Pt-H2SO4/Zr-mont morillonite:An efficient catalyst for the polymerization of octamethylcy-clotetrasiloxane,poly methylhydrosiloxane and hexamethyldisiloxane to low-hydro silicone oil☆

Yuedong Zhou,Fengfu Li,Junwei Liu,Zhi Yun*,Xia Gui

College of Chemistry and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

1.Introduction

After Dow Corning Corporation ful filled industrialized production of polysiloxane for the first time in the early of 1940s,polysiloxane has maintained a high development speed[1,2].With the increased quantity demand ofpolysilicon products in various fields ofsociety,low-hydro silicone oil,as one kind of polysiloxane polymers,is a significant chemical intermediate which is getting more and more concern[3-6].Various solid acid catalysts such as metal salts[7],metal oxides[8,9],heteropoly acids[9,10],clays[11],zeolite solid acids[12],and exchange resins[13,14]display excellent catalytic activity for low-hydro silicone oil.At present,the main preparation routing is based on the reaction of D4,D4Hand MM catalyzed by Mt[15].Although low cost and easy access[16,17],Mt has some drawbacks such as weak catalytic activity,poor repeatability,hard activation and so on[18,19].To overcome these defects and improve the catalytic performances of Mt in the reaction of polymerization,many efforts have been attempted.

Mtis nanoparticles with layered structures,and usually has comparatively small diffusion resistance which facilitates the diffusion of bulky organic molecules through their pores.The layers own net negative charge neutralized by cations such as Na+,K+,and Ca2+,which occupy the interlamellar space.The modification of Mtlies in the fact that these interlamellar cations can be easily substituted by other cations or other molecules[20].This provides large space for modifying the properties of clays like acidity,textile parameters,polarity and other characteristics that govern their performances as catalysts.Modified Mt catalysts are mainly applied in severaltypes of reactions:ring-opening reactions,addition reactions,condensation reactions and Diels-Alder reactions[17].For instance,Aicha et al.found an acid exchanged Mt called Maghnite-H+for the synthesis of per fluoroal kyl end-capped ethylene oxides via cationic ring-opening polymerization which reached a very high yield of 83.4%[21].Fernando et al.prepared a highly active Ti/Mt catalyst by using dipping method which enhanced selectivity from ethylene towards the production of PE-MMT nanocomposites[22].Issaadi et al.demonstrated H2SO4/Zr-Mt catalyst high and stable catalytic activity[23].However,these modified Mt catalysts do not have outstanding reproducibility.Many efforts have been dedicated to strengthening recyclability of modified Mt catalysts,most of them have complicated synthesis processes[24].Thus the invention of catalysts with high catalytic performances for the polymerization of D4,D4Hand MM is still the subject of our study[25].

As is generally known,Mt with catalytic activity is widely applied in the ring-opening of D4to silicone oil due to rich Brønsted acidity sites and weak oxidability[26,27].The catalyst deactivation of Mt results from coverage of Brønsted acidity sites and collapse of structure[28].Thus,researchers employ dopants like Pt,Pd and Ru to improve activity,stability and repetition of catalysts[29,30].Nevertheless,no scientific articles have reported Pt-H2SO4/Zr-Mt was utilized as an acidic catalyst in polymerization reaction of D4,MM and D4Hto low-hydro silicone oil up to now.

In this work,we prepared H2SO4,Pt and Zr modified or co-modified Mt catalysts with zirconium loading(5 wt%)via a facile one-step impregnation method,respectively.The physical and chemical characterizations of Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt catalysts were conducted,and the performances of H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt catalysts were explored in terms of the polymerization ofD4,D4Hand MM.The objective ofthis study is to develop a new kind of catalyst with strong Brønsted acidity and stable catalytic performances with high regeneration for polymerization of low-hydro silicone oil and compared with the commonly used catalysts for performance analysis.

2.Materials and Methods

D4(octamethylcy-clotetrasiloxane,with a mass fraction of 0.9999,made in America)and D4H(polymethylhydrosiloxane,with a mass fraction of 0.9999,made in America)were obtained from Dow-Corning Company.MM(hexamethyldisiloxane,with a mass fraction of 0.997,made in China),Mt(montmorillonite K-10,with a specific area of 240 m2·g-1,made in China),Zirconium hydroxide(Zr(OH)4,with a mass fraction of 0.97,made in China),Chloroplatinic acid solution(H2PtCl6,with a mass fraction of 0.998,made in China)and sulfuric acid(H2SO4,with a mass fraction of 0.98,made in China)were all obtained from Aladdin-Reagent Company in Shanghai.All components were used without further purification.

