NO178956B - Hydrocarbon synthesis catalyst and process for producing hydrocarbons from synthesis gas - Google Patents

Description

The present invention relates to a hydrocarbon synthesis catalyst and a method for producing hydrocarbons from synthesis gas. More particularly, the invention relates to a catalyst comprising cobalt on at least a predominantly titanium dioxide support, to which are added small amounts of silicon dioxide. The resulting catalyst is more active in terms of carbon monoxide conversion than catalysts that do not contain the added silicon dioxide and makes it possible to work with reduced temperatures in the hydrocarbon synthesis and thereby a reduction in methane selectivity and simultaneous increases in C5<+-> yield.

Methane is available in large quantities in many areas of the world. Some methane is generated from refining applications, while large amounts of methane, as the main component of natural gas, are found in deposits in various areas. Methane can be used as a gas, e.g. for heating purposes, and can be transported by pipeline or as liquefied gas over long distances. Where the use of methane as a gas is not economical, or the transport of methane requires the crossing of ocean areas, the methane can be converted into a liquid that is easier to transport, and can have a significantly higher value than methane gas.

Conversion of methane is normally carried out in a two-step process comprising reforming the methane to produce hydrogen and carbon monoxide, synthesis gas, and converting the synthesis gas to higher hydrocarbons, C5<+>, in a Fischer-Tropsch type reaction. Both steps in the method are well known, and can be easily illustrated: In the first step of US patents 1,711,036, 1,960,912 and 3,138,438, and the second step of US patents 4,477,595, 4,542,122; and 4,088,671.

The present invention relates to the second step, the well-known Fischer-Tropsch reaction, which is hereinafter referred to as hydrocarbon synthesis.

The present invention primarily relates to cobalt-containing catalysts for hydrocarbon synthesis, which are described to be useful in such reactions, either alone or together with other materials. What is not described in the art is the combination of steps required to produce a mixture that is new and has better catalytic activity properties compared to other cobalt-containing catalysts. These properties include: improved CO conversion, improved volumetric productivity, increased selectivity to C§<+ >and lower Cl^ production. US patent 4,542,122 describes a cobalt or cobalt-thorium oxide on titanium dioxide catalyst for hydrocarbon synthesis with a preferred ratio of rutile to anatase. US Patent 4,568,663 describes a high activity hydrocarbon synthesis catalyst comprising cobalt and rhenium on a titanium dioxide support. US patent 4,413,064 describes a catalyst on an alumina support with cobalt, ruthenium and a metal oxide of the group HIA or - the group I VB, e.g. thorium oxide. EP patent application 86/180,269A describes a cobalt catalyst on a silicon dioxide support to which is added a silicon compound addition of the catalyst metal.

A significant increase in the activity of cobalt catalysts on a titanium dioxide support can be achieved when silica is added to the catalyst. For the purposes of the present invention, activity can be measured as cobalt time yield (defined as the number of moles of carbon monoxide converted per hour per gram atom of cobalt in the catalyst), or as cobalt position-time yield (defined as the number of moles of carbon monoxide converted per hour per surface cobalt atom in the catalyst) . In both cases, activity increases above 40$, preferably above 50$, can be achieved by using the present invention.

According to the present invention, there is provided a hydrocarbon synthesis catalyst comprising cobalt in a catalytically active amount and possibly rhenium compounded with an inorganic refractory carrier comprising a greater proportion of titanium dioxide, characterized in that the catalyst includes up to 15 wt-# of the catalyst of silicon dioxide added to the catalyst as a silicon dioxide -precursor and decomposed to form the silicon dioxide.

Furthermore, according to the invention, a method for the production of hydrocarbons from synthesis gas is provided whereby hydrogen and carbon monoxide are reacted under hydrocarbon synthesis conditions over a catalyst, and this method is characterized by the use of a catalyst as defined above and in accompanying claim 1 or 2. Fig. 1 shows the weight change and rate of weight change for catalysts A, B and C at atmospheric pressure. The TG curve determined weight changes when the amount of cobalt oxide on a support is reduced in hydrogen from room temperature to 500°C at 6°/min. The DTG curve plots the rate of weight change with time as a function of temperature. Fig. 2 shows the effect on catalysts A and B of carbon enrichment, i.e. the tendency for carbon to grow on active positions, in a synthesis gas environment. Catalyst B shows an increase in weight due to carbon growth at a much higher temperature than for catalyst A, and shows that the presence of silicon dioxide reduces carbon growth.

