Partial Oxidation of Propane Using Dense Carbon Dioxide

By Andreas Martin and Boris Kerler*

The present work was aimed at investigating activity and selectivity of various catalysts for the partial oxidation of propane in sc-CO2 atmosphere. Catalytic experiments were performed in a stirred batch reactor. This paper reports on the used reactor systemand first results of a catalyst screening using different metal (oxide) catalysts as well as the variation of the reaction conditions.

1 Introduction

Supercritical fluids (SCFs) show a promising potential asreaction medium or reactant in chemical reactions due to theiroutstanding physicochemical properties, such as liquid-likedensities as well as solvent and heat transport behavior at gas-like diffusivities and dynamic viscosities [1±5]. An increasingnumber of publications in recent years confirms this trend,concerning for example hydrogenation reactions in supercrit-ical carbon dioxide (sc-CO2) [6], hydrolysis as well asdehydration reactions in sc-H2O [7], rearrangements insc-H2O [8] and also aminations in sc-NH3 [9]. Beside theapplication of their physicochemical properties, SCFs areoften simultaneously used as a reactant (e.g., sc-i-C4H10 [10],sc-H2O [7], sc-NH3 [9]).

Considerable progress could be noted in the last decadeusing SCF in heterogeneous catalysis [4]. But only rareliterature exists focussing on heterogeneously catalyzedoxidation of hydrocarbons at supercritical reaction conditions.For example, the toluene partial oxidation on a CoO/Al2O3

catalyst under sc-CO2 at 293±493 K was reported by Dooleyand Knopf [11], whereas Zhou et al. [12] suggested Pd/Al2O3

at temperatures of 618±663 K for total oxidation. Zhou andAkgerman [13] also used a Pt/TiO2 catalyst at 423±548 K forthe total oxidation of ethanol and acetaldehyde in sc-CO2. Fanet al. [10] reported on the partial oxidation of isobutane,directly using the hydrocarbon as SCF and a Pd/C catalyst at426 K. Gaffney and Sofranko [14] proposed Cu-catalysts forthe dihydroxylation of propene to propane-1,2-diol in sc-CO2

at temperatures of 393±423 K. However, the use of SCF inpartial oxidation reactions promises progress in increasing theselectivities of oxygenates due to their favorable properties.Especially, the liquid-like solvent power could effect a fasterdesorption of oxidation-sensitive reaction intermediates,which are more or less strongly adsorbed on the catalystsurface and become subject to consecutive oxidation, leadingto undesired total oxidation products.

The partial oxidation of propane in sc-CO2 was selected toevaluate this possible effect on heterogeneously catalyzedpartial oxidation reactions. The direct catalytic selectiveoxidation of light alkanes (i.e., propane) into their oxygenates(e.g., acetone, acrolein, acrylic acid) is of our special interestbecause it may contribute to the extensive research to replace

olefins as starting materials for the production of valuablechemicals. Until today, these reactions have often not led tosufficient high selectivities, e.g., the catalytic oxidation ofpropane to acrolein does not exceed selectivities of 60 %, evenat low conversions of 12 mol-% [15]. These results are mainlycaused by concurrent total oxidation of the formed reactionproducts (e.g., alkenes and oxygenates) under the rathersevere alkane activation conditions: The intermediatesremain adsorbed on the catalyst surface and are immediatelysubject to consecutive reactions, leading to the thermody-namically favored final products CO2, CO and H2O.

It was the aim of the present work to investigate activity andselectivity of various catalysts for the partial oxidation ofpropane in sc-CO2 atmosphere by using a stirred batchreactor. This paper reports on the used reactor system and firstresults of a catalyst screening using different metal (oxide)catalysts as well as the variation of the reaction conditions.

2 Experimental

The following catalyst samples were synthesized accordingto different methods (Tab. 1). (i) Transition metal oxide-containing catalysts (Co, Cu, Mn, Mo) were prepared by wetimpregnation of c-Al2O3, ZrO2 and SiO2 using aqueous metalsalt solutions with subsequent drying and calcination underair. Further Co-oxide catalysts were synthesized by precipi-tation and sol-gel process. (ii) Precious metal catalysts weresynthesized either by impregnation or precipitation. A Pt/SiO2 catalyst was prepared by impregnation of SiO2 usingH2PtCl6 ´ 6H2O and subsequent drying and reduction underhydrogen. A Pd/SiO2 catalyst was obtained by precipitationusing a H2PdCl4-SiO2 suspension and KOH.

