COAL BYSolubilizing Effect of Hydrogenation upon Aromatic-Derived Resins1 WILLIAM Ha CARMODY Carmody Research Laboratories, Inc., Springfield, Ohio
Recent examination of indene-coumarone polymers revealed that the entry of hydrogen into the molecular structure takes place in a well defined orderly manner. Three definite zones or regions have been established, into which hydrogen enters in unvarying sequence. The first zone is limited in extent and is distinguished as being that structural region about which oxidation, fulvenation, and discoloration occur. The second zone comprises polymer structures deep within the molecule; i t consists of those carbon atoms and unsaturation which are common to both the aromatic and the cyclopentadiene ring. Hydrogenation in this zone causes little change in the resin properties. The third zone comprises all the remaining carbon atoms and double bonds; this zone contains those
structural portions most distant from the cyclopentadiene rings. Hydrogenation of this outer zone causes great change in all the physical and chemical properties of the polymers. Complete saturation extending throughout the third zone destroys all tendencies of the polymers to associate in solution. The polymers are rendered almost chemically inactive, and they become easily soluble in petroleum cuts over a wide range. Ordinary hydrogenated indene-coumarone resins are rapidly giving way to newer improvements. Selectively hydrogenated polymers and modified indene polymers will eventually displace the earlier haphazard type of hydrogenated products, mainly on the basis of greater economy and performance superiority.
H
YDROGENATION as a processing step has been applied to every field of organic chemistry in which it has offered the slightest promise of beneficial result. Its application to the field of synthetic plastics is mainly a matter of the last five or six years. I n the case of coal-derived plastics this is due mainly to the fact that relatively few companies are engaged in this field and that their endeavors are centered mainly on production problems. The application of fundamental studies t o the field has generally followed a course faintly blazed and incompletely pioneered by Staudinger from 1926 to 1929 (6). Careful study of the hydrogenation of indene-coumarone polymers has proved that it is not of academic interest only but is a commercial necessity in the face of competition by plastics of other origin. Crude solvent naphtha from either coke or gas manufacture can yield resins susceptible to the present selective hydrogenation process, and their treatment is a matter Of eventual necessity; Controlled hydrogenation methods have been applied t o a variety of hydrocarbon Polymers from several sources. Results in all cases have indicated that the results are of value. Recent articles bear on the theoretical background and commercial practice of indenecoumarone hydrogenation @,4, 6). These researches revealed that a number of simultaneous hydrogenating reactions were in progress a t one time. Each was theoretically distinct from the other and was independent of molecular size. Facts indicated that various structural portions of the polymers reacted so as to give the appearance of a group of structural components acting as a single COlkCtive unit. Such a theoretical analysis is shown in Figure 1 It is to be understood that they have been only Partially separated and identified, and a t the same time that no evidence to the contrary has been found. Formula D repre-
sents one of the intermediates. Saturation of the double bond is indicated; and a t the same time there is no change in the aromatic structure. Continued absorption of hydrogen gives the totally saturated polymer of formula E. Further analysis of the reactions has been brought about by improvement in methods of hydrogenation. Application of new catalytic bodies comprised of mixtures of various metals and metal oxides has been instrumental in completing the work. Used separately or in mixtures, they have proved to be of great value, and will permit hydrogenation to predetermined degrees and a t selected points in the polymer molecules. Thus this research originally directed toward remedying the discoloration and afteryellowing of indene resins, has resulted in a marked advance in our knowledge of the mechanism of hydrogenation of indene polymers.
of Hydrogenation of Indene Polymers The route by which hydrogen enters into indene resin is known in an exact manner. Commercial indene resins have been recognized as consisting of a family of mixed polymers of varying molecular magnitude. Resin of the grade studied initially, for which there is a growing demand, averages close t o 775-800. The degree of polymerization is close to seven; that is, there are Seven monomer units in the chain. The total amount of hydrogen consumed can readily be calculated and expressed in any convenient manner. For example, 1 mole of a commercial resin with a melting point of 150" C. will absorb approximately 21 to 22 moles of hydrogen. If we number each of these moles from 1 to 21, we can demonstrate that they are introduced in three well defined groups. One mole of hydrogen comprises the first group, there are seven in the second, and fourteen make up the third group. Also, it has been discovered that the various Portions of the indene polymers have different degrees of reactivity towards hydrogen. By carrying out hydrogenation
1Previous papers in this series appeared in volume 32, pages 525, 684, 771, and 954 (1940).
