A selectivity study of 2,4-pentanediol hydrogenolysis combining experiments and computer simulation

Chemical Engineering Science 58 (2003) 4271–4285www.elsevier.com/locate/ces

A selectivity study of 2,4-pentanediol hydrogenolysis combiningexperiments and computer simulation

Keyi Wang, Martin C. Hawley∗, Todd D. FurneyDepartment of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824-1226, USA

Received 9 May 2002; received in revised form 12 May 2002; accepted 9 June 2003

Abstract

In the current work, the process of 2,4-PD hydrogenolysis is studied in a combined e8ort of experiments and computer simulation,to understand the mechanism and factors controlling the C–C vs. C–O bond cleavage selectivity of this process. It is expected that theresults from this study will be useful in control of the selectivity of sugar hydrogenolysis, as the bond cleavage mechanism in sugarhydrogenolysis is the same as in 2,4-pentanediol hydrogenolysis. In this capacity, the e8ects of reaction conditions, including temperature,base concentration, hydrogen pressure, catalyst amount and catalyst type, on the C–C vs. C–O bond cleavage selectivity of 2,4-pentanediolhydrogenolysis are investigated both experimentally and via simulation. The simulation study strengthens the experimental investigationin several aspects. It allows the process to be explored in a larger range of reaction conditions, the mechanism and factors controlling theselectivity to be understood at a deeper level, and the features of catalysts critical to improving the selectivity to be identi:ed. However,the experimental data are useful in providing a baseline for the simulation results to be compared.? 2003 Published by Elsevier Ltd.

Keywords: 2,4-pentanediol; Sugar hydrogenolysis; Catalysis; Selectivity; Simulation; Chemical processes

1. Introduction

Hydrogenolysis is a bond breaking reaction with additionof hydrogen. This reaction can be used to convert sugarsinto a spectrum of useful, value-added chemicals, includingglycerol, propylene glycol and ethylene glycol (Andrews& Klaeren, 1989; Clark, 1958; van Ling, Driessen, Piet &Vlugter, 1970). However, because both the C–C and C–Obonds of sugars are susceptible to cleavage in the reactionas shown below:

R3C− CR′3 + H2 → R3CH + HCR′

3;

R3C− OH + H2 → R3CH + H2O;

the products from sugar hydrogenolysis are fairly compli-cated. Besides the products mentioned above, other poly-hydric alcohols are also formed at the same time in thereaction. It is interesting and important as well to know how

∗ Corresponding author. Tel.: +1-517-355-5135;fax: +1-517-432-1105.

E-mail address: [email protected] (M. C. Hawley).

the selectivity of sugar hydrogenolysis can be controlled, ifthis process is to be commercialized.In the current work, 2,4-pentanediol (2,4-PD) is studied

as a model compound of sugars, to investigate the mecha-nism and factors controlling the C–C vs. C–O bond cleavageselectivity in hydrogenolysis. Our previous investigation(Wang, Furney & Hawley, 1995) has shown that theC–C and C–O bond cleavage reactions in sugar hydrogenol-ysis compete for a shared �-hydroxyl carbonyl precursorand the mechanism is the same as in 1,3-diol, including2,4-PD hydrogenolysis. It is expected that the insight ob-tained from studying 2,4-PD hydrogenolysis is applicable tosugar hydrogenolysis, for purpose of minimizing the C–Obond cleavage in the later reaction. As the C–O cleavage isto a great extent responsible for the complication of sugarhydrogenolysis products, minimizing C–O cleavage is ex-pected to make the reaction more selective. Reduction ofC–O cleavage in sugar hydrogenolysis is also expected toincrease the yield of glycerol. Glycerol is the highest-valuedmajor product of sugar hydrogenolysis, and it preserves allthe oxygen atoms in the starting sugar molecules.The major advantage of using 2,4-PD as a model com-

pound to study the C–C vs. C–O selectivity is that the

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4272 K. Wang et al. / Chemical Engineering Science 58 (2003) 4271–4285

Fig. 1. Reaction pathways of 2,4-pentanediol hydrogenolysis.

hydrogenolysis of 2,4-PD can only undergo one bond cleav-age reaction, to break either the C–C or the C–O bond, asshown in Fig. 1. These two reactions produce completelydi8erent products. Thus, the 2,4-PD having undergone eitherreaction can be easily quanti:ed from the products formed inthe reaction and the selectivity can be easily calculated. Ac-tually, the C–C vs. C–O selectivity of 2,4-PD hydrogenoly-sis can be calculated simply as the ratio of the sum of acetoneand isopropanol to the sum of 3-penten-2-one, 2-pentanoneand 2-pentanol. In contrast, the hydrogenolysis products ofreal sugars are very complicated, because of the multipleC–C and C–O bonds of sugars available for cleavage. Inaddition, it is also very diHcult to tie any reaction productsto a speci:c bond-cleavage reaction in sugar hydrogenoly-sis, due to the general existence of multiple pathways to thesame product.The study on 2,4-PD hydrogenolysis in the current work

has combined both experiments and computer simulation.The experiments are conducted to examine the e8ects of re-action conditions, such as temperature, hydrogen pressure,base concentration, catalyst amount and catalyst type, onthe C–C vs. C–O selectivity of 2,4-PD hydrogenolysis. Thecomputer simulation is carried out to rationalize the ex-perimental results and to understand the selectivity controlmechanism at a deeper level.

2. Experimental methods and results

In this work, 2,4-PD is hydrogenolyzed under various re-action conditions to investigate the e8ects of temperature,hydrogen pressure, base concentration, catalyst amount andcatalyst type on the C–C vs. C–O bond cleavage selectivity.The reaction is performed in a stainless steel batch reactorof 50 ml capacity. This reactor is equipped with a magnetic

stirring bar, and is placed in an oil bath. The oil bath isplaced on the top of a magnetic stirrer. Heating of the oilbath is conducted by an electrical heating coil emersed inthe oil bath. The oil bath is constantly stirred by a nitro-gen Iow bubbling through it, and its temperature is con-trolled by a temperature controller. The hydrogen pressurerequired by the reaction is provided by a hydrogen cylin-der. The high pressure regulator installed with the cylindercan provide a fairly accurate pressure control. To preventover-pressurization, a safety pressure relief valve is installedin the hydrogen supply line.To carry out the hydrogenolysis reaction, 2,4-PD, transi-

tion metal catalysts, sodium hydroxide and distilled waterof pre-determined amount are placed in the reactor. The re-actor is purged with hydrogen and then evacuated. After theabove steps, the reactor is pre-heated to a pre-determinedtemperature, and is then pressurized with hydrogen. There-upon, the reaction starts. During the reaction, samples areregularly taken from the reactor, and are analyzed with gaschromatography for the product composition. Thus obtainedproduct composition is recorded against the reaction time.The reaction is stopped after it is carried out long enough togive a reasonable conversion.Table 1 summarizes the range of the reaction conditions

explored in this work. A total of seven types of catalysts havebeen studied, all purchased from Aldrich. The properties ofthese catalysts are summarized in Table 2.The GC column used for product analysis is a DB624

non-polar column purchased from J&W Scienti:c Com-pany. The carrier gas is hydrogen (at a Iow rate of 1ml/min). The oven temperature is programmed so thatit stays at 40◦C for 1 min, then increases at a rate of40◦C=min and then stays at 260◦C for 3 min. The reten-tion times for the major products are: 1:43 min for ethanol,1:61 min for acetone, 1:69 min for isopropanol, 3:09 min for

