Industrial Kaolin Waste as Raw Material in the Synthesis of the SAPO-34 Molecular Sieve

2020 High-purity SAPO-34 was synthesized using industrial kaolin waste produced in the Amazon region as the main source of silicon and aluminium. The hydrothermal method was applied to investigate the effect of the Si/Al ratio and crystallization time on the phase formation and purity. The materials were characterized by X-ray fluorescence, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, which demonstrated that SAPO-34 with greater purity was obtained only for the composition whose Si/Al ratio was approximately 1, which was obtained by inserting Al 2 O 3 into the synthesis


Introduction
Silicoaluminophosphates (SAPOs) are a class of synthetic molecular sieves widely used in catalytic processes due to the acid characteristics of their active sites. SAPO-34 stands out for having moderate acid strength, which favours its use in various types of reactions 1 . SAPO-34 has been used with success in methanol-to-olefin (MTO) or dimethyl-ether-to-olefins (DTO) reaction 2,3 , selective catalytic reduction (NH 3 -SCR) of NO x 4 , membranes for CO 2 separations 5,6 , etc. Most SAPO-34 synthesis methodologies use hydrothermal processes by applying analytical reagents. Usually, silicon sol and Aerosil silica are used as sources of silicon with pseudoboehmite or aluminium isopropoxide as the starting source for aluminium which makes the process more expensive and limited 1 . Phosphoric acid, water, and an structure directing agent (SDA) complete the synthesis gel. Wang et al. 7 , for example, used phosphoric acid and silica sol and pseudoboehmite as precursors and triethylamine as a SDA in the synthesis of SAPO-34. Singh et al. 8 employed pseudoboehmite, Aerosil silica, and orthophosphoric acid as starting materials in the production of SAPO-34 and morpholine as an SDA.
The search for alternative sources is relevant to evaluate the possibility of synthesis of different molecular sieves. The Industrial kaolin waste is a reject from beneficiation stage produced in large-scale and deposited in open-pit lagoons. Due to its physicochemical characteristics it is highly promising to be used as a source of silicon and aluminium, besides being low cost and abundant. Hildebrando et al. 9 , Silva et al. 10 , and Maia et al. 11 used kaolin waste from the Amazon as a precursor of various zeolites and obtained excellent synthesis products.
In this current study, we have reported the synthesis of high quality SAPO-34 using the hydrothermal route with industrial kaolin waste as a new source of silicon and aluminum. The effect of the SiO 2 /Al 2 O 3 ratio with or without aluminum external source adjustment and crystallization time on the phase purity of the synthesized materials was investigated.

Chemicals and materials
The materials used in the synthesis were kaolin waste from the tailings pond of an industrial facility in the Amazon region, orthophosphoric acid (85%) (Neon), morpholine (C 4 H 9 NO, MW = 87.1 g/mol, EP = 129 ºC) (Synth), calcined aluminium hydroxide (Synth), and distilled water.

Synthesis of SAPO-34 samples
The process was initiated by calcination of the kaolin waste at 700 ºC for 2 h. SAPO-34 was prepared via a hydrothermal route with two molar ratios of SiO 2 /Al 2 O 3. The first considered the ratio of the industrial waste itself (SiO 2 /Al 2 O 3 ~ 2), and the second considered the compensation of the aluminium, performed by the insertion of aluminium hydroxide calcined at 700 ºC to meet the requirements for the synthesis composition established by the International Zeolite Association (IZA), which for SAPO-34 is Al 2 O 3 :1.08SiO 2 :1.06P:2R:66H 2 O in which R is the SDA. This study used morpholine as the SDA. Both methodologies were initiated with a mixture of metakaolin and morpholine, followed by the addition of phosphoric acid, distilled water, and calcined aluminium *e-mail: darllandorosario@gmail.com hydroxide when necessary. The system was agitated for 4 h and then sealed in an autoclave made of steel internally coated with PTFE at 200 °C at different time intervals. After the crystallization time, the autoclave was cooled to room temperature, and the synthesis products were filtered and washed until they achieved neutral pH and dried at 105 °C for 24 h.

