Shape-Selective Catalysis - American Chemical Society

Chapter 9

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Role of Shape Selectivity in n-Heptane Cracking and Aromatization Reaction on Modified ZSM-5 T. S. R. Prasada Rao, N. Viswanadham, G. Murali Dhar, and N. Ray Indian Institute of Petroleum, Dehra Dun 248 005, India ([email protected])

Cracking and aromatization reaction of n-heptane was carried out on ZSM-5 modified by three methods: They are (i) Extra framework alumina obtained by steaming at high temperature, (ii) Altering the pore size by coke deposition as a function oftimeon stream, and (iii) Presence of amorphous material in the zeolite. The effect of these modifications on the shape selective product p-xylene is investigated. In all the cases the shape selective properties are profoundly influenced. The presence of extra framework alumina, and coke increased the concentration of para xylene in the product. In the case of ZSM-5 containing amorphous material obtained by synthesizing the zeolite without the aid of template, the para xylene in the product was higher than that obtained over zeolite synthesized using the template. In all the cases the increase in shape selective product may be resulted by decrease in channel dimensions due to the presence of the extraneous materials in the channel. Pore size distribution studies on deactivated catalyst provided evidence for the operation of molecular traffic control (MTC) mechanism in the case of n-heptane aromatiztion reaction. Ever since weisz and co-workersfromMobil coined the word shape selectivity and demonstrated its commercial potential using small pore molecular sieves, shape selective properties of zeolites have been studied with great interest [/]. The availability of MFI type medium pore zeolites of ZSM family with pore sizes compatible with molecules in gasoline range has opened up new vistas in hydrocarbon transformations [2,3]. Based on their acidic and shape selective properties a number of processes were developed in the recent past [2]. Xylene isomerization, Methanol to gasoline, Distillate and lube dewaxing, M-forming, Mobil selective toluene disproportionation, manufacture of para diethyl benzene are 130

© 2000 American Chemical

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

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131 some of the many applications where shape selectivity concept has commercially exploited. Various types of shape selectivities that can be considered in ZSM-5 system are the reactant shape selectivity, this type of selectivity is observed when only part of the feed molecules are small enough to diffuse through the zeolite pores. Mobil middle distillate dewaxing and selecto-forming are good examples in this context. Another important type of shape selectivity is the product shape selectivity. The product shape selectivity occurs when some of the products formed with in the pores are too bulky to diffuse out to be observed as products. They are either converted to less bulky molecules or eventually deactivate the catalysts by pore blocking. Xylene isomerization, toluene methylation etc. are good examples in this connection. Yet another type of shape selectivity is transition-state shape selectivity, where certain reactions are prevented because corresponding transition-state would require more space than that is available in the zeolite pores or intersections. Since, neither reactant nor potential product molecules are prevented from diffusing, the reactions that proceed with smaller transition-state proceeds unhindered to alter the selectivity. Another variant of shape selectivity is molecular traffic control, which occurs in zeolites with more than one type of pore system [4]. The reactant molecules here may preferentially enter the catalyst through one of the pore systems, while the products diffuse out through the other type. Thus counter diffusion is minimized to maximize the selectivity. There have been extensive research efforts to understand the nature of shape selectivity in industrially important reactions like xylene isomerization, alkyl benzene disproportionation and alkylation, in spite of such extensive efforts the phenomenon is not well understood. The para selectivity is influenced by transitionstate, product selectivities, non selective reactions on external surface and, also by variations in the acidity and acid strength distribution. In addition higher quantities of shape selective products can be obtained by altering the pore dimensions by modification of the zeolite by extraneous matter present in the pores like extra framework alumina includes Ρ, B , M g or by altering the pore mouth by depositing silica by silylation. Yet another way of pore modification that is common in hydrocarbon transformations is by coke. The deposited coke can reduce the channel dimensions and there by increase the shape selectivity in favor of faster diffusing molecule. Amount of acidity and the distribution of acid strength of the sites, in both internal and external surface of the zeolite crystal is another factor that alters the shape selectivity by controlling side reactions or by non shape selective transformations on acid sites present on the external surface. There are numerous examples in this well studied area, a few examples relevant to the work presented will be discussed here. Kaeding et. al. [5] studied disproportionation of ethyl benzene, alkylation of toluene and ethyl benzene where they found that modification of the channels of ZSM-5 with oxides of Ρ and Β improves the selectivity for para isomer. They attributed this to the significantly higher diffusivity of para isomer due to decrease in channel dimensions. The diffusivity of para isomer in ZSM-5 is thousand times faster than other two isomers [6]. However, Chandawadker et. al. [7] reported that the presence of Β, Ρ and organic bases enhanced para selectivity by suppressing isomerization of primary product, para ethyl toluene. Kim et. al. [8] studied alkylation of ethyl benzene with



