KINETICS,CATALYSIS,AND REACTION ENGINEERING
Gas-Phase Conversion of Acetone to Methyl Isobutyl Ketone over Bifunctional Metal/Carbon Catalysts.2.Examination of the Hydrogenation Potential of Different Metals
Gerrit Waters,Oliver Richter,and Bettina Kraushaar-Czarnetzki*
Institute of Chemical Process Engineering CVT,Uni V ersity of Karlsruhe,Kaiserstrasse12,
D-76128Karlsruhe,Germany
The single-stage self-condensation and subsequent hydrogenation of acetone to methyl isobutyl ketone(MIBK)
in the gas phase was carried out using catalysts consisting of hydrogenating metals supported on active carbon
(Me/C).The reaction was conducted in a continuously operated,ideally backmixed Berty reactor at1MPa
and623K.Platinum,palladium,nickel,and copper were applied as metal components,and their hydrogenation
activity decreased in the order Pt>Pd>Ni>Cu.The impact of the hydrogenation reactions in the reaction
network can also be enhanced by increasing the molar ratio of hydrogen to acetone in the feed,and by
raising the metal content of the Me/C catalysts.A low hydrogenation activity negatively affects the acetone
conversion and promotes the production of mesityl oxide.Hydrogenation conditions being too severe may
favor the unwanted hydrogenation of acetone to2-propanol and of MIBK to methyl isobutyl carbinol,but
this effect is less detrimental to the MIBK selectivity than an unsufficient hydrogenation activity.The best
performance showed a Pt/C catalyst with0.5%m/m Pt and a Ni/C catalyst with a higher Ni loading(2.71%
m/m);the optimum H2/acetone feed ratio was0.5,which is the stoichiometric value for the idealized formation
of MIBK from acetone.Despite the higher metal content required,the Ni/C catalyst allows for a considerable
reduction of the costs for catalyst production.
1.Introduction
Methyl isobutyl ketone(MIBK)is an important industrial commodity produced through aldol condensation of acetone and subsequent hydrogenation.MIBK is predominantly used as a solvent and as an extracting agent.
Nowadays,the MIBK production is conducted in single-step processes using three-phase trickle-bed reactors at temperatures ranging from393to433K and hydrogen pressures around10 MPa.1,2The bifunctional catalysts consist of palladium supported on oxides or hydroxides,with the latter providing suitable acid-base properties.
Recent research,however,has focused on the development of a single-stage gas-phase process to furt
her simplify reactor technology and operation compared to the case of three-phase operation in a trickle-bed reactor.The catalysts investigated so far in the gas-phase conversion are based upon molecular sieves as supports such as Pt/H-ZSM-5,3-5Pd/SAPO-11and Pd/ AlPO-11,6Ni/ALPON,7Pt/Cu/H(Al)-ZSM-5,8and Pt/Cs-X and Pt/Na-X,9or they are produced with oxidic supports such as Cu/M I(M II)oxides2,Pd/Mg(Al)O,10Ni/CaO-C,11Cu/ MgO,12,13Pd/Na-MgO,14,15(Pd or Ni)/hydrotalcites,16and Ni/ MgO.17The reactions were typically studied using fixed-bed tubular reactors at atmospheric pressure and at temperatures that rarely exceed473K.Obviously,platinum,palladium,nickel, and copper are the preferred metals for implementing a catalytic hydrogenation activity.However,a systematic study comparing the hydrogenation potentials of the different metals has not been presented.This is the main purpose of our present paper. Previously,we reported on platinum supported on active carbon(Pt/C)as a new and suitable catalyst for single-stage processing of acetone to MIBK in the gas phase.There,we investigated the nature of the active sites responsible for the acid-base catalyzed aldol condensation of acetone by artificial alteration of the active carbon surface chemistry through oxidative pretreatments,while the type and content of the supported metal component(Pt)were kept constant.18In this study,we focus on one specific support prepared from com-mercial Norit R3Extra carbon extrudates,and we will vary the hydrogenation activity via type and amount of the supported metal and by changing the molar ratio of hydrogen to acetone in the feed.