K. Wang et al. / Chemical Engineering Science 58 (2003) 4271–4285 4273

Table 1Summary of Reaction Conditions

Reaction parameter Range

Temperature 180–240◦CBase concentration 0− 25× 10−3 mol NaOH/l H2OH2 Pressurea 3.0–7:0 MPaCatalyst amount 0.5–2 gCatalyst types Ni/silica–alumina, Ni/kieselguhr, copper chromite,

Ru/carbon, Raney Ni, Raney Cu, barium-promotedcopper chromite

aThe corresponding H2 concentration in the liquid phase is interpolatedas 100–200 (mM) at 220◦C in water from tabulated data in Young (1981,p. 308).

Table 2Summary of Catalyst Propertiesa

Catalyst Description

Nickel on silica–alumina Powder. Reduced and stabilized.Surface area 190 m2=g. 65 wt% Ni.

Nickel on kieselguhr Powder. Active and stabilized.Surface area 125 m2=g. 60 wt% Ni.

Copper chromite Powder. Merck 13,2662Ruthenium on carbon Powder. 5 wt% Ru.

Fieser 1,983; 5,574Raney nickel 50% slurry in water. pH¿ 9.

Merck 12,8284Raney copper 50% slurry in water. pH¿ 9.

Merck 12,2583Barium-promoted copper chromite Powder. 9.7% barium oxide.

Surface area 35 m2=g.Merck 13,2638

aAll catalysts were purchased from Aldrich. The descriptions are fromAldrich catalog.

Fig. 2. Experimental product concentration vs. reaction time pro:les, ex-perimental conditions: 220◦C, 5:0 MPa H2 pressure, 0:1 g Ni/kieselguhr,10−2 mol NaOH/1 H2O, 0:5 g starting 2,4-PD and 40 ml water. symbols:• 2,4-PD;� EtOH; ◦ acetone; 4 i-PrOH; 2-pentanone;∇ 2-pentanol.

2-pentanone, 3:19 min for 2-pentanol, and 4:68 min for 2,4-pentanediol.Fig. 2 presents the result from a typical experimental run.

This experiment is carried out at 220◦C and 5 MPa H2 pres-

Fig. 3. Selectivity vs. reaction time pro:le derived from Fig. 2.

sure, with Ni on kieselguhr as the catalyst. In the hydrogenol-ysis of 2,4-PD, two intermediates, acetone and 2-pentanone,are normally observed at signi:cant levels, in addition to theend products, ethanol, isopropanol and 2-pentanol. Otherintermediate products, including 4-hydroxy-2-pentanone,acetaldehyde and 3-penten-2-one, are detected during theinitial period of the reaction, but generally exist only atlow levels. These intermediate products at low levels arenot quanti:ed and therefore are not shown in Fig. 2. Fig.2 includes an initial heating period of about 15 min long.During this period, the reactor is heated from the initialroom temperature to the reaction temperature (220◦C inthis case). After this initial period, the reaction temper-ature is maintained constant, the reactor is pressurizedwith H2, and the reaction is considered started. However,products have been found at the end of this initial period,mainly because the :rst step of the reaction is between the2,4-PD substrate and the metal catalyst, which does notinvolve H2.Fig. 3 shows how the C–C vs. C–O selectivity changes

with the reaction time in the experiment just mentioned. Thisselectivity is de:ned as the ratio of the moles of C–C bondsto the moles of C–O bonds broken in the reaction. Sincethe amount of 3-penten-2-one is negligible in the reaction

Table 3E8ect of temperature on selectivity

Experiment series # 1a Experiment series # 2b

Temperature (◦C) Selectivity Temperature (◦C) Selectivity

180 1.38 — —200 1.42 200 1.06220 1.42 220 1.02240 1.45 240 1.10

aReaction Conditions: 5 MPa H2 pressure, 0:1 g Ni/kieselguhr,10−2 mol NaOH/l H2O, 0:5 g starting 2,4-PD and 40 ml water.bReaction conditions: 5:0 MPa H2 pressure, 0:25 g Raney Cu, 5 ×

10−3 mol NaOH/l H2O, 0:5 g starting 2,4-PD and 40 ml water.


Table 4E8ect of base concentration on selectivitya

Experiment series # 1b Experiment series # 2c

NaOH Added (10−3 mol/l H2O) selectivity NaOH Added (10−3 mol/l H2O) Selectivity

0 0.46 0 0.2610 1.42 5 1.0225 1.56 12.5 1.73

aIn all cases, base is added as 1 N aqueous NaOH solution.bReaction Conditions: 200◦C, 5:0 MPa H2 pressure, 0:1 g Ni/kieselguhr, 0:5 g starting 2,4-PD and 40 ml water.cReaction Conditions: 220◦C, 5:0 MPa H2 pressure, 0:25 g Raney Cu, 0:5 g starting 2,4-PD and 40 ml water.

product, the selectivity is calculated as

S =[acetone] + [isopropanol]

[2− pentanone] + [2− pentanol] :

As seen from Fig. 3, the selectivity is typically stabilized ata constant level after the initial period of the reaction. It isthis stabilized selectivity that is compared in the followingfor di8erent reaction conditions, to show their e8ects on thereaction selectivity of 2,4-PD hydrogenolysis.

E0ect of temperature. To investigate the e8ect of tem-perature, 2,4-PD is hydrogenolyzed under two sets of baseconditions with varying temperature. The selectivity, asTable 3 shows, is found to barely change with the tempera-ture, although increasing temperature greatly enhances thereaction rate.

E0ect of base concentration. The base concentration, asTable 4 shows, has a profound e8ect on the selectivity, aswell as on the reaction rate. Increase in the amount of baseadded to the reaction solution increases the reaction rate,and favors the cleavage of C–C bonds. This e8ect on theselectivity is most easily seen when the data from reactionswith base addition are compared with those from reactionswithout base addition. One thing that needs to be pointedout is that the base concentration, as indicated by the PHof the reaction solution, is generally found to drop from itsinitial concentration after the initial period of reaction.

