Platinum dehydrogenation catalyst

UOP Inc. is the key source of technology in this area, having numerous patents and over 70 units operating worldwide (12). The dehydrogenation catalyst is usually a noble metal such as platinum. Eor a typical conversion, the operating temperature is 300—500°C at 100 kPa (1 atm) (13) hydrogen-to-paraffin feed mole ratio is 5 1. 

The catalysts generally used in catalytic reforming are dual functional to provide two types of catalytic sites, hydrogenation-dehydrogenation sites and acid sites. The former sites are provided by platinum, which is the best known hydrogenation-dehydrogenation catalyst and the latter (acid sites) promote carbonium ion formation and are provided by an alumina carrier. The two types of sites are necessary for aromatization and isomerization reactions. 

AXB) shows time courees of amounts of evolved hydrogen and decalin conversions with caibon-supported platinum-based catalysts unda" supeiheated liquid-film conditions. Enhancement of dehydrogenation activities for decalin was realized by using fiiese composite catalysts. The Pt-W / C composite catalyst exhibited the hipest reaction rate at the initial stage, whereas the Pt-Re / C composite catalyst showed the second highest reaction rate in addition to low in sensitivity to retardation due to naphthaloie adsorbed on catalytic active sites [1-5], as indicated in Fig. 2(A) ). 

Platinum-based nanoparticles (e.g., Pt [1-15], Pt-Re [10,15], and Pt-W [5,6,15]) supported on granular activated carbon (KOH-activation, BET specific surface area 3100 m2/g, pore volume 1.78 cm3/g, average particle size 13 pm, average pore size 2.0 nm, Kansai Netsukagaku Co. Ltd. [32]) were mainly used as the dehydrogenation catalysts in the present study. 

Searching for a better catalyst than platinum to oxidize methanol to COad ° y be a new direction of the catalyst search. If such a catalyst is combined with a good catalyst for COad oxidation to C02> the overall catalytic activity may exceed that of platinum based catalysts. Platinum has catalytic activities for both reactions to some extent. For each reaction, however, platinum is not necessarily the best. Palladium may be a good catalyst to oxidize methanol to COad because it is known as a good catalysts for dehydrogenation of hydrocarbons. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

Hydroprocessing and special absorption techniques are utilized to remove sulfur and nitrogen from the reformer. If not removed through hydroprocessing, feedstock sulfur will be converted to H2S in the reformer. The H2S will then serve as a poison to the platinum reformer catalyst and diminish the dehydrogenation and dehydrocyclization reactions. When present, H2S can neutralize the acid sites on the catalyst diminishing the ability of the catalyst to promote isomerization, dehydrocyclization, and hydrocracking reactions. 

With higher hydrocarbons, the spectra depend upon the structure of the olefin [70], With platinum—silica catalysts, linear chain olefins tend to form dehydrogenated surface residues more readily than branched chain olefins, which give predominantly saturated adsorbed alkyl species. 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

Cyclization of alkylbenzenes is much slower over silica-alumina than over the platinum-containing catalysts. To clarify the successive steps of cyclization, Csicsery performed a set of experiments with 4-phenyl-1-butene, a dehydrogenation product of -butylbenzene (28). 

Note that each of these simple elementary reactions is reversible, and so the entire catalytic cycle is also reversible. This is known as the principle of microscopic reversibility. Consequently, if platinum is a good hydrogenation catalyst, then it must also be a good dehydrogenation catalyst. In fact, as we will see later, catalysts change only the reaction rate, not the equilibrium. Every catalyst catalyzes both the forward and the reverse reactions in the same proportions. In the above example, the reverse reaction is actually more interesting for industry, because propene is a valuable monomer for making poly(propylene) and other polymers. 

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [1] reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. 

Whilst the ability of platinum-based catalysts to effect the dehydrogenation of alkanes to the corresponding alkenes is well established [1-4], carbon laydown and consequential deactivation of the catalyst during the dehydrogenation reactions is a well known phenomenon... 

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

This early process is very capital and maintenance intensive and spurred improvements to catalysts and technology. The Oleflex process (UOP) has been commercialised to dehydrogenate propane to propylene using a platinum supported catalyst. Philips has developed a process using steam as a diluent and uses a tin-platinum catalyst. 

Preliminary results characterizing Sn (another spin-1/2 nucleus) have shown the potential of its NMR for investigation of the modifying function of tin in dehydrogenation catalysts based on supported platinum, palladium, and possibly nickel. Spectra have been obtained for Sn/Ni and Sn/Pd on silica, (with tin in excess in each) (188), and they show resonances that have not been clearly identified, in addition to those attributable to /3-tin. No additional reports have been published. 

In some cases a catalyst consists of minute particles of an active material dispersed over a less active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts, whose active ingredients are major amounts of other substances called promoters, which increase the activity. Examples of supported catalysts are the automobile-muffler catalysts mentioned above, the platinum-on-alumina catalyst used in petroleum reforming, and the vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid. On the other hand, the platinum gauze for ammonia oxidation, the promoted iron for ammonia synthesis, and the silica-alumina dehydrogenation catalyst used in butadiene manufacture typify unsupported catalysts. 

Lowering the energy hill, as we can see, decreases the energy of activation of the reverse reaction as well, and thus increases the rate of c/ehydrogenation. We might expect, therefore, that platinum, palladium, and nickel, under the proper conditions, should serve as dehydrogenation catalysts this is indeed the case. We are familiar with the fact that, although a catalyst speeds up a reaction, it does... 

Cortright RD, Dumesic JA (1995) L-zeoUte-supported platinum and platinum/tin catalysts for isobutane dehydrogenation. Appl Catal A Gen 129 101... 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

Dehydrogenation of cyclohexane can be achieved by heating with a metal catalyst such as platinum. Dehydrogenation may also be performed by quinone, which is reduced to hydroquinone in the process. 

Figure 2. Fouling data for dehydrogenation of methylcyclcdiexane to toluene over a platinum-alumina catalyst Ref [4]. Reproduced by permission of Academic Press, Inc.

A palladium-platinum-charcoal catalyst appears to be particularly effective for the dehydrogenative coupling of two benzene rings with formation of a third such ring, as in the conversion of o-terphenyl (8) to triphenylene (9), and of 1,1 -dinaphthyl (10) to perylene (II). The catalyst is prepared by adding 400 g. of granular... 

Non-conjugatively linked 9,9/-dibenzosilole polymers have also been reported [44,45]. In an early paper, a dehydrogenation catalyst, bis(l,5-cyclo-octadiene)palladium, was used in the synthesis of poly(9,9-dibenzosilole) 52 (Scheme 6) [45]. Recently, poly(dibenzosilole-vinylene)s 54 were obtained from the platinum-catalysed hydrosilylation of 9,9-dihydrodibenzosilole 51 with 9,9-diethynyldibenzosilole 53 (Scheme 6) [44]. 

---