NO150956B - PROCEDURE FOR REFORMING A MATERIAL CONTAINING ALIFATIC OXYGEN COMPOUNDS - Google Patents

Description

The invention relates to a method for transforming a material containing aliphatic oxygen compounds into a product containing aromatic hydrocarbons, i.e. petrol, i.e. hydrocarbon fuel for internal combustion engines.

Gasoline has a complete boiling range for C of approx. 138 to 221 oC depending on the particular mixture used. When sold, "gasoline" naturally contains additives, many of which are not hydrocarbons, but the present invention is not directed at such additives.

We have previously discussed, e.g. in BRD-off.skrift no. 2.4 38.252, that aromatic hydrocarbons can be obtained from oxygen-containing aliphatic compounds by means of a special group of catalysts. The present invention is based on our discovery that the amount of aromatic compounds thereby obtained can be significantly increased by using a special composition of the aliphatic compound. We have thus found that the yield of aromatic compounds is improved if the feed is a mixture of difficult-to-reform and easily-reformable oxygen-containing aliphatic compounds. The improvement is measured by the amount of moderate carbon that is converted to Cg to C^ aromatic compounds, and it will vary with the conditions under which the conversion is carried out.

The method according to the invention relates to the transformation of a material containing aliphatic oxygen compounds with the formula C H „ .pH_0 where n is the number of carbon atoms

n m-2p r 2

in the molecule and has a value of 1-6, m is the number of hydrogen atoms in the molecule and p is the number of oxygen atoms in the molecule, to a product containing aromatic hydrocarbons at temperatures of 260-649°C under pressure at a liquid-space velocity per hour of 0.5-50, in the presence of a crystalline aluminosilicate zeolite having a silica to alumina ratio of

12-3000 and a forced control index of 1-12, as a catalyst, and the method is characterized by the fact that a material is used which comprises a molar excess of such aliphatic oxygen-containing compounds where the ratio (<m>~<2P>^ exceeds 1, and a residual material of such aliphatic oxygen-containing compounds where the ratio (<m>~<2p>) is 1 or less.

n

In a preferred embodiment, the molar excess comprises a monohydric alcohol while the equilibrium comprises a carboxylic acid, but excellent results can be obtained when the molar excess comprises an alcohol (especially methanol), an ether, a ketone, a carboxylic acid ester and/or an aldehyde containing at least 3 carbon atoms, and the balance comprises a carboxylic acid,

a polyhydric alcohol, a carbohydrate, a carboxylic anhydride and/or an aldehyde containing less than 3 carbon atoms, e.g. acetic acid, acetaldehyde and/or formaldehyde. The method is preferably carried out at a temperature in the range of 315

to 482°C, and Zeolite ZSM-5 is preferred as zeolite.

The feed materials can of course be taken from any conventional source. However, in a further embodiment of the invention, the preparation of a suitable feed is integrated into the method, and a mixture of methanol and acetic acid is then obtained by reacting methanol with carbon monoxide at a temperature of 14 9 to 4 27°C in the presence of a carbonylation catalyst and the mixture is then brought together with the zeolite catalyst, and the carbonylation is preferably carried out up to the point where the mole ratio between methanol and acetic acid in the mixture feed is at least 12. A preferred carbonylation catalyst comprises rhodium, and it is desirable that it be promoted with iodine.

When carrying out the invention, a further step can be integrated, in that the methanol and carbon monoxide which are exposed > to said carbonylation reaction are developed by reaction of carbon monoxide and hydrogen at a temperature of 23 2 to 3 99°C over a methanol synthesis catalyst, such as one which includes zinc and/or copper. The carbon monoxide and hydrogen can be synthesis gas obtained by converting non-petroleum-containing fossil fuel by any of the techniques that have long been known in the industry.

When the invention is used in practice in a fully integrated form, i.e. by going from fossil fuel to synthesis gas to methanol to methanol/acetic acid to petrol, possibilities are created for recycling. Thus, parts of the carbon monoxide which is reacted over the methanol synthesis catalyst can be recycled from the outflow from the reaction over the zeolite. In addition, the fossil fuel can be subjected to conversion together with C2 hydrocarbon gas which is recycled from the outflow from the reaction over the zeolite.

