U.S. patent application number 13/652734 was filed with the patent office on 2013-04-18 for selective dehydration of alcohols to dialkylethers and integrated alcohol-to-gasoline processes.
This patent application is currently assigned to EXXONMOBIL RESEARCH AND ENGINEERING COMPANY. The applicant listed for this patent is TILMAN W. BEUTEL, STEPHEN J. McCARTHY. Invention is credited to TILMAN W. BEUTEL, STEPHEN J. McCARTHY.
Application Number | 20130096355 13/652734 |
Document ID | / |
Family ID | 47089187 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130096355 |
Kind Code |
A1 |
McCARTHY; STEPHEN J. ; et
al. |
April 18, 2013 |
SELECTIVE DEHYDRATION OF ALCOHOLS TO DIALKYLETHERS AND INTEGRATED
ALCOHOL-TO-GASOLINE PROCESSES
Abstract
The invention involves an integrated process for converting a
C.sub.1-C.sub.4 alcohol to gasoline and/or diesel boiling tinge
product, said process comprising: contacting a C.sub.1-C.sub.4
alcohol feed under selectively dehydrating conditions with a
catalyst comprising .gamma.-alumina which is substantially free of
terminal hydroxyl groups on tetrahedrally coordinated aluminum
sites of the catalyst to form a dialkylether dehydration product;
and contacting the dialkylether dehydration product with a zeolite
conversion catalyst under conversion conditions to form the
gasoline and/or diesel boiling range hydrocarbon product.
Inventors: |
McCARTHY; STEPHEN J.;
(Center Valley, PA) ; BEUTEL; TILMAN W.; (Neshanic
Station, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
McCARTHY; STEPHEN J.
BEUTEL; TILMAN W. |
Center Valley
Neshanic Station |
PA
NJ |
US
US |
|
|
Assignee: |
EXXONMOBIL RESEARCH AND ENGINEERING
COMPANY
Annandale
NJ
|
Family ID: |
47089187 |
Appl. No.: |
13/652734 |
Filed: |
October 16, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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61548064 |
Oct 17, 2011 |
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61548015 |
Oct 17, 2011 |
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61548038 |
Oct 17, 2011 |
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61548044 |
Oct 17, 2011 |
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61548052 |
Oct 17, 2011 |
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61548057 |
Oct 17, 2011 |
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Current U.S.
Class: |
585/310 |
Current CPC
Class: |
B82Y 40/00 20130101;
B01J 35/1014 20130101; B01J 35/1038 20130101; B01J 2229/186
20130101; B01J 37/06 20130101; B01J 37/28 20130101; B01J 35/002
20130101; C07C 1/24 20130101; B01J 29/00 20130101; C07C 1/22
20130101; B01J 35/1085 20130101; C10G 2400/04 20130101; B82Y 30/00
20130101; C07C 2/864 20130101; Y02P 30/20 20151101; B01J 37/0201
20130101; C07C 1/20 20130101; B01J 35/0026 20130101; B01J 35/1004
20130101; C10G 3/49 20130101; C10G 2400/02 20130101; B01J 21/04
20130101; B01J 37/04 20130101; B01J 2229/36 20130101; B01J 2229/42
20130101; B01J 37/0009 20130101; B01J 29/40 20130101; B01J 35/1042
20130101; B01J 35/1061 20130101; B01J 35/1019 20130101; C01B 39/54
20130101; Y02P 20/10 20151101; B01J 29/83 20130101; B01J 2229/37
20130101; C07C 41/09 20130101; C07C 41/09 20130101; C07C 43/043
20130101 |
Class at
Publication: |
585/310 |
International
Class: |
C07C 1/24 20060101
C07C001/24 |
Claims
1. An integrated process for converting a C.sub.1-C.sub.4 alcohol
to gasoline and/or diesel boiling range product, said process
comprising: contacting a C.sub.1-C.sub.4 alcohol feed under
selectively dehydrating conditions with a catalyst comprising
.gamma.-alumina which is substantially free of terminal hydroxyl
groups on tetrahedrally coordinated aluminum sites of the catalyst
to form a dialkylether dehydration product; and contacting the
dialkyl ether dehydration product with a zeolite conversion
catalyst under conversion conditions to form the gasoline and/or
diesel boiling range hydrocarbon product.
