Molecular sieve abstract
A method for characterizing molecular sieve catalysts by solid-state
nuclear magnetic resonance spectroscopy. In particular, the number
of acid and non-acid proton sites on molecular sieve catalysts are
quantitively determined by solid-state .sup.1H NMR magic angle spinning
spectroscopy in the presence of a spin-counting standard.
Molecular sieve claims
What is claimed is:
1. A solid state nuclear magnetic resonance method for characterizing
acid sites of a molecular sieve comprising: a) adding molecular
sieve and a spin-counting standard to a nuclear magnetic resonance
sample holder; b) acquiring a solid state nuclear magnetic resonance
spectrum of the molecular sieve in combination with the spin-counting
standard; and c) determining integrated areas for at least one signal
associated with the molecular sieve and at least one signal associated
with the spin-counting standard.
2. The method of claim 1 wherein the molecular sieve comprises
zeolite or non-zeolite molecular sieve.
3. The method of claim 2 wherein the non-zeolite molecular sieve
is selected from the group consisting of SAPO, MeSAPO, ALPO, MeALPO,
and combinations thereof.
4. The method of claim 3 wherein the molecular sieve is SAPO-34.
5. The method of claim 2 wherein the zeolite molecular sieve comprises
ZSM molecular sieve.
6. The method of claim 1 wherein the spin-counting standard comprises
polydimethylsiloxane.
7. The method of claim 1 wherein acquiring the solid state nuclear
magnetic resonance spectrum comprises acquiring a proton nuclear
magnetic resonance spectrum.
8. The method of claim 1 wherein acquiring the solid state nuclear
magnetic resonance spectrum comprises spinning the sample holder
between 4 and 40 kHz.
9. The method of claim 1 wherein acquiring the solid state nuclear
magnetic resonance spectrum comprises a Bloch decay sequence.
10. The method of claim 9 wherein the Bloch decay sequence comprises
a .pi./2 pulse width of 4 .mu.s.
11. The method of claim 1 wherein adding the molecular sieve and
spin-counting standard to the sample holder comprises adding a known
quantity of molecular sieve and a known quantity of spin-counting
standard.
12. The method of claim 1 wherein acquiring the solid state nuclear
magnetic resonance spectrum comprises acquiring more than one scan
wherein a time delay between scans is at least four times the spin-lattice
relaxation time of a nucleus of interest in the molecular sieve
and the spin-counting standard.
13. The method of claim 1 wherein acquiring the solid state nuclear
magnetic resonance spectrum comprises acquiring data using magic
angle spinning methods.
14. The method of claim 1 wherein adding the molecular sieve and
spin-counting standard to the sample holder further comprises adding
at least one spacer.
15. The method of claim 14 wherein adding the molecular sieve,
spin-counting standard, and at least one spacer comprises adding
the molecular sieve such that the molecular sieve is confined to
the middle third of the sample holder.
Molecular sieve description
FIELD OF THE INVENTION
[0001] The invention is directed to a method for characterizing
molecular sieve catalysts by solid-state NMR. In particular, the
number of active and inactive acid sites on molecular sieve catalysts
are characterized by solid-state .sup.1H NMR magic angle spinning
spectroscopy.
BACKGROUND OF THE INVENTION
[0002] Olefins, particularly light olefins, have been traditionally
produced from petroleum feedstocks by either catalytic or steam
cracking. Oxygenates, however, are becoming an alternative feedstock
for making light olefins, particularly ethylene and propylene. Promising
oxygenate feedstocks are alcohols, such as methanol and ethanol,
dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate,
and methyl formate. Many of these oxygenates can be produced from
a variety of sources including synthesis gas derived from natural
gas, petroleum liquids, and carbonaceous materials, including coal.
Because of these relatively inexpensive sources, alcohol, alcohol
derivatives, and other oxygenates have promise as an economical,
non-petroleum source for light olefin production.
[0003] Molecular sieve catalysts, such as the microporous, crystalline
zeolites and non-zeolites are known to promote the conversion of
oxygenates to light olefins. Numerous patents describe this catalytic
process: U.S. Pat. Nos. 4499327 and 4677242 to Kaiser; U.S.
