Solid acid alkylation catalysts

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. 

According to the open literature, other solid acid alkylation catalysts are generally susceptible to poisoning/deactivation by water and other common feed impurities (e.g., oxygenates, sulfur compounds, dienes, etc.), thus necessitating (potentially costly) feedstock pretreatment for their removal. In some cases, this requirement is further mandated by the potential corrosion problems associated with the use of halogens in the catalyst system. 

Several applications of the in situ decoking concept have appeared in the literature. One such application involves stabilizing the activity of solid acid catalysts such as in alkylation reactions. As reviewed elsewhere (55), numerous efforts aimed at developing solid acid alkylation catalysts and solid acid-based isobutane-olefin alkylation processes have been reported for more than three decades. However, to date, none of the solid alkylation catalysts has gained acceptance in industry for one or more of the following drawbacks rapid catalyst... 

A variety of reactions have been conducted. Catalysts based on noble metals on Deloxan amino poly siloxane supports have been used. Hitzler et al. (1998) have reported alkylation of mesitylene with propylene or wopropanol in SC propylene or CO2 using a solid acid Deloxan catalyst. Pesiri et al. (1998) have carried out selective epoxidation in SC CO2 with transition metal catalysts (V, Ti, Mo) and tert-BHPO high conversion and selectivity have been reported. 

The technology and chemistry of isoalkane-alkene alkylation have been thoroughly reviewed for both liquid and solid acid catalysts (15) and for solid acid catalysts alone (16). The intention of this review is to provide an up-to-date overview of the alkylation reaction with both liquid and solid acids as catalysts. The focus is on the similarities and differences between the liquid acid catalysts on one hand and solid acid catalysts, especially zeolites, on the other. Thus, the reaction mechanism, the physical properties of the individual catalysts, and their consequences for successful operation are reviewed. The final section is an overview of existing processes and new process developments utilizing solid acids. 

A continuous procedure for the alkylation of mesitylene and anisole with supercritical propene, or propan-2-ol in supercritical carbon dioxide, with a heterogeneous polysiloxane-supported solid acid Deloxan catalyst has been reported giving 100% selectivity for monoalkylation of mesitylene with 50% conversion at 250 °C and 150 bar by propan-2-ol in supercritical carbon dioxide. p-Toluenesulfonic acid monohydrate has been demonstrated as an efficient catalyst for the clean alkylation of aromatics using activated alkyl halides, alkenes or tosylates under mild conditions. Cyclohexene, for example, reacts with toluene to give 100% cyclohexyltoluenes (o m p-29 18 53) under these circumstances. 

Environmental and safety concerns regarding acid-based processes are promoting research and development efforts on solid-acid alkylation processes. Liquid catalysts pose possible risks to the environment, employees, and the general public from accidental atmospheric releases. Also, these acid... 

A second indirect alkylation process, InAlk, is also a solid catalyst process (Fig. 18.25).12 29 InAlk combines two commercially proven technologies polymerization and olefin saturation. Isobutylene is reacted with light olefins (C3-C5) in a polymerization reactor. The resulting mixture of iso-olefins is saturated in the hydrogenation reactor. Excess hydrogen is recycled and the product is stabilized to produce a paraffinic gasoline blending stream. Yet, new solid-acid alkylation processes face tech-... 

D Amico, V.J. et al. (2004) AlkyClean solid acid alkylation will it finally become a reality Akzo Nobel Catalysts Scope Symposium, Florence, Italy, June 21-23. 

In summary, the research work performed at the INL demonstrated that the introduction of supercritical cosolvents during the alkylation reaction did not result in improved or sustained catalytic performance. However, supercritical fluids in general and isobutane in particular were shown to be promising regenerants of some solid acid zeolite catalysts that may be utilized in isobutane/butene alkylation reaction. 

Poliakoff et al. introduced the supercritical phase to the Friedel-Crafts alkylation reaction by using SCCO2 or by making propene, one of the reactants, the supercritical fluid [56]. This heterogeneous supercritical phase reaction was conducted continuously and the selectivity was very high if a solid acid Deloxan catalyst was utilized. 

