Thermal Decomposition of Dimethyl Methylphosphonate over Manganese Oxide Catalysts

Article abstract of DOI:10.1006/jcat.2000.3126

The thermal oxidative decomposition of dimethyl methylphosphonate (DMMP) has been studied over amorphous manganese oxide (AMO) and Al₂O₃-supported manganese oxide catalysts. The reaction was carried out using air as the oxidant at te

Full text of DOI:10.1006/jcat.2000.3126

Journal of Catalysis 198, 66–76 (2001)
doi:10.1006/jcat.2000.3126, available online at http://www.idealibrary.com on
Thermal Decomposition of Dimethyl Methylphosphonate
over Manganese Oxide Catalysts
,
, ,1
Scott R. Segal, Lixin Cao, Steven L. Suib, † ‡ Xia Tang,§ and Sunita Satyapal§
U-60, Department of Chemistry, †Department of Chemical Engineering, and ‡Institute of Materials Science, University of Connecticut, Storrs,
Connecticut 06269-3060; and §United Technologies Research Center, 411 Silver Lane, East Hartford, Connecticut 06108
Received July 17, 2000; revised October 30, 2000; accepted November 28, 2000; published online February 7, 2001
moval of chemical warfare agents (CWAs). In the United
States alone, there are approximately 25,000 tons of chem-
ical weapons stockpiled (1). In order to reduce these stock-
piles safely, the development of methods alternate to incin-
eration is desired. Dimethyl methylphosphate (DMMP),
whose structure is shown in Fig. 1, is a widely used simulant
for CWAs and other toxic organophosphorous compounds
such as pesticides.
The thermal oxidative decomposition of dimethyl methylphos-
phonate (DMMP) has been studied over amorphous manganese
oxide (AMO) and Al2O3-supported manganese oxide catalysts. The
reaction was carried out using air as the oxidant at temperatures
between 200 and 400 C. The highest reaction rates occurred using
temperatures of 400 C. Gas chromatography (GC) was used to ex-
amine reactant DMMP and other gas phase products. DMMP was
found to oxidatively decompose over AMO and Al2O3-supported
manganese oxide catalysts. The highest activity was observed us-
ing a catalyst prepared by precipitation of AMO on Al2O3. During
the initial stages of reaction, DMMP was completely removed from
the gas phase. During this period DMMP was oxidized to CO2,
with no other gas phase products being observed. After a certain
period of time (5 min–8 h), DMMP reappeared in the gas phase.
The CO2 concentration then decreased and MeOH began to form,
indicative of hydrolysis of DMMP. These results indicate that de-
activation of catalysts occurs due to adsorbed P-species. Fourier
transform infrared (FTIR) spectroscopy and ion chromatography
(IC) were used to examine adsorbed products on the surface of the
catalysts. The IC analyses indicated that several products accumu-
late on the surface of the catalysts, including methyl methylphos-
phonate, methylphosphonic acid, and phosphoric acid. FTIR anal-
yses showed that DMMP bonds strongly to Mn Lewis acid sites
on the manganese oxide surface via phosphoryl oxygen. The bare
Al2O3 support was also examined in DMMP decomposition reac-
tions and showed high activity, with 100% DMMP removal from the
gas stream for over 15 h. The major products observed over Al2O3
were dimethyl ether and MeOH. No CO2 was observed, indicating
that DMMP is not oxidized over Al2O3. The GC, IC, and FTIR
results suggest that DMMP is dissociatively adsorbed over Al2O3.
Finally, the results for the thermal oxidation of DMMP over AMO
are compared to results previously obtained using photo-assisted
c
In our previous paper we have investigated the oxidative
photo-assisted decomposition ofDMMP over mixed valent,
amorphous manganese oxide (AMO) catalysts (2). This
work showed that under dark conditions (25 C), DMMP
strongly absorbs to the AMO surface. After irradiation
with UV-vis light, several gas phase products were formed.
These products included methanol and CO2. After a brief
period of high activity, the amounts of the products de-
creased significantly as the catalyst surface became poi-
soned byadsorbed phosphorousspecies. The spent catalysts
were extracted in H2O and the extracts were analyzed by
ion chromatography (IC). These results showed that sev-
eral products accumulate on the AMO surface, including
methyl methylphosphonate (MMP) and methyl phospho-
nic acid (MPA). Additionally, adsorbed DMMP was found
on the surface. Under these conditions no phosphoric acid
was detected, which indicated the incomplete decomposi-
tion of DMMP, MMP, and MPA. Fourier transform infrared
(FTIR) spectroscopy was also used to examine the nature
of the adsorbed species on the catalysts. These results con-
firmed the findings from the IC data.
The thermal decomposition of DMMP has been studied
over metals, including Mo(111) (3), Pt(111) (4), Pd(111)
(5), and Ni(111) (5), and metal oxides such as Fe2O3 (6),
MgO (7–9), La2O3 (10), Pt/Al2O3 (11), Fe2O3/Al2O3 (12),
Fe2O3/CaO/MgO (13), V2O3/MgO (13), and Cu-substituted
hydroxyapatite (14). We have recently found some very
active catalysts for DMMP decomposition based on VxOy
supported on Al2O3 and SiO2 (15). These catalysts showed
exceptionally high activity because they caused the forma-
tion of P2O5, which itself proved to be catalytically active for
further decomposition of DMMP. One of the main issues
oxidative methods.
2001 Academic Press
I. INTRODUCTION
The decomposition of organophosphorous compounds
continues to be of interest, due to the need for safe re-
1
To whom correspondence should be addressed.
66
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

THERMAL DECOMPOSITION OF DIMETHYL METHYLPHOSPHONATE
67
To this solution, 2.0 g of -Al2O3 was added and the so-
lution was stirred overnight. The next day, a dark brown–
black solid was observed. The sample was heated slowly
for several hours on a heating plate and then was placed in
an oven and dried in air at 120 C for 18 h. The final black
product was calcined at 400 C for 4 h. This sample will be
hereafter designated Mn/Al2O3.