Pat. Nos. 5095163 5191141 and 5126308 to Barger; as well
as many others. The general characteristics of zeolite and non-zeolite
molecular sieves which render them useful for oxygenate conversion
and other chemical transformations is their uniform, microporous,
crystalline structure and the acidity of the internal pore environment.
The uniform and distinctly shaped pores dictate how the reactant
interacts with the active site of the catalyst. The size of the
pores limits which reactants can react with the active site and
which products can exit the catalyst. It is this shape selectivity
and size exclusivity which governs the product selectivity of the
catalyst.
[0004] The crystalline zeolites comprise a corner-sharing AlO.sub.2
and SiO.sub.2 tetrahedra and are characterized by pore openings
of uniform dimension. However, it is the crystalline non-zeolites
that are generally utilized in the oxygenate conversion process.
In general, non-zeolites do not contain AlO.sub.2.sup.- tetrahedra
as essential molecular components of the molecular sieve framework.
For example, SAPO comprises a three-dimensional microporous framework
structure of [SiO.sub.2], [AlO.sub.2] and [PO.sub.2] corner-sharing
tetrahedra. The preferred SAPO catalyst for oxygenate conversion
will have a relatively low Si/Al ratio. In general, the lower the
Si/Al ratio, the lower the overall acidity of the catalyst, resulting
in lower amounts of light paraffins being produced during the conversion
process.
[0005] The catalytic activity in zeolites and non-zeolites occurs
at or near acidic sites on a bridging oxygen framework located within
the catalyst pores. Attached to these acidic sites are often relatively
labile hydrogens, also referred to as protons. During the conversion
process it is believed that the reactant, such as methanol, adsorbs
onto the sieve framework at or near these acid sites. The adsorbed
methanol then reacts with the acidic hydroxyl proton to form an
intermediate hydrocarbon fragment. The hydrocarbon fragments then
combine to form the desired light olefins and other hydrocarbon
products. In either case the transfer of hydrogen from the acid
sites of the catalyst to the reactant, and from the resulting adsorbed
hydrocarbon intermediates back to the acid site is vital to the
activity and selectivity of the catalyst, in particular a MTO catalyst.
The relative degree and manner in which this transferring of hydrogens
takes place is related to what is termed the acidity of the catalyst
active sites. Therefore, it is important to characterize catalysts,
particularly molecular sieve catalysts, by determining the number
and type (strength) of the various acid sites on a particular catalyst.
This information can then be catalogued and used to approximate
the catalytic activity of a related family of catalysts.
[0006] Farneth and Gorte have recently reviewed the methods used
to characterize both the strength and number of acid sites on a
molecular sieve catalyst. W. E. Farneth and R. J. Gorte, Chemical
Reviews, vol. 95 p. 615 1995. The most common techniques involve
adsorption or temperature programmed desorption (TPD) of NH.sub.3
and infrared (IR) spectroscopy. However, neither of these methods
are sufficiently reliable, as the NH.sub.3 TPD methods suffer from
multiple adsorbates per acid site. Quantification by infrared methods
is limited by a lack of knowledge of molar extinction coefficients
of the acidic hydroxyl groups and/or the respective adsorbate-proton
complexes. Also, peak resolution is typically poor in IR spectra
of solid catalysts because of the inherent breath of the OH absorption
band.
[0007] Currently, the preferred methods for quantifying Bronsted
sites involves combining TPD and thermogravimetric analysis (TGA)
of reactive amines like isopropylamine. D. J. Parrillo, Applied
Catalysis, vol. 67 p.107 1990. The amine acting like a reactant
is adsorbed at or near the acid site of a catalyst. The change in
weight is then used to quantify the number of such sites. TGA is
used to distinguish between the different types of acid sites since
the strength of the adsorbant interactions will differ between sites.
Others have used .sup.31P NMR magic angle spinning (MAS) in conjunction
with trimethylphosphine and trimethylphosphine oxide probe molecules
to distinguish between Lewis and Bronsted acidity. J. H. Lunsford
et al., J. American Chemical Society, vol.107 p.1540 1985; K.
