Synthesis of crystalline metal silicates having zeolite structure and study of their catalytic properties

Journal of Molecular Catalysis, 31 (1985)

355

355 - 370

SYNTHESIS OF CRYSTALLINE METAL SILICATES HAVING ZEOLITE STRUCTURE AND STUDY OF THEIR CATALYTIC PROPERTIES K. G. IONE, L. A. VOSTRIKOVA

and V. M. MASTIKHIN

Institute of Catalysis, Novosibirsk 630090 (Received

June 8,1984;

accepted

(U.S.S.R.)

February 11,1985)

Summary The possibility of isomorphous heterocharged substitution (IHS) of Si4+ by Me”+ cations of elements of Groups I - VIII (where n f 4) in crystalline non-aluminium silicates with the ZSM structure has been examined. The silicates were synthesized in hydrothermal conditions in the presence of Men+ and tetrabutylammonium bromide. The catalytic activity of the silicates in methanol conversion into hydrocarbons has been determined. The state of Me”+ in crystallization products was studied by EPR and NMR methods. As shown, the probability of Me”+ fixation in tetrahedral oxygen surroundings in the silicate framework can be predicted on the basis of the Pauling criterion. Acid-base catalytic activity was found only for silicates with Me”+ cations in IHS positions. Both the rate and selectivity of methanol conversion to hydrocarbons depend on the e/r of cations, because the number of Men+ which can be occluded in the IHS positions in the silicate framework depends on this parameter.

Introduction According to the theory of Pauling [ 11 and approaches developed by Tanabe [2], the capability of binary oxides to provide acid-base catalytic action results from incomplete compensation of cation charges by oxygen atoms of the environment. From this standpoint, any isomorphous heterocharged substitution (IHS) of the cations of inert oxides should lead to the formation of a catalytically active system. Crystalline silicates (zeolites) are well known as catalysts of acid-base action. The catalytic activity of these systems is attributed [3,4] to the excess of charge resulting from isomorphous substitution of Si044- by A104’ -. Crystalline aluminosilicates (zeolites) are widely used today as catalysts in petrochemistry, oil processing and organic synthesis. The various aspects of the synthesis and application of zeolites have been reported in numerous reviews and monographs. However, zeolites are only a particular case of the phenomenon because they represent only one type of IHS: 0304-5102/85/$3.30

@ Elsevier Sequoia/Printed

in The Netherlands

356

Si4+ 2 Al 3+. The possible p r ep aration of silicates having zeolite crystalline structure including MeOq8+“, where Men+ is a non-aluminium cation and n is different from 4, was .shown in [ 5 - 121. The catalytic effect of these species on reactions involving the acid-base mechanism has been revealed. For the development of this area it is necessary to prove that isomorphous substitution Si4+ z Me”+ does occur and does create the catalytic properties of silicates. This problem can be solved by simultaneous study of the coordination state of the non-aluminium cation in the silicate and its catalytic action. The probability of isomorphous substitution Si4+ 2 Me”+ and the stability of the Me”+ position in the tetrahedral oxygen surrounding can be predicted using the Pauling criterion (p) [ 11. In the simplified version where r, is the cation radius and r, is the radius of the oxygen P = rclr-3 9 ion. In accordance with [ 11, the tetrahedral surrounding should be stable for cations at 0.414 > p > 0.225. This group of cations includes only A13+, Mn4+, Ge4+, V4+, Cr’+, Si4+, P5+, Se6+ and Be2+. However, as shown in [ 131, any cation can be occluded in the isomorphous position by inorganic compounds, provided the concentration of this cation is low. In this work, silicates having the crystalline structure of a ZSM zeolite, with polycharged cations of Group I - VIII elements (0.7 > p > 0.2) introduced into the initial silica sol prior to hydrothermal synthesis, have been obtained and investigated.

Experimental Crystalline silicates were prepared by hydrothermal treatment of aqueous 30% silica sol containing tetrabutylammonium bromide, one of the salts of Group I - VIII elements and sodium hydroxide at 415 - 475 K for 1 - 30 days. A source of aluminium was not added to the initial mixture. The samples decationated with a 0.1 N solution of NH40H + NH4CI up to Na,O = 0.1 - 0.05 wt.% exhibited not less than 80 - 90% crystallinity. The SiO?:Me,O, ratio (where Me is the substituting cation) in the initial silica sol was varied from 3 0 to 1500. A DRF-I diffractometer was employed for X-ray analysis. The X-ray patterns were taken for the range of angles 3 - 15” using CuK, radiation. The degree of cry&Unity of the samples (Re, %) was determined by a comparison of the X-ray line intensities in the 10 - 15” range for the product formed, with silica&e-2 (SL) taken as a reference [ 141. The latter was synthesized by the method described above without addition of inorganic salts. The effective surface area of the silicates (8, m2 g-r) was calculated from the value of argon adsorption at the temperature of liquid nitrogen. 27A1 ‘iB, gBe, ‘lGa and 2gSi NMR spectra were registered using a Bruker-&P-300 spectrometer. In some cases, magic-angle spinning was used. Prior to recording the spectra, the samples were kept in a desiccator

