Silicalite-1/PDMS Hybrid Membranes on Porous PVDF Supports: Preparation, Structure and Pervaporation Separation of Dichlorobenzene Isomers

Round 1

Reviewer 1 Report

The study deals with the development of composite silicalite-1/PDMS/PVDF membranes for pervaporation separation of dichlorobenzene isomers. The topic of organic-organic separation in pervaporation, especially the separation of isomers is novel and not sufficiently explored yet, therefore, the manuscript will be interesting for researchers. The morphology and structure of the silicalite-1 zeolites and the silicalite-1/PDMS/PVDF hybrid membranes were characterized by a variety of methods: XRD, FTIR, SEM and BET. The study is logically organized and written in a good language. However, the results of pervaporation performance have to be reported more in detail. The paper can be published in Polymers after minor revision.

Comments:

1.Report, please, feed and permeate content in pervaporation experiments (when mixture of isomers is used).

2.Why a composite membrane with so thick selective layer was obtained? Report, please, additionally membrane normalized by thickness flux in pervaporation experiments for silicalite-1/α-Al2O3 and composite membranes to take into account membrane thickness.

3.Check, spelling throughout the manuscript and correct it.

4.Calculate, please, the thickness normalized pervaporation separation index for developed membranes to compare their effectiveness in pervaporation.

Author Response

1. Report, please, feed and permeate content in pervaporation experiments (when mixture of isomers is used).

     In the revised manuscript, feed and permeate content in pervaporation experiment were reported, and further marked in red font. The molar contents percentage of p-DCB both in p/o-DCB binary-isomer solutions and in p/m-DCB binary-isomer solutions were 50%.

2. Why a composite membrane with so thick selective layer was obtained? Report, please, additionally membrane normalized by thickness flux in pervaporation experiments for silicalite-1/α-Al₂O₃ and composite membranes to take into account membrane thickness.

   The main objective in our paper was to study the sillicate-1/PDMS/PVDF hybrid membranes with dense and continuous active layers without any defects for pervaporation separation of DCB isomer from their mixtures. Therefore, in the present experiment, the effect of thickness on DCB separation performance of the sillicate-1/PDMS/PVDF hybrid membranes has not been studied in detail.

   As we known, scraping technology was the main method for the preparation of organic and polymer hybrid membranes, especially zeolites/PDMS hybrid membranes. The thickness of the membranes could be easily controlled by adjusting the size of the spreading rod and the viscosity of the casting solutions. The main factor affecting the film thickness was the size of the spreading rod. In our experiment, we used a spreading rod, which could obtain a thickness of wet film about 50 μm. Then, after cross-linked under vacuum, the thick selective sillicate-1/PDMS layers were prepared on the surfaces of the porous PVDF supports.

   In the revised manuscript, thickness-normalized flux in pervaporation experiment for silicalite-1/α-Al₂O₃ membranes (of thickness about 5.3 μm) and sillicate-1/PDMS/PVDF hybrid membranes (of average thickness about 17.5 μm) have been taken into account, as shown in table 2.

Table 2. Thickness-normalized pervaporation flux for silicalite-1/α-Al₂O₃ membranes and sillicate-1/PDMS/PVDF hybrid membranes

3. Check, spelling throughout the manuscript and correct it.

   In the revised manuscript, spelling of the words throughout the manuscript have been checked and corrected carefully, and further marked in red font.

4. Calculate, please, the thickness normalized pervaporation separation index for developed membranes to compare their effectiveness in pervaporation.

   In the revised manuscript, the thickness-normalized pervaporation separation index for developed membranes has been calculated according to the reviewers’ advice. For silicalite-1/α-Al₂O₃ membranes (of thickness about 5.3 μm), the thickness-normalized pervaporation separation index of p-DCB in p/o-DCB binary-isomers solutions and of p-DCB in p/m-DCB binary-isomers solutions were 1.94 μm∙kg∙m^-2∙h^-1 and 2.78 μm∙kg∙m^-2∙h^-1, respectively. For sillicate-1/PDMS/PVDF hybrid membranes (of average thickness about 17.5 μm), the thickness-normalized pervaporation separation index of p-DCB in p/o-DCB binary-isomers solutions was 4.20 μm∙kg∙m^-2∙h^-1 and that of p-DCB in p/m-DCB binary-isomers solutions was 6.57 μm∙kg∙m^-2∙h^-1.

