acidity and basicity of metal oxide catalysts for formaldehyde reaction in supercritical water at...

Applied Catalysis A: General 245 (2003) 333–341

Acidity and basicity of metal oxide catalysts for formaldehydereaction in supercritical water at 673 K

Masaru Watanabea, Mitsumasa Osadab, Hiroshi Inomataa,∗,Kunio Araia,b, Andrea Krusec

a Research Center of Supercritical Fluid Technology, Tohoku University, 07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japanb Department of Chemical Engineering, Tohoku University, 07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

c Institut für Technische Chemie (ITC-CPV), Forschungszentrum Karlsruhe GmbH, P.O. 3640, 76021 Karlsruhe, Germany

Received 16 October 2002; received in revised form 5 December 2002; accepted 5 December 2002

Abstract

Formaldehyde (HCHO) reactions in supercritical water (673 K and 25–40 MPa) with and without acid and base catalysts(homogeneous: H2SO4 and NaOH, and heterogeneous: CeO2, MoO3, TiO2, and ZrO2) were conducted by use of batch reactors.Cannizzaro reaction (2HCHO+ H2O → CH3OH+ HCOOH) and self-decomposition of HCHO (HCHO→ CO+ H2) werefound to be primary reactions for all the cases and the contribution of each reaction depended on the condition. In the caseof the homogeneous systems, Cannizzaro reaction became dominant with increasing bulk hydroxyl ion (OH−). The simplenetwork model can well express the experimental results in the homogeneous conditions. We correlated the ratio of the yieldof CH3OH to that of CO (at 15 min) against bulk OH− in the homogeneous system. For elucidating acidity and basicity ofmetal oxide catalysts on HCHO reaction in supercritical water, OH− concentration on the metal oxide surface was calculatedby use of the above correlation and the following order was found: CeO2 > ZrO2 > MoO3 > TiO2 (rutile) > TiO2 (anatase).At the reaction condition, CeO2 and ZrO2 were base catalysts, and MoO3 and TiO2 were acid catalysts. The experimentalresults with the metal oxides can be expressed by the model that was developed under homogeneous systems, with the valuesof the OH− concentrations that were calculated from the correlation about the CH3OH/CO ratio at 15 min of reaction time.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Formaldehyde; Supercritical water; Hydroxyl ion; Metal oxide catalyst; Solid acid and base; CeO2; MoO3; TiO2; ZrO2

1. Introduction

Recently, supercritical water (SCW), that is waterabove its critical temperature (647 K) and pressure(22.1 MPa), has been noted to be a green chemical en-vironment for organic synthesis because many organicreactions occur without any catalyst[1–5]. Ikushimaet al.[1] reported that rearrangements of both pinacol

∗ Corresponding author. Tel.:+81-22-217-7282;fax: +81-22-217-7282.E-mail address: [email protected] (H. Inomata).

and Beckmann, which are normally reactions withstrong acid at ambient temperature, proceeded veryrapidly without any catalyst in SCW, in particular, ataround the critical point of water. Sato et al.[3] andChandler et al.[4,5] also found that a reaction whichrequires a strong acid catalyst, namely Friedel-Craftsalkylation between phenols and alcohols, occurred inSCW without a catalyst. Further, Cannizzaro-type re-action of benzaldehyde, which proceeds with a strongbase normally, was also confirmed to proceed in theabsence of any catalyst by Ikushima et al.[2]. Similarphenomena have been observed for hydrolysis (some

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334 M. Watanabe et al. / Applied Catalysis A: General 245 (2003) 333–341

ethers [6–13] and esters[14,15]) and dehydration[16–20]) in SCW, that is, hydrolysis and dehydrationthat normally need acid or base catalyst occur rapidlywithout catalyst in SCW. These organic synthesis andhydrolysis cases without catalyst provided the pos-sibility that SCW must be an acid or base catalystfor the reactions around the critical temperature andpressure[1,2].

