This invention relates to a method, apparatus and catalyst ffoorr ffoorrmmiinngg CC _?? hhyyddrrooccaarrbboonnss,, ii..ee.. eetthi ylene or ethane and ethylene from methane or natural gas

Background of the invention

The oxidative dimerization or "coupling" of methane to C hydrocarbons has been the object of numerous papers, patents and reviews during the last fifteen years (e.g. Keller and Bhasin 1982, Hinsen and Baerns 1983, Ito and Lunsford 1985, Lee and Oyama 1988, Jones et al 1986, Otsuka et al 1990, Eng and Stoukides 1991, Tsiakaras and Vayenas 1993, and references therein).

Several catalysts have been found which give selectivity to C_ hydrocarbons, hereafter denoted by S_ , higher than 90% at low 0C2%) CH, conversion, hereafter denoted by C-,-, . However it has been universally found (e.g. Keller and Bhasin 1982, Hinsen and Baerns 1983, Ito and

Lunsford 1985, Lee and Oyama 1988, Jones et al 1986, Otsuka et al 1990, Eng and Stoukides 1991, Tsiakaras and Vayenas 1993, and references therein) that the se¬ lectivity S _c decreases rapidly with increasing conversion C_ _u , so that the C„ hydrocarbon yield, hereafter denoted ^H _, ^ά by Y_ , which equals the product of methane conversion CC- _u H ₄ tim2es the selectivity SC_ ₂ had been always found to be less than 30%.

In a recent paper Aris and coworkers (Tonkovich, Carr

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and Aris 1993) have shown that Y_ can be increased up to 50% by using a Sm _? 0-. catalyst 2 in a simulated coun- tercurrent moving bed chromatographic reactor (SCMBCR).

The ethylene yield Y_ _u was up to 15% and the ethane yield Y _r „ up to 30% ^C 27 ^{H C} 2 ^H 6

oxygen than methane and therefore get easily converted to CO/CO- when their concentration becomes comparable to that of methane, i.e. for high methane conversions.

Consequently the observed improved C„ yield ( 50%) in the case of the simulated countercurrent moving bed chro¬ matographic reactor (SCMBCR, Tonkovich, Carr and Aris 1993) is due to the partial separation and removal of C _? hydrocarbons from the CH, . The same applies for the coun- tercurrent moving bed chromatographic reactor (Tonkovich, Carr and Aris 1993) .

The present invention provides the means for totally a- voiding the problem of the high reactivity of C~ hydro- carbons by trapping them quantitatively in a suitable adsorbent or absorbent material maintained at a lower temperature and thus permits obtaining, as shown in the examples, C_ hydrocarbon yield Y _r in excess of 80% and, more importantly, ethylene yield 2 _r „ in excess of 75%.

^C 2 ^H A

Brief description of the invention

This invention provides a means for converting methane or natural gas via partial oxidation to ethylene or etha- ne and ethylene and comprises a number n (n 1) of units each consisting of (I) a catalyst or catalytic reactor (II) an oxygen feedthrough and (III) a suitable adsorbent

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cr absorbent material maintained at a temperature signi¬ ficantly lower than that of the catalyst for cntinuous- ly and quantitatively trapping ethane and ethylene. The n units are connected in series and any portion of the gas leaving each unit can be recirculated, if desirable, to any previous unit via a recirculation pump.

The catalyst or catalytic reactor may comprise catalyst particles in a fixed or fluidized bed or it may comprise catalytic material coated on the reactor walls.

The oxygen feedthrough is necessary in each unit to en¬ sure that the methane to oxygen ratio in each catalyst unit can be controlled and maintained at desired levels. In general high methane to oxygen ratios favour high se¬ lectivity to C~ hydrocarbons and to ethylene. Oxygen is supplied either via a gas feedthrough or electrochemi- cally via a solid electrolyte cell (Vayenas et al 1990, 1992 and Tsiakaras and Vayenas 1993).

