Zeolites steric effect

PHOTOCHEMISTRY OF DIBENZYL KETONE ADSORBED ON SIZE/SHAPE SELECTIVE FAUJASITE ZEOLITES. STERIC EFFECTS ON PRODUCT DISTRIBUTIONS... 

Photochemistry of Dibenzyl Ketone Adsorbed on Size/Shape Selective Faujasite Zeolites Steric Effects on Product Distributions (N. J. Turro and Z. Zhang)... 

When supported complexes are the catalysts, two types of ionic solid were used zeolites and clays. The structures of these solids (microporous and lamellar respectively) help to improve the stability of the complex catalyst under the reaction conditions by preventing the catalytic species from undergoing dimerization or aggregation, both phenomena which are known to be deactivating. In some cases, the pore walls can tune the selectivity of the reaction by steric effects. The strong similarities of zeolites with the protein portion of natural enzymes was emphasized by Herron.20 The protein protects the active site from side reactions, sieves the substrate molecules, and provides a stereochemically demanding void. Metal complexes have been encapsulated in zeolites, successfully mimicking metalloenzymes for oxidation reactions. Two methods of synthesis of such encapsulated/intercalated complexes have been tested, as follows. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

When radicals with bulky substituents are found within the internal channels of the zeolite, their interactions may be inhibited by the narrow diameter of the channels, allowing them to persist for a long time (shown by electron spin resonance (ESR) spectroscopy). These steric constraints are known as a supramolecular steric effect. 

The steric effect observed in solution should be amplified in the zeolite if either of the models depicted in Fig. 4 operate because the sodium counterion should force the allylic substiment R to reside to an even greater extent on the face approached by singlet oxygen. This should lead to an increase in allylic hydroperoxy A at the expense of B in stark contrast to the experimental observation (e.g., the A/B ratio does not increase, but actually decreases from 2.4 to 1.58... 

Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids thus the energies associated with the cationic sites in crystalline zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties exhibited by sodalite and faujasite. 

A particularly detailed description was obtained for complexes of Co- A zeolite with mono-olefins (24). Steric effects due to methyl groups adjacent to the double bond resulted in the ligand strength spectrochemical series... 

The adsorptive separation is achieved by one of the three mechanisms steric, kinetic, or equilibrium effect. The steric effect derives from the molecular sieving property of zeolites. In this case only small and properly shaped molecules can diffuse into the adsorbent, whereas other molecules are totally excluded. Kinetic separation is achieved by virtue of the differences in diffusion rates of different molecules. A large majority of processes operate through the equilibrium adsorption of mixture and hence are called equilibrium separation processes. 

Paraxylene selectivity is a complex phenomenon which can only be observed at high loading and for bulky and weakly charged cations. This phenomenon is related to entropy effects which allow paraxylene to be more efficiently packed in the zeolite micropores. This leads to paraxylene/metaxylene selectivity over three and paraxylene/ethylbenzene over two, which is enough for a separation process. It is now established that steric effects are very important to make the adsorbent selective for paraxylene. These effects are driven by the size, the number and the position of the compensation cations. The charge of the cation also plays an important role by controlling these effects. 

The results pointed in another direction. The effects of steric congestion in the substrates are the most important factor. One way to enhance the selectivity would therefore be to amplify the steric effects by forcing the reaction to occur close to or on the surface of a solid acid catalyst. This was found to be a valid conclusion, and by using zeolites as catalysts the mode of ring closure could be partially controlled by a proper choice of zeolite. For certain substrates, the regioselectivity was reversed by changing the catalyst from Mordenite to Zeolite-Y [82]. 

For the linear alkanes studied (methane, ethane and propane), the fact that only one type of acid site can be represented with the 3T and 5T clusters should not be a major problem. As shown by the MD studies, because of their sizes, steric effects are of minor importance and these molecules have equal probability of visiting aU the distinct sites of the zeolite. In another words, for these molecules, as far as steric effects are concerned, the acid sites are all alike. Thus, the interaction between any of these molecules and the zeolite will depend mainly on the sites acidic strengths, which do not differ very much from each other. Therefore, for these molecules it is a reasonable approximation to treat aU the acid sites alike. However, for isobutane steric effects are more important and the molecule should be more sensitive to the type of the acid site. It will be easier for the isobutane molecule to approach the acid sites represented by 3T and 5T clusters than the one at the channels intersection, in the real zeolite, where it preferentially adsorbs. Therefore, for isobutane and other branched alkanes (and most probably for the large n-alkanes), the chemical reactions at the 3T and 5T clusters may take place artificially easier than in the real zeolite. 

