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AZERBAIJAN CHEMICAL JOURNAL № 2 2022 ISSN 2522-1841 (Online)
ISSN 0005-2531 (Print)
UDC 54.9:66.09:547.211
MECHANISM OF CATALYTIC ISOMERIZATION-DISPROPORTIONATION PROCESSING OF STRAIGHT-RUN GASOLINE
A.A.Iskenderova
Y.Mammadaliyev Institute of Petrochemical Processes, NAS of Azerbaijan
ayten_iskenderova@mail.ru
Received 30.10.2021 Accepted 27.11.2021
The isomerization-disproportionation conversion of straight-run gasoline over the Co/HZSM/SO42--ZrO2 (SZ) composite catalytic system at atmospheric hydrogen pressure has been studied. Optimal process conditions (1800C; H/CH=3; WHSV = 2.5 h-1) was established, under which the conversion of C8+ hydrocarbons is 77.3%, and the sum of high-octane gasoline components C5-C6 alkanes increases from 11% to 61%. The effect of C4-, C5-C6 products and H2O on this isomerization-disproportionation process have been studied. It is shown that, unlike the main C5-C6 products, side C4-products affect the stability of the process, and the catalyst is deactivated under the water's influence. Comparative studies of the conversion of a mixture of straight-run gasoline and C2H5OH showed that H2O vapor formed sites deactivate the centers located on the SZ. It has been established that the mechanism of isomerization-disproportionation conversion of straight-run gasoline with the participation of a composite catalytic system is realized through the conjugated interaction of red-ox centers contained on SZ with acid-base centers of Co/HZSM-5, responsible, respectively, for the formation of intermediate compounds [C4+C7] and their and their further hydro cleavage.
Keywords: straight-run gasoline, composite catalysts, catalytic system, conversion, intermediate, isom-erization-disproportionation conversion.
doi.org/10.32737/0005-2531-2022-2-28-33
Introduction
In the production of motor gasoline, there is a constant trend toward limiting the content of aromatic hydrocarbons in them, especially benzene and unsaturated hydrocarbons, while maintaining the high-octane characteristics of the fuel. An effective solution to this problem is associated with the development of the process of isomerization of n-paraffins [1,2]. In modern technological schemes for the production of gasoline, light naphtha is used as a raw material. The efficiency of the isomerization process aimed at obtaining high-octane gasoline components (isopentane and dimethylbutanes) depends on the recycling of low-octane n-pentane and hexanes, i.e., it reduces the individualization of the isomerization of the individual C5 and C6 components of light naphtha [3-5, 15]. Therefore, at present, the development of isomerization largely depends on the quality of the catalyst used in the process. Increasing the activity, selectivity, and stability of the isomerization catalyst are the main tasks facing oil refining [3, 5, 6].
Industrially preferred catalysts for the-process of paraffin isomerization are medium-temperature (280-3000C) zeolite [7, 8] and low-
temperature (90-2100C) anion-modified metal oxide catalysts, such as sulfated zirconium [1, 2]. The isomerization of C7+ hydrocarbons on these catalysts is limited by the fact that they are converted into by-product C1-C4 hydrocarbons or accumulate and contribute to the deactivation of the catalyst.
The use of high hydrogen pressures and the introduction of platinum into the composition of isomerizing industrial catalysts blocks the formation of hydrocarbon intermediates, which at low temperatures, accumulating on the catalyst surface, lead to its deactivation [9-14]. However, the conversion of mixtures of n-C6+:C4- and gas gasoline containing n-C6+ paraffin with the participation of composite catalysts containing zeolite and anion-modified components showed sufficient stability of the process at temperatures of 160-1800C and atmospheric pressure of hydrogen [16, 17]. In contrast to the usual skeletal isomerization of n-alkanes, the main products of the noted transformations of hydrocarbons are C5-C6 alkanes containing a significant amount of isocomponents. The conversion of straight-run gasoline on the marked catalytic systems is also accompanied mainly
by the formation of C5-C6 alkanes [18-20]. Taking into account the importance of C5-C6 alkanes as a raw material for the production of high-octane aromatic-free gasoline, it is of particular interest to establish the main patterns responsible for the conversion of straight-run gasoline with the participation of composite catalytic systems like Co/H-zeolite/SO42--ZrO2.
