Rudham, R.

Stockwell, A.

English

Rudham R.

Stockwell A.

University of Szeged

D E H Y D R A T I O N O F P R O P A N - 2 - O L O N Y Z E O L I T E S By R. RUDHAM and A. STOCKWELL Department of Chemistry, University of Nottingham, Nottingham NG7 2RD, England (Received 30th January, 1978) Studies of propan-2-ol dehydration made in a flow system operating at a total pressure of one-atmosphere, show that propene and di-isopropyl ether are readily produced by parallel reactions occurring at BRGNSTED acid centres within Y zeolites. In addition to HY; MgY, CaY, NiY, CuY, AgY and LaY were found to be active catalysts. Treatment with H2S greatly increased the activities-of NiY and CuY, had smaller effects on AgY and LaY, and none on MgY'and CaY. A reactioni mechanism involving carbonium and oxonium ion intermediates is discussed. Introduction \ Variations in alcohol structure, the nature and extent of cation exchange, and the susceptibility to base poisons show that carbonium ion intermediates are involved in alcohol dehydration on faujasitic zeolites [1—3]. Dehydration of propan-2-ol on -X and Y zeolites in microreactors [4] and recirculatory-flow reactors [5] yields propene and water, whereas di-z'jopropyl ether is also produced in continuous-flow reactors [6, 7]. From a study of propan-2-ol dehydration on a series of H X zeolites, GENTRY and RUDHAM [6] proposed an ionic mechanism involving one O I H group for propene formation and two O I H groups for di-wopropyl ether formation. JACOBS et al. [7]' investigated alkali cation exchanged X and Y zeolites, and showed that propene and di-wopropyl ether were formed by parallel reactions catalysed by weakly acidic OH groups. The OH groups were formed either by interaction of water with cations and the zeolite lattice, or by hydrolysis of polyvalent cation impurities. The generation, of acidic OH groups through cation hydrolysis is also considered to be the source of activity for propan-2-ol dehydration on a number of di- and trivalent cation, exchanged X and Y zeolites [4, 8—10]. Experimental Propan-2-ol dehydration was studied in a continuous-flow system in which the total pressure of alcohol and nitrogen diluent was one atmosphere [6]. A small number of infra-red measurements were made using a cell which could take the place of the reaction vessel, and had provisions for dehydrating samples at elevated tem-peratures. 2 8 2 R. R U D H A M A N D A. STOCKWELL The catalysts were prepared from binder-free NaX (B. D. H./Linde 13X, lot •0003940) or NaY (Linde SK—40, lot 3606-150) by cation exchange with aqueous solutions of analytical grade salts. Catalysts are designated MX-n or MY-«, where /7 represents the extent of exchange in terms of the number of exchange cations M per unit cell. Results and Discussion p• and Catalysis on HX and H Y Zeolites Catalysts were prepared by. calcination of either 10 mg or 100 mg samples •of NH4X and NH4Y for 16 hr at 623 K in a 1.67 cm3 s"1 flow of cylinder N2. Infra-red •examination of ~50mg, 2.5 cm pressed disc of NH4Y-14.0 subjected to identical calcination showed deammoniation to be complete. A band at 3640 cm - 1 superim-posed on a broad band between 3700—3250 cm - 1 and a band at 1642 c m - 1 indicated the presence of molecular water in addition to OH groups. Under these circumstances it is unlikely that any dehydroxylation occurs during calcination. A series of comparative experiments between X and Y zeolites were made with samples of HX-8.6 and HY-8.4. It was found that the rate of formation of t>oth di-i.TOpropyi ether (re,•molecules g - 