However, if one wanted to enhance and control anacidic and basic reaction in SCW, a catalyst would beuseful and effective. Because water dissociates intoH+ + OH− with an equilibrium constant at a temper-ature and a density, the amounts of H+ and OH− inpure water are the same for all temperatures and pres-sures. Therefore, if one wanted to make an increaseof only H+ concentration or make a huge increase ofH+ concentration in a system, one must add an acid inthe system. Indeed, some reactions, such as dehydra-tion of lower alcohols[17–20]and decarboxylation ofacetic acid[21], requires acid (for example, H2SO4)and base (such as KOH) for the increase of the reac-tion rate at around the critical point of water. Further-more, even for the reaction that can proceed withoutcatalyst, increase and control of the reaction rate canbe achieved by adding an acid and a base catalyst (forexample, the case of hydrolysis of methyl tert-butylether in SCW[13]). For the control of pH in water, ho-mogeneous acids (such as H2SO4) and bases (suchas NaOH and KOH) would be used normally, but theacid and base have a negative impact on the inner wallof a reactor and on the global environment. Thus wemust find an ecological friendly method for changingpH of the system, instead of using homogeneous acidand base catalyst.

One of the possibilities for green chemical controlof pH is to use a solid and base catalyst in SCW.However, there has been very little information aboutwhich sold acid and base catalyst can work in SCW,so far. Recently, we found that ZrO2 is a solid basecatalyst even in SCW through the study with aceticacid [21] and gasification of biomass[22]. Tomitaet al. [23] also reported that hydration of propyleneto 2-propanol, which usually needs an acidic cata-lyst, proceeded with MoO3 catalyst in SCW at aroundthe critical point of water. These results suggest thata metal oxide can work as an acid or a base cata-lyst in SCW, instead of a homogeneous acid or alkali.For selecting a metal oxide to a reaction, the acid-

ity and basicity of a metal oxide in SCW must beknown.

For elucidating acidity and basicity of a metal ox-ide in an atmosphere, an activity test through a re-action such as dehydration and dehydrogenation of2-propanol[24] is employed, for example, because thecontribution of dehydration and dehydrogenation of2-propanol can be controlled for acidity and basicityof a metal oxide[24]. In another study, we reportedthat reaction of HCHO in SCW is governed by the hy-droxide ion concentration[25]. If solid acidity and ba-sicity functioned as same as H+ and OH− in SCW, thereaction of HCHO could be controlled by solid acidand base catalyst. Thus, we can discover the acidityand basicity of the metal oxide through the study oncontrollability of HCHO reaction with a metal oxide.

In this study, we conducted HCHO reaction withand without catalyst using batch reactors. At first, fordeveloping a simple network model for HCHO reac-tion in SCW at 673 K, the effect of concentration ofHCHO and that of OH− on the reaction was examined.Metal oxides were selected in view of hydrothermaland thermal stability[26]; thus CeO2, MoO3, TiO2,and ZrO2 were used in this study. Through the com-parison between the network model and the experi-mental results, we would like to discuss the acidityand basicity of catalysts on the reaction pathway ofHCHO in SCW.

2. Experimental

Paraformaldehyde, which is a linear polymer ofHCHO, was used as a source of HCHO. Paraformalde-hyde, sodium hydroxide (NaOH), and sulfuric acid(H2SO4) were purchased from Wako Pure Chemi-cal and used without further purification. Pure wa-ter was obtained by with a water distillation appara-tus (Yamato Co., model WG-220). Ceria (CeO2) wasobtained from Merck. Titania (TiO2) (both anataseform and rutile form) and MoO3 were purchased fromWako Chemicals. CeO2, MoO3, and TiO2 (anatase andrutile) were used as received. Zirconia (ZrO2) cata-lyst was prepared by calcination of zirconium hydrox-ide (ZrO2·xH2O, which was purchased from NakaraiTesque Inc.), at 673 K for 3 h. The metal oxide cata-lysts that were used in the study are listed inTable 1along with information of structure and BET surface

M. Watanabe et al. / Applied Catalysis A: General 245 (2003) 333–341 335

Table 1Metal oxide catalysts used in the study

BET(m2/g)

Structure beforeHCHO reaction

Structure afterthe reaction

CeO2 1.7 Cubic CubicMoO3 4.9 Orthorombic OrthorombicTiO2(a) 47 Anatase AnataseTiO2(r) 6.8 Rutile RutileZrO2 68 Monoclinic and

tetragonal mixtureMonoclinic

area. Except for ZrO2, the structures of the metal ox-ides did not change from before to after the reaction.For ZrO2, the monoclinic and tetragonal mixture ofZrO2 before the reaction was changed into pure mono-clinic zirconia after the reaction. This structure changeof ZrO2 probably occurred very rapidly and the activ-ity of monoclinic ZrO2 was studied in this study, asin the previous study[27].

