PROCEDURE FOR REMOVING FROM SOX FROM FLUE GASES AND OTHER GAS FLOWS USING SORBENTIEN

The present invention relates to a process for removing the So_x components from a flue gas stream containing oxygen, sulfur dioxide and sulfur trioxide from the combustion of coal from a coal-fired boiler.

In particular, the present invention relates to the use of crystalline layered double hydroxide with an interlayer of an ion which oxidizes sulfur dioxide to sulfur trioxide.

In fossil-fuel-fired power plants, the sulfur content of the feed coal is oxidized during combustion to sulfur oxides (SO₂ and SO₃, commonly referred to as "SO_x"), which are released through stacks to the atmosphere, and are responsible for deposition as "acid rain". Analyses of flue gas produced by power plants burning coal before desulfurization, show 0.5% - 0.2% SO₂ and about 0.005% SO₃. Control of SO_x emission is mandated by the U.S. Environmental Protection Agency (EPA), and various studies are under way to develop methods for its removal from flue gas streams.

Formation of SO_x in combustion processes can be reduced by modifying the burner design and combustion system, by changing the operating conditions and by using fuels with lower sulfur contents. The most popular and inexpensive method of reducing SO_x emission is the addition of reactive dry sorbents with the fuel. Accordingly at present, SO_x removal is most often accomplished by using lime (CaO) or lime stone (CaCO₃). Several other basic sorbents like MgO, ZnO also are found to be effective in removing SO_x. For a review on dry sorbents see for example, Komppa, V., "Dry Adsorption Processes for Removal of SO_x and NO_x in Flue Gases - a review," Paperii ja Puu, 5, 401 to 405 (1986).

Use of Group 2 (formerly Group IIA) metal oxides such as magnesium and calcium oxides as SO_x sorbents has been disclosed in several patent disclosures and recent examples include U.S. Patent Nos. 3,835,031 and 3,699,037. Several other metal oxides of varying effectiveness as SO_x sorbents are described in U.S. Patent No. 4,153,534 which include oxides such as sodium, scandium, titanium, iron, chromium, molybdenum, manganese, cobalt, nickel, copper, zinc, cadmium, rare earth metals, and lead.

In typical coal-fired power plants the ground sorbent, for example lime or limestone, is added into boilers along with coal or sprayed into towers as a slurry to contact the flue gas. The SO₂ reacts with calcium hydroxide to form a calcium sulfite slurry which is then partially oxidized with air to calcium sulfate. In this way the sulfur oxides are retained as harmless solid compounds which can be removed from the stack gas by electrostatic precipitation or other standard methods. Such a process is potentially attractive for retrofitting existing power plants since no major structural alterations are required.

A major problem with this type of process is low utilization of the oxide sorbents. The rate of adsorption of SO_x declines rapidly with increasing conversion, due to mass transfer limitation and low reactivity of SO₂. Hence in the relatively short contact time available, only a small fraction of the sorbent reacts. In principle the problem of low utilization of the sorbents may be solved by reducing the particle size, but in practice, the particle size required for a reasonable level of utilization may be too small to achieve economically by conventional grinding or fragmentation methods.

Thermodynamic calculations indicate that the capture of sulfur trioxide with metal oxides is more favorable compared to sulfur dioxide. Several experimental results have suggested that catalytic oxidation of sulfur dioxide to sulfur trioxide can be beneficial for stack gas desulfurization. Kocaefe & Karman in Cand. J. Chem. Eng., 63, 971 to 977 (1985) has shown that the rate of reaction of SO₃ with Ca, Mg and ZnO is greater than that of sulfur dioxide with the same oxides under identical conditions. Furthermore, inclusion of Fe₂O₃ (as a SO₂ oxidation catalyst) leads to more effective utilization of the lime. The addition of a small amount of Fe₂O₃ gave both a more rapid initial uptake rate and a much higher final conversion of the lime (80-90%). In the absence of an oxidation catalyst the rate of SO₂ absorption declined sharply at about 70% conversion.

A similar approach has been employed in designing SO_x sorbents for fluid catalytic cracking (FCC) processing of petroleum. These sorbents, among other things, are mostly alkaline earth metal spinels containing one or more other metal components capable of oxidizing sulfur dioxide. For example, U.S. Patent Nos. 4,472,532 and 4,492,678 relate to the incorporation of iron, chromium, vanadium, manganese, gallium, boron, cobalt, platinum, and cerium as oxidation catalysts.

Therefore, in designing improved sorbents for SO_x removal, one must synthesize materials that will (i) oxidize SO₂ to SO₃, (ii) chemisorb the SO₃ formed, and (iii) be able to release the adsorbed SO_x for the regeneration of the sorbents or form stable materials for the safe deposition of the spent solid sorbents. The SO_x emitted from these spent sorbents can be captured safely and can be utilized in sulfuric acid or sulfur production.

European Patent Application EP-A 278 535 has recently described a catalyst composition suitable for the refining of heavy sulfur- and metal-containing petroleum feeds. Thus, the catalyst composition according to the disclosure contained a catalytically active zeolitic material such as ZSM-5, ZSM-11 etc. for the conversion of hydrocarbons, an anionic clay material with an LDH structure for the binding and removal of sulfur oxides, and a matrix material such as kaolin or alumina. Preferred catalyst compositions contained 1 to 30 percent amounts of anionic clay compositions, based on total catalyst composition.

