Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
131
Comparison of Chlorinated Solvent-Aluminum Reaction Inhibitors Wesley L. Archer Dow Chemical U.S.A., Inorganic Chemicals Department, Midland, Michigan 48640
Twenty-two compounds were evaluated a s inhibitors, at reflux temperatures, of the reaction with aluminum of 1,l-dichloroethane, 1,1,l-trichloroethane, 111,2-trichloroethane,and a solution of 10 vol % toluene in 90 vol 'YO methylene chloride. Minimum concentrations needed to inhibit the reaction are tabulated. Good I,l,l-trichloroethane-aluminum reaction inhibitors are not necessarily among the best inhibitors for other chlorinated solvents. Addition of an aromatic diluent, such a s toluene, to methylene chloride severely increases its corrosiveness toward aluminum. It is suggested that inhibition of the aluminum-methylene chloride reaction involves complexing of the chemisorbed aluminum chloride product with electronegative groups of the inhibitor. Successful inhibition then demands a complex that has only very limited solubility in the solvent system. Sixty compounds are ranked in order of their effectiveness in inhibiting reaction of the methylene chloride-toluene solvent system with aluminum.
Introduction Previous work by this laboratory (Archer, 1978) has established some of the relationships between structure and inhibition activity of a number of aluminum-l,l,ltrichloroethane reaction inhibitors. The present state of the art permits one to predict fairly accurately the inhibition activity of a particular structure as a l,l,l-trichloroethane-aluminum reaction inhibitor. Other chlorinated solvents can also cause severe corrosion of aluminum. Included in this group are 1,l-dichloroethane, 1,1,2-trichloroethane, and methylene chloride containing toluene as a co-solvent. We have found that a good l,l,l-trichloroethane-aluminumreaction inhibitor is not always the best choice for members of the latter group of solvents. The reported work details the inhibition activity of some better l,l,l-trichloroethane inhibitors in 1,l-dichloroethane, 1,1,2-trichloroethane,and the methylene chloride-toluene co-solvent system. A detailed analysis of structure-activity relationships of a large number of methylene chloride-toluene-aluminum reaction inhibitors is also presented. Experimental Section. Aluminum Stability Tests 1. Tests with 1,l-Dichloroethane, l,l,l-Trichloroethane, and 1,1,2-Trichloroethane. In these tests, 5 mL of inhibitor-containing solvent were refluxed with 0.5 g of 16-32 mesh pure aluminum pellets in open reaction tubes. The tubes, 33 cm long and 1 cm o.d., were placed in an oil bath that was held at 59 "C for the 1,l-dichloroethane,76 "C for l,l,l-trichloroethane, and 115 "C for the 1,1,2trichloroethane reflux tests. The upper portions of the reaction tubes extended through a water-cooled aluminum block that acted as a condenser. If no reaction (as evidenced by darkening of solvent and hydrogen chloride evolution) was observed in 24 h, the inhibitor was considered to be active at the test concentration. By observing the effect of different concentrations, a Minimum Active Concentration (MAC)-the minimum concentration needed to inhibit solvent-aluminum reaction-expressed as mol/L, was determined for the various inhibitors. Uninhibited, a t reflux temperatures, the l,l,l-trichloroethane-aluminum reaction begins within 3-5 min, 1,ldichloroethane-aluminum requires about 8 min, and 1,1,2-trichloroethane and aluminum react within 2 min. The results of these tests are summarized in Table I. 2. Tests with Mixtures of 907" Methylene Chloride and 10% Toluene. For these tests, the solvent consisted of a technical grade of methylene chloride (90 vol W )and 0019-7890/79/1218-0131$01 .OO/O
toluene (10 vol TO).One milliliter of a 0.05 mol/L solution of AlCl,, dissolved in 90/ 10 methylene chloride-toluene, was added to 0.5 g of 16-32 mesh pure aluminum pellets. The proper amount of test inhibitor was added and the solution volume adjusted to a total of 5 mL with pure 90,' 10 methylene chloride-toluene. The final A1C13 catalyst concentration in the test solution was 0.01 mol/L. The tubes were placed through the water-cooled aluminumblock condenser into a water bath maintained a t a temperature of 43 "C. With no inhibitor, 90% methylene chloride and 10% toluene required between 30 min and 3 h to exhibit a visible reaction with aluminum. The results of these tests are reported in Table 11. In a second test with mixtures of 90% methylene chloride and 10% toluene, 50 mL of the solvent mixture was refluxed with 0.5 g of a Reynolds 30-XD leafing aluminum powder contained in a 125-mL Erlenmeyer flask attached to a water-cooled condenser. An interaction between solvent and aluminum powder was indicated by a dark red solution. All of the uninhibited samples reacted within 1-2 days, as summarized in Table 111.
