Microporous carbon spheres derived from resorcinol-formaldehyde solutions. A new approach to coat supports

1 *Corresponding author. E-mail: [email protected] (present address: Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, R. Dr. Roberto Frias s/n, 4200-465 Porto, Portugal) Microporous carbon spheres derived from resorcinol-formaldehyde solutions. A new approach to coat supports Natalia Rey-Raap*, Sara F. Villanueva, J. Angel Menéndez and Ana Arenillas Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain Abstract Microporous carbon spheres of different morphology and porosity were synthesized from resorcinol-formaldehyde solutions by a simple and fast procedure. Polymeric spheres were shaped by means of microwave heating. Carbonization and activation with carbon dioxide were then applied to obtain the intended final carbon spheres. The influence of the pH, heating time and thermal treatments on the morphology and porosity of the carbon spheres was investigated. It was found that the size of the spheres, can be easily controlled during the synthesis process, specifically by modifying the pH of the precursor solution. An increase in the pH value from 3 to 5 led to carbon spheres with sizes of 4 µm and 3.5 µm, respectively, whereas time seemed to have no effect. These results have been attributed to the chemical mechanisms of the polymerization reaction. On the other hand, microporosity was tailored during the thermal treatments. Carbon spheres with surface areas of 630 m2/g and 1500 m2/g were obtained by applying carbonization and physical activation, respectively. Furthermore, the synthesis method proposed allows to obtained liquid polymerized inks that can be further used to coat ceramic supports by a simple spray-drying process, which enhances the potential of these materials for several applications. Keywords: carbon sphere, microwave-heating, spray-drying, coating 1. Introduction Over the last few decades, there has been an explosive growth in the synthesis, characterization and application of carbon spheres, due to the fact that their size, 2 morphology, porosity and chemical properties can be tailored for specific purposes. The final properties of carbon spheres are strongly influenced by the synthesis conditions under which they are prepared. Chemical vapour deposition (CVD), polymerization using templates, hydrothermal carbonization, inverse emulsion polymerization, extension of the Stöber method, etc. are some of the best-known examples of the wide list of methods studied. The reader is referred to a number of excellent reviews on these synthesis techniques, and on properties and applications of carbon spheres, which have already been published [1-5]. Another aspect worth bearing in mind, as it amends the final properties of carbon spheres, are the precursors employed, which comprise but are not limited to polymers, biomass-derived carbons, phenolic resins, benzene derivatives and other heterocyclic aromatic organic compounds [2, 4, 6]. Of these, phenolic resins have attracted most attention as carbon sphere precursors [7-17], and particularly resorcinol-formaldehyde (RF) precursor solutions which, as has been extensively reported, generally give rise to carbon gels [18-23]. A number of studies can be found in the literature focused on the use of RF solutions as precursors of carbon spheres, for the preparation of which several methods have been reported. The modified Stöber method has been one of the most employed techniques to prepare carbon spheres from RF solutions [8, 12, 13, 15, 16]. This method involves the use of additives such as ammonia, ethanol or templates, as well as long processing times. Another method that is widely used is that of inverse emulsion, where not only the use of additives is required but also a full control over the viscosity [11, 17]. Recently, hydrothermal treatments has made it possible to synthesize carbon spheres easily without the need for additives [9]. However, this process is still tedious as long times are required. In order to reduce the processing time, ultrasounds and microwave radiation have been employed as suitable techniques to obtain carbon spheres [7, 10, 14]. Although these methods allow the size and number of carbon spheres to be tailored via a rapid and easy route, the use of surfactants and full control of viscosity also seem to be essential requirements in these processes. Leaving aside the above mentioned drawbacks, each of these techniques leads to carbon spheres with different final properties, and hence, the choice of the appropriate synthesis method will depend on the application for which the carbon spheres are intended. The widely-recognized advantages of RF solutions as carbon precursors (which include the possibility of tailoring their porous properties or of doping them with metals and 3 heteroatoms [11, 24]), confer on them great potential for preparing carbon spheres that offer optimum performances in a