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Ind. Eng. Chem. Res. 1997, 36, 52-59
Structure of Sulfated Metal Oxides and Its Correlation with Catalytic Activity Dan Fraenkel* Engelhard Corporation, 101 Wood Avenue, Iselin, New Jersey 08830
Sulfated metal oxides (SMO’s) were prepared by thermal decomposition of metal sulfate salts at various temperatures in an attempt to relate their physicochemical properties with the decomposition temperature and the resultant catalytic activity. Work centered on SMO’s of Ti, Zr, Fe, Sn, and Al, which have been claimed in the literature to possess superacidity. In all cases, the BET surface area (SA) passed through a maximum as a function of sulfate decomposition temperature. Up to that maximum, a linear correlation was found in the case of Ti and Zr between SA and catalytic activity derived from literature cumene cracking data. Above the maximum, a close interrelation was revealed between SA, the average crystallite size (Dav), and the sulfate level (e.g., wt % SO4) by applying a simple, straightforward geometric model. From the specific surface area (SSA), i.e., surface area occupied by a single sulfate species, two saturated surface states of the sulfate have been found: a dilute state corresponding to an SSA of about 0.50 nm2, in the case of Ti, Zr, and Al, and a dense state (“close-packed”) of about 0.14 nm2, in the case of Fe and Sn. The results are discussed in light of the known literature and believed to indicate that SMO’s have well-organized surface structures and distinct sulfate centers that may act as strong acid catalytic sites. Specifically, sulfated zirconia was found (by elemental analysis and XPS) to have a surface metal-to-sulfur ratio of 7-8, and the sulfate groups can be modeled in terms of (O-)3SdO tripods, whether single or chained (e.g., surface “polysulfates”), as previously proposed based on IR and Raman studies. Introduction Sulfated oxides of various metals are known to possess catalytic activity by virtue of their strong surface acidity (for review, see Tanabe et al., 1989, 1990; Yamaguchi, 1990). Of particular interest are structures claimed to exhibit superacidity, such as sulfated zirconia, ZrO2/SO42-. Like fluorine-based solid and liquid superacids and some Friedel-Crafts systems, they effectively catalyze the commercially important isomerization of n-butane and n-pentane at as mild as ambient conditions (Tanabe et al., 1989, 1990; Yamaguchi, 1990; Hino et al., 1979; Hino and Arata, 1982), yet they do not contain any halogen and thus offer an environmentally more acceptable alternative to liquid and FriedelCrafts acids currently used for alkylate and oxygenate (e.g., MTBE) production. A great deal of effort has been devoted in recent years to studying sulfated metal oxides (SMO’s) and trying to explain their nature. Still the astonishingly high acidity and catalytic activity of SMO’s have largely remained a mystery. SMO’s are usually made by the precipitation-sulfation (PS) method: the respective metal hydroxide is precipitated from a salt solution by increasing the pH through addition of concentrated ammonium hydroxide; the filtered, washed, and dried solid is reacted with a sulfate source, e.g., sulfuric acid, and then calcined at elevated temperatures (normally above 700 K). The highly acidic (or “superacidic”) system is obtained at a narrow temperature range, allowing a loose, metastable oxide structure to be formed, e.g., the tetragonal phase in the case of ZrO2, with a few percent sulfate residing on its surface. Another apparent way of making SMO’s is by controlled thermal decomposition of a metal sulfate salt. Arata et al. (1990) made sulfated zirconia and titania * Correspondence address: Matheson Gas Products, Advanced Technology Center, 1861 Lefthand Circle, Longmont, CO 80501. S0888-5885(96)00196-0 CCC: $14.00
by decomposing the respective sulfate salts and claimed that the obtained systems were superacidic. The sulfate decomposition (SD) method avoids the complications associated with the PS method (type of starting salt, concentration and pH effects, various factors in the sulfation stage, etc.). As in the PS case, the eventual SMO product is shaped up by the calcination/decomposition temperature. However, in the SD method the SMO is an intermediate in the thermochemical transformation of a sulfate salt to the corresponding oxide, through eliminating the sulfate, essentially as free SO3. Thus, the SD-derived SMO product is created only when most of the sulfate has been removed, at temperatures usually higher than those needed for making an SMO of similar sulfate level by the PS method. On the other hand, an SD-derived SMO might be expected to be more ordered and structurally uniform than the parallel SMO obtained by the PS method. Surprisingly, only a little information exists