Shape-Selective Catalysis - American Chemical Society

Chapter 15

Enhanced para Selectivity in Alkylation and Isomerization Reactions over Silynated ZSM-5 Zeolite

Downloaded by MICHIGAN STATE UNIV on September 5, 2013 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch015

B. S. Rao, R. A. Shaikh, and Α. V. Ramaswamy Catalysis Division, National Chemical Laboratory, Pune 411 008, India (telephone and fax: +91 (212) 334761; e-mail: [email protected])

Controlled silynation of ZSM-5 by chemical vapor deposition (CVD) of tetraethyl orthosilicate, results in gradual alteration in the external surface and pore mouth. These samples are characterized by X R D , M A S - N M R , TPD of ammonia, FTIR and sorption techniques. The selectivity enhancement of 1,2,4-trimethylbenzene in the methylation of xylene, p-diethylbenzene in ethylbenzene disproportionation and p-xylene in m-xylene isomerization is attributed to the product shape selectivity, arising out of modification of ZSM-5.

Selectivity is one of the most important aspects to be considered in any catalytic process. Often the activity and selectivity are referred to together. The activity is associated with many aspects such as the reaction, reactants, reaction parameters, catalysts, inhibitors, etc. However, selectivity is associated with the catalyst and can be controlled by several ways such as structural, chemical, compositional changes, kinetic and energy considerations. New exploration in molecular sieve synthesis and increasing assemblage of novel structures has revealed catalytic chemistry and structure-activity relationship. A property frequently referred to in the catalytic behavior of zeolites is the shape selectivity, first coined by Weisz and Frilette (1). This property is more pronounced in medium pore pentasil zeolites. The dual pore system and the unique structure of ZSM-5 have enabled revolution in shape selective reactions (2,3), impressive commercial applications in petroleum refining (4) aromatics petrochemicals processing (5) and synthetic fuels (6,7). Molecular sieves represent a rich diversity of pore and channel sizes and shapes, channel

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Chemical Society

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

225


226 dimensions and connectivity and active site type concentration (8,9). The morphology and their crystallite size can also be varied (10,11). The zeolite sizes often match with those of the organic molecular dimensions. It is, therefore, possible to discriminate molecules on the basis of size. Only molecules whose dimensions are less than the critical diameter of the zeolites (pore diameter) can enter the pores and have access to internal catalytic sites to undergo reactions, then diffuse out and appear as product. A number of comprehensive reviews and critical discussions on the subject (12,13) with reference to the distinguished four categories of shape selectivity viz., reactant selectivity (14), product selectivity (15), restricted transition state selectivity (16) and to some extent, molecular traffic control (17), have appeared on medium pore zeolites like ZSM-5. This property of shape selectivity can further be enhanced by changes in the acidity, channel tortuosity, surface passivation and pore mouth narrowing (18-24). Even though wide pore zeolites like mordenite are reported to be shape selective (25), we restrict our discussions to the medium pore zeolites and to the product shape selectivity which often can be confused with transition shape selectivity. The product shape selectivity (26), which is the subject of this report, can be achieved by the modification of the catalyst or the reaction conditions. Khouw and Davis (27) have discussed shape and size selectivity as well as primary and secondary selectivity in zeolites which are relevant to this and will be discussed at the appropriate place. In zeolites, the product shape selectivity (28,29), occurs when some of the products formed within the pores are too bulky to diffuse out as observed product. They are either converted to less bulky molecules or deactivate the catalyst by blocking the pores. This type of selectivity is not only dependent on pore size but also on the crystal size of the catalyst particle (30-32). In the product, a substance having diffusion coefficient considerably higher than the others diffuses fast. For example, the diffusion coefficient of p-xylene in ZSM-5 is approximately 10 times higher than that of m- or o- xylene, hence it diffuses fast. Xylene isomerization and dewaxing are the best examples for product shape selectivity. In addition to crystallite size variation, the product shape selectivity can be enhanced by several modifications. The modifications include cation exchange (33), metal impregnation (34), liquid phase silynation (35) or C V D (36,37). We report here the modifications of ZSM-5 by controlled chemical vapor deposition of silicon alkoxide, and the catalytic activity and selectivity of the modified samples in the alkylation, disproportionation and isomerization reactions. 4

