3306
J . Phys. Chem. 1991, 95, 3306-3310
Vanadlum-Contalnlng ZSMS Zeolltes: Reaction between Vanadyl Trichloride and ZSMS/Slllcallte B. I. Whittington and J. R. Anderson* Chemistry Department, Monash University, Clayton, Victoria 31 68, Australia, and CSIRO Division of Materials Science and Technology, Clayton, Victoria 31 68, Australia (Received: September 20, 1990) Treatment with VOC13vapor (593-793 K) results in removal of silanol groups from silicalite and H-ZSM5 and formation of (=!SiO)3=V0 species that are relatively resistant to hydrolytic regeneration of silanols. With H-EMS, VOC13 treatment also removes mast Bransted acid sites associated with framework aluminum. This removal is only partly reversible on treatment with HzO: the irreversible and reversible parts are associated respectively with dealumination and with proton removal via binding of vanadium species at framework aluminum. There is no X-ray diffraction evidence for structural change as a result of these treatments. ESR revealed small amounts of paramagnetic vanadyl species in hydrated V/silicalite and V/ZSMS, attributed to reduction from Vs+ by HCl, possibly augmented with V/ZSMS by reaction with zeolite acidity. At 293 K the paramagnetic vanadyl is mobile in hydrated V/silicalite and immobile in hydrated V/ZSMS.
Introduction
The presence of transition-metal sites in pentasil zeolites has the potential for generating oxidation catalysts with size/shapeselective properties. In principle, the transition-metal atoms may be either in the zeolite framework (intrinsic to the zeolite lattice) or present as some foreign species, possibly bound chemically at the zeolite surface (extrinsic to the zeolite lattice). (When used without qualification, “surface” is taken to refer to the internal and external zeolite surface without distinction.) Two general routes are available for the preparation of transition-metal-containing zeolites-ab initio hydrothermal synthesis or postsynthesis modification. The established importance of vanadium compounds as catalysts for oxidation reactions’*2has made zeolites containing vanadium attractive candidates for study, and both hydrothermal ~ y n t h e s i sand ~ . ~ postsynthesis modification’f’ methods have been used. Although hydrothermal synthesis (using V3+) may introduce intrinsic vanadium, it has been shown that this product is subject to collapse of the zeolite framework under oxidative activation,’J’ presumably as a result of the oxidation of vanadium to V5+. The present study examines the postsynthesis treatment of the ZSMS/silicalite zeolites (silicalite refers to a ZSMS with an extremely low aluminum content) with gaseous VOCl, in order to try to obtain a product with adequate structural integrity under oxidative conditions. The reaction of VOCI, with zeolites has been previously mentioned in the patent literature.s*6 However, these patents give no information about the content, location, or nature of any incorporated vanadium. Some comparative results are also reported here for treatment of H-ZSM5 using TiC14or CrO2CI2. Experimental Section
Zeolites: Published methods were used for the preparation of H-ZSM59 and silicalite.1° Samples were characterized by standard methods of X-ray diffraction (XRD), scanning electron microscopy, and elemental analysis. Two samples of H-ZSM5 were used: sample A, aluminum content 52 mmo1/100 g, morphology of hexagonally terminated crystals, average crystal dimension ca. 3 rm; sample B, aluminum content 26 mmol/100 g, morphology of rectangular prisms, average crystal dimension ca. 0.6 rm. XRD data agreed with the standard literature for ZSM5.9 The silicalite sample had an (impurity) aluminum content of 0.7 mmo1/100 g, morphology of cubelike prisms with rounded corners, and evidence for some intergrowth; average crystal dimension ca. 4 pm. XRD data agreed with the standard literature for silicalite.’O Treatment with VOCI, was carried out in a quartz reactor (35-mm diameter) with the zeolite ( 5 g) carried as a layer on a quartz frit. The zeolite was first dehydrated at 793 K for 2 h in To whom correspondence should be addressed.
