I
J. B. MONTGOMERY', A. N. HOFFMANN, A. L. GLASEBROOK, and J. 1. THIGPEN Hercules Powder
Co., Wilmington, Del.
Catalytic Perhydrogenation of Rosin Perhydrogenated rosin should be valuable in applications where oxidation resistance and absence of color are important
FOR
a number of years the Hercules Powder Co. has produced a highly hydrogenated rosin which is prepared a t high temperatures and pressures over nickel catalysts, This material, Staybelite, is much more resistant to oxidation than natural rosin and has found uses in many fields. However, it contains some residual carbon-to-carbon unsaturation. In fact, with ordinary hydrogenation techniques and catalysts it is impossible to saturate rosin completely without causing considerable decarboxylation. The reason that rosin is so difficult to perhydrogenate (perhydrogenation here means the removal of all olefinic and aromatic unsaturation) can be deduced from structural formulas of the principal components of wood rosin based on the work of Harris (Z), Lawrence (4) and their associates, and current research in this laboratory (7, 6). One of the double bonds of the acids containing conjugated unsaturation, such as abietic acid, and the exocyclic double bonds of dextropimaric and isodextropimaric acids are easily reduced. The endocyclic double bonds of dihydroabietic acid and the pimaric acids are more difficult to hydrogenate, but they can be reduced over a nickel catalyst under severe conditions. Dehydroabietic acid, which contains a highly hindered aromatic ring, cannot be reduced a t a practical rate over conventional catalysts. Wood rosin also contains about 11% neutral hydrocarbons and alcohols. These neutral bodies contain about 1
,Deceased.
13% of the total unsaturation of the rosin. About one fifth of the unsaturation in the neutrals is in the form of highly hindered aromatics, probably similar in structure to dehydroabietic acid. Thus, rosin contains about 13% highly hindered aromatic rings which are very difficult to hydrogenate b y conventional techniques. Hydrogenation is made more difficult by the disproportionation of the abietic-type acids to dehydroabietic acid, which accompanies any catalytic reaction of rosin. Drastic hydrogenation over a nickel catalyst results in a product containing about 150/0 highly hindered aromatic rings. The acid group in rosin is alpha to a quaternary carbon atom and, therefore, undergoes decarboxylation relatively easily. Decarboxylation occurs by both a
"i 5
E
!b
thermal and a catalytic mechanism. These reactions severely limit the temperatures to which rosin can be exposed and the types of materials which can be used for hydrogenation catalysts. Equipment and Procedure
All batch experiments were carried out in a stainless steel rocking autoclave fitted with a microvalve for sampling. The general procedure consisted of charging,the rosin and catalyst into the autoclave, sealing, flushing with nitrogen, and pressure testing with hydrogen. T h e heatup was carried out under hydrogen pressure; some of the easily hydrogenated components of rosin were reduced during this period. Samples of the rosin were taken during the reaction. The extent of hydrogenation was measured by ultraviolet absorption determination of per cent abietic and dehydroabietic acids. Amount of decarboxylation was measured by change in acid number. Catalysts
Figure 1.
Evaluation of noble metal catalysts for hydrogenation of wood rosin Pressure, 5000 Ib./sq. inch. Palladium-rosin, 1-1 000
Temperature, 240' C.
