Journal of Natural Gas Chemistry 12(2003)5662

Properties and Characterization of Modified

HZSM-5 Zeolites

Renqing Lu1, Hejin Tangbo1, Qiuying Wang2, Shouhe Xiang1

1. Institute of New Catalytic Materials, Department of Material Chemistry, Nankai University, Tianjin, 300071, China

2. Catalytic Factory, Nankai University, Tianjin, 300071, China

[Manuscript received July 12, 2002; revised November 4, 2002]

Abstract: Physicochemical and catalytic properties of phosphorus and boron modified HZSM-5 zeolitestreated with 100% steam at 673 K were investigated. The acidity and distribution of acidic sites were stud-ied by infrared spectroscopy using pyridine as probe molecule and temperature programmed desorption(TPD) of ammonia. The structure of the samples was characterized by XRD, and the textural properties

of the catalysts were determined by nitrogen isothermal adsorption-desorption measurements and scanningelectron microscopy (SEM). The XRD results show that the modified samples have no novel crystallinephase, indicating a high dispersion of phosphorus and boron species. After treatment, the microporousvolume and surface area of the samples markedly decrease, implying the blockage of the channel. The nitro-gen adsorption-desorption measurements suggest that the isothermal type of all samples is a combinationof isothermal type I and IV, and all hysteresis loops resemble the H4-type in the IUPAC classification.

The total acidity of the modified samples, determined by pyridine adsorption IR and TPD of ammonia,decreases in contrast to that of the parent HZSM-5. The conversion of n-heptane over P and B steam-modified HZSM-5 is higher than that of P and B-modified HZSM-5 zeolites but lower than that of theparent HZSM-5.

Key words: HZSM-5 zeolite, steam treatment, phosphorus, boron, secondary pore, texture, crackingactivity

1. Introduction

The properties of catalysts are carefully tuned

for the desired catalytic process before use. Zeo-

lites are crystalline aluminosilicate, and their acid-

base properties depend on the aluminum content in

the framework. The adjustment of the acidity may

be realized by proper SiO2/Al2O3 molar ratio crys-

tallization, other elements replacing framework con-

stituents, or modification of the zeolite. Dealumina-

tion, the removal of framework aluminum from the

zeolite lattice, is a well known procedure for stabiliz-

ing zeolites and creating mesopores, which help over-

come diffusional problems in the zeolite micropores

[1]. ZSM-5 is a member of the pentasil family of high-

silica zeolites and, due to its unusual properties, has

found a wide range of applications as a catalytic ma-

terial. Modification of HZSM-5 zeolites by impreg-

nation with phosphoric or boric acid has been inves-

tigated because of its promising catalytic properties

in many reactions (e.g. conversion of methanol to hy-

drocarbons (MTH) [2,7,15,17,22], n-hexane cracking

[3,11], disproportionation of toluene [4,6,8,10], alkyla-

tion of toluene with methanol [46,10,13,15,16,20,21],

xylene isomerization [6], alkylation of benzene with

ethanol [9,23], alkylation of ethylbenzene [12], con-

version of methyl chloride to ethylene and propylene

Corresponding author. Tel: (022)23509932; E-mail: shxiang@public.tpt.tj.cn.On leave from Chemistry and Chemical Engineering College, University of Petroleum (East China), Dongying, Shandong

Province

Journal of Natural Gas Chemistry Vol. 12 No. 1 2003 57

[18,19]). To our knowledge, no attempts have been

made to combine phosphoric or boric acid modifica-

tion with steam treatment over HZSM-5 zeolites.

2. Experimental

2.1. Catalyst preparation

A template-free synthesized commercial HZSM-5

zeolite (SiO2/Al2O3=50), supplied by the Catalytic

Factory of Nankai University, was used as the start-

ing material (denoted parent 50). The zeolite material

was impregnated with an aqueous solution of phos-

phoric acid (H3PO4) in order to reach a 1%P content.

After being dried in air, the product was heated to

393 K in a mue furnace for 1 h. Then, the temper-

ature was increased to 823 K and held for 3 h. This

product was designated as P501. Another product,

B501, with a B content of 1% was prepared in a simi-

lar manner except for the replacement of H3PO4 with

H3BO3. Some P501 and B501 samples were treated

with 100% steam of 673 K for 4 h and designated as

P5014 and B5014, respectively.

2.2. Catalyst characterization

X-ray diffraction patterns were recorded on

D/max-2500 powder diffractometer using nickel-

filtered Cu K radiation (=0.1542 nm) and equipped

with a graphite monochromator. The step scans were

taken over a 2 range from 5 to 50o.

