[3] Metzler, P., Ph. D. Thesis, University of Kaiserslautern 1997.[4] Scott, T. C., Sep. Purif. Methods 18 (1989) pp. 65±109.[5] Scott, T. C., AIChE J. 33 (1987) No. 9, pp. 1557±1559.[6] Scott, T. C.; Sisson, W. G., Sep. Sci. & Tech. 23 (1988) pp. 1541±1550.[7] Bailey, A. G., Atomisation and Spray Technology 2 (1986) pp. 95±134.[8] Byers, C. H.; Perona, J. J., AIChE J. 34 (1988) No. 9, pp. 1577±1580.[9] He, W.; Chang, J. S.; Baird, M. H. I., J. Electrostatics 40 & 41 (1997)

pp. 259±264.[10] Sato, M., Formation of Uniformly-Sized Emulsions by Means of

Applied Electrostatic Field, in: Solvent Extraction (T. Sekine, Ed),Elsevier Science Publishers B.V., Amsterdam 1992, pp. 1435±1440.

This paper was also published in German in Chem. Ing. Tech. 73 (2001) No. 7,pp. 819±823.

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Effect of Pyridine Admixture onHeterogeneously Catalyzed PartialOxidation of Substituted MethylAromatics to their CorrespondingAldehydes

By Andreas Martin*, Ursula Bentrup and Gert-Ulrich Wolf

Dedicated to Prof. Dr. Bernhard Lücke on the occasion of his65th birthday

Vanadium-containing catalysts used in the partial oxidationof toluene and substituted toluenes, resp., to their aldehydesneed two different catalyst functions: (i) acidic surface sites(Lewis-acid sites) for reactant chemisorption and (ii) basicsites for an easier desorption of products. Therefore, thesimultaneous addition of nonoxidizable pyridine to the feedleads to an increased aldehyde selectivity at similar conversioncompared to runs without co-feeding of pyridine. This result ismainly due to a blockade of Brùnsted-acid sites of the catalystsurface. Additionally, the desorption of the desired productscould be sped up by an increased basicity of the catalystsurface. Beside catalyst properties, also different substituenteffects play an important role for chemisorption. For example,the addition of pyridine to a p-chlorotoluene-containingreaction feed does not enhance aldehyde selectivity, other-wise, pyridine addition to p-methoxytoluene-containing feedresults in drastically increased aldehyde selectivity.

1 Introduction

Vanadium-oxide-containing catalysts show an outstandingsignificance in industrial vapor phase oxidation reactions, e.g.for oxidation of olefins to saturated aldehydes, oxidation ofalkyl aromatics to carbonaceous acids and anhydrides as wellas for ammoxidation of methyl aromatics and methyl hetero

aromatics to their corresponding nitriles [e.g. 1±7]. Addition-ally, vapor phase processes demonstrate some crucial advan-tages compared with discontinuous liquid phase reactionsconcerning continuous processing, catalyst handling, masstransfer, homogeneity of the reaction mixture and last but notleast their environmentally benignity.

First commercial oxidation of aromatics based on vaporphase processes started in the first decades of the last century,mainly leading to acids and acid anhydrides that show stableoxidation states (e.g., phthalic anhydride production byoxidation of naphthalene over vanadium oxide catalysts in1916/17). This exemplary reaction is attributed to electrophil-ic-type oxidations whereas the more efficient conversion ofo-xylene to phthalic anhydride in the vapor phase belongs tothe nucleophilic reaction type [e.g. 8]. Otherwise, alsoindustrially important aromatic bulk oxygenates, like ter-ephthalic and benzoic acid, are produced by liquid phase side-chain oxidation of p-xylene and toluene, respectively [e.g. 8].

