Nature of Vanadium Species in Vanadium-Containing Silicalite and

Chapter 21

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 16, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch021

Nature of Vanadium Species in VanadiumContaining Silicalite and Their Behavior in Oxidative Dehydrogenation of Propane 1

G. Bellussi , G. Centi, S. Perathoner, and F. Trifirò Department of Industrial Chemistry and Materials, University of Bologna, Via le Risorgimento 4, 40136 Bologna, Italy V-containing silicalite samples prepared hydrothermally were characterized using a combination of physicochemical techniques and the activity of these silicalites in the oxidative dehydrogenation of propane using O andN Owas studied. FT-IR data on the mechanism of propane transformation in the presence of gaseous oxygen on these samples were also reported. The results indicate the presence of various types of vanadium species, and in particular, the formation of small amounts of a tetrahedral V species stabilized in its configuration by interaction with the framework. This species probably forms at defect sites and is the more active and selective species in the oxidative dehydrogenation ot propane. Activated oxygen species generated by the interaction ofO orN Owith reduced vanadium sites are suggested to be responsible for the selective transformation of propane to propylene. However, the good selectivity of this catalyst in propylene formation from propane is also connected to the type of chemisorption of the intermediate propylene on these tetrahedral V sites and to the relative inertness towards its further transformation due to the specific coordination environment of vanadium. In particular, the data indicate that propylene is coordinatively adsorbed as a π-complex with theCH group interacting with a nearlying weakly basic silanol. Infrared darta suggest that the rate of formation of propylene from propane in the presence of O is higher than the rate of its consecutive transformation to surface oligomers or acrolein and acrylic acid, probably intermediates to aromatics and COx, respectively. The model of the coordination environment and the peculiar characteristics of these tetrahedral V sites in the silicalite in relation to their selective behavior in propane oxidative dehydrogenation are discussed. 2

2

5+

2

2

5+

3

2

5+

Transition-metal substituted or modified zeolites are currently receiving increasing attention as gas-phase heterogeneous partial oxidation catalysts, because they offer the 1

Current address: Eniricerche, Via Maritano 26, San Donato Milanese (MI), Italy 0097-6156/93/0523-0281$06.00/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

282

CATALYTIC SELECTIVE OXIDATION


opportunity for well defined and isolated active centers in an ordered oxide matrix. Consequently, they can provide useful information about the structure-activity relationship in selective oxidation reactions. Furthermore, the inclusion of the transition metal in the zeolite framework or the stabilization effect due to the coordination at defined sites of the zeolite framework can stabilize the transition metal in an unusual coordination with a consequent change in its reactivity behavior. It is thus possible to expect a change in the specific catalytic behavior of the transition metal, in addition to possible effects of shape-selectivity. V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NO reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). x

Recently, interesting preliminary results have also been obtained in propane oxidative dehydrogenation to propylene (2). In this reaction, several advantages can be considered in having well defined vanadium atoms stabilized at specific coordination sites of the zeolite framework: i) atomically dispersed vanadium atoms, ii) a coordination of vanadium which limits alkene readsorption and activation through an allylic-type mechanism and iii) a coordination environment where the strength of bridging V-O-S (S = support) oxygen is sufficiently high to avoid Ο insertion on activated alkenes. In fact, in alkane oxidative dehydrogenation the more critical problem is not the selective activation of the alkane, but rather the inhibition of the consecutive transformation of the alkene formed. The factors cited favour this inhibition. Experimental V-containing silicalite (Al- and Na-free) samples were prepared hydrothermally and then treated with an ammonium acetate solution at room temperature in order to remove extralattice vanadium. Three samples with S1O2/V2O3 ratios of 117, 237 and 545, respectively, were prepared. Hereinafter these samples will be referred to as follows: V-SU117, V-SU237 and V-SU545. Details on the preparation procedure, and characterization of the samples have been reported previously (7,2). Catalytic tests were performed in an isothermal flow quartz reactor apparatus under atmospheric pressure, provided with on-line gas chromatographic (GC) analysis of the reagent and products by two GC instrument equipped with flame ionization and thermoconducibility detectors. The activity data reported refers to the behavior after at least two hours of time on stream, but generally the catalytic behavior was found to be rather constant in a time scale of around 20 hours. 1

Fourier-transform infrared (FT-IR) spectra (resolution 2 cm" ) were recorded with a Perkin-Elmer 1750 instrument in a cell connected to grease-free evacuation and gas manipulation lines. The self-supporting disk technique was used. The usual pretreatment of the samples was evacuation at 500°C.

Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

21. BELLUSSI ET AL.

Vanadium Species in Vanadium-Containing Silicalite

283


Results Characterization Data. A complete characterization of V- containing silicalite samples has been reported in a previous paper (/), but the main relevant aspects useful for a better understanding of the nature of vanadium species in V- silicalite samples and for the correlation with the catalytic behavior will be briefly summarized. The XRD powder patterns of V-containing silicalite samples indicate in all cases the presence of only a pentasyl-type framework structure with monoclinic lattice symmetry, characteristic of silicalite-1; no evidence was found for the presence of vanadium oxide crystallites. The analysis of cell parameters of VSU545 does not indicate significant modifications with respect to those found for pure silicalite-1. This is in agreement with that expected on the basis of the small amount of V atoms present in V-containing silicalite. 51

The wide-line V-NMR spectra of VSM17, VSH237 and VSU545 samples are characterized by a symmetrical sharp line centered at -480 ppm. However, in VSill 17 and possibly also in VSU237, the narrow symmetrical sharp line overlaps a broad signal in the 0-(-1000) ppm range which may suggest the presence in these samples of an additional V - species in an undefined environment. The line-shape observed for VSil samples can be interpreted as being due to the presence of V sites in a nearly symmetrical tetrahedral environment with relatively short (about 0.160- 0.165 nm) V-0 bonds, which, however, differ from those present in an orthovanadate such as Na V0 . The line-shape of this V tetrahedral species does not change after evacuation of the sample or exposure to moisture, whereas upon evacuation the submonolayer tetrahedral V species detected on S1O2 (7) or other oxides easily coordinate water molecules after exposure of the samples to ambient conditions and thus reform the more stable octahedral coordination. This indicates that vanadium is stabilized in nearly tetrahedral coordination by a direct specific interaction with the silicalite framework and the interaction is stronger than that observed for vanadium supported on silica. ESR analysis of VSill 17 indicates the presence of isolated nearly octahedral vanadyl species and of vanadium-oxide polynuclear species containing pairs of V ions and of V 0 ions. Compared to VSill 17, VSU237 and VSU545 are less heterogeneous and no polynuclear V-oxide species are present, but rather only isolated nearly octahedral VO sites. The ESR parameters are very close to those found for V 0 sites in samples obtained by solid-state reaction in air of V2O5 with H- ZSM5 (8) and can be attributed to isolated ions in the zeolite interior near to a charge- compensating (OH) site. The migration of V inside the zeolite and the spontaneous reduction of V to V occurring during calcination derives from the interaction of V with the strong Br0nsted groups associated with Al sites in the zeolite framework. The correspondence of ESR parameters of isolated V 0 species in V-containing silicalite samples with those observed in V-ZSM5 samples suggests that the vanadyl groups are probably inside the zeolitic channels of the silicalite and near to -OH groups. In VSU237 and VSU545 samples a new signal with hyperfine structure appears after 5 +

5 +

3

3

5 +

5 +

3 +

2 +

+

2 +

+

2 +


+

catalytic selective oxidation

284

reduction with H2 at 773 K. The signal can be attributed to ions in a distorted tetrahedral environment. A new signal with superhyperfine structure appears in the spectrum recorded at 77 Κ after admission at 293 Κ of small amounts of gaseous oxygen on the prereduced VSU545 sample and subsequent evacuation. The new signal overlaps the signal attributed to in a tetrahedral environment and indicates the formation of an O2 species by electron transferfromthe tetrahedral V " species to O2. 44