The BET specific surface area and porosity tex-

ture of each sample were determined by nitrogen ad-

sorption measurements at liquid nitrogen temperature

with an automatic Micromeritics ASAP 2400 appara-

tus. The samples were first degassed at 573 K for

approximate 6 h and then studied with a static volu-

metric technique.

IR measurements were carried out using pyridine

as the probe molecule, and the vibration spectra of

chemisorbed pyridine were recorded between 1,400

and 1,700 cm1. The samples were pressed into self-

supporting wafers 20 mm in diameter and heated to

673 K in a special IR cell under vacuum (0.04 Pa) for

1 h. After cooling to room temperature, excess pyri-

dine was adsorbed and outgassed at 433 K to elim-

inate the physisorbed pyridine. The concentrations

of Bronsted and Lewis sites able to retain pyridine

at 433 K were determined using the extinction coeffi-

cients and the adsorbance surface of the correspond-

ing bands at around 1,540 and 1,450 cm1, respec-

tively.

TPD patterns of chemisorbed ammonia were

recorded using a DuPont 2000 thermoanalyzer by

means of NH3 adsorption-desorption.

2.3. Catalytic activity measurements

The catalytic activity of samples in n-heptane

cracking was determined in a pulse microreactor (i.d.

4 mm) connected to a gas chromatograph. The reac-

tion was carried out with 0.2 g catalyst, a 30 cm3/min

N2 flow rate, a 2 l pulse and at 773 K. Before

the activity was measured, the catalyst was activated

in situ at 793 K for 1 h in dry nitrogen stream.

3. Results and discussion

3.1. Results of catalyst characterization

3.1.1. The measurement of physicochemical

properties

Specific surface area of the catalysts was com-

puted according to the BET method from the nitrogen

adsorption isotherms obtained at 77 K, taking a value

of 0.162 nm2 for the cross-section of the adsorbed N2molecule at that temperature. BET areas of the var-

ious samples are summarized in Table 1. The BET

surface area, microporous area and microporous vol-

ume of all modified samples significantly decreased

compared to the parent zeolite. Among the modi-

fied samples, BET surface area, mesoporous area and

mesoporous volume of the steam-treated P5014 and

B5014 are higher than those of P501 and B501, re-

spectively, resulting in diffusion benefits. However,

Table 1. Physicochemical properties of samples

BET surface area Micropore area Mesopore area Micropore volume Mesopore volumeSample

(m2/g) (m2/g) (m2/g) (cm3/g) (cm3/g)

Parent 50 388.5 303.1 85.4 0.1209 0.0429

P501 318.9 231.2 87.7 0.0940 0.0532

P5014 324.1 226.6 97.5 0.0925 0.0740

B501 323.3 251.3 72.0 0.1016 0.0324

B5014 340.2 233.7 106.5 0.0958 0.0731

58 Renqing Lu et al./ Journal of Natural Gas Chemistry Vol. 12 No. 1 2003

the microporous area and microporous volume of

steam-treated P5014 and B5014 are lower than those

of P501 and B501. These results suggest that the

channel of phosphorus and boron-modified samples

is occluded, and steam treatment may result in sec-

ondary pore formation.

3.1.2. X-ray powder analysis

The x-ray powder pattern peaks of the five sam-

ples, exhibited by all samples in the XRD patterns,

are typically of the MFI topology. The patterns in-

dicate that crystallinity was retained after treatment.

Also, there is no novel crystalline phase, which indi-

cates that phosphorus and boron species are highly

dispersed on the zeolites. The interaction between

ZSM-5 zeolites and phosphorus and boron species may

result in the peak split around 2=23o.

3.1.3. IR measurement of adsorbed pyridine

Information about the type of acid sites and their

distribution in the catalysts could be obtained from

the infrared spectra of pyridine adsorbed on the sam-

ples in the 1,4001,700 cm1 spectral region. The

acidity of the five samples is shown in Figure 1, and

a qualitative estimation of the band intensity ratio

representing pyridine adsorbed at Bronsted acid sites

and pyridine absorbed at Lewis acid sites is illustrated

in Table 2. As shown in Figure 1 and Table 2, the

parent zeolite possesses the largest number of both

Bronsted and Lewis acid sites of the five samples.

The reason for the difference between the Bronsted

and Lewis acid number in P501 and P5014 remains

unclear. Steam-treated B5014 and P5014 have a lower

acidity compared to B501 and P501, respectively.

