Ind. Eng. Chem. Res. 1999, 38, 4521-4524
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APPLIED CHEMISTRY Dean-Stark Apparatus Modified for Use with Molecular Sieves Edmund J. Eisenbraun* and Kirk W. Payne Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
A Dean-Stark apparatus, with the separation chamber modified to contain molecular sieves for scavenging water from an azeotrope, is described. This modification permits the efficient dehydration of an azeotrope, thereby greatly reducing the chance of water recycling to a reaction flask. The extent of water removal from recycling solvent was determined by Karl Fischer titration. Depending on the type of molecular sieve used, the pH of the water eluted at saturation ranged from 7.8 to 10.4. To demonstrate the effectiveness for removing water from recycling solvent and providing anhydrous reaction conditions, the apparatus was used to determine the water-loading capacity of a variety of molecular sieves and to effect the cyclodimerization of R-methylstyrene as well as 2-phenyl-2-propanol to 2,3-dihydro-1,1,3-trimethyl-3-phenyl-1Hindene (Chem. Abstracts 3910-35-8). Introduction The Dean-Stark apparatus was initially developed to measure the water content of petroleum products via azeotropic distillation.1 To chemists, the term DeanStark and the apparatus have become synonymous with the separation of water from other liquids and reaction mixtures.2 The apparatus has repeatedly been modified for varied and numerous applications (see current supply catalogs under Dean-Stark). It is widely used and frequently cited. Unfortunately, portions of some azeotropes may stubbornly cling as patches to the walls of the collection chamber and condenser. Also, the liquid in the collection chamber frequently remains cloudy, indicating incomplete separation and/or removal of water. As shown below, as much as 6%-8% of introduced water was not recovered as an azeotrope. This becomes objectionable when trace amounts of water are to be removed and/or there is a need to be certain that water does not recycle to the reaction vessel. The apparatus was modified for use with molecular (mol.) sieves to overcome these problems by thoroughly dehydrating the recycling solvent and thus maintaining anhydrous reaction conditions in a reaction vessel. Design of the Apparatus in Figure 1 A Dean-Stark apparatus was modified to include mol. sieve chamber A (22.7-mm i.d. × 210 mm, 85-mL total volume) of Figure 1. Chamber A was surrounded by water-cooled jacket B because of the exothermic response of dry mol. sieves to water. A coarse porosity, sintered glass, filter disk C prevents transport of particles to the reaction flask by a recycling solvent. Coarse porosity is essential because surface-wetting effects produced by an azeotrope may obstruct the recycle flow. The recycling solvent may be sampled for * To whom correspondence should be addressed. Phone: (405)744-6673. Fax: (405)744-6007. E-mail: eisenbr_osu@ osu.net.
Figure 1. Dean-Stark apparatus modified for use with mol. sieves.
analysis or withdrawn as an azeotrope through stopcock D. Needle valve E controls the return of recycling solvent and the level of solvent in chamber A. Sidearm F provides a minimum liquid level at approximately one-third of chamber A, and ensures an adequate free volume in the event that surface-wetting effects at the sintered disk interfere with the return of the recycling solvent. Further, the upward angle (22°) minimizes chance entry of water or azeotrope into the sampling area under C. In operation, a reaction flask is attached to inner joint G (S/T 24/40). The refluxing azeotrope passes through spray trap H, fitted with drain holes I
10.1021/ie9904044 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/09/1999
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Figure 2. Water breakthrough curves for 76 mL of various mol. sieves used for drying benzene.
