SELECTIVE ALKYLATION OF BENZENE WITH ETHANOL USING MODIFIED HZSM-5 TO PRODUCE ETHYLBENZENE

Abstract

Alkylation of benzene to produce ethylbenzene is a considerable importance due to the increasing demand of ethylbenzene and diethylbenzene in chemical industries. The demand of ethylbenzene is found to be higher due to its use in the manufacture of plastics and many petrochemicals. The commercial process for ethylbenzene from benzene and either ethylene or ethanol involves the vapor phase or liquid phase alkylation of benzene with ethylating agents over a variety of acidic zeolites. Several researchers have proposed the alternative catalytic reaction pathways for the production of ethylbenzene. The catalytic reaction which uses ethanol for benzene alkylation, instead of ethylene, would eliminate the ethylene production step and, therefore, leading to the commercial and environmental benefits in the ethylbenzene manufacturing. In addition to the intrinsic scientific interest, the direct use of ethanol (instead of ethylene) in the manufacture of ethylbenzene also has economic significance in those countries like Brazil and India, where biomass-derived alcohol is an additional feedstock for the manufacture of chemicals. Alkylation of benzene with ethanol yields ethylbenzene, polyalkylates, especially diethylbenzene (DEB), toluene and mixtures of xylenes as major products inside zeolite pores. The products subsequently diffuse out of the zeolite pores. Owing to its smaller size and very high diffusivity, ethylbenzene diffuses out of the pores at a very high rate in comparison to diethylbenzene. Several researchers have shown enhanced ethylbenzene selectivity of zeolites by the use of modifier agents such as rare earth metals. These modifier agents have been reported to partially block the pores and block the unselective active sites and inactivate the external active sites for secondary isomerization. Based on the fact that modification of a HZSM-5 zeolite may lead to (i) selectivity of the reactions based on the nature of the cation (ii) blockage of the zeolite pore mouth in proportion to the size of the exchanged ion, leading to shape selectivity for the desired product (ethylbenzene in the present case) and also (iii) greater stability of the exchanged zeolite than the unmodified form, the present work has been under taken. Alkylation reactions are known to be depending on the activity and selectivity of catalysts. Further developments of catalysts which can provide higher benzene conversion and ethylbenzene selectivity are necessary to ensure successful development of the industrial process for alkylation of benzene with ethanol. It is important to design highly stable, active and selective catalysts to perform the reaction at atmospheric pressure and 300-500oC temperature. ii Significant numbers of studies have been reported on the alkylation of benzene with ethanol using rare earth metals as modifying agent for HZSM-5. However, at the time of the start of work in this thesis (year 2012), no studies were reported on various possible application of boron, magnesium monometallic and bimetallic as modifying elements for HZSM-5 in the alkylation of benzene with ethanol for the formation of ethylbenzene. Therefore, in the present study boron, magnesium monometallic and boron-magnesium bimetallic catalyst having acidic sites for alkylation of benzene with ethanol were tested. The present work deals with the preparation, characterization and activity tests of the magnesium, boron monometallic and boron-magnesium bimetallic modified HZSM-5 for the alkylation of benzene with ethanol. The effect of support composition as well as metal loading on the catalyst was studied. To investigate the physico-chemical properties of fresh and used catalysts, various characterization techniques including BET surface area, X-ray diffraction (XRD), temperature programmed desorption (TPD), Scanning electron microscopy (SEM-EDAX), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Fourier transmission infrared spectroscopy (FTIR) were used. The effect of reaction temperature, benzene to ethanol ratio, amount of metal loading and different silicon to aluminium ratio HZSM-5 were studied to maximize ethylbenzene selectivity in the temperature range of 300-500oC. From the experimental results we observed that for HZSM-5 silicon to aluminium ratio 31 (SAR=31) with benzene to ethanol ratio 2:1 by volume ethylbenzene was the primary product while diethylbenzene, triethylbenzene, toluene and xylene mixtures also exist in the product. Highest selectivity of ethylbenzene (72.79%) and higher conversion of benzene (75.17%) was obtained by bimetallic catalyst (Mg(5%)-B(4%)-HZSM-5) at 500oC and 400oC respectively. Boron modified showed lower benzene conversion (62.2%) while magnesium modified showed approximately the same benzene conversion (71.7%) when compared to unmodified HZSM-5 (71.3%). The existence of abundant ethanol may facilitate the alkylation of benzene to produce ethylbenzene and then further alkylation to diethylbenzene and tri ethylbenzene. Our investigation demonstrates again that ethylbenzene was the primary product while diethylbenzene, triethylbenzene, toluene and xylene mixtures also exist in the product for magnesium monometallic modified HZSM-5 (SAR=90). The highest selectivity of ethylbenzene (71.14%) was obtained by 15%Mg-HZSM-5 while the lowest ethylbenzene selectivity (49.74%) was obtained by 10%Mg-HZSM-5 for 2:1 benzene to ethanol ratio by iii volume. For benzene to ethanol ratio 4:1 also the highest selectivity for ethylbenzene (67.59%) was observed by 15%Mg-HZSM-5 while the lowest selectivity of ethylbenzene (63.35%) was obtained by 5%Mg-HZSM-5. In terms of ethylbenzene yield all HZSM-5 catalysts modified by magnesium resulted approximately in the range of 34-42%. The highest conversion of benzene (71.82%) was obtained by 15%Mg-HZSM-5. Similar to magnesium monometallic HZSM-5 modified, boron modified HZSM-5 showed that ethylbenzene was the primary product while diethylbenzene, triethylbenzene, toluene and xylene mixtures also exist in the product. The highest selectivity of ethylbenzene (57.48%) was obtained by 15%B-HZSM-5 while the lowest ethylbenzene selectivity (42.00%) was obtained by10%B-HZSM-5 and 5%B-HZSM-5 for 4:1 benzene to ethanol ratio by volume. However, for benzene to ethanol ratio 2:1 all the catalysts showed approximately the same selectivity for ethylbenzene (48%). Except 15%B-HZSM-5, in terms of ethylbenzene yield both HZSM-5 catalysts modified by boron resulted on approximately 40.00%. The highest yield of ethylbenzene demonstrated by 15%B-HZSM-5 was 44.18% at 450oC for benzene to ethanol ratio 4:1 by volume. The highest conversion of benzene approximately (83%) was obtained by both 5%B-HZSM-5 and 10%Mg-HZSM-5. The existence of abundant ethanol may facilitate the alkylation of benzene to produce ethylbenzene and then further alkylation to diethylbenzene and tri ethylbenzene. Therefore, it would be essential to use lower ethylating agents. According to results obtained from experiments, ethylbenzene was the primary product while diethylbenzene, triethylbenzene, toluene and xylene mixtures also exist in the product. The highest selectivity of ethylbenzene (76.22%) was obtained by (Mg + B)-15%-HZSM-5 and the lowest ethylbenzene selectivity (49.15%) was obtained by (Mg + B)-5%-HZSM-5 using 2:1 benzene to ethanol ratio by volume. However, for benzene to ethanol ratio 4:1 (Mg+B)-5%-HZSM-5 and (Mg+B)-15%-HZSM-5 catalysts were showed approximately the same selectivity for ethylbenzene (67.01%). The highest yield of ethylbenzene demonstrated by (Mg+B)-5%-HZSM-5 was 43.63% at 450oC for benzene to ethanol ratio 4:1 by volume. The highest conversion of benzene approximately (77.01%) was obtained by (Mg + B)-5%-HZSM-5 for 4:1 benzene to ethanol ratio by volume.