Dual-form catalysts

The effect of the H-Beta ratio (y in wt%) in the dual-bed Pt/Z12(x) HB(y) catalyst system on the benzene purity at a reaction temperature (Tr) of 623 K is shown in Fig. 1. It is evident that the benzene purity gradually increased with increasing H-Beta ratio (Fig. la), eventually reaching a plateau value which meets the industrial specification of 99.85% at y 40 wt%. The effects of catalyst bed ratio on product yields are shown in Fig. lb. Comparing to the single-bed catalyst Pt/Z 12 (i.e., y = 0), the overall premium product yields of benzene and xylene (A68 yield) over the dual-bed catalyst Pt/Z12(x) HB(y) system reached an maximum at y 10 wt%. That the A68 yield dwindled and tetramethylbenzene (TEMB) increased with further increase in the H-Beta ratio may be attributed to the larger pore opening possessed by the bottom (H-beta) catalyst, which may provoke disproportionation of TMB to form tetramethylbenzene (TEMB) [8],... 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

As mentioned, cyclopentanes can be formed with monofunctional catalysts, and so even with dual function catalysts, one would expect some of the cyclopentanes to form via mechanisms associated with the platinum reactivity part of the dual functionality. 

While a majority of laboratory-scale dehydrocyclization studies involve carefully chosen feedstocks, often a single alkane, commercial operators use a naphtha fraction consisting of a complex mixture of hydrocarbons. At least some of these will be incapable of easily undergoing direct dehydrocyclization and need to be isomerized into reactive structures if aromatics are to be formed. The work of Davis suggests that the acidity of dual function catalysts is an important added factor in these isomerizations, one which likely complements the different set of isomerizations that may be catalyzed by the platinum function. 

In the reaction mechanisms described above the acidity of the catalyst plays an important role. Zeolites can be converted into the H+ form and as such are powerful catalysts for acid-catalyzed reactions. We discuss below some aspects of isomerization catalyst preparation to demonstrate factors which influence the activity of catalysts based on zeolites. In this discussion we are concerned with zeolite Y and mordenite. Data on paraffin isomerization over dual function catalysts besed on other zeolites are scarce, and no data have been published showing that materials like zeolite X, zeolite L, offretite, zeolite omega, or gmelinite can be converted into catalyst bases having an isomerization activity comparable with that of H-zeolite Y or H-mordenite. 

All side-chain isomers are formed in acid-catalyzed isomerization. Carbonium ions are the intermediates here. Over dual-function catalysts, such as platinum-on-alumina and platinum-on-silica-alumina, platinum increases the rate of isomerization by dehydrogenating alkanes to olefins. This facilitates the formation of carbonium ions. 

Dual- and multiple-form catalysts (connected cycles)... 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

Fg is the partial pressure of the intermediate (atm), dN/dt is the overall reaction rate (mol/s cm ), T is the reaction temperature (K), D is the diffusivity of the medium [cm /s], and R is the catalyst particle size (cm). The intimacy requirements in terms of particle size and partial pressure of intermediate have been calculated. This suggests the constraints that must be applied in forming a dual function catalyst. Guidance is provided for the possible mixing of two particles, for size reduction or the requirement that one catalyst component be added to the other in solution to obtain required intimacy. 

We have sought to determine by independent measurement individual activities characteristic of the (de)hydrogenation and of the acidic function of dual-function re-forming catalysts and to test for the existence of the type of relationship described above between these component activities and the over-all activity of the catalysts as seen in actual naphtha reforming. 

In almost the same period, Uraguchi and Terada reported the direct Mannich reaction of N-Boc-protected imines with acetyl acetone (Scheme 11.2) [5]. In their direct Mannich reaction, phosphoric acid also worked as a dual functional catalyst the Br0nsted acidic moiety of phosphoric acid catalyst Ic activated aldimines 5, and the Lewis basic site (phosphoryl oxygen) interacted with the O-H proton of the enol form of 6. As a result, the reaction proceeded under a chiral environment created by phosphoric acid 1, acetyl acetone, and aldimine through hydrogenbonding interactions to furnish optically active products 7. 

Figure 11 shows schematically how such sites may be formed on Mo/alumlna or CoMo/alumlna catalysts. The pair site contains a reduced metal next to an acidic metal cation, either Co or Mo. Prestimably this dual site can remove S, leaving an olefin or aromatic molecule attached to the cation. This Is then hydrogenated by hydrogen from the reduced metal component of the site. 

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