Toluene alkylation over ZSM-5 zeolite catalysts with skeletal and nonskeletal boron

Toluene alkylation over ZSM-5 zeolite catalysts with skeletal and nonskeletal boron L.Z. Chen and Y.Q. Feng

College of Chemical Engineering, Dalian University of Technology, Dalian, People's Republic of China. Three types of boron-containing ZSM-5 catalysts were prepared. Nonskeietal boron-containing catalyst (BZSM-5) was prepared by impregnating ZSM-5 with boric acid. B,AIZSM-5 zeolite catalyst containing skeletal boron was prepared by adding boric acid during the synthesis. Another partially dealuminated skeletal boron-containing zeolite (WBZ) was prepared by treating ZSM-5 with boric acid at high temperature. XRD and i.r. measurements of these catalysts show that all of them have the characteristics of ZSM-5. T.p.d. data were also measured in order to know their surface acidities. Toluene-ethanol alkylation reactions over these catalysts were studied. The p-ethyltoluene content in the alkylation product obtained over B,AIZSM-5 catalyst reached about 82%. On the other hand, the highest p-ethyltoluene content in the product over the WBZ catalyst was 93%, while the conversion rate was about 18%. The WBZ catalyst had good stability as well. Keywords: ZSM-5; modification; boron; alkylation; selectivity; toluene; ethanol

INTRODUCTION Shape-selective properties of pentasil ZSM-5 zeolite are well known, 1-5 but only the modified H-type ZSM-5 zeolites have para-selective properties for alkylation reactions. Therefore, modification is an effective way to improve shape-selectivity and stability of ZSM-5 zeolite catalysts. Various modification methods have been proposed; among them, ion impregnation is generally used. This paper discusses the preparation of nonskeletal boron-containing zeolite catalyst (BZSM-5) using ion impregnation, as well as the preparation of skeletal boron-containing zeolite catalyst (B,A1ZSM-5) by introducing boron during the synthesis. A new modification method is proposed, i.e., partial dealuminated boron-containing zeolite catalysts are prepared by the treatment of ZSM-5 zeolite with boric acid solution at high temperature. High activity and selectivity of toluene/ethanol alkylation reaction are found over these catalysts.

EXPERIMENTAL Preparation of catalysts Skeletal b o r o n - c o n t a i n i n g

B,A1ZSM-5 zeolite

Address correspondence to Dr. Chen at the College of Chemical Engineering, Dalian UniversiW of Technology, Dalian 116012, People's Republic of China. Received 30 April 1991; revised 7 Septem ber 1991 ; accepted 18 November 1991

© 1992 Butterworth-Heinemann

catalysts (SB) were prepared as follows: 1.3 g boric acid, 2.1 g aluminum sulfate, 1.2 g sulfuric acid (98%), and 62.2 g sodium silicate were used as raw materials; 32.4 g aqueous ammonia (25%) was used as templating agent; uniform gel was formed by mixing them at room temperature; after aging for 36 h at 363 K, the gel was poured into an autoclave and heated up to 438 + 4 K; and crystallization was carried out for 2 d. Afterward, the zeolite prepared was washed with water to a pH value 7-8, then filtered and dried by air at 373 K. H-type B,AIZSM-5 zeolite was prepared by treating SB with 1 N NH4NO~ at 358K and then deaminizing for 6 h at 783 K. By mixing the zeolite with 30% (wt) AluO3, pelletizing, and calcinating at 803 K, the resulting zeolite catalyst was H-type B,AIZSM-5. Nonskeletal boron-containing ZSM-5 catalyst (BZSM-5) was prepared by impregnating ZSM-5 catalyst with boric acid solution for 12 h at 353 K. Another group of catalysts (WBZ) was prepared by the treatment of HZSM-5 zeolite with 0.15 N boric acid solution and nitrogen (mole ratio = 3) in a flow condition at 813 K. At the same time, a set of catalysts (WHZ) was prepared under the same conditions, but pure water was used instead of boric acid solution.

Catalytic experiments Catalytic experiments were carried out in a fixedbed microflow reactor made of stainless steel. Reaction conditions were usually as follows: 1 g of catalyst, atmosphere pressure, 613 K reaction temperature,

ZEOLITES, 1992, Vol 12, April/May 347

Toulene alkylation over ZSM-5 catalysts with boron: L.Z. Chen and Y.Q. Feng Table 1 Cell dimensions of HZSM-5 and B,AIZSM-5 Sample

HZSM-5 B,AIZSM-5

a (A)

b (A)

c (A)

V (/~)

20.100 20.028

19.897 19.807

13.425 13.355

5371 5298

2 3 4

1.5 mole ratio of toluene/ethanol, and WHSV of toluene = 2.0 h -I. The conversion rate of toluene (C), the selectivity for p-ethyltoluene (S), and the total yield of ethyltoluene (Y) are defined as follows: C = (moles of toluene reacted/moles of toluene fed) x 100%; S = (moles ofp-ethyltoluene/moles of ethyltoluene) × 100%; and Y = (moles ofethyltoluene/moles of toluene fed) x 100%.

