hydrotalcite-type catalysts for narrow-range oxyethylation of 1-dodecanol using ethyleneoxide

Applied Catalysis A: General 249 (2003) 229–240

Hydrotalcite-type catalysts for narrow-range oxyethylationof 1-dodecanol using ethyleneoxide

Daehwan Kima, Chengzhe Huangb, Hongsun Leec, Insun Hanc, Sungkwon Kanga,Soohan Kwond, Jeongho Leee, Yohan Hane, Hyungrok Kime,∗

a Department of Chemistry, Chungnam National University, Daejeon 305-764, South Koreab Department of Chemical Engineering, Yanbian University, Yanji 133-002, PR Chinac Research Center, Dongnam Chemical Industries Ltd., Inchon 403-031, South Korea

d Department of Chemistry, Chungbuk National University, Chongju 361-763, South Koreae Korea Research Institute of Chemical Technology, Daejeon 305-606, South Korea

Received 7 November 2002; received in revised form 22 February 2003; accepted 4 March 2003

Abstract

Hydrotalcite-type catalysts were studied for the oxyethylation of 1-dodecanol. Hydrotalcite-type catalysts treated withdodecanoic acid are very active for the narrow-range oxyethylation of 1-dodecanol.

The narrow distribution of oxyethylene units in the dodecylethoxylates is elucidated by the constrained reaction on thesurface of hydrotalcite-type catalysts. In the proposed surface reaction scheme, the following points are presented. (1) Therelatively basic reactants, ethylene oxide and dodecylalcohol or dodecylethoxylates, are adsorbed on the relatively acidicAl3+ sites of the surface. (2) The adsorption of ethylene oxide is increasingly interfered with due to the occupation ofdodecylethoxylates of higher number of oxyethylene units on the isolated acidic sites of the catalyst. (3) The interferenceresults in the decrease of oxyethylation rate in each additional ethylene oxide insertion step, due to a smaller number ofthe ethylene oxide molecules adsorbed near the growing point of alcohol ethoxylates. (4) The difference in ethylene oxideinsertion rates with the numbers of oxyethylene units in the dodecylethoxylate products causes peaking of the distribution.

The large difference of oxyethylation activity over hydrotalcite-type catalysts of distinct Mg and Al composition ratios maybe correlated with the numbers of Al3+ sites at the closest distance from an Al3+ site, which can be deduced from the idealhexagonal or orthorhombic super lattice of Al3+ sites on the hydrotalcite surface layer.© 2003 Elsevier B.V. All rights reserved.

Keywords: Oxyethylation; 1-Dodecanol; Hydrotalcite; Ethyleneoxide

1. Introduction

The oxyethylation of aliphatic alcohol depicted inScheme 1has been utilized for the commercial pro-duction of non-ionic surfactants[1]. Similar types ofoxyethylations for other organic compounds having

∗ Corresponding author. Fax:+82-42-861-4245.

active hydrogens have been also applied in the pro-duction of various wetting and emulsifying agents[2].

In the oxyethylations of aliphatic alcohols, ho-mogeneous basic catalysts, such as NaOH, KOH orNaOCH3, are generally used to facilitate ethylene ox-ide (hereafter abbreviated as EO) insertion to the al-cohols at relatively low temperature and pressure[3].In this homogeneous type of alcohol oxyethylation,distributions of oxyethylene units (–CH2–CH2–O–)

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0926-860X(03)00297-7

230 D. Kim et al. / Applied Catalysis A: General 249 (2003) 229–240

Scheme 1. Oxyethylation of aliphatic alcohol. R: long-chain alkylgroup,n: mole ratio of EO/ROH,x: number of oxyethylene unitsin product mixture.

in the ethoxylated product mixture are much broaderthan the statistical Poisson-type distribution whichcan be calculated byEq. (1) [1,2,4]. The maincause of the broad distributions in the homogeneousoxyethylations has been explained as the formation ofintramolecular complexes between alkali metal ionsand the terminal oxygens of alcohol ethoxylate duringthe oxyethylation reactions. The phenomenon whichis known as the Weibull–Tornquist effect contributesin stabilizing ethoxylated oligomers and enhancingthe oxyethylation rate of the oligomers to broaden theproduct distribution[5,6].