2.Experimental Section
2.1.Catalyst Preparation and Characterization.All cata-lysts were prepared with Norit R3Extra(Lot No.610233)as the active carbon support.This commercial carbon support is delivered in the form of cylindrical extrudates with a diameter of3mm and lengths ranging between5and10mm.
Prior to the impregnation with metal salt solutions,the carbon extrudates were activated in a rotating kiln made of quartz (Carbolite HTR11/150).In a steady flow of300mL/min of carbon dioxide,the kiln temperature was raised at a rate of5 K/min until the final temperature of1073K was reached.This temperature was held for30h,resulting in a loss of37%mol/ mol(m/m)with respect to the original mass of the extrudates.
*To whom correspondence should be addressed.Tel.:+49-721-6083947.Fax:+49-721-6086118.E-mail:Kraushaar@cvt.uka.de.6111
Ind.Eng.Chem.Res.2006,45,6111-6117
10.1021/ie0601854CCC:$33.50©2006American Chemical Society
Published on Web08/03/2006
The impregnation of the carbon support was carried out with aqueous solutions of metal salts that exhibit a low decomposition temperature(see Table1).In all cases,10mL of solution per 1g of activated carbon was used.The concentrations of the impregnation solutions were adjusted such that the desired metal loading of the extrudates was obtained.The mixture of carbon extrudates and metal salt solution was stirred for16h before the water was removed using a rotary evaporator(Heidolph VV2000);the evaporation was conducted over4h at0.003MPa and308K.Since the surface area of the carbon particles per batch(typically30g with a total surface area of at least48000 m2)exceeded that of the evaporator(0.035m2)by a factor of 1.4×106,and because no visible salt residue was found on the walls of the evaporator vessel,it was assumed that the metals in the impregnation solution were deposited on the carbon completely.Further drying was performed in a furnace over4 h in air at393K and atmospheric pressure.
Prior to the catalytic experiments,the supported metal salts were decomposed and reduced in situ in the catalytic reactor. For this purpose,the fixed bed of particles was first heated in
flowing nitrogen(100mL/min,NTP)from ambient temperature to623K at a rate of2K/min.Then,the nitrogen was replaced by an equal flow of a H2/N2gas mixture containing10%v/v H2in which the reduction was accomplished for30min at623 K.
The specific surface areas and the size distributions of the micropores and small mesopores were determined from the adsorption and desorption isotherms of argon at77.35K on a Miromeritics ASAP2010device,using the Horvath-Kawazoe model for slit pores.
2.2.Reaction Unit and Experimental Conditions.The catalytic experiments were conducted in an automated,continu-ous flow unit with a backmixed reactor of Berty type equipped with on-line analyses and a catalytic afterburner.A scheme of the unit has been presented previously.18The reactor effluent was split,and a minor flow was analyzed by means of a gas chromatograph(GC;HP6890with ChemStation,column HP 19091N-133)equipped with both flame ionization and thermal conductivity detectors.The residual gas flow from the reactor outlet and the gas flow coming from the GC detectors were combined and sent to the catalytic afterburner for complete combustion with air.The on-line analyses of CO and CO2in the offgas of the afterburner by means of two infrared detectors enabled the continuous monitoring of the carbon balance and the detection of a possible accumulation of organics or coke on the catalyst.Typically,the basket of the Berty reactor contained about2g of the extrudates.
Liquid acetone(Merck KGaA,purity g99.8%)was fed via an HPLC pump(Gilson Type307)into a mixing section where evaporation and premixing with N2and H2was realized at473 K.All tubes upflow and do
wnflow to the reactor were taped and heated at473K to prevent condensation.The molar fraction of acetone in the feed was kept constant at a value of0.4in all experiments with nitrogen acting as diluent gas.The molar ratio of hydrogen to acetone,however,was varied between0.25and 1.5.In the case of H2/acetone)1.5,no nitrogen was added to the feed mixture.