0022-3654/9 1/2095-3306$02.50/0
a stream of dry nitrogen (120 cm3 m i d ) , following which the zeolite was treated for 1 h at 793 K with the nitrogen stream saturated with VOCl, vapor a t 293 K ( p V ~ ca. , , 2.1 kPa). At the end of this treatment, the sample was purged with dry nitrogen for 1 h at room temperature after initial rapid cooling from 793 K. In most cases, the treated zeolite samples were examined after shaking the zeolite overnight in 1 M HCl (35 cm3) at room temperature. X R D The following were used: Siemens MDS-500 diffractometer; Ni-filtered, Cu Ka radiation. Scanning electron microscopy: A Hitachi S45 ILB microscope was used. The samples, suspended in ethanol, were dispersed by ultrasonification before being placed on a graphite sample block. The specimen was coated with gold after drying. infrared spectra: FTIR spectra were recorded on a Mattson IRlOl 10 spectrometer; resolution 4 cm-I; scan accumulation were 128/spectrum. Self-supporting wafers, typically 30 mg an-*, used. The sample cell allowed in situ heating of the wafers, and subsequent treatment with metal halide vapor. For comparative purposes, the spectra are normalized to the sum of the areas of the 1998- and 1878-cm-’ peaks. ESR spectra: The ESR spectra were recorded in the X band (ca. 9.15 GHz) on a Varian E-12 spectrometer with a E-101 microwave bridge. Magnetic fields were calibrated against proton NMR frequencies, and microwave frequencies were measured with a direct-reading EIP 548A counter. ESR spectra were recorded at 77 or 293 K on unevacuated, hydrated samples. X-ray photoelectron spectroscopy: Measurements were made on a Vacuum Generators ESCALABS (A1 Ka excitation) a t a working pressure of ca. lo-* Pa. The intensity ratios %/AI and Si/V were obtained from the integral areas of the Si(2p), A1(2p), and V(2p3/*) peaks after linear background subtraction. From these ratios, the atom number ratios were obtained by using cross sections from the literature.” All samples were adhered to Scotch tape, except that which was pelleted in a die (pressure ca. 9000 kPa). The V(2p3/*)binding energies were determined by reference (1) Jsrgensen, K. A. Chem. Reu. 1989, 89, 431-458. (2) Gellings, P. J. Chemicol Soc. Spec. Rep. 1988, 7 , 105-124. (3) h i , T.; Medhanavyn, D.; Praserthdam, P.; Fukuda, K.; Ukawa. T.; Sakamoto, A.; Miyamoto, A. Appl. Catal. 1985, 18, 31 1-324. (4) Miyamoto, A.; Medhanavyn, D.; Inui, T. Appl. Coral. 1986, 28, 89-103. (5) Chang. C. D. U.S. Patent No. 4,273,753, 1981. (6) Chang, C. D.; Miale, J. N. US. Patent No. 4,576,805, 1986. (7) Cavani, F.; Trifiro, F.; Jiru, P.; Habersbergcr, K.; Tvaruzkova, Z. Zeolites 1988, 8, 12-1 8. (8) Habersberger. K.; Jim, P.; Tvaruzkova, Z.; Centi, G.; Trifiro, F. Reoct. Kinet. Coral. Lurt. 1989, 39, 95-100. (9) Chen, N. Y.; Miale, J. N.; Reagan, N. Y. US.Patent No. 4,112,056, 1978. ~.
(10) Anderson, J. R.; Foger, K.;Mole, T.; Rajadhyaksha, R. A.; Sanders, J . V. J . Carol. 1979, 58, 114-130. (1 1) Schofield, J. H. J. Electron Spectroscop. Related Phenom. 1976,8, 129-137.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3307
Vanadium-Containing ZSM5 Zeolites
TABLE I: Cwmoitioa of H-HSMS rsd S W d t e Treated witb VOCI, ~
~ _ _ _
elemental anaLL acidity/mmol Al/mmol V/mmol ('O0 sample (100 g)-I (100 g)-1 totar externale 52 47 0.30 ( I ) H-ZSM5(A) (2) H-ZSM5(A)-treated VOCI,' 30 51 25 0.41 (3) sample 2 + H202washb ND' 3 26 ND 26 24 1.81 (4) H-ZSMS(B) 6 18 1.38 (5) H-ZSMS(B)-treatedVOCIJO 12 (6) silicalite 0.7 2 0.11 49 14 0.41 (7) silicate-treated VOCIj' 0.7 (8) sample 7 + H202wash ND 3 2 ND oVOC13,793 K, 1 h; N2purge, 293 K, 1 h; acid wash. bVOC13-treated zeolite H202 (120 vol), 293 K, 17 h; water wash. CEstimatedaccuracy f l mmo1/100 g. dEstimatcd accuracy f 3 mmo1/100 g. @Estimatedaccuracy f0.02mmo1/100 g. fND, not determined.
+
to the adventitious carbon peak at 284.6 eV. Acidify measurement: Total zeolite acidity was measured by exhaustive Na+ exchange (1 M NaNO,, room temperature) and subsequent titration of the liberated acid (0.05 M NaOH). Surface acidity was measured by methylene blue adsorption,12 using a single-point determination at a methylene blue concentration of 2.45 X mmol g-l. Elemental analyses: Vanadium and aluminum contents were determined by a commercial analytical laboratory.
