Catalysis: Volume 26

by James J Spivey (Editor), K M Dooley (Editor), Yi-Fan Han (Editor)

Overview

Over 7000 papers are published in the field of catalysis each year. While the majority appear within a handful publications, keeping up with the literature can be difficult. Now in its 26th volume, the Specialist Periodical Report on Catalysis presents critical and comprehensive reviews of the hottest literature published over the last twelve months.

Industrial and academic scientists face increasing challenges to find cost-effective and environmentally sound methods for converting natural resources into fuels, chemicals and energy. This series is edited by two leading researchers in the field and provides a balanced and in-depth review of the modern approaches to these challenges, covering major areas of heterogeneous and homogenous catalysis, as well as specific applications of catalysis, such as NOx control, kinetics and experimental techniques, such as microcalorimetry.

With chapters detailing specific areas within the field, this series is a comprehensive reference for anyone working in Catalysis and an essential resource for any Library.

Product Details

ISBN-13:	9781782621188
Publisher:	RSC
Publication date:	03/17/2014
Series:	Specialist Periodical Reports - Catalysis , #26
Sold by:	Barnes & Noble
Format:	eBook
Pages:	286
File size:	9 MB

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Catalysis

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By Adeyiga Adeyinka, C. R. Apesteguía, Hakon Bergem, Francesca Lønstad Bleken, Edd A. Blekkan, Sara Boullosa-Eiras, De Chen, J. I. Di Cosimo, V. K. Díez, Marius Westgard Erichsen, C. Ferretti, Enrique García-Bordejé, Lenka Hannevold, Simon A. Kondrat, Karl Petter Lillerud, Rune Lødeng, Xiao-hua Lu, Zhiqiang Ma, Unni Olsbye, Xi Pan, M. F. R. Pereira, Magnus Rönning, James J. Spivey, Michael Stöcker, Nachal Subramanian, Stian Svelle, Stuart H. Taylor, Shewangizaw, Teketel, Jeroen van Bokhoven, Jian-guo Wang, Gui-lin Zhuang

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-78262-003-7

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CHAPTER 1

Basic catalysis on MgO: generation, characterization and catalytic properties of active sites

J. I. Di Cosimo, V. K. Díez, C. Ferretti and C. R. Apesteguía

DOI: 10.1039/9781782620037-00001

The generation, characterization and catalytic properties of MgO active sites were studied. MgO samples stabilized at different temperatures were used to control the distribution of surface base sites; specifically, MgO was calcined at 673 K, 773 K and 873 K (samples MgO-673, MgO-773 and MgO-873). The nature, density and strength of MgO base sites were characterized by temperature-programmed desorption of CO2 and infrared spectroscopy after CO2 adsorption at 298 K and sequential evacuation at increasing temperatures. MgO samples contained surface sites of strong (low coordination O2- anions), medium (oxygen in Mg2+-O2- pairs) and weak (OH- groups) basicity. The density of strong basic sites was predominant on MgO-673. The increase of the calcination temperature drastically decreased the density of strong base sites and to a lesser extent that of weak OH- groups, while slightly increased that of medium-strength base sites. The catalytic properties of MgO samples were proved for the aldol condensation of citral with acetone to yield pseudoionone, the hydrogen transfer reaction of mesityl oxide with 2-propanol to obtain the unsaturated alcohol 4-methyl-3-penten-2ol, and the synthesis of monoglycerides via the transesterification of methyl oleate with glycerol. The effect of calcination temperature on the MgO catalytic properties depended on the basicity re- quirements for the rate-limiting step of the base-catalyzed reaction. The activity for both the aldol condensation of citral with acetone and the glycerolysis of methyl oleate diminished with the MgO calcination temperature because these reactions were essentially promoted on strongly basic O2- sites. In contrast, the synthesis of 4-methyl-3-penten-2ol by the hydrogen transfer reduction of mesityl oxide with 2-propanol increased with calcination temperature because the reaction intermediate was formed on medium-strength Mg2+-O2- pair basic sites. Additional information on the role played by the MgO active sites on the kinetics of base-catalyzed reactions was obtained by performing molecular modeling studies on our MgO catalysts using Density Functional Theory (DFT) for the glycerolysis of methyl oleate, an unsaturated fatty acid methyl ester (FAME). The molecular modeling of glycerol and FAME adsorptions was carried out using terrace, edge and corner sites for representing the MgO (100) surface. In agreement with catalytic results, calculations predicted that dissociative chemisorption of glycerol with O–H bond breaking occurs only on strong base sites (edge sites) whereas nondissociative adsorption takes place on medium-strength base sites such as those of terrace sites. Results also indicated that glycerol was more strongly adsorbed than FAME. The glycerol/FAME reaction would proceed then through a mechanism in which the most relevant adsorption step is that of glycerol.