There is a need for sorbent compositions suitable for diminishing SO_x from flue gas streams particularly from coal-fired power plants. There is a need to develop sorbent compositions which give better SO_x uptake in shorter time duration to overcome the low utilization of common oxide sorbents such as CaO and MgO due to mass transfer limitation and low reactivity of SO₂.

LDHs are a group of anionic clay minerals. These have positively charged sheets of metal hydroxides, between which are located anions and some water molecules. Most common LDHs are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These clays have a structure similar to brucite [Mg(OH)₂] in which the magnesium ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In the LDHs, some of the magnesium is isomorphously replaced by a trivalent ion, such as Al³⁺. The Mg²⁺, Al³⁺, OH^- layers are then positively charged, necessitating charge balancing by insertion of anions between the layers.

One such anionic clay is hydrotalcite in which the carbonate ion is the interstitial anion, and has the idealized unit cell formula [Mg₆Al₂(OH)₁₆](CO₃)·4H₂O. However, the ratio of Mg/Al in hydrotalcite-like materials can vary between 1.7 and 4 and various divalent and trivalent ions may be substituted for Mg and Al. In addition, the anion which is carbonate in hydrotalcite, can be varied in synthesis by a large number of simple anions such as NO_3-, Cl-, OH-, SO₄^2- etc. These LDHs, based on their structure, fall into the Pyroaurite-Sjogrenite group, where brucite-like layers carrying a net positive charge alternate with layers in which the oxygen atoms of carbonate groups and water molecules are distributed on a single set of sites.

Hydrocalumite and related synthetic compounds also have a layered structure in which positively charged metal hydroxide layers alternate with the interlayers containing anions and water. The hydroxide layers contain specific combinations of metal ions derived from on one hand divalent calcium cations and on the other from trivalent cations of metals such as iron, or more particularly, aluminum. The interlayers contain anions such as OH^-, SO₄^2-, Cl^-, NO₃^- and, in particular CO₃^2-. The general formula for the group is [Ca₂M³⁺(OH)₆]X.yH₂O, where M³⁺ is a tripositive ion and typically Al³⁺, X is a singly charged anion or equal amounts of more highly charged ones, and y is between 2 and 6. As in the Pyroaurite-Sjogrenite group, principal layers alternate with inter-layers, the principal layers having the composition [Ca₂M³⁺(OH)₆]⁺ and the interlayers consisting of water molecules and anion X. However, because of the difference in size between the Ca²⁺ and Al³⁺ ions, the M²⁺:M³⁺ ratio is fixed at 2:1 and their arrangement is ordered. The only known natural mineral in the group is hydrocalumite the composition of which is approximately [Ca₂Al(OH)₆](OH)_0.75(CO₃)_0.125.2.5H₂O, but there are many synthetic analogues such as [Ca₂Fe(OH)₆](SO₄)_0.5.3H₂O, [Ca₂Al(OH)₆](OH).6H₂O etc.

The syntheses of LDHs are generally simple, and the so-called "precipitation method" is most popular. If a carbonate-containing product is desired, then the aqueous solution of magnesium and aluminum salts, i.e., nitrate, or chloride, is added to an aqueous solution of sodium hydroxide-carbonate with good mixing at room temperature. The resulting amorphous precipitate is then heated for several hours at 60 to 200°C to obtain a crystalline material. Washing and drying complete the synthesis in quantitative yield. By employing this precipitation method, replacement of all or part of Mg²⁺ with other M^IIions such as Ca²⁺, Zn²⁺, Cu²⁺ etc., or replacement of Al³⁺ with other M^III ions such as Fe³⁺, Cr³⁺ etc., is also possible.

Another important aspect of the synthesis of these materials is the variation of the nature of the interstitial anion. The preparation of hydrotalcite-like materials with anions other than carbonate in pure form requires special procedures, because LDH incorporates carbonate in preference to other anions. Most of the time the smaller anions are introduced to the LDH structure, via the precipitation method by using the desired anion solutions instead of carbonate. However, in these methods the synthesis has to be carried out in an anaerobic condition to prevent carbonate contamination from the atmospheric carbon dioxide. These methods of preparation of LDHs have been described in prior art publications, particular reference being made to the following review journal articles by S. L. Suib et al., in Solid State Ionics, 26, 77 to 86 (1988), and W. T. Reichle in CHEMTECH, 58 to 63 (1986).

Process for the synthesis of hydrotalcite-like clays also have been the subject of a number of patents.

U.S. Patent Nos. 3,796,792, 3,879,523 and 3,879,525 describe hydrotalcite-like derivatives with both cationic layer and anionic substitution including the smaller transition metal anions like CrO₄^2-, MoO₄^2- and Mo₂O₇^2-. Both composition and preparative methods are described, and the compositions are said to be useful for catalytic purposes, absorbents, desiccants and the like. Synthetic hydrotalcite-like derivatives with small anions, including anions of transition elements, and also large organic anions such as long chain aliphatic dicarboxylates, are shown to catalyze aldol condensation effectively.

Incorporation of larger anions, such as transition metal polyoxoanions into the LDH interlayer is not easy. This requires ion-exchange techniques subsequent to the LDH synthesis. Pinnavaia and Kwon in J. Am. Chem. Soc., 110, 3653 (1988) have demonstrated the pillaring of several polyoxometalles including V₁₀O₂₈^6- into the hyrotalcite structure containing Zn and Al metal ions in the layers. U.S. Patent No. 4,452,244 discloses the preparation of several polyoxometallate-LDHs. However, no XRD or analytical data were given to establish the purity of those materials. U.S. Patent No. 4,774,212 discloses the preparation of several Mg/Al hydrotalcite-like materials containing transition metal polyoxoanions.

