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SYNTHESIS AND CHARACTERIZATION OF A NOVEL SOLID ACID CATALYST FOR IMPROVED USE OF WASTE OIL FEEDSTOCK FOR BIODIESEL PRODUCTION
Impact/Purpose:
, and will be used to test the removal of free fatty acids in simulated waste vegetable oil systems. Student chemists will team with the members of the Green Team Organization at Radford University to complete this project. A special topics course devoted to green chemistry will be developed and the results from this project incorporated as novel experiments in our capstone Integrated Laboratory chemistry course.
, and will be used to test the removal of free fatty acids in simulated waste vegetable oil systems. Student chemists will team with the members of the Green Team Organization at Radford University to complete this project. A special topics course devoted to green chemistry will be developed and the results from this project incorporated as novel experiments in our capstone Integrated Laboratory chemistry course.Challenge Area: Materials and Chemicals
The interest in alternative fuels has increased dramatically in recent years due to the rising cost and the environmental concerns related to fossil fuels use. Biodiesel represents a non-toxic and carbon-neutral fuel representing one component in our strategic approach to reduce dependence on petroleum based fuels. While interest in biodiesel has increased in recent years, costs still remain high, particularly when using refined oil feed stocks. Concern exists, however, about the conversion of agricultural land and deforestation in developing countries for oil production to meet demand for biodiesel production. Waste oil feedstock including yellow and brown grease can be utilized, but processing is more difficult due to the high free fatty acid content.
We propose to meet the challenge of increasing the use of waste oil for biodiesel production through the development of novel carbon catalyst for the esterification of fatty acids, essentially removing them from waste feedstock. The catalyst will be non-toxic, inexpensive, and easy to produce using “green” methods, allowing adoption of the technology by not only interested businesses but by individuals in the growing biodiesel community both in the developed and developing world. Using a novel one-pot reactor, functionalized carbon will be synthesized in minutes using only sucrose and sulfuric acid. In this Phase I project, characterization will include the use of standard surface chemical techniques, and the conversion of oleic acid, a major fatty acid in soybean oil, to its corresponding methyl and ethyl ester will be evaluated at moderate temperatures using a micro-scale reactor. Fatty acid esterification will be monitored using an infrared spectroscopic technique requiring only micro-liter samples amounts. The catalytic activity will be optimized by varying the reaction conditions and post-reaction thermal treatments
Description:
Carbon Catalyst Synthesis - Sucrose was treated directly with excess sulfuric acid sulfuric acid (9:1 mol/mol, 25°C). A carbon foam (nearly 20 fold increase in bulk volume) was immediately formed. The foam was then washed until no sulfate was detected, dried, and sieved to varying particle sizes. Carbon made at room temperature is designated SCRT. Thermal treatment of the carbon was conducted for some samples under nitrogen at 155°C, 205°C, and 255°C and are designated as SC155, SC205, and SC255.
Figure 1. Granular carbon solid acid catalyst.
Catalyst Characterization
Scanning Electron Microscopy (SEM) - Samples were analyzed by SEM to characterize the surface topology. Figure 2 shows a SEM micrograph of the carbon surface at a low magnification illustrating a relatively smooth surface structure. The image insert is at a much higher magnification and clearly shows the presence of a sponge-like surface structure. Since transport of reactants and products in and out of the catalyst is key to its activity, fine tuning the pore structure is of utmost importance. Surface area analysis was performed and shows a very low surface area of less than 1 m2 /gram.
Figure 2. SEM micrograph of carbon at low
(5000x) and high (insert; 500,000x) magnification.
Determination of Strong and Weak Acid Sites – For a solid to be a promising candidate to replace sulfuric acid for the esterification of free fatty acids, it must have a significant number of strong acid catalytic sites (sulfonic acid groups). Also of interest is the number of weak acids that may alter the chemical environment near the active surface sites, which could possibly improve the catalytic activity in the presence of water. The number of strong acid sites in our carbon catalyst was evaluated through the determination of ion-exchange capacity (IEC) by measuring the amount of H+ exchanged with Na+. Carbon samples were equilibrated with 2.0 M NaCl overnight and then titrated with 0.01 M NaOH. To determine the presence of both strong and weak acid sites, samples were treated with a strong base solution to react with all sites, and then back titrated with a standard 0.1 M HCl solution. A large number of weak acid sites were found ranging from 6-7 mmol/g. The number of strong acid sites was found to range from 0.8-1.2 mmol/g which compares favorably to commercial solid acid catalysts.
