The gas phase oxidative dehydrogenation of propane over TS-1

Applied Catalysis A: General 209 (2001) 131–143

The gas phase oxidative dehydrogenation of propane over TS-1

Wolfgang Schuster, John P.M. Niederer, Wolfgang F. Hoelderich∗Department of Chemical Technology and Heterogeneous Catalysis,

University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany

Received 7 March 2000; received in revised form 26 July 2000; accepted 28 July 2000

Abstract

The oxidative dehydrogenation of propane to propene was studied on titanium and vanadium containing zeolites and nonzeolitic catalysts, and TS-1 was found to be the most active. In order to investigate the nature of the active site different batchesof TS-1 with varying amounts of titanium were examined and characterised with XRD, nitrogen sorption, UV–VIS, ICP-AES,NH3-TPD and pyridine adsorption. Also, the influence of the reaction parameters was investigated. The reaction rate was inde-pendent on the oxygen concentration in the feed. With increasing W/F ratios higher conversions could be obtained, but there wasa sharp drop in selectivity. The addition of water caused an increase in selectivity, probably due to a competitive adsorption onthe active sites. The reaction very likely takes place on the outer surface of the TS-1 crystallites on Lewis acid sites, and a sulfa-tion of the catalyst, which increases the acidity of these sites, resulted in a further increase of the catalytic activity. The maximumconversion obtained was 17% with a selectivity to propene of up to 74%. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Heterogeneous catalysis; Zeolites; Oxidative dehydrogenation; Partial oxidation of lower alkanes; Titanium silicalite; TS-1

1. Introduction

With the increasing world wide demand for olefinsexisting routes for their production might becomeinsufficient, making the development of alternativeways for the production of light olefins of industrialinterest [1]. A possible route is the direct dehydro-genation of an alkane to the corresponding olefin, andas an example the thermodynamical equilibrium forthe direct dehydrogenation of propane as a functionof the temperature was calculated (see Fig. 1: thecalculations were done with the computer programHCS Chemistry, Outokumpu Research). Clearly, withincreasing temperatures the equilibrium is shifted topropene; at 823 K for example the maximum yield

∗ Corresponding author. Tel.:+49-241-806560;fax: +49-241-8888291.E-mail address:[email protected] (W.F. Hoelderich).

of propene is 32%, whereas at 773 K the yield isonly 18%.

The main disadvantage of the direct dehydro-genation is the high temperature needed in orderto produce reasonable amounts of propene. In con-trast, the oxidative dehydrogenation, in which theformed hydrogen is selectively oxidised, is not re-stricted by the equilibrium composition of the directdehydrogenation. With a suitable catalyst and theappropriate amount of oxygen it should be possible,at least in theory, to completely convert propane intopropene.

In the past much effort was done in the field of theoxidative dehydrogenation of propane [2–4], as shift-ing the thermodynamic equilibrium to lower tempera-tures obviously is a very promising concept. Catalyststested in this reaction are mainly mixed metal oxides,more particularly based on transition metal oxides,rare earth metal oxides, metal phosphates and metal

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-860X(00)00749-3

132 W. Schuster et al. / Applied Catalysis A: General 209 (2001) 131–143

Fig. 1. Calculated equilibrium composition of the hydrogenation of propane based on the molar amounts of propane.

containing zeolites [1,5,6]. The main advantage ofthe latter is the possibility to incorporate several dif-ferent metals in the framework, thus, controlling thenature and the coordination of the active site. Vana-dium for example is probably tetrahedrally coordi-nated present in materials as VS-1 and VAPO-5, bothmaterials of which it has been shown that they areactive catalysts in the oxidative dehydrogenation ofpropane [7,8].

In the current study, various vanadium and titaniumbased materials are compared in a catalyst screen-ing for their activity in the oxidative dehydrogena-tion of propane. As the titanium containing zeoliteTS-1 showed the best results this catalyst was inves-tigated in more detail, varying the titanium content,the concentration of the reactants, the reaction tem-perature, the size of the catalyst pellets and the in-fluence of the sulfation of the catalyst, a treatmentwhich is known to influence the acidity of for examplemetal oxides.

