−論文−●A new approach to the preparation of MoTeVNb mixed oxide catalysts
for the oxidation of propane to acrylic acid
高分子材料研究所 モノマー・グループ
屠 新林 古田 尚正 住田 勇一 高橋 衛 新妻 裕志

工業触媒は一般的に原料成分の溶液から出発し、共沈を経て作られるが、適切な水溶性原料が存在しない場合、非水溶性原料の成分間の酸化還元反応（Redox均一化法）を利用し、均一化させる方法も知られている。
著者らは後者の方法を利用して高難度触媒反応のひとつとして知られるプロパンからアクリル酸への選択酸化に有望なMoSbVNbOx系触媒系を初めて見出した。一方、前記の酸化反応にもうひとつ有効な触媒系としてMoTeVNbOx系が知られ、溶液ルートで作られる。著者らはRedox均一化法が適切な水溶性原料が存在しない場合の次善策でなく、複雑な触媒系に最も有効であり、かつ汎用性のある方法であることを信じ、
MoTeVNbOx系にも適用できると考えた。
そこでMoTeVNb系溶液ルートにおいてTeの原料として使われるテルル酸の代わりに金属Teのサブミクロンの微粒子を用い、Mo、V成分間の酸化還元反応を利用し、均一化させた後、さらに電荷バランスや結晶相の形成条件を調整した結果、溶液ルートを大幅に凌ぐ性能を得ることに至った。本論文では、MoTeVNbOX系触媒の調製におけるRedox均一化法適用の結果と触媒の物性特徴などを記述するとともに、Redox均一化法が有効である原因について考察を加えた。

1. Introduction

therefore provides a diversity of intermediates to fit theformation of active phase 16）.

The one-step selective oxidation of propane into acrylic acidhas attracted widespread interest among researchers in bothacademic and industrial domains

Comparing with the antimony-based system, tellurium-1-16）. The most effective

based catalysts have been prepared with a soluble precursor

catalyst system for this reaction is the MoTeVNb mixed oxide

except for hydrothermal synthesis 4-5）. One motive of thiscatalyst, as first patented by Mitsubishi Chemicals 1-2）. A yieldstudy is therefore to investigate whether the non-solutionof 48.5% with 60.5% acrylic acid selectivity has been claimed

approach used in the Sb-based system is applicable tounder a reaction temperature of 380℃ and a steam/propane

tellurium-based catalysts and promotes their catalytic

ratio of 14. The high steam ratio condition is crucial to obtainperformance. For this purpose, a slurry method using finea high selectivity of acrylic acid, but is impractical due to themetallic tellurium sources and adding substances to generatecosts of downstream operations. Thus, a high performance byporosity has been designed and examined over the reaction of

the catalyst while operating under a low steam/propane ratio

direct oxidation of propane to acrylic acid.
This paper describes the performance of the catalysts andis desirable for the industrial application of this catalystdiscusses the role of each additive and the feature of thesystem.

catalysts.

As another promising catalyst system for the same reaction,
the authors revealed the effectiveness of a MoSbVNb mixed
catalyst using a slurry method 8）. For this catalyst system, the2. Experimental

adoption of a non-solution precursor through a heat treatment

2.1. Tellurium sourcesof a mixture of antimony trioxide, ammonium heptamolybdate

Fine metallic tellurium was synthesized by the heat（（NH4）
6MoO7O24-4H2O）and ammonium metavanadate（NH4VO3）

treatment of aqueous slurry composed of TeO2（MITSUWA8-9）

or by astepwise addition of ammonium heptamolybdate

8-11,14,16）

Chemicals, 80wt%）as reducing agent and distilled water at 80

provided optimal catalytic performance of catalyst. The non-℃ while stirring for 4h followed by rinsing of the remaining

solution precursor allows the existence of various species inhydrazine hydrate with distilled water 17）. For comparison,different valence states （in a non-equilibrium state） andmetallic tellurium（MITSUWA PURE CHEMICALS, 99.9%）,

followed by adding an oxidant to the slurry東亞合成グループ研究年報

PURE CHEMICALS, 99.9%）, hydrazine hydrate （WACO14TREND 2007 第10号

ZSM 100e instrument.

ball-mill ground metallic tellurium and telluric acid（MITSUWA
PURE CHEMICALS, 99.9%）were also used in the preparation