J. Sutovich, J. Catalysis, vol.183 p. 155 1999.
[0008] The present invention eliminates the need for a multistep
site analysis approach such as combining TPD and TGA. Also, the
invention eliminates the need to probe such sites by adding an adsorbant
molecule, such as an amine or phosphine to the catalyst. Though
the present invention can be used to investigate probe-acid site
interations, the invention does not require that a probe molecule
be used and a probe-acid site adsorption complex be formed. Also,
the present invention does not possess the limitations inherent
with TPD methods or IR spectroscopic methods briefly stated above.
SUMMARY OF THE INVENTION
[0009] The present invention is of a solid-state .sup.1H NMR method
that is used to characterize the active and inactive acid sites
of molecular sieve catalysts. In particular, the invention not only
assists in the identification of such sites, but more importantly,
quantifies the density (or concentration) of each site, e.g., the
number of acid sites per gram of catalyst by using a spin-counting
standard, preferably polydimethylsiloxane (PDMS). Thus, the present
invention can provide both qualitative and quantitative information
for any heterogeneous acid catalyst independent of the specific
crystallographic structure, pore size, pore/channel geometry, or
chemical content of the catalyst. Although the use of .sup.1H solid-state
NMR methods to investigate zeolite acidity is well known, there
are no known reports that use a spin-counting standard to quantify
Bronsted acidity in molecular sieve catalysts. Other NMR investigations
of molecular sieve acid sites include: H. H. P. Yiu et al., Catal.
Lett. vol. 59 p. 207 1999; H. Pfiefer et al., Zeolites, vol. 5
p. 274 1985; H. Pfeiffer J. Catalysis, vol. 127 p. 34 1991; J.
L. White et al, J. Am. Chem. Soc. vol. 114 p. 6182 1992; and H.
Liu et al J. Phys. Chem. B, vol. 103 p. 4786 1999.
[0010] The invention provides a method for characterizing molecular
sieve catalysts by solid-state NMR. In particular, the number of
acid and non-acid sites on the molecular sieve catalyst is quantitively
determined by solid-state .sup.1H NMR (MAS) spectroscopy using a
spin-counting standard. The solid state NMR method of the present
invention comprises: adding a known quantity of molecular sieve,
preferably a zeolite or non-zeolite molecular sieve, and a known
quantity of a spin-counting standard, preferably polydimethylsiloxane,
to a NMR sample holder; acquiring a solid state NMR spectrum, preferably
a .sup.1H NMR spectrum of the acid and non-acid protons of the molecular
sieve in combination with the protons of the spin-counting standard;
and determining the integrated areas for at least one signal associated
with the molecular sieve and at least one signal associated with
the spin-counting standard.
[0011] In the preferred embodiment a solid-state, NMR (MAS) spectrum
is acquired by using a Bloch decay pulse sequence, preferably with
a .pi./2 pulse. Preferably, the sample holder is spinned between
4 and 40 kHz, as the NMR data is acquired. Also, prior to determining
the integrated areas of the spectrum signals the NMR data is converted
by methods known in the art, preferably the NMR scan data is converted
by Fourier calculations. If more than one scan is taken of the sample,
then the time delay between scans is at least four times, preferably
at least ten times, the spin-lattice relaxation time of the nucleus
of interest in the molecular sieve and the spin-counting standard.
Also, it is preferred that at least one spacer be added to the sample
holder such that the molecular sieve is confined to the middle third
of the sample holder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention will be better understood by reference
to the Detailed Description of the Invention when taken together
with the attached drawings, wherein:
[0013] FIG. 1 is a schematic of a sample holder with two spacers
which restrict the sample molecular sieve to the middle third of
the holder;
[0014] FIG. 2a is a .sup.1H NMR (MAS) spectrum of SAPO-34 with
PDMS standard;
[0015] FIG. 2b is a .sup.1H NMR (MAS) spectrum of SAPO-34 without
PDMS standard;
[0016] FIG. 3 is a .sup.1H NMR (MAS) spectrum of relatively low
Si/Al ratio ZSM-5 with PDMS standard; and
[0017] FIG. 4 is a .sup.1H NMR (MAS) spectrum of relatively high
Si/Al ratio ZSM-5 with PDMS standard.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Solid state NMR is a spectroscopic technique that provides
information on the types of acid sites present and their surrounding
environment as well as the relative number of each type of acid
site, given that precautions are taken to avoid signal saturation
of one or more particular sites and the spin-counting standard.