Discussion of Results The inhibiting performance of 22 different compounds in four chlorinated solvents is presented in Table I. The inhibitors are ranked in order of increasing MAC values required to stabilize l,l,l-trichloroethane solvent in the presence of aluminum. It is apparent that a good l , l , l trichloroethane-aluminum reaction inhibitor is not necessarily the best choice for another chlorinated solvent. Greater concentrations of the better 1,1,1-trichloroethane inhibitors are required to prevent the 1,1,2-trichloroethane-aluminum reaction. Better inhibitors for the latter solvent include furfuryl alcohol, dimethyl oxalate, and 4-pyridinecarboxyaldehyde. 1,l-Dichloroethane is somewhat easier to stabilize by compounds like furfuryl alcohol, dimethyl oxalate, pyrazine, acetol, 4-pyridinecarboxyaldehyde, glycidol, or dioxane. The better inhibitors for methylene chloride-toluene contain an oxygen functional group, e.g., methoxy methyl acetate or dioxane. The compounds dimethoxymethane, 1,l-dimethoxyethane, and dimethyl carbonate have been patented for stabilizing the methylene chloridetoluene solvent system (Archer and Simpson, 1972). Inhibition Mechanism. The inhibitor, a Lewis base, probably functions by forming an insoluble complex with the metal chloride produced at initial microscopic reaction sites (Archer, 1978). This insoluble complex then affords 0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
132
Table I. -
Performance of Inhibitors in Four Chlorinated Solvents MACa values in mol/L inhibitor
-
CH,CCl,
furfuryl alcohol N,N'-dimethylpiperazine dimethyl oxalate 8-hydroxyquinoline 8-aminoquinoline pyrazine N, N, N ' , N ' -tetramethylethylenediamine adiponi trile acetol methoxymethyl acetate 4-pyridinecarboxyaldehyde 4-cyanopyridine p-cyanobenzaldehyde p-dimeth ylaminobenzaldehyde p-dithiane glycidol pyrrole 4,7-dihydro-l,3-dioxepin 4-ace tylpyridine dioxane propargyl alcohol nitromethane
a
0.009 0.013 0.02 0.03 0.03 0.03
CH,CI,
A1
80 50 90
toluene 10 20 50 10
90
10
* * * *
90
1. 2. 3. 4. 5. 6.
-
100 80 50
7. 8.
9.
20 50
-
100
AlC1,
*
* * *
0.08 0.08 0.13 ndb 0.10 0.08 ndb
0.03 0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04
0.20 0.02 nd
0.80 0.50 >1.0 0.08 0.40 nd >1.0
0.30 0.13
0.05 0.05 0.07 0.20
0.30
0.03
init. reacn time, min Xb
* * * * *
Xb Xb
30-180 YC 90 30
30 Xb
Note: Total test solvent was 5 m L with 0.5 g of aluminum pellets and 0.01 mol/L of AlC1,. X = no reaction before 72 h. Y = reaction after 9 6 h. Table 111. Reactive Systems Involving Methylene Chloride-Toluene Solutionsa and Reynolds 30-XD Leafing Aluminum Powder ~~
~
test no. 1.
2.
