A General Silica-Templating Synthesis of Alkaline Mesoporous

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A general silica-templating synthesis of alkaline mesoporous carbon catalysts for highly efficient HS oxidation at room temperature 2

Zixiao Zhang, Wuyou Jiang, Donghui Long, Jitong Wang, Wenming Qiao, and Licheng Ling ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13597 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 1, 2017

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ACS Applied Materials & Interfaces

A general silica-templating synthesis of alkaline mesoporous carbon catalysts for highly efficient H2S oxidation at room temperature Zixiao Zhang, Wuyou Jiang, Donghui Long∗, Jitong Wang, Wenming Qiao, Licheng Ling∗

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

ACS Paragon Plus Environment


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ABSTRACT: A general synthesis of alkaline mesoporous carbons (AMCs) is developed based on a simplified silica-templating method for room-temperature catalytic oxidation of H2S. The key to the success relies on dissolving the silica templates to create the inter-connected mesoporous structure as well as leaving parts of the alkaline products in the pores, both of them are prerequisites for H2S oxidation. By adjusting the alkaline etching degree and organic/inorganic ratio, the porosity and basicity of the AMC could be simultaneously tuned, allowing the AMCs direct use for H2S catalytic oxidation with an unprecedented removal capacities of 4.49±0.12 g/g. Such excellent catalytic performance should be attributed to the developed pore structure that stores the product sulfur, and the strong basicity that promotes the dissociation of H2S into HS- ions. Moreover, this simplified silica-templating method could be easily extended to the preparation of various silica templated mesoporous carbon catalysts. All these AMCs demonstrate a successful combination of low cost with high performance, which may well be the answer for the technical development of industrial H2S removal. KEYWORDS: mesoporous carbon, silica template, NaOH etching, catalytic oxidation, H2S removal


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1. Introduction Mesoporous carbons have attracted increasing attention because of their remarkable properties such as tunable textural structure, good electrical conductivity, and high mechanical stability1-3. These features make them very promising in various applications including adsorption4, purification5, catalysis6 and energy storage7. The most specific advantages over microporous carbons (i.e. activated carbons) are their easy accessibility of larger molecules to the inner body, which depends on the pore size and pore connectivity8-9. In the past two decades, great efforts have been made for the synthesis of mesoporous carbons with controllable structure and tunable pore size10-12. Particularly, templating methods involving hard or soft templates are popularly adopted due to the faithful control in pore size by simply adjusting the size of the templates13-14. Soft-templating method takes advantage of self-assembled surfactants as the pore-forming agents, which could directly yield mesoporous carbons with ordered structure15. However, the relatively small pore size and low pore volume usually limit their applications in catalysis and adsorption where larger pores and higher pore volume are necessary. On the other hand, hard-templating pathway has been succeeded in introducing higher pore volume as well as larger pore by reverse replication of the pre-formed porous silica templates16. Since the Knox’s group firstly reported the hard-templating method in early 1980’s17, a variety of mesoporous carbons have been successfully synthesized using various silica templates, such as SBA-series ordered mesoporous silica18-19, hierarchal porous silica20, silica sol21-22, etc. Nevertheless, the hard-templating method still suffers from apparent drawbacks due to the complicated preparation procedures, which generally involve three steps: (a) preparation of



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the nanostructured silica templates (e.g., colloidal silica and porous silica), (b) preparation of the silica/carbon composites, and (c) etching of the templates with HF or NaOH solution23-24. Apparently, the etching step is not environmentally friendly because it requires corrosive regents and repeated washing, which cause large amount of acid or alkaline wastewater that needs to be carefully treated. As a result, the complicated synthetic processes inevitably increase the total cost and hinder the large-scale preparation of mesoporous carbons for practical application. So far, it is still a big challenge to prepare low-cost mesoporous carbons with developed porosity and relatively large pores for industrial application. Hydrogen sulfide (H2S), as a highly toxic, corrosive and odorous pollutant, mainly originate from industrial operations such as coke ovens, sewage treatment, natural gas processing and petroleum refining25-26. The removal of H2S is greatly necessary for pollution control and operation safety within industrial processes. Among the developed methods for H2S removal, selectively catalytic oxidation of H2S to element sulfur is recognized as an effective way for dilute H2S-containing air steam, due to its low capital cost, high operation flexibility

and

environmental

friendliness27-32.

