High P-xylene Selectivity Research Articles

High p-xylene selectivity in aluminum-based metal–organic framework with 1-D channels

The separation of highly pure p-xylene (pX) from xylene isomers is an industrially important and challenging issue. Although simulated moving bed (SMB) processes using faujasite zeolites are currently used for pX separation, developing novel adsorbents with improved pX separation performances is strongly needed. In this study, an aluminum-based metal–organic framework (MOF), MIL-120(Al) with 1-D channels of approximately 7 Å, exhibited considerably high pX selectivities compared to xylene isomers (αpX/oX: ∼31; αpX/mX: ∼17; αpX/EB: ∼7.5; αpX/OME: ∼11), which are superior to reported values for other MOFs and zeolites under similar conditions. Such high selectivities may originate from the proper pore shape and size of MIL-120(Al). MIL-120(Al) also showed good cyclic adsorption properties as well as superior hydrothermal and chemical stabilities. Finally, a dynamic simulation showed that MIL-120(Al) could achieve approximately complete separation of pX from a xylene isomer mixture using an SMB process.

Selective toluene methylation to p-xylene: current status & future perspective

ABSTRACT Industrially, petroleum naphtha reforming and steam cracking are two major multistep and energy-intensive sources for p-xylene production. Additionally, p-xylene is also produced by selective toluene disproportionation, heavy (C9) aromatics trans-alkylation and isomerization of xylenes. Rapidly changing market dynamics from fuels to petrochemicals coupled with growing petrochemicals demand has led to the further exploration of economical and highly p-xylene selective processes by using surplus low-value aromatics. Methylation of toluene with methanol has been envisaged as one such potential opportunity to produce p-xylene. For the purpose, significant efforts have been made in the past forty years to develop suitable shape selective zeolite catalysts and technologies to make toluene methylation a highly p-xylene selective process. Current paper briefly highlights various advancements in the toluene methylation process development and aims to provide an extensive review of several works put up in the past to design suitable catalysts, catalyst’s limitations and developmental challenges for toluene methylation. Further, this paper provides an useful insight for two competing chemistries of toluene alkylation and side-by-side methanol reactions. Finally, motivated by competitive chemistry of methanol conversion reactions, existing catalytic limitations and novel synthesis opportunities in engineering shape selective zeolite catalysis, a way forward is proposed to arrive at a logical and futuristic catalyst design strategy for p-xylene production via toluene methylation. The proposed strategy is envisaged to offer low-to-moderate catalyst acidity, improved toluene methylation activity, high p-xylene selectivity and improved catalyst life which, otherwise, up till now have been the bottlenecks to develop distinctly economical toluene alkylation to p-xylene process.

Modification Strategy of ZSM-5 Zeolite for Enhancing p-Xylene Selectivity in Toluene Disproportionation

In this study, modified zeolite was used to improve p-xylene selectivity in toluene dis-proportionation process. ZSM-5 zeolite was modified by introducing the phosphorus/Germanium loading and vacuum UV treatment from He+O2 microwave plasma treatment. Results showed that the ZSM-5 zeolite impregnated with Germanium and phosphorus have high p-xylene selectivity (up to 33 %). A sharp decrease in the acidity properties was achieved after phosphorus loading. By Germanium loading, selectivity increased but conversion did not change much. Also, the highest p-xylene selectivity (47 %) was obtained by P/Ge ZSM-5 with VUV treatment at the highest WHSV of 85 h-1.

Selective Conversion of CO2 into para-Xylene over a ZnCr2 O4 -ZSM-5 Catalyst.

An oxide-zeolite (ZnCr2 O4 -ZSM-5) catalyst for directly converting CO2 to aromatics was designed and developed. It showed high PX/X (the C-mol ratio of p-xylene to all xylene) and PX/aromatics (the C-mol ratio of p-xylene to aromatics) ratios, which reached 97.3 and 63.9 %, respectively.

Acidity and catalyst performance of two shape-selective HZSM-5 catalysts for alkylation of toluene with methanol

Two shape-selective HZSM-5 catalysts with similar pore size were prepared by silicalite-1 coating or boron modification, respectively. These catalysts samples were characterized by XRD, SEM, NH3-TPD, Py-IR and N2 adsorption–desorption isotherms and tested in the alkylation reaction of toluene with methanol. The results indicated that the boron modification weakened the acidity but increased the total amount of acidity sites, while the silicalite-1 coating led to the decrease of the strong acidity and total acidity. Due to the specific shape-selectivity that the micropore architecture afforded and the increment of the weak acid percentage, both catalysts exhibited a high p-xylene selectivity over 98%. Moreover, the higher toluene conversion would be retained once the strong acid sites in the micropores were remained as many as possible under the premise of the shape-selectivity of pore size.