E0ect of hydrogen pressure. The e8ect of hydrogen pres-sure seems to vary with the catalyst used (Table 5). WithRaney Cu as the catalyst, increase in the hydrogen pressureincreases the selectivity; while with Ni on kieselghur as thecatalyst, the e8ect of hydrogen pressure appears not signif-icant. In the case of Raney Cu, hydrogen pressure is alsoobserved to have an inhibiting e8ect on the rate of 2,4-PDhydrogenolysis. This e8ect is consistent with the observa-tion made by Chang, Kuo, and Lee (1985) in hydrogenol-ysis of sobitol. It indicates that the H2 supply is typicallymore than suHcient in 2,4-PD hydrogenolysis, and there-fore that the reaction rate of 2,4-PD hydrogenolysis can notbe limited by the interphase transfer of H2 into the liquid. Itis believed that the 2,4-PD hydrogenolysis starts with a de-hydrogenation reaction between 2,4-PD and the metal cata-

Table 5E8ect of hydrogen pressure on selectivity


Pressure (MPa) Selectivity Pressure (MPa) Selectivity

3.0 1.50 3.0 0.805.0 1.42 5.0 1.027.0 1.49 7.0 1.15

aReaction conditions: 200◦C, 0:1 g Ni/kieselguhr, 10−2 mol NaOH/lH2O, 0.5 starting 2,4-PD and 40 ml water.bReaction conditions: 220◦C, 0:25 g Raney Cu, 0:2 ml 1 N NaOH;

0:5 g starting 2,4-PD and 40 ml water.

Table 6E8ect of catalyst amount on selectivity


Catalyst (g) Selectivity Catalyst (g) Selectivity

0.1 1.42 0.1 1.220.2 1.49 0.25 1.020.5 0.90 0.5 0.75

aReaction Conditions: 220◦C, 5:0 MPa H2 pressure, Ni/kieselguhr,10−2 mol NaOH/l H2O, 0.5 starting 2,4-PD and 40 ml water.bReaction conditions: 220◦C, 5:0 MPa H2 pressure, Raney Cu, 0:2 ml

1 N NaOH, 0:5 g starting 2,4-PD and 40 ml water.

lyst. It is also believed that H2 enters the reaction througha metal hydride formed in the reaction between H2 and themetal catalyst. This hydride is also the product of the de-hydrogenation reaction, and is consumed slowly in hydro-genation of unsaturated intermediate products. High level ofH2 leads to high level of metal hydride, which helps pushthe dehydrogenation reaction back to 2,4-PD. This explainswhy high H2 pressure actually slows down the 2,4-PD hy-drogenolysis process.

E0ect of catalyst amount. The e8ect of catalyst amountagain varies with the catalysts (Table 6). However, the gen-eral trend is that high catalyst concentration has an adversee8ect on the selectivity. With Raney Cu as the catalyst, theselectivity steadily decreases with the catalyst amount. WithNi on kieselghur as the catalyst, the e8ect of catalyst amountbecomes signi:cant only at a high level.


Table 7E8ect of catalyst type on selectivitya

Catalyst type Selectivity

Nickel on silica–alumina 1.12Nickel kieselguhr 1.18Copper chromite 1.21Ruthenium on carbon 1.36Raney nickel 1.46Raney copper 1.73Barium-promoted copper chromite 2.50

aAll reaction are run at 210◦C, 3:5 MPa H2 pressure, with 0:05 g metalcatalyst, 5 × 10−3 mol NaOH/l H2O, 0:5 g starting 2,4-PD and 40 mlwater.

E0ect of catalyst type. A good catalyst is considered tobe the key to achieve the control over the C–O cleavage.In this work, seven di8erent types of catalysts were inves-tigated. The results are presented in Table 7. As can beseen from this table, the selectivity varies widely with thetype of catalysts used. The best selectivity is furnished bybarium-promoted copper chromite, and is as high as 2.5,which is yet to be optimized against other reaction condi-tions. In contrast to the barium-promoted copper chromite,the plain copper chromite catalyst only provides a selectiv-ity of 1.21 in our experiments. It is interesting to see thedi8erence made by the presence of barium oxide. The ef-fect of the amount of barium oxide used in preparation ofthe catalyst may be another factor deserving study in thecontext of selectivity improvement.The experimental work produced useful information, but

it also raised new questions, such as “why the e8ect of hy-drogen pressure varies with the catalyst used?” and “whatcharacter of catalysts should be sought to improve the se-lectivity?” In addition, the experimental study provides lit-tle insight into why the selectivity changes with the reactionconditions as it does.To address the above questions, a simulation study was

conducted. In this study, the e8ects of base concentration,hydrogen pressure, catalyst amount and catalyst type weresimulated and compared against the experimental results, toprovide explanations to the experimentally observed phe-nomena and to help identify the features of catalysts criticalto selectivity improvement.The e8ect of temperature on selectivity is not as profound

as other variables. In this simulation work, the reactionswere considered to be isothermal at 220◦C.

3. Development of process model

At this time, the reaction pathway of 2,4-PD hydrogenol-ysis is clear to us. However, the pathway alone is not suH-cient to allow us to develop the process model. It does notprovide the reaction details on how the H2, the base catalystand the metal catalyst are involved in the reaction, althoughthese details are critical to process model development. Forthis reason, a more detailed reaction scheme of 2,4-PD hy-

drogenolysis is developed from the reaction pathway inFig. 1. This detailed reaction scheme is shown in Fig. 4.The considerations made in development of this scheme arediscussed brieIy in the following.In the absence of transitional metal catalysts, molecular

hydrogen is not reactive. Therefore, in hydrogenolysis of2,4-PD, the hydrogen must be activated by the metal cat-alyst before it possesses the ability to reduce any unsatu-rated compounds. The reaction of H2 with the metal catalystforms a metal hydride species, which has the ability to hy-drogenate both CKC and CKO double bonds (Collman,Hegedus, & Finke, 1987; Masters, 1981; Pignolet, 1983).The hydrogenation reaction between the metal hydride andthe unsaturated species must regenerate the metal catalyst,so that it can be re-used in the next cycle of catalysis. Thisreaction cycle has been used by many researchers to explainthe hydrogenation process of CKC and CKO bonds withmolecular hydrogen, and therefore is adopted in the currentwork to help elucidate the reaction details of 2,4-PD hy-drogenolysis.The hydrogenation of ketones and aldehydes by the metal

hydride yields alcohols and the metal catalyst. Since thisreaction is reversible, alcohols must also be able to reactwith the metal catalyst to become dehydrogenated and torelease a metal hydride. Therefore, it is further pictured inFig. 4 that the hydrogenolysis of 2,4-PD is initiated by thereaction of this starting compound with the metal catalyst,and that the metal hydride works as a hydrogen carrier totransport hydrogen from the 2,4-PD and the molecular H2to those unsaturated intermediate products.From Fig. 4, it may be noticed that the base catalyst