Figure 3. Water breakthrough curves using 76 mL (50.8 g) of 4A indicating mol. sieve.
and vapor vents J to control foaming, before proceeding past thermometer joint K (outer, S/T 24/40) to a water condenser at outer joint L (S/T 24/40). The condensed azeotrope and solvent flow into the mol. sieve chamber A for entrapment of water. The above changes resulted in a design reminiscent of a Soxhlet apparatus. However, Soxhlet apparatus usually are wide-bore, short-path devices in which the mol. sieve is totally immersed in solvent and the solvent periodically drains through a siphon whereas a DeanStark apparatus may continuously deliver recycling solvent. A Soxhlet apparatus, containing 5A mol. sieves, was shown to be effective in decreasing water content of saturated benzene.3 This study also shows that a Soxhlet apparatus with 5A mol. sieves provides a significant yield increase in esterification as compared to the use of a conventional Dean-Stark apparatus.3 The use of mol. sieves in esterification has become common.4 The current studies (Figures 2 and 3) show that the quantity of mol. sieves may be considerably reduced from those in esterification5 and ester exchange6 using a Soxhlet extractor. The apparatus of Figure 1 may be used with chamber A empty or charged with mol. sieves. General Procedure for Testing Mol. Sieves in the Apparatus of Figure 1 The apparatus of Figure 1 was fitted with a 125-mL, three-neck flask containing 80 mL of dry benzene and
a magnetic stir bar. The benzene level was marked on the wall of the reaction flask. Sensitive thermometers (0.1 °C) were added to a flask side neck and outer joint K to enable monitoring azeotrope formation and disappearance, as determined by changes in the boiling point between the solvent and its aqueous azeotrope (for benzene, 80 and 69.3 °C, respectively). Information about the azeotropes used in this study is available.7 A short, open-tube condenser fitted with a nitrogen inlet adapter was added to the third neck of the flask to serve as an entry port for water and replenishment benzene. Chamber A was filled with 76 mL of the dried (12 h at 300 °C/1 mm) mol. sieve. To maximize performance of the apparatus, the same volume (76 mL) of mol. sieve was used unless otherwise stated. Benzene (distilled and stored over dry 4A mol. sieves) was slowly added to fill chamber A until overflow appeared at sidearm F. Tests in which the liquid level was artificially raised using needle valve E showed that a higher level, including full immersion of the mol. sieve, did not increase the effectiveness of water absorption. A slow flow of nitrogen was initiated and the benzene was heated at reflux until the temperature stabilized. A blank sample (3.0 mL) was withdrawn through stopcock D. Water (2.00 mL) and replenishment benzene were added through the condenser sidearm. Reflux was continued for 30 min to fully remove the azeotrope from the reaction flask. Needle valve E was closed to prevent entry of azeotrope, a 3.0-mL sample withdrawn dropwise through stopcock D, and needle valve E reopened. Water, in 2.00-mL aliquots, was added and the sample-collection procedure continued at 30-min intervals until the azeotrope was observed in the collected sample. Water analyses were carried out using a Brinkman Instruments, Metrohm Automat 633 titration apparatus fitted with a 645 Multi-Dosimat. Prior to a Karl Fischer analysis, each 3.0-mL sample was diluted with 3.00 mL of dry methanol (ethanol in the case of cyclohexane) and thoroughly shaken to promote homogeneity. Three repetitions of a 2.00-mL aliquot were analyzed. Initially, Karl Fischer titrant was used because of numerous literature citations. However, Riedel-deHaen’s Hydranal titrant,8 containing imidazole as a substitute for pyridine, gave more reproducible data with considerably less