Characterization of the kaolin and metakaolin waste
The chemical composition of the industrial kaolin waste was obtained by XRF, and the material consisted mainly of silicon oxide (47.3%) and aluminium (37.0%). The iron and titanium oxides were also observed in percentages of 1.03% and 0.83%, respectively, with a loss on ignition of 13.84%.
The direct use of kaolin waste in the synthesis of molecular sieves was not indicated because the material was stable and led to the formation of structures with different crystalline phases and a very significant amount of amorphous material. Thus, it was necessary to modify the kaolin waste for its activation. This procedure was performed through a calcination process, which promoted rupture of the kaolinite structure, transforming it into an amorphous material a metakaolin more reactive material than the original material 11 . Figure 1 shows the diffractograms related to the kaolin and metakaolin wastes produced. The waste is characterized by the presence of peaks at the 12.4º and 24.9º positions, which are attributed to kaolinite (ICDD 01-083-0971), and the occurrence of anatase was also observed (ICDD 00-004-0477). The presence of anatase is common in kaolin in the Amazon region and is considered an impurity in the final product. Thus, separation occurs during processing, and anatase is released into the tailing ponds 12 . The transformation of kaolin into metakaolin necessary for its activation was observed by the XRD data, and the results are presented in Figure 1. The calcination step proved to be efficient, and the complete destruction of the kaolinite structure by the dehydroxylation reaction was observed; consequently, an amorphous material characterized by an elevation of the diffractogram background was produced 10,12 . Figure 2 presents the results obtained by the (Termogravimetry analysis) TGA and DSC analyses of the kaolin waste, and three thermal events are observed in the material: i) loss of mass starting at ~ 100 ºC attributed to moisture loss, followed by loss of mass until approximately 350 ºC due to dehydration of the material; ii) the largest loss of mass (~ 13.84%) at 514 ºC, indicated in the DSC curve, characterized by an endothermic peak (kaolinite dehydroxylation occurs in this region, transforming its structure into metakaolinite); and iii) an exothermic peak observed at 980 ºC, indicating the formation of a new crystalline phase (mullite) 10,12 .
Micrographs of the industrial waste from produced kaolin and metakaolin are shown in Figure 3a and 3b, respectively. For the industrial kaolin waste, pseudo-hexagonal clusters are observed, which characterize the presence of kaolinite, while for the metakaolin sample (red arrow) ( Figure 3b) these clusters separated, and pseudo-hexagonal morphologies were preserved 13 .

