MFI-Type Metallosilicates as Useful Tools to Clarify What Determines

Chapter 11

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MFI-Type Metallosilicates as Useful Tools to Clarify What Determines the Shape Selectivity of ZSM-5 Zeolites 1

T. Komatsu, J.-H. Kim , and T. Yashima Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan Ethylation of ethylbenzene and methylation of 2-methyl naphthalene on H Z S M - 5 are controlled by restricted transition state shape selectivity in the pore, yielding target molecules, pdiethylbenzene and 2,6-dimethylnaphthalene, respectively, as primary products. Weakening the acid strength and/or poisoning acid sites on the external surface of zeolite crystallites suppress the secondary isomerization of these primary products to improve the final selectivity to the target molecules. MFI-type metallosilicates having weaker acid strength than H Z S M - 5 and similar pore tortuosity to Z S M - 5 are of great advantage to differentiate the effects of acid strength and pore tortuosity on the shape selectivity.

Shape selectivity of zeolite catalysts is essentially determined by the relation between the sizes of zeolite pore and molecules of reactants, products and intermediates. In the case of alkylation of aromatic hydrocarbons, products such as dialkylbenzene and dialkylnaphthalene consist of isomers with different molecular dimensions, often resulting in the shape selective formation of the smaller isomers on zeolite catalysts. However, the isomerization of such smaller isomers into larger ones will sometimes make the shape selective catalysis ambiguous. Because the acid strength necessary for the alkylation and isomerization is different, that is, weak acid sites only catalyze the alkyla tion, the selectivity in the alkylation strongly depends on the acid strength of zeolites. Though modification of zeolites by the addition of metal oxides has been used to improve the shape selectivity, it will change the pore dimension and acid strength of zeolites, simultaneously. On the other hand, metallosili'Current address: Department of Chemical Technology, Chonnam National University, Kwangju 500-757, Korea.

162

© 2000 American Chemical Society

In Shape-Selective Catalysis; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

163


cates, whose acid strength is controlled by changing the metal cation in their framework, ideally possess the same structure as their aluminum analogue in zeolites, which accordingly provides the same diffusivity to the molecules inside the pore. Therefore, metallosilicates would be an effective tool to clarify the reason for the shape selectivity in the alkylation on zeolite catalysts. In this report, we used MFI-type metallosilicates as analogues to ZSM-5 in the ethylation of ethylbenzene and methylation of 2-methylnaphthalene to obtain the details in shape selectivity. Ethylation of Ethylbenzene Alkylation of alkylbenzene is one of the commercially important processes because j?ara-dialkylbenzene is one of the industrially useful chemicals. This reaction has been carried out using HZSM-5 zeolites modified with some metal oxides (1-6) to obtain high selectivity to />ara-isomers. Though many researchers have studied the factors governing the high para-selectivity of the modified HZSM-5 zeolites, a definite conclusion is not established yet. Kaeding et al. (3) have proposed that the high /?ara-selectivity of modified HZSM-5 zeolites for alkylation is due to so-called 'product selectivity', that is, the intracrystalline diffusivity of para-isomer is much higher than that of the other two isomers. In the report on the alkylation of toluene with ethanol (7), /?ara-isomer was formed selectively inside the ZSM-5 channels, while isomerization of the para-isomer proceeded on the external surface of zeolite crystallite. Therefore, the improvement in /?ara-selectivity by the modification with metal oxides was thought to result from the deactivation of acid sites on the external surface. On the other hand, we have proposed that the primary product in the alkylation of alkylbenzene is only /?ûr#-isomer because of so-called 'restricted transition-state selectivity' inside the ZSM-5 pore and that the other isomers are produced through the secondary isomerization of the primarily produced paraisomer (4,5). The modification by metal oxide would reduce the strength of acid sites, on which the secondary isomerization is accelerated. The influence of the secondary isomerization on the strong acid site has been discussed previously (8-11). Experimental. NaZSM-5 with Si/Al atomic ratio of 96 was prepared hydrothermally and transformed into HZSM-5 by NH -exchange and subsequent calcination in air (2). Metallosilicates with MFI structure, expressed as Me-MFI (Me=Ga, Fe and B), were prepared in a similar manner to HZSM-5 and had Si/Ga=64 (12), Si/Fe=56 (13% and Si/B=70 (14% respectively. Antimonosilicate (Sb-MFI, Si/Sb=120) and arsenosilicate (As-MFI, Si/As=92) were prepared by the atom-planting method using MFI silicalite (Si/Al>1000) and SbCl and AsCl , respectively (15,16% HZSM-5 and Me-MFI modified with Mg, Ρ or Β oxide were prepared by an impregnation method (4,5,17), and expressed as Mg(x)HZSM-5, where χ was the amount of Mg in wt%. The ethylation of ethylbenzene with ethanol was carried out with a +