1 Introduction

Alkaline earth metal oxides catalyze a variety of organic reactions requiring the cleavage of a C–H bond step and the formation of carbanion intermediates. In particular, pure and alkali-promoted MgO been shown to promote Cannizzaro and Tischenko reactions, Michael, Wittig and Knoevenagel condensations, transesterification reactions, double-bond isomerizations, self- and cross-condensation reactions, Henry reaction, alcohol coupling, and H2 transfer reactions. However, the MgO basicity needed for e?ciently promoting these reactions depend on the rate-limiting step requirements. MgO can be synthesized in a variety of presentation formats, including nanosheets, nanowires and nanoparticles, but its catalytic properties depend greatly on the preparation method. Nevertheless, most of reports on the preparation of magnesia deal with the effect of the synthesis method and conditions on the MgO structural and physical properties. Very few papers have attempted to tailor the distribution, density, and strength of surface base sites of MgO upon synthesis in order to design the catalyst surface to reaction requirements. More insight on the relationship between the synthesis procedure with the generation and control of MgO surface base sites is then required to improve the efficient use of this oxide in catalysis applications.

Detailed characterization of MgO base sites is crucial to establish correlations between the surface basic properties and the catalyst activity and selectivity for a given reaction. The most common methods for characterization of solid basicity are thermal programmed desorption (TPD) and infrared spectroscopy (IR) of preadsorbed probe molecules, and the use of test reactions. TPD studies provide information on the density and strength of base sites while additional insight on the base site nature is often obtained by IR characterization. Carbon dioxide has been largely employed as a probe molecule for evaluating the solid basicity by TPD and IR techniques although other acid molecules such as acetic acid have been also used. On the other hand, the test reactions most frequently used for characterizing the catalyst acid-base properties are the decomposition of alcohols, in particular 2-propanol, 2-butanol and 2-methyl-3-butyn-2-ol. In the case of 2-propanol, it is generally accepted that 2-propanol dehydration to propylene occurs on solid acids containing Brønsted acid sites via an E1 mechanism while on amphoteric oxides with acid-base pair sites propylene is obtained through a concerted E2 mechanism. On strong basic catalysts, 2-propanol is dehydrogenated to acetone via an E1cB anionic mechanism. Thus, the catalyst acid-base properties may be related to the propylene/acetone selectivity ratio. In contrast, test reactions have been used only in few cases for characterizing base site strength distributions on solid bases. For example, in a previous work, we proposed that on alkali-modified MgO catalysts 2-propanol decomposition to acetone and propylene takes place via a n E1cB mechanism in two parallel pathways sharing a common 2-propoxy intermediate; in this mechanism, the intermediate-strength base sites promote acetone formation, whereas high-strength base sites selectively yield propylene. Nevertheless, several studies have shown that the use of test reactions is not sensitive enough to establish a basicity scale of the catalysts.

Theoretical calculations of surface sites have been performed for exploration of MgO catalysis. In general, Density Functional Theory (DFT) calculations have shown to be a powerful tool to characterize the thermal stability of hydrated oxide surfaces. Regarding MgO catalysts, DFT studies on the structure of MgO surface defects have been carried out to establish the stability of surface OH groups for water and methanol adsorptions. Recently, combined IR and DFT studies have been performed in an attempt to specify the actual structure of the CO2 species adsorbed on magnesium oxide surface. Unfortunately, theoretical calculations to predict the relationship between the basic site nature and strength and the reaction mechanism have been done only for limited cases.

In this work we study the generation, characterization and catalytic properties of active sites on MgO catalysts. The base properties of MgO samples obtained from Mg(OH)2 decomposition were tuned by modify- ing the solid calcination temperature. The density and strength of MgO surface base sites were determined by TPD and IR spectroscopy of CO2 adsorbed at 298 K. The activity and selectivity of MgO samples were probed for the liquid-phase cross-aldol condensation of citral with acetone to obtain pseudoionones, the liquid-phase transesterification of methyl oleate with glycerol to yield monoglycerides, and the gas-phase hydrogen transfer reduction of mesityl oxide with 2-propanol toward 4-methyl-3-penten-2ol. Besides, we performed DFT calculations to obtain additional information on the role played by the MgO active sites on the kinetics of base-catalyzed reactions. Specifically, we present molecular modeling studies on our MgO catalysts for the glycerolysis of methyl oleate.

2 Experimental

2.1 Catalyst preparation

Magnesium oxide samples were prepared by hydration with distilled water of low-surface area commercial MgO (Carlo Erba, 99%, 27 m2/g). 250 ml of distilled water were slowly added to 25 g of commercial MgO and stirred at room temperature. The temperature was then raised to 353 K and stirring was maintained for 4 h. Excess of water was removed by drying the sample in an oven at 358 K overnight. The resulting Mg(OH)2 was decomposed in N2 (30 ml/min STP) to obtain high-surface area MgO which was then treated for 18 h in N2 either at 673, 773 or 873 K to give samples MgO-673, MgO-773 and MgO-873, respectively.

2.2 Catalyst characterization

The decomposition of Mg(OH)2 was investigated by differential thermal analysis (DTA) using a Shimadzu DT30 analyzer, by temperature programmed decomposition (TPDe) using a flame ionization detector with a methanation catalyst (Ni/Kieselghur) operating at 673 K and by X-ray diffraction (XRD) in a Shimadzu XD-D1 diffractometer equipped with Cu-Kα radiation source (λ = 0.1542 nm) and a high temperature chamber. Samples characterized by X-ray diffraction were heated at 5 K/min until 773 K, taking diffractograms at 298, 373, 573, 673 and 773 K.

Surface areas and pore volumes were measured by N2 physisorption at its boiling point using the BET method and Barret-Joyner-Halender (BJH) calculations, respectively, in an Autosorb Quantochrome 1-C sorptometer. The crystalline structure properties of MgO-x samples were determined by X-ray diffraction (XRD) using the instrument described above. Analysis was carried out using a continuous scan mode at 2°/min over a 2θ range of 20°–80°. Scherrer equation was used to calculate the mean crystallite size of the samples.

CO2 adsorption site densities and binding energies were determined from temperature-programmed desorption (TPD) of CO2 preadsorbed at room temperature. MgO-x samples were pretreated in situ in a N2 flow at its corresponding stabilization temperature (673, 773 or 873 K), cooled to room temperature, and then exposed to a mixture of 3% CO2/N2 until surface saturation was achieved (10 min). Weakly adsorbed CO2 was removed by flushing in N2 during 1 h. Finally, the temperature was increased to 773 K at 10 K/min. The desorbed CO2 was converted to methane by means of a methanation catalyst (Ni/Kieselghur) operating at 673 K and monitored using a flame ionization detector.

The chemical nature of adsorbed surface CO2 species was determined by infrared (IR) spectroscopy after CO2 adsorption at 298 K and sequential evacuation at increasing temperatures. Experiments were carried out using an inverted T-shaped cell containing the sample pellet and fitted with CaF2 windows. Data were collected in a Shimadzu FTIR Prestige-21 spectrometer. The absorbance scales were normalized to 20-mg pellets. Each sample was pretreated in vacuum at its corresponding stabilization temperature and cooled to room temperature, after which the spectrum of the pretreated catalyst was obtained. After admission of 5 kPa of CO2 to the cell at room temperature, the samples were evacuated at increased temperatures, and the resulting spectrum was recorded at room temperature. Spectra of the adsorbed species were obtained by subtracting the catalyst spectrum.

2.3 Catalytic testing

2.3.1 Cross-aldol condensation of citral with acetone. The cross-aldol condensation of citral (Millennium Chemicals, 95% geranial + neral) with acetone (Merck, p.a.) was carried out at 353 K under autogenous pressure (≈250 kPa) in a batch Parr reactor, using acetone/citral = 49 (molar ratio) and catalyst/(citral + acetone) = 1 wt% ratio. The reactor was assumed to be perfectly mixed and interparticle and intraparticle diffusional limitations were verified to be negligible. Reaction products were analyzed by gas chromatography in a Varian Star 3400 CX chromatograph equipped with a FID and a Carbowax Amine 30 M capillary column. Samples of the reaction mixture were extracted every 30 min and analyzed during the 6-h reaction. The main product of the citral/acetone reaction was pseudoionone, PS (cis- and trans-isomers). Moreover, di-acetone alcohol and mesityl oxide were simultaneously produced from self-condensation of acetone. Selectivities (Sj, mol of product j/mol of citral reacted) were calculated as Sj(%) = Cjx100/ΣCj, where Cj is the concentration of product j. Yields (ηj, mol of product j/mol of citral fed) were calculated as Zj= SjXCit, where XCit is the citral conversion.