2.4. Catalyst evaluationof catalysts.

The catalyst evaluations were performed using a fixed bed
2.2. Catalyst preparation

quartz tubular reactor （i.d. 8 mm）. The catalysts, once

The MoTeVNb catalysts, using fine metallic tellurium andpelletted and crushed into granules of 15-30 mesh, wereother metallic tellurium sources, were prepared following apacked into the reactor for evaluation. The reactions were17）. An aqueous solution of ammonium

carried out under 1 atm of pressure, a reaction temperature ofheptamolybdate （WAKO CHEMICALS, 99%）, ammonium

380℃, and a feed mixture with a molar ratio ofmetavanadate（WAKO CHEMICALS, 99%）and an aqueous

propane/air/steam=1/9.0/6.5 and a SV of 2400 h-1. Reactantssuspension of tellurium source were mixed and heated for theand products were analyzed using gas chromatography. The

required time, depending on the tellurium sources asconversion, selectivity and yield expressed in molar

described below. Blue slurry was formed following thepercentage form were calculated on a propane base.

slurry methodoxidation and reduction reactions among the Te, Mo and V
sources. Co-precipitation between an aqueous solution of

3. Results and discussion

niobium oxalate, prepared by heating an aqueous slurry of

3.1. Effect of variety of tellurium sources

Nb2O5・xH2O（CBMM, assay 80%）and oxalic acid（WACOFig. 1 shows SEM image of the tellurium sources used inthe preparation of catalysts.

CHEMICALS, 99.9wt%）with a molar ratio of oxalic acid to Nb
3.5, and the above blue slurry was found to occur when they
were mixed by stirring. Ammonium nitrate with a molar ratioof NH4NO3 to Mo 0.37 was added to samples Cat A, B, C,respectively, after mixing the niobium oxalate solution withthe slurry of the Mo, V, and Te sources. For samples from Cat
1 to 10, an aqueous ammonia and ammonium nitrate wereadded in amounts described in Table 2, respectively, before
and after mixing the niobium oxalate solution with the slurryof the Mo, V, and Te sources.
The slurry obtained was then evaporated to dryness at 120℃ for 3h followed by a pre-calcination at a temperature under300℃ for 1.5 h in a crucible and calcination under 590℃ for 1h
within a stainless steel tube which was sealed with small
amount of quartz wool and active carbon.
The MoTeVNb catalysts using telluric acid（Cat 11）wereprepared according to reference 2）.
2.3. Catalyst characterization
SEM images were measured with a JEOL JSM-6330F fieldemission scanning electron microscope. The powder X-raydiffraction（XRD）was measured via a continuous scanning（3°/min） method; using a Rigaku RINT 2000 automatic
diffractometer with Cu Kα radiation. Metallic Fe, whichshowed a major diffraction peak with 2θat 44.7°, was added asa reference substance to evaluate the crystallinity of thecatalyst. The BET surface area was determined with aHORIBA SA-6200 Series unit, while elemental analysis wasperformed through a wave dispersive XRF using a Rigaku

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The metallic tellurium used is composed of agglomerates
with size-distribution from 5-20μm. Ball-mill grinding in thepresence of hydrogen peroxide（H2O2/Te=0.7）crushed theagglomerates to their primary particle size of 0.5-2.0μm. On
the other hand, the reduction of TeO2 with hydrazine hydrate（hydrazine/Te=2.0）revealed a fiber-like shape with a uniform
section diameter of ca. 100 nm, and lengths from 400 to1000nm.

Fig. 2 shows the XRD patterns of the as-dried（at 50℃）
samples corresponding to Fig. 1. All the samples showed a setof diffraction lines with 2θ at 23.0°, 27.6°, 38.3°, 40.4°, 43.3°,45.9°, 49.6°which are consistent with hexagonal tellurium（JCPDS Card No. 36-1452）. Minor diffraction lines with 2θ at
26.2°, 29.9°, 48.6°, due to TeO2 impurity（JCPDS Card No.421365） were observed on samples derived from metallic
tellurium. No obvious diffraction lines owing to TeO2 phases
were detected for reduced tellurium indicating that thereduction of TeO2 had totally occurred.
Fig. 3 shows the XRD patterns of the as-dried catalyst
prepared with different tellurium sources. The patterns
indicate basically an amorphous property of the samples atthis stage. A weak diffraction line with 2θat 27.6°assigned tohexagonal tellurium can be seen for all the samples indicatingthat tellurium source partly remained as crystalline phaseeven after drying at 120℃ for 3 hours. Other minor impurity
phases with 2θ at 8.2°and 29.1°in the case of metallic
tellurium and ground tellurium are most likely due tohydrates of Mo and V compound.
Fig. 4 shows the XRD patterns of the as-calcined catalysts.
All the catalysts showedmajor diffraction lines with 2θat 22.1°,26.1°, 26.6°, 28.2°assigned to a hexagonal phase M2, and 2θat6.6°, 7.8°
, 22.2°, 27.2°assigned to an orthorhombic phase M1 2）.
The ratios of phase M1 to phase M2 as expressed by the ratioof intensity of line with 2θat 27.2°to that at 28°varied
depending on the tellurium source. The ratio is 0.06 for that
prepared with metallic tellurium, increasing to 0.35 and 1.73 forthat with ball-mill ground and reduced tellurium respectively.