The reactions were carried out in a SS 316 stain-less steel tube bomb reactor (inner volume of 6 cm3)[27]. The loaded amounts of paraformaldehyde were0.03–0.3 g (0.15–1.7 mol/l), and the loaded amounts ofwater were from 1.0 (0.17 g/cm3) to 3.1 g (0.52 g/cm3).By changing water density and adding an amount ofNaOH and H2SO4 in the system, we could make theOH− concentration range from about 1.0 × 10−12 to1.0 × 10−4 mol/l at 673 K. In the case of the exper-iments with solid catalysts (CeO2, MoO3, TiO2, andZrO2), 0.3 g of the catalyst was loaded. After sampleswere loaded, Ar gas was pressurized at about 1 MPato recover the gas product easily, after purging air in areactor. The reactor with the samples was submergedin a fluidized sand bath (Takabayashi Rico Co., modelTK-3) whose temperature was controlled to keep thereaction temperature (673 K). After a given reactiontime (3–30 min), the reactor was taken out of the bathand rapidly quenched in a water bath. After the reac-tor reached room temperature, it was connected to asyringe that was equipped with gas samplers to collectproduced gas and measure its volume. After the sam-pling of the produced gases, the reactor was openedand washed with pure water. Water-insoluble material(the solid catalyst) was separated by filtration with amembrane filter (1�m).

The identification and quantification of producedgas was conducted by GC-TCD (Shimadzu, modelGC-7A, and Hitachi, model GC163). An external stan-

dard (1-propanol) was added to the recovered watersolution for GC-FID analysis (Hewlett Packard, model6890). Some samples were analyzed by use of HPLCwith RI and UV detectors (JASCO, model Gulliver se-ries). The amounts of organic and inorganic carbon inthe water solution were evaluated using TOC (total or-ganic carbon detector, Shimadzu, model TOC-5000A).Surface areas of metal oxides were measured by sin-gle point BET method (Quantachrome Instruments,model ChemBET-3000), and are listed inTable 1.Structure of the solid catalysts before and after thereaction were analyzed by X-ray diffractometer (MacScience, model M18XHF-SRA) with Mo K� radia-tion (the results are also listed inTable 1).

Product yield was evaluated from carbon base. Hy-drogen yield was evaluated from hydrogen base of aloaded amount of paraformaldehyde. For all the exper-iments, H2, CO, CO2, CH3OH, HCHO, and HCOOHwere identified and quantified. The yield of HCOOHwas always less than 1 mol% and is not shown in thisstudy.

3. Result and discussion

3.1. Effect of HCHO concentration on HCHOreaction in SCW at 673 K

The effect of HCHO concentration on the reactionwas examined at 673 K, 0.35 g/cm3 of water density(30 MPa of water pressure at 673 K), and 3–30 min ofreaction time.Figs. 1–3show the results at HCHO con-centrations of 0.15, 0.54, and 1.7 mol/l, respectively.

At the lowest HCHO concentration (0.15 mol/l),while the yield of HCHO gradually decreased withtime, the yield of CH3OH, CO, CO2, and H2 increasedwith time, as shown inFig. 1. The yield of CH3OH be-came the highest amongst all the products after 10 minand finally reached 45%. CO2 and H2 were formedsimultaneously. The final value of CO2 and H2 yieldwas about 40%. The yield of CO increased up to 10%with reaction time.

