Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction

Patricia Horta-Fraijo; Elena Smolentseva; Andrey Simakov; Miguel José-Yacaman; Brenda Acosta

doi:10.1016/j.micromeso.2020.110707

Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction

Patricia Horta-Fraijo, Elena Smolentseva, Andrey Simakov, Miguel José-Yacaman, Brenda Acosta

Applied Physics and Materials Science

Research output: Contribution to journal › Article › peer-review

28 Scopus citations

Abstract

In the present work, the synthesis of silver species such as nanoparticles (NPs), cations and clusters finely dispersed in A4 zeolite (Ag 0.1 wt%) has been successfully obtained by microwave irradiation and ion-exchange method. The prepared samples were analysed by SEM, HR-TEM, XRD, XPS, UV–Vis spectroscopy, ICP-OES and evaluated in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Physicochemical characterization of samples revealed that ion-exchange method resulted in the main formation of Ag NPs (c.a. 5 nm) while the microwave irradiation promoted the formation of Ag NPs (c.a. 3 nm), Ag cations and clusters as well. The crystalline structure of microporous A4 zeolite was not altered. The sample prepared under microwave irradiation manifested a higher catalytic performance (by 7.5 times in TOF value) in comparison with the reference sample obtained via ion-exchange method. The catalytic activity of the sample prepared by microwave irradiation (TOF value of 396 min⁻¹/μmol-metal) was superior from reference systems reported in the literature. The later confirmed that used synthesis conditions influence the formation of highly active Ag species stabilized on the microporous structure of A4 zeolite. The high capacity for H₂ storage and efficient sorption for the nitroaromatics on A4 zeolite resulted in a promotional effect for the catalytic reduction of 4-NP. Undoubtedly, the obtained results revealed that the selection in the conditions applied for microwave-assisted synthesis and the components of the catalyst (Ag and A4 zeolite) offers an ultra-fast method for the synthesis of highly active catalysts.

Original language	English (US)
Article number	110707
Journal	Microporous and Mesoporous Materials
Volume	312
DOIs	https://doi.org/10.1016/j.micromeso.2020.110707
State	Published - Jan 2021

Keywords

4-Nitrophenol reduction
A4 zeolite
Ag nanoparticles
Ion-exchange
Microwave-assisted synthesis

ASJC Scopus subject areas

Chemistry(all)
Materials Science(all)
Condensed Matter Physics
Mechanics of Materials

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10.1016/j.micromeso.2020.110707

Cite this

@article{08d6c494168c4a7b95c75faa144b76a8,

title = "Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction",

abstract = "In the present work, the synthesis of silver species such as nanoparticles (NPs), cations and clusters finely dispersed in A4 zeolite (Ag 0.1 wt%) has been successfully obtained by microwave irradiation and ion-exchange method. The prepared samples were analysed by SEM, HR-TEM, XRD, XPS, UV–Vis spectroscopy, ICP-OES and evaluated in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Physicochemical characterization of samples revealed that ion-exchange method resulted in the main formation of Ag NPs (c.a. 5 nm) while the microwave irradiation promoted the formation of Ag NPs (c.a. 3 nm), Ag cations and clusters as well. The crystalline structure of microporous A4 zeolite was not altered. The sample prepared under microwave irradiation manifested a higher catalytic performance (by 7.5 times in TOF value) in comparison with the reference sample obtained via ion-exchange method. The catalytic activity of the sample prepared by microwave irradiation (TOF value of 396 min−1/μmol-metal) was superior from reference systems reported in the literature. The later confirmed that used synthesis conditions influence the formation of highly active Ag species stabilized on the microporous structure of A4 zeolite. The high capacity for H2 storage and efficient sorption for the nitroaromatics on A4 zeolite resulted in a promotional effect for the catalytic reduction of 4-NP. Undoubtedly, the obtained results revealed that the selection in the conditions applied for microwave-assisted synthesis and the components of the catalyst (Ag and A4 zeolite) offers an ultra-fast method for the synthesis of highly active catalysts.",

keywords = "4-Nitrophenol reduction, A4 zeolite, Ag nanoparticles, Ion-exchange, Microwave-assisted synthesis",

author = "Patricia Horta-Fraijo and Elena Smolentseva and Andrey Simakov and Miguel Jos{\'e}-Yacaman and Brenda Acosta",