FIG. 1. Structures of dimethyl methylphosphonate (DMMP), methyl
methylphosphonate (MMP), and methylphosphonic acid (MPA).
B. Catalytic studies. DMMP was purchased from
Aldrich and was used without further purification. A
schematic diagram of the reactor used for thermal DMMP
decomposition reactions is shown in Fig. 2. Air was used in
as the oxidant and was passed through a bubbler containing
liquid DMMP, which was kept in a water bath at 25 C. The
flow rate of air was kept between 30 and 50 mL/min. The
DMMP-saturated air was then passed through a glass tube
(ID = 1/400) containing either 50 or 100 mg of the catalyst
which was packed between glass wool plugs. Under these
conditions, the inlet DMMP concentration is 0.14 mol% .
The outlet lines (after the furnace) were heated to 120 C to
prevent condensation of DMMP and other products. Prior
to the DMMP decomposition reactions, the catalysts were
heated for 30 min in air at the temperature used in the par-
ticular experiment.
that plagues catalytic (photo and thermal) oxidation studies
of DMMP is catalyst poisoning due to adsorbed phospho-
rous species. Therefore, the sustained oxidative catalysis of
DMMP over metals and metal oxides is difficult to attain
unless temperatures of greater than 600 C are used (3).
In this work we have continued our investigation on the
decomposition of DMMP over manganese oxide catalysts.
The thermal oxidative decomposition of DMMP was stud-
ied over AMO catalysts between 200 and 400 C using air
as the oxidant. Additionally, several Al2O3-supported man-
ganese oxide samples were prepared, characterized, and
tested in thermal oxidative DMMP decomposition reac-
tions. After DMMP reactions, the catalysts were examined
using FTIR and IC to assist in identifying products and for
determining decomposition mechanisms.
C. GC analyses. Reactants and products were analyzed
using a Hewlett–Packard 5890 Series I gas chromato-
graph equipped with an automatic gas-sampling valve. A
Carbowax 20M capillary column with flame ionization de-
tection was used to analyze for DMMP, methanol (MeOH),
and dimethyl ether (DME). CO2 was analyzed using a GSC
Gas Pro capillary column with thermal conductivity detec-
tion. Calibration curves for MeOH, DME, and CO2 were
prepared from gas standards prepared in our laboratories.
II. EXPERIMENTAL
A. Preparation of materials. AMO was prepared by a
redox reaction involving the reduction of KMnO4 with ox-
alic acid. A solution containing 1.58 g of KMnO4 (Aldrich,
Milwaukee, WI) in 100 mL of distilled deionized water
(DDW) was mixed with a solution containing 2.26 g of ox-
alic acid (Fisher, Fair Lawn, NJ) in 100 mL of DDW. The
solution was allowed to mix for several hours, which then
yielded a dark brownish–black precipitate. The solid was fil-
tered and washed with DDW several times and then dried
in an oven at 110 C overnight. A portion of the sample was
calcined in air at 400 C for 4 h.
D. X-ray diffraction analyses. A Scintag Model PDS-
2000 - diffractometer was used for X-ray powder diffrac-
tion (XRD) experiments. Cu K radiation wasused at 45kV
and 40 mA. Sample scans were done at 5 2 /min.
Two different Al2O3-supported manganese oxide sam-
ples were prepared. The first sample was synthesized by
precipitation of AMO on -Al2O3. This was made by slowly
adding solutions of KMnO4 (1.58 g in 25 mL of DDW) and
oxalic acid (2.26 g in 25 mL of DDW) to a slurry containing
5.0 g of -Al2O3 (Davison, Chattanooga, TN) in 50 mL of
DDW. The mixture was stirred for 18 h, which yielded a
light-brown solid. The mixture was heated on a stir plate
at 60 C for 4 h. The sample was then placed in an oven
and heated in air for 18 h at 120 C. Finally, the light-brown
solid was calcined in air at 400 C for 4 h. This sample will
be hereafter designated AMO/Al2O3.
The second Al2O3-supported manganese oxide sample
was prepared by wet impregnation of Mn(NO3)2 6H2O
onto -Al2O3. A Mn2+ solution was prepared by dissolving
FIG. 2. Experimental setup used for thermal DMMP decomposition
1.16 g of Mn(NO3)2 6H2O (Aldrich) in 100 mL of DDW. reactions.

68
SEGAL ET AL.
E. Inductively coupled plasma (ICP) analyses. The
manganese concentrations in the catalysts were measured
using a Perkin–Elmer P40 ICP Analyzer. Prior to analy-
sis, the samples were dissolved in a small amount of HF
and were then diluted with distilled deionized water. Sev-
eral milliliters of HCl were added to assist in dissolving the
manganese.
F. FTIR analysis. Fourier transform infrared spectros-
copy (Nicolet Magna-IR 750) was used to examine surface
species on the catalysts after DMMP reactions. A DTGS
detector with a KBr beam splitter was used for analysis
over the entire mid-IR range (4000–400 cm 1). The samples
were pressed into KBr pellets (2.5% sample by weight), and
transmission spectra were collected.
G. Extraction analyses of spent AMO. Aqueous extrac-
tions of AMO after DMMP reactions were used to identify
methylphosphonic acid, methyl methylphosphonate, and
phosphates (PO34 ). The structures of MPA and MMP are
shown along with DMMP in Fig. 1. The aqueous extracts
were prepared by placing the spent AMO in H2O and treat-
ing it with ultrasound for 10 min. The solution was filtered
through 0.22- m filters and analyzed using a Dionex DX
500 ion chromatograph equipped with a CD 20 conductivity
detector and a Dionex AS4A-SC anion exchange column.
