CHAPTER 4. tert-butylation OF ETHYLBENZENE WITH tert-butyl ALCOHOL

Transcription

1 72 CHAPTER 4 tert-butylation OF ETHYLBENZENE WITH tert-butyl ALCOHOL 4.1 INTRODUCTION Production of dialkyl-substituted benzene compounds via alkylation, trans-alkylation or disproportionation of aromatic hydrocarbons is an important step in commercial chemical manufacturing processes (Ichioka et al 2000). Many di-alkyl substituted aromatics are commercially important, for example, p-xylene is used to produce terephthalic acid, a monomer for terylene (Patton and Seppi 1970, Chavan et al 2001), p-ethyltoluene is used to produce p-methylstyrene for poly-p-methylstyrene (Chu 1985, Burress 1986), p-diethylbenzene to produce divinylbenzene for cross-linked polystyrene (Kaeding et al 1987) and other p-alkylated ethylbenzenes to obtain p-alkylated styrenes for poly-p-alkylated styrenes. Alkylation of aromatics with olefins as alkylating agents over zeolites of 6-15 pores was described by Mattox and Arey (1959). X and Y zeolites loaded partly with rare earths were tested for alkylation of aromatics (Wise 1966). In many of these alkylation processes 1,3-isomer rather than 1,2- and 1,4-isomers were formed. Generally, in the alkylation of alkylaromatics 2, 4 and 6 positions are readily attacked by alkyl cations as the alkyl groups are o- and p-directive. If the alkyl groups are more bulky, p-substitution is enhanced with the suppression of o-substitution. But at higher temperatures 1,4-dialkyl products isomerise and enrich 1,3-dialkyl products. Since the formation of such an isomerised product demands more

2 73 space than the 1,4-dialkyl products, some external constraint to reduce the space is required to enhance 1,4-substituted products and to suppress its isomerisation. In addition, reducing the strength of their acid sites is an advantageous option to suppress isomerisation. Generally, the first demand can be easily satisfied with zeolites, especially with HZSM-5. They provide a selective constrained access to and egress from the intra-crystalline free space for molecules of size less than the pore size. As the zeolite possesses strong acid sites in addition to weak and medium acid sites, isomerisation of alkyl aromatics is a commonly observed phenomenon. But in the present study the HZSM-5 catalysts, synthesised in fluoride medium, were found to have only weak and medium acid sites without the stronger ones. Hence they could meet both the demands for selective formation of 1,4-dialkylated products. In addition, the crystals with large dimension enhance the formation of 1,4-isomer by suppressing isomerisation. The inverse relationship between the rate of isomerisation and crystal radius was also reported in the literature (Nayak and Riekert 1986, Paparatto et al 1987, Arsenova-Härtel et al 2000). Previously, the avoidance of o-substitution was reported over mesoporous materials (Umamaheswari et al 2002). The avoidance of o-substitution even in ethylation of ethylbenzene was also reported over ZSM-5 (Melson and Schüth 1997). The isomerisation of 1,4-diethylbenzene (1,4-DEB) to 1,3-diethylbenzene (1,3-DEB) was not an allowed one inside the pore as the kinetic diameter is 6.8 Å. The isomerisation of 4-tert-butyl ethylbenzene (4-t-BEB) to 3-tert-butyl ethylbenzene (3-t-BEB) (6.8 Å) may not occur inside the pore. Even if 3-t-BEB is formed in the channel intersection (9 Å) by isomerisation, it cannot diffuse out from it. As the zeolites synthesised in fluoride medium are devoid of strong acid sites, they are expected to be very well suited to selective

