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Ind. Eng. Chem. Prod. Res. Dev. 1004, 23, 449-452
Oligomerization vs. Methylation of Propene in the Conversion of Dimethyl Ether (or Methanol) to Hydrocarbons Rafael L. Espinoza Chemical Engineering Research Group-CSIR,
Pretoria 000 1, Republic of South Afrlca
Propene is the most reactive primary olefinic product formed when methanol or dimethyl ether (DME) reacts over an acid silica-alumina catalyst. Further conversion of propene occurs by methylation and oligomerization. Experiments with mixed DME-propene feeds at 1.7 bar, 150 to 380 O C , and mass hourly space velocities (MHSV) of 3.9 to 4.1demonstrate that for low to medium feed concentrations of DME (20to 80%) oligomerization is the dominant growth mechanism, in contrast with published reaction schemes. The oligomerization/methylation ratio can be related directly to the DME concentration.
Introduction For the conversion of methanol or dimethyl ether to hydrocarbons over zeolitic or amorphous silica-alumina catalysts it is now widely accepted that ethene and propene are the initial hydrocarbon products formed through parallel routes (van den Berg et al., 1980) by an intermolecular process (Perot et al., 1982);asymmetric ethers may be intermediates (Cormerais et al., 1980; Perot et al., 1982). The conversion then continues relatively fast by methylation of olefins (ethene being relatively unreactive) (Anderson et al., 1980; Espinoza et al., 1983; Dessau and LaPierre, 1982), paraffins (Chang and Chu, 1982), and aromatics (Dessau and LaPierre, 1982), oligomerization of olefins, cracking of particularly the larger hydrocarbons, and isomerization. There is disagreement about the relative rates of the reactions of the olefins in which the carbon chain is increased (methylation or oligomerization). Kaeding and Butter (1980) suggested that the presence of methanol, dimethyl ether, and water in the system suppresses the oligomerization markedly and various proposed reaction schemes have omitted oligomerization (Dessau and LaPierre, 1982). Others (Espinoza et d.,1983; Derouane et al., 1978) are of the opinion that methylation and oligomerization of the olefins are competitive, and that a fraction of the C4 hydrocarbons could be the result of secondary reactions of propene (Cormerais et al., 1980). The scope of the present work is to determine the relative contribution of methylation and oligomerization by deriving an approximate kinetic model meeting the requirements of the reactor design engineer. Some experiments with light olefin feeds were carried out to obtain the required data.
1.7 bar, an MHSV between 3.8 and 5.2, and temperatures in the range 160 to 390 “C. Results and Discussion The mechanism for the conversion of dimethyl ether (or methanol) to hydrocarbons could be represented in a simplified manner as 2CH3OH F? CH3OCH3 H2O
+
m{CH,=CH, nCH,OCH,
‘‘**e
\ V3(n - m){CH,=CHCH, + 3/,H,O}
Since ethylene is relatively unreactive compared with propylene, further growth of the hydrocarbon molecule proceeds mainly via propene by methylation: 2CH2=CHCH3 + CH30CH3-+ 2C4H8 2H20 oligomerization: 2CH2=CHCH,
-
+
C6H12
The larger hydrocarbons are subjected to isomerization, cracking, etc. To interpret the results of experiments with DMEcontaining feeds, it is then necessary to have an estimate of the amounts of propene and butenes formed by the cracking of the larger hydrocarbons. Propene and 1-Butene Feeds. The “scrambling” of hydrocarbon products-which takes place on acid catalysts in the presence of olefins is not usually highly selective, (except for branchiness constraints in small pores). Therefore, it was assumed that, irrespective of what olefin was fed, a similar product spectrum would arise provided the percentage conversion was substantial. For the range of experimental conditions used, the rates of formation of butene or propene (Figure 1) in the respective series of experiments were
Experimental Section The experimental equipment, analytical procedure, and the amorphous silica-alumina catalyst prepared in the presence of tetrapropylammonium ions have been described previously (Espinoza et al., 1983). The gas feeds were introduced to the reactor through Brooks mass flow controllers. The dimethyl ether (DME), propene, and 1-butene obtained from Matheson had purities of 99.87, 99.7, and 99.9%, respectively. Mixtures of DME and propene with 18% to 90% by mass of DME were used as feeds and the operating conditions were 1.7 bar, a mass hourly space velocity (MHSV) between 3.9 and 4.1, and temperatures ranging from 150 to 380 O C . Alternatively, propene or 1-butene were fed at O196-4327/84/ 1223-O449$01.50/0
+ H,O}
rc3 = 103e-4620/TrHC
(2)
where rc, and rcgare the rates of formation of butenes or propene, respectively, expressed in gg-’ catah-l, T is the reaction temperature in K, and rHC is the rate in gg-‘ catah-l at which hydrocarbons, other than the feed, report in the product. The correction required to represent the fact that, with an olefin feed, not all of that olefin in the product is unreacted feed but also cracking product, was 0
1984 American Chemical Society
450
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984
-* r
-30
\
-- 54 0 1
14
I5
17
16
I$
T
:Temperature
18
19
-30
:N
>
1
I5
16
17
I8
19
20
21
22
23
10' T-I
in K
Figure 1. Propene fraction from the cracking of C4"oligomers. -2 0
-90
T-' T
i
Tempeiotyre
in K
Figure 3. Methylation of propene (Ci in gg-' catvh-').
