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Energy, Environmental, and Catalysis Applications
Highly adsorptive and magneto-inductive Guefoams
(multifunctional guest-containing foams) for enhanced
energy-efficient preconcentration and management of VOCs
José Miguel Miguel Molina Jordá
ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b22858 • Publication Date (Web): 18 Feb 2020
Downloaded from pubs.acs.org on February 25, 2020
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ACS Applied Materials & Interfaces
Highly adsorptive and magneto-inductive Guefoams (multifunctional
guest-containing foams) for enhanced energy-efficient
preconcentration and management of VOCs
J.M. Molina-Jordá *
Department of Inorganic Chemistry, University of Alicante, Ap. 99, E-03080 Alicante,
Spain
University Materials Institute of Alicante, University of Alicante, Ap. 99, E-03080
Alicante, Spain
Keywords: multifunctional; guest; host; adsorption; magnetic induction; open-pore
foam, preconcentration, volatile organic compounds (VOCs).
* corresponding author:
[email protected]
Abstract
The design of multifunctional materials is a current demand for high-end technological
applications that need to combine different functions unable to be accomplished by a
single material. The aim of this work is to present, at first glance, a new family of
recently patented multifunctional porous materials developed by locating granular
phases with specific functionality (guests) within the cavities of open-pore cellular
materials (hosts) and, at second glance, the use of a set of these materials for the
preconcentration and management of volatile organic compounds (VOCs). These
materials (herein known as Guefoams, acronym for Guest-containing foams), present
host foams and guest phases that are not bonded and therefore allow fluids to pass
through. The processing method is the gas pressure infiltration of a host precursor into
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preforms containing particulate guest phases covered by a NaCl martyr coating, which
is later dissolved in water. The manuscript shows the manufacture and characterization
of a specific set of Guefoams composed of aluminum foams that incorporate both steel
particles and activated carbon particles as guest phases into the same material. These
guest phases make the materials highly adsorbent and susceptible to rapid desorption by
magnetic induction, two properties never achieved with traditional foams that transform
these materials into perfect candidates for preconcentration and energy-efficient
management of VOCs. The manuscript concludes with a discussion on advisable
properties to consider when exploring the use of these materials in the mentioned
applications.
1. Introduction
Integrating different, even contradictory or excluding functions into the same material
system is now a fundamental challenge that constitutes an interesting perspective for
many modern applications, seeking to improve the quality of human life and address
global challenges [1,2]. Given their wide variety of potential applications in recent
years, these so-called multifunctional materials have aroused great interest in the
scientific community. Multifunctional materials are often formed by a combination of
materials (multimaterials), each with one or more functionalities following a
characteristic design. Knowledge of the individual functions of materials themselves is
sometimes insufficient to predict the behavior of multimaterials in a given application
as their final properties emerge from the symbiosis of constituent material properties.
Designing and manufacturing new multifunctional multimaterials is one of the most
promising research lines in the coming years.
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This paper presents a new family of multifunctional materials that represent a promising
generation of porous materials with expanded functionalities. The original idea behind
these materials is to extend the limited functions of traditional foams, the properties of
which are normally limited by the nature of the material as well as by the shape,
geometry and pore size distribution. This innovative family of materials comes from
combining foam materials (host matrix) with functional phases that lodge within their
porous cavities (guest phases). Guest phases do not maintain any chemical or physical
union with the host matrix other than mere gravity-caused contact. These materials,
together with their manufacturing process and some revealing applications, are patentprotected [3,4] and henceforth referred to as Guefoams (Guest-containing foams) for
simplification purposes. Guest phases provide foams different functionalities, so their
intrinsic properties no longer limit them. This manuscript covers specific aspects of the
manufacturing process of these materials, based on the gas pressure infiltration of a host
precursor in preforms conformed by NaCl-coated guest phases. The infiltration process
must allow the porosity of the preform to be filled without leading to infiltration of the
coating, so that it can later be dissolved in water. For the sake of a specific application,
the article involves manufacturing and characterizing aluminum foams containing
different proportions of both activated carbon particles and steel spheres as guest
phases. This resulted in materials with high adsorption and rapid desorption capabilities
due to the previously unattained combination of large specific surface area and
magneto-inductive properties in any other cellular material. The manuscript presents a
complete study on the adsorption and desorption of butanol, a short-chain hydrocarbon
present in paint pigments and resins, in which it is shown that Guefoams contribute to
high preconcentration factors and are energy-efficient due to the low power
consumption of the inductive-assisted desorption process. The author believes that the
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presented ideas, as well as the covered examples and the final discussion on relevant
aspects of the applicability of these materials, can inspire the scientific community,
especially research groups investigating cellular materials, to design and conceive new
multifunctional Guefoams.
