(434d) Hydroformylation and Tandem Isomerization-Hydroformylation of Long-Chain Olefins: Mechanism, Kinetics and Optimal Reaction Control

Introduction
The
Hydroformylation of n-olefins using Rh-Bisphosphite catalysts (e.g.
Rh-Biphephos) for the production of terminal aldehydes is known to suffer from
side reactions, such as double-bond isomerization and
hydrogenation of the substrate [1]. However, the ability to isomerize the
n-olefins to their thermodynamic equilibrium composition allows such catalysts
to perform the transformation of complex n-olefin mixtures with internal
double-bond to valuable terminal aldehydes [2]. To exploit such processes on a
technical scale, it is necessary to understand the mechanism of the reactions
and tandem-effects in order to find a good kinetic model for reactor design and
operation with an optimal yield of terminal aldehydes. This motivated the
experimental and theoretical analysis of the hydroformylation reaction network including
isomerization and hydrogenation of 1-decene and iso-decenes with terminal and internal
double-bond, respectively. Based upon an extended reaction mechanism, detailed
kinetic models were derived including catalyst equilibria. Fitting the model to
experimental data was supported by parameter subset selection to avoid overparameterization
[3]. With this model, optimal reaction control strategies were generated.Experimental
The
kinetic experiments were performed with 1-decene and an equilibrium mixture of
iso-decenes in a thermomorphic multicomponent solvent system consisting of DMF,
dodecane and n-decenes. A Rh-Biphephos catalyst was used in all experiments
with a Rh-to-olefin ratio of 1:10000-1000. All reactions were realized in a 90
ml high pressure multiple batch reactor system (Parr Instrument Co.) under
careful consideration of the catalyst pretreatment at 15-18 bar syngas and a
molar Rh-to-ligand ratio of 1:3. Temperature, total Syngas pressure, partial
pressures of CO and H₂ as well as initial substrate concentrations
were varied systematically in 26 batch and semi-batch experiments. Gas
chromatography (GC) was used for quantification.

Calculations
The parameter
subset selection calculations and the parameter estimation were performed using
Matlab 2012a with standard solvers. Optimal reaction control strategies were
generated via maximizing the yield of undecanal subject to balance equations,
kinetics and thermodynamics with time dependent state variables (temperature,
partial pressures of CO/H₂) as degrees of freedom. This optimal
control problem was transformed to a large NLP using orthogonal collocation and
solved in AMPL.


Results
The
experimental results showed the expected high isomerization activity of the
catalyst as well as the highly regioselective hydroformylation. The activity
towards hydrogenation was always low. The presence of CO affected the active
catalyst and inhibited the isomerization reaction drastically, whereas the
hydroformylation is promoted by CO and H₂. Therefore, increasing the
syngas pressure lead to an increase in aldehyde yield if 1-decene was used as
substrate. However, increasing syngas pressures applied to an equilibrated
mixture of iso-decenes with internal double-bond counterintuitively decreased
the aldehyde yield. However, the ratio of linear to branched aldehydes remained
almost constant at about 97:3 in all cases. In total, 26 experiments including
batch and semi-batch mode were used for parameter estimation and 14 parameters
were estimated with low 95% confidence intervals (<10%). The resulting model
was able to describe all reactions under all reaction conditions with low
deviations (see Fig. 1).

Fig. 1: Selected model calibration results: a) Isomerization of 1-decene (Rh:Olefin
= 1:10000, 0 bar CO/H₂,

105 °C); b) Hydrogenation of 1/iso-decene mixture (Rh:Olefin = 1:10000, 10 bar
H2, 105 °C); c) Hydroformylation of 1-decene (Rh:Olefin = 1:10000, 10 bar CO/H₂,
105 °C); d) Tandem-Isomerization-Hydroformylation of an equilibrated iso-decene
mixture (Rh:Olefin = 1:1000, 5 bar CO/H₂, 105 °C).

The optimization results suggest to perform the reactions at low temperatures
and high syngas pressures for maximum aldehyde yield it if 1-decene is used as
substrate. The opposite is the case if an equilibrated mixture of iso-decenes
with internal double-bond is used as substrate (see Fig. 2) which is a result
of the coupling of isomerization and hydroformylation in the sense of a tandem-reaction.

Fig.
2: Selected optimal reaction control results for tandem
isomerization-hydroformylation (Rh:Olefin = 1:1000): a) Concentration-time
curves for maximum undecanal yield; b) Optimal temperature profile;

c) Optimal partial pressure profiles.


Acknowledgement
This work
is embedded in the collaborative research center „SFB/TRR-63 InPROMPT - Integrated
Chemical Processes in Liquid Multiphase Systems“ and the authors gratefully
thank the German Science Foundation for financial support.


Literature
[1] R. Franke
et al., Chemical Reviews 2012, 112, 5675−5732

[2] A. Behr et al., Journal of Molecular Catalysis A: Chemical 2003, 206,
179–184

[3] A. Jörke et al., Chemie Ingenieur Technik 2015, 87, 713–725