Materials and methods

3.2.1 Experimental: Autoclave reaction studies

Kinetic data for this study were obtained using a 100 mL autoclave with dead end operation (see Figure 3.2) and were supplied by Queen’s University Belfast⁴. The catalyst used was 4% Pt/TiO₂. Prior to reaction testing the catalyst was reduced in 1 bar H₂ at 333 K for 1 h.

Figure 3.2: Schematic for reaction testing equipment in this study⁵

4 Data supplied by Daly H., Thompson J.M., McManus I., Hardacre C. (CenTACat, Queen’s University Belfast) and Sedaie Bonab N. (Department of Chemical Engineering, University of Birmingham), August 2012

5 Schematic supplied from correspondence with Sedaie Bonab N., December 2012

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In all experiments, a slurry comprising 0.1 g catalyst (particle size 5 μm, mean pore size 25 Å) and 50 mL combined solvent and reactant was used. The starting reactant in all cases was PBN in the concentration range of 0.13 – 0.39 mol L^-1. H2 pressure and impeller speed, N, were kept constant at 5 bar and 1400 rpm respectively for all tests. This was chosen based on experiments shown in Figure 3.13 (Section 3.7.1), which explored the effect of stirrer speed on liquid-gas mass transport. All reaction tests were found to be performed in a kinetic rather than mass transfer controlled regime. Relevant experiments⁶ are shown in section 3.7.1 (appendix). All experimental runs lasted for 120 min with 2 cm³ liquid samples taken at 10, 20, 30, 60 and 120, min respectively. The samples were passed through a syringe filter to remove the catalyst before being diluted with solvent in a 1:10 ratio. The samples were analysed using a Perkin-Elmer Clarus 500 gas chromatograph equipped with a FID with an Agilent HP-5 column.

Four different solvents were tested whose properties are documented in Table 3.2.

The wide difference in characteristics between protic and aprotic polar solvents may assist in rationalising differences observed in reaction selectivity, rate constants and dominant adsorption constants estimated from the kinetic modelling process described in section 3.4.

Table 3.2: Properties of solvents used in this study. εdi denotes relative permittivity, μ denotes dipole moment, α1 denotes hydrogen bond donor parameter, α2 denotes

hydrogen bond acceptor parameter, kH denotes Henry’s Constant

Solvent Type ε_r^a μ (Debye) ^a α₁^a α₂^a k_H for H₂ at

a Values taken from Bertero et al., 2011.

b Values calculated using data from Bertero et al., 2011; Katayama and Nitta, 1978; Chang et al., 1991; Tsuji et al., 2005.

Table 3.3 summarises the three stage approach of the experimental programme to provide sufficient experimental data to build a mechanistically sound kinetic model that will

6 Sedaie Bonab N., Ph. D. thesis, The University of Birmingham, UK, currently in writing

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link selectivity and adsorption constants to choice of solvent. Series A, utilising multi-temperature data will be utilised for refinement of candidate models. This will both remove insignificant parameters from rate expressions and identify stronger models from the initial candidates. Series B will test the best model(s) further by probing their suitability over a range of starting concentrations. This will further discriminate remaining models and potentially shed additional fundamental understanding of reaction mechanism. The most suitable model(s) will finally be tested against experiments using a range of solvents (Series C) with the aim of demonstrating the link between solvation, selectivity and the dominant adsorption constant.

Table 3.3: Experimental series undertaken for this study

Series Variables Constants

B Effect of PBN concentration

- 0.13 – 0.39 mol L^-1 (5 points)

Parameter estimation within the kinetic models was carried out using Athena Visual Studio^© software⁷. The kinetic models tested within this work contain multiple parameters including some which are non-linear (e.g. activation energies in the Arrhenius equation). The Levenberg-Marquardt procedure, an indirect method for constrained optimisation of parameters, is appropriate for this problem (Marquardt, 1963). All response variables in the PBN hydrogenation reaction network are dependent on multiple reactions (see Figure 3.1) and so the models must be solved implicitly using a set of differential equations:

7 Athena Visual Studio 14.2, Stewart & Associates Engineering Software, Inc.

90 respect to each model parameter. In Eq. (3.2b) it can be seen that defining sensitivities as a function of time allows them to be solved alongside the main system differential equations, improving solver efficiency and performance.

To minimise cross-correlation between activation energy (Ea) and pre-exponential factor (Ai) parameters, a re-parameterised Arrhenius equation was used:

 373 K. The fitting process can be further improved by solving Ai,373 as an exponential term and lumping fitted value, Ea with constants Tbase and ideal gas constant, R (J K^-1 mol^-1) to

91 3.3 Results

3.3.1 Effect of temperature

Figure 3.3A shows the evolution of 4-phenyl-2-butanone concentration with time in a hexane solvent. PBN consumption increases with temperature; 10% conversion over 120 min is seen at 303 K through to 80% at 353 K.

In Figure 3.3B, selectivity is highest to the product CBN at all temperatures, formed via hydrogenation of the aromatic ring in PBN. This selectivity increases somewhat with temperature. Selectivity towards PBL (via ketone group hydrogenation) is considerably lower and declines with temperature. These temperature dependency observations suggest that the activation energy for aromatic ring hydrogenation is higher than that for ketone group hydrogenation, which agrees with the literature (Bergault et al., 1997).

