Quantitative carbon distribution analysis of hydrocarbons, alcohols and carboxylic acids in a Fischer-Tropsch product from a Co/TiO2 catalyst during gas phase pilot plant operation
Journal of Analytical Science and Technology volume 11, Article number: 42 (2020)
Comprehensive two-dimensional gas chromatography (GCxGC) analysis for 1-alcohols and gas chromatography–mass spectrometry (GC-MS) analysis for carboxylic acids, derivatised as their methyl esters, have been applied to liquid and wax Fischer-Tropsch (FT) hydrocarbon products. These methods in combination with conventional one-dimensional gas chromatography (GC) analysis of the aqueous, gaseous, liquid hydrocarbon and wax products plus conventional high-performance liquid chromatography (HPLC) analysis of the aqueous phase has allowed a quantitative distribution analysis of FT hydrocarbon and oxygenated products to be demonstrated for a Co/TiO2 catalyst operating in a fixed bed gas phase pilot plant utilising CANSTM catalyst carrier devices. The GC-MS method used is, to the best of our knowledge, the first application of this derivatisation route for the quantification of individual carboxylic acids in FT hydrocarbon product streams.
Whilst the hydrocarbons and oxygenates that were identified are known compounds formed during the low temperature, Co catalysed, FT process the combination of the multiple analysis techniques used has allowed a level of detail to be gained on the product composition that is seldom reported.
Additionally, 1H nuclear magnetic resonance spectroscopy (NMR) and 13C NMR analyses were used to quantify the average concentration of 1-olefin, cis- and trans-2-olefins, 1-alcohol and aldehyde as appropriate for the technique used. Comparison of GCxGC versus 1H NMR and GC-MS versus a KOH titration confirmed the applicability of the chromatographic methods for the quantitative analysis of FT oxygenated compounds. Long-chain 1-alcohols and carboxylic acids, ≥ C3, were found to be present at levels of 1/10th and 1/1000th that of hydrocarbons of equivalent carbon chain length respectively. The 1-olefin:n-paraffin ratio in the hydrocarbon liquid and wax products was found to decrease significantly with increasing carbon chain length and much more so than those of the 2-olefin or 1-alcohol.
Fischer-Tropsch (FT) synthesis is a well-known and much studied catalytic process for converting synthesis gas to hydrocarbon and oxygenate products having linear carbon chains ranging from C1 to over C100 (Day 2002; Khodakov et al. 2007). However, improvements to process design, catalyst efficiency and analytical methods for the detailed analysis of products are continuously being made.
BP and Johnson Matthey (JM) have been collaborating over many years to commercialise a propriety FT synthesis technology (Coe and Paterson 2019; Peacock et al. 2020). A tubular reactor demonstration plant was commissioned in Alaska in 2002 (Collins et al. 2006) using a Co/ZnO catalyst. This facility is the largest FT plant built in the US, producing 300 bbl/day of synthetic crude product from pipeline natural gas feedstock. When the plant was decommissioned, in 2009, it had exceeded all its performance goals related to catalyst productivity, hydrocarbon selectivity, carbon monoxide conversion, methane selectivity and catalyst lifetime. The original fixed bed tubular reactor technology was developed as a method of monetising stranded natural gas in remote locations but was typically only competitive at large scale (> 30,000 bbl/day) in areas with low natural gas prices and high oil prices.
Recent innovations in the BP/JM FT technology offers both small- and large-scale operations with good economics. In 2009, JM designed a novel catalyst carrier device (CANS™ carrier) inside a tubular reactor (Gamblin 2014), shown in Fig. 1, that allows for the use of small catalyst particles and, at the same time, BP developed a new second-generation catalyst. The combination of an improved catalyst and CANS™ reactor design produced a step change in FT performance achieving the advantages of both fixed-bed tubular reactors and slurry phase systems (Coe and Paterson 2019; Peacock et al. 2020). This step improvement in FT technology gives three times the catalyst productivity and halves the capital expenditure when compared with conventional multi-tubular fixed bed FT reactors. This simple to operate and cost advantaged technology can be utilised to economically convert synthesis gas, generated from sources such as municipal solid waste (MSW) and other renewable biomass, into long-chain hydrocarbons suitable to produce base oils, speciality waxes, diesel, and jet fuels. Fulcrum BioEnergy will use the technology in their new Sierra BioFuels Plant located in Storey County, Nevada. The Sierra plant will be the first commercial scale plant in the US to convert MSW feedstock, or household garbage that would otherwise be landfilled, into a low carbon, renewable transportation fuel (JM press release 2018).
Catalysts are required for the FT process to increase the rate of reaction and make the process industrially viable. The catalysts usually employed are based on Co or Fe, usually with modifiers added to optimise performance, and in the case of Co can be supported on oxides such as SiO2, Al2O3, TiO2 or on carbonaceous material (see review articles and references within by; Oukaci et al. 1999; Fu and Li, 2015; Gholami et al. 2020). Typically, Co catalysts operate at temperatures between 200 and 240 °C and pressures between 2 and 4 MPa, whereas Fe-based processes often use temperatures in excess of 300 °C and utilise the water-gas-shift reaction to control the syngas feed composition.