The eluent used was 1.8 mM Na2CO3/1.7 mM NaHCO3
buffer (app. pH 10).
In order to confirm the presence of MPA and MMP
in the IC analyses, standards were prepared and reten-
tion times were established. MPA was available commer-
cially (Aldrich), whereas MMP needed to be synthesized
in our laboratories. MMP was prepared according to litera-
ture procedures by partial hydrolysis of DMMP in aqueous
NaOH solution (16). In this procedure, 0.140 mol DMMP
was added to 150 mL of a 10% NaOH solution and refluxed
gently for 5 h. The solution was cooled slowly and acidified
to pH 2 with concentrated HCl. The solution was vacuum
distilled until a NaCl precipitate was observed in the distil-
lation flask. The mixture was extracted several times with
acetone and then filtered. The acetone extract was dried
over CaSO4 for several hours. The extract was refiltered
and placed in a rotovap until 15–20mL of liquid remained.
The sample was vacuum distilled and a distillate was col-
lected almost immediately. This distillation was necessary
to remove trace amounts of acetone in the mixture. Follow-
ing several more minutes of distillation, a small amount of
liquid ( 1 mL) was observed in the receiving flask. The liq-
uid was identified as MMP and was confirmed by 1H NMR
studies (17).
FIG. 3. Powder X-ray diffraction patterns for (A) AMO and (B)
AMO calcined at 400 C for 4 h.
is K0.6Mn0.93O2 (18). AMO has an average Mn oxidation
state of 3.5–3.6 (18), which indicates that AMO is a mixed
valent manganese oxide. The XRD patterns for AMO and
calcined AMO are shown in Fig. 3. These data show that
AMO is not entirely amorphous, as a few small and broad
peaks are seen. These include two peaks with d-spacings of
˚
6.92 and 2.42 A. A portion of the AMO material was cal-
cined at 400 C for 4 h. The XRD pattern for calcined AMO
shows that many new peaks are formed, indicating that the
material becomes crystalline during heating. In the calcined
AMO sample, peaks appear at 6.87, 4.86, 3.46, 3.10, 2.45,
˚
2.38, 2.30, 2.19, 2.15, 1.92, 1.82, 1.63, and 1.54 A. The labels
written on the XRD patterns denote phases and will be dis-
cussed in later sections. The broad peaks occurring between
15 and 25 2 are due to SiO2 (microscope slides), used to
support the powder samples.
B. AMO/Al2O3. After the KMnO4 solution is mixed
with the oxalic acid solution in the presence of Al2O3,
vigorous bubbling is observed, similar to that observed
with unsupported AMO syntheses. However, the color of
the AMO/Al2O3 sample is very different than that of un-
supported AMO. Unsupported AMO is essentially black,
while AMO/Al2O3 is light brown (even after calcination).
ICP analysis reveals that AMO/Al2O3 contains 6.18% Mn
by weight. The XRD pattern for calcined AMO/Al2O3 is
shown in Fig. 4. The XRD pattern shows a few broad peaks
˚
centered at 2.39 and 1.96 A.
C. Mn/Al2O3. Impregnation of the manganese(II) ni-
trate solution over Al2O3 results in the formation a black
solid. After calcination, ICP analysis reveals that the sam-
ple contains 6.52% Mn by weight. The X-ray pattern for
III. RESULTS
A. AMO preparation. AMO is formed through the re- calcined Mn/Al2O3 is shown in Fig. 5. This shows the for-
duction of KMnO4 by oxalic acid. From previous work we mation of several peaks, with d-spacings of 3.11, 2.41, 2.11,
˚
have shown that, after drying, the composition of AMO 1.97, 1.62, and 1.56 A.

THERMAL DECOMPOSITION OF DIMETHYL METHYLPHOSPHONATE
69
FIG. 4. Powder X-ray diffraction patterns for AMO/Al2O3 calcined
at 400 C for 4 h.
D. Thermal DMMP decomposition reactions over AMO.
It should first be noted that, in the absence of catalysts, no
measurable DMMP decomposition is observed at temper-
atures up to 400 C. In initial experiments, we examined the
decomposition of DMMP over unsupported AMO at tem-
peratures between 200 and 400 C. Plots showing the fate of
DMMP and formation of products are shown in Fig. 6. In
Fig. 6A, the data points seen on the left y-axis represent the
initial DMMP concentration, which is between 0.13 and
0.14 mol% . These data show initially that at all tempera-
tures DMMP is completely removed from the gas stream.
After a period of time, DMMP begins to reappear in the
gas stream. This period of time when 100% DMMP decom-
position occurs, typically toward the beginning of DMMP
decomposition reactions, is often referred to as “protection
time” (14). These data show that the protection time for
DMMP decomposition reactions increases with tempera-
ture. At 200 C, DMMP is only completely removed from
FIG. 6. Profile showing (A) fate of DMMP and formation of (B) CO2
and (C) MeOH as a function of time over AMO at different temperatures.
Reaction conditions: 50 mg AMO, air flow rate = 30 mL/min.
the gas stream for a few minutes. At 300 C, the protection
time increases to 30 min, while at 400 C the protection
time is almost 1 h.
The gas stream products that were detected included CO2
and MeOH. In Figs. 6B and 6C, the profiles for CO2 and
FIG. 5. Powder X-ray diffraction patterns for Mn/Al2O3 calcined at
400 C for 4 h.

70
SEGAL ET AL.
