Increment of activity of Pd(OH)2/C catalyst in order to improve the yield of high performance 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW)

Article abstract of DOI:10.1080/24701556.2017.1357593

CL-20 [2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW)] is a well-known high performance explosive. The reductive debenzylation of hexaazaisowurtzitane (HBIW) over supported palladium catalyst produces 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-h

Full text of DOI:10.1080/24701556.2017.1357593

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Increment of activity of Pd(OH)2/C catalyst in
order to improve the yield of high performance
2,4,6,8,10,12-hexanitrohexaazaisowurtzitane
(HNIW)
Farshad Fotouhi-Far, Hadis Bashiri, Masood Hamadanian & Mohammad
Hossein Keshavarz
To cite this article: Farshad Fotouhi-Far, Hadis Bashiri, Masood Hamadanian & Mohammad
Hossein Keshavarz (2017): Increment of activity of Pd(OH)2/C catalyst in order to improve the yield
of high performance 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW), Inorganic and Nano-
Metal Chemistry, DOI: 10.1080/24701556.2017.1357593
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1357593
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Increment of activity of Pd(OH)2/C catalyst in order to improve the yield of high
performance 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW)
Farshad Fotouhi-Far1, Hadis Bashiri*,1, Masood Hamadanian1 and Mohammad Hossein
Keshavarz*,2
1Department of physical chemistry, Faculty of chemistry, University of Kashan, Kashan, I. R.
Iran
2Department of Chemistry, Malek-ashtar University of Technology, Shahin-shahr P.O. Box
83145/115, Islamic Republic of Iran
*To whom correspondence should be addressed: (1) Prof. M. H. Keshavarz, E-mail:
mhkeshavarz@mut-es.ac.ir; keshavarz7@gmail.com; Tel: (0098) 031 45225071; Fax: (0098)
031 45225068; Tel: (0098)-0314-591-2728, Fax: (0098)-0314-522-0420;(2) Dr. H. Bashiri, E-
mail: hbashiri@kashanu.ac.ir; h.bashiri@ymail.com
Abstract
CL-20 [2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW)] is a well-known high
performance explosive. The reductive debenzylation of hexaazaisowurtzitane (HBIW) over
supported palladium catalyst produces 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-
hexaazaisowurtzitane (TADB). The effects of precipitation conditions of Pd(OH)2 and the
porosity of the activated carbon support on the activity of synthesized Pd(OH)2/C catalysts in the
debenzylation of HBIW are investigated here. The activity of Pd(OH)2/C catalyst is considerably
affected by the porosity of the activated carbon. The surface morphology of the synthesized
catalysts is studied by X-ray diffraction (XRD) profile analysis and transmission electronic
microscope (TEM). Metal dispersion of Pd(OH)2/C catalysts is changed by precipitation
conditions of Pd(OH)2.
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Keywords
Precipitation; Hexanitrohexaazaisowurtzitane; Debenzylation; Heterogeneous catalysis;
Palladium catalyst
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1. Introduction
Among different nitramines, hexanitrohexaazaisowurtzitane (HNIW) or CL-20 (2, 4, 6, 8,
10, 12 -hexanitro-2, 4, 6, 8, 10, 12 hexaazatetracyclo [5.5.0.03,11.05,9] dodecane) contains high
detonation performance [1]. Since CL-20 is superior oxidizer/explosive for using in conventional
high-energy propellants and explosive formulations, some efforts have been used to develop its
[3]
behaviors in the other fields, e.g. adsorption on a soil organic matter [2], thermal reactivity
,
[4]
fabrication of microcapsule
and corystal CL-20 and HMX (cyclotetramethylene-
tetranitramine) [5]. Due to importance of CL-20, several methods [6] have been developed that are
based on the condensation of glyoxal and benzyl amine (Figure 1). Among the synthesis steps of
HNIW (Figure 1), the reductive debenzylation of hexabenzylhexaazaisowurtzitane (HBIW) over
supported palladium catalyst is the most important step, which produces 2,6,8,12-tetraacetyl-
4,10-dibenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADB). Since the palladium catalyst rapidly
deactivates in this step, the costs of the production of intermediates and the HNIW are high.