in the literature on the relationship between structural parameters and physical properties, and the catalytic (or acidic) behavior of SMO’s. In particular, there is no known trend in the sulfate level and no established correlation with other factors. We have been interested in learning more about SMO’s, and through correlating their catalytic activity with their structural properties, gain better understanding, hopefully at the atomic level, on how these systems work. As a first step toward achieving this goal, we chose to use the SD method to generate various structures with different metals, obtained at different temperatures. All metals chosen for this study have been claimed to possess superacidity and superacid catalytic activity in their sulfated oxide forms (Matsuhashi et al., 1991). Through evaluating the obtained systems, we have tried to model their surface structure and elucidate the state(s) of sulfate on the surface. In limited cases, comparison has been made between SMO’s prepared by SD and those prepared by the PS method. We believe that the results © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 53
reported and discussed in this paper shed important light on some structural features of SMO’s hitherto not given adequate attention. Experimental Section Sample Preparation. SMO’s were prepared by the above two methods. Using the PS method, a metal salt, normally the nitrate, was dissolved in water and titrated with concentrated ammonium hydroxide (28%) up to a pH of >8; the precipitated hydroxide was filtered, washed, dried, and then treated at room temperature with dilute sulfuric acid (usually, 1 N), followed by drying and calcining. An example is sample PS-1 made as follows: 15.0 g of ZrO(NO3)2‚xH2O (tech., Aldrich) was dissolved in 120 mL of deionized water, and the solution was filtered over Whatman paper to remove undissolved particles. The solution was stirred magnetically, and NH4OH (∼28%) was added to it dropwise to a final pH of 8.7 (11.0 mL of the base). The precipitate was filtered, washed with deionized water, and dried at ∼105 °C for 3 days. The obtained solid, 6.4 g, was powdered and treated with 50 mL of 1 N H2SO4 under stirring for 1 h. After filtering, the solid was dried at ∼105 °C for 17 h, yielding 6.62 g of sulfated oxide. Calcination at 650 °C for 3 h gave 4.64 g of PS1. This sulfated zirconia sample was used in the XPS studies (see below and Table 3). Using the SD method, the metal sulfate was thermally decomposed (for 3-5 h) to allow a direct, singlestep production of the SMO; higher temperatures were usually required to obtain systems of parallel physicochemical and structural characteristics compared to those of the PS method. TGA curves were used in each case to estimate the temperature range at which the metal sulfate decomposes. Then, discrete temperatures were selected for the decomposition, covering the entire range of sulfate-to-oxide transformation. The sulfate salts used in the present study (SD method) were Al2(SO4)3‚x(14-18)H2O, assay >98%, from EM Science; FeSO4‚7H2O, >99.0%, from Fisher Scientific; and SnSO4, 95+%, TiOSO4‚xH2SO4‚xH2O, and Zr(SO4)2‚xH2O, 99.99%, all from Aldrich. Chemical and Physical Analysis. The obtained samples were chemically analyzed for metal (wet analysis) and sulfate content (sulfur assay) and characterized by nitrogen surface area measurementssBET and Matrix (“t-plot”)sas well as TGA (for composition, thermal stability), XRD (for crystallinity, crystal phase, average crystal size) and, in limited cases, SEM-TEM (for apparent crystal size, crystal morphology), and IR (results not reported here) and XPS spectroscopy. True densities were measured by the He pycnometer method. Surface area measurements were done on a Micromeritics ASAP 2400 instrument. X-ray diffraction analysis was performed on a Philips automated power PW1820 diffractometer using Cu KR1 radiation (1.5406 Å), operated at 45 kV and 40 mA, and equipped with a Philips 2 compensator and a graphite monochromator; spectra were run between 3 and 80° (2θ) at a step size of 0.02 (2θ) and a count time of 0.5 s. SEM images were obtained on a ISI-DS 130 dual-stage scanning electron microscope and TEM images on a Hitachi H600 STEM at 100 kV accelerating velocity. TGA curves were recorded on a 951 PL Thermal Sciences thermogravimetric analyzer. XPS analyses were performed on a Fisons/SSX 206 ESCA system using monochromatized Al KR (1486.6 eV) excitation, a spot size of 600 µm, and a pass energy of 100 V. Pressure in the analysis
Table 1. TGA Dataa temp °C reaction
onset
inflection
“pure oxide”b
(1) SnSO4 f SnO2 (2) TiOSO4‚H2O f TiO2 (3) FeSO4 f Fe2O3 (4) Zr(SO4)2 f ZrO2 (5) Al2(SO4)3 f Al2O3
400 ∼580 610 ∼550 760
472 616 675 706 854
530 670 710 760 900
a ∼30 mg sample, under air, heating rate 20 °C/min. position curve flattens.
