Quantitative measures of acidity in H-MFI by NH3 and H2O
1st Everton Santos,1 2nd Francisco Lemos,1 3rd M.A.N.D.A Lemos1,*, 4ndJ.C. Védrine,2
1CERENA,
Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, Lisboa, P-1049-001, Portugal
de Réactivité de Surface, Sorbonne Universités, Université P. & M. Curie, 4, Place Jussieu, Paris, F-75005, France
*Corresponding author: [email protected]
2Laboratoire
Keywords: Acidity; H-MFI; NH3-TPD; H2O-TPD
2. Experimental Part
Two types of experiments were performed: one with
ammonia as the molecule to be adsorbed and the other
with water, both experiments were done in duplicate.
The procedure for the adsorption of ammonia is
described elsewhere [8]. For the adsorption of water,
the catalyst was stored in a saturator with a constant
high humidity level. The zeolite is kept over a
saturated solution of calcium carbonate at room
temperature, with the purpose of ensuring that the
catalyst surface is fully saturated with water.
Desorption experiments were carried out in the
DSC/TGA apparatus. The analysis was conducted
under a nitrogen atmosphere with a continuous flow
rate of 80 mL min-1. The samples were heated up to
600 °C with a heating rate of 10 °C min-1. Prior to the
desorption step, the catalysts were heated for a period
of 15 min at 100 °C, for catalysts containing adsorbed
water, and at 150 ºC for catalysts with absorbed
ammonia in order to remove any physisorbed H2O or
NH3, respectively. From the TPD experiments the
catalyst acid strength distributions were obtained
resorting to a numerical deconvolution method,
described elsewhere [9].
3. Results and discussion
Figures 1 and 2 show the curves of TPD, plots of the
ammonia desorption and water desorption rate
respectively. Although the acid strength distribution
of the zeolite can be seen as a continuous function of
the adsorption energy, the sites were lumped into a set
of discreet energies with an energy grid that is
adequate to describe the continuous experimental
curves. So, each individual component obtained after
the deconvolution corresponds to a family of sites
characterized by the same acid strength. It is worth
mentioning that the grid used for the experiments
differ from water to ammonia so as to adapt to the best
fit of the experimental data.
SIMUL
1.6
dqi /dt (10-6 mol.s -1 .gz e o-1)
1. Introduction
A large number of characterization techniques have
been applied to describe zeolite acidity. Among them,
the temperature programmed desorption (TPD)
technique of pre-adsorbed bases such as ammonia or
pyridine, is one of the most commonly used [1, 2].
Several techniques have been proposed to circumvent
this problem and it has been shown that numerical
deconvolution of TPD curves provides quantitative
information about the distribution of the acid sites; an
improved numerical method has been proposed for
the deconvolution of TPD spectra (rate of ammonia
desorption versus temperature) enabling a
quantitative determination of the distribution of the
acid strength of zeolites [3-6].
Probe molecules commonly used as the basis for the
characterization of acidity are ammonia and pyridine,
but other molecules are being studied, not always in
the context of TPD, for example carbon monoxide,
carbon dioxide and water [7].
The present work studies the possibility of using
water as an acidity probe, in way that is similar to that
of ammonia: a molecule to be adsorbed for the
characterization of the acidity of the H-MFI catalyst
by the TPD curves deconvolution method.
EXP
1.4
45 KJ/mol
1.2
50 KJ/mol
1.0
60 KJ/mol
0.8
70 KJ/mol
0.6
80 KJ/mol
0.4
100 KJ/mol
0.2
120 KJ/mol
0.0
-0.2 100
200
300
400
500
Temperature (ºC)
600
140 KJ/mol
180 KJ/mol
250 KJ/mol
Figure 1. Ammonia desorption rate.
SIMUL
dqi /dt (10-6 mol.s -1 .gz e o-1)
3.5
EXP
3.0
40 KJ/mol
2.5
50 KJ/mol
2.0
60 KJ/mol
1.5
70 KJ/mol
1.0
80 KJ/mol
0.5
100 KJ/mol
120 KJ/mol
0.0
-0.5 100
200
300
400
500
600
Temperature (ºC)
140 KJ/mol
below 80 kJ.mol-1 corresponded to weak acid sites,
however for values equal to or greater than 80 kJ.mol1
correspond to strong acid sites [9].
It is observed from the results that the experiments are
reproducible and the total amount of sites compares
well between the two probe molecules.
180 KJ/mol
220 KJ/mol
Figure 2. Water desorption rate.
Table 1. Total amount and strength of acid sites.
NH3
dqi /dt (10-6 mol.s -1 .gzeo-1)
The results obtained for the amount of NH3 and H2O
molecules desorbed from each type of acid site, are
depicted in Figures 3 and 4 respectively. For each
molecule tested two experiments were conducted
under the same conditions to verify the
reproducibility of the technique.
400
300
200
100
0
45
50
60
70
80 100 120 140 180 250
Ei (kJ.mol-1 )
Experiment 1
Experiment 2
dqi /dt (10-6 mol.s -1 .gz e o-1)
Figure 3. Distribution of acid site strengths with NH3.
800
600
400
200
0
50
60
70
80 100 120 140 180 220
Ei (kJ.mol-1 )
Experiment 1
Experiment 2
Figure 4. Distribution of acid site strengths with H2O.
Although the TPD temperature profile used an
isothermal section to minimise physisorbed
molecules, it is likely that the weakest acidic sites
correspond to molecules that are still essentially
physically bound to the surface of the catalysts
(physical adsorption), as the activation energy for
desorption is quite close to the heat of vaporization of
the corresponding liquids. Strong acid sites form a
chemical bond with the molecules (chemical
adsorption), which is broken only at elevated
temperatures.
Table 1 shows the amount of total acidity for both
molecules adsorbed on the catalyst and the division
of these acid sites to weak and strong. In table 1 it was
considered that activation energies for desorption
Experiment 1
Experiment 2
H2O
Experiment 1
Experiment 2
Total acid sites
(10-6mol/gzeolite)
1863
1817
Weak
(%)
40.5
35.2
Strong
(%)
59.5
64.8
1715
1525
66.8
75.1
33.2
24.9
A significant difference is observed when comparing
the percentage of strong and weak acid sites between
the two adsorbed molecules in the catalyst, for the
ammonia the sample presents in its majority strong
acid sites, whereas for the water the reverse is
observed. This can be attributed to the fact that the
water molecule has a lower basicity when compared
to ammonia.
4. Conclusions
The acidic strength distribution of H-MFI catalyst can
be determined by TPD deconvolution for both
molecules as adsorbents (NH3 and H2O). The
differences that are observed can be associated with
the difference of basicity between the two probe
molecules. For total acidity, close values are observed,
demonstrating that it is feasible to substitute the NH3
molecule for H2O.
Acknowledgments
The authors would like to thank programme CNPq for the Grant
for Everton Santos.
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