Alkylation Of Isobutane Research Articles

Mechanism of the Alkylation of Isobutane by Butenes on Zeolite Catalysts

A key role has been proposed for primary carbocations in the initial and final steps of the alkylation of isobutane by butenes on zeolites. Secondary and tertiary carbocations play a role predominantly in steps involving intramolecular transformations.

Isobutane/2-Butene Alkylation Using Large-Pore Zeolites: Influence of Pore Structure on Activity and Selectivity

The alkylation of isobutane with 2-butene was performed in the liquid phase at 80°C and 300 psi. This study constitutes a detailed comparison under identical conditions of 12-membered pore zeolites with different dimensionalities, namely USY, mordenite, beta, LTL, and ZSM-12 zeolite. Particular attention was paid to the role of the structural characteristics of the catalysts. In the mechanism studies, very-low-olefin feed (i/o molar ratio of 1000, olefin WHSV of 0.01 h −1) was used to keep real alkylation without undesirable oligomerizations. For selectivity toward the individual TMP and DMH compounds, pore structure plays a significant role. At the initial stage (5 min on stream), it was found that the relative value for TMP selectivities described by the configurational ratio of 2,2,4-/2,2,3-TMP to 2,3,3-/2,3,4-TMP decreased in the following order: beta ≥ ZSM-12 > mordenite > USY ≥ LTL. Beta and ZSM-12 showed higher values due to their specific pore structure (the absence of large expansions at either the channel intersections or within the channel itself), in which bulky TMPs could diffuse out without structural restriction. In order to support this, the ratio of diffusivity for selected TMP isomers (2,2,4- to 2,3,4-TMP) was obtained. The value of diffusivity ratio of 2,2,4-TMP to 2,3,4-TMP decreased in the sequence beta > USY > LTL. This difference was reflected clearly in the different distribution of TMP isomers. The comparable trends were also observed for the selectivity of DMHs, ratio of 2,5-/2,4-/2,3-DMH to 3,4-DMH. Most of cracking products, 2-methyl butane, 2,3-dimethyl butane, and 2,3- and 2,4-dimethly pentane, were formed by type A β-scissions from [C 12H 25] + and [C 16H 33] +. For other studies with different feed conditions (i/o molar ratio of 98, olefin WHSV of 0.1 h −1), it was also observed that the pore structure of zeolites plays a significant role in the activity of olefin conversion. Beta and ZSM-12 were shown to outperform other zeolites because their favorable pore structures limited the accumulation of carbonaceous compounds that lead to pore plugging. This is in contrast to the common belief that three-dimensional zeolites are less susceptible to pore plugging than one-dimensional zeolites; we observed dramatically better coke tolerance over ZSM-12 (one dimensional) than USY (three dimensional). It was observed that samples synthesized with low Si/Al ratios resulted in higher activity and selectivity to the desired products. This was mainly because of the high hydrogen transfer (HT) capability of the low Si/Al samples. However, the dealumination procedure used strongly affected the catalyst performance; samples dealuminated with ammonium hexafluorosilicate were significantly less active and selective to TMPs than samples either dealuminated with HCl or synthesized directly. This was primarily because of the ability of the latter dealuminating agent to remove surface aluminum as well as partial pore blocking by the silicate salts deposited.

Isobutane/2-Butene Alkylation Using Large-Pore Zeolites: Influence of Pore Structure on Activity and Selectivity

