Hongwei Shan,Xiaodong Zhou,Hao Jiang,Yanjie Hu,Haibo Jiang,*,Chunzhong Li,*

1 Shanghai Engineering Research Center of Hierarchical Nanomaterials,Key Laboratory for Ultrafine Materials of Ministry of Education,School of Chemical Engineering,East China University of Science &Technology,Shanghai 200237,China

2 School of Materials Science and Engineering,East China University of Science &Technology,Shanghai 200237,China

3 State Key Laboratory of Chemical Engineering,School of Chemical Engineering,East China University of Science &Technology,Shanghai 200237,China

Keywords:Water glass Silica powder High pore volume Skeleton reinforcement Spray drying

ABSTRACT In this paper,a method composed of gelation of basic skeleton (first step) and skeleton reinforcement process (second step) was introduced to synthesize silica powder with high pore volume through the reaction between water glass and sulfuric acid.No organic solvents were involved in the entire preparation process and the final product was collected by spray drying.The effect of concentration of base solution,gelation point pH value and skeleton reinforcement time on the BET specific surface area and pore volume of the prepared silica powder were investigated intensively.The results show that,a basic skeleton with good dispersibility and high porosity was obtained when the concentration of base solution was0.1 mol·L-1 and the gelation pH value reached 6.5.Then the basic skeleton grew into a more uniform porous structure after 30 min skeleton reinforcement.Under these optimum conditions,silica powder prepared by skeleton reinforcement method had a BET specific surface area of 358.0 m2·g-1,and its pore volume reached 2.18 cm3·g-1,which was much higher than that of prepared by skeleton-free method(1.62 cm3·g-1) and by direct gelation method (0.31 cm3·g-1).

1.Introduction

Nowadays,silica powder becomes an important ingredient in many fields because of its low density,high pore volume,large specific surface area,high porosity,low refractive index and ultralow dielectric constant [1,2].Among these particular properties,silica powder with high pore volume can be used as matting agent,coating additive,adsorbent,thermal insulation aerogel,catalyst and drug carrier [3-8].

The sol-gel technique proposed by St?ber was a fundamental and important way to synthesize silica materials with high pore volume [9-11].Silicon alkoxide,such as tetraethyl orthosilicate(TEOS),was often used as precursor because of the high purity of final products and ethanol as byproduct [12,13].After adding acid or alkali catalysts,sol-gel process was carried out with two parts,namely hydrolysis and condensation reactions.Liet al.[14]tried to use TEOS and methyltrimethoxysilane(MTMS)as co-precursors to prepare silica aerogels,in which NH4OH concentration was adjusted as an important processing parameter.When NH4OH concentration reached 0.5 mol·L-1,the product had a specific surface area of 635.41 m2·g-1and the pore volume was 0.89 cm3·g-1.However,using silicon alkoxide such as TEOS as a precursor also has some disadvantages,such as expensive cost,hypotoxicity,and huge demand of organic solvents during the production process [15].In addition,due to the steric hindrance effect,the rate of organic silicon hydrolysis is low,resulting in a longer process cycle,which is not conducive to the large-scale industrial production [16].

Besides silicon alkoxide,water glass is another kind of common silicon precursor and its main component is sodium silicate.Although water glass isn’t as good as silicone in terms of purity,it’s much cheaper and reacts more quickly with acid,which is more popular used in industry.Heet al.[17]used water glass to prepare high-performance silica aerogel and also studied the effect of heat treatment temperature on the thermal conductivity and bulk density.Filipovicet al.[18] used water glass and TEOS as silicon sources,respectively,to prepare silica under the same conditions.Results show that processing parameters such as concentration,temperature and reaction time had different effects on the pore structure.Moreover,under identical conditions,water glassbased silica had a larger pore volume than TEOS-based silica.Huanget al.[19] used water glass,EtOH,HCl and HMDSO(hexamethyldisiloxane) as raw materials to prepare silica with a pore volume of 4.42 cm3·g-1and BET specific surface area of 855 m2·g-1.EtOH,as a nonignorable role of solvent,led to a decrease of density and an increase of BET (Brunauer,Emmett and Teller)specific surface area and pore volume to some extent.While,HMDSO and HCl were utilized to modify the gel surface.

