Para selectivity

Sulfonation andDiazonium Coupling. Like phenol, sahcylaldehyde reacts easily in these reactions (40,41), with a high para-selectivity. 

The reaction product of salicylaldehyde and a secondary aniline is the benzyUc alcohol, with total para-selectivity (57). The yield is 93%... 

By a modified procedure using polyethyleneglycol as complexing agent a para-selective reaction can be achieved. ... 

Traditional methods for bromination of toluene with bromine and a catalyst result in relatively low / ara-selectivity. For example, bromine in acetic acid gives rise to approximately a 4 1 mixture of the para- and ort/to-bromotoluenes (ref. 4). The para-selectivity is enhanced in trifluoroacetic acid so that approximately 90 % of the para-isomer is produced, but greater selectivity than this is unusual. 

Our own earlier work on the chlorination of toluene had been subject to similar constraints. In this case, chlorination with ferf-butyl hypochlorite had proved to be advantageous. In the presence of silica gel as catalyst the yield of chlorotoluenes was quantitative but the regioselectivity was more or less statistical (ref. 8). However, the use of proton-exchanged zeolite X allowed the production of chlorotoluenes with a para-selectivity of more than 90 % (Fig. 4) (ref. 9). No HCl is generated in this process since the by-product is tert-butanol, and there is no inhibition of the catalyst. Indeed, the catalyst can be reused if necessary. 

Increased para selectivity has been achieved by the use of polyethylene glycol Neumann, R. Sasson, Y. Synthesis, 1986, 569. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

The excellent high para-selectivity can be explained by the selective escape of p-xylene from the H-ZSM-5 catalyst and inhibition of isomerization on the external surface of catalysts by silicalite-1 coating. In addition to the high para-selectivity, toluene conversion was still high even after the silicalite-1 coating because the silicalite-1 layers on H-ZSM-5 crystals were very thin. 

The zeohte overgrowth has been reported for FAU on EMT zeohte [44] and MCM-41 on FAU zeohte [45]. On the other hand, in this study, zeohte layers were grown on the zeohte with the same framework structure, resulting in high coverage of ZSM-5 crystals with silicalite layers and high para-selectivity. The zeohte crystals with oriented thin layer on their external surface are expected to form a new class of shape-selective catalysts. 

Keggin type H3PW)2O40 is a stable, recyclable and effective catalyst for H2S04-free liquid phase nitration of bezene, chlorobenzene and toluene with nitric acid as a nitration agent. Higher para-selectivity of nitrotoluene was obtained, and the result implies that HPA can effectively catalyze the liquid phase nitration of various aromatics as an environmentally friendly nitration process. 

Acetic anhydride and trifluoroacetic anhydride have both been used in conjunction with nitric acid and zeolite (3. This system give excellent para selectivity in many cases.4 The improved selectivity is thought to occur as a result of nitration within the zeolite pores, which may restrict access to the ortho position see, e.g., Entry 7 in Scheme 11.1. 

Entry 5 is an example of nitration in acetic anhydride. An interesting aspect of this reaction is its high selectivity for the ortho position. Entry 6 is an example of the use of trifluoroacetic anhydride. Entry 7 illustrates the use of a zeolite catalyst with improved para selectivity. With mixed sulfuric and nitric acids, this reaction gives a 1.8 1 para ortho ratio. Entry 8 involves nitration using a lanthanide catalyst, whereas Entry 9 illustrates catalysis by Sc(03SCF3)3. Entry 10 shows nitration done directly with N02+BF4, and Entry 11 is also a transfer nitration. Entry 12 is an example of the use of the N02—03 nitration method. 

A Friedel-Crafts-type reaction of phenols under basic conditions is also possible. Aqueous alkaline phenol-aldehyde condensation is the reaction for generating phenol-formaldehyde resin.34 The condensation of phenol with glyoxylic acid in alkaline solution by using aqueous glyoxylic acid generates 4-hydroxyphenylacetic acid. The use of tetraalkylammonium hydroxide instead of sodium hydroxide increases the para-selectivity of the condensation.35 Base-catalyzed formation of benzo[b]furano[60]- and -[70]fullerenes occurred via the reaction of C60CI6 with phenol in the presence of aqueous KOH and under nitrogen.36... 

Selectivity to p-isopropyl toluene being close to 30 % was achieved with SSZ-33, SSZ-35 and Beta zeolites. This is connected with the 12-MR channels in SSZ-33 and Beta. In the case of SSZ-35 the presence of 18-MR cavities decreased the differences in the rate of transport of individual isopropyl toluene isomers. In contrast, ZSM-5 zeolite behaves as para-selective catalyst in this alkylation reaction, the selectivity to p-isopropyl toluene reached 76 % after 180 min of T-O-S. 