Enhancement of Bio-based para-Xylene Selectivity in Catalytic Fast Pyrolysis of Cellulose Using a Surface-modified Mg/P/HZSM-5 Catalyst

With the growing consumption of oil, the production of p-xylene(PX) from renewable biomass has gained significant attention recently. This work demonstrated that cellulose, a main component in lignocellulosic biomass, was directly converted into PX over the Mg/P surface-modified zeolites. The catalysts modified by the incorporation of P2O5 and MgO into HZSM-5(HZ) promoted the isomerization of m-/o-xylenes to p-xylene. The PX selectivity was greatly enhanced using the modified zeolites due to the deactivation of external surface and the adjustment of pore entrance. In addition, the addition of methanol to cellulose was beneficial to increase the selectivity of xylenes due to the alkylation reactions and the Diels-Alder reactions between cellulose-derived furans and methanol-derived olefins. The highest PX yield of 10.7%(molar fraction) with a high PX selectivity in xylenes(97.1%) was obtained over the 10%Mg/5%P/HZ catalyst. The reaction pathway for the formation of p-xylene was addressed according to the study of the key reactions and the characterization of catalysts.

Selective Toluene Disproportionation to Produce Para-Xylene over Modified ZSM-5

Industrial demand for p-xylene has been increasing in recent years and the major production is by toluene disproportionation. This research studied the effect of crystal size and the modification by silica deposition on the surface of the zeolite. Shape selective catalysts have been prepared by modifying two different ZSM-5 zeolites with different crystal sizes (0.5 µm, 5 µm) by chemical liquid deposition (CLD) using TEOS (tetraethyl orthosilicate) to deposit inert silica layers. The large ZSM-5 crystals modified with two cycles of silica showed high p-xylene selectivity at typically 84 % with very low conversion. Pressure effect on the reaction was investigated. Selectivity achieved was 40 – 66 % at conversions typically 4-20 % at 10 bar.

P-Xylene selectivity enhancement in methanol toluene alkylation by separation of catalysis function and shape-selective function

Catalyst reactivity and selectivity always compromise with each other in most reactions. HZSM-5 catalyst for the methanol toluene alkylation reaction suffers from active site loss when modified by elements such as Si, P, Mg and B to attain high p-xylene selectivity, incurring problems of low toluene conversion and rapid catalyst deactivation. In the present study, the separation of catalysis function and shape-selective function was achieved by epitaxial growing a Silicate-1 layer over HZSM-5 crystal to create an HZSM-5@S-1 catalyst. HZSM-5@S-1 was shown to possess less weak acid sites but more strong acid sites than the parent HZSM-5. With reduced contact time, the catalyst showed increased p-xylene selectivity up to 88% but no obvious degradation in toluene conversion. The enhancement of Silicate-1 shell to PX selectivity was less effective when applied to low Si/Al ratio ZSM-5 samples, due to the re-incorporation of aluminium leached from low Si/Al ratio parental ZSM-5 samples into the newly formed Silicate-1 framework. Benefiting from retaining most of the strong acid sites of HZSM-5, the catalyst showed high toluene conversion with increasing WHSV without apparent deactivation over a 130hours service life test.

Tailoring p-xylene selectivity in toluene methylation on medium pore-size zeolites

The p-xylene selectivity in toluene methylation was investigated over medium pore-size zeolites (H-ZSM5, H-ZSM11 and H-NU10) as well as after deposition of tetraethyl orthosilicate on the surface, isomorphous substitution of aluminum by iron and variation the crystal size. Higher reaction temperatures consistently resulted in higher p-xylene selectivity in the three reaction pathways for the formation of p-xylene during toluene methylation, i.e., methylation of toluene, isomerization of xylenes, further methylation and subsequent formation of xylenes after dealkylation of light hydrocarbons. The increased selectivity is attributed to the accumulation of slowly diffusing reaction products in the zeolite pores. Thus, the isomerization of m- and o-xylenes, as well as dealkylation of higher alkylated products influenced the xylene selectivity most strongly at high temperatures, while the intrinsic selectivity of the toluene alkylation dominates at lower reaction temperatures due to the absence of transport restrictions.

