J. Phys. Chem. 1993,97, 5678-5684
5678
Synthesis and Characterization of Ge-ZSM-5 Zeolites H. Kosslick, V. A. Tuan, and R. Fricke' Centre of Heterogeneous Catalysis, D- 12489 Berlin-Adlershof. Rudower Chaussee 5, Germany
Ch. Peuker and W. Pilz Wissenschafler-Integrationsprogramm, D- 12489 Berlin- Adlershof. Rudower Chaussee S, Geb.19.5, Germany
W. Storek Federal Institute for Materials Research and Testing, BAM, D- 12489 Berlin- Adlershof. Rudower Chaussee 6, Germany Received: November 18. 1992; In Final Form: February 16, 1993
Ge-ZSM-5 zeolites have been synthesized and characterized by SEM, n-hexane adsorption, XRD, MAS NMR, IR, and Raman spectroscopy. Despite the large size of the germanium atoms, up to 12.8 Ge/unit cell could be incorporated into the MFI framework. In order to rationalize this, a site preference of the germanium atoms in the lattice is proposed. Although a large amount of germanium is incorporated, only small changes in the unit cell parameters and lattice vibration frequencies are observed, suggesting that the T-T and T-O distances in the framework are nearly unchanged. However, the T-0-T deformation band is significantly broadened and shifted to higher wavenumber. From these findings it is deduced that the incorporation of germanium leads mainly to a decrease in the T-0-T angle.
Introduction The isomorphoussubstitution of silicon by other elementssuch as B,l-3 Al,b7 Ga,8-12 Fe,13-16 and in the framework of MFI-type zeolites (ZSM-5, silicalite I) has been reported. However, the isomorphous substitution by germanium is the subject of only a few works.20J' Although the isomorphous substitution of silicon by tetravalent germanium maintains the neutral charged zeolite lattice, Le., generates no Br~rnstedacid sites, this new type of zeolite is of special interest regarding its physicochemical properties. It opens the possibiIity of studying the influence of the isomorphous substitution on the crystal structure and its reflection in the physicochemical parameters as lattice constants and shifts of IR lattice vibration bands better than so far. The Ge-O distance in germanium oxide containing 4-fold coordinated germanium is 1.74 A, whereas the S i 4 distance is only 1.60A.22Additionally, germanium is chemically similar to silicon. Therefore, germanium is suggested to replace silicon in the MFI framework to a greater extent than other elements. A replacement of more than 30% was reported.2O In comparison the upper limit so far reached for aluminum is ca. 8%.23 Early synthesis attempts have yielded only some germaniumbearing ztolites.24~25 Often the coprecipitation of sodium germanate was observed. The coordination number n of an element M in a compound MX, is determined by the quotient of their radii Q = rM/rX. According to the Pauling rules the minimum valueof Qfor tetrahedral coordinationis 0.225 and for octahedral coordination 0.4 14.27,2s The corresponding value for oxygencoordinated germanium is ca. 0.3-0.41. Therefore, germanium in oxygen compounds can be coordinated both 4-fold and 6-fold. In the presence of sodium, which is an intrinsic mineralizing agent in zeolite synthesis gels, the formation of germanates containing octahedrally-coordinated germanium is preferred. Nevertheless, in an excellent work the successful synthesis of germanium-substitutedsilicalite I could be achieved by applying a newly developed synthesis route.20 However, the synthesis products were reported to contain crystals with highly distorted defectstructures. For this reason we haveoptimized the synthesis of GeZSM-5 and characterized the resulting samples by different physicochemicalmethods including 29SiMAS NMR, XRD, IR,
and Raman spectroscopy in order to study the influence of the isomorphous substitution on the zeolite framework.
