Synthesis and Properties of SBA-15 Modified with Non-Noble Metals

Modification of SBA-15 with non-noble metal leads to functional materials, which can be applied as gas sensors, adsorbents, and catalysts of various reactions. The new materials contain up to four various metals, which are deposited consecutively or simultaneously at various concentrations ranging from a fraction of 1% to an amount that is comparable with the mass of silica-support. These materials contain metals at various oxidation levels, usually as oxides, which occur in crystalline form (a typical crystallite size of about 10 nm matches the width of the SBA-15 channels), but in a few other materials, crystalline metal compounds have not been detected. Many researchers have provided detailed physicochemical characteristics of SBA-15 modified with non-noble metals by the means of various microscopic and spectroscopic techniques.


Introduction
This present review is devoted exclusively to non-noble metal-SBA-15 composites.These materials inherit periodic structures, high specific surface areas, and high thermal stabilities (up to about 700 • C) after SBA-15, and they show a wide variety of chemical properties that are inherited after the metallic components.Composites noble metal-SBA-15 were extensively studied, especially in the context of their catalytic properties.The noble metals were deposited on silica-only SBA-15 [1] or on non-noble metal-SBA-15 composites [2,3], but materials containing noble metals are outside the scope of the present review.There are specific problems in non-noble metal-SBA-15 composites, which need different approaches to noble metal-SBA-15 composites.For example, noble metals occur in an elementary (metallic) form, while non-noble metals occur in various chemical forms, which differ in the degree of oxidation of the metal (including but not limited to elementary metal), the degree of hydration, etc. Moreover on top of metal, oxygen, and hydrogen, composites with non-noble metals may contain other elements.Particular studies reported in the literature differ in many aspects, including the type of metal(s), the fraction of metal(s), the reaction being catalyzed, the availability of physico-chemical data obtained by different methods, etc.It is practically impossible to extract the effect of one variable from such a set of literature data.Impregnation with metal precursors often leads to a mixture of bulk metal oxides and silica, rather than to a real composite material.This is a real challenge in the synthesis of metal-SBA-15 composites, and various methods (e.g., addition of complexing agents) were used to avoid this problem.
The catalytic properties of SBA-15-metal composites were discussed (among many other materials) in several reviews.The catalysts containing non-noble metals, and various porous silicas (including SBA-15) are often considered as one class of materials.For example, Singh et al. [4] reviewed catalysts for reforming techniques.They especially emphasized the porous network of SBA-15 support as a factor limiting the growth of crystals of supported metal compounds.Very often, the catalysts containing noble and non-noble metals are considered as one class of materials [5].Usman et al. [6] reviewed catalysts of the dry reforming of methane, and discussed Ni-SBA-15 composites, among many other catalysts (chiefly Ni, Rh, and Pt supported on various oxides), and their study makes it possible to assess the influence of the support (SBA-15) on the catalytic activity, and to compare different supports.Ziolek and Sobczak [7] reviewed Nb-modified ordered silicas (including SBA-15) as supports of Cu, Ag, Au, and Pt, provided their physico-chemical characterization (XRD, X-rays diffraction, UV, ultraviolet-vis spectroscopy, H 2 -TPR, temperature-programmed desorption, FTIR, Fourier transform infrared spectroscopy, EPR, electron paramagnetic resonance), and discussed the role of the support in the catalytic activities of their catalysts (especially in oxidation of methanol).Debecker et al. [8] reviewed mesoporous mixed oxide catalysts, and they compared Ti-modified SBA-15 with several other catalysts.Akbari el al. [9] reviewed catalysts of oxidation reactions, and discussed Ni-, Cu-, Ga-, In-, W-, and Fe-SBA-15 composites among many other catalysts (metal oxides and composites thereof).
On top of catalysis, several other potential applications of non-noble metal-SBA-15 composites have been considered.Z1 = 2 et al. [10] summarized recent studies demonstrating the application of W-modified, Pd-and Sn-, Cr-and W-, and Ag-and Sn-modified SBA-15 (among other materials) as gas sensors (a broad range of target gases).Yang [11] applied Bi-modified SBA-15 as a material for the capture of iodine (especially of I-129) and its stable storage.A short section in this review is devoted to the adsorption properties of metal-modified SBA-15, but most of the presented results refer to their catalytic properties.

