Active silica, definition

As the surface tension of the pore liquid was lowered in these experiments, the total pore volume increased. Nitrogen adsorption indicated that the pores also became larger. This is an inevitable outcome of maintaining constant surface area (from the definition of a cylinder, or any other shape of pore, pore volume must be proportional to pore area multiplied by pore diameter). The least active silica in the series, which had the lowest pore volume, also contained most of its volume within small pores (less than 60 A in diameter). In contrast, the most active samples, which were dried by organic solvent extraction, had the highest total pore volumes and most of that volume was inside much larger (meso) pores. 

The silica-supported chromate can be activated directly to a very efficient ethylene polymerization catalyst by ethylene itself or by reduction under CO, to yield active Cr(ll) bisiloxy species, ](=SiO)2Cr] [8]. While the silsesquioxane Cr derivative on its own does not lead to an active polymerization catalyst under ethylene (albeit only low ethylene pressure were tested), the silsesquioxane chromate ester can yield an active polymerization catalyst by addition of methyl-aluminoxane as co-catalyst. Comparison between the two catalytic systems is therefore possible but suffers from the lack of molecular definition of the active homogeneous species obtained after activation with the alkylating agent (Scheme 14.11). 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

We observe again that the formation of hydrosilicates in the co-precipitation catalysts is deleterious for the activity, especially when montmoril-lonite is formed. The interaction causing the decrease in activity proves active even before a definite indication of hydrosilicate formation can be obtained, and it is interesting to note that partial deactivation occurs with a co-precipitation catalyst (8241), but not with a mixture type catalyst (8242). This fact appears to confirm the suggestion (see I, 5) that even a layer of silica on the Ni(OH)2 crystals during co-precipitation is harmful. 

If water was added to the 15% Si02 co-gel to fill the pore voids a partially recrystallized boehmite was formed with a surface area of 464 m /g and with a pore volume of 1.8 oc/g. If water was added to the 15% Si02 co-gel to form a slurry and then dried and calcined at 500 a partially recrystallized bodunite was formed with a surface area of 334 m /g. steam treatment at 760 of this second, small pore, bodunite-like silica-alumina resulted in no change in the surface area. The gas oil cracking activity of the steamed sample was definitely hi er than that for the amorphoias co-gel, i.e., a Micro Activity Test (MAT) Activity Number of 38 (see Table 1.). 

Because of the complex kinetics with Cr/silica, the term "activity can have many possible definitions. In this review, the word "activity is defined in the only way that has industrial meaning it is the polymer yield per unit of catalyst obtained in 1 h, including the induction time. Thus, it is an average rate integrated over 1 h. When other measurements are referred to here, they are so described specifically, such as the yield measured over another time interval, or the maximum rate obtained, or the "plateau rate," or turnovers per Cr atom, or turnovers per active Cr atom, etc. Still another term used is "productivity," which refers only to the polymer yield without reference to time. Therefore, in this section and the following sections, the observations made pertaining to "activity" apply to the above industrial definition. 

The average activity is also shown. This is the polymer yield at any specified time, divided by that time. Average activity is the value that is most often cited in comparison between catalysts. As the data of Figures 168 and 169 show, it depends on time. The kinetics observed with Cr/silica produces an average activity that rises with time, whereas the average activity of Cr/AlPC>4 drops with time. These two curves illustrate the difficulty of quoting one value to summarize the activity of any catalyst. In industrial operations, the residence time is usually about 1 h for slurry processes. Thus, as stated above, the polymer yield obtained in 1 h is the standard definition of activity used in this review. 

In the formation of valuable oxygen-containing compounds by the controlled or partial oxidation of hydrocarbons, such as benzene, two factors are of great importance, i.e., temperature and type of catahst. Other factors such as composition of hydrocarbon-air mixture and time of contact are also important. All of these factors are intimately related to each other and the successful operation of the process depends upon the control of each of them. It was early found that if mixtures of benzene apor and air in excess of tliat necessary for complete combustion were passed through heated tubes of such non-catalytic materials as iron, silica, aluminum, etc., and the temperature allowed to rise at will, only complete combustion products could be obtained, and no intermediate oxidation products could be isolated. On the other hand, if such mixtures of benzene vapor and air were passed over a catalyst such as platinum black, complete combustion also occurred but at a temperature far below that necessary in the empty tube made of non-catalytic material. However, only very small amounts of intermediate products could be obtained with such an active catalyst even when the temperatures were carefully controlled or the time of contact made very short. It is difficult to form any definite idea as to the temperatures which were actually attained by the reacting gases in most of the early experiments reported in the literature. Lack of uniformity in construction of reaction chambers, in displacement of cata-... 

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