Experimental Section
Materials. The samples were prepared under hydrothermal conditions by heating a starting gel having the molar composition: X G ~ O ~ . ~ S ~ O ~ . O . ~ P ~ ~ N B P ~where C H ~xN=H Z - H F - ~ 0.8, 0.4, and 0.165 and y = 1.2, 1.6, and 1.835, respectively. Tetrapropylammonium ions were used as organic structure directing agents. The gels were prepared as follows. A solution containingthe required amounts of silica sol (K 3010, Chemische Werke Bad Koestritz), tetrapropylammonium bromide (Merck, p.a.), methylamine (LaborchemieApolda, p.a.), and H F (Merck, 40%) was mixed under vigorous stirring until homogeneity was achieved (about 20 min). To this solution the necessary amount of GeC14 (Merck, synth. grade) was added dropwise under continued stirring. Then the pH of the gel was adjusted to a value of 9-9.5 by addition of methylamine, irrespective of the above given gel composition. The homogeneous gels were heated in Teflon-linedautoclaves for 18h at 443 K under static conditions and autogeneous pressure. Thereafter the autoclaves were quenched in cold water and the synthesisproducts were withdrawn immediately by filtration. The products were washed repeatedly with distilled water, dried, and calcined at 823 K in air for 4 h in order to remove the template. Methods. The scanning electron photographs were obtained on a TESLA B300 electron microscope. The n-hexane adsorption isotherm curves were measured at 293 K on a self-constructed McBain-typegravimetricunit connected to a vacuum-gas handling system. The sorption measurementswerecarried out in the range of relative pressures @ / p a )of n-hexane from 0.03 to 0.86. For this purpose the template-free hydrated sample was in-situ activated under vacuum at 673 K for several hours. The X-ray powder diffraction patterns of the samples were recorded on a HZG 4 diffractometer by using Ni-filtered Cu Kor radiation. The relative crystallinity of the samples was determined from the sum of the intensities of the MFI characteristic peaks between 26 = 23-25O, which were related toa silicalite I reference sample. The thermoanalytical measurementswere carried out on a MOM derivatograph (Hungarian Optical Factories) under the following
0022-3654/93/2097-5678S04.00/0 0 1993 American Chemical Society
Ge-ZSM-5 Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5679
conditions: 110-mg sample weight; flowing air; a heating rate of 5 K/min in alumina sample holders. Alumina was used as a reference sample. The 29SiMAS NMR spectra were obtained on a BRUKER MSL 400 instrument at 79.3 MHz. The spinning rate of the 7-mm standard zirconium dioxide rotor was about 5 KHz. Typically, 1500 and 4444 FIDs were accumulated with or without high-power proton decoupling (it was checked, there was no remarkable difference in the appearance of the spectra, but the S/N ratio was better without proton decoupling) and with pulse repetition times between 8 and 12 s. The chemical shifts are related to TMS by using Q8M8 (trimethylsilyl ester of double-4-ring silicate) as a secondary standard. Cross-polarization (CP from 'H to *9Si)spectra were also measured for two samples by using a contact time of 5 ms. The infrared spectra in the range of latticevibrations between 450 and 1400cm-I were measured on an BRUKER IFS 66 Fourier-Transform spectrometer by using the KBr technique. The diffuse reflectance measurements (DRIFT) were performed with the FTIR spectrometer IRF-180 (Centreof ScientificInstrumentations, Berlin). A homemade diffuse reflectance device with a heated sample cup was used. The Raman spectra were obtained on a DILOR XY spectrometer (Villeurbanne, France) using an Ar+ Laser (5 14.5 nm, 50 mW) ILA 120 (Carl Zeiss, Jena) and with a resolution of 4 cm-1.