The dehydrogenation reaction proceeds through the simultaneous elimination of the zeolitic proton and a hydride ion from the alkane molecule, giving rise to a transition state which resembles a carbenium ion plus an almost neutral H2 molecule to be formed. For the linear alkanes, the TS decomposes into an H2 molecule and the carbenium ion correspondent alkoxide. However, for the isobutane molecule the reaction follows a different path, the TS producing isobutene and H2. Most certainly the olefin elimination is flavored to the alkoxide formation due to steric effects as the t-butyl cation approaches the zeolite framework. The same mechanism is expected to be operative for other branched alkanes. 

It is clear from the first part of this chapter that the external medium has a major effect on the catalytic properties of TS-1 and other Ti-zeolites. Early rationahzation suggested that the adsorption of a protic molecule, ROH, had a stabilizing effect for Ti—OOH species. In this view, water and alcohol solvents influenced the catalytic performance through the electronic and steric effects exerted in the oxidation step by the ROH molecule present in the active species (Section 18.4.1.1) [62]. 

Kincaid and co-workers have exploited the steric effect in altering the excited-state photophysical properties of [Ru(bpy)2(daf)] +, where daf = 4,5 diazafluorene [32]. Aqueous solutions of this complex exhibit very weak luminescence, with lifetimes shorter than 10 ns at room temperature. Upon encapsulating this molecule into zeolite Y supercages, the fluorescence intensity increases by a factor of 100 and the excited-state lifetime is increased to 302 ns. 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

Kebarle. " Recently, Mota et al. calculated the potential energy surface of the C3H9+ cation [MP4(SDTQ)/6-311++G /MP2(full)/6-31G level]).The C-proponium cation (43) was again found to be of lowest energy but the complex xec-C ilh + H2 lies only 0.3 kcal mol above structure 43. The C-H bond lengths in the 3c-2e interactions are 1.272 and 1.188 A, whereas the C-C bond distance is 2.099 A.They also found that the interconversion between the -H-protonated cation (45) and the C-proponium cation (43) has no energy barrier. This may indicate that on zeolites where steric effects are important the primary, that is more accessible hydrogens are protonated initially to yield the... 

Fig. 15. Energy contribution to the transition-state energy due to sterical effects for hydride transfer reactions in three zeolites [74].

The mimic is prepared by sequential ion-exchanges with iron(ll) and Pd(ll) tetrammine cations followed by calcinations and reduction of the Pd(ll) to Pd(0) as previously described(14). A material with 2wt% Fe(ll) and 1wt% Pd(0) is used by immersing the dry zeolite solid in neat substrate alkane and then pressuring the reaction vessel with a 3 1 mixture of oxygemhydrogen. After shaking this mixture at room temperature for 4 hours the products are analyzed by capillary GC. As a control to assess the intrinsic selectivity of such a Pd/Fe system in the absence of steric effects of the zeolite, catalysts prepared with amorphous silico-aluminate supports were run for comparison. In these cases all reactions must take place at the particle surface since there is no interior pore structure available. In addition, comparison of reaction selectivities of this catalysts with our zeolite materials allows us to ascertain that the Fe active sites must be actually inside (and not on the exterior surface) of the zeolite crystallites. 

It seems that practical implementation of this type of selective catalysts will require a medium in which (very) polar products can be removed from the zeolite phase. Unfortunately, no attention has been paid in literature to such issues. On the contrary, some attention has been devoted to host modification after exchange of NaY with other alkali metal cations [37]. The cyclohexene epoxidation activity increases with decreasing size of the charge compensating cation pointing to the influence of steric effects or of electrostatic effects on the activity. In competitive experiments using cyclohexene and 1-octene as feed, the reactivity of the smaller substrate is suppressed, indicating that competitive sorption is involved as well [37],... 

---