Oxidation mechanism and antioxidative stabilization of polyhydric alcoholic esters as bases of high-temperature lubricants

0031-6458[7810701-0123507.50/0

Petrol. Chem. U.S.S.R. Vol. 18, pl}. 123-130. (~) Pergamon Press lad, 1979. 1)rlated In Poland

OXIDATION MECHANISM AND ANTIOXIDATIVE STABILIZATION OF POLYHYDRIC ALCOHOLIC ESTERS AS BASES OF HIGH-TEMPERATURE LUBRICANTS*

V. S. MARTEM'YANOV a n d M. 5[. KUKOVITSKI[ Bashkir Stat.o University

T111.~ interest in the study of oxidation of polyhydric alcoh<)lic and carboxylie aeid esters is due to their application as lubrieants in aviation technology. ]),e(luirements facing the thermal-oxidative stability of these oils continuously increase and research into et~icicnt, mcth(>ds of stabilization requires the knowlc(lgc of the mechanism of oxidat.ioll and spe(.ial features of this class of c<>mpolmds.

We selected dicthylene glycol dicaprylate (DEG])C) and pcntact'ylhritol tctravalcrate (PETV) for invcsligaiion. Kinetic r(.gularitics of initiated oxidation shows [I] that they undergo oxi(laliol~ 1)y a classical mechanism and the rate of initit~l stages of oxidatio~i is dcscribc(I 1)y

where k,~ -- thc ratc constant of chain extension, k G- rate constant of quadratic chain breakage on peroxide radicals, IV~ -- rate of initiation and R H -ester. The value of /~k~ ~ and the corresl)ol~db~g act~vat.ion energy for P E T V may characterize oxidation properties of an industrial pcntacrythritol ester mixture during appropriate purific~tion [2], whi(.h indicates the decisive effect on oxidation of the alcoholic part of ester. With an increase of temperature the role of degenerate chain branching increases and lhis restricts the rang<; of initiated extrication to a temperature of 150 °. A study of kinetic regularities of auto-oxidation of esters shows that dcgencratc chai~l branching takes place 1)v a monomolccuhlr reaction, rate constants of which were determined both for 1)E(ID(~' aIld PETV ill. A high eoncent.r~tioll of hydropcroxidc (I-[P) cvcn at ~ temperature of 200 ° (above 0.1 mole/l.) necessitates effc(.livc meas;urcs to control the brallching agent. 0

* Noflkhiiniya 18, No. 4, 53!)-545, 197,v,. Note: Neftekhimiya Vol. 18 N()s. 4 & 5 (~ontain pqpers prcsent,,:d at th(~ 3rd All-Uniou

Co-ordiqation Conference on Liquid Phase Oxidation of Organic Compounds hehi in Leningrad 3 1 6 December 1977. This volume of Petroleum Clmmistry U.S.S.B.. presents a selection of the papers given at that Conference. 123

124

V.S.

I~IARTEM'YANOV a n d

M. M. KUKOVITSKII

Auto-oxidation could be examined under kinetic conditions up to 200 ° for DEGDC and up to 220 ° for PETV. In the entire range of temperatures the mechanism of initial stages of oxidation remained amchangod. Initiated oxidation of esters in the presence of cumyl H P enabled rate constants of esters with cumyl peroxy radicals to be determined [3]. They were much lower than the total of partial rate constants of all CH bonds of csters determined [4] in the series of dicarboxylie acid monoesters or diesters. Not only C - - H bonds of the alcoholic residue, b u t also spacial C - - H bonds of the acid residue undergo specific deactivation which, according to a former .'-;tudy [5], is due to the multidipole interaction of polar groups of ester with a polar peroxy radical in the intermediate state. Therefore, the nature of ester itself, its polyhydric properties explain its reduced reactivity in oxidation. T I M E (I) AND CONCENTRATION ( I I ) ORDER OF THE REACTION (n) OF GAS FORMATION SVITII' AND ~'ITHOUT INIIIBITOR

H, Ester

--

I I if--

I

CO

',

InH

--

I

II

l

2

I

COl

InH II

-~l

,,

--

-H-

I-~-~-

I n I-I

1

II

I

Diethylene glycol dicaprylato Pentaerythritol tetravalerate

I

2

2>n>l

l (2) I 1

1

1

1

1

1

1

1

1

1

1

I

1 (2)

2

I (2)

1

1

I 1

Note: Roman numbers denote the method of determining eater: I.