MeOH are shown at the different temperatures used in creased from 1 h to only several minutes. The trends for
the DMMP decomposition reactions. The profile for CO2 MeOH and CO2 production are similar to results obtained
production shown in Fig. 6B indicates that large amounts in previous experiments; however, the MeOH concentra-
of CO2 are produced at 300 and 400 C, especially toward tion is slightly lower in these experiments.
the beginning of the reactions. At these temperatures the
initial concentrations of CO2 are around 0.20 mol% . The
concentration of CO2 then begins to decrease as a func-
E. Thermal DMMP decomposition reactions over Al2O3
and Al2O3-supported manganese oxides. A systematic
study of DMMP decomposition reactions over Al2O3 and
tion of reaction time. At 200 C, only low levels of CO2
Al2O3-supported manganese oxide materials was carried
( 0.05 mol% ) are observed toward the beginning of the re-
out at 400 C. The results for these experiments are shown in
actions. After approximately 2 h, no CO2 is detected at this
Fig. 8. The results shown in Fig. 8A indicate that -Al2O3 is
temperature.
quite active in DMMP decomposition reactions. Some ma-
The MeOH profiles for DMMP decomposition over
jor differences are observed compared with AMO catalysts.
AMO are shown in Fig. 6C. At 300 and 400 C, no MeOH
The most significant observation is the length of time in
is initially detected. After 30–45 min MeOH appears in the
which DMMP is completely removed from the gas stream.
gas stream, and its concentration for the most part increases
The protection time for Al2O3 is over 15 h. In contrast to re-
with time. At 300 C, the MeOH concentration reaches a
actions using AMO as the catalyst, no CO2 is formed in this
maximum at 0.020 mol% after 140 min. The MeOH con-
reaction. However, another product is observed through-
centration then begins to slowly decrease. This trend is dif-
out the reaction, which was identified as dimethyl ether.
ferent than that observed at 400 C. At 400 C, the MeOH
MeOH is also formed in this reaction, starting after 10 h.
concentration essentially increases with time. At 200 C, the
The concentration of MeOH increases rapidly to almost
profile for MeOH looks quite different than that observed
0.08 mol% after around 20 h.
with higher temperatures. Initially, the MeOH concentra-
In Fig. 8B the results for the DMMP decomposition re-
tion is zero and quickly reaches a maximum at 0.005 mol%
action over AMO/Al2O3 at 400 C are shown. The protec-
after only about 15 min. The MeOH concentration then
tion time is just over 8 h, followed by a slow increase in
decreases slowly with time.
the DMMP concentration. In contrast to results obtained
In another set of experiments, the decomposition of
using unsupported Al2O3, CO2 is produced in DMMP de-
DMMP was studied over AMO at 400 C using slightly dif-
composition reactions over AMO/Al2O3. The initial CO2
ferent conditions. In these experiments, the flow rate of air
concentration is 0.21 mol% . As with AMO samples, the
was increased from 30 to 50 mL/min. Also, the amount of
CO2 concentration then decreases as a function of reac-
catalyst was doubled from 50 to 100 mg. These changes in
tion time. Although not shown in the plot, a trace amount
the reaction conditions were found to cause a slight differ-
of DME was also observed throughout the reaction. The
ence in the catalytic results. In Fig. 7, the profiles for DMMP,
formation of MeOH begins to occur starting after 7.5 h.
MeOH, and CO2 are shown for a DMMP decomposition
The MeOH concentration then continues to increase as a
reaction over AMO using these new conditions. The main
function of time.
difference in these results compared to results obtained in
In Fig. 8C, the results for the DMMP decomposition re-
previous experiments is that the protection time has de-
action over Mn/Al2O3 are shown. These results are simi-
lar to those observed using AMO/Al2O3. The one differ-
ence here is that the protection time is somewhat shorter
( 4 h). Generally the same types and amounts of products
are formed in this reaction compared with the AMO/Al2O3
sample. Once again, trace amounts of DME were observed
throughout the entire reaction.
F. IR analyses. IR analyses were used to examine ad-
sorbed phosphorous species on the catalysts after DMMP
reactions. In order to identify these species, the IR data
for liquid DMMP were collected and the frequencies and
assignments are listed in Table 1. The spectrum was mea-
sured by placing a small drop of DMMP onto a KBr pellet.
Note that there are two important regions of the IR spec-
trum that are important for DMMP analysis. This includes
a high-frequency region between 3200 and 2600 cm 1 and
a low-frequency region between 1800 and 750 cm 1. The
higher frequency region contains peaks caused by methyl
group stretching, while the lower frequency region contains
FIG. 7. Profile showing fate of DMMP and the formation of products
in thermal DMMP decomposition reactions over AMO at 400 C. Reaction
==
conditions: 100 mg AMO, air flow rate = 50 mL/min. T = 400 C.
peaks caused by C–O, C–P, and P O stretching vibrations

THERMAL DECOMPOSITION OF DIMETHYL METHYLPHOSPHONATE
TABLE 1
71
IR Frequencies and Assignments for DMMP
1
Vibrational modea
Frequency (cm
)
a (CH3P)
a (CH3O)
s (CH3P)
s (CH3O)
s (CH3P)
2997
2956
2930
2854
1317
1250
1189
1062
1036
914
==
(P O)
(CH3O)
(CO)
(CO)
(CH3P)
(PO2)
(PO2)
(PC)
820
789
711
Note. , stretch; , deformation; , rock; a, anti-
symmetric; s, symmetric.
a Values for vibrational modes are taken from
Ref. (10).
FIG. 8. Profile showing fate of DMMP and the Formation of Prod-
ucts in Thermal DMMP decomposition reactions over (A) -Al2O3,
(B) AMO/Al2O3, and (C) Mn/Al2O3 at 400 C. Reaction conditions:100mg
catalyst, air flow rate = 50 mL/min. T = 400 C.
FIG. 9. IR spectra of (A) AMO before and after 6.5 h of DMMP rxn,
(B) -Al2O3 before and after 23 h of DMMP rxn, (C) AMO/Al2O3 before
and after 13.5 h, and (D) Mn/Al2O3 before and after 7 h of DMMP rxn.
T = 400 C. Note that in each series, the top spectrum is before rxn and
lower spectrum is after rxn.
as well as methyl deformation vibrations (10). The IR spec-
tra for all catalysts before and after DMMP decomposition
reactions at 400 C were measured and are shown in Fig. 9.