Moreover,

this

method

could

thermodynamically reduce H2S to sub-parts-per-million level even at room temperature. Our recent works showed that the alkaline mesoporous carbons (AMCs), prepared by the wet-impregnation of alkaline reagents into mesoporous carbon aerogels, possessed excellent catalytic performance towards H2S room-temperature oxidation with the breakthrough capacities of ~2 g H2S/g catalyst, almost 3-5 times higher than commercial activated carbon-based catalysts33. The alkaline impregnants have been confirmed as the promoters, which could greatly accelerate the dissociation of H2S into HS- ions and the consequent


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oxidation of HS- ions to elemental sulfur34. The high H2S removal capacities of AMCs could lead to less downtime and low replacement cost. If their preparation cost could be further reduced, these AMCs should stand up to real-world H2S applications. Herein, we demonstrate a facile silica-templating method to directly synthesize low-cost alkaline mesoporous carbons for highly efficient H2S catalytic oxidation. The synthesis strategy is the NaOH etching of the carbon-silica templates followed by filtration and drying, thus, eliminating the repeated washing and caustic impregnation steps. The obtained AMCs possess inter-connected mesoporous system replicated from the silica templates, as well as strong basicity due to the alkaline residuals. Therefore, these AMCs could be directly used as the catalysts for H2S oxidation, which exhibit outstanding H2S removal performance. This simplified silica-templating strategy has good generality and can be suited to various kinds of silica templates. The resultant AMCs demonstrate a successful combination of low cost with high performance, which may well be the answer for the technical development of industrial H2S removal.

2. Experimental 2.1 Preparation of the carbon-silica composites The

carbon-silica

composites

were

prepared

by

a

sol-gel

method

using

resorcinol/formaldehyde as the carbon precursors and colloidal silica as the hard templates, similar to our previous report35. In a typical synthesis, 9.7 g of resorcinol and 14.3 g of formalin (37 wt.% formaldehyde) were dissolved in 20 g deionized water. Then 70 g of colloidal silica sol (LUDOX SM-30, 30 wt.% SiO2) was added into above solution under



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stirring and the solution was diluted to 100 mL. After pre-polymerization at 40 oC for 1 h, the mixture was transferred to a sealed bottle and heated at 80 oC for 72 h. The carbon-silica composites were obtained after dried at 80 °C followed by pyrolysis at 800 °C for 3 h. The as-prepared carbon-silica composites were denoted as C/SiO2. In addition, a series of carbon-silica composites were prepared by adjusting the ratio of carbon precursors and colloidal silica to 0.5 and 1, respectively. Other carbon-silica composites, using ordered porous silica (SBA-15) and hierarchical porous silica (HPS) as the templates, were also prepared according to literatures18, 20. 2.2 General synthesis of the AMCs The alkaline mesoporous carbons (AMCs) were prepared by a simple NaOH etching method. In a typical preparation, 1 g of carbon-silica composites with particle size of 70-200 mesh were immersed into 20 g of NaOH solution (10 wt.%), and then the mixture were transferred to a sealed bottle and heated at 80 oC for 24 h. The samples were directly filtrated without any washing and dried at 120 oC for 10 h. The resultant alkaline mesoporous carbon catalysts were designated as AMC-10%. Also, NaOH solutions with different concentrations (5, 15 and 20 wt.%) were used to etch the carbon-silica composites. After exhaustion for H2S oxidation, a letter S was added to the sample name. 2.3 Characterization The morphologies of samples were observed under scanning electron microscopy (SEM, JEOL 7100F) and transmission electron microscopy (TEM, JEOL 2100F). The SEM mapping was observed under scanning electron microscopy (SEM, FEI Q-300). The surface composition of the catalysts was obtained from an Axis Ultra DLD X-ray photoelectron