b
Decom-
chamber was maintained at 2 × 10-7 Pa or less during data collection. Fisons X-Probe ESCA software was used for data collection and processing. Relative peak areas and Scofield sensitivity factors (Scofield, 1976) served for data quantification. Results Surface Area and Sulfate Level. All BET surface area (SA) values were practically identical with surface area values calculated by the t-plot method, indicating that the SMO materials obtained were all entirely compact (nonmicroporous). In all cases of SMO’s made by the SD method, the surface area clearly passed through a maximum as a function of decomposition temperature. An increase in SA occurred from the onset of the decomposition until only a minute amount of sulfate has remained (usually, a few percent as SO4). Then, the SA started to decrease with further loss of the residual sulfate. Thermogravimetric data are summarized in Table 1, and decomposition stages as well as some measured properties of the various products obtained are listed in Table 2. The Zr and Ti cases are presented in Figures 1 and 2, respectively, as SA and % SO4 plots against temperature; it is clearly seen that, as long as most of the sulfate salt is present, the SA is very low and constant and it starts to rise when about half of the sulfate content has been lost, passing through the maximum when over 90% of the original sulfate is decomposed. This pattern was observed also for the other metals. Figures 1 and 2 also present catalytic activity values derived from literature conversion data of cumene cracking over similar samples (Arata et al., 1990), assuming second-order cracking (i.e., inverse activity is 1/R ) 1/χ - 1, where χ is the fraction of cumene converted). In both cases, a nice, approximately linear correlation is observed between activity (R) and SA up to the SA maximum. Above-maximum SA values show a “hysteresis” behavior; namely, they exhibit a delayed response to the decline in activity. The system thus tends to preserve higher SA than expected from catalytic activity. Not being a combined surface area of the sulfate salt and of the SMO, as could be the case in the left-hand side of the SA curve, the high-temperature portion of the SA curve may reflect lattice sintering; it therefore could be expected to directly interrelate with crystal size and sulfur level, as indeed has been found (see below). X-ray Diffraction and Electron Microscopy. XRD spectra show the crystalline sulfate of both Zr and Ti up to the onset of decomposition and then fine-grain, low-density tetragonal zirconia and anatase, respectively. The anatase contains some phase impurity up to about 920 K, and the zirconia is mostly tetragonal at 980 K, mixed tetragonal and monoclinic at 1030 K, and all monoclinic at 1090 K. Sn transforms cleanly from the highly crystalline sulfate to essentially pure cas-
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Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
Table 2. Thermal Decomposition Results metal oxide ZrO2a ZrO2
TiO2
SnO2
Al2O3
Fe2O3
a
decomp temp, °C
crystalline phase
(650) 704 732 760 788 816 538 593 610 649 649 760 371 427 482 538 593 649 704 854 871 899 982 1038 671 688 710 760 816
mostly Tb mostly T mostly T T + some Mc
SA,
M Sd [Ti(SO4)2] Ae + trace Uf A + trace U A mainly Sg Ch
C Gi G G
Sample PS-1; see text. b Tetragonal. c Monoclinic. Sn3O4. h Cassiterite. i γ-Alumina.
d
m2/g
% SO4
SA/% SO4
73
2.94
24.8
115 102 71 25.8
4.05 3.63 2.11 0.84
28.4 28.1 33.6 30.7
50.1 54.4 54.2 43.6 34.6
1.23(?) 6.03 1.65 1.10 0.54
9.0 32.8 39.6 64.1
35.8 37.7 37.4 38.3 30.7 23.3 134 158 147 141 131 14.8 19.4 9.4 7.8 3.8
7.89 3.09 5.46 4.05 2.94 0.75 4.58 1.92(?) 4.08 0.81 0.45 1.55 3.54(?) 0.92 1.02 0.21
4.5 12.2 6.8 9.5 10.4 31.1 29.3
F, g/cm3 HB Pyc
3.63 5.6
est
Dav, nm calc
12.0 12.2
11.3 12.8
27.1
50.6
24.5
37.4
34.1
54.2
14.6
26.1
19.2
42.2
4.8 4.5
15.8 16.7
2.54 3.84
5.15 6.95 1.65
36.0 174 291 9.5 (5.5) 10.2 7.6 18.1
∼3.7 3.39 5.18
Sulfate. e Anatase. f Unknown phase. g SnSO4 + traces of cassiterite and possibly
Figure 1. Decomposition of zirconium sulfate (activity data from Arata et al., 1990).
siterite. In the Al case, the sulfate decomposes exclusively to γ-alumina, while in the case of Fe, within the decomposition range, only X-ray amorphous structures have been detected. The XRD line-broadening technique (Scherrer relation) was used to estimate average crystal sizes (Dav). As seen in Figure 3, Dav increases with decomposition temperature above the SA maximum but is apparently constant below that maximum, where it corresponds to fine grains of the sulfated oxide. This behavior is especially pronounced in the case of ZrO2. The XRD crystal size value of the sulfated zirconia sample prepared by the PS method (PS-1, calcined at 650 °C), 12.0 nm, is in excellent agreement with the data of the lowertemperature SD samples (up to 750 °C), as shown in Figure 3. This indicates, as also supported by the XPS results below, that systems obtained by both preparation methods might be structurally very similar. Electron micrographs of PS-1 are in good agreement with
Figure 2. Decomposition of titanium sulfate (same as Figure 1).
the XRD average size. TEM (120 KX, scale ∼190 000: 1) reveals crystallites of irregular shape, 6-10 nm in size, whereas SEM (23 kV, 30.3 KX, scale 32 000:1) depicts almost uniform arrays of fine grains, typically less than 50 nm in size. X-ray Photoelectron Spectroscopy. XPS was used to determine the location of the sulfate on the zirconia and to indicate possible differences owing to the preparation method used. Three samples were analyzed, PS-1, SD-650 (obtained by decomposing zirconium sulfate at 650 °C), and SD-730 (decomposition at 730 °C); they were all run in duplicates. Results are summarized in Table 3. The spectra show the sulfur to be present as sulfate in all cases and the Zr3d5/2 binding energy (BE) is consistent with Zr4+ for all samples. The Zr3d3/2 and Zr3d5/2 binding energies (B E’s) in PS-1 and in SD-730 are both shifted ∼1 eV from those of SD-650. This suggests a difference in the zirconium chemical environment between surface and bulk sulfate and points out that PS-1 and SD-730 may
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 55 Table 3. XPS Binding Energies and Elemental Analysis BE’s and BE intervals,a eV
elemental composition
sample
S 2p
Zr3d5/2
C 1s-S 2p
C 1s-Zr3d5/2
Zr3d5/2-S 2p
S/Zrb
% SO4c
% SO4d
A/Be
SD-650
169.4 169.3 169.5
183.6 183.7 182.6 182.7 182.7
101.0 100.9 102.0 f 101.9 101.9
14.2 14.2 13.1 13.2 13.2 13.3
1.41 1.33 0.14 0.13 0.12 0.11
51.7 50.0 9.0 9.3 8.3 7.9
51.8
169.5 169.4
115.2 115.3 115.1 f 115.1 115.2
1.0 0.97 2.2 2.3 2.8 2.7
SD-730 PS-1
4.1 2.9
a Refers to ubiquitous C 1s ) 284.6 eV. b XPS relative atom percent ratio. c Weight percent sulfate calculated from XPS relative atom percent data. d Weight percent sulfate assay. e Apparent (XPS)-to-bulk (assay) S ratio. f Anomalous C 1s peaks this second run.
