Shape-Selective Catalysis - American Chemical Society

Chapter 17

Recent Progress in Selective Catalytic Conversion of Polycyclic Hydrocarbons over Zeolite Catalysts

Downloaded by UNIV OF ARIZONA on November 20, 2012 | http://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch017

Chunshan Song Applied Catalysis in Energy Laboratory, and Department of Energy and Geo-Environmental Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802

This paper is an account of our recent work which demonstrates that high-value chemicals can be obtained from polycyclic hydrocarbons by shape-selective conversion over certain 12-MR or 10-MR zeolite catalysts or zeolite-supported metal catalysts. We are studying shape -selectivealkylation of naphthalene into 2,6-dialkylnaphthalene, ring -shift isomerization of sym-octahydrophenanthrene into sym -octahydroanthracene, shape-selective alkylation of biphenyl into 4,4' -dialkylbiphenyl, conformational isomerization of cis-decalin into trans-decalin, selective hydrogenation of naphthalene into either cis- or trans-decalin, and regio-selective hydrogenation of heteroatom -containing aromatic compounds. The products of such selective reactions are value-added chemicals, specialty chemicals, monomers of advanced polymer materials such as high-performance polyesters and liquid crystalline polymers, or components of advanced thermally stable aviation jet fuels for high-Mach aircraft.

Since the pioneering studies reported by Paul B. Weisz and coworkers at Mobil in 1960, shape selective conversion of acyclic and monocyclic compounds over various zeolites have been studied extensively, as summarized in many reviews (1-7). However, until recently, little attention has been paid to shape-selective catalytic conversion of polycyclic aromatic hydrocarbons (PAH). The needs for research on selective P A H conversion have been discussed recently (8-10). Briefly, recent years have witnessed significant growth of existing aromatic polymer materials and rapid development of advanced aromatic polymer materials such as engineering plastics, polyester resins and fibers, polyimides, and liquid crystalline polymers (LCPs). Table 1 shows some of these new polymer materials. The advanced polymer materials of interest include thermoplastic polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), thermotropic LCPs, and heat-resistant polymers (Table 1), in addition to well-known polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), and polyphenylene oxide (PPO). All these polymers require an one- to four-ring aromatic monomer with a specific structure (more linear ones in most cases). Therefore, shape248

© 2000 American Chemical Society

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

249

selective P A H conversion research can be applied for making chemicals from 1- to 4ring compounds that are rich in some industrial process streams (byproduct tars from coal carbonization or gasification, oil refinery streams, etc.). This article is an account of our recent research on selective conversion of polycyclic hydrocarbons.


Selective Synthesis of High-Value Chemicals Our attention on chemicals has focused on shape-selective catalytic synthesis of value-added chemicals from polycyclic aromatic compounds that are rich in coal liquids and some refinery streams. We are studying ring-shift isomerization of phenanthrene derivatives to anthracene derivatives (11-13), shape-selective alkylation of naphthalene (14-19), shape-selective alkylation of biphenyl (19,20), conformational isomerization of cis-decahydronaphthalene (11,21), and shapeselective hydrogénation of naphthalene (22,23), and as described below. Table 1. Structures of Some Important Aromatic Polymers Thermoplastic Polyesters

Thermotropic Polyester LCPs Ο

High-Temp Heat-Resistant Polymers Ο Ο

ο

Η

Η


250 Ring-Shift Isomerization. Phenanthrene and its derivatives are rich in various coal-derived liquids such as coal tars, but their industrial use is still very limited. On the other hand, anthracene and its derivatives have found wide industrial applications (8,9). We have found that some mordenite and ion-exchanged Y zeolite catalysts selectively promote the transformation of sym-octahydrophenanthrene (sym-OHP) to sym-octahydroanthraeene (sym-OHAn), which we call ring-shift isomerization (11), as shown in Scheme I. This reaction is in distinct contrast to the well-known ringcontraction isomerization which results in methylindane-type products. Table 2 shows some representative results (13). The properties of the Y-zeolite and mordenite catalysts in Table 2 are described elsewhere (11).


