Interconversion of unsaturated C4 nitriles under basic conditions II. Catalytic and FTIR study over basic zeolites

Interconversion of unsaturated C4 nitriles under basic conditions II. Catalytic and FTIR study over basic zeolites

ELSEVIER Applied Catalysis A: General 146 (1996) 331-338 Interconversion of unsaturated C 4 nitriles under basic conditions II. Catalytic and FTIR...

499KB Sizes 0 Downloads 5 Views

ELSEVIER

Applied Catalysis

A: General 146 (1996) 331-338

Interconversion

of unsaturated C 4 nitriles under basic conditions II. Catalytic and FTIR study over basic zeolites

Attila B&es a, Zolth

K6nya a, Istvh Hannus a, hp6d In-u-e Kiricsi a’*

Molnhr b,

aApplied Chemistry Department, hksef Attila Uniuersity, Rerrich Be’la te’r 1, H-6720 Szeged, Hungary b Department of Organic Chemistry, J&sef Attila University, D6m te’r 8, H-6720 Szeged, Hungan Received 2 January

1996; revised 19 April 1996; accepted

13 May 1996

Abstract A study was made

unsaturated C, nitriles (ally1 cyanide, Na/NaY, and for comparison on neutral NaY and acidic CaY catalysts at 623 K. Pulse experiments indicated the facile transformation of ally1 cyanide to crotononitrile and the low reactivities of crotononitrile and methacrylonitrile. FTIR studies of the adsorbed molecules revealed the possible involvement of anionic intermediates. crotononitrile

of the interconversion

and methacrylonitrile)

Keywords: Zeolite; Basic catalysts; Carbanion

over

of isomeric

strongly

basic

C, nitriles; Double bond migration;

Nitrile migration;

FTIR spectroscopy:

1. Introduction Among the base-catalysed heterogeneous catalytic transformations of hydrocarbons and hydrocarbon derivatives, the isomerization of unsaturated C, nitriles is regarded as one of the simplest. These reactions occur via a carbanionic mechanism under both homogeneous and heterogeneous conditions [ 11. Kurokawa et al. recently reported the rearrangement of methactylonitrile over solid basic catalysts [2]. We have started a complex study of the transformations of all three unsaturated C, nitriles. In preliminary papers, we have reported on

* Corresponding

author. Tel./fax:

0926-860X/96/$15.00 Copyright PZZ SO926-860X(96)00177-9

( + 36-62) 321 522; e-mail: [email protected] 0 1996 Elsevier Science B.V. All rights reserved.

332

A. B&s

et al./Applied

Catalysis A: General 146 (1996) 331-338

the transformations of ally1 cyanide to crotononitrile and methacrylonitrile in solution with butyllithium as a base [3] and over a sodium cluster-containing NaY zeolite which possessed strong basic sites [4-61. The results of a detailed study of the transformations in the solution phase were discussed in Part I (Ref. [7]). In the present paper, we report the results of interconversion of unsaturated C, nitriles over solid basic catalysts. Particular attention was paid to the generation and nature of the carbanionic intermediates.

2. Experimental

2.1. Preparation

of catalysts, materials used

Two zeolite catalysts were used in this study. Nay-FAU originated from Union Carbide Co., and is designated a neutral sample. The second, a basic sample, was prepared by impregnating NaY with NaN, in methanolic solution via a procedure described elsewhere [8,9]. When this sample was heated to 723 K, where the introduced NaN, decomposed, a catalyst with very strong basic sites was obtained [ 10,111. In some selected experiments, a Cay-FAU sample (Linde) was used for comparison. The basicity of the samples was characterized by using the I-butene double bond isomerization reaction, in which the ratio of the cis- and trans-2-butene formed is related to the basicity of the catalyst [12]. Analytical grade ally1 cyanide, crotononitrile and methacrylonitrile from Aldrich were used without further purification. 2.2. Reactor experiments For catalytic experiments, a pulse reactor with on-line GC product analysis was applied. Generally 0.1 g of catalyst was loaded into the reactor and was slowly heated to 723 K while the carrier gas helium flow was adjusted to 0.6 l/h. After treatment at 723 K for 2 h, the reactor was cooled to 623 K and pulses of reactants (1 pl> were dosed, while the effluent of the reactor was analysed with a Carlo-Erba Mod GV GC, on a 10% Carbowax 1000 on Chromosorb column (at 363 K, 1.5 l/h helium). 2.3. IR spectroscopy For the IR spectroscopic study, a Mattson Genesis FTIR spectrometer was used with the IR cell as the reactor. Self-supporting wafers with a thickness of 10 mg/cm2 were pressed from the catalysts and activated in a procedure almost identical to that applied in the reactor experiments. The only difference was that the heat treatment of the catalysts was performed in vacuum instead of flowing