Therefore, in this work, we studied the effect of products of the conversion of straight-run gasoline (SRG), water vapor, and ethanol on the stability of the operation of the Co/HZSM/SO42--ZrO2 catalytic system in the isomerization-disproportionation conversion of SRG to C5-C6 alkanes.
Experimental part
The conversion of SRG was studied on a laboratory flow type catalytic unit equipped with a quartz reactor in the presence of composite catalysts presented in Table 1, composition (wt%): sulfated zirconia (SZ)-15; Co or Ni, 0.4 (0.5); binder alumogel-15; the rest is H-zeolite or Al2O3. The procedure for synthesizing composite catalysts is described in detail in [18, 19].
Table 1. Composition of composite catalysts
Catalyst designation Catalyst composition
M-11 0.4%Co(HZSM)/SO42"(2%)-ZrO2
M-12 0.4%Co(HZSM)/SO42"(6%)-ZrO2
The volume of the catalyst loaded into the reactor was varied within 1-5 sm3 while maintaining the constancy of the process conditions given below. Before the experiment, the CC system was reduced with hydrogen at 3800C (2 h). The transformation of SRG on CC systems was studied by varying the temperature (T) in the range of 180-2200C, with a volumetric feed rate of SRG (WHSV-2.5 h-1) at atmospheric pressure and a linear hydrogen velocity of 2 l/h. For defini-teness, in the H2/CH ratio, the calculated average
molecular weight of the raw material (121) g was taken as the CH value. The reaction products were analyzed by gas chromatography using an Auto SystemXL chromatograph. The raw materials used were a straight-run gasoline fraction, a mixture of straight-run gasoline and C5-C6 alkanes (1:4), a mixture of straight-run gasoline and propane-butane fraction (1:1), and a mixture of the straight-run gasoline with ethanol (4:1).
Results and discussion
Contacting SRG with the synthesized Co/HZSM-5/SZ catalyst in a hydrogen atmosphere at 1800C leads to a noticeable enrichment of the catalyzate in low molecular weight C6 components and a decrease in the content of C8 paraffins.
A change in the linear rate of hydrogen supply uH2 leads to changes in the distribution of the products of the conversion of C7+ components of straight-run gasoline.
With an increase in uH2 from 10 to 30 mlmin-1, the conversion of high-molecular-weight components of straight-run gasoline and the yield of undesirable gaseous alkanes increase; an increase in the hydrocracking of C7+paraffins is observed. With a further increase in uH2 > 40 mlmin-1, the noted parameters slightly increase. The yield of target C5-C6 alkanes depending on uH2 has an extreme character with a maximum value in the range of 20-30 mlmin-1. Such a dependence of the yield of C5-C6 on uH2 is apparently a consequence of the overlap of different routes leading to their formation (Table 2).
The yield of gaseous C4- alkanes also depends on uH2.
An increase in the C4- yield in the range 10< uH2<30 mlmin-1 indicates the participation of hydrogen not only in the hydrocracking of C7+ paraffin's, but also as a carrier gas, which
Table 2. Hydroconversion of straight-run gasoline on the catalyst M-11, T=1800C, WHSV=2.5 h 1, t=30 min
uH2,mlxmin-1 Conversion of C8+ Catalyzate composition, wt.%
C4- C5-C6 C7 C8+
10 58.1 7.9 59.6(20.6)* 7.6 24.9
20 74.4 13.0 67.2(19.4)* 4.6 15.2
30 76.1 18.5 63.0(18.5)* 4.3 14.2
40 77.3 20.9 61.1(18.0)* 4.2 13.8
50 76.4 21.0 60.8(17.6)* 4.2 14.0
*total content of dimethylbutane in isohexane.
lecular hydrocarbons, the boiling temperatures of which exceed the temperature of the experiments. An increase in the hydrocracking activity of the catalyst by the zeolite component promotes the efficient splitting of the [C4+C7] intermediate into C7 products and, thereby, stabilizes the process, reducing the accumulation of HCD. The accumulation of HCD is a consequence of the formation of higher molecular weight products as a result of the interaction of straight-run gasoline with a composite catalyst. Based on the results obtained (Table 2), it can be assumed that the intermediates leading to the accumulation of HCD can be both monomolecular intermediates formed by high-molecular components of straight-run gasoline, and bimolecular ones formed with the participation of the initial C7+ components and their conversion products.
