Characterization and catalytic activity of non-hydrothermally synthesized saponite-like materials

91998ElsevierScienceB.V.All rightsreserved, Preparation of CatalystsVII B. Delmonet al., editors.

Characterization and catalytic synthesized saponite-like m a t e r i a l s

967

activity

of

non- hydrothermally

M. Sychev and R. Prihod'ko Faculty of Chemical Technology, National Technical University of Ukraine, "Kiev Polytechnic Institute" 252056, Kiev, pr. Peremogy 37, Ukraine Saponite-like materials were synthesized at 1 atmosphere and 363 K from Si/A1, Si/Cr and Si/Fe-gels and a solution containing urea and M 2+ nitrate (M 2+ =Zn and Mg) or mixture of M 2+ nitrates (M 2+= Zn, Mg and Cu) during 24 hours. The products were characterized by variety of methods. Incorporation of Cr and Fe in the tetrahedral positions and Zn, Mg or their combination with copper cations in the octahedral sheet could be established. The materials prepared possess acidic and basic/redox active sites depending on the chemical composition of both tetrahedral and octahedral sheets of saponite, as found from 2-propanol and 2methyl-3-butyne-2-ol (MBOH) decomposition. Their Al-exchanged forms display a high activity in alcohol dehydration. 1. INTRODUCTION The use of natural clays as acid catalysts has been known for a long time. However, their wide range catalytic application is still hampered mainly due to relatively low thermal stability and difficult control of chemical composition and textural properties. The synthetic clays offer the possibility to avoid these problems. Unfortunately the usually demanding hydrothermal treatment and long duration of the preparation [1] have severely limited their use. Recently, Vogels et al. [2] proposed the non-hydrothermal approach allowing synthesis of saponite, a tetrahedrally charged trioctahedral smectite. Its structure built from two SiO4 tetrahedral sheets (T) and one MgO6 octahedral sheet (O) arranged in a TOT sandwich. The synthetic saponites prepared through the above mentioned way displayed interesting catalytic properties in the Friedel-Krafts alkylation of benzene and were thermally stable [2]. Moreover, because of a mild condition and relatively easy preparation mode this method can by sealed-up making industrial application viable. Incorporation of transition-metal ions into the framework of molecular sieves provides the materials with versatile catalytic properties. The iron, chromium and copper containing zeolites have been assumed to be catalysts for a range of transformations [3-5]. Synthesis of zeolites isomorphously substituted with two heteroatoms was already reported [6] that gives a possibility to create different active sites in the same silicate matrix. Therefore, the incorporation of transition-

968 metal ions into the lattice of clay-like materials offers potemially a promising route for the preparation of new heterogeneous catalysts. The aim of this study was characterization of non-hydrothermally synthesized saponite-like materials by physical methods and catalytic tests from the standpoint of incorporation of the transition-metal ions in the silicate lattice. 2. EXPERIMENTAL Saponite-like materials were synthesized at 363 K and 1 atmosphere. The starting gel containing Si and AI, Fe or Cr (M 3+) was prepared by mixing a Na2SiO3 (Merck, 27 wt % SiO2) with solution of NaOH and respective M 3+ nitrate under vigorous stirring at 273 K for 1 h. The Si/M 3+ ratio was varied within 3.0 and 12. Next, a solution containing nitrate of octahedral ions (M 2+) and urea was added to the diluted staging gel and then the mixture was kept under stirring at 363 K for 24 hours. After synthesis completion, the solid products were separated, washed and dried at 383 K for 12 h. The Al-containing saponite with ratio of Si/AI=8.0 and octahedral Zn cations was prepared under the same conditions and used as a reference material. Apparatus used and conditions of measurements. XRD" DRON-3 diffractometer, CuKcz-radiation. XRF: high-vacuum device, Si(Li) detector (180 eV at 5.9 keV), sample weight=200 rag. EPR: SE/X 2544 spectrometer, X-band, operating at 293 K and 77 K, 27A1 MAS NMR: Brucker MSL 400 spectrometer, ffequency=104 MHz, rotation ffequency=15 kHz, repetition time=0.5 s, pulse width=0.6 Vts, chemical shift reported relative to [AI(H20)6] 3+. IR: Specord M80 spectrometer. N2-adsorption: Sorptomatic 1990 (Carlo Erba Instrumems), degassing at 403 K, 10-4 mbar, 5h. The MBOH conversion was performed as described elsewhere [7]. The 2propanol decomposition was conducted at 473 K using pulse microreactor. Prior to the testing, catalysts were preheated at 523 K in an Ar flow for 2h. First order rate constants for alcohol transformation were calculated according to ref. [8] and related to the catalyst surface unit. The selectivity is defined as follow: 100 x (moles product formed/moles alcohol decomposed at the same overall conversion) [9]. The samples with A1, Cr or Fe in the tetrahedral and Zn in the octahedral sheets were denoted as AIZnSP, CrZnSP and FeZnSP, respectively. The materials with Zn-Cu and Mg-Cu in the octahedral and Cr, Fe in the tetrahedral positions were referred to CrZn/CuSP, CrMg/CuSP, FeZn/CuSP, respectively. 3. RESULTS AND DISCUSSION The examination of samples prepared by XRD revealed the crystal structure corresponding to 2:1 trioctahedral clays, among them saponite, as deduced from the interlayer spacing and the position of (006) reflection at about 1.54 A (Fig. 1 and Table 1). The obtained XRD patterns agree well with those of synthetic saponite reported previously [3]. However, when the Si/M 3+ ratio in starting gel is

