Methanol to C2-C4 Olefins

The monetization of remote natural gas has been a key economic driver for catalysis research over the past 20 years. Significant reserves of natural gas exist in remote locations, distant from available gas pipehnes, which cannot be readily brought to market. The conversion of these resources to higher-valued, transportable products, such as methanol or polyolefins can allow the economical utilization of these stranded assets. Other low-valued natural gas streams, such as associated gas from oil production, could also provide feedstocks to such a technology. The conversion of remote gas, typically valued at US 0.50-1.50 per MMBTU, into polyolefins, valued at more than US 1000/t, via methanol has sparked the development of several MTO technologies. 

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SAPO-34, which has a narrow pore diameter, is highly selective for the conversion of methanol to C2-C4 olefins. Although the selectivity to aromatics on SAPO catalysts is lower than that on H-ZSM-5, the methanol conversion and the selectivity to C2-C4 olefins are 99 and 85 percent selectivity, respectively, on SAPO-34. Therefore, the yield of C2-C4 olefins is higher on SAPO-34 than on ZSM-5. SAPO-17 is active for the dehydration of methanol to light olefins." Anderson et al. also reported that SAPO-34 is highly selective for the formation of ethylene from methanol. ... 

Conversion of Methanol to Olefins. The conversion of methanol to C2-C4 olefins using the medium pore ZSM-5 and the small pore ZSM-34 catalysts has been described earlier in the literature. Selectivities for the production of small olefins of above 80% have been reported. 

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. 

Sugimoto et al. prepared Cr-silicalite-1 using morphohne (tetrahydro-1,4-oxazine) as template agent [195]. Incorporation of Cr was poorly supported by IR, XRD and EPR measurements in the H+-exchanged form, Cr(V) species were identified together with highly dispersed octahedrally coordinated Cr(III) species. TPD experiments showed a low Bronsted acidity, responsible for the high conversion of methanol to C2-C4 olefins [195]. 

The methanol transformations discussed precedingly can be modified to produce high amounts of light alkenes.437 454 474 475 The key to achieve this change is to prevent C2-C4 olefinic intermediates to participate in further transformations. Such decoupling of alkene formation and aromatization can be done by the use of small-pore zeolites or zeolites with reduced acidity. Reduced contact time and increased operating temperature, and dilution of methanol with water to decrease methanol partial pressure, are also necessary to achieve high alkene selectivities. This approach has led to the development of the MTO (methanol-to-olefin) process, which yields C2-C5 alkenes with about 80% selectivity. 

In our exploratory studies, we have now found good leads for yielding as much as 75-80% C2 C4 olefin product, which in turn can be converted to good quality distillate.( 5,6) We can now consider the co-production of gasoline and distillate from methanol as shown in Figure 15. Engineering and development studies are in the initial phases of evaluating such concepts. 

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... 

Various borosilicates have been reported in the methanol conversion process. In a study reported in 1984, Holderich gave details for the preparation of propene selectively from methanol using a borosilicate molecular sieve of the MFI structure type (10 ). Autocatalysis was observed when small amounts of olefin were added to the feed. Modification of the borosilicates using HF, HC1, or extrusion with amorphous silica-alumina led to changes in the observed product distribution to yield more C2-C4 olefins. Use of borosilicates of the MOR and ERI structure types for methanol conversion was reported by lone, et al. (28). The selectivity to olefins was improved for borosilicates with these structures relative to the silicate of the same structure and aluminum impurity level. 

Methanol Conversion to Olefins. - Chabazite, erionite, zeolite T, and ZK-5 have been used by Chang et al. for the conversion of methanol into olefins. The C2-C4 olefin concentration in the hydrocarbon fraction was always less than 60 wt% at 100% methanol conversion. It follows from Table 3 that the hydrocarbon fraction becomes richer in Cj-C olefins as the conversion of methanol decreases. That is because the conversion of olefins to paraffins is lower. Hydrocarbon fractions with more than 80 wt% of Cj-C olefins were attained with a dealuminated H-erionite, but the conversion of methanol was very low. 

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. 

Marchi and Froment observed that on dealuminated mordenite the selectivity changed with time on stream. The yield of C2-C4 olefins increased, while the yields of paraffins and aromatics dropped, even if the methanol conversion was maintained at 100% (see Figure 10). Coke would already be formed at early stages of the methanol conversion and would cover the strong acid sites, thus reducing the conversion of olefins to paraffins and aromatics. 

Recent advances have shown zeolites are effective in catalysing the direct conversion of synthesis gas to motor fuels. The MTO (methanol-to-olefins) process converts MeOH to C2-C4 alkenes and is also catalysed by ZSM-5. The development of a gallium-modified ZSM-5 catalyst (Ga-ZSM-5) has provided an efficient catalyst for the production of aromatic compounds from mixtures of C3 and C4 alkanes (commonly labelled LPG). 

Methanol Carbonylation. Metal-ion exchanged heteropoly acids (HPAs) of the general formula M[PWi204o] (M = a Group VIII metal) supported on Si02 have been found to be excellent catalysts for the vapor-phase carbonylation of methanol or DME to methyl acetate at 225°C and 1 atm total operating pressure (84). Experiments witii H3PM12O40 (M = Mo,W) carried out in a conventional flow reactor at 1 atm CO and 200-275°C have shown that methanol is converted into DME and small amount of C1-C5 saturated hydrocarbons (HCs) and C2-C4 olefins. No carbonylation products are detected under these... 

Methanol dehydrogenates to methyl formate over fresh WC and P-W2C powders with selectivities higher than 90% (109,110). The dominant side reaction is the decomposition to synthesis gas. Over WC and P-W2C modified with oxygen, methanol selectively dehydrates to dimethylether at 473 K and at higher reaction temperatures, C2-C4 olefins are produced (47). Thus, the dehydrodimerization of methanol apparently requires WC sites. These sites are titrated by chemisorbed oxygen. Thus, oxygen on the surface inhibits the formation of methyl formate and introduces a surface acid function WO that catalyzes dehydration by carbenium-ion type catalysis. 

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