1-s2.0-S0926860X01006391-main.pdf

Applied Catalysis A: General 218 (2001) 171–180

The dehydrogenation of 2-butanol over copper-based catalysts:optimising catalyst composition and determining

kinetic parameters

J.N. Keuler a,∗, L. Lorenzen a, S. Miachon b

a Department of Chemical Engineering, University of Stellenbosch, P.O. Box X1, Matieland 7602, South Africab IRC-CNRS, 2 Avenue A. Einstein, Villeurbanne 69626, France

Received 8 January 2001; received in revised form 4 March 2001; accepted 25 April 2001

Abstract

This work examines the dehydrogenation of 2-butanol over copper-based catalyst. The effects of support type (MgO andSiO2) and copper loading on methyl ethyl ketone (MEK) yield were studied. The effects of reaction temperature, 2-butanolfeed flow rate and catalyst particle size were also investigated. The highest MEK yields were obtained with a 15 wt.% copperon silica catalyst. The optimum catalyst was used to measure the kinetic parameters of the 2-butanol dehydrogenation reactionat temperatures from 190 to 280◦C. At higher temperatures catalyst deactivation took place. © 2001 Elsevier Science B.V.All rights reserved.

Keywords: 2-Butanol dehydrogenation; Catalyst optimisation; Kinetic parameters

1. Introduction

The industrially used alcohol dehydrogenation cata-lysts are copper and/or zinc-based [1]. Some oxidativedehydrogenation processes employ silver as a catalyst[1]. Copper-based catalysts can either be unsupportedor supported. Most are of the supported type, wherethe support provides a large surface area for the copperto be deposited on. Unsupported copper catalysts havea much smaller surface area. Catalyst supports can bebasic, acidic or both. The acidity of the support deter-mines whether the dehydration or the dehydrogena-tion reaction will be favoured. Basic supports (high

∗ Corresponding author. Present address: Sasol Technology,Andries Brink Building-B level, 1 Klasie Havenga Road,Sasolburg 1947, South Africa.E-mail address: [email protected] (J.N. Keuler).

pH) favour the dehydrogenation reaction, while acidicsupports (low pH) favour the dehydration reactions.Different catalyst supports were listed in [2,3]. Silica(basic) and alumina (acidic) have very high surfaceareas compared to the other oxides (typically in thehundreds of m2/g area). High copper surface areas canbe obtained by depositing copper on these supports.The activity of the catalyst is usually proportional tothe surface area of the active sites and thus a largecopper surface area will yield a more active catalyst.

The four basic techniques for preparing copper-based catalyst, namely, precipitation, urea hydrolysis,electroless plating and impregnation were discussedby Keuler [4]. The percentage copper on the sup-port has an effect on both reaction conversion andselectivity. Sivaraj and Kantarao [5] prepared coppersupported on �-alumina catalysts by a precipitationtechnique. For the 240 m2/g �-alumina support, a

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926 -860X(01 )00639 -1

172 J.N. Keuler et al. / Applied Catalysis A: General 218 (2001) 171–180

Nomenclature

F feed flow rate (mol/s)k′ reaction rate constant (mol/kg cat kPa h)K adsorption coefficient (kPa−1)Keq equilibrium constant (kPa)P pressure (kPa)r ′

A rate of generation for component Ain reaction (mol/kg cat h)

T temperature (K)W catalyst mass (kg)

SubscriptsA 2-butanolR MEKS hydrogen

copper loading of 20–25 wt.% gave an optimum cop-per surface area of about 40 m2/g catalyst. Changand Saleque [6] investigated cyclohexanol conver-sion using copper/�-alumina catalysts. Maximumconversions were obtained with copper loadings ofabout 12–17 wt.% (prepared by electroless plating),12–17 wt.% (prepared by precipitation) and about10 wt.% (prepared by impregnation). Chang andSaleque [7] obtained a maximum cyclohexanol con-version on copper/�-alumina with 18–20 wt.% copperloadings. For alumina-based catalysts, the selectiv-ity towards aldehyde or ketone formation increaseswith increasing copper loading [7,8]. The reason isthat the acidic centres, which reduce dehydrogena-tion selectivity, become covered and thus neutralised.Jeon and Chung [9] observed a continuous decreasein cyclohexanol conversion with an increase in cop-per loading on copper/silica catalysts. The selectivityremained constant (very high) over a wide range ofcopper values.