In recent years,some researchers have also tried to use microreactors to synthesize silica materials.By using water glass and sulfuric acid as raw materials,Zhanget al.[20] combined membrane dispersion microreactor with a stirred tank to synthesized silica powder.The pore volume of the final product reached 1.96 cm3·g-1and its specific surface area was 476.1 m2·g-1.With an effective mass transfer ability,the membrane microreactor created a relatively more uniform mixing of materials,thereby increasing supersaturation during the preparation of silica and shortening gelation time simultaneously.However,considering the filtration membrane with micron-sized pores as the dispersion medium is liable to cause blockage on account of the increase in solution viscosity during silica preparation.On the other hand,because of the surface tension of pore fluid,the pore structure of silica will be destroyed during the drying process,which can result in the decrease of pore volume [21].Both solvent exchange before drying process and supercritical drying method are effective solutions to this problem,which can greatly reduce the effect of surface tension,and silica with high pore volume finally can be prepared [22,23].Nonetheless,the complex process coupled with hazardous operating environment conditions and high costs has limited its application in mass production.Therefore,searching for a steady and environmental-friendly method for the preparation of high pore volume silica is always a concern for both industry and academia.

In our study,sulfuric acid and water glass were chosen as raw materials and the entire preparation process didn’t involve any organic solvents.Instead of supercritical drying technique and solvent exchange,the collapse of silica skeleton caused by the drying process was reduced by the skeleton reinforcement process.The basic skeleton with more pores was generated after the first step and the second step enabled a reinforced skeleton to prevent the structure disruption during a spray drying process.Influences of concentration,pH value and skeleton reinforcement time on the pore structure of final products were also studied.Through this work,we hope to provide a more efficient and low-cost process without organic solvents for the industrial production of silica materials with high pore volume.

2.Experimental

2.1.Silica preparation by skeleton reinforcement method

In order to obtain silica powder both with strengthened skeleton and high pore volume,a two steps process was introduced and the experimental procedure was shown in Fig.1.The first step was to prepare silica gel by using diluted water glass(Na2O·3.3SiO2) solution and sulfuric acid.As the base solution,diluted water glass solution with different concentration was kept stirring at 45 ℃,1.0 mol·L-1sulfuric acid was added into the base solution at a determined flow rate 1.0 ml·min-1.When the pH value of the base solution dropped to a certain range,the sulfuric acid flow was cut off immediately.After a certain aging time,the silica gel was obtained and treated as a basic skeleton in the following stage.

Then the temperature was raised from 45 ℃to 70 ℃with agitation.When the gel was completely dispersed and the solution exhibited good fluidity,skeleton reinforcement process was carried out:0.50 mol·L-1diluted water glass solution (in terms of Na2O) and 0.80 mol·L-1sulfuric acid were added simultaneously.The flow rate of the diluted water glass solution was fixed at 4.2 ml·min-1and the flow rate of sulfuric acid was adjusted to keep the pH value of the solution at 9.Different feeding time was conducted to explore the influence on the skeleton reinforcement.When the time was up,the water glass flow was firstly cut off and sulfuric acid was continuously added until the pH value dropped to about 4.Then the mixture solution was aged for 10 minutes,vacuum-filtered and washed with distilled water until the filtrate was free of.The filter cake was dispersed into water and formed slurry with a solid content of nearly 7% (mass).After agitating for 30 minutes,the slurry was dried by a spray dryer with an inlet air temperature of 160 ℃.Finally,dried silica powder was gained in a collection bottle.

2.2.Silica preparation by two other methods as contrast experiment

Direct gelation method was conducted by using 0.10 mol·L-1water glass solution (in terms of Na2O) as the base solution,and 1.0 mol·L-1sulfuric acid was added into it at a flow rate of 1.0 ml·min-1.The whole process was carried out under agitation at 45 ℃.When the pH value of the solution dropped to 6.5,the sulfuric acid flow was stopped,and the silica gel was obtained after aging for 40 minutes.Then the gel was dispersed,washed and spray-dried to achieve silica powder.

Skeleton-free method was different from skeleton reinforcement method because of the lack of formation of the basic skeleton.0.80 mol·L-1sulfuric acid was added into 0.10 mol·L-1water glass base solution with agitation to reduce pH value to 9.Then 0.50 mol·L-1water glass(4.2 ml·min-1)was simultaneously added together with sulfuric acid to fix the pH at 9.After 30 minutes,water glass was stopped adding and sulfuric acid was fed to acidify the solution to pH=4.The following treatment processes were consistent with that of the skeleton reinforcement method.

2.3.Characterization

Fig.1.Experimental procedure of skeleton reinforcement method.