Results Chemical characterization: Table I shows elemental analysis and acidity data for H-ZSM5 and silicalite samples both before and after treatment with VOCl, vapor. The small excess of total acidity Over total aluminum content for H-ZSM5 indicates a small amount of nonframework aluminum, a conclusion qualitatively confirmed by 27AlMAS-NMR. The total acidity of the silicalite is consistent within experimental error with the impurity aluminum content. Treatment of H-ZSM5 and silicalite with VOCI, vapor at 793 K introduced vanadium resistant to room-temperature washing with 1 M HCI. For H-ZSM5 the aluminum content was also reduced. Subsequent treatment with H202(1 20 vol) was sufficient to remove most of the remaining vanadium from the zeolites and thus makes these zeolites unsuitable for use as oxidation catalysts when H202 is used as the oxidant. The introduction of vanadium into silicalite resulted in the production of acidity although, on a molar basis, the vanadium content was considerably higher than the acidity. The total acidity of the VOCI,-treated H-ZSM5 similarly was considerably less than the sum of the residual aluminum and the introduced vanadium. XRD examination gave no evidence for structural change in the silicalite or ZSM5 as a result of these treatments. Infrared spectroscopy: The FTIR spectrum of H-ZSM5 (B) (Table I, sample 4) showed two peaks in the OH stretching region (Figure 1, curve a) after drying in situ under vacuum at 593 K for 4 h. The peak at 3605 cm-' is assigned to Br~nstedacid groups associated with framework aluminum,13J4while that at 3740 cm-I is assigned to silanol groups of low or negligible acidity present in the channels and on the external ~ u r f a c e . ' ~Further J~ heating of the sample at 593 K under vacuum resulted in a slow reduction in the intensity of the 3740-cm-' silanol peak (ca. 20% decrease in 8 h), probably resulting from dehydrative ring closure between neighboring silanol groups.I4 Treatment of the H-ZSM5 sample at 593 K with VOCl, vapor from an injected dose of 5 pL (liquid) of VOCI, (5.29 X mol, p v ~ca. ~ 430 , Pa) gave, after 10 min of equilibration time, the infrared spectrum shown in Figure 1, curve b. The infrared cell was then evacuated, and the procedure of VOCl, injection (5 p L (12) Handnck, 0.P.; Smith, T. D. J. Chem.Soc.,Faraday Trans. I 1988, 84,4191-4201. (13) Woolery, G. L.; Alemany. L. 8.; Dessau, R. M.; Chester, A. W. Zeolires 1986, 6, 14-16. (14) Darsau. R. M.; Schmitt. K. D.; Ken, G. T.; Woolery, G. L.; Alemany, L. B. J . Carol. 1981, 104, 484-489.
I
I
I
!
I
I
4000
3800
3600
3400
3200
3000
c m-l
Figure 1. FT infrared spectra of (a) H-ZSMS (AI, 26 mmol(100 and (b)-(d) H-ZSM5 after treatment at 593 K with increasing amounts of VOC13 vapor (for details, see text).
TABLE 11: Reactivity of Metal Chlorides witb H-ZSMS' by Infrared Spectroscopy
reactant treatment condtns VOCI, 593 K; pvocl, 3.5 kPa, 10 min TiCI, 643 K; hick = 3.2 kPa, 10 min CrO2CI2 693 K; pcroEll = 4.7 kPa, 10 min
site removal*/% Branstcd acid silanol (3605-cm-l (3740-cm-I band) band) 100 82 100 90 30 66
'H-ZSMS(B) (Table 1, sample 4), 26 mmol A1/100 g; evacuated in the infrared cell at 593 K before treatment with 40 p L of the metal halide. From integrated band intensities.
