Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Solid Phase Extraction of Bio-Oil Model Compounds and LigninDerived Bio-Oil Using Amine-Functionalized Mesoporous Silicas Baburam Sedai,† Jin Lin Zhou, Nansi Fakhri, Abdelhamid Sayari,* and R. Tom Baker*
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Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 30 Marie Curie, Ottawa, Ontario K1N 6N5, Canada
ABSTRACT: Lignin-derived pyrolysis bio-oils are a rich source of alkylphenols that could find application if a feasible separation method could be developed. In this work, bio-oil model compounds and lignin-derived bio-oil were used to evaluate commercially available silica gel and normal phase amine-functionalized silica, as well as three different amine-functionalized mesoporous silicas, as adsorbents. The adsorption capacities of all materials at maximum retention of 12 bio-oil model compounds were compared. Catechol showed maximum retention (193.3 mg/g, 1.75 mmol/g) in the presence of the best mesoporous absorbent, while bulkier guaiacol and benzophenone did not show any retention. The best adsorbent was also applied for the separation of three groups of phenolic compounds present in the lignin-derived bio-oil, as determined by GC/ MS analysis, namely, (i) methoxyphenols and alkylmethoxyphenols, (ii) phenol, alkylphenols, and hydroxyacids, and (iii) catechol. Adsorption studies showed that grafted triamine-functionalized mesoporous silica exhibited the best performance in separation of phenol-based compounds from bio-oils. KEYWORDS: Lignin, Pyrolysis bio-oils, Solid phase extraction, Amine-functionalized mesoporous silica
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INTRODUCTION There has been increasing interest over the past decade in utilizing lignocellulosic biomass for chemicals and energy production as a potential supplement for fossil fuels.1−9 The use of renewable biomass as a source of fuels and chemicals could also provide significant benefits to the environment.10,11 Lignin is one of the most abundant sources of aromatic organic macromolecules on Earth12,13 and is found as an integral part of the secondary cell wall of almost all dry land plants and some algae. It is typically extracted from cellulose fibers in wood using the Kraft pulping process, in which wood is converted into pulp by treatment with a mixture of sodium hydroxide and sodium sulfide.14 After several bleaching stages, the white paper-making pulp is separated, and the waste product containing the lignin, known as “black liquor”, is usually burned as a fuel. More recently, efficient processes have been developed for the separation of lignin from the black liquor,15 providing new opportunities for its valorization.16−19 Fast pyrolysis is a process that breaks down the complex macromolecules in biomass or lignin into low-molecular weight species in the absence of oxygen.20,21 The lignin is heated rapidly at 500−700 °C without oxygen, leading to a complex liquid (bio-oil) that can be separated into heavy and light oils by condensation at different temperatures.22 The heavy oil contains larger amounts of aromatic oligomers and © XXXX American Chemical Society
polymers, while the water and organic acid components largely constitute the light oil. The heavy bio-oil can be upgraded with hydrogen to provide a fuel or further processed to obtain aromatic chemicals.23 Biomass-derived bio-oils are known to be unstable with respect to further polymerization,24,25 but these characteristics can be improved by pyrolysis of isolated lignin that does not contain the cellulose or hemicellulose components of the wood.26,27 Solid phase extraction (SPE) is a powerful technique for the separation of specific compounds from complex mixtures.28,29 With SPE, many problems associated with liquid−liquid extraction such as incomplete phase separations, less-thanquantitative recoveries, and disposal of large quantities of organic solvents can be avoided.30 Moreover, SPE does not share the high energy requirements associated with distillation of high-boiling components in liquid−vapor separations31 or high cost of membrane separations.32 Selection of effective adsorbents for the specific chemicals within the bio-oil, however, is challenging. Although silica gels are the most common adsorbent materials, the use of surface-modified mesoporous silica adsorbents has been gaining popularity in Received: February 13, 2018 Revised: June 4, 2018
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DOI: 10.1021/acssuschemeng.8b00747 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering SPE.33−36 This is partly because of the wide variety of mesoporous silicas with high surface areas and well-defined mesoscale pores ranging from 2 to 30 nm in diameter.37−39 The surface properties of such silicas may be readily engineered by incorporating different functionalities via grafting or co-condensation using functionalized alkoxy- or chloro-organo-silanes.40 In particular, amine-functionalized mesoporous silica materials have been used for the selective adsorptive removal of phenols and aldehydes and ketones from simulated bio-oil.35 In other reports, amine-functionalized mesoporous silicas were found to be effective adsorbents for the selective removal of low-molecular weight volatile aldehydes from air.33,36 In addition, the selective removal of aldehydes from mixtures using other adsorbents, including activated carbon has also been reported.34 In this work, we employ mesoporous aminosilica-based adsorbents for the selective separation of phenolic compounds from both model compound mixtures and lignin-derived bio-oil. In particular, a comparison is made between regular and ordered mesoporous silicas containing different amine functionalities. To date, there have been no reports on the separation of phenol-based compounds from “real” bio-oil using aminefunctionalized mesoporous adsorbents.32,34,35 Herein, we compare several commercial and synthesized amine-functionalized mesoporous silicas for the separation of bio-oil model phenols and aldehydes. In addition, we compared them to a commercially available amine-functionalized silica (DSCNH2). Using the best adsorbent, we then developed a facile separation of three subclasses of phenols present in ligninderived heavy bio-oil.