3 74 p-tert-butylation of ethylbenzene. To verify this the same was carried out in the vapour phase over HZSM-5 catalysts synthesised in fluoride medium. 4-tert-Butylation of ethylbenzene is important, as its dehydrogenated product, namely, 4-tert-butyl vinylbenzene (4-t-BVB) is an important monomer for the commercial production of poly-4-tert-butyl styrene with low glass transition temperature (T g ). 4.2 CATALYTIC STUDIES Vapour phase tert-butylation of ethylbenzene with tert-butyl alcohol was studied over commercial HZSM-5(25), synthesised HZSM-5(25), HZSM-5(50) and HZSM-5(75) catalysts between 200 and 400 C. The reaction parameters were optimised in order to get maximum conversion and high selectivity to 4-t-BEB. The effects of temperature, feed ratio, weight hourly space velocity (WHSV) and time on stream on conversion and selectivity of products are discussed in the following sections Effect of Temperature tert-butylation of ethylbenzene with tert-butyl alcohol was carried out over commercial HZSM-5(25) at 200, 250, 300, 350 and 400 C. The products were found to be 1,2-DEB, 1,3-DEB, 1,4-DEB, 3-t-BEB, 4-t-BEB, 4-t-BVB and benzene. The conversion of ethylbenzene and the selectivity of the products are presented in Table 4.1. The conversion increased from 200 to 300 C but decreased at 350 and 400 C due to coke formation. Coke was observed even between 200 and 300 C but not as much as that observed between 350 and 400 C. The plausible pathway for tert-butylation of ethylbenzene is shown in Scheme 4.1. tert-butyl alcohol is chemisorbed on the Brönsted acid sites of zeolites to form tert-butyl cation. The electrophilic reaction between

4 75 ethylbenzene and tert-butyl cation gave 4-t-BEB. In addition to tert-butylation of ethylbenzene, disproportionation of ethylbenzene was also observed, but it was not a major reaction. The disproportionation of ethylbenzene occured as shown in the reaction Scheme 4.2. Ethylbenzene is chemisorbed on the Brönsted acid sites of ZSM-5 to form benzene and ethyl cation. The electrophilic reaction between ethyl cation and ethylbenzene gave diethylbenzene (DEB) isomers. The dispropotionated products viz., 1,2-DEB, 1,3-DEB and 1,4-DEB were obtained between 200 and 400 C. Hence, the ethyl cation formed by dealkylation of ethylbenzene might attack at the o- and p-positions of ethylbenzene to form 1,2-DEB and 1,4-DEB respectively. As the m-position of ethylbenzene is not favoured for electrophilic attack it is suggested to be formed by the isomerisation of 1,4-DEB or 1,2-DEB. It might occur particularly on the external surface. OH HZSM-5 Scheme 4.1 Formation of 4-tert-butyl ethylbenzene Si O H Al - Disproportionation C 2 H 5 DEB isomers Benzene Scheme 4.2 Disproportionation of ethylbenzene

5 Table 4.1 Effect of temperature on ethylbenzene conversion and products selectivity Catalyst HZSM-5(25) (Commercial) HZSM-5(25) HZSM-5(50) HZSM-5(75) Temperature Ethylbenzene Selectivity (%) ( C) conversion (%) 1,2-DEB 1,3-DEB 1,4-DEB 3-t-BEB 4-t-BEB 4-t-BVB Benzene Reaction condition: Feed ratio, ethylbenzene : tert-butyl alcohol, 1:1; WHSV 1.65 h -1 76

6 77 Since, the kinetic diameter of 1,2-DEB is 6.8 Å it cannot be formed inside the pore of HZSM-5. But 1,4-DEB could be formed inside the pore or on the external surface. 1,3-DEB isomer with its kinetic diameter of 6.8 Å may be formed by the isomerisation of 1,4-DEB isomer or 1,2-DEB on the external surface. Strong acid sites of the catalyst may be responsible for isomerisation. The selectivity of 1,2-DEB was very much low as it formed on the external surface and part of it isomerised to 1,3-DEB. Although the selectivity of 1,2-DEB was less, slight decrease in selectivity from 200 to 400 C is attributed to increase in steric hindrance for substitution at the o-position due to free rotation of the ethyl group about the C-C bond. At low temperature, therefore, the rotation might be slow but at high temperature it may be rapid thus leading to suppression of substitution at both o-position significantly. The formation of free benzene supported cracking of ethylbenzene. The selectivity of 4-t-BEB decreased with increase in temperature. It is the precursor for the formation of 3-t-BEB by isomerisation. The isomerisation of 4-t-BEB proceeds as shown in Scheme t-BEB is chemisorbed on the Brönsted acid sites of ZSM-5. The protonation of 4-t-BEB occurs on the carbon that carries the tert-butyl group. Reorganisation of bonds results in the migration of tert-butyl group to m-position to form 3-t-BEB. The latter product is shown to be thermodynamically more stable than the former. So its selectivity increased with increase in temperature. The molecular size of 3-t-BEB (6.8 Å) is also higher than the pore size of zeolite. Hence, the isomerisation of 4-t-BEB to 3-t-BEB might occur only on the external surface of the zeolite. The unexpected product was 4-t-BVB. So, there is dehydrogenation of ethyl group of 4-t-BEB to 4-t-BVB. Such dehydrogenation was reported due to presence of non-framework alumina in mesoporous AlMCM-41 (Umamaheswari et al 2002) but the present catalyst was devoid of such non-framework alumina. Hence dehydrogenation is suggested to occur by