However, it is important to increase the activity of catalyst. The activity of the supported
catalysts depends on availability of active sites in supported metal catalysts, as metal dispersion,
which participates in a desired catalytic reaction. Higher metal dispersion and more metal surface
sites produce smaller metal particles, having higher surface to volume ratio, which generally
relate to higher activity per mass of metal. The method of preparation of the catalyst and the
nature of support can strongly influence the metal dispersion as well as the activity of the
supported metal catalysts, which were demonstrated elsewhere [7].
[7c]
Simonov et al.
have done extensive research on the chemical processes, which occur
when H2PdCl4 from aqueous solution is adsorbed on the surface of carbon materials. They found
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that adsorption proceeds through two competitive pathways: reduction to form metallic Pd
particles and formation of π-complexes of PdCl2 with C = C fragments of the carbon matrix:
2
1
PdCl
C
Pd 2Cl
C
2 2HClH2PdCl4 

2HCl (1)
2C
They stated that Pd2+ (aq) ions are discharged by the “electron gas” of carbon accompanied by
adsorption of Cl ions. The C+ is a positively charged “hole” which appears at the carbon surface
when Pd2+ (aq) is reduced. The Cl ions compensate the charge through the formation of an
electric double layer on the carbon surface.
[7f]
Koskin and coworkers
studied the influence of the solvent nature, loadings of the
reactants, catalyst and cocatalyst as well as reaction temperature on debenzylation of HBIW.
Later they studied the influence of the synthesis methods and the nature of support of the Pd/C
catalyst on the debenzylation of HBIW [7g]
.
The purpose of this work is to investigate the effects of precipitation conditions of Pd(OH)2
including Pd content in catalyst, pH, temperature and time of Pd(OH)2 precipitation and the
porosity of the activated carbon support on the activity of catalyst in the reductive debenzylation
of HBIW. It will be shown that optimization of these variables can provide higher yield of
TADB. Moreover, the TADB yield will be considered as a criterion for assessing the activity of
samples of Pd(OH)2/C catalyst.
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2. Experiment
2.1. Pretreatment of activated carbon supports
The commercial activated carbons were supplied by Merck and used to prepare
Pd(OH)2/C catalysts. Activated carbon samples were treated by HCl 20% solution at 80°C for 4
hours. They were treated with acid to remove probable metal impurities from activated carbons.
2.2. Synthesis of Pd(OH)2/C catalysts
PdCl2 was supplied by Sigma-Aldrich. Na2CO3 and HCl were supplied by Merck. They
were used without any form of purification. The Pd(OH)2/C catalyst or unreduced palladium
catalyst was prepared via deposition-precipitation method [7a]. Acidic PdCl2 or H2PdCl4 solution
was used as precursor. To prepare H2PdCl4 solution, PdCl2 was dissolved in concentrated HCl
(37%), which was diluted with distilled water to the desired concentration. The mole ratio of
Pd:Cl was adjusted to 1:10 in the precursor solution. The precursor solution was added to
activated carbon, which was stirred with a magnetic stirrer at 30°C for 2 hours. While stirring
was continued, the pH of the mixture was slowly increased by addition of Na2CO3 (10%)
solution. H2PdCl4 was hydrolyzed in aqueous solution and converted to Pd(OH)2 on the activated
carbon supports. The Pd(OH)2/C catalyst or unreduced palladium catalyst was washed with
distilled water that was filtered through a sintered glass funnel and dried in oven at 50°C for 6
hours. In each run about 2 grams of Pd(OH)2/C catalyst was synthesized.
