HZSM-5 catalysts

Journal

of Molecular

Catalysis,

70 (1991) 11l-l 17

111

IR study of the active sites formed by H2 treatment of Ga/HZSM-5 catalysts V. Kanazirev, Institute

R. Dimitrova

of Organic Chemistry,

Bulgan’an

Academy

of Sciences,

Ill3

Sofia

(Bulgaria)

G. L. Price Department of Chemical 70803 (U.S.A.)

A. Yu. Khodakov, N, D. Zelinsky Moscow

Institute (U.S.S.R.)

Engineering,

Louisiana

L. M. Kustov of Organic

State

University,

Baton

Rouge,

LA

and V. B. Kazansky

Chemistry,

Academy

of Sciences

of the U.S.S.R.,

(Received March 3, 1991; accepted May 29, 1991)

Abstract Results obtained by IR spectroscopy using carbon monoxide as a probe molecule provide evidence for the formation of a new Ga active site after the hydrogen treatment of Ga/ HZSM-5 catalysts prepared by different techniques such as ball-milling, impregnation and substitution in the zeolite framework. A band at 2155 cm-’ arising after CO adsorption on reduced samples is assigned to a Ga+ -CO complex localized mainly in the interior of the zeolite crystallites.

Introduction Since the announcement of the Cyclar process for light paraffin aromatization [ 1 ] there has been a steadily growing interest in the investigation of the nature of the active sites and the mechanism of the catalytic action of ZSM-5 zeolites modified with gallium. A number of preparative techniques such as ion exchange [ 21, impregnation [ 3 1, substitution in the zeolite framework [ 41 and even mechanical mixtures [5] have been shown to be suitable procedures for the introduction of Ga into ZSM-6 zeolites. Moreover, the different preparative techniques generally produce catalysts with similar catalytic properties [ 6 ] . Considering that all of these preparative techniques can conceivably generate an intimate mixture of a gallium oxide phase with the zeolite, several authors have proposed a bifunctional (acid/gallium oxide) nature of the resulting catalysts (5, 7, S]. Le Van Mao et al. 19, 1O] have argued in favor of a theory based upon hydrogen back-spillover to explain the aromatization activity of gallium-containing zeolites, and have specifically excluded the possibility of gallium transfer into the zeolite host. Recent evidence, however, suggests that the gallium oxide phase interacts strongly with the acidic zeolite

0304-5102/91/$3.50

0 1991 - Elsevier Sequoia, Lausanne

112

in the presence of H, or under the operating conditions of light paraffin aromatization [ 11, 121. This interaction probably leads to the transfer of gallium species into the HZSM-5 zeolite and a simultaneous decrease in the content of zeolitic -OH acidic groups [ 13, 141. In the present paper, we report further IR investigations confirming that hydrogen treatment induces a process of gallium transfer into cationic positions of the HZSM-5 zeolite irrespective of the gallium catalyst preparative technique.

Experimental The samples used in this study are listed in Table 1 and described as follows: l HZSM-5 (Si02/A1203 = 25), an experimental Linde ELZ-105-6 zeolite. This sample is termed HZ1 . l HZSM-5 and NaZSM-5 (SiOJAlaOa = 38) zeolites, prepared by Leuna Werke (Germany), termed HZ2 and NaZ2. l two gallium catalysts containing 2 or 5 wt.% Ga, prepared by ball-milling P-Ga203 with HZ1 as described in [12], termed 2GaHZl and 5GaHZ1, respectively. l three gallium-containing samples prepared by the impregnation of aqueous gallium nitrate onto HZ2 (2% Ga), HZ2 (5% Ga) and NaZ2 (5% Ga) as described in [6] and termed 2GaHZ2, 5GaHZ2 and 5GaNaZ2, respectively. l P-Ga203 (HN5-grade, Ingal International). l A crystalline gallosilicate isostructural with ZSM-5 termed HGaL. Synthesis and characterization of this zeolite have been reported previously [15]. The diffuse reflectance IR spectra were measured on a Per-kin-Elmer 580 spectrophotometer using a home-made DR attachment [ 161. Before the spectral measurements, the samples were heated in vacuum at 840 K for 3 h. Room temperature adsorption of CO as a probe molecule was used. Sample TABLE

1

Catalyst samples Abbreviation

SiO+l,O,

Cation

Ga (wt.%)

Ga loading technique

HZ1 HZ2 NaZ2 2GaHZl 5GaHZl 2GaHZ2 5GaHZ2 5GaNaZ2 HGaL’

25 38 38 25 25 38 38 38 68b

H+ H+ H+ H+ H+ H+ H+ Na+ H+

0 0 0 2 5 2 5 5 4

ball-milled ball-milled impregnation impregnation impregnation framework

?l’his sample also contains extra-framework bSiOz/G+03 instead of SiO,/AI,O,.