7 78 carbenium/carbonium ion mechanism (Scheme 4.4). Proton is transferred to the methylene group of 4-t-BEB to form penta-coordinated transition state. It decomposes to form carbenium ion which subsequently releases an -hydrogen to form 4-t-BVB. But such dehydrogenation was not observed on ethylbenzene. Hence the p-tert-butyl group of 4-t-BEB may exhibit some activating influence to form 4-t-BVB. Since tert-butyl group is inductively electron-repelling, there may be enhanced electronic cloud on the methylene group of 4-t-BEB which protonates from the zeolite. Si O H Al - H H zeolite Scheme 4.3 Isomerisation of 4-t-BEB to 3-t-BEB H Si H 3 C CH 3 H3 C OH Al - CH 2 CH 2 H 2 C H Scheme 4.4 Carbenium/carbonium ion mechanism The same reaction was also studied over HZSM-5(25), HZSM-5(50) and HZSM-5(75) synthesised in fluoride medium. The results are also presented in the same table. These catalysts also showed similar variation like the commercial HZSM-5(25) in the conversion of ethylbenzene. The conversion increased from 200 to 300 C and decreased thereafter. HZSM-5(25) showed higher conversion than others due to more density of acid sites. Although HZSM-5(50) carried 50% less Brönsted acid sites than

8 79 HZSM-5(25), the conversion at 300 C was not 50% low. Hence, there may be decreased diffusional constrain for the products as well as for the reactants in HZSM-5(50) compared to HZSM-5(25). Similarly slightly higher conversion over HZSM-5(75) than the expected value is attributed to still decreased diffusional constrain compared to HZSM-5(50). This decrease of diffusional constrain is due to its high hydrophobicity. An important observation over HZSM-5 catalysts synthesised in fluoride medium showed complete absence of dehydrogenation and disproportionation. Hence absence of strong Brönsted acid sites may be the cause for the absence of such reactions. Disproportionation of ethylbenzene can occur either inside the pore or outside the pore. But the acid sites must be available for it. Since tert-butyl alcohol can be better chemisorbed than ethylbenzene on the Brönsted acid sites disproportionation of the latter might not occur significantly. The selectivity to 3-t-BEB increased with increase in temperature over all the catalysts. HZSM-5(75) showed slightly higher selectivity to 3-t-BEB than its family members but lower selectivity than commercial HZSM-5. Since it is more hydrophobic than its family members, it could better retain 4-t-BEB and promote isomerisation on the external surface. Though commercial HZSM-5 is more hydrophilic than others, its strong acidity might be the cause for the high selectivity to 3-t-BEB. The optimum temperature was 300 C since it gave high conversion Effect of WHSV Since isomerisation of 4-t-BEB occurred on the few Brönsted acid sites present on the external surface, it could be avoided or minimised by increasing the WHSV. The results of the effect of WHSV on ethylbenzene conversion and products selectivity are presented in Table 4.2. As expected, the isomerisation activity decreased with the increase in WHSV thus confirming our view. The optimum WHSV was found to be 1.65 h -1.