2. Experimental

Anatase, rutile, bariumtitanate (all Fluka) andpyrogenous TiO2 P25 (an anatase nucleus cov-ered with a rutile monolayer; kindly provided byDegussa-Hüls AG) were used as such. [V]-MCM-41was prepared according to Arnold et al. [9], VS-1according to Hong et al. [10], ZSM-5 (Si/Al = 40)and silicalite-1 according to Latourrette and Mag-nier [11], [Ti,B]-MFI according to Barsnick andHölderich [12], [Ti]-MCM-41 according to Niessenet al. [13] and [Ti]-BEA according to Rigutto et al.[14]. TS-1 samples with varying titanium contentwere prepared according to Taramasso et al. [15]and calcined using different temperature programs(see Table 1; the materials were heated with 1 or1.5 K/min up to 823 K. During the procedure forsamples B(1), C, D and E(1) the temperatures wereheld at 393 K for 2 h in order to dry the materials).Mo-USY was prepared by a single ion-exchange

W. Schuster et al. / Applied Catalysis A: General 209 (2001) 131–143 133

Table 1Characterisation data of the different TS-1 samples

TS-1 Si/Ti ratio (–) BET surface area (m2/g) Calcination procedure Relative crystallinity (%)

A 118.3 424 1.5 K/min 96B(1) 85.9 418 1.0 K/min (393 K) 96B(2) 85.1 –a 1.5 K/min 100C 45.4 415 1.0 K/min (393 K) 94D 100.3 424 1.0 K/min (393 K) 93E(1) 85.5 527 1.0 K/min (393 K) 54E(2) 85.1 526 1.5 K/min 56

a Not determined.

of USY (Degussa-Hüls AG) with an excess molyb-dic acid containing solution for 24 h at 353 K, fol-lowed by drying for 14 h at 383 K and calcining at823 K for 6 h. A 1 wt.% TiO2 on silicalite-1 wasprepared by stirring the zeolite for 1 h in a solutionof tetraethoxytitanate iniso-propanol, after whichthe iso-propanol was evaporated. After drying at392 K for 24 h the material was calcined at 823 K for6 h.

The materials were characterised with XRD(Siemens D5000), ICP-AES (Spectroflame D),N2-sorption (ASAP 2000), DR-UV–VIS (PerkinElmer Lambda 7) and ammonia TPD. The nature ofthe acid sites was investigated with pyridine adsorp-tion with FT-IR (Nicolet 510 P). Prior to adsorptionthe sample was calcined at 823 K for 6 h, grained intoa fine powder and pressed into a self supporting waferwhich was heated to 723 K for 12 h under vacuumin a home built high temperature cell. After coolingdown to room temperature pyridine was diffused intothe cell under vacuum for 30 s; it was desorbed atdifferent temperatures.

The catalytic test reactions were carried out in astainless tubular steel microreactor with an inner di-ameter of 9 mm. The temperatures in the reactor weremonitored with three thermocouples: two located di-rectly on the wall of the reactor, of which one at thebeginning of the catalyst bed and the other at the endof the bed. The latter was used to control the temper-ature of the reactor. A third thermocouple was placedin the centre of the furnace. Upon calcination the cat-alysts were pressed into pellets with a pressure of300×105 Pa; the 0.5–1.0 mm fraction was used in thereaction. The volume of the catalyst bed was 5 ml (forzeolitic materials 1.5–3.0 g).

Nitrogen (Westphalen 5.0), oxygen (Westphalen2.5) and propane (Gerling Holz 3.6) were fed intothe reactor under atmospheric pressure with a ro-tameter for each gas. The oxygen and nitrogen gasflows were calibrated before every run. Propane wasstored in a 750 ml stainless steel container whichwas weighed before and after the experiment. Dis-tilled water was fed into an evaporator with a pumplocated just before the reactor. The standard reac-tion conditions (unless indicated otherwise) were:T = 823 K, C3H8/O2/N2/H2O = 4/30/30/36,GHSV = 4000 h−1, mcat = 2.1–2.4 g, V cat = 5 ml,using catalyst TS-1(A) (see Table 1).

