Studies On Catalysts For The Isopropylation Of Naphthalene

2, 6-Dialkylnaphthalene is a valuable intermediate for advanced polymer materials displaying high thermal and mechanical stability, such as thermotropic liquid crystal polymers and so on. 2, 6-Dialkylnaphthalene can be prepared by alkylation of naphthalene with alkylating agents. The alkylation of naphthalene over various zeolites in liquid as well as vapour phase was extensively studied in the open literature. 12- Tungstophosphoric acid (PW) has been the target acid catalyst in many previous reports because of its strongest acidity among the Keggin series. However, pure PW in solid state is nonporous materials with very low surface areas (<10m²/g) and catalytic activity. In order to overcome this limitation, it is desirable to support PW on appropriate carriers to improve its surface area, and consequently, its catalytic activity and lifetime. Up to now, there are no reports on isopropylation of naphthalene over supported PW catalysts. Therefore, in this thesis, supported heteropoly acid catalysts, including PW/HM, PW/USY, PW/SBA-15, PW/SiO₂, PW/γ-Al₂O₃ and W/C were prepared by impregnation at ambient temperature. Their physical chemical properties such as the dispersion of PW, specific surface area, pore structure and acidity were characterized by X-ray diffraction (XRD), N₂ adsorption isotherm, infrared-ray spectrum (FT-IR), thermogravimetry-differential scanning calorimetry (TG-DSC) and temperature programmed desorption of ammonia (NH3-TPD). Their catalytic performances of the supported acid catalysts in the isopropylation of naphthalene were investigated in a batch reactor. The relations between the physicochemical properties of the catalysts and their activity and selectivity had been discussed in liquid phase alkylation of naphthalene with isopropanol. For comparison, several selected zeolite catalysts were also investigated in this reaction. The results of this thesis are summarized as follows.(1). The typical zeolite catalysts. Catalytic performance of USY, Hβ , HMCM-22> HM and HZSM-5 catalysts were investigated and compared in alkylation of naphthalene with isopropanol in a stainless-steel batch reactor, and their physical chemical properties were characterized by XRD and NH3-TPD techniques. It is revealed that both acidity and pore structure of the zeolite are the key factors influencing the catalytic behavior. The active centers with weak and medium acid strength and the 12-membered ring open pore structure facilitate the catalytic activity in the reaction, while the narrow unidimensional pore is favorable to the selectivity of desired product.(2). The dealuminated mordenite catalysts. The dealuminated H-mordenites (HM) modified by HC1 leaching, and deep hydrothermal treatment combined with HC1 leaching were prepared. 12-tungstophosphoric acid (PW) catalysts supported on dealuminated HM were obtained by impregnation at room temperature. The employed catalysts were characterized by XRD, FT-IR and NH3-TPD techniques, and their catalytic performances were tested in isopropylation of naphthalene in a 100 ml stainless-steel batch reactor. Lower acid amount and weaker acidity are observed for the HM samples after dealumination. However, the introducing of PW onto the dealuminated HM can improve the catalysts acidity apparently. It is also found that the catalytic activity and selectivity for 2,6-diisopropylnaphthalene (2,6-DIPN) increase greatly over the dealuminated HM catalysts; On the other hand, PW supported on dealuminated HM catalysts exhibits higher activities but slightly lower selectivity for 2,6-DIPN, compared with the dealuminated HM alone. Catalytic performances have also been discussed in relation with the pore structure and acidity of catalysts.(3). PW catalysts supported on USY. XRD and N2-adsorption results indicate that the various PW/USY catalysts retained the high surface area and micropore structure of USY with abundant secondary pore system. When the PW loading was as high as 30%, PW could highly disperse on the surface of USY because of the weak interaction between PW and pore wall of USY. Moreover, the thermal stability was improved as PW was supported on USY. On the other hand, both Bronsted and Lewis acid were observed on PW/USY catalysts. Comparing to pure PW and USY, PW/USY catalysts possess larger amount of acid sites. The experimental results show 10%PW/USY with the activation temperature of 573 K exhibited higher catalytic activity than other PW/USY catalysts because of its larger acid amount and appropriate medium strong acidity. Suitable reaction temperature was 43 3 K, and the molar ratio of naphthalene, isopropanol and cyclohexane was 1:2:10. Increasing of reaction time and isopropanol/naphthalene ratio resulted in increase in conversion, and had no effect or somewhat negative impact on the selectivity to p\(3'-DIPN. Compared with 40% PW/MCM-41 and 50% PW/SBA-15, 10%PW/USY exhibited higher catalytic activity with high selectivity at the same reaction conditions.