13C and 15N solid-state MAS NMR study of the conversion of methanol and ammonia over H-RHO and H-SAPO-34 microporous catalysts

Article abstract of DOI:10.1039/a709294f

¹³C and ¹⁵N MAS NMR have been used to study the conversion of methanol and ammonia over H-SAPO-34 and H-RHO using sealed glass ampoules as microreactors under static batch conditions. The product peaks were well resolved in the ¹³C NMR spectra whereas the ¹⁵N NMR spectra gave only a single broad peak making it impossible to follow the reaction by observing this nucleus. Both catalysts give the tetramethyammonium cation whereas only zeolite H-RHO produces the monomethylammonium cation.

Full text of DOI:10.1039/a709294f

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13C and 15N solid-state MAS NMR study of the conversion of
methanol and ammonia over H-RHO and H-SAPO-34 microporous
catalysts
Alan ThursÐeld,a,*¤ Michael W. Anderson,a John Dwyer,a Graham J. Hutchingsb,” and
Darren Leeb,°
a University of Manchester Institute of Science and T echnology, Department of Chemistry,
Sackville Street, PO Box 88, Manchester, UK M60 1QD
b L everhulme Centre for Innovative Catalysis, Department of Chemistry, University of L iverpool,
PO Box 147, L iverpool, UK L 69 3BX
13C and 15N MAS NMR have been used to study the conversion of methanol and ammonia over H-SAPO-34 and H-RHO using
sealed glass ampoules as microreactors under static batch conditions. The product peaks were well resolved in the 13C NMR
spectra whereas the 15N NMR spectra gave only a single broad peak making it impossible to follow the reaction by observing
this nucleus. Both catalysts give the tetramethyammonium cation whereas only zeolite H-RHO produces the mono-
methylammonium cation.
In this work we follow the reaction over zeolite RHO and
Introduction
H-SAPO-34 using 13C and 15N MAS (magic angle spinning)
The production of methylamines from ammonia (NH ) and
methanol (MeOH) is an industrially important and well
NMR on samples contained in sealed glass ampoules. This
method has successfully been used by Ernst and Pfeifer22 who
studied this reaction over ZSM-5 and mordenite using 13C
MAS NMR techniques.
3
studied reaction, as evidenced by the great wealth of literature
including a number of patents that give details of synthesis
procedures.1h4 A range of zeolites have been investigated for
their suitability for this reaction including: ZSM-5, T, Y, A
and erionite.5h10 Particular attention has been paid to morde-
nite,11h13 RHO14h18 and ZK-5.15,19,20 Some of these papers
include studies on modiÐcation of the catalyst, such as varying
the alkali metal content6 and the deposition treatment with
Experimental
Catalysts
Zeolite H-RHO and H-SAPO-34 were supplied by the Lever-
hulme centre for Innovative Catalysis at the University of
Liverpool. Both microporous materials used in this investiga-
tion were characterised before use by solid-state MAS NMR
27Al, 29Si, 31P, 23Na and 1H nuclei), X-ray di†raction, micro-
analysis and scanning electron microscopy (SEM) in order to
verify the sample quality (these results are reported in Table
silicon tetrachloride,12,13 Al O
and/or SiO 15 and
2
3
2
trimethylphosphite16 to reduce further the egress of TMA into
the product stream, and, in addition, to “capÏ any external
active sites to reduce non-selective reactions. A communica-
tion by Subrahmanyam et al.21 reported the production of the
three amines using CO (instead of methanol) and ammonia
over ZSM-5, mordenite, zeolite Y and SAPO-5 catalysts.
(
1
). The acidic form of zeolite RHO was prepared by two
ammonium sulfate exchanges with subsequent washing with
distilled water, followed by heating to 500 ¡C in dry air under
shallow-bed conditions using two temperature ramps. First at
¤
Present address: Departamento de Qu•
Aveiro, Aveiro 3810, Portugal.
Present address: Department of Chemistry, University of Wales,
Cardi†, PO Box 912, UK CF1 3TB.