357 TABLE 1 Conditions for recording NMR spectra of silicates (Ha = 7.04) No.

Cation in silicate

Recording frequency (Me)

Pulse duration (cls)

Reference chemical shift

Chemical shift found (ppm)

Recording range (kHa)

1 2 3 4 5

BJ+ Be2+ A13+ Ga3+ Si4+

96.276 42.11 78.182 91.495 59.610

5 1 0.5 1 0.5

IM H3B03 IM Na2BeOH4 IM Al(H20)e3+ IM Ga(NOa )s TMS

-23.3 -5.8 59 156 -113

100 50 100 200 30

above water at P/P,= 0.9 at 20 “C for 2 days. Positive chemical shifts (6) correspond to a shift to low field (Table 1). ESR* spectra of Cu2+, Eu3+, Mn2+ and V4+ were registered on a JES3BX apparatus in X- (X = 3 cm) and Q-bands (X = 8 mm) at 77 and 300 K, respectively. The values of the parameters were determined by comparison with DPPH in the X-region and with Mn2+ in MnO in the Q-band. ESR spectra of Fe3+ cations were registered on EP-LOOD Bruker spectrometer. The intensity of the signals in related units (Ii) were estimated by comparison of the spectra of the sample under investigation with the reference, taken simultaneously using a resonator of type TE,,. The catalytic tests were carried out using a pulse microcatalytic gradientless system with a vibrofluidized catalyst bed under conditions of ideal mixing at 653 - 723 K. The catalysts were pre-calcined in helium at 773 K for 2 h. The rate (K) of total conversion of methanol per unit concentration of methanol in the reaction mixture and per mol of Men’ in the silicate, as well as the selectivity toward the groups of individual hydrocarbons and dimethyl ether were determined. The experimental conditions were: reaction temperature 653, 683, 723 and 773 K; contact time 5 - 12 s; atmospheric pressure; inlet reaction mixture composition: 20 vol.% methanol, the remainder being helium.

Results and discussion Over the range of temperatures employed at 0H:Si02 = 0.1 - 0.2, the main crystalline phase of the solid product exhibited X-ray characteristics nearly identical with those of ZSM zeolite [ 15,161. The aluminium content in the silicates prepared without addition of aluminium salts was not more than 0.12%, which corresponded to a Si02:A1203 ratio > 1200. Some common features in the crystallization behaviour of silica gels with different additives have been observed. For example, the effective *ESR spectra of Fe3+, Cuzt and V4+ were taken by V. F. Ujudanov and V. Poluboyarov, while those of Eu3+ and Mn2+ were recorded by V. K. Ermolajev.

358

surface area of the samples depended linearly on the content of the zeolite phase in the precipitate obtained (Re, %). This means that under the synthesis conditions employed, both amorphous and crystalline non-zeolite phases of the final product were of minor porosity. At the same OH:SiO, ratio and the same crystallization time, the value of Re, % changed drastically as the temperature of crystallization increased in the 413 - 475 K region. An increase in the temperature of crystallization to 443 and 475 K resulted in an increase in the cr-quartz and crystobalite content in the final product. Under identical experimental conditions, the fraction of the zeohte phase in the crystallization product increased as the quantity of polycharged cations in the initial gel decreased (Table 2). In most cases the Si:Me”+ ratio was found to be much higher in a solid crystallization product than in the initial gel (Table 3). This means that in the course of the hydrothermal treatment, the greater part of Me”+ was. not occluded by the silicon-oxygen framework, nor was it removed from the silicate during washing and ion exchange. The only exceptions are silicates synthesized in the presence of triply- and doublycharged ions with cation radius 60.65 A (Fe3+, Cr3+, Coz+, Ga3+, A13+, B3+, Be2+) for which the Si4+: Men+ ratios in the solid product and in the initial gel were nearly identical even after decationation treatment (Table 3). The state of polycharged cations in silicates Aluminosilicates The 27A1 NMR spectrum of the sodium form of zeolites has a single line with a chemical shift of 59 + 1 ppm (signal Al-I) which was attributed TABLE 2 Relative content of the crystalline phase having ZSM zeolite structure in the products of silica sol crystallization, % mass Cation