3. 4. 5. 6.b
~
A1 powder, g 0.5 0.5 0.5 0.5 0.5 0.5
~
~~
iron filings, g 0.1 0.25 0.5
-
AlCl,, mol/L
-
-
0.01
-
-
no. of runs 2 5 2 3 1 3
90%-10% CH,Cl,/toluene
0.01 >1.0 0.08 nd >1.0 0.30 >1.0
Table 11. Initial Reaction Times of Methylene ChlorideToluene Solutions with Aluminum Pelletsa solvent, vol %
ClCH,CHCl,
0.013 0.40 0.08 0.30 0.17 0.04 0.10
0.30 Higher MAC value indicates lower activity as an inhibitor.
test no.
CH,CHCl,
days t o react >1,0.30 0.05 0.02 0.70
0.13 0.02 0.40 0.005 0.04 0.17
nd = n o t determined.
in any particular solvent. Removal of the complex from a metal surface by slowly dissolving in the solvent would be undesirable. Aromatic Diluents. Methylene chloride is not normally very corrosive toward common metals. For example, the dry, uninhibited solvent produces less than 1 mil per year penetration in corrosion of aluminum, iron, or zinc coupons a t reflux temperatures (Archer and Simpson, 1977). Addition of excess water does not increase the corrosion potential as much as it does with many of the other chlorinated solvents. However, addition of an aromatic diluent such as toluene, as used in aerosol aluminum paint concentrates, may increase severely the corrosion potential of methylene chloride. Trace amounts of aluminum chloride or ferric chloride produced by the normal reaction with a metal container serve as initiating agents of a reaction between the aromatic diluent and methylene chloride. This Friedel-Crafts reaction produces extensive alkylation of the aromatic ring. The resulting hydrogen chloride further attacks the metal, leading to increased rates in the alkylation reaction. The reaction may approach violent proportions if the metal involved is an aluminum powder, such as that used in aluminum-paint aerosol concentrates. A methylene chloride-aluminum pellet mixture that does not react within 72 h becomes reactive within 1.5 h when 0.01 mol/L of aluminum chloride is added (cf. examples 9 vs. 6 in Table 11). The probable reaction between metal and methylene chloride is 6CH2C12 + 2A1- 3ClCH2CHZCl + 2AlC13 (1) The aluminum chloride does not react with the solvent, as it does with l,l,l-trichloroethane (i.e., dehydrohalogenation of solvent). Initial reaction sites are covered with a rather insoluble salt product that provides a physical barrier between the two reactants. Addition of aluminum chloride facilitates attack of the protective aluminum oxide coating, followed by an increasing solvent-metal reaction rate. Aluminum chloride also increases the reactivity of methylene chloride-toluene mixtures toward the aluminum pellets (cf. examples 1, 2, and 3 with 4, 7, and 8 in Table 11). In this case aluminum chloride catalyzes the Frie-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979 133
(1) Figure 1. Effect on inhibition effectiveness of mono- and difunctional oxygen, sulfur, and nitrogen in a heterocyclic ring.