(hydroxide ion in this case) is not only involved in theretro-aldol and dehydration reactions as a real catalyst, butalso participates directly in the dehydrogenation of 2,4-PDand activation of H2. Participation of bases in the H2 ac-tivation and alcohol dehydrogenation are well documented(Collman et al., 1987). As the dehydrogenation step is mostlikely to be the rate limiting step in 2,4-PD hydrogenolysis,the above postulation is consistent with the fact that the2,4-PD hydrogenolysis rate is enhanced by the addition ofbases.It may also be noticed from Fig. 4 that all the reac-

tion steps in 2,4-PD hydrogenolysis, except the hydro-genation of 3-penten-2-one, are assumed to be reversible.Retro-aldolization and dehydration are well-known re-versible reactions (Neilsen & Houlihan, 1968). Hydro-genation of ketones and aldehydes and dehydrogenation ofalcohols are also known to be reversible, but hydrogena-tion of alkene is generally considered to be irreversible,because the reaction equilibrium lies far to the product end(Collman et al., 1987; Masters, 1981).To derive themathematicalmodel fromFig. 4, we assume:

(a) the reaction is carried out in a batch reactor, and thusthe product concentrations are functions of the reactiontime only;


Fig. 4. Detailed reaction scheme of 2,4-pentanediol hydrogenolysis.

(b) the reaction is carried out under isothermal conditions,and thus the reaction rate constants do not change withtime; and

(c) the concentration of H2 in the reaction solution is alsotime-independent, and is in equilibrium with the hydro-gen pressure.

The last assumption is based on the fact that the hydrogenol-ysis reaction take places under a constant hydrogen pressureand the solution is well mixed during the reaction.For the convenience of mathematical model development,

the reaction steps shown in Fig. 4 are all numbered (thenumbers are shown as the subscripts of the rate constantsfor corresponding reactions), and the chemical species aredesignated by letters. A mass balance on each of the speciespresent in Fig. 4 except H2 leads to

ddt[A] =−X1; (1)

ddt[B] = X1 − X2 − X5; (2)

ddt[C] = X2 − X3; (3)

ddt[D] = X3; (4)

ddt[E] = X2 − X4; (5)

ddt[F] = X4; (6)

ddt[G] = X5 − X6; (7)

ddt[I] = X6 − X7; (8)

ddt[J] = X7; (9)

ddt[M] =

ddt[OH] =− d

dt[MH]

= X1 + X3 + X4 + X6 + X7 − X8: (10)

In the above equations, [A], [B], [C], [D], [E], [F],[G], [I] and [J] are the molar concentration of 2,4-PD,4-hydroxy-2-pentanone, acetone, isopropanol, acetalde-hyde, ethanol, 3-penten-2-one, 2-pentanone and 2-pentanol,respectively. [M] is the molar concentration of the metalcatalyst, [MH] is the molar concentration of the metal hy-dride, and [OH] is the molar concentration of the base orhydroxide ion in the reaction solution. X1–X8 are the re-action Iuxes of Reactions No. 1–8 in Fig. 4, respectively.Assuming all these reactions are :rst order in the substratesand the catalysts, we further have

X1 = k1[A][M][OH]− k−1[B][MH]; (11)

X2 = k2[B][OH]− k−2[C][E][OH]; (12)

X3 = k3[C][MH]− k−3[D][M][OH]; (13)

X4 = k4[E][MH]− k−4[F][M][OH]; (14)

X5 = k5[B][OH]− k−5[G][OH]; (15)

X6 = k6[G][MH]; (16)

X7 = k7[I][MH]− k−7[J][M][OH]; (17)

X8 = k8[H][M][OH]− k−8[MH]; (18)


In the last equation, [H] is the molar concentration of H2dissolved in the reaction solution.At the beginning of the reaction, we may assume that all

the products, but the substrate (2,4-PD) and the metal andbase catalysts, have a zero concentration. Therefore,

[A] = [A]o; [M] = [M]o; [OH] = [OH]o (19)

and

[B] = [C] = [D] = [E] = [F]

= [G] = [I] = [J] = [MH] = 0 (20)

for t = 0.The above equations and initial conditions constitute the

process model. If all the reaction rate constants present inthe model are obtained, the model can be solved to give theproduct concentrations as a function of reaction time.Note that this model assumes an initial condition that is

diHcult to obtain in experiments. For mathematical sim-plicity, this model assumes that the reaction temperature isconstant throughout the reaction course and that the initialconcentration is zero for all end and intermediate products.In the experiment, however, the reactor is initially at roomtemperature, and is heated up to a pre-determined temper-ature and subsequently pressurized with hydrogen. There-upon, the reaction is considered started and the temperatureis maintained constant through the experiment. Becausethe reaction between 2,4-PD and the metal catalyst is in-evitable during the initial heating period, products havebeen found in the experiment before the reaction is con-sidered started. For this reason, although governed by thesame set of ordinary di8erential equations, the simulatedprocess cannot be compared literally with the experimentalprocess, even under the same reaction conditions.Nevertheless, the simulated process bears many impor-

tant features of the experimental process, as we will see laterin this paper, and is useful in rationalizing the experimentalresults, in providing insights into the mechanisms and fac-tors controlling the selectivity, and in identifying features ofcatalysts that are critical to improving the selectivity.

4. Determination of kinetic constants

As just mentioned, before any attempt to solve the processmodel, the reaction rate constants present in the model needto be determined. However, these rate constants were notdetermined directly from the experimental concentration vs.time data through statistical regression, due to the di8erencebetween the experimental and simulated processes as wehave just discussed, in addition to the mathematical diHcultyinvolved and the large amount of experimental data required.Therefore, in this work, we have adopted an unconven-

tional approach that combines thermodynamic analysis, ex-perimental data, and at times assumptions to determine therate constants. Because of some arbitrary measures taken inthis process, the rate constants thus determined are bound to

be inaccurate, and therefore no e8ort is made to determinethe statistical limits of error on these constants. However,the basic requirement of this approach is that the determinedrate constants should be thermodynamically consistent andbe within a reasonable range of absolute and relative magni-tude for the reactions they represent, and are therefore use-ful for the selectivity study in this work. In this approach,the experimental data have been exclusively taken from thepseudo-steady stage of the reaction, as only for this stage,can Eqs. (1)–(10) be used without considering the initialconditions to provide additional relations for solving the rateconstants.The rate constants are generally functions of reaction tem-

perature. However, we are only interested in knowing therate constants at 220◦C in this work, as the simulation studyis to be carried out at a constant temperature (220◦C) tosimulate the e8ects of reaction conditions other than tem-perature on the selectivity.Among the eight reactions shown in Fig. 4, reactions