~

krV[ROOH]', [FJ~

n = l , log [-F]~-'----D']-~ =0"43 k r , ' t 1

1

~= CRoori], ~=2.~®_[;],L,j - - [r]® =~¥...t II. W , o = k t . [ R O O H ] , '~ ( n = l 2),

Wro=lQr [ROOH],-.t k2t [RO01I]o 2

(n = 1 (2)).

A study of the kinetic laws of decomposition of ester H P in an inert atmosphere enabled us to establish that decomposition takes both by a monomoleccular and a chain mechanism [6]. Chain length of induced decomposition, according to temperature and initial ester varies between 4 and 44. The probabil;ty of radical yield from the cell appeared lower (by one order of magnitude for PETV), compared with well known probabilities for alkyl hydroperoxides, which points to the importance of non-radical decompositicn of ester HP. Both during auto-oxidation of esters and during decomposition of H P in an inert atmosphcre, hydrogen and carbon monoxide and dioxide are evolved, these being products of decomposition of HP; this is confirmed by extrapolation to zero of the ratio of the amount of gas given off to the anmunt of oxygen

Polyhydric alcohol esters

125

absorbed, or H P accumulated at a degree of the reaction tending to zero.* Taking the balance of oxygen absorbed during auto-oxidation, H P formed and the total of gases liberated shows that even at the earliest stages of formation of H I ) its decomposition is accompanied by gas evolution, up to half a mole of the total of gases being formed per mole of I I P decomposed. This is a special feature of ester HP, which is due to the proximity of the hydroperoxide group to ester oxygen. Since the gas ratio varies according to the degree of oxidation, composition typical of these initial conditions was determined b y extrapolation to zero degree of oxidation. Gas composition thus obtained depends on the type of ester and temperature and on partial oxygen pressure. Much lass hydrogen is formed in P E T V than in DEGDC, hydrogen content decreasing with temperature in the former and increasing, in the latter. CO~ content decreases with temperature, while CO content increases for DEGDC and the opposite is the case for PETV: CO S content increases with temperature and CO is practically independent of temperature. Oxygen suppresses CO formation and stimulates the formation of hydrogen and carbon dioxide. All these facts prove that gas formation is a reaction combining various directions of decomposition of ester H P and the study of kinetic regularities of gas formation may therefore be instrumental in understanding the mechanism of advanced stages of oxidation of esters. Kinetic rules of gas liberation during the decomposition of H P in inert atmosphere are different for various initial esters and gases. Interesting information may be obtained from a comparison of time orders of reactions obtained b y a transforamtion of the entire kinetic curve (method I, Table) with concentration orders obtained from the dependences of the initial reaction rate on the initial concentration of H P (method II, Table). Both relations were determined with and without an inhibitor -- 1,3,5-trimcthyl-2,4,6-tris-(3,5di-tert-butyl-4-hydroxybenzyl)- benzene (AO-40). For carbon dioxide both the time and concentration for H P is first order, whereby the inhibitor has practically no effect either on the rate, or the stoichiomctry of formation of C02. Consequently, the source of CO,, is a molecular product, the monomolecular decomposition of which results in a carboxyl radical. CO formation is characterized by a concentration of first order, this being exclusively so in the presence of an inhihitor, while for DEGDC H P without inhibitor a second ordcr is introduced. A feature most ch;,ractcristic of the formation of CO is a sudden reduction of the rate and st~;ichiometry of CO formation with the addition of an inhibitor. The formation of hydrogen is characterized by a concentration of sccond order both in the presence or absence of an inhibitor. For P E T V H P time is also of second order, while for DEGDC HP, it is first. The inhibitor has practically no effect on the rate or * Detailed experimental explanation of these results will be given in subsequent. publications.