note = "Funding Information: Patricia Horta Fraijo acknowledges to the PROMEP, Mexico , scholarship. Brenda Acosta thanks “C{\'a}tedras-CONACYT program”, Mexico , through project 767 . Elena Smolentseva acknowledges SENER-CONACYT, Mexico (project 117373 ). This research project was partially funded by CONACYT, Mexico (project A1-S-45958 ), UASLP, Mexico (Grants No. C18-FAI-05-01.01 and C19-FAI-05-66.66 ) and DGAPA–PAPIIT ( UNAM, Mexico ) (grant 206920 ). In addition, CONACYT, Mexico , financed the acquisition of infrastructure for the UV–Vis absorption and catalytic measurements through the projects 299818 and 302286 , respectively. Funding Information: Zeolites are widely used as support matrices for catalytic applications and adsorbents due to their unique porous structure that controls the transportation of molecules, acidity that promotes the adsorption of specific molecules to benefit the desired reaction and, the high surface area for wide metal NPs distribution, among others [ 5?8]. The mesoporous zeolites are preferentially used for catalytic application, mainly, because of their effective mass transport compared with microporous ones. However, the confinement of metal NPs into several zeolite frameworks depends on the zeolite porosity and structure [ 9?11]. Metal-based catalysts employing microporous zeolites as supports are characterised with a contribution of well-defined NPs, clusters and cations. In its turn, the high activity of the metal-based catalysts is correlated with the presence of cations or clusters [12], which can be preferentially formed inside the microporous zeolites structure, providing a large contribution of reaction sites with low coordination [11, 13?17]. Smolilo et al. assigned the catalytic activity of vanadium species in the oxidative dehydrogenation of propane to the presence of vanadium ions in the tetragonal coordination environment in the microporous faujasite zeolite [13]. Zhang et al. presented a novel synthesis method for the encapsulation of highly active and stable bimetallic Ni?Pt clusters into the silicate-1 zeolite. The complicated diffusion of ofm-xylene molecules inside the zeolite structure allows the enhancement of the selectivity in the steam reforming of n-dodecane reaction to produce H2 due to the clusters inside the zeolite framework [16]. Furthermore, the gradual disordering of NaA zeolite structure allows the systematic tuning of the molecular sieving effect, when small molecules such as CO2, N2, CH4 are used [18]. In fact, Xu et al. found that the pore size achieved for Ag supported on Linde Type A (LTA) membranes determines the H2 selectivity separation from H2/C3H8 mixture. Hence, the size of the interchangeable ion in LTA zeolite (where Ag was deposited) determined the available pore size for the mass transportation [19]. It was found by Sahn et al. that the catalytic performance of Pt clusters encapsulated into several LTA zeolites (3A, 4A and 5A, where the interchangeable ion is K, Na and Ca, respectively) in hydrogen combustion in a propene-saturated mixture was determined by pore selectivity and coking rate [20].The reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) is a model reaction widely studied to test the catalytic performance of catalysts in the safety transformation of toxic nitroaromatic compounds [27]. Although, steric restrictions are expectable due to the dimensions of 4-NP (0.8 nm in large and 0.45 nm in width), many reports devoted to the transformation of this molecule over metallic species stabilized on microporous zeolites have been reported. Pt [28], Fe and Cu [29] nanoparticles were formed on NaY zeolite supports (pore size <2 nm) and tested in the 4-NP reduction. In addition, MOF's hosting Cu species presented a remarkable catalytic activity attributed to the fast adsorption of 4-NP due to the ?-? stacking interaction between the reagent and the MOF surface [30]. Bimetallic Ni:Cu (0.1 wt%) NPs deposited on A4 zeolite were characterized with superior performance than the monometallic NPs for the catalytic reduction of 4-NP. This performance was assigned to the H2 storage properties exhibited by the zeolite support [31]. A composite formed by Ni stabilized on A3 zeolite and deposited on rGO was also studied [32]. Ag species (3.01 wt%) deposited on microporous BIFs supports were evaluated for the 4-NP reduction as well [33]. In the literature, there are some reports devoted to the microporous zeolite structure modification to obtain mesoporous materials and, by this way, promote their catalytic activity. A catalyst composed by a modified zeolite structure formed with ZSM5 zeolite and SBA15 mesoporous silica was used as a support for Au NPs deposition and tested in the 4-NP reduction. The catalytic performance of the obtained materials, Au/ZSBA15, was attributed to both the Au NPs size and the increase of the porosity, which passed from 0.45 nm for ZSM5 to 6 nm for ZSBA15 [34]. A4 zeolite was hydrothermally synthesised in the presence of Ag nanoparticles supported on carbon nanotubes. The final calcination of composite resulted in the removal of carbon accompanied by the dispersion of Ag on the internal structure of the zeolite and, the formation of mesopores [35].On the other hand, Ag nanoparticles supported on LTA zeolites have been widely studied in photoluminescence [36], luminescence [37], sensors [38], biocide agents [39], adsorbents [40], anticancer therapies [41]. In contrast, catalytic application of the composite