The study investigates the activity, selectivity, and stability of dealuminized H-Beta zeolite samples in the oligomerization of pent-1-ene. It was demonstrated that the yield of pentene oligomers primarily depends on the concentration of acid sites in the catalyst. At 110°C, the highest oligomer yield is achieved by an initial H-Beta (33 wt %), while samples subjected to citric acid treatment (H-Beta-1), steam heating (H-Beta-2), and combined treatment (H-Beta-3) are less active in oligomerization, isomers being yielded as the main product. The investigation of the functional dependence of monomer conversion rate and product composition on reaction conditions in the presence of the H-Beta-3 micro/mesoporous sample revealed that the pentene oligomer yield reaches 89% at 200°C, 10 wt % catalyst, and 5 h. Micro/mesoporous zeolites ensure a wider molecular weight distribution of oligomers. The examination of the H-Beta and H-Beta-3 stability in the oligomerization of pent-1-ene identified H-Beta-3 as the most stable sample.
Oligomerization of light olefins has a long and successful history of application in the production of environmentally friendly components of gasoline, jet fuel, and diesel fuel. An important advantage of these components is the absence of sulfur and aromatic hydrocarbons. Zeolites are of significant importance as heterogeneous oligomerization catalysts [1–6]. Although C3–C4 olefin oligomerization plants using zeolite catalysts have been in commercial operation since the 1980s [7–9], researchers still put much effort into the development of more effective zeolite-based catalysts.
Previously we have studied the oligomerization of pent-1-ene in the presence of different structural types of zeolite, such as FAU, OFF, MOR, BEA, MTW, and MFI in the H-form) . H-Beta was found to exhibit the highest reactivity, thus ensuring the highest (97–99%) pent-1-ene oligomer yield with an oligomerization degree of n = 2–6. The Beta’s high performance as a C3–C4 olefin oligomerization catalyst has also been demonstrated in [11–15].
A major disadvantage of zeolite catalysts is their rapid deactivation due to the blocking of their crystal lattice micropores by reaction products and coke. Furthermore, zeolite’s microporous structure precludes or limits the synthesis of bulk molecules more than 1 nm in diameter. One feasible technique to improve mass transfer in the microporous crystal lattice of zeolite, as well as to extend its stable service life, involves creating zeolites with secondary mesoporosity. Such micro/mesoporous zeolites can be prepared by post-synthetic treatment, such as dealumination with organic or inorganic acid solutions [16–19].
In this study, samples prepared by the dealumination of H-Beta with a citric acid solution before and after its high-temperature treatment in a steam environment were used as micro/mesoporous zeolites.
The purpose of this study was to investigate the activity, selectivity, and stability of dealuminated Beta zeolite samples in the course of pent-1-ene oligomerization.
Reagents and catalysts. 97% pent-1-ene from Acros was used for the study purposes. NH4-Beta zeolite (SiO2/Al2O3 = 40) manufactured by Zeolyst International was converted to the H-form by thermal treatment in air for 4 h at 540°C.
Sample H-Beta-1 was prepared by treatment of H-Beta with an 0.3 N citric acid solution at 85–90°C for 1 h followed by calcination at 540°C for 3 h. Sample H-Beta-2 was prepared by heat treatment of H-Beta in a 100% steam environment (steam heating) at 550°C for 3 h. Sample H-Beta-3 was prepared through three-step treatment of H-Beta: (1) steam heating at 540°C for 3 h; (2) treatment with a 0.3 N citric acid solution; and (3) air calcination at 550°C for 3 h.
Immediately before catalytic tests, the samples were heat-treated in air at 450°C for 4 h.
Catalyst characterization. The samples were tested for chemical composition by X-ray fluorescence analysis on a Shimadzu EDX-720/900HS instrument. The phase composition and crystallinity of the samples were determined by a matching technique on a Bruker D8 Advance diffractometer with monochromated CuKα radiation. Data were collected in the 2θ range of 5° to 40° with a step of 0.5° per minute and a counting time of 20 s/step. The crystallinity was calculated as a ratio of the total integral intensity of the crystalline phase to the total integral intensity of the crystalline and amorphous phases.