132

ethanol on MFI and M E L zeolites, and found that zeolites modified by steam, or coking or modified by oxides, exhibited much higher para selectivity. Toluene ethylation was also studied by Lonyi et. al. [9] on modified ZSM-5 containing oxides of Ρ, B , M g etc. and observed higher para selectivities and concluded that both change in diffusivity due to decrease in channel dimensions as a result of modifications, and alteration of acidity and acid strength distribution play an important role. Prasada Rao and co-workers reported increased selectivity for isomer, p-diethyl benzene in alkylation of ethyl benzene on silylated ZSM-5 [10]. It can be seen that both diffusivity modified by channel dimensions and change in acidity play important role in altering the shape selectivity. In some cases the presence of amorphous material in the zeolite may also effect the concentration of shape selective products due to promotion of side reactions. In this investigation the effect of extra framework alumina obtained by steaming the zeolites at different temperatures, coke formed during time-on-stream, and presence of amorphous materials in the zeolites, on η-heptane aromatization reaction are presented. The studies on deactivated catalysts also provided evidence for operation of molecular traffic control mechanism in the case of η-heptane aromatization reaction. Experimental ZSM-5 was synthesized following well known literature procedures [11]. The steam treatment of N H ZSM-5 was carried out in a fixed shallow bed reactor, the steaming was carried out with 100% water vapour, with water flow rate of 76ml/hr at different temperatures 300-600°C for 3h. The extra framework aluminum was removed by acid leaching with I N HC1 at 100°C for 2h. The samples after calcination at 480°C for 12 hours were used for activity evaluation studies. The η-heptane aromatization reaction was carried out in a fixed bed down flow reactor at 773°K, L H S V : 2h , N / H C : 2, pressure : 10 kg/cm TOS : 12h [4]. The pore volume and pore size distributions of the fresh and deactivated catalysts were evaluated on mieromeritics equipment using argon adsorption and Harvath and Kawazoe method. Both micro and mesopores were brought into single display by using density function theory. 4

-1

2

2

Results and Discussion Effect of foreign materials on shape selectivity has been examined by several investigators. Chen, Kaeding and Dwyer observed higher para xylene selectivities by modifying the ZSM-5 by phosphoric acid, boron compounds or coating the surface with a polymer [1,2]. Kaeding et. al. [75] observed increased selectivity for para isomer by modifying the ZSM-5 with B , M g and Silicon. They have also observed increase of p-ethyl toluene as a function of coke deposition. Kim et. al. [8] modified the ZSM-5 and ZSM-11 with coke or by steaming and found that the presence of coke or extra framework alumina increased para isomer in alkylation of ethyl benzene with ethanol. In this investigation the zeolite material was modified by steaming and mild coking, and the effect of these modifications on shape selectivity is studied. ZSM-5 zeolites containing three types of foreign materials viz. extra framework alumina, coke and amorphous materials were used in this investigation.


133 The extra framework alumina was obtained by steaming ZSM-5 at different temperatures under specified conditions mentioned in the experimental section. Fresh as well as samples steamed at different temperatures were analyzed by chemical analysis, S i N M R for evaluation of framework alumina, and total alumina. These results are presented in Table-I. The extra framework alumina was confirmed by A l N M R , spectra also. The coke on the catalyst was increased as a function of time on stream and the amount of coke deposited was analyzed. 2


2 7

Table-I Dependence of the Concentration of the Aluminum Types on the Steaming Severity of ZSM-5 Samples Steaming Concentration of al Species Per Unit Cell Severity ALF ALEF ASEFAL AIEFAL ΑΙχοτ (1) (2) (3) (4) (5) (6) (7) 4.4 Z-35 0°C 4.5 SÎZ-35