Experimental The C V D of silicon alkoxides like tetra ethylorthosilicate (TEOS) is well known and described by several authors (36,37). Niwa, et al., have reported in length the C V D technique and the mechanism of silynation (21,23). This method consists of depositing the desired substance in the form of vapor and calcining further to achieve the material of interest. Well-characterized ZSM-5 (So, Table I) was silynated as per the procedures described by Niwa, et.al. (21,23). The unmodified ZSM-5 was taken in a fixed bed reactor. The sample was activated at 773 Κ for 8 h in a flow of air. The temperature was slowly brought down to the reaction



227 temperature in a flow of dry nitrogen. A solution containing 5-7% (TEOS) in toluene and methanol in desired mole ratio was contacted with the sample until a silica breakthrough was noticed in the reactor effluent. This was slowly heated to 623 Κ in a flow of nitrogen to drive off and decompose the organic. The sample was calcined at 823 Κ for 10 h. This sample is denoted as S i . Ethylbenzene disproportionation was studied as a probe reaction. The reaction was stopped and the catalyst was regenerated and cooled to a temperature around 473 Κ and futher silynation was carried out to increase the silica deposition. The same procedure was repeated until the desired selectivity for p-diethylbenzene was obtained. Five samples designated as Si, S2, S3, S4, and S5 were thus prepared. These samples were characterized by sorption methods, XPS and M A S N M R techniques. The unit cell composition of the parent sample, amount of deposited silica and acidity changes by TPD are included in Table I along with the sorption data. Table-I: Amount of Silica deposited and equilibrium Sorption uptake of Silynated Catalysts (%, wt/wt). Sorbate Silica deposited Acidity Weak + (amount of medium desorbed strong NH3 wt%) n-Hexane Cyclohexane Ethylbenzene Water P-Xylene P-Ethyltoluene P-Diethylbenzene

So

Si

s

S3

s

S

0.0 2.10

1.80 2.20

4.60 2.35

5.60 2.59

9.90 2.00

12.5 2.33

0.75

0.80

0.84

0.87

0.92

0.83

11.50 2.00 10.9 7.70 13.60 11.80 9.70

11.10 1.50 10.20 7.63 13.20 11.30 9.10

10.50 1.10 9.80 6.59 12.40 10.60 8.80

10.60 0.46 9.50 6.90 12.80 10.30 8.10

9.50 0.40 7.84 5.76 10.80 9.00 7.90

8.70 0.35 6.40 5.70 9.20 7.90 7.70

2

4

5

Conditions: P/Po=0.5, Temp. 298K, Equilibrium time = 2h. Parent sample H/ZSM-5, SAR = 260 Unit cell composition Na 0.04 [ (Si02)95.27 (Αΐ2θ3)ο.73 ]1 llfeO

Catalytic evaluations were made in a down flow, all metal Catatest unit supplied by M/s Geomecanique, France. About 10 g of the catalyst was used in the study. The catalyst was activated in air for 4 h. at 773 Κ in N2 and then brought down to reaction temperature. Both gaseous and liquid products were collected at regular intervals and analyzed in a Shimadzu 15A G C fitted with a FID detector. Xylene Master capillary column (supplied by Shimadzu Corp.) was used for xylene isomers and a special column containing a mixed phase with Apiezon-L was used for


228 diethylbenzene isomer separation. Gaseous products were analyzed on porapak Q column (3 m long). Percent conversion and selectivity were calculated on the basis of yields of the individual components in the gas and in the liquid product. Results and Discussion Characterization of the samples


The modified samples were characterized by different techniques such as : XRD: X-ray diffraction patterns did not show any change with progressive modification with silica deposition from the parent zeolite, as can be seen from Figure 1. This shows that crystallinity is well maintained. SEM: The scanning electron micrographs did reveal changes in some samples. For 5.6 % silica deposited sample (S3) a few silica aggregates, and in case of sample with 12.5% deposition (S5) an accumulation like debris on the external surface were observed. The details of X R D and SEM are reported earlier (37) XPS: Since it is difficult to evaluate the contribution of the A l peaks to the composite Al -energy loss peak, using A l peak the surface concentration of aluminium was evaluated. The S i 0 /Α1 θ3 ratio was found to be 71,112,181,300 and >500 for samples Si to S5 respectively, whereas the bulk ratio is 260 for So and increase up to 290 for S (37a). Therefore, the XPS studies clearly indicate that the parent sample So is highly enriched with aluminium and with progressive silynation the surface was enriched with silica. 2 s

