N–H insertion reaction via an iron carbenoid from α-diazophenylpropionate and its application to the formal total synthesis of stizolobinic acid

Article abstract of DOI:10.1016/j.tet.2020.131619

The formal total synthesis of the non-proteinogenic amino acid stizolobinic acid was accomplished relying as key step on an iron carbenoid-based N–H insertion reaction. The N–H insertion of α-diazophenylpropionate and benzylamine derivative catalyzed by tetrakis(pentafluorophenyl)porphyrin iron (III) chloride afforded the desired N–H insertion product in 79% yield. This is the first example of an N–H insertion involving α-diazophenylpropionate and an aliphatic amine catalyzed by an iron(III) porphyrin complex.

Full text of DOI:10.1016/j.tet.2020.131619

Egyptian Journal of Petroleum (2014) 23, 79–86
Egyptian Petroleum Research Institute
Egyptian Journal of Petroleum
www.elsevier.com/locate/egyjp
www.sciencedirect.com
FULL LENGTH ARTICLE
Fuel rich and fuel lean catalytic combustion
of the stabilized confined turbulent gaseous
diffusion flames over noble metal disc burners
*
Amal S. Zakhary, Ahmed K. Aboul-Gheit, Salwa A. Ghoneim
Egyptian Petroleum Research Institute (EPRI), P.O. Box 9540, Nasr City, Cairo 11787, Egypt
Received 19 June 2013; accepted 28 October 2013
Available online 19 March 2014
KEYWORDS
Abstract Catalytic combustion of stabilized confined turbulent gaseous diffusion flames using
Pt/Al2O3 and Pd/Al2O3 disc burners situated in the combustion domain under both fuel-rich and
fuel-lean conditions was experimentally studied. Commercial LPG fuel having an average compo-
sition of: 23% propane, 76% butane, and 1% pentane was used. The thermal structure of these
catalytic flames developed over Pt/Al2O3 and Pd/Al2O3 burners were examined via measuring the
mean temperature distribution in the radial direction at different axial locations along the flames.
Under-fuel-rich condition the flames operated over Pt catalytic disc attained high temperature
values in order to express the progress of combustion and were found to achieve higher activity
as compared to the flames developed over Pd catalytic disc. These two types of catalytic flames dem-
onstrated an increase in the reaction rate with the downstream axial distance and hence, an increase
in the flame temperatures was associated with partial oxidation towards CO due to the lack of oxy-
gen. However, under fuel-lean conditions the catalytic flame over Pd catalyst recorded compara-
tively higher temperatures within the flame core in the near region of the main reaction zone
than over Pt disc burner. These two catalytic flames over Pt and Pd disc burners showed complete
oxidation to CO2 since the catalytic surface is covered by more rich oxygen under the fuel-lean
Catalytic combustion;
Fuel-rich;
Fuel lean;
Noble metal burners;
Thermal structure;
Oxidation products
condition.
ª 2014 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute.
Open access under CC BY-NC-ND license.
1. Introduction
Catalytic combustion or heterogeneous combustion had been
extensively investigated in recent years. The catalytic oxidation
*
Corresponding author.
E-mail address: salwaghoneim70@yahoo.com (S.A. Ghoneim).
Peer review under responsibility of Egyptian Petroleum Research
Institute.
of hydrocarbons became the focus of much basic and applied
catalysis research because of its increasing importance for bur-
ner’s design of industrial furnaces and the techniques of
power-generating gas turbines [1,2]. For these applications,
high temperature catalytic combustion was regarded as highly
efficient and clean energy system. It had been recognized that
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80
A.S. Zakhary et al.
noble metals possessed the highest catalytic activities that ini-
tiated the catalytic oxidation of fuels at relatively lower reac-
tion temperatures [3,4].
controlled primarily by the oxygen mass transfer to the cata-
lyst under fuel-rich condition and not by the fuel flow or the
fuel reactivity for all the fuels tested.
Many experimental investigations reviewed the effect of
using noble metals in the catalytic combustion domain. In a
confined turbulent flame the effect of using Pt/cAl2O3 catalytic
disc burner was examined at different axial distances over the
fuel jet nozzle [5]. This investigation revealed how the highly
efficient oxidative exothermic reactions in contact with the ac-
tive surface of platinum burner greatly enhanced the heat
evolved via catalytic ignition and hence, improved the stabil-
ization tendency to a great extent. Also, the progress of the
combustion process over the platinum sites had controlled
the combustion emission products therefore minimizing the
environmental pollution. Moreover, Appel et al. investigated
the catalytically stabilized turbulent combustion of fuel-lean
hydrogen/air pre-mixtures over platinum and found that
nearly half of the fuel was converted heterogeneously and
the remaining part was combusted in the post-catalyst [6].
More recently, the catalytic combustion of different hydro-
carbon fuels over platinum addressed the interaction between
homogenous and heterogeneous reactions. The experimental
study revealed that in the presence of platinum the reactions
are complex and highly dependent on the fuel used as well as
other parameters such as temperature, equivalence ratio and
Reynolds number of the gaseous fuel–air mixture [7]. Further-