2.1.Catalyst preparation

The preparation process of Zr-Mt with the molar ratio of Si/Zr=10 was as follows:at first,20 g of activated clay was dispersed in 30 ml of deionized water,followed by 2.1 g of Zr(OH)4.Then,the mixed solution was placed in a reaction kettle and stirred for 5 h at 80°C.After cooling to room temperature,the solid was filtered out and washed with distilled water three times.The resulting solid was placed in the oven at 80 °C for 12 h.At last,the catalyst was calcined at 300 °C for 6 h in dry air stream with a heating rate of 1 °C·min-1.

The H2SO4/Mt was pretreated as follows: firstly,20 g of Mt was dispersed in 100 ml of 10%dilute sulfuric acid solution under stirring for 2 h.Then the solid was filtered out and then washed with distilled water until the pH value of filter solution reached 6.The resulting solid was placed in an oven at 80°C for 12 h.At last,the catalyst was calcined at 300 °C for 6 h in dry air stream with a heating rate of 1 °C·min-1.

The H2SO4/Zr-Mt catalyst was prepared by taking Mt as carrier and Zr(OH)4as Zr source.The preparation process of H2SO4/Zr-Mt catalyst with the molar ratio of Si/Zr=10 was as follows:1 g of Zr(OH)4and 7.3 g of Mt were mixed into 100 ml of 10%dilute sulfuric acid solution under stirring for 2 h.Then the solid is filtered out and washed with distilled water until the pH value of filter solution reach 6.At last,the resultant sample was then dried at 80°C overnight and then calcined at 300°C for 4 h in dry air stream with a heating rate of 1 °C·min-1.

The preparation method of Pt-H2SO4/Zr-Mt was similar to H2SO4/Zr-Mt.At first,Mt,Zr(OH)4,H2SO4and H2PtCl6were added into a reaction kettle and stirred for 5 h at 80°C for 2 h.The obtained sample was filtered and washed with distilled water until the pH value offilter solution reached 6.At last,the obtained sample was dried at 80 °C overnight and then calcined at 300 °C for 4 h in dry air stream with a heating rate of 1 °C·min-1.The content of Pt loading was fixed at 2 wt%.

2.2.Catalyst characterization

The chemical composition of the Mt was determined using X-Ray Fluorescence(XRF)from WISDOM-6600 model.X-ray diffraction(XRD)experiments were conducted directly on the thin sheet with a Japan SmartLab X-ray diffraction meter powder diffractometer(40 kV,100 mA)(λ =0.183 nm)in a range from 10°to 90°at a rate of 20(°)·min-1.

The specific surface area,pore size distribution and pore volume of the catalyst were measured by the N2adsorption-desorption method on a V-Sorb 2800P nitrogen adsorption instrument.

NH3temperature-programmed desorption(NH3-TPD)was taken to determine the acidity of the samples.In the process,the sample was placed in the quartz reactor and heated from room temperature to 400 °C at a rate of 10 °C·min-1in pure He.Then,100 mg sample was pretreated at 100 °C and kept for 30 min.After cooling to 80 °C,NH3gas was fed to the reactor in the range from 100 °C to 700 °C for 30 min.At last,pure He was fed to the reactor to purge away any residual NH3for 30 min.

The IR spectra of adsorbed pyridine were carried out on a Shimadzu FTIR-8700 spectrometer.First,the samples were pressed into a selfsupporting sheet in a quartz-infrared tank which is pumped vacuum processing to achieve 10-2Pa.After cooling to room temperature to adsorb pyridine,the quartz-infrared tank was pretreated under vacuumfor2 h.Finally,the samples were degassed at250°C.The acidity sites of Brønsted and Lewis acid were calculated by using the integral intensities of the typical bands reported in the previous paper[31].

The Fourier transform infrared spectra(FT-IR)of the catalysts were recorded on a Nexus 870 FT-IR spectrometer.Samples were ground with spectral grade KBr(in a mass ratio of 1:10)to form a mixture for FT-IR measurement in the range of 4000-500 cm-1.