Si4+/Men+ in the initial gel

Cation

15

30

60

65 60

100 100

100 100

In’+ Fe3+

60

80

90

c!os+

-

amorph

60 80 60 60

100 100 60 90

20 70 60

50 90 60

70 100 70

Si4+/Mem in the initial gel 15

30

60

Me@:

$t+ Me'+: Sb5+ Me4+: $1 ;l-t Me3+: T13+ Nd3+ ‘Eu3+

50 -

cr s+ Es?: Me2+: Pb2+ Sn2+ Mn2+ Zn2+ CU2+ Be2+

40 90 50 50 -

amorph 40 -

60 80 80 90 70 75

80 100 95 100 125 100

50 70 60 60 80 100

50 80 90 100 80 100

359

TABLE 3 Values of Si4+/Mem in decationated crystallization 60; crystallization temperature 423 K Cation

Me6+: Mob+ Se6+ Me5+: Sb5+ Me4+: Zr4+ Ti4+ Me3+: T13+ Nd3+ Eu3+ In

3+

Si4+/Men+ in solid product

Re (%I

700 1000

70 100

260

90

1000 480

60 100

750 700 1300 330

70 100 75 80

products; initial silica gel Si4+/Men+ =

Cation

Fe3+ Cr3+ coJ+ Ga3+ A13+ B3+ Me2+: Pb2+ Sn2+ ;$I+ cu2+ Be2+

Si4+/Men+ in solid product

Re (%I

56 27 50 70 67

100 95 100 125 100 100

150 280 300 119 73 36

50 80 90 100 80 100

95

to aluminium atoms in tetrahedral coordination, i.e. in the zeolite framework in isomorphous Si4+ sites. The second signal (Al-2) with a chemical shift of -3 - 0 ppm appeared in the spectrum of decationated zeolites. It was attributed to aluminium atoms in octahedral coordination, i.e. in the cation sites outside the framework [ 17 - 191. For zeolites with Si0,:A120s > 30 the signal Al-2 in the region of the weak field is weak or absent, which indicates that the concentration of aluminium atoms in the cation positions in these zeolites is too small to be detected by NMR. The 2gSi NMR spectrum of these zeolites has an intense line with a chemical shift of -113 ppm, and weak lines near -102 and -115 ppm, which were ascribed to silicon atoms in Si(OAl)i and Si(OAl)O groups [20, 211. Bomsilicates ‘iB NMR spectra consist of two lines: a symmetric, narrow one (width ca. 1.7 kHz) with a chemical shift of -23.3 ppm and a broad one (Fig. 1). The quadrupole coupling constant for ‘iB nuclei in the trigonal oxygen environment is much larger than that for boron in tetrahedral oxygen coordination [ 221. On this basis we attributed the narrow line (1) to boron in tetrahedral coordination and the broad line (2), which exhibits a quadrupole splitting of the second order, to boron in the trigonal oxygen environment. The intensity of llB NMR signals depends upon the chemical composition, synthesis and treatment conditions of the borosilicates. In all cases the sum intensity of signals 1 and 2 corresponds to the total B20s content found by chemical analysis. As the OH:SiO, ratio in the boroncontaining gel increases, the content of ZSM-phase in the solid crystallization

360

I

100

I

I

50

0

-50

-100

8. ppm

Fig. 1. llB NMR spectra of borosilicates: (1) silicalite impregnated with HaB03; (2) borosilicates with ZSM structure before catalytic test; (3) after catalytic test; (4) glass.

product decreases. The intensity of the line with 6 = -23.3 ppm in “B NMR spectra also decreases (Table 4). On increasing the crystallization time from 48 to 144 h, the content of BzOs in the samples falls slightly, however the degree of crystallinity of the samples does not change. A slight decrease in the intensity of line (1) is detected, its chemical shift and linewidth remaining constant. The intensity of the line at S = -23.3 ppm dropped sharply after treatment of the samples in reaction conditions and after regeneration. Simultaneously, a broad line appeared (Fig. 2). Since the variations in the narrow signal intensity of NMR llB spectra depend upon the crystal phase content in the samples, it is possible to relate this signal to B3+ cations fixed in the crystal portion of a solid crystallization product. The decrease in intensity under thermochemical treatment shows that the Si-O-B bonds are not stable, and during the catalytic reaction-regeneration cycles part of B3+ cations are transferred from the framework into nonframework positions. The gBe NMR spectrum of beryllium silicate having a ZSM structure exhibits a single line, the chemical shift of which is 5.8 ppm compared to that of the ‘Be NMR spectrum of sodium beryllate. The spectrum differs from that of beryllium oxide (Fig. 3). The line intensity is proportional

361 TABLE 4 Influence of OH/Si02 in the initial mixture and of crystallization time (7) on the degree of crystallinity of boron silicates and on line intensity of the llB NMR spectrum; temperature of crystallization 150 ‘C, SiOz/BzOs = 60 Sample