1 s - 1 ) andpropene (rp, molecules g _ 1 s_ 1) on HY-8.4 between 372 and 400 K obeyed zero-order kinetics with respect to propan-2-ol pres-sure between 2.8 • 102 and 4.2 • 103 Pa. Similar experiments with HX-8.6 confirmed the small pressure dependencies previously observed [6]. Negligible error is introduced in assuming zero-order kinetics for both catalysts at low extents of reac-tion, when Te and rp become the zero-order rate constants kc and k ; The effect of temperature on k kp was investigated with both cata-lysts under conditions where extent of dehydration was <10% of 1.11 -103 Pa of alcohol in a total flow of 0.5 cm3 s -1,- Measurements covering- ~ 35 degrees gave excellent Arrhenius pilots, and values of the activation energies Ee, E nential factors k°, k°, kp at 377 K are given in Table I. The data clearly indicate that differences in 1 activity between HX-8.6 and HY-8.4 are controlled by energetic considerations rather than by diffe-rences in the concentration of active sites. The concentration of active 2 Q 3o Bronsted acid sites was thus deter-~ • / 1 mined by titrating working catalysts 10 x added base/molecules q , • j „ * . • ' ? by the progressive adsorption of Fig. 1. Progressive poisoning of HY-8.4 with: pyridine or piperidine. Figuré - 1 — pyridine at 381 K, o — piperidine at 379 K. shows plots obtained with HY-8.4, -0-40 0-35 0-30 _p, pre-expo-and k„ and DEHYDRATION OF PROPAN-2-OL 283 where, for each base, the curves for ke and kp intersect on the zero activity axis. Similar results were obtained for HX-8.6, and values for the concentration of active sites, taken to be the amount of base necessary for complete poisoning, are given Table I ' Comparison of HX and HY catalysts HX-8.6 HY-8.4 Di-isopropyl ether formation k» at 377 K/molecules g _ 1 s _ 1 ^/molecules g _ 1 s _ 1 EJ kJ mol- 1 3.98-1014 1.8,- JO30 113 2.24-10" 1.6- 1030 • 100 Propene formation k„ at 377 K/molecules g _ 1 s _ 1 . £°/molecules g"1 s _ 1 EJ kJ mol"1 2 . 8 2 - 1 0 1 5 1.6-10®= 121 6.17-1016 1 .5 -10 3 3 111 Brônsted acid site concentration/ 10"20-.x sites g"1 from NH4+ exchange from pyridine poisoning from piperidine poisoning 2.91 2.61 2.97 2.89 2.88 2.93 in Table I. The values obtained from the bases of different strength, and that calcula-ted from the NH^ content of the catalyst precursors, are closely similar. We conclude that all the hydroxyl groups produced by deammoniation are available, and are of sufficient acid strength, to catalyse the dehydration of propan-2-ol to di-z'sopropyl ether and propene. The selectivities for ether formation on HY-8.4, shown in Fig. 1 and defined as Se = 2ke/(2ke+kp) remain effectively constant with increasing base adsorption until very high coverages are achieved. This, together with the observa-tion that wide variations in reactant space velocity could not achieve a condition where ke>kp, show that propene and di-wopropyl ether are formed by parallel reac-tion paths. The effects of varying the extent of ion exchange have been investigated with a series of HY catalysts using 1.11 • 103 Pa of propan-2-ol in a total flow of 0.5 cm3 s _ 1 (a reactant mixture and flow rate used in all further experiments). Since values of Ee and Ep were all within 4 kJ mol - 1 of the respective mean values of 99 and 111 kJ mol -1 , catalytic activities can be directly compared. Values of ke and kp at 377 K, an experimental temperature common to all catalysts, are plotted against the extent of exchange in Fig. 2. Linear plots are obtained for both products on catalysts contain-ing more than 4 OH groups per