The nature of the thermal decomposition of LDHs especially the hydrotalcite-like materials, have been studied in detail. For example, upon thermolysis, hydrotalcite [Mg₆Al₂(OH)₁₆](CO₃).4H₂O loses weight in two stages. First, it loses the four interstitial water molecules when heated to 200°C, while retaining the skeletal hydroxide and the interlayer carbonate. Additional heating from 275°C to 450°C results in the simultaneous loss of hydroxyl groups and carbonate as water and carbon dioxide, respectively. These magnesium aluminum solid solutions have the sodium chloride type structure with cations deficiencies. Reichle in J. Catal. 101, 352 to 359 (1986) has shown that this heating of hydrotalcite was accompanied by an increase in the surface area from about 120 to about 230 m²/g (N₂/BET) and a doubling of pore volume (0.6 to 1.0 cm³/g, Hg intrusion). Further heating of these solid solutions to higher temperatures causes lowering of surface area as well as reactivity. At 1000°C, the formation of MgO and the spinel phase, MgAl₂O₄ has been observed.

JP-A-54056089 discloses a gasphase desulphurisation agent comprising a compound obtained by burning a crystalline compound having hydrotalcite-like structure.

It is the object of the present invention to provide a process for removing the So_x components from a flue gas stream using sorbent compositions which oxidize SO₂ to SO₃, remove the SO₃ and then are regeneratable for reuse. Said object is achieved by a process for removing the So_x components from a flue gas stream containing oxygen, sulfur dioxide and sulfur trioxide from the combustion of coal from a coal-fired boiler which comprises combusting the coal in the boiler to provide the flue gas stream and contacting said gas stream with a heated sorbent composition at 400 to 1000°C wherein said sorbent before being heated is selected from the group consisting of a layered double hydroxide composition of formula: [M_l-x^IIM_x^III(OH)₂](A^n-)_x/n.yH₂0 wherein M^II is a divalent metal cation; M^III is a trivalent metal cation selected from the group consisting of Group IIA, IIB and IIIA metals; A is a polyoxometalate anion or a metal oxalate anion of charge n which comprises at least one metal atom selected from the group consisting of main group metals and transition metals; x is between 0.8 to 0.12 and y is moles of water.

Figure 1 is a graph of a thermogravimetric analysis (TGA) plot for SO₂ uptake by [Mg₆Al₂(OH)₁₆](FeO₄).xH₂O as the layered double hydroxide (LDH).

Figure 2 is a graph showing the temperature dependence for SO₂ uptake by [Mg₆Al₂(OH)₁₆] (V₁₀O₂₈)_1/3.xH₂O.

Figure 3 is a graph of a TGA plot for SO₂ uptake by [Zn₂Al(OH)₆](SiV₃W₉O₄₀)_1/7.xH₂O and thermolysis of the spent sorbent.

Figure 4 is a graph comparing the rate of SO₂ uptake by different Mg containing sorbents prepared according to this invention and sorbents presently used in flue gas desulfurization.

Thermal decomposition of LDHs, leads to the formation of active metal oxides with fairly high basic character (pKa ≦35) and high surface area. These thermally treated materials should have exceptionally well-dispersed reactive metal centers, as judged from their catalytic properties. These properties have lead us to synthesize and to utilize thermally treated LDH materials as suitable sorbents for flue gas desulfurization. The SO₂ oxidation catalysts, (usually transition metals) and the SO₃ sorbents (usually a metal oxide from either group IA or IIA) can be easily incorporated into the layers or in the interlayers of LDH materials, using inexpensive starting materials.

Thus, the present invention relates to a process using layered double hydroxide compositions, more specifically hydrotalcite-like and hydrocalumite-like materials for the absorption of SO_x from flue gas streams. Also described herein is the incorporation of other metal components, preferably transition metal ions, capable of promoting the oxidation of sulfur dioxide to sulfur trioxide at calcination temperatures. These second metal components are incorporated in the form of metal polyoxoanions into the LDH by intercalation.

LDH sorbents react at different temperatures, particularly at 500 to 1000°C, with SO₂ such that the sorbents find particular use in diminishing the emissions of sulfur oxides from the coal-fired boilers.

Considered here are also ways of recycling the spent sorbents, (i) by removing the entrapped SO_x at high temperatures and/or (ii) by disposing of them as solid waste.

Both M^II and/or M^III comprise in total or in part metals (preferably from group IIA, IIB and IIIA) that form reactive basic oxides at calcination temperatures (preferably above 500°C) that are capable of reacting with SO_x. Accordingly, the preferred LDHs for use in the present invention comprise of these metals in the brucite-like [Mg(OH)₂-like] layers, in particular magnesium and aluminum. Other alkaline earth metal ions, such as calcium, strontium, barium and mixtures thereof may replace all or part of magnesium ions.