1"26

V. S. ~ARTEM'YANOV and M. M. KUKOVI'I"SKII

stoichiometry of formation of hydrogen from PETV HP and markedly reduces the rate of formation of hydrogen in DEGDC HP. A study of the effect of aldehyde and carboxylic acid additives on the rate and stoichiometry of gas formation during the decomposition of HP in inert atmosphere shows that de.canal increases tile rate a.nd stoichiometry of hydrogen formation, which is in agreement with the mechanisnl of hydrogen formation during the decomposition of primary alkylhydroperoxides [7]. At the same time decanal reduces the stoichiometry of CO,,. formation, ~pparently, by combining the source of its formation. The rate and stoichion~ctry of CO increase with decanal concentration, tile inhibitor having a practically exclusive effect on characteristics of CO formation. Valeric acid accelerates the formation of all gases Ul) to a certain limit, while stoichiometry only increases for CO_~, t!le difference in stoichiometry in inhibited and uninhibited experiments being very slight. I t follows form the foregoing that it is not carboxylic acid, but very probably per-ackl or per-ester, which is the source of CO~. Per-acid, whidl is similar to acid ]brming the ester, was detected by GLC durin~ mcthylati(m with diazomethane of the oxi(late obtained by initiated oxid.:~tion of P E T V (140°). Under these conditions the for/nation of methyl ester of wdcric acid is completed at 20 ° in 4 days, whereas methyhttio~t of valeric acid under t11o same conditions takes ph~eo in 30 rain. A study of the rate of methvlation in model systems containing per-va.leric a.nd per-eaprylic acid (the iatt.er was obtained without caprylie acid) shows that per-ackls during methylation are coltverted completely to methyl esters of acids only in 4 days (20'); this is, apparently, because of the slow reduction with diazomethane excess of methyl per-ester which ureter eonditioDs of GLC analysis breaks down in ~.he evaporator (the tail of the peak of solvent disappears after the tbrm~timl of methyl ester). From the difference betweml complete, amt rapid methylation we determined the concentration of per-a(:id. Under these conditions about a ([m~rter ot" tim oxygen absorbed in the init.ial st.ages of oxidation is in the form of pcrvaleric acid. '.Pile fur,nation of per-acid during oxidat.imt is also confirmed by spectrophotometric analysis with bcnzidine [S]. Under conditons of auto-oxi, h~tion of PI'7.t'V and DEGDC analysis of valeric and caprilic acids shows I]la.t these arc formed as products of direct decomposition of H.P of esters, lit is important to note that lower (formic and acetic) acids are not l)ro(htct.s directly formed fl'om H P, but are tin'reed by a nlecha.nsim lypical of advanced stages of oxidatkm of hydroca.rbot~s. (',eneralizing exp,wimental re.suits zmd using existing ideas about the mechanism of fonm~tion of hydrogen during the decompositiolt of primary alkylhydroperoxides [7J, the follow'ng system of tbrm~tion of breakdowll products of ester H P ma.y be 1)resentc(t, which reflects specific features of oxidation of esters, involving the formation of e-acyloxy-hydroperoxide and its specific methods of deeomi)osition. The main feature is the fact that in addition to conventional decomposition