The porous structure properties were determined by low-temperature nitrogen adsorption/desorption (77 K) on a Micromeritics ASAP-2020 sorptometer. A BET method was used for the calculation of specific surface, a BJH (Barrett–Joyner–Halendy) desorption branch for pore size distribution, and a BJH method for total pore volume. The micropore volume in the presence of mesopores was estimated using a t-method.
The acidic properties of the zeolite catalyst samples were examined by temperature-programmable desorption (TDP) of ammonia.
Oligomerization of pent-1-ene. Oligomerization of pent-1-ene was carried out in continuously rotating autoclaves in the temperature range of 110–200°C. The zeolite catalyst content was 5–20 wt % in an olefin equivalent. When the experiment was completed, the liquid-nitrogen-cooled reaction mixture was filtered from the catalyst and divided into an amylene fraction and an oligomer fraction. For this purpose, the reaction mixture was transferred into a two-neck flask connected to a receiver placed in a low-temperature bath (–80°C). Nitrogen was blown into the flask to extract the pentenes. These pentenes were collected in the receiver and tested by gas chromatography on a Crystal-Lux 4000M instrument equipped with a thermal conductivity detector (6 m packed column, 20% dibutyl phthalate/diatomaceous earth phase). The oligomers were tested by GLC on a Carlo Erba HRGS 5300 Mega Series chromatograph equipped with a flame ionization detector (25 m glass capillary column, SE-30 phase), and by high performance liquid chromatography on a Shimadzu LC-20 Prominence instrument equipped with a Plgel 500Å polystyrene column and a refractometric detector.
The products were identified by chromatography-mass spectrometry on a Fisons instrument with a chromatograph equipped with a DB-560 50 m quartz capillary column.
Spent catalysts were used repeatedly without regeneration to assess the stability of the samples.
The catalyst activity was estimated by the total pent-1-ene conversion rate (X, %):
where Xol is the conversion to oligomers; Xis is the conversion to isomers; C(C5=)0 and C(C5=)t are the C5 olefin concentrations before and after the reaction, respectively; and C(isoC5=)t is the total concentration of pent-1-ene isomers.
The selectivities (S, %) for isomers and pent-1-ene oligomerization products were estimated by the following formulas:
where Sol is the oligomer selectivity; Sis is the isomer selectivity; Ci and Si are the concentration in the reaction mixture and the selectivity, respectively, of the ith oligomerization product (i.e., dimers, trimers, oligomers n ≥ 4, or oligomers of cracking products); and γi is the yield of the ith product.
RESULTS AND DISCUSSION
Physicochemical properties of catalysts. Table 1 summarizes the physicochemical properties of the assayed zeolite samples.
The XRD crystallinity of the initial H-Beta is 98%. Remaining almost unchanged during the subsequent treatments, the crystallinity is near 98% for H-Beta-1, H-Beta-2, and H-Beta-3 (Fig. 1).
In aluminosilicates, aluminum atoms embedded in the silicate lattice are known to have a 27Al NMR MAS signal in the range of 50 to 60 ppm . Aluminum atoms not embedded in the crystal lattice can form extra-lattice Al2O3 microphases with a signal between 0 to 10 ppm.
The 27Al NMR spectrum of H-Beta has only one signal in the range of 50 to 60 ppm (Fig. 2), thus confirming the high crystallinity of the sample and indicating the absence of extra-lattice aluminum. Post-treatment of H-Beta forces a small amount of aluminum to leave the crystal lattice, as evidenced by the appearance of a weak signal between 0 and 10 ppm in the H-Beta-1, H-Beta-2, and H-Beta-3 spectra.
The post-synthetic dealumination predictably increases the SiO2/Al2O3 molar ratio in H-Beta-1 and H-Beta-3 (Table 1). According to elemental analysis, the aluminum content in H-Beta-2 does not change because the aluminum removed during steam heating remains in the zeolite framework channels.