The adsorbent or absorbent material can be in the form of granules in a fixed or fluidized bed and forms a mo¬ lecular sieve trap for ethane and ethylene. Two or more molecular sieve traps are used interchangeably in each unit so that one is trapping ethane and ethylene at a lower temperature (typically 25 to 100 C) while the other is releasing it at a higher temperature. Alternatively the trapping material is continuously recirculated be¬ tween the trapping zone which is maintained via cooling at a lower temperature (typically 25 to 100°C) and a de- sorption zone where the trapped ethane and ethylene are released via heating.

Brief description of the drawings

Fig. 1 Shows schematically an apparatus suitable for

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carrying out the process of this invention. The feed consists of methane or a methane-oxygen mix¬ ture. R ( 1 i<n) denotes a catalytic reactor, TR denotes a molecular sieve trap containing adsorbent or absorbent material .

Fig. 2 Shows schematically another version of the appa¬ ratus shown in Fig. 1 with movable molecular sieve traps.

Fig. 3 Shows schematically another version of the appa¬ ratus shown in Figures 1 and 2 with recirculating adsorbent or absorbent solid material.

Fig. 4 Shows a fluidised bed recirculating solid appa¬ ratus which is also suitable for carrying out the process of this invention. The adsorbent or absorbent material is being continuously recir¬ culated. The catalyst material is fixed on the reactor walls cr is also recirculated continu¬ ously.

Fig. 5 Shows schematically the version of the experimen¬ tal apparatus shown in Fig. 1 used to obtain the results presented in the Examples and in subse¬ quent figures; 6PV: six-port-valve; G.C.: gas chromatograph; T.C.: Thermocouple; MST: molecular sieve trap, APV: Four-port-valve, FCR: Fuel cell reactor, : working electrode; C: counter elec- trode; R: reference electrode; P/G: potentiostat- galvanostat, 3WV: three-way-valves, R.P.: recircula- tion pump, B.F.: bubble flowmeter. Numbers and symbols are also explained in Example 1.

Fig. 6 Shows typical results of the observed dependence

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of C„ selectivity and C„ yield on methane conver- sion for gas-phase supplied 0_ (F _n = 0.03 cm 3 STP/ min, triangles, example 2) and elec 2trochemically supplied 0„ (1= 7mA, squares, example 1); recir- culation flow rate 220 cm STP/min, T= 835°C, ini¬ tial methane partial pressure 20 kPa.

Fig. 7 refers to Example 3 and shows typical results of the dependence of C„ selectivity and C„ yield on CH, conversion at various levels of electrochemi¬ cal supply of oxygen I/4F, where I is the applied current.

Fig. 8 refers to Example 3 and shows typical results of the dependence of the selectivity to ethylene S _{r u}

°2 ^H 4 and to ethane S _r „ on the CH, conversion.

^L 2 ^M 6

Fig. 9 refers to Example 4 and to a pure Ag catalyst and depicts the dependence of C_ selectivity and yield on CH, conversion.

Fig. 10 refers to Example 5 and compares the performance of Y_0_-stabilized-Zr0 (triangles) and α-Al _p O_ (circles) as supports for a 20%wt Sm _p O.,-80% Ag ca- talyst. The figure shows the effect of CH, conver¬ sion on C _p selectivity and yield.

Fig. 11 refers to Example 6 and shows the dependence of

C _p selectivity and yield and of ethylene yield on methane conversion for low values of applied cur¬ rent (I=5mA) .

Detailed description of specific embodiments

This invention provides a method and an apparatus for

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converting methane or natural gas to ethylene or to C„ hydrocarbons, i.e. ethane and ethylene, which involves one or more successive contacts of a methane-oxygen-in¬ ert gas gaseous mixture with: a) A catalyst or electrocatalyst suitable for producing C _p hydrocarbons from methane. b) A suitable adsorbent or absorbent material which can selectively trap ethane and ethylene, thus separating them from unreacted methane and oxygen. As shown in Figs. 1, 2 and 3 each basic unit of the apparatus contains, in addition to the catalytic or electrocatalytic reactor and the two, or more, interchangeable molecular sieve traps, and an oxygen feedthrough for addition of supple¬ mentary oxygen.