2. Guefoams manufacturing process
Guefoams manufacturing process is based on the replication method [5,6], which is
conventionally practiced for the production of foams of several natures. In essence, the
replication method consists of filling the empty space of a martyr porous preform with a
material, usually by means of liquid infiltration, and then eliminating the preform. The
original martyr phase is replaced by a porous space that replicates its characteristics.
The most widespread replication process involves the use of preforms made up of
uniaxial or multiaxial pressure packed NaCl particles, which are liquid infiltrated by a
matrix precursor and then removed by water dissolution after precursor solidification.
The adaptation of this traditional process to the manufacture of foams with guest phases
(Guefoams) lies in the preparation of the particles that comprise the martyr preform.
These particles are multi-layered materials where the core is the guest phase, normally
with granular geometry, and the outermost material is a continuous NaCl phase
covering layer (or other suitable martyr material).
Figure 1a illustrates the manufacturing steps of Guefoam materials that contain one
single type of guest phase located in all cavities (Figure 1b). Alternatively, materials
with two (or more) guest phases of a different nature, located in a fraction of the
available cavities, can also be manufactured (Figure 1c). The main steps are as follows
(as shown in Figure 1a in order to manufacture the materials shown in Figures 1b and
1c):
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Figure 1 - (a) Manufacturing steps of Guefoams: (i) selection and coating of guest phases; (ii)
packaging; (iii) infiltration with liquid precursor; (iv) directional solidification; (v) machining;
(vi) removal of martyr material; (b) Guefoam with guest loading of 100% of one type of guest
phase; (c) Guefoam with guest loading < 100% of two types of guest phases.
A. Manufacture of the preform.
(i) Selection of one or two-types – eventually more types can be chosen – of guest
phases in a finely divided particle or fiber state, and coating them with one or
different sacrificial materials;
(ii) Packing of the coated guest phase(s) in a ceramic crucible suitable for
infiltration; the packed preform may contain massive particles of a sacrificial
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material to generate a fraction of pores containing no guest phases (sacrificial
materials of a single nature or of different natures can be used);
B. Infiltration of the packed preform.
(iii) Infiltration of the porous preform with a liquid precursor;
(iv) Solidification of the infiltrating liquid by means of a system allowing directional
cooling;
(v) Machining of the structural matrix;
C. Post-infiltration processing.
(vi) Removal of sacrificial material(s) by liquid dissolution or by controlled reaction
with a liquid or gas phase.
The resulting material is herein called Guefoam and consists of a host phase that
involves locating guest phases in a fraction or in all material cavities without host-guest
chemical bonding.
3. Experimental procedures
To manufacture Guefoams, high-purity aluminum (Al 99.999%), purchased from
Goodfellow Metals (Cambridge, UK), was used as the liquid infiltrating metal. Two
sets of particles were used as guest phases: activated carbon particles of the type Nuchar
RGC-30 (Chemical Division of Westvaco, Covington, VA, USA) with a 1 mm of
average diameter and low-carbon steel spheres (C: 0.1-0.2%; Mn: 0.6-0.9%; Si: 0.10.2%; S: max 0.5%; P: max 0.04%) with average diameters of 1, 1.5, 2 and 3 mm,
purchased from Redhill Precision (Lincoln, USA).
Guest phase particles were coated by spray deposition from a 20 wt.% NaCl solution;
this concentration produces the least defective coatings when using the coating device
described in the patents [3,4]. The NaCl solution was prepared from pure NaCl powder
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(99.9%) purchased from AppliChem GmbH (Germany). When required, guest phasefree NaCl spheres were produced by coating with NaCl (by the same spray deposition
procedure) raw NaCl particles with an average diameter of about 1mm until a final
diameter of approximately 1.4 mm was reached.
Particles (NaCl-coated guest phases of only one type or in combination with other
NaCl-coated guest phases and/or NaCl particles) were delicately packed into 18 mm
inner diameter graphite crucibles by repeatedly adding a small amount of powder,
which was compacted by vibrations [7]. Liquid aluminum infiltrations were performed
in a pressure chamber at 720ºC by means of a constant pressurization rate of 0.09 MPa/s
and up to variable maximum pressure values, depending on the specific material (see
[7] for infiltration details). After metal solidification, the sample was removed from the
mold and its surface was ground with SiC paper (400 grit). NaCl coating was dissolved
by two different procedures depending on the size of the sample. Large samples (18 mm
diameter) were immersed in hot water at 40ºC under magnetic stirring conditions for 6
minutes and then infiltrated with pressurized water following the procedure described in
[8-10]. Complete dissolution, as assessed by densitometry, is reached when samples
achieve stable weight within approximately 4-5 minutes. The total dissolution time is in
consequence about 10-11 minutes. For small samples (8 mm diameter), obtained by
cutting larger infiltrated pieces, it was enough to submerge them in hot water at 40ºC
under magnetic stirring conditions for a total of 10 minutes.