The IR data are summarized in Table 2.

72
SEGAL ET AL.
TABLE 2
Summary of IR Data for Catalysts before and after Thermal DMMP Decomposition Reactions at 400 C
1
1
Sample
Freq (cm
)
Vib modea
Sample
Freq (cm
)
Vib mode
AMO (before rxn)
3300
1622
500
(OH)
(H2O)
(Mn–O)
AMO/Al2O3 (before rxn)
3444
1638
1531
1386
779
(OH)
(H2O)
(H2O)
(H2O)
(Al–O)
(Al–O, Mn–O)
==
AMO (after rxn)
1238
1088
1029
925
(P O)
(PO3)
(PO2)
(CH3P)
(PC)
578
711
AMO/Al2O3 (after rxn)
3451
1639
1242
1150
916
(OH)
(H2O)
==
-Al2O3 (before rxn)
3470
1636
1540
1368
762
(OH)
(P O)
(H2O)
(H2O)
(H2O)
(Al–O)
(Al–O)
(PO3)
(PCH3)
(PO2)
(Al–O, Mn–O)
792
524
572
Mn/Al2O3 (before rxn)
Mn/Al2O3 (after rxn)
3459
1638
778
(OH)
(H2O)
(Al–O)
(Al–O, Mn–O)
-Al2O3 (after rxn)
3447
3001
2935
1321
1165
1108
918
802
668
477
(OH)
a (CH3P)
s (CH3P)
(CH3P)
619
==
(P O)
3459
1650
1419
1321
1204
1138
901
(OH)
(PO2)
(CH3P)
(PO2)
(Al–O)
(Al–O)
(H2O)
(CH3P)
(PCH3)
==
(P O)
(PO3)
(CH3P)
800
(PO2)
584
(Al–O, Mn–O)
a Values for vibrational modes are taken from Ref. (10). , stretch; , deformation; , rock; a, antisymmetric; s, symmetric.
G. Extraction analyses of spent catalysts. The results of
The IC data for thermal DMMP decomposition reactions
the IC analyses for extractions made on the catalysts after over manganese oxides show some major differences com-
thermal reactions are listed in Table 3. Quantitative data pared to results obtained for photoreactions. The most im-
for MMP could not be obtained for these experiments portant observation for catalysts after thermal reactions
because the MMP standard had reacted over time (con- is the formation of PO34 , especially when using AMO
verted to MPA). Therefore, MMP was only qualitatively and AMO/Al2O3. The formation of PO34 was not ob-
analyzed. Some problems were observed in these studies served during photoreactions of DMMP over AMO (2).
due to the presence of silica fibers from the glass wool used Small amounts of PO34 were also observed using -Al2O3;
during the thermal decomposition reactions. Therefore, however, the major product accumulated on the cata-
errors are likely in these data.
lyst is MPA (140.8 mg/g). For manganese-oxide-supported
Al2O3 catalysts, PO34 is observed in both samples, with the
AMO/Al2O3 catalyst having a greater amount of accumu-
lated PO34 (13.17 vs 2.13 mg/g). MMP was observed on all
catalysts after thermal DMMP decomposition reactions.
TABLE 3
Ion Chromatography (IC) Data for Catalysts after DMMP
Decomposition Reactions at 400 C
H. Turnover numbers (TON) for thermal DMMP decom-
position reactions. In order to get an idea whether ther-
mal DMMP decomposition reactions occur stoichiomet-
rically or catalytically over manganese oxide materials,
TONs were calculated for all of the catalysts. The results
of these calculations are given in Table 4. For comparison,
the TON for DMMP photoreactions over AMO is also
MPA
(mg/g)
Sample
PO34
2.48
21.04
13.17
2.13
MMP
Al2O3 (after 23 h)
AMO (after 6 h)
AMO/Al2O3 (after 13.5 h)
Mn/Al2O3 (after 6.4 h)
140.8
1.19
30.52
60.90
Trace
Trace
Some
Some

THERMAL DECOMPOSITION OF DIMETHYL METHYLPHOSPHONATE
73
TABLE 4
amount of Mn may be present in a lower oxidation state,
perhaps as Mn3+. We are currently examining this material
by X-ray photoelectron spectroscopy to assist in determin-
ing the average Mn oxidation state.
Mn/Al2O3, prepared by wet impregnation of man-
ganese(II) nitrate, leads to a black material, which after cal-
cination at 400 C forms a crystalline material as seen in the
Turnover Numbers (TON) for Catalysts
in DMMP Decomposition Reactions
Sample
TONa
AMO (photoreaction)
AMO (400 C)
AMO/Al2O3 (400 C)
Mn/Al2O3 (400 C)
Al2O3 (400 C)b
0.0529
0.179
1.22
1.87
0.0458
˚
XRD pattern in Fig. 5. Besides the peak at 1.97 A (which is
due to -Al2O3), the other peaks can all be assigned to py-
rolusite. The XRD data correlate to the black color, which
is common in MnO2 materials.
a TON is defined as (total mol products)/
(mol Mn)(h).
B. Catalytic data. The data of Fig. 6 indicate that
DMMP isdecomposed over AMO to form CO2 and MeOH.
At 200 C, only a small amount of reaction occurs as the
protection time is a few minutes. As the temperature is
increased to 300 and 400 C, a significant increase in the
amount of DMMP decomposition is observed. The forma-
tion of CO2 toward the beginning of the reaction suggests
that DMMP is completely oxidized. After approximately an
hour, a steadily increasing amount of MeOH and DMMP
appears in the gas stream, while the CO2 concentration be-
gins to decrease. This suggests that the catalyst surface is
being poisoned and that the oxidation mechanism shuts
off while a hydrolysis mechanism begins to dominate. Such
trends are frequently observed for DMMP decomposition
reactions over metal oxides (2, 11, 14).
b Mol Al substituted for mol Mn.
included. More will be said about these numbers in the
Discussion.
