Acid catalysts silica alumina

Solid acidic catalyst (silica-alumina, zeolite, and so on)... 

Solid acid catalyst (silica-alumina with rare earth metals, various other options) Temperature 260-450 C (500-845°F (solid or liquid contact)... 

Most importantly, biomass pyrolysis will be carried out at remote locations, and in distributed manner. Thus, the catalysts should be cheap and simple to use. Acidic clays, silica aluminas and H-FAU type zeolites are relatively cheap and robust materials, can be mixed easily with heat carriers, and used for pyrolysis. Efficient contact between the solids (catalyst and biomass) to maximize catalytic action is one of the challenges that need to be overcome. 

As another variation, the production of alkanes can be accomplished by modifying the support with a mineral acid (such as HCl) that is co-fed with the aqueous sorbitol reactant. In general, the selectivities to heavier alkanes increase as more acid sites are added to a non-acidic Pt/alumina catalyst by making physical mixtures of Pt/alumina and silica-alumina. The alkane selectivities are similar for an acidic Pt/silica-alumina catalyst and a physical mixture of Pt/alumina and silica-alumina components, both having the same ratio of Pt to acid sites, indicating that the acid and metal sites need not be mixed at the atomic level. The alkane distribution also shifts to heavier alkanes for the non-addic Pt/alumina catalyst when the pH of the aqueous sorbitol feed is lowered by addition of HCl. The advantages of using a solid acid are... 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Low temperatures in the alkylation reaction zone favor the isomerization of butylene-1 to butylene-2 during the alkylation reaction. Where low temperatures are not used for economic reasons, however, as is the case with many HF alkylation units, this isomerization is sometimes carried out in a separate unit and the isomerized product then charged to the alkylation unit. Catalysts for this isomerization reaction are phosphoric acid and silica-alumina. Some polymer is usually made when isomerizing with these catalysts, but when the polymer production is limited to diisobutylene, it will be alkylated to produce the trimethylpentanes. Any polymer made from normal butylenes, however, is an inferior feed for alkylation units. When this type of polymer is made, it is usually removed by fractionation before charging the isomerized butylene feed to the alkylation unit. 

As expected for silica-alumina as a mixed oxide (see also Section IV.B.5), the PyH+ and PyL species are observed simultaneously (160, 205,206,221-223). Two distinct types of Lewis acid sites could be detected (19b mode at 1456 and 1462 cm-1, respectively) on a specially prepared aluminum-on-silica catalyst (160). On water addition, the Lewis sites can be converted into Br nsted sites (160, 205, 221), The effect of Na+ ions on the acidity of silica-aluminas has been studied by Parry (205) and by Bourne et al. (160). It can be concluded from Parry s results that Na+ ions affect both types of acid sites, so that alkali poisoning does not seem to eliminate the Br nsted sites selectively. For quantitative determination of the surface density of Lewis and Br nsted acid sites by pyridine chemisorption, one requires the knowledge of at least the ratio of the extinction coefficients for characteristic infrared absorption bands of the PyH+ and PyL species. Attempts have been made to evaluate this ratio for the 19b mode, which occurs near 1450 cm-1 for the PyL species and near 1545 cm-1 for the PyH+ species (160,198,206,221,224,225). The most reliable value as calculated from the data given by Hughes and White (198) seems to be... 

Although all three Pd-Pt catalysts are mesoporous and have similar initial dispersions, their performance differs enormously. The activity decreases in the decreasing order of their A1 content, viz. ASA > MPSA > MPS. The replacement of Si by A1 in the silica skeleton and its subsequent charge unbalance are often used to explain the acidity in silica-alumina. It is expected that acidity will decrease in the same order, i.e. ASA > MPSA > MPS. This shows the impact of acidity on noble metal catalyzed deep HDS [5,6,8]. The texture is of secondary importance to the performance. 