Figure 3. Effect of temperature on SMO crystal size (estimated by the XRD line-broadening method).
be “true” SMO’s and almost identical in their surface electronic structure, whereas SD-650 is practically a bulk sulfate. Indeed, as shown in Table 3, comparing the sulfate levels as obtained by XPS with bulk analysis data confirms the above assumption (see also Figure 1). The apparent (XPS)-to-bulk (assay) S concentration for SD-650 is 1.0, agreeing with a bulk sulfate structure, whereas both the sulfate level and the S/Zr atom ratio of ∼1.4 indicate a mixed sulfate-oxysulfate, which could be approximated as ZrOSO4‚0.5Zr(SO4)2 or ZrO2‚2Zr(SO4)2. Thus, with respect to the starting sulfate, only one-third has decomposed and there is no clear zoning of sulfate-free zirconia. On the other hand, PS-1, with a S/Zr ratio of ∼0.12 and an apparent-to-bulk S ratio of ∼2.8, is a typical SMO with the sulfate concentrated on the zirconia surface. Assuming that all the sulfate of PS-1 is on the surface, the XPS measurement does not account for about 2/3 of the entire zirconia; that fraction of the zirconia can be considered as bulk pure zirconia beneath the sulfated zirconia layer that makes the crystallite “crust”. Idealizing the sulfated zirconia crystallite as a spherical particle (see below) with the sulfate “decorating” the zirconia core by an external layer of ∼0.5 nm thickness, if a typical crystallite size is 10 nm (see above), then the XPS beam penetration into the crystal is 1-1.5 nm, in fair agreement with known XPS penetrable distances. SD-730 is similar to PS-1 but has a 20% higher S/Zr ratio and a 20% lower apparent-to-bulk ratio. These differences are judged to be rather small, and the two samples are thus quite similar in composition and sulfate distribution. Smoothing the sulfur 2p curves has yielded 169.8 eV for the bulk sulfate (SD-650), 169.5 eV for PS-1, and 169.3 for the SD-730 sample. This indicates that the SMO sulfur atoms of PS-1 and SD730 are electronically indistinguishable and are perhaps somewhat less electronegative than the sulfur atoms of the zirconium sulfate salt.
Figure 4. Correlation between SA and % SO4.
Discussion SA-% SO4 Correlation. In Figure 4, SA results are correlated with the sulfate levels for all five metals. Chosen were only “pure” SMO systems, i.e., those not containing any detectable residual metal sulfate. With those samples, sulfate concentration was typically below 8%. Most data, therefore, reflect the behavior of SMO at and beyond the SA maximum. As demonstrated in Figure 4, there appear to be two distinct SA-sulfate correlations, both linear and starting from the origin (i.e., SA ) a[SO4]). A weaker dependence of SA on the sulfate level (a ) 8) is observed for Fe and Sn and a stronger dependence (a ) 30) for Zr, Ti, and Al. The first relationship reflects a “dense” surface sulfate structure, whereas the second can be attributed to a dilute sulfate “phase” on the oxide exterior. This, of course, is based on the presumption that in the pure SMO all the sulfate is located on the surface, as proposed in the literature for thermally decomposed iron sulfate (Yabe et al., 1979) and as strongly indicated by our own XPS measurement of ZrO2/SO42- (see above). As apparent from Table 2, some unplotted points would scatter significantly from the correlation lines in Figure 4, such as those of very low % SO4, corresponding to the highest decomposition temperatures. The “noncorrelating” higher-SA points at lower sulfate levels may reflect a “delayed” tendency of the SMO to sinter as a function of sulfate loss, as seen especially in the Al case; this contrasts markedly with the Zr behavior which clearly shows a collapse of the fine-grain structure (highSA tetragonal phase) with the disappearance of the sulfate. The Al behavior might be due to a stronger stabilizing effect of the sulfate on γ-alumina (Al) which retains a high SA at very high temperature in the presence of even minute amounts of sulfate. γ-Alumina does not transform to δ/θ-alumina at even >1000 °C (usually, this transformation occurs at 850-900 °C), keeping a SA corresponding, in the absence of sulfate, to a temperature 200 °C lower. As a result, the Al
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Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997
Figure 6. Modes of sulfate bonding: (a) chelating bidentate, (b) bridging bidentate, (c) bridging tridentate (“tripod”).