Evaluation of crystallization time and
SiO 2 /Al 2 O 3 ratio Figure 4a shows the X-ray diffraction of synthesis that used the ratio of SiO 2 /Al 2 O 3 ~ 2 originating from the kaolin waste itself. It is observed that during the first 5 h of crystallization, there was only the ALPO phase (ICDD 00-043-0563) and a significant amount of amorphous material. SAPO-34 was obtained only after 48 h of crystallization, in which the ALPO phase disappeared, and the amorphous material decreased. These results can be attributed to the formation mechanism of silicophosphates (SAPO), which are produced by the incorporation of silicon into the aluminophosphate (ALPO) structure, which can basically occur by three mechanisms: SM2 (Si 4+ → P +5 ), SM3 (2Si 4+ →Al 3+ + P +5 ), or a combination thereof. It is noteworthy that the insertion and distribution of Si in the ALPO structure is one of the main parameters that determine the final acid strength of the material and, consequently, its application 14 . Furthermore, the significant  presence of non-crystallized (amorphous) material is attributed to the high percentage of silicon in relation to the Al and P atoms in the reaction mixture, which results in an incomplete process of incorporation 7,8,15 and production of    The phases percentage as a function of crystallization time was roughly calculated according to the methodology described in 16 with modifications, with the results shown in Figure 5. The formation of SAPO-34 is favored when the crystallization time is increased, which in the time of 72h already only observes the formation of the crystalline phase SAPO-34. However, it is 38% pure mainly due to the presence of amorphous material. Figure 6 shows the micrograph of the synthesis without adjustment for 72 h. The presence of material with the cubic morphology characteristic of SAPO-34 (ICDD00-047-0429) can be observed; however, there is a significant amount of non-crystallized material, as observed by the XRD data. Figure 7 present the XRD results of the samples with the ratio of SiO 2 /Al 2 O 3 ~ 1 adjusted by the addition of aluminium in the synthesis gel. By comparing these results with those shown in Figure 2a, the adjustment of the ratio promotes the insertion of Si into the SAPO-34 structure, which is characterized by a sharp decrease of the background and consequent decrease of the amorphous material. The adjustment of the SiO 2 /Al 2 O 3 ratio also accelerates the incorporation reaction of Si into the ALPO structure because the SAPO-34 structure was observed with the chabazite structure (CHA) in all studied crystallization times.
The influence of crystallization time on phase is best observed by comparing the data shown in Figure 7 and 8. The use of the longer crystallization time of 72 h promoted a significant increase in the intensity of the peaks corresponding to the SAPO-34 phase ( Figure 8); in addition, no significant elevation of the background was observed (characteristic of amorphous material). Thus, there was a final material with high phase crystallinity and purity, as also observed in 17,18 . Figure 9 shows the micrographs obtained for the samples with the ratio of SiO 2 /Al 2 O 3 ~ 1, for which cubic morphologies are observed in all cases (red arrows), which is characteristic of SAPO-34; however, there is a small amount of metakaolin because the crystallization time is still not sufficient for the formation of the material. As time advanced, as observed in Figure 9a-d, the amount of non-crystallized material decreased, increasing the formation of cubes characteristic of SAPO-34.
The crystallization percentage as a function time for the synthesis with SiO 2 /Al 2 O 3 ratio was adjusted was determined through the methodology described in Bakhtiar et al. 16 , with changes considering the peak and background area. The obtained curve is shown in Figure 10 in which the highest crystallinity was approximately 98.6% for the 72 h period.
For the use of this material, its calcination is often necessary to promote pore clearance. Thus, the sample with the best crystallinity and purity result was determined by   verifying the thermal stability of its structure with the results obtained in Figure 11, 12 and 13.
The thermogravimetric analysis of the material is presented in Figure 11, in which three thermal events are observed: event I was related to water desorption from the surface of the material and hydration water occurring up to ~ 130 °C (~ 2%); event II (~ 3%) started at approximately 170 ºC and was attributed to the decomposition of the SDA (morpholine) and extended to ~ 420 ºC; and event III (~ 5%), originating after 420 ºC, was attributed to the volatilization of SDA waste 7,8 . Thus, it was concluded that the calcination temperature of 550 ºC was sufficient for the complete elimination of the produced materials. Figure 12 shows the diffractograms of the samples produced by adjusting the SiO 3 /Al 2 O 3 ratio and crystallization time of 72 h before and after the calcination process, which was performed at 550 ºC for 3 h in the static regime. The reduction in peak intensity in the calcined diffractogram shows a decrease in the material crystallinity. However, even though the peak intensity decreased, the molecular sieve structure remained intact, demonstrating that it had good thermal resistance. This result was also observed by Agarwal et al. 19 , who used a calcination temperature of 540 ºC under airflow. The mass yield of the synthesis was calculated based on the initial load of metakaolin, phosphoric acid and compensator of the Si/Al ratio with the final mass of the product obtained after calcination, reaching a yield of approximately 94.5%.
The micrograph shown in Figure 13 is related to the calcined sample, and the presence of cubic morphologies can be observed, demonstrating that even after thermal treatment, the sieve structure was preserved.

Conclusion
Industrial kaolin waste can be used as a low-cost alternative for the synthesis of SAPO-34. This molecular sieve was synthesized as the majority crystalline phase when the synthesis occurred with the adjustment of the SiO 2 /Al 2 O 3 ratio to ~1. When this occurs, the crystallinity of the material is high, and better formation is observed as the reaction time increases.
The thermal stability of the sample crystallized for 72 h was confirmed, in which the process of calcination at 550 °C    is sufficient to promote the cleaning of the material pores by eliminating the structure directing agent and its waste. Furthermore, the material produced maintained its crystalline structure after calcination, which demonstrates good thermal resistance, and it can be used for future catalytic applications and sorption processes.