No. 2 (retro-aldo) and No. 5 (dehydration) are, though atlow temperature, well-studied. From the kinetic data pub-lished by Jensen and Hashtroudi (1976), it is determinedthat k2 = 752 M−1 min−1, k−2 = 4480 M−2 min−1, k5 =14; 100 M−1 min−1 and k−5 = 6590 M−1 min−1 at 220◦C(see Appendix A for detail). The rate constants (11 in total)for the rest of the reactions shown in Fig. 4 vary with themetal catalyst involved, and are not available from litera-ture. Therefore, they are determined using information fromour 2,4-PD hydrogenolysis experiments.The 11 rate constants to be determined are not all indepen-

dent. They are bound together by thermodynamic relations.Of these relations, an obvious one is that, for any reversiblereaction, the ratio of the forward reaction rate constant tothe backward reaction rate constant is equal to the reactionequilibrium constant. Thus,

kik−i

= Ki; (21)

for i=1, 3, 4, 7 and 8. In the above equation, Ki is the equi-librium constant of reaction No. i. The reaction is assumedto take place under dilute conditions.The relation which is not so obvious is that the above

equilibrium constants are interdependent. These relationsbecome apparent when the following two equations are ex-amined:

R1CHOHR2 +M +OH−UGo;K� R1COR2 +MH +H2O;

R1CHOHR2 − H2(l)UGo;K� R1COR2;

The :rst equation in the above is a general representation ofthe dehydrogenation/hydrogenation reaction pairs shown inFig. 4. This equation can be obtained by adding ReactionNo. 8 in Fig. 4 to the second equation above. Therefore,

UGo =UGo +UGo8 ; (22)


Table 8Equilibrium constants of relevant dehydrogenation reactions

No. Alcohol Ketone/aldehyde UGo (kcal) K

1 2,4-PD 4-hydroxy-2-pentanol 12.54 2:81× 10−63 Isopropanol Acetone 12.54 2:81× 10−64 Ethanol Acetaldehyde 16.59 4:52× 10−87 2-Pentanol 2-Pentanone 12.54 2:81× 10−6

where UGo and UGo are the free energy changes of the :rstand second reactions above, respectively. From Eq. (22),we have

K = KK8; (23)

K and K are the equilibrium constants for the :rst and sec-ond reactions, respectively. Unlike UG and K , UGo and Kare well de:ned and are not dependent on the catalyst in-volved. Therefore, they can in principle be determined withcertainty.Values ofK’s relevant to 2,4-PD hydrogenolysis are listed

in Table 8. These equilibrium constants are estimated fromthe free energy data documented in Guthrie (1977) for ke-tones and aldehydes and in Young (1981) for hydrogen dis-solved in aqueous phase. Due to lack of free energy data athigh temperature, these equilibrium constants are calculatedusing free energies at 25◦C. Also because of lack of data,UGo’s for Reactions No. 1 and 7 are assumed to be thesame as Reaction No. 3. The rational is that these reactionsare all about the dehydrogenation of a secondary alcohol toa ketone.Combining Eq. (23) with Eq. (21), we obtain

k1 = K1K8k−1; (24)

k−3 = K3K8k3; (25)

k−4 = K4K8k4; (26)

k−7 = K7K8k7; (27)

k8 = K8k−8: (28)

In the above equations, K8 is unknown. Therefore, four re-lations exist among the 11 rate constants to be determined,which implies that, of these to-be-determined rate constants,only seven are independent. The remaining have to be de-termined from the seven independent rate constants chosen,in order to assure thermodynamical consistency.Besides the thermodynamic consistency, it is also impor-

tant for the determined rate constants to fall within a rea-sonable range, so that the simulated processes are as closeas possible to those actually occurring in our experiments.To achieve this, the best approach would be to accommo-date as much experimental information as possible into therate constant determination process. Therefore, data fromthe experiment presented previously in Fig. 2 are used in

the following to help determine the rate constants. That ex-periment was carried out at 220◦C.Fig. 2 shows how the concentration of 2,4-PD (A), ace-

tone (C), isopropanol (D), ethanol (F), 2-pentanone (I) and2-pentanol (J) changes with the reaction time in that ex-periment. The concentration of 4-hydroxy-2-pentanone (B),acetaldehyde (E) and 3-penten-2-one (G) is low (estimatedto be less than 1 mM) and is not determined in the exper-iment. The concentrations of metal catalyst (M), of metalhydride (MH) and of hydroxide ion (OH) are also undeter-mined, but at beginning of the reaction they are calculatedas 28, 0 and 10 mM, respectively, from the amounts of re-gents used in the experiment. The initial concentration of2,4-PD (A) is 120 mM. The H2 concentration in the reac-tion solution is found to be about 0:1 M in equilibrium with5 MPa hydrogen pressure at 220◦C (Young, 1981).From Fig. 2, it is seen that, after t=100 min, [C] and [I] are

quite steady. Considering also that [B], [E], [G] and [OH] arelow, we may assume that the reaction is at a pseudo-steadystate after the initial period. Thus from Eqs. (2), (3), (5),(7), (8) and (10), it is derived that

X2 = X3 = X4; (29)

X5 = X6 = X7; (30)

and

X1 = X8 = X2 + X5 (31)

From Fig. 2, X3 and X7 can be determined to be 8:85×10−5and 6:07 × 10−5 M min−1, respectively, at t = 175 min.All the other reaction Iuxes at t = 175 min are calculatedwith Eqs. (29) to (31) from X3 and X7, and are listed inTable 9, together with the product concentrations at t=175and 0 min. The concentrations of ethanol (F) and 2,4-PD(A) in Table 9 are re-calculated to make the mass balance.Data in Table 9 are to be used in the following in conjunctionwith Eqs. (11)–(18) to determine the reaction rate constants.From Eq. (11), we have

X1 = k1[A][M][OH]− k−1[B][MH];

= k−1[MH](k1k−1

[M][OH][MH]

[A]− [B]);

= k−1[MH](K1K8

[M][OH][MH]

[A]− [B]);


Table 9Summary of product concentrations and reaction Iuxes

t = 0 min t = 175 min Unit

[A] 120 28.0 mM[B] 0 Less than 1 mM mM[C] 0 5.89 mM[D] 0 48.0 mM[E] 0 Less than 1 mM mM[F] 0 53.9 mM[G] 0 Less than 1 mM mM[I] 0 6.26 mM[J] 0 31.7 mM[H] 100 100 mM[OH] 10 Unknown mM[M] 28 Unknown mM[MH] 0 Unknown mMX1 — 1:49× 10−4 M min−1

X2 — 8:85× 10−5 M min−1

X3 — 8:85× 10−5 M min−1

X4 — 8:85× 10−5 M min−1

X5 — 6:07× 10−5 M min−1

X6 — 6:07× 10−5 M min−1

X7 — 6:07× 10−5 M min−1

X8 — 1:49× 10−5 M min−1

Let

K0 = K8[M][OH][MH]