Addition of HCl can enhance acidity the chloride ion substitutes for an oxygen ion. The catalytic Brpnsted acid in silica-alumina and related catalysts can be very strong. It can have a strong tendency to donate protons (up to a Hammett function of Hq = —13.3). 

When using a bifunclional catalyst combining a hydrogenation function (Pl,Pd,Ni) and a Bransted acid support (silica-alumina, zeolite) good selectivities to cyclohexylbcnzene are obtained upon hydrogenating benzene [66], Apparently the intermediate cyclohexene alkylates benzene (present in excess). Cyclohexylbcnzene is of interest because it can be... 

Plank (46) has suggested an alternative explanation for the acidity of silica-alumina catalysts. From a study of the differences between silica and silica-alumina gels, he concluded that alumina always becomes a terminal group in the micelle structure, and that therefore isomorphous substitution of silicon with aluminum does not occur. According to his hypothesis, the aluminum ions in the terminal alumina groups are coordinated with hydroxyl and water in such a way as to retain their normal octahedral coordination ... 

As described in Section V,B,3, metal sulfates act as good catalysts for the formation of formalin from methylene chloride, but the reaction is accompanied by some side reactions when silica-alumina is used as the catalyst. This can also be explained by assuming that the strong acidity of silica-alumina leads to the occurrence of the side reaction. 

PS Acetophenone, benzophenone, enones, diketones, phenylacetaldehyde, sucdnimides, benzoyl peroxide, in chain peroxide linkages, hydroperoxides, polycyclic aromatic hydrocarbons, Fe derivatives, Co salts of fatty acids, AICI3, silica-alumina catalyst Hydrogen, benzene, conjugated double bonds, methane, ethylene, radicals, crosslinks Water, CO2, ketones, unsaturations, hydrtperoxides, radicals, chain scissions, quinomethane structures... 

PE/PP 380-430°C acid activated silica-alumina, ZSM-5, nonacidic mesoporous silica catalyst (folded sheet material) Increased 5neld of liquids 418... 

In accordance with the above definitions, a summarized list of solid acids and bases is given in Tables 1.1 and 1.2. The first group of solid acids in Table 1.1 includes naturally occurring clay minerals. The main constituents are silica and alumina. Various types of synthetic zeolites such as zeolites X,Y,A, ZMS-5, ZSM-11, etc. have been reported to show characteristic catalytic activities and selectivities. The well-known solid acid, synthetic silica-alumina, is listed in the seventh group, which also includes the many oxide mixtures which have recently been found to display acidic properties and catalytic activity. In the fifth and sixth groups are included many inorganic chemicals such as metal oxides, sulfides, sulfates, nitrates, phosphates and halides. Many have been found to show characteristic selectivities as catalysts. 

In the field of heterogeneous catalysts, silica-alumina ate widely used as acid supports or catalyst matrices. Their preparation involves a large number of unit operations, usualy present in catalyst manufacturing processes, such as precipitation or gelation, tiltration, drying, ion exchange... These unit... 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Acids are not limited to liquid (or gaseous) systems. Solid acids also play a significant role. Acidic oxides such as silica, silica-alumina, etc. are used extensively as solid acid catalysts. New solid acid systems that are stronger than those used conventionally are frequently called solid superacids. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

The product distrihution is influenced hy the catalyst properties as well as the various reaction parameters. The catalyst activity and selectivity are functions of acidity, crystalline size, silica/alumina ratio, and even the synthetic procedure. Since the discovery of the MTG process. 

The first commercial fluidized cracking catalyst was acid-treated natural clay. Later, synthetic. silica-alumina materials containing 10 lo... 

A conventional FCC unit can be an olefin machine with proper operating conditions and hardware. Catalysts with a low unit cell size and a high silica/alumina ratio favor olefins. Additionally, the addition of ZSM-5, with its lower acid site density and very high framework silica-alumina ratio, converts gasoline into olefins. A high reactor temperature and elimination of the post-riser residence time will also produce more olefins. Mechanical modification of the FCC riser for millisecond cracking has shown potential for maximizing olefin yield. 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

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