X-Ray Photoelectron Spectroscopy - To determine the chemical structure of the strong and weak surface acid sites, samples were analyzed using x-ray Photoelectron Spectroscopy (XPS). XPS is a high vacuum technique in which a surface is bombarded with x-rays causing electrons at the surface to be ejected (Figure 3). Electron energy is then analyzed and related to the surface atomic composition.
Figure 3. X-ray photoelectron spectroscopy
Results indicated that sulfonic acid groups were present as indicated by the sulfur photopeak at 168 eV (Figure 4). Other surface functional groups, including carboxylic acid groups, were also detected by examination of the carbon photopeak. The analysis revealed high surface concentrations of carboxylic acid groups (weak acids) and very low concentrations of sulfonic acid groups (strong acids) which is in agreement with IEC analysis.
Figure 4. Typical XPS analysis showing carbon and sulfur
surface functional groups.
Figure 5. Typical TGA results showing percent mass loss
versus temperature (left axis) and the rate of mass loss
(right axis).
Thermal Stability – For maximum effectiveness, catalysts should have the capacity to operate under a wide range of temperature conditions. Amberlyst, a commercially used solid acid catalyst, and other polymer based catalyst generally cannot be used at elevated temperatures (>120°C) due to degradation. Thermogravimetric analysis (TGA) was therefore used to evaluate the thermal stability of our carbon catalyst. As can be seen in Figure 5, the onset of degradation occurred at temperatures in excess of 250°C showing the potential of our catalyst to operate at much higher temperatures than found for many other polymer based catalysts. IEC, XPS, and TGA results for all carbon samples are summarized in Table 1 below:
1Results reported as meq/g 2Mbaraka et al. J. Catal. (219) 2003 329
Computational Chemistry - Currently, finding the best performance for a catalytic material involves the time consuming practice of screening large numbers of candidate materials. With the computational tools now available to chemists, predicting catalytic activity by modeling the catalytic reaction could reduce wasted time and material and is a key component in the drive to sustainable chemistry. In an attempt to better understand catalyst structure and function, the theoretical sub-group of our P3 team began work on modeling the structure for our carbon catalyst. This was done by optimizing a hypothetical geometry using a Gaussian 03 and GaussView03 software packages. The carbon structure was modeled on a geometry for carbon found in the literature with added SO3H groups that serve as the catalytically functional group.4, 5 Carboxylic acid functionality was also included as suggested by analysis of the XPS results. While still in the preliminary stages of development, the theoretical infrared spectra generated did allow us to better understand the absorption frequencies expected for this material.
Figure 6. Suggested carbon catalyst
structure
Catalyst Evaluation for Chemical Activity - Catalyst activity was evaluated using the esterification of oleic acid with methanol. The reaction scheme is shown in Figure 7.
Figure 7. Esterification reaction of oleic acid with methanol in the presence of a catalyst to
produce the methyl ester (biodiesel) and water.
A multi-channel micro-reactor was built using a custom-made heating block mounted to a horizontal orbital shaker. Vials were filled with methanol and oleic acid (10:1 molar ratio, 4 mL total volume) and allowed to equilibrate at 65°C before the reaction was initiated by adding 0.1g of the carbon catalyst. Samples (25 μL) were removed from the vials at varying times for 24 hours. The samples were then purged with nitrogen (65°C) to remove residual methanol from the sample. Samples (5μL) were tested for oleic acid and methyl ester content using attenuated total reflectance infrared spectroscopy (ATR). This novel technique requires very little sample for analysis and cleanup between samples is minimal. The analysis time is approximately one minute and much faster than traditional gas chromatography methods.
Figure 8. Nine channel micro-reactor
Two absorbance bands (1710 cm-1 and 1742 cm-1) were used to monitor the progress of the esterification reaction and typical ATR results are shown in Figure 9. The peak at 1710 cm-1 represents the carbonyl stretching frequency for oleic acid, and the peak at 1742 cm-1 is due to the stretching frequency for the fatty acid methyl ester. Fig. 9 shows ATR results for early, middle, and late reaction times and shows the shift from acid to methyl ester. Results showed that the technique was very effective for monitoring the reaction kinetics.
Figure 9. ATR results showing the conversion of fatty acid to
fatty acid methyl ester.
Carbon catalysts were evaluated and compared to commercial polymer based catalysts (Amberlyst and Nafion), and the results are shown in Figure 10. The carbon post-treated at 155°C performed better than all other treatments and clearly outperformed Amberlyst and Nafion. The 255°C carbon was found to have significantly lower catalytic activity than other carbon samples tested, possibly due to degradation which is in agreement with the TGA results at these temperatures.