wide variety of applications, such as adsorption [1, 25], ultrafiltration [26], catalysis [16] or electrochemical systems [9, 13, 15, 17]. In some of these applications carbon spheres are used as coating of surfaces, nanoparticles or supports and, in these cases, spray-drying methods are preferable. This process involves spraying the precursor solution onto a support, which is then dried to give rise to the formation of polymeric spheres on its surface [1]. Finally, the polymer thus obtained is carbonized to yield the intended carbon spheres. In order to use the spraydrying method, liquid precursor solutions are needed, which rules out many of the aforementioned methods. Furthermore, the studies so far published on the spraying method, again refer to the need for additives [27-29]. Consequently, although several attempts to achieve a facile synthesis methodology for carbon spheres have been made, there is still an ongoing need for an alternative low-cost and easily scalable method to prepare liquid polymerized precursors that could be used in spray-drying processes for producing carbon spheres. In the present work, microwave-assisted polymerization of resorcinol and formaldehyde was performed to obtain polymeric spheres, which were then transformed into carbon spheres. The novelty of the method proposed lies in the fact that i) the use of additives is avoided and ii) the synthesis time is greatly reduced. The process is thus simplified and become scalable. Furthermore, the method presented here has demonstrated to be an easy and suitable way to coat surfaces of ceramic solid catalysts, thereby extending the range of applications of carbon spheres prepared from RF precursor solutions. 2. Experimental 2.1. Preparation of resorcinol-formaldehyde inks and flakes The selected starting materials were resorcinol (Indspec, 99%), formaldehyde (Merck, 37 wt. % in water, stabilized by approx. 10 wt.% of methanol), deionized water (produced by a Millipore water system) and a solution of sodium hydroxide (NaOH 5 M solution prepared from solid NaOH (AnalaR Normapur, 99.9 %). 4 Resorcinol was first dissolved in deionized water in an unsealed glass beaker under magnetic stirring. After dissolution, formaldehyde was added and the mixture was stirred until a homogeneous solution was obtained. Finally, the pH value was adjusted by adding the NaOH solution. The concentration of each reagent was selected on the basis of results previously reported. These concentrations are reported in the literature to be related to the pH of the precursor solution, the dilution ratio (D) and the molar ratio between the resorcinol and the formaldehyde (R/F) [30]. It should be highlighted that the dilution ratio refers to the molar ratio between the total solvent and the main reagents. Rey-Raap et al. provided an interesting illustration of the combinations of pHD whereby the synthesis route via microwave heating did not lead to the formation of a solid material [18]. Based on that illustration, in the present work the precursor solutions were therefore prepared with pH values of 3 and 5, using the stoichiometric R/F molar ratio (i.e., 0.5) and a dilution ratio (D) fixed at 17. Once prepared, each precursor solution was placed in a multimode microwave oven (inlab design and constructed [31]) at 85 ºC. The heating time was selected on the basis of previous experiments performed, in some of which microwave energy consumption was recorded as a function of time [10, 19]. The time values selected were 2 and 3 hours to allow a polymeric ink (RF-ink) to be obtained instead of a wet-solid gel. Each polymeric RF-ink was air-dried at 85 ºC for 3 hours until a solid thin-layer (i.e. flake) was obtained. These materials were labelled RFS, in reference to resorcinolformaldehyde spheres, followed by the pH value and the duration of the microwave heating in hours. 2.2. Coating of the supports Coated supports were prepared by means of a spin-coating strategy based on the use of the precursor solution of resorcinol-formaldehyde gels prepared with a pH value of 3 and heated for 2 hours in the microwave oven. The supports used were monolithic catalyst substrates made of cordierite with a cell density of 200 cpsi and supplied by Corning Incorporated. Briefly, the supports were introduced into a glass flask containing 50 ml of hot RF-ink for 3 minutes. The impregnated support was then centrifuged for 15 seconds at 600 rpm and finally air-dried at 85 ºC for 3h. In order to determine the optimum number of spin-coating cycles required, one and five cycles were performed. 5 2.3. Carbonization and physical activation The RFS samples and coated supports were heated for 2h under a nitrogen atmosphere at 700 ºC and under a CO2 atmosphere at 1000 ºC in order to obtain carbonized (CS) and activated (ACS) carbon spheres, respectively. 