It is surprising that the method according to this invention under comparable reaction conditions gives a distribution of product components that hardly varies with variations of the feed materials, which depends on random differences in the ratio between the individual components in the product.

The special zeolite catalysts referred to here are taken from a special group of zeolites which exhibit some unusual properties. These zeolites cause profound conversions of aliphatic hydrocarbons to aromatic hydrocarbons in commercially desirable yields and they are usually very effective in alkylation, isomerization, disproportionation and other reactions involving aromatic hydrocarbons. Although they have an unusually low alumina content, i.e. high silica to alumina ratio, they are very active even at silica to alumina ratios exceeding 30. This activity is surprising since catalytic activity of zeolites is usually attributed to skeletal aluminum atoms and cations associated with these aluminum atoms. These zeolites retain their crystallinity for long periods of time despite the fact that the presence of steam even at high temperatures, which causes irreversible breakdown of the crystal skeleton of other zeolites, e.g. of type X and A. Furthermore, carbonaceous deposits, when such are formed, can be removed

by burning at higher temperatures than usual to renew the activity. In many environments, zeolites of this group exhibit very low coke-forming ability, and this leads to very long flow times between combustion regenerations.

An important feature of the crystal structure for this group of zeolites is that it provides forced access to,

and output from, the inner-crystalline free space due to the fact that it has a pore dimension that is greater than approx. 5 Ångstrøm and pore windows of approx. a size such as would be provided by 10-membered rings of oxygen atoms. It should of course be understood that these rings are those formed by regular arrangement of

the tetrahedron which constitutes the anlonic skeleton in the crystalline aluminum silicate, the oxygen atoms themselves being bound to silicon or aluminum atoms in the centers of the tetrahedra. In short, they have preferred zeolites that are useful as catalysts

this invention, this combination: a silicon <1 >dioxide to alumina ratio of at least 12, and a structure that provides forced access to the crystalline free space.;

The mentioned ratio between silicon dioxide and aluminum oxide can be determined by conventional analysis. This ratio is intended to mean, as closely as possible, the ratio in the solid anionic skeleton of the zeolite crystal and exclude aluminum in the binder or in anionic forms or other forms inside the channels. Although zeolites with silica to alumina ratios of at least 12 are useful, it is preferred to use zeolites having higher ratios of at least approx. 30. After activation, such zeolites acquire an intracrystalline sorption capacity for normal hexane that is greater than for water, ; i.e. they exhibit "hydrophobic" properties. It is believed that this hydrophobic property is advantageous in the present invention.

The zeolites useful as catalysts in this invention readily sorb normal hexane and have a pore size greater than about 5 Angstroms. Moreover, their structures must provide constrained access to some larger molecules. It is sometimes possible to judge from a known crystal structure whether such forced access exists. If, for example, the only pore window in a crystal is formed by 8-membered rings of oxygen atoms, then access for molecules with a larger cross-section than normal hexane is essentially excluded, and the zeolite is not of the desired type. Zeolites with windows of 10-membered rings are preferred, although excessive wrinkling and pore blocking can render these zeolites substantially ineffective overall. Zeolites with windows of 12-membered rings do not generally appear to provide sufficient coercivity to produce the beneficial transformations desired by the present invention, although structures are conceivable which, due to pore-i blocking or other reasons, may be useful.

