Accessibility and interaction of surface OH groups in microporous and mesoporous catalysts applied for vapor-phase Beckmann rearrangement of oximes

Article abstract of DOI:10.1007/s10562-010-0274-7

¹H and ²H MAS NMR spectroscopy were utilized for investigating the interaction of surface complexes upon adsorption of cyclohexanone and cyclododecanone oxime on MFI-type zeolites and [Al]SBA-15. Signals of hydrogen-bonded and proton

Full text of DOI:10.1007/s10562-010-0274-7

Catal Lett (2010) 135:91–97
DOI 10.1007/s10562-010-0274-7
Accessibility and Interaction of Surface OH Groups
in Microporous and Mesoporous Catalysts Applied
for Vapor-Phase Beckmann Rearrangement of Oximes
•
V. R. Reddy Marthala J. Frey M. Hunger
•
Received: 27 November 2009 / Accepted: 10 January 2010 / Published online: 2 February 2010
Ó Springer Science+Business Media, LLC 2010
2
Abstract 1H and H MAS NMR spectroscopy were uti-
lized for investigating the interaction of surface complexes
upon adsorption of cyclohexanone and cyclododecanone
oxime on MFI-type zeolites and [Al]SBA-15. Signals of
hydrogen-bonded and protonated complexes were identi-
fied and evidenced the diffusion of cyclohexanone oxime
into the micropores of silicalite-1 and ZSM-5 and limited
accessibility of acid sites in [Al]SBA-15 for cyclododeca-
none oxime.
materials, like siliceous, aluminum-containing, and nio-
bium incorporated MCM-41 [12, 13] as well as SBA-15
[14] and cross-linked TS-1 [15] have been investigated as
solid acid catalysts.
A number of recent in situ spectroscopic investigations
focused on the mechanism of the vapor-phase Beckmann
rearrangement of oximes [12, 16–20]. In these studies,
surface hydroxyl groups with different chemical properties
and local structures, such as isolated SiOH groups and
SiOH nests at the outer surface or pore mouth of zeolite
particles or located in mesopores as well as SiOH groups in
the neighbourhood of framework boron atoms and bridging
OH groups (SiOHAl) in aluminum-containing materials,
were discussed as catalytically active sites. Recent quan-
tum-chemical investigations indicate that silanol groups
play the key role under the above-mentioned candidates
[21]. As evidenced by 15N MAS NMR spectroscopy,
adsorption of cyclohexanone oxime at siliceous materials,
such as silicalite-1 or siliceous mesoporous SBA-15, is
accompanied by the formation of adsorbate complexes,
which are hydrogen-bonded to the N-atom of the reactant
cyclohexanone oxime [18–20]. In contrast, an N-proton-
ation of the reactant was found to occur upon adsorption of
this reactant at aluminum-containing materials, such as
H-ZSM-5 or [A]SBA-15, covered by acidic surface sites
[18–20].
Keywords Beckmann rearrangement Á Cyclohexanone
oxime Á Porous catalysts Á Acid sites Á Accessibility Á
Solid-state NMR spectroscopy
1 Introduction
Manufacturing of nylon-6 fibres requires e-caprolactam as
reactant, which is mainly produced via the liquid-phase
Beckmann rearrangement reaction [1–3]. However, the
conventional liquid-phase process has important disad-
vantages due to the production of large amounts of
ammonium sulfate as by-product and the corrosive reaction
medium caused by the use of sulphuric acid or oleum.
Environmentally benign new processes base on the vapor-
phase Beckmann rearrangement of cyclohexanone oxime
to e-caprolactam on solid catalysts [4, 5]. For the vapor-
phase Beckmann rearrangement reaction, various zeolites,
such as silicalite-1 [6], [B]ZSM-5 [7], ZSM-5 [8], MCM-
22 [9], Beta [10], and [B]Beta [11], and mesoporous
Generally, hydrogen bonding and protonation of adsor-
bate molecules leads to characteristic low-field shifts of the
1H MAS NMR signals caused by the contributing hydroxyl
protons [22–25]. In the case of the adsorption of non-
1
deuterated molecules, the H MAS NMR signals due to
hydroxyl groups on solid catalysts are overlapped by the
adsorbate signals. Not all reactants of interest, however,
are available in the fully deuterated state, which would
solve the above-mentioned problem. On the other hand,
V. R. Reddy Marthala Á J. Frey Á M. Hunger (&)
Institute of Chemical Technology, University of Stuttgart,
70550 Stuttgart, Germany
e-mail: michael.hunger@itc.uni-stuttgart.de
123