The basic aspects of the infiltration process were investigated by mercury porosimetry,
which was performed in a POREMASTER-60 GT porosimeter (Quantachrome
Instruments, Florida, USA) operating at a pressurization rate of 0.09 MPa/s and
pressures up to 45 kPa. Materials were characterized by their gas adsorption capacity
and magnetic performance. Nitrogen adsorption isotherms were collected at -196ºC in
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an Autosorb 6-b equipment of Quantachrome Instruments (Florida, USA) and analyzed
within the frame of the standard BET theory [11]. An AMH-DC-TB-S permeameter
from the commercial company Laboratorio Elettrofisico (Italy) was used to measure
magnetic properties, enabling large samples with cylindrical geometry. Additional
permeability and pressure drop measurements were made using a homemade device
already presented in [10] and equipped with 0.001 bar precision manometers at both
ends of the sample. Permeability was estimated with injection experiments using water
as fluid by monitoring water mass (g) with time (s) on a ±0.1 mg precision balance
(Precisa ES320A). The inlet air flow was regulated with a ±0.01 bar precision
manometer. Any negligible water mass loss due to evaporation from the collecting
container was discarded as the total time of each measurement was <2 min. Using the
same device, pressure drop was measured by injecting air as fluid. Air flow was
followed with an air flowmeter operating within the 0-30 l/min flow range (Ki Key
Instruments, Trevose, UK).
4. Results and discussion
4.1 Infiltration of NaCl-coated particle preforms
The experimental conditions of infiltration herein applied to manufacture Guefoams are
isobaric. The selected infiltration pressure therefore determines the metal saturation in
the porous preform. The infiltration pressure must be chosen to meet two objectives at
the same time: i) it must be sufficient to ensure a minimum metal volume fraction to
form a continuous host matrix; and ii) it must not exceed the minimum pressure to
initiate NaCl martyr coating infiltration. Compliance with both conditions requires the
use of infiltration pressures within a specific interval, which can be determined by
liquid metal infiltration studies. Figure 2a shows the results of mercury and aluminum
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infiltrations in preforms formed by 1 mm diameter steel spheres coated with
approximately 0.5 mm NaCl. Quasi-continuous results for mercury infiltration were
obtained at 25ºC with mercury porosimetry. The results corresponding to aluminum
were acquired from discrete infiltration experiments at 720ºC and at specific pressures
for which metal saturation was determined by densitometry after metal solidification.
The S-shape of the saturation-pressure curves in Figure 2a indicates that infiltration of
porous preforms with poorly wetting molten mercury and aluminum metals does not
occur in a single step, but rather as a gradual process in which the metal ingress
progresses as the applied pressure increases. The process is governed by the well-known
semi-empirical Brooks-Corey model [12]:
Seff = 1 ― 𝑆𝑚 =
1 ― 𝑉𝑚
1 ― 𝑉𝑟
=
𝑃𝑏
𝜆
()
𝑃
(1)
where Seff is the effective saturation, defined as the ratio of the volume fraction
occupied by the atmosphere (1-Vm, where Vm is the metal volume fraction) and the total
initial pore volume fraction (1-Vr, where Vr is the particle volume fraction). Sm is the
metal saturation, Pb is the minimum pressure, also called bubbling pressure, related to
the size of the largest pores forming a continuous network of channels in the porous
medium, P is the applied pressure and is the well-known "pore size distribution index"
evaluating the pore size distribution in the porous medium. When Equation (1) holds,
plotting log(Seff) versus log(P) must yield straight lines with slope (-). Figure 2b shows
such plots for the data in Figure 2a. Here we can see that, after nonlinear behavior at
low pressures, data become linear over most of the pressure range.