; (32)

and substitute it into the above equation. We get

X1 = k−1[MH](K1K0[A]− [B]): (33)

Similarly, from Eqs. (13), (14), (17) and (18) we may get

X3 = k3[MH]([C]− K3K0[D]); (34)

X4 = k4[MH]([E]− K4K0[F]); (35)

X7 = k7[MH]([I]− K7K0[J]); (36)

and

X8 = k−8[MH](K0[H]− 1): (37)

Combining Eqs. (33)–(37) with Eqs. (12), (15) and (16),there are now eight equations available in total for 11 un-knowns. These unknowns include six rate constants (k−1,k3, k4, k6, k7 and k8), four concentrations ([B], [E], [G] and[OH]) and a new variable Ko. [M] and [MH] are related to[OH] through the following equation:

[MH] = [M]o − [M] = [OH]o − [OH]; (38)

which is obtained by integration of Eq. (10). As the avail-able equations are not enough for unknowns, three of themhave to be assumed before others can be solved. In thefollowing, this liberty is to be taken to make sure that thedetermined [B], [E], [G] and [OH] conform with our experi-mental observations and that the relative magnitudes of thedetermined rate constants are consistent with our knowledgeof hydrogenation reactions.

If the dehydration (Reaction No. 5) were irreversibleduring hydrogenolysis, the maximum selectivity of 2,4-PDhydrogenolysis would be k2=k5 = 0:053, or the C–O bondcleavage would be tremendously favored over the C–C bondcleavage. The experiment shows otherwise. Therefore, thedehydration reaction must be reversible in reality in 2,4-PDhydrogenolysis. The retro-aldol reaction (Reaction No. 2)may also be reversible in 2,4-PD hydrogenolysis, dependingon the reaction conditions. In order to describe the degreethat the above reactions is reversed, a reversibility parame-ter may be de:ned for each of these reactions, as the ratioof the reverse reaction rate to the forward reaction rate, or

R2 =k−2[C][E][OH]k2[B][OH]

=k−2[C][E]k2[B]

; (39)

and

R5 =k−5[G][OH]k5[B][OH]

=k−5[G]k5[B]

: (40)

As later we will see, the reaction selectivity is closelyrelated to these two parameters. With the newly de:nedreversibility parameters, we may rewrite Eqs. (2) and (5) as

X2 = k2[B][OH](1− R2); (41)

and

X5 = k5[B][OH](1− R5): (42)

Dividing Eq. (41) by Eq. (42) and solving for R5 yields:

R5 = 1− k2X5k5X2

(1− R2) = 0:97 + 0:03R2: (43)

By de:nition, 0¡R2¡ 1. Hence, 0:97¡R5¡ 1, whichimplies that the dehydration reaction is highly reversible inthe 2,4-PD hydrogenolysis process.Also derivable from the above equations are:

[B] =X2

k2(1− R2)[OH] ; (44)

[E] =R2X5

k−2(1− R2)[C][OH] ; (45)

and

[G] =k5X2

k2k−5(1− R2)[OH] −X5

k−5[OH]: (46)

Thus, [B], [E] and [G] can be determined if [OH]and R2 are assumed. The PH of the reaction solutionhas been checked during the experiment to be around11. The retro-aldol reaction is speculated to be reversiblebut to a much less degree than the dehydration reac-tion. Let [OH] = 1 mM and R2 = 0:2. It is obtained that[B] = 0:147 mM, [E] = 0:838 mM and [G] = 0:305 mM,all less than 1 mM. From Eqs. (38) and (43), it is alsoobtained that [M]=19 mM, [MH]=9 mM and R5 =0:976.Now that all the product concentrations and reaction

Iuxes are known, the reaction rate constants of concern areready to be determined, providing that an appropriate value


Table 10Summary of determined reaction rate constants

k1 k2 k3 k4 k5 k6 k7 k8(M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1)

435 752 1.89 11.9 14100 22.1 1.16 76.2

k−1 k−2 k−3 k−4 k−5 k−6 k−7 k−8(M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (M−2min−1) (min−1)

67.4 4480 12.2 1.24 6590 0 7.48 3:32× 10−5

is assigned to Ko. The equations needed are derived fromEqs. (33) to (37) and Eq. (16), and are given as follows:

k−1 =X1

[MH](K1K0[A]− [B]) ; (47)

k3 =X3

[MH]([C]− K3K0[D]) ; (48)

k4 =X4

[MH]([E]− K4K0[F]) ; (49)

k6 =X6

[MH][G]; (50)

k7 =X7

[MH]([I]− K7K0[J]) ; (51)

and

k−8 =X8

[MH](K0[H]− 1) : (52)

Let Ko = 5000. k−1, k3, k4, k6, k7 and k−8 are calculatedas 67.1, 1.89, 11.9, 22.1, 1.16 and 3:22×10−5 M−1 min−1,respectively. In terms of their coordination capability withthe metal hydride, alkenes are better than aldehydes, aldehy-des are better than ketones, and small ketones are better thanbig ones (Collman et al., 1987; Masters, 1981). Therefore,it is expected that k6¿k4¿k3¿k7. The rate constants justdetermined are consistent with this expectation.With the rate constants just determined, k1, k−3, k−4,

k−7 and k8 can be further calculated from Eqs. (24)–(28).Table 10 summarizes all the rate constants determined inthis work. These rate constants are thermodynamically con-sistent, and are within a reasonable range of absolute andrelative magnitude for the reactions they represent. Mostimportantly, the simulation results obtained with these con-stants are comparable to those obtained experimentally, aswill be seen later. However, it needs to be pointed out that,although data from the experiment represented in Fig. 2(which is carried out with Ni on kieselguhr) are used in de-termining the rate constants in Table 10, these rate constantsdo not represent the Ni on kieselguhr catalyst, because ofthe arbitrary measures used in the rate constant determina-tion process to assign values to R2 and Ko. Nevertheless,they may represent a hypothetical catalyst not very di8erentfrom Ni on kieselguhr.

5. Simulation results and discussion

With the kinetic constants determined above, the processmodel of 2,4-PD hydrogenolysis, Eqs. (1)–(20), is solvedthrough numerical integration for various reaction condi-tions. Figs. 5–8 present the results from a typical simulated

Fig. 5. Simulated concentration pro:les of major products. simulationconditions: 220◦C, [H] = 100 mM, [M]o = 28 mM, [OH]o = 10 mM,[A]o = 120 mM.

Fig. 6. Simulated selectivity pro:le. S is calculated as the ratio of thesum of [C] and [D] to the sum of [G], [I] and [J].


Fig. 7. Simulated concentration pro:les of minor products.