92
V. R. Reddy Marthala et al.
deuteration of surface hydroxyl groups is possible and
opens the way for highly resolved 2H solid-state NMR
studies (2H: spin I = 1) of deuterated catalysts without and
after adsorption of non-deuterated molecules. In the present
vapor for 1 h. Subsequently, they were dehydrated at
473 K for 2 h. Upon repeating this procedure 5 times, the
materials were sealed in glass tubes. Utilizing this proce-
dure, deuteration degrees of 50% for D-ZSM-5 and 65%
for D-[Al]SBA-15 were obtained as determined by quan-
2
work, to the best of our knowledge first H high-speed
1
MAS NMR investigations of the hydroxyl groups of deu-
terated solid acid catalysts upon loading with adsorbate
molecules have been performed. By studying solid cata-
lysts with different chemical compositions and pore
diameters upon adsorption of oximes with different
molecular sizes, the accessibility and chemical interacting
of surface sites were indentified.
titative H MAS NMR spectroscopy.
Partially deuterated cyclohexanone oxime-D11 was
prepared as follows: In a 25 ml round-bottom flask, 1.00 g
of deuterated hydroxylamine-hydrochloride (ND2ODÁÁÁ
DCl) delivered by CDN Isotopes (Lot#B218P2) was dis-
solved in 4.28 g of deuterated water. Subsequently, 0.86 g
¨
of sodium acetate (Riedel-de Haen) was added and the
solution was heated to 313 K. Finally, 1.00 g of cyclo-
hexanone-D10 of CDN Isotopes (Lot#G126P13) was added
dropwise under stirring. After the complete addition, the
flask was sealed and shaken vigorously until the oxime
could be precipitated as a fine white powder. This cyclo-
hexanone oxime-D11 was collected by vacuum filtration
and dried in air for 24 h at room temperature. The obtained
cyclohexanone oxime-D11 reached a deuteration degree
of 83% as determined by quantitative 1H MAS NMR
spectroscopy.
2 Experimental
Silicalite-1 (nSi/nAl = 1,700), H-ZSM-5 (nSi/nAl = 14), and
aluminum-containing [Al]SBA-15 material (nSi/nAl = 9)
were prepared as described in Ref. [20]. The materials were
characterized by X-ray diffraction (Bruker D8), ICP/OES
(Varian), N2-adsorption (micrometrics ASAP 2010), and
solid-state NMR spectroscopy (Bruker MSL 400). The
XRD patterns of these catalysts were identical with the
patterns reported in the literature [26–28]. The physio-
chemical properties of the catalysts, such as the nSi/nAl ratio,
concentration of surface hydroxyl groups (nH), BET surface
areas, pore volume, and average pore diameters are given in
Table 1.
Before mixing the non-deuterated or deuterated catalysts
with the deuterated or non-deuterated cyclohexanone
oxime and cyclododecanone oxime, the oxime was evac-
uated at room temperature for 20 min. Then, the evacuated
reactants were mixed with the dehydrated (vide supra)
catalysts in a ratio of 1 : 1, i.e., one oxime molecule per OH
group, inside a glove box purged with dry nitrogen gas.
Subsequently, the catalyst/oxime mixtures were filled into
4 mm MAS NMR rotors without contact to air and heated
at 393–523 K for 20 min before the solid-state NMR
studies.
For dehydration, the catalyst materials were heated in
glass tubes at a vacuum line from 298 to 393 K with a rate of
0.5 K min-1 and maintained at 393 K for 3 h. Subsequently,
the temperature was increased from 393 to 723 K with a rate
of 1 K min-1 and maintained at the final temperature for
12 h. During this time, the materials were evacuated at a
pressure of p \ 10-2 mbar. Finally, the samples were cooled
to room temperature and sealed in the glass tubes.