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endpoint drifting and less objectionable odor. Instructions for sample preparation using this reagent are available.8 Comparison of the Apparatus in Figure 1 with a Recycle-Type, Dean-Stark Apparatus The apparatus of Figure 1, without mol. sieves, used to separate water (18 mL) and benzene (180 mL in a reaction flask and chamber A filled), under a slow stream of nitrogen, gave 16.5 mL of collected azeotrope (92% recovery of water). In comparison, a commercial recycle-type, Dean-Stark apparatus having a stopcock drain (Ace Glass Inc., Vineland, NJ, no. 7747), gave 17.0 mL of azeotrope (94% recovery of water). This difference in efficiency results from some retention of azeotrope by the sintered disk (C of Figure 1) and the residual pool of benzene in chamber A. With benzene, the liquid level in chamber A rises above the normal level when the azeotrope appears in chamber A. Addition of dried mol. sieve beads (4A, 8-12 mesh, 50.3 g, 76 mL) to chamber A and continuation of reflux caused the disappearance of the patches of azeotrope from the walls of the apparatus, clarification of the liquid pool, and return of a normal liquid level in chamber A. The recycle-type, Dean-Stark apparatus maintained a 0.023% water content for benzene whereas the modified apparatus (Figure 1), containing mol. sieves, maintained a 0.0076% water content. The aforementioned samples were withdrawn from the reaction flask. Water Removal from Solvents Using Mol. Sieves Because many reactions producing water are acidcatalyzed, it became a concern that recycled solvent, generally benzene, chlorobenzene, cyclohexane, or toluene, could carry alkali past the mol. sieve to the distillation flask and thus adversely influence a reaction. Further, because information regarding the water absorption capability of mol. sieves in this projected use was not available, it became a major objective to accurately determine the water-scavenging capability of mol. sieves. The mol. sieves tested were 1.6-mm pellets (AW-300 and AW-500) and 8-12-mesh beads (Sigma 4A w/15% indicator, Alltech 4A w/15% indicator, and Aldrich 3A). As shown in Figure 2, their effectiveness in benzene decreases in the following order: AW500 pellets, Sigma 4A, AW-300 (61.0 g, 76 mL), Alltech 4A, Aldrich 3A, AW-300 (50.3 g, 62 mL), and waterwashed and redried Sigma 4A. Water Retention by Mol. Sieves in Benzene Because water in contact with 4A mol. sieves is strongly alkaline (pH > 10), we attempted washing with water. Despite numerous requests, information about washing mol. sieves was not available from commercial sources, nor were satisfactory data found in the literature. Mol. sieves are fragile and must be carefully handled; hence, mixing with freshwater during washing was done by gently pouring the water/mol. sieves slurry alternately between two beakers. Washing the mol. sieve (100 g, 4A, beads, indicating type, 8-12 mesh, Sigma) with (40 × 300)-mL portions of distilled water caused a pH change from 10.0+ to ca. 9.5. The initial turbidity gradually decreased. Small particles, washed out from the beads, viewed under 30× magnification, changed shape from smooth lumps to sharp-edged pieces
Table 1. Reported and Found Water Capacities for Molecular Sieves water capacity (wt %) sieve type
UOP dataa
experimentalb
AW-300 AW-500 3A 4A
10 >17 21 22
24 60 31 47c
a Equilibrium water capacity at 17.5 mmHg/25 °C. b Saturation water capacity via the apparatus in Figure 1. c Average of the two types of 4A used.
