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Incorporation of Volatile Metal Compounds into Zeolitic Frameworks
159
INCORPORATION OF VOLATILE METAL COMPOUNDS INTO ZEOLITIC FRAMEWORKS P. FEJES, I. KIRICSI and I. HANNUS Appl. Chemistry Dept., J6zsef Attila Univ., Szeged (Hungary)
ABSTRACT Preparation methods were developed for introducing different 0- or halide-containing metal moieties into zeoli tic frameworks. The transformation of Bronsted acidity to Lewis acidity by the reaction [AI0
2}Wl;
+H20. [Al0
2]MO+
+ 2HCl
deserves special attention from the point of view of catalytic cracking, as no oxy~en vacancies are produced simultaneously (see Jacobs and Beyer (lJ). The halide ion-containing metal moieties exhibit noteworthy activity in cyclopropane isomerization. The specimens treated with SnCl are hyperactive in this reaction, revealing that allyl cations may be the active centres of the transformation. At elevated temperatures dealumination takes place; this does not necessarily lead to complete structural collapse if special care is taken. INTRODUCTION There are a few papers in the literature describing the surface treatment of Aerosil (Si0 2) samples with volatile metal halides [2-6] and alkyls [7-9] at relatively low temperatures (~ 600 K). It is presumed that the interactions taking place may be characterized by the following stoichiometric equations S-OH + MXn --- S-OMXn_l +HX and s-o S-OH 'MX +2HX + MXn - S-O"" n-2 S-OH
(1)
(2)
where S is a surface atom, M a metal atom, and X either a halide atom or an alkyl group. Aerosil samples modified with AICl), Bel) and AIMe) were tested in the cracking reaction of cumene (Hambleton et ale [6]) with the result that the activities were found to surpass that of the untreated Aerosil, but to lag behind those of the commercial aluminium silicates exhibiting Bronsted acidity.
160
A previous publication [lOJ has dealt with the dealumination of zeolitic samples using metal halides at elevated temperatures (above 700 K). It has been conjectured that prior to de alumination a halogen-containing moiety is incorporated into the zeolite according to the equation [Alo2'] H+ + MCI) -
[Al0
2] MCI~
+ HCI
(.3)
and in the form of a doubly charged species: [AI02'] MCI; + H+ -
2+ [AI02'] MCI + HCI
J
(4)
where the symbol [ ••• denotes framework constituents. At higher temperatures, charact ristic of the ions in question, a very interesting process takes place: a framework 0 2- ion from among the nearest neighbours of the aluminium leaves the lattice to join M; thereafter the halide ions undergo rearrangement leading to the production of oxy-halide clusters and a framework vacancy ("empty nest"): [AIO;] MC1
2+
-
AIOCI + MOCI + [ •••
J
(5)
In this paper the catalytic properties of zeolitic samples were tested after their surface treatment with volatile metal halides. alkyls and alcoholates within the temperature range of stability of the incorporated moieties. The degree of de alumination too was determined in the case of samples exposed to higher pretreatment temperatures than the respective limit of stability. EXPERIMENTAL The zeoli tic samples used in these experiments were NH 4(Na)Y (Linde) and NH (Norton) prepared from the Na forms 4(Na)-mordenite by ion exchange and heat-treated prior to the modifications. The surface treatment of the zeolite samples was carried out using solutions of the reactants in suitable solvents (method A) or gaseous substances introduced onto the.zeolit1c surface in a vacuum apparatus at low (mostly ambient) temperatures (method B). Another way of contacting the zeolite with the reactant consisted in purging the sample at higher temperatures with an indifferent carrier gas (mostly N2) containing the agent in low concentration (method C). The amount of aluminium released in the process of dealumination was determined by means of standard procedures. Possible structural changes in the modified specimens were checked via i.r. spectroscopy and X-ray diffraction. Whereas the catalytic activities of the Y zeolite samples were tested in the skeletal isomerization reaction of cyclo-
161
propane, the cracking of propane was chosen as the test reaction for the mordenite specimens. The kinetic investigations were carried out in a recirculatory flow reactor with g.c. product analysis. RESULTS a) Zeolitic samples modified at near ambient temperatures H-mordenite treated with AICI) according to method A (5 g Hmordenite refluxed with 1.33 g AIC1 in 50 cm3 ether in N2 atmos3 phere until no more HCI evolution could be observed) resulted in a dry sample after careful evaporation of the solvent and drying at 393 K. The product was very carefully wetted with water vapour in a desiccator for 24 hr, thereafter washed, and sUbsequently dried at 393 K. Figure 1. shows the i.r. spectra of surface-treated mordenite specimens in the region of lattice vibrations (a) prior to and (b) after wetting with water. The spectra are exactly the same, without any sign of (partial) lattice collapse. The same conclusion can be drawn from the respective simplified X-ray diffractograms in Fig. 2.