The pH value was measured during the synthesis of silica powder by using a pH electrode (LE438,Mettler Toledo).A transmission electron microscopy (TEM;JEM-1400,Japan) was employed to characterize the size of primary particles and the morphology of silica skeleton.The porous structure was observed by using a field emission scanning electron microscopy(FE-SEM;GeminiSEM 500,Germany).Specific surface area (based on BET method) and pore volume(based on BJH(Barrett,Joyner and Halenda)method)were collected by ASAP 2460 nitrogen adsorption-desorption apparatus (America) at 77 K.Especially,during the skeleton reinforcement process,the reaction solution was extracted at a certain interval and each sample was divided into two parts after being washed.One part was directly used in the TEM(transmission electron microscope)characterization,the other part was freeze-dried into powder for nitrogen adsorption-desorption subsequently.FTIR(Fourier transform infrared spectroscopy)spectra were recorded by Nicolet is50 FT-IR spectrometer (America) from 4000 cm-1to 400 cm-1.The XRD (X-ray diffractomer) data was measured over the 2θ range of 10°-80°viaa Rigaku D/Max X-ray Diffractometer(Japan).

3.Results and Discussion

3.1.Influences of the concentration of the base solution on the pore structures of silica

When preparing the basic skeleton,the concentration of the base solution was studied as a factor which had a great influence on the pore properties of the final silica powder and the results were listed in Table 1.As the concentration of the base solution increases from 0.05 to 0.20 mol·L-1,the specific surface area of the final powder also gradually raises from 272.2 to 439.0 m2·g-1,while the pore volume shows a trend of first increase and then decrease.Sample 2 shows the maximum pore volume of 2.18 cm3·g-1,when the base solution concentration reaches 0.10 mol·L-1.Fig.2 depicts the morphology of the basic skeleton prepared with different concentrations of the base solution.A fragile network skeleton with a few crosslinkage is formed at a very low concentration,as shown in Fig.2(a) and 2(b).When the concentration increases,the number of primary particles in TEM images explodes significantly and the network structure becomes much denser.However,if the concentration is too high,primary particles seriously agglomerate with each other and larger clusters are more favorable to generate as shown in Fig.2(f)and 2(h).It can also be known from Fig.2(d)that a more uniform pore structure is formed between primary particles when the concentration of the base solution is 0.10 mol·L-1.

Table 1BET specific surface area and pore volume of silica of various base solution concentration

The concept of supersaturation,which is connected with the concentration of reactants,plays a critical role in the precipitation reactions.Supersaturation is non-linearly related to the nucleation rate [24],which means when the system is at high supersaturation,explosive nucleation will appear in the solution,that is,nucleation is completed in an instant.Therefore,as the supersaturation improves,the number of primary particles in Fig.2 rises.The subsequent skeletal strengthening process is dominated by the heterogeneous growth mode of nucleation.Thus,the more nuclei burst in the first step,the less adequate particle growth will be in the second stage.This leads to a reduction in the size of the primary particles and an increase in the specific surface area of the final products.On the other hand,appropriately increasing the concentration of the base solution can accelerate the condensation rate of silicic acid.Then gel particles aggregate with loose structure can be obtained which is beneficial to the generation of an enhanced and porous structure after the skeleton reinforcement process,thereby improving its pore volume.However,when the concentration of the base liquid is too high,the connection points between gel particles grow in number,which leads to a dense network skeleton with inconspicuous pore structure and thus causing a decrease in pore volume.

3.2.Influences of the gelation point pH value on the pore structures of silica

The influence of pH value at gelation point on the microstructure parameters of basic skeleton were shown in Table 2 and Fig.3.When the final pH value of the first step increases,in other words,the prepared silica gel changes from acidic to alkaline(maintaining the skeleton reinforcement process unchanged),it has little impact on the specific surface area of the final powder,nearly 365 m2·g-1.However,the influence on the pore volume should be discussed in two aspects.If the silica gel synthesized in the first step is acidic (sample 5) or weak acidic (sample 6),the pore volume doesn’t change much.Otherwise,the pore volume of the final products will gradually diminish along with the higher degree of alkalinity.On the other hand,gelation time also changes with different pH values and the overall trend is similar to the Ntype curve mentioned in previous literature [25].At pH=6-7,the shortest gelation time is due to the highest rate of silicic acid polymerization.It can be seen that,setting the gelation point pH at 6.5 makes more sense for industrialization than gelation at pH=4.