The UV-visible diffuse reflectance spectra of VSU545 shows a well defined band centred at 26000 cm" and two further broad bands at about 38000 and 43000 cm" . VSill 17 shows these bands, as well as additional bands at about 30000 and 34000 cm" . The impregnation of pure silicalite or S1O2 with ammonium vanadate and subsequent calcination led, on the contrary, to quite different spectra. The presence of an intense absorption band at 26000 cm" is thus characteristic of V-containing silicalite prepared hydrothermally, and tentatively of V sites specifically interacting with the zeolite framework. The position of this band suggests the presence of a short (double) vanadium-oxygen bond. The second CT band, however, does not agree with that expected for a distorted octahedral or square-pyramidal environment, but rather is indicative of a nearly tetrahedral field. The interpretation of the UV-Visible DR spectra of VSU545 thus suggests the presence of a V -species characterized by a short vanadium-oxygen bond (about 0.16 nm) and three slightly longer (about 0.165-0.170 nm) V - 0 bonds, according to the correlation observed between ( V O 4 ) " charge-transfer transitions and V - 0 distance.


1

1

l

1

5+

3

UV-Visible diffuse reflectance spectra also show that vanadium is mainly present as V in V-containing silicalite samples. TPR and XPS results are in agreement with this conclusion. In addition, XPS data indicate that V is homogeneously dispersed in VSU545, whereas in VSill 17 part of the vanadium is segregated on the external surface of the silicalite samples. The interaction of vanadium with the silicaliteframeworkinduces modifications in the surface acidity properties that were characterized by IR spectroscopy using suitable probe molecules. The analysis of the IR hydroxyl stretching region does not provide evidence of specific differences between the V-containing silicalite (VSH545) and pure silicalite (Sil). In order to characterize further differences in the Br0nsted acidity, the adsorption of ammonia and pyridine on VSU545 and pure silicalite (Sil) was carried out. Additional tests were also performed using CD3CN and ί-butyl cyanide as probe molecules to differentiate between the presence of Lewis sites in internal or external positions of the zeolite crystals and surface heterogeneities. On the basis of (i) the higher steric hindrance of ί-butyl cyanide which does not allow its reaction with Lewis acid sites inside the zeolite crystals, and (ii) the presence of stronger additional sites in the V-containing silicalite as compared to pure silicalite (as shown by deuterated acetonitrile adsorption), it can be concluded that very weak Lewis acid sites are present on the external surface of both VSU545 and Sil, but additional stronger Lewis acid sites are present inside the zeolite channels in V-containing silicalite, and are reasonably related to vanadium sites. Ammonia temperature programmed desorption (NH3-TPD) results are in agreement with these conclusions. 5 +

Propane Oxidative Dehydrogenation on V-containing Silicalite. Reported in


21. BELLUSSI E T AL.



Figure 1 is the catalytic behavior of VSU545 in propane oxidative dehydrogenation to propylene. Selectivities to propylene in the range of60-80% are obtained up to propane conversions of about 20-25% and reaction temperatures up to around 450- 500°C. For higher reaction temperatures and conversions the selectivity decreases due both to the formation of carbon oxides and of aromatics. As compared to pure silicalite, a significant increase in both the selectivity to propylene and the activity in propane conversion is observed. As the amount of vanadium in the silicalite increases (VSill 17 vs. VSU545), the selectivity to propylene decreases with a parallel increase in the formation of C O (Figure 2). The global activity of the catalyst is higher in the samples containing higher amounts of vanadium (VSill 17), but the specific rate of propylene formation (moles of propylene formed per second and per V atom) is higher for VSU545 in which only tetrahedral V species are present. This is further evidences of the role this V species plays in the selective transformation of propane to propylene. (See also Figure 3.) x

5 +

In the absence of gaseous oxygen, the activity of the catalyst decreases considerably indicating the specific role of gaseous oxygen in the mechanism of propane activation. When N 2 O is used as the oxidizing agent instead of O2, the activity increases considerably (Fig. 4) (similar conversions are obtained for reaction temperatures around 100-150°C lower using N 2 O rather than O2) and the selectivity to propylene increases up to values in the 90-95% range. This is in agreement with the formation of the more active O" species by reaction of N 2 O with the reduced vanadium sites. The results in terms of selectivity are much better as compared to other Me-silicalites, confirming the peculiar role of vanadium in the formation of selective sites of propane oxidative dehydrogenation (2). The activity both in the presence of N 2 O or O 2 is higher than without oxidizing agents or in the absence of the catalyst (Fig. 4).

