Hydrocarbon conversion catalysts, pore

Evaluating Pore Structure and Morphology of Hydrocarbon-Conversion Catalysts... 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

The concentrations of hydrocarbons adsorbed in catalyst pores, especially of the heavier products, depend on their partial pressure in the catalyst bed. The partial pressure of the products at the same conversion and selectivity levels depends on the total pressure in the reactor. Therefore, Fischer-Tropsch synthesis at total pressure of 20 bar (tt CO=2) would result in a higher partial pressure of hydrocarbons. The higher partial pressure would lead to the condensation of reaction products which are normally in gaseous phase... 

The main purpose of this review is to demonstrate the usefulness of zeolite catalysts for selective synthesis of polycyclic specialty chemicals. This paper describes some new developments in the past several years on shape-selective conversion of polycyclic hydrocarbons over large-pore zeolites including mordenites and Y-zeolites and medium-pore molecular sieves. The results given here are selected examples showing the level of selectivity and conversion. It is not the intention to review all publications in this area, although many recent reports have been cited. 

The synthesis and characterization of the intermediate-silica zeolite ZSM-25 in the presence of sodium and tetraethylammonium cations is described. The overall characterization results of this study suggest that ZSM-25 is probably an 8-ring or constrained 10-ring pore material containing 4-rings as the smallest structural unit. However, the proton form of ZSM-25 was found to have a poor thermal stability, revealing a serious drawback in its applications as a shape-selective catalyst for acid-catalyzed hydrocarbon conversions. 

Roberts and co-workers have examined the use of sc-pentane and sc-hexane for FT synthesis over C0/AI2O3. At the same density (0.3 g/cm ) similar hydrocarbon product distribution was noted for each solvent, but CO conversion in pentane was higher due to the higher pressure required to achieve that density. The enhanced chain-growth probability in SCF-FT synthesis versus gas-phase FT synthesis has been credited to the improved solubility of heavy hydrocarbons and thus the increased availability of vacant sites for a-olefin readsorption and subsequent chain growth, and the elimination of the adsorption layer barrier (85). Further catalyst examination showed that neither catalyst pore radius nor pore volume significantly affected the catalyst activity or selectivity under supercritical conditions (86). These experiments also revealed a deviation in the product distribution from the ASF model which was dependent on the physical properties of the reaction mixture. Elbashir and co-workers have proposed an alternate model for SCF-FT synthesis that better accounts for the enhanced adsorp-tion/desorption phenomena observed in supercritical solvents (87). 

An increase in total pressure would generally result in condensation of hydrocarbons, which are normally in the gaseous state at atmospheric pressure. Higher pressures and higher carbon monoxide conversions would probably lead to saturation of catalyst pores by liquid reaction products [49]. A different composition of the liquid phase in catalyst pores at high syngas pressures could affect the rate of elementary steps and carbon monoxide and hydrogen concentrations. A series... 

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. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

The catalyst used for the conversion of methanol to gasoline is based on a new class of shape-selective zeolites (105-108), known as ZSM-5 zeolites, with structures distinctly different from other well-known zeolites. Apparently, the pore dimensions of the ZSM-5 zeolites are intermediate between those of wide-pore faujasites (ca. 10 A) and very narrow-pore zeolites such as Zeolite A and erionite (ca. 5 A) (109). The available structural data indicate a lattice of interconnecting pores all having approximately the same diameter (101). Hydrocarbon formation... 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

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