The dehydrogenation of sec-butyl alcohol (2-buta-nol) to yield methyl ethyl ketone (MEK) is an im-portant industrial process. The MEK is a widelyused industrial solvent. Perry and Chilton [10]listed six possible rate equations for solid-catalyseddehydrogenation reactions. Perona and Thodos [11]determined the kinetics of 2-butanol dehydrogenationbetween 340 and 400◦C and 1 atm, using a brasscatalyst (65 wt.% Cu, 35 wt.% Zn). Using a similarcatalyst, Thaller and Thodos [12] studied the reaction

between 290 and 370◦C at pressures up to 15 atm.Ford and Perlmutter [13] observed a change inreaction mechanism with temperature. Below about320◦C and above about 425◦C, the single site surfacereaction was rate limiting. In the temperature range inbetween, especially from 350 to 400◦C, the adsorp-tion of the alcohol was the controlling mechanism.

In the present study, a porous support impregnatedwith copper was used and the kinetic parameters for2-butanol dehydrogenation were determined at 1 atmbetween 190 and 280◦C. This paper forms part ofa larger project, where 2-butanol dehydrogenationwas modelled in a catalytic membrane reactor [4]. Toconstruct an accurate membrane reactor model, purekinetic data was required.

2. Experimental

Engelhard supplied the silica catalyst support(product code = C500-234). The purity was 99.5%silica and the BET surface area about 440 m2/g. The‘as received’ pellets were crushed and then sieved toobtain different particle size fractions. A commercialmagnesium oxide (MgO) powder from Merck (sur-face area = 27.4 m2/g) was mixed with a binder andpressed into extrusions. The extrusions were heatedto 1200◦C to agglomerate the powder particles. Theextrusions were then crushed and sieved into only a300–850 �m fraction. The BET surface area of theparticles was 16.7 m2/g. The BET surface area andchemisorption experiments were performed with aMicrometrics ASAP 2010.

Copper was deposited onto the silica support viaimpregnation. The low porosity of the MgO supportmade impregnation unsuitable hence adsorption wasused for depositing copper. The MgO support wasintroduced into a flask containing a copper nitratesolution (in distilled water) of a specific concentra-tion. The flask was placed on a magnetic stirrer andthe solution stirred for 2 h. Thereafter, the Cu–MgOparticles were filtered, washed and dried at 90◦C. Thecatalyst was calcinated at 500◦C and then reduced inhydrogen, in situ, at 350◦C for 2 h.

The silica support was dried at 200◦C for at least2–3 h and then stored in a desiccator. The dried par-ticles were then placed in heated copper nitrate solu-tions (in distilled water) of different concentrations.

J.N. Keuler et al. / Applied Catalysis A: General 218 (2001) 171–180 173

Fig. 1. Set-up used for testing the kinetics of the catalyst at the CNRS, France.

The copper solution was kept warm on a hotplatewhile the support was added. The support-solutionmixture was stirred throughout while adding the sup-port particles. The hotplate was kept at about 80◦Cto evaporate the remaining solution. The paste wasstirred every few minutes. When all the water hadevaporated, the catalyst was dried in an oven at 120◦Cfor at least 4 h. The catalyst was then calcinated at500◦C and reduced in situ in hydrogen at 350◦C for2 h. The copper concentrations on the catalysts weremeasured using atomic adsorption analysis.

Catalyst testing was performed in two stages.During the first stage, the optimum catalyst wasdetermined and during the second stage the kineticparameters of the optimised catalyst was determined.The first set of experimental tests was performedat the laboratories of the University of Stellenbosch(Stellenbosch, South Africa). The kinetic testing wasconducted at the laboratories of the IRC-CNRS (Insti-tut de Recherches sur la Catalyse, Centre National dela Recherche Scientifique) in Villeurbanne, France.Fig. 1 is the set-up used at the CNRS. A gas samplewas extracted at the sample point with a heated sy-ringe. The syringe was kept inside a stainless steel tubeand the temperature of the syringe was controlled atabout 110◦C. Carbon-containing products were anal-ysed on a HP 5850 gas chromatograph equipped witha FID detector. Two capillary columns, a 30 m HP

Innowax column and a 30 m HP Plot/Al2O3 column,were used in series. Hydrogen analysis was done on asimilar GC with a TCD detector. A Porapak Q columnand a molecular sieve column were operated in series.