In a broader sense this invention considers the use of these LDH sorbents in controlling the sulfur oxides from gas streams, more particularly from coal-fired boiler systems. These systems include a boiler, economizer and dust collectors such as electrostatic precipitator or bag filter house ("bag house"). The injection of the sorbents into these, particularly to the boiler (700-1000°C), along with the coal, or onto the electrostatic precipitators (hot side temp. 400-500°C) has been considered in this invention. Thus, the LDH sorbents were thermally treated in a temperature-programmed thermogravimetric balance at a temperature in the range of 500 to 1000°C in a stream of air or nitrogen, and SO_x gas was introduced. The amount of SO₂ that reacted with the sorbents was monitored as the weight uptake.

The reaction of the hydrotalcite sorbent, [Mg₆Al₂(OH)₁₆](CO₃)·xH₂O (abbreviated as Mg₃Al-LDH) with SO₂ provided a general description of the typical experimental method used to investigate the LDH reactivity. The hydrotalcite was heated to 700°C under a stream of air in a temperature controlled thermogravimetric balance at a rate of 5°C/min. The sample was calcined at 700°C for further one hour. During the above calcination process the sample lost weight due to the removal of CO₂ and H₂O. This sample was then exposed to SO₂-containing gas stream at 0.5% v/v concentration, at a flow rate of 200 ml/min. A weight gain of 6.2 % was observed. This corresponded to the amount of SO_x absorbed to form the metal sulfate, MgSO₄. The diffraction peaks observed in the X-ray diffraction pattern of the final product were due to crystalline MgSO₄, indicating that the magnesium sites were the reactive species at this temperature. The weight uptake observed corresponded to a 4.4% conversion of MgO to MgSO₄. However, this value is low compared to the other modified sorbents described later in this invention.

A third metal component is incorporated in the LDH to promote the oxidation of sulfur dioxide to sulfur trioxide. The third metal component is preferably a component of a metal selected from the transition metals, rare earth metals, and Group IVA in the periodic table. Some of the known transition metal and transition metal oxide catalysts that are suitable for SO₂ oxidation include, Pt, WO₃, Ag, Ag₃VO₄, Cu₃(VO₄)₂, V₂O₅, Fe₂O₃, TiO₂, CuO, CrO₃, MnO₂, PbO₂, MoO₃, CeO₂, Cr₂O₃, SnO₂ and ZnO. Platinum is an excellent oxidation catalyst, and other oxides such as vanadium pentoxide and iron oxides are also especially effective for catalyzing the oxidation of SO₂ to SO₃. See for example, Neuwmann et al in Z. Electrochem. 38, 304 to 310 (1932). The catalytic process on these oxides will involve the following steps: sorption of SO₂ to form a sulfite, oxidation of sulfite to sulfate, and sulfate decomposition with evolution of SO₃. Thus, for a particular metal oxide sorbent, the selection of a good metal oxide catalyst for SO₂ oxidation is very important. Requirements for a good catalyst can be compared to those for the SO₂ sorbent. For the catalyst, all three steps are surface reactions and should occur at the reaction temperature. For the SO₂ sorbent, the first two steps should occur as bulk reactions converting much of the sorbent to sulfate during sorption at the reaction temperature The last step should occur at a higher temperature.

Particularly good results were achieved as disclosed in this invention when transition metals, especially iron or vanadium, were introduced to the LDH as the third metal. These metals were incorporated into the LDH sorbent compositions by structural means during the synthesis. As disclosed in this invention, metal components are introduced into the interlayers between [M_l-x^IIM_x^III(OH)₂] layers in such a way, that a part or whole of A^n- in the LDH structure, [M_l-x^IIM_x^III(OH)₂](A_x/n)^n-·H₂O is replaced by anions containing sulfur dioxide oxidizing metals described earlier. Accordingly, the anions that contain metal ions, such as copper, zinc, cobalt, iron, cadmium, mercury, lead, manganese, tin, nickel, palladium, chromium, vanadium, manganese, gallium, boron, cobalt, and mixtures thereof may replace all or part of interlayer anion A^n- in the LDH structure.

Thus, the anions can be one or more from metal anionic complexes such as oxalates (ox), Fe(ox)₃^3-, simple oxo-anions such as, CrO₄^2-, FeO₄^2-, MnO₄^-, etc., or larger oxo-anions with higher charge such as, V₁₀O₂₈^6-, W₇O₂₄^6-, Mo₇O₂₄^6- etc. or anions like BVW₁₀O₄₀^7-, H₂W₁₂O₄₀^6-, SiV₃W₉O₄₀^7- polyoxometalates with Keggin-type structure or BCoW₁₁O₃₉^7-, SiW₁₁O₃₉^8-, PMo₂W₉O₃₉^7- polyoxometalates with defect Keggin structure or anions such as BCoW₁₂O₄₂^8- containing linked Keggin-type structure. Intercalation of these anions with different sizes, not only help SO₂ to oxidize to SO₃, but also introduce microporosity in the LDH structure and allow ready access of reacting SO₂ molecules.

The incorporation of guest anions into the hydrotalcite structure was carried out by replacing CO₃^2-from the interlayer as follows: The hydrotalcite material with suitable Mg/Al ratio was first calcined at 500°C and then hydrolyzed in aqueous solutions under anaerobic conditions to reform the hydrotalcite-like LDH containing OH^- as the interlayer anion. This OH^- in the interlayer can easily be replaced by wide variety of anions, such as FeO₄^2-, CrO₄^2-, Fe(ox)₃^3- etc., or larger anions such as V₁₀O₂₈^6-, W₇O₂₄^6- or much larger anions with Keggin structure such as H₂W₁₂O₄₀^6-, BVW₁₀O₄₀^7-, SiV₃W₉O₄₀^7- etc., or defect structure such as SiW₁₁O₃₉^9-, BCoW₁₁O₃₉^7-, etc. The products isolated showed X-ray diffraction peaks corresponding to crystalline phases with well-defined basal spacings (Table 1).

