Shinya
Kokuryo
*a,
Kazuya
Tamura
a,
Soshi
Tsubota
a,
Koji
Miyake
*ab,
Yoshiaki
Uchida
a,
Atsushi
Mizusawa
c,
Tadashi
Kubo
c and
Norikazu
Nishiyama
ab
aDivision of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. E-mail: skokuryo@cheng.es.osaka-u.ac.jp; kojimiyake@cheng.es.osaka-u.ac.jp
bInnovative Catalysis Science Division, Institute for Open and Transdisciplinary Research Initiatives (ICS-OTRI), Osaka University, Suita, Osaka 565-0871, Japan
cAC Biode Co., Ltd., 498-6 Iwakura Hanazono, Sakyo, Kyoto, 606-0024, Japan
First published on 8th May 2024
Recovering valuable products from waste materials has been critical in saving limited fossil resources. This study investigated the effects of the acid type in zeolites on the yields of light olefins during the catalytic cracking of polyolefins. Brønsted acid sites in zeolites promote the oligomerization of olefins via protonation, decreasing the light olefin yield. We prepared Lewis acidic Sn-Beta zeolites without Brønsted acids using a dealumination procedure. Sn-Beta zeolites exhibited a higher yield of light olefins (45%) than the pristine Beta zeolite (24%) on low-density polyethylene cracking. Our experiments revealed that Lewis acid sites promote light olefin production, and the absence of Brønsted acid sites dramatically inhibited the consecutive reactions, thereby increasing the light olefin yields. This study provides a new guideline for controlling the product distribution of polyolefin catalytic cracking.
To date, most studies have focused on Brønsted acids, as the cracking reaction of the polymer has been assumed to proceed via Brønsted acids,34,35 and the importance of Lewis acids has not been understood. However, our previous research revealed that Lewis acid sites also promote polymer cracking and decrease the decomposition temperature.36,37
To recycle the limited fossil resources, high-value products (e.g., light olefins and aromatics) should be produced selectively from polyolefin cracking. In particular, light olefins are important raw materials for producing polymers and various chemicals.38,39 Although product distributions can be controlled to some extent depending on the zeolitic framework type, and the yields of light olefins can be increased using zeolites with appropriate framework,40 the control of micropore structure is reaching its limits. In this study, we focused on the mechanism of the catalytic cracking of polyolefins and aimed to increase the yields of light olefins using a chemical approach.
Unlike the radical mechanism of thermal cracking, the catalytic cracking of polyolefins proceeds via a carbenium ion mechanism, as shown in Scheme 1.35 Brønsted and Lewis acids produce the carbenium ions via protonation and abstraction of a hydride ion, respectively. These carbenium ions are converted into shorter hydrocarbon chains via β-scission.
Brønsted acid sites promote the protonation of olefins, thereby converting light olefins into other hydrocarbon materials via hydrogen transfer, oligomerization, isomerization, and cracking. Thus, zeolites have been used as catalysts for the conversion of light olefins.41–43 Therefore, in the catalytic cracking of polymers, a decrease in the number of Brønsted acid sites is assumed to lead to an increase in the yield of light olefins. J. Lee et al. prepared lanthanum-containing zeolites and investigated the relationship between basicity and product distribution in C5 raffinate cracking.44 La species decrease the Brønsted acidity by covering the sites and increasing the basicity, increasing the selectivity for light olefins (ethylene and propylene). However, the decrease in Brønsted acidity and porosity by La-loading causes a decrease in catalytic activity and conversion of C5 raffinate; therefore, loading a solid base to zeolites is unsuitable for the catalytic cracking of polymers. As mentioned above, not only Brønsted acids but also Lewis acids promote the catalytic cracking of polymers. Therefore, we dealuminated zeolite Beta and incorporate Sn into the framework to prepare zeolites with only Lewis acids. The effect of Lewis acids and the absence of Brønsted acids on the yields of light olefins during low-density polyethylene (LDPE) cracking was examined.
