CA2040514C - Crystalline silicon enhanced aluminas - Google Patents

Abstract

This invention relates to novel silicon enhanced aluminas (SEAL) and processes to prepare them. The SEAL compositions have a bulk empirical for-mula of Al2-xSixO3Fx where x varies from about 0.01 to about 0.5. This SEAL
material has a three-dimensional pore structure with the pores having diameters in the range of about 20 to abut 300A, a crystal structure characteristic of alu-mina, and where the surface of the SEAL has a higher silicon concentration than the interior of the SEAL. The SEAL is prepared by contacting an alumina with a fluorosilicate salt. This SEAL can be calcined to give a calcined seal with a formula Al2-xSixO3Fy where x is as defined above and y varies from 0.01 to x.
The calcined SEAL contains both strong and weak acid sites. These SEAL
compositions are useful as hydrocracking catalysts.

Description

20~5 1~

t "NOVEL CRYSTALLINE SILICON ENHANCED ALUMINAS"

BACKGROUND OF THE INVENTION
Alumina is a well known catalyst support and a catalyst. It is also well known that the properties of alumina can be modified in various ways such as by cogelling with silica to form a silica-alumina. For example, U.S. Patent No.
4,758,330 discloses a silica-alumina support prepared by forming a hydrogel of alumina and adding to the hydrogel an alkali metal silicate. Additionally, U.S.
Patent No. 4,806,513 discloses prepari"g an alumina with a surface coating of silica and further treated with fluorine.
In contrast to this art, applicants have prepared a composition in which some of the aluminum atoms have been removed from the alumina lattice and silicon atoms have been added to the composition. Applicants have also found 15 that the silicon can be incorporated into the alumina framework. It has addi-tionally been found that the basic structure of the starting alumina is maintained in the silicon enhanced alumina (hereinafter SEAL). Finally the SEAL composi-tion also contains fluorine.
The SEAL compositions of this invention are prepared by cGnla.;ting an alumina with a fluorosilicate salt at reaction conditions to remove some of the aluminum atoms and enhance the alumina with silicon. Although the prior art discloses the use of fluorosi'icate salts, it is in regard to l,ealiny zeolites. Thus, U.S. Patent No. 4,576,711 disclosed contacting a Y-zeolite with an aqueous solution of ammonium hexafluorosilicate. Similarly, U.S. Patent No. 4,503,023 also disclQses dealumination of zeolites, in this case L~'-210. Other relevant prior art includes:
U.S. Patent No. 4,753,910 which discloses using a water soluble fluoride during or after the aluminum removal step in order to solubilize the aluminum fluoride which is produced during the aluminum removal (dealumination step).
U.s. Patent No. 4,711,770 discloses inserting silicon at~ms into the crystal lattice of an aluminosilicate zeolite by contactiny the zeolite with a fluoro-silicate salt at a pH of about 3 to 7 and at a rate to preserve at least 60% of the crystallinity of the 7eO''tl3. This patent also discloses "~a~erials which have de-- 2 2Q~O~ ~

fect s tes in the framework.
U.S. Patent No. 4,597,956 discloses a method of removing aluminum fluoride byproducts by contacting the aluminosilicate with a soluble aluminum co" "~ound such as aluminum 51 llf~t~.
s There is no ",ention in any of these r~ere"ces that one could prepare acrystalline SEAL con position by ~real",ent of a crystalline alumina with a fluo-rosi'ic~le salt. Applica"ts are the first to have synthesized such a novel compo-sition.

