Isomerization of a-Pinene Over Porous PhosphateHeterostructure Materials: Effects of Porosity and Acidity

Ying Li Æ Chen Wang Æ Hua Chen Æ Weiming Hua ÆYinghong Yue Æ Zi Gao

Received: 26 February 2009 / Accepted: 2 April 2009 / Published online: 15 April 2009

� Springer Science+Business Media, LLC 2009

Abstract A series of porous phosphate heterostructure

(PPH) samples with different content of silica intercalate

were prepared and characterized by XRD, N2 adsorption

and SEM. The total and accessible surface acidity of these

samples were measured by NH3 and 2,6-di-tert-butyl-pyr-

idine-TPD, respectively. Not the total acid sites but the

accessible ones play a crucial role in the a-pinene isom-

erization reaction. The number of the accessible aid sites is

strongly depended on the porosity of PPH samples. 68.1%

conversion of a-pinene with 51.9% yield of useful products

is achieved at 110 �C.

Keywords Porous phosphate heterostructure �a-Pinene isomerization � Porosity � Acidity

1 Introduction

Heterogeneous catalysis has succeeded in the production of

petrochemicals and other basic chemicals. Although there

are only a few industrial examples for using heterogeneous

catalysts in fine chemical reactions, increasing attention

has been paid in this area for advantages of safety, less

waste, ease of separation and reusability. Isomerization of

a-pinene, a conventional way to produce terpene products,

is carried out over treated TiO2 catalysts in industry [1, 2].

It is a typical acid-catalyzed reaction and the first step

of a-pinene rearrangement is to form a protonated

intermediate, which then transforms into tricyclic products

and monocyclic products by ring opening [3]. Among the

various products, camphene, tricyclene and limonene are of

the most importance for their widely application in fra-

grance and pharmacy industry. Since the catalytic perfor-

mance of the commercial catalyst is not effective enough,

many types of solid acids have been attempted in this

reaction, including layered clays [4, 5], zeolites [6, 7],

silica supported rare earth oxides [8, 9], sulfated zirconium

oxides [10, 11] and mesoporous materials such as Ga-SBA-

15 [12] and FSM-16 [13]. But the results were not satis-

fying due to the low activity or poor selectivity.

Metal oxide pillared zirconium phosphate is a kind of

well-known layered material. They are catalytically active

for a lot of acid-catalyzed reactions, such as dehydration of

isopropanol [14, 15], cumene cracking [15], and polymer-

ization of pyrrole [16]. The conventional way to synthesize

these pillared zirconium phosphates is so-called ion-

exchange method. The zirconium phosphate crystals were

first exfoliated with amine, then reacted with polynuclear

metal hydroxo and oxo complex ions through ion-exchange

reaction. After calcination, the intercalated precursors

formed the oxide pillars. Silica pillared zirconium phos-

phates have been prepared successfully by this method

using an organometallic pillar precursor, i.e., aminopropyl-

triethoxysilane [17].

Recently, a novel kind of porous phosphate hetero-

structure (PPH) material with a thermostable MCM-50

structure has been synthesized by a co-templated method

[18, 19]. A MCM-50 type lamellar zirconium phosphate

expanded with cationic surfactant molecule was first syn-

thesized, and then silica was inserted into its interlayer

space by cationic exchange of the surfactant guest with the

silica precursor. In this case, a supplementary neutral sur-

factant is necessary as cosurfactant for the correct

Y. Li � C. Wang � H. Chen � W. Hua � Y. Yue (&) � Z. Gao

Shanghai Key Laboratory of Molecular Catalysis and Innovative

Materials, Department of Chemistry, Fudan University,

200433 Shanghai, People’s Republic of China

e-mail: yhyue@fudan.edu.cn

123

Catal Lett (2009) 131:560–565

DOI 10.1007/s10562-009-9969-z

formation of silica galleries between the layers by the

hydrolysis and condensation of the silica precursor—tet-

raethylorthosilicate (TEOS). Silica pillared PPH material

has much larger surface area, as well as better porous

structure, than silica pillared zirconium phosphate synthe-

sized by the traditional ion-exchange method [18]. There-

fore, this kind of material is believed to be a promising

catalyst. Cu-PPHs and Ru-PPHs were reported in the

application of the selective catalytic reduction of NOx

and hydrotreating of aromatic hydrocarbons, respectively,

[20, 21]. However, there are few reports on its own cata-

lytic applications as a solid acid.

