GB2118570A - Hydrocarbon conversion - Google Patents

Abstract

Transalkylation of alkylaromatic hydrocarbons, especially toluene disproportionation, is carried out over a catalyst based on an open- framework crystalline silica ("Silicalite"). The open-framework crystalline silica is preferably one to which no metal cation has been deliberately added. The preferred toluene disproportionation reaction is preferably carried out at a temperature in the range 400 to 560 DEG C and at a low weight hourly space velocity, preferably less than 1 hr<-1>, for example in the range 0.2 to 0.7 hr<-1>.

Description

SPECIFICATION Hydrocarbon conversion This invention relates to the production of aromatic hydrocarbons by transalkylation of alkylaromatic hydrocarbons.

The transalkylation of alkylaromatic hydrocarbons has been discussed extensively in the technical literature and a number of transalkylation processes have been established or proposed. Such transalkylation processes include the transalkylation of ethylbenzene to produce diethylbenzenes and benzene, the reaction of toluene and trimethylbenzene to produce xylene, and the disproportionation of toluene to benzene and xylenes. Most attention has probably been paid to toluene disproportionation and in recent years a number of proposals have been made for carrying out the disproportionation both in the liquid phase and in the vapour phase. Over the years a number of catalysts have been proposed for use in transalkylation reactions including those of the Friedel-Craft type, silica-alumina catalysts and catalysts based on zeolites with or without the addition of various metal components.

We have now found that the transalkylation of alkyl-aromatic hydrocarbons can be effected using another type of catalyst.

According to the present invention a process for the transalkylation of an alkylaromatic hydrocarbon comprises contacting the alkylaromatic hydrocarbon or a mixture comprising it with a catalyst comprising an open-framework crystalline silica.

Open-frame work crystalline silicas have been prepared and reported in the literature. United States Patent Specification No. 4,073,865 discloses crystalline silica polymorphs and their preparation. Similarly, United States Patent Specification No. 4,061,724 describes a crystalline silica polymorph called 'silicalite' and its preparation.

"Open framework crystalline silica" as used herein refers to silicas having a rigid, threedimensional network of SiO4 tetrahedra in which the tetrahedra are crosslinked by the sharing of oxygen atoms. The open-framework crystalline silicas are substantially free of alumina, but they may contain minor amounts of alumina resulting from impurities in the starting materials or contamination of the manufacturing equipment. The silica/alumina mol ratio of the openframework crystalline silicas is typically greater than about 200:1. It is preferred to use in the process of this invention an open-framework crystalline silica to which no metal cation has been deliberately added. However, if desired, an open-framework crystalline silica containing a suitably catalytically active metal, for example a transition metal, for example nickel, may also be used.The open-framework crystalline silicas also have specific gravities, in the calcined form, of from about 1.50 to about 2.10 g/cc and a refractive index of about 1.3 to about 1.5.

Preferably the open-framework crystalline silica used in the process of the present invention comprises "Silicalite" as described in United States Patent No. 4,061,724. "Silicalite" has, as synthesized, a specific gravity at 25"C of 1.99 + 0.05 g/cc as measured by water displacement. In the calcined form (600"C in air for one hour), it has a specific gravity of 1.70 + 0.05 g/cc. With respect to the mean refractive index of "Silicalite" crystals, values obtained by measurement of the as synthesized form and the calcined form (600"C in air for one hour) are, respectively, 1.48 + 0.01 and 1.39 + 0.01.

Table 1 sets out the six strongest lines (i.e. interplanar spacing) in the X-ray powder diffraction pattern of "Silicalite" (600"C calcination in air for 1 hour). In Table 1, the abbreviations "s" and "vs" stand for "strong" and "very strong" respectively.

TABLE 1 d(A) Relative Insensity 11.1 +0.2 vs 10.0 +02 vs 3.85 + 0.07 vs 3.82 * 0.07 s 3.76 + 0.05 s 3.72 + 0.05 s Table 2 lists the data (as reported in US Patent No. 4,061,724) representing the X-ray powder diffraction pattern of a typical "Silicalite" composition containing 51.9 moles of SiO2 per mol of (tetrapropylammonium)20 prepared according to the method of U.S. Patent No.

