CC BY
38
CHEMICAL PROBLEMS 2020 no. 1 (18) ISSN 2221-8688
49
UDC -547.53:547525+665.652.4+661.183.6
ALKYLATION OF TOLUENE WITH ISOPROPANOL ON ZSM-5 TYPE ZEOLITE
MODIFIED BY RARE-EARTH METALS
N.M. Abdullayeva
Baku State University 23, Z.Khalilov str., AZ1148, Baku; e-mail:nigaramirova@yandex.ru
Received 11.12.2019
Abstract: Influence of rare-earth elements (La, Ho, Yb, Pr) on acidic, textural and catalytic properties of ZSM-5 high-silica zeolite in the reaction of toluene alkylation with isopropanol was studied in the temperature range of250-350 ° C in a flow unit of ideal displacement. It revealed that the nature of rare earth metal in zeolites ZSM-5 plays a crucial role in its para-selectivity and catalytic activity. Maximum selectivity with respect to cymols is achieved on a Yb modified zeolite and is 70.5%. The highest para-selectivity is displayed by La modified zeolite to make up 72.4%. It found that rise in selectivity for cymol is due to decrease in concentration of strong acid centers, rise in proportion of acid centers of moderate strength, as well as changes in the porous structure of the zeolite due to its chemical modification. Keywords: ZSM-5, alkylation, toluene, cymols, acidic centers, modification. DOi: 10.32737/2221-8688-2020-1-49-54
Introduction
Alkylaromatic hydrocarbons are widely introduction of zeolite catalysts for alkylation
used in petrochemical and organic syntheses. Isopropyltoluenes (cymols) obtained by alkylation of toluene with isopropanol in the presence of Friedel-Crafts acidic catalysts are used to produce iso- and terephthalic acids, cresols and other important products of organic synthesis [1,2]. Acid catalysts used in industry make it no possible to obtain a p-isomer enriched mixture of cymols. These catalysts have low stability, undergo no regeneration and pollute the environment. Conducting such a process at one stage on solid acid catalysts, corrosion-resistant and environmentally friendly is an important and urgent task. Therefore, much attention has recently been paid to the creation and
of aromatic hydrocarbons with olefins and alcohols [3-5]. Zeolite catalysts are replacing traditional homogeneous acid and heterogeneous oxide catalysts everywhere [68]. A very promising direction of zeolite alkylation was discovered due to the development of syntheses of high-silica zeolites such as ZSM-5 have a three-dimensional channel structure with two different channels and 10-membered oxygen rings with pores of size 0.53*0.56 nm [9].
The purpose of this report was to study the influence of rare-earth elements (REE) (La, Ho, Yb, Pr) on physicochemical and catalytic properties of ZSM-5 type zeolite in the alkylation of toluene with isopropanol.
Experimental part
For the study, a high-silica zeolite of the type of a ZSM-5 with a molar ratio SiO2 / AhO3 = 58 was used and then converted by ion exchange into the NH4-form according to the procedure described previously [10].
The H-form of the zeolite was obtained by thermal decomposition of the NH4-form at 500°C for 4 hours. Catalysts modified with 5.0 wt% REE were obtained by impregnating the H-forms of the zeolite with an aqueous solution of nitrates La, Ho, Yb, Pr at 80°C for
6 hours. Samples were dried in air for 16 hours, then 4 hours in an oven at 110°C, and finally calcined for 4 hours at 550°C.
The acidic properties of the modified zeolites were studied through ammonia thermal desorption. The porous structure of the samples was studied by means of low-temperature nitrogen adsorption at 77 K using the ASAP-2000 unit of Micromeritics.
Before the experiment, the catalysts were activated in air flow at a temperature of
www.chemprob.org
CHEMICAL PROBLEMS 2020 no. 1 (18)
50
N.M. ABDULLAYEVA
550°C for 1 hour, then lowered a temperature to 400°C and hydrogen was supplied for 1 hour. Experiments were carried out on a flow-type installation in a quartz reactor with a stationary catalyst bed and a volume of 4 cm3 at an atmospheric pressure in the presence of
hydrogen in the temperature range 250-350°C, bulk flow rate of 2h-1 at a molar ratio of C7H8: i-C3HyOH: H2 equal to 2: 1: 2. Analysis of the reaction products was performed by gas-liquid chromatography [10].