Some issues in Figures 2a and 2b are worth discussing. First, the double logarithmic
plots present a single slope domain that is identified for mercury and aluminum with
values of Hg = 4.9 and Al = 5.2, respectively. These values are fairly high compared to
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those obtained for particles with more irregular shape, in line with the values obtained
by other researchers for infiltrations of spherical particulate compacts [13]. A constant
lambda value for the entire pressure infiltration range indicates a homogeneous pore
size distribution of the metal-invaded pore space. At low pressure, metal is expected to
invade only the largest interconnected channels of the preform corresponding to the
largest voids between particles. Finer channel infiltration, including pores in the NaCl
coating (see Figure 2c for details of the porosity inherent in the NaCl coating) occurs at
high pressure. Since the present infiltrations are governed by single lambda values, the
pressure value at which the infiltration of NaCl coating begins cannot be accurately
determined. In order to explore this aspect, coatings were subjected to recrystallization
heat treatment (temperature rise for 2 h up to 795ºC, maintenance at this temperature for
2.5 h and, finally, natural cooling down to 25ºC) in which the pore size distribution of
the coatings was expected to vary by modifying the crystalline structure of NaCl (Figure
2d). Drainage curves for mercury and aluminum liquid metals were obtained for these
treated preforms (Figure 2e). On this occasion, two slope domains were determined
from double logarithmic representations (Figure 2f). From the beginning of infiltration
to an approximate metal saturation of 0.96 for mercury and 0.94 for aluminum, slopes
are identified with values of 1Hg= 5.2 for Hg and 1Al= 5.2 for Al, in accordance with
the values obtained by infiltration of untreated particle preforms. There is a second
lower-value slope that governs the last stages of infiltration and corresponds to 2Hg=
1.5 and 2Al= 1.9 for mercury and aluminum, respectively. At high pressure, a
concomitant infiltration of the finest spaces in the preform and pores in the NaCl
coating occurs. The lower values obtained after recrystallization indicate a more
complex pore size distribution of the NaCl coating. Given these results and the fact that
the soft recrystallization conditions used here do not significantly alter the average size
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of the coarse NaCl crystallites that define the largest pores in the NaCl coating and are
first infiltrated, it can be argued that NaCl coating infiltration only starts at pressures for
which the preform has been saturated with metal up to approximately 94-96%. For the
infiltration with aluminum of the particles considered herein, this is translated into
maximum infiltration pressures of approximately 25 kPa (0.25 bar). The first infiltration
stages of the NaCl coating may result from the metal invasion of extended defects, such
as coverage defaults or cracks (Figures 2g and 2h, respectively). Therefore, the
maximum infiltration pressure value can be increased either by NaCl coating procedures
which do not produce extended defects and lead to smaller pores or subsequent
treatments that cause the pores in the NaCl coating to shrink or close.
1,0
Hg
Al
0,8
metal saturation (-)
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0,6
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pressure (kPa)
(a)
(b)
100 m
(c)
100 m
(d)
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1,0
Hg
Al
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metal saturation (-)
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0,6
0,4
0,2
0,0
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5
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pressure (kPa)
(e)
(f)
500 m
250 m
(g)
(h)
Figure 2. (a) Drainage curves for infiltration of mercury and aluminum in preforms conformed
by packing 1 mm spherical steel particles coated with a NaCl layer of approximately 0.5 mm
thickness; (b) a double logarithmic representation of the results in (a); (c) and (d) are SEM
micrographs corresponding to the NaCl coating and the same coating after thermal treatment
for recrystallization, respectively; (e) and (f) correspond to their analogs (a) and (b),
respectively, obtained for samples in which the NaCl coating was subjected to recrystallization;
(g) and (h) are NaCl coating surface details showing some extended defects (coverage defaults
in g and coating cracks in g, in both cases highlighted with arrows). Linear regimes in (b) and
(f) were fitted by the equation log(Seff)=mlog(P)+n with the following results (R is the linear
regression coefficient) – data in (b): m=-4.938, n=4.350, R>0.99 for Hg and m=-5.234,
n=5.873, R>0.99 for Al; - data in (f): m=-5.243, n=4.789, R>0.99 for Hg when Sm<0.96; m=1.525, n=0.406, R>0.99 for Hg when Sm>0.96; m=-5.165, n=5.924, R>0.98 for Al when
Sm<0.94 and m=-1.937, n=1.508, R>0.98 for Al when Sm>0.94).
4.2 Structure of Guefoams: general structure and features of Al-[C+Fe] Guefoams
Guefoams take the general structure shown in Figure 3a: the host material forms a
porous structural matrix and the guest phases are located in all or part of the host
cavities, either randomly or in the desired positions. Figure 3a contains two drawings;
one is a two-dimensional sketch in which the lines represent interconnecting openings
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between pores; the other one is a three-dimensional drawing of a representative material
cell containing a cavity-housed guest phase. Since there is a minimal host-guest
interaction, limited to mere physical contact caused mainly by gravity force, virtually all
guest phase surfaces are functionally active (a particularly important issue in the herein
presented case of adsorbent guest phases, such as activated carbon).
!
1 mm
(a)
(b)
1 mm
(c)
(d)
200 m
(e)
(f)
Figure 3 – (a) Schematic diagram of the general structure of Guefoams; (b) SEM micrograph of
a NaCl-coated steel sphere; (c) SEM micrograph of NaCl-coated activated carbon particles; (d)
photograph of an Al-[C+Fe] Guefoam; and (e-f) optical and SEM micrographs, respectively,
showing the space gauge between aluminum skeleton foam and (e) steel sphere and (f) carbon
particle.