Prior to 2H solid-state NMR studies, the deuterated
catalysts D-ZSM-5 and D-[Al]SBA-15 were prepared via
the following procedure: The non-deuterated materials
were dehydrated at 723 K according to the above-men-
tioned route. Upon cooling to room temperature, the sam-
ples were kept at this temperature in contact with D2O
2
1H and H MAS NMR spectra of the dehydrated sam-
ples before and upon mixing with cyclohexanone oxime
were obtained on a Bruker MSL 400 spectrometer at res-
onance frequencies of 400.1 and 61.4 MHz and with
sample spinning rates of 8.0 kHz using 4 mm rotors and
25.0 kHz using 2.5 mm rotors, respectively. The spectra
were recorded after excitation with single pulses of the
lengths of 2.1 and 4.0 ls and with repetition times of 10
and 2 s, respectively.
Table 1 Physiochemical properties of the calcined catalysts used in this study
Catalyst
nSi/nAl
nH/mmol g-1
*
BET surface
area/m2 g-1
Pore volume/cm3 g-1
Average pore
diameter/nm
Silicalite-1
ZSM-5
1,700
14
0.31
1.12
1.29
1
365
338
672
0.20
0.16
1.54
0.55**
0.55**
8.97
[Al]SBA-15
9
* nH values were obtained by the quantitative H MAS NMR spectroscopy of dehydrated samples
** According to Ref. [26]
123

Interaction of Surface OH Groups in Porous Solids with Oximes
93
1.8
2.5
3 Results and Discussion
(a) unloaded silicalite-1
3.1 1H MAS NMR Investigations of Non-Deuterated
Silicalite-1 and H-ZSM-5 upon Adsorption
and Conversion of Partially Deuterated
Cyclohexanone Oxime-D11
silicalite-1 + C6(H,D)10NO(H,D)
9.3
(b) heated at 393 K
2
For supporting the discussion of H MAS NMR spectra of
the deuterated catalysts under study, at first 1H MAS NMR
investigations of the non-deuterated and unloaded as well as
oxime-loaded catalysts were performed. Figure 1a shows
the spectrum of dehydrated and unloaded silicalite-1 con-
sisting of a single signal at 1.8 ppm due to silanol groups.
After loading of silicalite-1 with partially deuterated (83%)
cyclohexanone oxime [C6(H,D)10NO(H,D)] and heating at
393–423 K (Fig. 1b, c), a strong signal occurred at ca.
2.5 ppm caused by the overlapping signals of residual ring
protons (ca. 17%) in cyclohexanon oxime [29]. The low-
field signal at 9.3 ppm can be explained by the hydroxyl
proton of the hydrogen-bonded oxime. The formation of
(c) heated at 423 K
6.5
(d) heated at 473 K
14.5 12.2
(e) heated at 498 K
these surface complexes was already observed in earlier 15
N
heated at 523 K
(f)
MAS NMR studies of oxime-loaded silicalite-1 [18–20].
Unfortunately, the signal of residual non-deuterated ring-
protons in cyclohexanone oxime at 2.5 ppm overlaps the
chemical shift range of surface hydroxyl groups of silica-
lite-1. This hinders the study of the effect of oxime
adsorption on the corresponding signals of OH groups
occurring at 1.8–4 ppm before the adsorption.
20
15
10
5
0
-5
-10
δ
1H / ppm
Fig. 1 1H MAS NMR spectra of dehydrated silicalite-1 recorded
before (a) and upon adsorption of partially deuterated cyclohexanon
oxime [C6(H,D)10NO(H,D)] and heating at temperatures of 393–
523 K (b–f)
Upon heating of the silicalite/oxime mixture at 473 and
498 K (Fig. 1d, e), the signal at 9.3 ppm became weaker
and another low-field signal at 12.2 ppm occurred, which
indicates the formation of hydrogen-bonded e-caprolactam
[18–20]. The very small hump at ca. 14.5 ppm may be
caused by few protonated e-caprolactam molecules formed
in the vicinity of the small number of framework aluminum
species in silicalite-1 (nSi/nAl = 1,700). The much stronger
and broad signal at ca. 6.5 ppm indicates the formation of
by-products, such as e-aminocapric acid and hydroxyl-
10.4 ppm has the same temperature-dependence like the
signal at 9.3 ppm in Fig. 1b–d, it was attributed to hydro-
gen-bonded cyclohexanone oxime. Based on earlier 15N
MAS NMR studies, the signals at 11.2–13.0 ppm are
assigned to protonated cyclohexanone oxime [18–20].