300°C/3hr

4.3

0.2

0.1

0.1

4.4

S Z-35

400°C/3hr

2.6

1.9

1.4

0.5

3.1

S3Z-35

500°C/3hr

2.1

2.4

2.0

0.4

2.5

S4Z-35

600°C/3hr

1.4

3.1

2.8

0.3

1.7

2

(1) (2) (3) (4) (5) (6) (7)

Zeolite samples : Z-35 (ZSM-5, SAR=35), and SiZ-35 to S Z-35 are the samples steamed at different temperatures Samples treated with steam at different temperatures Framework atoms per unit cell calculated from S i M A S N M R Extra framework alumina per unit cell Acid soluble extra framework alumina per unit cell Acid insoluble extra framework alumina per unit cell Total alumina per unit cell after the steaming and acid treatments 4

29

The synthesis without the aid of organic template resulted in formation of ZSM-5 zeolites with lower-crystallinity due to the presence of amorphous material, which is deduced by comparing the X R D crystallinity with the corresponding templated zeolite material containing same Si/Al ratio. The η-heptane aromatization reaction carried out in a fixed bed down flow reactor at 773°K, LHSV: 2h~ , N2/HC : 2 and pressure 10 kg/cm . A typical product distribution in η-heptane aromatization reaction is shown in Table-II. It can be seen that L P G range products and aromatics are obtained by a series of cracking, hydrogen transfer, oligomerization and cyclization reactions. The effects of shape selectivity were followed by detailed analysis of various xylenes formed. 1

2


134


Effect of Extra Framework Alumina (EFAL) On Shape Selectivity. The distribution of xylene isomers as a function of amount of extra framework aluminum in ZSM-5 is presented in Table-Ill. It can be noted that selectivity for p-xylene increases and selectivity for m-xylene and C9+ aromatics decreases which indicate that in addition to the enhanced shape selectivity, the side reactions also decreased due to acidity modification by dealumination. It is also clear that increase in extra framework alumina indeed increases para selectivity. Table-IV gives the amount of alumina in the channels of zeolite, and product distributions in n-heptane aromatization on ZSM-5 unsteamed, steamed and acid treated zeolites ZSM-5.

Table - II Typical Product Distribution in n- Heptane Aromatization Reaction Yields (wt. %)

Product Component Ci Methane

1.8

C2 Ethane

4.5

C 3 Propane

35.0

C4 Butanes

22.8

C + Paraffins

7.4

5

Aromatics

28.5

Benzene

1.7

Toluene

10.2

Xylenes

13.6 3.0

C9+ Aromatics U

1

z

Reaction conditions : Temperature = 500 C, WHSV = 6 hr" , Pressure =10 kg/cm , N / H C = 2, Catalyst = H-ZSM-5 (4.5 Al/U.C) 2

The conversion decreased on steamed zeolites as expected since the decrease of aluminum ions in the framework decreases the aromatization activity. The aromatic yields are also decreased in a similar manner. In steamed zeolite containing extra framework alumina and the same zeolite where extra framework aluminum is removed by acid treatment, both showed similar conversion and aromatic yields Table-IV. It is interesting to see how the aromatic distribution, particularly para xylene yield varies in these three cases. It can be noted that the para xylene concentration is more in the steamed zeolite, where in extra framework alumina (EFAL) is present, compared to either unsteamed or acid treated zeolite and o- xylene and m - xylene yields varied exactly in the opposite way. The fact that the unsteamed and acid treated zeolites have comparable xylene selectivities indicates that the enhanced para selectivity obtained is indeed due to the presence of extra framework alumina.