Present address: CRL, SpringÐelds Works, BNFL plc, Salwich,
Preston, Lancashire, UK PR4 0XJ.
mica, Universidade de
1
¡C min~1 to 105 ¡C, held for 2 h then at 2 ¡C min~1 to
”
500 ¡C and held for 12 h. 23Na MAS NMR indicated the com-
°
plete removal of sodium cations, which was corroborated by
microanalysis (\0.05 wt.%). The template in the SAPO-34
Table 1 Sample characterisation data of the H-SAPO-34 and H-RHO microporous catalysts in their Ðnal acidic forms
MAS NMR resonance/d
sample
unit cell composition
Si/Al
27Ala
29Sib
31Pc
1Hb
EFAld
morphologye
H-SAPO-34
H-RHO
H
H
[Si Al
.1 0.9 38.9 9.0 96]
P
O
]
0.26
4.2
33.4
55.3, 0.7
[95.3
[29.8
4.0f, 1.6g
4.2f, 2.5h
0%
16%
cubic
cubic
1
.5 1.5 5.6 4.8 24
Cs [Si Al O
[92.2, [97.6
8
[102.0, [108.7
External chemical shift references: a [Al(H O) ]3`; b TMS; c H PO . d From 27Al MAS NMR. e From SEM. f BrÔnsted acid sites. g Silanol
2
6
3
4
groups. h This signal has previously been assigned to a second type of BrÔnsted acid site.23
J. Chem. Soc., Faraday T rans., 1998, 94(8), 1119È1122
1119

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sample was removed under similar conditions to give the
acidic form. 1H MAS NMR conÐrmed that there were no
residual ammonium cations left after this calcination pro-
cedure (or AlOH/AlOOH groups caused by dealumination)
and that only BrÔnsted acid sites and silanol groups were
present. 27Al MAS NMR of the Ðnal form of the RHO sample
indicated the presence of 16% extra-framework aluminium
(
EFAl) ,with this information and knowing the bulk amount
of aluminium as indicated by microanalysis we obtained a
framework Si/Al ratio of 4.3. Information derived from the
2
9Si MAS NMR spectrum of this sample gave a Si/Al ratio of
4
.2, which is in close agreement to this value.
MAS NMR measurements
3C and 15N MAS NMR spectra were recorded on samples
1
sealed under vacuum in glass ampoules, typically 50 ^ 1 mg
of acid catalyst was used. Each sample was heated under
vacuum to 400 ¡C for 24 h to remove water. Following com-
plete dehydration of the sample, an aliquot of methanol
vapour and gaseous ammonia was introduced over the sample
and allowed to adsorb slowly. A stoichiometry of 1 : 1 meth-
anol to ammonia was used. The total adsorbate used was suf-
Ðcient to saturate the pores of H-RHO and H-SAPO-34.
1
3CH OH and 15NH were 99% enriched as supplied by
3
3
Aldrich, methanol was puriÐed by four freezeÈpumpÈthaw
cycles before use.
MAS NMR spectra were recorded using a Bruker MSL400
spectrometer in conjunction with a Chemagnetics double reso-
Fig. 2 13C MAS NMR spectra of the reaction of co-adsorbed meth-
anol and ammonia over H-RHO recorded with high-power decoup-
ling after the following sample pretreatments: heated to (a) 240 ¡C for
3
0 min; (b) 265 ¡C for 30 min; (c) 265 ¡C for 90 min; (d) 285 ¡C for 30
min; and (e) 400 ¡C for 60 min
nance “PencilÏ probe specially designed for stable spinning of
sealed samples at high spinning rates, all spectra were record-
ed at a sample spinning speed of 3 kHz. 13C MAS NMR
spectra were obtained with high-power proton decoupling
using a 13C p/2 pulse width of 4 ls and a repetition time of 10
s. Tetramethylsilane (TMS) was used as an external standard.
1
5N MAS NMR spectra were also recorded with high-power
proton decoupling using a p/2 pulse width of 10 ls and a
repetition time of 2 s, a saturated solution of NH 15NO was
4
3
used as the external standard. In order to follow the progress
of the methylamine synthesis the sealed sample (comprising
the acid catalyst and adsorbed methanol and ammonia) was
placed in a Carbolite furnace and heated to the required tem-
perature for a given period of time. Following this, the sample
was removed and allowed to cool to room temperature
thereby e†ectively quenching the reaction. The 13C and 15N
MAS NMR spectra were then recorded. All of the spectra pre-
sented within each stack plot are expanded as appropriate in
order to facilitate observation of low intensity signals.
