Platinum chromia catalyst

The dehydrocyclization of alkylaromatics was first described more than four decades ago. In 1936 Moldavskii and Kamusher reported the formation of naphthalene from u-butylbenzene on chromia at 475°C (7, 2). In 1945 Herington and Rideal reported the formation of indene from w-propyl-benzene over chromia-alumina (3). Platinum-containing catalysts were first used for these reactions in 1956 by Kazanskii and co-workers 4-6). 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

While the emphasis in this discussion is on the chemistry of aromatics production from the point of view of product distribution and mechanisms, it is necessary to describe the catalyst used briefly. Until about 20 years ago a number of different catalysts were used for catalytic reforming, including vanadium (126), molybdenum (47), cobalt-molybdenum (159), and chromia (3,41,85,100) catalysts. Since the time when platinum reforming catalysts were introduced to the industry (2, 24, 56,... 

Reactor Apparatus. The reactor systems used for obtaining data on n-dodecane dehydrogenation over molybdena-alumina and platinum-alumina catalysts and the data on reaction of bare supports with -dodo-cene have been described previously (12). The reactors used for obtaining data on n-dodecane dehydrogenation over chromia-alumina and... 

After a preliminary study of the properties of a number of platinum and nickel catalysts deposited on various carriers (activated carbon, chromia, alumina, molybdena, etc.) a platinum-alumina catalyst was selected for the present investigation with a 0.5 % Pt content. Cyclopentane (238.2 g) was passed over the catalyst at 0.43 hr. i space velocity, 20 atmospheres hydrogen pressure, and a temperature of 460 . As a result 184.3 grams of liquid product were obtained containing 9 % by volume of aromatics including benzene (81.9%), toluene, and p-xylene, and also w-pentane,... 

The catalytic dehydrogenation of lower alkanes was first developed more than fifty years ago using chromia/alumina systems [1]. Although there has been development of new processes [2 - 6], the catalyst technology has tended to remain with either modified chromia/alumina or modified platinum/alumina catalysts. Therefore it seemed appropriate to re-examine the possibility of using oxide systems other than chromia to effect the alkane to alkene transition. Supported vanadium pentoxide has been extensively studied for the oxidative dehydrogenation of propane to propene [7-10] but rarely for the direct dehydrogenation reaction [6]. 

Chemical exchange between hydrogen and steam (catalyzed by nickel—chromia, platinum, or supported nickel catalysts) has served as a pre-enrichment step in an electrolytic separation plant (10,70). If the exchange could be operated as a dual-temperature process, it very likely... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Davis (94b) aromatized several Cg and C9 hydrocarbons with a quaternary carbon atom over chromia- and platinum-on-alumina catalysts. Here the reactions of 1,1-dimethylcyclohexane, and 2,2- and 3,3-dimethylhexanes will be compared (Table V). 1,1-Dimethylhexane suffered demethylation predominantly over chromia and alkaline platinum however, with less alkaline platinum, isomerization to xylenes occurred. 

For focus, the discussion is centered on two nominal classes of oxidation catalysts—transition metal oxides, exemplified by chromia, and metals, exemplified by platinum and silver. 

Catalysts other than platinum catalysts have been considered for reforming. Included among these are certain metal oxides, e.g., chromia and molybdena. However, these catalysts are much less active than platinum and also less selective toward the reactions of most interest in reforming, at least under the conditions usually employed in commercial... 

The formation of aromatics by the catalytic dehydrocyclization of paraffins with chains of six or more carbon atoms has been known for some time. Certain oxides of the 5th and 6th subgroups of the periodic table, such as chromia and molybdena, were shown early to be particularly effective catalysts for the reaction. Consequently, most of the reported studies of the kinetics and mechanism of the reaction have been carried out using these catalysts (P6, H4, H5). Since the available data on the kinetics of dehydrocyclization over oxide catalysts have been reviewed by Steiner (S9) in 1956, only a brief summary of the work will be made here, primarily for the purpose of orientation. The relatively few kinetic data which have been reported for dehydrocyclization over the bifunctional platinum on acidic oxide catalysts will be discussed subsequent to this. 

It is interesting to compare the dehydrocyclization activity of platinum with that of chromia-alumina. Pines and Goetschel reacted different butyl-benzene isomers over acidic and nonacidic chromia-alumina catalysts between 480°C and 492°C (41). Dehydrocyclization is much slower over... 

C6-dehydrocyclization does not (50). As a consequence, over nonacidic platinum catalysts above 400°C, C6-dehydrocyclization predominates over C5-dehydrocyclization (27). Furthermore, the phenanthrene/anthracene ratio is independent of catalyst acidity (52). The effect of reaction temperature is, however, very interesting. Over platinum-on-carbon catalyst between 350°C and 400°C, more anthracene is formed than phenanthrene. Above 450°C phenanthrene is the main product (55). Phenanthrene is also the main product over chromia-alumina between 360°C and 440°C whereas, as seen above, anthracene is formed in this temperature range over platinum-on-carbon catalyst (54). 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

The composition of a reforming catalyst is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally molybdena-alumina, chromia-alumina, or platinum on a silica-alumina or alumina base. The nonplatinum catalysts are widely used in regenerative process for feeds containing, for example, sulfur, which poisons platinum catalysts, although pretreatment processes (e.g., hydrodesulfurization) may permit platinum catalysts to be employed. 

The conversion of cyclohexanes to aromatics is a classical dehydrogenation reaction which will readily take place on many transition metals and metal oxides. On chromia-alumina Herington and Eideal (S) have demonstrated the occurrence of cyclo-olefin intermediate products. Weisz and Swegler 25) have demonstrated the effect on benzene yield of allowing early diffusional escape of cyclo-olefin from the porous catalyst particle. Prater et al. 26) have developed evidence that cyclohexene occurs as a quasi-intermediate in aromatization catalysis over platinum catalyst also, although at a smaller concentration, because of a larger ratio of effective rate constants fe/Zci in the scheme... 

The discussion in this section pertains to alumina-supported platinum catalysts. The work by C.P. Poole and D.S. Maciver provides an extensive review of chromia-alumina catalysts... 

---