2s

2 p

2

2

5

2 7

MAS-NMR: A l M A S - N M R did not indicate any octahedral aluminium (-52 ppm) in any of the samples. Even after the acetyl acetone treatment there is no evidence of the peak around -52 ppm corresponding to the non-framework aluminium. FT-IR: FT-IR Studies indicated both Lewis and Bronsted acid sites. Bronsted sites increased with progressive silynations at the expence of Lewis sites (38). TPD: TPD of ammonia showed reduction in the weaker acid sites with silynation (38). Similarly, the sorption studies revealed a continuous decrease in the sorption volume with gradual silynation, which indicates the changes in the channels and in the pores (Table I).



Figure 1. XRD profiles of silynated samples



230 The general observation is that with the progressive silynation the surface of the zeolite is first passivated reducing the surface acidic sites, as evidenced by the reduction in non-selective reactions, and gradually blocking the channels and pores. Modified catalysts are used in three types of reactions to correlate the activity selectivity and the percent silica deposited. As pointed out earlier, the product shape selectivity and restricted shape selectivity is sometimes confused. The restricted transition state selectivity occurs when space within the zeolite is not sufficient to allow the larger or several transition state intermediates to form. This type of shape selectivity is independent of crystal size and activity of the catalyst, but depends on pore and cavity dimensions and on zeolite structure. For example, in transalkylation of dialkylbenzenes, the alkyl group is transferred from one molecule to another. The reaction is bimolecular and involves diphenylmethane type transition-state in case of molecules like xylene. The transition state complex can dissociate to smaller molecules, which can easily diffuse outside the zeolite channels or this can block the whole pore. Therefore, activity generally declines with time, unless hydrogénation activity is present. In toluene disproportionation reaction, a molecule each of benzene and xylene are formed by restricted transition-state due to steric hindrance inside the zeolite channel. However, in case of medium pore zeolites the formation of transition-state is very much restricted except for few molecules due to channel dimensions. But dealkylation can proceed. This dealkylated product can again react or realkylate with one of the reactants or the decomposed product to form the desired product as dictated by the reaction conditions. So, dealkylation, realkylation are also possible in medium pore zeolites. The disproportionation of toluene and ethylbenzene is interesting. It is possible to achieve benzene to xylene molar ratio of one, in case of toluene disproportionation, under optimum conditions with modified catalyst, indicating a predominance of transition state shape selectivity. However, in ethylbenzene reaction there is always an excess of benzene, which can only be explained by the dealkylation. The formation of diethylbenzene can be by dealkylation-realkylation as well as disproportionation (38). The thermodynamics indicate dealkylation to be more prominent than the disproportionation. Alkylation of xylenes with methanol The smallest 1,2,4 trimethylbenzene, also known as pseudocumene is an important starting material for the production of trimellitic acid/anhydride which is used in the manufacture of plasticisers, polyesteramides, epoxy resins, etc. (39). This is produced by the selective methylation of xylene. Not much literature is available on the catalytic methylation of xylenes except that of Namba, et al. (40). They reported very high selectivity (>99%) for 1,2,4 T M B during their methylation studies of individual xylene isomers over HZSM-5 zeolite. This was correlated to the shape selectivity and the ortho - para orientation of the alkylation reactions. De Simone (41) has claimed, with a crystalline borosilicate molecular sieve impregnated with Mg, as high as 92% yield of T M B in the aromatic fraction. Anuj Raj, et al. (42) have interpreted the selectivity in case of isomorphously substituted A l , Ga, Fe-MEL zeolites to the weaker acid sites. These zeolites compete with isomerization activity


231


by suppressing the alkylation reaction. Xylene reacts with methanol in presence of an acid catalyst to form tri- and tetra-alkylbenzenes. The selectivity of 1,2,4 T M B among the polyalkylbenzenes varies with the catalyst modification while the yield of the product depends on the strength and number of acid sites. However, the selectivity to tri- and tetra-methylbenzenes depends on the pore mouth narrowing by C V D of silica. The modification narrows the pore mouth and passivates the surface active sites. Table II summarizes the data on the conversions and selectivity to 1,2,4 T M B obtained on HZSM-5 samples with different levels of silylation. In S, silica deposited is less, hence conversion of xylene and selectivity of T M B and T T M B are similar to that of the parent unsilynated sample.

