The importance of short range interactions in the development of acidic and catalytic properties of H, Na - MoR

Catalysis Today, 5 (1989) 213-222 Elsevier Science Publishers B.V., Amsterdam

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THE IMPORTANCE OF SHORT RANGE INTERACTIONS IN THE DEVELOPMENT OF ACIDIC AND CATALYTIC PROPERTIES OF H, Na - MoR

I. BANKOS (x), J. VALYON, D. KALLO Central Research Institute for Chemistry of the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary

ABSTRACT Adsorption microcalorimetry and lR spectroscopy are used to study the nature, number, strength and accessibility of acid sites in H, Na-MoR samples, ion exchanged to different degrees. The differential molar adsorption heat and the number of ammonia molecules adsorbed are considered as a measure of the acid strength and of the number of acid sites, respectively. Sanderson electronegativity model allows to predict the trends of the variation of the average acid strength with the chemical composition. The average differential heat of NH3 adsorption (q) and the stretching frequency of OH groups (v) are in linear correlation with the intermediate electronegativity (Sint.). Catalytic activities for the isomerization and disproportionation of m-xylene are in better and closer correlation with the number of acid sites of different strength and type than with the intermediate electronegativity (Sint.).

INTRODUCTION The acidity of zeolites is widely investigated since their catalytic applications are based mainly on their acidic properties. Numerous methods, such as titrimetric, thermoanalytical and spectroscopic studies have been developed for this purpose. In this respect IR spectroscopy is of special importance [l]. However, there is no single method which could be used to characterize acidity in all respects i.e. , which would reveal at the same time the nature, number, strength, location and environment of acid sites [2]. For example, IR spectroscopic results reveal the nature of sites [l, 31 ; however it is rather difficult to quantify the data and to find a direct correlation (X) To

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correspondence

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addressed.

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with the catalytic behaviour. The measurement of adsorption heat of bases seems to be a more appropriate method for the quantitative determinatoin of the number of acid sites of different strength [4,5]. IR spectroscopy and calorimetry are used in the present work to characterize the acidity of H, Na-MoR, ion exchanged to different degrees. The acidity of the samples is correlated with their catalytic activity and selectivity for isomerization and disproportionation of m-xylene. The application of Sanderson electronegativity theory for zeolites allows to predict the average acid strength from the chemical composition only [6, 71. Experimental results are correlated with the data calculated by the Sanderson electronegativity principle.

EXPERIMENTAL Cations of Na-mordenite (“Zeolon loo”, Norton Co. , Massachusetts, USA) were ion exchanged for ammonium to 33,46, 83 and 98 % with NH4Cl solutions and activated at 823 K for 8 h in an in situ flow of hydrogen before catalytic measurements. Deammoniated samples for mordenite were thus prepared. The samples are noted M-33, M-46, M-83 and M-98. IR spectra were determined with a Perk&Elmer 577 spectrophotometer in the range of 1000-4000 cm-l. The samples were pressed without binder into wafers having a “thickness” of about 10 mg/cm2. Differential molar heat of adsorption of NH3 was measured by a Calvettype microcalorimeter. A volumetric vacuum apparatus connected with the calorimeter allows to determine the adsorbed amounts of ammonia. Samples were activated at 753 K for 40 h at 10-3 Pa. The calorimetric measurements were carried out at 573 K. The experimental details are describrd in [8]. Transformations of m-xylene were performed in an integral flow reactor at 573 K, at space velocities between 2.5 and 13.9 h-1, at hydrocarbon and hydrogen partial pressures of 0.5 MPa and 1.5 MPa, respectively. Gaseous and liquid products were analyzed with a JEOL 8 10 gas chromatograph. RESULTS The stretching frequency of acidic hydroxyl groups (v) and the average differential molar heat of NH3 adsorption (q) for all the sites are plotted against the intermediate electronegativity (Sint.) and the partial charge of hydrogen @Icharge) calculated in terms of Sanderson’s theory for the different H, Na-MoR samples (Figure 1).