2. The process of claim 1, wherein the .gamma.-alumina of the
selective dehydration catalyst has a normalized IR absorbance at
.about.3770 cm.sup.-1 of less than 10 cm/g.
3. The process of claim 1, wherein the selective dehydration
catalyst has an alpha value less than 1.
4. The process of claim 1, wherein the selective dehydration
catalyst, when used to isomerize 2-methyl-2-pentene at
.about.350.degree. C., approximately atmospheric pressure, and a
weight hourly space velocity of about 2.4, produces an isomerized
product in which the weight ratio of 2,3 dimethyl-2 butene to
4-methyl-2-pentene is less than 0.2.
5. The process of claim 1, wherein the selectively dehydrating
conditions include a temperature from about 250.degree. C. to about
500.degree. C. and a pressure from about 100 kPa to about 700
kPa.
6. The process of claim 1, wherein the C.sub.1-C.sub.4 alcohol feed
comprises less than 15 wt % water.
7. The process of claim 6, wherein the dialkylether product
contains less than 1 wt % of the corresponding alkane.
8. The process of claim 1, wherein the C.sub.1-C.sub.4 alcohol is
methanol, and wherein the dialkylether is dimethylether.
9. The process of claim 8, wherein the conversion step results in a
product comprising both gasoline and diesel boiling range
hydrocarbons.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/548,064, filed on Oct. 17, 2011, the entire
contents of which are hereby incorporated by reference herein.
[0002] This application also claims the benefit of related U.S.
Provisional Application Nos. 61/548,015, 61/548,038, 61/548,044,
61/548,052, and 61/548,057, each filed on Oct. 17, 2011, the entire
contents of each of which are hereby also incorporated by reference
herein. This application is also related to five other co-pending
U.S. utility applications, each filed on even date herewith and
claiming the benefit to the aforementioned provisional patent
applications, and which are entitled "Process for Producing
Phosphorus Modified Zeolite Catalysts", "Process for Producing
Phosphorus Modified Zeolite Catalysts", "Phosphorus Modified
Zeolite Catalysts", "Phosphorus Modified Zeolite Catalysts", and
"Phosphorus Modified Zeolite Catalysts", respectively, the entire
contents of each of which utility patents are hereby further
incorporated by reference herein.
FIELD OF THE INVENTION
[0003] This disclosure relates to the selective dehydration of
alcohols to dialkyl ethers, as well as integrated processes for
alcohol-to-gasoline formation including such selective dehydration
as first step.
BACKGROUND OF THE INVENTION
[0004] The selective dehydration of methanol to produce dimethyl
ether is a commercially important reaction as the first step in the
conversion of methanol to gasoline and diesel boiling range
hydrocarbons. Similarly, the selective dehydration of higher
alcohols, such as ethanol, is important in the synthesis of a
number of commercially significant dialkyl ethers, such as diethyl
ether.
[0005] As discussed in, for example, U.S. Pat. No. 4,536,485,
current processes for the selective dehydration of alcohols employ
a solid acid catalyst, such as alumina, silica, alumina-silica
mixtures and crystalline aluminosilicates, zeolites. However, these
solid acid catalysts frequently produce undesirable by-products,
such as coke, methane, carbon dioxide, and hydrogen, in addition to
the desired ether product. By-product formation typically reduces
selectivity and can trigger potentially dangerous temperature
excursions within an adiabatic reactor. Therefore, by-product
formation can be kept low by adjusting the water concentration of
the alcohol feed to control reactor outlet temperature. However,
increasing the water concentration of the alcohol feed can reduce
the equilibrium conversion of the alcohol per reactor pass and can
lower overall selective dehydration process efficiency.