more, the effect of catalytic combustion of gaseous turbulent
diffusion flame over a series of noble metal disc burners (Pt,
Pd and Pt + Pd) supported on cAl2O3 were experimentally
and mathematically studied [8]. These catalytic flames behave
in highly catalytic conditions and their catalytic enhancement
was found to be in the order: (Pt + Pd) > Pt > Pd. The ther-
mal distribution along these catalytic flames recorded high val-
ues due to the enhancing of fuel oxidation on the noble metal
sites in the reaction zone of the flame via improving homoge-
nous gas/heterogeneous surface reactions in the combustion
domain.
Fuel-rich catalytic combustion had been investigated in or-
der to demonstrate the successful technology of ultra-low emis-
sions for gas turbine which in the mean time, offered multiple
advantages, [9–12]. The catalytic combustion of methane un-
der lean and rich conditions over platinum and palladium cat-
alysts was investigated using dilute mixtures [13]. It had been
found that under lean conditions Pd was the more effective
catalyst. Pt containing catalysts had been found to be more ac-
tive as the reactant mixture was shifted from oxygen–rich to
methane–rich. The platinum catalysts were superior to palla-
dium in a fuel-rich gas mixture. Thus, platinum had a role as
a component in the catalyst for emission control of natural
gas vehicles. Also, the performance data of catalytic combus-
tion were presented for methane oxidation over platinum
group catalysts under fuel-rich and fuel-lean conditions [14].
These authors found that under fuel-lean conditions, Pd cata-
lyst was the most active, although deactivation occurred above
650 ꢀC. While under fuel-rich conditions, Pt catalyst was more
active above 600 ꢀC and acquired much higher activity of the
reaction rates through the catalytic combustion domain.
A rich catalytic lean burn combustion system was devel-
oped for the operation of natural gas as fuel and other
non-methane fuels [15]. For fuel-rich operation the reactor
performance was insensitive to the fuel reactivity, because
the reaction rate (heat release) upon the catalyst surface was
For methane or natural gas fuels, the catalyst activity was
significantly improved by operating the catalyst under fuel-rich
conditions as compared to fuel-lean conditions and therefore,
allowing a wider choice of catalyst materials. Fuel-rich meth-
ane combustion over Rh-La-MnO3 honeycomb catalysts was
developed as a preliminary conversion step in advanced com-
bustion system such as power turbine and utility burners for
reducing emissions [16]. The experimental results showed that
mixed Rh-La-MnO3 catalysts were suitable for the fuel-rich
applications. However, a progressive reduction of light-off
temperature and a parallel improvement of the catalytic partial
oxidation performance were observed by increasing Rh con-
tent in the Rh-La-MnO3 catalysts.
The present work investigated the effect of using Pt/cAl2O3
and Pd/cAl2O3 catalytic disc burners situated in the combus-
tion domain of confined turbulent stabilized gaseous diffusion
flames under fuel-rich and fuel-lean conditions. The thermal
structure of these catalytic flames developed over Pt and Pd
catalytic disc burners was examined by measuring the mean
temperature distribution in the radial direction at different ax-
ial locations along the flames. Also, the axis–symmetric distri-
butions of CO and CO2 along the flames were monitored
under the same conditions to clarify the catalytic combustion
process performance over the two current catalytic discs.
2. Experimental
The experimental setup (Fig. 1) was comprised of a vertical
cylindrical combustion chamber filled with an arrangement
supplying fuel and air. The combustion chamber (Fig. 2) is
150 mm in diameter, 5 mm thick and 1.0 m long. The combus-
tor was equipped with a thermal resistant glass window. The
fuel jet was discharged vertically through a nozzle of 2.5 mm
diameter connected at the centre of the fuel supply line in
the axial direction at the base of the combustor. Commercial
LPG fuel having an average composition of: 23% propane,
76% butane, and 1% pentane was used in all experiments.
Two catalytic disc burners of Pt and Pd over c-Al2O3 hav-
ing a diameter of 40 mm, 4 mm thick and perforated with 25
holes had been separately used as a catalytic flame burner.
The discs were placed at the base of the combustor at the spec-
ified supporting distance of 40 mm over the fuel jet nozzle.
These discs were made of c-Al2O3 with a surface area of
60m2gꢀ1. Each Al2O3 disc support was made by mixing c-
Al2O3 powder with a suitable binder then pasted and formed.
After drying at 110 ꢀC overnight, the disc was perforated by
drilling to acquire a suitable perforation (3 mm holes diame-
ter). The disc was then calcined at 400 ꢀC for 4 h. in a muffle
furnace. The heat treatment gave the highest crushing strength
while retaining the catalytic activity.
The 1st disc (Pt/ c-Al2O3) was impregnated with H2PtCl6
solution such that the Pt content was 0.0001 wt% of the disc.
The disc was again dried at 110 ꢀC overnight and calcined at
550 ꢀC for 4 h.
The 2nd disc (Pd/c-Al2O3) was prepared via wet impregna-
tion of an aqueous solution of Pd (NO3)2 containing 10ꢀ4 g of
Pd metal. The impregnation was adjusted to incorporate the
Pd containing solution on the external surface (1 mm depth).