The incorporation of the guest polyoxome talate anions into the Zn/Al-LDH was also carried out in a similar manner starting with an aqueous hot suspension of [Zn₂Al(OH)₆](NO₃)·xH₂O (referred to as Zn₂Al-LDH) and was found to undergo complete intercalative ion exchange reaction with aqueous solutions of polyoxometalate anions including alpha-[H₂W₁₂O₄₀]^6- and alpha-[SiV₃W₉O₄₀]^7-alpha-Keggin-type ions or defect Keggin-type ions such as SiW₁₁O₃₉^9-, BCoW₁₁O₃₉^7-. The anions with lower charge such as [PW₁₂O₄₀]^3- and [SiW₁₂O₄₀]^4- show no ion exchange whereas, intermediate anions show partial intercalation (e.g. [PCuW₁₁O₃₉(H₂O)]^5-). Furthermore, polyoxometalate anions with beta-Keggin structure such as beta-[SiV₃W₉O₄₀]^7- undergo partial intercalation. See Kwon, T. and Pinnavaia, T. J., "Pillaring of a Layered Double Hydroxide by Polyoxometalates with Keggin-Ion Structures", Chemistry of Materials, 1, 381 to 383 (1989).

The preparation of several polyoxometalate intercalated Zn/Al- and Mg/Al-LDH materials starting from corresponding nitrate or chloride containing LDH has been disclosed in U.S. Patent No. 4,454,244. However, in our hands, under the conditions mentioned the products formed were X-ray amorphous as judged by the absence of distinct Bragg reflections. Nevertheless, these polyoxometalate intercalated amorphous materials as well as partially intercalated LDHs described earlier also showed enhanced SO_x uptakes compared to their precursors.

In certain preferred embodiments discussed in this invention, hydrotalcite-like Mg₃Al-LDHs, when intercalated with Fe(ox)₃^3-, FeO₄^2-, V₁₀O₂₈^6-, W₇O₂₄^6- and Mo₇O₂₄^6-, gave better SO_x sorption than normal hydrotalcite (Table 2).

In these cases, iron, vanadium, tungsten and molybdenum oxides act as SO₂ oxidation catalysts. Similarly, Zn₂Al-LDH when intercalated with the same polyoxo anions (Table 2) also showed enhanced SO_x uptake relative to the Zn₂Al-NO₃ LDH precursor.

Furthermore, it was found that, Mg₃Al and Zn₂Al-LDH sorbents prepared with POM anions intercalated partially in the interlayers, as well as amorphous LDH-POM reaction products, also produce suitable SO_x sorbents. All these materials showed enhanced SO_x uptake relative to their precursor LDHs, [Zn₂Al(OH)₆](NO₃)·xH₂O or [Mg₃Al₂(OH)₁₆](OH)·xH₂O. For example the V₁₀O₂₈^6-intercalated crystalline Zn₂Al-LDH showed a weight uptake of 23.6% at 700°C under the conditions described earlier in this invention. The corresponding amorphous material showed a 20% weight uptake at this temperature. The crystalline sorbent formed by intercalating Zn₂Al-LDH with alpha-[SiV₃W₉O₄₀]^7- showed 5.78% weight uptake at 700°C with SO_x whereas, the partially intercalated sorbent with beta-[SiV₃W₉O₄₀]^7- in the interlayer showed 5.12% weight uptake under similar conditions.

In another embodiment, Mg₃Al-LDH intercalated with V₁₀O₂₈^6-, [Mg₆Al₂(OH)₁₆](V₁₀O₂₈)_1/3·xH₂O was subjected to air containing 0.5% SO₂ (v/v) in the temperature range 500-800°C. As evident by the uptake measurements (Figure 2), better SO_x sorption was seen in the temperature range 600-800°C.

In one embodiment the possibility of regenerating these spent LDH sorbents for recycling was considered. It was found that spent Zn₂Al-LDH sorbents after exposing to SO₂ at 700°C, release entrapped SO_x (and/or H₂SO₄) upon further thermal treatment, as evident by their weight loss (Table 3).

For example, a Zn/Al-LDH containing SiV₃W₉O₄₀^7-polyoxometalate anions showed a 5.78% weight uptake at 700°C in the presence of air containing 0.5% SO₂ (v/v). In the absence of SO₂ in a stream of air at 900°C, this spent sorbent (now in the sulfate form) released all of its bound SO_x to regain the initial weight prior to SO₂ treatment and to reform the oxide sorbent. This reformed sorbent had a similar activity as the initial sorbent for the reaction with SO_x (Figure 3). Thus, the SO_x sorbent can be recycled, if desired by appropriate control of absorption/desorption temperatures.

