Hydrogenated Monoterpenes Separation by Extractive Crystallization F. P. McCandless Department of Chemical Engineering, Montana State University, Bozeman, Mont. 59715
Hydrogenation of monoterpenes generally results in the formation of several compounds or isomers difficult to separate by conventional methods. Multistage extractive crystallization with thiourea was used as an effective method for separating the hydrogenation products of delta-3-carene, dipentene, and a-pinene. Data are presented on effects of feed composition, temperature, and thiourea to feed ratio on separation of 1,I ,4 trimethylcycloheptane from carane, cis-p-menthane from trans-p-menthane, and cis-pinane from trans-pinane. Results are analogous to distillation or liquid-liquid extraction with greatly increased “relative volatility” between components in the two phases in the adduct formation process.
T h e hydrogenation of monoterpenes generally results in the formation of several related compounds. When dipentene is hydrogenated, bot,h cis. and trans-p-methane are formed (Derfer, 1971), and a-pinene forms a mixture of cis- and trans-pinane (Cocker et al., 1966), although generally the cis isomer is favored. Cis-carane, 1,1,4-trimethylcycloheptaiie ( l , l , C T M C H ) , and 2,6-dimethyloctane (2,6-DNO) are formed when delta-3-carene is hydrogenated (Hannah, 1966; McFarliii, 1967). The closeness of the physical properties of these compouiids makes separation by dist’illation or other conventional methods difficult, if not impossible (Table I). If these materials are to be obtained in pure form, new methods must, be found for their separation. Extractive crystallization with thiourea is an effective method for making these separations. Extractive Crystallization
Extractive crystallization has developed into a powerful tool for the separation and purification of certain organic compounds. Formation of clathrates with Werner complexes is effective for the separation of aromatic isomers (Schaeffer e t al., 1957; deriadziteky and Hanotier, 1961). The formation of channel adducts with urea or thiourea permits the selective separation of certain classes of hydrocarbons from mixtures (Fetterly, 1964). The urea and thiourea adducts differ from clathrates in that guest hydrocarbons are trapped in twodimensional channels while the guests in the clathrates are enclosed by a three-dimensional cage structure. The term “channel adduct” was introduced to describe a particular type of molecular compound in which one component (the host) forms a channel structure in which another component (the guest) can be accommodated. The diameter of the channels formed places stringent structural limitations on the guest molecules which can be accommodated, and adducts will only form in the presence of suitable guests. Urea forms adducts with straight-chain paraffins, but thiourea forms adducts with branched-chain paraffins or naphthenic materials since the channel diameter is larger because of the bulky sulfur atoms in thiourea. Seleiiourea and tellurourea will also form adducts with certain hydrocarbons (Fetterly, 1950). This phenomenon has been used to separate broad classes of compounds; urea, commonly used to dewax petro406 Ind. Eng. Chem. Prod.
Res. Develop., Vol. 10, No.
4, 1971
leum fractions, is often overlooked in separating closely related isomers. Methods of Adduct Formation
The methods of adduct formation are well known and have been extensively described in the literature (Fetterly, 1964). Perhaps the most convenient laboratory method involves the contacting of the feed material with a near-saturated solution of thiourea in a suitable solvent and cooling the resulting solution. The needle-like adduct crystals are separated by filtration; a n y residual solution is washed from the crystals using a n inert solvent, and the adducted portion subsequently recovered. Other methods are possible. For example, if a solution of the react,ant in a neutral solvent contacts an excess of thiourea moistened with a thiourea solvent, adduct crystals will form on standing. Decomposition of the adduct and recovery of the thiourea and guest can also be effected in a number of ways. The simplest laboratory method involves dissolving the thiourea in water, thus freeing the guest which separates in a layer on top of the aqueous phase. The guest can also be removed by steam st,ripping, or the adduct can be dissolved in a solvent for both the guest and thiourea and the guest recovered by distillation. I n any case, the individual components can be recovered aiid recycled to a separation process. Experimental
The same preparation procedure applies to all adducts, but different feed compositions, complex to feed ratios and temperature, were used to determine the effect of these variables on the separation. A typical batch of adduct crystals was made as follows. A solution of thiourea was prepared by dissolving 12.5 grams of thiourea in 100 cc of methanol. Gentle heat was used to facilitate dissolution. Ten cc of the terpene feed mixture was added to this solution with &ring, aiid the resulting solut’ion placed on a cold plate and brought to the desired temperature while continuously stirring. Depending 011 the feed being studied, white needle-like crystals would form immediately on addition of t,he feed to the thiourea solution or a t various st,agesin t.he cooling process. The mixture was allowed to remain a t the desired temperature while stirring for about 8 hr. T h e adduct
Materials Table 1.