The reactant concentrations were varied in thefollowing molar ranges: propane 1–14%, oxygen0–80%, nitrogen 0–80% and water 0–75%. Finally,the influence of a sulfation of TS-1 was investigatedby treating TS-1 prior to the reaction with a 0.05 MH2SO4 solution at room temperature, and by an insitu treatment by feeding H2SO4 solutions with vary-ing concentrations in the reactor system for 2 h at823 K prior to the reaction.

The products were analysed on a gas chromato-graph Hewlett-Packard HP 6890 GC equipped witha TCD detector on a 3 m 1/8′′ Porapak QS packedcolumn and a 0.5 m 1/8′′ molecular sieve 5A packedcolumn.

3. Results and discussion

3.1. Characterisation

The best results in the catalytic screening wereobtained with the titanium containing MFI TS-1 (vide


infra). As we would like to focus on the extraor-dinary behaviour of this catalyst in the gas phaseoxidative dehydrogenation of propane, the character-isation of the other materials will not be discussedin detail in this paper. There were however no signi-ficant deviations in for example the crystallinity andthe compositions of the several different materialscompared to the data presented in the original papers,to which we would like to refer for a more in depthcharacterisation of the materials.

In order to obtain an clearer insight in the natureof the active sites of TS-1 several different batcheswere prepared or treated differently after their syn-thesis (see Table 1). All materials were highly crys-talline (the relative intensity (cps) of the reflection ofthe (0 5 1) plane was used as a measure for the crys-tallinity; catalyst B(2) was defined as 100%), exceptfor sample E, which was synthesised the same way assamples B, but was relatively poorly defined. The sili-con/titanium ratio of the materials (see Table 1) wasessentially the same as that of the synthesis gel, andwas varied between 45 and 120 in order to investigatethe influence of the number and the type of titanium

Fig. 2. DR-UV–VIS spectra of TS-1 catalysts A, B(1), B(2), D, E(1) and E(2).

sites on the catalytic performance, as it is known thatat low Si/Ti ratios the relative amount of extra frame-work TiO2 species will increase. Nitrogen sorption ex-periments of samples A–D revealed surface areas ofca. 420 m2/g. The surface area of catalyst E was sig-nificantly higher (over 525 m2/g).

The DR-UV–VIS spectra of the catalysts are com-pared in Fig. 2. Tetrahedrally coordinated frameworktitanium has a band at 200–212 nm, whereas foranatase (octahedrally coordinated titanium) a bandcan be found at 312–328 nm [16]. Extra frameworktitanium oxide species in TS-1 are believed to shiftthe first band into the direction of the band at around320 nm [17]. Clearly, in sample D there was essen-tially only framework titanium present, whereas insamples A and B also bands attributable to extraframework TiO2 species were found. There was aslight increase of the intensity of this band of sam-ple B(2) compared to B(1), which could be due tothe fast calcination procedure (no drying step, anda higher heating rate). The poorly defined samplesE(1) and E(2) contained relatively high amounts ofsmall extra framework titania clusters or octahedrally


Fig. 3. Ammonia TPD of catalysts A, B(1), and C.

coordinated titanium [18], as could be concluded fromthe broadness of the band at ca. 300–200 nm.

According to Bayevskaya and Baerns [19], alka-nes can be activated by a catalyst via three differ-ent mechanisms, namely: a redox-mechanism, anoxygen-surface-coverage or by Lewis acid sites. Asthe first two mechanisms seem to be unlikely ones forTS-1, the latter ones were very likely the active sites.Therefore, samples A, B(1) and C were investigatedwith NH3-TPD (see Fig. 3), which indeed revealedthe presence of weak Lewis acid sites.

According to Makarova et al. [20], these Lewisacid sites are unsaturated tetrahedrally coordinated ti-tanium atoms in TS-1. This was verified by examin-ing the catalysts with FT-IR after pyridine adsorption(Fig. 4). The interaction of pyridine with a Lewis acidsite leads to three typical bands in the IR-spectrum,namely at 1455, 1576 and 1621 cm−1, whereas theband at 1545 cm−1 for the pyridinium ion is typical fora Brønsted acid site [21]. Clearly, there was no bandattributable to the latter present in neither of the mate-rials, showing that the catalysts only contained Lewisacid sites, which was as to be expected for TS-1.