Materials and Methods
The chemical composition of the composites, preparation methods, and the chemical forms of the metals in the composites reported in the recent literature  are summarized in Table S1 (supplementary material).Each composite was given a unique code that is used in further discussions.The composites are ordered by their codes in the table.Our codes consist of the names of metallic elements and numbers, and they differ from the codes used for the same composites in previous publications.The list of the chemical elements used in the code does not fully describe the chemical composition of the composite, and only up to two elements are indicated in the codes, while certain composites in Table S1 contain three or even four metallic elements.An exhaustive list of metallic elements, in particular composites, is reported in the second column of Table S1.Most studies of SBA-15 metal composites report their quantitative compositions, but different authors express metal concentrations by different methods (mass ratio, molar ratio, etc.)In this study, we used a unified approach, and all concentrations were expressed in terms of Si-to-metal atomic (molar) ratios.The compositions reported in the original publications were re-calculated when necessary.These concentrations are reported in the third column of Table S1, and large numbers correspond to low metal concentrations.The preparation of the composites is briefly described in the fifth column.We only gave a short description: "one pot" for metals added in the course of synthesis of SBA-15, etc., and the metal precursor was specified.Long descriptions were avoided, and more details that can be found in the original literature cited in the last column are in the references therein.Empty cells denote that certain information was not applicable or not reported.
Most composites used as catalysts were obtained by the impregnation of the original SBA-15 with aqueous solutions of simple inorganic salts of certain metal(s), followed by calcination, and different metal concentrations in the composite were achieved by the adjustment of metal concentration in solution.A few composites were obtained by impregnation with solutions in non-aqueous solvents, or in the presence of complexing agents in solution.One-pot syntheses and CVD were far less popular than impregnation.
Composites based upon SBA-15 containing Ni (with or without other metals) were studied in recent publications more often than in composites containing other non-noble metals, followed by composites containing Co or Ce (with or without other metals), while composites without Ni, Co, or Ce were less popular.There was an obvious correlation between the catalytic properties of pure oxides of Ni, Co, and Ce, and catalytic properties of composites containing these oxides.The Si:metal atomic ratio in most catalysts was in the range of 5-30 in monometallic catalysts.In the catalysts containing more than one metal, the second (less abundant) metallic component was often added at very low concentrations (Si:metal atomic ratio > 100).Metals occurred in the form of oxides at various degrees of oxidation in most composites.Metallic Ni (pure or as an alloy with another metal) was found in a few composites.Spinel-type or perovskite-type mixed oxides were reported in a few composites.The formation of crystalline silicates was only observed in a few metal-SBA-15 composites containing Mg.
The methods used for the characterization of the composites (ordered by their codes), and the results obtained by nitrogen adsorption at its boiling point, and by XRD, are summarized in Table S2.The original acronyms used by the authors of cited papers are reported in Table S2, and sometimes one method is given various acronyms.The explanation of the codes of composites (detailed composition) can be found in Table S1.Most studies of SBA-15-metal composites report their specific surface areas, which are presented in the third column of Table S2.The SSA, specific surface area of original SBA-15 is given in the second column when available and applicable (e.g., there is no "original SBA-15" for one-pot synthesis).All SSA are rounded to 1 m 2 /g.The SSA is crucial as the property defining the ability of the composite to adsorb gaseous compounds.The size of crystallites and structures from XRD are reported in the fourth column.Several publications report the size of crystals of the metal compound from TEM, transmission electron microscopy (together with or instead of the size from XRD), but the size from microscopy is not reported in Table S2.In a few materials, the sizes from XRD and from TEM match, but in a few other composites, they do not.The size of the crystals has been emphasized as the parameter defining the catalytic activity of the composite.The fifth column of Table S2 shows a great variety of techniques other than SSA, and XRD.Actually, most abbreviations used in this review and summarized in the abbreviation list refer to various analytical techniques.Only a limited number of techniques were used in particular studies, and this can be explained by the limited availability of expensive equipment.Table S2 is a good illustration of two contradictory approaches used in catalytic studies.Several scientists prefer to experimentally study the catalytic activity of a series of catalysts in the reaction of interest first, and then follow this with physico-chemical characterization of the most promising specimens.The opposite approach is to provide detailed physico-chemical characterization of all specimens first, and then select promising specimens based on that characterization.
Table S2 shows that three to five different techniques (including SSA and XRD) were used in most studies to characterize particular composites.It should be emphasized that the term SBA-15 refers to a broad class of materials having a SSA in the range of 600-900 m 2 /g, and they also differ in their small-angle XRD patterns.Most composites had lower SSAs than the original SSA-15, but their SSAs were still very high (several hundred m 2 /g).A few composites showed exceptional behaviors.In Ce1, Ce3, Ce5, Ce11, Ce12, Ce13, and Ce15, the SSA was substantially higher than that of the original silica.These materials contained Ce as the only metallic component at very different concentrations (Si:Ce atomic ratios from 8 to 1000).In contrast La/Ni1, La/Ni2, and La/Ni3, had specific surface areas below 100 m 2 /g, that is, lower by an order of magnitude than the original silica.The sizes of the crystallites reported in the fourth column are often about 10 nm, and they match the diameters of the channels in the hexagonal network of SBA-15.This result confirms that SBA-15 confines the size of the crystals to the width of its channels.