Infrared Characterization of the Conversion of Propane on V- containing Silicalite. In order to obtain more information about the mechanism of propane transformation on the V-containing silicalite in the presence of gaseous oxygen, the nature of the adsorbed species formed after contact with a mixture of propane and oxygen was characterized by infrared spectroscopy. The study was carried out both at room temperature and at 200°C, but a better identification of the evolution of the adsorbed species was possible at the lower temperature. The study was carried out on VSU545 after activation by outgassing at 500°C into the IR cell. This pretreatment probably induces a partial reduction of the vanadium, analogous that observed by ESR spectroscopy after similar treatment (7). In the absence of gaseous oxygen, propane is only physisorbed on VSU545 at room temperature, as shown by the presence of V C H bands in the 3000-2800 cm" region and Ô C H bands in the 1300-1600 cm' region (Fig. 5b). Bands in the 2000-1600 cm' region are due to the overtones of fundamentals of skeletal vibrations of the silicalite framework and indicate that the crystalline structure is preserved after the evacuation procedure. In comparison to gaseous propane, there is an increase in the relative intensities of the v C H (2875 cm" ) and VasCH (2936 cm" ) and 5 CH (1370 cm* ) bands with respect to the main band at 2964 cm" (VasCI^). This modification is due to the weak interaction with the surface, but no shift in the frequency of the bands is observed. A slight perturbation in the V Q H region (Fig. 5) is also observed indicating 1

1

1

1

s

3

l

2

1

S

3

1


2

286


Select. (C basis), %

Conv. propane, %

100

Propyl. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 16, 2017 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch021

— CUC2 + C4+C5 Aromat. •^COx • C3 conv. »C-C3 Sil

390

410

430

450

470

490

510

530

550

Temperature,*C

Figure 1. Propane oxidative dehydrogenation to propylene on VSÎ1545. Exp. conditions: flow reactor tests with 2.8% C3, 8.4% O2 in helium. 4.2 g of catalyst with a total flow rate of 3.1 L/h (STP conditions).

117

237

545

SI02/V203 ratio

Figure 2. Comparison of the catalytic behavior of VSil samples in propane oxidative dehydrogenation to propylene. Conversion of propane and selectivity to propylene at 470 C. Exp. conditions as in Fig. 1. e


21. BELLUSSI ET AL.

Vanadium Species in Vanadium-Containing Silicalite 287

mola/aac.Vatoma (specific rat* of propylana form, par V atom)


0.041

100

200

300

400

500

S102/V203 ratio

Figure 3. Specific rate of propylene formation at 500°C per vanadium atom in VSil samples as a function of the S1O2/V2O3 ratio. Exp. conditions as in Fig. 1 Propana conv. or salact propylana, %

1201

300

350

400

450

500

550

Tamparaiura, "C

Figure 4. Comparison of the behavior of VSÎ1545 in propane oxidative dehydrogenation using N2O or O2 as oxidizing agents. Exp. conditions as in Fig. 1. The dotted lines represent the propane conversion and propylene selectivity observed in the absence of the catalyst (homogeneous gas phase). The activity of the catalyst in the absence of O2 or N2O is similar to that observed in the homogeneous gas phase, but the selectivity to propylene (around 50-60%) is lower.




288

'

4000

3000

»

1

1

1

•

1

*

•

2000 _ v, cm

*

1

1600

•

*

1

•

1200

»

«

•

1

800

«

400

-1

Figure 5. Infrared spectrum at room temperature of VSÎÎ545 after evacuation at 500° C (a) and after contact at room temperature with 50 torr of propane (b). Reported in the inset is the expansion in the 1300-1550 cm region of spectrum b after subtraction of spectrum a. The background spectrum due to gaseous species has been subtracted.