OH/SiOz ratio

r (h)

BzOsin silicate (wt.%)

S

(m2 g-i)

E)

Linewidth (kHz)

Relative intensity of llB NMR signal with 6 = -23.3 ppm

1

0.1

2

0.2

48 78 120 144 48 120

1.6 1.5 1.4 1.2 1.8 1.1

670 580 530 540 170 120

90 100 95 90 30 16

1.7 1.5 1.7 2.0 -

1.0 0.9 0.8 0.7 0.44 -

3

0.3

48

2.1

180

20

3.0

0.2

-22 3

wm

i

Fig. 2. llB NMR spectra of decationated borosilicates synthesized from gel with SiOz/ &Oa = 60: (1) before catalytic test, (2) after catalytic test, (3) after regeneration.

to the content of beryllium in the silicate as determined by chemical analysis. These data permit us to suggest that the major portion of Be2+ in the synthesized silicates is coordinated in the crystalline silicon-oxygen framework. The ‘lGa NMR spectrum has a single broad line with a chemical shift of 155 ppm (Fig. 4). There is a proportionality between the degree of crystallinity of the silicate and the intensity of the “Ga NMR signal. The value of the chemical shift corresponds to the gallium nuclei in the tetrahedral oxygen environment.

362

50p.p.m.

-58

Fig. 3. 9Be NMR spectra of beryllosilicates synthesized from gel with SiOZ/BeOZ ratio: (1) 120, (2) 60. Fig 4 ‘IGa NMR spectra of gallosilicates synthesized from gel with SiOz/Ga203 (1)‘60, (2) 30.

ratio:

The Fe3+ ESR spectrum has a very weak signal with g-factor 4.3 and two strong signals with g = 2.00 and 2.3. The relative intensity of the signal with g-factor 4.3 increased with increasing amounts of ZSM-phase in the solid product (Fig. 5). For the samples with equal degrees of crystallinity, increasing the Fe3+ content in silicates resulted in only a limited increase in the intensity of this signal (Fig. 5b). The relative intensity of the signal with g-factor 2 was independent of the degree of crystallinity of the samples (Fig. 5a), and increased with increasing Fe,03 content in the crystalline products (Fig. 5b). Upon washing the samples with a solution of 1 N HCl, the signals with g-factors 2 and 2.3 were first to vanish. In contrast, under water vapour treatment of the samples at 873 973 K, which caused the crystallinity of the silicates to drop sharply, the signal with g = 4.3 disappeared, and the intensity of the signal with g = 2.3 increased. These data support the assumption made in [25, 261, that the signal with g = 4.3 may be attributed to Fe3+ ions in tetrahedral oxygen coordination within the silicate framework, and the signals with g-factors 2 and 2.3 assigned to Fe3+ ions in octahedral oxygen coordination outside the silicate framework. The relative intensity of the signal with g-factor 4.3 is very low, which indicates that a small number of Fe3+ cations are fixed in the phase with zeolite structure. For all Mn-containing silicates having zeolite structure, the ESR spectra have an isotropic g-factor of 2.00, which represents the superposition of several (usually three) lines [ 121. According to the analysis made using the computer, the spectrum at 77 K can be attributed to a superposition of two signals of the Lore&z form, of 1250 and 450 oe in width, and a 6component spectrum with splitting between the third and fourth components (93 oe). Their relative contributions to the total intensity are 91%, 8.5% and 0.5%, respectively. The cations of the two former types

363

‘

b)

12.0

'2.0 30

10

30

20

1

IO

2

1

/ 20

(4

40

60

80

-

Re% IOC

,

WA % maSS

@I

Fig. 5. Dependence of relative intensities of the signals with g-factors 4.3 (1) and 2.0 (2) in ESR spectra of Fe”+ on (a) the content of crystal phase with ZSM structure, and on (b) the content of Fe203 in the solid hydrothermal crystallization product.

appear to be involved in a weak exchange interaction. For the ESR spectrum of the cations of the third type, the splitting between the components of the HFS is 90 - 98 oe. The splitting was improved when the recording temperature was lowered from 295 to 77 K upon transition into the Qregion. Such a spectrum can be ascribed to Mn2+ ions which are not coordinated in the zeolite framework. It should be noted that not all the manganese ions introduced into the sample can be detected by ESR methods. As ascertained by ESR spectra of Cu2+, V4+, Cr3+ and Eu ‘+ , the only state in which these cations can be observed in hydrated silicates is the octahedral oxygen surrounding, which corresponds to fixation outside the zeolite silicon-oxygen framework. Catalytic properties The catalytic properties of the synthesized metal silicates were compared with those of crystalline silicon oxide having zeolite structure (silicalite) which was used as the main reference sample. At 635 K under the conditions described above, no more than 50% methanol was converted on the silicalite. The main reaction product was dimethyl ether (88 - 99%); trace hydrocarbons were found. The low activity of the silicalite can be explained by the presence of small amounts of aluminium atoms which are known to impart acid-base properties to Si02. Similarly, Cr3+, Mn2+, EL?+ and In ‘+ silicates exhibited low activity and selectivity with respect to C1 - C i,, hydrocarbons. and high selectivity with respect to dimethyl ether (Table 5).