Results and its discussion
Table 1 presents data on the activity and isomerization of alkylation products, as well as
selectivity of HZSM-5 in the reaction of dehydration of isopropanol, oligomerization of
toluene alkylation with isopropanol. It is clear the resulting propylene followed by cracking
that as reaction temperature rises from 2500 C and flavoring the resulting olefin fragments.
to 350°C, the conversion of toluene increases As the reaction temperature increases to
from 8.8 to 27.1 mass%. The reaction products 330°C, the yield of by-products increases and
consist of benzene, cymols, propyltoluenes, C5 the content of p-cymol in the mixture of
+ aliphatic hydrocarbons and alkylaromatic cymols decreases from 43.8% to 33.7%, i.e.,
hydrocarbons C8 +. Alkylation is accompanied isomerization of p-cymol to m- and o-cymols
by side reactions of transalkylation and occurs.
Table 1. Composition of toluene alkylation products with isopropanol on HZSM-5
t,°C Conversion, wt.% Selectivity for products in catalyzate,% Isomeric composition of cymols,%
toluene isopropanol benzene cymols propyl- toluenes C5+ aliph. hydro- carb. ArH C8+ m p 0
250 8.8 94.2 0.3 52.5 33.3 12.4 1.5 50.8 43.8 5.4
300 19.5 99.1 2.5 56.4 30.2 8.8 5.1 53.2 40.3 6.5
330 27.1 99.7 3.1 55.8 29.0 6.2 5.9 58.2 33.7 8.1
300 310 3:0 330 340 350
Temperature,5 C
Fig. 1. Dependence of selectivity for cymols on the REE nature in the catalyst HZSM-5 at different temperatures.
Modification of REE zeolite significantly affects its activity and selectivity. Modification of HZSM-5 with metals amounting to 5.0 mass% significantly reduces the yield of by-products and raises the selectivity for p-cymol. It found that the modification of HZSM-5 by any of the REEs leads to rise both in the selectivity for cymols (Fig. 1) and the content of p-cymol in the catalyzate (Fig. 2).
The highest selectivity for cymols is observed on the Yb-modified catalyst, and at
350°C it makes up 70.5%. According to the selectivity of cymols formation depending on the modifying metal, the catalysts are arranged in a row as follows: Yb-HZSM-5> La-HZSM-5> Ho - HZSM-5> Pr - HZSM-5.
The difference in selectivities observed on Yb and La modified catalysts is 2.2%. As can be seen from Fig. 2, the highest content of p-cymol in a mixture of cymols is observed on a modified La catalyst, i.e. the highest p-selectivity is shown by the catalyst with 5% La / HZSM-5.
■5%Yb ■ 5%Ho •5%Fr
500
j10
>4P >40 350 Temperature^C
Fig. 2. Influence of the REE nature in the composition of HZSM-5 on the content of p-cymol in the catalyzate at different temperatures.
On this catalyst at 300°C, the selectivity for p-cymol is 72.4%. The rise in the reaction temperature from 300°C to 330°C significantly reduces the p-selectivity of modified catalysts. According to p-selectivity, depending on the modifying metal, the catalysts are arranged in the following row: La - HZSM-5> Ho -
Table 2. Acid characteristics of modified catalysts
HZSM-5> Yb - HZSM-5> Pr - HZSM-5.
Effect of the modification and REE nature on the activity and para-selectivity of the zeolite catalyst may be due to changes in the acidic and textural properties of catalysts as a result of modification.
Catalyst Tmax forms,°C desorption of ammonia Concentration of acid sites, (^kmol • g-1) x C1 /C2
Ti Tii C1 C2
HZSM-5 198 421 625 548 1.14
5%La HZSM-5 183 284 182 103 1.77
5%Ho HZSM-5 185 296 197 119 1.65
5%Yb HZSM-5 189 314 349 231 1.51
5%Pr HZSM-5 190 326 358 242 1.48
i, Tii -peak temperatures for forms I and I I Ci and C2 - concentrations of acid sites in forms (I) and (II).
CHEMICAL PROBLEMS 2020 no. 1 (18) ISSN 2221-8688 49
As the Table 2 shows, two peaks are observed in the thermally desorbed spectrum for the initial HZSM-5 spectrum which indicates the presence of two forms of ammonia desorption from the zeolite catalyst: a low-temperature peak in the region of 120-230°C with a maximum peak temperature Tmax = 198°C refers to the desorption of ammonia with weak Lewis acidic and high-temperature peak in the region of 300-500°C with Tmax = 421°C related to ammonia desorption mainly from strong Bronsted acid centers which are hydrogen ions of bridging hydroxyl groups [11-12]. The introduction of modifiers into HZSM-5 leads to significant changes in its thermal desorption spectra. The largest decrease of strong acid sites concentration is observed when lanthanum HZSM-5 is modified: the concentration of strong acid sites decreases from 548 |imol /g to 103 |imol /g.