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For the sake of structural characterization, two important parameters are defined:
GL(%) =
number of pores hosting a guest phase type
× 100
total number of pores
(2)
average volume of a guest specimen
× 100
average volume of its hosting pore
(3)
GO(%) =
where GL and GO refer to guest loading and guest occupation, respectively. In essence,
GL and GO are defined for each type of guest phase contained in the foam material. GL
is representative of the percentage of pores that host a certain guest phase. GO refers to
the percentage of the volume that a guest phase occupies in the cavity where it is hosted.
GL is controlled by the relative proportions of NaCl spheres containing guest phases to
those that are guest-free at the moment of preform preparation (see drawing in Figure
1). The thickness of the NaCl coating is the defining parameter for GO (in spherical
geometry GO=(r/R)3, where r and R are the average radii of the guest phase and the
NaCl-coated guest phase, respectively).
Figures 3b-c show the steel spheres (1.5 mm average diameter) and activated carbon
particles, respectively after NaCl coating, used in the manufacture of Al-[C+Fe]
Guefoams; that is, aluminum foams with different proportions of guest phases of both
steel spheres and activated carbon particles. These materials were fabricated at an
infiltration pressure of 22 kPa, a value low enough not to infiltrate the fine pores of the
NaCl martyr coating (other examples of selective preform infiltrations can be found in
[14,15], where infiltration pressures were properly selected to avoid infiltration of the
internal structure of NaCl consolidated particles). Figure 3d depicts a photograph of a
17 mm diameter and 32 mm long Guefoam sample in which all pores contain guest
phases, one half containing activated carbon particles and the other half containing steel
spheres. Figures 3e-f are optical and SEM micrographs, respectively, showing in detail
the remaining space between the host and guest phases. This gap can be filled with a
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static or moving fluid because the lack of host-guest chemical bonding causes the
interconnecting windows between pores to remain open.
4.3 Magnetic and adsorbent properties of Al-[C], Al-[Fe] and Al-[C+Fe] Guefoams
Table 1 gathers the properties of the Guefoam samples prepared in this work, where the
host phase is aluminum, and the nature of the guest phases (carbon particles, steel
spheres or combinations of both) and their guest loadings (GL) were varied. Samples
are referred to as Al-[C], Al-[Fe] or Al-[C+Fe] Guefoams, where C and Fe indicate the
activated carbon particles and steel spheres, respectively.
Table 1 – Characteristics and properties of manufactured Al foam (Al-0 sample) and Guefoams
of aluminum as the host phase and activated carbon particles (C), steel spheres (Fe) of 1.5 mm
average diameter, or mixture of both, as guest phases (samples Al-[C], Al-[Fe] and Al-[C+Fe],
respectively). GL refers to guest loading and GO to guest occupation. SBET (m2/cm3 or m2/g) is
the area obtained by nitrogen adsorption at -196ºC; Ms, Mr and Hc are saturation
magnetization (emu/g), remanent magnetization (emu/g) and magnetic coercitivity (Oe),
respectively.
GL (%) (1)
Sample code
C
Al-0
Al-[C]_1
Al-[C]_2
Al-[C]_3
Al-[Fe]_1
Al-[Fe]_2
Al-[Fe]_3
Al-[Fe]_4
Al-[C+Fe]_1
Al-[C+Fe]_2
Al-[C+Fe]_3
Al-[C+Fe]_4
Al-[C+Fe]_5
Al-[C+Fe]_6
0
25
50
100
0
10
50
75
10
10
50
Fe
GO (%)
(1)
C
Fe
Al
C
Fe
pores
0.37
0.00
0.09
0.18
0.36
0.00
0.00
0.00
0.00
0.04
0.18
0.27
0.04
0.04
0.18
0.00
0.00
0.00
0.00
0.05
0.24
0.38
0.54
0.05
0.05
0.05
0.27
0.41
0.27
0.63
0.54
0.44
0.27
0.58
0.39
0.25
0.09
0.54
0.40
0.31
0.32
0.19
0.18
0
10
45
70
100
10
10
10
50
75
50
58
-
-
87
58
Main property measured
Volume fraction (0.02)
SBET
(m2/cm3)
0.11
101
202
409
0.12
0.12
0.12
0.16
38.4
201
308
42.3
40.5
204
(m2/g)
0.11
91.5
173
312
0.08
0.04
0.03
0.03
25.9
126
185
13.3
9.54
62.0
Ms
Mr
Hc
48.0
109
128
132
47.5
44.2
41.6
112
128
106
0.08
0.51
0.79
1.10
0.09
0.08
0.09
0.54
0.76
0.47
8.28
9.96
10.32
10.45
7.87
7.70
7.72
9.19
9.52
9.43
In all cases, the volume fraction of aluminum is 0.37. This value agrees, given that
Seff0.95 at 22 kPa, with an effective packing of 0.61 for the volume fraction of the
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NaCl-coated guest particles measured when preparing the martyr preform. Raw (NaCluncoated) activated carbon and steel particles attain packing volume fractions of 0.56
and 0.61, respectively, by following the same packing method. This indicates that the
NaCl coating partially spheroidizes the irregular surface of the activated carbon
particles and increases their packing efficiency. At first glance, we can observe that the
Al-0 sample (sample manufactured by infiltration of NaCl spheres containing no guest
phase) exhibits a small surface area of approximately 0.11 m2/cm3, which is consistent
with the low values reported in previous findings for replicated carbon foams [8]. When
guest phases are present in all or part of the pores, the functionalities provided by each
appear rather independently, so that the properties of the materials derive from the
individual contributions of the present guest phases through their respective volume
fractions. This is certainly due to the net physical separation of guest phases and the
absence of chemical bonding with the host matrix.