215

DEGREE

Figure 1

30

40

6.11

0.12

OF ION

50

0.13

60

0.14

EXCHANGE,% 70

80

0.15

90

0.16

100

0.i7

upper scale : ELECTRONEGATIVITY

(Sint. )

lower

(Hdawe)

scale:

HYDROGEN

CHARGE

Variation of the frequency of acidic OH groups (v) and of the average differential

heat of NH3 adsorption (q) as a function of the

degree of ion-exchange, of the intermediate electronegativity (Sint.) and of the hydrogen charge (Hcharge) on H, Na-MoR samples.

The q values are weighted averages calculated from the plots of differential heat of NH3 adsorption vs. adsorbed amount (Figure 2). Experimental characteristics and those calculated from the chemical composition show linear correlation. The heat of adsorption decreases with increasing coverage for each sample indicating the energetic heterogeneity of the sorption sites (Figure 2). Some steps can be clearly distinguished on the curves. These steps correspond to groups of acid sites of different strength. On increasing the degree of ion exchange not only the number but also the acid strength of sites increase. The heat of adsorption of NH3 on Na-mordenite is around 80 W.mol-1 [8, 91. In this study we take into consideration only acid sites which bind NH3 with a heat of adsorption greater than 110 kJ.mol-1. Adsorption with heats between

Figure

50

2

1.5

between

110-160 kJ.mol-1.

of acid sites in lO2O.g1

out at 573 K, after a pretreatment

was carried

The columns of the table are : l- sample ; 2- number

M-46 t), M-33 (0). Adsorption

with ammonia

adsorption

heat

on M-98 (A), M-83 (o), at 7.53 K for 40 h.

amount

as a function

of ammonia

of the adsorbed

2.0

heat of adsorption

Differential

1.0

AMOUNT OF NH, ADSORBED (mmolg”)

0.5

217

1 lo-160 kJ.mol-1 is attributed to interactions with Brijnsted acid sites, because the amount of these sites increases with increasing degree of ion exchange (see column 2 of Table in Figure 2) ; furthermore, when increasing the pretreatment temperature, the number of the above sites is found to pass trough a maximum [lo, 111. Thus, with increasing pretreatment temperature the number of acidic OH groups formed by deammoniation increases: however, at higher temperatures dehydroxylation proceeds, resulting in a destruction of Brijnsted sites. Acid sites with adsorption heats of about 95-100 kJ.mol-1 and above 170 kJ.mol-1 can be attributed to the adsorption on Lewis acid centres [9-121. The intensity of the OH band around 3600 cm-l decreases upon adsorption of pyridine (Figure 3.A) whereas this band disappears completely upon adsorption of ammonia (5.32 kPa), (Figure 3.B). It should also be mentioned that the adsorption of pyridine shifts this OH band towards lower frequencies, whereas the adsorption of NH3 shifts the band towards higher frequencies. The isomerization (I) and disproportionation (D) of m-xylene, the changes in the number of acid sites of three groups [ 131 and the calculated values for characterization of the acid strength (Hcharge, Sint.) are shown as a function of ion exchange degree in Figure 4.A, B, and C respectively. Comparing Figure 4.A and C, it seems that only a qualitative relationship can be presented between the calculated mean acidity and the catalytic activity : a higher conversion of mxylene corresponds to a higher average acid strength. Comparison of Figure 4.A and B substantiates that there is a close connection between the number of acid sites of different strength and the isomerization (I) and of disproportionation (D) conversion of m-xylene, i.e. isomerization proceeds predominantly on weaker and medium Brijnsted acid sites (llO
DISCUSSION The average electronegativity (Sint.) and the hydrogen charge (Hcharge) calculated according to Sanderson’s model can be directly related to the acid strength of compounds. These theoretical values of the acid strength are in linear

218

correlation with those derived experimentally from IR and calorimetric measurements (see Fig. 1) which provide average values of the acid strength of the samples. On increasing the degree of ion exchange the experimental and theoretical average acid strength increases linearly, whereas the stretching frequency of hydroxyl groups decreases: however, these data usually only predict the trend of the change of the catalytic activity in acid catalysed reactions. As a matter of fact calculated Sint. and Hcharge values might not give informations about the energetic heterogeneity of acid sites contrary to differential adsorption calorimetry which presents the distribution of acid strengths (Figure 2). Furthermore catalytic properties are determined also by the accessibility of acid sites, which can be obtained, for instance, by IR spectroscopy.