(liquid)), equilibration, spectral acquisition, and evacuation repeated twice to give curves c and d in Figure 1. With increasing VOCI, treatment in the curve sequence a-d, Figure 1, there is a progressive reduction in the intensity of both the 3605- and the 3740-cm-l peaks. From the integrated peak intensities, it is estimated that in curve d all of the Br~nstedacid sites and ca. 85% of the silanol groups had been removed. The VOCI, clearly has access to most if not all of the channels of the H-ZSM5, as well as the external surface. A further 55 p L (liquid) of VOC13 was administered by injection to the zeolite at 593 K followed by equilibration and evacuation. Subsequent addition of 60 p L (liquid) of water to the treated sample at 593 K ( p H p= 27 kPa) gave an infrared spectrum showing no regeneration of the 3740-cm-l peak and regeneration of Bransted acid sites to an extent of ca. 60% of the original content of the untreated zeolite. The reactions of T U 4 and Cr02C12vapor with H-ZSMS(B) (Table I, sample 4) were also examined by F H R for comparison with the behavior of VOCI,. In each case, the H-ZSM5 sample, at 643 K for TiC14 and at 693 K for CrO2CI2,was treated for 10 min with the vapor from an injected dose of 40 p L of liquid halide (TiCI4, 3.64 X lo4 mol; Cr02C12,4.93 X lo4 mol). Data for the loss of the Br~nstedacid sites and of the silanol groups, obtained from the integrated peak intensities, are given in Table 11.
X-ray photoelectron spectroscopy: XPS data yielded values for the Si/V and Si/Al ratios in the zeolites and also provided a determination of the oxidation state of the vanadium. The XPS technique gives analytical results strongly weighted in favor of the accessible external surface of the specimen, and as a consequence, the results can be strongly dependent on the method of sample preparation. This may involve grinding or pelleting, which fractures the sample particles and exposes the internal surface.15J6 (15) Hughes, A. E.; Wilshier, K. G.; Sexton, B. A.; Smart, P. J . Carol.
1983,80, 221-227.
Whittington and Anderson
3308 The Journal of Physical Chemistry, Vol. 95, No. 8,1991 TABLE III: X-ray Photoelectron Spectroscopic Analysis of H-ZSMS and Silicalite Treated with VOCI, vanadium binding molar ratio energy, V(2p312)/eV sample V(2p312)leV ( S i W h m (Si/Al)xm H-ZSMS(A)' 61 powder 26 pelleted V / ZS M 5d a 106 powder pelleted 5 17.0 I69 74 V/silicalitee powder 5 17.2 17 b a V ( ~ P ' / ~signal ) not detectable; analytical vanadium content of sample 51 mmo1/100 g. bA1(2p) signal not detectable; analytical aluminum content of sample 0.7 mmo1/100 g. OH-ZSMS(A) (Table I, sample I ) , 52 mmol A1/100 g. dV/ZSM5 (Table I, sample 2): HZSM5(A) at 793 K treated with VOCI, for I h; N2purge at 293 K for 1 h; acid wash. CV/silicalite (Table I, sample 7): Silicalite at 793 K treated with V0Cl3 for 1 h; N2 purge at 293 K for 1 h; acid wash.
Y 1
I
~
I
I
~
~~
(16) Anderson, J. R.; Chang, Y.-F.; Hughes, A. E. Catal. Lett. 1989, 2, 279-286. (17) Larsson, R.; Folkesson, B.; SchBn, G. Chem. Scr. 1973, 3, 88-90. (18) Jonson, B.; Rebenstorf, B.; Larsson, R.; Andersson, S.L. T. J . Chem. Soc. Faraday Trans. I 1988, 84, 1897-1910. (19) Kucherov, A. V.; Slinkin, A. A. Zeolites 1987, 7, 583-584. (20) Sass, C. E.; Chen, X.; Kevan, L. J. Chem. Soc., Faraday Trans. 1990, 86, 189-191. (21) Che, M.; Canma, 8.; Gonzalez-Elipe,A. R. J. Phys. Chem. 1986,90, 61 8-621,
I
~
I
3000
2500
XPS results are given in Table 111 for V/ZSM5 and for V/ silicalite. The vanadium binding energy of 5 17.0-51 7.2 eV indicates the presence of Vs+ a t the surface of the specimens. Although these values are a little higher than 516.6 eV reported for V2O5,I7they agree well with the value of 517.0 eV for vanadium oxide on silica.l* No vanadium could be detected in the surface of the supported powder specimen of V/ZSM5 ((Si/V)xB = a). However, when this material was pelleted so as to allow examination of internal surfaces via particle fracture,I6 a value of (Si/V)xps = 169 was obtained. This Si/V is greater than that expected from bulk elemental analysis (Si/V = 31), indicating that in the pelleted sample the surface that was examined was partly the original vanadium-deficient external surface and partly the freshly fractured internal surface. From the data in Table 111, it is seen that