April 20, 2017 Published: May 3, 2017 5351
DOI: 10.1021/acssuschemeng.7b00659 ACS Sustainable Chem. Eng. 2017, 5, 5351−5359
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Comparison of XRD patterns of clay and metal(s)/clay catalysts.
Figure 2. FT-IR spectra of clay and metal(s)/clay catalysts.
substrates. In a report, the presence of oxide species in tungsten based catalysts promoted decarboxylation/decarbonylation, whereas carbide showed selectivity toward hydrodeoxygenation.32 Lercher and co-workers36−39 have extensively studied the hydroprocessing of microalgae oil. The conversion of stearic acid and microalgae oil into alkanes was achieved over Ni/HBeta catalyst with remarkable hydrodeoxygenation selectivity.36 Introduction of non-noble metals such as Ni, Co, and Mo in these catalysts is crucial for industrial applications; nevertheless the cost of solid support is also important. Naturally abundant clay40−43 and zeolites44−51 as solid support may serve this purpose well. Clays, a group of aluminosilicates, are modified by physical and chemical modifications.52−54 For instance, natural montmorillonite clay is treated with mineral acid for its improved adsorption and catalytic properties.52,53
Recently, we have reported the solvent free hydrogenation of highly unsaturated squalene into squalane catalyzed by Pdnanoparticle-intercalated natural clay.55 In a major advancement herein, a green and cost-effective catalytic system has been developed for the production of diesel-grade hydrocarbons from microalgae oil. The use of non-noble metals and naturally abundant clay as a solid support providing high reactivity and deoxygenation selectivity are some prominent features of this work. A very low metal leaching was observed during the hydroprocessing reaction, which resulted in good recyclablility of the catalysts. Clay based catalysts for microalgae oil upgradation have not been reported so far. For the current study, a series of nickel and cobalt containing mono- and bimetallic catalysts was prepared, and the activities of the catalysts in hydroprocessing were tested. Methyl oleate was selected as a model compound for initial deoxygenation studies. Serrano and co-workers56,57 studied the catalytic conversion of 5352
DOI: 10.1021/acssuschemeng.7b00659 ACS Sustainable Chem. Eng. 2017, 5, 5351−5359
Research Article
ACS Sustainable Chemistry & Engineering
Figure 3. TEM/HRTEM data of Co5/clay.
Figure 4. TEM/HRTEM data of Co2.5/clay.
adversely affected the performance of catalysts. The amount of metal species in catalysts was determined using ICP-AES analysis (Table S1). XRD patterns of clay (Figure S1) revealed its crystalline nature, and the characteristic peaks for montmorillonite clay were identified with some quartz content. Ni and Co peaks for (111) planes were observed for metal/clay catalysts (Figures S2−S8). A comparative XRD study revealed noteworthy changes on the {001}, {002}, and {003} planes of metal/clay materials (Figure 1). Thus, the wet impregnation under acidic conditions induced the formation of new layers followed by inclusion of metal ions. The FT-IR spectrum of clay showed the presence of O−H bonds at 3622 and 3415 cm−1 for the hydroxy groups of clay and interlayer water molecules (Figure 2). A peak at 1636 cm−1 was detected for O−H bending. For Si−O bonds, a broad and intense peak was detected at 1036 cm−1. The peak intensities were reduced at 3430 and 1636 cm−1 after wet impregnation due to dehydroxylation by nitric acid.59 Some changes were also observed for other peaks. During SEM analysis, the changes in morphology of clay were observed after wet impregnation (Figure S9). Elemental
methyl oleate and observed that Ni2P/SBA-15 provided better selectivity toward n-octadecane formation at low temperatures and high pressure (up to 60% selectivity) with low ester conversion, whereas Ni/SBA-15 gave n-heptadecane as a major product. However, the reaction was complete at ≥290 °C with reduced HDO selectivity. In another report, cobalt as the active phase on SBA-15 showed improved selectivity toward n-C18.58 The presence of other cracking products was observed in these cases.
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RESULTS AND DISCUSSION Nickel- and cobalt-based clay catalysts were prepared following a method reported earlier.19 The surface modification of clay and metal inclusion could be realized simultaneously during wet impregnation in dilute nitric acid. For bimetallic Ni−Co catalysts, cobalt was incorporated into NiO/clay (obtained after wet impregnation followed by calcination) by wet impregnation (see the Supporting Information). The presence of small amounts of oxidized metal species after subsequent hydrotreatment at 300 °C under hydrogen atmosphere could not be obviated. An attempt to increase the reduction temperature 5353
DOI: 10.1021/acssuschemeng.7b00659 ACS Sustainable Chem. Eng. 2017, 5, 5351−5359
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Hydrotreatment of Methyl Oleate with Metal(s)/Clay Catalystsa
selectivity (%) entry
catalyst
catalyst loading (wt %)
T (h)
BET surface area (m2 g−1)
acid density (mmol g−1)b
conv. (%)
n-C17
n-C18
1 2 3 4 5 6 7 8 9 10 11
Ni10/clay Ni7Co3/clay Ni5Co5/clay Ni3Co7/clay Co10/clay Co5/clay Co2.5/clay Co2.5/clay Co5/clay Co2.5/clay Co2.5/clay
10 10 10 10 10 20 40 30 20 40 40
6 6 6 6 6 6 6 8 8 8 5
51.275 50.205 48.131 46.032 42.162 47.521 51.419 51.419 47.521 51.419 51.419
0.289 0.282 0.131 0.034 0.046 0.125 0.201 0.201 0.125 0.201 0.201
100 100 100 100 100 100 100 100 100c 100c 100d
88 82 65 56 39 26 9 10 24 12 10
11 17 34 43 60 73 90 89 75 87 89
isomers/cracking trace trace trace trace trace trace trace trace trace trace trace
(
