Hydrofluoric Acid-Catalyzed Processes

Both sulfuric acid and hydrofluoric acid catalyzed alkylations are low temperature processes. Table 3-13 gives the alkylation conditions for HF and H2SO4 processes. One drawback of using H2SO4 and HF in alkylation is the hazards associated with it. Many attempts have been tried to use solid catalysts such as zeolites, alumina and ion exchange resins. Also strong solid acids such as sulfated zirconia and SbFs/sulfonic acid resins were tried. Although they were active, nevertheless they lack stability. No process yet proved successful due to the fast deactivation of the catalyst. A new process which may have commercial possibility, uses... 

Since the discovery of alkylation, the elucidation of its mechanism has attracted great interest. The early findings are associated with Schmerling (17-19), who successfully applied a carbenium ion mechanism with a set of consecutive and simultaneous reaction steps to describe the observed reaction kinetics. Later, most of the mechanistic information about sulfuric acid-catalyzed processes was provided by Albright. Much less information is available about hydrofluoric acid as catalyst. In the following, a consolidated view of the alkylation mechanism is presented. Similarities and dissimilarities between zeolites as representatives of solid acid alkylation catalysts and HF and H2S04 as liquid catalysts are highlighted. Experimental results are compared with quantum-chemical calculations of the individual reaction steps in various media. 

Although not commercialized, both Elf Atochem and Rn hm GmbH have pubUshed on development of hydrogen fluoride-catalyzed processes. Norsolor, since acquired by Elf Aquitaine, had been granted an exclusive European Hcense for the propylene-hydrogen fluoride technology of Ashland Oil (99). Rn hm has patented a process for the production of isobutyric acid in 98% yield via the isomerization of isopropyl formate in the presence of carbon monoxide and hydrofluoric acid (100). 

ReVAP [Reduced volatility alkylation process] A process for improving the safety of alkylation processes catalyzed by hydrofluoric acid. A proprietary additive curtails the emission of the acid aerosol, which forms in the event of a leak. Developed by Phillips Petroleum Company and Mobil Corporation and first installed at Wood Cross, UT, in 1996. See also Alkar. Chem. Mark. Rep., 1996, 250(3), 7. 

The early experimental work showed that the alkylation reactions could be carried out thermally and by using hydrofluoric acid or aluminum chloride as catalysts. Successful processes were developed with the hydrofluoric acid catalyst, and the first commercial HF unit started up on Christmas Day 1942, having a capacity of 1950 BPD (3). Other commercial HF alkylation units followed quickly. The thermal and aluminum chloride-catalyzed alkylation processes had limited commercial acceptance. 

The olefins in coal synthetic crude oil are reactive and tend to form gums and undesirable deposits and may be removed for chemical synthesis or may undergo alkylation processes to form highly branched, high-octane gasoline. Alkylation is normally catalyzed by sulfuric or hydrofluoric acid and involves the reaction of an olefin with an alkane ... 

Certain functional groups are inherently stable or unstable. For example, chloropolymers and fluoropolymers degrade relatively easily. Dehydrohalogenation removes a chloride or fluorine atom and the adjacent hydogen to form hydrochloric or hydrofluoric acid. The acids catalyze further degradation and attack processing equipment. Thus, chloropolymers, such as PVC, are always compoimded with heat stabilizers, and special coatings or materials are required for equipment in which chloropolymers and fluoropolymers will be processed. 

A third environmental issue concerns the generation of toxic waste solvents used to carry out various liquid acid catalytic conversions. Many of today s petroleum refining processes carry out acid-catalyzed isomerization and alkylation reactions using corrosive or toxic liquid acids such sulfuric acid and hydrofluoric acid. These solvents pose significant environmental concerns. Various solid acid materials have been targeted to replace these corrosive liquids. While there are at least three new patents on processes that can use solid acid catalysts, nearly all alkylation processes are still carried out using liquid acids. 

The reaction is acid catalyzed and proceeds by a mechanism that involves car-bocation species as the key intermediates. Two major process alternatives exists, using sulfuric acid and hydrofluoric acid as the catalyst, respectively ... 

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