The products of the FT reaction are complex (Shafer et al. 2019), more so from Fe-based catalysts, with paraffinic, olefinic, alcohol and carboxylic compounds being observed. Additionally, aldehydes, ketones and ester can also be formed. There are numerous papers describing methods for analysis of the gas phase (Bertoncini et al. 2009; Grobler et al. 2009; Seomoon et al. 2013; Pei et al. 2015), aqueous phase (Anderson and White, 1994; Grobler et al. 2009; Pei et al. 2015; Ma et al. 2019), liquid hydrocarbons and wax products (Bertoncini et al. 2009; Grobler et al. 2009; van der Westhuizen et al. 2010; Silva et al. 2011; Fernandes et al. 2015; Pei et al. 2015; Shafer et al. 2019) but in general the quantitative distribution analysis of alcohols and carboxylic acids versus carbon chain length is difficult to achieve when these compounds are present at the low levels normally expected in a cobalt-catalysed FT product. This can be due to (i) the sensitivity of analytical methods at low oxygenate concentrations, (ii) the challenge of keeping high boiling point compounds in the vapour phase and then chromatographically separating a complex mixture of components when using on-line gas chromatography (GC) and (iii) the difficulty in obtaining an acceptable mass balance of all product phases when combining on-line and off-line analyses (Xiao et al. 2017; Yang et al. 2017). Some success in quantifying C1 to C18 alcohols, formed over a silica-promoted Co catalyst designed to produce a high level of FT oxygenates, has been shown by combining data from gaseous, aqueous and liquid hydrocarbon samples when using simple on-line and off-line GC analyses, but no attempt was made to identify or quantify any carboxylic acids that might also have been present (Pei etc. 2015).
Furthermore, for a truly representative analysis across the full carbon range, the sampling of the FT products needs to be conducted only once the catalyst has been operating in a stable manner for a suitably long period of time and allowance has been made for effective purging of product traps and analysis lines. Achieving truly stable FT catalyst performance can take hundreds of hours before representative product samples are available. These requirements can be easily accommodated during pilot plant operation, such as described in our work, but may not be possible for many academic and small-scale micro-reactor studies.
Comprehensive two-dimensional chromatography (GCxGC) coupled with time-of-flight mass spectrometry has been used in many cases for the identification of trace components in FT products and sometimes quantification has also been given. Such examples show (i) the analysis of products from Co/Al2O3 operated under slurry conditions demonstrating an Anderson-Schulz-Flory (ASF) plot of carbon distribution over the C1 to C20 range for paraffins, olefins and oxygenates (Bertoncini et al. 2009); (ii) the quantification of 1-alcohols in liquid hydrocarbon from a Co/Al2O3 catalyst (Silva et al. 2011); (iii) the quantification of paraffins, olefins, aromatics and oxygenates in samples of oil made during high-temperature FT experiments (van der Westhuizen et al. 2010); (iv) the quantification of paraffins, olefins, alcohols, aldehydes and carboxylic acids up to C15 produced from a precipitated Fe catalyst in slurry operation (Grobler et al. 2009); and (v) the simultaneous identification and quantification of alcohols and carboxylic acids, as their trimethylsilyl derivatives, in FT syn-crude and distilled products (Fernandes et al. 2015). The use of supercritical fluid chromatography coupled with two-dimensional gas chromatography–mass spectrometry for the separation and identification of saturated, unsaturated, aromatic and oxygenate species in hydrocarbon samples derived from a high-temperature FT reaction has also been demonstrated (Potgieter et al. 2013), clearly indicating the diversity of class and number of organic compounds present in such products.
In this paper, we show the application of comprehensive two-dimensional gas chromatography (GCxGC) for the quantification of alcohols and gas chromatography-mass spectrometry (GC-MS) for the quantification of carboxylic acids, derivatised as their methyl esters, in liquid and wax hydrocarbon FT products. The latter method is, to the best of our knowledge, the first demonstrated application of this derivatisation method for the analysis and quantification of individual carboxylic acids in FT hydrocarbon product streams. GC-MS analysis, without the derivatisation step, has been reported for the identification of strongly adsorbed carboxylic acids present on a used Co/Al2O3 catalyst after FT operation (Peña et al. 2014). In that study, the used catalyst was first cleaned of paraffinic wax by a Soxhlet procedure and then the carboxylic acids, and other strongly adsorbed species, were extracted into a CH2Cl2:CH3OH solvent mixture for subsequent identification.
In our study, GCxGC and GC-MS methods in combination with conventional one-dimensional gas chromatography (1D-GC) analysis of the aqueous, gas, liquid hydrocarbon and wax products plus conventional high-performance liquid chromatography (HPLC) analysis of the aqueous phase has allowed a quantitative product distribution, from C1 to C100, to be demonstrated for a Co/TiO2 catalyst operating in a fixed bed gas phase pilot plant utilising the novel CANSTM carrier and reactor. Whilst the hydrocarbons and oxygenates that were identified are known compounds formed during the low temperature, Co catalysed, FT process the combination of the multiple analysis techniques used has allowed a level of detail to be gained on the product composition that is seldom reported.
Additionally, 1H NMR analysis was used to quantify the average concentration of 1-olefin, internal olefin, 1-alcohol and aldehyde compounds in the liquid hydrocarbon and wax products whilst 13C NMR and 13C distortionless enhancement by polarisation transfer (DEPT135) NMR were used to confirm the location of unsaturation of the internal olefins, and the ratio of cis and trans isomers. Comparison of GCxGC versus 1H NMR and GC-MS versus a KOH titration was also conducted in order to confirm the applicability of the chromatographic methods for quantitative analysis of FT oxygenated compounds.
Materials and methods
Production of Fischer-Tropsch products
A cobalt-based FT catalyst, supplied by Johnson Matthey PLC, with the active phase supported on TiO2 was loaded to several CANSTM carriers placed in series in a CANSTM reactor pilot plant. A schematic diagram of the pilot plant is shown in Fig. 2. After reduction using H2, syngas was added to the reactor and the temperature was ramped to achieve target CO conversion. Wax, light hydrocarbon liquid and aqueous phases were collected post-reactor under appropriate conditions for subsequent off-line analysis. The exit gas containing unreacted syngas and C1 to C6 hydrocarbons were recycled over the catalyst bed allowing any volatile olefins present the opportunity to further react to alkanes and oxygenates. A purge stream prevented inert compounds from building up in the recycle.
