Reaction of ammonia with surface species formed from acetone on a HZSM-5 zeolite

Reaction of ammonia with surface species formed from acetone on a HZSM-5 zeolite Z. Dolej~ek, J. Nov:ikov~i, V. Bos~i~ek, and L. Kubelkov~i

j. Heyrovsk~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia

Temperature-programmed conversion of preadsorbed acetone on HZSM-5 zeolite was studied using the mass spectrometric detection of the gaseous products, The effect of addition of ammonia to the surface complexes formed from acetone at mild temperatures on the composition of the products was investigated. The surface species, both with and without ammonia addition, were also characterized using 13C cross-polarization magic angle spinning nuclear magnetic resonance. It was found that the added ammonia reacts with the ketonic group of the surface intermediates and substantially affects the composition of the gaseous products. Keywords: Surface species from acetone on HZSM-5; reaction with ammonia

INTRODUCTION The acetone conversion over H forms of zeolites yields a wide scale of interesting products whose composition strongly depends on the properties of the individual zeolites.Z-s The reaction is assumed to proceed via a series of condensation, de(re)hydration, cracking, and aromatization steps. Infrared and n.m.r, studies 4-v revealed that the character of the surface species was changed when subjected to heating. At 150°C, a weakly adsorbed condensed species was found 4'7 that interacted with the zeolite bridging OH groups and changed between 150250°C into a complex of unsaturated cyclic ketones substituting skeletal hydroxyls, firmly bound to the zeolite oxygen. 4'6 This work was carried out to study the reactivity of the surface species with ammonia at 150°C, using temperature-programmed conversion (t.p.c.) of preadsorbed acetone. The gaseous products were analyzed mass spectrometrically, and the surface complexes were investigated using 13C crosspolarization magic angle spinning nuclear magnetic resonance (13C CP MAS n.m.r.).

EXPERIMENTAL NaZSM-5 zeolite (Si/A1 = 13.5) was supplied by the Research Institute for Oil and Hydrocarbon Gases, Czechoslovakia, and transformed into the acid form by acid leaching. Prior to the t.p.c., the sample (0.1 g) was treated in a vacuum of 10 -4 Pa at 400°C. Then, Address reprint requests to Dr. Dolej~ek at the J. Heyrovsk# Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Science, Dolej~kova 3, 182 23 Prague 8, Czechoslovakia. Received 23 April 1990; accepted 25 June 1990 © 1991 Butterworth-Heinemann

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ZEOLITE& 1991, Vol 11, March

0.2 mmol g-= of acetone (corresponding to 20% of the number of structural hydroxyls) was adsorbed onto the zeolite at room temperature for 30 rain and the gaseous conversion products were desorbed using linear heating (8°C min -Z) into the vacuum system of a Finnigan 400 quadrupole mass spectrometer. The mass spectra were recorded in 20-30°C steps. In the experiments in which the surface species were subjected to the reaction with ammonia, the temperature increase was interruped and 0.4 mmol g-l of ammonia was added and left to react for 30 rain at constant temperature. The t.p.c, was then resumed. The same procedure was performed in a "blank" experiment in which no ammonia was added during the same constant temperature interval. The effect of the amount of ammonia added (0.1-0.6 mmol g-l) was also checked and compared ,kith the desorption of the same amount of ammonia from the zeolite without preadsorbed acetone. Two labeled reactants were employed: acetone labeled with 13C in the carbonyl group (99.3 at%, supplied by MSD, Div. Merck Frost, Canada) and 15NH3. I'~NH3 was prepared by the deammonization of 15NH.I-KA zeolite after its dehydration. Labeled ammonia was introduced into the KA zeolite by ion exchange with 15NH4NO3 (96.8 at%, supplied by VEB, Germany). The samples for the n.m.r, measurements were prepared in the same way as those for the t.p.c.i.e., 0.2 mmol g- ~ of acetone-2-1SC was adsorbed at room temperature, heated to 150°C, and then either sealed off or left to react with ammonia (0.4 mmol g-~, sometimes also 0.1 mmol g-l) and sealed off after. The ampules were broken in an inert atmosphere and samples filled into the ceramic rotors of a Bruker-200 n.m.r, spectrometer. The 13C CP MAS

Ammonia and acetone surface species on HZSM-5: Z. Dolej#ek et al.

