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C. Jiménez-Sanchidrián, J.R. Ruiz / Applied Catalysis A: General 469 (2014) 367–372
Scheme 1. General mechanism for the Meerwein–Ponndorf–Verley reduction using Al(iPrO)3.
Thermogravimetric analyses were performed on a Setaram Set-
sys 12 instrument by heating in an argon atmosphere from 25 to
800 ◦C at 10 ◦C/min.
at 450 ◦C as catalysts for the Meerwein–Ponndorf–Verley reaction
of aldehydes and ketones with 2-propanol as hydrogen donor. The
results were compared to those provided by a magnesium oxide
widely used in this reaction [23–25].
BET surface areas, pore radii and pore volumes were calcu-
lated from nitrogen adsorption–desorption isotherms obtained at
−196 ◦C on a Micromeritics ASAP 2010 instrument. Samples were
outgassed in vacuo at 100 ◦C for 12 h prior to use. The amount of
CO2 chemisorbed on each solid was measured on a Micromeritics
2900 TPD/TPR analyser. Prior to analysis, samples were heated at
450 ◦C in argon stream for 1 h. Measurements were made at room
temperature by alternate passage or argon, and the same gas con-
taining 5% CO2, over the sample; the amount of chemisorbed CO2
was calculated as the difference between the first adsorption peak
(physisorbed plus chemisorbed CO2) and the arithmetic mean of
the adsorption and desorption peaks. Basicity was assessed under
the assumption that one molecule of CO2 was adsorbed at one basic
site. The number of basic sites found was thus a measure of basicity.
The amount of chemisorbed CO2 on each solid was measured
on a Micromeritics 2900 TPD/TPR analyser. Prior to analysis, sam-
ples were heated at 450 ◦C in argon stream for 1 h. Measurements
were made at room temperature by alternately passing argon and
the same gas containing 5% CO2 over each sample; the amount
of chemisorbed CO2 was calculated as the difference between the
first adsorption peak (physisorbed plus chemisorbed CO2) and the
arithmetic mean of the adsorption and desorption peaks. Basicity
was assessed under the assumption that one molecule of CO2 was
adsorbed at one basic site. The number of basic sites found was thus
a measure of basicity.
2. Experimental
2.1. Preparation of hydrotalcite-like compounds
Hydrotalcites containing Mg/Al or Mg/Al/Sn were prepared by
using a coprecipitation method described elsewhere [26]. In a typ-
ical synthetic run, a solution containing 0.3 mol of Mg(NO3)2·6H2O
and 0.15 mol of Al(NO3)3·9H2O in 250 mL of de-ionized water
(Mg/Al = 2) was slowly dropped over 500 mL of an Na2CO3 solu-
tion at pH 10 at 60 ◦C under vigorous stirring, the pH being
kept constant by adding appropriate volumes of 1 M NaOH dur-
ing precipitation. The suspension thus obtained was kept at
80 ◦C for 24 h, after which the solid was filtered and washed
with 2 L of de-ionized water. An Mg/Al/Sn hydrotalcite with
Mg(II)/[Al(III) + Sn(IV)] = 2 was obtained by following the same pro-
cedure but using appropriate amounts of Mg(II) and Al(III) nitrates,
and Sn(IV) chloride.
The HTs thus obtained were ion-exchanged with carbonate to
remove nitrate o chloride ions intercalated between layers. The
procedure involved suspending the solids in a solution containing
0.345 g of Na2CO3 in 50 mL of bidistilled, de-ionized water per gram
of HT at 100 ◦C for 2 h. Then, each solid was filtered off in vacuo and
washed with 200 mL of bidistilled, de-ionized water. The resulting
HTs were subjected to a second ion-exchange operation under the
same conditions. The exchanged Mg/Al solid was named HT-Mg/Al
and its Mg/Al/Sn counterpart HT-Sn (see Table 1). These solids were
calcined at 450 ◦C in the air for 8 h, using a temperature gradient of
1 ◦C/min.
2.3. Reaction conditions
The Meerwein–Ponndorf–Verley reaction was conducted in a
two-necked flask furnished with a condenser and a magnetic
stirrer. 2-propanol (0.06 mol) was treated with 0.003 mol of the
aldehyde and the reaction mixture heated at reflux temperature
with stirring (1000 rpm). The reaction was started by introducing
1 g of freshly calcined catalyst. The products of the reactions were
analyzed by GC–MS using an HP 5890 GC instrument furnished
with a Supelcowax 30 m × 0.32 mm column and an HP 5971 MSD
instrument.
The magnesium oxide used for comparison was prepared by cal-
cining commercial magnesium hydroxide in the air at 600 ◦C for
2 h. The calcined solid was rehydrated in refluxing water for 6 h to
obtain a new hydroxide that was re-calcined at 600 ◦C to obtain the
catalyst designated MgO-600.
2.2. Experimental techniques
Hydrotalcites and their calcination products were character-
ized by using various instrumental techniques. Thus, the metal
contents of the hydrotalcites were determined by inductively
coupled plasma-mass spectrometry on a Perkin-Elmer ICP-MS
instrument under standard conditions.
3.1. Characterization of catalysts
The solids used were characterized in previous work [17,26].
Table 1 shows the chemical composition and empirical formula of
each hydrotalcite and the magnesium oxide.
All catalysts were subjected to X-ray diffraction (XRD) analy-
sis to check for crystallinity. Powder patterns were recorded on a
Siemens D-5000 diffractometer using CuK␣ radiation. Scans were
performed over the 2Â range from 5◦ to 70◦, using a resolution of
0.02◦ and a count time of 2 s at each point.
The XRD patterns for both the Mg/Al and the Mg/Al/Sn solid
(Fig. 1) are typical of layered clay minerals.
Calcining Mg/Al hydrotalcite at 450 ◦C has been reported to
cause deep structural changes leading to the formation of perfectly
crystalline periclase MgO phases [27] the a values for which are