The set-up at Stellenbosch University differed in thefollowing way: Hastings flow controllers (HFC 202C)were used in stead of Brooks, the inner diameter ofthe quartz tube was 8 mm (10 mm outer diameter) anda Braun perfusion pump was used. The 2-butanol re-action products were analysed with a HP G1800A gaschromatograph, equipped with a mass spectrometerand flame ionisation detector. A 50 m capillary col-umn (50QGI.5/BPI PONA from SGE) was used.

3. Results and discussion of results

For catalyst optimisation experiments, the reactorwas operated as a plug flow reactor. Lower flow rateswere used to optimise the 2-butanol conversion. Whenthe kinetic parameters of the reaction was measured,the reactor was operated in the differential mode. Highfeed flow rates and small catalyst masses (typically<0.2 g) were used to limit the reaction conversion to<10%. The data presented in all three-dimensionalfigures were fitted by a smooth spline. Figures wereconstructed in the Statistica package.


Fig. 2. Total 2-butanol conversion for Cu on MgO catalysts as afunction of temperature and Cu% on the support.

3.1. 2-Butanol dehydrogenation overMgO supported catalysts

When 2-butanol reacted over Cu/MgO catalysts,the products were a mixture of butenes and MEK.There was <1% additional by-products. Figs. 2 and 3indicate total 2-butanol conversion and MEK yields asa function of temperature and copper loading on thecatalyst. In both cases, the values increased with in-creasing temperature. The total 2-butanol conversionwas the highest for pure MgO, because of the large

Fig. 3. MEK yield for Cu on MgO catalysts as a function oftemperature and Cu% on the support.

increase in butene yield in the absence of copper on thesupport. The addition of copper to the MgO supportsuppressed butene formation at all temperatures. Thereaction of 2-butanol to butene mainly took place onthe MgO. The 2-butanol to MEK reaction took placeon both the MgO and copper. With no Cu on MgO,similar amounts of butenes and MEK were formed atall temperatures. When copper was added to the sup-port, the MEK yield increased to an optimum yield at16.9 wt.% Cu on MgO (Fig. 3). For that catalyst, the to-tal BET surface area (9.3 m2/g) was about one-third ofthe value of the pure MgO support (27.3 m2/g). Furthercopper addition to the support resulted in a decline inMEK yield. Copper surface areas were not measureddue to the low BET values. Copper had much highercatalytic activity for MEK production than MgO did.

Figs. 4 and 5 show the effect of flow rate on MEKyield and selectivity for the best performing MgOsupported catalyst (16.9 wt.% Cu on MgO). TheW/F-ratio is the catalyst mass (in kg) divided by the2-butanol feed flow rate (in mol/s). The selectivitydeclined with an increase in temperature as morebutenes were formed. The longer residence time atthe low flow rates (high W/F values) improved theMEK yield (Fig. 4), but resulted in a sharp declinein selectivity (Fig. 5). The total 2-butanol conversiondropped from 65 to 35% when W/F decreased from206 to 51 kg s/mol. The higher flow rates meant thatinsufficient time was allowed for the 2-butanol to

Fig. 4. MEK yield for a 16.9 wt.% Cu on MgO catalyst as afunction of temperature and W/F-ratio.


Fig. 5. MEK selectivity for a 16.9 wt.% Cu on MgO catalyst as afunction of temperature and W/F-ratio.

fully react. This is clear in Fig. 4 where the MEKyield dropped as W/F decreased.

3.2. 2-Butanol dehydrogenation oversilica supported catalysts

Fig. 6 shows total 2-butanol conversion as a functionof temperature and copper loading. The main productwas MEK (Fig. 7) and the only significant by-productswere a mixture of butenes. Total 2-butanol conversionincreased sharply with an increase in temperature,

Fig. 6. Total 2-butanol conversion for Cu on silica catalysts as afunction of temperature and Cu% on the support.