The metal content of the catalysts was analyzed with ICP-OES (Optima 300 XL, Perkin-Elmer). The surface area wasdetermined using the BET method (Gemini III, Micromeri-tics). Additionally, the particle size of some parent sampleswas determined by transmission electron microscopy (BS 500,Tesla).

The catalytic experiments were performed in a stirred batchreactor (V = 305 ml, Tmax = 623 K, pmax = 207 bar; Series 4560M, Parr Instrument Co.) containing 2 ml of catalyst (dilutedwith 20 ml of glass beads) in a wire basket. The batch reactorwas equipped with a metering system for the reactants(propane, air, liquid carbon dioxide) and a heated samplingline for analysis of the reaction mixture by GC (Fig. 1). Theexperiments were carried out according to different proce-

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±

[*] Dr. A. Martin, Dipl.-Ing. B. Kerler, Institut für Angewandte ChemieBerlin-Adlershof e.V., Richard-Willstätter-Str. 12, D-12489 Berlin,Germany.

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dures: (i) For the catalyst screening a defined T-t-program wasadopted after propane, synthetic air and carbon dioxide hadbeen fed in a fixed molar ratio into the autoclave. Propaneconversion and oxygenate yield were determined at varyingreaction temperatures and pressures. (ii) For the investigationof the influence of the supercritical conditions on the reaction,

the partial oxidation was started by depressuriz-ing a defined amount of synthetic air at reactiontemperature into the reactor that was loaded withpremixed propane and CO2 up to the desiredpressure.

Critical points of propane plus CO2 mixtureswere determined before the catalytic testsusing the opalescence method [16] to guaran-tee the supercritical state during the catalytictests. Air was not included into these investiga-tion due to its total miscibility with supercrit-ical fluids.

The catalytic reactions were carried out underthe following standard conditions: Molar ratio ofpropane : air : CO2 = 1 : 2.3±2.9 : 68±108, p = 80±100 bar, T = 453±573 K. Reaction products as wellas unconverted propane were analyzed by on-linecapillary GC (FID). Oxygen consumption wasalso measured by GC (TCD).

3 Results and Discussion

Blank tests with inert material instead of catalysts, that werecarried out prior to the catalytic investigations, did not showany significant propane or oxygen consumption as well asoxygenate formation. Fig. 2 depicts the results of the catalystscreening, revealing that oxygenate traces (acetone) could bedetected only on Co3O4/c-Al2O3. The other catalysts alsoresult in propane as well as oxygen conversion but, however,partial oxidation products could not be found and this pointedto a complete total oxidation to carbon dioxide. Therefore,further investigations were based on Co-containing catalysts.

During a second screening step a catalyst support variationwas carried out. Aweaker chemisorption of intermediates andproducts was assumed to effect an improved oxygenateselectivity due to the decline of the support acidity (c-Al2O3

> ZrO2 > SiO2). Fig. 3 shows the results of the supportinfluence, revealing a significant increase of the oxygenateselectivity up to about 55 % at propane conversion of ca. 10±15%. Acetic acid that is formed via i-propanol and acetone in a

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Table 1. Synthesis (p ± precipitation, i ± impregnation, s-g ± sol-gel process), pretreatment andcharacterization data of the parent catalyst samples.

Sample Method CalcinationT/K; t/h

Me contentwt. -%

BET surface aream2 g±1

particle sizenm

Pd/SiO2 p 673; 2 (H2) 8.4 130 n.d.

Pt/SiO2 i 523; 2 (H2) 4.8 340 n.d.

CuO/c-Al2O3 i 523; 14 4.3 139 n.d.

MnO2/c-Al2O3 i 523; 14 2.5 142 n.d.

MoO3/c-Al2O3 i 523; 14 5.5 136 n.d.

Co3O4/c-Al2O3 i 473; 6 5.3 134 n.d.

Co3O4/SiO2 i 523; 14 3.8 310 n.d.