969 lower than 5.0, the formation of considerable amount of poorly crystallized phase was observed.

(ool)

t(ool) (060)

0

20

40

60

0

20

40

60

2 Thcta Figure 1. XRD patterns of saponite-like materials with atomic ratios Si/M3+=8.0 and Zn(Mg)/Cu=5.0: (1) CrZnSP, (2) CrZn/CuSP, (3) CrMgSP, (4) FeZnSP and (5) FeZn/CuSP. Hence, the content of M 3+ in tetrahedral sheets influences structural regularity of the samples. A higher crystallynity was established for the materials with atomic ratios of Si/M3+=8.0 and Zn(Mg)/Cu=5.0 and, therefore they were subjected for the further investigations. Among the products synthesized, the Zn-containing ones exhibit the sharpest XRD peaks (Fig.l) indicating the formation of relatively large crystals. A combination of Zn, Mg and Cu in the octahedral sheet results in some broadening of (001) reflection most probably due to a decrease of stacking. In the case of the Mg-containing samples, this phenomenon is less pronounced. For the A1ZnSP sample, a preferable location of AI3+ in tetrahedral positions was evidenced by A127 MAS NMR (strong resonance peak at 62-64 ppm). Nevertheless, a small amount of A1 cations is placed in the octahedral sheets what was detected by weak N M R peak at approximately 10 ppm. To study swelling of the materials prepared, their saturation with ethylene glycol vapour has been performed. As can be seen from data collected in Table 1, all the samples display an ability to swell. However, the influence of the nature of lattice constituting cations on swelling properties of saponite-like materials is not clear enough. Most likely, this feature is related to the layer charge that can vary with the composition of tetrahedral and octahedral sheets. A high cation exchange capacity established for all the samples prepared (Table l) agrees well that of synthetic saponite [10].

970 For all solids obtained, the IR spectra contain the lattice vibration bands at about 1050-, 986- 820-, 690-, 457-440-cm -1 characteristic of saponite [10]. Table 1 Cation exchange capacity (CEC) and basal spacing (001) evolution upon ethylene glyco! saturation of saponite-like materials synthesyzed . . . . dq01 d001, Sample CEC dQ01 d001, Sample CEC meq/g A EG, A meq/g A EG, A A1ZnSP 91 12.90 16.83 CrMg/CuSP 102 15.80 16.25 CrZnSP 105 12.24 14.90 FeZnSP 95 12.87 14.87 CrMgSP 96 15.40 16.56 FeZn/CuSP 107 12.07 15.15 CrZn/CuSP 119 13.05 14.65 (d001 EG) basal spacing after ethylene glycol saturation for 24 h. The ESR spectra of Na-forms of CrSP and FeSP samples recorded at 293 and 77 K revealed a broad line at g=1.98 and g=2.0, respectively. In the case of Crcontaining samples, the observed line can be assigned to Cr ions in trigonal of tetragonal symmetry in the saponite lattice or to the exchanged Cr 3+ cations [ 11]. For Fe-saponites, this signal can be attributed to Fe ions in tetrahedral symmetry [12]. To examine the case that those cations are partially located in the interlayer space, an additional ion-exchange with NH4 + ions was performed. As appeared, such treatment does not affect significantly both the line intensity and g-values thus indicating the absence of the detectable amount of Cr and Fe ions in the exchangeable positions. However, due to the large width of ESR signals our results do not allow a more detailed discussion. The N2 adsorption isotherms of AIZnSP and CrZnSP samples are almost of Type IV that is characteristic of mesoporous adsorbents (Figure 2). 250