untreated H-ZSM5 (A) gave a %/AI of 61 (powder specimen) and 26 (pelleted specimen). These values are to be compared with the Si/AI ratios expected from bulk elemental analysis (Si/AI = 30) and the total acidity measurement @/AI = 33). The data clearly indicate that the as-synthesized, acid-washed, ZSM5 had an external surface somewhat deficient in aluminum compared with the bulk. ESR spectroscopy: Although Vs+ is invisible in ESR, it is known that calcination of Vz05/H-ZSM5 in air forms a paramagnetic VOz+ species.19 Intensity enhancement of this V02+ signal by water vapor has been observed, and as this is greater than the intensity loss from line broadening due to oxygen,20 it is possible to obtain ESR spectra of the vanadium-treated zeolites by using hydrated, unevacuated samples. Indeed, coordination of water to V02+ can be sufficient to prevent its reaction with oxygen.21 By comparison with ESR measurements at 77 K on a frozen VOSO, solution, it is estimated that only ca.7% of the vanadium in the V/ZSMS and only ca. 3% in the V/silicalite samples were ESR-active at 77 K. The ESR spectra from the hydrated V/ ZSM5 (Table I, samples 2 and 5) and hydrated V/silicalite (Table I, sample 7 ) a t 293 and 77 K are shown in Figures 2 and 3, respectively. The presence of hyperfine splitting in these spectra indicates that the paramagnetic vanadium centres are well separated. The ESR spectral parameters from V/silicalite and V/ZSM5 are given in Table IV. The V/silicalite spectrum at 293 K is
I
Il I l I l~ l
~
l
,
~
4000
3500
Hlgauss
Figure 2. ESR spectra of hydrated V/ZSM5 (samples 2 and 5, Table I) at (a) 300 and (b) 77 K.
)
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I
3000
~
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I
3500
I
I
~
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4000
H haw Figure 3. ESR spectra of hydrated V/silicalite (sample 7, Table I) at (a) 300 and (b) 77 K.
isotropic and is similar to that observed for slowly tumbling V02+(aq) ions2*indicating only a weak interaction between the hydrated vanadyl and the silicalite. At 77 K the V/silicalite spectrum is anisotropic, due to quenching of the free vanadyl rotation. In contrast, the ESR spectra from V/ZSM5 are anisotropic at both 293 and 77 K and closely resemble the V/silicalite spectrum a t 77 K. All three of these anisotropic spectra closely resemble spectra previously reported at 47319 and 77 KZ0after calcination of V,0s/H-ZSM5 and are attributable to nonrotating vanadyl (e&, V02+) in the zeolite. The spectral temperature dependence observed with V/silicalite follows a trend similar to that reported for hydrated V 0 2 + exchanged into Y - ~ e o l i t e , ~ ~ although the spectrum at 300 K for the VO*+/Y-zeolite was only partially isotropic. An isotropic vanadium ESR spectrum has been observedz4at 20 K for a silica gel impregnated with NH4V03and subsequently (22) Campbell, R. F.; Freed, J. H. J . Phys. Chem. 1980,84,2668-2680. (23) Martini, G.; Ottaviani, M. F.; Seravalli, G. L.J. Phys. Chem. 1975, 79, 1716-1720. (24) Van Reijen, L. L.; Cossee, P. Discuss. Faraday Soc. 1966, 41, 277-289.
The Journal of Physical Chemistry, Vol. 95, No. 8, 1991 3309
Vanadium-Containing ZSM5 Zeolites
TABLE I V ESR Panmeten from H-ZSM5 a d Silicnlite Treated with VOCI:, ESR parameters sample VIZSM5"P V/ZSM 5'4 V/Silicalite*d
V/siIicaIiteb" V/siIicaIitec+'
VO*+(aq) V,O,/H-ZSMS'J VzO,/ H-ZSMYJ
V02+(aq) W+(ag) V/Si02 -*
meas temp/K
gl
gl
293 77 293 77 293 77 473 77 300 77 20
1.925 1.935
lad
1.975 1.980
201.9 203.6
1.976
201.7
( g ) = 1.965
1.932
75.3 ( a ) = 112.2
1.977 2.00 1.993
202.5 200.0 205.0
1.976 1.982
201.4 202.8
(g) = 1.964
1.933 1.922
ref this work this work this work this work this work this work 19 20 22 22 24
( a ) = 112.2
(g) = 1.965 1.935 1.93 1.937
lad 81.9 78.4
76.1 85 79.2 ( a ) = 115.7
75.5 77.8
'V/ZSMS (Table I, sample 2 or 5): H-ZSMS(A or B) at 793 K treated with VOCl, for 1 h; N2 purge at 293 K for 1 h; acid wash. bV/silicalite (Table I, sample 7): silicalite at 793 K treated with VOCI, for 1 h; N2purge at 293 K for 1 h; acid wash. cAs for b, but no acid wash. dHydrated sample. 'VZO, + H-ZSM5 ground together, calcined in air, 1023 K/4 h. fDehydrated sample. #As for d, calcined 723 K. *SiOzgel impregnated with aqueous NH,VO, solution, dried, and calcined in air at 773 K.
