Aromatics, methanol conversion products

COMPOSITION OF THE FRAC- AROMATICS OF THE VOLATILE PRODUCTS FROM METHANOL CONVERSION ON HZSM5 AT DIFFERENT REACTION TIMES 270 c. ... 

It is necessary, however, to maximize the intermediate olefin product at the expense of the aromatic/paraffin product which makes up the gasoline ( ). The olefin yield increases with increasing temperature and decreasing pressure and contact time. Judicious selection of process conditions result in high olefin selectivity and complete methanol conversion. The detailed effect of temperature, pressure, space velocity and catalyst silica/alumina ratio on conversion and selectivity has been reported earlier ( ). The distribution of products from a typical MTO experiment is compared to MTG in Figure 4. Propylene is the most abundant species produced at MTO conditions and greatly exceeds its equilibrium value as seen in the table below for 482 C. It is apparently the product of autocatalytic reaction (7) between ethylene and methanol (8). 

In this study, the crystallization temperature and the heating rate were varied using the milled precursor, and ZSM-5 crystals could be synthesized. For example, the temperature was elevated from 160 to 210°C with a constant heating rate of 0.2°C/min (Method 2). The crystals prepared by Methods 1 4 had about same BET-surface area of 385 11 ma /g and the XRD patterns of ZSM-5. The average size of crystals reduced from 8 urn for Method 1 to 1 ym for Method 4. The concentration profiles of Si and A1 from outside to inside the crystals became uniform with reducing size. The activity of methanol conversion, the yield of gasoline fraction, and the content of aromatics in the gasoline clearly increased for the product of Method 4 (Fig. 6). 

The reaction mechanism for the conversion of methanol to hydrocarbons over molecular sieve catalysts has been extensively investigated over the past 25 years. It is widely accepted that methanol conversion initially proceeds through equilibration with DME. Early work with ZSM-5 showed that light olefins are then the initial hydrocarbon products, followed by heavier olefins, paraffins and aromatics (Figure 12.5) (2). 

INITIAL STEPS IN METHANOL CONVERSION AND AN ALTERNATIVE HOMOLOGATION MECHANISM A small amount of methane (ca. 1C%) is formed in methanol conversion, and appears to be one of the first products formed (ref. 11). When a small amount of methanol is sorbed onto ZSM-5 zeolite, the lattice is methylated (ref. 5). Subsequent temperature-programmed desorption gives dimethyl ether and desorbed methanol first, then (at 250-300°C) methane (stable) and formaldehyde (unstable), and finally aromatic products (ref. 22-23). 

The following questions can in principle be addressed with spectroscopy (1) Zeolite synthesis what are the mechanisms of ZSM-5 synthesis and how do they influence the quality of the catalyst synthesized (2) Catalyst characterization what are the structure and composition of the zeolite, and what is the configuration of the active site for methanol conversion (3) How do methanol and dimethylether interact with the active sites i.e. what species are present in the catalyst in the initial stages of methanol conversion (4) What are the subsequent reaction pathways leading to the final alkane, alkene and aromatic products (5) What causes catalyst deactivation This question concerns both the temporary deactivation associated with coke formation, which can be reversed by oxidative regeneration, and the permanent deactivation which occurs after repeated deactivation-regeneration cycles. 

The relative distribution of para + meta aromatics in methanol conversion was increased over Mo exchanged ZSM-5 but not on the pyridine poisoned sample. The same increase trend was also observed in the disproportionation reaction over Mo exchanged zeolites (Fig. 3), thus a reasonable explanation is the presence of internal Mo. Both methanol and toluene conversions performed on zeolites are molecular shape selective processes. Internal Mo will create diffusional hindrances which will favour the formation of para aromatics (product selectivity). 

