Catalytic esterification of fatty acids using solid acid catalysts generated from biochar and activated carbon

Abstract

Reusable, solid acid carbon supported catalysts were generated from biomass by pyrolysis (400–500 °C) to generate a soft to hard carbon backbone (i.e., biochar) for addition of acidic functional groups. Acid catalysts were synthesized by sulfonating the biochar and wood derived activated carbon using concentrated H₂SO₄ at 100, 150 and 200 °C (12 h) and gaseous SO₃ (23 °C). Attenuated Total Reflectance, sulfur, and NH₃-TPD analysis of the sulfonated carbons indicated the presence of SO₃H groups on the 100 °C sulfonated biochar and activated carbon (AC), with higher active site densities (SO₃H density) for the SO₃ sulfonated material. The sulfonated carbons were tested for their ability to esterify free fatty acids with methanol in blends with vegetable oil and animal fat (5–15 wt.% FFA). Esterification of the fatty acids was typically complete (∼90–100% conversion) within 30–60 min at 55–60 °C (large methanol excess), but decreased with lower methanol to oil ratios using the biochar catalysts (e.g., 70%, 6 h, 20:1). Solid acid catalysts derived from wood based activated carbon had significantly higher activity compared to the biochar derived catalysts (e.g., 97%, 6 h, 6:1). Of the synthesized biochar catalysts, 400 °C pyrolyzedpine chip biochar, sulfonated at 100 °C, resulted in the highest reaction rate and lowest reduction in conversion (or deactivation) when reused multiple times. Drying the biochar catalysts for 1 h at 125 °C between uses maintained esterification activity, allowing the catalysts to be reused up to 7 cycles. For the SO₃ sulfonated AC catalyst, such a regeneration step was not required, as the fractional conversion of palmitic and stearic acid (5% FFA, 10:1, 3 h) remained >90% after 6 cycles.

Introduction

Recently there has been renewed interest in the synthesis of solid acid and base catalysts for producing biodiesel – primarily driven by the need to find environmentally benign catalysts to replace waste generating homogeneous acids and bases [1]. The renewed research appears to be driven by a need to replace sulfuric acid and sodium methoxide in the production of methyl esters (i.e., biodiesel) from free fatty acids (FFAs) and triglycerides. In the production of biodiesel, a commodity chemical, the use of acids and bases as unrecoverable catalysts generate large volumes of waste that must be treated, significantly adding to costs and the environmental impact of production.

The economics of biodiesel production are strongly linked to the feedstock cost [2], catalyst cost, and wastewater treatment. The raw material cost of the feedstock (e.g., soybean oil) for biodiesel production is significantly more than that of petroleum based diesel and represents the largest fraction of the cost for biodiesel production [2]. Thus, the high cost of refined vegetable oil, the inability to recover/reuse the catalysts, and waste formation due to use of homogeneous catalysts (e.g., H₂SO₄ and KOH) are all barriers to biodiesel commercialization.

Reusable solid acid catalysts would eliminate these barriers by allowing biodiesel production from low-quality feedstocks high in free fatty acids (FFAs), reducing catalyst cost, and eliminating the need for costly treatment of high and low pH streams. Solid acid catalysts could be used to esterify FFAs in inexpensive sources of triglycerides, such as yellow (<15% free fatty acid or FFA) or brown (>15% FFA) grease, rendered fat, and soapstock, followed by base catalysis to transesterify the glycerides.

This awareness has resulted in research on the synthesis and testing of heterogeneous acid catalysts for esterification. There have been recent reports on the generation of solid acid catalysts. Anion exchange resins (e.g., polystyrenesulfonic acid) have been used to esterify FFAs with a range of alcohols, yet are expensive and potentially unstable at high pH [3]. Perfluorinated alkanes supported on silicon oxides catalyze esterification [4], but again are expensive, environmentally unfriendly, unstable at high pH, and are generated from non-renewable carbon sources. Heteropolyacids impregnated/attached on/to zirconia have also been developed, but this support material (i.e., zirconia) is very expensive [5].

An option that has great possibility, but which has not been fully explored is the generation of acid catalysts supported on carbon for catalysis. Carbon supported catalysts have several distinct advantages over alumina or silica supported systems; they are stable under acidic basic conditions, can have very high surface area [200–1500 m²/g], renewable biomass sources can be used to generate the carbon, and the non-polar nature of the support matrix may reduce adsorption of polar molecules (e.g., water or glycerol) that can deactivate the catalyst.

Functionalized carbon (e.g., attached SO₃H groups) has been generated from refined sugars (pure cellulose, glucose and starch) and was demonstrated to catalyze the transesterification of oleic and stearic acid with ethanol [6]. Acidic functional groups have been attached to wood based activated carbon [7], but there has been limited testing of these materials as catalysts. Recently, solid carbon catalysts derived directly from biomass via fast pyrolysis biochars were explored as biodiesel catalysts, but there is limited data on esterification reaction kinetics and reuse capability of such derived catalysts [8].

The objectives of this research were to develop reusable, porous carbon supported acid catalysts for biodiesel generation using biochar generated from lignocellulosics (by slow pyrolysis) and to contrast esterification rates with activated carbon catalysts. It was theorized that low temperature (400–600 °C), slow pyrolysis of biomass would generate a highly cross-linked, multi-ringed, aromatic structure anchored to lignin that could be easily functionalized with catalytically active acidic groups. Based on the previous analysis of biochar generated by pyrolysis of pure biomass components (cellulose, hemicelluloses, and lignin) and whole biomass, it is theorized that lignin would undergo partial decomposition and hemicellulose and cellulose would undergo a series of thermal homolysis, hydrolysis, dehydration, and molecular rearrangement reactions to form a polymerized aromatic structure. Catalysts derived from biochar or lignocellulosics, contrary to refined carbohydrates, zeolites, and resins would be environmentally friendly (e.g., separation of glucose from biomass, mining required for zeolites, or use of petroleum feedstocks for synthesis of resins would not be required).

The first phase of this research focused on rapid screening of solid acid catalysts generated from biochars for esterification activity, with methanol in large excess. Screening was used to select an optimum biochar catalyst, a sulfonated pine chip biochar, based on fatty acid (FFA) conversion versus time data and reuse activity. Further research was then performed with the pine chip char catalyst in FFA spiked oil at lower methanol to oil ratios; biochars synthesized at 400 °C and sulfonated using H₂SO₄ at 100 °C gave optimum results. Research subsequently focused on biochars synthesized under these conditions and were compared to catalysts synthesized from wood based activated carbon. Although pine chip biochar catalysts generated the highest esterification rates, this particular catalyst was fragile and fragmented into fine powder in the batch, stirred reactions. Since we anticipated this would be problem at an industrial scale (e.g., in a packed-bed reactor), we focused on a biochar form that we theorized would not break down under shear stress and selected a granular peanut hull, smaller in size compared to large pelleted biomass forms to reduce mass transfer limitations. Our overall results, based on esterification reactions rates and turnover frequency or TOF (calculated from the reaction rate and SO₃H density of each catalyst) are presented in tabular form.