Surface sites, external

All real crystals have atoms which occupy external surface sites and which do not possess the correct number of nearest neighbors as a consequence, Thus, a surface is a scat of energy and is characterized by surface tension. Furthermore, internal surfaces exist, grain boundaries and twin boundaries across which atoms are incorrectly positioned. In a crystal of reasonable size—say 1 cubic centimeter, these two-dimensional defects, called surface defects, contain only about 1 atom in 106, a rather small fraction. Even so, surfaces are important attributes of solids. 

The above results are related to the structural properties of the clay minerals. In the case of kaolinite, the tetrahedral layers of adjacent clay sheets are held tightly by hydrogen bonds. Therefore, only readily available planar external surface sites exist for exchange. With smectite, the inner peripheral space is not held together by hydrogen bonds, but instead it is able to swell with adequate hydration and thus allow for rapid passage of ions into the interlayer. 

Overall, it can be concluded that zeolites, and more specifically MFI, are adequate catalysts for oligomerization of short chain olefins to produce gasoline and even diesel range fuels. Selectivity and catalyst life is strongly dependent on parameters such as crystallite size, Si/Al ratio, and poisoning of external surface sites. The introduction of some metals (Ni) can be helpful. 

Fraenkel, D. Role of external surface sites in shape-selective catalysis over zeolites, Ind. Eng. Chem. Res., 1990, 29, 1814-1821. 

The authors suggest that isomerization reactions occur predominantly on external surface sites of ZSM-5 and by using dealumination procedures that such sites are rendered inactive. Selectivities of 2,6-DMN and 2,7-DMN increased from 23.4 and 24.9 % before dealumination to 39.9 and 60 % after dealumination, respectively. The disproportionation reaction was proposed to occur in the pores of the zeolite. 

Among the ethyltoluenes, the para isomer is considered to be preferably formed in the alkylation reaction inside the zeolite channel system [14], whereas or//jo-ethyltoluene is mainly formed on external surface sites [15]. This view is completely in agreement with our findings that or/Ao-ethyltoluene formation is negligible even on the unmodified H-ZSM-5 and that it is entirely suppressed after passivation of the external surface by 12-tungstosilicic acid. The formation of small amounts of or/Ao-ethyltoluene over lOW-ZSM-5 agrees with the decrease in / ara-selectivity as explained for this sample above. 

In a poor solvent (cyclohexane), the clay lamella packages form aggregates due to adhesion of the secondary particles (a card house structure is formed). Furthermore, the value of dt is very close to the thickness of the silicate layer plus the cross sections of two alkyl chains. It follows that in a poor solvent, the alkyl chains are laying flat on the surface and no swelling takes place. In that case, the interstitial sites are not available for reactants and reaction proceeds on the external surface sites. 

Sugi and coworkers have reported detailed results on the effect of Si02/Al203 ratio of mordenite on naphthalene isopropylation with propylene [Kim et al., 1995a]. 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-diisopropylnaphthalene was formed in a much higher selectivity than 2,7-diisopropylnaphthalene, but the external surface sites contribute more to the non-selective reactions as well as coke formation. The same group also reported that the impregnation of cerium is an effective method for... 

The turnover numbers (TON) of phenol are 62.4 and 105.4 with a H2O2 efficiency of 28.9 wt.% and 48.9 wt.% for the Zr-Sil-2 samples A and B, respectively. A significant difference in the product distribution between these two runs is also observed. The catechol (CAT) to hydroquinone (HQ) ratios are 0.9 and 1.7 for Zr-Sil-2 (A) and (B) samples, respectively. A CAT/HQ ratio of 0.9 to 1.3 has been reported for titanium and vanadium silicate molecular sieves (TS-2 and VS-2) [13]. The samples synthesized using Zr(acac)4 show a nearly two fold activity in the reaction probably due to the smaller particle size. These results indicate that in the case of Zr-Sil-2 samples synthesized using ZrCU, the Zr " ions are well dispersed within the channels of the MEL structure while in the samples synthesized using Zr(acac)4, the hydroxylation occurs at the external surface as well, where a part of Zr species may be located. For small submicron crystals (<1 pm), external surface sites could be a significant fraction of the total surface area. If the external surface sites are catalytically either the same or more active than the intracrystalline active sites, then the shape selectivity of a zeolite could... 

When the reaction is carried out in methanol the reaction gives initialy equal amounts of hydroquinone and catechol but the selectivity increases with reaction time. This was explained assuming that catechol is produced on the external surface of the zeolite, whereas hydroquinone formation prevails at the internal catalytic sites by shape selectivity. After a short period, the activity of the external surface sites was killed by cocking and hydroquinone is obtained selectively. On the contrary, acetone prevents the formation of coke on the external surface by dissolving it. The selectivity of the reaction will then be the sum of external and internal catalysis. 

These results suggest that only external surface sites are active in the oxidation of isopropylbenzene. 

Movement of adsorbate across the external liquid film to the external surface sites on the adsorbent particle (film diffusion)... 

Zeolites and molecular sieves are crystalline porous metal oxides possessing enormous internal surface area (—1000 mVg), where every metal cation is either an internal or external surface site [3]. Zeolites are composed of an aluminosilicate framework of the type M ,/ [(A102)jc(Si02) ] zH20 (where n is the charge on the metal cation, which is typically Na, K, Ca ). Molecular sieves differ from the aluminosilicate zeolites because they possess framework atoms other than aluminum or silicon (P, Ga, Ti, V, etc.). Zeolites and molecular sieves possess a wide variety of properties pore size, pore shape, dimen-... 

---