The catalyst can be in the form of pellets, or extruda- tes, in a fixed bed reactor or fine particles in a flui¬ dized bed reactor (Fig. 4) or can be in the form of a film coated on the reactor walls. In this case the reac-

2 tor walls can be fabricated from an 0 -conducting solid electrolyte, such as yttria-stabilized-zirconia and oxy¬ gen can be supplied through the walls via application of a voltage between the catalyst film and a counter electrode film. In this case the voltage generated spon- taneously between the catalyst film and the counter el¬ ectrode, due to different oxygen activities, can be used

2 to transport oxygen (Vayenas et al 1992). Mixed 0 -elec¬ tronic conductors can also be used.

The catalyst material can be any solid material which catalyzes the oxidative coupling of methane (OCM) to e- thane and ethylene (e.g. Keller and Bhasin 1982, Hinsen and Baerns 1983, Ito and Lunsford 1985, Lee and Oyama 1988, Jones et al 1986, Otsuka et al 1990, Eng and Stou- kides 1991, Tsiakaras and Vayenas 1993). We have found that Ag or Ag mixed with 1%Ca0-doped-Sra_0-, is an excellent

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catalyst for the OCM reaction at high (larger than 2 to 1), CH, to O _p ratios as these material, give S_ selec¬ tivity values above 95% for low CH, conversions. Since these materials are also highly cnductive, and therefore can, whenever desirable, be used in conjunction with Y _p O_-stabilized-ZrO _p as well, as electrochemical oxygen feedthroughs, these materials are particularly suitable for the present invetion. The invention, however, refers to any material capable of catalyzing the OCM reaction.

The adsorbent or absorbent material in this invention can be any material capable of trapping selectively ethylene or ethane and ethylene without trapping methane and oxy¬ gen to any significant extent. We have found that mole- cular sieve 5A is very well suited for the present in¬ vention as it traps effectively ethane and ethylene at

_Q temperatures below 100 C and releases them reversibly at temperatures above 300 C. By using suitable adsorbent materials which trap ethylene more efficiently than etha- ne, such as e.g. molecular sieve 5A, it is possible to obtain very high ethylene yield values X _r „ (^75%) as shown on the Figures and Examples. 2 4

The present invention differs from all previous methods and apparatuses used to convert methane to ethylene or ethane and ethylene in that the gas mixture is brought continuously in repeated and successive contact with a catalyst at a high (>500 C) temperature and an adsorbent or absorbent material at a low «100 C) temperature with intermediate continuous addition of oxygen so that the desired product ethylene or the desired products ethane and ethylene are trapped quantitatively in the adsorbent or absorbent material and thus are not allowed to further react with oxygen and form carbon monoxide and carbon dioxide.

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The main differences of the present invention from the countercurrent moving bed chromatographic reactor (CMBCR) of Aris and coworkers (Tonkovich, Carr and Aris 1993 and references therein) are the following:

I. In the present invention the catalyst and the adsor¬ bent are not mixed as in the CMBCR and the adsorbent always operates at temperatures much lower (e.g. by 600 C lower) than that of the catalyst, which again does not apply to the CMBCR.

II. In the present invention, with the exception of Fig. 4 and claims 7, 8, 9 and 10 discussed at the end of this paragraph, the catalyst and the adsorbent are at fixed positions, contrary to the CMBCR where they move. Figure 4 and claims 7, 8, 9 and 10 refer to a recirculating solid fluidized bed reactor which is again totally different from the CMBCR.

III. In the present invention the feed gas (methane or methane plus oxygen) is supplied at one end of the reactor and not in the middle of the reactor as in the CMBCR. The present invention also involves side feed of oxygen along the reactor length which is not the case with the CMBCR.