All experiments were carried out at a fixed temperature of 623K and a fixed total pressure of1MPa because these reaction conditions turned out to be most favorable for high MIBK yields. Ideal backmixing in the reactor at these conditions can by ensured by applying a rotor speed of2600rpm or higher. The weight hourly space velocity(WHSV)was increased from1to9h-1.Data acquired within the first20h time on stream were not taken into consideration because some catalysts displayed an induction behavior.After a run time of20h at WHSV)1h-1,the catalysts showed a reasonably stable performance.All data reported here represent the arithmetic average value of seven to eight GC data points collected at one operational set of conditions over about4h.
3.Results and Discussion
3.1.Metal Loading.It has been reported before that the pH value of the metal salt solution used for the impregnation of a support may affect the final degree of the metal dispersion.23 The common explanat
ion is based on the fact that the surface of the solid in contact with a liquid is loaded with a positive or negative charge depending on the pH value.The metal salt solutions used here for the impregnation of the active carbon exhibit quite different pH values at equal salt concentrations. Figure1displays the pH values of the fresh solutions with equal concentrations of 2.6×10-3mol/L and of the slurries containing the carbon extrudates after CO2activation as a function of the contact time.The fresh solutions exhibit pH values ranging from5.5(Cu salt)up to11.8(Pt salt).However, the pH values quickly increase after addition of the activated carbon and approach a value that is not depending on the metal precursor but is rather impressed by the active carbon material itself.This effect is utilized for so-called mass titration measure-
Table1.Metal Precursor Salts Used
substance manufacturer decomposition temp,K
[Pt(NH3)4](OH)2‚n H2O
(56.6%m/m Pt)
Sigma-Aldrich554a
[Pd(NH3)4](CH3COO)2Sigma-Aldrich586b Ni(NO3)2‚6H2O Merck555c Cu(NO3)2‚3H2O STREM Chemicals455d a From ref19.b From ref20.c From ref21.d From ref
22.
Figure1.Evolution of pH values of impregnation solutions upon contact
with the active carbon support Norit R3Extra(2.6×10-3mol/L precursor
concentration).
Table2.Overview of Catalyst Types Used in the Catalytic
Experiments a
notation content Me,
%m/m
N Me/m active carbon,
mol/g
A BET,
m2/g
R3E activated in CO21601
R3E•Pt0.50Pt 2.6×10-51503
R3E•Pd0.24Pd 2.3×10-51497
R3E•Ni0.16Ni 2.6×10-51497
R3E•Cu0.16Cu 2.6×10-51497
R3E•48Ni 2.71Ni47.5×10-51385
R3E•48Cu 2.98Cu48.3×10-51313
a Sample R3E activated in CO2represents the base material from which
all metal-loaded catalysts were produced.
6112Ind.Eng.Chem.Res.,Vol.45,No.18,2006
ments that have been described in detail elsewhere.24,25We assume that a possible pH influence on the final metal dispersion can be neglected here.
Table 2provides an overview on the catalysts used in this study including the data of the metal-free carbon support after activation in CO 2for comparison.As demonstrated previously,18the activation in CO 2at 1073K causes the formation of new micro-,meso-,and macropores,and it results in a pronounced increase in the specific surface area of about 35%as related to the original Norit R3Extra carbon.The CO 2-activated extrudates were used as a base material for the preparation of all metal-loaded catalysts.The selection of Me/C catalysts comprises four samples loaded with the metals Pt,Pd,Ni,and Cu in similar amounts of about 2.6×10-5mol of metal per gram of support.The specific surface areas of these catalysts are almost identical.A second group of samples contains Ni or Cu,respectively,in about 18-fold higher molar loadings.
3.2.Effect of the Metal Type on the Catalytic Activity and Selectivity.The incentive for testing different hydrogenat-ing components for their applicability in the hydrogenation step after aldol condensation was derived from theoretical consid-erations based on the reaction scheme shown in Figure 2.The target product MIBK is the intermediate of a consecutive reaction in which it is both formed and consumed by hydro-genation.Apart from that,a parallel reaction path is possible representing the direct
hydrogenation of acetone to 2-propanol.Two different functional groups are subjected to hydrogenation.The hydrogenation of the highly reactive double carbon bond of mesityl oxide is mandatory for MIBK production and,hence,
has to be promoted.On the other hand,the hydrogenation of any carbonyl group will have a negative effect on the MIBK selectivity.
