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HPPO Process Technology A novel route to propylene oxide without
coproducts
Article in Chimica Oggi · March 2014
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sustainability/
green chemistry
Industry perspective
FRANZ SCHMIDT, MAIK BERNHARD, HEIKO MORELL, MATTHIAS PASCALY*
*Corresponding author
Evonik Industries AG, Advanced Intermediates – Innovation,
Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany
Matthias Pascaly
HPPO Process Technology
A novel route to propylene oxide
without coproducts
KEYWORDS: HPPO process, propylene oxide, hydrogen peroxide, titanium silicalite.
Abstract
The common industrial technologies for the conversion of propylene to propylene oxide have been
compared with a special focus on the direct oxidation using hydrogen peroxide. The HPPO process is an
economically and ecologically superior technology since there are no market dependencies of other coproducts and water is the
only waste product. The catalyst used in this process is a partly titanium substituted silica based zeolite called TS-1. The article
summarizes the most important information concerning the HPPO process.
INTRODUCTION
The oxidation of organic compounds is of vital importance
for the chemical industry. Besides basic oxidation reactions
such as bleaching processes (e.g. paper or laundry) also
oxidation reactions of chemicals are important: Epoxides
- especially ethylene and propylene oxide – are among the
major chemicals. Replacement of traditionally used halidebased oxidants (chlorine) by hydrogen peroxide provided a
new route to the desired oxidized products. Past demand for
hydrogen peroxide was due to the replacement of a
chlorine bleaching step in the paper industry or the
introduction of percarbonates as bleaching agent in
detergents. Today, the newly developed HPPO (hydrogenperoxide-to-propylene-oxide) process is one of the largest
consumers of hydrogen peroxide for the epoxidation of
propylene yielding propylene oxide on a titanium doped
zeolite without any coproducts. Also in this case, the
oxidative potential of hydrogen peroxide allows the
replacement of traditionally used chlorine-based oxidants
enabling a novel environmentally more benign process.
This article provides an overview on the HPPO technology
which is now state of the art for the industrial production of
propylene oxide. The article touches the catalyst TS-1, the
propylene oxide reaction as well as the process conditions.
Furthermore, a future perspective is given, based on the
market situation and in comparison to other processes.
approximately 7 Mt/a in the year 2010 at a production
capacity of approximately 8 Mt/a. Based on a total
market growth of around 5 % per year, the expected
values for demand and capacity in 2015 are nearly
at 9 and 10 Mt/a respectively. The growing markets are in
Asia and potentially in the Middle East (1).
There are several industrial routes to produce propylene
oxide, of which the chlorohydrin process (CH) is the
oldest one (2). Other indirect oxidation processes
coupled with coproducts are propylene oxide / styrene
monomer (PO/SM) and propylene oxide / methyl
tert-butyl ether (PO/MTBE) (3). Newer technologies are
based on an oxidation without a coproduct. One of
these technologies is the propylene oxide cumene
(PO/CU) process developed by Sumitomo (4). However,
the most promising way is the oxidation of propylene with
PROPYLENE OXIDE MARKETS AND PRODUCTION PROCESSES
Propylene oxide ranges on place eleven of all organic
chemicals being produced with a total demand of
Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014
Figure 1. Development of PO-technologies (* data based on (1)
and Evonik’s own estimates).
31
hydrogen peroxide (HPPO), independently developed
by Evonik/TKIS (ThyssenKrupp Industrial Solutions AG) and
BASF/Dow Chemical, respectively.
Figure 1 shows the percentaged PO production capacity
of the different processes in the past and an estimated
future trend (1). In recent years, the PO production
technology is observed to shift away from the formerly
standard chlorohydrin route. This shift takes place in favor
of the HPPO process.
However, the majority of the propylene oxide is currently
still produced via the Chlorohydrin (CH) route (Figure 2).
This process is performed generally in two-steps. In the
first step, intermediately generated hypochlorous acid
reacts with propylene resulting in two kinds of propylene
chlorohydrins. These chlorohydrins are subsequently
dehydrochlorinated by calcium hydroxide or sodium
hydroxide. Beside the aspired propylene oxide 2.1 tons
CaCl2 and 0.1 tons 1,2-dichloropropane are obtained as
byproducts per ton propylene oxide. This is the main
disadvantage of this process. For process optimization
Figure 2. Reactions of the industrial relevant propylene oxide
Ca(OH)2 can be replaced by NaOH. Subsequently the
producing processes.
generated NaCl is converted to NaOH and Cl2 via
electrolysis. This step reduces the salt load of the waste
achieved by Shell´s SMPO process using a heterogeneous
water, but increases investment and production costs
TiO2/SiO2 catalyst offering a more efficient catalyst
due to additional power consumption and required
separation in the epoxidation step (6).
purification of NaCl prior electrolysis (2).