The examination of the porous structure of the samples demonstrates that the initial H-Beta has a micropore volume of 0.23 cm3/g and a BET specific surface area of 625 m2/g. The presence of mesopores in the H-Beta (0.06 cm3/g) appears to result from intercrystalline mesoporosity. The acid treatment (H-Beta-1) reduces the micropore volume to 0.21 cm3/g, due to their partial destruction, and increases the mesopore volume by 50%. A similar occurrence is observed for steam heating (H-Beta-2). In this case, the volume of newly-formed mesopores is slightly higher than after acid treatment. The highest mesopore volume (0.17 cm3/g) is observed after combined treatment (H-Beta-3). In this case, as noted above, the crystal lattice is not destroyed.
Two peaks are observed in the NH3-TPD spectra, specifically a low-temperature peak with a maximum between 100–350°C, and a high-temperature peak with a maximum above 350°C. This indicates the presence of two types of acid sites in the assayed samples: weak acid sites corresponding to the low-temperature peak, and strong acid sites corresponding to the high-temperature peak. According to the ammonia TPD, the initial H-Beta has the highest concentrations of both weak and strong acid sites. Acid treatment of H-Beta slightly reduces the concentrations of weak and strong acid sites (H-Beta-1) due to the removal of aluminum atoms from the lattice with the simultaneous destruction of acid sites. Steam heating (H-Beta-2) leads to a two-fold reduction in the concentration of weak acid sites, and an almost five-fold reduction in strong acid sites. Such an abrupt drop cannot be explained only by the removal of more aluminum atoms from the zeolite lattice than in acid treatment. Most probably, alumina clusters formed during steam heating block the remaining acid sites. This assumption is confirmed by the almost three-fold increase in the strong acid site concentration during the subsequent acid treatment (H-Beta-3). Thus, the acid site concentration in the samples decreases in the following order: H-Beta > H-Beta-1 > H-Beta-3 > H-Beta-2.
Oligomerization of pent-1-ene in the presence of Beta zeolites. Under the assayed conditions, pent-1-ene is readily isomerized over zeolites into cis- and trans-pent-2-ene, while branching isomers are contained in very small amounts (below 1.0%). Oligomers are represented by dimers (C10H20), trimers (C15H30), tetramers (C20H40), pentamers (C25H50), and hexamers (C30H60) of pentenes. The newly-formed oligomers, as well as the initial pentenes, can undergo destruction at ≥150°C to form light hydrocarbons. These light hydrocarbons are further oligomerized to form C6–C9, C11–C14, and C16–C19 compounds (oligomers of cracking products, OCP).
Figure 3 graphically illustrates the results of pent-1-ene oligomerization over the above-described H-Beta samples before and after the dealumination by different methods.
The lowest pentene conversion (27%) is observed with the steam-heated sample (H-Beta-2). This is explained by the low concentration of strong acid sites in the catalyst. Pentene oligomers are also produced over this sample with a low yield of 6%. The isomer to oligomer yield ratio is 3.7. This is the highest value achieved for the assayed group of catalysts, which indicates the dominance of pentene isomerization in the presence of H-Beta-2.
The highest pentene conversion (79%) is ensured by H-Beta-1. This sample provides an isomer yield of 48% and a total oligomer yield of 30%.
Despite the similarity of pent-1-ene conversion rates for H-Beta and H-Beta-3 (60 and 57%, respectively), these products have substantially different compositions. The reaction mixture provided by the initial H-Beta primarily contains pentene oligomers (total yield of 33%), while the major reaction product in the presence of H-Beta-3 are pent-1-ene isomers (38%).
The review of the acidic properties of H-Beta samples vs. oligomer yields reveals that a decrease in the acid site concentration leads to a decline in the oligomer yield. Both the activity of Beta samples in oligomerization and the yield of pentene oligomers decrease in the following order: H-Beta > H-Beta-1 > H-Beta-3 > H-Beta-2.