Heating of the H-ZSM-5/oxime mixture to 478–523 K
(Fig. 2d–f) led to a strong decrease of the signals of pro-
tonated cyclohexanone oxime (11.2 and 13.0 ppm), while
few of the hydrogen-bonded reactants (10.4 ppm) remained
on the catalyst indicating the low catalytic activity of the
corresponding surface sites (SiOH). The most significant
change in the low-field range is the appearance of a new
signal at 14.2 ppm caused by the formation of protonated
e-caprolactam as observed in earlier 15N MAS NMR
investigations [18]. As already discussed in the case of
oxime-loaded silicalite-1, the signals at ca. 6.5 ppm are a
hint for the formation of by-products (vide supra).
1
amine [18, 20, 29]. A survey on the assigned H MAS
NMR signals and their chemical shift values is given in
Table 2.
In contrast to silicalite-1 (nSi/nAl = 1,700), the zeolite
H-ZSM-5 (nSi/nAl = 14) is characterized by a large num-
1
ber of acidic bridging OH groups causing the H MAS
NMR signal at 4.2 ppm in Fig. 2a. After adsorption of
partially deuterated cyclohexanone oxime on H-ZSM-5
and heating at 393–423 K (Fig. 2b, c), again a strong signal
of ring protons in cyclohexanon oxime appeared at ca.
2.5 ppm. Instead of the low-field signal occurring at
9.3 ppm in the spectrum of oxime-loaded silicalite-1,
however, the low-field range of the spectra consists of a
narrow signal at 10.4 ppm and two additional broad signals
at 11.2 and 13.0 ppm. Since the intensity of the signal at
Summarizing can be stated that adsorption of partially
deuterated cyclohexanone oxime on silicalite-1 and
H-ZSM-5 via hydrogen-bonding to SiOH groups leads
to 1H MAS NMR signals at 9.3–10.4 ppm. Protonation
of the cyclohexanone oxime at bridging OH groups of
H-ZSM-5 is accompanied by the occurrence of signals at
123

94
V. R. Reddy Marthala et al.
4.2
11.2–13.0 ppm. Upon conversion of this reactant to e-
caprolactam, 1H MAS NMR signals of the hydrogen-
bonded and the protonated reaction product can be
observed at 12.2 and 14.2 ppm, respectively. In the same
manner, also the interaction of non-deuterated oximes
with hydroxyl groups of deuterated solid catalysts should
(a) unloaded H-ZSM-5
1.8
H-ZSM-5 + C6(H,D)10NO(H,D)
11.2 10.4
2.5
2
lead to low-field H MAS NMR signals in the chemical
13.0
(b) heated at 393 K
shift range of
9–14 ppm indicating the accessibility of the correspond-
ing surface sites for adsorbate molecules and their
interactions.
6.5
heated at 423 K
(c)
2
3.2 High-Speed H MAS NMR Investigations of
Partially Deuterated ZSM-5 and [Al]SBA-15 upon
Adsorption of Non-Deuterated Cyclohexanone
Oxime and Cyclododecanone Oxime
heated at 473 K
(d)
The standard technique for performing 2H solid-state NMR
2
studies of the quadrupolar H nuclei with spin I = 1 in
14.2
heated at 498 K
(e)
solid catalysts and adsorbate complexes in the past decades
and until nowadays is the application of echo-sequences
with the use of static (no sample spinning) samples (see,
e.g., Refs. [30–34]). Often, this spectroscopic technique is
utilized for studying motional effects in the materials under
study. Similarly, the sideband intensities of 2H MAS NMR
spectra recorded with small to moderate spinning rates are
analyzed to obtain information on molecular mobilities
[35, 36]. On the other hand, there are only very few studies
heated at 523 K
(f)
20
15
10
5
0
-5
-10
δ
1H / ppm
Fig. 2 1H MAS NMR spectra of dehydrated H-ZSM-5 recorded
before (a) and upon adsorption of partially deuterated cyclohex-
anon oxime [C6(H,D)10NO(H,D)] and heating at temperatures of
393–523 K (b–f)
2
utilizing high-resolution H MAS NMR spectroscopy for
the investigation of deuterated surface hyxdroxyl groups or
adsorbate complexes on solid catalysts [37–39].