SUMMARY OF THE INVENTION
This invention relates to crystalline silicon enhanced alumina (SEAL) and processes for preparing the SEAL. Accordingly, one embodiment of this inven-tion is a non-homogeneous crystalline silicon enhanced alumina (SEAL) having a bulk empirical formula of Al2 xSixO3Fx where x varies from 0.01 to 0.5, the SEAL characterized in that it has a three-dime"sional pore structure with the pores having diameters in the range of 20 to 30Q~, has a crvstal structure characterialic of alumina, and where the surface of the SEAL has a higher CGI ,cenl- alion of silicon than the interior of the SEAL
Another embodiment of this invention is a crystalline SEAL having a bulk empirical formula of Al2 xSix03Fy where x varies from 0.01 to 0.5 y varies from 0.01 to x, the SEAL characterized in that the SEAL has both strong and weak acid sites, has a cr~stal structure characteristic of alumina, has a three-dimensional pore structure with the pores having a diameter in the range of 20 to 30aA, and where the surface of the SEAL has a higher concentration of silicon than the interior of the SEAL.
Yet another embodiment of this invention is a process for pre,uari~)y a non-homogeneous cr~stalline SEAL having the e"")irical formula A12 XSiX03FX
where x varies from 0.01 to 0.5 COrllpri5ing contacting a crystalline alumina with a solution of a fluorosilicate salt at a temperature and for a time effective toproduce a SEAL having a three-dimensional pore structure with the pores having diameters in the range of 20 to 30a~, having a crystal structure characte, ialic of the crystalline alumina, and where the surface of the SEAL has a higher concenlralion of silicon than the interior of the SEAL
This invention is a process for ~.repari,1g a non-homogeneous crystalline silicon ~nl,anced alumina (SEAL) having a bulk e""~;,ical formula of ~ 4 ~

Al2 xSix03Fy, where x varies from 0.01 to 0.5 and y varies from 0.01 to x, cG",~.rising con~at;ti"g a crystalline alumina with a solution COrltaining a fluorosilicate salt at a te"~perat~Jre and for a time effective to produce a cryslalline silicon enhanced product and calcining the product at a temperature 5 and for a time effective to form the SFAL com~osition, the SFAL characte,i~ed in that it has a three-di" ,ensional pore structure with the pores having dialnelers in the range of 20 to 300A, has both strong and weak acid sites, has a crystal structure characteristic of the crystalline alumina, and where the surface of the SEAL has a higher concen~,~ion of silicon than the interior of the SEAL.
Other objects and embodiments will become more apparent after a more detailed descri~tion of the invention.

DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a crystalline silicon elll,anced alumina and a method of preparing the crystalline silicon enl,anced alumina. The alumina 15 which constitutes the starting material may be any of the aluminas well known in the art such as boehmite, pseudoboehmite, gamma alumina, delta alumina, theta alumina and alpha alumina.
The starting aluminas which are treated to produce the subject crystalline SEAL may be in the form of a powder, sphere, extrudate, irregularly shaped 2 o particles, pills, etc. For ease of solids handling, it is prefer,ed to treat formed or shaped supports such as pellets, spheres, extrudates, rings, irregularly shaped pa, licles, etc. rather than powders. A particularly preferred shape is a small di-ameter sphere. These may be produced by the well known oil-drop method which comprises forming an alumina hydrosol by any of the techniques taught 25 in the art and preterably by reacting aluminum metal with hydrochloric acid;
combining the resulting hydrosol with a suitable gelling agent; and dropping theresultant mixture into an oil bath maintained at elevated temperatures. The droplets of the mixture remain in the oil bath until they set and form hydrogel spheres. The spheres are then continuously withdrawn from the oil bath and 3 0 typically subjected to specific aging and drying treatment in oil and an alnlnGni-acal solution to further improve their physical chara~Aerislics. The resulting aged and gelled pa, licles are then washed and dried at a relativefiy low te",per-ature of 200 to 300F (93-149C) and subjected to a calcination procedure at a temperature of 850 to 1300F for a period of 1 to 20 hours. This lrealrnent ao4û5 1 4 effect~ conversion of the alumina hydrogel to the corresponding crystalline gamma-alumina. Preferred carrier materials have an apparent bulk density of 0.3 to 0.7 g/cc and surface area characteristics such that the average pore diameter is between 20 and 300 An~sl,o,l,s, the pore volume is 0.1 to 1 cc/g.
Specific details regarding the oil-drop method may bs found in U.S. Patent No.
2,620,314.
The SEAL composition can be prepared by using the same general con-ditions and aqueous solutions used to remove aluminum and insert so-called "extraneous" silicon into zeolites. These conditions are set forth in U.S. Patent Nos. 4,597,956; 4,711,770 and 4,753,910. Other references which address sili-con substitution in zeolites are "Zeolite Chemistry V-Substitution of Silicon for Aluminum in Zeolites via Reaction with Aqueous Fluorosilicate" published at page 87 of Proceedings of 6th International Zeolite Conference, 1983, edited by David Olson; Butterworth, Guildford, U.K.; "Faujasites Dealuminated with Am-monium Hexafluorosilicate: Variables Affecting the Method of Preparation" by G.
Garralon et al. appearing at page 268 of Zeolites, Vol. 8, July 1988; and U.S.
Patent No. 4,610,856.
Accordingly, the process involves contacting the crystalline alumina with an aqueous solution of a fluorosilicate salt and preferably ammonium hexafluo-rosilicate. The contacting is carried out at a temperature of 10-125C and preferably 20 to 95C, with sufficient pressure to maintain liquid phase conditions. The pH of the so1ution should be in the range of 3 to 7 and preferably from 5 to 7. The amount of ammonium hexafluorosilicate (AFS) which is added can vary considerably, but usually the ratio of AFS:alumina is inthe range of 5 to 95 weight percent and preferably from 5 to 35 weight percent.
Typically the reaction is carried out by adding the solution of ammonium hexafluorosilicate to a slurry of the alumina to be treated. The addition can becarried out incrementally or continuously at a slow rate over a period of 30 minutes to 8 hours but preferably over a period of 30 minutes to 120 minutes.
3 o After the silicate solution has been added, the resultant mixture is stirred for an additional amount of time ranging from 1 to 4 hours and preferably from 1 to 2 hours. The resultant mixture is composed of the SEAL material, an insoluble by-product powder and a liquid phase. When the starting alumina is in the form of a shaped support such as spheres, the SEAL material can be separated from ~n4~5 11 4 the in~oluble by-product powder by ordinary physical means. However, when the starting alumina is in the form of a powder or small particulates, it is difficult to physically separate the desired product from the undesirable by-product. In this case, the combined solids are washed with a soluble aluminum salt, prefer-5 ably aluminum sulfate which solubilizes the by-product powder (which is primar-ily NH4AIF4). After the SEAL product is isolated, it is washed with water at a temperature of 25 to 50C and then dried at a temperature of 100 to 120C
for a time of 4 to 24 hours.
The reaction which takes place during the process is described by the lO following chemical equation.