94
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1942 FORMULA
C
,
FORMULA
4
?
separate and distinct regions in a truly selective manner. These definitely recognized regions have been termed “zones of hydrogen entry” since there appears to be no simpler or more descriptive term. Into these zones hydrogen can be introduced in groups of one, seven, and fourteen moles, respectively. It is suspected that the latter value can be resolved into two minor groups of similar properties as the reactions are brought under greater control. Table I summarizes a number of representative experiments by which such relations were established. The hydrogenator charge consisted of resin, catalyst composition, petroleum solvent, and hydrogen. Loss of hydrogen pressure during the reaction was converted to volume of gas consumed, and when expressed on a molal basis, this was correlated with the weight of polymer used.
.?
E
75
?
FIGURE1. REACTION B ILLUSTRATES SATURATION OF DOUBLE BOND ONLY; REACTIONC ILLUSTRATES AROMATICRING SATURATION; REACTIONS B AND C TAKXU PLACE CONCURRENTLY WITH RANEY NICKEL
reactions at a reduced rate, i t is possible to demonstrate the influence and characteristics of various portions of the molecular structure. Previous articles have described that, when the active Raney nickel catalyst is used, three reactions take place simultaneously: (a) hydrogenation (bleaching) of the fulvene structure, (b) saturation of the nonaromatic bond, and (c) saturation of the aromatic double bonds. As the reaction proceeds, improvement in color is accompanied by increased ease of solution in petroleum solvents. The course of experiments is often erratic and highly suggestive that parallel paths are being followed. Despite such a confused state of affairs, the final products in all cases are identical if they have received their full quota of hydrogen. Much light was thrown on the problem by following the course of hydrogenation quantitatively to determine the points of attachment of such hydrogen. The goal was reached in a relatively short time, and what began on a laboratory scale was soon converted to a pilot-plant process of exceptional commercial merit. Correlation of hydrogen absorbed, under different conditions, with stoichiometric requirements of the various structural sections of the polymers led t o a complete verification of the reactions. Selective hydrogenation c,atalysts were developed which, when used, directed the hydrogen into three
TABLEI. SUMMARY OF EXPERIMENTS Expt. No. Blank 235 234 204 225 224 195 237 191 207 203 205 249 217 261 247
Pressure, Lb. Sq. In. Initial 0 810 820 1000 940 925 1130 1000 1190 1070 1135 1130 1185 960 1190 1220
kina1 0 800 765 935 860 800 975 830 1000 860 890 745 710 440 650 500
Soly. in Moles Ha Petroleum Ma% Used er Mole Cut Temp., C. of%esin (85-126’C.) 0 220 226 160 226 220 225 230 224 223 222 223 178 220 212 216
0 0.18 1.07 1.10 1.48 2.12 2.86 3.04 3.54 3.90 4.65 6.54 8.04 8.82 ‘2.20 12.20
*
54 48 46 46 46 44 45 43 44 44 44 41
13
- 3 --34 10
ZONE 2 FORMULA D
0
Hz
I FORMULA
F
FIGURXU 2. STRUCTURAL FORMULAS AND ZONES OF HYDROGXUN ENTRY INTO INDENE POLYMERS DURING SELECTIVE AND DIRECTIVE HYDROGENATION
Inspection of the last two columns reveals an interesting relation. Change in the solubility property has been directly related to the entering hydrogen to show sharp breaks in the solubility values. Numerous experiments demonstrated that the catalysts were selective in their attack on the polymers, and different degrees of hydrogen receptivity is an established fact. This conception of zones is new in indene-coumarone resin chemistry. The generalization has been established for the dimer and tetramer of indene and coumarone and for an impure specimen of indene octamer. On mixtures of polymers such as make up the various commercial grades of indene resins it can be demonstrated that their general behavior during hydrogenation conforms with the idea of zones. With indene monomer itself the principle can be demonstrated. Zone 1is readily distinguished, while zones 2 and 3 are also distinct. Figure 2 illustrates the structure of an indene polymer, the progressive manner, and the three stages of hydrogenation. The original polymer, hydrogenation intermediates, and final
76
INDUSTRIAL AND ENGINEERING CHEMISTRY
