BENEMÉRITA UNIVERSIDAD AUTÓNOMA DE PUEBLA
FACULTAD DE CIENCIAS QUÍMICAS
Modificación y caracterización de crinoptilolita para reacciones catalíticas selectivas de oxidación
guion reducción.
Tesis profesional para obtener el titulo de Maestro en Ciencias Químicas
Presenta: Pérez Vélez, Roxana.
Asesor: Elizalde González, María de la Paz.
Puebla Pue Febrero 2011
Table of contents
Pages
1 Introduction and objectives 1
2 Theoretical part 4
2.1 Zeolites in heterogeneous catalysis 4
2.1.1 Clinoptilolite 6
2.1.2 Cu-Zeolites in oxidative carbonylation of methanol to dimethyl carbonate 8
2.2 Catalyst preparation methods 10
2.3.1 Incipient wetness impregnation 10
2.3.2 Cation exchange 12
2.3 Catalyst characterization 13
2.3.1 Ex situ methods 13
2.3.2 In situ and operando spectroscopic methods 16
3 Methodology 18
3.1 Preparation of the catalyst materials 18
3.1.1 Pretreatment of the support materials 18
3.1.2 Preparation of the catalyst materials by wetness impregnation 19
3.1.3 Preparation of the catalyst materials by cation exchange 19
3.2 Catalyst characterization 21
3.2.1 ICP-OES and thermal analysis (TG-DSC) 21
3.2.2 Spectroscopic methods (ATR-FTIR and UV-vis-DRS) 21
3.2.3 X-ray diffraction (XRD) 21
3.2.4 In situ FTIR spectroscopic characterization by adsorption of probe
molecules 22
3.2.4.1 Adsorption of CO 22
3.2.4.2 Adsorption of NO 23
3.2.4.3 Adsorption of pyridine 23
3.2.5 Monitoring of the oxidative carbonylation reaction of methanol 24
3.2.5.1 In situ FTIR spectroscopic investigation 24
3.2.5.2 Operando DRIFTS/UV-vis-DRS/MS 25
4 Results and discussion 26
4.1 Ex situ characterization 26
4.1.1 Catalyst composition 26
4.1.2 Spectroscopic characterization (ATR-FTIR and UV-vis-DRS) 29
4.1.3 Phase analysis by XRD 38
4.2 In situ FTIR spectroscopic characterization 42
4.2.1 Cu(I) species 42
4.2.2 Cu(II) species 49
4.2.3 Acidity 55
4.3 Monitoring of the oxidative carbonylation reaction of methanol 58
4.3.1 In situ FTIR spectroscopic investigation 58
4.3.2 Operando DRIFTS/UV-vis-DRS/MS 67
5 Summary and conclusions 71
6 References 74
7 Appendix 82
Introduction
Dimethyl carbonate (DMC) is an important gasoline additive and chemical.
Their production
is based principally in the phosgenation of methanol which is an
environmentally unfriendly
process [1]. One environmentally benign alternative process is the
oxidative carbonylation of
methanol for dimethyl carbonate synthesis, based on the catalytic gas
phase reaction of
methanol with carbon monoxide and oxygen. Several Cu containing
catalysts, mainly Cuexchanged
ZSM-5, Mordenite, X and Y zeolites [2-4], have been applied in this
reaction,
finding promising results the use of Cu+Y zeolite.
Clinoptilolite is the most abundant natural zeolite showing a framework
structure with a twodimensional
channel system. However, although clinoptilolite is a potential
heterogeneous
catalyst with a unique framework structure, only a few studies concerning
characterization of
transition-metal-modified clinoptilolites and their application in
heterogeneous catalyzed
reactions have been reported. The main reason of the lack of studies
seems to be the presence
of impurities, such as iron and quartz in the natural material which are
difficult to remove.
Furthermore, studies applying in situ spectroscopic methods to
characterize transition-metalmodified
clinoptilolites are rare too.
For this reason, the potential of various Cu-modified natural and
synthetic clinoptilolite
samples for DMC synthesis should be tested in this work while the
comprehensive
characterization of such materials mainly by different spectroscopic
methods was the main
topic. The Cu-modified materials were prepared by wetness impregnation
and cation
exchange. For comparison, a mesoporous (Al-MCM-41) and an aluminosilicate
material
(Al2O3/SiO2) as supports were additionally included in this study.
In the field of heterogeneous catalysis, it is important to get knowledge
of the chemical and
structural nature of the active sites. In this context besides the
preparation of the various
catalysts the crystalline phases and reducibility of Cu species were
analyzed by applying XRD
and several spectroscopic techniques, respectively. Further, mainly in
situ FTIR spectroscopic
investigations were made to evaluate the oxidation state of copper by
using different probe molecules. Thus, CO was used to analyze the Cu+
sites and NO to look at the Cu2+ sites.
Additionally, Bronsted and Lewis acid sites were determined by adsorption
of the probe
molecule pyridine.
To get knowledge about reaction intermediates and reaction products and
to elucidate the
reaction mechanism, the catalytic reaction was studied by in situ FTIR
spectroscopic
technique. For this purpose, a special homemade heatable transmission
cell was used enabling
the monitoring of the catalyst under reaction-like conditions. Whereas,
the analysis of the
reaction products as well as possible changes on the catalyst were
supervised by a
multitechnique operando set up allowing simultaneous DRIFTS/UV-vis/MS
measurements.