Syntheses of Disordered and A1-rich Hydrotalcite-like Compounds

-Hydrotalcite-like compounds, [Mg~ _xAlx(OH)2] x+ [xX-. n H20], where X= 1/2 CO32or OH , were prepared by hydrothermal syntheses at Pnzo = 100 MPa and T = 100~176 Starting materials were MgO, "y-A1203, H20 , and MgC204" 2H20. The synthesis depended on temperature, pressure, the A1/(A1 + Mg) ratio x, and the CO2 content of the starting material. Previously an A1 content of x = 0.33 was thought to be the upper limit in these double-layer compounds, but by using pressure the Al-content was increased to x = 0.44. Up to x = 0.33, ao decreased linearly to about 3.04 ~, but for x >0.33, ao remained nearly constant at this value. For the synthesized products the layer thickness c' varied between 7.40 and 7.57 ~ in contrast to the natural phases wherein c' varies from 7.60 to 7.80 ~. At higher temperatures CO2-free syntheses, i.e., those without Mg-oxalate, resulted in a disordered hydrotalcitelike phase. The transition temperature between the ordered and the disordered hydrotalcite-like phase depended on the Al-content, x. I N T R O D U C T I O N The carbona te -hydroxide [Mgl_~Alx(OH)2] x+ [xX 9 n HzO] x-, where X = l/z CO32or O H , occurs in two d imorph ic forms, rhombohedra l hydrota lc i te and hexagonal manassei te . Both are doublelayer structures. Rev iews o f c o m m o n l y non-s to ich iomet r i c doublelayer structures were made by A l l m a n n (1970) and Tay lor (1973). Hydrota lc i te and manasse i te are actually stacking po lymorphs that have the same basic substructure. They consist o f bruci tel ike layers Me(OH)2, in which Mg 2§ and AP § are r andomly dis t r ibuted a m o n g the octahedral posi t ions. In minerals , the va lue x = A1/ (A1 + Mg) varies be tween about 0.22 and 0.33. Because o f the different size o f the Mg 2§ and AP § ions, the lattice paramete r a is cont ro l led by x (a = 3.14 /~ for x = 0 in brucite, and a = 3.05 A for x = 0.33). The bruci telike layers are separated by d isordered (nearly l iquid) interlayers o f water molecules, CO32-, a n d / o r O H anions. Charge compensa t ion be tween the bruci tel ike layers and interlayers is real ized by hydrogen bonds, which also exist wi th in the interlayer. In the hydrota lc i te structure the layers are stacked with r h o m b o h e d r a l symmet ry ( B C . . . C A . . . A B . . . B C . . . for the brucite-like mainlayers) , and three doublelayers are present per uni t cell (c = 3c' --23.4/~) ; in the manasse i te structure they are stacked with hexagonal s y m m e t r y (BC . . . CB . . . BC . . . ) , and two doublelayers are present per unit cell (c = 2c' = 15.6/~). Fo r hydrota lc i te f rom Snarum, Norway, where x ~ 0.25, R o y et al. (1953) repor ted c' = 7 .63/~, whereas we found for the same mater ia l c' = 7.80/~. Fo r hydrota lc i te f rom Vezna, Czechoslovakia , where x = 0.33, C e r n ) (1963) repor ted c' = 7.62/~. The single crystals o f this mater ia l yielded c' = 7 .603 /~ (Al lmann and Jepsen, 1969). In nature hydrota lc i te and manasse i te are c o m m o n l y intergrown. Manasse i te general ly forms the core and hydrota lc i te . the outer part o f a grain. Thus, hydrota l cite appears to form later than the coexist ing manasseite and, presumably , at lower temperatures . To date, synthetic hydrotalc i tel ike c o m p o u n d s have been prepared hydro the rmal ly (Roy et al . , 1953), by coprecipi ta t ion f rom m i x e d MgC12-AIC13-solutions (Fei tknecht and Gerber , 1942; Gas tuche et al . , 1967), and f rom MgO-A12Oa-suspensions (Mascolo and Mar ino , 1980). Hydrota lc i te always fo rmed at lower tempera tures ; in some exper iments a d isordered phase was repor ted (Gastuche et al. , 1967), but manasse i te was neve r synthesized. E X P E R I M E N T A L In the present s tudy hydrotalc i tel ike c o m p o u n d s were prepared hydro the rmal ly at t empera tu res between 100 ~ and 350~ and a water pressure o f 100 MPa. A l u m i n a gel (,y-A1203) , M g O (freshly annea led at 1050~ MgC204 9 2H20, and dis t i l led and bo i led H 2 0 were used as starting materials . F r o m these oxides, mix tures wi th different Mg:A1 rat ios were prepared. T w o series o f exper iments were carr ied out: one wi th mix tures conta in ing CO2, and the o ther free f rom CO2, i.e., wi thou t MgC204 ' 2H20 . In bo th series the Mg:A1 rat io was va r i ed be tween x = 0.25 and x = 0.60. The react ion t imes var ied f rom 7 to 42 days. Exper imen t s were carr ied out in welded gold tubes o f about 4 m m internal d i amete r in Tut t le type cold-seal pressure vessels. At tempera tures above 200~ Moreytype pressure vessels were also used. Af ter the runs, the vessels were cooled to r o o m tempera tu re wi th in 10 to 15 min . Fo r the CO2-free series the gold tubes were opened in a g love-box conta in ing dry nitrogen. The products were Copyright 9 1986 The Clay Minerals Society 507 508 Pausch, Lohse, Schiirmann, and Allmann Clays and Clay Minerals