A123 + X(NH4)2siF6 ~ (A12 XSi~dO3FX + xNH4AlF4 ~ xNH4F
The value of x can range from 0.01 to 0.5. Chemical analysis shows that both silicon and fluorine are present in the alumina such that the alumina is en-hanced w~ith silicon. To determine the distribution of silicon, a sphere sample 15 was cut in half and the cross-section analyzed by scanning electron microscopy (SEM). The analysis showed that the silicon is concentrated in the outer one-fifth of the sphere. The distribution of the fluorine could not be detected using SEM. Separate samples were also analyzed by nuclear magnetic resonance (NMR). The fluorine NMR spectra are indicative of fluorine associated with sili-20 con, while the silicon NMR spectra are indicative of incompletely polymerizedsilica.
It should also be pointed out that not all of the silicon which is present in the reaction mixture is incorporated into the alumina. Analytical results indicate that 50 weight percent of the silicon added as ammonium hexafluorosilicate is 25 incorporated into the SEAL composition. A complete mass balance of the reaction has shown that the majorit~ of the fluoride ions are found in the fines as NH4AIF4 (when formed supports are used) and the reaction liquid as NH4F, with a small quantity of fluoride ions associated with the SEAL product. The amount of fluoride ions present in the SEAL is sufficient to charge balance all 3 o the silicon present in the SEAL. Ammonium fluoride is also present in the SEAL
The SEAL described above can be calcined at a temperature of 400 to 800C to give a SEAL with an empirical formula of (Al2 xSix)O3Fy where x is as defined above and y ranges from 0.01 to x. The SEAL that has been calcined is characterized in that it has less fluoride present in the structure than the .~