Fig. 2
E E
x:
I
I
I
I I
100
I I
I I
I
50
I
I;I 1200
800 ~
(em-')
400
5
I::
I I
10
I I I
I I
I I
I
I I I
15
I I I
20
I
I
I I I I I
I I
I I
I I I I
25
28
Fig. 1. Ir spectra of the treated samples: (8) prior to and (b) after wetting with water. Fig. 2. Simplified X-ray diagrams of the samples (solid line: original sample, dashed line: treated mordenite after wetting wi th water).
162
Another sample of H-mordenite was treated with Al(Et)3 in a vacuum apparatus at 473 K. The release of ethane (as monitored via the pressure change) is a good measure of the extent of reaction. After evacuation the fixed AI-alkyl was burned off at 773 K in 02 atmosphere to yield an AIO+ species in the exchange position. The rate of propane cracking (as revealed by the kinetic curves of the main reaction products in Fig. 3) was higher for the treated sample (b) than for the original specimen (a). A similar improvement in catalytic activity is observable in the case of dealuminated mordenites treated with Al(Et)3. These results whow the important role played by Al 20 in crack3 ing, even in a very dispersed form.
20
20
a
b
• C3H a o CH,
a
a,
• C 2H,
~
a.
o
• C2H 6
10
10
3
6
9
0
3
6
9
t (ks)
Fig. 3. Kinetic curves observed in the cracking of propane over tha original NaH-mordenite (a) and the NaH-mordenite treated with Al(Et)3 (b). (Reaction temperature 773 K; mass of catalyst 0.35g) Similar treatment of H(Na)Y with AICl in ether or CC1 4 leads 3 to similar results as in the case of H-mordenite (the retention of crystallinity was as high as 92% with the CC1 4 solvent). It is very interesting to note that a nearly quantitative change of the surface Br~nsted acidity into Lewis acidity caused a substantial decrease in the rate of skeletal isomerization of cyclopropane (see Fig. 4). The explanation of the reversal of catalytic activity in the case of propane cracking and cyclopropane isomerization might be very complex in nature: topological effects (the transformation
163
of Bronsted sites into sites of Lewis acidity is not quantitative when Al(Et)3 is used) and slight differences in the mechanisms of cracking and skeletal isomerization, the details of which are not yet fully understood, might be operative.
15 o a.. ~
Co
5
o
1,5
3
4,5
t (k s)
Fig. 4. Kinetic curves of cyclopropane isomerization observed at the untreated (a) and the treated (b) HY zeolite. (Reaction temperature 373 K, mass of catalyst 0,3 g; A , . cyclopropane, 0,. propylene). b) Surface treatment at elevated temperatures Method C applied with volatile metal halides above 650 K leads to dealumination with only a minor loss of crystallinity. The extent of dealumination depends on several factors [type of zeolite and reactant (thermodynamics, pore size and dimensions of reacting molecules), and temperatureJ; a few illustrative examples are summarized in Table 1. It is worth noting that Beyer attained almost complete dealumination in the case of HY, using SiC1 [l1J. 4 By reducing the temperature of surface modification below a critical limit (dependent on both zeolite and reactant), it is possible to reduce and even fully avoid dealumination. For samples treated with metal halides below 673 K (method C) an improvement in the catalytic activity in cyclopropane isomerization can be observed as compared with the untreated HY. The sample treated with SnC1 is hyperactive (full conversion of 4 cyclopropane into propylene in a few seconds, with a substantial
164
rise in reaction temperature). This exceptional activity can be rationalized in terms of production of allyl cations (regarded as active centres of cyclopropane isomerization), which undergo oxidation in an autocatalytic fashion: [Al02} 8n 4+ '(OH)~-' Cl- -
+6
[Al02} 8n 4+. 02-. (OH)- ·Cl-
CH 2
-IH+ \ CH2"CH2
non-classical ion -H 2 0 .