Rather than attempting to judge from the crystal structure whether or not a zeolite has the required coercivity, a simple determination of the "coercivity index" can be performed by continuously passing equal amounts of normal hexane and 3-methylpentane over a small sample, approximately 1 grams or less, of the zeolite at atmospheric pressure according to the following procedure. A sample of the zeolite, in the form of pellets or extrudate, is crushed to a particle size of approx. the size of coarse sand, and is placed in a glass tube. Before testing, the zeolite is treated with an air stream at 538 oC for at least 15 minutes. The zeolite is then flushed with helium and the temperature becomes

O o 0 > adjusted between 288 and 510 C to give a total conversion of between 10 and 60%. The mixture of hydrocarbons is passed over the zeolite at a liquid-space velocity per hour of 1 (ie 1 volume of liquid hydrocarbon per volume of catalyst per hour) together with a helium dilution to achieve a mole ratio of helium to total hydrocarbon amount of 4:1. After flowing for 20 minutes, a sample of the effluent is taken and analyzed, most conveniently by gas chromatography, to determine the remaining unchanged fraction for each of the two hydrocarbons.

The "forced management index" is calculated as follows:

The forced control index is approximately equal to the ratio between the constants for the degree of crack for the two hydrocarbons. Catalysts which are suitable for the present invention are those which make use of a zeolite having a coercivity index of from 1.0 to 12.0. Values for the forced control index (CI) for some typical zeolites, including some not within the scope of the present invention, are:

The forced control index described above is an important and even critical definition of such zeolites which are useful for catalyzing the present method. The real : nature of this parameter and the discussed technique by which it

is determined, however, makes it possible for a given zeolite to be tested under slightly different conditions and thereby obtain different coercive control indices. The coercive management index seems to vary somewhat with the severity of the execution (transformation).' It will therefore be understood that it may be possible to choose such test conditions that several forced control indices are obtained for a particular given zeolite, which may lie both outside and within the above-defined range of 1 to 12.

It should therefore be understood that when the value for the "coercive management index" is used here, it is an inclusive rather than an exclusive value. That is, a zeolite that is tested at any combination of conditions that falls within the test definition listed above and has a forced control index of 1 to 12 is considered to be included in the definition for the present catalyst regardless of whether it the same identical zeolite when tested under other defined conditions may give a value for the coercive control index that is outside of 1 to 12.

The group of zeolites defined here is, for example, ZSM-5, ZSM-11, ZSM-12, ZSM-21, ZSM-35 and ZSM-38. U.S. Patent Document No. 3,702,886 describes ZSM-5. ZSM-11 is described in U.S. Pat. Patent Document No. 3,709,979. ZSM-12 is described in U.S. Pat. Patent Document No. 3,832,449. French Patent No. 74-12078 describes methods for the preparation of mixtures designated as ZSM-21.

US Patents Nos. 4,016,245 and 4,046,859 be-

type ZSM-35 and ZSM-38 respectively.

The specific zeolites mentioned are, when they are prepared

in the presence of organic cations, generally substantially catalytically inactive, possibly because the inner crystalline free space is occupied by organic cations from the forming solution. They can be activated by heating in an inert atmosphere at 538°C for one hour, for example, followed by base yield with ammonium salts followed by calcination at 538°C in air. The presence of organic cations in the forming solution may not be absolutely essential for the formation of this particular type of zeolite, but the presence of these cations seems to favor the formation of this particular

zeolite type. More generally, it is desirable to activate this zeolite type by base extraction with ammonium salts followed by calcination in air at approx. 538°c for from 15 minutes to approx. 24 hours.

Natural zeolites can sometimes be converted to this zeolite type by various activation processes and other treatments, such as base yield, steaming, alumina extraction and calcination, either individually or in combinations. Natural minerals that can be treated in this way include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite and clinoptiolite. The preferred crystalline aluminosilicates are ZSM-5, ZSM-11, ZSM-12, ZSM-21, ZSM-35 and ZSM-38, and ZSM-5 is particularly preferred.

The zeolites which are used as catalysts in this invention can be in the hydrogen form or they can be base yielded or impregnated so that they contain an addition of ammonium or a meta11 cation. It is desirable to calcine the zeolite after base yield. The metal cations that may be present include any of the cations of the metals from Group I to Group VIII of the Periodic Table. However, when it concerns metals from group IA, the cation content should in no case be so great that it essentially eliminates the activity of the zeolite in the catalysis used in the present invention. For example, a fully sodium-yielded H-ZSM-5 appears to be very inactive in the form-selective transformations required by the present invention.

In a preferred aspect of this invention, as the zeolites which are useful as catalysts here, those which have a crystal skeleton density in the dry hydrogen form of o not substantially below approx. 1.6 g/cm 3. It has been found that the zeolites that satisfy all these three criteria are most desired. The preferred catalysts for use in this invention are therefore those which comprise zeolites which have a coercivity index as defined above of 1 to 12, a ratio between silicon dioxide and aluminum oxide of at least 12 and a dried crystal density of not significantly less than approx. l.6g// cm 3. The dry density for known structures can be calculated from the number of silicon plus aluminum atoms per 1000 cubic Angstroms, as indicated e.g. on page 19 of the article on zeolite structures by W.M. Meier. This paper, the entire content of which is hereby incorporated by reference, is included in the "Proceedings of the Conference on Molecular Sieves, London, April 1967", published by the Society of Chemical Industry, London, 1968. When the crystal structure is unknown, the crystal skeletal density can be determined by classic pycnometer technique. It can, for example, be determined by immersing the dry hydrogen form of the zeolite in an organic solvent that is not sorbed by the crystal. It is possible that the unusually persistent activity and stability of this group of zeolites is connected with their high crystalline anionic skeleton density of no less than approx. 1.6 g/cm 3. This high density must of course be associated with a relatively small amount of free space inside the crystal, and this can be expected to result in more stable structures. However, this free space seems to be important as a geometric site for catalytic activity.

In the following are indicated crystal skeleton densities for some typical zeolites, including some that are not within the scope of this invention:

Organic oxygenates useful in this invention are those whose formula can be written:

where n is the number of carbon atoms in the molecule, p is the number of oxygen atoms in the molecule and m is the number of hydrogen atoms in the molecule. Hardly reformable oxygenates, as the term is used here, are those where the size: is equal to or less than 1. Easily reformable oxygenates, as the term is used here, are those where this size is greater than 1 and they cannot be carboxylic acids.

Thus, short-chain aldehydes (one or two carbon atoms), all carboxylic acids and anhydrides, glycols, glycerol, carbohydrates and polyols, even if they are converted to highly aromatic gasoline as described in the aforementioned BRD-off.-script 2.4 38.252, act on a less satisfactorily with poorer catalyst process life than other compounds do However, these difficult-to-reform feedstocks can be converted to desired product mixtures, especially highly aromatic compounds throughout the gasoline range, in a synergistically better manner if the conversion is carried out as mentioned above but with admixture of easily converted oxygenates, such as alcohols, ethers, esters, long-chain aldehydes, ketones and their analogues.

In a preferred embodiment of this invention, the difficult-to-reform oxygenates consist of monocarboxylic acids and the easily-reformed oxygenates of monohydric alcohols. With a mixture of methanol and acetic acid, the feed should have a mole ratio between the former and the latter that is greater than 1, and most preferably greater than 2.

Carrying out this conversion using a mixed feed as mentioned above not only improves the process life of the catalyst and the yield of products, especially aromatic compounds, in the gasoline boiling range that can be obtained from the difficult to convert reactant, but also actually increases the yield of products, especially aromatic compounds,

in the gasoline boiling range at the expense of the proportion of product normally obtained from the conversion of the desired reactant, e.g. alcohol. Thus, conversion of acetic acid at 260

to 538°c over a ZSM-5 zeolite give a product which in the organ-

ical share comprises C1-aliphatic compounds and aromatic and aliphatic compounds. The catalyst is also coked and charred in a shorter time than expected. Conversion of methanol or dimethyl ether under the same conditions gives excellent yields of hydrocarbon products and exhibits long catalyst life and a slow build-up of little coke on the catalyst. Hydrocarbon pro-

the ducts are mainly in the gasoline boiling range and with some c4_ aliphatic compounds.