Due to the above mentioned drawbacks, intensive
An alternative is the so called Lummus process using
research was performed to develop coproduct free routes
tert-butyl hypochlorite and water to form tert-butanol
for the production of propylene oxide. For example, in the
and the propylene chlorohydrins (5). These chlorohydrins
Bayer-Degussa-process perpropionic acid is used as
are converted to propylene oxide using NaOH and the
oxidation agent (6). This process offers a high selectivity to
resulting NaCl is electrolyzed to NaOH and Cl2. Therefore,
propylene oxide and an efficient recycling of propionic
a total recycling to build up the required HOCl-carrier
acid. But a high price of H2O2 at the time of development
(tert-butyl hypochlorite) is possible. The drawback is the
prevented the commercialization of this process.
slower formation rate of propylene chlorohydrins using
The breakthrough regarding a direct oxidation and
tert-butyl hypochlorite.
therefore a coproduct-free method was achieved by ENI
The other industrially used propylene epoxidation
in the 1980s (7). Using a novel titanium silicalite-1 (TS-1)
processes can be divided into coproduct-producing (PO/
catalyst the direct oxidation of propylene with hydrogen
SM and PO/MTBE) and coproduct-free (PO/CU and HPPO)
peroxide was enabled without further oxidation agents (8).
processes and offer the opportunity of being chlorine free
Evonik and TKIS improved this process by developing a
(Figure 2). In the PO/SM and the PO/MTBE routes a
special TS-1 catalyst quality using an optimized process
precursor is used which is oxidized by readily available air
technology. On this basis, the HPPO process could be
or molecular oxygen. The intermediate hydroperoxide
developed and finally commercialized. This coproducttransfers the oxygen to the propylene resulting in
free process offers high specific propylene oxide yields,
propylene oxide and a primary coproduct, which is usually
resulting in low feedstock consumptions. A long catalyst
an alcohol. The challenge using the coproduct routes is to
lifetime is achieved by moderate reaction conditions
achieve a high selectivity for PO and receive an
which are enabled by the high-performance TS-1
additional benefit from selling the coproduct.
catalyst. Since the HPPO process is a stand-aloneDuring the last few years several processes were
technology, the product is independent from offering
developed using acetaldehyde, isobutane, isopentane,
coproducts on the market. Contrary to the chlorohydrin
cyclohexane, ethyl benzene and cumene as precursors
route the HPPO-process enables an environmentally
leading to different secondary coproducts (Table 1) (6).
friendly production due to a totally closed solvent and
However, all of these processes suffer from the need to
process the coproduct, preferentially by
obtaining a credit for supplementing it in the PO
production costs. Therefore, only the PO/SM and
PO/MTBE process yielding styrene monomer and
methyl tert-butyl ether as coproduct are
currently economically feasible (1).
Nevertheless, high amounts of styrene (690 kta)
and MTBE (830 kta) produced as coproducts in
a world scale PO-plant (300 kta) need to be
traded and the risk of an oversupply with
Table 1. Summary of possible precursors used for propylene oxide production via
styrene and MTBE could reduce the efficiency
indirect epoxidation routes with coproduct.
of such processes. An optimization was
32
Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014
feedstock cycle and the complete absence of chlorine.
Compared with the other state-of-the-art technologies,
the HPPO-process offers lower investment costs and
energy consumption. Furthermore, an additional benefit
can be achieved by recovering valuable byproducts like
propylene glycol which is obtained in the range
of 30 kg/t propylene oxide.
All these improvements meet the standard of a modern
sustainable process for propylene oxide and lead to the
startup of the first commercial plant of its kind at SKC in
Ulsan (Korea) under license of Evonik/TKIS with a starting
capacity of 100 kta (9). Several months later a further
HPPO-plant in Antwerp using the BASF/Dow technology
went on stream (10). The expansion of the HPPO plant in
Ulsan to 130 kta, Dow’s announcement for a further HPPO
plant in Saudi Arabia (11) after the startup of a plant
operated by Dow and SCM in Thailand (12) and the setup
of a further HPPO plant in Jilin (China) by Evonik/TKIS (13)
underline the potential of this modern process.
A further coproduct-free process in commercial
operation is the Sumitomo process. This process uses
cumene hydroperoxide as epoxidation intermediate,
which is obtained upon oxidation of cumene. Propylene
oxide and cumylalcohol are obtained in the epoxidation
stage. The latter is hydrogenated to cumene enabling a
complete recycling (4).