oxidation
[AI02].8n2+'(0H)-'Cl-
./:2
CH~CH
2
carboni~
+ t:::, isomerization
allyl cation as active centre
(6)
The water produced in the oxidation leads to the production of new Br~nsted centres
the acidity of which is increased substantially in the presence of the electrophilic halide ions. Hydrolysis of Cl- ions by water vapour reduces the activity. TABLE 1
Amounts of framework constituents removed by volatile metal halides Reagent TiC1 4 HgC12 FeCl.3
Temperature (K) 87.3 77.3 77.3
Al.3+ (mmol g-l) 0.8 0.95 0.1
Fe.3+ (;umOl g-l)
1.3.75
165
REFERENCES 1 2 3 4
P.A. Jacobs and H.K. Beyer, J. Phys. Chem., 83 (1979) 1174. J.B. Peri, J. Phys. Chem., 70 (1966) 2937. J.B. Peri and A.L. Hensley, J. Phys. Chem., 72 (1968) 2926. G.C. Armistead, A.J. Tyler, F.H. Hambleton, S.A. Mitchell and J.A. Hockey, J. Phys. Chem., 73 (1969) 3947. 5 R.J. Peglar, F.H. Hambleton and J.A. Hockey, J. Cata1., 20 (1971) 309. 6 F.H. Hambleton and J.A. Hockey, J. Catal., 20 (1971) 321. 7 G.C. Armistead and J.A. Hockey, Trans. Faraday Soc., 63 (1967) 2549. 8 M.L. Hair and W. Hertl, J. Phys. Chem., 73 (1969) 2372. 9 J. Kenawicz, P. Jones and J.A. Hockey, Trans. Faraday Soc., 67 (1971) 848. 10 P. Fejes, I. Kiricsi, I. Hannus, i. Kiss and Gy. Schobel, React. Kinet. Cata1. Lett., 14 (1980) 481. 11 H.K. Beyer and I. Belenkaja, in B. Imelik et al. (Eds.), Catalysis by zeolites, Elsevier, Amsterdam, 1980, p. 203.
INCORPORATION OF VOLATILE METAL COMPOUNDS INTO ZEOLITIC FRAMEWORKS P. FEJES, I. KIRICSI and I. HANNUS Appl. Chemistry Dept., J6zsef Attila Univ., Szeged (Hungary)
ABSTRACT Preparation methods were developed for introducing different 0- or halide-containing metal moieties into zeoli tic frameworks. The transformation of Bronsted acidity to Lewis acidity by the reaction [AI0
2}Wl;
+H20. [Al0
2]MO+
+ 2HCl
deserves special attention from the point of view of catalytic cracking, as no oxy~en vacancies are produced simultaneously (see Jacobs and Beyer (lJ). The halide ion-containing metal moieties exhibit noteworthy activity in cyclopropane isomerization. The specimens treated with SnCl are hyperactive in this reaction, revealing that allyl cations may be the active centres of the transformation. At elevated temperatures dealumination takes place; this does not necessarily lead to complete structural collapse if special care is taken. INTRODUCTION There are a few papers in the literature describing the surface treatment of Aerosil (Si0 2) samples with volatile metal halides [2-6] and alkyls [7-9] at relatively low temperatures (~ 600 K). It is presumed that the interactions taking place may be characterized by the following stoichiometric equations S-OH + MXn --- S-OMXn_l +HX and s-o S-OH 'MX +2HX + MXn - S-O"" n-2 S-OH
(1)
(2)
where S is a surface atom, M a metal atom, and X either a halide atom or an alkyl group. Aerosil samples modified with AICl), Bel) and AIMe) were tested in the cracking reaction of cumene (Hambleton et ale [6]) with the result that the activities were found to surpass that of the untreated Aerosil, but to lag behind those of the commercial aluminium silicates exhibiting Bronsted acidity.