4,061,724, and calcined in air at 600"C for one hour.

TABLE 2 d(A) Relative Intensity d(A) Relative Intensity 11.1 100 4.35 5 10.02 64 4.25 7 9.73 16 4.08 3 8.99 1 4.00 3 8.04 0.5 3.85 59 7.42 1 3.82 32 7.06 0.5 3.74 24 6.68 5 3.71 27 6.35 9 3.64 12 5.98 14 3.59 0.5 5.70 7 3.48 3 5.57 8 3.44 5 5.36 2 3.34 11 5.11 2 3.30 7 5.01 4 3.25 3 4.98 5 3.17 0.5 4.86 0.5 3.13 0.5 4.60 3 3.05 5 4.44 0.5 2.98 10 Crystals of "Silicalite" in both the as synthesised and calcined forms are orthorhombic and have the following unit cell parameters: a = 20.05 , b = 19.86 , c = 1 3.36A (all values + 0.1 ).

The pore diameter of "Silicalite" is about 6 Angstroms and its pore volume is 0.1 9 cc/gram as determined by adsorption. "Silicalite" adsorbs neopentane (6.2 A kinetic diameter) slowly at ambient room temperature.

As prepared, the "Silicalite" open-framework crystalline silica may contain residual traces of alkali metal. These traces can be removed by heating the silica in an aqueous solution of a suitable mineral acid, for example 1 to 5M hydrochloric acid, or of an ammonium salt, for example ammonium chloride, under reflux for 1 to 5 hours followed by calcination in air or nitrogen for some, typically 5, hours.

The open-framework crystalline silica such as "Silicalite" may be used in the form of a fine powder or in the form of aggregates having a size of at least 0.1 mm diameter for example extrudates, pellets, granules or spheres. Known methods of aggregation may be used to form the aggregates and if desired binders or diluents, for example silica, alumina or a clay, may be incorporated in the aggregates in amounts within the range 5 to 95% by weight of the total weight of aggregate.

In some circumstances, it may be desirable to activate the catalyst before use in the process of this invention and this is conveniently done by contacting the catalyst with nitrogen at elevated temperature, for example 500"C.

The process of this invention may be used to effect the transalkylation of an alkylaromatic hydrocarbon at transalkylation conditions including a temperature of 100"C to 700"C and a pressure of from atmospheric to 100 atmospheres (100 Kg/cm2). Suitable transalkylation reactions include the disproportionation of toluene to produce benzene and xylenes, the reaction of toluene and trimethylbenzene to produce xylene, the transalkylation of ethylbenzene to produce diethylbenzenes and benzene, the reaction of benzene with polyethylbenzenes to produce ethylbenzene.

The transalkylation reactions are preferably effected at a temperature in the range 200 to 600 C and at a pressure in the range atmospheric to 100 atmospheres (100Kg/cm2). It is preferred to use the catalyst as a fixed bed with contact times, expressed in terms of weight hourly space velocity, in the range 0.1 to 20 hr-1.

The process of the present application is particularly applicable to the disproportionation of toluene to benzene and xylenes. The feedstock for the process may comprise toluene alone or mixed with an inert gas or with polyalkylated benzenes, for example a hydrocarbon mixture known as "Aromasol H" (Registered Trade Mark). Preferred operating conditions for the disproportionation of toluene include a temperature in the range 400"C to 560"C more preferably in the range 410 to 520"C; a pressure in the range 10 to 100 psig (0.7 to 7.0 Kg/cm2); and a weight hourly space velocity in the range 0.1 to 0.8, more preferably in the range 0.2 to 0.7 hr-'. The reaction may be carried out in the presence or absence of hydrogen.

If desired, the transalkylation process of this invention may be carried out in the presence of a small proportion of steam. Steam levels of the order of 50 to 100ppm are preferred and the steam is conveniently added in admixture with the feed of hydrocarbon to the process.