[0006] According to the present invention, it has now been found
that C.sub.1-C.sub.4 alcohols, such as methanol, can be selectively
dehydrated to dialkylethers without significant byproduct formation
or reactor temperature excursions even at relatively low water
concentrations in the alcohol feed by selecting as the selective
dehydration catalyst a form of .gamma.-alumina that is
substantially free of terminal hydroxyl groups on tetrahedrally:
coordinated aluminum sites, as determined by the substantial
absence of an absorbance band at .about.3770 cm.sup.-1 in the IR
spectrum of the catalyst.
SUMMARY OF THE INVENTION
[0007] In one aspect, the invention resides in an integrated
process for converting a C.sub.1-C.sub.4 alcohol to gasoline and/or
diesel boiling range product, said process comprising: contacting a
C.sub.1-C.sub.4 alcohol feed under selectively dehydrating
conditions with a catalyst comprising .gamma.-alumina which is
substantially free of terminal hydroxy groups on tetrahedrally
coordinated aluminum sites of the catalyst to form a dialkylether
dehydration product; and contacting the dialkylether dehydration
product with a zeolite conversion catalyst under conversion
conditions to form the gasoline and/or diesel boiling range
hydrocarbon product.
[0008] Conveniently, the .gamma.-alumina of the selective
dehydration catalyst can exhibit an IR spectrum having a normalized
absorbance at .about.3770 cm.sup.-1 of less than 0.010 cm/mg.
[0009] Additionally or alternately, the selective dehydration
catalyst can have an alpha value of less than 1.
[0010] Additionally or alternately, the selective dehydration
catalyst, when used to isomerize 2-methyl-2-pentene at
.about.350.degree. C., approximately atmospheric pressure, and a
weight hourly space velocity of .about.2.4 hr.sup.-1, can produce
an isomerized product in which the weight ratio of 2,3 dimethyl-2
butene to 4-methyl-2-pentene is less than 0.2.
[0011] Additionally or alternately, the dehydration conditions can
include a temperature from about 250.degree. C. to about
500.degree. C. and a pressure from about 100 kPa to about 7000
kPa.
[0012] Additionally or alternately, the C.sub.1-C.sub.4 alcohol
feed can comprise less than 15 wt % water.
[0013] In a particular embodiment, the alcohol can comprise or be
methanol, and the dialkyl ether can thus comprise or be dimethyl
ether, in which case the conversion step advantageously forms both
gasoline and diesel boiling range hydrocarbons in its product.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a graph plotting methanol conversion and
dimethyl ether and methane production against temperature in the
methanol dehydration process of Example 1.
[0015] FIG. 2 shows a graph platting the weight fraction of methane
produced against temperature in the methanol dehydration process of
Example 1, with and without the addition of .about.10 wt % water to
the methanol feed.
[0016] FIG. 3 shows a graph plotting the weight fraction of methane
produced against temperature for the different .gamma.-alumina
catalysts employed in the methanol dehydration process of Example
2.
[0017] FIG. 4 shows a graph comparing the product slates obtained
with the different .gamma.-alumina catalysts employed in the
2-methyl-2-pentene isomerization process of Example 3.
[0018] FIG. 5 shows a graph comparing the concentration of Lewis
acids sites for the different .gamma.-alumina catalysts employed in
the methanol dehydration process of Example 2.
[0019] FIG. 6 compares the difference IR spectra for the
.gamma.-alumina catalysts employed in the methanol dehydration
process of Example 2 after pyridine adsorption at
.about.150.degree. C., followed by evacuation for .about.30 minutes
at .about.150''C, and after outgassing for .about.2 hours at
.about.450.degree. C. under vacuum.
[0020] FIG. 7 shows a plot of methane formation at
.about.440.degree. C. against concentration of hydroxyl groups at
.about.3771 cm.sup.-1 for the different .gamma.-alumina catalysts
employed in the methanol dehydration process of Example 2.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0021] Described herein is an integrated process for converting
C.sub.1-C.sub.4 alcohol to gasoline and/or diesel boiling range
product. As a first step, the integrated process comprises
selective dehydration of an alcohol to a dialkylether using a
catalyst comprising .gamma.-alumina that is substantially free of
terminal hydroxyl groups on tetrahedrally coordinated aluminum
sites of the selective dehydration catalyst. The second step can
advantageously include contacting the dialkylether dehydration
product with a zeolite conversion catalyst under conversion
conditions to form the gasoline and/or diesel boiling range
hydrocarbon product.