I.r. m e a s u r e m e n t I.r. measurements were carried out on a Shimadze IR 435 spectrometer. The wafers used were prepared by mixing H-type B,AIZSM-5 zeolites with a definite amount of KBr powder and pelletizing. To measure the characteristic skeletal vibration frequency of ZSM-5, the 400-2000 cm-range band was used. I.r. information of boron located on zeolite skeleton was obtained from the wafers of the same size prepared by weighing 8 mg H-type B,AIZSM-5 exactly and pelleting at the same pressure (scan range 1200-2000

cm-1).

a

J

JSoo 10oo J4oo 120o I0oo 800

RESULTS AND DISCUSSION Cell d i m e n s i o n s Cell dimensions of HZSM-5 and B,A1ZSM-5 were measured by XRD and are listed in Table 1. It is clear that the unit cell dimensions of B,AIZSM-5 are smaller than those of HZSM-5 as well as is the volume of the unit cell. This is evidence that boron has entered the zeolite framework. I.r. v i b r a t i o n o f zeolite s k e l e t o n I.r. spectra of the skeletal vibration of various Table 2

I.r. vibration frequencies ofzeolite skeleton

Sample

I.r. vibration frequency (cm -1)

HZSM-5 SB-2 SB-4 SB-5

348

1365 1376 1378

1225 1224 1225 1225

1090 1091 1093 1094

795 795 795 795

ZEOLITES, 1992, Vol 12, April/May

545 545 545 545

444 443 444 444

400

(cm-1) 1. HZSM-5

2. SB-2 3. SB-3 4. SB-4

b 2000

Measurement of NHs-t.p.d. Pretreated sample, 0.2 g was put into the sample cell, heated to 773 K for 2 h under pure nitrogen atmosphere to clean the catalyst surface, then cooled to 393 K. The system was purged with helium, ammonia was injected into it until saturation, and the helium rate was increased to 40 ml/min for 1 h to remove the physical adsorbed ammonia. The temperature was then increased to 823 K at the rate 32 K/min. T.p.d. spectra were obtained therefrom.

600

1800

160o

14oo

12oo

(cmd) Figure 1

I.r. spectra of zeolites

samples are shown in Figure I and their vibration frequencies are listed in Table 2. Framework i.r. spectra show that B,A1ZSM-5 zeolite exhibits the same characteristic bands of ZSM-5. Coudurier et al. 2 showed that the i.r. adsorption bands of the pentasil family are near 1230, 1100, 800, 560, and 455 cm-1; there are new bands at 1380, 970, 920, 700, and 670 cm-1 for the pentasil zeolite with boron in the framework. The strongest band at 1380 cm-1 cannot be detected in KBr wafers. For borates, a strong i.r. adsorption band in the region 1100-1400 cm -I characterizes the B-O asymmetric stretching vibration of boron in threefold coordination alone or in complex polymeric anions. When the next-nearest neighbors of boron atoms are progressively substituted by silicon atoms in the B203 framework, the B-O asymmetric.stretching frequency shifts from 1265 to 1380 cm-~. 6--s 'The adsorption band around 1090 cm -1, which is sensitive to the Si/AI ratio of framework elements, shifts to higher frequency. This phenomenon shows that the combined strength of A1-O bond, which has a relative small bond strength coefficient, is weaker than that of the Si-O bond. Therefore, the aluminum content on the zeolite skeleton will decrease as the boron amount added during the synthesis process increases; thus, the adsorption band near 1380 cm -1

Toulene alkylation over ZSM-5 catalysts with boron: L.Z. Chen and Y.Q. Feng

HZSM-5

also shifts to higher frequency, and the strength of the adsorption band increases. These results indicate that the more boron added during the synthesis, the more boron that is contained in the prepared zeolite.

Ii I

....

'~

BZSM-5 SB-2

w

Results of alkylation reaction over various catalysts Table 3 shows the results of the toluene-ethanol alkylation reaction over HZSM-5, BZSM-5, and B,AIZSM-5 catalysts. The measurements over SB catalysts show that the activity decreased and the para-selectivity increased gradually with the increase of skeletal boron. The alkylation reaction over ZSM-5 catalyst with nonskeletal boron shows high paraselectivity and relatively high activity. Although pore structure is an important factor that influences the para-selectivity of the zeolite catalyst, acid properties also play an important role. Because of the effect of modifiers, the amount of strong acid sites is reduced and some undesired side reactions such as isomerization are prohibited. Such a result is in favor of para-selectivity. 9'1° From the t.p.d, date shown in Table 4 and Figure 2, it is found that the total amount and the relative amount of strong acid on B,A1ZSM-5 zeolite catalysts decrease gradually with the increase of boron added; this may cause the para-selectivity of these catalysts to be enhanced. In Table 5, the comparison of the results of the aklylation reaction over the HZSM-5 zeolites modified by boric acid treatment and water-vapor treatment are presented. T h e para-selectivity of the

__.__ SB-4 ......

~stt

....

