Analytical Methods

3.6.1 Gas Chromatography (Mass Spectrometer)

The gas chromatography-mass spectrometry (GC-MS) method combines the principles of gas chromatography and mass spectrometry to predict the components present in liquid mixtures. The detection of components depends on the interaction between the molecules and the stationary phase of the column. The column retains the molecules as they travel through the length of the column and subsequently come off at different retention times which allows capture, ionization, acceleration, deflection and detection of ionized molecules separately. The liquid product mixtures from this study were analysed using an Agilent 7890A GC and Waters GCT Premier time of flight mass spectrometer (GC-TOFMS) equipped with a ZB-Wax

capillary column (30 m ×0.25 mm × 0.25 μm). The electron energy and source temperatures were 70 eV and 250 °C respectively.

Figure 3.2: A schematic of a gas chromatography-mass spectrometry setup (Wikipedia images).

3.6.2 Gas Chromatography (Flame Ionization Detector)

Liquid sample mixtures from experiments were quantified using a Trace GC Ultra gas chromatography equipped with a Flame Ionization Detector (FID) and 30 m × 0.25 mm × 0.25 μm ZB-Wax capillary column. This analysis uses helium (He) as a carrier gas, while hydrogen and air provide a flame for the FID to burn off hydrocarbons. The FID is mass sensitive and thus, detects the ionic sub-components based on the amount of signal obtained for individual components at different retention times with respect to the amount of hydrocarbons injected into the GC. Figure 3.3 shows the different components of a GC-FID.

The choice of a capillary column is highly significant for effective separation of components, and should be based on the following factors:

- Column I.D (depending on the size of the molecules).

- Stationary Phase (strongly depending on the sample polarity).

- Film Thickness (depending on the sample volatility).

- Column Length.

Figure 3.3: Schematic diagram of the gas chromatography-FID system (Zhu, 2014).

A suitable GC oven method was developed to ensure the column was operated within the temperature limits that would allow sufficient interaction of components with the column, and enhance separation from each other to result in sharp peaks within a reasonable time. The retention times that represent each compound are established by injecting known chemical standards into the GC which are used to identify the corresponding compounds from reaction mixtures by matching their retention times. The procedure for the sample amount and injection are developed for the auto sampler. Standard calibration plots were developed for the substrate and potential products as established from GC-MS analysis, which would allow the quantifications of analyte concentrations in the product mixture. An internal standard calibration method was used which involves spiking different known concentrations of the analyte of interest with a constant concentration of internal standard in a ratio of 10:1 to obtain standard curves (Appendix A). The calibration curves were developed by plotting the ratios of

the analyte to internal standard peak area versus analyte to internal standard concentration.

Calibration standards were mainly prepared at different concentrations in the range of 0.02 – 0.60 M for all the components. Also, the reaction mixtures from experiments were prepared and analysed by adding the same constant concentration of internal standard prior to injection into the GC. Butanol was used as a standard for experiments involving the hydrogenation of acetic and/or propanoic acids while propanol was used when acetic and/or butanoic acid was involved.

In addition, the internal standard method was used because it is more accurate and precise, and can compensate for errors due to instrumental and sample preparation. In this study, alcohols and esters formed during the hydrogenation of carboxylic acids (C2-C4) were analysed by the Trace GC Ultra (GC-FID). The GC equipment was operated under a constant pressure mode with a split flow of 181. The injection volume was 1 μl, both injector and detector temperatures were 250 °C, inlet pressure was 75 KPa, and the flow rate for helium (He) carrier gas was 1 mL/min. The GC oven temperature was held at 40 °C for 5 minutes and then subsequently heated up to 215 C at a ramp rate of 20 °C/min. Further heating to 220 °C was carried out with a ramping of 5 °C/min and held at 220 °C for 5 minutes.

3.6.3 Calculation of carboxylic acid conversion, alcohol yield, alcohol selectivity and ester selectivity

The experimental results investigated from the hydrogenation of carboxylic acids were obtained and presented in terms of conversion, alcohol yield, and alcohol and ester selectivities.

It should be noted that most of the reactants/products of interest are liquid so not usually of variable volume, apart from samples removed and insignificant amount converted to gas.

Conversion (%) = (Initial moles of acid−Final moles of acid

Initial moles of acid × 100 3.3

Selectivity (%) = Moles of desired product

Total moles of products formed × 100 3.4

Alcohol Yield (%) = Conversion (starting material) × Selectivity (desired product) ×

100 3.5

3.6.4. Mass Balance

Gaseous products are formed as a result of hydrogenation and decarboxylation reactions involving carboxylic acids. The mass balance showing the amount of gaseous and liquid products was obtained from the respective masses of the reactant and product mixtures.

Therefore, the total mass of gases formed was calculated by subtracting the mass of liquid products from the initial mass of the reactant mixture. The mass of catalyst at the start and end of reactions was assumed to be the same because coke deposit was not observed on the impeller after completion of reactions. The mass balance of liquid and gas products was calculated as the percentage of the mass of reactant mixture fed into the reactor using equations 3.6 and 3.7:

Liquid (wt. %) =^mf

m_i× 100 3.6

Gas (wt. %) = 100 − liquid yield (wt. %) 3.7

where mf is mass of product mixture and mi is the mass of reactant mixture.