2.3.2 Glycerolysis of methyl oleate. The transesterification of methyl oleate, FAME, (Fluka, W60.0%, with 86% total C18 + C16 esters as determined by gas chromatography) with glycerol (Aldrich, 99.0%,) was carried out at 493 K in a seven-necked cylindrical glass reactor that allows: separate loading of the two reactants and the catalyst, stirrer, thermocouple, in-out of inert gas to eliminate methanol of the gas phase, and periodical product sampling.

Glycerol/FAME molar ratio of 4.5 and a catalyst/FAME ratio (Wcat/n0FAME) of 30 g/mol were used. The reactor was operated in a semi-batch regime at atmospheric pressure under N2 (35 cm3/min). Liquid reactants were introduced into the reactor and flushed with nitrogen; then the reactor was heated to reaction temperature under stirring (700 rpm). Reaction products were α-and β-glyceryl monooleates (MG), 1,2- and 1,3-glyceryl dioleates (diglycerides) and glyceryl trioleate (triglyceride). Reactant and products were analyzed by gas chromatography in a SRI 8610C gas chromatograph equipped with a flame ionization detector, on-column injector port and a HP-1 Agilent Technologies 15 meter x 0.32 mm x 0.1 mm capillary column after silylation to improve compound detectability, as detailed elsewhere [49]. Twelve samples of the reaction mixture were extracted and analyzed during the 8-h catalytic run.

2.3.3 Hydrogen transfer reduction of mesityl oxide with 2-propanol. The gas-phase mesityl oxide/2-propanol reaction was conducted at 573 K and atmospheric pressure in a fixed bed reactor. MgO-x samples sieved at 0.35–0.42 mm were pretreated in N2 at the corresponding calcination temperatures for 1 h before reaction in order to remove adsorbed H2O and CO2. The reactants, mesityl oxide (Acros 99%, isomer mixture of mesityl oxide/isomesityl oxide = 91/9) and 2-propanol (Merck, ACS, 99.5%), were introduced together with the proper molar composition via a syringe pump and vaporized into flowing N2 to give a N2/IPA/MO = 93.4/ 6.6/1.3, kPa\ratio. Reaction products were analyzed by on-line gas chromatography in a Varian Star 3400 CX chromatograph equipped with a flame ionization detector and a 0.2% Carbowax 1500/80–100 Carbopack C column. Main reaction products from mesityl oxide conversion were identified as the two unsaturated alcohol isomers (UOL, 4-methyl-3-penten-2ol and 4-methyl-4-penten-2ol), isomesityl oxide, methyl isobutyl ketone, and methyl isobutyl carbinol.

(Continues...)

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Excerpted from Catalysis by Adeyiga Adeyinka, C. R. Apesteguía, Hakon Bergem, Francesca Lønstad Bleken, Edd A. Blekkan, Sara Boullosa-Eiras, De Chen, J. I. Di Cosimo, V. K. Díez, Marius Westgard Erichsen, C. Ferretti, Enrique García-Bordejé, Lenka Hannevold, Simon A. Kondrat, Karl Petter Lillerud, Rune Lødeng, Xiao-hua Lu, Zhiqiang Ma, Unni Olsbye, Xi Pan, M. F. R. Pereira, Magnus Rönning, James J. Spivey, Michael Stöcker, Nachal Subramanian, Stian Svelle, Stuart H. Taylor, Shewangizaw, Teketel, Jeroen van Bokhoven, Jian-guo Wang, Gui-lin Zhuang. Copyright © 2014 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents

Pyrolysis;
Water gas shift applied to bio-based syngas;
Catalyst preparation using supercritical fluid precipitation;
Carbons materials doped with heteroatoms;
Shape selectivity in zeolite catalysis. The Methanol to Hydrocarbons (MTH) reaction;
Computational Catalysis in Nanotubes