Catalytic Cracking
Catalytic cracking is the most widely deployed catalytic petroleum refining process. Nearly 35 wt.% of all gasoline is produced by cracking of gas oils and atmospheric resid over large pore ultrastable Y (USY) zeolite catalysts. The products include both fuel and petrochemical feedstocks. For many years, researchers have looked for larger pore alternatives to USY or to large pore materials to supplement the effectiveness of USY in fluidized catalytic cracking (FCC) particularly for processing heavy hydrocarbons. Aufdembrink et al.[26] examined MCM-41 alone and in combination with USY for catalytic cracking vacuum gas oils and atmospheric resids. The catalysts were mildly steamed to simulate equilibrated FCC regeneration conditions. Their performance comparisons showed that equilibrated MCM-41 was superior to amorphous silica-alumina both in its cracking activity and in its propensity for producing larger amounts of gasoline at equivalent coke yield. Comparisons at equivalent conversions to gasoline, distillate, and light gases, showed that MCM-41 was more selective for heavy oil conversion, again producing more gasoline than the amorphous silica-alumina. This is shown in Table 1.
Table 1 Comparison of MCM-41 and silica-alumina catalysts in catalytic cracking of Joliet sour heavy gas oil (516°C, fixed-fluidized bed reactor; 1 min on stream, catalyst/oil = 2 to 6)
|
|
Yields, wt.% |
|
|
|
Silica-aluminaa |
MCM-41a |
Difference |
|
|
Comparison at equivalent coke yields (=4 wt.%) |
|||
|
Coke, wt.% |
4.0 |
4.0 |
- |
|
Conversion, wt.% |
48.5 |
56.8 |
8.3 |
|
C4′s, vol.% |
9.7 |
13.3 |
3.6 |
|
C5′s, vol.% |
3.7 |
4.7 |
1.0 |
|
C5+ gasoline, wt.% |
32.6 |
37.2 |
4.6 |
|
Gasoline, RON |
92 |
92 |
- |
|
Light fuel oil, wt.% |
36.7 |
32.2 |
(4.5) |
|
Heavy fuel oil, wt.% |
14.7 |
11.0 |
(3.7) |
|
Comparison at equivalent conversion (=55 wt.%) |
|||
|
Conversion, wt.% |
55.0 |
55.0 |
- |
|
Coke, wt.% |
4.7 |
3.3 |
(1.4) |
|
C5′s, vol.% |
3.8 |
4.6 |
0.8 |
|
C5+ gasoline, wt.% |
34.9 |
36.0 |
1.1 |
|
Gasoline, RON |
92 |
92 |
- |
|
Light fuel oil, wt.% |
35.0 |
33.6 |
(1.4) |
|
Heavy fuel oil, wt.% |
13.1 |
11.3 |
(1.8) |
However, in a similar gas oil cracking comparison, MCM-41 was not nearly as active or as gasoline selective as USY, although it was more selective in converting the heavier fractions in fully formulated catalysts. This comparison is shown in Table 2.
Similar investigations by Corma et al.[27] suggested that fresh MCM-41 had unique cracking selectivities, producing significantly higher amounts of gasoline and less coke than USY, but that the selectivity disappeared once the material was steamed under simulated FCC regeneration conditions. Corma et al.[28] concluded that MCM-41 partially collapsed when steamed to produce a material resembling silica-alumina. The corollary was that MCM-41 lacked the hydrothermal stability needed for it to be useful as an FCC catalyst component.
Nickel and vanadium are present in small concentrations especially in heavier hydrocarbon feedstocks where they tend to degrade typical FCC catalyst performance as they accumulate on the catalyst. In a recent study, Balko et al.[29] found that MCM-41 could be used as an FCC catalyst component to very effectively trap and concentrate the metals so that they are much less deleterious to FCC catalyst performance. MCM-41, when used at low levels (5 to 30 wt.%) in conjunction with the USY zeolite, acted as a metals ”getter” and protected the cracking function by effectively passivating the metals. MCM-41 could be added as a component in the cracking catalyst particle or could be added as a separate particle.