To prepare precursor solution for Sn-Beta, 35 wt% TEAOH solution (Wako FUJIFILM), SnCl4·5H2O (Wako FUJIFILM), and deionized water were mixed to a concentration of 0.2 mol L−1 and 0.016 mol L−1 for TEAOH and Sn, respectively. An appropriate amount of precursor solution was added to the prepared deAl-Beta (0.5 g) to a mass ratio of Sn/Beta = x/100 (x = 2, 4, 6). The mixture was then placed in a Teflon-sealed autoclave and heated at 140 °C for 24 h. The obtained powder was washed by centrifugation with deionized water thrice, dried at 90 °C overnight, and then calcined at 550 °C for 6 h. The synthesized zeolites were named Sn-Beta(x).
EDS analysis was conducted to evaluate the composition of the zeolites (Table 1). After the dealumination procedure, the Al content was below the detection limit, indicating that the Al species in the zeolites were almost completely removed. The Sn content of the samples increased with increasing Sn content in the preparation solution. The morphologies of the samples were investigated by TEM (Fig. S1†). The acid treatment partly disrupted the zeolite structure, thereby deAl-Beta had a ragged surface. Moreover, the Sn incorporation procedure smoothened the surface of the zeolites because the procedure was performed under strong basic conditions and the SiO2 framework was partly dissolved. The porosities of the catalysts were evaluated by N2 adsorption. The pore volume of deAl-Beta was lower than that of the pristine Beta because of the decrease in its crystallinity, as shown in Fig. S2.† Moreover, Sn incorporation decreased the zeolite micropore volume, suggesting that the zeolite framework was partly dissolved and amorphous SiO2 occluded the zeolitic micropore, which is consistent with the changes observed in the TEM images.
Si/Al | Si/Sn | |
---|---|---|
Beta | 12.24 | — |
deAl-Beta | ∞ | — |
Sn-Beta(2) | ∞ | 492 |
Sn-Beta(4) | ∞ | 130 |
Sn-Beta(6) | ∞ | 119 |
The detailed chemical states of the Sn species in the synthesized zeolites were investigated using UV-vis spectroscopy. A reference SnO2 has a broad absorbance band at 210–280 nm,48 while all Sn-Beta samples have sharp peaks at 206 nm49 derived from charge transfer between O and isolated tetrahedral Sn4+, as shown in Fig. 2, indicating that Sn was incorporated into the zeolite frameworks. However, Sn-Beta(6) showed a slight peak at around 280 nm, indicating that the excess amount of Sn existed as extra-framework species.
Moreover, the intensity of the absorption band increases with increasing Sn content. The acidity of the samples was investigated by FT-IR spectroscopy using pyridine as a probe molecule (Fig. 3). Pyridine is a basic molecule that adsorbs on the acid sites of zeolites in various forms depending on the acid type. Therefore, the acidity of zeolites can be evaluated separately for each acid type. A peak at 1545 cm−1 derived from pyridine adsorbed on Brønsted acid sites50 was not observed, indicating that no Brønsted acid sites were present in the dealuminated samples. A peak at 1445 cm−1 corresponding to hydrogen-bonded pyridine50 was observed in deAl-Beta, indicating that the number of silanol groups increased by dealumination. Moreover, for Sn-Beta samples, peaks of coordinated pyridine on the Lewis acid sites (1450 cm−1)50 were observed, and the intensity of the peak became strong with the increase in the Sn content, indicating that the incorporated Sn species serve as Lewis acid sites. Additionally, NH3-TPD measurements revealed that Sn-Beta samples had strong Lewis acid sites derived from Sn sites (the peak top is 747 °C) as shown in Fig. S3.†
The activity of the samples in polyolefin cracking was evaluated by TG and GC using low-density polyethylene (LDPE) powder. First, the decomposition temperatures of LDPE with and without catalysts were measured by TG at a heating rate of 5 °C min−1 to 600 °C under a N2 atmosphere. The catalytic cracking of LDPE with Sn-Beta zeolites occurred at higher temperatures than that with pristine Beta zeolites because of the lack of active sites, as shown in Fig. 4. The Si/Sn ratio of Sn-Beta(6) is 119, while the Si/Al ratio of pristine Beta is approximately 12, as shown in Table 1; therefore, this difference in the number of acidic atoms led to an increase in the cracking temperature. However, the cracking temperature with Sn-Beta zeolites lowered with the increase in the Sn content, indicating that LDPE cracking was accelerated even by only Lewis acidity originating from Sn.