4

3

3


164 continuous flow reactor at 673 Κ under atmospheric pressure. Partial pressures of ethylbenzene and ethanol in helium carrier were both 20 kPa. Conversion and yield were calculated based on the amount of ethylbenzene reacted. The para-selectivity is defined as the fraction of para-isomer in diethylbenzene produced. For the selective poisoning of acid sites on the external surface of zeolite crystallites, the ethylation of ethylbenzene with ethanol was carried out under the same reaction conditions with 2,4-dimethylquinoline (2,4-DMQ, 0.075 ml h") co-fed with the reactant (18). The cracking of 1,3,5-triisopropylbenzene (1,3,5-ΤΠΡΒ) was performed in the presence or absence of 2,4-DMQ to know the extent of poisoning on the external surface (29). The effective pore dimension of each catalyst was estimated from the relative adsorption velocity of o-xylene. The catalyst (0.1 g) was set on a highly sensitive microbalance and evacuated at 823 Κ for 2 h. The gravi metric measurement was performed at 393 Κ with the vapor of o- or p-xylene (0.48 kPa). The acid properties of catalysts were examined by temperatureprogrammed desorption of ammonia (NH -TPD). The measurement of N H TPD was conducted, using a conventional static adsorption system connected to a quadrupole mass spectrometer through a high vacuum line. The catalyst was evacuated at 823 Κ for 1 h and exposed to 20 kPa of ammonia at 423 Κ for 30 min. After the evacuation at 423 Κ for 1 h, TPD was carried out by raising the temperature in vacuo from 298 to 823 Κ at a constant rate of 10 Κ min" .


1

3

3

1

Para-Selectivity in Ethylation of Ethylbenzene. Products in the reaction of ethylbenzene with ethanol on various HZSM-5 and Me-MFI catalysts were pand m-diethylbenzene as main products, benzene in a considerable amount, and 0-diethylbenzene in a trace amount, indicating that the ethylation as well as the dealkylation of ethylbenzene occurred under the reaction conditions. As shown in Table I, the para-selectivity of the catalyst was obtained at an almost constant yield of diethylbenzene (15-20%) by adjusting W/F. In the case of HZSM-5, the para-selectivity increased by the addition of Β-, P- and Mg-oxide though the catalytic activity decreased significantly. It is clear that all the Me-MFI catalysts exhibited the higher para-selectivity than HZSM-5. In particular, As-MFI gave the highest para-selectivity of 94% among the unmodified catalysts. The para-selectivity of Me-MFI catalysts was also improved by the modification with metal oxides. As a result, B(10)HZSM-5, B(5)Ga-MFI, B(l)Sb-MFI and B(3)Sb-MFI exhibited the perfect para-selectivity of 100%. Primary Product in Ethylation. In order to clarify the primary product in the ethylation, reactions were carried out on HZSM-5 with various W/F at 673 K. The distribution of diethylbenzene isomers is plotted against W/F as shown in Figure 1, where the yield of diethylbenzene was in the range of 326%. As W/F decreased to approach zero, the fraction of p-diethylbenzene increased to 100%, while that of m- and o-isomer decreased to 0%. This clearly indicates that the primary product in the ethylation on HZSM-5 is p-


165

Table I.