For a period of 5 h, at 1630 h on stream, the mass flow of the liquid and solid phases collected per unit time and the flow rate of the product gas were recorded to allow the analytical data from the gas, aqueous, liquid hydrocarbon and wax hydrocarbon phases to be combined to give a detailed view of the FT product distribution. The aqueous, liquid hydrocarbon and solid wax samples collected during this time are referenced as Aq1, LHcL1 and Wax1. The catalyst had good stability and at this time on stream the CH4 selectivity was 7.9% and C5+ selectivity was 85.0%.
Gas phase analysis of hydrocarbons
The gas phase was analysed by on-line gas chromatography using a PGC5000 GC consisting of 12 packed columns, one single-measurement thermal conductivity detector (TCD), one dual-measurement TCD, 6 air actuated solenoid valves to switch between columns and 4 carrier gas streams—one nitrogen, three hydrogen operating with an isothermal oven temperature of 100 °C. This allowed the complete analysis of CO, H2, N2, CO2 and C1 to C8 hydrocarbons.
Aqueous phase—analysis of 1-alcohols
The aqueous sample (Aq1) was analysed for C1–C10 linear alcohols by gas chromatography using a flame ionisation detector (FID). A model 7890A GC and G4513A automatic liquid sampler from Agilent Technologies was employed. The analysis was achieved using a CP-Wax52CB capillary column 60 m in length with an internal diameter of 0.25 mm and a stationary phase thickness of 0.25 μm supplied by Agilent Technologies. It was operated with an initial temperature of 40 °C held for 6 min then increased at 10 °C/min to 200 °C. Helium carrier gas at a flow of 1.5 mL/min was used. A sample volume of 0.2 μL was injected into a split injector at 200 °C with a split ratio of 30:1. Detection was by flame ionisation detector at 250 °C. Calibration was performed with standards prepared volumetrically using linear alcohols with purity > 98% supplied by Sigma-Aldrich. Dilutions were made with laboratory 18 MΩ water. An internal standard of propan-2-ol was added for quantification.
Aqueous phase—analysis of carboxylic acids
The aqueous sample (Aq1) was analysed for C1–C4 linear carboxylic acids by liquid chromatography using an ultraviolet (UV) detector. A model 1100 quaternary LC from Agilent Technologies was employed. Separation was achieved using a 300 mm Aminex 87H column with a diameter of 7.8 mm supplied by Bio-Rad Laboratories. The column was operated under isocratic conditions at ambient temperature with an aqueous mobile phase of 0.005 M sulfuric acid at a flow rate of 0.6 mL/min. A 25 μL injection volume was used together with UV detection at 210 nm and a bandwidth of 8 nm. Quantification was by external standard with response factors determined by calibration using standards prepared volumetrically in laboratory quality 18 MΩ water. Carboxylic acids were supplied by Sigma-Aldrich.
Light hydrocarbon liquid—analysis of hydrocarbons
The light hydrocarbon liquid sample (LHcL1) was analysed for a carbon distribution in the range C5–C50 by gas chromatography with flame ionisation detection. A model 6890 GC from Agilent Technologies was employed. The analysis was achieved using a VF-5HT metal capillary column 30 m in length with an internal diameter of 0.25 mm and a stationary phase thickness of 0.1 μm supplied by Agilent Technologies. It was operated with an initial temperature of 50 °C held for 4 min then increased at 10 °C/min to 430 °C and held for 20 min. Helium carrier gas at a flow of 2.3 mL/min was used. A sample volume of 0.1 μL was injected into a PTV injector at 400 °C with a split ratio of 120:1. Detection was by flame ionisation detector at 400 °C. Quantification was by an area normalisation method assuming an equivalent mass response for all components. Identifications were established by analysing Agilent Technologies reference mixtures GC Boiling Point Calibration Standard for SimDis #2 (5080-8678) and Boiling Point Calibration Standard #1 for simulated distillation (5080-8716).
Light hydrocarbon liquid—analysis of 1-alcohols
The light hydrocarbon liquid sample (LHcL1) was analysed for 1-alcohol distribution in the range C3–C17 by comprehensive two-dimensional gas chromatography (GCxGC) with flame ionisation detection. A model 7890A GC from Agilent Technologies equipped with an Agilent capillary flow technology flow modulator was employed. The analysis was achieved using a 30-m-long HP-5 capillary column in the first dimension (1D) with a stationary phase film thickness of 0.25 μm; the second dimension (2D) column was a 5-m-long HP-Innowax column with a phase thickness of 0.15 μm; both columns had an internal diameter of 0.25 mm and supplied by Agilent Technologies. The columns were operated with an initial temperature of 50 °C and increased at 5 °C/min to a final temperature of 240 °C where they were held for 20 min. Helium was used as the carrier gas with the first dimension and second dimension column flows set to 0.6 mL/min and 26.0 mL/min respectively. A modulation period and sampling time of 1.58 s and 1.45 s respectively was used. A sample volume of 0.3 μL was injected into a split injector at 240 °C with a split ratio of 50:1. The flame ionisation detector was operated at 250 °C. Calibration was performed with standards prepared volumetrically in cyclohexane using linear alcohols with purity > 98% supplied by Sigma-Aldrich. An internal standard of 2-methylbutan-1-ol was added for quantification.