6 ~$4G~

uncalcined SEAL. Sa",~!es of calc;ned SEAL tcalcined at 500C) co",positions were analyzed by ESCA (electron spect,oscopy for chemical analysis). Both whole spl ,eres and po, lions of ground up s~l ,eres were analyzed. Since ESCA
is a surface sensitive measurement, di~tere"ces in the concentration of an s ele."enl between the whole sphere and the ground sa",~'e are indicative of nonuniform distribution of the element. One sample analyzed by this technique showed that the silicon concent,~ion on the surface of the whole sphere was 1.6 times higher than the conce"t,dtion in the ground sample. This means that the surface of the sphere has a higher silicon cGncenlralion than the interior of 10 the sphere. In the case of fluorine the surface of the whole sphere contained1.3 times more fluorine than the ground sample indicative of a more uniform fluorine distribution. Although these analyses were performed on a calcined s~",pla, there is no indication that the silicon distribution is any different in the dried but uncalcined " ,aterial.
Without wishing to be bound by any one theory, one can propose the following based on the above physical characteri~alion. At the surface of the particle, e.g., sphere, there is sufficient fluorine to charge balance all the silicon (F/Si atomic ratio is 1.2). The remaining fluorine is probably associated with the ammonium ions which are probably more uniformly distributed throughout a 20 pal liClE; or sphere.
Calci"aliG, I of the SEAL compositions affects the ammonium and fluoride contenl of the SEAL. The amount of ammonium ions detected in the SEAL
composition decreases significantly such that at 800C the mass percent of ni-trogen is less than 0.1%. The fluorine coi ,ler,~ also decreases as the calcination 25 temperature is il ,creased.
The calcined SEAL is also characterized by its acidity. Acidity of an oxide can be measured by several known methods. In the present case acidity was measured by am",oi,ia temperature programmed desorption (NH3-TPD) and conversion of 1-heptene. The greater the ability of a material to crack the 1-heptene, the greater the acidity of the catalyst. Accordingly, a SEAL co""~o-sition calcined at 400C has been found to have a much higher cracking ability than the slailing alumina or an amorphous silica-alumina material. It is also observed that calcining the SEAL composition of this invention at 800C
decreases the cracking ability of the SEAL.
The NH3-TPD test of a SEAL ",aterial calcined at 400C shows the presence of weak acid sites and a number of very strong acid sites which do 7 ~ l 4 not release a",monia until ~realsr than 600C. In cont,asl, gamma alumina only shows weak acid sites as evidenced by r~lesse of ammonia at tel)")erd-tures less than 400C. Co"si~le"t with the 1-l,eplene test the NH3-TPD of a Sr~AL sample calc;n~d at 800C shows a redlJction in the number of both 5 strong and weak acid sites. Accordingly, it is ,~ refer, e.l to calcine the Sr~AL at a t~" ,perat-lre of 400 to 600C.
The NMR of the calcined SEAL also shows dif~erences from the dried SE~AL A SEAL composition which was calcined at 400C showed a silicon NMR spectrum resembling that of silica, while an 800C calcined SEAL
o composition showed a spectrum consislent with either depoly",eri~ation of sili-con or silicon in a more aluminum rich environment. Finally, infrared spec-troscopy (IR) data of an 800C calcined SEAL composition is consistent with the premise that the silicon has been incor~.oraled into the alumina lattice.
To further characterize the active species of the SEAL composition, the effect of fluorine on alumina was deter",ined. It was deter")i"ed that the addi-tion of fluorine to an amorphous silica-alumina support did increase the crack-ing ability of the support but nowhere near the activity of the calcined SEAL
co",positions. Again, without wishing to be bound by any single theory, it ap-pears that the active site of the SEAL composition is a silicon-fluorine species2 o partially attached or incorporaled into the alumina lattice.
The SEAL materials are also characterized in that they have the same crystal structure and pore structure as the sla, ling aluminas. The starting alu-minas have a three-dimensional pore structure with the pores having diarneter~
in the range of 20 to 30Q~. The fact that the SEAL l"a~erial has retained the 25 crystal structure of the s~allil-g alumina clearly shows that the charac~erislic alumina structure has not collapsed and accordingly the pore structure of the alumina has remained intact. This is important because if the pore structure collapses reactants would not be able to diffuse through the SEAL.
The SEAL malerials of this invention find applicalio" as hydrocarbon 30 conversion catalysts either as is or after dispersion of catalytic metals thereon.
For example, these malerials can be used both in cracking and hydrocracking processes. The SFAL may be used under well known hycJroc,acking condi-tions. Typically, these conditions include a temperalure in the range of 400 to1200F (204-649C) preferably between 600 and 950F (316-510C). Reac-35 tion pressures are in the range of atmospheric to 3,500 psig (24,132 kPa 9),prererably between 200 and 3000 psig (1379 - 20 685 kPa 9). Contact times 8 ~ ~ 4 ~ 5 ~ ~
.

