Molecular sieve abstract
Molecular sieve compositions having three-dimensional microporous
framework structures of CrO.sub.2 AlO.sub.2 and PO.sub.2 tetrahedral
oxide units are disclosed. These molecular sieves have an empirical
chemical composition on an anhydrous basis expressed by the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the molar amount of "R" present per mole of
(Cr.sub.x Al.sub.y P.sub.z)O.sub.2 ; and "x", "y"
and "z" represents the mole fractions of chromium, aluminum
and phosphorus, respectively, present as tetrahedral oxides. Their
use as adsorbents, catalysts, etc. is also disclosed.
Molecular sieve claims
We claim:
1. Process for converting a hydrocarbon feed to a hydrocarbon converted
product, which comprises contacting said hydrocarbon feed under
hydrocarbon converting conditions with a molecular sieve containing
chromium-aluminum-phosphorus oxide (CAPO), said molecular sieve
being a crystalline molecular sieve having intracrystalline pore
system selected from the group consisting of:
(a) crystalline molecular sieves having three-dimensional microporous
framework structures of CrO.sub.2.sup.n, AlO.sub.2 and PO.sub.2
tetrahedral units, where "n" has a value of -1 or +1
having an empirical chemical composition on an anhydrous basis expressed
by the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the molar amount of "R" present per mole of
(Cr.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero
to about 0.3; and "x", "y" and "z"
represent the mole fractions of chromium, aluminum and phosphorus,
respectively, present as tetrahedral oxides, said mole fractions
being such that they are within the hexagonal compositional area
defined by points A, B, C, D, E and F of FIG. 1; and
(b) crystalline molecular sieves having three-dimensional microporous
framework structures of CrO.sub.2.sup.n, AlO.sub.2 and PO.sub.2
tetrahedral units where "n" has a value of zero, having
an empirical chemical composition on an anhydrous basis expressed
by the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the molar amount of "R" present per mole of
(Cr.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero
to about 0.3; and "x", "y" and "z"
represent the mole fractions of chromium, aluminum and phosphorus,
respectively, present as tetrahedral oxides, said mole fractions
being such that they are within the pentagonal compositional are
defined by points G, H, I, J and K of FIG. 2.
2. Process according to claim 1 wherein the hydrocarbon conversion
process is cracking.
3. Process according to claim 1 wherein the hydrocarbon conversion
process is hydrocracking.
4. Process according to claim 1 wherein the hydrocarbon conversion
process is hydrogenation.
5. Process according to claim 1 wherein the hydrocarbon conversion
process is polymerization.
6. Process according to claim 1 wherein the hydrocarbon conversion
process is alkylation.
7. Process according to claim 1 wherein the hydrocarbon conversion
process is reforming.
8. Process according to claim 1 wherein the hydrocarbon conversion
process is hydrotreating.
9. Process according to claim 1 wherein the hydrocarbon conversion
process is isomerization.
10. Process according to claim 9 wherein the isomerization conversion
process is xylene isomerization.
11. Process according to claim 1 wherein the hydrocarbon conversion
process is dehydrocyclization.
12. Process according to claim 1 wherein the mole fractions of
chromium, aluminum and phosphorus present as tetrahedral oxides
are within the tetragonal compositional area defined by points a,
b, c and d of FIG. 3.
13. Process according to claim 1 wherein the mole fractions of
chromium, aluminum and phosphorus present as tetrahedral oxides
are within the hexagonal compositional area defined by points n,
o, p, q, r and s of FIG. 3.
14. Process according to claim 1 wherein the mole fractions of
chromium, aluminum and phosphorus present as tetrahedral oxides
are within the pentagonal compositional area defined by points e,
f, g, h and i of FIG. 4.
15. Process according to claim 1 wherein the mole fractions of
chromium, aluminum and phosphorus present as tetrahedral oxides
are within the tetragonal compositional area defined by points j,
k, l and m of FIG. 5.
16. Process according to claim 1 wherein, before being contacted
with said hydrocarbon, said molecular sieve is calcined at a temperature
sufficiently high to remove at least some of any organic templating
agent present in the intracrystalline pore system.