135 Table - III Effect of Extra Framework Alumina (EFAL) on Shape Selectivity EFAL / Unit Cell

0.0

1.9

2.4

3.1

p- Xylene

21.4

28.6

34.0

39.8

m - Xylene

18.6

17.5

16.0

15.5

C9+ Aromatics

23.5

22.0

18.6

16.0

Total

100.0

100.0

100.0

100.0


Selectivity (wt %)

Table - IV Effect of Extra Framework Alumina (EFAL) on Shape Selectivity Catalysts

Unsteamed

Steamed

Acid treated

Conversion

96.0

86.2

87.1

Aromatics Yield

29.0

25.6

24.3

Benzene

7.2

5.5

6.6

Toluene

28.0

26.7

29.7

Ethyl Benzene

1.8

2.1

1.9

Ρ - Xylene

13.5

19.0

14.0

m - Xylene

23.0

29.0

18.7

0-Xylene

11.7

10.3

16.2

C9+ Aromatics

23.5

22.0

18.6

Aromatic Distribution (wt%)

Effect of Coke Formation on Shape Selectivity. Formation of coke is invitable in hydrocarbon reactions even on the ZSM-5 zeolite which is known for its coke resistance especially in aromatization reactions. We have examined the effect of coke on ZSM-5 zeolite which is on stream for several hours. The aromatic distribution as a function of time on stream is analyzed and the selectivity to p-xylene is taken as a scale to measure the shape selectivity change due to coke formation. It can be seen from the data Table-V, that the p-xylene concentration in the product increases a function of time-on-stream. It can also be seen that ortho and meta isomer concentration decreases simultaneously. It is clearly indicated that isomerisation of ortho, meta xylenes into para isomer is indeed happening due to their lower diffusivities. The presence of coke on the catalyst is confirmed at the end of the experiment and the carbon analysis showed that there is 6 wt% coke on the catalyst at the end of the run. It appears that the coke reduces the channel dimensions by its presence and infact this decrease is responsible for the enhanced para selectivity.


136


Table - V Presence of Coke on Shape Selectivity Time - on - Stream (TOS) 6 hr 12 hr 24 hr Aromatic Distribution (wt%) Benzene Toluene Ethyl Benzene P-Xylene M - Xylene ο - Xylene C9+ Aromatics Total

8.0 30.0 1.5 14.2 22.5 11.0 12/8 100

5.0 28.5 2.5 17.5 23.0 10.0 13.8 100

5.6 25.0 2.0 21.0 21.0 9.5 15.9 100

To verify weather the pore dimensions are reduced due to coke formation, the pore size distributions of the coked catalysts were evaluated by gas adsorption, and the data is treated using density functional theory. Results showed that meadian pore diameter of the zeolite pore is 6.4 A for the fresh catalysts and the same for the deactivated catalysts is 5.7 A . This clearly shows there is infact decrease of pore diameter which has definitely contributed to the enhanced para selectivity. In order to underwtand the enhanced shape selectivity as a function of coke formation, three spent catalysts that were used in η-heptane cracking reaction were analysed to determine the amount of coke formed. Table V I provides the relevant data on product disturbution as function of reaction severity. The catalyst was on stream for 24 hrs at 425, 450 and 500°C, and the amount of coke and p-xylene formed at each condition are given in Table-VI. The incremental increase in p-xylene selectivity is caliculated from the selectivities at the start of the run and those at the end of the run (24 hrs). As the reaction temperature increases coke formation increased and so does para xylene selectivities. To further appreciate the increase in para selectivity with increase of coke, the incremental increase of p-xylene selectivity as a function of amount of coke formed is shown in Figure-1. One can clearly see the effect of coke on p-xylene selectivity. 0

0

Table-VI Effect of Reaction Severity on Coke Amount and Enhanced p-Selectivity Reaction Temp. (°C) 425 450 500 *Wt%ofeoke 1.54.0 6.0 Start End Aromatic Start End End of Start nrffntim of run Distribution (wt%) of run run of run 5.6 10.0 8.0 Β 8.0 4.0 3.5 20.0 25.0 Τ 25.6 28.0 25.0 30.0 1.2 2.0 2.0 2.0 1.5 EB 2.0 21.0 15.0 25.5 14.2 p-xyene 13.0 15.0 12.0 12.0 22.5 21.0 o-xylene 24.0 20.0 16.0 11.5 11.0 9.5 m-xylene 17.0 17.5 C+ 22.0 20.0 15.9 13.0 16.0 12.8 100 100 Total 100 100 100 1000 * Percent of coke formed (based on catalyst wt) after 24 hrs TOS 9