2.3.Catalytic reaction

2 g of catalyst was added in the mixture of 100 g of D4,9.7 g of D4H,and 3.38 g of MM at 55°C with strong agitation.After the reaction established equilibrium(reach stable viscosity),the mixture was cooled down,and the catalystwas separated by filtering later.Before obtaining the desired productM,MMand low viscosity silicone oil were removed by vacuum.

2.4.Product viscosity calculation

The products were analyzed by a NDJ-79 rotational viscometer equipped with a thermostat bath with a temperature of 25°C.The relationship between the kinematic viscosity and the average molar mass of the low-hydro silicone oil obtained by the equilibrium polymerization can be calculated by the following formula[32-34]:

where υ25°C(mm2·s-1)is the kinematic viscosity,Mwrepresents the average molar mass.

M yield was quantified as following equation:

3.Results and Discussion

3.1.Catalyst characterization

The XRF results of H2SO4/Mt and Mt samples are summarized in Table 1.The molar ratio of SiO2/Al2O3in H2SO4/Mt is larger than that of Mt.It reveals that these interlamellar cations(Na+,K+,Ca2+,etc.)ofMt can be substituted by cation H+after Mtwas acidified[20].Therefore,Mt can be served as an adequate catalyst carrier.

Table 1 Chemical composition of Mt and H2SO4/Mt by XRF

The high-angle XRD patterns of all the samples are shown in Fig.1.All the catalysts display strong peaks observed at 19.8°,26.5°,35.1°,50.2°and 61.8°which could be attributed to Mt.Peaks located at 19.8°and 35.98°could be ascribed to quartz as impurities in the material[35].Compared to the spectrum of Mt,the intensities of characteristic peaks of H2SO4/Mt decrease obviously because of cation H+replacing metal ions such as Ca2+,Na+and K+,which is in agreement with the results of Table 1.The intensities of characteristic Mt peaks in the XRD patterns of the H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt decrease and a new peak at 2θ =32.2°appears,indicating that Zr species are introduced in the Mt interlayer[36].In comparison to Zr-Mt,H2SO4/Zr-Mtand Pt-H2SO4/Zr-Mtsamples display stronger intensity of characteristic peaks at 2θ=32.2°because the zirconium ions are known to form tetrameric species in acidic solutions with the chemical formula[Zr4(OH)6(H2O)16]8+[37].Simultaneously,no other diffraction peak arising from crystalline Pt is observed in Fig.1,illustrating that the generated Pt nanoparticles are ultra fine,highly dispersed on the porous wall of Mt in Pt-H2SO4/Zr-Mt and the introduction of Pt species has no effect on the phase of H2SO4/Zr-Mt[38].

Fig.1.X-ray diffraction patterns of activated clay molecular sieves.

The datum of nitrogen adsorption-desorption about Mt,Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt is reported in Table 2.The original Mt specific surface area is 219 m2·g-1.After addition of zirconium and sulfuric acid,the specific surface areas of Zr-Mt and H2SO4/Mt respectively decrease to 136 m2·g-1and 173 m2·g-1due to the blockage of the channel[39].Compared with Mt,the specific surface area,pore volume and pore diameter of H2SO4-Zr/Mt increase obviously,suggesting that the introduction of sulfate groups in Zrpillared Mt increases its specific surface greatly[40].Simultaneously pore volume slightly decreases in Pt modified catalyst,pointing to a slight pore blockage of the H2SO4/Zr-Mt[41].

Table 2 Textural parameters of different samples

The NH3-TPD results of different samples are shown in Fig.2:The amount of acid sites is indicated by TPD adsorption which is calculated from absorption peak area,and is shown in Table 3.It is clear that Mt and H2SO4/Mt have only one TPD peak,indicating the presence of weak acid(600 °C-700 °C)in the catalysts[42].Zr-Mt displays a board peak from 230 °C to 330 °C,and exhibits a maximum desorption rate of ammonia at about 305°C,which is in agreement with Kooli et al.[43].Nevertheless,two peaks at 280 °C and 640 °C for the H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt are attributed to medium acid(200 °C-400 °C)of Zr and weak acid(600 °C-700 °C)of Mt respectively[42].The order of total acidities of the catalysts is H2SO4/Zr-Mt＞Pt-H2SO4/Zr-Mt＞Zr-Mt＞H2SO4/Mt＞Mt.

Fig.2.NH3-TPD spectra of Mt,Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mt,and Pt-H2SO4/Zr-Mt samples.