The crystallinity of the synthesized samples was checked by SEM photographs, n-hexane adsorption capacity, and XRD measurements. The three methods clearly demonstrate the good crystallinity of the synthesis products. The morphology of the crystals varies with increasing germanium content in the gel from hexagonal-likethin twinned pellets to crystals with spherical shape (Figure 1). This observation is in line with results obtained on Al-, Ga-, and Fe-containingZSM-5 zeolites, which show growing twinning with decreasing silicon-to-element ratio.29 Although germanium substitution creates no lattice charge, the same effect is observed. This may indicate that the different sizes of the incorporated elements could be the reason for the observed shape differences. Other factors like the pH value were kept constant.30 Additionally, an increasing size of the crystals with growing germanium content from 8 (Si/Ge = 11) to 16 pm (Si/Ge = 3/2) is observed. The n-hexane adsorption isotherm curves are shown in Figure 2. The shapes of the curves are not different from those of the original ZSM-5.3' They correspond to the type I.32 Already at low relative pressure the pores are saturated with n-hexane. The Ge-ZSM-5 samples crystallized from gels with a Si/Ge ratio of 11 and 4 exhibit an adsorption capacity of 1.25 and 1.2 mmol/g. Taking into account the increase in the lattice density by about 8%dueto the incorporation of the heavier germanium thesevalues are similar to those obtained for highly crystalline ZSM-5.33 However, the sample with maximum germanium content in the gel has a distinctly lower crystallinity. The n-hexane capacity is reduced by ca. 20%. The amount of mesopores (hysteresis) seems to increase with growing germanium content, which agrees with the increasing twinning of the crystals. The XRD powder pattems of the Ge-ZSM-5 samples (Figure 3) are very similar to that reported for ZSM-5 with respect to the positions and intensities of the observed reflecti0ns.3~ All reflections could be indexed. The low background and the wellresolved narrow reflections reveal the good crystallinity of the samples. With one exception no crystalline or amorphous byproducts are found. Only the sample crystallized with maximum germanium content in the gel seems to contain quartz impurities. This is deduced from theoccurrenceof a low-intensity peakat 28 = 26.6'. At this position, thereflectionwith thedistinctly highest intensity appears in the XRD pattern of quartz.3s However, the other lower intensity quartz reflections are not
C
IOpm Figure 1. SEM photographsof Ge-ZSM-5 zeolites crystallized from gels with different Si/Ge ratios: (a) 1 1 , (b) 4, (c) 3/2.
observed. Therefore, quartz should be present only in small quantities and should not cause the observed drastic decrease in the n-hexane adsorption capacity for this sample. In comparison with the other samples this zeolite shows a distinct decrease in the intensities of all reflections and a simultaneous increase of the background over the whole 8 range. Remarkably, the typical curvature in the range 28 = 15-30' caused by amorphous
-
Kosslick et al.
5680 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 c
! k u
*-----e----------
.c
F
9,
0.5
SI [ O G e ]
312
- adsorption
branch
---- desorption
branch
L
C 3
E
a
1
,
1.o Relative pressure / p/ps
0.5
Figure 2. n-Hexane adsorption isotherm curves of Ge-ZSM-5 zeolites.
1
8
-100
-110 -120 PPM Figure 5. 29SiMAS N M R spectra of Ge-ZSM-5 zeolites.
TABLE I: Unit Cell Parameters of Ge-ZSM-5 lattice constants/A Si/Gegel m
11 4 312
20ldeg
Figure3. XRD pattern of Ge-ZSM-5 zeolites. Asteriskdenotes trigonal
Ge02.
, IO i200
LOO
600
T e m p e r a t u r e /T
Figure 4. TG-DTA curve of Ge-ZSM-5 (Si/Ge in the gel = 4).
byproducts is absent.36 Hence, it is concluded that the observed loss in n-hexane adsorptioncapacity is a result of lattice distortion by the incorporated germanium atoms. This conclusion is further supported by TG curves of the as-synthesized samples (Figure 4). From thelossinweight in therangeoftemplatedecomposition between 673 and 773 K the template content was determined.