#Z$

523

62.3

723

#231

SB-5

f2:1

(K) Figure 2 T.p.d. spectra of zeolite catalysts

catalysts treated by boric acid and water vapor are both promoted, but the activity are both decreased. Furthermore, the para-selectivity increases and the activity decreases with the prolongation of treating time. However, the results for these two modification methods are different.

Comparison of the catalytic properties of various catalysts The shape-selective properties of catalysts prepared by different modification methods using boron as modifier are apparently different. These results are shown in Table 6. It is seen clearly that BZSM-5 has the highest para-selectivity and highest activity and that the para-selectivity of WBZ is higher than that of WHZ, while their activities are almost the

Table 3 Alkylation reaction over HZSM-5, BZSM-5, and SB catalysts

same.

Conversion rate of B203 + SiO2 + AI2Oa toluene Catalyst (mol ratio) (C %)

Yield (Y %)

selectivity (S %)

Table 5 Comparison of alkylation results over HZSM-5 zeolite catalysts modified by boric acid solution and water vapor treatments a

HZSM-5 BZSM-5 SB-1 SB-2 SB-3 SB-4 SB-5

43.00 21.18 38.78 36.67 27.89 25.91 24.41

46.60 98.00 44.64 47.09 59.76 75.16 82.17

B203

0.71 2.10 6.68 12.53 17.69

47.98 22.90 41.21 39.75 29.98 26.67 25.37

Para-

Reaction conditions: Temp. 613 K; toluene/ethanol = 1 (mol ratio); nitrogen/(toluene + ethanol) = 12 (mol ratio); WHSV = 2.0 l/h Table 4

Water-vapor treatment

Boric acid treatment

Treatment time (h)

C(%)

S(%)

C(%)

S(%)

1 3 5 12 18

28.43 25.80 20.10 18.76

76.60 85.62 90.51 91.34

27.54 21.23 18.03 18.04 14.54

82.50 90.05 91.11 93.65 93.41

~Reaction conditions are the same as in Table 3

T.p.d. data for catalysts with boron contained T.p.d. areas (cm 2) T.p.d. peak temp.

Ss

Ts (K)

Tw (K)

Sw ( T < 623 K)

Ss ( T > 623 K)

St

Sample HZSM-5 BZSM-5 SB-2 SB-4 SB-5

739 721 705 698

518 543 538 519 523

6.81 9.42 5.29 5.74 4.75

8.44 3.58 7.06 6.02 4.44

15.25 13.00 12.35 11.76 9.19

Adsorption amount (wt%)

Sw

n-hexane

c-hexane

1.24 0.38 1.33 1.05 0.93

9.14 5.00 9.76 9.55 9.44

8.08 4.63 7.73 7.59 7.59

Ts, Ss = T.p.d. peak temperature and area of strong acid sites; Tw, Sw = T.p.d. peak temperature and area of weak acid sites; St = total T.p.d. area

ZEOLITES, 1992, Vol 12, April/May

349

Toulene alkylation over ZSM-5 catalysts with boron: L.Z. Chen and Y.Q. Feng Table 6 Comparison of alkylation reaction over catalysts modified by various methods a Catalyst

C (%)

Y (%)

S (%)

HZSM-5 BZSM-5 SB WHZ WBZ

47.98 22.90 25.37 18.76 18.04

43.00 21.18 24.41 18.34 17.47

46.60 98.00 82.17 91.34 93.65

aReaction conditions are the same as in Table 3

Table.7 Stability of alkylation reaction over catalysts modified by various methods C (%) Reaction time (h) 1 3 5 7 9

HZSM-5

BZSM-5

WHZ

WBZ

28.12 12.07 5.96 -

20.70 11.11 7.17 1.71 -

19.21 15.08 8.10 4.36 -

14.64 12.68 11.13 10.82 10.10

Reaction conditions: WHSV = 4.1 h - l ; other conditions are the same as in Table 3

Table 7 presents the results of the alkylation reaction over various catalysts at high WHSV while other reaction conditions are unchanged. The best stability is obtained over the catalyst treated by boric acid solution. This is caused by the removal of unstable skeletal a l u m i n u m and the f r a m e w o r k is supplemented with boron. Therefore, the catalysts tre-

350

ZEOLITE& 1992, Vol 12, A p r i l / M a y

ated by boric acid solution not only have high alkylation activity and para-selectivity but also high stability. CONCLUSIONS I. ZSM-5 zeolite catalyst with nonskeletal boron prepared by impregnation has high para-selectivity, but it does not have high stability. 2. B,AIZSM-5 zeolite catalysts with skeletal boron are prepared by adding boron during the synthesis. The result of the alkylation reaction over these catalysts shows that the para-selectivity increases with the increase of skeletal boron. 3. A new modification method is suggested, i.e., partially dealuminated ZSM-5 zeolite catalysts are prepared by boric acid solution treatment. T h e alkylation reaction over these catalysts shows that their para-selectivities and stabilities are higher than that of B,A1ZSM-5 catalyst.

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