17. Process according to claim 1 wherein said crystalline molecular
sieve has a characteristic X-ray powder diffraction pattern which
contains at least the d-spacings set forth in one of the following
Tables A to H and J to V;
Molecular sieve description
FIELD OF THE INVENTION
The instant invention relates to processes for the use of a novel
class of crystalline microporous molecular sieves as adsorbents
and catalysts. The invention relates to novel chromium-aluminum-phosphorus-oxide
molecular sieves containing framework tetrahedral oxide units of
chromium, aluminum and phosphorus. These compositions may be prepared
hydrothermally from gels containing reactive compounds of chromium,
aluminum and phosphorus capable of forming framework tetrahedral
oxides, and preferably at least one organic templating agent which
functions in part to determine the course of the crystallization
mechanism and the structure of the crystalline product.
BACKGROUND OF THE INVENTION
Molecular sieves of the crystalline aluminosilicate zeolite type
are well known in the art and now comprise over 150 species of both
naturally occurring and synthetic compositions. In general the crystalline
zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2
tetrahedra and are characterized by having pore openings of uniform
dimensions, having a significant ion-exchange capacity and being
capable of reversibly desorbing an adsorbed phase which is dispersed
throughout the internal voids of the crystal without displacing
any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions which are not zeolitic,
i.e. do not contain AlO.sub.2 tetrahedra as essential framework
constituents, but which exhibit the ion-exchange and/or adsorption
characteristics of the zeolites are also known. Metal organosilicates
which are said to possess ion-exchange properties, have uniform
pores and are capable of reversibly adsorbing molecules having molcular
diameters of about 6 .ANG. or less, are reported in U.S. Pat. No.
3941871 issued Mar. 2 1976 to Dwyer et al. A pure silica polymorph,
silicalite, having molecular sieving properties and a neutral framework
containing neither cations nor cation sites is disclosed in U.S.
Pat. No. 4061724 issued Dec. 6 1977 to R. W. Grose et al.
A recently reported class of microporous compositions and the first
framework oxide molecular sieves synthesized without silica, are
the crystalline aluminophosphate compositions disclosed in U.S.
Pat. No. 4310440 issued Jan. 12 1982 to Wilson et al. These materials
are formed from AlO.sub.2 and PO.sub.2 tetrahedra and have electrovalently
neutral frameworks as in the case of silica polymorphs. Unlike the
silica molecular sieve, silicalite, which is hydrophobic due to
the absence of extra-structural cations, the aluminophosphate molecular
sieves are moderately hydrophilic, apparently due to the difference
in electronegativity between aluminum and phosphorus. Their intracrystalline
pore volumes and pore diameters are comparable to those known for
zeolites and silica molecular sieves.
In U.S. Pat. No. 4440871 there is described a novel class of
silicon-substituted aluminophosphates which are both microporous
and crystalline. The materials have a three dimensional crystal
framework of PO.sub.2.sup.+, AlO.sub.2.sup.- and SiO.sub.2 tetrahedral
units and, exclusive of any alkali metal or calcium which may optionally
be present, an as-synthesized empirical chemical composition on
an anhydrous basis of:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the moles of "R" present per mole of (Si.sub.x
Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3 the
maximum value in each case depending upon the molecular dimensions
of the templating agent and the available void volume of the pore
system of the particular silicoaluminophosphate species involved;
and "x", "y", and "z" represent the
mole fractions of silicon, aluminum and phosphorus, respectively,
present as tetrahedral oxides. The minimum value for each of "x",
"y", and "z" is 0.01 and preferably 0.02. The
maximum value for "x" is 0.98; for "y" is 0.60;
and for "z" is 0.52. These silicoaluminophosphates exhibit
several physical and chemical properties which are characteristic
of aluminosilicate zeolites and aluminophosphates.