[0022] The alcohol dehydration can generally take place at a
temperature ranging from about 250.degree. C. to about 500.degree.
C. and at a pressure from about 100 kPa to about 7000 kPa. At
higher operating temperatures, it has been found that the presence
of strong Lewis acidity on the selective dehydration catalyst can
lead to non-selective alcohol decomposition to coke, methane,
carbon dioxide, and hydrogen by-products, tending to result in loss
of yields, deactivation, and potentially dangerous temperature rise
in commercial reactors. By selecting an alumina without strong
Lewis acidity and substantially free of terminal [OH--]; groups on
the tetra-coordinated alumina sites (indicated by an absorbance
band at .about.3770 cm.sup.-1 in the pyridine IR spectra), alcohol
decomposition can be significantly reduced without significantly
negatively impacting the alcohol dehydration reaction. In
particular, it has been found that advantageous results were
obtained when the .gamma.-alumina of the selective dehydration
catalyst has a normalized IR absorbance at .about.3770 cm.sup.-1 of
less than 10 cm/g.
[0023] As used herein, the term "normalized absorbance at
.about.3770 cm.sup.-1" is used herein to mean a difference between
the absorbance centered at about 3770 cm.sup.-1 in the pyridine
FTIR spectrum of the selective dehydration catalyst comprising
.gamma.-alumina and the corresponding absorbance centered at about
3770 cm.sup.-1 in the background spectrum of the selective
dehydration catalyst comprising .gamma.-alumina, divided by the
weight of the sample per unit area. In this test, the background
FTIR spectrum is taken at a temperature T between .about.20.degree.
C. and .about.80.degree. C. after outgassing the sample at about
.about.450.degree. C. under vacuum, including a pressure P of about
1.3 mPa or less for at least .about.1 hour. The pyridine FTIR
spectrum is taken at the same temperature T after (i) outgassing
the sample at about 450.degree. C. under vacuum, including a
pressure P of about 1.3 mPa or less for at least .about.1 hour,
(ii) cooling the sample to about 150.degree. C., (iii) allowing the
sample to adsorb pyridine under a pyridine partial pressure of
about 18 torr (about 2.4 kPa) at about 150.degree. C. for at least
.about.20 minutes, (iv) evacuating the sample at about 150.degree.
C. and said pressure P for at least .about.20 minutes, and (v) then
cooling the sample to said temperature T.
[0024] The absence of strong Lewis acidity in the preferred
catalysts employed herein can be conveniently demonstrated by
investigating the product slate when the catalyst is used to
isomerize 2-methyl-2-pentene:
##STR00001##
[0025] Thus, as discussed by G. M. Kramer and G. B. McVicker, Acc.
Chem. Res. 19 (1986), p. 78, the presence of strong acid sites on
the catalyst can tend to favor the production of 2,3 dimethyl-2
butene:
##STR00002##
whereas sites of intermediate strength can tend to favor
3-methyl-2-pentene:
##STR00003##
and weak acid sites can tend to favor 4-methyl-2-pentene:
##STR00004##
[0026] In particular, it has been found that advantageous results
can be obtained in the present selective dehydration process using
an .gamma.-alumina, which, when employed to isomerize
2-methyl-2-pentene at 350.degree. C., atmospheric pressure, and a
weight hourly space velocity of .about.2.4 hr, can produce an
isomerized product in which the weight ratio of
2,3-dimethyl-2-butene to 4-methyl-2-pentene can be less than 0.2,
e.g., less than 0.05.
[0027] Preferably, the selective dehydration catalyst employed in
the present process can have an alpha value less than 1. Alpha
value can be a measure of the acid activity of a zeolite catalyst,
as compared with a standard silica-alumina catalyst. The alpha test
is described in U.S. Pat. No. 3,354,078; in the Journal of
Catalysis, v. 4, p. 527 (1965); v. 6, p. 278 (1966); and v. 61, p.