137

Figure 1. Enhanced Shape Selectivity Due to Coke Formation

Effect of Amorphous Material in Zeolites. The ZSM-5 zeolite was synthesized without the aid of template and also with the use of template in order to compare the relative performance of these materials, as a function of preparation method in nheptane aromatization reaction. The formation of ZSM-5 by both the methods is confirmed by X-ray diffraction method and percent crystallinity was also evaluated which showed that the zeolites synthesized without the aid of template are of lower crystallinity indicating that some amorphous silica-alumina is indeed present, probably inside the pores. In order to understand the effect of presence of this amorphous material on η-heptane conversion and aromatic distribution and particularly on the xylene products, a comparative data on zeolites prepared by both the methods, is presented in Table-VII. It can be seen that conversions are lower on the zeolite containing amorphous material. Table - VII Effect of Presence of Amorphous Material H-ZSM-5 templated

H-ZSM-5 Without Template

99.0

89.0

Benzene

7.2

3.5

Toluene

28.0

22.5

Ethyl Benzene

1.8

2.0

ρ - Xylene

13.5

16.0

m - Xylene

23.0

16.0

ο - Xylene

11.7

8.0

C9+ Aromatics

23.5

32.0

Total

100

100

Catalysts Percent Crystallinity Aromatic Distribution (wt. %)



138 The para xylene selectivities are higher on ZSM-5 containing amorphous material resulted by synthesizing without the aid of template. In this case it is difficult to attribute the enhanced selectivity to pore dimension modification alone, since the ZSM-5 prepared by the two methods differ considerably in acidity and also the presence of amorphous material promotes formation of C9+ aromatics [12]. In spite of these complications this can be taken as an example for alteration of shape selectivity due to presence of extraneous material. In all the cases above discussed V i z extra framework alumina, coke or amorphous materials, the shape selectivity is enhanced and this is manifested in the form of increased p-xylene selectivity, one of the main reason for such an observation is decrease in the channel dimensions and tortuosity due to the presence of such materials. However as pointed out by several authors [7,9], there are several other factors such as external acidity and change in internal acid strength distribution of ZSM-5, which also alters shape selective product formation. Most of the factors that decrease the channel dimensions such as extraneous materials like Ρ, AI2O3, MgO, amorphous material, polymers, silylation alter both internal and external acidity. From this data presented here it is not possible to choose between the two without further investigating the role of acidity. However it appears from the pore size data obtained on coked catalysts that pore size is indeed decreased. Similar thing is expected in the case of extra framework AI2O3 and amorphous material. It is our assessment that decrease in pore diameter is a major factor, while the effects due to acidity can not be ignored, in increasing the para selectivity in the products observed in η-heptane aromatization. Evidence for Molecular Traffic Control Mechanism. It is well known that ZSM-5 crystalline framework consists of two types of intersecting channel systems made up often membered ring openings. One channel system is sinusoidal and has 5.1 χ 5.5 A cross section. The other channel system has 5.3 χ 5.6 A openings which run straight and perpendicular to the first system [13] Figure 2. Investigations by Derouane and co-workers group [4] suggested : 1. Linear aliphatic molecules diffuse freely in ZSM-5 framework and can be adsorbed in both the channels. 2. Isoaliphatic compounds experience steric hindrence effects, which may restrict their diffusion in sinusoidal channel system. 3. Aromatic compounds and methyl substituted aliphatic compounds have strong preference for diffusion and / or adsorption in the linear elliptical channels. Implying that flat and large molecules will prefer to diffuse in wider elliptical channels. η-Heptane aromatization reaction on ZSM-5 catalyst consists of cracking, oligomerization, cyclization and other hydrogen transfer reactions. Applying the above mentioned observations by Derouane et. al. to the present reaction, it is clear that the reactant and/or the olefins formed by cracking, preferentially travel through sinusoidal channels and reach the intersections, where they undergo above mentioned reactions. Whereas, the aromatics and other bulkier products formed, diffuse out through linear channels of ZSM-5 catalyst. The experimental results that furnished an evidence for molecular traffic control in ZSM-5 during n-heptane aromatization reaction are discussed in the following. 0

0



139

PRODUCT

REACTANT

ZIG-ZAG CHANNELS * 5*1 X5-5 A

#

STRAIGHT CHANNELS " 5*3X5>6 Figure 2. Molecular traffic control (MTC) in ZSM-5 channels (adapted from reference 1).