Fig. 8. Simulated base concentration pro:le.

run of 2,4-PD hydrogenolysis. The assumed reaction con-ditions for this simulated run are identical to those for theexperimental run shown in Figs. 2 and 3, except that theinitial concentration of some end and intermediate productsmay not be zero in the experiment as assumed by the sim-ulated run, as the reactor has undergone a temperature risefrom ambient temperature to 220◦C in the experiment be-fore the reaction is considered started. Because of this dif-ference in initial conditions, also because of the di8erencebetween the hypothetical catalyst in the simulated run andthe Ni/kieselguhr in the experimental run, the concentra-tion and selectivity pro:les in Figs. 5 and 6 do not matchthose in Figs. 2 and 3 exactly. Nevertheless, they bear themajor features of the later pro:les. For example, the selec-tivity pro:le in Fig. 6 also becomes time-independent afterthe initial period of reaction. The concentration of acetone(C) and 2-pentanone (I) in the simulated run also shows anup-and-down character during the reaction, as in the exper-imental run. In addition to the information on the major hy-drogenolysis products, the simulation also reveals how theconcentration of the minor products and the concentration

Fig. 9. Simulated e8ect of base concentration on selectivity. simulationconditions: 220◦C, [H] = 100 mM, [M]o = 28 mM, [A]o = 120 mM,varying [OH]o.

Fig. 10. Simulated e8ect of catalyst concentration on selectivity. simulationconditions: 220◦C, [H] = 100 mM, [OH]o = 10 mM, [A]o = 120 mM,varying [M]o.

of the base catalyst (hydroxide ion) change during the re-action (Figs. 7 and 8). The minor products shown in Fig. 7seem at a higher level than that observed in the experimen-tal run, which, however, may also result from the catalystand initial condition di8erences that exist between the ex-perimental and the simulated runs.As in the experimental study, the e8ects of reaction con-

ditions on the selectivity are also investigated in the simu-lation study, and the results are presented in Figs. 9–12. Inthese :gures, the circles represent the reaction conditionsunder which the simulated runs are carried out, as well asthe stabilized selectivity obtained under these conditions.Fig. 9 shows how the selectivity is a8ected by the change

in the base concentration. As observed in our experiments,the simulation study also indicates that the selectivity in-creases with the base concentration. This e8ect is most pro-found at low base concentration, but diminishes as the baseconcentration gets high. Because of the limited range of base


Fig. 11. Simulated e8ect of hydrogen concentration on selectivity. simula-tion conditions: 220◦C, [M]o=28 mM, [OH]o=10 mM, [A]o=120 mM,varying [H].

Fig. 12. Simulated e8ect of k6 on selectivity. simulation conditions:220◦C, [H]=180 mM, [M]o=28 mM, [OH]o=10 mM, [A]o=120 mM,but varying k6.

concentration explored, the independence of the selectivityon the base concentration at high base concentration is notrevealed by the experiment.Fig. 10 shows how the selectivity is a8ected by the change

in the amount of catalyst used in the reaction. Again as inour experiments, the selectivity is found to decrease with thecatalyst amount. This e8ect remains over the whole range ofcatalyst concentration that has been explored. Lower catalystconcentration makes the reaction more selective, but it alsomakes the reaction slower.As for the e8ect of the hydrogen pressure (or the hydro-

gen concentration in the liquid phase) on the selectivity, oneset of experimental data (with Raney Cu as catalyst) showthat it increases with the hydrogen pressure, while the otherset of data (with Ni on Kieselguhr as catalyst) show that itis basically independent of the hydrogen pressure. The sim-ulation study reveals a more complete picture and providesa good explanation to these di8erent behaviors of hydrogen

pressure e8ect on the selectivity. It shows that, for a givencatalyst, the selectivity increases with the hydrogen pressureat low pressure (corresponding to a H2 concentration lessthan 110 mM in Fig. 11), is indi8erent to the change in hy-drogen pressure within the medium pressure range (corre-sponding to a H2 concentration between 110 and 150 mM inFig. 11), and decreases with the hydrogen pressure at highpressure (corresponding to a H2 concentration greater than150 mM in Fig. 11). These low, medium and high pressureranges are relative, and are catalyst-dependent. For RaneyCu, 7 MPa must be still in its low pressure range, while forNi on kieselguhr, 3–7 MPa must happen to be its mediumrange. This explains why with di8erent catalysts the selec-tivity behaves di8erently within the same hydrogen pressurerange.The e8ects of reaction conditions on the selectivity may

be understood at a deeper level with the help of some math-ematical relations relating the selectivity to the base concen-tration, the metal hydride concentration and the reversibilityparameter for the retro-aldol reaction. These equations maybe derived as follows. To start, we have

S =X2X5=k2(1− R2)[OH][B]k5(1− R5)[OH][B] =

k2(1− R2)k5(1− R5) : (53)

Assuming [G] is at a pseudo-steady state, we have

ddt[G] = k5[B][OH]− k−5[G][OH]− k6[G][MH] ∼= 0;

(54)

which leads to

[G][B]

=k5[OH]

k−5[OH] + k6[MH]: (55)

Substituting the above equation into Eq. (40) and rearrang-ing yields

1− R5 =(1 +

k6[MH]k−5[OH]

)−1: (56)

Further substituting Eq. (56) into Eq. (53), we obtain

S =k2k5(1− R2)

(1 +

k−5[OH]k6[MH]

)(57)

or

S =k2k5(1− R2) =

[1 +

k−5k6

([OH]0[MH]

− 1)]: (58)

Replace the rate constants in Eq. (56) with numbers. We get

S = 0:053(1− R2)[1 + 298

([OH]0[MH]

− 1)]; (59)

which provides additionally some quantitative ideas.Eqs. (58) and (59) clearly show that the selectivity will

increase as [OH]o increases, if other variables are kept un-changed. However, increase in [OH]o will inevitably leadto the increase in [MH]. The increase in [MH], as wellas the increase in R2, causes the selectivity to decrease.