395 (1980), each incorporated herein by reference as to that
description. The experimental conditions of the test used herein
included a constant temperature of .about.538.degree. C. and a
variable flow rate, as described in detail in the Journal of
Catalysis, v. 61, p. 395.
[0028] The selective alcohol dehydration process described herein
may be conducted in the presence of water, but generally the water
concentration of the alcohol feed can be lower than that required
with conventional catalysts, thereby advantageously increasing the
equilibrium conversion of the alcohol per reactor pass and/or
raising overall process efficiency. Typically, the C.sub.1-C.sub.4
alcohol feed can comprise less than 15 wt % water, for example less
than 10 wt % water, less than 7 wt % water, or less than 4 wt %,
water. Additionally or alternately, the C.sub.1-C.sub.4 alcohol
feed can comprise at least 0.5 wt % water, for example at least 2
wt % water.
[0029] The present process can be used to selectively dehydrate a
wide variety of alcohols to their corresponding dialkylethers,
although generally the process can be employed with n-alcohols
having from 1 to 6 carbons atoms, preferably 1 to 4 carbons,
especially methanol and/or ethanol. By using the selective
dehydration catalyst described herein, it can be possible to reduce
the amount of alkane by-product to less than 1 wt % of the
selective dehydration conversion products. In a particular
embodiment, the alcohol can comprise or be methanol, and the
dialkylether can comprise or be dimethylether. Generally, the
integrated process can further comprise a conversion step of
contacting at least part of the dialkylether dimethylether)
selective dehydration product with a zeolite conversion catalyst
under conversion conditions to form a gasoline and/or diesel
boiling range hydrocarbon product.
[0030] The invention can additionally or alternately include one or
more of the following embodiments.
Embodiment 1
[0031] An integrated process for converting a C.sub.1-C.sub.4
alcohol to gasoline and/or diesel boiling range product, said
process comprising: contacting a C.sub.1-C.sub.4 alcohol feed under
selectively dehydrating conditions with a catalyst comprising
.gamma.-alumina which is substantially free of terminal hydroxyl
groups on tetrahedrally coordinated aluminum sites of the catalyst
to form a dialkylether dehydration product; and contacting the
dialkylether dehydration product with a zeolite conversion catalyst
under conversion conditions to form the gasoline and/or diesel
boiling range hydrocarbon product.
Embodiment 2
[0032] The process of embodiment 1, wherein the .gamma.-alumina of
the selective dehydration catalyst has a normalized IR absorbance
at .about.3770 cm.sup.-1 of less than 10 cm/g.
Embodiment 3
[0033] The process of any one of the previous embodiments, wherein
the selective dehydration catalyst has an alpha value less than
1.
Embodiment 4
[0034] The process of any one of the previous embodiments, wherein
the selective dehydration catalyst, when used to isomerize
2-methyl-2-pentene at .about.350.degree. C., approximately
atmospheric pressure, and a weight hourly space velocity of about
2.4, produces an isomerized product in which the weight ratio of
2,3 dimethyl-2 butene to 4-methyl-2-pentene is less than 0.2.
Embodiment 5
[0035] The process of any one of the previous embodiments, wherein
the selectively dehydrating conditions include a temperature from
about 250.degree. C. to about 500.degree. C. and a pressure from
about 100 kPa to about 700 kPa.
Embodiment 6
[0036] The process of any one of the previous embodiments, wherein
the C.sub.1-C.sub.4 alcohol feed comprises less than 15 wt %
water:
Embodiment 7
[0037] The process of any one of the previous embodiments, wherein
the dialkylether product contains less than 1 wt % of the
corresponding alkane.
Embodiment 8
[0038] The process of any one of the previous embodiments, wherein
the C.sub.1-C.sub.4 alcohol comprises or is methanol, and wherein
the dialkylether comprises or is dimethylether.
Embodiment 9
[0039] The process of embodiment 8, wherein the conversion step
results in a product comprising both gasoline and diesel boiling
range hydrocarbons.
[0040] The invention will now be more particularly described with
reference to the following non-limiting Examples and the
accompanying drawings.
