(3) Metal salt (MX$_{2 }$ or MX$_{3}$ ) generating the protic acid (HX) by hydrolysis. 

[0080] The anion (X-) is preferably chlorine ion (Ci-), bromine ion (Br-), nitrate ion (NO$_{3 }$ -), perchlorate ion (ClO$_{4}$-), chlorate ion (ClO 3 -), and acetate ion (CH $_{3}$Coo-). X in general formula (2) represents corresponding atom, which is released as anion (X-) in aqueous solution, and protic acid (HX) is desirably HCl, HBr, HNO$_{3}$ , HC1O$_{4}$ , HC1O$_{3}$ , or CH$_{3}$COOH. However, it is only necessary that the protic acid is monovalent anion, and anions other than those listed above are not excluded regardless of whether they are organic or inorganic. 

[0081] R, R’, and R" of the amine acid salt (NRR’R".HX) are groups respectively represented by composition formula C,HpOq (t=0 to 6, p=l to 13, q=0 to 2). 

[0082] Of the amine acid salt used in Examples, three are selected to show how they are represented by this composi› tion formula. 

[0083] When all ofR, R’, andR" satisfyt=q=0and p=l, the formula becomes NH$_{3}$ .HX, which represents ammonium salt such as NH$_{4}$Cl and NH$_{4}$ClO$_{4}$ . 

[0084] When R satisfies t=2, p=5, and q=0, and both R’ and R" satisfy t=q=0 and p=l, the formula becomes C$_{2}$H$_{6}$NH$_{2}$ . HX, which represents diethylamine hydrochloride, one of amine acid salts, when X=Cl. 

[0085] When both Rand R’ satisfy t=2, p=5, and q=l 1, and R" satisfies t=q=0 and p= 1, the formula becomes C$_{2}$H$_{5}$ONH. HX, which represents diethanol hydrochloride, one of amine acid salt, when X=Cl. 

[0086] Specifically, R, R’, and R" are hydrogen, hydroxyl group or organic group. Here, organic group means alkyl group having carbon number of 1 to 6 substituted by alkyl group or hydroxyl group having carbon number of 1 to 6. 

[0087] Metal salt Mis one of the following metals: divalent Zn, Mg, and Ca, and trivalent Fe and Al. However, metal salts other than those listed above are not excluded, provided that they generate acid in an aqueous solution. 

[0088] Substituting carbonate ions contained in LDH by anions means that value b in formula (1) becomes larger than value b of the starting material. When all the carbonate ions have been substituted by anions ex-) completely, the obtained anion-exchanging LDH is represented by general formula (3) shown below: 

[Chemical formula 10] 

[0089] In formula (3) shown above, a represents numeric value range of 1.8sas4.2, z is 2(a+l), Q represents divalent metal. T represents trivalent metal, X represents an atom that turns into monovalent anion, and n represents the amount of water existing between layers. n changes within the following range depending on the humidity of the atmosphere: 0sns5. 

[0090] The anion-exchanging LDH represented by general formula (3) has high anion-exchanging property. General formula (3) is a chemical formula typically representing a case where ion exchange into anions (X-) has been completed most perfectly. By relaxing reaction conditions, partial sub› stitution into anions ex-) occurs, thus naturally generating a chemical compound containing residual carbonate ions. 

[0091] A case where complete substitution product is prac› tically not required and a case where partial substitution is practically necessary are included in the scope of the present invention. Specifically, it is only necessary that in formula (1 ), b increases to become larger than value b of the starting 

material and reach the desired value, and Qa T(OH)$_{2}$ ex-). nH$_{2}$O need not be strictly maintained. 

[0092] Supplemental description of anion-exchanging LDH will be given below. Generally, in an aqueous solvent, ions having large valence number (multivalent anions such as divalent and trivalent anions) or small-size ions have low anion-exchanging property. On the contrary, monovalent anions are known to have high anion-exchanging property. The larger the size of the ions, the higher the anion-exchang› ing property. In the present invention, X of the anion-ex› changing LDH represents one of the following anions: chlo› rine ion (Ci-), bromine ion (Br-), nitrate ion (NO$_{3 }$ -), perchlorate ion (ClO$_{4 }$-), chlorate ion (ClO 3 -), and acetate ion (CH$_{3}$Coo-). Since all of these are monovalent anions and large in size, affinity to LDH is low, and the LDH containing these has high anion-exchanging property. Consequently, anions other than those described above are not excluded, provided that they satisfy the similar conditions. 

[0093] Organic solvents having carbon number of 1 to 4, such as alcohols, acetone, tetrahydrofuran, are desirable. 

[0094] As alcohols, methanol, ethanol, n-propanol, i-pro› panol (2-propanol), n-butanol, and t-butanol can be used desirably. Since organic solvents having carbon number of 1 to 4 can be handled easily, are easily available, have high volatility and low boiling points, solvents can be removed easily after reaction. In addition, since there is no possibility that carbon dioxide in the air dissolves, thereby generating carbonate ions, secondary incorporation of carbonate ions between LDH layers does not occur. 

[0095] In an organic solvent containing an acidic com› pound, the amount of the acidic compound is calculated based on the amount of carbonate ions (CO$_{3 }$ 2 -) existing in carbon› ate ion-type LDH as the starting material. Since CO$_{3 }$ 2 - has a valence of -2, the amount of H+ (proton) required to com› pensate it completely is twice the molar quantity of the car› bonate ions, namely f=[H+]/(2x[CO$_{3 }$ 2 -]), which is inter› preted as valence-to-valence conversion. 

[0096] In the case of amine acid salt, value f was deter› mined, assuming that one proton was released from NRR’R". HX. In the case of acidic metal salt, value fwas determined, assuming that HX of a number of m were generated from MXm by hydrolysis. Since the minimum requisite for 100% substitution of co/- is f=l not only with the amount of protons but also with the amount of anions ex-) existing in the system, theoretically f must be 1 or higher to achieve com› plete substitution. 

(3) 

[0097] In an aqueous solvent, weight loss starts to occur before carbonate ions are substituted by other anions com› pletely. Meanwhile, since dissolution of LDH is suppressed more in an organic solvent than in an aqueous solvent, ion exchange occurs before weight loss starts to occur. 

[0098] In the case of decarbonation using protic acid and alcohol, acidic compound of f=l or higher is necessary for complete substitution. However, if the amount of acid is excessive, LDH dissolves even in alcoholic solvent, thus causing weight loss. The degree of weight loss varies depend› ing on the type of the alcohol and the acidic compound and water content, but generally, when f=l to 2, decarbonation is completed successfully without dissolution of LDH. In the case of decarbonation using amine acid salt and alcohol, f=2 to 3 is required, but since acidity is low, weight loss due to dissolution is not observed even if excessive amount of f=6 is added. 

