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Applied Thermal Engineering 50 (2013) 1590e1595

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Applied Thermal Engineering

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Zeolites direct synthesis on heat exchangers for adsorptionheat pumps

Lucio Bonaccorsi a,*, Luigi Calabrese a, Angelo Freni b, Edoardo Proverbio a, Giovanni Restuccia b

aDepartment of Industrial Chemistry and Materials Engineering, University of Messina, Contrada di Dio, 98166 Messina, ItalybCNR e Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano”, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy

a r t i c l e i n f o

Article history:Received 9 March 2011Accepted 17 October 2011Available online 25 October 2011

Keywords:Adsorption heat pumpCoatingsDirect synthesisZeolites

* Corresponding author. Tel.: þ39 0 903977245; faE-mail address: lbonaccorsi@unime.it (L. Bonaccor

1359-4311/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.applthermaleng.2011.10.028

a b s t r a c t

Improving performances of adsorption heat pumps involves developments in adsorbent materials aswell as in design of more efficient adsorbers and heat exchangers. The aim of the present work was tostudy some aspects of the “direct synthesis” of zeolites on metal surfaces for the preparation of coatedadsorbent beds for more efficient adsorption heat pumps. Different zeolites were grown on stainlesssteel, copper and aluminium and the results were analysed in terms of phenomena at the zeoliteemetalinterface, coating thickness, adhesion properties and protective effects. Direct synthesis, indeed, hasdemonstrated to be a valuable technique but influenced by requirements and limitations strictlydependent on the physicoechemical interaction between the material used for support and the zeolitecoating considered.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The preparation of films, membranes and coatings made ofzeolites has obtained in the last years a growing interest for newapplications in comparison to well-known and established fields ascatalysis, separation and adsorption processes. Traditionally,zeolites are mixed with a binder, generally clay, and are shaped aspellets or beads to be charged in reactors or vessels. Sensor tech-nologies, catalytic micro-reactors, low environmental impactapplications, however, are some examples where zeolites arepreferably employed as layers grown on a variety of differentsupports. Several advantages are recognized to zeolite coatingscompared to granules or similar, e.g. an improved accessibility tocatalytic sites, a more efficient heat and mass transfer through thezeolite layer, the possibility to grow membranes with virtually nopresence of macro-pores or large defects [1]. Moreover, zeolitecoatings can improve heat transfer in heat exchangers [2] or can actas antimicrobial layers [3] or anti-corrosive coatings in variousaggressive environments and operating conditions [4]. In a recentstudy, we have demonstrated by a dynamic model that zeolitecoated adsorbers for adsorption pumps show better performances,in terms of specific and volumetric powers, compared to otherconfigurations [5].

The most important commercial zeolites (zeolite A, zeolite Y,ZSM-5, etc.) have been synthesized over several ceramic supports,

x: þ39 0 90397464.si).

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such as alumina, quartz, cordierite as well as numerous metalsubstrates: aluminium, copper and stainless steel [1]. Depending onthe synthesis conditions and the surface properties, the charac-teristics of the zeolite coatings can be, to some extent, adapted tothe final use. In some applications, for instance in sensor technol-ogies or anti-corrosive surface treatments, the appeal is for verythin, regular, defect free coatings while in other cases, as in heatpumps or in adsorption applications in general, the opposite iswanted, i.e. relatively thick multiple layers of the active material. Inaddition, the support nature and the interface interaction, whichare responsible for bonding zeolite and substrate, are significantlyinfluenced by the chemical and physical conditions created duringthe synthesis process. Finally, Chakraborty et al. [6], using a newthermodynamic approach have demonstrated that the perfor-mance of adsorption systems can be improved by opportunelymodifying the pore structure of the adsorbent, a result which opensnew synthesis prospects.

In this paper, we have investigated some important features ofthe growth of zeolites on metal supports in view of the preparationof efficient adsorption beds for heat pumps applications, puttingparticular emphasis on the zeoliteemetal interaction and thecorrelated interface phenomena, on the coating thickness, theadhesion properties and the coating protective action. Twodifferent scenarios have been analyzed on the basis of our experi-mental results: aluminiumesilicate zeolites for high temperatureapplications and aluminiumephosphoresilicate zeolites for lowtemperature applications, both being grown by direct synthesis onstainless steel, copper and aluminium.