A satisfactory catalyst for the perhydrogenation of rosin must be extremely active, show its aGtivity under conditions which do not cause thermal degradation of rosin, and not itself catalyze the decarboxylation of rosin excessively. With these limitations in mind most conventional hydrogenation catalysts were evaluated. Only three catalysts, all noble metals, showed activity of the proper magnitude. The rates of hydrogenation of dehydroabietic acid with VOL. 50, NO. 3
MARCH 1958
3 13
Figure 2. Poisoning effect of metal ions on 5% palladiumon-charcoal catalyst
Figure 3. Poisoning effect of metal ions on 5% palladiumon-charcoal catalyst
Weight concentration basis. Palladium-rosin, 1-1 333 Temperature, 225' C. Pressure, 5000 Ib./sq. inch
Molar concentration basis. Palladium-rosin, 1-1 333 Temperature, 225' C. Pressure, 5000 Ib./sq. inch
five noble metals and nickel on equivalent supports are shown in Figure 1. The three noble metals of the sixth period of the periodic table are much more active than the metals of the eighth period. The three more active metals-rhodium, ruthenium, and palladium-are capable of completely saturating rosin. As ruthenium catalyzes the decarboxylation of rosin a t a somewhat greater rate than do the other two metals, it was concluded that rhodium and palladium are the two most desirable catalysts. The activity of the catalyst is affected by the support upon which it is carried and its state of subdivision. A noble metal supported on a powdered support was 10 to 100 times more active than the same metal supported on 20-mesh granules. This effect is undoubtedly a function of the surface area of the support. Catalyst life The high degree of catalyst activity required to saturate the rosin and the high cost of the noble metal catalyst dictated that the catalyst must retain a large proportion of its initial activity for a long time, Three factors overshad-
owed all othereffects in determiningcatalyst life : Recrystallization of the catalyst Permanent poisoning by contaminants introduced with the rosin or hydrogen Temporary poisoning by carbon oxides The recrystallization of metallic catalysts a t high temperatures is well known. I t is accompanied by either loss of catalytic activity or formation of dendritic growths, with subsequent mechanical degradation. Under the conditions in this work, about 260' C. under high hydrogen pressures, some recrystallization with loss of catalytic activity was noted. However, recrystallization was slow and did not limit catalyst life. The principal permanent poisons encountered in this process are: metals and high-boiling neutral materials in the rosin, metals from erosion and corrosion of equipment, sulfur and oil additives which enter the system with the hydrogen, and external contaminants which poison the catalyst before it enters the reactor. By careful handling and by precipitating the contaminants out of the hydrogen before use, the last two sources of poisoning were avoided. T o evaluate the poisoning effects of the metals which might enter the hydrogenation system with the rosin or be eroded from the equipment, the poisoning effect of a number of metal ions toward palladium catalysts was measured. The metal ions were introduced as the nitrates, because the nitrate ion was
inert toward this catalyst. Sulfur was introduced as hydrogen sulfide. I n Figure 2 the order of decreasing harmfulness of metal ions toward a palladium catalyst is shown. The ordinate, a, is the ratio of the reaction rate constant measured with a poisoned catalyst to the constant measured with an unpoisoned catalyst. I n all these cases the ratio of poison to palladium was 60 to 1,000,000 and the ratio of palladium to rosin was 750 to 1,000,000. Thus, a poison which has an a value of 0.5 will decrease the reaction rate by 50% when present to the extent of GO p.p.m. I n this work powdered 5% palladium-on-carbon catalyst was used and the reaction rate was measured a t 225" C. and 5000 pounds per square inch. In all cases the reaction rate was zero order in dehydroabietic acid concentration. These data show the extreme sensitivity of this catalyst to poisoning by metal ions and emphasize the necessity of protecting the catalyst from impurities. The relative effect of the poisons decreases from left to right in Figure 2. The order of relative poisoning is surprising. Aluminum, magnesium, and sodium are usually not considered toxic poisons. However, in this case, where extremely high catalyst activity is required, they are very objectionable. Figure 3 shows the effectiveness of these ions on a molar basis. The ordinate, p, is the ratio of the poisoned to the unpoisoned rate when 16.67 gram-moles of poisoning ion are present per 1,000,000
Figure 5. Effect of palladium - resin ratios on rate of hydrogenation of dehydroabietic acid in Resin 731 Figure 4. Effect of venting on hydrogenation rate of dehydroabietic acid in Resin 731