EXAMPLES
Example 1
[0041] .about.50-gram samples of a commercially available
.gamma.-alumina designated herein as catalyst 1 were tested in the
dehydration of .about.100% methanol at .about.55 prig (.about.380
kPag), .about.10 hr.sup.-1 WHSV, and a variety of temperatures of
.about.380.degree. C., .about.400.degree. C., .about.420.degree.
C., and .about.440.degree. C. The results are shown in FIG. 1. In
each case, the methanol was dehydrated to an equilibrium mixture of
dimethylether and methanol. However, a significant amount of
methane was formed from methanol decomposition, particularly at
higher temperatures.
[0042] The process was repeated with the addition of .about.10 wt %
water to the methanol feed, and the results are shown in FIG. 2,
which also plots the results for a silicon carbide control (no
methanol dehydration or decomposition). It can be seen that
increasing the water content of the methanol feed to .about.10 wt %
significantly reduced non-selective methanol decomposition and
methane formation. However, methane formation was still higher than
desired.
Example 2
[0043] The process of Example 1 was repeated, but using the three
different commercially available .gamma.-aluminas summarized in
Table 1 below to effect dehydration of .about.100% methanol. The
results are shown in FIG. 3 and demonstrate that the best results
appeared to be obtained with Catalyst 3.
TABLE-US-00001 TABLE 1 Alumina Catalyst 2 1 3 4 Shape Extrudate
Extrudate 1/8'' 1/16'' sphere Cylinder Surface area, m.sup.2/g ~216
~193 ~191 ~88 Particle density, g/cc ~1.15 ~1.28 ~1.13 ~1.33 Pore
volume, cc/g ~0.61 ~0.44 ~0.47 ~0.43 Alpha activity ~3.7 ~2.7 ~0.91
~2.2 TPA*, meq/g ~0.43 ~0.44 ~0.26 ~0.15 Trace metals by XRF, wt %
Na ~0.07 ~0.07 ~0.095 nd Si ~0.02 ~0.023 ~0.033 ~0.036 S nd ~0.003
~0.0061 ~0.0048 Cl nd nd nd nd Fe ~0.0026 ~0.015 ~0.01 ~0.0043
*TPAA = temperature programmed ammonia adsorption
[0044] Samples of the .gamma.-alumina, were screened for acidic
activity with n-hexane cracking measurements in a routine alpha
test at standard conditions (.about.100 torr hexane vapor pressure
in a He carrier gas flowing through a reactor held at
.about.1000.degree. F.). The alpha test consisted of four
evenly-spaced measurements over .about.30 minutes. The data was
plotted, and the relative rate of cracking was calculated relative
to silica-alumina, which is defined to have an alpha value of 1.
The results are shown in Table 1 above.
Example 3
[0045] The gas phase isomerization of 2-methyl-2-pentene (2M2P)
over each of the catalysts employed in Example 2 was studied in a
plug-flow reactor. An amount of .about.0.1 g of each catalyst was
pretreated in flowing helium for about 1 hour at .about.723 K prior
to the reaction. The reaction was initially conducted at .about.473
K for .about.1 hour, while a flow of .about.15 mL/min of .about.7
vol % 2M2P in helium at approximately atmospheric pressure was
passed over the catalyst. The feed was then switched to helium and
the catalyst cooled to .about.448 K and then to .about.423 K. The
catalyst samples were taken .about.10 minutes after switching on
the feed at .about.473 K, .about.448 K, and .about.423 K. The
products were analyzed by GC with an FID detector, and the results
are summarized in FIG. 4.
[0046] It can be seen that the Catalyst 1 appeared to be the most
acidic alumina, with about 50% of medium and strong acid sites,
whereas the alumina of Catalyst 3 appeared to have almost no strong
acid sites, and less acid sites overall than the other
aluminas.