[RO(–EO–)nH] = [ROH]0 e−u un

n!(1)

where R is the alkyl group, EO denotes ethyleneoxide,n is the number of oxyethylene groups,u the averagemole ratio of EO/ROH, –EO– denotes the oxyethyleneunit (–CH2–CH2–O–), and [ROH]0 is the initial ROHconcentration.

Regarding the oxyethylene unit distributions inthe aliphatic alcohol oxyethylations, the statisticalPoisson-type distribution is still far from satisfactorysince the Poisson-type distribution always accom-panies a considerable amount of free alcohol andless valuable alcohol ethoxylates which are eithertoo short or too long[2]. Recently, much attentionhas been focused on this field to reach the narroweroxyethylene distribution than the Poisson-type dis-tribution using various solid oxyethylation catalysts.The narrow-range alcohol ethoxylates (NRE) are sup-posed to be environmentally benign and suitable forproduction of high quality final surfactant products[7–9].

Hydrotalcite-type solids have been investigated asone group of catalytic materials for the narrow-rangeoxyethylation of aliphatic alcohols and esters[10,11].Hydrotalcite-type materials are a class of syntheticmixed metal layered hydroxides, generally describedby the formula [M1−x

2+Mx3+(OH)2][A x/m

m− ·nH2O], where x may vary from 0.17 to 0.33

depending on the particular combination of divalentM2+ and trivalent M3+ ions. Am− represents them-valent anion necessary to compensate the positivecharge of brucite-like hydroxide layer and locates be-tween mixed metal hydroxide layers[12]. Especially,hydrotalcite-type materials of a specific composi-tion of [Mg1−xAlx(OH)2][(CO3)x/2·nH2O] have beenwidely utilized for the oxyethylation researches be-cause of the unique acid–base bifunctional surfaceproperty generated by the isolated Al3+ acidic sitesin Mg(OH)2 type basic matrix[10].

At the early stage of the researches on the oxyethy-lation using hydrotalcite-type catalysts, the kinetic pa-rameters were more important than the distributionof ethoxylated products. The surface alkoxide group,–OR, which should be formed by the condensationbetween ROH, for example, heptanol, and surface hy-droxide, –OH, was considered as the initial activecatalytic species. The other reactant, EO was simplyproposed to be added to the surface –OR group inthe dissolved state without supporting evidence. Theoxyethylation rate was expressed as first-order with re-spect to the concentration of catalyst, alcohol and EO.The presence of the surface –OR species was provedby direct detection of ROH after carbonate ion treat-ment of the catalyst thoroughly cleaned with water[11].

Hydrotalcite-type catalysts were also appliedfor oxyethylation of 1-butanol in the synthesis of2-buthoxy ethanol. The preparations of commer-cially important mono-ethoxylate using homoge-neous base catalysts were always accompanied bythe co-production of by-product polyethoxylates. The10 times selective formation of mono-ethoxylate todi-ethoxylate on the hydrotalcite-type catalysts, mod-ified by a small amount of KOH, was demonstratedeven at 80% of EO conversion. The selective for-mation of mono-ethoxylate was explained by theincrease of surface basicity, which might induce thepreferred adsorption of 1-butanol rather than alcoholethoxylates of higher basicity during the reaction.The basic sites of the catalyst surface was consideredas the initial formation site of the surface alkox-ide species that undergoes nucleophilic attack onethylene oxide[13]. The profound mono-ethoxylateformations using other types of hydrotalcite catalystswere also reported. When hydrotalcite catalysts in-tercalated with polyoxometallates, such as chromate,

D. Kim et al. / Applied Catalysis A: General 249 (2003) 229–240 231

Scheme 2. Surface oxyethylation scheme of fatty acid ester on Mg/Al mixed oxide catalyst.

dichromate or pyrovanadate, were applied as cata-lysts for oxyethylation of 1-butanol, even 100% ofmono-ethoxylate selectivity was observed. The poly-oxometallate species seemed to be responsible for thehigh mono-oxyethylene butanol selectivity. However,the exact function of the polyoxometallates was notpresented[10].

Surface-modified hydrotalcite-type catalysts werealso used for the oxyethylation of fatty alcohols. Gen-erally, surface-modified hydrotalcite-type catalystswere claimed, in several patents, to be very active andto show narrow oxyethylene distributions comparedwith other types of solid catalysts, probably due to theimproved mixing of catalysts in reaction medium[14].