Ag nanoparticles on LTA zeolites has been only studied in the styrene epoxidation [42,43] and, curiously, just one for the reduction of 4-NP [35], both cases after porosity modification of LTA zeolite.In Table 1, there is a comparison of microwave-assisted synthesis conditions that were applied to obtain Ag NPs [56, 68?74]. In order to clarify the effect of the synthesis conditions (such as the presence of support, use of reducing agent, time and power of microwave irradiation) on the size of Ag NPs obtained, the results of the present work were compared with the literature data (Table 1). The comparison revealed that unsupported Ag NPs were relatively larger than supported ones, when a reductant agent was used. In contrast, when the reductant was omitted from the synthesis, unsupported Ag NPs were smaller than the supported ones. In addition, the chemical nature of the reducing agent also matters. For example, the use of glucose [45] or NaBH4 [50] as a reducing agent, at the same reaction time, results in the formation of unsupported Ag NPs with mean diameter of 65 nm and the supported ones with 8 nm, respectively. On the other hand, the increase of the reaction time or the microwave power irradiation promotes the growth of supported NPs. It was found that Ag NPs diameter raised from 7.4 to 45.9 nm, when the reaction time was increased from 300 to 1800 s [54], respectively, while the size passed from 3 to 20 nm at 1000 W [this work] and 1850 W [52] of irradiation power, respectively. In addition, the increase of Ag NPs size by the rise of power microwave irradiation is influenced by the support nature or the presence of reductant. The smallest Ag NPs for nanostructures based on rGO supports were obtained using L-Arginine as reducing agent and 900 W in microwave power [72]. Short exposition time of irradiation and 200 W of power were enough for the formation of Ag NPs with size 1.6 nm supported on SBA15 [71]. The effect of support nature and reductant in the stabilization of such small Ag NPs was evidenced. Furthermore, the synthesis without support and reducing agent resulted in Ag NPs with size less than 1 nm just by employing 15 s and 1000 W in power microwave irradiation [64]. In the current work, the conditions applied for the microwave-assisted synthesis were efficient for the formation of supported Ag NPs with size of 3 nm. No reductant was employed. The noticeable difference between Ag NPs diameter presented here and those obtained by Lara et al. (3 times larger) could be attributed to the presence of the support, which may favour the growth of the anchored Ag NPs and a deficient mass transport during the synthesis. In addition, supported Ag NPs obtained here were 2 times larger than those formed on SBA15 due to Na-PPA contribution as a reductant and stabilizing agent preventing the growth of the supported Ag NPs during the synthesis. In conclusion, the size of Ag NPs is under control of all the factors described here for the synthesis via microwave irradiation. A direct comparison between Ag NPs supported on microporous A4 zeolite, confirmed that prolonged exposition time for irradiation at low power led to the formation of relatively large Ag NPs [73].Fig. 8a shows the changes in the relative absorbance at 400 nm vs reaction time for pure A4 zeolite (in black), Ag/ZA4-IE (in red) and Ag/ZA4-MW (in blue) samples. It was observed that the pure A4 zeolite did not present any changes in absorbance at 400 nm, which implies no consumption of the reagent. In contrast, the Ag loaded samples manifested an intake of 4-NPi. Ag/ZA4-MW sample was characterised by a faster consumption of 4-NPi than Ag/ZA4-IE sample. Thus, it is clear that the presence of Ag promoted the reaction, while the synthesis method applied for the samples preparation affected the catalytic performance of samples. Indeed, the estimation of the kapp for both samples revealed a remarkable increase by ~12.5 times for Ag/ZA4-MW (kapp = 7.44 min?1) compared to Ag/ZA4-IE (kapp = 0.59 min?1) (see Fig. 8b). It is well known that the main factors affecting the 4-NP reduction rate are the metallic NPs size, the metal-support interaction, metal loading and the reagents molar ratio (4-NP:NaBH4) [105]. In the present work, the Ag/ZA4-MW and Ag/ZA4-IE samples were characterised with the same metal loading of 0.1 wt% (according to ICP results). In addition, the catalytic evaluation was performed using equal reagents molar ratio for both samples and the same zeolite. Therefore, the reaction rate may be directly related to the size and chemical state of Ag species (NPs, clusters and cations). Ag/ZA4-MW sample was characterised with a higher contribution of Ag clusters and cations compared to Ag/ZA4-IE sample. Besides, the sample prepared using microwave irradiation manifested the presence of Ag NPs with well-faceted surfaces and smaller size in comparison with Ag/ZA4-IE sample. Therefore, one can propose that such high content of coordinatively unsaturated Ag sites was responsible for mayor catalytic activity of Ag/ZA4-MW sample.The comparison of the results of catalytic performance in the 4-NP reduction to 4-AP for several Ag-based catalysts synthesised by microwave irradiation and other catalysts supported on ordered structures earlier described in the literature is presented in Table 2. The K (activity parameter), metal dispersion and turn over frequencies (TOF) values were estimated from the data reported by authors (Ag NPs size, metal loading in the