calcined at 773 K. In this case, the isotropy probably arises from the presence of isotropic V04&. However, it was observed24that hydration of the specimen yielded an anisotropic spectrum at 20 K,apparently due to conversion of V04' to immobile V02+. This result makes it improbable that the isotropic spectrum found for hydrated V/silicalite at 293 K could be due to VOt-.
Discussion to possess silanol groups of low or H-ZSM5 is neglgible acidity and Brornsted acidic groups associated with framework aluminum. It is therefore convenient to discuss the results from the treatment of H-ZSMS with VOCl, in terms of reaction at the silanol groups and at the Brornsted acid sites, using the reaction with silicalite to provide additional data for reaction at the silanol groups. Reaction at silanol groups: The initial mode of reaction of VOCI, with silanol groups plausibly occurs with the elimination of HCI and the formation of Si-0-V linkages according to reaction 1. The stoichiometry of this reaction has been analytically
( ~ i 0 ) , V ( = 0 ) ( 0 H ) , ~ , (1)
I1 confirmed for silica and Aero~il.~' Hydrolysis of the V-CI bonds in I introduces V-OH groups which are acidic,% the extent of this introduced acidity depending on the value of n ( 1 In I 3). The value of n in turn depends on the concentration and mutual disposition of the silanol groups. Data from the present work with VOC1,-treated silicalite are in general agreement with reaction 1 and allow some specification of the species. The data in Table I show that the molar ratio of (total V)/(total acidity) is 3.2. Thus a substantial proportion of the introduced vanadium does not carry an acidic O H group, and we therefore conclude that much of the vanadium is present as (=JiO)3=V0 groups with n = 3 in reaction 1. Since these samples had been washed in 1 M HCI at 293 K, this conclusion implies that most of the (=SiO),=VO groups are resistant to hydrolysis of the S i U V linkages under these conditions. Lesser amounts of =VO(OH) (n = 2) and -VO(OH)z (n = 1) are also probably present and would contribute to the observed acidity. The formation of (=!3iO)FVO groups requires three adjacent silanol groups,and this implies that the silanol groups tend to occur in clusters such as hydroxyl nests, rather than being uniformly distributed. A similar suggestion has previously been made in (25) Vedrine, J. C.; Auroux, A.; Bok, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.;van Hooff, J. H. C.; van den Berg, J. P.; Wolthuizen, J.
J. Coral. 1979, 59, 248-262. (26) Kol'tsov, S.1.: Malygin, A. A.; Volkova, A. N.; Aleskovskii, V. B. Zh. Fiz. Khim. 1973, 47, 988-991. (27) Jerschkewitz, H.-G.; Ehrhardt, K. 2.Anorg. Allg. Chem. 1983,502, 102-112. .. - ..-. (28) Phillips, C. S.G.; Williams, R. J. P. Inorganic Chemistry;Oxford: 1965 Vol. 1, p 518.