21. BELLUSSI ET AL.


a weak interaction of physisorbed propane with free silanols. Physisorbed propane is easily removed by evacuation at room temperature. In contact with both propane and oxygen, physisorbed propane is the main adsorbed species, but a new adsorbed species characterized by a band centred at 1623 cm" may be observed (Fig. 6B). Simultaneously a stronger perturbation in the V Q H region is noted (Fig. 6A). The band at 3728 cm" , attributed to free silanols (7), disappears leaving a less intense band at higher frequency (3742 cm" ). At the same time a broader band centred at about 3664 cm" appears. By evacuation at room temperature, all the adsorbed species disappear (only a weak band centred at 1623 cm" remains) and the original spectrum in the V Q H region is restored (Fig. 6, spectrum c). 1

1


1

1

1

The shift in the surface hydroxyl absorption band (about 65 cm" ) and the simultaneous presence of an adsorption band at 1623 cm" , reasonably attributed to C=C shifted by about 30 cm" in respect to Vc=c gaseous propylene due to the coordination to an unsaturated cation, indicates the formation of propylene coordinated to the surface in the form of a π-complex with the C H 3 group bonded to a hydroxyl group (9):

V

1

m

l n e

The spectrum is in agreement with that observed for room temperature adsorption on V-AI2O3 (9). It should be noted that acidic hydroxyls on the oxide surface usually give rise to stable hydrogen-bonded complexes with the involvement of the alkene double bond (9,10). When the acid strength of the hydroxyl is high as in ZSM zeolites, the complete transfer of the proton to the alkene to give a carbonium ion also occurs at room temperature. In the presence of transition metal oxides containing ions in high oxidation states ( V , Mo ) alkene interacts forming allyl-type species or alcoholate species (9,10). All these species giveriseto different IR bands than those observed for 5+

6+

VSU545 (Fig. 6).

Therefore, the formation of a π-complex of propylene bonded to a vanadium site and to a nearlying hydroxyl group after contact of a propane and oxygen mixture with activated VSU545 indicates some interesting aspects of the surface reactivity of V-containing silicalite: i) The interaction of gaseous oxygen with the activated VSU545 leads to the formation of dehydrogenation sites able to form propylene from propane even at room temperature. Reasonably, these oxidative dehydrogenating sites can be attributed to activated oxygen species such as (O2)" (77) formed by interaction of O2 with reduced V sites generated in the stage of evacuation of the zeolite before IR studies. ii) The formation of a π-bonded propylene instead of an alcoholate or allylic-type species as observed for example for room temperature propylene interaction with V-Ti0 (70), indicates the different reactivity of V sites in the silicalite with respect to V supported on T1O2 towards the reaction of Η-abstraction (allyl formation) or O-insertion (alcoholate formation). 2

Hi) The formation of a propylene π-complex with the CH3 group interacting with a vicinal OH group (see Scheme 1) indicates the presence of silanol groups lying near to the vanadium sites, in agreement with the data on the characterization of V-containing silicalite (7). In addition, the formation of this adsorbed species indicates the presence of a weak basic hydroxyl group, reasonably due to the inductive effect connected to the vicinal V sites interacting with the silicalite framework.


289

290



2

1340

3500 1740

3*00

v, cm '

Figure 6. (A) Infrared spectra in the V O H region of VSU545 after evacuation at 500 C (a), after contact at r.t. with 50 torr of propane and 1 torr of oxygen (b) and after subsequent r.t. evacuation (c). (B) IR spectra of the adsorbed species in the 1340-1740 cm" region after treatments as for Fig. 6A; the contributions due to evacuated sample and gaseous species have been subtracted from these spectra. e

ο 11/

O-Si si-

Si-

Ο /

Si

Si

\

y^o-s^cH, /

Si

Si

Scheme 1. Model of the room temperature chemisorption of intermediate propylene on VSU545.


21. BELLUSSI ET AL.


After 48 hours of contact of VSU545 with the propane and oxygen mixture, two poorly resolved bands appeared at 1684 and 1705 cm" as well as a shoulder centred at 1423 cm" (Fig. 7a). The same spectrum is obtained after 1 hour of contact at 200°C of the propane and oxygen mixture with VSU545 (Fig. 7b). The two bands at around 1700 cm" may be reasonably attributed to Vc=o different adsorbed species, probably acrolein and acrylic acid. In this compound, in fact, the v c = o b

Nature of Vanadium Species in Vanadium-Containing Silicalite and

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