The alkylation of isobutane with 2-butene was performed in the liquid phase at 80°C and 300 psi. This study constitutes a detailed comparison under identical conditions of 12-membered pore zeolites with different dimensionalities, namely USY, mordenite, beta, LTL, and ZSM-12 zeolite. Particular attention was paid to the role of the structural characteristics of the catalysts. In the mechanism studies, very-low-olefin feed (i/o molar ratio of 1000, olefin WHSV of 0.01 h −1 ) was used to keep real alkylation without undesirable oligomerizations. For selectivity toward the individual TMP and DMH compounds, pore structure plays a significant role. At the initial stage (5 min on stream), it was found that the relative value for TMP selectivities described by the configurational ratio of 2,2,4-/2,2,3-TMP to 2,3,3-/2,3,4-TMP decreased in the following order: beta ≥ ZSM-12 > mordenite > USY ≥ LTL. Beta and ZSM-12 showed higher values due to their specific pore structure (the absence of large expansions at either the channel intersections or within the channel itself), in which bulky TMPs could diffuse out without structural restriction. In order to support this, the ratio of diffusivity for selected TMP isomers (2,2,4- to 2,3,4-TMP) was obtained. The value of diffusivity ratio of 2,2,4-TMP to 2,3,4-TMP decreased in the sequence beta > USY > LTL. This difference was reflected clearly in the different distribution of TMP isomers. The comparable trends were also observed for the selectivity of DMHs, ratio of 2,5-/2,4-/2,3-DMH to 3,4-DMH. Most of cracking products, 2-methyl butane, 2,3-dimethyl butane, and 2,3- and 2,4-dimethly pentane, were formed by type A β-scissions from [C 12 H 25 ] + and [C 16 H 33 ] + . For other studies with different feed conditions (i/o molar ratio of 98, olefin WHSV of 0.1 h −1 ), it was also observed that the pore structure of zeolites plays a significant role in the activity of olefin conversion. Beta and ZSM-12 were shown to outperform other zeolites because their favorable pore structures limited the accumulation of carbonaceous compounds that lead to pore plugging. This is in contrast to the common belief that three-dimensional zeolites are less susceptible to pore plugging than one-dimensional zeolites; we observed dramatically better coke tolerance over ZSM-12 (one dimensional) than USY (three dimensional). It was observed that samples synthesized with low Si/Al ratios resulted in higher activity and selectivity to the desired products. This was mainly because of the high hydrogen transfer (HT) capability of the low Si/Al samples. However, the dealumination procedure used strongly affected the catalyst performance; samples dealuminated with ammonium hexafluorosilicate were significantly less active and selective to TMPs than samples either dealuminated with HCl or synthesized directly. This was primarily because of the ability of the latter dealuminating agent to remove surface aluminum as well as partial pore blocking by the silicate salts deposited.

Deactivation behavior of the catalyst in solid acid catalyzed alkylation: effect of pore mouth plugging

The deactivation of solid acid catalysts in liquid-phase alkylation of isobutane with butenes was investigated. The role of pore mouth plugging, in particular, was studied and it was found that it had a significant effect on the deactivation behavior. Simple explicit correlations were developed for catalyst lifetime in a CSTR, both in terms of time-on-stream and butene turnover per catalyst weight. The correlations were tested using available experimental data from the literature and a reasonable agreement was observed. It was shown that for a zeolitic catalyst the butene turnover per catalyst weight was proportional to the square root of catalyst loading, while it was inversely proportional to both the olefin feed concentration and the square root of feed flow rate.

The Effect of Supercritical Fluids on Solid Acid Catalyst Alkylation

The alkylation of isobutane with trans-2-butene was explored over six solid acid catalysts in the liquid, near-critical liquid, and supercritical regions through the addition of an inert cosolvent to the reaction feed mixture. The addition of supercritical cosolvents did not result in sustained catalytic alkylation activity. A modest improvement in product yield was obtained with the addition of methane in the modified-liquid region; however, catalyst longevity and product selectivity were decreased compared to cosolvent-free liquid conditions. This paper describes the catalyst screening and selection process, an exploration of catalyst performance with varying concentrations of methane, and an examination of the effects of seven supercritical fluids on catalyst performance. The catalysts included two zeolites, two sulfated metal oxides, and two Nafion catalysts. Three hydrocarbons, two fluorocarbons, carbon dioxide, and sulfur hexafluoride were explored as inert cosolvents added to the reaction mixture.

Acid soluble oil, by-product formed in isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid: Part I Acid soluble oil composition and its poisoning effect

The composition of acid soluble oil (ASO) formed during isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid (TfOH) was studied. Based on the data received, ASO was assumed to be a complicated mixture of unsaturated hydrocarbons with conjugated double bonds. Its main fragment is the substituted five-member cycle of the formula C 20H 36. ASOs formed (1) by interaction of 2-butene with TfOH; (2) during alkylation in the presence of liquid TfOH; or (3) by degradation of 2,2,4-trimethyl pentane, are practically identical and independent of preparation method. NMR and UV spectroscopy were used to study the interaction of ASO with TfOH. It was demonstrated that in the acid and hydrocarbon phases, a rather strong complex is formed with the composition ASO×2 TfOH. Such an interaction of ASO with TfOH explains the reason for the observed poisoning effect of ASO in the alkylation reaction.