Example 4
[0047] For IR measurements of adsorbed pyridine, samples of the
catalysts employed in Example 2 were around and pressed into thin
self-supporting wafers. Specific wafer weights ranged from about
20-35 mg/cm.sup.2. Each wafer was placed in an IR transmission cell
equipped with CaF.sub.2 windows. The IR cuvette could be heated and
evacuated. For the sample treatment, the cuvette was connected to a
high-vacuum manifold. A pressure of .about.2.times.10.sup.-6 torr
(.about.0.3 mPa) measured by an ion gauge could be achieved in the
manifold using a turbo molecular pump. The adsorption of pyridine
was carried out from a glass vial attached to the manifold. The
pyridine partial pressure was measured by a Baratron.TM.. For the
IR measurement, the sample cuvette was transferred into a
Nicolet.TM. 670 FTIR spectrometer. Spectra were taken at .about.2
cm.sup.-1 resolution accumulating approximately 512 scans.
[0048] Before loading each wafer, the IR cuvette was outgassed in
flowing air at a flow rate of about 50 ml/min for .about.2 hours at
.about.520.degree. C. in order to remove trace amounts of pyridine
that might be adsorbed on the walls of the cuvette from the
previous experiment. The catalyst wafer was then placed into the
regenerated cuvette and evacuated for .about.2 hours at
.about.450.degree. C. in order to remove physisorbed water and to
activate the sample. The sample cooled under vacuum to
.about.80.degree. C. under a final pressure of about
2.times.10.sup.-6 torr (.about.0.3 mPa). In this state, the cuvette
was disconnected from the manifold and a spectrum was taken from
the outgassed sample. This spectrum was referred to as background
spectrum.
[0049] For the adsorption of pyridine, the cuvette was reconnected
to the manifold, and pyridine was allowed to equilibrate with its
vapor for .about.20 minutes at room temperature (.about.23.degree.
C.) leading to pyridine partial pressures of about 18 torr
(.about.2.4 kPa). During this time the valve to the cuvette was
shut, and the sample was heated to .about.150.degree. C. After the
temperature of the cuvette reached .about.150.degree. C., the valve
to the liquid pyridine vial was closed, and pyridine vapor was
expanded into the ca. The sample was exposed to pyridine vapor at
.about.150.degree. C. for .about.30 minutes, then evacuated for
another .about.30 minutes at .about.150.degree. C., and
subsequently cooled to .about.80.degree. C. under vacuum. The final
pressure was between .about.5.times.10.sup.-6 and
.about.1.times.10.sup.-5 torr. A sample spectrum was taken of the
catalyst with adsorbed pyridine. The amount of pyridine adsorbed on
the sample after adsorption and evacuation of pyridine at
.about.150.degree. C. was referred to as the total amount of weakly
and strongly bonded pyridine corresponding to the amount of weak
and strong Lewis acid sites, respectively.
[0050] After the adsorption of pyridine at .about.150.degree. C.
and subsequent evacuation of the cell at .about.150.degree. C., the
cell was again reconnected to the manifold and continued to be
evacuated for .about.30 minutes at .about.450.degree. C. After
cooling down to .about.80.degree. C., another sample spectrum was
collected. The amount of pyridine left on the sample after
evacuation at .about.450.degree. C. was referred to as strongly
bonded pyridine corresponding to the amount of strong Lewis acid
sites.
[0051] The band for the 19b ring vibration of Lewis-bonded pyridine
with peak position between .about.1450 cm.sup.-1 and .about.1455
cm.sup.-1 was chosen for the evaluation of Lewis acid sites. The
baseline for this band was defined as a linear curve between the
minima to the high frequency and low frequency side of the band.
The Lewis band was integrated between limits at .about.1470
cm.sup.-1 and .about.1420 cm.sup.-1, and the integration limits
were set to coincide with the points defining the baseline. The
band for the 19a ring vibration of pyridinium ions with peak
position between .about.1540 cm.sup.-1 and .about.1545 cm.sup.-1,
which corresponded to Bronsted acid sites, was not observed in the
present study. The amount of Lewis-bonded pyridine per gram of
sample was calculated using Beer-Lambert's law given in equations
1) and 2):
A.sub.i/.epsilon..sub.i=c*d 1)
A.sub.i/.epsilon..sub.i=n/Q 2)
where A.sub.i: integrated absorbance [cm.sup.-1] .epsilon..sub.i:
integrated molar extinction coefficient [cm/.mu.mol] c:
concentration of pyridine adsorbed in the wafer [.mu.mol/cm.sup.3]
d: wafer thickness [cm] Q: geometric surface area of wafer
[cm.sup.2] n: amount of pyridine [.mu.mol]
[0052] The integrated molar extinction coefficient was adopted from
the literature to be .epsilon..sub.L.apprxeq.2.22 cm/.mu.mol for
Lewis-bonded pyridine. Reformulation of equation 1) and dividing by
the wafer mass, m [mg], yielded equation 3):
n/m=(A.sub.i*Q)/(.epsilon..sub.i*m) 3)
which expressed the amount of pyridine in mmol adsorbed per gram of
sample.