Another catalytic application of hydrotalcite-typematerials may be envisaged in the oxyethylation offatty methyl esters. The Al–Mg composite oxide cat-alysts prepared by the calcination of hydrotalcitesshowed acid–base bifunctional effect that results indissociative chemisorption of fatty methyl ester andinsertion of ethylene oxide to the chemisorbed –OCH3species, as depicted inScheme 2 [15,16]. However,in the proposed reaction steps of the oxyethylationon the calcined hydrotalcite catalysts, adsorptionof ethyleneoxide on the acidic Al3+ sites were notconsidered as in other previously discussed alcoholoxyethylations[11,13].

Even though various hydrotalcite-type catalystshave been applied in alcohol oxyethylations, the re-lationships between hydroxide layer compositionsand catalytic properties, such as activity and uniqueoxyethylene unit distribution, have not been seriouslyconcerned and are still not much understood. Espe-cially, the isolated acid site surface model, which isgenerally useful for providing the chemisorption siteof alkoxide –OR species, seems not to be adequateto reveal the relationship. Better understanding of therelationship is required for the design of new, com-mercially attractive hydrotalcite-type oxyethylationcatalysts.

In this paper, we report the catalytic properties ofhydrotalcite-type catalysts in 1-dodecanol oxyethyla-tion and we propose a surface oxyethylation schemethat is plausible for the explanations of the narrowoxyethylene unit distribution of the products andthe dependence of catalytic activity on the hydrox-ide layer compositions. For these purposes, we usewell-defined hydrotalcite-type materials and idealsurface models for the interpretation of catalytic prop-erties. As hydrotalcite-type catalysts for 1-dodecanolethoxylation, we prepare and characterize the fattyacid treated hydrotalcite-type materials that wereclaimed to be very active[14] and possibly intact intheir layered structure after the surface treatment.

2. Experimental

2.1. Materials

1-Dodecanol, dodecanoic acid, Na2CO3, NaOH,Mg(NO3)2·6H2O, and Al(NO3)3·9H2O, were ofreagent grade and were purchased from Junsei Chem.Co. (Japan). EO of 99.9% were supplied from HyundaiPetroleum Chem. Co. (South Korea) and used as re-ceived. Standard 1-dodecylethoxylates of 99% purityare supplied by Dongnam Chem. Co. (South Korea).

2.2. Catalyst preparation and characterization

2.2.1. Preparation of hydrotalcite-type materialsHydrotalcite-type material of desired formula

[Mg0.67Al0.33(OH)2][(CO3)0.17·4H2O] (hereafter ab-breviated as HT21 which denotes hydrotalcite-typematerial with 2:1 ratio of Mg/Al components) wasprepared by co-precipitation method at 40◦C fol-lowing the known procedure with slight modifica-tions [17]. To 300 ml of 0.90 M Na2CO3 solutionin 2.5 dm3 beaker was added 1 dm3 of mixed solu-tion of 0.75 mole Mg(NO3)2·6H2O and 0.375 mole


of Al(NO3)3·9H2O with strong mixing for 2 h; pH10 ± 0.5 was maintained during the co-precipitationreaction by dropping 2.25 M NaOH solution. Afteraddition of 1 dm3 of the Mg/Al mixed solution, theHT21 suspension was aged for 6 h with continu-ous stirring at the reaction temperature. The whitecake was isolated by filtration of the suspension andwashed five times with 1 dm3 portions of distilledwater. The cake was dried for 12 h in air circulatingoven at 100◦C to give HT21 white powder. An-other hydrotalcite material of the desired formulaof [Mg0.75Al0.25(OH)2][(CO3)0.13·4H2O] (hereafterabbreviated as HT31) was prepared following thesame procedure as in HT21 preparation except forusing 1 dm3 Mg/Al mixed solution of 0.75 moleMg(NO3)26H2O and 0.25 mole of Al(NO3)3·9H2O.

2.2.2. Dodecanoic acid treatmentTo 200 cm3 of 2-propanol and 5 g of HT21 or HT31,

0.5 g of dodecanoic acid is added at ambient temper-ature and refluxed for 30 min. The suspension is fil-tered after cooling to room temperature. The cake ofthe acid-treated HT21 or HT31 is re-suspended andfiltered three times more using 100 cm3 portions of2-propanol. The acid-treated HT21 or HT31 is driedin air and heat treated for 6 h at 250◦C in a quartztube furnace with flowing N2. The final dodecanoicacid-treated HT21 or HT31 (hereafter abbreviated asHT21-catalyst or HT31-catalyst which denotes cata-lyst derived from HT21- or HT31-type material) isstored under N2 atmosphere.