(4). PW catalysts supported on mesoporous silica SBA-15.12-Tungstophosphoric acid catalysts was highly dispersed on the SBA-15 support when the acid loading was as high as 60%, and meanwhile the Keggin structure of heteropolyanions could be retained. In addition, the thermal stability of the supported acid decreased byapproximately 100 K as compared with the pure acid. In isopropylation of naphthalene, SBA-15 supported acid catalysts showed much higher conversion and selectivity to DIPNs, P -IPN, P, P'-isomers than those of pure acid. The conversion of naphthalene increases substantially with the increase of PW loadings up to 50%. Moreover, the PW/SBA-15 catalyst is more catalytically active than PW/MCM-41 and PW/SiO2 catalysts in this reaction. For 50% PW/SBA-15, the conversion of naphthalene is 84.3%, with the high DIPN selectivity of 39.7%, high P, P'-selectivity of 81.8% and very high P -IPN selectivity of 100%. This result is comparable with that over the USY catalyst, with the conversion being much higher than those over HM and dealuminated HM catalysts. The high conversion of PW/SBA-15 catalyst in this reaction is attributed to their high surface areas, high dispersion of PW on the surface of support and the acidity originated from the Keggin structure of PW anions.(5). PW catalysts supported on silica gel SiO2. PW catalysts could be highly dispersed on the surface of silica gel support until the PW loadings reached as high as 30% by weight. Meanwhile, the 30%PW/ SiO2 catalyst exhibited higher catalytic activity because of its strong acidity and large acid amount. Moreover, it was found that the suitable reaction temperature and molar ratio of naphthalene to isopropanol were 573 K, and 1:3, respectively. Low concentration of reactants can increase the selectivity for p, P'-DIPN. The curve of distribution of products as a function of contact time indicates the feature of a consecutive reaction for the isopropylation of naphthalene over this catalyst. The key factor that affected the selectivity to 2,6-DIPN was reaction temperature in this reaction.(6). PW catalysts supported on aluminium oxide y-AkC^. 12-Tungstophosphoric acid catalysts was highly dispersed on the Y-AI2O3 support when the acid loading was as high as 40%, and meanwhile the Keggin structure of heteropolyanions could be retained. The 40%PW/y-Al2O3 catalyst has the highest acid amount and retained the high surface area. In isopropylation of naphthalene, Y-AI2O3 supported acid catalysts showed much higher conversion and selectivity to DIPNs, 3 -IPN, p, P'-isomers than those of pure acid. For 40% PW/Y-AI2O3, the conversion of naphthalene is 71.7%, with the high DIPN selectivity of 34.3%, high p, p'-selectivity of 82.7% and very high 3 -IPN selectivity of 92.8%. The conversion was much higher than those over HM and dealuminated HM catalysts. In addition, low PW loadings was advantageous to the selective production of 2,6-DIPN.(7). PW catalysts supported on active carbon. 12-Tungstophosphoric acid catalysts was highly dispersed on the active carbon support when the acid loading wasas high as 30%, and meanwhile the Keggin structure of heteropolyanions could be retained. In addition, 30%PW/C catalyst had highest acid amount and its surface area was decreased compared with active carbon support. In isopropylation of naphthalene, active carbon supported acid catalysts revealed much higher conversion and selectivity to DIPNs, P -IPN, p\ P'-isomers than those of pure acid. For 30% PW/C, the conversion of naphthalene is 86.0%, with the high DIPN selectivity of 38.6%, high p, P'-selectivity of 83.3% and very high P -IPN selectivity of 91.6%. This result is comparable with that over the USY catalyst, with the conversion being much higher than those over HM and dealuminated HM catalysts. Moreover, PW/C catalyst showed no selectivity to 2, 6-DIPN.Generally speaking, the pore structure and acidity of catalysts are the key factors that influence the catalytic performances, and the catalysts with larger acid amount, stronger acidity and wider pore sizes exhibit higher activity and selectivity for p-IPN and p,P'-DIPN. The selectivity for 2,6-DIPN is higher over the catalysts with smaller pore diameter because of the shape selectivity of catalyst pore. Catalytic activities decrease gradually over PW/HM, PW/USY, PW/SBA-15, PW/SiO2, PW/y-Al2O3, PW/C catalysts after several reaction cycles of isopropylation of naphthalene, probably due to the carbon deposition on the catalyst active acid sites arising from the improved catalyst acidity by loading PW on the supports. On the other hand, PW leaching into the polar solvent such as isopropanol and water is another reason for the decrease of catalytic activity.