reaction cell and kapp), as in Refs. [60]. Colloidal Ag NPs with a size of 65 and 40 nm were characterised with TOF values of 47 and 0.94 min?1?mol?1, respectively. The remarkable difference in the catalytic activity for colloidal Ag NPs could be attributed to the presence of trace amounts of reductant on the Ag NPs surface instead of the difference in NP size [45,46]. In the case of graphene oxide (GO) as support for Ag-based catalysts, the Ag-GO interaction was evident. Hong et al. use a two-step method to form and stabilize Ag NPs on GO by using sodium citrate as a stabilizing agent [49]. Nonetheless, the modification of GO leads to a notable loss in metal-support synergy, the difference may be noticed in from the TOF values, which varies from 195 to 5.8 min?1?mol?1 for Ag/GO [49] and Ag/rGO [50] catalysts, respectively. Such phenomena may be explained by the loss of active sites due to the detachment of Ag NPs from the support and their rapid agglomeration for modified GO. Indeed, the pre-reduction of GO led to the formation of a highly ordered graphitic surface [106], which could not be active enough to strongly attach Ag NPs. Besides, the Ag/rGO catalysts did not have a stabilizing agent on its surface, so the NPs in the reaction media may be easily agglomerated. In addition, Tian et al. find that the incorporation of N atoms to the rGO dramatically enhanced the Ag-support interaction and, consequently, the catalytic performance of the materials [50]. Unfortunately, the catalytic performance of Ag/rGO/CTN/Fe cannot be compared because the reagents molar ratio (4NP:NaBH4) is equal to 1; hence, the value of TOF may be influenced by the lack of hydrogen in the reaction media [52]. The catalytic performance of samples with zeolite supports depends on the ability of 4-NPi to be adsorbed, the mass transport limitations due to porosity and the activation of H2 on the metallic surface. TOF values summarized in Table 2 evidenced that the use of microporous structures as supports for the catalytic reduction of 4-NP must be carefully analysed. Most of the catalysts supported on microporous structures, listed in Table 2 were characterized with lower catalytic activity than those supported on mesoporous structures. It seems, that mass transport limitations are controlling the reaction velocity even more than the nanoparticle size. A comparison of the TOF values 1.45 min?1 ?mol metal for Ag (4.8 nm in nanoparticle size) supported on SBA15 mesoporous structure [48] with TOF value 0.61 min?1 ?mol metal for Ag supported BIF microporous ones [33] revealed the mass transport limitation over the nanoparticle size. Note, the microporous A4 zeolite was characterized with remarkable TOF value (134 min?1 ?mol metal) in comparison with the catalysts with BIF microporous structures, when Ag nanoparticles with 3 nm were deposited for both cases. The microporous to mesoporous modification of A4 zeolite resulted in superior TOF value (5.15 min?1 ?mol metal) than SBA15 mesoporous support. Thus, it seems that the support nature has noticeable influence on the catalytic performance of the samples with ordered structures. Indeed, the A4 zeolite is characterized by its H2 storage capability and efficient sorption for the nitroaromatic compounds [26,31]. The activity level of Ag NPs supported on zeolite type A was shown to be influenced by the Ag NPs size and the synthesis methodology applied (see Table 2). Wu et al. report the hydrothermal synthesis of 4A zeolite in the presence of Ag NPs deposited on carbon nanofibers. The carbon elimination by thermal decomposition leads to mesoporous 4A zeolite formation. As a result, Ag NPs obtained are internally dispersed inside the mesoporous 4A zeolite, denoted as Ag/P-4A-zeolite [35]. The TOF values for samples prepared in the present work were superior to that obtained for Ag/P-4A-zeolite (76 and 10 times for Ag/ZA4-MW and Ag/ZA4-IE, respectively). The superior TOF values of Ag/ZA4-MW and Ag/ZA4-IE microporous catalysts prepared in this work could be explained by the presence of fine Ag NPs.The application of microwave irradiation benefits the formation and stabilization of finely dispersed Ag species such as NPs (c.a. 3 nm), cations and clusters on the A4 zeolite under relatively soft conditions. The sample prepared under microwave irradiation, manifests a superior catalytic activity (TOF value of 396 min?1/?mol-metal) in the 4-NP reduction than reference systems reported in the literature. The use of microporous A4 zeolite as catalysts support resulted in a promotional effect for the catalytic reduction of 4-NP due to its H2 storage capability and high affinity for the nitroaromatic compounds sorption. The use of microwave irradiation offers a relatively ultra-fast and environmentally friendly method for the synthesis of multifunctional metallic NPs.Patricia Horta Fraijo acknowledges to the PROMEP, Mexico, scholarship. Brenda Acosta thanks ?C?tedras-CONACYT program?, Mexico, through project 767. Elena Smolentseva acknowledges SENER-CONACYT, Mexico (project 117373). This research project was partially funded by CONACYT, Mexico (project A1-S-45958), UASLP, Mexico (Grants No. C18-FAI-05-01.01 and C19-FAI-05-66.66) and DGAPA?PAPIIT (UNAM, Mexico) (grant 206920). In addition, CONACYT, Mexico, financed the acquisition of infrastructure for the UV?Vis absorption and catalytic measurements through the projects 299818 and 302286, respectively. Publisher Copyright: {\textcopyright} 2020 Elsevier Inc.",