46 36 59 210 57 108 112 304 1000 367 667 78 1040 91 72 143 164 1000

s+

> 1000

AP+

Bs+ Be2+ Ala+ Ga3+ Fe3+ Fe3+ Cr Mn2+ See+ Ti4+ Sbs+ zn2+ Ella+ Cu2+ Te6+ Pb2+ Sn4+ Ina+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

SiOz/ Me,G, in solid product

Modifying element

No.

85 75

120 100 85 95 90 100 90 90 80 75 70 90

100 70 80

Be (%)

47.5 0.40 69 1.10 82.5 0.40 96.0 0.50 93.5 0.70 84.0 3.5 86.0 0.5 45.6 7.3 2-5 66 0.25 2-5 18 18.4 5.8 58.3 25 21.5 CO and Hz are the main 6.0 3.0 41.0 0.2 25.3 12.3 29 -

CO, CH4

6.4 96.5 99.5 92.4 100.0 99.75 100.0 81.6 41.0 78.5 products 97.0 99.8 87.7 100

-

99.6 79.0 81

DME

1.7 -

51.2 80 46.5

76 29 39 37 40 41.2 31.9 57

-

0.5 3.2 21.3 0.4

2.4 5.3 1.1 1.2 0.9 3.5 1.2 23 -

CO, CH4

69.4 74 87.2 99.6 97.0 91.0 85.0 61 -

- Cl0

19.9 15.0 99.5 92.9 0.3 0.7 -

c2

hydrocarbons

97.5 96.8 78.7 99.2

48.8 20 51.2

97.9 -

88.1 24.9 50.3 1.0 6.6 37.4 75.2 -

DME

- Cl0

-

-

-

0.4

2.0

2.3

0.4

9.5 69.7 48.6 98.8 98.1 89.9 61.4 1.8 -

c2

hydrocarbons

Composition of reaction products (wt.%)

Methanol conversion

of reaction products

Methanol conversion (%)

Composition (wt.%)

Temperature 683 K

Temperature 653 K

Catalytic properties of crystalline metal silicates in methanol conversion

TABLE 5

365

Methanol conversion on all manganese-containing silicates studied was low (2 - 5%), dimethyl ether being the main reaction product. Note that low catalytic activity was observed for the sample in which 90% of the manganese was in the Mn 2+ state and was detectable by ESR methods; the sample in which 90% of manganese introduced was not detected by ESR also showed low activity. It was also found that on Pt4+, Zn2+, Pb2+, Cu2+ and MO’+ silicates, methanol decomposes at 2723 K to produce primarily CO and CH4 (up to 60 - 90% of all reaction products). This may result from the reduction of the cation oxide. Silicates of triplycharged cations B3+, A13+ (zeolite), Ga3+ and Fe’+, as well as beryllium silicate, exhibited significantly higher catalytic activity in methanol conversion to hydrocarbons. For silicates of triplycharged ions, the selectivity toward Ci - C,, hydrocarbons at 693 - 723 K was 60 - 98% (Table 5). On aluminosilicate catalysts (zeolites of the types ZSM-II and ZSM-5) the conversion of methanol was not less than 95 - 98% under the conditions described. The reaction products contained 22 - 38% light (C, - C4) and 60 70% liquid hydrocarbons. The Ci - C4 fraction included predominantly propane and butane, while the Cg+ fraction contained mainly aliphatic (aliph) and alkylaromatic (AA) compounds. In general, the total composition of the products was the same as that given in the literature by Chang [27]. As the aluminium content in zeolites was increased, the AA content in the reaction liquid products also increases. For example, at a ratio of Si02/A1203 = 38 the content of AA in the liquid hydrocarbon products was 46%, whereas at Si02/A1203 = 280, it reached only 22% [ 281. Introduction of copper or iron ions into aluminosilicates prior to hydrothermal synthesis caused a decrease in the AA content in the reaction products at the same content of Al203 in the silicate [ 291. A sharp change in the selectivity of the silicate was observed upon complete substitution of A13+ by Fe 3+. For example, on iron silicates containing only 0.12% of Al2O3, i.e. at the level of impurities, and more than 1.5% of Fe203, the content of AA in the liquid reaction products was < 15%. The catalytic properties of borosilicates which contain Al203 as an impurity (Si02/A1203 > 1000) differ drastically from those of aluminosilicates: at 653 K the reaction products contain mainly dimethyl ether and smaller amounts of olefins C2 - C,. The content of aromatic hydrocarbons in the reaction products using samples with Si02/B203 = 46 and 25 is < 2%. Using samples with Si02/B203 = 135, and with the silicalite, no cyclic compounds were detected. Beryllium silicates exhibit high activity and selectivity to hydrocarbons only at elevated temperatures (773 K). Figure 6 shows a plot of the rate of methanol conversion per mol of Me,O, in the silicate containing Me”+ cations (n = 2 - 3) and per unit concentration of methanol in the reaction mixture (K) uersus e/r, where e is the charge and r is the cation radius. The content of C2 - C, olefins (curve 4), dimethyl ether (curve 3), isoparaffins (curve 2) and aromatic compounds