Light hydrocarbon liquid and wax—analysis of carboxylic acids
The method used was based on the esterification of the organic acids to the methyl ester and measurement by GC or GC-MS (Martínez et al. 2012). Approximately 0.1 g of homogenised sample was accurately weighed (3 decimal places) in the bottom of a suitable culture tube and then 200 μL of an internal standard solution containing 9.3 μg/mL of 2-methylpentanoic acid in toluene was added as internal standard. This was followed by the methylating reagent (1 mL), prepared daily by carefully adding 2 mL of concentrated sulfuric acid to 18 mL of cooled, dry methanol. The culture tube was capped and heated to 80 °C for 60 min. After cooling to room temperature, 600 μL of heptane followed by 1.0 mL of 1 M aqueous sodium chloride solution was added. The tubes were capped and vortexed for 2 × 6 s periods. After standing to allow the layers to separate 200 μL of the top organic layer was removed, and further diluted with heptane if required. All reagents were supplied by Sigma-Aldrich. The prepared sample was analysed on a 7890A GC from Agilent Technologies with an Almsco BenchToF time of flight mass spectrometer supplied by Markes International. The analysis was achieved using an HP-5 capillary column 30 m in length with an internal diameter of 0.25 mm and a stationary phase with a thickness of 0.25 μm supplied by Agilent Technologies. It was operated with an initial temperature of 50 °C increasing at 8 °C/min to 320 °C and held for 10 min. Helium carrier gas at a flow of 1.5 mL/min was used. A sample volume of 1.0 μL was injected into a multi-mode inlet (MMI) injector at 200 °C with a split ratio of 30:1. Detection was by mass spectrometry with the electron ionisation source operated at 200 °C and an applied energy of − 70 eV. The transfer line temperature was 250 °C. Data was acquired at 8 spectra/s between m/z 34 to m/z 400. The methyl esters were quantified against the internal standard using the extracted ion chromatograms of m/z 74 and m/z 88 respectively. GC-MS response for the methyl esters was established by dilution in cyclohexane of a certified 1000 μg/mL C4 to C24 fatty acid methyl ester (FAME) standard and addition of methyl 2-methylpentanoate as internal standard. All materials were supplied by Sigma-Aldrich. The relative standard deviation (RSD) established for analyte response at a concentration of 10.0 μg/mL over 10 analyses was determined for the methyl esters of C6–C24 carboxylic acids (Additional file 1: Table S1). Finally, the linear response of selected FAMEs under the conditions used in the GC-MS analysis were validated with methyl 2-methylpentanoate, methyl hexanoate, methyl octadecanoate and methyl tetracosanoate being used for this purpose (Additional file 1: Figure S1).
Wax—analysis of hydrocarbons
The wax product (Wax1) was analysed for a carbon distribution in the range C8–C96 by gas chromatography with flame ionisation detection. A model 6890 GC from Agilent Technologies was employed. The analysis was achieved using an MXT-1HT metal capillary column, 5 m in length with an internal diameter of 0.53 mm and a stationary phase thickness of 0.2 μm supplied by Restek. It was operated with an initial temperature of 40 °C then increased at 10 °C/min to 420 °C. Helium carrier gas at a flow of 10 mL/min was used. A sample volume of 0.5 μL was injected into an on-column injector at 3 °C above the column oven temperature. Detection was by flame ionisation detector at 400 °C. Sample preparation involved taking approximately 0.01 g of homogenised sample and dissolving in 10 mL of cyclohexane by sonication and then heating gently in warm water. Data was interpreted after subtracting a thermal cycle blank run from the raw sample data to remove baseline drift caused by column bleed. Quantification was by an area normalisation method assuming an equivalent mass response for all components. Identifications were established by analysing Agilent Technologies reference mixtures GC Boiling Point Calibration Standard for SimDis #2 (5080-8678) and Boiling Point Calibration Standard #1 for simulated distillation (5080-8716).
Wax—analysis of 1-alcohols
The wax product (Wax1) was analysed for 1-alcohol distribution in the range C9–C17 by comprehensive two-dimensional gas chromatography (GCxGC) with flame ionisation detection. A model 7890A GC from Agilent Technologies equipped with an Agilent capillary flow technology flow modulator was employed. The analysis was achieved using a 30-m-long HP-5 capillary column in the first dimension (1D) with a stationary phase film thickness of 0.25 μm; the second dimension (2D) column was a 5-m-long HP-Innowax column with a phase thickness of 0.15 μm; both columns had an internal diameter of 0.25 mm and supplied by Agilent Technologies. The columns were operated with an initial temperature of 50 °C and increased at 5 °C/min to a final temperature of 240 °C where they were held for 20 min. Helium was used as the carrier gas with the first dimension and second dimension column flows set to 0.6 mL/min and 26.0 mL/min respectively. A modulation period and sampling time of 1.58 s and 1.45 s respectively was used. A sample volume of 0.3 μL was injected into a split injector at 240 °C with a split ratio of 50:1. The flame ionisation detector was operated at 250 °C. Calibration was performed with standards prepared volumetrically in cyclohexane using linear alcohols with purity > 98% supplied by Sigma-Aldrich. An internal standard of 2-methylbutan-1-ol was added for quantification.
Light hydrocarbon liquid—analysis of acidity by KOH titration
The method used was based on ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. The light hydrocarbon liquid (LHcL1) sample was miscible with the titration solvent of toluene and propan-2-ol at the sample volume (approximately 20 g) recommended in the ASTM method for low total acid number (TAN) values. This sample was titrated potentiometrically using 0.1 N alcoholic KOH with a Metrohm 848 Titrino autotitrator.
1H NMR—analysis of light hydrocarbon liquid
All NMR data was collected using a Bruker Avance 300 MHz instrument with Bruker Topspin acquisition software. Data processing was completed using Spectrus from ACD Labs. All reagents and consumables were supplied by Sigma-Aldrich. The 1H NMR spectra of a 10% v/v solution of light hydrocarbon liquid (LHcL1) in deuterated chloroform (CDCl3) was collected using a 2-s relaxation delay and 1000 scans.
1H NMR—analysis of wax
The 1H NMR spectra of a 0.1% w/w solution of wax (Wax1) in CDCl3 was collected using a 2-s relaxation delay and 1000 scans.