Polyhydric a!eohol esters

127

to radicals at the 0 - - 0 bond of the initial HP, peroxy compounds formed by an equilibrium mechanism from initial HP, undergo further decoml)osition. This is per-acid, which is formed along with aldehyde and ~-oxyalkylacyl peroxide, or for brevit.y, hydroxyperester. The formation of these prodnct.~ resembles advanced stages of oxidation of esters and ahlchydes [9]. A sight/leant prot)ortion of t h(." molecular decoml)osition of ttI ) via hydroxy per-ester explains the low probability of radical yiehl into the mass (below 0.1 for PETV ttP). The tbrmation of CO.., by a molecular mechanism is explained by the monomolecular decomposition of per-act(1 or hydroxy per-ester and radical dccarboxylation of acids is not typical of oxidati(m of ester itself. ttydrogcn is formed by a molecular mcchanisnl by a bimole(aflar reaction of H P with aldehyde or hydroxy l)cr-estcr, whereby with an equilibrium dist)laced in the direction of one of the components, the reaction takcs place as a pseu(lo-nmnonn)lccular reaction. The molecular method of CO formation is most probably a bimolccular reaction between ahlehydc and HP or per-acid, which is of first order for H P as a consequence of this cquilibrium. The system proposed explains the effect of aldehyde additives both on the increase in rate and the stoichiometry of hydrogen and CO formation and on the reduction of the stoichiomctry of CO: formation. In the al)sencc of inhibitor, chain decomposition of t i P is added to the reactions mentioned, whereby chain transfer through aldehyde, which markedly increases the rate of CO formation, is incorlaorated in the reaction of chain extension. In the presence of oxygen this reaction is inhibited, which is in agrccment with the experiment. For DEGDC thc radical method of hydrogen ibrmation may be used. The direct tbrmation from H P of the acid, from which ester is formed, is also explained within the framework of the system proposed. This system is not a mechanism substantiated in detail, however, it is in saiisiactory qualitative agreement with experimental results obtained, which form convincing evidence of the degradation of ester at the ester CO bond during oxidation at the stage of decomposition of a-acid hydroxyhydroperoxide. The effectivcness of anti-oxidizing compositions was examined using industrial ester of pentaerythritol and carboxylic acids of ~ C~-C0 fl'action (IEPECA). These investigations were aimed at explaining the a.pplication of a number of anti-oxidation inhibition effects, which have been studied in dctail for hydrocarbons at temperatures of the order of 100 ° and for IEPECA at 200 °. Regeneration of inhibitors in the process of chain breakage with hydroxypcroxide radicals [10] is the most promising method. We found that the anti-oxidative effect of aromatic amines is considerably intensified on adding polyhydric alcohols such as 1,2-dioxypropyl ester (diglycerin), 2-rnethylol-3,4-dioxytetrahydrofuran (xylitane), otriol, neopentyl glycol and pentacrythritol. The fact that we are dealing with the regeneration of the inhibitor with hydroxyperoxide radicals is confirmed by experiments

!

R"COOH + H20

ROOH -f- R ' C H O --* R ' C H (OH) OOCHR'OCOR" Reactions s u p p l e m e n t i n g the system in the absence o f an inhibitor

T

B.'COOH + H2-{" R'OCOCOR"

ROOH -i- R ' C I I (O.II) OOCOR" ---, IVCH (OIt) OOCHR'OCOR" -I- R"COOOH

/

R " C O 0 0 I I ~ R"CO0" + "OIt - - V

H~O + R ' C H O T ~ R ' H C (OH)2 + R"COOII R ' H + CO + H..O ROH R'CIIO -!- II,20 -i" ROII R ttI~ + co~ + H.,.O

R'CHO + P, OOH --, WCO" + H~O --]- RO" - - V

1 R'COOH -- R"COOH

R'COOH + H" + R'OCOCOR ~ H" + R H - * H~ + R" 2R" ( R " . R"') - . products o f r e c o m b i n a t i o n and disproportionation

B.O" -? R H - , BOH + R' .--" / R O t t + RO" R" -4- ROOIt --[.-, R H + R'C (OOH) OCOR" R'C (OOH) OCOR" --, "OH + R'OCOR ~ R ' C H O + RO' (R') - , R'CO + R O H (RH) R ' C O - - . R'" + CO R ' C H (OH) OOCHR'OCOR" + R O ' ( H O ' ) --. R ' ¢ (OH) OOCItR'OCOR" . - R O H (H~O)

RO" "4- 'OH ---, I-I~O 1 ROH ~ R'CttO + R"COOII

.---, R ' t I i CO -:- H.,O -- CO~ + P,"It R ' C H O -i- HOOOCR" ---, R'CO" -- I-I20 + R"CO0" --I ' " '~ 1~ . . . . . . . I_~ R'CHO -:- IL_O -'- R"COOH (ROOH) R ' C H ( 0 0 I I ) OCOR" ~. WCH (CH) OOCOR" - , R ' C H (OH) O' -',- 'OOCR" - , [ R ' H C (OH).,I :- CO., : l l " l l

R e a c t i o n s t a k i n g place in t h e presence of an i n h i b i t o r

SYSTEI~I F O R T H E FORMATION" O F P R O D U C T S O F D E C O I ~ I P O S I T I O N O F E S T E R H Y D R O P E R O X I D E ~