366

Fig. 6. Content of reaction products (wt.%) at reaction temperatures 683 - 723 K: curve 1 - aromatic compounds, 2 - isoparaffins, 3 - dimethyl ether, and 4 - olefins; rate of methanol conversion per mol of Me203 and per unit concentration of methanol: curve 5 - reaction temperature 653 K, 6 - reaction temperature 723 K. 0 In3+, A Fe3+, l Ga3+, A A13+, 0 J3e2+, 0 B3+.

(curve 1) in the products of methanol conversion on silicates (SiO*/Me,O, > 25) are also given. The dependences of Ig K pass through the maxima which correspond to gallium silicate (curve 5) and to aluminium silicate (curve 6). The lowest value of Ig K was obtained for indium and boron silicates. The maximum yields of C1 - Cl0 hydrocarbons, including olefins and aromatic compounds, are observed for aluminium and gallium silicates.

Discussion The present work supports the supposition that in the presence of nonaluminium polycharged cations Me”+ and at certain OH:SiO* ratios, silica can be crystallized into a product with the threedimensional crystalline structure of ZSM-5, -12. The content of the ZSM phase, crystallization kinetics, the number of occluded Me”+ cations and their state depend on the nature of the polycharged cation. The smaller the content of Me”+ in the initial silica sol, the higher the yield of the crystalline phase having ZSM structure. The inhibiting effect of polycharged cations on the crystallization process is revealed at Si4+:Me”+ < 15 - 30 (atom). This phenomenon is common for all cations. The quantity of Me”+ occluded by the silicate depends on the charge and radius of Me”+. For example, traces of Me6+, Me5+ and Me4+ were detected in solid crystallization products. For Me3+ and Me2+ the Si4+:Me”+ ratios in the initial gel and the solid product are nearly the s&e for cations with rM, > 0.65 A. Thus, when 80 - 100% of the solid product is crystallized in the form of ZSM, only triplycharged cations (Fe3+, Cr3+, Co3+, Ga3+, A13+, B3+) and doubly-charged cations (Cu2+ and Be2+) are occluded by the silicate in notable amounts.

367 TABLE 6 Values of Pauling criterion [ 1 ] for various cations* Critical values (P,,)

Coordination number at

Corresponding cations

P>Pcr

p > 0.732

8

Pb*+, Sn*+, Ti3+, Eu3+, Nd3+

0.732 >/I > 0.414

6

0.414

4

In’+, Mn*+, Zn*+, HP’, Cu*+, Sn4+, Fe3+, Mo6+, Ti4+, Pt4+, Cr3+, Ga3+, Sb5+, V4+ AI3’ Mn”+ Ge4+ Vs+ Si4+

>/J > 0.225

cr6+'p5+ ,,

0.225 > p > 0.147

3

&,a+

,

ie2+

'

'

B3+

Vahes of rMe+n are derived from [ 301.

According to the Pauling criterion [l] (Table 6), the stable tetrahedral environment for Me”+ with n < 3 can be expected only for A13+ and Be2+, for which 0.414 > per > 0.225. Close to these values are the per of Ga3+ and B3+ In fact, from our and literature NMR data one may conclude that A13+ and Be’+ are actually fixed in the tetrahedral oxygen surrounding in the silicate framework. Hydrolysis of the Al-O+% bond and elution of A13+ to the cation positions were detected for Si02:A1203 < 30. As ascertained by ESR spectroscopy, Cu2+, Mn’+, V4+, Cr3+ and Eu3+ ions in hydrated silicates are in an octahedral environment. The same is generally true of Fe j+ . B3+ cations in sodium silicates are fixed in the tetrahedral oxygen environment in an unstable state. Upon decationation and thermochemical treatments, a significant portion of the B3+ ions are removed from the framework. The catalytic conversion of methanol can proceed via two independent mechanisms involving centers exhibiting acid-base or redox action. If the silicate functions as a redox catalyst, CO, CH4 and CO2 should be expected as main reaction products. As shown in [ 271, methanol conversion on ZSM-type zeolites having acid-base properties is described by the general scheme: paraffins olefins --HzO, -Hz0 2CH30H < C2 - Cs aromatics CH,OCH, ___f +H20 cycloparaffins where the first step is the intermolecular dehydration of methanol, resulting in the formation of dimethyl ether. Subsequent dehydration of dimethyl ether leads to formation of olefins, predominantly C2 - C,; olefins, in turn, produce a mixture of paraffins, isoparaffins and aromatic compounds as a result of hydrogen redistribution reactions. These three main successive steps occur at different rates, and as a result, the products of intermediate reactions can be a predominant com-