333

A. B&es et al. /Applied Catalysis A: General 146 (1996) 331-338

helium. After the wafer had cooled to ambient temperature, the spectrum of the activated sample was recorded. Next, the adsorbates were introduced into the cell and their gas-phase spectra were taken. The temperature of the wafer in the presence of reactants was then increased to 623 K and maintained for a preselected period, and both the wafer spectrum and the gas-phase spectrum of the reaction products were registered.

3. Results and discussion

3.1. Reactor experiments In pulse reactor experiments, the conversion of each reactant showed the general feature of a small decrease in conversion with increasing pulse number. However, after a few pulses the degree of conversion reached a constant level and subsequently remained steady for several additional pulses. 3.2. Reactions

over Na / Nay-FAU

catalyst

This catalyst sample is characterized by very strong basic sites. Ally1 cyanide the most reactive compound of the three nitriles undergoes double bond migration as the characteristic transformation to yield isomeric crotononitriles (isomeric ratio 60:40) and gives a small amount of methacrylonitrile as a result of the migration of the cyano group. Crotononitrile (an isomeric mixture comprising 65% cis and 35% trans isomer), in contrast, is the least reactive under identical conditions (18% conversion), due to the conjugation of the multiple bonds. The cis/ trans ratio changed slightly to 61:39 during the transformation. Methacrylonitrile can be transformed only through the migration of the cyan0 group. It yields only isomeric crotononitriles in a ratio 62:38. The product distributions are seen in Table 1.

Table 1 Product distributions (in mol.-%) in the transformation methacrylonitrile (MCN) over basic (Na/NaY-FAU), catalysts

of ally1 cyanide (ACN), crotononitrile (CCN) and neutral (Nay-FAU) and acidic (Cay-FAU) zeolite

Catalysts

Na/NaY-FAU

Reactants

ACN

CCN

7

7

60

_

_

46 31 7 9

50 32 4 7

21 13 _ 6

43 21 36

61 34 5 _

54 33 13 _

93

18

40

64

5

87

MCN cis-CCN tram-CCN ACN Unidentified Conversion

(%)

Nay-FAU MCN

ACN

CaY-FAU CCN

ACN

334

A. Be’res et al./Applied

3.3. Reactions over Nay-FAU

Catalysis A: General 146 (1996) 331-338

catalyst

This catalyst is regarded as a neutral sample catalyzing only the interconversion of ally1 cyanide and crotononitrile. The relative reactivity of the two compounds is very similar to that over the strongly basic Na/NaY-FAU catalyst, Neither compound yielded any methacrylonitrile. 3.4. Reaction over Cay-FAU For comparison, the reaction of ally1 cyanide was studied over an acidic Cay-FAU zeolite. The reactivity of ally1 cyanide is as high as in the presence of the other two catalysts and it yields the isomeric crotononitriles. The product distribution is included in Table 1. 3.5. IR spectroscopic