{ =

i

~o

Figure 1 T.p.c. of preadsorbed acetone; effect of ammonia added at 150°C. Normalized products (with respect to fragmentation and ionization cross sections) without (A) and with (B) ammonia addition; acetone preadsorbed: 0.2 mmol g - l ; ammonia added: 0.4 mmol g-~. ( ) IB; (xxxx) propylene; ( ...... ) acetone; (. . . . ) CO2; ( . . . . ) aromatics C7-9; ( ~ ) allene; ( ~ ) acetonitrile

released without a pronounced maximum above 100°C and is not depicted because of the complicating factor of water in the MS background. The ion intensities were normalized with respect to fragmentation and ionization cross sections. It can be seen that the addition of ammonia substantially changed the composition of the products evolved during the resumed t.p.c. The lower temperature peak is completely missing, IB predominates in the higher temperature region, acetone and COz do not appear, much less aromatics and less p r o p y l e n e are released, more allene (methylacetylene) is evolved, and a new product, acetonitrile, is formed. The composition of the aromatic compounds is also changed, as can be seen in Figure 2: Without anamonia, the yield of C7, C8, and C,j aromatics is not very different, while with ammonia, the fraction of C8 and C,j compounds is much lower than that of toluene. This effect can be a resuh of poisoning of the acid centers with ammonia so that the acid-catalyzed condensation is restricted. The creation of C8_,~ aromatics seems to occur by subsequent alkylation, which is also supressed by poisoning of the acid centers. The consumption of ammonia by the surface species predominated the reaction with the surface hydroxyl groups. The desorption of anmlonia from the hydroxyls could be observed only with the doses of ammonia that were similar or higher than those of the preadsorbed acetone. The effect of ammonia addition on the yield of aromatics also appeared only if higher doses had been used. The reaction of anamonia with the surface species fi'om acetone is clearly visible in the l:~C CP MAS n.m.r, spectra

n.m.r, spectra were recorded at 50.32 MHz in a frequency range of 20 kHz. The cross-polarization experiment was carried out with a contact pulse of 2 ms and with an excitation FI/2 pulse of 5 ~ts. A repetition time of 4 s was used for accumulation of a satisfactory number of fid's. The spinning rate of 4.8 kHz was usually employed. RESULTS AND DISCUSSION

The desorption of the products of converted preadsorbed acetone starts under the above-mentioned experimental conditions at the temperature higher than 150°C. For that reason, the reaction of ammonia with the surface species was studied at 150°C just before release of the first products. Figure I shows the composition of products during the t.p.c, after the constant temperature pause (denoted by arrows in Figure IA and B), in a "blank" experiment without ammonia (A), and with the ammonia addition (B). The most abundant gaseous products were calculated u s i n g typical ions in the mass spectra: m/z = 58 for acetone, 56 for isobutene (IB), 44 for CO,_,, 42 for propylene, and 40 for allene, and Z 92 + 106 + 120 for C7, C8, and C9 aromatics, respectively. Small amounts of other aliphatic and alicyclic compounds released during the t.p.c, are not shown. Water was

200

4()0

' °C

Figure 2 C7-C9 aromates released during the t.p.c, of ammonia. Without ammonia (0.2 mmol g 1 of acetone): full lines, (O) C7; (l~) C8; (FI) C9; with ammonia added at 150°C (0.4 mmol g 1 of ammonia): dashed lines, (0) C7); (&) C8; ( I ) C9; peaks at m/z 92, 106, and 120 taken as C7, C8, and C9 aromatics, respectively

ZEOLITE& 1991, Vol 11, March

245

Ammonia and acetone surface species on HZSM-5: Z. Dolej#ek et ai.

[ 223.3

t

. 1211.3

200.0

III: Decomposition to I B (J:SC labeling demonstrated the composition of three C atoms from one acetone molecule and one C atom fi'om the methyl group of another acetonem), H20, acetone, and CO2: the latter species are probably formed via the reaction of two acetatelike species. IV: A partial release of IB and acetone into the gas phase; IB also reacts to form other aliphatic compounds (C2.3.5_7) as well as alicyclic and aromatic compounds (Cv-9); acetone partially reenters the new cycle; COe is released. V: A small fraction of acetone is dehydrated to allene (dehydration is the predominant primary reaction of higher ketones4). CH3 -.

+ / C = OH...lO~eo~ CH3 " ""

b

2112J9

I O II

CH3 -. C=CH-C

/

- C H 3 + 2HO~eoj + H20 N

CH,3 / II

] 23.9

c

CH~ \ /

CH3

300

200

!(}0

pb rn

Figure 3 ~3C CP MAS n.m.r spectra; effect of ammonia added at 150°C; 0.2 mmol g-~ of acetone preadsorbed and heated to 150°C: (a) no ammonia addition, NS (number of scans) 1600; (b) 0.1 mmol g-~ of ammonia added, NS = 1088; (c) 0.4 mmol g-1 of ammonia added, NS = 3200; (*) spinning side bands

(Figure 3): spectrum (a) corresponds to the species created from acetone in the absence of ammonia and is typical for the protonated carbonyl group 9 (signal at 223.3 ppm) and methyl group (28.3 ppm, less intensive due to the ~3C enrichment in the parent carbonyl group alone). After the reaction with ammonia (NHJpreadsorbed acetone ratio = 2: 1), the signal at 223.3 ppm disappears and a new signal at 200 ppm is formed with the simultaneous shift of the signal of the -CHa group to 23 ppm (spectrum c). We assigned this spectrum to the protonated form of dimethylimine in accordance with the published data. 9 Using the NH3/preadsorbed acetone ratio = 0.5, it can be seen that approximately one-half of the carbonyl group is converted to the imino group (spectrum b). The composition of the products, depicted in Figure IA, can correspond to the following set of reactions (Scheme I): I: Protonization of acetone (which follows from spectrum a, Figure 3). II: Condensation and dehydration to the mesityloxidelike species 4 (not detectable u n d e r our experimental conditions by 13C CP MAS n.m.r.).

