Smectite, acid-activated clay

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

In the early history of acid catalysis, clays had a significant utility. The acid leaching of natural smectite clays produced amorphous silica-alumina [Si-Al] materials with large surface areas and active acid sites, which were used as acid catalysts [31],... 

Montmorillonite is a layered smectite clay. Acid activation replaces the interlamellar cations with protons, leaches Al from octahedral layers resulting in increase of surface area, porosity and acidity. Clay is activated with a mineral acid for different time intervals. They are characterised by XRD, surface area and acidity by stepwise temperature desorption of ammonia Catalytic activity is studied on aniline alkylation reaction. 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

PCNs, acid-activated bentonites, organophilic montmorillonites, and pillared clays with designed micro- and mesoporosity (Giiven, 2009) find specific and selective applications as catalysts and catalyst carriers in the petrochemical industry, and also in environmental applications. The capabUily of smectites to absorb not... 

F. Kooli and P. C. M. M. Magusin, Adsorption of cetyltrimethyl ammonium ions on an acid-activated smectite and their thermal stability. Clay Minerals, 40 (2005), 233 3. 

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

The smectite clays exhibit catalytic activity related to the presence of add sites contained at the surface of the clay layer. The role the pillar plays in the catalytic activity of these materials has not been successfiiUy elucidated due to the overriding activity found in the clay itself. The aluminum pillar, for example, produces both Bronsted and Lewis acidity upon thermal treatment ... 

Smectite clays have three important properties related to catalytic activity, intercalation, swelling and cation exchange capacity. When smectites are immersed in water, both intercalation of water molecules and swelling occur. The suspended clay can also freely exchange its interlayer cations for other cations in solution. Interlayer water molecules are dissociated producing protons and exhibit Bronsted acidity (Reaction l).8... 

The discussion here will pertain primarily to smectites clays, since they comprise the bulk of catalytic applications. Both Lewis and Bronsted acidity are dependent upon a clay s hydration state, and therefore the activity may vary throughout the course of a reaction. Strong Bronsted acidity in smectites is derived... 

Pillared clays with pillars based on A1 and also different elements (e.g., Cr, Fe) or with mixed pillars have been extensively investigated in numerous catalytic reactions of hydrocarbons, principally in proton-catalyzed reactions. Catalytic aspects have been reviewed in several articles (3,91,92). Comparison of the catalytic performances of acid solids with potential catalytic applications preferably with a reference catalyst is a straightforward way to obtain a preliminary information on their efficiency. With this approach, the activity of Al-pillared materials prepared with different smectites and pillaring methods have been compared, using the hydroisomerization of linear paraffins over Pt-impregnated pillared samples. In these bifiinctional catalysts, the metal function is necessary to dehydrogenate the paraffin and rehydrogenate the branched olefins, and the protons aid the formation of carbenium ions, which isomerize via protonated cyclopropane... 

Nanoclays can be categorized into cationic and anionic types. Cationic nanoclays are based on smectite clays. An example is montmorillonite (MMT), a hydrated Al, Mg silicate that may contain cations such as Na+ and Ca++ between the anionic layers. In contrast, anionic clays contain cationic layers and anions such as Cr and in the interlayer space. Typical examples include layered double hydroxides (LDH) and hydrotalcite (HT), a mostly synthetic hydrated magnesium and aluminum carbonate salt. Whereas MMT is commonly used as a nanofiller to improve thermal, mechanical, and barrier properties, LDHs have many attractive properties that lead to application as surfactant adsorbents, biohybrid materials, antacid food formulations, acid neutralizers, and active pharmaceutical ingredients excipients [37, 13, 28, 14, 35]. 

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