unit cell, with values of Se showing, a monotonic increase from 0.36 for HY-4.3 to 0.47 for HY-16.6. This indicates that both propene and di-M»propyl ether formation involve a single OH group. To account for our observations we propose the reaction sequence shown in Fig. 3. Since water is present after deammoniation, OH groups are ionised (1) to form H 3 0 + ions associated with structural 0~ ions in the a-cages. At high propan-2--ol concentrations, proton transfer (2) gives an oxonium ion, which yields a carbonium ion by the Ej elimination of HaO (3). Subsequently, proton transfer (4) yields propene and H 3 0 + . Alternatively, interaction with propan-2-ol (5) results in an oxonium ion which, on proton transfer (6) yields di-wopropyl ether and H 3 0 + . In view of the, partial pressure of propan-2-ol being relatively high to that of the water produced, 2 8 4 R. R U D H A M A N D A. STOCKWELL 7 120 8-0 <e 40 i o 00 10 20 30 Na exchanged per unit cell Fig. 2. Activity for di-isopropyl ether and propene formation at 377 K as a function of the extent of ion exchange: • — k. on HY, O — kp on HY, à — ke on CuY, A — kP on CuY. O H . h 2 o h3O* & -( l) H-iO' : • C H 3 - C H O H - C H 3 O" C H 3 - C H O H 2 - C H 3 C H 3 - C H - C H 3 , O' H2o C H 3 - C H - C H 3 O - • C H 3 C H O H - C H 3 (CH3)2 C H Ô H C H ( C H 3 ) 2 Ô- + H2° CH3-CHOH2-CH3 Ô" • CH3-CH-CH3 O" H3Ô H2o h 2 o o-(CH 3 )2 -CH-OH-CH-(CH 3 )2 Ô" (2) -(3) - C H 3 - C H = C H 2 (5) • < C H 3 ) 2 - C H O - C H C H 3 ) 2 - - ( 5 ) Fig. 3. Reaction sequence/or di-isopropyl ether and propene formation on HY catalysts. possible alternatives to (4) and (6) may be proton transfer to propan-2-ol to give the oxonium ion produced in (2). Presumably, the water elimination (3) makes the largest contribution to the overall activation energy, and this accounts for the similarity in Ee and Ep. This mechanism, although basically similar to those previously suggested [6, 7], differs in that only a single site is involved in ether production. Furthermore, it obviates the interaction of positively charged carbonium and oxonium ions [6]. DEHYDRATION OF PROPAN-2-OL 285 Catalysis on CuY Zeolites CuY zeolites catalyse propan-2-ol dehydration, but, in contrast to HY, they took an appreciable time to achieve constant activity following changes in the reaction conditions. The activity also depended on the initial dehydration conditions, but the chosen prefreatment of 2 hr in flowing N2 (0.67 cm3 s - 1 ) at 523 K gave the optimum activity most rapidly. Infra-red examination of CuY-8.4 given this pretreat-ment indicated the presence of molecular water. Values of ke and kp at 377 K obtained at nine Cu2+ contents are shown in Fig. 2, where the activity consistently falls below that suggested by the hydrolysis of all Cu2+ according to: C U 2 + ( H 2 0 ) + 0 Z - = (CuOH)++O zH, (7) possibly followed by 2 (CuOH)+ + 2 0 2 H = (Cu—O—Cu)2+ + 2 CLH. (8) Although zero-order kinetics were obeyed, Ee increased from 82 to 122 kJ mol - 1 and Ep increased from 98 to 132 kJ mol - 1 with increasing Cu content. The experi-mental findings show that the concentration of Bronsted acid centres is critically dependent on cation distribution and the extent of cation hydrolysis. In addition to concentration effects with water and other molecules on ke and kp, Ee and Ep vary from those on HY by contributions from the J H values associated with changes of cation location and hydrolysis. Catalysis on Other Y Zeolites and the Effects of H2S Treatment Catalytic activities at 377 K have been determined for a number of Y