Finally, the TS-1 type catalysts did not suffer a lossof crystallinity or a loss of titanium after the reactions.

3.2. Catalysis

In Fig. 5, the results of the catalyst screening forthe oxidative dehydrogenation of propane to propeneare presented; the main by-product was CO2 for allmaterials. The dense phase titanates anatase, rutile,TiO2 (P25) and BaTiO3 showed both low conversionsand selectivities. The zeolitic materials silicalite-1,H-ZSM-5 (Si/Al = 40), 1.5 wt.% TiO2 on silicalite-1and Mo-USY (Si/Mo = 11.3) showed only low ac-tivities. On [Ti]-MCM-41 mainly carbon dioxide wasproduced as a result of the complete oxidation ofpropane. The titanium containing materials [Ti]-BEAand [Ti,B]-MFI could respectively convert 18 and13% of propane; the selectivity to propene howeverwas low (38 and 19%, respectively). Over VS-1 aconversion of 6% and a selectivity of circa 80%could be obtained, which was somewhat lower thanreported on by Cavani and Trifiro [1] (10 and 85%,respectively), but comparable to Centi and Trifiro [7],who found conversions of ca. 5% and selectivities ofup to 85%. This could possibly be attributed to thedifferent reaction conditions; especially the additionof water to the reaction feed could very well explainthe lower activity of the catalyst.


Fig. 4. FT-IR-spectra of the desorption of pyridine at different temperatures (a) after adsorption; (b) 1 h at room temperature; (c) 1 h at323 K; (d) 1 h at 373 K; and (e) 1 h at 473 K.

Fig. 5. Catalyst screening for the oxidative dehydrogenation of propane to propene.T = 823 K; molar ratios C3H8/O2/N2/H2O =5/25/25/45; GHSV= 1300 h−1; mcat = 1.4–8.0 g.


The best catalysts were TS-1 and [V]-MCM-41which combined relatively high yields with goodselectivities. At the chosen reaction conditions espe-cially TS-1 was highly selective (82%) and a conver-sion of up to 11% could be obtained. Therefore, TS-1was chosen as the catalyst for further experiments.

In Fig. 6, several TS-1 type catalysts having dif-ferent compositions or being subjected to differentcalcination procedures are compared in their activity,using somewhat different, better controlled reactionconditions than those used in the screening experi-ments. All materials were ca. 85% selective, exceptfor catalysts C and B(2) (77 and 67%, respectively)with conversions varying between 3 and 5%. It seemsthat the catalytic performance of the catalysts wasinfluenced by the type of titanium species present inthe TS-1 samples. Titaniumoxide on the outer surfacecould not be the most active species, as the activity ofthe silicalite-1 treated with TEOT was lower than thatof the TS-1 type catalysts. Since TEOT is too large to

Fig. 6. Conversion, selectivity and yield for the oxidative dehydrogenation of propane to propene over several differently prepared TS-1catalysts.T = 823 K; molar ratios C3H8/O2/N2/H2O = 4/30/30/36; GHSV= 4000 h−1; mcat = 2.1–2.4 g; V cat = 5 ml.

fit in the pores of an MFI, the titaniumoxide speciesof this material could only be present on the outer sur-face of the silicatite-1 crystals. This was an indicationthat the presence of tetrahedrally coordinated frame-work titanium was beneficial to the reaction [22].

As catalyst C had a high titanium content (2.9%)and catalyst B(2) was calcined using a higher heat-ing rate without preliminary drying, it could very wellbe that the amount of non-framework titanium ox-ide was of influence on the catalysts performance.However, catalyst E was not affected by the calcina-tion procedure, as could be shown with DR-UV–VIS,BET and XRD. The selectivity of this catalyst didnot change either. The results of these experimentswere still remarkable as all catalysts gave compara-ble selectivities and conversions. During the time onstream (max 6 h) there was no coke deposition andthe catalysts did not deactivate. Catalysts A, B(1) andC could be reused up to four times without a loss ofactivity.