G%03; see

[ 151.

113

reduction was accomplished in a static reactor under a hydrogen pressure of 6.5 kPa or in a flow reactor with HJHe mixture (130 cm3 mir-’ He and 30 cm3 min- ’ H2) at 840 K for 2 h. Results

and discussion

The spectra of CO adsorbed on HZl, the ball-milled samples and pGazO, are shown in Fig. 1. For the unreduced samples, three bands at 2230, 2195 and 2175 cm-’ are observed on HZl. The band at 2175 cm-’ is assigned to CO molecules adsorbed on bridged hydroxyl groups of the zeolite framework or residual Na’ ions, and the bands at 2230 and 2195 cm-’ are related to the interaction between CO and electron-accepting sites of the zeolite [17, 181. The reduced HZ1 spectrum is virtually the same as the unreduced spectrum. The spectra of both reduced and unreduced P-Ga203 show only a band at 2230 cm-‘, which we assign to the interaction between CO and the Ga3+ ions on the surface of P-Ga203 which are incompletely coordinated crystallographically. The spectra of CO adsorbed on unreduced 2GaHZl and unreduced 5GaHZl are almost identical to that of HZ1 (Fig. l), and it is reasonable to assume that the intensity of the band at 2230 cm-’ from the ball-milled zeolites is the sum of the adsorption of CO molecules on the electronaccepting sites of HZ1 and on the P-Ga203 phase. Reduced

Unreduced

2d30 Fig. 1. IFi spectra

2;30

of CO adsorbed H, reduction at 840 K (P(CO)=3.9

on HZl, 2GaHZ1, 5GaHZl and /3-G+0, before and after kPa). Wavenumbers in cm-‘.

114

The spectra of CO adsorbed on HGaL, HZ2 and the impregnated samples are presented in Fig. 2. For the unreduced samples, the interaction of CO and the electron-accepting sites is characterized by a band at 2190 cm- ’ for HGaL and by two bands at 22 10 and 2000 cm- ’ for HZ2. A comparison of the spectra of CO adsorbed on HZ1 and HZ2 shows some differences with respect to the electron-accepting sites. These differences are probably due to the different SiOJA1203 ratios of the samples as well as the peculiarities of the preparative procedure applied to each sample. The incorporation of gallium via impregnation of HZ2 leads to the appearance of a new band at 2230 cm-’ (Fig. 2, unreduced 2GaHZ2 and unreduced 5GaHZ2), which was noted in the spectrum of CO on P-GaaOa. The procedure of modifying HZSM-5 by impregnation with Ga(NO& probably results in Ga3+ cations preferentially localized on the external surface of the zeolite crystallites after calcination [S]. Therefore, we assign the 2230 cm-’ band to the interaction of carbon monoxide with the electron-accepting sites of the gallium oxide phase on the external surface of the zeolite crystallites. Evacuation of the unreduced samples at room temperature for - 15 mm results in the disappearance of all the CO bands, which is typical of physical adsorption. We conclude that either ball-milling HZ1 with pGaz03 or impregnation of HZ2 with Ga(NO,), followed by calcination leads

Reduced

Unreduced

-

HZ2-

-HGaL-

-2GaHZ2-

-5GaHZ2-

,I!5 2lf5

2130

Fig. 2. IR spectra of CO adsorbed on HZ2, HGaL, 2GaHZ2 and 5GaHZ2 before and after Hz reduction

at 840 K (P(CO)=3.9

kPa). Wavenumbers

in cm-‘.

