J. Phys. Chem. 1995,99, 4241-4246
4241
Acid Zeolites as Electron Acceptors. Generation of Xanthylium, Dibenzotropylium, and Fluorenylium Cations from Their Corresponding Hydrides through an Electron-Transfer Mechanism Maria Luz Cano, Avelino Coma,* Vicente Fornks, and Hermenegildo Garcia* Instituto de Tecnologia Quimica UPV-CSIC, Universidad P o l i t h i c a de Valencia, Apartado 22012, 46071 -Valencia, Spain Received: October 10, 1994@
Adsorption of xanthene (XH) and dibenzo[a,&ycloheptatriene onto large pore acid zeolites containing Bronsted sites gives rise to the formation of xanthylium (X+) and dibenzotropylium (DT+) ions, respectively. In the case of fluorene (FH) using acid Y and /3 zeolites, the same adsorption treatment did not allow the detection of the 9-fluorenylium cation (Ft),but instead the longer-lived radical cation FH'+ was identified. Interestingly, when FH was adsorbed within acid mordenite, the simultaneous presence of FIT+ and F+ was observed. Detection of FW+,together with the product distribution of the reaction of XH under electron-transfer conditions, suggests that X+, DT+, and F+ are formed from the corresponding radical cations. Concerning the nature of the zeolite oxidizing centers, it was established that formation of tricyclic carbenium ions does not take place on the nonacidic Na+ form of zeolites. By contrast, purely Bronsted zeolite HYD-W was found to be active for the generation of X+ and FIT+. On the other hand, selective neutralization of Bronsted sites with exhaustive Na+ exchange causes the deactivation of a sample in spite of the presence of unaltered residual Lewis sites.
Introduction Zeolites are crystalline aluminosilicates whose structures are formed by si0d4- and Mod5-tetrahedra sharing the Owing to the different valence between Si and Al, each framework A1 creates a negative charge in the lattice that requires the presence within the internal voids of chargebalancing cations to ensure the electroneutrality of the solid. The ionic nature of the bond between these countercations and the zeolite framework allows selectively the exchange of the cations originally introduced in the synthesis by any others (either organic or inorganic) without producing any alteration in the crystalline structure of the solid.5 Medium and large pore zeolites have been widely employed as molecular sieves to adsorb neutral organic molecule^.^^^ However, in spite of the fact that it can be anticipated that the negative charge of the lattice must provide a very convenient microenvironment for positively charged species, the use of zeolites as hosts to study and control the physical and chemical properties of organic cations in an inert environment is still quite limited. Recently, we have reported that 2,4,6-triphenylpyrylium cation7 can be prepared by ship-in-a-bottle synthesis within the supercages of the faujasites,* while the less space-demanding xanthylium cation (X+) can be readily incorporated within faujasite, mordenite, and ZSM-5 by treatment of 9-xanthydrol (XOH) with the H+ form of these ~eolites.~ Furthermore, we observed that using mordenite and Z S M J hosts the IR and W/ vis spectra of these composites remained essentially unchanged for a period of time longer than 1 year, while X+ can be detected in the case of faujasite for weeks. Since physisorbed HzO is also simultaneously present in all these samples, this observation evidences the dramatic stabilization due to the internal electrostatic fields experienced by the cationic guest.lO,ll This fact contrasts with the high value of the rate constant for the reaction of X+ and H20 in homogeneous liquid p h a ~ e . ' ~ - l ~ @Abstractpublished in Advance ACS Abstracts, February 15, 1995.
0022-3654/95/2099-4241$09.00/0
The extraordinary capability of zeolites to stabilize positively charged species has also been pointed out in the case of organic radical cations. Depending on their oxidation potential, radical cations can be generated spontaneously by mere adsorption of their neutral p r e c u r s ~ r s , ~or~ -photochemically ~~ in the presence23-25or even in the absence26-28of coadsorbed electron acceptors. An amazingly huge increase of the radical cation lifetime within zeolite media, which can even be physically separated from the contact ion pair by conventional ion e ~ c h a n g e ?has ~ been observed. Zeolites are among the most widely used heterogeneous cataly~ts?~-~l including large-scale applications in the petrochemical i n d u s t r i e ~ . In ~ ~ many of these cases, the reaction mechanisms are not yet completely understood. In view of this ability to generate radical cations, it is possible that cracking and other processes catalyzed by zeolites involve other reactive intermediates besides the generally accepted carbenium ions. In this context, EPR detection of radicals generated by adsorption of aliphatic hydrocarbons containing tertiary carbons within acid mordenites lend support to this h y p ~ t h e s i s .Although ~~ no experimental evidence was obtained, it was postulated that these radicals arise from the corresponding radical cations, carbenium ions being the final species. It is then of paramount importance to find out which are the zeolite active sites responsible for the formation of radical cations, and learn how to control them during zeolite synthesis or during postsynthesis chemical treatments. The exact location, nature, and structure of the zeolite oxidizing sites as well as the way how they are produced in the solids are still matters of controversy. From the relationship between the zeolite A1 content and the population of generated radical cation, it appears that A1 is somehow involved in the zeolite oxidizing sites.34 In the present work we report that cation stabilization combined with the electron acceptor ability results in the unprecedented formation of the moderately stable xanthylium (X+)35and dibenzotropylium (DT+)36cations by adsorption of the corresponding hydrides (XH and DTH) onto mono- and 0 1995 American Chemical Society
4242 J. Phys. Chem., Vol. 99, No. 12, 1995
Can0 et al.
CHART 1
FH
XH
TABLE 1: Main Physicochemical Parameters of the Acid Zeolites Used in This Work BET surface SUM ratio Br6nsted host area (m2/g) ratio to Lewis" NaY HY-21
HY-50 HY-100 HY-100-N
HYD
HYD-W
YB
NaMor HMor Si02-AI203
900 810 790 760 730 750 710 607 450
550 250
400 500 Wavelength (nm)
300
DTH
2.6 2.4 2.4 2.6 2.6 5.2 5.3 13 6.4 10 6
600
Figure 1. Diffuse reflectances (plotted as the inverse of reflectivity, 1/R) of the XH-HY-100 (a), XH-W (b), and XH-HMor (c) composites.
b 1.3:l 1.1:l 1:l 1:6 4: 1 > 1O:l 1:2 C
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