The partial oxidation of toluene or substituted methylaromatics to their corresponding aldehydes is much morecomplicated because the aldehydes themselves are consecu-tively oxidized very fast to acids and deeper oxidized products(e.g. anhydrides and quinones). For example, benzaldehyde isproduced in the vapor phase at rather low toluene conversionrates (10±20 % per pass) at short residence time (< 1 s); eventhen, it is only 40±60 % of the theoretical yield [9]. Due to thatrather poor performance of direct oxidation, so far, only littleis known on such vapor phase processes and varioussubstituted aromatic aldehydes are still manufactured intraditional way of the organic chemistry, for example byhydrolysis of benzal chlorides [9].

However, the following gives some examples on researchthat has been done in the last two decades in the field of thedirect oxidation of methyl aromatics to their correspondingaldehydes by the vapor phase. Chopra and Ramakrishna [10]reported on the oxidation of p-chlorotoluene to p-chloroben-zaldehyde on Bi,Mo-oxide catalysts with very high aldehydeselectivity of ca. 82 % at 93 % conversion (T = 723 K,O2 : N2 = 1, SV = 0.3 l/hg). However, these high yields couldnever been reproduced. A recent patent application byHoechst AG [11] has taken up this reaction again. The authorsapplied various catalyst compositions, mainly consisting ofV,Cs,Fe-oxides doped with various transition metals. Thereaction was carried out ca. 673±773 K (p-chlorotoluene : air =1 : 99, 2.5 ml/s at STP) and in dependence on the reactionconditions the p-chlorotoluene conversion reached valuesbetween 13 and 68 % but the p-chlorobenzaldehyde yield doesnot exceed ca. 20 %. Seko et al. [12,13] reported on the vaporphase oxidation of p-methoxytoluene to p-methoxybenzalde-hyde (anisaldehyde) on V2O5-P2O5 catalysts, additionallycontaining Cu(I)-oxide and K2SO4. The authors observed adecreasing catalyst activity by addition of phosphoric acid toV2O5 and simultaneously an increased aldehyde selectivitywas found. The addition of copper- and potassium-containingcompounds led to a further increase in selectivity (up toca. 70 %) at ca. 80 % conv/ersion. Ueshima and Saito [14]

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[*] Dr. A. Martin, Dr. U. Bentrup, Dipl.-Chem. G.-U. Wolf, Institut fürAngewandte Chemie Berlin-Adlershof e.V., Richard-Willstätter-Str. 12,D-12489 Berlin, Germany.

investigated the vapor phase oxidation of various toluenes(toluene, alkyl toluenes and p-methoxytoluene) on ratherhazardous V2O5-Tl2O5 catalysts (T = 703 K, toluene : air =1 : 99, 500 ml/min at STP). Catalyst activity and reactionselectivity strongly depend on the electronic properties(ionization potential) of the substituents. p-Methoxybenzal-dehyde selectivity was reached up to 75 % at ca. 83 %conversion. In general, it seems likely that the selectivity tosubstituted benzaldehydes is closely related to the basicproperties of the catalyst whereas the activity stronglydepends on the amount and strength of acid sites. Thisconnection and a sufficient surface basicity generated bycatalyst components or basic molecules adsorbed from thereaction mixture have to be considered in the search forimproved catalysts. Moreover, the nucleophilic properties ofdifferent reactants have also to be included in this thought.

A first example for this idea was rather unconsciously givenby Mathes et al. [15] fifty years ago. The authors reported onthe partial oxidation of methyl pyridines (picolines) to thecorresponding aldehydes in the vapor phase on V,Mo-oxidecatalysts. The aldehydes were obtained in good yields.Another example of an additional increase in basicity of thereaction system by the reactant was also given from our groupduring vapor phase oxidation of 4-picoline on (VO)2P2O7

catalyst [16]. The reactant is partially oxidized to the desiredaldehyde but simultaneously 4-picoline acts as adsorbate foracid surface sites in a defined temperature range. Such effect issimilar to the use of probe molecules for acid-base titration ofsolids by temperature-programmed desorption methods(TPD), e.g., the use of pyridine or ammonia for thecharacterization of acid surface sites. We observed 4-picolineconversion of ca. 80 % and aldehyde selectivity of max. 65 %was reached [16] at 693 K so far; carbon oxides wereexclusively formed as byproducts.