115

to a zeolite containing gallium oxide on the external surface of the samples, and no strong interaction between the zeolite and the modifying oxide occurs. As seen from the spectra of CO adsorbed on the ball-milled samples after reduction under static conditions, (Fig. 1, reduced 2GaHZl and reduced 5GaHZ2), an additional band at 2 155 cm-’ appears which is shifted 10 cm-’ from a known CO gas phase band. The intensity of the band increases with increasing gallium content. The band disappears upon evacuation but the removal process requires 420 K for 1 h, and it is therefore a strongly adsorbed CO species. If reduced 2GaHZl and reduced 5GaHZl are treated . . additionally in oxygen (P,, = 19.7 kPa) at 570 K prior to CO adsorption, the 2 155 cm- ’ band does not appear. Therefore, the 2 155 cm-’ band should be attributed to complexes of carbon monoxide and a reduced ionic state of gallium. The same band at 2155 cm- * is also present in the spectra of CO adsorbed on the reduced 2GaHZ2, 5GaHZ2 and HGaL samples (Fig. 2). In contrast, no band at 2155 cm-’ arises after CO adsorption on either /3Ga203 or GaNaZ2 (spectrum not shown) which were subject to the reductive pretreatment. These data, along with other physicochemical and catalytic investigations [12, 19, 201, suggest that the zeolite acidity is a necessary condition for initiating the process of transformation of the gallium state upon Ha treatment. The involvement of the acidic -OH groups of the zeolite in this process has recently been demonstrated by a strong reduction in the 3610 cm-’ band of the 5GaHZl sample as a result of H, reduction [141. Khodakov et al. report a band at 3618 cm-’ (attributed to acidic bridged hydroxyls of the gallosilicate) which was also reduced in intensity after regeneration of a sample used as a cataIyst for propane aromatization [ 15 1. Therefore, the gallosilicate sample may have been reduced by the propane reactant, though a decrease in -OH groups via dehydroxylation during the regeneration process is possible. XPS and EDAX measurements unambiguously show that a process of gallium transfer into HZSM-5 zeolite occurs as a result of Hz reduction of the ball-milled Ga20JHZSM-5 samples [ 13, 201. Finally, careful examination of the stoichiometry of the reduction process has confirmed a 1: 1, Ga:Al stoichiometric limit in the case of both impregnated [ 191 and ball-milled catalysts [12]. Taking into account all these observations, the assumption that the appearance of the band at 2155 cm- * is due to the formation of Gaf -CO complexes is reasonable. This assignment is in accordance with investigations of the electron-accepting sites in a number of zeolites [ 17, 181. Recent observations of Cu-containing Y zeolites have shown that the adsorption of CO under conditions similar to those used in the present investigation results in IR bands at 2156 and 2143 cm-’ due to Cu+ -CO carbonyl complexes differing in their location in the Y-type zeolite framework [ 2 11. Thus the capability of the HZSM-5 zeolite to stabilize a Ga+ oxidation state and accept Ga+ cations appears to be a driving force for the reaction: G%03 + 2H, -

G%O + 2Hz0

116

Fig. 3. IR spectrum of CO adsorbed on reduced 5GaHZ2 after TEMPO preadsorption kPa). Wavenumbers in cm-‘.

(P(C0)

= 3.9

by providing a sink for the product. Due to its mobility at high temperature, GazO penetrates quickly into the zeolite channels, reacting with acidic hydroxyl groups until the Gaz.OBsource is consumed or, at high Ga content, all protic acidic centers are replaced by Ga. It is likely that the formation of the GazO intermediate occurs on the external surface of the zeolite crystallites where Ga,O, is localized in the case of both ball-milled and impregnated samples (GaHZl and GaHZ2 series). In the H-form of the gallosilicate (HGaL), the reduction probably occurs in the zeolitic channels which contain non-framework gallium species. The assumption that the Ga+ cations are stabilized after the Hz reduction in the interior of the zeolite crystallites is further confirmed by experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) adsorption on the reduced 5GaHZl and 5GaHZ2 samples. The kinetic diameter of TEMPO molecules of about 11 w prevents them from penetrating into the zeolite channels, and consequently they can only interact with sites on the external surface of the crystallites. As depicted in Pig. 3, the intense band at 2155 cm-’ attributed to Ga’ -CO complexes is still present when the carbon monoxide adsorption is performed after coverage of the external zeolite surface by means of TEMPO preadsorption. Therefore, Ga+ species formed after the reduction are localized primarily in the zeolite channels. The relative concentrations of Ga’ cations in 5GaHZl and 5GaHZ2 samples which contain the same amount of Ga but differ in the SiO&1203 ratio of the HZSM-5 zeolite base can be estimated by a simple thermal desorption experiment. Both samples were reduced in H,/He flow at 840 K and, after CO adsorption, the temperature was raised to 470 K at which the band at 2155 cm- 1 completely disappears. The concentration of Ga+ calculated from this experiment is about 8 X 10” molecules g-i for 5GaHZl and 5 X 10” molecules g- ’ for 5GaHZ2. The ratio of these values is - 1.6, which corresponds closely to the Al content ratio of 5GaHZl and 5GaHZ2 samples, which is - 1.5.

Conclusions IR measurements of adsorbed CO reveal that the active state of Ga formed through hydrogen reduction of Ga/HZSM-5 catalysts prepared via different techniques is probably Ga+ stabilized in the interior of the zeolite

117

crystallites. Since hydrogen is always present under the operating conditions used for light paraffin aromatization, the present investigation points to the involvement of a new active site in the catalytic conversion process. Thus, theories directed toward a gallium oxide/acidic zeolite bifunctional catalytic mechanism need to be reevaluated.

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