Here we present a potentiometric titration technique for accurately quantifying carbonyl compounds in pyrolysis bio-oils.
Carbonyl compounds present in bio-oils are known to be responsible for bio-oil property changes upon storage and during upgrading. Specifically, carbonyls cause an increase in viscosity (often referred to as 'aging') during storage of bio-oils. As such, carbonyl content has previously been used as a method of tracking bio-oil aging and condensation reactions with less variability than viscosity measurements. Additionally, carbonyls are also responsible for coke formation in bio-oil upgrading processes. Given the importance of carbonyls in bio-oils, accurate analytical methods for their quantification are very important for the bio-oil community. Potentiometric titration methods based on carbonyl oximation have long been used for the determination of carbonyl content in pyrolysis bio-oils. Here, we present a modification of the traditional carbonyl oximation procedures that results in less reaction time, smaller sample size, higher precision, and more accurate carbonyl determinations. While traditional carbonyl oximation methods occur at room temperature, the Faix method presented here occurs at an elevated temperature of 80 °C.
While pyrolysis bio-oils are comprised of a large variety of compounds and chemical functional groups, quantification of carbonyl groups is especially important. Carbonyls are known to be responsible for the instability of bio-oil during both storage1 and processing2. The titration method presented here is a simple technique which can reliably quantify the total carbonyl content of bio-oils. Only aldehyde and ketone functional groups are quantified using this method; carboxylic acid and lactone groups are not quantified.
For analysis of bio-oils, quantification of carbonyl groups by titration has traditionally been accomplished using the method of Nicolaides3. This method has been commonly used in the bio-oil literature4,5,6,7. This is a simple procedure where carbonyls are converted to the corresponding oxime (see Figure 1). The liberated HCl reacts with pyridine to force the equilibrium to completion. The conjugate acid of pyridine is titrated with a known amount of NaOH (base titrant). The number of equivalents of NaOH used is stoichiometrically equivalent to the moles of carbonyl present in the bio-oil.
The Nicolaides method, however, has several limitations. It can require reaction times in excess of 48 hours to reach completion. This severely limits sample throughput. It utilizes pyridine, which is toxic. Sample weights of 1 to 2 g are required. Sample weight used is dependent on the amount of hydroxylamine HCl present and the carbonyl content of the sample. If initial estimates of the sample weight used are incorrect, the titration has to be repeated.
Faix et al.8 developed a method that has been modified here to address the issues of the Nicolaides method. The reaction is carried out at 80 °C for 2 hours, thereby increasing sample throughput. Pyridine has been replaced with triethanolamine, which is a less toxic chemical. The sample size can be reduced to 100 to 150 mg. The triethanolamine consumes the liberated HCl, driving the reaction to completion and the unconsumed triethanolamine is titrated directly. A secondary titration of the hydroxylamine is unnecessary. Comparison of these titration methods has shown that the Nicolaides method significantly underestimates carbonyl content of bio-oils9.
The method described here has been modified from the original method8 to be more applicable to the analysis of pyrolysis bio-oils. This method was developed for the analysis of raw pyrolysis bio-oils, but it has been successfully applied to other types of biomass-derived oils, including hydrotreated bio-oils. Additionally, this method has been used to monitor changes in carbonyl content during both aging and upgrading.
Caution: Please review all relevant material safety data sheets (MSDS) before beginning. Ethanol is flammable. All applicable chemical handling procedures should be followed, as well as all applicable waste disposable and handling procedures.
1. Reagent Solutions
2. Bio-oil Sampling and Handling
3. Analytical Procedure
4. Calculations
A typical titration curve consists of a single endpoint, as shown in Figure 2. Typical titrations for both a raw bio-oil sample, and a blank titration, are shown. As the endpoint lies at the inflection point in the titration curve; the endpoint can be easily identified by plotting the first derivative of the titration curve (shown on the right axis, dpH/dV, in Figure 2). Many automatic titration systems have software that calculates the derivative of the titration curve, which is sometimes referred to as endpoint recognition criteria (ERC).
Figure 1: Oximation Reaction. Reaction schematic showing the conversion of a carbonyl compound to the corresponding oxime.
Figure 2: Sample and Blank Titration Curves. Representative titration curves for a raw pyrolysis bio-oil, and a blank titration. First derivatives of the titration curves, dpH/dV, are also shown. Please click here to view a larger version of this figure.
