The present study investigates the aminothermal synthesis of SAPO-34 using various templates, including morpholine, diethylamine, triethylamine, triethanolamine, diglycolamine, and diisopropylamine, under static conditions. The catalyst's characteristics were assessed through XRD, SEM, and EDS analyses, and the catalytic behavior of the samples was studied in the MTO reaction. The results indicate that only morpholine leads to the formation of pure SAPO-34, and it is essential to regulate the crystallization time to prevent the formation of SAPO-20. Furthermore, the impact of the order of reactant addition and the intensity of stirring during aging on catalyst characteristics was investigated, and the outcomes were compared to those of the hydrothermally synthesized sample. The findings reveal that the aminothermal method reduces the crystallization time and enhances silica incorporation. However, this technique is recommended for the synthesis using polar templates or in the presence of polar additives. The utilization of non-polar templates such as DEA and TEA resulted in very thick pastes. The catalytic performance of the aminothermal samples was comparable to that of the hydrothermal samples, with a methanol conversion of over 98% and a selectivity to light olefins of approximately 80%. Nevertheless, the impact of preparation, aging, and post-treatment conditions on the hydrogen transfer index and alkanes production was significant.
Research Article
Comparison Of Aminothermal And Hydrothermal Synthesis Of SAPO-34: Impact Of Synthesis Conditions On Catalyst Characteristics And MTO Catalytic Performance
https://doi.org/10.21203/rs.3.rs-3133238/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 13 Sep, 2023
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SAPO-34
Template
Aminothermal
Hydrothermal
Amine
MTO
The escalating demand for everyday products such as plastic, rubber, and synthetic fibers has led to a pressing need for clean and sustainable production of light olefins. The Methanol-To-Olefin (MTO) process is a promising technology that converts methanol derived from renewable resources such as biomass, waste, and CO2 into high-value light olefins. SAPO-34, a microporous silicoaluminophosphate (SAPO), is a suitable catalyst for MTO due to its balanced acidity, appropriate shape selectivity, and high hydrothermal stability [1]. Although the hydrothermal method is the most well-known approach for SAPO synthesis, other techniques such as solvothermal [2][3], ionothermal [4][5], and dry gel conversion [6][7] have been proposed to achieve a cost-effective and environmentally friendly synthesis method.
In 2012, Fan et al. [8] introduced an aminothermal method for the synthesis of SAPO. The aminothermal approach is a subset of the solvothermal technique, wherein the template serves as both a solvent and a structure-directing agent, while the quantity of water in the synthesis mixture is kept to a minimum. Based on their discoveries, the utilization of triethylamine (TEA) at a temperature of 160°C for a duration of 48 hours resulted in the synthesis of SAPO-34 with good crystallinity and a zeolite yield of approximately 90%. Elevating the temperature to 200°C yielded SAPO-18. Additionally, the use of diethylamine (DEA) led to the formation of small-pore crystals of DNL-6. Furthermore, they proposed that environmentally sustainable methods could be achieved by recycling organic liquids obtained after the synthesis process and reusing them in the subsequent crystallization [8]. In the succeeding year, this research team documented the aminothermal production of SAPO-34 utilizing diisopropylamine (DIPA) and tetramethylethylenediamine (TMEDA), which resulted in rapid crystallization and a high solid yield. The catalytic performance of these samples was scrutinized in the MTO reaction. The light olefin selectivity commenced at values below 20 wt.% and progressively escalated to exceed 80 wt.% [9]. Wang et.al. conducted an investigation into the templating effects of various alkanolamines in the aminothermal system. Their results indicated that the use of diglycolamine and diisopropanolamine led to the synthesis of SAPO-34 crystals [10]. In the year 2016, they employed a HF-assisted aminothermal technique to produce SAPO-56 nanocrystals with a hollow structure using a dual-template strategy involving trimethylamine (TMA) and TEA [11]. In 2017, Gao et al.[12] examined the crystallization mechanism in the aminothermal system for a TEA-templated SAPO-34 and compared it with the hydrothermal method. The outcomes demonstrated that the strong interactions between protonated template and inorganic species in the alkaline media expedited the formation and assembly of an Al-P-O layered phase, which, in turn, accelerated the activation/depolymerization of the Al source and the generation of a substantial quantity of Al-O-P bonds [12]. In a related study, SAPO-18/34 hierarchical crystals were synthesized with the assistance of SAPO-34 seeds in the aminothermal system, and their performance was assessed in the MTO reaction. The resultant catalyst demonstrated an extended lifetime and superior selectivity compared to Sapo-18 in the MTO process [13]. Furthermore, in a recent investigation, CLO-SAPO and ECR-40 molecular sieves were produced in an aminothermal system employing N,N-diethyl ethanolamine as both a solvent and an organic structure-directing agent, and N,N,N-trimethylbutylammonium chloride as a co-template [14].
As evidenced by the existing literature, the aminothermal technique offers a viable means of structuring various SAPOs using a diverse range of amines. This approach not only reduces water consumption but also enables the reuse of residual amin solvents, thereby mitigating the economic and environmental challenges associated with large-scale production of these molecular sieves. However, the impact of various factors on catalyst properties during sample preparation has not been adequately investigated. The use of highly alkaline conditions and low water content during synthesis may result in sensitivity issues that must be taken into account. Therefore, this study aimed to investigate the static aminothermal synthesis of SAPO-34 using different amines and to examine the conditions of gel preparation, including the order of material addition, stirring intensity, aging time, and duration of crystallization in the aminothermal method. The characteristics of the resulting catalyst and its performance in MTO process were also evaluated and compared with those of the hydrothermally synthesized sample.
2-1- Materials
The Pseudoboehmite (70 wt.% Al2O3) used as the alumina source for all samples was obtained from the Iranian West Minerals Applied Research Center (IWMARC). The silica source consisted of colloidal silica (30 wt.% SiO2, Isatis, Iran) and tetraethyl orthosilicate (TEOS, Merck). Orthophosphoric acid (85 wt.%, Merck) was utilized as the source of phosphate. Morpholine (MOR), diethylamine (DEA), triethylamine (TEA), triethanolamine (TEOA), diglycolamine (DGA), and diisopropylamine (DIPA) were the sources of organic templates.
2-2- Catalyst preparation by hydrothermal method
The conventional SAPO-34, designated as SH, was synthesized using the static hydrothermal method with a molar composition of 1.0 Al2O3: 1.0 P2O5: 0.6 SiO2: 2 MOR: 52 H2O. Pseudoboehmite was added to a specific amount of deionized water, and then a diluted phosphoric acid solution was added dropwise to the mixture under vigorous stirring. Colloidal silica was slowly added to the solution, followed by the addition of morpholine. The resulting gel was stirred overnight at room temperature and then transferred into a 200-mL Teflon-lined stainless-steel autoclave. The mixture was then crystallized at 200°C for 48 hours under autogenous pressure.
2-3- Catalyst preparation by aminothermal method
In order to aminothermally synthesize SAPO-34, two different ingredient addition orders were utilized to prepare the gel with a molar composition of 1.0 Al2O3: 1.0 P2O5: 0.4 SiO2: 8 MOR: 15.7 H2O, as reported in [10]. For the preparation of SA1, pseudoboehmite was initially mixed with morpholine, followed by the dropwise addition of phosphoric acid and silica sol to the system, respectively. The resulting viscous gel was stirred under moderate rotation speed for one hour while being kept in a 45°C water bath, and then transferred to the autoclave and statically crystallized at 200°C for 36 hours. Sample SA2(i) was synthesized similarly to sample SA1, with the exception that silica sol was added first, followed by the addition of phosphoric acid to the gel. For further evaluation, aging conditions, crystallization time, and silica source were altered in sample SA2 (samples SA2(ii) and SA2(iii)). The synthesis conditions for all samples are detailed in Table 1.
| Sample name | Crystallization method | Order of adding | Aging condition | Crystallization time |
|---|---|---|---|---|
| SH | Hydrothermal | Al◊P◊Si◊MOR | Moderate magnetic stirring/18 h/room temperature | 48 h |
| SA1 | Aminothermal | MOR ◊Al◊P◊Si | Moderate mechanical stirring /1h/ 45 °C | 36 h |
| SA2(i) | Aminothermal | MOR ◊Al◊Si◊P | Moderate mechanical stirring /1h/ 45°C | 36 h |
| SA2(ii) | Aminothermal | MOR ◊Al◊Si◊P | strong mechanical stirring/1.5h/ 45°C | 36 h |
| SA2(iii) | Aminothermal | MOR ◊Al◊Si◊P | strong mechanical stirring/1.5h/45°C | 20 h |
The recovery of solid product was consistent for all samples. The autoclave was cooled and depressurized in cold water, and the pH was measured at room temperature. The as-synthesized samples were retrieved through centrifugation and thoroughly washed with water and ethanol to achieve a neutral pH. The solid products were subsequently dried at 100°C for 12 hours and calcined at 600°C for 8 hours.