Future Trends of propylene oxide production
Despite the existing processes the further development of
direct oxidation processes is ongoing. The use of N2O as
oxidation agent for propylene is under intense discussion
(14). Realizing this route would enable the use of a
coproduct, obtained during the production of adipic
acid. However, further optimization is necessary due to
the limited catalyst performance resulting in a low
selectivity and short catalyst lifetime. Additionally, the
local availability of N2O can prevent this process from
being economically realized.
A further possibility is the oxidation of propylene using
oxygen/hydrogen mixtures. This process can be performed
using bifunctional precious metal heterogeneous catalysts
such as Au/TiO2 or Au/TS-1 (15). Due to the in-situ
generation of hydrogen peroxide this method cannot be
classified as a direct oxidation of propylene.
The direct oxidation of propylene using oxygen is one of
the major challenges in heterogeneous catalysis. Due to
the huge activation energy (497 kJ/mol) of O2
dissociation and the high affinity of monooxygen to the
hydrogen atoms in allylic position, acrolein is
preferentially formed during oxidation of propylene.
Therefore, the direct oxidation of propylene is much
more difficult than ethylene oxidation, which is already
established on an industrial scale.
interconnected channel system of straight and sinusoidal
channels with pore diameters of 5.1 – 5.6 Å (8). Important
is the avoidance of non-tetrahedral coordinated titania
(TiO2) which is supposed to promote side reactions.
The ratio of framework incorporated titanium to extraframework species of titanium is a crucial factor for the
catalytic performance of the catalyst and can be
tailored via the synthesis route. Therefore, the choice of
raw materials and the right parameters during catalyst
synthesis are very important. A typical TS-1 synthesis starts
with the dissolution of a silica (SiO2) and a titanium
source in the presence of an organic template. The
obtained white powder is removed from the mother
liquor, dried and calcined. The most common templates
used are TPAOH (8) and TPABr (18,19). It is shown in
several publications that the catalytic activity is a
function of decreasing crystal size (20). The crystal size
itself usually depends on the ratio of template to silicon.
However, on an industrial scale it is important to minimize
the use or to reduce the costs of the template, since the
zeolite template is one of the cost driving factors during
the synthesis.
Another important aspect in the TS-1 synthesis is the purity
of the crystallographic phases. Hasenzahl et al. describe
in their patent the production of TS-1 from pyrogenic
mixed oxides produced via the aerosil process (21). Since
the mixture of silicon and titanium is already present in
the raw material, these pyrogenic mixed oxides result in
highly phase pure materials with increased catalytic
activity. However, the zeolite synthesis is only one crucial
part during catalyst synthesis. The powder needs to be
designed for an application in fixed bed reactors. The
physical and chemical properties of these formed
catalyst particles have a considerable effect on the
catalytic performance in the HPPO process.
REACTION
The catalyst used for the HPPO reaction is a titanium
silicalite-1 zeolite with framework type MFI. In the
structure Si-atoms are substituted by Ti-atoms forming the
catalytically active TiO4-centers (16). However, the
amount of titanium, which can be inserted into the
framework, is limited to about 3 wt.% TiO2 (17).
TS-1 comprises a microporous two-dimensional
There are several factors, having an impact on the
catalytic performance. The activation energy of the
epoxidation is with 26kJ/mol considerably high (22).
Therefore, a certain temperature has to be applied to
achieve the productive conversion. Typical reaction
temperatures range between 0–60 °C (23). On the other
side the epoxidation is an exothermic reaction
generating high temperatures (propylene oxide
formation enthalpy: ‑123kJ/mol) (24) which lower the
propylene oxide selectivity due to side reactions. The
ring-opening of propylene oxide to glycol or glycol ethers
is the major side reaction (2).
Haas et al. reported an optimized balance between H2O2
conversion and propylene oxide selectivity using a fixed
bed reactor. Running the process at ambient
temperatures, they could obtain almost full conversion
while keeping the selectivity very high (25). Other reactor
designs offer an intermediate external cooling to prevent
excessive side product formation (26-28). Also the reaction
pressure for the conversion of propylene to propylene
oxide is crucial for the reaction rate. On the one hand a
high pressure increases the solubility of gaseous propylene
in the solvent, on the other hand propylene is being
liquefied. Therefore, the reaction system either comprises a
gas-liquid-solid phase system or a liquid-liquid-solid phase
Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014
33
THE CATALYST
and there are several
approaches known in
the literature to restore
the activity of the
catalyst (38-40). One
approach is the
thermal treatment of
the deactivated
catalyst at elevated
temperatures in the
presence of an
oxidizing atmosphere
(40). The residuals are
broken down and
removed from the
catalyst. Another
possibility is the
regeneration using
liquids at ambient
temperatures
to restore the catalytic
activity (39).