measurements

3.5.1. Na/NaY-FAU sample Fig. 1 shows the spectra registered in the course of the adsorption and reactions of unsaturated C, nitriles. Spectra denoted a,,, c0 and m, were taken after the adsorption of ally1 cyanide, crotononitrile and methacrylonitrile, respectively, at 298 K, and those denoted a, c and m after treatment at 623 K. Substantial changes are to be seen in both the CN and the C=C vibration regions. In the spectra recorded after adsorption of the nitriles at 298 K, the bands due to the CN and C = C bonds are seen to be shifted as compared to their positions in the gas phase. In general, the spectra of adsorbates after treatment at 298 K are simpler than those at 623 K. The predominant bands appeared at 2267, 223 1 and 2240 cm-’ when ally1 cyanide, crotononitrile and methacrylonitrile were adsorbed at room temperature. After treatment at 623 K, the most intense bands occurred at 2233 cm-’ for ally1 cyanide, remained at 2231 cm- ’ for crotononitrile and shifted to 2233 cm- ’ for methacrylonitrile. This treatment led to a common new band with different intensities at 2295 cm-’ for each substrate. A similar feature was found in the range of the C=C bond absorption where the common band appeared at 1608 cm- ’. From the spectral changes, in agreement with the result of GC measurements, we concluded that ally1 cyanide was the most reactive adsorbate of the C, nitriles studied. 3.5.2. Nay-FAU sample Unlike Na/NaY-FAU, this sample contains no strong basic sites and therefore the transformations taking place are expected to be slower. Fig. 2 depicts spectra taken after the adsorption of ally1 cyanide and crotononitrile under identical conditions as for the Na/NaY sample. Ally1 cyanide exhibits a band at 2267 cm- ‘, due to the CN fundamental vibration. Upon treatment at 623 K, this band shifted to lower wavenumbers, displaying a feature characteristic of

335

A. Bkres et al. /Applied Catalysis A: General 146 (1996) 331-338

B /

20

w

----i-l

x ? a

1

aa

2295

2270

Wavenumber

2245 (l/cm)

2220

1620

1420

1520 Wavenumber

(l/cm)

Fig. 1, IR spectra, in the CN (A) and C=C, CH (B) vibration ranges, of surface compounds formed from unsaturated C, nitriles on Na/NaY-FAU. Spectra denoted a,, cO and m, were taken after treatment of ally1 cyanide, crotononitrile and methacrylonitrile in the presence of the catalyst at 298 K for 1 h, while those denoted a, c and m after treatment at 623 K for 1 h.

crotononitrile, while only a small shoulder remained at 2267 cm-‘. This behavior suggests that the intermediate(s) are similar in the adsorption of ally1 cyanide and crotononitrile. The same conclusion was drawn from the feature observed in the region of C=C bond vibrations, i.e. the single band at 1644

A. B&es et al./Applied

336

2300

2250 Wavenumber

Catalysis A: General 146 (1996) 331-338

2200 (l/cm)

0

1000

1500

Wavenumber

1400 (l/cm)

Fig. 2. IR spectra of surface compounds in the CN (A) and C=C, CH (B) vibration ranges formed from ally1 cyanide and crotononitrile on NaY-FAU. Spectra denoted a, and c,, were taken after treatment at 298 K, for 1 h while those denoted a and c after treatment at 623 K for 1 h.

cm-’ shifts to lower wavenumbers and splits into two bands due to the cis and trans isomers of crotononitrile after treatment at 623 K. This observation proves that surface transformation (double bond migration) occurred. This assumption is supported by the spectrum registered after the adsorption of crotononitrile. Crotononitrile adsorbed at room temperature exhibits bands at 2239 and 2225