The uptake of SO_x as a function of time for some of the Mg₃Al sorbents discussed in this invention is given in Figure 4. Compared to conventional basic sorbents such as MgO and Mg(OH)₂, the sorbents disclosed in this invention, especially when the interlayer anions containing iron in the layered double hydroxide structure, exhibit superior initial and overall SO_x sorptivity. Under specific reaction conditions, i.e., at 700°C in a gas stream containing 0.5% SO₂ and air, for example MgO was found to undergo 10.2% conversion of MgO sites to MgSO₄ and Mg(OH)₂ was found to undergo 14.0% conversion during a period of 1 hour. Incorporation of iron as Fe₂O₃ to MgO by (MgO:Fe₂O₃=3:1) by mixing enhanced the conversion to 36.5%. Conversely, similar mixing experiment with Mg(OH)₂ [Mg(OH)₂:Fe(OH)₃ = 3:1] showed reduced uptakes (9% conversion). Under the same set of reaction conditions Mg₃Al-LDH (Mg:Al=3:1) containing FeO₄^2- ions and Fe(ox)₃^3-anions in the interlayers as disclosed in this invention exhibited 85.2% and 67.8% conversion of Mg sites to MgSO₄, respectively. These values are much superior to MgO or Mg(OH)₂ with or without iron. The LDH sorbents intercalated with vanadium containing anions such as V₁₀O₂₈^6-, HVO₄^2-, SiV₃W₉O₄₀^7- and BVW₁₀O₄₀^7- also showed better SO_x uptake values (Table 2 and Figure 4). Moreover, these polyoxometalate anions intercalated LDH sorbents exhibited better initial SO_x uptake than MgO or Mg(OH)₂.

European patent EP-A 278 535, which was described herein earlier, disclosed hydrocarbon cracking catalyst compositions containing as a component, anionic clays with LDH structures, for the purpose of binding the SO_x liberated in the refining process, especially when processing high sulfur feeds. The LDH components incorporated many of the known SO₂ oxidation catalysts, including rare earth metal ions (e.g., La³⁺, Ce³⁺), noble metals (e.g., Pt, Rh) and transition metal ions (Cu²⁺, Fe³⁺, Mn²⁺, Cr³⁺). The rare earth and noble metal catalysts were preferred over the transition metal catalysts, in part, because of their greater reactivity. Also, it is known to those skilled in the art that transition metals, particularly iron, are undesirable constituents of petroleum cracking catalysts because they promote the formation of coke. However, iron is an economically attractive SO₂ oxidation catalyst for applications where coke formation is not a concern, such as in the reduction of SO_x from the flue gases of coal-burning power plants.

In the preferred invention described herein, we disclose in part that the effectiveness of transition metal ions (particularly iron and vanadium) in promoting SO₂ uptake by LDH materials depends substantially on the transition metal composition. The incorporation of transition metals into the interlayers of LDH structure affords an order of increase in SO_x absorption. This teaching is illustrated by the results presented in Table 2 for SO₂ uptake at 700°C by Mg₃Al and Zn₂Al-LDH compositions with different interlayer anions consisting of SO₂ oxidation catalysts. In the present invention, when the oxidation catalyst iron when incorporated to the interlayers of the LDH structure in the form of FeO₄^2- for example, showed an enhancement of SO_x uptake capability of hydrotalcite from 12.5% conversion of Mg sites to MgSO₄ to 85.2%. This is about 72.7% improvement of utilizing the Mg sites for SO uptake. Furthermore, incorporation of other iron containing anions such as Fe(ox)₃^3- as well as vanadium containing polyoxometalate anions showed significant improvements in SO_x uptake capabilities of the LDH sorbents. Thus, iron and vanadium are preferred as SO₂ oxidation catalyst for the sorbents disclosed in this invention which are fabricated to use in flue gas desulfurization processes.

Most of the sorbents disclosed in this invention contained Mg²⁺ as the M^II cation. Alternatively, Ca²⁺ containing LDHs such as hydrocalumite and its derivatives also could be used as sorbents for SO_x scrubbing after intercalating with said polyoxometalate. These materials should have similar or superior SO_x sorptivity as judged by reactivity of CaO with SO₂ and SO₃ (see above, background of the invention).

The metal-containing LDH useful as precursor for the preparation of compositions disclosed in the present invention may be synthesized from inexpensive starting materials. For example, hydrotalcite-like materials can be prepared starting with MgO and alumina (Al₂O₃) and hydrocalumite-like materials from CaO and alumina. Both CaO and MgO can be obtained by calcining the natural minerals such as Calcite (CaCO₃) and Magnesite (MgCO₃). Some of these layered double hydroxides, such as hydrotalcite, are commercially available and some may be naturally occurring. Moreover, methods for their synthesis are known in the art.

The SO₂ oxidation catalysts can be incorporated into the interlayers of the LDHs using wide variety of different metal containing anions other than the ones disclosed in this invention. These include, among other things, metal carbonyls such as V(CO)_6-, Fe₂(CO)₈^2-, Fe(CO)₄^2-, Fe₃(CO)₁₁^2-, Cr(CO)₅^2- etc., or metal halides such as VCl₄^-, CrCl₆^3-, W₂Cl₉^3-, FeCl₆^3- etc., or metal nitriles such as Mn(CN)₆^4-, W(CN)₈^4-, Fe(CN)₆^3- etc. or transition metal complex chelates such as metal-oxalates, Fe(ox)₂^2-, WO₂(ox)^-, Cr(ox)₃^3-, or metal-acetylacetonates etc.

It is known to those skilled in the art that some of the transition metals, particularly iron, is capable of oxidizing NO to NO₂. Thus, the transition metal-containing LDH sorbents, especially the iron-containing sorbents disclosed in this invention may be used to remove NO_x components from flue gas streams and other gas streams. In the gas streams, the calcined LDHs will react with NO_x components to form solid nitrates.