L. Bonaccorsi et al. / Applied Thermal Engineering 50 (2013) 1590e1595 1591

2. Experimental

2.1. Support preparation

Rectangular pieces (15 mm � 15 mm � 10 mm) cut fromstainless steel (AISI 304) copper and aluminium (Al 6061) foils werecleaned at room temperature with trichloroethylene in an ultra-sonic cleaner for 15 min, rinsed with distilled water and thentreated with 2.2 M KOH ethanol solution for 30 min at roomtemperature with stirring. After rinsing with distilled water, thesupports were immersed for 5 min in a concentrated HCl solutionwith stirring at room temperature and an additional washing with1 M NaOH solution at room temperature was carried out for 5 minfollowed by an abundant rinsing with distilled water. The supportswere, finally, immersed in deionized water for 15 min.

2.2. Aluminiumesilicate zeolites deposition

Two zeolites representative of the aluminium-silicate familywere grown by direct synthesis on metal supports: zeolite 4A andzeolite Y. The main difference between the two is the sili-conealuminium ratio in the crystalline framework which deter-mines the structure charge, the sodium content (both beingsynthesized in the sodium form) and consequently their wateraffinity. The zeolites were prepared using the same raw materials:sodium aluminate solution as aluminium source which wasprepared dissolving hydrate alumina (Riedel-de-Haen, Al2O3 65 w%) in caustic, sodium silicate solution (B.D.H. LLC, 27.5 w% SiO2 e

8 w% Na2O) as silicon source and caustic (Aldrich, NaOH 99 w%) assodium source. The composition used for zeolite 4A and for zeoliteY were respectively (molar composition): 3.9Na2O:1Al2O3:1.95-SiO2:274H2O and 5Na2O:1Al2O3:12SiO2:300H2O.

For the in-situ synthesis a common procedure was used,comprising a preliminary seeding of the metallic supports byimmersion in a colloidal suspension of submicronic precursorspreviously prepared of (molar) composition: 10.67Na2O:1A-l2O3:10SiO2:180H2O and then putting the supports on the bottomof a polytetrafluoroethylene (PTFE) reactor where the reactingmixture was immediately poured. The reactor was then closed andheated at 368 K for 4 h in case of zeolite 4A and for 8 h in case ofzeolite Y. In the deposition of zeolite Y on aluminium, however, thereaction mixture was aged at room temperature 24 h before thesupport immersion and the reaction time was reduced to 4 h.

Once the synthesis was completed, the support was removedfrom the reactor, rinsed with distilled water and dried at 80 �Covernight. The zeolite precipitated in the reactor was recovered aswell, filtered, rinsed, dried and checked by X-ray diffractometry.

2.3. Aluminiumephosphoresilicate zeolites deposition

In SAPO zeolites the reference structure is the AlPO framework,a three-dimensional network of AleOeP atoms, in which Si substi-tution is obtained to the detriment of the some phosphorus atoms.The synthesis environment, as well as raw materials, differs signif-icantly from the previous preparations. In these syntheses, indeed,an organic template is added to the reacting mixture to drive thestructure formation. The gel formulation 0.6SiO2:1Al2O3:1P2O5:70-H2O:0.5TEAOH was prepared by mixing aluminium isopropoxide(Aldrich, 98% Al(OC3H7)3) with an orthophosphoric acid solution,adding the template, tetraethylammoniumhydroxide (Fluka, 40%TEAOH), and then adding an aqueous solution of colloidal silica(Aldrich, 40% SiO2). The deposition was carried out by immersinga cleaned support directly in the reacting solution and then heatingup to 200 �C for 72 h in an autoclave (under autogenous pressure).Finally, the sample was removed from the autoclave, rinsed

accurately and dried at 80 �C. The zeolite formed during the processwas collected and tested by X-ray diffractometry.