a0
bo
co
20.1 114 20.1413 20.1519 20.1349
19.8308 19.8557 19.8844 19.8839
13.3939 13.4288 13.4503 13.4335
angle/deg unit cell vol/A3 89.445 89.378 89.397 89.489
5341.8 5370.4 5389.7 5378.0
Also the sample with maximum germanium content contains ca. 4 TPA ions per 96 T atoms as theoretically expected. The incorporation of germanium into the MFI framework is evidenced by the expansion of the unit cell for Ge-ZSM-5 in comparison to the silicaliteI referencesample (Table I). However, a Si/Ge gel ratio lower than 4 does not lead to a further unit cell increase. Interestingly, the observed expansion of the unit cell by about 0.3% is smaller than expected from the average increase of the mean T-O distance by about 1.3%. The mean T-O bond length should increase from 1.60 A in silicalite to 1.62 A in GeZSM-5 containing ca. 12 Ge/unit cell. The crystal symmetry of the template-containing samples is always orthorhombic. After calcination the symmetry is changed to monoclinic. In contrast to trivalent elements,37J*germanium incorporation does not change the symmetry from monoclinic to orthorhombic. This finding supports the more general validity of recently published results.54 The 29SiMAS NMR spectra of the Ge-ZSM-5 zeolites (Figure 5 ) consist of two main signals. One signal at -1 13 ppm is assigned to Si [OGe] groups. According to the known relationshipbetween thechemical shift ofthe29SiNMR signaland theelectronegativity of the next nearest neighbors of silicon nucleG9 the shoulder at -1 10 ppm is assigned to Si[ lGe] groups in the framework. This assignment is supported by the observed increase in the intensity of this shoulder and the simultaneous broadening of the NMR signals toward lower fields with growing germanium content. Interestingly the high-field part of the signal (third line at -1 16 ppm in the deconvoluted spectrum shown in Figure 6 ) is not affected. The latter is assigned to silicon atoms located in oxygen rings with large Si-O-Si angles.40, Therefore, it is concluded that the isomorphous substitution of silicon by germanium in the framework T-sites leads to an average decrease in the T-0-T
Ge-ZSM-5 Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5681 R' OD
-100
-110
-120
PPM
Figure 6. Line deconvolution of the 29SiMAS NMR spectrum of GeZSM-5 (Si/Ge = l l ) .
TABLE II: Determination of the Si/& Framework Ratio by Line Deconvolution of the "si NMR Spectra gel
Si/Ge framew peak
11
10.6
4
6.5
1 2 3 1 2 3
I3671 3732
LOO0
3700
3400
3100
Wavenumberlcm-' Figure 7. Hydroxyl vibration spectra of Ge-ZSM-5.
main peak ppm
linewidth/Hz area/% obsd/ppm
-110.8 -113.8 -116.6
299 232 181
-110.8 -113.7 -116.0
466 260 249
37.7 54.0 8.3 61.4 33.6 4.9
V,,SIOSI
I
V,SIOSI
,
vD5R
-113.6
-113.4
angles but Si-oSi groups with large angles are hardly disturbed. That would mean that germanium is not able to substitute silicon in all T-sites of the MFI framework although it is chemically similar to silicon. The observed signal-broadening shows additionally that the germanium incorporationleads to a considerable lattice distortion. The range of T-0-T angles and T-O distances should be increased. The 29Si MAS NMR spectra were deconvoluted by using the BRUKER Linesim program. The line fit shows that indeed only the three signals at -1 10 ppm (Si[ lGe] groups) and -1 13 ppm aswellasat-l16ppm(Si[OGe] groups) arenecessarytosimulate the observed spectra (Figure 6). This provides evidence that silicon atoms with more than one next nearest germanium are essentially absent, which is of interest concerning the discussion of possible framework sites of incorporated germanium atoms. Although a broad shoulder is observed at low field in the spectrum of a sample containing large amounts of germanium, the assumption of an additional line centered at -103 ppm for SiOH groups in defect sites4' does not improve the fit. However, as no distinct shoulder at this position is visible in the spectrum, the introduction of this signal into the line fit seems to be physically meaningless. From the intensity ratio of the 29SiNMR signals, it is possible to determine the Si/Ge ratio in the framework of the Ge-ZSM-5 ~eolites.9~ At low germanium content in the gel (from Si/Ge = to 11) there is observed a simultaneous increase of germanium incorporation with decreasing Si/Ge ratio of the gel (Table 11). If the Si/Ge ratio in the gel is further decreased, the Si/Ge framework ratio approaches a limit of 6.5 (ca. 12.8 Gelunit cell). The doubling of the germanium content in the gel from Si/Ge = 4 to Si/Ge = 312 leads only to an insignificant increase of germanium in the framework. Due to the line broadening the deconvolution of the NMR signal of this sample was not unambiguous. However, different iterations led to a Si/Ge framework ratio not lower than 6.5. The additional but small increase in the germanium framework content is indicated by the further broadening of the 2% MAS NMR signal and the observed changesin the OHvibration spectra. The incorporation of germanium gives rise to the appearance of a new absorption 0)
I
1250 1000 750 500 Wave number / c m
-'
Figure 8. IR lattice vibration spectra of Ge-ZSM-5 zeolites.
