(97h) Process Intensification of Aqueous Biphasic Hydroformylation of Mid Chain Olefin 1-Octene in a Jet Loop Reactor Creating Large Interfacial Areas

Process
Intensification of Aqueous Biphasic Hydroformylation of Mid-Chain Olefin
1-Octene in a Jet Loop Reactor Creating Large Interfacial Areas

Helge Warmeling, Arno Behr and Andreas J. Vorholt*

Chair
of Technical Chemistry, Department of Biochemical and Chemical Engineering

Technical
University of Dortmund, Dortmund, DE-44227, Germany

The hydroformylation is an atom-economic,
homogenously catalyzed reaction where an alkene is converted under syngas pressure
to form linear and branched aldehydes following figure 1. Most commonly the
reaction is catalyzed by rhodium complexes modified with steric phosphine
ligands to increase reactivity and selectivity. [1]

Fig. 1. Reaction
scheme of the hydroformylation of an terminal alkene
with prominent side reactions like isomerization and hydrogenation to alkanes.

Due to the high market price and even higher
price volatility of rhodium, an efficient catalyst recycle is eminent for an
economic process. A possible solution is to immobilize the rhodium in an
aqueous second phase by the use of the ligand triphenylphosphine
trisulfonate (TPPTS). Substrates diffuse into the
aqueous catalyst phase and react at the catalyst complex. Products can subsequently
be separated by a simple decantation step in an organic phase. This recycle concept works exceptional for short-chain
substrates but has its limitations for alkenes with C₆₊ because the substrate solubility in
the catalyst phase decreases significantly with increasing chain length. While
the majority of studies working on this limitation heavily focus on the use of
chemical additives like surfactants, co-solvents or phase transfer promoters to
increase solubility or interfacial surface only seldom procedural approaches
were investigated. [2] This work therefore focuses on an additive-free
procedural approach to enhance the reaction rates of the hydroformylation of
mid-chain olefin 1-octene in a phase contacting jet loop reactor.

Jet loop
reactors (JLR) are a promising tool in the intensification of catalytic
multiphase reactions as they provide large interfacial areas due to intense
mixing and therefore excellent mass transfer characteristics. The overcoming of
mass transfer limitations can be of significant economic interest for large
scale industrial applications. In contrast to other phase contacting reactor
concepts the jet loop reactor also offers a high energy efficiency which makes
it especially interesting in process intensification projects. [3]

The principle of operation is based on the
partial withdrawal and reinjection of the reaction medium through a jet nozzle
as can be seen in figure 2. The recycle stream sucks gaseous media via impulse
transfer into the jet stream and disperses it into the liquid phases to achieve
a fine gas-liquid-liquid distribution. The small gas
bubbles coagulate in an outer reactor ring called the riser and add additional
circulation momentum following the principle of an airlift pump. The reactor
itself features no moving parts and is hence particularly suitable for high
pressure applications like the hydroformylation.

Fig. 2. Scheme
of the jet loop reactor concept in biphasic homogenously catalyzed
hydroformylation of terminal alkenes. Intense mixing and therefore high
internal interfacial area is created through a partial external circulation of
the reaction medium through a nozzle.

In this work the jet loop reactor is compared
to a standard reactor type of the chemical industry, the stirred tank reactor
(STR). Both reactor types were build up on miniplant scale and were successively optimized. Reaction
conditions like temperature, pressure and syngas composition were kept in
standard industrial range while catalyst and ligand concentration were investigated
in preliminary tests. Surprisingly the reaction rates showed an inhibitory
effect in regards of catalyst concentration and the highest catalytic activity
and productivity were found at very low rhodium and ligand concentrations (c_Rh=0.00025 kmol/m³_aq)
with peak turn over frequencies (TOF) of up to 12500 in the JLR and 4500 in the
STR. Due to increased leaching the investigation was continued with higher
catalyst concentrations of c_Rh=0.0005 kmol/m³_aq and a ligand to rhodium ratio of 10.

While the jet loop reactor reached high
turnover frequencies up to 5500 h^-1 and space time yields (STY) up
to 55.5∙10^-2mol/m³h the stirred tank
reactor only achieved lower TOFs of up to 2200 h^-1 but with a
satisfying STY of up to 44∙10^-2mol/m³h as can be seen in Fig.
3. With a

3-pitched blade gassing stirrer the performance of the STR was improved up to a
TOF of 2600 h^-1 with a space time yield of 55.5∙10^-2mol/m³h.
The results show that in contrast to the STR the JLR is able to achieve excellent
phase dispersion even with relatively small amounts of aqueous catalyst phase
which results in a higher catalytic activity achieved with about a tenth of the
specific energy input (0.27 kW/m³ compared to 2.2 kW/m³).

Fig. 3. Catalytic activity (TOF) and productivity
(STY) of the aqueous biphasic hydroformylation of 1-octene in a jet loop
reactor compared to a stirred tank reactor at different stirrer speeds.

The results point out the potential of the
reactor setup compared to standard reactor types and also academic studies
using additives published in the literature. The direct connection between
specific energy input, created interfacial area and reaction rate is currently
subject of investigation. Based on calculations of the mass transfer coefficients
for all compounds, mass transfer resistance as the limiting step can be
excluded due to the relatively slow reaction rates compared to substrate
diffusion. The authors propose the creation of surface near film volume with
elevated substrate concentration as the cause of rate acceleration.

References

[1] Franke R, Selent
D, Börner A. Applied hydroformylation, Chem. Rev.
2012;11;5675-5732

[2] Obrecht
L., Kramer P., Laan W. Alternative approaches for the
aqueous–organic biphasic hydroformylation of higher alkenes, Catal. Sci. Technol. 2013;3;541-551

[3]
H. Warmeling, A. Behr, A. Vorholt, Jet loop reactors as a versatile reactor set
up - Intensifying catalytic reactions: A review, Chem. Eng. Sci., 149 (2016)
229-248