In this work, a series of PPH samples with different con-

tent of silica intercalate were prepared. Their textural and

acid properties were characterized by XRD, N2 adsorption,

SEM, NH3/2,6-di-tert-butyl-pyridine (DTBPy)-TPD. The

catalytic performance in the liquid phase isomerization of

a-pinene was tested and compared with that of the com-

mercial catalyst. The effects of the porosity and acidity of

PPH catalysts on the isomerization activity were also

discussed.

2 Experimental

2.1 Catalyst Preparation

Phosphate heterostructure (PPH) samples were prepared

following the procedures in the literature [18, 19]. Typi-

cally, 14.4 g cetyltrimethylammonium bromide (CTAB)

was dissolved in 100 mL n-propanol. 3.04 g 85 wt%

orthophosphoric acid and zirconium tetra-n-propoxide (70

wt% n-propanol solution) were then added dropwise under

vigorous stirring. The reaction mixture was stirred for

30 min. The resulting gel was separated by centrifugation,

washed first with the mixture of n-propanol/water and then

with distilled water, dried at 60 �C. The solid obtained is

denoted as CTAB–ZrP.

About 1 g of CTAB–ZrP was suspended in 100 mL

water and a certain amount of hexadecylamine in n-pro-

panol (35 g/L) solution was added as co-surfactant. After

being stirred for 1 day at room temperature, a solution

(50%, v/v) of TEOS in n-propanol was added. This sus-

pension was stirred for 3 days at room temperature. Then

the solid obtained was centrifuged, washed several times

with ethanol, dried at 60 �C, and finally calcined in air by

heating to 550 �C at a rate of 1.5 �C/min and maintaining

this temperature for 6 h to remove the surfactants com-

pletely. The obtained catalysts were denoted as ZrP–x–y,

where x and y are the hexadecylamine/P and TEOS/P molar

ratios used in the synthesis process.

Commercial TiO(OH)2 catalyst was provided by

Huayuan company and used as received.

2.2 Characterization of Catalysts

X-ray powder diffraction (XRD) patterns were recorded on

a Bruker D4 ENDEAVOR diffractometer using Cu Karadiation at 40 kV and 40 mA with a scan speed of 1�/min.

Scanning electron microscopic (SEM) images were

obtained on a Philips XL-30 scanning electron microscope.

The N2 adsorption/desorption isotherms were measured on

a Micromeritics ASAP2000 instrument at liquid N2 tem-

perature. Specific surface areas were calculated from the

adsorption isotherms by the BET method, and the pore size

distribution was calculated from the adsorption isotherm by

Cranston and Inkley method [22]. NH3-TPD and DTBPy-

TPD of the samples were carried out in a flow-type fixed-

bed reactor at ambient pressure. The catalysts were pre-

treated at 550 �C for 2 h in he flow. The NH3/DTBPy

adsorption temperature was 120 �C, and the temperature

was raised at a rate of 10 �C/min. The NH3 desorbed was

collected in a liquid N2 trap and detected by gas chroma-

tography, and the DTBPy desorbed was detected by on-line

gas chromatograph.

2.3 Activity Measurement

The isomerization of a-pinene was carried out at 110 �C in

a three necked round bottom flask fitted with a magnetic

stirrer, a thermometer and a reflux condenser. A mixture of

0.2 g catalyst, 5 mL a-pinene and 0.2 mL decane (used as

internal GC standard) was added to the flask. After reacting

for 10 h, the products were analyzed by a gas chromato-

graph equipped with a 30 m SE-30 capillary column

(30 m 9 0.25 mm 9 0.3 lm) and a flame ionization

detector.

The reaction data in the work were reproducible with a

precision of \5%.

3 Results and Discussion

3.1 Catalyst Characterization

A series of PPH samples with different content of silica

galleries were prepared by varying the hexadecylamine/P

and TEOS/P molar ratios. The XRD patterns of some

representative samples are shown in Fig. 1. A broad peak

with weak intensity was observed for almost all the sam-

ples, with d-spacing of 4–5 nm, indicating the formation of

silica galleries in the interlayer space of zirconium phos-

phate. This can be further confirmed by the platelike crystal

morphology of the ZrP–0.3–2 sample, as presented in

Fig. 2. No diffraction lines were observed in the high angle

region, indicating that coprecipitated silica is not present.