Representative titration curves are shown in Figure 2. A blank titration, as well as a titration for a pyrolysis oil sample, are shown. Furthermore, the first derivative of the titration curve (dpH/dV) is shown, which allows for easy recognition of the titration endpoint. The inset table on Figure 2 shows triplicate data for both pyrolysis oil and blank titrations, with average values and standard deviations. The endpoint values shown (in mL) are used in Section 4 to calculate the total carbonyl content (in mmol/g) in the pyrolysis oil sample. For the pyrolysis oil titration shown in Figure 2, the bio-oil sample mass was 0.1148 g, the acid concentration was 0.07032 mol/L, the blank endpoint was 13.041 mL, and the pyrolysis oil endpoint was 4.891 mL. This resulted in a carbonyl content of 4.992 mmol/g bio-oil.
During method development, no interferences were seen for ethyl acetate or acetic acid. We have found that the addition of alcohols causes interferences but it is dependent on chain length. The reason is as yet undetermined but may be related to solvent properties of the alcohol. It is worth noting that monosaccharides are measured using this method. The selectivity of this method was tested by using 1-butanol, 1-pentanol, tertiary butanol, 2-propanol, ethyl acetate, acetic acid, xylose and glucose as model compounds, representing alcohol, ester, carboxylic acid and carbohydrates in the bio-oil. This method has been used to reliably analyze at least 20 raw pyrolysis bio-oils, both fresh and aged up to 120 h at 80 °C. Additionally, several hydrotreated pyrolysis bio-oils have been successfully analyzed. The applicability of this method to a wide variety of bio-oil samples, combined with a high level of accuracy and reliability, may lend this method to other applications in the future. For example, carbonyl content may be used to replace viscosity in a bio-oil aging test.
In the original method, Faix et al.8 mixed the triethanolamine and hydroxylamine hydrochloride solutions to make a stock solution. This leads to the formation of triethanolamine•HCl which will result in a measurement error. In this method, titration of the sample should be done immediately following oximation (after 2 hours of stirring). If titration is delayed following oximation, triethanolamine can form triethanolamine•HCl, which will result in an error in the measurement.
An 80% solution of ethanol is not necessary for the transfer of the sample to the titration vessel; only a final volume of 80% ethanol is needed. As a model compound, 4-(benzyloxy)benzaldehyde (4-BBA) has a sufficiently high molecular weight to use approximately the same amount (~ 100 mg) as the bio-oil sample. While we suggest using 4-BBA as the carbonyl model compound, other compounds may be used in place of 4-BBA. Finally, blanks B and C are unnecessary if only organic acids are present in the bio-oil sample.
As discussed in the introduction, the Faix method presented here has many advantages over the traditional carbonyl titration method (the Nicolaides method). Recently, a comparison of Nicolaides and Faix titration methods has shown that the Nicolaides method significantly underestimates carbonyl content of bio-oils9. Additionally, in 2015 an inter-laboratory study was performed on a raw pyrolysis bio-oil, and this study included both Faix and Nicolaides titration methods10. It was found that the Faix carbonyl method presented here was especially reliable, with <5% relative standard deviation (RSD) variability between all participating laboratories. In comparison, the Nicolaides method showed an inter-laboratory variability of ~10% RSD. Finally, while this method was developed for the analysis of raw pyrolysis bio-oils, it has also been successfully used for carbonyl quantification in upgraded pyrolysis bio-oils.
The authors have nothing to disclose.
This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory. Funding provided by U.S. DOE Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.
Analytical balance | accurate to 0.1 mg | ||
dry block heater with magnetic stirrer, or hot water bath with magnetic stirrer | |||
Automatic titrator | We used a Metrohm Titrando 809 automatic titrator, though other equivalent systems are acceptable | ||
Deionized water | |||
Ethanol (reagent grade) | CAS # 64-17-5 | ||
Hydroxylamine hydrochloride | CAS # 5470-11-1 | ||
Triethanolamine | CAS #102-71-6 | ||
Hydrochloric acid (37%) | CAS # 7647-01-0 | ||
Sodium Carbonate (primary standard) | SigmaAldrich | 223484 | |
4-(benzyloxy)benzaldehyde | CAS # 4397-53-9 | ||
Dimethyl sulfoxide | CAS # 67-68-5 | ||
5 mL glass Reacti-vials with solid lid and teflon spinvane | Thermoscientific | TS-13223 | |
200 mL volumetric flask | |||
Volumetric or mechanical pipettes |