As shown inFig. 2, the trend of the product yield at0.54 mol/l was similar to that at 0.15 mol/l. However,the CH3OH yield at 0.54 mo/l was higher than that at0.15 mol/l and was about 50% at 30 min. The yield ofCO2 was always higher than that of H2. The differ-ences between the yield of CO2 and H2 were always


Fig. 1. Reaction time profile of the product distribution at 673 K,30 MPa, and [HCHO]= 0.15 mol/l.

more than 5%. The yield of CO was always less than10% at 0.54 mol/l of HCHO concentration.

At the highest concentration (1.7 mol/l), the yieldof HCHO rapidly decreased with time. The yields ofCH3OH, CO2, and H2 also increased rapidly with timeand reached a constant value, as shown inFig. 3. Theyield of CH3OH was always more than 50%. The yieldof CO2 was always higher than that of H2, as inFig. 2.The yield of CO2 was about 40% at all the reactiontime, while that of H2 was less than 20%. The yieldof CO was less than 5% at all the reaction times.



3.2. Effect of bulk OH− concentration on HCHOreaction in SCW at 673 K

The details of the effect of bulk OH− concentrationon HCHO reaction in SCW at 673 K can be found inanother study[25]. Here, we describe only the reac-tion results at 15 min of reaction time.Fig. 4 showsthe dependence of the product distribution at 15 minon bulk OH− concentration. Concentration of OH−and ion product of pure water were calculated bythe method of Marshall and Franck[28]. The OH−

Fig. 4. Effect of OH− concentration on the product yields at 673 K,15 min, and [HCHO]= 0.54 mol/l.


concentration with acid and base were obtained fromthe ion product of the water[28], the equilibrium con-stant[29,30] and the activity coefficient[31]. In thisstudy, we consider the experimental data at 1.9 <

−log[OH−] < 10.5. However, the carbon balances athigher OH− concentration (10−1.9 and 10−3.1 mol/l)were about 80 mol% and that at the lowest OH− con-centration inFig. 4(10−10.5 mol/l), namely acidic con-dition, was about 50 mol%. The low carbon balancemeans formation of some by-products other than H2,CO, CO2, HCOOH, and CH3OH. The reaction path-ways for production of the by-products are not takeninto consideration.

As shown inFig. 4, the yield of CH3OH graduallyincreased with increasing OH−; on the other hand, theyield of CO increased with a decrease of OH− concen-tration. Thus HCHO reactions in SCW are governedby the OH− concentration, as reported elsewhere[25].

3.3. Simple network model for HCHO reactionin SCW

Based on the above experimental results, we de-veloped a simple network model for HCHO reactionin supercritical water, as shown inFig. 5. The detailsof the simple network model shown inFig. 5 are de-scribed here.

CH3OH forms via Cannizzaro reaction in super-critical water, as suggested by Bröll et al.[32]. Inthe case of benzaldehyde which has no�-hydrogen,Cannizzaro-type reaction also proceeds in supercrit-ical water without catalyst[2]. Thus the Cannizzaroreaction can be taken into account for the model inSCW at 673 K.

2HCHO+ H2Ok1→CH3OH + HCOOH(Cannizzaro reaction) (1)

Fig. 5. Proposed HCHO reaction pathways in supercritical waterat 673 K.

wherek1 is the rate constant. In the model, the con-centration of water is considered to be constant, be-cause the amount of water (about 20 M) is significantlyhigher than that of the others. For the Cannizzaro reac-tion, we take OH− concentration into account becausethe yield of CH3OH, which is the product from theCannizzaro reaction, was strongly affected by OH−concentration, as shown inFig. 4.

According to the study by Yu and Savage[33],HCOOH decomposed via two pathways: one of themis decarboxylation (Eq. (2)) and the other is dehydra-tion (Eq. (3)):

HCOOHk2→H2 + CO2 (decarboxylation) (2)

HCOOHk3→H2O + CO (dehydration) (3)

Yu and Savage[33] reportedk2 and k3 and weemployed these rate constants in the model. Accord-ing to Yu and Savage, the pre-exponential factors ofk2 and k3 are 108.4 and 1011.6 s−1, respectively, andthe activation energies ofk2 and k3 are 115.3 and171.9 kJ/mol, respectively. At 673 K,k2 is 0.26 andk3is 1.85×10−2 s−1. Thus, under the experimental con-ditions, decarboxylation of HCOOH is predominant.