Co3O4/ZrO2 i 523; 14 3.9 15 n.d.

Co3O4/SiO2 i 523; 14 0.7 328 ³ 50

Co3O4/SiO2 i 523; 14 7.4 293 ³ 200

Co3O4/SiO2 p 523; 6 + 623; 6 2.4 345 250

Co3O4/SiO2 s-g 523; 6 + 623; 6 3.7 388 400

Co3O4/SiO2 p 523; 6 + 623; 6 3.6 252 n.d.

n.d. ± not determined

Figure 1. Flow sheet of the high-pressure batch reactor equipped with ametering system for gases and liquids (CO2) and a heated transfer line for GCanalysis.

Figure 2. Conversion of propane and selectivity of oxygenates and propene,respectively, using noble metal and transition metal oxides on supports(propane : air : CO2 = 1 : 2.3±2.9 : 98±108, p = 80±100 bar, T = 518 K, t = 5 h,catalyst volume = 2 ml; a) c-Al2O3-support, b) SiO2-support).

consecutive degradation reaction was the main product [17].Beside acetic acid also i-propanol, acetone and methanol werefound as products. However, the desired products, such asacrolein or acrylic acid, were not detected or in minor amountsonly. This product distribution points to a reaction mechanismthat includes the participation of free peroxy radicals [11],initiated by the redox pair Co(III)/Co(II).

Figure 3. Conversion of propane and selectivity of oxygenates and propene,respectively, using different Co3O4 support catalysts (propane : air : CO2 = 1 : 2.5: 101±106, p = 80±100 bar, T = 518 K, t = 5 h, catalyst volume = 2 ml).

The metal content of the used catalysts also influences theoxygenate selectivity. Fig. 4 depicts that the oxygenateselectivity increases with decreasing metal content. Firstinvestigations on the size of the cobalt oxide particles byTEM show that higher metal contents lead to larger particles(> 200 nm, 7.4 wt.-% Co). Otherwise, lower metal contentscause smaller particles (ca. 50 nm, 0.7 wt.-%) that promote theformation of oxygenates and prevent a consecutive oxidation,respectively. Beside the mentioned products, acrolein andn-propanol were additionally found using the catalyst with thesmall cobalt loading.

Figure 4. Conversion of propane and selectivity of oxygenates and propene,respectively, using precipitated Co3O4/SiO2 catalysts with different Coproportions (propane : air : CO2 = 1 : 2.6 : 68±69, p = 80±100 bar, T = 573 K,t = 6 h, catalyst volume = 2 ml).

Fig. 5 demonstrates the influence of the catalyst preparationmethod on the oxygenate selectivity. Different Co3O4/SiO2

catalysts of approximately equal cobalt proportions wereprepared by impregnation, precipitation and the sol-gelmethod. The highest oxygenate selectivities were obtainedusing the precipitated catalyst. By contrast, only pooroxygenate selectivities and propane conversions were foundfor the sample that was synthesized by the sol-gel method. Its

catalytic behavior may be explained by the inhomogeneouslydispersed, large metal oxide particles (up to 400 nm).Furthermore, it may also be concluded that the cobalt oxidehad partly been enclosed by the supporting material duringthe preparation and was no longer available for the reactants.So, sol-gel synthesis was ± against expectations ± even a lesssuitable method for the preparation of Co3O4/SiO2 thanprecipitation as well as impregnation. A deeper characteriza-tion of the catalyst specimens focussing on Co3O4 distribution,surface cobalt valence state and Co3O4 reducibility is stillunder study.

Figure 5. Conversion of propane and selectivity of oxygenates and propene,respectively, using differently prepared Co3O4/SiO2 catalysts (propane : air :CO2 = 1 : 2.5 : 101±112, p = 80±100 bar, T = 573 K, t = 6 h, catalyst volume = 2 ml).