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spectroscopy. The X-ray source was operated at 15 kV and 10 mA. The working pressure was lower than 2 × 10−8 Torr (1 Torr = 133.3 Pa). Nitrogen adsorption/desorption isotherms were measured at 77 K with a Quadrasorb SI analyzer. Before the measurements, the samples were degassed in vacuum at 473 K for 12 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area. The pore size distributions were derived from desorption branch by using the Barrett-Joyner-Halenda (BJH) model. Thermogravimetry (TG) analysis (TA Instrument Q600 Analyzer) of samples was carried out in air at flow rate of 100 mL/min. The samples were heated to 800 oC with a rate of 10 oC /min. The pH values were measured using S20 Seven Easy pH meter (METTLER TOLEDO) by adding 0.4 g of sample into 20 mL of deionized water and stirring for overnight to reach the equilibrium. 2.4 Desulphurization experiment Dynamic tests were carried out to evaluate capacities of the catalysts for H2S removal. 0.15 g of the samples were packed into a glass column (inner diameter of 8 mm). A simulated mixture (relative humidity 80% at 25 oC) containing 0.1% (1000 ppm) of H2S, 1% of O2 and balanced N2 was passed through the column of catalysts with a flow rate of 150 mL/min. The gas flow rates were controlled by a mass flow controller system (Aalborg GFM17A). The reaction temperature was controlled by a K-type thermocouple in the furnace and monitored by another K-type thermocouple axially centered in the reactor tube. The inlet and outlet gas (H2S) was analyzed by gas chromatograph (Shimadzu, GC2010) equipped with a flame



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photometry detector (FPD) permitting the detection levels as low as 0.5 ppm. In this test, the breakthrough concentration of H2S was defined as 50 ppm. The breakthrough and saturation capacities (g H2S/g catalyst), referred as QB and QS, respectively, were calculated by integration of area above the breakthrough curve, and from the H2S concentration in the inlet gas, flow rate, breakthrough time and the mass of material. 3. Results 3.1 Characterization of AMC

Scheme 1. Direct silica-templating synthesis of AMC

Owing to its facileness and low cost, the colloidal silica templating has been widely applied to prepare mesoporous carbons with high porosity21-22. Herein, we exemplify this method to illustrate the direct synthesis of the AMC catalysts for H2S oxidation, as shown in Scheme 1. Typically, the carbon-silica composites (C/SiO2) are prepared via a sol-gel process of resorcinol-formaldehyde and silica sol, followed by ambient drying and carbonization. The


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obtained C/SiO2 are immersed in NaOH solution at 80 oC to dissolve the silica templates. Then, the alkaline mesoporous carbons, denoted as AMC-x% (x% represents the NaOH concentration), are directly obtained by the simple filtration and drying.

Figure 1. TG curves of AMCs etched by different NaOH concentration and AMC-10%-rw.

Since no washing is performed, large amounts of alkaline compounds consists of etching product of SiO2 as well as excess NaOH should be retained within the as-obtained materials. Figure 1 shows the TG results of AMCs in air flow, which demonstrate 31.4 - 51.8 wt.% of ash contents, showing an increase with increasing the NaOH concentration from 5% to 20%. It should be noted that these ashes are mostly soluble, since they could be almost removed by further washing, as 0.2 wt.% ash determined for pure mesoporous carbons obtained by repeated washing (AMC-10%-rw). Such high content of soluble ashes, possibly in the form of NaOH and Na2SiO3, should contribute to strong basic character of AMCs. To confirm it, the basic character of the samples is measured by adding 0.4 g of dried samples into 20 mL of deionized water. The measured pH value (Table 1) increases from 9.7±0.2 to 12.3±0.2 with the increase of NaOH concentration, due to more alkaline residuals caused by the latter.



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Figure 2. N2 adsorption-desorption isotherms at 77 K (a) and resultant BJH pore size distribution curves (b) of AMCs etched by different NaOH concentration and AMC-10%-rw.

The pore structures of these AMCs are analyzed by N2 adsorption (Figure 2) and the resultant pore parameters are also listed in Table 1. All the AMCs show typical type II isotherms with broad hysteresis loops, indicating the wide distribution of pore size. At the low P/P0 region, the increases of N2 sorption could be attributed to the strong capillary condensation of N2 gas in small mesopores. And the broad hysteresis loops located at P/P0 of 0.5-0.9 are corresponded to the mesostructured characteristics. While the sharp increases of the adsorption quantity at P/P0 near to 1.0 reveal the existence of macropores within AMCs.