GC was used to analyze the product distribution during LDPE cracking with and without catalysts. First, the selectivities of gas, liquid + wax, and coke were calculated from a mass balance (Fig. 5a). The gas yield with pristine Beta was more than 80%, whereas that without catalysts was approximately 50%. In the case of deAl-Beta, the selectivity was almost the same as that of thermal cracking (noncatalytic). However, the gas yields increased to the same extent as in pristine Beta by the incorporation of Sn, indicating that Lewis acidic Sn sites promoted the cracking via the carbenium ion mechanism. Moreover, the amount of coke deposited on Sn-Beta zeolites was lower than that on pristine Beta, as shown in Fig. S4.† This is because the effect of the elimination of hydrogen transfer reaction that occurs at the Al sites, and the production of coke precursors such as aromatics was inhibited.51,52 The detailed distributions of the gaseous products were analyzed using GC, as shown in Fig. 5b. The selectivity of C6+ products with pristine Beta was lower than 30%, whereas that without catalysts was approximately 70%, and the gaseous product distributions with deAl-Beta was almost the same as that of pristine Beta. These results indicate that silanol groups in zeolites served as Lewis acids and promoted LDPE cracking. Moreover, the selectivity of C4 and C5 products with Sn-Beta(6) was higher than that of other Sn-Beta samples, suggesting that an excess amount of Sn species such as extra-framework Sn also proceeded the LDPE cracking.
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Fig. 5 (a) Yields by group and (b) gas product distribution in catalytic cracking of LDPE with Beta, deAl-Beta, and Sn-Beta(x). |
The selectivity for olefins and paraffins in C2–C4 products was calculated (Fig. 6a). The olefin selectivity was increased by dealumination because of the elimination of Brønsted acidity and further increased to nearly 90% by Sn incorporation. Moreover, the yields of light olefins increased by approximately 20% using Sn-Beta(6) compared to pristine Beta, as shown in Fig. 6b. These results indicate that the elimination of Brønsted acid sites leads to inhibition of olefin conversion in polyolefin catalytic cracking.
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Fig. 6 (a) Selectivity for olefins and paraffins and (b) yield of light olefins in catalytic cracking of LDPE with Beta, deAl-Beta, and Sn-Beta(x). |
Finally, we conducted a light olefin conversion test with pristine Beta and Sn-Beta(6) using isobutene as a model reactant. The reaction temperature was determined from the TG results. Ty is defined as the temperature at which y% of LDPE was decomposed (y = 20, 50, 60) as shown in Fig. S5.† The isobutene conversion test was conducted at the determined Ty with a fixed-bed reactor, and the conversion was calculated by taking into account all C4 olefin products. In the case of pristine Beta, the conversions were approximately 90% and most of the product was isobutane, indicating that the protonation of olefins by Brønsted acids was promoted (Fig. 7a). Meanwhile, the activity of Sn-Beta(6) for isobutene conversion was low, with a conversion rate of less than 40%, even though the reaction was conducted at higher temperatures than in the case of pristine Beta (Fig. 7b). Thus, the use of non-Brønsted acidic zeolites protects light olefins from conversion to other hydrocarbons.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy00317a |
This journal is © The Royal Society of Chemistry 2024 |