13C NMR—analysis of light hydrocarbon liquid
Quantitative 13C NMR of the light hydrocarbon liquid (LHcL1) was performed in a similar manner to that detailed by others (Burger et al. 2015). A 50% v/v solution of the hydrocarbon liquid in CDCl3 containing 60 mmol chromium (III) acetylacetonate was prepared and 2000 scans were collected using an inverse gated sequence with a relaxation delay of > 10 s.
13C DEPT 135 NMR—analysis of light hydrocarbon liquid
13C DEPT135 NMR (distortionless enhancement by polarisation transfer) shows signals due to CH3 and CH group as positive signals and those due to CH2 as negative signals. Quaternary carbons do not show any signal. The technique was useful in confirming the origin of signals present in the 13C NMR spectra. The 13C DEPT 135 analysis of the light hydrocarbon liquid (LHcL1) was performed using a 50% v/v solution of the sample in CDCl3. Further, 1000 scans were collected using relaxation delay of 2 s.
Results and discussion
Aqueous phase composition
The main product in the aqueous phase were the 1-alcohols from C1 to C10 with trace levels of methyl acetate, 2-methylpropan-1-ol and ethanoic acid also being observed in the GC chromatogram (Additional file 1: Figure S2). Quantification of the 1-alcohols is given in Table 1.
The chromatogram obtained from the analysis of carboxylic acids by HPLC (Additional file 1: Figure S3) shows the major acids to be methanoic and ethanoic acid with much lower levels of propanoic acid and butanoic acid. Quantifying any carboxylic acids with carbon numbers >C4 was not possible due to the sensitivity of the method and co-elution of other compounds at the longer retention times. The quantification of the acids is shown in Table 2.
Gas phase analysis
The relative concentration of the C1 to C6 gas phase organic hydrocarbon compounds relative to the methane are given in Table 3. Only the data recorded from the FID are shown since the other compounds present in the gas phase do not contribute to the hydrocarbon distribution analysis. The mass flow of each hydrocarbon was calculated from the absolute concentration in the gas phase and the volumetric flow of the gas and these data were used for input to the calculation of overall hydrocarbon product distribution.
Carbon distribution of light hydrocarbon liquid and wax phases
The GC chromatogram of the light hydrocarbon liquid (LHcL1) is shown in Fig. 3 and the insert gives an expanded view of the C11 region of the chromatogram. This shows the order of elution was iso-paraffins, 1-alcohol, 1-olefin, n-paraffin and trans-2-olefin and lastly cis-2-olefin. However, the carbon chain length of the 1-alcohol was not the same as the hydrocarbons in this region but was the Cn-2 alcohol. Achieving acceptable resolution of these species in the analysis of the wax sample (Wax1) was not possible (Additional file 1: Figure S4). The peak capacity of a chromatography separation is finite and co-elution of the 1-alcohol with the iso-paraffins became increasingly evident as carbon chain length increased and the number of potential isomers also increased.
Where it was possible to differentiate the n-paraffin, 1-olefin, 2-olefins and 1-alcohol peaks in the LHcL1 chromatogram, in the C7 to C14 range, a detailed integration of these peaks allowed their ratio to the corresponding n-paraffin to be quantified. Since the alcohol that eluted just before the n-paraffin had two carbons less, then this feature was accounted for when conducting the ratio analysis. The result shows that the 1-olefin decreased relative to the n-paraffin as the carbon number increased (Fig. 4a). This has been found in other studies of FT products (Bertoncini et al. 2009) and is attributed to the increasing likelihood of the 1-olefin being reabsorbed on the catalyst surface where it can lead to initiation of a new propagating alkyl chain before it can diffuse through the wax filled pores of the catalyst particle. This mechanism is not open to n-paraffins since they cannot undergo surface reattachment and participate in chain initiation due to unfavourable thermodynamics (Iglesia 1997). So, paraffins become an end-product of the chain growth process. Further secondary reactions of alcohols and carboxylic acids may be possible (de Klerk 2011) but their participation in any further chain re-initiation chemistry is less clear. Indeed, a 14C-labelled alcohol tracer study using ethanol and 1-propanol co-feeds suggests that the C–O bond of these alcohols is mostly stable on the cobalt surface under FT reaction conditions (Gnanamani et al. 2015). Certainly, in our analyses, the relationships of the 1-alcohol and 2-olefins to the n-paraffin were not the same as for the 1-olefin and these must either have a lower propensity to re-initiate new chains or be more difficult to re-adsorb on the active site than the 1-olefins. The same type of analysis was also conducted for the wax sample but using an analysis method suited for the separation of the light hydrocarbons in the C9 to C15 range, allowing the 1-olefins to be resolved from the n-paraffins. The 1-olefin:n-paraffin ratio in the wax was also found to decrease as the carbon number increased as shown in Fig. 4b.
1-alcohol content of light hydrocarbon liquid and wax phases
The analysis of the light hydrocarbon liquid (LHcL1) and wax (Wax1) materials by comprehensive two-dimensional GC (GCxGC) allowed an effective separation of the 1-alcohols from the hydrocarbon species. The GCxGC chromatogram of LHcL1 is shown in Fig. 5. This resulted in a more accurate quantification of the alcohols across a wider carbon range than could be achieved by the conventional GC method previously described. It was also apparent that the major products were n-paraffins, 1-olefins, 2-olefins and 1-alcohols with trace amounts of iso-paraffins. There were no other products detected in the two-dimensional GC analysis.
A comparison of the 1-alcohol quantified for C7–C14 carbon numbers in the LHcL1 sample by the conventional GC and comprehensive two-dimensional GC analyses showed that within this range, similar data were obtained (Additional file 1: Figure S5). However, the two-dimensional GC method generally gave slightly higher values. The one-dimensional GC method takes no account of the different detector mass response for oxygenated components which would lead to an under recovery for these components, in addition the quantification in the one-dimensional method could be further complicated by the possibility of coelution with iso-paraffins and difficulty in accurately integrating non-baseline resolved peaks.