connection with the alumination of ~ i l i c a l i t eand ~ ~ in . ~ its reaction with trimethyl~hlorosilane.~~ In the case of H-ZSM5, FTIR data in Figure 1 show that silanol groups (3740-cm-' band) are removed by treatment with VOCI, at 593 K, and further treatment with water vapor at the same temperature failed to regenerate most of the original silanol groups. By comparison, it has been reported32 from work using Aerosil at a lower treatment temperature (473 K) that about 45% of the reacted silanol groups are regenerable in a VOC13/H20 treatment. It is also knownZSthat silanol groups are regenerable by water vapor treatment at room temperature following thermal dehydroxylation of H-ZSM5 at 1173 K, making it improbable that a similar sort of dehydroxylation mediated by VOCI, could be responsible for the lack of silanol regenerability in the VOCI3-treated H-ZSM5 in the present work. The results obtained from VOCI, treatment of silicalite and H-ZSMS as summarized above indicate that under relatively high-temperature treatment conditions (593-793 K) the silanol groups react to form mainly ( S i O ) , = V O groups, which are resistant to hydrolysis of Si-0-V linkages in 1 M HCI at 293 K or in HzO vapor at 593 K. The data in Table IV show that the ESR signals from V/ silicalite come from (V@+). (Here and in the following discussion we use (VOz+) to indicate a species containing a VIv vanadyl group, without necessarily implying a free V02+ ion.) At 293 K an isotropic ESR spectrum was obtained from a hydrated sample and originated from a mobile (slowly rotating) molecule, although at 77 K the spectrum was anisotropic due to a lack of rotational freedom. An isotropic ESR spectrum requires molecular rotation about three axes, and this degree of rotational mobility places two requirements on the vanadium species: they cannot be bound to the silicalite by Si-O-V linkages, and the species must be small enough relative to the silicalite channels and/or channel intersections so that rotation is not restricted by steric effects. (This restriction could be lifted for those vanadium species at an external surface.) Examination of molecular models shows that an entity such as ( H 0 ) 2 V 0or its hydrate (H0)zVO(HzO)3would be small enough to rotate in a channel intersection, but the corresponding condensation/dehydration dimer (or any larger oligomer) would be too large for the required degree of rotation to be possible. Thus, although vanadium oxyacids normally occur as their condensation oligomers and have been so postulated in the Aerosil/V0Cl3 system after water treatment,27there is evidence in silicalite for the retention of a monomeric, paramagnetic vanadium species. (29) Chang, C. D.; Chu, C. T.-W.; Miale, J. N.; Bridger, R. F.; Calvert, R. B. J. Am. Chem.Soc. 1984, 106, 8143-8146. (30) Yamagishi, K.; Namba, S.;Yashima, T. J . Coral. 1990, I21,47-55. (31) Kraushaar, B.; Van de Ven, L. J. M.; de Haan, J. W.; Van Hooff, J . H. C. Stud. Surf. Sci. Catal. 1988, 37, 167-174. (32) Hanke, W.; Bienert, R.; Jerschkewitz, H.-G. 2.Anorg. Allg. Chem. 1975, 414, 109-129.
Whittington and Anderson
3310 The Journal of Physical Chemistry, Vol. 95, No. 8,1991
The V4+ paramagnetic species (e+, ( H 0 ) 2 V 0 or its hydrate (HO),VO(H,O),), are present only as a small proportion (a. 3%) of the vanadium in V/silicalite, consistent with the conclusion that most vanadium is present as (=SiO),=VO in which Si-0-V linkage is relatively resistant to hydrolysis. The V4+ species could be formed upon acid washing (1 M HCI at 293 K),although the presence of mobile (V02+)in the hydrated, as synthesized sample (Table IV) would suggest their formation upon interaction of the (=SiO),=VO with residual HCI generated in reaction 1. In both cases, we suggest that reduction to V4+ occurs by reaction with HCI either from the acid wash or as residual.33 (Reduction of Vs+ to V4+ by HCI is a known reaction (cf. ref 34).) Reaction at Bransted acid sites: Treatment of H-ZSMS at 7 9 3 K with VOCI, gave a (Si/Al)xPs of 106 for the powder specimen. This compares with the corresponding ratio of 61 for a powder specimen of the untreated zeolite (Table 111) and clearly indicates surface dealumination by VOCI,. This VOC1,-treated sample also gave (Si/Al)xps at 7 4 for the pelleted specimen, compared with a corresponding ratio of 26 for a pelleted specimen of the untreated zeolite (Table 111). This shows that the dealumination was not confined to the external surface but penetrated into the interior, a result confirmed by bulk elemental analytical data (Table I, sample 2). The FTIR data in Figure 1 show that treatment of H-ZSMS with VOCI, at 593 K resulted in the permanent removal of ca. 40% of the original Bransted acid sites, since these were unregenerable by water vapor treatment at 593 K. The remaining 60% were those that although removed by VOCI, treatment could be regenerated by water vapor treatment. Those sites that are removed permanently we associate with the loss of framework aluminum, while those that are regenerable we suggest are Bransted sites that have interacted with VOCI, without removal of aluminum from the framework and from which the vanadium species can be hydrolytically removed by high-temperature water vapor treatment. This hydrolytically reversible association between VOCI, and a Bransted acid center based on framework aluminum may be a precursor stage to dealumination, as indicated schematically in reaction 2. The exact nature of "(V=O)3+" in U
0 'AI=-
+
'(v=0)3+'
reaction 2 is unknown, although it could indicate a V5+hydroxyvanadyl species of unspecified degree of oligomerization or a species bound at the aluminum. The data in Table IV show that while the ESR signals from both the V/silicate and V/ZSMS come from species containing vanadyl (V02'), the hydrated V/ZSMS differs from V/silicalite in that the former gives spectra that are anisotropic at both 77 and 293 K. Anisotropy at 77 K is to be expected from lack of rotational mobility whatever the species, but from the presence of anisotropy at 293 K we conclude that in V/ZSM5 the (V02+) species is either bound or is large enough for rotation to be restricted in the zeolite channels for steric reasons. We do not have evidence to make a definite choice between these alternatives. However, we note that close association between paramagnetic vanadium and aluminum centers has previously been found in H-ZSMS calcined with V2Os by using electron spin echo modu(33) Malygin, A. A,; Volkova, A. N.; Kol'tsov, S.I.; Aleskovskii, V. B. Zh. Obshch. Khim. 1973, 43, 1436-1440. (34) Cotton, F. A.; Wilkinson, G . Advanced Inorganic Chemistry, 5th td.; Wiley: New York, 1988; p 612.
