Shape-selective catalyst

The new generations of zeolites and other microporous materials will start a new era for the petroleum processing, petrochemical, and chemical industries. These developments will also benefit our environment. Regenerable molecular sieves will replace corrosive and difficult-to-dispose-of catalysts. Shape selective processes can also generate less low-value byproducts and thus help us using our available resources more efficiently. Future shape selective catalysts and processes will be based on one or more of the foUowing ... 

De-aluminated mordenites were claimedto give more active and stable catalysts for toluene disproportionation than conventional H-mordenite. Becker, Karge, and StreubeP studied the alkylation of benzene with ethene and propene over an H-mordenite catalyst. Shape-selective catalysis was found because only ethylbenzene, w-diethylbenzene, p-diethylbenzene, cumene, p-di-isopropylbenzene, and m-di-isopropylbenzene were detected in the products neither o-diethylbenzenes nor higher alkylated products were found. The results are in agreement with earlier transalkylations over H-mordenite. Catalyst aging was caused by olefin polymerization. The selectivity of Be-mordenite... 

The use of an RI can also be useful in screening for selectivity differences. In many cases, the zeolite catalyst is modified by the insertion of various elements, the Si/Al ratio is changed, etc. In cases where catalyst shape selectivity is not considerably affected, the modification only changes catalyst activity in the same way that steaming or coking would. The apparent selectivity change is then only a function of activity. 

Because of the catalyst shape selectivity, most products are methyl-branched iso-olefins. Tn the Cg to CJ0 range, branched iso-olefins have good octane rating. In the CJ0 to CJ0 range isoparaffins have good distillate fuel properties after hydrogenation. 

FIGURE 10.7 Left Comparison of the fuel cell performance of a standard Pt/C catalyst, shape-selected nanopar-tides from TTAB and PVP synthesis and an impregnated system, right comparison of power densities between a standard Pt/C catalyst and the impregnated shape-selected sample normalized by Pt content. 

A characteristic aromatic constituent of ZSM-5-catalyzed MTG is durene (1,2,4,5-tetramethylbenzene), which is the predominant ClO aromatic, although it is not the thermodynamically favored tetramethylbenzene isomer. This is a consequence of catalyst shape selectivity, wherein the more bulky isomers can-... 

Cooper, B.H., Donnis, B.B.L., Moyse, B. 1986. Hydroprocessing conditions affect catalyst shape selection. OGJ 84(49) 39-44. 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Shape anisotropy Shape control Shape factors Shape-memory alloys Shape-selective catalysis Shape selectivity Sharpless catalyst Shaving cream Shaving creams... 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

Shape selective catalysts, such as ZeoHtes of the H-ZSM-5 type, are capable of directing alkyl groups preferentially to the para position (18). The ratio of the catalyst to the substrate also plays a role ia controlling the regiochemistry of the alkylations. For example, selective alkylation of anilines at the para position is achieved usiag alkylatiag ageats and AlCl ia equimolar ratio (19). 

A modified ZSM-5 catalyst has a unique shape-selective property for producing -ethyltoluene [622-96-8] selectively by the alkylation of toluene [108-88-3] with ethylene (54). j )-Ethyltoluene is an intermediate in the production of poly -methylstyrene) [24936-41-2] (PPMS), which is reported to have... 

The selective alkylation of toluene with methanol to produce -xylene as a predominant isomer can be achieved over shape-selective catalysts (99—101). With a modified ZSM-5 zeoHte catalyst, more than 99% -xylene in xylene isomers can be produced at 550°C. This -xylene concentration exceeds the equiHbrium concentration of 23% (99). The selective synthesis of -xylene using relatively low cost toluene is economically attractive however, this technology was not commercialized as of 1991. 

Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

To improve the yield of mono- and dimethylamines, a shape selective catalyst has been tried. Carhogenic sieves are microporous materials (similar to zeolites), which have catalytic as well as shape selective properties. Comhining the amorphous aluminum silicate catalyst (used for producing the amines) with carhogenic sieves gave higher yeilds of the more valuable MMA and DMA. ... 

The important property of ZSM-5 and similar zeolites is the intercrystalline catalyst sites, which allow one type of reactant molecule to diffuse, while denying diffusion to others. This property, which is based on the shape and size of the reactant molecules as well as the pore sizes of the catalyst, is called shape selectivity. Chen and Garwood document investigations regarding the various aspects of ZSM-5 shape selectivity in relation to its intercrystalline and pore structure. 

One example has used a manganese porphyrin and iodobenzene encapsulated within a dendrimer to bring about shape-selective epox-idation of alkenes. The important aspect of catalysts is that the reactants can move rapidly to the active site, and that the products can be removed rapidly from the active site and expelled from the dendrimer. 

At the low-molecular-weight end of the spectrum, a process newly commercialized by Mobil for converting methanol into gasoline has significantly expanded opportunities in C-1 chemistry— the upgrading of one-carbon molectrles to mrrlticarbon products. The process involves the use of ZSM-5, a shape-selective zeolite catalyst. (See "Zeolite and Shape-Selective Catalysts" in Chapter 9.)... 

The zeohte overgrowth has been reported for FAU on EMT zeohte [44] and MCM-41 on FAU zeohte [45]. On the other hand, in this study, zeohte layers were grown on the zeohte with the same framework structure, resulting in high coverage of ZSM-5 crystals with silicalite layers and high para-selectivity. The zeohte crystals with oriented thin layer on their external surface are expected to form a new class of shape-selective catalysts. 

The single-event microkinetic concept ensures the feedstock independence of the kinetic parameters [8]. Present challenges in microkinetic modelling are the identification of catalyst descriptors accounting for catalyst properties such as acidity [10,11] and shape selectivity [12,13]. 

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