Isomerization of a-Pinene Over PPH Materials 561

123

The N2 adsorption–desorption isotherm were measured

at 77 K. Type IV isotherms were observed for all the

samples prepared. Fig. 3 shows the N2 adsorption–

desorption isotherm and the pore size distribution of ZrP–

0.3–2, as a representative one. The textural properties of

these samples are summarized in Table 1. The CTAB–ZrP

sample calcined at 550 �C exhibits a very low BET surface

area (24 m2/g), while for those silica pillared ones, the

BET surface area and pore volume become much larger,

which are in the range of 434–768 m2/g and 0.20–

0.56 cm3/g, respectively, indicating the presence of silica

galleries between the interlayer space of the phosphate,

which results in a porous structure accessible to the N2

molecules. This porous structure changes with the pre-

parative conditions such as hexadecylamine/P and TEOS/P

ratios, since the BET surface area and pore volume

increase obviously with an increment of the hexadecyl-

amine/P ratio. As the TEOS/P ratio is increased, the BET

surface area and pore volume first increase and then

decrease.

3.2 Acidity Measurement

3.2.1 NH3-TPD

The acidity of the PPH samples was measured by NH3-

TPD method, and the results are given in Table 2. There is

only one asymmetric broad peak on the TPD profiles of all

the samples (not shown here), and the peak temperatures

are in the range of 245–264 �C, showing that the acid sites

of the samples are of weak–medium strength. The number

of acid sites on PPH samples depends on the preparative

conditions, which decreases with raising the hexadecyl-

amine/P ratio or the TEOS/P ratio (i.e., x or y values).

3.2.2 DTBPy-TPD

To characterize the acid sites accessible for bulky mole-

cules, 2,6-di-tert-butyl-pyridine (DTBPy) has been

employed as a base probe in the TPD measurement, since

the previous study [23] showed that the large DTBPy

molecules only adsorbed on the Brønsted acid sites located

on the external surface of zeolitic catalysts with 10-MR and

unidirectional 12-MR pore channel systems.

The results of DTBPy-TPD are quite different from

those of NH3-TPD. As shown in Table 2, the number of

desorbed DTBPy molecules, regarded as the accessible

acid sites, also changes with the preparative conditions.

However, the variation trend is not the same as that of the

total acid sites. With the increase of hexadecylamine/P and

TEOS/P ratios, the number of accessible acid sites first

increases and then decreases. A maximum value was

obtained when x and y reached 0.3 and 2, respectively.

Fig. 1 XRD patterns of some representative samples. a CTAB–ZrP;

b ZrP–0.3–1; c ZrP–0.3–2; d ZrP–0.4–2

Fig. 2 SEM image of the ZrP–0.3–2 sample

Fig. 3 Nitrogen adsorption (d)-desorption (s) isotherms of ZrP–

0.3–2 sample. The insert shows the pore size distribution

562 Y. Li et al.

123

This indicates that the ZrP–0.3–2 sample has the greatest

number of acid sites available by large molecules such as

DTBPy, although its total acid site number (measured by

NH3-TPD) is not the highest.

The above results can be explained by the mechanism of

the PPH material formation. As we know, the acidity of the

PPH material comes from the presence of free P–OH

groups located on the layers of zirconium phosphate. When

the silica pillars formed in the interlayer space by hydro-

lysis of TEOS, the P–OH groups are either neutralized or

covered by the silica pillars, leading to a decrease in the

number of the free P–OH groups and thus a concomitant

reduction of the acidity. If more silica precursors (TEOS)

were added, less free P–OH groups would be left after the

intercalation, resulting in less total acid sites as revealed by

NH3-TPD results. Meanwhile, adding more co-surfactant

(hexadecylamine) would make the silica precursor enter

into the interlayer space easier, which also resulted in the

decline of total acid sites.

On the other hand, not all the acid sites are accessible to

the bulky molecules. Only when a porous structure formed

between the interlayer space of the phosphate by the

intercalation of silica galleries, the free P–OH groups

located on the layers would be exposed, becoming acces-

sible for the bulky molecules. That’s the reason why the

number of the accessible acid sites increased with raising

the hexadecylamine/P or TEOS/P ratio though the number

of total acid sites decreased. This increasing tendency

would cease when most of the P–OH groups on the

phosphate layer were exposed. Further increase of the silica

pillars would reduce the accessible acid sites by the reac-

tion between the silica and the exposed P–OH groups.