Figure 4 displays the magnetization curves and the nitrogen adsorption capacity of some
samples in Table 1. The magnetization curves shown in Figure 4a are consistent with
the magnitude scale measured for composites with variable ferromagnetic spheres loads
[16-19]. Figure 4b is a magnification of Figure 4a for low magnetic fields in which the
characteristic hysteresis loops of ferromagnetic materials are distinguished. The curves
corresponding to Figure 4c are defined as Type II adsorption curves. The main
differences between the curves are primarily due to the amount of adsorbent in each
case. Figure 4d shows that the volume-specific surface areas (SBET in m2/m3) scale
linearly with the activated carbon volume fraction. This is not the case for mass-specific
surface areas, SBET in m2/g, as steel particles do not contribute to a substantial area rise
but significantly increase the final density of the material.
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(a)
(b)
(c)
(d)
Figure 4 – (a) Magnetization curves for some Guefoams containing steel as guest phase; (b)
details of the magnetization curves of (a) at low applied magnetic fields to determine magnetic
coercitivity Hc; (c) nitrogen adsorption isotherms (volume per mass unit versus relative
pressure P/Po where Po is the saturation pressure) for some Guefoams containing activated
carbon as guest phase; (d) the correlation between the specific surfaces measured by the BET
method from curves in (c) versus the volume fraction of activated carbon; the straight line
fitting the SBET (m2/cm3) data has a regression coefficient >0.99.
4.4 Application of Al-[C+Fe] Guefoams in portable VOCs preconcentration
devices
The Al-[C+Fe] Guefoams manufactured here may, among other applications, be used to
trap and preconcentrate contaminants such as short-chain hydrocarbons (phenol,
ethanol, butanol, etc.) normally present in synthetic paint pigments and resins. The
magnetic properties of some prepared Guefoams make them suitable for rapid adhesion
to magnetized surfaces of structural architectures and can also help them to be
recovered from fluid storage tanks where they may have been submerged. Magnetic
properties also allow these materials to be rapidly heated by magnetic induction [20,21]
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to efficiently desorb trapped substances. The device shown in Figure 5a was designed to
test the performance of these materials in real applications. This device gathers portable
characteristics due to its small dimensions and its 12V-battery autonomous electrical
supply. It consists mainly of a metal case covering the foam material (with dimensions
45 mm long and 8 mm diameter), which has a winding copper varnished wire around it.
This set is attached to an induction generator powered by the 12V AC battery and a
control plate containing a main switch, a maximum electrical current limiter and a
frequency selector. Figure 5b shows the heating speeds of different samples under
magnetic induction conditions in the device referred to for two powers (28W and 62W)
and in a 350W conventional electrical resistance furnace. It is clear from the findings
that magnetic induction generates higher heating speeds at much lower powers. In
addition, the heating speed of Al-[C+Fe] Guefoams (exemplified by sample Al[C+Fe]_6), which depends on frequency and induction power, is linear over the time
scales used here and much higher than that of conventional aluminum foam (Al-0
sample) (Figure 5b).
As proof of concept, butanol was adsorbed into the Al-[C+Fe]_6 sample by passing
through it nitrogen carrier gas flowing at room temperature and 450 ml/min with 0.3
mmol/l butanol inlet concentration Co. Butanol adsorption rate was followed with time
by the analysis of butanol concentrations (C) at the sample outlet measured by a microchromatograph (MTI P200H). The corresponding breakthrough curve (a C/Co versus
time representation) is shown in Figure 5c. Samples were then extracted from the device
for further characterization and subjected to a differential thermal gravimetry (DTG)
analysis using a thermo balance (Setaram TGA-92) coupled with a magnetic induction
system, which enables adjustable sample heating rates by operating at different
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induction powers and a fixed frequency of 200 kHz. A helium gas flow of 12 ml/min
was used during desorption.