as the washing proceeded. Apparently, some beads disintegrated; however, the main supply of beads remained smooth and round. The washed beads were dried in an oven at 100 °C and then 99 g were further dried at 300 °C/1 mmHg for 12 h. The second drying resulted in a weight loss of 10.2 g. As shown in Figure 2, this treatment greatly reduced (69%) the water absorption of the 4A mol. sieve. This effect coupled with the demonstrated waterscavenging abilities of AW-500 and AW-300, both having low pH for eluted water (7.8 and 8.8, respectively), show these mol. sieves to be a better choice if there is concern about possible contamination from eluted base entrained by the recycling solvent. Indicating-type, 4A mol. sieves were used to evaluate color change following the migration of water, as an azeotrope, through the mol. sieves in chamber A. However, migration of the azeotrope is easily observed in all the mol. sieves tested, so the use of indicating beads is not essential. Water Absorption by 4A Mol. Sieves in Different Solvents Benzene, chlorobenzene, or cyclohexane were used to determine the effect of different solvents on the water retention by 4A mol. sieve (Sigma) as shown in Figure 3. These beads performed comparably in benzene and chlorobenzene. However, in cyclohexane the loading capacity of this mol. sieve was reduced by 15%-20%. Table 1 shows a comparison of the mol. sieve data reported in UOP product information literature9 with the data from current usage of mol. sieves in scavenging water from recycling benzene. With the exclusion of water-washed 4A, the average water absorption capacity for the mol. sieves is 41%, with a maximum of 60% obtained with AW-500. Information about mol. sieves may be obtained from a manufacturer (UOP, Des Plains, IL) or suppliers (Aldrich Chemical Co., Milwaukee, WI; Alltech Associates, Inc., Deerfield, IL, or Sigma, St. Louis, MO). Mol. sieves is a broad topic (Scifinder Scholar lists 16086 references). It is described by a review10 and selected reference works.11-15 Dehydration and Cyclodimerization of 2-Phenyl-2-propanol (1) to 2,3-Dihydro-1,1,3-trimethyl-3-phenyl-1H-indene (4) via r-Methylstyrene (2) The reaction sequence in Scheme 1 was selected to demonstrate the effectiveness of the apparatus in a multistep process involving the formation of water as a byproduct and its removal as an azeotrope. The cyclodimerization step16 from R-methylstyrene (2) to 2,3-dihydro-1,1,3-trimethyl-3-phenyl-1H-indene (4)
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Scheme 1
was first verified by adding the intermediate (2) (11.8 g, 0.1 mol, steam distilled and dried over MgSO4), dry Amberlyst 15 (A-15, 1.18 g), dry cyclohexane (200 mL), and mol. sieve (76 mL, 50.0 g of dry AW-500) to the apparatus. Reflux was established and samples for capillary GLC analysis were withdrawn, filtered through a 1-cm pad of Dicalite over packed cotton in a Pasteur pipet, and gas chromatographed at 0.5-h intervals. These GLC studies showed cyclodimerization (formation of 4) was essentially complete at 1 h. After 3 h, the reaction mixture was cooled, filtered through Celite, steam distilled, and the steam condensate extracted with ether. Those fractions predominating in 4 spontaneously crystallized after extraction and removal of the solvent. Capillary GLC studies showed the initial fraction (0.75 g from 450 mL of steam condensate) contained only 4 preceded by a trace of 2. Admixture of this fraction with authentic 4 showed no depression in the melting point (50-52 °C).16 Fraction two of the steam condensate (3.1 L) gave 10.33 g of crystalline 4 in 97.9% purity accompanied by 2.1% of four closely spaced impurity peaks having m/z 354 corresponding to a trimer of 2. R-Methylstyrene (2) was absent. The overall yield of 4 (fractions one and two) was 92%. Fractions three and four gave 0.140 and 0.216 g of colorless oil from 4.5 and 4.2 L of steam condensate. These oils showed a decreasing concentration of 4 and increasing concentration of the m/z 354 mixture of trimers. The pot residue contained 0.156 g of sticky, colorless oil. Repetition of the experiment substituting 13.6 g (0.1 mol) of freshly distd 2-phenyl-2-propanol (1) for 2 gave a 90% yield of the expected 4 accompanied by the previously mentioned cluster of four higher-boiling