While it is obvious that the hydrogenation function must play a key role in optimizing the MIBK selectivity,its effect on the rate of acetone conversion is surprising at first sight.Figure 3shows that there is a clear connection between the initial catalyst activity and the type of metal used.The rate of acetone conversion increases from copper over nickel and palladium to platinum.As displayed in Figure 4,the selectivityto MIBK is promoted in the same sequence.
We believe that the main influencing factor affecting initial catalyst activity is the more or less pronounced deactivation by coking.To further clarify this assumption,Figure 5shows
the
Figure 2.Reaction scheme for aldol condensation of acetone and subsequent hydrogenation including the formation of
byproducts.
Figure 3.Effect of metal type at equal molar metal content on the BET surface related reaction rate of acetone at WHSV )1h -1and H 2/acetone )
0.5.
Figure 4.Effect of metal type at equal molar metal content on MIBK selectivity as a function of acetone conversion at H 2/acetone )
0.5.
Figure 5.Coking effect:mass increase (∆m )of the catalyst fill during reaction testing in relation to initial catalyst mass (m 0)and time on stream (tos).
Ind.Eng.Chem.Res.,Vol.45,No.18,20066113
mass increase of the catalyst fill during reaction measurement.A reference of this mass increase to the respective total value
of time on stream was necessary in this case because especially for the catalysts showing low activity (Cu,Ni)fewer data points were taken.
A comparison between Figures 3and 5makes clear that catalysts that accumulate higher amounts of coke residue during operation also show lower initial activity levels and vice versa.A possible explanation for the very different values of coking susceptibility observed can be given when comparing the data shown in Figure 5for the two catalysts containing nickel.Obviously a higher level of hydrogenation activity as imple-mented in this case by different amounts of metal loading (see also sections 3.4and 3.5)is an effective method to prevent coking.Therefore,it is concluded that for all catalysts shown in Figure 5both the coking and the initial activity are affected by their intrinsic hydrogenation activity.Concerning the mech-anism of coking in this reaction scheme,it appears reasonable to assume mesityl oxide (MO)to be the most important coke precursor.MO is subjected to both possible further aldol condensation (C d O group)and polymerization reactions (C d C group)even
tually leading to long-chain products capable of clogging the pore system.Figure 6correlates the MO selectivity to the acetone conversion for all catalysts included in Figure 5at a H 2/acetone ratio of 0.5.It can be seen that catalysts showing an improved coking resistance generally tend to produce less mesityl oxide during reaction testing.With regard to this finding it seems advisable to always prevent excessive formation of mesityl oxide in order to enhance catalyst stability.
It should be emphasized that the rates of acetone conversion as shown in Figure 3are related to the total specific surface areas of the catalysts rather than to the surface areas of the supported metals.This makes sense because both acid -base sites and metal sites are involved.On the other hand,this approach does not take into account possible differences in
the
Figure 6.Selectivity to mesityl oxide (MO)as a function of acetone conversion at equal H 2/acetone ratio of 0.5for different
catalysts.
Figure 7.Influence of molar H 2/acetone feed ratio on BET surface related reaction rate of acetone at WHSV )1h -1.All Me/C catalysts are loaded with the same molar metal
content.
Figure 8.Selectivity to MIBK as a function of acetone conversion at different molar H 2/acetone feed ratios over (a)R3E •Pt,(b)R3E •Pd,(c)R3E •Ni,and (d)R3E •Cu.
