REVERSE MOLECU LAR-SIZE SELECTIVITY AND AGING IN A ZEOLITE-CATALYZED A LKY LAT IO N P. B. VENUTO
AND L. A.
HAMILTON
Applied Research Division, Mobil Oil Gorp., Paulsboro, N .J . Practical catalyst lifetimes of 310 to 790 hours have been obtained in the continuous-flow alkylation of benzene with ethylene using rare earth cation-exchanged X-type molecular sieves. These catalysts, however, are subject to slow deactivation or aging with increasing time on stream. Deactivation is associated with the synthesis and occlusion of large molecules within the catalyst pores, a “reverse” molecular-size selectivity, in that molecules too large to escape are formed within the internal zeolite cavities. These conclusions are based on a comparison of the analyses of the highest boiling 0.1 weight % of liquid alkylation products passing from the catalyst, and the trapped intracrystalline aging products extracted from the deactivated catalyst sample. The similarity of the entrapped products to the reaction products when ethylene alone was passed over the catalyst is also demonstrated.
HERE is currently considerable interest in the use of molecuTlar sieves (crystalline aluminosilicates) as catalysts in the petroleum and petrochemical industry, and since Weisz, Frilette, and coworkers first reported cracking, alcohol dehydration, and hydrogenation using zeolite catalysts (Frilette et al., 1962; Weisz and Frilette, 1960; Weisz et al., 1962)) the field has rapidly expanded. The zeolites are unusual in that their intracrystalline active sites are accessible only to molecules whose size and shape permit sorption through the entry pores. These orifices range in size from 3 to 9 A., depending on the structure of the zeolite under consideration (Breck, 1964). Thus a highly selective form of catalysis based on sieving effects is possible. For example, Weisz et al. (1962) have shown that when Linde 5A sieve (5-A. pore diameter) was used as catalyst, selective cracking of normal paraffins, but not branched isomers, was observed ; similarly, 1-butanol, but not 2-methyl-1-propanol, was selectively dehydrated at moderate temperatures. Various cation-exchanged forms of the large pore (8 to 9 A,) faujasite family of molecular sieves have also shown intracrystalline catalytic activity for alkylation of aromatics with a wide variety of alkylating agents (Venuto et al., 1966a, 1966b; Venuto and Landis, 1966). T h e size-selective principle is operative although less dramatic in the open faujasite systems, except with bulky organic molecules. Good results have been obtained in the continuous-flow alkylation of benzene with ethylene to form ethylbenzene using a rare earth cation-exchanged X-type faujasite (Venuto et al., 1966b). At a benzene-ethylene molar ratio of 5, 500 p.s.i.g., and 204’ C., a practical catalyst lifetime of 790 hours has been observed, during which conversion of ethylene slowly decreased from 80% to about 50% at the end of the run. I n other cases, where the catalyst bed became overheated, or the pressure was allowed to decrease, catalyst deactivation was much more rapid. We present an example of a rare earth faujasite-catalyzed benzene ethylation reaction, wherein catalyst aging or deactivation is experimentally shown to be associated with the synthesis and trapping of large molecules within the catalyst, a “reverse” molecular size selectivity. The conclusions are based on a comparison of the analyses of the highest boiling 0.1 weight 7 0 of liquid alkylation products passing from the catalyst, and of the intracrystalline aging products within the deactivated catalyst.
190
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
The similarity of the entrapped products to the reaction products of ethylene alone over the catalyst is also demonstrated. Experimental
Reaction. A benzene ethylation was conducted in a continuous flow unit using a mixed rare earth X-type faujasite (REX) as catalyst. The rare earth mixture (gram atom %) was La (25.0), Ce (47.2)) Pr (5.9), Nd (19.3), Sm (1.9), and Gd (0.7). The details of such alkylations have been described (Venuto et al., 1966b). Approximately 1000 grams of ethylbenzene were produced per gram of catalyst before the catalyst became inactive after 310 hours of continuous operation. Liquid Alkylation Product. A sample of the total liquid product from this alkylation run was fractionally distilled, and the highest boiling 0.1 weight % (viscous reddish brown oil, b.p.> 280’ C.) analyzed. Deactivated Catalyst. Upon removal a t the end of the run, the deactivated catalyst was brown and contained 1870 carbon (C/H ratio = 0.74). Its infrared spectrum (Figure 1) showed C-H stretching and C-CHa deformation vibrations arising from entrapped organic matter. Ethylene-Treated REX Catalyst. A calcined R E X sample in a tubular glass reactor was exposed to a flow of ethylene (12 ml. per minute at 1 atm.) for 210 minutes at 213” C. The catalyst color changed from pale yellow to dark reddish brown during this period. The discharged catalyst showed a 19% increase in weight, with a C/H ratio of 0.68 for the
WAVELENGTH,
MICRONS