Results
Fig. 1 shows a stack plot of the 13C MAS NMR spectra
obtained after heating the reactants over H-SAPO-34 from
240 to 285 ¡C. After heating to 240 ¡C for 30 min all of the
products, including dimethylether (DME), and the tetra-
methylammonium cation (designated 4MA) can be observed
together with unreacted methanol. Their respective chemical
shift values (d) vary within a small range (this was also found
for H-RHO) and are: monomethylamine (MMA), or the
Fig. 1 13C MAS NMR spectra of the reaction of co-adsorbed meth-
anol and ammonia over H-SAPO-34 recorded with high-power
decoupling after the following sample pretreatments: heated to (a)
2
2
40 ¡C for 30 min; (b) 265 ¡C for 30 min; (c) 265 ¡C for 90 min; and (d)
85 ¡C for 30 min
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J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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MMAH` cation, at d \ 24.8È25.3, dimethylamine (DMA) at
d \ 34.5È34.8, trimethylamine (TMA) at d \ 44.7È45.2, 4MA
at d \ 56.6, DME at d \ 59.8 and unreacted methanol at
d \ 49.3È49.6. At higher temperatures methanol is completely
consumed with a corresponding increase in the product con-
centration. The 4MA peak is noticeably sharper than the
other lines in the stack plot.
For H-RHO (Fig. 2) we observe that products are produced
at 240 ¡C with the appearance of MMA, DMA and DME.
With increasingly severe reaction conditions methanol is con-
sumed and eventually all of the amines are produced. DME
ing to all of the methylamines and an upÐeld peak corre-
sponding to unreacted ammonia over H-SAPO-34. These 15N
MAS NMR spectra are representative for both samples.
Ammonia gave a resonance at d \ [357.3 on the H-SAPO-
34 sample and for H-RHO a band at d \ [358.5 was
observed (this band shifted to d \ [359.4 in the presence of
the reaction products for both catalysts). The four methyl-
amines gave unresolved resonances within the band centred at
d \ [351.3 over both catalysts. In spectra in which methyl-
amines were consumed to give hydrocarbons, CO and CO a
2
single 15N NMR resonance is observed at the original pre-
(
which is initially produced in signiÐcant quantities) is also
reaction chemical shift value (spectrum not shown) indicating
rejuvenation of ammonia.
consumed similarly to methanol until at 400 ¡C, where no
DME can be observed, there are also insigniÐcant amounts of
4
MA produced compared with the H-SAPO-34 sample. In the
Discussion
MMA region of the 13C NMR spectrum there are two peaks
assigned to MMA and the protonated cation, these assign-
ments are discussed later. The chemical shift values for this
sample are: MMAH` cation at d \ 24.3È24.8 and MMA at
d \ 26.9È27.5, DMA at d \ 34.8È35.2, TMA at d \ 45.7 and
The 13C NMR chemical shift values of the amines in both
catalysts are adequately separated so that complete resolution
of signals arising from the products is observed. These signals
are very close to the values obtained by Ernst and Pfeifer22
who used mordenite, ZSM-5 and alumina. Also in agreement
with their Ðndings is the signiÐcantly narrower linewidth of
the tetramethylammonium cation peak compared to the line-
widths of the bands assigned to mono-, di-, and tri-
methylamine.