It would of course be desirable to carry out this conversion in such a way as to increase the yield of hydrocarbons

in the gasoline-boiling range at the expense of the lighter C^ products. It is in fact an unexpected advantage of the co-conversion of lower alcohols and/or ethers etc. with acids, lower aldehydes and/or carbohydrates that not only the conversion of these latter compounds is improved, but that the ratio between the hydrocarbon product which is petrol and the lighter hydrocarbons has been significantly increased, even with regard to the already high yields of petrol obtained from alcohol and ether conversion.

It is also an important feature of this invention to use multi-component mixtures containing more than one readily converted and/or more than one difficult converted reactant. In reality, it may be most preferable to use a fully mixed feed, such as that obtained by controlled partial oxidation of propane, butane or naphtha in the vapor or liquid phase. Other sources for such mixtures of various light oxygenates include the Fischer-Tropsch process where synthesis gas, carbon monoxide and hydrogen are catalytically converted to a mixture of lower aliphatic organic oxygenated compounds including alcohols, ethers, aldehydes, ketones, etc.

We now return to what we have called "integrated" embodiments of the invention, and it can be said that it has been common industrial practice to carry out commercial methanol synthesis conversions up to a conversion of approx. 15 to 20 percent, separate unreacted carbon monoxide and hydrogen from the methanol formed and recycle the unreacted reactants to produce additional amounts of methanol. In our previously discussed process for making petrol, this methanol is then separately converted into petrol.

According to the present embodiments, the methanol synthesis product, comprising methanol and unreacted carbon monoxide, is not dissolved to recover the methanol, but is contacted with a carbonylation reaction catalyst at carbonylation temperatures to react some of the methanol with the unreacted carbon monoxide to form a mixture comprising methanol and acetic acid (and possibly other components such as methyl acetate). This mixture is then converted into a complex mixture of hydrocarbons including C₁- to C₁-monocyclic aromatic hydrocarbons by contact with the zeolite.

Conversion of carbon in the carbon monoxide reactant

to carbon in the C - to C. -monocyclic aromatic hydrocarbon-

6 10

products are increased by proceeding in accordance with this invention (with an intermediate carbonylation step) compared to recycling the carbon monoxide to be destroyed by methanol synthesis and aromatizing the methanol alone. All the components in the effluent from the carbonylation reaction are individually reformable over the zeolite catalysts into high-quality hydrocarbon gasoline, however, the mixture is reformable into a product that has an unexpectedly higher proportion of aromatic hydrocarbons than could be predicted from a consideration of the conversion yields that are -available from the individual reactants. Coal gasification to synthesis gas is usually carried out with oxygen and/or steam at approx. 538 to 1093°C, while the conversion of natural gas is carried out at approx. 538 to 831°C. The conversion of synthesis gas into a product comprising methanol is also well known commercial technology. This turnover, called metariol synthesis, is carried out at approx. 232 to 399°C and at 42 to 422 kg/cm 2 overpressure using a catalyst comprising zinc and/or copper. The reaction of methanol with carbon monoxide to produce acetic acid is known per se. It is carried out at approx. 149 to 42 7°C and at approx. 1.05 to 70.0 kg/cm 2 overpressure. Acid catalysts, such as phosphoric acid or boron trifluoride, have been used, and there are also rhodium salts in heterogeneous or homogeneous form together with an iodine carrier.

The conversion of methanol, acetic acid, methyl acetate, methyl formate or dimethyl ether is in itself known to be carried out at approx. 260 to 649°C using the special zeolite catalyst described above.

In contrast to the desired goal in the established carbonylation industry, in the carbonylation step it is not important how much acetic acid is formed or how pure it is, but rather how much total carbon from the fed synthesis gas has been bound to organic products. It is preferred that the molar ratio between methanol and acetic acid is kept at at least 12 in the carbonylation product. further, the description of a commercial embodiment using a rhodium-catalyzed carbonylation of methanol to form acetic acid, which description is found in the October 1971 issue of Chemical Technology, shows the use of hydroiodic acid as a carrier to convert methanol- the reactant of methyl iodide, and where the latter is proposed to be the real reactant. In the event that a halogen moiety is used as a promoter, as suggested, the methyl halide intermediate should be understood to be easily reformable over the special zeolite catalyst thereof to aromatic hydrocarbons. Thus, the yield of desirable C~ to C~Q monocyclic aromatic hydrocarbons will be further increased even if some of this carrier remains with the acetic acid-containing carbonylation product when such is fed to special zeolite conversion.