Fig. 7. MEK yield for Cu on silica catalysts as a function oftemperature and Cu% on the support.

but started levelling off at about 360◦C. For MEKproduction, there was an optimum temperature. Thistemperature was between 300 and 360◦C for averageW/F values. The optimum temperature did not varywith copper loading. The maximum MEK yield wasvery dependent on the copper content on the support.At low copper loadings the MEK yield was lower dueto butene formation. As the copper loading increased,butene formation decreased. Butene formation alsoincreased with temperature. Butene formation mainlytook place on the silica, while MEK formation tookplace on the copper. The optimum MEK yield oc-curred at 15 wt.% copper on silica. Other researchgroups [5–7] also found an optimum Cu% on theircatalysts, which resulted in maximum yields for thealcohol dehydrogenation reactions that they studied.

Figs. 8 and 9 show the MEK yield and selectivityfor the best performing catalyst (15 wt.% Cu on sil-ica) as a function of temperature and 2-butanol feedflow rate. The x- and y-axis in Fig. 9 were swappedto obtain a more visible fit to the data. With the stan-dard axis in Fig. 9, the trend in the data could notbe observed. As mentioned previously the yield in-creased with temperature. The optimum yield shiftedto higher temperatures when the flowrate increased(W/F decreased). For example,

• W/F = 206 kg s/mol: 93–91% yield at 270–300◦C,• W/F = 103 kg s/mol: 93–92% yield at 300–330◦C,• W/F = 51 kg s/mol: 89–87% yield at 330–360◦C.


Fig. 8. MEK yield for a 15 wt.% Cu on silica catalyst as a functionof temperature and W/F-ratio.

The selectivity towards MEK production (Fig. 9)decreased with an increase in temperature anddecreased very marginally with an increase in W/F.Longer residence times (lower feed rates) increasedbutene formation. At 390 ◦C, the selectivity variedbetween 83 and 86% for the different feed flow rates.

3.3. Comparing SiO2 and MgO supported catalysts

The MEK selectivities for the optimum MgO sup-ported catalyst were better than those for the optimum

Fig. 9. MEK selectivity for a 15 wt.% Cu on silica catalyst as afunction of temperature and W/F-ratio.

SiO2 supported catalyst. Pure MgO is >50% selectivetowards MEK production, while pure SiO2 is almost100% selective towards butene formation. If a similarfraction of support sites are not covered by copper, theMgO supported catalyst should have a higher selectiv-ity towards MEK production than the SiO2 supportedcatalyst. At 300◦C and below, the selectivities for theoptimum catalysts at the different flow rate tests var-ied as follows: MgO ≈ 100%; SiO2 > 93%.

The MEK yields were, however, much higher for theSiO2 supported catalysts, due to their higher supportarea as mentioned previously. At 300◦C and below, theyields at the different flow rate tests were as follows:MgO = 6–26%; SiO2 = 56–93%.

Further work was only performed on the SiO2 sup-ported catalysts due to the higher yields obtained atlower temperatures.

3.4. Effect of SiO2 support particle size on MEK yield

Figs. 10 and 11 show the effects of support particlesize on total 2-butanol conversion and on MEK yieldfor copper on silica catalysts. At 270◦C and above,catalyst particles up to 1180 �m gave similar valuesfor total 2-butanol conversion. The high conversionsindicate very little mass transfer resistance with anincrease in particle size. Particles of 3000 �m gavelower 2-butanol conversion due to channelling of thealcohol feed gas [4].

The MEK yields (Fig. 11) were the highest for the300–850 �m fraction. Smaller particles (150–300 �m)and particles ranging from 850–1180 �m gave higher

Fig. 10. Effect of catalyst particle size on total 2-butanolconversion.


Fig. 11. Effect of catalyst particle size on MEK yield percentage.

butene yields, which decreased the MEK yield.The BET surface area of the catalysts were similar(300 m2/g) and the copper surface areas determinedby H2 chemisorption were as follows:

• 150–300 �m: 3.8 m2/g Cu,• 300–850 �m: 3.8 m2/g Cu,• 850–1180 �m: 5.0 m2/g Cu.