The oligomer fraction produced over the initial microporous H-Beta contains primarily dimers (above 80%). The oligomers synthesized over micro/mesoporous samples exhibit a reduced dimer content and an increased proportion of higher-molecular-weight compounds. The dimer/trimer ratio is at its highest (6) in oligomers obtained over H-Beta and decreases in the presence of micro/mesoporous samples H-Beta-1, H-Beta-2, and H-Beta-3: 3.3, 2.7, and 2.8, respectively.
It should be noted that, under the given conditions (110°C, 10 wt % catalyst, 5 h), hardly any cracking products or their oligomers are formed (less than 0.5% yield).
Temperature elevation from 110°C to 150°C leads to a pent-1-ene conversion increase to 98–100% and to a 3.5-fold decline in pent-1-ene isomer selectivity (Fig. 4). The oligomer yield is doubled and reaches 82%. Further heating to 200°C increases the proportion of trimers and higher-molecular-weight compounds in the oligomers. The negative effect of high temperature includes the intensification of destruction processes and an increase in OCP content (from 1% at 150°C to 15% at 200°C).
At 180°C, oligomerization occurs most selectively and with the highest yield in the presence of 10–20% catalyst (Fig. 5). When the catalyst quantity is reduced to 5%, the reaction proceeds with a lower conversion (72%) and produces primarily isomers, rather than oligomers, of pentene.
The examination of H-Beta and H-Beta-3 stability during pent-1-ene oligomerization demonstrated that the highest stability is achieved by micro/mesoporous zeolite (Table 2). In the presence of H-Beta-3, the pent-1-ene conversion rate remained almost unchanged after four oligomerization cycles. With H-Beta, on the contrary, the monomer conversion had already declined in the second cycle.
Variations in the product composition are shown in Table 2. As a result of catalyst deactivation, the yields of pentene oligomers and OCP decline in each subsequent cycle, while the yield of isomers rises. As such, the oligomer yields decline faster with the microporous H-Beta than with the micro/mesoporous H-Beta-3. The proportion of dimers in total oligomers increases, while those of trimers and C20+ oligomers decrease.
The catalytic properties of H-Beta zeolite samples dealuminated by different methods were examined in the oligomerization of pent-1-ene. The examination revealed that the yield of pentene oligomers mainly depends on the concentration of acid sites in the catalyst and decreases in the following order: H-Beta > H-Beta-1 > H-Beta-3 > H-Beta-2 (110°C, 10 wt %, 5 h). In the presence of H-Beta-2, a sample prepared by steam heating of H-Beta and having the lowest acid site concentration, the dominant reaction is pent-1-ene isomerization. H-Beta, a microporous sample, promotes the formation of dimers as the main oligomerization product. Micro/mesoporous zeolites ensure a wider molecular weight distribution of oligomers, including dimers, trimers, tetramers, pentamers, and hexamers of pentene.
The study also enabled us to identify the conditions required to produce pentene oligomers with the maximum yield (89%) as ensured by the H-Beta-3 micro/mesoporous sample. These conditions are: 200°C, 10 wt %, 5 h.
It was demonstrated that the micro/mesoporous H-Beta-3 exhibits a higher stability in the oligomerization of pent-1-ene than the microporous H-Beta.
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The study described here was performed within the framework of the state assignment no. AAAA-A19-119022290006-2 on the topic “Zeolite Materials of Different Structural Type with High Crystallinity and Hierarchical Porous Structure as a New Generation of Catalysts for the Synthesis of Important Petrochemicals,” and with financial support from the Russian Foundation for Basic Research (research project no. 20-33-90120).
The authors declare no conflict of interest requiring disclosure in this article.
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Serebrennikov, D.V., Grigor’eva, N.G., Khazipova, A.N. et al. Oligomerization of Pent-1-ene in the Presence of Dealuminated Beta Zeolite Samples. Pet. Chem. (2021). https://doi.org/10.1134/S096554412103018X
- Beta zeolite