It is known that neither silicon nor silicon dioxide promotes hydrogenolysis of coke, or carbon deposits and the increase in activity cannot be explained on the basis of hydrogenation of carbon deposits on catalysts. Due to the fact that the increase in activity occurs immediately during the hydrocarbon synthesis, the silicon oxide can react with, or somehow bind up special positions on the surface of the carrier, titanium dioxide, which would otherwise promote the formation of carbon. The presence of silicon dioxide can therefore act to retard carbon formation on the catalyst. Indirect evidence suggests that the silicon dioxide exists in close proximity to cobalt and may even block some cobalt positions due to that the degree of cobalt dispersion, after the addition of the silicon dioxide, is somewhat less than the cobalt dispersion without the addition of silicon dioxide. Regardless of the theoretical explanation for the increased catalyst activity, the present invention is based on the presence of sufficient silicon dioxide to increase the catalyst activity over and above a catalyst without added silicon dioxide.

EP patent application 86/180269Å describes the addition of a silicon-containing compound to cobalt-containing catalysts on a silicon dioxide support and an increase in catalyst activity is indicated. It is stated that the siliceous compound prevents the subsequently applied cobalt from reacting with silicates present as a result of the use of a silica support and, therefore, is more available for use as a catalytic metal in hydrocarbon synthesis. The addition of the silicon-containing compound prevents the formation of hydroxy-silicates which effectively bind some of the cobalt metal.

The present invention uses a titanium dioxide-containing carrier and does not use a silicon dioxide-containing carrier, and the explanation for the increased activity in the aforementioned application does not apply to the present invention. Cobalt does not bind titanates in the way it binds to silicates in a silicon dioxide-containing carrier and chemisorption measurements show that the amount of available cobalt actually decreases when silicon dioxide is added to the catalyst according to the present invention.

Based on what is believed to happen in the present invention, any amount of silicon dioxide added to the catalyst will improve the catalyst activity. On the other hand, too much silica should not be added, because available cobalt is reduced sufficiently to counteract the benefit of adding silica, e.g. available cobalt can be reduced by masking with the silicon dioxide. Cobalt can also bind to silicates if too much silicon dioxide is added.

As mentioned, the silicon oxide additions are up to 15% by weight of the catalyst, and are preferably in the range 1-10% by weight, and more preferably 3-7% by weight.

Silica may be added to the catalyst with any suitable compound which will result in SiC>2 on decomposition, e.g. as an alkoxide solution (tetraethyl orthosilicate in methanol). Other precursors for silicon dioxide which are applicable in the present invention are described in EP patent application 86/180,269A.

The silicon dioxide or silicon dioxide precursor can be added to the support either before or after the addition of catalytic metals. Both methods will give the results described here, although addition of silicon dioxide or silicon dioxide precursor after addition of catalytic metals is preferred.

Catalysts that can be used in the present invention comprise cobalt or cobalt and thorium oxide on an inorganic oxide carrier containing a major amount of titanium dioxide. The catalyst may also contain a promoter metal, preferably rhenium, in an amount sufficient to provide a catalyst with a weight ratio of rhenium to cobalt greater than 0.01 to 1, preferably 0.025:1 to 0.1 to 1. The catalyst contains from 2 to 25 wt # cobalt, preferably from 5 to 20 wt # cobalt.

In general, the hydrocarbon synthesis reaction is carried out under conditions known in the art. The It:CO ratio is at least 0.5 up to 10, preferably from 0.5 to 4.0, and more preferably from 1.0 to 2.5. The gas chamber velocity per hour can vary from 100 to 5,000 v/hr/v, preferably from 300 to 1500 v/hr/v and the reaction temperatures can vary from 160 to 3300°C, preferably from 190 to 260°C, while the pressures are above 5.6 kg /cm<2> man.pressure, preferably from 5.6 to 42 kg/cm<2 >man.pressure, more preferably from 9.8 to 28 kg/cm<2>. The hydrocarbon synthesis results in the formation of hydrocarbons having carbon numbers ranging from C5 to C40 or higher. Preferably, the synthesized hydrocarbons are primarily or almost exclusively paraffins. While the reaction temperatures are generally in the range accepted for this type of reaction, it can be in the lower regions of this range, thereby reducing the amount of methane formed during the reaction.