3.3 Isomerization of a-Pinene

The activities of the PPH catalysts for a-pinene isomeri-

zation were investigated, and the results are summarized in

Table 3, together with that of the commercial TiO(OH)2

for comparison. During the reaction, several isomerization

products, such as tricyclene, camphene, limonene, terpi-

nolene, were found.

There is marked difference in the catalytic performance

of the PPH catalysts prepared using different hexadecyl-

amine/P and TEOS/P ratios. The activity of the catalysts

using the same TEOS/P ratio decreases in the order of ZrP–

0.3–2 [ ZrP–0.2–2 [ ZrP–0.4–2 [ ZrP–0.1–2 [ ZrP–0.5

–2, and that of the catalysts using the same hexadecyl-

amine/P ratio has the order of ZrP–0.3–2 [ ZrP–0.3–

3 [ ZrP–0.3–1 [ ZrP–0.3–4, which obviously does not

parallel the sequence of the number of total acid sites as

revealed by NH3-TPD data, though a-pinene isomerization

is a typical weak acid-catalyzed reaction. However, the

Table 1 Textural properties of

various ZrP–x–y samples

a Calcined at 550 �C for 6 hb Not detected

Catalyst BET surface

area (m2/g)

Pore

volume (cm3/g)

Average pore

diameter (nm)

CTAB–ZrPa 24 0.03 –b

ZrP–0.1–2 434 0.28 2.5

ZrP–0.2–2 473 0.28 2.8

ZrP–0.3–2 567 0.29 2.9

ZrP–0.4–2 582 0.37 3.3

ZrP–0.5–2 603 0.49 4.4

ZrP–0.3–1 444 0.20 2.6

ZrP–0.3–3 768 0.56 3.4

ZrP–0.3–4 693 0.42 3.4

Table 2 NH3-TPD and

DTBPy-TPD data of various

ZrP–x–y samples

Catalyst NH3-TPD DTBPy-TPD

Peak temperature (�C) NH3 desorbed

(mmol/g)

Peak temperature (�C) DTBPy desorbed

(mmol/g)

ZrP–0.1–2 245 3.1 258 0.58

ZrP–0.2–2 251 2.6 260 0.73

ZrP–0.3–2 254 1.7 279 0.82

ZrP–0.4–2 264 1.5 276 0.69

ZrP–0.5–2 250 1.3 253 0.55

ZrP–0.3–1 260 2.8 238 0.70

ZrP–0.3–3 254 1.1 285 0.75

ZrP–0.3–4 248 0.8 291 0.68

Isomerization of a-Pinene Over PPH Materials 563

123

order of the catalytic activity is quite in parallel with the

sequence of the number of the accessible acid sites as

revealed by DTBPy-TPD data, as illustrated in Fig. 4. This

indicates that not all the acid sites on the layers of PPH

samples but those exposed after intercalation which are

accessible for the reactants can catalyze the isomerization

reaction. Among all the PPH catalysts, ZrP–0.3–2 with the

most abundant accessible acid sites has the highest activity.

68.1% conversion of a-pinene with 51.9% yield of useful

products (tricyclene, camphene and limonene) is obtained

on this catalyst, much higher than that of the commercial

TiO(OH)2 catalyst under the same reaction conditions.

4 Conclusions

The PPH material is found to be an effective catalyst for

the liquid phase acid-catalyzed reaction of bulky mole-

cules. Much higher activity is achieved for the isomeriza-

tion of a-pinene over PHH catalysts as compared with the

commercial TiO(OH)2 catalyst. The activity of these cat-

alysts is strongly dependent on the preparative conditions,

since the porosity and acidity can be adjusted by the

amount of co-surfactant and silica source employed in the

synthesis process. A close comparison of the catalytic

activity for a-pinene isomerization with the acidity mea-

sured by NH3-TPD and DTBPy-TPD reveals that not all

the acid sites on the layer of PPH catalysts but those

exposed after intercalation which are accessible for the

reactants can catalyze the isomerization reaction. The

maximum conversion of a-pinene (68.1%) with 51.9%

yield of useful products (tricyclene, camphene and limo-

nene) is acquired over the ZrP–0.3–2 catalyst which pos-

sesses the greatest number of accessible acid sites as

measured by DTBPy-TPD.

Acknowledgments This work was supported by the State Basic

Research Project of China (2006CB806103), the National Natural

Science Foundation of China (20633030, 20773027 and 20773028)

and the Science & Technology Commission of Shanghai Municipality

(08DZ2270500).