2.3. Reductive debenzylation of HBIW
All solvents and reagents except HBIW were supplied by Merck, which were used without any
form of purification. HBIW was produced and purified in our laboratory. Debenzylation of
HBIW over Pd(OH)2/C catalyst was carried out according to the method of Wardle and
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coworkers [8] where their method was used in the related works[7f, 7g, 9]. The highest reported yield
for this reaction was about 90% (desired product). To investigate the effects of Pd(OH)2
precipitation conditions and the porosity of carbon support on catalyst activity, all catalytic
reactions were performed in the same conditions. For all catalytic runs, the mole ratio of
reactants and other conditions except Pd(OH)2/C catalysts were similar. HBIW (20 g),
bromobenzene (0.8 ml) and dry Pd(OH)2/C catalyst (1.4g) were added to a mixture of DMF
(156.5 g) and acetic anhydride (32.5 g) in a 1 liters semi batch stainless steel reactor. Pressure of
H2 gas and reactor temperature was adjusted respectively at 4 bar and 50°C during the reaction.
After 5 hours, the reaction was terminated by stopping H2 and the content of the reactor was
drained. The solid was filtered and washed with ethanol. The highest TADB yield was obtained
about 73%. After 4 hours from the beginning of the debenzylation reaction, the TADB yield
remains approximately constant. The reaction was appeared to no longer progress probably due
to deactivation of the active sites and the decreasing of HBIW surface concentration. The
unreacted HBIW as substrate and synthesized TADB as desired product were characterized by
spectroscopic (using FT-IR NICOLET 800 and H-NMR BRUKER 300 MHZ) and liquid
chromatographic analysis (using HPLC Waters 600 with a C-18 column and an acetonitrile/water
mixture in a ratio of 60: 40, as eluent). The relative standard deviation of TADB yield was
examined by repeating procedure fifth times for Pd(OH)2/C catalyst samples and was ±3%.
2.4. Catalyst characterization
The palladium content in catalyst and precursor solution were determined by a Perkin
Elmer 5300 DV inductively coupled plasma (ICP). To measure the palladium content in catalyst,
the catalyst samples were burnt in a furnace under O2 atmosphere at 900°C. Remaining catalyst
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sample was dissolved in aqua regia (a 1:3 mixture of nitric acid and hydrochloric acid) and the
palladium content in leached solution was measured by ICP. The detection limits of the method
was 3 mg/g and the relative standard deviation was lower than 2% (n = 5). The pH of the
solutions was measured with a standard pH electrode. The specific surface area and pore volume
of the synthesized palladium catalyst were measured by N2 adsorption at 77 K with a quanta
chrome gas sorption instrument. The chemisorption of H2 over Pd(OH)2/C catalyst at 303 K was
carried out by BEL Japan Inc. The samples were degassed in vacuum at 423 K for 4 hours before
measurements. The morphology of the catalyst was studied by Philips CM30 transmission
electronic microscope (TEM). Wide-angle X-ray diffraction (XRD) pattern of catalyst was
recorded on a Philips PW 1830 with CuK radiation (λ of 0.154178 nm at 40 kV and 30 mA). The
XRD scan was performed at ambient temperature with a 2θ range between 10o and 90o at a
scanning rate of 0.04° (2θ/ sec).
3. Results and Discussion
3.1. Effect of palladium content on catalyst activity
Figure 2 shows the plot of TADB yield versus palladium content in catalyst. The TADB
yield for the debenzylation reaction over catalyst with the palladium content of 3 wt% is
negligible. The TADB yield is increased significantly by increment of the palladium content to 7
wt%,. There is no further change in the TADB yield by increasing the palladium content of
catalyst to more than 13 wt%. When the active component is very expensive (as supported
palladium catalyst), the objective is a maximum active surface area per unit weight of the active
component. Therefore, active components will be applied to supports at low loadings [10]. The
reaction over catalyst with palladium content less than 3 wt% does not proceed to produce
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TADB. A probable reason is that HBIW molecules cannot be easily transferred to the active sites
[7g, 8]
of catalyst. Previous studies
have shown that debenzylation of HBIW occur over catalyst
with palladium content more than 5 wt%. Increasing the palladium content of catalyst to 7 wt%
causes the increment of the active surface of catalyst, which gives higher yield of TADB. The
probability of aggregation among palladium particles was increased by increment of the Pd
content of catalyst to more than 13 wt%. Despite of the further increment of the palladium
content of catalyst, the TADB yield has remained constant and therefore the activity of catalyst
was decreased.
