Chemistry of inorganic compounds – Zeolite
Patent
1998-04-20
2000-01-25
Bell, Mark L.
Chemistry of inorganic compounds
Zeolite
423713, 423714, 423715, 423716, 423718, 4233281, 4233282, 423DIG23, 502 85, 502 86, 502232, 502407, 502408, 95 90, 95143, 95147, 95902, C01B 3902, C01B 3944
Patent
active
060175087
DESCRIPTION:
BRIEF SUMMARY
In one aspect, this invention pertains to a process of modifying the porosity of aluminosilicates and silicas. In another aspect, this invention pertains to crystalline mesoporous aluminosilicate compositions.
It is known that porous aluminosilicates and silicas contain an array of pores or channels, and optionally, cages. The term "porosity" will henceforth refer to this collected array of pores, channels and cages. These solids may be amorphous or may have a periodic crystalline structure. The array of pores and channels and cages may be irregular, that is non-periodic, or alternatively, they may be regular or periodic. The pores or channels may be isolated or interconnected, and may be one, two or three dimensional. For the purposes of this invention a "micropore" is defined as a pore having a radius ranging from 2 Angstroms (.ANG.) to less than 10 .ANG., while a "mesopore" is defined as a pore having a radius ranging from 10 .ANG. up to 100 .ANG.. Pores having a radius larger than 100 .ANG. are typically classified as "macropores."
Microporous aluminosilicates and silicas are useful as catalyst supports and as selective adsorbents for purifying gases and for separating mixtures of chemicals and isomers. In addition, microporous aluminosilicates, for example zeolites, are well known as shape selective catalysts for a wide variety of industrial organic processes. Illustrative of such processes are the isomerization and cracking of aliphatic hydrocarbons and the alkylation and transalkylation of aromatic hydrocarbons. The utility of these microporous solids depends in part on the accessibility of organic reagents and catalytic metals to the micropores or cages. If access is limited or blocked, the utility of the aluminosilicate or silica may be hampered.
Increased accessibility to the micropores of an aluminosilicate zeolite is commonly effected by extracting the zeolite with a strong acid solution. The acid removes debris in the channels, such as extra-framework alumina, and may create mesoporosity by extracting some of the framework aluminum. This process is commonly referred to as "dealumination." An increase in the mesoporosity of a microporous aluminosilicate may correlate with its increased activity as a catalyst or adsorbent.
Disadvantageously, in small ring aluminosilicates and silicas, namely those composed of 8-rings or less, the interior of the micropores may be inaccessible to acids. Other aluminosilicates, such as those having a silica/alumina molar ratio of 2 or less may be unstable under acid extraction conditions. Other silicas and aluminosilicates, irrespective of pore size, may not respond to acid treatment. Ferrierite is an example of a 10-ring zeolite which does not readily dealuminate upon treatment with acid. Accordingly, in all of these systems it can be difficult to modify their porosity solely by acid extraction.
By way of example, the mineral bikitaite is a lithium aluminosilicate represented by the oxide formula Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2.2H.sub.2 O. The X-ray diffraction pattern of bikitaite, described by W. M. Meier and D. H. Olson in the Atlas of Zeolite Structure Types, 3rd edition, Butterworth-Heinemann, 1992, p. 60-61, reveals a monoclinic framework structure containing one-dimensional 8-ring channels having the dimensions 2.8 Angstroms (.ANG.) by 3.7 .ANG.. Disadvantageously, the lithium ions cannot be fully exchanged with hydrogen ions without destroying the bikitaite structure. The microporous channels in bikitaite are not accessible to small molecules, and bikitaite is not known to be an efficient adsorbent or catalyst.
A cesium aluminosilicate is known which is structurally related to, but not identical to bikitaite. The composition on an anyhydrous basis is represented by the following formula, as noted in U.S. Pat. No. 4,820,502: least in part by other cations, including hydrogen ions, ammonium ions and certain metal ions. Disadvantageously, the cesium ions, which are more plentiful than sodium ions, are not exchangeable. Consequently, the micropores of
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Garces Juan M.
Millar Dean M.
Bell Mark L.
Sample David
The Dow Chemical Company
Zuckerman Mariz F.
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