The concentration of strong acid sites on samples containing Yb and Pr is significantly higher than on samples containing La. When comparing the acidic and catalytic properties of modified catalysts, some assumptions can be made as follows: a decrease in the total number of acid sites leads
to an increase in the p-selectivity for p-cymol, and the ratio of the proportion of medium to strong acid centers plays an important role The best results are provided by medium-strength acid centers of Bronsted and Lewis which were formed after modification of zeolite HZSM-5 with lanthanum and holmium. The catalyst 5% La НZSМ-5 with its highest ratio of medium to strong acid sites (1.77) shows the highest selectivity for p-cymol.
The Table 3 presents textural properties of modified zeolites. It can be seen that the nature of the modifier significantly affects the porous structure of the zeolite. It is seen that as a result of the modification, the surface area, volume and radius of the pores decrease. Apparently, this is explained by the fact that during the preparation of the catalyst, part of the zeolite proton centers is exchanged for REE cations, as well as the deposition of REE oxides on the surface and in the channels of the zeolite, which leads to a change in the acid properties and geometry of the zeolite framework and, accordingly, the pore radius from Rmax 10-110 to 8-83 A, with which the activity and selectivity of zeolite systems are closely related.
Table 3. Textural characteristics of catalysts according to the BET methoc
Catalysts S, M2/r Vpore (CM3/r) Ref, Â Rmax, Â
Н-ZSМ-5 266.3 0.24 10-110 65
5% La-ZSМ-5 223.4 0.17 8-75 56
5% Yb-ZSМ-5 231.4 0.19 8-77 58
5% Ho-ZSМ-5 226.2 0.18 8-75 57
5% Pr-ZSМ-5 236.7 0.20 8-83 61
Thus, the REE nature in HZSM-5 zeolites plays a decisive role in its paraselectivity and catalytic activity which is due to changes in the acidic and textural properties of zeolites as a result of their chemical modification. It
found that the most active with respect to cymol is Yb-modified HZSM: the maximum selectivity for cymol is 70.5%. Maximum paraselectivity is observed on the modified lantan catalyst and is 72.4%.
References
1. Koshel G.N., Nesterova T.N., Rumyantseva
Yu.B., Kurganova E.A., Ivanova A.A.
Cymols. Obtaining cresols and their use. Vestnik MITHT, 2012, vol. 7, no. 6, pp. 5659. (In Russian).
2. Neatu F., Culic G., Florea M., Parvulescu V.I. and Cavani F. Synthesis of Terephthalic Acid by p-Cymene Oxidation using Oxygen: Toward a More Sustainable Production of Bio-Polyethylene
www.chemprob.org
CHEMICAL PROBLEMS 2020 no. 1 (18)
Terephthalate. ChemSusChem. 2016, vol. 27, no. 9, pp. 3102-3112. https://doi.org/10.1002/cssc.201600718
3. Thakur R., Gupta R.K. and Barman S. A comparative study of catalytic performance of rare earth metal-modified beta zeolites for synthesis of cymene. Chemical Papers. 2017, vol. 71, pp. 137-148. https://link. springer. com/article/10.1007/s1 1696-016-0071-x
4. Upadhyayula S. Gas phase toluene isopropylation over high silica mordenite. J. Chem. Sci. 2010, vol. 122, no. 4, pp. 613-619.
https://link. springer. com/article/10.1007/s1 2039-010-0096-6
5. Barman S., Maity S.K. and Pradhan N.C. Alkylation of toluene with isopropyl alcohol catalyzed by Ce-exchanged NaX zeolite. Chem. Eng. J. 2005, vol. 114, pp. 39-45.
https://doi.org/10.1016/j.cej.2005.08.014.
6. Odedairo T., Al-Khattaf S. and Jermy R. Influence of Mesoporous Materials Containing ZSM-5 on Alkylation and Cracking Reactions. J. Mol. Catal. A, Chemical. 2011, vol._345, pp. 21-36. D0I:10.1016/j.molcata.2011.05.015
7. Chiang T-C., Chan J-C., Tan C-S. Alkylation of Toluene with Isopropyl Alcohol over Chemical Liquid Deposition Modified HZSM-5 under Atmospheric and Supercritical Operations. Ind. Eng. Chem. Res. 2003, vol. 42, no. 7, pp. 1334-1340. https://pubs.acs.org/doi/abs/10.1021/ie0206 305
8. Janardhan H.L., Shanbhag G. and Halgeri A.B. Shape-selective catalysis by phosphate
modified ZSM-5: Generation of new acid sites with pore narrowing. Appl. Catal. A: Gen. 2014, vol. 471, pp. 12-18. https://doi.org/10.1016/j. apcata.2013.11.02 9
9. Antony Raj K.J., Padma Malar E.J. and Vijayaraghavan V.R. Shape-selective reactions with AEL and AFI type molecular sieves alkylation of benzene, toluene and ethylbenzene with ethanol, 2-propanol, methanol and t-butanol. J. Mol. Catal. A, Chemical. 2006, vol. 243, pp. 99-105. https://doi.org/10.1016/j. molcata.2005.07.0 40
10. Amirova N.M., Mamedov S.E., Gakhramanov T.O. The effect of the concentration of magnesium oxide on the physicochemical and catalytic properties of high-silica zeolites of the ZSM type in the alkylation reaction of toluene with isopropanol. Young scientist. 2018, no. 10, (196), pp. 8-11. (In Russian).