band at about 3670 cm-I, which is due to the formation of GeOH besides SiOH group.42 Indeed, the intensity ratio of the SiOH to GeOH vibration bands is decreased with the lowering of the Si/Ge ratio in the gel from 4 to 3/2 (Figure 7). Hence, the germanium framework content is increased. The infrared lattice vibration spectra of Ge-ZSM-5 zeolites are shown in Figure 8. As reported recently only a slight shift of theasymmetricT-0-Tvibration bandcould be detecteddespite the relative large extent of germanium incorporati~n.~~ Therefore, the spectra were repeatedly recorded by using the FTIR technique in order to get a more detailed picture about the influence of germanium incorporation on the lattice vibration bands. The main absorption bands are observed at ca. 1100 and 1225 cm-I. They are assigned to asymmetric T-0-T stretching vibrations. The absorption band at 800 cm-I is assigned to the symmetric T-0-T stretching vibrations and the absorption band at 450 cm-' to T-0-T deformation vibrations. A structurally-sensitive vibration band appears at 545 cm-1.43-44With increasing germanium content a shift of the asymmetric T-0-T vibration band from 1102 to 1090 cm-I is observed. At the same time an inverse shift of the symmetric T-0-T vibration band to higher wavenumber from 795 to 807 cm-I occurs. The change in t_he w?venymbej ratio of the T-0-T stretching vibration bands [vas - v s ] / [ v a s +vs] was found tobedirectlycorrelated withtheT-0-T angle in ~ilicates.~s This relation was also proven to be qualitatively
Kosslick et al.
5682 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
1
I
i
I
inverse shifts indicate a distinct decrease in the T-O-T angle caused by the germanium incorporation. Additionally, a successive increase in the width of the T-0-T deformation band in the directionto higher wavenumber is observed up to a germanium content of ca. 12 Ge/unit cell. The small increase of germanium incorporation going from Si/Ge = 4 to 3/2 results in an additional broadening of the deformation band, especially in the band maximum range. Hence, the incorporation of more than 12 germanium atoms is accompanied with a drastic lattice strain. This could explain the observed limited germanium incorporation. Furthermore, the formationof trigonal GeO2 (a-quartz structuretype) is observed by the appearance of characteristic bands at 439 and 512 ~ m - l . ~ 3
Discussion
I
1
1
1000
500
I
Wovenumber / cm-’
Figure 9. Raman spectra of Ge-ZSM-5zeolites. Asterisk denotes trigonal GeOz.