I
s 0 0 0
"O
t
a
I
a
-50
I
--76 ~o~ 0
15
,., 16
:
170 e - 4 6 9 5 ' 7 17
18
I
ccj
1JT 19- I
20
21
22
23
I
0 T -Temperature
in K
Figure 2. Oligomerization of propene (CQ- in gg-' catch-I).
included in the numerical constants of eq 1 and 2 after an iteration procedure involving the two sets of data. Mixed Propene and DME Feeds. In these experiments in which three major reactions were taking place (dehydration of DME, oligomerization of propene, and methylation of propene) the conversion of DME to hydrocarbons was less than the conversion by each of the other major reactions. Consequently, it was taken that all the C4 hydrocarbons observed in the product came either from methylation of propene or cracking of higher hydrocarbons; this was estimated to be valid for about 95% of the C4 hydrocarbons. The mass ratio of propene to butenes present in the product was between 5 and 100 for the entire range of experimental conditions; the conversion of butenes by oligomerization or methylation was neglected in comparison with the same processes with propene. There is no reason to believe that these processes are much faster for butene than for propene. The errors introduced by the approximations mentioned above counteract each other; the approximations are considered acceptable for the purpose of estimating the relative contributions of propene methylation and oligomerization. A step by step example of the calculation procedure is given in the Appendix. The oligomerization of propene was modeled as a bimolecular reaction and on this basis the experimental data on the mixed feed could be represented (Figure 2) by rol = 170e-4695ITCc32 (3) The rate equation arrived a t for methylation has no theoretical basis, but it fits the experimental data (Figure 3) as well as other empirical equations which were tried. It was selected because it simplifies the comparison with oligomerization r,, = 20.4e-4592/TC C32CDME (4)
loo
50
Mass % of DME
in the
feed
Figure 4. Relative rates of methylation and oligomerization as a function of feed composition.
Noting that the temperature response in eq 3 and 4 is virtually the same, and using the average temperature of 547 K, we may express the ratio of rmeand rol r ~ e / r o l = 0.15CDME (5) For the upper and lower part of the temperature range used this is considered a good approximation. Since the feed consists of mixtures of propene and DME CDME = 5% DME in feed x MHSV (6) Hence rme/rol= 0.15 X MHSV X 5% DME in feed (7) Equation 7 and the experimental data are shown in Figure 4. This figure demonstrates that the simple model proposed to predict the ratio of the oligomerization and methylation rates is acceptable. Oligomerization is clearly the dominant reaction for the concentration range of our experiments. In fixed bed tubular reactors, the concentration of reactants decreases with increasing bed length, while the concentration of the products increases. In the conversion of methanol-DME to hydrocarbons, the concentration of DME decreases with bed length. Consequently one may expect that methylation is an important hydrocarbon chain growth reaction in the first part of the catalyst bed, but as conversion progresses the oligomerization reaction takes over. If conversion is taken almost to completion, the oligomerization will be the predominant growth reaction over most of the catalyst bed. A discussion of which processing scheme is adequate for the maximization of oligomerization or methylation products is outside the scope of this paper. Obviously the type of reactor and how it is used will have a direct influence on the products. For example, a low DME conversion per pass with complete recycle of unreacted DME would favor methylation products, whereas the staged
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 451
Dimerization
Ether intermediates
ti
Methanol
Figure 5. Proposed scheme for the conversion of methanol to hydrocarbons. Table I. Gas Chromatograph Data compound
mass %
DME
20.56 0.65 0.19 69.04 0.91 8.64
methanol water propene C,' s other hydrocarbons