No.

Ethylation of Ethylbenzene with Ethanol on M F I Catalysts Diethylbenzene Yield/%

/%

HZSM-5 B(l)HZSM-5 B(3)HZSM-5 B(10)HZSM-5 P(l)HZSM-5 P(5)HZSM-5 Mg(18)HZSM-5

19.8 17.9 18.8 17.8 20.1 16.3 19.9

43.2 48.9 55.3 100.0 49.3 89.8 72.4

8 9 10 11 12

Ga-MFI Fe-MFI B-MFI Sb-MFI As-MFI

16.9 16.5 16.6 15.0 15.3

59.6 61.0 70.3 90.4 94.1

13 14 15 16 17 18

B(0.5)Ga-MFI B(l)Ga-MFI B(3)Ga-MFI B(5)Ga-MFI B(l)Sb-MFI B(3)Sb-MFI

18.5 17.5 16.7 17.1 15.2 15.6

67.1 77.3 94.3 100.0 100.0 100.0

1 2 3 4 5 6 7

Catalyst

Para-Selectivity

SOURCE: Adapted with permission from reference 6. Copyright 1991 Elsevier.

diethylbenzene. The primary products on H Y , which does not have significant shape selectivity for this reaction, were o- and ^-diethylbenzene, as was explained by ortho/para orientation (4). Therefore, at the initial stage of the reaction, HZSM-5 shows 'restricted transition-state selectivity' to depress the formation of odiethylbenzene. The increase in the fraction of m-isomer and the decrease in that of p-isomer with increasing W / F indicate the secondary isomerization of p-isomer into m-isomer. Although the molecular dimension of 0-isomer is almost the same as that of m-isomer, only a trace amount of oisomer was observed even at high W / F values. These results suggest that HZSM-5 has active sites for the isomerization of diethylbenzene and that it does not have 'product selectivity' due to the difference in diffusion of diethyl benzene isomers. From these results, it is obvious that the suppression of secondary isomerization w i l l be effective to achieve the higher para-selectivity on HZSM-5. A s shown in Table I, M e - M F I and modified HZSM-5 gave higher para-selectivity than HZSM-5. Therefore, they must have low activity for the secondary isomerization compared with HZSM-5. In the following sections, we try to clarify the reason for the very high para-selectivity of M e M F I and modified HZSM-5 resulting from this low activity for the isomeriza tion; the effects of acid sites on the external surface, pore tortuosity and acid strength are discussed.



166

log(l+W/F) Figure 1. Effect of contact time on the fraction of diethylbenzene isomer in ethylation of ethylbenzene on H Z S M - 5 . (Adapted with permission from reference 4. Copyright 1988 Chemical Society of Japan.)



167

Effect of Acid Sites on External Surface. Acid sites on the external surface of zeolite crystallites usually decrease the shape selectivity because they do not have significant steric effect by pore walls. In order to clarify the effect of the acid sites on the external surface on the para-selectivity, the ethylation of ethylbenzene with ethanol was carried out with co-feeding 2,4-DMQ. This base molecule selectively poisons the acid sites on the external surface of HZSM-5 because its molecular dimension is too large to enter the pores of ZSM-5 (18). The results are summarized in Table tt The complete poison ing of the acid sites on the external surface was confirmed by the complete deactivation of the catalysts for the cracking of 1,3,5-TIPB, which is too large to enter the pore of ZSM-5 and will be cracked only by the acid site on the external surface (19). HZSM-5 and Me-MFI poisoned with 2,4-DMQ were completely inactive for the 1,3,5-TEPB cracking, indicating the complete poisoning of external surface. Although the selective poisoning improved the para-selectivity of HZSM-5 and Me-MFI to some extent, the effect of oxide modification was much stronger to achieve 100% para-selectivity (Table Γ). These results indicate that the weak activity for secondary isomerization, which leads to the very high para-selectivity, is not caused mainly by the poisoning of the external surface but predominantly by the suppression of the isomerization activity inside the pores of ZSM-5.