3.2. Effect of pH of Pd(OH)2 Precipitate on catalyst activity
The activity of catalyst can be affected by the pH of precipitation [11]. Since all of the
synthesis conditions were similar except the pH of Pd(OH)2 precipitate, different acidic to basic
pH ranges were examined for Pd(OH)2 precipitate to prepare palladium catalyst. Figure 3 shows
the TADB yield for debenzylation reaction over the synthesized palladium catalyst versus the pH
of Pd(OH)2 precipitate. Catalyst precipitation conditions are: Pd content in precursor
solution/catalyst = 13 wt%, time = 5 hr, temperature = 50°C. As seen in Figure 3, the TADB
yield is negligible for the pH<3.0 of Pd(OH)2 precipitate. By increasing the pH of Pd(OH)2
precipitate to 9, the TADB yield increased significantly. For the pH>9.0 of Pd(OH)2 precipitate,
the TADB yield approximately was remained constant. Since a part of Pd2+ (aq) ions was
remained in the precursor solution for the pH<3.0 of Pd(OH)2 precipitate, the palladium content
in the catalyst can be decreased. By decreasing the palladium content in catalyst, the TADB yield
can be reduced. However, low pH of the Pd(OH)2 precipitate may raise the probability of co-
precipitation of Pd(OH)2 and PdCl2. It was reported that the co-precipitation of PdCl2 and
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Pd(OH)2 in low pH (<6) can enhance the reaction rate of debenzylation over the unreduced
palladium catalyst [12]
.
3.3. Effect of temperature of Pd(OH)2 precipitate on catalyst activity
Temperature of precipitation can affect the nucleation and growth of crystals, which may
influence the metal dispersion in the catalyst as well as the catalyst activity. Therefore, the effect
of the temperature of Pd(OH)2 precipitate on the TADB yield was also studied. Figure 4 shows
the TADB yield for the debenzylation of HBIW over the synthesized palladium catalyst at
several temperatures of Pd(OH)2 precipitate from 30 to 90°C. The results show that the TADB
yield was slightly affected by the temperature of Pd(OH)2 precipitate in this temperature range.
Hydrolysis rate of Pd2+(aq) ions in precursor is so high that the changes of Pd(OH)2 precipitate
temperature do not have major effects on the nucleation and growth of Pd(OH)2 colloids. Then,
Pd(OH)2 colloids was deposited on carbon surface under the effects of mass transfer phenomena.
3.4. Effect of time of Pd(OH)2 precipitate on catalyst activity
Palladium catalysts were synthesized at different precipitation times from 1 to 10 hours to
determine the appropriate time of Pd(OH)2 precipitate. Figure 5 shows the TADB yield for the
debenzylation of HBIW over palladium catalyst versus the time of Pd(OH)2 precipitate. As seen,
the TADB yield is relatively low for the time of Pd(OH)2 precipitate less than 1 hour.
Meanwhile, the TADB yield is increased with an increment of the time of Pd(OH)2 precipitate
until 5 hours. For above 5 hours, the TADB yield is remained nearly constant. For precipitation
reactions, hydrodynamic conditions have a major impact on the rates of nucleation as well as
agglomeration and the product quality [13]. Higher numbers of Pd(OH)2 particles are precipitated
on activated carbon support by increasing the time of Pd(OH)2 precipitate. Increment of
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palladium content of catalyst can increase the active surface of catalyst, which enhance the
TADB yield.