2.2.3. Characterizations of HT21, HT31,HT21-catalyst and HT31-catalyst

HT21, HT31, HT21-catalyst and HT31-catalystwere characterized using powder X-ray diffraction(Rigaku D/Max IIB, Cu K� radiation), ICP analysis(Jobinyvon, Jy Ultima C) for Mg and Al concen-tration determination, N2 BET surface area analysis(Micromeritics, ASAP2010) and scanning electronmicroscopy (Philips, XL30S EFG), following thespecific procedures of each analysis.

2.3. Oxyethylation and product analysis

2.3.1. Oxyethylation of 1-dodecanolOxyethylation was performed in a 200 cm3 batch-

type stainless steel stirred pressure reactor. The reactor

was equipped with a magnetic stirrer, an electric heat-ing/air cooling system and an 100 cm3 branch chamberfor quantitative liquid EO supply and was connectedto the vacuum/N2 purging unit.

A typical reaction procedure is illustrated as fol-lows, using our standard reaction parameters. The re-actor and head parts were pre-heated with electricair heating guns to 150◦C. Powdered HT21-catalyst(0.31 g) and 62.0 g (0.33 mole) of 1-dodecanol werecharged to the reactor and assembled while the partswere hot. After we evacuated the reactor, the reactorwas filled with N2 and heated to 120◦C. The reactorwas evacuated again for 20 min to remove residual wa-ter. The reactor was heated to 180◦C within 30 min.At 180◦C, EO was supplied to the reactor by openingthe needle valve of the EO storage chamber until 5 atmof reactor pressure was reached within 30 s. Duringthis step, about 5 cm3 of liquid EO was required to fillthe reactor. Starting from this point, reaction time wasmeasured at every 5.0 cm3 of EO consumption until50 cm3 (0.99 mole) of EO was transferred to the reac-tor. After the completion of EO supply, reaction wasstopped by purging the remaining EO in the reactorwith N2 gas at 180◦C. The reactor part was immergedin cold water to cool the liquid product below 30◦C.The reactor was disassembled after N2 purging andethoxylated product was collected for analysis.

2.3.2. Product analysisLiquid product was separated by filtration of crude

product and was analyzed using a gas chromatog-raphy method using SPB-OV column. The mixedethoxylated standard sample of known concentra-tion of 1-dodecylethoxylates was used for quantita-tive analysis of oxyethylene unit distribution. Freepolyethyleneoxide (PEG) was analyzed by the con-ventional solvent extraction method.

3. Results and discussion

3.1. Characterization of hydrotalcite-typematerials

Fig. 1 shows powder XRD patterns of HT21 andHT31. The XRD patterns of two samples are almostidentical to the typical pattern of layered crystallinehydrotalcite in peak positions and relative intensities.


Fig. 1. XRD patterns of HT21 (a) and HT31 (b).

In Fig. 2, scanning electron microscope pictures ofthe two samples are shown. The pictures show ap-parent similarity of the samples in crystal shape andsize without showing bulk amorphous phases. Thespecific surface areas of 89.6 and 89.8 m2/g for HT21and HT31 also support the similarity of the sam-ples. The relative Mg/Al atomic ratios of HT21 andHT31, determined by ICP method, are 2.05 and 3.01,respectively, as designed for the ideal distinct for-mula of [Mg0.67Al0.33(OH)2][(CO3)0.17·4H2O] and[Mg0.75Al0.25(OH)2][(CO3) 0.13·4H2O].

Fig. 3 compares the powder XRD patterns ofHT21-catalyst and HT31-catalyst derived from HT21and HT31 by dodecanoic acid modification andthermal treatment. The peak positions and intensityratios of the XRD patterns are also very similar tothose of corresponding HT21 and HT31 except forsome broadness after modification, probably due tothe loss of some regularity during the thermal treat-ment. The samed-spacing values (0.77 nm) of HT21-catalyst/HT21 pair and HT31-catalyst/HT31 pairmean that the intercalation of dodecanoate anion intoHT21 and HT31 materials can be ruled out and that theinterlayer regions of HT21-catalyst and HT31-catalystare intact after the surface and thermal modification.The N2 BET surface areas of HT21-catalyst andHT31-catalyst are 101.2 and 109.5 m2/g which also

imply the preservation of physical states. In addi-tion, Mg/Al compositional ratios of 2.09 and 3.27 forHT21-catalyst and HT31-catalyst are reasonably thesame values as in HT21 and HT31, which suggeststhat HT21-catalyst and HT31-catalyst might be use-ful enough to trace the compositional effects on thecatalytic properties of hydrotalcite-type materials.