These sorbents may be used, for example, in the form of particles of any suitable size and shape. Such particles may be formed by conventional techniques, such as spray drying, pilling, tableting, bead formation and the like.

In the coal-fired boiler application, the present sorbents may be added, either separately or with coal, to the combustion zone, (e.g., the boiler, temp. 700-1000°C) when combustion takes place. Sorbents then leave the combustion zone with coal ash and can be removed from the bag house. This process will in turn, provide enough contact time for the sorbents to react with SO_x from the flue gas streams. Thus the flue gas leaving the combustion zone/contacting zone systems have reduced amounts of sulfur oxide relative to the processing in the absence of present sorbents. If necessary, reacted sorbents can be separated from the ash, (especially the sorbents with Zn and Al, for the regeneration), e.g., by screening, density separation, or other well-known solid separation techniques. Moreover, the spent sorbents can be safely disposed without any serious environmental pollution, since SO_x in the spent sorbents is now in a thermally stable sulfate form. Furthermore, sorbents disclosed herein could be used in other processes such as hydrocarbon cracking processes where diminision of SO_x from gas streams are necessary.

The following examples will serve to illustrate certain embodiments of the herein disclosed invention.

The preparation of a hydrotalcite-like Mg₃Al-LDH is described in this example.

A solution of 12.8 g Mg(NO₃)₂·6H₂O and 9.4 g Al(NO₃)₃·9H₂O in 100 ml deionized water was added to a solution containing 14 ml 50% NaOH and 5 g Na₂CO₃ (anhydr.) in 200 ml distilled water. The addition was carried out very slowly over a period of 90 minutes, with vigorous stirring. Following the addition, the resulted heavy slurry was heated at 65±5°C for 18 hours with good mixing. The mixture was then cooled to room temperature, and the precipitate was separated by centrifugation. The solid was washed several times with deionized water until the washings were free of salts and then dried in air. The X-ray diffraction pattern of the dried solid corresponded to hydrotalcite and the basal spacing was found to be 7.78·10^-10 m (7.78 Å). Chemical analysis showed the Mg/Al ratio to be 3.2, very near the value expected for hydrotalcite with an idealized formula unit of [Mg₃Al(OH)₈](CO₃)_0.5·xH₂O.

By changing the amounts of Mg²⁺ and Al³⁺ salts used, one can prepare hydrotalcite-like materials with different Mg/Al ratios.

The preparation of [Zn₂Al(OH)₆]X.zH₂O (X = NO₃, Cl) is described in this example.

All the manipulations were carried out under a N₂ atmosphere and the solvents were pre-boiled for about 2 hours under N₂ before use.

To a 200 ml solution of 0.1M Al(NO₃)₃.9H₂O was added a 1.0 M solution of NaOH until the pH of the solution was 7. The white slurry was stirred for one hour, and a 200 ml solution of 0.3M Zn(NO₃)₂ was added drop-wise. The pH of the mixture was maintained at about 6.0, by adding NaOH during the addition. The resulting slurry was boiled for 24 hours under a nitrogen atmosphere. (Upon boiling this suspension for one week produced products with high crystallinity.) The product, [Zn₂Al(OH)₆]NO₃.zH₂O was washed several times with water by centrifugation, and dried in air. The X-ray diffraction powder pattern of the dried solid corresponded to a LDH structure powder pattern of the dried solid corresponded to a LDH structure with a basal spacing value of 7.7·10^-10 m (7.7 Å). Employing a similar method, the Cl^- derivative, [Zn₂Al(OH)₆]Cl.zH₂O can be prepared using AlCl₃ and MgCl₂.

By changing the amounts of Zn²⁺ and Al³⁺ salts used hyrotalcite-like materials with different Zn/Al ratios can be prepared.

A general method for the preparation of polyoxometalate intercalated Zn/Al hydrotalcite-like materials with the general formula [Zn₂Al(OH)₆](POM^n-)_l/n.xH₂O where POM is a polyoxometalate anion of charge n is described in this example.

A boiling solution containing about 5 mequiv. portion of Zn₂Al-X (X=NO₃, Cl) LDH prepared in Example 2, was added drop-wise to a stirred aqueous solution containing about 7.5 mequiv. of polyoxometalate anion. After the addition was complete, the pH of the resultant slurry was adjusted to about 6 by adding dilute HNO₃ acid. The slurry was stirred for about 1 hour and the solid product was isolated and washed thoroughly with water by centrifugation. The X-ray diffraction powder patterns of the dried solids correspond to a hyrotalcite-like layered structures, with polyoxometalate anions in the interlayer. The basal spacings are given in Table 1. Chemical analyses conformed to the structure Zn₂Al(OH)₆[POM^n-]_l/n.YH₂O, where POM represent the polyoxometalate with a charge of n.

The anions with lower charge such as [PW₁₂O₄₀]^3-and [SiW₁₂O₄₀]^4- show no ion exchange, whereas intermediate anions show partial intercalation (e.g. [PCuW₁₁O₃₉(H O)]^5-). Furthermore, polyoxometalate anions with beta-Keggin structure such as beta-[SiV₃W₉O₄₀]^7- undergo partial intercalation. The preparation of polyoxometalate intercalated Zn/Al-LDH materials from [Zn₂Al(OH)₆]NO₃.zH₂O according to the U.S. Patent No. 4,454,244 resulted in X-ray amorphous materials.