addition of DME to a set of catalyst beds in series would favor the oligomerization reaction. In the light of the above discussion, and from the results of previous work (Espinoza et al., 1983) a scheme is proposed for the conversion of methanol to hydrocarbons,as shown in Figure 5. In this scheme, the main primary hydrocarbon products, ethene and propene, are formed by parallel routes from an ether intermediate (van den Berg et al., 1980; Cormerais et al., 1980). Once formed, ethene is relatively unreactive (Cormerais et al., 1980; Anderson et al., 1980; Dessau and LaPierre, 1982) and its contribution to the further growth is small. The growth of the hydrocarbon chain is mainly by a C3+ step. The contribution of cracking to the product spectra is very important (Espinoza et al., 1983) and is shown in the bottom line of the figure. The isomerization and dehydrocyclization reactions are indicated by means of a circled arrow. Appendix The following example illustrates the calculation procedure, as well as the assumptions made, to find for each run: (i) the amount of propene and C4hydrocarbons from cracking, (ii) the conversion of DME to hydrocarbons, (iii) the amount of hydrocarbon products formed via oligomerization of propene, and (iv) the amount of hydrocarbon products formed via methylation of propene. (See Table
I.) Calculations. (a) Apparent Amount of HC Products. 1
CHCNP = 100 X (%C4 + 9O other HC) X MHSV
CHCMP= 0.37 g-g-' catmh-' (b) "True"Amount of HC Products. Some of the propene present in the exit stream is "second generation" propene, or propene from cracking of higher hydrocarbons. This propene should count as product for our purposes. Therefore CHC = C H C H p + C3= from cracking CHC =
CHC,mp X
+ 102.6e-4620/T)
(1
= 0.376 gg-' cabh-'
(c) C4Hydrocarbons from Methylation. C4hydrocarbons are assumed to come from either cracking or methylation reactions. Then C4,MET = C 4 , m T U - C4 from cracking =
100
X (mass
90C4 X MHSV) - (CHCX 31e-3380/T)
C~,MET = 0.016 eg-' cat-h-' (d) Water Mass Balance. Water consumed by the formation of methanol CH30CH3 H2O e 2CH3OH 46 18 2 X 32 Amount of methanol formed MHSV mass % methanol on exit stream X -100 0.025 g-g-' cat-h-' Water consumed by formation of methanol = 0.025 X 18/64 = 7.1 X gg-' catah-'. Water produced by the methylation of propene CH30CH3+2CH2=CHCH3 2C4H8+ H20 46 + 2 X42 -2X56 + 18 Water produced by 18 methylation of C3" = 0.016 X - = 112 2.57X gg-' catah-' Water produced by the dehydration of DME to HC water produced by dehydration of DME = (water present on exit stream) + (water consumed by methanol formation) (water produced by methylation of propene) water present on exit stream = 1/100 X mass 9O of H 2 0 on exit stream X MHSV = 7.37 X gg-' catoh-' water produced by the dehydration of DME to HC = 0.0119 g-g-' cat-h-' (e) Amount of Hydrocarbons Formed from the Dehydration of DME. 3CH30CH3 2CH24HCH3 + 3H20 3 X46 + 2 X42 + 3 X18 (or CH30CH3 CH2=CH2 + H20) hydrocarbon formed from dehydration of DME = 84 0.0119 X - = 0.0185 gg-' catch-' 54 (f) Amount of Hydrocarbons Formed by Oligomerization.
+
:.
+
-
+
45 2
Ind. Eng. Chem. Prod. Res. Dev. 1904, 23,452-454
CHCfrom oligomerization = (total HC products) (HC produced by dehydration of DME) (HC formed by methylation of propene) amount of hydrocarbons formed by oligomerization = 0.376 - 0.0185 - 0.016 = 0.342 gg-l cat-h-' Registry No. Dimethyl ether, 115-10-6;propene, 115-07-1; l-butene, 106-98-9methanol, 67-56-1.
Cormerals, F. X.; Perot, G.; Chevalier, F.; Gulsnet, M. J . Chem. Res. ( S ) , 1080, 362. Derouane, E. G.; Nagy, J. B.; Dejaifre, P.; van Hooff, J. H. C.; Spekman, B. P.: Vedrlne, J. C.; Naccache. C. J . Catal. 1978, 53,40. Dessau, R. M.;Lapierre, R. 8. J . Catel. 1982, 78, 136. Esplnoza, R. L.; Sander, C. M.; Mandersloot, W. G. B. Appl. Catal. 1983, 6, 11-26. Kaeding, W. W.; Butter, S. A. J . Catal. 1980, 6 1 , 155. Perot, G.; Cormerais, F. X.; Gulsnet, M. J . Chem. Res. ( S ) , 1982, 58. Perot, G.; Cormerais, F. X.; Gulsnet, M. J . Mol. Catel. 1082, 17, 255. Van den Berg, J. P.; Wolthulzen. J. P.; van Hooff. J. H. C. In "Proceedings, Vth Conference on Zeolites"; Naples, Italy, 1980 p 649.