3.5. Catalyst deactivation testing

Keuler [4] studied the effects of reduction tem-perature on catalyst stability and concluded that thecopper on silica catalyst should be reduced at 260◦Cin hydrogen for 1 h. That optimised reduction processwas used and not the process mentioned previously(2 h in hydrogen at 350◦C). Higher reduction tem-peratures resulted in a lower initial catalyst activitydue to a decrease in copper surface area by partialsintering. A 14.4 wt.% copper on silica catalyst wasemployed for the dehydrogenation of 2-butanol. Sta-bility was tested at 250 and 310◦C (see Fig. 12). Twodifferent runs were performed at 310◦C. Selectivitytowards MEK production was very high (>99%) atboth temperatures. At 250◦C, the catalyst was verystable over a 24 h period, but deactivation took placewhen the temperature was increased to 310◦C. Fur-ther, deactivation testing was not conducted.

Keuler [4] performed an intensive investigationinto the deactivation mechanisms of the same catalystfor ethanol dehydrogenation. Total organic carbonanalysis (TOC) was employed to detect coking atreaction temperatures ranging from 240 to 400◦C.

Fig. 12. MEK production rate as a function of time for a 14.4 wt.%Cu on silica catalyst.

The X-Ray diffraction analysis, transmission electronmicroscopy and copper surface area measurementswere used to determine the degree of sintering of thecatalyst at reaction temperatures ranging from 240 to400◦C. Deactivation occurred due to both sinteringand coking for ethanol dehydrogenation above 280◦C.Results of re-oxidation experiments indicated thatmost of the sintering occurred within the first 24 hof use. Addition of different amounts of chromiumand/or cobalt to copper [4] did not improve the hightemperature stability of the catalyst and furthermore,it reduced the selectivity towards the desired product.The same sintering effects that were present for theethanol catalyst should also be valid for this reaction,as the same catalyst was used.

3.6. Determining reaction rate masstransfer resistance

To obtain accurate kinetic data, the reaction mustbe operated in the region free from interphase masstransfer resistance. Mass transfer resistance is depen-dent on the particle Reynolds number, which in turnis a function of the linear gas velocity past the catalystparticles. The linear gas velocity can be increased toeliminate mass transfer resistance by increasing thefeed flow rate and/or decreasing the inside diameter ofthe quartz tube housing the catalyst. A quartz U-tubewith small inside diameter (4 mm) was used for allexperiments. Catalyst particles for kinetic testingvaried from 350 to 500 �m.

In the absence of interphase mass transfer resis-tance, the reaction rate will be independent of the feed


Fig. 13. The effect of 2-butanol feed flow rate on the MEKproduction rate.

flow rate and the lines on Fig. 13 will be horizontal.For the 2-butanol dehydrogenation reaction, therewas no significant interphase mass transfer resistance(Fig. 13). The MEK production rate remained fairlyconstant with an increase in 2-butanol feed flow rate atall temperatures tested. There were some fluctuationsin the MEK production rates, with a slight downwardtrend in production rates at the lower temperatures.Rates at different feed flows were averaged at eachtemperature. Raizada et al. [14] reported similar re-sults on interphase mass transfer resistance for thedehydrogenation of n-butanol over zinc oxide. Theyconcluded that bulk diffusion was not significant.

3.7. Determining kinetic parameters for2-butanol dehydrogenation

Perona and Thodos [11] determined reaction ki-netics for the dehydrogenation of 2-butanol between343 and 399◦C over solid brass spheres (65% copperand 35% zinc). Under those conditions, they foundthe desorption of hydrogen from a single site to berate limiting. Ford and Perlmutter [13] used a brass

Table 1Reaction rate parameters for 2-butanol dehydrogenationa

T (◦C) k′ (mol/kg cat h kPa) KA (kPa−1) KS (kPa−1) KR (kPa−1)

190 0.339 0.001433 −0.00129 0.11735220 0.772 0.001195 −0.00126 0.06075250 1.713 0.002987 −0.00225 0.05232280 3.342 0.003225 −0.00109 0.06432

a A: 2-butanol; S: hydrogen; R: MEK.