Reviewer 2 Report

This paper reports on the study of silicalite-1/PDMS/PVDF hybrid membranes for the pervaporation separation of dichlorobenzene isomers. There are several works in the literature with the use of silicalite-1/PDMS/PVDF hybrid membranes in pervaporation (mainly for the separation of water-alcohol mixtures). However, for the separation of dichlorobenzene isomers, these hybrid membranes are used for the first time. Few comments below can be considered for further refining of the manuscript.

1) In the Materials and Reagents section, the characteristics of the PVDF support are indicated. Among the characteristics is the diameter of 8 mm (line 126). However, the effective membrane diameter in the pervaporation module is 50 mm. What diameter is meant in this case for the PVDF support?

2) In Scheme 1, it is also desirable to depict temperature sensors and a safety trap in front of the vacuum pump, if any.

3) Line 184 – “condensed by liquid nitrogen (-176 ℃)”. The temperature of liquid nitrogen was indeed 176℃, not 196℃?

4) In the description for figure 1 (line 205), add the letters (a, b, c) to the figures. "SEM images (a), XRD patterns (b) and FTIR spectrums (c ) of silicalite-1 zeolites powders".

5) Line 401 – "channels (0.51x0.57 mm) and the b-directional straight channels (0.54 mm)". Most likely the size of the channels should be in "nm" units, not "mm".

6) In Figures 10 and 12, the permeate flux starts at point 0. What is the reason for this? After all, at 1, 2, 3 hours of the experiment, the permeate flux can be higher than with the data shown at 4-6 hours. It is better to present the permeate flux from the first data obtained (4-6 hours).

7) Figures 10-13 show the data for silicalite-1/a-Al2O3 membrane. These data were obtained in another work. You must indicate the source in the description of the figures. For example: "Figure 10. Pervaporation flux results of the silicalite-1/α-Al2O3 membranes [3] and the silicalite-1/PDMS/PVDF hybrid membranes in single-isomer solutions: (a) p-DCB; (b) o-DCB; (c) m-DCB.".

8) In the manuscript, hybrid silicalite-1/PDMS/PVDF membranes with different loading of silicalite-1 (5-30%) were obtained. However, for the pervaporation separation of dichlorobenzene isomers, a membrane with a silicalite-1 content of 10% was studied. It is of interest why this membrane was taken for the study of pervaporation separation? It often happens that with an increase in the loading of zeolite into the polymer material, the separation factor increases significantly with a stable or with a slight decrease of permeate flux. It would be interesting to show the dependence of the permeate flux on the loading of silicalite-1 in the membrane, if possible.

Author Response

This paper reports on the study of silicalite-1/PDMS/PVDF hybrid membranes for the pervaporation separation of dichlorobenzene isomers. There are several works in the literature with the use of silicalite-1/PDMS/PVDF hybrid membranes in pervaporation (mainly for the separation of water-alcohol mixtures). However, for the separation of dichlorobenzene isomers, these hybrid membranes are used for the first time. Few comments below can be considered for further refining of the manuscript.

1. In the Materials and Reagents section, the characteristics of the PVDF support are indicated. Among the characteristics is the diameter of 8 mm (line 126). However, the effective membrane diameter in the pervaporation module is 50 mm. What diameter is meant in this case for the PVDF support?

    There was a mistake about the diameter of the porous PVDF supports in the previously manuscript due to the authors’ carelessness during the writing of the essay. In the revised manuscript, the diameter of the porous PVDF supports has been corrected to 80 mm. In our experiment, in order to better prepare the sillicate-1/PDMS/PVDF hybrid membranes, we chose the porous PVDF supports with a larger size (80 mm) than that of the membrane module (50 mm). In the pervaporation experiment, then the sillicate-1/PDMS/PVDF hybrid membranes were cut to match the diameter of the membrane module. Hence, the effective diameter of the sillicate-1/PDMS/PVDF hybrid membranes during the pervaporation experiment was 50 mm.

2. In Scheme 1, it is also desirable to depict temperature sensors and a safety trap in front of the vacuum pump, if any.

    In the revised manuscript, the temperature sensor has been depicted in front of the vacuum pump. Generally, the vacuum pump also contained a safety trap, so I think it is not necessary to further depict a safety trap in front of the vacuum pump.

3. Line 184- “condensed by liquid nitrogen (-176 ℃)”. The temperature of liquid nitrogen was indeed 176℃, not 196℃?

    In the revised manuscript, the temperature of liquid nitrogen was corrected to

-196℃.

4. In the description for figure 1 (line 205), add the letters (a, b, c) to the figures. "SEM images (a), XRD patterns (b) and FTIR spectrums (c) of silicalite-1 zeolites powders".