Structure and Normal Boiling Points !Pt C
Name
Reference
Carane
162
Hannah (1966)
1,1,4-Trimethylcycloheptane
160-1
Hannah (1966)
2,6 Dimethyloctane
158,5
Hannah (1966)
cis-p-Menthane
168.5
Weast (1966)
trans-p-Menthane
169-70
Weast (1966)
cis-Pinane
166-7
Weast (1966)
trans-Pinane
was separated from the nonadducted portion by filtration, and the adduct crystals were allowed to dry for approximately 8 hr before being decomposed by adding water. The two phases separated in a separatory funnel. The nonadducted hydrocarbon was also liberated from the filtrate by the addition of water followed by phase separation. Analysis of the terpenes was carried out by gas-liquid chromatography utilizing a 30-ft column of carbowax 20011 on Chromsorb P. The identity of the various compounds was confirmed by the determination of their ir spectra (Mitzner et al., 1965).
Table II.
The carane, 1,1,4 trimethylcycloheptane, and 2,6 dimethyloctane were produced in an earlier study by the hydrogenation of delta-3-carene in a continuous-flow reactor (McFarlin, 1967). The p-menthane mixture was obtained from a commercial source (Newport Division, Tenneco Chemicals). The cis- and trans-pinane were produced by the hydrogenation of a-pinane by the author in a rocking bomb reactor. All other chemicals were reagent grade (Fisher Scientific Co.). Results
Initial exploratory tests showed t h a t the components of the systems in question formed thiourea adducts with the enrichment of one or more of the components in the included hydrocarbon. A more detailed study followed to determine the effect of some of the variables on the enrichment and recovery of the individual compounds. Carane-l,l,4-Trimethylcycloheptane
Preliminary Data on Separation of Hydrogenation Products of Carane
Feed composition 2,6-DMO lJ1,4-TMCH cis-p-M enthane Carane T
~ OC
+12.0 -5.0 -20.0 t12.0 +12.0 f12.0
~ Thiourea, ~ , g feed, cc
1.25 1.25 1.25 0.60 1.25 2.38
Separation
Further tests were run on the carane product to establish the effect of temperature and thiourea to feed ratio on the separation of the components (Table 11).These runs, with a constant solvent to feed ratio of 10: 1, indicated that both the amount of adduct and the separation efficiency (as measured by the residue composition) were increased by increasing the thiourea to feed ratio and decreasing the temperature. The particular feed material for these tests contained 5.1y0 2,6-DMO and about 1% of another impurity, tentatively identified as cis-p-menthane, in addition to the carane and 1,1,4-TRICH. Both of these impurities were nearly completely separated from the carane (residue) in one extraction stage. The formation of 2,6-Dh10 can be eliminated by hydrogenation conditions (AIcFarlin, 1967) ; therefore, subsequent tests on the carane system were made with feeds containing only carane and 1,1,4-TAICH. Because of the preliminary runs, a temperature of - 20°C, a thiourea to feed ratio of 1.25, and a solvent t o feed ratio of 10 were chosen for the other separation tests. This solvent-tothiourea ratio results in a near-saturated solution a t the boiling point of the mixture, and the thiourea-to-feed ratio chosen represents a compromise between more adduct formation and decreasing selectivity. Later tests established the equilibrium distribution of carane and lJ1,4-TAICH in the adduct, residue hydrocarbons remaining in the liquid phase (Figure l),and percent recovery