The effect of an increased basicity was also tried to apply forother oxidations, e.g., the oxidation of toluene to benzal-dehyde. The reaction was carried out on V,P-oxide catalysts(VPO) in the presence of pyridine, that is not oxidizable underthe chosen reaction conditions. The addition of an aqueouspyridine flow to the reaction feed results in a significantincrease in the selectivity of benzaldehyde at similar tolueneconversion [17]. Nag et al. [18] reported in the early 80s onsimilar runs but they used pulses of pyridine that were dosed toa toluene-air feed passing over a molybdenum oxide catalyst.They also observed an increase in the aldehyde selectivityduring pulsing but the conversion dropped to zero, all acidsites were blocked because of total poisoning. After someminutes of flushing by air and toluene feed the reaction startsagain due to desorption of pyridine from the acid sites. Thedesorption rate is mainly determined by the chosen temper-ature similar to the mentioned TPD experiments.

The aim of the present study was (i) to apply this processingto other reactants, especially substituted toluenes and (ii) tosynthesize metal oxide catalysts with enlarged basicity byadmixture of alkali metal cations. Furthermore, it was alsointeresting to investigate substituent effects of the used

reactants as well as the effect of size and basicity of differentalkali cations on the catalyst performance and to understandwhether phase transformations or solid-state reactions pro-ceed during reaction.

2 Experimental

Alkali-cation-containing V2O5 catalysts (MVO) were pre-pared by incipient wetness method with an aqueous M2SO4

solution (M = Li, Na, K, Rb, Cs). 50 ml of such a solution(0.01 mol K2SO4) were added to V2O5 (0.1 mol) andevaporated using a rotary evaporator at 343 K for 1 h. Afurther evaporation to dryness was carried out under vacuumat this temperature. The obtained product was dried overnightat 403 K.

Parent and used catalysts were characterized by X-raydiffractometry (XRD) to obtain knowledge on phase compo-sition and also to observe possible changes during reaction.The surface area of the catalytically employed materials wasdetermined by N2-physisorption at 77 K using the BETmethod. The total vanadium content and the averageoxidation state of vanadium in the samples were determinedby potentiometric titration with Fe2+ and Ce4+ solutionsaccording to a method described by Niwa and Murakami [19].

The catalytic runs were carried out in a common quartz-glass tube reactor (ca. 0.5 g catalyst) that was connected to agas- and liquid-metering system. The organic reactant was fedusing a saturator. The tube reactor was electrically heated by aradiation furnace (Tmax = 773 K). Toluene, p-chlorotoluene,p-methoxytoluene and p-tert.-butyltoluene were used asreactants. The product stream was trapped in ethanol by anice-cooled vessel. Product composition was analyzed by GC(Shimadzu, GC 17A) using benzonitrile as internal standard inoff-line mode (Shimadzu, Autosampler AOC 20i). Tolueneconversion (X/mol-%) and nitrile yield (Y/mol-%) weredetermined after attaining steady-state and given as anaverage of five analyses (�5 %). Total oxidation products(CO, CO2) were analyzed using nondispersive infraredanalyser (Rosemount, Binos 100 2M).

3 Results and Discussion

The XRD patterns of all parent samples revealed theexpected reflections of V2O5. Interestingly, the patterns of theK-, Rb- and Cs-containing samples showed additionalreflections belonging to a vanadate phase (MV3O8). Incontrast, LiVO and NaVO do not reveal such phase. Aftercatalytic tests the samples were again investigated by XRDbut it is surprising to note that the MV3O8 reflections arevanished and mixed-valent bronze-like alkali-cation-contain-ing vanadium oxides like MxV2O5-x (x = 0.3±0.5) wereidentified only in the case of K-, Rb- and Cs-containingcatalysts. The XRD patterns of LiVO and NaVO catalystsremained nearly unchanged and V2O5 could be seen as

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crystalline phase. These structural changes were also provenby FTIR spectroscopy which clearly showed the formation ofV(IV)=O proportions. The BET surface areas of the usedcatalyst specimens were found to vary between 5 and 7 m2/g.The analysis of the oxidation states of the parent samplesrevealed that the values of average oxidation states ofvanadium are in the range of 4.9±4.95; it could be concludedthat these parent samples are already partially reduced to amarginal extent. The oxidation states of the used samples areobserved to be lowered significantly, reaching values ofca. 4.81±4.91. Tab. 1 depicts a short survey on thesecharacterisation results.

