WO2010099885A1

WO2010099885A1 - Process for converting oxygenates to hydrocarbons

Info

Publication number: WO2010099885A1
Application number: PCT/EP2010/001111
Authority: WO
Inventors: Pablo Beato
Original assignee: Haldor Topsøe A/S
Priority date: 2009-03-05
Filing date: 2010-02-23
Publication date: 2010-09-10

Abstract

Process for converting oxygenates to hydrocarbons comprising contacting the oxygenates with a catalyst comprising an MFI zeolite been modified by desilication treatment to exhibit an FTIR spectrum characterised by an intensity peak ratio at 3726 ± 4 cm^-1 to 3745 ± 4 cm^-1 of between 0 and 1.

Description

Title : Process for Converting Oxygenates to Hydrocarbons

The present invention relates to a process for the conversion of oxygenates to hydrocarbon compounds. In particular, the invention provides a modified MFI zeolite for use in those processes.

MFI zeolites are known as effective catalysts in the conversion of oxygenates to olefins and hydrocarbons boiling in the range of gasoline. Typically, employed oxygenates for use as feed material comprise methanol, dimethyl ether and mixtures thereof being used in the so-called Methanol to Olefins (MTO) and Methanol to Gasoline (MTG) processes. Zeolites are crystalline aluminosilicates having a crystalline structure with regularly shaped intra-crystalline channels or pores of molecular dimensions. Zeolites with an MFI structure code have a three dimensional pore systems consisting of 10-ring channels that are intersected by zigzag channels. MFI zeolites comprise ZSM-5, Silicalite and metal (M) containing versions of the same, wherein M is not aluminum.

The ZSM-5 zeolite is the archetype catalyst for the conversion of oxygenates to hydrocarbons. Gasoline and olefins can be produced with high efficiency in the MTH and MTO processes. Despite its high efficiency deactivation of the zeolite catalyst through coking is a known problem in these processes and frequent regeneration of the catalyst is required.

The general object of this invention is therefore to pro- vide a process for the conversion of oxygenates to hydro- carbons with a prolonged run time by use of a modified MFI zeolite catalyst.

It is well known that solid silica is soluble under basic conditions. However, it has been early recognized that the dissolution behaviour of different crystalline forms of silica is rather complex.

As zeolites are basically made-up from silica, the addition of a base leads to a partial dissolution of the crystalline material. Earlier investigations have shown that the selective dissolution of crystalline material could be used to improve the catalytic properties of zeolites by decreasing the silica/alumina ratio (increase the acidity) and ex- change capacities of the resulting materials (R. M. Dessau, E. W. Valyosik, N. H. Goeke, Zeolites 12 (1992) 776). US patent No. 6,184,167 discloses the modification of the porosities in ZSM-5 by treatment with NaOH or Na₂CO₃ Base leaching of zeolites in combination with an acid post- treatment is mentioned in US patent No. 5,952,259.

In none of the aforementioned publications desilication has been used with the intention to specifically remove a certain type of micro-structural defects. Also none of these publications mention a direct correlation between such defects and the deactivation behaviour of the MFI catalyst.

Fourier transformed infrared spectroscopy (FTIR) is a useful tool for the characterization of zeolite catalysts. In particular, the spectral area between 3000 and 3800 cm^"1, which is the energy range for stretching vibrational modes of hydroxyl-groups is used to characterize the acidity and/or presence of extra-framework species in the zeolite structure. In a detailed FTIR study a correlation between the absence and presence of silanol defects and the catalyst deactivation rate has been established. In particular, it has been observed that ratio between the intensity of internal/defective silanols at 3726 ± 4 cm^"1 and isolated/external silanols at 3745 ± 4 cm^"1 is decisive for the deactivation rate of the catalyst. It has further been shown that the amount of internal/defective silanols can be reduced by a post-synthesis base treatment, also frequently called desilication, of the zeolite.

Pursuant to the above findings and observations, this invention is a process for converting oxygenates to hydrocar- bons comprising contacting the oxygenates with a catalyst comprising an MFI zeolite been modified by base treatment to exhibit an FT-IR spectrum characterised by an intensity peak ratio at 3726 ± 4 cm^"1 to 3745 ± 4 cm^"1 of between 0 and 1.