valid for zeolites.46With increasing germanium content this ratio varies from 16.16 to 14.15. From this finding a decrease in the averageT-O-T angle due to germaniumincorporation is deduced. In another series of samples (Si/Ge < 150) it could be shown that up toaSi/Geratioof 25 theobservedlatticevibrationspectra of Ge-ZSM-5 are very similar to the corresponding spectra of silicalite I.47.48If the germanium content is further increased, a new shoulder at 1030 cm-l arises in the spectra. The intensity of this shoulder is connected with the germanium content. Additionally a very weak absorption band appears at 670 cm-I. They are assigned to asymmetricand symmetricT-0-T Stretching vibrations of Si-O-Ge groups in the framework in analogy to the changesobserved in the spectra of SiOrGe02 glasses containing &16% The asymmetricand symmetric T-O-T vibration bands of Ge-O-Ge groups in GeO2 appear at 890 and 570 cm-l, re~pectively.5~The corresponding Ge-0 distance is 1.74 A and the T-0-T angle is 130°. The values for the Si-oSi bridges are 1.6 A and 140°.51 From the absence of these bands in the IR spectra, it follows that Ge-O-Ge bridges are unlikely to occur in Ge-ZSM-5. Also the vibration band of octahedrally coordinated GeOs arising at 720 cm-l is not observed. Therefore, extraframework germanium seems not to be present in the GeZSM-5 zeolites under consideration. The Raman spectra of Ge-ZSM-5 zeolitesin the range of lattice vibrations (Figure 9) show the most intensive bands in the range of Si-0-Si or Si-O-Ge deformation vibrations at 350-400 cm-1. Thisvibration is assigned to the motion of the oxygen atom along the bisecting line of the T-O-T angle.s2 The frequency of this vibration is very sensitive to the T - G T angle. A less intense band at 806 cm-l is assigned to the symmetric Si-oSi stretching vibrations. With increasing germanium content, a new band appears at ca. 685 cm-I. It is due to the presence of tetrahedrally coordinated germanium in the framework and is assigned to the corresponding symmetric Si-O-Ge stretching vibrations. The intensityof this band is increasedwith growing germanium content and reaches a maximum at Si/Ge = 4. The intensity ratio of the symmetricSi-04 t0Si-e band is not changed by additional increase of the germanium content in the gel. With increasing germanium content the T-O-T deformation band is shifted to higher wavenumber. At the same time the symmetric Si-oSi band is shifted to lower wavenumber by about 8 cm-1. These
The results of the physicochemicalcharacterization reveal that germanium is readily incorporated into the framework of the MFI structure. This is obviously a consequence of the close chemical similarity of the elements germanium and silicon. Additionally, no negative lattice charge is introduced into the framework, which might have an influence on the isomorphous substitution via energetic effacts.54 Up to an Si/Ge ratio of 11, linearity is found between the Si/Ge ratio in the gel and in the framework. Contrary to expectation, the Si/Ge framework ratio approaches a limit of 6.5. That means, only about 13% of the Si atoms, or 12.8 per unit cell, may undergo isomorphous substitution with germanium. After the incorporation of germanium, only Si[OGe] and Si[lGe] groups are observed in the 29SiMAS NMR spectra. Although the incorporation of germanium is accompanied by a distinct signal-broadeningthe highfield part of the signal assigned to Si atoms located in T-O-T bridges of large angles is affected only to a minor extent. These sites do not remarkably participate in the isomorphous substitution. Furthermore the IR spectra reveal the formation of SiM e bridges whereas, however, Ge-O-Ge bridges are not observed. These findings indicate that germanium is not statistically distributed over the T-sites in the Ge-ZSM-5 framework. Not all T-sites are preferred by the germaniumatoms as may be concluded from the similarity of both elements, the known isomorphism of Ge02and Si02,51and the variation of the GeO2 content in SiOfleOz glasses between 0 and lO0%1.~9This can be explained considering the equilibrium angles of Si-oSi and Ge-O-Ge bridges of 140° and 130°, respectively. Hence, the formation of large angles that is necessary for the crystallization of the MFI structure should be energetically favored for Si-oSi bridges as long as silicon is available in the synthesis. Indeed, the upper limit of the germanium framework content of about 12.8 atoms per unit cell found in this study can be rationalized by assuming a site preference of the germanium atoms in the MFI structure. The MFI structure differs from other Si02 structures by the great variation of T-O distances and T-O-Tangles.S5.56 Additionally, the mean T-O-T angleof 152O is distinctly larger than the equilibrium value of about 140O. Therefore, the incorporation of larger atoms into the MFI framework should be hindered by greater sterical constraints. Several theoretical investigations on ZSM-5 zeolites led to the prediction of preferential aluminum sites in the MFI framework. A preference of the positions 9 and 12 as well as 2, 5, and 6 located in the oxygen-4-ring, oxygen-6-ring, and oxygen-5-ring, respectively, was found (Figure The observed site differences are mainly due to differences in the zeolite framework energy. The larger A1 atoms should fit better into framework positions where the corresponding T-O bond lengthening and T-O-T angle lowering are easier to realize. Unfortunately, due to the low A1 content of the MFI frameworks, it was not possible to check the validity of computational results by XRD structural data. It is plausible to assume that the same structural constraints are valid for germanium. Germanium has
The Journal of Physical Chemistry, Vol. 97,No. 21, 1993 5683
Ge-ZSM-5 Zeolites
~ubstitution.~3 The resultson Ge-ZSM-5 show that these relations cannot be automatically applied to other systems. 23
15
Figure 10. Structural unit of the MFI-type framework.