HCHO decomposes into CO and H2 in gas phase[34–36], and thus the following reaction should betaken into consideration:

HCHOk4→H2 + CO (HCHO decomposition) (4)

In SCW, the water gas shift reaction proceeds[37–39]:

H2O + COk5→H2 + CO2 (water gas shift reaction)

(5)

Sato et al.[39] reported that the pseudo first-orderrate constant for CO disappearance is 103.9 s−1 for thepre-exponential factor and 98.3 kJ/mol for the activa-tion energy. In the model, the value reported by Satoet al. [39] was employed.

As shown in the experimental results (Figs. 1–4),the yield of H2 is almost always lower than that ofCO2. The above reactions show that the yield of H2must be higher than the yield of CO2 (Eqs. (2), (4)and (5)). Tsujino et al.[40] suggested that HCOOHis reacted with HCHO to produce CH3OH and CO2(hydride transfer reaction:Eq. (6)) in hydrothermalcondition.


HCOOH+ HCHOk6→CH3OH + CO2 (hydride transfer reaction) (6)

Through the hydride transfer reaction, HCOOH de-composes into CO2 without forming H2. Thus, this re-action can be a reason why the H2 yield is lower thanthat of CO2. Another possible reaction of the lowerH2 yield has been pointed out by some researchers[41,42]: that is, direct hydrogenation of HCHO intoCH3OH (Eq. (7)).

H2 + HCHOk7→CH3OH (hydrogenation of HCHO)

(7)

However, this reaction can be neglected becausewe verified the H2 did not affect the product distri-bution, through an experiment using H2 instead ofargon gas[25]. Further, the direct hydrogenation ofHCHO by H2 might not proceed in SCW withouta hydrogenation catalyst, such as a noble metal cat-alyst, as suggested by Marrone et al.[42]. There-fore, in this study, the hydrogenation of HCHO isneglected.

The disappearance rate of HCHO is calculated bythe following equation because all the reactions areassumed to depend on the concentration of each com-pound by first-order, except for OH− concentration.Here, the reaction order of OH− concentration for theCannizzaro reaction is set ton.

−d[HCHO]

dt= 2k1[HCHO]2[OH−]n + k4[HCHO]

+ k6[HCHO][HCOOH] (8)

The formation rates of the products (CH3OH,HCOOH, CO2, CO and H2) are described as follows:

d[CH3OH]

dt= k1[HCHO]2[OH−]n


d[HCOOH]

dt= k1[HCHO]2[OH−]n − k2[HCOOH]

− k3[HCOOH] − k6[HCHO][HCOOH]

(10)

d[CO2]

dt= k2[HCOOH] + k5[CO]


d[CO]

dt= k3[HCOOH] + k4[HCHO] − k5[CO] (12)

d[H2]

dt= k2[HCOOH] + k4[HCHO] + k5[CO] (13)

Eqs. (8)–(13)were solved by Euler’s method. Theunknown constants ofn, k1, k4, and k6 were deter-mined by fitting the experimental data. The constantsin the model are listed inTable 2. The calculatedresults are also shown inFigs. 1–4. As shown inFigs. 1–4, the model can express the experimentaldata.

As shown inTable 2, the reaction order for OH−concentration on the Cannizzaro reaction is 0.2, signif-icantly lower than unity. For this lower reaction order,the following reasons can be considered: (i) Canniz-zaro reaction is not an elementary reaction, that is, itis a summary of single reaction steps; (ii) one OH−can cause some reactions of single HCHO molecules,because it is a catalyst, as mentioned above; (iii) anOH− can cause reactions of another species suchas HCOOH or CO; (iv) the lower reaction order isdue to an experimental error, as suggested by Tayloret al. [13]. In a future work, we would like to studythe role of OH− for a reaction in SCW, by meansof microscopic experimental and/or computationaltechniques.