Table 2 Assignments of 1H MAS NMR signals observed upon
adsorption and conversion of cyclohexanone oxime on silicalite-1 and
zeolite H-ZSM-5
Via the above-mentioned solid-state NMR techniques,
the quadrupole coupling constants CQ of acid sites on solid
catalysts were determined. Values of CQ = 208–240 kHz
for deuterated bridging hydroxyl groups (SiODAl) in
zeolites [30, 37, 38] and CQ = 67–75 MHz for deuterated
silanol groups (SiOD) [30, 38] were obtained. The signif-
icant larger CQ values of deuterons in SiODAl groups in
comparison with those in SiOD groups are due to the effect
of framework aluminum atoms in the local structure of
SiODAl groups on the electric field gradient at the site of
d
1H/ppm
Assignmentsa
1.2–2.2
1.4–3.5
Silanol groups at the external and internal
surface of solid catalysts
Ring protons of cyclohexanone oxime
and e-caprolactam
3.6–4.3
ca. 6.5
Bridging OH groups of acidic zeolites
NH2 groups of by-products (e.g., e-aminocapric
acid, hydroxylamine)
2
the deuterons. Since the H MAS NMR spectrum of OD
9.3
Hydrogen-bonded cyclohexanone oxime
on silicalite-1
groups with CQ values of ca. 220 kHz is characterized by a
sideband pattern with an envelop having a width of ca.
330 kHz, high spinning speeds are required to decrease the
number of spinning sidebands and their intensity contri-
bution, i.e., for obtaining central lines with suitable inten-
10.4
Hydrogen-bonded cyclohexanone
oxime on H-ZSM-5
12.2
Hydrogen-bonded e-caprolactam
on silicalite-1
2
sities. In the present work, therefore, the H MAS NMR
11.2–13.0
14.2–14.5
a
Protonated cyclohexanone oxime
on H-ZSM-5
studies of deuterated catalysts before and upon adsorption
of reactant were performed with the sample spinning rate
of 25 kHz.
Protonated e-caprolactam
on silicalite-1 and H-ZSM-5
2
Most of the assignments were made on the base of earlier 15N MAS
NMR studies
In Figs. 3 and 4, the high-speed H MAS NMR spectra
of the zeolite D-ZSM-5 and the mesoporous material
123

Interaction of Surface OH Groups in Porous Solids with Oximes
95
Fig. 3 2H MAS NMR spectra
of deuterated and dehydrated
zeolite D-ZSM-5 recorded
before (a, b) and after
unloaded D-ZSM-5
(a)
*
3
*
adsorption of cyclohexanon
oxime [C6H10NOH] (c) and
cyclododecanone oxime
500
400
300
200
100
0
-100
-200
-300
-400
-500
[C12H22NOH] (d). While
spectrum (a) consists of central
line and the first left and right
spinning sidebands (marked by
asterisks), spectra (b–d) show
exclusively the central line
δ
2H / ppm
(c)
(b) unloaded D-ZSM-5
D-ZSM-5 + C6H10NOH
(d) D-ZSM-5 + C12H22NOH
5
2
12
3
9 - 12
3
20.0 10.0
0.0 -10.0
20.0 10.0
0.0 -10.0
20.0 10.0
0.0 -10.0
δ2H / ppm
Fig. 4 2H MAS NMR spectra
of the deuterated and
dehydrated mesoporous
(a) unloaded D-[Al]SBA-15
2
D-[Al]SBA-15 material
recorded before (a, b) and after
adsorption of cyclohexanon
oxime [C6H10NOH] (c) and
cyclododecanone oxime
[C12H22NOH] (d). While
spectrum (a) consists of the
central line and the first left
and right spinning sidebands
(marked by asterisks), spectra
(b–d) show exclusively the
central line
*
*
500
400
300
200
100
0
-100
-200
-300
-400
-500
δ
2H / ppm
(b) unloaded D-[Al]SBA-15
(c)
(d)
D-[Al]SBA-15 + C6H10NOH
D-[Al]SBA-15 + C12H22NOH
2
3
12
10
ca. 3
ca. 14
20.0 10.0
0.0 -10.0
20.0 10.0
0.0 -10.0
20.0 10.0
0.0 -10.0
δ2H / ppm
D-[Al]SBA-15 recorded before and upon adsorption of
oximes are shown. In the spectra depicted in Figs. 3a and
4a, the strong effect of aluminum species in the vicinity of
deuterons, such as in SiODAl groups (D-ZSM-5), on the
sideband intensity is obvious. The slight intensity differ-
ence between the left and right spinning sidebands is due to
the electronic filter utilized for improving the signal-to-
of H-ZSM-5 in Fig. 2a, the 2H MAS NMR signals of
D-ZSM-5 in Fig. 3b have significantly larger residual line
widths leading to a partial overlap of the resonances at 2
and 5 ppm.