In U.S. Pat. No. 4500651 there is described a novel class of
titanium-containing molecular sieves whose chemical composition
in the as-synthesized and anhydrous form is represented by the unit
empirical formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the moles of "R" present per mole of (Ti.sub.x
Al.sub.y P.sub.z)O.sub.2 and has a value of between zero and about
5.0; and "x", "y" and "z" represent
the mole fractions of titanium, aluminum and phosphorus, respectively,
present as tetrahedral oxides.
In U.S. Pat. No. 4567029 there is described a novel class of
crystalline metal aluminophosphates having three-dimensional microporous
framework structures of MO.sub.2 AlO.sub.2 and PO.sub.2 tetrahedral
units and having an empirical chemical composition on an anhydrous
basis expressed by the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the moles of "R" present per mole of (M.sub.x
Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; "M"
represents at least one metal of the group magnesium, manganese,
zinc and cobalt; "x", "y", and "z"
represent the mole fractions of the metal "M", aluminum
and phosphorus, respectively, present as tetrahedral oxides.
In U.S. Pat. No. 4544143 there is described a novel class of
crystalline ferroaluminophosphates having a three-dimensional microporous
framework structure of FeO.sub.2 AlO.sub.2 and PO.sub.2 tetrahedral
units and having an empirical chemical composition on an anhydrous
basis expressed by the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the moles of "R" present per mole of (Fe.sub.x
Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; and
"x", "y" and "z" represent the mole
fractions of the iron, aluminum and phosphorus, respectively, present
as tetrahedral oxides.
The instant invention relates to new molecular sieve compositions
having framework tetrahedral units of CrO.sub.2.sup.n, AlO.sub.2.sup.-
and PO.sub.2.sup.+ where "n" is -1 0 or +1.
DESCRIPTION OF THE FIGURES
FIG. 1 is a ternary diagram wherein parameters relating to the
instant compositions are set forth as mole fractions for when "n"
equals -1 or +1 as hereinafter discussed.
FIG. 2 is a ternary diagram wherein parameters relating to the
instant compositions are set forth as mole fractions for when "n"
equals zero, as hereinafter discussed.
FIG. 3 is a ternary diagram wherein parameters relating to preferred
compositions are set forth as mole fractions.
FIG. 4 is a ternary diagram wherein parameters relating to preferred
compositions are set forth as mole fractions.
FIG. 5 is a ternary diagram wherein parameters relating to preferred
compositions are set forth as mole fractions.
FIG. 6 is a ternary diagram wherein parameters relating to the
reaction mixtures employed in the preparation of the compositions
of this invention are set forth as mole fractions.
SUMMARY OF THE INVENTION
The instant invention relates to a new class of chromium-aluminum-phosphorus-oxide
molecular sieves having a crystal framework structure of CrO.sub.2.sup.n,
AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral oxide units where
"n" is -1 0 or +1. These new molecular sieves exhibit
ion-exchange, adsorption and catalytic properties and, accordingly,
find wide use as adsorbents and catalysts. The members of this novel
class of compositions have crystal framework structures of CrO.sub.2.sup.n,
AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral units and have an
empirical chemical composition on an anhydrous basis expressed by
the formula:
wherein "R" represents at least one organic templating
agent present in the intracrystalline pore system; "m"
represents the molar amount of "R" present per mole of
(Cr.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of zero to about
0.3; and "x", "y" and "z" represent
the mole fractions of chromium, aluminum and phosphorus, respectively,
present as tetrahedral oxides. These molecular sieve compositions
comprise crystalline molecular sieves having a three-dimensional
microporous framework structure of CrO.sub.2.sup.n, AlO.sub.2.sup.-
and PO.sub.2.sup.+ tetrahedral units where "n" is -1
0 or +1.
The molecular sieves of the instant invention will be generally
referred to by the acronym "CAPO" to designate the framework
of CrO.sub.2.sup.n, AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral
units. Actual class members will be identified by denominating the
various structural species which make up the CAPO class by assigning
a number and, accordingly, are identified as "CAPO-i"
wherein "i" is an integer. The given species designation
is not intended to denote a similarity in structure to any other
species denominated by a numbering system.