9 80 Table 4.2 Effect of WHSV on ethylbenzene conversion and products selectivity Feed rate (ml/h) WHSV (h -1 ) Ethylbenzene Selectivity (%) conversion (%) 3-t-BEB 4-t-BEB Reaction condition: Catalyst HZSM-5(25); temperature 300 C; feed ratio 1: Effect of Feed Ratio The feed ratio was altered in order to study its influence on conversion and selectivity and the results are presented in Table 4.3. The conversion decreased with increase of tert-butyl alcohol content in the feed. As the channel of HZSM-5 could be filled with large number of tert-butyl cations, they could offer high steric hindrance to diffusion of tert-butylated ethylbenzene. Hence high alcohol content in the feed particularly above feed ratio 1:1 may not be good for high conversion. However, when the ratio was changed to 2:1, the conversion increased since there may not be much free tert-butyl cations to offer hindrance to diffusion of 4-t-BEB. In other words close adsorption of tert-butyl cations should not be present for high rate of diffusion of 4-t-BEB. Hence, optimum ethylbenzene content in the feed may be better than other alternatives for enhanced conversion. Hence 2:1 was chosen as the optimum feed ratio. Though large crystal is generally suggested to be good for high shape selectivity, the present reaction does not depend on the shape selective features of zeolite. As the o-product is avoided due to steric hindrance and the m-product is kinetically not favoured the reaction gives preferentially p-product. Even if o- and m-products are formed in the channel intersection they cannot diffuse

10 81 out of the pore. So the m-product can be formed by the isomerisation of p-product only on the external surface. If the external surface carries few weak acid sites such isomerisation can be reduced significantly. Since HZSM-5 synthesised in fluoride medium is highly crystalline and carries weak and medium acid sites, its isomerisation activity may be minimum. In addition disproportionation is also very much suppressed. Table 4.3 Effect of feed ratio on ethylbenzene conversion and products selectivity Feed ratio WHSV (h -1 ) Ethylbenzene Selectivity (%) conversion (%) 3-t-BEB 4-t-BEB 1: : : : : Reaction condition: Catalyst HZSM-5(25); temperature 300 C; feed rate 1 ml/h Effect of Time on Stream The effect of time on stream on conversion and product selectivity was studied at 300 C with a feed ratio 2:1 and the results are illustrated in Figure 4.1. The conversion decreased with increase in time on stream but the decrease was not much rapid. This is due to medium pore size and absence of strong acid sites in HZSM-5 which suppressed coke formation inside the pore. The selectivity of 3-t-BEB slightly decreased with the increase in time on stream. This product was formed by the isomerisation of 4-t-BEB on the external surface acid sites. But coke formation was very much favoured on such external acid sites, as a result they are gradually deactivated and isomerisation is also gradually reduced. The decrease in the selectivity of

11 82 3-t-BEB and increase in the selectivity of 4-t-BEB confirmed that the latter is the precursor for the former. In addition, external acid sites deactivation may be more rapid than the sites lying inside the channels as the medium pore size may not encourage large space demanding coke formation. As 2-tert-butyl ethylbenzene was not obtained during the entire period of time on stream, o-positions may be sterically blocked for electrophilic substitution. Therefore m-product may be formed only on the external surface acid sites. The time on stream study illustrates that if the external acid sites are completely blocked by coke formation, the catalyst can give 100% selectivity to 4-t-BEB. Figure 4.1 Effect of time on stream on ethylbenzene conversion and products selectivity 4.3 CONCLUSION HZSM-5 synthesised in fluoride medium is better for selective alkylation of aromatics or alkyl aromatics. It prevented trans-alkylation, disproportionation and isomerisation as it does not have strong acid sites. The fluoride medium provided HZSM-5 crystals of large dimensions. Hence, there is significant improvement in product selectivity.

12 83 Coke formation is also suppressed because of medium pore size and weak and medium acid sites. Even if there are external acid sites, their activity to isomerisation is lost due to rapid coke formation on them. It is concluded that blocking such active sites, it could be possible to ensure 100% selectivity to p-alkylated product. Hence, HZSM-5 synthesised in fluoride medium is better than non-fluoride medium for selective alkylation of alkylaromatics. This synthesis medium could also be applied to synthesise other zeolites as it could generate weak and medium acid sites. This could ultimately suppress catalyst deactivation.