(a)
(b)
(c)
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1,65
1,70
1,75
1,80
1,85
1,90
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(f)
(g)
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Figure 5 – (a) Portable device for preconcentration and management of VOCs by adsorption
and rapid desorption through magnetic induction heating; (b) heating rate curves - surface
(R=4 mm) and core temperatures (R=0 mm) versus time - for samples Al-0 and Al-[C+Fe]_6
with dimensions of 45 mm long and 8 mm diameter under magnetic induction heating at
frequencies of 100 kHz and 200 kHz and powers of 28W and 62W; for comparison purposes,
the heating rate for the Al-[C+Fe]_6 sample under conventional electrical heating is included;
(c) the breakthrough curve for butanol adsorption for the Al-[C+Fe]_6 sample; (d) the
differential thermal desorption (DTG) curves for butanol desorption at different heating rates
for the Al-[C+Fe]_6 sample; and (e) the plot of 2ln(Tm)-ln(w) versus 1000/Tm, where Tm and w
are the maximum peak temperature in DTG and the heating rate, respectively - the straight line
fitting the data is 2ln(Tm)-ln(w)=6236ln(w)-1.262, with a regression coefficient >0.96; (f) the
concentration enrichment factor values attained with the Al-[C+Fe]_6 sample at different
heating rates together with some values achieved in bibliography for VOCs solid-gas extraction
systems using adsorption-desorption cycles; (g) the enrichment factor values obtained in
several adsorption-desorption cycles.
The DTG results (Figure 5d) confirmed that the butanol desorption process from the
activated carbon contained in Al-[C+Fe]_6 Guefoam was easily controlled by the
heating rate which, in turn, was determined by the magnetic induction capacity of the
sample. From the data obtained, the enthalpy for butanol desorption from the herein
used RGC-30 activated carbon was derived by the linear transformation proposed by
Cvetanovic and Amenomiya [22] and generalized in [23] for different experimental
conditions. It consists of plotting 2ln(Tm)-ln(w) versus (1/Tm), where Tm and w are the
temperature corresponding to the maximum desorption rate (peak temperature in DTG)
and the heating rate, respectively (Figure 5e). The value obtained for butanol desorption
from the RGC-30 activated carbon was 51.8 kJ/mol, which is in close agreement with
other values obtained for activated carbons generated by carbonizing wood of different
natures (coconut wood was the raw material employed for RGC-30 production). This
value was higher than butanol vaporization heat (approximately 43.7 kJ/mol [22]),
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suggesting that strongly endothermic processes occur during the desorption process, as
noted in previous works [24,25]. Integrating the curves in Figure 5d results in desorbed
butanol quantities at all heating speeds of approximately 104 mg, corresponding to
desorption percentages above 99% of the butanol quantities previously adsorbed.
Figure 5f shows the butanol concentration enrichment factor (EF) achieved for various
heating speeds. EF is a parameter widely used in analytical processes defined as the
ratio of the concentration of analyte (butanol) in the desorbed gas to the concentration
of analyte in the carrier gas prior to adsorption in the pre-concentrator device. Such
concentrations were determined with an MTI P200H microchromatograph. The major
enrichment factor corresponds to 33ºC/min, for which EF=150 is achieved. This value
is well above other values attained in the bibliography for preconcentration of VOCs in
solid-gas extraction systems using adsorption-desorption cycles, among which we
highlight the value of EF=60 achieved in the enrichment of butanol obtained by
fermentation [26] or the value of EF=32 attained in porphyrin-modified carbon
nanotube systems [27]. The EF results obtained for 1-butanol using an odor-measuring
system employing a mass sensitive sensor array based on quartz crystal microbalance
(QMB) [28] or the values attained with metal-organic frameworks for hazardous trace
gases [29] remain well below these values. One of the most interesting works published
on this topic is the microwave-assisted desorption of VOCs on activated carbon [30],
for which a preconcentration factor of only about 20 is deduced. Figure 5g presents the
concentration enrichment factor obtained for several adsorption-desorption cycles. The
values can be verified to remain approximately constant at intervals up to 20 cycles.
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5. Relevant aspects of the applicability of Guefoams as chemical preconcentrators
Guefoams are essentially open-pore foams and, as such, owe their greatest functionality
to their interconnected porous space, which allows fluids to pass through them. The
presence of guest phases may significantly alter the intrinsic fluid-dynamic
characteristics of these materials, such as their permeability or fluid pressure drop. To
explore this, aluminum foams and Guefoams of aluminum as host phase and steel
particles as guest phases were manufactured. Steel spheres were 2 or 3 mm in diameter,
each covered by NaCl until reaching a final 3.5 mm diameter. Two infiltration pressures
of 15 kPa and 22 kPa were explored. The characteristics and properties of the
manufactured specimens can be seen in Table 2.