Table 2BET specific surface area and pore volume of silica of various gelation point pH

The network skeleton of silica gel prepared at different final pH value was shown in Fig.3.When the final pH of the first step is less than 7,as shown in Fig.3(a) and 3(b),their network skeletons are similar,in which primary particles are well dispersed and agglomeration phenomenon can hardly be seen.Moreover,obvious porous structures are generated in both cases,which lays a solid foundation for subsequent skeleton reinforcement,leading to the final products with higher pore volume.In Fig.3(c) and 3(d),by contrast,primary particles gradually tend to form larger aggregates,which makes the porous structure less conspicuous and the network skeleton denser.The reason for this is that the solution is alkaline at the end of the first stage,and a part of raw materials haven’t been fully consumed.Further accumulation and dissolution take place on the particle surface and pores during the following aging process,giving rise to an increase in the number of connection points between particles.At last,proceeding skeleton reinforcement based on such a network skeleton causes the final silica powder to diminish in pore diameter and volume.

SEM images of the final silica powder correspond to the results of basic skeleton TEM images,BET specific surface area and pore volume,which can be seen from Fig.4.At pH=4.0 and 6.5,the internal three-dimensional network structures of the final silica powder are both highly developed and well dispersed.Clear and uniform porous structures are formed between silica particles,which also contribute to a relatively high pore volume(2.20 cm3·g-1).However,as shown in Fig.4(c) and 4(d),silica particles tend to agglomerate together and porous structures gradually begin to disappear,which causes a reduction in the pore volume of the final products.

Fig.2.TEM images of basic skeleton of various base solution concentration at different magnifications:(a),(b)0.05 mol·L-1;(c),(d)0.10 mol·L-1;(e),(f)0.15 mol·L-1;(g),(h)0.20 mol·L-1.

Fig.3.TEM images of basic skeleton of various gelation point pH:(a) pH=4.0;(b) pH=6.5;(c) pH=8.0;(d) pH=9.0.

3.3.Influences of the skeleton reinforcement time on the pore structures of silica

By tuning simultaneous feeding time in the second stage,the influence of the skeleton reinforcement process on the microstructure of silica particles were also studied.It can be seen from Fig.5a and 5b that the basic skeleton has a uniform pore structure and particles are also well dispersed.However,due to the small size of primary particles,the strength of network skeleton isn’t so high as to resist the shrinkage of porous structure during the drying process,so that its pore volume can be as low as 0.42 cm3·g-1,but the specific surface area reaches 592.3 m2·g-1,as shown in Table 3.

Table 3BET specific surface area and pore volume of silica during skeleton reinforcement process

Fig.4.SEM images of final silica powder of various gelation point pH:(a) pH=4.0;(b) pH=6.5;(c) pH=8.0;(d) pH=9.0.

At the early stage of the skeleton reinforcement process,in Fig.5(d) and (e),there still exist several rod-like particles with a low degree of sphericity.Furthermore,both particle size and the contact area between these particles start to increase gradually with the continuous feeding process.A reduction in the specific surface area (from 592.3 to 496.6 m2·g-1) can also be intuitively observed from the data,along with a rise in the pore volume(from 0.42 to 1.10 cm3·g-1)and the average diameter of primary particles(from 4.4 to 5.6 nm).When the skeleton reinforcement processes to about 20 minutes,almost all particles are spheroidal and the average particle diameter reaches 7.0 nm,which shows a certain degree of particle growth.During this time,primary particles gradually aggregate to form larger clusters,but the porous structure isn’t uniform enough.After continuous feeding for 30 minutes,it can be known from Fig.5(j)that the solid skeleton has the best dispersibility and sufficient connection points are generated between particles owing to the polymerization of silica surface hydroxyl groups.The network skeleton is well strengthened with uniform pores,high porosity and rare agglomeration,which can effectively suppress the shrinkage during the drying process.Compared Fig.5(n) with (k),the skeleton is severely coarsened and particles are also agglomerated at the same time.The porosity apparently declines because the network structure is filled with more particles.These changes result in not only a dense skeleton with a low pore volume but also an almost unchanged BET specific surface area and average diameter of primary particles.Therefore,if silica powder with a high pore volume is expected to be obtained,excessive skeleton reinforcement should be avoided.

3.4.Comparison between three preparation method of silica powder

In order to fully understand the effects of basic skeleton formation and skeleton reinforcement on the preparation of silica with large pore volume,skeleton reinforcement method is compared with two other methods,direct gelation method and skeletonfree method respectively.As can be seen from Fig.6(a),the XRD patterns of three different silica products are similar,showing a wide diffraction peak formed at 2θ=20°-30°,which indicates that these three kinds of silica powder are all amorphous.Fig.6(b)reveals information about the FT-IR spectra of silica materials with high similarity.A wide band observed at 3430 cm-1and 1630 cm-1are attributed to the O—H absorption peaks.The three bands at 1100 cm-1,800 cm-1and 468 cm-1are characteristic peaks of silica materials,which corresponds to the anti-symmetric stretching vibration of Si—O—Si,symmetric stretching vibration and bending vibration of Si—O,respectively.