140 2

The η-heptane aromatization reaction was carried out at 500°C, 10 kg/cm pressure with L H S V = 2h in a micro catalytic reactor containing 5 g of the catalyst, which is a mixture of ZSM-5 (4.5 Al/U.C) and alumina in 60:40 ratio. Synthesis, characterization and activity evaluation methods were described elsewhere [12]. Ultra pure nitrogen was used as carrier gas with N2/HC = 2. With a view to understand the effect of coke on pore volume and pore size distribution, the fresh H-ZSM-5 catalyst and the H-ZSM-5 catalyst that was 12 hrs on stream, were examined for pore size distribution using Argon adsorption and Harvath and Kawazoe method. Both micro and meso pore size distributions were brought onto single display by using density functional theory. The differential pore volume plot showed two peaks corresponding to sinusoidal channels and straight channels. In the case of used catalyst also similar display was observed, but the integrated areas of the peaks are smaller indicating loss of pore volume due to coke formed during the reaction Figure 3. From the data obtained using density functional theory shown in Tables-VIII & IX, the decrease in pore volumes and pore areas of zeolitic as well as non zeolitic portions were evaluated.


_1

Table-VIII Effect of Coke on Pore Size Distribution Properties

Fresh Catalyst

Maximum Pore Volume At Relative Pressure 0.30267 Median Pore Diameter

0.1503 cc/g 6.4 A

0

Used Catalyst 0.1131 cc/g 5.7 A

Percent Decrease 24.7 %

0

11.4%

υ

Analysis adsorptive = Argon at 87.05 Κ

Table-IX Effect of Coke on Pore Size Distribution of ZSM-5 Catalyst Properties 0

Volume in ZSM-5 pores (< 5.6 A ) 0

Area in ZSM-5 pores (< 5.6 A ) 0

Total volume in pores (< 300 A ) (Including binder) 0

Total area in pores (< 300 A ) (Including binder)

Fresh Catalysf

Used Catalyst

0.05165 cc/g

0.05069 cc/g

Percent Decrease 2%

300.562 cc/g

287.213 m2/g

4.4 %

0.32127 cc/g

0.20627 cc/g

35.8 %

32.872 m2/g

30.453 m2/g

7.4 %

υ

Analysis Gas : Argon @ 87.29 Κ

The data indicates that the non zeolitic pore volume decreased by 35.8%, while the zeolitic pore volume decreased only 7.4% in the case of zeolitic pores indicating significant portion of zeolite is not occupied by coke. Figure 4 shows the display of


141


FRESH CATALYST

USED

H>L 10

CATALYST

rfHwiflll 100

111

PORE WIDTH (ANGSTROM) Figure 3. Effect of coke formation on the pore size distribution of the catalyst. (Reproduced with permissionfromreference 16, copyright 1999 Elsevier)