The level ofR2 depends on levels of [OH]o and [MH], as wellas other factors. The simulation study reveals that, at lowbase concentration, R2 decreases as the [OH]o is increased.The combination of the e8ects of [OH]o and R2 overrides thee8ect of [MH]. As a result, the selectivity shows an overallincrease as the base concentration is increased. However,at high base concentration, R2 increases with [OH]o. Thee8ect of [OH]o is o8set by the combination of the e8ects of[MH] and R2. The overall result is a selectivity independentof base concentration.Increase in the amount of catalysts used simply increases

the metal hydride level in the solution. Change in R2 is mi-nor. Therefore, the selectivity decreases with the catalystamount. Increase in hydrogen pressure increases [MH] anddecreases R2. At low hydrogen pressure, R2 is very sensi-tive to the change in hydrogen pressure. As the pressure in-creases, it becomes less and less sensitive, which explainswhy the selectivity responds to the change in hydrogen pres-sure di8erently in di8erent pressure range.From Eq. (58), it is also seen that the selectivity should

increase as the rate constant k6 decreases. This deductionis con:rmed by the simulation results shown in Fig. 12.Since di8erent catalysts feature di8erent k6 values, the de-pendence of the selectivity on k6 explains why it varieswidely with the type of catalysts used. To improve the re-action selectivity, catalysts with small k6 value should beused. Fig. 12 suggests that the e8ect of k6 on the selectiv-ity is very profound. Therefore, the selectivity may be wellimproved by use of a good catalyst. A good catalyst impliesthat this catalyst possesses only a moderate capability to hy-drogenate the �,�-unsaturated ketone while still keeping areasonable capability to hydrogenate saturated ketones andaldehydes.Lowering the reaction temperature may also lead to a

reduced k6. However, it will also change other rate con-stants, including k2, k5 and k−5 in Eq. (58). As a result,the selectivity may not be improved by reduction of thereaction temperature. As a matter of fact, our experimentsindicate that the selectivity is not sensitive to the change intemperature, at least within the temperature range we haveexplored.

6. Conclusions

To summarize, the process of 2,4-PD hydrogenolysis wasstudied in a combined e8ort of experiments and computersimulation, to obtain a better understanding of the mecha-nism and factors controlling the C–C vs. C–O bond cleav-age selectivity. The e8ects of temperature, base concentra-tion, hydrogen pressure, catalyst amount and catalyst typewere investigated. It was found that temperature has littlee8ect on the selectivity; increasing the base concentrationcan increase the selectivity to certain extent; increase in theamount of catalyst used has an adverse e8ect on the selectiv-ity; and the selectivity varies widely with the type of catalyst

used. The e8ect of hydrogen pressure on the selectivity is themost interesting. At low pressure, the selectivity increaseswith the pressure; at medium pressure, the selectivity is notsensitive to the change in the pressure; while at high pres-sure, the selectivity decreases as the pressure is increased.Of the seven di8erent types of catalysts tested in our exper-iments, the best one is barium-promoted copper chromite,which furnishes a C–C vs. C–O bond cleavage selectivityof 2.5. The simulation study additionally points out that, toimproved the selectivity, a catalyst with a reasonable capa-bility to hydrogenate ketones and aldehydes but only moder-ate ability to hydrogenate �,�-unsaturated ketones should besought.

Notation

k1; k−1; : : : ; reaction rate constants de:ned in Fig. 4k8; k−8ki forward reaction rate constants of Reac-

tion No. 1 in Fig. 4k−i backward reaction rate constants of Reac-

tion No. 1 in Fig. 4K reaction equilibrium constantKi equilibrium constant of Reaction No. 1 in

Fig. 4K equilibrium constant of dehydrogenation

reactionR2 reversibility parameter of Reaction No. 2,

de:ned by Eq. (39)R5 reversibility parameter of Reaction No. 5,

de:ned by Eq. (40)S C–C vs. C–O selectivityt reaction timeX1; : : : ; X8 reaction Iux rate of Reaction No.1–8.

Special symbols

UGo free energy changeUGo free energy change of dehydrogenation re-

action[A] concentration of 2,4-pentanediol[A]o initial concentration of 2,4-pentanediol[B] concentration of 4-hydroxy-2-pentanone[C] concentration of acetone[D] concentration of isopropanol[E] concentration of acetaldehyde[F] concentration of ethanol[G] concentration of 3-penten-2-one[H] concentration of dissolved hydrogen[I] concentration of 2-pentanone[J] concentration of 2-pentanol[M] concentration of metal catalyst[M]o initial concentration of metal catalyst


Table 11Source data and derived rate constantsa

kobs × 104b khyd × 104b khyd=kdehyc kdehy × 104d kretro × 104d

40◦C 0.50 9.5 3.14 3.03 0.16850◦C 1.1 21 2.67 7.87 0.435

aUnit for all rate constants: s−1.bFrom Jensen and Hashtroudi (1976).cFrom Jensen and Carre (1974).dCalculated in this work.

Table 12Determined retro-aldol and dehydration rate constants

k5 (M−1min−1) k−5 (M−1min−1) k2 (M−1min−1) k−2 (M−2min−1)

40◦C 0.182 0.570 0.010150◦C 0.472 1.260 0.0261220◦C 14100 6590 752 4480

[MH] concentration of metal hydride[OH] concentration of base or hydroxide ion[OH]o initial concentration of base or hydroxide

ion

Appendix A.

This appendix documents the procedure used to derivethe rate constants of the retro-aldol and dehydration reac-tions of 4-hydroxy-2-pentanone, or the Reactions No. 2–5in Fig. 4. The basis of this derivation is the kinetic data ob-tained by Jensen and Hashtroudi (1976) on the hydration of3-penten-2-one.In Jensen and Hashtroudi’s experiments, 3-penten-2-one

was allowed to undergo the following reactions at 40◦C and50◦C in 0.1 N NaOH aqueous solution:

The hydration rate constant khyd was determined at 40◦Cand 50◦C, and was presented along with an observed rateconstant kobs. This observed rate constant is related to khyd,kdehy and kretro as follows:

kobs =khydkretro

kdehyd + kretro: (A.1)

Thus, if the reaction equilibrium constant for the hydra-tion/dehydration reaction pair, or the ratio of khyd to kdehy,is obtained, the dehydration rate constant kdehy may be cal-culated from khyd; and the retro-aldol rate constant kretro canthen be solved from Eq. (A.1). This is exactly what has beendone in this work. The source data and the calculation re-sults are both given in Table 11. The equilibrium constants

in Table 11 are from Jensen and Carre (1974), and are mea-sured in 2:57 M HClO4 solution.The rate constants listed in Table 11 are the products of

their counterparts in Fig. 4 with the concentration of hydrox-ide ion in the solution. As the hydroxide ion concentration atwhich the above rate constants are obtained are known (0.1M), the rate constants k5, k−5 and k2 in Fig. 4 can be calcu-lated from the rate constants in Table 11. The resulting rateconstants are presented in Table 12. The rate constants at220◦C in Table 12 are extrapolated from the rate constantsat 40 and 50◦C with the help of the Arrhenius equation. Toobtain the rate constant k−2, the equilibrium constant for theretro-aldol and aldol reaction pair is obtained to be 0.168.With this equilibrium constant, k−2 is calculated as listed inTable 12. The equilibrium constant used in the calculationis estimated from the free energy data at 25◦C in Guthrie(1977). Data at other temperature are not available.

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Jensen, J. L., & Carre, D. J. (1974). Kinetics and mechanism of reactionof 3-buten-2-one and related compounds in aqueous perchloric acid.Journal of Organic Chemistry, 39, 2103–2107.

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Documents

A selectivity study of 2,4-pentanediol hydrogenolysis combining experiments and computer simulation