lationZ0and from ESR "super-hyperfine" line ~plitting.'~Since the ESR spectra of these samples are anisotropic at 473 K,I9 it would not be unreasonable to propose a similar interaction between the paramagnetic vanadium and framework aluminum in our V/ZSMS. Alternatively, examination of molecular models shows that lower oligomers (e.g., dimer and trimer) of (HO),VO or its hydrate can adopt configurations that allow accommodation in the channels of H-ZSMS but with insufficient rotational freedom to allow an isotropic ESR spectrum. The level of the ESR-detectable species (V02+)observed from V/ZSMS (ca. 7% of total V) was greater than from V/silicalite (ca. 3% of total V), suggesting a possible source of (V@+) species from V/ZSMS additional to the HCI reduction of (=SiO),=VO groups operating with V/silicalite. Since there is previous evidence that strong Bransted acidity is necessary in the ZSM-5 structure for the generation of paramagnetic vanadyl from calcination with Vz0s,20~35 we suggest an additional reductive process of the sort indicated in reaction 3, which differs from the reduction previously suggested.20 2V02' + 2H+ 2V02+ + H20 + f / z 0 2 (3) +
As judged in terms of ease of removal of silanol groups and Bransted acid sites using infrared data, VOCI, and TiC14 are of roughly comparable effectiveness. However, both are much more effective than CrO2CI2(cf. Table 11).
Conclusions The reaction of VOCI, vapor with silicalite and H-ZSMS has been studied at 593-793 K by means of FTIR, XPS, ESR,acidity measurement, and elemental analysis. Silanol groups are removed from both silicalite and H-ZSMS with the probable formation of (=SiO),=VO species that are relatively resistant to hydrolytic regeneration of silanols. In addition, with H-ZSMS this VOC13 treatment removes most of the Bransted acid sites associated with framework aluminum. This removal of Brernsted acidity is only partly reversible on treatment with H20. It is concluded that the irreversible part is associated with dealumination, and it is suggested that the reversible part is associated with the removal of protons and the binding of vanadium-containing species a t framework aluminum centers. XRD examination gave no evidence for structural change in the silicalite or ZSMS as a result of these treatments. XPS examination showed that very little of the introduced vanadium is located at the external surface of the V/ZSMS, implying that VOCl, has ready access to the channel structure. Further, XPS examination of the zeolite interior via particle fracture showed that most of the introduced vanadium remains as Vs+. In both V/silicalite and V/ZSMS, ESR showed the presence of paramagnetic vanadyl species, probably formed by HCI reduction of vs+,although this ESR-active vanadium was only a small fraction of the total vanadium (ca. 3% for V/silicalite and ca. 7% for V/ZSMS). At 293 K the V/silicalite spectrum is isotropic due to a slow tumbling motion of the hydrated vanadyl species, indicating a weak interaction with the silicalite. Steric considerations suggest that these tumbling vanadyl species must be monomeric. At 77 K the spectrum is anisotropic due to quenching of this motion. With V/ZSMS, the ESR spectrum is anisotropic at both 293 and 77 K, indicating a relatively strong interaction between the hydrated vanadyl species and the zeolite, with binding probably occurring near framework aluminum. VOC13 and TiCI, are of roughly comparable effectiveness for the removal of silanol groups and acidic sites, but both are much more effective than CrO2CI2. Acknowledgment. We are grateful to A. E. Hughes for making the XPS measurements and to J. Warne for assistance with the ESR measurements. Registry No. VOCI,, 7727-18-6.
(35) Kucherov, A. V.; Slinkin, A. A.; Beyer, G . K.; Borbely, G . J. Chem.
SOC.,Faraday Trans. I 1989, 85, 2731-2141.