byproducts having m/z 354. In a separate experiment, purified (99.7+%) 4 (11.8 g) was shown to be stable to the aforementioned reaction conditions by heating at reflux in the presence of 1.18 g of A-15 for 25 h. The isolated product (97+% recovery) was shown by GLC studies (99.8+%) to be free of the previously mentioned impurities. On distillation, there was no polymeric residue. Materials The mol. sieves (AW-500, AW-300, 3A) were purchased from Aldrich Chemical Co., Milwaukee, WI. Other mol. sieves (4A, indicating) were purchased from Sigma Chemical Co., St. Louis, MO, and Alltech Associates, Inc., Deerfield, IL. Hydranal and Karl Fischer reagents were purchased from Aldrich. Conclusions Studies of the absorption of water using a DeanStark apparatus, modified to contain dried mol. sieves,
show that the water content of recycled solvent is decreased to 0.0076% as compared to that of a conventional recycle-type, Dean-Stark apparatus at 0.023%. It is worth noting that this average water content of recycling solvent was consistently maintained until the concentration rose asymptotically at the point of water breakthrough for the mol. sieves as shown in Figure 2. The study also provides information about the waterabsorption capacity of different mol. sieves, with AW500 being the most effective. While the modification of the Dean-Stark apparatus enables a significant reduction in the water content of recycled solvent, this change introduces the possibility of contaminating the recycled solvent with entrained alkali in the rare event that the water-loading capacity of the mol. sieve is prematurely compromised. However, the excellent performance of the acid-washed AW-500 mol. sieve, showing pH 7.8 for eluted water, provides an acceptable alternative. The overall result of the apparatus modification and the use of mol. sieves are to provide an effective means of water removal from solvents and from chemical reactions producing water. A broad usage is anticipated for this combination. Literature Cited (1) Dean, E. W.; Stark, D. D. A Convenient Method for the Determination of Water in Petroleum and Other Organic Emulsions. J. Ind. Eng. Chem. 1920, 12, 486. (2) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; Wiley & Sons: New York, 1968; pp 617-618. (3) Stern, R. L.; Bolan, E. N. Molecular Sieves in Esterification Reactions. Chem. Ind. 1967, 825. (4) Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; Wiley and Sons: New York, 1968; pp 703-705. See succeeding volumes as well. (5) Harrison, H. R.; Haynes, W. M.; Arthur, P.; Eisenbraun, E. J. Use of molecular sieves in the methyl esterification of carboxylic acids. Chem. Ind. 1968, 1568. (6) Roelofsen, D. P.; De Graaf, J. W. M.; Hagendoorn, J. A.; Verschoor, H. M.; Van Bekkum, H. Ester Interchange Using Molecular Sieves. Recueil 1970, 89, 193. (7) CRC Handbook of Chemistry and Physics, 79th ed.; Lide, D. R., Ed., CRC Press: New York, 1998-1999; pp 6-164-167. (8) Hydranal-Manual: Eugen Scholz Reagents for Karl Fischer Titration, available from Hydranal-Technical Center, 545 South Ewing, St. Louis, MO, 1997. (9) UOP Product literature titles: UOP Molecular Sieves 1989; MOLSIV Adsorbents 1991; UOP Type AW-300 Adsorbent Data 1971; UOP Type AW-500 Adsorbent Data 1971; UOP, 25 E. Algonquin Rd., Des Plaines, IL. (10) Breck, D. W. Crystalline Molecular Sieves. J. Chem. Educ. 1964, 41, 678. (11) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Use; Wiley & Sons: New York, 1974. (12) Szostak, R. Molecular Sieves: Principles of Synthesis and Identification VanNostrand Reinhold: New York, 1989. (13) Molecular Sieve Zeolites-I; Gould, R. F., Ed.; Advances in Chemistry Series 101; American Chemical Society: Washington, DC, 1971. (14) Molecular Sieve Zeolites-II; Gould, R. F., Ed.; Advances in Chemistry Series 102; American Chemical Society: Washington, DC, 1971. (15) Molecular Sieves; Gould, R. F., Ed.; Advances in Chemistry Series 121; American Chemical Society: Washington, DC, 1971. (16) Duncan, W. P.; Eisenbraun, E. J.; Taylor, A. R.; Keen, G. W. Synthesis of 2,3-Dihydro-3-methyl-1-(2-naphthyl)-1H-benz[e]indene and 1,1,3-Trimethyl-3-phenylindan. Org. Prep. Proc. Int. 1975, 7, 225.
Received for review June 7, 1999 Revised manuscript received August 20, 1999 Accepted August 23, 1999 IE9904044