2-4- Catalytic activity test
To investigate the catalysts' performances in the MTO reaction, a micro-scale stainless-steel fixed bed reactor operating at atmospheric pressure was utilized, as shown in Fig. 1. For each test, 2g of catalyst mixed with 3g of glass balls (< 0.4 mm) was loaded into the reactor. Prior to testing, the catalyst was pretreated under a dry N2 flow (60 ml/min) at 450°C for 1.5 hours, and then the temperature was adjusted to 400°C. An 80 wt.% aqueous methanol solution was fed by an HPLC pump (WHSV = 2 h− 1) and passed through a preheater. The vaporized reactant was directed through a heat traced line to the reactor. The reaction temperature was controlled by a PID controller and measured by a thermocouple placed at the center of the catalyst bed. The effluent gas was analyzed online using a YOUNGLIN 6000 gas chromatography equipped with an FID detector and a GS-GasPro column (60 m, 0.32 mm id). The conversion and selectivity were calculated based on the CH2 basis
2-5- Catalyst characterization
The X-ray diffraction (PXRD) patterns of the samples were collected using a Panalytical X’Pert diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 2θ range of 5°–80°. The average crystal size was determined based on the overall breadth of characteristic diffraction peaks at 2θ of 9.8° and 21°. The morphology and particle size of the SH, SA1, SA2(i), and SA2(ii) samples were analyzed using a field emission scanning electron microscope (FE-SEM, MIRA3 TESCAN, 15 kV), while a scanning electron microscope of Zeiss (DSM 960 A, 10kV) was utilized for the SA3(iii) sample. The particle size was measured in terms of the circular equivalent diameter of the projected area of each particle. Additionally, the elemental analysis was carried out using Energy dispersive X-ray (EDX).
3-1- Reaction gels:
The most apparent dissimilarity in the preparation of the reaction mixture in the aminothermal method compared to hydrothermal is the alteration in the rheology and pH of the gel. The pH values of the synthesis solution before and after the crystallization process were measured at room temperature. During the hydrothermal crystallization process, the pH value increases from 5.8 to approximately 9 due to the incorporation of precursor species into the framework. [15][16], while in the aminothermal treatment, the crystallization of SAPO occurs in an alkaline medium (~ 9.5) with a slight pH increase. Additionally, in contrast to the hydrothermal technique [17], the order of adding starting materials in the aminothermal method did not significantly affect the final pH of the reaction gel. The viscosity and rheology of the reaction mixture also differed between the two methods. In the hydrothermal method, a smooth gel was obtained due to the aqueous reaction media. However, the low amount of water in the aminothermal process resulted in a thick paste of a reaction mixture that was difficult to homogenize. At the stage when phosphoric acid is added to the gel, it begins to thicken. This may be attributed to the rapid interactions of phosphoric acid with depolymerized Al source facilitated by amine solution and the formation of an amorphous Al-O-P-based gel phase, which is the building unit of the SAPO crystal [18][12]. Therefore, adding H3PO4 to the SA1 solution before silica causes the formation of a thicker gel than SA2(i).
It is noteworthy that sample SA2(iii) with the silica source of TEOS had a smoother gel compared to SA2(i), despite having the same batch composition and gel preparation conditions. The reason for this lies in the presence of ethanol, which is a byproduct of the hydrolysis of TEOS. The polarity and interfacial tension between the solvent and reactants play a crucial role in the gel formation, as well as the nucleation and growth of crystals. Solvents with high polarity and high interfacial tension, such as water, prevent the strong interactions between reactants, thereby slowing down the polymerization/condensation reactions. As a result, the hydrothermal system forms a smoother synthesis gel than the aminothermal system. Ethanol as a polar additive in the amine media of sample SA2(iii) has a smoothing effect on the gel-based mixture.