142 incremental pore volume versus pore width plots. It can be seen that both micropores and mesopores are brought into the graph in Figure 3, using density functional theory, undergo significant changes as a result of coke formation. It is interesting to see that total pore volume in zeolite channels is more or less intact. However, the individual pore volume of the two channels change significantly Figure 4. In the fresh catalyst, the volume of large pores, viz straight channels is higher than that of the sinusoidal channels. In the case of used catalyst, the reverse is observed to be true. The change in volume of the two zeolite channels, observed in fresh and used ZSM-5 catalyst, without change in total zeolite pore volume suggests that, what ever pore volume lost in one type of pores is transferred to the other. How did this happen ? this can be understood by realizing that pore size distributions are evaluated using gas adsorption. The gas adsorption measures the diameter of the pores through which it enters. Therefore, in the fresh catalysts, where both the pores are accessible to gas adsorption, gives the real volume of the individual channels. The higher pore volume in linear channels compared to those of sinusoidal channels obtained in this study is in agreement with the earlier reports [4,14], where, based on the structure of ZSM-5 and Silicalite, it was determined that the total length of the pore system per unit cell is 8.8nm and, the individual length of the linear and elliptical channels is 5.9nm and 2.9nm respectively. In the case of used catalyst, the pore volume of straight channels decreased from 0.029 cc/g (in fresh catalyst) to 0.0095 cc/g (in the used catalyst) with a simultaneous increase in pore volume of sinusoidal channels from 0.025 cc/g (in the fresh catalyst) to 0.041 cc/g (in the used catalyst). No significant change in total zeolitic pore volume also can be seen from Figure 4. From these observations it is clear that, in the deactivated catalysts, the sinusoidal channels are predominantly accessible for gas adsorption. This also suggests that majority of straight channels are blocked by coke. It appears that the total gas is entering through sinusoidal channels and filling up the poremouth blocked straight channels through intersections, and this is keeping the total pore volume more or less intact. Guisnet and Magnoux in an excellent review on deactivation of MFI zeolites by coke formation with special reference to n-heptane cracking [15]. They have discussed that the deactivation is primarily due to three causes at various stages of deactivation (i) limitation of access of reactant molecules to the active site (ii) blockage of access to the active sties at intersection in which coke molecules are situated and (iii) blockage of access to the sites of the pores in which no coke molecules are present. In our case, since that total pore volume and area is more or less intact, it appears that these pores are blocked at the pore mouths. Since there is a small (2%) reduction in pore volume a small amount of blockage in the internal structure such as intersections cannot be completely ruled out. By similar argument we can say that most of the sinusoidal pores are still open after deactivation. Now, considering the aromatization reaction, the n-hepatne reactant molecules and/or the olefins generated from them, enter the zeolite through small sinusoidal pores and these molecules undergo reaction at channel intersections, and aromatics produced preferentially travel through straight channels. Since, these



143

aromatics and other bulkier products are also coke precursors, they can preferentially block these pores. It is also known that the pore blockage in ZSM-5 zeolites is likely to occur at or near pore mouths [15]. This pore blockage is occurring preferentially in linear channels, which means that large aromatic molecules and other coke precursors prefer to travel in the channels. By similar argument one can explain why the sinusoidal channels are not blocked. This result, in our opinion, can be taken as evidence for molecular traffic control (MTC) mechanism.

References 1. 2. 3.

Csicsry, S. M.; Zeolites 1984, 4, 202. Chen, N.Y.; and Garwood, W.E.; Catal. Rev.Sci.Eng., 1986, 28 (2&3). Ν. Y. Chen, W. W. Kaeding and F. G Dwyer, J. Am. Chem. Soc. 1979, 101, 6783.


144 4. 5. 6. 7.


8. 9. 10. 11. 12.

13. 14. 15. 16.

Derouane, E.G.; and Gabelica, Z.; J. Catal., 1980, 65, 486. Kaeding, W. W.; J. Catal., 1985, 95, 512. Chen, N.Y.; Garwood, W.E.; J. Catal., 1978, 52, 453. Chandawadker, K . H . ; Kulkarni, S. B.; and Ratnasamy, P.; Appl. Catal., 1982, 4, 287. Kim, J. W.; Namba, S.; and Yashima, T.; Stud. Surf. Sci. Catal., 1989, 46, 71. Lonyi, F.; Engelhardt, J.; and Kallo, D.; Zeolites, 1991, 11, 169. Halgeri, A . B.; Bhat, Y . S.; Unnikrishnan, S.; and Rao, T. S. R. Prasada.; ACS, Div. Pet. Chem. Preprints, 1991, 36(4), 772. Viswanadham, N.; Ph.D. Thesis, Roorkee University, India, 1996. Pradhan, A . R.; Viswanadham, N . ; Suresh, S.; Gupta, O. P.; Ray, N.; Murali. Dhar G.; Uma Shanker,.; and, Prasada. Rao T. S. R.; Catal. Lett., 1994, 28, 231. Szostak, R.; in "Introduction to Zeolite Science and Practice" H . Van Bekkum, E. M. Flanigen, J. C. Hensen (Editors) 1991, 153. Derouane, E. G.; in B . Imelik et. al. (Eds.) Catalysis by Zeolites. Elsevier, Amsterdam, 1980, p 5. Guisnet, M.; and Magnoux, P.; Appl. Catal. 1989, 54. 1. Viswanadham.N; Murali Dhar.G; and Prasada Rao, T.S.R; J.mol.Catal 1997, 125, L87-L90.


Shape-Selective Catalysis - American Chemical Society

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