Figure 5. SMO specific surface area (SSA) as a function of sulfate decomposition temperature.
individual behavior in Figure 4 may look more like a “flat” line (slope zero), but the reason why this SMO should, nevertheless, ideally be included in the correlation line is explained below. Specific Surface Area and Average S-S Distance. If the trends emerging from Figure 4 are real, then the dense and dilute sulfate structures are ideally uniform. The straight-line behaviors shown in Figure 4 can thus be interpreted in terms of a constant specific surface area (SSA) defined in this work as SA measured per single sulfate group, or per S atom. SSA (in nm2) is therefore calculated by the simple relation (see Appendix I)
SA SSA ) 0.016 [SO4]
(1)
where SA is given in m2/g and [SO4] in wt %. SSA values are plotted in Figure 5 against decomposition temperatures. As expected based on Figure 4, two constant values emerge, one of about 0.50 nm2 corresponding to the “dilute state” of surface sulfate (Ti, Zr, and Al) and the other of about 0.14 nm2 corresponding to the “dense state” (Fe and Sn). The average sulfatesulfate (S-S) distances, ∼0.7 nm in the dilute state and 0.37 nm in the dense state, are clearly outside the bonding range, but since uniform separation on the surface is unlikely, in reality, neighboring sulfate groups may lay quite close to each other. Sulfate coupling or polymerizing on the surface, leading to nonuniform S-S separation, is a reasonable possibility (see below). SSA values at too high temperatures (i.e., far beyond the SA maximum) are above the “constant” SSA (not shown for Zr) and are believed to be due to the destruction of the highly ordered surface sulfate state. It is plausible that the sulfate has a stabilizing effect on the surface that prevents sintering, especially in the Zr, Ti, and Fe cases, and with the gradual loss of sulfate, the oxide reorganizes itself through a controlled crystal size increase in order to maintain the same sulfate coating texture. This mechanism can prevail up to a temperature causing uncontrolled sulfate depletion, which is not instantly “compensated” any more by crystal structural changes; as a consequence, SSA in each case starts to increase (meaning loss of sulfate without parallel loss of SA). Figure 5 shows, however, that at least over a certain temperature intervalsabout 100 K or moresSSA is constant in spite of the rather dramatic changes in SA and sulfate level. This appears to be the first time that an SA-% SO4 correlation is suggested in SMO’s and concluded to bear structural significance.
Surface Sulfate Structure. It is tempting to explain the two surface sulfate states in terms of plausible sulfate surface structures, as drawn schematically in Figure 6. The bidentate sulfate, whether chelating or bridging, has been proposed in the literature based on IR analysis (Yamaguchi et al., 1986; Arata and Hino, 1988). A different analysis, however, has suggested the tripod state for Ti, Zr, and Al in the dehydrated state (Bensitel et al., 1988; Saur et al., 1986). Recently, the tripod sulfate in sulfated zirconia has been deduced from a combined IR-Raman study (Jehng and Wachs, 1993). To our knowledge, no attempt has been made so far to model SMO sulfate species based on physical measures (i.e., crystal size, surface area) and chemical analysis (assay, XPS) in order to gain a semiquantitative picture of the SMO surface. Crystalline solid oxides can be modeled by assuming a close-packed oxygen layer on the external surface at various crystallographic planes. Using the widely acceptable oxygen diameter of 0.27 nm (rO ) 1.35 Å), the effective SA occupied by a single oxygen atom on the surface is 0.073 nm2, corresponding to a square circumscribing the oxygen and whose parameter is the oxygen diameter. Fully dehydrated surface sulfate may be visualized as resulting from a fitting of SO3 molecules into an oxide surface containing oxygen vacancies (Yamaguchi et al., 1986). In one case, each SO3 molecule contributes one oxygen to the surface by filling a single vacancy and interacting with an “oxide oxygen” to form the bidentate species having two covalent bonds with “surface oxygens”. This may lead to the dense state, with the sulfate being associated with a total of four surface oxygens (3 of “oxide” type and 1 of “SO3”). However, the SA measurement may “miss” two surface oxygens because of the space taken by the sulfate group and thus “see” only the two other oxygens, or an area of 0.146 nm2. In another case of SO3 fitting into surface vacancies, one may visualize vacancy coalescence into double vacancies attracting two of the three SO3 oxygens, thus resulting in a tripod surface sulfate through bonding of the S with one oxide oxygen. This assembly of eight surface units, originally being six oxygens and two vacancies and after sulfate attachment becoming a surface entity containing a single tripod sulfate, may represent the dilute sulfate state. The single SdO oscillator is assumed to essentially block one oxide oxygen unit in the SA measurement, and the measured surface is therefore 0.073 × 7 ) 0.51 nm2. The tripod surface sulfate species, in contrast to the bidentate sulfate, has a second oxygen coordinating with a surface Zr atom and is thus formally electron-deficient. The less electronegative nature of the sulfur atom, as indicated by our XPS study, tends to support the existence of such a species. Through more spatial interaction, tripod sulfate may reduce its energy by returning, at least partially, to a closed-shell 6-electron state of the sulfur atom. Thus, tripod species may notbe isolated but
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 57 Table 4. SSA Values surface sulfate state dense dilute
SSA, nm2 Figure 6 (eq 1) calcda ∼0.14 ∼0.50
0.146b 0.51d
literature 0.15-0.30c 0.55e
a See text. b Two surface-exposed oxygens per sulfate. c For Fe, Tanaka et al., 1990. d Seven surface-exposed oxygens per sulfate. e For Ti, Chen and Yang, 1993.