Study of the alkylation of isobutane with n-butenes over WO 3/ZrO 2 strong solid acid. III. Effect of superfine ZrO 2 support on physico-chemical properties and catalytic behavior of the catalyst

Study of the alkylation of isobutane with n-butenes over WO 3/ZrO 2 strong solid acid. III. Effect of superfine ZrO 2 support on physico-chemical properties and catalytic behavior of the catalyst

Isobutane alkylation catalyzed by supported perfluoroalkanedisulphonic acids

Silica supported perfluoroalkanedisulphonic acids (PFAS-SiO2) catalyze the alkylation of isobutane with n-butene to yield mainly high-octane trimethylpentanes. PFAS-SiO2 activity is highly dependent on the PFAS employed, on the preparation method and on the reaction conditions adopted. 1H and 29Si MAS NMR spectroscopy were used to evidence the changes in surface silanols caused by impregnating silica with perfluoroethanedisulphonic acid. A possible active species is hypothesized on the basis of the data acquired through NMR spectroscopy.

Sulfuric Acid Alkylation of Isobutane with Butylenes in a Continuous–Flow Tubular Contactor

The results of pilot–industrial tests of sulfuric acid alkylation of isobutene from butane–butylene fractions in a tubular continuous–flow contactor operating in the turbulent regime with internal cooling are reported. After the time of contact (≈30 sec) of the reagents, an alkylate that satisfies the requirements for specifications is formed. Data from the chromatographic–mass spectrometric analysis of the alkylate are reported.

Promotion of Catalytic Properties of Sulfated Zirconium and Aluminum Oxides with Carbon Tetrachloride

The promoting effect of minor amounts of carbon tetrachloride on the catalytic activity and working life of sulfated zirconium and aluminum oxides is considered for the example of isobutane alkylation with butenes.

The influence of Si/Al ratios of synthesized beta zeolites for the alkylation of isobutane with 2-butene

Alkylation of isobutane with 2-butene was performed in the liquid phase at 80 °C over beta zeolites with Si/Al ratios ranging from 6 to 30. Two different feeds (isobutane/2-butene molar ratios of 20 and 98) were utilized with the same olefin WHSV of 0.5 h −1 to investigate the influence of feed compositions as well. The zeolites used in this study were synthesized directly in order to minimize the variations in structural characteristics such as crystallinity, BET area and pore volume. The relative hydrogen transfer activity of zeolites was determined using the n-hexane cracking reaction. The hydrogen transfer activity remains almost constant for zeolites with Si/Al ratios up to 15 and then decreases with any increase in Si/Al ratio. In contrast to the common belief that a higher aluminum content zeolite is more stable for this reaction, we observed optimum Si/Al ratios of beta zeolites. This unexpected result can be attributed to the existence of an optimal level of acidity, which minimizes the formation of heavy carbonaceous materials that deactivate the catalysts. With the loss of hydrogen transfer activity, bulky carbonaceous materials causing the deactivation of zeolite were formed by poly/oligomerization inside zeolite catalysts. Moreover, we found that the cracking activity plays some role in prohibiting the formation of these heavy materials. The result of this study indicates that both high hydrogen transfer capability and cracking activity are needed to maintain the stability of zeolite catalysts during alkylation. It is also observed that a significant change in selectivity of TMPs can be effected by changing feed conditions.

Isobutane alkylation: Recent developments and future perspectives

In the isobutane alkylation, alkylated gasoline is obtained which is a valuable blending component for the gasoline pool. Thereby the C 3–C 4 cut from the FCC units can be extensively used. Established technologies and recent developments will be reviewed and future perspectives will be given.

Alkylation of Isobutane with 1-Butene on a Solid Acid Catalyst in Supercritical Reaction Media

Alkylation of isobutane with n-butenes to form highly branched isoparaffins is an important process in gasoline manufacture. This reaction is currently catalyzed using either liquid hydrofluoric acid or sulfuric acid. Although these liquid acids rapidly catalyze the alkylation reaction, there are a number of drawbacks associated with their use. Solid acids have been studied extensively as possible catalysts for alkylation. However, with solid acid catalysts, the catalyst deactivates rapidly with time on stream. It is been hypothesized and shown that when the reaction is carried in supercritical media, the solid acid catalyst deactivation slows down considerably; however, high temperatures are needed to bring the reaction mixture to supercritical conditions, which results in the formation of undesired products. In this study, the effects of using different operation conditions for alkylation on a USY zeolite were investigated. It was observed that the deactivation rate was slower in supercritical-phase alkylation. Carbon dioxide was used as a diluent to lower the critical temperature of the reaction mixture in the ratio 27/9/1 for carbon dioxide/isobutene/1-butene. The use of diluent favored both alkylate selectivity and coke precursor removal.