product are shown in proper sequence. Formula C is ordinary indene resin of poor solubility and poor color stability. The outer double bond of the five-carbon cyclopentadiene ring is the sole component of zone 1 and is that point about which all activity centers ( 3 ) . One mole of hydrogen is sufficient to saturate fully the responsive points in the zone. Formula D is a polyindene with unsaturation in each of the indene units in such a position as to be common to both the five- and six-membered rings. Carbon atoms and double bonds in this relative position throughout all the units in the polymer are said t o comprise and to be in zone 2. Selective saturation of these zone 2 positions give rise to formula F, which is a newly discovered and isolated intermediate. Zone 3 is illustrated by the heavy bonds in formula F. Complete saturation of each of the units leads to formula E , which is the end of the hydrogenation series. These reaction paths were theoretically developed, but it remained for selective catalysts to demonstrate their actual existsnce. Hydropolymers corresponding to formulas D and F have been isolated. Hitherto only polymers shown in formulas C and E were known. The progressive and stepwise hydrogenation of indene polymers bears out the belief that indene is a true derivative of cyclopentadiene, and that similarity on paper to benzene is not to be too seriously considered. The paths by which hydrogenation is effected clearly demonstrate that the entry is accomplished by beginning in the five-membered ring, and that it sweeps entirely across it before progressing to other locations in the structure.
I‘
I,
ZONE IZONE ZONE ‘9
I
I 2
FIGURE 3. ZONESEXISTING IN INDENE POLYMERS, THE MAIN PROPERTY ATTRIBUTEDTO EACHZONE, AND THE GROUP OF ATOMSIX THE ZONEWHENADDITIONAL HYDROGEN Is ABSORBED
Figure 3 is a simplified structure of indene polymers. The dimeric form is shown; larger polymers have an extended linear dimension and fit within the pattern. Coumarone polymers are of comparative structure and are presumed to behave in a limited and similar manner. Zones are numbered from right t o left, in the order in which hydrogen enters the structures. Actual number of moles entering the respective zones is expressed in terms of degree of polymerization as follows: Zone 1 can absorb only one mole of hydrogen, zone 2 can absorb X moles of hydrogen, and zone 3 can absorb 22 moles of hydrogen. Zone 1 contains only one point of attack, since the corresponding locations in all other units have been saturated by
- 60
Vol. 34, No. 1
\ D
FIGERE 4. INFLUENCE OF INTRODUCED HYDROGE?; ox PRECIPITATIVX TEMPERATURE
polymer formation. This is true regardless of molecular size. To produce color-stable resin it is necessary t o satu. ate zone 1 completely to the maximum extent of only 1 mole. Polymers hydrogenated in zone 1 to an extent less than 100 per cent will undergo the fulvene reaction in a manner proportional t o the deficiency of hydrogenation. Resins made by the technique described earlier have been found t o be treated not in excess of 50 per cent in zone 1, and to be saturated nearly 95-98 per cent in the other zones combined (4). Zone 2 embraces all those carbon atoms which are common to both the five- and six-membered rings, and between which double bonds exist. The quantity of hydrogen entering this zone is strictly proportional to the molecular weights of the polymers involved, since each unit can absorb one mole of hydrogen. No very great change in either physical or chemical properties follows introduction of hydrogen into this zone. The polymer has the capacity to absorb hydrogen in this zone and to resist change in properties. This resistance is termed a “buffering effect”. The hydrogen atoms are introduced into zone 2 and deeply buried in the molecule, and are of almost negligible influence in their effect on melting point, solubility of the resin, and viscosity of solutions, etc. This is explainable on the probability that the properties of complex bodies are to a large extent determined by the nature of the terminal groups, or those groups likely t o be in close proximity to similar groups in adjacent resin molecules. Zone 3 is made up of four carbon atoms in the aromatic nucleus of each of the original indene units. These are most distant from the cyclopentadiene ring and can be considered as comprising an intermittent or interrupted chain of similarly situated carbon atoms. Profound change in all the physical and chemical properties results from saturation of this zone. The mist striking change is in solubility in petroleum solvents. The property of association has more or less been attributed to the aromatic ring, and is due to its peculiar type of unsaturation. Association tendencies are destroyed, and hence solubility in all solvents is greatly improved. Such solubility improvement continues with introduction of hydrogen until its full quota of hydrogen has been absorbed. Figure 3 illustrates the structural configurations and their main characteristics as derived from present hydrogenation