3.2. Oxyethylations of 1-dodecanol usingHT21-catalyst and NaOCH3 homogeneouscatalyst

Fig. 4 represents EO consumption rates forHT21-catalyst and NaOCH3 homogeneous catalyst.The average oxyethylation rates of HT21-catalyst andNaOCH3 catalyst are 9.7 and 18.7 g EO/gcat. min,respectively. At the beginning of the reaction, the rel-ative EO consumption rate of HT21-catalyst is muchfaster than in the homogeneous system. However, therate of HT21-catalyst system decreases slowly andthe rate of the homogeneous system increases withreaction time. The progressive increase of the EOconsumption rate in other homogeneous oxyethyla-tion systems has been also observed and explained bythe increase of EO solubility in the alcohol ethoxy-late solution[18]. The decrease of reaction rate withreaction time in the heterogeneous HT21-catalyst


Fig. 2. SEM images of HT21 (a) and HT31 (b).

system suggests that the dodecylethoxylates, pro-duced during the reaction, strongly interact with thecatalyst surface.

Fig. 5 shows the distributions of oxyethylene unitsin the product mixtures (for EO/alcohol mole ratio=3) using HT21-catalyst, NaOCH3 homogeneous cata-lyst and theoretical Poisson distribution. The distribu-tion of oxyethylene units using HT21-catalyst is muchnarrower than that of homogeneous NaOCH3 cata-lyst system and even narrower than the Poisson-typedistribution. In HT21-catalyst system, the amount ofunreacted 1-dodecanol in the final product mixturedecreases to 7 wt.% and reaches even lower content of1-dodecylethoxylates of higher numbers of oxyethy-

lene units than in the Poisson-type distribution. Com-paring the contents of 1-dodecanol ethoxylate of threeoxyethylene units, we see that HT21-catalyst sys-tem shows the highest value of 26.2 mole%, whichmeans that effective surface reaction steps for nar-rowing oxyethylene unit distribution are involved inthe HT21-catalyst oxyethylation system.

Figs. 6 and 7show effects of HT21-catalyst amountand EO pressure on the oxyethylation reaction rate.The initial oxyethylation rates are almost proportionalto the amount of HT21-catalyst and EO pressure.The linear dependency of reaction rate with catalystamount supports the conclusion that the surface ofHT21-catalyst is directly involved in the rate deter-mining step of the reaction. However, involvementof adsorbed EO on the catalyst surface can not beclaimed at this point, even though linear dependencyof EO consumption rate on the EO pressure is obvious.

3.3. Proposed surface reaction scheme to inducenarrow-range 1-dodecylethoxylates

Fig. 8 compares the changes of oxyethyleneunit distributions with EO insertion amount us-ing HT21-catalyst and Poisson-type distributions.It is confirmed that the peaked distributions of1-dodecylethoxylate of the corresponding oxyethy-lene units for each EO/alcohol mole ratio of 1.2, 2.4,and 3.0 gradually change toward a much narrower dis-tribution than the Poisson distribution, which impliesthat EO insertion rates of dodecylethoxylates of lowoxyethylene units should be faster than those of do-decylethoxylates of higher oxyethylene units over theentire reaction range. If they are the same, oxyethy-lene unit distribution should follow the Poisson-typedistribution.