2.4. Sample characterization All the samples, RFS and supports (dried, carbonized and activated), were first outgassed at 0.1 mbar and 120 ºC overnight in a Micromeritics VAcPrep 0.61 in order to remove humidity and other physisorbed gases. Their porous properties and morphology were then analyzed. Nitrogen adsorption-desorption isotherm analysis were performed at -196 ºC using a Micromeritics Tristar 3020 instrument. The BET surface area (SBET) and micropore volume (VDUB-N2) of the samples were determined by applying the BET equation and the Dubinin-Radushkevic method to the N2 adsorption isotherms, respectively. Envelope pycnometry (Geopyc1360 envelope density analyzer from Micromeritics) was also employed to further analyze other porous properties such as bulk density and porosity. All samples were synthesized and characterized in duplicate to ensure reproducibility The morphology of all the materials was examined using a Quanta FEG 650 scanning electron microscope. The samples were previously attached to an aluminum pin using conductive double-sided adhesive tape. An accelerating voltage of 25 kV and a secondary electron detector EDT (Everhart-Thornley) were used in all the analyses. This technique was also employed to determine the size of the sphere. However, coulter analysis (LS13320 Beckman fitted with an ALM water module) was also used to verify these data. Coulter analysis were performed in triplicate. 3. Results and discussion 3.1. Resorcinol-formaldehyde spheres (RFS) Reg a used isot h (S BE T mat e prov i ther e agre e b e o b Figu r refe r solu t RSF - The solu t an i n valu e (sa m poly m reac t con d a rdless of t h to prepar e h erms reve a T ≈1-2 m 2 / e rials have a i ded b y po r e fore, inap p e ment with b serve d , as r e 1. Scan n r ence to res o t ion and th e - 5-2h and ( d size of th e t ion, where a n crease in t h e of 3 (sa m m ples RFS - m erization t ions: i) t h d ensation o f h e pH val u e the RF-i n a l that all t / g). Howev e a bulk den s r es of large p reciable u n those obta i shown in F n ing electr o o rcinol-for m e duration d ) RSF-5-3 h e spheres i a s time see m h e pH val u m ples RFS- - 5-2h and reaction b h e additio n f the hydro x e of the pr e n ks, the res t he RFS sa m e r, the resu l s ity of 0.60 size fallin g n der nitrog e i ne d b y SE M igure 1. o n microsc m aldehyde s o f the mic r h . i s markedl y m s to have u e lea d s to s 3-2h and R RFS-5-3 h etween re s n of form a x ymethyl d e 6 ecursor sol u s ults obtain e a mples are lts of the R 0 .8 g/c m 3 a g exclusive l e n adsorpti o M images, w opy image s pheres, fol r owave he a y influenc e no effect ( s pheres wit R FS-3-3h) t h ). These s orcinol an d a ldehyde t o e rivatives f o u tion and t h ed from t h p ractically R F-inks stu d a nd a total p l y within t h o n analysis w here larg e s of organ i lowed by t h a ting: (a) R e d by the i Figure 1). W h sizes ran g t o 5-6 µm results c a d formald e o resorcin o o rmed (Fig u h e microw a e N 2 adso r non-micro p d ied also i n p orosity o f h e macropo r . These re s e voi d s b et w i c spheres h e pH valu e SF-3-2h, ( b i nitial pH o W hatever t h g ing from 6 when the p a n b e at t hyde, whi c o l and ii) u re 2). a ve heatin g r ption-deso r p orous ma t n dicate that f 50 %, pro r osity rang e s ults are in w een spher e labelled R F e of the pre c b ) RSF-3-3 h o f the pre c h e time sel e 6 -7 µm for p H is fixe d t ributed t o c h involve s the subse g time r ption t erials these b ably e an d , good e s can F S in c ursor h , (c) c ursor e cted, a pH d at 5 o the s two quent 7 Figure 2. Mechanism for the polymerization reaction of resorcinol with formaldehyde to form organic spheres. Initially, resorcinol anions are formed due to the abstraction of hydroxyl hydrogens, which generally allows the addition of formaldehyde in positions 2 and 4 (addition reaction) [30]. At the same time, the hydroxymethyl derivatives lose OH groups to form benzyl-type cations (condensation reaction). Each cation reacts with a benzene ring of another molecule giving rise to methylene and ether bonds. As the reaction proceeds, the number of bonds between the rings increases to form the polymer backbone, which leads to cross-linked polymer clusters, also named primary particles [18, 24]. Unlike in the synthesis of organic gels, these primary particles do not aggregate and crosslink with each other due to the high D value used to prepare the precursor solutions. Indeed, the large amount of water (high dilution ratio value, D = 17) increases the distance between primary particles, and hence, prevents the solution from reaching the gelation point [30]. Therefore, isolated organic spheres are formed, i.e. RF-inks (Figure 3a) instead of organic wet-solid gels (Figure 3b and 3d) are yielded. Once the RF-inks have been prepared, any excess solvent is removed. Solid materials composed exclusively of quasi non-interconnected micro-spheres, like those shown in Figure 3c, are then obtained. Fi g The age n prec u solu t appe depr o incr e Figu r poly m of r e carb