Oxygenates for use in the process comprise preferably methanol, dimethyl ether and mixtures thereof, which are conventionally synthesised from synthesis gas being derived from steam reforming of natural gas and basifying of coal and biomass, which are the predominant resources for the production C₅₊ fuels like gasoline and diesel fuel and olefins for various applications.

As mentioned above, an essential feature of the invention is the reduction of internal/defective silanol groups from the MFI zeolite catalyst employed in the inventive process. MFI zeolites being suitable catalysts for the invention are any of the zeolites having a MFI-structure type. The internal/defective silanol groups are reduced in those zeolites by desilication or post-synthesis treatment of the zeolite crystals with an alkaline solution or base treatment.

The base treatment needs to be tailored for each specific type/batch of zeolite to obtain optimum results. As reported in the literature, the Si/Al ratio and the Al distribution are very important for the efficiency of desili- cation. We found that further also the morphology and crystal defects of the sample have a major influence on the desilication behaviour. This means that even if two starting materials have the same XRD-pattern, similar Si/Al ratio and similar crystal size, the resulting ZSM-5 samples after desilication might have different textural and porous properties. Depending on e.g. ratio of mol hydroxyl ions per gram zeolite more or less silanol groups are removed from zeolite crystals.

Example 1:

Sample 2 was produced by suspending 30 g of the commercially available zeolite PZ-2/100 from Zeochem® (hereafter referred to as Sample 1) in a 0.3 M solution of NaOH, at a ratio of 0.0034 mol OH^" per gram zeolite. The suspension was stirred and heated up to 70°C for 30 minutes. The suspension was then quenched in an ice-bath and the zeolite particles separated from the suspension by centrifugation. After drying at room temperature, the sample was three times ion-exchanged using each time 350 mL from a 2 L aque- ous solution containing 270 g NH₄NO₃ and 128 g NH₄OH (25%) . Each ion-exchange cycle was performed at 80°C for 2 hours. Finally, the sample was calcined in static air by ramping with 2°C/min to 550°C for 5 hours.

Example 2 : Sample 3 was produced by suspending 30 g of the commercially available zeolite PZ-2/100 (Zeochem®) in a 0.3 M solution of NaOH at a ratio of 0.01 moles OH^" per gram zeolite. The suspension was stirred and heated up to 70°C for 30 minutes. The suspension was then quenched in an ice-bath and the zeolite particles separated from the suspension by centrifugation. After drying at room temperature, the sample was three times ion-exchanged using each time 350 mL from a 2 L aqueous solution containing 270 g NH₄NO₃ and 128 g NH₄OH (25%) . Each ion-exchange cycle was performed at 80°C for 2 hours. Finally, the sample was calcined in static air by ramping with 2°C/min to 550°C for 5 hours.

Example 3 :

Sample 4 was produced by suspending 30 g of the commer- cially available zeolite PZ-2/100 (Zeochem®) in a 0.3 M solution of NaOH, at a ratio of 0.01 moles OH^" per gram zeolite. The suspension was stirred and heated up to 70°C for 30 minutes. The suspension was then quenched in an ice-bath and left in the basic solution over night at room tempera- ture. The zeolite particles were separated from the suspension by centrifugation. After drying at room temperature, the sample was three times ion-exchanged using each time 350 mL from a 2 L aqueous solution containing 270 g NH₄NO₃ and 128 g NH₄OH (25%) . Each ion-exchange cycle was per- formed at 80⁰C for 2 hours. Finally, the sample was calcined in static air by ramping with 2°C/min to 550°C for 5 hours . Example 4 :

Sample 5 is an H-ZSM-5 sample with a Si/Al ratio of 140

(determined by ICP) . It was prepared by mixing 238,74 g of tetrapropylammonium hydroxide with 139,24 g of water. The mixture was placed in an ice-bath and stirred. 3,52 g of

A1(NO₃)₃ (Aldrich®) were added and after obtaining a clear solution, 293,49 g of tetraethylorthosilicate were added dropwise. The mixture was stirred for 20 hours while the ice-bad slowly warmed up until attaining room temperature. From the clear solution 264,26 g ethanol were distilled off at reduced pressure. The synthesis gel was transferred into a Teflon lined ceramic autoclave and then heated in a microwave oven to 180°C for 1 hour. Particles were recovered by centrifugation and dried at room temperature over night. To remove the template and obtain H-ZSM-5 the sample was calcined in static air by ramping with 2°C/min to 550°C for 5 hours .