Table II : Methylation of xylene S

Catalyst Si0 , % Temp.°C WHSV, h Xyl/MeOH Conv.Xyl.,% E-tmb(a), % E-ttmb(b), % 124tmb/a 1245ttm/b p-xyl/E-xyl. 2

1

0

0.0 450 2.5 4 32.7 60.2 22.0 50.5 40.9 22.3

s,

S

1.8 450 2.5 4 29.3 37.5 24.9 62.0 42.8 22.4

4.6 450 2.5 4 17.2 29.9 30.2 64.6 57.5 22.6

2

s

3

7.6 450 2.5 4 14.6 54.1 22.6 83.3 97.6 22.5

s

4

9.9 450 2.5 4 5.2 76.9 98.9 24.1

S

5

12.5 450 2.5 4 4.7

24.3

tmb = Trimethylbenzene, ttmb = tetramethylbenzene. However with increase of the degree of silynation gradual reduction in the formation of 1,3,5 and 1,2,3 T M B is noticed and selectively 1,2,4 T M B is formed at a silica level of 9.9%. At about 12.6% silica deposition no T M B formation was observed. Similarly the TTMB formation and selectivity of 1,2,4,5 TTMB was maximum at 5.6% deposition of silica. Thus, the cut off level for the selective formation of 1,2,4 T M B and 1,2,4,5 TTMB are 9.9 and 5.6 % silica. However unconverted xylene was noticed even at 12.6% silica. As described earlier the narrowing of the effective pore-mouth without much variation in the micropore volume is achieved by silynation, due to which the formation/reactions of molecules of higher molecular dimensions are restricted and p-isomers, being the smallest among the isomers of any compound diffuse out, can appear in large quantities in the product. Thus, the selective formation of 1,2,4 T M B and 1,2,4,5 TTMB is related to the modification of ZSM-5 by silynation. The p-xylene is in equilibrium concentration with other isomeric xylenes at this level of silica deposition indicating a need for higher level of modification for selective p-xylene formation by isomerization.


232


Ethylbenzene disproportionation P-diethylbenzene is a specialty chemical used as a desorbent in Parex process for the separation of para-xylene. Usually in the ethylbenzene disproportionation reaction or in the ethylation of ethylbenzene isomeric mixture of diethylbenzenes is produced. Two molecules of ethylbenzene react forming diethylbenzene and benzene over zeolite or any acid catalyst. The activity of the catalyst depends on the silica-alumina ratio of the catalyst. In this context, it is of great interest to study the reaction of ethylbenzene to form benzene and ethyl moiety, and preferential migration of ethyl group to another available reactant, in this case another ethylbenzene molecule. The competetion for accepting the alkyl group and transforming to diethylbenzene depends on the nature of the catalyst and the reaction conditions. Generally, dealkylation is accompanied by disproportionation and the ratio of benzene to diethylbenzene is a measure of this. When a modified catalyst, such as the silynated ZSM-5 is used, side reactions are controlled and the selectivity towards p-isomer increases. The selectivity is governed by the diffusivity of the molecules inside the crystal. As already mentioned decrease in crystal size decreases the p-selectivity and also the diffusivity of the molecules decreases with the increase in effective minimum dimensions of the molecule. Table III presents the experimental data from modified ZSM-5 with varied Si deposition on the reaction of ethylbenzene.