IV. The present invention, with the exception of Figure 4 and claims 7, 8, 9, 10 discussed above, consists of discrete units and not of a continuous moving bed.

V. In the present invention the gases are continuously recycled .

The main differences of the present invention from the simulated countercurrent moving bed chromatographic re¬ actor (SCMBCR) of Aris and coworkers (Tonkovich, Carr and Aris 1993) are the following:

I. In the present invention the gas feed is constant

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and continuous and not switched between various units and interrupted as in the case of the SCMBCR.

II. All catalyst beds are continuously in contact with reacting gas and therefore are in continuous opera- tion, contrary to the SCMBCR.

III. Product removal is carried out by raising the adsor¬ bent temperature and thus no large volumes of carrier gas are needed.

As a result of all the above main differences the present i Y v

The following working Examples provides a more detailed description of the invention.

Example 1

A porous catalyst film of total mass 150 mg consisting of 20wt% Sπi _p O- doped with 1 wt% CaO and of 80 wt% Ag was deposited on the inside wall of an 8 mol% Y _p 0_-stabilized- ZrO _p (YSZ) tube closed flat at one end as shown on Figure

5. Refering to Figure 5 the catalyst film 1 had a super-

2 ficial area of 10 cm . Two Ag films (denoted 2 and 3) were deposited on the outside walls of the YSZ tube and acted as counter and reference electrodes respectively.

The YSZ tube together with the Ag-Sm _p 0_-Ca0 catalyst thus formed the fuel-cell type catalytic reactor 4.

A recirculation pump 5 was used to recirculate the gas feed mixture comprising initially 20 mol% CH, in He be¬ tween the YSZ reactor and the absorbent molecular sieve trap 6 containing 5 gr of molecular sieve 5A. In addition to the catalytic reactor, the molecular sieve trap and the recirculation pump, the recirculation loop also

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comprised a 4-port-valve 7 and a 6-port-valve 8 with a gas-sampling-loop 9. By appropriate setting the needle- valves 10 and 11 the apparatus could be operated either as a continuous flow reactor with gas recycle or as a batch reactor with recirculation. In this example the batch mode of operation with recirculation was chosen. The oxygen was supplied electrochemically by applying a constant current I=7mA via a galvanostat-potentiostat between the catalyst film 1 and the counter electrode 2 Thus oxygen is transported from the ambient air to the catalyst at a rate I/4F, where F is Faraday's constant, equal to 1.8*10 ^" mol 0„/s. The Ag-Sm_0 ₃ -Ca0 catalyst films is maintained at T=835 C and now also acts as an electrocatalyst (Vayenas et al 1992, Tsiakaras and Va- yenas 1993) and catalyzes the partial oxidation of CH, to ethane and ethylene which are being continuously trapped in the molecular sieve trap 6 which is maintai¬ ned at T=30°C. The total volume of the recirculation

3 loop is 50 cm .

After operating the unit for a time t typically 5-300 min, the four-port-val e 7 is used so that the reactor is isolated from the loop and the temperature of the mo¬ lecular sieve trap is raised to T=400 C thus causing de- sorption of the adsorbed ethane and ethylene. The com¬ position of the gas present in the loop is then deter¬ mined via the gas chromatograph 12 and the methane con¬ version C and the y the measured gas composition. The carbon mass closure is then also checked between the initial and final compo¬ sition. By repeating the above experiment for various operating times t one varies the CH, conversion and thus obtains the results shown on Figure 6 (squares). Increa- sing methane conversion from zero to 95% causes a de¬ crease in C _p selectivity from 95% to 65% and an increase in C- yield from zero to 60%.

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Example 2

The same apparatus, catalyst and experimental procedure described in Example 1 is used but in the present case oxygen is not introduced via the YSZ solid electrolyte but from the gas phase via the needle valve 13 (Figure 5) at a flow rate of 3*10 -2 cm3 STP/min or equivalently

—8 2.2»10~ mol 0_/s, i.e. very similar to that used in

Example 1. The results are shown on Figure 6 (triangles) _a nd are almost identical to those obtained in Example 1.