Mechanistic Insights into the Zeolite-Catalyzed Isomerization and Disproportionation of m-Xylene

The mechanisms of m-xylene isomerization and disproportionation over 13 medium-pore zeolites and three large-pore ones are investigated. While H-TNU-10 and H-ZSM-57 with intersecting 10- and 8-ring channels were found to show considerably higher p/o ratios than H-ZSM-5, a commercial m-xylene isomerization catalyst, the GC-MS results from used zeolite catalysts demonstrate the intrazeolitic build-up of tri- and tetramethylated diphenylmethane species, whose existence during the m-xylene transformation over any acidic catalyst has not been experimentally verified until now. These dicyclic aromatic compounds were ascertained to serve as reaction intermediates of bimolecular m-xylene isomerization within the micropores not only of large-pore zeolites but also of medium-pore materials at temperatures lower than 523 K or so, once there are internal void spaces larger than 10-rings. Flushing experiments with used zeolites followed by GC-MS analyses strongly suggest that the high p-xylene selectivity found in som...

Unique catalytic performance of mesoporous molecular sieves containing zeolite units in transformation of m-xylene

Integrating ZSM-5 zeolite units into two different mesostructured materials, led to high p-xylene selectivity in the transformation of m-xylene. The activity and selectivity of both composite materials were compared with those of Al-MCM-41, Al-MCM-48 and the conventional microporous ZSM-5. A bimolecular pathway involving disproportionation and transalkylation, together with the presence of ZSM-5 zeolite units in the composite materials, was responsible for this high p-xylene selectivity. The catalysts were characterized by powder XRD, TGA, SEM, nitrogen sorption and FT-IR of pyridine adsorption. The selectivity towards p-xylene over the different catalysts under study follows the order: conventional ZSM-5 < ZSM-5/MCM-48 < ZSM-5/MCM-41. The highest p-xylene selectivity noticed over ZSM-5/MCM-41 composite material indicates more transalkylation of trimethylbenzene molecules with m-xylene. The apparent activation energies for the transformation of m-xylene over both composite materials were found to decrease as follows: E Z/48M-4 , E Z/41M-4 (transalkylation) > E Z/48M-3 , E Z/41M-3 (disproportionation) .

Shape selectivity of MWW-type aluminosilicate zeolites in the alkylation of toluene with methanol

We investigated the alkylation of toluene with methanol at 250 °C over MCM-22 and ITQ-2 zeolites possessing an MWW structure. Whereas ITQ-2, produced by the delamination of MCM-22 precursor, showed a poor p-xylene selectivity of 23% at 1.7% of the level of toluene conversion, a high p-xylene selectivity of 80% at 0.6% of the level of toluene conversion was obtained by poisoning the external surface of ITQ-2 with collidine. Similarly, MCM-22 treated with collidine showed a high p-xylene selectivity of 74% at 3.8% of the level of toluene conversion. We concluded that both interlayer and intralayer 10-membered ring (10MR) micropores in the MWW structure caused the shape-selective formation of p-xylene in the alkylation of toluene with methanol.

On the use of a catalytic H-ZSM-5 membrane for xylene isomerization

An H-ZSM-5 membrane was used in this work for xylene isomerization and for separation of p-xylene from the other isomers. MFI membranes (silicalite-1 and H-ZSM-5) were prepared by seeded growth on porous stainless steel supports. The membranes were tested for separation of the xylene isomers in the temperature range 100–400 °C. The H-ZSM-5 membrane showed higher p-xylene permselectivity ( β p/ o = 4.3 at 300 °C) and lower permeance to p-xylene (3.55 × 10 −8 mol m −2 s −1 Pa −1 at 300 °C) compared to the silicalite-1 membrane ( β p/ o = 2.1, permeance to p-xylene 7.4 × 10 −7 mol m −2 s −1 Pa −1 at 300 °C). However, both were not completely defect-free. The kinetics of xylene isomerization on the catalytically active H-ZSM-5 membrane and, for comparison, on a conventional H-ZSM-5 powder catalyst was studied in a gradient-free recycle reactor (Berty-type). Thereby the membrane showed higher activity than the conventional catalyst. The reaction kinetics was best described by a general triangular scheme. The H-ZSM-5 membrane was then used in a membrane reactor for xylene isomerization and the results were compared to those obtained in a packed-bed reactor using the conventional H-ZSM-5 catalyst. At 400 °C, a relative increase of the conversion of m-xylene by 15% was observed in the membrane reactor combined with higher p-xylene selectivity.