In addition, when at least one of the reactantsis involved in the oxyethylation in homogeneousstate, such a narrowing of oxyethylene unit distri-bution and the characteristic reaction rate decreasein HT21-catalyst system with reaction time can notbe expected. Thus, for the reaction scheme in whichhomogeneous EO is reacted with adsorbed dode-cylethoxylates, there is no reason to result in narrowerproduct distribution than the Poisson-type distribu-tion. In this situation, even broader oxyethylene unitdistribution is normally expected due to the pref-erential adsorption of dodecylethoxylates of higher


Fig. 3. XRD patterns of HT21-catalyst (a) and HT31-catalyst (b).

oxyethylene units on the catalyst surface.1 For theother case in which only EO is adsorbed on the cat-alyst and participates in oxyethylation with alcoholethoxylates of homogeneous state, increase of reac-tion rate with oxyethylation time and Poisson-typeproduct distribution are expected because of im-proved solubility of EO in the reaction medium, asin a homogeneous reaction system[18]. Therefore,it is reasonable to propose that the evolutions of twounique phenomena of decrease of EO consumptionrate and narrow distribution of oxyethylene units inthe products with reaction time should be attributedto the surface reaction between adsorbed 1-dodecanolor dodecylethoxylates and adsorbed EO on acidicsites. The involvement of adsorbed EO in the surfaceoxyethylation scheme has already been discussed indifferent types of heterogeneous acid–base bifunc-tional catalyst systems in which EO is adsorbed on

1 Unpublished result of following experiment. (1) 1.0 g ofHT21-catalyst is mixed with 5.0 g of separated dodecylethoxylateproduct at room temperature for 2 h. (2) Separation of liquid dode-cylethoxylate by centrifugation. (3) analyse liquid dodecylethoxy-late and compare oxyethylene unit distribution with that of before.Result: pattern of final oxyethylene unit distribution is shifted to-ward lower oxyethylene unit distribution by selective adsorptionof relatively higher dodecylethoxylates.

acidic site and alcohol is activated initially on basicsite to form alkoxy species[19].

Based on the characteristics of 1-dodecanoloxyethylation using HT21-catalyst, a very proba-ble reaction scheme can be proposed, as depictedin Fig. 9. In the proposed surface reaction scheme,the following are assumed. (1) Oxyethylation occurson the surface of HT21-catalyst which is composed

Fig. 4. EO consumption rates of HT21-catalyst and NaOCH3 cat-alyst. (a) 0.5 wt.% HT21-catalyst: EO/dodecanol mole ratio= 3.0,temperature= 180◦C, pressure= 5 atm. (b) 0.25 wt.% NaOCH3:EO/dodecanol mole ratio= 3.0, temperature= 180◦C, pressure= 5 atm.


Fig. 5. Distributions of oxyethylene units in dodecylethoxylatesof HT21-catalyst, NaOCH3 catalyst and distribution of Poissontype. (a) 0.5 wt.% HT21-catalyst: EO/dodecanol mole ratio= 3.0,temperature= 180◦C, pressure= 5 atm. (b) 0.25 wt.% NaOCH3:EO/dodecanol mole ratio= 3.0, temperature= 180◦C, pressure= 5 atm. (c) EO/dodecanol mole ratio= 3.0.

of isolated Al3+ acidic sites embedded on the do-decanoic acid-modified Mg(OH)2 type surface. (2)EO and 1-dodecanol or 1-dodecanol ethoxylates areadsorbed on the separated Al3+ sites for oxyethyla-tion. (3) Dodecanol ethoxylate of larger number ofoxyethylene units occupies a larger area for adsorp-tion. (4) –OR species, alkoxide or anionic terminalof alcohol ethoxylate on Al3+ acid site is the EO in-sertion point for oxyethylation, as discussed by otherresearchers[15,16]. (5) In the reaction step, oxyethy-lation proceeds by nucleophilic attack of alkoxide oralcohol ethoxylate toward adsorbed EO on the nearest

Fig. 6. Effect of HT21-catalyst amount on EO consumption rate.(a) 0.25 wt.% HT21-catalyst; (b) 0.50 wt.% HT21-catalyst; (c)1.0 wt.% HT21-catalyst. EO/dodecanol mole ratio= 3.0, temper-ature= 180◦C, pressure= 5 atm.

Fig. 7. Effect of EO pressure on EO consumption rate at (a)4 atm and (b) 6 atm. EO/dodecanol mole ratio= 3.0, temperature= 180◦C.

Al3+ acidic site to form another homologue alco-hol ethoxylate by epoxide ring opening and chargetransfer.

