Zeolites ethylene from methanol

A key step is the formation of a stable hydronium ion upon formation of dimethylether. The concept of Bronsted acid-Lewis base catalysis also allows us to understand the formation of ethylene from methanol, as formed in zeolite-catalysed reactions. A possible mechanism is sketched in Fig. 4.68. 

The catalyst is based on high levels of a ZSM-5 type zeolite which has been doped with a combination of phosphorus, magnesium and calcium. This type of formulation has been used to produce ethylene and propylene from methanol and is known to promote olefin formation from a wide variety of feeds. ... 

Small quantities of methanol and ethanol are sometimes added to the C3S in pipelines to protect against freezing because of hydrate formation. Although the beta zeolite catalyst is tolerant of these alcohols, removing them from the feed by a water wash may still be desirable to achieve the lowest possible levels of EB or cymene in the cumene product. Cymene is formed by the alkylation of toluene with propylene. The toluene may already be present as an impurity in the benzene feed, or it may be formed in the alkylation reactor from methanol and benzene. Ethylbenzene is primarily formed from ethylene impurities in the propylene feed. However, similar to cymene, EB can also be formed from ethanol. 

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... 

According to the (PEP) report, the ethylene manufacturing process from methanol involves two chemical steps, each of which occurs at close to 100% selectivity. In the first step, using Mobil Oil technology, methanol vapor was converted at 75% per pass to DME. The reaction takes place over a zeolite catalyst at temperatures close to 204°C and 1 atm pressure. 

This section reviews the Lurgi MTF technology for production of propylene from methanol on a proprietary ZSM-5 type catalyst MTPROP supplied from Slid Chemie [12,13]. Lurgi started the development of methanol to propylene (MTP ) in 1993. The proprietary zeolite-based MTP catalyst is exclusively supplied by Slid Chemie [47]. The process begins with a vapor phase dehydration of methanol to DME to produce an equilibrium mixture of DME, methanol, and steam. This mixture is then converted to propylene in a fixed-bed MTP reactor at 400-500°C in the presence of steam and recycled C -C olefins. By-products of the Lurgi MTP process are gasoline, LPG, and fuel gas. The ethylene could be recycled back or used as a copolymer. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

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. 

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. 

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. 

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-... 

The discussion of MTG kinetic effects just presented is generally applicable to MTO. Olefin selectivity is improved by decreasing methanol partial pressure, increasing temperature, and increasing zeoUte Si02/Al203. An additional effect, that of varying zeolite crystallite size, was reported by Howden et al [61], who found that when the crystallite size was reduced from 30 to 3 pm, ethylene selectivity increased. This was attributed to enhanced diffusivity of li t products, which reduces their opportunity for further reaction. 

Ceckiewicz, in Fourier transform infrared spectroscopy (FTIR) studies of methanol sorption on zeolite at 25 50°C, observed the propylene band pattern but failed to detect ethylene [96]. Upon temperature-programmed desorption, however, C -C olefins were identified. In the suggested MTO scheme (Fig. 16), the propylene was nominated as a primary product (ie, obtained directly from MeOH or DME), and the formation of the other products occurs via two parallel routes. 

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