rather coupled, forming disulfate surface species such as those previously suggested (Bensitel et al., 1988; Saur et al., 1986), or linearly chained (polymerized) to polysulfate. An alternative route to achieving tripod sulfate electron stabilization is through interaction with sorbate molecules such as water. The tripod model proposed here is consistent with a similar model postulated recently by White et al. (1995), based on IR and TG-MS studies. Those authors pointed out that surface zirconium atoms at the (001) plane of tetragonal zirconia coordinate with six different oxygens (instead of with eight in the bulk); the two vacancies, or coordination sites, left due to the structure termination (in the dehydrated state) could be filled by two SO3 oxygens. The sulfate thus becomes pentacoordinate with four covalent S-O bonds with the surface. It is plausible that tetracoordinate tripods equilibrate with pentacoordinate “tetrapods” to relieve their electron deficiency. At any rate, sulfate vacancy filling on the surface of tetragonal zirconia may, in fact, be the stabilization function causing the well-known inhibition of the tetragonal-monoclinic phase transformation of ZrO2. It should be noted that the surface model proposed here is coherent with the sulfate group being associated with four Zr atoms in the first surface layer (beneath the eight oxygens), whereas 3-4 more Zr atoms in the second, deeper layer are also detected by XPS, thus giving the XPS Zr/S ratio of 7-8 and a beam penetration of 1-1.5 nm (see above). Table 4 summarizes SSA data for the dense and dilute sulfate states. The above calculated values are in excellent agreement with those observed (Figure 5) based on eq 1 and in good accord with literature values for the Fe system (dense, assumed to exhibit bidentate sulfate) and the Ti system (dilute). Average Particle Size. Interestingly, in addition to the SA-% SO4 correlation, sulfate decomposition systems produced above the maximum SA temperature have been found to exhibit a rather straightforward Dav-SA correlation based on a simplified geometric structural model, as follows. Theoretically, if we assume that the oxide has a closepacked, nonmicroporous structure of spherical or cubic crystallites, then the average crystal size, Dav (in nm) is given by the expression,
Dav ) 6000/(SA)F
(2)
(see Appendix II) where SA is in m2/g and the density, F, in g/cm3. Dav can thus be calculated from SA and F, assuming that F is constant over the entire decomposition range of SMO’s beyond the maximum SA point. Experimental Dav (XRD-derived) values and handbook (HB) as well as measured (Pyc) F values are listed in Table 2. The measured F’s of SMO’s at the SA maximum are substantially lower than HB values of the respective oxide phases and are found to be close to the respective metal sulfates. Therefore, in using eq 2, F
Figure 7. SMO intrinsic cumene cracking activity (after Arata et al., 1990): Zr vs Ti.
in each case was approximated as the arithmetic average of the values for the oxide and sulfate. The fit between estimated (XRD) and calculated (eq 2) Dav values is shown in Table 2 to be quite good even though the calculated values tend to be 1.5-2 times larger than the estimated values. There might be a few reasons for this. First, the variation in F is quite substantial and the above approximation essentially introduces some inaccuracy in the calculation of Dav. Second, the XRD-derived values could be smaller than those based on SA since the SMO crust may be semicrystalline or have considerable crystallographic disorder; this sulfated oxide layer may be ∼1 nm in thickness and may influence more the smaller particles (hence the greater difference in the case of Al where the calculated Dav’s are 3 times larger than the estimated ones). Third, as discussed above, SA values may be somewhat smaller than those expected from the geometry of the measured particles due to the surface space taken by the sulfate group which masks 1-2 surface oxygens. An “occupancy factor” of 2/4 ) 0.5 is suggested (see above) for the dense state with bidentate sulfate, meaning that the “true” SA is twice the amount measured. This is applicable to Sn and Fe. In the case of the dilute state with the proposed tripod sulfate, the occupancy factor is 1/8 ) 0.125 and the true SA should be 8/7 ) 1.14 times larger than the measured SA. A correction treatment of eq 2 based on the above arguments brings the calculated and estimated Dav’s closer to each other. It is felt, however, that such a treatment is not necessary here to support the general argument of clear and rather straightforward interrelations between SA, % SO4, and Dav in SMO’s obtained by sulfate decomposition and that the point is made even with the uncorrected data as depicted in Table 2. Intrinsic Catalytic Activity. Since the SMO catalytic site is essentially associated with the surface sulfate entity and because the sulfate level correlates directly with SA, the intrinsic (or specific) catalytic activity of SMO’s can be expressed as activity (R) per surface unit. The behavior for Zr and Ti over the entire range of decomposition temperatures is shown in Figure 7. The intrinsic activity of decomposed titanium sulfate is high and constant during the formation of the SMO. In fact, the pure sulfate (500 °C) appears about as active as the SMO! For the pure SMO, this activity declines sharply between 625 and 675 °C even though the depletion in sulfate and loss of SA is very small at that temperature range. The intrinsic activity of the parallel zirconium system is 5-fold lower and extends over a broader temperature interval; this is because sulfated
