Catalytic cracking acidic nature

Caustics are widely used in petroleum refineries. Typical uses are to neutralize and to extract acidic materials that may occur naturally in crude oil, acidic reaction products that may be produced by various chemical treating processes, and acidic materials formed during thermal and catalytic cracking such as H2S, phenolics, and organic acids. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

Fundamental studies of catalytic cracking have led to the conclusion that the chief characteristics of the products may be traced to the primary cracking of the hydrocarbons in the feed stock and to the secondary reactions of the olefins produced both correspond to the ionic reaction mechanisms of hydrocarbons in the presence of acidic catalysts. The chemistry of both the hydrocarbons and catalysts dealt with here has advanced rapidly in the last decade. Nevertheless, much further exploration is required with respect to the nature of the catalyst and the properties of the hydrocarbons undergoing reaction. A promising field lies ahead for future research. 

The acidic nature of the SiO2 (AI2O3) catalysts over the whole range was explored by Thomas (78) using a titration procedure with potassium hydroxide as neutralizer. A general relationship was observed between the amount of catalytic cracking and acidity. His method of determining the acid nature of the catalyst has been criticized by Miesserov (79) whose work indicated that NaOH solutions reacted with other protons... 

The acidic nature of NiCaY after reduction of the metal can be illustrated by using the model reaction of cracking of cumene. Figure 3 shows the catalytic activity at various temperatures and the yields of the products. The catalyst possesses high activity even at 200°C, where the conversion is 20.5 mole %. At 400° C the activity increases and the conversion reaches 97.1 mole %. At 200°C dealkylation is accompanied by disproportionation with formation of diisopropylbenzene. With increasing temperature the disproportionation decreases, while hydrogenolysis of the alkyl chain is strongly increased. 

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. 

Feed stock for the first sulfuric acid alkylation units consisted mainly of butylenes and isobutane obtained originally from thermal cracking and later from catalytic cracking processes. Isobutane was derived from refinery sources and from natural gasoline processing. Isomerization of normal butane to make isobutane was also quite prevalent. Later the olefinic part of the feed stock was expanded to include propylene and amylenes in some cases. When ethylene was required in large quantities for the production of ethylbenzene, propane and butanes were cracked, and later naphtha and gas oils were cracked. This was especially practiced in European countries where the cracking of propane has not been economic. 

Synthetic zeolites are the most important materials used currently in industry for catalyst preparation. However, natural zeolites are not contemplated in catalyst manufacturing because of the impurities present in the natural raw materials nevertheless, in some reactions, such as the isomerization of hydrocarbons, this contamination does not affect the catalytic transformation therefore, acid natural zeolites can be used for this purpose [19]. Furthermore, acid clinoptilolites were tested for catalytic cracking with success [19,21,137-143], We have shown [19,21,138-143] that the acid clinoptilolite, used as catalyst in the reaction of ethanol dehydration, exhibits high selectivity for ethylene production due to steric restrictions imposed on the formation of diethyl ether. The scheme of the ethanol dehydration reaction is shown in Figure 9.18 [145],... 

At present more than 100 zeolitic structures (both natural and synthetic) have been reported and their number grows annually as new structures are continuously being discovered which opens up a wide range of possible applications [61, 62]. However, from a practical viewpoint, only a few zeolites are used as industrial catalysts such as Y, ZSM-5, Beta and mordenite (Table 3.1), mainly due to the cost and difficulties inherent to their preparation [60]. When zeolites are applied for the catalytic cracking of polymers, their microporous structure causes important diffusional and steric hindrances for the access of the bulky plastic molecules to the internal acid sites [5, 24]. 

Both natural clays and their alnminium oxide pillared analogues have also been tested for the catalytic cracking of polyethylene [49-51]. The clays investigated include mont-morillonite and saponite. They possess a layered structure which can be converted into a two-dimensional network of interconnected micropores by intercalation of molecular moieties. In the case of alnmininm pillared clays, these materials show a mild acidity... 

A common feature of these catalysts is their acidic nature (i.e., they all act as solid phase acids in the hot gas oil vapor stream). Synthetic silica/alumina catalyst composites, for example, have an acidity of 0.25 mEq/g distributed over an active surface area of some 500m /g). This acidity is the key feature that distinguishes catalytic cracking from straight thermal cracking. 

The chief sources of olefins are cracking operations, especially catalytic cracking. However, olefins can be produced by the dehydrogenation of paraffins butanes are dehydrogenated commercially to provide feeds to alkylation. Isobutane is obtained from crude oils, cracking operations, catalytic reformers, and natural gas. To supplement these sources, n-butane is sometimes isomer-ized. Only small concentrations of diolefins are permissible in feeds to alkylation, particularly for sulfuric add catalyst. Diolefins increase the consumption of acid. 

The first point in the list above does not mean that carriers, binders, or formulation aids must be nonreactive. Rather, there are appKcations in which a self-activity of these materials is desired. For instance, in hydrocracking and fluidized catalytic cracking, mesoporous matrices with acidic properties are employed to promote bottom cracking of heavy feed components that cannot penetrate the small pores of the embedded zeolite. It is important to find out whether the chemical nature of additives and materials used to shape active sites is beneficial or detrimental for the overall performance of the final catalyst body at the conditions of the catalyzed process. 

Catalysis by solid acids is of paramount importance in industrial chmnistry, namely due to its application in catalytic cracking, one of the most important processes in the world. However, despite its enormous importance, only recently have practical and quantitative relationship between the acidity of the catalyst and its catalytic activity began to appear, unlike homogeneous acid catalysis, which has made use of the Brdnsted relations for many years. The difficulties to be overcome are of various nature but it was found by some of the authors that Bronsted-like relationships also apply to solid acid catalysts [1]. 

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