L i t e r a t u r e Cited
Received for review August 15, 1983 Accepted January 11, 1984
Anderson. J. R.; Mole, T . ; Christov, V. J . Catat. 1980, 67, 477. Chang, C. D.; Chu, C. T. W.J . Catal. 1980, 7 4 , 203.
Preparation of Monoglycerides of Fatty Acids from Epichlorohydrin by Phase-Transfer Catalysis. Glycidyl Esters Abraham Aserln, Nloslm Garti,' and Yoel Sasson' Casali Institute of Applied Chemistty, School of Applled Science and Technolcgy, The Hebrew Unlvers@ of Jerusalem, 8 1804. Jerusalem, Israel
The exlsting lndustrlal reactions for the preparation of monoglycerides of fatty acids are based on acidic or basic catalysis at elevated temperatures and prolonged reaction times. The product is an equilibrium mixture of isomers which need further expensive and tedious molecular distillation. The following study presents a selective method for the preparation of pure monoglycerides of fatty acids. The reaction is based on treatment of epichlorohydrin with sodium stearate soap in the presence of various phase-transfer catalysts. Over 90% of pure monoglycerides are obtained in quantitative yields after short reaction time at relatively low temperatures. The effects of reactants ratio, catalyst type and concentration, temperature, and industrial aspects have been investigated.
Introduction
Glycerol esters of fatty acids have been available for many years and are produced by many companies. Several patents and papers have been published describing methods of manufacturing, properties, and applications. Glycerol esters of fatty acids are generally prepared in two different methods: esterification and transesterification. The esterification (Grummit et al., 1945; Hatman, 1962; Gros et al., 1964) is carried out between glycerol (2-3 mol) and fatty acids (1-1.3 mol) at elevated temperatures (170-240 "C) in the presence of acids or bases (0.5-1.5 wt %). The transesterification (Hilditch et al., 1937; Franzke et al., 1963; Zwiezykowski et al., 1972) is accomplished by treating glycerol (2-3 mol) and triglycerides (1-1.5 mol) a t similar conditions. The product in both methods contains 40-60% monoglycerides, 30-45% diglycerides, &15% triglycerides, 1-5% free fatty acids and/or their salts, and 2-10% glycerol. Since the required product is the monoglyceride, the mixture is molecular distilled to obtain 90-95 % of monoglycerides. Therefore, any method leading to the formation of pure monoglycerides in a one-step reaction from relatively low cost raw materials will have significant industrial advantage. As potential raw materials, one can consider the use of epichlorohydrin or glycidol for the preparation of pure monoglycerides. The direct reaction between epichlorohydrin and sodium stearate has been studied previously in the presence (Kester et al., 1948; Malkemus, 1959; Maerker et al., 1961; Carreau 1970; Martinez et al., 1973) and in the absence (Dalby, 1966) of catalysts. It was observed that the main difficulty in the process is the low solubility of the acid 0196-4321l84l1223-0452$01.50/0
salt in epichlorohydrin and in other apolar solvents. To overcome this problem a very large excess of epichlorohydrin (up to 16:l on a molar basis) has been used. The common catalyst which was applied is benzyltrimmetylammonium chloride. No explanation was given to the function of the catalyst in the reaction mechanism although it was proved that its presence is essential (Maerker et al., 1961). Based on the modern theories of phase transfer catalysis (Starks et al., 1978; Dehmlow et al., 1980) we believe that the quaternary ammonium salt assists in extracting the stearate anion into the organic phase via a liquid anionexchange mechanism (see Discussion). Since benzyltrimethylammonium chloride and even benzyltriethylammonium chloride are very poor extracting agents in phase transfer reactions due to low lipophilicity (Starks, 1971), we have examined the activity of an efficient phase-transfer catalyst in order to develop a practical procedure for the synthesis of glycidyl esters and monoglycerides. Experimental Section A. Materials. Epichlorohydrin (99%) was obtained
from Aldrich Chemical Co., Inc.; sodium stearate (98%), from Riedel-de-Haen. Tetramethylammonium chloride (TMAC), tetraethylammonium bromide (TEAB), tetrabutylammonium bromide (TBAB), and tetrahexylammonium bromide (THAB) were all pure grade (>98%) from Fluka AG. Pyridine (99%), hexamethyldisilazane, and trimethylchlorosilane were purchased from Sigma Chemical Co. B. Kinetics Runs. Experiments were carried out in a 500-mL glass reactor equipped with a mechanical stirrer, 0 1984 American Chemical Society