usual!y correspond to liquid hourly space velocities (LHSV) in the range of 0.1 hr~1 to 15 hr~1, preferably between 0.2 and 3 hr~1. Hydrogen circulation rates are in the range of 1,000 to 50,000 standard cubic feet (scf) per barrel of charge (178-8,888 std. m3/m3), preferably between 2,000 and 30,000 scf per barrel of charge (355-5,333 std. m3/m3). Suitable hydrotreating conditions are generally within the broad ranges of hydrocracking conditions set out above.
The reaction zone effluent is normally removed from the catalyst bed, subjected to partial condensation and vapor-liquid separation and then frac-tionated to recover the various components thereo~. The hydrogen, and if de-sired some or all of the unconverted heavier materials, are recycled to the re-actor. Alternatively, a two-stage flow may be employed with the unconverted material being passed into a second reactor. Catalysts of the subject invention may be used in just one stage of such a process or may be used in both reactor stages.
The SEAL materials of this invention may have dispersed thereon cat-alytic metals well known in the art and may be prepared according to the pro-cedure in U.S. Patent No. 4,422,959. The SEAL materials may also be combined with zeolites, clays, etc. in order to prepare a hydrocracking catalyst.

The following examples are presented in illustration of this invention and are not intended as undue limitations on the generally broad scope of the in-vention as set out in the appended claims.

EXAMPLE I
Heptene Cracking Test The following test procedure was used to evaluate the materials pre-pared in Examples 2-5. The heptene cracking test or the microreactor cracking test uses an electrically heated reactor which is loaded with 125 mg of 40-60 mesh (420-250 microns) particles of the catalyst to be tested. Each catalyst was dried in situ for 30 minutes at 200C using flowing hydrogen, and then sub-3 o jected to a reduction treatment of 550C in flowing hydrogen for one hour. The temperature of the reactor was then adjusted to 425C (inlet). The feed stream used to test the catalyst consists of hydrogen gas which is ~aturated with 1-heptene at 0C and atmospheric pressure. The feed stream was flowed over the catalyst at a flow rate of 500 cc/min. The effluent gas stream was analyzed .~

9 ~ 51 A

using a gas chro"l~logfapl,. What is repG,led in the exalnples that follow is weight ~ercei ll cracked product and selectivity for C3 + C4.

A sample of 1/16N s~heres of ga"""a-alumina prepared according to the s procedure in U.S. Patent No. 2,620,314 was ground to pass through a 40 mesh screen (420 micron). One hundred grams of this ma~erial was slurried in 1000 9 deioni~ed water and heated to 97C. A separate solution of 35.49 of ammonium hexafluorosilicate dissolved in 2009 deiorli~ed water was then added dropwise to the alumina slurry. This addition required 50 minutes during 10 which time the reaction mixture had a te",peralure in the range of 83-91C.
After the addition was complete the reaction mixture was digested at 90C for one hour. The solids were separated from the reaction mixture by filtration and then washed with one liter of 0.2M aluminum sulfate solution at 50C. This was followed by 400 mL deioni~ed water also at 50C. The solids were dried at 15 100C yielding 107.99 of mal~rial. The dried sample contained 2.86 mass percent F(as received (AR)), 0.725 mass percent N(AR) and 4.97 mass percent Si(AR). It had a loss on ignition (LOI) at 900C of 12.74%. A portion of this dried sample was calcined in air at 500C for 5 hours in a muffle oven. After calcination the sample contained 2.47 mass percent F(AR), 0.36 mass percent 20 N(AR) and 5.27 mass percent Si(AR). The mass percent reported is on an as received (AR) basis since calcination of the sample to remove all volatiles would remove the fluorine and nitrogen. The calcined sample had an LOI at 900C of 2.14%. The x-ray diffraction pattern of this material showed gamma-alumina as the only crystalline phase. The calcined sample was tested in the 1-he~.te"e 25 m croreactor test described in Example 1. This material showed 66 weight per- cent cracking with 95% selectivity to C3 + C4 products.