zeolites in which the extents of exchange of Na+ for other actions were approximately equivalent. In all cases the catalysts were dehydrated as with CuY, and no dehydro-genating activity to acetone was observed. Values of ke and kp given in Table II show Table II Effect of H..S treatment on dehydration activity at 377 K (values of ke and kp in 10_14-molecules g _ 1 s _ l ) HY-I 1.1 AgY-11.5 MgY-5.5 CaY-5.5 NiY-5.1 CuY-5.6 LaY-3.8 Before HoS k. 370 330 0.68 0.076 4.5 60 230 kP 930 790 2.2 0.71 25 190 490 S, 0.44 0.46 0.38 0.18 0.27 0.39 0.48 After H2S k. — 420 0.68 0.096 220 450 200 k„ — 910 2.2 0.93 680 1000 450 S„ — 0.48 0.38 0.17 0.39 0.47 0.47 a wide spread of activities, with only that of AgY-11.5 approaching HY-11.1. This most probably occurred through a substantial reduction of Ag+—Ag° accompanied by OzH formation in the initial stages of reaction. The Se values, apart from those of low activity catalysts which are subject to appreciable error, suggest the same mechanism as on HY zeolites. 2 8 6 R. R U D H A M A N D A. STOCK WELL: DEHYDRATION O F PROPAN-2-OL In view of the increase in cumene cracking activity observed following H2S treatment of certain ion-exchanged Y zeolites [11], we have determined activities following treatment of the catalysts with ~ 20 cm3 H2S for 15 min. The data in Table II show a large increase in the activities of NiY-5.1 and CuY-5.6, a smaller effect with AgY-11.5, no effect with MgY-5:5 and Ca-5.5, and even slight poisoning with LaY-3.8. Additional experiments also showed that there was no effect with NaY and HY-14. Such observations are'in accord with those of SUGIOKA et al. [11]. A series of experiments with H2S treated CuY-4.2 confirmed that the activity increase was associated with Bronsted acidity, since complete poisoning with pyridine readily occurred at 377 K. In addition, the source of the additional activity possessed similar stability to that of untreated CuY zeolites, since there was no diminution in activity following heating at 523 K for 2 hr in flowing nitrogen. We believe the increases in activity arise from hydroxy! groups generated in the reaction: M"+ +nOz +0.5«H2S - MeS0.5n + «OzH, (9) and that the neutral MeS0.5„ entity resists hydrolysis in the cases of AgY-11.5, NiY-5.1 and CuY-5.6. With AgY-11.5 both Ag° and Ag+ probably react with H2S, but only the Ag+ will generate additional OzH groups, so accounting for the smaller H,S effect with this catalyst. References [1] Venuto, P. B., P. S. Landis: Adv. Catalysis 18, 259 (1968). [2] Jacobs, P. A.: Carboniogenic Activity of Zeolites, Elsevier, Amsterdam, 1977, p. 99 and p. 131. [3] Rudham, R., A. Stock-well: Catalysis Vol. 1 (Specialist Periodical Report), The Chemical So-ciety, London, 1977, p. 111. [4] Stone, F. S., A. L. Agudo: Z. Phys. Cherh. N. F. 64, 161 (1969). [5] Levchuk, V. S., K. G. lone: Kinetika i Kataliz 13, 949 (1972). [6] Gentry, S. J„ R. Rudham: J. C. S. Faraday I 70, 1685 (1974). [7] Jacobs, P. A., M. Tielen, J. B. Uytterhoeven: J. Catalysis 50, 98 (1977). [8] Topchieva, K. V., H. S. Thoang: Kinetika i Kataliz 14, 1491 (1973). [9] Baile, M., A. Cortes, J. Soria: Anales de Quim. 70, 305 (1974). [10] Schoonheydt, R. A., L. J. Vandamme, P. A. Jacobs, J. B. Uytterhoeven: J. Catalysis 43, 292 (1976). [11] Sugioka, M., T. Hosotsubo, K. Aomura: J. C. S. Chem. Comm. 54 (1976). 

Dehydration of propan-2-ol on y zeolites

http://acta.bibl.u-szeged.hu/24080/1/phys_chem_024_fasc_001_002_281-286.pdf

Dehydration of propan-2-ol on y zeolites

Abstract

Similar works

Full text

Available Versions

University of Szeged