In general, the adsorption of the organic reactant (methylaromatic compound) on the catalyst surface proceeds via theinteraction of its �-electron system with surface Lewis-acidsites. The nucleophilic properties of the reactants areinfluenced by different substituents and, therefore, they affectthe strength of the chemisorption of the reactant on thecatalyst surface. Thus, the chemisorption strength mayincrease in the following order: p-chlorotoluene < toluene <p-tert.-butyltoluene < p-methoxytoluene. The generation ofan aldehyde group by methyl group partial oxidation leads toan additional electron-withdrawing effect that should speedup the desorption of the reaction product in the same order.Fig. 1 schematically presents these ideas.

Before testing alkali-cation-containing MVO catalysts,some runs on pure V2O5 with p-methoxytoluene as a reactantwere carried out for comparison. p-Methoxytoluene conver-sion was observed to increase up to ca. 75 mol-% with increasein reaction temperature from 613 to 653 K. The p-methox-ybenzaldehyde selectivity was found to be very poor (10±15 %) as expected; the exclusive byproducts were carbonoxides. Tab. 2 gives a summary of these results.

The evaluation of the catalytic results of the partialoxidation of various reactants (carried out on KVO catalystunder comparable reaction conditions) confirmed the as-sumption on the different effect of nucleophilic properties ofthe reactants (Fig. 2). p-Chlorobenzaldehyde could beobtained in a good selectivity up to 70 %, otherwise, theselectivities of the other aldehydes drastically decrease in theexpected order due to increasing nucleophilicity of thereactants and products. Thus, p-methoxybenzaldehyde canonly be obtained in a very poor selectivity of ca. 10 %. Thisresult is due to an intensive interaction of surface OH groups(Brùnsted-acid sites) and the carbonyl group (-C(H)=O) ofthe intermediately formed aldehyde via H-bonding as shownin detail by extensive in situ FTIR and ESR investigations [e.g.

17,20] on vanadium phosphates used as catalysts. TheseBrùnsted-acid sites are permanently generated on the catalystsurface due to a reaction of V-O-P and/or P-O-P bonds of aVPO catalyst with water that is formed during the oxidation asa reaction product. Such interaction leads to a strengthenedadsorption of the desired reaction products and, consequently,deeper overoxidation occurs that may lead to a considerableincrease in total oxidation.

The doping of V2O5 with alkali cations (e.g. K+) leads tosignificant decrease in activity but simultaneously, totaloxidation is suppressed compared to pure V2O5. For explana-tion, alkali cations can be positioned between V2O5 layers andmay hinder electron and oxygen transport through the bulk,moreover, new crystalline phases may be generated, as provenby the XRD investigations. In general, the electronicinteraction of the vanadyl sites as well as their redox potentialis altered due to these structural changes. Furthermore, theincreasing ionic radii of the incorporated cations cause aprogressive electronic and steric separation of active sites.However, an enhanced selectivity for stronger chemisorbedproducts (e.g. p-methoxybenzaldehyde) could not beobserved in comparison to pure V2O5 used as catalyst.

Therefore, it seems more likely that the addi-tional blockade of acidic surface sites may lead toincreased aldehyde selectivity as shown for thepartial oxidation of toluene [17]. Actually, suchan effect could be observed by addition of theoxidation-stable pyridine to the feed. The usedconcentration of pyridine shows a significantinfluence on the catalyst activity and reactionselectivity but it should be well-tuned, i.e., larger

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Table 1. Survey on some results of the physicochemical characterization.