INDUSTRIAL A N b E N G I N E E R I N G CHEMISTRY
January, 1942
APPLICABLETO STYRENE POLYMERS TABLE 11. RESULTS Styrene Polymer Derivative Original Dihydro D i n-hydro Di + 3n-hydro
+
Carbon,
Hydrogen i n Polymer, % '
% '
92.31 92.31
7.69 7.69 11,10+ 12.77+
+
-
58.90-
87.23 -
Solubility Characteristio Insol. i n boiling petroleum ether Insol. in boiling petroleum ether Limited soly. in petroleum ether Extremely sol. in petroleum ether
TABLE 111. RESULTS APPLICABLETO INDENE POLYMERS Extent of Hydrogenation
Carbon, Hydrogen,
%
%
Original indene resin
93.10
6.90
1 mole H2, in terminal unit in zone 1
92.87
7.13
7 moles Hn, into zone 2
91.29
8.71
14 moles Hz, into zone 3
58.29
11.71
Important Characteristics of End-Product Resin Yellows badly, has poor soly. in paraffins Yellowing reduced near1 to 0 ; sol improved b y agout 5-8' Yellowing eliminated soly. improved b y about &other
8:
- - r. -. .LAO
Yellowing eliminated aoly. improved so that n; pptn. occurs a t -65O C.
studies. Regions of polymers responsible for fulvenation, buffering, and solubilizing are indicated by arrows. The designation of zones is in keeping with the order of hydrogenation. The experimental conditions under which hydrogen enters the various zones is a measure of the stability of the regions involved. Reactions centering in zone 1-for example, fulvenationhave been known for years but are without exact explanation (3). These are mainly brought about by transient oxidation. The solitary double bond is selectively and completely saturated a t room temperature and with application of low hydrogen pressure. Zone 2 can be saturated by a combination of catalysts comprising Raney nickel and a variety of oxides. Zone 3 is difficult to saturate and requires nickel alone, high pressures, and temperatures up to 225' C. With the newer catalyst combinations it is feasible to saturate zone 1 completely, and the remaining zones may be saturated to any desired extent. It is obvious that hydrogen consumed in zone 2 is a total waste, as far as immediate benefit is concerned. Methods are being developed which will totally saturate zone 1, skip zone 2, and may be applied to zone 3 to any desired degree. Experimental evidence indicates that the zones react in their respective numerical orders. This definiteness and independence of zones is brought out in Figure 4. I n following the course of controlled hydrogen entry, the gradual solubility improvement can be used as reference point. Resin from a series of tests (Table I), dissolved in Stoddard solvent, precipitates a t specific temperatures. Plotting these precipitation values against moles of consumed hydrogen brings out exact relations which coincide with the expected results. Saturation of zone 1 causes only a slight drop in solubility value from the original value of 55' C. Introduction of the next 6.7 moles of hydrogen (corresponding to the average value of X ) causes little change in solubility. Introduction of the next 13.4 moles brings enormous change in the values. The precipitation value drops to about -65' C., where the polymer no longer separates, but the entire mass of solution solidifies or gels and the test is no longer applicable. Figure 4 shows that a t the point of addition of 18 moles of hydrogen the temperature of precipitation was still decreasing rapidly. The continued change in slope of curve caused by 22 moles of hydrogen suggests that it is the result of two overlapping reactions which are closely associated in nature. The ability to introduce hydrogen selectively and completely into zone 1 leads to interesting possibilities from an analytical viewpoint. Polymers can be quantitatively examined by this method, and the average molecular weight can
77