160
A previous publication [lOJ has dealt with the dealumination of zeolitic samples using metal halides at elevated temperatures (above 700 K). It has been conjectured that prior to de alumination a halogen-containing moiety is incorporated into the zeolite according to the equation [Alo2'] H+ + MCI) -
[Al0
2] MCI~
+ HCI
(.3)
and in the form of a doubly charged species: [AI02'] MCI; + H+ -
2+ [AI02'] MCI + HCI
J
(4)
where the symbol [ ••• denotes framework constituents. At higher temperatures, charact ristic of the ions in question, a very interesting process takes place: a framework 0 2- ion from among the nearest neighbours of the aluminium leaves the lattice to join M; thereafter the halide ions undergo rearrangement leading to the production of oxy-halide clusters and a framework vacancy ("empty nest"): [AIO;] MC1
2+
-
AIOCI + MOCI + [ •••
J
(5)
In this paper the catalytic properties of zeolitic samples were tested after their surface treatment with volatile metal halides. alkyls and alcoholates within the temperature range of stability of the incorporated moieties. The degree of de alumination too was determined in the case of samples exposed to higher pretreatment temperatures than the respective limit of stability. EXPERIMENTAL The zeoli tic samples used in these experiments were NH 4(Na)Y (Linde) and NH (Norton) prepared from the Na forms 4(Na)-mordenite by ion exchange and heat-treated prior to the modifications. The surface treatment of the zeolite samples was carried out using solutions of the reactants in suitable solvents (method A) or gaseous substances introduced onto the.zeolit1c surface in a vacuum apparatus at low (mostly ambient) temperatures (method B). Another way of contacting the zeolite with the reactant consisted in purging the sample at higher temperatures with an indifferent carrier gas (mostly N2) containing the agent in low concentration (method C). The amount of aluminium released in the process of dealumination was determined by means of standard procedures. Possible structural changes in the modified specimens were checked via i.r. spectroscopy and X-ray diffraction. Whereas the catalytic activities of the Y zeolite samples were tested in the skeletal isomerization reaction of cyclo-
161
propane, the cracking of propane was chosen as the test reaction for the mordenite specimens. The kinetic investigations were carried out in a recirculatory flow reactor with g.c. product analysis. RESULTS a) Zeolitic samples modified at near ambient temperatures H-mordenite treated with AICI) according to method A (5 g Hmordenite refluxed with 1.33 g AIC1 in 50 cm3 ether in N2 atmos3 phere until no more HCI evolution could be observed) resulted in a dry sample after careful evaporation of the solvent and drying at 393 K. The product was very carefully wetted with water vapour in a desiccator for 24 hr, thereafter washed, and sUbsequently dried at 393 K. Figure 1. shows the i.r. spectra of surface-treated mordenite specimens in the region of lattice vibrations (a) prior to and (b) after wetting with water. The spectra are exactly the same, without any sign of (partial) lattice collapse. The same conclusion can be drawn from the respective simplified X-ray diffractograms in Fig. 2.
Fig. 2
E E
x:
I
I
I
I I
100
I I
I I
I
50
I
I;I 1200
800 ~
(em-')
400
5
I::
I I
10
I I I
I I
I I
I
I I I
15
I I I
20
I
I
I I I I I
I I
I I
I I I I
25
28
Fig. 1. Ir spectra of the treated samples: (8) prior to and (b) after wetting with water. Fig. 2. Simplified X-ray diagrams of the samples (solid line: original sample, dashed line: treated mordenite after wetting wi th water).
162
Another sample of H-mordenite was treated with Al(Et)3 in a vacuum apparatus at 473 K. The release of ethane (as monitored via the pressure change) is a good measure of the extent of reaction. After evacuation the fixed AI-alkyl was burned off at 773 K in 02 atmosphere to yield an AIO+ species in the exchange position. The rate of propane cracking (as revealed by the kinetic curves of the main reaction products in Fig. 3) was higher for the treated sample (b) than for the original specimen (a). A similar improvement in catalytic activity is observable in the case of dealuminated mordenites treated with Al(Et)3. These results whow the important role played by Al 20 in crack3 ing, even in a very dispersed form.