Selectivity to the fully hydrogenated product, CBL, increases with both time and temperature. In Figure 3.3B, after 120 min, CBL selectivity appears to increase with temperature at the expense of PBL selectivity. PBL  CBL is an aromatic ring hydrogenation route, again demonstrating the prevalence of this reaction in a hexane solvent.

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Figure 3.3: A) Evolution of PBN concentration with reaction time at different reaction temperatures in hexane solvent. B) Selectivity of products at different temperatures at 10 min (closed symbols) and 120 min (open symbols) reaction time. Symbols denote: (♦,◊)

PBL, (■,□) CBN, (▲,∆) CBL. Each test result shown was only conducted once.

A first approximation of the kinetics of PBN hydrogenation to products can be made using Eq. (3.4) (below). Whilst this does not consider product selectivity or further product hydrogenation it is a pragmatic way of gauging the complexity of the system kinetics:

napp concentration measurement and napp is an apparent reaction order. Plotting a linear trend line through graphs of ln(r_{PBNproducts}) against ln([PBN]_t(n)) reveals n_app (line gradient) and can be ascertained for each reaction temperature.

In Table 3.4, n_app decreases with increasing reaction temperature. This immediately shows the kinetics of the reaction system is non-trivial. Evolution of n_app with temperature in a simple power law expression is a simple indicator that species adsorption will be required in the kinetic model rate expressions. Species adsorption is exothermic and its strength decreases with temperature which lowers napp in a power law expression. A further argument may be that hydrogenation of intermediates PBL and CBN may influence napp in this

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expression via active site competition. Whilst this cannot be ignored it is unlikely to be the sole influence on the high values of napp at lower temperatures where intermediate product formation is much lower.

Table 3.4: Apparent reaction order dependency on PBN at different temperatures using a simple linear fit

Reaction Temperature (K) n_app (-) R²

303 17.1 0.66

313 3.8 0.85

323 2.5 0.97

333 2.2 0.77

343 1.4 0.90

353 1.6 0.98

3.3.2 Effect of 4-phenyl-2-butanone concentration

Figure 3.4a shows the amount of PBN hydrogenated against time with five different starting concentrations. The profiles are very similar for all starting concentrations up to 60 min reaction time. This similarity is further shown in Figure 3.4b whereby all initial PBN hydrogenation rates as well as PBL and CBN formation rates appear to be invariant with starting PBN concentration. This effect has been previously seen for acetophenone hydrogenation (Bergault et al., 1997). It is noted that differences in the concentration profiles are evident after 120 min reaction time. This is chiefly for the experiments with 0.13 and 0.19 mol L^-1 PBN whereby the substrate is nearing complete conversion at this stage.

A zero order dependence of PBN concentration on reaction rate is unlikely as the individual concentration profiles over the reaction time show an apparent order that is always greater than 1. The observations in Figure 3.4b are consistent with a reaction mechanism that is rate determined, or at least strongly influenced, by product desorption.

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Figure 3.4: A) Evolution of PBN hydrogenated with reaction time using different starting concentrations of PBN at 343 K. B) Variation in initial rate behaviour against

starting concentration of PBN; (♦) PBN conversion, (■) PBL formation, (▲) CBN formation.

3.3.3 Effect of solvent choice

Figure 3.5 shows the reaction time concentration profiles for four different solvents.

These four solvents were chosen so that different categories of solvent can be compared, namely alkane (HEX), aromatic (TBT), primary alcohol (1-PrOH) and secondary alcohol (2-PrOH).

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Figure 3.5: Reaction time concentration profiles for different solvents at 343 K. Symbols denote: (♦) PBN, (■) PBL, (▲) CBN, (●) CBL. N.B.: Lines are to guide the eye

Some marked differences are seen, in particular between the aprotic apolar (HEX, TBT) and the protic solvents (1-PrOH, 2-PrOH). The former both show significant selectivity towards CBN (aromatics hydrogenation) across the entire reaction time. The protic solvents behave somewhat differently. 2-PrOH is ultimately more selective to PBL (ketone hydrogenation) after 120 minutes although both PBN hydrogenation pathways have comparable selectivity over the first 30 minutes of reaction. 1-PrOH is more selective towards CBN than PBL although this is less marked than with the aprotic apolar solvents.

The extent of PBN converted over 120 minutes is of the order HEX > TBT, 2-PrOH >

1PrOH. Figure 3.6 shows conversion follows an inverse relationship with k_H for H₂ solubility with each solvent. This suggests that a greater availability of hydrogen in the liquid phase results in an increased overall hydrogenation activity of the catalyst. This correlation does not hold for hydrogenation route selectivity however, suggesting other solvent effects may be prevalent.

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Figure 3.6: PBN Conversion plotted against Henry’s Constant for H2 solubility in each solvent experiment after (♦) 30 min and (■) 120 min. Solvent Henry’s Constants follow

the order HEX < TBT < 2-PrOH < 1-PrOH N.B.: Lines are to guide the eye.