Table III : Disproportionation of ethylbenzene Catalyst

S

Si0 , % Temp.°C WHSV, h' Conv.EB, % Sel.DEB, % Sel. p-DEB, % B/DEB, ratio 2

1

0

0.0 350 3.5 37.3 41.3 38.0 1.9

Si

S

1.8 350 3.5 35.4 42.5 68.4 1.7

4.6 350 3.5 33.0 43.1 78.1 1.6

2

s

3

7.6 350 3.5 30.3 43.6 95.2 1.65

s

4

9.9 250 3.5 29.2 43.5 99.8 1.6

S

5

12.5 350 3.5 20.6 51.9 99.8 1.3

The main products are benzene and diethylbenzene isomers, with small amounts of toluene, and C9 aromatics. With increase in silynation, fall in conversion and increase in p-selectivity is noticed from the data. However, it should be noted that the adsorption of ammonia does not indicate a loss in acidity. Therefore, it can be inferred that the decrease in conversion is due to the reacting molecule accessibility to the active sites inside the channels due to poremouth narrowing. The removal of the active sites on the external surface also helps in restricting the secondary reaction of paradiethylbenzene isomerisation.


233


Isomerisation of metaxylene: Xylene isomerisation is one of the industrial reaction where medium pore zeolites play an important role. The p-xylene demand increases every year by 6-8% and to meet this demand either new plants have to be set up or the existing processes should improve the p-xylene selectivity. Selective toluene disproportionation and use of modified catalysts are some of the methods for enhancing p-xylene yields. But isomerisation of xylenes always leads to equilibrium compositions. Improvements on the catalyst have resulted in enhancement of paraselectivity, but not as much as that in the case of paradiethylbenzene. The modifications carried out for the other two earlier describerd reactions are not sufficient to enhance the pselectivity in this reaction. Hence, further deposition of silica was done following the same C V D technique to obtain samples S and S with 15.4 % and 16.2 % silica respectively. Some of our recent results on the isomerization of m-xylene on the modified ZSM-5 are summerized in Table IV. 6

7

Table IV: Isomerization of m-xylene Catalyst

S

Si0 , % Temp.°C WHSV, h H / H C , mole Conv. m-xyl.,% p-/o-xylene Sel. p-xylene, % 2

1

2

0

0.0 375 5 1 51.5 0.9 48.0

s

5

12.5 375 5 1 43.4 1.05 54.2

s

6

15.4 375 5 1 36.6 1.50 69.8

S

7

16.2 375 5 1 30.2 1.74 87.6

The results indicate an increase in p-xylene concentration in the product and decrease in the o-xylene formation. The reduction in the conversion of m-xylene is in direct proportion to the decrease in the formation of o-xylene, which is controlled by the diffusivity due to pore mouth narrowing. Secondly, the secondary reaction of o-xylene formation from p-xylene is also controlled due to the passivation of the external surface of the catalyst sample. The results presented in Tables II-IV indicate that modification of ZSM-5 by chemical vapor deposition of silicon alkoxide improves the selectivity of the desired product. This is due to the passivation of the surface active sites. This passivation reduces the side reactions occurring on the surface sites. The silynation narrows the pore mouth opening, thereby facilitating the diffusivity of p-isomer in xylene isomerization, 1,2,4 and 1,2,4,5 methylbenzenes in xylene methylation and pdiethylbenzene in ethylbenzene disproportionation. Even though, a high selectivity


234

for the desired product is obtained, it may be noted that the percent silicon deposition varies from compound to compound and reaction to reaction. Thus, it is possible to achieve 88 % selectivity in p-xylene 98 % selectivity in p-DEB, 97 % selectivity in 1,2,4 T M B and 97 % selectivity in 1,2,4,5 TTMB with 16.2 %, 9.9 %., 9.9 % and 7.6 % deposition of S i 0 respectively. As pointed by Khou and Davis (27) the primary selectivity can be associated with the para-product formation by pore narrowing and diffusion phenomenon, whereas the secondary selectivity is due to the passivation of the external surface of the catalyst which prevents the secondary reaction so that the primary product selectivity is maintained. Downloaded by MICHIGAN STATE UNIV on September 5, 2013 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch015

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Acknowledgement:

The authors thank Mr. C.V. Kavedia and Dr. A.P. Budhkar for their help rendered during the experimentation and analysis.

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Shape-Selective Catalysis - American Chemical Society

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