/2

I’

4000

3500

WAVENUMBER Figure

3

Infrared

spectra

of hydroxyl

(cm-l ) groups

at 823 K for 7 h, [2] after pyridine kPa ; after ammonia kPa

141, 0.65 kPa

adsorption

on M-98 adsorption

[l] sample

evacuated

at 573 K and 1.33

at 573 K and 0.07 kPa [3], 0.39

[S] and 5.32 kPa

[6].

219

Figure 3 shows that a fraction of Briinsted sites is not accessible to pyridine, and thus not to m-xylene either. The decrease in the frequency of the OH band after pyridine adsorption indicates that centres in the narrow channels being not accessible for pyridine are stronger Briinsted acid sites than those accessible for pyridine. All the Briinsted sites can be poisoned by NH3 (Figure 3.B). Ammonia binds first to the stronger thereafter to the weaker acid sites. Therefore, the band around 3600 cm-l is shifted gradually toward higher and higher frequencies. The limits of application of the Sanderson model to predict catalytic properties are clearly seen from the plots shown in Figure 4. Only the number of acid centres of a definite type, as determined by calorimetry, can be directly correlated to the isomerization ans disproportionation of m-xylene. Taking into account that the strongest Bronsted acid sites are not accessible for m-xylene (see Figure 3.A) Bronsted acid sites of weak and medium-strength (110 < q ~150 kI/mole) and strong Lewis acid sites (170 < q ~185 kI/mole)should be considered for the above reactions. The linear correlation between the initial rates of mxylene disproportionation and the number of strong Lewis acid sites substantiates the decisive role of these centres in the reaction. We could not find such a strong correlation between the number of Brijnsted sites of weak and medium-strength and the initial rates of m-xylene isomerization. This may indicate that isomerization proceeds not only on the above sites but probably on strong Lewis centres too, or that the isomerization activity is at least influenced by the presence of the Lewis sites. Sanderson’s electronegativity depends on the chemical composition only and is independent of the structure. The Sanderson theory takes into account a very large number of atoms, therefore, informs about the average acid strength only. In transformations which proceed on acid sites of definite strength and type the acidity deduced by this macroscopic approach cannot correlate well with catalytic activity and selectivity. Differential calorimetry which can detect the short range effects on the acid sites was found to be very effective method in getting knowledge of catalytically active sites.

ACKNOWLEDGEMENT The authors wish to express their grateful acknowledgement to Dr. A.L. Klyachko and his co-workers (N.D. Zelinsky Institute of Organic Chemistry of USSR Academy of Sciences, Moscow, USSR) for careful calorimetric measurements and helpful discussions.

Figure 4

Cl

C

0.11

[I]

of m-xylene.

and disproportionation

isomei-ization (D)

B:

(0) (j (x)

110 e q c 150 kJ/mole-1 150 c q < 170 kJ/mole-1 170 c q c 185 kJ/mole-1

number of acid sites.

DEGREE OF ION EXCHANGE,%

70

5

r2 a

8 0.1%

0.15

x g

3 8 O-l7

I”

9

I

C:

4.2 t

ion exchange.

function of degree of

1.5 MPa hydrogen, samples pretreatment at 823 K.

as a

positive charge of hydrogen and electronegativity

Adsorption temperature : 573 K ; reaction temperature : 573 K ; partial pressures : 0.5 MPa m-xylene,

A :

/O

150-170

0

Ppdy/r /

30

0

I

I

I

/

110-150

B

h

221

8 ._

222

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