IV. DISCUSSION
A. Catalyst preparation and characterization. The aver-
age Mn oxidation state of 3.5–3.6 in AMO suggests that Mn
exists in mixed valencies, most likely a mixture of 3+ and 4+,
with some 2+ (18). Mixed valency is an important property
for redox catalysis and commonly occurs with manganese
oxide materials (19). Another property of AMO that con-
tributes to its applications in catalysis and adsorption is its
high surface area. BET surface area measurements indicate
that AMO has a surface area of 200 m2/g (20).
The poisoning of catalyst surfaces by POx species is com-
mon in catalytic oxidation studies of DMMP. The total ox-
idation of DMMP can be represented by (14)
The X-ray diffraction data shown in Fig. 3 indicate that
uncalcined AMO has some crystallinity. The peaks at 6.92 CH3(CH3O)2PO +(x+15)/2O2 →3CO2 +3/2H2O +POx .
˚
and 2.42 A match the two strongest peaks in the JCPDS
[1]
pattern for the mineral cryptomelane. Cryptomelane be-
longs to a family of porous, mixed valent manganese ox- The POx species can combine with H2O to form H3PO4,
ides known as octahedral molecular sieves. These mate- which contributes to catalyst poisoning. It is usually very
rials consist of edge- and corner-sharing MnO6 octahedra difficult to remove POx species in the gas phase. For exam-
that have tunnels on the order of molecular dimensions ple, Lee et al. observed that, even at temperatures as high
(21). The composition of cryptomelane is known to be as 500 C, phosphorous compounds could not be removed
KMn8O16 nH2O, which is reasonably close to the compo- from the surface of Cu-substituted hydroxyapatite catalysts
sition we determined for AMO (21). After calcination the (14).
XRD pattern indicates that AMO crystallizes further as py-
The data of Fig. 7 indicate that the DMMP decomposition
rolusite, a MnO2 phase is formed. This suggests that AMO reaction is affected by changes made in the experimental
is further oxidized during calcination. However, the com- conditions. The increase in flow rate from 30 to 50 mL/min
plete conversion of AMO to pyrolusite does not occur after causes a decrease in the rate of reaction. This is probably
4 h of calcination at 400 C, as several cryptomelane peaks due to the increase in residence time within the reactor.
are still observed.
With regard to the catalyst amount, one might expect that
The precipitation ofAMO on -Al2O3 leadsto the forma- doubling the catalyst amount would cause an increase in
tion of a light-brown material. Unlike AMO, this material the DMMP decomposition rate. However, it appears that
does not crystallize after 4 h of calcination at 400 C. The any increase in the DMMP decomposition rate caused by
XRD pattern shown in Fig. 4 indicates that the manganese increasing the amount of catalyst is offset by use of the
oxide phase is amorphous. The two peaks seen at 2.39 and higher flow rate.
˚
1.96 A are due to the -Al2O3 support. While we have not
In comparison to data obtained in our previous inves-
measured the average Mn oxidation state in this material, tigations concerning photo-assisted DMMP decomposi-
the light-brown color seems to suggest that a considerable tion reactions, the thermal activation of DMMP is much

74
SEGAL ET AL.
more effective. In that work we observed considerable to be detected by XRD indicating highly dispersed and high
adsorption of DMMP to the AMO surface under dark con- surface area MnOx.
ditions (at Room T), with small amounts of MeOH also
being produced (2). When the sample was irradiated with
C. IR, IC analyses and mechanism. The IR data for
UV-vis light, an initial burst of product formation (CO2 and AMO indicate that fresh AMO has a broad peak at
MeOH) was observed. This activity lasted for only several 3300 cm 1 and a smaller peak at 1622 cm 1, which are due
1
minutes, with the catalyst quickly deactivating due to poi- to O–H vibrations. The broad peak at 500 cm is caused
soning. In addition, we observed no PO34 on the surface of by Mn–O vibrations. After DMMP reaction, the peaks due
AMO after photoreactions, which indicated the incomplete to O–H vibrations decrease significantly, suggesting that
decomposition of DMMP (2).
DMMP reacts with surface hydroxyl groups. The new peaks
The DMMP decomposition data for -Al2O3 in Fig. 8a that appear in the lower portion of the spectrum (below
indicate that the support itself is very active in decompos- 1800 cm 1) indicate the presence of adsorbed phosphorous
1
==
ing DMMP, as a protection time of 15 h is observed. The species. The peak at 1250 cm is caused by the P O vi-
long protection time may be due to the large surface area bration. Since the peak is shifted toward a weaker energy
==
(291 m2/g) of the support. However, the mechanism for (compared with liquid DMMP), this suggests that the P
O
DMMP decomposition using -Al2O3 appears to be dif- moiety is involved in the bonding to the AMO surface. In
ferent than that observed for AMO catalysts. The major photo-assisted DMMP decomposition reactionsover AMO
==
difference is that when using -Al2O3 as a catalyst, no CO2 we observed similar trends (2). The interaction of P
O
is produced. This makes sense since no redox active metal with Mn is consistent with a Lewis acid/base type adsorp-
is present. Therefore, DMMP is not oxidized over -Al2O3. tion mechanism. The phosphoryl oxygen is electron-rich
Another observation is that dimethyl ether is produced, (16) and can act as a Lewis base, which can interact with
especially toward the beginning of the reaction. The for- Lewis acid sites on AMO (i.e., Mn3+, Mn4+). This leads to
==
mation of DME is probably caused by the dehydration the formation of Mn–O P bonds, which likely weaken the
==
of MeOH by the Al2O3 surface. DME has been observed
as the major product in MeOH oxidation reactions over
P
O bonds, consistent with the IR data.