An inorganic, refractory oxide is used as a carrier for the catalytic metals, comprising a major part of titanium dioxide, although other materials, e.g. aluminum oxide, may be present. Preferably, the support material is titanium oxide, and more preferably, the titanium oxide has a ratio of to and assumed to be at least 2:3, as determined by X-ray diffraction (ASTM D2730-78), preferably 2:3 to 100, or higher, more preferably from 4:1 to 100:1 or higher; e.g. 100$ rutile. The surface area of the carrier is generally less than 50 m<2>/g (BET).

Rhenium-cobalt/titanium dioxide catalysts exhibit high selectivity in the synthesis of liquid hydrocarbons from carbon monoxide and hydrogen. The catalysts used in the implementation of the present invention can be produced using techniques that are known in the field for the production of other catalysts. The catalyst can e.g. produced by gelation or cogelation techniques. However, the metals can be suitably deposited on a carrier material which has been prepared in advance in the form of pills, pellets, beads, extruded or sieved using the impregnation method. In the manufacture of catalysts, the metals are deposited from a solution on the carrier in pre-selected amounts to give the desired absolute amounts, and weight ratio between the respective metals, cobalt and rhenium. Cobalt and rhenium are conveniently combined with the support by contacting the support with a solution of a cobalt-containing compound, or salt, or not; rhenium-containing compound or salt, e.g. a nitrate, carbonate or the like. Optionally, cobalt and rhenium can be co-impregnated on the carrier. The cobalt and rhenium compounds used in the impregnation can be any organometallic or inorganic compounds which decompose to give cobalt and rhenium oxides on calcination, such as e.g. a cobalt or rhenium nitrate, acetate, acetylacetonate, naphthenate, carbonyl or the like. The amount of impregnation solution must be sufficient to completely wet the support, usually within the range from 1 to 20 times the volume of the support, depending on the concentration of the metal or metals in the impregnation solution. The impregnation treatment can be carried out under a wide range of conditions including ambient or elevated temperatures.

After the impregnation, the catalyst is dried by heating to a temperature above 30°C, preferably between 30 and 125°C, in the presence of nitrogen or oxygen, or both, or air, in a gas stream or under vacuum. It is necessary to activate the cobalt-titanium dioxide and the improved cobalt-titanium dioxide catalysts before use. Preferably, the catalyst is brought into contact with oxygen, air or another oxygen-containing gas at a temperature sufficient to oxidize cobalt, and convert cobalt to CO 3 O 4 . Temperatures above 150°C, and preferably above 200°C are satisfactory for converting cobalt to the oxide, but temperatures up to 500°C which can be used in regeneration of severely inactivated catalyst can generally be tolerated. Suitably, the oxidation of cobalt is achieved at temperatures varying from 150 to 300°C. Cobalt or cobalt and rhenium metals in the catalyst are then reduced. The reduction is carried out by contacting the catalyst, whether it is oxidized in advance or not, with a reducing gas, suitably with hydrogen or a hydrogen-containing gas stream at temperatures above 200°C, preferably above 300°C. Suitably, the catalyst is reduced at temperatures ranging from 200 to 500°C, and preferably from 300 to 450°C, for periods ranging from 0.5 to 24 hours at a pressure ranging from ambient to about 40 atmospheres. Hydrogen, or a gas containing hydrogen and inert constituents in mixture, is satisfactory for use in carrying out the reduction.

Examples

Catalyst manufacture ing

C0Re/ Ti02 fcatalvsator Al

Degusa P25 TiOg was calcined at 650°C for 16 hours and then sieved to a mesh size of 80-150. The carrier had a rutile content of 97%, a surface area of 14 m<2>/g and a pore volume of 0.17 cm<3>/g.

Cobalt and rhenium were deposited on this support from an acetone solution using a slurry technique. The slurry was dried at room temperature in a rotary evaporator and then in vacuum at 140°C for 16 hours. It was then calcined at 250°C for 3 hours and sieved again to remove fine particles. The catalyst was then reduced prior to the chemisorption and hydrocarbon synthesis measurements under conditions described below (flowing hydrogen 250-450°C). The Co and Re contents in the reduced catalyst were respectively 11.6 and 0.43-0.48% by weight (X-ray fluorescence).

