Aromatics, methanol conversion

Fig. 4 (right) Autocatalysis and retardation. Yields of paraffins, olefins, aromatics and "coke" as a function of duration of the experiment. Methanol conversion on HZSM5, 270 °C, Pqi oqu =2.5 bar, WHSV = 1 h. Differential coke yield abtained through internar standard). 

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). 

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). 

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 concept of molecular traffic control was proposed by Derouane and Gabelica (49) to explain the unexpected absence of counter-diffusion effects during methanol conversion over zeolites such as MFI presenting interconnected channels with different sizes and tortuosity the smallest molecules (e.g. methanol) would diffuse through the sinusoidal channels while the bulkiest (e.g. aromatics) exit through the slightly larger linear channels. 

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). 

To limit the formation of aromatics from methanol conversion, ZSM zeolites are sometimes modified by the addition of a Group VA element, for... 

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). 

The overall path of methanol conversion to hydrocarbons over ZSM-5 is illustrated in Fig. 2. Methanol and dimethyl ether (DME) form olefins, which are then converted to naphthenes, aromatics, and paraffins. Olefins initially react by oligomerization and methylation, and at increasing conversion olefins distribution is governed by kinetics. This effect, and the effects of process variables were summarized by Chang (ref. 14). The directional effects of process and catalyst variables on the MTO reaction are summarized in Table 3. 

Generally, catalyst and process variables which increase methanol conversion, decrease olefins yield. Process variables such as the use of an inert diluent may increase the spread between olefins formation and aromatization reactions (Fig. 2). Raising temperature increases methanol conversion and light olefins yield up to a point. However, higher temperatures have the disadvantage of increasing coke and light saturate yields, which lowers overall process efficiency. 

Since the measurement of on-line catalyst activity is difficult, we found it convenient to follow an on-line "reaction index" (RI), which is a selectivity ratio. The complex MTO reaction scheme can be presented schematically as A —> B —> C, where A represents methanol and DME, B - olefins, and C -aromatics and paraffins (Fig. 3). One particularly useful RI is the propane/propene ratio. Propene is the primary light olefin and propane represents paraffins. The propane/propene RI can be easily monitored by an on-line GC. We found that hydrocarbon selectivities correlate well with RI. For fixed hydrodynamics, it also correlates well with methanol conversion. 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

Influence of Space Time. Studies on the effect of space time on the methanol conversion and the hydrocarbon distribution clearly showed that Cj-Cg olefins are intermediates in the conversion of methanol to gasoline. This is illustrated in Table 8, which presents the yields of light olefins, aromatics and paraffins as a function of space time and methanol conversion. 

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