368

ponent of the mixture at the reactor outlet. Thus, the main products of methanol conversion following the acid-base mechanism can be either dimethyl ether, olefins or a mixture of hydrocarbons including aliphatic and aromatic compounds. The theories concerning the nature of the catalytically active acid sites of aluminosilicates (zeolites) assume the isomorphous substitution of silicon atoms in the silicon-oxygen framework by aluminium atoms, SiOg4-+ A1045- [31]. Hydroxyl groups or water molecules can be protonated by aluminium cations via the scheme:

\si/‘~Al/“\ W‘/

‘\/\/\

or [Si(O-Al),]“-.

t A13+(OH).

l

.H+

If this scheme is valid, isomorphous heterovalent substitution in the silicate framework by any non-4+ valent metal, Men+, is expected to result in the formation of catalytically active sites. Hence, the silicates containing Me”+ in the IHS position will exhibit acid-base catalytic activity in methanol transformations. On the other hand, silicates occluded by Me”+ outside the framework (in a finely divided oxide phase inside or outside the silicate crystal) will not accelerate the transformation of methanol into hydrocarbons. The overall activity and selectivity of silicates containing Me”+ cations in II-IS-positions will depend on the strength or the concentration of the catalytically active sites formed. The activity of the site with acid-base properties must depend on the strength of electron-acceptor interactions in the field of Me”+, i.e., to a rough approximation, on the value of its e/r. Consequently, if the activity of the silicate will depend on the nature of the cations in isomorphous positions it will increase with increasing e/r of these cations. But the concentration of Men+ in IHS positions, and hence the concentration of catalytically active sites, must depend on the e/r of the cations too. This may be explained by the following consideration: The silicon-oxygen framework will be noticeably distorted when the length of the Me-O bonds differs from that of the Si-0 bond. Hence, the change in e/r of the cation must lead to a change in the probability of hydrolysis of the Si-O-Me bond and in the probability of formation of hydrolysis products, Me,,O,(OH), . According to the order of variations of the bond lengths (B-O < Be-O < Si-0 < Al-O < Ga-0 < Fe-O < In-O) the largest distortions in the framework are to be expected for B-, In- and Fe-containing silicates. Owing to these phenomena, the actual quantity of cations in the silicate framework after thermochemical treatment may be much less than that found by chemical analysis and must change with variations of the cation e/r through the extrema. If the catalytic properties of silicates depend

primarily on the actual concentration of Me”+ in IHS positions, the rate and selectivity of methanol conversion will also change upon variations of e/r through extremum. In the series of samples containing B3+, Be’+, A13+, Ga3+, Fe3+ or In3+ cations, the greatest activities must be obtained for A13+and Ga3+ silicates. Our results indicate that on the silicalite, in which isomorphous substitution does not take place, hydrocarbons are not synthesized from methanol. Like silicalite, Cr3+, Mn2+ and Cu2+ silicates in which polycharged cations are fixed predominantly in the octahedral oxygen coordination, and thus are not involved in the silicon-oxygen framework, show no selectivity toward aliphatic reaction products. On silicates synthesized in the presence of Mob+, Pt4+, Co3+, Pb2+, Sn2+, Zn2+ or Cu2+, methanol decomposes to CO and Hz with subsequent formation of methane. Probably these cations, fixed either in the oxide phase or in the silicate framework, were reduced during the course of the reaction and then operated as active redox components. Aromatic hydrocarbons, paraffins and olefins are produced from methanol on silicates containing at least a part of the cations in the tetrahedral oxygen surrounding, namely, on Fe*, Ga3+, A13+, Be’+ and B3+ The activities of In3+- and B3+-containing silicates and the yields of hydrocarbons from methanol on them are very low; after regeneration they drop sharply. The dependence of log K and of selectivity on e/r for Me”+ with it < 3 is of a non-linear but very marked character. Two conclusions can therefore be made: (i) the catalytically active sites for reactions of the acid-base type are formed in the silicate only when polycharged cations are isomorphously fixed in its framework. The high catalytic activity obtained by Bragin et al. [ 321 for boron-containing zeolites toward the synthesis of aliphatic and aromatic hydrocarbons from methanol may be associated with the presence of aluminium cation impurities; (ii) the very marked dependence of the degree and selectivity of methanol conversion on e/r of the Me”+ cation introduced into the silicates reflects the change not in the strength of the acid centers produced but, predominantly, in their true concentration in the silicate framework.