Table Π.

Poisoning of acid sites on the external surface with 2,4-dimethyquinoline (DMQ) a

Catalyst

2,4-DMQ

Diethylbenzene Yield 1%

ParaSelectivity /%

TIPB Conversion /%

HZSM-5

without with

26.3 16.9

39.2 57.2

51.1 0.0

Ga-MFI

without with

33.1 29.9

36.7 40.2

22.9 0.0

Fe-MFI

without with

25.6 19.2

35.5 50.9

41.0 0.0

B-MFI

without with

16.6 8.7

70.3 83.0

5.5 0.0

Sb-MFI

without with

15.0 7.2

90.4 92.4

0.7 0.0

a

l,3,5-Triisopropylbenzene

Effect of Pore Tortuosity. Micropores of MFI structure often influences the internal diffusion of specific molecules, which may generate molecular shape selectivity. In order to clarify the effect of the pore tortuosity on the paraselectivity in the ethylation of ethylbenzene, the adsorption measurements of o-



168

and p-xylenes were carried out at 393 K, where these adsorbates did not react by itself. Figure 2 shows the typical o-xylene uptake curves for the modified HZSM-5 and Me-MFI catalysts. The initial rate of adsorption depended on the catalyst and the equilibrium was not achieved for every catalyst within 200 min of the adsorption. This adsorption behavior must reflect the pore tortuosity of each catalyst. The pore tortuosity was estimated by comparison with the adsorption rate of p-xylene, which has a smaller molecular size than that of oxylene. The adsorption of p-xylene on every catalyst reached the equilibrium within 30 min. The amount of p-xylene adsorbed at the equilibrium may correspond to the pore volume. From the results of o- and p-xylene adsorption experiments (5,16,17,20, 21), we determined 'time to reach 30% of amount of o-xylene adsorbed in infinite time', t , as a parameter of the pore tortuosity, as used by Mobil's researchers (22). Furthermore, we determined 'relative o-xylene adsorption velocities', V , as another parameter of the pore tortuosity obtained from the amount of o-xylene adsorbed in 180 min divided by that of p-xylene adsorbed in infinite time, where the amount of p-xylene adsorbed in infinite time may correspond to the pore volume. In the case of disproportionation of toluene, para-selectivity, fraction of p-xylene, decreased monotonously with V , suggesting the product selectivity (23). The relation between the para-selectivity in ethylation of ethylbenzene and the pore tortuosity, t and V A ? is shown in Figures 3 and 4, respectively. In the cases of parent (solid circle, No. 1) and modified (open circle, No. 2-7) HZSM-5, a close relation was observed both in Figures 3 and 4, that is, the para-selectivity increased with t and decreased with V . The paraselectivity of each parent (solid symbols, No. 8-12) and modified (open symbols, No. 13-18) Me-MFI was always higher than that of HZSM-5 catalysts when compared at the similar pore tortuosity. It is clear that a close relation between the para-selectivity and the pore tortuosity was never found for all the MFI catalysts examined here. It is doubtful that the para-selectivity in the ethylation of ethylbenzene on MFI catalysts is directly governed by the diffusivity of product molecules. Therefore, it would be concluded that the product selectivity is not the reason for the high para-selectivity in the ethylation of ethylbenzene on MFI catalysts. 03

R 0 A

ROA

03

RO

03

R 0 A

Effect of Acid strength. The effect of acid strength of MFI catalysts is finally examined because strong acid sites will be necessary for the isomerization of dialkylbenzenes but even weak acid sites will catalyze the alkylation of alkylbenzenes. TPD of adsorbed ammonia was used to estimate the acid strength of the MFI catalysts. In general, the peak temperature in an NH TPD profile corresponds intrinsically to the acid strength of the catalyst. However, it sometimes shifts to higher temperatures because of the readsorption of desorbed ammonia and/or the diffusion limitation (24). In this study, though every catalyst has the same MFI structure, its Si/metal ratio varies in the range of 56-120, indicating that the acid concentration differs significantly 3


