Hydrocarbons conversion over zeolites

Elementarf Steps of Hydrocarbon Conversion over Zeolites 429... 

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. 

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. 

The coke formation in methanol to hydrocarbons conversion over zeolite H-MFI was also studied with UV-RS. To distinguish between the signals that correspond to CH deformations and to CC stretches, experiments were carried out with deuterated methanol (CD3OH). The bands assignments used in this study are summarized in Table 2. It is concluded that cyclopentadienyl species are intermediates in the formation of polyaromatic hydrocarbons. By comparison with pure polynuclear aromatics UV-RS spectra, it is suggested that coke... 

These studies are rather uncommon, although, for instances. Van Santen et al [2], presented results of a computational study on the effect of differences in the strength of the acid site on the activation energy of the reaction steps for hydrocarbon conversion over a zeolite model cluster. In order to change the acid strength of the cluster, which was used in the calculations to mbdel the acid site, the terminal Si-H bonds were constrained to various lengths. Their... 

Mole s reaction path for olefin formation via toluene. Adapted from Mole T, Bett G, Seddon D. Conversion of methanol to hydrocarbons overZSM-5 zeolite an examination of the role of aromatic hydrocarbons using IScarbon- and deuterium-labeled feeds.] Catal 1983 84 435—45 Mole T, WhitesideJA, Seddon D. Aromatic co-catalysis of methanol conversion over zeolite catalysts.] Catal 1983 82 261-6. 

The conversion of methanol to hydrocarbons over zeolite H-beta. Micropor. Mesopor. Mater., 29,173-184. 

Chang, C.D. and Silvestri, A.J. (1977) The conversion of methanol and other 0-compounds to hydrocarbons over zeolite catalysts. J. Catal., 47, 249. 

Explain why, when 3-methyl pentane and n-hexane are cracked over zeolite A (Ca form) to produce smalier hydrocarbons, the percentage conversion for 3-methyl pentane is less than 1% whereas that for /r-hexane is 9.2%. 

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... 

Clearly the losses and gains of a particular species present in the product gasoline as compared to the parent feed gasoline can represent the balance of complex reactions. However, under the reaction conditions employed, it is not likely that there will be appreciable generation of C6+ hydrocarbons other than as intermediates, so that an examination of reactant losses in this region provides a reasonable comparison of reactant conversion over the two zeolites. 

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... 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

Figure 113. Conversion of nitrogen oxides and gaseous hydrocarbons reached over different NO.v-reduction catalyst formulations, as a function of the exhaust gas temperature (monolith catalyst with 62 cells cm dedicated NO t-reduction catalyst formulations with zeolites and with different active components, after laboratory oven-aging in air at a temperature of 1023 K for 16 hours light-off test in a model gas reactor at a space velocity of 50000 N11 h model gas simulates the exhaust gas composition of an IDl/NA passenger car diesel engine at medium load and speed, except for the hydrocarbon concentration, which was increased to reach yHc yNO, 3 1 (mol mol)).

Chang and Silvestri [lb], reporting the conversion of methanol to hydrocarbons over zeolite ZSM-5, also favored a carbene mechanism for initial C-C bond formation. However, the complicity of free carbenes was considered unlikely from an energetics viewpoint, and a concerted mechanism was proposed for carbene generation with concurrent sp3 insertion into MeOH or DME. Later, the possibility of a sequential mechanism was considered [14]. 

Under mild reaction conditions and particularly over zeolite of low aluminiun content at high temperature, the products of methanol conversion are olefins. Subsequent, less facile reactions convert the sorbed olefins into cycloalkanes, and to benzenoid hydrocarbons and alkanes. These subsequent reactions involve hydride anion transfer reactions and cyclization reactions of larger sorbed olefins (ref. 27). Figure A shows the product sequence. 

In the absence of molybdenum, the blank dehydrated zeolites showed no CO hydrogenation activity even up to 400°C. In contrast, measurable quantities of aliphatic hydrocarbons were detected over the molybdenum-zeolite catalysts at 300°C and above. Figs. 1-2 show the time dependence of CO conversion over MOii g HY and Mo g CsY at 300°C. The conversion and product distribution were dependent on the reaction conditions, a typical set of results is illustrated in Table 1. The molybdenum-zeolites prepared by adsorption and decomposition of Mo(C0)g resembled closely the alumina-supported molybdenum catalysts prepared by decomposing Mo(C0)g on alumina (ref. 13). The results obtained presently could not match the figures reported by Brenner et aK (ref. 8), but this could be due to the significant differences in the reaction conditions used by the above authors. However, a comparison with the silica-molybdena catalyst (prepared by impregnation of ammonium molybdate) clearly indicates that the molybdenum-zeolites were more active on per molybdenum basis. The improved activity is due to the presence of zerovalent molybdenum (for LaY and HY, residual zerovalent molybdenum were responsible for the activity). 

Zhao et al.2s have studied PP catalytic degradation over different zeolites by means of TGA measurements. The following order of activity was observed zeolite Y > mordenite > zeolite L. PP conversion over HY led to hydrocarbons concentrated in the range C4-C9, and some new compounds with cyclic structures were found compared to the thermal degradation. 

Ihe activation of hydrocarbons over zeolites is widely held to result from direct protonation at C-C or at C-H bonds (16) (17) as preposed for reaction in superacid media (18) (19). Present results (14) are exettplified by Fig (11) and Table 2. Frctti the limiting slopes of plots of weight selectivity against conversion (20) the products at zero conversion may be estimated (Scheme 1). 

C.D. Chang and A.J. Silvestri. The Conversion of Methanol and Other O-Com-pounds to Hydrocarbons over Zeolite Catalysts. J. Catal. 47 249 (1977). 

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