Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface
The calculations on the electronic and space structure have been carried out for both trimethyl phosphate molecule and that supported at silica surface within the frameworks of the SCF MO LCAO MNDO valence approximation. The routes of these systems destruction have been studied. The decomposition of...
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| Zitieren: | Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface / V.V. Kukueva, O.A. Kirilov, A.G. Grebenyuk, V.V. Lobanov // Поверхность. — 2002. — Вип. 7-8. — С. 158-162. — Бібліогр.: 10 назв. — англ. |
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Kukueva, V.V. Kirilov, O.A. Grebenyuk, A.G. Lobanov, V.V. 2017-11-20T18:49:37Z 2017-11-20T18:49:37Z 2002 Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface / V.V. Kukueva, O.A. Kirilov, A.G. Grebenyuk, V.V. Lobanov // Поверхность. — 2002. — Вип. 7-8. — С. 158-162. — Бібліогр.: 10 назв. — англ. XXXX-0106 https://nasplib.isofts.kiev.ua/handle/123456789/126363 The calculations on the electronic and space structure have been carried out for both trimethyl phosphate molecule and that supported at silica surface within the frameworks of the SCF MO LCAO MNDO valence approximation. The routes of these systems destruction have been studied. The decomposition of trimethyl phosphate molecule chemically immobilized onto fumed silica surface has been shown to occur with absorption of small heat quantity (~19.1 kcal/mol). This explains a high effectiveness of the use of silicas modified by organic phosphorus derivatives as active components of fire-extinguishing powders. en Інститут хімії поверхні ім. О.О. Чуйка НАН України Поверхность Surface chemistry of silica and related sorbents Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface Article published earlier |
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Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
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Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface Kukueva, V.V. Kirilov, O.A. Grebenyuk, A.G. Lobanov, V.V. Surface chemistry of silica and related sorbents |
| title_short |
Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
| title_full |
Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
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Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
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Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
| title_sort |
theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface |
| author |
Kukueva, V.V. Kirilov, O.A. Grebenyuk, A.G. Lobanov, V.V. |
| author_facet |
Kukueva, V.V. Kirilov, O.A. Grebenyuk, A.G. Lobanov, V.V. |
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Surface chemistry of silica and related sorbents |
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Surface chemistry of silica and related sorbents |
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2002 |
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English |
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Інститут хімії поверхні ім. О.О. Чуйка НАН України |
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The calculations on the electronic and space structure have been carried out for both trimethyl phosphate molecule and that supported at silica surface within the frameworks of the SCF MO LCAO MNDO valence approximation. The routes of these systems destruction have been studied. The decomposition of trimethyl phosphate molecule chemically immobilized onto fumed silica surface has been shown to occur with absorption of small heat quantity (~19.1 kcal/mol). This explains a high effectiveness of the use of silicas modified by organic phosphorus derivatives as active components of fire-extinguishing powders.
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XXXX-0106 |
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https://nasplib.isofts.kiev.ua/handle/123456789/126363 |
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Theoretical analysis of thermal destruction of dimethyl phosphates immobilized on dispersed silica surface / V.V. Kukueva, O.A. Kirilov, A.G. Grebenyuk, V.V. Lobanov // Поверхность. — 2002. — Вип. 7-8. — С. 158-162. — Бібліогр.: 10 назв. — англ. |
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2025-11-25T21:07:30Z |
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2025-11-25T21:07:30Z |
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1850545597211213824 |
| fulltext |
158
THEORETICAL ANALYSIS OF THERMAL DESTRUCTION
OF DIMETHYL PHOSPHATES IMMOBILIZED ON
DISPERSED SILICA SURFACE
V.V. Kukueva1, O.A. Kirilov1, A.G. Grebenyuk2, and V.V. Lobanov2
1Institute of Fire Security, Onoprienko Str. 8, 18034 Cherkasy, UKRAINE
2Institute of Surface Chemistry, National Academy of Sciences
Gen. Naumov Str. 17, 03680 Kyiv-164, UKRAINE
Abstract
The calculations on the electronic and space structure have been carried out for both
trimethyl phosphate molecule and that supported at silica surface within the frameworks of the
SCF MO LCAO MNDO valence approximation. The routes of these systems destruction have
been studied. The decomposition of trimethyl phosphate molecule chemically immobilized onto
fumed silica surface has been shown to occur with absorption of small heat quantity
(~19.1 kcal/mol). This explains a high effectiveness of the use of silicas modified by organic
phosphorus derivatives as active components of fire-extinguishing powders.