The main contact products of straight-run gasoline with the participation of a composite catalyst are C5-C6 (target) and C4- (side) alkanes. To establish their influence on the conversion of the C7+ components of the mixture, the conversion of mixtures of SRG:PBF and SRG :P-H was studied (Table 4).
Under the influence of high-molecular paraffins C8+, capable of undergoing transformations on composite catalysts, low-activity gaseous paraffins of the propane-butane fraction are involved in the process of formation of products with a large number of carbon atoms in the chain (C5-C7).
Table 3. HCD accumulation on SZ-containing catalysts during the conversion of straight-run gasoline, T=1800C
UH2 mlmin 1 Conversion of C8+ , % Time, min HCD C H2
g/gcat % g/gcat g/gcat
10 58.1 30 0.049 1.33 0.038 0.006
45.6 60 0.057 1.54 0.045 0.008
40 77.3 30 0.018 0.49 0.015 0.0032
72.5 60 0.026 0.72 0.023 0.0040
69.4 120 0.040 1.07 0.035 0.0060
Table 4. Influence of C6- conversion products of straight-run gasoline on the conversion of its C7+ components. T=1800C,WHSV=2.5 h1, t=30 min_
Composition of feedstock and catalyzate, wt.%
components feedstock SRG catalyzate feedstock SRG:PBF catalyzate feedstock SRG:PH catalyzate
n-C4 1.0 0.6 47.7 17.4 0.8 0.5
i-Cs 1.0 2.5 4.3 10.4 2.0 5.0
n-Cs 1.2 4.0 4.2 4.2 10.9 14.1
i-C6 5.0 11.2 3.0 16.0 4.9 12.2
n-C6 3.8 10.9 2.0 4.3 11.0 13.8
n-Cv 28.5 30.2 13.0 36.2 22.8 23.1
n-C8+ 59.5 40.6 25.8 11.5 47.6 31.4
facilitates the removal of products from the reaction zone and thereby accelerates the conversion of straight-run gasoline.
The stability of the functioning of the used catalyst depends on the conditions and duration of the experiments. In particular, other things being equal, a decrease in uH2 leads to a decrease in the stability of the functioning of Co/HZSM/SO42--ZrO2. The conversion of straight-run gasoline when it comes into contact with a composite catalytic system in a nitrogen environment, unlike hydrogen, leads to almost instantaneous deactivation of the catalyst.
The main cause of catalyst deactivation is the blocking of its active surface by hydrocarbon deposits (HCD). Intense accumulation of HCD during the reaction with uH2=10 mlmin1 is observed already in the first 30 minutes of operation (1.33%), which corresponds to the beginning of catalyst deactivation.
Additional accumulation of HCD (1.54%) leads to a sharp decrease in the activity of the catalyst. As a result of an increase in uH2 to 40 mlmin1, the accumulation of HCD is sharply reduced: in 120 min. is 1.05%. As follows from the data obtained (Table 3), the Co/HZSM-5/SZ sample in this case retains its stability over this time. Thus, the high content of hydrogen in the reaction space contributes to a significant decrease in the accumulation of HCD.
Taking into account the results of the elemental analysis of the sediment products, it can be assumed that HCD is saturated high-mo-
Taking into account the absence of Ci-C2 hydrocarbons in the conversion products of a mixture of SRG and PBF, it can be assumed that the formation of C5-C7 paraffins is preceded by the formation of bimolecular intermediates with the participation of C8+ and C4 hydrocarbons and their subsequent hydrocleavage.