about the same size as aluminum. If the postulated site preference of aluminum is adopted for germanium atoms, the upper limit of germanium atoms in the framework should be 2Olunit cell. Each unit cell contains four cross sectionswith the structural unit shownin Figure 10. However, thepositions2, 5,and6areadjacent sites of a single oxygen-5-ring and should not be occupied simultaneously due to the expected lattice strain. The simultaneous occupation of all three sites or of site 2 and 6 or site 2 and 5 requires the formation of Ge-O-Ge bridges. The simultaneous occupation of site 2 and 6 would establish Si[2Ge] groups. However, both Ge-O-Ge bridges and Si[2Ge] groups are not observed. Therefore, only one of these three positions in a single oxygen-5-ring should be occupied. In this way the available number of preferred framework sites is reduced to 12/ unit cell. This valueis in agreement with the experimental result. Additional incorporation of germanium into preferred oxygen5-ring sites leads to a considerable lattice strain. And this is indeed observed if the Si/Ge ratio in the gel is decreased to 3/2. The increase in the germanium content is small but indicated by the change in the intensities of the absorption bands of terminal SiOH and GeOH groups in the OH vibration spectra. However, the crystallinity of this sample is decreased by a considerable lattice distortion caused by the incorporation of the large germanium atoms. The latter is manifested in the observed drastic line-broadening of the Z9Si MAS NMR signal, the broadening of the T-0-T deformation band in the Raman spectrum, and the decrease in the intensity of the structure-sensitivevibration band at ca. 545 cm-' in the IR spectrum as well as in the decrease in the XRD crystallinity and n-hexane adsorption capacity. Surprisingly, these remarkable changes cause only small changes in the lattice parameter. The T-T distances are increased only to a low extent. This can beconcluded from thecomparatively low increase of the unit cell parameters and the only slight shift of the wavenumber of theasymmetricTUTstretchingvibration band by about 10 cm-I, although the Ge-0 bond is longer than the Si-O bond by about 6%. The Raman spectra reveal a decrease in the mean T-O-T angle. This decrease is also deduced from the inverse shift of the asymmetric and symmetric Si-OSi vibration bands in the IR spectra. All these findings suggest that the incorporation of germanium into the MFI framework results in a shift of the oxygen atoms perpendicular to the T-T axis. As the mean T-0-T angle in the MFI structure is far from the optimum value of 140°, such a shift should be also energetically favored. On the other hand the T-0 distance is increased so that the larger germanium atoms can fit into framework positions. Such a movement of the oxygen atoms would allow the maintenance of the Si-O and T-T distances after germanium incorporation, explaining the unexpected low changes in lattice parameters and IR wavenumbers. Summarizing, both Si04 and Ge04tetrahedra try to maintain their T-O distances in the MFI structure. This conclusion is supported by recent EXAFS data.58 As a consequence, the Si-oSi and Si-O-Ge stretching vibration bands are decoupled, resulting in a splitting of the corresponding absorption bands, however, only with small shifts of the wavenumbers. This behavior is completely different from the observations on faujasites. There was found a systematical shift of absorption bands with varying Si/Al ratio in the framework,which can be used to predict the degree of isomorphous