400

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300

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200

100

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0.0

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0.5

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0.0

0.5

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Relative pressure [P/Po] Figure 2. N2 adsorption-desorption isotherms for saponite-like materials prepared: (a) CrZnSP, (b) AIZnSP, (c) CrZn/CuSP and (d) FeZnSP.

971 The hysteresis loop for those samples is of Type H2, representative of ink-bottle shaped pores [13]. The same implies the Mg-containing materials (not shown). The shape of the isotherm of the CrZn/CuSP and FeZnSP samples exhibit a combination of Type I and Type IV with hysteresis loop closely to Type H4 reflecting the presence of slit-like pores in structure of this sample. All samples exhibit a relatively narrow pore size distribution (Dollimore-Heal method) centred at about 40 A. From Table 2 showing the main textural properties of the materials prepared and adsorption isotherms (Fig. 2), can be concluded that they are mainly mesoporous irrespective of the chemical composition. The samples with copper exhibit some mieroporosity indicating that incorporation of Cu ions in the octahedral sheet influences their texture (Table 2). Table 2 Chemical composition of silicate layer and the main textural properties of saponite-like materials pre-heated at 403 K . . . . . . . . Sa* Wm* Vmeso* Sample Si/M 3+ Si/M 3+ SBET (EPMA). . . . . ( X R F ) (ma/g) (ma/~) (cc/8) (cc/g) AIZnSP 7.93 7.94 189 192 -0.16 CrZnSP 7.92 7.90 223 246 -0.30 CrZn/CuS P 7.93 7.91 331 336 0.063 0.37 CrMgSP 7.91 7.87 420 436 -0.55 CrMg/CuSP 7.95 7.96 465 470 0.043 0.49 FeZnSP 7.93 7.94 283 296 -0.26 FeZn/CuSP 7.92 7.93 350 360. 0.084 0.38 Note: S, Vm, Vmeso are stands for specific surface area, micropore and mesopore volumes, respectively, * calculated by ets-method. The catalyst acid-base properties can be characterized by different methods, in particular by test-reactions, among them alcohol decomposition. To study the nature of active sites originated by the lattice, Na-form of the materials prepared was tested by means of 2-propanol and 2-methyl-3-butyne-2-ol (MBOH) decomposition. These alcohols were chosen as test molecules because it is known that MBOH dehydrogenation into acetone and acetylene probes the basicity while hydrogen subtraction from 2-propanol requires participation of the redox active sites [ 14]. The 2-propanol decomposition at 473 K over the samples with transition-metal ions results mainly in the alcohol dehydrogenation yielding acetone and hydrogen as products (Fig.3). The overall alcohol conversion is not considerably dependent on the nature of transition metal (Cr or Fe), as follow from Fig. 3A. In the case of the A1ZnSP material, the alcohol dehydration was the unique reaction. Incorporation of Mg cations in the octahedral positions of Cr- and Fe-containing samples leads to some increase of the dehydration activity that was reflected by the propene appearance (Fig. 3B). For Cu-containing samples, the dehydrogenation activity is somewhat higher than that of the samples without copper. This can be explained by the action of octahedral Cu cations exposed at the crystal edges.