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zirconia has its maximum SA at a much higher temperature. Here again, the sulfate appears to be as active, or almost so, as sulfate salt/SMO mixtures. The pure SMO loses activity rather dramatically when its temperature of formation increases beyond the SA maximum. The above behavior needs some clarification. Both sulfated zirconia and sulfated titania were claimed in the literature to be superacids (Arata et al., 1990) regardless of their method of preparation. Cumene cracking is an effective tool in probing surface acidity of solids by catalysis (and was used by the above authors as a diagnostic test for superacidity). According to our study, the SMO is superior to the starting sulfate not because of an intrinsic acidic property but rather due to the “extensive factor”; i.e., the sulfate catalytic centers are simply plentiful over the high-SA SMO as compared to the low-SA sulfate salt. It appears that both materials have catalytic sites of the same nature under the test reaction conditions and, if the metal sulfate salt does not exhibit superacidic behavior, the SMO derived from it does not either. Furthermore, the drop in activity occurs when the SMO still has a high sulfate level and rather large SA. It is noteworthy that both SD-730 and PS samples similar to PS-1 were found active (the PS samples were very active) in alkane conversion at ambient conditions. Thus, SD-730 when brought in contact with liquid pentane (1 mL over 0.4 g of sample in a sealed vial) gave after a prolonged time 1.0% conversion to a mixture containing 70% isopentane, 13% isobutane, 7% isohexane, and 10% others. PS samples under similar conditions gave 10-50% conversion to parallel mixtures. A detailed description and discussion of alkane reactions, including that of pentane, over sulfated zirconia has been presented (Fraenkel, 1994) and will be provided in a subsequent publication. The difference in catalytic activity in pentane conversion between the above types of samples, as well as between PS samples prepared using somewhat different recipes, indicates that subtle variations of the SMO structure may dramatically affect the outcome of the SMO catalytic action. Such effects may also be caused by slightly different handling precautions and reaction conditions. Nature of Acidity in SMO’s. The cumene cracking test of Arata et al. (1990) may have reflected Lewis acidity (nonsuperacidic) although those authors suggested the possible involvement of both Lewis and Brønsted sites. What supports the view of Lewis activity in the above case is (a) that the sulfate salt had the same intrinsic activity as the SMO and (b) that the activity strength order (RZr ,RTi) is the opposite of what was found in “superacidity” tests (Zr more active than Ti in butane and pentane isomerization). In another publication, Arata and Hino (1990) proposed that superacidity in SMO’s is exclusively of the Brønsted type. A tripod surface sulfate, as postulated in the present work (and in the literature), may become a Brønsted acid site by reaction with a water molecule which opens up one Zr-O-S bond to form (ZrO-)2S(dO)OH; the S(dO)OH may otherwise be a part of a di- or polysulfate complex on the surface. Conclusion When sulfate salts of Ti, Zr, Fe, Al, and Sn are thermally decomposed, the surface area (SA) exhibits a bell-shape behavior as a function of temperature (i.e., going through a maximum). Both the starting salt and the final oxide product have low SA, about 10-20 m2/g,
and the higher SA, peaking at 40-120 m2/g, is associated with a sulfated metal oxide (SMO), as an intermediate product of the decomposition. Decomposed sulfate salts of Ti and Zr possess acid catalytic activity as long as the surface sulfate species still exist. In sulfate salt/SMO mixtures, the intrinsic activity appears constant and equal to that of the undecomposed (but dehydrated and thermally activated) salt. The specific cumene cracking activity of the salt and the SMO is the same, and the activity order of Ti and Zr is reversed compared to their known superacid activity (in butane and pentane isomerization). This may indicate Lewis acid catalysis in cumene cracking, while Brønsted acidity may be more representative of the superacidic nature of SMO’s. Beyond the temperature of their SA maxima, all SMO’s of the present study exhibit an interrelation between SA, sulfate level (e.g., % SO4), and crystal size (e.g., Dav), indicating the existence of distinct highly ordered surface sulfate structures over a decomposition temperature interval of at least 100 °C. Two sulfate concentrations are concluded, corresponding to specific SA (SSA) of 0.14 (for Fe and Sn) and 0.50 nm2 (for Ti, Zr, and Al). The dense sulfate state seems to fit a surface bidentate structure, whereas the dilute state could reflect isolated tripod surface sulfate or chained sulfate groups (bi- or polysulfate), in agreement with earlier literature conclusions based on spectroscopic studies. Acknowledgment Thanks are due to Engelhard Corp. for allowing the publication of this work. I am indebted to Engelhard’s Chemical and Physical Analysis Department and Structure Characterization Department for analyzing and characterizing the SMO’s; special thanks are due to Tom Gegan for the XRD work and to Nancy Brungard for the XPS analysis. The technical assistance of Emberto Villanueva is greatly appreciated. Glossary a ) proportionality factor between SA and sulfate concentration [m2 g-1 (wt % SO4)-1] Dav ) average crystallite diameter (nm) SA ) surface area (m2 g-1) SSA ) specific surface area (nm2); surface occupied by a single sulfate group R ) second-order cumene cracking activity F ) density (g cm-3) χ ) fraction of cumene converted
