Acid site number

The liquid-phase dehydration of 1-hexanol and 1-pentanol to di-n-hexyl ether (DNHE) and di-n-pentyl ether (DNPE), respectively, has been studied over H-ZSM-5, H-Beta, H-Y, and other zeolites at 160-200°C and 2.1 MPa. Among zeolites with a similar acid sites concentration, large pore H-Beta and H-Y show higher activity and selectivity to ethers than those with medium pores, although activity of H-ZSM-5 (particularly in 1-pentanol) is also noticeable. Increased Si/Al ratio in H-Y zeolites results in lower conversion of pentanol due to reduced acid site number and in enhanced selectivity to ether. Selectivity to DNPE is always higher than to DNHE... 

The above considerations may give us some insight on the coke structure. Assuming that at low Pt loadings the metal-site number increases in proportion to the Pt content whereas the acid site number remains constant, then the number of the participating acid sites can be estimated from the increase of the monolayer coke due to increased metal loading. Let Abe... 

On the basis of the results obtained one may draw a conclusion that there exists a certain relationship between the concentration of acidic sites of different types of zeolite and Mo content. In this coimection, to produce a catalyst e diibiting a high activity in the process of methane dehydroaromatization, it is necessary to optimise the relationship between the acidic sites number of a zeolite and the number of active sites connected with different Mo forms. The highest methane conversion per one run and maximal yield of aromatic hydrocarbons are reached for the sample containing 4.0 mass% of Mo nanopowder. The development of the mesoporous zeolite structure is an important factor promoting the activity of Mo-ZSM-5 in the reactions of the formation of high-molecular aromatic compounds. 

In a study performed by Auroux et. al. [Ill] microcalorimetry experiments of ammonia and sulfur dioxide were performed in order to analyze the possible correlations between the acidity and basicity of the alkali-metal ion-exchanged X and Y zeolite structures and their catalytic properties. The catalytic results for the 4-methylpentan-2-ol conversion show that activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The activity was found to increase in order Cs > Rb > K > Na > Li for both X and Y zeolites. The dehydrogenation reaction occurs only on CsX + CS2O, which presents very strong basicity. The product selectivity of the reaction depends on both Lewis acidity and basicity Lewis basic or acidic sites of zeolites can be considered as acid-base pairs, in which both framework basic oxygens and neighboring cations are important. The selectivity ratio between the 1-alkene and (2-alkene+isomers) increases linearly with the ratio between basic and acid sites number, so2/ nh3, for both X and Y zeolites, as shown in Fig. 9.14. 

Fig. 9.14 1 -ene/(2-ene+isomers) selectivity ratio versus the basic/acidic site number for X samples left and for Y samples right... 

According to the obtained NH3 desorption temperatures and desorbed NH3 amounts (see Table 1 and fig.l), practically the same acid site number and the same acidity strength are obtained when the SG-SE and the SG methods are used. The NH3 desorption temperatures are respectively 147°C and 145°C. When Si amount is increased from 0 to 100%, the acidity strength diminishes and the acid site number augments sigrtificantly on the Si30 sample compared to that on the SiO and on the... 

Spectral studies at low temperatures enable us to broaden the number of test molecules for surface acidic sites and besides ammonia pyridine and nitriles, to use CO, NO and that do not adsorb at 300 K. 

Various analytical tests determine zeolite properties. These tests supply information about the strength, type, number, and distribution of acid sites. Additional tests can also provide information about surface area and pore size distribution. The three most common parameters governing zeolite behavior are as follows ... 

Sodium decreases the hydrothermal stability of the zeolite. It also reacts with the zeolite acid sites to reduce catalyst activity. In the regenerator, sodium is mobile. Sodium ions tend to neutralize the strongest acid sites. In a dealuminated zeolite, where the UCS is low (24.22°A to 24.25°A), the sodium can have an adverse affect on the gasoline octane (Figure 3-7). The loss of octane is attributed to the drop in the number of strong acid sites. 

The sodium in the E-cat is the sum of sodium added with the feed and sodium on the fresh catalyst. A number of catalyst suppliers report sodium as soda (Na20). Sodium deactivates the catalyst acid sites and causes collapse of the zeolite crystal structure. Sodium can also reduce the gasoline octane, as discussed earlier. 