The study also conducted aminothermal synthesis using various other amines, including TEOA, DGA, DEA, TEA, and DIPA, under the same conditions as sample SA2(i). The results showed that TEOA, as a polar solvent (more polar than morpholine), produced a smooth mixture, while DEA and TEA, with less polarity, formed a completely dry and lumpy gel after mixing with aluminum and phosphorus sources. The large-scale preparation of aminothermal reaction mixture for templates like DEA and TEA poses a serious challenge due to their high volatility and non-uniform dry gel formation. In such cases, the dry gel conversion method may be a more effective synthesis technique than aminothermal [19].
3-2- XRD results:
The XRD patterns obtained from all samples demonstrate the characteristic peaks of the CHA phase (Fig. 2). Sample SH, which underwent a longer crystallization time, exhibits more intense peaks compared to samples SA1 and SA2(i). The crystal size calculated from Scherer's equation for SH, SA1, and SA2(i) is 40, 40 and 37 nm, respectively. sample SH displays two shoulders at 2 thetas of 20.6° and 21.7°, which arise from tridymite impurity [20]. As reported in the literature, dense phases such as AlPO4-tridymite are formed in the first step of SAPO synthesis[21]. Therefore, the factors decreasing the crystallization rate facilitate the formation of tridymite and cristobalite impurity phases: the use of alumina and silica sources that have low reactivity, such as gibbsite and water glass, Al/P < 1, very dilute reaction mixture, and insufficient template [22][23][21][24]. It is worth noting that this impurity phase was not observed in any of aminothermally synthesized samples, which may be attributed to the higher amount of template and the alkaline synthesis solution.
Samples A1 and A2(i) were prepared under similar conditions except for the order of adding ingredients. XRD patterns of these samples confirm that the crystallinity of SAPO is affected by the order of adding materials in the aminothermal technique. The addition of phosphoric acid before silica in sample A1 leads to a thicker and drier gel compared to sample A2(i). A thick, viscous synthesis gel can reduce convection currents and mass transfer, leading to less nucleation and larger crystals [25][26]. Furthermore, due to the limitations of mass transfer in the aminothermal system, it was observed that increasing the duration and intensity of mixing during aging can significantly accelerate the growth of crystals and increase crystallinity (as observed in the XRD pattern of SA(ii)). Sample SA2(ii), synthesized in a shorter crystallization time, has a higher crystallinity than sample SH. This highlights the potential of the aminothermal method in promoting faster crystallization of SAPO structures than the hydrothermal method. The crystal size for samples SA(ii) and SA(iii) is 54 and 44 nm, respectively.
It is interesting to note that XRD patterns of samples SA1 and SA2(ii), which possess higher crystallinity and are subjected to conditions that facilitate faster crystallization, reveal the presence of an impurity phase of SAPO-20 (SOD). Furthermore, the color of these samples after calcination is considerably darker than that of the other samples. The incomplete calcination of these samples can be attributed to the very small size of SAPO-20 pores (i.e., 0.3 nm) in comparison to SAPO-34. The strong diffusion resistances in the structure of SAPO-20 make it difficult for oxygen to reach the morpholine molecules trapped in the cavities, thereby hindering their effective removal. To the best of our knowledge, the synthesis of SAPO-20 with morpholine has not been reported in the literature. The formation of this phase is favored by increasing the alkalinity of the reaction mixture, small amine molecules, high temperature, and longer crystallization time, as reported in [27][28]. Therefore, it can be inferred that the crystallization temperature of 200°C and the presence of large amounts of morpholine in the aminothermal method are the primary reasons for the formation of SAPO-20. This phase exhibits good stability at high crystallization temperatures, whereas for CHA structures such as SAPO-34, crystallization must be halted at the optimal time to prevent the formation of more stable phases. The disappearance of SAPO-20 peaks in SA(iii) confirms this claim.
Figure S1 displays the XRD patterns of the samples directed by other alkyl and alkanol amine templates. As evidenced, using DIPA resulted in the formation of SAPO-34 and SAPO-11 phases, while TEOA produced the structure of SAPO-5. The utilization of DGA, DEA, and TEA templates yielded dense phases. These outcomes are inconsistent with the findings reported in references [8] and [10], which may be due to different preparation conditions and static crystallization.
