Journal of Energy Chemistry 22(2013)245–250

Synthesis of SAPO-34/graphite composites for lowtemperature heat adsorption pumps

L. Bonaccorsia, L. Calabresea, E. Proverbioa, A. Frazzicab, A. Frenib,G. Restucciab, E. Piperopoulosa, C. Milonea∗

a. Department of Electronic Engineering, Chemistry and Industrial Engineering, Contrada di Dio, 98166 Messina, Italy;b. CNR-Institute for Advanced Energy Technologies “Nicola Giordano” Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy

[ Manuscript received October 25, 2012; revised December 21, 2012 ]

AbstractLow temperature heat adsorption pumps represent the innovative cooling systems, where cold is generated through adsorption/desorption cycleof water by a suitable adsorbent with good adsorption and high thermal conductive properties. In this work, the hydrothermal synthesis ofzeolite SAPO-34 on thermal conductive graphitic supports, aiming at the development of highly performing adsorbent materials, is reported.The synthesis was carried out using as-received and oxidized commercial carbon papers, and graphite plate. Composites were characterized byXRD, SEM and also by a thermogravimetric method, using a Cahn microbalance. The water adsorbing capacity showed typical S-shape trendand the maximum water loading was around 25 wt%, a value close to water adsorption capability of pure SAPO-34. These results are verypromising for their application in heat adsorption pumps.

Key wordscarbon composite; SAPO-34; hydrothermal synthesis; low temperature heat adsorption pump

1. Introduction

A heat pump is a machine able to transfer heat from anenvironment at lower temperature to another one at highertemperature. In reason of Clausius’ postulate of the secondlaw of thermodynamics which states that “it is not possible torealize a transformation whose only result is to transfer heatfrom a system at lower temperature to another one at highertemperature”, the external work of a compressor is necessary.

Heat adsorption pumps (HAPs) represent new ecologi-cal cooling systems where the electrically driven mechanicalcompressor is replaced by an adsorption/desorption cycle of arefrigerant, generally pure water, over a suitable solid adsor-bent.

The functioning of HAPs is schematically reported in Fig-ure 1. Upon absorption of the amount of heat Qc from the en-vironment water evaporates, the vapor is then adsorbed overthe adsorbent material contained in a heat exchanger that givesuseful heat (Qout). The cycle continues with the adsorbentmedium regeneration, providing energy (Qin); water vaporliquefies in the condenser (giving useful heat Qλ) and reachesthe evaporator through the lamination valve for a new cycle.

HAPs coefficient of performance (COP) is defined as:COPcooling = Qc/Qin.

Low temperature HAPs represent a very interesting cool-ing system because they allow using low temperature Qin(<120 ◦C) that is normally rejected to the environment andlargely contributes to the overall energy and exergy dissipa-tion. These pumps, for example, can be advantageously cou-pled with solar cells to realize the so called “solar cooling”system that use the radiation heat of sun to drive the thermo-dynamic cycle of the adsorption pump.

The search for efficient adsorbent materials having amoderate hydrophilicity, i.e. regenerable at low tempera-ture (80–150 ◦C), and high water adsorption capacity is themain goal for the development of a low temperature adsorp-tion cooling-unit. Since the beginning, zeolite/water vapoursystem has been one of the most studied and tested adsor-bent/adsorbate pair in heat pumping/air conditioning applica-tions [1,2]. However, traditional zeolites, i.e. belonging tothe alumina-silicate family require high temperatures for re-generation. Recent studies have evidenced the possible appli-cation of zeotype materials like alumino-phosphates (AlPOs)and alumino-phospho-silicates (SAPOs) that have similar

∗ Corresponding author. Tel: +39-90-3977242; Fax: +39-90-3977464; E-mail: cmilone@unime.itThis work was partially funded by “Fondo per la Ricerca per il Sistema Elettrico-AdP MSE-CNR”.

Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

246 L. Bonaccorsi et al./ Journal of Energy Chemistry Vol. 22 No. 2 2013

behaviour of zeolites but lower desorption temperatures [3,4]and fast adsorption kinetics [5]. SAPO-34, more than oth-ers, shows adsorption properties at low equilibrium temper-atures and pressures that are particularly appropriate to heatadsorption pumps [6]. A disadvantage presented by zeolites isthe low thermal conductivity which compromises heat trans-fer efficiency “to” and “from” the heat exchanger. Therefore,there is a need of new engineered materials joining sorbentand heat transfer properties. With this respect, the use ofzeolite-graphite hybrids represents a valid strategy. Wang etal. [7] report several examples of composite which made com-pressing grains of expanded graphite with salts. In these com-posites, however, it was observed an improvement in terms ofthermal conductivity but a contemporaneous rise in resistanceto water vapour mass transfer [2,8]. In this paper, the studyon the preparation of SAPO-34/graphite composite throughdirect growth of SAPO-34 on different graphitic supports bymeans of hydrothermal synthesis is reported. The obtainedcomposites should maintain the permeable structure of car-bon tissues, so that the resistance to vapour diffusion throughthe zeolite layers would not be significantly increased. Theadsorption capacity of the prepared composites is evaluated.

Figure 1. Heat adsorption pump’s functioning

2. Experimental

2.1. Materials

Commercial graphitic materials such as carbon felt (CF)(supplied by Freudenberg FCCT, Freudenberg H-2315), car-bon papers (CP) (supplied by Ballard, AvCarb EP40), andgraphite plate (GP) (supplied by Graphtek LLC) were usedas supports.

SAPO-34 was deposited over the carbonaceous supportby hydrothermal synthesis according to the following pro-cedure: the reacting mixture was prepared by mixing aproper amounts of aluminium isopropoxide (Al(OC3H7)3,98%, Aldrich), orthophosphoric acid, tetraethylammonium-

hydroxide (TEAOH, 40%, Fluka) as template with an aque-ous solution of colloidal silica (SiO2, 40%, Aldrich) in orderto get the final gel composition: 0.6 SiO2 : 1Al2O3 : 1P2O5 :70H2O : 0.5TEAOH

A square section (20×20 mm) of the carbon support wassoaked into the suspension and the closed reactor was heatedat 200 ◦C under auto-generated pressure. Due to differentphysical properties of the material (thickness, density) shownin Table 1, the whole mass of support was 0.037 g for CF,0.024 g for CP and 0.2728 g for GP.

The synthesis time was 72 h. After synthesis, the samplewas removed, accurately rinsed with water and dried at 80 ◦Covernight. Finally, it was calcined at 450 ◦C for 12 h in orderto remove TEAOH.

Synthesis was carried out on as-received (CF, CP and GP)and oxidized (300 ◦C for 12 h) carbonaceous materials (CF-OX, CP-OX and GP-OX).

2.2. Samples characterization

Structural characterization of carbon composites was car-ried out by X-ray diffraction (ITALSTRUCTURE APD 2000,Cu Kα radiation, 40 kV and 30 mA) and their morphologywas evaluated by scanning electron microscopy (SEM, JEOL5600LV operated at 20 kV). Semi-quantitative zeolite compo-sition was measured by the coupled energy dispersive micro-analysis (EDS) system (OXFORD INSTRUMENTS).

The adsorption isobars of carbon composites were eval-uated through measuring the water uptake by thermo-gravimetric technique based on the use of a Cahn 2000microbalance in the typical range of pressures (P = 9.8–25.5 mbar) and temperatures (30 ◦C 6 T 6150 ◦C) of a “lowtemperature” adsorption cycle [9]. In a typical measurement,about 50 mg sample was loaded in vacuum-tight vessel of thebalance and then degassed under vacuum for 24 h. The bal-ance vessel was, then, connected to a thermostated evaporatorin order to maintain the water vapor pressure constant duringthe analysis. Once the water adsorption equilibrium reached,as monitored by the increase of sample weight, the temper-ature was raised stepwise in the range of 30 ◦C–150 ◦C. Ateach temperature the water uptake was calculated as:

wt (%) =m(P H2O,T )

m0×100

where, m(P H2O,T) is the mass of water adsorbed at fixed waterpartial pressure and temperature; m0 is the dry sample weight.

3. Results and discussion

The main characteristics of carbonaceous materials usedfor the composite preparation, as provided by the suppliers,are reported in Table 1. Carbon papers CF and CP showed aclose thickness, however, CP showed lower surface and bulkdensity. GP is mainly closed packed graphite with typical highbulk density value.