The range of alcohols analysed by comprehensive two-dimensional chromatography could be extended to higher carbon numbers with an alternative column set capable of higher temperature operation (e.g. HP-5/BPX-50) than we used along with the suitable temperatures, gas flows and modulation conditions required. A severe limitation to the range of chromatography conditions that can be employed in GCxGC is introduced with the use of simple flow modulation, in particular the column dimensions and flow rates used in the second dimension and the modulation period. Cryogenic modulation, whilst usually more capital intensive and often requiring additional services such as liquid cryogens, allows optimisation closer to ideal chromatographic conditions.
Carboxylic acid content of light hydrocarbon liquid and wax phases
The concentration of organic acids in the Fischer-Tropsch hydrocarbon liquids and wax materials studied were typically two orders of magnitude lower than that of the primary alcohols. These would therefore have been present below the limit of quantification for the GCxGC approach used for the analysis of alcohols. Whilst extraction methods could be used to concentrate the acid analytes, a derivatisation method was chosen to provide the additional benefit of improved chromatography for the separation of methyl esters over that which could be achieved for carboxylic acids.
The method applied to the analysis of carboxylic acids was based on that used successfully for the quantification of organic acids in milk (Martinez et al. 2012). Methylation of the carboxylic acids was quantitatively achieved using H2SO4/methanol with subsequent concentration and recovery of the product methyl esters by addition of a small quantity of heptane. GC-MS analysis of the heptane phase allowed for low levels of the methyl esters to be accurately quantified and therefore giving the concentration of the carboxylic acids in the parent samples for both the light hydrocarbon liquid and wax materials.
Methyl esters of linear carboxylic acids characteristically give an intense peak in the electron impact ionisation spectra corresponding to m/z = 74 due to the McLafferty rearrangement ion (Takayama 1995; rearrangement reaction is shown in Additional file 1: Figure S6). In addition to the McLafferty ion, there are a series of related ions formed having the general formula [(CH2)nCOOCH3]+ where n = 2 gives the next most abundant peak at m/z = 87.
The GC-MS chromatogram of a heptane extract showing the total ion current, background subtracted, is given in Fig. 6. This also shows the mass spectra obtained from the C11 methyl ester against that of the reference spectra. The peaks at m/z = 74 and m/z = 87 are clearly visible and were the most intense peaks present. Analysing the chromatogram using the McLafferty rearrangement ion, m/z = 74, allows for a significant increase in the signal/noise improving the detection limit and thereby the quantification of the ester.
The GC-MS method was demonstrated to give a satisfactory analysis of carboxylic acids in FT liquid hydrocarbon and wax products by (i) measuring the linearity of response of selected FAMEs, including that of the internal standard, under the conditions used in the GC-MS analysis; (ii) quantifying the relative standard deviation (RSD) for the methyl esters of C6–C24 carboxylic acids over 10 analyses; (iii) estimating the recovery of hexanoic acid, decanoic acid and octadecanoic acid standards during the derivatization reaction; and (iv) conducting duplicate sample preparations for analysis. The results of these investigations (Additional file 1: Figure S1 and Tables S1, S2 and S3) showed that the method was reproducible, had a linear response to methyl esters of carboxylic acids spanning the C6 to C24 range, showed close to 100% recovery for C6, C10 and C18 carboxylic acids in the derivatization reaction and also gave reproducible analysis for a real FT liquid hydrocarbon product.
Summary of chromatographic analyses of light hydrocarbon liquid and wax samples
The concentration of the hydrocarbons (n-paraffins, iso-paraffins, olefins), 1-alcohols and carboxylic acids in the light hydrocarbon liquid (LHcL1) and wax (Wax1) products are summarised in Tables 4 and 5, along with the analysis method used. The combined concentration of the paraffin and olefin for each carbon number plus the individual concentrations of the 1-olefin and 2-olefin, where chromatographic separation from the n-paraffin and iso-paraffins was possible, are also given. Hydrocarbons with carbon numbers > 30 have been combined, although these were reported separately up to C96.
Full Fischer-Tropsch hydrocarbon and oxygenate product distributions
The products of the FT synthesis reaction are hydrocarbons and oxygenate products having linear carbon chains ranging from C1 to over C100. The balanced equations for the paraffinic, olefinic, alcoholic and carboxylic acid products of the FT process are given in Eqs. 1 to 4.
Many mechanisms have been proposed for the FT reaction and this remains an active area for research and debate (Krylova 2014). A simplified reaction network based on an oxygenate mechanism can be used to show the potential routes to forming all the class of compounds that are found in gas, liquid and wax FT products (de Klerk 2011). This type of mechanism is illustrated in Fig. 7 and shows the initiation by hydrogenation of adsorbed CO to form a surface methylene group, the stepwise growth in the carbon chain length by CO insertion, hydrogenation of intermediate oxygenates to remove oxygen as water and then further CO insertion. Finally, products are liberated from the active site either by desorption (aldehydes and olefins), by addition of water (carboxylic acids) or by addition of hydrogen (alcohols and paraffins). The reactions can, however, be kinetically described by the step-growth polymerisation of C1 surface species where the product distributions can be explained by the ASF growth probability factor, α, (van de Laan and Beenackers, 1999; de Klerk 2011). Usually, this growth probability factor is calculated from the analysis of hydrocarbons and oxygenates with ≥ C3 carbon atoms, since the C1 and C2 products do not adhere to the same growth kinetics. The value of α is obtained as the gradient of a plot of Log (mol. fraction of compound with Cn) versus Cn where Cn is the carbon number of the compound.