3-3- SEM and EDX results
As depicted in Fig. 3, the SEM images reveal that all samples consist of cubic crystal particles, which are the characteristic shape of SAPO-34. Samples SH and SA1 exhibit a smooth surface, with an average particle size of 3.3 and 2.2 µm, respectively. Conversely, in samples SA2(ii) and SA2(iii), which were aged under vigorous stirring, crystalline particles with broken surfaces and an average size of 5.8 and 9.3 µm, respectively, were observed. These interfacial and surface defects can result in an increase in Lewis acid sites within the structure [29]. Surprisingly, the particle size of SA2(iii) is greater than that of the other samples, despite having a significantly shorter crystallization time. According to the literature, the rapid hydrolysis of TEOS in the hydrothermal system leads to a higher nucleation rate and, consequently, smaller particle size compared to silica sol [30]. This contradicts the results observed in the aminothermal method. As previously stated in section 3.1, the liberation of ethanol from TEOS alters the solution's behavior and enhances the uniformity of the gel. This, in turn, can improve mass transfer within the system and promote crystal growth.
The results of the EDS analysis, which detail the elemental composition, are presented in Table 2. The (Si + P)/Al ratio was greater than unity for all samples except for SA1, indicating the incorporation of silica in the structure via SM2–SM3 mechanism combination and the formation of silica island [31]. The molar composition of SA1 suggests the existence of extraframework aluminum, which may be due to the distinct order of material addition. The quantity of silica present in the sample synthesized with DIPA (Table S1) is lower than that of samples synthesized with morpholine. This is likely due to the smaller size of morpholine (90.39 A³) in comparison to DIPA (124.94 A³). The small size of the amine results in more amine being trapped within the cages, necessitating a greater amount of silica to balance the protonated amines [10]. Furthermore, P/Al > 1 observed in this sample, which may be attributed to the presence of terminal P = O and P-OH groups within the SAPO structure. This is likely a result of the reduction in dissociation of H3PO4 and subsequent reduction in condensation with aluminum species, which increases the likelihood of forming terminal P-O bonds [32].
3-4- Catalytic performance
The catalytic efficacy of all samples was assessed in the MTO reaction at a temperature of 400°C and a WHSV of 2 h− 1. All samples synthesized with morpholine (both aminothermal and hydrothermal) exhibited a methanol conversion exceeding 98% throughout the catalyst's active life. Figure 4 illustrates the product distribution of the samples when selectivity to light olefins is at its maximum value. The columns display selectivity values to light olefins (C2=, C3=, and C4=) and light alkanes (C2 and C3). In all samples, the selectivity to ethylene was greater than that of propylene, although this ratio was slightly higher for hydrothermal samples than aminothermal samples. According to the literature, the C2=/C3 = ratio is dependent on the intermediates formed within the catalysts, which are influenced by structural factors such as cage size, crystal size, density and strength of acid sites, and operating factors such as reaction temperature and methanol throughput [33]. Propane was the dominant alkane across all SAPO-34 samples. DeLuca et al. suggest that alkylcarbenium stability plays a significant role in alkene hydrogenation. Due to the greater stability of secondary and tertiary alkylcarbenium ions compared to primary carbenium ions, hydrogenation barriers for propylene and butylene are lower than for ethylene [34]. However, structural constraints and diffusion resistances limit the production of butane, resulting in propane being the primary alkane observed in the MTO process on SAPO-34.
The catalytic efficacy of the hydrothermal sample was compared to the aminothermally synthesized samples with two distinct addition orders (Fig. 5). As demonstrated, the performance of the aminothermal samples in the MTO process is comparable to that of the hydrothermal sample. The highest selectivity to light olefins for all three samples exceeds 80%, with the exception that sample SA2(i) displays a consistent trend, while for samples SH and SA1, the selectivity to light olefins is initially low due to alkene hydrogenation and alkane production. However, as the reaction progresses, the selectivity to alkenes increases, and subsequently, with catalyst deactivation (concomitant with the appearance of DME), the selectivity value drops sharply. SA1 exhibits greater stability compared to SH and SA2(i), demonstrating over 6 hours of high activity. The performance of the sample synthesized using DIPA is also depicted in Figure S2. Due to its moderate acidic properties and low silica content, this sample exhibits a consistent trend in selectivity towards olefins. However, the presence of the SAPO-11 phase results in a lower yield of light olefins in comparison to the MOR-templated sample. Owing to the larger pore size of SAPO-11(4.0 × 6.5 Å) in comparison to SAPO-34 (3.8 Å), butylene was produced in greater quantities than propylene and ethylene on this catalyst, which contrasts with the outcomes observed in other samples.
