Methanol-ethylene, conversion zeolites

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

Early attempts to convert methanol into olefins were based on the zeolite ZSM-5. The Mobil MTO process was based on the fluidised bed version of the MTG technology. Conversion took place at about 500°C allegedly producing almost complete methanol conversion. However, careful reading of the patent Uterature indicates that complete methanol conversion may not have been achieved by this means. Because of incomplete conversion, there would be a necessity to strip methanol and dimethyl ether from water and hydrocarbon products in order to recycle unconverted methanol. In this variant, the total olefin yield is less than 20% of the products of which ethylene is a minor but not insignificant product. The major product is gasoUne. Ethylene is difficult to process and has to be treated specially. Claims that it is possible that ethylene can be recycled to extinction conflict with the known behaviour of ethylene in zeolite catalyst systems and have to be viewed with some suspicion. 

The conversion of simple organic molecules (e.g. methanol, ethanol or ethylene) can also be monitored by the use of combined TG-DTA. For instance such an analysis, applied to ethylene conversion on the acid form of ZSM-5, enabled the transformation to be interpreted in terms of five different reaction steps [25]. Another example of thermal analysis application to the study of the development of a catalyzed reaction is the use of isothermal TG for investigating the kinetics of coke deposition in inner or external zeolitic sites and its subsequent removal by oxidation in air [25]. 

Yamaguchi et al. [143] studied the methylation of ethylene with methanol to propylene over MFI zeolites with different heteroatoms. They found that an efficient catalyst based on MFI with weak acidity prevents the formation of ethylene oligomers. It was demonstrated that the MeOH could be completely converted to hydrocarbons at 500°C with a selectivity of about 50-60% to propylene over a weak acid catalyst in presence of ethylene. The ethylene conversion under such conditions was nearly zero. The authors suggested that the enhancement of propylene selectivity was caused by the methylation of ethylene with methanol. 

The conversion of methanol to ethylene consists of the dehydration of methanol on acidic zeolite with the formation of an equilibrium mixture containing methanol and... 

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

The conversion of ethylene on a fresh zeolite HZSM-5 catalyst, which had not been used beforehand for methanol conversion, led to the spectra shown in Fig. 37c. The MAS NMR spectrum consists of signals at 14, 24, and 34 ppm caused by alkyl groups of cyclic compounds. Furthermore, a broad signal in the chemical shift range of alkenic and aromatic compounds appeared at ca. 120 ppm. The UV/Vis spectrum consists of bands similar to those shown in Fig. 37b and an additional weak band at ca. 450 nm. The latter may be attributed to condensed aromatics or trienylic carbenium ions (301). A weak shoulder observed at ca. 400 nm is an indication for the formation of hexamethylbenzenium ions (302). 

The zeolite-catalyzed methanol conversion technology, whether the desired product is gasoline, diesel, jet fuel or ethylene for petrochemicals, will provide new opportunities for synfuels in the coming decades. 

Two variants of the process are available, one maximising ethylene and the other propylene. The performance appears to be similar to that of the conversion of methanol to olefins using small pore zeolites. Such systems suffer from high methane yield (which has to be recycled back to a reformer) and high coke yields. The formation of olefins is promoted by using crude methanol, which can contain up to about 17% water. 

A catalytic process was developed which selectively produces a hydrocarbon product, comprised principally of ethylene and propylene. The catalyst used for this process was a crystalline aluminosilicate zeolite of the erionite-offretite family, designated as ZSM-34. The conversion conditions included temperatures of 260—540°C, pressures of 0.1—30 atm, and WHSV of 0.1—30. The feed consisted essentially of methanol and dimethyl ether, with at least 0.25 moles of water per mole of organic charge, owing to the fact that although ZSM-34 is a very effective catalyst for selectively converting methanol to lower olefins, its selectivity is markedly improved by the presence of added water to the feed. This process was directed toward enhancing the... 

The synthesis of olefins from methanol using aluminophosphate molecular sieve catalysts was studied [76], Process studies were conducted in a fluid-ized-bed bench-scale pilot plant unit utilizing small-pore silicaluminophosph-ate catalyst synthesized at Union Carbide. These catalysts are particularly effective in the catalytic conversion of methanol to olefins, when compared to the performance of conventional aluminosilicate zeolites. The process exhibited excellent selectivities toward ethylene and propylene, which could be varied considerably. Over 50 wt% of ethylene and 50 wt% propylene were synthesized on the same catalyst, using different combinations of temperatures and pressures. These selectivities were obtained at 100% conversion of methanol. Targeting light olefins in general, a selectivity of over 95% C2-C4 olefins was obtained. The catalyst exhibited steady performance and unaltered... 

Givens et al. used erionite, TMA-offretite, zeolite T, and ZSM-34 as catalysts for methanol conversion. They claim that the use of steam as diluent enhanced the selectivity for ethylene. The results obtained after 2 hours on stream showed that methanol conversion was higher when a 30/70 wt% methanol-water mixture was fed (see Table 6). This may be due to the fact that deactivation by coke was more rapid when a nondiluted methanol feed was used. Hydrocarbon fractions with up to 90 wt% of C2-C4 olefins were attained for the case that ZSM-34 zeolite was used as catalyst. 