The other bands observed at 1238, 1088, 1029, 925, and
-Al2O3 at temperatures up to 225 C (22). It appears that 711 cm 1 are due to P–C vibrations, methyl deformations,
once the catalyst becomessaturated with P-species(marked and different types of P–O vibrations. While the IR data
by the appearance of DMMP and MeOH in the gas phase), indicate the presence of P–CH3, it appears as if initially,
the Al2O3 surface is no longer able to dehydrate MeOH (during the protection time) P–CH3 is oxidized. This seems
and therefore MeOH begins to appear in the gas stream. likely because of IC data, which indicate the formation of
The formation of DME and MeOH over -Al2O3 is similar PO34 on the catalyst surface. This could be produced only
to results obtained in the destructive adsorption of DMMP if DMMP is completely oxidized. Perhaps once the surface
over MgO (7–9).
The catalytic results for DMMP decomposition over the additional DMMP is no longer oxidized.
is saturated with PO34 , the surface becomes poisoned and
two Al2O3-supported manganese oxide samples shown in
The production of MeOH is indicative of a hydrolysis
Figs. 8B and 8C indicate protection times which are inter- mechanism. Most likely, hydrolysis of the methoxy groups
mediate between that observed using AMO and the bare occurs by abstraction of a hydrogen from the surface hy-
support. In both samples, CO2 is observed, indicating that droxyls, which then evolves as MeOH. Since no C–O vibra-
DMMP is oxidized. Similar to results obtained using AMO, tions are observed under these conditions, this suggests that
after several hours the CO2 levels decrease and the MeOH at 400 C all methoxy groups are lost. This is consistent with
concentration increases. From these data it is clear that the results observed for many metal oxides in the literature (10,
use of the high surface area support aids in increasing the 11, 14). Another observation is that the bands in the higher
1
amount of DMMP decomposition (compared with unsup- frequency portion of the IR spectrum (3200–2600 cm
)
ported AMO). The protection time for AMO/Al2O3 is al- are absent. Therefore, the decomposition mechanism over
most twice that of the Mn/Al2O3 catalyst. We are not sure AMO is similar to that observed in photo-assisted decom-
why this longer protection time occurs with AMO/Al2O3. position reactions, except that here the reaction proceeds
However, it appears that this may be due to differences in further with loss of all methoxy groups to oxidation to CO2.
the nature of the manganese species. Perhaps differences
The IR data for -Al2O3 indicate that the fresh support
in the manganese oxidation state, dispersion, and surface has a strong, broad peak centered at 3470 cm 1, which is
area contribute to the differences in the catalytic data. We due to O–H vibrations. The peaks at 1636, 1540, and 1368
are currently using BET methods to examine the catalysts’ are also due to O–H vibrations. These peaks disappear after
surface areas to assist in explaining these trends. The XRD reaction with DMMP, which indicates that they react with
data for AMO/Al2O3 indicate that the MnOx phase is amor- DMMP. Broad peaks centered at 762 and 570 cm 1 are due
phous. This may suggest that the crystallite size is too small to Al–O vibrations. The IR spectrum of Al2O3 after DMMP

THERMAL DECOMPOSITION OF DIMETHYL METHYLPHOSPHONATE
75
reactions indicates the formation of several new peaks in obtained for TONs of supported manganese oxide catalysts
both the lower and the higher frequency regions. Since the (1.22 and 1.87).
1
1
==
P
O stretch is shifted by almost 100 cm to 1165 cm ,
this suggests that DMMP interacts very strongly with Al2O3
and that the bond is weakened considerably. As with AMO
catalysts, the peaks at 1321, 1108, 918, and 802 cm 1 indicate
V. CONCLUSIONS
DMMP is thermally decomposed over AMO in the pres-
the presence of P–C, P–O, and C–H bonds. Again, since ence of air to form CO2 and MeOH in the gas phase
no C–O peaks are observed, this suggests that all of the and MMP, MPA, and PO34 on the catalyst surface. The
methoxy groups are lost.
temperature is the key parameter in affecting the rate
Unlike AMO, a few peaks are observed in the higher of reaction. The highest rates were obtained at 400 C.
frequency region of the spectrum. These include peaks at Turnover numbers for thermal DMMP decomposition re-
3001 and 2935 cm 1, which are due to methyl stretches actions over AMO at 400 C were several orders of mag-
in the PCH3 group. Since no methyl stretches were ob- nitude greater than turnover numbers for photo-assisted
served for OCH3 groups, further evidence is provided that reactions. The rates of thermal decomposition reactions
all methoxy groups are lost during the reaction. The rea- were also found to be dependent on catalyst amount and
son why the high frequency peaks are observed with reactant flow rates. The mechanism of thermal decomposi-
-Al2O3 and not with AMO may be due to the fact that tion over AMO occurs similarly to that in photo-assisted
the reaction was run for a longer time (23 vs 6.5 h) and reactions, except that at the temperatures used in these
that -Al2O3 has a higher surface area, leading to a higher experiments all methoxy groups are lost, presumably to
concentration of adsorbed species. No CO2 was observed oxidation to CO2. The presence of PO34 on the AMO sur-
to form in the gas phase during reactions. However, the IC face after reaction at 400 C indicates the complete oxida-
data do indicate the presence of small amounts of PO43 . We tion of DMMP (P–CH3 oxidation), especially during the
are not sure why this happens. The major product observed protection time. After several hours, the catalyst becomes
on Al2O3 after thermal DMMP decomposition reactions is poisoned, and hydrolysis of DMMP occurs (formation of
MPA. These results are consistent with data in the litera- MeOH).
ture, whereby DMMP is not oxidized over Al2O3; rather it
dissociatively adsorbs on Al2O3, with loss of both methoxy active materials for thermal DMMP decomposition reac-
groups (at 400 C) to form adsorbed MPA (10, 16). tions. The bare support had the longest protection time of
Both Al2O3 and Al2O3-supported manganese oxides are
The IR spectra for the AMO/Al2O3 and Mn/Al2O3 cata- all catalysts. Major products formed over Al2O3 included
lysts exhibit similar trends to that observed using AMO. Af- DME and MeOH. No CO2 wasproduced when usingAl2O3,
ter reaction with DMMP, both samples show a similar shift indicating that DMMP is not oxidized. The mechanism of
==
in the P O stretch toward lower energies. Similar peaks decomposition over Al2O3 appears to occur by dissociative
are observed in the P–C, P–O, and methyl deformation re- adsorption of DMMP.
gions of the lower frequency portion of the spectra. Also,
The most active catalystsfor DMMP decomposition reac-
no C–O peaks are observed. These results suggest that a tions were those prepared by supporting manganese oxide
similar mechanism of DMMP decomposition occurs using on Al2O3, especially by precipitation of AMO onto Al2O3.