Results and Discussion
In spite of their high SSAs, very few studies demonstrated possible application of non-noble metal-SBA-15 composites as adsorbents.A few examples are presented in Table 1.
Apparently, the ability of Ni-SBA-15 to adsorb hydrogen was the only adsorption property of the non-noble metal-SBA-15 composites, which has attracted a substantial amount of attention of scientists over the recent few years.This process was studied over a wide range of temperatures, ranging from the boiling point of nitrogen to the limit of thermal stability of SBA-15.• a series of catalysts used to study one reaction at the same conditions, • one catalyst used to study one reaction at various conditions (temperature, time of equilibration),

•
one catalyst used to study multiple reactions, and combinations thereof.The results are also presented in different formats in the original publications.In Table 2, we limited ourselves to the presentation of a few selected details.The reaction is briefly described in the second column, and the temperature t is given in the third column.In the studies performed at various temperatures with the same catalysts, the results obtained at each temperature are presented as a separate entry in the table.In parallel reactions (the same substrates give different products), two different approaches appear in the literature.In several papers, the overall conversion % is reported, followed by the selectivities for particular products (which add up to 100%).In other publications, the conversion percentages to certain products are reported, which may but not necessarily add up to 100%.In Table 2, we used the convention used by the authors of particular original papers.In the studies with various equilibration times, the conversion percentage in the fourth column is followed by the equilibration time in parentheses.
A broad variety of chemical reactions have been studied, as shown in the second column of Table 2.Many studies were performed at high temperatures (>700 • C), which are higher than the limit of long-term thermal stability for SBA-15.Although the presence of metallic elements may improve the thermal stability of SBA-15, the degradation of the catalyst and the change of its chemical and physical properties in the course of the catalytic reactions performed at >700 • C is very likely.Therefore, processes occurring at lower temperatures are more promising in the view of possible practical applications.Unfortunately, the temperature is critical for a substantial conversion rate in many reactions.For example, in the conversion of carbon dioxide to carbon oxide and hydrogen catalyzed by Zr/Ce2, the increase in the temperature from 620 to 750 • C improved the conversion rate from 58% to 100%.The problem is that the later temperature rise leads to degradation of the catalyst.Thus, we have a difficult choice between a high conversion rate and a long catalyst lifetime.This problem occurs in many other examples presented in Table 2; e.g., with the conversion of methane to hydrogen as catalyzed by Ni63 and Ni64.We would also like to emphasize that the results of thermal analysis may be misleading in the assessment of long-term thermal stability.Many publications report the results of fast scans (10 K/min or more), which only reflect short-term stability, and they substantially overrate the long-term thermal stability.Fortunately many reactions, e.g., of syngas to methane catalyzed by Ni20-Ni22, show a 100% or an almost 100% conversion at temperatures as low as 400 • C, when the SBA-15 support is stable against thermal degradation.The studies of the effect of the preparation method on the catalytic activity are rare.Two catalysts containing the same amount of Ca, and with very similar specific surface areas, obtained by the one-pot route on the one hand, and by wet impregnation on the other were studied, and the former was superior as a catalyst of reaction of diphenyl carbonate with isosorbide to poly(isosorbide carbonate) at 240 • C [18].Three series of catalysts containing the same amounts of Co and Mo obtained by wet impregnation with different solutions were studied, and the materials obtained by impregnation with solutions containing EDTA, ethylenediaminetetraacetic acid were superior as catalysts of reaction of hydrodesulfurization of dibenzothiophene at 300 • C, as compared with materials obtained by impregnation with