Catalyst Vanadium Phase composition Oxidation number BET% parent used parent used m2/g

LiVO 52.3 V* V* 4.95 4.91 7.2NaVO 51.9 V* V* 4.95 4.85 6.5KVO 47.2 V*, KV3O8 V*, K0.5V2O5 4.93 4.89 5.2RbVO 47.9 V*, RbV3O8 V*, Rb0.4V2O5 4.94 4.81 6.1CsVO 46.9 V*, CsV3O8 V*, Cs0.3V2O5 4.90 4.84 4.7V*±V2O5

Cl Cl

H 3 C

H 3 C

H 3 C

O H C

O H C

O H C

O C H 3 O C H 3

basicity

electron acceptor

electron donor

H 3 C O H C

(nucleophilicity)

Figure 1. Electronic properties of reactants and products during partialoxidation of various toluenes to the corresponding aromatic aldehydes.

Table 2. Partial oxidation of p-methoxytoluene to p-methoxybenzaldehyde onpure V2O5 (p-methoxytoluene : O2 : N2 = 1 : 89 : 316; 0.55 g catalyst).

Temperature Conversion SelectivityK mol% %613 20.6 12.7633 36.5 13.9653 73.4 13.3

proportion of pyridine in the reaction mixture isalso expected to block Lewis sites to a greaterextent leading to a considerable drop in activityand therefore, this should be avoided.

Fig. 3 depicts a comparison of catalytic data ofthe partial oxidation of p-methoxytoluene onLiVO, KVO and CsVO catalyst samples withoutand in the presence of pyridine at ca. 628±638 K.It can be clearly seen that the admixture ofpyridine causes a significant increase in productselectivity. Furthermore, it is obvious that thisselectivity enhancement depends on the kind of the used alkalication, i.e., the aldehyde selectivity increases in the followingorder: Li < Na << K,Rb,Cs (the results obtained on NaVO andRbVO are not shown in Fig. 3). The increase of the basicity ofthe reaction system causes an easier product desorption fromthe catalyst surface, leading to increased aldehyde yields to aconsiderable degree; the product selectivity could be in-creased by a factor of ca. 2.5.

0

20

40

60

80

100

conversion

selectivity

%

LiVO

LiVOCsVO

CsVOKVO

KVO

+ pyridine

Figure 3. Comparison of p-methoxytoluene conversion and p-methoxyben-zaldehyde selectivity on LiVO, KVO and CsVO catalysts at reactiontemperatures of 628±638 K without and in the presence of pyridine (molarratio p-methoxytoluene : O2 : N2 : (Py : H2O) = 1 : 72 : 256 (: 5 : 21), 0.5 g catalysts,W/F = ca. 0.6 ghmol±1).

Tab. 3 presents the best results obtained during the partialoxidation of methyl aromatics so far, using alkali-cation-containing catalysts compared to pure V2O5. p-Chloroben-zaldehyde can be synthesized on KVO catalyst in high

selectivity. The addition of pyridine does not lead to a furtherincrease in aldehyde formation, in contrast, small amounts ofpyridine in the feed cause a decline in p-chlorotolueneconversion. As a possible reason, it seems likely that pyridineblocks Lewis-acid sites being essential for reactant chemi-sorption. p-Chlorotoluene shows a rather weak interactionwith the surface (Lewis-acid sites) due to its electronicproperties. This could lead to a preferred blockade of suchsites by pyridine and the activity drops due to a decliningnumber of active sites. Similar conversion and aldehydeselectivity can be reached during partial oxidation of p-tert.-butyltoluene, but it could be seen that significant proportionsof pyridine are necessary. The same effect was observed usingp-methoxytoluene as feed, the conversion could be increasedup to ca. 75 % and an aldehyde selectivity over 60 % is reachedby addition of pyridine. Even on the rather unselective V2O5

(see Tab. 2) in the presence of pyridine a surprisingly highaldehyde selectivity at high conversion p-methoxytoluene ofhas been reached.

