Acid sites accessibility

In order to correlate chemieal analysis of aetivated membrane surface with other characteristic parameters, tangential streaming potential measurements at a constant NaCl concentration (5x10 " M) but different pHs were carried out with the DT200 sample and analysed using the local dissociation model [68-69], which allows the determination of the pKa and the munber of acid sites accessible on the membrane surface (N ), and their comparison with the same parameter for PAO polyamide/polysulfone composite membrane (the un-modified base membrane). The obtained values and surface roughness are ... 

P.F. Siril, D.R. Brown et al.. Acid site accessibility in sulfonated polystyrene acid catalysts calorimetric study of NH3 adsorption from flowmc gas stream. J. Mol. Catal. A Chem. 252, 125-131 (2006)... 

Both methods suggest that the chemical structure of A A (cis double bonds connected by two single bonds) allows the fatty acid to access the cyclooxygenase active site of PGHS-1 through a narrow hydrophobic channel and to bind in a shape favorable for the cyclooxygenation reaction. 

Activity in n-butane isomerization reaction of various alkaline salts of H3PW12O40 and H4SiWi2O40 was shown to be strongly dependent on the strength and number of accessible protons whereas the stability with time on stream was correlated to the presence of mesoporosity. For the liquid iC4/C4 continuous alkylation reaction, the strength and the number of acid sites appeared less important than the existence of mesoporosity indicating that the diffusion of the reactants and of the products plays an important role in this reaction. 

The most active samples for n-C4 isomerization, (NH4)2.4P and Csi.gP, showed opposite reactivities in liquid alkylation. The first one gave rise to a high production of TMP while the second one was only initially slightly active. The main difference between these two samples concerned their porosity (NH4)2.4P was mesoporous while Csi.gP was mainly microporous. Then, one may suggest that the presence of mesoporosity is essential for the accessibility of the reactants to the acid sites and the desorption of the products. As a consequence the catalytic activity seems more governed by the textural features than by the acidity. As a general trend, the samples which were, at the same time, active and stable for the alkylation reaction, exhibited a mesoporosity equivalent to about 40 m. g-i. 

Shakhashiri, Bassam. Chemical of the week phosphoric acid, University of Wisconsin-Madison Web site. Available online. URL http //scifun.chem.wisc.edu/CHEMWEEK/PDF/ Phosphoric Acid.pdf. Accessed on March 17, 2008. 

The results of the catalyst testing are shown in Table 3. The data listed in the table show, that on a per proton basis, catalyst A (based on 7% DVB) has higher activity as compared to resin materials, crosslinked with 12% DVB. This result is in accord with the finding by Petrus et al.,3 that at temperatures higher than 120 °C the hydration is under into particle diffusion limitation and as such, a more flexible polymeric matrix will provide better access to the acidic sites. On a dry weight basis, catalyst D showed the highest activity, which correlates well with the high acid site density found for this resin (Table 2). On a catalyst volume basis, catalyst A has the best performance characteristics followed by catalyst D. 

A highly stable and shielded polyplex should circulate in the blood stream without undesired interactions until it reaches the target cell. At that location, specific interactions with the cell surface should trigger intracellular uptake. While lipid membrane interaction is undesired at the cell surface, it should happen subsequently within the endosomal vesicle and mediate polyplex delivery into the cytosol. During or after intracellular transport to the site of action, the polyplex stability should be weakened to an extent that the nucleic acid is accessible to exert its function. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

Add groups are introduced by sulfonation, e.g., with sulfuric add. If all the benzene rings are monosulfonated, an acid capacity of the order of 5 mmol gca, 1 should be obtained. The sulfonic acid sites are situated in the easily accessible macropores, and also inside the gel-type microspheres [26]. 

The size of the probe molecule should be selected so that it is accessible to the sites of interest, for example, internal versus external acid sites in a zeolite. 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based... 

In 1996, Kim and coworkers reported for the first time on the use of a polyami-doamine (PAMAM) dendrimer [Gl] as a soluble support for organic synthesis (Fig. 7.5) [37]. Advantages of PAMAM are its commercial availability and its high symmetry, which provides uniform site accessibility (in lower generations) and facilitates NMR interpretation. By attaching 4-hydroxymethylbenzoic acid (HMB) to... 

Needle like nanoparticles of Mg-Al mixed spinel catalysts synthesized under hydrothermal conditions were used for the synthesis of 1-methylimidazole by the gas phase imidazole methylation with methanol performed at atmospheric pressure [113]. High yield and selectivity to desired product were obtained at a temperature range between 320 and 350°C. It was proposed that one of the nitrogen atoms participates in the bonding of the imidazole with the basic site of the catalyst, and the second nitrogen atom is accessible for the reaction with electrophilic methyl species formed from methanol on acid site of the catalysts. 

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