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The catalyst prepared with metallic tellurium showed extra

a further improvement in the catalytic performance wasdiffraction peaks with 2θat 14.0°, 24.9°which is assigned to theobtained. Among them, an increase in the amount ofMoO3 phase （JCPDS Card No. 5-508）; indicating phaseammonium nitrate combined with the addition of aqueous

separation of Mo with the Nb component. The catalyst withammonia and adjustment of the elemental composition wereball-mill ground tellurium and tellurium reduced from TeO2

found to be most effective. The catalytic performance andshowed almost no diffraction lines due to this phase.

change in the physicochemical properties were summarized

The performance of the catalysts was summarized in Table

in Table 2. A 48% yield with 76% selectivity of acrylic acid was

1. In the case involving use of the metallic tellurium source,

obtained under optimal conditions. These included thethe performance depended on the duration of heat treatment

addition of ammonium nitrate and aqueous ammonia in aof metallic tellurium with Mo and V sources. The optimal

molar ratio of 0.52 to Mo component, the addition of aqueous

heating time was 12 hours, but the yield of acrylic acid was asammonia before the Mo, V, Te slurry was mixed with niobium

low as 3.8%. Ball-mill grinding of metallic tellurium shortenedoxalatethe time to one hour and increased the yield to 23.9%, while

MoTe0.14V0.26Nb0.16 O3.87. As shown in Table 2, with thethe catalyst with reduced tellurium exhibited a 37% yieldchange of the amount of ammonium nitrate and aqueous

without heat treatment. These results indicate a positive effectammonia, the BET. specific surface area, the ratio of XRD

andusing

anelemental

compositionofof a small tellurium particle size and negative effects ofpeak intensity of 27.2°to 28.2 °of 2θ, the crystallinity in term

reactions among Mo, V, Te sources. During the heatof the relative diffraction intensity of 22.2°of 2θto that oftreatment process, the occurrence of oxidation and reductionreference substance of Fe（44.7°of 2θ）, and the averagereactions among Te, Mo

6+and V5+

valence state number of Mo, V, Te and Nb components varied

species trigger thesuggesting a very complicated phenomenon.

dispersion of tellurium and the formation of a blue slurry;Tocontaining reductive Mo compound generally called15）

understandtherelationshipbetweenthe. The dispersion of tellurium is desirable,

physicochemical properties and the catalytic performance of

while the presence of Molybdenum blue, which tends tothe catalyst, correlations between them were examined. As

molybdenum blue15）, is unfavorable forshown in Fig 5, excellent linear relationships between thethe incorporation of Nb via precipitation. The XRD data forconversion of propane and the ratio of XRD peak intensity ofthe presence of the MoO3 phase when using metallic

27.2°to 28.2 °of 2θ, the conversion of propane per BET.

tellurium supported the above assumption. By comparing thespecific surface area and the ratio of XRD peak intensity of

XRD pattern, the performance of the catalyst can be seen to27.2°to 28.2 °of 2θwere obtained indicating a dominating and

be closely connected with the ratio of M1 to M2; suggesting

independent effect of phase constitution and a positive effectthat fine metallic tellurium improves catalytic performance byof surface area on the performance of catalyst. On the otherenhancing the formation of phase M1.

hand, relatively poorcondense into large polyoxometalates

correlations of conversion to otherproperties suggest their indirect relationship with the activity.
3.2. Effect of the addition of ammonium nitrate
and aqueous ammonia
By optimizing parameters for the preparation of the catalyst,