Possibly the different TS-1 type catalysts performedsimilar because the conversions at the chosen reac-tion conditions were low. At higher conversions a bet-ter comparison should be possible, and therefore, theinfluence of the reaction parameters on the activitywere investigated, using catalyst A as the standardcatalyst.

An increase in conversion can usually be obtainedby either increasing the temperature or by lowering thespace velocity. For partial oxidation reactions usuallycatalysts with a low surface area are used for a bettercontrol of both residence time and temperature. In thiswork a zeolitic system was used, which has a high sur-face area due to its porosity. In order to compensatefor possible effects in the residence time distributionhigher space velocities were used, e.g. by the addi-tion of water to the system which additionally couldinfluence the adsorption–desorption behaviour of thecatalytic system.

One of the advantages of an oxidative dehydrogena-tion over a pure dehydrogenation is that it is possible tocarry out the reaction at lower temperatures. Whereasdehydrogenations are typically done at temperaturesof ca. 923 K, oxidative dehydrogenations are usuallydone at ca. 823 K, a temperature at which good con-

Fig. 7. Influence of propane partial pressure in the oxidative dehydrogenation of propane over catalyst TS-1(A).T = 823 K; molar ratiosC3H8/O2/N2/H2O = 1–14/29–33/29–33/28–33; GHSV= 3000 h−1; mcat = 2.31 g; V cat = 5 ml.

versions can be obtained without a strong drop in theselectivity due to the formation of CO2. At lower tem-peratures a strong drop in the activity could be ob-served: at 673 K there was no conversion of propaneat all. In a blind experiment using standard reactionconditions with the catalyst zone of the reactor filledwith quartz wool instead of catalyst also no conver-sion could be detected.

While keeping the space velocity constant the molarfraction of propane was varied (see Fig. 7). The high-est conversion of 19% with a selectivity to propene ofover 70% could be found at low propane concentra-tions. An increase of the propane partial pressure re-sulted in a decrease of the conversion; the selectivityhowever was constant. There was no complete oxy-gen consumption under these reaction conditions: asthe reactant molar ratio of propane to oxygen was atleast 2 and combustion to carbon dioxide with com-plete oxygen consumption would convert 40% of thepropane, is was clear that only part of the oxygen wasconsumed.

The influence of the oxygen partial pressure wasalso investigated (see Fig. 8). The space velocity waskept constant by adapting the nitrogen partial pressureand the oxygen/propane ratio was varied between


Fig. 8. Influence of the oxygen partial pressure pressure in the oxidative dehydrogenation of propane over catalyst TS-1(A).T = 823 K;molar ratios C3H8/O2/N2/H2O = 7/0–81/81–0/12; GHSV= 5000 h−1; mcat = 2.31 g; V cat = 5 ml.

0.4 and 12.0. Even at an oxygen concentration of80 mol%, that is no nitrogen present, no total oxida-tion occurred.

The selectivity to propene was higher than 70% forall cases. The low increase of the conversion couldbe an indication for mass transport limitations, prob-lems in the adsorption–desorption cycle, an insuffi-cient amount of active sites or a blockage of the activesites. The latter was highly unlikely, as no coke for-mation could be detected. Also, TS-1 is an MFI typestructure, which has a three-dimensional pore system,which should make the influence of pore blocking if itoccurs of less importance because of the accessibilityof the pore system. In a comparison test, where cata-lyst C was used instead of catalyst A, similar profiles(not shown here) as presented in Fig. 8 were found,with somewhat lower selectivities and conversions.

Replacing propane with propene using standardreaction conditions led to a 70% propene conver-sion with a selectivity to CO2 of 95%. Assuminga consecutive reaction pathway of the transforma-tion of propane to propene, followed by the com-plete oxidation to carbon dioxide, these experimentsclearly indicated that the deep oxidation was not thelimiting step, which means that, once the olefin is

generated on the catalyst, a further oxidation is pos-sible. The adsorption of the propene therefore hadto be fast, and the rate limiting step in the oxida-tive dehydrogenation over TS-1 therefore probablywas the activation of the alkane. If mass transportlimitations could be ruled out profiles as shown inFig. 8 could be interpreted as that the activation ofthe alkane was independent of the oxygen partialpressure.