Once formed, carbenium ions can form a number of different reactions. The nature and strength of the catalyst acid sites influence the extent to which each of these reactions occur. The three dominant reactions of carbenium ions are ... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

The catalyst acidity is determined by the number of acid sites and their acid strength. The total concentration of acid sites, C<, can be obtained from independent TPD measurements. The average acid strength of the sites is characterized by the alkene standard protonation enthalpy,, and is typically determined by regression using reference... 

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

A1-MCM-41-P were distributed mainly over C5-C20 hydrocarbons and were quite similar to those over A1-MCM-41-D in spite of the differences in the number of their acid sites. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

The nature (Bronsted or Lewis centers), the number, and the strength of the acidic sites of the Pd/Al203 and Pd/Zr02 solids have been checked using infixed spectroscopy of adsorbed pyridine and thermoprogrammed desorption of ammonia. 

For determination of the number of protonic sites and Lewis acid sites on the surface, the integrated absorbances of the bands at 1450 cm-i (due to pyridine chemisorbed on Lewis acid sites, L-Py) and 1490 cm i (due to both the L-Py and pyridine chemisorbed on protonic acid sites, B-Py) were used with the tangent background for all samples. The values obtained were normalized to the weight of the sample wafer. To obtain the apparent absorption coefficients of the bands, a known amount of pyridine was adsorbed on the sample, and the absorbance of each band was measured. Then, a small quantity of water which is sufficient to convert all Lewis acid sites into protonic acid sites was introduced into the IR-... 

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 activity and decay behaviour of the different porous heteropolycompounds were compared in two reactions requiring strong acid sites the n-butane isomerization and the isobutane/2-butene alkylation. Although these two reactions are important in the petroleum refining industry, n-butane isomerization is often used as a "test reaction" since it is known that this reaction requires very strong acid sites and only a limited number of oxides are active in this reaction, under mild conditions (T = 473 K). 

This interpretation is further confirmed by the results obtained in the liquid-phase methylation of m-cresol with Mg/Fe mixed oxides having different Fe contents Mgi.xFexOi+i/2x. In these catalysts the number of Lewis acid sites was proportional to the Fe content [4], Figure 4 plots the selectivity parameters and the number of Lewis acid sites as a function of the Mg/Fe ratio in catalysts (selectivity was calculated at very low m-cresol conversion, thus in the absence of any consecutive reaction). 

Increasing the number of acid sites, the C-methylation becomes largely preferred over the O-methylation (the QC-mcthylation ratio increased), while between the two ortho-C-methylated compounds, 2,5-DMP becomes more preferred than 2,3-DMP, in agreement with the calculated energies for the corresponding transition states. Disagreement between results obtained in gas-and liquid-phase methylation with the Mg/Fe/O catalyst (Table 2) concerns the ortho/para-C-methylation ratio (which in liquid phase was not varied when the Fe content was increased), and the 2,5-DMP/2,3-DMP ratio (which in gas phase... 

Figure 4 Effect of the Mg/Fe atomic ratio in Mg/Fe/O catalysts on the number of acid sites, as determined by pyridine adsorption (o), on the Q C-methylation ratio (v), on the ortho/para-C-methylation ratio (a), and on the 2,5-DMP/2,3-DMP selectivity ratio (/ ), in the liquid-phase methylation of m-cresol [4],...

The condensation of acetone can also occur over acidic sites as shown by a number of authors [1,9], Generally, when this occurs other products are formed such as isobutene and acetic acid, by the cracking of DAA. Additionally mesitylene can be formed by the internal 2,7-aldol condensation of 4,6-dimethylhepta-3,5-dien-2-one which is in turn obtained by the aldol condensation of MO with a deprotonated acetone molecule [7, 8], As these species are not observed we can concluded that any acidic sites on the silica support are playing no significant role in the condensation of acetone. 

In coals alkylated in this manner, the number of acidic sites is substantially reduced, and acid-base associations are virtually precluded. Extracts from alkylated coals should, therefore, be amenable to GPC fractionation. Such fractionation, conducted on Bio Beads S-Xl and S-X2, results in separation by molecular weight and indicates that both benzene and chloroform extracts contain substantial amounts of high ( 6000) and fairly low (560-640) molecular weight fractions (Figure 2). While the extract yields from non-reductively ethylated vitrinite increase in the order benzene extr. chloroform extr. - pyrid. extr > the molecular weights determined by VPO in pyridine, decrease in this order. 

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