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The BET surface area and pore volume decrease from 307±4 to 133±4 m2/g and 0.66±0.01 to 0.24±0.01 cm3/g, respectively, with the increase of NaOH concentration from 5% to 20%. As the same pristine carbon-silica composite is used, the decreased porosity should be due to more alkaline ashes retained in the porous structure. On the contrast, the pure mesoporous carbons obtained by repeated washing (AMC-10%-rw) show much higher BET surface area (683±5 m2/g) and larger pore volume (1.61±0.02 cm3/g).

Figure 3. SEM (a), TEM (b) and SEM elemental mapping (c) images of AMC-10%.

The typical pore morphology of the AMC is further observed by SEM and TEM images, as shown in Figure 3 a and b. Even with 37.2 wt.% of residual of alkaline ashes, the AMC possesses obviously a developed 3-D porous network comprised of interconnected mesopores and macropores. Here the mesopores should be replicated from the single silica nanoparticle,



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while the large mesopores and even macropores may originate from the aggregation of silica nanoparticles. Furthermore, the SEM elemental mappings in Figure 3c indicate that the sample contains plenty of Na and Si species (7.5 and 4.2 wt.%, respectively), which are homogenously dispersed into the mesoporous network. The molar ratio of Na/Si is about 2.17, indicating the co-existing of NaOH and Na2SiO3.

Table 1. Porosity parameters and sulfur capacities of the samples pH

SBETa

Vtb

Dc

Q Bd

QSe

value

(cm2/g)

(cm3/g)

(nm)

(g H2S/g)

(g H2S/g)

AMC-10%-rw

6.8±0.1

683±5

1.61±0.02 16±0

< 0.01

< 0.01

AMC-5%

9.7±0.2

307±4

0.66±0.01 16±0

0.65±0.03

1.15±0.05

AMC-10%

10.4±0.2

232±4

0.51±0.01 13±0

1.80±0.06

2.28±0.08

AMC-15%

11.3±0.2

149±4

0.34±0.01 12±0

1.83±0.06

2.83±0.09

AMC-20%

12.3±0.2

133±4

0.24±0.01 10±0

1.87±0.05

3.25±0.09

AMC-0.5-10%

10.9±0.2

180±5

0.41±0.01

9±0

2.65±0.09

4.49±0.12

AMC-1-10%

9.9±0.2

389±5

0.97±0.01 12±0

1.01±0.04

1.64±0.06

AMC-SBA-10%

11.6±0.2

30±1

0.13±0.01 17±0

0.55±0.02

1.75±0.06

AMC-HPS-10%

10.9±0.2

239±4

0.51±0.01

2.07±0.09

3.67±0.012

Samples

a

9±0

BET specific surface area. b Total pore volume (P/P0=0.985). c BJH desorption average pore

size. d Breakthrough sulfur capacity. e Saturation sulfur capacity. The data are expressed as the average values and the standard errors are less than 5%.

3.2 The catalytic performance over AMCs


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Figure 4. (a) H2S breakthrough curves over the AMCs etched by different NaOH concentration, AMC-10%-rw and commercial catalyst. (b) The effect of NaOH concentration on the gravimetric capacity (columns) and volumetric capacity (line). The data are expressed as the average values and the standard errors are less than 5%.

The catalytic oxidation of H2S over these AMCs is performed in a packed-bed reactor under atmospheric pressure at 25 oC. And a commercially available activated carbon catalyst (3018-JT, Dalian Purtec Chemical Technology Co.) is used as reference. The breakthrough curves are plotted in Figure 4a and the resultant H2S breakthrough and saturation capacities are listed in Table 1. All the AMCs show long periods during which no H2S is detected in the outlet gas, demonstrating that all of the H2S is captured by the AMCs and converted into oxidation products on the active sites. After breakthrough, the breakthrough curves steadily



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increase to 1000 ppm, which should be due to the gradual deactivation of the AMCs caused by more oxidation products generated and covered on the active sites. Even though the shapes of the breakthrough curves are similar, the less steep curve obtained at higher NaOH concentration indicates the slower deactivation rate. In addition, no other tail gases such as SO2 and COS are detected during the catalytic process, demonstrating high selectivity for oxidation of H2S to elemental sulfur. In a controlled experiment, pure mesoporous carbons with repeated washing are also evaluated, which show an instant breakthrough of H2S with negligible removal capacity (

A General Silica-Templating Synthesis of Alkaline Mesoporous

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