DETAILED DESCRIPTION OF THE INVENTION
The instant invention relates to a new class of chromium-aluminum-phosphorus-oxide
molecular sieves comprising a crystal framework structure of CrO.sub.2.sup.n,
AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral oxide units, where
"n" is -1 0 or +1. These new molecular sieves exhibit
ion-exchange, adsorption and catalytic properties and, accordingly,
find wide use as adsorbents and catalysts.
In forming the reaction mixture from which the instant molecular
sieves are formed the organic templating agent can be any of those
heretofore proposed for use in the synthesis of conventional zeolite
aluminosilicates. In general these compounds contain elements of
Group VA of the Periodic Table of Elements, particularly nitrogen,
phosphorus, aresenic and antimony, preferably nitrogen or phosphorus
and most preferably nitrogen, which compounds also contain at least
one alkyl or aryl group having from 1 to 8 carbon atoms. Particularly
preferred compounds for use as templating agents are the amines,
quaternary phosphonium and quaternary ammonium compounds, the latter
being represented generally by the formula R.sub.4 X.sup.+ wherein
"X" is nitrogen or phosphorus and each R is an alkyl or
aryl group containing from 1 to 8 carbon atoms. Polymeric quaternary
ammonium salts such as [(C.sub.14 H.sub.32 N.sub.2)(OH).sub.2 ].sub.x
wherein "x" has a value of at least 2 are also suitably
employed. The mono-, di- and tri-amines are advantageously utilized,
either alone or in combination with a quaternary ammonium compound
or other templating compound. Mixtures of two or more templating
agents can either produce mixtures of the desired CAPOs or the more
strongly directing templating species may control the course of
the reaction with the other templating species serving primarily
to establish the pH conditions of the reaction gel. Representative
templating agents include tetramethylammonium, tetraethylammonium,
tetrapropylammonium or tetrabutylammonium ions; tetrapentylammonium
ion; di-n-propylamine; tripropylamine; triethylamine; triethanolamine;
piperidine; cyclohexylamine; 2-methylpyridine; N,N-dimethylbenzylamine;
N,N-dimethylethanolamine; choline; N,N'-dimethylpiperazine; 14-diazabicyclo(222)octane;
N-methyldiethanolamine, N-methylethanolamine; N-methylpiperidine;
3-methylpiperidine; N-methylcyclohexylamine; 3-methylpyridine; 4-methylpyridine;
quinuclidine; N,N'-dimethyl-14-diazabicyclo(222)octane ion; di-n-butylamine,
neopentylamine; di-n-pentylamine; isopropylamine; t-butylamine;
ethylenediamine; pyrrolidine; and 2-imidazolidone. Not every templating
agent will direct the formation of every species of CAPO, i.e.,
a single templating agent can, with proper manipulation of the reaction
condition, direct the formation of several CAPO compositions, and
a given CAPO composition can be produced using several different
templating agents.
The reactive phosphorus source is preferably phosphoric acid, but
organic phosphates such as triethyl phosphate may be satisfactory,
and so also may crystalline or amorphous aluminophosphates such
as the AlPO.sub.4 composition of U.S. Pat. No. 4310440. Organo-phosphorus
compounds, such as tetrabutylphosphonium bromide, do not apparently
serve as reactive sources of phosphorus, but these compounds may
function as templating agents. Conventional phosphorus salts such
as sodium metaphosphate, may be used, at least in part, as the phosphorus
source, but are not preferred.
The preferred aluminum source is either an aluminum alkoxide, such
as aluminum isoproproxide, aluminum chlorhydrol (Al.sub.2 (OH).sub.5
Cl.2.5H.sub.2 O) or pseudoboehmite. The crystalline or amorphous
aluminophosphates which are a suitable source of phosphorus are,
of course, also suitable sources of aluminum. Other sources of aluminum
used in zeolite synthesis, such as gibbsite, sodium aluminate and
aluminum trichloride, can be employed but are not preferred.
The reactive source of chromium can be introduced into the reaction
system in any form which permits the formation in situ of a reactive
form of chromium, i.e., reactive to form the framework tetrahedral
oxide unit of chromium. Compounds of chromium which may be employed
include oxides, alkoxides, hydroxides, chlorides, bromides, iodides,
nitrates, sulfates, carboxylates (e.g., acetates) and the like.