3. Results and discussion

3.1. Interface phenomena

The direct grow of aluminiumesilicate zeolites on metalsurfaces is generally facilitated by the natural tendency of the metalsurface to be covered by hydroxyl groups in an aqueous solution.Metals are, indeed, always covered by a layer of oxide which ishydrated in his outer surface, according to the equilibrium:

MOþ H2O4MðOHÞ2In an aqueous solution, however, hydroxyl groups bring a charge

according to the following reactions:

M� OHþHþ4M� OHþ2 (1)

M� OHþ OH�4M� O� þH2O (2)

which can give rise to a superficial net charge if the solution pHdiffers from the point of zero charge (PZC) defined as the pH atwhich the surface charge equals zero (for amore rigorous definitionconsult [7]). If the solution pH is less than the PZC the surface excesscharge is positive, negative in the opposite case (pH > PZC). Thesynthesis mixture for the zeolite deposition, actually, is a rathercomplex environment for the presence of several electrolytespecies in solution (aluminate, silicate and sodium ions) whichwere not considered in the ideal situation of equations (1) and (2).However, in aluminiumesilicate zeolite syntheses the pH valuewasalways higher than 12, higher than PZC (z8/9) of the majority ofthe oxi/hydroxides generally formed on stainless steel, copper andaluminium [7,8]. In such conditions, it is acceptable to expect themetal supports to be negatively charged on the surface althoughthe presence of several anions and cations in solution. Beingsodium Naþ ions the predominant cations in solution, it can behypothesized that Naþ ions are specifically adsorbed at the metalinterface, enabling the surface nucleation of zeolite crystals. It is, infact, known that solvated sodium ions act like template moleculesaround which silicate and aluminate anions cluster to form thepolyhedral units of the zeolite’s network [9,10]. However, sucha mechanism of surface nucleation and growth has to competeagainst the simultaneous nucleation of zeolite crystals in thesolution bulk. These crystals precipitate over the support giving riseto a porous and less cohesive coating, poorly attached to thesurface. The zeolite coating nucleated and grown on the surface, onthe other hand, shows better characteristics because attached tothe support by chemical bonds. To induce the growing of zeolitecrystals on metals, several strategies have been proposed in thepast [11e13] but the most used is the preliminary seeding ofsupport surfaces [12,14]. The aim is to make the support the pref-erential site for the crystal growth by adsorbing on its surfacea number of seeding sites that, by reducing the induction (nucle-ation) time [9,10], accelerate the zeolite deposition against thenucleation in the solution mass. In our experience, in case ofstainless steel or copper supports the seeding has always givena significant improvement in terms of surface coverage and coatingbonding at oxide/hydroxide interface (Fig. 1).

The zeolite deposition on aluminium, instead, is made moredifficult by the support dissolution, because the metal and theprotective oxide are corroded by the synthesis environment. Zeolite4A direct coating resulted almost impossible on aluminium owingto the fast support dissolution. In zeolite Y synthesis, instead, the

Fig. 1. Zeolite 4A on stainless steel: not seeded (a), seeded (b).

L. Bonaccorsi et al. / Applied Thermal Engineering 50 (2013) 1590e15951592

reaction environment is less caustic and the deposition was ob-tained by a long ageing time before the support immersion. Thepre-seeding and the long ageing time accelerated the zeolitegrowth on the aluminium surface (the deposition timewas reducedto 4 h) and once a first zeolitic layer formed, the substratewasmoreprotected so the metal corrosion almost arrested and a zeolitecoating could grow (Fig. 2).

In the deposition of SAPO zeolites the synthesis environment isdifferent because the solution pH at the end of the raw materialsaddition isz8, i.e. a value near to the PZC of several oxi/hydroxidesand then not aggressive for the majority of the metallic supports.For such pH value, metal surfaces are still negatively charged(equation (2)) favouring the template adsorption, which is now theammonium ion TEAþ. However, the surface charge is low and theadhesion of zeolite coatings on stainless steel and copper was poor.On aluminium, instead, a better adhesion was observed (Fig. 3),which was attributed to the active role developed by thealuminium oxi/hydroxide layers in forming chemical bonds at thezeolite/metal interface. On aluminium the surface oxide layer isstable in these synthesis conditions and the zeolite deposition ismuch easier for SAPO than for aluminiumesilicate zeolites, in thesense that none a particular synthesis method or pre-seeding isneeded.