Figure 1. Total acidity of samples measured by FT-

IR

3.1.4. TPD of ammonia

TPD profiles of the parent HZSM-5 and modi-

fied HZSM-5 are shown in Figure 2. The two-peak

pattern is well documented for HZSM-5 [24], indicat-

ing the existence of weak and strong acid sites in the

parent ZSM-5 zeolite. The profiles of modified sam-

ples reveal similar patterns as the parent sample, but

the peak intensity of weak and strong acid sites in the

modified samples markedly decreased. The significant

decrease in the acidity of the single-phosphorus mod-

ified P501 and the single-boron modified B501 may

be explained by the phosphorus and boron species

combining with bridging hydroxyl Al(OH)Si groups

[2]. After steam treatment at 673 K, some framework

aluminum atoms are partially hydrolysed to form non-

tetrahedrally symmetric aluminum atoms, which act

as a strong electron withdrawal centers for the re-

maining tetrahedral framework aluminum atoms thus

creating stronger Bronsted acids [26]. Table 2 shows

the acidity of all samples, strongly suggesting the de-

crease in acidity of the modified samples in contrast to

Parent50. The acidity of the samples determined by

the pyridine adsorbed IR method is lower than that

of their counterparts determined by TPD of ammo-

nia. This difference may be the results of differences

in the molecular size of these bases, i.e. the smaller

molecules of ammonia may penetrate through more

pores than the larger molecules of pyridine.

Figure 2. Ammonia TPD profiles of samples

Journal of Natural Gas Chemistry Vol. 12 No. 1 2003 59

Table 2. Ratio of the B to L acid site intensity

(denoted Ratio) and acidity measured by

TPD of NH3 (denoted TPD)

Sample Ratio TPD (mmol/g)

Parent 50 1.643 0.79

P501 3.910 0.71

P5014 3.211 0.68

B501 1.321 0.60

B5014 1.144 0.54

3.1.5. Porosity measurement

The porous structure of all samples was deter-

mined by N2 adsorption-desorption measurements,

and the nitrogen isotherm for the samples is illus-

trated in Figure 3. According to IUPAC [25], the

shape of the adsorption isotherm can be classified into

one of six groups. Of these, the most common are type

I (Langmuir) isotherms for purely microporous solids,

and type IV for mesoporous goods in which capillary

condensation takes place at higher pressures of adsor-

bate as well as a hysteresis loop. As is shown in Figure

3, the adsorption volume at very low relative pres-

sures (p/p0

60 Renqing Lu et al./ Journal of Natural Gas Chemistry Vol. 12 No. 1 2003

B5014 reveals some cracks and faults that appeared

on the surface of steam-treated B5014 and P5014.

This shows the formation of secondary pores and this

formation is an important explanation for the higher

heptane cracking activity because cracking is often

limited by diffusion inside the micropores of the zeo-

lite [27]. P501 has few cracks or faults, and this may

be because of the stronger acidity of H3PO4 than that

of H3BO3. This clarified that the mesoporous volume

of P501 is higher than that of B501 and the heptane

cracking activity of P501 is higher than that of B501.

Figure 4. SEM pictures of all samples: (a) parent, (b) enlarged parent, (c) P501, (d) P5014, (e) B501, (f)

B5014

Journal of Natural Gas Chemistry Vol. 12 No. 1 2003 61

3.2. Activity of n-heptane cracking over the

catalyst

To further study the activity of catalysts,

n-heptane cracking was used as a test reac-

tion. The results of an n-heptane cracking

conversion over the catalysts are presented in

Table 3. It reveals an order of conversion

(parent50=B5014>P5014>P501>B501) that shows

no correlation to the acidity determined by pyridine

adsorption and TPD-NH3. The activity of steam-

treated phosphorus and boron-modified samples is

higher than that of single-phosphorus and boron-

modified samples. This may be the result of steam

enhancement of the BET surface area and mesoporous

area as well as proper steam treatment enhancement

of acid strength. The selectivity of products is also

shown in Table 3. It can be seen that B5014 shows the

highest C=3 selectivity, while B501 shows the highest

C=4

selectivity. According to the acidic results calcu-

lated by TPD of ammonia and FT-IR, the acidity of

steam-treated P5014 and B5014 is slightly lower than

that of P501 and B501, respectively. This suggests

a decrease in diffusion constraints brought about by

the creation of mesopores in the steam-treated sam-

ples (as seen from the enhancement of the mesoporous

area and mesoporous volume of the steam-treated

samples). In addition, higher acid site strength may

contribute to the activity enhancement.

