Determination of Carbonyl Functional Groups in Bio-oils by Potentiometric Titration: The Faix Method
Summary
Here we present a potentiometric titration technique for accurately quantifying carbonyl compounds in pyrolysis bio-oils.
Abstract
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.
Introduction
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.
Protocol
Representative Results
Discussion
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 …
Divulgations
The authors have nothing to disclose.
Acknowledgements
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.
Materials
| 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 |
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