Scheme I.

sym-OHP

sym-OHAn

The reaction mechanism for the ring-shift isomerization has not yet been clarified. We proposed a possible mechanistic explanation of sym-OHP to symOHAn conversion on zeolite (11), as shown in a modified version in Scheme II. The first requirement would be the appropriate adsorption of sym-OHP on the catalyst surface. After the adsorption, the reaction is likely initiated by the protonation of the central aromatic ring in sym-OHP on Bronsted acid sites. However, the protonated intermediate could lead to several different products due to ring-opening cracking, alkyl chain isomerization, and subsequent cracking. In fact, the cracking reactions occur extensively on H - Y at >250°C, as can be seen from Table 2. Therefore, the second step is dependent upon whether or not the positive charge is stabilized. For the ring-shift isomerization, the cationic intermediate should be stabilized by nearby anionic sites. Scheme II.

sym-OHAn

ZO

H


251


Since zeolite acidity is associated with the aluminum ions, certain level of density of acid sites is required for effective ring-shift isomerization. From the above considerations, good zeolite catalysts should possess appropriate pore structure and the desired density and strength of the acidic sites for ring-shift isomerization of symOHP. This suggests that highly Al-deficient zeolite may not be suitable for this reaction. The fact that a hydrogen mordenite HML8 displayed higher selectivity than many others examined at 250 °C in both aliphatic and aromatic solvents suggests that it has the characteristics closer to those required for the reaction. The selectivity and activity of the catalysts also depend on the reaction conditions. Under mild conditions, some zeolites can afford over 90% selectivity to sym-OHAn with high conversion of sym-OHP (11,13). This could provide a cheap route to anthracene and its derivatives, which are valuable chemicals in demand, from phenanthrene that is rich in liquids from coal. Possible uses of sym-OHAn include the manufacturing of anthracene (for dyestuffs), anthraquinone (pulping agent), and pyromellitic dianhydride (the monomer for polyimides such as Du Pont's Kapton) (8). Table 2. Ring-Shift Isomerization of sym-OHP over Zeolite Catalysts in the Presence of Mesitylene Solvent % Select OHP Yieldls(% of OHP) Conv SymTHAn THP Ring-shift (%) OHAn HY 250°C-1 h 72.7 22.1 3.4 5.3 31.1 LaHY 5.5 30.7 250°C-1 h 73.4 3.6 21.8 NiHY 4.5 87.9 250°C-1 h 51.5 46.9 1.2 HML8 93.1 250°C-0.5 h 50.3 48.8 1.0 1.7 HML8 250°C-2 h 91.9 51.9 49.3 1.3 1.7 HML8 91.2 200°C-0.5 h h 13.1 0.1 14.7 1.4 HML8 300°C- 0.5 h 75.6 55.4 2.1 3.7 42.7 HM30A 250°C-1 h 76.2 55.1 2.1 2.5 42.9 a) The feed originally contained 91% sym-OHP, 2.9% sym-OHAn, and 6.1% others. Catalyst ID

Run Conditions

3

Shape-selective alkylation of naphthalene. Until recently, only limited attention has been paid to shape selective alkylation of two-ring aromatics such as naphthalene and biphenyl. Due to the demand for monomers for making the advanced polymer materials such as PEN and PBN, 2,6-dialkyl substituted naphthalene (2,6DAN) is needed now for making the monomers for PEN, PBN and LCPs. In some refinery streams such as LCOs and in tars or liquids derived from coal, naphthalene and its derivatives are major components. Shape-selective alkylation over molecular sieve catalysts can produce 2,6-DAN. There are ten possible D A N isomers. The β,β-selective alkylation over molecular sieve catalysts (Scheme III) can produce 2-alkylnaphthalene, 2,6-, 2,7-, and 2,3-DAN. The key challenge is to obtain 2,6-DAN with high selectivity, which means increasing the ratio of 2,6/2,7-DAN. Several recent papers described shape-selective isopropylation. Katayama and coworkers (24) reported preferential formation of 2,6diisopropylnaphthalene (2,6-DIPN) using mordenite. On the other hand, Moreau and coworkers have observed that 2,6- and 2,7-disubstituted products are formed in equal yields with a ratio of around 1 when using mordenite and Y-zeolite catalysts for


252 isopropylation with isopropyl bromide and for cyclohexylation with cyclohexyl bromide (25).