Figure 1. Infrared spectrum (KBr disk) of typical deactivated catalyst after benzene-ethylene alkylation reaction Peaks arising from entrapped organic matter denoted b y arrows; inset shows C-H region under high resolution
occluded organic matter. Such catalysts have been shown to be deactivated for benzene-ethylene alkylation (Venuto et al., 1966b). Catalyst Dissolution Technique. Deactivated catalyst (5 grams) was stirred in a flask with 75 ml. each of 2 N HC1 and chloroform for 0.5 hour a t 25' C . T h e greenish black organic extract, after separation from the green aqueous layer, was washed three times with 10% sodium hydroxide, and three times with water, and dried over anhydrous sodium sulfate. After removal of the solvent in vacuo, 1 gram of benzene-soluble black tar (osmometric molecular weight 485, C / H ratio 0.793) was obtained. The tar showed numerous ultraviolet absorption bands in the 230- to 350-mP region and a complex, poorly resolved N M R spectrum. T h e products were separated into fractions by preparative scale gas chromatography (Figure 3A) and analyzed spectroscopically. Similar dissolution-extraction procedures and analyses were performed on the ethylene-treated R E X catalyst. T h e viscous, oily extract showed an osmometric molecular weight of 490. Analyses. A temperature-programmed F & M Model 720 dual-column chromatograph was used for gas chromatographic analyses. Runs were made with either 12-foot, '/4-inch 0.d. or %foot, '/Z-inch 0.d. stainless steel columns of 2OT0 silicone gum rubber on Chromosorb. Infrared analyses were run on Perkin-Elmer 421 or Infracord spectrophotometers; ultraviolet spectra on a Cary 14 spectrophotometer; nuclear magnetic resonance (NMR) spectra a t 60 Mc. per second on a Varian A-60 spectrometer using carbon tetrachloride solutions; and mass spectra at 7 and 70 e.v. on a Consolidated Electrodynamics Corp. Model 21-103 spectrometer. Results
Properties of Highest Boiling 0.1 Weight yo of Liquid Alkylation Product. Table I shows the mass spectroscopic analysis, on the basis of C,H,,-, groups, of the hydrocarbon products in this fraction. For this extremely complex mixture, an average (osmometric) molecular weight of 250 was obtained, to (218. and the most predominant carbon numbers were Over 35 weight yo of the products was a mixture of polyalkylbenzenes, in which C M H ~ Jidentified , as hexaethylbenzene, was dominant. Moderate amounts of diphenylmethanes, diphenylethanes, etc. (CnH2,-14), smaller amounts of indanes, Tetralins, etc. (C,HZ,-a), and a variety of other complex aromatic compounds constituted the remainder of the products. T h e mass spectroscopic interpretations were consistent with infrared, ultraviolet, and NMR data. T h e absence of significant amounts of condensed polycyclic aromatics in this liquid product was shown by ultraviolet analysis. Large amounts of hexaethylbenzene have been isolated from heavy ends in the Alkar process (benzene-ethylene alkylation with supported phosphoric acid-boron trifluoride catalyst) (Grote and Gerald, 1960). Also, in the alkylation of benzene
I
f
43.83
Table 1. Mass Spectroscopic Analysis of Highest Boiling 0.1 Weight % of Total Liquid Alkylation Product
Group
CnHzn-s8. Indanes, TetraIlns, etc. C,H2,- 1 0 . Octahydroanthracenes, etc. C,Hzn- 11. Dodecahydronaphthacenes, et.c. C,Hr,- 14. Diphenyls, diphenylmethanes, diphenylethanes, etc. C,HZ,- 1 6 . Dihydroanthracene, fluorene, etc. C,Hz,- 18. Other polyhydroaromatics
CIBHZ~-C~&S
12.71
C I S H ~ & Z ~ H ~ ~ 10.66 C1sH24-CZoH28
11.11
Cl6H18 CI~HZZ C15H161 C17Hz0, and Ci g H z I-CZIH3 4 CieHi 6-C24H32
10.42)
4'69321, 8 1 6.70 5.09
C I ~ H ~ Z - C Z ~ H ~ 3~. 3 8 100.00
with ethylene over aluminum chloride, Sherwood (1953) has described the formation of polyalkylated benzenes and condensation products such as 1,l-diphenylethane and 9,10-dimethyldihydroanthracene. I n Figure 2, the gas chromatogram of the product mixture outlined in Table I is shown. The arrows show the retention times (from use of internal standards) of n-hexadecane (b.p. 286" C.) and hexaethylbenzene (b.p. 298' C.) for reference. About 84y0 of the species present boil between 286" and about 305" C., and another 12.670 boil a t a temperature only slightly higher. Thus, there appears to be a relatively abrupt cutoff in boiling range-and molecular weight-of the highest boiling 0.1% of liquid alkylation product. Analysis of Intracrystalline Trapped Products from "Aged" Catalyst. The black organic tar originally entrapped within the zeolite internal pore system was analyzed after extraction from the catalyst. T h e extract was a complex mixture of high molecular weight (average 485) aromatic compounds. Its gas chromatogram is shown in Figure 3, top. For reference, the retention times of 1,3,5-triethyIbenzene and hexaethylbenzene are indicated by arrows. The brackets
II 1.0
I
Sfiecies
Weight % of 0.7% Composite Fraction
TIME
(TEMPERATURE)
LL
x
LL
-
0
Figure 2. Gas chromatogram of highest boiling 0.1 wt. tion product Temperature programmed (5.6' C./min.) from 6 3 ' to areas (wt. 70) corrected for attenuation
345' C.