.<

©

N

N

0

z

H

g:

o'a

Polyhydric alcohol esters

129

with 2,5-diphenylquinone, in the presence of which (4.4× l0 -3 mole/l.) the rate of oxidation of I E P E C A at 180 ° decreases 20-fold on adding diglycerin (2 × 10-2 mole/l.). It is interesting to note that even branched phenols in the presence of polyhydric alcohols inhibit oxidation more effectively. When t > 2 2 0 ° diffusion restrictions during oxidation of PETV increaae the stationary concentration of alkyl radicals in relation to the stationary concentration of peroxide radicals; consequently, the antioxidative composition may be reinforced with an active alkyl radical accepter. Testing a number of nitrones shows that they are weak inhibitors of oxidation, but their inhibiting action is intensified with polyhydric alcohols. An attempt to combine nitrones, as alkyl radical accepters with aromatic amines, as peroxide radical" accepters showed a synergism of their joint action, this effect depending both on the nature of amine and on the structure of nitrone and reaching a limiting value with a given concentration ratio of amine and nitrone. The anti-oxidative effect of aromatic amines may be intensified with a combined effect, with the simultaneous addition of cobalt stearate and a polyhydric alcohol--etriol. The former, apparently, operates by a mechanism explained previously [11] as a result of complex formation with amine; etriol, apparently, regenerates amine ir~ the process of chain breakage on hydroxyperoxide radicals. In most cases the arylamine--CoSt~--polyhydric alcohol system shows noticeable synergism in relation to binary systems and functions much more satisfactorily than an individual inhibitor. As indicated, a specific feature of oxidation of esters is the formation of acids and per-acids at the stage of decomposition of the primary molecular product of oxidation of ~-aeyloxy hydroperoxide. This feature of the mechanism of oxidation of esters is not taken into account in most well-known antioxidative compositions of ester oils. At the same time the decomposition of per-acid, for example, by simple neutralization prevents its breakdown to radicals and therefore reduces the rate of initiation and consequently, also the rate of oxidation of ester, generally. In fact, the addition of alkali metal stearates to IEPECA results in induction periods in oxidation; the higher the basicity of the metal ( K > N a > B a ) and the higher its concentration, the longer these induction periods. Potassium tert-butylate introduced as a synergetic additive to arylamine in an ester oil, apparently, fulfils the same function [12]. Metals of variable valency do not have a marked effect on the rate of oxidation of IEPECA [13]. However, it is important to note that metals which can undergo two-electron transfer (Sn II; Sn IV; Pb II) are among metals with an inhibiting effect. Their effect is, apparently, due to the heterolytic reduction of per-acid as a branching agent. The initial valence form of the metal is of no significance since a reducing agent (aldehyde), as well as an oxidizing agent (HP, per-acid) are present in the oxidized ester. We showed that the introduction of polyhydric alcohol to IEPECA oxidized, containing metal compounds of variable valency increases the anti-oxidative stability of

130

V. S. MARTEM'YANOVand M. M. KUKOWTSKII

oil, increasing the inhibiting effect of some metals and reducing the catalytic a c t i v i t y of others. Therefore, using the informatioa concerning the mechanism of oxidation o f polyh~'dric alcoholic esters and the experience of anti-oxi(l~tive stabilization of oils 1)repare(l from them, requir(~ments t h a t have to be satisfied by the additive composition whit~h ])reve~tts tlwrm:~l-oxidativc degradation of ester oil, m a y bc formulated. I t shouhl contain the following compolwnts: a) those which effectively accept both peroxide and alkyl radicals, regeneration of the inhibitor ii~ the pro('ess of" chain breakage being the most, promising m e t h o d of increasing this efficiency; b) decomposing peroxi(le comt)ounds w i t h o u t fl'cc radical f o r m a t i o n at a r a t e considerably ex(.ccding the rate of degenerate chain branching; c) c o m p o n e n t s p r e v e n t i n g the decomposition of the additives mentioned b y the action of acids and per-acids via neutralization, or a n y other rapid heterolytic interaction; on ttm other ban(I, additives shouht possibly retain t h e i r anti-oxidative eft'cot in the presence c f low concentrations (up to l0 -z mo]e/l.) of carboxylic acids and per-aci(Is. A complex s t u d y of these re(llliremcnts with the simultaneous solution o f problems of compatibility o1' components both with the oil base and with each o t h e r m a y be a guideline in research into anti-oxidative compositions added to ester lubricants.

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Izv. AN SSSR, Scr. khim., 2179, 1976 3. G. G. AGLIULLINA, V. S. MARTEM'YNOV, T. I. YELISEYEVA and Ye. T. DENISOV,

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