169

100

Fe-MFI

'οο βο

^^=======

ε

Ga-MFI HZSM-5


_——

B-MFI Sb-MFI

— —

^—^^B(3)HZSM-5

B(10)HZSM-5 ο 0

50

100

150

200

Adsorption time /min Figure 2. Change in the amount of o-xylene adsorbed on catalysts. (Adapted with permissionfromreference 20. Copyright 1991 Elsevier.) 100

< 12

14

80

10

9

S Ο

60 •S

I S «5

40

g

•

• 11

Δ

\13 Ο

Ο

2

5

ο 3

1

20

10

15

20

to.3 /min Figure 3. Relation between para-selectivity in ethylation of ethylbenzene and pore tortuosity, to , of catalysts. Number: see Table I. 3

(Adapted with permission from reference 6. Copyright 1994 Elsevier.)


170 depending on the catalyst. Therefore, the peak temperature may not abso lutely correspond to the acid strength. N H - T P D measurements were carried out in vacuo by using a very small amount (18 mg) of catalyst to minimize the effect of readsorption. We used the peak temperature in N H - T P D , T , as a parameter of the acid strength of catalysts. Figure 5 shows the typical profiles of N H - T P D for H Z S M - 5 and M e - M F I . The peak in the profile for all the M e - M F I catalysts appeared at a temperature lower than that for H Z S M - 5 . A s was expected, the modification of H Z S M - 5 and M e - M F I with oxide lowered the peak temperature. It is clear that the acid strength of M e - M F I and that of the modified zeolites are weaker than that of H Z S M - 5 . The relation between the para-selectivity in the ethylation of ethyl benzene and the acid strength, T , is shown in Figure 6. A close relation is observed for all the catalysts, that is, the weaker the acid strength, the higher the para-selectivity. This indicates that the para-selectivity in the ethylation on catalysts with M F I structure is strongly governed by their acid strength. 3

3

m a x


3

m a x

Generation of Para-Selectivity. The primary product in the ethylation of ethylbenzene on catalysts with M F I structure is p-diethylbenzene because of the restricted transition-state selectivity. Secondary isomerization of p-isomer into m-isomer lowers the para-selectivity of the catalysts. Only strong acid sites catalyze this isomerization, while the ethylation readily occurs on weak acid sites. Metallosilicates with M F I structure and H - Z S M - 5 modified with metal oxides have weak acid strength compared with H - Z S M - 5 . Therefore, on these catalysts, primarily produced p-isomer does not suffer the isomeriza tion to obtain high para-selectivity. It is concluded that the para-selectivity is caused by the restricted transition-state selectivity and the absence of strong acid sites. Methylation of 2-Methy lnaphthalene As was revealed above, metallosilicates are useful tools to clarify what gener ates the shape selectivity of zeolite catalysts. Next we studied on the shape selective formation of dimethylnaphthalene ( D M N ) again using M F I type metallosilicates. Naphthalene derivatives having substituents at C-2 and C-6 positions are important raw materials for manufacturing sophisticated polymers of special functions. Methylation of naphthalene (25-27) and transalkylation of methylnaphthalene (28-30) at these positions, therefore, have been studied using shape-selective zeolite catalysts in industrial prospects as well as for the further understanding of shape selective nature of zeolite catalysis. Fraenkel et al. (25,26) have carried out the methylation of naphthalene and 2-methylnaphthalene (2-MN) with methanol on various zeolites and found that H Z S M - 5 is highly selective for the alkylation at β-position to form 2,6- and 2,7dimethylnaphthalene ( D M N ) compared with H Y and H-mordenite. They attributed the high selectivity of H Z S M - 5 to the shape-selective sites called 'half-cavities' located on the external surface of H Z S M - 5 crystallites. On the other hand, Weitkamp and Neuber (27) have studied the methylation of 2 - M N


171

100 12

£

8 0

Δ

Ο


I

7

*Δ 9

3

6 0

Ο Ο Ο 5

40

I

10

14

2

13

^ 8

• 1

20

0.2

0.4

0.6

0.8

VROA

Figure 4. Relation between para-selectivity in ethylation of ethylbenzene and pore tortuosity, V , of catalysts. Number: see Table I. (Adapted with permission from reference 6. Copyright 1994 Elsevier.) R 0 A

Figure 5.

