Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 83, No. 1, 129-136and Kluwer Academic Publishers, Dordrecht (2004)

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RKCL4473

SYNTHESIS OF o-PHENYLPHENOL FROM CYCLOHEXANONEOVER PLATINUM SUPPORTED ON CALCINED Mg/Al

HYDROTALCITE

Piaoping Yang, Zhenlü Wang, JianFeng Yu, Qingsheng Liuand Tonghao Wu*

College of Chemistry, Jilin University, Changchun, 130023, P. R. China

Received December 15, 2003In revised form March 5, 2004

Accepted March 12, 2004

Abstract

Mg/Al mixed oxides used as supports for platinum catalysts with Mg/Al molarratios of 2-6 were obtained by thermal decomposition method. The effects ofcompositions and catalytic properties were studied by TPD, BET, XRD and TG-DTA techniques. Synthesis of o-phenylphenol (OPP) from Dimer wasinvestigated over Pt/CHT catalysts and compared with Pt/Al2O3. These catalystsshowed a conversion of 93.8% and a selectivity of 87.9%.

Keywords: Hydrotalcite, OPP, Dimer dehydrogenation, Mg/Al ratios

INTRODUCTION

o-Phenylphenol (OPP) is one of the most important fine chemical industrialproducts, widely used in the fields of dyeing, antiseptics, surface activator andthermal stabilizer, etc. OPP is generally prepared by a two-step process fromcyclohexanone. First, cyclohexanone is condensed by acid catalysts in liquidphase to form 2-(1-cyclohexenyl) cyclohexanone (Dimer). Second, Dimer isremoved from the reaction mixture and then dehydrogenated in the presence ofa suitable dehydrogenation catalyst to yield the OPP.

______________________∗Corresponding author. E-mail: yangpiaoping@sina.com.cn

130 PIAOPING YANG et al.: HYDROTALCITE

Layered double hydroxides are anionic clays in which divalent cationswithin brucite-like layers are replaced by trivalent cations. The resultingpositive charge is compensated by hydrated anions located in the interlayerspace between two brucite sheets [1-4]. Because of their high surface area,phase purity, basic surface properties, and structural stability [1, 2],hydrotalcite-derived mixed oxides are widely used as antacids, ion exchangers,absorbents, catalysts and catalyst supports. To our knowledge, there is no information available using calcinedhydrotalcites as supports for the dehydrogenation to OPP. In this study, wepresent the preparation and characterization of hydrotalcites, calcinedhydrotalcites, Pt/CHTx catalysts, as well as their catalytic properties for OPPsynthesis.

EXPERIMENTAL

Catalyst preparation

Hydrotalcite

Hydrotalcite (HT) was prepared by the method described by Reichle et al.[5]. Solution A was prepared by dissolving 125 g Mg(NO3)2·6H2O (0.5 mol)and 94 g Al(NO3)3·9H2O (0.25 mol) in 350 mL distilled water. Solution B wasobtained by blending 140 g of 50% aqueous NaOH (1.75 mol) and 50 g ofanhydrous Na2CO3 (0.472 mol) in 50 mL distilled water. At 313 K, solution Awas slowly added into B within about 3 h under vigorous stirring. After that theslurry was kept at 308 K for about 18 h under good stirring and then cooled andwashed thoroughly with distilled water until the pH was neutral. The whitecrystalline material was dried in an oven at 353 K for 24 h. This is termed asHT2. HT2 was then calcined in air at 823 K for 18 h to obtain thecorresponding Mg/Al mixed oxides (CHT2). HT3, HT4 and HT6 were preparedfollowing the same procedure as the HT2.

Pt/CHTx and Pt/Al2O3

All CHT powers were impregnated with an aqueous solution ofH2PtCl6·6H2O for 12 h, filtered, and then dried at 353 K under vacuum. Aftercalcined in air flow at 673 K for 2 h, the powers were reduced in H2 flow at 673K for 2 h (flow rate: 16.7 cm3 min-1, ramp: 0.5 K min-1). These catalysts weredesignated as Pt/CHTx. The preparation of Pt/Al2O3 catalyst was described in

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detail by previous reference [6], and its principle was basically the same as forPt/CHTx.

Catalysts characterization

BET surface areas of the samples were obtained in an ASAP 2010apparatus. The elements compositions of the samples were determined by ICP-PLASMA 1000. X-ray diffractions (XRD) were recorded in a Shimadzu RD-6000 diffractometer operated at 40 kV and 30 mA, using CuK� ��������� �

0.1542 nm). TG-DTA was carried out in a Shimadzu instrument at a heatingrate of 5 K min-1 from room temperature to 1073 K in air. Temperature-programmed desorption of CO2 (TPD-CO2) was carried out with a self-designed apparatus.