From the analysis obtained (Table 4) it follows that the products of the reaction of C5-C6 paraffins, have little effect on the conversion of C7+ components of straight-run gasoline. The conversion of C8+ components of SRG and SRG:C5-C6 (4:1) is 31.8 and 34%, respectively, and the total yield of C5-C6 is 17.6 and 16.0%. Therefore, it can be assumed that C5-C6 products have practically no effect on the conversion of straight-run gasoline. Under the influence of C4 products, the conversion of C8+ noticeably changes. The transformation of a mixture of SRG:PBF is characterized not only by an increased conversion of C8+ components (55.4% in Table 3) but also by an intensive involvement of gaseous alkanes in the process with an increased formation of both C5-C6 and C7 alkanes. The high conversion of C4- and C8+ components of the SRG:PBF mixture and the formation of C5+ products indicate the formation of bimolecular intermediates C4- - C8+ and their subsequent cleavage [19].
Thus, the C7+ components of SRG are capable of transformation, both monomolecularly with the formation of C4 products, and in parallel with their involvement in the formation of [C4+C7] and their hydrocleavage. The presence of water vapor in the reaction zone within 5% has a complete deactivating effect on the conversion of straight-run gasoline with the participation of a composite catalyst. To elucidate the effect of water vapor on the active centers of the individual components of the used catalyst with their participation, we studied the acid-base re-
action of the conversion of ethanol into diethyl ether and ethylene, which proceeds with the release of water molecules.
Table 5. Conversion of ethanol and a mixture of SRG:C2H5OH (4:1) on M-12 and its components. T=1800C, WHSV = 2.5 h-1, > 40 mlmin-1, t=60 min
Catalyst Conversion Conversion of SRG:
sample of C2H5OH, C2H5OH, %
% SRG C2H5OH
SO42--ZrO2 trace - -
Co/HZSM-5 12.0 - 9.5
M-12 12.2 - 8.8
From the results obtained (Table 5) it follows that SZ turned out to be practically inactive in the process of C2H5OH conversion under the conditions of SRG conversion. In contrast to SZ, Co/HZSM-5 exhibits sufficient activity and stability both in the individual conversion of C2H5OH and in mixtures with SRG. Therefore, the formation of water molecules has an inhibitory effect on the centers located on the SZ. At the same time, it follows from the results obtained (Table 5) that the primary activation of the C7+ components of SRG proceeds with the participation of active sites that do not exhibit acid-base properties located on the SZ. Such centers can be electrophilic oxygen atoms that are part of the modifying SO42- anion. As a result of the interaction of water molecules, the resulting SZ, such centers either lose their electrophilicity or are blocked for interaction by C7+ hydrocarbons. At the same time, the centers located on Co/HZSM-5 are independent of the action of water molecules and retain their activity in the acid-base type reaction, which includes the hydrocracking of C7+ type alkanes on SZ, there is an equilibrium of the type (1) with the participation of which the initial activation and isomerization of natural gas paraffins occurs (2)
Stabilization of I proceeds on SZ' with the participation of RH molecules contained in the
RH + [O] C>: : SZ
Stabilized alkanes in the case of RH=H2 and process conditions can be isolated as iso-merization products or undergo hydrocracking with the participation of the zeolite component. In the case of low activity of the cracking component, the accumulation of intermediate products will contribute to the deactivation of the composite catalyst.
Inactivated as a result of reduction (2), SZ is reactivated again by oxidation.
SZ +- HîO SZ + H2
Thus, the isomerization-disproportionation transformation of straight-run gasoline is a consequence of the synergy of the catalytic properties of the SZ and Co/HZSM-5 components of the composite catalyst, which occurs in the temperature range of 140-1800C.
Conclusion
The process of isomerization-dispropor-tionation of straight-run gasoline with the participation of the composite catalytic system Co/HZSM/SO42--ZrO2 is realized through the conjugate interaction of red-ox centers contained on SZ with acid-base centers of Co/HZSM-5, responsible, respectively, for the formation of intermediate compounds and their hydrolysis.