o acti v ther m 3.2. C The sphe r the s effe c sam p g ure 3. Pict u polymeriz a n ts, which a u rsor solut i t ion) was a arance of o tonation o e asing the p r e 1. There m eric sphe r e agents, sp e o n spheres v ation proc e m al stabilit y C arbonize d effect of t h r es was ev a ize of the c c t while th e p les CS-3-2 u res of the r SEM ima g a tion reacti o a ccelerates i ons prepar e a dded to s hydroxym e o f resorcin o H, and this fore, it can r es of diffe r e cially pH a by mean s e sses. Thes y and poros i d and activ a h e thermal a luated. As e arbon sphe r e size of th h and CS-5 r esorcinolf g es of the R F o n shown the additi o e d had an i o me samp l e thyl deri v o l. As a c o results in t h be said th a r ent sizes, o a nd D. Th e s of suitab l e thermal p i ty. ted sphere s treatments e xpected, t h r es was si m e spheres d -2h, in Fig u 8 f ormaldehy F spheres ( c in Figure 2 o n reactio n i nitial pH v l es, to inc r v atives is o nsequence h e formati o a t the RF-i n o btained by e se RF sph e l e thermal p ost-treatm e s on the m o h e effect of m ilar to that d ecreases w u re 4a and 4 d e in k (a) a c ) and a car b 2 may be c n [24, 30]. v alue of 3, r ease the i n favored at , the addit o n of spher e n ks prepare d selecting t h e res can be treatments e nts will p r o rphology a the pH an d of the orga n w ith pH, as 4 b , respecti v nd a wet-s o b on gel (d) . c atalyzed b In the pr e so a basic n itial pH v high pH i on reactio n e s of small s d lead to m h e appropri a easily be t such as c r ovide car b a nd porosi t microwav e n ic samples can be se e v ely. o lid gel (b) a . b y basic o r esent stud y agent (a N v alue to 5 . values vi a n is favor e s ize, as sho w m aterials m a a te concent r t ransfor m e d c arbonizati o b on sphere s t y of the c a e heating ti m s , i.e. time h e n b y com p a nd r acid y , the N aOH . The a the e d by w n in a de of r ation d into o n or with a rbon m e on h as no p aring Figu r b y t h CS3 The infe r arou n valu e coul t sphe r of t h high l smal met h time s impl e The d matt e poly m prev i effe c Org a oxy g thos e inter a r e 4. Scann h e pH valu e 3 -2h and (b ) size of the r red by co m n d a mean v e of 3 and t er analysis r es prepare d h e method l ight that s o ler sizes, w h ods to obt s [12, 15, ementation d ecrease in er , which o m eric sphe r i ously repo r c ts, the act i a nic sphere g enated gr o e with oxy g actions wi t ing electro n e of the pre c ) CS-5-2h; a carbon sp h m paring Fig u v alue of 4 µ 5, respecti v of the car b d via m ore proposed o me studie s w hich mig h ain those s 16] and and increa s the size o f o ccurs mai r es. This p r te d in lite r i vation pro c s are com p o ups [23]. I g enate gro u t h CO 2 th a n microsco p c ursor solut i a nd (c) SE M h eres decre a u re 1 and 4. µ m and fro m v ely. Thes e b onized sph complex p r in the pre s s also sho w h t be an a s maller sph e ven surfa s es thei r pr o the carbon n ly above p rocess als o r ature [12, 2 c ess leads t p osed of p r I t has bee n u ps in thei r a t lead to c 9 p y images o i on and the M image of a ses slightl The size o m 5-6 µm t o e results ar e h eres, show n r ocesses [1 1 s ent work. w that it is p a dvantage f eres requir e a ctants [14, o duction co spheres ca n 500 ºC [2 4 o contribut e 2 1]. Furthe r t o a certai n r imary par t n widely d e r backbone c ertain de g o f carbon s p duration o f activated s p y during t h f the spher e o around 3. 5 e in agree m n in Figure 1 , 17], whi c Nonethele p ossible to f or some a e solvents 16], whi c st. n be attribu t 4 ], causin g e s to the f o r more, in a d n degree o f t icles with e monstrate d structure s g ree of sin t p heres label l f the micro w p heres AS3 h e carboniz a e s decrease s 5 µm for sa m ent with t h S1, and wi t h undersco r ss, it is a l prepared c a a pplication s [7, 13, 15] c h hinders t ed to the r e g a slight s o rmation o f d dition to t h f merging b a great a m d that poly m s uch as et h t ering of t h l ed CS, fol l w ave heati n 3 -2h. a tion proce s from 6-7 µ a mples wit h h ose obtain e t h that of c a r es the rele v l so import a a rbon sphe r s . Howeve r ] , long syn t their ind u e lease of v o s hrinkage o f micropor e t he carboni z b etween sp h m ount of s u y mers, espe h ers, have s h e spheres l owed n g: (a) ss, as µ m to a pH e d by a rbon v ance a nt to r es of r , the t hesis u strial o latile o f the e s, as z ation h eres. u rface cially s t r ong [32]. Mi F croporous Natalia Re y Institut o F igure S1. C carbon sp h y -Raap*, S a o Nacional d C arbon sph e h eres deri v new appr o a ra F. Villa n d el Carbón, e re size dis t 16 v ed from r e o ach to co a n ueva, J. A n CSIC, Ap a t ribution m e e sorcinol-f o a t supports n gel Mené n a rtado 73, 3 e asured by c o rmaldeh yd n dez and A n 3 080 Ovie d c oulter cou n d e solution n a Arenilla s d o, Spai n n ter analysi s. A s s.