As shown in Fig.6(c),silica prepared by direct gelation method has the largest specific surface area,nearly 580 m2·g-1.In contrast,via skeleton reinforcement method,silica powder with a relatively small specific surface area of 358 m2·g-1can be produced,which also reflects an increase of particle size due to the mode of heterogeneous nucleation growth during the reinforcement process.Also,silica prepared by the skeleton-free method has a centered value of specific surface area (410 m2·g-1).On the other hand,it’s worth noting that the fluid contained in pores is expelled during the drying process and then generates a large capillary force acting on the solid skeleton.This may cause severe shrinkage and collapse of the porous structure,resulting in a decrease of the pore volume[26].From the right side of Fig.6(c),it can be known that a large gap exists between the pore volume of these three different silica products,and its trend is exactly opposite to that of the specific surface area.The skeleton reinforcement method can manufacture silica powder with a quite high pore volume of 2.18 cm3·g-1,which is over 7 times that of prepared by direct gelation method.This result benefits from the skeleton reinforcement process based on the basic skeleton,which means the internal solid skeleton is strengthened in the process of particle growth.Therefore,without organic solvent exchange,the same effect can also be achieved by performing an appropriate skeleton reinforcement process.Although skeleton-free method also includes a simultaneous feeding procedure,the pore volume of the corresponding product (1.62 cm3·g-1) is still smaller than that obtained by the skeleton reinforcement method and the main reason for this is the lack of basic skeleton.When large amounts of seed particles exist in the system,subsequent feeding of raw materials is used for the particle growth to a greater extent,and few new particles are generated.In other words,the probability of heterogeneous nucleation is greater than that of the homogeneous nucleation [27].Simultaneous feeding procedure without a basic skeleton as seed particles will result in the appearance of numerous small particles in the porous structure,continuously coarsening the skeleton in the following reactions,which reduces the pore size and the pore volume.

Fig.5.Primary particle size distributions and TEM images at different magnifications of silica particles during skeleton reinforcement process:(a),(b),(c)T0;(d),(e),(f)T10;(g),(h),(i) T20;(j),(k),(l) T30;(m),(n),(o) T40.

The N2adsorption-desorption isotherms of silica powder prepared by three different methods are also demonstrated in Fig.6.According to the reference [28],these three curves present typical IV type isotherm characteristics because of the existence of an obvious hysteresis loop,which also means the prepared silica products are all mesoporous materials.Furthermore,the hysteresis loops in Fig.6(d) belongs to type H2,while the hysteresis loop in Fig.6(e) and 6(f) both belong to type H3,indicating that the pore volume and porous structure have changed during the skeleton reinforcement process and the slit-shaped pores are eventually formed[29].Finally,Fig.6(g)-6(i)show the microstructures of silica powder prepared by these three different methods and the results are consistent with the above data.

Fig.6.Different characterization of silica prepared by various methods:(a)XRD;(b)FT-IR;(c)specific surface area and pore volume;(d)N2 adsorption-desorption isotherms of silica prepared by direct gelation method;(e) N2 adsorption-desorption isotherms of silica prepared by skeleton-free method;(f)N2 adsorption-desorption isotherms of silica prepared by skeleton reinforcement method;(g)TEM photo of silica prepared by direct gelation method;(h)TEM photo of silica prepared by skeleton-free method;(i)TEM photo of silica prepared by skeleton reinforcement method.

4.Conclusions

In this paper,silica powder with a BET specific surface area of 358 m2·g-1and a high pore volume of 2.18 cm3·g-1was successfully synthesizedviaa skeleton reinforcement method.From the TEM/SEM images and characterization of pore properties,the influences of the base solution concentration,gelation pH value and skeleton reinforcement time on the growth of silica particles were also investigated.As a result,different products could be obtained by appropriate adjustment of these parameters.Moreover,as the two parts of the skeleton reinforcement method,gelation of skeleton and skeleton reinforcement process were both of great significance to the synthesis of silica,which was reflected by the comparison with two other preparation methods.The main purpose of this study is to provide a controllable,low-polluting and high-efficiency method for the industrial production of silica with high pore volume.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21838003,91834301,21878092),the Shanghai Scientific and Technological Innovation Project(18JC1410600),the Social Development Program of Shanghai(17DZ1200900,18DZ2252400),and the Innovation Program of Shanghai Municipal Education Commission,and the Fundamental Research Funds for the Central Universities (222201718002).