The analysis of the gas phase, aqueous phase, light hydrocarbon liquid and wax phases (Tables 1, 2, 3, 4 and 5) along with the volumetric or mass flows per unit time of these phases allows the compilation of the detailed and quantitative ASF distribution for hydrocarbons, 1-alcohols and carboxylic acids. This is shown in Fig. 8 for carbon numbers up to C40. The overall ASF α number for the hydrocarbons between C10 to C40 was 0.92 and there was some loss in recovery of C4 in the gas phase, and some loss of C7 fraction was evident in the light hydrocarbon fraction.
Parallel lines with slopes having the same gradient as the ASF fit for the hydrocarbons have been overlaid on the 1-alcohol and carboxylic acid distributions with these being allowed to fit to their C3 carbon values. This indicates that the probability of chain propagation for 1-alcohols was similar to that of the hydrocarbons for C3+ alcohols and that this was also possibly the case for the carboxylic acids, at least in the C3 to C17 range, although there was a poor recovery of acids between C5 and C10. Since the GC-MS method had already been shown to give close to 100% recovery for hexanoic and decanoic acids in a liquid hydrocarbon phase analysis, then this poor overall carboxylic acid recovery in the C5 to C10 range was suspected to be due to the non-quantification of these acids in the aqueous phase HPLC analysis. This could have been due to the low sensitivity of the HPLC method coupled with the very low concentration of the acids expected in aqueous phase. To confirm this hypothesis, an estimation was made of the concentrations of the C5 to C10 acids that would have been required in the aqueous phase to allow a fit to the expected acid ASF distribution. These data are compared in Table 6 to reference data for the measured solubility of linear carboxylic acids in aqueous media at ambient temperature (Bell 1973). This confirmed that the acids should have had the required solubility in the aqueous phase and that the poor recovery was likely due to the low sensitivity of the HPLC analysis. Whilst not pursued in our work, a method that could be investigated for improving the quantification of high molecular weight carboxylic acids at very low concentrations in the aqueous phase would be to conduct a neutralisation step using NaOH followed by low-temperature evaporation of the aqueous phase, acidification and then extraction into an organic phase possibly as the methyl ester as described here for the carboxylic acid determination in FT wax and light hydrocarbon materials.
The FT product distribution had a higher level of methane than would be expected from an extrapolation of the hydrocarbon C3+ ASF distribution, as usually observed in Co-based FT catalysis (de Klerk 2011). There were also significant levels of methanol, ethanol, formic acid and ethanoic acid which were above the extrapolation of the C3+ ASF distribution for the 1-alcohols and carboxylic acids. These observations were replicated in micro-reactor tests, using the same catalyst type, where aqueous and wax products were collected post-reactor for off-line analysis using the same analytical methods. It is expected that C1 and C2 products may not fit the ASF distribution since they are either independent of the polymerisation kinetics or are heavily influenced by the re-adsorption and subsequent reactions of ethylene (van de Laan and Beenackers 1999).
The recycling of the gas phase product led to some reincorporation and hydrogenation of the light olefins over the catalyst giving a disconnect to the expected distribution of the olefins compared with the LHcL1 and Wax1 products, which were not recycled. The paraffin and olefin distributions for the combined products are shown in Fig. 9 and clearly show a disconnect between the absolute values of olefins of the light hydrocarbons (C3–C5) and those of the hydrocarbon liquid and wax (C7–C14).
1H NMR, 13C NMR and 13C DEPT NMR analysis of light hydrocarbon liquid and wax
Previous work (Speight et al. 2011) demonstrated 1H NMR for FT wax speciation whilst both 13C NMR and 1H NMR have been used for the analysis of FT liquid hydrocarbon products (Cookson and Smith 1989). We applied similar methods of analysis to our light hydrocarbon liquid and wax samples. The potential of 1H NMR for quantification of alcohol and olefin class compounds and average carbon number for long-chain hydrocarbons was confirmed by analysis of the C14 standards, tetradecane, 1-tetradecene and 1-tetradecanol in CDCl3 solvent. All standards returned average carbon numbers of between 13.5 and 14.1 and with a class compound group value of 0.98 per carbon chain for both 1-tetradecene and 1-tetradecanol, these being close to the expected value of 1.00 (Additional file 1: Figure S7 and Table S4). There was no evidence for alcohols or olefins in the analysis of the tetradecane standard. The reproducibility of the 1H NMR analysis for FT products was demonstrated by preparing three solutions of a wax sample with each of these being prepared and analysed on separate days. The 1H NMR spectra and concentration of each class groups present in each of the three samples are given in Additional file 1: Figure S8 and Table S5.
The solution 1H NMR analysis of the LHcL1 and Wax1 samples confirmed that the major classes of compounds present were 1-alcohol, 1-olefin and internal olefin (Fig. 10). The 1H NMR analysis could not distinguish the position of the internal double bond, i.e. whether this was in the 2-position or located further into the hydrocarbon chains, but the 13C NMR and 13C DEPT 135 NMR were able to confirm that the internal olefins were the cis- and trans-2-olefins, in approximately equal amounts, as shown in Fig. 11. The peaks labelled 1 and 2 are due to the 1-olefins, those labelled 3 and 4 are from the trans-2-olefins and those labelled 5 and 6 are from the cis-2-olefins. The peak labelled 8 was due to the terminal methyl group associated with the 2-olefins. The complete assignments of the 1H NMR and 13C NMR peaks are given in Tables 7 and 8 along with their input into the calculations used to give average carbon number and average concentration of the class groups, as described in Additional file 1.
Comparison of class group concentrations by NMR and chromatographic methods
A summary of the class group concentrations in the LHcL1 and Wax1 samples are given in Tables 9 and 10. This shows that 1H NMR analysis gave higher values for 1-olefin, 2-olefin and 1-alcohol compared to 1D-GC and GCxGC which is to be expected due the problems already discussed with GC sensitivity and peak resolution. The average carbon numbers derived from 1H NMR and 1D-GC were similar.