Wells–Dawson heteropolyacid supported on silica: isobutane alkylation with C4 olefins

The alkylation of isobutane with C4 olefins is studied, using heteropolyacids (HPA) with Wells–Dawson (WD) structure supported on silica (WD/SiO 2). The catalytic performance of these catalysts is compared with a lanthanum-exchanged Y-zeolite catalyst. The loading of the HPA on silica was varied between 9 and 28 wt.%. These catalysts have activity for trimethylpentanes (TMP) production. The selectivity towards these products is not as high as in the case of the lanthanum containing Y-zeolite. The acidity of WD/SiO 2 catalysts increases as the loading increases, as seen by 1 H MAS-NMR. Correspondingly, a better TMP production is observed. The increase both in acidity and in the TMP production as a function of the WD content is more noticeable at low loading. The coke formed during the reaction requires high temperatures, 550°C approximately, in order to be fully removed with an oxygen containing carrier gas. The temperature-programmed oxidation (TPO) profile of this coke displays two peaks, the first one between 80 and 300°C associated with hydrocarbons that are released upon heating, and the second between 300 and 550°C, associated with coke that changed its structure during the heating. A regeneration at intermediate temperatures, e.g. 300°C, removes the coke that corresponds to the first peak, but does not restore the initial activity. If the regeneration is carried out at higher temperatures, e.g. 500°C, most of the coke is removed, but this treatment leads to changes in the structure of the HPA, as indicated by FTIR and MAS-NMR analysis. Regeneration with O 3 at low temperature (125°C) is effective both for coke removal and to recover the catalytic activity.

Mesoporous Aluminosilicates with Ordered Hexagonal Structure, Strong Acidity, and Extraordinary Hydrothermal Stability at High Temperatures

Highly ordered hexagonal mesoporous aluminosilicates (MAS-5) with uniform pore sizes have been successfully synthesized from assembly of preformed aluminosilcate precursors with cetyltrimethylammonium bromide (CTAB) surfactant. The aluminosilicate precursors were obtained by heating, at 100--140 degrees C for 2--10 h, aluminasilica gels at the Al(2)O(3)/SiO(2)/TEAOH/H(2)O molar ratios of 1.0/7.0--350/10.0--33.0/500--2000. Mesoporous MAS-5 shows extraordinary stability both in boiling water (over 300 h) and in steam (800 degrees C for 2 h). Temperature-programmed desorption of ammonia shows that the acidic strength of MAS-5 is much higher than that of MCM-41 and is comparable to that of microporous Beta zeolite. In catalytic cracking of 1,3,5-triisopropylbenzene and alkylation of isobutane with butene, MAS-5 exhibits greater catalytic activity and selectivity, as compared with MCM-41 and HZSM-5. The MAS-5 samples were characterized with infrared, UV--Raman, and NMR spectroscopy and numerous other techniques. The results suggest that MAS-5 consists of both mesopores and micropores and that the pore walls of MAS-5 contain primary and secondary structural building units, similar to those of microporous zeolites. Such unique structural features might be responsible for the observed strong acidity and high thermal stability of the mesoporous aluminosilicates with well-ordered hexagonal symmetry.

Studies on supported triflic acid in alkylation

A thermally stable solid acid catalyst has been prepared by treating amorphous silica gel with trifluoromethanesulphonic (triflic) acid (TFA) at 150°C in a closed glass vessel. A solvent such as 1,1,2-trichloro-2,2,1-trifluoro-ethane (Freon 113) can be conveniently used to dissolve TFA. The obtained material is an active catalyst in the alkylation of isobutane with n-butenes to yield high-octane gasoline components. In the reaction it behaves like a truly solid catalyst not leaching TFA in the solution for about 24 h. The catalyst characterisation has been performed by 1H- and 19F-NMR and IR spectroscopy.