Besides, it is worth mentioning that despite ionic liquids having the appropriate composition might be an alternative to the less environment friendly H2SO4 and HF acids, it would be much more interesting from the practical viewpoint to immobilize these ionic liquids on solid supports so as the resulting catalyst contains ionic complexes in which either anion or cation is covalently bonded to the porous carrier (70,71). In this respect, it has been shown that the imidazolium-type ionic liquid immobilized on a high surface area (946 m /g) ordered mesoporous silica (Si-MCM-41) performed relatively well for the alkylation reaction at temperatures around 80° C, with a selectivity to isooctanes of about 60 wt% (72). Nevertheless, as usually occurs in solid acid alkylation, the activity of immobilized IL catalyst starts to fall at a certain time on stream after which butenes dimerization instead of alkylation becomes the predominant reaction (72). 

Fixed bed and liquid riser type reactors are found in the majority of the proposed commercial solid acid alkylation processes, as will be detailed hereinafter. Nevertheless, other reactor configurations have been proposed for alkylation processes employing solid catalysts. For instance, a spouted bed reactor is constituted by a gas flow propelled riser, an annular downcomer where part of the solid and liquid are recycled to the reactor inlet and a hydrocyclon in which the solid catalyst is collected to be regenerated in continuous at a neighboring unit. Recently, a spouted bed reactor equipped with a fluidized-bed catalyst regenerator has been designed to be used in solid alkylation processes, employing a Pt/S0x(Zr-Ti)02... 

There is one very recent example of application of the rigorous approach outlined here. It deals with the deactivation in the solid acid alkylation process for the production of high octane gasoline [Martinis and Froment, 2006]. The kinetic modeling of the reaction between 1-butene and i-butane on a Y-zeolite catalyst was expressed in terms of elementary steps and the kinetics were written in terms of single events, discussed in Chapters 1 and 2. It was found that the... 

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... 

The major disadvantage of the alkylation process is that acid is consumed in considerable quantities (up to 100 kg of acid per ton of product). Hence, solid acids have been explored extensively as alternatives. In particular, solid super acids such sulfated zirconia SO/ IZr02) show excellent activities for alkylation, but only for a short time, because the catalyst suffers from coke deposition due to oligomerization of alkenes. These catalysts are also extremely sensitive to water. 

Another recent new application of a microporous materials in oil refining is the use of zeolite beta as a solid acid system for paraffin alkylation [3]. This zeolite based catalyst, which is operated in a slurry phase reactor, also contains small amounts of Pt or Pd to facilitate catalyst regeneration. Although promising, this novel solid acid catalyst system, has not as yet been applied commercially. 

As shown in Table 3, after a pretreatment performed at 333 K, the activity of the K3P sample increased with time on stream (TOS), giving rise to a high production of dimethylhexanes (DMH) and of olefins (Cg" ). After a dehydratation performed at 423 K, the conversion of C4= and the selectivities towards TMP were initially high. As generally observed in the aliphatic alkylation reaction with solid acids, the decrease of the catalyst activity was accompanied by a concomitant decrease of the selectivity in TMP and an increase of the selectivities in DMH and olefins (C4 dimerization) indicating... 

Tanabe and Hdlderich (1999) have given an extensive statistical survey of industrial processes using solid acids/bases as catalysts. Over 300 solids and bases have been covered. A variety of reactions like alkylation, isomerization, amination, cracking, and etherification with catalysts like zeolites, oxides, complex oxides, phosphates and ion-exchange resins have been covered. Over 120 industrial processes are referred with 180 different catalysts. 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Figure 2 also includes a comparative experiment, where the solid acid catalyst is a sample of non-fluorided (but calcined), acidic mordenite. Here we see a) a significant loss of alkylation activity with time on stream and b) a measurably lower... 

HETACAT An alkylation process using a solid acid catalyst. Not commercialized as of 1997. 

This contribution is an in-depth review of chemical and technological aspects of the alkylation of isobutane with lightalkenes, focused on the mechanisms operative with both liquid and solid acid catalysts. The differences in importance of the individual mechanistic steps are discussed in terms of the physical-chemical properties of specific catalysts. The impact of important process parameters on alkylation performance is deduced from the mechanism. The established industrial processes based on the application of liquid acids and recent process developments involving solid acid catalysts are described briefly. 2004 Elsevier Inc. 

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