Anderson et al. showed that the active sites involved in the conversion of methanol on zeolites are not Lewis acids. Wolthuizen et al., ° however, presented evidence that the presence of Lewis-acid sites enhances the polymerization of ethylene. This is in agreement with the results obtained with HY zeolites,where reaction of ethylene at 80 °C was observed only after dehydroxylation of the Bronsted acid sites into Lewis acid sites. At higher temperatures, ethylene is well-known to react on catalysts with strong Bronsted acid sites.Sayed and Cooney reported the involvement of aluminum Lewis sites in the formation of dimethylether. Haber and Szybalska observed that, when ethanol is converted on boron aluminum phosphates, only dehydration to ethylene takes place on the Bronsted acid sites, whereas, on Lewis acid-base centers, ethanol is mainly dehydrated to diethylether. 

Influence of Catalyst Preparation. Pebrine reported on the influence of the synthesis conditions of HZSM-5 on the selectivity toward light olefins. Synthesizing ZSM-5 in the presence of a tetra-urea-cobalt(II) complex resulted in an ethylene yield of 24.3 wt% of the hydrocarbon fraction at 43.7% methanol conversion, whereas the conventionally prepared ZSM-5 yielded only 18 wt% ethylene at the same conditions and conversion. Heering et al. mentioned that the conversion of dimethylether on ZSM-5 catalysts crystallized from a sodium-free gel with 1,6-dicunino-hexane as organic base was more selective toward both ethylene and propylene than on a conventionally prepared zeolite in the sodium form from a gel containing tetrapropylammonium. 

The crystal size of the zeolites has been found to be an important factor influencing the conversion of methanol to light olefins. With the use of crystals of 1-2 p a higher selectivity for the production of C2 C olefins and particularly ethylene has been... 

Balkrishnan et al. suggested that, even though the tortuosity in the zeolite channels is increased, the observed higher selectivity toward Cj-C olefins in the methanol conversion on P-modified ZSM-5 is due more to a change in the acidity than to any steric effect. According to Cai et al., ° phosphorus affects both Bronsted and Lewis sites of various acid strength. As a result, an increased ethylene selectivity was attained. 

The seak for cheaper feedstocks has made the researches and industrials to look the possibilities of C1-C4 alkanes. Much work is being done on the conversion of methane to methanol, ethane and ethylene of ethane to vinil chloride, and propane to acrylonitrile. Butane has been already successful oxidized to maleic anhydride. Zeolites can also contribute to upgrade C1-C4 paraffins through the reactions presented in Scheme 1. 

SAPOS also show a weaker acidity than zeolites and are selective catalysts for olefin formation. In this sense, SAPO-34 can produce ethylene and propylene with selectivities as high as 85% for methanol conversion levels of 100% (223). 

More recently, the conversion of methanol to C2 C olefins has been also reported using aluminophosphate-based molecular sieves. Surprisin y, in contrast to zeolite catalysts where best results were obtained with the medium pore ZSM-5, the best results with aluminophosphate catalysts have been described with the small pore SAPO-34 as catalyst. This molecular sieve, with a crystal structure belonging to the chabazite family, produces ethylene, propylene, and butenes with 90% or even higher selectivity. According to the data, methanol can be converted with emphasis to ethylene or to propylene as principal products by using an appropriate choice of reaction conditions (Table 6). Practical process development efforts for the conversion of methanol to C2-C4 olefins have been reported using SAPO-34 catalyst in a fluid-bed configuration. 

Improvements in chemical processes are very often based on the discovery or development of new catalysts or adsorbents. One particularly exciting example in the field of zeolite catalysis is the replacement of the formerly used amorphous silica-aliunina catalysts in fluid catalytic cracking (FCC) of vacuiun gas oil by rare earth exchanged X-type zeoUtes [1]. This resulted in considerably improved yields of the desired gasoUne and, hence, a much more efficient utilization of the crude oil. Fiuther examples are the introduction of zeolite HZSM-5 as catalyst in the synthesis of ethylbenzene from benzene and ethylene [2], for xylene isomerization [3] and for the conversion of methanol to high-... 

Zeolite NU-87, if containing Bronsted-acid sites, is an active catalyst for a large variety of acid catalyzed reactions hke toluene disproportionation, alkylation of benzene with ethylene, amination of methanol to methylamines etc. [51]. Moreover, it was found to possess interesting shape selective properties in the conversion of m-xylene [52] and of polynuclear aromatics, e.g. methylnaphtha-lenes [53]. On non-acidic (i.e. Cs+-exchanged) zeolite NU-87, loaded with small amounts of platinum, n-alkanes like n-hexane or n-octane can be dehydrocycliz-ed in high yields to the corresponding aromatics [54]. 

Experiments with conversion of MeOH in the presence of benzene [109,123], alkylation of ethylene with toluene [124], and alkylation of ethylene with ethylbenzene [125], over zeolite catalysts, show high yields and conversions to the corresponding ethylated analogs. This suggests that ethylene oligomerization and methylation (alkylation of ethylene by methanol) reactions are not very important under MTO conditions. So, the ethylene is relatively inert compared to other olefins under MTO conditions. 

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