Adsorption of cyclohexanone oxime (diameter of
0.65 nm [40]) on D-ZSM-5 is accompanied by the occur-
rence of a low-field 2H MAS NMR signal at 12 ppm
(Fig. 3c). Assuming same interactions for adsorption of
deuterated cyclohexanone oxime on non-deuterated ZSM-5
(Fig. 2b) like for adsorption of the non-deuterated reactant
on the deuterated zeolite (Fig. 3c), the above-mentioned 2H
2
noise ratio. In the H MAS NMR spectra of D-ZSM-5 and
D-[Al]SBA-15 in Figs. 3b and 4b, the signals at 2 ppm and
5 ppm have to be attributed to SiOD and SiODAl groups,
respectively. In comparison with the 1H MAS NMR signals
123

96
V. R. Reddy Marthala et al.
MAS NMR signals at 12 ppm indicates protonation of the
2
adsorbate molecules. In this case, the results of H MAS
4 Conclusions
2
NMR spectroscopy demonstrate that a significant content
of the acidic bridging hydroxyl groups (SiOHAl and
SiODAl) in ZSM-5 are accessible for cyclohexanone
oxime, which supports the diffusion of the adsorbate
molecules into the pores of this material.
1H MAS NMR and high-speed H MAS NMR spectros-
copy were utilized for studying the accessibility and
interaction of surface hydroxyl groups on microporous
MFI-type zeolites (pore diameter of ca. 0.55 nm) and
mesoporous [Al]SBA-15 material (pore diameter of ca.
9 nm) for cyclohexanone oxime and cyclododecanone
oxime with molecular diameters of 0.65 and 0.9 nm,
respectively. Deuteration of cyclohexanone oxime up to an
H/D-exchange degree of 83% and application of this oxime
as probe molecule and reactant for vapor-phase Beckmann
rearrangement on silicalite-1 and H-ZSM-5 led to 1H MAS
NMR spectra with reasonable resolution in the low-field
range, which is the characteristic chemical shift range of
signals caused by hydrogen-bonded and protonated adsor-
bate complexes.
Upon adsorption of cyclododecanone oxime (diameter
of 0.9 nm [41]) on D-ZSM-5, most of the deuterated
hydroxyl groups do not interact with the adsorbate mole-
cules and only a small content of the deuterated hydroxyl
groups contribute to the low-field shoulder at 9–12 ppm
(Fig. 3d). Considering the molecular diameters of cyclo-
hexanone oxime and cyclododecanone oxime of 0.65 and
0.9 nm, respectively, the latter molecule do not fit into the
pores of ZSM-5 and interacts exclusively with hydroxyl
groups at the outer surface and at pore mouths. The low-
field shoulder in Fig. 3d covering the chemical shift range
of 9–12 ppm indicates protonation of a small number of
adsorbed cyclododecanone oxime molecules. Hence, some
of the strongly acidic SiODAl groups of D-ZSM-5 must
be accessible for cyclododecanone oxime, i.e., these few
SiODAl groups are located at the outer surface of the
zeolite particles or at pore mouths.