year = "2021",

month = jan,

doi = "10.1016/j.micromeso.2020.110707",

language = "English (US)",

volume = "312",

journal = "Microporous and Mesoporous Materials",

issn = "1387-1811",

publisher = "Elsevier",

}

TY - JOUR

T1 - Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction

AU - Horta-Fraijo, Patricia

AU - Smolentseva, Elena

AU - Simakov, Andrey

AU - José-Yacaman, Miguel

AU - Acosta, Brenda

N1 - Funding Information: Patricia Horta Fraijo acknowledges to the PROMEP, Mexico , scholarship. Brenda Acosta thanks “Cátedras-CONACYT program”, Mexico , through project 767 . Elena Smolentseva acknowledges SENER-CONACYT, Mexico (project 117373 ). This research project was partially funded by CONACYT, Mexico (project A1-S-45958 ), UASLP, Mexico (Grants No. C18-FAI-05-01.01 and C19-FAI-05-66.66 ) and DGAPA–PAPIIT ( UNAM, Mexico ) (grant 206920 ). In addition, CONACYT, Mexico , financed the acquisition of infrastructure for the UV–Vis absorption and catalytic measurements through the projects 299818 and 302286 , respectively. Funding Information: Zeolites are widely used as support matrices for catalytic applications and adsorbents due to their unique porous structure that controls the transportation of molecules, acidity that promotes the adsorption of specific molecules to benefit the desired reaction and, the high surface area for wide metal NPs distribution, among others [ 5?8]. The mesoporous zeolites are preferentially used for catalytic application, mainly, because of their effective mass transport compared with microporous ones. However, the confinement of metal NPs into several zeolite frameworks depends on the zeolite porosity and structure [ 9?11]. Metal-based catalysts employing microporous zeolites as supports are characterised with a contribution of well-defined NPs, clusters and cations. In its turn, the high activity of the metal-based catalysts is correlated with the presence of cations or clusters [12], which can be preferentially formed inside the microporous zeolites structure, providing a large contribution of reaction sites with low coordination [11, 13?17]. Smolilo et al. assigned the catalytic activity of vanadium species in the oxidative dehydrogenation of propane to the presence of vanadium ions in the tetragonal coordination environment in the microporous faujasite zeolite [13]. Zhang et al. presented a novel synthesis method for the encapsulation of highly active and stable bimetallic Ni?Pt clusters into the silicate-1 zeolite. The complicated diffusion of ofm-xylene molecules inside the zeolite structure allows the enhancement of the selectivity in the steam reforming of n-dodecane reaction to produce H2 due to the clusters inside the zeolite framework [16]. Furthermore, the gradual disordering of NaA zeolite structure allows the systematic tuning of the molecular sieving effect, when small molecules such as CO2, N2, CH4 are used [18]. In fact, Xu et al. found that the pore size achieved for Ag supported on Linde Type A (LTA) membranes determines the H2 selectivity separation from H2/C3H8 mixture. Hence, the size of the interchangeable ion in LTA zeolite (where Ag was deposited) determined the available pore size for the mass transportation [19]. It was found by Sahn et al. that the catalytic performance of Pt clusters encapsulated into several LTA zeolites (3A, 4A and 5A, where the interchangeable ion is K, Na and Ca, respectively) in hydrogen combustion in a propene-saturated mixture was determined by pore selectivity and coking rate [20].The reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) is a model reaction widely studied to test the catalytic performance of catalysts in the safety transformation of toxic nitroaromatic compounds [27]. Although, steric restrictions are expectable due to the dimensions of 4-NP (0.8 nm in large and 0.45 nm in width), many reports devoted to the transformation of this molecule over metallic species stabilized on microporous zeolites have been reported. Pt [28], Fe and Cu [29] nanoparticles were formed on NaY zeolite supports (pore size <2 nm) and tested in the 4-NP reduction. In addition, MOF's hosting Cu species presented a remarkable catalytic activity attributed to the fast adsorption of 4-NP due to the ?-? stacking interaction between the reagent and the MOF surface [30]. Bimetallic Ni:Cu (0.1 wt%) NPs deposited on A4 zeolite were characterized with superior performance than the monometallic NPs for the catalytic reduction of 4-NP. This performance was assigned to the H2 storage properties exhibited by the zeolite support [31]. A composite formed by Ni stabilized on A3 zeolite and deposited on rGO was also studied [32]. Ag species (3.01 wt%) deposited on microporous BIFs supports were evaluated for the 4-NP reduction as well [33]. In the literature, there are some reports devoted to the microporous zeolite structure modification to obtain mesoporous materials and, by this way, promote their catalytic activity. A catalyst composed by a modified zeolite structure formed with ZSM5 zeolite and SBA15 mesoporous silica was used as a support for Au NPs deposition and tested in the 4-NP reduction. The catalytic performance of the obtained materials, Au/ZSBA15, was attributed to both the Au NPs size and the increase of the porosity, which passed from 0.45 nm for ZSM5 to 6 nm for ZSBA15 [34]. A4 zeolite was hydrothermally synthesised in the presence of Ag nanoparticles supported on carbon nanotubes. The final calcination of composite resulted in the removal of carbon accompanied by the dispersion of Ag on the internal structure of the zeolite and, the formation of