20
20
a
b
• C3H a o CH,
a
a,
• C 2H,
~
a.
o
• C2H 6
10
10
3
6
9
0
3
6
9
t (ks)
Fig. 3. Kinetic curves observed in the cracking of propane over tha original NaH-mordenite (a) and the NaH-mordenite treated with Al(Et)3 (b). (Reaction temperature 773 K; mass of catalyst 0.35g) Similar treatment of H(Na)Y with AICl in ether or CC1 4 leads 3 to similar results as in the case of H-mordenite (the retention of crystallinity was as high as 92% with the CC1 4 solvent). It is very interesting to note that a nearly quantitative change of the surface Br~nsted acidity into Lewis acidity caused a substantial decrease in the rate of skeletal isomerization of cyclopropane (see Fig. 4). The explanation of the reversal of catalytic activity in the case of propane cracking and cyclopropane isomerization might be very complex in nature: topological effects (the transformation
163
of Bronsted sites into sites of Lewis acidity is not quantitative when Al(Et)3 is used) and slight differences in the mechanisms of cracking and skeletal isomerization, the details of which are not yet fully understood, might be operative.
15 o a.. ~
Co
5
o
1,5
3
4,5
t (k s)
Fig. 4. Kinetic curves of cyclopropane isomerization observed at the untreated (a) and the treated (b) HY zeolite. (Reaction temperature 373 K, mass of catalyst 0,3 g; A , . cyclopropane, 0,. propylene). b) Surface treatment at elevated temperatures Method C applied with volatile metal halides above 650 K leads to dealumination with only a minor loss of crystallinity. The extent of dealumination depends on several factors [type of zeolite and reactant (thermodynamics, pore size and dimensions of reacting molecules), and temperatureJ; a few illustrative examples are summarized in Table 1. It is worth noting that Beyer attained almost complete dealumination in the case of HY, using SiC1 [l1J. 4 By reducing the temperature of surface modification below a critical limit (dependent on both zeolite and reactant), it is possible to reduce and even fully avoid dealumination. For samples treated with metal halides below 673 K (method C) an improvement in the catalytic activity in cyclopropane isomerization can be observed as compared with the untreated HY. The sample treated with SnC1 is hyperactive (full conversion of 4 cyclopropane into propylene in a few seconds, with a substantial
164
rise in reaction temperature). This exceptional activity can be rationalized in terms of production of allyl cations (regarded as active centres of cyclopropane isomerization), which undergo oxidation in an autocatalytic fashion: [Al02} 8n 4+ '(OH)~-' Cl- -
+6
[Al02} 8n 4+. 02-. (OH)- ·Cl-
CH 2
-IH+ \ CH2"CH2
non-classical ion -H 2 0 .
oxidation
[AI02].8n2+'(0H)-'Cl-
./:2
CH~CH
2
carboni~
+ t:::, isomerization
allyl cation as active centre
(6)
The water produced in the oxidation leads to the production of new Br~nsted centres
the acidity of which is increased substantially in the presence of the electrophilic halide ions. Hydrolysis of Cl- ions by water vapour reduces the activity. TABLE 1
Amounts of framework constituents removed by volatile metal halides Reagent TiC1 4 HgC12 FeCl.3
Temperature (K) 87.3 77.3 77.3
Al.3+ (mmol g-l) 0.8 0.95 0.1
Fe.3+ (;umOl g-l)
1.3.75
165
REFERENCES 1 2 3 4
P.A. Jacobs and H.K. Beyer, J. Phys. Chem., 83 (1979) 1174. J.B. Peri, J. Phys. Chem., 70 (1966) 2937. J.B. Peri and A.L. Hensley, J. Phys. Chem., 72 (1968) 2926. G.C. Armistead, A.J. Tyler, F.H. Hambleton, S.A. Mitchell and J.A. Hockey, J. Phys. Chem., 73 (1969) 3947. 5 R.J. Peglar, F.H. Hambleton and J.A. Hockey, J. Cata1., 20 (1971) 309. 6 F.H. Hambleton and J.A. Hockey, J. Catal., 20 (1971) 321. 7 G.C. Armistead and J.A. Hockey, Trans. Faraday Soc., 63 (1967) 2549. 8 M.L. Hair and W. Hertl, J. Phys. Chem., 73 (1969) 2372. 9 J. Kenawicz, P. Jones and J.A. Hockey, Trans. Faraday Soc., 67 (1971) 848. 10 P. Fejes, I. Kiricsi, I. Hannus, i. Kiss and Gy. Schobel, React. Kinet. Cata1. Lett., 14 (1980) 481. 11 H.K. Beyer and I. Belenkaja, in B. Imelik et al. (Eds.), Catalysis by zeolites, Elsevier, Amsterdam, 1980, p. 203.