In the proposed oxyethylation scheme, the spe-cific effects of heterogeneous HT21-catalyst can bedemonstrated. Thus, the differences in the numbersof oxyethylene units of alcohol ethoxylates and theiradsorptoin properties can be manipulated for theexplanation of the narrowness of oxyethylene unit dis-tribution and the slow decay of 1-dodecanol oxyethy-lation rate with reaction time. In the surface reactionscheme ofFig. 9, with increasing the number ofoxyethylene units in an dodecylethoxylate adsorbedon the catalyst surface, the group size of Al3+ acidsites of the surface occupied by the oxyethylene unitsof the dodecylethoxylate also increases and the num-ber of acid sites for EO adsorption near the growingpoint (the terminal ethoxide of dodecylethoxylate)should decrease. The significant decrease in thenumber of acid sites near the growing point is ex-pected, especially for the dodecylethoxylates of rel-atively large number of oxyethylene units. Inversely,the oxyethylation of dodecylethoxylates containingsmaller number of oxyethylene units should be rel-atively facile and the peaking of oxyethylene unitdistribution can be justified. The proposed surface re-action scheme can be also applied for the explanationof reaction rate decay with reaction time simply byconsidering the decreasing number of acid sites forEO adsorption in total with the progress of reaction.


Fig. 8. Comparisons of oxyethylene unit distributions ofHT21-catalyst (1) with EO insertion amount and Poisson-type dis-tribution (2). (a) EO/dodecanol mole ratio= 1.2; (b) EO/dodecanolmole ratio= 2.4; (c) EO/dodecanol mole ratio= 3.0.

3.4. Correlation between oxyethylation activity andnumbers of Al3+ sites of closest distance in idealhydrotalcite-type surfaces

Figs. 10 and 11show the oxyethylene unit distri-butions and EO consumption rates for the oxyethy-lation of 1-dodecanol using HT21-catalyst andHT31-catalyst of different Mg/Al compositional ra-tios. The oxyethylene unit distributions of the twoHT-catalyst systems are almost the same. However,the ratio of reaction rates of the two catalyst systems

is about 4, showing the average EO consumption ratesof 9.7 and 2.4 g EO/gcat. min for HT21-catalyst andHT31-catalyst, respectively.

The similarity in the oxyethylene unit distributionsof the two catalyst systems strongly supports the claimthat the difference of EO consumption rates does notoriginate from the type of surface active sites, but fromthe number of sites. However, the oxyethylation rateratio of 4 between HT21-catalyst and HT31-catalystcan not be explained by some difference in either phys-ical properties, such as surface area and microscopicshape nor simple Mg/Al compositional ratio of thecatalysts. Here, it should be mentioned that the differ-ence in the number of Al3+ sites that are consideredas the adsorption sites for the basic reactants is only1 site per 12 metal ion sites between HT21 and HT31hydrotalcite-type surfaces of their ideal compositions,corresponding to only 8.3% difference (4 Al sites outof 12 (Mg/Al) sites in HT21, and 3 Al sites out of 12(Mg/Al) sites in HT31).

In the interpretation of the large oxyethylation ac-tivity difference between the two catalysts systems,types of combinations among the acid sites and theirnumbers of specific combinations have to be seriouslyconsidered because both basic reactants require acidsites for adsorption and close interactions, which arerelated with arrays of acid sites on the surface. Espe-cially, for the hydrotalcite-type materials, there havebeen several arguments in which the surface arraysof metal ion components are speculated to be regu-lar [20,21]. Therefore, it is reasonable to differentiateHT21-catalyst from HT31-catalyst by comparing thepossible arrangements of acidic sites of their specificcompositions.

The surface of HT21- or HT31-type hydrotalcitematerial can be simplified as a hexagonal lattice whichis composed of M2+ and M3+ ion lattice points. In lo-cating M3+ ions in the hexagonal lattice-type surface,a simple limiting rule has been applied, in which M3+ions are managed to fill the lattice points far from otherM3+ points to reduce repulsive energy between thehigher positive point charges[20,22]. Thus, even forthe hydrotalcite-type materials of different M2+/M3+composition ratios, the six nearest lattice points fromone M3+ site are expected to be occupied by M2+ions to result in some regularity, possibly forming atwo-dimensional super lattice-type distribution at spe-cific compositions[21,22].


Fig. 9. Proposed surface reaction scheme for dodecanol oxyethylation on HT21-catalyst (R: dodecyl group).

The ideal hexagonal lattice surfaces with super lat-tice regularity, which have similar M2+/M3+ compo-sitional ratios to those of HT21- and HT31-type mate-rials, were already presented by other researchers andare depicted inFig. 12a and b [23]. Even though nostructural evidence for the super lattice formation forHT21- or HT31-type hydrotalcite material has beenreported until now, it seems very useful to adopt thesuper lattice-type arrays to compare possible Al3+ ionarrangements in HT21-catalyst and HT31-catalyst.