Adduct, g thiourea, g
2,6-DMO
0.64 0.89 0.99 0.056 0.64 0.91
12.0 8.9 9.3 12.5 12.0 8.4
Percent 5.1 16.9 Tr 78.0
Adduct composition 1,1,4-TMCH p-Menthane
32.4 28.4 28.6 36.6 32.4 25.4
7.6 7.3 6.5 15.8 7.6 6.1
Carane
2,6-DMO
48.0 55.1 55.7 35.1 48.0 60.0
1.9 1.5 1.3 5.2 1.9 0.8
Residue composition 1,1,4-TMCH p-Menthane
10.7 6.1 5.1 16.7 10.7 6.0
Tr Tr Tr Tr Tr Tr
Carane
87.4 92.4 93.6 78.0 87.4 93.2
~~
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 4, 1971
407
g 80 n
9 z_
w 60
z
U I
t-
40
d
- THEORETICAL & 2.72 0
u, 0
EXPERIMENTAL DATA TRANS- PINAME
d 20
0 X lJ,4 TMCH IN RESIDUE
%CIS P-MENTHANE IN RESIDUE
Figure 1. Equilibrium distribution between adduct and residue for 1 ,1,4 trimethylcycloheptane and carane
Figure 3. Equilibrium distribution between adduct and residue for cis-p-menthane, trans-p-menthane, and franspinane, cis-pinane
of 1,1,4-TMCH in the adduct (Figure 2) by forming adducts
tion. With this value and the equilibrium relationship, the recovery of 1,1,4-TMCH in the adduct (defined as the percentage of lJ1,4-ThlCHin the feed included in the adduct) was calculated as a function of the feed composition and compared with the experimentally determined recovery (Figure 2 ) .
with various feed compositions. Feeds were made by mixing materials previously separated by several successive extractive crystallization steps. Equilibrium between the lJ1,4-TMCHin the adduct (solid phase) and that in the residue (liquid phase) closely resembles a n ideal two-component hydrocarbon-equilibrium diagram between liquid and vapor (Figure 1). Because of this resemblance, a separation factor or “relative volatility”
was defined and calculated based on the experimental points, giving a range of values between 8.0 and 8.4. With the average value of 8.16, the equilibrium curve was calculated and plotted as the solid line in Figure 1. Nearly a constant volume of hydrocarbons was included in the adduct (0.49 cc/cc feed), regardless of the feed composi-
L
cis- and trans-p-Menthane and cis- and trans-Pinane Separation
The same separation procedure proved successful with mixtures of cis- and trans-p-menthane (Figures 3 and 4). The same temperature and solvent and feed and thiourea ratios successful in separating the carane system were arbitrarily used. The cis isomer was enriched in the adduct; however, the separation factor between the cis- and trans-p-menthane mas considerably lower than between 1,1,4-TRlCH and carane. The experimental separation factor varied from 2.6-2.8, with the average being 2.72. I n addition to the p-menthane tests, several runs were made using a pinane feed. The trans isomer was the enriched
I
w
+* i
!401
;
20
d
0’
Q
h
I
20
’ 40
’ 60
’
80
I
‘
E 2ol d
100
%1,1,4 TMCH IN FEED
Figure 2. Recovery of 1,1,4 trimethylcycloheptane in adduct as function of feed composition 408 Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 4, 1971
THEO(IETICAL d.2.72 EXPERIMENTAL DATA
I %ClSP-MENTHANE IN F E E D
Figure 4. Recovery of cis-p-menthane in adduct as function of feed composition
Table 111.