Within the micropores of a zeolitic crystal usuallymass transport limitation occurs [23]. The intercrys-tallite mass transport may be limited by the geometry(form) of the pellet and/or the pellet size. In order toinvestigate the influence of the latter several differentcatalyst pellet sizes were tested, keeping the space ve-locities constant at 5000 and 8000 h−1 (see Fig. 9).With the higher space velocity the transport barriers inthe macro–meso-pores of the pellet, if existent, shouldbe less dominant. It was found that the conversion wasessentially independent of the pellet size. At a GHSVof 8000 h−1 the selectivity was also independent of thepellet size. At a space velocity of 5000 h−1 the optimalpellet size was 0.5–1.0 mm, which might be due to ahigher pressure drop when smaller pellet sizes wereused, or to mass transport phenomena when larger pel-


Fig. 9. The influence of the pellet size in the oxidative dehydrogenation of propane with a constant amount of catalyst TS-1(A).T = 823 K;molar ratios C3H8/O2/N2/H2O = 7/39/39/15; GHSV= 5000–8000 h−1; mcat = 2.0 g; V cat = 5 ml.

lets were used. However, the effect was only minor,and will be regarded as non existent, indicating thatthere was essentially no mass transport hindrance inthe macro- and mesopores of the catalyst. This couldalso be an indication that the reaction mainly tookplace on the outer surface of the catalyst crystallitesor only in the outer layer of the microporous system,since the mass transport within the pores of a zeoliteis always limited [23]. Taking the behaviour of TS-1with an increasing oxygen partial pressure into ac-count it seemed most likely that the reaction rate waslimited by the amount of active sites.

In Fig. 10, the influence of the W/F ratio(Weight/Flow ratio; catalyst weight over the total flowof the reactants), which was varied either by increas-ing the amount of catalyst or by lowering the totalgas flow keeping the concentrations constant, on theconversion and selectivity is presented.

As expected the conversion increased with higherW/F ratios. The selectivity however dropped sharplyto 45% at a conversion of 30%, similar to the be-haviour of multicomponent catalysts observed by Zan-

thoff et al. [24]. Because of instrumental limitations ofthe pump used to feed water it was not possible to fur-ther increase the W/F ratio. It was, therefore, decidedto study higher W/F ratios without feeding water intothe system (see Fig. 11).

A similar behaviour could be observed: at a W/F ra-tio of 26 g h/mol the conversion increased up to 50%;the selectivity however dropped to 11%. Higher W/Fratios correspond with higher residence times, whichexplained the low selectivity for the partial oxidationproduct. It is also clear that the addition of watercaused an increase of the selectivity. Unfortunately,the conversion dropped in the presence of water. Thismight very well be caused by a competitive adsorptionof the water and the reactant molecules at the activesite, thus, reducing the mean residence time of eachmolecule at the active site and the overall accessibleamount of active sites.

As all TS-1 samples revealed similar results in thecatalytic experiments, the activation of the alkanecould very well be the rate limiting step. Lewis acidtetrahedrally coordinated titanium on the outer surface


Fig. 10. The influence of the W/F ratio in the oxidative dehydrogenation of propane over catalyst TS-1(A).T = 823 K; molar ratiosC3H8/O2/N2/H2O = 6/40/40/14.

Fig. 11. The influence of the W/F ratio in the oxidative dehydrogenation of propane without water over catalyst TS-1(A).T = 823 K;molar ratios C3H8/O2/N2 = 8/25/67.