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BIRBA§AQOVULMA BENZININ KATALITIK IZOMERLO§MO-DISPROPORSIALA§MA PROSESININ
MEXANiZMi
A.0.iskand3rova
Hidrogenin atmosfer tszyiqinds Co/HZSM/SO42"-ZrO2 (SZ) Kompozit katalitik sisteminin içtiraki ils birbaçaqovulma benzinin izomerlsçms-disproporsialaçma çevrilmssi tsdqiq edilmiçdir. Optimal proses çsraiti (1800С; Н2/СН=3; h.s.=2,5 s-1) yara-dilmiçdir ki, bu çsraitds C8+ karbohidro-genlsrinin çevrilmssi 77,3% tsçkil edir vs yukssk oktanli benzin komponentlsri C5-C6 alkanlarinin ciximi 11%-dsn 61%-э qsdsr artir. Reaksiya mshsullari C4-, C5-C6 mshsullarinin vs H2O-nun bu prosess tssiri ôyronilmiçdir. Gôstsrilmiçdir ki, ssas C5-C6 mshsullarindan fsrqli olaraq slavs C4- mshsullari prosesin dayaniqligina tssir edir, katalizator iss suyun tssiri ils deaktiv olur. Birbaçaqovulma benzin vs etanol qançiginin çevrilmssinin muqayissli tsdqiqatlari gostsrdi ki, alinan H2O buxari SZ-ds yerlsçsn msrkszlsri deaktiv edir. Musyysn edilmiçdir ki, kompozit katalitik sistemin içtiraki ils birbaçaqovulma benzinin izomerlsçms-disproporsialaçma çevrilmssi mexanizmi SZ-ds olan red-oks msrkszlsrinin, Co/HZSM-5-in turçu-qslsvi msrkszlsri ils konyuqasiya olunmuç qarçiliqli tssiri yolu ils hsyata keçirilir, muvafiq olaraq araliq birlsçmslsrin [C4-+C7] smsls gslmssins vs onlarin hidroparçalanmasina ssbsb olur.
Açar sozlzr: birbaçaqovulma benzin, kompozit katalizator, katalitik sistem, konversiya,intermediat, izomerh§m3-disprosiala§ma cevrilm3.
МЕХАНИЗМ КАТАЛИТИЧЕСКОЙ ИЗОМЕРИЗАЦИОННО-ДИСПРОПОРЦИОННОЙ ПЕРЕРАБОТКИ
ПРЯМОГОННОГО БЕНЗИНА
А.А.Искендерова
Исследовано изомеризационно-диспропорционное превращение прямогонного бензина с участием композиционной каталитической системы Co/HZSM/SO42--ZrO2 (SZ) при атмосферном давлении водорода. Установлены оптимальные условия процесса (1800С; Н2/СН=3; О.С. = 2,5ч-1), при которых конверсия С8+ углеводородов составляет 77.3%, а сумма высокооктановых компонентов бензина С5-С6 алканов повышается от 11 до 61%. Изучено воздействие на изомеризационно-диспропорционный процесс С4-, С5-С6 продуктов реакции и Н2О. Показано, что в отличии от основных С5-С6 продуктов, побочные С4- углеводороды влияют на стабильность процесса, а под воздействием воды происходит дезактивация катализатора. Сопоставительные исследования превращения смеси прямогонного бензина и этанола показали, что образующиеся пары Н2О дезактивирют центры, расположенные на SZ. Установлено, что механизм изомеризационно-диспропорционного превращения прямогонного бензина с участием композиционной каталитической системы реализуется посредством сопряжённого взаимодействия ред-окс центров SZ, с кислотно -основными центрами Co/HZSM-5, ответственных, соответственно, за образование промежуточных соединений [С4- +С7] и их дальнейшее гидрорасщепление.
Ключевые слова: прямогонный бензин, композиционный катализатор, каталитическая система, конверсия, интермедиат, изомеризационно-диспропорционное превращение.
