Catalytic cracking protonic acidity

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 presence of catalyst is necessary in the catalytic cracking process. Acid cracking catalysts produce carbonium ions by protons addition to olefins or abstracting of hydride ions from hydrocarbon molecules ... 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Sie, S.T. (1993) Acid-catalyzed cracking of paraffinic hydrocarbons 2. Evidence for protonated cyclopropane mechanism from catalytic cracking experiments. Ind. Eng. Chem. Res., 32, 397. 

The acidic centers, both the Br0nsted and Lewis types, are generated by the isomorphous substitution of trivalent aluminum for a tetravalent silicon in the silica lattice.61-63 Additionally, carbocations may be formed by protonation of alkenes,43,52,56,61 which explains their higher reactivity in catalytic cracking. 

A central feature of the mechanism that accounts for the catalytic cracking of hydrocarbons by appropriately cation exchanged zeolites is the formation of carbonium ions (also designated carbocations and alkylcarbenium ions) as intermediates. Many other reactions for which aluminosilicates, be they clay-or zeolite-based, also predicate (320) the existence of carbonium ion intermediates, formed usually by proton donation from Bronsted acid sites, have been discussed earlier (Section III,K). 

Neutrality is satisfied by a cation (e.g., M+) which is usually Na+ derived from the salts used in the synthesis. When the cation is exchanged with a proton an acid site is created. This is the key active site for catalytic cracking reactions. The first exchange is with NH4+ which when heat-treated decomposes to NH3 and the H+ is retained on the zeolite. The acid zeolite is designated HZ... 

However frequently the support material does have a very important function. This is particularly so when the support acts as a (Bronsted) acid or a base. An example is in the catalytic cracking, alkylation, and isomerization of hydrocarbons (Section 5.2.6) The role of the transition metal is in oxidation or hydrogen transfer reactions while the support, for example acidic oxides such as aluminosilicates, act to protonate, rearrange and dehydrate organic species. 

In their protonated forms, zeolites are widely employed in the oil and petrochemical industries, in processes such as the conversion of alcohols to gasoline, catalytic cracking, isomerization and alkylations of hydrocarbons [3]. These chemical reactions most probably involve proton transfer from the acidic site of the zeolite to the organic substrate. In the case of hydrocarbons, this transfer gives rise to carbenium (I) or carbonium (II) ions as intermediates or transition states. 

Zeolites find major applications in catalysis. A form of the zeolite FAU is, for example, an active catalyst component in catalytic cracking of heavy hydrocarbons to produce motor gasoline and diesel. The catalyst activity arises from its Bronsted acidity, which in turn comes from the presence in the stmcture of protons attached to bridging oxygen atoms. Protons can be introduced by ion exchange of anunonium cations, followed by calcination to remove NH3 and generate the acid form of the zeolite. The process is more complex... 

Results suggest that the OH groups absorbing near 3630 cm" are primarily responsible for cracking activity. Hence, the decrease in their concentration caused by more severe calcination appears to account for the decrease in catalytic activity. The acidity or protonation ability of these groups, as measured by pyridine adsorption, would be expected to be the prime variable. Correlations of catalytic activity with Bronsted acidity have been reported previously (20, 21, 24, 25). Results from the... 

Two factors which are not very well understood are proton transfer and hydride-ion transfer as they concern the catalyst. Both of these factors are important steps in the reactions which have been proposed to explain the chemistry of catalytic cracking. How can these be accounted for on the basis of the structural dynamics proposed in this paper It is believed that the transfer of protons can occur on the surface of the catalyst by minor structural changes in 7-alumina. The catalyst being largely a network of oxygen ions can accommodate and, thereby, transfer protons somewhat in the same way that water does. For hydride ion, it is believed that it can be accommodated as depicted in reaction (2). The six-coordinated aluminum ion is a potential Lewis acid. Paraffin... 

Discussions of OH groups in the context of catalysis normally focus on their role as active centers in a number of reactions. The work by Haag et al. (94) constitutes a classic example the authors estahhshed a linear relationship between the concentration of aluminum in HZSM-5 (which imphes an equal concentration of bridging hydroxyls) and the activity for cracking of -hexane. It was concluded that aU protonic acid sites in the zeohte are characterized by the same turnover frequency. Many other correlations between catalytic properties of materials and the strength and/or density of their Bronsted acid sites are well estabHshed. We will not discuss this aspect in detail and recommend instead a number of recently pubhshed reviews (59,60,87). Two more points are worth mentioning. One point is that the cooperative action of Bronsted and Lewis acid sites has been demonstrated. The second is that, of course, OH groups must not necessarily be involved in a catalytic conversion in fact, they can even block the catalyt-icaUy active sites. 

Whilst solid acids such as zeolites and other aluminosilicates have been the subject of considerable study in the context of alkane chemistry (e.g. catalytic cracking and alkene isomerisation), the design of more versatile materials which can be used in the fine chemical industry has been less thoroughly researched. However, the need for the production of solid acids to replace traditional protonic acids such as hydrogen fluoride, phosphoric and sulfuric acids in liquid phase processes is an increasingly important goal. Some progress has been made in this area and the commercial product Envirocat EPIC [19] provides an excellent example of a... 

---