Temperature, 200" C. Pressure, 5000 Ib./sq. inch
Palladium-rosin, 1-1 333 Temperature, 200' C. Pressure, 5000 Ib./sq. inch
3 14
INDUSTRIAL A N D ENGINEERING CHEMISTRY
L Iy
p. w
TIME AT OPERATING CONDITIONS, HOURS
PERHYDROGENATION OF R O S I N
2S 0
0 OW5
0 0010
0 0015
'/c
1.0 I
o/-
I 0 0020
- ROSIL dlIi0 Figure 6. Effect of catalyst concentration on rate of hydrogenation of dehydroabietic acid Temperature, 200' C. Pressure, 5000 Ib./sq. inch Catalyst, 5% Pd on C antilog (1.75 X 108 catalyst concentration) 1.O
0
,o
0 3c
0 20
WEIGHT PER CENT P A L U D I U M BASED ON ROSIN
PALLWIUII
KO =
'
Figure 7. Effect of catalyst concentration on decarboxylation. Hydrogenation of N-Wood rosin at 260' C., 5000 Ib./sq. inch
-
6.2
grams of palladium. Here the order of poisoning is less surprising and it is obvious that the apparent poisoning effect of aluminum, magnesium, and sodium in Figure 2 is magnified by their low molecular weight. The data in Figure 3 were derived from Figure 2 by assuming Maxted's law (5)-that in this range the effectiveness of a catalyst poison is directly proportional to its concentcation. This assumption was valid in the case of mercury. Even relatively large deviations from Maxtcd's law would not change the order of poisoning shown here. Both very high boiling and very volatile neutral constituents of rosin, when added to dehydroabietic acid, decreased catalyst life. I t was concluded, that to obtain an economical catalyst life the rosin must be purified before use, access of external contaminants must be denied, and certain materials of construction avoided. The temporary poisons encountered in this work were carbon monoxide and carbon dioxide, both known to poison active catalysts. These oxides are formed during the reaction by decarboxylation of rosin. Any measures which decreased rosin decarboxylation invariably increased hydrogenation rate and, presumably, catalyst life. A study of the rate of decarboxylation of rosin showed that, for the noble metals, the efficacy of a metal as a hydrogenation catalyst is roughly inversely proportional to the amount of decarboxylation which it causes. This suggests that rhodium and palladium appear better than ruthenium as hydrogenation catalysts not because they inherently have more catalytic activity, but because they cause less decarboxylation of the rosin under reaction conditions and, therefore, "poison themselves" with carbon oxides less than other catalysts. These effects of the carbon oxides can best be avoided by venting the gas in the
reactor and replacing it with fresh hydrogen periodically. As shown in Figure 4, the rate of hydrogenation of Resin 731, a disproportionated rosin containing about 55% dehydroabietic acid, can be increased fourfold by periodic venting during reaction.
Process Variables
This study indicated that a satisfactory catalyst was available for commercial preparation of perhydrogenated rosin. Quantitative data on the effect of process variables on the reaction rate and product properties were then obtained, to design a continuous pilot plant. The only detectable reactions occurring under these conditions are hydrogenation and decarboxylation, and the important process variables are catalyst concentration, reaction temperature and pressure, and agitation. The effect of agitation has been discussed (3). This work was done in Aminco-type rockers agitated through a 45" angle a t 42 cycles per minute.
Because all other constituents of rosin are hydrogenated about 100 times as fast as dehydroabietic acid, the rate of perhydrogenation of rosin is determined exclusively by the rate of hydrogenation of this acid. Therefore, the rate of hydrogenation of dehydroabietic acid using Resin 731 was studied under a variety of conditions to determine the effect of catalyst concentration, pressure, and temperature upon the rates of hydrogenation and decarboxylation. Figure 5 shows that the rate of hydrogenation of dehydroabietic acid is zero order in acid concentration during most of the reaction under these conditions. The rate of decarboxylation was approximately first order in acid concentration. However, as a maximum of 8% of the rosin was decarboxylated in these studies, the exact kinetic order of decarboxylation had little effect upon the reaction rate. Figure 6 shows the effect of concentration of powdered 570 palladium-qn-carbon catalyst upon the rate of hydrogenation. The deviations from a first-order dependence of reaction rate upon catalyst
y
g
200-
1 55. 0 --
v \ 8
;
100 -
io; p i 0
L 4 0
-
5
L 243 00 $ 0 $ 2 0
3
--
\ \O
1 0 -
0
\
-
280°C
270-c
IbC"
L io0
250°C j
O I"P'C
4 0
2
dm
Figure 8. Rate of hydrogenation of dehydroabietic acid in Resin 7 3 1 '
Palladium-resin. 1-1333 Pressure, 5000 Ib./sq. inch VOL. 50, NO. 3
MARCH 1958
315
2000
PREESLRE
Figure 10. Effect of pressure on rate of hydrogenation of dehydroabietic acid in Resin 731 Palladium-resin. 1-1 333. Temperature, 200' C.