Table 3. Product selectivity and conversion of heptane cracking

Product selectivity (%)Sample

C1-C2 C3 C=3 C4 C=4

C+5

Conversion (%)

Parent 50 11.3 31.5 8.0 16.2 4.6 28.4 100

P501 10.9 30.5 5.7 17.3 4.9 30.7 93

P5014 12.5 36.5 6.1 17.9 4.2 22.9 94

B501 11.3 31.0 4.8 19.0 7.6 26.3 59.2

B5014 9.9 32.5 8.5 13.4 3.7 32.0 100

4. Conclusions

The BET surface area, microporous area and mi-

croprous volume of modified samples decreased pro-

nouncedly in contrast to Parent50. Phosphorus and

boron species were highly dispersed over the HZSM-

5 (as suggested by XRD and SEM). The acidity of

treated samples (measured by FT-IR and TPD of am-

monia) pronouncedly decreased. The isothermal type

of all samples is a complex of type I and IV, while

hysteresis loops belong to the H4 type. The heptane

cracking activity of a phosphorus or boron-modified

sample is lower than that of the parent zeolite. The

activity of steam-treated P5014 is higher than that

of only phosphorus-modified P501, while the activity

of steam-treated B5014 is remarkably enhanced com-

pared to B501.

Acknowledgements

Financial support from Catalytic Key Laboratory

of China Petroleum and Natural Gas Group Corpo-

ration (University of Petroleum) was greatly appre-

ciated. We thank the National Science Foundation

Committee for Grant NSFC 20233030.

References

[1] Bertea L, Kouwenhoven H W, Prins R. Appl Catal A,

1995, 129(1): 229

[2] Kaeding W W, Butter S A. J Catal, 1980, 61(1): 155

[3] Vinek H, Rumplmayr G, Lercher J A. J Catal, 1989,

115(2): 291

[4] Ashton A G, Batmanian S, Dwyer J et al. J Catal,

1986, 34(1): 73

[5] Nunan J, Cronin J, Cunningham J. J Catal, 1984,

87(1): 77

[6] Young L B, Butter S A, Kaeding W W. J Catal, 1982,

76(2): 418

[7] Vedrine J C, Auroux A, Dejaifve P et al. J Catal,

1982, 73(1): 147

[8] Kaeding W W, Chu C, Ying L B et al. J Catal, 1981,

69(2): 392

[9] Chandawar K H, Kulkarni S B, Ratnasamy P. Appl

Catal, 1982, 4: 287

[10] Chen N Y, Kaeding W W, Dwyer F G. J Am Chem

Soc, 1979, 101(2022): 6783

[11] Li D, Tao L, Zhang Y et al. Shiyou Huagong

(Petrochem Technol), 1990, 19(3): 449

[12] Kaeding W W. J Catal, 1985, 95(2): 512

[13] Kaeding W W, Chu C, Ying L B et al. J Catal, 1981,

67(1): 159

62 Renqing Lu et al./ Journal of Natural Gas Chemistry Vol. 12 No. 1 2003

[14] Kim J H, Namba S, Yashima T. Bull Chem Soc Jpn,

1988, 61(4): 1051

[15] Derouane E G, Dejaifve P, Gabelica Z et al. Chem

Soc, 1981, 72(2): 331

[16] Chen N Y. J Catal, 1988, 114(1): 17

[17] Dehertog W J H, Froment G F. Appl Catal, 1991,

71(1): 153

[18] Sun Y, Campbell S M, Lunsford J H et al. J Catal,

1993, 143(1): 32

[19] Lersch P, Bandermann F. Appl Catal, 1991, 75(1):

133

[20] Cavallaro S, Pino L, Tsiakaras P. Zeolites, 1987, 7(5):

408

[21] Giordano N, Pino L, Cavallaro S et al. Zeolites, 1987,

7(2): 131

[22] Suzuki K, Kiyozumi Y, Matsuzaki K. Appl Catal,

1988, 39(2): 315

[23] Romannikov V N, Tissler A T, Thome R. React Kinet

Catal Lett, 1993, 51(1): 125

[24] Richter M, Janchen J, Jerschkewitz H-G et al. J Chem

Soc Faraday Trans, 1991, 87(9): 1461

[25] Sing K S W, Everett D H, Haul R A W et al. Pure

Appl Chem, 1985, 57(4): 603

[26] Lago R M, Haag W O, Mikovsky R J et al. In: Mu-

rakami Y, Lijima A, Ward J W eds. New Develop-

ments in Zeolite Science and Technology: Proceedings

of the 7th International Zeolite Conference. Amster-

dam: Elsevier, 1986. 677

[27] Sato K, Nishimura Y, Shimada H. Cata Lett, 1999,

60: 83