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Synthesis of Adsorbent Materials. Functionalization of Silica by Postsynthesis Grafting: P10-TRI. The preparation procedure was as follows: 1.0 g of CARiACT P10 silica was dried under vacuum at 100 °C for 1 h. Then 30 mL of toluene was added, and the suspension was stirred for 10 min. Subsequently, 0.2 g of water was added, and the mixture was heated to 85 °C. After 15 min of stirring, 3 mL (11.7 mmol) of triaminosilane was added, and the mixture was stirred at the same temperature for 20 h. The solution was filtered, and the resultant solid was washed with toluene and dried at 50 °C for 2 h. Functionalization of Silica Material by Co-Condensation: Synthesis of SBA-15MONO. Here, 4.0 g of P123 block copolymer and 8 g KCl were dissolved in 30 mL of water and 120 mL of a 2 M HCl solution at 40 °C. Then, 8.50 g (40.8 mmol) of TEOS was added and prehydrolyzed for 2 h before adding 1.08 g (6.0 mmol) of APTS which corresponds to an APTS/TEOS molar ratio of 0.15. The mixture was stirred at the same temperature for 20 h and then heated at 100 °C for 24 h under static conditions. The white solid product was collected by filtration, dried in air at room temperature for 24 h, and then extracted with ethanol (140 mL/g) at 100 °C for 24 h to remove the organic template. To ensure complete removal of the template, 1 g of the ethanol-extracted material was stirred in 50 mL of aqueous 0.1 M HCl for 1 h, filtered, and dried, then neutralized in 50 mL of 0.1 M NaHCO3 for 1 h and finally filtered and dried under vacuum at 100 °C for 3 h. Synthesis of SBA-15TRI. The same procedure as for the synthesis of SBA-15MONO was used, except that triaminosilane (1.62 g, 6.1 mmol) was used instead of APTS. Characterization of Adsorbents. Nitrogen Adsorption Measurements. The structural properties of all materials were determined by nitrogen adsorption at 77 K using a Micromeritics ASAP 2020 volumetric apparatus (Table 1). Prior to each analysis, the samples
Table 1. Structural Properties of Silica Gel and AmineFunctionalized Silicas
EXPERIMENTAL SECTION
Materials and Methods. Tetraethylorthosilicate (TEOS), used as the silica source for SBA-15; triblock poly(ethylene oxide)-βpoly(propylene oxide)-β-poly(ethylene oxide) copolymer Pluronic P123 (MW = 5800), used as the structure-directing agent; and functionalization agents, 3-amino-propyltrimethoxysilane (APTS) and trimethoxysilylpropyldiethylene-triamine (referred to here as triaminosilane) were all purchased from Sigma-Aldrich. Chromium(III) acetylacetonate, cyclohexanol, and tetramethylphospholane were also purchased from Sigma-Aldrich. Pyridine, acetonitrile, and chloroform were purchased from Fisher Scientific. High purity gases were provided by BOC, Canada. Deuterated solvents were acquired from Cambridge Isotopes and dried with molecular sieves. Pyridine was dried with calcium hydride and further dried with molecular sieves. Gases and all other reagents were used without further purification. The heavy bio-oil sample22 (
