Effects of Seed Crystals on the Growth and Catalytic Performance of TS-1

membranes

Article Eﬀects of Seed Crystals on the Growth and Catalytic Performance of TS-1 Zeolite Membranes

Wenjuan Ding, Sitong Xiang, Fei Ye, Tian Gui, Yuqin Li, Fei Zhang, Na Hu, Meihua Zhu * and Xiangshu Chen *

State-Province Joint Engineering Laboratory of Zeolite Membrane Materials, Institute of Advanced Materials, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China; [email protected] (W.D.); [email protected] (S.X.); [email protected] (F.Y.); [email protected] (T.G.); [email protected] (Y.L.); [email protected] (F.Z.); [email protected] (N.H.) * Correspondence: [email protected] (M.Z.); [email protected] (X.C.)

Received: 14 February 2020; Accepted: 7 March 2020; Published: 13 March 2020

Abstract: Dense and good catalytic performance TS-1 zeolite membranes were rapidly prepared on porous mullite support by secondary hydrothermal synthesis. The properties of seed crystals were very important for the preparation of high-catalytic performance TS-1 zeolite membranes. Inﬂuences of seed crystals (Ti/Si ratios, size, morphology, and zeolites concentration of the seed suspension) on the growth and catalytic property of TS-1 zeolite membranes were investigated in details. High Ti/Si ratio, medium-size, and morphology of the seed crystals were critical for preparing the high-performance TS-1 zeolite membrane. Compared with the bi-layer TS-1 zeolite membrane (inner and outer of the mullite tube), the mono-layer TS-1 zeolite membrane had a better catalytic performance for Isopropanol IPA oxidation with H2O2. When the Ti/Si ratio, size, and morphology of the TS-1 zeolites were 0.030, 300 nm, ellipsoid, and the zeolites concentration of the seed suspension was 5%, the IPA conversion, and ﬂux through the TS-1 zeolite membrane were 98.23% and 2.58 kg m 2 h 1, respectively. · − · − Keywords: TS-1 zeolite membrane; seed crystals; catalytic oxidation; mono-layer

1. Introduction Taramasso et al. prepared the titanium silicalite-1 (TS-1) with MFI structure by the hydrothermal synthesis in 1983, which was a breakthrough in the zeolites ﬁeld [1]. TS-1 zeolites could catalyze the liquid-phase oxidation of a variety of organic compounds to oxygenated products actively, which has been focused on the TS-1/H2O2 reaction system for the environmentally benign and outstanding oxidation selectivity [2–7]. Separation and recovery of the TS-1 zeolites from the liquid-phase reactions are a major problem, which is limited to the industrial application of TS-1 zeolites. TS-1 zeolite membrane could avoid the separation and recovery processes [8–10], which is a novel and good catalyst for H2O2 oxidations with some advantages, such as mild conditions (low reaction temperature and room pressure), environment-friendly, and energy-saving [11,12]. Compact and good catalytic performance TS-1 membranes have been successfully prepared by in-situ or secondary growth hydrothermal synthesis [13–15]. Secondary growth hydrothermal synthesis, a layer of seeded zeolites is coated on suitable support before crystallization, which is an eﬀective way for shortening the crystallization time and preparing high-quality zeolite membranes [16–20]. In addition, the secondary growth could suppress the transformation from nucleation to other phase crystals. It is well known that the seeded crystals play an important role in the membrane formation.

Membranes 2020, 10, 41; doi:10.3390/membranes10030041 www.mdpi.com/journal/membranes Membranes 2020, 10, 41 2 of 11 b-oriented TS-1 zeolite membranes were prepared by the secondary growth method, which had a high reproducibility and excellent catalytic performance for the n-hexane oxyfunctionalization [16]. Zhu et al. first prepared the Ti-MWW zeolite membrane by secondary hydrothermal synthesis, and the membranes showed good catalytic performance for phenol hydroxylation [17]. Liu et al. synthesized the highly oriented, thin, and hierarchically porous TS-1 zeolite membrane by secondary hydrothermal treatment [18]. Wang et al. synthesized the Ti-containing membrane using nanosized silicalite-1 particles as seeds by hydrothermal treatment; the membranes showed large proportions of tetrahedrally coordinated titanium and excellent hydrothermally stability [19]. Recently, the high catalytic performance TS-1 zeolite membrane with the ultra-dilute synthetic solution by the secondary hydrothermal synthesis in our previous study, which greatly shortened the crystallization time and preparation cost of the membrane [20]. In order to improve the catalytic performance of the membranes, effects of the seed crystals (Ti/Si ratios, size, morphology, and zeolites concentration of the seed suspension) on the growth and catalytic performance of the TS-1 zeolite membranes are studied in the present work. Besides, influences of the bi-layer/mono-layer TS-1 zeolite layer on the catalytic performance of the TS-1 zeolite membrane were investigated in this work.

2. Experimental Section

2.1. Materials The silicon sources, titanium sources, and organic structure-directing agent (SDA) were tetraethyl orthosilicate (Sigma-Aldrich, TEOS, MO, USA, 98 wt%), tetra-tert-butyl orthotitanate (TCI, TTIP, Tokyo, Japan, 97 wt%), and tetrapropylammonium hydroxide (TCI, Tokyo, Japan, TPAOH, 10 wt% in H2O). The porous mullite tubes (Noritake Co., Nagoya, Japan, OD: 12 mm, ID: 9 mm, average pore size: 1.3 µm, length: 100 mm) were used as supports. Six types of MFI zeolite crystals were used as seeded crystals for preparing TS-1 zeolite membranes, which were numbered Z-1 (ellipsoid, ca. 230 nm, Ti/Si = 0); Z-2 (ellipsoid, ca. 300 nm, Ti/Si = 0.022); Z-3 (ellipsoid, ca. 300 nm, Ti/Si = 0.030); Z-4 (agglomerate, ca. 540 nm, Ti/Si = 0.030); Z-5 (petals, ca. 540 nm, Ti/Si = 0.030); Z-6 (reunion, ca. 750 nm, Ti/Si = 0.030), respectively.

2.2. Preparation of TS-1 Zeolite Membrane The seed crystals were coated on the mullite support by dip-coating. Seed crystals were dispersed into ethanol and formed 2–7 wt% zeolite crystals suspension, and the solution was ultrasonicated to a uniform solution. The mullite supports were seeded with TS-1 zeolite crystals suspensions before hydrothermal synthesis. The support was immersed in the zeolite crystals suspension for 40 s by two times, the seeded support was placed and dried at 85 ◦C ovens. It was noted that the pipe mouth of the mullite support was blocked during the seeding procedure, which could prevent the zeolite crystals to enter into the inner surface of the support. The molar composition of the precursor synthesis solution was SiO2: 0.035 TiO2: 0.25 TPAOH: 120 H2O and the preparation procedure of the precursor synthesis solution was identical with our previous work [20]. The seeded supports were still plugged with Teﬂon rods during crystallization procedures in this study. Thereafter, the precursor synthesis solution and seeded supports were sealed in a titanium autoclave at 423 K for 24 h. The membranes were washed, dried and calcined after hydrothermal synthesis. The calcination was carried out at 500 ◦C for 10 h; the heating and cooling rate were 0.25 ◦C/min.

2.3. Characterizations X-ray diﬀraction (XRD, Rigaku Ultima IV,Tokyo, Japan) with Cu-Kα radiation, Fourier transformed infrared spectroscopy (FT-IR, JASCO FT/IR-6100, Tokyo, Japan), ultraviolet, visible spectroscopy (UV-vis, JASCO, V-700, Tokyo, Japan) and ﬁeld-emission scanning electron microscopy (FE-SEM, Tokyo, Japan) were characterized the structure and morphology of TS-1 zeolites and membranes. Energy Dispersive X-Ray Spectroscopy (EDX, Bruker Quantax200, Bruker, Germany) and inductively coupled plasma Membranes 2020, 10, x FOR PEER REVIEW 3 of 12

Membranes 2020, 10, 41 3 of 11 (FE-SEM, Tokyo, Japan) were characterized the structure and morphology of TS-1 zeolites and membranes. Energy Dispersive X-Ray Spectroscopy (EDX, Bruker Quantax200, Bruker, Germany) atomicand inductively emission coupled spectroscopy plasma (ICP-AES, atomic emission Varian 725ES, spectroscopy California, (ICP-AES, USA) were Vari measuredan 725ES, the California, element compositionUSA) were measured of zeolites. the element composition of zeolites. Oxidation ofof isopropanolisopropanol (IPA) (IPA) and and H 2HO2Owas2 was carried carried in ain 500 a mL500 membranemL membrane reactor reactor with awith reﬂux a apparatus,reflux apparatus, and the and reaction the reaction device, device, as illustrated as illustra in ourted previousin our previous study [ 20study]. Firstly, [20]. Firstly, 5.27 g IPA, 5.27 96.34g IPA, g H96.342O2 g(30%) H2O and2 (30%) 254.39 and g 254.39 H2O were g H added2O were into added the membrane into the membrane reactor; the reactor; TS-1 zeolite the TS-1 membrane zeolite wasmembrane introduced was introduced into the membrane into the reactor.membrane The reacto waterr. bathThe water was heated bath was to 70 he◦atedC by to the 70 water °C by bath. the Thewater reaction bath. The equation reaction was equation as follows. was as follows.

O OH TS-1 zeolite membrane 2H O + H2O2 + 2

The total ﬂuxflux (Q) and IPA conversionconversion ((CCIPAIPA) were calculated as the following:

Q = W/(A t) (1) Q = W/(A ×× t) (1)

IPA acetone IPA acetone CC = nnacetone/(n/(n + n+ nacetone) × 100) 100%% (2) (2) IPA IPA × where W, t,, andand A were thethe totaltotal weightweight ofof thethe permeatepermeate (kg),(kg), collecting interval (h), and eeffectiveﬀective 22 surface area ofof TS-1TS-1 zeolitezeolite membranemembrane (m(m),), nnacetoneacetone andand nIPAIPA werewere the the molar molar contents contents of acetone and IPA inin thethe permeatepermeate mixture,mixture, respectively. respectively. It It was was noted noted that that the the acetone acetone selectivity selectivity of of IPA IPA oxidation oxidation by theby the TS-1 TS-1 zeolite zeolite membrane membrane was was almost almost 100% 100% in in this this work. work. The The oxidationoxidation productproduct waswas periodically collected and analyzed by gas chromatography (GC,(GC, Shimadzu,Shimadzu, GC-2014C,GC-2014C, Tokyo,Tokyo, Japan).Japan).

3. Results and Discussion Six types of of TS-1/Silicalite-1 TS-1/Silicalite-1 zeolites zeolites were were applied applied as as seed seed crystals crystals (Z-1, (Z-1, Z-2, Z-2, Z-3, Z-3, Z-4, Z-4, Z-5 Z-5 and and Z- Z-6)6) for for preparing preparing TS-1 TS-1 zeolite zeolite membranes. membranes. Table Table 11 summarizedsummarized the the properties of thethe seedseed crystalscrystals (morphology, size,size, andand TiTi/Si),/Si), andand FigureFigure1 1 presented presented SEM SEM images images of of these these zeolites. zeolites. ZeoliteZeolite Z-1Z-1 waswas the typical silicalite-1, which was ellipsoid with an averageaverage size ofof ca.ca. 230 nm.nm. Zeolites Z-2 and Z-3 exhibited well-deﬁnedwell-defined edges and regular ellipsoidellipsoid morphology (ca. 300 nm), while the Ti/Si Ti/Si ratios of the zeolites Z-2 and Z-3 were 0.022 and 0.030 0.030 by by ICP ICP characterization. characterization. Besides, Besides, in in order order to to study study effects eﬀects of morphology on the growth of TS-1 zeolite membrane,membrane, zeolites Z-4 (ca.(ca. 540 nm, agglomerate), Z-5 (ca. 540 nm, petals) Z-6 (ca. 750 750 nm, nm, reunion) reunion) were were used as seed crystals, wh whoseose Ti/Si Ti/Si ratio were the sameMembranes as zeolites 2020, 10 Z-3., x FOR PEER REVIEW 4 of 12

Table 1. Textural properties of Silicalite-1/TS-1 zeolites.

No. Morphology Size 2 (nm) Ti/Si 1Ratio Z-1 ellipsoid 230 0 Z-2 ellipsoid 300 0.022 Z-3 ellipsoid 300 0.030 Z-4 agglomerate 540 0.030 Z-5 petals 540 0.030 Z-6 reunion 750 0.030 Note:1 determined by Inductively coupled plasma atomic emission spectroscopy (ICP), 2 size of a single crystal block.

Figure 1. SEM images of Silicalite-1/TS-1 zeolites. (a) Z-1; (b) Z-2; (c) Z-3; (d) Z-4; (e) Z-5; (f) Z-6. Figure 1. SEM images of Silicalite-1/TS-1 zeolites. (a) Z-1; (b) Z-2; (c) Z-3; (d) Z-4; (e) Z-5; (f) Z-6.

3.1. Effect of Seed Crystals Ti/Si Ratio Table 2 summarized the preparation conditions and catalytic performance of TS-1 zeolite membranes in this work. Water was the only by-product of isopropanol/hydrogen peroxide by TS-1 zeolite membranes, and all the membranes showed good perm-selectivity for the acetone by pervaporation (PV) nearly 100%. Figures 2 and 3 showed the XRD patterns, surface, and cross-section SEM images of the TS-1 zeolite membranes with different Ti/Si ratio seed crystals.

Table 2. Catalytic performance and Ti/Si ratio of the TS-1 zeolite membranes with different seeded crystals.

Seed Suspension Membrane Q CIPA Seed Seed Concentration Ti/Si Ratio No. [kg∙m−2∙h−1] [%] No. (wt%) M-1 Z-1 5% 0.024 1.79 37.78 M-2 Z-2 5% 0.036 1.90 81.88 M-3 Z-3 5% 0.055 2.58 98.18 M-4 Z-4 5% 0.027 1.84 69.02 M-5 Z-5 5% 0.0319 1.97 86.70 M-6 Z-6 5% 0.024 2.80 64.89 M-7 Z-3 2% 0.025 1.06 68.72 M-8 Z-3 3% 0.035 1.29 82.56 M-9 Z-3 7% 0.030 Leak Leak M-10 * Z-3 5% 0.051 1.98 93.27 Note: * M-10 is the bi-layer TS-1 zeolite membrane.

Membranes 2020, 10, 41 4 of 11

Table 1. Textural properties of Silicalite-1/TS-1 zeolites.

No. Morphology Size 2 (nm) Ti/Si 1 Ratio Z-1 ellipsoid 230 0 Z-2 ellipsoid 300 0.022 Z-3 ellipsoid 300 0.030 Z-4 agglomerate 540 0.030 Z-5 petals 540 0.030 Z-6 reunion 750 0.030 Note: 1 determined by Inductively coupled plasma atomic emission spectroscopy (ICP), 2 size of a single crystal block.

3.1. Eﬀect of Seed Crystals Ti/Si Ratio Table2 summarized the preparation conditions and catalytic performance of TS-1 zeolite membranes in this work. Water was the only by-product of isopropanol/hydrogen peroxide by TS-1 zeolite membranes, and all the membranes showed good perm-selectivity for the acetone by pervaporation (PV) nearly 100%. Figures2 and3 showed the XRD patterns, surface, and cross-section SEM images of the TS-1 zeolite membranes with diﬀerent Ti/Si ratio seed crystals.

Table 2. Catalytic performance and Ti/Si ratio of the TS-1 zeolite membranes with diﬀerent seeded crystals.

Seed Suspension Membrane 2 1 Ti/Si Ratio Q [kg m h ] CIPA [%] No. Seed No. Seed Concentration (wt%) · − · − M-1 Z-1 5% 0.024 1.79 37.78 M-2 Z-2 5% 0.036 1.90 81.88 M-3 Z-3 5% 0.055 2.58 98.18 M-4 Z-4 5% 0.027 1.84 69.02 M-5 Z-5 5% 0.0319 1.97 86.70 M-6 Z-6 5% 0.024 2.80 64.89 M-7 Z-3 2% 0.025 1.06 68.72 M-8 Z-3 3% 0.035 1.29 82.56 M-9 Z-3 7% 0.030 Leak Leak M-10 * Z-3 5% 0.051 1.98 93.27 Membranes 2020, 10, x FOR PEER REVIEWNote: * M-10 is the bi-layer TS-1 zeolite membrane. 5 of 12

Figure 2. XRD patterns of TS-1 zeolite membranes with diﬀerent Ti/Si ratio seeded crystals. (a) M-1; Figure 2. XRD patterns of TS-1 zeolite membranes with different Ti/Si ratio seeded crystals. (a) M-1; (b) M-2; (c) M-3. (b) M-2; (c) M-3.

Figure 3. Surface and cross-section SEM images for TS-1 zeolite membranes with different Ti/Si ratio seed crystals, (a,b) M-1; (c,d) M-2; (e,f) M-3.

As given in Figure 2, all diffraction peaks of the membranes M-1, M-2, M-3 were the MFI zeolite structure (2θ = 7.8°, 8.8°, 23.2°, 23.8°) and mullite support diffraction peaks, which indicated that all membranes had pure MFI phase [21–24]. Besides, Figure 3 presented that the mullite surface was fully covered with the TS-1 zeolites. Table 2 summarized the Ti/Si ratio of the membranes by EDX characterization, which was increased with the Ti/Si ratio of the seeded crystals. The Ti/Si ratio of the membrane M-1 was low (0.024, Table 2), the catalytic performance of the membrane M-1 was poor, the total flux and IPA conversion of membrane M-1 were only 1.79 kg·m−2·h−1 and 37.78%. The accompanying TS-1 zeolites of the TS-1 zeolite membranes were collected in this work, the number of the accompanying zeolites of the membranes M-2 and M-3 were S-2 and S-3. Figure 4 showed the FT-IR spectra of the TS-1 zeolites. The peak at ca. 960 cm−1 of the spectra was ascribed to the interaction between the stretching vibration of [SiO4] unit and titanium in neighboring coordination sites, which was an evidence of the vibration of Si–O–Ti bond in the zeolite framework. As given in Figure 4, the zeolite S-3 had a higher tetrahedral titanium adsorption peak (ca .960 cm−1) than the zeolite S-2. Besides, the M-3 had a higher Ti/Si ratio than the M-2 by EDX characterization (Table 2), the total flux of the membrane M-3 and IPA conversion of oxidation were up to 2.58 kg·m−2·h−1 and 98.18%, respectively. Hence, the high Ti/Si

Membranes 2020, 10, x FOR PEER REVIEW 5 of 12

MembranesFigure2020 2., 10 XRD, 41 patterns of TS-1 zeolite membranes with different Ti/Si ratio seeded crystals. (a) M-1;5 of 11 (b) M-2; (c) M-3.

FigureFigure 3. 3.Surface Surface and and cross-section cross-section SEM SEM images images forfor TS-1TS-1 zeolitezeolite membranesmembranes withwith didifferentﬀerent Ti Ti/Si/Si ratio ratio seedseed crystals, crystals, ( a(a,b,b)) M-1; M-1; ( c(,cd,d)) M-2; M-2; ( e(,ef,)f) M-3. M-3.

AsAs given given in in Figure Figure2 ,2, all all di diffractionffraction peaks peaks of of the the membranes membranes M-1, M-1, M-2, M-2, M-3 M-3 were were the the MFI MFI zeolite zeolite structurestructure (2 (2θθ= =7.8 7.8°,◦, 8.8 8.8°,◦, 23.2 23.2°,◦, 23.823.8°)◦) andand mullitemullite supportsupport didiffractionffraction peaks, peaks, which which indicated indicated that that all all membranesmembranes had had pure pure MFI MFI phase phase [21 [21–24].–24]. Besides, Besides, Figure Figure3 presented 3 presented that thethat mullite the mullite surface surface was fully was coveredfully covered with the with TS-1 the zeolites. TS-1 zeolites. TableTable2 2summarized summarized the the Ti Ti/Si/Si ratio ratio of of the the membranes membranes by by EDX EDX characterization, characterization, whichwhich waswas increasedincreased with with the the Ti /Ti/SiSi ratio ratio of theof the seeded seeded crystals. crystals. The TiThe/Si Ti/Si ratio ofratio the of membrane the membrane M-1 was M-1 low was (0.024, low Table(0.024,2), Table the catalytic 2), the performancecatalytic performance of the membrane of the membrane M-1 was poor, M-1 the was total poor, flux the and total IPA flux conversion and IPA 2 1 ofconversion membrane of M-1membrane were only M-1 1.79 were kg onlym− 1.79h− kg·mand− 37.78%.2·h−1 and The37.78%. accompanying The accompanying TS-1 zeolites TS-1 zeolites of the · · TS-1of the zeolite TS-1 membraneszeolite membranes were collected were collected in this work,in this the work, number the number of the accompanying of the accompanying zeolites zeolites of the membranesof the membranes M-2 and M-2 M-3 and were M-3 S-2 were and S-3.S-2 and Figure S-3.4 Figureshowed 4 theshowed FT-IR the spectra FT-IR of spectra the TS-1 of zeolites.the TS-1 1 Thezeolites. peak atThe ca. peak 960 cmat ca.− of960 the cm spectra−1 of the was spectra ascribed wasto ascribed the interaction to the interaction between the between stretching the stretching vibration ofvibration [SiO4] unit of [SiO and4 titanium] unit and in titanium neighboring in neighboring coordination coordination sites, which sites, was which an evidence was an of evidence the vibration of the ofvibration Si–O–Ti of bond Si–O–Ti in the bond zeolite in framework.the zeolite framework. As given in As Figure given4, thein Figure zeolite 4, S-3 the had zeolite a higher S-3 had tetrahedral a higher 1 titaniumtetrahedral adsorption titanium peakadsorption (ca. 960 peak cm (ca− ) .960 than cm the−1) zeolitethan the S-2. zeolite Besides, S-2. Besides, the M-3 the had M-3 a higherhad a higher Ti/Si ratioTi/Si thanratio the than M-2 the by M-2 EDX by characterizationEDX characterization (Table (T2),able the 2), total the flux total of flux the of membrane the membrane M-3 and M-3 IPA and 2 1 conversionIPA conversion of oxidation of oxidation were were up to up 2.58 to 2.58 kg m kg·m− h−2·hand−1 and 98.18%, 98.18%, respectively. respectively. Hence, Hence, the the high high Ti Ti/Si/Si · · ratio seed crystals were a favor for preparing fine crystals layer and good catalytic performance TS-1 zeolite membrane in this work. Table3 presented the amounts of seeded crystals, seeded support, and membrane M-3. ca. 0.62 g seeded crystals were attached to the support by dip-coating. After crystallization, washing, drying, and calcination, the weight of the zeolite layer was ca. 0.75 g.

Table 3. Amount of the support, seeded support, and ﬁnal membrane M-3.

Support (g) Seeded Support (g) Amount of Seed (g) Membrane (g) Amount of Zeolite (g) 9.44 10.06 0.62 10.19 0.75 Membranes 2020, 10, x FOR PEER REVIEW 6 of 12 ratio seed crystals were a favor for preparing fine crystals layer and good catalytic performance TS-1 zeolite membrane in this work. Table 3 presented the amounts of seeded crystals, seeded support, andMembranes membrane 2020, 10, M-3. 41x FOR ca.PEER 0.62 REVIEW g seeded crystals were attached to the support by dip-coating. After6 of 12 11 crystallization, washing, drying, and calcination, the weight of the zeolite layer was ca. 0.75 g. ratio seed crystals were a favor for preparing fine crystals layer and good catalytic performance TS-1 zeolite membrane in this work. Table 3 presented the amounts of seeded crystals, seeded support, and membrane M-3. ca. 0.62 g seeded crystals were attached to the support by dip-coating. After crystallization, washing, drying, and calcination, the weight of the zeolite layer was ca. 0.75 g.

Figure 4. FT-IR spectra of the accompanying TS-1 zeolites: ( a)) S-2; S-2; ( b) S-3. 3.2. Effect of Seed Crystals Morphology Table 3. Amount of the support, seeded support, and final membrane M-3. Morphology and size of the seeded crystals had a great influence on the growth and performance Support Seeded Support Amount of Seed Membrane Amount of Zeolite of the zeolite membraneFigure 4. byFT-IR secondary spectra of hydrothermal the accompanying synthesis TS-1 zeolites: [19,25 ].(a The) S-2; TS-1(b) S-3. zeolite membranes (g) (g) (g) (g) (g) (M-3, M-4, M-5, and M-6, Table2) were prepared with di fferent morphology and size TS-1 zeolites 9.44 10.06 0.62 10.19 0.75 (Z-3, Z-4, Z-5, andTable Z-6, 3. Table Amount1). Figures of the support,5 and6 were seeded the support, XRD patterns, and final surfacemembrane and M-3. cross-section SEM images of the membranes M-4, M-5, and M-6. 3.2. EffectSupport of Seed CrystalsSeeded Morphology Support Amount of Seed Membrane Amount of Zeolite XRD(g) patterns of theses(g) TS-1 zeolite membranes(g) (M-4, M-5, and(g) M-6) indicated that the(g) characteristic diffractionMorphology9.44 peaks belonged and size10.06 of to the seeded MFI-type crystals zeolites 0.62 had di ffa ractiongreat influence peaks. 10.19 IPA on conversionthe growth ofand 0.75 the performance membranes wasof the high zeolite by the membrane medium sizeby seco andndary morphology hydrothermal TS-1 zeolites synthesis membrane [19,25]. (M-3, The 98.18%,TS-1 zeolite M-5, membranes 86.70%); the (M-3,3.2.fine Effect seed M-4, crystalsof M-5, Seed andCrystals was M-6, conducive Morphology Table 2) to were prepare prepared good catalytic with different performance morphology TS-1 zeolite and membranesize TS-1 zeolites in this work.(Z-3, Z-4, The Z-5, aggregated and Z-6, zeolites Table Z-41). Figures and Z-6 5 had and a large6 were size; the therefore, XRD patterns, the mullite surface support and cross-section was difficult SEMto adsorb Morphologyimages large of the aggregated and membranes size of seeded the M-4, seeded crystals M-5, crystals and from M-6. had the a suspension. great influence Hence, on the there growth were and many performance large size ofzeolites the zeolite on the membrane membranes, by andsecondary the surface hydrothermal of the membranes synthesis (M-4[19,25]. and The M-6) TS-1 were zeolite relatively membranes rough (Figure(M-3, M-4,6a,b,e,f). M-5, and The particlesM-6, Table on 2) the were surface prepared of the with membrane different were morphology large, the and contact size area TS-1 with zeolites the (Z-3,reactant Z-4, molecules Z-5, and becameZ-6, Table small, 1). Figures and the catalytic5 and 6 were performance the XRD of patterns, the membranes surface wasand poor.cross-section SEM images of the membranes M-4, M-5, and M-6.

Figure 5. XRD patterns of TS-1 zeolite membranes with different morphology of seeded crystals. (a) M-3; (b) M-4; (c) M-5; (d) M-6.

Figure 5. XRD patterns of TS-1 zeolite membranes with diﬀerent morphology of seeded crystals. Figure 5. XRD patterns of TS-1 zeolite membranes with different morphology of seeded crystals. (a) (a) M-3; (b) M-4; (c) M-5; (d) M-6. M-3; (b) M-4; (c) M-5; (d) M-6.

MembranesMembranes2020 2020,,10 10,, 41 x FOR PEER REVIEW 77 ofof 1112

FigureFigure 6.6.Surface Surface and and cross-section cross-section SEM imagesSEM images for TS-1 for zeolite TS-1 membranes zeolite membranes with diﬀerent with morphology different ofmorphology seeded crystals. of seeded (a,b) crystals. M-4; (c,d ()a M-5;,b) M-4; (e,f )(c M-6.,d) M-5; (e,f) M-6.

3.3. Effect of Seed Crystals Suspension Concentration XRD patterns of theses TS-1 zeolite membranes (M-4, M-5, and M-6) indicated that the characteristicThe mullite diffraction support waspeaks seeded belonged with to TS-1 the zeolitesMFI-type by zeolites dip-coating; diffraction the concentration peaks. IPA conversion of seeded crystalsof the membranes suspension was criticalhigh by for the preparing medium dense size and TS-1 morphology zeolite membrane TS-1 zeolites [26]. Figure membrane7 presented (M-3, the98.18%, surface M-5, and 86.70%); cross-sectional the fine seed images crystals of thewas seeded conducive supports to prepare with good different catalytic seeds performance concentration TS- suspension.1 zeolite membrane All mullite in supportthis work. surface The aggregated was fully covered zeolites with Z-4 theand fine Z-6 and had ellipsoid a large size; Z-3 zeolites, therefore, while the themullite thickness support of thewas seed difficult crystals to adsorb layer was large increased aggregated with seeded the concentration crystals from of the the suspension. seeded crystals Hence, in thethere suspension. were many As large shown size in zeolites Table2 on, the the membranes membranes, M-7, and M-8, the surface M-3, and of M-9the membranes were prepared (M-4 with and diM-6)fferent were concentration relatively rough seed (Figure suspension, 6a,b,e,f). and The the concentrationparticles on the of surf seededace of crystals the membrane suspension were was large, 2%, 3%,the 5%,contact and area 7%. Whenwith the the reactant concentration molecules of seeded became crystals small, suspension and the catalytic was 5%, theperformance membrane of M-3 the hadmembranes the highest was catalytic poor. performance in this work. Figure8 displayed the XRD patterns of these TS-1 zeolite membranes, the diffraction patterns indicated that the random orientation of the TS-1 zeolite membrane3.3. Effect of was Seed formed Crystals on Suspension the porous Concentration mullite support, and intensity of the MFI structural diffraction peaksMembranesThe was mullite2020 increased, 10, xsupport FOR with PEER thewas REVIEW seed seeded crystals with suspension TS-1 zeolites concentration. by dip-coating; the concentration of seeded8 of 12 crystals suspension was critical for preparing dense TS-1 zeolite membrane [26]. Figure 7 presented the surface and cross-sectional images of the seeded supports with different seeds concentration suspension. All mullite support surface was fully covered with the fine and ellipsoid Z-3 zeolites, while the thickness of the seed crystals layer was increased with the concentration of the seeded crystals in the suspension. As shown in Table 2, the membranes M-7, M-8, M-3, and M-9 were prepared with different concentration seed suspension, and the concentration of seeded crystals suspension was 2%, 3%, 5%, and 7%. When the concentration of seeded crystals suspension was 5%, the membrane M-3 had the highest catalytic performance in this work. Figure 8 displayed the XRD patterns of these TS-1 zeolite membranes, the diffraction patterns indicated that the random orientation of the TS-1 zeolite membrane was formed on the porous mullite support, and intensity of the MFI structural diffraction peaks was increased with the seed crystals suspension concentration.

FigureFigure 7. 7. SurfaceSurface andand crosscross sectionalsectional imagesimages ofof seededseeded supportsupport withwith didifferentﬀerent seededseeded crystalscrystals concentrationconcentration suspension suspension (Z-3). (Z-3). ( a(,a,bb)) 2%; 2%; ( c(,c,dd)) 3%; 3%; ( e(,e,ff)) 5%; 5%; ( g(g,h,h)) 7%. 7%.

Figure 8. XRD patterns of TS-1 zeolite membranes with with different seeded crystals concentration suspension. (a) 2%, M-7; (b) 3%, M-8; (c) 5 catalytic %, M-3; (d) 7%, M-9.

As shown in Figure 3e,f, and Figure 9, the TS-1 zeolite crystals layer thickness and morphology of the membranes (M-7, M-8, M-3, and M-9) were dependent on the seeded crystals suspension concentration. When the concentration of seeded crystals suspension was 5%, the zeolite layer thickness of the membrane M-3 was ca. 18 μm, and the membrane M-3 had the highest catalytic performance for the much catalytic active sites [27] because the support surface had a certain adsorption limitation and saturation of zeolite crystals. Even though the seeded crystals suspension concentration was reached 7%, excessive zeolite crystals of the support surface were dissolved into the precursor gel, and the large zeolites and intercrystalline pores were formed on the membrane M- 9 surface, which had poor performance for the IPA oxidization (Table 2).

Membranes 2020, 10, x FOR PEER REVIEW 8 of 12

Figure 7. Surface and cross sectional images of seeded support with different seeded crystals Membranes 2020, 10, 41 8 of 11 concentration suspension (Z-3). (a,b) 2%; (c,d) 3%; (e,f) 5%; (g,h) 7%.

Figure 8. XRD patterns of TS-1 zeolite membranes with with diﬀerent seeded crystals concentration Figure 8. XRD patterns of TS-1 zeolite membranes with with different seeded crystals concentration suspension. (a) 2%, M-7; (b) 3%, M-8; (c) 5 catalytic %, M-3; (d) 7%, M-9. suspension. (a) 2%, M-7; (b) 3%, M-8; (c) 5 catalytic %, M-3; (d) 7%, M-9. As shown in Figure3e,f, and Figure9, the TS-1 zeolite crystals layer thickness and morphology As shown in Figure 3e,f, and Figure 9, the TS-1 zeolite crystals layer thickness and morphology of the membranes (M-7, M-8, M-3, and M-9) were dependent on the seeded crystals suspension of the membranes (M-7, M-8, M-3, and M-9) were dependent on the seeded crystals suspension concentration. When the concentration of seeded crystals suspension was 5%, the zeolite layer concentration. When the concentration of seeded crystals suspension was 5%, the zeolite layer thickness of the membrane M-3 was ca. 18 µm, and the membrane M-3 had the highest catalytic thickness of the membrane M-3 was ca. 18 μm, and the membrane M-3 had the highest catalytic performance for the much catalytic active sites [27] because the support surface had a certain adsorption performance for the much catalytic active sites [27] because the support surface had a certain limitation and saturation of zeolite crystals. Even though the seeded crystals suspension concentration adsorption limitation and saturation of zeolite crystals. Even though the seeded crystals suspension was reached 7%, excessive zeolite crystals of the support surface were dissolved into the precursor gel, concentration was reached 7%, excessive zeolite crystals of the support surface were dissolved into and the large zeolites and intercrystalline pores were formed on the membrane M-9 surface, which had the precursor gel, and the large zeolites and intercrystalline pores were formed on the membrane M- poorMembranes performance 2020, 10, x forFOR the PEER IPA REVIEW oxidization (Table2). 9 of 12 9 surface, which had poor performance for the IPA oxidization (Table 2).

Figure 9. Surface and cross-section SEM images for the TS-1 membranes prepared with diﬀerent seeded Figure 9. Surface and cross-section SEM images for the TS-1 membranes prepared with different crystal suspension concentrations. (a,b) 2%, M-7; (c,d) 3%, M-8; (e,f) 7%, M-9. seeded crystal suspension concentrations. (a,b) 2%, M-7; (c,d) 3%, M-8; (e,f) 7%, M-9. 3.4. Comparison of Bi-Layer and Mono-Layer TS-1 Zeolite Membranes 3.4. Comparison of Bi-Layer and Mono-Layer TS-1 Zeolite Membranes In our previous study, the bi-layer TS-1 zeolite membrane had a good catalytic performance for In our previous study, the bi-layer TS-1 zeolite membrane had a good catalytic performance for IPA and H2O2 [20]. As shown in Figure 10a,b, the out and inner support surface were fully covered by theIPA dense and TS-1H2O2 zeolite [20]. As layer. shown Mass in transferFigure 10a,b, resistance the out was and depended inner support on the thickness surface were of the fully membrane covered layer,by the the dense preparation TS-1 zeolite of the layer. mono-layer Mass membranetransfer resistance was a favor was fordepended increasing on thethe membranethickness of ﬂux. the membrane layer, the preparation of the mono-layer membrane was a favor for increasing the membrane flux. Because the nano zeolite crystals could easily enter the mullite support (average pore size, ca. 1.3 μm) and grow into a bi-layer TS-1 zeolite membrane, which could increase the mass transfer resistance of the membrane. In order to prepare a thin TS-1 zeolite membrane layer, the mullite tubes were added with the PTFE rods during the crystallization process in this work. As shown in Figure 10c,d, a dense and fine TS-1 zeolite layer was formed on the outer surface of the support; there were only a few seed crystals were scattered on the inner surface of the support, which could not form a continuous zeolite layer. Table 2 exhibited the catalytic performance of the bi-layer (M-10) and mono-layer (M-3) TS-1 zeolite membranes, the flux and IPA conversion of the membrane M-10 was only 1.98 kg·m−2·h−1 and 93.27%. The membrane M-3 had the plenty of fine TS-1 zeolite crystals and a thin mono-layer zeolite layer, the flux and IPA conversion of were reached 2.58 kg·m−2·h−1 and 98.23%.

Figure 10. Outer (a and c) and inner (b and d) surface images of the bi-layer (M-10, a and b) and mono- layer (M-3, c and d) TS-1 zeolite membranes.

Membranes 2020, 10, x FOR PEER REVIEW 9 of 12

Figure 9. Surface and cross-section SEM images for the TS-1 membranes prepared with different seeded crystal suspension concentrations. (a,b) 2%, M-7; (c,d) 3%, M-8; (e,f) 7%, M-9.

3.4. Comparison of Bi-Layer and Mono-Layer TS-1 Zeolite Membranes In our previous study, the bi-layer TS-1 zeolite membrane had a good catalytic performance for IPA and H2O2 [20]. As shown in Figure 10a,b, the out and inner support surface were fully covered Membranesby the dense2020, 10 TS-1, 41 zeolite layer. Mass transfer resistance was depended on the thickness of9 ofthe 11 membrane layer, the preparation of the mono-layer membrane was a favor for increasing the Becausemembrane the flux. nano Because zeolite crystalsthe nano could zeolite easily crystals enter co theuld mullite easily enter support the (averagemullite support pore size, (average ca. 1.3 poreµm) andsize, grow ca. 1.3 into μm) a bi-layer and grow TS-1 into zeolite a bi-layer membrane, TS-1 whichzeolite could membrane, increase which the mass could transfer increase resistance the mass of thetransfer membrane. resistance In order of the to membrane. prepare a thin In TS-1order zeolite to prepare membrane a thin layer, TS-1 thezeolite mullite membrane tubes were layer, added the withmullite the tubes PTFE rodswere duringadded thewith crystallization the PTFE rods process during in thisthe work.crystallization As shown process in Figure in 10thisc,d, work. a dense As andshown fine in TS-1 Figure zeolite 10c,d, layer a wasdense formed and fine on theTS-1 outer zeolite surface layer of was the support;formed on there the were outer only surface a few of seed the crystalssupport; were there scattered were only on a the few inner seed surface crystals of were the support,scattered which on the could inner notsurface form of a the continuous support, zeolite which layer.could Tablenot form2 exhibited a continuous the catalytic zeolite performancelayer. Table 2 of exhibited the bi-layer the catalytic (M-10) and performance mono-layer of (M-3)the bi-layer TS-1 (M-10) and mono-layer (M-3) TS-1 zeolite membranes, the flux and IPA conversion of the membrane2 1 zeolite membranes, the flux and IPA conversion of the membrane M-10 was only 1.98 kg m− h− and −2 −1 · · 93.27%.M-10 was The only membrane 1.98 kg·m M-3·h had and the 93.27%. plenty of The fine membrane TS-1 zeolite M-3 crystals had the and plenty a thin of mono-layer fine TS-1 zeolitezeolite crystals and a thin mono-layer zeolite layer, the flux and 2IPA1 conversion of were reached 2.58 layer, the flux and IPA conversion of were reached 2.58 kg m− h− and 98.23%. kg·m−2·h−1 and 98.23%. · ·

Figure 10. Outer (a,c) and inner (b,d) surface images of the bi-layer (M-10, a and b) and mono-layer Figure 10. Outer (a and c) and inner (b and d) surface images of the bi-layer (M-10, a and b) and mono- (M-3, c and d) TS-1 zeolite membranes. layer (M-3, c and d) TS-1 zeolite membranes. Table4 summarized the catalytic performance of five pieces pf mono-layer TS-1 zeolite membrane with the zeolite Z-3. Clearly, the membranes had a similar catalytic performance, which showed the mono-layer TS-1 membrane preparation had good reproducibility in this work. Besides, titanosilicate stability was important for the TS-1 zeolite membrane [28]. The repeatable catalytic performance of the mono-layer and bi-layer membranes (M-3 and M-10) was studied in this work (Table5). The IPA conversion of the two membranes showed no obvious changes, indicating that the catalysts were robust catalysts because the structure destruction, loss of framework Ti, or the coke deposition of the catalyst [29,30], both the flux of the membranes were decreased with the running times, and the declining of the bi-layer TS-1 membrane flux (M-10) was larger than the mono-layer TS-1 membrane (M-3). The mono-layer TS-1 zeolite membrane presented the better catalytic performance and repeatability than the bi-layer zeolite membrane, particularly the flux.

Table 4. Catalytic performance of TS-1 zeolite membranes in IPA/H2O2 mixture at 70 ◦C.

No. Q (kg m 2 h 1) C (%) · − · − M-3 2.58 98.18 M-11 2.63 97.34 M-12 2.30 93.87 M-13 2.48 96.64 M-14 2.18 97.29

Note: The molar composition of the synthesis gel of the TS-1 zeolite membrane was SiO2: 0.035 TiO2: 0.25 TPAOH: 120 H2O, the crystallization temperature and time were 150 ◦C and 1 d. Membranes 2020, 10, 41 10 of 11

Table 5. Repeatable catalytic performance of TS-1 zeolite membranes (M-3 and M-10).

M-3 M-10 Running Times Q (kg m 2 h 1) C (%) Q (kg m 2 h 1) C (%) · − · − · − · − 1 2.58 98.23 1.98 93.27 2 2.45 98.18 1.82 93.04 3 2.34 97.91 1.62 93.65

4. Conclusions In summary, a facile and green approach to produce acetone by a mono-layer TS-1 zeolite membrane catalyst had been developed. The effects of seed crystals on the growth and properties of TS-1 zeolite membranes were investigated. When the zeolite seed crystal had a suitable Ti/Si ratio, size, morphology and seed suspension concentration (0.030, 300 nm, ellipsoid, 5%, respectively), a dense and mono-layer TS-1 zeolite membrane was fully covered on the seeded mullite support, the IPA conversion and the flux of TS-1 zeolite membrane were 98.23% and 2.58 kg m 2 h 1. · − · − Author Contributions: Conceptualization, M.Z. and X.C.; Methodology, M.Z.; Software, W.D.; Validation, W.D., S.X. and F.Y.; Formal Analysis, T.G.; Investigation, Y.L.; Resources, F.Z.; Data Curation, W.D.; Writing—Original Draft Preparation, W.D. and M.Z.; Writing—Review & Editing, W.D. and M.Z.; Visualization, N.H.; Supervision, M.Z.; Project Administration, X.C.; F unding Acquisition, M.Z. and X.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 21868012 and 21968009), Jiangxi Provincial Department of Science and Technology (20171BCB24005, 20181ACH80003, 20192ACB80003 and 20192BBH80024). Conflicts of Interest: The authors declare no conflict of interest.

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