Composition of Adducts for Various Systems
System
Thiourea/guest, mole ratio
p-Menthane l,lI4-TRICH, carane Piiiane
4 2 4.5 4 6
species in the adduct’,and the separation factor was about the same as that between the p-menthane isomers (Figure 3). Discussion
T h a t the experimentally determined equilibrium data show that the tendency for adduct formation is in the order, cis-pmenthane > trans-p-methane > 1,1,4-T?\ICH > caraiie > trans-pinane > cis-pinane, is further indicated by the adduct composition for the different systems (Table 111). The composit8ions(Table 111) which compare favorably wit’h those reported by other workers for cyclohexane derivatives (Fet,terly, 1964), together with a study of molecular models of t8hedifferent components, suggest that selectivity is in some way based on molecular size in the same order as the t’endency for adduct. formation. However, electronic configuration, van der Waal’s forces, or hydrogen bonding must also be important because p-cymene, which has about t’hesame molecular dimensions as p-menthane (hexahydro-p-cymene), does not form a n adduct with thiourea. Extensive research beyond the scope of this study would be required to define further the basis for selectivity. Nevertheless, practical use of t’his separation scheme should be possible. The selectivity in the adduct formation is astonishing when t,he requirements for separation of the components of the t’hree systems by distillation are considered. For example, no separation was obt,ained when the carane, 1,1,4-TIlICH mixture was distilled in a laboratory distillation column containing 40 theoretical plates a t a reflux ratio of 20: 1 (McFarlin, 1967). Alt,hough no tests were made on the p-menthane and pinaiie syst,ems, t,hese separations are probably as difficult by distillation. Hence, one extractive crystallization stage is probably equal t’o a distillation column with hundreds of theoret,ical plates. Extractive crystallization with urea has been successfully applied to petroleum separation problems where only one stage is required. For example, in a urea-dewaxing operat’ion, straight-chained paraffins are removed from branched-chain and naphthenic constituents. One stage is sufficient since the
other constituents do not form a n adduct with urea. However, in the systems studied in this research, both constituents form adducts with the thiourea, and separation depends on the tendency toward equilibrium in the two phases. Under these conditions, several extraction stages are required for nearly complete separation which could be accomplished by successive crystallizations. This scheme, used in the laboratory to separate a gallon of a 50y0 cis-trans-p-menthane mixture with 98y0 products, is extremely tedious and time consuming and is impractical except for exceptional cases. For this reason, a continuous multistage system would be necessary to separate even moderate quantities of the components. When the similarity of extractive crystallization to distillation is considered, it is easy to visualize a continuous multistage extraction unit which will be the subject of a future study. Acknowledgment
The author expresses his thanks to R. C. IlIcGihon, B. C. Sager, A. R. Fedoruk, and R. I. Orr, undergraduate students in Chemical Engineering a t Nontana State University, for substantial contributions to this research. Nomenclature CY
=
2 =
y
=
separation factor mole fraction in residue mole fraction in adduct
SUBSCRIPTS 1 2
= =
component enriched in adduct component enriched in residue
literature Cited
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dertadzitzky, P., Hanotier, J., Ind. Eng. Chem. Process Des. Develop., 1 , 10-14 (1961).
Derfer. J., Glidden-Durkee CorD., Jacksonville, Fla.. Drivate communication, 1971. Fetterly, L. C., ‘%on-Stoichiometric Compounds,” L. llandelcorn, Ed., Academic Press, ?Jew York, X.Y., 1964, Chap. 10, _
I
I
_
pp 491-567.
Fetterly, L. C., U. S. Patent 2,520,716, Aug. 29, 1950. Hannah, hl. A., PhD thesis. Montana State Universitv. Bozeman, RIont:, 1966.’ McFarlin, H. E., blS thesis, RIontana State University, Bozeman, hlont.. 1967. Mitzner,‘B. Rl., Theimer, E. T., Freeman, S. K., A p p l . Spectrosc., “
I
19 (60), 169-85 (1965).
Schaeffer, W. D., Dorsey, W. S., Skinner, D. A., Christian, C. E., J. Amer. Chem. SOC.,79, 5870-5 (1957). Weast, R. C., Ed., “Handbook of Chemistry and Physics,” Chemical Rubber Co., Cleveland, Ohio, 1966. RECEIVED for review April 16, 1971 ACCEPTED June 11, 1971
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