Positional Isomerization of Dialkylnaphthalenes: A Comprehensive Interpretation of the Selective Formation of 2,6-DIPN over HM Zeolite Gyula Tasi,* ,²,‡ Fujio Mizukami, ² Istva ´ n Pa ´ linko ´ , § Makoto Toba, ² and A Ä kos Kukovecz ²,‡ Department of Surface Chemistry, National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan, Department of Applied and EnVironmental Chemistry, UniVersity of Szeged, H-6720 Szeged, Rerrich B. te ´ r 1, Hungary, and Department of Organic Chemistry, UniVersity of Szeged, H-6720 Szeged, Do ´ m te ´ r 8, Hungary ReceiVed: September 21, 2000; In Final Form: December 13, 2000 A new ab initio method was developed to supply reliable molecular dimensions for catalytic studies. For the DIPN isomers, calculations at correlated level revealed that the 2,6-isomer has the best molecular shape and dimensions concerning molecular transportation in the channel system of mordenite (M) zeolite. Adsorption rate measurements supported this theoretical finding. According to the ab initio calculations performed at correlated level, the 2,6- and 2,7-DIPN molecules may transform into each other via 1,2-isopropyl shift at an appropriate temperature in the main channel of mordenite. Isomerization reactions of 2,6-DIPN carried out over HM at high temperatures resulted in 2,7-DIPN in the reaction mixture supporting the theoretical results. Theoretical and experimental studies revealed that the selective formation of 2,6-DIPN over HM zeolite is the result of diffusion-controlled shape-selective catalysis, i.e., product selectivity is operative in this case. Introduction Acid-catalyzed alkylation of naphthalene with various alky- lating agents (alkenes, alcohols, etc.) is of great importance from industrial point of view. 1 Of dialkylnaphthalenes, the 2,6 isomers are starting materials for preparing liquid crystal polymers. 2 Accordingly, many efforts for their selective synthesis are demonstrated in the literature. 3-11 For the selective preparation of 2,6-diisopropylnaphthalene (DIPN) and 2,6-di(tert-butyl)- naphthalene (DTBN), HM and HY zeolites, 3,5,6,11 respectively, were found to be the best catalysts. Dealumination to some extent, i.e., decrease in acidity, was observed to further increase their catalytic activities. 6,7 Up to this point, for the rationalization of the experimental results molecular mechanics 11 and semiem- pirical quantum chemical calculations 9,10 were applied. The selective formation of the 2,6-isomer was explained by either product selectivity 11,12 or spatial transition state selectivity 3,13 or restricted electronic transition state selectivity. 9,10 Recently, we performed a detailed study concerning the positional isomerization of monoalkylnaphthalenes at various theoretical levels. 14 Ab initio quantum chemical calculations at correlated level revealed that intramolecular migrations are allowed for the methyl, ethyl and isopropyl groups. Moreover, the process becomes easier as the size of the alkyl group increases. In contrast, the bulky tert-butyl group transfers exclusively intermolecularly. For the methyl group, we have also studied the possibility of an intermolecular transfer. Calculations with three-body basis set superposition error correction at correlated level revealed that this transfer requires very high activation energy and, thus, it is of low probability compared to the intramolecular one. In this contribution we prove theoretically as well as experimentally that in contrast with the dimethylnaphthalene (DMN) isomers, 15 the DIPN isomers do not form equilibrium groups. Ab initio calculations at correlated level reveal that the  f ′ intramolecular migrations become energetically more and more favorable with the increasing size of the migrating alkyl group: methyl f ethyl f isopropyl. At an appropriate temperature the 2,6- and the 2,7-DIPN molecules may freely transform to one another via intramolecular  f ′ isopropyl group migration in the main channel of HM zeolite. Formation of the corresponding transition state complex is not restricted spatially. Results of adsorption rate measurements of the mixture of the DIPN isomers and the calculated ab initio molecular dimensions prove that among the DIPN isomers the 2,6 one moves the easiest, i.e., diffuses the fastest in the main channel of mordenite. Experimental Section For the adsorption measurements, 0.5 g of a mixture of some DIPN isomers (Kureha Chemical Co., KMC-113) and 0.2 g of n-octadecane (internal standard) were dissolved in 10.0 g of n-undecane solvent. 2.0 g of commercial NaM zeolite (Tosoh Chemicals, HSZ-640NAA) outgassed at 473 K for 2 h in high vacuo was then added to the solution, which was kept at 353 K in a silicone oil bath and stirred continuously. Samples for analysis were taken every 5 min and treated as described below. The result of a typical adsorption experiment can be seen in Figure 1. For the isomerization reaction of 2,6-DIPN over HM, 5.0 g of 2,6-DIPN and 1.0 g of n-octadecane (internal standard) were dissolved in 50.0 g of n-undecane. After the analysis of the initial mixture, the solution was transferred into a 100 cm 3 stainless steel autoclave and mixed with 5.0 g of commercial HM zeolite (Tosoh Chemicals, HSZ-660HOA) outgassed at 573 K for 2 h in high vacuo. The reaction mixture was stirred (700 rpm) at 513 K for 5 h, then the reaction was stopped and a sample was withdrawn for analysis. Results of a typical isomerization experiment are shown in Table 1. ² Department of Surface Chemistry. ‡ Department of Applied and Environmental Chemistry. § Department of Organic Chemistry. 6513 J. Phys. Chem. A 2001, 105, 6513-6518 10.1021/jp003428t CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001