3.5. Effect of the activated carbon porosity on the activity of catalyst
Since the surface functional groups of the activated carbon supports affect the yield of
heterogeneous catalytic reactions, the differences in activity for catalysts on supports of different
origin should be ascribed to differences in their porous structure [7e]. Therefore, it is important to
study the effect of the porosity of the activated carbon supports on activity of the synthesized
catalysts. Three types of commercial activated carbon were chosen as catalyst supports, which
were used to prepare palladium catalysts. Table 1 shows the characteristics of the prepared
catalysts. As seen in Table 1, the BET surface and cumulative micropore surface areas (m2/g),
respectively, for Pd(OH)2/C1 sample are considerably more than those for the other two catalysts.
For three Pd(OH)2/C samples at 303 K, the amount of chemisorbed hydrogen was approximately
equal (Table 1). In other words, the metal dispersion is the same in catalysts. However, in similar
conditions, the TADB yield was considerably different for each Pd(OH)2/C samples. By
assuming a similar metal dispersion for the three samples, decreasing TADB yield over
Pd(OH)2/C1 and Pd(OH)2/C3 with respect to Pd(OH)2/C2 may attribute to their micropore surface
area. Since HBIW is relatively large molecule, it cannot be easily transferred into micropores and
a portion of mesopores in the catalyst. Hence, a fraction of the active sites on catalyst surface is
not accessible in the catalytic debenzylation of HBIW.
3.6. Electron microscopy imaging and XRD analysis
Activity of the supported catalysts depends largely on metal dispersion. The wide-angle
XRD patterns and TEM images were used to explain some effects of the Pd(OH)2 precipitate
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conditions on the mean size of metal particles in catalyst and the activity of catalysts. Figure 6a
and b show the XRD pattern of catalysts, which were prepared without and with the addition of
Na2CO3 solution into the precursor slurry, respectively. The peaks positioned at 2θ° = 34.26°,
42.61°, 55.01°, 59.81° and 72.48° correspond to the lattice planes of the palladium oxide
hydrate[14]. Meanwhile, the peaks positioned at 2θ° = 40.42°, 46.79°, 68.43° and 82.35° match to
the lattice planes of the palladium metal [15]. These results show that Pd metal and PdO.H2O
phases may exist in both catalyst samples. However, the peaks corresponding to the PdO.H2O
and the Pd metal are very weak for the prepared catalyst without and with Pd(OH)2 precipitate,
respectively (Figures 6a and b). Thus, Pd2+ (aq) ions in acidic precursor solution, without the
addition of Na2CO3 solution, were mainly deposited on the activated carbon surface in metallic
phase[7a, 7c]. However, a main portion of Pd2+ (aq) ions was converted into Pd(OH)2 by addition
of Na2CO3 solution to the precursor slurry. Then, Pd(OH)2 was precipitated on the activated
carbon surface.
The evaluated crystallite dimension, that was deduced from Scherrer’s equation [16], applied
on the PdO.H2O (101) and Pd (111) diffraction peaks of XRD patterns for the prepared catalyst
without and with Pd(OH)2 precipitates, respectively (Figure 6a and b). According to the
Scherrer’s equation, the average crystallite size for the prepared catalyst without Pd(OH)2
precipitate (>10 nm) is greater than that of with Pd(OH)2 precipitate (<3 nm). Palladium
dispersion for the supported Pd/C catalyst increases relatively when the Pd2+ (aq) ions are
directly deposited on the activated carbon surface in the precursor solutions[7a]
.
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Figure 7 shows the TEM images for both catalysts. The mean size of palladium particles for
the prepared catalyst without Pd(OH)2 precipitate (Figure 7a) is obviously larger than that of the
prepared catalyst with Pd(OH)2 precipitate (Figure 7b). The TEM images and XRD patterns of
catalysts confirm that the mean diameters of palladium crystallites and particles are increased
relatively when Pd2+ (aq) ions are directly deposited on the activated carbon surface. However, a
large portion of Pd2+ (aq) ions is hydrolyzed to form Pd(OH)2 with small particle size, which is
well-dispersed on the activated carbon surface.