Table 2 – Characteristics and properties of the manufactured Al foams (Al-x-y sample) and
Guefoams of aluminum as the host phase and steel spheres (Fe) as guest phases (Al-[Fe]_x-y
samples). x and y in the sample code refer to the steel spheres diameter (in mm) and infiltration
pressure (in kPa), respectively (two nominal average diameters - 2 and 3 mm - and two
infiltration pressures - 15 and 22 kPa - were used). The average pore size of each sample was
3.5 mm. GL refers to guest loading and GO to guest occupation. K is permeability in m2 and P
is pressure drop in kPa/m, measured at different airflow velocities v (in m/s).
Sample code
Al-0-15
Al-0-22
Al-[Fe]_2-15
Al-[Fe]_2-22
Al-[Fe]_3-15
Al-[Fe]_3-22
GL (%)
(1)
GO (%)
(1)
Fe
Fe
0
0
16
100
53
Main property measured
Volume fraction (0.02)
Al
Fe
pores
K
0.20
0.37
0.21
0.37
0.19
0.37
0.00
0.00
0.09
0.10
0.32
0.33
0.80
0.63
0.70
0.53
0.49
0.30
5.5610-11
2.7910-11
1.7810-11
1.0610-11
1.0510-11
3.3510-12
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8.46
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19.9
65.0
P/L
v = 0.025
13.0
27.2
31.0
59.1
56.6
172
v = 0.05
32.0
67.1
78.9
141
129
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(a)
(b)
Figure 6. (a) Permeability versus guest occupation (GO in %) and (b) the relative pressure
drop versus superficial velocity for samples in Table 2.
Figure 6a depicts sample permeability values in Table 2 as a function of guest
occupation. This graph shows that permeability decreases as infiltration pressure and
guest occupation increases. Dependence on infiltration pressure is easily understood as
higher infiltration pressures generate higher metal saturations in the NaCl particulate
preform (see Equation 1). Consequently, narrower interconnection channels between
pores are generated after NaCl particle dissolution. Such channels determine the
permeability of these open-pore media in conventional foams (see experimental results
and analytical models in [31]). Permeability in Guefoams is even lower than in their
analogous conventional foams due to the lower volume of interconnected porosity
through which flow can pass. This latter effect can also be qualitatively deduced from
the models described in [31]. As a result of the drop in permeability due to the presence
of guest phases, an increase in the pressure drop required to make fluid pass through the
porous space of Guefoams is expected. This behavior is seen in Figure 6b, where
pressure drop is plotted according to airflow velocity.
Permeability and pressure drop in foam materials are determining factors for their use as
chemical preconcentrators. The presence of guest phases generates some resistance to
fluid passage that must be taken into account. Indeed, far from being a disadvantage, it
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can become a clear advantage since a decrease in permeability has been shown to
increase the fluid dwelling time within the porous foam skeleton. This has resulted in
better heat transfer between circulating fluid and solid phases [32], which can improve
the homogeneity of the temperature distribution in the material, thereby favoring the
thermal desorption process. At present, an exhaustive characterization of permeability,
pressure drop and heat transfer coefficient is being carried out on Guefoams of different
characteristics.
6. Conclusions
In summary, this manuscript presents a new family of multifunctional materials
consisting of host open-cell foam materials with cavities locating functional guest
specimens and their use for preconcentration and management of VOCs. Lack of hostguest chemical interactions ensures pore connectivity to enable fluid transport while
maintaining guest phase functionalities. Adequate host and guest compositions may
bring new property combinations uncovered by any foam material and definitely open
the way to challenge new technological frontiers. For each particular system, the
infiltration process followed in the manufacture of these materials must be studied
thoroughly because the host phase precursor must not infiltrate the NaCl coatings so
that they can later be dissolved in water. In the present case, the maximum infiltration
pressure was determined at 25 kPa, a pressure from which the infiltration of NaCl
coatings made by spray coating begins. As proof of concept, the article entails the
manufacture, characterization and testing of aluminum foams with carbon and steel
guest phases (Al-[C+Fe] Guefoams). These materials are highly capable of adsorbing
volatile organic compounds (VOCs) such as butanol (the compound studied in this
manuscript), and can desorb them by fast and low-power magnetic induction heating to
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become excellent and energy-efficient preconcentrators and managers of VOCs. Based
on their significance for the applicability of these materials, certain characteristic
parameters such as permeability or pressure drop on fluid passage are discussed. It was
found that the permeability decreased with the percentage of guest occupation, while the
pressure drop increased, two characteristics that could enhance the heat exchange
between the circulating fluid and the solid phases, thereby enabling a faster and more
efficient desorption process. Essentially, the new Guefoam family materials feature
extended properties hitherto unattained in conventional open-pore foams and offer
tremendous potential to impact new system performance in current and future
applications requiring advanced multifunctional materials.
Acknowledgement
The author acknowledges financial support from the Spanish “Agencia Estatal de
Investigación” (AEI) and European Union (FEDER funds) through grant MAT201677742-C2-2-P.
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Miguel Jordá