N H - T P D profiles. 3



172

on HZSM-5 and HZSM-11, and proposed that the high selectivity to 2,6- and 2,7-DMN is caused by the product shape selectivity, that is, the reaction occurs inside the pore of these zeolites. In the case of the transalkylation of 2-MN, Matsuda et al. (30) have claimed that the high selectivity to 2,6- and 2,7-DMN on dealuminated HZSM-5 is caused by its shape selectivity inside the pore. We have carried out the methylation of 2-MN with methanol using HZSM-5 and MFI-type metallosilicates as catalysts to obtain 2,6-DMN (31). The reaction path was first examined taking account of the contribution of the acid sites inside and outside the zeolite crystallites. Then we will discuss on the factors determining the selectivity and a possible way to obtain 2,6-DMN with high selectivity. Experimental. ZSM-5 (Si/Al=43), Ga-MFI (Si/Ga=42), Fe-MFI (Si/Fe=37) and B-MFI (Si/B=70) were prepared by hydrothermal synthesis. Sb-MFI (Si/Sb=120) was prepared by the atom-planting method (15). These were transformed into proton type by the usual ion-exchange technique. Boron oxide added HZSM-5 was prepared by the impregnating HZSM-5 with an aqueous solution of boric acid followed by the calcination in air. The methylation of 2-MN with methanol was carried out with a continuous flow reactor under atmospheric pressure at 723 K. Partial pressures of 2-MN and methanol in helium carrier were 6.2 and 14.1 kPa, respectively. Conversion and selectivity were calculated based on the amount of 2-MN reacted. Similar techniques were taken for the selective poisoning of the acid sites on the external surface, the adsorption of o-xylene, and NH -TPD. 3

Methylation of 2-MN on HZSM-5. Main products in the reaction of 2-MN with methanol on HZSM-5 at 723 Κ were isomers of D M N and 1-MN though small amounts of naphthalene, ethylnaphthalene and trimethylnaphthalene were observed. Among ten isomers of DMN, 2,6- and 2,7-DMN were produced with relatively high fractions. Other DMNs produced were 1,2-, 1,3-, 1,6-, 1,7- and 2,3-isomers. The high fractions of 2,6- and 2,7-DMN imply the shape selective catalysis of HZSM-5 as was reported for the alkylation of naphthalene derivatives on HZSM-5 (25-27). Data obtained at 30 min on stream were discussed hereafter because of the gradual deactivation. Reaction path. As shown in Figure 7, the effect of contact time on the fraction of D M N isomers was examined to clarify the reaction path. When W/F was extrapolated to zero, only four isomers of DMN were observed, i.e., 2,6- (initial fraction of 50%), 2,7- (39%), 2,3- (6%) and 1,2-DMN (5%), indicating that these are the primary products in DMN. As W/F increased, the fractions of 2,6- and 2,7-DMN decreased and those of 1,6-, 1,7- and 1,3-DMN increased in the opposite manner. This can be explained by the secondary isomerization of the initially produced 2,6- and 2,7-DMN into the latter isomers. The slight increase in the fractions of initially produced 1,2- and 2,3-DMN indicate the secondary isomerization of 2,6- and 2,7-DMN also into these isomers. Figure 7 indicates that the reaction rate of methylation is much faster


Ο

1

450

'

'

500

550

» 600

Tmax in N H s - T P D / K Figure 6. Relation between para-selectivity in ethylation of ethylbenzene and acid strength, T , of catalysts. Number: see Table I. (Adapted with permission from reference 6. Copyright 1994 Elsevier.) m a x

60 r

»-

MFI-Type Metallosilicates as Useful Tools to Clarify What Determines

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