Bromodesilylation T-methoxy-l-indanones. Cyclization contrary to the normal para-selectivity of anisole derivatives can be effected by temporary use of an ort/jo-trimethylsilyl group introduced by directed orf/io-metallation (11,75). Thus the anisole derivative 1 undergoes bromodesilylation and hydrolysis to provide 2. This product undergoes cyclization to 3 in good yield on conversion to the lithio salt followed by bromine-lithium exchange (8,65-66). 

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. 

As is apparent from the previous discussion on toluene disproportionation,the observation of high p-selectivity in STDP requires a dramatic change in selectivity. First, the primary product must be directed to be highly para-selective. Secondly, the subsequent isomerization of the primary p-xylene product must be selectively inhibited ... 

Various ways to modify ZSM-5 catalyst in order to induce para-selectivity have been described. They include an increase in crystal size (15,17,20) and treatment of the zeolite with a variety of modifying agents such as compounds of phosphorus (15,18), magnesium (15), boron (16), silicon (21), antimony (20), and with coke (14,18). Possible explanations of how these modifications may account for the observed selectivity changes have been presented (17) and a mathematical theory has been developed (22). A general description of the effect of diffusion on selectivity in simple parallel reactions has been given by Weisz (23). 

We find that the degree of para-selectivity obtainable depends uniquely on the activity and diffusion characteristics of the catalyst, independent of how these properties are obtained. While we will discuss these relationships with regard to STDP, the principles involved are generally applicable to those reactions over ZSM-5 where dialkylaromatic products are formed. 

The general characteristics of toluene disproportionation are summarized by the data presented in Figure 8. With standard HZSM-5 catalyst, as indicated by the lowest curve, the xylenes produced contain essentially an equilibrium concentration of the para isomer (24%) and exceed it only slightly at low conversion. The other curves result from a variety of HZSM-5 catalysts modified in different ways and to different degrees. It is apparent that a wide range of para-selectivities can be obtained. At increasing toluene conversions, the para-selectivity decreases for all catalysts. 

Condition 2 The para-selectivity is increased by an increase in kj and in the crystal size (r) and by a reduction in the diffusivity. The applicability of this relationship will be illustrated below. 

It is seen that high para-selectivity should result from mass transport inhibition for formation of the undesirable m-/o-xylenes. 

In view of the difficulty of measuring the diffusivity of o-xylene at the reaction temperature, 482°c, we have used the diffusivity determined at 120°C. For a series of ZSM-5 catalysts, the two D-values should be proportional to each other. Para-xylene selectivities at constant toluene conversion for catalysts prepared from the same zeolite preparation (constant r) with two different modifiers are shown in Figure 11. The large effect of the modifier on diffusivity, and on para-selectivity, is apparent. 

Para-selectivity for a wide variety of ZSM-5 preparations of comparable activity are shown in Figure 12. These data include results for unmodified HZSM-5 s of varying crystal size as well as chemically modified HZSM-5 s. Since the activity of these catalysts is nearly identical, these data clearly establish the major role of diffusion in the para-xylene content of the xylenes produced in TDP. We have examined in more detail the effect of the concentration of one of these chemical modifiers, MgO. 

The beneficial effect of catalyst activity, k, can also be seen from the temperature dependence of the para-selectivity. Comparing the selectivity for the same, unmodified ZSM-5 catalyst at 550°C and 600 °C in Figure 10 shows the advantage of the... 

Rigorous treatment of the para-selectivity requires a knowledge of the intrinsic value of the rate constant for all the reactions involved and of the absolute value of the crystal size and of the diffusivity, all under reaction conditions. These values are obtainable only with considerable difficulty and effort. As has been mentioned, the 30 percent sorption time for o-xylene at 120°c, t0.3/ is proportional to the actual values, r2/D. 

Thus, the critical value determining para-selectivity r2k/D can be replaced by two readily measured quantities, a and t0.3 ... 

If the data in Figure 13 are replotted against a.t0.3 as a pseudo Thiele-modulus, a single curve describing all the catalysts (Figure 15) results. Thus, the para-selectivity of a catalyst can be readily predicted from this empirical correlation and a knowledge of two basic catalyst properties, activity and diffusion time. Furthermore, these data are in full agreement with the model advanced above, which describes para-selectivity in terms of a classical diffusion-reaction interplay. 

It has also been shown that the selectivity features of para-selective catalysts can be readily understood from an interplay of catalytic reaction with mass transfer. This interaction is described by classical diffusion-reaction equations. Two catalyst properties, diffusion time and intrinsic activity, are sufficient to characterize the shape selectivity of a catalyst, both its primary product distribution and products at higher degrees of conversion. In the correlative model, the diffusion time used is that for o-xylene adsorption at... 

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