70of
liquid alkyla-
Numbers above peaks show
VOL. 6
NO. 3
SEPTEMBER 1 9 6 7
191
Top. Black tar extracted from deactivated REX catalyst Bottom. Viscous oil extracted from REX catalyst ofter reaction with ethylene alone at 21 2’ C. Temperature programmed ( 1 1 C./min.) from 75’ to 350’ C.
’
A TIME
B (TEMPERATURE)
-
C
Figure 3. Gas chromatograms
show the three fractions (A, B, and C) that were separated for analysis by preparative-scale gas chromatography. Fraction A consisted largely of a series of highly alkylated benzenes; B consisted mainly of a series of polyalkylnaphthalenes; C was a complex mixture of polyalkylnaphthalenes and alkylated higher condensed polycyclics. Previously, it was found that polyalkylbenzenes, polyalkylnaphthalenes, and higher condensed polycyclic aromatics were formed within the catalyst pores when ethylene alone was passed over R E X at 213’ C. (Venuto et al., 1966a). The chromatographic profile of the extracted intracrystalline products from the laboratory-scale control run with ethylene is shown in Figure 3, bottom. The over-all profile is similar to that of the products extracted from the catalyst deactivated in the benzene-ethylene alkylation (Figure 3, top). Moreover, when subjected to an identical procedure of preparative-scale gas chromatography, the ethylene-derived product showed fraction-for-fraction correspondence in major species type with that from the aged alkylation catalyst. These results suggest that, at least in part, intracrystalline reactions of ethylene alone are responsible for deactivation of R E X catalyst during the alkylation reaction. Discussion
The aging process associated with the alkylation of benzene with ethylene over R E X catalyst a t 213’ C. showed the following salient features: The highest boiling 0.1 weight yoliquid product was rich in polyalkylbenzenes and hydroaromatics, but devoid of condensed polycyclic aromatics. This fraction showed an average molecular weight of 250, with an abrupt cutoff in products boiling higher than about 305’ C. O n the other hand, the tarry intracrystalline extract was rich in polyalkylbenzenes and condensed (polynuclear) aromatics, with an average molecular weight almost twice that of the liquid products. Molecules such as hexaethylbenzene (molecular weight 246) ~ CISmolecules with molecular weight near 250 and other C I to
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l & E C P R O D U C T RESEARCH A N D DEVELOPMENT
are typical of the species found in the highest boiling 0.1 weight yo liquid product. The approximate molecular diameters of these species are all fairly near 10 A. They would thus pass easily in and out of the 10-A. size pores of faujasite (Mair and Shamaiengar, 1958). Within the catalyst pores, however, an extremely complex series of side reactions of ethylene and possibly polyalkylbenzenes has occurred. Polymerization, hydrogen-transfer, and dehydrocyclization processes have resulted in the formation of condensed polynuclear aromatics. From small building blocks with high diffusivity, bulky molecules were formed with such great steric requirements that diffusion rates through the narrow pore entrance were greatly retarded. The aromatics tended to concentrate within the pore system and inevitably reacted further. Eventually, such high molecular weight, rigid, hydrogen-deficient species were formed that escape from the cavity became impossible. Thus, a sharp limit in the size of the species in the liquid product was observed. Inspection of the chromatogram in Figure 2 shows a “tailing off)’in peaks that indicates the escape of small amounts of higher molecular weight products. This is not surprising, since at the temperatures of most catalytic reactions, organic molecules possess considerable energy, and the zeolite lattice itself is not totally rigid (Breck, 1964). literature Cited
Breck, D. W., J . Chem. Educ. 41, 678 (1964). Frilette, V. J., Weisz, P. B., Golden, R. L., J . Catalysis 1, 301 (1962). Grote, H. W., Gerald, C. F., Chem. Erie. Pro.gr. 56, 60 (1960). Mair, B. J., Shamaiengar, M., Anal. C&m. 36, 276 (1958). Sherwood, P. W., Petrol. Refiner 32, 97 (1953). Venuto, P. B., Hamilton, L. A., Landis, P. S., J . Catalysis 5 , 484 (1966a). Venuto, P. B., Hamilton, L. A , , Landis, P. S., Wise, J. J., J . CatalyJis 5 , 81 (1966b). Venuto, P. B., Landis, P. S., J . Catalysis 6 , 237 (1966). Weisz, P. B., Frilette, V. J., J . Phys. Chem. 64, 382 (1960). Weisz, P. B., Frilette, V. J., Maatman, R. W., Mower, E. B., J . Catalysis 1, 307 (1962). RECEIVED for review March 24, 1967 ACCEPTED May 5, 1967
