Isomerization/disproportionation conversion

Zeolites are integral components of petrochemical refineries that produce benzene, xylene isomers, ethylbenzene and cumene. These aromatics must be high in purity for downstream conversion to polyesters and styrenic or phenolic based plastics. Catalytic processes for producing aromatics employ zeolites for isomerization, disproportionation, transalkylation, alkylation, and dealkylation. 

Humphries et al. [92,93] gave an overview of different test reactions used to characterize the acidity of zeolites. The reactions included cracking, isomerization, disproportionation, and alcohol dehydration as well as hydride transfer reactions involving cyclohexene conversion. In the following sections, we will discuss each class of these test reactions for their ability to give information about the nature, concentration, and strength of the active sites involved in acid-catalyzed reactions. 

Di-t-butylethene, a component also used as an additive for synthetic gasoline, is obtained from isobutene and ethene with a dual isomerization-disproportionation catalyst [20]. The process begins with dimerization in a reactor for conversion of isobutene to 2,4,4-trimethyl-1-pentene this is followed by isomerization-disproportionation in a bifunctional unit (isomerization of 2,4,4-trimethyl-1-pentene to 2,4,4-tri-methyl-2-pentene and conversion of the latter into di-r-butylethene) (Fig. 6). A by-product of the process, 2,3-dimethyl-2-butene, is recirculated to the disproportionation unit to be cleaved with ethene to isobutene, which is reintroduced into the process. When neohexene (3,3-dimethyl-1-butene) is employed as the starting material in this process, the installation consists solely of the disproportionation and fractionation units. 

Mobil s Low Pressure Isomerization Process (MLPI) was developed in the late 1970s (123,124). Two unique features of this process are that it is Operated at low pressures and no hydrogen is used. In this process, EB is converted to benzene and diethylbenzene via disproportionation. The patent beheved to be the basis for the MLPI process (123) discusses the use of H-ZSM-5 zeoHte with an alumina binder. The reaction conditions described are start-of-mn temperatures of 290—380°C, a pressure of 273 kPa and WHSV of 5—8.5/h. The EB conversion is about 25—40% depending on reaction conditions, with xylene losses of 2.5—4%. The PX approach to equiHbrium is about 99 ndash 101%. The first commercial unit was Hcensed in 1978. A total of four commercial plants have been built. 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent Hterature (125), the catalyst used in this process is beHeved to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equiHbrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocatbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

The structure of SSZ-35 (IZA structure code STF) as viewed in the [001] direction is shown in Fig. 17. The dimensions of the 10-MR structures are 5.5 x 6.1 A and the diameter of the 18-MR structures is 12.5 x 9 A. This pore structure is in contrast to the structure of SSZ-44 (IZA structure code SFF) shown in Fig. 18, where the 10-MR structures are nearly spherical (5.8 A) and the 18-MR structures are slightly larger (12.9 x 9 A). These small differences in pore size apparently translate into startling differences in reactivity. A study of m-xylene conversion shows a high degree of isomerization versus disproportionation, which is consistent with a 10-MR pore system (47). The interesting data is the para to ortho selectivity in the isomerization products, where SSZ-44 exhibited a higher para/ortho... 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

The primary product will be rich in the para isomer if initial m-and o-xylene diffuse out of the zeolite crystal at a lower rate (Dm q/t2) than that of their conversion to p-xylene (kj) and the latter s diffusion (Dp/r2). Conversion of the para-rich primary product to secondary product low in p-xylene is minimized when the actual, observed rate of isomerization (kj g is lower than the rate of toluene disproportionation (kD). 

Figure 13.28 Meto-xylene disproportionation isomerization selectivity ratio over various zeolites at 31 7-318°C and 10% conversion [64].

It was concluded at this point that zeolites with a very spacious pore system, such as faujasites or ZSM-20, are inappropriate catalysts for the isomerization of 1-methyl-naphthalene. Subsequently, a zeolite with much narrower pores was tested, viz. HZSM-5. Pertinent results are shown in Fig. 2. At 300 C, the conversion is low and even a temperature increase of 100 °C does not bring about a considerable increase in conversion. We presume that the reaction of 1-methylnaphthalene in HZSM-5 is controlled by diffusioiL There were practically no side reactions such as cracking, dealkylation or transalkylation, in other words XjMHp Y2.M.NP "e identical. This is at variance with the results of Matsuda et al. [21] who did observe some disproportionation on their H2SM-5 sample at 300 °C. More work is needed to elucidate the reasons for this different catalytic behavior of various samples of HZSM-5. As a whole, zeolite ZSM-5 was discarded at this stage due to its too narrow pore system. 

Further proof was obtained by comparing the catalytic activity for the disproportionation and isomerization of trimethyl benzene. As the isomerization is expected to obey first order kinetics, the microporous effect would appear more in disproportionation than in isomerization. Here, 1,2,3-trimethyl benzene (1,2,3-TrMB) was used as reactant instead of 1,2,4-TrMB, since isomerization conversion of 1,2,4-TrMB was too small to discuss the change in... 