The catalytic performance of catalysts can be significantlyinfluenced by mass transport limitations at elevated pressures.Especially an increased coking of catalyst surfaces is observedbeside an intensified running of nonselective parallel orconsecutive reaction pathways. These effects could besuppressed by processing in the near- or supercritical region[e.g. 1]. An increased mass transport with markedly higherSherwood numbers and the liquid-like solvent power of thedense phase might be the main reason. Rather strongadsorbed reaction intermediates can desorb from the catalystsurface more easily. This could be a helpful advantage forheterogeneously catalyzed oxidation reactions that oftensuffer from selectivity limitations due to a fast consecutivetotal oxidation of partial oxidation products. Intensiveresearch on the adsorption/desorption behavior of differentreactants and products on catalyst surfaces in the sub- andsupercritical state using supercritical fluid chromatography(SFC) are still under study.

To investigate the influence of the supercritical reactionconditions on the propane partial oxidation, the secondexperimental method was used, that means propane andCO2 were fed into the batch reactor and heated up to thedesired reaction temperature before air was depressurizedinto the reactor. Fig. 6 depicts the conversion/selectivitybehavior of a precipitated Co3O4/SiO2 catalysts in depen-dence on the reaction pressure covering the sub- andsupercritical region at a constant reaction temperature. Theresults show that at pressures below ca. 40 bar only a very smallconversion of propane can be observed. A significantconversion of 20 % and oxygenate selectivities of 25 % werefound for the first time at ca. 60 bar increasing further up to

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selectivities of 40 % in the near-critical region. Above thecritical point higher reaction pressures did not markedlyimprove the oxygenate selectivities. These results confirm theassumption that the intermediates desorb more easily fromthe catalyst surface with higher pressures/densities due to animproved solubility power of the SCF for the oxygenates and abetter mass transport from the solid catalyst to the densephase. Therefore, total oxidation of the partially oxidizedsurface intermediates is avoided and improved selectivitiescan be yielded.

Figure 6. Conversion of propane and selectivity of oxygenates and propene,respectively, using a 2.4 wt.-% Co3O4/SiO2 catalyst at various reaction pressuresup to the supercritical region (propane : air : CO2 = 1 : 5±7 : 100±124, T = 553 K,t = 2 h, catalyst volume = 2 ml; Sh/Sh(26 bar) quotient of Sherwood numbers(normalizedto26bar,r/r(26bar)quotientofCO2density(normalizedto26bar).

Additionally, first tests were carried out admixing so-calledªentrainerº substances, such as methanol or water [18]. Thesesubstances are also completely solved in the homogeneoussupercritical fluid phase in dependence on their concentrationand reaction conditions. In contrast to CO2, methanol andwater are expected to enable a stabilization of ionicintermediates that could lead to improved selectivities of theproducts of ionic reaction mechanism; otherwise the admix-ture of entrainers causes an increased polarity of the reactionmixture that could lead to an increased solubility of the ratherpolar oxygenates. First results confirm the latter effect: theoxygenate selectivity can be doubled at similar propaneconversion in the presence of 0.5 mol-% water (Tab. 2).However, an alteration in the product spectrum could not beobserved.

Table 2. Effect of entrainer compounds on the selectivity of partial oxygenatesand propene during the oxidation of propane in sc-CO2 (catalyst: Co3O4/SiO2

(precipitation), 3.6 wt.-% Co, propane : air : CO2 = 1 : 6 : 106, p = 120 bar,T = 553 K, t = 2 h).

Entrainer X (propane)mol %

S (oxygenates)%

S (propene)%

without 22 14 4

methanol (1 mol-%) 19 16 6

water (0.5 mol-%) 19 32 20

4 Conclusions

The partial oxidation of propane to oxygenates in sc-CO2

confirms the expected improvement of the selectivity ofpartial oxygenates in the near-critical region due to anincreased solvent power of the dense phase. The obtainedresults will be verified in a continuously running high-pressureflow reactor setup. Beside cobalt-oxide-containing solidsredox catalysts will be tested. The potential of SCF and ourgained knowledge will be applied in the synthesis of finechemicals and specialities, e.g., in partial oxidation of morecomplex molecules in the near future.

Acknowledgement

The authors like to thank Prof. Dr. M. Baerns for theconception of this work and the stimulating discussions, Ms M.Hartelt for technical assistance and Dr. U. Schülke for the helpwith preparation of the catalyst samples. The financial supportby the Federal Ministry of Education and Research, Germany,as well as by the Senate of Berlin is greatly acknowledged(project 03 C 3012 0).

Received: May 11, 2000 [CET 1238]

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