Example 5 : Sample 6 was produced by suspending 1 g of sample 5 in a

0.5 M solution of NaOH, at a ratio of 0.005 moles OH^" per gram zeolite. The suspension was stirred and heated up to 75°C for 60 minutes. The suspension was then quenched in an ice-bath and the zeolite particles separated from the sus- pension by centrifugation. After drying at room temperature, the sample was three times ion-exchanged using each time 50 mL from a 0,1 M solution of NH₄NO₃. Each ion- exchange cycle was performed at 75°C for 2 hours. Finally, the sample was calcined in static air by ramping with 2°C/min to 550°C for 24 hours. Example 6 :

Sample 7 was produced by suspending 1 g of sample 5 in a

0.2 M solution of NaOH, at a ratio of 0.01 moles OH^" per gram zeolite. The suspension was stirred and heated up to 75°C for 5 hours. The suspension was then quenched in an ice-bath and the zeolite particles separated from the suspension by centrifugation. After drying at room temperature, the sample was three times ion-exchanged using each time 50 mL from a 0,1 M solution of NH₄NO₃. Each ion- exchange cycle was performed at 75°C for 2 hours. Finally the sample was calcined in static air by ramping with 2°C/min to 550⁰C for 24 hours.

Example 7 : The thus prepared samples 1 to 7 were tested in the reaction of methanol to hydrocarbons following the gas composition of the reactor gas exit by a mass spectrometer (BalzersGAM 400 or Balzers ThermoStar) . To determine the conversion capacity of a zeolite based catalyst in the MTH reaction, the reactor was filled with a 150 mg sample of the catalyst (sieve fraction 150-300 μm) and exposed to a feed of about 15 % methanol in N2 at 350⁰C at 15 bar g, using a total flow of 307 or 350 NmI /min. These conditions correspond to a WHSV of about 3 g_Meosi/g_cath. The methanol concentration in the reaction exit gas was determined by following m/e=32 and m/e=46 signals during the experiment. The m/e=32 and m/e=4β signals reflect the amount of methanol and DME, respectively, without interference of any hydrocarbon or other compound present in the system. The con- version capacity is defined as the amount of methanol converted per gram of catalyst before complete deactivation occurs. Complete deactivation is defined as the state where no hydrocarbons are formed anymore.

FTIR experiments were performed in transmission mode at 2 cm^"1 resolution. The measurements were carried out on self supporting pellets, activated in high vacuum, oxidized and out gassed at 450°C for 2h. Spectra were collected at room temperature in He atmosphere.

The FTIR spectra of samples 1 to 4 are shown in Fig. 1.

The FTIR spectra of samples 5 to 7 are shown in Fig. 2.

In Fig. 3 the intensity peak ratio at 3726 ± 4 cm^"1 to 3745 ± 4 cm^"1 of the samples are plotted against the inverse of the conversion capacity. The sample number is attached to the data points. To further illustrate the validity of the correlation, Fig. 3 contains more samples (unlabeled data points) that have been treated in a similar way as samples 1 and 5.

Claims

1. Process for converting oxygenates to hydrocarbons comprising contacting the oxygenates with a catalyst compris- ing an MFI zeolite been modified by desilication treatment to exhibit an FTIR spectrum characterised by an intensity peak ratio at 3726 ± 4 cm^"1 to 3745 ± 4 cm^"1 of between 0 and 1.

2. Process of claim 1, wherein the oxygenates comprise methanol and/or dimethyl ether.

3. Process of claim 1, wherein the hydrocarbons comprise olefins .

4. Process of claim 1, wherein the hydrocarbons comprise C₅₊ hydrocarbons.

5. Process of claim 1, wherein the MFI zeolite comprises ZSM aluminosilicates.

6. Process according to anyone of the preceding claims, wherein the desilication treatment comprises base treatment.