With respect to the acidity of solid surfaces one has to distinguish
between two independent quantities: 1) S.sub.a, the strength of
acidity, which is defined as the ease of proton (hydrogen) transfer
from a surface site to an adsorbant (Bronsted acidity) or of an
electron pair transfer from an adsorbant, such ammonia, to a surface
site Lewis acidity); and 2) A.sub.a, the concentration of the respective
acid sites.
[0019] The NMR signal is characterized by its relative position
in the overall spectrum, i.e by its chemical shift (.delta.), and
the area of the signal is used to quantify the relative concentration
of a particular site in the molecular sieve. An absolute concentration
can be determined by using an internal spin-counting standard. Thus,
if one knows the concentration of the spin-counting standard in
or around the sample, then the concentration of each particular
acid site can be determined. To measure resonance chemical shifts
in solid samples and to distinguish one acid site from another,
line narrowing techniques, such as magic angle spinning (MAS), is
utilized.
[0020] The application of NMR (MAS) to zeolites, non-zeolites,
and related catalysts has become one of the most promising directions
of research in catalyst development. See, J. M. Thomas et al. Angew.
Chemie. vol. 22 p. 259 (1983). For example, since each particular
acid site will interact differently with a given adsorbate, such
as ammonia or pyridine, information as to the type and degree of
acidity for the acid site can be qualitatively determined. Adsorption
studies in zeolites and non-zeolites are of particular importance
because hydrogen bonded adsorption complexes are proposed intermediates
in the catalytic transformation of hydrocarbons. For example, it
has been reported that methanol forms an extended adsorption complex
with the Bronsted protons in H-ZSM-5. G. Mirth et al., J. Chemical
Society Faraday Trans. vol. 86 p. 3039 1990. .sup.1H NMR (MAS)
has also been used to differentiate Bronsted acid sites from surface
silanol groups and non-framework (external) hydroxyl groups. See,
G. Engelhardt, High Resolution Solid-State NMR of Silicates and
Zeolites; chapters 6 and 7 Wiley and Sons, New York 1987.
[0021] Molecular sieve comprises a three-dimensional microporous
crystal framework structure of [SiO.sub.2], [AlO.sub.2] and [PO.sub.2]
corner sharing tetrahedral units. In general, silicoaluminophosphate
molecular sieves comprise a molecular framework of corner-sharing
[SiO.sub.2], [AlO.sub.2], and [PO.sub.2] tetrahedral units. The
silicoaluminophosphate molecular sieves are synthesized by hydrothermal
crystallization methods generally known in the art. See, for example,
U.S. Pat. Nos. 4440871; 4861743; 5096684; and 5126308 the
methods of making of which are fully incorporated herein by reference.
This type of framework is effective in converting various oxygenates
into olefin products.
[0022] If a silicoaluminophosphate molecular sieve is used for
the conversion of oxygenates a relatively low Si/Al ratio is preferred.
In general, the lower the Si/Al ratio, the lower the C.sub.1-C.sub.4
saturates selectivity, particularly propane selectivity. A Si/Al
ratio of less than 0.65 is desirable, with a Si/Al ratio of not
greater than 0.40 being preferred. A Si/Al ratio of not greater
than 0.20 is most preferred. Also, the preferred silicoaluminophosphate
molecular sieves used in the oxygenate conversion process comprise
8 10 or 12 membered ring structures. These ring structures can
have an average pore size ranging from 3.5-15 angstroms. More preferred
are the small pore SAPO molecular sieves having an average pore
size of less than 5 angstroms, preferably an average pore size ranging
from about 3.5 to 5 angstroms. These pore sizes are typical of molecular
sieves having 8 membered rings.
[0023] The [PO.sub.2] tetrahedral units within the framework structure
of the molecular sieve of this invention can be provided by a variety
of compositions. Examples of these phosphorus-containing compositions
include phosphoric acid, organic phosphates such as triethyl phosphate,
and aluminophosphates. The phosphorous-containing compositions are
mixed with reactive silicon and aluminum-containing compositions
under the appropriate conditions to form the molecular sieve.
