Synthesis of tert-Butyl Peroxypivalate using a Microreactor
dence time of 1.5 s, and a reaction temperature of 408C. The
corresponding space-time yield was 469000 gLÀ1 hÀ1. After ex-
traction (see Experimental Section) of the separated organic
phase, a 7% loss in yield took place, resulting in a space-time
yield of 420000 gLÀ1 hÀ1, which is an order of magnitude
higher than for the aforementioned processes and demon-
strates the potential of this concept.
reactor setup using 9 orifices with a distance of 5 cm at a reac-
tion temperature of 408C resulted in a space-time yield of
420000 gLÀ1 hÀ1, which is orders of magnitude higher than the
space-time yield for a triple cascaded batch reactor process
(190 gLÀ1 hÀ1).
Experimental Section
General
Conclusions
KOH (puriss. p. a., Reag. Ph.) and TBHP (68 wt% solution in water)
were purchased from Aldrich and used in the preparation of fresh
aqueous solutions of KOH (22.7 wt%) and KTBP (2.65 molLÀ1). Ace-
tonitrile (Rotisolv HPLC 99.9%) and NaOH (99% p. a.) were pur-
chased from Carl-Roth, 1n aqueous HCl (Titrisol) from Merck, extra-
pure NaC2H3O2·3H2O from Riedel-de-Haꢂn, and NaHCO3 (@99.7%
puriss. p. a.) from Fluka. The orifices used were CUCPK Unions
(tubing OD=1.59 mm; i.d.=0.25 mm) from Nordantec GmbH.
The shift from batch to continuous processing for the deproto-
nation step was successfully demonstrated with a simple mixer
tube setup connected upstream of the second reaction step.
The process performance was determined indirectly by the
amount of TBPP produced, since the direct measurement of
the deprotonation was not possible. The continuous deproto-
nation showed similar performance compared to the TBPP for-
mation by which the deprotonation was done in batch mode.
The concept of an orifice setup is introduced here as one pos-
sibility to re-emulsify the biphasic reaction mixture of the
second reaction step. A flow velocity of 0.34 msÀ1, which equa-
tes to an energy density of 3.5ꢁ105 JmÀ3, is needed to break
up the biphasic reaction mixture into smaller droplets. The
shortening of orifice distances was shown to significantly
shorten the reaction time (down to 0.5 s giving a yield of
ca. 70% by HPLC, non-isolated). There is evidence that the ca-
pillary length between the orifices only contributes a minor
amount to the yield obtained (as compared to the total reactor
length), but rather is needed for heat removal. The influence
of increased reaction temperature to open a novel process
window for the TBPP synthesis was demonstrated by two dif-
ferent setups, distinguishing in the distances of the emulsifica-
tion units. By using a setup with a distance of 52 cm from ori-
fice to orifice, an optimum of reaction performance was found
at a reaction temperature of 508C with a corresponding yield
of roughly 75% by HPLC (non-isolated). The increase to higher
reaction temperatures had no significant impact on reaction
performance, but instead caused undesired gas formation. De-
pending on the setups used, and thus depending on the pro-
vided interfacial area the maximum reaction temperature that
can be used without gas formation is between 408C and 508C.
The difference of the investigated setup variations was dimin-
ished by higher reaction temperatures. This diminution can be
a result of changed physical properties and thus decreased dif-
ferences in created interfacial area, or by internal hot spots,
which counteract the effect of increased reaction temperature.
Further investigations are needed to find an optimum combi-
nation of orifice geometry and distances. The implementation
of this orifice concept into a microreactor design is expected
to improve the TBPP process significantly and thus further in-
vestigations should help to optimize this single-channel micro-
reactor setup and lead to a specially designed microreactor for
the synthesis of TBPP. Finally, an approximate benchmark be-
tween an industrial process performed in a 3 cascaded batch
reactor process with a single reactor volume of 350 L, a micro-
reactor process, and two single-channel microreactor setups
has been established. Processing in the single-channel micro-
In the caterpillar micromixer, mixing occurs due to splitting and re-
combination of the fluid streams. A more detailed discussion of
this principle is given in Ref. [30]. Mixer 1=CPMM-V.1.2-R300/12
from IMM; mixer 2=CPMM-V1.2-R300/12-PEEK-prefla from IMM.
Process parameters
Batch deprotonation of TBHP: For the preparation of KTBP in
batch mode, a freshly prepared 22.7 wt% aqueous solution of KOH
in a standard laboratory 3-necked flask was immersed in a water
bath to maintain a reaction temperature 20–258C. The amount of
KOH was set to be in a slight excess of 1.12 equivalents based on
TBHP. A 68 wt% aqueous solution of TBHP was added in portions,
in such a way that the temperature of the reaction mixture did not
exceed 258C. The concentration of the resulting aqueous KTBP so-
lution was 2.65 molLÀ1 (31 wt%).
Continuous deprotonation of TBHP: For the continuous deprotona-
tion step, both reactants—KOH (22.7 wt%) and TBHP (68 wt%)—
were fed with HPLC-pumps (Knauer K-501 with a 10 mL stainless
steel pump head) into a caterpillar micromixer (CPMM-V.1.2-R300/
12 from IMM) connected with a reaction time section (Teflon or flu-
orinated ethylene propylene (FEP)) capillary; OD=1.59 mm; i.d.=
0.25 mm, L=110 cm). Both feed streams were preheated and
cooled using a 1/16’’ Teflon capillary. The outlet and both inlet
temperatures were controlled in line, using a Type K miniature
thermocouple. The feed rates were adjusted in such a way that the
KOH was in a slight excess of 1.12 equivalents based on TBHP
(TBHP feed rate=3.1 mLminÀ1
; KOH (22.7 wt%) feed rate=
5.02 mLminÀ1; PivCl feed rate=2.2 mLminÀ1). The heat of reaction
À1
was 23 kJmol , resulting in an adiabatic temperature rise of 25 K
TBHP
À1
À1 [6]
for a solution of 6.78 molL
and a solution of 4.85 molL
.
TBHP
KOH
Conversion of KTBP and PivCl to TBPP: The premixing step for the
conversion of KTBP with PivCl was done by using a caterpillar mi-
cromixer (CPMM-V1.2-R300/12-PEEK-prefla from IMM). Re-emulsifi-
cation was done using orifices with an i.d. of 0.25 mm made out of
polyether ether ketone (PEEK). The number of orifices and their dis-
tances were varied. An overview on the setups used is given in
Table 1. The PivCl was fed via a Knauer Smartline 1000 with a
10 mL titanium pump head. The necessary residence time was pro-
vided by a 1.59 mm FEP capillary with an i.d. of 0.8 mm. The heat
À1
of reaction was 126 kJmol , resulting in an adiabatic temperature
PivCl
À1
rise of 72 K for a solution of 2.65 molL and a solution of
KTBP
À1 [6]
8.04 molL
.
PivCl
ChemSusChem 2011, 4, 392 – 398
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
397