Fuel rich and fuel lean catalytic combustion
81
Figure 1 Experimental set up.
À
Á
mꢁfuel
mꢁair
experimental
À
Á
Overall Conventional / ¼
¼ 1
mꢁfuel
mꢁair
stoichiometry
A modification was introduced by Schmidt and coworkers
[17] including the modified equivalence ratio as:
/
Modified / ¼
1 þ /
This modification had the advantage that it considered
equal weight on the fuel-lean and the fuel-rich sides of the igni-
tion condition. Unlike the overall conventional equivalence ra-
tio Ø, the modified equivalence ratio Ø was bounded between
0 and 1.0, and was symmetric about the stoichiometric point
(0.5) having an equal range of values on both the fuel-lean
(Ø < 0.5) and fuel-rich (Ø > 0.5) sides. Two gas compositions
were selected for testing, one at (Ø = 0.25) in the ‘‘middle ‘‘of
the fuel-lean ‘‘mixture composition space’’, and the other at
(Ø = 0.75) in the ‘‘middle’’ of the fuel-rich mixture composi-
tion space.
3. Results and discussion
Figure 2 Combustor and burner arrangements.
The performance data are presented in Figs. 3–10 for the cat-
alytic combustion of stabilized gaseous turbulent diffusion
flames on Pt/c-Al2O3 and Pd/c-Al2O3 catalytic disc burners
under both fuel-rich and fuel-lean conditions. The thermal
structure of the stabilized catalytic flames was determined via
measurements of the temperature profile at different axial loca-
tions over the catalytic discs. The temperature profiles were
characterized by steep gradients due to chemical combustion
(oxidation) reactions and flame boundary layer where the heat
transfer occurred.
Results of the mean temperature distributions of the
stabilized turbulent diffusion catalytic flames developed over
Pt/c-Al2O3 and Pd/c-Al2O3 catalysts in the radial direction
under fuel-rich condition at the modified equivalence
ratio (Ø/1 + Ø = 0.75) are shown in Figs. 3 and 4.
In Fig. 3, the thermal structure of the stabilized catalytic
flames was developed over Pt/Al2O3 under fuel rich condition
at Ø/1 + Ø = 0.75 and had been characterized by the radial
distribution of the measured mean temperature at different
axial distances over the disc. The results showed comparatively
high temperature values within the flame core at the near
upstream distance within the central flame region, at
Each catalytic disc was then held at the combustor position
in each experiment.
Detailed local measurements of the mean temperature were
examined along the stable flames in the presence of each
catalytic disc burner. The temperature measurements had been
obtained by using a thermocouple probe (platinum/platinum–
13% rhodium, 0.1 mm diameter wire). The thermocouple
output voltage was integrated over a period of 20 s using a
microprocessor integrator. Also, the axial distribution of CO
and CO2 concentrations had been achieved along the flame
developed over the catalytic disc burners. The concentrations
were obtained through the use of stainless steel water cooled
sampling probe with 1.0 mm inner diameter and 6.0 mm outer
diameter. Samples of the combustion products were condi-
tioned and measured using on-line gas analyzer (Anapol EU-
200/4).
For each catalytic disc burner, the developed flame was
tested under both fuel-rich and fuel-lean conditions. These test
conditions were usually defined in the combustion literature by
the term of overall equivalence ratio, Ø, which was usually
defined in the combustion literature as:

82
A.S. Zakhary et al.
Figure 3 Radial mean temperature distribution over Pt/Al2O3
disc for fuel-rich condition, Ø/1 + Ø = 0.75 at different axial
distance x.
Figure 4 Radial mean temperature distribution over Pd/Al2O3
disc for fuel-rich condition, Ø/1 + Ø = 0.75 at different axial
distance x.
X = 30 mm indicating 400 ꢀC with respect to Pd disc (Fig. 4).
The temperature profile attained maximum values at the edges
of this zone then dropped radially outwards. Evidently, under
fuel-rich mixture, the flame core was hot at the early upstream
region in the main reaction zone of the stabilized turbulent
combustion domain. A gradual increase of the axial location
over the Pt catalytic disc, the radial mean temperature distribu-
tion along the flame expressed the progress of the combustion
process demonstrating much higher activity. A higher reaction
activity under fuel-rich condition could be assumed to provide
reduced catalyst light-off temperature for catalytic flames
operating over the Pt disc.
In Fig. 4, the radial mean temperature distribution of the
catalytic stabilized flame over the Pd disc at different axial
locations proceeded in the same trend as in the use of Pt disc
but showed comparatively lower activity. At the early up-
stream distance over Pd disc the temperature recorded
300 ꢀC at X = 30 mm within the flame core. However, the
chemical reactions of the combustion process proceeded with
lower active rates over the Pd disc in the combustion applica-
tion under fuel-rich condition.
Fig. 5 shows the axial mean temperature distribution of the
catalytic stabilized flames developed over Pt/Al2O3 and Pd/
Al2O3 catalytic disc burners under fuel-rich condition, (Ø/
1 + Ø = 0.75). The temperature distribution of these two sta-
bilized catalytic flames exhibited similar trends; a gradual in-
crease of temperature as a function of the axial distance,
reached a maximum due to the progress of the combustion fol-
lowed by a gradual decline. The maximum mean temperature
of the catalytic flame developed over Pt is about 1280 ꢀC at
X = 115 mm whereas the maximum mean temperature of cat-
alytic flame over Pd was about 980 ꢀC at X = 140 mm. It was

Fuel rich and fuel lean catalytic combustion
83
Figure 6 Axial distribution of CO% vs. axial distance at fuel-
rich condition, Ø/1 + Ø = 0.75.
Figure 5 Axial mean temperature distribution vs. axial distance
at fuel-rich condition, Ø/1 + Ø = 0.75.
Pt/Al2O3 and Pd/Al2O3 disc burners at different axial locations
are shown in the Figs. 7 and 8, respectively. Over the Pt disc
burner, (Fig. 7), the catalytic flame was acquired at near up-
stream locations, X = 30, 45, 60, 75, the main reaction zone
enclosed the flame core and was characterized by compara-
tively low temperature due to the presence of the unreacted
mixture under the fuel-lean condition. Because of the excess
oxygen, the activation of the fuel on the catalyst surface was
a limiting step, while the adsorption of oxygen is fast and
proceeded without significant activation barrier. The catalyst
surface was therefore, covered by oxygen as adsorbent of O2
molecules on the Pt particles. Some forms of metal oxide
species could be assumed to contribute also to this reaction.
Nevertheless, the temperature profiles at the downstream loca-
tions indicated a gradual increase in the main values. Because
Pt had been found to be activated above 600 ꢀC [5], a gradual
increase of the reaction rate was observed, i.e. the thermal
energy increased.
evident that the chemical reaction proceeded with lower rates
over Pd disc reaching the maxima at a further downstream dis-
tance in the catalytic combustion domain.
Fig. 6 shows the axial distribution of, CO%, at different ax-
ial locations along the stabilized flames over the two catalytic
discs under fuel-rich condition (Ø/1 + Ø = 0.75). Pt and Pd
catalysts showed the highest activity to partial oxidation to
CO. The lack of oxygen over the catalysts provided CO which
correspondingly increased with increasing the temperature as
shown in Fig. 5. As the catalyst temperature increased the CO
desorption rate increased which manifested the increase of the
functionalization towards the partially oxidized product, CO.
Under the fuel-lean condition at a modified equivalence ra-
tio (Ø/1 + Ø = 0.25), the radial mean temperature distribu-
tion for the two stabilized catalytic flames operated over