4
6.1, 4MA at d \ 56.6, DME at d \ 59.5È60.5 and unreacted
methanol at d \ 49.3È49.8. Heating this sample to 500 ¡C
spectrum not shown) resulted in the production of alkanes in
(
the region of d \ [11È25 and also CO and CO . The aro-
matic region was broad and poorly deÐned. At 600 ¡C only
2
methane, ethane CO and CO were observed (spectra not
shown), these assignments were conÐrmed by the J-coupled
A particularly interesting feature of the results in this work
is the two signals in the MMA region of the 13C MAS NMR
spectrum for zeolite RHO. A previous study by Vega and
Luz24 concerning the adsorption of the three methylamines
over H-RHO gave values almost identical to those reported in
this work (MMA/MMAH` at d \ 24.4, DMA at d \ 34.1 and
TMA d \ 46.424). The band at d \ 24.4 was assigned to
MMAH` therefore we assign the upÐeld band in Fig. 2 at
d \ 24.3È24.8 to the cation and the downÐeld band to physi-
sorbed MMA. These values vary from zeolite to zeolite, for
example Meinhold et al.25 found that physisorbed MMA and
the MMAH` cations give resonances at d \ 31 and 26.4,
respectively, over H-ZSM-5. No evidence of the protonated
forms of DMA or TMA was found. The close proximity of the
2
spectra in which a quintet and quartet were observed for the
Ðrst two signals, respectively, and two singlets for the later
two.
Fig. 3 shows two 15N MAS NMR spectra typical of those
recorded in this work, one of unreacted ammonia at room
temperature and the other showing a single peak correspond-
1
3C NMR chemical shift values of methylamines and their
respective protonated forms coupled with the broad 15N MAS
NMR peak add a degree of uncertainty to this assignment.
A variable temperature study of MMA adsorbed on H-Y
discovered that at room temperature a 1H NMR resonance
intermediate between a value for MMA and MMAH` was
produced from species undergoing rapid proton transfer.26 At
[
140 ¡C this exchange process was frozen out and two reso-
nances were observed for MMA and the MMAH` cation. It
is possible that such an exchange process is occurring in
H-RHO but is slow enough to give rise to the two signals on
the NMR timescale at room temperature. Although less likely
we cannot rule out completely the possibility that the two
signals arise from MMA adsorbed on two adsorption sites
such as EFAl and BrÔnsted acid sites, however Meinhold et
al.25 found no evidence of amines interacting with EFAl in
H-ZSM-5. A more interesting scenario, which takes into con-
sideration the structure of zeolite RHO, is that one of the
signals arises from MMA, which has entered the double 8-ring
prisms during heating of the sample. Upon cooling a propor-
tion of the MMA becomes trapped within the double 8-ring
prisms.
No 13C MAS NMR data are currently available in the liter-
ature for the three methylamines adsorbed on H-SAPO-34
and so it is possible that the signal assigned to MMA in this
work might correspond to MMAH`, as the chemical shift
values observed are close to the MMAH` cation chemical
shift values found for H-RHO.
Fig. 3 Representative 15N MAS NMR spectra of the reaction of co-
adsorbed methanol and ammonia over H-SAPO-34 recorded with
high-power decoupling after the following sample pretreatments: (a)
room temperature showing unreacted protonated ammonia; and (b)
heated to 265 ¡C for 30 min showing a signiÐcant amount of unre-
acted protonated ammonia and a downÐeld resonance assigned to the
three amine products and the 4MA cation
Pre-reaction NH adsorbed on H-SAPO-34 gave rise to a
3
signal at d \ [357.3 in the 15N NMR spectrum correspond-
ing to the formation of ammonium cations produced by
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
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protonation of NH by the BrÔnsted acid sites. NH adsorbed
amine products and therefore provided no futher insight into
the reaction.
3
3
on H-RHO gave a band at d \ [358.5, which is assigned to
ammonium cations involved in hydrogen bonding to adjacent
ammonium cations.27 This explanation Ðts in with the prox-
imity of BrÔnsted acid sites relative to each other, in
H-SAPO-34 the ammonium cations are isolated whereas in
H-RHO there are eight BrÔnsted acid sites per unit cell that
can facilitate the interaction between neighbouring ammon-
ium species.
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At around 240 ¡C zeolite H-RHO and H-SAPO-34 convert
methanol and ammonia to methylamines under static batch
conditions using a 1 : 1 ratio of the two reactants, in addition
both catalysts form the tetramethylammonium cation but
only H-RHO forms the monomethylammonium cation. At
higher temperatures amines produced by H-RHO are cracked
and the reactive species give hydrocarbons and ammonia.
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Paper 7/09294F; Received 24th December, 1997
1122
J. Chem. Soc., Faraday T rans., 1998, V ol. 94