If we accept the super lattice-type arrays for HT21-and HT31-type materials and the surface oxyethyla-

Fig. 10. Comparison of oxyethylene unit distributions ofHT21-catalyst (a) and HT31-catalyst (b). 0.5 wt.% HT21-catalystor HT31-catalyst, EO/dodecanol mole ratio= 3.0, temperature= 180◦C, pressure= 5 atm.

tion scheme in which two reactants are adsorbed ondifferent acidic sites as described inScheme 2, theclosest Al3+–Al3+ combination which has “d” separa-tion distance should be important in the estimation ofoxyethylation rate because the distance between Al3+acidic sites and number of the shortest Al3+–Al3+combination are the main factors. In the HT21-typesurface array of Al3+ ions, the number of the short-est Al3+–Al3+ combination from an Al3+ site is 6, asshown inFig. 12a. For the case of HT31-type surfacearray, the number is 0 or 2 depending on the types ofAl3+ super lattice, as shown inFig. 12b. It is not pos-sible to decide which array is more probable for thereal HT31-type surface. However, it is clear that the

Fig. 11. Comparison of EO consumption rates of HT21-catalyst (a)and HT31-catalyst (b). 0.5 wt.% HT21-catalyst or HT31-catalyst,EO/dodecanol mole ratio= 3.0, temperature= 180◦C, pressure= 5 atm.


Fig. 12. Ideal hexagonal lattice surfaces with M3+ ion super lattice regularity. (a) M2+/M3+ ratio = 2.0; (b) M2+/M3+ ratio = 3.0. Numberof the closest distance “d” combination is 6 for M2+/M3+ ratio = 2.0 and 2 (orthorhombic) or 0 (hexagonal) for M2+/M3+ ratio = 3.0.

number ratio of the shortest Al3+–Al3+ combinationof the HT21-type array to that of HT31-type array islarger than 3 for either case.

Here, it may be suggested that the large 1-dodecanoloxyethylation activity ratio of 4:1 between HT21-catalyst and HT31-catalyst, which can not be ex-plained by the differences in key physical properties, isrelated with the theoretical number ratio of the shortestAl3+–Al3+ site combination of two ideal HT-type ma-terials. In the ideal surface model of hydrotalcite-typematerial, the shortest Al3+–Al3+ separation distance“d” is estimated to be about 0.51 nm, which is asomewhat large distance for the insertion reaction[24]. However, by considering the layered hydroxidesurface on which the adsorption of alkoxide or EOcan be rather diffuse around Al3+ sites, and expand-ing the adsorption sites to the adjacent hydroxidesites, we can justify the proposed distance-sensitiveEO insertion reactions on the HT21- and HT31-typecatalysts.

4. Conclusions

Hydrotalcite-type catalysts were studied for theoxyethylation of 1-dodecanol. Hydrotalcite-type cata-lysts treated with dodecanoic acid are very active forthe narrow-range oxyethylation of 1-dodecanol.

The narrow distribution of oxyethylene units in thedodecylethoxylates is elucidated by the constrained re-action on the surface of hydrotalcite-type catalysts. Inthe proposed surface reaction scheme, the followingpoints are presented. (1) The reactants of basic charac-teristics, ethylene oxide and dodecylalcohol or dode-cylethoxylates, are adsorbed on the different Al3+ sitesof relatively acidic characteristics on the surface. (2)The adsorption of ethylene oxide is increasingly inter-fered with due to the occupation of dodecylethoxylatesof higher numbers of oxyethylene units on the isolatedAl3+ acidic sites of the catalyst. (3) The interferenceresults in the decrease of oxyethylation rate in each ad-ditional ethylene oxide insertion step due to a smaller


number of the ethylene oxide molecules adsorbed nearthe growing point of alcohol ethoxylates. (4) The dif-ference in ethylene oxide insertion rates with the num-bers of oxyethylene units in the dodecylethoxylateproducts causes peaking of the distribution.

The large activity difference in HT21-catalyst andHT31-catalyst of distinct compositions may be corre-lated with the numbers of Al3+ sites at the closest dis-tance from an Al3+ site, which can be deduced fromsuper lattices of the ideal HT-type surface models.

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