Moreover, a comparison was made between the catalytic behavior of SA2 samples synthesized under different aging conditions and crystallization durations (Fig. 6). The presence of SAPO-20 and coarse particle size in SA2(ii) samples increased the diffusion resistance, leading to accelerated coke formation and catalyst deactivation. Although sample SA2(iii) exhibited good stability, its selectivity to alkanes was initially high (approximately 45%). As the reaction progressed, the trend for alkanes decreased while that for olefins increased, indicating that alkanes are derived from olefins and that strong acid sites play a crucial role in the hydrogenation reaction of olefins. As these sites gradually deactivate and coke forms, the rate of alkane production decreases.
In general, two primary factors have been identified as contributing to the formation of alkanes. The first factor is high diffusion resistance and residence time, which create favorable conditions for secondary reactions [35] [36] [37] [38]. Research indicates that olefins have a greater tendency to interact with internal acidic hydroxyl groups than aromatics and paraffins[39]. The second factor is the high acid density in SAPO-34. Results demonstrate that a high silica incorporation in the structure increases the likelihood of alkane formation [36] [40] [37] [38]. Recently, the role of Lewis acid sites (LAS) in hydrogen transfer has also been established [41] [42]. LAS in SAPOs can exist as threefold-coordinated Al or silicon and/or extraframework Al. It is also believed that the adsorption of methanol on the Brønsted acid site can induce frustrated Lewis pairs and create a synergetic effect in catalyzing the cascade reaction [43][44][45].
As evident from the results, SA2(iii) exhibited a higher amount of silica (acid density) and particle size (diffusion resistance) compared to the other samples, leading to increased alkane formation. However, intense post-treatment involving washing and calcination to remove amines from the structure can result in the formation of Lewis acids, which necessitates further investigation in this regard.
In this study, SAPO-34 was synthesized using the aminothermal method under static conditions with different amine templates. The results indicated that only the MOR template led to the production of pure SAPO-34, while the DIPA template favored the formation of the SAPO-11 impurity phase, and TEOA directed the formation of pure SAPO-5 crystals. DEA-, DGA-, and TEA-templated samples also showed dense phases. Additionally, the physiochemical characteristics and catalytic behavior of the MOR-directed sample were compared between aminothermal and hydrothermal methods. The alkaline media in the aminothermal system increased the crystallization rate and eliminated tridymite impurity phases which were observed in hydrothermal methods. However, the SAPO-20 phase was observed as a competing phase next to MOR-templated SAPO-34, which can be prevented by adjusting the crystallization time and aging conditions. On the other hand, the insufficiency of water resulted in the formation of dry thick paste in less polar systems, leading to a lack of uniformity in the synthesis mixture. Therefore, it is recommended to employ polar amines for an aminothermal system or conduct synthesis in the presence of polar additives, such as ethanol. Due to the low polarity and high volatility of DEA and TEA, the dry gel method is suggested, although further studies are required to compare it with aminothermal synthesis. The findings indicate that the addition of phosphoric acid is a crucial step that governs the rheology of the reaction mixture, and thus, the sequence of adding materials plays a role in the crystallinity and purity of aminothermally synthesized crystals. The silica source had also a significant impact on the synthesis process. In contrast to the hydrothermal method, the use of TEOS resulted in larger crystals than silica sol under the same conditions, which could be attributed to the effect of ethanol as a byproduct of TEOS hydrolysis.
The catalytic performance of the aminothermally synthesized samples in the MTO reaction was found to be comparable to that of the hydrothermally synthesized sample. Methanol conversion was above 98% in all samples, with a maximum selectivity to light olefins of approximately 80%. The lifetime of the catalyst varied between 310 and 410 minutes, independent of the synthesis method. The hydrogen transfer index was significant in some aminothermal samples, which can be attributed to the large crystal size, high silica incorporation, and presence of Lewis acid sites. The source of silica, order of addition of materials, intensity of stirring, crystallization time, and post-treatment conditions were found to play a significant role in this regard.
Acknowledgment
The authors gratefully acknowledge the support from National Petrochemical Company, Research and Technology Company (NPC-RT) for supporting this work.
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No competing interests reported.