Appendix I: Calculation of SSA The wt % SO4 divided by the formula weight (96) and multiplied by Avogadro’s number gives the number of S species in 100 g of SMO; this is simply 6.25 × 1021 % SO4. The surface area of 100 g of SMO is 100 SA (m2/ g), given in m2, and thus 1020 SA (m2/g) in nm2. The specific surface area, SSA (in nm2), is therefore
SSA )
1020SA 6.25 × 1021 % SO4
(1-I)
which simplifies to eq 1. Appendix II: Calculation of Dav Idealizing the crystallites as perfect spheres, the effective average crystal size is given by the average
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diameter of the spheres, Dav, and since SMO’s are nonmicroporous compact bulk oxide systems, Dav can readily be calculated from the measured SA. Given in cm2/g, the surface area, SA, is S/m, with S (in cm2) being the surface of an average sphere with Dav (in cm), equal to πDav2, and m (in g) being the particle mass, VF ) πDav3F/6. Thus
Dav ) 6/(SA)F
(1-II)
and expressing Dav in nm and SA in m2/g leads to eq 2. It should be noted that the same expression is obtained by assuming cubic particles (since the surface of the particle divided by its volume is again 6/Dav, here Dav being the cube parameter). Literature Cited Arata, K.; Hino, M. Synthesis of Solid Superacid of Tungsten Oxide Supported on Zirconia and Its Catalytic Action. Proceedings of the 9th International Congress Catalysis, Calgary, 1988; Chemical Institute of Canada: Ottawa, Canada, 1988; Vol. 4, p 1727. Arata, K.; Hino, M. Solid Catalyst Treated with Anion. XVIII. Benzoylation of Toluene with Benzoyl Chloride and Benzoic Anhydride Catalyzed by Solid Superacid of Sulfate-Supported Alumina. Appl. Catal. 1990, 59, 197. Arata, K.; Hino, M.; Yamagata, N. Acidity and Catalytic Activity of Zirconium and Titanium Sulfates Heat-Treated at High Temperature. Solid Superacid Catalysts. Bull. Chem. Soc. Jpn. 1990, 63, 244. Bensitel, M.; Saur, O.; Lavalley, J.-C.; Morrow, B. A. An Infrared Study of Sulfated Zirconia. Mater. Chem. Phys. 1988, 19, 147. Chen, J. P.; Yang, R. T. Selective Catalytic Reduction of NO with NH3 on SO42-/TiO2 Superacid Catalyst. J. Catal. 1993, 139, 277. Fraenkel, D. Catalytic Performance of Sulfated Zirconia. 1994 Spring Symposium of the Catalysis Society of Metropolitan New York, Lehigh University, Bethlehem, PA, March 9, 1994. To be published. Hino, M.; Arata, K. Conversion of Pentane to Isopentane and Isopentane to Isobutane Catalyzed by a Solid Superacid in the Vapor Phase. React. Kinet. Catal. Lett. 1982, 19, 101.
Hino, M.; Kobayashi, S.; Arata, K. Reactions of Butane and Isobutane Catalyzed by Zirconium Oxide Treated with Sulfate Ion. Solid Superacid Catalyst. J. Am. Chem. Soc. 1979, 101, 6439. Jehng, J.-M.; Wachs, I. E. The Effect of Surface Sulfate Species on the Molecular Structure and Reactivity of the Supported Vanadium Oxide Catalysts. Poster presented at the 1993 Spring Symposium of the Catalysis Society of Metropolitan New York, Lehigh University, Bethlehem, PA, March 17, 1993. Matsuhashi, H.; Hino, M.; Arata, K. Synthesis of Solid Superacid of Silica Treated with Sulfuryl Chloride. Catal. Lett. 1991, 8, 269 and references therein. Saur, O.; Bensitel, M.; Saad, A. B. M.; Lavalley, J. C.; Tripp, C. P.; Morrow, B. A. The Structure and Stability of Sulfated Alumina and Titania. J. Catal. 1986, 99, 104. Scofield, J. H. Hartree-Slater Subshell Photoionization CrossSections at 1254 and 1487 eV. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases: Their Catalytic Properties; Studies in Surface Science Catalysis 51; Kodensha/Elsevier: Tokyo/New York, 1989. Tanabe, K.; Hattori, H.; Yamaguchi, T. Surface Properties of Solid Superacids. Crit. Rev. Surf. Chem. 1990, 1, 1. Tanaka, T.; Itagaki, A.; Zhang, G.; Hattori, H.; Tanabe, K. Selective Conversion of 4-Isopropenyl-1-methylcyclohex-1-ene to 4-Isopropylidenyl-1-methylcyclohex-1-ene over SO42-/ZrO2. J. Catal. 1990, 122, 384. White, R. L., Sikabwe, E. C., Coelho, M. A., Resasco, D. E. Potential Role of Penta-Coordinated Sulfur in the Acid Site Structure of Sulfated Zirconia. J. Catal. 1995, 157, 755. Yabe, K.; Arata, K.; Toyoshima, I. X-Ray Photoelectron Spectroscopic Studies of Iron Oxide Catalysts Prepared from the Calcination of Iron Sulfates at High Temperature. J. Catal. 1979, 57, 231. Yamaguchi, T. Recent Progress in Solid Superacid. Appl. Catal. 1990, 61, 1. Yamaguchi, T.; Jin, T.; Tanabe, K. Structure of Acid Sites on Sulfur-Promoted Iron Oxide. J. Phys. Chem. 1986, 90, 3148.
Received for review April 1, 1996 Revised manuscript received October 15, 1996 Accepted October 22, 1996X IE9601965 X Abstract published in Advance ACS Abstracts, December 1, 1996.