    In the revised manuscript, the letters (a, b, c) have been added to the description for Figure 1. “SEM images, XRD patterns and FTIR spectrums of silicalite-1 zeolites powders” has been replaced by “SEM images (a), XRD patterns (b) and FTIR spectrums (c) of silicalite-1 zeolites powders”.

5. Line 401- "channels (0.51x0.57 mm) and the b-directional straight channels (0.54 mm)". Most likely the size of the channels should be in "nm" units, not "mm".

    There were some mistakes about the size of the zeolite channels in the previously manuscript due to the authors’ carelessness during the writing of the essay. In the revised manuscript, the “channels (0.51x0.57 mm) and the b-directional straight channels (0.54 mm)” has been corrected to “channels (0.51x0.57 nm) and the b-directional straight channels (0.54 nm)”.

6. In Figures 10 and 12, the permeate flux starts at point 0. What is the reason for this? After all, at 1, 2, 3 hours of the experiment, the permeate flux can be higher than with the data shown at 4-6 hours. It is better to present the permeate flux from the first data obtained (4-6 hours).

    The point at t=0 represented the beginning of the pervaporation experiment.

    Due to the permeation flux is too small, it is impossible to obtain a sufficient amount of liquid in a shorter time interval under the current experimental scale. For the convenience of operation and test, the time of sampling was set at 4-6 h. However, I firmly believe that it is better to present the permeate flux from the first data point, like at 1 h or 2 h or 3 h.

     In the revised manuscript, instead of the permeate flux from the point at t=0, the permeate flux from the first data obtained 4-6 h was presented in Figure 10 and 12.

7. Figures 10-13 show the data for silicalite-1/α-Al₂O₃ membrane. These data were obtained in another work. You must indicate the source in the description of the figures. For example: "Figure 10. Pervaporation flux results of the silicalite-1/α-Al₂O₃ membranes [3] and the silicalite-1/PDMS/PVDF hybrid membranes in single-isomer solutions: (a) p-DCB; (b) o-DCB; (c) m-DCB.".

    In the revised manuscript, the source was respectively inserted into the description of the figures 10-13.

8. In the manuscript, hybrid silicalite-1/PDMS/PVDF membranes with different loading of silicalite-1 (5-30%) were obtained. However, for the pervaporation separation of dichlorobenzene isomers, a membrane with a silicalite-1 content of 10% was studied. It is of interest why this membrane was taken for the study of pervaporation separation? It often happens that with an increase in the loading of zeolite into the polymer material, the separation factor increases significantly with a stable or with a slight decrease of permeate flux. It would be interesting to show the dependence of the permeate flux on the loading of silicalite-1 in the membrane, if possible.

    In industrial application, membranes with dense and continuous active layers without any defects could exhibit excellent separation performance.

    In one hand, only the silicalite-1 zeolites content no more than 10%, can we obtain the dense and continuous active silicalite-1/PDMS layers without any longitudinal cracks and other defects on the porous PVDF supports in this study. When the silicalite-1 zeolites content exceeded 10%, the surfaces of the active silicalite-1/PDMS layers became rougher and silicalite-1 zeolites aggregated to form pile pores, and also the thickness of the active silicalite-1/PDMS layers became thicker. The 1,3,5-triisopropylbenzene (TIPB,～0.85 nm) pervaporation experiment indicated that large amount of TIPB was detected in the permeate solutions when using the silicalite-1/PDMS/PVDF hybrid membranes with higher silicalite-1 content (>10%). The result demonstrated that the silicalite-1/PDMS/PVDF hybrid membranes of higher zeolites content (>10%) possessed poor structural integrity. Therefore, the silicalite-1/PDMS/PVDF hybrid membranes with higher silicalite-1 content (>10%) were not conducive to the separation of DCB mixtures. In other hand, under the premise of the membranes with better structural integrity, the higher the silicalite-1 zeolites content, the better the DCB isomers separation performance of the membranes. The 1,3,5-triisopropylbenzene pervaporation experiment further showed extremely small amount of TIPB was detected in the permeate solutions, indicating the silicalite-1/PDMS/PVDF hybrid membranes with lower silicalite-1 content (≤10%) of excellent structural integrity. Thus, the silicalite-1/PDMS/PVDF hybrid membranes with a zeolites content of 10% was taken for the study of pervaporation separation of DCB isomers. However, I firmly believe that it is important to show the dependence of the permeate flux on the loading of silicalite-1 zeolites in detail in our future research.