SipQ promoted CoRe/ TiO? fcatalysts B and Cl

SiC>2 was deposited on unreduced CoRe/TiOg [catalyst A] by initial moisture impregnation with a solution of tetraethoxysilane (TEOS) in methanol in an inert atmosphere (N2). TEOS was cleaved by treatment with water-saturated He (40 Torr H2O) while the temperature was increased from 25 to 400°C at 4°C/min. and kept at this latter temperature for 13 hours. The catalysts were reduced at 250-450°C for 2-14 hours prior to the chemisorption and hydrocarbon synthesis experiments. Two SiOg contents were investigated: 4.0 wt% [catalyst B] and 5.2 wt%

[catalyst C], on the basis of fully reduced catalysts. Their cobalt content was 10.8-11.1% by weight. The amounts of SiO2 were lower than calculated from the amount of impregnation solution, respectively 5.0 and 9.0% by weight, which indicates some sublimation of the TEOS material from the carrier during the steam treatment.

Water-treated CoRe/ Ti02 fcatalystor Dl

A portion of catalyst A was pretreated in water-saturated He (40 toss HgO) following the procedure used for catalysts B and C, but without SiO2~tii setting to ensure that any observed effects of added SiOg are not caused by the pretreatment procedure. The catalyst was then reduced as described above.

Catalvsator characterization

K. iemisorps. ion

Molecular hydrogen was used as a probe for surface cobalt atoms in hydrocarbon synthesis catalysts.

Dihydrogen uptake was measured in a static chemisorption unit of gas, pumped by means of diffusion and coarse pumps, isolated from the system with traps of liquid nitrogen, and capable of a dynamic vacuum of IO-<7> torr. Pre-reduced and passivated samples were re-reduced in flowing dihydrogen (200 cm<3> (STP )/g-cat-min) for 1-2 hours at 200-800°C and then evacuated to less than 10~<6> Torr in 0 .5-1 hour at a temperature sufficient to remove all chemisorbed hydrogen (> 250°C). The samples were then cooled to the absorption temperature (25 to 150°C) and isotherms were measured at 3 to 5 hydrogen pressures between 100 and 700 Torr. A countersorption isotherm was sometimes measured by evacuating the sample to 10"^ Torr at the absorption temperature for 0.5 hour and then measuring the hydrogen uptake again between 100 and 600 Torr. Adsorption and countersorption isotherms were extrapolated to zero pressure to up- reach the total and weak chemisorption uptakes, respectively.

Dispersions were calculated from hydrogen uptake and from the cobalt content in the samples, assuming a stoichiometry between hydrogen and surface cobalt atoms of 1:1. Dispersions were converted to surface average crystallite sizes (d), assuming hemispherical crystallites, using:

where D is the fractional dispersion, assuming that cobalt crystallites form in fcc structures with a randomized distribution of (111), (110) and (100) crystallographic planes.

Thermogravimetric analysis method

A Mettler TA 2000C calorimeter measured both weight changes (TG) and rates of weight change (DTG) in H2, CO or 1:1 H2/C0 mixtures, at atmospheric pressure. Peaks in the obtained weight curve correspond to maximum rates of weight change. Gas flows were set at 100 cm<3> (STP)/min. and 150 mg catalyst samples were used. A temperature program of 6°C/min was used. as standard heating speed. All samples were reduced in Hg from room temperature to 500°C. After the Hg treatment, the sample was cooled and treated in a 1:1 Hg/CO mixture. The growth of an amorphous carbon phase was used to determine the intimate contact and strong interactions between cobalt and silicon dioxide. Powder X-ray diffraction spectra, taken before and after the experiments, confirmed the phases present.

Hydrocarbon synthesis process

The kinetics at steady state and residence time studies were measured in a fixed bed reactor with plug flow at 180-230°C, 100-2,050 kPa and H2/C0 of 2/1 using 2-110 g of catalyst. The catalysts were reduced in hydrogen at 250-450°C, cooled to synthesis temperature and subjected to Hg/CO supply. All data reported were obtained after at least 24 hours of flow. Standard conditions are 200°C, 2050 kPa and H2/C0 of 2/1. The products were analyzed by capillary and packed column gas chromatography and GC/MS, using dinitrogen as an internal standard. C2o<+->fordelinSer ^ e obtained by gas chromatograph i and gel permeation chromatography.