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reactions at temperatures exceeding 260℃. Therefore, in thiscase, Ammonium nitrate possibly undergoes a rapiddecomposition leading to the porous texture of the precursor.
The porosity inhibits the sintering of the formed crystalline
leading to a low extent of agglomeration and a relatively highsurface area. As for the effect of the aqueous ammonia, itprabobly neutralizes the oxalic acid in slurry and thus preventthe ammonium nitrate from hydrolysis.
The manner by which ammonium nitrate and aqueous
ammonia affect the phase constitution, however, remains
unclear. One assumption is that the porous texture over alarge surface area of the precursor, generated by the additionFig. 6 shows SEM images of the as-precalcinated and as-of ammonium nitrate, may affect the crystallization process by

calcined catalysts prepared with and without the addition ofincreasing diffusion resistance. Since phase M2 crystallizes

ammonium nitrate and aqueous ammonia respectively.

more slowly than M1 20）, the diffusion resistance may favor

Without the addition, although the precursor contains somethe formation of phase M1.

pore structure, it is a dense solid on the whole.
3.3. The comparison of catalysts prepared

Corresponding to the precursor, the as-calcined catalyst
exhibited an irregular shape and a particles subject towith telluric acid and fine metallic tellurium

considerable agglomeration. The addition of ammonium

Table 3 summarizes the catalytic performance and some

nitrate and aqueous ammonia prompted the appearance ofphysicochemical properties of catalysts prepared with telluric

small submicroscopic pores, generating the continously

acid and fine metallic tellurium（reduced tellurium）. Those

porous texture of the precursor. Ammonium nitrate is known

prepared with telluric acid gave a 22.1% yield of acrylic acid,to undergo explosive decomposition due to inner-molecular

lower than a half of that using the reduced tellurium. Adding

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catalysts.

ammonium nitrate and aqueous ammonia and adjustment of
the elemental composition did not improve the catalytic

In addition, one drawback concerning the use of solubleperformance. Therefore, the above results indicate that thetelluric acid for the preparation of the catalyst has been

non-solution approach; using fine metallic tellurium combined

pointed out. Oliver and co-workers reported that telluric acidwith the addition of ammonium nitrate and aqueous ammonia,tends to form an Anderson type molybdotellurate with Mo

is superior to that using soluble telluric acid.

source at a PH of above 4.5, which negatively affects

The above result appears contradictory to the general

performance 19）. The low solubility of metallic tellurium

principle of the necessity of homogeneity for the catalyst

probably minimizes the formation of impurities between theprecursor. However, the low melting point of metallic

tellurium and Mo components and favors the incorporation oftellurium（near 460℃ ）and a high vapor pressure of TeO2 are

the niobium component. This assumption is consistent withunique. It is known that atoms or ions are mobile for bulk-to-

the high ratio of Nb to Te of the optimal catalyst using finesurface migration when the temperature is raised tometallic tellurium comparing with that using telluric acid.

[18]

Tammann temperature（ca. half the melting point） . Thus a

Although the details remain under investigation, the non-diffusion process of the tellurium species accompanying thesolution property of precursors derived from fine metallic

solid state reactions in oxidation and reduction mode among

tellurium would be important for the catalytic performance.

the Mo, V and Te species possibly occurs before and/or

4. Conclusions

during the pre-calcined steps at 300℃. The sublimation of thetellurium species at calcination step under a temperature ofThis work demonstrated the effectiveness of a non-solution

590℃ will aid its incorporation into the cryastalline phase ofprecursor prepared with fine metallic tellurium for the東亞合成グループ研究年報

19TREND 2007 第10号

19）J.M.Oliver, J.M.Lopez Nieto, P.Botella, and A.Mifsud,preparation of MoTeVNb mixed oxide catalyst aimed at theselective oxidation of propane to acrylic acid. The combined

Appl.Catal. A 257, 67（2004）.

use of fine metallic tellurium with the addition of porosity20）Unpublished data of the authors.

generating substances such as ammonium nitrate andaqueous ammonia allows the enrichment of the orthorhombic
phase M1 on a fixed elemental composition of mixed oxide. It
also provides uniform particles of crystalline with a low levelof agglomeration and a high surface area. The mixed oxide
catalyst obtained exhibited a 48% acrylic acid yield（63.4%conversion and 75.6% selectivity） under a reaction
temperature of 380℃ and a low steam/propane ratio of 6.5.

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