References 1 2 3 4

L. Pauling, The Nature of Chemical Bond, Goskhimizdat, Moscow, 1947. K. Tanabe, Catal. Sci. Tech., 2 (1981) 232. D. W. Breck, Zeolite Molecular Sieues, Wiley-Interscience, New York, London, 1974. J. Ward, in J. Rabo (ed.), Zeolite Chemistry and Catalysis, A.C.S. Monograph, American Chemical Society, Washington, DC, 1976. 5 K. G. Ione, L. A. Vostrikova, E. A Paukshtis, E. N. Yurchenko and V. G. Stepanov, Dokl. Akad. Nauk SSSR, 261 (1981) 1160. 6 K. G. Ione, L. A. Vostrikova and ,V. G. Stepanov, U.S.S.R. Certificate of authorship No. 1092 141 (1981) 1984.

370 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Fr. Pat. 2 471 950 (1981) to M. Taramasso, G. Perego and B. Notari. Get-. Pat. 28 31611 (1981) to L Marosi, L Stabenow and M. Schwarzmann. U.S. Pat. 4 299 808 (1981) to M. R. Klotz. Ger. Pat. 30 06 47101 (1981) to L. Marosi, L Stabenow and M. Schwarzmann. K. G. Ione, L. A Vostrikova, A. V. Petrova and V. M. Mastikhin, Proc. 8th Int. Congr. Catal., West Berlin, 1984, IV-619. L. A. Vostrikova, V. K. Ermolaev and K. G. Ione, React. Kinet. Cotal. Lett., 25 (1984) 1. E. S. Makarov, Zzomorfizm Atomov v Kristalhkh, Atomizdat, Moscow, 1973. U.S. Pat. 4 073 865 (1978) to E. M. Flanigen and R. L Patton. U.S. Pat. 3 702 886 (1972) to R. J. Arguer and G. R. Landolt. U.S. Pat. 3 804 746 (1974) to Pochen Chu. V. G. Stepanov, V. M. Mastikhin and K. G. Ione, Zzv. Akad. Nauk SSSR, Ser. Khim., 3 (1982) 619. K. G. Ione, V. G. Stepanov, V. N. Romannikov and S. S. Shepelev, Khim. Tverd. Topl., 6 (1982) 35. K. G. Ione, V. G. Stepanov, G. V. Echevskii, E. A. Paukshtis and A A. Shubin, Zeolites, 4 (1984) 114. J. B. Nagy, Z. Gabelica, G. Debras, P. Bodart, E G. Derouane and P. A. Jacobs, J. Mol. Catal, 20 (1983) 327. J. B. Nagy, Z. Gabelica, E. G. Derouane and P. A. Jacobs, Chem. Lett., (1982) 2003. H. M. Kriz, S. G. Bishop and P. J. Bray, J. Chem. Phys., 49 (1968) 557. C. A. Fyfe, G. G. Gobbi, J. S. Hartman, R E. Lenkinski and J. II O’Brien, J. Ma@. Reson, 47 (1982) 1,168. B. J. Klinowski, R Subramaniar and J. M. Thomas, J. Chem. Sot., Faraday Trans., 178 (1982) 7, 1025. E. G. Derousne, M. Mestdagh and L Vielvoye, J. CatoZ., 33 (1974) 2, 169. J. C. J. Bart, N. Burr&xi, F. Csriati, M. Petrera and C. Zipeih, Zeolites, 3 (1983) 6, 226. L Chang, Catal. Rev. Sci. Eng., 25 (1983) 1. K. G. Ione, G. V. Echevskii and G. N. Nosyreva, J. Catal., 85 (1984) 284. L A. Vostrikova, K. G. Ione, G. V. Echevskii and G. N. Nosyreva, Izv. Akad. Nauk SSSR, Ser. Khim., (1985), in press. G. B. Bokii, Kristallokhimiya, Moscow State University, 1960. P. A Jacobs, Carboniogenic Activity ofZeoZites, Elsevier, Amsterdam, 1977. 0. V. Bragin, T. V. Vasina, B. K. Nefedov, V. H. Lugovikova, T. V. Aiekseeva and Kh. M. Minachev, Zzv. Akad. Nauk SSSR, Ser. Khim., 5 (1981) 1179.