Journal of Energy Chemistry Vol. 22 No. 2 2013 247

Table 1. Main characteristics of the as-received carbon materials

Thickness Surface density Bulk densityCode

(µm) (g/m2) (kg/m3)

CF 210 95 450

CP 200 60 300

GP 310 − 2200

SEM analysis (Figures 2a, 2c, 2e) show that CF wasformed by carbon fibres with diameter of 5 µm somehow tightentangled (Figure 2a), whilst CP is mainly constituted by freestanding thicker fibres (10 µm) (Figure 2c) which account forthe lower surface and bulk density with respect to CF. GP (Fig-ure 2e) mainly shows a highly compacted structure made bytightly interacting graphite crystallites with almost absence ofintra-particles voids, which was in agreement with the highestbulk density of the material (Table 1).

As known, hydrothermal synthesis of zeolite evolves

through an initial nucleation step; when nuclei reach a stabledimension crystal growth takes place, depleting nutrients insolution. To drive the formation of zeolite crystals directly ona support avoiding the crystallization in solution, nucleationhas to occur over the support surface. Therefore it should bewetted by the reacting solution and should adsorb the precur-sor molecules. Carbon supports are mostly hydrophobic, thenit is expected to be scarcely wetted in polar medium such asthe synthesis solution. In order to overcome this drawback,zeolite synthesis was also carried out over carbon supportspreviously oxidized at 300 ◦C for 12 h. This is a very com-mon procedure for increasing the oxygenated functionalitiesthen the wettability of carbon surface [10]. Moreover, takinginto account that oxygen treatment mainly introduces basicgroups (pKa = 10±0.2) over the carbon surface [11,12], thismethod is the most suitable one for creating adsorption sitesfor TEA cations which act as template in zeolite synthesis.

Figure 2. SEM images of as-received graphitic supports and corresponding composites. (a) CF, (b) SAPO-34/CF, (c) CP, (d) SAPO-34/CP, (e) GP, (f) SAPO-34/GP

248 L. Bonaccorsi et al./ Journal of Energy Chemistry Vol. 22 No. 2 2013

XRD analysis of all carbon based composites (as-receivedand oxidized ones) confirms that zeolite coating was alwayscomposed by crystalline SAPO-34 and no impurities were ob-served [13] (Figure 3).

SEM analysis evidences that the direct synthesis ofSAPO-34 on as-received carbon supports resulted in a limitedsurface coverage (Figures 2b, 2d, 2f). Carbon fibres depictedin Figures 2(b) and 2(d) were indeed loosely covered by ze-olite crystals. Moreover, the presence of large aggregates ofSAPO-34 clearly suggests that, as expected, they mainly formin solution by bulk growth rather than nucleation and growthon fibres surfaces due to the low wettability. Similar phe-nomenon occurred for GP support (Figure 2f).

A significant improvement of composites morphologywas obtained over the oxidized carbon supports. Indeed, asshown in Figures 4(a), 4(c) and 4(e), SAPO-34 coating wasalmost homogeneously distributed over the entire carbon sur-faces. In the case of CF-OX and CP-OX, carbon fibres ap-peared completely coated by a homogeneous thickness of ze-olite (Figures 4a, 4d) and no zeolite free powder entrapped in

the inter-fibres voids was visible. As a difference with com-posites obtained over as-received supports, the coatings were

Figure 3. X-ray diffraction pattern of SAPO-34/CP after deposition. Refer-ence line peaks of zeolite SAPO-34 are also reported [13]

Figure 4. SEM images of composites prepared over oxidized graphitic supports. (a, b) SAPO-34/CF-OX, (c, d) SAPO-34/CP-OX, (e) SAPO-34/GP-OX

Journal of Energy Chemistry Vol. 22 No. 2 2013 249

formed of condensed crystals aggregates where the typical cu-bic morphology of SAPO-34 particles was almost missed (in-set of Figure 4d). This result suggests that an high densityof nuclei forms over fibres surface, which favours the inter-condensation of zeolite crystals [14].

Deposition of SAPO-34 over closely packed GP-OX wasalso significantly improved by the oxidation pre-treatment, asshown in Figure 4(e), where it is evident the complete cov-erage of the support surface. However, some morphologicaldifferences can be envisaged with respect to the zeolite mor-phology obtained on CF-OX and CP-OX composites. On theformer, indeed, well defined cubic-shaped crystals of largersize (up to several microns) were clearly observable (insetof Figure 4f). At light of previous consideration it is likelythat surface nucleation over GP-OX occurred at lower ex-tent, so fewer crystals accrued, due to the absence of inter-condensation effects.

It is noteworthy that, despite the calcination procedure at450 ◦C for 12 h, no significant coating detachments betweenzeolite coating and carbon occurred. This results strongly sup-port that SAPO-34 directly grows on the carbon surfaces.