Maximum C _p yield is again 60%.

Example 3

The same apparatus, catalyst and experimental procedure described in Example 1 was used and the effect of applied constant current I was investigated. As shown on Fig. 7 increasing I, i.e. increasing the rate I/4F of oxygen supply causes a small decrease in C~ selectivity and yield. This is also shown on Figure 8 which shows addi¬ tionally the variation in the selectivity to ethylene

S„ _u and to ethane S- „ . Increasing conversion of CH, v ^C ia2 6increasing operatio2n ^H 4time causes a decrease in S_ „ because ethylene is well known to form via oxidative 2 6 dehydrogenation of C _? H, which is the primary product of the OCM reaction. It is clear from this Figure that the C_H, formed in the reactor is not trapped 100% in the molecular sieve trap but some of it recirculates and forms C_H, . The results shown on Fig. 8 for high CH, conversions are of significant practical importance, since C-H, is more valuable than C-H, .

Example 4

The same apparatus and experimental procedure described in Example 1 is used but in this case the catalyst is pure Ag (mass 129 mg, apparent surface area 18 cm )

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deposited on the YSZ solid electrolyte. The results are shown on Figure 9 for three different values of the applied current I. Maximum C„ yield is 59%. This shows in conjunction with example 1 that the addition of Sm _? 0- (1% CaO) has only a small beneficial effect in the Ag catalyst performance.

Example 5

The same apparatus, catalyst and experimental procedure of Example 2 is used and two different catalyst supports are examined under conditions of gas-phase oxygen addi¬ tion: Yttria-stabilized-zirconia (YSZ) as in Example 2 (triangles Figure 10) and α-Al _p 0_ (circles, Figure 10). Maximum C„ yield is again near 60% and the catalyst sup¬ port plays only a minor role in catalyst performance with YSZ being slightly better than α-Al _p O-.

Example 6

The same apparatus and experimental procedure described in Example 1 is used but in this case, as in example 4, the catalyst is pure Ag (mass 129 mg, apparent surface area 18 cm ) deposited on the YSZ solid electrolyte. In the present case a small current is applied (I=5mA). The molecular sieve trap contains 5 gr of molecular sieve 5A, Figure 11 shows the observed dependence of C _p selecti¬

8 as the methane conversion increases from zero to 93% correspondingly. The C _p yield increases from zero to 81% while the ethylene yield increases from zero to 78%. These are by far the highest C„ and ethylene yields re¬ ported in the open or patent literature for the OCM re- action.

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Literature cited

Keller, G.E. and Bhasin, M.M., J.Catal. 7_3, 9 (1982)

Hinsen, W. and Baerns, M. , Chem.Z. 107, 223 (1983)

Ito, T. and Lunsford, J.H., J.Phys.Chem. 8_7_, 301 (1983)

Jones, C.A., Leonard, J.J. and Sofranko, J.A., U.S. Pa- tent 4,567,307 (1986)

Lee, J.S. and Oyama, S.T. Catal .Rev .-Sci .Eng. 3_0_(2), 249 (1988)

Otsuka, K., Suga, K. and Yamanaka, I., Catal.Today 6_, 587 (1990)

Eng, D. and Stoukides, M. , Catal .Re .-Sci .Eng. _3_3- 375 (1991)

Tonkovich, A.L., Carr, R.W. and Aris, R., Science 262 , 221 (1993) and references therein.

Vayenas, C.G., Bebelis, S. and Ladas, S., Nature (London) 3_4_3, 625 (1990)

Vayenas, C.G., Bebelis, S., Yentekakis, I.V. and Lintz, H.-G. Catalysis Today JJ_(3), 303 (1992) and references therein.

Tsiakaras, P. and Vayenas, C.G., J.Catal. 14 , 333 (1993)

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