3.2. Coating thickness

The layer thickness is one of the major points in zeolite coatingsfor heat pumping applications. The direct synthesis, indeed, has theadvantage to produce an intimate contact between the adsorbent

Fig. 2. Zeolite deposition on aluminium: comparison between a

and the heat exchanger surfaces increasing thermal conductivitymore than in all other configurations of zeolite beds. The mainlimitation is the effective quantity which can be deposited bya synthesis process, being the deposition time generally limited bythe zeolite formation process. Synthetic zeolites, in fact, are meta-stable phases which precipitate from supersaturated reactionmixtures under the action of temperature and/or pressure. Forprolonged synthesis times, not only the zeolite growth is almoststopped (the crystal formation shows an S-shaped curve againsttime [15]) but the precipitated zeolite evolves through more stablephases which are usually considered impurities for not being ableto adsorb vapours. The zeolite mass formed in a synthesis batch isproportional to the raw material quantities added, but not all thezeolite crystals grow over the support and a concurrent zeolitenucleation and growth has to be expected in the solution bulk. Asalready discussed, the deposition is usually catalysed, for exampleby seeding, although soon the surface is covered by the first layers,the catalytic action is slowed down and the formation of crystals insolution starts again to compete against the coating thickening. Apossibility to increase the zeolite mass is to repeat the depositionprocess several times, but even in this case significant limitationsare expected. The synthesis reaction is a time consuming process,especially in case of SAPO zeolites, and the risk of contaminatingthe coating with unwanted phases increases with the number oftreatments, therefore more than two direct syntheses are usuallynot convenient.

Several examples in literature attest thicknesses in the range ofhundreds of microns for zeolite coatings grown onmetal substratesby particular synthesis strategies [16e18]. Even if not all authors

corroded (a; zeolite 4A) and a coated (b; zeolite Y) support.

Fig. 3. Zeolite SAPO on stainless steel (a) and aluminium (b).

L. Bonaccorsi et al. / Applied Thermal Engineering 50 (2013) 1590e1595 1593

agree on the optimal coating thickness [19,20], values between 200and 300 mm should be considered critical because a further coatingthickening could be detrimental to vapour diffusion and masstransport. On the other hand, low thicknesses lower the zeolite/metal ratio which reduces the heat pump efficiency in terms of COP[21,22]. For this reason, the development of in-situ grown coatingswill be possible if new, high-surface, heat exchangers are available.Recently, in fact, several efforts were made on depositing zeoliteson foamed metal, metal sponges and sintered metal fibres [14,16].

3.3. Coating adhesion

The bonding between metal and zeolite can be very intense ifa chemical reaction is produced at the support interface during thezeolite growth. From this point of view, metals that are reactive inthe synthesis environment are preferred, if the surface reaction canbe controlled. The aluminate ions, which form due to the surfaceoxide dissolution and cause the aluminium corrosion (see par 3.1),can be part of the growing zeolitic structures creating an intensechemical bond between zeolite and metal support [16]. Once thefirst layer formed at metal surface, the coating thickening proceedsby several mechanisms, such as the accretion of crystals for nutri-ents intake from the solution and/or the inclusion in the layer ofprecipitated crystals (secondary growth mechanism) [15]. The finalresult is the formation of a compact coating with a typicalmorphology consisting of aggregates of intergrown crystals denselypacked. The evaluation of adhesive properties of these coatings hasnot been really well defined until now and the few citations found

Fig. 4. Aluminium coated sample. Evaluation grid befo

in literature do not clearly explain the test methodology adopted[16,18]. We have used for our evaluations the standard test foradhesion ASTM D3359, which consists in making a grid on thecoated surface by a cutting tool, applying an adhesive tape and thenremoving the tape after 90 s (Fig. 4). The measurement consists ina qualitative comparison with a reference evaluation grid, whichhelp to estimate the coating quantity detached. The results ob-tained with zeolites grown on aluminium supports have demon-strated the good adhesion properties of such coatings, being almostany part of the tested surface wholly detached by the tape peelaction (Fig. 4).