Isobutane alkylation with C 4 olefins on a sulfonic solid acid catalyst system based on laminar clays

Natural clays were modified by means of the controlled attack of strong Brönsted acids in aqueous solution. The variations of the surface acidity of the solids, followed by IR of adsorbed pyridine and hammett indicators, indicated a population of Brönsted type sites above 800 μ mol/g, while the surface area increased from 23 up to 105 m 2/g. A series of solids (ATZ-L, ATZ-A and ATZ-G) was activated using trifluoromethanesulfonic acid (CF 3SO 3H), then evaluated at the bench scale in a fixed bed reactor, both in liquid and gas phase, using an iC 4/C 4 = molar ratio, which led to 5.5% trimethylpentanes (TMP) against 35.9% of C 8 = and 53.4% of heavier C 9 + products for the catalysts (ATZ-L) having higher acid strength, i.e. Ho≈−9.3. For the solids having a moderate surface acid strength (Ho≈−4.4) the activation rate after 12 h runs was about 70%, but the solids having a higher acid strength, i.e. Ho=−5.6 and −9.3, showed a deactivation rate almost null after 24 h runs.

Comparison of protonated zeolites with various dimensionalities for the liquid phase alkylation of i-butane with 2-butene

This study focused on the effects of structure on the catalytic performance of various three-, two-, and one-dimensional zeolites. The alkylation of isobutane with 2-butene was carried out under both mild and severe conditions with respect to the feed flow rate. It was observed that zeolite beta outperformed the other zeolites, exhibiting high olefin conversion and alkylate selectivity. For the olefin WHSV used in this work, LTL and USY deactivated rapidly. It is proposed that their fast deactivation was a result of the periodic expansions of their respective pore structures. In contrast to the behavior of other zeolites, the C8 selectivities almost remained high and constant over LTL. The one-dimensional ZSM-12 was found to outperform in terms of activity many of the samples in this study because of its unique pore structure. A study of the carbonaceous material formed inside the zeolites pores discovered that their composition also depended on the zeolite pore structure. The results of this study indicate that the ability of certain zeolites to inhibit the formation of coke precursors because of their particular pore structures is an important parameter.

Performance of potassium 12-tungstophosphoric salts as catalysts for isobutane/butene alkylation in subcritical and supercritical phases

Potassium salts of H 3PW 12O 40 heteropolyacid have been synthesised with different K contents and studied for their acid and catalytic properties in liquid phase isobutane alkylation by but-2-ene or but-1-ene. Subcritical and supercritical conditions have been chosen for comparison in a batch reactor. The influence of the protonic site density has been studied. High acid density was observed to be detrimental for akylation reaction which was explained by increased olefin dimerisation, the main side reaction responsible of catalyst poisoning. A K 2.6H 0.4P sample was tested in a batch reactor in liquid subcritical, near supercritical and in low and high density supercritical isobutane/butene mixture (418 K, P=4–9 MPa ). An increase in the quality of the alkylate with the density of the supercritical phase was observed which almost counterbalanced the negative effect of the high temperature necessary to reach supercritical isobutane conditions. In the presence of porous alkaline salts of 12-tungstophosphoric acid, alkylation to oligomerisation ratio is significantly improved over solids with a low density of strong Brønsted sites and by the use of high density supercritical isobutane conditions, the latter preserving hydride transfer and trimethyl pentane (TMP) formation.

Isobutane/butene alkylation: regeneration of solid acid catalysts

The regeneration of Y-zeolite catalysts used during the isobutane alkylation reaction is studied. Coke is characterized by temperature-programmed techniques and measuring the H/C ratio. The coke deposited under supercritical conditions is very similar (TPO profile and amount) to the coke deposited in liquid phase. The regeneration was carried out in many ways. Air was used for heating with at a low rate, and holding the temperature at low values for long times. Platinum was incorporated in the zeolite to catalyze the coke combustion or to provide an additional route for coke gasification, such as hydrogenation. Ozone was used to remove most of the coke, followed by a second step with H 2 or He. Hydrogen peroxide was studied as an alternative low-temperature oxidation compound. It was found that the ozone treatment, followed by a hydrogen treatment, is an effective way to regenerate this microporous catalyst. The ozone not only eliminates a large fraction of the coke but also changes the characteristics of the small amount of coke left on the catalyst, making it easier to be burnt. The treatment with hydrogen peroxide at 90°C also removes a large fraction of coke, but without changing significantly the characteristics of the coke left after the treatment.