4 Conclusions

Summarizing the results, it could be shown that a simulta-neous addition of pyridine to the methyl aromatic reactantcontaining feed leads to an increased aldehyde selectivity atsimilar conversion compared to runs without co-feeding ofpyridine. This result is mainly due to a blockade of Brùnsted-acid sites of the catalyst surface. Additionally, the desorption ofthe desired products could be sped up by an increased basicityof the catalyst surface. As a conclusion, the partial oxidation oftoluene and substituted toluenes, resp., to their aldehydes onvanadium-containing catalysts needs two different catalystfunctions: (i) acidic surface sites (Lewis-acid sites) for reactantchemisorption and (ii) basic sites for speeding up thedesorption of products. Fig. 4 demonstrates these ideas andmay explain the different reaction steps: (a) The chemisorptionof a methyl aromatic reactant occurs via its p-electron systemon the catalyst surface that must contain Lewis-acid sites andbulk oxygen as oxidant. (b) The formation of the aldehyde byoxidation of the methyl group also leads to the formation ofwater that interacts with the catalyst surface and surface OHgroups are formed. H-bonding between carbonyl oxygen ofthe aldehyde and surface OH groups causes a strengthenedchemisorption. Additionally, oxygen vacancies are formed that

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0

20

40

60

80

selectivity

conversionp -Cl-toluene

p -tert.-C4H9-toluene

p -CH3O-toluenetoluene

%

Figure 2. Conversion of toluene as well as substituted reactants and selectivity ofthe corresponding aldehydes on KVO catalyst at reaction temperatures of631±678 K (molar ratio toluene : O2 : N2 = 1 : 6±30 : 115±320, 0.5 g catalyst).

Table 3. Heterogenously catalyzed partial oxidation of substituted toluenes (p-chlorotoluene(CTOL), p-tert.-butyltoluene (BTOL) und p-methoxytoluene (MTOL)) to the correspondingaldehydes.

Reactant Catalyst T Molar ratio Conversion SelectivityK Reactant : O2 : N2 : Py : H2O mol% %

CTOL KVO 678 1 : 6 : 220 24.4 73.4CTOL KVO 668 1 : 6 : 220 : 0.4 : 40 20.1 68.4BTOL KVO 638 1 : 97 : 346 : 37 : 168 17.5 53.8BTOL KVO 633 1 : 103 : 364 : 39 : 177 13.8 59.3MTOL KVO 673 1 : 97 : 343 : 37 : 167 60.5 62.8MTOL V2O5 653 1 : 81 : 289 : 31 : 141 74.4 62.9

will be filled up by bulk oxygen; in the end, the oxygendeficiency of the catalyst is permanently compensated by gasphase oxygen. (c) The formed acidic OH groups of the catalystsurface are blocked by pyridine that may lead to an easierdesorption of the products. Beside catalyst properties, alsodifferent substituent effects play an important role forchemisorption. For example, the addition of pyridine to thep-chlorotoluene reaction feed does not enhance aldehydeselectivity, otherwise, pyridine addition to p-methoxytolueneoxidation results in drastically increased aldehyde selectivity.

Acknowledgement

The authors thank Mrs. Elke Oliev and Mrs. Heidi Frenchfor experimental assistance. This work was supported by theFederal Ministry of Education and Research, Germany(project 03C0279).

Received: July 16, 2001 [K 2853]

References

[1] Baiker, A., Chimia 50 (1996) No. 3, pp. 65±73.[2] Haber, J., in: Perspectives in Catalysis (J. M. Thomas, K. I. Zamaraev,

Eds.), Blackwell Scientific Publications, Oxford 1992, pp. 371±385.

[3] Babel, D. K.; Shanker, R. B. R.; Bakore, G. V., J. Sci. Ind. Res. 43 (1984)pp. 250±260.

[4] Mori, K.; Miyamoto, A.; Murakami, Y., J. Chem. Soc., Faraday Trans. 183 (1987) pp. 3303±3315.