Hydrocarbon synthesis rates are reported as cobalt-normalized rates (cobalt-time yields), defined as the number of moles of CO converted per hour per grams of cobalt in the catalysts, as position-normalized rates (position-time yields), defined as the number of molecules of CO that are converted per hour per surface cobalt atom in the catalysts, and as volumetric rates, defined as the volume of CO that is converted per volume of catalyst per hour. Hydrocarbon selectivities are reported on a carbon atom basis, as a percentage of the converted CO appearing as a given product. Reported chain growth probabilities are asymptotic values, obtained from the constant slope of Flory deposits for C35-C-Loø hydrocarbons •

Example 1

Effect of SiOg addition on the hydrocarbon synthesis performance of CoRe/TiOo

The addition of small amounts of SiO2 (4-6 wt-56) to CoRe/TiO22 increases the cobalt time yield despite the decrease in cobalt dispersion that occurs during the pretreatment necessary to cleave the SiO2 precursor (Tables 1 and 2). The apparent intrinsic activity of a cobalt surface atom (position-time yield) increases twice with the addition of 4-6 wt% SiO 2 . The hydrocarbon synthesis selectivity is almost unchanged by the SiO2 addition. The CH4 selectivity is 5.0-5.4% by weight and the C5<+->selectivity is 88.8-90% for these catalysts. The olefin content increases with SiO2 addition for C5<+->hydrocarbons. Treatment of C0Re/TiO22 with He/E2O at 400°C reduced cobalt dispersion to the level measured for SiO2-containing samples subjected to the same pretreatment. The cobalt time yield is lower than again CoRe/Ti02, due to the lower dispersion, but the position-time yield is identical.

Retention of activity is at least equal to that of CoRe/TiO 2 catalysts that do not contain silica. Because of. however, their high initial activity maintains SiO2-promoted catalysts at a higher level of productivity throughout the cycle.

Example 2

Effect of SiOg addition on reduction and carbon deposition properties for CoRe/TiOo

The addition of SiOg to CoRe/TiOg did not affect its reduction behavior. The temperature-programmed reduction profiles and the degree of reduction at 450°C were identical in SiOg-activated and unactivated samples (Fig. 1).

Carbon deposition on the catalysts was dramatically inhibited by SiOg addition (Fig. 2). Addition of 4 wt% SiOg delays the onset of carbon deposition from 370°C to 500°C during the temperature-programmed treatment with Hg/CO mixtures (1/1 ratio). Similar results were obtained at higher SiOg additions.

These data suggest that the role of SiOg is not to improve the reducibility of cobalt oxide precursors or to prevent the formation of cobalt titanates during catalyst preparation, pretreatment and use in hydrocarbon synthesis. The action of SiOg during carbon deposition suggests that SiOg can prevent the short-term inactivation observed on these catalysts during the first few hours in a Hg/CO environment. The effect is to increase the apparent site activity by maintaining surface cobalt atoms available during hydrocarbon synthesis.

It is known that SiOg is adsorbed to strong acid positions in AI2O3, modifies hydroxyl groups and acid positions in fused silica tubes and prevents carbon deposition on the walls of stainless steel reactors. It is believed that SiOg titrates or modifies particular positions on CO/TiOg catalysts, which reduces their activity for carbon formation. Decoration of the cobalt surface by SiOg and inhibition of carbon deposition by the accompanying bond in available cobalt surface size is unlikely. If this were the case, addition of SiO22 would have reduced the hydrogen uptake and apparent dispersion, more than the He/HgO treatment did (Table 1).

A decrease in catalyst acidity upon SiOg addition is consistent with the observed decrease in internal olefin and branched product selectivity for the (^-hydrocarbons when SiOg was introduced into the cobalt catalyst on the TiOg support (Table 3). Internal olefins are branched products, and are usually compounds with double bonds and skeletal isomerization of primary α-olefin products on metal and oxide catalysts.

Claims (4)

1. Hydrocarbon synthesis catalyst comprising cobalt in a catalytically active amount and optionally rhenium compounded with an inorganic refractory support comprising a greater proportion of titanium dioxide, characterized in that the catalyst includes up to 15% by weight of the catalyst of silicon dioxide added to the catalyst as a silicon dioxide precursor and decomposed to form the silicon dioxide. 2. Catalyst according to claim 1, characterized in that silicon dioxide is present in an amount in the range 1-10 weight-*. 3. Process for producing hydrocarbons from synthesis gas whereby hydrogen and carbon monoxide are reacted under hydrocarbon synthesis conditions over a catalyst, characterized in that a catalyst as defined in claims 1 or 2 is used. 4. Method according to claim 3, characterized in that the process temperature varies from 160 to 300°C, the ratio of hydrogen to carbon monoxide varies from 0.5 to 10 and the gas space velocity varies from 100 to 5000 v/h/v.