Oligomerization Catalysts
Pelrine et al.[30,31] evaluated a chromium-impregnated MCM-41 as an oligomerization catalyst for the production of high viscosity synthetic lubricants. Evaluations were carried out in a fixed bed reactor using 1-decene and reaction temperatures ranging from 120°C to 182°C at LHSV= 1.9 to 2.0 hr~1 The analysis showed that MCM-41 could produce a significantly higher viscosity product than, for example, a commercial Cr/SiO2 polymerization catalyst under the same reaction conditions. The same reaction catalyzed by chromium acetate-impregnated and calcined MCM-41 was used to demonstrate the concept and utility of functionalized MCM-41.[32] Bhore et al.[33] extended the oligomerization concept to Ni/MCM-41 catalysts for dimerization of lower molecular weight olefins. The principal application was for C3 to C10 olefin dimerization primarily to produce gasoline. In their study the performance of Ni/MCM-41 catalysts compared favorably to Ni-based Dimersol® catalysts.
Le et al.[34] used MCM-41 to selectively react the C3-C5 olefins in a mixed stream of lower molecular weight (MW) olefins and paraffins with the intent of producing a heavier oligomer stream that could be readily separated from the lower MW paraffin-enriched stream. Reaction conditions used in the study were 120°C, 10.3 MPa, and LHSV=1.8 hr~1 The resulting oligomers were highly branched and could be converted to tertiary olefins suitable for a number of applications via subsequent selective disproportionation or cracking over, for example, a zeo-litic catalyst. Specific applications are for the production of tertiary C4 and C5 olefins for subsequent paraffin-ole-fin alkylation or oxygenate production.[35]
Bhore et al.[36] investigated metals-free MCM-41 for the oligomerization of olefins for the production of higher molecular weight products as, for example, fuels, lubricants, fuel additives, and detergents. For acid-catalyzed propylene oligomerization, they found MCM-41 particularly selective for trimer and tetramer synthesis, materials that are well suited for clean gasoline. Reaction temperatures ranged from 40°C to 250°C and pressures ranged from 0.1 to 13 MPa.
Table 2 Comparison of 35% MCM-41 and 35% USY catalystsa in catalytic cracking of Joliet sour heavy gas oil (516°C, fixed-fluidized bed reactor; 1 min on stream, catalyst/oil = 2 to 6)
|
\ |
|
Yields, wt.% |
|
|
USYb |
MCM-41c |
Difference |
|
|
Equivalent conversion |
|||
|
Conversion, vol.% |
65.0 |
65.0 |
- |
|
Coke, wt.% |
2.4 |
6.0 |
3.6 |
|
C4‘s, vol.% |
14.2 |
16.1 |
1.9 |
|
C5‘s, vol.% |
10.2 |
9.7 |
(0.5) |
|
C5+ gasoline, wt.% |
43.2 |
37.4 |
(5.8) |
|
Gasoline, RON |
92.1 |
91.6 |
(0.5) |
|
Light fuel oil, wt.% |
28.1 |
28.9 |
0.8 |
|
Heavy fuel oil, wt.% |
9.8 |
8.3 |
(1.5) |
Hydrocracking
MCM-41 alone, or in combination with zeolites such as USY, has been examined as the active component in vacuum gas oil and lube hydrocracking catalysts. Degnan et al.[37-39] examined the performance of NiW-impreg-nated USY/MCM-41/Al2O3 catalysts and found them to be superior in activity and comparable in selectivity to several commercial distillate selective hydrocracking catalysts. The work was further extended to hydrocracking heavier feedstocks to produce lubricants.[40-42] Fig. 5 compares the hydrocracking activity of NiW/MCM-41 and a conventional NiW/fluorided alumina lube hydro-cracking catalyst for conversion of heavy slack wax at 13.7 MPa, 1 LHSV. The comparison, at equivalent conditions of 13.8 MPa, LHSV=1 hr~ \ shows that MCM-41 is more active for conversion of this heavy hydrocarbon feed.