[0024] The [AlO.sub.2] tetrahedral units within the framework structure
can be provided by a variety of compositions. Examples of these
aluminum-containing compositions include aluminum alkoxides such
as aluminum isopropoxide, aluminum phosphates, aluminum hydroxide,
sodium aluminate, and pseudoboehmite. The aluminum-containing compositions
are mixed with reactive silicon and phosphorus-containing compositions
under the appropriate conditions to form the molecular sieve.
[0025] The [SiO.sub.2] tetrahedral units within the framework structure
can be provided by a variety of compositions. Examples of these
silicon-containing compositions include silica sols and silicium
alkoxides such as tetra ethyl orthosilicate. The silicon-containing
compositions are mixed with reactive aluminum and phosphorus-containing
compositions under the appropriate conditions to form the molecular
sieve.
[0026] Substituted SAPOs can also be characterized by the present
invention. These compounds are generally known as MeSAPOs or metal-containing
silicoaluminophosphates. The metal can be alkali metal ions (Group
IA), alkaline earth metal ions (Group IIA), rare earth ions (Group
IIIB, including the lanthanoid elements: lanthanum, cerium, praseodymium,
neodymium, samarium, europium, gadolinium, terbium, dysprosium,
holmium, erbium, thulium, ytterbium and lutetium; and scandium or
yttrium) and the additional transition cations of Groups IVB, VB,
VIB, VIIB, VIIIB, IB, and IIB. Preferably, the Me represents metals
such as Zn, Mg, Mn, Co, Ni, Ga, Fe, Ti, Zr, Ge, Sn, and Cr.
[0027] Suitable silicoaluminophosphate molecular sieves characterized
by the present invention include SAPO-5 SAPO-8 SAPO-11 SAPO-16
SAPO-17 SAPO-18 SAPO-20 SAPO-31 SAPO-34 SAPO-35 SAPO-36 SAPO-37
SAPO-40 SAPO-41 SAPO-42 SAPO-44 SAPO-47 SAPO-56 the metal
containing forms thereof, and mixtures thereof. Preferred are SAPO-17
SAPO-18 SAPO-34 SAPO-35 SAPO-44 and SAPO-47 particularly SAPO-34
including the metal containing forms thereof, and mixtures thereof.
As used herein, the term mixture is synonymous with combination
and is considered a composition of matter having two or more components
in varying proportions, regardless of their physical state.
[0028] An aluminophosphate (ALPO) molecular sieve can also be characterized
by the invention. Aluminophosphate molecular sieves are crystalline
microporous oxides which can have an AlPO.sub.4 framework. They
can have additional elements within the framework, typically have
uniform pore dimensions ranging from about 3 angstroms to about
10 angstroms, and are capable of making size selective separations
of molecular species. More than two dozen structure types have been
reported, including zeolite topological analogues. A more detailed
description of the background and synthesis of aluminophosphates
is found in U.S. Pat. No. 4310440 which is incorporated herein
by reference in its entirety. Preferred ALPO structures are ALPO-5
ALPO-11 ALPO-17 ALPO-18 ALPO-31 ALPO-34 ALPO-36 ALPO-37 and
ALPO-46.
[0029] The ALPOs can also include a metal substituent in its framework.
Preferably, the metal is selected from the group consisting of magnesium,
manganese, zinc, cobalt, and mixtures thereof. These materials preferably
exhibit adsorption, ion-exchange and/or catalytic properties similar
to aluminosilicate, aluminophosphate and silica aluminophosphate
molecular sieve compositions. Members of this class and their preparation
are described in U.S. Pat. No. 4567029 incorporated herein by
reference in its entirety. The metal containing ALPOs are sometimes
referred to by the acronym as MeAPO. Also in those cases where the
metal "Me" in the composition is magnesium, the acronym
MAPO is applied to the composition. Similarly ZAPO, MnAPO and CoAPO
are applied to the compositions which contain zinc, manganese and
cobalt respectively. To identify the various structural species
which make up each of the subgeneric classes MAPO, ZAPO, CoAPO and
MnAPO, each species is assigned a number and is identified, for
example, as ZAPO-5 MAPO-11 CoAPO-34 and so forth.