After adsorption of partially deuterated cyclohexanone
1
oxime on silicalite-1, the H MAS NMR spectra indicated
the hydrogen-bonding of the surface complexes (9.3 ppm).
Also upon conversion of cyclohexanone oxime to e-capro-
lactam on silicalite- 1, the hydrogen bonding was the
dominating interaction of the surface complexes (12.2 ppm)
1
and only a very weak H MAS NMR signal of very few
2
The H MAS NMR spectrum recorded after adsorption
protonated e-caprolactam molecules was found (14.5 ppm).
In contrast, for adsorption and conversion of cyclohexanone
oxime on zeolite H-ZSM-5, mainly the 1H MAS NMR
signals due to surface complexes of protonated cyclohexa-
none oxime (11.2–13.0 ppm) and protonated e-caprolactam
(14.2 ppm) were observed. The above-mentioned findings
for the interaction of surface complexes formed upon
adsorption and conversion of cyclohexanone oxime support
the result of earlier 15N MAS NMR studies on this reaction
system.
As an important advantage of high-speed 2H MAS NMR
studies of the deuterated catalysts loaded with non-deu-
terated oximes, the direct observation of surface OD groups
and of the effects of adsorbate molecules are possible. In
of cyclohexanone oxime on the mesoporous D-[Al]SBA-15
material (Fig. 4c) is very similar to that obtained upon
2
adsorption on D-ZSM-5 (Fig. 3c). Again, a low-field H
MAS NMR signal at 12 ppm occurs, which is a hint for
protonation of the adsorbate molecule. A weak shoulder at
the high-field flank of the latter signal is due to the for-
mation of hydrogen-bonded cyclohexanone molecules.
Only few surface hydroxyl groups are not accessible for the
adsorbate molecules and cause the weak signal at ca.
3 ppm.
According to the pore diameter of D-[Al]SBA-15 of ca.
9 nm (see Tab. 1), cyclododecanone oxime is able to dif-
fuse into the mesopores of the above-mentioned material.
In agreement with this assumption, the 2H MAS NMR
spectrum of D-[Al]SBA-15 loaded with cyclododecanone
oxime shows a strong signal at 10 ppm indicating an
interaction of the corresponding adsorbate molecules with
surface sites via hydrogen bonding (Fig. 4d). Surprisingly,
however, a large content of the surface hydroxyl groups is
not accessible for cyclododecanone oxime and causes the
signal intensities at ca. 3 ppm. In addition, the content of
cyclododecanone oxime molecules, which are protonated
upon adsorption on D-[Al]SBA-15, is much lower than
for cyclohexanone oxime (signals at 12–14 ppm). Hence,
a significant number of acidic hydroxyl groups on the
D-[Al]SBA-15 material under study are located in small
mesopores or in micropores, which are not accessible for
cyclododecanone oxime molecules.
1
comparison with the H MAS NMR spectra in this work,
2
larger residual line widths occurred in the H MAS NMR
spectra, which may be caused by the crucial adjustment of
the magic angle in the latter case. Upon adsorption of
cyclohexanone oxime on deuterated ZSM-5 and [Al]SBA-
2
15, H MAS NMR signals at 12 ppm occurred, which is a
hint to the formation of protonated surface complexes. For
both materials under study, microporous zeolite D-ZSM-5
and mesoporous D-[Al]SBA-15 material, a significant
content of the surface OD were involved in the formation
of adsorbate complexes. This observation indicates that
cyclohexanone oxime diffuses into the pores of the
microporous zeolite ZSM-5. Adsorption of cyclododeca-
none oxime on D-ZSM-5, on the other hand, was found to
affect only a small content of surface hydroxyl groups
123

Interaction of Surface OH Groups in Porous Solids with Oximes
97
leading to a broad low-field shoulder at 9–12 ppm in the 2H
MAS NMR spectrum. The corresponding OD groups
should be located at the outer particle surface and at pore
mouths. Also in the case of cyclododecanone oxime
adsorption on D-[Al]SBA-15, not all OD groups are
accessible for the adsorbate molecules. Especially the
stronger acidic hydroxyl groups in the vicinity of alumi-
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