A. B.&es et al. /Applied Catalysis A: General I46 (1996) 331-338

331

and a small shoulder at 2267 cm-‘, revealing the presence of cis and tram isomers and the formation of some ally1 cyanide. Treatment at 623 K resulted in an increase in intensity of the band at 2267 cm-‘, reflecting the enhanced formation of an ally1 cyanide-like surface intermediate. Identical changes were observed in the C=C stretching range. Besides the doublet, a shoulder attributed to ally1 cyanide formed on the surface appeared at 1644 cm-‘. The similarity of the features of the doublet due to cis and tram isomers in the two cases is noteworthy. The most obvious experimental evidence of the surface transformation is seen in the region of CH bond vibrations at around 1380 cm-l. It appears that the transformations of C, nitriles in the presence of strong bases are similar, but not necessarily identical to the reactions in the solution phase, due to the lack of solvation effects for solid basic catalysts. On solids, the molecules first adsorbed can convert to other adsorbed molecules or isomers and react with the adsorbates, leading to the formation of dimers, etc. In solution, the possibility of the molecules taking part in such transformations is limited, since solvent molecules isolate the intermediates, hindering their further transformations. The observed spectral changes, therefore, should not be identical. The spectra of the adsorbed phase are characterized by smaller shifts as compared with the spectra in the solution phase. This indicates that interactions between surface sites and intermediates in the adsorbed phase are different from those in the solution phase. The reactivities of the three different C, nitriles vary in the sequence ally1 cyanide > crotononitrile > methacrylonitrile. From ally1 cyanide, both crotononitrile and methacrylonitrile are formed. From methacrylonitrile, only isomeric crotononitriles and unidentified side-products are formed over the strongly basic catalyst. No transformation of methacrylonitrile is observed over the less basic NaY catalyst. These differences are easily rationalized in terms of molecular structure and the energetics of the transformations taking place. Because of the conjugation of the two multiple bonds, crotononitrile and methacrilonitrile are more stable than ally1 cyanide. The difficulty of CN migration (which requires carbon-carbon bond breaking) as compared with the facile nature of double bond migration explains the high reactivity of ally1 cyanide and the low reactivity of methacrylonitrile. The spectral changes are more complex than those observed in the reactor experiments. In Part I (Ref. [7]), we proposed and experimentally identified the anionic intermediates in the solution phase. Here, for the solid catalyst also a common band appeared at 2295 cm-’ in the CN vibration range upon the adsorption of each substrate. In addition, as Fig. 1B shows, a band at 1608 cm-’ was found as common absorption in the C=C region. It follows from this that these absorptions may be characteristic of the common, probably anionic surface intermediate that we are looking for.

cm-’

338

A. B&s

et al/Applied

Catalysis A: General 146 (1996) 331-338

4. Conclusions Isomer-k unsaturated C, nitriles are interconverted over a strongly basic Na/NaY-FAU catalyst. In contrast, the neutral Nay-FAU zeolite promotes only double bond migration, i.e. interconversion of ally1 cyanide and crotononitrile. The appearance of common bands in the CN and C=C absorption regions (2295 cm-’ and 1608 cm- ‘, respectively) strongly suggests the involvement of a common anionic intermediate, which can be identified and characterized by these absorptions.

Acknowledgements Financial support from National Research Foundation of Hungary (OTKA No. T/016941) is gratefully acknowledged.

References [l] H. Pines and W.M. Stalick, Base-Catalyzed Ractions of Hydrocarbons and Related Compounds, Academic Press, New York, 1977, Chap. 2, p. 99. 121 H. Kurokawa, S. Nakamura, W. Ueda, Y. Morikawa and T. Ikawa, J. Catal., 141 (1993) 94. 131 Z. K6nya. I. Hannus, A. Molnar and I. Kiricsi, Mikrochim. Acta, 1996, accepted. [4] I. Hannus, H. FGrster, Gy. Tasi, I. Kiricsi and A. Molttar, J. Mol. Struct., 348 (1995) 345. [5] A. B&es, Z. K6nya, I. Hannus, A. Moln&r and I. Kiricsi, Book of Abstracts, ZEOCAT’95, Int. Symp. Catal. Micropor. Mater., Szombathely, Hungary, 1995. [6] I. Hannus, I. Kiricsi, A. B&es, J. B.Nagy and H. Wrster, Stud. Surf. Sci. Catal., 98 (1995) 81. [7] Z. K6nya, I. Hannus, A. Mom&r and I. Kiricsi, Appl. Catal. A, 146 (1996) 323. [8] I. Kiricsi, I. Hannus, A. Kiss and P. Fejes, Zeolites, 2 (1982) 247. [9] L.R.M. Martens, P.J. Grobet and P.A. Jacobs, Nature, 315 (1985) 568. [lo] L.R.M. Martens, P.J. Grobet, J.M. Vermeiren and P.A. Jacobs, Stud. Surf. Sci. Catal., 28 (1986) 935. [ill A. B&es, I. Hannus and I. Kiricsi, React. Kinet. Catal. L&t., 56 (1995) 55. [12] W. 0. Parker, Jr., Stud. Surf. Sci. Catal., 94 (1995) 568.