Especially preferred sources of chromium are chromium(III) orthophosphate,
chromium(III) acetate and chromium acetate hydroxide (Cr.sub.3 (OH).sub.2
(CH.sub.3 COO).sub.7).
While not essential to the synthesis of CAPO compositions, stirring
or other moderate agitation of the reaction mixture and/or seeding
the reaction mixture with seed crystals of either the CAPO species
to be produced or a topologically similar aluminophosphate, aluminosilicate
or molecular sieve composition, facilitates the crystallization
procedure.
After crystallization the CAPO product may be isolated and advantageously
washed with water and dried in air. The as-synthesized CAPO generally
contains within its internal pore system at least one form of the
templating agent employed in its formation. Most commonly the organic
moiety is present, at least in part, as a charge-balancing cation
as is generally the case with as-synthesized aluminosilicate zeolites
prepared from organic-containing reaction systems. It is possible,
however, that some or all of the organic moiety is an occluded molecular
species in a particular CAPO species. As a general rule the templating
agent, and hence the occluded organic species, is too large to move
freely through the pore system of the CAPO product and must be removed
by calcining the CAPO at temperatures of 200.degree. C. to 700.degree.
C., preferably about 350.degree. C. to about 600.degree. C., to
thermally degrade the organic species. In a few instances the pores
of the CAPO product are sufficiently large to permit transport of
the templating agent, particularly if the latter is a small molecule,
and accordingly complete or partial removal thereof can be accomplished
by conventional desorption procedures such as carried out in the
case of zeolites. It will be understood that the term "as-synthesized"
as used herein does not include the condition of the CAPO phase
wherein the organic moiety occupying the intracrystalline pore system
as a result of the hydrothermal crystalline process has been reduced
by post-synthesis treatment such that the value of "m"
in the composition formula
has a value of less than 0.02. The other symbols of the formula
are as defined hereinabove. In those preparations in which an alkoxide
is employed as the source of chromium, aluminum or phosphorus, the
corresponding alcohol is necessarily present in the reaction mixture
since it is a hydrolysis product of the alkoxide. It has not been
determined whether this alcohol participates in the synthesis process
as a templating agent. For the purposes of this application, however,
this alcohol is arbitrarily omitted from the class of templating
agents, even if it is present in the as-synthesized CAPO material.
Since the present CAPO compositions are formed from CrO.sub.2
AlO.sub.2 and PO.sub.2 tetrahedral units which, respectively, have
a net charge of "n" (-1 0 or +1), -1 and +1 the matter
of cation exchangeability is considerably more complicated than
in the case of zeolitic molecular sieves in which, ideally, there
is a stoichiometric relationship between AlO.sub.2.sup.- tetrahedra
and charge-balancing cations. In the instant compositions, an AlO.sub.2.sup.-
tetrahedron can be balanced electrically either by association with
a PO.sub.2.sup.+ tetrahedron or a simple cation such as an alkali
metal cation, a proton (H.sup.+), a cation of chromium present in
the reaction mixture, or an organic cation derived from the templating
agent. Similarly, an CrO.sub.2.sup.- tetrahedron can be balanced
electrically by association with PO.sub.2.sup.+ tetrahedra, a cation
of chromium present in the reaction mixture, organic cations derived
from the templating agent, a simple cation such as an alkali metal
cation, a proton (H.sup.+), or other divalent or polyvalent metal
cations introduced from an extraneous source. It has also been postulated
that non-adjacent AlO.sub.2.sup.- and PO.sub.2.sup.+ tetrahedral
pairs can be balanced by Na.sup.+ and OH.sup.- respectively [Flanigen
and Grose, Molecular Sieve Zeolites-I, ACS, Washington, DC (1971)].