Figure 10. Percent conversion versus time for the kinetic studies of
the carbon catalysts and commercial solid acid catalysts.
The data was fit to a pseudo-homogeneous kinetic model which is often used for esterification reactions catalyzed by polymer based catalysts. The initial rate of the reaction was determined and is shown in Fig. 10.
Removal of Fatty Acids Using a Model Waste Oil System - The primary goal of the Phase I project was to investigate the use of our solid acid carbon catalyst for efficiency in esterification reactions and particularly for the removal of free-fatty acids (FFA) from waste vegetable oil. A simulated waste oil system was designed using soy based vegetable oil (ADM) and oleic acid (at 15 wt. %). Tests were conducted using excess methanol at 65°C with a carbon loading of 12% (155-SC) and the total reaction volume was 2 - 4 milliliters. For analysis, samples were purged with nitrogen to remove methanol and titrated using standard methods to determine the fatty acid content. The experiment was repeated using Amberlyst and Nafion. Figure 11 shows a plot of free fatty acid content as a function of time for both the carbon and commercial catalysts. Results show that the carbon catalyst is very efficient in removing free fatty acids through esterification and this removal can occur at moderate temperatures and ambient pressure. Results also show that the carbon performs more effectively than the common petroleum-based commercial catalysts investigated (Amberlyst and Nafion). This is quite impressive considering the lower cost and more sustainable method of producing a catalyst made from a completely renewable resource.
Figure 11. Percent fatty acid removed comparing the carbon
catalyst with commercial polymer based catalysts.
Integration of P3 Concepts as an Educational Tool
Dissemination - A key component of our team’s efforts has been the dissemination of our findings on sustainable catalyst development to the broader scientific community. With a matching internal grant from Radford University (RU), students were able to begin the P3 research in the spring of 2007. The early success of the project resulted in four presentations being given at national meetings related to this work:
S.R. Hash, C.S. Estes and H. F. Webster “Synthesis and Characterization of a Novel Solid Acid Catalyst for Improved Use of Waste Oil Feedstock for Biodiesel Production”, 11th Green Chemistry & Engineering Conference, Washington D.C., June 2007 (poster)
S.R. Hash, C.S. Estes and H. F. Webster “Synthesis and Characterization of a Novel Solid Acid Catalyst for Biodiesel Production”, 11th Green Chemistry & Engineering Conference, Washington D.C., June 2007 (poster; winner of a $1500 travel grant for best poster)
S.R. Hash, C.S. Estes and H. F. Webster “Esterification of Fatty Acids using a Novel Solid Acid Carbon Catalyst”, 11th Green Chemistry & Engineering Conference, Washington D.C., June 2007 (oral)
S.R. Hash, C.S. Estes and H. F. Webster “Synthesis and characterization of a novel solid acid catalyst for improved biodiesel production”, 234th National ACS Meeting, Boston, August, 2007 (oral)
In addition to dissemination of our research to the scientific community, we have advocated the principles of green chemistry and sustainability in both the local community and in the classroom. This research, in conjunction with the Center for Environmental Studies at RU was presented at the Clean Valley Summit in Roanoke, Virginia on November 2, 2007. A short presentation was delivered to high school students on the importance of utilizing an environmentally responsible scientific approach to today’s energy needs. Also in the Fall of 2007, a group of children (ages 10-13) were invited to RU for a biofuel workshop to learn about the importance of alternative fuels and were given a brief overview of what biodiesel is and how it is made. They then made biodiesel in the laboratory with the help of the P3 team. When finished, they poured their product right into the tank of a diesel car.
Figure 12. Workshop students making
biodiesel
Integrated Laboratory – As part of the curriculum for chemistry majors, our Department requires students to complete an integrated lab course (CHEM403:404) that combines aspects of the traditional chemistry disciplines (organic, analytical, physical, and inorganic) in a research-like environment. In the Fall of 2007, two projects incorporating the principles of green chemistry and specifically the P3 project results were included in the course. In the first project, students investigated the possibility of using the carbon catalyst synthesized in this study to remove copper from solution. In a second project, students investigated the use of heterogeneous catalysts including our carbon for the transesterification of methyl acetate with butanol to produce butyl acetate, an important industrial solvent. While investigating real research problems is an important part of this course, students were also introduced to the concepts of heterogeneous versus homogeneous catalysis, green chemistry, and the role of chemistry in solving many of the barriers to sustainability.