In the conversion of methanol to gasoline on ZSM-11, much less Ci-Cj products are observed than with ZSM5 while the Cg -aliphatics are more abundant. Whereas on ZSM-5 the aromatics fraction is mainly composed of xylenes, on ZSM-11 more aromatics are produced. Harrison et al. defined in this context product selectivity as the ratio of xylenes to trimethylbenzenes. The ratio varied from 2.5 to 0.5 for the conversion of methanol on ZSM-5 and ZSM-11, respectively. The difference in shape selectivity was also illustrated by the (m+p) o-xylene ratio, which was 13 on HZSM-5 but only 3 on HZSM-11. This was attributed to the relative dimensions of the channel intersections, 50% of which are 30% larger in ZSM-11, and to the difference in relative length of the straight (ZSM-5, ZSM-11) and tortuous channels (ZSM-5). Furthermore, since ZSM-11 contains only one type of channel, the molecular traffic control shape selectivity should, in principle, not occur in this catalyst. This provides a possible explanation for the higher yield of polysubstituted aromatics methanol reactant and aromatic products cannot avoid counterdiffusion as in ZSM-5 and this increases the probability of alkylation of the aromatics. 

SAPO-5, MAPO-5, and MeAPO-5 molecular sieves are also active catalysts for methanol conversion into hydrocarbons. However, high concentrations of aromatics can also be obtained on these molecular sieves. In SAPO-5, the selectivity toward olefins can be improved by decreasing the Si/Al ratio, therefore, the concentration of strong acid sites. Incorporating bivalent elements to the aluminophosphate freunework also modifies the acid properties. Cations like and Co " " lead to active catalysts for methanol conversion, but the production of aromatics is high so that the olefin selectivity is lower. 

Methanol and Wood Conversion Product Classes. Methanol has been used in this screening work to ascertain catalyst activity. The methanol relative product distribution on an active, pure catalyst is shown in Figure lA. (Table II gives the identification of the ions observed). No methanol (m/z 31 and 32) breakthrough was observed, and the first formed product, dimethyl ether (m/z 45 and 46), has been consumed to form a mixture of C2 to Cg olefins and toluene, xylene, and trimethyl-benzene. Note the lack of benzene and alkanes. With lower space velocities and higher methanol partial pressure, the alkenes are known to disproportionate to branched alkanes and to form more aromatics (11). The absence of products above m/z 120 indicates the well-known shape selectivity of the catalyst. 

A hypothesis for the synergistic behavior is that methanol, in the presence of wood conversion products, does not have to undergo the dimethyl ether conversion step to alkenes, but rather can react directly with the reactive wood-derived products to alkylate aromatics. This increase in methylated benzenes would be expected to occur at the expense of the normal slate of products formed from pure methanol. 

Figure 3.52 depicts the dependence of methanol conversion and product composition on contact time as contact time increases, (reciprocal of the catalyst load), the yield of gasoUne and aromatics increases. (In English terminology, the catalyst load is defined as (LHSV) liquid hourly space velocity ). 

A decrease of the partial pressure also tends to enhance the olefins formation at low temperature by suppressing the aromatization reaction. Thus, olefins yields are maximized by operating at partial methanol conversions and by using steam as inert diluents. Even at reduced methanol conversions, however, the production of aromatics is still significant. 

Svelle et al. proposed to separate the Cj alkene cycle and the aromatics/ethylene cycle. However, the authors say that it is probably not the case for H-ZSM-5, as the constant production of aromatics during methanol conversion means that some of the Cj alkenes continuously form new aromatics [93]. This means that for H-ZSM-5 the aromatics/ethylene cycle caimot run without the alkene cycle. Inversely, on the basis of the low reactivity of ethylene toward methanol relative to that of propylene and butenes, the contribution to the alkenes involving ethylene is very small and might not be required for the Cj alkene cycle to occur [93],... 

Mobil MTG and MTO Process. Methanol from any source can be converted to gasoline range hydrocarbons using the Mobil MTG process. This process takes advantage of the shape selective activity of ZSM-5 zeoHte catalyst to limit the size of hydrocarbons in the product. The pore size and cavity dimensions favor the production of C-5—C-10 hydrocarbons. The first step in the conversion is the acid-catalyzed dehydration of methanol to form dimethyl ether. The ether subsequendy is converted to light olefins, then heavier olefins, paraffins, and aromatics. In practice the ether formation and hydrocarbon formation reactions may be performed in separate stages to faciHtate heat removal. 

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