mesopores [35].On the other hand, Ag nanoparticles supported on LTA zeolites have been widely studied in photoluminescence [36], luminescence [37], sensors [38], biocide agents [39], adsorbents [40], anticancer therapies [41]. In contrast, catalytic application of the composite Ag nanoparticles on LTA zeolites has been only studied in the styrene epoxidation [42,43] and, curiously, just one for the reduction of 4-NP [35], both cases after porosity modification of LTA zeolite.In Table 1, there is a comparison of microwave-assisted synthesis conditions that were applied to obtain Ag NPs [56, 68?74]. In order to clarify the effect of the synthesis conditions (such as the presence of support, use of reducing agent, time and power of microwave irradiation) on the size of Ag NPs obtained, the results of the present work were compared with the literature data (Table 1). The comparison revealed that unsupported Ag NPs were relatively larger than supported ones, when a reductant agent was used. In contrast, when the reductant was omitted from the synthesis, unsupported Ag NPs were smaller than the supported ones. In addition, the chemical nature of the reducing agent also matters. For example, the use of glucose [45] or NaBH4 [50] as a reducing agent, at the same reaction time, results in the formation of unsupported Ag NPs with mean diameter of 65 nm and the supported ones with 8 nm, respectively. On the other hand, the increase of the reaction time or the microwave power irradiation promotes the growth of supported NPs. It was found that Ag NPs diameter raised from 7.4 to 45.9 nm, when the reaction time was increased from 300 to 1800 s [54], respectively, while the size passed from 3 to 20 nm at 1000 W [this work] and 1850 W [52] of irradiation power, respectively. In addition, the increase of Ag NPs size by the rise of power microwave irradiation is influenced by the support nature or the presence of reductant. The smallest Ag NPs for nanostructures based on rGO supports were obtained using L-Arginine as reducing agent and 900 W in microwave power [72]. Short exposition time of irradiation and 200 W of power were enough for the formation of Ag NPs with size 1.6 nm supported on SBA15 [71]. The effect of support nature and reductant in the stabilization of such small Ag NPs was evidenced. Furthermore, the synthesis without support and reducing agent resulted in Ag NPs with size less than 1 nm just by employing 15 s and 1000 W in power microwave irradiation [64]. In the current work, the conditions applied for the microwave-assisted synthesis were efficient for the formation of supported Ag NPs with size of 3 nm. No reductant was employed. The noticeable difference between Ag NPs diameter presented here and those obtained by Lara et al. (3 times larger) could be attributed to the presence of the support, which may favour the growth of the anchored Ag NPs and a deficient mass transport during the synthesis. In addition, supported Ag NPs obtained here were 2 times larger than those formed on SBA15 due to Na-PPA contribution as a reductant and stabilizing agent preventing the growth of the supported Ag NPs during the synthesis. In conclusion, the size of Ag NPs is under control of all the factors described here for the synthesis via microwave irradiation. A direct comparison between Ag NPs supported on microporous A4 zeolite, confirmed that prolonged exposition time for irradiation at low power led to the formation of relatively large Ag NPs [73].Fig. 8a shows the changes in the relative absorbance at 400 nm vs reaction time for pure A4 zeolite (in black), Ag/ZA4-IE (in red) and Ag/ZA4-MW (in blue) samples. It was observed that the pure A4 zeolite did not present any changes in absorbance at 400 nm, which implies no consumption of the reagent. In contrast, the Ag loaded samples manifested an intake of 4-NPi. Ag/ZA4-MW sample was characterised by a faster consumption of 4-NPi than Ag/ZA4-IE sample. Thus, it is clear that the presence of Ag promoted the reaction, while the synthesis method applied for the samples preparation affected the catalytic performance of samples. Indeed, the estimation of the kapp for both samples revealed a remarkable increase by ~12.5 times for Ag/ZA4-MW (kapp = 7.44 min?1) compared to Ag/ZA4-IE (kapp = 0.59 min?1) (see Fig. 8b). It is well known that the main factors affecting the 4-NP reduction rate are the metallic NPs size, the metal-support interaction, metal loading and the reagents molar ratio (4-NP:NaBH4) [105]. In the present work, the Ag/ZA4-MW and Ag/ZA4-IE samples were characterised with the same metal loading of 0.1 wt% (according to ICP results). In addition, the catalytic evaluation was performed using equal reagents molar ratio for both samples and the same zeolite. Therefore, the reaction rate may be directly related to the size and chemical state of Ag species (NPs, clusters and cations). Ag/ZA4-MW sample was characterised with a higher contribution of Ag clusters and cations compared to Ag/ZA4-IE sample. Besides, the sample prepared using microwave irradiation manifested the presence of Ag NPs with well-faceted surfaces and smaller size in comparison with Ag/ZA4-IE sample. Therefore, one can propose that such high content of coordinatively unsaturated Ag sites was responsible for mayor catalytic activity of Ag/ZA4-MW sample.The comparison of the results of catalytic performance in the 4-NP reduction to 4-AP for several Ag-based catalysts synthesised by microwave irradiation and other catalysts supported on ordered structures earlier described in the literature is presented in Table 2. The K (activity