[5] Rizayev, R. G.; Mamedov, E. A.; Vislovskii, V. P.; Sheinin, V. E., Appl.Catal. A: General 83 (1992) pp. 103±140.

[6] Sheldon, R. A.; de Heij, N., in: The Role of Oxygen in Chemistry andBiochemistry (W. Ando, Y. Moro-oka, Eds.), Stud. in Org. Chem.,Elsevier, Amsterdam, 33 (1988), pp. 243±256.

[7] Sheldon, R. A., Chemtech (1991) pp. 566±76.[8] Weissermel, K.; Arpe, H. J., Industrielle Organische Chemie, VCH,

Weinheim 1994.[9] Brühne, F.; Wright, E., in: Ullmann's Encyclopedia of Industrial

Chemistry, 6th ed., 1998, electronic release (benzaldehyde entry).[10] Chopra, B.; Ramakrishna, V., Ind. Chem. J., Annu. (1972) pp. 38±49.[11] Borchert, H.; Gerdau, T.; Weiguny, J., Verfahren zur Herstellung von

Halogenbenzaldehyden, European Pat. EP 0723949A1 (1996).[12] Seko, H.; Tokuda, Y.; Matsuoka, M., Nippon Kagaku Kaishi (1979) 4,

pp. 558±559.[13] Matsuoka, M.; Seko, H., Process for Preparing Anisaldehyde, U. S.

Patent 4054607 (1977).[14] Ueshima, M.; Saito, N., Chem. Lett. (1992) pp. 1341±1344.[15] Mathes, W.; Sauermilch, W.; Klein, T., Chem. Ber. 84 (1951) pp. 452±458.[16] Martin, A.; Lücke, B.; Förster, A.; Niclas, H.-J., React. Kinet. Catal. Lett.

43 (1991) pp. 583±588.[17] Martin, A.; Bentrup, U.; Lücke B.; Brückner, A., Chem. Commun.

(1999) pp. 1169±1170.[18] Nag, N. K.; Fransen, T.; Mars, P., J. Catal. 68 (1981) pp. 77±85.[19] Niwa, M.; Murakami, Y., J. Catal. 76 (1982) pp. 9±16.[20] Martin, A.; Bentrup, U.; Brückner, A.; Lücke, B., Catal. Lett. 59 (1999)

pp. 61±65.

This paper was also published in German in Chem. Ing. Tech. 74 (2002) No. 1+2.

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New Hydrocracking Catalysts Based onMesoporous Al-MCM-41 Materials*

By Andreas Klemt and Wladimir Reschetilowski**

1 Introduction

Hydrocracking is beside catalytic cracking a key process inthe processing of heavy crude fractions and will becomeincreasingly important, due to its flexibility [1]. Hydrocrack-ing catalysts are bifunctional systems, which consist of anacidic support and a hydrogenating/dehydrogenating metalcomponent. A commonly used combination is Ni/Mo depos-ited on an alumosilicate support. The optimal ratio of thedifferent active sites and the accessibility are important for thecatalytic properties of the hydrocracking catalysts. Mesopo-rous Al-MCM-41 materials are interesting in terms of theiruse as active supports for hydrocracking catalysts and as

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H 3 C R

O O

C

R

O

H

C R O

H

OH

N O H

O

OH

OH

Lewis-acid site surface oxygen

O-vacancy H-bonding

OH-blockade

b)

a)

c)

Figure 4. Chemisorption and reaction steps of methyl aromatics on a vanadiumoxide catalyst surface during their partial oxidation to aldehydes.

±

[*] Lecture presented by A. Klemt at the ACHEMA 2000, Frankfurt/Main(Germany), May 23, 2000.

[**] Dr. rer. nat. A. Klemt, CRI KataLeuna GmbH Catalysts, Prof. Dr. rer. nat.habil. W. Reschetilowski, Dresden University of Technology, Institute ofIndustrial Chemistry, D-01062 Dresden, Germany.

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