Introduction
Any burning is first of all a chemical reaction of the combustible mixture components
accompanied by heat emitting and creating of disperse kind mass-transfer processes in gas.
That is why all the phenomena of burning and fire-extinguishing are closely connected with
ideas and rules of chemical kinetics and first of all with the concept of the rate of chemical
reaction running under non-isothermal conditions. Then the heat evaluated due to reaction
accelerates the latter, in according to the Arrenius rule. The availability of such a correlation is
a high degree characteristic of the most of burning phenomena. Owing to this correlation
between mixture heating due to reaction and acceleration of the latter a heat self-accelerated
avalanche arises that is often perceived as a self-ignition. However it has been shown in [1]
that self-ignition has not consistently a heat nature but there is another type of self-ignition
based on a chain chemical oxidation reaction. A chain branched process gives rise to active
particles – radicals in the substances, their quantity being quickly rising in the time as a
sequence of chain branching. Thereafter as the flame spreads they play a role of active sites
stimulating chemical process what is a characteristic of the chain mechanism of chemical
reaction. A suitable partner can be selected for almost each active radical, interaction between
them resulting in the formation of less reactive particle (the latter is most often a molecule in
the singlet state) followed by the chain rupture. In particular, just this can take place on
addition of some inorganic or organic substances showing inhibiting properties into the
hydrogen flame that can be performed conditionally as a totality of active sites (O·, H·, OH·,
HO2
·). An active site can create two new second chains with definite probability when enters
the reaction of main chain continuation. When branched, these second chains form tertiary
chains ad so on. Thus, the quantity of chains should arise in the time. Simultaneously all these
chains should be interrupted, for example, when an active particle meets the inhibitor thermal
decomposition products [1].
159
Background
The mechanism of the hydrogen flame suppression with disperse compositions can be
caused either by a heat interaction of solid and gaseous phases (the physical factor – a decrease
in the temperature of burning mixture) or by chemical inhibiting, or by their combination (as a
result of the chain rupture the heat evaluation ends and the temperature is reduced and
becomes below the ignition level). There is a reason to believe that the high action
effectiveness of the fire-extinguishing powders is conditioned mainly by chemical factors rather
by than physical ones [2]. In other words, the powder supply into the ignition zone should
result in increasing the reaction active site ruin rate. Moreover, a recombination reaction of
“hot” molecules and radicals can take place on solid powder particle. The solid particle plays a
role of an effective reservoir for their excitation energy dissipation. It should be expected that
the less ordered is the particle structure, the more wide is the spectrum of the energy
(rotational, vibration, electric) absorption of “hot” atoms and molecules. In principle, the
powder action can be caused by both heterogeneous ruin rate of reactive chain carriers and
homogeneous reactions following the transition of the powder particle decomposition products
into the gas phase. There are considerable differences in the conformities to natural laws of the
flame suppression that are conditioned by differences in the chain carrier types reacting with
the inhibitor. The effect of extinguishing consists in the deactivation of the flame active sites
(FAS) Y· (O·, H·, CH3
·, OH·, HO2
·, and so on) at the surface of fire-extinguishing powder
particle that is in the flame torch or in the gas phase including the thermal decomposition
products of inhibiting agent [3].
Basing on comparing the rate of chain branching process with that of the hydrogen
atom ruin in the reactions with decomposition products of fire-extinguishing powders, an
assumption was made [4] on the availability of combined homogeneous-heterogeneous
inhibiting mechanism. Moreover, it should be taken into consideration that the dissociation
energy of adsorbed molecule may be lesser than that of dissociation of the same molecule in
gas phase. If the dissociation activation energy is considered to be close to that of dissociation
itself or to coincide with it, the rate of molecule dissociation on surfaces should be greater that
that in gas phase what is conditioned by chemical sorption processes. In this case the surface
should play a role of a catalyst of radical recombination reactions. It is important for the
further elucidation of the character of interaction of the flame active sites with substances-
inhibitors to compare the energy of the probable dissociation paths of both isolated and
immobilized molecules.