84
A.S. Zakhary et al.
Figure 7 Radial mean temperature distribution over Pt/Al2O3
disc at fuel-lean condition, Ø/1 + Ø = 0.25 at different axial
distance x.
Figure 8 Radial mean temperature distribution over Pd/Al2O3
disc at fuel-lean condition, Ø/1 + Ø = 0.25 at different axial
distance x.
On the other hand, Fig. 8 represents the radial mean tem-
perature distribution of the two stabilized catalytic flames over
Pd disc burner at different axial locations under fuel-lean
condition (Ø/1 + Ø = 0.25). A closer inspection of the results
revealed that the mean temperature values at early upstream
distance within the flame core were comparatively higher with-
in the main reaction zone as compared to the catalytic flame
over Pt disc in the same case of fuel-lean condition. The Pd cat-
alyst was the most active under this lean condition, although,
Lyubovsky et al. [14], reported deactivation above 650 ꢀC, i.e.
the reaction rate decreased.
developed over Pt and Pd disc burners at different locations
under fuel-lean condition (Ø/1 + Ø = 0.25). At these condi-
tions, the two flames behaved in the same manner, a gradual
increase of temperature with respect to the axial distance;
reached a maximum followed by a gradual decay. The mean
temperature values for the flames operated over Pd catalytic
disc burner at upstream distances from X = 30 up to 75 mm
recorded higher values than the flames over Pt disc. Pd catalyst
was the most active at fuel-lean condition. Lyubovsky et al.
[14] confirmed this result especially within the range up to
650 ꢀC. This is remarkably noticed within the flame core at
the main reaction zone. A maximum temperature of 960 ꢀC
Fig. 9 includes a relation of the axis-symmetric mean tem-
perature distribution of the two stabilized catalytic flames

Fuel rich and fuel lean catalytic combustion
85
Figure 10 Axial distribution of CO2% vs. axial distance at fuel-
lean condition, Ø/1 + Ø = 0.25.
4. Conclusions
The following conclusions were derived from the obtained
results:
Figure 9 Axial mean temperature distribution vs. axial distance
at fuel-lean condition, Ø/1 + Ø = 0.25.
1- The thermal structure of the stabilized turbulent gaseous
diffusion flames developed over two catalytic disc burn-
ers, Pt/c-Al2O3 and Pd/c-Al2O3 indicated higher activity
under fuel-rich than under fuel-lean conditions.
2- The catalytic flame operating over Platinum disc burner
attained comparatively higher temperature values at
early upstream region of the main reaction zone and
reached the peak thermal values at nearer axial location
as compared to the flame developed over Palladium disc
burner under fuel-rich condition.
3- The stabilized flames over Pt and Pd catalytic discs
showed high activity towards partial oxidation to CO
due to the lack of oxygen over the disc, which were
increased with increasing the temperature under fuel-
rich conditions.
is attained at X = 115 mm followed by gradual decay up to
225 mm. The recorded maximum temperature along the flame
over Pt disc is 1025 ꢀC at X = 140 mm. Smith et al. [15]
reported that Pt is active above 600 ꢀC in all conditions.
Fig. 10 shows the axial distribution of CO2% along the two
stabilized catalytic flames at different axial distance over
Pt and Pd disc burner under fuel-lean condition (Ø/
1 + Ø = 0.25). The axial distribution of CO2% behaved in a
similar manner as the axial thermal distribution in Fig. 9. Since
the catalytic surface was covered by excess oxygen under
fuel-lean conditions, the fuel dissociated with high rates and
immediately was oxidized to CO2 and H2O.

86
A.S. Zakhary et al.
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at early upstream distance within the catalytic
flame core operating over the Pd disc burner were
higher as compared to the catalytic flames over the
Pt disc.
5- The two catalytic flames over Pt and Pd disc burners
showed the complete oxidation to CO2 since the
catalytic surface was covered by excess oxygen under
fuel-lean condition.
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