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ZEOLITES, 1991, Vol 11, March

O t II C =CH-CH! OH

CH3

--+ 2 IB + (CHa)CO + C O 2 Jr- H2O 2 III IV

IB gas phase

(CH3).CO C2.a.5-7 gas C 7 - 9 phase

new cycle

CO2

Ho

gas phase

gas phase

CHa V

+ " C = O H . . . IO,.~oL CH3 " "

--+ C3H4 + H20 + HOz'~ol N

SCHEME 1 The addition of ammonia, which changes the product composition (Figure IB), reacts with the protonized acetone (spectrum b and c in Figure 3) and most probably also with the mesityloxidelike species (Scheme 2): I': To an iminolike species (a Schiff-base). II': A condensed species of mesityloxide character (not detected by I'~C CP MAS n.m.r, under our experimental conditions similarly to species II). III': Species I' (and II') is more stable than is species I (and II) and is decomposed above 250°C to IB, ammonia, and acetonitrile. IV': Subsequent reactions of IB are supressed most

Ammonia and acetone surface species on HZSM-5: Z. Dolej#ek et al.

i,

II'

CH3 ~ + / H /" ... C = N ,. . . . I Ozeol CH~ H " + NH2 II CH3 ,.C = CH -CCH3 CH3

p r o b a b l y d u e to p o i s o n i n g o f acid active c e n t e r s by a m m o n i a ( a c e t o n e a n d CO,~ a r e n o t f o r m e d as the k e t o n i c g r o u p s have r e a c t e d with a m m o n i a ) • V ' : T h e d e h y d r a t i o n o f species I ' to a l l e n e is m o r e effective t h a n that o f species I (Figure IB). T h e r e a c t i o n o f a m m o n i a with the s u r f a c e i n t e r m e d i a t e s o f a c e t o n e t r a n s f o r m a t i o n c a n t h u s assist in e l u c i d a t i o n o f the a c e t o n e r e a c t i o n pathway.

/

I Ozeol \

CH~ III'

"/" C =

2

REFERENCES

H ../

CH3

H

• • • I O,.col " /

--~ IB + N H 3 + C H 3 C N + 2 HOzcol N

CH3 /" C

= CH

+ NH2 II - C -

CH3

CH3 •

f

1%eol

•

IB + C H 3 C N + H O ~ o l IV'

0 CH3

V'

+ ..H " C = N CH3 H

1 Chang, C.D. and Silvestri, A.J.J. Catal. 1977, 47, 249 2 Chang, C.D., Lang, W.H. and Bell, W.K., in Catalysis of Organic Reactions (Ed• W.R• Moser) 1981, p. 73 3 Servotte, Y., Jacobs, J. and Jacobs, P.A. in Proceedings of the International Symposium on Zeolite Catalysis Si6fok, Hungary, 1985, Acta Physica et Chemica Szegediensis, Szeged, 1985, p. 609 4 Novfikovfi, J., Kubelkovfi, L•, Jir~, P., Beran, S• and Nedomovfi, K., in Proceedings of the International Symposium on Zeolite Catalysis, Si6fok, Hungary, 1985, Acta Physica et Chemica Szergediensis, Szeged, 1985, p. 561 5 Nov&kovfi, J., Kubelkovfi, L. and Dolej~ek, Z. J. Mol. Catal. 1987, 39, 195 6 Kubelkovfi, L., Cejka, J., Novfikovfi, J., Bosfi6ek, L., Jirka, I., and Jiru•, in Zeolites, Facts, Figures, Future (Eds. P.A. Jacobs, and R.A. van Santen) Elsevier, Amsterdam, 1989, p. 1203 7 Bosfi6ek, V. and Kubelkov& L. Zeolites 1990, 10 64; Bosfi6ek, V., unpublished 8 Salvapati, G.S., Ramanamurty, K.V. and Janardanarao, M. J. Mol. Catal. 1989, 54, 9 9 0 l a h , G•A• and Donovan, D.J.J. Org. Chem. 1978, 43, 860 10 Novfikovfi, J. and Kubelkovfi, L., submitted

.. . . I O~ol "

o

C3H4 + N H ~ + HO~'~ol

SCHEME 2

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247