Fig. 12. The influence of a 2 h sulfation of catalyst TS-1(A) at 823 K prior to the oxidative dehydrogenation of propane.T = 823 K; molarratios C3H8/O2/N2/H2O = 4/28/28/40; GHSV= 2000 h−1; mcat = 2.1 g; V cat = 5 ml.

of the zeolite crystals could very well be the activesite. It is for several reasons unlikely that a large partof the conversion takes place inside the TS-1 crystals.Inside the micropores there will always be a (strong)diffusion limitation, which causes a high residencetime of the propene formed, which would then beconverted to CO2 and possibly even coke. As therewas indeed only a small amount of CO2 formed, andno coke at all was formed, this was an indication thatonly a minor part of the microporous system took partin the reaction. Furthermore, the high flow will limitthe interparticle mass transport limitation, making itlikely that the major part of the conversion took placeon the outer surface of the zeolite crystals, or in theouter microporous layer of the TS-1 crystallites.

The Brønsted and Lewis acidity of metal oxides(e.g. ZrO2, Fe2O3, TiO2, SiO2, Al2O3) can be changedby a sulfation [25,26]. These SO4

2−/MexOy materialswere often described as having superacidity, althoughnowadays it is believed that these materials have anacid strength comparable to that of protonic zeolites,and there are now several alternative proposals forthe extraordinary behaviour of these systems [26].It also seems that hydrated materials are Brønstedacid, which can be converted to Lewis acid sites by

a dehydration [26]. As we assume that the here in-vestigated oxidative dehydrogenation needs Lewisacid sites, and as a zeolite essentially consists out ofSiO2 and, more specifically, in this case SiO2/TiO2(TS-1), we tested if a similar behaviour could be ob-served in the reaction by sulfating the catalyst in situprior to the reaction by leading increasing amountsof evaporated sulphuric acid through the catalyst bedat 823 K (see Fig. 12), or by a liquid phase sulfationof the catalyst with a dilute H2SO4 solution prior tothe reaction. At the reaction conditions the sulfatedsites will indeed be dehydrated, thus, forming Lewisacid sites as described by Brown and Hargreaves[26].

Clearly, with increasing amounts of sulphuric acidin the feed prior to the reaction, the conversion alsoincreased, whereas the selectivity remained constant,which is an indication that the reaction was mainly in-fluenced by the type and the amount of the acid sites.Upon the liquid phase sulfation (also prior to the re-action; the material was not affected by the treatment)using the same reaction conditions the conversion in-creased to 17% with a selectivity of 74%, which againis in support of a limitation by the amount of acidsites.


4. Conclusions

The activity of titanium and vanadium containingzeolitic and non-zeolitic materials in the oxidative de-hydrogenation of propane to propene was investigated.Especially over [V]-MCM-41 and TS-1 high selectiv-ities with good conversions were obtained, with CO2being the main by product resulting from the completeoxidation.

The reaction system was optimised using TS-1 asthe catalyst. The propane and oxygen partial pressurehad no influence on the selectivity, and the mass trans-port limitation in the macro- and mesopores could beneglected. The addition of water caused a decrease inthe conversion, but increased the selectivity, probablydue to a competitive adsorption of the water moleculesand the reactant molecules on the active site. The re-action probably takes place on the outer surface of theTS-1 crystallites on Lewis acid sites. The activation ofpropane on these sites was probably the rate limitingstep, assuming a reaction pathway in which propaneis initially converted to propene or CO2, followed bya deep oxidation of propene to CO2. An increase inthe residence time led to higher conversions, com-bined with lower selectivities because of the forma-tion of the thermodynamically more favourable CO2.The best results obtained over TS-1 were selectivitiesof up to 82% at a conversion of 11%.

Although it is assumed that the reaction takes placeon Lewis acid sites on the outer surface of the TS-1crystallites the exact reaction mechanism nor the ex-act active site are not yet clear. For example, neitherthe titanium content nor the crystallinity were of in-fluence on the catalysts performance. Titanium oxidespecies on the crystallite outer surface could also notbe the active site, as the activity of with TEOT treatedsilicalite-1 was lower than for the TS-1 type materials.However, increasing the Lewis acidity by a sulfationof TS-1 in both the gas phase and the liquid phaseprior to the reaction resulted in an increase of the con-version of up to 17% with a selectivity of ca. 74%,which are the best results up to now.

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