Concentration are probably caused by a combination of two factors. At very low catalyst concentrations the catalyst poisons (both metals and neutral) in the rosin have a disproportionately large effect upon reaction rate; a synthetic blend of the acidic constituents of rosin is hydrogenated much more rapidly at low catalyst concentration than natural wood rosin, but not if a large amount of catalyst is present. At high catalyst concentrations diffusion of hydrogen to the catalyst becomes the rate-controlling step, and addition of more catalyst has little effect upon the observed reaction rate. The empirical expression shown in Figure 6 expresses the dependence of hydrogenation rate upon catalyst concentration adequately for design purposes. The effect of catalyst concentrations on the rate of decarboxylation of rosin is shown in Figure 7. The solid line is the best mean-square fit of the data. If the experimental line is extrapolated to 0% catalyst (dotted line), the indicated decarboxylation rate coincides exactly with the rate of homogeneous decarboxylation of rosin at this temperature-l.O% per hour. I n the temperature range of 150' to 250" C. the rate of hydrogenation of dehydroabietic acid increases about 24% for each IO" C. increase in temperature (Figure 8). This corresponds to an apparent activation energy of 10.4 kcal. per mole. Each 10" C. increase in reaction temperature doubles the rate of the catalytic decarboxylation of rosin corresponding
Color (U.S. rosin grade)
A% dehydro -
(Antilog 1.75 X lo3 C.C - 1 ) (1) _A _acid _ _no. - (5.0 X 106 e -8,060 X hour T (1.0 2.1 C.C.) (2) where T = temperature, C. (423' to 563' K.)
0
-)
+
Stay belit eQ X
Hydrogen absorption, % 1.4-1 5 0.7-0.8 Min. 76 80-85 Softening point, C. 162 160-163 Acid No. 45-55 1-2 Abietic acid, % Dehydroabietic acid, 70 10-15 10-15 Oxygen absorption, % 9.53 0.2 Approximately 40% of total unsaturation removed by hydrogenation. O
3 16
INDUSTRIAL AND ENGINEERING CHEMISTRY
50
81,O
I&
P
hydrogen pressure, lb./sq. inch (200 to 8000) C.C. = ratio of Pd to rosin (0 to 0.002) =
The table below compares the physical properties of N-wood rosin, commercially hydrogenated rosin, and perhydrogenated rosin. Because it is colorless and extremely resistant to oxidation while still retaining the other physical properties of rosin, perhydrogenated rosin can be utilized in formulations where subsequent oxidation of the end product causes undesirable effects such as darkening, loss of tack, or odor. Evaluations have shown that the properties of perhydrogenated rosin are valuable in lacquer, adhesive, and chewing gum formulations. I n fact, an automobile painted with a lacquer utilizing a perhydrogenated rosin in its formulation has shown very good gloss and color retention after five years of weathering. However, perhydrogenated rosin is not yet available in commercial quantities.
Literature Cited (1) Enos, H. I., Jr., Skolnik, H., Divisions
hour
N
LB
Figure 11. Effect of pressure on rate of hydrogenation of dehydroabietic acid in Resin 731 Palladium-resin. 1-1 333. Temperature, 200' C.
to an apparent activation energy of 37 kcal. per mole (Figure 9). O n the basis of these data, 260' C. was chosen as the operating temperature. At this temperature the relatively rapid hydrogenation rate is accompanied by insufficient decarboxylation to cause concern. Figure 10 shows the actual rate curves for pressures from 200 to 8000 pounds per square inch at 260' C. The initial hydrogenation rate of dehydroabietic acid at pressures above 200 pounds per square inch is zero order in acid concentration. At pressures above 200 pounds per square inch the initial rate of hydrogenation increases in direct proportion to the hydrogen pressure-that is, the reaction is first order in hydrogen pressure within the limits shown here (Figure 11). As expected, the rate of decarboxylation is independent of hydrogen pressure. Equations 1 and 2 summarize in analytical form the effect of catalyst concentration and reaction temperature and pressure upon the rates of hydrogenation and decarboxylation of rosin.
K-Wood Rosin
coo0
1903
TIMZ AT O P E R A T I S C CONDITIONS, HOURS
Perhydrogenated Rosin Colorless 0.0-0.05
Min. 79 Min. 158 0.0