Figure 3. Schematic scheme of Evonik/TKIS’s HPPO process.
system with a propylene rich phase and a solvent rich
phase. It is obvious that the concentration of propylene in
the liquid phase has a direct influence on the reaction
rate. The HPPO technology usually operates at pressures
above 15 bar (23).
The solvent used for the epoxidation of propylene has a
fundamental impact on the reaction rate. An OH-group
is mandatory for formation of the five-membered ring,
which is mechanistically necessary for the propylene
oxide production (29). Clerici et al. showed that using
methanol (being the smallest alcohol) results in the
highest activity for the epoxidation (30).
By using methanol not only higher conversion rates could
be achieved, but also an increased selectivity towards
propylene oxide (31). Corma et al. showed that the
polarity of the solvent is also very important (32). While
using a more hydrophilic Ti-Beta catalyst, aprotic solvents
like acetonitrile are superior, whereas using a
hydrophobic TS-1, protic solvents like methanol are the
solvents of choice. However, water as the smallest protic
solvent is not suitable since the solubility of propylene is
very low and the concentration of propylene at the
titanium center in the channels of TS-1 would be
insufficient.
Intense research has been carried out to use basic or
non-basic additives to improve the selectivity (33-35). The
basic additives are poisoning the acid sites of the TS-1,
moderating their activity. Furthermore unwanted side
reactions such as ring opening of propylene oxide are
diminished. The decomposition of hydrogen peroxide to
oxygen and water is another side reaction. Due to higher
temperatures in the catalyst bed, especially hot spots,
hydrogen peroxide tends to decompose (36, 37). It has
also been reported in the literature that non-tetrahedral
coordinated species of titanium enhance the
catalytically decomposition of hydrogen peroxide (37).
The deactivation of the catalyst accompanied by the loss
of catalytic activity is a challenge in heterogeneous
catalysis. The main reason for the deactivation of the HPPO
catalyst is caused by pore blockage due to side product
formation. However, the blockage of active sites is reversible
34
PROCESS DESIGN
The literature describes several reactor concepts for the
HPPO process (25-28). For laboratory and catalyst
research purposes, batch reactors proved to be the most
suitable solution. Upon scale up, packed bed reactors,
trickle bed reactors and heat exchanger reactors are
commonly used. The Evonik/TKIS technology employs a
trickle bed reactor operated at appropriate reaction
temperature (25). Besides a highly selective catalyst, an
efficient reaction temperature control is essential to
suppress side reactions and to ensure a high PO
selectivity.
A simplified process flow diagram with all reaction and
purification steps is shown in Figure 3 (41). The reactor is
fed with H2O2, propylene and a solvent. After the
reaction the residual propylene is separated in a
consecutive flash and purge gas system. The remaining
product stream is lead to a preseparation unit, where
an enriched propylene oxide containing stream is
separated from a water solvent mixture. The obtained
product stream is further purified via a propylene
stripper and a distillation column. The resulting
propylene oxide is very pure (polymer grade). The
remaining solvent/water mixture is purified and
recycled. The recycled propylene as well as, the
purified solvent is lead back to the reaction mixture
enabling integrated solvent recycling leaving only
water as a byproduct (42).
CONCLUSION
Propylene oxide ranges on place eleven of all organic
chemicals produced worldwide. It is one of the most
important epoxides currently used in industry. To serve
these market needs several industrial routes to produce
propylene oxide – with and without coproducts – have
been commercially established in the past. From the
coproduct producing processes only the PO/SM and
PO/MTBE process yielding styrene monomer
Chimica Oggi - Chemistry Today - vol. 32(2) March/April 2014
and methyl tert-butyl ether as coproduct are currently
economically feasible.
The HPPO process is an economically and ecologically
state-of-the-art technology being coproduct-free and
with water as the only waste product. The heart of the
HPPO process is the TS-1 catalyst system showing high
activity, improved selectivity for propylene oxide and low
H2O2 decomposition. Excellent process control of the
catalyst manufacturing is the vital necessity for creating
this high performing catalyst system.
2.
3.
4.
5.
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13.
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20.
22.
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24.
25.
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28.
29.
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on MICroreaCtIon teChnology / IMret13
Fundamentals: fluidics, mixing, mass & heat transfer
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