During use, the activity of the catalyst will decline and it is preferred to regenerate the catalyst with an oxygen-containing gas. The concentration of oxygen in the regeneration gas should be kept at a level, which avoids the development of large exotherms. For example, in the initial stages of regeneration a gas containing 5% air and 95% nitrogen may be used, the maximum temperature in the catalyst bed being limited to 520"C. As regeneration proceeds, the concentration of oxygen in the gas may be progressively increased until air only is fed. Finally, the temperature of the catalyst bed may be raised further, for example to 550"C.

The process according to this invention is further illustrated by the following Examples.

EXAMPLE 1 A quantity (30g) of pure "Silicalite" open-framework crystalline silica (a Union Carbide product, designated S115) in the form of 3mm extrudates ws loaded into a glass tubular laboratory reactor and heated in air at 500"C for 1 6 hours. The catalyst was then purged with nitrogen at 500"C.

Toluene (99% pure) was passed over the extrudates at 500"C and at a space velocity of 0.13 hr-' for 1 73 hours. The compositions of the products obtained at intervals during this period are given in Table 3.

TABLE 3 Time on line (hr) 4 24 48 96 144 173 PRODUCT Benzene (% w/w) 20.1 19.4 18.6 17.1 13.5 13.2 Toluene (% w/w) 57.4 57.6 58.9 61.9 64.4 70.1 Paraxylene (% w/w) 5.1 5.4 5.4 5.4 6.1 5.6 Metaxylene(%w/w) 11.0 11.3 11.0 10.2 10.0 7.5 Orthoxylene (% w/w) 5.1 5.3 4.9 4.6 4.3 3.0 Ethylbenzene (% w/w) 0.1 0.1 0.1 0.1 0.1 0.0 C9+Aromatics(%w/w) 1.0 0.9 1.0 0.8 1.5 0.5 Conversion of toluene (%) 42.5 42.2 40.9 38.0 35.5 29.8 EXAMPLE 2 Extrudates (3mm) of "Silicalite" (Union Carbide Co's S115) bound with 15% silica were crushed and aggregates of 250-500y selected. These aggregates (0.669) were loaded into a microreactor and, after flushing with nitrogen at 500"C, toluene vapour was passed over the catalyst bed at around atmospheric pressure.The catalyst bed was maintained at 500"C. By far the major products were xylenes and benzene. The following results were obtained at a weight hourly space velocity of 0.31 hr-1.

TABLE 4 Time on Stream Toluene Conversion 123 hrs 31.7% 145 hrs 30.0% 167 hrs 28.7% EXAMPLE 3 Extrudates (3mm) of "Silicalite" (Union Carbide Co's S11 5) bound with 15% silica were crushed and aggregates of 250 to 500y selected. These aggregates (0.629) were loaded into a microreactor and, after flushing with hydrogen while raising the temperature to 475"C, a feedstock containing toluene and hydrogen in a mole ratio of 0.80:1 was passed over the catalyst bed, the toluene being vaporised and mixed with hydrogen gas prior to entry to the reactor.The pressure in the reactor was 100 p.s.i.g. (7.0 Kg/cm2) and the catalyst bed was maintained at 475"C. The following results were obtained at a weight hourly space velocity which averaged at 0.67 hrs ' by analysis of the products collected over the intervening periods.

TABLE 5 Time on stream (hr) Toluene conversion (%) 19.5 25.5 44.1 25.4 60.0 24.9 67.1 25.3 95.1 24.0 116.0 24.0 130.0 23.4 EXAMPLE 4 Extrudates (3mm) of "Silicalite" (Union Carbide Co's S1 1 5) bound with 20% alumina were crushed and aggregates of 250 to 500y selected. These aggregates (0.719) were loaded into a microreactor and, after flushing with nitrogen at 475 C, toluene vapour was passed over the catalyst bed at around atmospheric pressure. The catalyst bed was maintained at 475 C. By far the major products were benzene and xylenes, C9+ aromatics in the product stream being less than 0.3 mol%. The following results were obtained at a weight hourly space velocity of 0.2, averaging throughout the run.