The CAPO compositions of the present invention may exhibit cation-exchange
capacity when analyzed using ion-exchange techniques heretofore
employed with zeolitic aluminosilicates and have pore diameters
which are inherent in the lattice structure of each species and
which are at least about 3 .ANG. in diameter. Ion exchange of CAPO
compositions is ordinarily possible only after any organic moiety
derived from the template, present as a result of synthesis, has
been removed from the pore system. Dehydration to remove water present
in the as-synthesized CAPO compositions can usually be accomplished,
to some degree at least, in the usual manner without removal of
the organic moiety, but the absence of the organic species greatly
facilitates adsorption and desorption procedures. The CAPO materials
have various degrees of hydrothermal and thermal stability, some
being quite remarkable in this regard, and function well as molecular
sieve adsorbents and hydrocarbon conversion catalysts or catalyst
bases.
In preparing the CAPO composition it is preferred to use a stainless
steel reaction vessel lined with an inert plastic material, e.g.,
polytetrafluoroethylene, to avoid contamination of the reaction
mixture. In general, the final reaction mixture from which each
CAPO composition is crystallized is prepared by forming mixtures
of less than all of the reagents and thereafter incorporating into
these mixtures additional reagents either singly or in the form
of other intermediate mixtures of two or more reagents. In some
instances the reagents admixed retain their identity in the intermediate
mixture and in other cases some or all of the reagents are involved
in chemical reactions to produce new reagents. The term "mixture"
is applied in both cases. Further, it is preferred that the intermediate
mixtures as well as the final reaction mixtures be stirred until
substantially homogeneous.
X-ray patterns of reaction products are obtained by X-ray analysis
using either: (1) copper K-alpha radiation, Siemens Type K-805 X-ray
sources and computer interfaced Seimen's D-500 X-ray powder diffractometers,
available from Seimens Corporation, Cherry Hill, N.J.; or (2) standard
X-ray powder diffraction techniques. When the standard technique
is employed the radiation source is a high-intensity, copper target,
X-ray tube operated at 50 Kv and 40 ma. The diffraction pattern
from the copper K-alpha radiation and graphite monochromator is
suitably recorded by an X-ray spectrometer scintillation counter,
pulse height analyzer and strip chart recorder. X-ray patterns are
obtained using flat compressed powder samples which are scanned
at 2.degree. (2 theta) per minute, using a two second time constant.
All interplanar spacings (d) in Angstrom units are obtained from
the position of the diffraction peaks expressed as 2.theta. where
.theta. is the Bragg angle as observed on the strip chart. Intensities
are determined from the heights of diffraction peaks after subtracting
background, "I.sub.o " being the intensity of the strongest
line or peak, and "I" being the intensity of each of the
other peaks.
As will be understood by those skilled in the art the determination
of the parameter 2 theta is subject to both human and mechanical
error, which in combination, can impose an uncertainty of about
.+-.0.4.degree. on each reported value of 2 theta. This uncertainty
is, of course, also manifested in the reported values of the d-spacings,
which are calculated from the 2 theta values. This imprecision is
general throughout the art and is not sufficient to preclude the
differentiation of the present crystalline materials from each other
and from the compositions of the prior art. In some of the X-ray
patterns reported, the relative intensities of the d-spacings are
indicated by the notations vs, s, m, w and vw which represent very
strong, strong, medium, weak and very weak, respectively.
In certain instances the purity of a synthesized product may be
assessed with reference to its X-ray powder diffraction pattern.
Thus, for example, if a sample is stated to be pure, it is intended
only that the X-ray pattern of the sample is free of lines attributable
to crystilline impurities, not that there are no amorphous materials
present.
The molecular sieves of the instant invention may be characterized
by their X-ray powder diffraction patterns and as such may have
one of the X-ray patterns set forth in the following Tables A through
V, wherein said X-ray patterns are for the as-synthesized form unless
otherwise noted. In most cases, the pattern of the corresponding
calcined form will also fall within the relevant Table. However,
in some cases the removal of the occluded templating agent which
occurs during calcination will be accompanied by sufficient relaxation
of the lattice to shift some of the lines slightly outside the ranges
specified in the relevant Table. In a small number of cases, calcination
appears to cause more substantial distortions in the crystal lattice,
and hence more significant changes in the X-ray powder diffraction
pattern. |