4. Conclusion
In this study, the Pd(OH)2/C catalyst was prepared using deposition-precipitation method.
The palladium content in catalyst should be more than 7 wt% to obtain a considerable yield for
the desired product TADB. Optimal values for pH and time of Pd(OH)2 precipitate for the
synthesis of palladium catalyst were found to be about 9 and 5 hours, respectively. The results
showed that the TADB yield was slightly influenced by the temperature of Pd(OH)2 precipitate in
a temperature range of 30 to 90°C. It was also shown that the activity of Pd(OH)2/C catalyst was
considerably affected by the porosity of activated carbon supports. The TEM images and XRD
patterns of catalysts showed that the mean diameter of palladium crystallites and particles
increased relatively when Pd2+ (aq) ions were directly deposited on activated carbon surface.
After addition of Na2CO3 solution to the precursor solution, small size and well-dispersed
palladium were precipitated on the activated carbon surface.
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Acknowledgement
We would like to thank the research committee of Kashan University for supporting this
work.
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Table 1. The characteristics of Pd(OH)2/C1, Pd(OH)2/C2 and Pd(OH)2/C3 catalyst
Characteristics
catalyst
of
Pd(OH)2/C1
Pd(OH)2/C2
Pd(OH)2/C3
The volume of
150
630
560
70
66
34
149
82
monolayer (cm3/g)
BET surface area
(m2/g)
286
111
175
Cumulative micropore
surface area (m2/g)
Cumulative wide pore
surface area (m2/g)
Ratio of cumulative
micropore surface
area to BET surface
area
67
0.89
7.0
0.39
7.2
0.55
7.1
Pd content in catalyst
(wt. %)
Chemisorbed hydrogen
(cm3(STP)/g) at 303
K
16.5
39.0
17.2
64.4
16.9
48.5
TADB yield (wt. %)
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Figure 1. The synthesis process steps of hexanitrohexaazaisowurtzitane
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80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
Pd in Catalyst (wt%)
Figure 2. The TADB yield versus the Pd content in catalyst (catalyst precipitation conditions: pH
= 12, time = 5 h, temperature = 50°C)
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80
60
40
20
0
0
3
6
9
12
15
pH
Figure 3. The TADB yield versus the pH of Pd(OH)2 precipitate (catalyst precipitation
conditions: Pd content in precursor solution/catalyst = 13 wt%, time = 5 h, temperature = 50°C)
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80
60
40
20
0
0
20
40
60
80
100
Temprature (°C)
Figure 4. The TADB yield versus temperature of precipitation of Pd(OH)2 (catalyst precipitation
conditions: Pd content in precursor solution/catalyst = 13 wt%, time = 5 h, pH = 12)
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80
70
60
50
40
30
20
10
0
0
5
10
Time (hour)
15
Figure 5. The TADB yield versus time of precipitation of Pd(OH)2 (catalyst precipitation
conditions: Pd content in precursor solution/catalyst = 13 wt%, temperature = 50°C, pH = 12)
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Figure 6. Wide-angle XRD patterns for: (a) the prepared catalyst without Pd(OH)2 precipitate (Pd
content in catalyst = 3.5 wt%) and (b) the prepared catalyst with Pd(OH)2 precipitate (Pd in
catalyst = 10 wt%)
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18
16
14
12
10
8
30
25
20
15
10
5
6
4
2
0
0
1
2
3
4
5
5
10
15
20
25
D(nm)
D(nm)
(a)
(b)
Figure 7. The TEM images for: (a) the prepared catalyst without Pd(OH)2 precipitate (Pd content
in catalyst = 3.5 wt %) and (b) the prepared catalyst with Pd(OH)2 precipitate (Pd content in
catalyst = 10 wt%)
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