Scheme III

2-AN

2,6-DAN

2,7-DAN

2,3-DAN

The results from our laboratory show that by using partially dealuminated mordenite catalysts, regioselective alkylation of naphthalene can be achieved with over 65% selectivity to 2,6-DIPN by using isopropanol (14) with 2,6-DIPN/2,7-DIPN ratio of about 3 or using propylene (15-19) as the alkylating agent with 2,6-DIPN/2,7DIPN ratio of >2. Compared to parent mordenites, the partially dealuminated protonform mordenites are more effective as shape-selective catalysts for isopropylation of naphthalene. The results on the effects of dealumination have been reported recently (15-19, 24, 26). It has been indicated that 2,6-DIPN is slightly smaller than 2,7-DIPN (14). Horsley and coworkers (27) have shown by computer simulation that the diffusion of 2,6-DIPN inside mordenite pore channel is easier than that of 2,7-DIPN, accounting at least partially for the observed selectivity of mordenite for 2,6-DIPN and higher 2,6-DIPN/2,7-DIPN ratio. We also found some simple and effective methods for enhancing the shape selectivity by using water and dealuminated mordenite (15,17,18). Table 3 shows some typical results for naphthalene alkylation with isopropanol as the alkylating agent in the presence of mesitylene solvent (11). Table 3. Isopropylation of Naphthalene with Isopropanol over Zeolites at 250°C Catalyst

Structure Type

Naph Yield (% of Naph) % Isomer Select. Conv IPN MIPN %2-IPN % 2,6-DIPN ID (%) HY Y Zeolite 38.2 90.4 33.8 16.0 9.5 HM30A Mordenite 27.6 84.1 88.6 67.0 15.9 a) The alkylation was conducted in the presence of mesitylene solvent. a

Sugi and coworkers (28) have reported detailed results on the effect of ratio of mordenite on naphthalene isopropylation with propylene. They analyzed not only the bulk of the reaction products, but also the products trapped in the pores of mordenite after the isopropylation. The results reveal that inside the mordenite pore channel 2,6-DIPN was formed in a much higher selectivity than 2,7DIPN, but the external surface sites contribute more to the non-selective reactions as S1O2/AI2O3



253

well as coke formation. The same group also reported (29) that the impregnation of cerium is an effective method for the deactivation of external acid sites of hydrogen mordenite, which leads to improved selectivity to 2,6-DIPN, up to 70%. We have shown that the reactivity of 2,6-DIPN is substantially lower on dealuminated mordenite than on parent mordenite (16). The selective naphthalene methylation has also been reported in literature (30, 31). Fraenkel and coworkers (30) first published on selective naphthalene methylation over ZSM-5 type catalysts in 1986, but 2,6- and 2,7-isomers were not separated. Komatsu and Yashima (31) recently reported on the selective formation of 2,6dimethylnaphthalene (2,6-DMN) from methylation of 2-methylnaphthalene with methanol on HZSM-5 and metallosilicates with MFI structure. They demonstrated that isomorphous substitution of Al by other elements such as Β and Fe and deactivation of external surface (by using basic nitrogen compound) can increase the selectivity to 2,6-DMN. They concluded that in order to obtain 2,6-DMN in high selectivity, it is effective to weaken the acid strength while keeping the pore dimension of MFI structure constant (or, wider, if possible), which can be achieved by using Fe-MFI as a catalyst. Their results probably represent the level of selectivity in 2-MN methylation that has been achieved and reported in open literature so far using HZSM-5 and metallosilicates with MFI structure. Based on the literature, it is difficult for methylation of naphthalene over medium-pore zeolites to reach the same level of selectivity achieved in isopropylation to 2,6-DAN with mordenite catalysts. Shape-selective Alkylation of Biphenyl. Biphenyl and its derivatives are present in some refinery streams and in coal-derived liquids, although at concentrations lower than those of naphthalene derivatives. Shape-selective alkylation of biphenyl can produce 4,4-dialkyl substituted biphenyl (4,4'-DAB), the starling material for monomer of some LCP materials represented by Xydar. Partially dealuminated proton-form mordenite can be used as shape-selective catalyst for isopropylation of biphenyl (Scheme IV). Lee and Garces first published (32) on the effect of dealumination of mordenite on selective biphenyl isopropylation in 1989. They have demonstrated the beneficial effect of dealumination foi selective formation of 4,4*- diisopropylbiphenyl (4,4*DIPB). Sugi and coworkers have carried out a series of studies on biphenyl isopropylation over mordenites (33,34). They have reported on the influence of propylene pressure (35), effects of S1O2/AI2O3 ratio of mordenites, on shapeselectivity and coke deposition (36), and impact of cerium exchange of sodium mordenite (37). It was shown in our report that dealumination of some commercial mordenites by acid treatmentfirstincreases then decreases their activity, but increases their selectivity toward 4,4*-DIPB in isopropylation with propylene (19). Scheme IV

Shape-Selective Catalysis - American Chemical Society

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