[0030] Molecular sieves catalysts that are mixed with inert matrix
materials, such as clays, or binders, such as aluminochlorhydrol
can also be characterized by the present invention. Materials which
can be blended with the molecular sieve can be various inert or
catalytically active materials, or various binder materials. These
materials include compositions such as kaolin and other clays, various
forms of rare earth metals, metal oxides, other non-zeolite catalyst
components, zeolite catalyst components, alumina or alumina sol,
titania, zirconia, magnesia, thoria, beryllia; quartz, silica or
silica or silica sol, and mixtures thereof. These components are
also effective in reducing, inter alia, overall catalyst cost, acting
as a thermal sink to assist in heat shielding the catalyst during
regeneration, densifying the catalyst and increasing catalyst strength.
It is particularly desirable that the inert materials that are used
in the catalyst to act as a thermal sink have a heat capacity of
from about 0.05 to about 1 cal/g-.degree. C., more preferably from
about 0.1 to about 0.8 cal/g-.degree. C., most preferably from about
0.1 to about 0.5 cal/g-.degree. C.
[0031] Other molecular sieve materials can also be characterized
by the invention either separately or when they are mixed with other
catalytic components. Structural types of small pore molecular sieves
that are suitable for use in this invention include AEI, AFT, APC,
ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO,
KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, THO, and substituted
forms thereof. Structural types of medium pore molecular sieves
that are suitable for use in this invention include MFI, MEL, MTW,
EUO, MTT, HEU, FER, AFO, AEL, TON, and substituted forms thereof.
These small and medium pore molecular sieves are described in greater
detail in the Atlas of Zeolite Structural Types, W. M. Meier and
D. H. Olsen, Butterworth Heineman, 3rd ed., 1997 the detailed description
of which is explicitly incorporated herein by reference. Preferred
molecular sieves which can be combined with a silicoaluminophosphate
catalyst include ZSM-5 ZSM-34 erionite, and chabazite.
[0032] Also, catalytic materials that are subjected to a variety
of treatments to achieve the desired physical and chemical characteristics,
such as hydrothermal treatment, calcination, acid treatment, base
treatment, milling, ball milling, grinding, spray drying, and combinations
thereof, can also be characterized by the present invention. In
other words, the acidity of the catalyst can be characterized by
the present invention following any step of the preparative process.
Thus, for example, one could investigate how various calcination
procedures or how metal incorporation affects the catalyst's acidic
properties.
[0033] The preferred method of placing a catalyst sample 12 in
the sample holder 10 for NMR data acquisition is shown in FIG. 1.
The positioning of the sample in this way minimizes field inhomogeneties
across the sample volume. Field inhomogeneties distort the signal
response from that portion of the sample that is not centered about
the receiver coil of the NMR probe. The result is an inaccurate
count of acid sites in the total sample. The problem of field inhomogeneity
is discussed in detail for a variety of commercial probes by Campbell
et al., J. Magn. Reson., vol. 112A, p.225 1995. The excitation
profile is different for different probe vendors, but in general,
is optimum at the center or middle third of the sample holder (rotor).
Therefore it is preferred that the catalyst sample be confined to
the middle third of the rotor volume in the present invention. For
example, when a spectrum of hexamethylbenzene (HMB) and PDMS was
obtained in which the HMB was placed outside the middle third of
the rotor the absolute number of hydrogen in the HMB sample was
11% less than expected. However, when the HMB spectrum was obtained
in which the HMB was confined to the middle third volume element
of the rotor, as shown in FIG. 1 then the measured signal for the
HMB was 98% of the expected value.
[0034] For the reasons stated above, the preferred embodiment of
the invention comprises positioning the sample 12 within the middle
third of the sample bolder (rotor) 10 volume. It is also preferred
that the spin-counting standard 14 also be positioned in the middle
third of the sample holder 10 volume. Spacers 16 known in the art,
are positioned within the sample holder 10 as shown to confine the
sample and spin-counting standard to the desired position. The sample
holder also comprises a drive tip 18.
[0035] PDMS was chosen as the preferred spin-counting standard
because it is an inert, solid material which is easily loaded into
the sample rotor and which does not interact with the catalyst.