tube (60% copper and 40% zinc) as catalyst and car-ried out the dehydrogenation reaction at temperaturesbetween 316 and 427◦C. From 350 to 400◦ alcoholadsorption was rate limiting, while at both higher andlower temperatures the single site surface reactionwas rate limiting. Thaller and Thodos [12] performedexperiments with smaller brass catalyst particles(50–60 mesh; 65% copper and 35% zinc) with a largersurface area. Below 300◦C the reaction was dual site,surface reaction controlling, while at higher tempera-tures the reaction was dual site, hydrogen desorptioncontrolling. As a first approximation, kinetic data for2-butanol dehydrogenation from 190 to 280◦C and1 atm was fitted to the reaction mechanism describedby Eq. (1). Reaction rate data in this study indicateda strong inverse quadratic relationship between theobserved reaction rate and the MEK partial pressure.

r ′A = k′(PA − PRPS/Keq)

(1 + KAPA + KRPR + KSPS)2(1)

with A, R, S = 2-butanol, MEK, hydrogen.More detail on calculating the different coefficients

were supplied in Keuler [4]. Table 1 lists the differentcalculated reaction rate parameters. The trends in thek′ and KR values were in line with Peloso et al. [15],except at 280◦C where KR showed an increase insteadof a decrease.

Very little catalyst deactivation took place at 250◦Cand below (see Fig. 12), but at 310◦C (Fig. 12) signifi-cant deactivation took place. From initial experimentsit could be concluded that some deactivation tookplace at 280◦C, which resulted in lower measured re-action rates and a distortion in the kinetic data. BothKA and KS (the adsorption coefficients for 2-butanoland hydrogen) were negligible compared to the ad-sorption coefficient of MEK (KR). When adsorptiontook place, the reaction rate slowed down. This wasbecause diffusion resistance of the feed molecules to


the active sites increased. Negative hydrogen adsorp-tion coefficients indicated an increase in reaction rates.

The reasons for the increase in reaction rate withhydrogen in the feed have been documented forother dehydrogenation reactions and is not unex-pected where coking tends to deactivate the catalysts.Sheintuch and Dessau [16] cited many referenceswhere hydrogen was co-fed with either an alco-hol or an alkane and where hydrogen improved thedehydrogenation activity. Hydrogen in the feed streamreduces coking [16] and it reduces the partial pressureof the alkane or the alcohol, which is favourable forhigher conversions [1].

The equilibrium constant for 2-butanol dehydro-genation was taken from Kolb and Burwell [17] andthe units were transformed from atm to kPa. Combin-ing the equilibrium equitation with Arrhenius fits ofk′ and KR yielded the following kinetic expression:

−r ′A =

8.290 × 105 × e−6903/T (PA − PRPS/

(3.538 × 108 × e−7100/T ))

(1 + 8.804 × 10−5 × e3298/T × PR)2(2)

with pressures in kPa and temperatures in K. The eq-uitation is valid for temperatures from 190 to 250◦C(463–523 K).

4. Conclusions

The reaction of 2-butanol over MgO and SiO2,impregnated with copper, yielded MEK and butenes.MEK was the main product (except for catalystswithout copper), with a mixture of butenes as theby-product. MgO supported catalysts gave low MEKyields due to their low BET surface area. For silicasupported catalysts, there was an optimum copperconcentration on the support (15 wt.%), which gavethe highest MEK yields. Silica support particles inthe range of 300–850 �m gave the highest MEKyields. Smaller or larger particles produced increas-ing amounts of butenes. For a 15 wt.% Cu on silicacatalyst, the selectivity towards MEK production wasclose to 100% at 240◦C, but declined to between 83and 86% at 390◦C.

The catalyst was stable at 250◦C over a 24 hperiod, but deactivated at 310◦C. For the 2-butanoldehydrogenation reaction, there was little interphase

mass transfer resistance. The 2-butanol and hydrogenadsorption coefficients were negligible compared tothe MEK adsorption coefficient.

Acknowledgements

We want to acknowledge Sasol and the FRD (both inSouth Africa) for their financial contributions towardsthe project. We want to acknowledge the Directiondes Relations Internationales of CNRS (France) forfinancial support.

References

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