6114Ind.Eng.Chem.Res.,Vol.45,No.18,2006
metal dispersion.To our knowledge,there are no established methods that enable the comparison of the surface areas of supported Pt,Pd,Ni,and Cu at equal conditions,and even at the actual temperatures and hydrogen pressures of MIBK formation as applied here.It remains uncertain,hence,to which extent each of the factors,metal type and metal dispersion,is influencing the reaction rate.Common experience suggests, however,that a qualitative ranking of the metals with respect to their activity in the hydrogenation of double carbon bonds in the order Pt>Pd>Ni>Cu is justified.
3.3.Influence of the Molar Ratio of Hydrogen to Acetone in the Feed.In the experiments reported above,a molar feed ratio of H2to acetone of0.5was applied,which represents the stoichiometric value for the idealized reaction of acetone to MIBK.Because the catalysts loaded with nickel,copper,and palladium displayed an insufficient hydrogenation activity at this feed composition,the H2/acetone ratio
was increased to1.5 for these materials.In the case of the Pt/C catalyst,both H2/ acetone values of1.5and0.25were tested.It should be noticed that the acetone concentration in the feed and the total pressure were kept constant in these experiments,whereas the concentra-tions of hydrogen and of the carrier gas nitrogen were altered. Figure7displays the results in terms of the surface area related rate of the acetone conversion.The rate of the initial acetone conversion increases with increasing hydrogen partial pressure. This effect is most pronounced in case of the noble metals Pt and Pd,while the catalysts containing Cu or Ni show only very limited response to the altered feed composition.
The plots in Figure8show the impact of the H2/acetone feed ratio on the selectivities to MIBK as a function of the acetone conversion.Over the Pt-containing catalyst(Figure8a),the
maximum levels of the MIBK selectivity decrease with increas-ing ratio H2/acetone in the feed,and they shift to higher conversion levels.Here,an substoichiometric hydrogen supply (H2/acetone)0.25)has a beneficial effect on the maximum MIBK selectivity because the direct hydrogenation to2-propanol is prevented.The other metals,in contrast,have a lower hydrogenation activity,and higher hydrogen partial pressures should be applied to increase the MIBK selectivity.
This behavior can be explained by considering the reaction scheme depicted in Figure2.It is clear that
an increase of MIBK selectivity by means of additional H2supply can only be effective when mesityl oxide is available.If this is not the case, as,for example,when using the catalyst containing Pt at H2/ acetone)0.5(Figure6),an increased H2/acetone ratio will only promote overhydrogenation of MIBK to MIBC and,even more pronounced,the direct hydrogenation of acetone to 2-propanol.
We suppose that this general explanation also holds for the copper catalyst.However,raising the H2/acetone feed ratio from 0.5to1.5was not sufficient to show this effect(Figure8d). Higher feed ratios beyond1.5were not installed because this would have caused a concomitant decrease in the acetone feed concentration.
3.4.Effect of the Amount of Supported Metal.As reported above,the Pt/C and Pd/C catalysts exhibit a superior activity and MIBK selectivity as compared to Ni/C and Cu/C catalysts of equal metal loading.Aiming at an enhancement of the hydrogenation function,two catalysts with about18-fold higher contents of Ni or Cu,respectively,were prepared.As shown in Figure9,the higher metal loadings result in an increase in the rate of acetone conversion.Using Cu,this effect is small,but the catalyst with the higher Ni loading exhibits an activity even higher than that of the Pt/C catalyst.Likewise,the selectivity to MIBK is boosted when the Ni content is increased.The data in Figure10indicate that the MIBK selectivities of sample R3E•48Ni containing2.71%m/m Ni and of sample R3E•Pt with a loading o
f0.5%m/m Pt are comparable.The copper catalysts,in contrast,are found inapt to compete even at increased metal content.
From the results shown,the use of nickel is highly recom-mended over platinum for economic reasons.An estimate of the materials costs for catalysts R3E•Pt and R3E•48Ni indicates that,despite the larger amount of nickel required,the above Ni/C catalyst is about60times less expensive.
3.5.Hydrogenation Potential and Product Distribution. The effect of the hydrogenation activity on the product distribution can best be demonstrated by comparing the perfo-mances of the two Ni/C catalysts with different nickel contents. The corresponding ,the product selectivities at various acetone conversion levels,are displayed in Figures11and12 and refer to equal reaction conditions.