Comparison of the conversion of carbon monoxide to r 6 - to C1,0-monocyclic aromatic hydrocarbons via methanol synthesis and the special zeolite conversion based on recycling unreacted carbon monoxide to methanol synthesis and based on a carbonylation unit process as described here shows a remarkable advantage for the latter process. The carbon yield for such aromatic compounds is increased from approx. 30

to approx. 80% in the execution of this invention.

It can be attributed to this invention that.that product

which is finally formed by conversion with the special aromatizing zeolite catalyst described above, comprises water and C^~ to approx. C^-hydrocarbons throughout the range. It is common for this product to have a stoichiometric distribution of oxygen and carbon in the feed, that is to say that essentially all the oxygen in the organic part of the feed is converted to water and essentially all the carbon in the organic part is converted to hydrocarbons . Some small amounts of carbon oxides can be formed, but these are in pollution concentrations.

If the conversion of the organic feed is less than

100%, the distribution of carbon and oxygen seems to be generally substantially proportional. Of the hydrocarbon product, the main amount is made up of the usually liquid fraction with C 5+ and the smaller amount is made up of the usually gaseous fraction of C4~- Of the C^+ fraction, the main amount is monocyclic aromatic hydrocarbons, C, to C.^- It it is possible to increase the olefin proportion of the product to 6 10

expense of aromatic compounds by increasing space velocity or by other means discussed elsewhere. In this situation, 1

where a strong olefin-containing product is desired, it is preferred to operate with the higher ratios between methanol and acetic acid. Alternatively, lower ratios of methanol to acetic acid appear to favor the formation of aromatic compounds.

These aspects of this invention will be illustrated in the following examples in which parts are parts by weight and percentages are parts by weight unless otherwise expressly stated. The following table sets out the results of four comparative tests which were carried out side by side under essentially identical conditions. The temperature was 371°C, the pressure was 1 atmosphere, the space velocity was 1 LHSV and the catalyst was H-ZSM-5 with 35% A^O^ as binder.

In the following example 5, a mixture consisting of 57% C, -C alkanols, 3% C 1 -C 1 -alkanes, 11% C 1 -C -alkanones

lb 2. 4 od

and 29% C2-C5 alkanoic acids converted to a mixed hydrocarbon product, as indicated, using a HZSM-5 catalyst. The conditions were 1 atmosphere pressure, 1 LHSV, 371°c and 3 hours total flow time.

Claims (4)

1. Process for transforming a material containing aliphatic oxygen compounds with the formula C H<_>~. pH-O J3 n m-2p r 2 where n is the number of carbon atoms in the molecule and has a value of 1-6, m is the number of hydrogen atoms in the molecule and p is the number of oxygen atoms in the molecule, to a product containing aromatic hydrocarbons at temperatures of 260-649 °C under pressure at a liquid-space velocity per hour of 0.5-50, in the presence of a crystalline aluminosilicate zeolite having a ratio between silicon dioxide and aluminum oxide of 12-3000 and a forced control index of 1-12, as a catalyst, characterized in that it. a material is used which comprises a molar excess of such aliphatic oxygen-containing compounds where the ratio (m~2p) exceeds 1, and a residual material of such aliphatic oxygen-containing compounds where the ratio (mn~2P> is 1 or less, 2. Method as stated in claim 1, characterized in that a molar excess comprising a monohydric alcohol is used and that the rest of the material comprises a carboxylic acid. 3. Method as stated in claim 1, characterized in that a molar excess is used which comprises a monohydric alcohol and that the remainder comprises an aldehyde with less than 3 carbon atoms. 4. Method as stated in claim 2 or 3, characterized in that an excess comprising methanol is used and the remainder comprises acetic acid and/or acetaldehyde.