More importantly, the .sup.1H linewidth is extremely narrow (.DELTA..upsilon..sub.1/2.apprxeq.1-
20 Hz) because PDMS has a very low glass transition temperature.
Further, the chemical shift for the methyl hydrogens is 0 ppm, which
is removed from any peaks associated with most zeolite and non-zeolite
catalysts. As a result, because there is no overlap between the
spin-counting standard and the peaks associated the acid sites of
the catalyst, integration and deconvolution of the sample peaks,
if necessary, is relatively straightforward.
[0036] FIG. 2a depicts a .sup.1H NMR (MAS) spectrum of a SAPO-34
catalyst with PDMS. The spectrum of a SAPO-34 catalyst without PDMS
is depicted in FIG. 2b. The SAPO-34 catalyst exhibits a peak at
3.8 ppm for the Bronsted acid sites, and a smaller, broader peak
near 1.8 ppm for the non-acidic Al--OH and P--OH hydroxyl groups.
A comparison of the spectra in FIG. 2 indicates there is no change
in the spectral characteristics of the SAPO peaks upon addition
of the PDMS.
[0037] The spectrum in FIG. 2b was deconvoluted into three peaks:
the Bronsted acid site centered at 3.8 ppm; the PDMS peak centered
at 0 ppm; and the non-acidic hydroxyls centered at 1.6 ppm. Based
on the integration of the peaks, the SAPO-34 sample in FIG. 2a has
a Bronsted acid site density of about 1.32 mmoles per gram of catalyst,
and a non-acidic site density of about 0.1 mmols per gram of catalyst
or about 7% of the total acid sites in the catalyst. Repeated measurements
with the same SAPO-34 sample gave a standard deviation of about
4% for the Bronsted site density. The intensity of the first order
spinning sidebands were also included in the calculation, while
higher orders were neglected.
[0038] FIG. 3 and FIG. 4 depict the .sup.1H NMR (MAS) spectra of
a low and high Si/Al ratio ZSM-5 catalyst with PDMS, respectively.
As shown, the 4.2 ppm bridging hydroxyl peak is larger for the low
Si/Al sample, FIG. 3. The assignments for the room temperature spectrum
of ZSM-5 follow those of J. L. White et al., J. Am. Chem. Soc.,
vol.114 p. 6182 1992 in that two distinct Bronsted acid sites
exist. In addition to the 4.2 ppm peak, an additional peak near
6 ppm is also observed; it is the sum of these two resonances which
are used to quantify the total Bronsted acidity in the ZSM-5 catalyst.
Presumably, this second downfield acid site arises from the hydrogen
bonding of a Bronsted hydroxyl group with a nearest neighbor oxygen.
For the catalysts shown in FIG. 3 and FIG. 4 the Bronsted acid
densities are estimated to be 0.44 and 0.86 mmoles per gram of catalyst,
respectively.
[0039] This invention will be better understood with reference
to the following example, which is intended to illustrate a specific
embodiment within the overall scope of the invention as claimed.
EXAMPLE 1
[0040] Solid-state .sup.1H NMR (MAS) spectra were acquired at about
500 MHz, and at spinning speeds of 8-12 kHz using a 4-mm spinning
system. A one-pulse Bloch decay acquisition sequence (.pi./2 pulse
widths were 3.8 .mu.s) was used for all the measurements. Typically,
16 scans were collected using a recycle time of 70 seconds. This
delay time was chosen to avoid signal saturation, and thus insure
more accurate peak areas. The spin-counting standard, PDMS, has
a T.sub.1H=2.2 sec, while the longest T.sub.1H for any measured
molecular sieve catalyst was about 2 sec. All spectra were acquired
at room temperature.
[0041] In a typical measurement, a known quantity of the PDMS spin-counting
standard was added to the 4-mm MAS NMR rotor (of known weight) prior
to packing the catalyst. Following a stepwise "activation"
of the catalyst to remove moisture using a vacuum line, the MAS
rotor was packed with a known quantity of catalyst inside an inert
atmosphere glove box. The final temperature of the stepwise dehydration
was 300-400.degree. C. The catalyst weight inside the rotor was
obtained by difference. Typically, 5-10 mg of catalyst and 50-150
.mu.g of PDMS were used in each measurement.
[0042] Having now fully described the invention, it will be appreciated
by those skilled in the art that the invention can be performed
within a wide range of parameters within what is claimed, without
departing from the spirit of the invention. |