Toluene disproportionation and transalkylation are important industrial processes in the manufacture of p-xylene. Toluene disproportionation [Eq. (5.73)] transforms toluene into benzene and an equilibrium mixture of isomeric xylenes. The theoretical conversion of toluene is 55%. Commercial operations are usually run to attain 42 18% conversions. In conventional processes308 309 324 325 alumina-supported noble metal or rare-earth catalysts are used in the presence of hydrogen (350-... 

The species RNO participates in five types of reactions pertinent here. At high NO pressures, it acts as a catalyst for the conversion of NO to N2 + N02. It is regenerated in the reaction chain lengths to 100 have been achieved. At the same time, it is also consumed by a reaction first-order in NO to produce N20 and RO radicals. At lower NO pressures, dimerization of RNO is an important process. Another possibility for RNO species with an a hydrogen atom is isomerization to the oxime R =NOH. This reaction proceeds with considerable activation energies for small radicals and, thus, may not be important at room temperature. If the pressure of NO is sufficiently low, so that all the radicals are not scavenged by NO, RNO may react with the radicals, either by addition or disproportionation. 

The reactions using the three NO-containing complexes all showed equilibrium conversion to 2-butene and 3-hexene in 1 hr. The cis/trans ratios for all olefins were also at their equilibrium values (the initial 2-pentene was 48% trans, 52% cis). With the complex Mo(CO)4(bipy) there was observable disproportionation although the conversion was quite small. Some double-bond isomerization was observed with this system (1.2% 1-pentene present). The last complex of Table III also gave a trace of disproportionation, some double-bond isomerization (1.6% 1-pentene), and cis/trans isomerization (equilibrium ratio of cis/trans 2-pentene). 

The formation of 5,6-dihydro-2,4,6-trimethyl-l,3,5-dithiazine, 2,4,6-trimethyl-1,3,5-trithiane, and 3,5-d ime thy1-1,2,4-trithiolane by heating of acetaldehyde, hydrogen sulfide, and ammonia was outlined by Takken and coworkers (36) and is summarized in Figure 4. Under oxidative conditions, dialkyltrithiolanes are formed at low pH there is conversion to trialkyltrithianes at elevated temperature isomerization into trisulfides occurs, which compounds disproportionate into di and tetrasulfides and in the presence of ammonia, dithiazines are formed. These compounds and the conditions for their formation are of extreme importance for the production of desirable meat flavors. 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

Figure 17.4 Isomerization and disproportionation of pentane on S0j/Zr02 at 0°C and H-mordenite at 200°C conversion (O), butane ( ), isobutane ( ), isopentane ( ), hexanes (A),...

Sastre et aL [112] studied the isomerization of m-x>iene over OfBretite and observed monotonical increase in m-xylene conversion upon exdumge of the K -cations. This was ascribed to the increase of the concentration of the protons and the increase in accessibility of the pores, which resulted in a higher selectivity for the isomerisation reaction at the e q>aise of the disproportionation reactioiL Only a sHght increase in the p-xyloie in the fraction of oitho-and para-xylene was observed. Over Beta a maximum activity for the xylene isomersation was observed and this was explained by either a pos le existence of a eigistic effect between extra-framework aluminium and the fitunework Bronsted acid sites or a concentration effect [113]. 

F. and the conditions mentioned, a minimum partial pressure of about 60 p.s.i. is required to prevent disproportionation. Under milder conditions, somewhat lower partial pressures will serve, but conversion is much lower. Hydrogen pressures above 100 p.s.i. tend to suppress isomerization. A total reactor pressure of 300 p.s.i. is sufficient to obtain the desired hydrogen partial pressure at operating conditions. 

The conversion of cyclohexanol on acid sites in zeolites and boralites is composed of two steps dehydration (to cyclohexene and water) and consecutive reactions of cyclohexene skeletal isomerization and disproportionation. Our IR and catalytic studies have shown that the dehydration occurs on both strong and weak Bronsted sites. On the other hand, only the strong Bronsted acid sites are required for isomerization and disproportionation. This observation may be used to propose a new ipethod for investigation of heterogeneity of acid sites in zeolites by a simple catalytic test. 

The title reaction has been studied on HY, HM, HZSM-5 and HBeta zeolites under standard conditions. The reaction pathway involves many parallel and/or successive steps. The reactant can undergo dealkylation, isomerization and disproportionation in various relative ratios, depending on the nature of the catalyst and on the reaction conditions. The influence of pressure was investigated. It was found that, generally, the activity and stability of the catalyst increase with increasing pressure. Products distribution, which strongly depends on the nature of the zeolite, is also affected by pressure. Low conversion (< 10%) runs were also performed at different temperatures to evaluate the activation energy values of the reactions. 

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