Hydroisomerization catalyst

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

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. 

A new composite containing montmorillonite and zeolite Beta is prepared by in situ crystallization. Nano-zeolite Beta grows on montmorillonite. The composite possesses a dual system of micropore, originated from zeolite Beta, and mesopore of size around 50nm, due to the abundance of interspace formed by montmorillonite laminaes. Compared with catalyst MoNi/Beta, more i-C8 is produced on catalyst MoNi/composite, when n-Cg is used for feedstock for hydroisomerization. This results from the high diffusion created by composite and the short channel of nano-size zeolite Beta. 

Hydroisomerization of n-hexadecane on Pt/HBEA bifunctional catalysts effect of the zeolite crystallites size on the reaction scheme. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

An increase in the zeolite crystallites size would very likely produce substantial changes in the physicochemical properties of the catalyst and consequently on the selectivity for hydroisomerisation. Since the effect of the zeolite crystallites size in the nanoscale range cannot be predicted theoretically, n-hexadecane hydroisomerization was carried out on PtHBEA catalysts with different zeolite crystallites sizes. 

Hydroisomerization of n-octane over Pt-containing micro/mesoporous catalysts obtained by recrystallization of zeolites BEA and MOR was investigated in the temperature range of 200-250 °C under 1-20 bar. Composite materials showed remarkably high activity and selectivity with respect to both pure microporous and pure mesoporous materials. The effect is due to high zeolitic acidity combined with improved accessibility of active sites and transport of bulky molecules provided by mesopores. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

In addition to this, solid acid catalysts can also be used in the hydroisomerization cracking of heavy paraffins, or as co-catalysts in Fischer-Tropsch processes. In the first case, it could also be possible to transform inexpensive refinery cuts with a low octane number (heavy paraffins, n-Cg 20) to fuel-grade gasoline (C4-C7) using bifunctional metal/acid catalysts. In the last case, by combining zeolites with platinum-promoted tungstate modified zirconia, hybrid catalysts provide a promising way to obtain clean synthetic liquid fuels from coal or natural gas. 

Shell Gas B.V. constructed a 1987 mVd (12,500 bbl/d) F-T plant in Malaysia that started operations in 1994. The Shell Middle Distillate Synthesis (SMDS) process uses natural gas as the feedstock to fixed-bed reactors containing cobalt-based catalyst. The heavy hydrocarbons from the F-T reactors are converted to distillate fuels by hydrocracking and hydroisomerization. The quality of the products is very high, the diesel fuel having a cetane number in excess of 75 with no sulfur. 

Soualah, A., Lemberton, J.L., Pinard, L., Chater, M., Magnoux, P., and Moljord, K. (2008) Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts effect of the zeolite strucmre on the product selectivity and on the reaction mechanism. Appl. Catal. A., 336, 23-28. 

Allain, J.F., Magnoux, P., Schulz, P., and Guisnet, M. (1997) Hydroisomerization of n-hexane over platinum mazzite and platinum mordenite catalysts kinetics and mechanism. 

Chao, K.-) Wu, H.-C., and Leu, L-J. (1996) Hydroisomerization oflight normal paraffins over series of platinum-loaded mordenite and beta catalysts. Appl. Catal. A., 143, 223-243. 

Hydrocracking and hydroisomerization are related bond breaking and rearrangement processes which rely on the use of dual function catalysts operating under high hydrogen pressure to achieve their objectives. In fact, they share the same fundamental mechanistic steps and differ mainly in the degree to which some... 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Table II. Hydroisomerization of n-Pentane Influence of Silica-Alumina Molar Ratio of Activity of Pd-H-Mordenite Catalysts...

While the rate of cleavage is given by temperature, acidity of the catalyst and concentration of i-alkyl cations, the rate of desorption is assumed to be enhanced by the steady state concentrations of n-alkenes, i. e., a high dehydrogenation activity of the catalyst favors hydroisomerization. This is the concept of competitive chemisorption which in ideal bifunctional catalysis keeps the residence times of the alkylcarbenium ions low. 

Figure 5. Reaction scheme for hydroisomerization of n-alkanes on bifunctional catalysts and possible modes of cleavage...

The chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

Our study was undertaken specifically to investigate the processing of a typical raffinate to produce either high yields of LPG or isobutane as well as to determine the octane improvement in the C5+ fraction due to hydroisomerization. A 0.7 wt % Pd-15 wt % Ni-SMM catalyst was used for all the experimentation. 

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