E)(AMPLE 3 A 250 mg sample of gamma-alumina obtained by grinding 1/16 spheres as per Example 2 was tested according to the procedure set forth in Example 1 30 with the exception that the flow rate of feed gas was 1000 cc/min. instead of500 cc/min. This is equivalent to testing 125 mg of catalyst at a feed rate of 500 cc/min. Analysis of this alumina showed that no silicon or fluorine was present.This sample cracked only 0.34 weight percent of the feed and the selectivity for ~Q~1 4 C3 + C4 products was only 40%.

A sample of 1/16' spl,eres of gamma-alumina prepared as per U.S.
Patent No. 2,620,314 was t,ealed with a"""onium hexafluorosilicate in this ex-5 ample. The use of the spherical form of the alumina facilitates se~.aralion of theproduct from the fines. Spherical ga"""a-alumina,110 9, was slurried in 1000 9 deionized water and heated to 62C. A solution of 70.09 ammonium hexafluorosilicale in 400 9 deioni~ed water was added dropwise to the alumina slurry over the course of 6 hours. The reaction temperature reached 84C after o the first hour and was maintained in the 82-84C range for the rest of the addi-tion. After addilion of the ammonium hexafluorosilic~te was completed, the mixture was digested for one hour at 83C before the liquid and fines were de-canted. The spheres were washed with 5 liters of deionized water and dried at 100C, yielding 135 9. The spheres were calcined at 500C for 5 hours in a muffle oven. The calc;ned ",alerial contained 4.76 mass percent Si(AR), 1.88 mass percent F(AR) and had an LOI at 900C of 21.46%. This material exhib-ited a cracking activity in the 1-heptene microreactor test of Example 1 of 34 weight percent. The C3 + C4 selectivity was 95%.

A 1109 sample of 1/16 spheres of gar"",a-alumina prepared as per U.S. Patent No. 2,620,314 was slurried in 1000 9 deionized water and heated to 80-90C. A solution of a",lno"-;Jm hexafluorosilicate, 7.00 9 in 200 9 cieiol)ked water was added to the slurry over a 3 hour period, after which time the reaction mixture was digested for an ~dcli~ional 2 hours at 86C. The spheres were separated from the reactiorl mixture by decantation, washed with 5.5 liters of deionized water and then dried at 100C. Finally, dried spheres were calcined in a muf~e oven for 5 hours at 500C. The product contained 0.92 mass percent Si(AR) and 1.88 mass percent F(AR), and had an LOI at 900C of 8.27%. The calcined sample was tested using the procedure in Example 1 and 3 o showed a cracking conversion of 15% and a C3 + C4 selectivity of 93%.

Claims (5)

1. A non-homogeneous crystalline silicon enhanced alumina (SEAL) having a bulk empirical formula of Al2-xSixO3Fy where x varies from 0.01 to 0.5,y varies from 0.01 to x, the SEAL characterized in that it has both strong and weak acid sites, has a crystal structure characteristic of alumina, has a three-dimensional pore structure with pores having diameters in the range of 20 to 300.ANG., and where the surface of the SEAL has a higher concentration of silicon than the interior of the SEAL.

2. the SEAL of Claim 4 where the SEAL has the crystal structure of gamma-alumina.

3. A process for preparing the SEAL of Claim 1 or 2 comprising contacting a crystalline alumina with a solution of a fluorosilicate salt at a temperature and for a time effective to produce a SEAL having a three-dimensional pore structure with the pores having diameters in the range of 20 to 300.ANG., having a crystal structure characteristic of the crystalline alumina, and where the surface of the SEAL has a higher concentration of silicon than the interior of the SEAL.

4. The process of Claim 3 where the product of the contacting step is calcined at a temperature of 400 to 800°C for about 1 to about 5 hours.

5. A process for the conversion of a hydrocarbon comprising contacting the hydrocarbon with the SEAL of Claim 1 or 2 at hydrocarbon conversion conditions.