solutions containing citrate or without complexing agents [37].All catalysts studied in [37] had similar specific surface areas, and they contained β-CoMoO 4 .Four catalysts containing similar amounts of Cu and Ni, obtained by wet impregnation on the one hand, and by precipitation with carbonate or with urea on the other, were studied as catalysts of conversion of cinnamaldehyde to hydrocinnamyl alcohol at 130 • C and of oxidation of carbon oxide to carbon dioxide at 160 • C [40].The catalyst obtained by precipitation with urea was superior to other catalysts in the oxidation of carbon oxide, and the catalyst obtained by precipitation with carbonate was more selective than other catalysts in the conversion of cinnamaldehyde to hydrocinnamyl alcohol.Interestingly enough, the efficient catalysts obtained by precipitation had lower SSA than less-efficient catalysts obtained by impregnation [40].Three catalysts containing similar amounts of Ni, obtained by different methods were studied as catalysts of conversion of carbon dioxide to carbon oxide and hydrogen at 500 and 600 • C, and of the conversion of methane to hydrogen at 500 and 700 • C [46].The catalyst obtained in mixed suspension with urea and ascorbic acid was more efficient than other catalysts in both reactions.Again, the most efficient catalysts had lower SSA than the less efficient catalysts [46].Four catalysts containing the same amount of Ni, obtained by wet impregnation in the presence and absence of different complexing agents were studied as catalysts of conversion of CH 4 to H 2 and of the conversion of CO 2 to CO, both reactions at 600 and 800 • C [56].The catalyst were equally efficient at 800 • C, and the catalyst obtained in absence of complexing agents was less efficient than catalysts obtained in the presence of complexing agents at 600 • C, although the effect was not very significant.Five series of catalysts containing the same amounts of Ni obtained by wet impregnation in the presence and absence of EDTA were studied as catalysts of conversion of naphthalene to tetralin on the one hand, and to cis-decalin on the other, at 300 • C [57].Tetralin was the main product with catalysts synthesized in absence of complexing agents, especially with materials with low Ni concentration, and cis-decalin was the main product, with catalysts synthesized in the presence of EDTA, especially with materials with high Ni concentration.The catalysts synthesized in the presence of EDTA had lower specific surface areas than their counterparts containing the same amount of Ni, but synthesized in absence of complexing agents.Three catalysts containing the same amounts of Ni and Ce, obtained by wet impregnation at different conditions (ultrasounds, reflux) were studied as catalysts of methane reforming with CO 2 for hydrogen and syngas production at 600 and 750 • C [24].The catalyst obtained by reflux was superior to the other catalysts, although the effect was not very significant.

Conclusions
Non-noble metal-SBA-15 composites have been extensively studied as compared with their counterparts, based on other types of ordered mesoporous silicas (MCM-41, FSM-16, KIT-6, etc.).In spite of their high specific surface areas, their potential applications as adsorbents have attracted little attention.In contrast, their catalytic properties were demonstrated in many studies.Composites containing Ni (with or without other metals) are especially promising as catalysts of numerous redox reactions.Not much work was done on the effect of the preparation method on the catalytic properties.Very likely, catalysts having the same composition as the materials already examined, but prepared in different ways (e.g., by wet impregnation in the presence of complexing agents) may show superior catalytic activities to the results in reported in Table 2. Further studies in this direction are greatly desired.

Table 1 .
SBA-15 and SBA-15-metal composites as adsorbents.Several examples of catalytic activity of SBA-15-metal composites are presented in Table2.A few authors studied the catalytic activity of the original SBA-15 (no metal added) as a reference.Silica-only SBA-15 are presented in the upper part of Table2with codes None (none is for no metal added).The other materials are ordered by their codes, as in the other tables.Different authors have used different research strategies:
* Error in the original paper.