Catalysts, bifunctional reforming acidic

Some of the typical reforming reactions catalyzed by the bifunctional, metal-acidic oxide catalysts, along with a specific example of each, are listed below ... 

In the naphtha reforming process, a bifunctional metal-acid catalyst is used. During the coinnercial operation, coke is deposited on both catalytic functions producing their deactivation (refs, 1-3). When activity and selectivity are decreased to values where the operation is not convenient from the economical... 

The deactivation of bifunctional reforming catalysts is mainly due to the deposition of coke on the metal and the acid sites. Coking on the metallic function is responsible for the rapid initial deactivation which is also accompanied by changes in the selectivity of products formed on its surface. The acidic function deactivates more gradually with time as a result of the add site fouling by coke coverage. 

The reformation of lower paraffins to aromatics has been studied for about 20 yr by using zeolite catalysts. Recently, an excellent review was published of lower alkane transformation to aromatics on ZSM-5 zeolites [2]. From the studies of the mechanism of this reaction, it has been suggested that the bifunctional catalysts, having solid acidity and dehydrogenation activity, can effectively promote the aromatization of lower paraffins[3-6]. It has been reported that ZSM-5 and ZSM-11 are excellent solid acid catalysts [7] and the transition metals [8], Ga [9] and Zn [9] show high dehydrogenation activity in this reaction. In the case of bifunctional... 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Pt-alumina is a profoundly subtle and possibly unique system for bifunctional reforming catalysis. The Pt crystallite sizes which gamma alumina can stabilise contain high proportions of sites upon which selective catalysis can take place in addition, the acidity of gamma alumina can be promoted and controlled for selective carbenium chemistry in-situ. Further enhancement of Pt selectivity has been achieved by the addition of a second element, notably Re, Ir and Sn. Both ensemble and electronic models have been put forward to explain the effects, and have been vigorously discussed in the open literature. Geometric effects tend to be preferred for Pt-Re and Pt-Sn, but inevitably in such multivariate systems, there is still room for debate. The commercial success of these catalyst systems depends on their ability... 

The individual analysis of the important reactions that occur in a catalytic reforming reactor is important to understand the influence of the operating conditions on the quality of the reformate. There are several reviews where this topic is analyzed, for reactions occurring on metals or on bifunctional (metal-acid) catalysts (4-13). 

Straight-run gasoline is composed primarily of alkanes and cycloalkanes with only a small fraction of aromatics, and has a low ON of about 50. The ON is improved by catalytic reforming of n-paraffins and cycloalkanes into branched alkanes and aromatics. The main reactions are isomerization (w- to iso-), cycli-zation, dehydrogenation, and dehydrocyclization. The bifunctional catalyst has an acidic function to catalyze isomerization and cyclization and a dehydrogenation function that requires an active metal site. Typically, platinum is used as the metal and AI2O3 for the acidity. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Takanabe, K. Aika, K.-I. Inazu, K. T. B. Seshan, K. Lefferts, L., Steam reforming of acetic acid as a biomass derived oxygenate Bifunctional pathway for hydrogen formation over Pt/ZrOz catalysts. Journal of catalysis 2006,243(2), 263-269. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

As seen in Figure 2.2 and from the corresponding discussion, dehydrocyclization is a key reaction in forming aromatic compounds.307 A study comparing dehydrocyclization over mono- and bifiinctional catalysts at atmospheric pressure and high pressure representative of naphtha reforming conditions concludes that primary aromatic products at all pressures are formed by direct six-carbon ring formation.313 Over bifunctional catalysts the acid-catalyzed cyclization is more rapid... 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

Recently, extensive efforts have been made to synthesize liquid hydrocarbons from biomass feedstocks. In 2004, Dumesic and co-workers reported that a clean stream of alkanes could be produced by aqueous phase reforming of sorbitol over a bifunctional catalyst. The sugar is repeatedly dehydrated using a solid acid catalyst and then hydrogenated using a precious metal catalyst such... 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

Typical reforming catalyst are bifunctional, with a metal function (Pt, Pt-Re, Pt-Ir, etc] and an acid function (chlorided alumina). During operation, along with the desired reactions, the deposition of carbonaceous material occurs over the metal and the acid sites (1). This coke is the most important factor affecting the lifetime of the catalyst. As a consequence of this deposition, the reactor temperature must be raised to maintain the same octane number In the reformate. The partially deactivated catalyst has different selectivity than the fresh one. This suggests that the coke is deposited to a different extent on the metallic and on the acid function, and that the more demanding reactions are preferentially deactivated (2). 

On the other hand, there is the question of the close relationship between coke formation (catalyst stability) and aromatics cyclization (catalyst selectivity) over the acid sites present in the zeolites. On naphtha reforming, coke formation is a bifunctional reaction requiring the dehydrogenation capacity of the metallic function and the condensation capacity of the acidic function. Therefore, it is interesting to... 

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