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DOI: 10.1002/cssc.201501518
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Synthesis of C4 and C8 Chemicals from Ethanol on MgO-
Incorporated Faujasite Catalysts with Balanced
Confinement Effects and Basicity
Lu Zhang,[a] Tu N. Pham,[a] Jimmy Faria,[b] Daniel Santhanaraj,[a] Tawan Sooknoi,[a]
Qiaohua Tan,[a] Zheng Zhao,[a] and Daniel E. Resasco*[a]
A new type of catalyst has been designed to adjust the basici-
ty and level of molecular confinement of KNaX faujasites by
controlled incorporation of Mg through ion exchange and pre-
cipitation of extraframework MgO clusters at varying loadings.
The catalytic performance of these catalysts was compared in
the conversion of C2 and C4 aldehydes to value-added prod-
ucts. The product distribution depends on both the level of
acetaldehyde conversion and the fraction of magnesium as ex-
traframework species. These species form rather uniform and
highly dispersed nanostructures that resemble nanopetals.
Specifically, the sample containing Mg only in the form of ex-
changeable Mg2+ ions has much lower activity than those in
which a significant fraction of Mg exists as extraframework
MgO. Both the (C6 +C8)/C4 and C8/C6 ratios increase with addi-
tional extraframework Mg at high acetaldehyde conversion
levels. These differences in product distribution can be attrib-
uted to 1) higher basicity density on the samples with extrafra-
mework species, and 2) enhanced confinement inside the zeo-
lite cages in the presence of these species. Additionally, the
formation of linear or aromatic C8 aldehyde compounds de-
pends on the position on the crotonaldehyde molecule from
which abstraction of a proton occurs. In addition, catalysts
with different confinement effects result in different C8
products.
Introduction
Although biorefineries can produce bioethanol on a large
scale,[1] the low return on investment, which is characteristic of
commodities, challenges their economic profitability and ulti-
mately the widespread use of bioethanol as a transportation
fuel. Therefore, chemical conversion of bioethanol into more
valuable commodity chemicals is a highly desirable option for
its utilization.[2] The integration of energy and chemical produc-
tion in biorefineries by combining existing fermentation facili-
ties and chemical industrial production seems to be a crucial
aspect in the development of the technology.[3]
their selectivity to C4 products to minimize excessive conden-
sation to ꢀC6 products. In this scenario, zeolites appear to be
potentially attractive catalysts to maximize the desirable prod-
uct due to their enhanced shape and size selectivity. Adjusta-
ble molecular confinement when using zeolites as catalysts is
a unique tool offered by zeolites to enhance selectivity. Previ-
ously, we investigated the aldol condensation of acetaldehyde
over faujasite zeolites with varying silicon/aluminum ratios and
different types of alkali-metal cations.[21] We found that the C4/
(C6 +C8) product ratio depended on a variety of factors, such
as strength, density, and accessibility of basic sites. It was also
shown that base-catalyzed condensation (C4 and ꢀC6 prod-
ucts) dominated at temperatures around 2308C, whereas acid-
catalyzed acetalization prevailed below 1808C. Because high
reaction temperatures may lead to side products and faster de-
activation, we investigate herein the modification of faujasite
zeolites to improve their activity at low temperatures.
For example, the conversion of ethanol to crotonaldehyde is
a reaction that has been investigated extensively.[4–8] Crotonal-
dehyde is a highly reactive a,b-unsaturated aldehyde that can
be used to produce the industrially valuable crotyl alcohol
through chemoselective hydrogenation.[3] The two main steps
in the conversion of ethanol into crotonaldehyde include dehy-
drogenation to acetaldehyde and aldol condensation to the C4
product. The latter step is conducted on either acid or basic
catalysts.[9–20] One of the key requirements of these catalysts is
Due to its high basicity, MgO is an effective basic catalyst for
aldol condensation, but, in general, it tends to generate exces-
sive over-condensation products. For instance, acetaldehyde
on MgO results in low C4/(C6 +C8) product ratios as the conver-
sion increases. Therefore, herein, we have investigated novel
compositions of MgO-modified faujasites, which exhibit the
desirable combination of the required basic properties of MgO
for the aldol reaction with the confinement benefits of the fau-
jasite zeolites, which inhibit excessive condensation. This work
is motivated by numerous previous investigations that have
demonstrated the incorporation of metal or metal oxide clus-
ters inside zeolite cavities.[22–32] Herein, we have used several
[a] Dr. L. Zhang, Dr. T. N. Pham, Dr. D. Santhanaraj, Prof. T. Sooknoi,
Dr. Q. Tan, Z. Zhao, Prof. D. E. Resasco
School of Chemical, Biological, and Materials Engineering
University of Oklahoma, Norman, OK 73019 (USA)
[b] Dr. J. Faria
Abengoa Research, C/Energía Solar no. 1
Palmas Altas, Seville 41014 (Spain)
Supporting Information for this article can be found under http://
ChemSusChem 2016, 9, 736 – 748
736
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim