Direct Synthesis of Zeolites from a Natural Clay, Attapulgite - ACS

Research Article pubs.acs.org/journal/ascecg

Direct Synthesis of Zeolites from a Natural Clay, Attapulgite Xing-Yang Li,†,‡ Yao Jiang,† Xiao-Qin Liu,*,† Li-Ying Shi,† Dong-Yuan Zhang,† and Lin-Bing Sun*,† †

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China ‡ College of Biological and Chemical Engineering, Anhui Polytechnic University, Middle Beijing Road, Wuhu 241000, Anhui, China S Supporting Information *

ABSTRACT: Presently, chemical Si/Al sources are predominantly used as raw materials for the synthesis of zeolites in spite of their high cost. Here, we report for the first time the direct synthesis of a ZSM-5 zeolite by using a natural clay, attapulgite (ATP), as the lowcost and environmentally benign Si/Al source through a vaporinduced transformation (VIT) method, in which the vapor diffuses into ATP and interacts with the framework. In a crystallization recipe, ATP is well present with a pristine crystal structure, and the yield of zeolite reaches 96%. The resultant zeolite exhibits welldefined crystallinity and porosity, which are comparable to the counterpart synthesized from traditional chemical Si/Al sources. In the synthetic process, the formation of Si−F species plays an important role, which promotes the transformation of an ATP crystal to a zeolite crystal. Meanwhile, the metal elements (e.g., Al, Fe, and Mg) of ATP exist in the zeolite products, and at least part of them are located in the zeolite framework, which endows the zeolite with excellent catalytic activity. Direct synthesis of various zeolite crystals including TON, MOR, and *BEA from ATP is also realized via the VIT method. Our work provides a green and economic alternative for the synthesis of zeolites using natural clays instead of chemical raw materials. KEYWORDS: Direct synthesis, Natural clay, Zeolite, Si/Al sources, Vapor-induced transformation

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structure.17−19 Its crystal structure can be described as two bands of SiO4 tetrahedra linked by metal ions (i.e., Al, Mg, and Fe) in octahedral coordination (Scheme S1).20−22 Si and Al elements are basic components of ATP which are the main raw materials for zeolites synthesis. Notably, ATP contains a small amount of Fe and Mg (Table S1), which are important potential heteroatoms of zeolites. Moreover, ATP has an appropriate Si/Al ratio and similar Si−O tetrahedra bonds to zeolites, which makes it a promising candidate in replacing chemical raw materials for zeolites. More importantly, the price of ATP is approximately 200 USD per ton, which is only 1/5 of the price of chemical Si/Al raw materials.15,23 Therefore, attempts have made to synthesize zeolites from natural clay minerals: Pan et al. successfully synthesized HZSM-5 from natural clay, kaolin, which was subjected to heat treatment and acid treatment in turn; the obtained HZSM-5 zeolite showed good catalytic performances for the MTO reaction.24 Rectorite, a natural aluminosilicate mineral, was also used as a starting material for the synthesis of zeolite after being alkali-fused at 900 °C.25 Recently, the direct synthesis of characteristic zeolites (including ZSM-5,9 USY,26 and TES-1027) using traditional

INTRODUCTION Zeolites, a family of microporous silica-, aluminosilicate-, or titanosilicate-based crystals, have been widely used in industrial processes as catalysts, adsorbents, and additives.1−8 Due to their unique pore structure, high thermal and hydrothermal stability, and tunable active sites, the applications of zeolites have been continuously expanded. For example, one application of the prominent zeolite, ZSM-5, is catalytic conversion of methanol to gasoline and propene (known as MTG and MTP).9 Due to the unique microporous structure of AlPOs and SAPOs, these zeolites have been employed in the commercialized methanol to olefins (MTO).10,11 ZSM-22 with one-dimensional channels has been used as a catalyst for hydroisomerization reactions and a methanol to hydrocarbon (MTH) process.12,13 It is worth noting that the regular feedstocks of zeolites are chemical Si and Al sources (e.g., sodium silicate, silica, aluminum sulfate, and sodium aluminate) at present, regardless of their high cost. In industry, the cost percentage of chemical raw materials for zeolites synthesis is around 40%.14−16 Also, these Si and Al chemicals are manufactured from natural Si and/or Al minerals, which is associated with the production of a large amount of wastes. Despite a great challenge, direct synthesis of zeolites from low-cost and green raw materials is extremely desirable. Attapulgite (ATP) is a natural Si/Al-containing clay with nanoscaled, fiber-shaped clusters, and a 2:1 phyllosilicate © 2017 American Chemical Society

Received: April 2, 2017 Revised: May 16, 2017 Published: June 5, 2017 6124

DOI: 10.1021/acssuschemeng.7b01001 ACS Sustainable Chem. Eng. 2017, 5, 6124−6130

ACS Sustainable Chemistry & Engineering

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chemical Si/Al sources by the hydrothermal method has been reported. However, previously reported clays were destructively activated by strong acid, high temperature (>1073 K), or alkali melting to extract the Si and/or Al sources where the crystal of the clay is no longer present in the reported recipe.28−31 In the work, the “direct synthesis” highlights that the clay (ATP) does not require deep activation before the crystallization process, which is directly used with its pristine crystal structure. Corresponding to direct synthesis, the “indirect synthesis” means that the clays were destructively activated before the crystallization process in order to obtain the effective ingredients. To our knowledge, direct synthesis of zeolite from natural clay (ATP) with pristine crystal structure has never been reported. For now, zeolites are commonly prepared using the hydrothermal method, in which water is usually used as the solvent. Nonetheless, several issues exist in hydrothermal synthesis, such as low yield, large amount of wastewater, and high autogenous pressure. Previously, great effort has been devoted to ameliorate these issues.5,32−35 More recently, Xiao’s group reported a solvent-free route for zeolites synthesis, which leads to decreased waste but increased yield. The method involves mixing, grinding, and heating solid raw materials in the absence of solvent.4,36−39 In comparison with conventional hydrothermal synthesis, these methods involving less or no solvents are highly potential for scale-up and industrial application.40−42 Here, we report for the first time the direct synthesis of zeolites from the natural clay ATP by using a vapor-induced transformation (VIT) method (Scheme 1). In the vapor

RESULTS AND DISCUSSION

As a proof-of-concept, the synthesis of an MFI-type zeolite, ZSM-5, from ATP was first attempted. In a typical process, ATP, SDA (tetrapropylammonium bromide, TPABr), and NH4F were ground mechanically, followed by heating in a vapor (e.g., ammonia or water) atmosphere generated at 180 °C for 48 h (Scheme 1). Synthetic parameters including the molar ratio of F−/SiO2 (FSR), the ratio of TPA+/SiO2 (TSR), and the types of vapor were optimized, achieving up to 96% zeolite yield based on SiO2. The high yield can be ascribed to the use of ATP with a well-preserved pristine crystal structure as the Si/Al source. The Si−O tetrahedra bonds in a pristine ATP crystal are similar to those in zeolites, which facilitates the formation of zeolite frameworks but is absent in conventional chemical Si/Al sources. Therefore, the zeolite yield from ATP was enhanced obviously (Table S3). Various techniques were employed to characterize the resultant zeolite. The XRD pattern presents the typical peaks associated with an MFI-type structure whose characteristic diffraction peaks are at 7−9° and 22−25° and that of ATP (typically at 2θ of 8.4°) disappears (Figure 1a). The SEM image of a ZSM-5 zeolite (Figure 1b) exhibits almost perfect crystals with coffin-shaped morphology, which is fundamentally different from the fiber-shaped morphology of ATP (Figure 1d).43−46 Compared with a reference zeolite, the ZSM-5 zeolite displays high relative crystallinity up to 106% (Figure S1). The N2 adsorption−desorption isotherm of ZSM-5 zeolite is a typical Langmuir adsorption curve classified into type I and that of ATP is type II (Figure 1c). Correspondingly, the BET surface area and micropore volume of the ZSM-5 zeolite are calculated to be 402 m2/g and 0.13 cm3/g, respectively, which are comparable to the counterpart synthesized from chemical Si/Al sources (Table S4).45,47−49 The characteristic band of a pentasil framework at 550 cm−1 is observed clearly in IR spectra (Figure S2). Furthermore, a peak at 1225 cm−1 is formed, which is attributed to the asymmetric stretching vibration of the T−O bond.49 To inspect the generalizability of our strategy, three kinds of ATP from different regions were used as the Si/ Al sources (Table S2). XRD patterns of the zeolites synthesized from different ATPs (Figure S3) present the characteristic diffraction peaks of the ZSM-5 zeolite. ATPs from different regions with different compositions can be used as Si/Al sources to synthesize zeolites through the VIT method. On the basis of the above results, it is safe to say that ZSM-5 zeolite is synthesized successfully from an ATP crystal without the use of any extra Si and Al sources. In the process of synthesis, the factors involving the types of vapor and fluoride, FSR, and TSR affect the crystallization of zeolite greatly. Figure S4 shows XRD patterns of the products with different vapors and fluorides. In the presence of vapor (either H2O or NH3·H2O) and fluoride (either NaF or NH4F), the ZSM-5 zeolite can be formed. In the absence of vapor, however, no zeolite products are observed. Apparently, the vapor plays a crucial role in the formation of zeolite. The highest crystallinity of ZSM-5 zeolite (106%) is obtained in NH3·H2O-NH4F combination, and the relative crystallinity for H2O-NH4F, NH3·H2O-NaF, and H2O-NaF combination are 43%, 51%, and 31%, respectively. Figures S5 and S6 display XRD patterns of the products at different FSR and TSR. Accordingly, the optimal ranges of FSR and TSR for the ZSM-5 zeolite are 0.8−1.5 and 0.03−0.15, respectively. Figure S7 shows XRD patterns of zeolites synthesized at different

Scheme 1. Direct Synthesis of Zeolite Crystal from ATP Crystal by the VIT methoda

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M refers to Al, Mg, or Fe.

atmosphere generated at elevated temperatures, the ATP crystal can be transformed to various zeolite crystals (including MFI, TON, MOR, and *BEA) in the presence of a structuredirecting agent (SDA) and fluoride with a high yield of 96%. This yield is obviously higher than that from conventional chemical Si/Al sources (30−80%). The obtained zeolites show high crystallinity and abundant porosity, which are comparable to their analogues synthesized from chemical Si/Al sources. More importantly, the present strategy can produce H-type zeolites directly without an ion-exchange process. These properties endow the resultant zeolites with good catalytic performance in alkylation reactions. 6125


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Figure 1. (a) XRD patterns of ATP and ZSM-5, (b) SEM image of ZSM-5, (c) N2 adsorption−desorption isotherms of ATP and ZSM-5, and (d) SEM image of ATP.

crystallization temperatures. The ZSM-5 zeolite can be obtained at the range of 140−190 °C. With an increase in temperature, the relative crystallinity was gradually enhanced. At 140 and 160 °C, the relative crystallinity of ZSM-5 are 42% and 56%, respectively. When the temperature was increased from 180 to 190 °C, no significant change in the relative crystallinity was observed. From the perspective of lower energy consumption, the appropriate crystallization temperature is 180 °C. To understand the NH3·H2O vapor-induced transformation, DTG and TG curves of the resulted ZSM-5 zeolite were recorded. DTG curves (Figure S8a) display a peak at around 450 °C attributed to the decomposition of TPA+ in all samples. Interestingly, a peak at 487 °C, which is associated with the decomposition of NH4+ ions, is observed only on a ZSM-5 zeolite resulted from NH3·H2O-NH4F combination. The overall weight loss of the ZSM-5 zeolite is 13.6% in which weight loss of NH4+ is 1.8% (Figure S8c) and that of the others is 10.5% (Figure S8b). Higher weight loss of the zeolite indicates that more cations (TPA+/NH4+) are in demand to neutralize the excessive framework negative charges due to the introduction of Al or other heteroatoms.50,51 On the basis of these results, it is safe to say that the NH4+ ions enter into the channels of the zeolite to balance the framework negative charges. XRD patterns of ZSM-5 zeolites at different transformation times are displayed in Figure 2a. Before transformation, ATP adopted as raw material well preserves its pristine crystal structure and the fiber-shaped morphology (Figure S9). XRD patterns of each raw material and the grinded mixture are shown in Figure S10. Notably, new diffraction peaks at 15.7° and 21.8° are observed, which are associated with (NH4)2SiF6 (JCPDS no. 03-0097) resulting from the interaction between ATP and NH4F. It is clear that grinding the starting raw materials actuates a chemical reaction rather than a simple physical mixture.4 At the transformation time of 4 h, the peaks associated with the starting raw solids disappear; meanwhile,

Figure 2. (a) XRD patterns of ATP as well as ZSM-5 crystallized for different times. (b) 19F, (c) 29Si, and (d) 27Al MAS NMR spectra of samples crystallized for different times. The reference sample is Silicalite-1.

the characteristic peaks of the ZSM-5 zeolite are observed. The transformation process is up to the utmost limit at 48 h, where the relative crystallinity runs up to maximum (Figure S1). The fiber-shaped ATP are observed from SEM images of the samples transformed for 0, 2, and 4 h, and the morphology is almost invisible at 6 h. When the crystallization time is more than 12 h, the coffin-shaped ZSM-5 crystals can be identified obviously (Figure S11). To examine the transformation process, 27Al, 29Si, and 19F MAS NMR spectra were recorded (Figures 2b−d). Due to the strong quadrupole interactions, only the central transition can 6126


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ACS Sustainable Chemistry & Engineering be observed in the 27Al MAS NMR spectra.52 Therefore, we qualitatively discuss the changes of the NMR spectra. For the product crystallized for 0 h, the band at −122.4 ppm (19F spectrum) gets a feedback in the 29Si spectrum at −189.3 ppm, and both of the bands are assigned to SiF62− ions, as confirmed by XRD patterns (Figure S10).53,54 With an increase in time, the solid is transformed gradually. To be specific, when the transformation time is 16 h, a new peak at −63.7 ppm appears in19F spectra (Figure 2b), which is assigned to F− ions in the [415262] cage of the MFI structure, and the peak at −122.4 ppm is weakened.55,56 Furthermore, as for 29Si NMR spectra (Figure 2c), a predominately bulk band at −114.8 ppm is observed at 48 h, which is assigned to Q4 silica species [Si(SiO)4], and the peak at −189.3 ppm becomes weak and then disappears over 16 h. This conversion of 29Si spectra demonstrates the progressive transformation of the SiF62− ions into the Q4 silica species, and the transformation is completed at 48 h.33,47,57,58 Probably, the formation of Si−F species drives the crystallization of the ZSM-5 crystal. As observed from 27Al NMR spectra (Figure 2d), the band at −1.5 ppm is dominant in the starting sample at 0 h, which is associated with octahedral coordinated aluminum. Notably, when the transformation time reaches 16 h, the sample shows XRD characteristic peaks associated with the ZSM-5 zeolite, and a new peak at 53.1 ppm appears in 27Al NMR spectra, which is associated with tetrahedral lattice Al in the ZSM-5 framework, indicating the low crystallinity of zeolite (59.4%).59−61 The higher content of Q4 silica species the ZSM-5 product has, the higher the crystallinity is. With the crystallization time increasing from 16 to 48 h, the intensity of XRD peaks and 27Al NMR spectra at 53.1 ppm further increase, suggesting the successful transformation from octahedrally coordinated Al in ATP to a tetrahedral environment (Al in the ZSM-5 framework). At the crystallization time of 48 h, 27Al NMR spectra give a tiny band associated with octahedrally coordinated Al, which shows that overwhelmingly most of octahedrally coordinated Al has been converted to tetrahedrally coordinated Al. When the crystallization time is over 48 h, there is no obvious change in XRD patterns, indicating that the transformation from ATP to ZSM5 is basically finished. In consideration that the ZSM-5 product is in a state of solid power throughout crystallization, we think that the transformation mechanism is in accordance with the solid-state synthesis.62,63 Taking into consideration that ATP comprises Si as well as metals (Al, Fe, and Mg), the fate of metals is examined by various methods including EDS mapping, XPS, and UV−vis spectra. First, chemical composition of a ZSM-5 zeolite is characterized by EDS mapping (Figure S12), and the results show the presence of Si, Al, Mg, and Fe with homogeneous distribution. The existence of Al, Mg, and Fe in addition to Si is confirmed by XRF (Table S1). However, no F was observed in the EDS mapping. To investigate the fate of fluorine, we compared the XRD patterns of the washed and unwashed assynthesized ZSM-5 (Figure S13). The extra peaks of the unwashed ZSM-5 at 13.8°, 25.6°, and 40.6° are attributed to the AlF3 complexes (JCPDS no. 43-0435). The diffraction peaks of the complexes, however, were not observed in the washed ZSM-5. Furthermore, the Al 2p XPS spectrum of ZSM5 shows a peak at 75.7 eV (Figure S14), which corresponds to Al−O bonds in the ZSM-5 framework. The Fe 2p spectrum of ZSM-5 has two signals at 726.4 and 713.5 eV (Figure S15), which are associated with Fe 2p1/2 and Fe 2p3/2, respectively. The Mg 1s binding energy of ZSM-5 is at 1304.6 eV (Figure

S16). Moreover, compared with Silicalite-1, the UV−vis spectra of the ZSM-5 zeolite (Figure S17) displays a strong adsorption at 258 nm, which is associated with ligand to metal charge transfer involving oxygen to 4-coordinated Fe3+ p-d charge transfer in [FeO4]− tetrahedra. Furthermore, three weak bands at ca. 372, 407, and 436 nm are observed in the magnified spectrum, which are assigned to the weak spin forbidden d-d transitions of the Fe3+ ion in tetrahedral symmetry.64,65 The weak band at 220 nm can be attributed to Mg2+ at tetrahedral framework sites. On the basis of these results, it is safe to say that metals exist in the ultimate products, and at least part of them locate in the zeolite framework. Furthermore, the Si/Al ratio of the ZSM-5 zeolite from XPS and XRF was compared. Due to the close Si/Al ratio from the two methods, Al should be uniformly distributed through the crystallites (Table S1). In addition, other heteroatom sources could be used as the raw materials. Under proper synthetic conditions, these heteroatoms might be possible to incorporate into the framework of zeolites. Though these adatoms such as Fe and Mg may in fact limit potential applications of the zeolites, for the specific system, heteroatom-containing zeolites have a beneficial effect. For example, Fe-ZSM-5 is active in numerous reactions of industrial and environmental relevance, such as selective oxidations with N2O66 and selective catalytic reduction of NOx.67 Mg-containing zeolites could be applied for the Knoevenagel condensation reaction.68 For our direct synthesis strategy, the useful composition of ATP including heteroatoms (Fe and Mg) was utmost utilized as the raw materials, so the zeolite yield was enhanced obviously. For all that, to expand the scope of application and weaken the environmental issue, we are going to continue working on ways of decreasing/replacing heteroatoms or doing this without fluorides. The surface acidic properties were probed by NH3-TPD. In comparison with ATP, two desorption peaks are observed in the NH3-TPD profile of the ZSM-5 zeolite centered at 190 and 320 °C, which can be assigned to weak and strong acid sites, respectively (Figure S18).60,63,69 However, the NH3-TPD profile of ATP is flat without distinct desorption peaks. For the ZSM-5 zeolite, the total amount of NH3 desorption is calculated to be 0.18 mmol g−1. To evaluate the acidic catalytic performance of the ZSM-5 zeolite, the alkylation reaction of toluene with benzyl bromide is conducted (Figure S19). The conversion of benzyl bromide over ATP is only 29.8% at 180 min; it is worthy of note that the conversion over ZSM-5 is as high as 97.1%. These results demonstrate that the synthesized ZSM-5 zeolite exhibits strong acidic sites due to Al and/or Fe ions in the zeolite framework. Various zeolites with TON, MOR, and *BEA topology can also be synthesized from ATP directly. Their XRD patterns and SEM images are displayed in Figure 3 and indicate high crystallinity of the products. In IR spectra, the double-ring characteristic peaks32 (Figure S20) of the TON zeolite are discovered at 552 and 645 cm−1. The pentasil framework characteristic band (Figure S23) of the MOR zeolite is observed at 550 cm−1. The characteristic bands (Figure S26) of the *BEA zeolite are at 519 and 567 cm−1. In the UV−vis spectra (Figures S21, S24, and S27), intense adsorption bands are observed at 215, 255, and 250 nm for the three zeolites, respectively, which indicates that heteroatoms from ATP could be introduced to the zeolite framework. The surface areas of TON, MOR, and *BEA zeolites are 149, 428, and 521 m2/g calculated by N2 adsorption−desorption isotherms (Figures S22, S25, and S28). Their microporous volumes are 0.07, 0.16, 6127


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XRD, SEM, IR, TG/DTG, element mapping, XPS, UV− vis, NH3-TPD, and N2 adsorption−desorption isotherms of the different samples. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 25 83587177. [email protected] (X.-Q. *Tel.: +86 25 83587177. [email protected] (L.-B.

Fax: +86 25 83587191. E-mail: Liu). Fax: +86 25 83587191. E-mail: Sun).

ORCID

Yao Jiang: 0000-0002-2316-8274 Lin-Bing Sun: 0000-0002-6395-312X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work by the National High Technology Research and Development Program of China (863 Program, 2013AA032003), National Natural Science Foundation of China (21576137, 21676138, and 51572004), Distinguished Youth Foundation of Jiangsu Province (BK20130045), Fok Ying-Tong Education Foundation (141069), National Basic Research Program of China (973 Program, 2013CB733504), and Project of Priority Academic Program Development of Jiangsu Higher Education Institutions.

Figure 3. XRD patterns of (a) TON, (c) MOR, and (e) *BEA and SEM images of (b) TON, (d) MOR, and (f) *BEA.

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and 0.20 cm3/g, respectively. These properties are comparable with those of zeolites obtained via traditional chemical Si/Al sources (Table S5).70−73

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CONCLUSIONS In summary, zeolites with MFI, TON, MOR, and *BEA structures have been successfully synthesized from the natural clay ATP crystal directly, for the first time. In the VIT process, the solid gel was obtained by grinding ATP, fluoride, and SDA before crystallization. Notably, such strategy is distinguishable from steam-assisted conversion, where the solvent is necessary for the preparation of homogeneous gels, then vaporized to obtain the dry gels. In our recipe, the crystal of ATP is well preserved with a pristine structure, which is the essential difference from the traditional hydrothermal method where the clay is no longer present in the crystallization recipe. Interestingly, the Al ions from ATP can be incorporated into zeolite frameworks, the Fe and Mg ions of ATP exist in the ultimate products, and at least part of them locate in the zeolite framework. H-type zeolites are obtained without ion exchange, which endows zeolites with high catalytic performance in alkylation. Compared with chemical Si/Al sources, direct synthesis of zeolites from ATP has prominent advantages with low cost and high zeolite yield. Our work may open a new door for direct synthesis of zeolites from natural clay resources and is highly potential for industrial application at a large scale.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01001. 6128


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Direct Synthesis of Zeolites from a Natural Clay, Attapulgite - ACS

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