Al2O3-supported manganese oxide catalysts as with AMO. Turnover numbers for reactions over these materials were
The main difference is the use of Al2O3 causes an increase the highest (>1), suggesting that the reaction proceeds cata-
in the protection time, which is attributable to the higher lytically. Mechanistically, the reactions appear to occur sim-
surface area and increased dispersion of the redox active ilarly to those performed over AMO. However, poisoning
manganese.
by phosphorous species still occurs with these materials,
The turnover numbers shown in Table 4 indicate that the leading to reduced DMMP decomposition activity. The poi-
thermal decomposition of DMMP over AMO (400 C) is soning of solid catalysts by phosphorous-containing species
several orders of magnitude greater than TONs for pho- continues to be problematic and remains a challenge for
toreactions. The most active catalysts are the manganese- future work in catalytic decomposition studies of chemical
oxide-supported Al2O3 materials. Both of these catalysts warfare agents.
result in TONs greater than 1, suggesting that the reaction
It may be interesting to see what effects adding vanadium
occurs catalytically over these materials. Al2O3 appears to to either manganese oxide or supported manganese oxide
be a good support for DMMP decomposition reactions due materials has on the DMMP decomposition activity. Recent
to its: (a) reactivity toward DMMP, (b) high surface area, results from our lab clearly indicate the superiority of VxOy
and (c) good dispersion of manganese. There also appears supported on Al2O3 and SiO2 for DMMP decomposition
to be a synergistic effect in DMMP decomposition activ- as protection times >100 h were observed at 450 C (15).
ity by supporting manganese oxide on Al2O3, as TONs for Additionally, the use of higher temperatures (>500 C) in
Al2O3 (0.0458) and AMO (0.179) do not add up to values DMMP decomposition studies may be beneficial for the

76
SEGAL ET AL.
formation of gaseous P-species species which could prolong
the activity of the manganese oxide catalysts.
8. Li, Y. X., Koper, O., Atteya, M., and Klabunde, K. J., Chem. Mater. 4,
323–330 (1992).
9. Lin, S. T., and Klabunde, K. J., Langmuir 1, 600–605 (1985).
10. Mitchell, M. B., Sheinker, V. N., and Mintz, E. A., J. Phys. Chem. B
101, 11192–11203 (1997).
11. Tzou, T. Z., and Weller, S. W., J. Catal. 146, 370–374 (1994).
12. Tesfai, T. M., Sheinker, V. N., and Mitchell, M. B., J. Phys. Chem. B
102, 7299–7302 (1998).
13. Jiang, Y., Decker, S., Mohs, C., and Klabunde, K. J., J. Catal. 180, 24–
35 (1998).
ACKNOWLEDGMENTS
We thank the U.S. Army and United Technologies Research Center
(East Hartford, CT) for this work. This work was supported by the U.S.
Army Research Office, under Contract DAAH04-96-C-0067.
14. Lee, K. W., Houalla, M., Hercules, D. M., and Hall, W. K., J. Catal.
145, 223–231 (1994).
REFERENCES
15. Cao, L., Segal, S. R., Suib, S. L., Tang, X., and Satyapal, S., J. Catal.
194, 61–70 (2000).
1. Obee, T. N., and Satyapal, S., J. Photochem. Photobiol A Chem. 118, 16. Templeton, M. K., and Weinberg, W. H., J. Am. Chem. Soc. 107, 97–
45–51 ((1998). 108 (1985).
2. Segal, S. R., Suib, S. L., Tang, X., and Satyapal, S., Chem. Mater. 11, 17. Christol, H., Levy, M., and Marty, C., J. Org. Chem. 12, 459–470 (1968).
1687–1695 (1999).
3. Smentkowski, V. S., Hagans, P., and Yates, J. T., J. Phys. Chem. 92,
18. Chen, J., Lin, J. C., Purohit, V., Cutlip, M. B., and Suib, S. L., Catal.
Today 33, 205–214 (1997).
6351–6357 (1988).
4. Henderson, M. A., and White, J. M., J. Am. Chem. Soc. 110, 6939–
19. Kapteijn, F., Singoredjo, L., Andreini, A., and Moulijn, J. A., Appl.
Catal. B Environ. 3, 173–189 (1994).
6842 (1988).
20. Lin, J. C., Chen, J., Suib, S. L., Cutlip, M. B., and Frehaut, J. D., J. Catal.
161, 659–666 (1996).
5. Guo, X., Yoshinobu, J., and Yates, J. T., J. Phys. Chem. 94, 6839–6842
(1990).
21. Segal, S. R., Suib, S. L., and Foland, L., Chem. Mater. 9, 2526–2532
(1997).
22. Ozkan, U. S., Kueller, R. F., and Moctezuma, E., Ind. Eng. Chem. Res.
29, 1136–1142 (1990).
6. Hedge, R. I., and White, J. M., Appl. Surf. Sci. 28, 1–10
(1987).
7. Li, Y. X., and Klabunde, K. J., Langmuir 7, 1388–1393 (1991).