972 0.45,

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ii

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2

4

1

2

3

4

5

6

7

sample

n m time [h]

Figure 3. (A) 2-propanol decomposition versus rtm-time and (B) selectivity for acetone and propene formation at 40% conversion for samples: (1) A1ZnSP, (2) CrMgSP, (3) CrZn/CuSP, (4) CrMg/CuSP, (5) CrZnSP, (6) FeZnSP, (7) FeZn/CuSP. The MBOH transformation undergoes the acid and base catalyzed cleavage reaction over the catalysts studied giving 3-methyl-3-butene-2-one (MIPK), acetylene and acetone (Fig. 4). 0.45 r

~ ~ i acetone ~/'//~] M IP K

A 80

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.

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.

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6

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sample

Figure 4. (A) MBOH decomposition versus run time and (B) selectivity for acetone and M I P K formation at 60% conversion for samples: (1) CrMgSP, (2) CrMg/CuSP, (3) CrZn/CuSP, (4) AIZnSP, (5) CrZnSP, (6) FeZn/CuSP, (7) FeZnSP.

973 Likewise 2-propanol decomposition, octahedral copper cations play a role in contributing the catalysts dehydrogenation activity (Fig. 4B). To elucidate a possible participation Na-exehangeable cations in the alcohol dehydrogenation, the catalysts containing transition-metal ions were ionexchanged with Cs +. For 2-propanol, no significant variations in the overall alcohol decomposition and selectivity were found. In the case of MBOH such treatment does not affected the activity whereas catalyst acidic function was completely suppressed resulting in the dehydrogenation activity increases. From the above, one can conclude that the 2-propanol dehydrogenation is linked to the lattice redox active sites such as Cr, Fe and Cu. By comparison, on basic catalysts without redox properties acetone formation from this alcohol only occur at high temperature (573 K) [15]. However, a participation of the acidic sites in 2-propanol dehydrogenation cannot be excluded. It was shown [15], that its dehydrogenation proceeds through a concerted mechanism involving the acid active centres action. The MBOH dehydrogenation seems to proceeds over redox and basic active sites originated by lattice constituting and exchangeable cations without participation of acidic sites. Due to this reasol~ the elucidation of surface acidity by means of MBOH dehydration appears to be realistic. For this alcohol, the dehydration activity (expressed as ratio between rate constants of the products formation) of the samples follows the order: A1SP> CrSP>_ FeSP. This sequence agrees well with the expected Bronsted acidity of bridging hydroxyls Si(OH)M 3+. It is known that incorporation of trivalent cations with increasing ion radius into the silicalite framework leads to decreasing acid Bronsted eentres [16]. Considering the similarity in symmetry of M 3+ ions in the silicalite framework and in the saponite lattice, it might be concluded that the introduced Fe and Cr cations occupy mainly the tetrahedral positions. The ion exchange of interlayer Na + cations with A13+ results in the increase of the total 2-propanol and MBOH conversion over the catalysts, dramatically decreasing their dehydrogenation activity. The achieved activity is remarkably higher than that of Al-montmorillonite and comparable with that of acid-leached commercial montmorillonite K10 (Fluka). 4. CONCLUSIONS It might be said that the results derived from XRD, IR, swelling tests, ionexchange and N2-adsorption indicate the formation of saponite-like structure thereby evidencing the preferable incorporation of Cr and Fe in the tetrahedral and combined insertion of Zn or Mg and Cu cations in the octahedral positions under synthesis procedure applied. Nevertheless, the presence of some M 3+ cations in the octahedral positions camlot be ruled out and additional study is necessary to elucidate this case. The non-hydrothermal synthesis approach enables combined introduction of Zn or Mg and Cu cations in the octahedral positions thus allowing isomorphous substitution of those cations by each other. The synthesized materials are mainly mesoporous with high surface area, differing in the pore shape. They possess both acidic and basie/redox active sites amount of which can be altered by

974 varying of the lattice composition and/or an additional ion exchange. Furthermore, all materials prepared being ion-exchanged with A13+cations displayed a high dehydration activity in alcohol decomposition. 5. ACKNOWLEDGEMENTS

The work was supported in part by the Ukrainian Ministry of Education. The authors gratefully thank Mrs. N. Kostoglod (NTUU, Kiev, Ukraine) for help and Mr. E.M. van Oers (TUE, Eindhoven, The Netherlands) for the adsorption measurements. REFERENCES

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