del-Crafts reaction between methylene chloride and toluene. Dissolving of the initial salt product at the metal-solvent interface encourages further and faster metal attack. Addition of an organic diluent such as toluene, that is capable of complexing with the aluminum chloride, also increases metal attack by salt product removal. The aromatic diluent also reacts directly with methylene chloride to create new metal corrosion problems. Reference to Table I11 shows that the combination of 90% methylene chloride and 10% toluene diluent reacted with aluminum powder within 2 days (example 5), while refluxing aluminum powder with pure methylene chloride for 14 days failed to initiate a reaction (example 6). Although aluminum chloride was not necessary for a reaction to occur in less than 2 days (example 5), the addition of the Lewis acid in example 4 produced a shorter induction period (less than 1 day) in all three attempts. Increasing the amount of iron filings (cf. example 1 vs. 2) also shortened the induction time. Conclusions Inhibition of the aluminum-methylene chloride reaction appears to involve complexing of the chemisorbed aluminum chloride product with electronegative group(s) of the inhibitor. Inhibitor molecular size and electronegative nature determine the solubility of the resultant complex. Successful inhibition (i.e., formation of an effective physical barrier a t the reaction site) demands a complex that has only limited solubility in the solvent system. Loss of inhibitor complex from the metal surface by dissolving in the solvent promotes the accompanying Friedel-Crafts reaction. Sixty compounds were tested with the methylene chloride-toluene solvent mixture in the presence of aluminum pellets in order to determine the effect of various functional groups on inhibitor activity. The minimum inhibitor concentration (mol/L) necessary to inhibit the aluminum reaction for 24 h was taken as the minimum active concentration (MAC). The various compounds, ranked in order of increasing MAC values, are presented in Table IV. A lower MAC value denotes a more effective inhibitor. Heterocyclic Elements. Figure 1 illustrates the effect on inhibition effectiveness of oxygen, sulfur, or nitrogen in a heterocyclic ring, as well as the differences in activity
0.005
I
Figure 2. Relative inhibition activity of saturated compounds and their unsaturated analogues.
0.0°5
It
CH,CN
Figure 3. Inhibitor effectiveness of mono- and difunctional inhibitor structures.
between mono- and difunctional heteroatoms. Some difunctional inhibitors are more effective; e.g., p-dithiane is active a t 0.02 mol/L, while 0.07 mol/L of pentamethylene sulfide is required for the same activity. Dioxane is somewhat more active than tetrahydropyran, and pyrazine is less active than the mononitrogen analogue, pyridine. Comparison of difunctional heterocyclics shows oxygen more active than sulfur, which is, in turn, more active than nitrogen. Unsaturation generally decreases inhibition activity, as shown in Figure 2. Tetrahydrofuran is some ten times more active than the unsaturated analogue, furan, and somewhat more active than 2,5-dihydrofuran. A smaller difference is observed between the tetrahydrothiophene and thiophene derivatives, while the unsaturated nitrogen heterocyclic, pyridine, is more active than the saturated piperidine. The behavior of the nitrogen derivatives parallels the results with l,l,l-trichloroethane,where the unsaturated structure is always more active than the corresponding saturated structure. Aliphatic Inhibitors. The differences in activity of mono- and difunctional compounds containing carbonyl or nitrile groups is the reverse of the behavior seen in l,l,l-trichloroethane; see Figure 3. In methylene chlo-
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Ind. Eng. Chern. Prod. Res. Dev., Vol. 18, No. 2, 1979
Table IV. Inhibitors for Methylene Chloride-Toluene Solution Reaction with Aluminum Pellets
-
test compound
MAC
r
MAC,a mol/L
Aliphatic Oxygen Linkage Compounds dimethoxymethane 0.005 dimethyl carbonate 0.01 1,l-dimethoxyethane 0.017 1,l-dimethoxycyclohexane 0.02 methoxybenzene 0.03 trimethyl orthoformate 0.05 1,2-dimethoxyethane 0.06 methoxyacetone 0.06 p-dimethoxybenzene 0.06 methanol 0.50 0.80 2,2-dimethoxypropane Heterocyclic Oxygen Compounds 1,4-dioxane 0.005 te trahydropyran 0.008 tetrahydrofuran 0.013 2-methyltetrahydro furan 0.02 2,5-dihydrofuran 0.03 0.08 furfuryl alcohol furan 0.08 dioxolane 0.08 trioxane 0.20 2-methylfuran 0.30 glycidol 0.70 dihydropyran 0.80 Heterocyclic Nitrogen or Sulfur Compounds N-methylmorpholine 0.013 pyridine 0.02 p-dithiane 0.02 te trahydropyrrole 0.03 4-cyanopyridine 0.05 1,4-thioxane 0.06 pentamethylene sulfide 0.07 tetrahydrothiophene 0.07 pyrazine 0.08 N,N'-dimethylpiperazine 0.08 pi per id ine 0.08 8-aminopuinoline 0.10 pyrrole 0.13 thiophene 0.30 4-pyridinecarboxaldehyde 0.30 4-acetylpyridine 0.40 piperazine (reaction with solvent) >0.50 Carbonyl Group Compounds methyl acetate 0.013 propargyl formate 0.017 0.02 methoxymethyl acetate acetone 0.02 cyclohexanone 0.02 dimethylformamide 0.05 0.05 p-dimethylaminobenzaldeh yde acetophenone 0.06 2,4-pentanedione 0.07 dimethyl sulfoxide 0.08 dimethyl oxalate 0.13 acetol 0.13 1,4-cyanohexanedione 0.17 2,5-hexanedione 0.20 methyl pyruvate 0.20 p-cyanobenzaldehyde >0.30 Miscellaneous Compounds acetonitrile 0.008 n-propylamine 0.02 0.04 propargyl alcohol malononi trile 0.08 ni tromethane 0.17 (I MAC is minimum active concentration-the minimum concentration required to inhibit solvent-aluminum reaction.