The preparation of a Mg/Al LDH of the formula, [Mg₆Al₂(OH)₁₆]OH.xH₂O LDH from hydrotalcite is described in this example.

A sample of hydrotalcite, prepared according to Example 1, was calcined at 500°C for three hours. A 5 g portion or this sample was pulvarized and suspended in a 200 ml of hot (65°C) degassed deionized water to form a white slurry. The resulting slurry was then stirred vigorously at 65°C for one hour under an atmosphere of nitrogen, to form the hydroxide derivative [Mg₃Al(OH)₈]OH.xH₂O. The resulting slurry was cooled to room temperature and volume was adjusted to 250 ml with deionized water. The X-ray diffraction powder pattern of the dried solid corresponded to a hydrotalcite structure. The basal spacing was found to be 7.76·10^-10 m (7.76 Å).

This example describes the general method adopted in preparing the polyoxometallate-intercalated Mg₃Al-LDHs.

A solution containing about 25 mmol portion of Mg₃Al LDH-OH slurry prepared according to Example 4 was added dropwise to stirred aqueous solution containing 40 mmols of polyoxometalate anion under an atmosphere of nitrogen. The resulting slurry was stirred at ambient temperature for about 18 hours and the solid product was isolated and washed thoroughly with water by centrifugation. The X-ray diffraction powder patterns of the dried solids corresponded to the hyrotalcite-like structure with intercalated polyoxometalates. The basal spacings are given in Table 1.

The uptake of SO_x by various LDH sorbents was determined by thermogravimetric analysis using a Cahn Model TG-121 thermogravimetric analyzer.

Approximately 50-mg portions of the sorbent was placed on a quartz pan in the thermogravimetric balance. Subsequent treatment of the sample was carried out in a three step procedure.

1. Step 1: Under a flow of air as a carrier gas (200ml/min), sample was allowed to equilibrate at 25°C for 15 minutes and slowly heated (5°C/min) to the calcining temperature, typically 700°C. The sample was maintained at this temperature for an additional 1 hour.
2. Step 2: SO₂ gas (0.5%) then was introduced into the carrier gas at the temperature and the weight was monitored for a 1 hour period. For the more reactive sorbents a rapid initial weight uptake of SO_x was observed, especially with sorbents containing iron. The weight increase corresponded to the amount of SO₃ absorbed by the calcined sample (Table 2). For a typical TGA plot see Figure 1.
3. Step 3: Passage of SO₂ into the carrier gas was ceased and the sample weight at reaction temperature was monitored for another 1 hour. This step was carried out in order to determine the thermal stability of the metal sulfate products formed after the reaction with SO_x. All of the samples containing Mg and Al in the layers showed little or no weight loss, whereas most of the samples that contained Zn in the layers showed a significant weight loss (Table 3).

Hydrotalcite-like materials intercalated with metal polyoxoanions as in Examples 3 and 5 were tested for SO_x sorption at 700°C according to the procedure of Example 6. The results are given in Table 2. LDHs that contain Mg and Al show better SO_x adsorption than that contain Zn and Al. The preferred polyoxoanion is FeO₄^2-, which showed more than 85% conversion of Mg sites to MgSO₄ when exposed to SO₂. However, polyoxoanions containing V, W and Mo also show enhanced SO_x adsorption, compared to free hydrotalcite. Compared to polyoxometalates intercalated Mg₃Al-LDHs, the corresponding Zn₂Al-LDH formed metastable products with SO_x. Upon further calcining in the absence of SO₂ (Step 3, Example 6), most of these spent sorbents lost weight (Table 3). Accordingly some of these spent sorbents can be regenerated for further use, (See Example 9).

The hydrotalcite-like material intercalated with V₁₀O₂₈^6- according to Example 4, was tested for SO_x sorption at different calcining temperatures according to the procedure of Example 6. Results are given in Figure 2. Preferred calcining temperatures were in the range 550-800°C where 32% to 52% conversion of Mg sites to MgSO₄ was observed. At very high temperatures (>900°C) the uptake was very low.

Some of the Zn₂Al-LDHs intercalated with polyoxometalates according to Example 5, were regenerated after exposing to SO₂ and tested for the SO₂ re-adsorption using the following procedure.

Approximately a 50-mg portion of the Zn₂Al LDH intercalated with SiV₃W₉O₄₀^8- was tested for SO_x uptake using the Steps 1 and 2 in the procedure given in Example 7, and sample was further treated as follows:

1. Step 3: Passage of SO₂ into the carrier gas was ceased and the sorbent was heated to 800°C (5°C/min) and the temperature was maintained at this value for an additional 30 minutes and cooled down to 700°C (-5°C/min).
2. Step 4: SO₂ gas (0.5%) was again introduced into the carrier gas at 700°C for a 1 hour period.
3. Step 5: Passage of SO₂ was ceased and the reaction temperature was kept at 700°C for another 1 hour period.

During Step 2, when SO₂ was introduced for the first time a weight uptake of 5.6% was observed (Figure 3). Almost all (90%) of this weight gain was lost in Step 3 at higher temperatures from this spent sorbent, indicating the release of the absorbed SO_x. The SO₃ uptake by this regenerated sorbent was 6.4% in Step 4 indicating that this recovered material was as good as the virgin sorbent.