The general trends discussed in the following have been observed with the other catalysts as well and,therefore,have exemplary character.When the hydrogenation activity is low (Figure11),mesityl oxide(MO)produced by aldol condensation is prevented from extensive further hydrogenation to MIBK. As a consequence,MO appears as a main product.High MO concentrations,in turn,are accompanied by high selectivities to isophorone,which is formed from MO and acetone in a secondary aldol condensation.On the other hand,a high hydrogenation activity(Figure12)favors the overhydrogenation
of MIBK to MIBC and the undesired hydrogenation of acetone to2-propanol.The formation of consecutive products from 2-propanol(propene and propane)is also slightly enhanced.In the case of catalyst R3E•48Ni it is obvious that these drawbacks of high hydrogenation activity are minor.In addition,it
should Figure9.Effect of metal content on BET surface related reaction rate of acetone at WHSV)1h-1and H2/acetone)
0.5.
Figure10.Effect of metal content on MIBK selectivity as a function of acetone conversion at H2/acetone)0.5.
Ind.Eng.Chem.Res.,Vol.45,No.18,20066115
be kept in mind that catalysts with a high hydrogenation potential generally display a higher overall activity in the conversion of acetone.4.Conclusions
Platinum,palladium,copper,and nickel were tested for their applicability as hydrogenation components supported on a commercial active carbon in the aldol condensation and hydrogenation of acetone to methyl isobutyl ketone.At similar molar contents of supported metal,Pt proved to be best suited followed by Pd,Ni,and Cu in that sequence.
The impact of the hydrogenation within the reaction network can be altered through the molar ratio of hydrogen to acetone in the feed,the metal type,and through the amount of metal supported.Using nickel,in particular,it could be shown that a higher metal loading can compensate for the lower intrinsic hydrogenation activity.A Ni/C catalyst with 2.71%m/m Ni exhibits an overall performance very similar to
that of a Pt/C catalyst containing 0.5%m/m Pt.In this way,a significant reduction of the costs for the catalyst production can be achieved.
The optimum value of the H 2/acetone ratio with regard to MIBK selectivity strongly depends on the nature and amount of hydrogenation metal.Any increase in H 2supply can only have a positive effect on MIBK selectivity when there is mesityl oxide present for hydrogenation.If this is not the case,an increase of the H 2/acetone ratio only results in a loss of MIBK selectivity due to overhydrogenation to MIBC and direct hydrogenation of acetone.
active下载The hydrogenation activity has a major impact not only on the MIBK selectivity but also on the rate of the acetone conversion.When the hydrogenation activity is low,mesityl oxide production is enhanced with a concomitant negative influence on MIBK selectivity.Also,experimental results indicate a connection between high MO selectivity and ag-gravated catalyst deactivation by coking.Acknowledgment
G.W.thanks the state of Baden-Wu ¨rttemberg,Germany,for his grant in the context of a “Landesgraduiertenstipendium”.Symbols and Abbreviations
A BET )specific surface area measured by means of BET method,m 2/g
m i )mass of species i ,kg m ˘i )mass flow of species i ,kg/h N ˙i )molar flow of species i ,mol/h p )pressure,Pa
S i )reactor selectivity to species i )(νacetone /νi )[N ˙i ,out /(N ˙in -N ˙out )acetone ]
T )temperature,K
WHSV )weight hourly space velocity,h -1)m ˘acetone,in /m catalyst X acetone )conversion of acetone
νi )stoichiometric coefficient of species i DAA )diacetone alcohol
MIBC )methyl isobutyl carbinol MIBK )methyl isobutyl ketone MO )mesityl oxide R3E )Norit R3E Literature Cited
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Figure 11.Product distribution obtained over R3E •Ni at four different acetone conversions;H 2/acetone )
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Figure 12.Product distribution obtained over R3E •48Ni at four different acetone conversions;H 2/acetone )0.5.
6116Ind.Eng.Chem.Res.,Vol.45,No.18,2006
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