From the next section, we will evaluate an acidityand basicity of a metal oxide for HCHO reaction inSCW. For doing that, it is convenient that an effec-tive OH− concentration to the reaction can be cor-related by a product distribution. As described theabove, HCHO reaction is controlled by OH− concen-tration. Thus, as shown inFig. 4, the yield of CH3OHwas higher than that of CO at higher OH− concen-tration, on the other hand, the formation of CO be-came dominant at lower OH− concentration. Thus theratio of the yield of CH3OH to that of CO can tell

Table 2Reaction order and rate constants of the model at 673 K

n 0.2k1 (l1.2/(mol1.2 s)) 0.3k2 (s−1) 0.26k3 (s−1) 1.85 × 10−2

k4 (s−1) 2.5 × 10−4

k5 (s−1) 1.87 × 10−4

k6 (l/(mol s)) 1.0


us the amount of OH− available for the reaction inthe system. In other words, the CH3OH/CO ratio ata reaction time relates to OH− concentration that canparticipate to the reaction. If acidity and basicity ofmetal oxides can function as same as H+ and OH− inSCW, we must evaluate the acidity and basicity of themetal oxide for HCHO reaction through the experi-mental CH3OH/CO ratio. Using the model, we corre-lated the CH3OH/CO ratio at 15 min of reaction timewith OH− concentration and obtained the followingequation:

CH3OH

CO= 63 exp(0.26 log[OH−]) (14)

Here, we defined OH− concentration calculated bythe above correlation as “effective OH− concentra-tion”.

3.4. Effect of metal oxides on HCHO reaction insupercritical water

We conducted the experiments for examining of theeffect of some metal oxides on HCHO reaction inSCW (673 K and 30 MPa; the loaded amount of allmetal oxide catalysts was 0.3 g). CeO2, MoO3, TiO2(both anatase and rutile), and ZrO2 are used in thisstudy because they are stable even in SCW, from theviewpoints of hydrothermal and thermal stability[26].

The recovered compounds were H2, CO, CO2,CH3OH, unreacted HCHO, and a small amount ofHCOOH with all the metal oxides. As with the re-sults without a solid catalyst, the yield of HCOOHis not shown here because it was less than 1 mol%.The yield of CH3OH and CO depended on the kindof a metal oxide as well as on OH− concentration inthe absence of a metal oxide catalyst. For evaluatingthe acidity and basicity on the metal oxide catalysts(CeO2, MoO3, TiO2 (both anatase and rutile), andZrO2), we employed the CH3OH/CO ratio with thecatalyst. In the absence of a solid catalyst at 673 Kand 30 MPa (0.35 g/cm3 of water density, the bulkOH− concentration is 10−8 mol/l [28,31]); thus theCH3OH/CO ratio is calculated as to be 7.9 by the cor-relation (Eq. (14)). In the presence of CeO2 and ZrO2,the CH3OH/CO ratios were higher than 7.9. On theother hand, the CH3OH/CO ratios were lower than7.9 with MoO3 and TiO2 (both anatase and rutile).

Table 3Effective OH− concentration for HCHO reaction at 673 K and0.35 g/cm3 of water density

CH3OH/CO Log ([OH−1]) (mol/l)

CeO2 16 −5MoO3 3.7 −11TiO2(a) 1.7 −14TiO2(r) 2.8 −12ZrO2 8 −8

Thus, it was suggested that CeO2 and ZrO2 are basicand MoO3 and TiO2 (both anatase and rutile) areacidic catalysts for HCHO reaction in SCW at 673 K.Using the CH3OH/CO ratios with the solid catalysts,we calculated the effective OH− concentration byuse of the correlation (Eq. (14)). The calculated OH−concentration (effective OH− concentration) is shownin Table 3with the CH3OH/CO ratio.