parameter), metal dispersion and turn over frequencies (TOF) values were estimated from the data reported by authors (Ag NPs size, metal loading in the reaction cell and kapp), as in Refs. [60]. Colloidal Ag NPs with a size of 65 and 40 nm were characterised with TOF values of 47 and 0.94 min?1?mol?1, respectively. The remarkable difference in the catalytic activity for colloidal Ag NPs could be attributed to the presence of trace amounts of reductant on the Ag NPs surface instead of the difference in NP size [45,46]. In the case of graphene oxide (GO) as support for Ag-based catalysts, the Ag-GO interaction was evident. Hong et al. use a two-step method to form and stabilize Ag NPs on GO by using sodium citrate as a stabilizing agent [49]. Nonetheless, the modification of GO leads to a notable loss in metal-support synergy, the difference may be noticed in from the TOF values, which varies from 195 to 5.8 min?1?mol?1 for Ag/GO [49] and Ag/rGO [50] catalysts, respectively. Such phenomena may be explained by the loss of active sites due to the detachment of Ag NPs from the support and their rapid agglomeration for modified GO. Indeed, the pre-reduction of GO led to the formation of a highly ordered graphitic surface [106], which could not be active enough to strongly attach Ag NPs. Besides, the Ag/rGO catalysts did not have a stabilizing agent on its surface, so the NPs in the reaction media may be easily agglomerated. In addition, Tian et al. find that the incorporation of N atoms to the rGO dramatically enhanced the Ag-support interaction and, consequently, the catalytic performance of the materials [50]. Unfortunately, the catalytic performance of Ag/rGO/CTN/Fe cannot be compared because the reagents molar ratio (4NP:NaBH4) is equal to 1; hence, the value of TOF may be influenced by the lack of hydrogen in the reaction media [52]. The catalytic performance of samples with zeolite supports depends on the ability of 4-NPi to be adsorbed, the mass transport limitations due to porosity and the activation of H2 on the metallic surface. TOF values summarized in Table 2 evidenced that the use of microporous structures as supports for the catalytic reduction of 4-NP must be carefully analysed. Most of the catalysts supported on microporous structures, listed in Table 2 were characterized with lower catalytic activity than those supported on mesoporous structures. It seems, that mass transport limitations are controlling the reaction velocity even more than the nanoparticle size. A comparison of the TOF values 1.45 min?1 ?mol metal for Ag (4.8 nm in nanoparticle size) supported on SBA15 mesoporous structure [48] with TOF value 0.61 min?1 ?mol metal for Ag supported BIF microporous ones [33] revealed the mass transport limitation over the nanoparticle size. Note, the microporous A4 zeolite was characterized with remarkable TOF value (134 min?1 ?mol metal) in comparison with the catalysts with BIF microporous structures, when Ag nanoparticles with 3 nm were deposited for both cases. The microporous to mesoporous modification of A4 zeolite resulted in superior TOF value (5.15 min?1 ?mol metal) than SBA15 mesoporous support. Thus, it seems that the support nature has noticeable influence on the catalytic performance of the samples with ordered structures. Indeed, the A4 zeolite is characterized by its H2 storage capability and efficient sorption for the nitroaromatic compounds [26,31]. The activity level of Ag NPs supported on zeolite type A was shown to be influenced by the Ag NPs size and the synthesis methodology applied (see Table 2). Wu et al. report the hydrothermal synthesis of 4A zeolite in the presence of Ag NPs deposited on carbon nanofibers. The carbon elimination by thermal decomposition leads to mesoporous 4A zeolite formation. As a result, Ag NPs obtained are internally dispersed inside the mesoporous 4A zeolite, denoted as Ag/P-4A-zeolite [35]. The TOF values for samples prepared in the present work were superior to that obtained for Ag/P-4A-zeolite (76 and 10 times for Ag/ZA4-MW and Ag/ZA4-IE, respectively). The superior TOF values of Ag/ZA4-MW and Ag/ZA4-IE microporous catalysts prepared in this work could be explained by the presence of fine Ag NPs.The application of microwave irradiation benefits the formation and stabilization of finely dispersed Ag species such as NPs (c.a. 3 nm), cations and clusters on the A4 zeolite under relatively soft conditions. The sample prepared under microwave irradiation, manifests a superior catalytic activity (TOF value of 396 min?1/?mol-metal) in the 4-NP reduction than reference systems reported in the literature. The use of microporous A4 zeolite as catalysts support resulted in a promotional effect for the catalytic reduction of 4-NP due to its H2 storage capability and high affinity for the nitroaromatic compounds sorption. The use of microwave irradiation offers a relatively ultra-fast and environmentally friendly method for the synthesis of multifunctional metallic NPs.Patricia Horta Fraijo acknowledges to the PROMEP, Mexico, scholarship. Brenda Acosta thanks ?C?tedras-CONACYT program?, Mexico, through project 767. Elena Smolentseva acknowledges SENER-CONACYT, Mexico (project 117373). This research project was partially funded by CONACYT, Mexico (project A1-S-45958), UASLP, Mexico (Grants No. C18-FAI-05-01.01 and C19-FAI-05-66.66) and DGAPA?PAPIIT (UNAM, Mexico) (grant 206920). In addition, CONACYT, Mexico, financed the acquisition of infrastructure for the UV?Vis absorption and catalytic measurements through the projects 299818 and 302286, respectively. Publisher Copyright: © 2020 Elsevier Inc.