Table 3 Acidities of Mt,Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt

Fig.3 illustrates the pyridine-FT-IR spectra of Mt,H2SO4/Mt,Zr-Mt,H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt after pyridine adsorption.The polymerization efficiency of low-hydro silicone oil is associated with the Brønsted acid content of the catalyst.In addition,the peak in the range of 1540-1548 cm-1is ascribed to pyridine adsorption which was used to calculate the content of Brønsted acid[44].The amount of Brønsted acid and Lewis acid sites is measured by the integral of the spectra,and the results are shown in Table 3.With few acid sites in Mt,the concentration of Brønsted acid in Zr-Mt catalyst is higher than that of Mt because Zr atoms replace the Si atoms in Mt[45].These Brønsted acid sites are associated with the change in the electron density of Sibecause the electronegativity charge imbalance caused by the introduction of Zr atoms near negatively charged silicon weakens the SiO--H bond[43].Meanwhile,H2SO4loading obviously increases the Brønsted acid sites of the catalysts.Furthermore,it is observed that the number of the Lewis acid sites decreases after Pt loading in the support,which can be explained by the coverage of Pt on the Lewis acid sites.Although the totalacidity of the Pt-H2SO4/Zr-Mtdecreases slightly[46],the amount of Brønsted acid sites increases.The increase of Brønsted acid content in Pt-H2SO4/Zr-Mt is ascribed to polarization or hydrolysis of Pt2+[47].Besides,the total acidity measured by FT-IR spectra is well consistent with the NH3-TPD results.

Fig.3.The pyridine-FT-IR spectra ofMt,Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mtand Pt-H2SO4/Zr-Mt.

3.2.Catalytic activity

3.2.1.Effect of reaction temperature

Fig.4.Reaction velocity of polymerization over Pt-H2SO4/Zr-Mt catalyst at different reaction temperatures.Reaction conditions:100 g of D4,6,71 g of,1.96 g of MM,and 3.2 g of catalyst.

Effect of reaction temperature on the polymerization of D4,MM andwas investigated over Pt-H2SO4/Zr-Mt catalyst(Fig.4).The increase of temperature has a beneficial effect on the polymerization reaction of D4,MM and.As expected,the polymerization speed of D4,MM andenhances obviously with increasing temperature from 20°C to 80°C.Relationship between theoretical viscosity and molecular weight of the product is calculated by the formula(1).The boiling point of MM is 97 °C.When the temperature is greater than 100 °C,part of MM as a blocking agent in the reactor will transfer to the gaseous state,which results in reduction of polyatomic MM as a blocking agent.So the molecular weight of the product is higher than the theoretical calculated molecular weight and the product viscosity of the product is greater than theoretical viscosity[32-34].

3.2.2.Effect of catalyst dosage

The polymerization reaction was carried out at different catalyst dosages ranging from 1.8 wt%to 7.2 wt%(Fig.5).It can be seen from Fig.5,when the amount of catalyst is 1.8 wt%,the viscosity reaches the equilibrium value of 100 mPa·s,which indicates that the catalyst does not occur a large number of inactivation during polymerization of D4,and MM.The equilibrium velocity of the product increases with dosage,reaching an equilibrium velocity value of 100 mPa·s at 7.2 wt%within 4 h,the molecular weight of the product is the maximum as well.The reaction velocity improves as the catalyst dosage increases,but the reaction velocity did not change significantly from 7.2 wt%to 9 wt%.So 7.2 wt%of the Pt-H2SO4/Zr-Mt catalyst was chosen as the optimum catalyst amount for the polymerization reaction.

3.2.3.Stability of Pt-H2SO4/Zr-Mt

Deactivation of catalysts occurs when the pores of catalysts are blocked.Thus,as Fig.6 shows,the activity evolution as a function of the time on stream(TOS)was investigated over H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt at 60°C.It can be seen that the stability of Pt-H2SO4/Zr-Mt is much higher than that of H2SO4/Zr-Mt,the yield of low-hydro silicone oil over the H2SO4/Zr-Mt brings out an obvious decrease from 93%to 42%.Nevertheless,the yield of low-hydro silicone oil the Pt-H2SO4/Zr-Mt catalyst remains at 79%after reaction of 50 h,which is slightly lower than that in the first run(93%),indicating an outstanding stability of the catalyst.It demonstrates that the Pt-H2SO4/Zr-Mtis capable of efficient polymerization reaction for long-lasting and regeneration application.