Reaction studies

2-(1-cyclohexenyl)cyclohexanone (Dimer) was prepared fromcyclohexanone by adol condensation reaction. The Dimer dehydrogenation wascarried out in a fixed bed tubular reactor at atmospheric pressure. Reaction runswere performed on 20~40 mesh catalysts. The Dimer was introduced mixedwith hydrogen at a constant rate using a syringe pump into the upper zone ofthe reactor packed with SiO2 pellets maintaining the reaction temperature forpreheating and vaporization. Before reaction, the catalysts were reduced in situin flowing hydrogen at 673 K for 3 h and then cooled down to reactiontemperature. The exit stream from the reactor was collected for analysis usinggas chromatography (Shimadzu GC-14B).

RESULT AND DISCUSSION

Characterization of the catalysts

Influence of the chemical composition on the structure and properties of theHTx and CHTx

The structural formulas given in Table 1 were established from the chemicalanalysis of the samples. The values of c and a parameters were calculated from(003) and (110) XRD peaks. The surface areas and porosities are presented inTable 2.

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Table 1

Chemical composition and crystallographic a and c parameters of different Mg/Al hydrotalcitessamples

Samples Composition c (Å) a (Å)

HT2 Mg0.684Al0.312(OH)2(CO3)0.17,0.85H2O 22.849 3.048HT3 Mg0.748Al0.259(OH)2(CO3)0.130,0.73H2O 23.323 3.061HT4 Mg0.756Al0.191(OH)2(CO3)0.121,0.75H2O 23.773 3.076HT6 Mg0.787Al0.133(OH)2(CO3)0.118,0.76H2O 24.236 3.090

Table 2

Surface properties of the catalysts

Samples Mg/Al Surface area Pore diameter range Pore volume(molar ratios) (m2/g) (nm) (cm3/g)

CHT2 2 265 20-27 0.832CHT3 3 238 15-35 0.653CHT4 4 221 16-35 0.645CHT6 6 178 14-33 0.637

Table 3

The effect of Mg/Al ratio on the basicities of CHTx

Sample Mg/Al Amount adsorbed at RT (mL/g)

MgO 10.5CHT6 6 5.56CHT4 4 6.33CHT3 3 7.32CHT2 2 5.81

The surface areas of these samples calcined at 823 K shows a decreasingtrend with increment of Mg/Al ratios. When the molar ratios are equal to or lessthan 4, the surface areas of calcined hydrotacites (CHTs) are greater than 200m2/g. However, it decreases sharply as the ratio of Mg/Al increases beyond 4.The results are consistent with the literature [7]. The effect of the Mg/Al ratios on the basicity of calcined Mg/Alhydrotalcites is also presented in Table 3. The basic site densities were

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measured by TPD of CO2. From Table 3, the basic site density of MgOdecreased when small amounts of Al were added, then it increases with Mg/Alratio, reaching a maximum at Mg/Al = 3. These results are in agreement withthe literatures [8, 9].

0 10 20 30 40 50 60 70 80 90

HT2

HT3

HT4

HT6

Inte

nsity

2θ

Fig. 1. XRD patterns of uncalcined hydrotalcites (HTx)

XRD analysis

Typical XRD patterns of some uncalcined HT samples are shown in Fig. 1.A well-crystallized HT structure is present in all the samples, exhibiting sharpand symmetrical reflections. The difference in intensities from one sample toanother indicates different degrees of crystallinity. The crystallinity of thematerial as evidenced by the sharpness of (003) and (006) reflections indicatesthat HT3 is highly crystalline. The peak intensity is low though sharp for HT6and HT2. No shift in the base line is observed ruling out the possibility of thepresence of any amorphous phase in the samples having high Mg2+ or Al3+

contents. The values of the unit cell parameters, with the c parametercorresponding to three times the thickness of the expanded brucite-like layerand the a corresponding to two times of d110 (Table 1) decrease with increasingaluminium content and which can be explained by the substitution of layerMg2+ ions by smaller Al3+ ion [10]. The parameters a and c calculated for ahexagonal cell match well with the literature [11].

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10 20 30 40 50 60 70 80

823K

723 K

623 K

uncalcined

In

tens

ity

2θ

Fig. 2. XRD patterns of HT3 calcined with different temperature

Figure 2 shows the XRD patterns of calcined HT at different temperatures.Diffraction patterns of CHT samples show no residual hydrotalcite orhydroxide phase, confirming that heating at 823 K leads to completedecomposition of HT samples. A poorly crystalline MgO phase was detected inall CHT samples, but no crystalline AlOy phase was observed. This suggeststhat Al3+ cations remain closely associated with the MgO structure afterdecomposition of HT samples within the entire composition range of our study.