y
3
CH,OCOCH,
I
0.1 -
0.05
-
I
0°'
t
t
at
0.8rnoler/E
CHI
CHIOCH,CH,OCH,
CHIO
HOCH, iH1
CH,OCH,OCH,
Figure 4. Effect of minor structural differences on inhibition activity.
ride-toluene mixtures the monofunctional cyclohexanone is about ten times more active than the 1,4-cyclohexanedione, while acetonitrile is likewise more active than the dinitrile. Acetone has an MAC value of 0.02 while a diketone such as 2,4-pentanedione has an activity of 0.07. In methylene chloride-toluene systems many monofunctional structures are preferred over the corresponding difunctional structure. Structures with aliphatic oxygen linkages constitute an interesting group of inhibitors. Dimethoxymethane, an acetal, is the most active, with an MAC value of 0.005 mol/L. Figure 4 depicts the drastic effect of minor structural differences on inhibition activity. Substitution of a methyl group for one methane hydrogen changes the activity of dimethoxymethane from 0.005 t o 0.017. Substitution of a second methyl group into the structure gives 2,2-dimethoxypropane, with an MAC value of 0.80. An additional carbon between ether linkages, i.e., 1,2dimethoxyethane, gives an activity of 0.06 mol/L-some ten times less active than dimethoxymethane. A group of compounds with aliphatic oxygen linkages, as well as related compounds, tested for inhibition activity, is listed in Table IV. Comparisons of the activities of inhibitors in both methylene chloride-toluene and l,l,l-trichloroethanepoint to a general observation-the better l,l,l-trichloroethane inhibitors (e.g., see Table I) are not the best structures for inhibiting methylene chloride. Strongly electronegative groups, particularly two such groups, do not normally afford the best inhibitor for the aluminum-methylene chloride-toluene system. Perhaps the resultant highly polar complex is sufficiently soluble to initiate a Friedel-Crafts reaction with its accompanying complications. The inhibiting ability of simple functional groups is ranked below (where the basic structure is methyl or a similar group) (best inhibitor) -0 CN > C(=O)-0 = 0-C(=O)-0 > C(=O) > NO2 > OH In summary, the following general observations apply to the effectiveness of methylene chloride-toluene solvent inhibitors: (1)0 > S > N; (2) RX > XRX (in many cases); (3) saturated structured > unsaturated structure (in many cases); and (4) an inhibitor of only average effectiveness in CH,CCl, will usually perform better in CH2C12. Selection of an inhibitor for a chlorinated solvent will involve factors other than just the inhibitor activity. Other
Ind. Eng.
important requirements include inhibitor availability, price, and boiling point. Adequate vapor phase protection and inhibitor distillation recovery is provided when the selected inhibitor and solvent boiling points are similar. The selected inhibitor must also not present any toxicity problems in the planned solvent use. The inhibitor must afford adequate inhibition a t low concentrations (