TABLE 6 Time on stream (hr) Toluene conversion (%) 10 34.5 20 34.0 50 33.8 70 33.5 90 32.2 110 31.3 130 30.3 1 ZD 30.0 17 29.6 190 29.5 250 26.3 300 23.0 330 22.0 EXAMPLE 5 Extrudates (3mm) of "Silicalite" (Union Carbide Co's S11 5) containing 0.6 wt% nickel and 20% alumina binder were crushed and aggregates of 250 to 500p selected. These aggregates (0.669) were loaded into a microreactor and, after flushing with hydrogen while raising the temperature to 475 C, a feedstock containing toluene and hydrogen in a mole ratio of 1:1.19 was passed over the catalyst bed, the toluene being vaporised and mixed with hydrogen gas prior to entry to the reactor.The pressure in the microreactor was 90 p.s.i.g. (6.3 Kg/cm2) and the catalyst bed was maintained at 475 C. The following results were obtained at a weight hourly space velocity which averaged 0.654 hr-' by analysis of the products collected over the intervening periods.

TABLE 7 Time on stream (hr) Toluene conversion (%) 18 35.5 22.5 35.0 40 35.2 47 34.6 65 35.0 143 35.7 These examples illustrate that catalysts based on "Silicalite" open-framework crystalline silica retain their performance in toluene disproportionation with little change for on-stream periods of several days. For efficient full-scale plant operation it is desirable to mintain nearly constant the conversion of the feed hydrocarbon. For example, a conversion in the range 25 to 45% is convenient when disproportionating toluene. Near constant conversion may be conveniently achieved by gradually increasing the temperature as the run progresses.

Claims (11)

1. A process for the transalkylation of an alkylaromatic hydrocarbon which comprises contacting the alkylaromatic hydrocarbon or a mixture comprising it with a catalyst comprising an open-framework crystalline silica, as hereinbefore defined.

2. A process as claimed in claim 1 in which the open-framework crystalline silica comprises "Silicalite".

3. A process as claimed in claim 1 or 2 in which the open-framework crystalline silica is used in the form of aggregates of at least 0.1 mm diameter.

4. A process as claimed in claim 3 in which a binder or diluent is incorporated in the aggregates in amounts within the range 5 to 95% by weight of the total aggregate.

5. A process as claimed in any one of the preceding claims in which the catalyst is activated by contact before use with nitorgen at elevated temperature.

6. A process as claimed in any one of the preceding claims in which the open-framework crystalline silica used is one to which no metal cation has been deliberately added.

7. A process as claimed in any one of the preceding claims in which the transalkylation reaction is effected at a temperature in the range 200 to 600"C, a pressure in the range from atmospheric to 100 atmospheres (100 Kg/cm2) and at a weight hourly space velocity in the range 0.1 to 20 hr-'.

8. A process as claimed in any one of the preceding claims in which the transalkylation reaction is the disproportionation of toluene to form benzene and xylenes.

9. A process as claimed in claim 8 in which the disproportionation reaction is carried out at a temperature in the range 400 to 560"C.

10. A process as claimed in claim 8 or 9 in which the disproportionation is carried out a pressure in the range 10 to 100 p.s.i.g. (0.7 to 7.0 Kg/cm2g).

11. A process as claimed in any one of claims 8 to 10 in which the disproportionation is carried out at a weight hourly space velocity of 0.1 to 0.8 hr-'.

1 2. A process as claimed in claim 11 in which the disproportionation is carried out at a weight hourly space velocity of 0.2 to 0.7 hr- 1 3. A process as claimed in any one of the preceding claims in which the process is carried out in the presence of hydrogen.

1 4. A process for the transalkylation of an alkylaromatic hydrocarbon substantially as hereinbefore described with reference to any one of examples 1 to 5.

1 5. A process for the disproportionation of toluene substantially as hereinbefore described with reference to any one of examples 1 to 5.