The 1H NMR data also supports the assessment from the 1D-GC data showing 1-olefin to decrease more significantly with increasing carbon number compared to the 2-olefin and the 1-alcohol. The average number of each class group per carbon chain has been calculated for the LHcL1 and Wax1 samples and these are summarised along with ratio of the Wax1:LHcL1 values in Table 11. These ratios were 0.20 for the 1-olefin, 0.77 for the 2-olefin and 0.78 for the 1-alcohol indicating that the higher Mw material (Wax1) had lower values that the lower Mw material (LHLc1).
A series of wax and light hydrocarbon liquid samples produced on pilot plants and waxes produced on micro-reactors, all again from Co based TiO2 catalysts, were analysed by 1H NMR and GC-GC for 1-alcohols and the results are compared in Additional file 1: Figure S9. This showed that the 1H NMR analysis consistently gave slightly higher values than the GCxGC analysis which was likely caused by the limited carbon range of the latter analysis. However, this also confirmed the robustness of the GCxGC analysis over the carbon range of the analysis.
Comparison of acidity of light hydrocarbon liquid by GC-MS and KOH titration methods
Acidity analysis of a light hydrocarbon liquid (LHc2), from an earlier pilot plant trial of a Co-based TiO2 catalyst, was quantified by both GC-MS and KOH titration methods. This confirmed that both analytical techniques gave similar acidity results when this was calculated on a common unit basis, e.g. mg KOH/g as shown in Table 12. The calculation used for converting the carboxylic acids concentrations determined from the GC-MS analysis to an equivalent acidity value expected from a KOH titration is given in Eq. 5 and assumes that monobasic carboxylic acids are neutralised effectively by a molar equivalent of KOH.
The conclusion from these results was that the acidity present in FT products made over the Co/TiO2 catalyst was due to linear carboxylic acids with little, or no, contribution from any other acidic function since the GC-MS method used was specific to the carboxylic acid class group.
Summary and conclusions
Comprehensive two-dimensional gas chromatography analysis (GCxGC) of 1-alcohols and gas chromatography–mass spectrometry (GC-MS) analysis of derivatised carboxylic acids, as their methyl esters, have demonstrated accurate quantification of Fischer-Tropsch hydrocarbon liquid and wax products.
The GCxGC and GC-MS methods in combination with conventional one-dimensional GC analysis of the aqueous, gas, liquid hydrocarbon and wax products plus conventional HPLC analysis of the aqueous phase allowed a detailed and quantified class compound distribution to be demonstrated for a Fischer-Tropsch product from a Co-based TiO2 catalyst operating in a fixed bed gas phase pilot plant. Comparison of GCxGC versus 1H NMR and GC-MS versus KOH titration confirmed the applicability of the chromatographic methods for quantitative analysis of FT oxygenated compounds.
Whilst the hydrocarbons and oxygenates that were identified are known compounds formed during the low temperature, Co catalysed, FT process the combination of the multiple analysis techniques used has allowed a level of detail to be gained on the FT product composition that is seldom reported.
Typically, the long-chain 1-alcohols and carboxylic acids were found to be present at levels of 1/10th and 1/1000th that of hydrocarbons of equivalent carbon chain length respectively.
Additionally, 1H NMR and 13C NMR analyses were used to quantify the average class compounds concentration of 1-olefin, cis- and trans-2-olefins, 1-alcohol and aldehyde as appropriate for the technique used.
The 1-olefin:n-paraffin ratio in the hydrocarbon liquid and wax products was found to decrease significantly with increasing carbon chain length in both phases and much more so than those of the 2-olefin or 1-alcohol.
Availability of data and materials
All data generated or analysed during this study are included in this published article.
Municipal solid waste
One-dimensional gas chromatography
Comprehensive two-dimensional gas chromatography analysis
Gas chromatography–mass spectrometry
High-performance liquid chromatography
- 1H NMR:
Proton nuclear magnetic resonance
- 13C NMR:
Carbon 13 nuclear magnetic resonance
Distortionless enhancement by polarisation transfer
Total acid number
Fatty acid methyl ester
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Relative standard deviation of response established for FAME analytes at a concentration of 10.0 μgmL-1 over 10 analyses in GC-MS analysis. Table S2. Recovery of carboxylic acid standards in GC-MS analysis. Table S3. Example of a duplicate sample preparations of a liquid hydrocarbon for carboxylic acid analysis. Table S4. 1H NMR of C14 standards, calculation of average carbon number and class compound per carbon chain. Table S5. Concentration of class compounds in repeat analyses of a wax sample. Figure S1. Linearity of response of selected FAMEs in GC-MS analysis. Figure S2. GC chromatogram of aqueous phase, Aq1. Figure S3. HPLC chromatogram of aqueous phase, Aq1. Figure S4. 1D GC analysis of wax, Wax1. Figure S5. Comparison of 1-alcohols in LHcL1 using 1D-GC and GCxGC. Figure S6. McLafferty rearrangement of long chain methyl ester to form the McLafferty ion. Figure S7. 1H NMR of C14 standards, 1%w/w in CDCl3. Figure S8. Repeatability of 1H NMR analysis of a wax sample. Figure S9. Comparison of 1H NMR and GCxGC analysis of 1-alcohols in FT waxes and light hydrocarbons.
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Partington, R., Clarkson, J., Paterson, J. et al. Quantitative carbon distribution analysis of hydrocarbons, alcohols and carboxylic acids in a Fischer-Tropsch product from a Co/TiO2 catalyst during gas phase pilot plant operation. J Anal Sci Technol 11, 42 (2020). https://doi.org/10.1186/s40543-020-00235-5
- Fischer-Tropsch products
- Comprehensive two-dimensional gas chromatography
- Gas chromatography–mass spectrometry
- Carboxylic acid
- 1H NMR
- 13C NMR