Influence of Fe or Zn loading method on toluene methylation over MFI-type zeolite catalysts

Toluene methylation with methanol was investigated on MFI-type zeolite catalysts containing Fe or Zn within the range of 0-2% by weight as an active component. The catalytic performances were compared on catalysts to which Fe or Zn was introduced by different methods, i.e., ion-exchanged and incorporation methods. The prepared catalysts were characterized by XRD, XRF, BET, FTIR and pyridine adsorption technique on in-situ FTIR. The results showed that the incorporated samples, H-Fe, Al-silicate (Si/Fe=150) and H-Zn, Al-silicate (Si/Zn=150), exhibited catalytic activity and xylene selectivities approximately equivalent to those from the ion-exchanged samples, Fe(0.8)/H-MFI and Zn(1.0)/H-MFI, containing nearly the same amount of Fe or Zn. The higherp-xylene selectivity was achieved with H-Fe, Al-silicate (Si/Fe=150) and H-Zn, Al-silicate (Si/Zn=150) because of Bronsted acid strengths weaker than Fe(0.8)/H-MFI and Zn(1.0)/H-MFI. Therefore, the isomerization ofp-isomer produced primarily was suppressed on the incorporated catalysts better than the ion-exchanged ones.

Catalytic activity of a zeolite disc synthesized through solid-state reactions

ZSM-5 zeolite in a disc-shaped form was synthesized by a new method in which reactions occurred in the solid state. The proton form of the zeolite was prepared by ion exchange of the sodium ions of the disc-shaped pellet in hydrochloric acid solution. The catalyst prepared was characterized by SEM, thermogravimetry, nitrogen adsorption and ammonia TPD. High p-xylene selectivity was observed for the new disc-shaped ZSM-5 zeolite catalyst in the alkylation of toluene with methanol. This catalytic feature was attributed to the inherent character of the zeolite, since its structure consisted of inter-grown individual HZSM-5 crystals forming secondary mesopores which promote the high gas permeability of the zeolite disc.

ChemInform Abstract: Separation of p‐Xylene and Ethylbenzene from C8 Aromatics Using Medium‐Pore Zeolites.

The author discusses medium-pore ZSM-5 zeolites found to be excellent for the separation of p-xylene and ethylbenzene from C/sub 8/ aromatics. Their adsorption capacity and selectivity for p-xylene were high. The selectivity for p-xylene over ethylbenzene was 5.5 at the optimum SiO/sub 2//Al/sub 2/O/sub 3/ of 700. The competitive adsorption capacity for p-xylene from a simulated reformer product was 120 mg/g. The p-xylene selectivity increased sharply and then leveled off when p-xylene loadings reached 50 and 90 mg/g, respectively. The high p-xylene selectivity and its dependence on p-xylene loading are believed to be related to the unique packing of p-xylene in the crystalline cavities.

Combined catalytic and infrared study of the modification of H-ZSM-5 with selected poisons to give high p-xylene selectivity

The methylation of toluene with methanol has been compared over unmodified Zeolite HZSM-5 and over HZSM-5 samples that were partially or fully poisoned with preadsorbed quinoline or trimethylphosphite (TMP). The hydroxyl stretching region of the HZSM-5 zeolites was examined using infrared spectroscopy. Application of this technique to unmodified HZSM-5 showed that exposure to D 2O vapor caused deuteration of both the internal hydroxyls (3605 cm −1) and the external hydroxyls (3740 cm −1). Quinoline was shown to react with only the external hydroxyls whereas TMP reacted with and poisoned both the internal and external hydroxyls when adsorbed at room temperature. Desorption of quinoline was accompanied by dehydroxylation. Studies with a continuous-flow microcatalytic reactor demonstrated that HZSM-5 samples modified by preadsorption with quinoline or TMP prior to measurement of the activity profiles for alkylation exhibited diminished catalytic activity relative to unmodified HZSM-5 samples. However, they showed increased selectivity toward xylenes and toward p-xylene in particular. High para selectivity is attributed to the selective poisoning of the surface hydroxyls. TMP was found to be the more efficient poisoning agent and yielded the higher para selectivity, which is explicable in terms of more effective reaction with the surface hydroxyls.