According to the ASTM D3359 test, the coatings on aluminiumhave scored the value 5B that corresponds in the reference grid tothe condition of “optimal adhesion”. On stainless steel and coppersurfaces, instead, the adhesion test has given results from “scarceadhesion” (on stainless steel) to “sufficient adhesion” (on copper).

3.4. Coating protective action

In the operating condition of an adsorption heat pump, the heatexchanger in contact with the zeolite material is exposed to anaggressive environment for the presence of water vapour undervacuum. Cause the deaerated atmosphere, the oxide layer, whichnaturally develops on metal surfaces and acts as anti-corrosivebarrier, is not able to regenerate in case of damage, exposingdirectly the metal to the water vapour attack. For aluminium thesituation can be worse, because the passive oxide film can deteri-orate by dissolution during the pump operation, causing the metal

re (a) and after the test (b); polarized-light image.

1.0E-06 1.0E-04 1.0E-02 1.0E+00-1.4

-1

-0.6

-0.2

0.2

0.6

I [mA/cm^2]

E [

V v

s SC

E]

Stainless Steel

Coated StSt

1.0E-06 1.0E-04 1.0E-02 1.0E+00-1.4

-1

-0.6

-0.2

0.2

0.6

I [mA/cm^2]

E [

V v

s SC

E]

Coated Al

Aluminiuma b

Fig. 5. Polarization curves for stainless steel (a) and aluminium supports (b), uncoated (grey line) and zeolite coated (black line).

L. Bonaccorsi et al. / Applied Thermal Engineering 50 (2013) 1590e15951594

corrosion with evolution of hydrogen as a result of cathodic reac-tions [23]. In view of the growth of zeolitic layers over the originalmetal oxide, a key point is to evaluate if the zeolite coating operatesa protective action or, instead, could accelerate the support corro-sion. The electrochemical activity of zeolites, indeed, is known fromseveral years and is at the basis of some applications such as zeolitemodified electrodes or corrosion-resistant coatings [24,25].

We performed several tests in an electrochemical cell with thezeolite coated support as working electrode immersed in a NaClsolution (3.5 w%). These experiments demonstrated that the zeolitecoating was able to protect both stainless steel and aluminiumsupports against corrosion, or, in other words, passivated the metalsurfaces. In Fig. 5 are presented the polarization curves for stainlesssteel (Fig. 5a) and aluminium (Fig. 5b) supports. In each graph aredepicted the curves of the current density (I) vs. the electro-chemical potential (E) in the two cases: the support as is (greycurve) and the support coated with zeolites (black curve). The shiftof the polarization (black) curves for the coated supports to higherelectrochemical potentials and to smaller current densitiescompared to the polarization (grey) curves for the uncoated metalsis the prove of the protective action exerted by the zeolite coatings.More experiments are needed, however, to simulate the conditionscreated in a heat pump operation cycle, but these first resultsconfirm that zeolites coatings are able to protect the metallic base ifgood coverage and good adhesion properties are reached.

4. Conclusions

The direct growth of zeolites on the heat exchanger surfaces isa promising technique for increasing performance of adsorptionheat pumps. Objective of this study was to highlight some featuresof the deposition process that are important, as well as thermo-dynamic and kinetic properties of the adsorbent material, in thedevelopment of new advanced adsorbers. It has been shown theessential role of interface phenomena in determining the nature ofsurface bonds between the zeolite coating and the metal support:surface coverage, adhesion and mechanical properties, more thanthickness, should be considered for a valuable coating. Adhesiontests and corrosion tests, indeed, were introduced for a moreextensive analysis of the coating properties. Our investigation hashighlighted that promising results can be obtained if the metallicsupport is opportunely pre-treated before the synthesis process.Furthermore, interactions generated during in-situ reactions on thesupport surface determine several important characteristics of thefinal coating. Finally, pre-requisite for the development of

a successful technology based on in-situ growth of zeolite coatingsis the contemporaneous development of high-surface supportsmade of high-conductive light materials for advanced heatexchangers.

Acknowledgements

The present work was partially funded by “Fondo per la Ricercaper il Sistema Elettrico - AdP MSE-CNR”.

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