Zeolites hydrocarbon conversion

Hydrocarbon Conversion Catalyzed by Zeolites 15 Periodic approach... 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

Only scant information is available about the influence of coke formation on the alkylation mechanism. It has been proposed that, similar to the conjunct polymers in liquid acids, heavy unsaturated molecules participate in hydride transfer reactions. However, no direct evidence was given for this proposition (69). In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced from an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

As an illustration, the isomerization of 1-butene adsorbed on NaGeX or mixed tin-antimony oxides has been carried out. In the methanol to hydrocarbon conversion on the shape selective H-ZSM-5 zeolite, the surface methylation could be observed, the role of... 

Elementarf Steps of Hydrocarbon Conversion over Zeolites 429... 

Control of the multitude of pathways which feed molecules can take is the primary objective of aU catalyst and process developments. The work covered in this chapter focuses primarily on describing the approaches in material and catalysis development which have led to major advances in zeolite application in hydrocarbon conversion. The breaking and formation of carbon-carbon and carbon-hydrogen bonds constitute the majority of the chemical transformations involved here with the less prevalent, but very important, breaking of carbon bonds with sulfur, nitrogen and oxygen taking place in parallel. 

The third and last part of the book (Chapters 12-16) deals with zeolite catalysis. Chapter 12 gives an overview of the various reactions which have been catalyzed by zeolites, serving to set the reader up for in-depth discussions on individual topics in Chapters 13-16. The main focus is on reactions of hydrocarbons catalyzed by zeolites, with some sections on oxidation catalysis. The literature review is drawn from both the patent and open literature and is presented primarily in table format. Brief notes about commonly used zeolites are provided prior to each table for each reaction type. Zeolite catalysis mechanisms are postulated in Chapter 13. The discussion includes the governing principles of performance parameters like adsorption, diffusion, acidity and how these parameters fundamentally influence zeolite catalysis. Brief descriptions of the elementary steps of hydrocarbon conversion over zeolites are also given. The intent is not to have an extensive review of the field of zeolite catalysis, but to select a sufficiently large subset of published literature through which key points can be made about reaction mechanisms and zeolitic requirements. 

A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics of formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra-methylammonium ions and exhaustive treatments of the zeolite with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared and utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. 

Catalytic activity measurements. Alkali metal ion X and Y zeolites show no activity for hydrocarbon conversions involving carbonium ion intermediates. Ward (210) showed that the decomposition of cumene to benzene and propylene did not occur over Na—Y zeolite at 260°C, whereas extensive conversion took place with H—Y and alkaline earth ion-exchanged forms. Similarly, isomerization of o-xylene at 250°C did... 

In hydrocarbon conversion over zeolite catalysts, the formation and retention of heavy products (carbonaceous compounds often called coke ) is the main cause of catalyst deactivation. 5X 77 XI1 These carbonaceous compounds may poison or block the access of reactant molecules to the active sites. Moreover, their removal, carried out through oxidation treatment at high temperature, often causes a decrease in the number of accessible acid sites due to, e.g., zeolite dealumination or sintering of supported metals. 

Another important zeolite catalyst is the so-called bifunctional catalyst. The thermal reduction of zeolites previously exchanged with metals is the method currently used for the preparation of bifunctional catalysts for hydrocarbon conversion. The bifunctional zeolite catalysts are composed of both acidic sites and metal clusters. The preparation methods of these catalysts encompass three steps ion exchange, calcinations, and reduction, (Section 3.2.1.4) [123,127],... 

Zeolites and other oxide-based solid acid catalysts are used in hydrocarbon conversion reactions with enormous economic impact. Their lifetime is, however, very... 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced firom an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

The advantages of UV Raman speetroscopy in avoiding fluoreseenee from eoke deposits and eatalyst impurities and interference from luminescence at elevated temperatures were mentioned above. When it became apparent that the fluorescence problem was avoided, measurements were attempted of hydrocarbons adsorbed in catalytically active zeolites and during hydrocarbon conversions under catalytic reaction conditions. The spectra suggested that interference from sample damage caused by the ultraviolet laser was a serious problem. 

The formation of hydrocarbons from methanol catalyzed by zeolite H-MFI has been investigated extensively 60,61). As with many hydrocarbon conversions, the catalytic activity of the methanol-to-hydrocarbons reaction decreases over time as a result of the buildup of high-molecular-weight carbonaceous deposits (coke). UV Raman spectroscopy was employed to characterize the accumulation and chemical nature of deposited hydrocarbons as a function of time and reaction temperature with both methanol and dimethyl ether as reactants and with zeolite MFI of various Si/Al atomic ratios as catalysts the first account of this work reported results for a zeolite MFI with low acid content (Si/Al = 90) (62). Both polyolefin and a cyclopentadienyl species were observed as intermediates during the formation of polyaromatic retained hydrocarbons. These observations strongly confirm the mechanism of coke formation proposed by Schulz and Wei (63) involving aromatic formation via a five-membered ring... 

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. 

Hydrocarbon conversion processes as potential candidates for using zeolite membranes... 

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