PY - 2021/1

Y1 - 2021/1

N2 - In the present work, the synthesis of silver species such as nanoparticles (NPs), cations and clusters finely dispersed in A4 zeolite (Ag 0.1 wt%) has been successfully obtained by microwave irradiation and ion-exchange method. The prepared samples were analysed by SEM, HR-TEM, XRD, XPS, UV–Vis spectroscopy, ICP-OES and evaluated in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Physicochemical characterization of samples revealed that ion-exchange method resulted in the main formation of Ag NPs (c.a. 5 nm) while the microwave irradiation promoted the formation of Ag NPs (c.a. 3 nm), Ag cations and clusters as well. The crystalline structure of microporous A4 zeolite was not altered. The sample prepared under microwave irradiation manifested a higher catalytic performance (by 7.5 times in TOF value) in comparison with the reference sample obtained via ion-exchange method. The catalytic activity of the sample prepared by microwave irradiation (TOF value of 396 min−1/μmol-metal) was superior from reference systems reported in the literature. The later confirmed that used synthesis conditions influence the formation of highly active Ag species stabilized on the microporous structure of A4 zeolite. The high capacity for H2 storage and efficient sorption for the nitroaromatics on A4 zeolite resulted in a promotional effect for the catalytic reduction of 4-NP. Undoubtedly, the obtained results revealed that the selection in the conditions applied for microwave-assisted synthesis and the components of the catalyst (Ag and A4 zeolite) offers an ultra-fast method for the synthesis of highly active catalysts.

AB - In the present work, the synthesis of silver species such as nanoparticles (NPs), cations and clusters finely dispersed in A4 zeolite (Ag 0.1 wt%) has been successfully obtained by microwave irradiation and ion-exchange method. The prepared samples were analysed by SEM, HR-TEM, XRD, XPS, UV–Vis spectroscopy, ICP-OES and evaluated in the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). Physicochemical characterization of samples revealed that ion-exchange method resulted in the main formation of Ag NPs (c.a. 5 nm) while the microwave irradiation promoted the formation of Ag NPs (c.a. 3 nm), Ag cations and clusters as well. The crystalline structure of microporous A4 zeolite was not altered. The sample prepared under microwave irradiation manifested a higher catalytic performance (by 7.5 times in TOF value) in comparison with the reference sample obtained via ion-exchange method. The catalytic activity of the sample prepared by microwave irradiation (TOF value of 396 min−1/μmol-metal) was superior from reference systems reported in the literature. The later confirmed that used synthesis conditions influence the formation of highly active Ag species stabilized on the microporous structure of A4 zeolite. The high capacity for H2 storage and efficient sorption for the nitroaromatics on A4 zeolite resulted in a promotional effect for the catalytic reduction of 4-NP. Undoubtedly, the obtained results revealed that the selection in the conditions applied for microwave-assisted synthesis and the components of the catalyst (Ag and A4 zeolite) offers an ultra-fast method for the synthesis of highly active catalysts.

KW - 4-Nitrophenol reduction

KW - A4 zeolite

KW - Ag nanoparticles

KW - Ion-exchange

KW - Microwave-assisted synthesis

UR - http://www.scopus.com/inward/record.url?scp=85096381789&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85096381789&partnerID=8YFLogxK

U2 - 10.1016/j.micromeso.2020.110707

DO - 10.1016/j.micromeso.2020.110707

M3 - Article

AN - SCOPUS:85096381789

SN - 1387-1811

VL - 312

JO - Microporous and Mesoporous Materials

JF - Microporous and Mesoporous Materials

M1 - 110707

ER -

Ag nanoparticles in A4 zeolite as efficient catalysts for the 4-nitrophenol reduction

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