Conclusion Although germanium is chemically similar to silicon, only a partial isomorphous substitution of silicon for germanium is observed. The upper limit of germanium incorporation is found to be about 12 per unit cell. This limit can be rationalized by assuming an occupation of energetically-favored T-sites by germanium as recently predicted for aluminum from theoretical computations. The Ge-ZSM-5 framework consists of Si[OGe] and Si[ lGe] groups. Beside Si-OSi bridges, Si-O-Ge bridges are formed. Ge does not substitute silicon in sites of large T-O-T angles. These findings predict a site preference of the germanium atoms in the MFI framework. The incorporation of germanium affects mainly the T-0-T angles in the first coordination sphere of the germanium atoms. They decrease. The T-T distance seems to be only slightly affected. In consequence, the bridging oxygens of the Si-O-Ge groups may be shifted into the channels. This would lead to changes of the pore shapes. The Ge-ZSM-5 system is very suitable for the investigation of structural effects of isomorphous substitution. The lack of lattice charge and its hydrophobicity minimize the disturbing effects of interstitial cations and water adsorption. And the effects of isomorphous substitution are detected in the physicochemicalcharacterization only when the Si/Ge ratios are 11 or below. This ratio is difficult to achieve by isomorphous substitution with trivalent elements. References and Notes (1) Tielen, M.; Gallen, M.;Jacobs, P. A. Proc. Int. Conf. Zeolite Catalysis; Siofok, Hungary; May 1985;p 1. (2) de Ruiter, R.; Jansen, J. C.; van Bekkum, H. Zeolites 1992,12,56. (3) Kutz, N . A. In Perspectives in ZeoliteMolecularScience; Flank, W. H., White, T. E., Eds.; ACS Symposium Series 368;American Chemical Society, Washington, DC, 1988;p 532. (4) Argauer, R. J.; Landoldt US.Patent 1972,3,702,886. (5) Gabelica, Z.;Blom, N.; Derouane, E. G. Appl. Catal. 1983,5,227. (6) Chu, C. T. W.; Chang, C. D. J . Phys. Chem. 1985,89,1569. (7) van Koningsveld, H.; Jansen, J. C.; van Bekkum, H. Zeolites 1990, 10. ~. , 235. (8) Simmons, D. K.; Szostak, R.; Agrawal, P. K.; Thomas, T. L. J. Catal. 1987,106, 287. (9) Handreck, G. P.;Smith, T. D. J . Chem. Sot.,Faraday Trans. 1 1989, 85,3215. (10) Gricuskofke, T. J.; Gerte, R. J.; Kokotailo, G. T. Appl. Catal. 1989, 54. 177. (1 1) Bayedse, C. R.: van ddr Pol, A. J. H. P.; van Hoof, J. H. C. Appl. Catal. 1991,72,81. (12) Kosslick, H.; Richter, M.; Tuan, A. V.; Parlitz, B.; Szulzewsky, K.; Fricke, R. In Zeolite Chemistry and Catalysis; Jacobs, P. A., Jaeger, N . I., Kubelkova. L.. Wichterlova. B.. Eds.: Elsevier: Amsterdam, 1991;P 108. (13) Handreck, G. P.; Smith, T. D. J . Chem.Soc.,Faraday Trans.i1989, 85. .., 3195. (14) Szostak, R.; Nair, V.; Thomas, T. L. J . Chem. Sot., Faraday Trans. I1987.83, 487. (15) Kotasthane, A. N.; Shiralkar, V. P.; Hegde, S. G.; Kulkarni, S.B. Zeolites 1986,6, 253. (16) Vorbeck, G.; Richter, M.; Fricke, R.; Parlitz, B.; Schreier, E.; Szulzewsky, K.; Zibrowius, B. In Catalysis and Adsorption by Zeolites; Ohlmann, G., Pfeifer, H., Fricke, R., Eds.; Elsevier: Amsterdam, 1991;p ~~
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