be estimated. Likewise if the molecular weight be determined with certainty, we can demonstrate by selective methods that only one double bond is present. Determination of unsaturation by means of halogen leads to values which fluctuate widely on either side of one double bond. The development of selectivity by choice of catalysts reduces operating difficulties and leads to economies of highest commercial value. Resins can now be hydrogenated to meet specifications covering color stability and precipitation value. Color-stable resins with a solubility of 0' C. can be produced with a saving in excess of 60 per cent hydrogen. Eight moles of hydrogen will give such a value. With nickel alone, as by early technique, and random introduction of hydrogen to the extent of 40 per cent, the yellowing tendency is but 20-25 per cent eliminated. Laboratory tests with the new catalysts have indicated that zone 1 may be completely saturated at only a fraction of the cost encountered with the use of Raney nickel or any other type of metallic catalyst. ,Figure 4 indicates that resins may be hydrogenated in such manner as to cause the finished product to fall anywhere on the experimentally derived curve. Resin between A and B will still show afteryellowing, and be of.poor solubility. Resins between B and C have practically no tendency to afteryellow and have the approximate initial Door Solubility. Resins following C and extending towards D (complete hydrogenation) do not afteryellow and have greatly improved solubility. It is obvious that resins from A to B and from C to D are of importance. Experimentally, when zone 1 is saturated, a repeat treatment on the same material shows p r a c t i c a l l y zero hydrogen I' consumption, whereas on the first time through, exactly one mole of hydrogen is consumed. This is in distinct contrast to Raney nickel. When a quantity of hydrogen is consumed which corresponds to one mole of hydrogen, this same quantity can be repeatedly introduced until the total amount approximates the theoretical value for FIGURE 5. ZONESOF HYI)ROGEN h9 the material examined. No THEYEXISTIN STYRENE selected control of hydrogenaPOLYMERS tion can be enforced upon indene polymers where metallic nickel is used as catalyst. The present selective catalysts that have been reported and the conception of zones of hydrogen entry are not limited to indene polymers. They can be applied to styrene, indene, coumarone, to a limited degree to cyclopentadiene, and to certain naphthalene derivatives. The conformity of styrene polymers is graphically shown in Figure 5. A summary of results as applied to styrene and polymers is shown in Tables I1 and 111.
'
Nature and Preparation of Catalysts The application of oxide catalysts to a number of hydrogenations was suggested by the works of Ipatiev as early as 1910. More recently Adkins (1) and Calingaert and Edgar (9) reported upon the application of oxide catalysts to a wide variety of organic materials. The application of such mate-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
78
rials to polymers seems to have been overlooked in commercial work. The reaction between carbon monoxide and hydrogen in the production of industrial solvents catalyzed by mixed oxides has been extensively reported. Such catalysts have been used mainly with reactants in the gas phase and a t high temperatures and pressures. The synthetic materials resulting are of relatively simple composition. The first application of oxide catalysts to polymeric materials were highly successful since these resins are relatively easy to activate a t the double bond and are responsive t o activated hydrogen. The additional fact that mixed oxides are sometimes dehydrogenation catalysts made them exceptionally useful in the present application since one must operate at temperatures not greatly in excess of 200" C. to avoid appreciable depolymerization. Various oxides, such as copper oxide, iron and nickel oxides, and complex combinations thereof with chromium and magnesium oxides as promoters, serve in the present technique as applied to resins. Catalysts containing any of the oxide materials are effective. The degree of effectiveness will range from a trivial introduction of hydrogen up to that required for the introduction of exactly one mole of hydrogen.
Choice of Solvent in the Hydrogenation Indene resin is soluble with difficulty in light petroleum benzine and forms stable solutions only when heated above 50" C. The exact temperature required to obtain solution is determined to some extent by the concentration of the resin solution being made. During hydrogenation the solubility becomes progressively better, and the viscosity of the solution decreases. Any number of solvents might be used for the purpose. For example, cycloparaffins, aliphatic hydrocarbons, esters, ethers, and aromatic acid esters have been examined. Satisfactory results are obtained in selective hydrogenation, and none of them interfere with the theoretical
5. 2
TABLE IV. Expt. No.