Work by Apelian et al.[40] and Marler and Maz-zone[41-42] also showed that MCM-41 could be combined with a strong hydrogenation function to subsequently hydroisomerize the heavy hydrocrackate to produce high-quality, low pour point lubricants. In a similar study, Corma et al.[43] compared the mild hydro-cracking performance of NiMo (12 wt.% MoO3, 3 wt.% NiO) supported on MCM-41 with that of amorphous silica-alumina and USY zeolite catalysts with the same Ni and Mo loadings. The feedstock was vacuum gas oil. The MCM-41 catalyst was superior to the other catalysts in hydrodesulfurization, hydrodenitrogenation, and hy-drocracking activities in single-stage hydrocracking. When a hydrotreating stage was used in front of the hydrocracking stage, the USY catalyst became more active than the MCM-41 catalyst, but MCM-41 was still significantly more active than the amorphous silica-alumina catalyst. Most importantly, the MCM-41 catalyst distillate selectivity was better than that of USY and was very similar to the silica-alumina catalyst. A number of other studies have also pointed to MCM-41 as a superior distillate selective hydrocracking catalyst.[44-45]
Hydrode metallat ion
Vanadium, nickel, iron, and other trace metals pose problems in processing heavy oils because they foul catalysts and cause undesirable side reactions. Kresge et al.[46] showed that MCM-41 (d-spacing >1.8 nm) was very active for removing trace metals from petroleum residua and shale oils. Nickel- or molybdenum-impregnated MCM-41 extrudates (MCM-41/Al2O3) were found to be particularly selective for the removal of iron, vanadium, nickel, and even arsenic. Shih also found that staging MCM-41 materials of different pore sizes, with the largest pore size material positioned first to see the oil, was a particularly effective strategy for hydrodemetalla-tion.[47] Fig. 6, taken from Shih’s patent, shows the effect of MCM-41 pore diameter on metals uptake effectiveness using hydrodesulfurization (HDS) activity as a basis for comparison. As metals accumulate on the catalyst they tend to poison HDS activity. The comparison shows that MCM-41 with 8.0-nm pores has a greater metals capacity than smaller pore MCM-41.
Fig. 5 Comparison of the hydrocracking activities of NiW/MCM-41 and NiW/fluorided alumina demonstrating the superior cracking activity of the MCM-41 catalyst.
Fig. 6 Hydrodemetallation activity comparison—effect of MCM-41 pore diameter.
Hydrogenation
Given its large surface area (>600 m2/g) and its large concentration of silanol groups that are easily function-alized or ion exchanged, MCM-41 is an obvious choice as a support material for metals in both precious and base metal hydrogenation catalysts. Evaluations by Baker et al.[48] and Degnan et al.[49] showed that MCM-41 was more active than other conventional supports for long-chain olefin and heavy aromatic hydrogenation. The specific applications cited are for polyalphaolefin (PAO) saturation for synthetic lube hydrofinishing and for alkylaromatics hydrogenation for color and viscosity index (VI) improvement. Similarly, Borghard et al.[50] demonstrated that metal impregnated MCM-41 is a very active catalyst for the saturation of highly aromatic feedstocks under relatively mild hydrogenation conditions. Hydro-treating highly aromatic cracked distillate stocks with NiMo- or CoMo/MCM-41 or other strong hydrogenation metals supported on MCM-41 to produce low aromatic distillates, e.g., for high-quality diesel fuels, is described by Apelian et al.[51]
Similarly, Corma et al.[52] found that MCM-41 provided an excellent medium for dispersion of Pt particles, and that the Pt/MCM-41 catalysts were superior in overall hydrogenation activity for naphthalene saturation when compared to Pt supported on amorphous silica-alumina, zeolite USY, g-alumina, and silica. These investigators demonstrated that sulfur tolerance was a strong function of molecular sieve aluminum content. Pt supported on USY and Al-rich MCM-41 was superior in sulfur tolerance to Pt located on the other supports. They were able to confirm the sulfur tolerance in the hydrogenation of a mildly hydrotreated light cycle oil (LCO) containing approximately 70 wt.% aromatics and 400 ppm sulfur.
Hydroisomerization
When combined with a strong hydrogenation function (e.g., Pt or Pd) MCM-41 is an effective long-chain paraffin isomerization catalyst once trace nitrogen and sulfur compounds are removed. This is shown in the aforementioned hydroprocessing patents[42,48] where Marler and Mazone demonstrated that MCM-41 could be used to improve the viscometric properties of hydro-processed or synthetic lubricating oils. DelRossi et al.[53] extended the hydroisomerization studies to lower molecular weight hydrocarbon feeds and found that noble metal MCM-41 catalysts are both active and selective for isomerization of C4 to C8 paraffins. Similar results were obtained by Chaudhari et al.[54] in their analysis of noble-metal-impregnated MCM-41 for n-hexane isomerization.