[0053] The concentration of a particular Lewis acid-Bronsted base
pair site containing a hydroxyl group with an OH stretching
frequency between .about.3770 cm.sup.-1 and .about.3772 cm.sup.-1
was determined from the change in absorbance at .about.3771
cm.sup.-1 upon adsorption of pyridine. Normalization by the
specific wafer weight yielded equation 4:
c[OH.about.3771 cm.sup.-1]=A[.about.3771
cm.sup.-1,py.about.150.degree. C.]-A[.about.3771
cm.sup.-1,vac.about.450.degree. C.]/(m/Q) 4)
[0054] The difference spectrum [py.about.150.degree.
C.]-[vac.about.450.degree. C.] produced a minimum at the position
of the OH band at .about.3771 cm.sup.-1, relative to the high
frequency region neighboring the OH stretching regime. The
intensity of the .about.3771 cm.sup.-1 band was defined as the
difference in absorbance between the minimum at .about.3771
cm.sup.-1 and the baseline at .about.3850 cm.sup.-1. The
corresponding concentration of the hydroxyl groups at .about.3771
cm.sup.-1, as defined in equation 4, has units of [cm/mg
sample].
[0055] FIG. 5 shows the amount of the total number of Lewis acid
sites after pyridine adsorption and evacuation at
.about.150.degree. C., and that of strongly bonded pyridine
evaluated after subsequent evacuation at .about.450.degree. C. It
can be seen that the total number of Lewis sites increased in the
order of Catalyst 4<Catalyst 3<Catalyst 2.apprxeq.Catalyst 1,
while the amount of strong Lewis acid sites followed the order of
Catalyst 4.apprxeq.Catalyst 3<Catalyst 2.apprxeq.Catalyst 1. No
Bronsted acid sites were detected by IR of adsorbed pyridine.
[0056] FIG. 6 shows the difference spectra in the range of OH
stretching frequencies of the alumina samples formed by subtracting
the background spectra after outgassing for .about.2 hours in
vacuum at .about.450.degree. C. from the sample spectra taken after
adsorption of pyridine and subsequent evacuation at
.about.150.degree. C.
[0057] Pyridine was observed to interact by H-bonding with the
hydroxyl group at .about.3771 cm.sup.-1, producing a new OH band
for the perturbed hydroxyl group which was shifted to lower
frequencies. The H-bonding led to the disappearance of the band at
.about.3771 cm.sup.-1. Consequently, H-bonding produced a negative
band at the position of the unperturbed OH stretching frequency at
.about.3771 cm.sup.-1 in the difference spectrum between the sample
spectrum taken after pyridine adsorption at .about.150.degree. C.
and the background spectrum taken after outgassing at
.about.450.degree. C.
[0058] FIG. 7 shows a correlation between the concentration of the
hydroxyl group at .about.3771 cm.sup.-1 and the wt % methane made
over the alumina in the DME reactor at .about.440.degree. C. The
activity of the catalyst to convert methanol into methane increased
approximately linearly with the concentration of the hydroxyl group
at .about.3771 cm.sup.-1.
[0059] While the present invention has been described and
illustrated by reference to particular embodiments, those of
ordinary skill in the art will appreciate that the invention lends
itself to variations not necessarily illustrated herein. For this
reason, then, reference should be made solely to the appended
claims for purposes of determining the true scope of the present
invention.
* * * * *