TG and DTA analysis

TGA-DG data for HT are shown in Table 4. These catalysts showed twostages of weight loss that is characteristic of HT. Each weight loss isaccompanied by an endothermal transformation. With an increase in the valueof Mg/Al ratio, the relative intensity of the lower temperature endothermic peakbetween 476 and 523 K, corresponding to the loss of interlayer watermolecules, decreases and shifts towards lower temperature (for HT2 Tmax = 521K and for HT6 Tmax = 476 K, respectively). This indicates that the quantity ofinterlayer water and the strength with which it is bound to the carbonate anions

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and the hydroxyl sheet increase with the increases of aluminium content insamples. The second weight loss is observed between 679 and 688 K and isattributed to the removal of condensed water molecules and carbon dioxidefrom the carbonate anion present in brucite layer. The second peak usuallyappears broad because of the simultaneous loss of water and carbon dioxide.From the literature [12], it is noticed that initially CO2 is lost, followed by theloss of water molecules in brucite sheet.

Table 4

Thermal analysis results

Sample First weight Temperature Second weight Temperatureloss (%) (K) loss (%) (K)

HT2 18.18 521 26.23 688HT3 16.46 496 26.31 683HT4 16.34 488 26.22 678HT6 16.45 476 26.49 679

Catalytic tests

The catalytic results for OPP synthesis over 0.5% Pt/CHTx catalysts aregiven in Table 5, and they were compared with the catalysts of Pt supported onacidic Al2O3.

Table 5

Effects of Mg/Al molar ratio on OPP synthesis, for 0.5%Pt/CHTxa catalysts.Reactions conditions: T = 623 K, p = 1 atm, LHSV = 0.33, and H2/Dimer = 300

x

2 3 4 6 Al2O3

Conversion (%) 91.5 93.8 85.6 84.7 82.0

Selectivity (%)OPP 85.0 87.9 80.0 75.3 15.4o-Cyclohexylphenol 3.0 1.7 5.5 9.6 19Biphenyl 4.6 2.2 6.9 7.2 19.4Phenol 3.3 3.2 3.8 4.3 39Phenylcyclohexane 1.6 1.8 1.9 2.1 2.6C11

b 1.3 1.9 1.5 1.4 0.8Unknown 1.2 1.3 0.4 0.1 3.8

a x: Mg/Al molar ratiosb Compounds with molecular weights of 146 and 148

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Under these conditions, OPP and OCHP are the main products in the catalyticdehydrogenation of Dimer. Pt/Al2O3 shows that lowest conversion of Dimerand selectivity for OPP. The phenomena are attributed to the acidity of aluminaby Neri [10]. From Table 2 it can be seen that the BET surface area of CHT3 issmaller than CHT2, but CH3 shows the highest basicity (Table 3), and thePt/CHT3 catalyst shows the highest conversion and selectivity. These resultsallude that the basicities of supports play an important role in affecting theactivities of the dehydrogenation reaction as well as the BET surface areas.

CONCLUSION

More than the surface area, the brucite structure and the basicity wereresponsible for the activity of dehydrogenation. The densities of surface basicsites on Mg/Al mixed oxides depended on the compositions. CHT3, which hadlarge surface area and basicity catalyst, showed good Dimer dehydrogenationactivity with 93.8% conversion and 87.9% selectivity for OPP. It could be usedas a good support for platinum catalyst.

REFERENCES

1. A. Schutz, P. Bileon: J. Solid State Chemistry, 68, 360 (1987).2. F. Cavani, F. Triffiro, A. Vaccari: Catal. Today, 11, 173 (1991).3. A. Sugier, E. Freund: U.S. Patent 4,112,110, Institut Français du Pétrole, 1978.4. J.J. Alcaraz, B.J. Arena, R. Gillispie, J.S. Holmgren, in Proceedings, 15th Meeting of the

North American Catalysis Society. Chicago, 1997, p.14.5. W.T. Reichle: J. Catal., 94, 547 (1985).6. B. Rebours, J.B. d’Espinose de la Caillerie, O. Clause: J. Am. Chem. Soc., 116, 1707

(1994).7. Y.Z. Chen, C.W. Liaw, L.I. Lee: Applied Catal. A: 177, 1 (1999).8. C.T. Fishel, R. Davis: Langmuir, 10, 159 (1994).9. T. Kanno, M. Kobayashi, in “Acid-Base catalysts II “(M. Misono and Y. Ono, Eds.),

p. 207. Kodansha-Elsevier, Tokyo 1994.10. G. Neri, A.M. Viso, A. Donato, C. Milone, M. Malentacchi, G. Gubitosa: Appl. Catal. A ,

110, 830 (1994).11. Brindely, S. Kikkava: Am. Mineral., 64, 836 (1979).12. L. Pesic, S. Salipurovic, V. Markovic, D. Vucelic, W. Kagunya, W. Jones: J. Mater.

Chem., 2, 1069 (1992).