Fig. 6 shows the plot of the product yields againstthe effective OH− concentration (Table 3) with themetal oxides. In this figure, the results calculated withthe simple network model are also shown. The trendsof the increase of CH3OH yield and the decrease ofCO yield with increasing the effective OH− concentra-tion are the same for the experimental results and forthe model. However, the discrepancy of HCHO yieldbetween the experimental results and the model at

Fig. 6. Relationship between the effective OH− concentration andthe product yields at 673 K, 15 min, 0.35 g/cm3 of water density,and [HCHO]= 0.54 mol/l.


the lower OH− concentration (MoO3 and TiO2 (bothanatase and rutile)) is very huge. The discrepancy mayindicate the presence of another reaction pathway thatconsumes HCHO. At this moment, we consider onlythe Cannizzaro reaction (Eq. (1)) and the HCHO de-composition (Eq. (4)) as primary reaction pathways ofHCHO reaction in SCW (at 673 K and 0.35 g/cm3 ofwater density).

Next, we would like to consider the reason whyCeO2 and ZrO2 are basic and MoO3 and TiO2 areacidic catalysts for HCHO reaction in SCW. The acid-ity or basicity of a metal oxide is related to the na-ture of the metal ion such as the electronegativity[43].The generalized electronegativity of a metal ion (χi )is calculated by the following equation[43]:

χi = (1 + 2Z)χ0 (15)

whereZ is the charge of a metal ion andχ0 the elec-tronegativity of a neutral atom (Z = 0). In this study,we employed the value of electronegativity of neu-tral atom given by Sanderson[44]. Fig. 7 shows therelationship between the effective OH− concentrationto HCHO reaction and the electronegativity of themetal ion of the metal oxides. One of the reasons forthe difference of OH− concentration on rutile TiO2and anatase TiO2 may be the difference of the sur-face area (Table 1). The effective OH− concentrationdecreases to a minimum and then slightly increaseswith increasing the electronegativity of the metal ion.This trend is similar to the relationship between theheat of hydration of metal oxides and the electroneg-ativity of metal ion [43]. Further, Shimizu reported

Fig. 7. Relationship between electronegativity of metal ion andthe effective OH− concentration.

that the surface of metal oxide has hydroxyl groups(base site) and oxy-acid groups (acid site) due to thedissociative adsorption of water[45]. According toShimizu [45], hydroxyl groups exist on a metal ox-ide surface up to about 13 of electronegativity of themetal ion, while oxy-acid group exist over 13. Then,on CeO2 and ZrO2, hydroxyl group (base site) is as-sumed to be rich, on the other hand, the oxy-acidgroup (acid site) is assumed to be rich on MoO3 andTiO2. Recently, Busca[46] summarized the relation-ship between acidity and basicity of metal oxide cat-alyst and the properties of metal ion. The results sug-gested that high acidity on a metal oxide surface ap-pears when the radius of a metal ion is small and itselectronegativity is large. On the contrary, strong ba-sicity appears when the radius of a meal ion is largeand its electronegativity is small. Such insistence byBusca[46] is basically the same as that by Shimizu[45].

In this study, a large amount of water always existedin the system with the metal oxide catalysts. Thus weconsider that the acidity and basicity of metal oxidecatalysts strongly depends on the interaction betweenwater and the metal oxide. As suggested by Shimizu[45] and Busca[46], acidity and basicity of the metaloxide catalysts is found to be related to the electroneg-ativity of the metal ion in this study. Consequently, atthis moment, we consider that water is activated as hy-droxyl and/or oxy-acid group on the metal oxide sur-face and then acidity and basicity for HCHO reactionappeares in SCW at 673 K and 0.35 g/cm3 of waterdensity.

4. Conclusion

The experiments of HCHO reaction in SCW (673 K)were conducted by use of batch type reactors. HCHOwas mainly converted into CH3OH through a Can-nizzaro reaction at a high OH− concentration (basiccondition), but into CO through self-decomposition ata low OH− concentration (acidic condition). We de-veloped a simple network model for HCHO reactionin SCW and can express the experimental data by themodel.

With MoO3 and TiO2 (both anatase and rutile), COformation was enhanced due to their acidity. On theother hand, in the presence of CeO2 and ZrO2, the


Cannizzaro reaction was promoted due to basicity ontheir surfaces. This trend can be explained by elec-tronegativity of metal ion. This means that the acid-ity and basicity on a metal oxide surface in SCW areprobably due to dissociative absorption of water onthe surface.

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acidity and basicity of metal oxide catalysts for formaldehyde reaction in supercritical water at...

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