There are widely used phosphorus organic compounds among various chemical
additives to fire-extinguishing powders. Thus, trimethyl phosphate (TMP) has been shown [5]
to be an effective burning inhibitor. Firstly the reaction mechanism of phosphorus oxides with
radicals – flame active sites was studied by Twarowski [6] who examined a catalytic effect of
phosphorus oxides on the H· and OH· recombination reaction. It was shown in the same work
by an experiment that on addition of TMP into flame the HPO2
·, PO2
· and PO· particles
(formed due to thermal decomposition of trimethyl phosphate) are the most active inhibiting
ones. But no qualitative data (deactivation energy, rate of recombination reaction) were
obtained concerning the interaction of these particles with the flame active sites. Obviously it is
connected, to our mind, with the complexity of experimental measurements at elevated
temperatures.
Earlier it was shown in the experimental studies on the investigation of thermal stability
of phosphorus-containing fumed silica [7] that a presence of chemically sorbed phosphorus
compounds at silica surface bound with silicon atoms via oxygen bridge leads to the
considerable diminishing of the C-H bond strength in the ºSi-O-PH(=O)-CH3 groups. This fact
160
testifies an opportunity of to use trimethyl phosphate immobilized onto silica surface in order
to diminish the heat of decomposition of the fire-extinguishing powder.
Models and Method
In the case when direct experimental investigations are difficult or impossible due to
one or other cause theoretical approaches (in particular, methods of modern quantum
chemistry) become an invaluable tool for obtaining qualitative data. The rupture energies are
calculated in the given work of both free TMP molecule and that grafted to fumed silica
surface simulated by a HO-Si[OSi(OH)3]3 cluster within the frameworks of the SCF MO
LCAO MNDO valence approximation [8] according to the technique described in [9].
Results and Discussion
The heat effects (DH°298) of the isolation reactions of methyl and oxymethyl particles
bearing electric charge of –1, 0, and +1 from free TMP molecule in the ground state are
performed in the Table 1. These effects were computed as differences between the heat of
formation of the parent molecule and the sum of analogous values of the destruction products,
the charge and spin state of the system destroyed being kept.
Table 1. Enthalpies (DH°298) of the destruction reactions of free trimethyl phosphate
molecule (kcal/mol).
N Destruction Products Reaction Enthalpies
1 OPO(OCH3)2
· + CH3
· 319.0
2 OPO(OCH3)2
+ + CH3
— 328.8
3 OPO(OCH3)2
— + CH3
+ 220.3
4 OP(OCH3)2
· + OCH3
· 143.5
5 OP(OCH3)2
+ + OCH3
— 343.6
6 OP(OCH3)2
— + OCH3
+ 194.9
It is seen from the table considered that in isolated TMP molecule the rupture of P-O
bond followed by the formation of two radicals
OP(OCH3)3 ® OPO(OCH3)2
· + CH3
·
is somewhat greater (DH°298 = 319 kcal/mol). Hence, the probability of the methyl radical
formation (that is one of active sites of hydrocarbon flame) is very low when the inhibitor
molecule disintegrates. This is another weighty argument in favor of the opportunity of TMP
use as an inhibiting component of fire-extinguishing compositions.
Accordingly to the data we obtained the ionizing potential of the OCH3
· particles is
relatively small (7.3 eV) (experimental value is of 8.2 [10]) what testifies the easiness of the
unpaired electron isolation from the OCH3
· radical, this electron is capable in its turn to
interact with surface active sites and so to deactivate them. Thus, the data obtained for the
TMP destruction in gas phase testify the easiness of radical particles formation that can make
an inhibiting impact along with free electrons generated due to ionizing of radicals.