11. Kazansky V.B., Borovkov V.Yu., Serykh A.I., van Santen R.A. and Anderson B.G. Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation. Catal. Lett. 2000, vol. 66, pp. 39-47. https://link. springer. com/article/10.1023/A %3A1019031119325
12. Gong T., Zhang X., Bai T., Zhang Q., Tao L., Qi M., Duan C. and Zhang L. Coupling Conversion of Methanol and C4 Hydrocarbon to Propylene on La-Modified HZSM-5 Zeolite Catalysts. Ind. Eng. Chem. Res. 2012, vol. 51, pp. 1358913598. DOI: 10.1021/ie300515z
NADiR TORPAQ METALLARIiLd MODiFiKASiYA OLUNMU§ ZSM-5 NÖVLÜ SEOLiTiNigTiRAKINDA TOLUOLUNiZOPROPANOLLA ALKiLLd$MdSi
N.M. Abdullayeva
Baki Dövlat Universiteti AZ 1148, Baki, Z. Xdlilov küg.,23; e-mail: nigaramirova@yandex.ru
Axin tipli qurguda 250-350OC temperatur intervalinda toluolun izopropanolla alkilla§masi reaksiyasinda nadir torpaq elementlarinin (La,Ho,Yb,Pr) tabiatinin yüksak silisiumlu ZSM-5 növlü seolitin tur§u, tekstur va katalitik xassalarina tasiri öyranilmi§dir. Müayyan olmu§dur ki,
54
N.M. ABDULLAYEVA
ZSM-5 seolitda nadir torpaq elementlarinin tdbidti onun para-segiciliyina va katalitik aktivliyina mühüm daracada tasir edir. Cimollara gora maksimal segicilik Yb ila modifikasiya olunmu§ seolitda ba§ verir va 70.5% ta§kil edir. Para-segiciliya gora daha yüksak segicilik La ila modifikasiya olunmu§ seolit gostarir(72.4%). Müayyan olunmu§dur ki, kimyavi modifikasiya naticasinda p-cimola gora segiciliyin artmasi güvvatli tur§u markazlarinin azalmasi, orta tur§u markazlarinin payinin artmasi va seolitin masamali qurulu§unun dayi§masi ila alaqalidir. Agar sozlw. ZSM-5, alkilla§ma, toluol, izopropanol, cimol, tur§u markazlari, modifikasiya.
АЛКИЛИРОВАНИЕ ТОЛУОЛА ИЗОПРОПАНОЛОМ НА ЦЕОЛИТЕ ТИПА ZSM-5, МОДИФИЦИРОВАННОГО РЕДКОЗЕМЕЛЬНЫМИ МЕТАЛЛАМИ
Н.М. Абдуллаева
Бакинский государственный университет AZ 1148, Баку, ул. З.Халилова, 23; e-mail: nigaramirova@yandex. ru
В интервале температур 250-350° С в проточной установке идеального вытеснения изучено влияние природы редкоземельных элементов (La, Ho, Yb, Pr) на кислотные, текстурные и каталитические свойства высококремнеземного цеолита типа ZSM-5 в реакции алкилирования толуола изопропанолом. Установлено, что природа редкоземельного металла в цеолитах ZSM-5 играет определяющую роль в его пара-селективности и каталитической активности. Максимальная селективность по отношению к цимолам достигается на цеолите, модифицированном Yb и составляет 70.5%. Наибольшую пара-селективность проявляет цеолит, модифицированный La и составляет 72.4%. Установлено, что возрастание селективности по цимолу связано с уменьшением концентрации сильных кислотных центров, увеличением доли кислотных центров средней силы, а также изменением пористой структуры цеолита в результате его химического модифицирования.
Ключевые слова: ZSM-5, алкилирование, толуол, изопропанол, цимолы, кислотные центры, модифицирование.