The amount of zeolite deposited per gram of carbona-ceous support decreased in the order of CF-OX>CP-OX>GP-OX.

The observed differences mainly reflected the variation ofthe exposed surface area of supports, which in turn rules thezeolite nucleation.

General behaviour is found that carbon fibres contain-ing samples (CF-OX and CP-OX) showed a larger amount ofloaded SAPO-34 (>100 wt%) with respect to compact poly-crystalline GP-OX (7 wt%). This result is mainly understoodin terms of different morphology of the samples, i.e. exposedsurface area. Indeed the highly compacted structure of GP-OX (Figure 4e) with almost absence of intra-particles voidsexposed a much lower surface than that of carbon fibres (Fig-ures 4a and 4c).

Among the highest surface area samples, CF-OX and CP-OX, the lowering of loaded SAPO-34 (180 wt% and 100 wt%respectively) agreed with the decrease of fibres mass present-ing in the square section of carbon support used in preparationprocedure (0.037 g and 0.024 g respectively).

It is important to note that by in-situ growth process thepermeable structure of carbon tissues was preserved, so resis-tance to vapor diffusion through the zeolite layers would notbe significantly increased.

3.1. Adsorption properties

In order to complete the characterization of composites,it is very important to measure the adsorption capability interms of water uptake in comparison with the adsorption prop-erties of SAPO-34. The thermogravimetric technique allowedto measure adsorption equilibrium isobars of zeolite coatingsin a configuration as close as possible to the actual application.At this purpose, the water uptake values for coated CF-OXsupports which show the highest amount of deposited zeolite,

and pure zeolite powders are shown in Figure 5. It should bementioned that CP-OX and GP-OX composites behave simi-larly.

All the curves showed typical S-shape trend of zeoliteSAPO-34, evidencing a sharp change of water uptake ina narrow range of temperatures (30 ◦C−90 ◦C); besides, anearly complete zeolite regeneration at temperatures lowerthan 100 ◦C occurred. Moreover, the extent of water uptakeof composites, around 25 wt%, was very close with typicalwater adsorption capability of SAPO-34. It is noteworthy thatthe contribution to the water uptake of the oxidized carbona-ceous supports, if any, should be lower than 1 wt%, whichis the maximum value measured on the un-coated sample at30 ◦C and water vapor pressure of 25.5 mbar.

These results demonstrate that carbon-based SAPO-34composites maintained the principal adsorption characteris-tics of the original zeolite. The only significant difference wasthe slope of the isobar curves when water uptake reached thehighest values. For composites, in fact, it seems that isobariccurves did not reach a plateau, suggesting that the adsorptionequilibrium was not as fast as the case of pure zeolite.

The reason for the observed behaviour could lie in theinterference of capillary condensation phenomena at low tem-perature within voids created by the support fibres stacking.However, a possible capillary condensation has no influenceon coating adsorption performances.

Figure 5. Adsorption isobars of SAPO-34/CF-OX and pure SAPO-34 pow-der

4. Conclusions

In this paper a new thermal conductive adsorbent materialfor low temperature heat adsorption pumps, is proposed. Thecomposite material was made through growing SAPO-34 oncommercial graphite fibres and plate by in-situ hydrothermalsynthesis. The intrinsic hydrophobicity of carbon materialshas been decreased by a preliminary oxidation treatment. Thecomposites have been characterized by XRD and SEM anal-ysis. On oxidized supports, SAPO-34 coating appeared morehomogeneous. Carbon fibres-shaped support looked com-pletely coated by a homogeneous thickness of zeolite, where

250 L. Bonaccorsi et al./ Journal of Energy Chemistry Vol. 22 No. 2 2013

the typical cubic morphology of SAPO-34 particles was al-most missed, whilst on packed graphite plate, well definedcubic-shaped crystals of zeolite were clearly observable, sincethe absence of inter-condensation effects limited the nucle-ation. The water adsorbing capacity of composites, measuredby thermogravimetric method, showed typical S-shape trendand adsorption characteristics of the original SAPO-34, indi-cating that they are particularly appropriate for a low temper-ature regenerative cycle.

AcknowledgementsThe present work was partially funded by “Fondo per la Ricerca

per il Sistema Elettrico-AdP MSE-CNR”. The authors are gratefulto Dr. Enza Passalacqua for supplying some carbon materials.

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