Olefin Disproportionation
Higher molecular weight olefins can be converted to lower, more highly branched and often more valuable lower molecular weight olefins through disproportion-ation. The process is not used widely but has the potential for providing incremental lower molecular olefins as a feedstock for paraffin-olefin alkylation for fuels or for petrochemical applications. Le and Thompson[55,56] determined that MCM-41 was an attractive cracking catalyst for the conversion of light olefinic gasoline to propylene and isobutylene.
Light Olefin-Paraffin Alkylation
MCM-41 is an attractive support for either Lewis or Bronsted acids used in the alkylation of C4 to C12 isoparaffins with <C10 olefins.[57,58] The principal application of low molecular weight isoparaffin-olefin alkyl-ation is for the production of high-octane C5 to C12 isoparaffins for gasoline. These studies highlighted the ability of MCM-41 to concentrate the acid and increase its effectiveness by nearly an order of magnitude over the free acid as measured by the amount of alkylate produced at the same volumetric acid to oil ratio. Typical reaction conditions for both Bronsted (H2SO4) and Lewis (BF3) acid/MCM-41 catalyzed paraffin-olefin alkylation were – 20°C to 200°C, 0.7 to 1.4 MPa, and an isobutane/2-butene molar ratio of 10:1. The reactions were carried out in batch reactors for a preset duration.
Kresge et al.[59] and DelRossi et al.[60] found that the performance of MCM-41 for this application could be improved if the activity of MCM-41 could be increased through the addition of heteropoly acids via, for example, impregnation with phosphotungstic acid, H3PW12O40. In batch experiments with 50:1 isobutane/2-butene molar ratio, 121°C, 3.4 MPa, incorporation of 50 to 75 wt.% of the heteropoly anion, via impregnation, significantly improved both the catalyst activity and the yield of desirable trimethylpentane (TMP) and dimethylhexane (DMH) isomers. Comparison catalysts prepared by impregnation of equivalent loadings of H3PW12O40 onto high surface silica and high surface alumina did not produce nearly equivalent activities or TMP and DMH yields. This comparison is shown in Table 3.
Table 3 Comparison of isoparaffin-olefin alkylation selectivities and activities of high surface area heteropoly acid catalysts effect of supports
|
Conditions: Isobutane/2-butene ratio=50:1; |
3.4 MPa; 121°C; |
; batch autoclave |
|
|
|
Support |
MCM-41 |
SiO2 |
(Cab-O-Sil) |
Alumina |
|
Support surface area, m2/g |
738 |
159 |
207 |
|
|
H3PW12O40, wt.% |
75 |
75 |
75 |
|
|
Olefin conversion, wt.% |
87 |
60 |
66 |
|
|
Yield, gram C5+/gram 2-C4 = |
1.1 |
1.0 |
1.2 |
|
|
Total product distribution, wt.% |
||||
|
C5-C7 |
9.1 |
2.8 |
5.5 |
|
|
C8 |
57.2 |
69.7 |
58.8 |
|
|
C9 |
34.0 |
27.5 |
35.7 |
|
|
C8 product distribution, wt.% |
||||
|
TMP (trimethylpentane) |
23.9 |
13.8 |
13.2 |
|
|
DMH (dimethylhexane) |
35.8 |
25.3 |
28.0 |
|
|
Unknown |
40.3 |
60.9 |
58.9 |
|
|
TMP/DMH |
0.7 |
0.5 |
0.5 |
|
|
TMP/ (C8-TMP) |
0.3 |
0.2 |
0.2 |
|
PETROCHEMICAL CATALYSIS
Aromatics Alkylation
MCM-41 is an effective catalyst for olefin alkylation of single-ring aromatics under milder conditions than are used in, for example, ZSM-5. Le[61] evaluated MCM-41 combined with an alumina binder for the production of ethylbenzene via benzene alkylation and found that the same ethylene conversion could be achieved at a reaction temperature that was 40°C lower than with ZSM-5. Polyalkylated benzene yields were higher, but MCM-41 produced no undesirable xylenes. Conditions were 2.1-3.4 MPa and a benzene/ethylene (molar) ratio=10:1.
Le[62] also identified MCM-41 as an attractive catalyst for single- and multiring aromatic alkylation with higher molecular weight olefins. Higher molecular weight olefins are used to alkylate aromatics to produce linear alkyl benzenes (LABs). Linear alkyl benzenes are the basis for linear alkyl benzene sulfonate (LAS). Linear alkyl benzene sulfonate is used widely as a large component in liquid detergents. Le’s work is focused on the alkylation of mono- and polycyclic aromatics for lube additive and detergent applications.