References

1. Korotov SV, Vyrodov VA, Afanas’eva EV, Kolesov AI, Ma-

slakova ZL, Oblivantseva TD, Cheirkov PK, Zhurovlev PI,

Minaeva OI (1969) USSR Patent 238541

2. Afanas’eva EV, Vyrodov VA, Korotov SV (1970) Nauch Tr

135:11

3. Wstrach VP, Barnum LH, Garber M (1957) J Am Chem Soc

79:5786

Table 3 Reaction data of

various catalysts for the

isomerization of a-pinene

a Useful products including

tricyclene, camphene and

limonene

Catalyst Conversion (%) Selectivity (%) Yield (%)a

Tricyclene Camphene Limonene Terpinolene Others

TiO(OH)2 10.6 8.5 60.0 10.9 \0.1 20.6 8.4

ZrP–0.1–2 23.5 3.3 39.0 28.8 5.8 23.2 16.7

ZrP–0.2–2 49.7 3.1 44.5 32.6 7.7 12.2 39.9

ZrP–0.3–2 68.1 2.9 43.4 29.9 9.7 14.1 51.9

ZrP–0.4–2 36.1 2.9 42.1 33.2 8.0 13.8 28.2

ZrP–0.5–2 15.4 2.7 40.3 30.9 6.9 18.7 11.5

ZrP–0.3–1 45.2 3.2 43.1 32.1 8.2 13.5 35.4

ZrP–0.3–3 56.9 3.2 44.4 32.1 8.0 12.3 45.3

ZrP–0.3–4 36.4 3.6 56.6 25.6 4.9 9.4 31.2

Fig. 4 Correlation between the conversion of a-pinene and the

amount of desorbed DTBPy over various catalysts

564 Y. Li et al.

123

4. Besgun N, Ozkan F, Gunduz G (2002) Appl Catal A: Gen

224:285

5. Mukesh KrY, Chintansinh DC, Jasra RV (2004) J Mol Catal A:

Chem 216:51

6. Allahverdiev AI, Gunduz G, Murzin DY (1998) Ind Eng Chem

Res 37:2373

7. Gunduz G, Dimitrova R, Yilmaz S, Dimitrov L, Spassova M

(2005) J Mol Catal A: Chem 225:253

8. Yamamoto T, Tanaka T, Funabaki T, Yoshida S (1999) J Phys

Chem B 102:5830

9. Yamamoto T, Matsuyama T, Tanaka T, Funabaki T, Yoshida S

(2000) J Mol Catal A: Chem 155:271

10. Ecormier MA, Wilson K, Lee AF (2003) J Catal 215:57

11. Ecormier MA, Lee AF, Wilson K (2005) Micropor Mesopor

Mater 80:301

12. Jarry B, Launay F, Nogier JP, Montouillout V, Gengembre L,

Bonardet JL (2006) Appl Catal A 309:177

13. Yamamoto T, Tanaka T, Funabaki T, Yoshida S (1998) J Phys

Chem B 102:5830

14. Tang Y, Zhang H, Wang X, Gao Z (1997) Chem J Chinese Univ

18:1337

15. Tang Y, Xu JS, Gao Z (1998) Chinese J Catal 57:354

16. Maireles-Torres P, Olivera-Pastor P, Rodriguez-Castellon E,

Jimenez-Lopez A, Tomlinson AAG (1992) J Incl Phenom 14:327

17. Roziere J, Jones DJ, Cassagneau T (1991) J Mater Chem 1:1081

18. Jimenez-Jimenez J, Rubio-Alonso M, Quesada DE, Rodrıguez-

Castellon E, Jimenez-Lopez A (2005) J Mater Chem 15:3466

19. Jimenez-Jimenez J, Maireles-Torres P, Olivera-Pastor P, Rodrı-

guez-Castellon E, Jimenez-Lopez A (1997) Langmuir 13:2857

20. Moreno-Tost R, Oliveira ML, Eliche-Quesada D, Jimenez-

Jimenez J, Jimenez-Lopez A, Rodrıguez-Castellon E (2008)

Chemosphere 72:608

21. Eliche-Quesada D, Oliveira ML, Jimenez-Jimenez J, Rodrıguez-

Castellon E, Jimenez-Lopez A (2006) J Mol Catal A: Chem

255:41

22. Cranston RW, Inkley FA (1957) Adv Catal 9:143

23. Corma A, Fornes V, Forni L, Maquez F, Martinez-Triguero J,

Moscottiy D (1998) J Catal 179:451

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