59-A 59 167 64
Hydrogenator Charge Catalyst alone, no organic material Catalyst with petroleum benzine Catalyst with bqnzene Pure indene with aromatic solvent
115
Initial Hg Pressure, Lb. 1185
Hydro en UseJ, Go. Nil
100
1185
Nil
100 100
1000 1433
Nil 11,700
The most difficult test of selective hydrogenation is exemplified in experiment 64 where it is shown that a clean-cut performance of catalyst was carried out. Substitution of indene polymers of all magnitudes in place of monomeric indene leads t o similar selective results. The hydrogen consumed corresponded to that required for indene. Under similar experimental conditions employing Raney nickel, an amount of hydrogen approaching a sevenfold volume would have been consumed.
Constancy of Hydrogen Consumed under Different Pressures It is known that the use of Raney nickel enables increasing amounts of hydrogen to be introduced into indene polymers by continued increase of hydrogen pressure, or by the maintenance of an initial high pressure throughout the reaction. Metal oxide catalysts act in such a manner as to reach their peak a t low temperatures and do not exceed this theoretical value (Figure 6).
m
1000
1400
IN.
FIGCRE 6. ILLUSTRATION THATA RELATIVELY Low PRESSURE INTRODUCES THE THEORETICAL QUANTITY OF HYDROGEN, AND THAT FURTHER INCREASP~ IN
PRESSURE DOES NOT INTRODUCE^ ADDITIONAL HYDROGEN
goal of about 5.2 per cent hydrogen consumption being obtained. Economy directs the use of low-cost paraffin, and to obtain this end, the resin, solvent, and catalysts are charged into the pressure vessel. Slow warming completes solution; hydrogenation is benun with agitation and the amlication of pressure, and is continued until absorption is complete.
-
SELECTIVITY OF OXIDE CATALYSTB Temp., C.
5
HYOROGEN P R E S S U R E , LB./SQ.
Selectivity of Catalysts (Nonhydrogenation of Aromatic Rings) Numerous experiments indicate that the described catalysts do not readily attack unsaturation residing within aromatic rings. The more useful catalysts limit their activity to unsaturation of the alkene type, described in detail by Adkins and demonstrated by numerous examples. However, it has been found that catalysts of the mixed oxide type will attack unsaturation existing as part of certain ring structures, in addition to any unsaturation existing outside of such rings. A number of laboratory tests demonstrate the absolute selectivity shown by oxide catalysts. Table IV summarizes such results.
Vol. 34, No. 1
v
I L
Application of Selective Process Hydrogen Required, Go. Nil
Selective oxide catalysts can be used in several ways with a number of commercial resins, depending on the extent of treatment reNil quired; oxide catalysis can be carried out as a separate step before a follow-up treatment Nil 11,760 with Raney nickel. It can be used simultaneously with nickel or as a separate follow-up step by itself. Oxide catalyst operates in low temperature ranges and will complete its role a t values as low as 75" C. Nickel catalvsts are most active on plastics of the indene type a t temperatures much higher, and hence a wide variety of operating conditions are made possible by combinations of these different temperature factors. When both metal oxide catalysts and nickel are used in the same charge, each catalyst will function independently. In such a manner full advantage of the new technique can be employed with absolutely no change in customary operating technique or plant equipment. All that is required is to charge in the proper amount of catalyst; it is recovered simultaneously with the Raney metal.
Literature Cited. (1) (2) (3) (4) (6) (6)
Adkins, "Reaotionu of H y d r o g e n " , 1937. Calingaert and E d g a r , IND. ENG.CHIOM., 26, 878 (1934). C a r m o d y , W. H., Ibid., 32, 525 (1940).
Ibid., 32, 684 (1940).
Ibid., 32, 771 (1940).
H., Swiss p a t e n t 121,817 (1926); Helv. Chim. Acta, 12, 962 (1929).
Staudinger,