PHASE TRANSFER CATALYSTS
Hellring and Beck[63] found that MCM-41 containing a stabilized onium ion, such as cetyltrimethylammonium (CTMA) cation, could effectively catalyze dual-phase reactions within the MCM-41 pores. To demonstrate the concept, Hellring and Beck reacted water insoluble bromopentane with potassium iodide in an agitated aqueous medium to produce iodopentane. Addition of CTMA-MCM-41 to the stirred dual-phase mixture greatly increased the reaction rate. The results implied that CTMA, which is amphiphillic, is able to concentrate bro-mopentane and increase its interaction with iodide, which is closely associated with the CTMA cation.
CATALYTIC DECOMPOSITION OF NITROGEN OXIDES
Vanadium- and titanium-impregnated high surface area alumina or silica is a commercially attractive selective catalytic reduction (SCR) catalyst for NOx reduction. Beck et al.[64] compared several TiV/MCM-41 samples with conventional metal-oxide-based SCR catalysts and found the MCM-41 materials were comparable in both NOx removal selectivity and activity. The ability of the high surface area of MCM-41 to support large amounts of highly dispersed titanium and vanadium was believed to account for the high activity of the MCM-41 catalyst.
APPLICATION OF MCM-41 TO SEPARATIONS
MCM-41 can be used to separate at least one component from a mixture of gaseous or liquid components as described in the patent of Beck et al.[65] The examples provided in this reference relate mainly to separation of higher and lower viscosity components of a mixture. The concept of using MCM-41 alone or as a composite in a membrane or as an active component in chromatographic separation media is described by Herbst et al.[66] Among the contemplated uses are size exclusion applications such as the bioseparation of endotoxins or pyrogens. In another study, Kuehl[67] examined MCM-41 as a selective sorbate for the removal of large molecules such as polynuclear aromatics (PNAs) from fluids and gases. The applications range from wastewater and catalyst regeneration gas cleanup to the reduction of PNAs in hydrocarbon fuels.
OTHER APPLICATIONS
A wide array of other MCM-41 applications have been investigated, each based on the unique composition, uniform pore size, and extraordinarily high surface area of the M41S family of materials. While the reviews by On et al.[21] and Zhao et al.[22] have described many of these, we want to highlight three applications to illustrate the breadth of potential applications outside the realm of catalysts and adsorbents. The first relates to the use of MCM-41 and M41S materials as key components in microoptical and microelectronic applications. Beck et al.[68] determined that M41S materials, when processed to include quantum clusters of semiconducting inorganic or organic compounds, have unique nonlinear optical properties useful as frequency mixers, frequency doublers, and parametric amplifiers. Extended uses are for beam steering, optical switching, and image processing.
In the second application, MCM-41 is used as a central component in a sensor device.[69] Electrical, optical, or gravimetric systems designed around MCM-41 detect, for example, the presence of specific gases, changes in pH, or the presence of metal ions. The examples describe a range of biological, chemical, and physical sensor applications including the selective detection of benzene, ammonia, nickel, formaldehyde, and carbon monoxide.
Finally, Ozin et al.[70] describe the use of M41S (nick-el/platinum)-yittria-zirconia M41S materials as thermally stable electrode materials in solid oxide fuel cells. The mesoporous materials, which are synthesized in aqueous media using glycometallates and metal complexes, have the highest known surface area of any form of (metal)-yittria-stabilized zirconia and thus improve the efficiency of solid oxide fuel cells.
CONCLUSION
For many new materials, traversing the path from discovery to commercial application can take as long as a decade. This was certainly true for MCM-41, the most widely studied representative of a new class of materials known as the M41S family. Commercialization of MCM-41 required approximately 10 years as issues surrounding scale-up of the synthesis, raw materials selection, and post-processing had to be addressed. However, the major challenge was associated with identification of an application where the performance advantages justified the risks and costs associated with the targeted commercialization. Initial commercializations of truly new materials are, by their very nature, expensive and associated with a large degree of uncertainty. The initial application bears all of the developmental costs which normally represents a significant hurdle in the development of any truly ”step out” material.
Now that MCM-41 is commercial, we anticipate that several new applications will be pursued. Work on meso-porous materials such as M41S continues in many laboratories around the world as witnessed by the consistent increase in both M41S-related patents and publications since the discovery of MCM-41, the first member of this family, in 1989.