161
Analogous calculations on the TMP molecule destruction energy were carried out for
the system simulated for its chemically sorbed state at the fumed silica surface (see Fig.). The
ºSi-O-PO(CH3)2 bond appeared to be the most weak one, the corresponding value of the
rupture energy being only of 19.1 kcal/mol (see Table 2).
Table 2. Enthalpies (DH°298) of the destruction reactions of dimethyl phosphate
groups immobilized on silica surface (kcal/mol).
N Destruction Products Reaction Enthalpies
1 [(HO)3SiO]3SiO· + OP(OCH3)2
· 19.1
2 [(HO)3SiO]3SiO— + OP(OCH3)2
+ 193.6
3 [(HO)3SiO]3SiO+ + OP(OCH3)2
— 226.2
P
Fig. A schematic presentation of the cluster (CH3O)2PO-OSi[OSi(OH)3]3
As a result of thermal destruction of the phosphorus-containing fumed silica in gas
phase the OP(CH3)2
· radical appears that make an inhibiting influence on the flame active sites,
and an inhibiting site is generated at the solid surface with a high degree of spin density
localizing (at the surface oxygen atom (0.82)). This site is capable to capture and deactivate
practically all the known types of flame active sites because the energy they yield dissipates
O Si C H
162
effectively along the bonds of solid support. This fact argues for the perspective of silica use as
a support for the prolongation of inhibitor action.
Conclusions
Thus, a theoretical investigation by quantum chemical calculations not only gives an
opportunity to foresee real ways of decomposition of the active substance of fire-extinguishing
powders, but also proves a greater activity of the inhibitor grafted onto silica surface than that
of isolated state what can be used for the prolongation of inhibitor action via immobilization
onto fumed silica. Such kind calculations give a real opportunity to examine complicated
physicochemical processes at molecular level for selection of the most effective conditions of
the flame inhibition.
References
1. Semenov N.N. Chain Reactions. – Moscow: Nauka, 1986. – 535 p. (In Russian).
2. 2. Bezarashvili G.S., Lordkipanidze D.N., Dzotsenidze Z.G., Gordeziani D.G., and
Zizova L.A. Role of chemical and thermal factors at heterogeneous inhibiting various
flames // Doklady AN SSSR. – 1987. – V.293, N4. – P.892-895.
3. Krasnyanskiy M.G. Fire-Extinguishing and Explosion-Suppressing Powders. –
Donetsk: Donbass, 1990. -111 p. (In Russian).
4. Baratov A.N., Dobrikov V.V., and Shamonin V.G. On role of homogeneous factors at
inhibiting methane-air flame with powders // Khimicheskaya Fizika. – 1988. – V.7, N6.
– P.827-831.
5. Hastie J.W. and Bonnell D.W. Molecular Chemistry of Inhibited Combustion Systems
// National Bureau of Standards Report N NBSIR, 80-2169.
6. Twarowski A. The Influence of phosphorus oxides and acids on rate of H + OH
recombination // Combust. Flame. – 1994. – V.94. – P.91-107.
7. Bogatyrev V.M. and Chuiko A.A. Study on interaction of di- and triethyl phosphite
with dispersed silica surface by IR-spectroscopy method // Ukr. Khim. Zhurnal. – 1986.
– V.52, N5. – P.470-471.
8. Dewar M.J.S. and Thiel W. Ground states of molecules. 38. MNDO method.
Approximation and parameters // J. Amer. Chem. Soc. – 1977. – V.99, N10. –
P.4899-4907.
9. Chuiko A.A. and Gorlov Yu.I. Surface Chemistry of Silica: Surface Structure, Active
Sites, Sorption Mechanisms. – Kiev: Naukova Dumka, 1992. – 248 p. (In Russian).
10. Rupture Energies of Chemical Bonds, Ionizing Potentials, and Electron Affinities /
P.V. Gurvich, T.V. Karachentsev, V.N. Kondratiev. – Moscow: Nauka, 1971. – 351 p.
(In Russian).
Abstract
Abstract
Introduction
Background
Results and Discussion
N
Destruction Products
Destruction Products
Conclusions
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