Fig.5.Reaction velocity of polymerization at different amount of catalyst.Reaction conditions:100 g of D4,6.71 g of D4H,1.96 g of MM,and 60°C.

Fig.6.Yield of low-hydro silicon oil by using different samples.Reaction conditions:100 g of D4,6,71 g of1.96 g of MM,3 g of catalyst,and 60°C.

3.2.4.Regeneration test of deactivated Pt-H2SO4/Zr-Mt

Regeneration of deactivated Pt-H2SO4/Zr-Mt is an important research topic in the catalytic polymerization of D4,D4Hand MM to lowhydro silicone oil.Accordingly,the regeneration test was carried out to study the reusability of deactivated Pt-H2SO4/Zr-Mt.After the longterm stability test(50 h),the used Pt-H2SO4/Zr-Mt was regenerated by washing with mixed solution of sulfuric acid and MM,then dried in a vacuum oven at 80 °C for 3 h and calcined at 300 °C for 4 h[48].Fig.7 shows the yield of low-hydro silicone oil catalyzed by fresh and regenerated Pt-H2SO4/Zr-Mt catalysts.It is worth our attention that the regenerated Pt-H2SO4/Zr-Mt catalyst also exhibits high catalytic performance.Compared to the fresh Pt-H2SO4/Zr-Mt,the regenerated Pt-H2SO4/Zr-Mt shows almost the same catalytic activity after 35 h,although faster deactivation was shown after 35 h.

Fig.7.The comparison of the stability of fresh and regenerated Pt-H2SO4/Zr-Mt.Reaction conditions:100 g of D4,6,71 g of D4H,1.96 g of MM,3 g of catalyst,and 60°C.

FT-IR measurement of fresh and regenerated Pt-H2SO4/Zr-Mt was conducted in order to study the structural stability of the catalyst.The results of the fresh and regenerated Pt-H2SO4/Zr-Mt are shown in Fig.8.The broad peaks of adsorption band around 3441 cm-1and 1390 cm-1respectively represent characteristic peaks of O--H stretching in fresh and regenerated Pt-H2SO4/Zr-Mt[49],while the corresponding bending mode at 1631 cm-1is attributed to the deformation vibration of adsorbed water when substitution of Al for Si is low[50].The peak at 1037 cm-1corresponds to the perpendicular Si--O--Si antisymmetric vibration[51],and the bands at 618,518 and 470 cm-1are ascribed to Si--O bending vibration[52].Unfortunately,we cannot see the lower frequency(＜400 cm-1)region and thus the effect of the inter layered cations(＜100 cm-1)cannot be identified.Comparing with FT-IR spectrum of fresh Pt-H2SO4/Zr-Mt,regenerated Pt-H2SO4/Zr-Mt showed a new band at 2977 cm-1which is stretching vibration of methyl indicating that a trace amount of silicone oil exists on the catalyst surface.In addition,a Si--O bending vibration of regenerated Pt-H2SO4/Zr-Mt at 678 cm-1disappeared.

Fig.8.FT-IR spectra of fresh and regenerated Pt-H2SO4/Zr-Mt catalysts.

4.Conclusions

We investigated the catalytic behaviors of Mt,Zr-Mt,H2SO4/Mt,H2SO4/Zr-Mt and Pt-H2SO4/Zr-Mt in the polymerization of D4,MM and D4H.The results indicated Pt-H2SO4/Zr-Mtwas the most effective catalyst in ring-opening of D4in this study.In comparison to H2SO4/Zr-Mt,the porous structure of the Pt-H2SO4/Zr-Mt remained almost unchanged,and the total acidity of the Pt-H2SO4/Zr-Mt decreased while the amount of Brønsted acid sites increased,which can accelerate the rate of openloop reaction.In addition,the Pt-H2SO4/Zr-Mt showed better stability compared to H2SO4/Zr-Mt.After 50 h of reaction,the yield of lowhydro silicone oil reduced from 93%to 78%using Pt-H2SO4/Zr-Mt.However,H2SO4/Zr-Mt displayed an obvious decrease in yield from 93%to 42%,which was due to pore blockage,structure collapse and partial proton escape of the catalyst.Pt-H2SO4/Zr-Mt can be regenerated under the conditions set and exhibited well reusability in the polymerization of D4,MM and D4Hto low-hydro silicone oil.

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